Electrically evoked dendritic pH transients in rat cerebellar Purkinje cells

Our aim was to test the hypothesis that depolarization-induced intracellular pH (pH(i)) shifts in restricted regions (dendrites) of mammalian neurones might be larger and faster than those previously reported from the cell soma. We used confocal imaging of the pH-sensitive dye, HPTS, to measure pH changes in both the soma and dendrites of whole-cell patch-clamped rat cerebellar Purkinje cells. In the absence of added CO2-HCO3-, depolarization to +20 mV for 1 s caused large (similar to0.14 pH units) and fast dendritic acid shifts, whilst the somatic acidifications were significantly smaller (similar to0.06 pH units) and slower. The pHi shifts were smaller in the presence of 5 % CO2-25 mM HCO3--buffered saline (similar to0.08 pH units in the dendrites and similar to0.03 pH units in the soma), although a clear spatiotemporal heterogeneity remained. Acetazolamide (50 muM) doubled the size of the dendritic acid shifts in the presence of CO2-HCO3-, indicating carbonic anhydrase activity. Removal of extracellular calcium or addition of the calcium channel blocker lanthanum (0.5 mM) inhibited the depolarization-evoked acid shifts. We investigated more physiological pH(i) changes by evoking modest bursts of action potentials (similar to10 s duration) in CO2-HCO3--buffered saline. Such neuronal firing induced an acidification of similar to0.11 pH units in the fine dendritic regions, but only similar to0.03 pH units in the soma. There was considerable variation in the size of the pH(i) shifts between cells, with dendritic acid shifts as large as 0.2-0.3 pH units following a 10 s burst of action potentials in some Purkinje cells. We postulate that these large dendritic pH(i) changes (pH microdomains) might act as important signals in synaptic function.