Emerging roles of long non‐coding RNAs in neuropathic pain

Neuropathic pain, a type of chronic and potentially disabling pain resulting from primary injury/dysfunction of the somatosensory nervous system and spinal cord injury, is one of the most intense types of chronic pain, which incurs a significant economic and public health burden. However, our understanding of its cellular and molecular pathogenesis is still far from complete. Long non‐coding RNAs (lncRNAs) are important regulators of gene expression and have recently been characterized as key modulators of neuronal functions. Emerging evidence suggested that lncRNAs are deregulated and play pivotal roles in the development of neuropathic pain. This review summarizes the current knowledge about the roles of deregulated lncRNAs (eg, KCNA2‐AS, uc.48+, NONRATT021972, MRAK009713, XIST, CCAT1) in the development of neuropathic pain. These studies suggested that specific regulation of lncRNAs or their downstream targets might provide novel therapeutic avenues for this refractory disease.     

Regulation of increased glutamatergic input to spinal dorsal horn neurons by mGluR5 in diabetic neuropathic pain

Diabetic neuropathic pain is associated with increased glutamatergic input in the spinal dorsal horn. Group I metabotropic glutamate receptors (mGluRs) are involved in the control of neuronal excitability, but their role in the regulation of synaptic transmission in diabetic neuropathy remains poorly understood. Here we studied the role of spinal mGluR5 and mGluR1 in controlling glutamatergic input in a rat model of painful diabetic neuropathy induced by streptozotocin. Whole-cell patch-clamp recordings of lamina II neurons were performed in spinal cord slices. The amplitude of excitatory post-synaptic currents (EPSCs) evoked from the dorsal root and the frequency of spontaneous EPSCs (sEPSCs) were significantly higher in diabetic than in control rats. The mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) inhibited evoked EPSCs and sEPSCs more in diabetic than in control rats. Also, the percentage of neurons in which sEPSCs and evoked EPSCs were affected by MPEP or the group I mGluR agonist was significantly higher in diabetic than in control rats. However, blocking mGluR1 had no significant effect on evoked EPSCs and sEPSCs in either groups. The mGluR5 protein level in the dorsal root ganglion, but not in the dorsal spinal cord, was significantly increased in diabetic rats compared with that in control rats. Furthermore, intrathecal administration of MPEP significantly increased the nociceptive pressure threshold only in diabetic rats. These findings suggest that increased mGluR5 expression on primary afferent neurons contributes to increased glutamatergic input to spinal dorsal horn neurons and nociceptive transmission in diabetic neuropathic pain.     

Intracranial portion of the trochlear nerve and dorsal oblique muscle composition in dog: a structural and ultrastructural study

In the present investigation the right intracranial portion of the trochlear nerves and dorsal oblique muscle of the right ocular globe were removed from six adult dogs and analyzed by light and electron microscopy. Unmyelinated fibers were observed in the analyzed nerves. The number, diameter, area, and density of myelinated fibers were determined, as were corresponding axon area and diameter and myelin sheath thickness. Frequency histograms of myelin sheath thickness and fiber size show a bimodal distribution with a similar proportion of large and small fibers. Muscle samples were taken from the central portion of the muscle belly, subsequently frozen, cut, and stained with m-ATPase at pH 4.6. Fibers were classified as Type 1 or Type 2 according to their reaction to the m-ATPase and detailed morphologic and morphometric studies were made. The muscles showed two clearly distinct layers, a central layer and a peripheral layer, chiefly composed of Type 2 fibers. The fibers in the central layer were larger in size than those in the peripheral layer.     

Recent evidence for activity-dependent initiation of sympathetic sprouting and neuropathic pain

Traumatic injury or inflammatory irritation of the peripheral nervous system often leads to persistent pathophysiological pain states. It has been well-documented that, after peripheral nerve injury or inflammation, functional and anatomical alterations sweep over the entire peripheral nervous system including the peripheral nerve endings, the injured or inflamed afferent fibers, the dorsal root ganglion (DRG), and the central afferent terminals in the spinal cord. Among all the changes, ectopic discharge or spontaneous activity of primary sensory neurons is of great clinical interest, as such discharges doubtless contribute to the develop-ment of pathological pain states such as neuropathic pain. Two key sources of abnormal spontaneous activity have been identified following peripheral nerve injury: the injured afferent fibers (neuroma) leading to the DRG, and the DRG somata. The purpose of this review is to provide a global account of the abnormal spontaneous activity in various animal models of pain. Particular attention is focused on the consequence of peripheral nerve injury and localized inflammation. Further, mechanisms involved in the generation of spontaneous activity are also reviewed; evidence of spontaneous activity in contributing to abnormal sympathetic sprouting in the axotomized DRG and to the initiation of neuropathic pain based on new findings from our research group are discussed. An improved understanding of the causes of spontaneous activity and the origins of neuropathic pain should facilitate the development of novel strategies for effective treatment of pathological pain.     

