Nerve growth factor contributes to the up-regulation of growth-associated protein 43 and preprotachykinin A messenger RNAs in primary sensory neurons following peripheral inflammation

Early-life cisplatin exposure induces neuroinflammation and chemotherapy-induced neuropathic pain

Chemotherapy-induced neuropathic pain (CINP) is a common adverse health-related comorbidity that manifests later in life in patients with paediatric cancer. Current analgesia is ineffective, aligning closely with our lack of understanding of CINP. The aim of this study was to investigate how cisplatin induces nerve growth factor (NGF)-mediated neuroinflammation and nociceptor sensitisation. In a rat model of cisplatin-induced survivorship pain, cisplatin induced a neuroinflammatory environment in the dorsal root ganglia (DRG), demonstrated by NGF-positive macrophages infiltrating into the DRG. Cisplatin-treated CD11b- and F4/80-positive macrophages expressed more NGF compared to those treated with vehicle control. Mouse primary DRG sensory neuronal cultures demonstrated enhanced NGF-dependent TRPV1-mediated nociceptor activity after cisplatin treatment. Increased nociceptor activity was also observed when cultured mouse DRG neurons were treated with conditioned medium from cisplatin-activated macrophages. Elevated nociceptor activity was inhibited in a dose-dependent manner by an NGF-neutralising antibody. Intraperitoneal administration of the NGF-neutralising antibody reduced cisplatin-induced mechanical hypersensitivity and aberrant nociceptor intraepidermal nerve fibre density. These findings identify that a monocyte- or macrophage-driven NGF-TrkA pathway is a novel analgesic target for adult survivors of childhood cancer.

Neurochemical mechanism of muscular pain: Insight from the study on delayed onset muscle soreness

We reviewed fundamental studies on muscular pain, encompassing the characteristics of primary afferent fibers and neurons, spinal and thalamic projections, several muscular pain models, and possible neurochemical mechanisms of muscle pain. Most parts of this review were based on data obtained from animal experiments, and some researches on humans were also introduced. We focused on delayed-onset muscle soreness (DOMS) induced by lengthening contractions (LC), suitable for studying myofascial pain syndromes. The muscular mechanical withdrawal threshold (MMWT) decreased 1-3 days after LC in rats. Changing the speed and range of stretching showed that muscle injury seldom occurred, except in extreme conditions, and that DOMS occurred in parameters without muscle damage. The B2 bradykinin receptor-nerve growth factor (NGF) route and COX-2-glial cell line-derived neurotrophic factor (GDNF) route were involved in the development of DOMS. The interactions between these routes occurred at two levels. A repeated-bout effect was observed in MMWT and NGF upregulation, and this study showed that adaptation possibly occurred before B2 bradykinin receptor activation. We have also briefly discussed the prevention and treatment of DOMS.

Neuropeptides and neurohormones in immune, inflammatory and cellular responses to ultraviolet radiation

Humans are exposed to varying amounts of ultraviolet radiation (UVR) through sunlight. UVR penetrates into human skin leading to release of neuropeptides, neurotransmitters and neuroendocrine hormones. These messengers released from local sensory nerves, keratinocytes, Langerhans cells (LCs), mast cells, melanocytes and endothelial cells (ECs) modulate local and systemic immune responses, mediate inflammation and promote differing cell biologic effects. In this review, we will focus on both animal and human studies that elucidate the roles of calcitonin gene-related peptide (CGRP), substance P (SP), nerve growth factor (NGF), nitric oxide and proopiomelanocortin (POMC) derivatives in mediating immune and inflammatory effects of exposure to UVR as well as other cell biologic effects of UVR exposure.

Local anaesthesia decreases nerve growth factor induced masseter hyperalgesia

The aim of this investigation was to evaluate the effects of local anaesthesia on nerve growth factor (NGF) induced masseter hyperalgesia. Healthy participants randomly received an injection into the right masseter muscle of either isotonic saline (IS) given as a single injection (n = 15) or an injection of NGF (n = 30) followed by a second injection of lidocaine (NGF + lidocaine; n = 15) or IS (NGF + IS; n = 15) in the same muscle 48 h later. Mechanical sensitivity scores of the right and left masseter, referred sensations and jaw pain intensity and jaw function were assessed at baseline, 48 h after the first injection, 5 min after the second injection and 72 h after the first injection. NGF caused significant jaw pain evoked by chewing at 48 and 72 h after the first injection when compared to the IS group, but without significant differences between the NGF + lidocaine and NGF + IS groups. However, the mechanical sensitivity of the right masseter 5 min after the second injection in the NGF + lidocaine group was significantly lower than the second injection in the NGF + IS and was similar to the IS group. There were no significant differences for the referred sensations. Local anaesthetics may provide relevant information regarding the contribution of peripheral mechanisms in the maintenance of persistent musculoskeletal pain.

Nerve reinnervation and itch behavior in a rat burn wound model

In this study, we investigated whether postburn itch in rats, after a full thickness burn, is correlated to the nervous reinnervation of the burn wound area. For this purpose, we determined scratching duration (expressed as second/hour) at 24 hours, 2, 4, 8, and 12 weeks postburn and combined this with immunohistochemistry for protein gene product 9.5 (PGP9.5) to identify all nerve fibers, calcitonin gene related peptide (CGRP) to identify peptidergic fibers, tyrosine hydroxylase (TH) for sympathetic fibers, and growth-associated protein 43 (GAP-43) for regrowing fibers. We found a modest, but highly significant, increase in scratching duration of all burn wound rats from 3 to 12 weeks postburn (maximally 63 ± 9.5 second/hour compared to sham 3.1 ± 1.4 second/hour at 9 weeks). At 24 hours postburn, all nerve fibers had disappeared from the burn area. Around 4 weeks postburn PGP 9.5- and CGRP-immunoreactive nerve fibers returned to control levels. TH- and GAP-43-IR nerve fibers, which we found to be almost completely colocalized, did not regrow. No correlation was found between scratching duration and nervous reinnervation of the skin. The present results suggest that in rat, like in human, burn wound healing will induce increased scratching, which is not correlated to the appearance of nervous reinnervation.

NGF and Its Receptors in the Regulation of Inflammatory Response

There is growing interest in the complex relationship between the nervous and immune systems and how its alteration can affect homeostasis and result in the development of inflammatory diseases. A key mediator in cross-talk between the two systems is nerve growth factor (NGF), which can influence both neuronal cell function and immune cell activity. The up-regulation of NGF described in inflamed tissues of many diseases can regulate innervation and neuronal activity of peripheral neurons, inducing the release of immune-active neuropeptides and neurotransmitters, but can also directly influence innate and adaptive immune responses. Expression of the NGF receptors tropomyosin receptor kinase A (TrkA) and p75 neurotrophin receptor (p75NTR) is dynamically regulated in immune cells, suggesting a varying requirement for NGF depending on their state of differentiation and functional activity. NGF has a variety of effects that can be either pro-inflammatory or anti-inflammatory. This apparent contradiction can be explained by considering NGF as part of an endogenous mechanism that, while activating immune responses, also activates pathways necessary to dampen the inflammatory response and limit tissue damage. Decreases in TrkA expression, such as that recently demonstrated in immune cells of arthritis patients, might prevent the activation by NGF of regulatory feed-back mechanisms, thus contributing to the development and maintenance of chronic inflammation.

