MODULATION OF AMPA/KAINATE RECEPTORS BY CYCLOTHIAZIDE INCREASES CYTOPLASMIC FREE CA2+ AND CA-45(2+) UPTAKE IN BRAIN NEURONS

alpha-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-induced Ca2+ responses, and their modulation by cyclothiazide, were investigated in two functional assays of Ca2+ channel activity. AMPA produced a marked increase in cytoplasmic free Ca2+ levels ([Ca2+](i)) in single cortical neurons, whereas no such effects of AMPA could be observed in intact cerebellar granule neurons. In monolayer cultures of cortical cells, cyclothiazide caused a pronounced enhancement of AMPA-induced stimulation of Ca-45(2+) uptake, whereas similar studies in cerebellar granule neurons revealed only a weak potentiation of AMPA-induced Ca-45(2+) uptake. Higher concentrations of cyclothiazide alone produced [Ca2+](i) oscillations as well as an increase of basal Ca-45(2+) uptake in cortical neurons, whereas no such effects were obtained in cerebellar granule neurons. Our data indicate that AMPA receptors located on cortical and cerebellar granule neurons, respectively, may differ in their permeability to Ca2+ and that this difference is markedly potentiated following the application of cyclothiazide.