The action of dopamine on neurones of the caudate nucleus.

1. Dopamine applied iontophoretically to neurones of the caudate nucleus of cats caused excitation of some (9% of those encountered) and depression of others (60%). Some cells have been found affected both by dopamine and by acetylcholine.2. The effects of dopamine could be prevented by the previous iontophoretic administration of phenoxybenzamine, but not by dichloroisopropylnoradrenaline.3. Responses evoked in caudate neurones by electrical stimulation of substantia nigra were depressed by dopamine. No evidence for enhancement of the effects of nigral stimulation through the application of dopamine were detected.4. Stimulation of nucleus centromedianus thalami depressed firing of caudate neurones.5. The hypothesis that dopamine may act as an inhibitory synaptic transmitter within the caudate is put forward.