Basal ganglia outflow pathways and circling behaviour in the rat

The role of efferents in substantia nigra pars reticulata in the mediation of circling behaviour in the rat has been studied by means of lesions designed to interrupt these pathways or to damage nigral projection areas. The behavioural model used was the circling rodent with a prior 6-hydroxydopamine lesion of the left nigro-striatal pathway in which amphetamine induced ipsiversive rotation and apomorphine induced contraversive rotation. Removal of the left fronto-parietal cortex caused only a transient decrease in drug-induced rotation. An electrolytic lesion of the left, right or both parafascicular thalamic nuclei did not alter circling behaviour. Electrolytic lesioning of the left ventromedial thalamus decreased apomorphine-induced contraversive circling whereas a lesion of the right ventromedial thalamus decreased amphetamine-induced ipsiversive rotation. Bilateral electrolytic lesions of the ventromedial thalamus did not alter drug-induced circling. Unilateral or bilateral electrolytic lesioning of the medial superior colliculus did not alter the rotational response to apomorphine or amphetamine. However, an electrolytic lesion interrupting the dorsal tegmental decussation reduced apomorphine-induced circling but not amphetamine-induced circling. That a critical role for the nigro-thalamic and nigro-tectal pathways is not involved in the mediation of circling behaviour was confirmed by placing knife cuts so as to separate these structures from the substantia nigra; such lesions failed to alter the contraversive rotation induced by the ipsilateral injection of muscimol into substantia nigra pars reticulata. Electrolytic lesions of the ipsilateral nucleus reticularis gigantocellularis or kainic acid lesions of the ipsilateral nucleus tegmenti pedunculopontinus did not alter drug-induced circling in animals with a prior 6-hydroxydopamine nigral lesion. In contrast, an ipsilateral lesion of the midbrain periaqueductal grey matter and adjacent midbrain reticular formation (the angular complex) decreased apomorphine-induced contraversive rotation in such animals, while bilateral lesions reduced both apomorphine-and amphetamine-induced circling; in each case the postural component of rotation was abolished. Unilateral kainic acid lesions of the angular complex in naive animals caused ipsiversive rotation which was enhanced by apomorphine. Unilateral kainic acid lesions of the angular complex with an ipsilateral 6-hydroxydopamine nigral lesion caused reversal of the previous contraversive rotation to apomorphine, and enhanced amphetamine-induced ipsiversive rotation.(ABSTRACT TRUNCATED AT 400 WORDS)