Characterization of the ORL1 receptor on adrenergic nerves in the rat anococcygeus muscle

1 Nociceptin, the endogenous ORL1 receptor agonist inhibited the motor resp onse to electrical-field stimulation in the rat anococcygeus muscle. This e ffect was characterized using the peptide ligands acetyl-Arg-Tyr-Tyr-Arg-Tr p-Lys-NH2 (Ac-RWRWK-NH2), acetyl-Arg-Tyr-Tyr-Arg-Ile-Lys-NH2 (Ac-RYYRIK-NH2 ) and [Phe(1)psi(CH2-NH)Gly(2)]nociceptin(1-13)NH2 ([F/G]NC(1-13)NH2), and the non-selective opioid antagonist naloxone benzoylhydrazone (NalBzOH). 2 Nociceptin produced a concentration-dependent inhibition of the adrenergi c motor response to electrical-field stimulation (EC50 19 nM, pEC(50) 7.7 /- 0.1, n = 8), but the response to exogenous noradrenaline (0.2-1 mu M) wa s unaffected. The inhibitory nerve response was not affected by up to 1 mu M nociceptin. 3 After inhibition of nitric oxide synthase (N-omega-nitro-L-arginine 100 m u M), and in the presence of peptidase inhibitors, nociceptin produced full inhibition of the pure adrenergic motor response (EC50 4 nM; pEC(50) 8.4 /- 0.1, E-max 98.3 +/- 1.2%, n = 12). Ac-RYYRWK-NH2 was a potent partial-ag onist (pEC(50) 9.0 0.1, E-max 66.4 +/- 5.2; n = 11) but the efficacy of Ac- RYYRIK-NH2 (EC50 8.0 +/- 0.2, E-max 36.7 +/- 3.5; n = 12) was lower and the peptide could be tested as an antagonist (pA(2) 9.01). 4 [F/G]NC(1-13)NH2 and NalBzOH had little or no efficacy and were competiti ve antagonists with pKB values of 7.4 (95% c.l. 7.1, 7.7) and 6.9 (95% c.l. 6.7, 7.1) respectively. Both increased the response to field stimulation a t high concentrations, suggesting the release of an endogenous agonist for the ORL1, receptor during stimulation. 5 Rat nocistatin did not affect the response to electrical-held stimulation , nor did it modify the inhibitory action of nociceptin. 6 Our findings suggest there is a significant endowment of ORL1, receptors on sympathetic terminals of the rat anococcygeus, where nociceptin mediates a powerful inhibitory effect on adrenergic neuromuscular transmission.