Xendorphin B1, a novel opioid-like peptide determined from a Xenopus laevis brain cDNA library, produces opioid antinociception after spinal administration in amphibians

Abstract

Prodynorphins (PDYNs) from the African clawed frog (Xenopus laevis), originally described as 'proxendorphins', are novel members of the family of opioid-like precursor polypeptides and were recently discovered based on polymerase chain reaction (PCR) isolates from a Xenopus brain cDNA library. This amphibian prodynorphin was found in two isoforms, (Xen)PDYN-A and (Xen)PDYN-B, consisting of 247 and 279 amino acids, respectively. Each prepropeptide contains five potential opioid-like peptides, collectively named xendorphins. One of these, xendorphin B1 ((Xen)PDYN-B sequence 96-111: YGGFIRKPDKYKFLNA), is a hexadecapeptide that displaced [3H]naloxone and the radiolabelled kappa opioid, [3H]dynorphin A (1-17), with nanomolar affinity from rat brain membranes. Using the acetic acid pain test, the present study examined the antinociceptive effects of spinally administered xendorphin B1 in amphibians. Xendorphin B1 produced a long-lasting and dose-dependent antinociceptive effect in the Northern grass frog (Rana pipiens) with an ED50 value of 44.5 nmol/frog. The antinociceptive effects of xendorphin B1 were significantly blocked by pretreatment with the non-selective opioid antagonist, naltrexone. This is the first report of the in vivo characterization of a non-mammalian prodynorphin-derived peptide and suggests that xendorphin peptides may play a role in the modulation of noxious information in vertebrates.

Figure 1

Time course of the antinociceptive effect of intraspinal xendorphin B1 (XEN B1) in amphibians. Doses used are given in the legend on the graph (in nmol/animal). Data points plotted as MPE +S.E.M (standard error of the mean). N=6 animals per dose and for the saline group.

Figure 2

Log dose-response curves of the antinociceptive effect of intraspinal xendorphin B1 compared with the mu opioid DAMGO, the delta opioid, DPDPE, and the kappa opioid, U50488. Drug groups are denoted in the legend on the graph (XEN B1 is xendorphin B1). Data points are plotted as mean peak effect (MPE + S.E.M.) of individual animals grouped for each agent and dose. N= 6–8 animals per dose. Selective opioid agonist curves are from previous studies [22] and provided here for comparison.

Figure 3

Naltrexone antagonism of the antinociceptive effects of intraspinal xendorphin B1. Time course of the antinociceptive effect of intraspinal xendorphin B1 in amphibians. Treatment group are denoted as Saline (5 μl per animal, i.s.), Naltrexone (100 nmol/g, s.c.), XEN B1 (xendorphin B1, 30 nmol/frog, i.s.) and XEN B1 + NAL (Naltrexone at100 nmol/g, s.c. given 1 h before xendorphin B1 at 30 nmol/frog, i.s). Data points plotted as maximum MPE (+S.E.M.) values across the time course. N=6 animals per treatment group. Asterisk (*) denotes significantly different from SAL group at P<0.01; plus sign (+) denotes significantly different than XEN B1 group at P<0.01 (one-way ANOVA followed by Newman-Keuls post-hoc test).