Animal models of pain: progress and challenges

Thermosensitive transient receptor potential (TRP) channel agonists and their role in mechanical, thermal and nociceptive sensations as assessed using animal models

INTRODUCTION

The present paper summarizes research using animal models to investigate the roles of thermosensitive transient receptor potential (TRP) channels in somatosensory functions including touch, temperature and pain. We present new data assessing the effects of eugenol and carvacrol, agonists of the warmth-sensitive TRPV3, on thermal, mechanical and pain sensitivity in rats.

METHODS

Thermal sensitivity was assessed using a thermal preference test, which measured the amount of time the animal occupied one of two adjacent thermoelectric plates set at different temperatures. Pain sensitivity was assessed as an increase in latency of hindpaw withdrawal away from a noxious thermal stimulus directed to the plantar hindpaw (Hargreaves test). Mechanical sensitivity was assessed by measuring the force exerted by an electronic von Frey filament pressed against the plantar surface that elicited withdrawal.

RESULTS

Topical application of eugenol and carvacrol did not significantly affect thermal preference, although there was a trend toward avoidance of the hotter surface in a 30 vs. 45°C preference test for rats treated with 1 or 10% eugenol and carvacrol. Both eugenol and carvacrol induced a concentration-dependent increase in thermal withdrawal latency (analgesia), with no significant effect on mechanosensitivity.

CONCLUSIONS

The analgesic effect of eugenol and carvacrol is consistent with previous studies. The tendency for these chemicals to increase the avoidance of warmer temperatures suggests a possible role for TRPV3 in warmth detection, also consistent with previous studies. Additional roles of other thermosensitive TRP channels (TRPM8 TRPV1, TRPV2, TRPV4, TRPM3, TRPM8, TRPA1, TRPC5) in touch, temperature and pain are reviewed.

Evaluation of dynamic weight bearing for measuring nonevoked inflammatory hyperalgesia in mice

BACKGROUND

Animal models in pain research have suggested that inclusion of both evoked and nonevoked behavioral measures is needed to better reflect the human pain experience. Individuals with chronic pain are known to experience spontaneous pain, in addition to pain after exposure to an external stimulus. Recently, the dynamic weight bearing (DWB) apparatus was developed to assess for nonevoked hyperalgesia by capturing weight bearing and surface distribution in the paws of mice after acute inflammation.

OBJECTIVES

The aim of this study was to evaluate the DWB test as a measure of nonevoked hyperalgesia.

METHODS

The experimental group received an intraplantar injection in the left hind paw of the inflammatory agent--complete Freund's adjuvant (CFA)--whereas the vehicle control group received a saline injection and the naive control group had no treatment. Calipers and a plethysmometer were used to verify inflammation and the hot-plate test was used as a measure for stimulus-evoked hyperalgesia. Data were collected at baseline; 3 hours; and 1, 3, and 7 days after injection.

RESULTS

Mice injected with CFA showed a statistically significant higher mean paw thickness and volume displacement compared with the vehicle and naive control groups. In the hot-plate testing, CFA-treated mice showed lower response temperature at 7 days compared with the other groups. On the DWB test, CFA-treated mice showed a reduction in the ipsilateral paw load and surface area compared with the contralateral paw load at Days 1, 3, and 7.

DISCUSSION

Mice with inflammation showed alterations in weight bearing as well as increased thermal hyperalgesia in comparison with control groups. These findings support the use of the DWB test as a tool for measuring nonevoked inflammatory hyperalgesia in a mouse model.

Initial evaluation of nighttime restlessness in a naturally occurring canine model of osteoarthritis pain

Chronic pain due to osteoarthritis (OA) can lead to significant disruption of sleep and increased restlessness. Our objective was to assess whether naturally occurring canine OA is associated with nighttime restlessness and so has potential as a model of OA-associated sleep disturbance. The study was designed as a two-part prospective masked, placebo-controlled study using client-owned dogs (Part A n = 60; Part B n = 19). Inclusion criteria consisted of OA-associated joint pain and mobility impairment. The primary outcome measure for both parts was nighttime accelerometry. In Part B, quality of sleep was assessed using a clinical metrology instrument (Sleep and Night Time Restlessness Evaluation Score, SNoRE). Part A included dogs receiving two weeks of non-steroidal anti-inflammatory drug (NSAID) preceded with two weeks of no treatment. Part B was a crossover study, with NSAID/placebo administered for two weeks followed by a washout period of one week and another two weeks of NSAID/placebo. Repeated measures analysis of variance was used to assess differences between baseline and treatment. There were no significant changes in accelerometry-measured nighttime activity as a result of NSAID administration. SNoRE measures indicated significant improvements in aspects of the quality of nighttime sleep that did not involve obvious movement. These results reflect the few similar studies in human OA patients. Although accelerometry does not appear to be useful, this model has potential to model the human pain-related nighttime sleep disturbance, and other outcome measures should be explored in this model.

The pain of pain: challenges of animal behavior models

Berend Olivier has had a long-standing interest in the utility of animal models for a wide variety of therapeutic indications. His work has spanned multiple types of models, blending ethological, or species typical and naturalistic behaviors, along with methodologies based on learned behavior. He has consistently done so, from an analytical as well as predictive perspective, and has made multiple contributions while working in both the pharmaceutical industry and within an academic institution. Although focused primarily on psychiatric disorders, Berend has conducted research in the area of pain in humans and in animals, demonstrating an expansive appreciation for the breadth, scope and significance of the science and applications of the discipline of pharmacology to these diverse areas. This review focuses on the use of animal models in pain research from the perspective of the long-standing deficiencies in the development of therapeutics in this area and from a preclinical perspective where the translational weaknesses have been quite problematic. The challenges confronting animal models of pain, however, are not unique to this area of research, as they cut across several therapeutic areas. Despite the deficiencies, failures and concerns, existing animal models of pain continue to be of widespread use and are essential to progress in pain research as well as in other areas. Although not focusing on specific animal models of pain, this paper seeks to examine general issues facing the use of these models. It does so by exploring alternative approaches which capture recent developments, which build upon principles and concepts we have learned from Berend's contributions, and which provide the prospect of helping to address the absence of novel therapeutics in this area.

