Pain In Rodents Research Articles

Discovery of (R)-1-(7-chloro-2,2-bis(fluoromethyl)chroman-4-yl)-3-(3-methylisoquinolin-5-yl)urea (A-1165442): a temperature-neutral transient receptor potential vanilloid-1 (TRPV1) antagonist with analgesic efficacy.

The synthesis and characterization of a series of selective, orally bioavailable 1-(chroman-4-yl)urea TRPV1 antagonists is described. Whereas first-generation antagonists that inhibit all modes of TRPV1 activation can elicit hyperthermia, the compounds disclosed herein do not elevate core body temperature in preclinical models and only partially block acid activation of TRPV1. Advancing the SAR of this series led to the eventual identification of (R)-1-(7-chloro-2,2-bis(fluoromethyl)chroman-4-yl)-3-(3-methylisoquinolin-5-yl)urea (A-1165442, 52), an analogue that possesses excellent pharmacological selectivity, has a favorable pharmacokinetic profile, and demonstrates good efficacy against osteoarthritis pain in rodents.

Antinociceptive properties of N-Mannich bases derived from 3-substituted pyrrolidine-2,5-dione in the formalin model of persistent pain in mice

BackgroundAccumulated data indicate that anticonvulsants possess antinociceptive properties in rodent pain models. In view of the anticonvulsant activity demonstrated previously among N-Mannich bases derived from 3-mono- (1–6) and 3,3-disubstituted pyrrolidine-2,5-diones (7–14) their analgesic activity has been investigated in the formalin model of tonic pain in mice. MethodsThe compounds 1–14 were tested at doses equal to the respective ED50 values obtained earlier in the MES test. 0.5% formalin solution was given as intraplantar injections into the hind paw of the mouse and the duration of the nocifensive response was counted in drug-treated and vehicle-treated animals in the acute and the late phases of the test. ResultsA significant antinociceptive activity was observed for majority of the compounds. In the first phase of the test all the active compounds, except for 9–11, reduced the duration of the licking response up to 88% (compounds 2 and 6; p<0.001). In the late phase the 1–3, 5, 6, 9 and 14 were the most effective agents and their analgesic activities ranged from 92 to 100%. ConclusionsThe results of the research indicate that some of the investigated compounds reduced effectively either both phases of the test or were able to attenuate pain during only the acute or late phase of the formalin test. These properties, which are particularly strong in case of the compounds 1–3, 5, 6, 9 and 14, might be relevant for the development of novel analgesic-active compounds and their possible use in neuropathic pain syndromes.

Optimized inhibitors of soluble epoxide hydrolase improve in vitro target residence time and in vivo efficacy.

Diabetes is affecting the life of millions of people. A large proportion of diabetic patients suffer from severe complications such as neuropathic pain, and current treatments for these complications have deleterious side effects. Thus, alternate therapeutic strategies are needed. Recently, the elevation of epoxy-fatty acids through inhibition of soluble epoxide hydrolase (sEH) was shown to reduce diabetic neuropathic pain in rodents. In this report, we describe a series of newly synthesized sEH inhibitors with at least 5-fold higher potency and doubled residence time inside both the human and rodent sEH enzyme than previously reported inhibitors. These inhibitors also have better physical properties and optimized pharmacokinetic profiles. The optimized inhibitor selected from this new series displayed improved efficacy of almost 10-fold in relieving pain perception in diabetic neuropathic rats as compared to the approved drug, gabapentin, and previously published sEH inhibitors. Therefore, these new sEH inhibitors could be an attractive alternative to treat diabetic neuropathy in humans.

Central P2Y12 receptor blockade alleviates inflammatory and neuropathic pain and cytokine production in rodents

