Mouse Model Of Pain Research Articles

Comparing Three Formulations of Buprenorphine in an Incisional Pain Model in Mice.

This study compared the therapeutic effects in mice of 3 different formulations of buprenorphine. These formulations were standard buprenorphine hydrochloride (Bup-HCL) and 2 different extended-release buprenorphine formulations (Bup-ER and Ethiqa-XR [Bup-XR]). Drugs were evaluated based on their ability to attenuate thermal hypersensitivity in a mouse plantar incisional pain model. We hypothesized that Bup-HCL would attenuate postoperative thermal hypersensitivity at 20 min after administration, and that Bup-ER and Bup-XR would attenuate thermal hypersensitivity at 40 min after administration. Male C57BL6/J mice were randomly assigned to 1 of 4 treatment groups: 1) saline, 5 mL/kg SC, once; 2) Bup-HCL, 0.1 mg/kg SC, once; 3) Bup-ER, 1 mg/kg, SC, once; and 4) Bup-XR, 3.25 mg/kg, SC, once. Thermal hypersensitivity was assessed on the day before surgery and again on the day of surgery at 20, 40, 60, 90, and 120 min after drug administration. Thermal hypersensitivity after surgery was not different among the Bup-HCL, Bup-ER and Bup-XR groups at any timepoint. In addition, all buprenorphine treatment groups showed significantly less thermal hypersensitivity after surgery than did the saline group. Subjective observations suggested that mice that received Bup-ER or Bup-XR became hyperactive after drug administration (83 and 75% of mice tested, respectively). Our results indicate that Bup-HCL, Bup-ER, or Bup-XR attenuate thermal hyper- sensitivity related to foot incision by 20 min after administration.

Tyrosine Kinase Receptor ErbB4 in Advillin-Positive Neurons Contributes to Inflammatory Pain Hypersensitivity in Mouse DRG.

Inflammatory pain is a common type of pathological pain. Although the dorsal root ganglion (DRG) is key to pathogenesis of inflammatory pain, the underlying specific molecular and cellular mechanisms remain unclear. In this study, we used mouse models of acute or chronic inflammatory pain, induced by formalin or complete Freund' s adjuvant (CFA), respectively, to explore whether tyrosine kinase receptor ErbB4 participates in the pathogenesis of inflammatory pain. Firstly, we found that both the expression of Neuregulin 1 (Nrg1) and phosphorylation of ErbB4 receptor were upregulated in DRG after inflammatory pain, implying the activation of ErbB4 in DRG. Using ErbB4-mutant mice, we found reduced pain sensitivity of mice when ErbB4 gene expression was largely ablated; furthermore, ErbB4 deletion decreased the inflammatory pain hypersensitivity of either formalin- or CFA-induced mouse models. Moreover, the pain sensitivity was reduced in mice with specific deletion of ErbB4 on advillin-positive neurons within DRG. Importantly, pain hypersensitivity also decreased in Advillin-Cre;ErbB4-/- cKO mice after formalin- or CFA-induced inflammatory pain. Finally, gene quantification differential expression analysis, using RNAseq technology in combination with GO and KEGG enrichment analysis, suggested that calcium signaling pathway possibly mediated the roles of ErbB4 on DRG sensory neurons in inflammatory pain models. Together, these results indicate that ErbB4 on advillin-positive sensory neurons enhances inflammatory pain sensitivity, providing new clues towards the pathogenic mechanisms of inflammatory pain.

Evaluation of Antinociceptive Activity of Ficus Religiosa Root Extract in Swiss Albino Mice

Background 
 Ficus religiosa, commonly known as peepal, is widely distributed in Indian subcontinent. It has been used as an antiepileptic, aphrodisiac, analgesic, anti-inflammatory and laxative in traditional medicine.
 Objective To explore the analgesic effect of aqueous root extract of Ficus religiosa using thermal and chemical models of pain in swiss albino mice.
 Method The aqueous aerial root extract of Ficus religiosa was prepared using soxhlet apparatus. The anti-nociceptive effect of the extract at doses of 50 and 100 mg/kg was evaluated using peripheral (acetic acid-induced abdominal writhing), spinal (tail flick) and supra-spinal (hot plate) behavioral models of pain. All data were presented as Mean ± SEM. Statistical differences between Ficus religiosa (50 and 100 mg/kg) and standard control groups were evaluated using Mann-Whitney U test.
 Result There was significant dose dependent increase in the mean reaction time compared to the vehicle control in hot plate and tail- flick test. In acetic acid induced writhing test, mice treated with Ficus religiosa (50 and 100 mg/kg) exhibited significant dose-dependent decrease in the mean number of writhes (57.45% and 79.20% respectively) compared to the vehicle control. The activity of Ficus religiosa extract at doses of 50 and 100 mg/kg was equipotent to Standard control (Morphine and Indomethacin) used in different test models.
 Conclusion The extract of Ficus religiosa possesses both central and peripheral analgesic activity thus validating the traditional use of this plant in the management of pain.

