Comparison of operant escape and reflex tests of nociceptive sensitivity

Quantification of stimulus-evoked tactile allodynia in free moving mice by the chainmail sensitivity test

Chronic pain occurs at epidemic levels throughout the population. Hypersensitivity to touch, is a cardinal symptom of chronic pain. Despite dedicated research for over a century, quantifying this hypersensitivity has remained impossible at scale. To address these issues, we developed the Chainmail Sensitivity Test (CST). Our results show that control mice spend significantly more time on the chainmail portion of the device than mice subject to neuropathy. Treatment with gabapentin abolishes this difference. CST-derived data correlate well with von Frey measurements and quantify hypersensitivity due to inflammation. Our study demonstrates the potential of the CST as a standardized tool for assessing mechanical hypersensitivity in mice with minimal operator input.

Intrathecal non-viral interleukin-10 gene therapy ameliorates neuropathic pain as measured by both classical static allodynia and a novel supra-spinally mediated pain assay, the Two-Arm Rodent Somatosensory (TARS) task

Intrathecal delivery of interleukin-10 (IL-10) gene therapy has been reported to be effective in suppressing pain enhancement in a variety of rodent models. However, all publications that have tested this treatment have relied upon measures of static allodynia (von Frey test) and thermal hyperalgesia (Hargreaves test). As this plasmid DNA IL-10 (pDNA-IL10) therapeutic approach is now in human clinical trials for multiple pain indications, including intrathecal delivery for human neuropathic pain, it is important to consider the recent concerns raised in the pain field that such tests reflect spinal rather than supraspinal processing of, and responsivity to, noxious stimuli. Consequently, this raises the question of whether intrathecal pDNA-IL10 can reverse established neuropathic pain when assessed by a test requiring supraspinal, rather than solely spinal, mediation of the behavioral response. The present study utilizes the rat sciatic chronic constriction injury (CCI) model of neuropathic pain to compare the expression of static allodynia with that of cognitively controlled choice behavior in a two-arm maze, adapted from Hayashida et al. (2019). This modification, termed the Two-Arm Rodent Somatosensory (TARS) task, provides rats free choice to reach a desired goal box via a short "arm" of the maze with tactile probes as flooring versus a longer "arm" of the maze with a smooth surface. Here we demonstrate that static allodynia and avoidance of the nociceptive flooring in TARS develop in parallel over time, and that both behaviors also resolve in parallel following intrathecal pDNA-IL10 gene therapy. Details for the construction and use of this new maze design are also provided. Together, this study documents both: (a) the important finding that intrathecal IL-10 gene therapy does indeed resolve neuropathic pain as measured by a supraspinally-mediated behavioral task, and (b) a new, supraspinally-mediated task that allows behavioral assessments across weeks and allows the analysis of both development and resolution of neuropathic pain by therapeutic interventions. As such, the TARS operant behavior task is an improvement over other approaches such as the mechanical conflict-avoidance system which have difficulties demonstrating development and reversal of pain behavior in a within-subject design.

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

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

The Use of an FR1 Schedule Operant Approach-Avoidance Paradigm to Measure the Aversiveness of Neuropathic and Inflammatory Pain

Pain is a subjective, private, yet universal phenomenon that depends on a unique combination of sensory, affective, and evaluative characteristics. Although preclinical models have been used to understand much of pain physiology, the inability to communicate with animals limits affective and evaluative feedback and has constrained traditional behavioral methods to adequately represent and study the multidimensional pain experience. Therefore, this study sought to characterize the affective component of pain within a novel operant approach-avoidance paradigm (AAP) to determine which type of pain (inflammatory and neuropathic) may be more aversive. To reveal the possible differences in pain aversiveness within the AAP paradigm, animals received bilateral inflammatory and neuropathic pain conditions and were given the choice to a) forgo appetitive reward by not receiving noxious stimulus of either inflammatory or neuropathic conditions or b) receive noxious stimulus in exchange for an appetitive reward. Although all pain conditions produced significant hypersensitivity, the AAP results revealed there was no preference in the stimulation of a specific paw in the bilateral pain conditions. The finding suggests that despite unique clinical pain characteristics for inflammatory and neuropathic conditions, the lack of observable differences in the pain conditions may not necessarily equate to the overall similarity in aversiveness, but rather that the fixed ratio (FR1) paradigm presentation allowed appetitive reward to be more salient, highlighting the complexities of competing motivational drives of pain and hunger when satiating hunger is always guaranteed. Thus, future studies should seek to further tease apart this relationship with a different schedule and food-controlled methodologies. The development of such preclinical approaches can thoroughly investigate the intricacy of competing drives and likely reveal important information regarding the complexity of pain, enhancing our understanding of pain perception in individuals suffering from comorbid pain states.

