Actions of kappa, sigma and partial mu narcotic receptor agonists on rat brain acetylcholine turnover

Cholinergic neurons which possess mu and delta receptors and/or which are regulated by neurons possessing these receptors, do not appear to have kappa receptors. Only at doses far exceeding (16--32 x) the analgesic ED50 of the prototype kappa antagonist ethylketazocine is acetylcholine turnover depressed in the cortex and hippocampus. This suggests that such activity may involve occupation of mu and/or delta receptors at high doses. The lack of activity of the sigma agonist SKF 10047 also indicates a possible absence of sigma receptors on cholinergic neurons. The partial mu antagonist buprenorphine was found to be a unique narcotic agent in that it behaved as a classical mu agonist at low doses, but as an antagonist at high doses.