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pH modulation of Ca2+-dependent afterhyperpolarizations in rat hippocampal neurons Kelly, Tony

Abstract

In the present study, I examined the effects of changes in extracellular pH (pH₀) on the fast, medium and slow afterhyperpolarizations (AHPs) in rat hippocampal neurons and assessed the contributions of changes in Ca²⁺ influx and intracellular pH (pH[sub i]) to the effects observed. Initially, sharp microelectrode recordings were employed to confirm previous findings that decreases and increases in pH₀ inhibit and augment, respectively, the fast, medium and slow AHPs in CA1 pyramidal neurons in hippocampal slices. Whole-cell patch-clamp recordings were then used to extend these findings and provide the first descriptions of the effects of changes in pH₀ and pH[sub i] on the currents underlying the medium AHP (m/AHP) and slow AHP •(s/[sub AHP]). The results of these experiments indicated not only that changes in pH[sub i] could modulate the AHPs independent of changes in Ca²⁺ influx but also that the relative contributions of changes in Ca²⁺ influx and pH[sub i] to the modulation of the medium and slow AHPs by changes in pH₀ were dependent on the direction of the pH₀ change. Whereas low pH₀ -induced reductions in 9-4- the AHPs and their underlying currents were dependent on decreases in Ca influx rather than decreases in pH[sub i], the augmented AHPs observed at high pH₀ in large part reflected increases in pH[sub i] rather than increases in Ca influx. The latter observations were confirmed through the use of a novel technique for the simultaneous measurement of pH[sub i], intracellular free calcium concentration ([Ca²⁺][sub i]) and membrane potential in patch-clamped cultured hippocampal neurons. The final series of experiments examined the possibility that the high pH₀-induced augmentation of the slow AHP might act to limit the increase in neuronal excitability typically observed during increases in pH₀. The sensitivity of AHPs to changes in pH₀ and pH[sub i] has a number of implications for neuronal function under physiological and pathophysiological conditions.

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