Figure 5.

Proton buffering limits the effect of feedback inhibition. A, Time course fluorescence intensity traces demonstrating the significant impact of increased pH buffering on the effects of both by 50 μm kainate (left column) and 50 μm CNQX (right column; drug applications indicated by the black bars below each trace; elevated [K⁺] depolarizing stimuli as per Fig. 2). Bathing medium containing both 32.5 mm bicarbonate () and 4 mm HEPES reduced the effects elicited by kainate and CNQX. Medium containing 20 mm HEPES also attenuated the effects of kainate. HEPES at 20 mm completely abolished the effects of CNQX. An additional increase to 30 mm HEPES caused an even greater attenuation of the kainate-induced reduction of the Ca²⁺ signal. Calibration: 90 s, 0.1 ΔF/F. B, Histogram showing the mean ± SEM effects of pH buffering on the response ratios obtained with kainate and CNQX. The decrease of the response ratio attributable to kainate was significantly attenuated by the addition of 32.5 mm bicarbonate or an increase in HEPES from 4 to 20 mm. An additional increase in HEPES to 30 mm led to an even greater attenuation of the kainate-induced decrease in response ratio. Bicarbonate significantly reduced the effect of CNQX, whereas 20 mm HEPES completely abolished it. Open bars show control data from Figure 4 (4 mm HEPES).