**Abstract**

Depression and anxiety currently rank as the second and fifth most common causes worldwide of years lived with disability-a reality that has intensified the search for new treatments. There are many studies of herbal extracts and secondary metabolites from plants used in traditional medicine due to their antidepressant and anxiolytic properties. Clinical and preclinical studies about some of the mechanisms of action of metabolites like alkaloids, terpenes, flavonoids, and sterols, among others, have documented effects similar to those produced by clinically effective drugs. These metabolites have shown anxiolytic and antidepressant effects in various experimental models of anxiety by interacting with γ-aminobutyric acid subtype A receptors (GABAA-receptors) and by stimulating the serotonergic, noradrenergic, and dopaminergic neurotransmitter systems. These pharmacological effects can be attributed to plant metabolites that share structural similarities with monoamines, which allow them to bind to receptors. The objective of this chapter is to summarize the various mechanisms of action that have been identified in secondary metabolites with anxiolytic and antidepressant properties. Terpenes, alkaloids, flavonoids, and sterols can interact at different levels of the neurotransmission systems involved in the neurobiology of anxiety and depression, suggesting their potential for treating these mental illnesses.

**Keywords:** antidepressant, anxiolytic, active metabolites, plant extracts, herbal medicines

### **1. Introduction**

According to the Global Burden of Disease, depression and anxiety are currently the second and fifth most common causes worldwide of years lived with disability in both sexes in the age range of 15–49 years [1]. In 2015, 4.4% (322 million people) of the world's population suffered from depressive disorders, while 3.6% (264 million) were affected by anxiety [2]. In that year, the World Health Organization (WHO) estimated that by 2020 depression would be the second leading cause of

disability; thus, its prediction has been confirmed. Depression is characterized by persistent sadness and a loss of interest in activities that an individual normally enjoys, accompanied by periods of at least 2 weeks marked by the inability to perform everyday activities [2]. Anxiety, in turn, is defined as an emotion expressed in response to stressful, dangerous, or unfamiliar situations, or some unidentified factor, that is, the feeling of unease, distress, or dread that one feels in the face of a significant event. A certain level of anxiety is necessary to keep us alert and aware, but for those who suffer from anxiety disorders, it can be totally debilitating [3]. Current pharmacological treatments for depressive disorders are mainly based on selective serotonin reuptake inhibitors (SSRIs), serotonin (5-HT) and noradrenaline (NE) reuptake inhibitors (SNRIs), and monoamine oxidase inhibitors (MAOIs), all of which act by increasing short-term levels of neurotransmitters in the brain. One consequence of treatment is the desensitization of receptors, for example, 5-HT1A, with a downregulation of autoreceptors, but no changes in the postsynaptic receptors, which leads to the recovery of neuronal activity in the long term [4]. These changes are associated with the long latency to the onset of antidepressant effects. However, up to 70% of depressed patients have residual symptoms [5], and few options exist for transitioning treatment-resistant sufferers to alternative therapies that operate through distinct mechanisms [6]. It is important to note that conventional antidepressants produce significant side effects, such as nausea and vomiting, insomnia, agitation, fatigue, sedation, sexual dysfunction, headaches, and weight gain, which contribute to poor patient compliance and, in some cases, abandonment of treatment [7]. This occurs under such anxiolytic treatments as benzodiazepine (a GABAA receptor agonist) and SSRIs [8] and is the main cause of the increasing demand for alternative medicines, such as medicinal plants, to alleviate the symptoms of these psychiatric disorders. However, reports of adverse reactions to products of this kind have increased [9], leading WHO to publish the document, "The WHO's traditional medicine strategy: 2014-2023", which outlines a global approach to fomenting the appropriate integration, regulation, and supervision of natural substances. This paper will be useful in countries seeking to develop a proactive policy toward this important and expanding area of health care and will contribute to the use of herbal medicines of proven quality, safety, and efficacy, providing quality medical care to all people [10]. Recent decades have witnessed efforts to gather scientific evidence that validates the efficacy of plants commonly used for their antidepressant and anxiolytic properties [11], but research has been insufficient because of the wide range of plants available worldwide. We lack solid scientific data on the neurochemical pathways and mechanisms of action of medicinal plants or their active metabolites because few clinical studies have addressed these issues. Also, reports of adverse reactions to medicinal plants [9] may reflect the broad variety of active metabolites they contain, thus highlighting the need for preclinical and clinical studies that evaluate the possible biological activity of compounds isolated from plants or standardized crude extracts, their mechanisms of action, and possible toxicity.

*extracts, fractions, isolated compounds, or derivatives using ex vivo, in vitro, and in vivo assays (e.g., preliminary pharmacological screening, classic animal models of anxiety like the light dark box (LDB) or elevated plus-maze (EPM) tests, etc., or the forced swim test (FST) and tail suspension tests (TST), among others).* These tools have allowed researchers to analyze the possible metabolites responsible for the anxiolytic or antidepressant properties of plants used by different populations, and identify how their mechanisms of action affect the functioning of the central nervous system (CNS). Their studies contribute to advancing scientific understanding of the neurobiology of depression and anxiety, and to developing new

*Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant…*

pharmacological treatments that may favorably impact public health.

*DOI: http://dx.doi.org/10.5772/intechopen.90919*

overstimulation that may generate undesirable collateral effects.

and (ii) standard chemical tests performed with specific metabolites.

**2. Terpenes with antidepressant effects**

of food and water ingested were reported.

**87**

Plants used in traditional medicine contain compounds in their secondary metabolism [13] such as alkaloids, phenols, sterols, carbohydrates, tannins, terpenes, and phytoalexins, all of which have important biological activities [14]. The most widely studied metabolites are terpenes, flavonoids, alkaloids, and sterols, whose mechanisms of action stimulate the serotonergic, noradrenergic, dopaminergic, or GABAergic neurotransmission systems, acting on receptors or the synthetic pathways of neurotransmitters and their transporters. However, they may also stimulate other neurotransmission systems. For example, terpenes can stimulate at the same time serotonergic, dopaminergic, and noradrenergic neurotransmission systems [12] that can produce a similar effect on mood regulation, perhaps leading to an

This chapter reviews and discusses the findings from research on several metabolites of medicinal plants that have shown potential anxiolytic and antidepressant activities once screened for their biological mechanisms at various levels: receptor, transporter, synthesis, gene, protein, or metabolic. The studies analyzed were identified by a preliminary search in PubMed, Scopus and Ovid for articles on (i) the dose effects and possible mechanisms of action of metabolite(s) isolated from parts of plants with previously identified anxiolytic or antidepressant effects;

Terpenes are formed by the union of isoprene units (5 C atoms). Their classification depends on the number of units they contain: 10 C terpenes (two units) are called monoterpenes, while 15 C terpenes (three units) are called sesquiterpenes, and those with 20 C are diterpenes. Triterpenes have 30 C, tetraterpenes have 40 C, and polyterpenes are those with over 8 isoprene units. Studies have evaluated the effect of terpenes isolated from plants, including rosmanol from *Rosmarinus officinalis*, ursolic acid, and oleanolic acid; carnosol from *Artemisa indica*; and linalool and β-pinene from *Litsea glaucescens*. All these terpenes have proven antidepressant effects. Abdelhalim et al. [15] isolated rosmanol, an ethyl acetate diterpene, from *R. officinalis*. A single acute dose of 30 or 100 mg/kg i.p. of rosmanol in male Swiss mice produced an antidepressant effect on the FST and TST. The 100-mg/kg dose produced an effect similar to that of a 60-mg/kg dose of imipramine on the FST. Their study also tested the acute toxicity of administering 50, 150, and 200 mg/kg, i.p., of rosmanol. Some signs of toxic effects on grooming behavior were observed, as well as hyperactivity, sedation, respiratory arrest, convulsions, and locomotor activity. However, no cases of lethality or variations in the amount

Other terpenes with antidepressant properties include phenolic diterpene, carnosol, and pentacyclic triterpenoids like betulinic, oleanolic, and ursolic acids.

Fajemiroye et al. [12] proposed a hypothetical model for identifying medicinal plant extracts and phytoconstituents with anxiolytic and/or antidepressant properties that is currently used by most researchers: *(i) select medicinal plants with anxiolytic and/or antidepressant potential based on local reports; (II) prepare standard crude extracts; (III) perform phytochemical studies that include sequential partitioning of crude extracts, purification and isolation of phytoconstituents, chemical elucidation or characterization of the isolates, structural modifications or syntheses of new compounds based on chemical structure of their isolates; and (IV) conduct pharmacological analyses of the anti-anxiety and antidepressant properties of the standard crude*

*Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant… DOI: http://dx.doi.org/10.5772/intechopen.90919*

*extracts, fractions, isolated compounds, or derivatives using ex vivo, in vitro, and in vivo assays (e.g., preliminary pharmacological screening, classic animal models of anxiety like the light dark box (LDB) or elevated plus-maze (EPM) tests, etc., or the forced swim test (FST) and tail suspension tests (TST), among others).* These tools have allowed researchers to analyze the possible metabolites responsible for the anxiolytic or antidepressant properties of plants used by different populations, and identify how their mechanisms of action affect the functioning of the central nervous system (CNS). Their studies contribute to advancing scientific understanding of the neurobiology of depression and anxiety, and to developing new pharmacological treatments that may favorably impact public health.

