Morphine preexposure facilitates morphine place preference and attenuates morphine taste aversion.

Repeated morphine preexposure has been reported to enhance measures of morphine reward (conditioned place preference; CPP) and attenuate measures of morphine aversion (conditioned taste aversion; CTA). These effects are generally independently assessed, limiting the ability to determine if the enhancing and attenuating effects of morphine exposure are mediated by a common factor. To assess any potential relationship between these two effects, the present study examined the impact of morphine preexposure on these motivational properties of morphine using a combined CTA/CPP procedure in which the same animals receive concurrent taste and place conditioning. Specifically, male Sprague-Dawley rats were preexposed to morphine [5 mg/kg; subcutaneously (sc)] or equivolume drug vehicle. Following preexposure, animals were given saccharin to drink and injected with morphine sulfate (1 or 5 mg/kg sc) or drug vehicle (CTA). Immediately thereafter, they were placed on one side of a two-compartment chamber (CPP). On the next day, they were given water followed by injections of the drug's vehicle and then placed in the other compartment. There were four such conditioning cycles after each of which a CTA and CPP test were given. While preexposure to morphine attenuated morphine-induced CTAs, morphine-induced CPPs were enhanced within the same animals. These effects of morphine preexposure were dose- and time-dependent and parallel. These data indicate that the attenuating and sensitizing effects of morphine preexposure on taste aversions and place preferences, respectively, could be mediated by a common mechanism, although other possibilities for these effects of morphine preexposure remain.