The grimace scale reliably assesses chronic pain in a rodent model of trigeminal neuropathic pain

A novel animal model of neuropathic corneal pain-the ciliary nerve constriction model

Introduction

Neuropathic pain arises as a result of peripheral nerve injury or altered pain processing within the central nervous system. When this phenomenon affects the cornea, it is referred to as neuropathic corneal pain (NCP), resulting in pain, hyperalgesia, burning, and photoallodynia, severely affecting patients' quality of life. To date there is no suitable animal model for the study of NCP. Herein, we developed an NCP model by constriction of the long ciliary nerves innervating the eye.

Methods

Mice underwent ciliary nerve constriction (CNC) or sham procedures. Safety was determined by corneal fluorescein staining to assess ocular surface damage, whereas Cochet-Bonnet esthesiometry and confocal microscopy assessed the function and structure of corneal nerves, respectively. Efficacy was assessed by paw wipe responses within 30 seconds of applying hyperosmolar (5M) saline at Days 3, 7, 10, and 14 post-constriction. Additionally, behavior was assessed in an open field test (OFT) at Days 7, 14, and 21.

Results

CNC resulted in significantly increased response to hyperosmolar saline between groups (p < 0.0001), demonstrating hyperalgesia and induction of neuropathic pain. Further, animals that underwent CNC had increased anxiety-like behavior in an open field test compared to controls at the 14- and 21-Day time-points (p < 0.05). In contrast, CNC did not result in increased corneal fluorescein staining or decreased sensation as compared to sham controls (p > 0.05). Additionally, confocal microscopy of corneal whole-mounts revealed that constriction resulted in only a slight reduction in corneal nerve density (p < 0.05), compared to naïve and sham groups.

Discussion

The CNC model induces a pure NCP phenotype and may be a useful model for the study of NCP, recapitulating features of NCP, including hyperalgesia in the absence of ocular surface damage, and anxiety-like behavior.

Elevated Serotonin in Mouse Spinal Dorsal Horn Is Pronociceptive

Serotonergic neurons in the rostral ventral medulla (RVM) contribute to bidirectional control of pain through modulation of spinal and trigeminal nociceptive networks. Deficits in this pathway are believed to contribute to pathologic pain states, but whether changes in serotonergic mechanisms are pro- or antinociceptive is debated. We used a combination of optogenetics and fiber photometry to examine these mechanisms more closely. We find that optogenetic activation of RVM serotonergic afferents in the spinal cord of naive mice produces mechanical hypersensitivity and conditioned place aversion (CPA). Neuropathic pain, produced by chronic constriction injury of the infraorbital nerve (CCI-ION), evoked a tonic increase in serotonin (5HT) concentrations within the spinal trigeminal nucleus caudalis (SpVc), measured with liquid chromatography-tandem mass spectroscopy (LC-MS/MS). By contract, CCI-ION had no effect on the phasic serotonin transients in SpVc, evoked by noxious pinch, and measured with fiber photometry of a serotonin sensor. These findings suggest that serotonin release in the spinal cord is pronociceptive and that an increase in sustained serotonin signaling, rather than phasic or event driven increases, potentiate nociception in models of chronic pain.

Protease-Activated Receptor 2 (PAR2) Expressed in Sensory Neurons Contributes to Signs of Pain and Neuropathy in Paclitaxel Treated Mice

Chemotherapy-induced peripheral neuropathy (CIPN) is a common, dose-limiting side effect of cancer therapy. Protease-activated receptor 2 (PAR2) is implicated in a variety of pathologies, including CIPN. In this study, we demonstrate the role of PAR2 expressed in sensory neurons in a paclitaxel (PTX)-induced model of CIPN in mice. PAR2 knockout/wildtype (WT) mice and mice with PAR2 ablated in sensory neurons were treated with PTX administered via intraperitoneal injection. In vivo behavioral studies were done in mice using von Frey filaments and the Mouse Grimace Scale. We then examined immunohistochemical staining of dorsal root ganglion (DRG) and hind paw skin samples from CIPN mice to measure satellite cell gliosis and intra-epidermal nerve fiber (IENF) density. The pharmacological reversal of CIPN pain was tested with the PAR2 antagonist C781. Mechanical allodynia caused by PTX treatment was alleviated in PAR2 knockout mice of both sexes. In the PAR2 sensory neuronal conditional knockout (cKO) mice, both mechanical allodynia and facial grimacing were attenuated in mice of both sexes. In the DRG of the PTX-treated PAR2 cKO mice, satellite glial cell activation was reduced compared to control mice. IENF density analysis of the skin showed that the PTX-treated control mice had a reduction in nerve fiber density while the PAR2 cKO mice had a comparable skin innervation as the vehicle-treated animals. Similar results were seen with satellite cell gliosis in the DRG, where gliosis induced by PTX was absent in PAR cKO mice. Finally, C781 was able to transiently reverse established PTX-evoked mechanical allodynia. PERSPECTIVE: Our work demonstrates that PAR2 expressed in sensory neurons plays a key role in PTX-induced mechanical allodynia, spontaneous pain, and signs of neuropathy, suggesting PAR2 as a possible therapeutic target in multiple aspects of PTX CIPN.

An automated rat grimace scale for the assessment of pain

Pain is a complex neuro-psychosocial experience that is internal and private, making it difficult to assess in both humans and animals. In pain research, animal models are prominently used, with rats among the most commonly studied. The rat grimace scale (RGS) measures four facial action units to quantify the pain behaviors of rats. However, manual recording of RGS scores is a time-consuming process that requires training. While computer vision models have been developed and utilized for various grimace scales, there are currently no models for RGS. To address this gap, this study worked to develop an automated RGS system which can detect facial action units in rat images and predict RGS scores. The automated system achieved an action unit detection precision and recall of 97%. Furthermore, the action unit RGS classifiers achieved a weighted accuracy of 81-93%. The system's performance was evaluated using a blast traumatic brain injury study, where it was compared to trained human graders. The results showed an intraclass correlation coefficient of 0.82 for the total RGS score, indicating that the system was comparable to human graders. The automated tool could enhance pain research by providing a standardized and efficient method for the assessment of RGS.

Rat Grimace Scale as a Method to Evaluate Animal Welfare, Nociception, and Quality of the Euthanasia Method of Wistar Rats

Refinement of experimental procedures in animal research has the objective of preventing and minimizing pain/distress in animals, including the euthanasia period. This study aimed to evaluate pain associated with six methods of euthanasia in Wistar rats (injectable, inhalational, and physical), by applying the Rat Grimace Scale (RGS), comparing the scores, and determining the method with the highest score that might indicate pain for laboratory rodents. Sixty adult male and female Wistar rats were used and assigned to six treatments: pentobarbital, CO2, decapitation, isoflurane, ketamine + xylazine, and ketamine + CO2. Video recording to assess the RGS scores was performed in four events: basal: 24 h before the procedure; Ti1: three minutes before the procedure; Ti2: during the application of the euthanasia method; and Ti3: immediately after the application until LORR. The main findings of this study showed that, during Ti2, decapitation and ketamine + xylazine had the highest scores (0.6 ± 0.26 and 0.6 ± 0.16, respectively) (p < 0.0001), while at Ti3, CO2 (0.9 ± 0.18) and isoflurane (1.2 ± 0.20) recorded the highest scores (p < 0.0001). According to the present results, decapitation and ketamine + xylazine elicited short-term acute pain, possibly due to tissue damage caused by both methods (injection and guillotine). In contrast, isoflurane's RGS scores recorded during Ti3 might be associated with nociception/pain due to the pungency of the drug or to the pharmacological muscle relaxant effect of isoflurane. Further research is needed to establish a comprehensive study of pain during euthanasia, where RGS could be used minding the limitations that anesthetics might have on facial expression.

