REPRODUCTIVE EXPERIENCE REDUCES HALOPERIDOL-INDUCED PROLACTIN SECRETION IN FEMALE RATS

The present study examined possible changes in the dopaminergic regula tion of prolactin (PRL) secretion which have been reported to occur in reproductively experienced female rats. In the first experiment, fema le rats which had raised a litter to weaning and age-matched nulliparo us controls were ovariectomized and challenged sequentially 2 days apa rt with a low (0.2 mg/kg) and high (1.0 mg/kg) dose (intravenously) of haloperidol (HAL), a dopamine antagonist. Blood samples were collecte d via intra-atrial catheters, and plasma samples were assayed for PRL content by radioimmunoassay. Whereas HAL stimulated increases in plasm a PRL concentrations in both nulliparous and primiparous animals, sign ificantly higher levels of plasma PRL, were present after both doses o f HAL in the nulliparous rats. A second experiment investigated the ro le of lactation in this change in the ability of HAL to stimulate incr eases in circulating PRL levels. Separate sets of age-matched primipar ous (pups removed on day 1 of lactation) and nulliparous rats were cha llenged with two doses of HAL 2 weeks after gonadectomy. In contrast t o the effect of pregnancy and lactation found in the first experiment, pregnancy and parturition in the absence of lactation failed to alter the female's sensitivity to HAL. The PRL responses in the two groups were identical at the low HAL dose and similar after the high HAL dose . These findings demonstrate that a single prior pregnancy and lactati on, but not pregnancy alone, significantly reduce the ability of HAL t o elevate circulating plasma PRL concentrations. Decreased circulating PRL levels in reproductively experienced females, therefore, may resu lt in part from increased endogenous dopaminergic activity/tone.