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Received June 27, 2002
Accepted after revision July 26, 2002
1 Department of Physiology, University of Cambridge, Downing Street, Cambridge CB2 3EG, UK
* To whom correspondence should be addressed. E-mail: dw212{at}cam.ac.uk.
Our aim was to test the hypothesis that depolarization-induced intracellular pH (pHi) 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 (~0.14 pH units) and fast dendritic acid shifts, whilst the somatic acidifications were significantly smaller (~0.06 pH units) and slower. The pHi shifts were smaller in the presence of 5 % CO2-25 mM HCO3--buffered saline (~0.08 pH units in the dendrites and ~0.03 pH units in the soma), although a clear spatiotemporal heterogeneity remained. Acetazolamide (50 µM) 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 pHi changes by evoking modest bursts of action potentials (~10 s duration) in CO2-HCO3--buffered saline. Such neuronal firing induced an acidification of ~0.11 pH units in the fine dendritic regions, but only ~0.03 pH units in the soma. There was considerable variation in the size of the pHi 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 pHi changes (pH microdomains) might act as important signals in synaptic function.
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