Persistent or chronic pain is the primary reason people seek medical care, yet current therapies are either limited in efficacy or cause intolerable side effects. Diverse mechanisms contribute to the basic phenomena of nociceptor hyperexcitability that initiates and maintains pain. Two prominent players in the modulation of nociceptor hyperexcitability are the transient receptor potential vanilloid type 1 (TRPV1) ligand-gated ion channel and the voltage-gated potassium channel, Kv7.2/3, that reciprocally regulate neuronal excitability. Across many drug development programs targeting either TRPV1 or Kv7.2/3, significant evidence has been accumulated to support these as highly relevant targets; however, side effects that are poorly separated from efficacy have limited the successful clinical translation of numerous Kv7.2/3 and TRPV1 drug development programs. We report here the pharmacological profile of 3 structurally related small molecule analogues that demonstrate a novel mechanism of action (MOA) of dual modulation of Kv7.2/3 and TRPV1. Specifically, these compounds simultaneously activate Kv7.2/3 and enable unexpected specific and potent inhibition of TRPV1. This in vitro potency translated to significant analgesia in vivo in several animal models of acute and chronic pain. Importantly, this specific MOA is not associated with any previously described Kv7.2/3 or TRPV1 class-specific side effects. We suggest that the therapeutic potential of this MOA is derived from the selective and specific targeting of a subpopulation of nociceptors found in rodents and humans. This efficacy and safety profile supports the advancement of dual TRPV1-Kv7.2/3 modulating compounds into preclinical and clinical development for the treatment of chronic pain.

Pain is one of the major public health burdens worldwide, however, conventional analgesics are often ineffective. Capsaicin-the active compound of Capsicum species, being responsible for their pungency-has been part of traditional medicine long ago. Capsaicin is a natural agonist of the Transient Receptor Potential Vanilloid 1 receptor-localized on capsaicin-sensitive sensory neurons and strongly involved in pain transmission-, and has been in focus of analgesic drug research for many years. In this study, we aimed to develop a sustained release transdermal patch (transdermal therapeutic system, TTS) combining the advantages of low-concentration capsaicin and diclofenac embedded in an innovative structure, as well as to perform complex preclinical investigations of its analgesic effect.

Drug delivery properties of the TTS were investigated with Franz cell and flow-through cell tests. Analgesic effect of the TTS was examined in in vivo models of acute postoperative and inflammatory, chronic neuropathic and osteoarthritic pain.

Modified silicone polymer matrix-based TTS containing low-concentration capsaicin and diclofenac has been developed, releasing both compounds according to zero-order kinetics. Moreover, capsaicin and diclofenac facilitated the liberation of each other. Combined TTS significantly reduced acute postoperative and inflammatory pain, as well as chronic neuropathic and osteoarthritic pain. Interestingly, in acute postoperative and chronic osteoarthritic pain, capsaicin prolonged and potentiated the pain-relieving effect of diclofenac.

New generation combined low-concentration capsaicin-diclofenac containing TTS can be an effective therapeutic tool in acute and chronic pain states involving neuropathic and inflammatory components.

In the clinical settings, patients often develop opioid-induced hyperalgesia (OIH) after utilization of high dose intra-operative remifentanil. It is widely considered that systemic α2 agonists, including dexmedetomidine (DEX), have the potential to mitigate postoperative pain and minimize the needs for opioid, thus leading to a decrease in the incidence of hyperalgesia. However, the regulating method remains ambiguous. Recent studies have shown that DEX can alleviate spinal nerve injury via regulating P2 X 4. Although the effects of DEX on remifentanil-induced hyperalgesia (RIH) have been previously reported, the specific mechanisms remain to be fully elucidated. The objective of our study was to investigate the potential of intraperitoneal injections of DEX in attenuate RIH in rats through the modulation of P2 X 4Rs and brain-derived neurotrophic factor (BDNF) in spinal cord. The findings of this study indicate that intraperitoneal administration of DEX at a dosage of 50 µg/kg could alleviate mechanical allodynia and thermal hyperalgesia, as demonstrated through a behavioral test. Moreover, DEX suppressed the enhancement of P2 X 4 and BDNF expression induced by RIH. Furthermore, the structure of synaptic clefts caused by RIH showed improvement to a certain extent after DEX treatment, as shown using TEM transmission electron microscopy. In summary, we examined the protective effect of DEX on remifentanil-induced hyperalgesia. The findings indicates that the reduced expression of P2 X 4 and decreased synthesis and release of BDNF may be responsible for the analgesic processes. This study would provide a new perspective and strategy for the pharmacological treatment on RIH.

Background Postoperative sleep disturbances and pain are common, negatively impacting recovery and quality of life. While various preventive strategies exist, the role of preoperative exercise in mitigating these effects remains underexplored. Objective This study evaluates the efficacy of preoperative exercise as a prehabilitation strategy to reduce postoperative sleep disturbances and pain in a rat model. Methods Male Wistar rats were divided into three groups: postoperative pain (PO) without preoperative exercise (N-group), PO with preoperative exercise (P-group), and a sham-operated control (S-group). Sleep patterns, including sleep duration and quality, were analyzed using EEG over a 72-hour period, starting at 8:00 a.m. on the first day of the experiment. Additionally, pain thresholds were assessed using the von Frey and Hargreaves tests. Results Compared to the N-group, the P-group exhibited reduced wake time and increased non-rapid eye movement (NREM) sleep duration. Additionally, the N-group showed increased wake time and decreased NREM sleep duration compared to the S-group, whereas no significant differences were observed between the P- and S-groups. The thermal allodynia test indicated a higher pain threshold in the P-group than in the N-group, although both remained lower than the S-group. Conclusions Our study demonstrates the efficacy of preoperative exercise as a nonpharmacological intervention for reducing postoperative sleep disturbances and alleviating pain. These findings highlight the potential benefits of prehabilitation for patients undergoing surgery.

The perinatal period encompasses a critical window for neurodevelopment that renders the brain highly responsive to experience. Trauma, such as intimate partner violence (IPV) and early life stress/neglect, during this period negatively affects physical and mental health outcomes, including increasing ones risk for chronic pain. Although epigenetic programming likely contributes, the mechanisms that drive the relationship between perinatal trauma and adverse health outcomes, are not fully understood. Therefore, we explored the relationship between perinatal trauma (in utero exposure to IPV and/or early life neglect) and socio-emotional functioning, nociceptive sensitivity, and transcriptomic changes within the prefrontal cortex (PFC) and hypothalamus in dams and their adolescent offspring. Rat dams were randomly assigned to an IPV (i.e., combined mild traumatic brain injury and strangulation) or sham procedure during pregnancy. Following birth, offspring were subsequently assigned the early life neglect or control paradigm. In adolescence, offspring received a plantar incision or sham injury. Perinatal trauma altered nociception and emotional functioning in a sex-dependent manner when combined with the surgical procedure. We identified transcriptomic changes related to DNA transcription and expression within the PFC and hypothalamus of the dams. Examination of the offspring transcriptome highlighted impairment in immune regulation, dysfunction in stress-reactivity, as well as microglia activation. We also identified altered expression of genes associated with chronic pain. This demonstrates that perinatal trauma modifies offspring behaviour, including nociceptive sensitivity. We provide insight into the mechanisms that contribute to the chronification of pain, thereby informing future research targeted at the generation of prevention and therapeutic strategies. PERSPECTIVE: Perinatal trauma impaired cognitive, socio-emotional, and pain processing in offspring, while also inducing changes in gene expression, in both mothers and offspring. The findings highlight possible mechanisms responsible for intergenerational transmission of risk for chronic pain and provide targets for therapeutics which could potentially reverse perinatal-trauma induced epigenetic change.

