CYCLOTHIAZIDE MODULATES AMPA RECEPTOR-MEDIATED INCREASES IN INTRACELLULAR FREE CA2-BRAIN( AND MG2+ IN CULTURED NEURONS FROM RAT)

We investigated the modulation of lpha-amino-3-hydroxy-5-methylisoxazo le-4-propionic acid (AMPA)-induced increases in intracellular free Ca2 + ([Ca2+](i)) and intracellular free Mg2+ ([Mg2+](i)) by cyclothiazide and GYKI 52466 using microspectrofluorimetry in single cultured rat b rain neurons. AMPA-induced changes in [Ca2+](i) were increased by 0.3- 100 mu M cyclothiazide, with an EC(50) value of 2.40 mu M and a maximu m potentiation of 428% of control values. [Ca2+](i) responses to gluta mate in the presence of N-methyl-D-aspartate (NMDA) receptor antagonis ts were also potentiated by 10 mu M cyclothiazide. The response to NMD A was not affected, demonstrating specificity of cyclothiazide for non -NMDA receptors. Almost all neurons responded with an increase in [Ca2 +](i) to both kainate and AMPA in the absence of extracellular Na+, an d these Na+-free responses were also potentiated by cyclothiazide, GYK I 52466 inhibited responses to AMPA with an IC50 value of 12.0 mu M. T en micromolar cyclothiazide significantly decreased the potency of GYK I 52466. However, the magnitude of this decrease in potency was not co nsistent with a competitive interaction between the two ligands. Cyclo thiazide also potentiated AMPA- and glutamate-induced increases in [Mg 2+](i). These results are consistent with the ability of cyclothiazide to decrease desensitization of non-NMDA glutamate receptors and may p rovide the basis for the increase in non-NMDA receptor-mediated excito toxicity produced by cyclothiazide.