Evaluation of different drug classes on transient sciatic nerve injury-depressed marble burying in mice

Neurobehavioral toxicity of Cold plasma activated water following oral gavage in mice

Cold plasma-activated water (PAW) is a novel technology that was recently used in biomedical research; Despite its potential, PAW's safety remains inadequately assessed. The study explores the impact of PAW on behavioral responses and brain tissue histopathology in mice. Ten-week-old female albino mice were divided into three groups each containing 10 mice (5 replicates, 2 mice/cage) and received either distilled water (DW), or distilled water exposed to cold atmospheric plasma (CAP) for 3 min (PAW-3), or 15 min (PAW-15) by oral gavage in a dose of 200 μL/mice (3 times/week) for four weeks. PAW exhibited altered physicochemical properties compared to DW. Mice exposed to PAW demonstrated reduced burrowing activity, marble burying ability, and novel object recognition compared to controls, indicating potential neurobehavioral alterations. PAW-treated groups displayed notable histological lesions in brain tissues, including nerve cell necrosis, vascular congestion, and Purkinje cell degeneration, confirming neurotoxic effects. Positive reactions for NF-κB and iNOS in brain tissues of PAW-treated mice corroborated the histopathological findings, suggesting neuroinflammation and oxidative stress. The study highlights the need for further investigation into PAW's safety profile and optimal treatment protocols to mitigate potential neurobehavioral toxicity in biomedical research.

Characterization of commercially available murine fibrosarcoma NCTC-2472 cells both in vitro and as a model of bone cancer pain in vivo

For many cancer patients tumor burden negatively impacts quality of life due to associated pain onset. Neuropathic pain is commonly associated with late cancer stages, and is resultant of tumor metastasis to bone, herein referred to as cancer-induced bone pain. Given the severe impact on quality of life and clinical treatment strategies focusing on symptom management, novel therapeutics are needed to alleviate cancer-induced bone pain and/or reduce cancer burden. In the current study we characterized a commercially available murine fibrosarcoma cell line, NCTC-2472 in vitro, which can be used to assess the capacity of novel compounds to impact cellular viability. We found that dimethyl sulfoxide, a known cytotoxic agent and common drug preparation compound, significantly decreased cell viability in a dose-related manner. We then characterized the in vivo tumor development and associated pain behavior characteristics following implantation of NCTC-2472 fibrosarcoma into male and female C3H/HeJ mice. The C3H/HeJ strain was utilized as these mice are syngeneic with NCTC-2472 fibrosarcoma and their use reduces potential implantation failure. We found that tumor development in mice resulted in the development of mechanical allodynia but not thermal hyperalgesia. Gabapentin, a clinically relevant analgesic, produced dose-related mechanical allodynia reversal. These studies provide further characterization of a cancer-induced bone pain model that can be used to examine novel compounds as anti-cancer and analgesic therapeutics.

Effects of Repeated Treatment with the Monoacylglycerol Lipase Inhibitor MJN110 on Pain-Related Depression of Nesting and Cannabinoid 1 Receptor Function in Male and Female Mice

MJN110 inhibits the enzyme monoacylglycerol lipase (MAGL) to increase levels of the endocannabinoid 2-arachidonoylglycerol , an endogenous high-efficacy agonist of cannabinoid 1 and 2 receptors (CB1/2R). MAGL inhibitors are under consideration as candidate analgesics, and we reported previously that acute MJN110 produced partial antinociception in an assay of pain-related behavioral depression in mice. Given the need for repeated analgesic administration in many pain patients and the potential for analgesic tolerance during repeated treatment, this study examined antinociceptive effects of repeated MJN110 on pain-related behavioral depression and CB1R-mediated G-protein function. Male and female ICR mice were treated daily for 7 days in a 2 × 2 design with (a) 1.0 mg/kg/d MJN110 or its vehicle followed by (b) intraperitoneal injection of dilute lactic acid (IP acid) or its vehicle as a visceral noxious stimulus to depress nesting behavior. After behavioral testing, G-protein activity was assessed in lumbar spinal cord (LSC) and five brain regions using an assay of CP55,940-stimulated [35S]GTPɣS activation. As reported previously, acute MJN110 produced partial but significant relief of IP acid-induced nesting depression on day 1. After 7 days, MJN110 continued to produce significant but partial antinociception in males, while antinociceptive tolerance developed in females. Repeated MJN110 also produced modest decreases in maximum levels of CP55,940-induced [35S]GTPɣS binding in spinal cord and most brain regions. These results indicate that repeated treatment with a relatively low antinociceptive MJN110 dose produces only partial and sex-dependent transient antinociception associated with the emergence of CB1R desensitization in this model of IP acid-induced nesting depression. SIGNIFICANCE STATEMENT: The drug MJN110 inhibits monoacylglycerol lipase (MAGL) to increase levels of the endogenous cannabinoid 2-arachidonoylglycerol and produce potentially useful therapeutic effects including analgesia. This study used an assay of pain-related behavioral depression in mice to show that repeated MJN110 treatment produced (1) weak but sustained antinociception in male mice, (2) antinociceptive tolerance in females, and (3) modest cannabinoid-receptor desensitization that varied by region and sex. Antinociceptive tolerance may limit the utility of MJN110 for treatment of pain.