Previously, abobotulinumtoxinA (aboBoNT-A) injected intraoperatively resulted in effective, but delayed post-surgical analgesia in pigs. Here, we explore the efficacy of preemptively administered aboBoNT-A in intact animals on pain and associated behaviors following a full-skin-muscle incision and retraction surgery on the lower back. AboBoNT-A (200 U/animal) or saline, distributed across ten points, were injected around anticipated incision 15, 5, or 1 day before surgery via ID route (part A) or 15 days before surgery via ID, intramuscular (IM) or subcutaneous (SC) routes (part B). We assessed mechanical sensitivity (withdrawal force; WF), distress behavior score (DBS), and latency to approach the investigator before and after surgery for 7 days.AboBoNT-A, injected ID 15 days before surgery, didn't alter any baseline behaviors, but resulted in 5-fold increases in WF, 75% reduction in DBS and 70% reduction in approach latencies (all p < 0.01). Injections 5 days before surgery led to similar effects, albeit with a fewer animals reaching thresholds, while those made 1 day before surgery were less effective. SC and IM injections were ineffective. Thus, aboBoNT-A administered ID 15 days before surgery represents the most optimal condition for postoperative analgesia. These findings warrant for clinical investigation of preemptively administered aboBoNT-A in postsurgical pain.

Deeper understanding of the mechanisms of postoperative pain is critical for developing more effective pain management strategies. The present animal study explored the function of four p38 mitogen-activated protein kinase (MAPK) subclasses (α, β, γ, and δ) in dorsal root ganglion (DRG) in the development of post-incisional pain hypersensitivity.

The amount of p38 MAPK subclass mRNA in the DRG of male Sprague-Dawley rats was quantified using real-time PCR. Localization of p38 MAPK expression was analyzed by immunohistochemistry using subclass-selective antibodies. The effects of a p38α MAPK inhibitor on plantar incision-induced pain hypersensitivity was assessed using behavioral tests to measure mechanical and thermal sensitivity. The impact of the inhibitor on phosphorylated p38 MAPK expression was also analyzed by immunohistochemistry.

Four p38 MAPK subclass mRNA were identified in the DRG, with p38α, β, and γ MAPK showing significant expression. p38α and γ MAPK were identified in the DRG neurons, whereas p38β MAPK was distributed in satellite glial cells. Selective inhibition of p38α MAPK reduced both mechanical and thermal hypersensitivity following plantar incision. Treatment with the p38α MAPK inhibitor decreased the expression of phosphorylated p38 MAPK in the DRG.

These results demonstrated the distinct roles of p38 MAPK subclasses in the DRG, with p38α MAPK playing a dominant role in the development of pain hypersensitivity after tissue injury. Targeting p38α MAPK might be a promising therapeutic strategy for managing postoperative pain.

Persistent post-surgical pain (PPSP) occurs in a proportion of patients following surgical interventions. Research suggests that specific microbiome components are important for brain development and function, with recent studies demonstrating that chronic pain results in changes to the microbiome. Consumption of a high fat, high sugar (HFHS) diet can drastically alter composition of the microbiome and is a modifiable risk factor for many neuroinflammatory conditions. Therefore, we investigated how daily consumption of a HFHS diet modified the development of PPSP, brain structure and function, and the microbiome. In addition, we identified significant correlations between the microbiome and brain in animals with PPSP. Male and female rats were maintained on a control or HFHS diet. Animals were further allocated to a sham or surgery on postnatal day (p) p35. The von Frey task measured mechanical nociceptive sensitivity at a chronic timepoint (p65-67). Between p68-72 rats underwent in-vivo MRI to examine brain volume and diffusivity. At p73 fecal samples were used for downstream 16 s rRNA sequencing. Spearman correlation analyses were performed between individual microbial abundance and MRI diffusivity to determine if specific bacterial species were associated with PPSP-induced brain changes. We found that consumption of a HFHS diet exacerbated PPSP in adolescents. The HFHS diet reduced overall brain volume and increased white and grey matter density. The HFHS diet interacted with the surgical intervention to modify diffusivity in numerous brain regions which were associated with specific changes to the microbiome. These findings demonstrate that premorbid characteristics can influence the development of PPSP and advance our understanding of the contribution that the microbiome has on function of the brain-microbiota-gut axis.

Pain resulting from tissue damage, including surgical incision, is often only partially responsive to standard treatments focusing on inflammation, suggesting additional mechanisms are involved. Tissue damage leads to expression in dorsal root ganglion (DRG) sensory neurons of genes associated with axonal injury and regeneration, most notably activating transcription factor 3 (ATF3) and GAP-43. ATF3 expression is associated with sensitization of cellular physiology and enhanced amplitude/duration of a nociceptive reflex. It is unclear how tissue damage leads to these changes in the sensory neurons, but it could include direct damage to the tissue-innervating axons and inflammation-associated retrograde biochemical signalling. Using the CTM reflex to map innervation fields, we examined the necessity and sufficiency of incision, inflammation, and axonal conduction for induction of ATF3 in response to skin incision. Incision outside the innervation field, but close enough to induce inflammation inside the innervation field, was not sufficient to induce ATF3 expression in the field-innervating DRG. Incision inside the innervation field led to strong expression of ATF3. Anti-inflammatory treatments did not prevent this induction of ATF3. In rodent models of repeated injury - a major etiological factor for chronic pain - ATF3 expression was synergistically-increased and the threshold for paw-withdrawal to mechanical stimulation was significantly decreased for an extended duration. Together, these results suggest that actual damage to axons innervating the skin is both necessary and sufficient for induction of ATF3, expression of which appears additionally increased by repeated injury. Further, pre-treatment of the nerves innervating the incised skin with bupivacaine, a local anesthetic commonly used to reduce surgical pain, did not prevent induction of ATF3, indicating that conduction of action potentials is not necessary for induction of ATF3. We also determined that closure of incision with surgical glue significantly reduced incision-induced expression of GAP-43. Intriguingly, treatment with polyethylene glycol (PEG), known to enhance membrane integrity after injury among other effects, reduced incision-associated ATF3 expression and electrophysiological changes. These results suggest that pain resulting from tissue damage may arise from a mix of ATF3-/axonal-damage-associated mechanism as well as ATF3-independent inflammation-related mechanisms and therefore require a mix of approaches to achieve more complete control.

Chronic pain after surgery, also known as chronic postsurgical pain (CPSP), is recognized as a significant public health issue with serious medical and economic consequences. Current research on CPSP underscores the significant roles of both peripheral and central sensitization in pain development and maintenance. Peripheral sensitization occurs at the site of injury, through the hyperexcitability of nerve fibers due to surgical damage and the release of inflammatory mediators. This leads to increased expression of pronociceptive ion channels and receptors, such as transient receptor potential and acid-sensing ion channels (ASIC), enhancing pain signal transmission. Central sensitization involves long-term changes in the central nervous system, particularly in the spinal cord. In this context, sensitized spinal neurons become more responsive to pain signals, driven by continuous nociceptive input from the periphery, which results in an enhanced pain response characterized by hyperalgesia and/or allodynia. Key players in this process include N-methyl-D-aspartate receptor and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, along with proinflammatory cytokines and chemokines released by activated glia. These glial cells release substances that further increase neuronal excitability, maintaining the sensitized state and contributing to persistent pain. The activation of antinociceptive systems is required for the resolution of pain after surgery, and default in these systems may also be considered as an important component of CPSP. In this review, we will examine the clinical factors underlying CPSP in patients and the mechanisms previously established in preclinical models of CPSP that may explain how acute postoperative pain may transform into chronic pain in patients.

