**5. The decision-making unit: the brain**

The discrimination and integration among the odor molecules and the exchange of electrochemical information consequently influence the neuronal decision-making abilities of the brain system [58]. When an animal is given preference for food, several decisions can be made such as whether to eat or not, what to eat, and when to eat, which not only depends on the internal physiological condition but also relies on the biological clock of the respective animal. In the case of mosquito species, making a choice among the different available foods requires a fine-tuning of the nasal system and strong integration of the decision-making machinery. The availability of diverse nature of blood-feeding hosts not only makes the decision-making process more complex but also has an impact on mosquito survival, fitness, and fecundity [59].

#### **5.1 Structural basis of signal processing**

The knowledge about insect olfactory coding is strongly rooted in the fruit fly *Drosophila melanogaster*. Over the last two decades, the cellular and molecular bases of *Drosophila* olfaction have been studied well with the assistance of varied genetic tools. The three milestones of olfaction have been documented comprehensively in the fruit fly on how odor information is received, concatenated, and processed by the peripheral and central nervous systems, respectively [60, 61]. Apart from that, "the parallel olfactory processing" and "feature detection" mechanism has also been unlocked in honey bee brain and sphinx moth, respectively [62–64]. Several studies on *Drosophila* and other insects (*Manduca sexta and Bombyx mori*) suggested that the primary brain structures responsible for receiving initial information of odor are the antennal lobes (ALs) [62, 63, 65, 66]. These antennal lobes consist of a specific number of spherical condensed neuropil structures, which are known as glomeruli. Depending on the nature and sex of the insect species, the number of glomeruli varied between 50 and 200, whereas each respective species possess the same number of glomeruli having identical features (shape, size, location) [67, 68]. Olfactory receptor neurons that express a particular type of receptor on their dendrites project their axons into the same glomerulus [8, 45, 67, 69]. Furthermore, each glomerulus is housed with the arms of the local interneurons (LNs) and the dendrites of the projection neurons (PNs) [69]. Thus, within the antennal lobe, a synaptic connection is formed between olfactory receptor neurons and antennal lobe interneurons. From the antennal lobe, the olfactory information is transmitted to a higher brain center by the projection neurons [8, 69, 70] (**Figure 4**). Horizontal innervation of the local interneurons within the glomerulus facilitates interglomerular communication. The primary neurotransmitter found to communicate between local interneurons is the gamma-aminobutyric acid (GABA) which facilitates the generation of Na+ -mediated action potential in response to olfactory stimulation [8].

**81**

**Figure 4.**

*signal flow.*

*Neuro-Olfactory Regulation and Salivary Actions: A Coordinated Event for Successful…*

Vertically arranged distinct fiber tract of the projection neurons connects the ALs to the higher brain centers such as calyces of the mushroom body and the lateral horn of the protocerebrum [8], where olfactory information is integrated with other sensory cues. The cell bodies of the PNs are located at the periphery of the antennal lobe glomeruli and their axons spread in the higher brain center. The branching pattern of PNs is either uniglomerular or multiglomerular [69]. The functional characterization of odor coding properties of individual ORN targeting each glomerulus revealed the existence of strong sexual dimorphism between male and female AL glomeruli which lead to a highly specialized odorant response towards general odorants and sex pheromones. PN odor response is not identical to the ORN odor response. The majority of the PNs are broadly tuned with respect to the general odors and send their dendritic arbor into the ordinary glomeruli (OG) (both uniglomerular and multiglomerular) which respond vigorously during the odor onset [69, 70]. This results from the high convergence of ORNs expressing the same odorant receptor into a single glomerulus. Generally, ORNs project its axon into a particular glomerulus, and PNs receive input from all of the ORN axons entering into that cognate glomerulus. As a result, the signal gets amplified many folds which makes the PNs very sensitive to small changes in the presynaptic ORN input. But the PNs associated with sex pheromone target different regions of the lateral horn in a sexually dimorphic manner, and thus the same pheromone elicits distinct behavior in males and females [32, 68, 71]. Most of the ORNs to PN synapses are cholinergic, and PNs respond more strongly to the fluctuating amount of

