A refinement to the formalin test in mice

IUPHAR review: Navigating the role of preclinical models in pain research

Chronic pain is a complex and challenging medical condition that affects millions of people worldwide. Understanding the underlying mechanisms of chronic pain is a key goal of preclinical pain research so that more effective treatment strategies can be developed. In this review, we explore nociception, pain, and the multifaceted factors that lead to chronic pain by focusing on preclinical models. We provide a detailed look into inflammatory and neuropathic pain models and discuss the most used animal models for studying the mechanisms behind these conditions. Additionally, we emphasize the vital role of these preclinical models in developing new pain-relief drugs, focusing on biologics and the therapeutic potential of NMDA and cannabinoid receptor antagonists. We also discuss the challenges of TRPV1 modulation for pain treatment, the clinical failures of neurokinin (NK)- 1 receptor antagonists, and the partial success story of Ziconotide to provide valuable lessons for preclinical pain models. Finally, we highlight the overall success and limitations of current treatments for chronic pain while providing critical insights into the development of more effective therapies to alleviate the burden of chronic pain.

Endomorphin-2 analogs with C-terminal esterification display potent antinociceptive effects in the formalin pain test in mice

Previously, we have investigated three C-terminal esterified endomorphin-2 (EM-2) analogs EM-2-Me, EM-2-Et and EM-2-Bu with methyl, ethyl and tert-butyl ester modifications, respectively. These analogs produced significant antinociception in acute pain at the spinal and supraspinal levels, with reduced tolerance and gastrointestinal side effects. The present study was undertaken to determine the analgesic effects and opioid mechanisms of these three analogs in the formalin pain test. Our results demonstrated that intracerebroventricular (i.c.v.) administration of 0.67-20 nmol EM-2 analogs EM-2-Me, EM-2-Et and EM-2-Bu produced dose-dependent antinociceptive effects in both phase Ⅰ and phase Ⅱ of formalin pain. EM-2-Me and EM-2-Bu displayed more potent antinociception than morphine. Especially, EM-2-Bu exhibited the highest antinociception in phase Ⅱ of formalin pain, with the ED50 value being 2.1 nmol. Naloxone (80 nmol, i.c.v.) completely antagonized the antinociceptive effects of EM-2-Me, EM-2-Et and EM-2-Bu (20 nmol, i.c.v.) in both phase I and phase Ⅱ of formalin pain, suggesting a central opioid mechanism. Nevertheless, the antinociception induced by EM-2-Me might be involved in the release of dynorphin A, which subsequently acted on κ- opioid receptor. EM-2-Bu produced the antinociception probably by the direct activation of both μ- and δ-opioid receptors. EM-2-Me, EM-2-Et and EM-2-Bu also produced significant analgesic effects after peripheral administration, and the central opioid receptors were involved. Furthermore, EM-2-Bu had no influence on the locomotor activity after i.c.v. injection. The present investigation demonstrated that C-terminal esterified modifications of EM-2 will be beneficial for developing novel therapeutics in formalin pain.

The role of l -arginine/NO/cGMP/K ATP channel pathway in the local antinociceptive effect of berberine in the rat formalin test

Berberine is an isoquinoline alkaloid naturally produced by several types of plants. Berberine has extensive pharmacological effects, such as anti-diabetic, anti-inflammatory, and antioxidant effects. In the current study, we assess the antinociceptive effects of berberine and its association with the l -arginine ( l -Arg)/NO/cGMP/K ATP channel pathway via intraplantar administration in rats. To examine the antinociceptive properties of berberine, the formalin test was conducted. The number of rat paw flinches was counted for an h. l -Arg (precursor of nitric oxide, 3-30  μ g/paw), l -NAME (NO synthase inhibitor, 10 and 100  μ g/paw), methylene blue (guanylyl cyclase inhibitor, 100 and 200  μ g/paw), and glibenclamide (ATP-sensitive potassium channel blocker, 10 and 30  μ g/paw) were locally injected, respectively, into the right hind paws of rats as a pre-treatment before berberine injection to understand how the l -Arg/NO/cGMP/K ATP pathway plays a role in the antinociceptive effect of berberine. The ipsilateral injection of berberine into the right paw (0.1-10 0   μ g/paw) showed a dose-dependent antinociceptive effect in both the first and second phases of the formalin test, almost similar to morphine (25  μ g/paw). Intraplantar injection of l -Arg (30 µg/paw) increased the antinociceptive effect of berberine in the second phase. In addition, injection of l -NAME, methylene blue, and glibenclamide caused a reduction in the antinociceptive effect of berberine throughout the second phase in a dose-dependent manner. However, the antinociceptive effects of berberine in the first phase of the rat formalin test were not affected by this pathway. As a novel local antinociceptive agent, berberine can exert a peripheral antinociceptive effect via the l -Arg/NO/cGMP/K ATP channel pathway.

