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 pHi 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. ring. Examined in the presence of 1 mu M TTX and 5 mM TEA with 10 mM Hepes in the recording pipette, NH3 and TMeA increased pHi 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 pHi 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.