Correlation between analgesia and the decrease of acetylcholine turnover rate in cortex and hippocampus elicited by morphine, meperidine, viminol R2 and azidomorphine.

In rats, an ED50 for analgesia of morphine, meperidine, viminol R2 or azidomorphine decreases the turnover rate of acetylcholine (TRACh) in cortex and hippocampus. These four analgetics fail to change to TRACh in striatum when given in a dose range from ED30 for analgesia up to a cataleptic dose. Viminol S2, a nonanalgesic stereoisomer of vimonol R2, fails to decrease the TRACh in cortex and hippocampus. Naltrexone, an opiate antagonist, also fails to change the cortical and hippocampal TRACh but it antagonizes the decrease in cortical and hippocampal TRACh elicited by the four analgetics. Since the ED50 of these four analgetics fails to change the TRACh in striatum which contains a high density of opiate receptors and intrinsic cholinergic neurons, but decreases the TRACh in hippocampus and cortex which contain a low density of opiate receptors, it can be inferred that opiate receptors are not exclusively involved in the regulation of TRACh. However, the results suggest that certain cholinergic pathways participate in the mediation of analgesia.