Plants used in traditional medicine contain compounds in their secondary metabolism [13] such as alkaloids, phenols, sterols, carbohydrates, tannins, terpenes, and phytoalexins, all of which have important biological activities [14]. The most widely studied metabolites are terpenes, flavonoids, alkaloids, and sterols, whose mechanisms of action stimulate the serotonergic, noradrenergic, dopaminergic, or GABAergic neurotransmission systems, acting on receptors or the synthetic pathways of neurotransmitters and their transporters. However, they may also stimulate other neurotransmission systems. For example, terpenes can stimulate at the same time serotonergic, dopaminergic, and noradrenergic neurotransmission systems [12] that can produce a similar effect on mood regulation, perhaps leading to an overstimulation that may generate undesirable collateral effects.

This chapter reviews and discusses the findings from research on several metabolites of medicinal plants that have shown potential anxiolytic and antidepressant activities once screened for their biological mechanisms at various levels: receptor, transporter, synthesis, gene, protein, or metabolic. The studies analyzed were identified by a preliminary search in PubMed, Scopus and Ovid for articles on (i) the dose effects and possible mechanisms of action of metabolite(s) isolated from parts of plants with previously identified anxiolytic or antidepressant effects; and (ii) standard chemical tests performed with specific metabolites.

#### **2. Terpenes with antidepressant effects**

Terpenes are formed by the union of isoprene units (5 C atoms). Their classification depends on the number of units they contain: 10 C terpenes (two units) are called monoterpenes, while 15 C terpenes (three units) are called sesquiterpenes, and those with 20 C are diterpenes. Triterpenes have 30 C, tetraterpenes have 40 C, and polyterpenes are those with over 8 isoprene units. Studies have evaluated the effect of terpenes isolated from plants, including rosmanol from *Rosmarinus officinalis*, ursolic acid, and oleanolic acid; carnosol from *Artemisa indica*; and linalool and β-pinene from *Litsea glaucescens*. All these terpenes have proven antidepressant effects. Abdelhalim et al. [15] isolated rosmanol, an ethyl acetate diterpene, from *R. officinalis*. A single acute dose of 30 or 100 mg/kg i.p. of rosmanol in male Swiss mice produced an antidepressant effect on the FST and TST. The 100-mg/kg dose produced an effect similar to that of a 60-mg/kg dose of imipramine on the FST. Their study also tested the acute toxicity of administering 50, 150, and 200 mg/kg, i.p., of rosmanol. Some signs of toxic effects on grooming behavior were observed, as well as hyperactivity, sedation, respiratory arrest, convulsions, and locomotor activity. However, no cases of lethality or variations in the amount of food and water ingested were reported.

Other terpenes with antidepressant properties include phenolic diterpene, carnosol, and pentacyclic triterpenoids like betulinic, oleanolic, and ursolic acids.

disability; thus, its prediction has been confirmed. Depression is characterized by persistent sadness and a loss of interest in activities that an individual normally enjoys, accompanied by periods of at least 2 weeks marked by the inability to perform everyday activities [2]. Anxiety, in turn, is defined as an emotion expressed in response to stressful, dangerous, or unfamiliar situations, or some unidentified factor, that is, the feeling of unease, distress, or dread that one feels in the face of a significant event. A certain level of anxiety is necessary to keep us alert and aware, but for those who suffer from anxiety disorders, it can be totally debilitating [3]. Current pharmacological treatments for depressive disorders are mainly based on selective serotonin reuptake inhibitors (SSRIs), serotonin (5-HT) and noradrenaline (NE) reuptake inhibitors (SNRIs), and monoamine oxidase inhibitors (MAOIs), all of which act by increasing short-term levels of neurotransmitters in the brain. One consequence of treatment is the desensitization of receptors, for example, 5-HT1A, with a downregulation of autoreceptors, but no changes in the postsynaptic receptors, which leads to the recovery of neuronal activity in the long term [4]. These changes are associated with the long latency to the onset of antidepressant effects. However, up to 70% of depressed patients have residual symptoms [5], and few options exist for transitioning treatment-resistant sufferers to alternative therapies that operate through distinct mechanisms [6]. It is important to note that conventional antidepressants produce significant side effects, such as nausea and vomiting, insomnia, agitation, fatigue, sedation, sexual dysfunction, headaches, and weight gain, which contribute to poor patient compliance and, in some cases, abandonment of treatment [7]. This occurs under such anxiolytic treatments as benzodiazepine (a GABAA receptor agonist) and SSRIs [8] and is the main cause of the increasing demand for alternative medicines, such as medicinal plants, to alleviate the symptoms of these psychiatric disorders. However, reports of adverse reactions to products of this kind have increased [9], leading WHO to publish the document, "The WHO's traditional medicine strategy: 2014-2023", which outlines a global approach to fomenting the appropriate integration, regulation, and supervision of natural substances. This paper will be useful in countries seeking to develop a proactive policy toward this important and expanding area of health care and will contribute to the use of herbal medicines of proven quality, safety, and efficacy, providing quality medical care to all people [10]. Recent decades have witnessed efforts to gather scientific evidence that validates the efficacy of plants commonly used for their antidepressant and anxiolytic properties [11], but research has been insufficient because of the wide range of plants available worldwide. We lack solid scientific data on the neurochemical pathways and mechanisms of action of medicinal plants or their active metabolites because few clinical studies have addressed these issues. Also, reports of adverse reactions to medicinal plants [9] may reflect the broad variety of active metabolites they contain, thus highlighting the need for preclinical and clinical studies that evaluate the possible biological activity of compounds isolated from plants or standardized crude

*Behavioral Pharmacology - From Basic to Clinical Research*

extracts, their mechanisms of action, and possible toxicity.

**86**

Fajemiroye et al. [12] proposed a hypothetical model for identifying medicinal plant extracts and phytoconstituents with anxiolytic and/or antidepressant properties that is currently used by most researchers: *(i) select medicinal plants with anxiolytic and/or antidepressant potential based on local reports; (II) prepare standard crude extracts; (III) perform phytochemical studies that include sequential partitioning of crude extracts, purification and isolation of phytoconstituents, chemical elucidation or characterization of the isolates, structural modifications or syntheses of new compounds based on chemical structure of their isolates; and (IV) conduct pharmacological analyses of the anti-anxiety and antidepressant properties of the standard crude*


**Metabolite**

**89**

**Terpenes**

Rosmanol

Ethyl acetate extract of

i.p. 30 and 100 mg/kg a

Male Swiss mice (20–

•

Forced

Antidepressant

 effect with

NE

[15]

*DOI: http://dx.doi.org/10.5772/intechopen.90919*

swim test

both doses 100 mg/kg like with

> • Tail

suspension

imipramine

Antidepressant

both doses

 effect with

test

30 g)

single dose

*Rosmarinus officinalis*

(Rosemary)

Linalool

Found shrub such as *Litsea*

i.p. 100 mg/kg a single

Male ICR mice

•

Forced

Antidepressant

 effect with

Serotonergic

by 5-HT1A receptors

Noradrenergic

by NE

*Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant…*

α2-adrenoceptor

 mechanism

 mechanism

[22]

[21]

swim test

100 mg/kg Antidepressant

 effect with

> • Tail

suspension

100 and 200 mg/kg

test

(27–33 g) Male Swiss mice

(30–40 g)

dose

i.p. (10, 50, 100, and

200 mg/kg, a single dose)

*glaucescens* in these studies

used chemical standard

()-Linalool

(1S)-()-β-pinene

Found shrub such as *Litsea*

i.p. 100 mg/kg a single

Male ICR mice

•

Forced

Antidepressant

 effect with

Serotonergic

by 5-HT1A

Noradrenergic

by activation of the β-

adrenoceptor

noradrenergic

neurotransmission

Dopaminergic

by activation of D1

receptors

 mechanisms

 and regulate

 mechanism

receptors

 mechanism

[22]

swim test

100 mg/kg

(27–33 g)

dose

*glaucescens*

**Extract from plant/part**

**Posology** 

**way, doses, and duration**

**of treatment)**

**(administration**

**Experimental**

**Experimental**

**Identified effect**

**Mechanisms**

 **of action**

 **References**

**model**

**subject**


#### *Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant… DOI: http://dx.doi.org/10.5772/intechopen.90919*

**Metabolite**

**88**

**Alkaloids**

Harmine Mitragynine Evodiamine

Evodiamine

*fructus*, *Evodia rutaecarpa*

Benth., Rutaceae)

> Protopine

> Protopine

was synthesized from

Protopine

*scandens*

*Dactylicapnos*

hydrochloride

Doses of 3.75 mg/kg,

Male BALB/cj mice

• Tail

Antidepressant

 effects

Inhibitor of serotonin

[71]

transporter and

noradrenaline

 transporter

suspension

with 30 mg/kg

test

(20–24 g)

7.5 mg/kg and 30 mg/kg

 (*Evodia*

v.o 10–20 mg/kg for

Male Sprague–

•

Sucrose

Antidepressant-like

 effect

NE

[69]

preference

in the CUMS model

test

•

Forced

swim test

Dawley rats (180–

220 g)

14 days

Separated and purified

i.p 5, 10, and 30 mg/kg a

Male mice

• Tail

Antidepressant

 effect with

Modulating

[67]

*Behavioral Pharmacology - From Basic to Clinical Research*

neuroendocrine

 axis HPA

suspension

10 and 30 mg/kg

test

•

Forced

swim test

single dose

from

*Mitragyna speciosa*

Harmine

i.p. 15 mg/kg/day for

Stressed rats CUMS

•

Sucrose

Antidepressant-like

 effect

NE

[62]

NE

preference

in the CUMS model

test

Antidepressant

 effect with

> •

Forced

both doses

> swim test

(60 days old)

No stressed rats

(60 days old)

