Abstract

The technique of direct intraventricular injection in the conscious mouse was used to study the pharmacologic effects of codeine, morphine, and norcodeine, respectively. It was found that the depression of the tail-flick response to a thermal stimulus required doses of codeine that were more than 100 times greater than for morphine. Moreover, while the effects of morphine were antagonized by pretreatment with nalorphine, the effects of codeine were not. It was further found that norcodeine resembled codeine in its effect on the tail-flick response with respect to both the size of the dose required and the pharmacologic indifference to nalorphine. Norcodeine, however, was about six times more toxic than codeine and about twice as toxic as morphine similarly injected. In no case was toxicity reduced by pretreatment of the mice with nalorphine, and no antagonism between codeine and morphine was seen in any of the effects studied.

It is suggested that in the mouse when codeine is given by the usual routes the major pharmacologic effects are achieved only after the release and distribution of its demethylated metabolites. This suggestion is supported by the results of using C¹⁴ labeled codeine preparations which show that when codeine is injected subcutaneously in mice a considerable fraction of the dose is rapidly O-demethylated to morphine and N-demethylated to norcodeine.