Metabotropic glutamate receptors in superficial laminae of the rat dorsal horn

Light and electron microscopic immunocytochemistry were employed here to sh ow the distribution of metabotropic glutamate receptors (mGluRs) mGluR2/3 a nd mGluR5 in laminae I and II of the dorsal horn, to identify their pre- an d postsynaptic location, and to test colocalization with gamma-aminobutyric acid (GABA). mGluR2/3 was mainly in the inner part of lamina II; mGluR5 wa s mainly in laminae I and II. Electron microscopy showed that bath mGluR2/3 and mGluR5 were in perikarya, dendrites, and vesicle-containing profiles. Two main morphological types of primary afferent terminals can be distingui shed in the superficial laminae: C1, likely to be endings of unmyelinated f ibers, and C2, of small myelinated fibers. Quantitative data show that only a small fraction of C2s stained for either receptor; more common were immu nopositive dendrites postsynaptic to these terminals, and most common were appositions between C2s and mGluR5 immunopositive dendrites. Vesicle-contai ning profiles were characteristically apposed to primary afferent terminals , mainly C2s. Immunopositivity for mGluRs, especially mGluR2/3, was present in vesicle-containing profiles apposed to C2, none to C1, and about half o f the profiles immunostained for either receptor were also stained for GABA . The presence of presynaptic and postsynaptic mGluRs in both inhibitory an d excitatory interneurons may contribute to complex processing of fast and slow responses to peripheral input in superficial laminae. As selective ago nists of mGluRs may modulate GABA release, the present demonstration of mGl uRs in GABAergic terminals of presumed interneurons suggests that facilitat ory effects may involve a mechanism of disinhibition. (C) 1999 Wiley-Liss, Inc.