Role of Ca2+ stores in metabotropic L-glutamate receptor-mediated supralinear Ca2+ signaling in rat hippocampal neurons

Abstract

The role of metabotropic L-glutamate (mGlu) receptors in supralinear Ca2+ signaling was investigated in cultured hippocampal cells using Ca2+ imaging techniques and whole-cell voltage-clamp recording. In neurons, but not gila, global supralinear Ca2+ release from intracellular stores was observed when the mGlu receptor agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) was combined with elevated extracellular K+ levels (10.8 mM), moderate depolarization (15-30 mV), or NMDA (3 μM). There was a delay (2-8 min) before the stores were fully charged, and the enhancement persisted for a short period (up to 10 min) after removal of the store-loading stimulus. Studies with the mGlu receptor antagonist 2-methyl-6-(phenylethynyl)-pyridine demonstrated that these effects were mediated by activation of the mGlu5 receptor subtype. The L-type voltage-gated Ca2+ channel antagonist nifedipine (10 μM) substantially reduced responses to DHPG obtained in the presence of elevated extracellular K+ but not NMDA. This suggests that the Ca2+ that is required to load the stores can enter either through L-type voltage-gated Ca2+ channels or directly through NMDA receptors. The findings that both depolarization and NMDA receptor activation can facilitate mGlu receptor Ca2+ signaling adds considerable flexibility to the processes that underlie activity-dependent changes in synaptic strength. In particular, a temporal separation between the store-loading stimulus and the activation of mGlu receptors could be used as a recency detector in neurons.