Progress in animal models of trigeminal neuralgia

OBJECTIVE

This review aims to systematically summarize the methods of establishing various models of trigeminal neuralgia (TN), the scope of application, and current animals used in TN research and the corresponding pain measurements, hoping to provide valuable reference for researchers to select appropriate TN animal models and make contributions to the research of pathophysiology and management of the disease.

DESIGN

The related literatures of TN were searched through PubMed database using different combinations of the following terms and keywords including but not limited: animal models, trigeminal neuralgia, orofacial neuropathic pain. To find the maximum number of eligible articles, no filters were used in the search. The references of eligible studies were analyzed and reviewed comprehensively.

RESULTS

This study summarized the current animal models of TN, categorized them into the following groups: chemical induction, photochemical induction, surgery and genetic engineering, and introduced various measurement methods to evaluate animal pain behaviors.

CONCLUSIONS

Although a variety of methods are used to establish disease models, there is no ideal TN model that can reflect all the characteristics of the disease. Therefore, there is still a need to develop more novel animal models in order to further study the etiology, pathological mechanism and potential treatment of TN.

Parabrachial Nucleus Activity in Nociception and Pain in Awake Mice

The parabrachial nuclear complex (PBN) is a nexus for aversion and for the sensory and affective components of pain perception. We have previously shown that during chronic pain PBN neurons in anesthetized rodents have amplified activity. We report a method to record from PBN neurons of behaving, head-restrained mice while applying reproducible noxious stimuli. We find that both spontaneous and evoked activity are higher in awake animals compared with urethane anesthetized mice. Fiber photometry of calcium responses from calcitonin-gene-related peptide-expressing PBN neurons demonstrates that these neurons respond to noxious stimuli. In both males and females with neuropathic or inflammatory pain, responses of PBN neurons remain amplified for at least 5 weeks, in parallel with increased pain metrics. We also show that PBN neurons can be rapidly conditioned to respond to innocuous stimuli after pairing with noxious stimuli. Finally, we demonstrate that changes in PBN neuronal activity are correlated with changes in arousal, measured as changes in pupil area.SIGNIFICANCE STATEMENT The parabrachial complex is a nexus of aversion, including pain. We report a method to record from parabrachial nucleus neurons of behaving mice while applying reproducible noxious stimuli. This allowed us to track parabrachial activity over time in animals with neuropathic or inflammatory pain. It also allowed us to show that the activity of these neurons correlates with arousal states and that these neurons can be conditioned to respond to innocuous stimuli.

Optogenetic cortical spreading depolarization induces headache-related behaviour and neuroinflammatory responses some prolonged in familial hemiplegic migraine type 1 mice

BACKGROUND

Cortical spreading depolarization (CSD), the neurophysiological correlate of the migraine aura, can activate trigeminal pain pathways, but the neurobiological mechanisms and behavioural consequences remain unclear. Here we investigated effects of optogenetically-induced CSDs on headache-related behaviour and neuroinflammatory responses in transgenic mice carrying a familial hemiplegic migraine type 1 (FHM1) mutation.

METHODS

CSD events (3 in total) were evoked in a minimally invasive manner by optogenetic stimulation through the intact skull in freely behaving wildtype (WT) and FHM1 mutant mice. Related behaviours were analysed using mouse grimace scale (MGS) scoring, head grooming, and nest building behaviour. Neuroinflammatory changes were investigated by assessing HMGB1 release with immunohistochemistry and by pre-treating mice with a selective Pannexin-1 channel inhibitor.

RESULTS

In both WT and FHM1 mutant mice, CSDs induced headache-related behaviour, as evidenced by increased MGS scores and the occurrence of oculotemporal strokes, at 30 min. Mice of both genotypes also showed decreased nest building behaviour after CSD. Whereas in WT mice MGS scores had normalized at 24 h after CSD, in FHM1 mutant mice scores were normalized only at 48 h. Of note, oculotemporal stroke behaviour already normalized 5 h after CSD, whereas nest building behaviour remained impaired at 72 h; no genotype differences were observed for either readout. Nuclear HMGB1 release in the cortex of FHM1 mutant mice, at 30 min after CSD, was increased bilaterally in both WT and FHM1 mutant mice, albeit that contralateral release was more pronounced in the mutant mice. Only in FHM1 mutant mice, contralateral release remained higher at 24 h after CSD, but at 48 h had returned to abnormal, elevated, baseline values, when compared to WT mice. Blocking Panx1 channels by TAT-Panx₃₀₈ inhibited CSD-induced headache related behaviour and HMGB1 release.

CONCLUSIONS

CSDs, induced in a minimally invasive manner by optogenetics, investigated in freely behaving mice, cause various migraine relevant behavioural and neuroinflammatory phenotypes that are more pronounced and longer-lasting in FHM1 mutant compared to WT mice. Prevention of CSD-related neuroinflammatory changes may have therapeutic potential in the treatment of migraine.

Foramen lacerum impingement of trigeminal nerve root as a rodent model for trigeminal neuralgia

Trigeminal neuralgia (TN) is a classic neuralgic pain condition with distinct clinical characteristics. Modeling TN in rodents is challenging. Recently, we found that a foramen in the rodent skull base, the foramen lacerum, provides direct access to the trigeminal nerve root. Using this access, we developed a foramen lacerum impingement of trigeminal nerve root (FLIT) model and observed distinct pain-like behaviors in rodents, including paroxysmal asymmetric facial grimaces, head tilt when eating, avoidance of solid chow, and lack of wood chewing. The FLIT model recapitulated key clinical features of TN, including lancinating pain-like behavior and dental pain-like behavior. Importantly, when compared with a trigeminal neuropathic pain model (infraorbital nerve chronic constriction injury [IoN-CCI]), the FLIT model was associated with significantly higher numbers of c-Fos-positive cells in the primary somatosensory cortex (S1), unraveling robust cortical activation in the FLIT model. On intravital 2-photon calcium imaging, synchronized S1 neural dynamics were present in the FLIT but not the IoN-CCI model, revealing differential implication of cortical activation in different pain models. Taken together, our results indicate that FLIT is a clinically relevant rodent model of TN that could facilitate pain research and therapeutics development.

Grimace scale assessment during Citrobacter rodentium inflammation and colitis development in laboratory mice

Introduction

Bacterial infections and chronic intestinal inflammations triggered by genetic susceptibility, environment or an imbalance in the intestinal microbiome are usually long-lasting and painful diseases in which the development and maintenance of these various intestinal inflammations is not yet fully understood, research is still needed. This still requires the use of animal models and is subject to the refinement principle of the 3Rs, to minimize suffering or pain perceived by the animals. With regard to this, the present study aimed at the recognition of pain using the mouse grimace scale (MGS) during chronic intestinal colitis due to dextran sodium sulfate (DSS) treatment or after infection with Citrobacter rodentium.

Methods

In this study 56 animals were included which were divided into 2 experimental groups: 1. chronic intestinal inflammation (n = 9) and 2. acute intestinal inflammation (with (n = 23) and without (n = 24) C. rodentium infection). Before the induction of intestinal inflammation in one of the animal models, mice underwent an abdominal surgery and the live MGS from the cage side and a clinical score were assessed before (bsl) and after 2, 4, 6, 8, 24, and 48 hours.

