Nociceptive-specific Neurons Research Articles

Biphasic Hormetic-like Effect of Lebecetin, a C-type Lectin of Snake Venom, on Formalin-induced Inflammation in Mice.

Integrins, important extracellular matrix (ECM) receptor proteins, are affected by inflammation and can participate in the maintenance of many painful conditions. Although they are ubiquitous and changeable across all cell types, the roles of these cell adhesion molecules in pathological pain have not been fully explored. We evaluated the effects of the subcutaneous injection of lebecetin, a C-type lectin isolated from Macrovipera lebetina snake venom, previously reported to inhibit α5β1 and αv integrin activity, on different components of inflammation induced by the formalin administration in the hind paw of mice. The formalin-induced nocifensive behavior, edema, and histopathological changes in the hind paw associated with cytokine, iNOS, and COX2 expression, nociceptive-specific neuron activity, and microglial activation analysis in the spinal cord were evaluated in mice receiving vehicle or lebecetin pretreatment. Lebecetin inhibited the nocifensive responses in the formalin test, related edema, and cell infiltration in the injected paw in a biphasic, hormetic-like, and dose-dependent way. According to that hormetic trend, a reduction in pro-inflammatory cytokines IL-6, IL-8, and TNF-alpha and upregulation of the anti-inflammatory cytokine IL-10 in the spinal cord were found with the lowest doses of lebecetin. Moreover, COX2 and iNOS expression in serum and spinal cord followed the same biphasic pattern of cytokines. Finally, nociceptive neurons sensitization and activated microglia were normalized in the dorsal horn of the spinal cord by lebecetin. These findings implicate specific roles of integrins in inflammation and tonic pain, as well as in the related central nervous system sequelae.

Network Model of Nociceptive Processing in the Superficial Spinal Dorsal Horn Reveals Mechanisms of Hyperalgesia, Allodynia, and Spinal Cord Stimulation.

The spinal dorsal horn (DH) processes sensory information and plays a key role in transmitting nociception to supraspinal centers. Loss of DH inhibition during neuropathic pain unmasks a pathway from non-nociceptive Aβ-afferent inputs to superficial dorsal horn (SDH) nociceptive-specific (NS) projection neurons, and this change may contribute to hyperalgesia and allodynia. We developed and validated a computational model of SDH neuronal circuitry that linked non-nociceptive Aβ-afferent inputs in lamina II/III to a NS projection neuron in lamina I via a network of excitatory interneurons. The excitatory pathway and the NS projection neuron were in turn gated by inhibitory interneurons with connections based on prior patch clamp recordings. Changing synaptic weights in the computational model to replicate neuropathic pain states unmasked a low-threshold excitatory pathway to NS neurons similar to experimental recordings. Spinal cord stimulation (SCS) is an effective therapy for neuropathic pain, and accumulating experimental evidence indicates that NS neurons in the SDH also respond to SCS. Accounting for these responses may inform therapeutic improvements, and we quantified responses to SCS in the SDH network model and examined the role of different modes of inhibitory control in modulating NS neuron responses to SCS. We combined the SDH network model with a previously published model of the deep dorsal horn (DDH) and identified optimal stimulation frequencies across different neuropathic pain conditions. Finally, we found that SCS-generated inhibition did not completely suppress model NS activity during simulated pinch inputs, providing an explanation of why SCS does not eliminate acute pain.

Insular cortical descending projections facilitate neuronal responses to noxious but not innoxious stimulation in rat trigeminal spinal subnucleus caudalis

The insular cortex (IC) receives orofacial nociceptive information. Pyramidal neurons in IC layer V send their axons to various brain regions, such as the trigeminal spinal subnucleus caudalis (Sp5C), parabrachial nucleus, and periaqueductal gray. However, little information has been available about the functions of these descending projections from the IC. This study aimed to elucidate the effect of IC → Sp5C on neuronal spike firings responding to noxious and innoxious stimuli to the face of the rat receiving an injection of adeno-associated virus encoding modified channelrhodopsin-2 (ChR2) fused to mCherry under the control of the human synapsin promotor. We classified Sp5C neurons responding to mechanical stimuli into three groups: low-threshold (LT), nociceptive specific (NS), and wide dynamic range (WDR) neurons, which respond to innoxious stimuli (brushing) only, noxious mechanical stimuli (pinching) only, and both noxious and innoxious stimuli, respectively. Neuronal activities of IC neurons were activated by photostimulation (repetitive pulses at 20 Hz for 5 Hz) to the IC that consistently induced action potentials in IC layer V pyramidal neurons. LT neurons showed comparable spike firing rates to brushing the facial skin before and during ChR2 activation induced by photostimulation. In contrast, NS neurons showed an increase in their firing frequency to pinching during ChR2 activation. On the other hand, WDR neurons increased their Sp5C neuronal firing to pinching during ChR2 activation without changing their firing rates to innoxious mechanical stimuli. These results suggest that the IC descending projections facilitate nociception by increasing Sp5C neuronal activities responding to noxious mechanical stimuli.

