Ultrastructural Evidence for Oxytocin and Oxytocin Receptor at the Spinal Dorsal Horn: Mechanism of Nociception Modulation

Oxytocin is a hypothalamic neuropeptide involved in the inhibition of nociception transmission at spinal dorsal horn (SDH) level (the first station where the incoming peripheral signals is modulated). Electrophysiological, behavioral, and pharmacological data strongly support the role of this neuropeptide and its receptor (the oxytocin receptor, OTR) as a key endogenous molecule with analgesic properties. Briefly, current data showed that oxytocin release from the hypothalamus induces OTR activation at the SDH, inducing selective inhibition of the nociceptive Aδ- and C-fibers (probably peptidergic) activity, but not the activity of proprioceptive fibers (i.e. Aβ-fibers). The above inhibition could be a direct presynaptic mechanism, or a mechanism mediated by GABAergic interneurons. However, the exact anatomical localization of oxytocin and OTR remains unclear. In this context, the present study set out to analyze the role of OTRs, GABAergic cells and CGRP fibers in the SDH in rats by using electron microscopy. Ultrastructural analyses of the SDH tissue show that: (i) oxytocin and OTR are found in asymmetrical synapsis; (ii) OTR is found in GABAergic interneurons (near unmyelinated fibers), CGRPergic fibers and glial cells; (iii) whereas oxytocin is present in supraspinal descending projection fibers. These anatomical data strongly support the notion that oxytocin released at the SDH could presynaptically inhibit the nociceptive input from the peripheral primary afferent fibers. This inhibitory action could be direct or use a GABA interneuron. Furthermore, our findings that OTR is exhibited in glial tissue at the SDH requires further exploration in nociception assays.

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.

Why It Is Important to Consider the Effects of Analgesics on Sleep: A Critical Review

We review the known physiological mechanisms underpinning all of pain processing, sleep regulation, and pharmacology of analgesics prescribed for chronic pain. In particular, we describe how commonly prescribed analgesics act in sleep-wake neural pathways, with potential unintended impact on sleep and/or wake function. Sleep disruption, whether pain- or drug-induced, negatively impacts quality of life, mental and physical health. In the context of chronic pain, poor sleep quality heightens pain sensitivity and may affect analgesic function, potentially resulting in further analgesic need. Clinicians already have to consider factors including efficacy, abuse potential, and likely side effects when making analgesic prescribing choices. We propose that analgesic-related sleep disruption should also be considered. The neurochemical mechanisms underlying the reciprocal relationship between pain and sleep are poorly understood, and studies investigating sleep in those with specific chronic pain conditions (including those with comorbidities) are lacking. We emphasize the importance of further work to clarify the effects (intended and unintended) of each analgesic class to inform personalized treatment decisions in patients with chronic pain. © 2021 American Physiological Society. Compr Physiol 11:1-31, 2021.

JI017 Attenuates Oxaliplatin-Induced Cold Allodynia via Spinal TRPV1 and Astrocytes Inhibition in Mice

Oxaliplatin, a well-known chemotherapeutic agent, can induce severe neuropathic pain, which can seriously decrease the quality of life of patients. JI017 is an herb mixture composed of Aconitum carmichaelii, Angelica gigas, and Zingiber officinale. Its anti-tumor effect has been reported; however, the efficacy of JI017 against oxaliplatin-induced allodynia has never been explored. Single oxaliplatin injection [6 mg/kg, intraperitoneal, (i.p.)] induced both cold and mechanical allodynia, and oral administration of JI017 (500 mg/kg) alleviated cold but not mechanical allodynia in mice. Real-time polymerase chain reaction (PCR) analysis demonstrated that the upregulation of mRNA of spinal transient receptor potential vanilloid 1 (TRPV1) and astrocytes following oxaliplatin injection was downregulated after JI017 treatment. Moreover, TRPV1 expression and the activation of astrocytes were intensely increased in the superficial area of the spinal dorsal horn after oxaliplatin treatment, whereas JI017 suppressed both. The administration of TRPV1 antagonist [capsazepine, intrathecal (i.t.), 10 μg] attenuated the activation of astrocytes in the dorsal horn, demonstrating that the functions of spinal TRPV1 and astrocytes are closely related in oxaliplatin-induced neuropathic pain. Altogether, these results suggest that JI017 may be a potent candidate for the management of oxaliplatin-induced neuropathy as it decreases pain, spinal TRPV1, and astrocyte activation.

Mammalian target of rapamycin signaling pathway is involved in synaptic plasticity of the spinal dorsal horn and neuropathic pain in rats by regulating autophagy

Unveiling the etiology and the underlying mechanism of neuropathic pain, a poorly treated disease, is essential for the development of effective therapies. This study aimed to explore the role of mammalian target of rapamycin (mTOR) signaling in autophagy-mediated neuropathic pain. We established a spared nerve injury (SNI) model in adult male SD rats by ligating the common peroneal nerve and tibial, with the distal end cutoff. The paw withdrawal threshold (PWT) and C/A-fiber evoked field potentials were determined by electrophysiologic tests at day 0 (before operation), day 7 and day 14 postoperation, and SNI significantly increased field potentials (P < 0.05). Immunohistochemistry and western blots using spinal cord tissues showed that the expressions of GluR1, GluR2, Beclin-1, p62, mTOR and 4EBP1 were significantly increased after SNI (all P < 0.05), whereas the expressions of LC3 and LAMP2 were significantly decreased after SNI (all P < 0.05). Rapamycin efficiently counteracted the effect of SNI and restored the phenotypes to the level comparable to the sham control. In conclusion, rapamycin inhibits C/A-fiber-mediated long-term potentiation in the SNI rat model of neuropathic pain, which might be mediated by activation of autophagy signaling and downregulation of GluRs expression.

Effects of fadolmidine, an α2 -adrenoceptor agonist, as an adjuvant to spinal bupivacaine on antinociception and motor function in rats and dogs

α2 -Adrenoceptor agonists such as clonidine and dexmedetomidine are used as adjuvants to local anesthetics in regional anesthesia. Fadolmidine is an α2 -adrenoceptor agonist developed especially as a spinal analgesic. The current studies investigate the effects of intrathecally administered fadolmidine with a local anesthetic, bupivacaine, on antinociception and motor block in conscious rats and dogs. The antinociceptive effects of intrathecal fadolmidine and bupivacaine alone or in combination were tested in the rat tail-flick and the dog's skin twitch models. The durations of motor block in rats and in dogs were also assessed. In addition, the effects on sedation, mean arterial blood pressure, heart rate, respiratory rate and body temperature were evaluated in telemetrized dogs. Concentrations of fadolmidine in plasma and spinal cord were determined after intrathecal and intravenous administration in rats. Co-administration of intrathecal fadolmidine with bupivacaine increased the magnitude and duration of the antinociceptive effects and prolonged motor block without hypotension. The interaction of the antinociceptive effect was synergistic in its nature in rats. Concentration of fadolmidine in plasma was very low after intrathecal dosing. Taken together, these studies show that fadolmidine as an adjuvant to intrathecal bupivacaine provides enhanced sensory-motor block and enables a reduction of the doses of both drugs. The results indicate that co-administration of fadolmidine with intrathecal bupivacaine was able to achieve an enhanced antinociceptive effect without hypotension and could thus represent a suitable combination for spinal anesthesia.

Clavulanic Acid Attenuating Effect on the Diabetic Neuropathic Pain in Rats

Diabetic neuropathy is one of the most common complications of diabetes mellitus. Excess glutamate release and oxidative stress are hypothesized to be involved in the pathophysiology of diabetes-induced neuropathy. This study was designed to investigate the effect of clavulanic acid (CLAV), a competitive beta-lactamase inhibitor, on the streptozocin (STZ)-induced neuropathic pain and possible mechanisms in the spinal cord of rats. Male Wistar rats were divided into naive group; control group which got a single dose of STZ (50 mg/kg, i.p.), as a model of diabetic neuropathic pain; prophylactic groups: animals received CLAV (10, 20 and 40 mg/kg, i.p.) 1 week after STZ for 10 days; and therapeutic group: animals received 20 mg/kg CLAV, 21 days after STZ for 10 days. Study of pain behaviors was started on days 0, 7, 14, 21, 28, 35 and 42 after STZ. The expression of the glutamate transport 1 (GLT1), genes of oxidative stress including inducible nitric oxide synthase (iNOS), proinflammatory cytokine, tumor necrosis factor alpha (TNF-α), as well as genes involved in the apoptosis including bcl2, bcl2-associated x (bax) were measured in the spinal cord tissue by Real Time PCR, on day 42. On day 21 post injection of STZ, diabetic animals showed significant mechanical allodynia, cold allodynia and thermal hyperalgesia. CLAV in all doses of 10, 20 and 40 mg/kg reduced symptoms of allodynia and hyperalgesia, in both prophylactic and therapeutic regimens. While iNOS, TNF-α, bax/bcl2 were found significantly overexpressed in spinal cord of diabetic animals, their expression in animals received CLAV had been reduced. In contrast, GLT1 that had decreased in the spinal cord of diabetic animals, significantly increased in those received CLAV. CLAV was found a promising candidate for reliving neuropathic pain in diabetes mellitus. Such beneficial effect of CLAV could be, in part, attributed to the increased expression of GLT 1, inhibition of nitrosative stress, anti-inflammation, and inhibition of some apoptotic mediators followed by administration into diabetic animals.