Results

The highest clinical score as well as the highest live MGS was detected 2 hours after surgery and almost no sign of pain or severity were detected after 24 and 48 hours. Eight weeks after abdominal surgery B6-Il4/Il10-/- mice were treated with DSS to trigger chronic intestinal colitis. During the acute phase as well as the chronic phase of the experiment, the live MGS and a clinical score were evaluated. The clinical score increased after DSS administration due to weight loss of the animals but no change of the live MGS was observed. In the second C57BL/6J mouse model, after infection with C. rodentium the clinical score increased but again, no increased score values in the live MGS was detectable.

Discussion

In conclusion, the live MGS detected post-operative pain, but indicated no pain during DSS-induced colitis or C. rodentium infection. In contrast, clinical scoring and here especially the weight loss revealed a decreased wellbeing due to surgery and intestinal inflammation.

C781, a β-Arrestin Biased Antagonist at Protease-Activated Receptor-2 (PAR2), Displays in vivo Efficacy Against Protease-Induced Pain in Mice

Given the limited options and often harmful side effects of current analgesics and the suffering caused by the opioid crisis, new classes of pain therapeutics are needed. Protease-activated receptors (PARs), particularly PAR2, are implicated in a variety of pathologies, including pain. Since the discovery of the role of PAR2 in pain, development of potent and specific antagonists has been slow. In this study, we describe the in vivo characterization of a novel small molecule/peptidomimetic hybrid compound, C781, as a β-arrestin-biased PAR2 antagonist. In vivo behavioral studies were done in mice using von Frey filaments and the Mouse Grimace Scale. Pharmacokinetic studies were done to assess pharmacokinetic/pharmacodynamic relationship in vivo. We used both prevention and reversal paradigms with protease treatment to determine whether C781 could attenuate protease-evoked pain. C781 effectively prevented and reversed mechanical and spontaneous nociceptive behaviors in response to small molecule PAR2 agonists, mast cell activators, and neutrophil elastase. The ED50 of C781 (intraperitoneal dosing) for inhibition of PAR2 agonist (20.9 ng 2-AT)-evoked nociception was 6.3 mg/kg. C781 was not efficacious in the carrageenan inflammation model. Pharmacokinetic studies indicated limited long-term systemic bioavailability for C781 suggesting that optimizing pharmacokinetic properties could improve in vivo efficacy. Our work demonstrates in vivo efficacy of a biased PAR2 antagonist that selectively inhibits β-arrestin/MAPK signaling downstream of PAR2. Given the importance of this signaling pathway in PAR2-evoked nociception, C781 exemplifies a key pharmacophore for PAR2 that can be optimized for clinical development. PERSPECTIVE: Our work provides evidence that PAR2 antagonists that only block certain aspects of signaling by the receptor can be effective for blocking protease-evoked pain in mice. This is important because it creates a rationale for developing safer PAR2-targeting approaches for pain treatment.

Sex differences in the role of parabrachial in nociception and pain in awake mice.. [PMID: 36993729 PMCID: PMC10055376 DOI: 10.1101/2023.03.22.533230] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The parabrachial nucleus is a nexus for aversion, and for the sensory and affective components of pain. In anesthetized rodents with chronic pain, parabrachial neurons have amplified activity. Both spontaneous and evoked activity are considerably higher in awake, compared to anesthetized animals. Parabrachial neurons are rapidly conditioned to respond to innocuous stimuli, after pairing with nociceptive stimuli. In neuropathic or inflammatory pain, parabrachial responses remain amplified for at least 6 weeks, in parallel with increased pain metrics. Calcium responses from CGRP- expressing parabrachial neurons in males demonstrate responses to nociceptive stimuli, and amplified activity in inflammatory pain. In females these neurons evoke smaller responses at baseline, and only small increases in neuropathic pain. This sex difference may relate to our finding that, in females, a small percentage of neurons expresses CGRP RNA. Finally, we show that changes in parabrachial activity are correlated with in arousal, measured as changes in pupil size.

Assessing Orofacial Pain Behaviors in Animal Models: A Review

Orofacial pain refers to pain occurring in the head and face, which is highly prevalent and represents a challenge to clinicians, but its underlying mechanisms are not fully understood, and more studies using animal models are urgently needed. Currently, there are different assessment methods for analyzing orofacial pain behaviors in animal models. In order to minimize the number of animals used and maximize animal welfare, selecting appropriate assessment methods can avoid repeated testing and improve the reliability and accuracy of research data. Here, we summarize different methods for assessing spontaneous pain, evoked pain, and relevant accompanying dysfunction, and discuss their advantages and disadvantages. While the behaviors of orofacial pain in rodents are not exactly equivalent to the symptoms displayed in patients with orofacial pain, animal models and pain behavioral assessments have advanced our understanding of the pathogenesis of such pain.

Preclinical Studies of Posttraumatic Headache and the Potential Therapeutics

Posttraumatic headache (PTH) attributed to traumatic brain injury (TBI) is a secondary headache developed within 7 days after head injury, and in a substantial number of patients PTH becomes chronic and lasts for more than 3 months. Current medications are almost entirely relied on the treatment of primary headache such as migraine, due to its migraine-like phenotype and the limited understanding on the PTH pathogenic mechanisms. To this end, increasing preclinical studies have been conducted in the last decade. We focus in this review on the trigeminovascular system from the animal studies since it provides the primary nociceptive sensory afferents innervating the head and face region, and the pathological changes in the trigeminal pathway are thought to play a key role in the development of PTH. In addition to the pathologies, PTH-like behaviors induced by TBI and further exacerbated by nitroglycerin, a general headache inducer through vasodilation are reviewed. We will overview the current pharmacotherapies including calcitonin gene-related peptide (CGRP) monoclonal antibody and sumatriptan in the PTH animal models. Given that modulation of the endocannabinoid (eCB) system has been well-documented in the treatment of migraine and TBI, the therapeutic potential of eCB in PTH will also be discussed.

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.

The neurobiology of pain and facial movements in rodents: Clinical applications and current research

One of the most controversial aspects of the use of animals in science is the production of pain. Pain is a central ethical concern. The activation of neural pathways involved in the pain response has physiological, endocrine, and behavioral consequences, that can affect both the health and welfare of the animals, as well as the validity of research. The strategy to prevent these consequences requires understanding of the nociception process, pain itself, and how assessment can be performed using validated, non-invasive methods. The study of facial expressions related to pain has undergone considerable study with the finding that certain movements of the facial muscles (called facial action units) are associated with the presence and intensity of pain. This review, focused on rodents, discusses the neurobiology of facial expressions, clinical applications, and current research designed to better understand pain and the nociceptive pathway as a strategy for implementing refinement in biomedical research.

A model-specific simplification of the Mouse Grimace Scale based on the pain response of intraperitoneal CCl4 injections

Despite its long establishment and applicability in mice pain detection, the Mouse Grimace Scale still seems to be underused in acute pain detection during chronic experiments. However, broadening its applicability can identify possible refinement approaches such as cumulative severity and habituation to painful stimuli. Therefore, this study focuses on two main aspects: First, five composite MGS criteria were evaluated with two independent methods (the MoBPs algorithm and a penalized least squares regression) and ranked for their relative importance. The most important variable was used in a second analysis to specifically evaluate the context of pain after an i.p. injection (intervention) in two treatment groups (CCl₄ and oil (control)) at fixed times throughout four weeks in 24 male C57BL/6 N mice. One hour before and after each intervention, video recordings were taken, and the MGS assessment was performed. In this study, the results indicate orbital tightening as the most important criterion. In this experimental setup, a highly significant difference after treatment between week 0 and 1 was found in the CCl₄ group, resulting in a medium-sized effect (W = 62.5, p value < 0.0001, r_CCl4 = 0.64). The oil group showed no significant difference (week 0 vs 1, W = 291.5, p value = 0.7875, r_control = 0.04). Therefore, the study showed that the pain caused by i.p. injections was only dependent on the applied substance, and no significant cumulation or habituation occurred due to the intervention. Further, the results indicated that the MGS system can be simplified.

