ANTERIOR-PITUITARY RELEASE OF PROLACTIN IS INHIBITED BY EXPOSURE TO SHORT PHOTOPERIOD

The potential regulatory sites responsible for the decrease of circula ting prolactin (PRL) levels shown by many photoperiodic mammals follow ing prolonged exposure to short days was investigated in Siberian hams ters that had been maintained under a stimulatory, long-day photoperio d, and in hamsters that had been shifted to a nonstimulatory, short-da y photoperiod for 8-10 weeks. The ability of anterior pituitary fragme nts (AP) from each of these groups to release prolactin was evaluated in pituitary tissue cultured alone and also in pituitary tissue co-cul tured with hypothalamic fragments (HF); using a perifusion tissue cult ure system. The perfusate from these cultures was collected every 1/2 h for 8 h, and was assayed for basal levels of prolactin using radioim munoassay. 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-cult ure with either long- or short-day HF did not induce significant chang es in basal PRL release, Similarly, co-culture with short-day HF did n ot significantly alter PRL release in short-day APs, However, there wa s a significant increase in release when short-day APs were co-culture d with long-day HF, These results suggest a direct effect of photoperi od on PRL synthesis and/or release at the level of the pituitary. Howe ver, 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 abil ity of vasoactive intestinal peptide (VIP) to stimulate PRL release fr om long- and short-day APs. Results indicated that the ability of VIP to stimulate PRL release is both photoperiod and dose dependent.