Endogenous nerve growth factor regulates the sensitivity of nociceptors in the adult rat

Nerve growth factor (NGF) has a well characterized role in the development of the nervous system and there is evidence that it interacts with nociceptive primary afferent fibres. Here we applied a synthetic tyrosine kinase A IgG (trkA-IgG) fusion molecule for 10–12 days to the innervation territory of the purely cutaneous saphenous nerve in order to bind, and thereby neutralize endogenous NGF in adult rats. Using neurophysiological analysis of 152 nociceptors we now show that sequestration of NGF results in specific changes of their receptive field properties. The percentage of nociceptors responding to heat dropped significantly from a normal 57% to 32%. This was accompanied by a rightward shift and a reduced slope of the stimulus response function relating the intracutaneous temperature to the neural response. The number of nociceptors responding to application of bradykinin was also significantly reduced from a normal of 28% to 8%. In contrast, the threshold for mechanical stimuli and the response to suprathreshold stimuli remained unaltered, as did the percentage of nociceptors responding to noxious cold. The reduced sensitivity of primary afferent nociceptors was accompanied by a reduction in the innervation density of the epidermis by 44% as assessed with quantitative immunocytochemical analysis of the panaxonal marker PGP 9.5. This demonstrates that endogenous NGF in the adult specifically modulates the terminal arborization of unmyelinated fibres and the sensitivity of primary afferent nociceptors to thermal and chemical stimuli in vivo.     

Histamine-induced itch converts into pain in neuropathic hyperalgesia.

Physiologically, itch and pain are transmitted in separate specific peripheral C-units and central afferent pathways. Some neuropathic pain patients with intact but sensitized (irritable) primary C-nociceptors have spontaneous pain, heat hyperalgesia, static and dynamic mechanical hyperalgesia. The question was whether cutaneous histamine application induces pain in these patients. For comparison histamine was applied into normal skin experimentally sensitized by capsaicin. Histamine application in the capsaicin-induced primary or secondary hyperalgesic skin did not change the intensity and quality of capsaicin pain. Itch was profoundly inhibited. Conversely, histamine application in neuropathic skin induced severe increase in spontaneous burning pain but no itch. In neuropathies irritable nociceptors may express histamine receptors or induce central sensitization to histaminergic stimuli so that itch converts into pain.     

Human microneurography and intraneural microstimulation in the study of neuropathic pain

Psychophysical experiments in combination with microneurography and intraneural microstimulation in awake human subjects have yielded some useful information on somatosensory functions under normal and pathologic conditions. Normally, pain is signaled by nociceptive afferents, and tactile sensations are evoked from activation of low-threshold mechanoreceptors. Following tissue injury, nociceptors are sensitized, and their enhanced responsiveness correlates with hyperalgesia to heat and in some cases to mechanical stimuli. In addition, ongoing activity in sensitized nociceptive afferents may lead to central sensitization in such a way that normally nonpainful gentle stroking the skin evokes pain from activation of low-threshold mechanoreceptors. This particular change in signal processing in the central nervous system is restored when the ongoing nociceptive input is interrupted, whereas other forms of central sensitization can outlast the duration of the nociceptive input. © 1993 John Wiley & Sons, Inc.     

Acute injury, inflammation, and pain

Post-traumatic analgesia may depend on presynaptic inhibition, by a restriction of the input from damaged tissues into the central nervous system. With the development of inflammation, primary afferent depolarization decreases. Persistent irritation of nociceptors evoked by tissue inflammation can lower the pain threshold and cause hyperalgesia.     

Effects of carrageenan and morphine on acute inflammation and pain in Lewis and Fischer rats