Evaluation of renal injury and function biomarkers, including symmetric dimethylarginine (SDMA), in the rat passive Heymann nephritis (PHN) model

Symmetric dimethylarginine (SDMA) is a serum biomarker of excretory renal function which consistently correlates with glomerular filtration rate (GFR) across multiple species including rats, dogs, and humans. In human and veterinary clinical settings SDMA demonstrates enhanced sensitivity for detection of declining renal function as compared to other serum biomarkers, but application in preclinical study designs thus far has been limited. The purpose of this study was to determine the performance of serum SDMA in a rat passive Heyman nephritis model of glomerulopathy. In addition to SDMA other biomarkers of excretory renal function were measured including serum creatinine (sCr), blood urea nitrogen (BUN), and cystatin C along with creatinine clearance. Urinary renal biomarkers including microalbumin (μALB), clusterin (CLU), cystatin C, kidney injury marker-1 (KIM-1), neutrophil gelatinase-associated lipocalin (NGAL), and osteopontin (OPN) were also measured. PHN was induced using commercial sheep anti-Fx1A serum. Tissue, serum, and urine were collected from groups of control and anti-Fx1A-treated animals for biomarker evaluation, hematology, urinalysis, serum biochemistry, and histologic examination of kidney. Over the course of a 28-day study, concentrations of the urinary biomarkers μALB, CLU, cystatin C, NGAL, KIM-1 and the serum biomarker cystatin C increased significantly in anti-Fx1A-treated rats as compared to controls but no significant increase in serum SDMA, sCr, BUN, or creatinine clearance were noted in anti-Fx1A-treated rats. Given lack of direct GFR measurement or significant change in the renal function biomarkers sCr, BUN, and creatinine clearance, it is unclear if GFR differed significantly between control and anti-Fx1A-treated rats in this study, though urinary biomarkers and histopathologic findings supported renal injury in anti-Fx1A-treated rats over the time course investigated. This study is among the first to investigate serum SDMA in a rat model relevant to preclinical safety assessment and serves to inform future experimental designs and biomarker selection when evaluation of glomerular injury is of priority.

Measuring Mouse Somatosensory Reflexive Behaviors with High-speed Videography, Statistical Modeling, and Machine Learning

Objectively measuring and interpreting an animal's sensory experience remains a challenging task. This is particularly true when using preclinical rodent models to study pain mechanisms and screen for potential new pain treatment reagents. How to determine their pain states in a precise and unbiased manner is a hurdle that the field will need to overcome. Here, we describe our efforts to measure mouse somatosensory reflexive behaviors with greatly improved precision by high-speed video imaging. We describe how coupling sub-second ethograms of reflexive behaviors with a statistical reduction method and supervised machine learning can be used to create a more objective quantitative mouse "pain scale." Our goal is to provide the readers with a protocol of how to integrate some of the new tools described here with currently used mechanical somatosensory assays, while discussing the advantages and limitations of this new approach.

Goofballing of Opioid and Methamphetamine: The Science Behind the Deadly Cocktail

Globally, millions of people suffer from various substance use disorders (SUD), including mono-and polydrug use of opioids and methamphetamine. Brain regions such as the cingulate cortex, infralimbic cortex, dorsal striatum, nucleus accumbens, basolateral and central amygdala have been shown to play important roles in addiction-related behavioral changes. Clinical and pre-clinical studies have characterized these brain regions and their corresponding neurochemical changes in numerous phases of drug dependence such as acute drug use, intoxication, craving, withdrawal, and relapse. At present, many studies have reported the individual effects of opioids and methamphetamine. However, little is known about their combined effects. Co-use of these drugs produces effects greater than either drug alone, where one decreases the side effects of the other, and the combination produces a prolonged intoxication period or a more desirable intoxication effect. An increasing number of studies have associated polydrug abuse with poorer treatment outcomes, drug-related deaths, and more severe psychopathologies. To date, the pharmacological treatment efficacy for polydrug abuse is vague, and still at the experimental stage. This present review discusses the human and animal behavioral, neuroanatomical, and neurochemical changes underlying both morphine and methamphetamine dependence separately, as well as its combination. This narrative review also delineates the recent advances in the pharmacotherapy of mono- and poly drug-use of opioids and methamphetamine at clinical and preclinical stages.

A functional subdivision within the somatosensory system and its implications for pain research

Somatosensory afferents are traditionally classified by soma size, myelination, and their response specificity to external and internal stimuli. Here, we propose the functional subdivision of the nociceptive somatosensory system into two branches. The exteroceptive branch detects external threats and drives reflexive-defensive reactions to prevent or limit injury. The interoceptive branch senses the disruption of body integrity, produces tonic pain with strong aversive emotional components, and drives self-caring responses toward to the injured region to reduce suffering. The central thesis behind this functional subdivision comes from a reflection on the dilemma faced by the pain research field, namely, the use of reflexive-defensive behaviors as surrogate assays for interoceptive tonic pain. The interpretation of these assays is now being challenged by the discovery of distinct but interwoven circuits that drive exteroceptive versus interoceptive types of behaviors, with the conflation of these two components contributing partially to the poor translation of therapies from preclinical studies.

A Model in Female Rats With Phenotypic Features Similar to Interstitial Cystitis/Bladder Pain Syndrome

This report describes methodological and exploratory investigations of the zymosan-induced neonatal bladder inflammation (NBI) model of interstitial cystitis/bladder pain syndrome (IC/BPS) in female rats. These results validate and extend the currently employed model by evaluating critical timepoints for obtaining treatment effects and identified that a second insult as an adult including repeat intravesical zymosan, intravesical lipopolysaccharide, acute footshock stress, neuropathic nociception (facial) or somatic inflammation (hindpaw) all resulted in magnified visceromotor responses to urinary bladder distension (UBD) in rats which had experienced NBI when compared with their controls. NBI also resulted in increased tone and reactivity of pelvic floor musculature to UBD, as well as increased responsiveness to intravesical potassium chloride solutions, abnormal anxiety measures (elevated plus maze) and an increased number of submucosal petechial hemorrhages following 30 min of hydrodistension of the bladder. These phenotypic findings have correlates to the clinical features of IC/BPS in humans and so support use of this model system to examine mechanisms of and treatments for IC/BPS.

Measurement properties of grimace scales for pain assessment in non-human mammals: a systematic review

ABSTRACT

Facial expressions of pain have been identified in several animal species. The aim of this systematic review was to provide evidence on the measurement properties of grimace scales for pain assessment. The protocol was registered (SyRF#21-Nov-2019) and the study is reported according to the PRISMA guidelines. Studies reporting the development, validation, and the assessment of measurement properties of grimace scales were included. Data extraction and assessment were performed by two investigators, following the Consensus-based Standards for the Selection of Health Measurement Instruments (COSMIN) guidelines. Six categories of measurement properties were assessed: internal consistency, reliability, measurement error, criterion and construct validity, and responsiveness. Overall strength of evidence (high, moderate, low) of each instrument was based on methodological quality, number of studies and studies' findings. Twelve scales for nine species were included (mice, rats, rabbits, horses, piglets, sheep/lamb, ferrets, cats and donkeys). Considerable variability regarding their development and measurement properties was observed. The Mouse, Rat, Horse and Feline Grimace Scales exhibited high level of evidence. The Rabbit, Lamb, Piglet and Ferret Grimace Scales and Sheep Pain Facial Expression Scale exhibited moderate level of evidence. The Sheep Grimace Scale, EQUUS-FAP and EQUUS-Donkey-FAP exhibited low level of evidence for measurement properties. Construct validity was the most reported measurement property. Reliability and other forms of validity have been understudied. This systematic review identified gaps in knowledge on the measurement properties of grimace scales. Further studies should focus on improving psychometric testing, instrument refinement and the use of grimace scales for pain assessment in non-human mammals.

