δ-Opioid Mechanisms for ADL5747 and ADL5859 Effects in Mice: Analgesia, Locomotion, and Receptor Internalization

N,N-diethyl-4-(5-hydroxyspiro[chromene-2,4'-piperidine]-4-yl) benzamide (ADL5859) and N, N-diethyl-3-hydroxy-4-(spiro[ chromene-2,4'-piperidine]-4-yl) benzamide (ADL5747) are novel delta-opioid agonists that show good oral bioavailability and analgesic and antidepressive effects in the rat and represent potential drugs for chronic pain treatment. Here, we used genetic approaches to investigate molecular mechanisms underlying their analgesic effects in the mouse. We tested analgesic effects of ADL5859 and ADL5747 in mice by using mechanical sensitivity measures in both complete Freund's adjuvant and sciatic nerve ligation pain models. We examined their analgesic effects in delta-opioid receptor constitutive knockout (KO) mice and mice with a conditional deletion of delta-receptor in peripheral voltage-gated sodium channel (Nav) 1.8-expressing neurons (cKO mice). Both ADL5859 and ADL5747, and the prototypical delta agonist 4-[(R)-[(2S, 5R)-4-allyl-2,5-dimethyl-piperazin-1-yl]-(3-methoxyphenyl) methyl]-N,N-diethyl-benzamide (SNC80) as a control, significantly reduced inflammatory and neuropathic pain. The antiallodynic effects of all three delta-opioid agonists were abolished in constitutive delta-receptor KO mice and strongly diminished in delta-receptor cKO mice. We also measured two other well described effects of delta agonists, increase in locomotor activity and agonist-induced receptor internalization by using knock-in mice expressing enhanced green fluorescence protein-tagged delta receptors. In contrast to SNC80, ADL5859 and ADL5747 did not induce either hyperlocomotion or receptor internalization in vivo. In conclusion, both ADL5859 and ADL5747 showed efficient pain-reducing properties in the two models of chronic pain. Their effects were mediated by delta-opioid receptors, with a main contribution of receptors expressed on peripheral Nav1.8-positive neurons. The lack of in vivo receptor internalization and locomotor activation, typically induced by SNC80, suggests agonist-biased activity at the receptor for the two drugs.