Effect of a Novel, Orally Active Matrix Metalloproteinase-2 and -9 Inhibitor in Spinal and Trigeminal Rat Models of Neuropathic Pain

AIMS

To study the effects of a novel matrix metalloproteinase-2 (MMP-2) and MMP-9 inhibitor, AQU-118, on mechanical allodynia in the spinal nerve ligation (SNL) model of neuropathic pain and the chronic constriction injury of the infraorbital nerve (CCI-IoN) model of neuropathic orofacial pain.

METHODS

Five groups of SNL rats were given daily oral doses of AQU-118 (5, 10, 20 mg/kg), gabapentin (100 mg/kg), or vehicle (0.5% methylcellulose) and then paw withdrawal threshold was measured with von Frey filaments (VF). Three groups of CCI-IoN rats were given daily oral doses of either AQU-118 (40 mg/kg), gabapentin (100 mg/kg), or vehicle (0.5% methylcellulose) and then mechanical allodynia was measured with facial VF and non-reflex-based orofacial stimulation test (OFST) assay. Naïve rats were also tested for the effect of AQU-118 (40 mg/kg) on basal sensitivity to mechanical stimulation/locomotive activity.

RESULTS

Mechanical allodynia in SNL rats was attenuated by gabapentin (100 mg/kg) and AQU-118 (in a dose-dependent manner). Mechanical allodynia in CCI-IoN rats was also attenuated (in an equipotent manner) by both AQU-118 (40 mg/ kg) and gabapentin (100 mg/kg) as measured by both facial VF and OFST assay. Upon cessation of either AQU-118 or gabapentin, VF-related responses in both models and OFST assay times reverted to levels observed in vehicle-treated rats. No statistically significant change was observed in locomotive activity/paw withdrawal threshold by AQU-118 (40 mg/kg) in naïve rats.

CONCLUSION

The results demonstrated that oral AQU-118 attenuates mechanical allodynia in both neuropathic pain models and with efficacies that mirror gabapentin at the 40 mg/kg dose used in the CCI-IoN model but without effect on basal sensitivity to mechanical stimulation/locomotive activity. These findings support a possible role for MMP-2/-9 in the etiology of neuropathic pain and also suggest that inhibition strategies represent a viable treatment option.

Sensory Neurons, Ion Channels, Inflammation and the Onset of Neuropathic Pain

Neuropathic pain often fails to respond to conventional pain management procedures. here we review the aetiology of neuropathic pain as would result from peripheral neuropathy or injury. We show that inflammatory mediators released from damaged nerves and tissue are responsible for triggering ectopic activity in primary afferents and that this, in turn, provokes increased spinal cord activity and the development of ‘central sensitization’. Although evidence is mounting to support the role of interleukin-1β, prostaglandins and other cytokines in the onset of neuropathic pain, the clinical efficacy of drugs which antagonize or prevent the actions of these mediators is yet to be determined. basic science findings do, however, support the use of pre-emptive analgesia during procedures which involve nerve manipulation and the use of anti-inflammatory steroids as soon as possible following traumatic nerve injury.

Effects of μ-opioid receptor agonists in assays of acute pain-stimulated and pain-depressed behavior in male rats: role of μ-agonist efficacy and noxious stimulus intensity

Pain is associated with stimulation of some behaviors and depression of others, and μ-opioid receptor agonists are among the most widely used analgesics. This study used parallel assays of pain-stimulated and pain-depressed behavior in male Sprague-Dawley rats to compare antinociception profiles for six μ-agonists that varied in efficacy at μ-opioid receptors (from highest to lowest: methadone, fentanyl, morphine, hydrocodone, buprenorphine, and nalbuphine). Intraperitoneal injection of diluted lactic acid served as an acute noxious stimulus to either stimulate stretching or depress operant responding maintained by electrical stimulation in an intracranial self-stimulation (ICSS). All μ-agonists blocked both stimulation of stretching and depression of ICSS produced by 1.8% lactic acid. The high-efficacy agonists methadone and fentanyl were more potent at blocking acid-induced depression of ICSS than acid-stimulated stretching, whereas lower-efficacy agonists displayed similar potency across assays. All μ-agonists except morphine also facilitated ICSS in the absence of the noxious stimulus at doses similar to those that blocked acid-induced depression of ICSS. The potency of the low-efficacy μ-agonist nalbuphine, but not the high-efficacy μ-agonist methadone, to block acid-induced depression of ICSS was significantly reduced by increasing the intensity of the noxious stimulus to 5.6% acid. These results demonstrate sensitivity of acid-induced depression of ICSS to a range of clinically effective μ-opioid analgesics and reveal distinctions between opioids based on efficacy at the μ-receptor. These results also support the use of parallel assays of pain-stimulated and -depressed behaviors to evaluate analgesic efficacy of candidate drugs.

Improving and accelerating drug development for nervous system disorders

Advances in the neurosciences have placed the field in the position where it is poised to significantly reduce the burden of nervous system disorders. However, drug discovery, development, and translation for nervous system disorders still pose many unique challenges. The key scientific challenges can be summarized as follows: mechanisms of disease, target identification and validation, predictive models, biomarkers for patient stratification and as endpoints for clinical trials, clear regulatory pathways, reliability and reproducibility of published data, and data sharing and collaboration. To accelerate nervous system drug development, the Institute of Medicine's Forum on Neuroscience and Nervous System Disorders has hosted a series of public workshops that brought together representatives of industry, government (including both research funding and regulatory agencies), academia, and patient groups to discuss these challenges and offer potential strategies to improve the translational neuroscience.

Antihyperalgesic effects of imidazoline I(2) receptor ligands in rat models of inflammatory and neuropathic pain

BACKGROUND AND PURPOSE

A new imidazoline I2 receptor ligand, CR4056, is effective for chronic inflammatory pain and diabetic neuropathy. However, it is unclear whether other I2 receptor ligands have similar effects and whether antinociceptive tolerance develops with repeated treatment.