Patterns of cognitive decline and somatosensory processing in a mouse model of amyloid accumulation

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Despite copious amyloid plaques, 5XFAD mice show modest signs of cognitive decline.

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At ages 2 to 13 months old 5XFAD mice show no signs of sensory or pain dysfunctions.

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5XFAD mice may not be a valid model for pain abnormalities in the context of AD.

Pain and cognitive decline increase with age. In particular, there is a troubling relationship between dementia and pain, with some studies showing higher prevalence and inadequate treatment of pain in this population. Alzheimer’s disease (AD) is one of the most common causes of dementia in older adults. Amyloid plaques are a hallmark of AD. The downstream processes these plaques promote are believed to affect neuronal and glial health and activity. There is a need to better understand how the neuropathological changes of AD shape neural activity and pain sensitivity. Here, we use the 5XFAD mouse model, in which dense amyloid accumulations occur at early ages, and in which previous studies reported signs of cognitive decline. We hypothesized that 5XFAD mice develop sensory and pain processing dysfunctions. Although amyloid burden was high throughout the brain, including in regions involved with sensory processing, we identified no functionally significant differences in reflexive or spontaneous signs of pain. Furthermore, expected signs of cognitive decline were modest; a finding consistent with variable results in the literature. These data suggest that models recapitulating other pathological features of Alzheimer’s disease might be better suited to studying differences in pain perception in this disease.

Potential Molecular Targets for Treating Neuropathic Orofacial Pain Based on Current Findings in Animal Models

This review highlights potential molecular targets for treating neuropathic orofacial pain based on current findings in animal models. Preclinical research is currently elucidating the pathophysiology of the disease and identifying the molecular targets for better therapies using animal models that mimic this category of orofacial pain, especially post-traumatic trigeminal neuropathic pain (PTNP) and primary trigeminal neuralgia (PTN). Animal models of PTNP and PTN simulate their etiologies, that is, trauma to the trigeminal nerve branch and compression of the trigeminal root entry zone, respectively. Investigations in these animal models have suggested that biological processes, including inflammation, enhanced neuropeptide-mediated pain signal transmission, axonal ectopic discharges, and enhancement of interactions between neurons and glial cells in the trigeminal pathway, are underlying orofacial pain phenotypes. The molecules associated with biological processes, whose expressions are substantially altered following trigeminal nerve damage or compression of the trigeminal nerve root, are potentially involved in the generation and/or exacerbation of neuropathic orofacial pain and can be potential molecular targets for the discovery of better therapies. Application of therapeutic candidates, which act on the molecular targets and modulate biological processes, attenuates pain-associated behaviors in animal models. Such therapeutic candidates including calcitonin gene-related peptide receptor antagonists that have a reasonable mechanism for ameliorating neuropathic orofacial pain and meet the requirements for safe administration to humans seem worth to be evaluated in clinical trials. Such prospective translation of the efficacy of therapeutic candidates from animal models to human patients would help develop better therapies for neuropathic orofacial pain.

Neuropathic pain: Spotlighting anatomy, experimental models, mechanisms, and therapeutic aspects

The International Association for the Study of Pain defines neuropathic pain as "pain arising as a direct consequence of a lesion or disease affecting the somatosensory system". The associated changes can be observed in the peripheral as well as the central nervous system. The available literature discusses a wide variety of causes as predisposing for the development and amplification of neuropathic pain. Further, key interactions within sensory pathways have been discovered, but no common molecular mechanism leading to neuropathic pain has been identified until now. In the first part of this review, the pain mediating lateral spinothalamic tract is described. Different in vivo models are presented that allow studying trauma-, chemotherapy-, virus-, and diabetes-induced neuropathic pain in rodents. We furthermore discuss approaches to assess neuropathic pain in these models. Second, the current knowledge about cellular and molecular mechanisms suggested to underlie the development of neuropathic pain is presented and discussed. A summary of established therapies that are already applied in the clinic and novel, promising approaches closes the paper. In conclusion, the established animal models are able to emulate the diversity of neuropathic pain observed in the clinics. However, the assessment of neuropathic pain in the presented in vivo models should be improved. The determination of common molecular markers with suitable in vitro models would simplify the assessment of neuropathic pain in vivo. This would furthermore provide insights into common molecular mechanisms of the disease and establish a basis to search for satisfying therapeutic approaches.

Repetitive stress in mice causes migraine-like behaviors and calcitonin gene-related peptide-dependent hyperalgesic priming to a migraine trigger

Migraine is one of the most disabling disorders worldwide but the underlying mechanisms are poorly understood. Stress is consistently reported as a common trigger of migraine attacks. Here, we show that repeated stress in mice causes migraine-like behaviors that are responsive to a migraine therapeutic. Adult female and male mice were exposed to 2 hours of restraint stress for 3 consecutive days, after which they demonstrated facial mechanical hypersensitivity and facial grimace responses that were resolved by 14 days after stress. Hypersensitivity or grimace was not observed in either control animals or those stressed for only 1 day. After return to baseline, the nitric oxide donor sodium nitroprusside (SNP; 0.1 mg/kg) elicited mechanical hypersensitivity in stressed but not in control animals, demonstrating the presence of hyperalgesic priming. This suggests the presence of a migraine-like state, because nitric oxide donors are reliable triggers of attacks in migraine patients but not controls. The stress paradigm also caused priming responses to dural pH 7.0 treatment. The presence of this primed state after stress is not permanent because it was no longer present at 35 days after stress. Finally, mice received either the calcitonin gene-related peptide monoclonal antibody ALD405 (10 mg/kg) 24 hours before SNP or a coinjection of sumatriptan (0.6 mg/kg). ALD405, but not sumatriptan, blocked the facial hypersensitivity due to SNP. This stress paradigm in mice and the subsequent primed state caused by stress allow further preclinical investigation of mechanisms contributing to migraine, particularly those caused by common triggers of attacks.

Methods Used and Application of the Mouse Grimace Scale in Biomedical Research 10 Years on: A Scoping Review

The Mouse Grimace Scale (MGS) was developed 10 years ago as a method for assessing pain through the characterisation of changes in five facial features or action units. The strength of the technique is that it is proposed to be a measure of spontaneous or non-evoked pain. The time is opportune to map all of the research into the MGS, with a particular focus on the methods used and the technique's utility across a range of mouse models. A comprehensive scoping review of the academic literature was performed. A total of 48 articles met our inclusion criteria and were included in this review. The MGS has been employed mainly in the evaluation of acute pain, particularly in the pain and neuroscience research fields. There has, however, been use of the technique in a wide range of fields, and based on limited study it does appear to have utility for pain assessment across a spectrum of animal models. Use of the method allows the detection of pain of a longer duration, up to a month post initial insult. There has been less use of the technique using real-time methods and this is an area in need of further research.