Assessment of spinal cord relative vulnerability in C4-C5 compressive cervical myelopathy using multi-modal spinal cord evoked potentials and neurological findings

Objective: The correlation between the progression of spinal cord lesions using spinal cord evoked potentials (SCEPs) and neurological findings are unclear. The purpose is to electrophysiologically evaluate relative vulnerability of spinal cord in patients with compressive cervical myelopathy (CCM) at C4-C5 intervertebral level using SCEPs and correlate the progression of spinal cord lesions with neurological findings.Design: Retrospective study.Setting: Yamaguchi University Hospital.Participants: 36 patients.Methods: SCEPs following median nerve stimulation (MN-SCEPs), ulnar nerve stimulation (UN-SCEPs), transcranial electric stimulation (TCE-SCEPs), and spinal cord stimulation (SC-SCEPs) were intraoperatively recorded. MN-SCEPs are mediated by posterior horns (4, 5 layers), UN-SCEPs by the Burdach tract, TCE-SCEPs by the lateral corticospinal tract, and SC-SCEPs by the Goll tract. We evaluated the neurological findings (numbness, tactile sense and pain sense in the C6 area, tactile sense in the lower extremities, and triceps tendon reflex [TTR]).Results: The incidence of electrophysiological and clinical abnormalities decreased in the order of UN-SCEPs (100%), TCE-SCEPs (94.4%), MN-SCEPs (77.8%), and SC-SCEPs (69.4%), and in the order of numbness (100%), pain sense (97.2%), TTR (91.7%), tactile sense in the C6 area (83.3%), and tactile sense in the lower extremities (70.0%), respectively.Conclusions: The relative vulnerability of spinal cord occurred in the order of the Burdach tract, the lateral corticospinal tract, posterior horns (4, 5 layers), and the Goll tract in most patients with CCM at the C4-C5 intervertebral level.

The prolactin receptor long isoform regulates nociceptor sensitization and opioid-induced hyperalgesia selectively in females

Pain is more prevalent in women for reasons that remain unclear. We have identified a mechanism of injury-free nociceptor sensitization and opioid-induced hyperalgesia (OIH) promoted by prolactin (PRL) in females. PRL signals through mutually inhibitory long (PRLR-L) and short (PRLR-S) receptor isoforms, and PRLR-S activation induces neuronal excitability. PRL and PRLR expression were higher in females. CRISPR-mediated editing of PRLR-L promoted nociceptor sensitization and allodynia in naïve, uninjured female mice that depended on circulating PRL. Opioids, but not trauma-induced nerve injury, decreased PRLR-L promoting OIH through activation of PRLR-S in female mice. Deletion of both PRLR-L and PRLR-S (total PRLR) prevented, whereas PRLR-L overexpression rescued established OIH selectively in females. Inhibition of circulating PRL with cabergoline, a dopamine D2 agonist, up-regulated PRLR-L and prevented OIH only in females. The PRLR-L isoform therefore confers protection against PRL-promoted pain in females. Limiting PRL/PRLR-S signaling pharmacologically or with gene therapies targeting the PRLR may be effective for reducing pain in a female-selective manner.

Optogenetic Manipulations of Amygdala Neurons Modulate Spinal Nociceptive Processing and Behavior Under Normal Conditions and in an Arthritis Pain Model

The amygdala is an important neural substrate for the emotional–affective dimension of pain and modulation of pain. The central nucleus (CeA) serves major amygdala output functions and receives nociceptive and affected–related information from the spino-parabrachial and lateral–basolateral amygdala (LA–BLA) networks. The CeA is a major site of extra–hypothalamic expression of corticotropin releasing factor (CRF, also known as corticotropin releasing hormone, CRH), and amygdala CRF neurons form widespread projections to target regions involved in behavioral and descending pain modulation. Here we explored the effects of modulating amygdala neurons on nociceptive processing in the spinal cord and on pain-like behaviors, using optogenetic activation or silencing of BLA to CeA projections and CeA–CRF neurons under normal conditions and in an acute pain model. Extracellular single unit recordings were made from spinal dorsal horn wide dynamic range (WDR) neurons, which respond more strongly to noxious than innocuous mechanical stimuli, in normal and arthritic adult rats (5–6 h postinduction of a kaolin/carrageenan–monoarthritis in the left knee). For optogenetic activation or silencing of CRF neurons, a Cre–inducible viral vector (DIO–AAV) encoding channelrhodopsin 2 (ChR2) or enhanced Natronomonas pharaonis halorhodopsin (eNpHR_3.0) was injected stereotaxically into the right CeA of transgenic Crh–Cre rats. For optogenetic activation or silencing of BLA axon terminals in the CeA, a viral vector (AAV) encoding ChR2 or eNpHR_3.0 under the control of the CaMKII promoter was injected stereotaxically into the right BLA of Sprague–Dawley rats. For wireless optical stimulation of ChR2 or eNpHR_3.0 expressing CeA–CRF neurons or BLA–CeA axon terminals, an LED optic fiber was stereotaxically implanted into the right CeA. Optical activation of CeA–CRF neurons or of BLA axon terminals in the CeA increased the evoked responses of spinal WDR neurons and induced pain-like behaviors (hypersensitivity and vocalizations) under normal condition. Conversely, optical silencing of CeA–CRF neurons or of BLA axon terminals in the CeA decreased the evoked responses of spinal WDR neurons and vocalizations, but not hypersensitivity, in the arthritis pain model. These findings suggest that the amygdala can drive the activity of spinal cord neurons and pain-like behaviors under normal conditions and in a pain model.

Novel and Emerging Electrophysiological Biomarkers of Diabetic Neuropathy and Painful Diabetic Neuropathy

PURPOSE

Diabetic peripheral neuropathy (DPN) is the most common complication of diabetes. Small and large peripheral nerve fibers can be involved in DPN. Large nerve fiber damage causes paresthesia, sensory loss, and muscle weakness, and small nerve fiber damage is associated with pain, anesthesia, foot ulcer, and autonomic symptoms. Treatments for DPN and painful DPN (pDPN) pose considerable challenges due to the lack of effective therapies. To meet these challenges, there is a major need to develop biomarkers that can reliably diagnose and monitor progression of nerve damage and, for pDPN, facilitate personalized treatment based on underlying pain mechanisms.

METHODS

This study involved a comprehensive literature review, incorporating article searches in electronic databases (Google Scholar, PubMed, and OVID) and reference lists of relevant articles with the authors' substantial expertise in DPN. This review considered seminal and novel research and summarizes emerging biomarkers of DPN and pDPN that are based on neurophysiological methods.

FINDINGS

From the evidence gathered from 145 papers, this submission describes emerging clinical neurophysiological methods with potential to act as biomarkers for the diagnosis and monitoring of DPN as well as putative future roles as predictors of response to antineuropathic pain medication in pDPN. Nerve conduction studies only detect large fiber damage and do not capture pathology or dysfunction of small fibers. Because small nerve fiber damage is prominent in DPN, additional biomarkers of small nerve fiber function are needed. Activation of peripheral nociceptor fibers using laser, heat, or targeted electrical stimuli can generate pain-related evoked potentials, which are an objective neurophysiological measure of damage along the small fiber pathways. Assessment of nerve excitability, which provides a surrogate of axonal properties, may detect alterations in function before abnormalities are detected by nerve conduction studies. Microneurography and rate-dependent depression of the Hoffmann-reflex can be used to dissect underlying pain-generating mechanisms arising from the periphery and spinal cord, respectively. Their role in informing mechanistic-based treatment of pDPN as well as facilitating clinical trials design is discussed.

IMPLICATIONS

The neurophysiological methods discussed, although currently not practical for use in busy outpatient settings, detect small fiber and early large fiber damage in DPN as well as disclosing dominant pain mechanisms in pDPN. They are suited as diagnostic and predictive biomarkers as well as end points in mechanistic clinical trials of DPN and pDPN.

