Discriminative learning occasioned by the administration of a dopamine agonist

The repeated administration of psychostimulants usually brings about a progressive increment of the behavioral responses that they induce. We examined to what extent this sensitization is due to an associative learning process.

The dopamine agonist apomorphine elicits stereotyped pecking in pigeons, a response that increases with successive intramuscular injections. We tested whether this sensitized pecking would be discriminatively directed at environmental stimuli that had been present during the sensitization phase.

In a preliminary experiment we identified a pair of stimulus compounds that attracted an equal number of apomorphine peck responses. During discrimination training naive pigeons were exposed on 5 days to both a cage furnished with one of these stimuli after having been injected with apomorphine and to a cage furnished with the other stimuli after having been injected with saline. Then the birds were administered apomorphine (or saline) and tested in a cage that offered both compound stimuli simultaneously. A discrimination reversal training and renewed tests followed.

The tests under apomorphine and saline showed that the pecking by the pigeons was virtually exclusively aimed at the specific environmental stimuli under which the sensitization to apomorphine had taken place. This discriminative stimulus control was reversed after the pigeons had been retrained with converse stimulus compound allocations.

The sensitized apomorphine pecking of pigeons was subject to close control by environmental stimuli. The results thus support the hypothesis that the sensitization to psychostimulants may be due to a conditioning process. The conditioning occasioned by apomorphine injections in birds could be a useful model for the study of sensory-motor learning processes.