MODULATION OF PROLACTIN RECEPTORS IN THE RAT HYPOTHALAMUS IN RESPONSETO CHANGES IN SERUM CONCENTRATION OF ENDOGENOUS PROLACTIN OR TO OVINEPROLACTIN ADMINISTRATION

Specific binding of I-125-labeled rat prolactin (I-125-rat PRL) to hyp othalamic membranes was studied in Sprague-Dawley rats after ovine PRL administration and in relation to rat PRL serum variations induced by ectopic pituitary implants or by drugs which stimulate (domperidone) or inhibit (bromocriptine) PRL release. Repeated treatments with ovine PRL markedly increased specific binding values of I-125-rat PRL to hy pothalamic membranes of female rats. Repeated treatments with domperid one also increased specific PRL binding in the hypothalamus. This effe ct was associated with an increase in PRL serum levels. Similar result s were obtained in male rats after renal pituitary implants which resu lted in a state of chronic hyperprolactinaemia. In contrast, a subchro nic treatment with bromocriptine decreased specific PRL binding in the hypothalamus and concomitantly caused a sharp reduction in PRL serum levels. Scatchard analysis of data obtained from competition curves sh owed that the variations in the level of PRL binding to hypothalamic m embranes were related to the number of PRL binding sites but not to th e dissociation constant (K-d), which was unaffected by different treat ments or by pituitary implantation. These results demonstrate a correl ation between circulating concentrations of PRL and number of its rece ptors in the rat hypothalamus and give further support to the hypothes is that these binding sites may have a specific functional role in reg ulating the homeostasis of pituitary PRL secretion.