Mechanisms Of Astrocytic Glutamate Release

IntroductionMost synapses in CNS are covered by astrocytes, which respond to neuronal activity by increases in cytosolic Ca²⁺ . In return, astrocytes can modulate the strength of synaptic transmission by release of excitory neurotransmitter glutamate. Astrocytic glutamate release is a receptor-mediated Ca²⁺-dependent process, thought to share several features with regulated excytosis. However, the mechanism of calcium-dependent astrocytic glutamate release is largely unknown. Astrocytes possess volume sensitive channels, which permeate cytosolic glutamate upon activation by cell swelling. Previously, the involve-ment of volume sensitive channels in calcium-dependent release has been over-looked because volume sensitive channels are not regulated by intracellular calcium. In this study we have investigated the possibility of glutamate release through volume sensitive channels upon receptor stimulation.Materials and methodsWe used confocal vertical sectioning imaging, Coutler counter, and fluorescence dilution techniques to demonstrate astrocytic swelling during Ca²⁺ signaling events. 2-photon laser scanning revealed receptor-mediated swelling of fine processes of astrocytes in vivo. Examinations on glutamate and other osmolytes efflux by fluorescent enzymatic assay, HPLC, and whole cell patch clamping all showed characteristics of volume sensitive channel activation.ResultsWe showed: 1) Astrocytes respond to receptor stimulation by volume chan-...