ARTICLES

Aminergic and thermoregulatory mechanims in hypothalamic regulation of growth hormone in cats

In conscious cats intravenous infusion of L-dopa (20 mg/kg) increased plasma growth hormone (GH), hydrocorticosterone (17-OHCS), and glucose while lowering midbrain (core) temperature (Tmb). Peripheral blockade of aromatic amino acid decarboxylase by L-alpha-hydrazino-methyldopa (MK 486) abolished only L-dopa hyperglycemia, indicating CNS stimulation by de novo catecholamines caused endocrine and temperature changes. Phenoxybenzamine but not pimozide pre-treatment abolished GH release after L-dopa. Apomorphine infusion alone did not alter basal GH. These data indicate central alpha-adrenergic mediation of GH release. Norepinephrine (NE) injected (5 or 10 microgram) into the preoptic anterior hypothalamus (POAH) (7 of 10 cases) and medial hypothalamus (17 of 30) raised GH while lowering Tmb, in agreement with alpha-adrenergic mediation. Responsive sites were distributed in ventral and dorsal medial hypothalamus and extended into the thalamus. Cooling of the POAH by implanted thermode raised GH in association with shivering, vasoconstriction, and rise in Tmb. Correlation between GH release and change in Tmb after L-dopa, central NE injection, and POAH cooling suggests hypothalamic integration of GH release with thermoregulation. Intravenous infusion of 5-hydroxytryptophan after MK 486 raised GH and 17-OHCS, suggesting serotonergic mechanisms also control GH and ACTH.