δ-Opioid receptor agonists produce antinociception and [35S]GTPγS binding in μ receptor knockout mice
Introduction
Many studies indicate that δ-opioid receptors mediate antinociception. δ-opioid receptor-selective compounds have been shown to produce antinociception that can be blocked by δ-selective antagonists Heyman et al., 1987, Porreca et al., 1987, Shah et al., 1994 but not by μ-selective antagonists (Heyman et al., 1987). Prolonged agonist exposure produces antinociceptive tolerance to δ-opioids without significant cross-tolerance to μ-opioid receptor-selective agonists (Mattia et al., 1991). These data support a direct role for δ-opioid receptors in mediating antinociception. Furthermore, treatment of animals with antisense oligodeoxynucleotides targeted at the cloned δ-opioid receptor selectively attenuated the antinociception produced by δ-opioid, but not μ- or κ-opioid agonists, suggesting that δ receptor selective agonists such as [d-Ala2, Glu4]deltorphin act predominantly through δ receptors Tseng et al., 1994, Bilsky et al., 1996, Standifer et al., 1997.
Germ line mutation of opioid receptor genes have produced transgenic mice which lack specific opioid receptors; these models have been used to evaluate the role of opioid receptors in physiological and pathological function. It was recently reported that mice with null mutation of the cloned μ-opioid receptor (MOR) (−/− mice) did not exhibit antinociceptive response to two putative δ-opioid receptor selective agonists, Tyr-d-Ser(O-t-butyl)-Gly-Phe-Leu-Thr-(O-t-butyl) (BUBU) Matthes et al., 1996, Matthes et al., 1998 and [d-Pen2, d-Pen5]enkephalin (DPDPE) (Sora et al., 1997a). These observations led to the postulation that antinociceptive efficacy of δ-opioid receptor selective agonists requires the presence of μ-opioid receptors in the central nervous system. On the other hand, it may also be plausibly argued that the ligands used in this case are not highly selective for δ receptors as previously thought. The present study aims to test the hypothesis that in the −/− mice the δ-opioid receptors are functionally fully active and mediate antinociception. We have determined the activation of G proteins by δ-opioid selective agonists in both brain and spinal cord membranes prepared from control or −/− mice, and the antinociceptive actions of δ-selective agonists in both control and −/− mice. Our data indicate that the δ-opioid receptor remains functionally coupled to G proteins and produces antinociceptive actions in −/− mice. Thus, expression of μ-opioid receptors is not required for δ-opioid receptor functions.
Section snippets
Generation of μ-opioid receptor knockout mice
The generation of μ-opioid receptor knockout mice in a C57/129Sv genetic background has been previously described (Sora et al., 1997b). Briefly, a 16.5-kb DNA fragment encoding part of the 5′ flanking region, the first exon and part of the first intron of the μ-opioid receptor was isolated from a mouse λ library. A deletion mutation of the fragment was produced in which the translation initiation site was omitted and this fragment was subcloned into the pPGKneo expression vector. Embryonic stem
Results
We examined G-protein activation by the μ-opioid receptor-selective agonist DAMGO, and the opioid agonists DPDPE, pCl-DPDPE, SNC80 and DELT in whole brain membranes prepared from control and −/− mice. These latter drugs have previously been shown to be highly δ-opioid receptor-selective Kramer et al., 1993, Knapp et al., 1996. Activation was determined as drug-induced binding of [35S]GTPγS to membranes (Fig. 1). In membranes prepared from control mouse brains, mean maximal responses of 118,
Discussion
Recent reports using μ-opioid receptor knockout mice have suggested that δ-opioid receptor-mediated antinociception is dependent upon the μ receptor Matthes et al., 1996, Matthes et al., 1998, Sora et al., 1997a. Such findings were unexpected in light of an extensive literature supporting antinociception mediated by the δ-opioid receptor. We thus compared δ-opioid receptor-mediated antinociception and G protein activation in control and −/− mice using several δ-selective agonists. The
Acknowledgements
This research was supported in part by USPHS Grants from the NIDA (DA06284) and the Arizona Disease Control Research Commission.
References (19)
- et al.
Opioid δ-receptor involvement in supraspinal and spinal antinociception in mice
Brain Res.
(1987) - et al.
Intrathecal morphine in mice: a new technique
Eur. J. Pharmacol.
(1980) - et al.
The effect of naltrindole on spinal and supraspinal delta opioid receptors and analgesia
Life Sci.
(1994) - et al.
The μ-opioid receptor is necessary for [d-Pen2, d-Pen5]enkephalin-induced analgesia
Eur. J. Pharmacol.
(1997) - et al.
Antisense oligodeoxynucleotide to a δ-opioid receptor selectively blocks the spinal antinociception induced by δ-, but not μ- or κ-opioid agonists in the mouse
Eur. J. Pharmacol.
(1994) - et al.
Mu and delta receptors: their role in analgesia and in the differential effects of opioid peptides on analgesia
Life Sci.
(1982) - et al.
Characterization of antinociception to opioid receptor selective agonists after antisense oligodeoxynucleotide-mediated “knock-down” of opioid receptors in vivo
J. Pharmacol. Exp. Ther.
(1996) - et al.
Opioid efficacy in a C6 glioma cell line stably expressing the delta opioid receptor
J. Pharmacol. Exp. Ther.
(1997) - et al.
Modulation of the potency and efficacy of mu-mediated antinociception by delta agonists in the mouse
J. Pharmacol. Exp. Ther.
(1990)
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