The activation mechanism of rat vanilloid receptor 1 by capsaicin involves the pore domain and differs from the activation by either acid or heat

Abstract

The recently cloned rat vanilloid receptor, VR1, can be activated by capsaicin, acid, and heat. To determine the molecular mechanisms facilitating channel opening in response to these stimuli, VR1 and six channels containing charge neutralization point mutations surrounding the putative channel pore domain were expressed and characterized in Xenopus laevis oocytes. Steady-state dose-response relationships, current-voltage relationships, ionic selectivities, and single-channel properties were recorded using voltage-clamp techniques. Three of the mutant channels are significantly more sensitive to capsaicin than is wild-type VR1, whereas none differed in their activation by acidic pH or temperature. Furthermore, one of the mutants has lost all positive cooperativity for capsaicin activation (Hill coefficient ≅ 1, VR1 ≅ 2), is much more selective for Ca 2+ , and exhibits a lower efficacy for acid than for capsaicin activation. Single-channel recordings show that capsaicin- and acid-activated channels have the same conductance, that the three mutants with increased capsaicin sensitivity exhibit higher open probabilities at submaximal capsaicin concentrations, and that the gating properties of capsaicin activation differ from those of acid activation. These data indicate that VR1 undergoes conformational changes upon capsaicin binding that it does not undergo in response to activation by protons or thermal stimuli. Furthermore, these structural rearrangements include the putative pore domain and reveal the location of an intracellular domain that contributes to the positive cooperativity seen for capsaicin activation.