Abstract
Summary
In order to study the time relationship and role of dopamine (DA) in feedback inhibition of prolactin (PRL) by PRL, (a), ovine PRL (oPRL) (4 mg/kg) was given to male rats 1,4, and 12 hr before restraint stress; and (b) oPRL was given together with pimozide (2 mg/kg), a specific dopami-nergic receptor blocker. Four hours after injection of oPRL, basal levels of serum rat PRL were significantly reduced. Restraint stress elevated serum PRL levels about tenfold; pretreatment with ovine PRL prevented about 50% of this rise when given 4 hr but not 1 or 12 hr prior to stress. Blockade of dopamine receptors by administration of pimozide produced a six-fold rise in serum prolactin levels by 2 and 4 hr after injection. o-PRL given in combination with pimozide blocked only 25% (p ⋍ 0.05) of the increase of endogenous PRL by pimozide at 2 and 4 h. These observations suggest that (a) the “short-loop” inhibitory feedback on PRL release requires more than 1 but less than 4 hr to become active, and (b) a dopaminergic mechanism is mainly responsible for inhibition of PRL release by PRL.
We are grateful to Dr. Allen Tucker, Department of Dairy Science, Michigan State University, for performing the ovine PRL radioimmunoassays.
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