Measure Of Spontaneous Pain Research Articles

Use of the Rat Grimace Scale to Evaluate Visceral Pain in a Model of Chemotherapy-Induced Mucositis.

Simple SummaryMucositis is a painful and often debilitating condition associated with cancer treatment. Management of associated symptoms is an important clinical consideration. Animal models are used in mucositis research to model the condition in humans in order to develop novel therapeutic agents to relieve symptoms. Previous animal studies have focused on disease severity and outcomes, but often failed to measure pain. The rat grimace scale (RGS) is a validated observational measure used to gauge pain levels experienced by rats. The aim of this study was to assess the rat grimace scale in a rat model of mucositis, and to examine whether changes in clinical signs and anxiety reflected the grimace responses recorded. We also aimed to determine whether the responses were pain-specific by administering potent opioid painkilling agents. In the present study rat grimace scores did not change significantly between treatments. Development of reliable pain assessment methods in animal models is urgently required to improve model relevance to human clinical practice, in addition to safeguarding animal welfare. The rat grimace scale (RGS) is a measure of spontaneous pain that evaluates pain response. The ability to characterize pain through a non-invasive method has considerable utility for numerous animal models of disease, including mucositis, a painful, self-limiting side-effect of chemotherapy treatment. Preclinical studies investigating novel therapeutics for mucositis often focus on pathological outcomes and disease severity. These investigations fail to measure pain, in spite of reduction of pain being a key clinical therapeutic goal. This study assessed the utility of the RGS for pain assessment in a rat model of mucositis, and whether changes in disease activity index (DAI) and open field test (OFT) reflected the grimace responses recorded. Sixty tumor-bearing female Dark Agouti rats were injected with either saline or 5-Fluourouracil alone, or with co-administration of opioid analgesics. Whilst differences in DAI were observed between treatment groups, no difference in RGS scores or OFT were demonstrated. Significant increases in grimace scores were observed across time. However, whilst a statistically significant change may have been noted, the biological relevance is questionable in terms of practical usage, since an observer is only able to score whole numbers. Development of effective pain assessment methods in animal models is required to improve welfare, satisfy regulatory requirements, and increase translational validity of the model to human patients.

(224) An Open-Source Deep Neural Network to Assess Spontaneous Pain in Freely-Moving Mice using Facial Expressions

There is a dire need for new non-opioid analgesics. However, efforts to develop new pain medicines have thus far met with limited success. This failure is partly due to an overreliance on evoked pain measures in preclinical models. Most preclinical models do not measure spontaneous pain—the main symptom of chronic pain in humans. Here we show our progress towards developing and validating a fully-automated and user-friendly spontaneous pain measurement tool by training a machine learning model to detect and quantify facial grimacing in mice. The original Mouse Grimace Scale (MGS) was developed to train researchers to detect pain in freely moving mice by quantifying characteristic facial expressions, or “grimacing”. To increase the rate of adoption for this spontaneous pain measure, we recently adapted and trained a general object classification convolutional neural network (Google's Inception v1.0TM) to detect and classify the presence or absence of grimace in still images of freely-moving white mice. We call this recently published model the automated Mouse Grimace Scale, aMGS. Here we show our continuing efforts to further improve our model using new convolutional neural architecture to detect grimacing in mice of various coat colors and across different laboratory settings. Unlike our first aMGS program, our newest model can track mouse faces of various colors using continuous video input, rather than relying on still images to determine pain status. We also show the results of our latest validation experiments, including grimace assessment in mice following laparotomy, a common post-surgical pain assay. The rapidity and utility of our proposed automated pain classifier has the potential to greatly increase the adoption of the mouse grimace as a reproducible and clinically-relevant pain measure, and allow researchers to objectively and rapidly assess tonic and chronic pain states in a variety of mouse models using a measure of spontaneous pain.

Mineralocorticoid Antagonist Improves Glucocorticoid Receptor Signaling and Dexamethasone Analgesia in an Animal Model of Low Back Pain.

Low back pain, a leading cause of disability, is commonly treated by epidural steroid injections that target the anti-inflammatory glucocorticoid receptor (GR). However, their efficacy has been controversial. All currently used epidural steroids also activate the pro-inflammatory mineralocorticoid receptor (MR) with significant potency. Local inflammation of the dorsal root ganglia (DRG), a rat model of low back pain, was used. This model causes static and dynamic mechanical allodynia, cold allodynia and guarding behavior (a measure of spontaneous pain), and activates the MR, with pro-nociceptive effects. In this study, effects of local Dexamethasone (DEX; a glucocorticoid used in epidural injections), and eplerenone (EPL; a second generation, more selective MR antagonist) applied to the DRG at the time of inflammation were examined. Mechanical and spontaneous pain behaviors were more effectively reduced by the combination of DEX and EPL than by either alone. The combination of steroids was particularly more effective than DEX alone or the model alone (3-fold improvement for mechanical allodynia) at later times (day 14). Immunohistochemical analysis of the GR in the DRG showed that the receptor was expressed in neurons of all size classes, and in non-neuronal cells including satellite glia. The GR immunoreactivity was downregulated by DRG inflammation (48%) starting on day 1, consistent with the reduction of GR (57%) observed by Western blot, when compared to control animals. On day 14, the combination of DEX and EPL resulted in rescue of GR immunoreactivity that was not seen with DEX alone, and was more effective in reducing a marker for satellite glia activation/neuroinflammation. The results suggest that EPL may enhance the effectiveness of clinically used epidural steroid injections, in part by enhancing the availability of the GR. Thus, the glucocorticoid-mineralocorticoid interactions may limit the effectiveness of epidural steroids through the regulation of the GR in the DRG.

