A rat model of full thickness thermal injury characterized by thermal hyperalgesia, mechanical allodynia, pronociceptive peptide release and tramadol analgesia

Identifying Stress-Exacerbated Thermal-Injury Induced MicroRNAs

Using a model of combat and operational stress reaction (COSR), our lab recently showed that exposure to an unpredictable combat stress (UPCS) procedure prior to a thermal injury increases pain sensitivity in male rats. Additionally, our lab has recently shown that circulating extracellular vesicle-microRNAs (EV-miRNAs), which normally function to suppress inflammation, were downregulated in a male rat model of neuropathic pain. In this current study, male and female rats exposed to UPCS, followed by thermal injury, were evaluated for changes in circulating EV-miRNAs. Adult female and male Sprague Dawley rats were exposed to a UPCS procedure for either 2 or 4 weeks. Groups consisted of the following: nonstress (NS), stress (S), NS + thermal injury (TI), and S + TI. Mechanical sensitivity was measured, and plasma was collected at baseline, throughout the UPCS exposure, and post-thermal injury. EV-miRNA isolation was performed, followed by small RNA sequencing and subsequent data analysis. UPCS exposure alone resulted in mechanical allodynia in both male and female rats at specific time points. Thermal-injury induction occurring at peak UPCS resulted in increased mechanical allodynia in the injured hind paw compared to thermal injury alone. Differential expression of the EV-miRNAs was observed between the NS and S groups as well as between NS + TI and S + TI groups. Consistent differences in EV-miRNAs are detectable in both COSR as well as during the development of mechanical sensitivity and potentially serve as key regulators, biomarkers, and targets in the treatment of COSR and thermal-injury induced mechanical sensitivity. PERSPECTIVE: This article presents the effects of unpredictable combat stress and thermal injury on EV-contained microRNAs in an animal model. These same mechanisms may exist in clinical patients and could be future prognostic and diagnostic biomarkers.

Exploring combat stress exposure effects on burn pain in a female rodent model

In the military, constant physiological and psychological stress encountered by Soldiers can lead to development of the combat and operational stress reaction (COSR), which can effect pain management. Similar effects are seen in other populations subjected to high levels of stress. Using a model of COSR, our lab recently showed that four weeks of stress prior to an injury increases pain sensitivity in male rats. With the roles of women in the military expanding and recent studies indicating sex differences in stress and pain processing, this study sought to investigate how different amounts of prior stress exposure affects thermal injury-induced mechanosensitivity in a female rat model of COSR. Adult female Sprague Dawley rats were exposed to the unpredictable combat stress (UPCS) procedure for either 2 or 4 weeks. The UPCS procedure included exposure to one stressor each day for four days. The stressors include: (1) sound stress for 30 min, (2) restraint stress for 4 h, (3) cold stress for 4 h, and (4) forced swim stress for 15 min. The order of stressors was randomized weekly. Mechanical and thermal sensitivity was tested twice weekly. After the UPCS procedure, a sub-set of rats received a thermal injury while under anesthesia. The development of mechanical allodynia and thermal hyperalgesia was examined for 14 days post-burn. UPCS exposure increased mechanosensitivity after two weeks. Interestingly, with more stress exposure, females seemed to habituate to the stress, causing the stress-induced changes in mechanosensitivity to decrease by week three of UPCS. If thermal injury induction occurred during peak stress-induced mechanosensitivity, after two weeks, this resulted in increased mechanical allodynia in the injured hind paw compared to thermal injury alone. This data indicates a susceptibility to increased nociceptive sensitization when injury is sustained at peak stress reactivity. Additionally, this data indicates a sex difference in the timing of peak stress. Post-mortem examination of the prefrontal cortex (PFC) showed altered expression of p-TrkB in 4-week stressed animals given a thermal injury, suggesting a compensatory mechanism. Future work will examine treatment options for preventing stress-induced pain to maintain the effectiveness and readiness of the Warfighter.

