Guarding pain and spontaneous activity of nociceptors after skin versus skin plus deep tissue incision

Widespread hyperexcitability of nociceptor somata outlasts enhanced avoidance behavior after incision injury

ABSTRACT

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.

Efficacy of Preoperative Exercise in Prehabilitation for Preventing Postoperative Sleep Disturbances and Pain: An Experimental Rat Model Study

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.

Mechanisms of chronic postsurgical pain

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.

Synaptic-like coupling of macrophages to myofibers regulates muscle repair

Peripheral injury responses essential for muscle repair and nociception require complex interactions of target tissues, immune cells and primary sensory neurons. Nociceptors and myofibers both react robustly to signals generated from circulating immune cells, which promote repair, growth, and regeneration of muscle while simultaneously modulating peripheral sensitization. Here, we found that macrophages form a synaptic-like contact with myofibers to hasten repair after acute incision injury and to facilitate regeneration after major muscle damage. Transient chemogenetic activation of macrophages enhanced calcium dependent membrane repair, induced muscle calcium waves in vivo , elicited low level electrical activity in the muscles and enhanced myonuclear accretion. Under severe injury, macrophage activation could also modulate pain-like behaviors. This study identifies a novel mechanism by which synaptic-like functions of macrophages impacts muscle repair after tissue damage.

EGFR Tyrosine Kinase Inhibitor Lazertinib Activates a Subset of Mouse Sensory Neurons Via TRPA1

Lazertinib (JNJ-73841937, YH25448) is a mutant-selective irreversible epidermal growth factor receptor tyrosine kinase inhibitor targeting both the T790M and activating mutation while sparing wild-type epidermal growth factor receptor. Paresthesia is one of the most common adverse events seen with lazertinib treatment, suggesting that lazertinib could affect the sensory nervous system. However, the mechanism of action for this paresthesia remains unclear. In this study, we investigated whether and how lazertinib affects peripheral sensory neurons. Through Fura-2-based calcium imaging and whole-cell patch clamp recording in primary-cultured dorsal root ganglion (DRG) neurons from adult mice, we found that application of lazertinib elicits spontaneous calcium responses in a subset of small-to-medium-sized neurons. Moreover, lazertinib induced spontaneous firings and hyperexcitability in a subset of transient receptor potential vanilloid 1-lineage DRG neurons and sensitized transient receptor potential ankyrin 1 (TRPA1) response, while sparing transient receptor potential vanilloid 1 response. Lazertinib-responsive neurons were also responsive to capsaicin, further supporting that lazertinib selectively activates nociceptive neurons. Lazertinib-induced calcium responses were pharmacologically blocked with HC-030031 (TRPA1 antagonist) and MDL-12330A (adenylyl cyclase inhibitor), suggesting that lazertinib activates sensory neurons through indirect activation of TRPA1. However, unlike vincristine which produces peripheral neuropathy by axonal degeneration, lazertinib did not cause neurite fragmentation in cultured DRG neurons. Finally, intraplantar injection of lazertinib induced TRPA1-dependent pain-like behaviors in vivo. Collectively, our data suggest a direct effect of lazertinib on nociceptive sensory neurons via TRPA1 selective mechanisms, which could be a putative mechanism of lazertinib-induced sensory abnormalities in clinical patients. PERSPECTIVE: This article presents a TRPA1-dependent, lazertinib-induced activation of mouse sensory neurons in vitro and lazertinib-induced pain-like behaviors in vivo. The same mechanisms may underlie the clinical condition, suggesting that TRPA1 could be a potential therapeutic target to manage lazertinib-induced paresthesia.

Widespread latent hyperactivity of nociceptors outlasts enhanced avoidance behavior following incision injury

Nociceptors with somata in dorsal root ganglia (DRGs) exhibit an unusual readiness to switch from an electrically silent state to a hyperactive state of tonic, nonaccommodating, low-frequency, irregular discharge of action potentials (APs). Ongoing activity (OA) during this state is present in vivo in rats months after spinal cord injury (SCI), and has been causally linked to SCI pain. OA induced by various neuropathic conditions in rats, mice, and humans is retained in nociceptor somata after dissociation and culturing, providing a powerful tool for investigating its mechanisms and functions. An important question is whether similar nociceptor OA is induced by painful conditions other than neuropathy. The present study shows that probable nociceptors dissociated from DRGs of rats subjected to postsurgical pain (induced by plantar incision) exhibit OA. The OA was most apparent 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 latent hyperactivity persisted for at least 3 weeks, whereas behavioral indicators of affective pain - hindpaw guarding and increased avoidance of a noxious substrate in an operant conflict test - persisted for 1 week or less. An unexpected discovery was latent OA in neurons from thoracic DRGs that innervate dermatomes distant from the injured tissue. The most consistent electrophysiological alteration associated with OA was enhancement of DSFs. Potential in vivo functions of widespread, low-frequency nociceptor OA consistent with these and other findings are to amplify hyperalgesic priming and to drive anxiety-related hypervigilance.

Diabetes-Induced Amplification of Nociceptive DRG Neuron Output by Upregulation of Somatic T-Type Ca2+ Channels

The development of pain symptoms in peripheral diabetic neuropathy (PDN) is associated with the upregulation of T-type Ca2+ channels (T-channels) in the soma of nociceptive DRG neurons. Moreover, a block of these channels in DRG neurons effectively reversed mechanical and thermal hyperalgesia in animal diabetic models, indicating that T-channel functioning in these neurons is causally linked to PDN. However, no particular mechanisms relating the upregulation of T-channels in the soma of nociceptive DRG neurons to the pathological pain processing in PDN have been suggested. Here we have electrophysiologically identified voltage-gated currents expressed in nociceptive DRG neurons and developed a computation model of the neurons, including peripheral and central axons. Simulations showed substantially stronger sensitivity of neuronal excitability to diabetes-induced T-channel upregulation at the normal body temperature compared to the ambient one. We also found that upregulation of somatic T-channels, observed in these neurons under diabetic conditions, amplifies a single action potential invading the soma from the periphery into a burst of multiple action potentials further propagated to the end of the central axon. We have concluded that the somatic T-channel-dependent amplification of the peripheral nociceptive input to the spinal cord demonstrated in this work may underlie abnormal nociception at different stages of diabetes development.

Green Light Exposure Elicits Anti-inflammation, Endogenous Opioid Release and Dampens Synaptic Potentiation to Relieve Post-surgical Pain

Light therapy improves multiple conditions such as seasonal affective disorders, circadian rhythm dysregulations, and neurodegenerative diseases. However, little is known about its potential benefits in pain management. While current pharmacologic methods are effective in many cases, the associated side effects can limit their use. Non-pharmacological methods would minimize drug dependence, facilitating a reduction of the opioid burden. Green light therapy has been shown to be effective in reducing chronic pain in humans and rodents. However, its underlying mechanisms remain incompletely defined. In this study, we demonstrate that green light exposure reduced postsurgical hypersensitivity in rats. Moreover, this therapy potentiated the antinociceptive effects of morphine and ibuprofen on mechanical allodynia in male rats. Importantly, in female rats, GLED potentiated the antinociceptive effects of morphine but did not affect that of ibuprofen. We showed that green light increases endogenous opioid levels while lessening synaptic plasticity and neuroinflammation. Importantly, this study reveals new insights into how light exposure can affect neuroinflammation and plasticity in both genders. Clinical translation of these results could provide patients with improved pain control and decrease opioid consumption. Given the noninvasive nature of green light, this innovative therapy would be readily implementable in hospitals. PERSPECTIVE: This study provides a potential additional therapy to decrease postsurgical pain. Given the safety, availability, and the efficacy of green light therapy, there is a significant potential for advancing the green light therapy to clinical trials and eventual translation to clinical settings.