P26. Tibia fracture in mice as a model of Complex Regional Pain Syndrome (CRPS)

Introduction Complex Regional Pain Syndrome (CRPS) occurs after limb trauma, and is accompanied by inflammatory symptoms, motor abnormalities, sensory changes, pain and hyperalgesia. Aim We want to establish a mouse fracture model for CRPS in our lab resembling the symptoms seen in humans. Methods Wild type (WT) mice underwent tibial fracture and casting for 3 weeks. After cast removal, the post-traumatic inflammatory symptoms were tested and observed over time. We measured paw temperature, paw thickness, weight bearing of the paws, mechanical allodynia with von Frey hairs, heat pain thresholds with the Hargreaves method and cold allodynia with acetone. Results Mice reliably generated hindpaw warmth, edema, mechanical, hot and cold allodynia, as well as unweighting (as a measure of spontaneous pain); which regularly resolved after 10 weeks. Conclusion We have developed a model for a standard closed experimental fracture in the mouse tibia and have been able to quantify the complex post-traumatic symptoms. Our next goal is to use this model for the induction of an exaggerated posttraumatic response which is the underlying pathophysiology of human CRPS. Supported by the Hopp-Foundation.

Measures of Spontaneous and Movement-Evoked Pain Are Associated With Disability in Patients With Whiplash Injuries

This study examined the degree to which measures of spontaneous and movement-evoked pain accounted for shared or unique variance in functional disability associated with whiplash injury. The study also addressed the role of fear of movement as a mediator or moderator of the relation between different indices of pain and functional disability. Measures of spontaneous pain, single-point movement-evoked pain, repetition-induced summation of activity-related pain (RISP), and fear of movement and disability were obtained on a sample of 142 individuals who had sustained whiplash injuries. Participants' pain ratings, provided after lifting a weighted canister, were used as the index of single-point movement-evoked pain. RISP was computed as the increase in pain reported by participants over successive lifts of 18 weighted canisters. Measures of functional disability included physical lift tolerance and self-reported disability. Hierarchical regression analyses revealed that measures of single-point movement-evoked pain and RISP accounted for significant unique variance in self-reported disability, beyond the variance accounted for by the measure of spontaneous pain. Only RISP accounted for significant unique variance in lift tolerance. The results suggest that measures of movement-evoked pain represent a disability-relevant dimension of pain that is not captured by measures of spontaneous pain. The clinical and conceptual implications of the findings are discussed. PerspectiveThis study examined the degree to which measures of spontaneous and movement-evoked pain accounted for shared or unique variance in functional disability associated with whiplash injury. The findings suggest that approaches to the clinical evaluation of pain would benefit from the inclusion of measures of movement-evoked pain.

TREK2 Expressed Selectively in IB4-Binding C-Fiber Nociceptors Hyperpolarizes Their Membrane Potentials and Limits Spontaneous Pain

Ongoing/spontaneous pain behavior is associated with ongoing/spontaneous firing (SF) in adult DRG C-fiber nociceptors (Djouhri et al., 2006). Causes of this SF are not understood. We show here that conducting (sometimes called uninjured) C-nociceptors in neuropathic pain models with more hyperpolarized resting membrane potentials (Ems) have lower SF rates. Understanding the control of their Ems may therefore be important for limiting pathological pain. We report that TREK2, a leak K(+) channel, is selectively expressed in IB4 binding rat C-nociceptors. These IB4(+) C-neurons are ∼10 mV more hyperpolarized than IB4(-) C-neurons in vivo (Fang et al., 2006). TREK2 knockdown by siRNA in these neurons in culture depolarized them by ∼10 mV, suggesting that TREK2 is responsible for this ∼10 mV difference. In vivo, more hyperpolarized C-nociceptor Ems were associated with higher cytoplasmic edge-TREK2 expression (edge-TREK2). Edge-TREK2 decreased in C-neurons 7 d after axotomy, and their Ems depolarized by ∼10 mV. This again supports a contribution of TREK2 to their Ems. These relationships between (1) Em and TREK2, (2) SF rate and Em, and (3) spontaneous pain behavior and C-nociceptor SF rate suggested that TREK2 knockdown might increase spontaneous pain. After CFA-induced inflammation, spontaneous foot lifting (a measure of spontaneous pain) was (1) greater in rats with naturally lower TREK2 in ipsilateral small DRG neurons and (2) increased by siRNA-induced TREK2 knockdown in vivo. We conclude that TREK2 hyperpolarizes IB4 binding C-nociceptors and limits pathological spontaneous pain. Similar TREK2 distributions in small DRG neurons of several species suggest that these role(s) of TREK2 may be widespread.

Analgesic activity of metabotropic glutamate receptor 1 antagonists on spontaneous post-operative pain in rats

Activation of metabotropic glutamate (mGlu) receptors has previously been shown to play a role in inflammatory or neuropathic pain states. However, the role of mGlu type 1 receptors in post-operative pain remains to be investigated. In the present study, effects of potent and selective mGlu1 receptor antagonists A-841720, A-794282, A-794278, and A-850002 were evaluated in a skin incision-induced post-operative pain model in rats. Post-operative pain was examined 2 h following surgery using weight-bearing difference between injured and uninjured paws as a measure of spontaneous pain. In this model, A-841720, A-794282, A-794278, and A-850002 induced significant attenuation of spontaneous post-operative pain behavior, with ED50s of 10, 50, 50, and 65 μmol/kg i.p., respectively. Depending on the compound, significant motor side effects were also observed at 3 to 10 fold higher doses. These results support the notion that mGlu1 receptor activation plays a significant role in nociceptive transmission in post-operative pain, though motor impairment may be a limiting factor in developing mGlu1 receptor antagonists as novel analgesics.