Neuropathic corneal pain (NCP) is a debilitating condition affecting millions of people worldwide. Despite their critical importance, currently available animal models for NCP research are limited by complex surgeries with high-risk strategy. To advance fundamental understanding of NCP, we developed a novel rodent model that explores both structural and functional mechanisms of the disease, offering a comprehensive approach.

By uplifting (2-3 mm transversely) the long ciliary nerve (LCN) with gentle force (0.09 ± 0.02 newton [N]) and pressure (0.18 ± 0.05 MPa), our pulled nerve model mimics human NCP conditions and was investigated alongside normal control, sham control, and full transection groups. Specifically, we quantified the NCP status by establishing a relationship between pain perception and chemical sensitivity, using Stevens' Power Law concept.

Following surgery, the temporal patterns of heightened pain perception showed consistent trends across different stimulus methods, suggesting that von Frey and chemical tests could effectively evaluate pain progression. The discernable differences in Alpha values (exponent) of the pain-perception curves across the normal control, pulled nerve, and full transection groups (0.175 ± 0.035, 0.235 ± 0.015, and 0.275 ± 0.005, respectively) demonstrate the model's sensitivity to changes in NCP status. Histological analysis revealed LCN elongation, thickening, and corneal alterations in pulled nerve models, with reduced satellite glial cells (SGCs) in trigeminal ganglion compared to the normal control models. Krt16 gene expression was significantly upregulated following pulled nerve surgery.

Our model not only delineates the pathological landscape of NCP but also promises to accelerate the development of targeted therapies.

Introduction: Post-surgical pain arises following a clinical operation, often persisting throughout recovery. While current treatments reduce pain, repeated use increases the probability of adverse events. monoacylglycerol lipase (MAGL) inhibition has previously been shown to produce analgesia, either through CB1 or CB2 mechanisms, dependent on the underlying pain phenotype. Thus, this study investigated the analgesic potential of inhibiting MAGL, alone and in combination with the analgesic non-steroidal anti-inflammatory drug (NSAID) diclofenac sodium in a model of post-surgical pain. Methods: Male and female C57BL/6J mice were subjected to hindpaw incision (HPI) surgery. Mechanical allodynia, climbing, grip strength, and thermal preference were measured 24 h following HPI. The dose-dependent anti-allodynic effects of the MAGL inhibitors (irreversible MAGL inhibitor [JZL184] and selective MAGL inhibitor [MJN110]) and the NSAID diclofenac, as well as the additive potential of combined MAGL and cyclooxygenase (COX) inhibition, were assessed. Selective antagonists of CB1 and CB2 receptors were used to challenge the cannabinoid-receptor mechanism of JZL184. Similarly, the anti-allodynic effects of the CB2-selective agonist (LY2828360) were tested. JZL184 was administered repeatedly to determine tolerance. Finally, hindpaw cytokines were quantified via multiplex ELISA 24 h after HPI surgery. Results: Approximately 24 h post-surgery, the MAGL inhibitors JZL184 (≥4 mg/kg) or MJN110 (≥5 mg/kg), as well as the NSAID diclofenac sodium (≥16.67 mg/kg), attenuated HPI-induced mechanical allodynia, as assessed with von Frey filaments. The anti-allodynic effects of JZL184 (40 mg/kg) were blocked by pre-treatment of the CB2 antagonist SR144528 (3 mg/kg) but not the CB1-selective antagonist rimonabant (SR141716A; 3 mg/kg), suggesting a CB2-mediated mechanism of anti-allodynia via MAGL inhibition. Similarly, LY2828360 (3 mg/kg) reduced HPI-induced allodynia. Moreover, when administered repeatedly, the anti-allodynic effects of JZL184 (8 mg/kg) persisted and did not undergo tolerance. A separate cohort was administered a sub-analgesic dose of JZL184 (1 mg/kg), diclofenac sodium (1.85 mg/kg), or both compounds concurrently. This subthreshold JZL184 and diclofenac sodium combination attenuated HPI-induced allodynia, suggesting an additive drug interaction. Finally, HPI per se increased pro-inflammatory cytokine levels, which were unaltered by MAGL inhibition despite the anti-allodynia assessed behaviorally. Conclusion: These data support simultaneously targeting endocannabinoids and COX enzymes as a potential post-operative pain management approach.

Acute injuries can progress into painful states that endure long after healing. The mechanism underlying this transition remains unclear, but metabolic adaptations to the bioenergy demands imposed by injury are plausible contributors. Here we show that peripheral injury activates AKT/mTORC1 in afferent segments of the mouse spinal cord, redirecting local core metabolism toward biomass production while simultaneously suppressing autophagy-mediated biomass reclamation. This metabolic shift supports neuroplasticity, but creates a resource bottleneck that depletes critical spinal cord nutrients. Preventing this depletion with a modified diet normalizes biomass generation and autophagy and halts the transition to chronic pain. This effect, observed across multiple pain models, requires activation of the nutrient sensors, sirtuin-1 and AMPK, as well as restoration of autophagy. The findings identify metabolic reprogramming as a key driver of the progression to pain chronicity and point to nutritional and pharmacological interventions that could prevent this progression after surgery or other physical traumas.

The concomitant epidemics of chronic pain and opioid misuse in the United States have led to a call for novel analgesics with limited abuse potential. Previously, we have shown that co-delivery of a novel combination targeting both μ- and δ-opioid receptors in the peripheral and central nervous systems can produce synergistic analgesia. Loperamide, a peripherally restricted μ-opioid agonist, and oxymorphindole, a δ-opioid receptor partial agonist, synergize in multiple mouse models of hyperalgesia. We predicted this effect would generalize across species and therefore assessed this combination for analgesic synergy in a mouse model of post-incisional hypersensitivity. In mice, oxymorphindole and loperamide displayed significant analgesic synergy. Similar synergy was observed with N-benzyl-oxymorphindole and loperamide. In cross-bred pigs, we compared the analgesic effects of either morphine alone or the combination of oxymorphindole and loperamide or the combination of N-benzyl-oxymorphindole and loperamide. Both combinations showed increased potency as compared to morphine sulfate and effectively reduced hypersensitivity following injury without side effects. From these data we conclude that the combination of oxymorphindole and loperamide or the combination of N-benzyl-oxymorphindole and loperamide reverse incisional hyperalgesia, likely by acting in the periphery, in a large animal model without adverse effects on respiration or heart rate. PERSPECTIVE: This article presents novel opioid combinations, the μ-opioid agonist loperamide with a δ-opioid agonist, either oxymorphindole (OMI) or N-benzyl-oxymorphindole (BOMI), that relieve pain in mice and pigs without adverse side effects. These therapies could help clinicians manage pain in patients while reducing overall opioid burden and limiting side effects.

Depressive disorder is a chronic, recurring, and potentially life-endangering neuropsychiatric disease. According to a report by the World Health Organization, the global population suffering from depression is experiencing a significant annual increase. Despite its prevalence and considerable impact on people, little is known about its pathogenesis. One major reason is the scarcity of reliable animal models due to the absence of consensus on the pathology and etiology of depression. Furthermore, the neural circuit mechanism of depression induced by various factors is particularly complex. Considering the variability in depressive behavior patterns and neurobiological mechanisms among different animal models of depression, a comparison between the neural circuits of depression induced by various factors is essential for its treatment. In this review, we mainly summarize the most widely used behavioral animal models and neural circuits under different triggers of depression, aiming to provide a theoretical basis for depression prevention.

