Nav1.8, TRPV1 and TRPA1 as possible targets of ambroxol when used for topical treatment of neuropathic pain

Sodium channels and the capsaicin receptor TRPV1 in epidermal sensory nerve endings are established targets for topical treatment of neuropathic pain. The secretolytic ambroxol induces effective topically analgesia, a property that may involve a preferential block of Nav1.8. We here examined to what extent ambroxol inhibits human and rat (r) Nav1.8, but also the human (h) orthologues of the irritant receptors TRPV1 and TRPA1 by performing whole-cell patch clamp recordings. Ambroxol-induced tonic inhibition was stronger on rNav1.8 (IC₅₀ 18 µM) than on hNav1.8 (IC₅₀ 279 µM) and TTX-sensitive Na⁺ channels (IC₅₀ 76 µM). Non-inactivating currents of both hNav1.8 and rNav1.8 displayed a higher sensitivity to ambroxol than transient currents. A weak but concentration-dependent activation of hTRPV1 and hTRPA1 exceeded an unspecific increase in intracellular calcium by high concentrations of ambroxol. While the activation mechanism for hTRPV1 involves the vanilloid-binding domain, residues required for menthol-sensitivity of hTRPA1 seems to dictate ambroxol-sensitivity. Ambroxol also inhibited capsaicin-induced currents on hTRPV1 in a concentration-dependent and partly reversible manner. This inhibition was independent of intracellular calcium and preserved on the non-desensitizing mutant hTRPV1-Y672K. Currents induced by mustard oil or carvacrol on hTRPA1 exhibited a concentration-dependent reduction by ambroxol that was stronger on outward as compared to inward currents. We conclude that inhibition of Nav1.8 by ambroxol exhibits a pronounced species-specificity with rNav1.8 > hNav1.8. Furthermore, high concentrations of ambroxol modulate hTRPV1 and hTRPA1 in a manner that might contribute to topical analgesia. PERSPECTIVE: Topically applied ambroxol is a well-known off the label approach for treatment of focal neuropathic pain, but it still lacks a mechanistic foundation. This in vitro study presents human Nav1.8, TRPA1 and TRPV1 as possible pharmacological targets of ambroxol in sensory neurons.