A new automated method of pain scoring in the formalin test in rats

Synchronized cluster firing, a distinct form of sensory neuron activation, drives spontaneous pain

Spontaneous pain refers to pain occurring without external stimuli. It is a primary complaint in chronic pain conditions and remains difficult to treat. Moreover, the mechanisms underlying spontaneous pain remain poorly understood. Here we employed in vivo imaging of dorsal root ganglion (DRG) neurons and discovered a distinct form of abnormal spontaneous activity following peripheral nerve injury: clusters of adjacent DRG neurons firing synchronously and sporadically. The level of cluster firing correlated directly with nerve injury-induced spontaneous pain behaviors. Furthermore, we demonstrated that cluster firing is triggered by activity of sympathetic nerves, which sprout into DRGs after injury, and identified norepinephrine as a key neurotransmitter mediating this unique firing. Chemogenetic and pharmacological manipulations of sympathetic activity and norepinephrine receptors suggest that they are necessary and sufficient for DRG cluster firing and spontaneous pain behavior. Therefore, blocking sympathetically mediated cluster firing may be a new paradigm for treating spontaneous pain.

Further exploration of the structure-activity relationship of dual soluble epoxide hydrolase/fatty acid amide hydrolase inhibitors

Fatty acid amide hydrolase (FAAH) is a membrane protein that hydrolyzes endocannabinoids, and its inhibition produces analgesic and anti-inflammatory effects. The soluble epoxide hydrolase (sEH) hydrolyzes epoxyeicosatrienoic acids (EETs) to dihydroxyeicosatetraenoic acids. EETs have anti-inflammatory and inflammation resolving properties, thus inhibition of sEH consequently reduces inflammation. Concurrent inhibition of both enzymes may represent a novel approach in the treatment of chronic pain. Drugs with multiple targets can provide a superior therapeutic effect and a decrease in side effects compared to ligands with single targets. Previously, microwave-assisted methodologies were employed to synthesize libraries of benzothiazole analogs from which high affinity dual inhibitors (e.g. 3, sEH IC₅₀ = 9.6 nM; FAAH IC₅₀ = 7 nM) were identified. Here, our structure-activity relationship studies revealed that the 4-phenylthiazole moiety is well tolerated by both enzymes, producing excellent inhibition potencies in the low nanomolar range (e.g. 6o, sEH IC₅₀ = 2.5 nM; FAAH IC₅₀ = 9.8 nM). Docking experiments show that the new class of dual inhibitors bind within the catalytic sites of both enzymes. Prediction of several pharmacokinetic/pharmacodynamic properties suggest that these new dual inhibitors are good candidates for further in vivo evaluation. Finally, dual inhibitor 3 was tested in the Formalin Test, a rat model of acute inflammatory pain. The data indicate that 3 produces antinociception against the inflammatory phase of the Formalin Test in vivo and is metabolically stable following intraperitoneal administration in male rats. Further, antinociception produced by 3 is comparable to that of ketoprofen, a traditional nonsteroidal anti-inflammatory drug. The results presented here will help toward the long-term goal of developing novel non-opioid therapeutics for pain management.

An Effective and Safe Enkephalin Analog for Antinociception

Opioids account for 69,000 overdose deaths per annum worldwide and cause serious side effects. Safer analgesics are urgently needed. The endogenous opioid peptide Leu-Enkephalin (Leu-ENK) is ineffective when introduced peripherally due to poor stability and limited membrane permeability. We developed a focused library of Leu-ENK analogs containing small hydrophobic modifications. N-pivaloyl analog KK-103 showed the highest binding affinity to the delta opioid receptor (68% relative to Leu-ENK) and an extended plasma half-life of 37 h. In the murine hot-plate model, subcutaneous KK-103 showed 10-fold improved anticonception (142%MPE·h) compared to Leu-ENK (14%MPE·h). In the formalin model, KK-103 reduced the licking and biting time to ~50% relative to the vehicle group. KK-103 was shown to act through the opioid receptors in the central nervous system. In contrast to morphine, KK-103 was longer-lasting and did not induce breathing depression, physical dependence, and tolerance, showing potential as a safe and effective analgesic.

Machine learning-based automated phenotyping of inflammatory nocifensive behavior in mice

The discovery and development of new and potentially nonaddictive pain therapeutics requires rapid, yet clinically relevant assays of nociception in preclinical models. A reliable and scalable automated scoring system for nocifensive behavior of mice in the formalin assay would dramatically lower the time and labor costs associated with experiments and reduce experimental variability. Here, we present a method that exploits machine learning techniques for video recordings that consists of three components: key point detection, per frame feature extraction using these key points, and classification of behavior using the GentleBoost algorithm. This approach to automation is flexible as different model classifiers or key points can be used with only small losses in accuracy. The adopted system identified the behavior of licking/biting of the hind paw with an accuracy that was comparable to a human observer (98% agreement) over 111 different short videos (total 284 min) at a resolution of 1 s. To test the system over longer experimental conditions, the responses of two inbred strains, C57BL/6NJ and C57BL/6J, were recorded over 90 min post formalin challenge. The automated system easily scored over 80 h of video and revealed strain differences in both response timing and amplitude. This machine learning scoring system provides the required accuracy, consistency, and ease of use that could make the formalin assay a feasible choice for large-scale genetic studies.

The Antinociceptive Activities of Certain NSAIDS Combinations in Murine Orofacial Test

Pain models are mostly in rodents and between them formalin orofacial test allow discrimination among antinociception and anti-inflammation. This assay use a formalin solution injected into the upper right lip of each mouse which produces two periods of pain separated by an inactive period. The aims of the present study were to evaluate, by means of the isobolographic analysis, the antinociception and anti-inflammatory activities of the following NSAIDs: dexketoprofen, diclofenac, piroxicam and metamizole in an orofacial. The NSAIDs administered intraperitoneally produced a dose-dependent activity with the following order of potency of the rubbing behavior, in phase I: diclofenac>dexketoprofen>piroxicam>metamizole and in the phase II: metamizole>diclofenac>piroxicam>dexketoprofen. The coadministration of NSAIDs resulted in a synergistic interaction, which according to the value of the potency of the combination (II) presents the following range: dexketoprofen plus metamizole>dexketoprofen plus diclofenac>dexketoprofen plus piroxicam, in phase I and dexketoprofen plus metamizole>dexketoprofen plus piroxicam>dexketoprofen plus diclofenac, on the phase II. Data obtained in this work corroborate that NSAIDs alone or in combination inducing activities by additional mechanism of action supplementary to inhibition of COXs. This fact represent a novel approach that could be used as multimodal management of orofacial pain, since with this treatment strategies, by the reduction of doses, can help to diminish side effects of other dugs such opioids.

