title: Coupling of metabotropic glutamate receptor 8 to N-type Ca2+ channels in rat sympathetic neurons.
abstract: Group III metabotropic glutamate receptors (mGluRs; mGluR4, 6, 7, and 8) couple to the Galpha(i/o)-containing G protein heterotrimers and act as autoreceptors to regulate glutamate release, probably by inhibiting voltage-gated Ca(2+) channels. Although most mGluRs have been functionally expressed in a variety of systems, few studies have demonstrated robust coupling of mGluR8 to downstream effectors. We therefore tested whether activation of mGluR8 inhibited Ca(2+) channels. Both L-glutamate (L-Glu) and l-2-amino-4-phosphonobutyric acid (L-AP4), a selective agonist for group III mGluRs, inhibited N-type Ca(2+) current in rat superior cervical ganglion neurons previously injected with a cDNA encoding mGluR8a/b. L-AP4 was approximately 100-fold more potent (IC(50) = 0.1 microM) than L-Glu ( approximately 10 microM), but it had efficacy similar to that of L-Glu ( approximately 50% maximal inhibition). The potency and efficacy of L-AP4 and L-Glu were similar for both splice variants. Agonist-induced inhibition was abolished by pretreatment with (R,S)-alpha-cyclopropyl-4-phosphonophenylglycine, a selective group III mGluR antagonist, and pertussis toxin. Deletion of either a calmodulin (CaM) binding motif in the C terminus or the entire C terminus of mGluR8 did not affect mGluR8-mediated response. Our studies indicate that both mGluR8a and 8b are capable of inhibiting N-type Ca(2+) channel, suggesting a role as presynaptic autoreceptors to regulate neuronal excitability. The studies also imply that the potential CaM binding domain is not required for the mGluR8-mediated Ca(2+) channel inhibition and the C terminus of mGluR8a is dispensable for receptor coupling to N-type Ca(2+) channels.
authors.affiliation: Laboratory of Molecular Physiology, National Institute on Alcohol Abuse and Alcoholism/NIH, Room TS-06, 5625 Fishers Lane, Bethesda, MD 20892-8815, USA.
authors.forename: Juan
authors.initials: J
authors.lastname: Guo
authors.forename: Stephen R
authors.initials: SR
authors.lastname: Ikeda
mesh_terms: D000320:Adrenergic Fibers / Q000187:drug effects / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D002121:Calcium Channel Blockers / Q000378:metabolism / Q000494:pharmacology; D020864:Calcium Channels, N-Type / Q000378:metabolism* / Q000502:physiology; D002460:Cell Line; D004305:Dose-Response Relationship, Drug; D018698:Glutamic Acid / Q000378:metabolism / Q000494:pharmacology; D006801:Humans; D008297:Male; D051379:Mice; D008969:Molecular Sequence Data; D011422:Propionates / Q000378:metabolism / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D018094:Receptors, Metabotropic Glutamate / Q000235:genetics / Q000378:metabolism*
chemical_list: C114785:2-amino-4-phosphono-propinate; D002121:Calcium Channel Blockers; D020864:Calcium Channels, N-Type; D011422:Propionates; D018094:Receptors, Metabotropic Glutamate; C111435:metabotropic glutamate receptor 8; D018698:Glutamic Acid