Abstract
The mechanisms of Ca^2+^ responses evoked in hippocampal glial cells in situ, by local application of glutamate and by synaptic activation, were studied in slices from juvenile rats using the membrane permeant fluorescent Ca^2+^ indicator fluo‐3AM and confocal microscopy. Ca^2+^ responses induced by local application of glutamate were unaffected by the sodium channel blocker tetrodotoxin and were therefore due to direct actions on glial cells. Glutamate‐evoked responses were significantly reduced by the L‐type Ca^2+^ channel blocker nimodipine, the group I/II metabotropic glutamate receptor antagonist (S)‐α‐methyl‐4‐carboxyphenylglycine (MCPG), and the N‐methyl‐D‐aspartate (NMDA) receptor antagonist (±)2‐amino‐5‐phosphonopentanoic acid (APV). However, glutamate‐induced Ca^2+^ responses were not significantly reduced by the non‐NMDA receptor antagonist 6‐cyano‐7‐nitro‐quinoxaline‐2,3‐dione (CNQX). These results indicate that local application of glutamate increases intracellular Ca^2+^ levels in glial cells via the activation of L‐type Ca^2+^ channels, NMDA receptors, and metabotropic glutamate receptors. Brief (1 s) tetanization of Schaffer collaterals produced increases in intracellular Ca^2+^ levels in glial cells that were dependent on the frequency of stimulation (≥50 Hz) and on synaptic transmission (abolished by tetrodotoxin). These Ca^2+^ responses were also antagonized by the L‐type Ca^2+^ channel blocker nimodipine and the metabotropic glutamate receptor antagonist MCPG. However, the non‐NMDA receptor antagonist CNQX significantly reduced the Schaffer collateral‐evoked Ca^2+^ responses, while the NMDA antagonist APV did not. Thus, these synaptically mediated Ca^2+^ responses in glial cells involve the activation of L‐type Ca^2+^ channels, group I/II metabotropic glutamate receptors, and non‐NMDA receptors. These findings indicate that increases in intracellular Ca^2+^ levels induced in glial cells by local glutamate application and by synaptic activity share similar mechanisms (activation of L‐type Ca^2+^ channels and group I/II metabotropic glutamate receptors) but also have distinct components (NMDA vs. non‐NMDA receptor activation, respectively). Therefore, neuron‐glia interactions in rat hippocampus in situ involve multiple, complex Ca^2+^‐mediated processes that may not be mimicked by local glutamate application. Hippocampus 2001;11:132–145. © 2001 Wiley‐Liss, Inc.