Effects of cycle stage on regionalised galanin, galanin receptors 1–3, GNRH and GNRH receptor mRNA expression in the ovine hypothalamus

The neurotransmitter galanin has been implicated in the steroidogenic regulation of reproduction based on work mainly conducted in rodents. This study investigated the temporal changes in the expression of galanin and its three receptor isoforms and GNRH and GNRHR mRNA in specific hypothalamic nuclei known to be involved in the regulation of reproductive cyclicity, namely the medial pre-optic area (mPOA), the rostral mPOA/organum vasculosum of the lamina terminalis, the paraventricular nucleus and the arcuate nucleus using an ovine model. Following synchronisation of their oestrous cycles, tissues were collected from ewes at five time points: the early follicular, mid follicular (MF) and late follicular phases and the early luteal and mid luteal phases. The results indicated significant differences in regional expression of most of the genes studied, with galanin mRNA expression being highest during the MF phase at the start of the GNRH/LH surge and the expression of the three galanin receptor (GalR) isoforms and GNRH and its receptor highest during the luteal phase. These findings are consistent with a role for galanin in the positive feedback effects of oestradiol (E2) on GNRH secretion and a role for progesterone induced changes in the pattern of expression of GalRs in the regulation of the timing of E2's positive feedback through increased sensitivity of galanin-sensitive systems to secreted galanin.