Effects of pH changes on calcium-mediated potentials in rat hippocampal neurons in vitro

Abstract

The effects of changes in extra- and intracellular pH (pH o and pH i, respectively) on potentials mediated by the influx of Ca 2+ ions were investigated in intracellular “current-clamp” recordings from CA1 pyramidal neurons in rat hippocampal slices. In neurons which exhibited a “regular-spiking” discharge in response to depolarizing current injection at pH 7.3, perfusion with pH 7.7 medium led to the development of burst firing. Conversely, neurons which were “burst-firing” at pH 7.3 became regular spiking upon exposure to pH 6.9 medium. In addition, the rebound depolarization following a current-evoked hyperpolarization to >−60 mV, which in part reflects activation of a low-voltage-activated Ca 2+ conductance, was reduced at pH o 6.9 and enhanced at pH o 7.7. Neither the burst firing pattern of discharge nor the augmented rebound depolarization observed during perfusion with pH 7.7 medium was due to the reduction in [Cl −] o consequent upon the increase in [HCO − 3] o at a constant P CO2. The magnitudes of the fast afterhyperpolarization which follows a single depolarizing current-evoked action potential and the slow afterhyperpolarization which follows a train of action potentials were attenuated and enhanced, respectively, during perfusion with pH 6.9 and pH 7.7 media, compared with responses obtained at pH 7.3. Reducing pH i at a constant pH o (by exposure to pH 7.3 HCO 3 −/CO 2-free medium buffered with 30 mM HEPES) also attenuated fast and slow afterhyperpolarizations. In tetrodotoxin- and tetraethylammonium-poisoned slices, perfusion with pH 6.9 and pH 7.7 media reduced and increased, respectively, the magnitude of current-evoked Ca 2+-dependent depolarizing potentials and their associated slow afterhyperpolarizations, compared to responses obtained at pH 7.3. In contrast, reducing pH i at a constant pH o elicited only a small reduction in the magnitude of Ca 2+ spikes but markedly attenuated the subsequent slow afterhyperpolarization. The results suggest that, in rat CA1 hippocampal pyramidal neurons, Ca 2+-dependent depolarizing potentials mediated by the influx of Ca 2+ ions through voltage-activated Ca 2+ channels are sensitive to changes in pH o. These effects of changes in pH o are not dependent upon changes in pH i consequent upon the changes in pH o. Changes in pH o also affect the magnitudes of fast and slow afterhyperpolarizations mediated by Ca 2+-dependent K + conductances. In these cases, however, the effects of changes in pH o are mimicked by changes in pH i at a constant pH o, suggesting in turn that the effects of changes in pH o on fast and slow afterhyperpolarizations may be mediated both by changes in Ca 2+ influx (reflecting mainly changes in pH o) and by direct effects of changes in pH i (consequent upon changes in pH o) on Ca 2+-dependent K + conductances.