Responses of motoneurones to electrophoretically applied dopamine.

1. The effect of electrophoretically applied dopamine upon motoneurone excitability has been investigated. Field potentials originating from antidromically activated motoneurones were recorded from the ventral horn of the rat lumbar spinal cord. 2. Field potentials showed an increase in amplitude following electrophoretic application of dopamine. Dopamine was shown to be less potent than noradrenaline and 5-hydroxytryptamine in producing these changes. 3. Measurement of the transport number of dopamine suggest that the relatively low potency of dopamine cannot be attributed to differences in ionic mobilities between the amines. 4. Electrophoretic application of alpha-flupenthixol was shown to discriminate between dopamine and 5-hydroxytryptamine responses. Dopamine responses were profoundly reduced. 5. Electrophoretically applied alpha-flupenthixol also discriminated between dopamine and noradrenaline. Noradrenaline responses were consistently potentiated by alpha-flupenthixol. The possibility is discussed that dopamine may not merely be a precursor for noradrenaline in the rat spinal cord.