Sham surgeries for central and peripheral neural injuries persistently enhance pain-avoidance behavior as revealed by an operant conflict test

Studies using rodent models of neuropathic pain use sham surgery control procedures that cause deep tissue damage. Sham surgeries would thus be expected to induce potentially long-lasting postsurgical pain, but little evidence for such pain has been reported. Operant tests of voluntary behavior can reveal negative motivational and cognitive aspects of pain that may provide sensitive tools for detecting pain-related alterations. In a previously described operant mechanical conflict test involving lengthy familiarization and training, rodents freely choose to either escape from a brightly lit chamber by crossing sharp probes or refuse to cross. Here, we describe a brief (2-day) mechanical conflict protocol that exploits rats' innate exploratory response to a novel environment to detect persistently enhanced pain-avoidance behavior after sham surgeries for 2 neural injury models: thoracic spinal cord injury and chronic constriction injury of the sciatic nerve. Pitting the combined motivations to avoid the bright light and to explore the novel device against pain from crossing noxious probes disclosed a conflicting, hyperalgesia-related reluctance to repeatedly cross the probes after injury. Rats receiving standard sham surgeries demonstrated enhanced pain-like avoidance behavior compared with naive controls, and this behavior was similar to that of corresponding chronic constriction injury or spinal cord injury rats weeks or months after injury. In the case of sham surgery for spinal cord injury, video analysis of voluntary exploratory behavior directed at the probes revealed enhanced forepaw withdrawal responses. These findings have important implications for preclinical investigations into behavioral alterations and physiological mechanisms associated with postsurgical and neuropathic pain.

The Operant Plantar Thermal Assay: A Novel Device for Assessing Thermal Pain Tolerance in Mice

Pain is a multidimensional experience of sensory-discriminative, cognitive, and affective processes; however, current basic research methods rely heavily on response to threshold stimuli, bypassing the supraspinal processing that ultimately gives rise to the pain experience. We developed the operant plantar thermal assay (OPTA), which utilizes a novel, conflict-based operant task requiring evaluation and active decision-making to obtain reward under thermally aversive conditions to quantify thermal pain tolerance. In baseline measures, male and female mice exhibited similar temperature preferences, however in the OPTA, female mice exhibited greater temperature-dependent tolerance, as defined by choice time spent in an adverse thermal condition to obtain reward. Increasing reward salience (4% vs 10% sucrose solution) led to increased thermal tolerance for males but not females. To determine whether neuropathic and inflammatory pain models alter thermal tolerance, animals with chronic constriction injury (CCI) or complete Freund's adjuvant (CFA), respectively, were tested in the OPTA. Surprisingly, neuropathic animals exhibited increased thermal tolerance, as shown by greater time spent in the reward zone in an adverse thermal condition, compared with sham animals. There was no effect of inflammation on thermal tolerance. Administration of clonidine in the CCI model led to increased thermal tolerance in both injured and sham animals. In contrast, the non-steroidal anti-inflammatory meloxicam was anti-hyperalgesic in the CFA model, but reduced thermal pain tolerance. These data support the feasibility of using the OPTA to assess thermal pain tolerance to gain new insights into complex pain behaviors and to investigate novel aspects of analgesic efficacy.