Overview of Neurological Mechanism of Pain Profile Used for Animal "Pain-Like" Behavioral Study with Proposed Analgesic Pathways

Pain is the most common sensation installed in us naturally which plays a vital role in defending us against severe harm. This neurological mechanism pathway has been one of the most complex and comprehensive topics but there has never been an elaborate justification of the types of analgesics that used to reduce the pain sensation through which specific pathways. Of course, there have been some answers to curbing of pain which is a lifesaver in numerous situations-chronic and acute pain conditions alike. This has been explored by scientists using pain-like behavioral study methodologies in non-anesthetized animals since decades ago to characterize the analgesic profile such as centrally or peripherally acting drugs and allowing for the development of analgesics. However, widely the methodology is being practiced such as the tail flick/Hargreaves test and Von Frey/Randall-Selitto tests which are stimulus-evoked nociception studies, and there has rarely been a complete review of all these methodologies, their benefits and its downside coupled with the mechanism of the action that is involved. Thus, this review solely focused on the complete protocol that is being adapted in each behavioral study methods induced by different phlogogenic agents, the different assessment methods used for phasic, tonic and inflammatory pain studies and the proposed mechanism of action underlying each behavioral study methodology for analgesic drug profiling. It is our belief that this review could significantly provide a concise idea and improve our scientists' understanding towards pain management in future research.

Developing Improved Translational Models of Pain: A Role for the Behavioral Scientist

The effective management of pain is a longstanding public health concern. Although opioids have been frontline analgesics for decades, they also have well-known undesirable effects that limit their clinical utility, such as abuse liability and respiratory depression. The failure to develop better analgesics has, in some ways, contributed to the escalating opioid epidemic that has claimed tens of thousands of lives and has cost hundreds of billions of dollars in health-care expenses. A paradigm shift is needed in the pharmacotherapy of pain management that will require extensive efforts throughout biomedical science. The purpose of the present review is to highlight the critical role of the behavioral scientist to devise improved translational models of pain for drug development. Despite high heterogeneity of painful conditions that involve cortical-dependent pain processing, current models often feature an overreliance on simple reflex-based measures and an emphasis on the absence, rather than presence, of behavior as evidence of analgesic efficacy. Novel approaches should focus on the restoration of operant and other CNS-mediated behavior under painful conditions.

Assessing complex movement behaviors in rodent models of neurological disorders

Behavioral phenotyping is a crucial step in validating animal models of human disease. Most traditional behavioral analyses rely on investigator observation of animal subjects, which can be confounded by inter-observer variability, scoring consistency, and the ability to observe extremely rapid, small, or repetitive movements. Force-Plate Actimeter (FPA)-based assessments can quantify locomotor activity and detailed motor activity with an incredibly rich data stream that can reveal details of movement unobservable by the naked eye. This report describes four specific examples of FPA analysis of behavior that have been useful in specific rat or mouse models of human neurological disease, which show how FPA analysis can be used to capture and quantify specific features of the complex behavioral phenotypes of these animal models. The first example quantifies nociceptive behavior of the rat following injection of formalin into the footpad as a common model of persistent inflammatory pain. The second uses actimetry to quantify intense, rapid circling behaviors in a transgenic mouse that overexpresses human laminin α5, a basement membrane protein. The third example assesses place preference behaviors in a rat model of migraine headache modeling phonophobia and photophobia. In the fourth example, FPA analysis revealed a unique movement signature emerged with age in a digenic mutant mouse model of Tourette Syndrome. Taken together, these approaches demonstrate the power and usefulness of the FPA in the examination and quantification of minute details of motor behaviors, greatly expanding the scope and detail of behavioral phenotyping of preclinical models of human disease.

The major histocompatibility complex genes impact pain response in DA and DA.1U rats

Our recent studies have shown that the difference in basal pain sensitivity to mechanical and thermal stimulation between Dark-Agouti (DA) rats and a novel congenic DA.1U rats is major histocompatibility complex (MHC) genes dependent. In the present study, we further used DA and DA.1U rats to investigate the role of MHC genes in formalin-induced pain model by behavioral, electrophysiological and immunohistochemical methods. Behavioral results showed biphasic nociceptive behaviors increased significantly following the intraplantar injection of formalin in the hindpaw of DA and DA.1U rats. The main nociceptive behaviors were lifting and licking, especially in DA rats (P<0.001 and P<0.01). The composite pain scores (CPS) in DA rats were significantly higher than those in DA.1U rats in both phases of the formalin test (P<0.01). Electrophysiological results also showed the biphasic increase in discharge rates of C and Aδ fibers of L5 dorsal root in the two strains, and the net change of the discharge rate of DA rats was significantly higher than that of DA.1U rats (P<0.05). The mechanical thresholds decreased after formalin injection in both strains (P<0.01), and the net change in the mechanical threshold in DA was greater than that in DA.1U rats (P<0.05). The expression of RT1-B, representation of MHC class II molecule, in laminae I-II of L4/5 spinal cord in DA rats was significantly higher than that in DA.1U rats in the respective experimental group (P<0.05). These results suggested that both DA and DA.1U rats exhibited nociceptive responses in formalin-induced pain model and DA rats were more sensitive to noxious chemical stimulus than DA.1U rats, indicating that MHC genes might contribute to the difference in pain sensitivity.

Gene Silencing in Skin After Deposition of Self-Delivery siRNA With a Motorized Microneedle Array Device

Despite the development of potent siRNAs that effectively target genes responsible for skin disorders, translation to the clinic has been hampered by inefficient delivery through the stratum corneum barrier and into the live cells of the epidermis. Although hypodermic needles can be used to transport siRNA through the stratum corneum, this approach is limited by pain caused by the injection and the small volume of tissue that can be accessed by each injection. The use of microneedle arrays is a less painful method for siRNA delivery, but restricted payload capacity limits this approach to highly potent molecules. To address these challenges, a commercially available motorized microneedle array skin delivery device was evaluated. This device combines the positive elements of both hypodermic needles and microneedle array technologies with little or no pain to the patient. Application of fluorescently tagged self-delivery (sd)-siRNA to both human and murine skin resulted in distribution throughout the treated skin. In addition, efficient silencing (78% average reduction) of reporter gene expression was achieved in a transgenic fluorescent reporter mouse skin model. These results indicate that this device effectively delivers functional sd-siRNA with an efficiency that predicts successful clinical translation.