EXPERIMENTAL APPROACH

The Von Frey filament test was used to measure mechanical hyperalgesia and the plantar test to measure thermal hyperalgesia in rats injected with complete Freund's adjuvant (CFA) treatment or had undergone surgery to induce chronic constriction injury (CCI), models of inflammatory pain and peripheral neuropathic pain respectively. The effects of morphine and I2 receptor ligands, 2-BFI, BU224, tracizoline and CR4056, 3.2-32 mg·kg(-1) , i.p., on hyperalgesia or affective pain (as measured by a place escape/avoidance paradigm) were studied in separate experiments.

KEY RESULTS

Morphine and the I2 receptor ligands (2-BFI, BU224 and tracizoline) all dose-dependently attenuated mechanical and thermal hyperalgesia in CFA-treated rats. The anti-hyperalgesic effects of 2-BFI in CFA-treated and CCI rats were attenuated by the I2 receptor antagonist idazoxan. The combination of 2-BFI and morphine produced additive effects against mechanical hyperalgesia in CFA-treated rats. Repeated treatment (daily for 7-9 days) with 2-BFI or CR4056 did not produce antinociceptive tolerance in CFA-treated or CCI rats. Morphine and the I2 receptor ligands (2-BFI, BU224 and CR4056) were all effective at attenuating place escape/avoidance behaviour in CFA-treated rats.

CONCLUSIONS AND IMPLICATIONS

Imidazoline I2 receptor ligands have antihyperalgesic effects in rat models of inflammatory and neuropathic pain and may represent a new class of pharmacotherapeutics for the management of chronic pain.

Molecular signatures of mouse TRPV1-lineage neurons revealed by RNA-Seq transcriptome analysis

Disorders of pain neural systems are frequently chronic and, when recalcitrant to treatment, can severely degrade the quality of life. The pain pathway begins with sensory neurons in dorsal root or trigeminal ganglia, and the neuronal subpopulations that express the transient receptor potential cation channel, subfamily V, member 1 (TRPV1) ion channel transduce sensations of painful heat and inflammation and play a fundamental role in clinical pain arising from cancer and arthritis. In the present study, we elucidate the complete transcriptomes of neurons from the TRPV1 lineage and a non-TRPV1 neuroglial population in sensory ganglia through the combined application of next-gen deep RNA-Seq, genetic neuronal labeling with fluorescence-activated cell sorting, or neuron-selective chemoablation. RNA-Seq accurately quantitates gene expression, a difficult parameter to determine with most other methods, especially for very low and very high expressed genes. Differentially expressed genes are present at every level of cellular function from the nucleus to the plasma membrane. We identified many ligand receptor pairs in the TRPV1 population, suggesting that autonomous presynaptic regulation may be a major regulatory mechanism in nociceptive neurons. The data define, in a quantitative, cell population-specific fashion, the molecular signature of a distinct and clinically important group of pain-sensing neurons and provide an overall framework for understanding the transcriptome of TRPV1 nociceptive neurons.

PERSPECTIVE

Next-gen RNA-Seq, combined with molecular genetics, provides a comprehensive and quantitative measurement of transcripts in TRPV1 lineage neurons and a contrasting transcriptome from non-TRPV1 neurons and cells. The transcriptome highlights previously unrecognized protein families, identifies multiple molecular circuits for excitatory or inhibitory autocrine and paracrine signaling, and suggests new combinatorial approaches to pain control.

Broad-spectrum analgesic efficacy of IBNtxA is mediated by exon 11-associated splice variants of the mu-opioid receptor gene

μ-Opioids remain vastly important for the treatment of pain, and would represent ideal analgesics if their analgesic effects could be separated from their many side effects. A recently synthesized compound, iodobenzoylnaltrexamide (IBNtxA), acting at 6-transmembrane (6-TM) splice variants of the μ-opioid receptor gene, was shown to have potent analgesic actions against acute, thermal pain accompanied by a vastly improved side-effect profile compared to 7-TM-acting drugs such as morphine. Whether such analgesia can be seen in longer-lasting and nonthermal algesiometric assays is not known. The current study demonstrates potent and efficacious IBNtxA inhibition of a wide variety of assays, including inflammatory and neuropathic hypersensitivity and spontaneous pain. We further demonstrate the dependence of such analgesia on 6-TM μ-opioid receptor variants using isobolographic analysis and the testing of Oprm1 (the μ-opioid receptor gene) exon 11 null mutant mice. Finally, the effect of nerve damage (spared nerve injury) and inflammatory injury (complete Freund's adjuvant) on expression of μ-opioid receptor variant genes in pain-relevant central nervous system loci was examined, revealing a downregulation of the mMOR-1D splice variant in the dorsal root ganglion after spared nerve injury. These findings are supportive of the potential value of 6-TM-acting drugs as novel analgesics.

Human sensory neurons: Membrane properties and sensitization by inflammatory mediators

Biological differences in sensory processing between human and model organisms may present significant obstacles to translational approaches in treating chronic pain. To better understand the physiology of human sensory neurons, we performed whole-cell patch-clamp recordings from 141 human dorsal root ganglion (hDRG) neurons from 5 young adult donors without chronic pain. Nearly all small-diameter hDRG neurons (<50 μm) displayed an inflection on the descending slope of the action potential, a defining feature of rodent nociceptive neurons. A high proportion of hDRG neurons were responsive to the algogens allyl isothiocyanate (AITC) and ATP, as well as the pruritogens histamine and chloroquine. We show that a subset of hDRG neurons responded to the inflammatory compounds bradykinin and prostaglandin E2 with action potential discharge and show evidence of sensitization including lower rheobase. Compared to electrically evoked action potentials, chemically induced action potentials were triggered from less depolarized thresholds and showed distinct afterhyperpolarization kinetics. These data indicate that most small/medium hDRG neurons can be classified as nociceptors, that they respond directly to compounds that produce pain and itch, and that they can be activated and sensitized by inflammatory mediators. The use of hDRG neurons as preclinical vehicles for target validation is discussed.

