Coupling of metabotropic glutamate receptor 8 to n-type Ca2+ channels in rat sympathetic neurons

Group III metabotropic glutamate receptors (mGluRs; mGluR4, 6, 7, and 8) couple to the G alpha(i/o)-containing G protein heterotrimers and act as autoreceptors to regulate glutamate release, probably by inhibiting voltage-gated Ca2+ 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 Ca2+ 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 Ca2+ current in rat superior cervical ganglion neurons previously injected with a cDNA encoding mGluR8a/b. L-AP4 was similar to 100-fold more potent (IC50 = 0.1 mu M) than L-Glu (similar to 10 mu M), but it had efficacy similar to that of L-Glu (similar to 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 Ca2+ 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 Ca2+ channel inhibition and the C terminus of mGluR8a is dispensable for receptor coupling to N-type Ca2+ channels.