Non-peptidergic primary afferents are presynaptic to neurokinin-1 receptor immunoreactive lamina I projection neurons in rat spinal cord

CD34 distribution in C-fiber low threshold mechanoreceptors in the mouse dorsal root ganglion and spinal cord

CD34 is a well-known cell marker of hematopoietic stem/ progenitor cells, endothelial cells, and fibrocytes. In the peripheral nervous system, a certain type of primary sensory neuron C-fiber low threshold mechanoreceptors (C-LTMRs) are reported to express CD34 mRNA. Here, we investigated the distribution of CD34 protein among putative C-LTMRs (pC-LTMR) using pC-LTMR markers such as VGLUT3 and TH in the dorsal root ganglion (DRG) and spinal cord. CD34 was frequently observed in DRG neurons double-positive for VGLUT3 and TH and single-positive for VGLUT3 in C8 and L4 levels, however, in C4 and L1 levels most of CD34-positive DRG neurons were demonstrated to be double-positive for VGLUT3 and TH. As for the termination, CD34-positive DRG neurons terminated in the ventral part of inner lamina II (lamina IIiv). At C4 and L1 levels of the dorsal horn, CD34 was observed in the entire region of lamina IIiv, however, in C8 and L4 levels of the dorsal horn CD34 was not detected in the medial part of lamina IIiv, which receives neural inputs from DRG neurons that innervate palm or sole skin. These results indicate that CD34 is expressed in pC-LTMRs and suggest that CD34 may play a role in providing C-LTMRs with a specific sensation by maintaining neural circuits.

Central Neuropathic Pain Development Modulation Using Coffee Extract Major Polyphenolic Compounds in Spinal-Cord-Injured Female Mice

It was recently shown that coffee polyphenolic extract exerts preventive effects on central neuropathic pain development, but it is unknown whether its beneficial effects are associated with only one of its major polyphenolic compounds or if the whole extract is needed to exert such effects. The main objective of this study was to determine whether the separate administration of major polyphenols from coffee extract exerts preventive effects on the development of central neuropathic pain in mice compared with the effects of the whole coffee extract. Thus, spinal-cord-injured female ICR-CD1 mice were daily treated with either coffee extract or its major polyphenolic compounds during the first week, and reflexive and nonreflexive pain responses were evaluated within the acute phase of spinal cord injury. In addition, the injury-induced gliosis and dorsal horn sprouting were evaluated with immunohistochemistry. The results showed that the coffee extract prevented spinal cord injury-induced neuropathic pain, whereas its major polyphenolic compounds resulted in reflexive pain response attenuation. Both preventive and attenuation effects were associated with gliosis and afferent fiber sprouting modulation. Overall, the results suggested that coffee extract effects may be associated with potential synergistic mechanisms exerted by its major polyphenolic compounds and not by the sole effect of only one of them.

The Impact of Activity-Based Interventions on Neuropathic Pain in Experimental Spinal Cord Injury

Physical activity-based rehabilitative interventions represent the main treatment concept for people suffering from spinal cord injury (SCI). The role such interventions play in the relief of neuropathic pain (NP) states is emerging, along with underlying mechanisms resulting in SCI-induced NP (SCI-NP). Animal models have been used to investigate the benefits of activity-based interventions (ABI), such as treadmill training, wheel running, walking, swimming, and bipedal standing. These activity-based paradigms have been shown to modulate inflammatory-related alterations as well as induce functional and structural changes in the spinal cord gray matter circuitry correlated with pain behaviors. Thus far, the research available provides an incomplete picture of the cellular and molecular pathways involved in this beneficial effect. Continued research is essential for understanding how such interventions benefit SCI patients suffering from NP and allow the development of individualized rehabilitative therapies. This article reviews preclinical studies on this specific topic, goes over mechanisms involved in SCI-NP in relation to ABI, and then discusses the effectiveness of different activity-based paradigms as they relate to different forms, intensity, initiation times, and duration of ABI. This article also summarizes the mechanisms of respective interventions to ameliorate NP after SCI and provides suggestions for future research directions.