[Nerve distribution and density in the canine hip joint capsule. Comparison of healthy and dysplastic hip joints]

OBJECTIVE

The hip-joint capsule is exposed to increased tension forces during canine hip dysplasia, resulting in inflammation of the capsular tissue. It has been postulated that inflammation is associated with an increased nerve-distribution density. Therefore, it could be supposed that the nerve-distribution density in the hip-joint capsule is higher in dogs with dysplastic hip compared to healthy dogs.

MATERIAL AND METHODS

In 16 Labrador Retriever dogs that had been euthanised due to unrelated reasons, the hip joints were classified as normoplastic (group 1, n = 18) or dysplastic (group 2, n = 14) based on radiography. Following staining of the capsular nerve fibres by the Sihler method, histological specimens of the hip-joint capsules were scanned. By subdividing each specimen into 10 quadrants numbered from dorsomedial (Q01) to craniodorsolateral (Q10), the ratio of black to white pixels was calculated digitally for each specimen and each quadrant by using a semiautomatic image analysis. Statistical analysis was performed using an independent t-test.

RESULTS

Comparison of the mean values of each quadrant showed a significantly higher (p < 0.03) nerve distribution density for the craniodorsolateral quadrant (Q10) in group 2 when compared to group 1. Mean nerve-distribution density for all quadrants combined was not significantly different between the two groups.

CONCLUSION

The increase in nerve-distribution density of the craniodorsal region of the hip-joint capsule in dogs with dysplastic hip could be the result of increased tension forces on this area following hip-joint dysplasia. The craniodorsal region of the hip-joint capsule is an important origin of pain and coxarthrosis in canine hip dysplasia.

CLINICAL RELEVANCE

The results provide the pathophysiological basis for the efficacy of hip-joint denervation. Denervation of the cranial region of the acetabular rim is essential to reduce capsular inflammation and joint-related pain in canine hip dysplasia.

Role of nerve growth factor in pain

Nerve growth factor (NGF) was first identified as a substance that is essential for the development of nociceptive primary neurons and later found to have a role in inflammatory hyperalgesia in adults. Involvement of NGF in conditions with no apparent inflammatory signs has also been demonstrated. In this review we look at the hyperalgesic effects of exogenously injected NGF into different tissues, both human and animal, with special emphasis on the time course of these effects. The roles of NGF in inflammatory and neuropathic conditions as well as cancer pain are then reviewed. The role of NGF in delayed onset muscle soreness is described in more detail than its other roles based on the authors' recent observations. Acute effects are considered to be peripherally mediated, and accordingly, sensitization of nociceptors by NGF to heat and mechanical stimulation has been reported. Changes in the conductive properties of axons have also been reported. The intracellular mechanisms so far proposed for heat sensitization are direct phosphorylation and membrane trafficking of TRPV1 by TrkA. Little investigation has been done on the mechanism of mechanical sensitization, and it is still unclear whether mechanisms similar to those for heat sensitization work in mechanical sensitization. Long-lasting sensitizing effects are mediated both by changed expression of neuropeptides and ion channels (Na channels, ASIC, TRPV1) in primary afferents and by spinal NMDA receptors. Therapeutic perspectives are briefly discussed at the end of the chapter.

Imaging neuroinflammation in vivo in a neuropathic pain rat model with near-infrared fluorescence and ¹⁹F magnetic resonance

Chronic neuropathic pain following surgery represents a serious worldwide health problem leading to life-long treatment and the possibility of significant disability. In this study, neuropathic pain was modeled using the chronic constriction injury (CCI). The CCI rats exhibit mechanical hypersensitivity (typical neuropathic pain symptom) to mechanical stimulation of the affected paw 11 days post surgery, at a time when sham surgery animals do not exhibit hypersensitivity. Following a similar time course, TRPV1 gene expression appears to rise with the hypersensitivity to mechanical stimulation. Recent studies have shown that immune cells play a role in the development of neuropathic pain. To further explore the relationship between neuropathic pain and immune cells, we hypothesize that the infiltration of immune cells into the affected sciatic nerve can be monitored in vivo by molecular imaging. To test this hypothesis, an intravenous injection of a novel perfluorocarbon (PFC) nanoemulsion, which is phagocytosed by inflammatory cells (e.g. monocytes and macrophages), was used in a rat CCI model. The nanoemulsion carries two distinct imaging agents, a near-infrared (NIR) lipophilic fluorescence reporter (DiR) and a ¹⁹F MRI (magnetic resonance imaging) tracer, PFC. We demonstrate that in live rats, NIR fluorescence is concentrated in the area of the affected sciatic nerve. Furthermore, the ¹⁹FF MRI signal was observed on the sciatic nerve. Histological examination of the CCI sciatic nerve reveals significant infiltration of CD68 positive macrophages. These results demonstrate that the infiltration of immune cells into the sciatic nerve can be visualized in live animals using these methods.

Loss of neurons in rostral ventromedial medulla that express neurokinin-1 receptors decreases the development of hyperalgesia

It is well known that neurons in the rostral ventromedial medulla (RVM) are involved in descending modulation of nociceptive transmission in the spinal cord. It has been shown that activation of neurokinin-1 receptors (NK-1Rs) in the RVM, which are presumably located on pain facilitating ON cells, produces hyperalgesia whereas blockade of NK-1Rs attenuates hyperalgesia. To obtain a better understanding of the functions of NK-1R expressing neurons in the RVM, we selectively ablated these neurons by injecting the stable analog of substance P (SP), Sar(9),Met(O2)(11)-Substance P, conjugated to the ribosomal toxin saporin (SSP-SAP) into the RVM. Rats received injections of SSP-SAP (1 μM) or an equal volume of 1 μM of saporin conjugated to artificial peptide (Blank-SAP). Stereological analysis of NK-1R- and NeuN-labeled neurons in the RVM was determined 21-24 days after treatment. Withdrawal responses to mechanical and heat stimuli applied to the plantar hindpaw were determined 5-28 days after treatment. Withdrawal responses were also determined before and after intraplantar injection of capsaicin (acute hyperalgesia) or complete Freund's adjuvant (CFA) (prolonged hyperalgesia). The proportion of NK-1R-labeled neurons in the RVM was 8.8 ± 1.3% in naïve rats and 8.1 ± 0.8% in rats treated with Blank-SAP. However, injection of SSP-SAP into the RVM resulted in a 90% decrease in NK-1R-labeled neurons. SSP-SAP did not alter withdrawal responses to mechanical or heat stimuli under normal conditions, and did not alter analgesia produced by morphine administered into the RVM. In contrast, the duration of nocifensive behaviors produced by capsaicin and mechanical and heat hyperalgesia produced by capsaicin and CFA were decreased in rats pretreated with SSP-SAP as compared to those that received Blank-SAP. These data support our earlier studies using NK-1R antagonists in the RVM and demonstrate that RVM neurons that possess the NK-1R do not play a significant role in modulating acute pain or morphine analgesia, but rather are involved in pain facilitation and the development and maintenance of hyperalgesia.

