Susceptibility of spontaneous sympathetic outflow and sympathetic reflexes to depression by clonidine

The hypotensive activity of clonidine has been attributed to inhibition of sympathetic outflow. The present series of experiments examined the relative influence of clonidine on spontaneous outflow and various sympathetic reflexes in anesthetized cats. Clonidine resulted in dose-related decreases in the former. Increases in efferent sympathetic activity mediated by baroreceptors and supraspinally integrated potentials evoked in the splanchnic nerve by somatic nerve stimulation were inhibited to a lesser extent. Splanchnic potentials evoked by spinal cord stimulation in spinal animals were also diminished. In contrast, baroreceptor induced reflex decreases in sympathetic outflow were only minimally affected. These experiments indicate that clonidine can inhibit central sympathetic mechanisms at various levels including the spinal cord but that considerable differences exist in the susceptibility of sympathetic outflow and various reflexes to depression by the compound.