In order to investigate the role of GABAergic neurotransmission within the reticular part of substantia nigra (SNR) in the switching of motor patterns and the performance of movements, cats trained to walk on the running belt of a treadmill at constant speed were subjected to three different tests: (1) a food dispenser test measuring the animals' capacity to switch motor patterns in order to get access to food during walking; (2) an obstacle test measuring the animals' capacity to switch motor patterns in reaction to incoming obstacles; (3) EMG recording of two representative antagonistic muscles of the hindlimb during walking on the treadmill. Local injection of a moderate dose of the GABA antagonist picrotoxin (PTX; 250-500 ng/0.5 μl) into the SNR disrupted the animals' capacity to switch motor patterns in the food dispenser test, but not in the obstacle test. These animals displayed normal EMG patterns during walking. Higher doses of intranigral injections of PTX, however, impaired the execution of movements per se as detected by an increased number of 'faults' in the obstacle test and pathological EMG patterns during walking. These experiments support the view that (1) the SNR plays a distinct role for switching motor patterns; (2) the SNR is involved in the control of movements per se; (3) the degree of motor disorder depends on the degree of pathology within this brain structure.
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