An IL-1 receptor antagonist blocks a morphine-induced attenuation of locomotor recovery after spinal cord injury
Research highlights
► Morphine undermines locomotor recovery and increases pain symptoms after SCI. ► IL-1ra pre-treatment blocks the morphine-induced attenuation of locomotor recovery. ► The IL-1ra administered alone attenuated locomotor recovery after spinal injury. ► IL-1ra treatment blocks the development of at-level pain symptoms after SCI. ► IL-1ra pre-treatment reduces weight gain after SCI.
Introduction
Our previous studies suggest that morphine, administered in the acute phase of a spinal contusion injury, significantly undermines recovery of function and increases the expression of paradoxical pain symptoms in the chronic phase of injury (Hook et al., 2007, Hook et al., 2009). Despite producing analgesia, a single dose of systemic morphine administered one day after a moderate contusion injury, led to allodynic responses to innocuous sensory stimuli (three weeks later), decreased sensory function, decreased weight gain, and increased lesion size when compared with vehicle-treated controls (Hook et al., 2007). Intrathecal morphine significantly attenuated the recovery of locomotor function, decreased weight gain, increased the incidence of autophagia, and increased lesion size rostral to the injury site (Hook et al., 2009). These data suggest that caution is warranted when administering morphine in the acute phase of a spinal cord injury (SCI). For SCI patients faced with a lifetime of intractable pain, however, simply removing morphine as a potential analgesic is not an option. It is essential that we further our understanding of the consequences of, and molecular mechanisms engaged by, commonly used opiate analgesics.
Recent studies have implicated the immune system in the development of the ‘paradoxical’ pain observed with morphine administration (Watkins et al., 2005, Watkins et al., 2007, Scholz and Woolf, 2007). These studies have shown that repeated morphine administration activated microglia and astrocytes (Song and Zhao, 2001, Cui et al., 2006, Raghavendra et al., 2002, Tai et al., 2006), and increased TNFα, IL-1β and IL-6 expression in the spinal cord (Johnston et al., 2004, Raghavendra et al., 2002, Tai et al., 2006). These pro-inflammatory cytokines block the analgesic effects of opioids (Gul et al., 2000, Szabo et al., 2002), and have been linked to the development of paradoxical pain symptoms. Indeed, at a cellular level, IL-1β has been shown to facilitate substance-P release from primary afferent neurons in the spinal dorsal horn (Inoue et al., 1999), increase tyrosine phosphorylation of the NMDA receptor NR2B subunit (Viviani et al., 2003), increase surface expression of AMPA receptors (Stellwagen et al., 2005), and has been linked to inhibition of glutamate transporters such as GLT-1 and GLAST (Tai et al., 2006, Prow and Irani, 2008). Subsequent increases in neuroexcitability, with the potentiation of the glutamate signal, underlies the development of central sensitization, a mechanism that results in increased pain reactivity and may exacerbate the secondary neuronal death seen after spinal injury. Also, the pro-inflammatory cytokines, TNFα and IL-1β, activate NF-κB, which is the transcriptional regulator of the synthesis of the inducible form of nitric oxide synthase (iNOS) (Conti et al., 2007). Increased iNOS activity elevates basal nitric oxide (NO) levels, which may also exacerbate the secondary neuronal death seen after spinal injury (Conti et al., 2007). Clearly, the changes in pain reactivity, increased lesion size, and the decreased recovery of function observed with morphine administration may be due to elevations of pro-inflammatory cytokine levels.
The effects of morphine on the immune system are particularly relevant in a spinal contusion injury, which is characterized by inflammation. Pro-inflammatory cytokine levels are significantly upregulated 1, 3, and 6 h after a contusion injury, returning to background levels after 1–3 days (Yang et al., 2004, Yang et al., 2005, Wang et al., 2005, Pineau and Lacroix, 2007). In the early stages of injury, pro-inflammatory cytokines appear to have beneficial effects as they are involved in the regulation of leukocyte recruitment and microglial activation. However, they have also been linked to cytotoxic effects (Merrill and Benveniste, 1996, Gruol and Nelson, 1997, Knoblach et al., 1999, Nesic et al., 2001). Indeed, Yang et al. (2005) suggest that there is a concentration-dependent relationship, and critical balance, between the beneficial and toxic effects of pro-inflammatory cytokines. As morphine increases the expression of pro-inflammatory cytokines, we hypothesize that superimposing this analgesic on the vulnerable contusion site may push the injured system past an adaptive inflammatory response to cytotoxic levels thereby increasing the expression of paradoxical pain symptoms and cell death. To test this hypothesis, the initial experiments confirmed that acute morphine administration increased pro-inflammatory cytokine levels at the site of injury. This effect was observed with both systemic (Experiment 1) and intrathecal (Experiment 2) morphine administration. Experiment 3 then examined whether co-administration of an IL-1 receptor antagonist (IL-1ra) and morphine would block the morphine-induced attenuation of recovery of function. In Experiment 3, both morphine and the IL-1ra were applied directly onto the spinal cord, thereby focusing the loci of changes on the vulnerable contusion site. We found that the IL-1ra did prevent the effects of morphine on recovery of locomotor function and blocked the development of at-level neuropathic symptoms. However, subjects treated with the IL-1ra alone also displayed a dose-dependent attenuation of weight gain after injury, and increased tissue loss across the extent of the lesion.
