Nociceptive primary afferents: they have a mind of their own

Inhibitory modulation of action potentials in crayfish motor axons by fluoxetine

The goal of this report is to explore how K2P channels modulate axonal excitability by using the crayfish ventral superficial flexor preparation. This preparation allows for simultaneous recording of motor nerve extracellular action potentials (eAP) and intracellular excitatory junctional potential (EJP) from a muscle fiber. Previous pharmacological studies have demonstrated the presence of K2P-like channels in crayfish. Fluoxetine (50 µM) was used to block K2P channels in this study. The blocker caused a gradual decline, and eventually complete block, of motor axon action potentials. At an intermediate stage of the block, when the peak-to-peak amplitude of eAP decreased to ∼60%-80% of the control value, the amplitude of the initial positive component of eAP declined at a faster rate than that of the negative peak representing sodium influx. Furthermore, the second positive peak following this sodium influx, which corresponds to the after-hyperpolarizing phase of intracellularly recorded action potentials (iAP), became larger during the intermediate stage of eAP block. Finally, EJP recorded simultaneously with eAP showed no change in amplitude, but did show a significant increase in synaptic delay. These changes in eAP shape and EJP delay are interpreted as the consequence of depolarized resting membrane potential after K2P channel block. In addition to providing insights to possible functions of K2P channels in unmyelinated axons, results presented here also serve as an example of how changes in eAP shape contain information that can be used to infer alterations in intracellular events. This type of eAP-iAP cross-inference is valuable for gaining mechanistic insights here and may also be applicable to other model systems.

Toward a brighter constellation: multiorgan neuroimaging of neural and vascular dynamics in the spinal cord and brain

Significance

Pain comprises a complex interaction between motor action and somatosensation that is dependent on dynamic interactions between the brain and spinal cord. This makes understanding pain particularly challenging as it involves rich interactions between many circuits (e.g., neural and vascular) and signaling cascades throughout the body. As such, experimentation on a single region may lead to an incomplete and potentially incorrect understanding of crucial underlying mechanisms.

Aim

We aimed to develop and validate tools to enable detailed and extended observation of neural and vascular activity in the brain and spinal cord. The first key set of innovations was targeted to developing novel imaging hardware that addresses the many challenges of multisite imaging. The second key set of innovations was targeted to enabling bioluminescent (BL) imaging, as this approach can address limitations of fluorescent microscopy including photobleaching, phototoxicity, and decreased resolution due to scattering of excitation signals.

Approach

We designed 3D-printed brain and spinal cord implants to enable effective surgical implantations and optical access with wearable miniscopes or an open window (e.g., for one- or two-photon microscopy or optogenetic stimulation). We also tested the viability for BL imaging and developed a novel modified miniscope optimized for these signals (BLmini).

Results

We describe "universal" implants for acute and chronic simultaneous brain-spinal cord imaging and optical stimulation. We further describe successful imaging of BL signals in both foci and a new miniscope, the "BLmini," which has reduced weight, cost, and form-factor relative to standard wearable miniscopes.

Conclusions

The combination of 3D-printed implants, advanced imaging tools, and bioluminescence imaging techniques offers a coalition of methods for understanding spinal cord-brain interactions. Our work has the potential for use in future research into neuropathic pain and other sensory disorders and motor behavior.

Towards a Brighter Constellation: Multi-Organ Neuroimaging of Neural and Vascular Dynamics in the Spinal Cord and Brain

Significance

Pain is comprised of a complex interaction between motor action and somatosensation that is dependent on dynamic interactions between the brain and spinal cord. This makes understanding pain particularly challenging as it involves rich interactions between many circuits (e.g., neural and vascular) and signaling cascades throughout the body. As such, experimentation on a single region may lead to an incomplete and potentially incorrect understanding of crucial underlying mechanisms.

Aim

Here, we aimed to develop and validate new tools to enable detailed and extended observation of neural and vascular activity in the brain and spinal cord. The first key set of innovations were targeted to developing novel imaging hardware that addresses the many challenges of multi-site imaging. The second key set of innovations were targeted to enabling bioluminescent imaging, as this approach can address limitations of fluorescent microscopy including photobleaching, phototoxicity and decreased resolution due to scattering of excitation signals.

Approach

We designed 3D-printed brain and spinal cord implants to enable effective surgical implantations and optical access with wearable miniscopes or an open window (e.g., for one- or two-photon microscopy or optogenetic stimulation). We also tested the viability for bioluminescent imaging, and developed a novel modified miniscope optimized for these signals (BLmini).

Results

Here, we describe novel 'universal' implants for acute and chronic simultaneous brain-spinal cord imaging and optical stimulation. We further describe successful imaging of bioluminescent signals in both foci, and a new miniscope, the 'BLmini,' which has reduced weight, cost and form-factor relative to standard wearable miniscopes.

Conclusions

The combination of 3D printed implants, advanced imaging tools, and bioluminescence imaging techniques offers a new coalition of methods for understanding spinal cord-brain interactions. This work has the potential for use in future research into neuropathic pain and other sensory disorders and motor behavior.

Practice of Peritoneal Adhesions in Osteopathic Medicine: Part 2

Peritoneal adhesions are an unwanted and frequent event following abdominal surgery, with a response rate that can reach 100%. The adhesions can be symptomatic, becoming a source of pain and discomfort for the patient, or asymptomatic, with possible chronic or acute visceral dysfunction. The article reviews what the diagnostic strategies are and discusses what could be the causes that lead to chronic pain in the presence of adhesions. The text reports the knowledge of the literature on the manual treatment of adhesions and illustrates possible symptoms that are not easily recognized by the clinician. To conclude, the article proposes osteopathic manual approaches derived from clinical experience and from what has been explained about the formation of peritoneal adhesions. Research must make further efforts to identify not only the causes triggering the formation of peritoneal neogenesis but also seek the most appropriate non-invasive treatments to help the patient.