Cannabinoid receptor 1 positive allosteric modulator (GAT229) attenuates cisplatin-induced neuropathic pain in mice

Chemotherapy-induced peripheral neuropathy (CIPN) is one of chemotherapies’ most often documented side effects. Patients with CIPN experience spontaneous burning, numbness, tingling, and neuropathic pain in their feet and hands. Currently, there is no effective pharmacological treatment to prevent or treat CIPN. Activating the cannabinoid receptor type 1 (CB1) by orthosteric agonists has shown promising results in alleviating the pain and neuroinflammation associated with CIPN. However, the use of CB1 orthosteric agonists is linked to undesirable side effects. Unlike the CB1 orthosteric agonists, CB1 positive allosteric modulators (PAMs) don’t produce any psychoactive effects, tolerance, or dependence. Previous studies have shown that CB1 PAMs exhibit antinociceptive effects in inflammatory and neuropathic rodent models. This study aimed to investigate the potential benefits of the newly synthesized GAT229, a pure CB1 PAM, in alleviating neuropathic pain and slowing the progression of CIPN. GAT229 was evaluated in a cisplatin-induced (CIS) mouse model of peripheral neuropathic pain (3 mg/kg/d, 28 d, i.p.). GAT229 attenuated and slowed the progression of thermal hyperalgesia and mechanical allodynia induced by CIS, as evaluated by the hotplate test and von Frey filament test. GAT229 reduced the expression of proinflammatory cytokines in the dorsal root ganglia (DRG) neurons. Furthermore, GAT229 attenuated nerve injuries by normalizing the brain-derived neurotrophic factor and the nerve growth factor mRNA expression levels in the DRG neurons. The CB1 receptor antagonist/inverse agonist AM251 blocked GAT229-mediated beneficial effects. According to our data, we suggest that CB1 PAMs might be beneficial in alleviating neuropathic pain and slowing the progression of CIPN.

Water extract of Notopterygium incisum alleviates cold allodynia in neuropathic pain by regulation of TRPA1

Ethnopharmacological relevanceNeuropathic pain can be debilitating and drastically affects the quality of life of those patients suffering from this condition. The Chinese herb Notopterygium incisum Ting ex H.T. Chang has long been used to disperse “cold”. One under examined clinical feature of neuropathic pain is sensitivity to cold. Patients with neuropathic pain or arthritis usually describe a worsening of symptoms during the winter. Aims of this studyWe proposed to test the hypothesis that Notopterygium incisum has a positive effect on the cold sensitivity found in neuropathic pain. Materials and methodsIn this study, we established chronic constriction injury (CCI) and cisplatin induced neuropathic pain mice models. Behavioral experiments and physiological examination methods were employed to investigate the effect of water extract of Notopterygium incisum (WN) on cold pain. ResultsWe found WN reduced cold pain and allyl isothiocyanate (AITC, Transient Receptor Potential A1 (TRPA1 agonist)) induced pain. WN inhibited AITC induced calcium response in HEK 293 cells transfected with TRPA1 and dorsal root ganglion (DRG) neurons. Moreover, we found that oral administration of WN reduced cold allodynia and mechanical allodynia caused by (CCI) and cisplatin induced neuropathic pain. We also observed that oral administration of WN decreased responses to AITC in DRG neurons as well as expression of TRPA1 in the WN treated neuropathic pain model. ConclusionsThe present study provide evidence that Notopterygium incisum alleviates cold allodynia in CCI and cisplatin induced neuropathic pain mouse models. WN alleviated neuropathic pain induced cold allodynia via directly modulating TRPA1. Our findings identify WN as a promising candidate for treating neuropathic pain that highlights a new mechanism of Notopterygium incisum on ‘disperse cold’.

