Preclinical Pain Research: Can We Do Better?

Prazosin as an Adjuvant to Increase Effectiveness of Duloxetine in a Rat Model of Oxaliplatin-Induced Peripheral Neuropathy

OBJECTIVES

Duloxetine, the only American Society of Clinical Oncology (ASCO) treatment recommended for chemotherapy-induced peripheral neuropathy (CIPN) in cancer survivors, is not effective for 40% of survivors. This study examined the ability of a duloxetine-prazosin combination to prevent the development of allodynia and hyperalgesia in a rat model of oxaliplatin-induced peripheral neuropathy (OPIN).

METHODS

Female (n = 24) and male (n = 41) rats were started on duloxetine (15 mg), prazosin (2 mg), or a duloxetine-prazosin combination one week prior to administration of the chemotherapy drug, oxaliplatin, and continued the duloxetine-prazosin combination for 32 days. Behavioral testing for mechanical allodynia and mechanical hyperalgesia was done with selected von Frey filaments over the course of the study.

RESULTS

Overall percent paw withdrawal for rats that received the duloxetine-prazosin combination was significantly lower in female (p < .001 for both conditions) and male (p = .029 for allodynia; p < .001 for hyperalgesia) than those that received water. No significant posttreatment differences were found for allodynia or hyperalgesia between rats treated with duloxetine and rats that received the duloxetine-prazosin combination in either sex.

CONCLUSIONS

These finding provide preliminary evidence that a duloxetine-prazosin combination can prevent the posttreatment development of allodynia and hyperalgesia in both male and female rats; however, the results suggest that the duloxetine-prazosin combination is no more efficacious than duloxetine alone in preventing chronic OIPN.

IMPLICATIONS FOR NURSING PRACTICE

The profession of nursing is built on clinical practice supported by scientific research. The current study addressed the clinical practice problem of prevention and management of painful OIPN, which is a priority area in oncology nursing.

Kv3.1 Interaction with UBR5 Is Required for Chronic Inflammatory Pain

Chronic inflammatory pain caused by neuronal hyperactivity is a common and refractory disease. Kv3.1, a member of the Kv3 family of voltage-dependent K+ channels, is a major determinant of the ability of neurons to generate high-frequency action potentials. However, little is known about its role in chronic inflammatory pain. Here, we show that although Kv3.1 mRNA expression was unchanged, Kv3.1 protein expression was decreased in the dorsal spinal horn of mice after plantar injection of complete Freund's adjuvant (CFA), a mouse model of inflammatory pain. Upregulating Kv3.1 expression alleviated CFA-induced mechanical allodynia and heat hyperalgesia, whereas downregulating Kv3.1 induced nociception-like behaviors. Additionally, we found that ubiquitin protein ligase E3 component n-recognin 5 (UBR5), a key factor in the initiation of chronic pain, binds directly to Kv3.1 to drive its ubiquitin degradation. Intrathecal injection of the peptide TP-CH-401, a Kv3.1 ubiquitination motif sequence, rescued the decrease in Kv3.1 expression and Kv currents through competitive binding to UBR5, and consequently attenuated mechanical and thermal hypersensitivity. These findings demonstrate a previously unrecognized pathway of Kv3.1 abrogation by UBR5 and indicate that Kv3.1 is critically involved in the regulation of nociceptive behavior. Kv3.1 is thus a promising new target for treating inflammatory pain.

IUPHAR review: Navigating the role of preclinical models in pain research

Chronic pain is a complex and challenging medical condition that affects millions of people worldwide. Understanding the underlying mechanisms of chronic pain is a key goal of preclinical pain research so that more effective treatment strategies can be developed. In this review, we explore nociception, pain, and the multifaceted factors that lead to chronic pain by focusing on preclinical models. We provide a detailed look into inflammatory and neuropathic pain models and discuss the most used animal models for studying the mechanisms behind these conditions. Additionally, we emphasize the vital role of these preclinical models in developing new pain-relief drugs, focusing on biologics and the therapeutic potential of NMDA and cannabinoid receptor antagonists. We also discuss the challenges of TRPV1 modulation for pain treatment, the clinical failures of neurokinin (NK)- 1 receptor antagonists, and the partial success story of Ziconotide to provide valuable lessons for preclinical pain models. Finally, we highlight the overall success and limitations of current treatments for chronic pain while providing critical insights into the development of more effective therapies to alleviate the burden of chronic pain.

Right posterior insular epidural stimulation in rats with neuropathic pain induces a frequency-dependent and opioid system-mediated reduction of pain and its comorbid anxiety and depression

Neuropathic pain (NP) is a sensory, emotional, and persistent disturbing experience caused by a lesion or disease of the somatosensory system which can lead when chronic to comorbidities such as anxiety and depression. Available treatments (pharmacotherapy, neurostimulation) have partial and unpredictable response; therefore, it seems necessary to find a new therapeutical approach that could alleviate most related symptoms and improve patients 'emotional state'. Posterior Insula seems to be a potential target of neurostimulation for pain relief. However, its effects on pain-related anxiety and depression remain unknown. Using rats with spared nerve injury (SNI), this study aims to elucidate the correlation between NP and anxio-depressive disorders, evaluate potential analgesic, anxiolytic, and antidepressant effects of right posterior insula stimulation (IS) using low (LF-IS, 50 Hz) or high (HF-IS, 150 Hz) frequency and assess endogenous opioid involvement in these effects. Results showed positive correlation between NP, anxiety, and depression. LF-IS reversed anhedonia and despair-like behavior through pain alleviation, whereas HF-IS only reduced anhedonia, all effects involving endogenous opioids. These findings support the link between NP and anxio-depressive disorders. Moreover, IS appears to have analgesic, anxiolytic and antidepressant effects mediated by the endogenous opioid system, making it a promising target for neurostimulation.

Mechanisms and treatments of chronic pain after traumatic brain injury

While pain after trauma generally resolves, some trauma patients experience pain for months to years after injury. An example, relevant to both combat and civilian settings, is chronic pain after traumatic brain injury (TBI). Headache as well as pain in the back and extremities are common locations for TBI-related chronic pain to be experienced. TBI-related pain can exist alone or can exacerbate pain from other injuries long after healing has occurred. Consequences of chronic pain in these settings include increased suffering, higher levels of disability, serious emotional problems, and worsened cognitive deficits. The current review will examine recent evidence regarding dysfunction of endogenous pain modulatory mechanisms, neuroplastic changes in the trigeminal circuitry and alterations in spinal nociceptive processing as contributors to TBI-related chronic pain. Key pain modulatory centers including the locus coeruleus, periaqueductal grey matter, and rostroventromedial medulla are vulnerable to TBI. Both the rationales and existing evidence for the use of monoamine reuptake inhibitors, CGRP antagonists, CXCR2 chemokine receptor antagonists, and interventional therapies will be presented. While consensus guidelines for the management of chronic post-traumatic TBI-related pain are lacking, several approaches to this clinically challenging situation deserve focused evaluation and may prove to be viable therapeutic options.