Operant nociception in nonhuman primates

The effective management of pain is a longstanding public health concern. Morphine-like opioids have long been front-line analgesics, but produce undesirable side effects that can limit their application. Slow progress in the introduction of novel improved medications for pain management over the last 5 decades has prompted a call for innovative translational research, including new preclinical assays. Most current in vivo procedures (eg, tail flick, hot plate, warm water tail withdrawal) assay the effects of nociceptive stimuli on simple spinal reflexes or unconditioned behavioral reactions. However, clinical treatment goals may include the restoration of previous behavioral activities, which can be limited by medication-related side effects that are not measured in such procedures. The present studies describe an apparatus and procedure to study the disruptive effects of nociceptive stimuli on voluntary behavior in nonhuman primates, and the ability of drugs to restore such behavior through their analgesic actions. Squirrel monkeys were trained to pull a cylindrical thermode for access to a highly palatable food. Next, sessions were conducted in which the temperature of the thermode was increased stepwise until responding stopped, permitting the determination of stable nociceptive thresholds. Tests revealed that several opioid analgesics, but not d-amphetamine or Δ(9)-THC, produced dose-related increases in threshold that were antagonist sensitive and efficacy dependent, consistent with their effects using traditional measures of antinociception. Unlike traditional reflex-based measures, however, the results also permitted the concurrent evaluation of response disruption, providing an index with which to characterize the behavioral selectivity of antinociceptive drugs.

Supraspinal TRPV1 modulates the emotional expression of abdominal pain

The transient receptor potential vanilloid receptor type-1 (TRPV1) is critically involved in peripheral nociceptive processes of somatic and visceral pain. However, the role of the capsaicin receptor in the brain regarding visceral pain remains elusive. Here, we studied the contribution of TRPV1 to abdominal pain transmission at different nociceptive pathway levels using TRPV1 knock-out mice, resiniferatoxin-mediated deletion of TRPV1-positive primary sensory neurons, and intracerebral TRPV1 antagonism. We found that constitutive genetic TRPV1 deletion or peripheral TRPV1 deletion reduced acetic acid-evoked abdominal constrictions, without affecting referred abdominal hyperalgesia or allodynia in an acute pancreatitis model of visceral pain. Notably, intracerebral TRPV1 antagonism by SB 366791 significantly reduced chemical and inflammatory spontaneous abdominal nocifensive responses, as observed by reduced expressions of nociceptive facial grimacing, illustrating the affective component of pain. In addition to the established role of cerebral TRPV1 in anxiety, fear, or emotional stress, we demonstrate here for the first time that TRPV1 in the brain modulates visceral nociception by interfering with the affective component of abdominal pain.

Assessment of morphine-induced hyperalgesia and analgesic tolerance in mice using thermal and mechanical nociceptive modalities

Opioid-induced hyperalgesia and tolerance severely impact the clinical efficacy of opiates as pain relievers in animals and humans. The molecular mechanisms underlying both phenomena are not well understood and their elucidation should benefit from the study of animal models and from the design of appropriate experimental protocols.

We describe here a methodological approach for inducing, recording and quantifying morphine-induced hyperalgesia as well as for evidencing analgesic tolerance, using the tail-immersion and tail pressure tests in wild-type mice. As shown in the video, the protocol is divided into five sequential steps. Handling and habituation phases allow a safe determination of the basal nociceptive response of the animals. Chronic morphine administration induces significant hyperalgesia as shown by an increase in both thermal and mechanical sensitivity, whereas the comparison of analgesia time-courses after acute or repeated morphine treatment clearly indicates the development of tolerance manifested by a decline in analgesic response amplitude. This protocol may be similarly adapted to genetically modified mice in order to evaluate the role of individual genes in the modulation of nociception and morphine analgesia. It also provides a model system to investigate the effectiveness of potential therapeutic agents to improve opiate analgesic efficacy.

Human models of pain for the prediction of clinical analgesia

Human experimental pain models are widely used to study drug effects under controlled conditions. However, efforts to improve both animal and human experimental model selection, on the basis of increased understanding of the underlying pathophysiological pain mechanisms, have been disappointing, with poor translation of results to clinical analgesia. We have developed an alternative approach to the selection of suitable pain models that can correctly predict drug efficacy in particular clinical settings. This is based on the analysis of successful or unsuccessful empirical prediction of clinical analgesia using experimental pain models. We analyzed statistically the distribution of published mutual agreements or disagreements between drug efficacy in experimental and clinical pain settings. Significance limits were derived by random permutations of agreements. We found that a limited subset of pain models predicts a large number of clinically relevant pain settings, including efficacy against neuropathic pain for which novel analgesics are particularly needed. Thus, based on empirical evidence of agreement between drugs for their efficacy in experimental and clinical pain settings, it is possible to identify pain models that reliably predict clinical analgesic drug efficacy in cost-effective experimental settings.

Preclinical assessment of pain: improving models in discovery research

To date, animal models have not sufficiently "filtered" targets for new analgesics, increasing the failure rate and cost of drug development. Preclinical assessment of "pain" has historically relied on measures of evoked behavioral responses to sensory stimuli in animals. Such measures can often be observed in decerebrated animals and therefore may not sufficiently capture affective and motivational aspects of pain, potentially diminishing translation from preclinical studies to the clinical setting. Further, evidence indicates that there are important mechanistic differences between evoked behavioral responses of hypersensitivity and ongoing pain, limiting evaluation of mechanisms that could mediate aspects of clinically relevant pain. The mechanisms underlying ongoing pain in preclinical models are currently being explored and may serve to inform decisions towards the transition from drug discovery to drug development for a given target.

The effect of intranasal oxytocin administration on acute cold pressor pain: a placebo-controlled, double-blind, within-participants crossover investigation

BACKGROUND

This study examined the effect of synthetic oxytocin delivered intranasally on acute pain sensitivity using a placebo-controlled, double-blind, within-participant crossover design.

METHODS

Thirty-seven (18 were male) pain-free young adults underwent two laboratory sessions separated by 1 week. Each session consisted of baseline, administration, second baseline, pain, and recovery phases, completed in a fixed order. Participants were given an intransal administration of 40 IU oxytocin or placebo. Blood pressure and heart rate (HR) were measured at 1-minute intervals throughout each phase. Pain was induced by submersing the nondominant hand in cold (2°C) water. Pain threshold, intensity, unpleasantness, and Short-Form McGill Pain Questionnaire-2 pain descriptors were rated immediately after pain testing. Mood was assessed using visual analog scales after baseline, second baseline, and pain phases. The second laboratory session was identical to the first, with the exception that a different nasal spray was administered.

