Standard analgesics reverse burrowing deficits in a rat CCI model of neuropathic pain, but not in models of type 1 and type 2 diabetes-induced neuropathic pain

Strategies for Measuring Non-Evoked Pain in Preclinical Models of Neuropathic Pain: Systematic Review

The development of new analgesics for neuropathic pain treatment is crucial. The failure of promising drugs in clinical trials may be related to the over-reliance on reflex-based responses (evoked pain) in preclinical drug testing, which may not fully represent clinical neuropathic pain, characterized by spontaneous non-evoked pain (NEP). Hence, strategies for assessing NEP in preclinical studies emerged. This systematic review identified 443 articles evaluating NEP in neuropathic pain models (mainly traumatic nerve injuries in male rodents). An exponential growth in NEP evaluation was observed, which was assessed using 48 different tests classified in 12 NEP-related outcomes: anxiety, exploration/locomotion, paw lifting, depression, conditioned place preference, gait, autotomy, wellbeing, facial grooming, cognitive impairment, facial pain expressions and vocalizations. Although most of these outcomes showed clear limitations, our analysis suggests that conditioning-associated outcomes, pain-related comorbidities, and gait evaluation may be the most effective strategies. Moreover, a minimal part of the studies evaluated standard analgesics. The greater emphasis on evaluating NEP aligning with clinical pain symptoms may enhance analgesic drug development, improving clinical translation.

Effects of NADPH Oxidase Isoform-2 (NOX2) Inhibition on Behavioral Responses and Neuroinflammation in a Mouse Model of Neuropathic Pain

NADPH oxidase isoform-2 (NOX2) has been implicated in the pathophysiology of neuropathic pain (NP), mostly through the modulation of neuroinflammation. Since it is also accepted that some neuroimmune mechanisms underlying NP are sex-dependent, we aimed to evaluate the effects of early systemic treatment with the NOX2-selective inhibitor (NOX2i) GSK2795039 on behavioral responses and spinal neuroinflammation in spared nerve injury (SNI)-induced NP in male and female mice. Mechanical sensitivity was evaluated with the von Frey test, while general well-being and anxiety-like behavior were assessed with burrowing and light/dark box tests. Spinal microglial activation and cytokines IL-1β, IL-6, and IL-10, as well as macrophage colony-stimulating factor (M-CSF) were evaluated by immunofluorescence and multiplex immunoassay, respectively. NOX2i treatment reduced SNI-induced mechanical hypersensitivity and early SNI-induced microglial activation in both sexes. SNI-females, but not males, showed a transient reduction in burrowing activity. NOX2i treatment did not improve their burrowing activity, but tendentially reduced their anxiety-like behavior. NOX2i marginally decreased IL-6 in females, and increased M-CSF in males. Our findings suggest that NOX2-selective inhibition may be a potential therapeutic strategy for NP in both male and female individuals, with particular interest in females due to its apparent favorable impact in anxiety-like behavior.

Pain Recognition in Rodents

Available methods for recognizing and assessing pain in rodents have increased over the last 10 years, including the development of validated pain assessment scales. Much of this work has been driven by the needs of biomedical research, and there are specific challenges to applying these scales in the clinical environment. This article provides an introduction to pain assessment scale validation, reviews current methods of pain assessment, highlighting their strengths and weaknesses, and makes recommendations for assessing pain in a clinical environment.

