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 δ-
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 δ-
opioid receptor constitutive knockout (KO) mice and mice with a conditional deletion of δ-receptor in peripheral
voltage-gated sodium channel (Nav)1.8-expressing neurons (cKO mice). Both
ADL5859 and
ADL5747, and the prototypical δ 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 δ-
opioid agonists were abolished in constitutive δ-receptor KO mice and strongly diminished in δ-receptor cKO mice. We also measured two other well described effects of δ agonists, increase in locomotor activity and agonist-induced receptor internalization by using knock-in mice expressing enhanced green fluorescence
protein-tagged δ 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 δ-
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.