Elsevier

Brain Research

Volume 1695, 15 September 2018, Pages 78-83
Brain Research

Research report
Differential effects of left and right neuropathy on opioid gene expression in lumbar spinal cord

https://doi.org/10.1016/j.brainres.2018.05.043Get rights and content

Highlights

  • Opioid gene expression is differentially affected by left and right neuropathy.

  • Expression is coordinated between ipsi- and contralesional sides.

  • Prodynorphin is upregulated in the ipsilesional dorsal spinal domain.

  • Signs of mechanical allodynia and Pdyn correlate in ventral but not dorsal domains.

  • Opioid genes may have multiple roles in pain control and motor reflexes.

Abstract

The endogenous opioid system (EOS) controls the processing of nociceptive stimuli and is a pharmacological target for opioids. Alterations in expression of the EOS genes under neuropathic pain condition may account for low efficacy of opioid drugs. We here examined whether EOS expression patterns are altered in the lumbar spinal cord of the rats with spinal nerve ligation (SNL) as a neuropathic pain model. Effects of the left- and right-side SNL on expression of EOS genes in the ipsi- and contralateral spinal domains were analysed. The SNL-induced changes were complex and different between the genes; between the dorsal and ventral spinal domains; and between the left and right sides of the spinal cord. Prodynorphin (Pdyn) expression was upregulated in the ipsilateral dorsal domains by each the left and right-side SNL, while changes in expression of μ-opioid receptor (Oprm1) and proenkephalin (Penk) genes were dependent on the SNL side. Changes in expression of the Pdyn and κ-opioid receptor (Oprk1) genes were coordinated between the ipsi- and contralateral sides. Withdrawal response thresholds, indicators of mechanical allodynia correlated negatively with Pdyn expression in the right ventral domain after right side SNL. These findings suggest multiple roles of the EOS gene products in spinal sensitization and changes in motor reflexes, which may differ between the left and right sides.

Introduction

The endogenous opioid system (EOS) is the pharmacological target for opioid analgesics. It is involved in the development and maintenance of pathological pain conditions including neuropathic pain. Treating neuropathic pain with opioids is often discouraging due to their lowered efficacy and high-doses required for opioid therapy (Bleeker, et al., 2001, Kupers et al., 1991, Law et al., 2000, MacPherson, 2002, Obara et al., 2004, Ossipov et al., 1995). Alteration in expression of the EOS genes (Mayer et al., 1999, Ossipov et al., 2000, Przewlocki and Przewlocka, 2001) causing rewiring of the nociceptive neural circuits in neuropathic pain conditions may account in part for the reduced efficacy of opioid drugs (Arner and Meyerson, 1988, Bleeker, et al., 2001, Kupers et al., 1991).

The EOS includes the proenkephalin (Penk), prodynorphin (Pdyn) and proopiomelanocortin (Pomc) genes that give rise to enkephalins, dynorphins and endorphins, respectively. These peptides act via δ-, κ- and µ-opioid receptors encoded by the Oprd1, Oprk1 and Oprm1 genes, respectively. The most consistent adaptive response in neuropathic animals is the upregulation of Pdyn gene transcription and dynorphin expression (Wagner et al., 1993, Wang et al., 2001). Pdyn mRNA levels were increased in the dorsal lumbar spinal cord after chronic constriction injury (Obara et al., 2009), and dynorphin immunoreactivity was elevated in the ipsilateral dorsal quadrant of the lumbar spinal cord in rats with ligations of the L5 and L6 spinal nerves (Kajander et al., 1990, Malan et al., 2000). Enhanced dynorphin expression may be pronociceptive (Caudle and Isaac, 1988, Wang et al., 2001) and promote morphine tolerance (Vanderah et al., 2000). The molecular basis for these non-opioid receptor mediated effects may formation of giant transient pores in plasma membrane causing profound long-lasting neuronal depolarisation (Maximyuk et al., 2015). On the other hand, dynorphins may block nociceptive signalling by activating κ-opioid receptors (Xu et al., 2004).

