Conditioned circling in rats: bilateral involvement of the mesotelencephalic dopamine system demonstrated following unilateral 6-hydroxydopamine lesions.

High rates of conditioned circling have previously been associated with a bilateral augmentation of striatal dopamine metabolism. These results suggest that both striata subserve this response. The present experiment further assessed this possibility by determining the effects of unilateral 6-hydroxydopamine lesions of the mesotelencephalic dopamine system on conditioned circling. Rats were initially trained to circle in their preferred direction for water reinforcement. Upon establishment of this response, they received unilateral lesions at the level of the lateral hypothalamus either contralateral or ipsilateral to the reinforced direction of circling. Reinforced responding was virtually abolished in rats with contralateral lesions. In contrast, rats lesioned ipsilateral to the direction of reinforced circling exhibited only a 50% decrease in rate of reinforced responding. Non-reinforced responding was increased only in rats with contralaterally placed lesions. Following 5 postoperative test sessions, the experimental contingencies were reversed. 'Ipsilaterally lesioned' rats were now required to circle away from their lesion whereas 'contralaterally lesioned' rats had to turn towards their lesion. The 'Contralateral' group acquired the reversal, such that reinforced responding occurred more frequently than non-reinforced responding. However, reinforced rates of responding did not reach preoperative rates. Conversely, 'ipsilaterally lesioned' rats could not learn to turn contraversively and now made more non-reinforced than reinforced responses. These findings suggest that conditioned circling is mediated by a bilateral involvement of the mesotelencephalic dopaminergic systems. However, the specific role of each side appears to be dependent upon its relationship with the direction of circling emitted.