Mechanisms of Below-Level Pain Following Spinal Cord Injury (SCI)

Mechanisms of below-level pain are discoverable as neural adaptations rostral to spinal injury. Accordingly, the strategy of investigations summarized here has been to characterize behavioral and neural responses to below-level stimulation over time following selective lesions of spinal gray and/or white matter. Assessments of human pain and the pain sensitivity of humans and laboratory animals following spinal injury have revealed common disruptions of pain processing. Interruption of the spinothalamic pathway partially deafferents nocireceptive cerebral neurons, rendering them spontaneously active and hypersensitive to remaining inputs. The spontaneous activity among these neurons is disorganized and unlikely to generate pain. However, activation of these neurons by their remaining inputs can result in pain. Also, injury to spinal gray matter results in a cascade of secondary events, including excitotoxicity, with rostral propagation of excitatory influences that contribute to chronic pain. Establishment and maintenance of below-level pain results from combined influences of injured and spared axons in the spinal white matter and injured neurons in spinal gray matter on processing of nociception by hyperexcitable cerebral neurons that are partially deafferented. A model of spinal stenosis suggests that ischemic injury to the core spinal region can generate below-level pain. Additional questions are raised about demyelination, epileptic discharge, autonomic activation, prolonged activity of C nocireceptive neurons, and thalamocortical plasticity in the generation of below-level pain. PERSPECTIVE: An understanding of mechanisms can direct therapeutic approaches to prevent development of below-level pain or arrest it following spinal cord injury. Among the possibilities covered here are surgical and other means of attenuating gray matter excitotoxicity and ascending propagation of excitatory influences from spinal lesions to thalamocortical systems involved in pain encoding and arousal.

Comparisons of motor and sensory abnormalities after lumbar and thoracic contusion spinal cord injury in male rats

Rodent models of contusion spinal cord injury (SCI) are widely studied for the mechanisms underlying functional deficits after SCI. Yet, how does lesion level affect SCI-induced motor and sensory dysfunctions remains unclear. Using a computer-controlled impactor (Impact One™, Leica) and the same parameters (diameter, 2.0 mm; Speed: 4.0 m/s; Depth: 1.5 mm; Dwell time: 0.1 s), we produced contusions at mid-thoracic (T10) and rostral-lumbar (L2) spinal cord in male rats, and compared locomotor and sensory dysfunctions within the same experimental setting. The time courses of locomotor deficit were comparable between thoracic (n = 8) and lumbar (n = 7) SCI rats, but the severity was greater after thoracic SCI especially during the first week post-injury, as indicated by the lower Basso, Beattle and Bresnahan open-field locomotion scores. Both groups showed similar heightened avoiding response (hyper-reactivity) to mechanical stimulation applied at the hindpaws from day 21-56 post-injury, as indicated by decreased paw withdrawal thresholds. Compared to lumbar SCI, thoracic SCI induced a greater decrease of paw withdrawal latency in hot-plate test from day 28-56 post-injury. In contrast, lumbar SCI rats showed a greater reduction of avoidance threshold to mechanical stimulation at the girdle region, and larger overgroomed area than thoracic SCI rats at day 14 post-injury. Thus, thoracic SCI may induce greater motor deficits and hindpaw heat hyper-reactivity than did lumbar SCI. In contrast, lumbar SCI may elicit greater at-level mechanical hyper-reactivity and overgrooming behavior than thoracic SCI. Future study needs to examine the specific pathological changes underlying different dysfunctions in two SCI models.

Pros and Cons of Clinically Relevant Methods to Assess Pain in Rodents

The primary objective of preclinical pain research is to improve the treatment of pain. Decades of research using pain-evoked tests has revealed much about mechanisms but failed to deliver new treatments. Evoked pain-tests are often limited because they ignore spontaneous pain and motor or disruptive side effects confound interpretation of results. New tests have been developed to focus more closely on clinical goals such as reducing pathological pain and restoring function. The objective of this review is to describe and discuss several of these tests. We focus on: Grimace Scale, Operant Behavior, Wheel Running, Burrowing, Nesting, Home Cage Monitoring, Gait Analysis and Conditioned Place Preference/ Aversion. A brief description of each method is presented along with an analysis of the advantages and limitations. The pros and cons of each test will help researchers identify the assessment tool most appropriate to meet their particular objective to assess pain in rodents. These tests provide another tool to unravel the mechanisms underlying chronic pain and help overcome the translational gap in drug development.