Monitoring the delicate operations of surgical robots via ultra-sensitive ionic electronic skin

The arrival of surgical robots in high-end medical equipment is a landmark, and the realization of tactile sensation a major challenge in this important cutting-edge research field. Aiming to address this issue, we present ultra-sensitive ionic electronic skin in the form of flexible capacitive pressure sensors, which incorporate multistage bionic microstructures in ion gels for the purpose of monitoring the delicate operations of surgical robots. Significantly, the ionic skin exhibits an ultra-high sensitivity of 9484.3 kPa^-1 (<15 kPa), and the sensitivity remains higher than 235 kPa^-1 in the wide range of 15-155 kPa. The device has also achieved a detection limit as low as 0.12 Pa or, equivalently, 0.31 mg, fast response within 24 ms, and high robustness (loading/unloading for 5000 cycles without fatigue). The sensor facilitates the challenging task of tele-operated robotic threading, which exceeds the human tactile perception limit when threading a needle. We have also confirmed that ionic skin can be used in robot-assisted invasive surgery, such as incision/resection of tissues and suturing of wounds, providing tactile information to surgeons to improve operation success rates. The flexible ionic skin is capable of conforming to the various shapes of robotic manipulators, thus has great promise for applications in robotic dexterous manipulation, prosthetics and human-machine interfaces.

Examination of the contribution of Nav1.7 to axonal propagation in nociceptors

ABSTRACT

Nav1.7 is a promising drug target for the treatment of pain. However, there is a mismatch between the analgesia produced by Nav1.7 loss-of-function and the peripherally restricted Nav1.7 inhibitors, which may reflect a lack of understanding of the function of Nav1.7 in the transmission of nociceptive information. In the periphery, the role of Nav1.7 in transduction at nociceptive peripheral terminals has been comprehensively examined, but its role in axonal propagation in these neurons is less clearly defined. In this study, we examined the contribution of Nav1.7 to axonal propagation in nociceptors using sodium channel blockers in in vivo electrophysiological and calcium imaging recordings in mice. Using the sodium channel blocker tetrodotoxin (TTX) (1-10 µM) to inhibit Nav1.7 and other tetrodotoxin-sensitive sodium channels along the sciatic nerve, we first showed that around two-thirds of nociceptive L4 dorsal root ganglion neurons innervating the skin, but a lower proportion innervating the muscle (45%), are blocked by TTX. By contrast, nearly all large-sized cutaneous afferents (95%-100%) were blocked by axonal TTX. Many cutaneous nociceptors resistant to TTX were polymodal (57%) and capsaicin sensitive (57%). Next, we applied PF-05198007 (300 nM-1 µM) to the sciatic nerve between stimulating and recording sites to selectively block axonal Nav1.7 channels. One hundred to three hundred nanomolar PF-05198007 blocked propagation in 63% of C-fiber sensory neurons, whereas similar concentrations produced minimal block (5%) in rapidly conducting A-fiber neurons. We conclude that Nav1.7 is essential for axonal propagation in around two-thirds of nociceptive cutaneous C-fiber neurons and a lower proportion (≤45%) of nociceptive neurons innervating muscle.

Objective and Quantitative Evaluation of Spontaneous Pain-Like Behaviors Using Dynamic Weight-Bearing System in Mouse Models of Postsurgical Pain

Background

The paucity of objective and reliable measurements of pain-like behaviors has impeded the translatability of mouse models of postsurgical pain. The advanced dynamic weight-bearing (DWB) system enables evaluation of spontaneous pain-like behaviors in pain models. This study investigated the suitability and efficiency of the DWB system for assessing spontaneous pain-like behaviors and analgesic therapies in murine models of postsurgical pain.

Methods

Male adult C57BL/6JJcl mice were subjected to multiple surgical pain models with distinct levels of invasiveness, including a superficial incisional pain model involving only hind paw skin incision, deep incisional pain model that also involved incision and elevation of the underlying hind paw muscles, and orthopedic pain model involving tibial bone fracture and fixation with a pin (fracture and pinning [F/P] model). Spontaneous pain-like behaviors post-surgery were evaluated using weight distribution, pawprint area of the operated paw in the DWB system, and guarding pain score. Mechanical hypersensitivity was assessed using the von Frey test. The therapeutic effects of analgesics (diclofenac and buprenorphine for the deep incision model and diclofenac for the F/P model) were evaluated using the DWB system and von Frey test.

Results

The von Frey test demonstrated contradictory results between superficial and deep incisional pain models. The DWB system captured weight distribution changes in the operated hind paw, in accordance with the invasiveness and time course of wound healing in these surgical pain models. The reduction in weight-bearing on the operated paw correlated with guarding score, degree of paw swelling, and local expression of inflammatory mediators. DWB enabled accurate evaluation of the pharmacological effects of analgesics for detecting attenuation of surgery-induced weight-bearing changes in these models.

Conclusion

The DWB system serves as an objective and reliable method for quantifying pain-like behaviors and evaluating the therapeutic effects of analgesics in mouse models of postsurgical pain models.

Local effects of natural alkylamides from Acmella oleracea and synthetic isobutylalkyl amide on neuropathic and postoperative pain models in mice

Neuropathic and postoperative pain are clinical conditions that impair the patient's quality of life. The current pharmacotherapy of both painful states is ineffective and accompanied by several side effects. In order to develop new therapeutics targets, the secondary metabolites of plants have been extensively studied. Acmella oleracea ("jambu") is a native plant from the Amazon region and rich in alkylamides, bioactive compounds responsible for inducing anesthetic and chemesthetic sensations. We previously demonstrated that the intraplantar administration of an hexanic fraction (HF) rich in alkylamides from jambu and the synthetic isobutylalkyl amide (IBA) at 0.1 μg/20 μL can promote antinociceptive and anti-inflammatory effects. Thus, this study aimed to evaluate the local effect of HF and IBA (0.1 μg/20 μL) on neuropathic (partial sciatic nerve ligation, PSNL) and postoperative pain (plantar incision surgery, PIS) models in mice. Seven days after the PSNL, the mechanical (von Frey test) and cold (acetone-evoked evaporative cooling) allodynia, and digital gait parameters were analyzed. The intraplantar HF and IBA treatments attenuated the mechanical and cold allodynia as well as the static (max. Contact and print area) and dynamic (stand duration) parameters of digital gait analyses. On the day after PIS, the mechanical allodynia, heat hyperalgesia (hot plate, 52 ± 0.1°C), and spontaneous nociception scores were evaluated. Topical treatment with HF reduced the mechanical allodynia, heat hyperalgesia, and spontaneous nociception scores. In contrast, IBA treatment only partially reduced the mechanical allodynia. In summary, the local treatment with HF was effective on both neuropathic and postoperative pain, as opposed to IBA, which only had an effect on neuropathic pain.

The “WWHow” Concept for Prospective Categorization of Post-operative Severity Assessment in Mice and Rats

The prospective severity assessment in animal experiments in the categories' non-recovery, mild, moderate, and severe is part of each approval process and serves to estimate the harm/benefit. Harms are essential for evaluating ethical justifiability, and on the other hand, they may represent confounders and effect modifiers within an experiment. Catalogs and guidelines provide a way to assess the experimental severity prospectively but are limited in adaptation due to their nature of representing particular examples without clear explanations of the assessment strategies. To provide more flexibility for current and future practices, we developed the modular Where-What-How (WWHow) concept, which applies findings from pre-clinical studies using surgical-induced pain models in mice and rats to provide a prospective severity assessment. The WWHow concept integrates intra-operative characteristics for predicting the maximum expected severity of surgical procedures. The assessed severity categorization is mainly congruent with examples in established catalogs; however, because the WWHow concept is based on anatomical location, detailed analysis of the tissue trauma and other intra-operative characteristics, it enables refinement actions, provides the basis for a fact-based dialogue with authority officials and other stakeholders, and helps to identify confounder factors of study findings.

