Temporal resolution of activity-dependent pH shifts in rat hippocampal slices

1. The rise time of activity dependent extracellular pH shifts was measured in the CA1 stratum radiatum of rat hippocampal slices by recording pH sensitive fluorescence of a fluorescein-conjugated dextran. Optical data were compared with simultaneous pH microelectrode recordings. 2. The pH shifts generated by CO₂ or by stimulation of the Schaffer collaterals were paralleled by shifts in fluorescence emissions at 535 nm when the probe was excited with 490-nm light (ΔF⁴⁹⁰). Emissions at 535 nm induced by 440-nm light were unchanged in these paradigms. 2. A train of three stimuli at 100 Hz was repeated at 30-s intervals and the stimulus-triggered ΔF⁴⁹⁰ was averaged. The mean rise time of the ΔF⁴⁹⁰ was 69 ± 24 (SE) ms (range 20- 200 ms, n = 6). The mean increase in emission was 0.75 ± 0.22% of baseline, associated with a pH microelectrode response of +0.06 ± 0.02 unit pH. 4. These data demonstrate that synaptically evoked alkaline transients develop within tens of milliseconds. The occurrence of the alkalinization in the same time frame as excitatory postsynaptic currents indicates that these pH shifts arise with sufficient speed to modulate synaptic transmission.