Assessing the aversive nature of pain with an operant approach/avoidance paradigm

Preclinical pain assessments can be criticized for failing to adequately characterize the human clinical pain experience. Although recent assessments have improved upon this shortcoming, there are still significant limitations. One concern is that current procedures fail to examine underlying motivational drives related to pain. Therefore, we used a novel approach-avoidance paradigm that allowed a rat to either satisfy hunger or avoid noxious stimulation to reveal prioritizing of motivational drives. The operant paradigm utilized a single lever that the animal pressed for appetitive reward (approach). The lever press was associated with mechanical stimulation of an inflamed paw induced by subcutaneous injection of carrageenan (avoidance). The results revealed that carrageenan-injected animals had a significant suppression of lever pressing and, in addition, had a longer latency to approach and press a lever for appetitive reward. The pattern of operant behavioral responses indicates that the motivation to avoid pain superseded the motivation to alleviate hunger. Utilization of approach-avoidance paradigms, such as this one, can allow researchers to unravel the complexities of the pain experience with the goal of enhancing translation to clinical efficacy.

Neuropathic pain and spasticity: intricate consequences of spinal cord injury

STUDY DESIGN

The 2016 International Spinal Cord Society Sir Ludwig Guttmann Lecture.

OBJECTIVES

The aim of this review is to identify different symptoms and signs of neuropathic pain and spasticity after spinal cord injury (SCI) and to present different methods of assessing them. The objective is to discuss how a careful characterization of different symptoms and signs, and a better translation of preclinical findings may improve our understanding of the complex and entangled mechanisms of neuropathic pain and spasticity.

METHODS

A MEDLINE search was performed using the following terms: 'pain', 'neuropathic', 'spasticity', 'spasms' and 'spinal cord injury'.

RESULTS

This review identified different domains of neuropathic pain and spasticity after SCI and methods to assess them in preclinical and clinical research. Different factors important for pain description include location, onset, pain descriptors and somatosensory function, while muscle tone, spasms, reflexes and clonus are important aspects of spasticity. Similarities and differences between neuropathic pain and spasticity are discussed.

CONCLUSIONS

Understanding that neuropathic pain and spasticity are multidimensional consequences of SCI, and a careful examination and characterization of the symptoms and signs, are a prerequisite for understanding the relationship between neuropathic pain and spasticity and the intricate underlying mechanisms.

Application of the Rat Grimace Scale as a Marker of Supraspinal Pain Sensation after Cervical Spinal Cord Injury

Experimental models of neuropathic pain (NP) typically rely on withdrawal responses to assess the presence of pain. Reflexive withdrawal responses to a stimulus are used to evaluate evoked pain and, as such, do not include the assessment of spontaneous NP nor evaluation of the affective and emotional consequences of pain in animal models. Additionally, withdrawal responses can be mediated by spinal cord reflexes and may not accurately indicate supraspinal pain sensation. This is especially true in models of traumatic spinal cord injury (SCI), wherein spastic syndrome, a motor disorder characterized by exaggeration of the stretch reflex that is secondary to hyperexcitability of the spinal reflex, can cause paroxysmal withdrawals not associated with NP sensation. Consequently, the aim of this study was to utilize an assessment of supraspinal pain sensation, the Rat Grimace Scale (RGS), to measure both spontaneous and evoked NP after a contusion SCI at cervical level 5 in adult male rats. Spontaneous and evoked pain were assessed using the RGS to score facial action units before and after the application of a stimulus, respectively. Rodents exhibited significantly higher RGS scores at week 5 post-injury as compared to baseline and laminectomy controls before the application of the stimulus, suggesting the presence of spontaneous NP. Additionally, there was a significant increase in RGS scores after the application of the acetone. These data suggest that the RGS can be used to assess spontaneous NP and determine the presence of evoked supraspinal pain sensation after experimental cervical SCI.