LQM10, a guanylhydrazone derivative, reduces nociceptive and inflammatory responses in mice

In the present study, we examined the antinociceptive and anti-inflammatory activities of a guanylhydrazone derivative, (E)-(3,5-di-tert-butyl-4-hydroxybenzylidene)-2-guanylhydrazone hydrochloride (LQM10), in mice. The antinociceptive effect was determined by assessing behavioural responses in different pain models, while anti-inflammatory activity was examined in carrageenan-induced pleurisy. Intraperitoneal LQM10 administration reduced the acetic acid-induced nociceptive behaviour, a phenomenon that was unaltered by pretreatment with yohimbine, atropine, naloxone or glibenclamide. In the formalin assay, LQM10 reduced nociceptive behaviour only in the second phase, indicating an inhibitory effect on inflammatory pain. LQM10 did not alter the pain latency in the hot plate assay and did not impact the locomotor activity of mice in the rotarod assay. In the carrageenan-induced pleurisy assay, LQM10 treatment inhibited critical events involved in inflammatory responses, namely, leucocyte recruitment, plasma leakage and increased inflammatory mediators (tumour necrosis factor Like Properties of Chalchones and Flavonoid Derivatives [TNF]-α and interleukin [IL]-1β) in the pleural exudate. Overall, these results indicate that LQM10 exhibits antinociceptive effects associated with peripheral mechanisms and anti-inflammatory activity mediated via a reduction in leucocyte migration and proinflammatory mediators, rendering this compound a promising candidate for treating pain and inflammatory process.

Segmental Upregulation of ASIC1 Channels in the Formalin Acute Pain Mouse Model

BACKGROUND

Hindpaw injection of formalin in rodents is used to assess acute persistent pain. The response to formalin is biphasic. The initial response (first minutes) is thought to be linked to inflammatory, peripheral mechanisms, while the latter (around 30 min after the injection), is linked to central mechanisms. This model is useful to analyze the effect of drugs at one or both phases, and the involvement of ion channels in the response. Acid-sensing ion channels (ASICs) regulate synaptic activities and play important roles in pain conditions. Recently, psalmotoxin-1 (Pctx-1), a toxin that inhibits ASIC1a-constituted channels, and antisense ASIC1a-RNA, intrathecal administered in mice were shown to affect both phases of the test.

METHODS

The mouse formalin test was performed on C57/BL6 7- to 9-week-old mice. Behavioral tests were conducted and tissue was extracted to detect proteins (ASIC1 and pERK) and ASIC1-mRNA and mir485-5p levels.

RESULTS

The injection of formalin was accompanied by an increase in ASIC1 levels. This was detected at the contralateral anterior cingulate cortex (ACC) compared to the ipsilateral side, and both sides of the ACC of vehicle-injected animals. At the spinal cord and dorsal root ganglia, ASIC1 levels followed a gradient stronger at lumbar (L) 3 and decreased towards L5. Gender differences were detected at the ACC; with female mice showing higher ASIC1a levels at the ACC. No significant changes in ASIC1-mRNA levels were detected. Evidence suggests ASIC1 upregulation depends on regulatory microRNAs.

CONCLUSION

This work highlights the important role of ASIC1 in pain and the potential role of pharmacological therapies aimed at this channel.

Mechanisms Underlining Inflammatory Pain Sensitivity in Mice Selected for High and Low Stress-Induced Analgesia-The Role of Endocannabinoids and Microglia

In this work we strived to determine whether endocannabinoid system activity could account for the differences in acute inflammatory pain sensitivity in mouse lines selected for high (HA) and low (LA) swim-stress-induced analgesia (SSIA). Mice received intraplantar injections of 5% formalin and the intensity of nocifensive behaviours was scored. To assess the contribution of the endocannabinoid system, mice were intraperitoneally (i.p.) injected with rimonabant (0.3–3 mg/kg) prior to formalin. Minocycline (45 and 100 mg/kg, i.p.) was administered to investigate microglial activation. The possible involvement of the endogenous opioid system was investigated with naloxone (1 mg/kg, i.p.). Cannabinoid receptor types 1 and 2 (Cnr1, Cnr2) and opioid receptor subtype (Oprm1, Oprd1, Oprk1) mRNA levels were quantified by qPCR in the structures of the central nociceptive circuit. Levels of anandamide (AEA) and 2-arachidonoylglycerol (2-AG) were measured by liquid chromatography coupled with the mass spectrometry method (LC-MS/MS). In the interphase, higher pain thresholds in the HA mice correlated with increased spinal anandamide and 2-AG release and higher Cnr1 transcription. Downregulation of Oprd1 and Oprm1 mRNA was noted in HA and LA mice, respectively, however no differences in naloxone sensitivity were observed in either line. As opposed to the LA mice, inflammatory pain sensitivity in the HA mice in the tonic phase was attributed to enhanced microglial activation, as evidenced by enhanced Aif1 and Il-1β mRNA levels. To conclude, Cnr1 inhibitory signaling is one mechanism responsible for decreased pain sensitivity in HA mice in the interphase, while increased microglial activation corresponds to decreased pain thresholds in the tonic inflammatory phase.

