Pharmacological characterization of acute pain behavior produced by application of endothelin-1 to rat sciatic nerve

When applied to rat sciatic nerve, endothelin-1 (ET-1) induces acute pain behavior that is blocked by coadministration of an endothelin A (ET(A)) receptor-selective antagonist. To characterize the pharmacology of this effect, we applied (1) different concentrations (40 to 800 μmol/L) of ET-1 to the surface of sciatic nerve, (2) a second dose of ET-1, 1 to 24 hours after the initial dose, (3) an ET(A) receptor-selective antagonist (BQ-123) to nerve after induction of pain behavior by ET-1, or (4) ET-1 intraneurally to determine, respectively, the dose-dependence, pharmacokinetics, desensitization, reversibility, and likely tissue site of action for ET-1 in inducing pain behavior. In vitro and in vivo experiments also were performed to determine the degree of saturable binding of ET(A) and ET(B) receptors, and to detect correlations between this binding and pain behavior. Maximum hindpaw flinch frequency was observed at a concentration of 400 μmol/L ET-1, with estimated EC₅₀ of 250 to 300 μmol/L for events assayed at 10 and 15 minutes after drug application. Repeat application of 400 μmol 1 to 24 hours after initial application produced no flinching, suggesting persistent behavioral desensitization to the effects of ET-1. Complete desensitization to 400 μmol/L ET-1 also was observed 75 minutes after an initial application of 100 μmol/L ET-1, a concentration that produced minimal flinching, suggesting that desensitization occurred secondary to receptor occupancy. Extraneural administration of BQ-123 12 minutes after ET-1-induced hindpaw flinching had begun reversed this pain behavior within 5 to 10 minutes, implying reversibility of ET-1 binding and the potential clinical usefulness of such agents. Intraneural ET-1 also induced flinching, but at one tenth the dose used in extraneural experiments and with an earlier peak effect, pointing to an intraneural site of action for extraneural ET-1. Although saturable (BQ-123-sensitive) binding was measured in vitro in 4 and 40 nmol/L ET-1 solutions (B(max) ~ 10 fmol/mg wet nerve), extraneural application of 400 μmol/L ET-1 in vivo resulted in too high an ET-1 content (1 to 4 pmol/mg wet nerve) for any saturable binding to be detected. These results establish that acute pain behavior induced by ET-1 applied to rat sciatic nerve is dose-dependent, subject to desensitization with repeat application, reversible, and likely caused by actions within the intraneural compartment.