GABAergic synaptic interactions in the substantia nigra.

The substantia nigra receives a strong GABAergic input from the ipsilateral striatum and globus pallidus. Nigral GABAergic synaptic interactions have been described in the pars compacta (SNC) and pars reticulata (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 dopaminergic neurons in each of the divisions of the substantia nigra by employing a double-labeling immunocytochemical technique at the light and electron microscope levels. Glutamic acid decarboxylase (GAD)-containing terminals make symmetrical synaptic contacts with dopaminergic neurons in the SNC and SNR. Neurons that contain GAD also receive a GABAergic input in the SNR and SNL. The proportion of GAD-GAD contacts appears to be highest in the SNL where virtually all GAD-positive terminals are found to be in synaptic contact with or apposed to GAD positive profiles. This study demonstrates a strong GABAergic input onto nigral dopaminergic neurons and GABAergic neurons in the SNR and SNL. This GABAergic influence which is ostensibly striatal or pallidal in origin is particularly prominent in relation to the SNL-mediated nigro-collicular pathway.