Section snippets
Subjects
The subjects were male Sprague–Dawley rats obtained from Harlan (Houston, TX). They were approximately 90–110 days old (300–350 g) and were individually housed in Plexiglas bins [45.7 (length) × 23.5 (width) × 20.3 (height) cm] with food and water continuously available. To facilitate access to the food and water, extra bedding was added to the bins after surgery and long mouse sipper tubes were used so that the rats could reach the water without rearing. Subjects were weighed on the same days that
Experiment 1
Day 1 BBB scores were indicative of a moderate contusion injury: prior to morphine treatment, subjects assigned to the morphine and saline treatment groups, respectively, displayed mean (±SEM) converted BBB scores of 2.0 ± 0.56 and 2.75 ± 0.58 (equivalent to untransformed BBB scores of 2.69 ± 0.76 and 3.56 ± 0.89, respectively).
There was a significant main effect of systemic morphine treatment on levels of IL-1β in spinal tissue taken from the injury site 24 h after drug treatment (F(1, 14) = 23.50, p <
Discussion
As found in our previous studies, intrathecal morphine applied in the acute phase of SCI significantly undermined recovery of locomotor function, and increased the expression of symptoms of paradoxical pain in the chronic phase of injury. These adverse effects of morphine appear to depend on activation of the IL-1 receptor. Expression levels of IL-1β in the injured spinal cord were significantly elevated 24 h after systemic morphine administration compared with vehicle controls. Similarly,
Conflicts of interest statement
All authors declare that there are no conflicts of interest.
Acknowledgments
This study was supported by NS041548 and HD058412 to James Grau and Michelle Hook, and Mission Connect, a project of the TIRR foundation. A portion of the data from this study has been previously presented in abstract form.
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2021, Biomedicine and PharmacotherapyMorphine increases macrophages at the lesion site following spinal cord injury: Protective effects of minocycline
2019, Brain, Behavior, and ImmunityCitation Excerpt :The subsequent release of pro-inflammatory cytokines in the spinal cord has been shown to oppose opioid analgesia and facilitate pain (Hutchinson et al., 2008a; Johnston et al., 2004). Importantly, in our SCI model, we have shown that morphine administration results in increased pro-inflammatory cytokine expression at the injury site, and that blocking the interleukin-1 (IL-1) receptor during treatment with this analgesic prevents the morphine-induced attenuation of locomotor recovery (Hook et al., 2011). However, we have also shown that non-classic opioid binding, using [+]-morphine, does not decrease recovery of locomotor function (Aceves et al., 2016), and blocking the TLR4 receptor with LPS-RS did not prevent the morphine induced attenuation of recovery (unpublished data).
Morphine amplifies mechanical allodynia via TLR4 in a rat model of spinal cord injury
2016, Brain, Behavior, and ImmunityCitation Excerpt :In addition, we have recently discovered that TLR4 is also activated by a range of clinically relevant yet structurally diverse classes of opioids (Grace et al., 2015; Hutchinson et al., 2011; Wang et al., 2012). This is important because virtually all SCI patients are treated with opioids soon after injury, whether en route to the hospital or during early care for acute trauma (Hook et al., 2011). Given that both endogenous danger signals due to SCI and morphine could converge at TLR4, the first goal of this study was to determine whether morphine amplifies SNAP, and would recapitulate the multiple pre-clinical (Grace et al., 2016; Hook et al., 2007, 2009; Loram et al., 2012) and clinical (Hansen et al., 2005; Salengros et al., 2010; Trevino et al., 2013; van Gulik et al., 2012) examples of the deleterious effects of opioids given shortly after trauma.