*Schematic presentation of flow of odor signals from the environment to the central nervous system. Odor molecules of diverse nature (highlighted as multicolored small circle) bind to their respective receptors present on the olfactory receptor neurons of antennae of mosquitoes. Then the initial signal of odor is transmitted to the antennal lobe (AL), and from the AL the signal is transmitted to higher brain centers, i.e., mushroom body (MB) and lateral horn (LH) for signal processing and decision-making. The red arrow indicates the path of* 

*DOI: http://dx.doi.org/10.5772/intechopen.90768*

*Neuro-Olfactory Regulation and Salivary Actions: A Coordinated Event for Successful… DOI: http://dx.doi.org/10.5772/intechopen.90768*

#### **Figure 4.**

*Sino-Nasal and Olfactory System Disorders*

involves the ionotropic pathway [8, 27, 57].

**5. The decision-making unit: the brain**

**5.1 Structural basis of signal processing**

messengers. In contrast, another study indicated the entanglement of G protein and the synthesis of cAMP, IP3, and other secondary messengers that consequently induce the downstream effector enzymes and also affect the membrane potential through activating the co-receptor protein [27, 56]. The resultant change in the membrane potential/permeability by either process causes the generation and propagation of action potentials along the ORN axon membrane towards the antennal lobes. In contrast to the rapid ionotropic pathway, the G protein-mediated metabotropic pathway is slower. However, it plays an important role when the odor cues are present in lower concentration, whereas high concentration directly

The discrimination and integration among the odor molecules and the exchange of electrochemical information consequently influence the neuronal decision-making abilities of the brain system [58]. When an animal is given preference for food, several decisions can be made such as whether to eat or not, what to eat, and when to eat, which not only depends on the internal physiological condition but also relies on the biological clock of the respective animal. In the case of mosquito species, making a choice among the different available foods requires a fine-tuning of the nasal system and strong integration of the decision-making machinery. The availability of diverse nature of blood-feeding hosts not only makes the decision-making process more complex but also has an impact on mosquito survival, fitness, and fecundity [59].

The knowledge about insect olfactory coding is strongly rooted in the fruit fly *Drosophila melanogaster*. Over the last two decades, the cellular and molecular bases of *Drosophila* olfaction have been studied well with the assistance of varied genetic tools. The three milestones of olfaction have been documented comprehensively in the fruit fly on how odor information is received, concatenated, and processed by the peripheral and central nervous systems, respectively [60, 61]. Apart from that, "the parallel olfactory processing" and "feature detection" mechanism has also been unlocked in honey bee brain and sphinx moth, respectively [62–64]. Several studies on *Drosophila* and other insects (*Manduca sexta and Bombyx mori*) suggested that the primary brain structures responsible for receiving initial information of odor are the antennal lobes (ALs) [62, 63, 65, 66]. These antennal lobes consist of a specific number of spherical condensed neuropil structures, which are known as glomeruli. Depending on the nature and sex of the insect species, the number of glomeruli varied between 50 and 200, whereas each respective species possess the same number of glomeruli having identical features (shape, size, location) [67, 68]. Olfactory receptor neurons that express a particular type of receptor on their dendrites project their axons into the same glomerulus [8, 45, 67, 69]. Furthermore, each glomerulus is housed with the arms of the local interneurons (LNs) and the dendrites of the projection neurons (PNs) [69]. Thus, within the antennal lobe, a synaptic connection is formed between olfactory receptor neurons and antennal lobe interneurons. From the antennal lobe, the olfactory information is transmitted to a higher brain center by the projection neurons [8, 69, 70] (**Figure 4**). Horizontal innervation of the local interneurons within the glomerulus facilitates interglomerular communication. The primary neurotransmitter found to communicate between local interneurons is the

gamma-aminobutyric acid (GABA) which facilitates the generation of Na+

action potential in response to olfactory stimulation [8].