Less consistent and even controversial data were reported for other EOS genes (Hughes and Smith, 1994, Obara et al., 2009, Plantinga et al., 1992, Smith et al., 1994, van der Kraan et al., 1999). No changes and downregulation of the µ-opioid receptor (Oprm1) gene expression were identified (deGroot et al., 1997, Obara et al., 2009, Pol et al., 2006, Porreca et al., 1998, Zhang et al., 1998), while µ-opioid receptor protein and µ-receptor binding were increased in the lumbar spinal cord in animal models of neuropathic pain (Stevens et al., 1991, Truong et al., 2003). Either no changes or decreases in δ-opioid receptor (Oprd1) gene expression in the spinal cord after chronic constriction injury or sciatic nerve ligation were detected (Besse et al., 1992, Obara et al., 2009, Pol et al., 2006, Robertson et al., 1999, Stevens et al., 1991, Stone et al., 2004). The lack of changes or an increase in Oprk1 gene expression were also reported (Besse et al., 1992, Obara et al., 2009, Pol et al., 2006, Stevens et al., 1991). Expression of Penk mRNA in the lumbar spinal cord was down-regulated after sciatic nerve ligation (Obara et al., 2009), or not changed following complete transection of the sciatic nerve or sciatic nerve crush (Draisci et al., 1991).

In this study we re-examined whether the EOS expression patterns undergo adaptive changes in the ipsi- and contralesional domains of lumbar spinal cord of rats with spinal nerve ligation (SNL), a widely used neuropathic pain model (Chung et al., 2004). SNL-induced neuropathic pain and changes in processing of nociceptive and non-nociceptive stimuli in the spinal cord are controlled by the EOS (Podvin et al., 2016, Sun et al., 2017, Tseng et al., 2017). The opioid receptors are expressed in the dorsal and ventral spinal domains at similar levels (Kononenko et al., 2017) and may be involved in regulation of nociceptive motor reflexes. Therefore, the EOS mRNA levels in the ventral domains were also analysed. The left- and right-side unilateral neuropathy differently activates ipsilateral amygdala neurons, and produces different effects on anxiety and cognitive functions (Goncalves and Dickenson, 2012, Leite-Almeida et al., 2012). This side specificity may be underlined by lateralization of the dynorphin – κ-opioid receptor system in the amygdala (Nation et al., 2018, Xie et al., 2017). The expression patterns of the EOS genes were found to be strongly lateralized in the cervical spinal cord. Interregional co-expression patterns were side specific while intraregional co-expression profiles were differently affected by left- and right-side unilateral body injury (Kononenko et al., 2017). Therefore, in the present study, we also examined whether the left- and right-side SNL differently affects the balance in expression of EOS genes between ipsi- and contralesional spinal domains. The overall design included analyses of four groups of animals including two SNL groups with the left- and right-side nerve injury, and two groups with left- and right-side sham-operation (SO).

Section snippets

Results

First, we examined whether effects of SNL on paw withdrawal thresholds differed significantly from those of SO, suggesting behavioural signs of mechanical allodynia correlative with neuropathic pain. Second, we analysed the EOS expression levels in the left (L) and right (R) halves of dorsal and ventral domains (L-DD, L-VD, R-DD, and R-VD) in the left- and right-side SNL and SO groups. Third, we evaluated whether molecular parameters analysed are associated with the hind paw withdrawal

Discussion

Patterns of the SNL induced changes in expression of the EOS genes were complex and different among the genes; between the dorsal and ventral spinal domains; between the left and right operations; and between the spinal cord sides. The robust effect was upregulation of Pdyn in the ipsilesional DD after the left and right-side SNL. Other significant effects were dependent on the side of nerve ligation, and different in left and right sides of the spinal cord. Thus μ-opioid receptor (Oprm1) was

Animal treatments and surgery

Male Sprague Dawley rats (Harlan, Indianapolis, IN) weighing between 225 and 300 g were maintained in cages in a climate-controlled room on a 12 h light/dark cycle with ad libitum access to food and water. All testing procedures were performed in accordance with the policies and recommendations of the International Association for the Study of Pain and the National Institutes of Health guidelines for the handling and use of the laboratory animals and were approved by the Institutional Animal

Acknowledgments

This work was supported by Grants from the Swedish Science Research Council (VR), the Swedish Council for Working Life and Social Research (FORTE), and the Swedish Research Council Formas (to G.B.); the Swedish Institute (Visby Program; to O.K. and G.B.). The authors declare no conflicts of interest.

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