Cyclovirobuxine D, a cardiovascular drug from traditional Chinese medicine, alleviates inflammatory and neuropathic pain mainly via inhibition of voltage-gated Cav3.2 channels.

Cyclovirobuxine D (CVB-D), the main active constituent of traditional Chinese medicine Buxus microphylla, was developed as a safe and effective cardiovascular drug in China. B. microphylla has also been used to relieve various pain symptoms for centuries. In this study, we examined and uncovered strong and persistent analgesic effects of cyclovirobuxine D against several mouse models of pain, including carrageenan- and CFA-induced inflammatory pain and paclitaxel-mediated neuropathic hypersensitivity. Cyclovirobuxine D shows comparable analgesic effects by intraplantar or intraperitoneal administration. Cyclovirobuxine D potently inhibits voltage-gated Cav2.2 and Cav3.2 channels but has negligible effects on a diverse group of nociceptive ion channels distributed in primary afferent neurons, including Nav1.7, Nav1.8, TRPV1, TPRA1, TRPM8, ASIC3, P2X2 and P2X4. Moreover, inhibition of Cav3.2, rather than Cav2.2, plays a dominant role in attenuating the excitability of isolated dorsal root ganglion neurons and pain relieving effects of cyclovirobuxine D. Our work reveals that a currently in-use cardiovascular drug has strong analgesic effects mainly via blockade of Cav3.2 and provides a compelling rationale and foundation for conducting clinical studies to repurpose cyclovirobuxine D in pain management.

TIAM1-mediated synaptic plasticity underlies comorbid depression-like and ketamine antidepressant-like actions in chronic pain.

Chronic pain often leads to depression, increasing patient suffering and worsening prognosis. While hyperactivity of the anterior cingulate cortex (ACC) appears to be critically involved, the molecular mechanisms underlying comorbid depressive symptoms in chronic pain remain elusive. T cell lymphoma invasion and metastasis 1 (Tiam1) is a Rac1 guanine nucleotide exchange factor (GEF) that promotes dendrite, spine, and synapse development during brain development. Here, we show that Tiam1 orchestrates synaptic structural and functional plasticity in ACC neurons via actin cytoskeleton reorganization and synaptic N-methyl-d-aspartate receptor (NMDAR) stabilization. This Tiam1-coordinated synaptic plasticity underpins ACC hyperactivity and drives chronic pain-induced depressive-like behaviors. Notably, administration of low-dose ketamine, an NMDAR antagonist emerging as a promising treatment for chronic pain and depression, induces sustained antidepressant-like effects in mouse models of chronic pain by blocking Tiam1-mediated maladaptive synaptic plasticity in ACC neurons. Our results reveal Tiam1 as a critical factor in the pathophysiology of chronic pain-induced depressive-like behaviors and the sustained antidepressant-like effects of ketamine.

Diosgenin Exerts Analgesic Effects by Antagonizing the Selective Inhibition of Transient Receptor Potential Vanilloid 1 in a Mouse Model of Neuropathic Pain.

Diosgenin is a botanical steroidal saponin with immunomodulatory, anti-inflammatory, anti-oxidative, anti-thrombotic, anti-apoptotic, anti-depressant, and anti-nociceptive effects. However, the effects of diosgenin on anti-nociception are unclear. Transient receptor potential vanilloid 1 (TRPV1) plays an important role in nociception. Therefore, we investigated whether TRPV1 antagonism mediates the anti-nociceptive effects of diosgenin. In vivo mouse experiments were performed to examine nociception-related behavior, while in vitro experiments were performed to examine calcium currents in dorsal root ganglion (DRG) and Chinese hamster ovary (CHO) cells. The duration of capsaicin-induced licking (pain behavior) was significantly reduced following oral and intraplantar administration of diosgenin, approaching levels observed in mice treated with the TRPV1 antagonist N-(4-tertiarybutylphenyl)-4-(3-cholorphyridin-2-yl) tetrahydropyrazine-1(2H)-carbox-amide. Additionally, oral administration of diosgenin blocked capsaicin-induced thermal hyperalgesia. Further, diosgenin reduced capsaicin-induced Ca2+ currents in a dose-dependent manner in both DRG and CHO cells. Oral administration of diosgenin also improved thermal and mechanical hyperalgesia in the sciatic nerve constriction injury-induced chronic pain model by reducing the expression of TRPV1 and inflammatory cytokines in DRG cells. Collectively, our results suggest that diosgenin exerts analgesic effects via antagonism of TRPV1 and suppression of inflammation in the DRG in a mouse model of neuropathic pain.

