Pharmacological evidence for stimulation of growth hormone secretion by a central noradrenergic system in dogs

The role of brain catecholamines in the regulation of growth hormone secretion was investigated in pentobarbital-anesthetized dogs by using drugs which modify the function of adrenergic neurons and receptors. Intravenous administration of L-dopa produced a prompt, statistically significant increase in plasma growth hormone concentration. This response was not significantly reduced by blockade of peripheral dopa decarboxylase activity with carbidopa. Clonidine, an alpha-agonist which penetrates the brain, increased plasma growth hormone secretion. Norepinephrine, epinephrine, dopamine and isoproterenol, catecholamines which do not penetrate the blood-brain barrier, failed to affect plasma growth hormone concentration when administered intravenously. Apomorphine did not produce a statistically significant increase in plasma growth hormone concentration when administered directly into the the third ventricle, and pimozide failed to abolish the increase in plasma growth hormone produced by L-dopa. The increase in plasma growth hormone concentration produced by intravenous L-dopa and clonidine was prevented by administration of phentolamine or phenoxybenzamine directly into the third ventricle. The response to L-dopa was also abolished by intraventricular procaine. In dogs in which central beta-adrenergic blockade was produced by intraventricular L-propranolol, the growth hormone response to L-dopa was greater than it was in control dogs treated with intraventricular D-propranolol. The data indicate that in pentobarbital anesthetized dogs, the increase in growth hormone secretion produced by L-dopa is mediated by norepinephrine, rather than dopamine, that the site of action of the norepinephrine is central, above the median eminence and inside the 'blood-brain barrier', and that the norepinephrine acts via alpha-adrenergic receptors.