Preferential block of inactivation-deficient Na⁺ currents by capsaicin reveals a non-TRPV1 receptor within the Na⁺ channel

Abstract 

Capsaicin elicits burning pain via the activation of the vanilloid receptor (TRPV1). Intriguingly, several reports showed that capsaicin also inhibits Na⁺ currents but the mechanisms remain unclear. To explore this non-TRPV1 action we applied capsaicin to HEK293 cells stably expressing inactivation-deficient rat skeletal muscle Na⁺ mutant channels (rNav1.4-WCW). Capsaicin elicited a conspicuous time-dependent block of inactivation-deficient Na⁺ currents. The 50% inhibitory concentration (IC₅₀) of capsaicin for open Na⁺ channels at +30mV was measured 6.8±0.6μM (n=5), a value that is 10–30 times lower than those for resting (218μM) and inactivated (74μM) wild-type Na⁺ channels. On-rate and off-rate constants for capsaicin open-channel block at +30mV were estimated to be 6.37μM⁻¹s⁻¹ and 34.4s⁻¹, respectively, with a calculated dissociation constant (K_D) of 5.4μM. Capsaicin at 30μM produced ∼70% additional use-dependent block of remaining rNav1.4-WCW Na⁺ currents during repetitive pulses at 1Hz. Site-directed mutagenesis showed that the local anesthetic receptor was not responsible for the capsaicin block of the inactivation-deficient Na⁺ channel. Interestingly, capsaicin elicited little time-dependent block of batrachotoxin-modified rNav1.4-WCW Na⁺ currents, indicating that batrachotoxin prevents capsaicin binding. Finally, neuronal open Na⁺ channels endogenously expressed in GH₃ cells were as sensitive to capsaicin block as rNav1.4 counterparts. We conclude that capsaicin preferentially blocks persistent late Na⁺ currents, probably via a receptor that overlaps the batrachotoxin receptor but not the local anesthetic receptor. Drugs that target such a non-TRPV1 receptor could be beneficial for patients with neuropathic pain.

Keywords: Capsaicin, Voltage-gated sodium channel, Open-channel block, Analgesia, Use-dependent block