Animal models of pain and migraine in drug discovery

Preclinical research activities in relation to pain typically involve the 'holy trinity' of nociceptive, inflammatory and neuropathic pain for purposes of target validation and defining target product profiles of novel analgesic compounds. For some reason it seems that headache or migraine are rarely considered as additional entities to explore. Frontline medications used in the treatment of, for example, inflammatory pain, neuropathic pain and migraine (NSAIDs versus pregabalin/duloxetine versus triptans) reveal distinct differences in pathophysiology that partially explain this approach. Nevertheless, for many patients enduring chronic pain, regardless of aetiology, high unmet needs remain. By focusing more on commonalities shared between neuropathic pain and headache disorders such as migraine, drug discovery efforts could be spread more efficiently across a larger indication area. Here, some of the most commonly used models and methods employed within 'pain and migraine' drug development will be presented. Recent advances within these disciplines suggest that, with the addition of a few extra carefully chosen ancillary models and/or endpoints, the relative value in terms of resources used, reciprocal flow of information and net worth of a 'typical' package could be increased substantially for the pain and migraine fields.

Altered nocifensive behavior in animal models of autism spectrum disorder: The role of the nicotinic cholinergic system

Caretakers and clinicians alike have long recognized that individuals with autism spectrum disorder (ASD) can have altered sensory processing, which can contribute to its core symptoms. However, the pathobiology of sensory alterations in ASD is poorly understood. Here we examined nocifensive behavior in ASD mouse models, the BTBR T+Itpr3tf/J (BTBR) and the fragile-X mental retardation-1 knockout (Fmr1-KO) mice. We also examined the effects of nicotine on nocifensive behavior given that nicotine, a nicotinic cholinergic receptor (nAChR) agonist that has antinociceptive effects, was shown to improve social deficits and decrease repetitive behaviors in BTBR mice. Compared to respective controls, both BTBR and Fmr1-KO had hyporesponsiveness to noxious thermal stimuli and electrical stimulation of C-sensory fibers, normal responsiveness to electrical stimulation of Aβ- and Aδ-fiber, and hyperresponsiveness to visceral pain after acetic acid intraperitoneal injection. In BTBR, nicotine at lower doses increased, whereas at higher doses, it decreased hotplate latency compared to vehicle. In a significantly different effect pattern, in control mice, nicotine had antinociceptive effects to noxious heat only at the high dose. Interestingly, these nocifensive behavior alterations and differential responses to nicotine antinociceptive effects in BTBR mice were associated with significant downregulation of α3, α4, α5, α7, β2, β3, and β4 nAChR subunits in several cerebral regions both, during embryonic development and adulthood. Taken together, these findings further implicate nAChRs in behaviors alterations in the BTBR model and lend support to the hypothesis that nAChRs may be a target for treatment of behavior deficits and sensory dysfunction in ASD.

Neuropathic pain following traumatic spinal cord injury: Models, measurement, and mechanisms

Neuropathic pain following spinal cord injury (SCI) is notoriously difficult to treat and is a high priority for many in the SCI population. Resolving this issue requires animal models fidelic to the clinical situation in terms of injury mechanism and pain phenotype. This Review discusses the means by which neuropathic pain has been induced and measured in experimental SCI and compares these with human outcomes, showing that there is a substantial disconnection between experimental investigations and clinical findings in a number of features. Clinical injury level is predominantly cervical, whereas injury in the laboratory is modeled mainly at the thoracic cord. Neuropathic pain is primarily spontaneous or tonic in people with SCI (with a relatively smaller incidence of allodynia), but measures of evoked responses (to thermal and mechanical stimuli) are almost exclusively used in animals. There is even the question of whether pain per se has been under investigation in most experimental SCI studies rather than simply enhanced reflex activity with no affective component. This Review also summarizes some of the problems related to clinical assessment of neuropathic pain and how advanced imaging techniques may circumvent a lack of patient/clinician objectivity and discusses possible etiologies of neuropathic pain following SCI based on evidence from both clinical studies and animal models, with examples of cellular and molecular changes drawn from the entire neuraxis from primary afferent terminals to cortical sensory and affective centers. © 2016 Wiley Periodicals, Inc.

