Sexual differences in the stimulatory effects of bombesin on tuberoinfundibular dopaminergic neurons

Abstract

The purpose of this study was to (1) examine the effects of central administration of bombesin on the activity of tuberoinfundibular dopaminergic (DA) neurons in male and female rats, and (2) determine if sexual differences in the responsiveness of these neurons to bombesin were due to the presence of prolactin or gonadal steroids. The activity of tuberoinfundibular DA neurons was estimated by measuring the concentrations of the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in terminals of these neurons in the median eminence. The effects of bombesin on the secretion of prolactin and α-melanocyte stimulating hormone (αMSH), and the activities of nigrostriatal, mesolimbic, and periventricular-hypophysial Da neurons were also determined in gonadally intact male and female rats. Central administration of bombesin (10 ng/rat; i.c.v.) decreased prolactin secretion in gonadally intact male and female rats, but only in males was this associated with an increase in the activity of tuberoinfundibular DA neurons. In contrast, bombesin increased the activity of periventricular-hypophysial DA neurons terminating in the intermediate lobe of both male and female rats, and this was associated with a decrease in αMSH secretion in both sexes. Bombesin had no effect on the activities of nigrostriatal, mesolimbic, or periventricular-hypophysial DA neurons terminating in the neural lobe in either sex. The loss of endogenous gonadal hormones following ovariectomy rendered tuberoinfundibular DA neurons responsive to the stimulatory effects of bombesin, whereas immunoneutralization of endogenous prolactin following administration of prolactin antiserum had no effect on the inability of bombesin to alter the activity of these neurons in female rats. Estrogen administration to both ovariectomized and ovariectomized + prolactin antiserum-treated rats reinstated the lack of responsiveness of tuberoinfundebular DA neurons to bombesin, whereas neither orchidectomy nor testosterone administration to orchidectomized male rats altered the stimulatory effects of bombesin on the activity of tuberoinfundibular DA neurons. Taken together, these results reveal that sexual differences in the responsiveness of tuberoinfundibular DA neurons to the stimulatory effects of bombesin are due, at least in part, to the prolactin-independent effects of estrogen.