In this study the role of P2Y12 receptors (P2Y12R) was explored in rodent models of inflammatory and neuropathic pain and in acute thermal nociception. In correlation with their activity to block the recombinant human P2Y12R, the majority of P2Y12R antagonists alleviated mechanical hyperalgesia dose-dependently, following intraplantar CFA injection, and after partial ligation of the sciatic nerve in rats. They also caused an increase in thermal nociceptive threshold in the hot plate test. Among the six P2Y12R antagonists evaluated in the pain studies, the selective P2Y12 receptor antagonist PSB-0739 was most potent upon intrathecal application.P2Y12R mRNA and IL-1β protein were time-dependently overexpressed in the rat hind paw and lumbar spinal cord following intraplantar CFA injection. This was accompanied by the upregulation of TNF-α, IL-6 and IL-10 in the hind paw. PSB-0739 (0.3mg/kg i.t.) attenuated CFA-induced expression of cytokines in the hind paw and of IL-1β in the spinal cord. Subdiaphragmatic vagotomy and the α7 nicotinic acetylcholine receptor antagonist MLA occluded the effect of PSB-0739 (i.t.) on pain behavior and peripheral cytokine induction. Denervation of sympathetic nerves by 6-OHDA pretreatment did not affect the action of PSB-0739. PSB-0739, in an analgesic dose, did not influence motor coordination and platelet aggregation. Genetic deletion of the P2Y12R in mice reproduced the effect of P2Y12R antagonists on mechanical hyperalgesia in inflammatory and neuropathic pain models, on acute thermal nociception and on the induction of spinal IL-1β.Here we report the robust involvement of the P2Y12R in inflammatory pain. The anti-hyperalgesic effect of P2Y12R antagonism could be mediated by the inhibition of both central and peripheral cytokine production and involves α7-receptor mediated efferent pathways.

Studying ongoing and spontaneous pain in rodents--challenges and opportunities.

The measurement of spontaneous ongoing pain in rodents is a multiplex issue and a subject of extensive and longstanding debate. Considering the need to align available rodent models with clinically relevant forms of pain, it is of prime importance to thoroughly characterize behavioral outcomes in rodents using a portfolio of measurements that are not only stimulus-dependent but also encompass voluntary behavior in unrestrained animals. Moreover, the temporal course and duration of behavioral tests should be taken into consideration when we plan our studies to measure explicit chronic pain, with a particular emphasis on performing longitudinal studies in rodents. While using rodents as model organisms, it is also worth considering their circadian rhythm and the influence of the test conditions on the behavioral results, which are affected by social paradigms, stress and anxiety. In humans, general wellbeing is closely related to pain perception, which also makes it necessary in rodents to consider modulators as well as readouts of overall wellbeing. Optimizing the above parameters in study design and the new developments that are forthcoming to test the affective motivational components of pain hold promise in solving inconsistencies across studies and improving their broad applicability in translational research. In this review, we critically discuss a variety of behavioral tests that have been developed and reported in recent years, attempt to weigh their benefits and potential limitations, and discuss key requirements and challenges that lie ahead in measuring ongoing pain in rodent models.

Discovery of MK-4409, a Novel Oxazole FAAH Inhibitor for the Treatment of Inflammatory and Neuropathic Pain.

We report herein the identification of MK-4409, a potent and selective fatty acid amide hydrolase (FAAH) inhibitor. Starting from a high throughput screening (HTS) hit, medicinal chemistry efforts focused on optimizing of FAAH inhibition in vitro potency, improving the pharmacokinetic (PK) profile, and increasing in vivo efficacy in rodent inflammatory and neuropathic pain assays.