Secondary damage and neuroinflammation in the spinal dorsal horn mediate post-thalamic hemorrhagic stroke pain hypersensitivity: SDF1-CXCR4 signaling mediation.

Central post-stroke pain (CPSP) is an intractable neuropathic pain, which can be caused by primary lesion of central somatosensory system. It is also a common sequelae of the thalamic hemorrhagic stroke (THS). So far, the underlying mechanisms of CPSP remain largely unknown. Our previous studies have demonstrated that SDF1-CXCR4 signaling in the hemorrhagic region contributes to the maintenance of the THS pain hypersensitivity via mediation of the thalamic neuroinflammation. But whether the spinal dorsal horn, an initial point of spinothalamic tract (STT), suffers from retrograde axonal degeneration from the THS region is still unknown. In this study, neuronal degeneration and loss in the spinal dorsal horn were detected 7 days after the THS caused by intra-thalamic collagenase (ITC) injection by immunohistochemistry, TUNEL staining, electron microscopy, and extracellular multi-electrode array (MEA) recordings, suggesting the occurrence of secondary apoptosis and death of the STT projecting neuronal cell bodies following primary THS via retrograde axonal degeneration. This retrograde degeneration was accompanied by secondary neuroinflammation characterized by an activation of microglial and astrocytic cells and upregulation of SDF1-CXCR4 signaling in the spinal dorsal horn. As a consequence, central sensitization was detected by extracellular MEA recordings of the spinal dorsal horn neurons, characterized by hyperexcitability of both wide dynamic range and nociceptive specific neurons to suprathreshold mechanical stimuli. Finally, it was shown that suppression of spinal neuroinflammation by intrathecal administration of inhibitors of microglia (minocycline) and astrocytes (fluorocitrate) and antagonist of CXCR4 (AMD3100) could block the increase in expression levels of Iba-1, GFAP, SDF1, and CXCR4 proteins in the dorsal spinal cord and ameliorate the THS-induced bilateral mechanical pain hypersensitivity, implicating that, besides the primary damage at the thalamus, spinal secondary damage and neuroinflammation also play the important roles in maintaining the central post-THS pain hypersensitivity. In conclusion, secondary neuronal death and neuroinflammation in the spinal dorsal horn can be induced by primary thalamic neural damage via retrograde axonal degeneration process. SDF1-CXCR4 signaling is involved in the mediation of secondary spinal neuroinflammation and THS pain hypersensitivity. This finding would provide a new therapeutic target for treatment of CPSP at the spinal level.

Effects of Early Life Stress or Inflammation on Spinal Dorsal Horn Neuronal Responses to Perineal Mechanical Stimulation in Male Mice

This study characterized responses of dorsal horn neurons to perineal mechanical stimulation in adult male C57Bl/6J mice after neonatal maternal separation (NMS) or neonatal pelvic inflammation (NPI). For NMS, pups were removed from their home cage 4 hrs daily on P1-P21; controls were left undisturbed. For NPI, pups were anesthetized on P12 and Zymosan A (10 mg/ml, 20 ul) was injected into the pelvic floor musculature (Zymo); controls were exposed to anesthesia (Anes). At 12-15 weeks of age, mice were anesthetized and prepared for in vivo spinal recordings. Single-unit extracellular recordings were discriminated from background, converted into uniform pulses, and saved to computer as peristimulus-time histograms. Von Frey filaments (15-180 g) were applied to the perineum and evoked activity was calculated as total activity during 10-sec stimulus minus spontaneous activity in 10-sec prior to stimulus. Responses to non-noxious input and to a heterotopic noxious conditioning stimulus (HCNS) were determined for each neuron (Type I inhibited >20%, all others Type II). A total of 23 control and 17 NMS neurons were recorded. Chi-square showed no difference in the proportions of wide dynamic range (WDR) and nociceptive specific (NS) neurons between groups. Linear regression showed no difference in the slopes for WDR or NS neurons between groups. The proportion of Type I neurons was significantly reduced by NMS (Chi-square, p<0.05). A total of 9 Anes and 17 Zymo neurons were recorded. Chi-square showed no difference in the proportions of WDR and NS neurons, but the slopes for WDR (p<0.05) and NS (p<0.01) neurons were different between groups. NPI did not change the proportions of Type I neurons (Chi-square, NS). Early life stress and inflammation differentially affect subpopulations of dorsal horn neurons responsive to UBD. Changes in gain and the effect of HNCS reflect alterations in spinal transmission consistent with development of hypersensitivity.