Pharmacological blockade of neurokinin1 receptor restricts morphine-induced tolerance and hyperalgesia in the rat

OBJECTIVES

The most notable adverse side effects of chronic morphine administration include tolerance and hyperalgesia. This study investigated the involvement of dorsal root ganglion (DRG) protein kinase Cɛ (PKCɛ) expression during chronic morphine administration and also considered the relationship between DRG PKCɛ expression and the substance P- neurokinin1 receptor (SP- NK1R) activity.

METHODS

Thirty-six animals were divided into six groups (n=6) in this study. In the morphine and sham groups, rats received 10 µg intrathecal (i.t.) morphine or saline for eight consecutive days, respectively. Behavioral tests were performed on days 1 and 8 before and after the first injections and then 48 h after the last injection (day 10). In the treatment groups, rats received NK1R antagonist (L-732,138, 25 µg) daily, either alone or 10 min before a morphine injection, Sham groups received DMSO alone or 10 min before a morphine injection. Animals were sacrificed on days 8 and 10, and DRG PKCɛ and SP expression were analyzed by western blot and immunohistochemistry techniques, respectively.

RESULTS

Behavioral tests indicated that tolerance developed following eight days of chronic morphine injection. Hyperalgesia was induced 48 h after the last morphine injection. Expression of SP and PKCɛ in DRG significantly increased in rats that developed morphine tolerance on day 8 and hyperalgesia on day 10, respectively. NK1R antagonist (L-732,138) not only blocked the development of hyperalgesia and the increase of PKCɛ expression but also alleviated morphine tolerance.

CONCLUSIONS

Our results provide evidence that DRG PKCɛ and SP-NK1R most likely participated in the generation of morphine tolerance and hyperalgesia. Pharmacological inhibition of SP-NK1R activity in the spinal cord suggests a role for NK1R and in restricting some side effects of chronic morphine. All experiments were performed by the National Institute of Health (NIH) Guidelines for the Care and Use of Laboratory Animals (NIH Publication No. 80-23, revised1996) and were approved by the Animal Ethics Committee of Shahid Beheshti University of Medical Sciences, Tehran, Iran (IR.SBMU.MSP.REC.1396.130).

Experimental models for controlled burn injuries in rats: a systematic analysis of original methods and burn devices

AIM

Despite a wide variety of models found in literature, choosing the right one can be difficult as many of them are lacking precise methodology. This study aims to analyze and compare original burn models in terms of burn device and technique, parameters, and wound depth assessment.

METHODS

A systematic search was performed according to PRISMA guidelines on studies describing original experimental burn models in rats. The adapted PICO formula and ARRIVE checklist were followed for inclusion and assessment of quality of studies. Characteristics of animals, burn technique, burn parameters and method of histological confirmation of burn depth were recorded.

RESULTS

Twenty-seven studies were included in the final analysis. Most studies used direct contact with skin for burn infliction (n=20). The rat's dorsum was the most common site (n=18). Ten studies used manually controlled burn devices, while ten designed automatic burn devices with control over temperature (n=10), exposure time (n=5), and pressure (n=5). Most studies (n=7) used a single biopsy taken from the center of the wound to confirm burn depth immediately after burn infliction.

CONCLUSION

From the wide variety of burn models in current literature, our study provides an overview of the most relevant experimental burn models in rats aiding researchers to understand what needs to be addressed when designing their burn protocol. Models cannot be compared as burn parameters variate significantly. Assessment of burn depth should be done in a standardized, sequential fashion in future burn studies to increase reproducibility.

A novel excisional wound pain model for evaluation of analgesics in rats

Background

Management of pain from open wounds is a growing unmet healthcare need. However, the models available to study pain from wounds or to develop analgesics for the patients suffering from them have primarily relied on incisional models. Here, we present the first characterized and validated model of open wound pain.

Methods

Unilateral full-skin excisional punch biopsy wounds on rat hind paws were evaluated for evoked pain using withdrawal responses to mechanical and thermal stimulation, and spontaneous pain was measured using hind paw weight distribution and guarding behavior. Evaluations were done before wounding (baseline) and 2-96 hours post-wounding. The model was validated by testing the effects of buprenorphine and carprofen.

