Anterior pituitary release of prolactin is inhibited by exposure to short photoperiod

The potential regulatory sites responsible for the decrease of circulating prolactin (PRL) levels shown by many photoperiodic mammals following prolonged exposure to short days was investigated in Siberian hamsters that had been maintained under a stimulatory, long-day photoperiod, and in hamsters that had been shifted to a nonstimulatory, short-day photoperiod for 8-10 weeks. The ability of anterior pituitary fragments (AP) from each of these groups to release prolactin was evaluated in pituitary tissue cultured alone and also in pituitary tissue co-cultured with hypothalamic fragments (HF); using a perifusion tissue culture system. The perfusate from these cultures was collected every 1/2 h for 8 h, and was assayed for basal levels of prolactin using radioimmunoassay. For AP tissue cultured alone, there was a robust reduction in prolactin release by the fragments harvested from short-day housed animals. In AP tissue harvested from long-day exposed animals, co-culture with either long- or short-day HF did not induce significant changes in basal PRL release, Similarly, co-culture with short-day HF did not significantly alter PRL release in short-day APs, However, there was a significant increase in release when short-day APs were co-cultured with long-day HF, These results suggest a direct effect of photoperiod on PRL synthesis and/or release at the level of the pituitary. However, the altered responsiveness of short-day pituitaries could be the result of previous, chronic inhibitory hypothalamic input during short-day exposure, A follow-up study was conducted to investigate the ability of vasoactive intestinal peptide (VIP) to stimulate PRL release from long- and short-day APs. Results indicated that the ability of VIP to stimulate PRL release is both photoperiod and dose dependent.