Physiological properties of the lamina I spinoparabrachial neurons in the mouse.

Spinal cord lamina I neurons receiving dense input from nociceptors and projecting to the parabrachial area at the ponto-mesencephalic junction form the major ascending pain-related pathway in rodents. Lamina I spinoparabrachial (SPB) neurons have never been characterized in mice, despite the growing and extensive use of this species to understand the contribution of lamina I SPB neurons in chronic pain. The electrophysiological properties of lamina I SPB neurons recorded here in anaesthetized mice are comparable to those of rat or cat, forming a nociceptive and thermoreceptive pathway. It was confirmed 'on line' that lamina I SPB neurons that normally encode noxious stimuli can receive input from low threshold mechanoreceptors in certain conditions. The present work indicates that the study of lamina I SPB neurons in vivo could take advantage of the use of genetically modified mice. Ongoing studies investigating the role of lamina I projection neurons in the generation of chronic pain are mainly based on the use of genetically modified mice. However, lamina I projection neurons have never been physiologically characterized in this species. The present work aimed to fill this gap, and to assess the effect of spinal 'disinhibition' that may occur in chronic pain states on the responses of these neurons to light touch. Seventy lamina I spinoparabrachial (SPB) neurons were characterized in anaesthetized mice. These neurons showed low central conduction velocities (<12.4ms-1 ) and wide range of responses. Fifty-six neurons responded equally to noxious mechanical and thermal (heat) stimuli (16% responded consistently to light touch). Modality-specific neurons responded preferentially to thermal (cold) stimuli (n=10) and pinch (n=2), or specifically to heat (n=2). Spinal bicuculline and strychnine application induced responses to brush in half of the neurons tested, confirming directly the potential connection between low threshold mechanoreceptors and nociceptive-specific neurons, responsible for mechanical allodynia. Remarkably, the effect of the treatment was highly variable and apparently independent of the initial profile of the neurons. The present data confirm that mice lamina I SPB neurons have the expected characteristics to form a nociceptive and thermoreceptive pathway, but they constitute a highly heterogeneous group. The differential effect of spinal disinhibition observed here suggests that a subgroup of lamina I SPB neurons might be responsible for abnormal pain in pathological conditions, and emphasizes the importance of in vivo recording, a neglected approach.

Decanoic acid attenuates the excitability of nociceptive trigeminal primary and secondary neurons associated with hypoalgesia.

BackgroundAcute application of decanoic acid (DA) in vivo suppresses the excitability of spinal trigeminal nucleus caudalis (SpVc) wide dynamic range (WDR) neurons associated with the short-term mechanical hypoalgesia via muscarinic M2 receptor signaling; however, the effect of DA on nociceptive trigeminal ganglion (TG) and SpVc nociceptive-specific (NS) neuronal excitability under in vivo conditions remains to be determined. The present study investigated whether this effect could be observed in naive rats.ResultsExtracellular single-unit recordings were made from TG and SpVc NS neurons of pentobarbital-anesthetized rats in response to orofacial noxious mechanical stimuli. DA inhibited the mean firing frequency of both TG and SpVc NS neurons, reaching a maximum inhibition of discharge frequency within 1–5 minutes and reversing after approximately 10-minutes; however, this DA-induced suppression of SpVc NS neuronal firing frequency did not occur in rats administered with methoctramine intravenously prior to stimulation.ConclusionThis in vivo study indicated that firing of TG and SpVc NS neurons induced by mechanical hypoalgesia through peripheral M2 receptors could be inhibited by acutely administered DA, implicating the potential of DA in the future treatment of trigeminal pain.PerspectiveThis article presents that the acute DA application suppresses the excitability of TG and SpVc NS neurons associated with mechanical hypoalgesia via peripheral M2 receptor signaling, supporting DA as a potential therapeutic agent in complementary and alternative medicine for the attenuation of nociception.