Cutaneous tissue damage induces long-lasting nociceptive sensitization and regulation of cellular stress- and nerve injury-associated genes in sensory neurons

Tissue damage is one of the major etiological factors in the emergence of chronic/persistent pain, although mechanisms remain enigmatic. Using incision of the back skin of adult rats as a model for tissue damage, we observed sensitization in a nociceptive reflex enduring to 28days post-incision (DPI). To determine if the enduring behavioral changes corresponded with a long-term impact of tissue damage on sensory neurons, we examined the temporal expression profile of injury-regulated genes and the electrophysiological properties of traced dorsal root ganglion (DRG) sensory neurons. The mRNA for the injury/stress-hub gene Activating Transcription Factor 3 (ATF3) was upregulated and peaked within 4 DPI, after which levels declined but remained significantly elevated out to 28 DPI, a time when the initial incision appears healed and tissue-inflammation largely resolved. Accordingly, stereological image analysis indicated that some neurons expressed ATF3 only transiently (mostly medium-large neurons), while in others it was sustained (mostly small neurons), suggesting cell-type-specific responses. In retrogradely-traced ATF3-expressing neurons, Calcium/calmodulin-dependent protein kinase type IV (CAMK4) protein levels and isolectin-B4 (IB4)-binding were suppressed whereas Growth Associated Protein-43 (GAP-43) and Neuropeptide Y (NPY) protein levels were enhanced. Electrophysiological recordings from DiI-traced sensory neurons 28 DPI showed a significant sensitization limited to ATF3-expressing neurons. Thus, ATF3 expression is revealed as a strong predictor of single cells displaying enduring pain-related electrophysiological properties. The cellular injury/stress response induced in sensory neurons by tissue damage and indicated by ATF3 expression is positioned to contribute to pain which can occur after tissue damage.

Trigeminal neuroplasticity underlies allodynia in a preclinical model of mild closed head traumatic brain injury (cTBI)

Post-traumatic headache (PTH) following TBI is a common and often persisting pain disability. PTH is often associated with a multimodal central pain sensitization on the skin surface described as allodynia. However, the particular neurobiology underlying cTBI-induced pain disorders are not known. These studies were performed to assess trigeminal sensory sensitization and to determine if sensitization measured behaviorally correlated with detectable changes in portions of the trigeminal sensory system (TSS), particularly trigeminal nucleus, thalamus, and sensory cortex. Thermal stimulation is particularly well suited to evaluate sensitization and was used in these studies. Recent advances in the use of reward/conflict paradigms permit use of operant measures of behavior, versus reflex-driven response behaviors, for thermal sensitization studies. Thus, to quantitate facial thermal sensitization (allodynia) in the setting of acute TBI, the current study utilized an operant orofacial pain reward/conflict testing paradigm to assess facial thermal sensitivity in uninjured control animals compared with those two weeks after cTBI in a rodent model. Significant reductions in facial contact/lick behaviors were observed in the TBI animals using either cool or warm challenge temperatures compared with behaviors in the normal animals. These facial thermal sensitizations correlated with detectable changes in multiple levels of the TSS. The immunohistochemical (IHC) studies revealed significant alterations in the expression of the serotonin (5-HT), neurokinin 1 receptor (NK1R), norepinephrine (NE), and gamma-aminobutyric acid (GABA) in the caudal trigeminal nucleus, thalamic VPL/VPM nucleus, and sensory cortex of the orofacial pain pathways. There was a strong correlation between increased expression of certain IHC markers and increased behavioral markers for facial sensitization. The authors conclude that TBI-induced changes observed in the TSS are consistent with the expression of generalized facial allodynia following cTBI. To our knowledge, this is the first report of orofacial sensitization correlated with changes in selected neuromodulators/neurotransmitters in the TSS following experimental mild TBI.

Mechanical Conflict System: A Novel Operant Method for the Assessment of Nociceptive Behavior

A new operant test for preclinical pain research, termed the Mechanical Conflict System (MCS), is presented. Rats were given a choice either to remain in a brightly lit compartment or to escape to a dark compartment by crossing an array of height-adjustable nociceptive probes. Latency to escape the light compartment was evaluated with varying probe heights (0, .5, 1, 2, 3, and 4 mm above compartment floor) in rats with neuropathic pain induced by constriction nerve injury (CCI) and in naive control rats. Escape responses in CCI rats were assessed following intraperitoneal administration of pregabalin (10 and 30 mg/kg), morphine (2.5 and 5 mg/kg), and the tachykinin NK1 receptor antagonist, RP 67580 (1 and 10 mg/kg). Results indicate that escape latency increased as a function of probe height in both naive and CCI rats. Pregabalin (10 and 30 mg/kg) and morphine (5 mg/kg), but not RP 67580, decreased latency to escape in CCI rats suggesting an antinociceptive effect. In contrast, morphine (10 mg/kg) but not pregabalin (30 mg/kg) increased escape latency in naive rats suggesting a possible anxiolytic action of morphine in response to light-induced fear. No order effects following multiple test sessions were observed. We conclude that the MCS is a valid method to assess behavioral signs of affective pain in rodents.

