Postinjury stimulation triggers a transition to nociplastic pain in mice

Deciphering nociplastic pain: clinical features, risk factors and potential mechanisms

Nociplastic pain is a mechanistic term used to describe pain that arises or is sustained by altered nociception, despite the absence of tissue damage. Although nociplastic pain has distinct pathophysiology from nociceptive and neuropathic pain, these pain mechanisms often coincide within individuals, which contributes to the intractability of chronic pain. Key symptoms of nociplastic pain include pain in multiple body regions, fatigue, sleep disturbances, cognitive dysfunction, depression and anxiety. Individuals with nociplastic pain are often diffusely tender - indicative of hyperalgesia and/or allodynia - and are often more sensitive than others to non-painful sensory stimuli such as lights, odours and noises. This Review summarizes the risk factors, clinical presentation and treatment of nociplastic pain, and describes how alterations in brain function and structure, immune processing and peripheral factors might contribute to the nociplastic pain phenotype. This article concludes with a discussion of two proposed subtypes of nociplastic pain that reflect distinct neurobiological features and treatment responsivity.

Low-Frequency Stimulation of Trpv1-Lineage Peripheral Afferents Potentiates the Excitability of Spino-Periaqueductal Gray Projection Neurons

High-threshold dorsal root ganglion (HT DRG) neurons fire at low frequencies during inflammatory injury, and low-frequency stimulation (LFS) of HT DRG neurons selectively potentiates excitatory synapses onto spinal neurons projecting to the periaqueductal gray (spino-PAG). Here, in male and female mice, we have identified an underlying peripheral sensory population driving this plasticity and its effects on the output of spino-PAG neurons. We provide the first evidence that Trpv1-lineage sensory neurons predominantly induce burst firing, a unique mode of neuronal activity, in lamina I spino-PAG projection neurons. We modeled inflammatory injury by optogenetically stimulating Trpv1+ primary afferents at 2 Hz for 2 min (LFS), as peripheral inflammation induces 1-2 Hz firing in high-threshold C fibers. LFS of Trpv1+ afferents enhanced the synaptically evoked and intrinsic excitability of spino-PAG projection neurons, eliciting a stable increase in the number of action potentials (APs) within a Trpv1+ fiber-induced burst, while decreasing the intrinsic AP threshold and increasing the membrane resistance. Further experiments revealed that this plasticity required Trpv1+ afferent input, postsynaptic G protein-coupled signaling, and NMDA receptor activation. Intriguingly, an inflammatory injury and heat exposure in vivo also increased APs per burst, in vitro These results suggest that inflammatory injury-mediated plasticity is driven though Trpv1+ DRG neurons and amplifies the spino-PAG pathway. Spinal inputs to the PAG could play an integral role in its modulation of heat sensation during peripheral inflammation, warranting further exploration of the organization and function of these neural pathways.

Can Myofascial Trigger Points Involve Nociplastic Pain? A Scoping Review on Animal Models

Nociplastic pain is a non-specific, regional pain lasting more than three months, characterised by the onset of hypersensitivity, despite no clear evidence of tissue damage. It is a relatively new classified type of pain. As a result, there has not yet been much work describing its precise modelling. The mechanism of its formation needs to be clearly explained. Authors point out that the occurrence of myofascial trigger points (MTrPs) can lead to this type of pain as one possibility. This paper summarises the available literature on modelling nociplastic pain and MTrPs. It complies with studies describing animal model creation and presents the results of performed experiments. The literature search was conducted in December 2022 and included the following databases: PubMed, Scopus, and Web of Science. In this scoping review, six studies were included. Two described the creation of animal models of nociplastic pain, one adapted old models to nociplastic pain, and three described the modelling of MTrPs. This is the first paper pointing in the possible direction of detecting and studying the correlation between MTrPs and nociplastic pain in animal models. However, there is currently insufficient evidence to describe MTrPs as nociplastic, as few studies with animal models exist.