Increased anxiety-like behaviors in rats experiencing chronic inflammatory pain

For many patients, chronic pain is often accompanied, and sometimes amplified, by co-morbidities such as anxiety and depression. Although it represents important challenges, the establishment of appropriate preclinical behavioral models contributes to drug development for treating chronic inflammatory pain and associated psychopathologies. In this study, we investigated whether rats experiencing persistent inflammatory pain induced by intraplantar injection of complete Freund's adjuvant (CFA) developed anxiety-like behaviors, and whether clinically used analgesic and anxiolytic drugs were able to reverse CFA-induced anxiety-related phenotypes. These behaviors were evaluated over 28 days in both CFA- and saline-treated groups with a variety of behavioral tests. CFA-induced mechanical allodynia resulted in increased anxiety-like behaviors as evidenced by: (1) a significant decrease in percentage of time spent and number of entries in open arms of the elevated-plus maze (EPM), (2) a decrease in number of central squares visited in the open field (OF), and (3) a reduction in active social interactions in the social interaction test (SI). The number of entries in closed arms in the EPM and the distance traveled in the OF used as indicators of locomotor performance did not differ between treatments. Our results also reveal that in CFA-treated rats, acute administration of morphine (3mg/kg, s.c.) abolished tactile allodynia and anxiety-like behaviors, whereas acute administration of diazepam (1mg/kg, s.c) solely reversed anxiety-like behaviors. Therefore, pharmacological treatment of anxiety-like behaviors induced by chronic inflammatory pain can be objectively evaluated using multiple behavioral tests. Such a model could help identify/validate alternative potential targets that influence pain and cognitive dimensions of anxiety.

Subtype-selective GABAA receptor mimetics--novel antihyperalgesic agents?

Agonists at the benzodiazepine-binding site of ionotropic gamma-aminobutyric acid (GABA(A)) receptors are in clinical use as hypnotics, anxiolytics, and anticonvulsants since the early 1960. Analgesic effects of classical benzodiazepines have occasionally been reported in certain subgroups of patients suffering from chronic pain or after spinal delivery through intrathecal catheters. However, these drugs are generally not considered as analgesics but should in fact be avoided in patients with chronic pain. Recent evidence from genetically modified mice now indicates that agents targeting only a subset of benzodiazepine (GABA(A)) receptors should provide pronounced antihyperalgesic activity against inflammatory and neuropathic pain. Several such compounds have been developed recently, which exhibit significant antihyperalgesia in mice and rats and appear to be devoid of the typical side-effects of classical benzodiazepines.

Lectin extracted from Canavalia grandiflora seeds presents potential anti-inflammatory and analgesic effects

Neutrophil migration is responsible for tissue damage observed in inflammatory diseases and is also implicated in inflammatory nociception. The use of lectins has been demonstrated to be effective in different activities including anti-inflammatory, antimicrobial, and in cancer therapy. In this study, we addressed the potential use of a lectin from Canavalia grandiflora seeds (ConGF) to control neutrophil migration and inflammatory hypernociception. Pretreatment of the animals intravenously (15 min before) with ConGF inhibited neutrophil migration to the peritoneal cavity in a dose-dependent fashion confirmed by an inhibition of rolling and adhesion of leukocytes by intravital microscopy. Another set of experiments showed that pretreatment of the animals with ConGF inhibited the mechanical hypernociception in mice induced by the i.pl. injection of carrageenan or formalin. This anti-nociceptive effect correlated with an effective blockade of neutrophil influx, as assessed by the hind paw tissue myeloperoxidase levels. Furthermore, ConGF had important inhibitory effects on the mouse carrageenan-induced paw edema. In addition, animals treated with ConGF showed inhibition of cytokines release. In conclusion, we demonstrated that the lectin ConGF inhibits neutrophil migration and mechanical inflammatory hypernociception.

Models of nociception: hot-plate, tail-flick, and formalin tests in rodents

Experimental models of pain include tests of response thesholds to high intensity stimuli (acute pain tests) and changes in spontaneous or evoked behavioral responses in animals with peripheral injury or inflammation (persistent pain models). Acute thermal pain is modeled by the hot-plate and tail-flick test, while persistent pain can be modeled by the formalin test. This unit presents protocols for all three of these tests, including preparation of animals (rats or mice), administration of a compound being tested for its analgesic properties and data collection.

Nefopam and ketoprofen synergy in rodent models of antinociception

Combinations of analgesics with different mechanisms of action offer the possibility of efficient analgesia with a decrease in side effects as a result of reduced dosages of one or both compounds. Based on a clinical observation of synergism between nefopam, a centrally acting non-opioid that inhibits monoamines reuptake, and ketoprofen, a non-steroidal anti-inflammatory drug, the objective of this study was to further explore this antinociceptive synergy in four distinct animal models of pain (both drugs were administered subcutaneously). Strong antinociceptive properties were observed in the mouse writhing abdominal test with ED50 values of 2.56+/-0.38 and 1.41+/-0.41 mg/kg for nefopam and ketoprofen, respectively. In the inflammatory phase of the mouse formalin test, both compounds significantly inhibited the licking time of the injected hind-paw with ED50 of 4.32+/-0.17 mg/kg for nefopam and 49.56+/-15.81 mg/kg for ketoprofen. Isobolographic analysis revealed that this drug combination is synergistic in the formalin test and additive in the writhing test. In rat carrageenan-induced tactile allodynia, single administration of nefopam or ketoprofen only partially reduced allodynia. Combination of low analgesic doses of nefopam (10 or 30 mg/kg) with low analgesic doses of ketoprofen (30 or 100 mg/kg) significantly reduced or reversed allodynia, with a more pronounced anti-allodynic effect and a longer duration efficacy. In a rat model of postoperative thermal hyperalgesia induced by incision, co-administration of nefopam at a low analgesic dose (10 mg/kg) with ketoprofen at non-analgesic doses (30 or 100 mg/kg) showed the appearance of a strong anti-hyperalgesic effect, maintained during at least 3 h. In conclusion, co-administration of nefopam with ketoprofen is synergistic, and should allow either to increase their analgesic efficacy and/or to reduce their side effects.