Pre-Synaptic GABAA in NaV1.8+ Primary Afferents Is Required for the Development of Punctate but Not Dynamic Mechanical Allodynia following CFA Inflammation

Hypersensitivity to mechanical stimuli is a cardinal symptom of neuropathic and inflammatory pain. A reduction in spinal inhibition is generally considered a causal factor in the development of mechanical hypersensitivity after injury. However, the extent to which presynaptic inhibition contributes to altered spinal inhibition is less well established. Here, we used conditional deletion of GABA_A in NaV1.8-positive sensory neurons (Scn10a^Cre;Gabrb3^fl/fl) to manipulate selectively presynaptic GABAergic inhibition. Behavioral testing showed that the development of inflammatory punctate allodynia was mitigated in mice lacking pre-synaptic GABA_A. Dorsal horn cellular circuits were visualized in single slices using stimulus-tractable dual-labelling of c-fos mRNA for punctate and the cognate c-Fos protein for dynamic mechanical stimulation. This revealed a substantial reduction in the number of cells activated by punctate stimulation in mice lacking presynaptic GABA_A and an approximate 50% overlap of the punctate with the dynamic circuit, the relative percentage of which did not change following inflammation. The reduction in dorsal horn cells activated by punctate stimuli was equally prevalent in parvalbumin- and calretinin-positive cells and across all laminae I–V, indicating a generalized reduction in spinal input. In peripheral DRG neurons, inflammation following complete Freund’s adjuvant (CFA) led to an increase in axonal excitability responses to GABA, suggesting that presynaptic GABA effects in NaV1.8⁺ afferents switch from inhibition to excitation after CFA. In the days after inflammation, presynaptic GABA_A in NaV1.8⁺ nociceptors constitutes an “open gate” pathway allowing mechanoreceptors responding to punctate mechanical stimulation access to nociceptive dorsal horn circuits.

Spinal Excitatory Dynorphinergic Interneurons Contribute to Burn Injury-Induced Nociception Mediated by Phosphorylated Histone 3 at Serine 10 in Rodents

The phosphorylation of serine 10 in histone 3 (p-S10H3) has recently been demonstrated to participate in spinal nociceptive processing. However, superficial dorsal horn (SDH) neurons involved in p-S10H3-mediated nociception have not been fully characterized. In the present work, we combined immunohistochemistry, in situ hybridization with the retrograde labeling of projection neurons to reveal the subset of dorsal horn neurons presenting an elevated level of p-S10H3 in response to noxious heat (60 °C), causing burn injury. Projection neurons only represented a small percentage (5%) of p-S10H3-positive cells, while the greater part of them belonged to excitatory SDH interneurons. The combined immunolabeling of p-S10H3 with markers of already established interneuronal classes of the SDH revealed that the largest subset of neurons with burn injury-induced p-S10H3 expression was dynorphin immunopositive in mice. Furthermore, the majority of p-S10H3-expressing dynorphinergic neurons proved to be excitatory, as they lacked Pax-2 and showed Lmx1b-immunopositivity. Thus, we showed that neurochemically heterogeneous SDH neurons exhibit the upregulation of p-S10H3 shortly after noxious heat-induced burn injury and consequential tissue damage, and that a dedicated subset of excitatory dynorphinergic neurons is likely a key player in the development of central sensitization via the p-S10H3 mediated pathway.

Microglia Are Indispensable for Synaptic Plasticity in the Spinal Dorsal Horn and Chronic Pain

Spinal long-term potentiation (LTP) at C-fiber synapses is hypothesized to underlie chronic pain. However, a causal link between spinal LTP and chronic pain is still lacking. Here, we report that high-frequency stimulation (HFS; 100 Hz, 10 V) of the mouse sciatic nerve reliably induces spinal LTP without causing nerve injury. LTP-inducible stimulation triggers chronic pain lasting for more than 35 days and increases the number of calcitonin gene-related peptide (CGRP) terminals in the spinal dorsal horn. The behavioral and morphological changes can be prevented by blocking NMDA receptors, ablating spinal microglia, or conditionally deleting microglial brain-derived neurotrophic factor (BDNF). HFS-induced spinal LTP, microglial activation, and upregulation of BDNF are inhibited by antibodies against colony-stimulating factor 1 (CSF-1). Together, our results show that microglial CSF1 and BDNF signaling are indispensable for spinal LTP and chronic pain. The microglia-dependent transition of synaptic potentiation to structural alterations in pain pathways may underlie pain chronicity.