[Nociplastic pain in research and practice : Overview of biopsychosocial principles, possibilities and difficulties]

Traditionally, two mechanistic pain categories were distinguished: nociceptive and neuropathic pain. After the definitions of these two mechanistic descriptors were refined more precisely in the International Association for the Study of Pain (IASP) taxonomy in 2011, a large group of patients remained whose pain could not be assigned to either of the two categories. Nociplastic pain was therefore proposed as a third mechanistic descriptor in 2016. This review article presents the current state of the integration of nociplastic pain into research and clinical practice. In particular, the possibilities and difficulties of applying this concept are addressed from a human and animal experimental research perspective.

Protocol for Development and Validation of Multivariable Prediction Models for Chronic Postsurgical Pain Following Video-Assisted Thoracic Surgery

Purpose

Chronic postsurgical pain (CPSP) is a common complication after thoracic surgery and associated with long-term adverse outcomes. This study aims to develop two prediction models for CPSP after video-assisted thoracic surgery (VATS).

Methods and Analysis

This single-center prospective cohort study will include a total of 500 adult patients undergoing VATS lung resection (n = 350 for development and n = 150 for external validation). Patients will be enrolled continuously at The First Affiliated Hospital of Soochow University in Suzhou, China. The cohort for external validation will be recruited in another time period. The outcome is CPSP, which is defined as pain with the numerical rating scale score of 1 or higher 3 months after VATS. Univariate and multivariable logistic regression analyses will be performed to develop two CPSP prediction models based on patients' data of postoperative day 1 and day 14, respectively. For internal validation, we will use the bootstrapping validation technique. For external validation, the discrimination capability of the models will be assessed using the area under the receiver operating characteristic curve, and the calibration will be evaluated using the calibration curve and Hosmer-Lemeshow goodness-of-fit statistic. The results will be presented in model formulas and nomograms.

Conclusion

Based on the development and validation of the prediction models, our results contribute to early prediction and treatment of CPSP after VATS.

Trial Registration

Chinese Clinical Trial Register (ChiCTR2200066122).

What Do We Know about Nociplastic Pain?

Nociplastic pain is a recently distinguished type of pain, distinct from neuropathic and nociceptive pain, and is well described in the literature. It is often mistaken for central sensitization. Pathophysiology has not been clearly established with regard to alteration of the concentration of spinal fluid elements, the structure of the white and gray matter of the brain, and psychological aspects. Many different diagnostic tools, i.e., the painDETECT and Douleur Neuropathique 4 questionnaires, have been developed to diagnose neuropathic pain, but they can also be applied for nociplastic pain; however, more standardized instruments are still needed in order to assess its occurrence and clinical presentation. Numerous studies have shown that nociplastic pain is present in many different diseases such as fibromyalgia, complex regional pain syndrome type 1, and irritable bowel syndrome. Current pharmacological and nonpharmacological treatments for nociceptive and neuropathic pain are not entirely suitable for treating nociplastic pain. There is an ongoing effort to establish the most efficient way to manage it. The significance of this field has led to several clinical trials being carried out in a short time. The aim of this narrative review was to discuss the currently available evidence on pathophysiology, associated diseases, treatment possibilities, and clinical trials. It is important that physicians widely discuss and acknowledge this relatively new concept in order to provide optimized pain control for patients.

Intraoperative Spinal Cord Stimulation Mitigates Central Sensitization After Spine Surgery in Mice

STUDY DESIGN

Double-blinded, prospective laboratory animal study.

OBJECTIVE

To examine whether intraoperative spinal cord stimulation (SCS) inhibits the development of spine surgery-induced hypersensitivity.

SUMMARY OF BACKGROUND DATA

Managing postoperative pain after spine surgery is challenging, and as many as 40% of patients may develop failed back surgery syndrome. Although SCS has been shown to effectively reduce chronic pain symptoms, it is unknown whether intraoperative SCS can mitigate the development of central sensitization that causes postoperative pain hypersensitivity and potentially leads to failed back surgery syndrome after spine surgery.