Onge CM, Lee YK, Li M, Rice KC, Zhang Y. Effects of Selective and Mixed-Action Kappa and Delta Opioid Receptor Agonists on Pain-Related Behavioral Depression in Mice

We recently developed a series of nalfurafine analogs (TK10, TK33, and TK35) that may serve as non-addictive candidate analgesics. These compounds are mixed-action agonists at the kappa and delta opioid receptors (KOR and DOR, respectively) and produce antinociception in a mouse warm-water tail-immersion test while failing to produce typical mu opioid receptor (MOR)-mediated side effects. The warm-water tail-immersion test is an assay of pain-stimulated behavior vulnerable to false-positive analgesic-like effects by drugs that produce motor impairment. Accordingly, this study evaluated TK10, TK33, and TK35 in a recently validated assay of pain-related behavioral depression in mice that are less vulnerable to false-positive effects. For comparison, we also evaluated the effects of the MOR agonist/analgesic hydrocodone (positive control), the neurokinin 1 receptor (NK1R) antagonist aprepitant (negative control), nalfurafine as a selective KOR agonist, SNC80 as a selective DOR agonist, and a nalfurafine/SNC80 mixture. Intraperitoneal injection of dilute lactic acid (IP lactic acid) served as a noxious stimulus to depress vertical and horizontal locomotor activity in male and female ICR mice. IP lactic acid-induced locomotor depression was alleviated by hydrocodone but not by aprepitant, nalfurafine, SNC80, the nalfurafine/SNC80 mixture, or the KOR/DOR agonists. These results suggest that caution is warranted in advancing mixed-action KOR/DOR agonists as candidate analgesics.

Role of mu opioid receptor (MOR) agonist efficacy as a determinant of opioid antinociception in a novel assay of pain-depressed behavior in female and male mice

Introduction

Intermediate efficacy mu opioid receptor (MOR) agonists have potential to retain analgesic effectiveness while improving safety, but the optimal MOR efficacy for effective and safe opioid analgesia is unknown. Preclinical assays of pain-depressed behavior can assess effects of opioids and other candidate analgesics on pain-related behavioral depression, which is a common manifestation of clinically relevant pain and target of pain treatment. Accordingly, the present study goal was to validate a novel assay of pain-depressed locomotor behavior in mice and evaluate the role of MOR efficacy as a determinant of opioid analgesic effects and related safety measures.

Methods

Male and female ICR mice were tested in a locomotor chamber consisting of 2 compartments connected by a doorway that contained a 1-inch-tall barrier. Dependent measures during 15-min behavioral sessions included crosses between compartments (which required vertical activity to surmount the barrier) and total movement counts (which required horizontal activity to break photobeams in each compartment).

Results and Discussion

Intraperitoneal injection of lactic acid (IP acid) produced a concentration- and time-dependent depression of both endpoints. Optimal blockade of IP acid-induced behavioral depression with minimal motor impairment was achieved with intermediate-efficacy MOR treatments that also produced less gastrointestinal-transit inhibition and respiratory depression than the high-efficacy MOR agonist fentanyl. Sex differences in treatment effects were rare. Overall, these findings validate a novel procedure for evaluating opioids and other candidate analgesic effects on pain-related behavioral depression in mice and support continued research with intermediate-efficacy MOR agonists as a strategy to retain opioid analgesic effectiveness with improved safety.

