PHARMACOLOGICAL CHARACTERIZATION OF METABOTROPIC GLUTAMATE RECEPTORS IN SEVERAL TYPES OF BRAIN-CELLS IN PRIMARY CULTURES

Several cDNAs coding for metabotropic glutamate receptors (mGluR1-7) h ave now been isolated. mGluR1 and -5 are positively coupled to phospho lipase C, whereas mGluR2, -3, -4, -6, and -7 are negatively coupled to adenylyl cyclase (AC) when they are expressed in Chinese hamster ovar y or baby hamster kidney cells. However, the exact transduction mechan isms of these receptors in their natural environment remain to be dete rmined. In a previous work, we demonstrated that striatal neurons in p rimary culture expressed a mGluR that is negatively coupled to AC and that has a pharmacology different from that of mGluR2. In the present study, the pharmacology of mGluRs negatively coupled to AC in several neuronal types and in glial cells was compared with the pharmacology o f mGluR2, -3, and -4. Like striatal neurons, cerebral cortical neurons express a mGluR that is able to inhibit AC both in intact cells and i n membrane preparations, via a pertussis toxin-sensitive G protein. Th is mGluR has a pharmacological profile similar to that of mGluR3, beca use quisqualate is active at relatively low concentrations (EC(50) < 1 00 mu M). Similar experiments revealed that cerebellar granule cells e xpressed mGluR2-like and mGluR4-like receptors. Striatal glial cells a lso expressed a mGluR negatively coupled to AC via a pertussis toxin-s ensitive G protein. However, only glutamate and aspartate, and not qui squalate, 2-(carboxycyclopropyl)glycine, trans-1-aminocyclopentane-1,3 -dicarboxylate, or L-2-amino-4-phosphonobutyrate, were agonists for th is glial mGluR. This pharmacology is different from that of any cloned mGluR. Reverse transcription associated with polymerase chain reactio n revealed that mGluR2 and mGluR3 mRNAs are present in striatal, corti cal, and cerebellar neurons but not in striatal glial cells. Interesti ngly, mGluR4 mRNA was found at a high level in cerebellar granule cell s and at a lower level in cortical neurons and glial cells. However, t he mGluR4-specific agonist L-2-amino-4-phosphonobutyrate was found to inhibit AC very slightly in granule cells only. In conclusion, our dat a show that mGluR2- and mGluR3-like receptors can directly inhibit AC in neurons, and they raise the question of whether mGluR4 is really ne gatively coupled to AC in its normal environment. We also present evid ence for a new mGluR subtype expressed in glial cells.