RESULTS

Participants reported lower pain intensity (50.57 [20.94] versus 56.73 [20.12], p = .047), pain unpleasantness (47.00 [27.24] versus 55.78 [22.46], p = .033), and Short-Form McGill Pain Questionnaire-2 pain descriptors (53.38 [31.18] versus 60.92 [31.17], p = .031) and higher pain threshold (45.70 [59.55] versus 38.35 [59.12], p = .040) after oxytocin administration relative to placebo. There was a nasal spray by phase interaction on HR (p = .006). Pain-related increase in HR was attenuated by oxytocin nasal spray. Systolic and diastolic blood pressure increased during pain testing but were unaffected by nasal spray.

CONCLUSIONS

These results suggest that oxytocin can lead to decreased acute pain sensitivity.

Epigenetic modification of DRG neuronal gene expression subsequent to nerve injury: etiological contribution to complex regional pain syndromes (Part I)

DRG is of importance in relaying painful stimulation to the higher pain centers and therefore could be a crucial target for early intervention aimed at suppressing primary afferent stimulation. Complex regional pain syndrome (CRPS) is a common pain condition with an unknown etiology. Recently added new information enriches our understanding of CRPS pathophysiology. Researches on genetics, biogenic amines, neurotransmitters, and mechanisms of pain modulation, central sensitization, and autonomic functions in CRPS revealed various abnormalities indicating that multiple factors and mechanisms are involved in the pathogenesis of CRPS. Epigenetics refers to mitotically and meiotically heritable changes in gene expression that do not affect the DNA sequence. As epigenetic modifications potentially play an important role in inflammatory cytokine metabolism, neurotransmitter responsiveness, and analgesic sensitivity, they are likely key factors in the development of chronic pain. In this dyad review series, we systematically examine the nerve injury-related changes in the neurological system and their contribution to CRPS. In this part, we first reviewed and summarized the role of neural sensitization in DRG neurons in performing function in the context of pain processing. Particular emphasis is placed on the cellular and molecular changes after nerve injury as well as different models of inflammatory and neuropathic pain. These were considered as the potential molecular bases that underlie nerve injury-associated pathogenesis of CRPS.

Emerging targets in neuroinflammation-driven chronic pain

Current analgesics predominately modulate pain transduction and transmission in neurons and have limited success in controlling disease progression. Accumulating evidence suggests that neuroinflammation, which is characterized by infiltration of immune cells, activation of glial cells and production of inflammatory mediators in the peripheral and central nervous system, has an important role in the induction and maintenance of chronic pain. This Review focuses on emerging targets - such as chemokines, proteases and the WNT pathway - that promote spinal cord neuroinflammation and chronic pain. It also highlights the anti-inflammatory and pro-resolution lipid mediators that act on immune cells, glial cells and neurons to resolve neuroinflammation, synaptic plasticity and pain. Targeting excessive neuroinflammation could offer new therapeutic opportunities for chronic pain and related neurological and psychiatric disorders.

Review of preclinical studies on treatment of mucositis and associated pain

Oral mucositis is a significant problem in cancer patients treated with radiation or chemotherapy, often hindering definitive cancer treatment. For patients with oral mucositis, pain is the most distressing symptom, leading to loss of orofacial function and poor quality of life. While oral mucositis has been well-described, its pathophysiology is poorly understood. Oral health professionals treating patients with mucositis have almost no effective therapies to treat or prevent oral mucositis. The purpose of this review is to (1) describe the current preclinical models of oral mucositis and their contribution to the understanding of mucositis pathophysiology, (2) explore preclinical studies on therapies targeting mucositis and discuss the clinical trials that have resulted from these preclinical studies, and (3) describe the proposed pathophysiology of oral mucositis pain and preclinical modeling of oral mucositis pain.

Experimenter effects on behavioral test scores of eight inbred mouse strains under the influence of ethanol

Eight standard inbred mouse strains were evaluated for ethanol effects on a refined battery of behavioral tests in a study that was originally designed to assess the influence of rat odors in the colony on mouse behaviors. As part of the design of the study, two experimenters conducted the tests, and the study was carefully balanced so that equal numbers of mice in all groups and times of day were tested by each experimenter. A defect in airflow in the facility compromised the odor manipulation, and in fact the different odor exposure groups did not differ in their behaviors. The two experimenters, however, obtained markedly different results for three of the tests. Certain of the experimenter effects arose from the way they judged behaviors that were not automated and had to be rated by the experimenter, such as slips on the balance beam. Others were not evident prior to ethanol injection but had a major influence after the injection. For several measures, the experimenter effects were notably different for different inbred strains. Methods to evaluate and reduce the impact of experimenter effects in future research are discussed.

Soluble epoxide hydrolase inhibition is antinociceptive in a mouse model of diabetic neuropathy

Neuropathic pain is currently an insufficiently treated clinical condition. There remains a critical need for efficacious therapies without severe side effects to treat the uniquely persistent and tonic pain of neuropathy. Inhibitors of the soluble epoxide hydrolase (sEH) enzyme that stabilize endogenous epoxy fatty acids have demonstrated antihyperalgesia in clinical chronic inflammatory pain and modeled neuropathic pain. Recently, the conditioned place preference assay has been used to evaluate the tonic nature of neuropathy in several animal models. The current experiments use the conditioned place preference assay alongside withdrawal thresholds to investigate the antihyperalgesic efficacy of sEH inhibitors in a murine model of diabetic neuropathy. Here, the sEH inhibitor trans-4-[4-(3-trifluoromethoxyphenyl-1-ureido)-cyclohexyloxy]-benzoic acid (t-TUCB) at 10 mg/kg induced a robust place preference in diabetic neuropathic mice representative of pain relief. Importantly, this effect was absent both in control mice and in sEH-knockout mice at the same dose, indicating that t-TUCB is not positively reinforcing or rewarding. When compared to gabapentin, t-TUCB elicited a similar degree of withdrawal threshold improvement without the same degree of spontaneous locomotion decline in neuropathic mice. Overall, these experiments show that inhibiting the sEH enzyme attenuates chronic pain and offers an alternative to current side-effect-limited therapies to meet this clinical need.

