Background: A large number of patients suffer from neuropathic pain, and systemic therapy often remains ineffective while inducing severe side effects. Topical therapy with the TRPV1-agonist capsaicin is an established alternative, and the identification of co-therapeutics that modulate TRPV1 may be a promising approach to reduce the dose of capsaicin while maintaining efficacy. Here, we aimed to determine if the antidepressant amitriptyline displays properties rendering it a potential co-therapeutic agent. Methods: We performed patch clamp and calcium imaging experiments on HEK293T cells expressing human (h) TRPV1 as well as on dorsal root ganglion (DRG) neurons from adult mice. Results: Amitriptyline induced an increase in intracellular calcium in both HEK293T and mouse DRG neurons expressing TRPV1. Patch clamp experiments revealed a concentration-dependent activation of hTRPV1 by amitriptyline that was also evident in cell-free inside-out patches. When hTRPV1 was fully activated by capsaicin, amitriptyline induced concentration-dependent and partly reversible current inhibition. In contrast, amitriptyline potentiated small responses to capsaicin, heat and protons. We also found that amitriptyline desensitized hTRPV1 to capsaicin. This effect was reduced by the intracellular application of the strong calcium chelator BAPTA. Furthermore, the non-desensitizing mutant hTRPV1-Y672K displayed a reduced amitriptyline-induced desensitization. Conclusions: Our data showed that amitriptyline can activate, sensitize, desensitize and even inhibit TRPV1. Together with its property as a strong local anesthetic, our data suggest that amitriptyline may be a promising adjunct to topical capsaicin.

The activation of TRPV1 and TRPA1 by UVA light is a complex process involving channel modulation by reactive oxygen species (ROS). The present study describes staurosporine and midostaurin, two protein kinase inhibitors, as photosensitizers that can modulate the activity of TRPV1 and TRPA1 in a UVA light-dependent manner. Patch-clamp and calcium imaging were used to investigate effects of staurosporine and midostaurin on recombinant human (h) TRPV1 and TRPA1 in HEK 293T cells and on native mouse dorsal root ganglion (DRG) cells. Staurosporine applied alone did not induce channel activation, but co-application with UVA light activated both TRPV1 and TRPA1. Staurosporine with UVA light also potentiated TRPV1-mediated membrane currents induced by heat and protons. Midostaurin induced the UVA light-independent activation and sensitization of TRPV1 and TRPA1, and this effect was strongly potentiated by UVA light. Effects induced by both staurosporine and midostaurin were reversed by the reducing agent dithiothreitol (DTT). Midostaurin induced a calcium influx in TRPA1-expressing DRG neurons. Our results show that staurosporine and midostaurin modulate the activity of TRPV1 and TRPA1 channels in the presence of UVA light. These photosensitizing properties can be relevant when staurosporine is used for in vitro experiments, and they may account for the phototoxic side effects of midostaurin.