Sexual Differences in the Sensitivity of Tuberoinfundibular Dopamine Neurons to the Actions of Prolactin

Studies were undertaken to investigate the observed sexual differences in tuberoinfundibular dopamine (DA) neuronal activity. The ability of prolactin to differentially alter the activity of tuberoinfundibular DA neurons in gonadectomized male and female rats was determined by measuring the rate of accumulation of dihydroxyphenylalanine (DOPA) after inhibition of DOPA decarboxylase with NSD 1015 in the median eminence, the terminal region of these neurons. The rate of DOPA accumulation in control female rats was 2–3 times greater than that in male rats, confirming previous observations. Estradiol benzoate (25 µg/kg s.c. × 3 days), which increases serum prolactin, increased the rate of DOPA accumulation in the median eminence of both male and female rats. This effect was not observed in hypophysectomized rats, confirming that the estradiol-induced increase in DOPA accumulation is not due to direct action of estrogen, but to an indirect effect via increases in prolactin secretion. It was unexpectedly found, however, that hypophysectomy per se dramatically reduced the rate of DOPA accumulation in the median eminence of female but not male rats. Tuberoinfundibular neurons in the hypophysectomized female rats did, however, retain the capacity to respond to intracerebroventricular (ICV) administered prolactin (1 µg). Multiple injections of the DA agonist, bromocriptine, which like hypophysectomy decreases circulating concentrations of prolactin also selectively decreased the rate of DOPA accumulation in the median eminence of female rats with little effect in the male. These data suggest that the tuberoinfundibular neurons in the female are more sensitive to circulating levels of prolactin than those in the male. To test this possibility, the capacity of prolactin to stimulate DOPA accumulation in the median eminence was compared in male and female rats. Rat prolactin (0.1–10µg) was administered ICV via previously implanted cannulas. When measured 12 h after prolactin administration, DOPA accumulation in the median eminence of female rats was significantly increased at a lower dose than in the male (0.1 vs. 1.0 µg). This difference in sensitivity to prolactin in male and female rats was clearly demonstrated in rats which were first pretreated with bromocriptine and then the effect of graded doses of ICV prolactin determined. In these animals, doses of prolactin as low as 0.03 µg stimulated DOPA accumulation in the median eminence of female rats, while the minimal effective dose in the male rat was unchanged (1.0 µg). These results indicate that the sensitivity of tuberoinfundibular DA neurons to prolactin is markedly different in male and female rats.