Systematic review and meta-analysis of studies in which burrowing behaviour was assessed in rodent models of disease-associated persistent pain

ABSTRACT

Burrowing behaviour is used to assess pain-associated behaviour in laboratory rodents. To gain insight into how models of disease-associated persistent pain and analgesics affect burrowing behaviour, we performed a systematic review and meta-analysis of studies that assessed burrowing behaviour. A systematic search in March 2020 and update in September 2020 was conducted in 4 databases. Study design characteristics and experimental data were extracted, followed by a random-effects meta-analysis. We explored the association between burrowing and monofilament-induced limb withdrawal. Dose response relationship was investigated for some analgesics. Forty-five studies were included in the meta-analysis, in which 16 model types and 14 drug classes were used. Most experiments used rat (79%) and male (72%) animals. Somatic inflammation and trauma-induced neuropathy models were associated with reduced burrowing behaviour. Analgesics (nonsteroidal anti-inflammatory drug and gabapentinoids) attenuated burrowing deficits in these models. Reporting of measures to reduce risk of bias was unclear except for randomisation which was high. There was not a correlation ( R2 = 0.1421) between burrowing and monofilament-induced limb withdrawal. Opioids, gabapentin, and naproxen showed reduced burrowing behaviour at high doses, whereas ibuprofen and celecoxib showed opposite trend. The findings indicate that burrowing could be used to assess pain-associated behaviour. We support the use of a portfolio of composite measures including spontaneous and stimulus-evoked tests. The information collected here could help in designing experiments involving burrowing assessment in models of disease-associated pain.

Severity Assessment in Rats Undergoing Subarachnoid Hemorrhage Induction by Endovascular Perforation or Corresponding Sham Surgery

INTRODUCTION

Animal models for preclinical research of subarachnoid hemorrhage (SAH) are widely used as much of the pathophysiology remains unknown. However, the burden of these models inflicted on the animals is not well characterized. The European directive requires severity assessment-based allocation to categories. Up to now, the classification into predefined categories is rather subjective and often without underlying scientific knowledge. We therefore aimed at assessing the burden of rats after SAH or the corresponding sham surgery to provide a scientific assessment.

METHODS

We performed a multimodal approach, using different behavior tests, clinical and neurological scoring, and biochemical markers using the common model for SAH of intracranial endovascular filament perforation in male Wistar rats. Up to 7 days after surgery, animals with SAH were compared to sham surgery and to a group receiving only anesthesia and analgesia.

RESULTS

Sham surgery (n = 15) and SAH (n = 16) animals showed an increase in the clinical score the first days after surgery, indicating clinical deterioration, while animals receiving only anesthesia without surgery (n = 5) remained unaffected. Body weight loss occurred in all groups but was more pronounced and statistically significant only after surgery. The analysis of burrowing, open field (total distance, erections), balance beam, and neuroscore showed primarily an effect of the surgery itself in sham surgery and SAH animals. Only concerning balance beam and neuroscore, a difference was visible between sham surgery and SAH. The outcome of the analysis of systemic and local inflammatory parameters and of corticosterone in blood and its metabolites in feces was only robust in animals suffering from larger bleedings. Application of principal component analysis resulted in a clear separation of sham surgery and SAH animals from their respective baseline as well as from the anesthesia-only group at days 1 and 3, with the difference between sham surgery and SAH being not significant.

DISCUSSION/CONCLUSION

To our knowledge, we are the first to publish detailed clinical score sheet data combined with advanced behavioral assessment in the endovascular perforation model for SAH in rats. The tests chosen here clearly depict an impairment of the animals within the first days after surgery and are consequently well suited for assessment of the animals' suffering in the model. A definitive classification into one of the severity categories named by the EU directive is yet pending and has to be performed in the future by including the assessment data from different neurological and nonneurological disease models.

Peripheral Neuropathy Phenotyping in Rat Models of Type 2 Diabetes Mellitus: Evaluating Uptake of the Neurodiab Guidelines and Identifying Future Directions

