Endothelin-1, unlike the selective endothelin ETB receptor agonist sarafotoxin S6c, causes nociception in the rat when injected intraarticularly into the naive knee-joint. By using selective antagonists, the present study further characterizes the receptors underlying the articular nociceptive actions of endothelin-1, as well as the possible contribution of endogenous endothelins towards nociception induced by carrageenan or E. coli lipopolysaccharide (LPS) in this tissue. Nociception was evaluated by placing the animal for 1 min each hour on a revolving (3 rpm) cylinder and measuring the increase in time the hindpaw of the limb affected by the intra-articular (i.a.) injection of the nociceptive agent, failed to touch its metallic surface (i.e, paw elevation time, PET). In naive joints, endothelin-1 (120 pmol) increased the area under the PET curve (AUC 0-6 h, in arbitrary units) from 61 +/- 3 (control) to 156 +/- 12. This nociceptive effect was reduced by prior intravenous (i.v.) injection of the mixed ETA/ETB receptor antagonist bosentan (by 54 and 73% with 10 and 30 mg/kg) or i.a. administration of the selective ETA receptor antagonist BQ-123 (cyclo [D-Asp-Pro-D-Val-Leu]; by congruent to 45% with 10 or 30 nmol), but was unaffected by the selective ETB receptor antagonist BQ-788 (N-cis-2,6-dimethyl-piperidinocarbonyl-L-gamma-methyl-leucyl-D-1-methoxycarbonyl-tryptophanil-D-norleucine; 10 nmol). Prior joint challenge with carrageenan (300 mu g) 72 h beforehand (i.e. priming) rendered the joint more sensitive to nociception induced by either endothelin-1 or sarafotoxin S6c (15, 30 and 60 pmol). Responses elicited by endothelin (30 pmol) in the primed joint were sensitive to inhibition by either BQ-123 or BQ-788 (each causing congruent to 80% inhibition at 10 nmol). Priming also enhanced PET responses to carrageenan itself and to LPS (1 mu g) markedly and persistently, increasing the area under the curve (AUC 0-12 h, in arbitrary units) from 241 +/- 19 to 409 +/- 50 and from 312 +/- 40 to 466 +/- 25, respectively (P < 0.05), without changing that measured following vehicle injection (from 121 +/- 3 to 117 +/- 4). Bosentan (up to 30 mg/kg, i.v.) failed to modify nociception caused by carrageenan or LPS in naive joints, by carrageenan in the primed joint, or control PET responses; LPS-induced nociception in the primed joint, however, was inhibited by 52 to 56% by bosentan (3 or 10 mg/kg) or 59% by local injection of the selective endothelin ETB receptor antagonist BQ-788 (10 nmol, i.a.), but was unaffected by the selective endothelin ETA receptor antagonist BQ-123. Thus, nociception induced by endothelin-1 in the naive joint is mediated largely via endothelin ETA receptors, whereas both ETA and ETB receptors contribute to its action in the carrageenan-primed joint. Furthermore, LPS-induced nociception in the primed joint is mediated to a large extent via endothelin release and activation of ETB receptors within the joint itself. These findings may be relevant to the etiology of pain underlying chronic arthritic disease in humans. (C) 1998 International Association for the Study of Pain. Published by Elsevier Science B.V.
