Hippocampal AstrocytesIn SituRespond to Glutamate Released from Synaptic Terminals

Abstract

A long-standing question in neurobiology is whether astrocytes respond to the neuronal release of neurotransmitters in vivo. To address this question, acutely isolated hippocampal slices were loaded with the calcium-sensitive dye Calcium Green-1 and the responses of the astrocytes to electrical stimulation of the Schaffer collaterals were monitored by confocal microscopy. To confirm that the responsive cells were astrocytes, the slices were immunostained for the astrocytic marker glial fibrillary acidic protein. Stimulation of the Schaffer collaterals (50 Hz, 2 sec) resulted in increases in the concentration of intracellular calcium ([Ca 2ϩ ] i ) in the astrocytes located in the stratum radiatum of CA1. The astrocytic responses were blocked by the sodium channel blocker tetrodotoxin, the voltage-dependent calcium channel blocker -conotoxin-MVIIC, and the selective metabotropic glutamate receptor antagonist ␣-methyl-4-carboxyphenylglycine (MCPG). These results suggest that the astrocytic responses were induced by stimulation of metabotropic glutamate receptors on the astrocytes by neuronally released glutamate. The astrocytic responses to neuronal stimulation were enhanced in the presence of the K ϩ channel antagonist 4-aminopyridine (4-AP). Inhibition of the astrocytic responses in the presence of 4-AP required the presence of both MCPG and the ionotropic glutamate receptor antagonist kynurenic acid. These results suggest that higher levels of neuronal activity result in stimulation of both metabotropic and ionotropic glutamate receptors on the astrocytes. Overall, the results indicate that hippocampal astrocytes in situ are able to respond to the neuronal release of the neurotransmitter glutamate with increases in [Ca 2ϩ ] i .

Models of astrocytic Ca2+ dynamics and epilepsy

“…Additionally, elevated Ca 2+ propagated in waveforms among many astrocytes suggesting a long-distance communication between these cells. Similar responses have been subsequently observed in cultured organotypic [12] and acute slices from the rat hippocampus [13] and visual cortex [14]; astrocytes responded to the neuronal communication where glutamate was released into and escaped (spilled-over) from the synaptic cleft.…”

Activity-Dependent Structural Plasticity of Perisynaptic Astrocytic Domains Promotes Excitatory Synapse Stability

“…Increasing evidence suggests the existence of bidirectional interactions between neurons and astrocytes at excitatory synapses, which has been coined the ''tripartite synapse'' [4]. Astrocytes display activity-mediated Ca 2+ responses in vitro [5,6] and in vivo [7,8]. Ca 2+ signals can trigger the release of gliotransmitters (glutamate, D-serine, ATP), which in turn regulate synaptic transmission [2].…”

Differences in Stroke Between White, Hispanic, and Native American Patients

“…Signals may be transmitted to astrocytes from neurons by direct electrical coupling 5,6 or by binding of released neurotransmitters, such as glutamate, to receptors on astrocytes. 7 Either mechanism, as will be discussed, can lead to increases in the release of vasoactive metabolites of AA from astrocytes. Astrocytes express functional metabotropic and ionotropic glutamate receptors that mediate a number of cellular signaling events including elevation of intracellular Ca 2ϩ , 8 -11 activation of phospholipase C, 9 activation of cell-cell signaling, 12,13 and release of AA into the cytosol.…”