Climbing behavior by mice as an endpoint for preclinical assessment of drug effects in the absence and presence of pain

This study evaluated climbing in mice as a tool to assess the expression and treatment of pain-related behavioral depression in male and female ICR mice. Mice were videotaped during 10-min sessions in a vertical plexiglass cylinder with wire mesh walls, and "Time Climbing" was scored by observers blind to treatments. Initial validation studies demonstrated that baseline climbing was stable across repeated days of testing and depressed by intraperitoneal injection of dilute lactic acid (IP acid) as an acute pain stimulus. Additionally, IP acid-induced depression of climbing was blocked by the positive-control non-steroidal anti-inflammatory drug (NSAID) ketoprofen but not by the negative control kappa opioid receptor agonist U69593. Subsequent studies examined effects of single-molecule opioids (fentanyl, buprenorphine, naltrexone) and of fixed-proportion fentanyl/naltrexone mixtures (10:1, 3.2:1, and 1:1) that vary in their efficacy at the mu opioid receptor (MOR). Opioids administered alone produced a dose- and efficacy-dependent decrease in climbing, and fentanyl/naltrexone-mixture data indicated that climbing in mice is especially sensitive to disruption by even low-efficacy MOR activation. Opioids administered as a pretreatment to IP acid failed to block IP acid-induced depression of climbing. Taken together, these findings support the utility of climbing in mice as an endpoint to evaluate candidate-analgesic effectiveness both to (a) produce undesirable behavioral disruption when the test drug is administered alone, and (b) produce a therapeutic blockade of pain-related behavioral depression. The failure of MOR agonists to block IP acid-induced depression of climbing likely reflects the high sensitivity of climbing to disruption by MOR agonists.

Salt as a non-caloric behavioral modifier: A review of evidence from pre-clinical studies

Though excess salt intake is well-accepted as a dietary risk factor for cardiovascular diseases, relatively little has been explored about how it impacts behavior, despite the ubiquity of salt in modern diets. Given the challenges of manipulating salt intake in humans, non-human animals provide a more tractable means for evaluating behavioral sequelae of high salt. By describing what is known about the impact of elevated salt on behavior, this review highlights how underexplored salt's behavioral effects are. Increased salt consumption in adulthood does not affect spontaneous anxiety-related behaviors or locomotor activity, nor acquisition of maze or fear tasks, but does impede expression of spatial/navigational and fear memory. Nest building is reduced by heightened salt in adults, and stress responsivity is augmented. When excess salt exposure occurs during development, and/or to parents, offspring locomotion is increased, and both spatial memory expression and social investigation are attenuated. The largely consistent findings reviewed here indicate expanded study of salt's effects will likely uncover broader behavioral implications, particularly in the scarcely studied female sex.

(E)-3-furan-2-yl-N-phenylacrylamide (PAM-4) decreases nociception and emotional manifestations of neuropathic pain in mice by α7 nicotinic acetylcholine receptor potentiation

Clinical intervention of pain is often accompanied by changes in affective behaviors, so both assays of affective and sensorial aspects of nociception play an important role in the development of novel analgesics. Although positive allosteric modulation (PAM) of α7 nicotinic acetylcholine receptors (nAChRs) has been recognized as a novel approach for the relief of sensorial aspects of pain, their effects on affective components of pain remain unclear. Therefore, we investigated whether PAM-4, a highly selective α7-nAChR PAM, attenuates inflammatory and neuropathic pain, as well as the concomitant depressive/anxiety comorbidities. The anti-nociceptive activity of PAM-4 was assessed in mice using the formalin test and chronic constriction injury (CCI)-induced neuropathic pain model. The anxiolytic- and antidepressant-like activity of PAM-4 was evaluated using the marble burying test and forced swimming test. Acute systemic administration of PAM-4 dose-dependently reversed formalin-induced paw licking behavior and CCI-induced mechanical allodynia without development of any motor impairment. PAM-4 reversed the decreased swimming time and number of buried marbles in CCI-treated mice, suggesting that this ligand attenuates chronic pain-induced depression-like behavior and anxiogenic-like effects. The effects of PAM-4 were inhibited by the α7-selective antagonist methyllycaconitine, indicating molecular mechanism mediated by α7-nAChRs. Indeed, electrophysiological recordings showed the PAM-4 enhances human α7 nAChRs with higher potency and efficacy compared to rat α7 nAChRs. These findings suggest that PAM-4 reduces both sensorial and affective behaviors induced by chronic pain in mice by α7-nAChR potentiation. PAM-4 deserves further investigations for the management of chronic painful conditions with comorbidities.