Results

Pain responses to all tests increased within 2 hours post-wounding and were sustained for at least 4 days. Buprenorphine caused a reversal of all four pain responses at 1 and 4 hours post-treatment compared to 0.9% saline (P < 0.001). Carprofen decreased the pain response to thermal stimulation at 1 (P ≤ 0.049) and 4 hours (P < 0.011) post-treatment compared to 0.9% saline, but not to mechanical stimulation.

Conclusions

This is the first well-characterized and validated model of pain from open wounds and will allow study of the pathophysiology of pain in open wounds and the development of wound-specific analgesics.

Analgesic and Antiallodynic Effects of 4-Fluoro-N-(4-Sulfamoylbenzyl) Benzene Sulfonamide in a Murine Model of Pain

INTRODUCTION

Physical, chemical, thermal injuries along with infectious diseases lead to acute pain with associated inflammation, being the primary cause of hospital visits. Moreover, neuropathic pain associated with diabetes is a serious chronic disease leading to high morbidity and poor quality of life.

OBJECTIVE

Earlier multiple sulphonamides have been reported to have an antinociceptive and antiallodynic profile. 4-Fluoro-N-(4-sulfamoylbenzyl) Benzene Sulfonamide (4-FBS), a synthetic sulfonamide with reported carbonic anhydrase inhibitory activity, was investigated for its potential effects in mice model of acute and diabetic neuropathic pain.

METHODS AND RESULTS

4-FBS was given orally (p.o.) one hour before the test and then mice were screened for antinociceptive activity by using the tail immersion test, which showed significant antinociceptive effect at both 20 and 40 mg/kg doses. To explore the possible mechanisms, thermal analgesia of 4-FBS was reversed by the 5HT3 antagonist ondansetron 1mg/kg intraperitoneally (i.p.) and by the µ receptor antagonist naloxone (1 mg/kg i.p.), implying possible involvement of serotonergic and opioidergic pathways in the analgesic effect of 4-FBS. Diabetes was induced in mice by a single dose of streptozotocin (STZ) 200 mg/kg i.p. After two weeks, animals first became hyperalgesic and progressively allodynic in the fourth week, which was evaluated through behavioral parameters like thermal and mechanical tests. 4-FBS at 20 and 40 mg/kg p.o. significantly reversed diabetes-induced hyperalgesia and allodynia at 30, 60, 90, and 120 minutes.

CONCLUSION

These findings are significant and promising while further studies are warranted to explore the exact molecular mechanism and the potential of 4-FBS in diabetic neuropathic pain.

Dual-Acting Peripherally Restricted Delta/Kappa Opioid (CAV1001) Produces Antinociception in Animal Models of Sub-Acute and Chronic Pain

Background

The development of highly efficacious alternatives to mu-opioid analgesics represents an urgent unmet medical and public health need. In the presence of inflammation both delta- and kappa-opioid agonists, acting on peripheral sensory neurons, mediate analgesia. The dual-acting, peripherally restricted kappa/delta-opioid agonist, CAV1001, was tested in four rodent pain models.

Methods

Experiment 1 – Formalin testing in mice. Three doses (1–10 mg/kg) of CAV1001 or ICI204448 at 30 minutes were tested after formalin injection. Spontaneous nocifensive responses were video recorded. Experiment 2 – Complete Freund’s Adjuvant (CFA)-induced arthritis. CFA was injected into the ankle joint of rats. Joint compression thresholds (JCT) were measured. CAV1001 was compared to celecoxib. Experiment 3 – Spinal nerve ligation (SNL) in rats. Paw compression thresholds (PCT) were measured. CAV1001 was compared to gabapentin. Experiment 4 – MMRT-1 bone cancer implantation into the rat tibia. Weight-bearing was assessed. CAV1001 was compared to morphine.

