The effect of neuronal morphology and membrane-permeant weak acid and base on the dissipation of depolarization-induced pH gradients in snail neurons

Neuronal depolarization causes larger intracellular pH (pH_i) shifts in axonal and dendritic regions than in the cell body. In this paper, we present evidence relating the time for collapse of these gradients to neuronal morphology. We have used ratiometric pH_i measurements using 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) in whole-cell patch-clamped snail neurons to study the collapse of longitudinal pH gradients. Using depolarization to open voltage-gated proton channels, we produced alkaline pH_i microdomains. In the absence of added mobile buffers, facilitated H⁺ diffusion down the length of the axon plays a critical role in determining pH_i microdomain lifetime, with axons of ∼100 μm allowing pH differences to be maintained for >60 s. An application of mobile, membrane-permeant pH buffers accelerated the collapse of the alkaline-pH gradients but, even at 30 mM, was unable to abolish them. Modeling of the pH_i dynamics showed that both the relatively weak effect of the weak acid/base on the peak size of the pH gradient and the accelerated collapse of the pH gradient could be due to the time taken for equilibration of the weak acid and base across the cell. We propose that appropriate weak acid/base mixes may provide a simple method for studying the role of local pH_i signals without perturbing steady-state pH_i. Furthermore, an extrapolation of our in vitro data to longer and thinner neuronal structures found in the mammalian nervous system suggests that dendritic and axonal pH _i are likely to be dominated by local pH_i-regulating mechanisms rather than simply following the soma pH_i.