The present study used inbred, histocompatible Fischer 344 (FIS) and Lewis (LEW) rats to begin to explore the role of the hypothalamic-pituitary-adrenal (HPA) axis in the immune processes and pain behavior associated with the carrageenan model of acute hindpaw inflammation. Because the HPA axis contributes in part to morphine's analgesic and immunomodulatory properties, the present study also assessed the effects of morphine in carrageenan-inflamed LEW and FIS rats. The results showed that carrageenan-induced hindpaw swelling and pain behavior were greater in FIS than in LEW rats. The enhanced hindpaw swelling in FIS rats correlated with an increase in myeloperoxidase (MPO; a measure of neutrophils) in the inflamed hindpaw. FIS rats showed lower circulating levels of TNFalpha, higher IL-6 levels, and similar IL-1beta and nitric oxide levels, when compared to LEW rats. Morphine produced a significant decrease in carrageenan-induced hindpaw swelling and MPO in both strains, but morphine did not significantly alter circulating cytokine/mediator levels. Morphine's analgesic effects were greater in the inflamed than the noninflamed hindpaw, and they did not correlate with morphine's anti-inflammatory effects. In fact, low doses of morphine produced a mechanical allodynia and hyperalgesia in the noninflamed hindpaw of FIS, but not LEW, rats. These results suggest a positive relationship between HPA axis activity and acute inflammation and inflammatory pain. In contrast, little evidence is provided for HPA axis involvement in morphine's anti-inflammatory or analgesic effects.     

Xenon prevents inflammation-induced delayed pain hypersensitivity in rats

Rats received an intraplantar carrageenan injection for inducing hind paw inflammation. After 1 h 45 min, they were exposed to medical air (air group), xenon 25% (Xe-25 group) or 50% (Xe-50 group) for 1 h 45 min. Mechanical nociceptive threshold was evaluated on experimental day and once daily for 1 week. Beyond the well-known antinociceptive effect of xenon, the delayed hyperalgesia observed for 4 days after carrageenan injection was strongly reduced in Xe-25 group and totally suppressed in Xe-50 group on the inflamed hind paw. Moreover, delayed hyperalgesia on the noninflamed hind paw was totally suppressed for both the xenon concentrations. These results show that xenon, beyond its antinociceptive effects, may be a fruitful therapeutic strategy to limit the development of pain sensitization after tissue injury.     

Antidromic nerve stimulation in monkey does not sensitize unmyelinated nociceptors to heat

Spread of sensitization of nociceptors has been proposed as the mechanism for the secondary hyperalgesia in the region that surrounds a cutaneous injury. One possibility, tested in this study, is that antidromically propagated action potentials in nociceptors result in the release of substances at their cutaneous arborization which sensitize adjacent nociceptors. Using standard teased-fiber techniques, we recorded from single C-fiber nociceptive afferents innervating hairy skin in the monkey. Heat testing was performed before and after electrical stimulation of the parent nerve at strengths sufficient to activate C-fibers. The heat thresholds and total response to the heat sequences were not changed by the electrical stimulation. We conclude that antidromic stimulation of C-fibers in the monkey does not sensitize C-fiber nociceptors to heat stimuli.     

Spinal administration of capsazepine inhibits noxious evoked responses of dorsal horn neurons in non-inflamed and carrageenan inflamed rats

Vanilloid VR1 receptors are located in the dorsal horn of the spinal cord. The aim of the present study was to determine the role of spinal vanilloid receptors (VR1) during nociceptive processing in control and inflamed rats. Effects of spinal administration of capsazepine (0.5-30 microM/50 microl), a competitive VR1 antagonist, on innocuous and noxious evoked responses of spinal neurones were studied in halothane anaesthetised rats. Transcutaneous electrical-evoked neuronal responses of spinal neurones were recorded in control and carrageenan (2%, 3 h) inflamed rats. Spinal application of capsazepine did not significantly alter Abeta-fibre evoked responses of neurones, however Adelta-fibre evoked responses were significantly inhibited by capsazepine in both non-inflamed and carrageenan inflamed rats (30 microM: non-inflamed 31+/-8% of control, P<0.01: carrageenan-inflamed 43+/-6% of control, P<0.01). Similarly, the evoked C-fibre mediated post-discharge responses of spinal neurones in non-inflamed and carrageenan inflamed rats were reduced by capsazepine (30 microM: non-inflamed 41+/-14% of control, P<0.01: carrageenan-inflamed 31+/-9% of control, P<0.01). These results demonstrate a role of spinal VR1 receptors during noxious, but not innocuous transmission, at the level of the spinal cord. The degree of effect of capsazepine on evoked neuronal responses was similar in control and inflamed rats, suggesting that the role of spinal VR1 receptors is not altered following short-term peripheral inflammation. Our data suggest that following noxious peripheral stimulation, spinal VR1 receptors are activated, but the endogenous ligands mediating this effect remain to be elucidated.     