Neural population dynamics reveal that motor-targeted intraspinal microstimulation preferentially depresses nociceptive transmission in spinal cord injury-related neuropathic pain

The purpose of this study is to determine whether intraspinal microstimulation (ISMS) intended to enhance voluntary motor output after spinal cord injury (SCI) modulates neural population-level spinal responsiveness to nociceptive sensory feedback. The study was conducted in vivo in three cohorts of rats: neurologically intact, chronic SCI without behavioral signs of neuropathic pain, and chronic SCI with SCI-related neuropathic pain (SCI-NP). Nociceptive sensory feedback was induced by application of graded mechanical pressure to the plantar surface of the hindpaw before, during, and after periods of sub-motor threshold ISMS delivered within the motor pools of the L5 spinal segment. Neural population-level responsiveness to nociceptive feedback was recorded throughout the dorso-ventral extent of the L5 spinal segment using dense multi-channel microelectrode arrays. Whereas motor-targeted ISMS reduced nociceptive transmission across electrodes in neurologically intact animals both during and following stimulation, it was not associated with altered nociceptive transmission in rats with SCI that lacked behavioral signs of neuropathic pain. Surprisingly, nociceptive transmission was reduced both during and following motor-targeted ISMS in rats with SCI-NP, and to an extent comparable to that of neurologically intact animals. The mechanisms underlying the differential anti-nociceptive effects of motor-targeted ISMS are unclear, although they may be related to differences in the intrinsic active membrane properties of spinal neurons across the cohorts. Nevertheless, the results of this study support the notion that it may be possible to purposefully engineer spinal stimulation-based therapies that afford multi-modal rehabilitation benefits, and specifically that it may be possible to do so for the individuals most in need - i.e., those with SCI-related movement impairments and SCI-NP.

Motor-targeted spinal stimulation promotes concurrent rebalancing of pathologic nociceptive transmission in chronic spinal cord injury

Electrical stimulation of spinal networks below a spinal cord injury (SCI) is a promising approach to restore functions compromised by inadequate excitatory neural drive. The most translationally successful examples are paradigms intended to increase neural transmission in weakened yet spared motor pathways and spinal motor networks rendered dormant after being severed from their inputs by lesion. Less well understood is whether spinal stimulation is also capable of reducing neural transmission in pathways made pathologically overactive by SCI. Debilitating spasms, spasticity, and neuropathic pain are all common manifestations of hyperexcitable spinal responses to sensory feedback. But whereas spasms and spasticity can often be managed pharmacologically, SCI-related neuropathic pain is notoriously medically refractory. Interestingly, however, spinal stimulation is a clinically available option for ameliorating neuropathic pain arising from etiologies other than SCI, and it has traditionally been assumed to modulate sensorimotor networks overlapping with those engaged by spinal stimulation for motor rehabilitation. Thus, we reasoned that spinal stimulation intended to increase transmission in motor pathways may simultaneously reduce transmission in spinal pain pathways. Using a well-validated pre-clinical model of SCI that results in severe bilateral motor impairments and SCI-related neuropathic pain, we show that the responsiveness of neurons integral to the development and persistence of the neuropathic pain state can be enduringly reduced by motor-targeted spinal stimulation while preserving spinal responses to non-pain-related sensory feedback. These results suggest that spinal stimulation paradigms could be intentionally designed to afford multi-modal therapeutic benefits, directly addressing the diverse, intersectional rehabilitation goals of people living with SCI.

Identification of Potential Visceral Pain Biomarkers in Colon Exudates from Mice with Experimental Colitis: An Exploratory In Vitro Study

Chronic visceral pain (CVP) is extremely difficult to diagnose, and available analgesic treatment options are quite limited. Identifying the proteins secreted from the colonic nociceptors, or their neighbor cells within the tube walls, in the context of disorders that course with visceral pain, might be useful to decipher the mechanism involved in the establishment of CVP. Addressing this question in human with gastrointestinal disorders entails multiple difficulties, as there is not a clear classification of disease severity, and colonic secretion is not easy to manage. We propose using of a murine model of colitis to identify new algesic molecules and pathways that could be explored as pain biomarkers or analgesia targets. Descending colons from naïve and colitis mice with visceral hyperalgesia were excised and maintained ex vivo. The proteins secreted in the perfusion fluid before and during acute noxious distension were evaluated using high-resolution mass spectrometry (MS). Haptoglobin (Hp), PZD and LIM domain protein 3 (Pdlim3), NADP-dependent malic enzyme (Me1), and Apolipoprotein A-I (Apoa1) were increased during visceral insult, whilst Triosephosphate isomerase (Tpi1), Glucose-6-phosphate isomerase (Gpi1), Alpha-enolase (Eno1), and Isoform 2 of Tropomyosin alpha-1 chain (Tpm1) were decreased. Most identified proteins have been described in the context of different chronic pain conditions and, according to gene ontology analysis, they are also involved in diverse biological processes of relevance. Thus, animal models that mimic human conditions in combination with unbiased omics approaches will ultimately help to identify new pathophysiological mechanisms underlying pain that might be useful in diagnosing and treating pain. PERSPECTIVE: Our study utilizes an unbiased proteomic approach to determine, first, the clinical relevance of a murine model of colitis and, second, to identify novel molecules/pathways involved in nociception that would be potential biomarkers or targets for chronic visceral pain.

Age-related changes in peripheral nociceptor function

Pain and pain management in the elderly population is a significant social and medical problem. Pain sensation is a complex phenomenon that typically involves activation of peripheral pain-sensing neurons (nociceptors) which send signals to the spinal cord and brain that are interpreted as pain, an unpleasant sensory experience. In this work, young (4-5 months) and aged (26-27 months) Fischer 344 x Brown Norway (F344xBN) rats were examined for nociceptor sensitivity to activation by thermal (cold and heat) and mechanical stimulation following treatment with inflammatory mediators and activators of transient receptor potential (TRP) channels. Unlike other senses that decrease in sensitivity with age, sensitivity of hindpaw nociceptors to thermal and mechanical stimulation was not different between young and aged F344xBN rats. Intraplantar injection of bradykinin (BK) produced greater thermal and mechanical allodynia in aged versus young rats, whereas only mechanical allodynia was greater in aged rats following injection of prostaglandin E₂ (PGE₂). Intraplantar injection of TRP channel activators, capsaicin (TRPV1), mustard oil (TRPA1) and menthol (TRPM8) each resulted in greater mechanical allodynia in aged versus young rats and capsaicin-induced heat allodynia was also greater in aged rats. A treatment-induced allodynia that was greater in young rats was never observed. The anti-allodynic effects of intraplantar injection of kappa and delta opioid receptor agonists, salvinorin-A and D-Pen²,D-Pen⁵]enkephalin (DPDPE), respectively, were greater in aged than young rats, whereas mu opioid receptor agonists, [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) and morphine, were not effective in aged rats. Consistent with these observations, in primary cultures of peripheral sensory neurons, inhibition of cAMP signaling in response to delta and kappa receptor agonists was greater in cultures derived from aged rats. By contrast, mu receptor agonists did not inhibit cAMP signaling in aged rats. Thus, age-related changes in nociceptors generally favor increased pain signaling in aged versus young rats, suggesting that changes in nociceptor sensitivity may play a role in the increased incidence of pain in the elderly population. These results also suggest that development of peripherally-restricted kappa or delta opioid receptor agonists may provide safer and effective pain relief for the elderly.