Intraplantar injection of glutamate evokes peripheral adenosine release in the rat hind paw: involvement of peripheral ionotropic glutamate receptors and capsaicin-sensitive sensory afferents

Glutamate receptors have been identified on the peripheral terminals of both primary sensory afferents and sympathetic post-ganglionic neurons, and activation of these receptors produces peripheral sensitization and enhances nociception. Adenosine is an endogenous agent that has a regulatory effect on pain. In brain and spinal cord, adenosine release can be promoted by excitatory amino acids. In the present study, we used in vivo microdialysis to determine whether glutamate also can release adenosine in peripheral tissues. Rats were anesthetized with pentobarbital and microdialysis probes were implanted into the subcutaneous tissue of the plantar aspect of the rat hind paw. Subcutaneous injection of glutamate (50 microL, 0.3-100 micromol) evoked a short-lasting adenosine release immediately following drug injection. Co-administration of either the N-methyl-D-aspartate (NMDA) receptor antagonist, dizocipine maleate (MK-801, 1 nmol) or the non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline (CNQX, 10 nmol) with glutamate blocked such release, suggesting an involvement of peripheral ionotropic glutamate receptors in this response. Systemic pre-treatment with capsaicin, a neurotoxin selective for unmyelinated sensory afferents, significantly reduced glutamate-evoked peripheral adenosine release, but release was not affected by systemic pre-treatment with 6-hydroxydopamine, a neurotoxin selective for sympathetic nerve efferents. Neither MK-801 nor CNQX blocked 5% formalin-evoked adenosine release, suggesting adenosine release by formalin is not secondary to ionotropic glutamate receptor activation. We conclude that administration of glutamate evokes peripheral adenosine release, and that peripheral ionotropic glutamate receptors on unmyelinated sensory afferents are involved in such release. The released adenosine may provide a negative feedback control on nociception.     

Evidence that gabapentin reduces neuropathic pain by inhibiting the spinal release of glutamate

Gabapentin is an anticonvulsant that successfully treats many neuropathic pain syndromes, although the mechanism of its antihyperalgesic action remains elusive. This study aims to help delineate gabapentin's antihyperalgesic mechanisms. We assessed the effectiveness of gabapentin at decreasing mechanical and cold hypersensitivity induced in a rat model of neuropathic pain. Thus, we compared the effectiveness of pre‐ or post‐treatment with systemic or intrathecal (i.t.) gabapentin at reducing the development and maintenance of the neuropathic pain symptoms. Gabapentin successfully decreased mechanical and cold hypersensitivity, both as a pretreatment and post‐treatment. Furthermore, both i.t. and systemic administration of gabapentin were effective in reducing the behavioral hypersensitivity; however, the i.t. administration was superior to the systemic. We also examined gabapentin's effects at inhibiting hindpaw formalin‐induced release of excitatory amino acids (EAAs) in the spinal cord dorsal horn (SCDH) both in naïve rats and in rats with neuropathic pain. We present the first evidence that gabapentin reduces the formalin‐induced release of both glutamate and aspartate in SCDH. Furthermore, i.t. gabapentin reduces the enhanced noxious stimulus‐induced spinal release of glutamate seen in neuropathic rats. These data suggest that gabapentin reduces neuropathic pain symptoms by inhibiting the release of glutamate in the SCDH.     

Update on the pathobiology of neuropathic pain

Nerve injury or dysfunction in the peripheral and central nervous systems are the leading causes for the development of neuropathies, which are frequently associated with allodynia and hyperalgesia. Treatment of these disorders is often unsatisfactory due to side effects or insufficient analgesia of the currently available drugs. Therefore, elucidating the molecular mechanisms of neuropathic pain is an important prerequisite for the rational development of novel analgesic drugs for the therapy of neuropathic pain. Several proteomic approaches have been performed to explore protein modifications in the nervous system associated with neuropathies in different animal models, which might contribute to the detection of new drug targets. Furthermore, there are proteomic studies investigating human cerebrospinal fluid from patients suffering from neuropathies. The results of these studies and the potential clinical value of the proteomic data are summarized and discussed in this review.     

