Effect of gamma-aminobutyric acid on growth hormone and prolactin secretion in man: influence of pimozide and domperidone.

To investigate the possibility that GH release induced by gamma-aminobutyric acid (GABA) in man is mediated by a dopaminergic mechanism(s), we evaluated the effect of two antidopaminergic compounds, pimozide and domperidone, on the plasma GH response to acute GABA administration (5 g, orally). Only the former compound can freely cross the blood-brain barrier. In 9 normal volunteers, GABA caused a significant increase of plasma GH levels (P < 0.0001 vs. GH levels in a group of 14 controls). In these subjects, pimozide (6 mg/day, orally, for 4 days) significantly blunted the GH elevation induced by GABA (P < 0.01). Unlike pimozide, in 8 additional subjects, domperidone (4 mg injected iv immediately before GABA administration) did not influence the GABA-induced GH response. GABA did not alter either baseline or pimozide-stimulated plasma PRL levels. Likewise, it did not significantly modify the brisk PRL rise after domperidone injection. These findings are consistent with the hypothesis that GABA-stimulated GH secretion is mediated via dopamine release at a suprapituitary level. With regard to PRL secretion, no GABA-dopamine interactions are readily apparent.