Possible involvement of 5-lipoxygenase metabolite in itch-associated response of mosquito allergy in mice

This study investigated endogenous mediators involved in mosquito allergy-associated itching in mice. An intradermal injection of an extract of mosquito salivary gland elicited marked scratching in sensitized mice. The 5-lipoxygenase inhibitor zileuton (100 mg/kg), the 5-lipoxygenase activating peptide inhibitor MK-886 (10 mg/kg), and the glucocorticoid betamethasone 17-valerate (3 mg/kg) inhibited the scratching. The scratching was not affected by the cyclooxygenase inhibitors indomethacin and ketoprofen, the TP prostanoid receptor antagonist SQ-29548, the leukotriene B(4) antagonist ONO-4057, the cysteinyl leukotriene antagonist pranlucast, the leukotriene D(4) antagonist MK-571, the platelet-activating factor antagonist CV-3988, the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester, the H(2) histamine-receptor antagonist cimetidine, the H(1) histamine-receptor antagonist terfenadine plus cimetidine, and cypoheptadine that blocks the 5-HT(1/2) serotonin receptors. Zileuton (100 mg/kg) inhibited the increased activity of the cutaneous nerve branch induced by an intradermal injection of the extract, suggesting the peripheral action. Zileuton and MK-886 (10 and 100 microM) did not affect high K(+)-induced increase in intracellular Ca(2+) concentration in cultured dorsal root ganglion neurons. The results suggest that 5-lipoxygenase metabolite(s) other than leukotriene B(4) and cysteinyl leukotrienes are involved in mosquito allergy-associated itching.

A novel automated method for measuring the effect of analgesics on formalin-evoked licking behavior in rats

The behavioral assessment of pain is essential for the analysis of pain mechanisms and the evaluation of analgesic drugs. The formalin test is one of such methods widely used as a model of injury-induced pain in rodents. This test is manually demanding and the recording of results is left to the subjectivity of the experimenters. Thus we developed a novel automated method to estimate the pharmacological response in formalin-induced licking behavior in rats using a multicolor detection technique. Two color markers were preliminarily applied to rats-yellow dye on the mouth and fluorescent green tape on the right hind paw. Behaviors of the animals were recorded from both above and below the subject, by a dual-view digital video camera system. After injection with formalin into the hind paw, rats exhibited a biphasic display of licking behavior. Licking time was measured by the sum of frames where the distance between these markers was less than an appropriate threshold of distance (TD). The split-plot analysis of variance demonstrated that the sum of squares of differences in licking time between manual and automated measurement was minimized when TD = 20mm. In addition, frames in which moving velocity of these markers is less than 2.5mm/s was neglected for calculation in order to eliminate sedative effect on the recorded data. On these conditions, subcutaneous administration of morphine in rats dose-dependently decreased formalin-elicited nociceptive responses. These results suggest that under optimal conditions the automated technique when applied to pharmacological studies are more reliable and efficient than if they are manually recorded.

Lonchocarpus sericeus lectin decreases leukocyte migration and mechanical hypernociception by inhibiting cytokine and chemokines production

In this study, we tested the potential use of a lectin from Lonchocarpus sericeus seeds (LSL), to control neutrophil migration and inflammatory hypernociception (decrease of nociceptive threshold). Pretreatment of the animals intravenously (15 min before) with LSL inhibited neutrophil migration to the peritoneal cavity in a dose-dependent fashion confirmed by an inhibition of rolling and adhesion of leukocytes by intravital microscopy. We also tested the ability of the pretreatment with LSL to inhibit neutrophil migration on immunised mice, and it was observed that a strong inhibition of neutrophil migration induced by ovoalbumin in immunized mice. Another set of experiments showed that pretreatment of the animals with LSL, inhibited the mechanical hypernociception in mice induced by the i.pl. injection of OVA in immunized mice and of carrageenan in naïve mice, but not that induced by prostaglandin E(2) (PGE(2)) or formalin. This anti-nociceptive effect correlated with an effective blockade of neutrophil influx, as assessed by the hind paw tissue myeloperoxidase levels. In addition, we measured cytokines (TNF-alpha and IL-1beta) and chemokines (MIP-1alpha [CCL3] and KC [CXCL1]) from the peritoneal exudates and i.pl. tissue. Animals treated with LSL showed inhibition of cytokines and chemokines release in a dose-dependent manner. In conclusion, we demonstrated that the inhibitory effects of LSL on neutrophil migration and mechanical inflammatory hypernocicepetion are associated with the inhibition of the production of cytokines and chemokines.

Antinociceptive effect from Ipomoea cairica extract

Ipomoea cairica L. Sweet (Convolvulaceae) is used in Brazilian folk medicine for the treatment of rheumatism and inflammations. Ipomoea cairica ethanolic extract (100, 300, 1000 and 3000 mg/kg; per os) induced dose-dependent reduction of response in the formalin test inflammatory phase in mice. The same dose range did not modify neurogenic pain in formalin test, tail-flick reflex latency, carrageenan-induced paw edema, and Rota-Rod test motor performance. From the bio-active fraction 3,5-di-O-caffeoylquinic acid and 4,5-di-O-caffeoylquinic acid were obtained. These compounds have been previously reported to have analgesic and antioxidative effects. A possible explanation for the antinociception is that somehow the compounds present in the extract reduced the release of pro-nociceptive mediators unrelated to carrageenan-induced edema, such as histamine. Interestingly, caffeoylquinic acid derivatives have been reported to inhibit histamine release on in vitro models. The isolated caffeoylquinic acids could explain, at least in part, the antinociceptive effect of Ipomoea cairica polar extract.

Is endogenous d-serine in the rostral anterior cingulate cortex necessary for pain-related negative affect?

