Neuropharmacological studies on the nigro-striatal and raphe-striatal system in the rat

The responses of single neostriatal neurones to substantia nigra (SN) and dorsal raphe nucleus (DRN) stimulation and iontophoretic administration of several drugs were studied in urethane-anaesthetised rats. Stimulation of the SN-evoked excitation followed by inhibition in striatal neurones. In some cells only inhibition of firing was evoked indicating that there may be separate nigrostriatal inhibitory and excitatory pathways. DRN stimulation evoked mainly inhibition of striatal cell firing. The activity of most neurones responding to SN and DRN stimulation was depressed by iontophoretically administered dopamine, 5-hydroxytryptamine and GABA and increased by acetylcholine. Studies with antagonist revealed that alpha-flupenthixol reduced responses to dopamine and 5-hydroxytryptamine and inhibition evoked by SN and DRN stimulation. Bicuculline methochloride only reduced responses to GABA. Methysergide selectively reduced responses to 5-hydroxytryptamine and also reduced DRN-but not SN-evoked inhibition. It was concluded that the SN-evoked inhibition was probably mediated by dopamine and DRN-evoked inhibition by 5-hydroxytryptamine.