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

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

Bifunctional Peptidomimetic G Protein-Biased Mu-Opioid Receptor Agonist and Neuropeptide FF Receptor Antagonist KGFF09 Shows Efficacy in Visceral Pain without Rewarding Effects after Subcutaneous Administration in Mice.

There is still an unmet clinical need to develop new pharmaceuticals for effective and safe pain management. Current pharmacotherapy offers unsatisfactory solutions due to serious side effects related to the chronic use of opioid drugs. Prescription opioids produce analgesia through activation of the mu-opioid receptor (MOR) and are major contributors to the current opioid crisis. Multifunctional ligands possessing activity at more than one receptor represent a prominent therapeutic approach for the treatment of pain with fewer adverse effects. We recently reported on the design of a bifunctional MOR agonist/neuropeptide FF receptor (NPFFR) antagonist peptididomimetic, KGFF09 (H-Dmt-DArg-Aba-βAla-Bpa-Phe-NH2), and its antinociceptive effects after subcutaneous (s.c.) administration in acute and persistent pain in mice with reduced propensity for unwanted side effects. In this study, we further investigated the antinociceptive properties of KGFF09 in a mouse model of visceral pain after s.c. administration and the potential for opioid-related liabilities of rewarding and sedation/locomotor dysfunction following chronic treatment. KGFF09 produced a significant dose-dependent inhibition of the writhing behavior in the acetic acid-induced writhing assay with increased potency when compared to morphine. We also demonstrated the absence of harmful effects caused by typical MOR agonists, i.e., rewarding effects (conditioned-place preference test) and sedation/locomotor impairment (open-field test), at a dose shown to be highly effective in inhibiting pain behavior. Consequently, KGFF09 displayed a favorable benefit/side effect ratio regarding these opioid-related side effects compared to conventional opioid analgesics, such as morphine, underlining the development of dual MOR agonists/NPFFR antagonists as improved treatments for various pain conditions.

Selective androgen receptor modulator microparticle formulation reverses muscle hyperalgesia in a mouse model of widespread muscle pain.

Chronic pain is a significant health problem associated with disability and reduced quality of life. Current management of chronic pain is inadequate with only modest effects of pharmacological interventions. Thus, there is a need for the generation of analgesics for treating chronic pain. Although preclinical and clinical studies demonstrate the analgesic effects of testosterone, clinical use of testosterone is limited by adverse androgenic effects. Selective androgen receptor modulators (SARMs) activate androgen receptors and overcome treatment limitations by minimizing androgenic side effects. Thus, we tested whether daily soluble SARMs or a SARM-loaded microparticle formulation alleviated muscle hyperalgesia in a mouse-model of widespread pain (male and female C57BL/6J mice). We tested whether the analgesic effects of the SARM-loaded microparticle formulation was mediated through androgen receptors by blocking androgen receptors with flutamide pellets. In vitro and in vivo release kinetics were determined for SARM-loaded microparticles. Safety and toxicity of SARM treatment was determined using serum cardiac and liver toxicity panels, heart histology, and conditioned place preference testing. Subcutaneous daily SARM administration, and 2 injections, 1 week apart, of SARM-loaded microparticles alleviated muscle hyperalgesia in both sexes and was prevented with flutamide treatment. Sustained release of SARM, from the microparticle formulation, was observed both in vitro and in vivo for 4 weeks. Selective androgen receptor modulator treatment produced no cardiac or liver toxicity and did not produce rewarding behaviors. These studies demonstrate that SARM-loaded microparticles, which release drug for a sustained period, alleviate muscle pain, are safe, and may serve as a potential therapeutic for chronic muscle pain.