Neuronal-Immune Cell Units in Allergic Inflammation in the Nose

Immune cells and immune-derived molecules, endocrine glands and hormones, the nervous system and neuro molecules form the combined tridirectional neuroimmune network, which plays a significant role in the communication pathways and regulation at the level of the whole organism and local levels, in both healthy persons and patients with allergic rhinitis based on an allergic inflammatory process. This review focuses on a new research paradigm devoted to neuronal-immune cell units, which are involved in allergic inflammation in the nose and neuroimmune control of the nasal mucociliary immunologically active epithelial barrier. The categorization, cellular sources of neurotransmitters and neuropeptides, and their prevalent profiles in constituting allergen tolerance maintenance or its breakdown are discussed. Novel data on the functional structure of the nasal epithelium based on a transcriptomic technology, single-cell RNA-sequencing results, are considered in terms of neuroimmune regulation. Notably, the research of pathogenesis and therapy for atopic allergic diseases, including recently identified local forms, from the viewpoint of the tridirectional interaction of the neuroimmune network and discrete neuronal-immune cell units is at the cutting-edge.

Advancing the Understanding of Acupoint Sensitization and Plasticity Through Cutaneous C-Nociceptors

Acupoint is the key area for needling treatment, but its physiology is not yet understood. Nociceptors, one of the responders in acupoints, are responsible for acupuncture manipulation and delivering acupuncture signals to the spinal or supraspinal level. Recent evidence has shown that various diseases led to sensory hypersensitivity and functional plasticity in sensitized acupoints, namely, acupoint sensitization. Neurogenic inflammation is the predominant pathological characteristic for sensitized acupoints; however, the underlying mechanism in acupoint sensitization remains unclear. Recent studies have reported that silent C-nociceptors (SNs), a subtype of C nociceptors, can be “awakened” by inflammatory substances released by sensory terminals and immune cells under tissue injury or visceral dysfunction. SNs can transform from mechano-insensitive nociceptors in a healthy state to mechanosensitive nociceptors. Activated SNs play a vital role in sensory and pain modulation and can amplify sensory inputs from the injured tissue and then mediate sensory hyperalgesia. Whether activated SNs is involved in the mechanism of acupoint sensitization and contributes to the delivery of mechanical signals from needling manipulation remains unclear? In this review, we discuss the known functions of cutaneous C nociceptors and SNs and focus on recent studies highlighting the role of activated SNs in acupoint functional plasticity.

Neurogenic inflammation as a novel treatment target for chronic pain syndromes

Chronic pain syndrome is a heterogeneous group of diseases characterized by several pathological mechanisms. One in five adults in Europe may experience chronic pain. In addition to the individual burden, chronic pain has a significant societal impact because of work and school absences, loss of work, early retirement, and high social and healthcare costs. Several anti-inflammatory treatments are available for patients with inflammatory or autoimmune diseases to control their symptoms, including pain. However, patients with degenerative chronic pain conditions, some with 10-fold or more elevated incidence relative to these manageable diseases, have few long-term pharmacological treatment options, limited mainly to non-steroidal anti-inflammatory drugs or opioids. For this review, we performed multiple PubMed searches using keywords such as "pain," "neurogenic inflammation," "NGF," "substance P," "nociception," "BDNF," "inflammation," "CGRP," "osteoarthritis," and "migraine." Many treatments, most with limited scientific evidence of efficacy, are available for the management of chronic pain through a trial-and-error approach. Although basic science and pre-clinical pain research have elucidated many biomolecular mechanisms of pain and identified promising novel targets, little of this work has translated into better clinical management of these conditions. This state-of-the-art review summarizes concepts of chronic pain syndromes and describes potential novel treatment strategies.

The Emerging Pro-Algesic Profile of Transient Receptor Potential Vanilloid Type 4

Transient receptor potential vanilloid type 4 (TRPV4) channels are Ca²⁺-permeable non-selective cation channels which mediate a wide range of physiological functions and are activated and modulated by a diverse array of stimuli. One of this ion channel's least discussed functions is in relation to the generation and maintenance of certain pain sensations. However, in the two decades which have elapsed since the identification of this ion channel, considerable data has emerged concerning its function in mediating pain sensations. TRPV4 is a mediator of mechanical hyperalgesia in the various contexts in which a mechanical stimulus, comprising trauma (at the macro-level) or discrete extracellular pressure or stress (at the micro-level), results in pain. TRPV4 is also recognised as constituting an essential component in mediating inflammatory pain. It also plays a role in relation to many forms of neuropathic-type pain, where it functions in mediating mechanical allodynia and hyperalgesia.Here, we review the role of TRPV4 in mediating pain sensations.

Renal Denervation Exacerbates LPS- and Antibody-induced Acute Kidney Injury, but Protects from Pyelonephritis in Mice

BACKGROUND

Renal denervation (RDN) is an invasive intervention to treat drug-resistant arterial hypertension. Its therapeutic value is contentious. Here we examined the effects of RDN on inflammatory and infectious kidney disease models in mice.

METHODS

Mice were unilaterally or bilaterally denervated, or sham operated, then three disease models were induced: nephrotoxic nephritis (NTN, a model for crescentic GN), pyelonephritis, and acute endotoxemic kidney injury (as a model for septic kidney injury). Analytical methods included measurement of renal glomerular filtration, proteinuria, flow cytometry of renal immune cells, immunofluorescence microscopy, and three-dimensional imaging of optically cleared kidney tissue by light-sheet fluorescence microscopy followed by algorithmic analysis.

