Attenuation of the aversive and analgesic effects of morphine by repeated administration: Different mechanisms

Rats were repeatedly injected with a low (8 mg/kg) or high (40 mg/kg) dose of morphine during the preexposure phase of the experiment. They then received both taste-aversion training and morphine-induced analgesia assessment. These tasks were presented in the same environment or in a different environment from that associated with the drug during preexposure. Morphine preexposure attenuated morphine-induced taste aversion conditioning, whether or not the conditioning was conducted in the same environment as morphine preexposure. In contrast, the display of analgesic tolerance was more pronounced in the preexposed environment than in the alternative environment. These results suggest that repeated morphine administrations attenuate the aversive and analgesic effects of the drug by different mechanisms.