Alpha 2-adrenoceptor-mediated inhibition of prolactin release in suckling- or fenfluramine-induced hyperprolactinemia.

It has recently been shown that the specific and selective alpha 2-antagonist idazoxan (IDZ) displays prolactin-lowering activity on hyperprolactinemia induced in the rat either by suckling or serotonergic drugs. In an attempt better to understand the role of alpha 2-adrenoceptors under the above conditions, experiments were carried out to compare the effects of IDZ with that of the classic alpha 2-antagonist yohimbine (YOH), and also of the alpha 2-agonists clonidine (CLO) and B-HT 920, on prolactin (PRL) release during lactation and in hyperprolactinemia induced in male rats by the serotonergic drug fenfluramine (FEN). In lactating rats, both alpha 2-agonists decreased PRL release; this effect was enhanced by prior separation of the animals from their pups for several hours. A decrease of plasma PRL levels was also induced by IDZ but not by YOH, which tended further to increase hyperprolactinemia. In male rats treated with FEN, IDZ and CLO, a significant decrease of plasma PRL was produced, but YOH further enhanced PRL secretion. It is concluded that the alpha 2-agonists tested and also the alpha 2-antagonist IDZ display a unique inhibitory activity on PRL release during suckling or serotonergic-induced hyperprolactinemia.