The effects of pH on the interaction between capsaicin and the vanilloid receptor in rat dorsal root ganglia neurons

1 The vanilloid receptor of sensory neurons is a polymodal nociceptor sensi tive to capsaicin, protons, heat and anandamide. Although it is known that interaction occurs between these different mediators the mechanism by which this occurs is poorly understood. In this study capsaicin elicited current s were recorded from vanilloid receptors found in adult rat isolated dorsal root ganglia (DRG) neurons under conditions of varying pH and the mechanis m whereby protons can modulate this capsaicin response investigated. 2 Under whole-cell voltage clamp. modulating extracellular pH shifted the p osition of the capsaicin log(concentration)-response curve. Acidification f rom pH 9.0 to pH 5.5 lowered the EC50 values from 1150+/-250 nM to 5+/-2 nM with coincident change in the mean apparent slope factor from 2.3+/-0.3 to 0.9+/-0.2 and no change in maximal response. 3 The magnitude of the potentiation seen on reducing extracellular pH was n ot significantly affected by changes in extracellular calcium and magnesium concentration. 4 The response to capsaicin was not potentiated by a reduction in intracell ular pH suggesting a site of action more accessible from the extracellular than the intracellular side of the membrane. 5 Potentiation by low pH was voltage independent indicating a site of actio n outside the membrane electric field. 6 At the single channel level, reducing extracellular pH increased channel open probability but had no significant effect on single channel conductanc e or open time. 7 These results are consistent with a model in which, on reducing extracell ular pH, the vanilloid receptor in rat DRG neurons, changes from a state wi th low affinity for capsaicin to one with high affinity, coincident with a loss of cooperativity. This effect, presumed to be proton mediated, appears to involve one or more sites with pK(a) value 7.4-7.9, outside the membran e electrical field on an extracellularly exposed region of the receptor pro tein.