Effects of clonidine and yohimbine on parasympathetic reflex salivation and vasodilatation in cat SMG.

The present experiments were designed to examine the site of action of the alpha 2-adrenoceptor agonist, clonidine, and the alpha 2-antagonist, yohimbine, on the salivary secretion response to parasympathetic and sympathetic stimulation in the cat submandibular gland (SMG). The parasympathetic reflex salivary secretion was significantly affected by both clonidine and yohimbine (clonidine reduced and yohimbine increased the amount of saliva secreted) while the reflex vasodilator response was not affected by either agent. No degree of methacholine-induced salivation was affected by prior treatment with either clonidine or yohimbine. The salivation elicited by facial nerve root stimulation was markedly reduced by clonidine but potentiated by yohimbine. Prior treatment with scopolamine completely abolished the salivary secretion induced by either methacholine or facial nerve root stimulation. Clonidine and yohimbine had no effect on SMG salivation elicited by sympathetic stimulation. These results indicate that the alpha 2-agonist and antagonist act specifically on parasympathetic salivary fibers but have no effect on parasympathetic vasodilator fibers. They thus suggest that postganglionic parasympathetic innervation is differentiated into salivary and vasodilator fibers.