Suppression of a Somatosympathetic Reflex by the γ-Aminobutyric Acid Agonist Muscimol and by Clonidine

gamma-Aminobutyric acid (GABA) and GABA agonists, e.g., muscimol, reduce blood pressure and sympathetic outflow and inhibit the "carotid occlusion reflex." In contrast, muscimol exerted only marginal effects on postural reflexes in a prior study. The somatosympathetic reflex, i.e., potentials evoked in sympathetic nerves in response to sensory nerve stimulation, is a useful model for studying centrally acting drugs. Effects of muscimol on the reflex were examined in anesthetized normotensive rats. At doses which had previously been shown to reduce blood pressure but to produce only minimal attenuation of postural reflexes in conscious hypertensive rats, muscimol, administered intracerebroventricularly, reduced blood pressure and inhibited the somatosympathetic reflex in the present study. The time course of the evoked potential was not altered. Baroreceptor activation and intracerebroventricular clonidine also suppressed the reflex. The inhibitory effect of muscimol but not that of clonidine was prevented by pretreatment with the GABA antagonist bicuculline. Thus, the marked suppression of the somatosympathetic reflex by muscimol and by clonidine, in contrast to minimal effects on postural reflexes, point to the selectivity of their central inhibitory actions.