The antinociceptive potency of N-arachidonoyl-dopamine (NADA) and its interaction with endomorphin-1 at the spinal level

The endogenous N-arachidonoyl-dopamine (NADA) activates both transient receptor potential vanilloid1 (TRPV1) and cannabinoid-1 (CB1) receptors. The goal of this study was to characterize the antinociceptive potential of NADA on inflammatory thermal hyperalgesia in rats at spinal level, and to determine its interaction with endomorphin-1 (EM) at the spinal level. The effects of NADA and EM on thermal hyperalgesia were evaluated in rats with a unilateral hind paw carrageenan-induced inflammation. Intrathecal injection of either EM (0.03-10 mu g) or NADA (1.5-50 mu g) caused dose-dependent antihyperalgesia, but NADA was 5.4 times less potent than EM. The antihyperalgesia caused by 15 mu g NADA was inhibited by the TRPV1 antagonist AMG9810, but not by CB1 antagonist/inverse agonist AM 251, whereas the effect of 50 mu g NADA was decreased by both drugs. Co-administration of EM with NADA in 1:15 and 1:50 ratios produced a short-lasting potentiation, but isobolographic analysis for the whole investigated period revealed additive interaction between the two endogenous ligands. The results show that both TRPV1 and CB1 receptor activation play a substantial role in the antinociceptive effects of NADA at spinal level, while co-administration of NADA with EM did not show potentiation. (C) 2011 Elsevier Inc. All rights reserved.