RESULTS

Unilateral RDN increased glomerular filtration in denervated kidneys, but decreased it in the contralateral kidneys. In the NTN model, more nephritogenic antibodies were deposited in glomeruli of denervated kidneys, resulting in stronger inflammation and injury in denervated compared with contralateral nondenervated kidneys. Also, intravenously injected LPS increased neutrophil influx and inflammation in the denervated kidneys, both after unilateral and bilateral RDN. When we induced pyelonephritis in bilaterally denervated mice, both kidneys contained less bacteria and neutrophils. In unilaterally denervated mice, pyelonephritis was attenuated and intrarenal neutrophil numbers were lower in the denervated kidneys. The nondenervated contralateral kidneys harbored more bacteria, even compared with sham-operated mice, and showed the strongest influx of neutrophils.

CONCLUSIONS

Our data suggest that the increased perfusion and filtration in denervated kidneys can profoundly influence concomitant inflammatory diseases. Renal deposition of circulating nephritic material is higher, and hence antibody- and endotoxin-induced kidney injury was aggravated in mice. Pyelonephritis was attenuated in denervated murine kidneys, because the higher glomerular filtration facilitated better flushing of bacteria with the urine, at the expense of contralateral, nondenervated kidneys after unilateral denervation.

Neurogenic substance P-influences on action potential production in afferent neurons of the kidney?

We recently showed that a substance P (SP)–dependent sympatho-inhibitory mechanism via afferent renal nerves is impaired in mesangioproliferative nephritis. Therefore, we tested the hypothesis that SP released from renal afferents inhibits the action potential (AP) production in their dorsal root ganglion (DRG) neurons. Cultured DRG neurons (Th11-L2) were investigated in current clamp mode to assess AP generation during both TRPV1 stimulation by protons (pH 6) and current injections with and without exposure to SP (0.5 µmol) or CGRP (0.5 µmol). Neurons were classified as tonic (sustained AP generation) or phasic (≤ 4 APs) upon current injection; voltage clamp experiments were performed for the investigation of TRPV1-mediated inward currents due to proton stimulation. Superfusion of renal neurons with protons and SP increased the number of action potentials in tonic neurons (9.6 ± 5 APs/10 s vs. 16.9 ± 6.1 APs/10 s, P < 0.05, mean ± SD, n = 7), while current injections with SP decreased it (15.2 ± 6 APs/600 ms vs. 10.2 ± 8 APs/600 ms, P < 0.05, mean ± SD, n = 29). Addition of SP significantly reduced acid-induced TRPV1-mediated currents in renal tonic neurons (− 518 ± 743 pA due to pH 6 superfusion vs. − 82 ± 50 pA due to pH 6 with SP superfusion). In conclusion, SP increased action potential production via a TRPV1-dependent mechanism in acid-sensitive renal neurons. On the other hand, current injection in the presence of SP led to decreased action potential production. Thus, the peptide SP modulates signaling pathways in renal neurons in an unexpected manner leading to both stimulation and inhibition of renal neuronal activity in different (e.g., acidic) environmental contexts.

Experimentally induced spine osteoarthritis in rats leads to neurogenic inflammation within neurosegmentally linked myotomes

Naturally occurring spine osteoarthritis is clinically associated with the manifestation of chronic inflammatory muscle (myofascial) disease. The purpose of this study was to investigate the causal association between experimentally induced spine osteoarthritis and neurogenic inflammatory responses within neurosegmentally linked myotomes. Wistar Kyoto rats were randomly assigned to spine facet compression surgery (L4-L6) or sham surgery. Animals exposed to facet compression surgery demonstrated radiographic signs of facet-osteoarthritis (L4-L6 spinal levels) and sensory changes (allodynia, thermal hyperalgesia) at 7, 14 and 21 days post-intervention, consistent with the induction of central sensitization; no radiologic or sensory changes were observed after sham surgery. Increased levels of proinflammatory biomarkers including substance P (SP), calcitonin gene related peptide (CGRP), protease-activated receptor-2 (PAR2) and calcium/calmodulin dependent protein kinase II (CaMKII) were observed post-surgery within neurosegmentally-linked rectus femoris (L2-L5) muscle when compared to the non-segmentally linked biceps brachii (C4-C7) muscle; no differences were observed between muscles in the sham surgery group. These findings offer novel insight into the potential role of spine osteoarthritis and neurogenic inflammatory mechanisms in the pathophysiology of chronic inflammatory muscle (myofascial) disease.

Neurogenic tachykinin mechanisms in experimental nephritis of rats

We demonstrated earlier that renal afferent pathways combine very likely “classical” neural signal transduction to the central nervous system and a substance P (SP)–dependent mechanism to control sympathetic activity. SP content of afferent sensory neurons is known to mediate neurogenic inflammation upon release. We tested the hypothesis that alterations in SP-dependent mechanisms of renal innervation contribute to experimental nephritis. Nephritis was induced by OX-7 antibodies in rats, 6 days later instrumented for recording of blood pressure (BP), heart rate (HR), drug administration, and intrarenal administration (IRA) of the TRPV1 agonist capsaicin to stimulate afferent renal nerve pathways containing SP and electrodes for renal sympathetic nerve activity (RSNA). The presence of the SP receptor NK-1 on renal immune cells was assessed by FACS. IRA capsaicin decreased RSNA from 62.4 ± 5.1 to 21.6 ± 1.5 mV s (*p < 0.05) in controls, a response impaired in nephritis. Suppressed RSNA transiently but completely recovered after systemic administration of a neurokinin 1 (NK1-R) blocker. NK-1 receptors occurred mainly on CD11⁺ dendritic cells (DCs). An enhanced frequency of CD11c⁺NK1R⁺ cell, NK-1 receptor⁺ macrophages, and DCs was assessed in nephritis. Administration of the NK-1R antagonist aprepitant during nephritis reduced CD11c⁺NK1R⁺ cells, macrophage infiltration, renal expression of chemokines, and markers of sclerosis. Hence, SP promoted renal inflammation by weakening sympathoinhibitory mechanisms, while at the same time, substance SP released intrarenally from afferent nerve fibers aggravated immunological processes i.e. by the recruitment of DCs.

