**3. Serotonin and aggression**

Serotonin, as an ancient chemical, is a key neurotransmitter. It plays a critical role in shaping social responses by regulating both basic (proactive) behaviors

**35**

stressors presented.

*Gut-Brain Axis: Probiotic,* Bacillus subtilis*, Prevents Aggression via the Modification…*

disorders in humans and various experimental animals [62, 63, 159, 160].

and reduces aggressive behaviors in primates and rodents [178, 179].

(such as feeding, drinking, and sexuality) and reactive behaviors (fearfulness, anxiety, and cognition) including aggressive behaviors [155, 156] and mood disorders [157, 158]. Abnormalities of blood and brain levels of 5-HT, 5-HIAA (its metabolite, 5-hydroxyindoleacetic acid), tryptophan, and its receptors have been used as major indicators or targets in the diagnoses and treatments of psychiatric and compulsive

In the CNS, 5-HT functions to inhibit aggression, thereby controlling domestic behaviors [161–163]. The 5-HT deficiency theory of aggression is driven from the negative correlation between the changes of the CNS 5-HT and aggressiveness in humans [164, 165], non-human primates [166], rodents [25, 167], and chickens [24, 168]. Aggressive animals have low levels of 5-HT in the brain, including in the hypothalamus [169–171]. Experimental increase of 5-HT and/or 5-HIAA in the brain, such as in the lateral hypothalamus and amygdala, blocks or retracts killing behavior in rodents [172, 173]. Hypothalamic injection of a 5-HT1a agonist inhibits aggression in male hamsters [174]. Depletion of brain 5-HT in TPH2 mutant mice marks aggression and lowers habituation in novel environments [149]. In addition, 5-HT1BR knockout mice show increased aggression and impulsivity [175, 176]. The implication of 5-HT successfully relieves the depression syndromes in humans [177]

In the peripheral system, however, pathophysiological roles of 5-HT in behavioral and motivational regulations are unclear. Reduced, elevated, and unchanged blood 5-HT concentrations have all been reported in association with behavioral dysfunctions, including aggressiveness [180, 181]. The conflicting data from different investigations could be related to the differences in species of animals, behavioral evaluations, and/or stressors used as well as duration and frequency of

**4. Chicken as an animal model for social stress and related aggression**

Human emotional susceptibility in an inter-group is associated with individual differences in the functions of the hormonal and neurochemical systems in response to internal and external stimulations [182, 183]. Various social and biological factors are associated with the development and expression of aggressive behaviors, including environmental, genetic, cognitive, hormonal, and neurotransmitter circumstances [184]. Aggression and associated mental illness are an emerging public health problem [29]. Animal models are critical for investigating the potential biological processes involved in human aggression and mental disorders. Although there are dissimilarities between humans and chickens, as indicated, the neural circuitry for aggression and social behavior appear to be evolutionarily conserved across the vertebrates [185]; chickens have been used as an animal model in various clinical and psychopharmacological studies, such as anxiety, depression, and aggression [186, 187]. A commercial hen can have more than 300 offspring with similar genetic characteristics during her lifetime, and chicks can be hatched without maternal condition effects. Previous studies have reported that birds' brain possess a core "social behavioral network" which is humongous to the social behavioral network of mammals [188]. There are evidences that the central nuclei involved in moodiness in avian, at least in part, are morphofunctional homologous to the mammalian nuclei [189], such as the hypothalamus [190], nucleus taeniae (homolog to the amygdala of mammals, [191, 192]), and Raphe nucleus [193]. These nuclei exert similar cognitive abilities and consciousness [194] with capability of plasticity in response to environmental stimulations [195]. In addition, there are similar distributions of neurotransmitter receptors, including serotonergic receptors

*DOI: http://dx.doi.org/10.5772/intechopen.86775*

*Gut-Brain Axis: Probiotic,* Bacillus subtilis*, Prevents Aggression via the Modification… DOI: http://dx.doi.org/10.5772/intechopen.86775*

(such as feeding, drinking, and sexuality) and reactive behaviors (fearfulness, anxiety, and cognition) including aggressive behaviors [155, 156] and mood disorders [157, 158]. Abnormalities of blood and brain levels of 5-HT, 5-HIAA (its metabolite, 5-hydroxyindoleacetic acid), tryptophan, and its receptors have been used as major indicators or targets in the diagnoses and treatments of psychiatric and compulsive disorders in humans and various experimental animals [62, 63, 159, 160].

In the CNS, 5-HT functions to inhibit aggression, thereby controlling domestic behaviors [161–163]. The 5-HT deficiency theory of aggression is driven from the negative correlation between the changes of the CNS 5-HT and aggressiveness in humans [164, 165], non-human primates [166], rodents [25, 167], and chickens [24, 168]. Aggressive animals have low levels of 5-HT in the brain, including in the hypothalamus [169–171]. Experimental increase of 5-HT and/or 5-HIAA in the brain, such as in the lateral hypothalamus and amygdala, blocks or retracts killing behavior in rodents [172, 173]. Hypothalamic injection of a 5-HT1a agonist inhibits aggression in male hamsters [174]. Depletion of brain 5-HT in TPH2 mutant mice marks aggression and lowers habituation in novel environments [149]. In addition, 5-HT1BR knockout mice show increased aggression and impulsivity [175, 176]. The implication of 5-HT successfully relieves the depression syndromes in humans [177] and reduces aggressive behaviors in primates and rodents [178, 179].

In the peripheral system, however, pathophysiological roles of 5-HT in behavioral and motivational regulations are unclear. Reduced, elevated, and unchanged blood 5-HT concentrations have all been reported in association with behavioral dysfunctions, including aggressiveness [180, 181]. The conflicting data from different investigations could be related to the differences in species of animals, behavioral evaluations, and/or stressors used as well as duration and frequency of stressors presented.
