Differential role of GABAA and GABAB receptors in two distinct output stations of the rat striatum: studies on the substantia nigra pars reticulata and the globus pallidus

The role of GABAA and GABAB receptors in the substantia nigra pars reticulata and the globus pallidus in turning behaviour of rats was studied. Unilateral injection of the GABAA receptor agonist muscimol (25 and 50 ng) into the substantia nigra pars reticulata elicited contralateral pivoting, namely tight head-to-tail turning marked by abnormal hindlimb backward stepping. This effect was GABAA receptor specific, since it was dose-dependent and prevented by co-administration of the GABAA receptor antagonist bicuculline (100 and 200 ng) which alone did not elicit turning behaviour. Unilateral injection of the GABAB receptor agonist baclofen (100 and 200 ng) into the substantia nigra pars reticulata also produced contralateral pivoting. This effect was GABAB receptor specific, since it was dose-dependent and inhibited by the GABAB receptor antagonist CGP 55845 (200 ng) which alone did not elicit turning behaviour. In contrast, unilateral injection of bicuculline (100 and 200 ng) into the globus pallidus produced contralateral circling, namely turning marked by normal stepping. This effect was GABAA receptor specific, since it was dose-dependent and prevented by muscimol (50 ng), which alone did not elicit turning behaviour. Unilateral injection of baclofen (100 and 200 ng) into the globus pallidus dose-dependently produced ipsilateral pivoting; this effect was inhibited by CGP 55845 (200 ng), which alone did not elicit turning behaviour. The present study demonstrates that GABAA and GABAB receptors in the globus pallidus and the substantina nigra pars reticulata play differential roles in the production of turning behaviour. This study underlines the notion that the two types of turning, namely pivoting and circling, are valid tools to map out the information flow across the basal ganglia.