PERSPECTIVE

These experiments demonstrate antihyperalgesia in a murine chronic pain model mediated by inhibiting the sEH enzyme. The results of this study indicate that inhibiting the sEH is a promising alternative for blocking chronic pain.

A review of the Institute of Medicine's analysis of using chimpanzees in biomedical research

We argue that the recommendations made by the Institute of Medicine's 2011 report, Chimpanzees in Biomedical and Behavioral Research: Assessing the Necessity, are methodologically and ethically confused. We argue that a proper understanding of evolution and complexity theory in terms of the science and ethics of using chimpanzees in biomedical research would have had led the committee to recommend not merely limiting but eliminating the use of chimpanzees in biomedical research. Specifically, we argue that a proper understanding of the difference between the gross level of examination of species and examinations on finer levels can shed light on important methodological and ethical inconsistencies leading to ignorance of potentially unethical practices and policies regarding the use of animals in scientific research.

Improving the translation of analgesic drugs to the clinic: animal models of neuropathic pain

Neuropathic pain remains an area of considerable unmet clinical need. Research based on preclinical animal models has failed to deliver truly novel treatment options, questioning the predictive value of these models. This review addresses the shortcomings of rodent in vivo models commonly used in the field and highlights approaches which could increase their predictivity, including more clinically relevant assays, outcome measures and animal characteristics. The methodological quality of animal studies also needs to be improved. Low internal validity and incomplete reporting lead to a waste of valuable research resources and animal lives, and ultimately prevent an objective assessment of the true predictivity of in vivo models.

Psychometric assessment of the Rat Grimace Scale and development of an analgesic intervention score

Our limited ability to assess spontaneous pain in rodent models of painful human conditions may be associated with a translational failure of promising analgesic compounds in to clinical use. If measurement of spontaneous pain behaviours can be used to generate an analgesic intervention score their use could expand to guide the use of analgesics, as mandated by regulatory bodies and ethical and welfare obligations. One such measure of spontaneous pain, the Rat Grimace Scale (RGS), has recently been described and shown to exhibit reliability. However, reliability of measurement scores is context and content specific, and further testing required to assess translation to a heterogenous setting (different model, raters, environment). The objectives of this study were to perform reliability testing with the Rat Grimace Scale in a heterogenous setting and generate an analgesic intervention score for its use. In a randomised, blinded study, sixteen adult female rats received one of three analgesia treatments (0.05 mg/kg buprenorphine subcutaneously, 1 mg/kg meloxicam subcutaneously, 0.2 mg/kg oral buprenorphine in jelly) peri-operatively (telemetry unit implantation surgery). Rats were video-recorded (before, 1-6 and 12 hours post-operatively) and images collected for independent scoring by three blinded raters using the RGS, and five experts based on "pain/no pain" assessment. Scores were used to calculate inter- and intra-rater reliability with an intraclass correlation coefficient and generate an analgesic intervention score with receiver operating characteristic curve analysis. The RGS scores showed very good inter- and intra-rater reliability (0.85 [0.78-0.90 95% CI] and 0.83 [0.76-0.89], respectively). An analgesic intervention threshold of greater than 0.67 was determined. These data demonstrate that the RGS is a useful tool which can be successfully employed in a heterogenous setting, and has the potential to guide analgesic intervention.

Relief of pain induced by varicella-zoster virus in a rat model of post-herpetic neuralgia using a herpes simplex virus vector expressing enkephalin

Acute and chronic pain (post-herpetic neuralgia or PHN) are encountered in patients with herpes zoster that is caused by reactivation of varicella-zoster virus (VZV) from a state of neuronal latency. PHN is often refractory to current treatments, and additional strategies for pain relief are needed. Here we exploited a rat footpad model of PHN to show that herpes simplex virus (HSV) vector-mediated gene delivery of human preproenkephalin (vHPPE) effectively reduced chronic VZV-induced nocifensive indicators of pain. VZV inoculated at the footpad induced prolonged mechanical allodynia and thermal hyperalgesia that did not develop in controls or with ultraviolet light-inactivated VZV. Subsequent footpad administration of vHPPE relieved VZV-induced pain behaviors in a dose-dependent manner for extended periods, and prophylactic vector administration prevented VZV-induced pain from developing. Short-term pain relief following low-dose vHPPE administration could be effectively prolonged by vector re-administration. HPPE transcripts were increased three- to fivefold in ipsilateral ganglia, but not in the contralateral dorsal root ganglia. VZV hypersensitivity and its relief by vHPPE were not affected by peripheral delivery of opioid receptor agonist or antagonist, suggesting that the efficacy was mediated at the ganglion and/or spinal cord level. These results support further development of ganglionic expression of enkephalin as a novel treatment for the pain associated with Zoster.

Chronic at-level thermal hyperalgesia following rat cervical contusion spinal cord injury is accompanied by neuronal and astrocyte activation and loss of the astrocyte glutamate transporter, GLT1, in superficial dorsal horn

Neuropathic pain is a form of pathological nociception that occurs in a significant portion of traumatic spinal cord injury (SCI) patients, resulting in debilitating and often long-term physical and psychological burdens. While many peripheral and central mechanisms have been implicated in neuropathic pain, central sensitization of dorsal horn spinothalamic tract (STT) neurons is a major underlying substrate. Furthermore, dysregulation of extracellular glutamate homeostasis and chronic astrocyte activation play important underlying roles in persistent hyperexcitability of these superficial dorsal horn neurons. To date, central sensitization and astrocyte changes have not been characterized in cervical SCI-induced neuropathic pain models, despite the fact that a major portion of SCI patients suffer contusion trauma to cervical spinal cord. In this study, we have characterized 2 rat models of unilateral cervical contusion SCI that behaviorally result in chronic persistence of thermal hyperalgesia in the ipsilateral forepaw. In addition, we find that STT neurons are chronically activated in both models when compared to laminectomy-only uninjured rats. Finally, persistent astrocyte activation and significantly reduced expression of the major CNS glutamate transporter, GLT1, in superficial dorsal horn astrocytes are associated with both excitability changes in STT neurons and the neuropathic pain behavioral phenotype. In conclusion, we have characterized clinically-relevant rodent models of cervical contusion-induced neuropathic pain that result in chronic activation of both STT neurons and astrocytes, as well as compromise in astrocyte glutamate transporter expression. These models can be used as important tools to further study mechanisms underlying neuropathic pain post-SCI and to test potential therapeutic interventions.