A Female-Specific Role for Calcitonin Gene-Related Peptide (CGRP) in Rodent Pain Models

We aimed to investigate a sexually dimorphic role of calcitonin gene-related peptide (CGRP) in rodent models of pain. Based on findings in migraine where CGRP has a preferential pain-promoting effect in female rodents, we hypothesized that CGRP antagonists and antibodies would attenuate pain sensitization more efficaciously in female than male mice and rats. In hyperalgesic priming induced by activation of interleukin 6 signaling, CGRP receptor antagonists olcegepant and CGRP_8-37 both given intrathecally, blocked, and reversed hyperalgesic priming only in females. A monoclonal antibody against CGRP, given systemically, blocked priming specifically in female rodents but failed to reverse it. In the spared nerve injury model, there was a transient effect of both CGRP antagonists, given intrathecally, on mechanical hypersensitivity in female mice only. Consistent with these findings, intrathecally applied CGRP caused a long-lasting, dose-dependent mechanical hypersensitivity in female mice but more transient effects in males. This CGRP-induced mechanical hypersensitivity was reversed by olcegepant and the KCC2 enhancer CLP257, suggesting a role for anionic plasticity in the dorsal horn in the pain-promoting effects of CGRP in females. In spinal dorsal horn slices, CGRP shifted GABA_A reversal potentials to significantly more positive values, but, again, only in female mice. Therefore, CGRP may regulate KCC2 expression and/or activity downstream of CGRP receptors specifically in females. However, KCC2 hypofunction promotes mechanical pain hypersensitivity in both sexes because CLP257 alleviated hyperalgesic priming in male and female mice. We conclude that CGRP promotes pain plasticity in female rodents but has a limited impact in males.SIGNIFICANCE STATEMENT The majority of patients impacted by chronic pain are women. Mechanistic studies in rodents are creating a clear picture that molecular events promoting chronic pain are different in male and female animals. We sought to build on evidence showing that CGRP is a more potent and efficacious promoter of headache in female than in male rodents. To test this, we used hyperalgesic priming and the spared nerve injury neuropathic pain models in mice. Our findings show a clear sex dimorphism wherein CGRP promotes pain in female but not male mice, likely via a centrally mediated mechanism of action. Our work suggests that CGRP receptor antagonists could be tested for efficacy in women for a broader variety of pain conditions.

2022 AAHA Pain Management Guidelines for Dogs and Cats

These updated guidelines present a practical and logical approach to the assessment and management of acute and chronic pain in canine and feline patients. Recognizing pain is fundamental to successful treatment, and diagnostic guides and algorithms are included for assessment of both acute and chronic pain. Particularly for chronic pain, capturing owner evaluation is important, and pain-assessment instruments for pet owners are described. Expert consensus emphasizes proactive, preemptive pain management rather than a reactive, "damage control" approach. The guidelines discuss treatment options centered on preemptive, multimodal analgesic therapies. There is an extensive variety of pharmacologic and nonpharmacologic therapeutic options for the management of acute and chronic pain in cats and dogs. The guidelines include a tiered decision tree that prioritizes the use of the most efficacious therapeutic modalities for the treatment of acute and chronic pain.

Innovations and advances in modelling and measuring pain in animals

Best practices in preclinical algesiometry (pain behaviour testing) have shifted over the past decade as a result of technological advancements, the continued dearth of translational progress and the emphasis that funding institutions and journals have placed on rigour and reproducibility. Here we describe the changing trends in research methods by analysing the methods reported in preclinical pain publications from the past 40 years, with a focus on the last 5 years. We also discuss how the status quo may be hampering translational success. This discussion is centred on four fundamental decisions that apply to every pain behaviour experiment: choice of subject (model organism), choice of assay (pain-inducing injury), laboratory environment and choice of outcome measures. Finally, we discuss how human tissues, which are increasingly accessible, can be used to validate the translatability of targets and mechanisms identified in animal pain models.

fNIRS brain measures of ongoing nociception during surgical incisions under anesthesia

Significance: Functional near-infrared spectroscopy (fNIRS) has evaluated pain in awake and anesthetized states. Aim: We evaluated fNIRS signals under general anesthesia in patients undergoing knee surgery for anterior cruciate ligament repair. Approach: Patients were split into groups: those with regional nerve block (NB) and those without (non-NB). Continuous fNIRS measures came from three regions: the primary somatosensory cortex (S1), known to be involved in evaluation of nociception, the lateral prefrontal cortex (BA9), and the polar frontal cortex (BA10), both involved in higher cortical functions (such as cognition and emotion). Results: Our results show three significant differences in fNIRS signals to incision procedures between groups: (1) NB compared with non-NB was associated with a greater net positive hemodynamic response to pain procedures in S1; (2) dynamic correlation between the prefrontal cortex (PreFC) and S1 within 1 min of painful procedures are anticorrelated in NB while positively correlated in non-NB; and (3) hemodynamic measures of activation were similar at two separate time points during surgery (i.e., first and last incisions) in PreFC and S1 but showed significant differences in their overlap. Comparing pain levels immediately after surgery and during discharge from postoperative care revealed no significant differences in the pain levels between NB and non-NB. Conclusion: Our data suggest multiple pain events that occur during surgery using devised algorithms could potentially give a measure of "pain load." This may allow for evaluation of central sensitization (i.e., a heightened state of the nervous system where noxious and non-noxious stimuli is perceived as painful) to postoperative pain levels and the resulting analgesic consumption. This evaluation could potentially predict postsurgical chronic neuropathic pain.

Sympathectomy decreases pain behaviors and nerve regeneration by downregulating monocyte chemokine CCL2 in dorsal root ganglia in the rat tibial nerve crush model

ABSTRACT

Peripheral nerve regeneration is associated with pain in several preclinical models of neuropathic pain. Some neuropathic pain conditions and preclinical neuropathic pain behaviors are improved by sympathetic blockade. In this study, we examined the effect of a localized "microsympathectomy," ie, cutting the gray rami containing sympathetic postganglionic axons where they enter the L4 and L5 spinal nerves, which is more analogous to clinically used sympathetic blockade compared with chemical or surgical sympathectomy. We also examined manipulations of CCL2 (monocyte chemoattractant protein 1), a key player in both regeneration and pain. We used rat tibial nerve crush as a neuropathic pain model in which peripheral nerve regeneration can occur successfully. CCL2 in the sensory ganglia was increased by tibial nerve crush and reduced by microsympathectomy. Microsympathectomy and localized siRNA-mediated knockdown of CCL2 in the lumbar dorsal root ganglion had very similar effects: partial improvement of mechanical hypersensitivity and guarding behavior, reduction of regeneration markers growth-associated protein 43 and activating transcription factor 3, and reduction of macrophage density in the sensory ganglia and regenerating nerve. Microsympathectomy reduced functional regeneration as measured by myelinated action potential propagation through the injury site and denervation-induced atrophy of the tibial-innervated gastrocnemius muscle at day 10. Microsympathectomy plus CCL2 knockdown had behavioral effects similar to microsympathectomy alone. The results show that local sympathetic effects on neuropathic pain may be mediated in a large part by the effects on expression of CCL2, which in turn regulates the regeneration process.

