Tonic, phasic, and integrator components of psychophysical responses to topical capsaicin account for differences of location and sex.

We reanalyze data of Frot et al on sex and location differences in the pain response to topical capsaicin using a dynamic model developed from responses to oral capsaicin. This model considers the pain response as the sum of 3 underlying component processes: a phasic component, a tonic component, and an integrator component. Sex differences in response to stimulation of both the cheek and ankle could be accounted for by a greater gain in the tonic mechanism. Responses to ankle stimulation showed a greater integrator component than responses to cheek stimulation, a negligible phasic component, and required a time delay. This analysis demonstrates that the model is applicable to responses to stimuli outside the oral cavity and that it can explain differences due to location and sex, in addition to explaining sensitization, desensitization, and individual differences in earlier studies. Application of this model in future genetic studies, for instance, would be more appropriate than the use of the peak response or the response at an arbitrarily determined time.

CONCLUSIONS

This dynamic model provides insight into individual differences in sensitivity to vallinoid receptor-activating compounds including capsaicin, and it may be useful for the identification of subgroups of patients with differential responsiveness to therapeutic topical capsaicin. A similarly derived model might prove useful for the analysis of development of chronic pain.