| The discovery of glutamate induced Ca{dollar}sp{lcub}rm 2+{rcub}{dollar} waves and oscillations within cultured astrocyte monolayers led to the speculation that such phenomena occur in situ, being triggered by synaptically released glutamate. We have tested this hypothesis within the hippocampal formation by stimulating two of the intrinsic excitatory connections: the mossy fiber and Schaffer collateral pathways. Through the use of laser confocal microscopy and the fluorescent Ca{dollar}sp{lcub}rm 2+{rcub}{dollar} indicator Fluo-3, we have shown that astrocytes respond to neuronal activity with oscillations in their cytoplasmic Ca{dollar}sp{lcub}rm 2+{rcub}{dollar} concentration and both inter- and intracellular Ca{dollar}sp{lcub}rm 2+{rcub}{dollar} waves. These astrocytic phenomena dependent upon neuronal activity as tetrodotoxin completely abolishes the response, and in addition appear to result from a synaptically released substance, most likely the excitatory neurotransmitter glutamate since the non-specific antagonist kynurenic acid is able to block the response. Furthermore, these behaviors are common to several hippocampal preparations including dissociated cell culture, organotypic slice and acutely isolated tissue. By comparing astrocytic response characteristics such as latency, oscillatory period and wave velocity, we conclude that these properties vary little between the preparations examined (e.g. latency ranges from 2 to 6 s, period ranges from 14 to 32 s, and velocity ranges from 5 to 60 {dollar}mu{dollar}m/s). Hence neuron to astrocyte signaling and inter-astrocytic signaling both appear to be basic properties within the hippocampus, and perhaps the entire nervous system. |
