AMPA RECEPTOR ACTIVATION IS RAPIDLY TOXIC TO CORTICAL ASTROCYTES WHENDESENSITIZATION IS BLOCKED

Although cultured astrocytes express functional glutamate receptors, t hey are generally resistant to excitotoxic cell death. We explored the role of receptor desensitization in glutamate-mediated astrocyte inju ry. In cultures of type 1 astrocytes from mouse neocortex, brief appli cation of AMPA evoked small, rapidly desensitizing inward currents, wh ereas kainate evoked small, sustained currents. Neither agonist increa sed cytosolic calcium, and astrocyte toxicity occurred only after 24 h r exposure to high (500-1000 mu M) concentrations of kainate but not t o AMPA or glutamate. Cyclothiazide, a drug that selectively blocks AMP A receptor desensitization, greatly potentiated AMPA- or kainate-gated currents and intracellular calcium elevation. Coapplication of 10-100 mu M cyclothiazide with glutamate, AMPA, or kainate produced widespre ad astrocyte cell death within 2 hr of application. The enhancement of toxicity by cyclothiazide, which alone was not toxic, was concentrati on-dependent for each of the tested agonists (EC(50) 30-100 mu M) and was blocked by further addition of the selective AMPA/kainate antagoni st 2,3-dioxo-6-nitro-7-sulfamoylbenzo(f)quinoxaline (NBQX). NMDA cause d no injury even in the presence of cyclothiazide. Cyclothiazide-enhan ced injury varied with the age of astrocyte cultures; the maximal effe ct occurred at similar to 2 weeks in vitro, and little death was seen after 4 weeks. Type 1 astrocytes express AMPA-type glutamate receptors that are unmasked by reducing their desensitization with cyclothiazid e. Although overactivation of AMPA receptors can be rapidly lethal to astrocytes, rapid desensitization normally limits this toxicity. The e xtent of AMPA receptor desensitization may be an important determinant of glial vulnerability to excitotoxic insults.