神经病理性痛的交感—感觉耦联作用

周围组织和神经受到损伤引起自发性疼痛、触刺激诱发痛和痛觉过敏等慢性痛症状。交感神经系统通过发展异常交感功能,或者通过影响传入神经异常活动参与上述的病理性变化,进而造成神经病理性痛。本文对目前关于交感－感觉耦联作用及其受体、细胞内和神经机制进行综述。     

The role of sodium channels in neuropathic pain

Our knowledge of the ion channels, receptors and signalling mechanisms involved in pain pathophysiology, and which specific channels play a role in subtypes of pain such as neuropathic and inflammatory pain, has expanded considerably in recent years. It is now clear that in the neuropathic state the expression of certain channels is modified, and that these changes underlie the plasticity of responses that occur to generate inappropriate pain signals from normally trivial inputs. Pain is modulated by a subset of the voltage-gated sodium channels, including Nav1.3, Nav1.7, Nav1.8 and Nav1.9. These isoforms display unique expression patterns within specific tissues, and are either up- or down-regulated upon injury to the nervous system. Here we describe our current understanding of the roles of sodium channels in pain and nociceptive information processing, with a particular emphasis on neuropathic pain and drugs useful for the treatment of neuropathic pain that act through mechanisms involving block of sodium channels. One of the future challenges in the development of novel sodium channel blockers is to design and synthesise isoform-selective channel inhibitors. This should provide substantial benefits over existing pain treatments.     

A new combination of methods for the localization,identification, and three-dimensional reconstruction of the sensory endings of articular afferents characterized by electrophysiology

A combination of methods is described to identify and reconstruct corpuscular and non-corpuscular sensory endings of group II and group III nerve fibers following functional examination by electrophysiology. Afferent units activated by electrical stimulation of the medial articular nerve of the cat's knee were analyzed by single fiber recordings and characterized by their responsiveness to mechanical stimuli. The receptive fields of the units were closely demarcated by fine needles when the responses elicited by insertion of the needles were being recorded. After fixation, the tissue around the demarcated field was dissected and histologically processed. Series of semithin sections were cut from the embedded tissue blocks containing the receptive fields. Corpuscular endings of group II fibers and peripheral myelinated group III nerve fibers, presumably corresponding to the characterized units, were identified by light microscopy of semithin sections and localized within the demarcated area. Non-corpuscular endings were identified by electron microscopy of ultrathin sections cut in alternation with, or after re-embedding of, semithin sections. Morphometric analysis of ultrathin section series allowed the measurement of parameters such as the mean axon diameter and the organelle content of the sensory endings. The methods described are appropriate for collecting data that correlate the structural and functional characteristics of sensory endings in deep tissues.     

The pattern of lateral-line afferents in urodeles

Summary The organization of posterior and anterior afferents of the lateralline system was studied in several species of urodeles by means of transganglionic transport of horseradish peroxidase. The afferents of each lateral-line nerve form distinct fascicles in the medullary alar plate. Each of the two branches of the anterior lateral-line nerve is organized in two long and one short fascicles. The posterior lateral-line afferents form only two long fascicles. Each ordinary neuromast is supplied by only two afferents, which run in the two ventral medullary fiber bundles. It is suggested that afferents to hair cells displaying one type of polarity form together one bundle, but those contacting hair cells polarized in the opposite way form the second ventral bundle of one lateral-line branch. Thus, the lateral-line afferents may be organized in a directotopic fashion.The short dorsal fascicle formed only by the anterior lateral-line afferents receives fibers exclusively from small pit organs. Each pit organ is supplied by only one afferent. Anatomically, these pit organs resemble in many respects the electroreceptive ampullary organs of certain fish.Neurons labeled retrogradely via the anterior lateral-line nerve afferents have been attributed to the nervus trigeminus or facialis. In addition to the posterior lateral-line afferents, only few centrifugally projecting neurons were labeled. These neurons are discussed as efferents to the posterior lateral-line neuromasts.     

