MELATONIN RECEPTORS IN THE BRAIN AND PITUITARY-GLAND OF HYPOTHALAMOPITUITARY DISCONNECTED SOAY RAMS

In Soay rams in which the pituitary gland has been surgically separate d from the hypothalamus, blood prolactin concentrations vary in respon se to changes in photoperiod and the administration of melatonin, as i n intact animals, providing evidence that melatonin acts within the pi tuitary gland to control prolactin secretion. In this study the presen ce of potentially functional melatonin receptors in the pars tuberalis and zona tuberalis (PT/ZT) of hypothalamo-pituitary disconnected (HPD ) Soay rams is confirmed using both in vitro autoradiography with the ligand 2-(I-125)- iodomelatonin and in situ hybridization for the mela tonin receptor. There was no effect of the HPD operation on the patter n and quantity of 2-(I-125)iodomelatonin binding in the brain demonstr ating that this binding is independent of hypothalamic regulation. The possibility that melatonin may control prolactin secretion directly v ia specific receptors on lactotrophs was investigated using dual in si tu hybridization with a (S-35) labelled probe for the ovine melatonin receptor (Mel1a(b)) and a Digoxigenin labelled probe for ovine prolact in. Melatonin receptor gene expression was observed in the PT/ZT in bo th intact and HPD rams, however, there was no colocalization with pro[ actin gene expression; only in the ZT was there a close association be tween cells expressing the melatonin receptor and lactotrophs. The res ults provide strong support for the view that melatonin acts via the P T/ZT to mediate the effects of photoperiod on the seasonal cycle in pr olactin secretion.