Diabetic peripheral neuropathy (DPN) affects over half of type 2 diabetes mellitus (T2DM) patients, with an urgent need for effective pharmacotherapies. While many rat and mouse models of T2DM exist, the phenotyping of DPN has been challenging with inconsistencies across laboratories. To better characterize DPN in rodents, a consensus guideline was published in 2014 to accelerate the translation of preclinical findings. Here we review DPN phenotyping in rat models of T2DM against the 'Neurodiab' criteria to identify uptake of the guidelines and discuss how DPN phenotypes differ between models and according to diabetes duration and sex. A search of PubMed, Scopus and Web of Science databases identified 125 studies, categorised as either diet and/or chemically induced models or transgenic/spontaneous models of T2DM. The use of diet and chemically induced T2DM models has exceeded that of transgenic models in recent years, and the introduction of the Neurodiab guidelines has not appreciably increased the number of studies assessing all key DPN endpoints. Combined high-fat diet and low dose streptozotocin rat models are the most frequently used and well characterised. Overall, we recommend adherence to Neurodiab guidelines for creating better animal models of DPN to accelerate translation and drug development.

Burrowing behaviour of rats: Strain differences and applicability as well-being parameter after intracranial surgery

In mice, burrowing is considered a species-typical parameter for assessing well-being, while this is less clear in rats. This exploratory study evaluated burrowing behaviour in three rat strains during training and in the direct postoperative phase after complex intracranial surgery in different neuroscience rat models established at Hannover Medical School or Aachen University Hospital. Male Crl:CD (SD; n = 18), BDIX/UlmHanZtm (BDIX; n = 8) and RjHan:WI (Wistar; n = 35) rats were individually trained to burrow gravel out of a tube on four consecutive days. Thereafter, BDIX rats were subjected to intracranial injection of BT4Ca cells and tumour resection (rat glioma model), SD rats to injection of 6-hydroxydopamine (6-OHDA) or vehicle (rat Parkinson's disease model) and Wistar rats to endovascular perforation or sham surgery (rat subarachnoid haemorrhage (SAH) model). Burrowing was retested on the day after surgery. During training, BDIX rats burrowed large amounts (mean of 2370 g on the fourth day), while SD and Wistar rats burrowed less gravel (means of 846 and 520 g, respectively). Burrowing increased significantly during training only in Wistar rats. Complex surgery, that is, tumour resection (BDIX), 6-OHDA injection (SD) and endovascular perforation or sham surgery for SAH (Wistar) significantly reduced burrowing and body weight, while simple stereotactic injection of tumour cells or vehicle did not affect burrowing. Despite the training, burrowing differed between the strains. In the direct postoperative phase, burrowing was reduced after complex surgery, indicating reduced well-being. Reduced burrowing was accompanied with postoperative weight loss, a validated and recognised quantitative measure for severity assessment.

Effects of Ketoprofen and Morphine on Pain-Related Depression of Nestlet Shredding in Male and Female Mice

A primary goal in pain treatment is restoration of behaviors that are disrupted by pain. Measures of pain interference indicate the degree to which pain interferes with activities in pain patients, and these measures are used to evaluate the effects of analgesic drugs. As a result of the emphasis on the expression and treatment of functional impairment in clinical settings, preclinical pain researchers have attempted to develop procedures for evaluation of pain-related functional impairment in laboratory animals. The goal of the present study was to develop and validate a low cost procedure for the objective evaluation of pain-related depression of home cage behavior in mice. On test days, a 5 × 5 cm Nestlet was weighed prior to being suspended from the wire lid of the home cage of individually housed male and female ICR mice. Over the course of experimental sessions, mice removed pieces of the suspended Nestlet, and began to build a nest with the material they removed. Thus, the weight of the pieces of Nestlet that remained suspended at various time points in the session provided an indicator of the rate of this behavior. The results indicate that Nestlet shredding was stable with repeated testing, and shredding was depressed by intra-peritoneal injection of 0.32% lactic acid. The non-steroidal anti-inflammatory drug ketoprofen blocked 0.32% lactic acid-induced depression of shredding, but did not block depression of shredding by a pharmacological stimulus, the kappa opioid receptor agonist U69,593. The μ-opioid receptor agonist morphine did not block 0.32% lactic acid-induced depression of shredding when tested up to doses that depressed shredding in the absence of lactic acid. When noxious stimulus intensity was reduced by decreasing the lactic acid concentration to 0.18%, morphine was effective at blocking pain-related depression of behavior. In summary, the data from the present study support consideration of the Nestlet shredding procedure for use in studies examining mechanisms, expression, and treatment of pain-related functional impairment.

