Effects of clonidine and yohimbine on plasma cyclic nucleotide levels in clonidine-naive and clonidine-treated mice.
Effects of clonidine and yohimbine on plasma cyclic nucleotide levels were investigated in both clonidine-naive and clonidine-treated male mice. Clonidine increased plasma cyclic GMP but decreased slightly cyclic AMP levels in clonidine-naive mice. Clonidine treatment for 10-14 days in the drinking water did not decrease the cyclic GMP response to clonidine indicating that no tolerance develops to the effect of clonidine on plasma cyclic GMP. alpha 2-Agonists, such as clonidine, oxymetazoline and naphazoline, were more potent than phenylephrine, an alpha 1-agonist, in increasing cyclic GMP, although azepexole, a weak alpha 2-agonist, had no effect. Inhibition of clonidine-induced increase in plasma cyclic GMP by yohimbine, hexamethonium and atropine, but not by prazosin suggests that the effect of clonidine is mediated by the central alpha 2-adrenoceptors, activating the muscarinic receptor-linked guanylate cyclase through the stimulation of vagal activity. Yohimbine increased plasma cyclic AMP levels in clonidine-naive mice. Inhibition of this effect by hexamethonium and propranolol suggests that yohimbine increases plasma cyclic AMP through increasing the sympathetic tone. The increase in plasma cyclic AMP elicited by yohimbine was potentiated by chronic clonidine treatment. Enhancement of the cyclic AMP effect of yohimbine found in clonidine-treated mice may be regarded as a precipitated withdrawal symptom and indicate development of dependence on clonidine.