Introduction
Purotoxin 1, a component from the venom of Geolycosa spiders, exerts selective inhibitory action on P2X3 receptors, which opens up new prospects in the studies of structural/functional peculiarities of receptor-channel complexes Р2Х3 as a peripheral link of the nociception system. Until now, in the purinoreceptor family, there was no selective modulators capable of selectivity affecting only one of the subtypes. Thus, Purotoxin 1 may be the first compound that can influence exclusively the Р2Х3 receptor-channel complex.
Pharmacologic action
Purotoxin 1 is a peptide built of 35 amino acid residues, according to the primary structure. Among the 35 amino acid residues, eight are cysteines building four disulfide bonds. By a dramatic concentration-dependent prolongation of P2X3 desensitization removal, Purotoxin 1 exerts its inhibitory action. The inhibition is reversed back to potentiation as the period of agonist applications increases, in the continuous presence of Purotoxin 1. Desensitized P2X3 receptors acquire a high-affinity binding site for the agonist and its unbinding mirrors the rate of recovery from desensitization. The preliminary data indicated that Purotoxin 1 did not compete for this site acting allosterically.
Function
The effects of Purotoxin 1 on purinergic receptor-channel complexes Р2Х2, Р2Х3 and Р2Х2/3 was tested. The results suggested that Purotoxin 1 is a highly selective blocker of exclusively Р2Х3 receptors. Purotoxin 1 could influence ion currents in primary sensory neurons of mammals (neurons of the dorsal root ganglia, DRGs), which were mediated by activation of purinergic receptors Р2Х3. Besides, Purotoxin 1 produced a small potentiating effect on the current generated by P2X3 receptors. Nevertheless, Purotoxin 1 greatly decreased subsequent responses of P2X3 to ATP, when acting on the desensitized receptors. Both the effects were totally reversible.
Moreover, the effects of Purotoxin 1 on behavioral reactions of rats in inflammatory pain models was evaluated. The antinociceptive effect was observed following intraplantar injection of Purotoxin 1 into the inflamed hind paw of rats under both carrageenan-and complete Freund adjuvant-induced thermal hyperalgesia, in the Hargreaves plantar test. Purotoxin 1 was also effective in reducing the number of nocifensive events which was triggered by the injection of capsaicin or formalin. The results showed that nocifensive behavior was significantly reduced in the second but not in the first phase of the formalin assay.
Pharmacokinetics and metabolism
Purotoxin 1 has the high affinity for these receptors (ЕK50 = 12 nM). The currents mediated by the functioning of Р2Х3 receptor-channel complexes entirely underwent the predictable full suppression (total blocking) under the influence of 100 nM Purotoxin 1 which meant the concentration (100 nМ) is saturating. Except Р2Х3, there was no tested voltage- and ligand-operated ion channels expressed in the membranes of DRG neurons of rats, sensitive to the action of 100 nМ Purotoxin 1. The fact indicated the exclusively high selectivity of Purotoxin 1 directed only toward purine receptors of the Р2Х3 subtype.
References:
1. Grishin, E. V., Savchenko, G. A., Vassilevski, A. A., Korolkova, Y. V., Boychuk, Y. A., Viatchenko‐Karpinski, V. Y., ... & Voitenko, N. V. (2010). Novel peptide from spider venom inhibits P2X3 receptors and inflammatory pain. Annals of neurology, 67 (5), 680-683.
2. Kabanova, N. V., Vassilevski, A. A., Rogachevskaja, O. A., Bystrova, M. F., Korolkova, Y. V., Pluzhnikov, K. A., ... & Kolesnikov, S. S. (2012). Modulation of P2X3 receptors by spider toxins. Biochimica et Biophysica Acta (BBA)-Biomembranes, 1818 (11), 2868-2875.
3. Savchenko, G. A., Volkova, T. M., Vasilevskii, A. A., Korol'kova, Y. V., Grishin, E. V., Boichouk, Y. A., & Krishtal, O. A. (2011). Purinergic Membrane Receptors as Targets for the Effect of Purotoxin 1, a Component of Venom of Spiders from the Geolycosa Genus. Neurophysiology, 42 (6), 387-391.