Neonatal repetitive needle pricking: plasticity of the spinal nociceptive circuit and extended postoperative pain in later life

Repetitive exposure of neonates to noxious events is inherent to their health status monitoring in neonatal intensive care units (NICU). Altered basal nociception in the absence of an injury in later life has been demonstrated in ex-NICU children, but the impact on pain hypersensitivity following an injury in later life is unknown. Also, underlying mechanisms for such long-term changes are relatively unknown. The objective of this study is to investigate acute and long-term effects of neonatal repetitive painful skin-breaking procedures on nociception and to investigate plasticity of the nociceptive circuit. The repetitive needle prick animal model was used in which neonatal rats received four needle pricks into the left hind paw per day during the first postnatal week and control animals received nonpainful tactile stimuli. Repetitive needle pricking during the first week of life induced acute hypersensitivity to mechanical stimuli. At the age of 8 weeks, increased duration of postoperative hypersensitivity to mechanical stimuli after ipsilateral hind paw incision was shown in needle prick animals. Basal nociception from 3 to 8 weeks of age was unaffected by neonatal repetitive needle pricking. Increased calcitonin gene-related peptide expression was observed in the ipsilateral and contralateral lumbar spinal cord but not in the hind paw of needle prick animals at the age of 8 weeks. Innervation of tactile Aβ-fibers in the spinal cord was not affected. Our results indicate both acute and long-term effects of repetitive neonatal skin breaking procedures on nociception and long-term plasticity of spinal but not peripheral innervation of nociceptive afferents.

Atopic keratinocytes induce increased neurite outgrowth in a coculture model of porcine dorsal root ganglia neurons and human skin cells

Skin of patients suffering from atopic eczema displays a higher epidermal nerve fiber density, associated with neurogenic inflammation and pruritus. Using an in vitro coculture system, allowing a spatially compartmented culture of somata from porcine dorsal root ganglion neurons and human primary skin cells, we investigated the influence of dermal fibroblasts and keratinocytes on neurite outgrowth. In comparison with dermal fibroblasts, keratinocytes induced more branched and less calcitonin gene-related peptide (CGRP)-immunoreactive nerve fibers. By adding neutralizing antibodies, we showed that nerve growth factor (NGF) and glial cell-line-derived neurotrophic factor (GDNF) are pivotal neurotrophic factors of skin cell-induced neurite outgrowth. Keratinocytes and dermal fibroblasts secreted different ratios of neurotrophic factors, influencing morphology and CGRP immunoreactivity of neurites. To investigate changes of the peripheral nervous system in the pathogenesis of atopic eczema in vitro, we analyzed neurite outgrowth mediated by atopic skin cells. Atopic keratinocytes produced elevated levels of NGF and mediated an increased outgrowth of CGRP-positive sensory fibers. Our results demonstrate the impact of dermal fibroblasts and keratinocytes on skin innervation and emphasize the role of keratinocytes as key players of hyperinnervation in atopic eczema.

The expression of tumor necrosis factor-α and CD68 in high-intensity zone of lumbar intervertebral disc on magnetic resonance image in the patients with low back pain

STUDY DESIGN

Several recent studies suggest a high-intensity zone (HIZ) within the posterior annulu of lumbar intervertebral disc seen on T2-weighted magnetic resonance (MR) images represents the inflammation in the annulus, an annular tear, and/or vascular granulation tissue. It has not be reported yet whether there is the expression of tumor necrosis factor (TNF-α) and inflammatory cells appear in HIZ of intervertebral disc on MR images.

OBJECTIVE

To study whether HIZ is a specific signal for the inflammatory reaction of painful intervertebral disc.

SUMMARY OF BACKGROUND DATA

The presence of HIZ signal within the posterior annulus seen on sagittal T2-weighted spin-echo magnetic imaging sequences has already been used in the diagnosis of painful annular tears in the last decade. There have been studies suggesting that the presence of HIZ reflects inflammation in the annulus, an annular tear, and/or vascular granulation tissue.

METHODS

Twenty-six consecutive patients with low back pain underwent MR images and discography (age range = 26-65 years; mean age = 47.5 years; 16 men, 10 women). In all the patients, HIZ appeared in the involved intervertebral discs on T2-weighted MR images. Lumbar discography was usually performed on each patient for the discs L3-L4, L4-L5, and L5-S1. All patients received posterior lumbar interbody fusion procedures. The intervertebral disc specimens contained HIZ were excised en bloc during posterior interbody fusion. The distribution of TNF-α and CD68 in the intervertebral disc specimens within HIZ on MR images from 26 consecutive patients with low back pain and in the intervertebral disc specimens from five fresh cadavers were observed.

RESULTS

The histologic study of the consecutive sagittal slices of the HIZ showed a lot of proliferated small round cells and fibroblasts. There were a lot of TNF-α positive cells and some CD68 positive cells in HIZ and the number of TNF-α and CD68 positive cells in HIZ was significantly higher than that in the annulus fibrosus around HIZ and in the control (P < 0.05).

CONCLUSION

The results of this study indicate that HIZ may be a specific signal for the inflammatory reaction of painful intervertebral disc.

Persistent inflammation alters the density and distribution of voltage-activated calcium channels in subpopulations of rat cutaneous DRG neurons

The impact of persistent inflammation on voltage-activated Ca(2+) channels in cutaneous DRG neurons from adult rats was assessed with whole cell patch clamp techniques, sqRT-PCR and Western blot analysis. Inflammation was induced with a subcutaneous injection of complete Freund's adjuvant (CFA). DiI was used to identify DRG neurons innervating the site of inflammation. Three days after CFA injection, high threshold Ca(2+) current (HVA) density was significantly reduced in small and medium, but not large diameter neurons, reflecting a decrease in N-, L- and P/Q-type currents. This decrease in HVA current was associated with an increase in mRNA encoding the α2δ1-subunit complex, but no detectable change in N-type subunit (Ca(V)2.2) mRNA. An increase in both α2δ1 and Ca(V)2.2 protein was detected in the central nerves arising from L4 and L5 ganglia ipsilateral to the site of inflammation. In current clamp experiments on small and medium diameter cutaneous DRG neurons from naïve rats, blocking ∼40% of HVA current with Cd(2+) (5μM), had opposite effects on subpopulations of cutaneous DRG neurons (increasing excitability and action potential duration in some and decreasing excitability in others). The alterations in the density and distribution of voltage-activated Ca(2+) channels in subpopulations of cutaneous DRG neurons that develop following CFA injection should contribute to changes in sensory transmission observed in the presence of inflammation.

Signaling pathways that mediate nerve growth factor-induced increase in expression and release of calcitonin gene-related peptide from sensory neurons

Nerve growth factor (NGF) can augment transmitter release in sensory neurons by acutely sensitizing sensory neurons and by increasing the expression of calcitonin gene-related peptide (CGRP) over time. The current study examined the intracellular signaling pathways that mediate these two temporally distinct effects of NGF to augment CGRP release from sensory neurons. Growing sensory neurons in 30 or 100 ng/mL of NGF for 7 days increases CGRP content and this increase augments the amount of CGRP that is released by high extracellular potassium. Overexpressing a dominant negative Ras, Ras(17N) or treatment with a farnesyltransferase inhibitor attenuates the NGF-induced increase in CGRP content. Conversely, overexpressing a constitutively active Ras augments the NGF-induced increase in content of CGRP. Inhibiting mitogen activated protein kinase (MEK) activity also blocks the ability of NGF to increase CGRP expression. In contrast to the ability of chronic NGF to increase peptide content, acute exposure of sensory neurons to 100 ng/mL NGF augments capsaicin-evoked release of CGRP without affecting the content of CGRP. This sensitizing action of NGF is not affected by inhibiting Ras, MEK, or PI3 kinases. In contrast, the NGF-induced increase in capsaicin-evoked release of CGRP is blocked by the protein kinase C (PKC) inhibitor, BIM and the Src family kinases inhibitor, PP2. These data demonstrate that different signaling pathways mediate the alterations in expression of CGRP by chronic NGF and the acute actions of the neurotrophin to augment capsaicin-evoked release of CGRP in the absence of a change in the content of the peptide.

