title: Modulation of group III metabotropic glutamate receptors by hydrogen ions.
abstract: Protons act as neuromodulators and produce significant effects on synaptic transmission. The molecular basis of neuromodulation by extracellular protons is partially explained by their effects on certain neurotransmitter receptors and ion channels. The metabotropic glutamate receptors (mGluRs) are a family of eight receptor subtypes that are widely expressed throughout the mammalian CNS. In this study, the effects of physiologically relevant changes in extracellular pH were examined in mammalian cells expressing the mGluR subtypes: human mGluR1a, mGluR4a, mGluR5d or mGluR8b. The signal transduction coupling properties of mGluR4a and mGluR8b were switched from the adenylate cyclase (G(i)) pathway to the phospholipase C (G(q)) pathway by coexpression of a promiscuous G protein. Fluorometric imaging plate reader was used to measure changes in cytoplasmic calcium concentrations in response to agonist. Extracellular acidification from pH 8.0 to pH 6.5 progressively diminished mGluR4 responsiveness to the agonists L-glutamate and (2S,1'S,2'R)-2-(carboxycyclopropyl)glycine (L-CCG-I), and this inhibition was characterized by insurmountable antagonism. By comparison, extracellular acidification did not significantly alter mGluR8 responses to agonists. Furthermore, agonist activation of mGluR1a and mGluR5d was virtually unaffected by changes in pH. Because mGluR4 is expressed presynaptically and its activation inhibits the release of neurotransmitters such as glutamate and GABA, we propose that the net effect of proton inhibition of mGluR4 would be to reverse or prevent that suppression of neurotransmitter release. As such, local decreases in pH could have significant effects on the regulation of transmitter release and synaptic tone via modulation of mGluR4.
authors.affiliation: NPS Pharmaceuticals, Salt Lake City, Utah 84108, USA. clevinthal@qthera.com
authors.forename: Cynthia
authors.initials: C
authors.lastname: Levinthal
authors.forename: Lisa
authors.initials: L
authors.lastname: Barkdull
authors.forename: Pam
authors.initials: P
authors.lastname: Jacobson
authors.forename: Laura
authors.initials: L
authors.lastname: Storjohann
authors.forename: Bradford C
authors.initials: BC
authors.lastname: Van Wagenen
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Stormann
authors.forename: Lance G
authors.initials: LG
authors.lastname: Hammerland
mesh_terms: D000262:Adenylyl Cyclases / Q000378:metabolism; D000494:Allosteric Regulation / Q000187:drug effects; D000600:Amino Acids, Dicarboxylic / Q000819:agonists; D000818:Animals; D002118:Calcium / Q000819:agonists / Q000378:metabolism / Q000494:pharmacology; D002460:Cell Line; D002478:Cells, Cultured; D045604:Extracellular Fluid / Q000378:metabolism; D018698:Glutamic Acid / Q000378:metabolism / Q000494:pharmacology; D006801:Humans; D006863:Hydrogen-Ion Concentration / Q000187:drug effects*; D007424:Intracellular Fluid / Q000378:metabolism; D018377:Neurotransmitter Agents / Q000378:metabolism / Q000494:pharmacology; D011522:Protons; D018094:Receptors, Metabotropic Glutamate / Q000819:agonists / Q000037:antagonists & inhibitors / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects; D010738:Type C Phospholipases / Q000378:metabolism; D014981:Xenopus; D005680:gamma-Aminobutyric Acid / Q000187:drug effects / Q000378:metabolism
chemical_list: D000600:Amino Acids, Dicarboxylic; D018377:Neurotransmitter Agents; D011522:Protons; D018094:Receptors, Metabotropic Glutamate; C035261:(alpha-carboxycyclopropyl)glycine; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid; D010738:Type C Phospholipases; D000262:Adenylyl Cyclases; D002118:Calcium