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.

Divergent profiles of fentanyl withdrawal and associated pain in mice and rats

Opioid abuse has devastating effects on patients, their families, and society. Withdrawal symptoms are severely unpleasant, prolonged, and frequently hinder recovery or lead to relapse. The sharp increase in abuse and overdoses arising from the illicit use of potent and rapidly-acting synthetic opioids, such as fentanyl, highlights the urgency of understanding the withdrawal mechanisms related to these drugs. Progress is impeded by inconsistent reports on opioid withdrawal in different preclinical models. Here, using rats and mice of both sexes, we quantified withdrawal behaviors during spontaneous and naloxone-precipitated withdrawal, following two weeks of intermittent fentanyl exposure. We found that both mice and rats lost weight during exposure and showed increased signs of distress during spontaneous and naloxone precipitated withdrawal. However, these species differed in their expression of withdrawal associated pain, a key contributor to relapse in humans. Spontaneous or ongoing pain was preferentially expressed in rats in both withdrawal conditions, while no change was observed in mice. In contrast, withdrawal associated thermal hyperalgesia was found only in mice. These data suggest that rats and mice diverge in how they experience withdrawal and which aspects of the human condition they most accurately model. These differences highlight each species' strengths as model systems and can inform experimental design in studies of opioid withdrawal.

Optogenetic Spreading Depression Elicits Trigeminal Pain and Anxiety Behavior

OBJECTIVE

Cortical spreading depression (SD) is an intense depolarization underlying migraine aura. Despite the weight of evidence linking SD to the pain phase of migraine, controversy remains over a causal role of SD in cephalgia because of the invasive nature of previous SD induction methods. To overcome this problem, we used a novel minimally invasive optogenetic SD induction method and examined the effect of SD on behavior.

METHODS

Optogenetic SD was induced as a single event or repeatedly every other day for 2 weeks. End points, including periorbital and hindpaw mechanical allodynia, mouse grimace, anxiety, and working memory, were examined in male and female mice.

RESULTS

A single SD produced bilateral periorbital mechanical allodynia that developed within 1 hour and resolved within 2 days. Sumatriptan prevented periorbital allodynia when administered immediately after SD. Repeated SDs also produced bilateral periorbital allodynia that lasted 4 days and resolved within 2 weeks after the last SD. In contrast, the hindpaw withdrawal thresholds did not change after repeated SDs suggesting that SD-induced allodynia was limited to the trigeminal region. Moreover, repeated SDs increased mouse grimace scores 2 days after the last SD, whereas a single SD did not. Repeated SDs also increased thigmotaxis scores as a measure of anxiety. In contrast, neither single nor repeated SDs affected visuospatial working memory. We did not detect sexual dimorphism in any end point.

INTERPRETATION

Altogether, these data show a clinically congruent causal relationship among SD, trigeminal pain, and anxiety behavior, possibly reflecting SD modulation of hypothalamic, thalamic, and limbic mechanisms. ANN NEUROL 2021;89:99-110.

Association of Intranasal and Neurogenic Dural Inflammation in Experimental Acute Rhinosinusitis

Background

Nasal cavity and sinus disorders, such as allergic rhinitis, rhinosinusitis, or certain anatomical defects, are often associated with transient or ongoing headaches. On the other hand, migraine headache patients often exhibit pain referral over the area of nasal sinuses and typical nasal autonomic symptoms involving congestion and rhinorrhea. Mechanism for convergence of nasal or sinus disorders and headaches is unknown. Herein, we examined the association of sino-nasal inflammatory pain with common preclinical indicators of trigeminovascular system activation such as dural neurogenic inflammation (DNI) and neuronal activation in brainstem nociceptive nuclei.

Methods

Nasal and paranasal cavity inflammation and pain was induced by formalin (2.5%/10 μl) or capsaicin (0.1%/10 μl) instillation at the border of maxillary sinus and nasal cavity in rats. Quantification of inflammation of nasal mucosa and DNI was performed by spectrophotometric measurement of Evans blue - plasma protein complex extravasation. Pain behavior was quantified by rat grimace scale (RGS). Nociceptive neuronal activation in caudal part of spinal trigeminal nucleus (TNC) was assessed by c-Fos protein immunohistochemistry.

Results

Capsaicin and formalin administered into rat nasal cavity increased plasma protein extravasation in the nasal mucosa and dura mater. Intensity of plasma protein extravasation in nasal mucosa correlated with extravasation in dura. Similarly, facial pain intensity correlated with nociceptive neuronal c-Fos activation in the TNC.

Conclusion

Present data show that inflammatory stimuli in deep nasal and paranasal structures provoke distant intracranial changes related to trigeminovascular system activation. We hypothesize that this phenomenon could explain overlapping symptoms and comorbidity of nasal/paranasal inflammatory disorders with migraine.

The Utility of Grimace Scales for Practical Pain Assessment in Laboratory Animals

Simple Summary

Grimace scales for laboratory animals were first reported ten years ago. Yet, despite their promise as pain assessment tools it appears that they have not been implemented widely in animal research establishments for clinical pain assessment. We discuss potential reasons for this based on the knowledge gained to date on their use and suggest avenues for further research, which might improve uptake of their use in laboratory animal medicine.

Abstract

Animals’ facial expressions are widely used as a readout for emotion. Scientific interest in the facial expressions of laboratory animals has centered primarily on negative experiences, such as pain, experienced as a result of scientific research procedures. Recent attempts to standardize evaluation of facial expressions associated with pain in laboratory animals has culminated in the development of “grimace scales”. The prevention or relief of pain in laboratory animals is a fundamental requirement for in vivo research to satisfy community expectations. However, to date it appears that the grimace scales have not seen widespread implementation as clinical pain assessment techniques in biomedical research. In this review, we discuss some of the barriers to implementation of the scales in clinical laboratory animal medicine, progress made in automation of collection, and suggest avenues for future research.

Grimace Scores: Tools to Support the Identification of Pain in Mammals Used in Research

The 3Rs, Replacement, Reduction and Refinement, is a framework to ensure the ethical and justified use of animals in research. The implementation of refinements is required to alleviate and minimise the pain and suffering of animals in research. Public acceptability of animal use in research is contingent on satisfying ethical and legal obligations to provide pain relief along with humane endpoints. To fulfil this obligation, staff, researchers, veterinarians, and technicians must rapidly, accurately, efficiently and consistently identify, assess and act on signs of pain. This ability is paramount to uphold animal welfare, prevent undue suffering and mitigate possible negative impacts on research. Identification of pain may be based on indicators such as physiological, behavioural, or physical ones. Each has been used to develop different pain scoring systems with potential benefits and limitations in identifying and assessing pain. Grimace scores are a promising adjunctive behavioural technique in some mammalian species to identify and assess pain in research animals. The use of this method can be beneficial to animal welfare and research outcomes by identifying animals that may require alleviation of pain or humane intervention. This paper highlights the benefits, caveats, and potential applications of grimace scales.

