Opposite rotation induced by dopamine agonists in rats with unilateral lesions of the globus pallidus or substantia nigra.

Normal rats with a unilateral ibotenic acid lesion of substantia nigra pars reticulata (SNR, n = 12) or globus pallidus (GP, n = 12) were challenged systemically with the mixed dopaminergic agonist apomorphine (0.5 and 1.5 mg/kg) and the indirect acting d-amphetamine (1.5 mg/kg). The low dose of apomorphine produced a weak contralateral rotation only in the SNR-lesioned group, which showed an intense ipsilateral rotation following the administration of the higher dose. GP-lesioned rats also showed ipsilateral rotation after the high dose of apomorphine. d-Amphetamine produced ipsilateral rotation in GP-lesioned rats, contrasting with a vigorous contralateral rotation in SNR-lesioned rats. The unexpected opposite rotation after apomorphine and d-amphetamine, observed only in SNR-lesioned animals, indicates that the role of SNR in basal ganglia functions is less clear and more complex than what is expected from our current model of basal ganglia circuitry and functions. On the other hand, the GP lesion resulted in a consistent and predictable ipsilateral rotation after both apomorphine and d-amphetamine, indicating a more determinant effect on the output of the basal ganglia than heretofore believed. Our results may contribute to the recently expressed views challenging the established model of basal ganglia organisation.