The weak bases NH3 and trimethylamine inhibit the medium and slow afterhyperpolarizations in rat CA1 pyramidal neurons

The weak bases NH₃ and trimethylamine (TMeA), applied externally, are widely used to investigate the effects of increasing intracellular pH (pH_i) on neuronal function. However, potential effects of the compounds independent from increases in pH_i are not usually considered. In whole-cell patch-clamp recordings from rat CA1 pyramidal neurons, bath application of 1–40 mM NH₄Cl or TMeA HCl reduced resting membrane potential and input resistance, inhibited the medium and slow afterhyperpolarizations (AHPs) and their respective underlying currents, mI_ahp and sI_ahp, and led to the development of depolarizing current-evoked burst firing. Examined in the presence of 1 M TTX and 5 mM TEA with 10 mM Hepes in the recording pipette, NH₃ and TMeA increased pH_i and the magnitudes of depolarization-evoked intracellular [Ca²⁺] transients, Ca²⁺-dependent depolarizing potentials, and inward Ca²⁺ currents but reduced the slow AHP and sI_ahp. When internal H⁺ buffering power was raised by including 100 mM tricine in the patch pipette, the effects of NH₃ and TMeA to increase pH_i and augment Ca²⁺ influx were attenuated whereas the reductions in the slow AHP and sI_ahp (as well as membrane potential and input resistance) were maintained. The findings indicate that increases in pH_i contribute to the increases in Ca²⁺ influx observed in the presence of NH₃ and TMeA but not to the reductions in membrane potential, input resistance or the magnitudes of AHPs. The results have implications for the interpretation of data from experiments in which pH_i is manipulated by the external application of NH₃ or TMeA.