Isoliquiritigenin, an active ingredient of Glycyrrhiza, elicits antinociceptive effects via inhibition of Na channels.

Glycyrrhiza extract has been used for the treatment of oral and gastric ulcers, but the analgesic mechanism remains unknown. In the present study, we investigated the effects of isoliquiritigenin, an active ingredient of Glycyrrhiza, on Na channels in vitro and nociceptive behaviors in vivo. In an autopatch-clamp study, isoliquiritigenin inhibited the currents of Na1.1, Na1.3, Na1.6, Na1.7, and Na1.8 in a channel expression system. In small- and medium-sized cultured trigeminal ganglion neurons, the compound suppressed Na currents in many neurons (78%) and K currents in all neurons, dose-dependently. In current-clamp mode, isoliquiritigenin blocked action potential generation in many neurons (64%), but it conversely accelerated action potential generation in the remaining neurons. The opposing effects on action potentials were reproduced in a computational simulation of a modified Hodgkin-Huxley-based model, based on the electrophysiological data. In behavioral experiments, local treatment with isoliquiritigenin suppressed nociceptive behaviors in response to oral ulcer development or nociceptive TRP channel agonists in the oral mucosa and hind paw. These results suggest that isoliquiritigenin exerts an analgesic effect predominantly via inhibitory action on Na channels on sensory nociceptive fibers. This pharmacological mechanism indicates that isoliquiritigenin is useful for pain relief and provides scientific evidence for Glycyrrhiza at the ingredient level.