Nerve injury-induced upregulation of apolipoprotein E in dorsal root ganglion participates in neuropathic pain in male mice

Apolipoprotein E (ApoE) is an apolipoprotein involved in lipid metabolism and is primarily responsible for lipid transport and cholesterol homeostasis in the central nervous system (CNS). The aim of this study is to explore the role of ApoE in the pathological development of neuropathic pain. First, we examined the location of ApoE in the dorsal root ganglion (DRG) and spinal cord in male mice using immunohistochemistry, and found that ApoE was predominantly expressed in DRG satellite glial cells (SGCs) and macrophages and spinal cord astrocytes. Using a spinal nerve ligation (SNL)-induced neuropathic pain mouse model, we found that nerve injury caused an increase in ApoE expression in the injured DRGs, but not in the spinal cord after SNL surgery. Furthermore, we observed reduced SNL-induced pain hypersensitivity in ApoE knockout mice compared to wild-type mice. Moreover, an antisense oligonucleotide (ASO) targeting the Apoe gene sequence, which was microinjected into the DRG or administered intrathecally, not only reduced ApoE expression in DRG but also attenuated SNL-induced pain hypersensitivity. Finally, we found that a tyrosine kinase receptor AXL, which was previously demonstrated to contribute to neuropathic pain, may mediate ApoE function under neuropathic pain condition. In conclusion, our data suggest that ApoE in DRG promote pain hypersensitivity via the DRG membrane receptor AXL in neurons under neuropathic pain conditions. This study revealed a novel mechanism between lipid homeostasis and neuropathic pain.

VTA-NAc glutaminergic projection involves in the regulation of pain and pain-related anxiety.

Besides the established role of dopamine neurons and projections in nociceptive stimuli, the involvement of ventral tegmental area (VTA) glutamatergic projections to nucleus accumbens (NAc) in pain remains unknown. In the present study, we aimed to examine the role of VTA glutamatergic projections to NAc in painful stimuli and its related behavioral changes. Unilateral chronic constrictive injury (CCI) of sciatic nerve or intraplantar hind paw injections (i.pl.) of complete Freund's adjuvant (CFA) were used to develop pathological pain models in wild-type and VGluT2-Cre mice. The involvement of VTA glutamatergic neurons with projections to NAc in CCI-induced pain model was noted by c-Fos labeling and firing rate recordings. Pain response and pain-related behavior changes to the artificial manipulation of the VTA glutamatergic projections to NAc were observed by Hargreaves tests, von Frey tests, open field tests, elevated maze tests, and sucrose preference tests. Glutamatergic neurons in VTA had efferent inputs to shell area of the NAc. The CCI pain model significantly increased neuronal activity and firing rate in VTA glutamate neurons with projections to NAc. The photoinhibition of these glutamatergic projections relieved CCI-induced neuropathic pain and CFA-induced acute and chronic inflammatory pain. Moreover, pathological neuropathic pain-induced anxiety and less sucrose preference were also relieved by inhibiting the VTA glutamatergic projections to NAc. Together, glutamatergic inputs from VTA to NAc contribute to chronic neuropathic and inflammatory pain and pain-related anxiety and depressive behaviors, providing a mechanism for developing novel therapeutic methods.

Cannabidiol and Beta-Caryophyllene in Combination: A Therapeutic Functional Interaction

Cannabis contains over 500 distinct compounds, which include cannabinoids, terpenoids, and flavonoids. However, very few of these compounds have been studied for their beneficial effects. There is an emerging concept that the constituents of the cannabis plant may work in concert to achieve better therapeutic benefits. This study is aimed at determining if the combination of a minor cannabinoid (cannabidiol, CBD) and a terpene (beta-caryophyllene, BCP) works in concert and if this has any therapeutic value. We used an inflammatory pain model (formalin) in mice to test for any functionality of CBD and BCP in combination. First, we determined the analgesic effect of CBD and BCP individually by establishing dose-response studies. Second, we tested the analgesic effect of fixed-ratio combinations and monitored any adverse effects. Finally, we determined the effect of this combination on inflammation. The combination of CBD and BCP produces a synergistic analgesic effect. This effect was without the cannabinoid receptor-1 side effects. The analgesic effect of CBD and BCP in combination involves an inflammatory mechanism. The combination of these two constituents of the cannabis plant, CBD and BCP, works in concert to produce a therapeutic effect with safety profiles through an inflammatory mechanism.

Involvement of Brn3a-positive spinal dorsal horn neurons in the transmission of visceral pain in inflammatory bowel disease model mice.