Characterization and pharmacological modulation of noci-responsive deep dorsal horn neurons across diverse rat models of pathological pain.

This overview compares the activity of wide dynamic range (WDR) and nociceptive specific (NS) neurons located in the deep dorsal horn across different rat models of pathological pain and following modulation by diverse pharmacology. The data were collected by our group under the same experimental conditions over numerous studies to facilitate comparison. Spontaneous firing of WDR neurons was significantly elevated (>3.7 Hz) in models of neuropathic, inflammation, and osteoarthritic pain compared with naive animals (1.9 Hz) but was very low (<0.5 Hz) and remained unchanged in NS neurons. WDR responses to low-intensity mechanical stimulation were elevated in neuropathic and inflammation models. WDR responses to high-intensity stimuli were enhanced in inflammatory (heat) and osteoarthritis (mechanical) models. NS responses to high-intensity stimulation did not change relative to control in any model examined. Several therapeutic agents reduced both evoked and spontaneous firing of WDR neurons (e.g., TRPV1, TRPV3, Nav1.7, Nav1.8, P2X7, P2X3, H3), other targets affected neither evoked nor spontaneous firing of WDR neurons (e.g., H4, TRPM8, KCNQ2/3), and some only modulated evoked (e.g, ASIC1a, Cav3.2) whereas others decreased evoked but affected spontaneous activity only in specific models (e.g., TRPA1, CB2). Spontaneous firing of WDR neurons was not altered by any peripherally restricted compound or by direct administration of compounds to peripheral sites, although the same compounds decreased evoked activity. Compounds acting centrally were effective against this endpoint. The diversity of incoming/modulating inputs to the deep dorsal horn positions this group of neurons as an important intersection within the pain system to validate novel therapeutics. NEW & NOTEWORTHY Data from multiple individual experiments were combined to show firing properties of wide dynamic range and nociceptive specific spinal dorsal horn neurons across varied pathological pain models. This high-powered analysis describes the sensitization following different forms of injury. Effects of diverse pharmacology on these neurons is also summarized from published and unpublished data all recorded under the same conditions to facilitate comparison. This comprehensive overview describes the function and utility of these neurons.

Decreased spontaneous activity and altered evoked nociceptive response of rat thalamic submedius neurons to lumbar vertebra thrust.

The thalamus is a central structure important to modulating and processing all mechanoreceptor input destined for the cortex. A large number of diverse mechanoreceptor endings are stimulated when a high velocity low amplitude thrust is delivered to the lumbar spine during spinal manipulation. The objective of this study was to determine if a lumbar thrust alters spontaneous and/or evoked nociceptive activity in medial thalamic submedius (Sm) neurons. Extracellular recordings were obtained from 94 thalamic Sm neurons in 54 urethane-anesthetized adult Wistar rats. Spontaneous activity was recorded 5min before and after an L5 control (no thrust) and thrust (85% rat body weight; 100ms) procedure. In a subset of responsive nociceptive-specific neurons, mean changes in noxious-evoked response (10-s pinch with clip; 795g) at three sites (tail, contra- and ipsilateral hindpaw) were determined following an L5 thrust. Mean changes in Sm spontaneous activity (60s bins) and evoked noxious response were compared using a mixed model repeated measures ANOVA with Bonferroni post hoc t tests and paired t tests, respectively. Compared to control, spontaneous Sm activity decreased 180-240s following the lumbar thrust (p<0.005). Inhibitory evoked responses were attenuated in the contralateral hindpaw following an L5 thrust compared to control (p<0.05). No other changes in spontaneous or noxious-evoked Sm activity were found. A delayed, but prolonged suppression of spontaneous Sm activity along with changes in noxious-evoked inhibitory responses in the contralateral hindpaw following lumbar vertebra thrust suggest that thalamic submedius neurons may play a role in central pain modulation related to manual therapy intervention.

