Sensitization to apomorphine in pigeons is due to conditioning, subject to generalization but resistant to extinction

Apomorphine (Apo) administration induces a persistent bout of pecking in pigeons and other birds. Repeated injections of Apo in pigeons lead to sensitization, i.e. the pecking response to a particular dose increases up to a dose-dependent asymptotic level. It is also known that Apo-induced pecking can be classically conditioned to the cage environment where the animals experience the effect of the drug. Here we address the question of whether, and to what extent, the sensitization effect arises as a consequence of a conditioning or of a pharmacological process. An extinction experiment demonstrated that an extinction procedure supposed to be effective in inhibiting the conditioned pecking response was not effective in suppressing the sensitization to Apo, thus casting provisional doubt on the conditioning hypothesis. However, a conditioning experiment demonstrated that the sensitization effect undoubtedly involved an important component of conditioning to an experimental cage environment, but also suggested that there was an additional component possibly not due to learning. A generalization experiment, however, showed that this second component was very probably due to a stimulus generalization effect deriving from conditioning to the home cage, suggesting that learning can account for most, if not all, of the increase in Apo-induced pecking and that an exclusively pharmacological sensitization process plays, at best, a minor role. The apparent contrast between the results of the first experiment, indicating that the sensitization is not affected by inhibitory conditioning, and the results of the last two experiments, suggesting that the sensitization is due to excitatory conditioning, can be resolved by assuming that Apo induces a drug-state-dependent conditioning. These results are related to findings and arguments concerning the sensitization to psychostimulant drugs in mammals.