Preclinical Neuropathic Pain Assessment; the Importance of Translatability and Bidirectional Research

For patients suffering with chronic neuropathic pain the need for suitable novel therapies is imperative. Over recent years a contributing factor for the lack of development of new analgesics for neuropathic pain has been the mismatch of primary neuropathic pain assessment endpoints in preclinical vs. clinical trials. Despite continuous forward translation failures across diverse mechanisms, reflexive quantitative sensory testing remains the primary assessment endpoint for neuropathic pain and analgesia in animals. Restricting preclinical evaluation of pain and analgesia to exclusively reflexive outcomes is over simplified and can be argued not clinically relevant due to the continued lack of forward translation and failures in the clinic. The key to developing new analgesic treatments for neuropathic pain therefore lies in the development of clinically relevant endpoints that can translate preclinical animal results to human clinical trials. In this review we discuss this mismatch of primary neuropathic pain assessment endpoints, together with clinical and preclinical evidence that supports how bidirectional research is helping to validate new clinically relevant neuropathic pain assessment endpoints. Ethological behavioral endpoints such as burrowing and facial grimacing and objective measures such as electroencephalography provide improved translatability potential together with currently used quantitative sensory testing endpoints. By tailoring objective and subjective measures of neuropathic pain the translatability of new medicines for patients suffering with neuropathic pain will hopefully be improved.

The role of P2X4 receptor in neuropathic pain and its pharmacological properties

Neuropathic pain (NPP) is a common symptom of most diseases in clinic, which seriously affects the mental health of patients and brings certain pain to patients. Due to its pathological mechanism is very complicated, and thus, its treatment has been one of the challenges in the field of medicine. Therefore, exploring the pathogenesis and treatment approach of NPP has aroused the interest of many researchers. ATP is an important energy information substance, which participates in the signal transmission in the body. The P2 × 4 receptor (P2 × 4R) is dependent on ATP ligand-gated cationic channel receptor, which can be activated by ATP and plays an important role in the transmission of information in the nervous system and the formation of pain. In this paper, we provide a comprehensive review of the structure and function of the P2 × 4R gene. We also discuss the pathogenesis of NPP and the intrinsic relationship between P2 × 4R and NPP. Moreover, we explore the pharmacological properties of P2 × 4R antagonists or inhibitors used as targeted therapies for NPP.

Antihyperalgesic effects of Meteorin in the rat chronic constriction injury model: a replication study

Careful attention to study design, bioactive material, and drug exposure was used in replication of a single study supporting efficacy of Meteorin in experimental neuropathic pain.

Data from preclinical research have been suggested to suffer from a lack of inherent reproducibility across laboratories. The goal of our study was to replicate findings from a previous report that demonstrated positive effects of Meteorin, a novel neurotrophic factor, in a rat model of neuropathic pain induced by chronic constriction injury (CCI). Notably, 5 to 6 intermittent subcutaneous (s.c.) injections of Meteorin had been reported to produce reversal of mechanical allodynia/thermal hyperalgesia after injury, wherein maximum efficacy of Meteorin was reached slowly and outlasted the elimination of the compound from the blood by several weeks. Here, we evaluated the efficacy of Meteorin in reversing hindpaw mechanical hyperalgesia and cold allodynia in male, Sprague-Dawley rats with CCI. Nociceptive behavior was monitored before and after CCI, and after drug treatment until day 42 after injury. Systemic administration of recombinant mouse Meteorin (0.5 and 1.8 mg/kg, s.c.) at days 10, 12, 14, 17, and 19 after CCI produced a prolonged reversal of neuropathic hypersensitivity with efficacy comparable with that obtained with gabapentin (100 mg/kg, orally). Despite some protocol deviations (eg, nociceptive endpoint, animal vendor, testing laboratory, investigator, etc.) being incurred, these did not affect study outcome. By paying careful attention to key facets of study design, using bioactive material, and confirming drug exposure, the current data have replicated the salient findings of the previous study, promoting confidence in further advancement of this novel molecule as a potential therapy for neuropathic pain.