Modification and Delivery of Enkephalins for Pain Modulation

Chronic pain negatively affects patient's quality of life and poses a significant economic burden. First line pharmaceutical treatment of chronic pain, including NSAIDs or antidepressants, is often inefficient to reduce pain, or produces intolerable adverse effects. In such cases, opioids are frequently prescribed for their potent analgesia, but chronic opioid use is also frequently associated with debilitating side effects that may offset analgesic benefits. Nonetheless, opioids continue to be widely utilized due to the lack of effective alternative analgesics. Since their discovery in 1975, a class of endogenous opioids called enkephalins (ENKs) have been investigated for their ability to relieve pain with significantly reduced adverse effects compared to conventional opioids. Their low metabolic stability and inability to cross biological membranes, however, make ENKs ineffective analgesics. Over past decades, much effort has been invested to overcome these limitations and develop ENK-based pain therapies. This review summarizes and describes chemical modifications and ENK delivery technologies utilizing ENK conjugates, nanoparticles and ENK gene delivery approaches and discusses valid lessons, challenges, and future directions of this evolving field.

Characterization of Fragile X Mental Retardation Protein expression in human nociceptors and their axonal projections to the spinal dorsal horn

Fragile X Mental Retardation Protein (FMRP) regulates activity-dependent RNA localization and local translation to modulate synaptic plasticity throughout the central nervous system. Mutations in the FMR1 gene that hinder or ablate FMRP function cause Fragile X Syndrome (FXS), a disorder associated with sensory processing dysfunction. FXS premutations are associated with increased FMRP expression and neurological impairments including sex dimorphic presentations of chronic pain. In mice, FMRP ablation causes dysregulated dorsal root ganglion (DRG) neuron excitability and synaptic vesicle exocytosis, spinal circuit activity, and decreased translation-dependent nociceptive sensitization. Activity-dependent, local translation is a key mechanism for enhancing primary nociceptor excitability that promotes pain in animals and humans. These works indicate that FMRP likely regulates nociception and pain at the level of the primary nociceptor or spinal cord. Therefore, we sought to better understand FMRP expression in the human DRG and spinal cord using immunostaining in organ donor tissues. We find that FMRP is highly expressed in DRG and spinal neuron subsets with substantia gelatinosa exhibiting the most abundant immunoreactivity in spinal synaptic fields. Here, it is expressed in nociceptor axons. FMRP puncta colocalized with Nav1.7 and TRPV1 receptor signals suggesting a pool of axoplasmic FMRP localizes to plasma membrane-associated loci in these branches. Interestingly, FMRP puncta exhibited notable colocalization with calcitonin gene-related peptide (CGRP) immunoreactivity selectively in female spinal cord. Our results support a regulatory role for FMRP in human nociceptor axons of the dorsal horn and implicate it in the sex dimorphic actions of CGRP signaling in nociceptive sensitization and chronic pain.

HomeCageScan analysis reveals ongoing pain in Fabry rats

HomeCageScan (HCS) is an automated behavioral scoring system that can be used to classify and quantify rodent behaviors in the home cage. Although HCS has been used for a number of inducible models of severe pain, little has been done to test this system in clinically relevant genetic disease models associated with chronic pain such as Fabry disease. Rats with Fabry disease exhibit mechanical hypersensitivity, however, it is unclear if these rodents also exhibit ongoing non-evoked pain. Therefore, we analyzed HCS data from male and female rats with Fabry disease. Using hierarchical clustering and principal component analysis, we found both sex and genotype differences in several home cage behaviors. Additionally, we used hierarchical clustering to derive behavioral clusters in an unbiased manner. Analysis of these behavioral clusters showed that primarily female Fabry animals moved less, spent less time caring for themselves (e.g., less time spent grooming and drinking), explored less, and slept more; changes that are similar to lifestyle changes observed in patients with long lasting chronic pain. We also show that sniffing, one of the exploratory behaviors that is depressed in Fabry animals, can be partly restored with the analgesic gabapentin, suggesting that depressed sniffing may reflect ongoing pain. Therefore, this approach to HCS data analysis can be used to assess drug efficacy in Fabry disease and potentially other genetic and inducible rodent models associated with persistent pain.

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.

How different experimental models of secondary hyperalgesia change the nociceptive flexion reflex

OBJECTIVE

In this neurophysiological study in healthy humans, we assessed how central sensitization induced by either high-frequency stimulation (HFS) or topical capsaicin application modulates features of the RIII reflex response. The ability of these stimuli to engage the endogenous pain modulatory system was also tested.

METHODS

In 26 healthy participants we elicited an RIII reflex using suprathreshold stimulation of the sural nerve. Subsequently HFS or capsaicin were applied to the foot and the RIII reflex repeated after 15 minutes. Contact heating of the volar forearm served as the heterotopic test stimulus to probe activation of the endogenous pain modulatory system.

RESULTS

HFS significantly reduced the pain threshold by 29% and the RIII reflex threshold by 20%. Capsaicin significantly reduced the pain threshold by 17% and the RIII reflex threshold by 18%. Both HFS and capsaicin left RIII reflex size unaffected. Numerical Rating Scale (NRS) pain scores elicited by the heterotopic noxious heat stimulus were unaffected by capsaicin and slightly increased by HFS.

CONCLUSIONS

HFS and capsaicin similarly modulated the pain threshold and RIII reflex threshold, without a concomitant inhibitory effect of the endogenous pain modulatory system.

SIGNIFICANCE

Our neurophysiological study supports the use of the RIII reflex in investigating central sensitization in humans.

The N13 spinal component of somatosensory evoked potentials is modulated by heterotopic noxious conditioning stimulation suggesting an involvement of spinal wide dynamic range neurons

OBJECTIVES

Although somatosensory evoked potentials (SEPs) after median nerve stimulation are widely used in clinical practice, the dorsal horn generator of the N13 SEP spinal component is not clearly understood. To verify whether wide dynamic range neurons in the dorsal horn of the spinal cord are involved in the generation of the N13 SEP, we tested the effect of heterotopic noxious conditioning stimulation, which modulates wide dynamic range neurons, on N13 SEP in healthy humans.

METHODS

In 12 healthy subjects, we performed the cold pressor test on the left foot as a heterotopic noxious conditioning stimulus to modulate wide dynamic range neurons. To verify the effectiveness of heterotopic noxious conditioning stimulation, we tested the pressure pain threshold at the thenar muscles of the right hand and recorded SEPs after right median nerve stimulation before, during and after the cold pressor test.

RESULTS

The cold pressor test increased pressure pain threshold by 15% (p = 0.04). During the cold pressor test, the amplitude of the N13 component was significantly lower than that recorded at baseline (by 25%, p = 0.04).

DISCUSSION

In this neurophysiological study in healthy humans, we showed that a heterotopic noxious conditioning stimulus significantly reduced N13 SEP amplitude. This finding suggests that the N13 SEP might be generated by the segmental postsynaptic response of dorsal horn wide dynamic range neurons.

