The cholinergic system and nociception in the primate: interactions with morphine

In Experiment 1 the shock titration task was used to evaluate the antinoceptive properties of 5 different classes of cholinergic compounds in the rhesus monkey. Only scopolamine and high doses of physostigmine were effective in elevating the shock threshold. The apparent antinociceptive effect of physostigmine, however, was difficult to separate from its nonspecific behavioral depressant effect and was probably not related to an increase in cholinergic tone. Experiment 2 examined the interaction of morphine with arecoline, scopolamine and physostigmine. Only scopolamine (0.05 and 0.1 mg/kg) and high doses of physostigmine (0.1 mg/kg) interacted with morphine in the shock titration paradigm. The multiplicative interaction of morphine with scopolamine was confirmed in Experiment 3 over a wider range of doses. It was concluded that morphine and the cholinergic compounds produce antinociceptive effects through different mechanisms of the pain system.