Pain assessment in animal models: do we need further studies?

In the last two decades, animal models have become important tools in understanding and treating pain, and in predicting analgesic efficacy. Although rodent models retain a dominant role in the study of pain mechanisms, large animal models may predict human biology and pharmacology in certain pain conditions more accurately. Taking into consideration the anatomical and physiological characteristics common to man and pigs (median body size, digestive apparatus, number, size, distribution and communication of vessels in dermal skin, epidermal-dermal junctions, the immunoreactivity of peptide nerve fibers, distribution of nociceptive and non-nociceptive fiber classes, and changes in axonal excitability), swines seem to provide the most suitable animal model for pain assessment. Locomotor function, clinical signs, and measurements (respiratory rate, heart rate, blood pressure, temperature, electromyography), behavior (bright/quiet, alert, responsive, depressed, unresponsive), plasma concentration of substance P and cortisol, vocalization, lameness, and axon reflex vasodilatation by laser Doppler imaging have been used to assess pain, but none of these evaluations have proved entirely satisfactory. It is necessary to identify new methods for evaluating pain in large animals (particularly pigs), because of their similarities to humans. This could lead to improved assessment of pain and improved analgesic treatment for both humans and laboratory animals.

Analgesic efficacy and safety of DALDA peptide analog delivery to the brain using oil-in-water nanoemulsion formulation

PURPOSE

The main objective of this study was to develop and evaluate therapeutic efficacy and safety following systemic delivery of a peptide analgesic into the CNS using an oil-in-water nanoemulsion system.

METHODS

We have formulated a safe and effective, omega-3 rich polyunsaturated fatty acid containing oil-in-water nanoemulsion formulation, for encapsulating and delivering chemically-modified DALDA, a potent mu-opioid peptide analogue, to the CNS. One of the challenges with CNS delivery is the lack of a non-invasive bioanalytical technique to confirm CNS uptake and therapeutic efficacy. Using blood oxygen-level dependent (BOLD) functional magenetic resonance imaging (fMRI), we provide quantitative evidence of nanoemulsion-based delivery and analgesic activity of DALDA analogue in capsaicin-induced awake rat model of pain.

RESULTS

Nanoemulsion formulation effectively encapsulated the modified analgesic peptide and demonstrated efficacy in the capsaicin- pain induced functional magnetic resonance imaging model in rodents. Preliminary safety evaluations show that the nanoemulsion system was well tolerated and did not cause any acute negative effects.

CONCLUSIONS

Overall, these results show tremendous opportunity for the development of modified peptide analgesic-encapsulated nanoemulsion formulations for CNS delivery and therapeutic efficacy.

Pain and depression comorbidity: a preclinical perspective

Pain and depression are two highly prevalent and deleterious disorders with significant socioeconomic impact to society. Clinical observations have long recognized the co-existence and interactions of pain and depression. However, the underlying mechanisms of pain-depression comorbidity and their dynamic interactions remain largely unknown. Preclinical animal studies may provide critical information for the understanding of this important comorbidity. This review analyzed the current preclinical evidence of interactions between pain and depression, which generally supports the causative relationship of the two conditions. In addition, the analysis proposed to apply domain interplay concept in future model development of pain-depression comorbidity and mechanism studies. The application of spectrum-centered animal models will better the understanding of pain-depression dyad and foster the development of more effective therapeutic strategies.

A review of the studies using buprenorphine in cats

Pain management is a crucial component of feline medicine and surgery. This review critically evaluates studies using buprenorphine in cats and highlights the clinical application of the opioid in this species. The pharmacokinetic-pharmacodynamic (PK-PD) modeling of IV buprenorphine has been best described by a combined effect compartmental/receptor association-dissociation model with negative hysteresis. Therefore, plasma concentrations of the drug are not correlated with analgesia, and clinicians should not expect to observe pain relief immediately after drug administration. In addition, a ceiling effect has not been demonstrated after administration of clinical doses of buprenorphine in cats; dosages of up to 0.04 mg/kg have been reported. The route of administration influences the onset, duration, and magnitude of antinociception and analgesia when using this drug in cats. At clinical dosages, the SC route of administration does not appear to provide adequate antinociception and analgesia whereas the buccal route has produced inconsistent results. Intravenous or IM administration at a dosage of 0.02-0.04 mg/kg is the preferred for treatment of pain in the acute setting. A literature search found 14 clinical trials evaluating buprenorphine sedation, analgesia, or both in cats. There were 22 original research studies reporting the antinociceptive effects of buprenorphine by means of thermal threshold, mechanical threshold, or both, minimal alveolar concentration, or PK-PD. Individual variability in response to buprenorphine administration has been reported, indicating that buprenorphine may not provide sufficient analgesia in some cats. Pain assessment is important when evaluating the efficacy of buprenorphine and determining whether additional analgesic treatment is needed.

Antihyperalgesic effects of a novel muscarinic agonist (LASSBio-873) in spinal nerve ligation in rats

New chemicals or adjuvants with analgesic effects on chronic pain are needed and clinically relevant due to the limited number of effective compounds that possess these characteristics. LASSBio-873, a pyrazolo[3,4-b]pyrrolo[3,4-d]pyridine derivative, activates muscarinic cholinergic receptors and has potent analgesic effects on acute and inflammatory pain. The present study evaluated the therapeutic and prophylactic effects of oral administration of LASSBio-873 in a spinal nerve ligation (SNL) model of chronic peripheral nerve injury. LASSBio-873 (100 mg/kg) inhibited the development of thermal hyperalgesia and mechanical allodynia when administered once daily for 7 consecutive days after SNL surgery and reversed these symptoms. LASSBio-873 treatment did not alter rat behaviour in open field testing measured during the first 24 h after administration and again after 7 continuous days administration. The analgesic effect of LASSBio-873 was inhibited by intrathecal methoctramine, an M2 receptor antagonist, implicating the muscarininc M2 receptor signalling pathway in the drug's action. These results reinforce the potential of LASSBio-873 as a possible prototype for the development of more effective alternatives for the treatment of neuropathic pain.

