Local hypothalamic adrenoceptor activation in rat: α₁ inhibits and α₂ stimulates growth hormone secretion

We made stereotaxic microinjections of adrenoceptor agonists and the catecholamine-releasing agent, tyramine, into the preoptic anterior hypothalamic area (PO/AHA) or the medial basal hypothalamus (MBH) of unstressed rats. Growth hormone (GH) plasma concentrations were measured serially before and after intrahypothalamic injections. Noradrenaline and phenylephrine inhibited GH secretion wherever injected but were effective at lower doses in the PO/AHA. Clonidine stimulated GH secretion at both sites, at several doses in the MBH and only at one dose in the PO/AHA. Tyramine inhibited GH when injected in the PO/AHA, but not in the MBH. We conclude: (a) α₁ inhibition is predominant over α₂ stimulation of GH on or near somatostatin neurons; (b) α₂ stimulation predominates over α₁ inhibition of GH on or near GRF neurons, and (c) endogenous catecholamines in the PO/AHA have a predominantly inhibitory effect on GH secretion.