MATERIALS AND METHODS

Mice were randomly stratified into three experimental groups: (1) sham surgery, (2) laminectomy alone, and (3) laminectomy plus SCS. Secondary mechanical hypersensitivity was measured in hind paws using von Frey assay one day before and at predetermined times after surgery. In addition, we also performed a conflict avoidance test to capture the affective-motivational domain of pain at selected time points postlaminectomy.

RESULTS

Mice that underwent unilateral T13 laminectomy developed mechanical hypersensitivity in both hind paws. Intraoperative SCS applied to the exposed side of the dorsal spinal cord significantly inhibited the development of hind paw mechanical hypersensitivity on the SCS-applied side. Sham surgery did not produce any obvious secondary mechanical hypersensitivity in the hind paws.

CONCLUSIONS

These results demonstrate that spine surgery for unilateral laminectomy induces central sensitization that results in postoperative pain hypersensitivity. Intraoperative SCS after laminectomy may be able to mitigate the development of this hypersensitivity in appropriately selected cases.

Microglia polarization in nociplastic pain: mechanisms and perspectives

Nociplastic pain is the third classification of pain as described by the International Association for the Study of Pain (IASP), in addition to the neuropathic and nociceptive pain classes. The main pathophysiological mechanism for developing nociplastic pain is central sensitization (CS) in which pain amplification and hypersensitivity occur. Fibromyalgia is the prototypical nociplastic pain disorder, characterized by allodynia and hyperalgesia. Much scientific data suggest that classical activation of microglia in the spinal cord mediates neuroinflammation which plays an essential role in developing CS. In this review article, we discuss the impact of microglia activation and M1/M2 polarization on developing neuroinflammation and nociplastic pain, besides the molecular mechanisms engaged in this process. In addition, we mention the impact of microglial modulators on M1/M2 microglial polarization that offers a novel therapeutic alternative for the management of nociplastic pain disorders. Illustrating the mechanisms underlying microglia activation in central sensitization and nociplastic pain. LPS lipopolysaccharide, TNF-α tumor necrosis factor-α, INF-γ Interferon gamma, ATP adenosine triphosphate, 49 P2Y12/13R purinergic P2Y 12/13 receptor, P2X4/7R purinergic P2X 4/7 receptor, SP Substance P, NK-1R Neurokinin 1 receptor, CCL2 CC motif ligand 2, CCR2 CC motif ligand 2 receptor, CSF-1 colony-stimulating factor 1, CSF-1R colony-stimulating factor 1 receptor, CX3CL1 CX3C motif ligand 1, CX3XR1 CX3C motif ligand 1 receptor, TLR toll-like receptor, MAPK mitogen-activated protein kinases, JNK jun N-terminal kinase, ERK extracellular signal-regulated kinase, iNOS Inducible nitric oxide synthase, IL-1β interleukin-1β, IL-6 interleukin-6, BDNF brain-derived neurotrophic factor, GABA γ-Aminobutyric acid, GABAR γ-Aminobutyric acid receptor, NMDAR N-methyl-D-aspartate receptor, AMPAR α-amino-3-hydroxy-5-methyl-4-isoxazolepropi-onic acid receptor, IL-4 interleukin-4, IL-13 interleukin-13, IL-10 interleukin-10, Arg-1 Arginase 1, FGF fibroblast growth factor, GDNF glial cell-derived neurotrophic factor, IGF-1 insulin-like growth factor-1, NGF nerve growth factor, CD Cluster of differentiation.

