Localization of metabotropic glutamate receptor 7 mRNA and mGluR7a protein in the rat basal ganglia

Metabotropic glutamate receptors (mGluRs) coupled to G-proteins have important roles in the regulation of basal ganglia function. We have examined the localization of the mGluR7 mRNA and mGluR7a protein in the basal ganglia of the rat. Strong mGluR7 hybridization signals are found in cerebral cortex and striatum, but much less intense signals are present in other components of the basal ganglia. Abundant mGluR7a immunoreactivity was found in striatum, globus pallidus (GP), and substantia nigra pars reticulata (SNr). Examination using confocal microscopy together with dendritic and presynaptic markers as well as studies in lesion models provided evidence for the presence of mGluR7a on presynaptic terminals in all three structures. Electron microscopic studies confirmed the presence of mGluR7a in axon terminals in both the striatum and the GP and also revealed the presence of mGluR7a at postsynaptic sites in both of these regions. Our data demonstrate that mGluR7a is located not only on presynaptic glutamatergic terminals of the corticostriatal pathway, where it may serve as an autoreceptor, but also on terminals of striatopallidal and striatonigral projections, where it may modulate the release of gamma-aminobutyric acid (GABA). The presence of mGluR7 at these multiple sites in the basal ganglia suggests that this receptor has a particularly crucial role in modulating neurotransmitter release in major basal ganglia pathways.