7 days

i.p. 5, 10, and 15 mg/kg/

day for 7 days

**Extract from plant/part**

**Posology** 

**way, doses, and duration**

**of treatment)**

**(administration**

**Experimental**

**Experimental**

**Identified effect**

**Mechanisms**

 **of action**

 **References**

**model**

**subject**


**Metabolite**

**91**

Ursolic acid

Crude extract of stems and

leaves of *officinalis* and identified in

ethyl acetate fraction

> Ursolic acid

Chemical standard

 0.001 and 0.1 mg/kg

 Swiss mice (35–45 g,

• Tail

Antidepressant

 effects

Suggest mechanism

[19]

serotonergic

of 5-HT and activation of

5-HT1A

*Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant…*

 by synthesis

suspension

with 0.1 mg/kg alone and

test

in combination

 with

pretreatment

100 mg/kg, i.p., 4 days 0.001 mg/kg of ursolic acid

and 5 mg/kg of fluoxetine

produced a

pharmacological

Terpinene-4-ol

*Origanum majorana*

The OMEO was made up

Male mice (20–30 g) •

Forced

Antidepressant

 effect with

Dopaminergic

by activation of D1 and D2

receptors

Serotonergic

by activation of 5-HT1A

and 5-HT2A increases 5-HT synthesis

Noradrenergic

by activation of α1 and α2-

adrenoceptors

They regulate brain

monoamine

neurotransmitters

 mechanism

receptors,

 mechanisms

 mechanisms

[20]

swim test

40 and 80 mg/kg of OMEO

80 mg/kg de OMEO was

more effective

of 24 compounds

Terpinene-4-ol

γ-terpinene (12.88%) Transsabinenehydrate

(8.47%)

α-terpinene (7.98%)

sabinene (6.21%) α-terpineol (5.25%)

α-terpinolene

sabinenehydrate

β-phellandrene

ρ-cymene (2.32%)

trans-caryophyllene

(2.31%)

(E)-p-menth-2-en-1-ol

 (2.64%)

 (2.92%)

 (3.36%) Cis-

 (32.69%)

essential oil (OMEO)

*γ*-terpinene

Transsabinenehydrate

*α*-terpinene

*α*-terpinolene

Cis-sabinenehydrate

*β*-phellandrene

*ρ*-cymene

trans-caryophyllene

(E)-p-menth-2-en-1-ol

bicyclogermacrene

*β*-myrcene

 synergism

 with PCPA

55–60 days old)

of either sex homogeneously

distributed

*Rosmarinus*

**Extract from plant/part**

**Posology** 

**way, doses, and duration**

**of treatment)**

p.o. 0.1, 1, and 10 mg/kg,

Male Swiss mice (20–

•

Forced

Antidepressant

 effect with

NE

[18]

Suggest a probable

dopaminergic

 mechanism

*DOI: http://dx.doi.org/10.5772/intechopen.90919*

swim test

10 mg/kg similar to

bupropion (10 mg/kg)

Antidepressant

 effect with

by D1 and D2 receptor

• Tail suspension

test

0.01 and 0.1 mg/kg were

similar to fluoxetine (10 mg/kg), imipramine (1 mg/kg), and bupropion

(10 mg/kg)

30 g, 60–70 days old)

in a single dose

p.o. 0.001, 0.01, 0.1, 1, and

10 mg/kg, in a single dose

**(administration**

**Experimental**

**Experimental**

**Identified effect**

**Mechanisms**

 **of action**

 **References**

**model**

**subject**


*Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant… DOI: http://dx.doi.org/10.5772/intechopen.90919*

**Metabolite**

**90**

Ursolic acid

Methanol extract of *Artemisa indica* in the

chloroform fraction

(Mugwort)

Oleanolic acid

Carnosol

Carnosol

Crude extract of stems and

leaves of *officinalis* and identified

and isolation of the hexane

fraction (carnosol) and of

the ethyl acetate fraction

(betulinic acid)

*Rosmarinus*

p.o. 0.01, 0.1, 1, and

Male Swiss mice (45–

• Tail

Carnosol produced an

NE

[16]

suspension

antidepressant

 effect with

test

0.01 and 0.1 mg/kg, while

betulinic acid only with

10 mg/kg

50 g, 60–70 days old)

10 mg/kg, in a single dose

p.o. 0.1, 1, and 10 mg/kg,

in a single dose

> Betulinic acid

**Extract from plant/part**

**Posology** 

**way, doses, and duration**

**of treatment)** i.p. 10, 30, and 100 mg/kg

Male Swiss mice

•

Forced

Antidepressant

 effect with

Suggest a GABAergic

[17]

mechanisms

GABAA

receptors

 in α1β2γ2L

> swim test

all doses of three

metabolites without effect

> • Tail

suspension

in locomotor activity

100 mg/kg of ursolic acid

was similar to imipramine

(60 mg/kg) Antidepressant

all doses of three

metabolites

 effect with

*Behavioral Pharmacology - From Basic to Clinical Research*

test

(20–30 g)

in a single dose for

metabolite

independently

**(administration**

**Experimental**

**Experimental**

**Identified effect**

**Mechanisms**

 **of action**

 **References**

**model**

**subject**


**Metabolite**

**93**

Quercetin

Genistein

Baicalin Naringin

Commercial

 Naringin

i.p. 2.5, 5, and 10 mg/kg,

Male Swiss mice

•

Forced

Antidepressant

 effects

 Maybe by increased neuro-

[44]

antioxidant and cholinergic

activities and it

significantly

 decreased

malondialdehyde

nitrite suggesting the

involvement

nitrosative pathways

 of oxidative/

concentrations,

 and

swim test

once daily for 7 days

(Sigma-Aldrich)

Commercial

(Nanjing, China)

 flavonoid

60 mg/kg

Male ICR mice

•

Sucrose

Antidepressant

 effect

 Through a mechanism to

promote the

of neurons, and the

transformation

neurons and their survival

via the Akt/FOXG1

pathway

 into mature

differentiation

[43]

*Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant…*

preference

test

Commercial

Lang

Biotechnology)

 Genistein (Ze

p.o. 5–45 mg/kg, (once per

Male ICR mice

•

Forced

Antidepressant

 effect with

Genistein was potentiated

[33]

by co-treatment

8-OH-DPAT

receptor agonist)

 (5-HT1A

 with

> swim test

15 and 45 mg/kg

45 mg/kg is similar to

> • Tail

suspension

imipramine 15 mg/kg

test

day for 3 weeks)

4'-O-glucoside

Bulbs of *Allium cepa* var

**Extract from plant/part**

**Posology** 

**way, doses, and duration**

**of treatment)**

 p.o. 50 and 100 mg/kg

Swiss albino mice of

•

Forced

Antidepressant

 effect with

Effect might be attributed

[36]

to its anti-oxidant properties, MAO-A

inhibition, and consequent

increase in brain 5-HT

levels

*DOI: http://dx.doi.org/10.5772/intechopen.90919*

swim test

50 and 100 mg/kg

either sex

extract. Once daily for

7 days

**(administration**

**Experimental**

**Experimental**

**Identified effect**

**Mechanisms**

 **of action**

 **References**

**model**

**subject**


*Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant… DOI: http://dx.doi.org/10.5772/intechopen.90919*

**Metabolite**

**92**

**Extract from plant/part**

**Posology** 

**way, doses, and duration**

**of treatment)**

(2.25%)

Bicyclogermacrene

(1.60%)

β-myrcene (1.49%)

These anointed for 92.37%

of the yield, while the

other detected components represented <1.0% in each

case.

10, 20, 40, and 80 mg/kg of OMEO in a single dose

> Hesperidin

Quercetin Kaempferitrin

Isolation of aerial parts of

1.0, 5.0, 10, and 20 mg/kg

Male Swiss Webster

• Tail

Antidepressant

 effect with

The activation of 5HT1A

[95]

receptors and the synthesis

of 5-HT are mandatory to

produce effect of

noradrenergic

by α2- adrenoceptor

agonism

 mechanism

suspension

5.0, 10, and 20 mg/kg

doses

test

•

Forced

swim test

mice

doses

*Justicia spicigera*

Commercial

(Sigma Chemical)

 flavonoid

25 mg/kg, for 14 days

Bulbectomized

 mice • Tail suspension

25 mg/kg

Antidepressant

 effect with

Lipid

content (LOOH) levels

were reversed by

quercetin;

like effects seem to occur

by modulation of

glutamate and nitric oxide

antidepressant-

hydroperoxide

[37]

test

•

Forced

swim test

Commercial

(Sigma Chemical)

 flavonoid

i.p 0.01, 0.1, 0.3, and 1 mg/

Male Swiss mice

• Tail

Antidepressant

 effect with

Increase in BDNF

[42]

*Behavioral Pharmacology - From Basic to Clinical Research*

concentrations

hippocampus

 in the

suspension

all doses evaluated

test

kg, for 21 days

**(administration**

**Experimental**

**Experimental**

**Identified effect**

**Mechanisms**

 **of action**

 **References**

**model**

**subject**


**Metabolite**

**95**

β-Amirine

Hexane–ethyl

extracts from *heptaphyllum*

 acetate

p.o. 1, 2.5, and 5 mg/kg

 Male Swiss mice (20–

•

Forced

Antidepressant

 effect with

NE

[94]

*DOI: http://dx.doi.org/10.5772/intechopen.90919*

swim test

dose 2.5 and 5 mg/kg

30 g)

*Protium*

α-Amirine

*NE = no explorated.*

**Table 1.** *Secondary metabolites*

 *with* 

*antidepressant*

 *properties in preclinical*

 *study.*

*Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant…*

**Extract from plant/part**

**Posology** 

**way, doses, and duration**

**of treatment)**

**(administration**

**Experimental**

**Experimental**

**Identified effect**

**Mechanisms**

 **of action**

 **References**

**model**

**subject**

*Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant… DOI: http://dx.doi.org/10.5772/intechopen.90919*


