Temporal analysis of naloxone attenuation of morphine-induced taste aversion

In a dose-response study, 7.5 mg/kg of naloxone produced maximal attenuation of conditioned taste aversion to saccharin induced by 10 mg/kg of morphine. Naloxone was administered immediately after the morphine in this study. In a second experiment, naloxone still caused a significant attenuation of taste aversions when administered with a 1 hr delay after morphine, but not after delays of 4 or 8 hr. These results suggest that behavioral consequences of morphine which peak during the first hr after injection (analgesia, catalepsy, and depression of intracranial self-stimulation) are not correlated with the aversive effect of morphine. Nor can the aversiveness of morphine be attributed to withdrawal effects. Only the facilitative actions of morphine occurring 1 to 4 hr after injection, including the facilitation of intracranial self-stimulation, are temporally correlated with the naloxone-sensitive aversive effect. Thus, a temporal analysis cannot be used to dissociate the paradoxical positive reinforcement and aversive effects of morphine. Rather, the temporal correlation between the two opposite motivational effects of morphine serves to emphasize the nature of this paradox.