Efficacy of Intrathecal Morphine in a Model of Surgical Pain in Rats

Grimace scale, burrowing, and nest building for the assessment of post-surgical pain in mice and rats-A systematic review

Several studies suggested an informative value of behavioral and grimace scale parameters for the detection of pain. However, the robustness and reliability of the parameters as well as the current extent of implementation are still largely unknown. In this study, we aimed to systematically analyze the current evidence-base of grimace scale, burrowing, and nest building for the assessment of post-surgical pain in mice and rats. The following platforms were searched for relevant articles: PubMed, Embase via Ovid, and Web of Science. Only full peer-reviewed studies that describe the grimace scale, burrowing, and/or nest building as pain parameters in the post-surgical phase in mice and/or rats were included. Information about the study design, animal characteristics, intervention characteristics, and outcome measures was extracted from identified publications. In total, 74 papers were included in this review. The majority of studies have been conducted in young adult C57BL/6J mice and Sprague Dawley and Wistar rats. While there is an apparent lack of information about young animals, some studies that analyzed the grimace scale in aged rats were identified. The majority of studies focused on laparotomy-associated pain. Only limited information is available about other types of surgical interventions. While an impact of surgery and an influence of analgesia were rather consistently reported in studies focusing on grimace scales, the number of studies that assessed respective effects was rather low for nest building and burrowing. Moreover, controversial findings were evident for the impact of analgesics on post-surgical nest building activity. Regarding analgesia, a monotherapeutic approach was identified in the vast majority of studies with non-steroidal anti-inflammatory (NSAID) drugs and opioids being most commonly used. In conclusion, most evidence exists for grimace scales, which were more frequently used to assess post-surgical pain in rodents than the other behavioral parameters. However, our findings also point to relevant knowledge gaps concerning the post-surgical application in different strains, age levels, and following different surgical procedures. Future efforts are also necessary to directly compare the sensitivity and robustness of different readout parameters applied for the assessment of nest building and burrowing activities.

Macrophage Migration Inhibitory Factor (MIF) Makes Complex Contributions to Pain-Related Hyperactivity of Nociceptors after Spinal Cord Injury

Neuropathic pain is a major, inadequately treated challenge for people with spinal cord injury (SCI). While SCI pain mechanisms are often assumed to be in the CNS, rodent studies have revealed mechanistic contributions from primary nociceptors. These neurons become chronically hyperexcitable after SCI, generating ongoing electrical activity that promotes ongoing pain. A major question is whether extrinsic chemical signals help to drive ongoing electrical activity after SCI. People living with SCI exhibit acute and chronic elevation of circulating levels of macrophage migration inhibitory factor (MIF), a cytokine implicated in preclinical pain models. Probable nociceptors isolated from male rats and exposed to an MIF concentration reported in human plasma (1 ng/ml) showed hyperactivity similar to that induced by SCI, although, surprisingly, a 10-fold higher concentration failed to increase excitability. Conditioned behavioral aversion to a chamber associated with peripheral MIF injection suggested that MIF stimulates affective pain. A MIF inhibitor, Iso-1, reversed SCI-induced hyperexcitability. Unlike chronic SCI-induced hyperexcitability, acute MIF-induced hyperexcitability was only partially abrogated by inhibiting ERK signaling. Unexpectedly, MIF concentrations that induced hyperactivity in nociceptors from naive animals, after SCI induced a long-lasting conversion from a highly excitable nonaccommodating type to a rapidly accommodating, hypoexcitable type, possibly as a homeostatic response to prolonged depolarization. Treatment with conditioned medium from cultures of DRG cells obtained after SCI was sufficient to induce MIF-dependent hyperactivity in neurons from naive rats. Thus, changes in systemic and DRG levels of MIF may help to maintain SCI-induced nociceptor hyperactivity that persistently promotes pain.SIGNIFICANCE STATEMENT Chronic neuropathic pain is a major challenge for people with spinal cord injury (SCI). Pain can drastically impair quality of life, and produces substantial economic and social burdens. Available treatments, including opioids, remain inadequate. This study shows that the cytokine macrophage migration inhibitory factor (MIF) can induce pain-like behavior and plays an important role in driving persistent ongoing electrical activity in injury-detecting sensory neurons (nociceptors) in a rat SCI model. The results indicate that SCI produces an increase in MIF release within sensory ganglia. Low MIF levels potently excite nociceptors, but higher levels trigger a long-lasting hypoexcitable state. These findings suggest that therapeutic targeting of MIF in neuropathic pain states may reduce pain and sensory dysfunction by curbing nociceptor hyperactivity.

