Glutamate Release through Volume-Activated Channels during Spreading Depression

Abstract

Volume-sensitive organic anion channels (VSOACs) in astrocytes are activated by cell swelling and are permeable to organic anions, such as glutamate and taurine. We have examined the release of glutamate through VSOACs during the propagation of spreading depression (SD). SD was induced by bath application of ouabain in hippocampal brain slices and was monitored by imaging intrinsic optical signals, a technique that provides a measure of cellular swelling. The onset of SD was associated with increased light transmittance, confirming previous studies that cellular swelling occurs during SD. NMDA receptor antagonists, either noncompetitive (MK-801, 10-50 microM) or competitive (CGS-17355, 100 microM), reduced the rate of propagation of SD, indicating that glutamate release contributes to SD onset. SD still occurred in zero Ca(2+)-EGTA (0-Ca(2+)-EGTA) solution, a manipulation that depresses synaptic transmission. HPLC measurements indicated that, even in this solution, there was significant glutamate release. Two lines of experiments indicated that glutamate was released through VSOACs during SD. First, 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), a blocker of VSOACs, depressed the rate of propagation of SD in a manner similar to NMDA antagonists. Second, NPPB inhibited the release of glutamate during SD in 0-Ca(2+)-EGTA external solution. These results indicate that cellular swelling during SD causes the activation of VSOACs and the release of glutamate by permeation through this channel. Cellular swelling is a result of neuronal activity and is observed during excitotoxicity. Therefore, glutamate release from VSOAC activation could occur under conditions of cell swelling and contribute to excitotoxic damage.

Cell swelling and ion redistribution assessed with intrinsic optical signals

“…This method might even be too slow to measure the regulatory volume decrease which is to a large extend mediated through astrocytic volume-sensitive organic anion channels. These anion channels activate within a few seconds following exposure to an osmotic stress (Basarsky et al 1999) The present data favour the glial cells as the major cell type involved in regulation of extracellular volume under physiological conditions. Thus the widening of the extracellular space in superficial cortical layers depends on cell coupling as shown by pharmacological manipulations and are suggested to be due to potassium syphoning through the glial network.…”

RepeatedN-Acetylcysteine Administration Alters Plasticity-Dependent Effects of Cocaine

“…Aside from system x c Ϫ, numerous mechanisms may be capable of releasing glutamate into the extrasynaptic compartment including vesicular release from astrocytes (Bezzi et al, 2004;Zhang et al, 2004), nonvesicular release from astrocytic gap junction hemichannels (Ye et al, 2003), volume-sensitive organic anion channels (Strange et al, 1996;Basarsky et al, 1999), or hydrolysis of N-acetylaspartylglutamate (Blakely et al, 1988;Zhou et al, 2005). Thus, models of brain functioning in the normal and diseased states need to account for the existence of high, steady-state levels of glutamate in the extrasynaptic space.…”

Mechanisms of glutamate release from astrocytes

“…Another study showed that glutamate released from astrocytes contributed to spreading depression in hippocampal slices. This glutamate was released in Ca 2+ free medium but blocked by 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), a Cl − channel inhibitor, indicating that glutamate could have been released through VRACs (Basarsky et al, 1999). Liu et al (2006) discerned swelling induced glutamate release from two different anion channels, the volume-sensitive outwardly rectifying (VSOR) chloride channels and the maxi-anion channels.…”