Poor dietary intake is associated with peripheral pain sensitization and postoperative pain. Given the limited research on diet and pain, it is essential to examine the possible analgesic effects of dietary interventions in preclinical studies.

This study aimed to elucidate the role of high-fat diet (HFD) on pain sensitivity and postoperative pain, and determine the potential effects of modulating branched-chain amino acids (BCAA) intake on pain phenotypes.

Four-week-old male mice were fed a purified control diet (CD) or HFD for 10 wk, followed by a hind paw incision. Four-week-old male mice were initially fed a CD or HFD for 8 wk, then switched to the high or low BCAA diet, and underwent a hind paw incision at 10 wk of these diets. Pain behaviors were assessed. Several proinflammatory genes in the lumbar dorsal root ganglion (DRG) were detected by quantitative polymerase chain reaction. Immunohistochemistry was used to estimate nerve fiber density at the incision site. Two-tailed unpaired Student's t-test, 2-way repeated-measures analysis of variance (ANOVA) with Bonferroni posttests, and 1-way ANOVA with Tukey's multiple comparisons test were used for data analysis.

HFD consumption induced pain sensitization and worsened postoperative pain in male mice (P < 0.0001). In CD group, mice switching to high or low BCAA diet displayed minor impacts on pain phenotypes. In HFD mice, switching to high BCAA diet exacerbated hyperalgesia and postsurgical pain (P < 0.05), leading to proinflammatory responses in the DRG and the reduction of nerve fiber density near the incision site on day 3 postsurgery (P < 0.05); whereas low BCAA diet intake alleviated these effects (P < 0.05).

High BCAA intake has negative impacts on pain sensitivity and postoperative pain in HFD-fed mice. Reducing dietary BCAA may be a novel nonpharmacological therapeutic to relieve pain in individuals on a conventional HFD.

Post-surgical pain affects millions each year, hindering recovery and quality of life. Surgical procedures cause tissue damage and inflammation, leading to peripheral and central sensitization, resulting in pain at rest or mechanical and heat hyperalgesia. In a rat model for post-surgical pain, spinal GABAergic transmission via GABAA receptors reduces mechanical hypersensitivity but has no effect on pain at rest. While fMRI studies show consistent brain activity changes during mechanical stimulation in post-surgical pain, central processing of pain at rest and the role of spinal GABAergic circuits on surgical pain processing is currently unclear. The aim of this study was to evaluate the influence of an acute surgical incision injury, a proxy for post-surgical pain, on the cerebral processing of pain at rest and mechanical hypersensitivity, and to assess the influence of spinal GABAA-circuits on this processing. In rats, a unilateral incision affected sensorimotor and thalamo-limbic subnetworks at rest and following mechanical stimulation, indicating changes in neural processing relevant to pain at rest and mechanical hypersensitivity in post-surgical pain. Enhancing spinal GABAergic tone increased functional connectivity (FC) in parts of these subnetworks during mechanical stimulation, but not at rest, highlighting spino-cerebral interactions in pain regulation relevant for mechanical hypersensitivity and potentially the transisition to chronic pain after surgery but likely not for pain at rest. These findings underscore the complex and interconnected nature of brain networks in post-surgical pain processing, and provide insights into potential spinal targets for pharmacological intervention to alleviate post-surgical pain and prevent it's chronification.

Armadillidium vulgare Latreille (AV), the dried body of pillbug, was originally described in Shennong's Classic of Materia Medica. As a common analgesic in animal-based traditional Chinese medicine, it is mainly used to relieve pain, promoting diuresis, relieving fatigue and so on. Our work demonstrated that AV could alleviate various types of acute and chronic pain including neuropathic pain (NP). And transcriptome sequencing analysis revealed that AV could suppress CXCL10 to alleviate NP, however, the upstream mechanisms governing CXCL10 synthesis remain vague.

The research's goal was to identify the mechanism via which AV regulates CXCL10 to ameliorate NP.

Chronic constriction injury (CCI) to the sciatic nerve was used to induce the NP model 14 days following surgery. To identify cell signaling pathways, various approaches were used, including transcriptome sequencing, western blotting, immunofluorescence, as well as ELISA. The in vitro assay involved the cultivation of neuron PC12 cells and astrocyte C6 cells.

Both in vivo and in vitro results demonstrated that IL-12/IL-18 enhanced IFN-γ production in spinal neurons, which acted on IFN-γ receptors on neurons and astrocytes to upregulate CXCL10 expression in these cells, illustrating the pivotal role of IL-12 in the crosstalk between neurons and astrocytes. The role of IL-12 in pain regulation was elucidated for the first time within the nervous system. Additionally, its synergistic interaction with IL-18 on the downstream IFN-γ-CXCL10 pathway dramatically altered the activation of neurons and astrocytes. And AV could suppress CXCL10 to alleviate NP by mediating the IL-12-IFN-γ-IFNGR signaling pathway.

We explored a new target for NP by regulating neuron-astrocyte crosstalk and provided a theoretical basis for AV in clinical use.

Safe, effective pain management remains one of the biggest challenges following surgical procedures. Despite widespread recognition of this problem and advances in the mechanistic understanding of pain signaling, post-surgical pain is often undermanaged, with opioid use remaining the clinical standard. As an alternative to current oral, systemic treatments, a degradable bupivacaine-loaded poly(ester urea) (PEU) thin film has been developed to deliver bupivacaine directly to the site of injury over an extended duration. The dose and duration of bupivacaine delivery is controlled using polymer composition and bupivacaine concentration. Systemic bupivacaine concentrations are more than an order of magnitude lower when delivered locally versus intravenous injection. Tissue analysis showed that the majority of bupivacaine is deposited into subcutaneous tissue directly surrounding the implant. Bupivacaine concentration in soft tissue around the implant are 30-fold higher than plasma values, indicating that release from PEU implants remains localized. Bupivacaine-loaded PEU films are assessed into two established mouse models for diabetic neuropathic pain and post-surgical incisional pain. In each model, bupivacaine eluting PEU films effectively block pain for 3-5 days before returning to baseline levels without loss of motor function and without signs of neurotoxicity.

Approximately one-third of postoperative patients are troubled by postoperative pain. Effective treatments are still lacking. The aim of this study is to investigate the role of brain-derived neurotrophic factor (BDNF)-VGF (non-acronymic) in dorsal root ganglia (DRG) in postoperative pain. Pain behaviors were assessed through measurements of paw withdrawal threshold (PWT) and paw withdrawal latency (PWL). Transcriptome analysis was conducted to identify potential targets associated with postoperative pain. Western blotting, immunofluorescence, and ELISA were employed to further detect macrophage activation as well as the expression of BDNF, VGF, TNF-α, IL-1β, and IL-6. Results showed that plantar incision induced both mechanical and thermal hyperalgesia. Transcriptome analysis suggested that plantar incision caused upregulation of BDNF and VGF. The expressions of BDNF and VGF were upregulated in isolectin B4-positive (IB4+) and calcitonin gene-related peptide-positive (CGRP+) neurons, rather than neurofilament 200-positive (NF200+) neurons. The activation of BDNF-VGF pathway upregulated expression of IL-6, TNF-α, and IL-1β and promoted the activation of macrophages. In conclusion, BDNF-VGF pathway aggravates acute postoperative pain by promoting macrophage activation and pro-inflammatory cytokine production, which may provide a new target for the treatment of postoperative pain.

