QUANTITATIVE STUDIES OF THE ANTAGONISM BY NALORPHINE OF SOME OF THE ACTIONS OF MORPHINE‐LIKE ANALGESIC DRUGS

Abstract

A quantitative investigation has been made of the antagonism by nalorphine of the analgesia and lenticular opacity produced in mice by a number of compounds. ED50 values have been obtained for each drug in the absence and in the presence of increasing doses of nalorphine, and from these, appropriate dose-ratios have been calculated. It has been possible to derive the equivalent of a pA₂ value for each drug with nalorphine and, since these are almost identical, it may be concluded that all the drugs combine with similar receptors. Nalorphine antagonizes both actions by competing for the receptors. It was not possible to antagonize quantitatively the analgesic action of pethidine with nalorphine, although the lenticular effect could be abolished. The effect of nalorphine on the change in skin temperature in mice induced by some of the analgesic drugs was also investigated.