Expression of hypothalamic GnRH-I mRNA in the female turkey at different reproductive states and following photostimulation

In birds, changes in hypothalamic gonadotropin-releasing hormone-I (GnRH-I) content and release are correlated with reproductive stages. This study examined the distribution and expression level of GnRH-I mRNA in anatomically discrete hypothalamic nuclei throughout the turkey reproductive cycle and following photostimulation. GnRH-I mRNA expression was determined using in situ hybridization in non-photostimulated (NPS), egg-laying (LAY), incubating (INC) and photorefractory (REF) hens. Overall, GnRH-I mRNA expression was greatest in the nucleus commissurae pallii (nCPa) and around the organum vasculosum lamina terminalis (OVLT), with less expression observed in the nucleus septalis lateralis (SL), cortico-habenula cortico-septum area, and within the nucleus preopticus medialis. GnRH-I mRNA expression was significantly increased in nCPa, OVLT, and SL after NPS hens (6L:18D) were exposed to a 30 or 90 min pulse of light beginning 14 h after first light (dawn). GnRH-I mRNA abundance within nCPa, OVLT and SL was greater in LAY than in NPS and INC hens, while mRNA expression was least in REF hens. These results indicate that GnRH-I mRNA expression in birds is sensitive to light stimulation during the photosensitive period and can be used to more precisely characterize their different reproductive stages.