Dopaminergic agonists differentially affect open-field activity of rats with A10 lesions

Dopaminergic systems appear to exert considerable control over locomotor activity. Although dopamine neurons are located in relatively close proximity within the mesencephalon, their axons project to more diffuse areas, perhaps reflecting some underlying heterogeneity in their function. The purpose of this study was to determine whether dopamine agonists differentially affect activity by acting upon distinct dopamine systems. Bilateral radio-frequency lesions of area A10 in rats failed to affect spontaneous open-field behavior over a 1-month postoperative period. When injected with 1 mg/kg of apomorphine, however, experimental rats more than doubled their activity as compared to the response of sham-operated controls. In contrast, no difference between the two groups of animals was observed in terms of increased activity following 3 mg/kg of either d-amphetamine or methylphenidate. These results are consistent with previous work indicating the involvement of ventromedial mesencephalic dopamine somata in the control of locomotor activity. The data suggest, however, that systems in addition to the dopaminergic mesolimbic projection are responsible, in part, for the hyperactivity elicited by d-amphetamine or methylphenidate.