Altered neuropathic pain behaviour in a rat model of depression is associated with changes in inflammatory gene expression in the amygdala

The association between chronic pain and depression is widely recognized, the comorbidity of which leads to a heavier disease burden, increased disability and poor treatment response. This study examined nociceptive responding to mechanical and thermal stimuli prior to and following L5‐L6 spinal nerve ligation (SNL), a model of neuropathic pain, in the olfactory bulbectomized (OB) rat model of depression. Associated changes in the expression of genes encoding for markers of glial activation and cytokines were subsequently examined in the amygdala, a key brain region for the modulation of emotion and pain. The OB rats exhibited mechanical and cold allodynia, but not heat hyperalgesia, when compared with sham‐operated counterparts. Spinal nerve ligation induced characteristic mechanical and cold allodynia in the ipsilateral hindpaw of both sham and OB rats. The OB rats exhibited a reduced latency and number of responses to an innocuous cold stimulus following SNL, an effect positively correlated with interleukin (IL)‐6 and IL‐10 mRNA expression in the amygdala, respectively. Spinal nerve ligation reduced IL‐6 and increased IL‐10 expression in the amygdala of sham rats. The expression of CD11b (cluster of differentiation molecule 11b) and GFAP (glial fibrillary acidic protein), indicative of microglial and astrocyte activation, and IL‐1β in the amygdala was enhanced in OB animals when compared with sham counterparts, an effect not observed following SNL. This study shows that neuropathic pain‐related responding to an innocuous cold stimulus is altered in an animal model of depression, effects accompanied by changes in the expression of neuroinflammatory genes in the amygdala .     

Serotonin in pain and analgesia

The purpose of this article is to summarize recent findings on the role of serotonin in pain processing in the peripheral nervous system. Serotonin (5-hydroxtryptamine [5-HT]) is present in central and peripheral serotonergic neurons, it is released from platelets and mast cells after tissue injury, and it exerts algesic and analgesic effects depending on the site of action and the receptor subtype. After nerve injury, the 5-HT content in the lesioned nerve increases. 5-HT receptors of the 5-HT₃ and 5-HT_2A subtype are present on C-fibers. 5-HT, acting in combination with other inflammatory mediators, may ectopically excite and sensitize afferent nerve fibers, thus contributing to peripheral sensitization and hyperalgesia in inflammation and nerve injury.     

电压门控钠通道在神经病理性痛中的作用

电压门控钠通道(VGSC)在神经病理性痛的发生和维持中起重要作用。非特异性的通道阻断剂是神经病理性痛的一种治疗手段,但由于可能产生严重的副作用而限制了其使用。最近研究揭示了几种主要在外周感觉神经系统中表达的VGSC的亚型与神经病理性痛密切相关,发展特异性的通道亚型阻断剂将成为治疗神经病理性痛的重要研究方向。     

The role of voltage-gated sodium channels in neuropathic pain

Use-dependent inhibitors of voltage-gated sodium channels (VGSC) are important therapeutic tools for chronic pain management, but are limited by possible severe side effects. Recent studies have provided much new information on the function of several voltage-gated sodium channels that are predominantly expressed in peripheral sensory neurons, and on their possible link to pathological pain states arising from injuries to the sensory nerve. The use of antisense oligonucleotides to target specific channel subtypes shows that the functional localization of the channel subtype Na(V)1.8 after nerve injury is essential for persistent pain states. The putative roles of Na(V)1.3 and Na(V)1.9 in neuropathic pain are also discussed. These studies may form a basis for developing inhibitors to target specific channel subtype(s) for use in chronic pain treatment.     

Discovery and evaluation of novel FAAH inhibitors in neuropathic pain model

Conceptual design and modification of urea moiety in chemotype PF-3845/04457845, the bench marking irreversible inhibitor of fatty acid amide hydrolase (FAAH), led to discovery of a novel nicotinamide-based lead 12a having reversible mechanism of action. Focused SAR around the pyridine heterocycle (Ar) in 12a (Tables 1 and 2) resulted into four shortlisted compounds, (?)-12a, (?)-12i, (?)-12l–m. The required (?)-enantiomers were obtained via diastereomeric resolution of a novel chiral dissymmetric intermediate 15. Based on comparative profile of FAAH potency, metabolic stability in liver microsome, liability of inhibiting major hCYP450 isoforms, rat PK, and brain penetration ability, two SAR optimized compounds, (?)-12l and (?)-12m, were selected for efficacy study in rat model of chemotherapy-induced peripheral neuropathy (CIPN). Both the compounds exhibited dose related antihyperalgesic effects, when treated with 3–30?mg/kg po for 7?days. The effects at 30?mg/kg are comparable to that of PF-04457845 (10?mg/kg) and Tramadol (40?mg/kg).