Behavioral Battery for Testing Candidate Analgesics in Mice. II. Effects of Endocannabinoid Catabolic Enzyme Inhibitors and ∆9-Tetrahydrocannabinol

Enhanced signaling of the endocannabinoid (eCB) system through inhibition of the catabolic enzymes monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH) has received increasing interest for development of candidate analgesics. This study compared effects of MAGL and FAAH inhibitors with effects of ∆9-tetrahydrocannabinol (THC) using a battery of pain-stimulated, pain-depressed, and pain-independent behaviors in male and female mice. Intraperitoneal injection of dilute lactic acid (IP acid) served as an acute visceral noxious stimulus to stimulate two behaviors (stretching, facial grimace) and depress two behaviors (rearing, nesting). Nesting and locomotion were also assessed in the absence of IP acid as pain-independent behaviors. THC and a spectrum of six eCB catabolic enzyme inhibitors ranging from MAGL- to FAAH-selective were assessed for effectiveness to alleviate pain-related behaviors at doses that did not alter pain-independent behaviors. The MAGL-selective inhibitor MJN110 produced the most effective antinociceptive profile, with 1.0 mg/kg alleviating IP acid effects on stretching, grimace, and nesting without altering pain-independent behaviors. MJN110 effects on IP acid-depressed nesting had a slow onset and long duration (40 minutes to 6 hours), were blocked by rimonabant, and tended to be greater in females. As inhibitors increased in FAAH selectivity, antinociceptive effectiveness decreased. PF3845, the most FAAH-selective inhibitor, produced no antinociception up to doses that disrupted locomotion. THC decreased IP acid-stimulated stretching and grimace at doses that did not alter pain-independent behaviors; however, it did not alleviate IP acid-induced depression of rearing or nesting. These results support further consideration of MAGL-selective inhibitors as candidate analgesics for acute inflammatory pain. SIGNIFICANCE STATEMENT: This study characterized a spectrum of endocannabinoid catabolic enzyme inhibitors ranging in selectivity from monoacylglycerol lipase-selective to fatty acid amide hydrolase-selective in a battery of pain-stimulated, pain-depressed, and pain-independent behaviors previously pharmacologically characterized in a companion paper. This battery provides a method for prioritizing candidate analgesics by effectiveness to alleviate pain-related behaviors at doses that do not alter pain-independent behaviors, with inclusion of pain-depressed behaviors increasing translational validity and decreasing susceptibility to motor-depressant false positives.

Untapped endocannabinoid pharmacological targets: Pipe dream or pipeline?

It has been established that the endogenous cannabinoid (endocannabinoid) system plays key modulatory roles in a wide variety of pathological conditions. The endocannabinoid system comprises both cannabinoid receptors, their endogenous ligands including 2-arachidonoylglycerol (2-AG), N-arachidonylethanolamine (anandamide, AEA), and enzymes that regulate the synthesis and degradation of endogenous ligands which include diacylglycerol lipase alpha (DAGL-α), diacylglycerol lipase beta (DAGL-β), fatty acid amide hydrolase (FAAH), monoacylglycerol lipase (MAGL), α/β hydrolase domain 6 (ABHD6). As the endocannabinoid system exerts considerable involvement in the regulation of homeostasis and disease, much effort has been made towards understanding endocannabinoid-related mechanisms of action at cellular, physiological, and pathological levels as well as harnessing the various components of the endocannabinoid system to produce novel therapeutics. However, drug discovery efforts within the cannabinoid field have been slower than anticipated to reach satisfactory clinical endpoints and raises an important question into the validity of developing novel ligands that therapeutically target the endocannabinoid system. To answer this, we will first examine evidence that supports the existence of an endocannabinoid system role within inflammatory diseases, neurodegeneration, pain, substance use disorders, mood disorders, as well as metabolic diseases. Next, this review will discuss recent clinical studies, within the last 5 years, of cannabinoid compounds in context to these diseases. We will also address some of the challenges and considerations within the cannabinoid field that may be important in the advancement of therapeutics into the clinic.

Antinociceptive Effects of Kappa-Opioid Receptor Agonists

Preclinical models that assess "pain" in rodents typically measure increases in behaviors produced by a "pain stimulus." A large literature exists showing that kappa opioid receptor (KOR) agonists can decrease these "pain-stimulated behaviors" following many different pain stimuli. Despite showing apparent antinociceptive properties in these preclinical models, KOR agonists failed as analgesics in clinical trials. Recent studies that assessed decreases in behavior due to a pain stimulus show that KOR agonists are not effective in restoring these "pain-depressed behaviors" to normal levels, which agrees with the lack of effectiveness for KOR agonists in clinical trials. One current explanation for the failure of previous KOR agonists in clinical trials is that those agonists activated beta-arrestin signaling and that KOR agonists with a greater bias for G protein signaling will be more successful. However, neither G protein-biased agonists nor beta-arrestin-biased agonists are very effective in assays of pain-depressed behavior, which suggests that novel biased agonists may still not be effective analgesics. This review provides a concise account of the effectiveness of KOR agonists in preclinical models of pain-stimulated and pain-depressed behaviors following the administration of different pain stimuli. Based on the previous results, it may be appropriate to include both behaviors when testing the analgesic potential of KOR agonists.