#### **Table 1.**

*Secondary metabolites with antidepressant properties in preclinical study.*

**Metabolite**

**94**

Fisetin **Sterols**

Fucosterol β-Sitosterol **α-Spinasterol**

Toronto Research

i.p. 1 and 2 mg/kg

 Male albino Swiss

•

Forced

Antidepressant

 effect with

TRPV1 antagonistic

 effects [90]

> swim test

dose 1 and 2 mg/kg

mice (23–25 g)

Chemicals Inc., Canada

Ethanol extract of

i.p. 10, 20, and 30 mg/kg,

Male Kun Ming mice

•

Forced

Antidepressant

 effect with

Increase in CNS 5-HT, NA

[93]

β-sitosterol

β-sitosterol increases

5-HIAA levels

swim test

dose 20 mg/kg and 200 mg/kg sterols total

> • Tail

suspension

like fluoxetine

test

(20

 2 g)

for 7 days

*Sargassum horneri*

Ethanol extract of

i.p. 10, 20, 30, and

Male Balb/e mice

•

Forced

Antidepressant

 effect with

Increase in CNS 5HT, NA,

[92]

*Behavioral Pharmacology - From Basic to Clinical Research*

and BDNF levels

> swim test

dose 20 and 30 mg/kg like

fluoxetine

• Tail suspension

test

(20

 2 g)

40 mg/kg

*Sargassum fusiforme* (algas)

Commercial

 Fisetin

p.o. 5 mg/kg, with

Male adult ICR mice • Tail suspension

Antidepressant

 effect

 Maybe by activation of

[40]

TrkB signaling in the

hippocampus

pro-neurogenesis

fisetin in the

 effects of

hippocampus

 suggesting

test

•

Forced

swim test

1–2 weeks of treatment

(Sigma-Aldrich)

**Extract from plant/part**

**Posology** 

**way, doses, and duration**

**of treatment)**

**(administration**

**Experimental**

**Experimental**

**Identified effect**

**Mechanisms**

 **of action**

 **References**

**model**

**subject**


**Metabolite**

**97**

Koumine Berberine

alkaloid)

**Terpenes**

Rosmanol

Ethyl acetate extract of

i.p. 1, 10, 30, and

Male Swiss

•

Elevated

Anxiolytic effect with 10,

Suggest a GABAergic

[15]

*Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant…*

mechanisms

receptors PTZ (20 mg/kg),

but not Flumazenil (2.5 mg/kg) blocked the

anxiolytic effect of 10 mg/

kg of rosmanol in the

elevated plus maze

NE

[28]

 in GABAA

plus maze

30, and 100 mg/kg

mice

100 mg/kg

Only one

administration

(20–30 g)

•

Light/dark

10 and 30 mg/kg like

box

diazepam (1 mg/kg)

*Rosmarinus*

(Rosemary)

Linalool oxide

Frequently found

Inhalation of linalool

Male Swiss

•

Elevated

Anxiolytic effect with all

> plus maze

doses without effect in

mice

(20–30 g)

•

Light/dark

coordination

 motriz

box

•

Rota-rod

test

oxide emulsion 0.65%,

1.25%, 2.5%, and 5.0%

w/w.

7 min of exposure to the

inhalation chamber

aromatic plants such as

*Lavandula angustifolia* Mill., *Melissa officinalis* L.,

Rosmarinus

and

DC

*Cymbopogon*

 *citratus*

 officinalis L.,

 *officinalis*

(isoquinoline

Berberine

hydrochloride

 v.o. 100 mg/kg/day for

Male Wistar

•

Elevated

Anxiety-like

addiction

 behaviors in

Modulation

 of

[59]

neuropeptide

its receptor

 oxytocin and

plus maze

rats

(200–250 g)

21 days

Separated and purified

s.c. 0.167, 0.5, or 1.5 mg/

Male Wistar

• Vogel

Anxiolytic effect with all

NE

[74]

*DOI: http://dx.doi.org/10.5772/intechopen.90919*

conflict test

doses

rats

(6–8 weeks

and

180–220 g)

kg Only one

administration

from

*Gelsemium elegans*

**Extract from plant/part**

**Posology**

**Experimental**

**Experimental**

**Identified effect**

**Mechanisms**

 **of action**

 **References**

**model**

**(administration**

**doses and duration of**

**treatment)**

 **way,**

**subject**


### *Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant… DOI: http://dx.doi.org/10.5772/intechopen.90919*

**Metabolite**

**96**

**Alkaloids**

Gelsemine

Isolated from *elegans* Benth

*Gelsemium*

s.c. gelsemine, koumine

Male mice

•

Elevated

Anxiolytic activity with all

Mechanism

involved in the glycine

receptor

*Behavioral Pharmacology - From Basic to Clinical Research*

 may be

[72]

(24–26 g)

plus maze

doses

•

Light/dark

box

and gelsevirine 0.4, 2,

and 10 mg/kg

s.c. gelsenicine

20 μg/kg

 0.8, 4, or

Koumine

Gelsevirine

Gelsemine

Hydroalcoholic

*Gelsemium elegans* Benth

 solution of

i.p. 500 μ<sup>l</sup> (10–6, 10–10,

Male Sprague–

•

Elevated

Anxiolytic activity with

NE

[73]

10–6 and 10–10

M

Dawley rats

plus maze

> (250–300 g)

or 10–14 M) for 7 days

**Extract from plant/part**

**Posology**

**Experimental**

**Experimental**

**Identified effect**

**Mechanisms**

 **of action**

 **References**

**model**

**(administration**

**doses and duration of**

**treatment)**

 **way,**

**subject**


**Metabolite**

**99**

**Flavonoids**

Quercetin

Flowers and bracts of *Tilia*

All i.p. mixes of quercetin

(20 mg/kg), (2 mg/kg), and rutin

(15.70 mg/kg)

isoquercitrin

mice

test

•

Elevated

plus maze

during the hole-board

an increase in time spent at

the open-side arms in the

plus-maze

 test,

Male CD-1

•

Hole-board

Anxiolytic-like

producing a significant

diminution

 in head dips

 effects

The GABAergic

serotonergic

Flumazenil

 and

WAY100635,

anxiolytic-like

the flavonoid mixture in the plus-maze test, while

WAY100635

significant decrease in the

number of explorations

the hole-board test

 in

*Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant…*

 showed a

 effects of

 inhibited the

 receptors.

*DOI: http://dx.doi.org/10.5772/intechopen.90919*

 and

involvement

 of

[46]

*americana* L.

Rutin

Isoquercitrin

Formononetin

Formononetin

*Trifolium pratense L.*

 from

25 mg/kg for 8

C57BL/6 male

•

Elevated

Formononetin

CFA-induced

behaviors in mice

 anxiety-like

 relieved

A mechanism

inhibition of hyperexcitability

inflammation

basolateral amygdala is

suggested through the

inhibition of NMDA

receptor and CREB-

binding protein (CBP)

 in the

 and

 based on the

[47]

plus maze

mice

consecutive

 days

**Extract from plant/part**

**Posology**

**Experimental**

**Experimental**

**Identified effect**

**Mechanisms**

 **of action**

 **References**

**model**

**(administration**

**doses and duration of**

**treatment)**

 **way,**

**subject**


*Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant… DOI: http://dx.doi.org/10.5772/intechopen.90919*

**Metabolite**

**98**

Ursolic acid

Methanol extract of whole

i.p. 1, 10, 30, and

Male Swiss

•

Elevated

Anxiolytic effect with 10,

Flumazenil

blocked the anxiolytic effect of 10 mg/kg of the

three metabolites elevated plus maze test

Suggest a GABAergic

mechanisms

GABAA

receptors

 in

α1β2γ2L

*Behavioral Pharmacology - From Basic to Clinical Research*

 in

 (2.5 mg/kg)

[17]

plus maze

30, and 100 mg/kg of the

mice (20–

100 mg/kg

Only one

administration

30 g)

•

Light/dark

three metabolites

 without

box

effect in locomotor activity

30 and 100 mg/kg of the

three metabolites diazepam (1 mg/kg)

 are like

*Artemisa indica* in the

chloroform

(Mugwort)

 fraction

> Oleanolic acid

Carnosol

Songorine

The chloroform obtained from the aerial

part of wolfsbane (*A.*

*barbatum* Pers.)

> p-Cymene + thymol

 Ethyl acetate extract of

i.p. 3 mg/kg

Male CD-1

•

Hole-board

Anxiolytic effect

NE

[24]

mice (25–30g)

test

•

Elevated

plus maze

*Lippia graveolens* leaves

 extract

p.o. 2.5 and 0.25 mg/kg,

Male CBA/

• Vogel

Anxiolytic activity with

NE

[29]

CaLac mice

conflict test

0.25 mg/kg produced

exceeding that of

phenazepam

sedative effect

 Without

(20–22 g)

for 5 days

**Extract from plant/part**

**Posology**

**Experimental**

**Experimental**

**Identified effect**

**Mechanisms**

 **of action**

 **References**

**model**

**(administration**

**doses and duration of**

**treatment)**

 **way,**

**subject**


**Metabolite**

**101**

Genistein 6-Methoxyflavanone

Commercial

 6-

i.p. 10, 30, 50, and

BALB/c mice

•

Elevated

6-methoxyflavanone

 (10,

α1-subunit containing

[56]

*DOI: http://dx.doi.org/10.5772/intechopen.90919*

GABAA sedative action of the 6-

methoxyflavanone

receptor mediated

plus maze

30, and 50 mg/kg) spent

> •

Staircase

appreciably

 longer in the

test

open and arms and on the

central platform like

diazepam. In staircase test,

both diazepam and

flavonoid 6-MeOF (10 and

*Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant…*

30 mg/kg) reduced the

incidence of rearing without decreasing the

number of steps ascended

> Rutin

Commercial

 Rutin

(i.p.) 30, 100, 300, 562,

Male Wistar

•

Elevated

Anxiolytic-like

 effects in

Anxiolytic-like

 effects are

[57]

 mg/kg)

partly modulated by

GABAA basolateral amygdala.