Cardiac sensory afferents modulate susceptibility to anxio-depressive behaviour in a mouse model of chronic heart failure

AIM

Impairments in cerebral structure and cognitive performance in chronic heart failure (CHF) are critical components of its comorbidity spectrum. Autonomic afferents that arise from cardiac sensory fibres show enhanced activity with CHF. Desensitization of these fibres by local application of resiniferatoxin (RTX) during myocardial infarction (MI) is known to prevent cardiac hypertrophy, sympathetic hyperactivity and CHF. Whether these afferents mediate cerebral allostasis is unknown.

METHODS

CHF was induced by myocardial infarction. To evaluate if cardiac afferents contribute to cerebral allostasis, RTX was acutely applied to the pericardial space in controls (RTX) and in MI treated animals (MI/RTX). Subjects were then evaluated in a series of behavioural tests recapitulating different symptoms of depressive disorders. Proteomics of the frontal cortices (FC) was performed to identify contributing proteins and pathways responsible for behavioural allostasis.

RESULTS

Desensitization of cardiac afferents relieves hallmarks of an anxio/depressive-like state in mice. Unique protein signatures and regulatory pathways in FCs isolated from each treatment reveal the degree of complexity inherent in the FC response to stresses originating in the heart. While cortices from the combined treatment (MI/RTX) did not retain protein signatures from the individual treatment groups, all three groups suffer dysregulation in circadian entrainment.

CONCLUSION

CHF is comorbid with an anxio/depressive-like state and ablation of cardiac afferents relieves the despair phenotype. The strikingly different proteomic profiles observed in FCs suggest that MI and RTX lead to unique brain-signalling patterns and that the combined treatment, potentially through destructive interference mechanisms, most closely resembles controls.

Does muscular or mental fatigue have an influence on the nociceptive flexion reflex? A randomized cross-over study in healthy people

BACKGROUND

The nociceptive flexion reflex (NFR) is a spinally-mediated withdrawal reflex occurring in response to noxious stimuli and is used as an electrophysiological marker of spinal nociception. Although it is well-documented that the NFR is subject to powerful modulation of several personal factors, the effects of experimentally induced fatigue on the NFR have not yet been examined. Hence, this study aimed to characterize if and how fatigue affects spinal nociception in healthy adults.

METHODS

The NFR of 58 healthy people was measured prior to and following rest and two fatiguing tasks performed in randomized order. The NFR was elicited by transcutaneous electrical stimulation of the sural nerve and objectified by electromyographic recordings from the biceps femoris muscle. An isokinetic fatiguing protocol was used to induce localized muscle fatigue of the hamstrings. The modified incongruent Stroop-word task was used to provoke mental fatigue. A linear mixed model analysis was performed to assess the influence of fatigue on the NFR.

RESULTS

Low-to-moderate levels experimentally induced localized muscle and mental fatigue did not affect the NFR in healthy adults. These results suggest that descending pain inhibitory processes to dampen spinal nociception are resistant to the effects of localized muscle and mental fatigue.

CONCLUSIONS

The relative robustness of the NFR to fatigue may be beneficial in both clinical and research settings where the influence of confounders complicates interpretation. Furthermore, the findings possibly help enhance our understanding on why even demanding cognitive/physical exercise-based treatment programs form effective treatment strategies for patients with chronic pain.

SIGNIFICANCE

The present study unraveled that low-to-moderate levels experimentally induced localized muscle and mental fatigue did not affect the NFR. These results suggest that descending pain inhibitory processes to dampen spinal nociception are resistant to the effects of localized muscle and mental fatigue. This relative robustness of the NFR may be beneficial in a clinical setting in which the evaluation of spinal nociception that is unaffected by clinical symptoms of fatigue may be useful (e.g. chronic fatigue syndrome, cancer-related fatigue, etc.).

Leptin Contributes to Neuropathic Pain via Extrasynaptic NMDAR-nNOS Activation

Leptin is an adipocytokine that is primarily secreted by white adipose tissue, and it contributes to the pathogenesis of neuropathic pain in collaboration with N-methyl-D-aspartate receptors (NMDARs). Functional NMDARs are a heteromeric complex that primarily comprise two NR1 subunits and two NR2 subunits. NR2A is preferentially located at synaptic sites, and NR2B is enriched at extrasynaptic sites. The roles of synaptic and extrasynaptic NMDARs in the contribution of leptin to neuropathic pain are not clear. The present study examined whether the important role of leptin in neuropathic pain was related to synaptic or extrasynaptic NMDARs. We used a rat model of spared nerve injury (SNI) and demonstrated that the intrathecal administration of the NR2A-selective antagonist NVP-AAM077 and the NR2B-selective antagonist Ro25-6981 prevented and reversed mechanical allodynia following SNI. Administration of exogenous leptin mimicked SNI-induced behavioral allodynia, which was also prevented by NVP-AAM077 and Ro25-6981. Mechanistic studies showed that leptin enhanced NR2B- but not NR2A-mediated currents in spinal lamina II neurons of naïve rats. Leptin also upregulated the expression of NR2B, which was blocked by the NR2B-selective antagonist Ro25-6981, in cultured dorsal root ganglion (DRG) neurons. Leptin enhanced neuronal nitric oxide synthase (nNOS) expression, which was also blocked by Ro25-6981, in cultured DRG cells. However, leptin did not change NR2A expression, and the NR2A-selective antagonist NVP-AAM077 had no effect on leptin-enhanced nNOS expression. Our data suggest an important cellular link between the spinal effects of leptin and the extrasynaptic NMDAR-nNOS-mediated cellular mechanism of neuropathic pain.

The Distinct Functions of Dopaminergic Receptors on Pain Modulation: A Narrative Review

Chronic pain is considered an economic burden on society as it often results in disability, job loss, and early retirement. Opioids are the most common analgesics prescribed for the management of moderate to severe pain. However, chronic exposure to these drugs can result in opioid tolerance and opioid-induced hyperalgesia. On pain modulation strategies, exploiting the multitarget drugs with the ability of the superadditive or synergistic interactions attracts more attention. In the present report, we have reviewed the analgesic effects of different dopamine receptors, particularly D1 and D2 receptors, in different regions of the central nervous system, including the spinal cord, striatum, nucleus accumbens (NAc), and periaqueductal gray (PAG). According to the evidence, these regions are not only involved in pain modulation but also express a high density of DA receptors. The findings can be categorized as follows: (1) D2-like receptors may exert a higher analgesic potency, but D1-like receptors act in different manners across several mechanisms in the mentioned regions; (2) in the spinal cord and striatum, antinociception of DA is mainly mediated by D2-like receptors, while in the NAc and PAG, both D1- and D2-like receptors are involved as analgesic targets; and (3) D2-like receptor agonists can act as adjuvants of μ-opioid receptor agonists to potentiate analgesic effects and provide a better approach to pain relief.

Management of Musculoskeletal Pain: An Update with Emphasis on Chronic Musculoskeletal Pain

Musculoskeletal pain is a challenging condition for both patients and physicians. Many adults have experienced one or more episodes of musculoskeletal pain at some time of their lives, regardless of age, gender, or economic status. It affects approximately 47% of the general population. Of those, about 39–45% have long-lasting problems that require medical consultation. Inadequately managed musculoskeletal pain can adversely affect quality of life and impose significant socioeconomic problems. This manuscript presents a comprehensive review of the management of chronic musculoskeletal pain. It briefly explores the background, classifications, patient assessments, and different tools for management according to the recently available evidence. Multimodal analgesia and multidisciplinary approaches are fundamental elements of effective management of musculoskeletal pain. Both pharmacological, non-pharmacological, as well as interventional pain therapy are important to enhance patient’s recovery, well-being, and improve quality of life. Accordingly, recent guidelines recommend the implementation of preventative strategies and physical tools first to minimize the use of medications. In patients who have had an inadequate response to pharmacotherapy, the proper use of interventional pain therapy and the other alternative techniques are vital for safe and effective management of chronic pain patients.

Reorganization of synaptic inputs to spinal dorsal horn neurons in neuropathic pain

Peripheral nerve injuries produce a variety of negative structural and functional changes in the central terminal sites of damaged axons, as well as the injured primary afferents. Such changes have been shown to be involved in the development of neuropathic pain, which includes abnormal pain sensations such as allodynia and hyperalgesia. Since the spinal dorsal horn is the first central site where signals from peripheral sensory nerves are transmitted and shows a variety of changes after peripheral nerve injury or chronic inflammation of peripheral tissues, it is one of the most important sites contributing to the mechanisms underlying the development of neuropathic pain. The functional disruption of inhibitory interneurons and glial activation in the spinal dorsal horn after peripheral nerve injury cause reorganization of neuronal circuits and changes in the excitability of second-order neurons. These events are involved in the development or maintenance of neuropathic pain. Here, we describe the interactions of primary afferents, interneurons, and glial cells that may cause reorganization of synaptic inputs to spinal dorsal horn neurons after peripheral nerve injury.

