HIPPOCAMPAL ASTROCYTES IN-SITU RESPOND TO GLUTAMATE RELEASED FROM SYNAPTIC TERMINALS

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 ca lcium-sensitive dye Calcium Green-1 and the responses of the astrocyte s to electrical stimulation of the Schaffer collaterals were monitored by confocal microscopy. To confirm that the responsive cells were ast rocytes, the slices were immunostained for the astrocytic marker glial fibrillary acidic protein. Stimulation of the Schaffer collaterals (5 0 Hz, 2 sec) resulted in increases in the concentration of intracellul ar calcium ([Ca2+](i)) in the astrocytes located in the stratum radiat um of CA1. The astrocytic responses were blocked by the sodium channel blocker tetrodotoxin, the voltage-dependent calcium channel blocker o mega-conotoxin-MVIIC, and the selective metabotropic glutamate recepto r antagonist alpha-methyl-4-carboxyphenylglycine (MCPG). These results suggest that the astrocytic responses were induced by stimulation of metabotropic glutamate receptors on the astrocytes by neuronally relea sed glutamate. The astrocytic responses to neuronal stimulation were e nhanced in the presence of the K+ 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 recept or antagonist kynurenic acid. These results suggest that higher levels of neuronal activity result in stimulation of both metabotropic and i onotropic glutamate receptors on the astrocytes. Overall, the results indicate that hippocampal astrocytes in situ are able to respond to th e neuronal release of the neurotransmitter glutamate with increases in [Ca2+](i).