Flumazenil

antagonized

systemic rutin

 the effects of

 partly

receptors in the

plus maze

rutin (300–1000

significantly

manner increased (3–6-fold) the

number of entries to the

open arms and the time

spent in this significantly

increased in a dose-

dependent manner

dependently

 and dose

rats

and 1000 mg/kg

microinjected

basolateral amygdala

(16 nmol/4 μl,

intracerebral)

 into the

(Sigma-Aldrich)

of both sex

100 mg/kg

methoxyflavanone

(Sigma-Aldrich)

Commercial

 Genistein

i.p. 2–8 mg/kg, for 7 days Sprague– Dawley male

plus maze

were produced by genistein (2–8 mg/kg)

•

Elevated

Anxiolytic-like

 effects

Significantly

total time spent in open

arms in a

manner

dose-dependent

 increased

[53]

rats

(Sigma-Aldrich)

**Extract from plant/part**

**Posology**

**Experimental**

**Experimental**

**Identified effect**

**Mechanisms**

 **of action**

 **References**

**model**

**(administration**

**doses and duration of**

**treatment)**

 **way,**

**subject**


*Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant… DOI: http://dx.doi.org/10.5772/intechopen.90919*

**Metabolite**

**100**

Theaflavins

TF40, a crude theaflavin

p.o 1 or 5 mg/kg

DDY male

•

Elevated

5 mg/kg theaflavins

 show

Theaflavin increased the

[48]

levels of 3,4dihydroxyphenylacetic

acid (DOPAC) and the

ratios of DOPAC/DA

(DOPAC +

acids)/DA indicating DA

turnover, in the frontal

cortex

GABAA partial, picrotoxin (1 mg/kg), did

not block the anxiolytic-

like effects of chrysin

pretreatment

 with

receptor activation

[49]

 and

homovanillic

*Behavioral Pharmacology - From Basic to Clinical Research*

plus maze

anxiolytic-like

 effects in

> •

Light/dark

both models. In EPM, the

box

time spent in the open

arms was significantly increased, while the time

spent and the number of

entries in the light box

increased

mice

theaflavins,

for 5 days for EPM and

once a day for 6 days for

LDB

Chrysin

Commercial

 Chrysin

(0.5, 1, 2, and 4 mg/kg)

 Adult female

•

Light/dark

2 and 4 mg/kg produced

box

anxiolytic-like

 effects.

Wistar rats in

a model of

•

Elevated

Increased the total time

surgical

plus maze

spent in the light

compartment

the long-term absence of

ovarian hormones. With

respect to the elevated plus

maze, chrysin (2 mg/kg) increased the time spent

on the open arms

> Puerarin

Commercial

 Puerarin

p.o. 30, 60, and 120 mg/

Male Sprague–

•

Elevated

Anxiolytic-like

 effects

It's suggested that puerarin

[50]

(60 and 120 mg/kg, i.g.) produced an increase of

allopregnanolone

serotonin (5-HT) in the

prefrontal cortex

 and

Dawley rats

plus maze

were produced by puerarin (60 and 120 mg/

• Vogel conflict test

kg, i.g)

kg

(Sigma-Aldrich)

 in rats with

menopause

(Sigma-Aldrich)

 once a day

extract

**Extract from plant/part**

**Posology**

**Experimental**

**Experimental**

**Identified effect**

**Mechanisms**

 **of action**

 **References**

**model**

**(administration**

**doses and duration of**

**treatment)**

 **way,**

**subject**


 *study.* Carnosol generated an antidepressant-like effect at doses of 0.01 and 0.1 mg/kg [16] on TST, and 10, 30, and 100 mg/kg, i.p., on TST and FST, as did the oleanolic and ursolic acids. Meanwhile, 100 mg/kg of ursolic acid showed an effect similar to that of imipramine at 60 mg/kg [17]. Betulinic acid at 10 mg/kg was evaluated in male mice on TST [16]. None of those metabolites had effects on locomotor activity. Ursolic acid at doses of 0.01 and 0.1 mg/kg produced an effect similar to that of fluoxetine (10 mg/kg), imipramine (1 mg/kg), and bupropion (10 mg/kg) on TST [18]. Only the 10-mg/kg dose had an antidepressant effect on FST, which was similar to that of bupropion at 10 mg/kg. Studies exploring the mechanism of action of ursolic acid found the involvement of D1 and D2 receptors and a pharmacological synergism with bupropion at 1 mg/kg, p.o. (dual dopamine/noradrenaline reuptake inhibitor, DDNRI) [18], 5 mg/kg of fluoxetine, or 2 mg/kg of reboxetine (SRNI) [19]. They also observed a related increase in noradrenaline and dopamine synthesis on TST [19]. These findings are noteworthy because they suggest the possibility that

*Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant*

various mechanisms of action may be stimulated by the same metabolite.

<sup>1</sup> and D

abundant in OMEO were terpinene-4-ol (32.69%),

*DOI: http://dx.doi.org/10.5772/intechopen.90919*

α2-adrenoceptors [22], while (1S)-(

—especially high doses of monoterpenes

**3. Terpenes with anxiolytic effects: preclinical research**

the dopaminergic receptors D

summary). (R)-(

the cerebral cortex [22].

However, due to the terpene

and

systems

action involved.

**103**

A study of the essential oil of *Origanum majorana* (OMEO) identified 14 component terpenes that represented 92.32% of yield. The five components that were more

2, the noradrenergic

)-linalool produced antidepressant effects in male mice on TST at

sabinene hydrate (8.47%), α-terpinene (7.98%), and sabinene (6.211%). Administered in a single acute dose of 40 or 80 mg/kg, OMEO increased swimming and climbing times in male mice on FST. These findings were interpreted as showing an antidepressant-like effect. In that study [20], pretreatment with antagonist drugs demonstrated that the terpenes act through several mechanisms: first, by activating

and the 5-HT1A and 5-HT2A receptors, and, second, by activating or increasing 5-HT synthesis and monoamine vesicular storage involved in reducing the immobility time produced by 80 mg/kg of OMEO [20]. A pharmacological synergism at the combined subthreshold OMEO doses of 10 mg/kg plus fluoxetine or imipramine (5 mg/kg, i.p.) was also seen. It reduced immobility time but increased swimming and climbing times, similar effects to those produced by OMEO at 80 mg/kg [20]. (A more detail view about the OMEO components can be reviewed at **Tables 1** and

a single dose of 100 or 200 mg/kg, i.p., and on FST when administered 3 times at 100 mg/kg, i.p. [21]. That effect was produced by activation of the 5-HT1A receptor

)-

adrenoceptors, 5-HT1A and D1-receptors, and noradrenergic neurotransmission in

effects as serotonergic syndrome need to be explored. The monoterpenic oxide, 1,4 cineole, for example, produced a prodespair effect at doses of 200 mg/kg, i.p., on FST and 400 mg/kg, i.p., and FST and TST, but did not induce any significant deficit in motor coordination on the rota-rod test (RRT). It did, however, have an anxiolytic effect at a dose of 400 mg/kg in male mice evaluated in EPM that was not associated with any sedative effect [23]. These findings require additional study in light of potential depressor effects on the CNS, and to elucidate the mechanisms of

Anxiolytic properties have also been attributed to terpenes. A combination of two monoterpenoids, p-cymene + thymol, both at doses of 3mg/kg i.p., produced

effect after three applications at a dose of 100 mg/kg, i.p., by activating

γ-terpinene (12.88%), trans-

α2-adrenoceptor,

β - **2** as

*…*

α1- and

β-pinene produced an antidepressant

—possible collateral or undesirable

's capacity to stimulate several neurotransmission

#### *Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant… DOI: http://dx.doi.org/10.5772/intechopen.90919*

Carnosol generated an antidepressant-like effect at doses of 0.01 and 0.1 mg/kg [16] on TST, and 10, 30, and 100 mg/kg, i.p., on TST and FST, as did the oleanolic and ursolic acids. Meanwhile, 100 mg/kg of ursolic acid showed an effect similar to that of imipramine at 60 mg/kg [17]. Betulinic acid at 10 mg/kg was evaluated in male mice on TST [16]. None of those metabolites had effects on locomotor activity. Ursolic acid at doses of 0.01 and 0.1 mg/kg produced an effect similar to that of fluoxetine (10 mg/kg), imipramine (1 mg/kg), and bupropion (10 mg/kg) on TST [18]. Only the 10-mg/kg dose had an antidepressant effect on FST, which was similar to that of bupropion at 10 mg/kg. Studies exploring the mechanism of action of ursolic acid found the involvement of D1 and D2 receptors and a pharmacological synergism with bupropion at 1 mg/kg, p.o. (dual dopamine/noradrenaline reuptake inhibitor, DDNRI) [18], 5 mg/kg of fluoxetine, or 2 mg/kg of reboxetine (SRNI) [19]. They also observed a related increase in noradrenaline and dopamine synthesis on TST [19]. These findings are noteworthy because they suggest the possibility that various mechanisms of action may be stimulated by the same metabolite.

