Membrane potential and intracellular Ca2+ oscillations activated by mGluRs in hippocampal stratum oriens/alveus interneurons

Metabotropic glutamate receptors (mGluRs) are expressed heterogeneously in hippocampal interneurons, and their signal transduction cascades remain largely unclear. We characterized an oscillatory response activated by the mGluR agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) in hippocampal interneurons of stratum oriens-alveus (OA) with simultaneous whole cell current-clamp recordings and intracellular Ca2+ imaging with confocal microscopy. 1S,3R-ACPD induced oscillatory membrane depolarizations and rises in intracellular Ca2+ that persisted in tetrodotoxin and were blocked by the antagonist of group I and II mGluRs (S)-alpha-methyl-4-carboxyphenylglycine. Membrane depolarizations and intracellular Ca2+ rises were blocked by extracellular Cd2+ and in Ca2+-free medium. mGluR responses therefore required Ca2+ influx via voltage-gated Ca2+ channels. 1S,3R-ACPD responses were also antagonized by depleting intracellular stores with thapsigargin and ryanodine, indicating that Ca2+ release from intracellular stores was also necessary. These data suggest that oscillatory responses generated by group I/II mGluRs involve a coupling of Ca2+ entry through voltage-gated Ca2+ channels and Ca2+ release from internal stores. In contrast, 1S,3R-ACPD evoked only smaller depolarizations and intracellular Ca2+ rises, with no oscillations, in other hippocampal interneurons located in or near stratum lacunosum-moleculare. Thus mGluR-mediated oscillatory responses are specifically expressed in certain interneuron subtypes. This heterogeneous expression of glutamate and Ca2+ signaling pathways in specific interneurons may be relevant to their selective vulnerability to excitotoxicity.