Ontogeny of non-NMDA glutamate receptors in rat barrel field cortex: I. Metabotropic receptors

The ontogeny of metabotropic excitatory amino acid receptors (mGluRs) in rat barrel field cortex was characterized by using receptor autoradiography and immunocytochemistry to test the hypothesis that changes in mGluR expression coincide with the emergence of somatotopic patterns in this region. On postnatal days 1 (P1) and 3, [3H]glutamate binding to mGluRs was not distributed in a somatotopic pattern. By P5, mGluRs exhibited a whisker-related pattern, with higher densities of mGluRs in barrel centers than in surrounding cortex. Between P5 and P14 and at P60, the overall binding density remained higher in barrels than in surrounding cortex. At P60, a somatotopic pattern of binding was not apparent. The majority of mGluR sites in the barrel field were blocked by the metabotropic agonist trans-1-aminocyclopentane-1,3-dicarboxylic acid but were not significantly displaced by quisqualate. Immunocytochemical studies of phosphoinositide-linked mGluRs, mGluR5 and mGluR1alpha, showed that the developmental expression of mGluR5 mirrored that of the pattern of autoradiographically labeled mGluRs. The immature barrel field (ages P5-P14) was enriched in mGluR5, with greater concentrations of mGluR5 immunoreactivity in barrels than in surrounding cortex. Within barrel centers, mGluR5 was localized within the neuropil, on the surfaces of cell bodies and dendrites in layer IV. A somatotopic pattern of mGluR5 immunoreactivity persisted into adulthood, although the pattern was less pronounced after P14. In contrast, mGluR1alpha was never localized in a somatotopic pattern in barrel field cortex. We conclude from the developmental localization of mGluRs that the spatiotemporal regulated expression of these receptors may influence barrel maturation and plasticity.