A study of the essential oil of *Origanum majorana* (OMEO) identified 14 component terpenes that represented 92.32% of yield. The five components that were more abundant in OMEO were terpinene-4-ol (32.69%), γ-terpinene (12.88%), transsabinene hydrate (8.47%), α-terpinene (7.98%), and sabinene (6.211%). Administered in a single acute dose of 40 or 80 mg/kg, OMEO increased swimming and climbing times in male mice on FST. These findings were interpreted as showing an antidepressant-like effect. In that study [20], pretreatment with antagonist drugs demonstrated that the terpenes act through several mechanisms: first, by activating the dopaminergic receptors D1 and D2, the noradrenergic α1- and α2-adrenoceptor, and the 5-HT1A and 5-HT2A receptors, and, second, by activating or increasing 5-HT synthesis and monoamine vesicular storage involved in reducing the immobility time produced by 80 mg/kg of OMEO [20]. A pharmacological synergism at the combined subthreshold OMEO doses of 10 mg/kg plus fluoxetine or imipramine (5 mg/kg, i.p.) was also seen. It reduced immobility time but increased swimming and climbing times, similar effects to those produced by OMEO at 80 mg/kg [20]. (A more detail view about the OMEO components can be reviewed at **Tables 1** and **2** as summary). (R)-()-linalool produced antidepressant effects in male mice on TST at a single dose of 100 or 200 mg/kg, i.p., and on FST when administered 3 times at 100 mg/kg, i.p. [21]. That effect was produced by activation of the 5-HT1A receptor and α2-adrenoceptors [22], while (1S)-()-β-pinene produced an antidepressant effect after three applications at a dose of 100 mg/kg, i.p., by activating βadrenoceptors, 5-HT1A and D1-receptors, and noradrenergic neurotransmission in the cerebral cortex [22].

However, due to the terpene's capacity to stimulate several neurotransmission systems—especially high doses of monoterpenes—possible collateral or undesirable effects as serotonergic syndrome need to be explored. The monoterpenic oxide, 1,4 cineole, for example, produced a prodespair effect at doses of 200 mg/kg, i.p., on FST and 400 mg/kg, i.p., and FST and TST, but did not induce any significant deficit in motor coordination on the rota-rod test (RRT). It did, however, have an anxiolytic effect at a dose of 400 mg/kg in male mice evaluated in EPM that was not associated with any sedative effect [23]. These findings require additional study in light of potential depressor effects on the CNS, and to elucidate the mechanisms of action involved.

### **3. Terpenes with anxiolytic effects: preclinical research**

Anxiolytic properties have also been attributed to terpenes. A combination of two monoterpenoids, p-cymene + thymol, both at doses of 3mg/kg i.p., produced

**Metabolite**

**102**

Viscosine

**Sterols**

α-Spinasterol

β-Amirine

Hexane–ethyl

from

*Protium*

*heptaphyllum*

 acetate

p.o. 10, 25, and 50 mg/kg Male Swiss

mice

•

Elevated

Anxiolytic effect with dose

Mechanisms

by GABAA

subunit α1

receptors in the

 GABAérgic

[94]

plus maze

10–25 mg/kg like

diazepam

(20–30 g)

α-Amirine

*NE = no explorated.*

**Table 2.** *Secondary metabolites*

 *with anxiolytic properties in preclinical*

 *study.*

Toronto Research

i.p. 1 and 2 mg/kg

 Male albino

•

Elevated

No effects were found

NE

[90]

plus maze

(0.5, 1, and 2 mg/kg)

Swiss mice

(23–25 g)

•

Light/dark

box

Chemicals Inc., Canada

*Dodonaea viscosa* (Linn)

 i.p. 10, 30, and

Male Swiss

•

Elevated

Viscosin increased the %

Anxiolytic effect of

[58]

viscosine are likely mediated via its positive

allosteric modulatory

action of GABA at

different GABAA

subtypes

receptor

*Behavioral Pharmacology - From Basic to Clinical Research*

entries and % time spent in

plus maze

> •

Light/dark

the open arms

box

mice

100 mg/kg

**Extract from plant/part**

**Posology**

**Experimental**

**Experimental**

**Identified effect**

**Mechanisms**

 **of action**

 **References**

**model**

**(administration**

**doses and duration of**

**treatment)**

 **way,**

**subject**

anxiolytic effects on the hole-board test (HBT) and EPM [24]. Studies have also demonstrated that ()-myrtenol, a monoterpenoid alcohol, produced an anxiolytic effect on EPM at doses of 25, 50, and 75 mg/kg, i.p., though only the 25- and 50-mg/kg doses did so on LDB. On both tests, the anxiolytic effect of 25 mg/kg of ()-myrtenol was mediated by GABAA receptors [25]. In another study, rosmanol produced an anxiolytic effect at doses of 10, 30, and 100 mg/kg in EPM and LDB, and the 10- and 30-mg/kg doses showed an effect similar to those of diazepam at 1 mg/kg [15]. This mechanism of action acts on GABAA receptors at a distinct site from the high-affinity benzodiazepine-binding region [15]. Triterpenes as ursolic acid, oleanolic acid, and carnosol produced anxiolytic effects at doses of 10, 30, and 100 mg/kg in EPM and LDB. The effect of the 30- and 100-mg/kg doses of these three metabolites was similar to that of diazepam at 1 mg/kg. No significant effect was seen on locomotor activity. We know that this effect was produced through GABAA receptors of the α1β2γ2L conformation because 2.5 mg/kg of flumazenil blocked the anxiolytic effects of 10 mg/kg of all three in EPM [17].

neurobiology of depression and anxiety. Their results [33] also demonstrate that central depletion of 5-HT reversed the antidepressant effect of genistein, suggesting a critical role of the serotonergic system, specifically through 5-HT1A receptors. It is important to note that these results on the serotonergic metabolic ratio (5-HIAA/5- HT) may be dependent on gonadal hormones. Ovariectomized rats (OVX, surgical removal of both ovaries) showed reduced immobility times on FST after administration of genistein (10 mg/kg, p.o. [34], or by i.m.) [35] for 14 days, but the downward tendency of the serotonergic metabolic ratio caused by FST was only

*Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant…*

antidepressant-like effects on FST at the same dose of genistein (10 mg/kg) but in non-ovariectomized rats. That effect was more marked in the metestrus and diestrus phases of the estral cycle, which are characterized by low plasmatic concentrations of ovarian hormones, than during the proestrus and estrus stages with their characteristically high hormone concentrations. This suggests that these hormones

A similar effect on the serotonergic system and MAO-A activity was found with quercetin 4'-O-glucoside or quercetin administered at doses of 10 and 20 mg/kg, p.

o., for 7 days in Swiss albino mice of both sexes. These substances produced antidepressant-like effects in mice on FST as well as those subjected to unpredictable, chronic mild stress (CUMS, a mouse model designed to induce depression) and subsequently evaluated on FST. In that study, a 20-mg/kg dose of quercetin 4'-O-glucoside showed a similar effect to that of fluoxetine at 20 mg/kg, p.o., on FST, with or without prior exposure to CUMS [36]. A consequence of CUMS on the sucrose preference test (SPT, a test used to study anhedonia rodents, the main symptom of depression in humans) is a decrease in the consumption of a sweetened solution. In these sense, both doses of quercetin 4'-O-glucoside reverted this effect, and interpreted as being antidepressant. Other consequences of CUMS are metabolic, for example, excessive production of reactive oxygen species that was evidenced by higher brain thiobarbituric acid reactive species (TBARS). Compromising endogenous anti-oxidants, like reduced glutathione (GSH), enhances MAO-A activity in the brain and, consequently, depletes monoamine levels there, especially serotonin 5-HT. The effects observed in that study were blocked by 21 days of treatment with 10 and 20 mg/kg of quercetin 4'-O-glucoside [36], suggesting a possible mechanism of action with an antioxidant effect that impedes ROS production. Another study [37] found that 10 mg/kg of quercetin administered for 14 days reduced immobility time on TST, but not FST, while doses of 25 and 50 mg/kg produced this effect in female mice on both tests. The mechanisms of action were explored on TST, where i.c.v. administration of N-methyl-Daspartate (NMDA at 0.1 pmol/site) and L-arginine (at 750 mg/kg, i.p., a nitric oxide

inhibitor) blocked the antidepressant effect of quercetin. Hence, the

antidepressant-like effect of quercetin may involve inhibiting NMDA receptors to decrease intracellular calcium that, in turn, inhibits the protein calmodulin, which then inhibits neuronal nitric oxide synthase to decrease nitric oxide levels (NO). This hypothesis is supported by the finding that administering methylene blue (a NO synthase inhibitor) at 20 mg/kg, i.p., and soluble guanylate cyclase or 7-nitroindazole (another NO synthase inhibitor) at 50 mg/kg, i.p., improved quercetin's antidepressant-like effect on TST. This indicates that the antidepressant

effect may be dependent on limiting NO synthesis, either by inhibiting the enzyme or by reducing NO production, perhaps via decreased cyclic guanosine monophosphate (cGMP), since sildenafil (a phosphodiesterase 5 selective inhibitor

A model of depression induced by olfactory bulbectomy (OB, surgical removal of the olfactory bulbs) reduced the latency to immobility and increased immobility

that increases cGMP levels) also canceled this effect [37].

**105**

evident in the hippocampus [34]. Sapronov and Kasakova [35] found

play a significant role in the antidepressant effect of genistein.