Effect of Selective Lesions of Nucleus Accumbens µ-Opioid Receptor-Expressing Cells on Heroin Self-Administration in Male and Female Rats: A Study with Novel Oprm1-Cre Knock-in Rats

The brain µ-opioid receptor (MOR) is critical for the analgesic, rewarding, and addictive effects of opioid drugs. However, in rat models of opioid-related behaviors, the circuit mechanisms of MOR-expressing cells are less known because of a lack of genetic tools to selectively manipulate them. We introduce a CRISPR-based Oprm1-Cre knock-in transgenic rat that provides cell type-specific genetic access to MOR-expressing cells. After performing anatomic and behavioral validation experiments, we used the Oprm1-Cre knock-in rats to study the involvement of NAc MOR-expressing cells in heroin self-administration in male and female rats. Using RNAscope, autoradiography, and FISH chain reaction (HCR-FISH), we found no differences in Oprm1 expression in NAc, dorsal striatum, and dorsal hippocampus, or MOR receptor density (except dorsal striatum) or function between Oprm1-Cre knock-in rats and wildtype littermates. HCR-FISH assay showed that iCre is highly coexpressed with Oprm1 (95%-98%). There were no genotype differences in pain responses, morphine analgesia and tolerance, heroin self-administration, and relapse-related behaviors. We used the Cre-dependent vector AAV1-EF1a-Flex-taCasp3-TEVP to lesion NAc MOR-expressing cells. We found that the lesions decreased acquisition of heroin self-administration in male Oprm1-Cre rats and had a stronger inhibitory effect on the effort to self-administer heroin in female Oprm1-Cre rats. The validation of an Oprm1-Cre knock-in rat enables new strategies for understanding the role of MOR-expressing cells in rat models of opioid addiction, pain-related behaviors, and other opioid-mediated functions. Our initial mechanistic study indicates that lesioning NAc MOR-expressing cells had different effects on heroin self-administration in male and female rats.SIGNIFICANCE STATEMENT The brain µ-opioid receptor (MOR) is critical for the analgesic, rewarding, and addictive effects of opioid drugs. However, in rat models of opioid-related behaviors, the circuit mechanisms of MOR-expressing cells are less known because of a lack of genetic tools to selectively manipulate them. We introduce a CRISPR-based Oprm1-Cre knock-in transgenic rat that provides cell type-specific genetic access to brain MOR-expressing cells. After performing anatomical and behavioral validation experiments, we used the Oprm1-Cre knock-in rats to show that lesioning NAc MOR-expressing cells had different effects on heroin self-administration in males and females. The new Oprm1-Cre rats can be used to study the role of brain MOR-expressing cells in animal models of opioid addiction, pain-related behaviors, and other opioid-mediated functions.

Gonadal hormone-dependent nociceptor sensitization maintains nociplastic pain state in female mice

ABSTRACT

Nociplastic pain conditions develop predominantly in women. We recently established a murine nociplastic pain model by applying postinjury thermal (40°C) stimulation to an injured (capsaicin-injected) area, triggering a transition to a nociplastic pain state manifesting as persistent mechanical hypersensitivity outside of the previously injured area. The nociplastic pain state was centrally maintained by spinal microglia in males but peripherally by ongoing afferent activity at the previously injured area in females. Here, we investigated whether gonadal hormones are critical for the development of this peripherally maintained nociplastic pain state in females. Although the transition to a nociplastic pain state still occurred in ovariectomized females, the pain state was maintained neither by ongoing afferent activity at the previously injured area nor by spinal microglia. Estradiol reconstitution a week before the injury plus postinjury stimulation, but not after the transition had already occurred, restored the development of peripherally maintained nociplastic mechanical hypersensitivity in ovariectomized females. G protein-coupled estrogen receptor antagonism during the transition phase mimicked ovariectomy in gonad-intact females, whereas the receptor antagonism after the transition gradually alleviated the nociplastic mechanical hypersensitivity. At the previously injured area, afferents responsive to allyl isothiocyanate (AITC), a TRPA1 agonist, contributed to the maintenance of nociplastic mechanical hypersensitivity in gonad-intact females. In ex vivo skin-nerve preparations, only AITC-responsive afferents from the nociplastic pain model in gonad-intact females showed ongoing activities greater than control. These results suggest that gonadal hormones are critical for peripherally maintained nociplastic pain state in females by sensitizing AITC-responsive afferents to be persistently active.