Afferent renal innervation in anti-Thy1.1 nephritis in rats

Afferent renal nerves exhibit a dual function controlling central sympathetic outflow via afferent electrical activity and influencing intrarenal immunological processes by releasing peptides such as calcitonin gene-related peptide (CGRP). We tested the hypothesis that increased afferent and efferent renal nerve activity occur with augmented release of CGRP in anti-Thy1.1 nephritis, in which enhanced CGRP release exacerbates inflammation. Nephritis was induced in Sprague-Dawley rats by intravenous injection of OX-7 antibody (1.75 mg/kg), and animals were investigated neurophysiologically, electrophysiologically, and pathomorphologically 6 days later. Nephritic rats exhibited proteinuria (169.3 ± 10.2 mg/24 h) with increased efferent renal nerve activity (14.7 ± 0.9 bursts/s vs. control 11.5 ± 0.9 bursts/s, n = 11, P < 0.05). However, afferent renal nerve activity (in spikes/s) decreased in nephritis (8.0 ± 1.8 Hz vs. control 27.4 ± 4.1 Hz, n = 11, P < 0.05). In patch-clamp recordings, neurons with renal afferents from nephritic rats showed a lower frequency of high activity following electrical stimulation (43.4% vs. 66.4% in controls, P < 0.05). In vitro assays showed that renal tissue from nephritic rats exhibited increased CGRP release via spontaneous (14 ± 3 pg/mL vs. 6.8 ± 2.8 pg/ml in controls, n = 7, P < 0.05) and stimulated mechanisms. In nephritic animals, marked infiltration of macrophages in the interstitium (26 ± 4 cells/mm²) and glomeruli (3.7 ± 0.6 cells/glomerular cross-section) occurred. Pretreatment with the CGRP receptor antagonist CGRP_8-37 reduced proteinuria, infiltration, and proliferation. In nephritic rats, it can be speculated that afferent renal nerves lose their ability to properly control efferent sympathetic nerve activity while influencing renal inflammation through increased CGRP release.

Expression and effect of sodium-potassium-chloride cotransporter on dorsal root ganglion neurons in a rat model of chronic constriction injury

Sodium-potassium-chloride cotransporter 1 (NKCC1) and potassium-chloride cotransporter 2 (KCC2) are associated with the transmission of peripheral pain. We investigated whether the increase of NKCC1 and KCC2 is associated with peripheral pain transmission in dorsal root ganglion neurons. To this aim, rats with persistent hyperalgesia were randomly divided into four groups. Rats in the control group received no treatment, and the rat sciatic nerve was only exposed in the sham group. Rats in the chronic constriction injury group were established into chronic constriction injury models by ligating sciatic nerve and rats were given bumetanide, an inhibitor of NKCC1, based on chronic constriction injury modeling in the chronic constriction injury + bumetanide group. In the experiment measuring thermal withdrawal latency, bumetanide (15 mg/kg) was intravenously administered. In the patch clamp experiment, bumetanide (10 µg/µL) and acutely isolated dorsal root ganglion neurons (on day 14) were incubated for 1 hour, or bumetanide (5 µg/µL) was intrathecally injected. The Hargreaves test was conducted to detect changes in thermal hyperalgesia in rats. We found that the thermal withdrawal latency of rats was significantly decreased on days 7, 14, and 21 after model establishment. After intravenous injection of bumetanide, the reduction in thermal retraction latency caused by model establishment was significantly inhibited. Immunohistochemistry and western blot assay results revealed that the immune response and protein expression of NKCC1 in dorsal root ganglion neurons of the chronic constriction injury group increased significantly on days 7, 14, and 21 after model establishment. No immune response or protein expression of KCC2 was observed in dorsal root ganglion neurons before and after model establishment. The Cl^– (chloride ion) fluorescent probe technique was used to evaluate the change of Cl^– concentration in dorsal root ganglion neurons of chronic constriction injury model rats. We found that the relative optical density of N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide (a Cl^– fluorescent probe whose fluorescence intensity decreases as Cl^– concentration increases) in the dorsal root ganglion neurons of the chronic constriction injury group was significantly decreased on days 7 and 14 after model establishment. The whole-cell patch clamp technique revealed that the resting potential and action potential frequency of dorsal root ganglion neurons increased, and the threshold and rheobase of action potentials decreased in the chronic constriction injury group on day 14 after model establishment. After bumetanide administration, the above indicators were significantly suppressed. These results confirm that CCI can induce abnormal overexpression of NKCC1, thereby increasing the Cl^– concentration in dorsal root ganglion neurons; this then enhances the excitability of dorsal root ganglion neurons and ultimately promotes hyperalgesia and allodynia. In addition, bumetanide can achieve analgesic effects. All experiments were approved by the Institutional Ethics Review Board at the First Affiliated Hospital, College of Medicine, Shihezi University, China on February 22, 2017 (approval No. A2017-169-01).

Involvement of serotonergic and opioidergic systems in the antinociceptive effect of ketamine-magnesium sulphate combination in formalin test in rats

BACKGROUND

Ketamine and magnesium sulphate showed synergic interaction in the tail-immersion test and additive interaction in the rat formalin test. Aim of study was to evaluate the influence of serotonergic and opioidergic system of this combination in the formalin test in rats.

METHODS

Antinociceptive activity was assessed by the formalin test in male Wistar rats (200-250 g). Antagonists (naloxone and methysergide) were administrated 5 min before and magnesium sulphate 5 min after ketamine injection. Formalin (2.5%, 100 μL) was injected into the right hind paw surface (intraplantar) of rats 5 min after ketamine/magnesium combination. Data were recorded as the total time spent in pain related behavior after the injection of formalin or vehicle (0.9% NaCl).

RESULTS

In the intermediate phase of the formalin test, methysergide at a dose of 0.2 mg/kg did not have any effect, but at doses of 0.5 and 1 mg/kg it had a pronociceptive effect. Methysergide (0.2, 0.5 and 1 mg/kg) inhibited the antinociceptive effect of ketamine-magnesium sulphate combination. In the intermediate phase, naloxone at a dose of 0.2 mg/kg did not have any effect, but at a dose of 3 mg/kg it produced a pronociceptive effect. Naloxone (0.2 and 3 mg/kg) antagonized the antinociceptive effect of the ketamine (5 mg/kg)-magnesium sulphate (5 mg/kg) combination.