Alpha lipoic acid attenuates evoked and spontaneous pain following surgical skin incision in rats

Our previous studies have implicated CaV3.2 isoform of T-type Ca2+ channels (T-channels) in the development of postsurgical pain. We have also previously established that different T-channel antagonists can alleviate in vivo postsurgical pain. Here we investigated the analgesic potential of another T-channel blocker and endogenous antioxidant molecule, α-lipoic acid (ALA), in a postsurgical pain model in rats. Our in vivo results suggest that single and repetitive intraperitoneal injections of ALA after surgery or preemptively, significantly reduced evoked mechanical hyperalgesia following surgical paw incision. Furthermore, repeated preemptive systemic injections of ALA effectively alleviated spontaneous postsurgical pain as determined by dynamic weight-bearing testing. We expect that our preclinical study may lead to further investigation of analgesic properties and mechanisms of analgesic action of ALA in patients undergoing surgery.

Functional Characterization of Ovine Dorsal Root Ganglion Neurons Reveal Peripheral Sensitization after Osteochondral Defect

Knee joint trauma can cause an osteochondral defect (OD), a risk factor for osteoarthritis (OA) and cause of debilitating pain in patients. Rodent OD models are less translatable because of their smaller joint size and open growth plate. This study proposes sheep as a translationally relevant model to understand the neuronal basis of OD pain. A unilateral 6-mm deep OD was induced in adult female sheep. Two to six weeks after operation, lumbar dorsal root ganglia (DRG) neurons were collected from the contralateral (Ctrl) and OD side of operated sheep. Functional assessment of neuronal excitability and activity of the pain-related ion channels transient receptor potential vanilloid receptor 1 (TRPV1) and P2X3 was conducted using electrophysiology and Ca²⁺ imaging. Immunohistochemistry was used to verify expression of pain-related proteins. We observed that an increased proportion of OD DRG neurons (sheep, N = 3; Ctrl neurons, n = 15, OD neurons, n = 16) showed spontaneous electrical excitability (Ctrl: 20.33 ± 4.5%; OD: 50 ± 10%; p = 0.009, unpaired t test) and an increased proportion fired a greater number of spikes above baseline in response to application of a TRPV1 agonist (capsaicin) application (Ctrl: 40%; OD: 75%; p = 0.04, χ² test). Capsaicin also produced Ca²⁺ influx in an increased proportion of isolated OD DRG neurons (Ctrl: 25%; OD: 44%; p = 0.001, χ² test). Neither protein expression, nor functionality of the P2X3 ion channel were altered in OD neurons. Overall, we provide evidence of increased excitability of DRG neurons (an important neural correlate of pain) and TRPV1 function in an OD sheep model. Our data show that functional assessment of sheep DRG neurons can provide important insights into the neural basis of OD pain and thus potentially prevent its progression into arthritic pain.

Inhibiting endocytosis in CGRP+ nociceptors attenuates inflammatory pain-like behavior

The advantage of locally applied anesthetics is that they are not associated with the many adverse effects, including addiction liability, of systemically administered analgesics. This therapeutic approach has two inherent pitfalls: specificity and a short duration of action. Here, we identified nociceptor endocytosis as a promising target for local, specific, and long-lasting treatment of inflammatory pain. We observed preferential expression of AP2α2, an α-subunit isoform of the AP2 complex, within CGRP⁺/IB4^- nociceptors in rodents and in CGRP⁺ dorsal root ganglion neurons from a human donor. We utilized genetic and pharmacological approaches to inhibit nociceptor endocytosis demonstrating its role in the development and maintenance of acute and chronic inflammatory pain. One-time injection of an AP2 inhibitor peptide significantly reduced acute and chronic pain-like behaviors and provided prolonged analgesia. We evidenced sexually dimorphic recovery responses to this pharmacological approach highlighting the importance of sex differences in pain development and response to analgesics.

The authors show the endocytotic adaptor subunit called AP2A2 is differentially expressed in CGRP⁺ nociceptors. Locally inhibiting nociceptor endocytosis with a lipidated AP2 inhibitor peptide reduces acute and chronic pain-like behaviour in mice and rats, indicating prolonged analgesia.

Intrathecal Injection of GRIP-siRNA Reduces Postoperative Synaptic Abundance of Kainate Receptor GluK2 Subunits in Rat Dorsal Horns and Pain Hypersensitivity

The mechanisms underlying postoperative pain differ from the inflammatory or neuropathic pain. Previous studies have demonstrated that intrathecal α-amino-3-hydroxy-5-methy-4-isoxazole propionate (AMPA) -kainate (KA) receptor antagonist inhibits the guarding pain behavior and mechanical hyperalgesia, indicating a critical role of spinal KA receptors in postoperative pain hypersensitivity. However, how the functional regulations of spinal KA receptor subunits are involved in the postoperative pain hypersensitivity remains elusive. Therefore, in the current study, we investigated the synaptic delivery of spinal KA receptor subunits and the interaction between KA receptor subunits and glutamate receptor-interacting protein (GRIP) during the postoperative pain. Our data indicated that plantar incision induced the synaptic delivery of GluK2, but not GluK1 or GluK3 in ipsilateral spinal cord dorsal horns. The co-immunoprecipitation showed an increased GluK2 -GRIP interaction in ipsilateral dorsal horn neurons at 6 h post-incision. Interestingly, Intrathecal pretreatment of GRIP siRNA increased the paw withdrawal thresholds to mechanical stimuli and decreased the cumulative pain scores in the paws ipsilateral to the incision at 6 h post-incision. Additionally, Intrathecal pretreatment of GRIP siRNA reduced the synaptic abundance of GluK2 in ipsilateral spinal dorsal horn at 6 h after plantar incision. In general, our data have demonstrated that the GluK2- GRIP interaction-mediated synaptic abundance of GluK2 in dorsal horn neurons plays an important role in the postoperative pain hypersensitivity. Disrupting the GluK2- GRIP interaction may provide a new approach for relieving postoperative pain.

Nonsurgical mouse model of endometriosis-associated pain that responds to clinically active drugs

Endometriosis is an estrogen-dependent inflammatory disease that affects approximately 10% of women. Debilitating pelvic or abdominal pain is one of its major clinical features. Current animal models of endometriosis-associated pain require surgery either to implant tissue or to remove the ovaries. Moreover, existing models do not induce spontaneous pain, which is the primary symptom of patients with chronic pain, including endometriosis. A lack of models that accurately recapitulate the disease phenotype must contribute to the high failure rate of clinical trials for analgesic drugs directed at chronic pain, including those for endometriosis. We set out to establish a murine model of endometriosis-associated pain. Endometriosis was induced nonsurgically by injecting a dissociated uterine horn into a recipient mouse. The induced lesions exhibited histological features that resemble human lesions along with an increase in proinflammatory cytokines and recruitment of immune cells. We also observed the presence of calcitonin gene-related peptide-, TRPA1-, and TRPV1-expressing nerve fibers in the lesions. This model induced mechanical allodynia, spontaneous abdominal pain, and changes in thermal selection behavior that indicate discomfort. These behavioral changes were reduced by drugs used clinically for endometriosis, specifically letrozole (aromatase inhibitor) and danazol (androgen). Endometriosis also induced neuronal changes as evidenced by activation of the NF-κB signaling pathway in TRPA1- and TRPV1-expressing dorsal root ganglion neurons. In conclusion, we have established a model of endometriosis-associated pain that responds to clinically active drugs and can, therefore, be used to identify novel therapies.

Plantar incision with severe muscle injury can be a cause of long-lasting postsurgical pain in the skin

AIMS

Although chronic local inflammation in deeper tissues after skin wound healing might produce chronification of acute postsurgical pain, its mechanisms have not been fully elucidated. We hypothesized that muscle injury and severe inflammation would prolong acute postsurgical pain by its central nervous system mechanisms.

MAIN METHODS

After approval of the Animal Care Committee, experiments were performed in Male Sprague-Dawley rats weighing 250-300 g. Plantar incision and plantar incision combined with cryoinjury of the plantar flexor digitorum brevis muscle were made in the plantar incision group and muscle injury group, respectively. Pain-related behaviors were assessed, and inflammatory cells were isolated from injured muscle and analyzed by flow cytometry. Spinal microglial activation was assessed with Iba-1 staining.

