Enhanced δ-opioid receptor-mediated antinociception in μ-opioid receptor-deficient mice
Introduction
Endogenous opioid peptides and their receptors are integral components of neural circuits by which cognitive, emotional and attentional aspects of the pain experience modulate pain transmission at spinal levels (Fields and Basbaum, 1994), Opioids act through three classes of receptors referred to as the μ-opioid, δ-opioid and κ-opioid receptor types. The genes encoding these receptors have been cloned and all three receptors can mediate opioid-induced analgesia (Kieffer, 1999). Recently, mice lacking μ-opioid receptors have been generated by homologous recombination in several laboratories by disrupting exon 1 Schuller et al., 1997, Sora et al., 1997b, Tian et al., 1997, exon 2 (Matthes et al., 1996) or exons 2 and 3 (Loh et al., 1998). In each of these knockout strains, selective μ-opioid receptor ligands were ineffective in modulating pain in tail flick and hot plate models of acute or transient pain. Most strains of μ-opioid receptor-deleted or knockout mice did not differ from their wild-type or heterozygous littermates in size, development, apparent fertility and locomotion in novel environments. No compensatory changes were seen in either δ- or κ-opioid receptor binding or distribution, G-protein activation or peptide message levels Matthes et al., 1996, Matthes et al., 1998, Sora et al., 1997b, Narita et al., 1999.
We have taken advantage of the known plasticity of the spinal cord in models of persistent inflammation and pain to study changes in opioid receptor function in mice with a μ-opioid receptor deletion. The injection of inflammatory agents such as complete Freund's adjuvant into the hindpaw of the rat produces an intense inflammation and hyperalgesia that is limited to the injected paw Iadarola et al., 1988, Hylden et al., 1989. We successfully adapted this model to transgenic mice in order to study endogenous opioid mechanisms after inflammation and to examine the effects of selective opioid receptor ligands on opioid function. In contrast to previous studies in transient pain models, we demonstrated that μ-opioid receptors may be involved in mechanisms important for the persistence of inflammatory hyperalgesia and that compensatory changes in endogenous and exogenous antinociceptive mechanisms are present in μ-opioid receptor knockout mice as compared to their wild-type and heterozygous littermates. Some of these findings have been previously published in abstract form (Qiu et al., 1998).
Section snippets
Animal preparation
Animals were bred under standard conditions in a 12 h dark–light cycle. The genetic background of μ-opioid receptor knockout animals was 1:1 hybrids from 129/SvEv and C57BL/6J mouse strains (Sora et al., 1997b) and F2 or F3 knockout, heterozygous and wild-type offspring of heterozygous intercrosses were used for behavioral and pharmacological testing. The animals were 12–16 weeks of age and equal numbers of males and females were used. All mice were genotyped by southern blot analysis (Sora et
Complete Freund's adjuvant-induced hyperalgesia in μ-opioid receptor knockout (−/−), heterozygous (+/−) and wild-type (+/+) mice
We first examined the effects of an intraplantar injection of the inflammatory agent, complete Freund's adjuvant, on baseline paw withdrawal latencies, a measure of nocifensive behavior. As shown in Fig. 1, baseline paw withdrawal latencies before complete Freund's adjuvant injection ranged from 8 to 9 s depending on the genotype. Baseline paw withdrawal latencies from male and female mice were separated and appeared to be similar depending on genotype (Fig. 1). A two factor ANOVA revealed
Discussion
Our major findings indicate that the life-long deletion of μ-opioid receptors leads to different types of phenotypic change in μ-opioid receptor knockout mice in response to acute or transient pain as compared to the murine adaptation of the complete Freund's adjuvant-induced inflammation model of persistent pain Hargreaves et al., 1988, Iadarola et al., 1988. μ-Opioid receptor knockout mice without inflammation were hyperalgesic in response to transient painful stimuli, as previously reported
Acknowledgements
This study was supported by grants from the National Institute on Drug Abuse (NIDA)(DA10275), the National Institute of Dental and Craniofacial Research (DE11964), and the NIDA intramural research program.
References (36)
- et al.
Enhancement of spinothalamic neuron responses to chemical and mechanical stimuli following combined microiontophoretic application of N-methyl-d-aspartic acid and substance P
Pain
(1991) - et al.
A new and sensitive method for measuring thermal nociception in cutaneous hyperalgesia
Pain
(1988) - et al.
Lack of cross-tolerance between U69,593 and bremazocine suggests κ-opioid receptor multiplicity in mice
Eur. J. Pharmacol
(1993) - et al.
Intrathecal morphine in mice: a new technique
Eur. J. Pharmacol.
(1980) - et al.
Enhancement of receptive fields of spinal lamina I projection neurons in rats with unilateral adjuvant-induced inflammation: the contribution of dorsal horn mechanisms
Pain
(1989) - et al.
Differential activation of spinal cord dynorphin and enkephalin neurons during hyperalgesia: evidence using cDNA hybridization
Brain Res.
(1988) Opioids: first lessons from knockout mice
Trends Pharmacol. Sci.
(1999)- et al.
μ-opioid receptor knockout in mice: effects on ligand-induced analgesia and morphine lethality
Mol. Brain Res.
(1998) - et al.
Transgenic studies of pain
Pain
(1998) - et al.
Excitatory and inhibitory amino acids and peptide-induced responses in acutely isolated rat spinal dorsal horn neurons
Neurosci. Lett.
(1989)