A new mechanistic approach for the treatment of chronic neuropathic pain with nitrous oxide integrated from a systems biology narrative review

The limitations of the currently available treatments for chronic neuropathic pain highlight the need for safer and more effective alternatives. The authors carried out a focused review using a systems biology approach to integrate the complex mechanisms of nociception and neuropathic pain, and to decipher the effects of nitrous oxide (N2O) on those pathways, beyond the known effect of N2O on N-methyl-D-aspartate receptors. This review identified a number of potential mechanisms by which N2O could impact the processes involved in peripheral and central sensitization. In the ascending pathway, the effects of N2O include activating TWIK-related K+ channel 1 potassium channels on first-order neurons, blocking voltage-dependent calcium channels to attenuate neuronal excitability, attenuating postsynaptic glutamatergic receptor activation, and possibly blocking voltage-dependent sodium channels. In the descending pathway, N2O induces the release of endogenous opioid ligands and stimulates norepinephrine release. In addition, N2O may mediate epigenetic changes by inhibiting methionine synthase, a key enzyme involved in DNA and RNA methylation. This could explain why this short-acting analgesic has shown long-lasting anti-pain sensitization effects in animal models of chronic pain. These new hypotheses support the rationale for investigating N2O, either alone or in combination with other analgesics, for the management of chronic neuropathic pain.

Pain pathways and potential new targets for pain relief

Pain is an unpleasant sensory and emotional experience that affects a sizable percentage of people on a daily basis. Sensory neurons known as nociceptors built specifically to detect damaging stimuli can be found throughout the body. They transmit information about noxious stimuli from mechanical, thermal, and chemical sources to the central nervous system and higher brain centers via electrical signals. Nociceptors express various channels and receptors such as voltage-gated sodium and calcium channels, transient receptor potential channels, and opioid receptors that allow them to respond in a highly specific manner to noxious stimuli. Attenuating the pain response can be achieved by inhibiting or altering the expression of these pain targets. Achieving a deeper understanding of how these receptors can be affected at the molecular level can lead to the development of novel pain therapies. This review will discuss the mechanisms of pain, introduce the various receptors that are responsible for detecting pain, and future directions in pharmacological therapies.

Noble metal sensitized invasive porous bioelectrodes: advanced medical device for enhanced neuronal activity and chronic alcohol treatment

Invasive bioelectrodes are widely used as an effective treatment for several acute and chronic diseases. In earlier work using high surface area invasive porous bioelectrodes evaluated in an animal model of alcoholism withdrawal, we demonstrated significantly improved electrophysiological and behavioral responses. In this study, we further modify the surface of these invasive porous bioelectrodes with noble metal (Ag, Au, Pt) nanoparticles. Compared to both conventional and porous bioelectrodes, noble metal sensitized invasive porous bioelectrodes show markedly increased low threshold (LT) and wide dynamic range (WDR) neuronal activity. In particular, Pt-sensitized invasive porous bioelectrodes show the highest WDR neuronal activity only upon insertion. In addition, Ag-sensitized invasive porous bioelectrodes, whose surface area is about 37 times greater than that of conventional bioelectrodes, show improved electrochemical properties with higher LT and WDR neuronal activity when stimulated. In an animal model of chronic alcoholism, using normal and alcohol-treated Sprague-Dawley (SD) rats evaluated with the elevated plus maze (EPM) test, the Ag-sensitized invasive porous bioelectrodes show about 20% higher open arms time. These results suggest that these noble metal-sensitized invasive bioelectrodes may offer improved therapeutic outcomes for the treatment of chronic alcoholism, and given these enhanced electrophysiological properties, for other conditions as well.

Chronic Treatment With Hydrogen Sulfide Donor GYY4137 Mitigates Microglial and Astrocyte Activation in the Spinal Cord of Streptozotocin-Induced Diabetic Rats

Long-term diabetic patients suffer immensely from diabetic neuropathy. This study was designed to investigate the effects of hydrogen sulfide (H2S) on peripheral neuropathy, activation of microglia, astrocytes, and the cascade secretion of proinflammatory cytokines in the streptozotocin (STZ)-induced peripheral diabetic neuropathy rat model. STZ-induced diabetic rats were treated with the water-soluble, slow-releasing H2S donor GYY4137 (50 mg/kg; i.p.) daily for 4 weeks. Antiallodynic/antihyperalgesic activities were evaluated using different tests and histopathological changes and the expression of proinflammatory cytokines in the spinal cord were examined. GYY4137 treatment produced neuroprotective effects in the spinal cord of diabetic animals and modulated their sensory deficits. The treatment decreased allodynia (p &lt; 0.05) and mechanical hyperalgesia (p &lt; 0.01) and restored thermal hyperalgesia (p &lt; 0.001) compared with diabetic rats. The treatment decreased the microglial response and increased astrocyte counts in spinal cord gray and white matter compared with untreated diabetic rats. Proinflammatory cytokines were reduced in the treated group compared with diabetic rats. These results suggest that H2S has a potentially ameliorative effect on the neuropathic pain through the control of astrocyte activation and microglia-mediated inflammation, which may be considered as a possible treatment of peripheral nerve hypersensitivity in diabetic patients.

A Conceptual Model of Biopsychosocial Mechanisms of Transition from Acute to Chronic Postsurgical Pain in Children and Adolescents

Acute and chronic pain are highly prevalent and impactful consequences of surgery across the lifespan, yet a comprehensive conceptual model encompassing biopsychosocial factors underlying acute to chronic pain transition is lacking, particularly in youth. Building on prior chronic postsurgical pain models, we propose a new conceptual model of biopsychosocial mechanisms of transition from acute to chronic postsurgical pain. This review aims to summarize existing research examining key factors underlying acute to chronic postsurgical pain transition in order to guide prevention and intervention efforts aimed at addressing this health issue in children. As pain transitions from acute nociceptive pain to chronic pain, changes in the peripheral and central nervous system contribute to the chronification of pain after surgery. These changes include alterations in sensory pain processing and psychosocial processes (psychological, behavioral, and social components), which promote the development of chronic pain. Patient-related premorbid factors (eg, demographic factors, genetic profile, and medical factors such as premorbid pain) may further modulate these changes. Factors related to acute injury and recovery (eg, surgical and treatment factors), as well as biological response to surgery (eg, epigenetic, inflammatory, and endocrine factors), may also influence this process. Overall, longitudinal studies examining temporal pathways of biopsychosocial processes including both risk and resiliency factors will be essential to identify the mechanisms involved in the transition from acute to chronic pain. Research is also needed to unravel connections between the acute pain experience, opioid exposure, and sensory pain processing during acute to chronic pain transition. Furthermore, future studies should include larger and more diverse samples to more fully explore risk factors in a broader range of pediatric surgeries. The use of conceptual models to guide intervention approaches targeting mechanisms of transition from acute to chronic pain will significantly advance this field and improve outcomes for children and adolescents undergoing surgery.

Efficacy and Safety of Magnesium for the Management of Chronic Pain in Adults: A Systematic Review

Chronic pain is a highly prevalent and complex health problem that is associated with a heavy symptom burden, substantial economic and social impact, and also, very few highly effective treatments. This review examines evidence for the efficacy and safety of magnesium in chronic pain. The previously published protocol for this review was registered in International Prospective Register of Systematic Reviews (PROSPERO), MEDLINE, EMBASE, and Cochrane Central Register of Controlled Trials (CENTRAL) databases were searched until September 2018. We included randomized controlled trials (RCTs) comparing magnesium (at any dose, frequency, or route of administration) with placebo using participant-reported pain measures. A total of 9 RCTs containing 418 participants were included. Three studies examined neuropathic pain (62 participants), 3 examined migraines (190 participants), 2 examined complex regional pain syndrome (86 participants), and 1 examined low back pain with a neuropathic component (80 participants). Heterogeneity of included studies precluded any meta-analyses. No judgment could be made about safety because adverse events were inconsistently reported in the included studies. Evidence of analgesic efficacy from included studies was equivocal. However, reported efficacy signals in some of the included trials provide a rationale for more definitive studies. Future, larger-sized trials with good assay sensitivity and better safety assessment and reporting, as well as careful attention to formulations with optimal bioavailability, will serve to better define the role of magnesium in the management of chronic pain.

