Distribution and change in number of gonadotropin-releasing hormone-1 neurons following activation of the photoneuroendocrine system in the chick, Gallus gallus

The photoneuroendocrine system (PNES) of chicks was activated by transferring birds to a long photoperiod and by giving them a diet supplemented with sulfamethazine (SMZ), a compound that augments the effect of long-day photostimulation. We wished to determine (1) the number of gonadotropin-releasing hormone-1 (GnRH-1) neurons in each identified nucleus (n.) in the subpallium and diencephalon and the major terminal fields (TFs) of GnRH-1 neurons, and (2) the effect of SMZ on the immunoreactive expression of GnRH-1 in perikarya. Four groups of birds were exposed to one of two light treatments, viz., light:dark (LD) cycles of LD20:4 or LD8:16, and given one of two rations, viz., control or one supplemented with SMZ (n=5/treatment). After 3 days, chicks were anesthetized, and their brains were prepared for immunocytochemistry with an antibody identifying GnRH-1 neurons. Seven areas or nuclei contained GnRH-1 neurons: paramedial septal n., preoptic periventricular n./periventricular hypothalamic n., bed n. of the pallial commissure (NCPa), parvocellular lateral and medial septal n., lateral septum near the ventral horn of the lateral ventricle, parvocellular lateral anterior thalamic n., and displaced thalamic neurons. Six TFs of GnRH neurons were found including the organum vasculosum of lamina terminalis (OVLT), preoptic recess (POR), hypothalamic recess (HR), lateral septum adjacent to the ventral horn of the lateral ventricle (SL-VLvh) associated with the choroid plexus, subseptal organ (SSO), and external zone of the median eminence. The extensive TFs for GnRH-1 neurons in the OVLT, POR/HR, SL-VLvh, and SSO suggested that a large amount of the peptide was secreted into the ventricular system. The NCPa responded to the photoperiod and SMZ treatments combined, with a significant increase in GnRH-1 cell number compared with birds fed control diets and exposed to a short-day photoperiod. More than 73% of GnRH-1 neurons resided in the septal region of the subpallium and not in the preoptic hypothalamic region characteristic of several mammalian species. Thus, instead of the traditional descriptor hypothalamo-pituitary-gonadal axis, either the septal- or subpallial-pituitary-gonadal axis may be more appropriate for describing the neuroendocrine axis related to gonadal function in birds.