Dopaminergic involvement in stimulus selection : a behavioural study

People with schizophrenia show deficits in the inhibitory processes controlling stimulus selection. Overshadowing and the Kamin blocking effect provide examples of stimulus selection. Overshadowing refers to the phenomenon whereby greater learning accrues to the more salient stimulus of a compound presentation. Kamin blocking occurs when prior learning to a single stimulus in stage 1 disrupts learning to an added stimulus in stage 2, when both stimuli are presented in compound. Both paradigms measure the ability to select relevant and deselect irrelevant information from the stimulus environment, and are dependent upon cognitive processes hypothesised to be disrupted in schizophrenic patients. The study of the neural basis of such stimulus phenomena provides a putative model with construct validity for the information processing deficits seen in schizophrenia. A disturbance in central dopaminergic activity is thought to underlie the cognitive deficits of schizophrenia. In the present study, the effect of dopaminergic manipulations on Kamin blocking and overshadowing was assessed in the rat.;The indirect dopamine agonist d-amphetamine (1.0 mg/kg, i.p.) disrupted Kamin blocking when administered at stage 2 conditioning. Amphetamine also abolished overshadowing when given at conditioning. It was concluded that amphetamine effects in both paradigms may consist of a selective increase in learning to the less salient stimulus.;The DA antagonist haloperidol (0.2 mg/kg, i.p.) failed to reverse the attenuating effects of amphetamine (1.0 mg/kg i.p.) on overshadowing. However, haloperidol (0.2 mg/kg & 0.5 mg/kg, i.p.) successfully reversed amphetamine-induced hyperlocomotion. Neither the selective D2 receptor antagonist raclopride (0.5 mg/kg, i.p.) nor the selective D2 receptor antagonist sulpiride (50 & 100 mg/kg, i.p.) restored overshadowing in amphetamine - treated animals.;SCH23390 (0.05 mg/kg, i.p.), the selective D1 receptor antagonist, restored overshadowing in amphetamine - treated rats. The D1 receptor agonist SKF 38393 (5 mg/kg i.p.) disrupted overshadowing in a manner similar to that observed following amphetamine treatment.