Characterization of two alternatively spliced forms of a metabotropic glutamate receptor in the central nervous system of the rat

Prolonged induction of c-fos in neuropeptide Y- and somatostatin-immunoreactive neurons of the rat dentate gyrus after electroconvulsive stimulation

Induction of c-fos mRNA and Fos was studied in the hilus and granular layer of the dentate gyrus at various times up to 24 h after single electroconvulsive stimulation (ECS) using in situ hybridization and immunocytochemistry. In both areas of the dentate gyrus, a prominent induction of c-fos mRNA and Fos was observed. Compared to the granular layer, however, c-fos mRNA and Fos in hilar cells reached maximum later and remained elevated considerably longer. Several neurochemically distinct populations of hilar neurons have been described, some of which contain neuropeptide Y (NPY) and/or somatostatin (SS). Using double-labelling immunocytochemistry, we examined to what extent Fos was induced in these hilar neurons after ECS. Although a minor population of non-NPY non-SS cells displayed Fos induction early after ECS, prolonged induction of Fos almost exclusively occurred in NPY or SS neurons. The Fos-immunoreactive NPY or SS neurons only amounted to about 50% of the total hilar population of NPY or SS neurons. The present observations suggest that a subpopulation of hilar NPY and SS neurons may be central to the actions of electroconvulsive seizures in the dentate gyrus.

Metabotropic glutamate receptor subtype 1A activates adenylate cyclase when expressed in baby hamster kidney cells

1. The coupling of subtype mGluR1a of the metabotropic glutamate receptor family to phosphoinositide hydrolysis, cAMP-formation and arachidonic acid release was characterized when the receptor was expressed in baby hamster kidney (BHK) cells. 2. When measuring cAMP-formation in BHK cells expressing mGluR1a, the rank order of agonist potency as well as the efficacy of mGluR1a antagonists was comparable to what has been observed when measuring mGluR1a-mediated phosphoinositide (PI)-hydrolysis. 3. However, while the presence of extracellular calcium increased the efficacy of quisqualate to stimulate phosphoinositide hydrolysis no significant effects were observed when measuring quisqualate-induced cAMP-formation. 4. Pretreatment of mGluR1a-expressing cells with pertussis toxin increased quisqualate-induced cAMP-formation in contrast to the observed partial inhibition of PI-hydrolysis by pertussis toxin. Cholera toxin increased cAMP-formation in BHK cells but showed no effects on PI-hydrolysis. 5. While quisqualate also stimulated [3H]-arachidonic acid release from BHK cells expressing mGluR1a this effect may be secondary to activation of phospholipase C. 6. These data further suggest that mGluR1a is coupled to PI-hydrolysis as well as cAMP-formation via different G-proteins which can be discriminated by their sensitivity to pertussis toxin.

The metabotropic glutamate receptors, mGluR2 and mGluR3, show unique postsynaptic, presynaptic and glial localizations

Glutamate neurotransmission involves numerous ionotropic and metabotropic glutamate receptor types in postsynaptic, presynaptic and glial locations. Distribution of the metabotropic glutamate receptors mGluR2 and mGluR3 was studied with an affinity-purified, characterized polyclonal antibody made from a C-terminus peptide. This antibody, mGluR2/3, recognized both mGluR2 and mGluR3, but did not cross-react with any other type of metabotropic glutamate receptor except for a very slight recognition of mGluR5. Light microscope distribution of the antibody binding sites in the nervous system matched the combined distributions of messenger RNA for mGluR2 and mGluR3. For example, dense staining seen in the accessory olfactory bulb and cerebellar Golgi cells matched high levels of mGluR2 messenger RNA in these structures, while moderately dense staining in the reticular nucleus of the thalamus and light to moderate staining in glia throughout the brain matched significant levels of mGluR3 messenger RNA in these structures. In the rostral olfactory structures, the densest stained neurons belonged to presumptive "necklace olfactory glomeruli." In the hippocampus, staining was densest in the neuropil of the stratum lucidum/pyramidale, stratum lacunosum/moleculare, hilus and middle third of the molecular layer of the dentate gyrus. Ultrastructural studies of the cerebral cortex, hippocampus and caudate-putamen revealed significant staining in postsynaptic and presynaptic structures and glial wrappings of presumptive excitatory synapses; frequently, this staining was concentrated in discrete patches at or near active zones. In the hippocampus, presynaptic staining appeared to be concentrated in terminals of two populations of presumptive glutamatergic axons: mossy fibers originating from granule cells and perforant path fibers originating from the entorhinal cortex. These data suggest that populations of mGluR2 and/or mGluR3 receptors are localized differentially in synapses, i.e. those in and near the postsynaptic and presynaptic membranes and in glial wrappings of synapses, in several regions of the brain. In addition, we provide immunocytochemical evidence of mGluR2 or mGluR3 receptors in presynaptic terminals of glutamatergic synapses. Thus, mGluR2 and mGluR3 are found in various combinations of presynaptic, postsynaptic and glial localizations that may reflect differential modulation of excitatory amino acid transmission.