The spinal dorsal horn plays a crucial role in the transmission and processing of somatosensory information. Although spinal neural circuits that process several distinct types of somatic sensations have been studied extensively, those responsible for visceral pain transmission remain poorly understood. In the present study, we analyzed dextran sodium sulfate (DSS)-induced inflammatory bowel disease (IBD) mouse models to characterize the spinal dorsal horn neurons involved in visceral pain transmission. Immunostaining for c-fos, a marker of neuronal activity, demonstrated that numerous c-fos-positive cells were found bilaterally in the lumbosacral spinal dorsal horn, and their distribution was particularly abundant in the shallow dorsal horn. Characterization of these neurons by several molecular markers revealed that the percentage of the Pit1-Oct1-Unc86 domain (POU domain)-containing transcription factor Brn3a-positive neurons among the c-fos-positive neurons in the shallow dorsal horn was 30%-40% in DSS-treated mice, which was significantly higher than that in the somatic pain model mice. We further demonstrated by neuronal tracing that, within the shallow dorsal horn, Brn3a-positive neurons were more highly represented in spino-solitary projection neurons than in spino-parabrachial projection neurons. These results raise the possibility that Brn3a-positive spinal dorsal horn neurons make a large contribution to visceral pain transmission, part of which is mediated through the spino-solitary pathway.

Up-regulation of HCN2 channels in a thalamocortical circuit mediates allodynia in mice.

Chronic pain is a significant problem that afflicts individuals and society, and for which the current clinical treatment is inadequate. In addition, the neural circuit and molecular mechanisms subserving chronic pain remain largely uncharacterized. Herein we identified enhanced activity of a glutamatergic neuronal circuit that encompasses projections from the ventral posterolateral nucleus (VPLGlu) to the glutamatergic neurons of the hindlimb primary somatosensory cortex (S1HLGlu), driving allodynia in mouse models of chronic pain. Optogenetic inhibition of this VPLGlu→S1HLGlu circuit reversed allodynia, whereas the enhancement of its activity provoked hyperalgesia in control mice. In addition, we found that the expression and function of the HCN2 (hyperpolarization-activated cyclic nucleotide-gated channel 2) were increased in VPLGlu neurons under conditions of chronic pain. Using in vivo calcium imaging, we demonstrated that downregulation of HCN2 channels in the VPLGlu neurons abrogated the rise in S1HLGlu neuronal activity while alleviating allodynia in mice with chronic pain. With these data, we propose that dysfunction in HCN2 channels in the VPLGlu→S1HLGlu thalamocortical circuit and their upregulation occupy essential roles in the development of chronic pain.

Desulfovibrio confers resilience to the comorbidity of pain and anxiety in a mouse model of chronic inflammatory pain.

Patients with chronic pain frequently suffer from anxiety symptoms. It has been well established that gut microbiota is associated with the pathogenesis of pain and anxiety. However, it is unknown whether the gut microbiota, particularly the specific bacteria, play a role in the comorbidity of chronic pain and anxiety. Chronic inflammatory pain was induced in mice by a single injection of complete Freund's adjuvant (CFA). Mice were then separated into anxiety-susceptible and anxiety-resilient phenotypes by hierarchical clustering analysis of behaviors. Fecal samples were collected to perform 16S rRNA gene sequencing. Chronic diazepam intervention served as a therapeutic strategy and its effect on the composition of gut microbiota was also determined. α-Diversity and β-diversity both showed significant differences among the groups. A total of 12 gut bacteria were both altered after CFA injection and reversed by chronic diazepam treatment. More importantly, the pain hypersensitivity and anxiety-like behaviors were relieved by chronic diazepam treatment. Interestingly, we also found that Desulfovibrio was increased in anxiety-resilient group compared to control and anxiety-susceptible groups. Abnormal composition of gut microbiota plays an essential role in chronic pain as well as in anxiety. Besides, the increased level of Desulfovibrio in anxiety-resilient mice indicated its therapeutic effects on the comorbidity of pain and anxiety. Collectively, targeting gut microbiota, especially increasing the Desulfovibrio level, might be effective in the alleviation of chronic pain-anxiety comorbidity.

A neural circuit for the suppression of feeding under persistent pain.

