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

Apomorphine (Apo) administration induces a persistent bout of pecking in pi geons and other birds. Repeated injections of Apo in pigeons lead to sensit ization, i.e. the pecking response to a particular dose increases up to a d ose-dependent asymptotic level, It is also known that Ape-induced pecking c an be classically conditioned to the cage environment where the animals exp erience the effect of the drug. Here we address the question of whether, an d to what extent, the sensitization effect arises as a consequence of a con ditioning or of a pharmacological process. An extinction experiment demonst rated that an extinction procedure supposed to be effective in inhibiting t he conditioned pecking response was not effective in suppressing the sensit ization to Ape, thus casting provisional doubt on the conditioning hypothes is. However, a conditioning experiment demonstrated that the sensitization effect undoubtedly involved an important component of conditioning to an ex perimental cage environment, but also suggested that there was an additiona l component possibly not due to learning, A generalization experiment, howe ver, showed that this second component was very probably due to a stimulus generalization effect deriving from conditioning to the home cage, suggesti ng that learning can account for most, if not all, of the increase in Apo i nduced pecking and that an exclusively pharmacological sensitization proces s plays, at best, a minor role. The apparent contrast between the results o f the first experiment, indicating that the sensitization is not affected b y inhibitory conditioning, and the results of the last two experiments, sug gesting that the sensitization is due to excitatory conditioning, can be re solved by assuming that Apo induces a drug-state-dependent conditioning. Th ese results are related to findings and arguments concerning the sensitizat ion to psychostimulant drugs in mammals. (C) 1999 Lippincott Williams & Wil kins.