Noradrenaline hyperalgesia is mediated through interaction with sympathetic postganglionic neurone terminals rather than activation of primary afferent nociceptors

In hyperalgesic states, observed commonly as a major symptom of tissue inflammation or after central or peripheral nerve injury, non-noxious stimuli produce pain and noxious stimuli are perceived as more painful than usual. The mechanisms underlying the generation of hyperalgesia are not known. In patients with causalgia (burning pain and severe hyperalgesia after a nerve injury) activation of sympathetic post-ganglionic neurones or application of noradrenaline to painful skin exacerbates pain and hyperalgesia while sympathectomy may afford complete relief. One suggestion is that noradrenaline released from sympathetic post-ganglionic neurons increases the discharge of damaged small-diameter afferents by a direct action on the primary afferents. Here we present a new model for noradrenaline-sensitive hyperalgesia and demonstrate that the site of action of noradrenaline is not on the primary afferents but rather is presynaptic on the sympathetic post-ganglionic terminals.     

Vanilloid receptor-1 is essential for inflammatory thermal hyperalgesia

The vanilloid receptor-1 (VR1) is a ligand-gated, non-selective cation channel expressed predominantly by sensory neurons. VR1 responds to noxious stimuli including capsaicin, the pungent component of chilli peppers, heat and extracellular acidification, and it is able to integrate simultaneous exposure to these stimuli. These findings and research linking capsaicin with nociceptive behaviours (that is, responses to painful stimuli in animals have led to VR1 being considered as important for pain sensation. Here we have disrupted the mouse VR1 gene using standard gene targeting techniques. Small diameter dorsal root ganglion neurons isolated from VR1-null mice lacked many of the capsaicin-, acid- and heat-gated responses that have been previously well characterized in small diameter dorsal root ganglion neurons from various species. Furthermore, although the VR1-null mice appeared normal in a wide range of behavioural tests, including responses to acute noxious thermal stimuli, their ability to develop carrageenan-induced thermal hyperalgesia was completely absent. We conclude that VR1 is required for inflammatory sensitization to noxious thermal stimuli but also that alternative mechanisms are sufficient for normal sensation of noxious heat.     

Bradykinin and nerve growth factor release the capsaicin receptor from PtdIns(4,5)P2-mediated inhibition.

Tissue injury generates endogenous factors that heighten our sense of pain by increasing the response of sensory nerve endings to noxious stimuli. Bradykinin and nerve growth factor (NGF) are two such pro-algesic agents that activate G-protein-coupled (BK2) and tyrosine kinase (TrkA) receptors, respectively, to stimulate phospholipase C (PLC) signalling pathways in primary afferent neurons. How these actions produce sensitization to physical or chemical stimuli has not been elucidated at the molecular level. Here, we show that bradykinin- or NGF-mediated potentiation of thermal sensitivity in vivo requires expression of VR1, a heat-activated ion channel on sensory neurons. Diminution of plasma membrane phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) levels through antibody sequestration or PLC-mediated hydrolysis mimics the potentiating effects of bradykinin or NGF at the cellular level. Moreover, recruitment of PLC-gamma to TrkA is essential for NGF-mediated potentiation of channel activity, and biochemical studies suggest that VR1 associates with this complex. These studies delineate a biochemical mechanism through which bradykinin and NGF produce hypersensitivity and might explain how the activation of PLC signalling systems regulates other members of the TRP channel family.     