Neural changes in acute arthritis in monkeys. II. Increased glutamate immunoreactivity in the medial articular nerve

Glutamate and other excitatory amino acids have been shown to play a key role in nociception and the hyperalgesia associated with the acute inflammatory response. In an effort to understand more fully the role of Glu in this process, we determined that there is Glu in a percentage of axons in the medial articular nerve (MAN) of monkeys, a source of preterminal afferent fibers innervating the knee joint. After induction of the experimental knee joint inflammation with a kaolin/carrageenan mixture, comparison was made of the percentage of Glu positive axons in the MAN on the side of the inflammation versus the contralateral MAN using post-embedding immunogold electron microscopic methods. A doubling in the percentage of Glu-containing axons was observed on the side of the experimental arthritis as compared to the MAN of the other side or of uninjected controls. Glu positive axons were unmyelinated or were included in the small, thinly myelinated group in control nerves. Following induction of the inflammation, axonal diameter measurements revealed an increase in Glu content primarily in the small, thinly myelinated axons, which correspond to the group III afferent fibers. These increases were observed in the anesthetized preparation only when injection of kaolin/carrageenan was combined with joint flexion and mechanical stimulation. The dramatic increase in percentages of fibers stainable for Glu in the MAN following the induction of inflammation suggests that Glu content is greatly increased in the afferent fibers and may be a major contributor to the enhanced responses of sensory neurons in inflammatory states such as arthritis.     

Intrathecally injected neurotrophins and the release of substance P from the rat isolated spinal cord

The aim of this study was to investigate the effects of intrathecally delivered trophic molecules nerve growth factor (NGF), neurotrophin-3 (NT-3) and glial cell line-derived neurotrophic factor (GDNF) on substance P (SP) release and content in the rat spinal cord and SP content in sciatic nerve. SP release was assayed with an in vitro dorsal roots-attached spinal cord preparation, in which the roots were stimulated at A- or C-fibre strength, and SP levels were measured by radioimmunoassay (RIA). NGF but not NT-3 and GDNF treatment caused a significant increase in basal SP outflow; NGF, NT-3 but not GDNF increased C-fibre stimulation-evoked SP release, and capsaicin superfusion-induced SP release. The increase in C-fibre stimulation-evoked SP release over basal outflow was greater in NGF- than NT-3-treated cords, and nociceptive threshold testing showed that intrathecal NGF, but not NT-3 or GDNF treatment was associated with thermal hyperalgesia. There was no detectable A-fibre stimulation-induced SP release from any group as well as no change in SP content in the sciatic nerve and spinal cord. Systemic treatment with the NGF-sequestering fusion protein trkA-IgG significantly inhibited electrically or capsaicin-evoked SP release without affecting basal outflow and SP content in spinal cord and sciatic nerve. These results suggest that: (i) NGF tonically regulates the central synaptic function of SP-containing primary afferents; (ii) increased SP-release from the spinal cord is not necessarily associated with behavioural hyperalgesia as in NT-3-treated rats there was increased SP release but no detectable hyperalgesia; and (iii) because A-fibre stimulation failed to increase SP release in any group, these neurotrophins are unlikely to be responsible for the de novo upregulation of SP in large afferents seen after peripheral inflammation or nerve injury.     

PKC在角叉菜胶致炎引起的脊髓背角神经元敏感化中的作用

组织损伤或炎症引起的脊髓背角神经元兴奋性的变化同痛觉过敏、痛觉超敏、自发痛等病理过程密切相关。本实验大鼠足底注射角叉菜胶后 ,脊髓背角神经元发生敏感化 ,自发放电及对伤害性刺激的诱发反应明显增强。背角局部经微透析给予蛋白激酶 C(PKC)非特异性抑制剂氯丙嗪 (CPZ)或特异性抑制剂 H- 7后 ,自发及诱发反应均明显回降 ,提示 PKC激活参与中枢敏感化的形成与维持     

Induction of long‐term potentiation at spinal synapses by noxious stimulation or nerve injury