Bradykinin and nerve growth factor play pivotal roles in muscular mechanical hyperalgesia after exercise (delayed-onset muscle soreness)

Unaccustomed strenuous exercise that includes lengthening contraction (LC) often causes delayed-onset muscle soreness (DOMS), a kind of muscular mechanical hyperalgesia. The substances that induce this phenomenon are largely unknown. Peculiarly, DOMS is not perceived during and shortly after exercise, but rather is first perceived after approximately 1 d. Using B(2) bradykinin receptor antagonist HOE 140, we show here that bradykinin released during exercise plays a pivotal role in triggering the process that leads to muscular mechanical hyperalgesia. HOE 140 completely suppressed the development of muscular mechanical hyperalgesia when injected before LC, but when injected 2 d after LC failed to reverse mechanical hyperalgesia that had already developed. B(1) antagonist was ineffective, regardless of the timing of its injection. Upregulation of nerve growth factor (NGF) mRNA and protein occurred in exercised muscle over a comparable time course (12 h to 2 d after LC) for muscle mechanical hyperalgesia. Antibodies to NGF injected intramuscularly 2 d after exercise reversed muscle mechanical hyperalgesia. HOE 140 inhibited the upregulation of NGF. In contrast, shortening contraction or stretching induced neither mechanical hyperalgesia nor NGF upregulation. Bradykinin together with shortening contraction, but not bradykinin alone, reproduced lasting mechanical hyperalgesia. We also showed that rat NGF sensitized thin-fiber afferents to mechanical stimulation in the periphery after 10-20 min. Thus, NGF upregulation through activation of B(2) bradykinin receptors is essential (though not satisfactory) to mechanical hyperalgesia after exercise. The present observations explain why DOMS occurs with a delay, and why lengthening contraction but not shortening contraction induces DOMS.

Contralateral paw sensitization following injection of endothelin-1: effects of local anesthetics differentiate peripheral and central processes

Subcutaneous injection of the peptide endothelin-1 (ET-1) into the rat's footpad is known to cause rapid, transient ipsilateral mechanical and thermal sensitization and nocifensive hind paw flinching. Here we report that local injection of ET-1 (2 nmoles) into one hind paw slowly sensitizes the contralateral paw to chemical and mechanical stimulation. There was a 1.5-2-fold increase in the hind paw flinching response, over that from the first injection, to a second injection of the same dose of ET-1 delivered 24 h later into the contralateral paw. A similar increase in the number of flinches during the second phase of the response to formalin also occurred in the contralateral paw 24 h after ET-1. The contralateral paw withdrawal threshold to von Frey hairs was lowered by approximately 55% at 24 h after ipsilateral ET-1 injection. ET-1 injected s.c. at a segmentally unrelated location, the nuchal midline, caused no sensitization of the paws, obviating a systemic route of action. Local anesthetic block of the ipsilateral sciatic nerve during the period of initial response to ipsilateral ET-1 prevented contralateral sensitization, indicating the importance of local afferent transmission, although ipsilateral desensitization was not changed. These findings suggest that peripheral ET-1 actions lead to central sensitization that alters responses to selected stimuli.

Role of neurotrophin signalling in the differentiation of neurons from dorsal root ganglia and sympathetic ganglia

Manipulation of neurotrophin (NT) signalling by administration or depletion of NTs, by transgenic overexpression or by deletion of genes coding for NTs and their receptors has demonstrated the importance of NT signalling for the survival and differentiation of neurons in sympathetic and dorsal root ganglia (DRG). Combination with mutation of the proapoptotic Bax gene allows the separation of survival and differentiation effects. These studies together with cell culture analysis suggest that NT signalling directly regulates the differentiation of neuron subpopulations and their integration into neural networks. The high-affinity NT receptors trkA, trkB and trkC are restricted to subpopulations of mature neurons, whereas their expression at early developmental stages largely overlaps. trkC is expressed throughout sympathetic ganglia and DRG early after ganglion formation but becomes restricted to small neuron subpopulations during embryogenesis when trkA is turned on. The temporal relationship between trkA and trkC expression is conserved between sympathetic ganglia and DRG. In DRG, NGF signalling is required not only for survival, but also for the differentiation of nociceptors. Expression of neuropeptides calcitonin gene-related peptide and substance P, which specify peptidergic nociceptors, depends on nerve growth factor (NGF) signalling. ret expression indicative of non-peptidergic nociceptors is also promoted by the NGF-signalling pathway. Regulation of TRP channels by NGF signalling might specify the temperature sensitivity of afferent neurons embryonically. The manipulation of NGF levels "tunes" heat sensitivity in nociceptors at postnatal and adult stages. Brain-derived neurotrophic factor signalling is required for subpopulations of DRG neurons that are not fully characterized; it affects mechanical sensitivity in slowly adapting, low-threshold mechanoreceptors and might involve the regulation of DEG/ENaC ion channels. NT3 signalling is required for the generation and survival of various DRG neuron classes, in particular proprioceptors. Its importance for peripheral projections and central connectivity of proprioceptors demonstrates the significance of NT signalling for integrating responsive neurons in neural networks. The molecular targets of NT3 signalling in proprioceptor differentiation remain to be characterized. In sympathetic ganglia, NGF signalling regulates dendritic development and axonal projections. Its role in the specification of other neuronal properties is less well analysed. In vitro analysis suggests the involvement of NT signalling in the choice between the noradrenergic and cholinergic transmitter phenotype, in the expression of various classes of ion channels and for target connectivity. In vivo analysis is required to show the degree to which NT signalling regulates these sympathetic neuron properties in developing embryos and postnatally.

The density of nociceptive SP- and CGRP-immunopositive nerve fibers in the dura mater lumbalis of rats is enhanced after laminectomy, even after application of autologous fat grafts

A considerable number of patients complain about pain after lumbar surgery. The spinal dura mater has been debated as a possible source of this pain. However, there is no information if laminectomy influences the nociceptive sensory innervation of the dura. Therefore, we quantitatively evaluated the density of SP- and CGRP-immunopositive nerve fibers in the dura mater lumbalis in an animal model of laminectomy. Twelve adult Lewis rats underwent laminectomy, in six of them the exposed dura was covered by an autologous fat graft. Further six animals without surgical treatment served as controls. Six weeks after surgery, the animals were perfused and the lumbar dura was processed immunohistochemically for the detection of CGRP- and SP-containing nerve fibers. In controls, the peptidergic nerve fibers were found predominantly in the ventral but rarely in the dorsal dura mater lumbalis. After laminectomy, the density of SP- and CGRP-immunopositive neurons significantly increased in ventral as well as in dorsal parts of the dura. Axonal spines could be observed in some cases at the site of laminectomy. The application of autologous fat grafts failed to inhibit the significant increase in the density of peptidergic afferents. Thus, we have provided the first evidence that laminectomies induce an increase in the density of putative nociceptive SP- and CGRP-immunopositive neurons in the lumbar dura mater ascribable to an axonal sprouting of fine nerve fibers. This effect was not prevented by using autologous fat grafts. It is conceivable that the neuronal outgrowth of nociceptive afferents is a cause of low back pain observed after lumbar surgery.