Functional activation of NMDA receptors requires co-activation of glutamate- and glycine-binding sites. D-serine is considered to be an endogenous ligand for the glycine site of NMDA receptors. Using a combination of a rat formalin-induced conditioned place avoidance (F-CPA) behavioral model and whole-cell patch-clamp recording in rostral anterior cingulate cortex (rACC) slices, we examined the effects of d-amino acid oxidase (DAAO), an endogenous D-serine-degrading enzyme, and 7-chlorokynurenate (7Cl-KYNA), an antagonist of the glycine site of NMDA receptors, on pain-related aversion. Degradation of endogenous D-serine with DAAO, or selective blockade of the glycine site of NMDA receptors by 7Cl-KYNA, effectively inhibited NMDA-evoked currents in rACC slices. Intra-rACC injection of DAAO (0.1 U) and 7Cl-KYNA (2 and 0.2 mM, 0.6 microL per side) 20 min before F-CPA conditioning greatly attenuated F-CPA scores, but did not affect formalin-induced acute nociceptive behaviors and electric foot shock-induced conditioned place avoidance. This study reveals for the first time that endogenous D-serine plays a critical role in pain-related aversion by activating the glycine site of NMDA receptors in the rACC. Furthermore, these results extend our hypothesis that activation of NMDA receptors in the rACC is necessary for the acquisition of specific pain-related negative emotion. Thus a new and promising strategy for the prevention of chronic pain-induced emotional disturbance might be raised.

Efficacy of antiepileptic isomers of valproic acid and valpromide in a rat model of neuropathic pain

Antiepileptic drugs (AEDs) are often utilized in the treatment of neuropathic pain. The major AED valproic acid (VPA) is of particular interest as it is thought to engage a variety of different neural mechanisms simultaneously. However, the clinical use of VPA is limited by two rare but life-threatening side effects: teratogenicity and hepatotoxicity. We synthesized VPA's corresponding amide: valpromide (VPD), two of VPAs isomers and their corresponding amides; valnoctic acid (VCA), valnoctamide (VCD), diisopropyl acetic acid (DIA), diisopropylacetamide (DID), and VPD's congener: N-methyl-VPD (MVPD). VCD, DID and VPD are nonteratogenic, potentially nonhepatotoxic, and exhibit better anticonvuslant potency than VPA. In this study, we assessed the antiallodynic activity of these compounds in comparison to VPA and gabapentin (GBP) using the rat spinal nerve ligation model of neuropathic pain (SNL, Chung model). VCA and MVPD were inactive. However, VPD (20-100 mg kg(- 1)), VCD (20-100 mg kg(- 1)) and DID (20-90 mg kg(- 1)) produced dose-related reversal of tactile allodynia with ED50 values of 61, 52 and 58 mgkg(- 1), respectively. All the amides were more potent than VPA (ED50=269 mgkg(- 1)). The antiallodynic effect of VPA, VPD, VCD and DID was obtained at plasma concentrations of 125, 24, 18 and 7 mg l(- 1), respectively, with a good pharmacokinetic-pharmacodynamic correlation and a minimal lag response. VCD and DID were found to have minimal motor and sedative side effects at analgesic doses, and were equipotent to GBP, currently the leading drug in neuropathic pain treatment. Consequently, VCD and DID have potential to become new drugs for the treatment of neuropathic pain.

Involvement of cholecystokininergic systems in anxiety-induced hyperalgesia in male rats: behavioral and biochemical studies

Keeping in mind the increased pain complaints reported in anxious or depressive patients, our goal was to investigate in rats the consequences of an experimentally provoked state of anxiety/depression on pain behavior and on its underlying mechanisms. We therefore used a model of social defeat consisting of a 30 min protected confrontation followed by a 15 min physical confrontation, repeated during 4 d, that elicited symptoms close to those observed in humans with anxiety or depression. Indeed, 5 d later, animals subjected to social-defeat confrontation were characterized by a decrease of sweet-water consumption and of body weight, and a hyperactivity of the hypothalamic-pituitary-adrenal axis, suggesting that the social-defeat procedure induced a prolonged state of anxiety. Rats subjected to the social-defeat procedure showed an enhanced nociceptive behavior to the subcutaneous administration of formalin, 5 d after the last confrontation session. Because chronic treatment with the established anxiolytic chlordiazepoxide (10 mg.kg(-1).d(-1)) prevented hyperalgesia, this strongly suggested that this experimental procedure might be a suitable animal model of "anxiety-induced hyperalgesia." Hyperalgesia associated with anxiety not only was related to a significant increase of CCKLM [cholecystokinin (CCK)-like material] in frontal cortex microdialysates but also was prevented by a CCK-B receptor antagonist [4-[[2-[[3-(1H-indol-3-yl)-2-methyl-1-oxo-2[[(tricyclo[3.3[12,17]dec-2-yloxy)-carbonyl]amino]-propyl]amino]-1-phenyethyl]amino]-4-oxo-[R-(R*, R*)]-butanoate N-methyl-D-glucamine (CI-988)] (2 mg/kg), strongly supporting the involvement of central CCKergic systems in these phenomena. Finally, combined treatments with CI-988 and morphine completely suppressed pain-related behavior, supporting the idea that the association of both compounds might represent a new therapeutic approach to reduce the increase of pain complaints highly prevalent among anxious or depressive patients.

Efficacy of antiepileptic tetramethylcyclopropyl analogues of valproic acid amides in a rat model of neuropathic pain

Antiepileptic drugs (AEDs) are widely utilized in the management of neuropathic pain. The AED valproic acid (VPA) holds out particular promise as it engages a variety of different anticonvulsant mechanisms simultaneously. However, the clinical use of VPA is limited by two rare but potentially life-threatening side effects: teratogenicity and hepatotoxicity. We have synthesized several tetramethylcyclopropyl analogues of VPA amides that are non-teratogenic, and are likely to be non-hepatotoxic, and that exhibit good antiepileptic efficacy. In the present study we have assessed the antiallodynic activity of these compounds in comparison to VPA and gabapentin (GBP) using the rat spinal nerve ligation (SNL) model of neuropathic pain. TMCA (2,2,3,3-tetramethylcyclopropanecarboxylic acid, 100-250 mg/kg), TMCD (2,2,3,3-tetramethylcyclopropanecarboxamide, 40-150 mg/kg), MTMCD (N-methyl-TMCD, 20-100 mg/kg), and TMCU (2,2,3,3-tetramethylcyclopropanecarbonylurea, 40-240 mg/kg) all showed dose-related reversal of tactile allodynia, with ED(50) values of 181, 85, 41, and 171 mg/kg i.p., respectively. All were more potent than VPA (ED(50)=269 mg/kg). An antiallodynic effect was obtained for TMCD, MTMCD and TMCU at plasma concentrations as low as 23, 6 and 22 mg/L, respectively. MTMCD was found to be non-toxic, non-sedative and equipotent to gabapentin, currently the leading AED in neuropathic pain treatment. Tetramethylcyclopropyl analogues of VPA amides have potential to become a new series of drugs for neuropathic pain treatment.