The medullary dorsal reticular nucleus as a relay for descending pronociception induced by the mGluR5 in the rat infralimbic cortex

Metabotropic glutamate receptor 5 (mGluR5) activation in the infralimbic cortex (IL) induces pronociceptive behavior in healthy and monoarthritic rats. Here we studied whether the medullary dorsal reticular nucleus (DRt) and the spinal TRPV1 are mediating the IL/mGluR5-induced spinal pronociception and whether the facilitation of pain behavior is correlated with changes in spinal dorsal horn neuron activity. For drug administrations, all animals had a cannula in the IL as well as a cannula in the DRt or an intrathecal catheter. Heat-evoked paw withdrawal was used to assess pain behavior in awake animals. Spontaneous and heat-evoked discharge rates of single DRt neurons or spinal dorsal horn wide-dynamic range (WDR) and nociceptive-specific (NS) neurons were evaluated in lightly anesthetized animals.Activation of the IL/mGluR5 facilitated nociceptive behavior in both healthy and monoarthritic animals, and this effect was blocked by lidocaine or GABA receptor agonists in the DRt. IL/mGluR5 activation increased spontaneous and heat–evoked DRt discharge rates in healthy but not monoarthritic rats. In the spinal dorsal horn, IL/mGluR5 activation increased spontaneous activity of WDR neurons in healthy animals only, whereas heat-evoked responses of WDR and NS neurons were increased in both experimental groups. Intrathecally administered TRPV1 antagonist prevented the IL/mGluR5-induced pronociception in both healthy and monoarthritic rats.The results suggest that the DRt is involved in relaying the IL/mGluR5-induced spinal pronociception in healthy control but not monoarthritic animals. Spinally, the IL/mGluR5-induced behavioral heat hyperalgesia is mediated by TRPV1 and associated with facilitated heat-evoked responses of WDR and NS neurons.

Effects of High-Dose Capsaicin on TMD Subjects: A Randomized Clinical Study.

Temporomandibular joint disorder (TMD) is a complex musculoskeletal disorder that presents with pain, limited jaw opening, and abnormal noises in the temporomandibular joint. Despite the significant impact that TMD has in terms of suffering and financial burden, relatively few new treatments have emerged; therefore, development of novel treatments to treat TMD pain remains a high priority. The rationale of this study was to use a double-blind, vehicle-controlled clinical trial to evaluate the effects of a high-concentration (8%) capsaicin cream on TMD. This is based on the hypothesis that targeting TRP vanilloid subfamily member 1 (TRPV1) for pain control may provide a novel method for pain relief in TMD patients. TRPV1 is primarily expressed on a population of nociceptive-specific neurons and provides a candidate target for the development of pain treatments. Capsaicin is the primary agonist for TRPV1 and has been used previously in relatively low doses (0.025% to 0.075%) as a therapeutic for a variety of pain disorders, including postherpetic neuralgia and osteoarthritis; however, analgesic efficacy remains equivocal. TMD and healthy control subjects were assigned to either an active capsaicin or vehicle control group. The treatments were applied for 2 h and then removed. Quantitative sensory testing (QST) was completed prior to drug application (baseline), 2 h after drug application, and 1 wk later. Perceived pain intensity was measured using a visual analog scale (VAS) following capsaicin or vehicle cream application. Significantly lower pain was reported in the week after application in the capsaicin-treated TMD subjects. For QST measures, there was a decreased thermal pain threshold 2 h after capsaicin application for both the control and TMD groups, but this resolved within a week. Capsaicin had no effect on pressure pain threshold or mechanical sensitivity in both TMD and healthy individuals. This study demonstrates that 8% topical capsaicin therapy is a relatively safe, simple, and effective treatment for patients with TMD. Knowledge Transfer Statement: This study evaluated a novel topical capsaicin therapy for reducing orofacial pain. The results of this study can be used to provide another treatment option for patients with TMD.

UNDERSTANDING THE REGIONAL SPECIFICITY OF NEURONS IN THE TRIGEMINAL SUBNUCLEUS CAUDALIS AND SPINAL DORSAL HORN

Neurons in the trigeminal subnucleus caudalis (Vc) and spinal cord dorsal horn (SDH) play important roles in modulating and relaying pain signals to the higher centers of the central nervous system (CNS). Morphologically, many aspects including a laminated structure, cytoarchitecture and cellular elements in these two central regions are very similar [1,2,3,4] . Most nociceptive afferents with a small-diameter terminate in the superficial laminae of Vc and SDH. Functionally, sensory neurons in Vc and SDH respectively receive nociceptive inputs from the orofacial and other somatic regions, and convey the inputs to higher brain centers [1,2,3,4,5] . According to the mechanoreceptive properties, sensory neurons in both the Vc and SDH are classified into three types. They are low-threshold mechanoreceptive (LTM), wide-dynamic range (WDR) and nociceptive-specific neurons (NS) [1,2,3,4,5] . Because of these similarities Vc has been considered as analogous to SDH [4,6] . However, some differences between Vc and SDH have also been reported. For example, the distribution pattern of substance P (SP) and calcitonin gene related peptide (CGRP) in Vc of adult animals is different from that in SDH. In SDH there is no dual representation of peripheral regions, which can be found in rostral and caudal Vc. A transitional zone found between Vc and trigeminal subnucleus interpolaris plays a special role in sensorimotor function. However, no such region is found in the spinal system [5,7,8] . CNS neurons are regionalized. They are organized as groups through their circuitry. Although it is known that differences in functions among these groups in the adult CNS are predetermined during the development of the neuroepithelium [9,10,11,12,13] , understanding how the regional specificity is developed in the CNS is still a big challenge in neuroscience research. It has recently been found that ninhydrin-reacting small molecules released locally are involved in the region-specific regulation of neuronal development in cultured Vc and SDH neurons [14,15] . In this review we focused on present knowledge about the region-specific regulation of neuronal development in these two CNS regions associated with the transmission of nociceptive signals.