**80**

*Schematic presentation of flow of odor signals from the environment to the central nervous system. Odor molecules of diverse nature (highlighted as multicolored small circle) bind to their respective receptors present on the olfactory receptor neurons of antennae of mosquitoes. Then the initial signal of odor is transmitted to the antennal lobe (AL), and from the AL the signal is transmitted to higher brain centers, i.e., mushroom body (MB) and lateral horn (LH) for signal processing and decision-making. The red arrow indicates the path of signal flow.*

Vertically arranged distinct fiber tract of the projection neurons connects the ALs to the higher brain centers such as calyces of the mushroom body and the lateral horn of the protocerebrum [8], where olfactory information is integrated with other sensory cues. The cell bodies of the PNs are located at the periphery of the antennal lobe glomeruli and their axons spread in the higher brain center. The branching pattern of PNs is either uniglomerular or multiglomerular [69]. The functional characterization of odor coding properties of individual ORN targeting each glomerulus revealed the existence of strong sexual dimorphism between male and female AL glomeruli which lead to a highly specialized odorant response towards general odorants and sex pheromones. PN odor response is not identical to the ORN odor response. The majority of the PNs are broadly tuned with respect to the general odors and send their dendritic arbor into the ordinary glomeruli (OG) (both uniglomerular and multiglomerular) which respond vigorously during the odor onset [69, 70]. This results from the high convergence of ORNs expressing the same odorant receptor into a single glomerulus. Generally, ORNs project its axon into a particular glomerulus, and PNs receive input from all of the ORN axons entering into that cognate glomerulus. As a result, the signal gets amplified many folds which makes the PNs very sensitive to small changes in the presynaptic ORN input. But the PNs associated with sex pheromone target different regions of the lateral horn in a sexually dimorphic manner, and thus the same pheromone elicits distinct behavior in males and females [32, 68, 71]. Most of the ORNs to PN synapses are cholinergic, and PNs respond more strongly to the fluctuating amount of

odors in the odor plume [8, 32, 68]. Next, the PNs form synaptic connections with Kenyon cells, neurons of the protocerebrum, and mushroom bodies. Odor information from multiple glomeruli finally merges into narrowly tuned Kenyon cells which affect memory formation [65].

Despite the knowledge about the neuronal firing path during odor transmission, a pilot question arises in mosquito neurobiology on how discrete sensory inputs integrate and translate into varied behavior. An in-depth understanding of the neuronal circuitry involved in olfactory signaling and decision-making in mosquitoes is limited due to the absence of established neurogenetic methods. A recent study by Olena Riabinina et al. suggested that an integration of the olfactory and gustatory signals commenced within the antennal lobe and subesophageal zone of the brain, respectively [72]. Furthermore, Clement Vinauger et al. reported that despite having a synergistic effect during mosquito navigation, the visual and odor modulation is asymmetric and processed by distinct loci of the brain, where olfaction always works preceding to visual selection. But detailed understanding of the molecular and neurophysiological bases of mosquito olfactory behaviors and crucial decisionmaking events in the brain needs further research.

#### **5.2 Molecular physiology of neuronal signal processing**

Mosquitoes are well known for their plasticity in host preference. The selection of host species for blood meal uptake is skewed depending on the availability of the preferred host, the quality of blood meal, and the defensive behavior of the host. Apart from the neuronal firing and neurotransmitter-mediated signal transmission, the molecular factors of the brain are shown to play a crucial role in olfactory learning, neuronal decision-making, and memory formation in insects [73, 74]. The diverse neuromodulators that include neurotransmitters, neuropeptides, neurohormones, and biogenic amines facilitate the nervous system to transduce varied signals and thus enable the insects to manage the complex behavioral events with amazing accuracy [8, 11]. Neurotransmitters are the primary and potent neurochemicals that make synaptic connections between neurons and thus relay information from presynaptic cells to postsynaptic cells. The crucial neurotransmitters in the insect chemosensory system are acetylcholine, gamma-aminobutyric acid (GABA), and nitrous oxide (NO) [17, 75, 76]. Our ongoing study has shown that blood meal intake causes dynamic changes in the neurotransmitter abundance within the brain, suggesting their possible contribution in cognition and foodassociated memory formation in adult female mosquitoes [77]. Furthermore, we also showed that the gut of the mosquitoes also can synthesize neurotransmitters and play a crucial role in gut-brain-axis communication during metabolic switch (sugar-fed condition to blood-fed condition) and thus modulate neuronal decisionmaking process [77].

