This project proposes a neurophysiological and neuropharmacological study of the caudate nucleus in a model of Parkinson's disease produced by the interruption of the nigrostriatal pathway in cats. Extending our current studies we intend to compare the effects of acetylcholine, dopamine, serotonin and other drugs with the effects of stimulating different fiber systems converging on this nucleus. We thereby wish to test the hypotheses that the normal functioning of the caudate nucleus depends on a balance between cholinergic, dopaminergic and serotoninergic inputs and that Parkinson's disease is due to a disturbance of this balance. We will use multibarreled micropipettes in order to apply drugs microiontophoretically near single neurons of the caudate nucleus and to record the spontaneous firing and the synaptic responses of these neurons. The neuronal responses will be enhanced by the computation of poststimulus histograms.