Capsaicin and menthol reduce excitability of insular pyramidal neurons via TRP-independent sodium channel inactivation.

The insular cortex integrates interoceptive and exteroceptive signals to support sensory perception, affective processing, and motivated behaviors. Capsaicin and menthol, prototypical agonists of TRPV1 and TRPM8 thermosensitive transient receptor potential (TRP) channels, are widely used to probe peripheral somatosensory pathways. However, these amphiphilic compounds can also modulate neuronal excitability through mechanisms independent of canonical TRP signaling. This study investigated their effects on intrinsic excitability in layer 2/3 pyramidal neurons of the mouse insular cortex using whole-cell patch-clamp recordings in acute brain slices. Bath application of capsaicin or menthol strongly suppressed repetitive action potential firing evoked by depolarizing current injections and increased the current threshold required to elicit action potentials, with minimal effects on resting membrane potential, input resistance, or spike threshold. These suppressive effects persisted in the presence of selective TRPV1 and TRPM8 antagonists, indicating a TRP-independent mechanism. Voltage-clamp recordings showed no substantial change in the voltage dependence of Na⁺ channel activation. In contrast, both compounds induced a clear hyperpolarizing shift in steady-state inactivation of voltage-gated Na⁺ channels, thereby reducing channel availability at subthreshold potentials. Additionally, both compounds reduced burst firing frequency under hyperexcitable ionic conditions. These results demonstrate that capsaicin and menthol suppress cortical pyramidal neuron excitability primarily by stabilizing the inactivated state of voltage-gated Na⁺ channels, independent of canonical TRP signaling. The findings highlight direct actions on Na⁺ channels that should be considered when interpreting the effects of these compounds in central neurons beyond their established role as selective TRP agonists.