Neuromodulators include the neuropeptide and biogenic amines, which have an intense effect on mosquito chemosensation, feeding, social behavior, circadian rhythm, and also maintenance of general physiological homeostasis [11, 17, 78–81]. Usually, these neuromodulators are produced by the specialized neurosecretory cells and released into the local vicinity of the brain circuits and in the hemolymph. Both neuropeptides and biogenic amines modulate the response through G protein-coupled receptor signaling pathway [11, 79]. Two important amine neuromodulators are dopamine and serotonin which are found to modulate mosquitoes' learning and memory response. The immunoreactive neurons of serotonin and dopamine innervate all the glomeruli of AL and higher brain regions such as lateral horn and mushroom body, indicating their role in memory formation [73]. Apart from the biogenic amines, 28 neuropeptides have been predicted from the genome

**83**

**Table 1.**

**6. The action machinery: salivary gland**

*Neuro-Olfactory Regulation and Salivary Actions: A Coordinated Event for Successful…*

3. Allatotropin Stimulates juvenile hormone biosynthesis

7. Ecdysis triggering hormone Trigger ecdysis during larval and pupal molting

8. Eclosion hormone Function not known in mosquitoes

2. Allatostatin A and C Regulate juvenile hormone biosynthesis and gut motility

4. CCHamide 1 A nutrient-responsive hormone in *Drosophila* but function not known in mosquitoes

6. Diuretic hormone Myotropic activity, regulation of Malpighian tubule for fluid

10. Insulin-like peptide Elevate carbohydrate and lipid storage, female reproduction,

12. Neuropeptide F Inhibition of anterior midgut peristalsis in larval stage

secretion, osmoregulation, and diuresis

vitellogenesis, hemocyte differentiation, blood meal digestion

Induces ecdysone production and egg development

1. Adipokinetic hormone Mobilizes stored carbohydrate

5. Corazonin Cardioactive peptide

9. FMRFamide Heart contraction

14. Prothoracicotropic hormone Regulates metamorphosis 15. Pyrokinin Regulation of diuresis

16. Short neuropeptide F Regulation of host-seeking behavior 17. Sulfakinin Function not known in mosquitoes 18. Tachykinin Function not known in mosquitoes

11. Leukokinin Diuresis

13. Ovary ecdysteroidogenic hormone

*List of peptide hormones and their possible functions.*

database of *Aedes aegypti* through the bioinformatics approach [82]. Among them, short neuropeptide F (sNPF) was found to play a crucial role in mosquito feeding and inhibition of host-seeking behavior following blood feeding [83, 84]. Recent studies provide contrasting evidence that either sNPF is synthesized from male accessory gland and transferred to the female during mating or female's own sNPF titer is increased in the hemolymph after consumption of blood meal significantly and reduces host-seeking behavior in adult females. While the functions of different neurohormones have been studied in many insects, functional studies in mosquitoes are limited. The wide distribution of peptide hormones throughout the mosquito body from the neurosecretory cells of the brain (corpora allata, corpus cardiacum) to the endocrine cells of the gut enable them to perform diverse function in mosquito physiology such as (1) regulation of metabolism, (2) maintenance of physiological homeostasis, (3) metamorphosis and eclosion, (4) osmoregulation, and (5) regulation of vitellogenesis and gonotropic cycle [11]. **Table 1** summarizes the name of peptide hormones in mosquitoes and their possible functions.

A successful feeding event of mosquitoes is regulated by the synchronized action of mosquito navigation and food choice decision which finally tuned

*DOI: http://dx.doi.org/10.5772/intechopen.90768*

**Sl. no. Peptide hormone name Function**


*Neuro-Olfactory Regulation and Salivary Actions: A Coordinated Event for Successful… DOI: http://dx.doi.org/10.5772/intechopen.90768*

#### **Table 1.**

*Sino-Nasal and Olfactory System Disorders*

affect memory formation [65].

making events in the brain needs further research.

