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

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Abstract
The vanilloid receptor of sensory neurons is a polymodal nociceptor sensitive 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 currents were recorded from vanilloid receptors found in adult rat isolated dorsal root ganglia (DRG) neurons under conditions of varying pH and the mechanism whereby protons can modulate this capsaicin response investigated. Under whole‐cell voltage clamp, modulating extracellular pH shifted the position of the capsaicin log(concentration)‐response curve. Acidification from 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. The magnitude of the potentiation seen on reducing extracellular pH was not significantly affected by changes in extracellular calcium and magnesium concentration. The response to capsaicin was not potentiated by a reduction in intracellular pH suggesting a site of action more accessible from the extracellular than the intracellular side of the membrane. Potentiation by low pH was voltage independent indicating a site of action outside the membrane electric field. At the single channel level, reducing extracellular pH increased channel open probability but had no significant effect on single channel conductance or open time. These results are consistent with a model in which, on reducing extracellular pH, the vanilloid receptor in rat DRG neurons, changes from a state with 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 pKa value 7.4 – 7.9, outside the membrane electrical field on an extracellularly exposed region of the receptor protein. British Journal of Pharmacology (2001) 132, 899–908; doi:10.1038/sj.bjp.0703900