Human metabotropic glutamate receptor 1: mRNA distribution, chromosome localization and functional expression of two splice variants

We have isolated clones from a human brain cDNA library coding for two splice variants of mGluR1. The combined sequences account for 6864 base pairs (bp) of the approximately 7 kilobase (kb) transcript seen on northern blots of human brain mRNA. The distribution of mGluR1 mRNA in human brain is similar to that found in rat brain and the gene for mGluR1 is localized on human chromosome 6. The mRNA for mGluR1 alpha contains an open reading frame that codes for a 1194 amino acid protein, which is slightly smaller than rat mGluR1 alpha. The major structural difference noted between the human and rat mGluR1 is a deletion of 21 nucleotides which would result in the loss of seven amino acids in the middle of a proline- and glutamine-rich region of the C-terminal tail of mGluR1 alpha. The 85 bp exon that results in the HmGluR1 beta splice variant was found to code for a protein with two amino acid differences compared to the rat receptor. Functional and pharmacological characterization of heterologously expressed human mGluR1 was performed using electrophysiological recordings from Xenopus oocytes and calcium imaging in HEK-293 cells. No major differences were found in the response of human mGluR1 to typical agonists and antagonists compared to the rat, or in the behavior of the two splice variants.

Developmental change from inhibition to facilitation in the presynaptic control of glutamate exocytosis by metabotropic glutamate receptors

We have addressed the role of presynaptic metabotropic glutamate receptors in the control of glutamate release from cerebrocortical nerve terminals. The metabotropic glutamate receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid enhances the release evoked by a submaximal depolarization in the presence of low concentrations of arachidonic acid and in a staurosporine-sensitive manner. In contrast, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid and L(+)-2-amino-4-phosphonobutyrate inhibit the release evoked by a maximal depolarization, in the absence of arachidonic acid and by a staurosporine-insensitive mechanism. Interestingly, the effects of the metabotropic glutamate receptors that inhibit glutamate release are only observed in the nerve terminals from young rats (one to three weeks), while the facilitatory effects are better seen in latter developmental stages (three to four weeks) and adult (two to three months) rats, coinciding with the development of the maximal capacity of glutamate uptake. These results indicate the existence of important developmental changes in the presynaptic control of glutamate release.

Rapid agonist mediated phosphorylation of the metabotropic glutamate receptor 1 alpha by protein kinase C in permanently transfected BHK cells

Clonal BHK cells permanently transfected with the metabotropic glutamate receptor 1 alpha (mGluR1 alpha), which is coupled to phospholipase C, were used to study the phosphorylation state of the receptor. Cells were labelled with 32PO4(3-), lysed, the receptor immunoprecipitated with specific anti-peptide antibodies and the immunoprecipitates analysed by SDS-PAGE followed by autoradiography. A significant basal level of receptor phosphorylation was observed which was rapidly and transiently increased in response to agonist activation of the receptor. This agonist effect was found to be dose dependent with a rapid time course and could be abolished by the specific PKC inhibitor Ro318220, suggesting that PKC was responsible for the agonist mediated phosphorylation of the receptor.

NMDA receptor expression in the mouse cerebellar cortex

A detailed, light microscopic study on the distribution of the N-methyl- D-aspartate receptor subunit 1 (NMDAR1) was carried out with immunohistochemistry and in situ hybridization on the cerebellar cortex of the mouse. With a monoclonal antibody, labeling of Purkinje cell bodies varied from intense to negative, while heavy dendritic staining was limited to the proximal dendrites (unlike the rat, which also had heavily stained distal dendrites). In the granular layer, the cell bodies and and the dendritic shafts of Golgi II cells were only moderately stained, but very intense labeling was associated with granule cell bodies, and with their dendrites and dendritic endings in the glomeruli. The mossy and climbing fibers were negative. In situ hybridization with a cRNA probe showed levels and spatial distributions of NMDAR1 mRNA consistent with the immunolabeling pattern, in that signals were strongest in the granular and Purkinje cell layers and relatively low or absent in the molecular layer and white matter. The findings are consistent with the hypothesis that NMDAR1 may be especially well concentrated at the synaptic target sites of the mossy and climbing fibers. In the mouse, NMDAR1 at the parallel fiber sites associated with Purkinje cell spiny branchlets may differ from the rat in its level of expression or in its molecular configuration.