Distribution of metabotropic glutamate 2 and 3 receptors in the rat forebrain: Implication in emotional responses and central disinhibition

The receptor localization of metabotropic glutamate receptors (mGlu) 2 and 3 was examined by using in situ hybridization and a well-characterized mGlu2-selective antibody in the rat forebrain. mGlu2 was highly and discretely expressed in cell bodies in almost all of the key regions of the limbic system in the forebrain, including the midline and intralaminar structures of the thalamus, the association cortices, the dentate gyrus of the hippocampus, the medial mammillary nucleus, and the lateral and basolateral nuclei of the amygdala. Moreover, presynaptic mGlu2 terminals were found in most of the forebrain structures, especially in the lateral part of the central nucleus of the amygdala, and the CA1 region of the hippocampus. Although some overlaps exist, such as in the hippocampus and the amygdala, the expression of mGlu3 mRNA, however, appeared to be more disperse, compared with that of mGlu2 mRNA. These distribution results support previous behavioral studies that the mGlu2 and 3 receptors may play important roles in emotional responses. In addition to its expression in glia, mGlu3 was distinctively expressed in cells in the GABAergic reticular nucleus of the thalamus. Local infusion of a non-selective mGlu2/3 agonist, LY379268, in the reticular nucleus of the thalamus, significantly reduced GABA release, suggesting that mGlu3 may also play a role in central disinhibition.