In order to further assess the alterations which might underly behavioral deficits associated with a reduced dopaminergic transmission, the effects of apomorphine at doses thought to stimulate dopaminergic autoreceptors were studied on rat operant behavior.
Low doses of apomorphine caused a reward deficit when animais were shifted from continuons reinforcement to fixed ratio schedules of food delivery (fig. 1). This effect could be accounted for by a decreased ability of secondary reinforcers to sustain responding and/or by a disruption of cognitive processes (Table 1). The apomorphine-induced reward deficit in the fixed ratio 4 schedule was reversed by “disinhibitory” neuroleptics including amisulpride, pimozide, pipotiazine and sulpiride, at low to moderate doses. Conversely, “conventional” neuroleptics such as chlorpromazine, fluphenazine, haloperidol, metoclopramide and thioridazine were found inactive in reversing the deficit caused by apomorphine (fig. 2). Results obtained after lesion of dopaminergic neurons by 6-hydroxydopamine suggested that the behavioral deficit induced by apomorphine was related not so much to a reduction in dopaminergic activity in given restricted areas such as the VTA (fig. 3), the nucleus accumbens (fig. 4) or the prefrontal cortex (fig. 5), as to a functional imbalance between mesolimbic and mesocortical dopaminergic systems.