Site of gamma-aminobutyric acid (GABA)-mediated inhibition of growth hormone secretion in the rat.

The effect of altering gamma-aminobutyric acid (GABA) activity on growth hormone (GH) secretion of freely moving chronically cannulated male rats was studied. Systemic injection of the GABA agonist muscimol (2 mg/kg i.v.) inhibited the anticipated secretory episode. Increasing brain GABA levels by gamma-acetylenic-GABA (50 mg/kg i.v.) also inhibited the expected GH rise. When injected before the expected secretory episode, the GABA receptor antagonist bicuculline methiodide (2 mg/kg i.v.) triggered an early secretory peak. GABA and muscimol failed to change GH secretion by cultured anterior pituitary cells. When the somatostatin input to the stalk-median eminence region was interrupted by an anterolateral cut around the medial basal hypothalamus, the GH level was steadily increased and muscimol injections caused a prompt decrease of plasma GH levels. These results are consistent with the hypothesis that GABAergic tonic inhibition participates in the control of GH secretion and that GABA inhibits spontaneous GH release by inhibiting the secretion of a GH-releasing factor.