Enkephalinergic Circuit Involved in Nociceptive Modulation in the Spinal Dorsal Horn

Enkephalin (ENK) has been implicated in pain modulation within the spinal dorsal horn (SDH). Revealing the mechanisms underlying ENK analgesia entails the anatomical and functional knowledge of spinal ENK-ergic circuits. Herein, we combined morphological and electrophysiological studies to unravel local ENK-ergic circuitry within the SDH. First, the distribution pattern of spinal ENK-ergic neurons was observed in adult preproenkephalin (PPE)-GFP knock-in mice. Next, the retrograde tracer tetramethylrhodamine (TMR) or horseradish peroxidase (HRP) was injected into the parabrachial nucleus (PBN) in PPE-GFP mice. Immunofluorescent staining showed I-isolectin B4 (IB4) labeled non-peptidergic afferents were in close apposition to TMR-labeled PBN-projecting neurons within lamina I as well as PPE-immunoreactivity (-ir) neurons within lamina II. Some TMR-labeled neurons were simultaneously in close association with both IB4 and PPE-ir terminals. Synaptic connections of these components were further confirmed by electron microscopy. Finally, TMR was injected into the PBN in adult C57BL/6 mice. Whole-cell patch recordings showed that δ-opioid receptor (DOR) agonist, [D-Pen^2,5]-enkephalin (DPDPE, 1 µM), significantly reduced the frequency of miniature excitatory postsynaptic current (mEPSC) and decreased the activity of TMR-labeled neurons. In conclusion, spinal ENKergic neurons receive direct excitatory inputs from primary afferents, which might be directly recruited to release ENK under the condition of noxious stimuli; ENK could inhibit the glutamatergic transmission towards projecting neurons via presynaptic and postsynaptic DORs. These morphological and functional evidence may explain the mechanisms underlying the analgesic effects exerted by ENK within the SDH.

Spinal cord projection neurons: a superficial, and also deep, analysis

Today there are extensive maps of the molecular heterogeneity of primary afferents and dorsal horn interneurons, yet there is a dearth of molecular and functional information regarding the projection neurons that transmit pain and itch information to the brain. Additionally, most contemporary research into the spinal cord and medullary projection neurons focuses on neurons in the superficial dorsal horn; the contribution of deep dorsal horn and even ventral horn projection neurons to pain and itch processing is often overlooked. In the present review we integrate conclusions from classical as well as contemporary studies and provide a more balanced view of the diversity of projection neurons. A major question addressed is the extent to which labeled-lines are maintained in these different populations or whether the brain generates distinct pain and itch percepts by decoding complex convergent inputs that engage projection neurons.

Electrophysiological Actions of N/OFQ

Whilst the nociceptin/orphanin FQ (N/OFQ) receptor (NOP) has similar intracellular coupling mechanisms to opioid receptors, it has distinct modulatory effects on physiological functions such as pain. These actions range from agonistic to antagonistic interactions with classical opioids within the spinal cord and brain, respectively. Understanding the electrophysiological actions of N/OFQ has been crucial in ascertaining the mechanisms by which these agonistic and antagonistic interactions occur. These similarities and differences between N/OFQ and opioids are due to the relative location of NOP versus opioid receptors on specific neuronal elements within these CNS regions. These mechanisms result in varied cellular actions including postsynaptic modulation of ion channels and presynaptic regulation of neurotransmitter release.

ATP-Gated P2X3 Receptors Are Specialised Sensors of the Extracellular Environment

P2X3 receptors are ion channels expressed by autonomic and sensory nerves and specialised in transducing extracellular ATP signals. Structural data, together with functional and biochemical studies, suggest that conformational changes of P2X3 receptors upon agonist binding influence downstream intracellular molecular mechanisms relevant for neuronal responses. Activity of P2X3 receptors is implicated in pain, itch, asthma, cardiovascular dysfunction and other pathologies. The study of these receptors has therefore a large potential in the field of drug development and interdisciplinary efforts could clarify molecular mechanisms controlling P2X3 receptor function in different physiological or pathological contexts.

Neuronal networks and nociceptive processing in the dorsal horn of the spinal cord

The dorsal horn (DH) of the spinal cord receives a variety of sensory information arising from the inner and outer environment, as well as modulatory inputs from supraspinal centers. This information is integrated by the DH before being forwarded to brain areas where it may lead to pain perception. Spinal integration of this information relies on the interplay between different DH neurons forming complex and plastic neuronal networks. Elements of these networks are therefore potential targets for new analgesics and pain-relieving strategies. The present review aims at providing an overview of the current knowledge on these networks, with a special emphasis on those involving interlaminar communication in both physiological and pathological conditions.

