The natural product argentatin C attenuates postoperative pain via inhibition of voltage-gated sodium and T-type voltage-gated calcium channels.

BACKGROUND AND PURPOSE: Postoperative pain occurs in as many as 70% of surgeries performed worldwide. Postoperative pain management still relies on opioids despite their negative consequences, resulting in a public health crisis. Therefore, it is important to develop alternative therapies to treat chronic pain. Natural products derived from medicinal plants are potential sources of novel biologically active compounds for development of safe analgesics. In this study, we screened a library of natural products to identify small molecules that target the activity of voltage-gated sodium and calcium channels that have important roles in nociceptive sensory processing. EXPERIMENTAL APPROACH: Fractions derived from the Native American medicinal plant, Parthenium incanum, were assessed using depolarization-evoked calcium influx in rat dorsal root ganglion (DRG) neurons. Further separation of these fractions yielded a cycloartane-type triterpene identified as argentatin C, which was additionally evaluated using whole-cell voltage and current-clamp electrophysiology, and behavioural analysis in a mouse model of postsurgical pain. KEY RESULTS: Argentatin C blocked the activity of both voltage-gated sodium and low-voltage-activated (LVA) calcium channels in calcium imaging assays. Docking analysis predicted that argentatin C may bind to NaV 1.7-1.9 and CaV 3.1-3.3 channels. Furthermore, argentatin C decreased Na+ and T-type Ca2+ currents as well as excitability in rat and macaque DRG neurons, and reversed mechanical allodynia in a mouse model of postsurgical pain. CONCLUSION AND IMPLICATIONS: These results suggest that the dual effect of argentatin C on voltage-gated sodium and calcium channels supports its potential as a novel treatment for painful conditions.

Stereospecific Effects of Benzimidazolonepiperidine Compounds on T-Type Ca2+ Channels and Pain.

T-type calcium channels activate in response to subthreshold membrane depolarizations and represent an important source of Ca2+ influx near the resting membrane potential. These channels regulate neuronal excitability and have been linked to pain. For this reason, T-type calcium channels are suitable molecular targets for the development of new non-opioid analgesics. Our previous work identified an analogue of benzimidazolonepiperidine, 5bk, that preferentially inhibited CaV3.2 channels and reversed mechanical allodynia. In this study, we synthesized and screened a small library of 47 compounds derived from 5bk. We found several compounds that inhibited the Ca2+ influx in DRG neurons of all sizes. After separating the enantiomers of each active compound, we found two compounds, 3-25-R and 3-14-3-S, that potently inhibited the Ca2+ influx. Whole-cell patch clamp recordings from small- to medium-sized DRG neurons revealed that both compounds decreased total Ca2+. Application of 3-14-3-S (but not 3-25-R) blocked transiently expressed CaV3.1-3.3 channels with a similar IC50 value. 3-14-3-S decreased T-type, but not N-type, Ca2+ currents in DRG neurons. Furthermore, intrathecal delivery of 3-14-3-S relieved tonic, neuropathic, and inflammatory pain in preclinical models. 3-14-3-S did not exhibit any activity against G protein-coupled opioid receptors. Preliminary docking studies also suggest that 3-14-3-S can bind to the central pore domain of T-type channels. Together, our chemical characterization and functional and behavioral data identify a novel T-type calcium channel blocker with in vivo efficacy in experimental models of tonic, neuropathic, and inflammatory pain.