CONCLUSION

The results of the present study suggest that serotonergic and opioidergic systems are involved, at least in part, in the antinociceptive effect of the ketamine-magnesium sulphate combination in the model of inflammatory pain in rats.

Changes in inflammatory plasma proteins from patients with chronic pain associated with treatment in an interdisciplinary multimodal rehabilitation program – an explorative multivariate pilot study

It has been suggested that alterations in inflammation molecules maintain chronic pain although little is known about how these factors influence homeostatic and inflammatory events in common chronic pain conditions. Nonpharmacological interventions might be associated with alterations in inflammation markers in blood. This study of patients with chronic pain investigates whether an interdisciplinary multimodal rehabilitation program (IMMRP) was associated with significant alterations in the plasma pattern of 68 cytokines/chemokines 1 year after rehabilitation and whether such changes were associated with clinical changes. Blood samples and self-reports of pain, psychological distress, and physical activity of 25 complex chronic pain patients were collected pre-IMMRP and at 12-month follow-up. Analyses of inflammatory proteins (cytokines/chemokines/growth factors) were performed directly in plasma using the multiplex immunoassay technology Meso Scale Discovery. This explorative pilot study found that 12 substances, mainly pro-inflammatory, decreased after IMMRP. In two other relatively small IMMRP studies, four of these proinflammatory markers were also associated with decreases. The pattern of cytokines/chemokines pre-IMMRP was associated with changes in psychological distress but not with pain or physical activity. The present study cannot impute cause and effect. These results together with the results of the two previous IMMRP studies suggest that there is a need for larger and more strictly controlled studies of IMMRP with respect to inflammatory markers in blood. Such studies need to consider responders/non-responders, additional therapies, involved pain mechanisms and diagnoses. This and the two other studies open up for developing biologically measurable outcomes from plasma. Such biomarkers will be an important tool for further development of IMMRP and possibly other treatments for patients w ith chronic pain.

Pituitary Adenylate Cyclase-Activating Polypeptide: 30 Years in Research Spotlight and 600 Million Years in Service

Emerging from the depths of evolution, pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptors (i.e., PAC1, VPAC1, VPAC2) are present in multicellular organisms from Tunicates to humans and govern a remarkable number of physiological processes. Consequently, the clinical relevance of PACAP systems spans a multifaceted palette that includes more than 40 disorders. We aimed to present the versatility of PACAP1-38 actions with a focus on three aspects: (1) when PACAP1-38 could be a cause of a malfunction, (2) when PACAP1-38 could be the cure for a malfunction, and (3) when PACAP1-38 could either improve or impair biology. PACAP1-38 is implicated in the pathophysiology of migraine and post-traumatic stress disorder whereas an outstanding protective potential has been established in ischemia and in Alzheimer’s disease. Lastly, PACAP receptors could mediate opposing effects both in cancers and in inflammation. In the light of the above, the duration and concentrations of PACAP agents must be carefully set at any application to avoid unwanted consequences. An enormous amount of data accumulated since its discovery (1989) and the first clinical trials are dated in 2017. Thus in the field of PACAP research: “this is not the end, not even the beginning of the end, but maybe the end of the beginning.”

Lateral epicondylalgia: A primary nervous system disorder

Lateral epicondylalgia (LE) is the most common chronic painful condition affecting the elbow in the general population. Although major advances have been accomplished in recent years in the understanding of LE, the underlying physiopathology is still a reason for debate. Differences in clinical presentation and evolution of the symptoms among patients, suggest the need for revisiting the current knowledge about subjacent mechanisms that attempt to explain pain and functional loss. Previous models have suggested that the condition is mainly a degenerative tendinopathy, associated with changes in pain pathways and the motor system. The hypothesis of this work is that LE is the clinical manifestation of a primary nervous system disorder, characterized by an abnormal increase in neuronal activity and a subsequent loss of homeostasis, which secondarily affects the musculoskeletal tissues of the elbow-forearm-hand complex. A new model for LE is presented, supported by an in-deep analysis of basic sciences, epidemiological and clinical studies.

Plasma Protein Pattern Correlates With Pain Intensity and Psychological Distress in Women With Chronic Widespread Pain

Objectives: Although generalized muscle pain, tiredness, anxiety, and depression are commonly present among chronic widespread pain (CWP) patients, the molecular mechanisms behind CWP are not fully elucidated. Moreover, the lack of biomarkers often makes diagnosis and treatment problematic. In this study, we investigated the correlation between pain intensity, psychological distress, and plasma proteins among CWP patients and controls (CON). Methods: The plasma proteome of CWP (n = 15) and CON (n = 23) was analyzed using two-dimensional gel electrophoresis. Orthogonal Partial Least Square analysis (OPLS) was used to determine proteins associated with pain intensity (numeric rating scale) in CWP and psychological distress (Hospital and Depression Scale, HADS) in CWP and CON. Significant proteins were identified by MALDI-TOF and tandem MS. Results: In CWP, pain intensity was associated with plasma proteins mostly involved in metabolic and immunity processes (e.g., kininogen-1, fibrinogen gamma chain, and ceruloplasmin), and psychological distress was associated with plasma proteins related to immunity response, iron ion, and lipid metabolism (e.g., complement factor B, complement C1r subcomponent, hemopexin, and clusterin). Discussion: This study suggests that different plasma protein patterns are associated with different pain intensity and psychological distress in CWP. Proteins belonging to the coagulation cascade and immunity processes showed strong associations to each clinical outcome. Using the plasma proteome profile of CWP to study potential biomarker candidates provides a snapshot of ongoing systemic mechanisms in CWP.