Changes in spinal cord reflexes after cross-anastomosis of cutaneous and muscle nerves in the adult rat

Evidence exists that the specification of afferent nerves and their central connections in the embryo may depend in part on influences from the peripheral target innervated. We have now investigated whether such peripheral determination persists in the adult rat using the unmyelinated afferent system of C fibres, which differ chemically in the adult depending on their target. We have previously shown that if the cutaneous sural nerve and the muscle gastrocnemius nerve are cross-anastomosed so that they grow to each other's target, the A fibres establish functional endings and the C fibres change their chemistry to that which is appropriate for the new target. Here we report that in normal adult rats, a short train of stimuli to the cutaneous sural nerve produced a brief facilitation of the flexion reflex, lasting on average only 5 min, whereas similar stimulation of the gastrocnemius-muscle nerve enhanced this reflex for an average of 54 min. In cross-anastomosed animals, stimulation of the gastrocnemius nerve (innervating skin) induced a brief potentiation of the flexion reflex, lasting on average only 3 min. By contrast, stimulation of sural nerve (innervating muscle) produced a potentiation of this reflex lasting 57 min. Thus the ability of adult afferent nerves to potentiate the flexion reflex depends on the target with which they make contact. We propose that tissue-specific factors influence some of the central actions of primary afferent neurons in the adult.     

Peripheral nerve injury triggers central sprouting of myelinated afferents.

The central terminals of primary afferent neurons are topographically highly ordered in the spinal cord. Peripheral receptor sensitivity is reflected by dorsal horn laminar location: low-threshold mechanoreceptors terminate in laminae III and IV (refs 2, 3) and high-threshold nociceptors in laminae I, II and V (refs 4,5). Unmyelinated C fibres, most of which are nociceptors, terminate predominantly in lamina II (refs 5, 7). There is therefore an anatomical framework for the transfer of specific inputs to localized subsets of dorsal horn neurons. This specificity must contribute to the relationship between a low-intensity stimulus and an innocuous sensation and a noxious stimulus and pain. We now show that after peripheral nerve injury the central terminals of axotomized myelinated afferents, including the large A beta fibres, sprout into lamina II. This structural reorganization in the adult central nervous system may contribute to the development of the pain mediated by A-fibres that can follow nerve lesions in humans.     

A stomatin-domain protein essential for touch sensation in the mouse

Touch and mechanical pain are first detected at our largest sensory surface, the skin. The cell bodies of sensory neurons that detect such stimuli are located in the dorsal root ganglia, and subtypes of these neurons are specialized to detect specific modalities of mechanical stimuli. Molecules have been identified that are necessary for mechanosensation in invertebrates but so far not in mammals. In Caenorhabditis elegans, mec-2 is one of several genes identified in a screen for touch insensitivity and encodes an integral membrane protein with a stomatin homology domain. Here we show that about 35% of skin mechanoreceptors do not respond to mechanical stimuli in mice with a mutation in stomatin-like protein 3 (SLP3, also called Stoml3), a mammalian mec-2 homologue that is expressed in sensory neurons. In addition, mechanosensitive ion channels found in many sensory neurons do not function without SLP3. Tactile-driven behaviours are also impaired in SLP3 mutant mice, including touch-evoked pain caused by neuropathic injury. SLP3 is therefore indispensable for the function of a subset of cutaneous mechanoreceptors, and our data support the idea that this protein is an essential subunit of a mammalian mechanotransducer.     

A neuronal mechanism for sensory gating during locomotion in a vertebrate

The response of the foot to touch during walking depends on whether it is in the air or on the ground. In most animals, reflex responses to external stimuli are similarly adapted to their timing in the locomotor cycle, but there is only fragmentary information about the neural mechanisms involved. In arthropods, reflex modulation can occur in the sensory receptors themselves and in neurons that discharge during locomotion. By recording with dye-filled microelectrodes from neurons in the spinal cord of frog embryos, we describe reflex modulation at the level of sensory interneurons. Sensory inputs from skin receptors excite a specific class of spinal sensory interneuron whose activity leads to reflex bending of the body away from the stimulus. During swimming, these inputs are gated by rhythmic postsynaptic inhibition, so that sensory drive reaches motor neurons only at phases in the locomotor cycle when the resulting contraction would likewise turn the embryo away from the stimulated side. Such gating of sensory pathways could be a general feature of all locomotor systems where responses to sensory stimuli need to be adapted to the phase of locomotion.     