Intrathecal morphine injections in lumbar fusion surgery: Case-control study

INTRODUCTION

Intrathecal morphine (ITM) is a well-known and extensively used method for analgesia in various surgical fields; however, its relevance in spine surgery is debated given the conflicting results in the literature. The aim of this study was to investigate the opioid-sparing effect of ITM after lumbar fusion.

METHODS

This retrospective study involves two consecutive series of patients undergoing posterior lumbar fusion. The first cohort (control group, n=30) received the standard analgesia protocol while the second cohort (ITM Group, n=30) had the standard protocol supplemented with ITM (100μg of morphine hydrochloride). Morphine consumption, pain assessment (VAS), specific complications and postoperative recovery data were collected.

RESULTS

Consumption of morphine at 24hours and 48hours postoperatively was lower in the ITM group than the control group (p<0.001 and p=0.004). The pattern was similar for pain on VAS at H6, H24 and H36 (p=0.001; p=0.003 and p=0.01). The patients in the ITM group were able to get out of bed faster than the controls (1.13 days vs 1.83 days, p=0.002) and the discharge was earlier in the ITM group (5.1 days vs. 6.2 days, p=0.002). There was no difference in morphine-specific complications between the two groups.

CONCLUSION

Adding ITM to the analgesia protocol for lumbar fusion provides better management of postoperative pain, without increasing early complications, and it accelerates the recovery process after surgery.

LEVEL OF EVIDENCE

IV.

Development of a novel analgesic for neuropathic pain targeting brain-derived neurotrophic factor

Effective treatment of neuropathic pain is challenging as its underlying mechanism remains largely unknown. Recently, the participation of brain-derived neurotrophic factor (BDNF) in neuropathic pain has been attracting increased attention. BDNF binds to a member of the tyrosine kinase receptor family, the TrkB receptor, that is specific for BDNF and is the transmembrane receptor on the posterior horn of spinal cord. In the present study, we purified two proteins that included the BDNF-binding domain of TrkB (eTrkB) and eTrkB coupled with a liposomal outer surface (liposomal eTrkB) in order to inhibit the BDNF-TrkB pathway in neuropathic pain. Results of the pull-down assay showed that eTrkB was bound to BDNF. We investigated the neuropathic pain suppression effect of this purified protein by its intrathecal administration in a rat neuropathic pain model. Mechanical and thermal hyperalgesia induced by L5 lumbar nerve ligation was markedly suppressed by treatment with eTrkB protein. Furthermore, we showed a prolonged algetic inhibition by liposomal eTrkB protein treatment. In conclusion, this study suggests that eTrkB, which sequesters endogenous BDNF and inhibits the BDNF-TrkB pathway, may prove to be a novel analgesic to treat neuropathic pain.

The development and use of facial grimace scales for pain measurement in animals

The measurement of pain in animals is surprisingly complex, and remains a critical issue in veterinary care and biomedical research. Based on the known utility of pain measurement via facial expression in verbal and especially non-verbal human populations, "grimace scales" were first developed a decade ago for use in rodents and now exist for 10 different mammalian species. This review details the background context, historical development, features (including duration), psychometric properties, modulatory factors, and impact of animal grimace scales for pain.