Pain and itch are regulated by a diverse array of neuropeptides and their receptors in superficial laminae of the spinal cord dorsal horn (DH). Neuropeptide Y (NPY) is normally expressed on DH neurons but not sensory neurons. By contrast, the Npy2r receptor (Y2) is expressed on the central and peripheral terminals of sensory neurons but not on DH neurons. Neurophysiological slice recordings indicate that Y2-selective agonists inhibits spinal neurotransmitter release from sensory neurons. However, behavioral pharmacology studies indicate that Y2 agonists exert minimal changes in nociception, even after injury. Additional discrepancies in the behavioral actions of the Y2-antagonist BIIE0246 - reports of either pronociception or antinociception - have now been resolved. In the normal state, spinally-directed (intrathecal) administration of BIIE0246 elicits ongoing nociception, hypersensitivity to sensory stimulation, and aversion. Conversely, in the setting of nerve injury and inflammation, intrathecal BIIE024 reduced not only mechanical and thermal hypersensitivity, but also a measure of the affective dimension of pain (conditioned place preference). When administered in chronic pain models of latent sensitization, BIIE0246 produced a profound reinstatement of pain-like behaviors. We propose that tissue or nerve injury induces a G protein switch in the action of NPY-Y2 signaling from antinociception in the naïve state to the inhibition of mechanical and heat hyperalgesia in the injured state, and then a switch back to antinociception to keep LS in a state of remission. This model clarifies the pharmacotherapeutic potential of Y2 research, pointing to the development of a new non-opioid pharmacotherapy for chronic pain using Y2 antagonists in patients who do not develop LS.

Perioperative Paradoxical sleep deprivation (PSD) is associated with postoperative hyperalgesia. However, the clinical therapeutic strategies for PSD-induced postoperative hyperalgesia are limited. Electroacupuncture (EA) has been used for attenuating many types of pain, including neuropathic pain and inflammatory pain, but its effect on PSD-induced postoperative hyperalgesia is still unclear, and its analgesia mechanism should be further explored. In this study, we designed to investigate the possible mechanism of PSD-induced postoperative hyperalgesia and the effect of EA on PSD-induced postoperative hyperalgesia, and whether the mechanism is related to the BDNF/TrkB signaling pathway mediated by α7nAChR in the spinal cord. The paw withdrawal thermal latency (PWTL) and paw withdrawal mechanical threshold (PWMT) of rats were used to detect PSD-induced hyperalgesia. The expression of α7nAChR, BDNF, TrkB and KCC2 in the spinal cord were evaluated by Western blot and immunofluorescence. The results showed that preoperative 24 h PSD significantly decreased the PWTL and PWMT. The expression of α7nAChR and KCC2 significantly downregulated in the spinal cord of PSD-induced postoperative hyperalgesia rats, the opposite was observed for BDNF and TrkB expression. Moreover, intrathecal injection of alpha-bungarotoxin (α-BGT), a selective antagonist for α7nAChR, not only aggravated the pain hypersensitivity, but also demonstrated a further decrease of α7nAChR and KCC2 expression and a further increase of BDNF and TrkB expression. EA stimulation increased the PWTL and PWMT values of PSD-induced postoperative hyperalgesia rats, significantly upregulated α7nAChR and KCC2 expression, and significantly downregulated BDNF and TrkB expression. Moreover, intrathecal injection of α-BGT suppressed the analgesic effect of EA, inhibited the enhancement of α7nAChR and KCC2 expression and the reduction of BDNF and TrkB expression induced by EA. In conclusion, our study indicated that 24 h PSD can cause postoperative hyperalgesia, and the mechanism may be related to the disorder of α7nAChR mediated BDNF/TrkB-KCC2 signaling pathway. EA can alleviate postoperative hyperalgesia induced by PSD, which may be related to its effect in activating α7nAChR, inhibiting the expression of BDNF/TrkB, and up-regulating the expression of KCC2 in the spinal cord.

Neuropeptide Y (NPY) Y2 receptor (Y2) antagonist BIIE0246 can both inhibit and facilitate nociception. The authors hypothesized that Y2 function depends on inflammation or nerve injury status.

The authors implemented a battery of behavioral tests in mice of both sexes that received (1) no injury; (2) an incision model of postoperative pain; (3) a spared nerve injury model of neuropathic pain; and (4) a latent sensitization model of chronic postsurgical pain. In addition to Y2 gene expression assays, spinal Y2 G-protein coupling was studied with guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPγS) binding assays.

The authors report that intrathecal BIIE0246 increased mechanical and cold hypersensitivity, produced behavioral signs of spontaneous nociception and itch, and produced conditioned place aversion and preference in normal, uninjured mice. BIIE0246 did not change heat hypersensitivity or motor coordination. Conditional (sensory neuron-specific) Y2 deletion prevented BIIE0246-induced mechanical and cold hypersensitivity, nocifensive behaviors, and aversion. Both conditional deletion and pharmacologic blockade of Y2 reduced mechanical and thermal hypersensitivity after incision or nerve injury. SNI did not change the sensitivity of Y2 G-protein coupling with the Y2 agonist peptide YY (3-36) (PYY3-36), but increased the population of Y2 that effectively coupled G-proteins. Intrathecal PYY3-36 failed to reduce spared nerve injury- or incision-induced hypersensitivity in C57BL/6N mice. Incision did not change Npy2r gene expression in dorsal root ganglion.

The authors conclude that Y2 at central terminals of primary afferent neurons provides tonic inhibition of mechanical and cold nociception and itch. This switches to the promotion of mechanical and thermal hyperalgesia in models of acute and chronic postsurgical and neuropathic pain, perhaps due to an increase in the population of Y2 that effectively couples to G-proteins. These results support the development of Y2 antagonists for the treatment of chronic postsurgical and neuropathic pain.

Postoperative pain management is challenging. We used mice with a transverse laparotomy to study tactile allodynia measured by the von Frey test, pain at rest measured by facial pain expressions detected by an artificial intelligence algorithm, and movement-induced pain measured by reductions in exploratory activity. The standard analgesics morphine and ibuprofen induced distinct patterns of outcome-dependent effects. Whereas morphine was more effective in reversing pain at rest compared to tactile allodynia, it was unable to alter movement-induced pain. Ibuprofen showed comparable effects across the three outcomes. Administered together, the compounds induced synergistic effects in the three aspects of postoperative pain, mirroring the known advantages of multimodal analgesia used in clinical practice. We explored the impact of neuroimmune interactions using a neutrophil depletion strategy. This reversed pain at rest and movement-induced pain, but did not alter cutaneous sensitivity. Non-peptidergic (IB4+) and peptidergic (CGRP+) nociceptors are segregated neuronal populations in the mouse. We tested the effects of gefapixant, an antitussive drug targeting non-peptidergic nociceptors through P2X3 antagonism, and olcegepant, an antimigraine drug acting as a CGRP antagonist. Both compounds reversed tactile allodynia, while only gefapixant reversed pain at rest, and none of them reversed movement-induced pain. In conclusion, tactile allodynia, pain at rest, and movement-induced pain after surgery have different pharmacological profiles, and none of the three aspects of postoperative pain can predict the effects of a given intervention on the other two. Combining these measures provides a more realistic view of postoperative pain and has the potential to benefit analgesic development.