Localization of TRPV1 and P2X3 in unmyelinated and myelinated vagal afferents in the rat

The vagus nerve is dominated by afferent fibers that convey sensory information from the viscera to the brain. Most vagal afferents are unmyelinated, slow-conducting C-fibers, while a smaller portion are myelinated, fast-conducting A-fibers. Vagal afferents terminate in the nucleus tractus solitarius (NTS) in the dorsal brainstem and regulate autonomic and respiratory reflexes, as well as ascending pathways throughout the brain. Vagal afferents form glutamatergic excitatory synapses with postsynaptic NTS neurons that are modulated by a variety of channels. The organization of vagal afferents with regard to fiber type and channels is not well understood. In the present study, we used tract tracing methods to identify distinct populations of vagal afferents to determine if key channels are selectively localized to specific groups of afferent fibers. Vagal afferents were labeled with isolectin B4 (IB4) or cholera toxin B (CTb) to detect unmyelinated and myelinated afferents, respectively. We find that TRPV1 channels are preferentially found in unmyelinated vagal afferents identified with IB4, with almost half of all IB4 fibers showing co-localization with TRPV1. These results agree with prior electrophysiological findings. In contrast, we found that the ATP-sensitive channel P2X3 is found in a subset of both myelinated and unmyelinated vagal afferent fibers. Specifically, 18% of IB4 and 23% of CTb afferents contained P2X3. The majority of CTb-ir vagal afferents contained neither channel. Since neither channel was found in all vagal afferents, there are likely further degrees of heterogeneity in the modulation of vagal afferent sensory input to the NTS beyond fiber type.

Limited changes in spinal lamina I dorsal horn neurons following the cytotoxic ablation of non-peptidergic C-fibers

BACKGROUND

Non-peptidergic nociceptive neurons are a sub-population of small diameter primary sensory neurons that comprise approximately 50 % of the C fiber population. Together with the peptidergic sub-population, they transmit nociceptive information from the periphery to the superficial dorsal horn of the spinal cord. Despite the numerous studies investigating the role of the non-peptidergic primary afferents, their role in normal nociception and in pain remains poorly understood. Our lab has previously demonstrated that, in rat models of neuropathic and inflammatory pain, there is a de novo expression of substance P receptors (NK-1r) by lamina I pyramidal projection neurons, a neuronal population that normally does not express these receptors.

RESULTS

In this study, we used a ribosomal toxin, saporin, conjugated to the lectin IB4 to selectively ablate the non-peptidergic nociceptive C fibers, to investigate if the loss of these fibers was enough to induce a change in NK-1r expression by lamina I projection neurons. IB4-saporin treatment led to the permanent ablation of the IB4-positive afferents but also to a small non-significant reduction in CGRP-positive afferents. An overall increase in immunoreactivity for the NK-1r was observed in lamina I projection neurons, however, the lack of non-peptidergic afferents did not increase the number of lamina I pyramidal projection neurons immunoreactive for the receptor.

CONCLUSIONS

Our results demonstrate that the deletion of the non-peptidergic afferents, at the L4-L5 spinal levels, is not sufficient to trigger the de novo expression of NK-1r by projection pyramidal neurons but increases the expression of NK-1r in fusiform and multipolar projection neurons. Furthermore, our data suggest that a neuropathic component is essential to trigger the expression of NK-1r by pyramidal neurons.

Regulation of Neurotrophin-3 and Interleukin-1β and Inhibition of Spinal Glial Activation Contribute to the Analgesic Effect of Electroacupuncture in Chronic Neuropathic Pain States of Rats

Growing evidence indicates that neurotrophin-3, interleukin-1β, and spinal glia are involved in neuropathic pain derived from dorsal root ganglia to spinal cord. Electroacupuncture is widely accepted to treat chronic pain, but the precise mechanism underlying the analgesic effect of EA has not been fully demonstrated. In this study, the mechanical withdrawal threshold and thermal withdrawal latency were recorded. We used immunofluorescence and western blots methods to investigate the effect of EA on the expression of NT-3 and IL-1β in DRG and spinal cord of CCI rats; we also examined the expression of spinal GFAP and OX-42 in spinal cord. In present study, the MWT and TWL of CCI group rats were lower than those in the Sham CCI group rats, but EA treatment increased the pain thresholds. Furtherly, we found that EA upregulates the expression of NT-3 in DRG and spinal cord of CCI rats, while EA downregulates the expression of IL-1β. Additionally, immunofluorescence exhibited that CCI-induced activation of microglia and astrocytes was inhibited significantly by EA treatment. These results demonstrated that the analgesic effect of EA may be achieved through promoting the neural protection of NT-3 as well as the inhibition of IL-1β production and spinal glial activity.

