Abstract

Group III metabotropic glutamate receptors (mGluRs; mGluR4, 6, 7, and 8) couple to the Gα_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 ∼100-fold more potent (IC₅₀ = 0.1 μM) than l-Glu (∼10 μM), but it had efficacy similar to that of l-Glu (∼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)-α-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.