*DOI: http://dx.doi.org/10.5772/intechopen.90919*

Although most terpenes have a GABAergic mechanism, their action may also occur through the serotonergic system, as evidenced in the study by Costa et al. [26]. In an experiment with male rats, those authors identified that acute administration of 5 mg/kg, p.o., or 14-day repeat doses (1 mg/kg/day), of the essential oil of ripe fruits of *C. aurantium* (Rutaceae) (whose chemical composition includes 98.66% limonene, 0.53% β-myrcene, 0.41% β-pinene, and 0.41% unidentified compounds) produced an anxiolytic-like effect in LDB that was mediated by the serotonergic system through 5-HT1A receptors (WAY 100635 0.5 mg/kg, i.p.), not the GABAergic system (flumazenil 2 mg/kg, i.p.). That study also analyzed the antidepressant effect on FST after oral and inhaled treatment, but found that it did not modify immobility. Their results suggest that distinct mechanisms of action exist for the anxiolytic and antidepressant effects [26]. In this regard, some of the terpenes in certain essential oils produce anxiolytic effects and are often used in aromatherapy to reduce anxiety in animals and humans [27]. Inhaling emulsions of linalool oxide (a monocyclic alcohol) at 0.65, 1.25, 2.5, and 5.0% w/w during 7 min of exposure in the inhalation chamber, for example, produced anxiolytic-like effects in EPM and LDB, but no significant motor deficit on RRT [28].

Finally, songorine, a C20 diterpenoid alkaloid, produced an anxiolytic-like effect at 0.25 mg/kg v.o. for 5 days with greater efficacy than phenazepam on Vogel's conflict test (VCT) [29]. For the terpenes described so far, we know that both the serotonergic and GABAergic systems are involved in their mechanisms of action, and these are the same systems that are activated by other groups of metabolites, such as flavonoids and sterols (see **Tables 1** and **2** for summaries).

#### **4. Flavonoids with antidepressant effects: preclinical research**

Flavonoids are the most widely studied active metabolites (for a broad review of research, see German-Ponciano et al. [30]). Genistein is an isoflavone that can cross the blood-brain barrier in mice [31] and rats [32]. Acute oral administration of 5, 15, and 45 mg/kg genistein in male mice did not reduce immobility time on FST or TST, but chronic, dose-dependent administration for 21 days produced antidepressantlike effects on both tests, without affecting locomotor activity [33]. This effect was associated with increased NA and 5-HT concentrations in the hippocampus and frontal cortex, and of 5-HT in the hypothalamus, though it decreased the 5-HTIAA/ 5-HT ratio in the hippocampus and frontal cortex. These results suggest an inhibition effect of genistein on MAO-A in the hippocampus, frontal cortex, and hypothalamus and on MAO-B in the hippocampus, three brain structures involved in the *Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant… DOI: http://dx.doi.org/10.5772/intechopen.90919*

neurobiology of depression and anxiety. Their results [33] also demonstrate that central depletion of 5-HT reversed the antidepressant effect of genistein, suggesting a critical role of the serotonergic system, specifically through 5-HT1A receptors. It is important to note that these results on the serotonergic metabolic ratio (5-HIAA/5- HT) may be dependent on gonadal hormones. Ovariectomized rats (OVX, surgical removal of both ovaries) showed reduced immobility times on FST after administration of genistein (10 mg/kg, p.o. [34], or by i.m.) [35] for 14 days, but the downward tendency of the serotonergic metabolic ratio caused by FST was only evident in the hippocampus [34]. Sapronov and Kasakova [35] found antidepressant-like effects on FST at the same dose of genistein (10 mg/kg) but in non-ovariectomized rats. That effect was more marked in the metestrus and diestrus phases of the estral cycle, which are characterized by low plasmatic concentrations of ovarian hormones, than during the proestrus and estrus stages with their characteristically high hormone concentrations. This suggests that these hormones play a significant role in the antidepressant effect of genistein.

A similar effect on the serotonergic system and MAO-A activity was found with quercetin 4'-O-glucoside or quercetin administered at doses of 10 and 20 mg/kg, p. o., for 7 days in Swiss albino mice of both sexes. These substances produced antidepressant-like effects in mice on FST as well as those subjected to unpredictable, chronic mild stress (CUMS, a mouse model designed to induce depression) and subsequently evaluated on FST. In that study, a 20-mg/kg dose of quercetin 4'-O-glucoside showed a similar effect to that of fluoxetine at 20 mg/kg, p.o., on FST, with or without prior exposure to CUMS [36]. A consequence of CUMS on the sucrose preference test (SPT, a test used to study anhedonia rodents, the main symptom of depression in humans) is a decrease in the consumption of a sweetened solution. In these sense, both doses of quercetin 4'-O-glucoside reverted this effect, and interpreted as being antidepressant. Other consequences of CUMS are metabolic, for example, excessive production of reactive oxygen species that was evidenced by higher brain thiobarbituric acid reactive species (TBARS). Compromising endogenous anti-oxidants, like reduced glutathione (GSH), enhances MAO-A activity in the brain and, consequently, depletes monoamine levels there, especially serotonin 5-HT. The effects observed in that study were blocked by 21 days of treatment with 10 and 20 mg/kg of quercetin 4'-O-glucoside [36], suggesting a possible mechanism of action with an antioxidant effect that impedes ROS production. Another study [37] found that 10 mg/kg of quercetin administered for 14 days reduced immobility time on TST, but not FST, while doses of 25 and 50 mg/kg produced this effect in female mice on both tests. The mechanisms of action were explored on TST, where i.c.v. administration of N-methyl-Daspartate (NMDA at 0.1 pmol/site) and L-arginine (at 750 mg/kg, i.p., a nitric oxide inhibitor) blocked the antidepressant effect of quercetin. Hence, the antidepressant-like effect of quercetin may involve inhibiting NMDA receptors to decrease intracellular calcium that, in turn, inhibits the protein calmodulin, which then inhibits neuronal nitric oxide synthase to decrease nitric oxide levels (NO). This hypothesis is supported by the finding that administering methylene blue (a NO synthase inhibitor) at 20 mg/kg, i.p., and soluble guanylate cyclase or 7-nitroindazole (another NO synthase inhibitor) at 50 mg/kg, i.p., improved quercetin's antidepressant-like effect on TST. This indicates that the antidepressant effect may be dependent on limiting NO synthesis, either by inhibiting the enzyme or by reducing NO production, perhaps via decreased cyclic guanosine monophosphate (cGMP), since sildenafil (a phosphodiesterase 5 selective inhibitor that increases cGMP levels) also canceled this effect [37].

A model of depression induced by olfactory bulbectomy (OB, surgical removal of the olfactory bulbs) reduced the latency to immobility and increased immobility

anxiolytic effects on the hole-board test (HBT) and EPM [24]. Studies have also demonstrated that ()-myrtenol, a monoterpenoid alcohol, produced an anxiolytic effect on EPM at doses of 25, 50, and 75 mg/kg, i.p., though only the 25- and 50-mg/kg doses did so on LDB. On both tests, the anxiolytic effect of 25 mg/kg of ()-myrtenol was mediated by GABAA receptors [25]. In another study, rosmanol produced an anxiolytic effect at doses of 10, 30, and 100 mg/kg in EPM and LDB, and the 10- and 30-mg/kg doses showed an effect similar to those of diazepam at 1 mg/kg [15]. This mechanism of action acts on GABAA receptors at a distinct site from the high-affinity benzodiazepine-binding region [15]. Triterpenes as ursolic acid, oleanolic acid, and carnosol produced anxiolytic effects at doses of 10, 30, and 100 mg/kg in EPM and LDB. The effect of the 30- and 100-mg/kg doses of these three metabolites was similar to that of diazepam at 1 mg/kg. No significant effect was seen on locomotor activity. We know that this effect was produced through GABAA receptors of the α1β2γ2L conformation because 2.5 mg/kg of flumazenil

*Behavioral Pharmacology - From Basic to Clinical Research*

blocked the anxiolytic effects of 10 mg/kg of all three in EPM [17].

in EPM and LDB, but no significant motor deficit on RRT [28].

such as flavonoids and sterols (see **Tables 1** and **2** for summaries).

**104**

**4. Flavonoids with antidepressant effects: preclinical research**

Although most terpenes have a GABAergic mechanism, their action may also occur through the serotonergic system, as evidenced in the study by Costa et al. [26]. In an experiment with male rats, those authors identified that acute administration of 5 mg/kg, p.o., or 14-day repeat doses (1 mg/kg/day), of the essential oil of ripe fruits of *C. aurantium* (Rutaceae) (whose chemical composition includes 98.66% limonene, 0.53% β-myrcene, 0.41% β-pinene, and 0.41% unidentified compounds) produced an anxiolytic-like effect in LDB that was mediated by the serotonergic system through 5-HT1A receptors (WAY 100635 0.5 mg/kg, i.p.), not the GABAergic system (flumazenil 2 mg/kg, i.p.). That study also analyzed the antidepressant effect on FST after oral and inhaled treatment, but found that it did not modify immobility. Their results suggest that distinct mechanisms of action exist for the anxiolytic and antidepressant effects [26]. In this regard, some of the terpenes in certain essential oils produce anxiolytic effects and are often used in aromatherapy to reduce anxiety in animals and humans [27]. Inhaling emulsions of linalool oxide (a monocyclic alcohol) at 0.65, 1.25, 2.5, and 5.0% w/w during 7 min of exposure in the inhalation chamber, for example, produced anxiolytic-like effects

Finally, songorine, a C20 diterpenoid alkaloid, produced an anxiolytic-like effect