Neuronal hyperexcitability in the ventral posterior thalamus of neuropathic rats: modality selective effects of pregabalin.

Neuropathic pain represents a substantial clinical challenge; understanding the underlying neural mechanisms and back-translation of therapeutics could aid targeting of treatments more effectively. The ventral posterior thalamus (VP) is the major termination site for the spinothalamic tract and relays nociceptive activity to the somatosensory cortex; however, under neuropathic conditions, it is unclear how hyperexcitability of spinal neurons converges onto thalamic relays. This study aimed to identify neural substrates of hypersensitivity and the influence of pregabalin on central processing. In vivo electrophysiology was performed to record from VP wide dynamic range (WDR) and nociceptive-specific (NS) neurons in anesthetized spinal nerve-ligated (SNL), sham-operated, and naive rats. In neuropathic rats, WDR neurons had elevated evoked responses to low- and high-intensity punctate mechanical stimuli, dynamic brushing, and innocuous and noxious cooling, but less so to heat stimulation, of the receptive field. NS neurons in SNL rats also displayed increased responses to noxious punctate mechanical stimulation, dynamic brushing, noxious cooling, and noxious heat. Additionally, WDR, but not NS, neurons in SNL rats exhibited substantially higher rates of spontaneous firing, which may correlate with ongoing pain. The ratio of WDR-to-NS neurons was comparable between SNL and naive/sham groups, suggesting relatively few NS neurons gain sensitivity to low-intensity stimuli leading to a "WDR phenotype." After neuropathy was induced, the proportion of cold-sensitive WDR and NS neurons increased, supporting the suggestion that changes in frequency-dependent firing and population coding underlie cold hypersensitivity. In SNL rats, pregabalin inhibited mechanical and heat responses but not cold-evoked or elevated spontaneous activity.

Loss of inhibitory tone on spinal cord dorsal horn spontaneously and nonspontaneously active neurons in a mouse model of neuropathic pain.

Plasticity of inhibitory transmission in the spinal dorsal horn (SDH) is believed to be a key mechanism responsible for pain hypersensitivity in neuropathic pain syndromes. We evaluated this plasticity by recording responses to mechanical stimuli in silent neurons (nonspontaneously active [NSA]) and neurons showing ongoing activity (spontaneously active [SA]) in the SDH of control and nerve-injured mice (cuff model). The SA and NSA neurons represented 59% and 41% of recorded neurons, respectively, and were predominantly wide dynamic range (WDR) in naive mice. Nerve-injured mice displayed a marked decrease in the mechanical threshold of the injured paw. After nerve injury, the proportion of SA neurons was increased to 78%, which suggests that some NSA neurons became SA. In addition, the response to touch (but not pinch) was dramatically increased in SA neurons, and high-threshold (nociceptive specific) neurons were no longer observed. Pharmacological blockade of spinal inhibition with a mixture of GABAA and glycine receptor antagonists significantly increased responses to innocuous mechanical stimuli in SA and NSA neurons from sham animals, but had no effect in sciatic nerve-injured animals, revealing a dramatic loss of spinal inhibitory tone in this situation. Moreover, in nerve-injured mice, local spinal administration of acetazolamide, a carbonic anhydrase inhibitor, restored responses to touch similar to those observed in naive or sham mice. These results suggest that a shift in the reversal potential for anions is an important component of the abnormal mechanical responses and of the loss of inhibitory tone recorded in a model of nerve injury-induced neuropathic pain.

D-aspartate modulates nociceptive-specific neuron activity and pain threshold in inflammatory and neuropathic pain condition in mice.