(362) The effects of voluntary wheel running on pain

Chronic pain is a major public health problem, afflicting more of the general population than cardiovascular disease, diabetes, and cancer combined. Despite the prevalence of chronic pain, clinically prescribed analgesics with high efficacy often have undesirable off-target effects. Exercise is commonly prescribed to chronic pain patients and gives rise to analgesia in some cases. Preclinical animal models support a conserved mechanism for exercise-induced analgesia as exercise attenuates hypersensitivity in rodent inflammatory and neuropathic pain models. The cellular mechanisms that mediate the therapeutic effects of exercise are not well characterized, though a role for neurotrophic factors has been suggested. Exercise significantly increases neurotrophic factor levels in the central nervous system, and in turn, is associated with enhanced functional plasticity. We hypothesize that analogous changes in the peripheral nervous system underlie exercise-induced analgesia. Understanding these mechanisms may aid development of novel analgesics. To investigate the behavioral effects of exercise in mice, we are testing the effects of voluntary wheel running on thermal and mechanical sensitivity and in acute pain models, as compared to sedentary control animals. To elucidate the cellular mechanisms underlying the effects of exercise, neurotrophic factor levels in dorsal root ganglia and spinal cord are being quantified biochemically as candidate signaling molecules involved in exercise-induced analgesia. These studies will enhance our understanding of both the molecular effects exercise, and more broadly, our comprehension of pain modulation. Supported by NINDS grant R01NS042595. Chronic pain is a major public health problem, afflicting more of the general population than cardiovascular disease, diabetes, and cancer combined. Despite the prevalence of chronic pain, clinically prescribed analgesics with high efficacy often have undesirable off-target effects. Exercise is commonly prescribed to chronic pain patients and gives rise to analgesia in some cases. Preclinical animal models support a conserved mechanism for exercise-induced analgesia as exercise attenuates hypersensitivity in rodent inflammatory and neuropathic pain models. The cellular mechanisms that mediate the therapeutic effects of exercise are not well characterized, though a role for neurotrophic factors has been suggested. Exercise significantly increases neurotrophic factor levels in the central nervous system, and in turn, is associated with enhanced functional plasticity. We hypothesize that analogous changes in the peripheral nervous system underlie exercise-induced analgesia. Understanding these mechanisms may aid development of novel analgesics. To investigate the behavioral effects of exercise in mice, we are testing the effects of voluntary wheel running on thermal and mechanical sensitivity and in acute pain models, as compared to sedentary control animals. To elucidate the cellular mechanisms underlying the effects of exercise, neurotrophic factor levels in dorsal root ganglia and spinal cord are being quantified biochemically as candidate signaling molecules involved in exercise-induced analgesia. These studies will enhance our understanding of both the molecular effects exercise, and more broadly, our comprehension of pain modulation. Supported by NINDS grant R01NS042595.

Characterisation of pain-related behaviours in association with joint pathology in an 8-week antigen-induced arthritis model

s / Osteoarthritis and Cartilage 22 (2014) S7–S56 S37 articular antigen leads to additional pain-related behaviours. This approach should enable us to dissect relationships between specific pain pathways and joint pathology. 54 PAIN SIGNALLING IN RODENT JOINTS BY SERINE PROTEASES J.J. McDougall, A. Reid, F.A. Russell. Dalhousie Univ., Halifax, NS, Canada Purpose: Serine proteases modulate pain by either activating or disarming protease-activated receptors (PARs). The aim of the present study was to assess the effect of two serine proteases (cathepsin G and neutrophil elastase) on knee joint mechanosensitivity and pain in rodents. Methods: Electrophysiological recordings from knee joint primary afferents were carried out in deeply anaesthetised male Wistar rats in response to normal and hyper-rotation of the knee. Animals were treated with either normal or denatured cathepsin G (1ng 10mg; close intra-arterial injection) and nerve recordings repeated. In separate experiments, the effect of intra-articular injection of human neutrophil elastase (1-50mg rat; 1-5mg mouse) on pain behaviour in awake and unrestrained Wistar rats and c57bl/6 mice was determined by measurement of hindlimb incapacitance (weight bearing) and von Frey hair tactile allodynia. Results: Peripheral administration of cathepsin G caused a dosedependent reduction in joint mechanosensitivity compared to boiled enzyme, which had no discernible effect (P 0.8). As expected, KOOS is more strongly correlated with physical subscales of the SF-36 across different conditions, interventions and languages. While KOOS demonstrates large effect sizes post-surgery, future studies should prioritise evaluation of longitudinal validity to adhere with updated recommendations for evaluation of measurement properties. 56 ALL-CAUSE MORTALITY AND SERIOUS CARDIOVASCULAR EVENTS IN PEOPLE WITH HIP AND KNEE OSTEOARTHRITIS: A POPULATION BASED COHORT STUDY G.A. Hawker yz, R. Croxford x, A.S. Bierman y, P.J. Harvey yz, B. Ravi y, I. Stanaitis z, L.L. Lipscombe yz. yUniv. of Toronto, Toronto, ON, Canada; zWomen’s Coll. Hosp., Toronto, ON, Canada; x Inst. for Clinical Evaluative Sci. (ICES), Toronto, ON, Canada Purpose: Osteoarthritis (OA) is under-treated in part due to the high coprevalence of other chronic conditions. Approximately 90% of people aged 65þ years with symptomatic OA have at least one additional chronic condition; OA and cardiovascular disease, CVD, are among the most common dyads seen in clinical practice. CVD, in particular, may be perceived as precluding the use of OA therapies (e.g. NSAIDs). Inadequately treated, people with OA may manage their OA by avoiding activities, like walking, that exacerbate symptoms. Reduced physical activity may further increase these individuals’ CVD risk. We therefore examined the contribution of OA disability to other CVD risk factors, including prior history of a CVD event, on risk for CVD events. Methods: This retrospective cohort study used data from a population cohortwith symptomatic hip and kneeOA that was recruited from 199698 through a screening survey of 100% of the population aged 55þ years in twoOntario regions (n1⁄42225hadOAconfirmedonexamination andxray). The baseline survey assessed: socio-demographics; smoking; selfreportedheight,weight andNSAIDuse; comorbidity (includingphysician diagnosis and treatment in thepast year forhighbloodpressure, diabetes, andheartproblems); andmentalhealth status.OAdisabilitywas assessed using theWOMAC function subscale, useof awalkingaid (yes/no) and the Health Assessment Questionnaire walking disability subscale (0–3; higher scores indicate greater disability). The current study used baseline surveys linked to provincial health administrative databases.

