The manipulation of basal ganglia circuit function may be possible by the strategic placement of genes designed specifically to alter normal function. Recent studies by Kaplitt et al. have used gene therapy to change the normal excitatory input of the STN-GPi projection to an inhibitory gamma amino butyric acid (GABA)ergic projection. Rats with chemical lesions in the substantia nigra were treated with viral delivery of glutamic acid decarboxylase (GAD) via an AAV vector into the STN. They subsequently showed a reversal of SNr responses to STN stimulation with excitatory and inhibitory ratios changing from 83% and 6% to 17% and 78%, respectively. This was associated with a 65% decrease in amphetamine-induced rotational behavior (106). A clinical study is now underway, exploring the role of this potent new therapy in the treatment of PD (107).
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