The binding of 3H-quinuclidinyl benzilate to cholinergic muscarinic receptors in rat brain region following chronic treatment with morphine and during abstinence.

Chronic morphine treatment (subcutaneous pellet implantation of four morphine pellets) caused significant tolerance to morphine analgesia as observed at zero and 6 hrs. after pellet removal. However, such treatment failed to elicit any changes in striatal muscarinic receptors using (3H)-QNB binding studies. At 24 hours after pellet removal there was significant development of physical dependence but the affinity (Kd) as well as density (Bmax) of striatal muscarinic receptors remained unaffected. Although naloxone caused marked precipitation of abrupt withdrawal in morphine tolerant rats, it also failed to produce any changes in striatal muscarinic receptors. Results indicate that any cholinergic involvement in opiate tolerance and physical dependence does not involve an effect on muscarinic receptors.