Pain signalling in rodent joints by serine proteases

s / Osteoarthritis and Cartilage 22 (2014) S7–S56 S37 articular antigen leads to additional pain-related behaviours. This approach should enable us to dissect relationships between specific pain pathways and joint pathology. 54 PAIN SIGNALLING IN RODENT JOINTS BY SERINE PROTEASES J.J. McDougall, A. Reid, F.A. Russell. Dalhousie Univ., Halifax, NS, Canada Purpose: Serine proteases modulate pain by either activating or disarming protease-activated receptors (PARs). The aim of the present study was to assess the effect of two serine proteases (cathepsin G and neutrophil elastase) on knee joint mechanosensitivity and pain in rodents. Methods: Electrophysiological recordings from knee joint primary afferents were carried out in deeply anaesthetised male Wistar rats in response to normal and hyper-rotation of the knee. Animals were treated with either normal or denatured cathepsin G (1ng 10mg; close intra-arterial injection) and nerve recordings repeated. In separate experiments, the effect of intra-articular injection of human neutrophil elastase (1-50mg rat; 1-5mg mouse) on pain behaviour in awake and unrestrained Wistar rats and c57bl/6 mice was determined by measurement of hindlimb incapacitance (weight bearing) and von Frey hair tactile allodynia. Results: Peripheral administration of cathepsin G caused a dosedependent reduction in joint mechanosensitivity compared to boiled enzyme, which had no discernible effect (P 0.8). As expected, KOOS is more strongly correlated with physical subscales of the SF-36 across different conditions, interventions and languages. While KOOS demonstrates large effect sizes post-surgery, future studies should prioritise evaluation of longitudinal validity to adhere with updated recommendations for evaluation of measurement properties. 56 ALL-CAUSE MORTALITY AND SERIOUS CARDIOVASCULAR EVENTS IN PEOPLE WITH HIP AND KNEE OSTEOARTHRITIS: A POPULATION BASED COHORT STUDY G.A. Hawker yz, R. Croxford x, A.S. Bierman y, P.J. Harvey yz, B. Ravi y, I. Stanaitis z, L.L. Lipscombe yz. yUniv. of Toronto, Toronto, ON, Canada; zWomen’s Coll. Hosp., Toronto, ON, Canada; x Inst. for Clinical Evaluative Sci. (ICES), Toronto, ON, Canada Purpose: Osteoarthritis (OA) is under-treated in part due to the high coprevalence of other chronic conditions. Approximately 90% of people aged 65þ years with symptomatic OA have at least one additional chronic condition; OA and cardiovascular disease, CVD, are among the most common dyads seen in clinical practice. CVD, in particular, may be perceived as precluding the use of OA therapies (e.g. NSAIDs). Inadequately treated, people with OA may manage their OA by avoiding activities, like walking, that exacerbate symptoms. Reduced physical activity may further increase these individuals’ CVD risk. We therefore examined the contribution of OA disability to other CVD risk factors, including prior history of a CVD event, on risk for CVD events. Methods: This retrospective cohort study used data from a population cohortwith symptomatic hip and kneeOA that was recruited from 199698 through a screening survey of 100% of the population aged 55þ years in twoOntario regions (n1⁄42225hadOAconfirmedonexamination andxray). The baseline survey assessed: socio-demographics; smoking; selfreportedheight,weight andNSAIDuse; comorbidity (includingphysician diagnosis and treatment in thepast year forhighbloodpressure, diabetes, andheartproblems); andmentalhealth status.OAdisabilitywas assessed using theWOMAC function subscale, useof awalkingaid (yes/no) and the Health Assessment Questionnaire walking disability subscale (0–3; higher scores indicate greater disability). The current study used baseline surveys linked to provincial health administrative databases.