Flavonoids are the most widely studied active metabolites (for a broad review of research, see German-Ponciano et al. [30]). Genistein is an isoflavone that can cross the blood-brain barrier in mice [31] and rats [32]. Acute oral administration of 5, 15, and 45 mg/kg genistein in male mice did not reduce immobility time on FST or TST, but chronic, dose-dependent administration for 21 days produced antidepressantlike effects on both tests, without affecting locomotor activity [33]. This effect was associated with increased NA and 5-HT concentrations in the hippocampus and frontal cortex, and of 5-HT in the hypothalamus, though it decreased the 5-HTIAA/ 5-HT ratio in the hippocampus and frontal cortex. These results suggest an inhibition effect of genistein on MAO-A in the hippocampus, frontal cortex, and hypothalamus and on MAO-B in the hippocampus, three brain structures involved in the

at 0.25 mg/kg v.o. for 5 days with greater efficacy than phenazepam on Vogel's conflict test (VCT) [29]. For the terpenes described so far, we know that both the serotonergic and GABAergic systems are involved in their mechanisms of action, and these are the same systems that are activated by other groups of metabolites,

time on FST and TST. This was accompanied by an increase in the levels of the markers of oxidative stress, for example, 116% in the case of lipid hydroperoxide content (LOOH) in the hippocampus. This effect was reverted by 52.25% by administering 25 mg/kg of genistein in the content of LOOH, as observed on the immobility on FST and TST. In sham rats only (*i.e.*, animals subjected to the same surgical procedure but without resection of the olfactory bulbs), genistein reduced glutathione (GSH) levels, in that study by 65.94%. The authors [37] explained that "the reduction of GSH levels caused by OB and, surprisingly, quercetin, can be explained by the fact that glutathione peroxidase, in addition to reducing H2O2, decreases lipid and nonlipid hydroperoxides at the expense of GSH, causing it to become oxidized and giving rise to glutathione disulfide. Therefore, it is suggested that LOOH activated glutathione peroxidase which, in turn, oxidized GSH to normalize LOOH levels"*.* In this area, increased levels of the markers of oxidative stress in major depression have been associated with poor response to antidepressant treatment [38]. Therefore, a therapy that reduces the levels of markers of oxidative stress and produces antidepressant effects could be a promising form of treatment.

antidepressant and antioxidant effects is naringin, which reduced immobility on FST at doses of 2.5, 5, and 10 mg/kg given for 7 days. The antidepressant effect of these doses correlated with enhanced cholinergic transmission due to a decrease in the activity of the enzyme acetylcholinesterase and of the antioxidant defense systems caused by higher GSH levels, as well as an increase in the activity of

*Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant…*

Studies have demonstrated that naringin inhibits lipid peroxidation and nitrosative processes by reducing levels of ROS and nitrogen species [44]. Finally, the extract of *Cirsium japonicum* at doses of 200 and 400 mg/kg has shown the ability to reduce immobility time on FST in a similar manner to that of 5 mg/kg of the antidepressant imipramine. A major component of this plant is the flavonoid luteolin, which at doses of 5 and 10 mg/kg produced a similar effect to that of the complete extract, likely through a positive modulating effect on the GABAA receptor complex. This was proven in an *in vitro* study where extracts of both *Cirsium japonicum* and luteolin increased Cl influx in an effect impeded by pretreatment

The varied mechanisms seen in flavonoids make them an important object of study, especially in the search for side effect-free treatments that can compromise their effectiveness or produce toxicity by interacting with other medications or

A particularly important fact concerning the potency of the biological activity of plants is that it depends on several factors, for instance, the part of the plant used, the region where it is gathered, the season, and harvesting time, among others [46]. For example, in male mice evaluated by HBT and EPM, a single dose of 100 mg/kg i.p. of the methanolic extract of inflorescences of *Tilia americana* var. mexicana collected in Morelia, Mexico, produced a more effective anxiolytic effect than those gathered in Honey, Puebla, though the leaves collected in Honey were more effective than those from Morelia or Santa María Ahuacatitlan, Mexico. These three Mexican states are located at different elevations with distinct humidity and soil types. That study quantified quercetin, rutin, and isoquercitrin in the inflorescences and leaves, determining that the concentrations of these substances differed with the part of the plant used and the collection area [46]. It also tested several standard commercial flavonoids: kaempferol (10 mg/kg), quercetin (20 mg/kg), astragalin (10 mg/kg), isoquercitrin (2 mg/kg), quercetin (10 mg/kg), and rutin (15.7 mg/kg), and a mixture of flavonoids (MIX) composed of quercetin 20 mg/kg, rutin 15.7 mg/kg, and isoquercitrin 2 mg/kg and quercetin (20 mg/kg). Results showed that a mixture of quercitin (20 mg/kg), rutin (15.70 mg/kg), and isoquercitrin (2 mg/kg) produced an anxiolytic effect in male mice tested in HBT and EPM [46] by reducing the number of head-dippings but increasing the time spent in the open arms, respectively. Finally, upon testing the anxiolytic effect of the methanolic extract of *Tilia americana* var. mexicana, those authors found that this produced an effect in EPM through the participation of GABA/BDZ (flumazenil 5 mg/kg) and 5HT1A serotonergic receptors (WAY 100635 0.5 mg/kg), though they were not involved in the

Another flavonoid known to have anxiolytic effects is formononetin, an active metabolite of traditional Chinese medicine red clover (*Trifolium pratense L.).* Wang et al. [47] observed that administering 25 mg/kg of this metabolite to male mice once daily for 8 days blocked the anxiogenic effect on the open field test (OFT) produced by administering Freund's complete adjuvant (CFA), reduced the time

superoxide dismutase (SOD) and catalase (CAT) in mice brains.

with bicuculline, a competitive GABAA receptor antagonist [45].

food. This is another area of research that remains to be explored.

**5. Flavonoids with anxiolytic effects**

*DOI: http://dx.doi.org/10.5772/intechopen.90919*

anxiolytic effect on HBT [46].

**107**

Additional mechanisms of the antidepressant action of flavonoids have been explored. Administering chrysin for 14 days at a dose of 20 mg/kg, for example, increased grooming time in male OB C57B/6J mice evaluated on the splash test (ST), where 200 ml of a 10% sucrose solution is squirted on the mouse's snout to initiate grooming behavior. Here, greater grooming time is considered an antidepressant effect. Doses of 5 and 20 mg/kg impeded an increase in immobility time by these OB mice on FST, but increased 5-HT and brain-derived neurotrophic factor concentrations in the hippocampus [39]. In another study, fisetin administered at 5 mg/kg v.o. increased activation of the tropomyosin kinase B receptor (TrkB) by signaling and increasing its phosphorylation in the hippocampus. This suggests that fisetin produced pro-neurogenesis [40] related to its antidepressant effect on FST and TST after 1 or 2 weeks of treatment with a relatively short therapeutic latency compared to clinically-effective antidepressants. Fisetin also reversed depressionlike behaviors induced by spatial restraint stress in mice evaluated on FST and TST [40]. Other studies have found that the chemical standard dihydromyricetin activated the ERK1/2-CREB pathway and increased glycogen synthase kinase-3 beta (GSK-3β) phosphorylation at ser-9 with upregulation of BDNF expression in the hippocampus, while inhibiting neuroinflammation. These findings may be related to the antidepressant effect seen on TST and FST after once-daily administration of 10 and 20 mg/kg, v.o. for 3 days, but not after a single acute dose [41]. Interestingly, dihydromyricetin reverted the depressogenic effect caused by CUMS in mice subjected to SPT and FST, or TST, only after administration of once daily for 7 days, but not 3 days [41]. Another flavonoid analyzed is hesperidin, which increased BDNF levels in the hippocampus after administration of once daily for 21 days (0.3 and 1 mg/kg, i.p.). These doses produced an antidepressant effect on TST similar to fluoxetine (32 mg/kg i.p.) and imipramine (15 mg/kg, i.p.). Another research has also verified that when applied acutely (1 mg/kg after 30 min) or chronically (0.1, 0.3, and 1 mg/kg for 21 days) hesperidin significantly decreased nitrate/nitrite (NOX) levels in the hippocampus of mice, suggesting a possible inhibition of the L-arginine-NO-cGMP pathway [42].

Another flavonoid that has shown effects on the CNS is baicalin, which may promote neuronal differentiation through neuronal maduration and ensure their survival via the associated Akt/FOXG1 pathway, which stimulates dendrite elongation. This is related to findings that indicated that, after 6 weeks of treatment, a 60-mg/kg dose of baicalin had an effect similar to that of fluoxetine (15 mg/kg, v.o.), because it reverted the decrease of sucrose intake on SPT and the increase in immobility on TST produced by CUMS [43]. Another flavonoid that associates

#### *Neuropharmacology of Secondary Metabolites from Plants with Anxiolytic and Antidepressant… DOI: http://dx.doi.org/10.5772/intechopen.90919*

antidepressant and antioxidant effects is naringin, which reduced immobility on FST at doses of 2.5, 5, and 10 mg/kg given for 7 days. The antidepressant effect of these doses correlated with enhanced cholinergic transmission due to a decrease in the activity of the enzyme acetylcholinesterase and of the antioxidant defense systems caused by higher GSH levels, as well as an increase in the activity of superoxide dismutase (SOD) and catalase (CAT) in mice brains.

Studies have demonstrated that naringin inhibits lipid peroxidation and nitrosative processes by reducing levels of ROS and nitrogen species [44]. Finally, the extract of *Cirsium japonicum* at doses of 200 and 400 mg/kg has shown the ability to reduce immobility time on FST in a similar manner to that of 5 mg/kg of the antidepressant imipramine. A major component of this plant is the flavonoid luteolin, which at doses of 5 and 10 mg/kg produced a similar effect to that of the complete extract, likely through a positive modulating effect on the GABAA receptor complex. This was proven in an *in vitro* study where extracts of both *Cirsium japonicum* and luteolin increased Cl influx in an effect impeded by pretreatment with bicuculline, a competitive GABAA receptor antagonist [45].

The varied mechanisms seen in flavonoids make them an important object of study, especially in the search for side effect-free treatments that can compromise their effectiveness or produce toxicity by interacting with other medications or food. This is another area of research that remains to be explored.