Local anesthetics (LA), as part of multimodal analgesia, have garnered significant interest for their role in delaying the initiation of opioid therapy, reducing postoperative opioid usage, and mitigating both hospitalization duration and related expenses. Despite numerous endeavors to extend the duration of local anesthetic effects, achieving truly satisfactory long-acting analgesia remains elusive. Drawing upon prior investigations, vesicular phospholipid gels (VPGs) emerge as promising candidates for extended-release modalities in small-molecule drug delivery systems. Therefore, we tried to use the amphiphilicity of phospholipids to co-encapsulate levobupivacaine hydrochloride and meloxicam, two drugs with different hydrophilicity, to obtain a long-term synergistic analgesic effect. Initially, the physicochemical attributes of the formulation were characterized, followed by an examination of its in vitro release kinetics, substantiating the viability of extending the release duration of the dual drugs. Sequentially, in vivo investigations encompassing pharmacokinetic profiling and assessment of analgesic efficacy were undertaken, revealing a prolonged release duration of up to 120 h and attainment of optimal postoperative analgesia. Subsequently, inquiries into the mechanism underlying synergistic analgesic effects and safety evaluations pertinent to the delivery strategy were pursued. In summation, we successfully developed a promising formulation to achieve long-acting analgesia.

Nociceptors with somata in dorsal root ganglia (DRGs) readily switch from an electrically silent state to a hyperactive state of tonic, nonaccommodating, low-frequency, irregular discharge of action potentials (APs). Spontaneous activity (SA) during this state is present in vivo in rats months after spinal cord injury (SCI) and has been causally linked to SCI pain. Intrinsically generated SA and, more generally, ongoing activity (OA) are induced by various neuropathic conditions in rats, mice, and humans and are retained in nociceptor somata after dissociation and culturing, providing a powerful tool for investigating its mechanisms and functions. The present study shows that long-lasting hyperexcitability that can generate OA during modest depolarization in probable nociceptors dissociated from DRGs of male and female rats is induced by plantar incision injury. OA occurred when the soma was artificially depolarized to a level within the normal range of membrane potentials where large, transient depolarizing spontaneous fluctuations (DSFs) can approach AP threshold. This hyperexcitability persisted for at least 3 weeks, whereas behavioral indicators of affective pain-hind paw guarding and increased avoidance of a noxious substrate in an operant conflict test-persisted for 1 week or less. The most consistent electrophysiological alteration associated with OA was enhancement of DSFs. An unexpected discovery after plantar incisions was hyperexcitability in neurons from thoracic DRGs that innervate dermatomes distant from the injured tissue. Potential in vivo functions of widespread, low-frequency nociceptor OA consistent with these and other findings are to contribute to hyperalgesic priming and to drive anxiety-related hypervigilance.

The shortfall in new analgesic agents is a major impediment to reducing reliance on opioid medications for control of severe pain. In both animals and man, attenuating nociceptive transmission from primary afferent neurons with a μ-opioid receptor agonist yields highly effective analgesia. Consequently, deeper molecular characterization of human nociceptive afferents expressing OPRM1, the μ-opioid receptor gene, is a key component for advancing analgesic drug discovery and understanding clinical pain control. A co-expression matrix for the μ-opioid receptor and a variety of nociceptive channels as well as δ- and κ-opioid receptors is established by multiplex in situ hybridization. Our results indicate an OPRM1-positive population with strong molecular resemblance to rodent peptidergic C-nociceptors associated with tissue damage pain and an OPRM1-negative population sharing molecular characteristics of murine non-peptidergic C-nociceptors. The empirical identification of two distinct human nociceptive populations that differ profoundly in their presumed responsiveness to opioids provides an actionable translational framework for human pain control.

More than 80% of patients may experience acute pain after a surgical procedure, and this is often refractory to pharmacological intervention. The identification of new targets to treat postoperative pain is necessary. There is an association of polymorphisms in the Cav2.3 gene with postoperative pain and opioid consumption. Our study aimed to identify Cav2.3 as a potential target to treat postoperative pain and to reduce opioid-related side effects.

A plantar incision model was established in adult male and female C57BL/6 mice. Cav2.3 expression was detected by qPCR and suppressed by siRNA treatment. The antinociceptive efficacy and safety of a Cav2.3 blocker-alone or together with morphine-was also assessed after surgery.

Paw incision in female and male mice caused acute nociception and increased Cav2.3 mRNA expression in the spinal cord but not in the incised tissue. Intrathecal treatment with siRNA against Cav2.3, but not with a scrambled siRNA, prevented the development of surgery-induced nociception in both male and female mice, with female mice experiencing long-lasting effects. High doses of i.t. SNX-482, a Cav2.3 channel blocker, or morphine injected alone, reversed postoperative nociception but also induced side effects. A combination of lower doses of morphine and SNX-482 mediated a long-lasting reversal of postsurgical pain in female and male mice.

Our results demonstrate that Cav2.3 has a pronociceptive role in the induction of postoperative pain, indicating that it is a potential target for the development of therapeutic approaches for the treatment of postoperative pain.

Mammalian zinc ectopeptidases have significant functions in deactivating neurological and hormonal peptide signals on the cell surface. The identification of Opiorphin, a physiological inhibitor of zinc ectopeptidases that inactivate enkephalin, has revealed its strong analgesic effects in both chemical and mechanical pain models. Opiorphin achieves this by increasing the transmission of endogenous opioids, which are dependent on the body's own opioid system. The function of opiorphin is closely linked to the rat sialorphin peptide, which inhibits pain perception by enhancing the activity of naturally occurring enkephalinergic pathways that depend on μ- and δ-opioid receptors. Opiorphin is highly intriguing in terms of its physiological implications within the endogenous opioidergic pathways, particularly in its ability to regulate mood-related states and pain perception. Opiorphin can induce antidepressant-like effects by influencing the levels of naturally occurring enkephalin, which are released in response to specific physical and/or psychological stimuli. This effect is achieved through the modulation of delta-opioid receptor-dependent pathways. Furthermore, research has demonstrated that opiorphin's impact on the cardiovascular system is facilitated by the renin-angiotensin system (RAS), sympathetic ganglia, and adrenal medulla, rather than the opioid system. Hence, opiorphin shows great potential as a solitary candidate for the treatment of several illnesses such as neurodegeneration, pain, and mood disorders.

Millions of people undergo surgical procedures each year with many developing postsurgical pain. Dynamic allodynia can arise when, for example, clothing brushing close to the surgical site elicits pain. The allodynia circuits that enable crosstalk between afferent tactile inputs and central pain circuits have been studied, but the peripheral tactile drive has not been explored.

Investigate the innervation of the skin in the rat plantar hindpaw skin-muscle incision model.

Incision increased epidermal thickness and cell layers and reduced intraepidermal nerve fibre density, identified with PGP9.5 immunostaining. Strikingly, Collagen IV immunostaining revealed the development of dermal protrusions, oriented towards the incision site, that were reminiscent of the dermal papillae that exist in glabrous footpads. S100 immunostaining for lamellar Schwann cells revealed the presence of novel tactile corpuscles (S100-positive bulb) within incision-induced putative dermal papillae. The occurrence of these novel tactile corpuscles coincided with behavioural observations of dynamic allodynia. Tactile corpuscles require brain-derived neurotrophic factor- tropomyosin receptor kinase B (BDNF-TrkB) signalling to form during development, and an increase in BDNF-immunostaining intensity was observed close to the incision site. Local acute administration of TrkB-Fc, to block BDNF-TrkB signalling, reduced, by approximately 50%, both tactile corpuscle size (S100+ bulb area) and dynamic allodynia.

