Ibuprofen modulates tetrodotoxin-resistant persistent Na⁺ currents at acidic pH in rat trigeminal ganglion neurons

Non-steroidal anti-inflammatory drugs (NSAIDs) are widely used to relieve various symptoms such as headache, arthralgia, and dental pain. While the primary mechanism of NSAID-based pain relief is the inhibition of cyclooxygenase-2, several NSAIDs also modulate other molecular targets related to nociceptive transmission such as voltage-gated Na⁺ channels. In the present study, we examined the effects of NSAIDs on persistent Na⁺ current (I_NaP) mediated by tetrodotoxin-resistant (TTX-R) Na⁺ channels in small-to medium-sized trigeminal ganglion neurons using a whole-cell patch-clamp technique. At clinically relevant concentrations, all propionic acid derivatives tested (ibuprofen, naproxen, fenoprofen, and flurbiprofen) preferentially inhibited the TTX-R I_NaP. The inhibition was more potent at acidic extracellular pH (pH 6.5) than at normal pH (pH 7.4). Other NSAIDs, such as ketorolac, piroxicam, and aspirin, had a negligible effect on the TTX-R I_NaP. Ibuprofen both accelerated the onset of inactivation and retarded the recovery from inactivation of TTX-R Na⁺ channels at acidic extracellular pH. However, all NSAIDs tested in this study had minor effects on voltage-gated K⁺ currents, as well as hyperpolarization-activated and cyclic nucleotide-gated cation currents, at both acidic and normal extracellular pH. Under current-clamp conditions, ibuprofen decreased the number of action potentials elicited by depolarizing current stimuli at acidic (pH 6.5) extracellular pH. Considering that extracellular pH falls as low as 5.5 in inflamed tissues, TTX-R I_NaP inhibition could be a mechanism by which ibuprofen and propionic acid derivative NSAIDs modulate inflammatory pain.