Elsevier

Neuroscience

Volume 101, Issue 3, 15 November 2000, Pages 709-717
Neuroscience

Disparate spinal and supraspinal opioid antinociceptive responses in β-endorphin-deficient mutant mice

https://doi.org/10.1016/S0306-4522(00)00422-XGet rights and content

Abstract

The role of endogenous opioid systems in the analgesic response to exogenous opiates remains controversial. We previously reported that mice lacking the peptide neurotransmitter β-endorphin, although unable to produce opioid-mediated stress-induced antinociception, nevertheless displayed intact antinociception after systemic administration of the exogenous opiate morphine. Morphine administered by a peripheral route can activate opioid receptors in both the spinal cord and brain. However, β-endorphin neuronal projections are confined predominantly to supraspinal nociceptive nuclei. Therefore, we questioned whether the absence of β-endorphin would differentially affect antinociceptive responses depending on the route of opiate administration. Time- and dose–response curves were obtained in β-endorphin-deficient and matched wild-type C57BL/6 congenic control mice using the tail-immersion/withdrawal assay. Null mutant mice were found to be more sensitive to supraspinal (i.c.v.) injection of the μ-opioid receptor-selective agonists, morphine and d-Ala2-MePhe4-Gly-ol5 enkephalin. In contrast, the mutant mice were less sensitive to spinal (i.t.) injection of these same drugs. Quantitative receptor autoradiography revealed no differences between genotypes in the density of μ, δ, or κ opioid receptor binding sites in either the spinal cord or pain-relevant supraspinal areas.

Thus we report that the absence of a putative endogenous ligand for the μ-opioid receptor results in opposite changes in morphine sensitivity between discrete areas of the nervous system, which are not simply caused by changes in opioid receptor expression.

Section snippets

β-Endorphin mutant mice

The gene targeting vector, POMCX*4, and production of β-endorphin null mutant mice are described fully in Rubinstein et al.32 In brief, a point mutation was introduced by site-directed mutagenesis into exon 3 of the POMC gene to generate a premature translational stop codon. The resultant truncated prohormone lacks the carboxyl-terminal 31 amino acids composing β-endorphin, but is expressed at normal levels and correctly processed to adrenocorticotropic hormone (ACTH), melanocyte stimulating

Baseline tail-withdrawal latencies and effect of vehicle injection

Combining data from all experiments, a significant KO>WT difference in baseline 49°C tail-withdrawal latency was observed (3.0±0.1 vs 2.7±0.1 s, respectively; t338=2.65, P<0.005). This small genotypic difference had not been seen previously; the present study, however, provided far greater statistical power owing to the very large number of total subjects. Indeed, the difference was significant in only one individual experiment in the present study (WT vs KO, i.c.v. DAMGO).

Repeated-measure

Discussion

In this study we demonstrate reciprocal alterations in supraspinal and spinal opioid antinociception in null mutant mice lacking β-endorphin. KO mice were found to be more and less sensitive, respectively, than their WT counterparts to antinociception from i.c.v. and i.t. morphine. Although morphine, especially at high doses, can bind to δ- and κ-opioid receptors,36 the phenomenon described presently was shown to be selectively related to μ antinociception since parallel results were obtained

Conclusions

Although the antinociceptive actions of systemically administered morphine were unaltered in mice lacking β-endorphin, opposing effects were seen in the mutant mice when morphine was specifically injected into the supraspinal or spinal compartments. Relative to WT mice of the same C57BL/6 congenic background, KO mice displayed increased antinociceptive sensitivity to i.c.v. morphine, but decreased sensitivity to i.t. morphine. Both phenomena are apparently related to the μ-opioid receptor,

Acknowledgements

The authors thank C. Fjeld, C. MacPherson and S. Lapostolle for maintaining and genotyping the mutant mouse colony and Y. Fang for invaluable advice on receptor autoradiography. This work was supported by R01 DE12735 and R29 DA11394 (J.S.M.), start-up funding from Furman University (J.E.G.), F32 DA05841 (M.D.H.), VA Merit Review No. 350 (J.K.B.) and P01 DK55819 (M.J.L.).

References (47)

  • J.S. Mogil et al.

    Mu-opiate receptor binding is up-regulated in mice selectively bred for high stress-induced analgesia

    Brain Res.