Antihyperalgesic/Antinociceptive Effects of Ceftriaxone and Its Synergistic Interactions with Different Analgesics in Inflammatory Pain in Rodents

The β-lactam antibiotic ceftriaxone stimulates glutamate transporter GLT-1 expression and is effective in neuropathic and visceral pain models. This study examined the effects of ceftriaxone and its interactions with different analgesics (ibuprofen, celecoxib, paracetamol, and levetiracetam) in somatic and visceral pain models in rodents. The effects of ceftriaxone (intraperitoneally/intraplantarly), analgesics (orally), and their combinations were examined in the carrageenan-induced paw inflammatory hyperalgesia model in rats (n = 6-12) and in the acetic acid-induced writhing test in mice (n = 6-10). The type of interaction between ceftriaxone and analgesics was determined by isobolographic analysis. Pretreatment with intraperitoneally administered ceftriaxone (10-200 mg/kg per day) for 7 days produced a significant dose-dependent antihyperalgesia in the somatic inflammatory model. Acute administration of ceftriaxone, via either intraperitoneal (10-200 mg/kg) or intraplantar (0.05-0.2 mg per paw) routes, produced a significant and dose-dependent but less efficacious antihyperalgesia. In the visceral pain model, significant dose-dependent antinociception of ceftriaxone (25-200 mg/kg per day) was observed only after the 7-day pretreatment. Isobolographic analysis in the inflammatory hyperalgesia model revealed approximately 10-fold reduction of doses of both drugs in all examined combinations. In the visceral nociception model, more than 7- and 17-fold reduction of doses of both drugs was observed in combinations of ceftriaxone with ibuprofen/paracetamol and celecoxib/levetiracetam, respectively. Ceftriaxone exerts antihyperalgesia/antinociception in both somatic and visceral inflammatory pain. Its efficacy is higher after a 7-day pretreatment than after acute administration. The two-drug combinations of ceftriaxone and the nonsteroidal analgesics/levetiracetam have synergistic interactions in both pain models. These results suggest that ceftriaxone, particularly in combinations with ibuprofen, celecoxib, paracetamol, or levetiracetam, may provide useful approach to the clinical treatment of inflammation-related pain.

How are we going to make progress treating bladder pain syndrome? ICI-RS 2013

To look at the current state of knowledge in bladder pain syndrome and ascertain how we can make advances in the near term. A compendium of the ideas presented at the International Consultation on Incontinence Research Society 2013 meeting of clinicians and basic scientists. The meeting included the following topics: potential connection between defined and undefined IC/BPS; association between psychiatric disorders and IC/BPS; rationale for multimodal therapy approach in IC/BPS; and issues of a placebo control in human studies. Translational research studies are still in need of improved animal models to study IC/BPS mechanisms and development of novel methods to objectively measure bladder pain in rodents. The need to try and develop better clinical therapies will best be met by proper phenotyping of this heterogeneous population and avoiding premature publication of clinical trials that are anecdotal and do not include randomized placebo control populations. Patients with Hunner's lesions should be identified prior to or in the course of clinical trials so that results in this subgroup can be evaluated.

Blockade of peripheral P2Y1 receptors prevents the induction of thermal hyperalgesia via modulation of TRPV1 expression in carrageenan-induced inflammatory pain rats: Involvement of p38 MAPK phosphorylation in DRGs