Interleukin-1beta-mediated induction of Cox-2 in the CNS contributes to inflammatory pain hypersensitivity

Inflammation causes the induction of cyclooxygenase-2 (Cox-2), leading to the release of prostanoids, which sensitize peripheral nociceptor terminals and produce localized pain hypersensitivity. Peripheral inflammation also generates pain hypersensitivity in neighbouring uninjured tissue (secondary hyperalgesia), because of increased neuronal excitability in the spinal cord (central sensitization), and a syndrome comprising diffuse muscle and joint pain, fever, lethargy and anorexia. Here we show that Cox-2 may be involved in these central nervous system (CNS) responses, by finding a widespread induction of Cox-2 expression in spinal cord neurons and in other regions of the CNS, elevating prostaglandin E2 (PGE2) levels in the cerebrospinal fluid. The major inducer of central Cox-2 upregulation is interleukin-1beta in the CNS, and as basal phospholipase A2 activity in the CNS does not change with peripheral inflammation, Cox-2 levels must regulate central prostanoid production. Intraspinal administration of an interleukin-converting enzyme or Cox-2 inhibitor decreases inflammation-induced central PGE2 levels and mechanical hyperalgesia. Thus, preventing central prostanoid production by inhibiting the interleukin-1beta-mediated induction of Cox-2 in neurons or by inhibiting central Cox-2 activity reduces centrally generated inflammatory pain hypersensitivity.     

A molecular mechanism for long-term sensitization in Aplysia

Sensitization of the gill- and siphon-withdrawal reflex in Aplysia is thought to result from a set of molecular processes with different time courses: short-term sensitization is explained by cyclic AMP-dependent modulation of ion-channel function in sensory neurons lasting minutes; memory that endures for hours or longer, by the expression and distribution within the neurons of new gene products. Because gene induction and axonal transport are relatively slow, there may also be a need for a distinct form of intermediate memory to bridge the short- and long-term processes. We now report that a protocol producing long-term sensitization results in a decrease in the amount of regulatory subunits of the cAMP-dependent protein kinase in animals 24 h after training, with no effect on the catalytic subunit. The loss appears to be post-translational. Because a decrease in the ratio of regulatory to catalytic subunits would result in elevated kinase activity after cAMP has returned to its unstimulated concentration in sensory cells, it could be the molecular mechanism of intermediate memory.     

Peripheral modulation of mechano-sensitivity in primary afferent neurons

Considerable attention has centered recently on the changes in neuron excitability and synaptic efficacy caused by certain biogenic amines and neuropeptides. These neuromodulators act at a wide variety of both central and peripheral targets, and bring about diverse biological results. In sensory pathways, modulation occurs at central input synapses of the primary afferents and at peripheral terminals of efferents. This study was undertaken to look at non-synaptic modulation of membrane potentials in peripheral sensory endings of identifiable receptors. Using intracellular recording from the three primary afferent fibres of a recently described simple crustacean stretch receptor, which lacks centrifugal control, we observed in vitro modulation of the sensory response by three neuroactive substances known to be present in vivo. Two neuroamines, serotonin and octopamine, depressed receptor potentials and impulse discharge whereas the pentapeptide proctolin enhanced both these components of the sensory response. The peripheral sensory modulation reported here for a lobster mechano-receptor may occur in many animal groups and sensory systems.     

Mustard oils and cannabinoids excite sensory nerve fibres through the TRP channel ANKTM1

Wasabi, horseradish and mustard owe their pungency to isothiocyanate compounds. Topical application of mustard oil (allyl isothiocyanate) to the skin activates underlying sensory nerve endings, thereby producing pain, inflammation and robust hypersensitivity to thermal and mechanical stimuli. Despite their widespread use in both the kitchen and the laboratory, the molecular mechanism through which isothiocyanates mediate their effects remains unknown. Here we show that mustard oil depolarizes a subpopulation of primary sensory neurons that are also activated by capsaicin, the pungent ingredient in chilli peppers, and by Delta(9)-tetrahydrocannabinol (THC), the psychoactive component of marijuana. Both allyl isothiocyanate and THC mediate their excitatory effects by activating ANKTM1, a member of the TRP ion channel family recently implicated in the detection of noxious cold. These findings identify a cellular and molecular target for the pungent action of mustard oils and support an emerging role for TRP channels as ionotropic cannabinoid receptors.     