Age-Dependent Changes in the Inflammatory Nociceptive Behavior of Mice

The processing of pain undergoes several changes in aging that affect sensory nociceptive fibers and the endogenous neuronal inhibitory systems. So far, it is not completely clear whether age-induced modifications are associated with an increase or decrease in pain perception. In this study, we assessed the impact of age on inflammatory nociception in mice and the role of the hormonal inhibitory systems in this context. We investigated the nociceptive behavior of 12-month-old versus 6–8-week-old mice in two behavioral models of inflammatory nociception. Levels of TRP channels, and cortisol as well as cortisol targets, were measured by qPCR, ELISA, and Western blot in the differently aged mice. We observed an age-related reduction in nociceptive behavior during inflammation as well as a higher level of cortisol in the spinal cord of aged mice compared to young mice, while TRP channels were not reduced. Among potential cortisol targets, the NF-κB inhibitor protein alpha (IκBα) was increased, which might contribute to inhibition of NF-κB and a decreased expression and activity of the inducible nitric oxide synthase (iNOS). In conclusion, our results reveal a reduced nociceptive response in aged mice, which might be at least partially mediated by an augmented inflammation-induced increase in the hormonal inhibitory system involving cortisol.

Baclofen reversed thermal place preference in rats with chronic constriction injury

Chronic constriction injury to the sciatic nerve was used as an animal model of neuropathic pain. Instead of frequently used reflex-based tests we used an operant thermal place preference test to evaluate signs of neuropathic pain and the effect of baclofen administration in rats with neuropathy. Chronic constriction injury was induced by four loose ligations of the sciatic nerve. Thermal place preference (45 °C vs. 22 °C and 45 °C vs. 11 °C) was measured after the ligation and after the administration of baclofen in sham and experimental rats. Rats with the chronic constriction injury spent significantly less time on the colder plate compared to sham operated animals at the combination 45 °C vs. 11 °C. After administration of baclofen (10 mg/kg s.c.), the aversion to the colder plate in rats with chronic constriction injury disappeared. At the combination 45 °C vs. 22 °C, no difference in time spent on colder and/or warmer plate was found between sham and experimental animals. These findings show the importance of cold allodynia evaluation in rats with chronic constriction injury and the effectiveness of baclofen in this neuropathic pain model.

Sex differences in a Murine Model of Complex Regional Pain Syndrome

Complex Regional Pain Syndrome (CRPS) is a major cause of chronic pain after surgery or trauma to the limbs. Despite evidence showing that the prevalence and severity of many forms of chronic pain, including CRPS, differ between males and females, laboratory studies on sex-related differences in animal models of CRPS are not available, and the impact of sex on the transition from acute to chronic CRPS pain and disability are unexplored. Here we make use of a tibia fracture/cast mouse model that recapitulates the nociceptive, functional, vascular, trophic, inflammatory and immune aspects of CRPS. Our aim is to describe the chronic time course of nociceptive, motor and memory changes associated with fracture/cast in male and female mice, in addition to exploring their underlying spinal mechanisms. Our behavioral data shows that, compared to males, female mice display lower nociceptive thresholds following fracture in the absence of any differences in ongoing or spontaneous pain. Furthermore, female mice show exaggerated signs of motor dysfunction, deficits in fear memory, and latent sensitization that manifests long after the normalization of nociceptive thresholds. Our biochemical data show differences in the spinal cord levels of the glutamate receptor NR2b, suggesting sex differences in mechanisms of central sensitization that could account for differences in duration and severity of CRPS symptoms between the two groups.