Reg-2 expression in dorsal root ganglion neurons after adjuvant-induced monoarthritis

Reg-2 is a secreted protein that is expressed de novo in motoneurons, sympathetic neurons, and dorsal root ganglion (DRG) neurons after nerve injury and which can act as a Schwann cell mitogen. We now show that Reg-2 is also upregulated by DRG neurons in inflammation with a very unusual expression pattern. In a rat model of monoarthritis, Reg-2 immunoreactivity was detected in DRG neurons at 1 day, peaked at 3 days (in 11.6% of DRG neurons), and was still present at 10 days (in 5%). Expression was almost exclusively in the population of DRG neurons that expresses the purinoceptor P2X(3) and binding sites for the lectin Griffonia simplicifolia IB4, and which is known to respond to glial cell line-derived neurotrophic factor (GDNF). Immunoreactivity was present in DRG cell bodies and central terminals in the dorsal horn of the spinal cord. In contrast, very little expression was seen in the nerve growth factor (NGF) responsive and substance P expressing population. However intrathecal delivery of GDNF did not induce Reg-2 expression, but leukemia inhibitory factor (LIF) had a dramatic effect, inducing Reg-2 immunoreactivity in 39% of DRG neurons and 62% of P2X(3) cells. Changes in inflammation have previously been observed predominantly in the neuropeptide expressing, NGF responsive, DRG neurons. Our results show that changes also take place in the IB4 population, possibly driven by members of the LIF family of neuropoietic cytokines. In addition, the presence of Reg-2 in central axon terminals implicates Reg-2 as a possible modulator of second order dorsal horn cells.

Pancreatic pain

Abdominal pain is an important clinical symptom in pancreatic diseases. There is increasing evidence that pain in chronic pancreatitis and pancreatic cancer is triggered by pancreatic neuropathy. Damage to intrapancreatic nerves seems to support the maintenance and exacerbation of neuropathic pain. In chronic pancreatitis, intrapancreatic nerves are invaded by immune cells. This observation led to the hypothesis that neuro-immune interactions play a role in the pathogenesis of chronic pancreatitis and the accompanying abdominal pain syndrome. Similarly, pancreatic cancer cells infiltrate the perineurium of local nerves, which may in part explain the severe pain experienced by the patients. Furthermore, perineural invasion extending into extrapancreatic nerves may preclude curative resection and thus often leads to local recurrence. In recent years, the involvement of a variety of neurotrophins and neuropeptides in the pathogenesis of pancreatic pain was discovered. This review summarises recent data on the mechanisms of neuropathy and pain generation in pancreatic disorders.

Spinal subarachnoid adrenal medullary transplants reduce hind paw swelling and peripheral nerve transport following formalin injection in rats

Previous studies have demonstrated that adrenal medullary chromaffin cells transplanted into the spinal subarachnoid space significantly reduced pain-related behavior following hind paw plantar formalin injection in rats. The data suggests a centrally mediated antinociceptive mechanism. The spinal transplants may have effects on sciatic nerve function as well. To address this, the current study examined the effects of spinal adrenal transplants on hind paw edema and the anterograde transport of substance P (SP) that occur following formalin injection. Robust formalin-evoked edema, as well as hind paw flinching, was observed in striated muscle control-transplanted rats, which were not observed in adrenal-transplanted rats. To visualize transport of SP, the sciatic nerve was ligated ipsilateral to formalin injection and the nerve was processed 48 h later for immunocytochemistry. A significant formalin-induced accumulation of SP immunoreactivity (IR) was observed proximal to the ligation in control-transplanted rats. In contrast, there was significantly less SP IR observed from nerve of adrenal-transplanted rats, suggesting a diminution of anterograde axoplasmic transport by adrenal transplants. The change in SP IR may have been due to an alteration of transport due to formalin injection, thus, transport was visualized by the accumulation of growth-associated protein 43 (GAP43) at the ligation site. Formalin injection did not significantly increase proximal accumulation of GAP43 IR, indicating that formalin does not increase anterograde transport. Surprisingly, however, adrenal transplants significantly diminished GAP43 IR accumulation compared to control-transplanted rats. These data demonstrate that spinal adrenal transplants can attenuate the formalin-evoked response by modulating primary afferent responses.

Plasticity of gene expression in injured human dorsal root ganglia revealed by GeneChip oligonucleotide microarrays

Root avulsion from the spinal cord occurs in brachial plexus lesions. It is the practice to repair such injuries by transferring an intact neighbouring nerve to the distal stump of the damaged nerve; avulsed dorsal root ganglia (DRG) are removed to enable nerve transfer. Such avulsed adult human cervical DRG ( [Formula: see text] ) obtained at surgery were compared to controls, for the first time, using GeneChip oligonucleotide arrays. We report 91 genes whose expression levels are clearly altered by the injury. This first study provides a global assessment of the molecular events or "gene switches" as a consequence of DRG injuries, as the tissues represent a wide range of surgical delay, from 1 to 100 days. A number of these genes are novel with respect to sensory ganglia, while others are known to be involved in neurotransmission, trophism, cytokine functions, signal transduction, myelination, transcription regulation, and apoptosis. Cluster analysis showed that genes involved in the same functional groups are largely positioned close to each other. This study represents an important step in identifying new genes and molecular mechanisms in human DRG, with potential therapeutic relevance for nerve repair and relief of chronic neuropathic pain.

Effects of high-affinity nerve growth factor receptor inhibitors on symptoms in the NC/Nga mouse atopic dermatitis model

BACKGROUND

Nerve growth factor (NGF) is an important substance in the skin, where it modulates nerve maintenance and repair. However, the direct link between NGF and pruritic diseases such as atopic dermatitis is not yet fully understood. Our previous study showed that NGF plays an important role in the pathogenesis of atopic dermatitis-like skin lesions in NC/Nga mice. NGF mediates its effects by binding to two classes of transmembrane receptors, a high-affinity receptor (tropomyosin-related kinase A, TrkA) and a low-affinity receptor (p75).

OBJECTIVES

To determine the significance of NGF receptors in the pathogenesis of atopic dermatitis, the effects of TrkA inhibitors AG879 and K252a on the symptoms of NC/Nga mice were evaluated.

METHODS

Male NC/Nga mice with severe skin lesions were used. AG879 or K252a was applied to the rostral part of the back of mice five times a week. The dermatitis score for the rostral back was assessed once a week. The scratching behaviour was measured using an apparatus, MicroAct (Neuroscience, Tokyo, Japan). Immunofluorescence examinations were made in the rostral back skin for nerve fibres, NGF and TrkA receptor.

RESULTS

Repeated applications of AG879 or K252a significantly improved the established dermatitis and scratching behaviour, and decreased nerve fibres in the epidermis. NGF was observed more weakly in keratinocytes, and a lower expression of TrkA was observed in stratum germinativum of the epidermis of mice treated with AG879 or K252a compared with those treated with vehicle.

CONCLUSIONS

We suggest that NGF plays an important role in the pathogenesis of atopic dermatitis-like skin lesions via the high-affinity NGF receptor. These findings provide a new potential therapeutic approach for the amelioration of symptoms of atopic dermatitis.