Modulation of the N13 component of the somatosensory evoked potentials in an experimental model of central sensitization in humans

The N13 component of somatosensory evoked potential (N13 SEP) represents the segmental response of dorsal horn neurons. In this neurophysiological study, we aimed to verify whether N13 SEP might reflect excitability changes of dorsal horn neurons during central sensitization. In 22 healthy participants, we investigated how central sensitization induced by application of topical capsaicin to the ulnar nerve territory of the hand dorsum modulated N13 SEP elicited by ulnar nerve stimulation. Using a double-blind placebo-controlled crossover design, we also tested whether pregabalin, an analgesic drug with proven efficacy on the dorsal horn, influenced capsaicin-induced N13 SEP modulation. Topical application of capsaicin produced an area of secondary mechanical hyperalgesia, a sign of central sensitization, and increased the N13 SEP amplitude but not the peripheral N9 nor the cortical N20-P25 amplitude. This increase in N13 SEP amplitude paralleled the mechanical hyperalgesia and persisted for 120 min. Pregabalin prevented the N13 SEP modulation associated with capsaicin-induced central sensitization, whereas capsaicin application still increased N13 SEP amplitude in the placebo treatment session. Our neurophysiological study showed that capsaicin application specifically modulates N13 SEP and that this modulation is prevented by pregabalin, thus suggesting that N13 SEP may reflect changes in dorsal horn excitability and represent a useful biomarker of central sensitization in human studies.

Systematic review and meta-analysis of cannabinoids, cannabis-based medicines, and endocannabinoid system modulators tested for antinociceptive effects in animal models of injury-related or pathological persistent pain

ABSTRACT

We report a systematic review and meta-analysis of studies that assessed the antinociceptive efficacy of cannabinoids, cannabis-based medicines, and endocannabinoid system modulators on pain-associated behavioural outcomes in animal models of pathological or injury-related persistent pain. In April 2019, we systematically searched 3 online databases and used crowd science and machine learning to identify studies for inclusion. We calculated a standardised mean difference effect size for each comparison and performed a random-effects meta-analysis. We assessed the impact of study design characteristics and reporting of mitigations to reduce the risk of bias. We meta-analysed 374 studies in which 171 interventions were assessed for antinociceptive efficacy in rodent models of pathological or injury-related pain. Most experiments were conducted in male animals (86%). Antinociceptive efficacy was most frequently measured by attenuation of hypersensitivity to evoked limb withdrawal. Selective cannabinoid type 1, cannabinoid type 2, nonselective cannabinoid receptor agonists (including delta-9-tetrahydrocannabinol) and peroxisome proliferator-activated receptor-alpha agonists (predominantly palmitoylethanolamide) significantly attenuated pain-associated behaviours in a broad range of inflammatory and neuropathic pain models. Fatty acid amide hydrolase inhibitors, monoacylglycerol lipase inhibitors, and cannabidiol significantly attenuated pain-associated behaviours in neuropathic pain models but yielded mixed results in inflammatory pain models. The reporting of criteria to reduce the risk of bias was low; therefore, the studies have an unclear risk of bias. The value of future studies could be enhanced by improving the reporting of methodological criteria, the clinical relevance of the models, and behavioural assessments. Notwithstanding, the evidence supports the hypothesis of cannabinoid-induced analgesia.

Automated home-cage for the evaluation of innate non-reflexive pain behaviors in a mouse model of inflammatory pain

The failure to develop analgesic drugs is attributed not only to the complex and diverse pathophysiology of pain in humans but also to the poor experimental design and poor preclinical assessment of pain. Although considerable efforts have been devoted to overcoming the relevant problems, many features of the behavioral pain assessment remain to be characterized. For example, a decreased locomotor activity as a common presentation of pain-like behavior has yet to be described. Studies on mice experimentally induced with carrageenan have provided opportunities to explore pain-related behaviors in automated home-cage monitoring. Through this approach, the locomotor activities of mice with carrageenan-induced inflammatory pain can be precisely and objectively captured. Here, we found that the mobile behaviors of mice reduced, and their immobility increased, indicating that carrageenan induction in mice caused a significant decrease in locomotor activity. These non-reflexive pain behaviors were strongly correlated with the reflexive pain behaviors measured via von Frey and plantar tests. Furthermore, the pharmacological intervention using indomethacin improved the locomotor activity of mice with carrageenan-induced pain. Thus, the analysis of the locomotor activity in automated home-cage monitoring is useful for studying the behavioral analgesia and the pharmacological screening of analgesic drugs. The combined evaluation of reflexive and non-reflexive pain behaviors enhances the translational utility of preclinical pain research in rodents.

Fabry disease pain: patient and preclinical parallels

ABSTRACT

Severe neuropathic pain is a hallmark of Fabry disease, a genetic disorder caused by a deficiency in lysosomal α-galactosidase A. Pain experienced by these patients significantly impacts their quality of life and ability to perform everyday tasks. Patients with Fabry disease suffer from peripheral neuropathy, sensory abnormalities, acute pain crises, and lifelong ongoing pain. Although treatment of pain through medication and enzyme replacement therapy exists, pain persists in many of these patients. Some has been learned in the past decades regarding clinical manifestations of pain in Fabry disease and the pathological effects of α-galactosidase A insufficiency in neurons. Still, it is unclear how pain and sensory abnormalities arise in patients with Fabry disease and how these can be targeted with therapeutics. Our knowledge is limited in part due to the lack of adequate preclinical models to study the disease. This review will detail the types of pain, sensory abnormalities, influence of demographics on pain, and current strategies to treat pain experienced by patients with Fabry disease. In addition, we discuss the current knowledge of Fabry pain pathogenesis and which aspects of the disease preclinical models accurately recapitulate. Understanding the commonalities and divergences between humans and preclinical models can be used to further interrogate mechanisms causing the pain and sensory abnormalities as well as advance development of the next generation of therapeutics to treat pain in patients with Fabry disease.

Mexican Plants and Derivates Compounds as Alternative for Inflammatory and Neuropathic Pain Treatment-A Review

Despite the availability of many anti-pain drugs, in the form of NSAIDs, steroids, gabapentinoids, opioids, and antidepressants, in this study we address the natural compounds belonging to the group of Mexican medicinal plants or "Mexican folk medicine", used for pain management in Mexico. Our interest in this subject is due to the growing idea that "natural is harmless" and to the large number of side effects exhibited in pharmacotherapy. The objective of this review was to document the scientific evidence about Mexican medicinal plants and their derivatives used for inflammatory and neuropathic pain treatment, as well as the mechanisms of action implicated in their antinociceptive effects, their possible adverse effects, and the main pharmacological aspects of each plant or compound. Our data review suggested that most studies on Mexican medicinal plants have used inflammatory experimental models for testing. The anti-pain properties exerted by medicinal plants lack adverse effects, and their toxicological assays report that they are safe to consume; therefore, more studies should be performed on preclinical neuropathic pain models. Moreover, there is no convincing evidence about the possible mechanisms of action involved in the anti-pain properties exerted by Mexican plants. Therefore, the isolation and pharmacological characterization of these plant derivatives' compounds will be important in the design of future preclinical studies.