Hypothermia During General Anesthesia Interferes with Pain Assessment in Laboratory Rats (Rattus norvegicus)

Accurate pain assessment methods are necessary to ensure animal welfare and reliable data collection in animal research. The Rat Grimace Scale (RGS), a facial expression pain scale, allows effective identification of pain. However, the potential confounds of this method remain mostly unexplored. General anesthesia, which is used in many laboratory procedures, suppresses thermoregulation and results in hypothermia. We investigated the effects of isoflurane-induced hypothermia on RGS scores. Twenty (10 male and 10 female) Sprague-Dawley rats each received 30 min of anesthesia, followed by 30 min of observation after the return of sternal recumbency. Rats were randomized to receive warming with an electric heating pad or no warming during both periods. Unwarmed rats became hypothermic within 15 min after isoflurane exposure began and returned to normothermia within 15 min after returning to sternal recumbency. Warmed rats did not deviate from the normothermic range. The RGS scores of unwarmed rats were significantly higher than baseline levels for 3 h after anesthesia and were higher than those of warmed rats at 5 and 180 min after anesthesia. Hypothermia resulted in a larger proportion of rats crossing a predetermined analgesic intervention threshold. Our findings show that hypothermia induced by isoflurane anesthesia presents a confound to accurate RGS scoring. These results emphasize the importance of maintaining normothermia to avoid inflated pain scores and to obtain accurate pain assessment.

Development of a Mouse Pain Scale Using Sub-second Behavioral Mapping and Statistical Modeling

Rodents are the main model systems for pain research, but determining their pain state is challenging. To develop an objective method to assess pain sensation in mice, we adopt high-speed videography to capture sub-second behavioral features following hind paw stimulation with both noxious and innocuous stimuli and identify several differentiating parameters indicating the affective and reflexive aspects of nociception. Using statistical modeling and machine learning, we integrate these parameters into a single index and create a "mouse pain scale," which allows us to assess pain sensation in a graded manner for each withdrawal. We demonstrate the utility of this method by determining sensations triggered by three different von Frey hairs and optogenetic activation of two different nociceptor populations. Our behavior-based "pain scale" approach will help improve the rigor and reproducibility of using withdrawal reflex assays to assess pain sensation in mice.

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.

Overview of Neurological Mechanism of Pain Profile Used for Animal "Pain-Like" Behavioral Study with Proposed Analgesic Pathways

Pain is the most common sensation installed in us naturally which plays a vital role in defending us against severe harm. This neurological mechanism pathway has been one of the most complex and comprehensive topics but there has never been an elaborate justification of the types of analgesics that used to reduce the pain sensation through which specific pathways. Of course, there have been some answers to curbing of pain which is a lifesaver in numerous situations-chronic and acute pain conditions alike. This has been explored by scientists using pain-like behavioral study methodologies in non-anesthetized animals since decades ago to characterize the analgesic profile such as centrally or peripherally acting drugs and allowing for the development of analgesics. However, widely the methodology is being practiced such as the tail flick/Hargreaves test and Von Frey/Randall-Selitto tests which are stimulus-evoked nociception studies, and there has rarely been a complete review of all these methodologies, their benefits and its downside coupled with the mechanism of the action that is involved. Thus, this review solely focused on the complete protocol that is being adapted in each behavioral study methods induced by different phlogogenic agents, the different assessment methods used for phasic, tonic and inflammatory pain studies and the proposed mechanism of action underlying each behavioral study methodology for analgesic drug profiling. It is our belief that this review could significantly provide a concise idea and improve our scientists' understanding towards pain management in future research.

The cellular basis of protease-activated receptor 2-evoked mechanical and affective pain

Protease-activated receptor 2 (PAR2) has long been implicated in inflammatory and visceral pain, but the cellular basis of PAR2-evoked pain has not been delineated. Although PAR2-evoked pain has been attributed to sensory neuron expression, RNA-sequencing experiments show ambiguous F2rl1 mRNA detection. Moreover, many pharmacological tools for PAR2 are nonspecific, acting also on the Mas-related GPCR family (Mrg) that are highly enriched in sensory neurons. We sought to clarify the cellular basis of PAR2-evoked pain. We developed a PAR2-conditional knockout mouse and specifically deleted PAR2 in all sensory neurons using the PirtCre mouse line. Our behavioral findings show that PAR2 agonist-evoked mechanical hyperalgesia and facial grimacing, but not thermal hyperalgesia, are dependent on PAR2 expression in sensory neurons that project to the hind paw in male and female mice. F2rl1 mRNA is expressed in a discrete population (~4%) of mostly small-diameter sensory neurons that coexpress the Nppb and IL31ra genes. This cell population has been implicated in itch, but our work shows that PAR2 activation in these cells causes clear pain-related behaviors from the skin. Our findings show that a discrete population of DRG sensory neurons mediate PAR2-evoked pain.

Spinal Mobilization Prevents NGF-Induced Trunk Mechanical Hyperalgesia and Attenuates Expression of CGRP

Introduction

Low back pain (LBP) is a complex and growing global health problem in need of more effective pain management strategies. Spinal mobilization (SM) is a non-pharmacological approach recommended by most clinical guidelines for LBP, but greater utilization and treatment optimization are hampered by a lack of mechanistic knowledge underlying its hypoalgesic clinical effects.

Methods

Groups of female Sprague-Dawley rats received unilateral trunk (L5 vertebral level) injections (50 μl) of either vehicle (phosphate-buffer solution, PBS; VEH) or nerve growth factor (NGF; 0.8 μM) on Days 0 and 5 with or without daily L5 SM (VEH, NGF, VEH + SM, VEH + SM). Daily passive SM (10 min) was delivered by a feedback motor (1.2 Hz, 0.9N) from Days 1 to 12. Changes in pain assays were determined for mechanical and thermal reflexive behavior, exploratory behavior (open field events) and spontaneous pain behavior (rat grimace scale). On Day 12, lumbar (L1–L6) dorsal root ganglia (DRG) were harvested bilaterally and calcitonin gene-related peptide (CGRP) positive immunoreactive neurons were quantified from 3 animals (1 DRG tissue section per segmental level) per experimental group.

Results

NGF induced bilateral trunk (left P = 0.006, right P = 0.001) mechanical hyperalgesia and unilateral hindpaw allodynia (P = 0.006) compared to the vehicle group by Day 12. Additionally, we found for the first time that NGF animals demonstrated decreased exploratory behaviors (total distance traveled) and increased grimace scale scoring compared to the VEH group. Passive SM prevented this development of local (trunk) mechanical hyperalgesia and distant (hindpaw) allodynia, and normalized grimace scale scores. NGF increased CGRP positive immunoreactive neurons in ipsilateral lumbar DRGs compared to the VEH group ([L1]P = 0.02; [L2]P = 0.007) and SM effectively negated this increase in pain-related neuropeptide CGRP expression.

Conclusion

SM prevents the development of local (trunk) NGF-induced mechanical hyperalgesia and distant (hindpaw) allodynia, in part, through attenuation of CGRP expression in lumbar DRG sensory neurons. NGF decreases rat exploratory behavior and increases spontaneous pain for which passive SM acts to mitigate these pain-related behavioral changes. These initial study findings suggest that beginning daily SM soon after injury onset might act to minimize or prevent the development of LBP by reducing production of pain-related neuropeptides.

