GABA and the nigrothalamic pathway

Abstract In the rat, electrolytic or kainate lesions of one substantia nigra caused a small reduction in the levels of GABA, glutamic acid decarboxylase and glutamate in the ipsilateral medial thalamus. Nigral stimulation gave similar extracellular synaptic responses in projection identified thalamic VM or PF neurones. These comprised a short latency inhibition (mean 5.1 msec) often following a brief excitation, and a longer latency inhibition (mean 22.3 msec) often following a more sustained excitatory burst. Iontophoretic bicuculline selectively blocked the effects of GABA, but not glycine or 5-hydroxytryptamine, on these thalamic neurones, and also blocked both short and long latency synaptic inhibition. Intranigral injection of muscimol massively excited some PF thalamic neurones. The contralateral circling behaviour induced by intranigral muscimol was potentiated by intrathalamic microinjections of picrotoxin and attenuated by ethanolamine-O-sulphate. Thus, GABA appears to be the main inhibitory transmitter of the nigrothalamic pathway and for longer latency inhibition in the medial thalamus where it plays a key role in mediating some circling behaviours.