Adrenergic and Serotoninergic Control of Growth Hormone Secretion in Adult Male Rats1

Plasma radioimmunoassayable growth hormone (GH) levels measured in adult male rats anesthetized for 2 hr with urethane, were found to be lower and strikingly more uniform than those of animals decapitated while awake. Rats prepared in this manner, and bearing a cannula chronically implanted in a lateral ventricle of the brain, were used for the determination of the effect, on plasma GH levels, of the intraventricular administration of catecholamines, adrenergic receptor blockers, and serotonin (5- HT). While saline (10 μl/rat) or noradrenaline (1.0 μg/rat) did not induce any significant changes in plasma GH, dopamine (1.0 Mg/rat) significantly reduced plasma GH levels. This dopaminergic inhibition is apparently exerted through hypothalamic beta-adrenergic receptors, since it is abolished by the previous administration of propranolol (5O μg/rat), a specific beta-adrenergic blocker. The injections of 5-HT (1.0 μg/rat) were followed by a highly significant rise of plasma GH; this effect was abolished by the previous administration of phenoxybenzamine (50 μg/rat), an alpha-adrenergic blocker, which is also able to block tryptaminergic receptors. When injected alone, propranolol was inactive, while phenoxybenzamine strikingly reduced plasma GH levels. These data suggest that rat GH is under the antagonistic control of a central beta-dopaminergic inhibiting, and a central tryptaminergic stimulating mechanism. Central alpha-adrenergic receptors seem on the contrary to be inactive. It is concluded that adult male rats anesthetized with urethane represent a very useful preparation for the study of rat GH secretion and it is suggested that discrepancies observed between experimental results obtained in various laboratories might be due to differences in the way animals are prepared before blood collection. (Endocrinology90: 1231, 1972)