The central serotonergic system is involved in various functions such as instinct, emotion, and cognitive functions in mammals. However, it is not clear how much molecular and functional conservation the central serotonergic system has in vertebrates. It has become clear that birds have advanced cognitive functions comparable to mammals. Therefore, we attempted to elucidate the molecular and anatomical structure of the central serotonergic system using the chicken (Gallus gallus domesticus) as an avian model.

First, we determined the location of the dorsal raphe (DR) nucleus and median raphe (MR) nucleus in the chicken using the expression distribution of the chicken orthologs of serotonin transporter (SERT) and tryptophan hydroxylase 2 (TPH2) in the brainstem. Next, we clarified the serotonin receptors (5-HT receptors, 5-HTRs) expressed in serotonergic neurons contained in the DR and MR (Fujita, et al., 2022a). Our data indicate that the molecular properties of serotonergic neurons are evolutionarily well conserved between birds and mammals. In addition, we comprehensively elucidated the expression regions of almost all 5-HTR genes in the chicken telencephalon (Fujita, et al., 2020; 2022b). Our data has comprehensively revealed the brain regions and receptors modulated by serotonin in the avian telencephalon, making it possible to access the understanding of the neural circuits that govern cognition and emotion modulated by the serotonergic system in birds.