An Amygdalo-Parabrachial Pathway Regulates Pain Perception and Chronic Pain

The parabrachial (PB) complex mediates both ascending nociceptive signaling and descending pain modulatory information in the affective/emotional pain pathway. We have recently reported that chronic pain is associated with amplified activity of PB neurons in a rat model of neuropathic pain. Here we demonstrate that similar activity amplification occurs in mice, and that this is related to suppressed inhibition to lateral parabrachial (LPB) neurons from the CeA in animals of either sex. Animals with pain after chronic constriction injury of the infraorbital nerve (CCI-Pain) displayed higher spontaneous and evoked activity in PB neurons, and a dramatic increase in after-discharges, responses that far outlast the stimulus, compared with controls. LPB neurons in CCI-Pain animals showed a reduction in inhibitory, GABAergic inputs. We show that, in both rats and mice, LPB contains few GABAergic neurons, and that most of its GABAergic inputs arise from CeA. These CeA GABA neurons express dynorphin, somatostatin, and/or corticotropin releasing hormone. We find that the efficacy of this CeA-LPB pathway is suppressed in chronic pain. Further, optogenetically stimulating this pathway suppresses acute pain, and inhibiting it, in naive animals, evokes pain behaviors. These findings demonstrate that the CeA-LPB pathway is critically involved in pain regulation, and in the pathogenesis of chronic pain.SIGNIFICANCE STATEMENT We describe a novel pathway, consisting of inhibition by dynorphin, somatostatin, and corticotropin-releasing hormone-expressing neurons in the CeA that project to the parabrachial nucleus. We show that this pathway regulates the activity of pain-related neurons in parabrachial nucleus, and that, in chronic pain, this inhibitory pathway is suppressed, and that this suppression is causally related to pain perception. We propose that this amygdalo-parabrachial pathway is a key regulator of both chronic and acute pain, and a novel target for pain relief.

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.

Evoked and spontaneous pain assessment during tooth pulp injury

Injury of the tooth pulp is excruciatingly painful and yet the receptors and neural circuit mechanisms that transmit this form of pain remain poorly defined in both the clinic and preclinical rodent models. Easily quantifiable behavioral assessment in the mouse orofacial area remains a major bottleneck in uncovering molecular mechanisms that govern inflammatory pain in the tooth. In this study we sought to address this problem using the Mouse Grimace Scale and a novel approach to the application of mechanical Von Frey hair stimuli. We use a dental pulp injury model that exposes the pulp to the outside environment, a procedure we have previously shown produces inflammation. Using RNAscope technology, we demonstrate an upregulation of genes that contribute to the pain state in the trigeminal ganglia of injured mice. We found that mice with dental pulp injury have greater Mouse Grimace Scores than sham within 24 hours of injury, suggestive of spontaneous pain. We developed a scoring system of mouse refusal to determine thresholds for mechanical stimulation of the face with Von Frey filaments. This method revealed that mice with a unilateral dental injury develop bilateral mechanical allodynia that is delayed relative to the onset of spontaneous pain. This work demonstrates that tooth pain can be quantified in freely behaving mice using approaches common for other types of pain assessment. Harnessing these assays in the orofacial area during gene manipulation should assist in uncovering mechanisms for tooth pulp inflammatory pain and other forms of trigeminal 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.

The Study of Pain in Rats and Mice

Pain is a clinical syndrome arising from a variety of etiologies in a heterogeneous population, which makes successfully treating the individual patient difficult. Organizations and governments recognize the need for tailored and specific therapies, which drives pain research. This review summarizes the different types of pain assessments currently being used and the various rodent models that have been developed to recapitulate the human pain condition.

Sex differences in the expression of calcitonin gene-related peptide receptor components in the spinal trigeminal nucleus

BACKGROUND AND PURPOSE

Calcitonin gene-related peptide (CGRP) plays an important role in migraine pathophysiology. CGRP acts primarily by activating a receptor composed of 3 proteins: calcitonin receptor-like receptor (CLR), receptor activity-modifying protein 1 (RAMP1), and receptor component protein (RCP). We tested the hypothesis that sex differences exist in protein levels of two key components of this CGRP receptor: CLR and RCP.

METHODS

We used specific antibodies to assess baseline protein levels of CLR and RCP in the spinal trigeminal nucleus caudalis (SpVc) and upper cervical spinal cord of both male and female rats. We also tested if manipulations that knock-down the expression of RCP in SpVc, using locally-mediated gene transfer of short hairpin RNA (shRNA), ameliorate pain in an animal model of intracranial migraine-like pain induced by chemical noxious stimulation of the meninges. To assess pain, we used tests of ongoing pain (rat face grimace test and freezing behavior) and tests of facial mechanical hypersensitivity and allodynia.

RESULTS

There was no difference in CLR levels between male and female animals (p > 0.11) in SpVc and the upper cervical cord. However, female animals exhibited greater baseline levels of RCP (up to 3-fold higher) compared to males (p < 0.002). The knock-down of RCP expression in SpVc attenuated mechanical facial allodynia induced by chemical noxious stimulation of the meninges, but had little effect on ongoing pain behaviors in female and male animals.

CONCLUSIONS

RCP is an integral component of the CGRP receptor and may play a key role in mediating CGRP induced central sensitization after noxious stimulation of the meninges. RCP expression in the SpVc and upper cervical cord is sexually dimorphic, with higher levels of expression in females. This dimorphism may be related to the increased incidence of migraines in females-a hypothesis that should be tested in the future.

In search of a rodent model of placebo analgesia in chronic orofacial neuropathic pain

All treatments are given in a context, suggesting that conditioning cues may significantly influence therapeutic outcomes. We tested the hypothesis that context affects placebo analgesia in rodents. To produce neuropathic pain in rats, we performed chronic constriction injury of the infraorbital nerve. We then treated the rats daily, over a seven day period, with injections of either fentanyl or saline, with or without associated conditioning cues; a fourth group received no treatment. On the eighth day, we replaced fentanyl with saline to test for conditioned placebo analgesia. We tested the effects of treatment by measuring sensitivity to mechanical stimuli and grimace scale scores. We found no significant differences in either of these outcomes among the four experimental groups. These findings suggest that chronic, neuropathic pain in rats may not be susceptible to placebo analgesia.

Differences between Dorsal Root and Trigeminal Ganglion Nociceptors in Mice Revealed by Translational Profiling

Nociceptors located in the trigeminal ganglion (TG) and DRG are the primary sensors of damaging or potentially damaging stimuli for the head and body, respectively, and are key drivers of chronic pain states. While nociceptors in these two tissues show a high degree of functional similarity, there are important differences in their development lineages, their functional connections to the CNS, and recent genome-wide analyses of gene expression suggest that they possess some unique genomic signatures. Here, we used translating ribosome affinity purification to comprehensively characterize and compare mRNA translation in Scn10a-positive nociceptors in the TG and DRG of male and female mice. This unbiased method independently confirms several findings of differences between TG and DRG nociceptors described in the literature but also suggests preferential utilization of key signaling pathways. Most prominently, we provide evidence that translational efficiency in mechanistic target of rapamycin (mTOR)-related genes is higher in the TG compared with DRG, whereas several genes associated with the negative regulator of mTOR, AMP-activated protein kinase, have higher translational efficiency in DRG nociceptors. Using capsaicin as a sensitizing stimulus, we show that behavioral responses are greater in the TG region and this effect is completely reversible with mTOR inhibition. These findings have implications for the relative capacity of these nociceptors to be sensitized upon injury. Together, our data provide a comprehensive, comparative view of transcriptome and translatome activity in TG and DRG nociceptors that enhances our understanding of nociceptor biology.SIGNIFICANCE STATEMENT The DRG and trigeminal ganglion (TG) provide sensory information from the body and head, respectively. Nociceptors in these tissues are critical first neurons in the pain pathway. Injury to peripheral neurons in these tissues can cause chronic pain. Interestingly, clinical and preclinical findings support the conclusion that injury to TG neurons is more likely to cause chronic pain and chronic pain in the TG area is more intense and more difficult to treat. We used translating ribosome affinity purification technology to gain new insight into potential differences in the translatomes of DRG and TG neurons. Our findings demonstrate previously unrecognized differences between TG and DRG nociceptors that provide new insight into how injury may differentially drive plasticity states in nociceptors in these two tissues.