L-R4W2

The arginine-rich hexapeptide R4W2 is a stereoselective antagonist at the vanilloid receptor 1: a Ca2+ imaging study in adult rat dorsal root ganglion neurons.[Pubmed:12023528]

J Pharmacol Exp Ther. 2002 Jun;301(3):981-6.

Vanilloid receptors (VR) integrate various painful stimuli, e.g., noxious heat, acidic pH, capsaicin, and resiniferatoxin (RTX). Although VR antagonists may be useful analgesics, the available agents capsazepine and ruthenium red lack the necessary potency and selectivity. Recently, submicromolar concentrations of the arginine-rich hexapeptide RRRRWW-NH(2) (R(4)W(2)) blocked VR-mediated ionic currents in a Xenopus expression system in a noncompetitive and nonstereoselective manner. Here, VR-antagonistic effects of L-R(4)W(2) and D-R(4)W(2), hexapeptides consisting entirely of L- and D-amino acids, were characterized in native adult rat dorsal root ganglion neurons using [Ca(2+)](i) imaging (Fura-2/acetoxymethyl ester). Fura-2 fluorescence ratio (R) was increased by RTX and capsaicin by 0.473 +/- 0.098 unit above basal levels of 0.903 +/- 0.011 (R(max), 2.289 +/- 0.031; R(min), 0.657 +/- 0.007) in a concentration-dependent manner (log EC(50): RTX, -10.04 +/- 0.05, n = 10; capsaicin, -6.60 +/- 0.10, n = 11). Agonist concentration-response curves were shifted to the right by L- and D-R(4)W(2) (0.1, 1, and 10 microM each) and by capsazepine (3, 10, 30, and 100 microM), whereas their maximal effects and slopes remained unaffected, indicating competitive antagonism. Schild analysis for L-R(4)W(2) yielded apparent dissociation constants of 4.0 nM (RTX) and 3.7 nM (capsaicin), and slopes smaller than unity (RTX, 0.38; capsaicin, 0.42). Apparent dissociation constants and slopes for D-R(4)W(2) and capsaicin were 153 nM and 0.67 versus 4.1 microM and 1.19 for capsazepine and capsaicin. Thus, VR-mediated effects in native dorsal root ganglion neurons were antagonized by L-R(4)W(2) > D-R(4)W(2) > capsazepine (order of potency). In conclusion, the R(4)W(2) hexapeptide is a potent, stereospecific, and (probably) competitive VR antagonist, although an allosteric interaction cannot be completely ruled out.

Arginine-rich peptides are blockers of VR-1 channels with analgesic activity.[Pubmed:10996311]

FEBS Lett. 2000 Sep 15;481(2):131-6.

Vanilloid receptors (VRs) play a fundamental role in the transduction of peripheral tissue injury and/or inflammation responses. Molecules that antagonize VR channel activity may act as selective and potent analgesics. We report that synthetic arginine-rich hexapeptides block heterologously expressed VR-1 channels with submicromolar efficacy in a weak voltage-dependent manner, consistent with a binding site located near/at the entryway of the aqueous pore. Dynorphins, natural arginine-rich peptides, also blocked VR-1 activity with micromolar affinity. Notably, synthetic and natural arginine-rich peptides attenuated the ocular irritation produced by topical capsaicin application onto the eyes of experimental animals. Taken together, our results imply that arginine-rich peptides are VR-1 channel blockers with analgesic activity. These findings may expand the development of novel analgesics by targeting receptor sites distinct from the capsaicin binding site.