Results

In Phase 2 of the formalin model, CAV1001 (1 mg/kg) significantly reduced pain behaviors to a degree comparable to the peripherally restricted kappa-opioid agonist, ICI204448 (10 mg/kg). CAV1001 (10 mg/kg) effectively eliminated pain behaviors associated with phase 2. In the CFA-induced arthritis model, a significant increase in JCTs, similar to the comparator celecoxib, was observed with CAV1001 at 1 mg/kg at 2 hours; CAV1001 (10 mg/kg) was effective at 1 hour. In the SNL model, both the comparator gabapentin and CAV1001 (5 mg/kg) significantly reduced PCT at 2 hours, but at 4 hours, the CAV1001 thresholds improved to baseline. CAV1001 10 mg/kg significantly improved weight bearing at 4-hour post-dosing compared to baseline following MMRT-1 implantation.

Conclusion

CAV1001 demonstrated efficacy in several different preclinical pain models. Time- and dose-dependent differences in the efficacy of CAV1001 amongst these rodent pain models parallel the degree of underlying inflammation.

Clinical Management of Pain in Rodents

The use of effective regimens for mitigating pain remain underutilized in research rodents despite the general acceptance of both the ethical imperative and regulatory requirements intended to maximize animal welfare. Factors contributing to this gap between the need for and the actual use of analgesia include lack of sufficient evidence-based data on effective regimens, under-dosing due to labor required to dose analgesics at appropriate intervals, concerns that the use of analgesics may impact study outcomes, and beliefs that rodents recover quickly from invasive procedures and as such do not need analgesics. Fundamentally, any discussion of clinical management of pain in rodents must recognize that nociceptive pathways and pain signaling mechanisms are highly conserved across mammalian species, and that central processing of pain is largely equivalent in rodents and other larger research species such as dogs, cats, or primates. Other obstacles to effective pain management in rodents have been the lack of objective, science-driven data on pain assessment, and the availability of appropriate pharmacological tools for pain mitigation. To address this deficit, we have reviewed and summarized the available publications on pain management in rats, mice and guinea pigs. Different drug classes and specific pharmacokinetic profiles, recommended dosages, and routes of administration are discussed, and updated recommendations are provided. Nonpharmacologic tools for increasing the comfort and wellbeing of research animals are also discussed. The potential adverse effects of analgesics are also reviewed. While gaps still exist in our understanding of clinical pain management in rodents, effective pharmacologic and nonpharmacologic strategies are available that can and should be used to provide analgesia while minimizing adverse effects. The key to effective clinical management of pain is thoughtful planning that incorporates study needs and veterinary guidance, knowledge of the pharmacokinetics and mechanisms of action of drugs being considered, careful attention to individual differences, and establishing an institutional culture that commits to pain management for all species as a central component of animal welfare.

A novel rat model of extremity trauma for prehospital pain management research

BACKGROUND

Pain management is important in prehospital care of patients with extremity trauma (ET). The goal of this study was to establish a rat model of ET for prehospital pain research and validate it using pain behaviors and analgesics.

METHODS

Rats were anesthetized using isoflurane, and ET was induced in one hindlimb via clamping retrofemoral tissues for 30 seconds, followed by closed fibula fracture. Rats regained consciousness after ET. Pain responses in the injured hindlimb to thermal hyperalgesia (paw withdrawal latency [PWL]), mechanical allodynia (paw withdrawal pressure [PWP]), and weight bearing (WB) were determined before and 90 minutes after ET. Morphine (2 mg/kg), fentanyl (10 μg/kg), sufentanil (1 μg/kg), ketamine (5 mg/kg), or vehicle (saline) were then administered via intravenous (i.v.) injection, followed by PWL, PWP, and WB assessments at 10 minutes, 40 minutes, 80 minutes, and 120 minutes after analgesia.

RESULTS

After ET, PWL, PWP, and WB were significantly decreased by 61 ± 4%, 64 ± 8%, and 65 ± 4%, respectively, compared with pre-ET values. These pain behaviors were maintained for 3 hours to 4 hours. Compared with the saline group, opioid analgesics significantly increased PWL for at least 80 minutes, with sufentanil exhibiting the highest analgesic effect. An increase in PWL was only observed at 10 minutes after ketamine. The PWP was transiently increased with opioid analgesics for 10 minutes to 40 minutes, but was not changed with ketamine. Weight bearing was improved with opioid analgesics for at least 2 hours, but only for up to 80 minutes with ketamine.