Pain Chemogenomics Knowledgebase (Pain-CKB) for Systems Pharmacology Target Mapping and Physiologically Based Pharmacokinetic Modeling Investigation of Opioid Drug-Drug Interactions

More than 50 million adults in America suffer from chronic pain. Opioids are commonly prescribed for their effectiveness in relieving many types of pain. However, excessive prescribing of opioids can lead to abuse, addiction, and death. Non-steroidal anti-inflammatory drugs (NSAIDs), another major class of analgesic, also have many problematic side effects including headache, dizziness, vomiting, diarrhea, nausea, constipation, reduced appetite, and drowsiness. There is an urgent need for the understanding of molecular mechanisms that underlie drug abuse and addiction to aid in the design of new preventive or therapeutic agents for pain management. To facilitate pain related small-molecule signaling pathway studies and the prediction of potential therapeutic target(s) for the treatment of pain, we have constructed a comprehensive platform of a pain domain-specific chemogenomics knowledgebase (Pain-CKB) with integrated data mining computing tools. Our new computing platform describes the chemical molecules, genes, proteins, and signaling pathways involved in pain regulation. Pain-CKB is implemented with a friendly user interface for the prediction of the relevant protein targets and analysis and visualization of the outputs, including HTDocking, TargetHunter, BBB predictor, and Spider Plot. Combining these with other novel tools, we performed three case studies to systematically demonstrate how further studies can be conducted based on the data generated from Pain-CKB and its algorithms and tools. First, systems pharmacology target mapping was carried out for four FDA approved analgesics in order to identify the known target and predict off-target interactions. Subsequently, the target mapping outcomes were applied to build physiologically based pharmacokinetic (PBPK) models for acetaminophen and fentanyl to explore the drug-drug interaction (DDI) between this pair of drugs. Finally, pharmaco-analytics was conducted to explore the detailed interaction pattern of acetaminophen reactive metabolite and its hepatotoxicity target, thioredoxin reductase.

Persistent Rheb-induced mTORC1 activation in spinal cord neurons induces hypersensitivity in neuropathic pain

The small GTPase Ras homolog enriched in the brain (Rheb) can activate mammalian target of rapamycin (mTOR) and regulate the growth and cell cycle progression. We investigated the role of Rheb-mediated mTORC1 signaling in neuropathic pain. A chronic constriction injury (CCI) model was dopted. CCI induced obvious spinal Rheb expression and phosphorylation of mTOR, S6, and 4-E-BP1. Blocking mTORC1 signal with rapamycin alleviated the neuropathic pain and restored morphine efficacy in CCI model. Immunofluoresence showed a neuronal co-localization of CCI-induced Rheb and pS6. Rheb knockin mouse showed a similar behavioral phenotype as CCI. In spinal slice recording, CCI increased the firing frequency of neurons expressing HCN channels; inhibition of mTORC1 with rapamycin could reverse the increased spinal neuronal activity in neuropathic pain. Spinal Rheb is induced in neuropathic pain, which in turn active the mTORC1 signaling in CCI. Spinal Rheb-mTOR signal plays an important role in regulation of spinal sensitization in neuropathic pain, and targeting mTOR may give a new strategy for pain management.

Manganese-enhanced MRI reveals changes within brain anxiety and aversion circuitry in rats with chronic neuropathic pain- and anxiety-like behaviors

Chronic pain often predicts the onset of psychological distress. Symptoms including anxiety and depression after pain chronification reportedly are caused by brain remodeling/recruitment of the limbic and reward/aversion circuitries. Pain is the primary precipitating factor that has caused opioid overprescribing and continued overuse of opioids leading to the current opioid epidemic. Yet experimental pain therapies often fail in clinical trials. Better understanding of underlying pathologies contributing to pain chronification is needed to address these chronic pain related issues. In the present study, a chronic neuropathic pain model persisting 10 weeks was studied. The model develops both anxiety- and pain-related behavioral measures to mimic clinical pain. The manganese-enhanced magnetic resonance imaging (MEMRI) utilized improved MRI signal contrast in brain regions with higher neuronal activity in the rodent chronic constriction trigeminal nerve injury (CCI-ION) model. T1-weighted MEMRI signal intensity was increased compared to controls in supraspinal regions of the anxiety and aversion circuitry, including anterior cingulate gyrus (ACC), amygdala, habenula, caudate, ventrolateral and dorsomedial periaqueductal gray (PAG). Despite continuing mechanical hypersensitivity, MEMRI T1 signal intensity as the neuronal activity measure, was not significantly different in thalamus and decreased in somatosensory cortex (S1BF) of CCI-ION rats compared to naïve controls. This is consistent with decreased fMRI BOLD signal intensity in thalamus and cortex of patients with longstanding trigeminal neuropathic pain reportedly associated with gray matter volume decrease in these regions. Significant increase in MEMRI T2 signal intensity in thalamus of CCI-ION animals was indication of tissue water content, cell dysfunction and/or reactive astrogliosis. Decreased T2 signal intensity in S1BF cortex of rats with CCI-ION was similar to findings of reduced T2 signals in clinical patients with chronic orofacial pain indicating prolonged astrocyte activation. These findings support use of MEMRI and chronic rodent models for preclinical studies and therapeutic trials to reveal brain sites activated only after neuropathic pain has persisted in timeframes relevant to clinical pain and to observe treatment effects not possible in short-term models which do not have evidence of anxiety-like behaviors. Potential improvement is predicted in the success rate of preclinical drug trials in future studies with this model.

Possible implications of animal models for the assessment of visceral pain

Acute pain, provoked generally after the activation of peripheral nociceptors, is an adaptive sensory function that alerts the individual to avoid noxious stimuli. However, uncontrolled acute pain has a maladaptive role in sensory activity leading to development of a chronic pain state which persists even after the damage is resolved, or in some cases, in the absence of an initial local acute injury. Huge numbers of people suffer from visceral pain at least once during their life span, leading to substantial health care costs. Although studies reporting on the mechanism of visceral pain are accumulating, it is still not precisely understood. Therefore, this review aims to elucidate the mechanism of visceral pain through an evaluation of different animal models and their application to develop novel therapeutic approaches for treating visceral pain. To assess the nociceptive responses in viscera, several visceral pain models such as inflammatory, traction, stress and genetic models utilizing different methods of measurement have been devised. Among them, the inflammatory and traction models are widely used for studying the visceral pain mechanism of different disease conditions and post-operative surgery in humans and animals. A hapten, 2,4,6-trinitrobenzene sulfonic acid (TNBS), has been extensively used as an inflammatory agent to induce visceral pain. The traction model seems to cause a strong pain stimulation and autonomic reaction and could thus be the most appropriate model for studying the underlying visceral pain mechanism and for probing the therapeutic efficacies of various anesthetic and analgesics for the treatment of visceral pain and hyperalgesia.

Phospholipase A2 Inhibitor-Loaded Phospholipid Micelles Abolish Neuropathic Pain

Treating persistent neuropathic pain remains a major clinical challenge. Current conventional treatment approaches carry a substantial risk of toxicity and provide only transient pain relief. In this work, we show that the activity and expression of the inflammatory mediator secretory phospholipase-A₂ (sPLA₂) enzyme increases in the spinal cord after painful nerve root compression. We then develop phospholipid micelle-based nanoparticles that release their payload in response to sPLA₂ activity. Using a rodent model of neuropathic pain, phospholipid micelles loaded with the sPLA₂ inhibitor, thioetheramide-PC (TEA-PC), are administered either locally or intravenously at the time of painful injury or 1-2 days afterward. Local micelle administration immediately after compression prevents pain for up to 7 days. Delayed intravenous administration of the micelles attenuates existing pain. These findings suggest that sPLA₂ inhibitor-loaded micelles can be a promising anti-inflammatory nanotherapeutic for neuropathic pain treatment.

Deepening the Mechanisms of Visceral Pain Persistence: An Evaluation of the Gut-Spinal Cord Relationship

The management of visceral pain is a major clinical problem in patients affected by gastrointestinal disorders. The poor knowledge about pain chronicization mechanisms prompted us to study the functional and morphological alterations of the gut and nervous system in the animal model of persistent visceral pain caused by 2,4-dinitrobenzenesulfonic acid (DNBS). This agent, injected intrarectally, induced a colonic inflammation peaking on day 3 and remitting progressively from day 7. In concomitance with bowel inflammation, the animals developed visceral hypersensitivity, which persisted after colitis remission for up to three months. On day 14, the administration of pain-relieving drugs (injected intraperitoneally and intrathecally) revealed a mixed nociceptive, inflammatory and neuropathic pain originating from both the peripheral and central nervous system. At this time point, the colonic histological analysis highlighted a partial restitution of the tunica mucosa, transmural collagen deposition, infiltration of mast cells and eosinophils, and upregulation of substance P (SP)-positive nerve fibers, which were surrounded by eosinophils and MHC-II-positive macrophages. A significant activation of microglia and astrocytes was observed in the dorsal and ventral horns of spinal cord. These results suggest that the persistence of visceral pain induced by colitis results from maladaptive plasticity of the enteric, peripheral and central nervous systems.

