GABA-ergic and dopaminergic mechanisms in gonadotrophin secretion in males--effects of baclofen and metoclopramide.

The GABA analogue, baclofen, and the dopamine antagonist, metoclopramide, were studied with respect to their effects on basal and LRH-induced LH and FSH release in 6 normal male volunteers. Basal gonadotrophin secretion was unchanged following the administration of baclofen or metoclopramide given alone or in combination. LRH-stimulated LH release was significantly blunted after metoclopramide administration in the baclofen pre-treated volunteers. Serum LH concentration (mean +/- SD) in the control phase was 30.1 +/- 17.2 mIU/ml and was 19.4 +/- 9.6 mIU/ml after baclofen plus metoclopramide (P less than 0.02). LRH-stimulated values, however, were unaffected by baclofen or metoclopramide when the drugs were given alone. LRH-stimulated FSH release was not significantly influenced by baclofen or metoclopramide given alone or in combination. Basal Prl secretion increased significantly when baclofen and metoclopramide were given separately and in combination. Basal Prl concentration (mean +/- SD) increased from 14 +/- 2 ng/ml to 18.7 +/- 4.8 ng/ml after baclofen and to 111.5 +/- 31.9 ng/ml after metoclopramide (P less than 0.01). The rise in serum Prl concentration, however, was not significantly different when measured after metoclopramide alone (111 +/- 31.9 ng/ml) or after metoclopramide and baclofen (112 +/- 33.3 ng/ml). It is proposed that GABA and dopamine exert opposing effects on LH secretion in normal men.