KEY FINDINGS

Mechanical hyperalgesia from day 5 to day 8 and spontaneous pain-related behavior from day 3 to day 7 were significantly greater in the muscle injury group than in the plantar incision group (P < 0.05), whereas there was no significant difference between the two groups in thermal hyperalgesia. In the muscle injury group, the number of inflammatory cells on day 4 was significantly larger and spinal Iba-1 expression levels on days 4 and 7 were significantly higher than those in the plantar incision group (P < 0.05).

SIGNIFICANCE

Surgical injury in deep tissues accompanying severe muscle inflammation induced prolonged postsurgical pain in the healing wound of the skin not by the persistence of muscle inflammation but by a central mechanism involving microglial activation at the level of the spinal cord.

The Effect and Possible Mechanism of Intradiscal Injection of Simvastatin in the Treatment of Discogenic Pain in Rats

To study the effect of intradiscal injection of simvastatin on discogenic pain in rats and its possible mechanism, 30 adult female rats were used in this experiment. Twenty rats were randomly divided into sham operation group (Control group), intervertebral disk degeneration group (DDD group), intervertebral disk degeneration + hydrogel group (DDD + GEL group), and intervertebral disk degeneration + simvastatin group (DDD + SIM group). The mechanical pain threshold and cold sensation in rats were measured. The contents of NF-kappa B1, RelA, GAP43, SP, CGRP, TRPM 8, IL-1β, and TNF-α in the intervertebral disk (IVD), the corresponding contents of dorsal root ganglion (DRG) and plantar skin GAP43 and TRPM 8 were quantitatively detected by PCR. The corresponding IVDs were stained to detect their degeneration. There was no significant difference in the mechanical pain threshold between the groups at each time point. From the first day to the 8th week after surgery, the cold-sensing response of the DDD group was significantly higher than that of the Control group (P < 0.05). At 7 and 8 weeks postoperatively, the cold-sensing response of the DDD + SIM group was significantly lower than that of the DDD + GEL group (P < 0.05). The levels of NF-κB1, RelA, GAP43, SP, CGRP, TRPM8, IL-1β, and TNF-α in the IVD of DDD + SIM group were significantly lower than those in DDD group (P < 0.05). The content of GAP43 and TRPM8 in rat plantar skin decreased significantly and TRPM8 in DRG decreased significantly (P < 0.05).

Stay or go? Neuronal activity in medial frontal cortex during a voluntary tactile preference task in head-fixed mice

The decision to move is influenced by sensory, attentional, and motivational cues. One such cue is the quality of the tactile input, with noxious or unpleasant sensations causing an animal to move away from the cue. Processing of painful and unpleasant sensation in the cortex involves multiple brain regions, although the specific role of the brain areas involved in voluntary, rather than reflexive movement away from unpleasant stimuli is not well understood. Here, we focused on the medial subdivision of secondary motor cortex, which is proposed to link sensory and contextual cues to motor action, and tested its role in controlling voluntary movement in the context of an aversive tactile cue. We designed a novel, 3D-printed tactile platform consisting of innocuous (grid) and mildly noxious (spiked) surfaces (50:50 % of total area), which enabled monitoring neuronal activity in the medial frontal cortex by two-photon imaging during a sensory preference task in head-fixed mice. We found that freely moving mice spent significantly less time on a spiked-surface, and that this preference was eliminated by administration of a local anesthetic. At the neuronal level, individual neurons were differentially modulated specific to the tactile surface encountered. At the population level, the neuronal activity was analyzed in relation to the events where mice chose to "stop-on" or "go-from" a specific tactile surface and when they "switched" surfaces without stopping. Notably, each of these three scenarios showed population activity that differed significantly between the grid and spiked tactile surfaces. Collectively, these data provide evidence that tactile quality is encoded within medial frontal cortex. The task pioneered in this study provides a valuable tool to better evaluate mouse models of nociception and pain, using a voluntary task that allows simultaneous recording of preference and choice.

Serotonin enhances depolarizing spontaneous fluctuations, excitability, and ongoing activity in isolated rat DRG neurons via 5-HT4 receptors and cAMP-dependent mechanisms

Ongoing activity in nociceptors, a driver of spontaneous pain, can be generated in dorsal root ganglion neurons in the absence of sensory generator potentials if one or more of three neurophysiological alterations occur - prolonged depolarization of resting membrane potential (RMP), hyperpolarization of action potential (AP) threshold, and/or increased amplitude of depolarizing spontaneous fluctuations of membrane potential (DSFs) to bridge the gap between RMP and AP threshold. Previous work showed that acute, sustained exposure to serotonin (5-HT) hyperpolarized AP threshold and potentiated DSFs, leading to ongoing activity if a separate source of maintained depolarization was present. Cellular signaling pathways that increase DSF amplitude and promote ongoing activity acutely in nociceptors are not known for any neuromodulator. Here, isolated DRG neurons from male rats were used to define the pathway by which low concentrations of 5-HT enhance DSFs, hyperpolarize AP threshold, and promote ongoing activity. A selective 5-HT₄ receptor antagonist blocked these 5-HT-induced hyperexcitable effects, while a selective 5-HT₄ agonist mimicked the effects of 5-HT. Inhibition of cAMP effectors, protein kinase A (PKA) and exchange protein activated by cAMP (EPAC), attenuated 5-HT's hyperexcitable effects, but a blocker of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels had no significant effect. 5-HT₄-dependent PKA activation was specific to DRG neurons that bind isolectin B4 (a nonpeptidergic nociceptor marker). 5-HT's effects on AP threshold, DSFs, and ongoing activity were mimicked by a cAMP analog. Sustained exposure to 5-HT promotes ongoing activity in nonpeptidergic nociceptors through the G_s-coupled 5-HT₄ receptor and downstream cAMP signaling involving both PKA and EPAC.

The Protective Effects of Pre- and Post-Administration of Micronized Palmitoylethanolamide Formulation on Postoperative Pain in Rats

Background: Postoperative pain (PO) is a common form of acute pain. Inadequate PO treatment is an important health problem, as it leads to worse outcomes, such as chronic post-surgical pain. Therefore, it is necessary to acquire new knowledge on PO mechanisms to develop therapeutic options with greater efficacy than those available today and to lower the risk of adverse effects. For this reason, we evaluated the ability of micronized palmitoylethanolamide (PEA-m) to resolve the pain and inflammatory processes activated after incision of the hind paw in an animal model of PO. Methods: The animals were subjected to surgical paw incision and randomized into different groups. PEA-m was administered orally at 10 mg/kg at different time points before or after incision. Results: Our research demonstrated that the pre- and post-treatment with PEA-m reduced the activation of mast cells at the incision site and the expression of its algogenic mediator nerve growth factor (NGF) in the lumbar spinal cord. Furthermore, again at the spinal level, it was able to decrease the activation of phospho-extracellular signal-regulated kinases (p-ERK), ionized calcium binding adaptor molecule 1 (Iba1), glial fibrillary acidic protein (GFAP), and the expression of brain-derived neurotrophic factor (BDNF). PEA-m also reduced the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) spinal pathway, showing a protective effect in a rat model of PO. Conclusion: The results obtained reinforce the idea that PEA-m may be a potential treatment for the control of pain and inflammatory processes associated with PO. In addition, pre- and post-treatment with PEA-m is more effective than treatment alone after the surgery and this limits the time of taking the compound and the abuse of analgesics.

