Inhibition of Gonadotropin and Prolactin Release by Dopamine: Effect of Endogenous Estradiol Levels*

Abstract

The influence of endogenous estradiol (E₂) levels on gonadotropin and PRL sensitivity to dopamine (DA) infusion (4 μg/kg/min) was assessed at different stages of the follicular phase of the menstrual cycle. Basal LH and FSH levels were comparable in day 2 and day 12 subjects, and despite a 4-fold increase in E₂ concentration, the inhibition of LH by DA was small and quantitatively similar and there was no discernible effect on FSH in either group. In marked contrast, day 14 subjects with an elevated basal LH level exhibited a dramatic increase in the sensitivity of LH and FSH to DA inhibition. Further, a remarkable rebound release for LH but not FSH occurred on the termination of DA infusion. There was a significant correlation between basal LH and response to DA (r = 0.979). This unique increase in response to DA at a time when hypothalamic LRF secretion is assumed to be elevated suggests that DA may exert its effect by inhibiting LRF release. The inhibition of PRL release by DA is correlated with endogenous E₂ levels (r = 0.685) as well as basal PRL levels (r = 0.878). Rebound release of PRL occurs in all three groups of women on termination of the DA infusion, but the magnitude was greatest in Day 14 subjects with the highest endogenous E₂ levels. These data suggest that while E₂ seems to augment the sensitivity of PRL inhibition by DA, its does not seem to directly influence gonadotropin sensitivity to DA inhibition. The selective hypersensitivity of both LH and FSH to DA observed on the day before midcycle LH peak is consistent with a reduction in LRF neuronal inhibition by tuberoinfundibular DA neurons at this time.