D-Aspartate (D-Asp) is a free D-amino acid found in the mammalian brain with a temporal-dependent concentration based on the postnatal expression of its metabolizing enzyme D-aspartate oxidase (DDO). D-Asp acts as an agonist on NMDA receptors (NMDARs). Accordingly, high levels of D-Asp in knockout mice for Ddo gene (Ddo −/−) or in mice treated with D-Asp increase NMDAR-dependent processes. We have here evaluated in Ddo −/− mice the effect of high levels of free D-Asp on the long-term plastic changes along the nociceptive pathway occurring in chronic and acute pain condition. We found that Ddo −/− mice show an increased evoked activity of the nociceptive specific (NS) neurons of the dorsal horn of the spinal cord (L4–L6) and a significant decrease of mechanical and thermal thresholds, as compared to control mice. Moreover, Ddo gene deletion exacerbated the nocifensive responses in the formalin test and slightly reduced pain thresholds in neuropathic mice up to 7 days after chronic constriction injury. These findings suggest that the NMDAR agonist, D-Asp, may play a role in the regulation of NS neuron electrophysiological activity and behavioral responses in physiological and pathological pain conditions.

PC1, a non-peptide PKR1-preferring antagonist, reduces pain behavior and spinal neuronal sensitization in neuropathic mice

Peripheral neuropathy is characterized by abnormal pain responses triggered by the release of several mediators and neuronal hyperexcitability at the spinal cord level. Emerging evidence indicates that the enhanced activity of dorsal horn neurons requires communication with glia and microglia, cells that are physiologically involved in neuronal wellbeing. Prokineticins (PKs), which include PK1 and PK2, represent a novel family of chemokines characterized by a unique structural motif comprising five disulfide bonds. They are expressed in the peripheral and central nervous system. PKs bind two G protein coupled receptors, PKR1 and PKR2, and participate in the regulation of several biological processes, including pain sensation. This study aimed to investigate the anti-nociceptive effect of PC1, a non-peptide PKR1-preferring antagonist, in a mouse model of neuropathic pain. To do this, we assessed the activity of spinal cord nociceptive neurons as well as astrocyte and microglia phenotypes after repeated administration of PC1 in vivo. PC1 treatment strongly delayed the development of thermal hyperalgesia and tactile and mechanical allodynia. It also reduced spinal microglial and glial activation 8 days post injury in spared nerve injury (SNI) mice. Neuropathic mice showed an increased level of PK2 protein in the spinal cord, mostly in astrocytes. PC1 treatment completely reversed the increased responsiveness to mechanical stimuli, the decreased threshold of neuronal activation, and the increased spontaneous activity that were observed in nociceptive specific (NS) neurons of SNI mice.

The somatostatin receptor 4 agonist J-2156 reduces mechanosensitivity of peripheral nerve afferents and spinal neurons in an inflammatory pain model

Somatostatin (SST) is a peptide hormone that regulates the endocrine system and affects neurotransmission via interaction with G protein-coupled SST receptors and inhibition of the release of different hormones. The aim of this study was to investigate whether the analgesic properties of the selective SSTR4 agonist J-2156 are mediated via peripheral and/or spinal receptors.Effect on mechanical hyperalgesia in the Complete Freund׳s Adjuvant (CFA) model was measured after intraperitoneal application of J-2156. Electrophysiological neuronal recordings were conducted 24h after injection of CFA or vehicle into the paw of Wistar rats. Mechanosensitivity of peripheral afferents of the saphenous nerve as well as of spinal wide dynamic range (WDR) and nociceptive-specific (NS) neurons were measured after systemic or spinal application of J-2156.In CFA animals J-2156 dose dependently reduced hyperalgesia in behavioral studies. The minimal effective dose was 0.1mg/kg. Mechanosensitivity of peripheral afferents and spinal neurons was significantly reduced by J-2156. NS neurons were dose dependently inhibited by J-2156 while in WDR neurons only the highest concentration of 100µM had an effect. In sham controls, J-2156 had no effect on neuronal activity.We demonstrated that J-2156 dose-dependently reduces peripheral and spinal neuronal excitability in the CFA rat model without affecting physiological pain transmission. Given the high concentration of the compound required to inhibit spinal neurons, it is unlikely that the behavioral effect seen in CFA model is mediated centrally. Overall these data demonstrated that the analgesic effect of J-2156 is mediated mainly via peripheral SST4 receptors.

