Effect of neonatal prolactin deficiency on prepubertal tuberoinfundibular and tuberohypophyseal dopaminergic neuronal activity.

PRL is present in milk of lactating rats and, when ingested by the pups, can pass through the gut and enter the systemic circulation. The present study tested whether suppression of normal milk PRL intake by the pups during the early postpartum period affects the subsequent level of activity of the tuberoinfundibular dopamine (DA) system. Maternal plasma and milk PRL levels were depressed in lactating rats by daily injections of bromocriptine. Such treatment during postpartum days 2-5, but not during postpartum days 9-12, resulted in a marked suppression of DA turnover in the median eminence and an elevation of the plasma PRL concentration in the offspring when measured at 30-35 days of age. Simultaneous infusion of ovine PRL by osmotic minipump reversed the effects of neonatal bromocriptine treatment. DA turnover in the neurointermediate lobe was unaffected by any treatment. Basal plasma PRL levels in 3- to 5-day-old pups were not reduced by maternal bromocriptine treatment, indicating that bromocriptine was not secreted into the milk in sufficient amounts to depress endogenous PRL secretion in the pups. These results suggest that the normal activity of the tuberoinfundibular DA system, but not that of the tuberohypophyseal DA system, of the rat may be impaired if milk PRL levels are reduced during a critical postpartum period.