Full-length ArticleMorphine amplifies mechanical allodynia via TLR4 in a rat model of spinal cord injury
Introduction
Spinal cord injury (SCI) is the leading cause of central neuropathic pain (CNP) and is often intractable to treatment. Current therapies, including opioids, provide only ∼50% pain relief in 1 out of 2–3 SCI patients (Finnerup et al., 2001). Although opioids are one of the most effective analgesics for CNP (Woller and Hook, 2013), recent rodent and clinical studies have shown that opioids administered soon after trauma can be detrimental to health and recovery in some circumstances (Grace et al., 2016, Hook et al., 2007, Hook et al., 2009, Loram et al., 2012, Salengros et al., 2010).
In order to study central neuropathic pain, we have developed and optimized a unilateral low thoracic (T13)/high lumbar (L1) dorsal root avulsion model of SCI that does not cause paralysis, urinary tract infections/retention, autotomy, or other non-pain relevant aspects that can complicate the study of this phenomenon. Our model, termed SNAP (Spinal Neuropathic Avulsion Pain), creates physical damage to the superficial laminae of the sensory dorsal horn and robust, reliable, below-level bilateral hindpaw mechanical allodynia for ∼9 weeks (Ellis et al., 2014, Wieseler et al., 2010).
Toll like receptor 4 (TLR4) is a critical mediator of neuropathic pain arising from peripheral nerve injury, and is widely expressed in the nervous system, including by DRG neurons, endothelial cells, microglia and astrocytes (Due et al., 2012, Grace et al., 2014, Nicotra et al., 2012). After trauma, endogenous danger signals are released by stressed, damaged, and dying/dead cells and signal via TLR4 (Kofler and Wiley, 2011, Nicotra et al., 2012). 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, Hook et al., 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. As TLR4 is widely expressed in the nervous system, we also assessed whether the combination of SNAP and morphine could amplify the expression of a range of neuroinflammatory markers that are downstream of TLR4, as an indication of engagement of this pathway. The second goal was to identify whether TLR4 was indeed a critical mediator of morphine-potentiated SNAP-allodynia.
Section snippets
Animals and ethics
Pathogen-free male Sprague-Dawley rats (325–350 g; Harlan Laboratories, Madison, WI, USA) were used for all experiments. The rats were allowed a minimum of 1 week to habituate to the colony room before initiating the experiment. Rats were pair-housed prior to surgery and then single-housed after surgery to avoid further spinal cord damage by a cagemate, given removal of overlying vertebral bone. Standard rat chow and water were available ad libitum. Housing was in a temperature-controlled room
Administering a weeklong course of morphine beginning 24 h post-surgery amplifies SNAP allodynia
In order to determine if a short course of morphine early in the post-trauma period would amplify SNAP allodynia, morphine (10 mg/kg s.c.) or vehicle was administered once a day for one week beginning 24 h post-surgery. No baseline (BL) differences were observed between groups in either ipsilateral (Fig. 2A) or contralateral (Fig. 2B) paw withdrawal thresholds. Morphine significantly enhanced SNAP allodynia for both the ipsilateral (Fig. 2A; time x treatment: F12,75 = 0.62, p = 0.8; time: F3,75 = 0.64, p
Discussion
Although opioids have long been accepted as one of the most effective analgesics, there is emerging evidence that opioids may also be detrimental for pain. While opioids create pain suppression via activation of classical opioid receptors on neurons, opioids simultaneously induce pain enhancement via activation of TLR4 receptors (Grace et al., 2015, Watkins et al., 2009). Together with the release of endogenous danger signals after SCI, opioids may interact to create enduring enhancement of
Acknowledgements
This work was supported by NIH (USA) grants DE021966 and DA023132 (LRW). Synthesis and purification of (+)-naltrexone was supported by the NIH Intramural Research Programs of the National Institute on Drug Abuse (NIDA) and the National Institute of Alcohol Abuse and Alcoholism (NIAAA). Peter M Grace was a NHMRC (Australia) CJ Martin Fellow [ID: 1054091] and American Australian Association Sir Keith Murdoch Fellow. Mark R. Hutchinson was a NHMRC (Australia) CJ Martin Fellow (ID 465423;
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