A Thermal Place Preference Test for Discovery of Neuropathic Pain Drugs

Developing potent non-opioid pain medications is an integral part of the battle to conquer both chronic pain and the current opioid crisis. Although most screening approaches use in vitro surrogate targets, in vivo screening of analgesic candidates is a necessary preclinical step in drug discovery. Here, we report the design of a new automated behavioral testing apparatus based on the principle of a thermal place preference test (TPPT). This new design can detect, quantify, and differentiate behavioral responses to cold stimuli between sham and chronic constriction injury (CCI) rodents with up to 12 animals tested simultaneously. At an optimized temperature pair of 12.5 °C vs 30.0 °C (±0.5 °C), the TPPT design has captured the antinociceptive effects of morphine and pregabalin on CCI rats in individual 10 min tests. Moreover, it can differentiate analgesic effects by morphine or pregabalin from anxiolytic effects by diazepam. The results, along with the relatively low cost to construct the apparatus and moderately high throughput, make our TPPT design applicable for behavioral studies of chronic pain in rodents and for high-throughput in vivo screening of the next generation of pain medications.

The search for translational pain outcomes to refine analgesic development: Where did we come from and where are we going?

Pain measures traditionally used in rodents record mere reflexes evoked by sensory stimuli; the results thus may not fully reflect the human pain phenotype. Alterations in physical and emotional functioning, pain-depressed behaviors and facial pain expressions were recently proposed as additional pain outcomes to provide a more accurate measure of clinical pain in rodents, and hence to potentially enhance analgesic drug development. We aimed to review how preclinical pain assessment has evolved since the development of the tail flick test in 1941, with a particular focus on a critical analysis of some nonstandard pain outcomes, and a consideration of how sex differences may affect the performance of these pain surrogates. We tracked original research articles in Medline for the following periods: 1973-1977, 1983-1987, 1993-1997, 2003-2007, and 2014-2018. We identified 606 research articles about alternative surrogate pain measures, 473 of which were published between 2014 and 2018. This indicates that preclinical pain assessment is moving toward the use of these measures, which may soon become standard procedures in preclinical pain laboratories.

Cancer-induced Bone Pain Impairs Burrowing Behaviour in Mouse and Rat

BACKGROUND

Cancer-induced bone pain remains a serious public health concern, with a need for translational behavioural tests in order to assess nociception in preclinical models of this condition. Burrowing is an innate, ethologically relevant rodent behaviour that has been proven sensitive to chronic pain conditions. Herein, we studied for the first time whether burrowing performance is altered in preclinical models of cancer-induced bone pain.

MATERIALS AND METHODS

Mice and rats were inoculated with syngeneic breast cancer cells. Bone degradation was radiographically evaluated and nociception was assessed in limb-use and burrowing tests.

RESULTS

Cancer-bearing rodents showed reduced relative bone density and limb-use scores, confirming disease development. Burrowing performance decreased over time in both rodent models.

CONCLUSION

Burrowing performance was reduced in both rodent models, indicating that the burrowing test is a relevant and reproducible behavioural test for assessing disease development in both mouse and rat models of cancer-induced bone pain.