Modeling Complex Orthopedic Trauma in Rodents: Bone, Muscle and Nerve Injury and Healing

Orthopedic injury can occur from a variety of causes including motor vehicle collision, battlefield injuries or even falls from standing. Persistent limb pain is common after orthopedic injury or surgery and presents a unique challenge, as the initiating event may result in polytrauma to bone, muscle, and peripheral nerves. It is imperative that we understand the tissue-specific and multicellular response to this unique type of injury in order to best develop targeted treatments that improve healing and regeneration. In this Mini Review we will first discuss current rodent models of orthopedic trauma/complex orthotrauma. In the second section, we will focus on bone-specific outcomes including imaging modalities, biomechanical testing and immunostaining for markers of bone healing/turnover. In the third section, we will discuss muscle-related pathology including outcome measures of fibrosis, muscle regeneration and tensile strength measurements. In the fourth section, we will discuss nervous system-related pathology including outcome measures of pain-like responses, both reflexive and non-reflexive. In all sections we will consider parallels between preclinical outcome measures and the functional and mechanistic findings of the human condition.

Dorsal horn disinhibition and movement-induced behaviour in a rat model of inflammatory arthritis

OBJECTIVES

Alterations beyond joint inflammation such as changes in dorsal horn (DH) excitability contribute to pain in inflammatory arthritis (IA). More complete understanding of specific underlying mechanisms will be important to define novel targets for the treatment of IA pain. Pre-clinical models are useful, but relevant pain assays are vital for successful clinical translation. For this purpose, a method is presented to assess movement-induced pain-related behaviour changes that was subsequently used to investigate DH disinhibition in IA.

METHODS

IA was induced by intra-articular injection of complete Freund's adjuvant (CFA) in male rats, and weight distribution was assessed before and after walking on a treadmill. To confirm increased activity in nociception-related pathways, fos expression was assessed in the superficial DH, including in nociceptive neurons, identified by neurokinin 1 (NK1) immunoreactivity, and interneurons. Inhibitory terminal density onto NK1+ neurons was assessed and lastly, a cohort of animals was treated for 3 days with gabapentin.

RESULTS

At 4 weeks post-CFA, walking reduced weight distribution to the affected joint and increased DH fos expression, including in NK1+ neurons. Neuronal activity in inhibitory cells and inhibitory terminal density on NK1+ neurons were decreased in CFA-treated animals compared with controls. Treatment with gabapentin led to recovered behaviour and DH neuronal activity pattern in CFA-treated animals.

CONCLUSION

We describe an assay to assess movement-induced pain-related behaviour changes in a rodent IA model. Furthermore, our results suggest that disinhibition may contribute to pain related to movement in IA.

Clinical relevance of chronic neuropathic pain phenotypes in mice: A comprehensive behavioral analysis

Despite a large number of preclinical studies performed each year, the safe and effective therapeutic interventions for chronic pain are scant. Therefore, it appears that pre-clinical modeling requires a systematically organized behavioral test paradigm to quantify the response of animals for a specific pain state. The present study, therefore, conceptualized a test battery to evaluate the behavioral changes in mice following neuropathic pain. We employed sciatic nerve chronic constriction injury (CCI) in C57BL/6 J mice to model chronic pain state. Mice were monitored for thermal hyperalgesia and grip strength for 30 days. Subsequently, mice underwent a behavioral test battery consisting of the nociceptive threshold, the affective and cognitive functions and motor coordination, and strength. Our results showed that CCI mice are insensitive to thermal stimuli. However, nerve-injured mice showed significant changes in neuromuscular coordination, basal anxiety, and hedonic state. Such impaired neuromuscular coordination is indicative of disability rather than the actual pain phenotype. While using the digital gait analysis, our study revealed rationales for the insensitivity of CCI mice to thermal stimuli. Our results suggest that the predictive validity of the CCI model necessitates a comprehensive behavioral test battery to select the clinically relevant and measurable phenotype to quantify chronic neuropathic pain.

The influence of rat strain on the development of neuropathic pain and comorbid anxio-depressive behaviour after nerve injury

Back-translating the clinical manifestations of human disease burden into animal models is increasingly recognized as an important facet of preclinical drug discovery. We hypothesized that inbred rat strains possessing stress hyper-reactive-, depressive- or anxiety-like phenotypes may possess more translational value than common outbred strains for modeling neuropathic pain. Rats (inbred: LEW, WKY, F344/ICO and F344/DU, outbred: Crl:SD) were exposed to Spared Nerve Injury (SNI) and evaluated routinely for 6 months on behaviours related to pain (von Frey stimulation and CatWalk-gait analysis), anxiety (elevated plus maze, EPM) and depression (sucrose preference test, SPT). Markers of stress reactivity together with spinal/brain opioid receptor expression were also measured. All strains variously developed mechanical allodynia after SNI with the exception of stress-hyporesponsive LEW rats, despite all strains displaying similar functional gait-deficits after injury. However, affective changes reflective of anxiety- and depressive-like behaviour were only observed for F344/DU in the EPM, and for Crl:SD in SPT. Although differences in stress reactivity and opioid receptor expression occurred, overall they were relatively unaffected by SNI. Thus, anxio-depressive behaviours did not develop in all strains after nerve injury, and correlated only modestly with degree of pain sensitivity or with genetic predisposition to stress and/or affective disturbances.

Emotional and cognitive impairments in the peripheral nerve chronic constriction injury model (CCI) of neuropathic pain: A systematic review

BACKGROUND AND OBJECTIVE

Emotional and cognitive impairments are common comorbidities of chronic neuropathic pain that significantly impact the quality of life of patients. While the nociceptive components of the peripheral nerve chronic constriction injury (CCI) animal model have been extensively analyzed, data related to the development of mood and cognitive disorders, and especially its impact on female rats remains fragmented. We systematically reviewed the literature analyzing the methods used to induce and evaluate the development of emotional- and cognitive-like impairments and sex-specific differences in the CCI model.

DATABASES AND DATA TREATMENT

We searched PubMed, Google Scholar and Web of Science from inception to September 30th, 2019, and a total of 44 papers were considered eligible for inclusion. We included animal studies assessing nociception, locomotion, anxious-like, depressive-like and cognitive behaviours after the CCI induction.

RESULTS

The overall quality of the studies was considered moderate to high. Overall, the induction of CCI leads to the development of emotional impairments, namely anxiety- and depressive-like behaviours, as well as cognitive impairments. With the majority of the studies using male subjects, the lack of evidence on female animals prevents the evaluation of sex-specific differences.

CONCLUSIONS

This review supports the development of an anxiodepressive-like phenotype, associated with cognitive impairments, in CCI-induced animals. These results support the use of this animal model for the study of the mechanisms underlying these comorbidities, as well as a screening tool for the development/repurposing of drugs that tackle both the neuropathy-induced nociceptive and emotional impairments, such as tricyclic antidepressants. Importantly, our review also highlights the need for studies performed in female rodents as these are almost non-existent.

