Gene therapy targets in Parkinson disease. Excitatory connections from the cortex stimulate striatal neurons. Dopamine release regulates two populations of striatal neurons inversely: neurons that project directly to the GPi from the striatum are stimulated, and neurons that project to the GPi via the globus pallidus pars externa and STN are inhibited. Therefore, dopamine inhibits thalamic activity, which disinhibits the cortex and allows movement to occur. In PD, loss of dopaminergic neurons eliminates this cortical activation and inhibits movement. a | Vector injection into the caudate for the expression of dopamine producing enzymes replaces PD-related dopamine loss. b | Neurturin expression in the striatum and substantia nigra might preserve dopamine neurons, and enhance their function. c | Delivery of GAD to the STN induces GABA production, changing the STN input to the GPi from excitatory to inhibitory. GAD expression, therefore, reverses the abnormal increase in STN activity that occurs in PD, reducing the abnormally high GPi activity that prevents movement. Abbreviations: AADC, aromatic amino acid decarboxylase; GCH1, GTP cyclohydrolase 1; GABA, γ-aminobutyric acid; GAD, glutamic acid decarboxylase; GPi, globus pallidus pars interna; PD, Parkinson disease; STN, subthalamic nucleus; TH, tyrosine hydroxylase.