Spared nerve injury rats exhibit thermal hyperalgesia on an automated operant dynamic thermal escape task

Well-established methods are available to measure thermal and mechanical sensitivity in awake behaving rats. However, they require experimenter manipulations and tend to emphasize reflexive behaviors. Here we introduce a new behavioral test, with which we examine thermal sensitivity of rats with neuropathic injury. We contrast thermal hyperalgesia between spared nerve injury and chronic constriction injury rats. This device is a fully automated thermal sensitivity assessment tool designed to emphasize integrated learned responses to thermal painful and non-painful stimuli that are applied dynamically to a surface on which the animal is standing. It documents escape behavior in awake, unrestrained animals to innocuous and noxious heating of the floor where the animal is located. Animals learn to minimize pain by escaping to the opposite non-heated side; escape latency is recorded. On this device, thermal stimulus-response curves showed > 6°C leftward shift in both groups of neuropathic rats. In contrast, when these animals were tested on hotplate the stimulus-response shift was < 2°C. Spared nerve injury rats showed even less evidence for thermal hyperalgesia when thermal sensitivity was tested by measuring paw withdrawal to infrared heating, plantar test. The implications of test dependent magnitude of thermal hyperalgesia are discussed from the viewpoint of the tests used, as well as the animal models studied. It is argued that the dynamic thermal operant task reveals the relevance of the neuropathic injury associated pain-like behavior in relation to the whole organism.

Validation of a simple automated movement detection system for formalin test in rats

AIM

To investigate the validity and sensitivity of an automatic movement detection system developed by our laboratory for the formalin test in rats.

METHODS

The effects of systemic morphine and local anesthetic lidocaine on the nociceptive behaviors induced by formalin subcutaneously injected into the hindpaw were examined by using an automated movement detection system and manual measuring methods.

RESULTS

Formalin subcutaneously injected into the hindpaw produced typical biphasic nociceptive behaviors (agitation). The mean agitation event rate during a 60-min observation period increased linearly following increases in the formalin concentration (0.0%, 0.5%, 1.5%, 2.5%, and 5%, 50 microL). Systemic application of morphine of different doses (1, 2, and 5 mg/kg) 10-min prior to formalin injection depressed the agitation responses induced by formalin injection in a dose-dependent manner, and the antinociceptive effect induced by the largest dose (5 mg/kg) of morphine was significantly antagonized by systemic application of the opioid receptor antagonist naloxone (1.25 mg/kg). Local anesthetic lidocaine (20 mg/kg) injected into the ipsilateral ankle subskin 5-min prior to formalin completely blocked the agitation response to formalin injection. These results were comparable to those obtained from manual measure of the incidence of flinching or the duration time of licking/biting of the injected paw.

CONCLUSION

These data suggest that this automated movement detection system for formalin test is a simple, validated measure with good pharmacological sensitivity suitable for discovering novel analgesics or investigating central pain mechanisms.

Contributions of the anterior cingulate cortex and amygdala to pain- and fear-conditioned place avoidance in rats

The pain experience includes a sensory-discriminative and an affective-emotional component. The sensory component of pain has been extensively studied, while data about the negative affective component of pain are quite limited. The anterior cingulate cortex (ACC), and amygdala are thought to be key neural substrates underlying emotional responses. Using formalin-induced conditioned place avoidance (F-CPA) and electric foot-shock conditioned place avoidance (S-CPA) models, the present study observed the effects of bilateral excitotoxic (quinolinic acid 200 nmol/microl) lesions of the ACC and amygdala on pain and fear induced negative emotion, as well as on sensory component of pain. In the place-conditioning paradigm, both intraplantar (i.pl.) injection of formalin and electric foot-shock produced conditioned place avoidance. Excitotoxin-induced lesion of either the ACC or amygdala significantly reduced the magnitude of F-CPA. However, the decrease in the magnitude of S-CPA occurred only in the amygdala, but not ACC lesioned animals. Neither ACC nor amygdala lesion significantly changed formalin-induced acute nociceptive behaviors. These results suggest that the amygdala is involved in both pain- and fear-related negative emotion, and the ACC might play a critical role in the expression of pain-related negative emotion.

Antinociception by tricyclic antidepressants in the rat formalin test: differential effects on different behaviours following systemic and spinal administration

The present study (1) examined analgesic effects of systemically and spinally administered antidepressants (ADs) on phase 2 flinching and biting/licking behaviours in the rat formalin test, a model considered to be of greater relevance to clinical pain than acute threshold tests, and (2) determined whether motor or anti-inflammatory effects contributed to such actions. Systemic administration of amitriptyline (3-20 mg/kg) produced a dose-related enhancement of flinching behaviours, while at the same time suppressing biting/licking behaviours. Imipramine (except for 20 mg/kg), nortriptyline, desipramine and fluoxetine had no significant effect on flinching behaviours, while producing a dose-related suppression of biting/licking behaviours. When administered spinally, either by acute lumbar puncture or via chronically implanted intrathecal cannulas, amitriptyline similarly augmented flinching behaviours. When given by lumbar puncture, amitriptyline suppressed biting/licking behaviours, but when intrathecal cannulas were used, this behaviour was not expressed in the formalin group. Other ADs also suppressed biting/licking behaviours without affecting flinching when given by lumbar puncture. Effects on paw volume were determined at the end of behavioural testing. Systemic administration of all ADs produced a dose-related reduction in paw volume. Spinal administration of nortriptyline by lumbar puncture also reduced paw volume, but for other agents, the reduction was not significant. Motor effects were noted qualitatively throughout these experiments, and considered in relation to nociceptive behaviours. These results indicate (a) a marked dissociation between the effects of systemic ADs on flinching and biting/licking behaviours in the formalin test, (b) spinal efficacy of ADs that essentially reproduces effects seen with systemic administration when given by lumbar puncture, (c) a lack of causality between anti-inflammatory effects of ADs and their analgesic properties in the formalin test, and (d) a contribution of motor effects to analgesic actions at higher doses affecting biting/licking but not flinching behaviours.