Neurotrophin-induced preprotachykinin-A gene promoter modulation in organotypic rat spinal cord culture

To study regulation of the preprotachykinin-A gene promoter, we utilised a biolistic gene transfer protocol to deliver a DNA construct that incorporates a portion of the preprotachykinin-A gene promoter and an enhanced green fluorescent protein reporter gene into neonatal rat spinal cord organotypic slices. The ability of the neurokinin-1 receptor agonist [Sar9,Met(O2)11]-substance P, nerve growth factor and brain derived neurotrophic factor to modulate positively preprotachykinin-A gene promoter construct activity, as indicated by de novo enhanced green fluorescent protein expression, was determined. Treatment of organotypic slices with [Sar9, Met(O2)11]-substance P (10 microm, P < 0.05), nerve growth factor (200 ng/mL, P < 0.001) or brain derived neurotrophic factor (200 ng/mL, P < 0.02) significantly increased the proportion of cytomegaloviral promoter-DsRed transfected cells (used to visualise total transfected cells) that co-expressed enhanced green fluorescent protein. The distribution of enhanced green fluorescent protein/DsRed-positive neurones across spinal laminae was broadly in line with the known distribution of spinal Trk and neurokinin-1 receptors. These data suggest a modulated activity of the preprotachykinin-A gene promoter in spinal neurones in vitro by substance P and/or neurotrophins. The functional consequences of such transcriptional changes within central peptidergic circuitry and their relevance to chronic pain are considered.

Cannabinoid mechanisms of pain suppression

A large body of literature indicates that cannabinoids suppress behavioral responses to acute and persistent noxious stimulation in animals. This review examines neuroanatomical, behavioral, and neurophysiological evidence supporting a role for cannabinoids in suppressing pain at spinal, supraspinal, and peripheral levels. Localization studies employing receptor binding and quantitative autoradiography, immunocytochemistry, and in situ hybridization are reviewed to examine the distribution of cannabinoid receptors at these levels and provide a neuroanatomical framework with which to understand the roles of endogenous cannabinoids in sensory processing. Pharmacological and transgenic approaches that have been used to study cannabinoid antinociceptive mechanisms are described. These studies provide insight into the functional roles of cannabinoid CB1 (CB1R) and CB2 (CB2R) receptor subtypes in cannabinoid antinociceptive mechanisms, as revealed in animal models of acute and persistent pain. The role of endocannabinoids and related fatty acid amides that are implicated in endogenous mechanisms for pain suppression are discussed. Human studies evaluating therapeutic potential of cannabinoid pharmacotherapies in experimental and clinical pain syndromes are evaluated. The potential of exploiting cannabinoid antinociceptive mechanisms in novel pharmacotherapies for pain is discussed.

Effects of anti-nerve growth factor antibody on symptoms in the NC/Nga mouse, an atopic dermatitis model

Nerve growth factor (NGF) is an important substance in the skin, where it can modulate nerve maintenance and repair. However, the direct link between NGF and pruritic disease such as atopic dermatitis is not yet fully understood. To determine whether NGF plays a major role in atopic dermatitis and in the development or maintenance of skin lesions, we performed a study using NC/Nga mice and compared mice with and without skin lesions. Our examinations of the NC/Nga mice sought to detect nerve fibers in the epidermis, measured serum and skin NGF content, and observed skin NGF by immunohistochemistry staining. We also examined the effects of anti-NGF antibody on dermatitis symptoms in NC/Nga mice. In these mice, nerve fibers were significantly increased in the epidermis of lesioned skin, and the NGF content of the serum and skin was significantly elevated. Anti-NGF antibodies significantly inhibited the development and proliferation of skin lesions and epidermal innervation and significantly inhibited any growth in scratching but did not ameliorate scratching already developed. Our findings suggest that NGF plays important roles in the pathogenesis of atopic dermatitis-like skin lesions and that inhibiting the physiological effects of NGF or suppressing increased NGF production may prevent or even moderate the symptoms of atopic dermatitis.

Activation of NK1 receptor of trigeminal root ganglion via substance P paracrine mechanism contributes to the mechanical allodynia in the temporomandibular joint inflammation in rats

The aim of this study was to investigate whether under in vivo conditions, temporomandibular joint (TMJ) inflammation alters the excitability of Abeta-trigeminal root ganglion (TRG) neuronal activity innervating the facial skin by using extracellular electrophysiological recording with multibarrel-electrodes. Complete Freund's adjuvant (CFA) was injected into the rat TMJ. Threshold for escape from mechanical stimulation applied to the whisker pad area in inflamed rats (2 days) was significantly lower than that in control rats. A total of 36 Abeta-TRG neurons responding to electrical stimulation of the whisker pad was recorded in pentobarbital-anesthetized rats. The number of Abeta-TRG neurons with spontaneous firings and their firing rate in TMJ inflamed rats were significantly larger than those in control rats. The firing rates of their spontaneous activity in the Abeta-TRG neurons were current-dependently decreased by local iontophoretic application of an NK1 receptor antagonist (L-703,606) in inflamed, but not non-inflamed rats. Their spontaneous activities were current-dependently increased by local iontophoretic application of substance P (SP) in control and inflamed rats. The mechanical response threshold of Abeta-TRG neurons in inflamed rats was significantly lower than that in control rats. The mechanical response threshold in inflamed rats after iontophoretic application of L-703,606 was not different from that in control rats. These results suggest that TMJ inflammation modulate the excitability of Abeta-TRG neurons innervating the facial skin via paracrine mechanism due to SP released from TRG neuronal cell body. Such a SP release may play an important role in determining the trigeminal inflammatory allodynia concerning the temporomandibular disorder.

A proximal E-box modulates NGF effects on rat PPT-A promoter activity in cultured dorsal root ganglia neurones

The rat preprotachykinin A (rtPPTA) promoter fragment spanning -865+92, relative to the major transcriptional start, has previously been demonstrated to be nerve growth factor (NGF) responsive in primary cultures of rat dorsal root ganglion (DRG) neurones [Harrison, P.T., Dalziel, R.G., Ditchfield, N.A., Quinn, J.P., 1999. Neuronal-specific and nerve growth factor-inducible expression directed by the preprotachykinin-A promoter delivered by an adeno-associated virus vector. Neuroscience 94, 997-1003]. In this communication, we demonstrate that an E box element at -60, in part, regulates the activity of this rtPPT-A promoter fragment in DRG neurones in response to NGF. Differential regulation of the promoter is observed in the presence or absence of NGF when the E Box site is present. Under basal conditions binding of proteins to this -60 element may antagonise promoter activity. Hence, in the absence of NGF, mutation of the -60 E box increased reporter gene expression. Further, comparison of levels of reporter gene expression supported by both WT and mutated promoter indicate that in the presence of NGF the -60 E box element also plays a role as an activator domain. This represents a novel mechanism for NGF regulation of rtPPT-A. Similarly, an important role for this signalling pathway was observed in neonate rat DRG neuronal cultures, which require NGF for their survival, namely mutation of the -60 element resulted in higher levels of reporter gene expression.