Phα1β, a dual blocker of TRPA1 and Cav2.2, as an adjuvant drug in opioid therapy for postoperative pain

Opioids are the "gold standard" treatment for postoperative pain, but these drugs also have limiting adverse effects. Thus, adjuvant drugs might be useful in opioid therapy for postoperative pain. The aim of the present study was to evaluate the effect of Phα1β, a dual blocker of Cav2 and TRPA1 channels, on antinociceptive and adverse actions of morphine in a model of postoperative pain. Phα1β (100-300 pmol/site) or morphine (3-10 mg/kg), alone, largely reduced postoperative nociception. However, Phα1β (100 pmol/site) or morphine (10 mg/kg) also produced motor impairment. Lower doses of Phα1β (30 pmol/site) or morphine (1 mg/kg), that did not have an effect alone, showed antinociceptive effect when concomitantly administrated. Moreover, co-administration of Phα1β (30 pmol/site) with morphine (1 or 10 mg/kg) was unable to cause motor impairment. Preoperative repeated treatment with morphine increased the expression of Cav2 and TRPA1 channels in spinal cord, and caused tolerance and withdrawal syndrome, which were reversed with a single injection of Phα1β (30 pmol/site). When injected postoperatively, escalating doses of morphine worsened postoperative hyperalgesia, induced tolerance, and withdrawal syndrome. Similarly, Phα1β (30 pmol/site) reversed these adverse effects. Single or repeated morphine caused constipation, which was not altered by Phα1β. Thus, a low dose of Phα1β potentiated the analgesia, and reversed some adverse effects of morphine on operated mice, indicating the potential use of this agent as an adjuvant drug in opioid therapy for postoperative pain.

Sham surgeries for central and peripheral neural injuries persistently enhance pain-avoidance behavior as revealed by an operant conflict test

Studies using rodent models of neuropathic pain use sham surgery control procedures that cause deep tissue damage. Sham surgeries would thus be expected to induce potentially long-lasting postsurgical pain, but little evidence for such pain has been reported. Operant tests of voluntary behavior can reveal negative motivational and cognitive aspects of pain that may provide sensitive tools for detecting pain-related alterations. In a previously described operant mechanical conflict test involving lengthy familiarization and training, rodents freely choose to either escape from a brightly lit chamber by crossing sharp probes or refuse to cross. Here, we describe a brief (2-day) mechanical conflict protocol that exploits rats' innate exploratory response to a novel environment to detect persistently enhanced pain-avoidance behavior after sham surgeries for 2 neural injury models: thoracic spinal cord injury and chronic constriction injury of the sciatic nerve. Pitting the combined motivations to avoid the bright light and to explore the novel device against pain from crossing noxious probes disclosed a conflicting, hyperalgesia-related reluctance to repeatedly cross the probes after injury. Rats receiving standard sham surgeries demonstrated enhanced pain-like avoidance behavior compared with naive controls, and this behavior was similar to that of corresponding chronic constriction injury or spinal cord injury rats weeks or months after injury. In the case of sham surgery for spinal cord injury, video analysis of voluntary exploratory behavior directed at the probes revealed enhanced forepaw withdrawal responses. These findings have important implications for preclinical investigations into behavioral alterations and physiological mechanisms associated with postsurgical and neuropathic pain.

Contribution of T-Type Calcium Channels to Spinal Cord Injury-Induced Hyperexcitability of Nociceptors

A hyperexcitable state and spontaneous activity of nociceptors have been suggested to play a critical role in the development of chronic neuropathic pain following spinal cord injury (SCI). In male rats, we employed the action potential-clamp technique to determine the underlying ionic mechanisms responsible for driving SCI-nociceptors to a hyperexcitable state and for triggering their spontaneous activity. We found that the increased activity of low voltage activated T-type calcium channels induced by the injury sustains the bulk (∼60-70%) of the inward current active at subthreshold voltages during the interspike interval in SCI-nociceptors, with a modest contribution (∼10-15%) from tetrodotoxin (TTX)-sensitive and TTX-resistant sodium channels and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. In current-clamp recordings, inhibition of T-type calcium channels with 1 μm TTA-P2 reduced both the spontaneous and the evoked firing in response to current injections in SCI-nociceptors to a level similar to sham-nociceptors. Electrophysiology in vitro was then combined with the conditioned place preference (CPP) paradigm to determine the relationship between the increased activity of T-type channels in SCI-nociceptors and chronic neuropathic pain following SCI. The size of the interspike T-type calcium current recorded from nociceptors isolated from SCI rats showing TTA-P2-induced CPP (responders) was ∼6 fold greater than the interspike T-type calcium current recorded from nociceptors isolated from SCI rats without TTA-P2-induced CPP (non-responders). Taken together, our data suggest that the increased activity of T-type calcium channels induced by the injury plays a primary role in driving SCI-nociceptors to a hyperexcitable state and contributes to chronic neuropathic pain following SCI.SIGNIFICANCE STATEMENT Chronic neuropathic pain is a major comorbidity of spinal cord injury (SCI), affecting up to 70-80% of patients. Anticonvulsant and tricyclic antidepressant drugs are first line analgesics used to treat SCI-induced neuropathic pain, but their efficacy is very limited. A hyperexcitable state and spontaneous activity of SCI-nociceptors have been proposed as a possible underlying cause for the development of chronic neuropathic pain following SCI. Here, we show that the increased activity of T-type calcium channels induced by the injury plays a major role in driving SCI-nociceptors to a hyperexcitable state and for promoting their spontaneous activity, suggesting that T-type calcium channels may represent a pharmacological target to treat SCI-induced neuropathic pain.

Localized sympathectomy reduces peripheral nerve regeneration and pain behaviors in 2 rat neuropathic pain models

Previous studies have shown that the peripheral nerve regeneration process is linked to pain in several neuropathic pain models. Other studies show that sympathetic blockade may relieve pain in some pain models and clinical conditions. This study examined reduction in peripheral nerve regeneration as one possible mechanism for relief of neuropathic pain by sympathetic blockade. A "microsympathectomy," consisting of cutting the gray rami containing sympathetic postganglionic axons where they enter the L4 and L5 spinal nerves, reduced mechanical hypersensitivity in 2 different rat neuropathic pain models. In the spinal nerve ligation model, in which some functional regeneration and reinnervation of the ligated spinal nerve can be observed, microsympathectomy reduced functional and anatomical measures of regeneration as well as expression of growth-associated protein 43 (GAP43), a regeneration-related protein. In the spared nerve injury model, in which functional reinnervation is not possible and the futile regeneration process results in formation of a neuroma, microsympathectomy reduced neuroma formation and GAP43 expression. In both models, microsympathectomy reduced macrophage density in the sensory ganglia and peripheral nerve. This corroborates previous work showing that sympathetic nerves may locally affect immune function. The results further highlight the challenge of improving pain in neuropathic conditions without inhibiting peripheral nerve regeneration that might otherwise be possible and desired.

The role of the spinal cyclooxygenase (COX) for incisional pain in rats at different developmental stages

BACKGROUND

Cyclooxygenase enzymes (COX)-1 and COX-2 are important targets for pain relief after surgery, but the spinal contribution of both isoforms is still unclear, e.g., from a developmental point of view. Here, we studied changes of spinal COX-1 and COX-2 expression and their functional relevance in rats of different ages for pain-related behaviour after incision.

METHODS

Mechanical paw withdrawal thresholds (PWT) were assessed before and after incision and after intrathecal administration (IT) of SC-560 (COX-1 inhibitor) or NS-398 (COX-2 inhibitor) in rats aged 5, 14 and 28 days (P5, P14, P28). Furthermore, spinal expressions of COX m-RNA and proteins were investigated.

RESULTS

In P5 rats, only IT-administered NS-398 but not SC-560 significantly reversed the decreased PWT after incision. In P14 rats, none of the substance modified PWT, and in P28 rats, only SC-560 increased PWT. Spinal COX-2 mRNA and protein were increased in P5 but not in P14 and P28 rats after incision. Whereas COX-2 is located in spinal neurons, COX-1 is mainly found in spinal microglia cells.

