A comparison of muscarinic cholinergic involvement in the antinociceptive effects of morphine and clonidine in the mouse.

The antinociceptive effects (AE) of morphine and clonidine were examined in the mouse tail immersion test (48 degrees C). The interactions of these two agents with cholinergic (muscarinic) and anticholinergic (antimuscarinic) drugs were examined with respect to antinociceptive effect. Physostigmine, administered peripherally and intracerebroventricularly (i.c.v.) increased the AE of both morphine and clonidine. Atropine sulphate (but not atropine methylnitrate) antagonized the AE of both morphine and clonidine in a dose-dependent manner. Oxotremorine (OTMN) produced an AE that was additive with that of morphine and clonidine without being synergistic. These findings are discussed in terms of possible cholinergic mechanisms involved in the AEs produced by morpine and clonidine.