Activity-dependent targeting of TRPV1 with a pore-permeating capsaicin analog

Abstract

The capsaicin receptor TRPV1 is the principal transduction channel for nociception. Excessive TRPV1 activation causes pathological pain. Ideal pain mangement requires selective inhibition of hyperactive pain-sensing neurons, but sparing normal nociception. We sought to determine whether it is possible to use activity-dependent TRPV1 agonists to identify nerves with excessive TRPV1 activity, as well as exploit the TRPV1 pore to deliver charged anesthetics for neuronal silencing. We synthesized a series of permanently charged capsaicinoids and found that one, cap-ET, efficaciously evoked TRPV1-dependent entry of Ca 2+ or the large cationic dye YO-PRO-1 comparably to capsaicin, but far smaller electrical currents. Cap-ETinduced YO-PRO-1 transport required permeation of both the agonist and the dye through the TRPV1 pore and could be enhanced by kinase activation or oxidative covalent modification. Moreover, cap-ET reduced capsaicin-induced currents by a voltage-dependent block of the pore. A low dose of cap-ET elicited entry of permanently charged Na + channel blockers to effectively suppress Na + currents in sensory neurons presensitized with oxidative chemicals. These results implicate therapeutic potential of these unique TRPV1 agonists exhibiting activity-dependent ion transport but of minimal pain-producing risks.activity-dependent capsaicinoids | hyperalgesia | ion permeation | selective analgesia C apsaicin, a small lipophilic molecule from hot chili peppers, acts on sensory neurons by opening the TRPV1 channel. TRPV1 is activated by noxious temperatures (>43°C) and serves as an integrator for major pain-producing signals (1, 2). Inflammatory hyperalgesia is dramatically reduced in mice lacking TRPV1 (3, 4). Besides being an attractive target for pain management, TRPV1 regulates autonomic function, such as body temperature, blood vessel tone, and release of transmitters from sensory nerves (5-9). TRPV1 activated by capsaicin at a concentration far below the pain-producing threshold triggers neuropeptide release (7) and production of other second messengers (10). In light of the complexity of TRPV1 actions in physiology, one major challenge in developing TRPV1-based pain treatment is to target therapeutic compounds to selectively inhibit hyperactive nociceptive neurons while sparing nerves of normal thresholds for pain detection.Capsaicin is among the most powerful chemical agonists of TRPV1. Being small and hydrophobic, capsaicin crosses the plasma membrane readily to reach its intracellular ligand-binding site on TRPV1 (11, 12), leading to channel activation and cation permeation. TRPV1 activation rapidly depolarizes nerves to evoke acute pain sensation and allows Ca 2+ entry to initiate downstream signaling events such as neuropeptide release or production of other second messengers. Nevertheless, TRPV1 activation facilitates cellular transport of organic cations such as tetraethylammonium (TEA), N-methylglucamine (NMG) (13), and the quaternary ammonium Na + channel blocker QX-314 (14) or even larger fluor...

Transient Receptor Potential Channels as Drug Targets: From the Science of Basic Research to the Art of Medicine

“…For example, TRPV1 is sensitized by chemical messengers or signaling pathways during inflammation . 784 Nilius and Szallasi Augmented permeation of sensitized TRPV1 by the activity-dependent agonist Cap-ET allows the targeted delivery of the cationic anesthetic Na + channel blocker QX-314, taming neurons with hyperactive TRPV1 but sparing normal nociception (Li et al, 2011a). Sensitization itself seems to depend on the interaction of TRPV1 with the scaffolding protein AKAP79 (Zhang et al, 2008d); indeed, molecules that interfere with this interaction exhibit analgesic potential Fischer et al, 2013).…”

“…This elegant approach affords selective targeting of TRPV1-expressing sensory neurons (Binshtok et al, 2007(Binshtok et al, , 2009). This approach was recently further refined by identifying such TRPV1 agonists (e.g., Cap-ET) that recognize and selectively target already sensitized TRPV1 channels in inflamed tissues and augment the uptake of QX-314 (Li et al, 2011a). It is hoped that this innovative approach may tame hyperactive TRPV1 in diseased tissues and spare normal nociception.…”

Analgesic transient receptor potential vanilloid‐1‐active compounds inhibit native and recombinant T‐type calcium channels

“…Capsaicin is used as a topical analgesic in “low‐concentration” creams (0.1% or ~3 mM) that have poor efficacy in the treatment of neuropathic pain (Derry & Moore, ) or improved efficacy “high concentration” patches (8% or ~260 mM; Noto, Pappagallo, & Szallasi, ). These doses are several fold higher (>50,000, lower limit) than those required to activate the human isoform of TRPV1 (EC 50 ~0.05–0.3 μM; Li, Wang, Chuang, Cohen, & Chuang, ). At such high concentrations, off‐target effects become significant and a clear mechanism for its analgesic effects is difficult to ascertain.…”

TRP Channels and Pruritus

“…Another novel strategy could be the activity dependent targeting of TRPV1; this may be reached by using pore permeable capsaicin analogs (e.g. permanently charged capsaicinoids) which target and desensitize only (hyper)active channels [259,260].…”