Neuro-immune interactions in allergic diseases: novel targets for therapeutics

Recent studies have highlighted an emerging role for neuro-immune interactions in mediating allergic diseases. Allergies are caused by an overactive immune response to a foreign antigen. The peripheral sensory and autonomic nervous system densely innervates mucosal barrier tissues including the skin, respiratory tract and gastrointestinal (GI) tract that are exposed to allergens. It is increasingly clear that neurons actively communicate with and regulate the function of mast cells, dendritic cells, eosinophils, Th2 cells and type 2 innate lymphoid cells in allergic inflammation. Several mechanisms of cross-talk between the two systems have been uncovered, with potential anatomical specificity. Immune cells release inflammatory mediators including histamine, cytokines or neurotrophins that directly activate sensory neurons to mediate itch in the skin, cough/sneezing and bronchoconstriction in the respiratory tract and motility in the GI tract. Upon activation, these peripheral neurons release neurotransmitters and neuropeptides that directly act on immune cells to modulate their function. Somatosensory and visceral afferent neurons release neuropeptides including calcitonin gene-related peptide, substance P and vasoactive intestinal peptide, which can act on type 2 immune cells to drive allergic inflammation. Autonomic neurons release neurotransmitters including acetylcholine and noradrenaline that signal to both innate and adaptive immune cells. Neuro-immune signaling may play a central role in the physiopathology of allergic diseases including atopic dermatitis, asthma and food allergies. Therefore, getting a better understanding of these cellular and molecular neuro-immune interactions could lead to novel therapeutic approaches to treat allergic diseases.

Morphological and functional diversity of first-order somatosensory neurons

First-order somatosensory neurons transduce and convey information about the external or internal environment of the body to the central nervous system. They are pseudo unipolar neurons with cell bodies residing in one of several ganglia located near the central nervous system, with the short branch of the axon connecting to the spinal cord or the brain stem and the long branch extending towards the peripheral organ they innervate. Besides their sensory transducer and conductive role, somatosensory neurons also have trophic functions in the tissue they innervate and participate in local reflexes in the periphery. The cell bodies of these neurons are remarkably diverse in terms of size, molecular constitution, and electrophysiological properties. These parameters have provided criteria for classification that have proved useful to establish and study their functions. In this review, we discuss ways to measure and classify populations of neurons based on their size and action potential firing pattern. We also discuss attempts to relate the different populations to specific sensory modalities.

Occlusion of blood flow attenuates exercise-induced hypoalgesia in the occluded limb of healthy adults

Animal studies have demonstrated an important role of peripheral mechanisms as contributors to exercise-induced hypoalgesia (EIH). Whether these same mechanisms contribute to EIH in humans is not known. In the current study, pain thresholds were assessed in healthy volunteers ( n = 36) before and after 5 min of high-intensity leg cycling exercise and an equivalent period of quiet rest. Pressure pain thresholds (PPTs) were assessed over the rectus femoris muscle of one leg and first dorsal interosseous muscles (FDIs) of both arms. Blood flow to one arm was occluded by a cuff throughout the 5-min period of exercise (or rest) and postexercise (or rest) assessments. Ratings of pain intensity and pain unpleasantness during occlusion were also measured. Pain ratings during occlusion increased over time (range, 1.5 to 3.5/10, all d > 0.63, P < 0.001) similarly in the rest and exercise conditions ( d < 0.35, P > 0.4). PPTs at all sites were unchanged following rest (range, −1.3% to +0.9%, all d < 0.05, P > 0.51). Consistent with EIH, exercise significantly increased PPT at the leg (+29%, d = 0.69, P < 0.001) and the nonoccluded (+23%, d = 0.56, P < 0.001) and occluded (+8%, d = 0.19, P = 0.003) unexercised arms. However, the increase in the occluded arm was significantly smaller ( d = −1.03, P < 0.001). These findings show that blocking blood flow to a limb during exercise attenuates EIH, suggesting that peripheral factors contribute to EIH in healthy adults.

NEW & NOTEWORTHY This is the first demonstration in humans that a factor carried by the circulation and acting at the periphery is important for exercise-induced hypoalgesia. Further understanding of this mechanism may provide new insight to pain relief with exercise as well as potential interactions between analgesic medications and exercise.

Evidence for a novel, neurohumoral antinociceptive mechanism mediated by peripheral capsaicin-sensitive nociceptors in conscious rats

Stimulation of capsaicin-sensitive peripheral sensory nerve terminals induces remote anti-inflammatory effects throughout the body of anesthetized rats and guinea-pigs mediated by somatostatin. As somatostatin has also antinociceptive effects, the study aimed at investigating whether similar remote antinociceptive effects can be demonstrated in awake animals. In conscious rats, nociceptive nerve endings of the right hind paw decentralized by cutting the sciatic and saphenous nerves 18h before were chemically stimulated, and drop of the noxious heat threshold (heat hyperalgesia) induced by prior (18h before) plantar incision was measured on the contralateral, left hind paw using an increasing-temperature water bath. 18h after nerve transection, mustard oil-evoked plasma extravasation was not significantly reduced in the right hind paw as tested by in vivo fluorescence imaging. Applying agonist of either transient receptor potential vanilloid 1 (TRPV1) or transient receptor potential ankyrin 1 (TRPA1) receptor (capsaicin or mustard oil, respectively) to the nerve-transected paw inhibited the plantar incision-induced drop of the noxious heat threshold on the contralateral paw. The onset of these remote antihyperalgesic effects was 10-20min. A similar contralateral inhibitory effect of capsaicin or mustard oil treatment was observed on neuropathic mechanical hyperalgesia evoked by partial sciatic nerve injury 2days before nerve transection and measured by a Randall-Selitto apparatus. The remote thermal antihyperalgesic effect was prevented by chronic (5days) denervation or local capsaicin desensitization of the stimulated paw; reduced by intraperitoneally applied antagonist of somatostatin (cyclosomatostatin) or opioid receptors (naloxone). The response was mimicked by intraperitoneally applied somatostatin and associated with a 72±27% increase in plasma somatostatin-like immunoreactivity that was absent after chronic (5days) denervation. In conclusion, chemical activation of decentralized peripheral capsaicin-sensitive nociceptors evokes remote antihyperalgesic responses initiated outside the central nervous system and mediated by somatostatin and endogenous opioids.