    (1994)
  • J.S. Mogil et al.

    Nociceptive and morphine antinociceptive sensitivity of 129 and C57BL/6 inbred mouse strains: implications for transgenic knock-out studies

    Eur. J. Pain

    (1997)
  • J.S. Mogil et al.

    Heritability of nociception. I. Responses of eleven inbred mouse strains on twelve measures of nociception

    Pain

    (1999)
  • M. Narita et al.

    Evidence for the existence of the β-endorphin-sensitive “ε-opioid receptor” in the brain: the mechanisms of ε-mediated antinociception

    Jpn. J. Pharmac.

    (1998)
  • M.H. Ossipov et al.

    Characterization of supraspinal antinociceptive actions of opioid delta agonists in the rat

    Pain

    (1995)
  • D. Paul et al.

    Different receptor subtypes mediate spinal and supraspinal analgesia in mice

    Eur. J. Pharmac.

    (1989)
  • S.C. Roerig et al.

    Tolerance to morphine analgesia: decreased multiplicative interaction between spinal and supraspinal sites

    Brain Res.

    (1984)
  • S.J. Slowe et al.

    Quantitative autoradiography of μ-,δ- and κ1 opioid receptors in κ-opioid receptor knockout mice

    Brain Res.

    (1999)
  • A.E. Takemori et al.

    Enkephalin antinociception in mice is mediated by δ1- and δ2-opioid receptors in the brain and spinal cord, respectively

    Eur. J. Pharmac.

    (1993)
  • T.L. Yaksh

    Spinal opiate analgesia: characteristics and principles of action

    Pain

    (1981)
  • T.L. Yaksh et al.

    Narcotic analgesics: CNS sites and mechanisms of action as revealed by intracerebral injection techniques

    Pain

    (1978)
  • T. Yamamoto et al.

    Nociceptin/orphanin FQ: role in nociceptive information processing

    Prog. Neurobiol.

    (1999)
  • A. Zangen et al.

    Nociceptive stimulus induces release of endogenous β-endorphin in the rat brain

    Neuroscience

    (1998)
  • Cited by (20)

    • β-endorphin regulates alcohol consumption induced by exercise restriction in female mice

      2016, Alcohol
      Citation Excerpt :

      These data, adding to a body of both clinical and basic evidence (cf. del Arbol et al., 1995; Thiagarajan, Mefford, & Eskay, 1989), support the idea that β-endorphin modulates endocrine and behavioral components of the stress response and may contribute to an increased susceptibility for heavy drinking. Moreover, the use of low-endorphin heterozygous mice (βE-HT) in our study enables a more nuanced analysis for the role of this peptide beyond what has been shown in earlier studies only comparing knockouts and wild-types (Mogil et al., 2000; Racz et al., 2008) and may better model the human condition. β-endorphin produces its effects by acting on μ, δ, and κ opioid receptors, binding preferentially to the μ receptor (Hallberg & Nyberg, 2003).

    • Nicotine anxiogenic and rewarding effects are decreased in mice lacking β-endorphin

      2009, Neuropharmacology
      Citation Excerpt :

      The change in the spontaneous locomotor responses of mice lacking β-endorphin further emphasizes the role played by β-endorphin in the control of locomotion. The enhancement of the spontaneous nociceptive threshold of β-endorphin KO mice was mainly revealed in the tail-immersion test, in agreement with previous results (Mogil et al., 2000). This enhanced nociceptive threshold might reflect a compensatory regulation of other neurobiological mechanisms involved in pain control.

    • Exploring the opioid system by gene knockout

      2002, Progress in Neurobiology
      Citation Excerpt :

      Finally, morphine analgesia, hyperlocomotion, reward and withdrawal were unchanged in the Pdyn mutant (Zimmer et al., 2001), indicating that major morphine effects do not require the presence of endogenous prodynorphin peptides. Similarly systemic morphine analgesia was unchanged in the absence of βend (Rubinstein et al., 1996), while subtle modifications were noted when morphine was administered intracerebroventricular (i.c.v.) or intrathecal (i.t.) (Mogil et al., 2000). The analysis of in vivo responses of opioid receptor-deficient mice to kappa agonists has provided results consistent with the previous pharmacology (Table 4).

    View all citing articles on Scopus
    View full text