Study of innervation, sensory neuropeptides, and serotonin in murine contact allergic skin

Density of nerve fibers, axonal growth, calcitonin gene-related peptide (CGRP), and substance P, and serotonin immunoreactivity as well as concentration were all determined in a murine model of contact allergy. Female Balb/c mice were sensitized on the back with oxazolone and 6 days later challenged with the same antigen on the dorsal surface of the ears, while control mice received the vehicle only. Then, 24 hr postchallenge, one ear was processed for immunohistochemical staining, while the other was frozen and processed for gas chromatography-mass spectrometry or radioimmunoassay (RIA). Protein gene product 9.5 (PGP 9.5) positive nerve fibers showed a tendency to increase in inflamed ears versus control ears in epidermis as well as the dermis. Growth-associated protein-43 (GAP-43) positive fibers in the epidermis were increased (p < .01) in inflamed ears, compared with control ears, as was the case for the dermal fibers, indicating increased axonal growth. Total (epidermis and dermis) numbers of CGRP and substance P positive nerve fibers tended to increase in the inflamed skin in contrast to control skin. In contrast, RIA demonstrated a lower (p < .05) concentration of CGRP in the inflamed ears compared with controls and a tendency for substance P to decrease in concentration in eczematous ears versus controls. There was no difference in serotonin concentration, or in the number of serotonin positive mast cells, between the inflamed and control skin, whereas semiquantification of serotonin positive platelets showed an increase in the inflamed (+/+) compared with control ears (+). Our results indicate that 24 hr after being challenged with the antigen, at the peak of murine skin inflammation, axonal growth, sensory neuropeptides, as well as serotonin may be involved.

Temporomandibular Joint Inflammation Potentiates the Excitability of Trigeminal Root Ganglion Neurons Innervating the Facial Skin in Rats

The aim of this study was to test the hypothesis that temporomandibular joint (TMJ) inflammation alters the excitability of trigeminal root ganglion (TRG) neurons innervating the facial skin, by using behavioral, electrophysiological, molecular, and immunohistochemical approaches. Complete Freund’s adjuvant (CFA) was injected into the rat TMJ to produce inflammation. The threshold for escape from mechanical stimulation applied to the orofacial area in TMJ-inflamed rats was significantly lower than that in naïve rats. The TRG neurons innervating the inflamed TMJ were labeled by 2% Fluorogold (FG) injection into the TMJ. The number of FG-labeled substance P (SP)-immunoreactive neurons in the inflamed rats was significantly increased compared with that in the naïve rats. On the other hand, medium- and large-diameter TRG neurons (>30 μm) innervating the facial skin were labeled by FG injection into the facial skin. In the FG-labeled cutaneous TRG neurons, the occurrence of SP (100 nM) induced membrane depolarization in inflamed rats (medium: 73.3%, large : 85.7%) was larger than that in the naïve rats (medium: 29.4%, large : 0%). In addition, SP application significantly increased the firing rate evoked by depolarizing pulses in the neurons of inflamed rats compared with those of naïve rats. Quantitative single-cell RT-PCR analysis showed the increased expression of mRNA for the NK1 receptor in FG-labeled TRG neurons in inflamed rats compared with that in naïve rats. The numbers of SP and NK1 receptors/neurofilament 200 positive immunoreactive TRG neurons innervating the facial skin (FG-labeled) in the inflamed rats were significantly increased compared with those seen in naïve rats. These results suggest that TMJ inflammation can alter the excitability of medium- and large-diameter TRG neurons innervating the facial skin and that an increase in SP/NK1 receptors in their soma may contribute to the mechanism underlying the trigeminal inflammatory allodynia in the TMJ disorder.

Wound pain: the need for a more understanding approach

Wound pain is hard to measure, and its treatment has suffered from a lack of knowledge on the part of health-care professionals. This paper describes current approaches, the mechanisms of pain and suggests strategies for improved care.

Disc inflammation potentially promotes axonal regeneration of dorsal root ganglion neurons innervating lumbar intervertebral disc in rats

STUDY DESIGN

The expression of growth-associated protein 43 (GAP-43), a marker of axonal growth, in the dorsal root ganglion (DRG) neurons innervating the lumbar intervertebral disc was assessed using the retrograde tracing method and immunohistochemistry.

OBJECTIVES

To study whether disc inflammation affects GAP-43 expression in DRG neurons innervating the disc in rats.

SUMMARY AND BACKGROUND DATA

Persistent inflammation and nerve ingrowth into the inner layer of degenerated discs can be a cause of discogenic pain. Although the presence of GAP-43-expressing nerve fibers in painful discs has been reported, the expression of GAP-43 in DRG neurons innervating the disc has not been studied.

METHODS

Seven days after the application of Fluoro-Gold to the L5-L6 disc, 50 microL of saline (n = 10, control group) or complete Freund's adjuvant (n = 10, inflammatory group) was applied to the disc in rats. Ten days after the Fluoro-Gold application, T13-L5 DRGs were double-stained with GAP-43 and either calcitonin gene-related peptide or isolectin B4 (IB4).

RESULTS

The percentage of Fluoro-Gold-labeled neurons that were positive for GAP-43 was significantly higher in the inflammatory group (44%) than in the control group (24%, P < 0.001). In both groups, the majority of GAP-43-positive neurons were small and positive for calcitonin gene-related peptide but not IB4.

CONCLUSIONS

The present results suggest that disc inflammation potentially promotes axonal growth of DRG neurons innervating the disc. In light of the strong correlation between the expression of calcitonin gene-related peptide and nerve growth factor receptor, it is most likely that nerve growth factor-sensitive DRG neurons extend their axons following disc inflammation.

A model of organotypic rat spinal slice culture and biolistic transfection to elucidate factors that drive the preprotachykinin-A promoter

The tachykinin substance P (SP) is a neuropeptide that is expressed in some nociceptive primary sensory afferents and in discrete populations of spinal cord neurons. Expression of spinal SP and the preprotachykinin-A (PPT-A) gene that encodes SP exhibits plasticity in response to conditions such as peripheral inflammation but the mechanisms that regulate expression are poorly understood. We have developed a spinal cord organotypic culture system that is suitable for the analysis of PPT-A gene promoter activity following biolistic transfection of recombinant DNA constructs. Spinal cord organotypic slices showed good viability over a 7-day culture period. Immunostaining for phenotypic markers such as NeuN and beta-III tubulin demonstrated preservation of neurons and their structure, although there was evidence of axotomy-induced down-regulation of NeuN in certain neuronal populations. Neurokinin-1 receptor (NK-1R) immunostaining in laminae I and III was similar to that seen in acute slices. Biolistic transfection was used to introduce DNA constructs into neurons of these organotypic cultures. Following transfection with a construct in which expression of enhanced green fluorescent protein (EGFP) is controlled by the PPT-A promoter, we showed that induction of neuronal activity by administration of a forskolin analogue/high K(+) (10 microM/10 mM) for 24 h resulted in a fourfold increase in the number of EGFP-positive cells. Similarly, a twofold increase was obtained after treatment with the NK-1R-specific agonist [Sar(9),Met (O(2))(11)]-substance P (10 microM). These data demonstrate the usefulness of this model to study physiological and pharmacological factors relevant to nociceptive processing that can modulate PPT-A promoter activity.