**5.2 Molecular physiology of neuronal signal processing**

odors in the odor plume [8, 32, 68]. Next, the PNs form synaptic connections with Kenyon cells, neurons of the protocerebrum, and mushroom bodies. Odor information from multiple glomeruli finally merges into narrowly tuned Kenyon cells which

a pilot question arises in mosquito neurobiology on how discrete sensory inputs integrate and translate into varied behavior. An in-depth understanding of the neuronal circuitry involved in olfactory signaling and decision-making in mosquitoes is limited due to the absence of established neurogenetic methods. A recent study by Olena Riabinina et al. suggested that an integration of the olfactory and gustatory signals commenced within the antennal lobe and subesophageal zone of the brain, respectively [72]. Furthermore, Clement Vinauger et al. reported that despite having a synergistic effect during mosquito navigation, the visual and odor modulation is asymmetric and processed by distinct loci of the brain, where olfaction always works preceding to visual selection. But detailed understanding of the molecular and neurophysiological bases of mosquito olfactory behaviors and crucial decision-

Despite the knowledge about the neuronal firing path during odor transmission,

Mosquitoes are well known for their plasticity in host preference. The selection of host species for blood meal uptake is skewed depending on the availability of the preferred host, the quality of blood meal, and the defensive behavior of the host. Apart from the neuronal firing and neurotransmitter-mediated signal transmission, the molecular factors of the brain are shown to play a crucial role in olfactory learning, neuronal decision-making, and memory formation in insects [73, 74]. The diverse neuromodulators that include neurotransmitters, neuropeptides, neurohormones, and biogenic amines facilitate the nervous system to transduce varied signals and thus enable the insects to manage the complex behavioral events with amazing accuracy [8, 11]. Neurotransmitters are the primary and potent neurochemicals that make synaptic connections between neurons and thus relay information from presynaptic cells to postsynaptic cells. The crucial neurotransmitters in the insect chemosensory system are acetylcholine, gamma-aminobutyric acid (GABA), and nitrous oxide (NO) [17, 75, 76]. Our ongoing study has shown that blood meal intake causes dynamic changes in the neurotransmitter abundance within the brain, suggesting their possible contribution in cognition and foodassociated memory formation in adult female mosquitoes [77]. Furthermore, we also showed that the gut of the mosquitoes also can synthesize neurotransmitters and play a crucial role in gut-brain-axis communication during metabolic switch (sugar-fed condition to blood-fed condition) and thus modulate neuronal decision-

Neuromodulators include the neuropeptide and biogenic amines, which have an intense effect on mosquito chemosensation, feeding, social behavior, circadian rhythm, and also maintenance of general physiological homeostasis [11, 17, 78–81]. Usually, these neuromodulators are produced by the specialized neurosecretory cells and released into the local vicinity of the brain circuits and in the hemolymph. Both neuropeptides and biogenic amines modulate the response through G protein-coupled receptor signaling pathway [11, 79]. Two important amine neuromodulators are dopamine and serotonin which are found to modulate mosquitoes' learning and memory response. The immunoreactive neurons of serotonin and dopamine innervate all the glomeruli of AL and higher brain regions such as lateral horn and mushroom body, indicating their role in memory formation [73]. Apart from the biogenic amines, 28 neuropeptides have been predicted from the genome

**82**

making process [77].

*List of peptide hormones and their possible functions.*

database of *Aedes aegypti* through the bioinformatics approach [82]. Among them, short neuropeptide F (sNPF) was found to play a crucial role in mosquito feeding and inhibition of host-seeking behavior following blood feeding [83, 84]. Recent studies provide contrasting evidence that either sNPF is synthesized from male accessory gland and transferred to the female during mating or female's own sNPF titer is increased in the hemolymph after consumption of blood meal significantly and reduces host-seeking behavior in adult females. While the functions of different neurohormones have been studied in many insects, functional studies in mosquitoes are limited. The wide distribution of peptide hormones throughout the mosquito body from the neurosecretory cells of the brain (corpora allata, corpus cardiacum) to the endocrine cells of the gut enable them to perform diverse function in mosquito physiology such as (1) regulation of metabolism, (2) maintenance of physiological homeostasis, (3) metamorphosis and eclosion, (4) osmoregulation, and (5) regulation of vitellogenesis and gonotropic cycle [11]. **Table 1** summarizes the name of peptide hormones in mosquitoes and their possible functions.