EEG frequency tagging to dissociate the cortical responses to nociceptive and nonnociceptive stimuli.

Whether the cortical processing of nociceptive input relies on the activity of nociceptive-specific neurons or whether it relies on the activity of neurons also involved in processing nonnociceptive sensory input remains a matter of debate. Here, we combined EEG "frequency tagging" of steady-state evoked potentials (SS-EPs) with an intermodal selective attention paradigm to test whether the cortical processing of nociceptive input relies on nociceptive-specific neuronal populations that can be selectively modulated by top-down attention. Trains of nociceptive and vibrotactile stimuli (Experiment 1) and trains of nociceptive and visual stimuli (Experiment 2) were applied concomitantly to the same hand, thus eliciting nociceptive, vibrotactile, and visual SS-EPs. In each experiment, a target detection task was used to focus attention toward one of the two concurrent streams of sensory input. We found that selectively attending to nociceptive or vibrotactile somatosensory input indistinctly enhances the magnitude of nociceptive and vibrotactile SS-EPs, whereas selectively attending to nociceptive or visual input independently enhances the magnitude of the SS-EP elicited by the attended sensory input. This differential effect indicates that the processing of nociceptive input involves neuronal populations also involved in the processing of touch, but distinct from the neuronal populations involved in vision.

Effect of Spinal Manipulation Thrust Duration on Trunk Mechanical Activation Thresholds of Nociceptive-Specific Lateral Thalamic Neurons

ObjectiveThe objective of this preliminary study was to determine if high-velocity, low-amplitude spinal manipulation (HVLA-SM) thrust duration alters mechanical trunk activation thresholds of nociceptive-specific (NS) lateral thalamic neurons. MethodsExtracellular recordings were obtained from 18 NS neurons located in 2 lateral thalamic nuclei (ventrolateral [n = 12] and posterior [n = 6]) in normal anesthetized Wistar rats. Response thresholds to electronic von Frey anesthesiometer (rigid tip) mechanical trunk stimuli applied in 3 lumbar directions (dorsal-ventral, 45° caudal, and 45° cranial) were determined before and immediately after the delivery of 3 HVLA-SM thrust durations (time control 0, 100, and 400 milliseconds). Mean changes in mechanical trunk activation thresholds were compared using a mixed model analysis of variance. ResultsHigh-velocity, low-amplitude spinal manipulation duration did not significantly alter NS lateral thalamic neurons' mechanical trunk responses to any of the 3 directions tested with the anesthesiometer. ConclusionsThis study is the first to examine the effect of HVLA-SM thrust duration on NS lateral thalamic mechanical response thresholds. High-velocity, low-amplitude spinal manipulation thrust duration did not affect mechanical trunk thresholds.

A selective α2 B adrenoceptor agonist (A-1262543) and duloxetine modulate nociceptive neurones in the medial prefrontal cortex, but not in the spinal cord of neuropathic rats.

The noradrenergic system contributes to pain modulation, but the roles of its specific adrenoceptors are still being defined. We have identified a novel, potent (rat EC50 = 4.3 nM) and selective α2B receptor agonist, A-1262543, to further explore this adrenoceptor subtype's contribution to pathological nociception. Systemic administration of A-1262543 (1-10 mg/kg, intraperitoneal) dose-dependently attenuated mechanical allodynia in animals with a spinal nerve ligation injury. To further explore its mechanism of action, the activity of nociceptive neurones in the spinal cord and medial prefrontal cortex (mPFC) were examined after injection of 3 mg/kg of A-1262543 (intravenous, i.v.). These effects were compared with duloxetine (3 mg/kg, i.v.), a dual noradrenaline (NA) and serotonin (5-HT) reuptake inhibitor. Systemic administration of A-1262543 or duloxetine did not alter the spontaneous or evoked firing of spinal wide dynamic range and nociceptive-specific neurones in the neuropathic rats, indicating that neither compound engaged spinal, peripheral or descending pathways. In contrast to the lack of effect on spinal neurones, both A-1262543 and duloxetine reduced the evoked and spontaneous firing of 'pain-responsive' (PR) neurones in the mPFC. Duloxetine, but not A-1262543, also inhibited the firing of pain non-responsive (nPR) neurones in the mPFC probably reflecting duloxetine's contribution to modulating non-pain endpoints. These data highlight that activation of the α2B adrenoceptor as well as inhibiting NA and 5-HT reuptake can result in modulating the ascending nociceptive system, and in particular, dampening the firing of PR neurones in the mPFC.