Intrathecal Administration of Nanoclusters for Protecting Neurons against Oxidative Stress in Cerebral Ischemia/Reperfusion Injury

Oxidative stress is one of the important mechanisms in cerebral ischemia/reperfusion (I/R) injury. Antioxidants with high brain accumulation are highly desired to help prevent cerebral I/R injury. Herein, intrathecal injection of polyoxometalate (POM) nanoclusters as nano-antioxidants with preferential brain uptake were applied for neuronal protection in cerebral I/R injury. Using powerful positron emission tomography imaging, the uptake of nano-antioxidants in the brain was non-invasively and real-timely monitored. Our results demonstrated that POM nanoclusters rapidly reached the ischemic penumbra after intrathecal injection and effectively scavenged reactive oxygen species (ROS) for inhibiting oxidative stress. The infarct size was reduced, and neurological function was restored in cerebral I/R injury rat models. As a proof-of-concept, the intrathecal injection of nano-antioxidants is an excellent therapeutic strategy to ameliorate cerebral I/R injury.

Effects of Intraperitoneal and Intrathecal Morphine Analgesia on the Expression of μ-Opioid Receptors in Bone Cancer Pain Rats

Backgrounds

This study compared analgesic effects and μ-opioid receptor expression levels during long-term intraperitoneal and intrathecal treatment in a bone cancer pain rat.

Methods

Twenty-four female Sprague-Dawley rats were injected Walker 256 tumor cells into the femur to create a bone cancer pain model. The control group was injected with saline intraperitoneally and intrathecally. The intraperitoneal group was injected with morphine intraperitoneally and saline intrathecally. The intrathecal group was injected saline intraperitoneally and morphine intrathecally. Changes in pain threshold, μ-opioid receptor expression levels in spinal cord, and tumor tissue were compared between 3 groups.

Results

The intrathecal morphine group and the intraperitoneal group showed no difference in analgesia effects (P > .05). Western blot and immunohistochemical staining of μ-opioid receptors demonstrated that its level in the intrathecal group was significantly lower than the intraperitoneal group (P < .05) and without significant difference with the control group (P > .05). The expression levels of μ-opioid receptor in the spinal cord tissue did not reveal a difference among these 3 groups (P > .05).

Conclusion

Intrathecal group and intraperitoneal group showed significant difference in μ-opioid receptor expressions although with no difference in analgesia effects. Long-term intrathecal morphine administration provided similar analgesia compared to systemic morphine.

Evans Blue Reduces Neuropathic Pain Behavior by Inhibiting Spinal ATP Release

Upon peripheral nerve injury, vesicular ATP is released from damaged primary afferent neurons. This extracellular ATP subsequently activates purinergic receptors of the spinal cord, which play a critical role in neuropathic pain. As an inhibitor of the vesicular nucleotide transporter (VNUT), Evans blue (EB) inhibits the vesicular storage and release of ATP in neurons. Thus, we tested whether EB could attenuate neuropathic pain behavior induced by spinal nerve ligation (SNL) in rats by targeting VNUT. An intrathecal injection of EB efficiently attenuated mechanical allodynia for five days in a dose-dependent manner and enhanced locomotive activity in an SNL rat model. Immunohistochemical analysis showed that EB was found in VNUT immunoreactivity on neurons in the dorsal root ganglion and the spinal dorsal horn. The level of ATP in cerebrospinal fluid in rats with SNL-induced neuropathic pain decreased upon administration of EB. Interestingly, EB blocked ATP release from neurons, but not glial cells in vitro. Eventually, the loss of ATP decreased microglial activity in the ipsilateral dorsal horn of the spinal cord, followed by a reduction in reactive oxygen species and proinflammatory mediators, such as interleukin (IL)-1β and IL-6. Finally, a similar analgesic effect of EB was demonstrated in rats with monoiodoacetate-induced osteoarthritis (OA) pain. Taken together, these data demonstrate that EB prevents ATP release in the spinal dorsal horn and reduces the ATP/purinergic receptor-induced activation of spinal microglia followed by a decline in algogenic substances, thereby relieving neuropathic pain in rats with SNL.

