The contribution of classical conditioning to tolerance to the antinociceptive effects of ethanol.

In rats, ethanol increases the latency of the tail-flick reflex to radiant heat. Three experiments examined the contribution of classical conditioning to the acquisition of tolerance to this antinociception. Experiment 1 showed that the antinociception produced by ethanol was dose dependent. The results of Experiment 2 demonstrated that rats exposed to a series of ethanol injections paired with a distinctive environment developed tolerance to this antinociception. In Experiment 3, tolerance was more pronounced in animals that had been exposed to ethanol and tested in the distinctive environment than in animals that had received ethanol in a nondistinctive environment. In contrast to previous reports in the literature, these results show that animals need not practice the tail-flick reflex while intoxicated in order to develop tolerance. Additionally, the data suggest that classical conditioning may contribute to tolerance to the antinociceptive effects of ethanol.