K+ Channels in Primary Afferents and Their Role in Nerve Injury-Induced Pain

Sensory abnormalities generated by nerve injury, peripheral neuropathy or disease are often expressed as neuropathic pain. This type of pain is frequently resistant to therapeutic intervention and may be intractable. Numerous studies have revealed the importance of enduring increases in primary afferent excitability and persistent spontaneous activity in the onset and maintenance of peripherally induced neuropathic pain. Some of this activity results from modulation, increased activity and /or expression of voltage-gated Na⁺ channels and hyperpolarization-activated cyclic nucleotide–gated (HCN) channels. K⁺ channels expressed in dorsal root ganglia (DRG) include delayed rectifiers (K_v1.1, 1.2), A-channels (K_v1.4, 3.3, 3.4, 4.1, 4.2, and 4.3), KCNQ or M-channels (K_v7.2, 7.3, 7.4, and 7.5), ATP-sensitive channels (K_IR6.2), Ca²⁺-activated K⁺ channels (K_Ca1.1, 2.1, 2.2, 2.3, and 3.1), Na⁺-activated K⁺ channels (K_Ca4.1 and 4.2) and two pore domain leak channels (K_2p; TWIK related channels). Function of all K⁺ channel types is reduced via a multiplicity of processes leading to altered expression and/or post-translational modification. This also increases excitability of DRG cell bodies and nociceptive free nerve endings, alters axonal conduction and increases neurotransmitter release from primary afferent terminals in the spinal dorsal horn. Correlation of these cellular changes with behavioral studies provides almost indisputable evidence for K⁺ channel dysfunction in the onset and maintenance of neuropathic pain. This idea is underlined by the observation that selective impairment of just one subtype of DRG K⁺ channel can produce signs of pain in vivo. Whilst it is established that various mediators, including cytokines and growth factors bring about injury-induced changes in DRG function and excitability, evidence presently available points to a seminal role for interleukin 1β (IL-1β) in control of K⁺ channel function. Despite the current state of knowledge, attempts to target K⁺ channels for therapeutic pain management have met with limited success. This situation may change with the advent of personalized medicine. Identification of specific sensory abnormalities and genetic profiling of individual patients may predict therapeutic benefit of K⁺ channel activators.

Challenging the challenge: A randomized controlled trial evaluating the inflammatory response and pain perception of healthy volunteers after single-dose LPS administration, as a potential model for inflammatory pain in early-phase drug development

BACKGROUND AND AIMS

Following an infection, cytokines not only regulate the acute immune response, but also contribute to symptoms such as inflammatory hyperalgesia. We aimed to characterize the acute inflammatory response induced by a human endotoxemia model, and its effect on pain perception using evoked pain tests in two different dose levels. We also attempted to determine whether combining a human endotoxemia challenge with measurement of pain thresholds in healthy subjects could serve as a model to study drug effects on inflammatory pain.

METHODS AND RESULTS

This was a placebo-controlled, randomized, cross-over study in 24 healthy males. Twelve subjects were administered a bolus of 1 ng/kg LPS intravenously, and twelve 2 ng/kg LPS. Before days of placebo/LPS administration, subjects completed a full study day without study drug administration, but with identical pain threshold testing. Blood sampling and evoked pain tests (electrical burst and -stair, heat, pressure, and cold pressor test) were performed pre-dose and at frequent intervals up to 10hr post-dose. Data were analysed with a repeated-measures ANCOVA. For both dose levels, LPS induced an evident acute inflammatory response, but did not significantly affect any of the pain modalities. In a post-hoc analysis, lowering of pain thresholds was observed in the first 3 h after dosing, corresponding with the peak of the acute inflammatory response around 1-3 h post-dose.

CONCLUSION

Mild acute systemic inflammation, as induced by 1 ng/kg and 2 ng/kg LPS intravenous administration, did not significantly change pain thresholds in this study. The endotoxemia model in combination with evoked pain tests is not suitable to study acute inflammatory hyperalgesia in healthy males.

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.

Differential methylation and expression of genes in the hypoxia-inducible factor 1 signaling pathway are associated with paclitaxel-induced peripheral neuropathy in breast cancer survivors and with preclinical models of chemotherapy-induced neuropathic pain

BACKGROUND

Paclitaxel is an important chemotherapeutic agent for the treatment of breast cancer. Paclitaxel-induced peripheral neuropathy (PIPN) is a major dose-limiting toxicity that can persist into survivorship. While not all survivors develop PIPN, for those who do, it has a substantial negative impact on their functional status and quality of life. No interventions are available to treat PIPN. In our previous studies, we identified that the HIF-1 signaling pathway (H1SP) was perturbed between breast cancer survivors with and without PIPN. Preclinical studies suggest that the H1SP is involved in the development of bortezomib-induced and diabetic peripheral neuropathy, and sciatic nerve injury. The purpose of this study was to identify H1SP genes that have both differential methylation and differential gene expression between breast cancer survivors with and without PIPN.

METHODS

A multi-staged integrated analysis was performed. In peripheral blood, methylation was assayed using microarray and gene expression was assayed using RNA-seq. Candidate genes in the H1SP having both differentially methylation and differential expression were identified between survivors who received paclitaxel and did (n = 25) and did not (n = 25) develop PIPN. Then, candidate genes were evaluated for differential methylation and differential expression in public data sets of preclinical models of PIPN and sciatic nerve injury.

RESULTS

Eight candidate genes were identified as both differential methylation and differential expression in survivors. Of the eight homologs identified, one was found to be differential expression in both PIPN and "normal" mice dorsal root ganglia; three were differential methylation in sciatic nerve injury versus sham rats in both pre-frontal cortex and T-cells; and two were differential methylation in sciatic nerve injury versus sham rats in the pre-frontal cortex.

CONCLUSIONS

This study is the first to evaluate for methylation in cancer survivors with chronic PIPN. The findings provide evidence that the expression of H1SP genes associated with chronic PIPN in cancer survivors may be regulated by epigenetic mechanisms and suggests genes for validation as potential therapeutic targets.

"Bridging the Gap" Everything that Could Have Been Avoided If We Had Applied Gender Medicine, Pharmacogenetics and Personalized Medicine in the Gender-Omics and Sex-Omics Era

Gender medicine is the first step of personalized medicine and patient-centred care, an essential development to achieve the standard goal of a holistic approach to patients and diseases. By addressing the interrelation and integration of biological markers (i.e., sex) with indicators of psychological/cultural behaviour (i.e., gender), gender medicine represents the crucial assumption for achieving the personalized health-care required in the third millennium. However, ‘sex’ and ‘gender’ are often misused as synonyms, leading to frequent misunderstandings in those who are not deeply involved in the field. Overall, we have to face the evidence that biological, genetic, epigenetic, psycho-social, cultural, and environmental factors mutually interact in defining sex/gender differences, and at the same time in establishing potential unwanted sex/gender disparities. Prioritizing the role of sex/gender in physiological and pathological processes is crucial in terms of efficient prevention, clinical signs’ identification, prognosis definition, and therapy optimization. In this regard, the omics-approach has become a powerful tool to identify sex/gender-specific disease markers, with potential benefits also in terms of socio-psychological wellbeing for each individual, and cost-effectiveness for National Healthcare systems. “Being a male or being a female” is indeed important from a health point of view and it is no longer possible to avoid “sex and gender lens” when approaching patients. Accordingly, personalized healthcare must be based on evidence from targeted research studies aimed at understanding how sex and gender influence health across the entire life span. The rapid development of genetic tools in the molecular medicine approaches and their impact in healthcare is an example of highly specialized applications that have moved from specialists to primary care providers (e.g., pharmacogenetic and pharmacogenomic applications in routine medical practice). Gender medicine needs to follow the same path and become an established medical approach. To face the genetic, molecular and pharmacological bases of the existing sex/gender gap by means of omics approaches will pave the way to the discovery and identification of novel drug-targets/therapeutic protocols, personalized laboratory tests and diagnostic procedures (sex/gender-omics). In this scenario, the aim of the present review is not to simply resume the state-of-the-art in the field, rather an opportunity to gain insights into gender medicine, spanning from molecular up to social and psychological stances. The description and critical discussion of some key selected multidisciplinary topics considered as paradigmatic of sex/gender differences and sex/gender inequalities will allow to draft and design strategies useful to fill the existing gap and move forward.

