Sensitization to apomorphine in pigeons: a multifactorial conditioning process.

Apomorphine (apo), an unspecific direct dopamine agonist, elicits an intense and lasting pecking bout in pigeons. Apo yielded orderly dose-response functions, and repeated administrations led to sensitization. Strain and individual differences in sensitivity to apo were at least partly due to genetic factors. However, a strong cage-context dependency of the sensitization, which is indicative of conditioning, occurred in both pigeon strains studied. Apo-induced pecking and sensitization also occurred in total darkness. Pigeons could be conditioned to discriminate between an apo state and a non-apo state. A small dose of apo was effective as a conditioned stimulus when paired with a high dose as an unconditioned stimulus. The conditioned response (CR) was strongly specific to the context in which the sensitization to apo took place. The resistance to extinction of the CR could be increased through an oversensitization treatment. The incremental responses arising during the sensitization treatment and the CRs shown afterward by individual pigeons correlated significantly. The sensitization to apo in pigeons is well accounted for by a conditioning schema in which an interoceptive drug state is a conditional conditioned stimulus for the full expression of the incremental response. Variants of the scheme might also account for the sensitization of rodents to psychostimulants. A neural model that embodies the characteristics of the conditioning scheme has been proposed.