Next generation behavioral sequencing for advancing pain quantification

With symptoms such as spontaneous pain and pathologically heightened sensitivity to stimuli, chronic pain accounts for about 20% of physician visits and up to 2/3 of patients are dissatisfied with current treatments. Much of our knowledge on pain processing and analgesics has emerged from behavioral studies performed on animals presenting the same symptoms under pathological conditions. While humans can verbally describe their pain, studies on rodents have relied on behavioral assays providing non-exhaustive characterization or altering animals' original sensitivity through repetitive stimulations. The emergence of what we term "next-generation behavioral sequencing" is now permitting us to quantitatively describe behavioral features on millisecond to minutes long timescales that lie beyond easy detection with the unaided eye. Here, we summarize emerging videography and computational based behavioral approaches that have the potential to significantly improve pain research.

Identifying genetic determinants of inflammatory pain in mice using a large-scale gene-targeted screen

ABSTRACT

Identifying the genetic determinants of pain is a scientific imperative given the magnitude of the global health burden that pain causes. Here, we report a genetic screen for nociception, performed under the auspices of the International Mouse Phenotyping Consortium. A biased set of 110 single-gene knockout mouse strains was screened for 1 or more nociception and hypersensitivity assays, including chemical nociception (formalin) and mechanical and thermal nociception (von Frey filaments and Hargreaves tests, respectively), with or without an inflammatory agent (complete Freund's adjuvant). We identified 13 single-gene knockout strains with altered nocifensive behavior in 1 or more assays. All these novel mouse models are openly available to the scientific community to study gene function. Two of the 13 genes (Gria1 and Htr3a) have been previously reported with nociception-related phenotypes in genetically engineered mouse strains and represent useful benchmarking standards. One of the 13 genes (Cnrip1) is known from human studies to play a role in pain modulation and the knockout mouse reported herein can be used to explore this function further. The remaining 10 genes (Abhd13, Alg6, BC048562, Cgnl1, Cp, Mmp16, Oxa1l, Tecpr2, Trim14, and Trim2) reveal novel pathways involved in nociception and may provide new knowledge to better understand genetic mechanisms of inflammatory pain and to serve as models for therapeutic target validation and drug development.

Analgesic, anti-inflammatory and acute oral toxicity profile of leaf and bark extracts of Albizia procera

Background

Pain and inflammation are associatory events in cancer, diabetes, cardiovascular diseases, arthritis and other chronic diseases. Corticosteroids, non-steroidal anti-inflammatory drugs exert potential side effects on long term use. This study was aimed to investigate the acute oral toxicity, anti-inflammatory and analgesic activities of leaf and bark extracts of Albizia procera in experimental animal models.

Methods

Ethyl acetate, ethanol, and hydroalcoholic extracts of Albizia procera (leaf and bark) were subjected for acute oral toxicity, anti-inflammatory and analgesic screening. Carrageenan and cotton pellet granuloma models were used to assess acute and chronic anti-inflammatory effects, respectively. Intraplanar formalin test was used to assess the analgesic activity.

Results

All the extracts of Albizia procera were found to be well-tolerated up to 2000 mg/kg in female rats. Ethanolic leaf (ETLE) and bark (ETBE) of Albizia procera showed anti-inflammatory actions. But, only ETBE produced significant protection in chronic inflammation and analgesic activity.

Conclusion

In summary, Albizia procera possess significant anti-inflammatory and analgesic properties. This study adds evidence on the traditional use of Albizia procera plant for treating painful inflammatory disorders.