An automated flinch detecting system for use in the formalin nociceptive bioassay

The biphasic display of paw-flinch behavior in the rat after injection of formalin into the dorsum of the hind paw is used for the screening of anti-hyperalgesic agents. Described and characterized here is a less labor-intensive system for counting flinch activity by detecting movement of a small metal band placed on the formalin-injected paw. A signal is generated as the band breaks the electromagnetic field of a loop antenna located under the rat and processed through an algorithm that determines flinch activity using 1) amplitude, 2) zero-voltage crossing, and 3) signal duration. Flinches are summed and stored over a selected collection interval throughout the assay for later analysis. Studies have validated the measures with respect to 1) system stability over time; 2) system-to-“practiced observer” correlation on flinch detection, r 2 = 0.94; 3) system variables including time of day, sex, age, and body weight; and 4) 50% effective dose values similar to those previously reported for intrathecal morphine and the NMDA antagonist MK-801.

Effects of rapid eye movement (REM) sleep deprivation on pain sensitivity in the rat

The relationship between pain and sleep seems to be reciprocal: if pain may interrupt or disturb sleep, poor sleep can also influence pain perception. However the influence of sleep disturbances on pain sensitivity remain poorly investigated. The aim of this study was to assess the effect of REM sleep deprivation on the reaction of rats subjected to different noxious stimuli. In each experiment 16 Wistar male rats were randomly assigned to two groups: controls (n=8), and REM sleep deprived rats (n=8). REM sleep deprivation was elicited using the 'inverted flower pot' technique. Four different experiments were performed to assess the sensitivity to mechanical (vocalization threshold in paw pressure), thermal (tail withdrawal latency in hot water immersion), electrical (envelope of 2nd peep in tail shock test) and chemical (analgesic behavior in formalin test) noxious stimuli. All experiments were performed over a 5-day period with baseline (day 1, day 2) in a dry environment and REM sleep deprivation (day 3, day 4 and day 5) in a wet environment. Under wet conditions, vocalization threshold in the paw pressure test (-20%, P=0.005), and tail withdrawal latency in the hot water immersion test (-21%, P=0.006) were significantly lower, and the envelope of 2nd peep in the tail electrical shock was significantly greater (+78%, P=0.009), in REM sleep deprived rats compared to controls. However, under wet conditions the mean duration of nociceptive behaviors in the formalin test did not differ between the two groups. In conclusion, REM sleep deprivation induces a significant increase in the behavioral responses to noxious mechanical, thermal and electrical stimuli in rats.

[Acute pain measurement in animals. Part 1]

OBJECTIVE

To describe tests of nociception which appear in the "pre-clinical" literature.

DATA SOURCES

References obtained by computerized bibliographic research (Medline) and the authors' personal data.

DATA SYNTHESIS

Ethical problems arising from the study of the pain in awake animals, problems arising from the choice of stimulus and stimulus parameters and the quantification of responses are presented. Pain in animals can be estimated only by examining their reactions, but at the same time, the existence of a reaction does not necessarily mean that there is a concomitant sensation. A description of the signs of pain in mammals is proposed. A noxious stimulus can be defined by its physical nature, its site of application and what has previously happened to the tissues at this site. Electrical stimulation short-circuits the process of transduction at free nerve endings and is not specific; however it has the advantage that it can be applied suddenly and briefly and thus results in synchronised signals in the relevant primary afferent fibres which can be differentiated into A delta and C fibres. Heat selectively stimulates thermoreceptors and nociceptors, but the low calorific power of conventional stimulators restricts their usefulness. Radiant sources have the disadvantage of emitting waves in the visible and the adjacent infrared spectra, for which the skin is a poor absorber and good reflector. Thermodes have the disadvantage of activating mechanoreceptors and thermoreceptors simultaneously; furthermore, their capacity for transferring heat depends on the quality of contact with skin and thus on the pressure with which they are applied. These problems can be overcome by using CO2 lasers but even today, the cost of these is a major disadvantage. Chemical stimuli differ from those mentioned above by the progressive onset of their effectiveness, their duration of action and the fact that they are of an inescapable nature. Experimental models employing chemical stimuli are undoubtedly the most similar to acute clinical pain. A wide spectrum of reactions are observed in nociceptive tests, but in almost every case they involve motor responses. After defining the ideal characteristics of a nociceptive test, tests based on the use of short duration and longer duration stimuli are presented. In tests of phasic pain, reactions are evoked by thermal (tail-flick test, hot-plate test), mechanical or electrical (flinch-jump test, vocalisation test) stimuli. Tests of tonic pain employ injections of algogenic agents intradermally (formalin test) or intraperitoneally (writhing test) or even the dilation of hollow organs. All these tests will be critically appraised in a subsequent paper [1].

CONCLUSION

The tail-flick and hot-plate tests are the most used, but there is an increasing recourse to the formalin test and tests involving foot withdrawal after mechanical stimulation.

Automated behavioural analysis in animal pain studies

The assessment of the effectiveness of analgesics is strongly based on observational data from behavioural tests. These tests are interesting and give a quantification of the effect of the drugs on the whole animal but their use is subject to several difficulties: (i) many results are difficult to analyse as they only correspond to the evaluation of a reflex response; (ii) the tests dealing with more integrated responses are also more difficult to use and closely depend on the experimenter's subjectivity. If automation is widely used in a lot of research fields, this is not the case in behavioural pharmacology. Yet, it can contribute to optimize the tests. The use of signal processing devices allows the automated (and thus objective) measurement of behavioural reactions to nociceptive stimulation (amplitude of a reflex, vocal emission intensity). Mechanical devices based on a computer-driven dynamic force detector allows the recording of some pain behaviours. Video image analysis allows the quantification of more complex behaviours (nociception-induced specific motor behaviours) as well as meaningful information during the same experimentation (exploratory behaviour, total motor activity, feeding behaviour). Moreover, these methods make it possible to obtain a more objective measurement, to reduce animal-experimenter interactions, to ease system use, and to improve effectiveness. The prospects to work in this field are multiple: continuation of the attempts at an automation of the behaviours specifically induced by chronic pain; development of real animal pain monitoring based on analysis of specific and non-specific behavioural modifications induced by pain. In this context, the automation of the behavioural analysis is likely to make possible real ethical progress thanks to an increase in the test's effectiveness and a real taking into account of animal's pain. Nevertheless, there are some limits due to characteristics of the behavioural expression of nociception and technological problems.