In humans, persistent pain often leads to decreased appetite. However, the neural circuits underlying this behaviour remain unclear. Here, we show that a circuit arising from glutamatergic neurons in the anterior cingulate cortex (GluACC) projects to glutamatergic neurons in the lateral hypothalamic area (GluLHA) to blunt food intake in a mouse model of persistent pain. In turn, these GluLHA neurons project to pro-opiomelanocortin neurons in the hypothalamic arcuate nucleus (POMCArc), a well-known neuronal population involved in decreasing food intake. In vivo calcium imaging and multi-tetrode electrophysiological recordings reveal that the GluACC → GluLHA → Arc circuit is activated in mouse models of persistent pain and is accompanied by decreased feeding behaviour in both males and females. Inhibition of this circuit using chemogenetics can alleviate the feeding suppression symptoms. Our study indicates that the GluACC → GluLHA → Arc circuit is involved in driving the suppression of feeding under persistent pain through POMC neuronal activity. This previously unrecognized pathway could be explored as a potential target for pain-associated diseases.

Synthesis of 2-(2-(4-thioxo-3H-1,2-dithiole-5-yl) phenoxy)ethyl)isoindole-1,3-thione, a novel hydrogen sulfide-releasing phthalimide hybrid, and evaluation of its activity in models of inflammatory pain

Hydrogen sulfide (H2S) is a gaseous mediator that modulates several physiological and pathological processes. Phthalimide analogues, substances that have the phthalimide ring in the structure, belong to the group of thalidomide analogues. Both H2S donors and phthalimide analogues exhibit activities in models of inflammation and pain. As molecular hybridization is an important strategy aiming to develop drugs with a better pharmacological profile, in the present study we synthesized a novel H2S-releasing phthalimide hybrid, 2-(2-(4-thioxo-3H-1,2-dithiole-5-yl) phenoxy)ethyl)isoindole-1,3-thione (PTD-H2S), and evaluated its activity in models of inflammatory pain in mice. Per os (p.o.) administration of PTD-H2S (125 or 250 mg/kg) reduced mechanical allodynia induced by carrageenan and lipopolysaccharide. Intraperitoneal (i.p.) administration of PTD-H2S (25 mg/kg), but not equimolar doses of its precursors 5-(4-hydroxyphenyl)-3H-1,2-dithiole-3-thione (14.2 mg/kg) and 2-phthalimidethanol (12 mg/kg), reduced mechanical allodynia induced by lipopolysaccharide. The antiallodynic effect induced by PTD-H2S (25 mg/kg, i.p.) was more sustained than that induced by the H2S donor NaHS (8 mg/kg, i.p.). Previous administration of hydroxocobalamin (300 mg/kg, i.p.) or glibenclamide (40 mg/kg, p.o.) attenuated PTD-H2S antiallodynic activity. In conclusion, we synthesized a novel H2S-releasing phthalimide hybrid and demonstrated its activity in models of inflammatory pain. PTD-H2S activity may be due to H2S release and activation of ATP-sensitive potassium channels. The demonstration of PTD-H2S activity in models of pain stimulates further studies aiming to evaluate H2S-releasing phthalimide hybrids as candidates for analgesic drugs.

Sural hypersensitivity after nerve transection depends on anatomical differences in the distal tibial nerve of mice and rats

IntroductionVarious mouse and rat models of neuropathic pain after nerve injury exist. Whilst some models involve a proximal nerve lesion or ligation of the sciatic trifurcation in mice and rats, others consists of a transection or ligation of distal nerves at the tibial bifurcation in mice or rats. The level of nerve cut directly affects the magnitude of hypersensitivity, and anatomical differences between mice and rats might therefore impact the development of hypersensitivity after distal tibial nerve transection as well. MethodsThe bifurcation of the distal tibial nerve into the medial and lateral plantar nerve (MPN and LPN), and the presence of anatomical differences in sural and tibial nerve distribution between mice and rat was evaluated. Sural mechanical sensitivity after transection of the MPN or whole tibial nerve was assessed using von Frey test until 8 weeks after surgery in 48 rats and 16 mice. ResultsThe bifurcation of the tibial nerve into the MPN and LPN is situated proximal to the ankle in both mice and rats. The sural nerve joins the LPN in mice, but not in rats. A proximal communicating branch is present between the LPN and MPN in rats, but not in mice. MPN transection in mice caused hypersensitivity of the hindpaw innervated by the sural nerve, but not in rats. In rats, sural hypersensitivity only developed when both MPN and LPN were cut. ConclusionInter-species variation in nerve anatomy should be taken in consideration when performing surgery to induce plantar hypersensitivity in rodents.