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Melatonin Receptors in the Brain and Pituitary Gland of Hypothalamo‐Pituitary Disconnected Soay Rams
Author(s) -
Williams L. M.,
Lincoln G. A.,
Mercer J. G.,
Barrett P.,
Morgan P. J.,
Clarke I. J.
Publication year - 1997
Publication title -
journal of neuroendocrinology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.062
H-Index - 116
eISSN - 1365-2826
pISSN - 0953-8194
DOI - 10.1046/j.1365-2826.1997.00625.x
Subject(s) - pars tuberalis , medicine , endocrinology , melatonin , prolactin , pineal gland , melatonin receptor , biology , pituitary gland , hypothalamus , prolactin cell , receptor , pinealocyte , endocrine gland , hormone
In Soay rams in which the pituitary gland has been surgically separated from the hypothalamus, blood prolactin concentrations vary in response to changes in photoperiod and the administration of melatonin, as in intact animals, providing evidence that melatonin acts within the pituitary gland to control prolactin secretion. In this study the presence 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‐( 125 I)‐ iodomelatonin and in situ hybridization for the melatonin receptor. There was no effect of the HPD operation on the pattern and quantity of 2‐( 125 I)iodomelatonin binding in the brain demonstrating that this binding is independent of hypothalamic regulation. The possibility that melatonin may control prolactin secretion directly via specific receptors on lactotrophs was investigated using dual in situ hybridization with a ( 35 S) labelled probe for the ovine melatonin receptor (Mel1a b ) and a Digoxigenin labelled probe for ovine prolactin. Melatonin receptor gene expression was observed in the PT/ZT in both intact and HPD rams, however, there was no colocalization with prolactin gene expression; only in the ZT was there a close association between cells expressing the melatonin receptor and lactotrophs. The results provide strong support for the view that melatonin acts via the PT/ZT to mediate the effects of photoperiod on the seasonal cycle in prolactin secretion.