EFFECT OF PROTONS ON THE INWARD CURRENT EVOKED BY CAPSAICIN IN ISOLATED DORSAL-ROOT GANGLION-CELLS

Capsaicin excites a subset of dorsal root ganglion (DRG) neurons by in ducing an inward current. We have examined this inward current in resp onse to high concentrations of protons in the external medium. The exp eriments were done in freshly dissociated DRG cells under whole-cell v oltage-clamp conditions. The amplitude of the current evoked by capsai cin (200-300 nM) was increased with increasing concentrations of proto ns (pH 6.9-6.3). Increasing the acidification of the external medium f rom a physiological pH of 7.3 to 6.3 enhanced 7-fold the current induc ed by capsaicin (300 nM). Cells unresponsive to capsaicin in a physiol ogical pH were also unresponsive to capsaicin in an acidic solution. T here was a progressive decrease in the amplitude of the capsaicin-evok ed current when the drug was repeatedly applied either at a physiologi cal pH or an acidic pH of 6.3. Exposure of the cell to acidic solution s of capsaicin did not subsequently increase an evoked inward current when capsaicin was applied at a physiological pH nor did it lead to an inward current evoked by proton ions where there had been none. Our r esults suggest that increased proton concentrations in the extracellul ar medium could play an important modulatory role in the function of c hemosensitive sensory neurons.