CONCLUSION

Our ET model includes long bone fracture and soft tissue injury, but no fixation surgery, mimicking prehospital ET. Our model produces acute, steady, and reproducible trauma-related pain behaviors, and is clinically relevant regarding the pain behaviors and established responses to common analgesics. This model of acute pain due to ET is ideal for prehospital pain management research.

Revisiting Tramadol: A Multi-Modal Agent for Pain Management

Tramadol-an atypical opioid analgesic-has a unique pharmacokinetic and pharmacodynamic profile, with opioidergic, noradrenergic, and serotonergic actions. Tramadol has long been used as a well-tolerated alternative to other drugs in moderate pain because of its opioidergic and monoaminergic activities. However, cumulative evidence has been gathered over the last few years that supports other likely mechanisms and uses of tramadol in pain management. Tramadol has modulatory effects on several mediators involved in pain signaling, such as voltage-gated sodium ion channels, transient receptor potential V1 channels, glutamate receptors, α₂-adrenoceptors, adenosine receptors, and mechanisms involving substance P, calcitonin gene-related peptide, prostaglandin E₂, and proinflammatory cytokines. Tramadol also modifies the crosstalk between neuronal and non-neuronal cells in peripheral and central sites. Through these molecular effects, tramadol could modulate peripheral and central neuronal hyperexcitability. Given the broad spectrum of molecular targets, tramadol as a unimodal analgesic relieves a broad range of pain types, such as postoperative, low back, and neuropathic pain and that associated with labor, osteoarthritis, fibromyalgia, and cancer. Moreover, tramadol has anxiolytic, antidepressant, and anti-shivering activities that could improve pain management outcomes. The aim of this review was to address these issues in the context of maladaptive physiological and psychological processes that are associated with different pain types.

Involvement of brain-derived neurotrophic factor (BDNF) in chronic intermittent stress-induced enhanced mechanical allodynia in a rat model of burn pain

BACKGROUND

Reports show that stressful events before injury exacerbates post-injury pain. The mechanism underlying stress-induced heightened thermal pain is unclear. Here, we examined the effects of chronic intermittent stress (CIS) on nociceptive behaviors and brain-derived nerve growth factor (BDNF) system in the prefrontal cortex (PFC) and hypothalamus of rats with and without thermal injury.

RESULTS

Unstressed rats showed transient mechanical allodynia during stress exposure. Stressed rats with thermal injury displayed persistent exacerbated mechanical allodynia (P < 0.001). Increased expression of BDNF mRNA in the PFC (P < 0.05), and elevated TrkB and p-TrkB (P < 0.05) protein levels in the hypothalamus were observed in stressed rats with thermal injury but not in stressed or thermally injured rats alone. Furthermore, administration of CTX-B significantly reduced stress-induced exacerbated mechanical allodynia in thermally injured rats (P < 0.001).

CONCLUSION

These results indicate that BDNF-TrkB signaling in PFC and hypothalamus contributes to CIS-induced exacerbated mechanical allodynia in thermal injury state.

Burn Pain: A Systematic and Critical Review of Epidemiology, Pathophysiology, and Treatment

Objective

This review aims to examine the available literature on the epidemiology, pathophysiology, and treatment of burn-induced pain.

Methods

A search was conducted on the epidemiology of burn injury and treatment of burn pain utilizing the database Medline, and all relevant articles were systemically reviewed. In addition, a critical review was performed on the pathophysiology of burn pain and animal models of burn pain.

Results

The search on the epidemiology of burn injury yielded a total of 163 publications of interest, 72 of which fit the inclusion/exclusion criteria, with no publications providing epidemiological data on burn injury pain management outcomes. The search on the treatment of burn pain yielded a total of 213 publications, 14 of which fit the inclusion/exclusion criteria, highlighting the limited amount of evidence available on the treatment of burn-induced pain.

Conclusions

The pathophysiology of burn pain is poorly understood, with limited clinical trials available to assess the effectiveness of analgesics in burn patients. Further studies are needed to identify new pharmacological targets and treatments for the effective management of burn injury pain.

