Effects of central GABA receptors activation on catecholamine secretion in rats.

We investigated the effects of muscimol, the GABAA receptor agonist, and baclofen, the GABAB receptor agonist, injected into the third cerebral ventricle on plasma epinephrine (E) and norepinephrine (NE) levels in anesthetized rats. Baclofen (0.4-5 nmol) increased plasma NE levels in a dose dependent manner but did not affect plasma E levels. Muscimol (2.5 nmol) affected neither plasma E nor NE levels. Concomitant injection of muscimol (2.5 nmol) with baclofen (5 nmol) attenuated the baclofen (5 nmol)-induced NE secretion. These findings suggest that activation of GABAB receptors in the central nervous system (CNS) stimulates the sympathetic nervous system but not the adrenal medullary response. In contrast, activation of GABAA receptors in the CNS affects neither the sympathetic nervous system nor the adrenal medullary response, but inhibits the sympathetic neural activity induced by activation of GABAB receptors in anesthetized rats.