Supraspinal NMDA and non-NMDA receptors are differentially involved in the production of antinociception by morphine and beta-endorphin administered intracerebroventricularly in the formalin pain model

Our previous studies have demonstrated that supraspinal glutamate receptors are differentially involved in the antinociception induced by morphine and beta-endorphin given intracerebroventricularly (i.c.v.) in the tail-flick and hot-plate tests. The formalin pain test was used in the present study. Injection of mice with formalin solution (2%, 10 microl) into the hindpaw intraplantarly produced the first (0-5 min) and second (20-40 min) phases of formalin responses. The formalin responses in the both phases were attenuated dose-dependently by morphine (0.125-1 microg) or beta-endorphin (0.125-1 microg) administered i.c.v. 5 min before. The antinociceptive effect of morphine was slightly more potent in the second phase whereas the effect of beta-endorphin was more pronounced in the first phase. MK-801 (0.1-1 microg), a non-competitive NMDA receptor antagonist, and CNQX (0.05-0.5 microg), a non-NMDA antagonist, given i.c.v., produced antinociceptive effect in the both phases, but only in a partial manner. Both MK-801 (0.05 microg) and CNQX (0.01 microg), at the dose which had no intrinsic effect, reversed the antinociceptive effect of beta-endorphin (1 microg) observed during the second, but not the first, phase partially but significantly. However, the antinociceptive effect of morphine (1 microg) was not affected by the same dose of MK-801 or CNQX given i.c.v. Our results indicate that, at the supraspinal level, both NMDA and non-NMDA receptors are involved in the production of antinociception induced by supraspinally administered beta-endorphin, but not morphine, in the formalin pain model.

Scoring the mouse formalin test: validation study

The formalin test is a well-established model for assessing nociceptive processes and analgesic drug effects. Previous studies have provided statistical validation of optimal procedures for conducting and scoring the rat formalin test. In the mouse model, formalin concentration has been subjected to validation studies. The present research extended previous work by subjecting two additional parameters - the behaviors that should be scored and the optimal interval for second-phase formalin response - to empirical validation. Five behavioral (formalin-induced favoring, lifting, and biting/licking, rearing and locomotion) and two physiological measures (paw weight and paw thickness increases reflective of formalin-induced inflammation) were examined under four formalin concentrations (Exp. 1: 0.5, 1.0, 5.0, and 10.0%/25µl formalin) or under four morphine doses (Exp. 2: 0.0, 1.0, 5.0, and 10.0 mg/kg, s.c. with 5% formalin concentration). Multiple regression analyses defined the optimal second-phase formalin response in outbred, Swiss-Webster mice as 15-35 min post formalin injection, and revealed that the second-phase response is best characterized by the cumulative time spent biting/licking the injected paw. Finally, paw physiological measures provided convergent evidence of nociceptive and antinociceptive processes in the mouse formalin test. Copyright 1998European Federation of Chapters of the International Association for the Study of Pain.

Improving the efficiency of the formalin test

A time-sampling method that allows up to eight rats to be tested simultaneously in the formalin test is described and compared to the continuous rating method. Time sampling the behavioural response to formalin every 1 or 2 min produces scores that are essentially identical to continuous rating for both the formalin concentration effect relationship and the morphine dose effect relationship, with no loss of statistical power. The most important advantage of the method is that it allows data on other aspects of the rats' behaviour, such as behavioural state and the side effects of drugs to be scored during the formalin test. Formalin injection produces a dose-dependent decrease in locomotor and exploratory activity. The activity pattern of rats is normalized at morphine doses that produce about a 50% reduction in pain, while morphine doses high enough to completely suppress the pain response are accompanied by considerable sedation. The use of the jackknifing procedure to obtain unbiased estimates of the variability of parameters estimated from dose effect relationships is also described.

Pharmacological validation of an automated method of pain scoring in the formalin test in rats

In 1997, we described a new automated method of scoring the pain behaviors in the formalin test. The algic behavior was automatically measured with the help of a video-analysis system. The time during which the animal grooms, licks, or bites itself was used as the parameter of pain. In the present study, we tested various analgesics to realize a pharmacological validation of the system. The effect of opiate analgesic (morphine, i.v.), nonsteroidal anti-inflammatory drugs (paracetamol, i.v., piroxicam, i.v., indomethacin, i.v.), antidepressant drugs (clomipramine, desipramine, nortryptyline, and paroxetine, i.p.), and serotonin (i.t.) were analyzed. A dose of 1.25 mg/kg of morphine induced a decrease in the scores of phases 1 and 2. Naloxone (0.25 mg/kg) reversed the effect of morphine (2.5 mg/kg). A 20-mg/kg dose of indomethacin induced a decrease in the second phase, and paracetamol induced a decrease in both phases (analgesic doses were 400 mg/kg and 200 mg/kg for first and second phases, respectively). Piroxicam had no effect on the pain scores. Clomipramine, desipramine, and paroxetine at a dose of 5 mg/kg induced a significant decrease in the second phase. Nortriptyline had no effect on the pain scores. A dose of 75 microg of serotonin induced a decrease in both phases 1 and 2. This study demonstrated that this system shows a good pharmacological sensitivity, although it is lower than that of manual assessment.

Pharmacological characterization of capsaicin-induced body movement of neonatal rat

In neonatal rats, nociceptive responses induced by capsaicin were characterized pharmacologically. Capsaicin, injected subcutaneously (s.c.), induced body movement including scratching and struggling responses, and the responses were quantified by using a device composed of an audio speaker as a detector. The capsaicin-induced body movement was inhibited by a tachykinin NK1-receptor antagonist RP-67580 with an ID50 value of 3.5 mg/kg, s.c. Opioid analgesics, morphine, buprenorphine and pentazocine, also inhibited the body movement with ID50 values of 0.085, 0.0079 and 0.92 (mg/kg, s.c.), respectively. Non-steroidal anti-inflammatory drugs, indomethacin, ibuprofen and acetaminophen, did not exert any effect on the capsaicin-induced body movement. Neither the sedative diazepam nor the sedative chlorpromazine inhibited the body movement. It is concluded that the capsaicin-induced body movement in neonatal rats, which is considered to be nociceptive responses mediated by substance P, is sensitive to centrally acting analgesics with micro-opioid receptor agonist activity.