Inhibition of the morphine withdrawal syndrome by a novel muscarinic antagonist (4-DAMP).

It has been recognized for many years that central cholinergic neurons are susceptible to inhibition by opiates and that during withdrawal their firing rates are enhanced. Nevertheless, classical nonselective muscarinic receptor antagonists have not been demonstrated to provide consistent inhibition of withdrawal symptoms in humans or in animal models. The purpose of this study was to determine whether selective blockade of central M1 or M2 muscarinic receptor subtypes could provide inhibition of naloxone precipitated withdrawal symptoms in morphine dependent rats. As with earlier human studies, both cardiovascular and behavioral measures of withdrawal were quantitated. The selective M2 receptor antagonist 4-DAMP was significantly more effective than the M1 antagonist pirenzepine in reducing both cardiovascular and behavioral symptoms. These results are consistent with a role for cholinergic neurons in the expression of certain morphine withdrawal symptoms and suggest that future therapies might be targeted towards central M2 receptors.