Changes in spinal cord hemodynamics reflect modulation of spinal network with different parameters of epidural stimulation

In this study functional ultrasound (fUS) imaging has been implemented to explore the local hemodynamics response induced by electrical epidural stimulation and to study real-time in vivo functional changes of the spinal cord, taking advantage of the superior spatiotemporal resolution provided by fUS. By quantifying the hemodynamics and electromyographic response features, we tested the hypothesis that the temporal hemodynamics response of the spinal cord to electrical epidural stimulation could reflect modulation of the spinal circuitry and accordingly respond to the changes in parameters of electrical stimulation. The results of this study for the first time demonstrate that the hemodynamics response to electrical stimulation could reflect a neural-vascular coupling of the spinal cord. Response in the dorsal areas to epidural stimulation was significantly higher and faster compared to the response in ventral spinal cord. Positive relation between the hemodynamics and the EMG responses was observed at the lower frequencies of epidural stimulation (20 and 40 Hz), which according to our previous findings can facilitate spinal circuitry after spinal cord injury, compared to higher frequencies (200 and 500 Hz). These findings suggest that different mechanisms could be involved in spinal cord hemodynamics changes during different parameters of electrical stimulation and for the first time provide the evidence that neural-vascular coupling of the spinal cord circuitry could be related to specific organization of spinal cord vasculature and hemodynamics.

Plasma Concentrations of Select Inflammatory Cytokines Predicts Pain Intensity 48 Hours Post-Shoulder Muscle Injury

OBJECTIVES

The relationship between elevated inflammatory cytokine levels and peak pain intensity following acute musculoskeletal injury has not been fully elucidated in high risk subgroups. Identifying the role that these cytokines have on pain responses may help with developing tailored therapeutic approaches.

METHODS

Data were collected from 54 participants who were vulnerable to a robust pain response and delayed recovery following musculoskeletal injury. Participants completed baseline active and resting pain measurements and a blood draw before an exercised induced shoulder muscle injury. Participants returned at 24 and 48 hours postinjury for follow-up pain measurements and blood draws. Blood plasma was analyzed for interleukin (IL)-1β, IL-6, IL-8, IL-10, and tumor necrosis factor α. Pearson bivariate correlations were performed between cytokines and pain measurements to identify candidate variables for stepwise multiple linear regression predicting pain intensity reports.

RESULTS

Pearson bivariate correlation identified 13/45 correlations between inflammatory cytokines and resting pain intensity and 9/45 between inflammatory cytokines and active pain (P<0.05, r≥0.3 or r≤-0.3). This led to 5 stepwise multiple linear regression models, of which 4 met the statistical criterion (P<0.0167); including IL-10 baseline plasma concentrations predicting active pain (r=0.19) and resting pain (r=0.15) intensity 48 hours postinjury. IL-6 and IL-10 plasma concentrations at 48 hours were respectively associated with active and resting pain at 48 hours.

DISCUSSION

These findings suggest that elevated concentrations of inflammatory cytokines, specifically IL-10 (at baseline and 48 h) and IL-6 (at 48 h), may play a role in heightened pain responses following exercise-induced muscle injury.

Conditional Lpar1 gene targeting identifies cell types mediating neuropathic pain

LPA1 is one of six known receptors (LPA1-6) for lysophosphatidic acid (LPA). Constitutive Lpar1 null mutant mice have been instrumental in identifying roles for LPA-LPA1 signaling in neurobiological processes, brain development, and behavior, as well as modeling human neurological diseases like neuropathic pain. Constitutive Lpar1 null mutant mice are protected from partial sciatic nerve ligation (PSNL)-induced neuropathic pain, however, the cell types that are functionally responsible for mediating this protective effect are unknown. Here, we report the generation of an Lpar1flox/flox conditional null mutant mouse that allows for cre-mediated conditional deletion, combined with a PSNL pain model. Lpar1flox/flox mice were crossed with cre transgenic lines driven by neural gene promoters for nestin (all neural cells), synapsin (neurons), or P0 (Schwann cells). CD11b-cre transgenic mice were also used to delete Lpar1 in microglia. PSNL-initiated pain responses were reduced following cre-mediated Lpar1 deletion with all three neural promoters as well as the CD11b promoter, supporting involvement of Schwann cells, central and/or peripheral neurons, and microglia in mediating pain. Interestingly, rescue responses were nonidentical, implicating distinct roles for Lpar1-expressing cell types. Our results with a new Lpar1 conditional mouse mutant expand an understanding of LPA1 signaling in the PSNL model of neuropathic pain.

Neuroprotective effect of novel celecoxib derivatives against spinal cord injury via attenuation of COX-2, oxidative stress, apoptosisand inflammation

A novel series of celecoxib derivatives were synthesized and evaluated for cyclooxygenase (COX-1/COX-2) inhibitory activities for benefit in spinal cord injury (SCI). The title compounds were synthesized by conventional methods in good yields and subsequently tested for inhibitory activity against COX-1/COX-2. The most potent COX-2 inhibitor among the tested derivatives was further assayed for protective effect against experimental SCI of Sprague-Dawley rats. The designed compounds showed considerable inhibition of COX-2 as compared to COX-1 revealing compound 7m as most potent inhibitor of COX-2 isoenzyme (IC₅₀ = 0.04 µM). The expression of mitochondrial apoptotic genes (Bcl-2 and Bax) together with COX-2 and iNOS was restored near to normal as evidenced by western blot analysis in SCI rats. Taken altogether, compound 7m was identified as most potent inhibitor of COX-2. It also showed protective action against SCI via attenuation of COX-2, oxidative stress and apoptosis and inflammation in Male Sprague-Dawley rats.

Distribution and chemical coding of phoenixin-immunoreactive nerve structures in the spinal cord of the pig

Phoenixin (PNX) is a newly described peptide found in both neural and non-neural tissues. Until now, no attempts have been made to investigate the expression of PNX in the nervous system of animals other than laboratory rodents, in which an enzyme immunoassay revealed the highest quantity of the substance in the spinal cord. Since the domestic pig, due to its anatomical and histological resemblance to humans, is often used as an animal model in biomedical investigations, the present study was designed to examine PNX-immunoreactivity in the spinal cords of female pigs (n=5). The spinal cords were dissected and divided into the cervical, thoracic, lumbar, sacral and coccygeal segments, which were sectioned transversally into 10-μm-thick serial sections. The sections from each spinal cord segment were processed for double-labelling immunohistochemistry using antibodies against PNX in a mixture with those against calcitonin gene-related peptide (CGRP), substance P (SP) or choline acetyltransferase (CHAT). The PNX-immunoreactivity had a similar distribution in the grey matter of all the spinal cord sections examined and was mainly observed in varicose nerve fibres (NF) that formed a dense plexus in laminae I and II of the dorsal horn. Nearly all of the PNX-immunoreactive NF stained also for CGRP or SP and, interestingly, many of them were CHAT-positive. The present study has provided for the first time the detailed information on the arrangement and chemical features of nerve structures expressing PNX-immunoreactivity in the spinal cord of a large mammal. The exact function of PNX in the spinal cord is not known yet. However, the distribution pattern and immunohistochemical characteristics of PNX-IR NF clearly suggest that this peptite most likely plays a role in spinal noxious signalling.

Opioid Alternatives in Spine Surgery: A Narrative Review

Adequate analgesia is known to improve outcomes after spine surgery. Despite recent attention highlighting the negative effects of narcotics and their addiction potential, opioids have been the mainstay of management for providing analgesia following spine surgeries. However, side effects including hyperalgesia, tolerance, and subsequent dependence restrict the generous usage of opioids. Multimodal analgesia regimens acting through different mechanisms offer significant opioid sparing and minimize the side effects of individual drugs. Hence, they are being increasingly incorporated into enhanced recovery protocols. Multimodal analgesia includes drugs such as N-methyl-D-aspartate antagonists, nonsteroidal anti-inflammatory drugs and membrane-stabilizing agents, neuraxial opioids, local anesthetic infiltration, and fascial compartment blocks. Analgesia started before the painful stimulus, termed preemptive analgesia, facilitates subsequent pain management. Both nonsteroidal anti-inflammatory drugs and neuraxial analgesia have been conclusively shown to reduce opioid requirements after spine surgery, and there is a resurgence of interest in the use of low-dose ketamine or methadone. Neuraxial narcotics offer enhanced analgesia for a longer duration with lower dosage and side effect profiles compared with systemic opioid administration. Fascial compartment blocks are increasingly used as they provide effective analgesia with fewer adverse effects. In this narrative review, we will discuss multimodality analgesic regimens incorporating opioid-sparing adjuvants to manage pain after spine surgery.