Surgery induces the development of novel tactile corpuscles in the incision surround, in a BDNF-TrKB-dependent manner, that contributes to postsurgical tactile-evoked pain.

This study explores the potential of a nanomedicine approach, using Leu-enkephalin-squalene nanoparticles (LENK-SQ NPs) for managing long-lasting pain. It was observed that the nanomedicine significantly improved the pharmacological efficacy of the Leu-enkephalin, a fast metabolized neuropeptide, in a rat model of acute inflammatory pain, providing local analgesic effect, while minimizing potential systemic side effects by circumventing central nervous system. The LENK-SQ NPs were tested in a rat model of postoperative pain (Brennan's rodent plantar incision model) using continuous infusion via Alzet® pump, with an additional bolus injection. The analgesic activity was assessed through stimulus-evoked methods, such as the von Frey and Hargreaves tests. Both mechanical and thermal hyperalgesia were significantly reduced at days 2 and 3 post-incision. An additional pharmacokinetic study was conducted, showing that LENK-SQ NPs allowed a sustained circulation of the neuropeptide under its prodrug form. On the other hand, the biodistribution of fluorescently labelled LENK-SQ NPs revealed their selective accumulation in the incised paw within the first hour post administration, followed by a disassembly of the NPs, starting 24 h later. The study proposes the following multi-step mechanism for the anti-nociceptive pharmacological activity of LENK-SQ NPs: (i) protection of the neuropeptide from metabolization into the bloodstream, (ii) targeted accumulation of the nanoparticles within the incised painful tissue and (iii) gradual release of LENK at the onset of the inflammatory process, leading to the observed analgesic activity.

Spinal opioids have mixed efficacy and their adverse effects force treatment cessation of postoperative pain. Consequently, there is an ongoing search for new therapeutic strategies. Here, we evaluated the analgesic efficacy of intrathecal UCM707, an anandamide reuptake inhibitor, and morphine combination. Firstly, we assessed the effects of morphine (1, 5 and 10 μg), UCM707 (75 μg) and its combination in the hot plate. Then, morphine + UCM707 at sub-effective doses was evaluated in a rat post-incisional pain model. In addition, μ-, CB1r-, CB2r- and TRPV1-antagonists were pre-administered before the combination. Activation of μ-opioid and CB1r, and Cnr1, Cnr2, Oprm1 and TRPV1 expressions were evaluated in the lumbar sacra and periaqueductal grey by [35 S]-GTPγS binding autoradiography and qPCR studies. In the hot plate, morphine (1 μg) and UCM707 (75 μg) induced a more robust analgesic effect than each drug alone. Morphine plus UCM707 did not modify μ-opioid nor CB1 receptor function in the PAG or LS. Cnr1 and TRPV1 expression increased in the lumbar sacra (LS). Morphine plus UCM707 significantly reduced post-incisional pain at 1 and 4 days after surgery. Cnr1, Cnr2 and TRPV1 expressions increased in the LS. Blockade of μ-opioid receptor reduced combination effects on days 1 and 4. CB1r- and CB2r-antagonism reduced morphine + UCM707 effects on days 1 and 4, respectively. CB1r and TRPV1-antagonism improved their antinociceptive effects on day 4. These results revealed a synergistic/additive analgesic effect of UCM707 and morphine combination controlling postincisional pain. CB1r, CB2r and TRPV1 contribute differently as central sensitization occurs.

One of the methods for pain management involves the use of local anesthesia, which numbs sensations in specific body regions while maintaining consciousness.

Considering the certain limitations (e.g., pain, the requirement of skilled professionals, or slow passive diffusion) of conventional delivery methods of local anesthetics, developing alternative strategies that offer minimally invasive yet therapeutically effective delivery systems is of great concern for ophthalmic regional anesthesia.

In this study, a rapidly dissolving cambered microneedle (MNs) patch, composed of poly(vinylpyrrolidone) (PVP) and hyaluronic acid (HA) and served as a delivery system for lidocaine (Lido) in local anesthesia, was developed taking inspiration from the mosquito proboscis's ability to extract blood unnoticed. The lidocaine-containing MNs patch (MNs@Lido) consisted of 25 microneedles with a four-pronged cone structure (height: 500 μm, base width: 275 μm), arranged in a concentric circle pattern on the patch, and displays excellent dissolubility for effective drug delivery of Lido. After confirming good cytocompatibility, MNs@Lido was found to possess adequate rigidity to penetrate the cornea without causing any subsequent injury, and the created corneal pinhole channels completely self-healed within 24 h. Interestingly, MNs@Lido exhibited effective analgesic effects for local anesthesia on both heel skin and eyeball, with the sustained anesthetic effect lasting for at least 30 min.

These findings indicate that the mosquito proboscis-inspired cambered MNs patch provides rapid and painless local anesthesia, overcoming the limitations of conventional delivery methods of local anesthetics, thus opening up new possibilities in the treatment of ophthalmic diseases.

Tropomyosin receptor kinases (Trks) are receptor tyrosine kinases activated by neurotrophic factors, called neurotrophins. Among them, TrkA interacts with the nerve growth factor (NGF), which leads to pain induction. mRNA-display screening was carried out to discover a hit compound 2, which inhibits protein-protein interactions between TrkA and NGF. Subsequent structure optimization improving phosphorylation inhibitory activity and serum stability was pursued using a unique process that took advantage of the peptide being synthesized by translation from mRNA. This gave peptide 19, which showed an analgesic effect in a rat incisional pain model. The peptides described here can serve as a new class of analgesics, and the structure optimization methods reported provide a strategy for discovering new peptide drugs.

Chronobiological approaches have emerged as tools to study pain and inflammation. Although time-of-day effects on the expression of pain after injury have been studied, it remains unaddressed whether the timing of the injury itself can alter subsequent pain behaviors. The aim of this study was to assess postsurgical pain behaviors in a mouse hind paw incision assay in a circadian-dependent manner. Incisions were made at one of four equally spaced time points over a 24-hour period, with evoked and spontaneous pain behaviors measured using the von Frey mechanical sensitivity test, Hargreaves' radiant heat paw-withdrawal test, and the Mouse Grimace Scale. Algesiometric testing was performed in C57BL/6 mice prior to and at multiple time points after incision injury, at the same time of day, until pain resolution. No statistically significant differences were observed between groups. This study adds to the literature on circadian rhythms and their influence on pain in the pursuit of more biologically informed pre- and postoperative care.

Postoperative pain remains one of the most common complaints after surgery, and appropriate treatments are limited.

We therefore investigated the effect of the anti-nociceptive properties of magnesium sulfate (MgSO4), an N-methyl-D-aspartate (NMDA) receptor antagonist, on incision-induced postoperative pain and peripheral and central nervous system inflammation.

We found that local MgSO4 administration dose-dependently increases paw withdrawal latency, indicating reduced peripheral postoperative pain. Furthermore, MgSO4 inhibited the expression of interleukin-1β (IL-1β) and inducible nitric oxide synthase (iNOS) and phosphorylation of the NMDA receptor NR1 subunit in injured paw tissue and significantly attenuated microglial and astrocytic activation in the ipsilateral lumbar spinal cord dorsal horn.

Locally administered MgSO4 has potential for development as an adjunctive therapy for preventing central nociceptive sensitization.