Pregabalin attenuates long-lasting post-inflammatory nociplastic mechanical sensitization in mice

Nociplastic pain, the most recently proposed mechanistic descriptor of chronic pain, is the pain resulting from an altered nociceptive system and network without clear evidence of nociceptor activation, injury or disease in the somatosensory system. As the pain-associated symptoms in many patients suffering from undiagnosed pain would result from the nociplastic mechanisms, it is an urgent issue to develop pharmaceutical therapies that would mitigate the aberrant nociception in nociplastic pain. We have recently reported that a single injection of formalin to the upper lip shows sustained sensitization lasting for more than 12 days at the bilateral hindpaws, where there is no injury or neuropathy in rats. Using the equivalent model in mice, we show that pregabalin (PGB), a drug used for treating neuropathic pain, significantly attenuates this formalin-induced widespread sensitization at the bilateral hindpaws, even on the 6 day after the initial single orofacial injection of formalin. On the 10th day after formalin injection, the hindlimb sensitization before PGB injection was no more significant in mice receiving daily PGB injections, unlike those receiving daily vehicle injections. This result suggests that PGB would act on the central pain mechanisms that undergo nociplastic changes triggered by initial inflammation and mitigate widespread sensitization resulting from the established changes.

Dehydrocorydaline alleviates sleep deprivation-induced persistent postoperative pain in adolescent mice through inhibiting microglial P2Y12 receptor expression in the spinal cord

During adolescence, a second period of central nervous system (CNS) plasticity that follows the fetal period, which involves sleep deprivation (SD), becomes apparent. SD during adolescence may result in abnormal development of neural circuits, causing imbalance in neuronal excitation and inhibition, which not only results in pain, but increases the chances of developing emotion disorders in adulthood, such as anxiety and depression. The quantity of surgeries during adolescence is also consistently on the rise, yet the impact and underlying mechanism of preoperative SD on postoperative pain remain unexplored. This study demonstrates that preoperative SD induces upregulation of the P2Y12 receptor, which is exclusively expressed on spinal microglia, and phosphorylation of its downstream signaling pathway p38Mitogen-activated protein/Nuclear transcription factor-κB (p38MAPK/NF-κB)in spinal microglia, thereby promoting microglia activation and microglial transformation into the proinflammatory M1 phenotype, resulting in increased expression of proinflammatory cytokines that exacerbate persisting postoperative incisional pain in adolescent mice. Both intrathecal minocycline (a microglia activation inhibitor) and MRS2395 (a P2Y12 receptor blocker) effectively suppressed microglial activation and proinflammatory cytokine expression. Interestingly, supplementation with dehydrocorydaline (DHC), an extract of Rhizoma Corydalis, inhibited the P2Y12/p38MAPK/NF-κB signaling pathway, microglia activation, and expression of pro-inflammatory cytokines in the model mice. Taken together, the results indicate that the P2Y12 receptor and microglial activation are important factors in persistent postoperative pain caused by preoperative SD in adolescent mice and that DHC has analgesic effects by acting on these targets.

Spinal GABAergic disinhibition allows microglial activation mediating the development of nociplastic pain in male mice