CONCLUSION

Our results demonstrate a possible developmental transition from COX-2 to COX-1 activation. Whereas in adult rats spinal COX-1 but not COX-2 is involved in pain-related behaviour after incision, it seems opposite in P5 rats. Interestingly, in P14, neither COX-1 nor COX-2 seems to play a role. This switch may relate to altered neuronal/microglia activation. Our findings indicate specific mechanisms to pain after incision that are age-dependent and may guide further research improving paediatric pain management.

SIGNIFICANCE

Postoperative pain in pediatric patients after surgery is still poorly controlled; this might contribute to long-lasting alteration in the nociceptive system and prolonged chronic pain. Here we show a possible developmental switch in the COX-dependent pathway for nociceptive spinal transmission that may explain why pain management in young children needs to be related to age-dependent mechanisms.

Adaptive mechanisms driving maladaptive pain: how chronic ongoing activity in primary nociceptors can enhance evolutionary fitness after severe injury

Chronic pain is considered maladaptive by clinicians because it provides no apparent protective or recuperative benefits. Similarly, evolutionary speculations have assumed that chronic pain represents maladaptive or evolutionarily neutral dysregulation of acute pain mechanisms. By contrast, the present hypothesis proposes that chronic pain can be driven by mechanisms that evolved to reduce increased vulnerability to attack from predators and aggressive conspecifics, which often target prey showing physical impairment after severe injury. Ongoing pain and anxiety persisting long after severe injury continue to enhance vigilance and behavioural caution, decreasing the heightened vulnerability to attack that results from motor impairment and disfigurement, thereby increasing survival and reproduction (fitness). This hypothesis is supported by evidence of animals surviving and reproducing after traumatic amputations, and by complex specializations that enable primary nociceptors to detect local and systemic signs of injury and inflammation, and to maintain low-frequency discharge that can promote ongoing pain indefinitely. Ongoing activity in nociceptors involves intricate electrophysiological and anatomical specializations, including inducible alterations in the expression of ion channels and receptors that produce persistent hyperexcitability and hypersensitivity to chemical signals of injury. Clinically maladaptive chronic pain may sometimes result from the recruitment of this powerful evolutionary adaptation to severe bodily injury. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.

Effect of Thoracic Epidural Anesthesia in a Rat Model of Phrenic Motor Inhibition after Upper Abdominal Surgery

WHAT WE ALREADY KNOW ABOUT THIS TOPIC

WHAT THIS ARTICLE TELLS US THAT IS NEW: BACKGROUND:: One important example of impaired motor function after surgery is diaphragmatic dysfunction after upper abdominal surgery. In this study, the authors directly recorded efferent phrenic nerve activity and determined the effect of the upper abdominal incision. The authors hypothesized that phrenic motor output would be decreased after the upper abdominal incision; it was also hypothesized that blocking sensory input from the incision using thoracic epidural anesthesia would diminish this incision-induced change in phrenic motor activity.

METHODS

Efferent phrenic activity was recorded 1 h to 10 days after upper abdominal incision in urethane-anesthetized rats. Ventilatory parameters were measured in unanesthetized rats using whole-body plethysmography at multiple time points after incision. The authors then determined the effect of thoracic epidural anesthesia on phrenic nerve activity and ventilatory parameters after incision.

RESULTS

Phrenic motor output remained reduced by approximately 40% 1 h and 1 day after incision, but was not different from the sham group by postoperative day 10. One day after incision (n = 9), compared to sham-operated animals (n = 7), there was a significant decrease in spike frequency area-under-the-curve (median [interquartile range]: 54.0 [48.7 to 84.4] vs. 97.8 [88.7 to 130.3]; P = 0.0184), central respiratory rate (0.71 [0.63 to 0.79] vs. 0.86 [0.82 to 0.93]/s; P = 0.0460), and inspiratory-to-expiratory duration ratio (0.46 [0.44 to 0.55] vs. 0.78 [0.72 to 0.93]; P = 0.0023). Unlike humans, a decrease, not an increase, in breathing frequency has been observed after the abdominal incision in whole-body plethysmography. Thoracic epidural anesthesia attenuated the incision-induced changes in phrenic motor output and ventilatory parameters.

CONCLUSIONS

Upper abdominal incision decreased phrenic motor output and ventilatory parameters, and this incision-induced impairment was attenuated by thoracic epidural anesthesia. The authors' results provide direct evidence that afferent inputs from the upper abdominal incision induce reflex inhibition of phrenic motor activity.

NOD-like receptor protein 3 inflammasome drives postoperative mechanical pain in a sex-dependent manner

Postoperative pain management continues to be suboptimal because of the lack of effective nonopioid therapies and absence of understanding of sex-driven differences. Here, we asked how the NLRP3 inflammasome contributes to postoperative pain. Inflammasomes are mediators of the innate immune system that are responsible for activation and secretion of IL-1β upon stimulation by specific molecular signals. Peripheral IL-1β is known to contribute to the mechanical sensitization induced by surgical incision. However, it is not known which inflammasome mediates the IL-1β release after surgical incision. Among the 9 known inflammasomes, the NLRP3 inflammasome is ideally positioned to drive postoperative pain through IL-1β production because NLRP3 can be activated by factors that are released by incision. Here, we show that male mice that lack NLRP3 (NLRP3) recover from surgery-induced behavioral and neuronal mechanical sensitization faster and display less surgical site inflammation than mice expressing NLRP3 (wild-type). By contrast, female NLRP3 mice exhibit minimal attenuation of the postoperative mechanical hypersensitivity and no change in postoperative inflammation compared with wild-type controls. Sensory neuron-specific deletion of NLRP3 revealed that in males, NLRP3 expressed in non-neuronal cells and potentially sensory neurons drives postoperative pain. However, in females, only the NLRP3 that may be expressed in sensory neurons contributes to postoperative pain where the non-neuronal cell contribution is NLRP3 independent. This is the first evidence of a key role for NLRP3 in postoperative pain and reveals immune-mediated sex differences in postoperative pain.

Comparison of the peripheral antinociceptive effect of somatostatin with bupivacaine and morphine in the rodent postoperative pain model

BACKGROUND AND OBJECTIVES

Infiltration of surgical wound with local anaesthetics attenuate postoperative pain. However, side effects can also occur. Somatostatin (SST) and its analogues like octreotide reportedly reduce peripheral sensitisation. The current study evaluates peripherally mediated antinociceptive effect of SST in a rat model of postoperative pain. This was compared with bupivacaine and morphine under identical experimental conditions.

DESIGN

Randomised vehicle-controlled blind study.

SETTING

Pain research laboratory, All India Institute of Medical Sciences, New Delhi from February 2014 to July 2017.

EXPERIMENTAL SUBJECT

Rodent hind paw incision model.

INTERVENTIONS

Sprague-Dawley rats were subjected to incision and one of the following drugs administered into the open wound once by a micropipette: SST (10, 30 or 100 μg), bupivacaine (3, 10, 30, 50 or 100 μg) or morphine (100 μg). Antinociceptive effect of SST was further evaluated for its reversibility, site of action, effect on spinal c-fos expression and blood glucose level. The site of action of morphine was also investigated.

MAIN OUTCOME MEASURE

Nociception was estimated by nonevoked (guarding behaviour) and evoked (mechanical allodynia and thermal hyperalgesia) pain behaviours between 2 h and days 4 to 7.

