Temporal Relationship and Role of Dopamine in "Short-Loop" Feedback of Prolactin

Abstract

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.