Previously we developed a murine model in which postinjury stimulation of an injured area triggers a transition to a nociplastic pain state manifesting as persistent mechanical hypersensitivity outside of the previously injured area. This hypersensitivity was maintained by sex-specific mechanisms; specifically, activated spinal microglia maintained the hypersensitivity only in males. Here we investigated whether spinal microglia drive the transition from acute injury-induced pain to nociplastic pain in males, and if so, how they are activated by normally innocuous stimulation after peripheral injury. Using intraplantar capsaicin injection as an acute peripheral injury and vibration of the injured paw as postinjury stimulation, we found that inhibition of spinal microglia prevents the vibration-induced transition to a nociplastic pain state. The transition was mediated by the ATP-P2X4 pathway, but not BDNF-TrkB signaling. Intrathecally injected GABA receptor agonists after intraplantar capsaicin injection prevented the vibration-induced transition to a nociplastic pain state. Conversely, in the absence of intraplantar capsaicin injection, intrathecally injected GABA receptor antagonists allowed the vibration stimulation of a normal paw to trigger the transition to a spinal microglia-mediated nociplastic pain state only in males. At the spinal level, TNF-α, IL-1β, and IL-6, but not prostaglandins, contributed to the maintenance of the nociplastic pain state in males. These results demonstrate that in males, the transition from acute injury-induced pain to nociplastic pain is driven by spinal microglia causing neuroinflammation and that peripheral injury-induced spinal GABAergic disinhibition is pivotal for normally innocuous stimulation to activate spinal microglia.

Effect of deuteration on the single dose pharmacokinetic properties and postoperative analgesic activity of methadone

Although methadone is effective in the management of acute pain, the complexity of its absorption-distribution-metabolism-excretion profile limits its use as an opioid of choice for perioperative analgesia. Because deuteration is known to improve the pharmacokinetic, pharmacodynamic and toxicological properties of some drugs, here we characterized the single dose pharmacokinetic properties and post-operative analgesic efficacy of d9-methadone. The pharmacokinetic profiles of d9-methadone and methadone administered intravenously to CD-1 male mice revealed that deuteration leads to a 5.7- and 4.4-fold increase in the area under the time-concentration curve and maximum concentration in plasma, respectively, as well as reduction in clearance (0.9 ± 0.3 L/h/kg vs 4.7 ± 0.8 L/h/kg). The lower brain-to-plasma ratio of d9-methadone compared to that of methadone (0.35 ± 0.12 vs 2.05 ± 0.62) suggested that deuteration decreases the transfer of the drug across the blood-brain barrier. The estimated LD50 value for a single intravenous dose of d9-methadone was 2.1-fold higher than that for methadone. Moreover, d9-methadone outperformed methadone in the efficacy against postoperative pain by primarily activating peripheral opioid receptors. Collectively, these data suggest that the replacement of three hydrogen atoms in three methyl groups of methadone altered its pharmacokinetic properties, improved safety, and enhanced its analgesic efficacy.

Development and validation of a multivariable prediction model for early prediction of chronic postsurgical pain in adults: a prospective cohort study

BACKGROUND

Early identification of patients at risk of developing chronic postsurgical pain (CPSP) is an essential step in reducing pain chronification in postsurgical patients. We aimed to develop and validate a prognostic model for the early prediction of CPSP including pain characteristics indicating altered pain processing within 2 weeks after surgery.

METHODS

A prospective cohort study was conducted in adult patients undergoing orthopaedic, vascular, trauma, or general surgery between 2018 and 2019. Multivariable logistic regression models for CPSP were developed using data from the University Medical Centre (UMC) Utrecht and validated in data from the Erasmus UMC Rotterdam, The Netherlands.

RESULTS

In the development (n=344) and the validation (n=150) cohorts, 28.8% and 21.3% of patients reported CPSP. The best performing model (area under the curve=0.82; 95% confidence interval [CI], 0.76-0.87) included preoperative treatment with opioids (odds ratio [OR]=4.04; 95% CI, 2.13-7.70), bone surgery (OR=2.01; 95% CI, 1.10-3.67), numerical rating scale pain score on postoperative day 14 (OR=1.57; 95% CI, 1.34-1.83), and the presence of painful cold within the painful area 2 weeks after surgery (OR=4.85; 95% CI, 1.85-12.68). Predictive performance was confirmed by external validation.

CONCLUSIONS

As only four easily obtainable predictors are necessary for reliable CPSP prediction, the models are useful for the clinician to be alerted to further assess and treat individual patients at risk. Identification of the presence of painful cold within 2 weeks after surgery as a strong predictor supports altered pain processing as an important contributor to CPSP development.