Insights Into Spinal Dorsal Horn Circuit Function and Dysfunction Using Optical Approaches

Somatosensation encompasses a variety of essential modalities including touch, pressure, proprioception, temperature, pain, and itch. These peripheral sensations are crucial for all types of behaviors, ranging from social interaction to danger avoidance. Somatosensory information is transmitted from primary afferent fibers in the periphery into the central nervous system via the dorsal horn of the spinal cord. The dorsal horn functions as an intermediary processing center for this information, comprising a complex network of excitatory and inhibitory interneurons as well as projection neurons that transmit the processed somatosensory information from the spinal cord to the brain. It is now known that there can be dysfunction within this spinal cord circuitry in pathological pain conditions and that these perturbations contribute to the development and maintenance of pathological pain. However, the complex and heterogeneous network of the spinal dorsal horn has hampered efforts to further elucidate its role in somatosensory processing. Emerging optical techniques promise to illuminate the underlying organization and function of the dorsal horn and provide insights into the role of spinal cord sensory processing in shaping the behavioral response to somatosensory input that we ultimately observe. This review article will focus on recent advances in optogenetics and fluorescence imaging techniques in the spinal cord, encompassing findings from both in vivo and in vitro preparations. We will also discuss the current limitations and difficulties of employing these techniques to interrogate the spinal cord and current practices and approaches to overcome these challenges.

Preemptive Infiltration with Betamethasone and Ropivacaine for Postoperative Pain in Laminoplasty or Laminectomy (PRE-EASE): study protocol for a randomized controlled trial

BACKGROUND

Laminoplasty and laminectomy have been used for decades for the treatment of intraspinal space-occupying lesions, spinal stenosis, disc herniation, injuries, etc. After these procedures, patients often experience severe postoperative pain at the surgical site. Intense immediate postoperative pain after many spinal procedures makes its control of utmost importance. Preemptive injection of local anesthetics can significantly reduce postoperative pain during rest and movement; however, the analgesic effect is only maintained for a relatively short period of time. Whether betamethasone combined with local anesthetic for laminoplasty or laminectomy has better short-term and long-term effects than the local anesthetic alone has not been reported yet.

METHODS

The PRE-EASE trial is a prospective, randomized, open-label, blinded endpoint, single-center clinical study including 116 participants scheduled for elective laminoplasty or laminectomy, with a 6 months' follow-up process. Preemptive local infiltration with betamethasone and ropivacaine (treatment group) or ropivacaine alone (control group) throughout the entire thickness of the planned incision site will be performed by the surgeon prior to making the incision. The primary outcome will be the cumulative butorphanol consumption within the first 48-h postoperative period.

DISCUSSION

This study will add significant new knowledge to the effect and feasibility of preemptive local infiltration of betamethasone for postoperative pain management in laminoplasty and laminectomy.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT04153396. Registered on 6 November 2019.

Selected pathobiological features and principles of pharmacological pain management

Pain induced by inflammation and nerve injury arises from abnormal neural activity of primary afferent nociceptors in response to tissue damage, which causes long-term elevation of the sensitivity and responsiveness of spinal cord neurons. Inflammatory pain typically resolves following resolution of inflammation; however, nerve injury-either peripheral or central-may cause persistent neuropathic pain, which frequently manifests as hyperalgesia or allodynia. Neuralgias, malignant metastatic bone disease, and diabetic neuropathy are some of the conditions associated with severe, often unremitting chronic pain that is both physically and psychologically debilitating or disabling. Therefore, optimal pain management for patients with chronic neuropathic pain requires a multimodal approach that comprises pharmacological and psychological interventions. Non-opioid analgesics (e.g., paracetamol, aspirin, or other non-steroidal anti-inflammatory drugs) are first-line agents used in the treatment of mild-to-moderate acute pain, while opioids of increasing potency are indicated for the treatment of persistent, moderate-to-severe inflammatory pain. N-methyl D-aspartate receptor antagonists, antidepressants, anticonvulsants, or a combination of these should be considered for the treatment of chronic neuropathic pain. This review discusses the various neural signals that mediate acute and chronic pain, as well as the general principles of pain management.

Peripheral nitric oxide signaling directly blocks inflammatory pain

Pain is a classical sign of inflammation, and sensitization of primary sensory neurons (PSN) is the most important mediating mechanism. This mechanism involves direct action of inflammatory mediators such as prostaglandins and sympathetic amines. Pharmacologic control of inflammatory pain is based on two principal strategies: (i) non-steroidal anti-inflammatory drugs targeting inhibition of prostaglandin production by cyclooxygenases and preventing nociceptor sensitization in humans and animals; (ii) opioids and dipyrone that directly block nociceptor sensitization via activation of the NO signaling pathway. This review summarizes basic concepts of inflammatory pain that are necessary to understand the mechanisms of peripheral NO signaling that promote peripheral analgesia; we also discuss therapeutic perspectives based on the modulation of the NO pathway.

Impaired EAT-4 Vesicular Glutamate Transporter Leads to Defective Nocifensive Response of Caenorhabditis elegans to Noxious Heat

In mammals, glutamate is an important excitatory neurotransmitter. Glutamate and glutamate receptors are found in areas specifically involved in pain sensation, transmission and transduction such as peripheral nervous system, spinal cord and brain. In C. elegans, several studies have suggested glutamate pathways are associated with withdrawal responses to mechanical stimuli and to chemical repellents. However, few evidences demonstrate that glutamate pathways are important to mediate nocifensive response to noxious heat. The thermal avoidance behavior of C. elegans was studied and results illustrated that mutants of glutamate receptors (glr-1, glr-2, nmr-1, nmr-2) behaviors was not affected. However, results revealed that all strains of eat-4 mutants, C. elegans vesicular glutamate transporters, displayed defective thermal avoidance behaviors. Due to the interplay between the glutamate and the FLP-18/FLP-21/NPR-1 pathways, we analyzed the effectors FLP-18 and FLP-21 at the protein level, we did not observe biologically significant differences compared to N2 (WT) strain (fold-change < 2) except for the IK602 strain. The data presented in this manuscript reveals that glutamate signaling pathways are essential to elicit a nocifensive response to noxious heat in C. elegans.

Human experimental pain models

Pain is an unpleasant sensory experience, associated with existing or potential tissue damage. It has also strong cognitive and emotional components. Stimuli that causes pain goes through process of nociception, which includes transduction, transmission, modulation and perception of said stimuli. Depending on the type of stimuli, we can classify human experimental pain models into mechanical, electrical, thermal and chemical. Information about pain mechanisms can be obtained from the following: 1) in vitro studies, 2) animal experiments, 3) human experimental pain studies and 4) clinical studies. Chosing the appropriate method for pain evaluation is a key step in the design of pain studies. Combining it with different electro-physiological and imaging methods, it can provide better objectivity and quantification of pain mechanisms. Focus in experimental pain studies is slowly shifting from static parameters of pain, such as pain threshold and maximum tolerance, to dynamic parameters, which can give us valuable insight in function of endogenous analgesic systems. This can be done using conditioned pain modulation. Using experimental pain on healthy voulenteers is key step in switching from animal models to clinical studies, foremost for validization of data from animals, making them important in translational research. Results from experimental pain studies can help us in understanding nociceptive mechanisms of acute and chronic pain, alongside development of new therapeutic modalities.

Magnetic resonance spectroscopy across chronic pain disorders: a systematic review protocol synthesising anatomical and metabolite findings in chronic pain patients

BACKGROUND

Chronic pain is pain greater than 3 months duration that may result from disease, trauma, surgery, or unknown origin. The overlap between the psychological, behavioural, and management aspects of pain suggest that limbic brain neurochemistry plays a role in chronic pain pathology. Proton magnetic resonance spectroscopy (1H-MRS) can evaluate in vivo brain metabolites including creatine, N-acetylaspartate, myo-inositol, choline, glutamate, glutamine, and gamma-aminobutyric acid in chronic pain; however, a comprehensive systemic review of metabolite expression patterns across all brain areas has yet to be performed.