Antinociceptive and Anti-Inflammatory Effects of Recombinant Crotamine in Mouse Models of Pain

Crotamine, a toxin found in the venom of the South American rattlesnake Crotalus durissus terrificus, has been reported to have antinociceptive effects. We purified recombinant crotamine expressed in Escherichia coli and investigated its antinociceptive and anti-inflammatory effects using the hot-plate test, acetic-acid-induced writhing method, and formalin test in mice. Recombinant crotamine was administered intraperitoneally (0.04–1.2 mg kg⁻¹) or intraplantarly (0.9–7.5 μg 10 μL⁻¹) before the tests. The paw volume was measured with a plethysmometer. To evaluate the antagonistic and anti-inflammatory effects of naloxone, subcutaneous naloxone (4 mg kg⁻¹) or intraplantar naloxone (5 μg 10 μL⁻¹) was administered before recombinant crotamine. For tumor necrosis factor (TNF)-α assays, blood was drawn 3 h after formalin injection and measured using enzyme-linked immunosorbent assay. Intraperitoneal and intraplantar recombinant crotamine had antinociceptive and anti-inflammatory effects, neither of which were affected by pre-treatment with naloxone. The mean serum TNF-α levels were significantly lower in the intraperitoneal recombinant crotamine (0.4 and 1.2 mg kg⁻¹) or intraplantar (2.5 and 7.5 μg 10 μL⁻¹) recombinant crotamine groups than in the saline group and were not affected by naloxone pre-treatment. In conclusion, recombinant crotamine possesses significant antinociceptive and anti-inflammatory effects that do not appear to be related to the opioid receptor. The antinociceptive and anti-inflammatory effects of intraperitoneal or intraplantar recombinant crotamine are related to TNF-α.

Thiophene-Based Compounds with Potential Anti-Inflammatory Activity

Rheumatoid arthritis, arthrosis and gout, among other chronic inflammatory diseases are public health problems and represent major therapeutic challenges. Non-steroidal anti-inflammatory drugs (NSAIDs) are the most prescribed clinical treatments, despite their severe side effects and their exclusive action in improving symptoms, without effectively promoting the cure. However, recent advances in the fields of pharmacology, medicinal chemistry, and chemoinformatics have provided valuable information and opportunities for development of new anti-inflammatory drug candidates. For drug design and discovery, thiophene derivatives are privileged structures. Thiophene-based compounds, like the commercial drugs Tinoridine and Tiaprofenic acid, are known for their anti-inflammatory properties. The present review provides an update on the role of thiophene-based derivatives in inflammation. Studies on mechanisms of action, interactions with receptors (especially against cyclooxygenase (COX) and lipoxygenase (LOX)), and structure-activity relationships are also presented and discussed. The results demonstrate the importance of thiophene-based compounds as privileged structures for the design and discovery of novel anti-inflammatory agents. The studies reveal important structural characteristics. The presence of carboxylic acids, esters, amines, and amides, as well as methyl and methoxy groups, has been frequently described, and highlights the importance of these groups for anti-inflammatory activity and biological target recognition, especially for inhibition of COX and LOX enzymes.

Formalin Itch Test: Low-Dose Formalin Induces Histamine-Independent, TRPA1-Mediated Itch in Mice

Chronic itch is a common distressing symptom of many diseases, which reduced patient's quality of life. The mechanistic study on itch and screening for new anti-itch drugs require the development of new pre-clinical itch animal models. Herein, we established an acute itch model by intradermal (i.d.) injection of low-dose formalin into the neck or cheek in mice. In mice, i.d. injection of formalin (0.1–5%) in the nape of the neck evoked robust scratching behavior in a dose-dependent manner and the dose–response curves showed an inverted “U” shape. I.d. injection of formalin (0.3–0.6%) into the cheek evoked scratching in mice but wiping in rats, while formalin (1.25–5%) induced mixed wiping and scratching behavior in both mice and rats. Further, we found that 0.3% formalin-induced scratching was histamine-independent and significantly attenuated by transient receptor potential ion channel A1 (TRPA1) inhibitor (HC030031) or in TRPA1 knockout (KO) mice, but not affected by transient receptor potential ion channel V1 (TRPV1) inhibitor (capsazepine) or in TRPV1 KO mice. Additionally, 0.3% formalin-induced up-regulation of phosphorylation of extracellular regulated protein kinases (p-ERK) in the dorsal root ganglion (DRG) and scratching were suppressed by intrathecal injection of MEK inhibitor U0126 in mice. Incubation of 0.03% formalin induced the accumulation of intracellular reactive oxygen species (ROS) in the cultured DRG-derived cell line ND7-23, and formalin-induced itch was suppressed by antioxidants in mice. Finally, perfusion of 0.03% formalin induced elevation of intracellular calcium in a subset of primary cultured DRG neurons of mice. Thus, these results indicate that low-dose formalin induced non-histaminergic itch by activation of TRPA1 in mice, which may be employed as a useful acute itch model for screening potential anti-itch drugs.

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.