δ-Opioid receptor agonists produce antinociception and [35S]GTPγS binding in μ receptor knockout mice

We examined the effects of [D-Pen²,D-Pen⁵]enkephalin (DPDPE), [D- Ala²,Glu⁴]deltorphin (DELT), and (+)-4-[(αR)-α((2S,5R)-4-Allyl-2,5- dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80) on [³⁵S]GTPγS binding in brain membranes prepared from μ-opioid receptor knockout (-/-) mice. The potency and maximal response (E(max)) of these agonists were unchanged compared to control mice. In contrast, while the potency of [D-Pen²,pCl-Phe⁴,D-Pen⁵]enkephalin (pCl-DPDPE) was not significantly different, the E(max) was reduced as compared to controls. In the tail-flick test, intracerebroventricular (i.c.v.) or intrathecal (i.th.) DELT produced antinociceptive effects in -/- mice with potency that did not differ significantly from controls. In contrast, the antinociceptive potency of i.c.v. and i.th. DPDPE was displaced to the right by 4- and 9-fold in -/- compared to control mice, respectively. Reduced DPDPE antinociceptive potency in -/- mice, taken together with reduced DPDPE- and pCl-DPDPE- stimulated G protein activity in membranes prepared from -/- mice, demonstrate that these agonists require μ-opioid receptors for full activity. However, because DELT mediated G protein activation and antinociception were both comparable between -/- and wild type mice, we conclude that the μ-opioid receptor is not a critical component of δ-opioid receptor function. (C) 2000 Elsevier Science B.V.