Local Resiniferatoxin Induces Long-Lasting Analgesia in a Rat Model of Full Thickness Thermal Injury

Objective

Opioid-based analgesics are a major component of the lengthy pain management of burn patients, including military service members, but are problematic due to central nervous system-mediated side effects. Peripheral analgesia via targeted ablation of nociceptive nerve endings that express the transient receptor potential vanilloid channel 1 (TRPV1) may provide an improved approach. We hypothesized that local injection of the TRPV1 agonist resiniferatoxin (RTX) would produce long-lasting analgesia in a rat model of pain associated with burn injury.

Methods

Baseline sensitivities to thermal and mechanical stimuli were measured in male and female Sprague-Dawley rats. Under anesthesia, a 100 °C metal probe was placed on the right hind paw for 30 seconds, and sensitivity was reassessed 72 hours following injury. Rats received RTX (0.25 μg/100 μL; ipl) into the injured hind paw, and sensitivity was reassessed across three weeks. Tissues were collected from a separate group of rats at 24 hours and/or one week post-RTX for pathological analyses of the injured hind paw, dorsal spinal cord c-Fos, and primary afferent neuropeptide immunoreactivity.

Results

Local RTX reversed burn pain behaviors within 24 hours, which lasted through recovery at three weeks. At one week following RTX, decreased c-Fos and primary afferent neuropeptide immunoreactivities were observed in the dorsal horn, while plantar burn pathology was unaltered.

Conclusions

These results indicate that local RTX induces long-lasting analgesia in a rat model of pain associated with burn. While opioids are undesirable in trauma patients due to side effects, RTX may provide valuable long-term, nonopioid analgesia for burn patients.

Nerve reinnervation and itch behavior in a rat burn wound model

In this study, we investigated whether postburn itch in rats, after a full thickness burn, is correlated to the nervous reinnervation of the burn wound area. For this purpose, we determined scratching duration (expressed as second/hour) at 24 hours, 2, 4, 8, and 12 weeks postburn and combined this with immunohistochemistry for protein gene product 9.5 (PGP9.5) to identify all nerve fibers, calcitonin gene related peptide (CGRP) to identify peptidergic fibers, tyrosine hydroxylase (TH) for sympathetic fibers, and growth-associated protein 43 (GAP-43) for regrowing fibers. We found a modest, but highly significant, increase in scratching duration of all burn wound rats from 3 to 12 weeks postburn (maximally 63 ± 9.5 second/hour compared to sham 3.1 ± 1.4 second/hour at 9 weeks). At 24 hours postburn, all nerve fibers had disappeared from the burn area. Around 4 weeks postburn PGP 9.5- and CGRP-immunoreactive nerve fibers returned to control levels. TH- and GAP-43-IR nerve fibers, which we found to be almost completely colocalized, did not regrow. No correlation was found between scratching duration and nervous reinnervation of the skin. The present results suggest that in rat, like in human, burn wound healing will induce increased scratching, which is not correlated to the appearance of nervous reinnervation.

Design and Testing of an Experimental Steam-Induced Burn Model in Rats

Background

Most of the current models for experimental burns pose difficulties in ensuring consistency and standardization.

Aim of Study

We aimed to develop an automated, reproducible technique for experimental burns using steam-based heat transfer.

Methods

The system developed for steam exposure was based on a novel, integrated, computer-controlled design. Three groups of rats were exposed to steam for 1, 3, and 7 seconds. The lesions were evaluated after 20 minutes, 48 hours, and 72 hours after burn induction.

Results

One-second steam application produced a superficial second-degree burn; three-second application induced deep second-degree burn; and seven-second application led to a third-degree burn.

Conclusion

The high level of automation of our integrated, computer-controlled system makes the difference between our system and other models, by ensuring the control of the duration of exposure, temperature, and pressure and eliminating as many potential human generated errors as possible. The automated system can accurately reproduce specific types of burns, according to histological assessment. This model could generate the reproducible data needed in the study of burn pathology and in order to assess new treatments.

