GABAERGIC SYNAPTIC-INTERACTIONS IN THE SUBSTANTIA-NIGRA

The substantia nigra receives a strong GABAergic input from the ipsila teral striatum and globus pallidus. Nigral GABAergic synaptic interact ions have been described in the pars compacta (SNC) and pars reticulat a (SNR) but not in the pars lateralis (SNL). The SNR and particularly the SNL are the nodal points of the GABAergic nigrotectal pathway. The present study analyzes the synaptic connections of GABAergic and dopa minergic neurons in each of the divisions of the substantia nigra by e mploying a double-labeling immunocytochemical technique at the light a nd electron microscope levels. Glutamic acid decarboxylase (GAD)-conta ining terminals make symmetrical synaptic contacts with dopaminergic n eurons in the SNC and SNR. Neurons that contain GAD also receive a GAB Aergic input in the SNR and SNL. The proportion of GAD-GAD contacts ap pears to be highest in the SNL where virtually all GAD-positive termin als are found to be in synaptic contact with or apposed to GAD positiv e profiles. This study demonstrates a strong GABAergic input onto nigr al dopaminergic neurons and GABAergic neurons in the SNR and SNL. This GABAergic influence which is ostensibly striatal or pallidal in origi n is particularly prominent in relation to the SNL-mediated nigro-coll icular pathway.