Molecular mechanisms of nociception

The sensation of pain alerts us to real or impending injury and triggers appropriate protective responses. Unfortunately, pain often outlives its usefulness as a warning system and instead becomes chronic and debilitating. This transition to a chronic phase involves changes within the spinal cord and brain, but there is also remarkable modulation where pain messages are initiated - at the level of the primary sensory neuron. Efforts to determine how these neurons detect pain-producing stimuli of a thermal, mechanical or chemical nature have revealed new signalling mechanisms and brought us closer to understanding the molecular events that facilitate transitions from acute to persistent pain.     

Sensory neuron sodium channel Nav1.8 is essential for pain at low temperatures

Sensory acuity and motor dexterity deteriorate when human limbs cool down, but pain perception persists and cold-induced pain can become excruciating. Evolutionary pressure to enforce protective behaviour requires that damage-sensing neurons (nociceptors) continue to function at low temperatures. Here we show that this goal is achieved by endowing superficial endings of slowly conducting nociceptive fibres with the tetrodotoxin-resistant voltage-gated sodium channel (VGSC) Na(v)1.8 (ref. 2). This channel is essential for sustained excitability of nociceptors when the skin is cooled. We show that cooling excitable membranes progressively enhances the voltage-dependent slow inactivation of tetrodotoxin-sensitive VGSCs. In contrast, the inactivation properties of Na(v)1.8 are entirely cold-resistant. Moreover, low temperatures decrease the activation threshold of the sodium currents and increase the membrane resistance, augmenting the voltage change caused by any membrane current. Thus, in the cold, Na(v)1.8 remains available as the sole electrical impulse generator in nociceptors that transmits nociceptive information to the central nervous system. Consistent with this concept is the observation that Na(v)1.8-null mutant mice show negligible responses to noxious cold and mechanical stimulation at low temperatures. Our data present strong evidence for a specialized role of Na(v)1.8 in nociceptors as the critical molecule for the perception of cold pain and pain in the cold.     

The principle of temperature-dependent gating in cold- and heat-sensitive TRP channels

The mammalian sensory system is capable of discriminating thermal stimuli ranging from noxious cold to noxious heat. Principal temperature sensors belong to the TRP cation channel family, but the mechanisms underlying the marked temperature sensitivity of opening and closing ('gating') of these channels are unknown. Here we show that temperature sensing is tightly linked to voltage-dependent gating in the cold-sensitive channel TRPM8 and the heat-sensitive channel TRPV1. Both channels are activated upon depolarization, and changes in temperature result in graded shifts of their voltage-dependent activation curves. The chemical agonists menthol (TRPM8) and capsaicin (TRPV1) function as gating modifiers, shifting activation curves towards physiological membrane potentials. Kinetic analysis of gating at different temperatures indicates that temperature sensitivity in TRPM8 and TRPV1 arises from a tenfold difference in the activation energies associated with voltage-dependent opening and closing. Our results suggest a simple unifying principle that explains both cold and heat sensitivity in TRP channels.     

阿片类物质的外周镇痛作用

就外周阿片受体的分布和阿片类物质的外周镇痛作用及机制的相关研究进展进行综述.阿片类物质的镇痛作用通过中枢和外周两种机制,研究表明:阿片类物质的外周镇痛作用是通过外周阿片受体起作用的.初级感觉神经元内及其外周末梢上存在着各类阿片受体,在炎症及神经损伤的情况下,阿片受体合成增加并转运到神经末梢或者受损部位,增加传导痛觉冲动...     

