Conditioning of and contextual sensitization to apomorphine-induced climbing in mice: evidence against the habituation hypothesis

Several predictions of the habituation hypothesis of conditioned drug effects were tested by looking at contextual sensitization to apomorphine-induced climbing in mice (Mus musculus). Mice were first sensitized to that effect after 9 daily injections of 0.4 mg/kg apomorphine in the test context. Other mice received the same treatment outside the test context. On Day 10, all mice were challenged with either saline (conditioned drug effects test) or apomorphine (contextual sensitization test). On both tests, the levels of climbing of mice that received apomorphine paired with the test context during the intermittent treatment were significantly higher than those of mice that were experiencing the test context for the first time (unexposed mice). Also, the rate of extinction in conditioned mice did not parallel the rate of habituation in the unexposed mice. Results contradict the habituation hypothesis of conditioned drug effects and contextual sensitization.