PAIN AND INFLAMMATION. PART 1. PATHOGENETIC ASPECTS

The relief of suffering, which is associated with a rapid and complete elimination of painful sensations, is the most important challenge facing physicians of many specialties. It is obvious that it can be solved only when you understand clearly the processes governing the development and chronization of pain. Inflammation, a universal adaptive mechanism that always accompanies damage to living tissues, plays a key role. Part 1 of this review considers the main stages of development of an inflammatory response, beginning with primary damage accompanied by the release of molecules acting as an alarm and ending with the deployment of a complete picture of the inflammatory response with the involvement of many cell elements and the overexpression of cytokines and proinflammatory mediators. The biological basis of the peripheral and central nociceptive sensitization phenomenon that is rigidly associated with inflammation is presented. Particular emphasis is placed on the possible natural completion of the inflammatory response, on the adaptive mechanisms regulating this process and on the reasons that prevent this and determines inflammation chronization.

Exploring the Mechanisms of Exercise-Induced Hypoalgesia Using Somatosensory and Laser Evoked Potentials

Exercise-induced hypoalgesia is well described, but the underlying mechanisms are unclear. The aim of this study was to examine the effect of exercise on somatosensory evoked potentials, laser evoked potentials, pressure pain thresholds and heat pain thresholds. These were recorded before and after 3-min of isometric elbow flexion exercise at 40% of the participant's maximal voluntary force, or an equivalent period of rest. Exercise-induced hypoalgesia was confirmed in two experiments (Experiment 1–SEPs; Experiment 2–LEPs) by increased pressure pain thresholds at biceps brachii (24.3 and 20.6% increase in Experiment 1 and 2, respectively; both d > 0.84 and p < 0.001) and first dorsal interosseous (18.8 and 21.5% increase in Experiment 1 and 2, respectively; both d > 0.57 and p < 0.001). In contrast, heat pain thresholds were not significantly different after exercise (forearm: 10.8% increase, d = 0.35, p = 0.10; hand: 3.6% increase, d = 0.06, p = 0.74). Contrasting effects of exercise on the amplitude of laser evoked potentials (14.6% decrease, d = −0.42, p = 0.004) and somatosensory evoked potentials (10.9% increase, d = −0.02, p = 1) were also observed, while an equivalent period of rest showed similar habituation (laser evoked potential: 7.3% decrease, d = −0.25, p = 0.14; somatosensory evoked potential: 20.7% decrease, d = −0.32, p = 0.006). The differential response of pressure pain thresholds and heat pain thresholds to exercise is consistent with relative insensitivity of thermal nociception to the acute hypoalgesic effects of exercise. Conflicting effects of exercise on somatosensory evoked potentials and laser evoked potentials were observed. This may reflect non-nociceptive contributions to the somatosensory evoked potential, but could also indicate that peripheral nociceptors contribute to exercise-induced hypoalgesia.

Decoding a neural circuit controlling global animal state in C. elegans

Brains organize behavior and physiology to optimize the response to threats or opportunities. We dissect how 21% O₂, an indicator of surface exposure, reprograms C. elegans' global state, inducing sustained locomotory arousal and altering expression of neuropeptides, metabolic enzymes, and other non-neural genes. The URX O₂-sensing neurons drive arousal at 21% O₂ by tonically activating the RMG interneurons. Stimulating RMG is sufficient to switch behavioral state. Ablating the ASH, ADL, or ASK sensory neurons connected to RMG by gap junctions does not disrupt arousal. However, disrupting cation currents in these neurons curtails RMG neurosecretion and arousal. RMG signals high O₂ by peptidergic secretion. Neuropeptide reporters reveal neural circuit state, as neurosecretion stimulates neuropeptide expression. Neural imaging in unrestrained animals shows that URX and RMG encode O₂ concentration rather than behavior, while the activity of downstream interneurons such as AVB and AIY reflect both O₂ levels and the behavior being executed.

DOI:http://dx.doi.org/10.7554/eLife.04241.001

From humans to worms, animals must respond appropriately to environmental challenges to survive. Starving animals must conserve energy while they seek food; animals that encounter a predator must fight or flee. These responses involve the animals re-programming their bodies and behavior, and, in humans, are thought to coincide with feelings or emotions such as ‘hunger’ and ‘fear’. Understanding these states in humans is difficult, but studies of simpler animals may provide some insights.

The microscopic worm Caenorhabditis elegans offers a unique advantage to these studies because it has the most precisely described nervous system of any animal. The worm lives in rotting fruit, but it avoids the fruit's surface, perhaps because there is an increased risk of it drying out or being eaten by predators. Microbes that grow within the rotting fruit reduce the oxygen level below the 21% oxygen found in the surrounding air, and so one strategy that C. elegans uses to avoid surface exposure is to continuously monitor the oxygen concentration. If the worm senses that the oxygen level is approaching 21%, which suggests it is nearing the surface, it reverses and turns around. If it cannot find a lower-oxygen environment, the worm switches to continuous rapid movement until it locates such an environment, and adapts its body for surface exposure.

Laurent, Soltesz et al. sought to understand the circuit of neurons that controls this switch. Monitoring gene expression in the worms revealed that specific oxygen-sensing neurons help generate the widespread changes that occur in the worm's body. These neurons also control the switch in the worm's behavior. Sensory neurons relay signals to downstream neurons that act on muscles to alter behavior. Neurons typically communicate with other neurons via specific connections; but neurons can also release signaling molecules, which act like ‘wireless’ signals and can affect many other cells. Laurent, Soltesz et al. showed that both kinds of signaling are needed to change the worm's behavior, and suggest that the release of signaling molecules may explain the widespread effects of 21% oxygen on the worm.

Laurent, Soltesz et al. then monitored the activity of neurons in freely moving worms, and found that some neurons appear to encode and relay specific sensory information. Other neurons encode the behavior the animal is performing, and yet others can encode both kinds of information. To confirm which neurons control particular behavioral responses, Laurent, Soltesz et al. measured changes in the worm’s behavior after destroying or altering specific cells, or while they used light-based techniques to artificially excite or inhibit specific neurons.

At a simple level the worm's response to 21% oxygen resembles the response of a mammal to a dangerous environment: both become more aroused, change how they respond to other sensory cues, and adapt both their bodies and behavior. As such, C. elegans provides a great model to explore at a small and accessible scale how changes in animals' states are generated.

DOI:http://dx.doi.org/10.7554/eLife.04241.002