Discriminative learning occasioned by the administration of a dopamine agonist

Rationale: The repeated administration of psychostimulants usually brings a bout a progressive increment of the behavioral responses that they induce. We examined to what extent this sensitization is due to an associative lear ning process. Objectives: The dopamine agonist apomorphine elicits stereoty ped pecking in pigeons, a response that increases with successive intramusc ular injections. We tested whether this sensitized pecking would be discrim inatively directed at environmental stimuli that had been present during th e sensitization phase. Methods: In a preliminary experiment we identified a pair of stimulus compounds that attracted an equal number of apomorphine p eck responses. During discrimination training naive pigeons were exposed on 5 days to both a cage furnished with one of these stimuli after having bee n injected with apomorphine and to a cage furnished with the other stimuli after having been injected with saline. Then the birds were administered ap omorphine (or saline) and tested in a cage that offered both compound stimu li simultaneously. A discrimination reversal training and renewed tests fol lowed. Results: The tests under apomorphine and saline showed that the peck ing by the pigeons was virtually exclusively aimed at the specific environm ental stimuli under which the sensitization to apomorphine had taken place. This discriminative stimulus control was reversed after the pigeons had be en retrained with converse stimulus compound allocations. Conclusions: The sensitized apomorphine pecking of pigeons was subject to close control by e nvironmental stimuli. The results thus support the hypothesis that the sens itization to psychostimulants may be due to a conditioning process. The con ditioning occasioned by apomorphine injections in birds could be a useful m odel for the study of sensory-motor learning processes.