Vasodepressor responses to [D-Ala2]-endomorphin 2 (TAPP) are mediated by an L-NAME-sensitive mechanism in the rat

Endomorphin 1 and 2 are newly discovered endogenous ligands for the mu-opioid receptor. We recently showed that endomorphin 1 and 2 have vasodepressor activity, and in this study, responses to a novel endomorphin analog [D-Ala2]-endomorphin 2 (TAPP) were investigated in the systemic vascular bed of the rat. Intravenous injections of TAPP, endomorphin 1, and endomorphin 2 decreased systemic arterial pressure in a dose-related manner. Decreases in systemic arterial pressure in response to TAPP were similar to vasodepressor responses to endomorphin 1 and 2 and were not altered by passage of time. Decreases in systemic arterial pressure in response to TAPP, endomorphin 1, and endomorphin 2 were attenuated by the opioid receptor antagonist naloxone (2 mg/kg, i.v.) when the vasodepressor response to the ORL1-receptor agonist nociceptin (orphanin FQ) was not altered. Decreases in systemic arterial pressure in response to TAPP, endomorphin 1 and 2, and acetylcholine were attenuated by the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME; 50 mg/kg, i.v.) when decreases in systemic arterial pressure in response to nociceptin and calcitonin gene-related peptide (CGRP) were not altered. These results indicate that TAPP, endomorphin 1, and endomorphin 2 decrease systemic arterial pressure by a naloxone-sensitive mechanism and suggest that the vasodepressor response to TAPP, endomorphin 1 and 2, but not nociceptin, is mediated by the release of nitric oxide.