Characterization of Cancer-Induced Nociception in a Murine Model of Breast Carcinoma

Severe and poorly treated pain often accompanies breast cancer. Thus, novel mechanisms involved in breast cancer-induced pain should be investigated. Then, it is necessary to characterize animal models that are reliable with the symptoms and progression of the disease as observed in humans. Explaining cancer-induced nociception in a murine model of breast carcinoma was the aim of this study. 4T1 (10⁴) lineage cells were inoculated in the right fourth mammary fat pad of female BALB/c mice; after this, mechanical and cold allodynia, or mouse grimace scale (MGS) were observed for 30 days. To determine the presence of bone metastasis, we performed the metastatic clonogenic test and measure calcium serum levels. At 20 days after tumor induction, the antinociceptive effect of analgesics used to relieve pain in cancer patients (acetaminophen, naproxen, codeine or morphine) or a cannabinoid agonist (WIN 55,212-2) was tested. Mice inoculated with 4T1 cells developed mechanical and cold allodynia and increased MGS. Bone metastasis was confirmed using the clonogenic assay, and hypercalcemia was observed 20 days after cells inoculation. All analgesic drugs reduced the mechanical and cold allodynia, while the MGS was decreased only by the administration of naproxen, codeine, or morphine. Also, WIN 55,212-2 improved all nociceptive measures. This pain model could be a reliable form to observe the mechanisms of breast cancer-induced pain or to observe the efficacy of novel analgesic compounds.

Performance of behavioral assays: the Rat Grimace Scale, burrowing activity and a composite behavior score to identify visceral pain in an acute and chronic colitis model

Supplemental Digital Content is Available in the Text.

The ability of spontaneous behavioral assays to assess chronic pain is limited. In colitis, RGS identified acute and chronic pain, burrowing identified acute pain.

Introduction:

The Rat Grimace Scale (RGS), a facial expression scale, quantifies the affective component of pain in rats. The RGS was developed to identify acute and inflammatory pain, and applicability in acute and chronic visceral pain is unknown. The dextran sulfate sodium (DSS) colitis model is commonly used in rats, but pain is rarely assessed, instead, disease progression is monitored with the Disease Activity Index (DAI; assessing fecal blood, stool consistency, and weight loss).

Objectives:

The aim of this study was to assess whether the RGS and 2 additional behavioral tools (composite behavior score [CBS] and burrowing) could identify pain in an acute and chronic DSS colitis model.

Methods:

Male and female Sprague-Dawley rats were block randomized to (1) acute colitis (4 days DSS in drinking water); (2) chronic colitis (4 days DSS, 7 days water, and 3 days DSS); or (3) control (14 days water). Disease Activity Index, RGS, CBS, and burrowing assessments were performed daily.

Results:

Rat Grimace Scale scores increased as DAI scores increased during both acute and chronic phases. Burrowing only decreased during the acute phase. By contrast, CBS scores did not increase significantly during either colitis phase.

Conclusions:

These data show that the RGS and burrowing did not decrease in a sustained manner during chronic phase visceral pain, and that variables assessed in the DAI are indicative of pain. This suggests that the RGS can be applied to a wider range of pain types and chronicity than originally suggested. These findings increase the application of the RGS as a pain scale and welfare improvement tool.

Neuronal and glial factors contributing to sex differences in opioid modulation of pain

