Evidence for a GABAergic projection from the substantia nigra to the ventromedial thalamus and to the superior colliculus of the rat.

Unilateral intranigral infusion of kainic acid (1.5 microgram) produced neuronal loss in the lateral two-thirds of the nigra while sparing axons en passage. Fink-Heimer silver impregnation revealed dense terminal degeneration in the nigra itself (both in the compacta and in the reticulata) and in areas of non-dopaminergic nigral projection such as the ventromedial (VM) nucleus of the thalamus, the superior colliculus and the reticular formation; only spare terminal degeneration was found in areas of dopaminergic projection such as the caudate and septum. In order to clarify the nature of the transmitter of the nigrothalamic and nigrocollicular neurons, the activity of glutamic decarboxylase (GAD), the marker of cholinergic neurons, was measured in the VM and ventrobasal (VB) thalamus and in the nigra of each side, 7 days after unilateral intranigral injection of kainic acid. GAD activity was reduced significantly in the VM-thalamus (-33%), in the superior colliculus (-40%) and in the substantia nigra (-18%) but not in the VB-thalamus of the lesioned side. CAT remained unchanged in these areas. Similar results were obtained in the thalamus and in the superior colliculus after electrocoagulative lesions of the nigra. The results indicate the existence of a nigrothalamic and of a nigrocollicular GABAergic pathway. This projection might play an important role in motor coordination and gaze control.