Sound-stress-induced altered nociceptive behaviors are associated with increased spinal CRFR2 gene expression in a rat model of burn injury

Sound stress (SS) elicits behavioral changes, including pain behaviors. However, the neuronal mechanisms underlying SS-induced pain behaviors remain to be explored. The current study examined the effects of SS on nociceptive behaviors and changes in expression of the spinal corticotropin-releasing factor (CRF) system in male Sprague Dawley rats with and without thermal pain. We also studied the effects of SS on plasma corticosterone and fecal output. Rats were exposed to 3 days of SS protocol (n = 12/group). Changes in nociceptive behaviors were assessed using thermal and mechanical pain tests. Following the induction of SS, a subgroup of rats (n = 6/group) was inflicted with thermal injury and on day 14 postburn nociceptive behaviors were reassessed. Spinal CRF receptor mRNA expression was analyzed by semiquantitative reverse transcription polymerase chain reaction (RT-PCR). In addition, plasma corticosterone and spinal CRF concentrations were quantified using enzyme-linked immunosorbent assay (ELISA). Increased defecation was observed in SS rats. SS produced transient mechanical allodynia in naive rats, whereas it exacerbated thermal pain in thermally injured rats. Spinal CRFR2 mRNA expression was unaffected by stress or thermal injury alone, but their combined effect significantly increased its expression. SS had no effect on plasma corticosterone and spinal CRF protein in postburn rats. To conclude, SS is capable of exacerbating postburn thermal pain, which is linked to increased CRFR2 gene expression in the spinal cord. Future studies have to delineate whether attenuation of CRFR2 signaling at the spinal level prevents stress-induced exacerbation of burn pain.

Peripheral Neuropathy and Nerve Compression Syndromes in Burns

Peripheral neuropathy and nerve compression syndromes lead to substantial morbidity following burn injury. Patients present with pain, paresthesias, or weakness along a specific nerve distribution or experience generalized peripheral neuropathy. The symptoms manifest at various times from within one week of hospitalization to many months after wound closure. Peripheral neuropathy may be caused by vascular occlusion of vasa nervorum, inflammation, neurotoxin production leading to apoptosis, and direct destruction of nerves from the burn injury. This article discusses the natural history, diagnosis, current treatments, and future directions for potential interventions for peripheral neuropathy and nerve compression syndromes related to burn injury.

Endogenous Opiates and Behavior: 2015

This paper is the thirty-eighth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2015 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.

Effects of kynurenic acid analogue 1 (KYNA-A1) in nitroglycerin-induced hyperalgesia: Targets and anti-migraine mechanisms

Background Trigeminal sensitization represents a major mechanism underlying migraine attacks and their recurrence. Nitroglycerin (NTG) administration provokes spontaneous migraine-like headaches and in rat, an increased sensitivity to the formalin test. Kynurenic acid (KYNA), an endogenous regulator of glutamate activity and its analogues attenuate NTG-induced neuronal activation in the nucleus trigeminalis caudalis (NTC). The anti-hyperalgesic effect of KYNA analogue 1 (KYNA-A1) was investigated on animal models specific for migraine pain. Aim Rats made hyperalgesic by NTG administration underwent the plantar or orofacial formalin tests. The effect of KYNA-A1 was evaluated in terms of nocifensive behavior and of neuronal nitric oxide synthase (nNOS), calcitonin gene-related peptide (CGRP) and cytokines expression in areas involved in trigeminal nociception. Results KYNA-A1 abolished NTG-induced hyperalgesia in both pain models; NTG alone or associated to formalin injection induced an increased mRNA expression of CGRP, nNOS and cytokines in the trigeminal ganglia and central areas, which was reduced by KYNA-A1. Additionally, NTG caused a significant increase in nNOS immunoreactivity in the NTC, which was prevented by KYNA-A1. Conclusion Glutamate activity is likely involved in mediating hyperalgesia in an animal model specific for migraine. Its inhibition by means of a KYNA analogue modulates nNOS, CGRP and cytokines expression at peripheral and central levels.