Morphine remains one of the most widely prescribed opioids for alleviation of persistent and/or severe pain; however, multiple preclinical and clinical studies report that morphine is less efficacious in females compared to males. Morphine primarily binds to the mu opioid receptor, a prototypical G-protein coupled receptor densely localized in the midbrain periaqueductal gray. Anatomical and physiological studies conducted in the 1960s identified the periaqueductal gray, and its descending projections to the rostral ventromedial medulla and spinal cord, as an essential descending inhibitory circuit mediating opioid-based analgesia. Remarkably, the majority of studies published over the following 30 years were conducted in males with the implicit assumption that the anatomical and physiological characteristics of this descending inhibitory circuit were comparable in females; not surprisingly, this is not the case. Several factors have since been identified as contributing to the dimorphic effects of opioids, including sex differences in the neuroanatomical and neurophysiological characteristics of the descending inhibitory circuit and its modulation by gonadal steroids. Recent data also implicate sex differences in opioid metabolism and neuroimmune signaling as additional contributing factors. Here we cohesively present these lines of evidence demonstrating a neural basis for sex differences in opioid modulation of pain, with a focus on the PAG as a sexually dimorphic core of descending opioid-induced inhibition and argue for the development of sex-specific pain therapeutics.

A deep neural network to assess spontaneous pain from mouse facial expressions

Grimace scales quantify characteristic facial expressions associated with spontaneous pain in rodents and other mammals. However, these scales have not been widely adopted largely because of the time and effort required for highly trained humans to manually score the images. Convoluted neural networks were recently developed that distinguish individual humans and objects in images. Here, we trained one of these networks, the InceptionV3 convolutional neural net, with a large set of human-scored mouse images. Output consists of a binary pain/no-pain assessment and a confidence score. Our automated Mouse Grimace Scale integrates these two outputs and is highly accurate (94%) at assessing the presence of pain in mice across different experimental assays. In addition, we used a novel set of “pain” and “no pain” images to show that automated Mouse Grimace Scale scores are highly correlated with human scores (Pearson’s r = 0.75). Moreover, the automated Mouse Grimace Scale classified a greater proportion of images as “pain” following laparotomy surgery when compared to animals receiving a sham surgery or a post-surgical analgesic. Together, these findings suggest that the automated Mouse Grimace Scale can eliminate the need for tedious human scoring of images and provide an objective and rapid way to quantify spontaneous pain and pain relief in mice.

Analgesics in Small Mammals

Managing pain effectively in any species is challenging, but small mammals present particular problems. Methods of pain assessment are still under development in these species, so the efficacy of analgesic therapy cannot be evaluated fully. Methods of assessing abdominal pain are established; however, applying these can be challenging. Alternative methods, using assessment of facial expression, may be more applicable to a range of painful procedures and across species. Multimodal and preventive analgesic strategies are most likely to be effective. Although data on analgesic dose rates are limited, sufficient information is available to enable analgesia to be provided safely.

Endogenous Opiates and Behavior: 2016

This paper is the thirty-ninth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2016 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

Sciatic nerve injury following analgesic drug injection in rats: A histopathological examination

OBJECTIVE:

Sciatic nerve neuropathy can be observed following intramuscular gluteal injections. The histopathological examination of sciatic nerve damage following intramuscular injection in the gluteal region for acute pain treatment is not feasible in humans due to the inability to dissect and examine the nerve tissue. To overcome this issue, we used a rat model for demonstrating damage to the sciatic nerve tissue after the application of commonly used drug injections.

METHODS:

We investigated possible damage following the intramuscular injection of diclofenac, lornoxicam, morphine, and pethidine in a rat model based on histopathological characteristics such as myelin degeneration, axon degeneration, epineurium degeneration, fibrosis, epineurium thickening, perineurium thickening, lymphocyte infiltration, vacuolization, and edema.

RESULTS:

All the analgesic drugs used in our study induced histopathological changes in the sciatic nerve. Anti-S100 positivity, showing nerve damage, was found to be the lowest in the group treated with diclofenac. Neurotoxic effects of diclofenac on the sciatic nerve were greater than those of the other drugs used in the study. Lornoxicam induced the least histopathological changes in the nerve.

CONCLUSION:

Diclofenac induced severe nerve damage not only after direct injection in the sciatic nerve but also after injection in the area around the nerve. Thus, we recommend restricting the use of intramuscular gluteal injections of diclofenac. Intramuscular use of morphine and pethidine should also be overviewed.