Microglial Modulation as a Target for Chronic Pain: From the Bench to the Bedside and Back

With a widespread opioid epidemic and profound biopsychosocial implications, chronic pain is a multifaceted public health issue requiring urgent attention. The treatment of chronic pain is particularly important to anesthesiologists given our unique role as perioperative physicians and pain medicine specialists. The present review details the recent shift from a neuronal theory of chronic pain to one that includes complex neuron-glia interactions. In particular, we highlight microglia, the myeloid-lineage cells of the central nervous system, as initiators of a postinjury neuroimmune response that contributes to the acute to chronic pain transition. We discuss ever-advancing preclinical studies, wherein significant success has been made through pharmacologic and genetic modulation of microglia, and we emphasize where these approaches have made the transition to the clinical realm. Furthermore, we highlight the most current, novel efforts to visualize glial activation in vivo using positron emission tomography and improve the diagnosis of chronic pain through radiotracer binding of specific targets, like the 18 kDa translocator protein in microglia and myeloid-lineage cells. Our rapidly advancing knowledge about microglia and their involvement in pain suggests that the era of glial-targeted therapeutics is just beginning so long as we refocus our attention on optimizing preclinical studies using a clinically informed approach, before translation.

Front and hind paw differential analgesic effects of amitriptyline, gabapentin, ibuprofen, and URB937 on mechanical and cold sensitivity in cisplatin-induced neuropathy

Cisplatin is a widely used platinum-derived antineoplastic agent that frequently results in peripheral neuropathy. Therapeutic strategies for neuropathic pain are limited and characterized by variable efficacy and severe adverse effects. Clinical translation of novel analgesics has proven difficult with many agents demonstrating preclinical efficacy failing in clinical trials. Preclinical studies frequently assess pain behaviors in the hind paws; however, the front paws have a greater degree of the fine sensorimotor functions characteristically damaged by chemotherapy-induced neuropathy. This is the first study to assess pain responses in the front paws. Here, we test the hypothesis that mouse front paws exhibit pain-related alterations in mechanical and thermal (cold) sensitivity in a murine model of cisplatin-induced neuropathy and that pharmacological treatment with amitriptyline, gabapentin, ibuprofen, and URB937 normalize pain behaviors in the front and hind paws. Cold (acetone withdrawal latencies) and mechanical (von Frey withdrawal thresholds) sensitivity were significantly increased and decreased respectively in both the front and the hind paws following initiation of weekly systemic (intraperitoneal) cisplatin injections (5 mg/kg). For the hind paws, systemic administration of amitriptyline (30 mg/kg), gabapentin (100 mg/kg), ibuprofen (0–10 mg/kg), or URB937 (0–10 mg/kg) resulted in a decrease in acetone withdrawal latencies and increase in von Frey withdrawal thresholds with return to normal values at the highest doses tested. For the front paws, return to baseline values for the highest doses was found for cold allodynia but not mechanical allodynia, where the highest doses failed to return to baseline values. These results indicate that mouse front paws exhibit pain-related changes in cisplatin-induced neuropathy and that drug effects can vary based on testing stimulus and location. This suggests that front paw responses across multiple modalities provide reliable and accurate information about pain-related drug effects. Future studies should be aimed at elucidating the mechanisms underlying these differential effects.

Reporting preclinical anesthesia study (REPEAT): Evaluating the quality of reporting in the preclinical anesthesiology literature

Poor reporting quality may contribute to irreproducibility of results and failed 'bench-to-bedside' translation. Consequently, guidelines have been developed to improve the complete and transparent reporting of in vivo preclinical studies. To examine the impact of such guidelines on core methodological and analytical reporting items in the preclinical anesthesiology literature, we sampled a cohort of studies. Preclinical in vivo studies published in Anesthesiology, Anesthesia & Analgesia, Anaesthesia, and the British Journal of Anaesthesia (2008-2009, 2014-2016) were identified. Data was extracted independently and in duplicate. Reporting completeness was assessed using the National Institutes of Health Principles and Guidelines for Reporting Preclinical Research. Risk ratios were used for comparative analyses. Of 7615 screened articles, 604 met our inclusion criteria and included experiments reporting on 52 490 animals. The most common topic of investigation was pain and analgesia (30%), rodents were most frequently used (77%), and studies were most commonly conducted in the United States (36%). Use of preclinical reporting guidelines was listed in 10% of applicable articles. A minority of studies fully reported on replicates (0.3%), randomization (10%), blinding (12%), sample-size estimation (3%), and inclusion/exclusion criteria (5%). Statistics were well reported (81%). Comparative analysis demonstrated few differences in reporting rigor between journals, including those that endorsed reporting guidelines. Principal items of study design were infrequently reported, with few differences between journals. Methods to improve implementation and adherence to community-based reporting guidelines may be necessary to increase transparent and consistent reporting in the preclinical anesthesiology literature.

Core Outcome Measures in Preclinical Assessment of Candidate Analgesics

All preclinical procedures for analgesic drug discovery involve two components: 1) a "pain stimulus" (the principal independent variable), which is delivered to an experimental subject with the intention of producing a pain state; and 2) a "pain behavior" (the principal dependent variable), which is measured as evidence of that pain state. Candidate analgesics are then evaluated for their effectiveness to reduce the pain behavior, and results are used to prioritize drugs for advancement to clinical testing. This review describes a taxonomy of preclinical procedures organized into an "antinociception matrix" by reference to their types of pain stimulus (noxious, inflammatory, neuropathic, disease related) and pain behavior (unconditioned, classically conditioned, operant conditioned). Particular emphasis is devoted to pain behaviors and the behavioral principals that govern their expression, pharmacological modulation, and preclinical-to-clinical translation. Strengths and weaknesses are compared and contrasted for procedures using each type of behavioral outcome measure, and the following four recommendations are offered to promote strategic use of these procedures for preclinical-to-clinical analgesic drug testing. First, attend to the degree of homology between preclinical and clinical outcome measures, and use preclinical procedures with behavioral outcome measures homologous to clinically relevant outcomes in humans. Second, use combinations of preclinical procedures with complementary strengths and weaknesses to optimize both sensitivity and selectivity of preclinical testing. Third, take advantage of failed clinical translation to identify drugs that can be back-translated preclinically as active negative controls. Finally, increase precision of procedure labels by indicating both the pain stimulus and the pain behavior in naming preclinical procedures.