Parecoxib increases muscle pain threshold and relieves shoulder pain after gynecologic laparoscopy: a randomized controlled trial

Objectives

Postlaparoscopic shoulder pain (PLSP) remains a common problem after laparoscopies. The aim of this study was to investigate the correlation between pressure pain threshold (PPT) of different muscles and PLSP after gynecologic laparoscopy, and to explore the effect of parecoxib, a cyclooxygenase-2 inhibitor, on the changes of PPT.

Materials and methods

The patients were randomly allocated into two groups; group P and group C. In group P, parecoxib 40 mg was intravenously infused at 30 minutes before surgery and 8 and 20 hours after surgery. In group C, normal saline was infused at the corresponding time point. PPT assessment was performed 1 day before surgery and at postoperative 24 hours by using a pressure algometer at bilateral shoulder muscles (levator scapulae and supraspinatus) and forearm (flexor carpi ulnaris). Meanwhile, bilateral shoulder pain was evaluated through visual analog scale score at 24 hours after surgery.

Results

Preoperative PPT level of the shoulder, but not of the forearm, was significantly and negatively correlated with the intensity of ipsilateral PLSP. In group C, PPT levels of shoulder muscles, but not of forearm muscles, decreased after laparoscopy at postoperative 24 hours. The use of parecoxib significantly improved the decline of PPT levels of bilateral shoulder muscles (all P<0.01). Meanwhile, parecoxib reduced the incidence of PLSP (group P: 45% vs group C: 83.3%; odds ratio: 0.164; 95% confidence interval: 0.07–0.382; P<0.001) and the intensity of bilateral shoulder pain (both P<0.01).

Conclusion

Preoperative PPT levels of shoulder muscles are closely associated with the severity of shoulder pain after gynecologic laparoscopy. PPT levels of shoulder muscles, but not of forearm muscles, significantly decreased after surgery. Parecoxib improved the decrease of PPT and relieved PLSP.

Progress of clinical practice on the management of burn-associated pain: Lessons from animal models

Opioid-based analgesics provide the mainstay for attenuating burn pain, but they have a myriad of side effects including respiratory depression, nausea, impaired gastrointestinal motility, sedation, dependence, physiologic tolerance, and opioid-induced hyperalgesia. To test and develop novel analgesics, validated burn-relevant animal models of pain are indispensable. Herein we review such animal models, which are mostly limited to rodent models of burn-induced, inflammatory, and neuropathic pain. The latter two are pain syndromes that provide insight into the pain caused by systemic pro-inflammatory cytokines and direct injury to nerves (e.g., after severe burn), respectively. To date, no single animal model optimally mimics the complex pathophysiology and pain that a human burn patient experiences. No currently available burn-pain model examines effects of pharmacological intervention on wound healing. As cornerstones of pain and wound healing, pro-inflammatory mediators may be utilized for insight into both processes. Moreover, common clinical concerns such as systemic inflammatory response syndrome and multiple organ dysfunction remain unaddressed. For development of analgesics, these aberrations can significantly alter the potential efficacy and/or adverse effects of a prescribed analgesic following burn trauma. We therefore suggest that a multi-model strategy would be the most clinically relevant when evaluating novel analgesics for use in burn patients.

Prior stress exposure increases pain behaviors in a rat model of full thickness thermal injury

Thermal burns among individuals working in highly stressful environments, such as firefighters and military Service Members, are common. Evidence suggests that pre-injury stress may exaggerate pain following thermal injury; however current animal models of burn have not evaluated the potential influence of pre-burn stress. This sham-controlled study evaluated the influence of prior stress exposure on post-burn thermal and mechanical sensitivity in male Sprague-Dawley rats. Rats were exposed to 20 min of inescapable swim stress or sham stress once per day for three days. Exposure to inescapable swim stress (1) increased the intensity and duration of thermal hyperalgesia after subsequent burn and (2) accelerated the onset of thermal hyperalgesia and mechanical allodynia after subsequent burn. This stress-induced exacerbation of pain sensitivity was reversed by pretreatment and concurrent treatment with the serotonin-norepinephrine reuptake inhibitor (SNRI) duloxetine. These data suggest a better understanding of mechanisms by which prior stress augments pain after thermal burn may lead to improved pain treatments for burn survivors.