Use-dependent long-term potentiation of synaptic strength (LTP) is an intensively studied model for learning and memory in vertebrates. Induction of LTP critically depends on the stimulation parameters of presynaptic fibres with synchronous high-frequency bursts being most effective at many central synapses. It is, however, not known whether naturally occurring discharge patterns may induce LTP and whether LTP has any biological function in sensory systems. Here we have investigated the LTP of excitatory synaptic transmission between primary afferent C-fibres, many of which are nociceptors, and neurons in rat superficial spinal dorsal horn. LTP that lasted for 4–6 h could not only be induced by electrical stimulation of sural nerve but also by natural stimulation of heat-, mechano- or chemosensitive nociceptors in the skin or by acute nerve injury. Maintenance of LTP was not affected when afferent nerves were cut 1 h or 5 min after noxious skin stimulation, indicating that an ongoing afferent barrage is not required. Natural noxious stimuli induced LTP in animals which were spinalized but were ineffective in intact animals. Thus, induction of LTP is suppressed by tonically active supraspinal descending systems. We conclude that the natural non-synchronized discharge patterns that are evoked by noxious stimulation may induce LTP and that this new form of LTP may be an underlying mechanism of afferent induced hyperalgesia.     

Discharge characteristics of fine medial articular afferents at rest and during passive movements of inflamed knee joints

After induction of an experimental knee joint inflammation, the activity of single Group III and IV afferent units in the medial articular nerve of the cat was recorded at rest and during passive movements. the properties of these units were compared to those sampled from normal knee joints. The proportion of units displaying resting discharges was higher in the inflamed group. The frequency of discharges was also higher. The receptive fields were larger than those in the control units. Passive movements in the normal working range of the joint activated many more units in the inflamed joint than in the control sample. We conclude that joint inflammation sensitizes articular nociceptors to be active not only at rest, but also during normally innocuous joint movements.     

Contribution of the activation of satellite glia in sensory ganglia to pathological pain

Peripheral tissue injury/inflammation can alter the properties of somatic sensory pathways, resulting in behavioral hypersensitivity and pathological and/or chronic pain, including increased responses to pain caused by both noxious stimuli (hyperalgesia) and normally innocuous stimuli (allodynia). Although there are increasing reports that glia in the spinal cord contribute to the maintenance of pathological pain, recent evidence suggests that activation of satellite glia in sensory ganglia may also play an important role in the development of hyperalgesia and allodynia. There is evidence that non-synaptically released chemical mediators derived from both neurons and satellite glia may trigger chronic pain via autocrine and/or paracrine mechanisms and that augmented excitability of primary afferent neurons results in changes in central pain-signaling neurons (central sensitization). The focus of the present review is on the contribution of the activation of satellite glia in sensory ganglia to pathological pain. In addition, we discuss potential therapeutic targets in satellite glia–neuronal interactions for the prevention of pathological pain.     

Differential effects of morphine, dopamine and prolactin administered iontophoretically on arcuate-ventromedial hypothalamic neurons

After induction of an experimental knee joint inflammation, the activity of single Group III and IV afferent units in the medial articular nerve of the cat was recorded at rest and during passive movements. The properties of these units were compared to those sampled from normal knee joints. The proportion of units displaying resting discharges was higher in the inflamed group. The frequency of discharges was also higher. The receptive fields were larger than those in the control units. Passive movements in the normal working range of the joint activated many more units in the inflamed joint than in the control sample. We conclude that joint inflammation sensitizes articular nociceptors to be active not only at rest, but also during normally innocuous joint movements.     

TRPV1 antagonist, A-889425, inhibits mechanotransmission in a subclass of rat primary afferent neurons following peripheral inflammation

TRPV1 (transient receptor potential vanilloid family type 1) is a nonselective cation channel that is activated and/or sensitized by noxious heat, protons, and other endogenous molecules released following tissue injury. In addition, a role for TRPV1 in mechanotransmission is emerging. We have recently reported that a selective TRPV1 receptor antagonist, A-889425, reduces mechanical allodynia and spinal neuron responses to mechanical stimulation of complete Freund's adjuvant (CFA)-inflamed rat hind paws. The population of peripheral nerve fibers through which TRPV1 antagonists mediate their effect on mechanotransmission have not yet been described. The objective of this study was to characterize TRPV1-mediated modulation of mechanically evoked activity in sensory axons innervating rat hind paws. We used an in vitro skin-nerve preparation to record neural activity from single axons isolated from rat tibial nerve. Single fibers were classified by conduction velocity, mechanical threshold, and stimulus-response relationships. We used A-889425 to investigate uninjured and inflamed skin afferent neuron populations to evoked mechanical stimulation. Application of A-889425 had no effect on the mechanical responsiveness of Aδ and C-fiber units innervating uninjured skin. In contrast, A-889425 inhibited responses of slowly conducting Aδ fiber units to noxious mechanical stimulation in a population of axons innervating CFA-inflamed hind paws. These data support a role for TRPV1 in mechanotransmission following peripheral inflammation, and highlight the importance of a distinct subclass of primary afferent neurons in mediating this effect.     

