Figure 4.

Potentiation of AMPA-evoked [Na⁺]_i transients in cultured cerebellar granule neurons exposed to increasing doses of either cyclothiazide or IDRA-21. [Na⁺]_i is measured by SFBI-fluorescence as described in the Materials and Methods section. The SFBI-loaded cells were incubated for 3 min in the presence of 1 μM nitrendipine, 10 μM dizocilpine, and 1 μM tetrodotoxin prior to the application of AMPA (10 μM). The level of [Na⁺]_i in the cells before the application of AMPA is 15 ± 1.2 mM. This basal value is subtracted from the values obtained after application of AMPA (▪) or AMPA plus modulators (○ = IDRA-21; ▴ = cyclothiazide). The dose-response relationship for IDRA-21 and cyclothiazide was constructed on the same culture dish by applying for 2 min AMPA to cells preincubated (1–2 min) with increasing concentrations of the modulators. After reaching the maximal fluorescence intensity, the cells were washed sufficient time (≈2 min) for the complete return of the fluorescence intensity ratio to baseline values. At the end of each experimental session, the cells were washed with Locke’s solution, and after 10, min the [Na⁺]_i calibration was performed as described in Materials and Methods. Each value is the mean ± SEM of the last six experiments. ∗, P < 0.01 when values for AMPA/cyclothiazide and AMPA/IDRA-21 were compared with values of AMPA alone. The slopes of the two regression curves are significantly different (P < 0.01).