A simplified up-down method (SUDO) for measuring mechanical nociception in rodents using von Frey filaments

Background

The measurement of mechanosensitivity is a key method for the study of pain in animal models. This is often accomplished with the use of von Frey filaments in an up-down testing paradigm. The up-down method described by Chaplan et al. (J Neurosci Methods 53:55–63, 1994) for mechanosensitivity testing in rodents remains one of the most widely used methods for measuring pain in animals. However, this method results in animals receiving a varying number of stimuli, which may lead to animals in different groups receiving different testing experiences that influences their later responses. To standardize the measurement of mechanosensitivity we developed a simplified up-down method (SUDO) for estimating paw withdrawal threshold (PWT) with von Frey filaments that uses a constant number of five stimuli per test. We further refined the PWT calculation to allow the estimation of PWT directly from the behavioral response to the fifth stimulus, omitting the need for look-up tables.

Results

The PWT estimates derived using SUDO strongly correlated (r > 0.96) with the PWT estimates determined with the conventional up-down method of Chaplan et al., and this correlation remained very strong across different levels of tester experience, different experimental conditions, and in tests from both mice and rats. The two testing methods also produced similar PWT estimates in prospective behavioral tests of mice at baseline and after induction of hyperalgesia by intraplantar capsaicin or complete Freund’s adjuvant.

Conclusion

SUDO thus offers an accurate, fast and user-friendly replacement for the widely used up-down method of Chaplan et al.

Genome-wide transcriptional profiling of skin and dorsal root ganglia after ultraviolet-B-induced inflammation

Ultraviolet-B (UVB)-induced inflammation produces a dose-dependent mechanical and thermal hyperalgesia in both humans and rats, most likely via inflammatory mediators acting at the site of injury. Previous work has shown that the gene expression of cytokines and chemokines is positively correlated between species and that these factors can contribute to UVB-induced pain. In order to investigate other potential pain mediators in this model we used RNA-seq to perform genome-wide transcriptional profiling in both human and rat skin at the peak of hyperalgesia. In addition we have also measured transcriptional changes in the L4 and L5 DRG of the rat model. Our data show that UVB irradiation produces a large number of transcriptional changes in the skin: 2186 and 3888 genes are significantly dysregulated in human and rat skin, respectively. The most highly up-regulated genes in human skin feature those encoding cytokines (IL6 and IL24), chemokines (CCL3, CCL20, CXCL1, CXCL2, CXCL3 and CXCL5), the prostanoid synthesising enzyme COX-2 and members of the keratin gene family. Overall there was a strong positive and significant correlation in gene expression between the human and rat (R = 0.8022). In contrast to the skin, only 39 genes were significantly dysregulated in the rat L4 and L5 DRGs, the majority of which had small fold change values. Amongst the most up-regulated genes in DRG were REG3B, CCL2 and VGF. Overall, our data shows that numerous genes were up-regulated in UVB irradiated skin at the peak of hyperalgesia in both human and rats. Many of the top up-regulated genes were cytokines and chemokines, highlighting again their potential as pain mediators. However many other genes were also up-regulated and might play a role in UVB-induced hyperalgesia. In addition, the strong gene expression correlation between species re-emphasises the value of the UVB model as translational tool to study inflammatory pain.

Criticality and degeneracy in injury-induced changes in primary afferent excitability and the implications for neuropathic pain

Neuropathic pain remains notoriously difficult to treat despite numerous drug targets. Here, we offer a novel explanation for this intractability. Computer simulations predicted that qualitative changes in primary afferent excitability linked to neuropathic pain arise through a switch in spike initiation dynamics when molecular pathologies reach a tipping point (criticality), and that this tipping point can be reached via several different molecular pathologies (degeneracy). We experimentally tested these predictions by pharmacologically blocking native conductances and/or electrophysiologically inserting virtual conductances. Multiple different manipulations successfully reproduced or reversed neuropathic changes in primary afferents from naïve or nerve-injured rats, respectively, thus confirming the predicted criticality and its degenerate basis. Degeneracy means that several different molecular pathologies are individually sufficient to cause hyperexcitability, and because several such pathologies co-occur after nerve injury, that no single pathology is uniquely necessary. Consequently, single-target-drugs can be circumvented by maladaptive plasticity in any one of several ion channels.

DOI:http://dx.doi.org/10.7554/eLife.02370.001

Although the pain associated with an injury is unpleasant, it normally serves an important purpose: to make you avoid its source. However, some pain appears to arise from nowhere. Frustratingly, this type of pain, known as neuropathic pain, does not respond to common painkillers and is thus very difficult to treat.

The neurons that transmit pain and other sensory information do so using electrical signals. In response to a stimulus, ions travel through channels in the membrane of a neuron, which leads to a change in the electrical potential of the membrane. When this change is large enough, a voltage spike is produced: this signal is ultimately transmitted to the brain.

When certain neurons fire too easily or too often, neuropathic pain can arise. This hyperexcitability can make something painful feel even worse, or it can make things hurt that shouldn’t. To prevent this, extensive research has been devoted to identify drugs that target particular types of ion channels and block them. However, despite the discovery of many promising drugs, those drugs have been frustratingly ineffective in clinical trials.

Using simulations and experiments, Ratté et al. have examined the behavior of a type of neuron that normally conducts information about touch, but the brain sometimes misinterprets this information as pain. Increasing the flow of ions through the cell membrane in these simulations eventually causes a ‘tipping point’ to be crossed, which triggers a dramatic, discontinuous change in spiking pattern. However, as several different types of ion channels contribute to the current, there are several different ways in which the tipping point can be crossed.

This ability to produce the same result by multiple means is a common feature of complex systems. Known as degeneracy, it makes systems more robust, as a given result can still be achieved if one particular attempt to achieve this result fails. The work of Ratté et al. helps to explain why drugs that target just one type of ion channel may fail to relieve neuropathic pain: maladaptive changes in any one of several other ion channels may circumvent the therapeutic effect.

DOI:http://dx.doi.org/10.7554/eLife.02370.002