Actylcholine output from the cerebral cortex,choline uptake and muscarinic receptors in morphine-dependent,freely-moving rats

Spontaneous acetylcholine (ACh) output from the cerebral cortex, choline high affinity uptake and [³H]-QNB binding to muscarinic receptors in the cerebral cortex and caudate nucleus in freely moving rats made morphine-dependent by morphine pellet subcutaneous implantation were investigated before and during naloxone-induced withdrawal syndrome. The frequency and intensity of the withdrawal signs were also assessed.No significant change in ACh output was found in tolerant rats when compared with that of placebopellet implanted rats. During naloxone-induced withdrawal syndrome a 60% increase in ACh output occurred.In rats made dependent after a large septal lesion or treated for ten days with calcium gluconate (10 mg/kg i.m.) no increase in ACh output was found during the withdrawal syndrome. The intensity of some of its signs was also reduced.During the withdrawal syndrome a marked increase in choline high affinity uptake in the cerebral cortex and caudate nucleus was detected.The affinity of muscarinic receptors (K_D) for [³H]-QNB was significantly increased in the cerebral cortex and caudate nucleus of morphine-dependent rats before naloxone administration. It returned to normal during the withdrawal syndrome. In the caudate nucleus the number of binding sites (B_max) was decreased before and after the withdrawal syndrome.These findings emphasize the role of cholinergic mechanisms in opiate addiction.