Spatial and temporal pattern of changes in the number of GAD65-immunoreactive inhibitory terminals in the rat superficial dorsal horn following peripheral nerve injury

Inhibitory interneurons are an important component of dorsal horn circuitry where they serve to modulate spinal nociception. There is now considerable evidence indicating that reduced inhibition in the spinal dorsal horn contributes to neuropathic pain. A loss of these inhibitory neurons after nerve injury is one of the mechanisms being proposed to account for reduced inhibition; however, this remains controversial. This is in part because previous studies have focused on global measurements of inhibitory neurons without assessing the number of inhibitory synapses. To address this, we conducted a quantitative analysis of the spatial and temporal changes in the number of inhibitory terminals, as detected by glutamic acid decarboxylase 65 (GAD65) immunoreactivity, in the superficial dorsal horn of the spinal cord following a chronic constriction injury (CCI) to the sciatic nerve in rats. Isolectin B4 (IB4) labelling was used to define the location within the dorsal horn directly affected by the injury to the peripheral nerve. The density of GAD65 inhibitory terminals was reduced in lamina I (LI) and lamina II (LII) of the spinal cord after injury. The loss of GAD65 terminals was greatest in LII with the highest drop occurring around 3–4 weeks and a partial recovery by 56 days. The time course of changes in the number of GAD65 terminals correlated well with both the loss of IB4 labeling and with the altered thresholds to mechanical and thermal stimuli. Our detailed analysis of GAD65+ inhibitory terminals clearly revealed that nerve injury induced a transient loss of GAD65 immunoreactive terminals and suggests a potential involvement for these alterations in the development and amelioration of pain behaviour.

Pregabalin alters nociceptive behavior and expression level of P2X3 receptor in the spinal dorsal horn in a rat model induced by chronic compression of the dorsal root ganglion

P2X3 receptors are present in the spinal dorsal horn (SDH) and play an essential role in the regulation of nociception and pain. Pregabalin (PGB) has been used as a new antiepileptic drug in the treatment of neuropathic pain. However, it is unclear whether PGB-induced analgesia was associated with the P2X3 receptor in SDH. Here, rats were randomly divided into four groups (n = 12 per group), including 2 sham operation groups, which was treated by normal saline (Sham + NS group) or PGB (Sham + PGB group), other 2 groups with chronic compression of the dorsal root ganglion, a normal saline-treated CCD group (CCD+NS group), and a PGB-treated CCD group (CCD + PGB group). A rat model of neuropathic pain was used by compressing the right L4 and L5 dorsal root ganglia. Each group was evaluated using the mechanical withdrawal threshold (MWT). The mRNA and protein levels of the P2X3 receptor in the ipsilateral SDH were measured by RT-PCR, western blot, and immunofluorescence on 14 day after CCD operation. CCD rats showed the highest mechanical hyperalgesia and the lowest pain threshold in the four groups. Simultaneously, CCD rats showed higher P2X3 mRNA and protein expression in ipsilateral side of the SDH than the sham operation rats. However, the MWT was increased and expression of P2X3 mRNA and protein in the ipsilateral SDH in CCD rats was decreased 3 days after PGB treatment. Thus, PGB may partially reverse mechanical hyperalgesia in CCD rats by inhibiting P2X3 receptor expression in the ipsilateral SDH.

Neural processing of itch

While considerable effort has been made to investigate the neural mechanisms of pain, much less effort has been devoted to itch, at least until recently. However, itch is now gaining increasing recognition as a widespread and costly medical and socioeconomic issue. This is accompanied by increasing interest in the underlying neural mechanisms of itch, which has become a vibrant and rapidly-advancing field of research. The goal of the present forefront review is to describe the recent progress that has been made in our understanding of itch mechanisms.

Spinal μ and δ opioids inhibit both thermal and mechanical pain in rats

The expression and contribution of μ (MOPR) and δ opioid receptors (DOPR) in polymodal nociceptors have been recently challenged. Indeed, MOPR and DOPR were shown to be expressed in distinct subpopulation of nociceptors where they inhibit pain induced by noxious heat and mechanical stimuli, respectively. In the present study, we used electrophysiological measurements to assess the effect of spinal MOPR and DOPR activation on heat-induced and mechanically induced diffuse noxious inhibitory controls (DNICs). We recorded from wide dynamic range neurons in the spinal trigeminal nucleus of anesthetized rats. Trains of 105 electrical shocks were delivered to the excitatory cutaneous receptive field. DNICs were triggered either by immersion of the hindpaw in 49°C water or application of 300 g of mechanical pressure. To study the involvement of peptidergic primary afferents in the activation of DNIC by noxious heat and mechanical stimulations, substance P release was measured in the spinal cord by visualizing neurokinin type 1 receptor internalization. We found that the activation of spinal MOPR and DOPR similarly attenuates the DNIC and neurokinin type 1 receptor internalization induced either by heat or mechanical stimuli. Our results therefore reveal that the activation of spinal MOPR and DOPR relieves both heat-induced and mechanically induced pain with similar potency and suggest that these receptors are expressed on polymodal, substance P-expressing neurons.