RESULTS

Nociception was maximum 2 h after incision. SST (10 to 100 μg) significantly attenuated guarding behaviour between 2 h and day 2. A delayed inhibitory effect was observed on allodynia. Bupivacaine (10 to 100 μg doses) similarly decreased guarding score up to day 2 though evoked pain behaviours were relatively unaffected. In contrast, morphine produced a potent but transient inhibitory effect on guarding score at 2 h, which was mediated by both peripheral and central opioid receptors. The antinociceptive effect of SST was peripherally mediated by type 2 receptors and was associated with decreased c-fos staining. Blood glucose level was unaltered.

CONCLUSION

Guarding behaviour, which likely represents pain-at-rest following surgery, was attenuated by both bupivacaine and SST to comparable extents. This novel peripherally mediated antinociceptive effect of SST needs further evaluation.

Interleukin 1β inhibition contributes to the antinociceptive effects of voluntary exercise on ischemia/reperfusion-induced hypersensitivity

Issues of peripheral circulation have been increasingly suggested as an underlying cause of musculoskeletal pain in many conditions, including sickle cell anemia and peripheral vascular disease. We have previously shown in our model of transient ischemia and reperfusion (I/R) injury of the forelimb that individual group III and IV muscle afferents display altered chemosensitivity and mechanical thresholds 1 day after injury. Functional alterations corresponded to increased evoked and spontaneous pain-related behaviors and decreased muscle strength and voluntary activity-all actions that echo clinical symptoms of ischemic myalgia. These behavioral and physiological changes appeared to originate in part from the action of increased interleukin 1β (IL1β) in the injured muscles at its upregulated IL1 receptor 1 within the dorsal root ganglion. Here, we describe that two days of voluntary wheel running prior to I/R blocks both injury-induced IL1β enhancement and the subsequent development of ischemic myalgia-like behaviors. Furthermore, the protective effects of 2 days prior exercise on the I/R-evoked increases in pain-related behaviors were also paralleled with systemic injection of the IL1 receptor antagonist during I/R. Interleukin 1 receptor antagonist treatment additionally prevented the I/R-induced changes in mechanical and chemical sensitivity of individual primary muscle afferents. Altogether, these data strengthen the evidence that transient I/R injury sensitizes group III and IV muscle afferents via increased IL1β in the muscles to stimulate ischemic myalgia development. Targeting IL1β may, therefore, be an effective treatment strategy for this insidious type of muscle pain.

Depiction of Oral Tumor-Induced Trigeminal Afferent Responses Using Single-Fiber Electrophysiology

Considerable gap in knowledge exists about the mechanisms by which oral tumors regulate peripheral sensory fibers to produce pain and altered sensations. To address this gap, we used a murine model of oral squamous cell carcinoma (OSCC) of the tongue to investigate changes in response properties of trigeminal afferent neurons. Using this model, we developed an ex vivo method for single neuron recordings of the lingual nerve from isolated tongue tissue. Our data demonstrated that the tongue tumor produced increased spontaneous firing of lingual fibers compared to control as well as produced mechanical hypersensitivity and reduced von Frey thresholds of C- and A-slow-high-threshold mechanoreceptors (HTMR) fibers but had no effect on C-LTMR, A-slow-LTMR and A-fast lingual fibers. Mechanically-insensitive fibers were also detected in lingual afferents of the control group, that were significantly decreased in tumor-bearing preparations. Collectively, using single fiber electrophysiology of lingual sensory fibers, we show that human OSCC tumors sensitize peripheral trigeminal nerve terminals, providing a unique opportunity to study mechanisms of oral cancer pain.

Lesion development is modulated by the natural estrous cycle and mouse strain in a minimally invasive model of endometriosis

Many rodent models of endometriosis are invasive, involving surgery to implant donor endometrial tissue into recipient animals. Moreover, few studies have compared and contrasted lesions between rodent strains and estrous stages without exogenous hormone manipulation. This is despite extensive data demonstrating that genetic and hormonal factors can influence endometriosis progression. Here, we have refined a minimally invasive model of endometriosis using naturally cycling mice (donor and recipient matched for cycle phase) to investigate lesion development in two different strains (C57BL/6 and BALB/c), induced in estrous stages of high and low estrogen (proestrus or estrus, respectively), and with varying amounts of donor endometrial tissue (7.5-40 mg), injected intraperitoneally. The overall probability of developing endometriosis-like lesions was higher in proestrus than estrus, and increased with greater masses of donor tissue. Similarly, the total number of lesions (0-3) increased from 7.5 to 40 mg, and was significantly greater in proestrus C57BL/6 mice but not BALB/cs. The dominant lesion type also differed between mouse strains; C57BL/6 mice were more likely to develop dense-type lesions, whereas BALB/c mice developed a greater proportion of cystic type. These data further support a role for estrogen in the development of endometriosis, and that genetic variance can influence the degree and characteristics of lesions. Our minimally invasive model would be beneficial for studies with outcome measurements particularly sensitive to incisional injury, such as pain, or alterations to sex hormones, including fertility.

Role of NaV1.6 and NaVβ4 Sodium Channel Subunits in a Rat Model of Low Back Pain Induced by Compression of the Dorsal Root Ganglia

Low back pain is a common cause of chronic pain and disability. It is modeled in rodents by chronically compressing the lumbar dorsal root ganglia (DRG) with small metal rods, resulting in ipsilateral mechanical and cold hypersensitivity, and hyperexcitability of sensory neurons. Sodium channels are implicated in this hyperexcitability, but the responsible isoforms are unknown. In this study, we used siRNA-mediated knockdown of the pore-forming Na_V1.6 and regulatory Na_Vβ4 sodium channel isoforms that have been previously implicated in a different model of low back pain caused by locally inflaming the L5 DRG. Knockdown of either subunit markedly reduced spontaneous pain and mechanical and cold hypersensitivity induced by DRG compression, and reduced spontaneous activity and hyperexcitability of sensory neurons with action potentials <1.5 msec (predominately cells with myelinated axons, based on conduction velocities measured in a subset of cells) 4 days after DRG compression. These results were similar to those previously obtained in the DRG inflammation model and some neuropathic pain models, in which sensory neurons other than nociceptors seem to play key roles. The cytokine profiles induced by DRG compression and DRG inflammation were also very similar, with upregulation of several type 1 pro-inflammatory cytokines and downregulation of type 2 anti-inflammatory cytokines. Surprisingly, the cytokine profile was largely unaffected by Na_Vβ4 knockdown in either model. The Na_V1.6 channel, and the Na_Vβ4 subunit that can regulate Na_V1.6 to enhance repetitive firing, play key roles in both models of low back pain; targeting the abnormal spontaneous activity they generate may have therapeutic value.

Mechanisms of acute and chronic pain after surgery: update from findings in experimental animal models

PURPOSE OF REVIEW

Management of postoperative pain is still a major issue and relevant mechanisms need to be investigated. In preclinical research, substantial progress has been made, for example, by establishing specific rodent models of postoperative pain. By reviewing most recent preclinical studies in animals related to postoperative, incisional pain, we outline the currently available surgical-related pain models, discuss assessment methods for pain-relevant behavior and their shortcomings to reflect the clinical situation, delineate some novel clinical-relevant mechanisms for postoperative pain, and point toward future needs.

RECENT FINDINGS

Since the development of the first rodent model of postoperative, incisional pain almost 20 years ago, numerous variations and some procedure-specific models have been emerged including some conceivably relevant for investigating prolonged, chronic pain after surgery. Many mechanisms have been investigated by using these models; most recent studies focussed on endogenous descending inhibition and opioid-induced hyperalgesia. However, surgical models beyond the classical incision model have so far been used only in exceptional cases, and clinical relevant behavioral pain assays are still rarely utilized.

SUMMARY

Pathophysiological mechanisms of pain after surgery are increasingly discovered, but utilization of pain behavior assays are only sparsely able to reflect clinical-relevant aspects of acute and chronic postoperative pain in patients.