Stimulation of PAR-2 excites and sensitizes rat cutaneous C-nociceptors to heat

Proteinase-activated receptor 2 (PAR-2) is expressed on many nociceptive neurons. Application of PAR-2 agonists has been shown to induce behavioral signs of hyperalgesia. We investigated effects of the rat PAR-2 agonist SLIGRL-NH2 in the isolated rat skin-saphenous nerve preparation. SLIGRL-NH2 (100 microM) excited 20% of all C-fiber nociceptors tested. In addition, C-fiber nociceptors were sensitized to heat after SLIGRL-NH2 application resulting in an increase in response magnitude and a decrease of heat threshold. The PAR-2-inactive control peptide LRGILS-NH2 had no effect. The mechanical sensitivity of C-fibers was not affected by SLIGRL-NH2. PAR-2-mediated excitation and sensitization of primary nociceptors may contribute to PAR-2-mediated hyperalgesia.     

Contribution of activated interleukin receptors in trigeminal ganglion neurons to hyperalgesia via satellite glial interleukin-1beta paracrine mechanism

The present study investigated whether under in vivo conditions, inflammation alters the excitability of nociceptive Adelta-trigeminal ganglion (TRG) neurons innervating the facial skin via a cytokine paracrine mechanism. We used extracellular electrophysiological recording with multibarrel-electrodes in this study, and complete Freund's adjuvant (CFA) was injected into the rat facial skin. The threshold for escape from mechanical stimulation applied to the whisker pad area in inflamed rats (2 days after CFA injection) was significantly lower than that in control rats. A total of 45 Adelta-nociceptive-TRG neurons responding to electrical stimulation of the whisker pad were recorded in pentobarbital-anesthetized rats. The number of Adelta-TRG neurons with spontaneous firings and their firing rate in inflamed rats were significantly larger than those in control rats. The firing rates of the Adelta-TRG neuronal spontaneous activity were current-dependently decreased by local iontophoretic application of an interleukin I receptor type I antagonist (IL-1ra) in inflamed rats, but not in controls, and current-dependently increased by iontophoretic application of interleukin 1beta (IL-1beta) in both control and inflamed rats. IL-1ra also inhibited Adelta-TRG neuron activity evoked by mechanical stimulation in the inflamed rats. The mechanical threshold of nociceptive-TRG neurons in inflamed rats was significantly lower than that in control rats, but was not significantly different between control and inflamed rats after application of an IL-1ra. These results suggested that inflammation modulates the excitability of nociceptive Adelta-TRG neurons innervating the facial skin via IL-1beta paracrine action within trigeminal ganglia. Such an IL-1beta release could be important in determining trigeminal inflammatory hyperalgesia.     

Lumbar 5 ventral root transection-induced upregulation of nerve growth factor in sensory neurons and their target tissues: a mechanism in neuropathic pain

We have previously demonstrated that profound and persistent neuropathic pain as displayed by mechanical and cold allodynia and thermal hyperalgesia can be produced by a lumbar 5 ventral root transection (L5 VRT) model in adult rats in which only the motor nerve fibers were injured without axotomy of sensory neurons. However, the underlying mechanisms remain to be determined. In this study, by examining its changes in expression and by inhibiting its functions using a neutralizing antibody, we have investigated whether nerve growth factor (NGF), a neurotrophic factor known to have a function in regulating nerve injury-induced pain, is involved in the development of neuropathic pain induced by L5 VRT. Motor nerve injury by L5 VRT resulted in a de novo expression of NGF mRNA in a subpopulation of small sensory neurons and pericellular satellite cells in ipsilateral L5 dorsal root ganglion. NGF protein expression was also increased by sensory neurons with various sizes and by keratinocytes in the target tissue ipsilateral skin. Systemic administration of NGF antiserum twice within 17 days markedly attenuated L5 VRT-induced mechanical allodynia but not the cold allodynia and thermal hyperalgesia. These findings suggest that NGF is an important pain mediator in the generation of mechanical sensitivity induced by L5 VRT.