A sex difference in the stimulatory afferent regulation of tuberoinfundibular dopaminergic neuronal activity

The involvement of afferent neuronal systems in the maintenance of basal and prolactin-stimulated tuberoinfundibular dopaminergic (TIDA) neuronal activity was examined in female and male rats. The synthesis and turnover of dopamine (DA) was measured in the median eminence, the terminal region of the TIDA neurons, to estimate the activity of these neurons. Complete and retrochiasmatic deafferentations of the mediobasal hypothalamus were made 7 days prior to experimentation to either completely isolate the TIDA neurons from the rest of the brain or to interrupt neuronal connections from rostral brain regions to the TIDA neurons, respectively. Both complete and retrochiasmatic deafferentations decreased the basal rate of DA synthesis and turnover in the median eminence of female, but not of male rats. These results suggest that neuronal afferents originating rostral to the mediobasal hypothalamus stimulate TIDA neurons in the female but not in the male rat. Intracerebroventricular administration of rat prolactin increased DA synthesis in the median eminence of both sham and retrochiasmatic deafferentiated female and male rats showing that the stimulatory action of prolactin is not blocked by retrochiasmatic deafferentation. Ovariectomy reduced the rate of DA synthesis in the median eminence but retrochiasmatic deafferentation did not cause a further decrease in ovariectomized rats. These results suggest that retrochiasmatic deafferentation and ovariectomy may remove a stimulatory input to the TIDA neurons which is mediated through a common afferent neuronal pathway. These afferent influences do not appear to be operational in the adult male rat since retrochiasmatic deafferentation did not reverse the castration-induced increase in the rate of DA synthesis in the median eminence of male rats.