The dopaminergic system in the brain of the native Thai chicken, Gallus domesticus: localization and differential expression across the reproductive cycle

Dopamine (DA) has a pivotal role in avian prolactin (PRL) secretion, acting centrally through D₁ DA receptors to stimulate PRL secretion by operating through vasoactive intestinal peptide (VIP). DA also inhibits PRL secretion by activating D₂ DA receptors at the pituitary level. This study was designed to investigate the distribution of DA neurons in the native Thai chicken, utilizing tyrosine hydroxylase (TH) as a marker for dopaminergic neurons. The differential expression of hypothalamic TH immunoreactive (TH-ir) neurons was also compared across the reproductive cycle. The results revealed that TH-ir neurons and fibers were found throughout the brain of the laying hen and were predominantly located within the diencephalon and mesencephalon. The observed distribution pattern of TH immunoreactivity was consistent with that reported previously in several avian species. However, changes in the number of TH-ir neurons in the nucleus intramedialis (nI) were observed across the reproductive cycle and correlated directly with variations in PRL levels. The population of TH-ir neurons in the nI increased significantly during the egg incubation period, where circulating PRL levels were the greatest. This study indicates, for the first time, that an association exists between DA neurons and the regulation of the reproductive system in the native Thai chicken. There is a paucity of information about the reproductive neuroendocrine regulation of tropical non-seasonally breeding avian species and it is suggested that the differential expression of DA neurons in the nI might play a role in the control of VIP secretion and subsequent PRL release in such birds.