Although previous reports have suggested that P2Y1 receptors (P2Y1Rs) are involved in cutaneous nociceptive signaling, it remains unclear how P2Y1Rs contribute to peripheral sensitization. The current study was designed to delineate the role of peripheral P2Y1Rs in pain and to investigate potential linkages to mitogen-activated protein kinase (MAPK) in DRGs and Transient Receptor Potential Vanilloid 1 (TRPV1) expression in a rodent inflammatory pain model. Following injection of 2% carrageenan into the hind paw, expressions of P2Y1 and TRPV1 and the phosphorylation rates of both p38 MAPK and ERK but not JNK were increased and peaked at day 2 post-injection. Blockade of peripheral P2Y1Rs by the P2Y1R antagonist, MRS2500 injection (i.pl, D0 to D2) significantly reduced the induction of thermal hyperalgesia, but not mechanical allodynia. Simultaneously, MRS2500 injections suppressed upregulated TRPV1 expression and DRG p38 phosphorylation, while pERK signaling was not affected. Furthermore, inhibition of p38 activation in the DRGs by SB203580 (a p38 inhibitor, i.t, D0 to D2) prevented the upregulation of TRPV1 and a single i.t injection of SB203580 reversed the established thermal hyperalgesia, but not mechanical allodynia. Lastly, to identify the mechanism of action of P2Y1Rs, we repeatedly injected the P2Y1 agonist, MRS2365 into the naïve rat's hind paw and observed a dose-dependent increase in TRPV1 expression and p38 MAPK phosphorylation. These data demonstrate a sequential role for P2Y1R, p38 MAPK and TRPV1 in inflammation-induced thermal hyperalgesia; thus, peripheral P2Y1Rs activation modulates p38 MAPK signaling and TRPV1 expression, which ultimately leads to the induction of thermal hyperalgesia.

Gabapentin reverses central pain sensitization following a collagenase-induced intrathalamic hemorrhage in rats

PurposeThe treatment of central neuropathic pain remains amongst the biggest challenges for pain specialists. The main objective of this study was to assess gabapentin (GBP), amitriptyline (AMI), and carbamazepine (CARBA) for the treatment of a rodent central neuropathic pain model.MethodsMale Sprague Dawley rats were trained on the rotarod, Hargreaves, Von Frey and acetone behavioral tests, and baseline values were obtained prior to surgery. A stereotaxic injection of either a collagenase solution or saline was made in the right ventral posterolateral thalamic nucleus. The rats were tested on days 2, 4, 8, and 11 postsurgery. They were retested at regular intervals from day 15 to day 25 postsurgery, after oral administration of either the vehicle (n=7 and n=8 rats with intracerebral injections of collagenase and saline, respectively) or the different drugs (GBP [60 mg/kg], AMI [10 mg/kg], CARBA [100 mg/kg]; n=8 rats/drug).ResultsA significant decrease in the mechanical thresholds and no change in heat threshold were observed in both hind limbs in the collagenase group, as we had previously shown elsewhere. Reversal of the mechanical hypersensitivity was achieved only with GBP (P<0.05). AMI and CARBA, at the dosages used, failed to show any effect on mechanical thresholds. Transient cold allodynia was observed in some collagenase-injected rats but failed to be statistically significant.ConclusionIntrathalamic hemorrhaging in the ventrolateral thalamic nucleus induced a bilateral mechanical allodynia, which was reversed by GBP but not AMI or CARBA.

Optimization of a Pain Model: Effects of Body Temperature and Anesthesia on Bladder Nociception in Mice

Interstitial cystitis/bladder pain syndrome (IC/BPS) is a debilitating urological condition that is resistant to treatment and poorly understood. To determine novel molecular treatment targets and to elucidate the contribution of the nervous system to IC/BPS, many rodent bladder pain models have been developed. In this study we evaluated the effects of anesthesia induction and temperature variation in a mouse model of bladder pain known as urinary bladder distension (UBD). In this model compressed air is used to distend the bladder to distinct pressures while electrodes record the reflexive visceromotor response (VMR) from the overlying abdominal muscle. Two isoflurane induction models are commonly used before UBD: a short method lasting approximately 30 minutes and a long method lasting approximately 90 minutes. Animals were anesthetized with one of the methods then put through three sets of graded bladder distensions. Distensions performed following the short anesthesia protocol were significantly different from one another despite identical testing parameters; this same effect was not observed when the long anesthesia protocol was used. In order to determine the effect of temperature on VMRs, animals were put through three graded distension sets at 37.5 (normal mouse body temperature), 35.5, and 33.5°C. Distensions performed at 33.5 and 35.5°C were significantly lower than those performed at 37.5°C. Additionally, Western blot analysis revealed significantly smaller increases in spinal levels of phosphorylated extracellular-signal regulated kinase 2 (pERK2) following bladder distension in animals whose body temperature was maintained at 33.5°C as opposed to 37.5°C. These results highlight the significance of the dynamic effects of anesthesia on pain-like changes and the importance of close monitoring of temperature while performing UBD. For successful interpretation of VMRs and translation to human disease, body temperature should be maintained at 37.5°C and isoflurane induction should gradually decrease over the course of 90 minutes.