METHODS AND ANALYSIS

Online databases including PubMed/MEDLINE, Google Scholar, EMBASE, the Cochrane Library, OVID, and PsycINFO will be searched for articles relating to 1H-MRS and chronic pain. Study inclusion criteria will include ages of between 18 and 65 years with a definite diagnosis of chronic pain, no comorbidities, clearly stated brain volumes of interest, and imaging protocols, with comparisons to healthy controls. Two reviewers will extract data relating to volumes of interest, metabolites, study participant demographics, diagnostic method and pain scores, treatments and duration of treatment, scanner information, 1H-MRS acquisition protocols, and spectral processing software. Where possible, volumes of interest will be reassigned as regions of interest consistent with known regional anatomical and functional properties to increase the power and relevance of the analysis. Statistical analyses will then be conducted using STATA. A central common pathway may exist for chronic pain due to the behavioural manifestations and management similarities between its different types. The goal of this systemic review is to generate a comprehensive neurochemical theory of chronic pain in different brain compartments.

SYSTEMATIC REVIEW REGISTRATION

This study is registered with PROSPERO CRD42018112640.

Histamine H4 receptor stimulation in the locus coeruleus attenuates neuropathic pain by promoting the coeruleospinal noradrenergic inhibitory pathway

The locus coeruleus (LC) adrenergic nuclei constitute a pain-control inhibitory system nucleus implicated in descending modulation of pain through the action on spinal α₂-adrenoceptors. Histaminergic innervation from the tuberomammillary nucleus of the LC increases firing of noradrenergic neurons and might contribute to pain control. Here we evaluated the contribution of LC histaminergic innervation in descending modulation of neuropathic hypersensitivity, by investigating the role of the histamine H₄ receptor subtype in a mouse model of neuropathic pain. Intra LC administration of the H₄ agonist VUF 8430 attenuated mechanical and thermal allodynia of mice that underwent spared nerve injury (SNI). Similarly, histamine in the LC showed mechanical and thermal anti-hypersensitivity. Pretreatment of LC with JNJ 10191584 (H₄ antagonist) prevented the beneficial effect of VUF 8430 and histamine on nociceptive behaviour. Comparable results were obtained after intrathecal administration of drugs. The intrathecal administration of the α₂-adrenoceptor agonist clonidine ameliorated mechanical and thermal allodynia in SNI mice. The clonidine-induced anti-hypersensitivity effect was prevented by intra LC pretreatment with JNJ 10191584. In addition, clonidine failed to suppress neuropathic pain in H₄ deficient mice. LC H₄ receptors showed a ubiquitous distribution within LC, a neuronal localization and H₄ immunostaining was detected on noradrenergic neurons expressing phosphorylated cAMP response element-binding protein (CREB), a marker of neuronal activation. Under pain pathological conditions H₄ stimulation might promote the activation of the coeruleospinal noradrenergic neurons that exert an inhibitory control over spinal dorsal horn neuronal excitability. Thus, histamine H₄ receptor stimulation may represent a perspective for neuropathic pain management.

The effects of kinesiology taping on experimentally-induced thermal and mechanical pain in otherwise pain-free healthy humans: A randomised controlled repeated-measures laboratory study

Background

Kinesiology taping (KT) is used to manage musculoskeletal-related pain. There is a paucity of physiological studies evaluating the effect of KT on stimulus-evoked experimental pain.

Objective

To investigate the effect of KT (applied to lumbar region) on cutaneous somatosensation to noxious and innocuous stimuli in humans with a non-sensitised normally functioning nociceptive system using quantitative sensory testing (QST).

Methods

Fifty-four participants were randomised to one of three interventions: (i) KT (ii) standard ‘rigid’ taping (ST) (iii) sham taping (ShT). QST measurements were taken at lumbar sites pre-intervention (T1), during-intervention (T2) and during-intervention (T3) in the following sequence: warm-detection-threshold (WDT), heat-pain-threshold (HTPh), heat-pain-tolerance (HPTo), mechanical-detection-threshold (MDT), mechanical-pain-threshold (MPT) and pressure-pain-threshold (PPT).

Results

Mixed ANOVA revealed statistically significant interaction between Intervention and Time on MDT (p < .0005) and MPT (p < .0005) but not on WDT (p = .09), HPTh (p = .09), HPTo (p = .51) and PPT (p = .52) datasets. There was no significant simple main effect of Intervention on MDT at T2 (p = .68) and T3 (p = .24), and MPT at T2 (p = .79) and T3 (p = .54); post-hoc tests found KT and ST groups had higher (but non-significant) MDT and MPT than the ShT group. There was a significant simple main effect of Time on MDT and MPT for KT (p < .0005) and ST (p < .0005) groups; post-hoc tests found significant increases in MDT and MPT at T3 and T2 compared with T1 in both KT and ST groups. There was no significant simple main effect of Time on MDT (p = .13) nor MPT (p = .08) for the ShT group.

Conclusion

Taping, irrespective of the elasticity, may modulate cutaneous mechanosensation. KT, ST and ShT seemed to have similar influence on cutaneous thermal and deep pressure nociception.

A Review of the Effects of Pain and Analgesia on Immune System Function and Inflammation: Relevance for Preclinical Studies

One of the most significant challenges facing investigators, laboratory animal veterinarians, and IACUCs, is how to balance appropriate analgesic use, animal welfare, and analgesic impact on experimental results. This is particularly true for in vivo studies on immune system function and inflammatory disease. Often times the effects of analgesic drugs on a particular immune function or model are incomplete or don't exist. Further complicating the picture is evidence of the very tight integration and bidirectional functionality between the immune system and branches of the nervous system involved in nociception and pain. These relationships have advanced the concept of understanding pain as a protective neuroimmune function and recognizing pathologic pain as a neuroimmune disease. This review strives to summarize extant literature on the effects of pain and analgesia on immune system function and inflammation in the context of preclinical in vivo studies. The authors hope this work will help to guide selection of analgesics for preclinical studies of inflammatory disease and immune system function.

Remote Analgesic Effects Of Conventional Transcutaneous Electrical Nerve Stimulation: A Scientific And Clinical Review With A Focus On Chronic Pain

BACKGROUND

Transcutaneous electrical nerve stimulation (TENS) is a safe, noninvasive treatment for chronic pain that can be self-administered. Conventional TENS involves stimulation of peripheral sensory nerves at a strong, non-painful level. Following the original gate-control theory of pain, stimulation is typically near the target pain. As another option, remote stimulation may also be effective and offers potential advantages.

OBJECTIVE

This narrative review examines mechanisms underlying the remote analgesic effects of conventional TENS and appraises the clinical evidence.

METHODS

A literature search for English-language articles was performed on PubMed. Keywords included terms related to the location of TENS . Citations from primary references and textbooks were examined for additional articles.

RESULTS

Over 30 studies reported remote analgesic effects of conventional TENS. The evidence included studies using animal models of pain, experimental pain in humans, and clinical studies in subjects with chronic pain. Three types of remote analgesia were identified: at the contralateral homologous site, at sites distant from stimulation but innervated by overlapping spinal segments, and at unrelated extrasegmental sites.

CONCLUSION

There is scientific and clinical evidence that conventional TENS has remote analgesic effects. This may occur through modulation of pain processing at the level of the dorsal horn, in brainstem centers mediating descending inhibition, and within the pain matrix. A broadening of perspectives on how conventional TENS produces analgesia may encourage researchers, clinicians, and medical-device manufacturers to develop novel ways of using this safe, cost-effective neuromodulation technique for chronic pain.

Domains of Chronic Low Back Pain and Assessing Treatment Effectiveness: A Clinical Perspective

Nonspecific chronic low back pain (CLBP) is a common clinical condition that has impacts at both the individual and societal level. Pain intensity is a primary outcome used in clinical practice to quantify the severity of CLBP and the efficacy of its treatment; however, pain is a subjective experience that is impacted by a multitude of factors. Moreover, differences in effect sizes for pain intensity are not observed between common conservative treatments, such as spinal manipulative therapy, cognitive behavioral therapy, acupuncture, and exercise training. As pain science evolves, the biopsychosocial model is gaining interest in its application for CLBP management. The aim of this article is to discuss our current scientific understanding of pain and present why additional factors should be considered in conservative CLBP management. In addition to pain intensity, we recommend that clinicians should consider assessing the multidimensional nature of CLBP by including physical (disability, muscular strength and endurance, performance in activities of daily living, and body composition), psychological (kinesiophobia, fear-avoidance, pain catastrophizing, pain self-efficacy, depression, anxiety, and sleep quality), social (social functioning and work absenteeism), and health-related quality-of-life measures, depending on what is deemed relevant for each individual. This review also provides practical recommendations to clinicians for the assessment of outcomes beyond pain intensity, including information on how large a change must be for it to be considered "real" in an individual patient. This information can guide treatment selection when working with an individual with CLBP.