Dopaminergic control of circadian and pulsatile pituitary thyrotropin release in women

Dopamine (DA) inhibits pituitary TSH release, but its role as a regulator of circadian and pulsatile TSH secretion is not clear. Accordingly, we studied the 24-h TSH secretory patterns in seven normal women in the early follicular phase of their cycles before and during DA receptor blockade by metoclopramide (MCP). Serum TSH was measured by a highly sensitive (0.05 mU/L) RIA at 15-min intervals for 48 h during sequential 24-h saline and 24-h MCP infusions (30 micrograms/kg.h). Sleep was confirmed by electroencephalogram between 2300-0700 h. All women had a nocturnal rise of TSH, independent of sleep, which began in the late afternoon and reached a peak (acrophase) after midnight during the saline infusion. This circadian periodicity was composed of a series of TSH pulses with greater magnitude and frequency during nocturnal hours. Infusion of MCP had no effect on pulse frequency, but the pulse amplitude increased (P less than 0.05), especially at night. As a consequence, the circadian excursion of TSH, as assessed by cosinor function, was exaggerated. The mean acrophase amplitude and mesor levels increased (P less than 0.05), but the nadir and acrophase times did not change. These findings suggest that DA is an inhibitor of TSH pulse amplitude throughout the 24-h biological clock. By inference, the neuroendocrine mechanism(s) that underlies the nocturnal increase in TSH secretion is not due to decreased dopaminergic inhibition.