Deficit in voluntary wheel running in chronic inflammatory and neuropathic pain models in mice: Impact of sex and genotype

Patients with chronic pain report decreased general activity and emotional distress. Therefore, the development of various animal models that encompass different aspects of pain are crucial for the discovery of genetic differences and the assessment of novel analgesics to improve quality of life. C57BL/6J and DBA/2J mice received unilateral intraplantar injections of 100 % CFA, paclitaxel, or CCI surgery to compare their distance traveled in a voluntary wheel running assay, paw edema diameter, and mechanical sensitivity. Mechanical withdrawal thresholds were lower in both strains of mice that received CFA when compared to their vehicle. However, a decrease in distance traveled was observed in CFA-treated C57BL/6J but not DBA/2J mice. In a separate group, chemotherapy agent paclitaxel 8 mg/kg, i.p. was administered to both strains of mice to induce CIPN which was confirmed by lower mechanical thresholds in paclitaxel-treated mice compared to vehicle-treated mice. Only female C57BL/6J mice showed attenuation of distance traveled following treatment, whereas male C57BL/6J and DBA/2J mice did not. Lastly, C57BL/6J mice underwent chronic constriction injury (CCI) or sham surgery to observe the impact of another chronic neuropathic pain model in wheel running assay. CCI mice showed a gradual decrease in mechanical withdrawal threshold and a decrease in distance traveled compared to sham 5 days following the procedure. Comparing these chronic inflammatory and neuropathic pain models in different mouse strains may help us better understand genetic differences underlying pain perception and its impact on reflexive and nonreflexive outcome measures.

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

BACKGROUND AND OBJECTIVE

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

DATABASES AND DATA TREATMENT

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

RESULTS

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

CONCLUSIONS

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

Endogenous Opiates and Behavior: 2018

This paper is the forty-first consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2018 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (2), the roles of these opioid peptides and receptors in pain and analgesia in animals (3) and humans (4), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (5), opioid peptide and receptor involvement in tolerance and dependence (6), stress and social status (7), learning and memory (8), eating and drinking (9), drug abuse and alcohol (10), sexual activity and hormones, pregnancy, development and endocrinology (11), mental illness and mood (12), seizures and neurologic disorders (13), electrical-related activity and neurophysiology (14), general activity and locomotion (15), gastrointestinal, renal and hepatic functions (16), cardiovascular responses (17), respiration and thermoregulation (18), and immunological responses (19).

C57BL/6 substrain differences in formalin-induced pain-like behavioral responses

Substantial evidence from preclinical models of pain suggests that basal and noxious nociceptive sensitivity, as well as antinociceptive responses to drugs, show significant heritability. Individual differences to these responses have been observed across species from rodents to humans. The use of closely related C57BL/6 inbred mouse substrains can facilitate gene mapping of acute nociceptive behaviors in preclinical pain models. In this study, we investigated behavioral differences between C57BL/6 J (B6 J) and C57BL/6 N (B6 N) substrains in the formalin test, a widely used tonic inflammatory pain model, using a battery of pain-related phenotypes, including reflexive tests, nesting, voluntary wheel running, sucrose preference and anxiety-like behavior in the light/dark test at two different time points (1-h and 24-h). Our results show that these substrains did not differ in reflexive thermal and mechanical responses at the 1-h time point. However, B6 N substrain mice showed increased sensitivity to spontaneous pain-like behaviors. In addition, B6 N substrain continued to show higher levels of mechanical hypersensitivity compared to controls at 24-h. indicating that mechanical hypersensitivity is a more persistent pain-related phenotype induced by formalin. Finally, no sex differences were observed in our outcome measures. Our results provide a comprehensive behavioral testing paradigm in response to an inflammatory agent for future mouse genetic studies in pain.