MicroRNA-1224 Splicing CircularRNA-Filip1l in an Ago2-Dependent Manner Regulates Chronic Inflammatory Pain via Targeting Ubr5

Dysfunctions of gene transcription and translation in the nociceptive pathways play the critical role in development and maintenance of chronic pain. Circular RNAs (circRNAs) are emerging as new players in regulation of gene expression, but whether and how circRNAs are involved in chronic pain remain elusive. We showed here that complete Freund's adjuvant-induced chronic inflammation pain significantly increased circRNA-Filip1l (filamin A interacting protein 1-like) expression in spinal neurons of mice. Blockage of this increase attenuated complete Freund's adjuvant-induced nociceptive behaviors, and overexpression of spinal circRNA-Filip1l in naive mice mimicked the nociceptive behaviors as evidenced by decreased thermal and mechanical nociceptive threshold. Furthermore, we found that mature circRNA-Filip1l expression was negatively regulated by miRNA-1224 via binding and splicing of precursor of circRNA-Filip1l (pre-circRNA-Filip1l) in the Argonaute-2 (Ago2)-dependent manner. Increase of spinal circRNA-Filip1l expression resulted from the decrease of miRNA-1224 expression under chronic inflammation pain state. miRNA-1224 knockdown or Ago2 overexpression induced nociceptive behaviors in naive mice, which was prevented by the knockdown of spinal circRNA-Filip1l. Finally, we demonstrated that a ubiquitin protein ligase E3 component n-recognin 5 (Ubr5), validated as a target of circRNA-Filip1l, plays a pivotal role in regulation of nociception by spinal circRNA-Filip1l. These data suggest that miRNA-1224-mediated and Ago2-dependent modulation of spinal circRNA-Filip1l expression regulates nociception via targeting Ubr5, revealing a novel epigenetic mechanism of interaction between miRNA and circRNA in chronic inflammation pain.SIGNIFICANCE STATEMENT circRNAs are emerging as new players in regulation of gene expression. Here, we found that the increase of circRNA-Filip1l mediated by miRNA-1224 in an Ago2-dependent way in the spinal cord is involved in regulation of nociception via targeting Ubr5 Our study reveals a novel epigenetic mechanism of interaction between miRNA and circRNA in chronic inflammation pain.

The analgesic efficacy of morphine varies with rat strain and experimental pain model: implications for target validation efforts in pain drug discovery

Background

Translating efficacy of analgesic drugs from animal models to humans remains challenging. Reasons are multifaceted, but lack of sufficiently rigorous preclinical study design criteria and phenotypically relevant models may be partly responsible. To begin to address this fundamental issue, we assessed the analgesic efficacy of morphine in three inbred rat strains (selected based on stress reactivity and affective/pain phenotypes), and outbred Sprague Dawley (SD) rats supplied from two vendors.

Methods

Sensitivity to morphine (0.3–6.0 mg/kg, s.c.) was evaluated in the hot plate test of acute thermal nociception, the Complete Freund's Adjuvant (CFA) model of inflammatory‐induced mechanical hyperalgesia, and in a locomotor motility assay in male rats from the following strains; Lewis (LEW), Fischer (F344), Wistar Kyoto (WKY), and SD's from Envigo and Charles River.

Results

F344 and SD rats were similarly sensitive to morphine in hot plate and CFA‐induced inflammatory hyperalgesia (Minimum Effective Dose (MED) = 3.0 mg/kg). WKY rats developed a less robust mechanical hypersensitivity after CFA injection, and were less sensitive to morphine in both pain tests (MED = 6.0 mg/kg). LEW rats were completely insensitive to morphine in the hot plate test, in contrast to the reversal of CFA‐induced hyperalgesia (MED = 3.0 mg/kg). All strains exhibited a dose‐dependent reduction in locomotor activity at 3.0–6.0 mg/kg.

Conclusion

Sensory phenotyping in response to acute thermal and inflammatory‐induced pain, and sensitivity to morphine in various inbred and outbred rat strains indicates that different pathophysiological mechanisms are engaged after injury. This could have profound implications for translating preclinical drug discovery efforts into pain patients.

Significance

The choice of rat strain used in preclinical pain research can profoundly affect the outcome of experiments in relation to (a) nociceptive threshold responses, and (b) efficacy to analgesic treatment, in assays of acute and tonic inflammatory nociceptive pain.

Deficits in Burrowing Behaviors Are Associated With Mouse Models of Neuropathic but Not Inflammatory Pain or Migraine

Burrowing, or the removal of material from an enclosed tube, is emerging as a prominent means of testing changes in a voluntary behavior in rodent models of various pain states. Here, we report no significant differences between male and female mice in terms of burrowing performance, in a substantially shorter time frame than previous reports. We found that the color of the burrow tube affects the variability of burrowing performance when tested in a lit room, suggesting that light aversion is at least a partial driver of this behavior. Spared nerve injury (SNI; as a model of neuropathy) impairs burrowing performance and correlates with enhanced mechanical sensitivity as assessed by von Frey filaments, as well as being pharmacologically reversed by an analgesic, gabapentin. Loss of the SNI-induced burrowing deficit was observed with daily testing post-surgery, but not when the testing interval was increased to 5 days, suggesting a confounding effect of daily repeat testing in this paradigm. Intraplantar complete Freund’s adjuvant (as a model of inflammatory pain) and systemic nitroglycerin (as a model of migraine-like symptoms) administration did not induce any burrowing deficit, indicating that assessment of burrowing behavior may not be universally suitable for the detection of behavioral changes across all rodent pain models.

Parathyroid hormone-related peptide activates and modulates TRPV1 channel in human DRG neurons

Parathyroid hormone-related peptide (PTHrP) is associated with advanced tumor growth and metastasis, especially in breast, prostate and myeloma cancers that metastasize to bones, resulting in debilitating chronic pain conditions. Our recent studies revealed that the receptor for PTHrP, PTH1R, is expressed in mouse DRG sensory neurons, and its activation leads to flow-activation and modulation of TRPV1 channel function, resulting in peripheral heat and mechanical hypersensitivity. In order to verify the translatability of our findings in rodents to humans, we explored whether this signalling axis operates in primary human DRG sensory neurons. Analysis of gene expression data from recently reported RNA deep sequencing experiments performed on mouse and human DRGs reveals that PTH1R is expressed in DRG and tibial nerve. Furthermore, exposure of cultured human DRG neurons to PTHrP leads to slow-sustained activation of TRPV1 and modulation of capsaicin-induced channel activation. Both activation and modulation of TRPV1 by PTHrP were dependent on PKC activity. Our findings suggest that functional PTHrP/PTH1R-TRPV1 signalling exists in human DRG neurons, which could contribute to local nociceptor excitation in the vicinity of metastatic bone tumor microenvironment.

