Variability and selectivity of anterior pituitary response to dopamine agonists throughout the normal menstrual cycle

To clarify whether there is a variation of dopamine effect throughout the normal menstrual cycle, 24 studies were performed during the follicular, periovulatory, and luteal phase in seven ovulatory women. The subjects were studied for 24 hours after receiving two different dopamine agonists, 2.5 mg of bromocriptine in one cycle and 50 micrograms of pergolide in a subsequent cycle. Baseline plasma luteinizing hormone, follicle-stimulating hormone, prolactin, and thyrotropin were followed through time, and the dynamic responses to gonadotropin-releasing hormone and thyrotropin-releasing hormone before and at 6 and 22 hours after medication were studied. Since the results obtained with both agonists were similar, the data have been combined in a single group. Baseline luteinizing hormone levels (but not follicle-stimulating hormone) were significantly suppressed (p less than 0.01) during the follicular phase only, and the plasma luteinizing hormone and follicle-stimulating hormone response to gonadotropin-releasing hormone was not affected by the agonists in any of the three cycle phases. Baseline plasma prolactin was suppressed equally (p less than 0.005) in all phases of the cycle, and the response to thyrotropin-releasing hormone was similarly suppressed in all phases only at 6 hours (p less than 0.002). Baseline thyrotropin also was suppressed (p less than 0.01) in all phases but the degree of inhibition was greater in the luteal than in the follicular phase (p less than 0.05). The response to thyrotropin-releasing hormone was inhibited, with the smallest response seen at 22 hours (p less than 0.01). In conclusion, these results suggest that the modulatory effect of dopamine on pituitary hormone secretion is variable and selective throughout the normal menstrual cycle. The greatest inhibition is on prolactin release, which is similar in all phases, followed by thyrotropin, which is greater in the luteal phase, and then by luteinizing hormone in the follicular phase only; it has no effect on follicle-stimulating hormone release.