Nonivamide possessed similar anti-inflammatory potential as capsaicin and t-pellitorine. In U-937 macrophages, the tested compounds exploited an anti-inflammatory effect by inhibiting the EC-LPS induced activation of the MAPK pathway. In addition, the TRP channel activation plays a role in the anti-inflammatory capacity of capsaicin and nonivamide.

For a long time, the focus of trigeminal chemoperception has rested almost exclusively on TRP channels. However, two-pore domain (K2P) potassium channels have recently been identified as targets for substances associated with typical trigeminal sensations, such as numbing and tingling. In addition, they have been shown to be modulated by several TRP agonists. We investigated whether the pungent substances piperine, capsaicin, 6-gingerol and polygodial have an effect on human K2P channels. For this purpose, we evaluated the effects of these pungent substances on both wild-type and mutant K2P channels by means of two-electrode voltage-clamp experiments using Xenopus laevis oocytes. All four pungent substances were found to inhibit the basal activity of TASK-1 (K2P 3.1), TASK-3 (K2P 9.1), and TRESK (K2P 18.1) channels. This inhibitory effect was dose-dependent and, with the exception of polygodial on TASK-1, fully reversible. However, only piperine exhibited an IC50 similar to its reported EC50 on TRP channels. Finally, we observed for TASK-3 that mutating H98 to E markedly decreased the inhibition induced by piperine, capsaicin, and 6-gingerol, but not by polygodial. Our data contribute to the relatively sparse knowledge concerning the pharmacology of K2P channels and also raise the question of whether K2P channels could be involved in the pungency perception of piperine.

Primary sensory afferents of the dorsal root and trigeminal ganglia constantly transmit sensory information depicting the individual’s physical and chemical environment to higher brain regions. Beyond the typical trigeminal stimuli (e.g. irritants), environmental stimuli comprise a plethora of volatile chemicals with olfactory components (odorants). In spite of a complete loss of their sense of smell, anosmic patients may retain the ability to roughly discriminate between different volatile compounds. While the detailed mechanisms remain elusive, sensory structures belonging to the trigeminal system seem to be responsible for this phenomenon. In order to gain a better understanding of the mechanisms underlying the activation of the trigeminal system by volatile chemicals, we investigated odorant-induced membrane potential changes in cultured rat trigeminal neurons induced by the odorants vanillin, heliotropyl acetone, helional, and geraniol. We observed the dose-dependent depolarization of trigeminal neurons upon application of these substances occurring in a stimulus-specific manner and could show that distinct neuronal populations respond to different odorants. Using specific antagonists, we found evidence that TRPA1, TRPM8, and/or TRPV1 contribute to the activation. In order to further test this hypothesis, we used recombinantly expressed rat and human variants of these channels to investigate whether they are indeed activated by the odorants tested. We additionally found that the odorants dose-dependently inhibit two-pore potassium channels TASK1 and TASK3 heterologously expressed In Xenopus laevis oocytes. We suggest that the capability of various odorants to activate different TRP channels and to inhibit potassium channels causes neuronal depolarization and activation of distinct subpopulations of trigeminal sensory neurons, forming the basis for a specific representation of volatile chemicals in the trigeminal ganglia.

Two-pore domain K+ (KCNK, K2P) channels underlie the “leak” (background) potassium conductance in many types of excitable cells. They oppose membrane depolarization and cell excitability. These channels have been reported to be modulated by several physical and chemical stimuli. The compound 2-aminoethoxydiphenyl borate (2-APB) was originally described as an inhibitor of IP3-induced Ca2+ release but has been shown to act as either a blocker or an activator for several ion channels. Here, we report the effects of this compound on members of the TREK (TWIK related K+ channels) subfamily of human KCNK channels. We injected Xenopus laevis oocytes with cRNAs (complementary RNAs) encoding several KCNK channels and measured their response using the two-electrode voltage clamp technique. 2-APB was found to be an effective activator for all members of the TREK subfamily (hKCNK2, hKCNK4, and hKCNK10), with the highest efficacy in hKCNK10. We also found that 2-APB was able to activate these channels in cell-excised patches of HEK293 (human embryonic kidney 293) cell transfected with hKCNK4 or hKCNK10, demonstrating direct activation. TREK channels are widely expressed in the central nervous system and peripheral tissues, where they play roles in several key processes. However, little is known regarding their pharmacology; therefore, the identification of a common, stable and inexpensive agonist should aid further investigations of these channels. Additionally, 2-APB has been used to study native receptors in cell systems that endogenously express members of the TREK subfamily (e.g., rat dorsal root ganglia); our results thus warn against the use of 2-APB at high concentrations in these systems.

Extracts from Glycyrrhiza are traditionally used for the treatment of insomnia and anxiety. Glabridin is one of the main flavonoid compounds from Glycyrrhiza glabra and displays a broad range of biological properties. In the present work, we investigated the effect of glabridin on GABAA receptors. For this purpose, we employed the two-electrode voltage-clamp technique on Xenopus laevis oocytes expressing recombinant GABAA receptors. Through this approach, we observed that glabridin presents a strong potentiating effect on GABAA α1β(1−3)γ2 receptors. The potentiation was slightly dependent on the β subunit and was most pronounced at the α1β2γ2 subunit combination, which forms the most abundant GABAA receptor in the CNS. Glabridin potentiated with an EC50 of 6.3±1.7 µM and decreased the EC50 of the receptor for GABA by approximately 12-fold. The potentiating effect of glabridin is flumazenil-insensitive and does not require the benzodiazepine binding site. Glabridin acts on the β subunit of GABAA receptors by a mechanism involving the M286 residue, which is a key amino acid at the binding site for general anesthetics, such as propofol and etomidate. Our results demonstrate that GABAA receptors are strongly potentiated by one of the main flavonoid compounds from Glycyrrhiza glabra and suggest that glabridin could contribute to the reported hypnotic effect of Glycyrrhiza extracts.