Electrophysiological evidence for the existence of excitatory fibres in the caudato-nigral pathway in the cat.

The electrophysiological effect of stimulation of the caudate nucleus on cat nigral neurones was investigated under conditions of the strong inhibitory influence of GABA being blocked by intravenously administered picrotoxin. At 10-20 min after picrotoxin injection, the caudate-evoked inhibition of nigral neurones was markedly diminished and instead the caudate-evoked excitatory responses of the same neurones appeared. These caudate-evoked excitatory responses were antagonized by intravenously administered baclofen, but not by atropine. These results open up the possibility that the caudate-evoked excitation of the nigral neurones may be mediated by substance P.