Localization of GABA receptors in the basal ganglia.

The majority of neurons in the basal ganglia utilize GABA as their principal neurotransmitter and, as a consequence, most basal ganglia neurons receive extensive GABAergic inputs derived from multiple sources. In order to understand the diverse roles of GABA in the basal ganglia it is necessary to define the precise localization of GABA receptors in relation to known neuron subtypes and known afferents. In this chapter, we summarize data on the ultrastructural localization of ionotropic GABA(A) receptors and metabotropic GABA(B) receptors in the basal ganglia. In each of the regions of the basal ganglia that have been studied, GABA(A) receptor subunits are located primarily at symmetrical synapses formed by GABAergic boutons, where they display a several-hundred-fold enrichment over extrasynaptic sites. In contrast, GABA(B) receptors are widely distributed at synaptic and extrasynaptic sites on both presynaptic and postsynaptic membranes. Presynaptic GABA(B) receptors are localized on striatopallidal, striatonigral and pallidonigral afferent terminals, as well as glutamatergic terminals derived from the cortex, thalamus and subthalamic nucleus. It is concluded that fast GABA transmission mediated by GABA(A) receptors in the basal ganglia occurs primarily at synapses whereas GABA transmission mediated by GABA(B) receptors is more complex, involving receptors located at presynaptic, postsynaptic and extrasynaptic sites.