Chronic pain patients' perspectives of medical cannabis

Medical cannabis (MC) is used for a variety of conditions including chronic pain. The goal of this report was to provide an in-depth qualitative exploration of patient perspectives on the strengths and limitations of MC. Members of MC dispensaries (N = 984) in New England including two-thirds with a history of chronic pain completed an online survey. In response to "How effective is medical cannabis in treating your symptoms or conditions?," with options of 0% "no relief" to 100% "complete relief," the average was 74.6% ± 0.6. The average amount spent on MC each year was $3064.47 ± 117.60, median = $2320.23, range = $52.14 to $52,140.00. Open-ended responses were coded into themes and subthemes. Analysis of answers to "What is it that you like most about MC?" (N = 2592 responses) identified 10 themes, including health benefits (36.0% of responses, eg, "Changes perception and experience of my chronic pain."), the product (14.2%, eg, "Knowing exactly what strain you are getting"), nonhealth benefits (14.1%), general considerations (10.3%), and medications (7.1%). Responses (N = 1678) to "What is it that you like least about MC?" identified 12 themes, including money (28.4%, eg, "The cost is expensive for someone on a fixed income"), effects (21.7%, eg, "The effects on my lungs"), the view of others (11.4%), access (8.2%), and method of administration (7.1%). These findings provide a patient-centered view on the advantages (eg, efficacy in pain treatment, reduced use of other medications) and disadvantages (eg, economic and stigma) of MC.

Electrical peripheral nerve stimulation relieves bone cancer pain by inducing Arc protein expression in the spinal cord dorsal horn

Objective

The analgesic effect on chronic pain of peripheral nerve stimulation (PNS) has been proven, but its underlying mechanism remains unknown. Therefore, this study aimed to assess the analgesic effect of PNS on bone cancer pain in a rat model and to explore the underlying mechanism.

Materials and methods

PNS on sciatic nerves with bipolar electrode was performed in both naïve and bone cancer pain model rats. Then, the protein levels of activity-regulated cytoskeleton-associated protein (Arc), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid–type glutamate receptor 1 (GluA1), and phosphate N-methyl-d-aspartic acid-type glutamate receptor subunit 2B (pGluNR2B) in spinal cord were evaluated by immunohistochemistry and Western blotting. Thermal paw withdraw latency and mechanical paw withdraw threshold were used to estimate the analgesic effect of PNS on bone cancer pain. Intrathecal administration of Arc shRNA was used to inhibit Arc expression in the spinal cord.

Results

PNS at 60 and 120 Hz for 20 min overtly induced Arc expression in the spinal cord, increased thermal pain thresholds in naïve rats, and relieved bone cancer pain; meanwhile, 10 Hz PNS did not achieve those results. In addition, PNS at 60 and 120 Hz also reduced the expression of GluA1, but not pGluNR2B, in the spinal cord. Finally, the anti-nociceptive effect and GluA1 downregulation induced by PNS were inhibited by intrathecal administration of Arc shRNA.

Conclusion

PNS (60 Hz, 0.3 mA) can relieve bone-cancer-induced allodynia and hyperalgesia by upregulating Arc protein expression and then by decreasing GluA1 transcription in the spinal cord dorsal horn.

Spontaneous painful disease in companion animals can facilitate the development of chronic pain therapies for humans

OBJECTIVE

To outline the role that spontaneous osteoarthritis (OA) in companion animals can play in translational research and therapeutic pharmacological development.

OUTLINE

Narrative review summarizing the opportunities and limitations of naturally occurring, spontaneous OA as models of human OA pain, with a focus on companion animal pets. The background leading to considering inserting spontaneous disease models in the translational paradigm is provided. The utility of this model is discussed in terms of outcome measures that have been validated as being related to pain, and in terms of the potential for target discovery is outlined. The limitations to using companion animal pets as models of human disease are discussed.

CONCLUSIONS

Although many steps along the translational drug development pathway have been identified as needing improvement, spontaneous painful OA in companion animals offers translational potential. Such 'models' may better reflect the complex genetic, environmental, temporal and physiological influences present in humans and current data suggests the predictive validity of the models are good. The opportunity for target discovery exists but is, as yet, unproven.

Pharmacological validation of voluntary gait and mechanical sensitivity assays associated with inflammatory and neuropathic pain in mice

The urgent need for more effective analgesic treatment options has prompted a re-evaluation of the behavioral tests used to assess pain in pre-clinical research, with an emphasis on inclusion of more voluntary, un-evoked behavioral assessments of pain. In order to validate voluntary gait analysis and a voluntary mechanical conflict-avoidance assay, we tested mouse models of neuropathy (spared nerve injury) and inflammation (complete Freund's adjuvant) alongside reflexive measures of mechanical and thermal hypersensitivity. To establish whether the observed changes in behavioral responses were pain-related, known analgesics (buprenorphine, gabapentin, carprofen) were also administered. Spared nerve injury persistently altered several gait indices, whereas complete Freund's adjuvant caused only transient changes. Furthermore, known analgesics could not reverse these gait changes, despite demonstrating their previously established efficacy in reflexive measures of mechanical and thermal hypersensitivity. In contrast, the mechanical conflict-avoidance assay demonstrated aversion in mice with neuropathy and inflammation-induced hypersensitivity, which could both be reversed by analgesics. We conclude that voluntary gait changes in rodent neuropathic and inflammatory pain models are not necessarily indicative of pain-related adaptations. On the other hand, mechanical conflict-avoidance represents a valid operant assay for quantifying pain-related behaviors in mice that can be reversed by known analgesics.

Inflammation and nerve injury minimally affect mouse voluntary behaviors proposed as indicators of pain

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Inflammation suppressed wheel running and locomotion, and impaired gait in mice.

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Nerve injury gave rise to gait deficits that are likely motor-, not pain-related.

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Changes in wheel running or gait were unrelated to the degree of hypersensitivity.

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Neither inflammation nor nerve injury altered social interactions or anxiety-like behavior.

It has been suggested that the lack of rodent behavioral assays that represent the complexities of human pain contributes to the poor translational record of basic pain research findings. Clinically, chronic pain interferes with patient mobility and physical/social activities, and increases anxiety symptoms, in turn negatively impacting quality of life. To determine whether these behaviors are similarly influenced by putative pain manipulations in rodents, we systematically evaluated wheel running, locomotion, gait, social interaction, and anxiety-like behavior in models of inflammation and nerve injury in adult C57BL6/J male mice. We demonstrate that inflammation and nerve injury differentially affect voluntary behaviors while mice are hypersensitive to mechanical stimuli. Bilateral Complete Freund’s Adjuvant (CFA)-induced inflammation transiently suppressed wheel running and locomotion and also induced gait deficits. In contrast, spared nerve injury (SNI) altered gait and impaired gross motor coordination. SNI-induced gait changes were not reversed by the analgesic PD123319, an angiotensin II type 2 receptor antagonist, and are therefore likely to be motor-related rather than pain-related. Neither CFA nor SNI significantly altered social interaction or elicited general anxiety-like behavior. Our findings suggest that in contrast to humans, mobility and physical/social activities are minimally altered, if at all, in mice following inflammation or nerve injury.