“Bedside-to-Bench” Behavioral Outcomes in Animal Models of Pain: Beyond the Evaluation of Reflexes

Despite the myriad promising new targets and candidate analgesics recently identified in preclinical pain studies, little translation to novel pain medications has been generated. The pain phenotype in humans involves complex behavioral alterations, including changes in daily living activities and psychological disturbances. These behavioral changes are not reflected by the outcome measures traditionally used in rodents for preclinical pain testing, which are based on reflexes evoked by sensory stimuli of different types (mechanical, thermal or chemical). These measures do not evaluate the impact of the pain experience on the global behavior or disability of the animals, and therefore only consider a limited aspect of the pain phenotype. The development of relevant new outcomes indicative of pain to increase the validity of animal models of pain has been increasingly pursued over the past few years. The aim has been to translate “bedside-to-bench” outcomes from the human pain phenotype to rodents, in order to complement traditional pain outcomes by providing a closer and more realistic measure of clinical pain in rodents. This review summarizes and discusses the most important nonstandard outcomes for pain assessment in preclinical studies. The advantages and drawbacks of these techniques are considered, and their potential impact on the validation of potential analgesics is evaluated.

Substance P-Saporin for Bone Cancer Pain in Dogs

Substance P-Saporin for Bone Cancer Pain in Dogs

Discovery of a selective Na V 1.7 inhibitor from centipede venom with analgesic efficacy exceeding morphine in rodent pain models

Loss-of-function mutations in the human voltage-gated sodium channel NaV1.7 result in a congenital indifference to pain. Selective inhibitors of NaV1.7 are therefore likely to be powerful analgesics for treating a broad range of pain conditions. Herein we describe the identification of µ-SLPTX-Ssm6a, a unique 46-residue peptide from centipede venom that potently inhibits NaV1.7 with an IC50 of ∼25 nM. µ-SLPTX-Ssm6a has more than 150-fold selectivity for NaV1.7 over all other human NaV subtypes, with the exception of NaV1.2, for which the selectivity is 32-fold. µ-SLPTX-Ssm6a contains three disulfide bonds with a unique connectivity pattern, and it has no significant sequence homology with any previously characterized peptide or protein. µ-SLPTX-Ssm6a proved to be a more potent analgesic than morphine in a rodent model of chemical-induced pain, and it was equipotent with morphine in rodent models of thermal and acid-induced pain. This study establishes µ-SPTX-Ssm6a as a promising lead molecule for the development of novel analgesics targeting NaV1.7, which might be suitable for treating a wide range of human pain pathologies.

Advantageous safety profile of a dual selective alpha2Cagonist/alpha2Aantagonist antinociceptive agent

A selective α2C -adrenoceptor (AR) agonist was developed for the treatment of neuropathic pain. The objective was to dissociate analgesic activity from cardiovascular and sedative side effects commonly observed with nonselective agents. A 2-amino-oxazoline derivative (compound A), identified as a dual α2C -AR agonist/α2A -AR antagonist in in vitro-binding assays, exhibited in vivo efficacy in rodent pain models. Its safety profile was compared with that of clonidine in six different in vivo models. Contrary to clonidine, compound A did not induce hypotension in pentobarbital-anesthetized rats, in conscious spontaneous hypertensive rats, or in telemetered dogs. Both agents induced similar dose-dependent decreases in heart rate in dogs and rats. In anesthetized pithed rats, clonidine showed dose-dependent hypertension and inhibited electrical nerve stimulation-induced tachycardia at doses close to its efficacious doses in the mouse formalin test, while compound A had much weaker vasoconstrictive and antichronotropic effects. Finally, in a mouse Irwin test, no sedation was observed with compound A at 30-fold its ED50 in the mouse formalin test, while sedative effects of clonidine started from three-fold its ED50 . These data confirm the advantageous safety profile of the new dual α2C -AR agonist/α2A -AR antagonist agent vs. the nonselective agonist clonidine.

