CHARACTERIZATION OF 2 ALTERNATIVELY SPLICED FORMS OF A METABOTROPIC GLUTAMATE-RECEPTOR IN THE CENTRAL-NERVOUS-SYSTEM OF THE RAT

Amplification of complementary DNA by the polymerase chain reaction an d anti-peptide antibodies were used to characterize the expression of two alternatively spliced forms of a metabotropic glutamate receptor ( mGluR1 alpha and mGluR1 beta) in the central nervous system of the rat . Polymerase chain reaction analysis showed that mGluR1 alpha was the predominate of the two forms in the cerebellum, diencephalon, mesencep halon, olfactory bulb and brainstem, while mGluR1 beta was the major f orm present in the hippocampus. Approximately equal amounts of the two receptors were expressed in the cerebral cortex, septum and striatum. Immunochemical analyses of the two receptors were conducted in the ra t cerebellum and hippocampus. An mGluR1 alpha-specific antibody labell ed a protein with a relative molecular weight of 146,000 on immunoblot s of the hippocampus and cerebellum. Immunoblot analysis of the develo pmental expression of mGluR1 alpha in the hippocampus and cerebellum d emonstrated that in both structures, the levels of mGluR1 alpha were a t or near their maximum levels in the adult brain. In contrast, two mG luR1 beta-specific antibodies failed to detect mGluR1 beta on immunobl ots of brain tissue, thus precluding an immunocytochemical analysis of this receptor. Although low levels of a higher-molecular weight prote in, possibly a dimeric form of mGluR1 beta were seen with one of the m GluR1 beta-specific antibodies, we hypothesize that some of the mGluR1 beta present in brain tissue may undergo proteolytic cleavage of the carboxy terminus. Immunocytochemical analysis of mGluR1 alpha showed t hat very high levels of this receptor were expressed in Purkinje cell bodies and dendrites. In the granule cell layer, some Golgi neurons we re immunostained. The granule cells were not labelled. In the hippocam pus, mGluR1 alpha immunoreactivity was present in interneurons of the stratum oriens and the dentate hilar region. Double-labelling studies demonstrated that these interneurons were also immunopositive for the neuropeptide somatostatin. The presence of mGluR1 alpha in cells of th e hippocampus that are associated with the release of somatostatin, su ggest that this receptor could play a role in regulating hippocampal e xcitability in both normal and epileptic tissues.