Breaking points on a progressive ratio schedule reinforced by intravenous apomorphine increase daily following 6-hydroxydopamine lesions of the nucleus accumbens

It has been shown previously that 6-hydroxydopamine (6-OHDA)-induced lesions of the nucleus accumbens cause extinction of cocaine self-administration behavior, yet fail to affect the rate of apomorphine self-administration on a Fixed Ratio 1 schedule of reinforcement. Since the dopamine (DA) receptors in this nucleus should become supersensitive, and since these receptors are thought to mediate the reinforcing effects of apomorphine, then some change in apomorphine self-administration would be expected. We have therefore reinvestigated the effect of 6-OHDA lesions on apomorphine self-administration using a progressive ratio schedule of reinforcement. Rats were trained to self-administer apomorphine (0.033 mg/inj, IV) on a schedule in which the response requirements increased after each reinforcement. The breaking point was defined as the final ratio completed during each daily session. Sham lesions had no effect on the breaking points, and a steady increase in the breaking point was observed in the 6-OHDA-lesioned animals which may parallel the development of DA receptor supersensitivity.