A Review of Pain Assessment Methods in Laboratory Rodents

Ensuring that laboratory rodent pain is well managed underpins the ethical acceptability of working with these animals in research. Appropriate treatment of pain in laboratory rodents requires accurate assessments of the presence or absence of pain to the extent possible. This can be challenging some situations because laboratory rodents are prey species that may show subtle signs of pain. Although a number of standard algesiometry assays have been used to assess evoked pain responses in rodents for many decades, these methods likely represent an oversimplification of pain assessment and many require animal handling during testing, which can result in stress-induced analgesia. More recent pain assessment methods, such as the use of ethograms, facial grimace scoring, burrowing, and nest-building, focus on evaluating changes in spontaneous behaviors or activities of rodents in their home environments. Many of these assessment methods are time-consuming to conduct. While many of these newer tests show promise for providing a more accurate assessment of pain, most require more study to determine their reliability and sensitivity across a broad range of experimental conditions, as well as between species and strains of animals. Regular observation of laboratory rodents before and after painful procedures with consistent use of 2 or more assessment methods is likely to improve pain detection and lead to improved treatment and care-a primary goal for improving overall animal welfare.

Toward evidence-based severity assessment in rat models with repeated seizures: II. Chemical post-status epilepticus model

OBJECTIVE

Considering the complexity of neuronal circuits and their epilepsy-associated alterations, epilepsy models cannot be completely replaced by in vitro experimental approaches. Decisions about ethical approval of in vivo studies require a thorough weighing of the animal's burden and the benefit regarding the expected gain in knowledge.

METHODS

Based on combined behavioral, biochemical, and physiological analyses, we assessed the impact on animal well-being and condition in different phases of the pilocarpine post-status epilepticus (SE) model in rats.

RESULTS

As a consequence of SE, increased levels of impairment were evident in the early postinsult phase and late chronic phase, whereas only mild impairment was observed in the interim phase. Parameters that stood out as sensitive indicators of animal distress include burrowing, which proved to be affected throughout all experimental phases, saccharin preference, fecal corticosterone metabolites, heart rate, and heart rate variability.

SIGNIFICANCE

The cumulative burden with temporary but not long-lasting phases of more pronounced impairment suggests a classification of severe as a basis for laboratory-specific prospective and retrospective evaluation. Among the parameters analyzed, burrowing behavior and saccharin preference stand out as candidate parameters that seem to be well suited to obtain information about animal distress in epileptogenesis models.

Toward evidence-based severity assessment in rat models with repeated seizures: III. Electrical post-status epilepticus model

OBJECTIVE

Ethical approval of experiments in chronic epilepsy models requires a careful balancing of the expected gain-in-knowledge with the level of distress. Thus recommendations for evidence-based severity assessment and classification are urgently needed for preclinical epilepsy research.

METHODS

Therefore, we have completed a comprehensive analysis of alterations in behavioral, biochemical, and physiological parameters in a rat electrical post-status epilepticus model. Selected parameters were repeatedly analyzed during different experimental phases to obtain information about the level of distress throughout the course of the model.

RESULTS

Behavioral patterns comprised an increase in activity along with a reduction in risk assessment behavior, active social interaction, saccharin preference as well as nonessential, but evolutionary-determined behavior such as nest building and burrowing. Among the biochemical parameters, fecal corticosterone metabolites proved to be increased in different phases of the experiment. In the early post-insult phase, this increase was reflected by elevated serum corticosterone concentrations. Telemetric recordings demonstrated increases in home cage activity and heart rate in selected experimental phases but argued against relevant changes in heart rate variability. Comparison between animals with tethered or telemetric recordings including a principal component analysis revealed differences between both groups.

SIGNIFICANCE

The present findings further confirm that burrowing behavior and saccharin preference might serve as valid parameters for severity assessment in chronic epilepsy models. Considering the course of alterations providing evidence for a more pronounced level of distress in the early phase following status epilepticus (SE), we suggest a classification of the electrical post-SE model as severe. This suggestion may serve as a guidance for laboratory-specific evaluations. Comparison between data from animals with tethered and telemetric recordings indicated an impact of the mode of recordings. However, further research is necessary to analyze the validity of telemetry as a putative refinement measure.