Differential effects of pharmacological manipulations of central alpha 1- and alpha 2-adrenergic receptors on the secretion of thyrotropin and growth hormone in male rats.

Abstract

The effects of pharmacological manipulations of the two subtypes of central a-adrenergic receptors, designated α1 and α2, respectively, on the secretion of TSH and GH were tested in conscious unrestrained male rats. Intraventricular (ivt) administration of the α1-receptor agonist, methoxamine, induced a rapid and dose-related lowering of the plasma levels of both hormones, which was antagonized by systemic administration of the α1 antagonist, prazosin. Intraventricular administration of phenylephrine, another α1 agonist, also depressed plasma levels of both hormones. In contrast, the administration of relatively small doses (50 or 12.5 μg/kg, iv) of the α2 agonist, clonidine, induced large, dose-related secretory surges of both TSH and GH. The TSH/GH-stimulating effect of clonidine was inhibited in a dose-related fashion by the Α2 antagonist, yohimbine. Prazosin was ineffective against the TSH-stimulating effect, but it partially suppressed the GH secretory response, which may indicate involvement of a mi...