Effects of morphine and its analogues on m-cholinoceptor binding affinity in rat brain

The effects of morphine and its analogues (fentanyl, pentazocine. meperidine, buprenorphine. naloxone and nalOi'phine) on [3H]QNB binding to' muscarinic cholinoceptOTS of the rat brain was studied in vitro. Scatchard analysis revealed that the affinity of the binding was dose-dcpendently inhibited by morpFiine lO-JOC liM without significant effect on the total number of the binding sites. Fentanyl was more potent than morphine in blocking [3HlQNB binding while naloxone, nalorphine and buprenorphine were ineffective. Neither nicotinic cholinoceptor agoriist and antagonists nor the putative monoamine neurotransmitters exhibited any significant effects. The functional relevance of the effect of morphine and possibly endogenous opioids on the activities of cholinoceptors in vivo was discussed.