EVIDENCE THAT PROLACTIN MEDIATES THE STIMULATORY EFFECTS OF ESTROGEN ON TUBEROINFUNDIBULAR DOPAMINE NEURONS IN FEMALE RATS

The effects of ovariectomy and estrogen on prolactin secretion and/or the activity of tuberoinfundibular dopamine (TIDA) neurons were examined by either concurrently measuring concentrations of prolactin in plasma and 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence of female rats or by determining the rate of DA synthesis (accumulation of 3,4-dihydroxyphenylalanine (DOPA) after the administration of a decarboxylase inhibitor) in the median eminence. For comparison, concentrations of alpha-melanocyte-stimulating hormone (alpha-MSH) in plasma and DOPAC in the intermediate lobe of the pituitary (an index of the activity of tuberohypophysial DA neurons) were also determined. Ovariectomy produced a time-dependent decrease in the accumulation of DOPA and the concentrations of DOPAC in the median eminence and prolactin in plasma with maximal effects occurring by 7 days. Estrogen administration to ovariectomized rats increased plasma prolactin and median eminence DOPAC concentrations to levels comparable to those in diestrous controls. In contrast, neither ovariectomy nor estrogen replacement altered the concentrations of alpha-MSH in plasma or DOPAC in the intermediate lobe. Administration of the DA agonist bromocriptine blocked the ability of estrogen to increase plasma prolactin and median eminence DOPAC concentrations. Also, administration of antiserum to rat prolactin blocked the stimulatory action of estrogen on median eminence DOPAC concentrations. Taken together, these results indicate that the stimulatory effect of estrogen on the activity of TIDA neurons is mediated by prolactin.