Fatty Acid Amide Hydrolase-Dependent Generation of Antinociceptive Drug Metabolites Acting on TRPV1 in the Brain

The discovery that paracetamol is metabolized to the potent TRPV1 activator N-(4-hydroxyphenyl)-5Z,8Z,11Z,14Z-eicosatetraenamide (AM404) and that this metabolite contributes to paracetamol’s antinociceptive effect in rodents via activation of TRPV1 in the central nervous system (CNS) has provided a potential strategy for developing novel analgesics. Here we validated this strategy by examining the metabolism and antinociceptive activity of the de-acetylated paracetamol metabolite 4-aminophenol and 4-hydroxy-3-methoxybenzylamine (HMBA), both of which may undergo a fatty acid amide hydrolase (FAAH)-dependent biotransformation to potent TRPV1 activators in the brain. Systemic administration of 4-aminophenol and HMBA led to a dose-dependent formation of AM404 plus N-(4-hydroxyphenyl)-9Z-octadecenamide (HPODA) and arvanil plus olvanil in the mouse brain, respectively. The order of potency of these lipid metabolites as TRPV1 activators was arvanil = olvanil>>AM404> HPODA. Both 4-aminophenol and HMBA displayed antinociceptive activity in various rodent pain tests. The formation of AM404, arvanil and olvanil, but not HPODA, and the antinociceptive effects of 4-aminophenol and HMBA were substantially reduced or disappeared in FAAH null mice. The activity of 4-aminophenol in the mouse formalin, von Frey and tail immersion tests was also lost in TRPV1 null mice. Intracerebroventricular injection of the TRPV1 blocker capsazepine eliminated the antinociceptive effects of 4-aminophenol and HMBA in the mouse formalin test. In the rat, pharmacological inhibition of FAAH, TRPV1, cannabinoid CB1 receptors and spinal 5-HT3 or 5-HT1A receptors, and chemical deletion of bulbospinal serotonergic pathways prevented the antinociceptive action of 4-aminophenol. Thus, the pharmacological profile of 4-aminophenol was identical to that previously reported for paracetamol, supporting our suggestion that this drug metabolite contributes to paracetamol’s analgesic activity via activation of bulbospinal pathways. Our findings demonstrate that it is possible to construct novel antinociceptive drugs based on fatty acid conjugation as a metabolic pathway for the generation of TRPV1 modulators in the CNS.

Discovery Versus Translation: Do we Need Different Strategies?

249 ISSN 1758-1869 10.2217/PMT.13.21 © 2013 Future Medicine Ltd Pain Manage. (2013) 3(4), 249–252 There is little doubt that we have experienced far too many quiet moments when it comes to talking about the success of translational pain research. Over the past 5 years a number of articles have been written explaining how good we are at preclinical discovery, and yet, over a period of nearly two decades, we have little to show for it in terms of significant clinical advances in the development of more effective interventions [1]. Perhaps the most compelling commentary on this topic is the growing economic burden of pain and the staggering number of people, from young adults to the elderly, affected by chronic pain [2,3]. These statistics beg the question: if our research strategy is on the right track, then why are the economic and prevalence metrics escalating at a rate faster than at any time in history and why are providers relying on the same basic drug classes to treat pain that have been in place for over a decade? Given a not-so-successful track record in translational research, one is tempted to ask the question, “Are we really that stubborn?” or, as some have asked, “Are we just smart people making dumb mistakes?” working with the hope that if we keep plugging away we will eventually discover a magic bullet. First, we need to appreciate the fact that when dealing with chronic pain there are no magic bullets. Second, we should acknowledge that our basic strategy of problem-solving, starting with the development and assessment of preclinical models right through to the design of clinical trials, hasn’t changed significantly in 20 years. When one considers that all preclinical models of chronic pain reliably produce the desired behavioral effects, and that these models have provided a much improved understanding of the putative mechanisms responsible for different pain conditions, the obvious question is “why has not all the research productivity been validated with a positive clinical outcome?” One possible answer is that for all the discoveries we have made studying pain in rodents, there seems to be little evidence of meaningful parallels with the human condition. It is also possible that what we have been studying for the past 30 years applies primarily to acute pain and that all preclinical models are distant relatives to the more clinically relevant condition of chronic pain. One cannot avoid the unthinkable possibility that we might also