The Effects of Morphine, Baclofen, and Buspirone Alone and in Combination on Schedule-Controlled Responding and Hot Plate Antinociception in Rats

Better therapeutic options are needed for pain. Baclofen, buspirone, and morphine are characterized as having analgesic properties. However, little is known about potential interactions between analgesic effects of these drugs when combined. Furthermore, it is not known if the magnitude of these potential interactions will be similar for all drug effects. Thus, we tested the effects of these drugs alone and in combination for their capacity to produce thermal antinociception and to decrease food-maintained responding. Four male and four female Sprague-Dawley rats responded for food under a fixed-ratio 10 schedule; afterward they were immediately placed on a 52°C hot plate. Morphine, baclofen, and buspirone were examined alone and in 1:1 combinations, based upon ED₅₀ values. Morphine and baclofen effects were evaluated with the opioid antagonist naltrexone and the GABA_B antagonist (3-Aminopropyl)(diethoxymethyl)phosphinic acid (CGP35348), respectively. Morphine, baclofen, and buspirone dose dependently decreased operant responding, with the calculated ED₅₀ values being 7.09, 3.42, and 0.57 mg/kg, respectively. The respective antinociception ED₅₀ values were 16.15, 8.75, and 2.20 mg/kg. Analysis of 1:1 combinations showed the effects of morphine plus baclofen to decrease schedule-controlled responding and to produce thermal antinociception were synergistic. Effects of morphine plus buspirone and baclofen plus buspirone to decrease schedule-controlled responding were additive. Effects of the two combinations to produce thermal antinociception were synergistic. Naltrexone and CGP35348 antagonized the effects of morphine and baclofen, respectively. Synergistic antinociceptive effects, in conjunction with additive effects on food-maintained responding, highlight the therapeutic utility of opioid and non-opioid drug combinations.

Core Outcome Measures in Preclinical Assessment of Candidate Analgesics

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

Brain-Permeant and -Impermeant Inhibitors of Fatty Acid Amide Hydrolase Synergize with the Opioid Analgesic Morphine to Suppress Chemotherapy-Induced Neuropathic Nociception Without Enhancing Effects of Morphine on Gastrointestinal Transit

Opioid-based therapies remain a mainstay for chronic pain management, but unwanted side effects limit therapeutic use. We compared efficacies of brain-permeant and -impermeant inhibitors of fatty acid amide hydrolase (FAAH) in suppressing neuropathic pain induced by the chemotherapeutic agent paclitaxel. Paclitaxel produced mechanical and cold allodynia without altering nestlet shredding or marble burying behaviors. We compared FAAH inhibitors that differ in their ability to penetrate the central nervous system for antiallodynic efficacy, pharmacological specificity, and synergism with the opioid analgesic morphine. (3'-(aminocarbonyl)[1,1'-biphenyl]- 3-yl)-cyclohexylcarbamate (URB597), a brain-permeant FAAH inhibitor, attenuated paclitaxel-induced allodynia via cannabinoid receptor 1 (CB₁) and cannabinoid receptor 2 (CB₂) mechanisms. URB937, a brain-impermeant FAAH inhibitor, suppressed paclitaxel-induced allodynia through a CB₁ mechanism only. 5-[4-(4-cyano-1-butyn-1-yl)phenyl]-1-(2,4-dichlorophenyl)-N-(1,1-dioxido-4-thiomorpholinyl)-4-methyl-1H-pyrazole-3-carboxamide (AM6545), a peripherally restricted CB₁ antagonist, fully reversed the antiallodynic efficacy of N-cyclohexyl-carbamic acid, 3'-(aminocarbonyl)-6-hydroxy[1,1'- biphenyl]-3-yl ester (URB937) but only partially reversed that of URB597. Thus, URB937 suppressed paclitaxel-induced allodynia through a mechanism that was dependent upon peripheral CB₁ receptor activation only. Antiallodynic effects of both FAAH inhibitors were reversed by N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251). Antiallodynic effects of URB597, but not URB937, were reversed by 6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indol-3-yl](4-methoxyphenyl)methanone (AM630). Isobolographic analysis revealed synergistic interactions between morphine and either URB597 or URB937 in reducing paclitaxel-induced allodynia. A leftward shift in the dose-response curve of morphine antinociception was observed when morphine was coadministered with either URB597 or URB937, consistent with morphine sparing. However, neither URB937 nor URB597 enhanced morphine-induced deficits in colonic transit. Thus, our findings suggest that FAAH inhibition may represent a therapeutic avenue to reduce the overall amount of opioid needed for treating neuropathic pain with potential to reduce unwanted side effects that accompany opioid administration.