Yokukansan, a traditional Japanese medicine (Kampo), has been widely used to treat neurosis, dementia, and chronic pain. Previous in vitro studies have suggested that Yokukansan acts as a partial agonist of the 5-HT1A receptor, resulting in amelioration of chronic pain through inhibition of nociceptive neuronal activity. However, its effectiveness for treating postoperative pain remains unknown, although its analgesic mechanism of action has been suggested to involve serotonin and glutamatergic neurotransmission. This study aimed to investigate the effect of Yokukansan on postoperative pain in an animal model.

A mouse model of postoperative pain was created by plantar incision, and Yokukansan was administered orally the day after paw incision. Pain thresholds for mechanical and heat stimuli were examined in a behavioral experiment. In addition, to clarify the involvement of the serotonergic nervous system, we examined the analgesic effects of Yokukansan in mice that were serotonin-depleted by para-chlorophenylalanine (PCPA) treatment and intrathecal administration of NAN-190, 5-HT1A receptor antagonist.

Orally administered Yokukansan increased the pain threshold dose-dependent in postoperative pain model mice. Pretreatment of para-chlorophenylalanine dramatically suppressed serotonin immunoreactivity in the spinal dorsal horn without changing the pain threshold after the paw incision. The analgesic effect of Yokukansan tended to be attenuated by para-chlorophenylalanine pretreatment and significantly attenuated by intrathecal administration of 2.5 µg of NAN-190 compared to that in postoperative pain model mice without para-chlorophenylalanine treatment and NAN-190 administration.

This study demonstrated that oral administration of Yokukansan has acute analgesic effects in postoperative pain model mice. Behavioral experiments using serotonin-depleted mice and mice intrathecally administered with a 5-HT1A receptor antagonist suggested that Yokukansan acts as an agonist at the 5-HT1A receptor, one of the serotonin receptors, to produce analgesia.

Lidocaine is the most commonly used local anesthetic worldwide, known for its rapid onset and moderate duration of anesthesia. However, it is short-lived and does not effectively promote effective topical anesthesia in the oral cavity when used alone. Our aim was to investigate whether an approximate 50% encapsulation of lidocaine in poly(ε-caprolactone) nanocapsules (LDC-Nano) would be able to increase its permeation and analgesic efficacy and reduce cytotoxicity. In this study, we characterized LDC-Nano and conducted MTT tests with HaCaT cells to assess their in vitro cytotoxicity. Additionally, in vitro permeation assays across the pig esophageal epithelium and the anesthetic efficacy of the hind paw incision model in rats were performed. Plain lidocaine (LDC) was compared with LDC-Nano and lidocaine hydrochloride plus epinephrine (LDC-Epi). The physicochemical characteristics of LDC-Nano were satisfactory (pH: 8.1 ± 0.21; polydispersity index: 0.08 ± 0.01; mean diameter (nm): 557.8 ± 22.7; and encapsulation efficiency (%): 51.8 ± 1.87) and remained stable for up to 4 months. LDC-Nano presented similar in vitro cytotoxicity to LDC but was higher than LDC-Epi (LD50: LDC = 0.48%; LDC-Nano = 0.47%; and LDC-Epi = 0.58%; p < 0.0001). Encapsulation increased the permeability coefficient about 6.6 times and about 7.5 the steady-state flux of lidocaine across the mucosal epithelium. Both encapsulation and epinephrine improved anesthesia duration, with epinephrine demonstrating superior efficacy (100% of animals were anesthetized up to 100, 30, and 20 min when LDC-Epi, LDC-nano, and LDC were used, respectively). Although LDC-Epi demonstrated superior in vivo anesthetic efficacy, the in vitro permeation and cytotoxicity of LDC-Nano indicate promising avenues for future research, particularly in exploring its potential application as a topical anesthetic in the oral cavity.

Buprenorphine in an extended-release formulation intended for use in laboratory subjects is frequently administered to rats to provide extended analgesia without repeated handling. While levels of buprenorphine may persist in serum once extended-release buprenorphine has been introduced, exposure to opioids can cause opioid tolerance or opioid-induced hypersensitivity. This work examined the analgesic duration and efficacy of a single administration of extended-release buprenorphine intended for use in laboratory subjects in models of inflammatory pain and post-operative pain and the development of opioid tolerance in rat. After subcutaneous administration of 1 mg/kg extended-release buprenorphine, analgesic efficacy did not persist for the expected 72 hours. No changes were observed in mechanical thresholds in the hindpaws that were contralateral to the injury, suggesting a lack of centrally mediated opioid-induced hypersensitivity. To determine whether opioid tolerance arose acutely after one exposure to extended-release buprenorphine, we conducted the warm water tail flick assay; on Day 1 we administered either saline or extended-release buprenorphine (1 mg/kg) and on Day 3 we quantified the standard buprenorphine dose-response curve (0.1-3 mg/kg). Rats previously given extended-release buprenorphine displayed decreased analgesic responses after administration of standard buprenorphine as compared to the robust efficacy of standard buprenorphine in control subjects. Males appeared to show evidence of acute opioid tolerance, while females previously exposed to opioid did not demonstrate a decreased response at the doses examined. Taken together, these results suggest that opioid tolerance arises quickly in male rats after exposure to the extended-release formulation of buprenorphine. This tolerance may account for the brief period of antinociception observed.

Delayed emergence from general anaesthesia, opioid-induced sedation, and opioid-induced respiratory depression is associated with perioperative complications. We characterised the preclinical effects of the orexin receptor 2 (OX2R)-selective agonist danavorexton (TAK-925) on emergence from anaesthesia and reversal of fentanyl-induced sedation, respiratory depression, and analgesia.

Emergence from isoflurane- or propofol-induced anaesthesia and fentanyl-induced sedation were investigated by righting reflex, rotarod, and electroencephalography in rats or monkeys. Fentanyl-induced respiratory depression was assessed by arterial blood gas analysis and whole-body plethysmography in rats and monkeys. Analgesia was evaluated using formalin- and skin incision-induced pain models in rats.

Danavorexton shortened emergence from isoflurane- or propofol-induced anaesthesia and from fentanyl-induced sedation at 1 (P=0.005), 3 (P=0.006), and 3 mg kg-1 s.c. (P=0.022), respectively, by righting reflex in rats. Danavorexton (10 mg kg-1 s.c.) accelerated recovery from isoflurane-, propofol- and fentanyl-induced motor impairment in separate rotarod tests in rats (P=0.008, P=0.007, P=0.017, respectively), and reversed anaesthesia and fentanyl-induced delta-power increases. Danavorexton shortened emergence (return of righting reflex) from isoflurane- or propofol-induced anaesthesia at 1 (P=0.002) and 3 mg kg-1 (P=0.004), respectively, in cynomolgus monkeys. Danavorexton (10 mg kg-1 s.c.) reversed fentanyl-induced increase in Pco2 (P=0.006), and decrease in Po2 (P=0.015) and pH (P<0.001) in rats, and at 3 mg kg-1 s.c. reversed fentanyl-induced increase in Pco2 (P=0.007), and decrease in Po2 (P=0.013) and SO2 (P=0.036) in monkeys. Danavorexton increased minute volume and tidal volume in fentanyl-treated animals. Danavorexton at ≤10 mg kg-1 s.c. did not compromise fentanyl analgesia in rat formalin- and skin incision-induced pain models.

Danavorexton promoted recovery from anaesthesia and fentanyl-induced sedation, and antagonised fentanyl-induced respiratory depression without compromising fentanyl analgesia.