大鼠丘脑中央下核神经元对不同刺激的反应

目的：观察丘脑中央下核（Sm)神经元对伤害性机械刺激,手针和电针刺激的反应特性。方法：在麻醉大鼠,用玻璃微电极细胞外记录的方法,观察Sm神经元对上述刺激的反应。结果：大多数对手针刺激反应的神经元一般地也对伤害性皮肤或肌肉机械刺激发生反应,这些神经元也对电针刺激穴位发生反应,其反应随刺激强度、串长或重复次数的增加而加强,揭示针刺与伤害性机械刺激具有相同的属性。结论：Sm可能在针感形成及针刺兴奋细胞纤维产生的镇痛中起重要作用。     

Regulation of leukocyte migration by activation of the leukocyte adhesion molecule-1 (LAM-1) selectin.

A central feature of host defence is the ability of leukocytes to enter tissues in response to immune or inflammatory stimuli. The leukocyte adhesion molecule-1 (LAM-1) regulates the migration of human leukocytes by mediating the binding both of lymphocytes to high endothelial venules of peripheral lymph nodes and of neutrophils to endothelium at inflammatory sites. As lymphocytes and neutrophils express the same LAM-1 protein, it is not clear how lineage-specific differences in leukocyte migration are controlled. We now report that the affinity of LAM-1 for a carbohydrate-based ligand, PPME, is dramatically increased following lymphocyte and neutrophil activation by lineage-specific stimuli. In addition, activation of lymphocytes by physiological stimuli enhanced LAM-1-dependent binding to high endothelial venules. Thus, transient changes in LAM-1 affinity after leukocyte stimulation probably directly influence leukocyte migration.     

Spontaneous and evoked activity of fetal primary afferents in vivo

The first movements of the fetus are apparently random and spontaneous. Their onset coincides with the growth of dorsal root afferents into the spinal cord and it is possible that they are not simply a result of spontaneous motoneuron activity but are reflex responses to sensory stimulation. It is not clear what stimuli could normally evoke such reflexes because nothing is known of the properties of primary afferent neurons in the fetus. I have investigated this by making recordings from single dorsal root ganglion cells in fetal rats in vivo. The afferents have small, defined receptive fields and respond to mechanical stimulation of skin or muscle at intensities that might occur in utero. Many of them are also chemosensitive. Unlike postnatal afferents they display background activity which peaks at the same age as fetal movements. Repeated stimulation causes long-lasting increases of both background and evoked activity. Such sensory input is likely to have a considerable influence on fetal movements and on the development of spinal cord connections.     

促代谢型谷氨酸受体mGluR5参与膀胱的生理与伤害性感受

促代谢型谷氨酸受体(mGluRs)是一类G蛋白偶联受体,参与中枢神经系统的突触可塑性和学习记忆等过程。一些研究资料表明,mGluRs还在躯体痛的外周信号转导和信号传递中起重要作用,但其在内脏感受中的作用尚不清楚。本研究的目的在于考察Ⅰ组促代谢型谷氨酸受体(mGluR5)是否参与膀胱的生理与伤害性感受。在用戊巴比妥钠(50mg/kg,ip)麻醉的大鼠,mGluR5拮抗剂MPEP(3.0mg/kgiv)能明显提高排尿反射的容积阈值,并能减弱快速充胀膀胱引起的腹肌收缩反应,提示mGluR5参与膀胱的生理和伤害性感受过程。在取自正常小鼠的膀胱/盆神经模型,MPEP(0.1￣100μM)对充胀膀胱引起的传入神经活动没有明显影响,而在环磷酰胺(125mg/kgip)致间质性膀胱炎(IC)的小鼠,MPEP(0.3μm)能抑制高阈值膀胱传入神经放电。在清醒、自由活动的IC小鼠,MPEP(1μmol/kg,ip)还明显抑制其后肢对机械刺激的反应。上述结果表明,mGluR5可能并不参与正常膀胱的外周机械性感受的信号转导,但可能参与膀胱生理和伤害性感受信息向脊髓和在中枢内的传递过程;外周和中枢mGluR5在实验性间质性膀胱炎时有上调现象,故可能是治疗内脏感觉过敏的一个潜在药物作用靶点。