Sensory neuropeptide mRNA up-regulation is bilateral in periodontitis in the rat: a possible neurogenic component to symmetrical periodontal disease

Periodontal disease is a common multifactorial chronic inflammatory disease in humans. In inflammatory conditions that are known to be associated with changes in nociception, such as arthritis, the neuronal expression of the proinflammatory neuropeptides, substance P and calcitonin gene-related peptide is altered. In this study the expression of these neuropeptides' mRNAs has been studied in an inflammatory model that shows no behavioural evidence of altered nociception. Periodontitis was induced in male rats by intragingival injection of lipopolysaccharide adjacent to the second right mandibular molar. The animals were killed at various times after lipopolysaccharide injection and right and left trigeminal ganglia and brain were processed for in situ hybridization for beta-preprotachykinin and alpha-calcitonin gene-related peptide mRNAs. Expression of both neuropeptide mRNAs was significantly increased only in small neurons in the mandibular division of the trigeminal ganglion ipsilateral to the LPS injection from 3 to 10 days postinjection. Neuropeptide mRNA expression was also significantly increased in the contralateral trigeminal ganglion at day 10. No significant changes in neuropeptide mRNA levels were seen in the maxillary and ophthalmic divisions of the trigeminal ganglia or in the trigeminal mesencephalic nucleus. The up-regulation of substance P and CGRP mRNAs in periodontal disease suggests that this is associated with the inflammatory process rather than nociception, as this disease does not appear to result in altered nociception in either rats or humans. The contralateral alteration in neuropeptide mRNA expression suggests a role for neurogenic mechanisms in the development of periodontal disease.

Nerve terminals extend into the temporomandibular joint of adjuvant arthritic rats

The innervation of the temporomandibular joint (TMJ) has attracted particular interest because of the close association with complex mandibular movement. Although the pathological changes of disk innervation may have a crucial role in the development of TMJ pain, the innervation of the TMJ disk by experimentally induced arthritis has rarely been examined in detail. Arthritic rats were induced by injection with 0.1ml solution of Complete Freund's adjuvant (CFA). We investigated three-dimensional distribution of nerve fibers in the TMJ disk using immunohistochemistry for protein gene product-9.5 (PGP-9.5) and calcitonin gene-related peptide (CGRP) in naive and arthritic rats. To clarify the possible role of nerve growth factor (NGF) and its receptor on changes in peripheral innervation of the TMJ, the expressions of trkA and p75 receptor in trigeminal ganglia were examined. Although PGP-9.5 and CGRP immunoreactive (ir) fibers were seen in the peripheral part of the TMJ disk, they were not seen in its central part. The total length and the length density of PGP-9.5 ir and CGRP ir nerve fibers increased in arthritic rats. The innervation area of fibers proliferating in the rostro-medial part merged with that of fibers in the rostro-lateral part in the arthritic rats. In addition, the ratio of trkA- and p75-positive small- and medium-sized cells increased in trigeminal ganglia. It is assumed that increasing innervation of the TMJ disk may be important for the pathophysiology of TMJ pain. NGF and its receptors are likely involved in pathological changes of the TMJ disk.

Neonatal inflammation and primary afferent terminal plasticity in the rat dorsal horn

Abnormal or excessive activity related to pain and injury in early life may alter normal synaptic development and lead to changes in somatosensory processing. The aim of the current study was to define the critical factors that determine long-term plasticity in spinal cord afferent terminals following neonatal inflammation. Hindpaw inflammation was produced in neonatal rat pups with 5 or 25 microl 2% carrageenan, and 5 or 25 microl complete Freund's adjuvant (CFA). All groups displayed a clear inflammatory response that recovered in 2 weeks in all but the 25 microl CFA group, who had persistent chronic inflammation confirmed by histological examination of the paw at 8 weeks. The 25 microl CFA group was also the only group that displayed a significant expansion of the sciatic and saphenous nerve terminal field in lamina II of the dorsal horn at 8 weeks, using wheat-germ agglutinin-horse radish peroxidase transganglionic labelling. This effect was not accompanied by changes in dorsal root ganglion (DRG) cell number, expression of activating transcription factor 3 (ATF3), or alterations in calcitonin gene related peptide (CGRP) or isolectin B4 binding; and was not mimicked by partial nerve damage. No long-term change in mechanical or thermal behavioural sensory thresholds was seen in any group. Lower dose CFA caused an acute, reversible expansion of terminal fields in lamina II in neonatal animals, while CFA did not produce this effect in adults. The duration and effect of neonatal inflammation is therefore dependent on the type and volume of inflammatory agent used. The expansion of afferent terminals in lamina II following neonatal CFA inflammation is maintained into adulthood if the inflammation is also maintained, as seen following 25 microl CFA. This effect is not seen in adult animals, emphasising the plasticity of the nervous system early in development.

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.

Topical and peripherally acting analgesics

Acute nociceptive, inflammatory, and neuropathic pain all depend to some degree on the peripheral activation of primary sensory afferent neurons. The localized peripheral administration of drugs, such as by topical application, can potentially optimize drug concentrations at the site of origin of the pain, while leading to lower systemic levels and fewer adverse systemic effects, fewer drug interactions, and no need to titrate doses into a therapeutic range compared with systemic administration. Primary sensory afferent neurons can be activated by a range of inflammatory mediators such as prostanoids, bradykinin, ATP, histamine, and serotonin, and inhibiting their actions represents a strategy for the development of analgesics. Peripheral nerve endings also express a variety of inhibitory neuroreceptors such as opioid, alpha-adrenergic, cholinergic, adenosine and cannabinoid receptors, and agonists for these receptors also represent viable targets for drug development. At present, topical and other forms of peripheral administration of nonsteroidal anti-inflammatory drugs, opioids, capsaicin, local anesthetics, and alpha-adrenoceptor agonists are being used in a variety of clinical states. There also are some clinical data on the use of topical antidepressants and glutamate receptor antagonists. There are preclinical data supporting the potential for development of local formulations of adenosine agonists, cannabinoid agonists, cholinergic ligands, cytokine antagonists, bradykinin antagonists, ATP antagonists, biogenic amine antagonists, neuropeptide antagonists, and agents that alter the availability of nerve growth factor. Given that activation of sensory neurons involves multiple mediators, combinations of agents targeting different mechanisms may be particularly useful. Topical analgesics represent a promising area for future drug development.

Influence of peripheral inflammation on growth-associated phosphoprotein (GAP-43) expression in dorsal root ganglia and on nerve recovery after crush injury

An experimental study was performed to investigate the influence of the inflammation in peripheral target tissue on growth-associated phosphoprotein (GAP-43) expression in dorsal root ganglia (DRG) and on recovery of crushed nerve. Fifty-four male Wistar rats were used for this study. The sciatic nerve was operatively crushed unilaterally with an aneurysm clip. Inflammation in peripheral target tissue was induced by injection of complete Freund's adjuvant (CFA) at 1 week before crush. In crushed or crushed with arthritis rats DRGs were examined in immunohistochemistry for GAP-43 and the sciatic nerves were observed in Epon embedded sections with toluidine blue stain. In addition, electrophysiological studies of the nerves were performed to evaluate the recovery of function. Immunohistochemical studies showed the ratio of GAP-43 immunopositive cells in crushed with arthritis rats was significantly lower than that in crushed rats at 1 week after crush (P<0.01). Electrophysiological studies at 4 weeks after crush showed functional nerve recovery in crushed with arthritis rats was significantly suppressed compared with that in crushed rats (P<0.01). Histological studies showed the mean diameter of the axons in crushed with arthritis rats was significantly smaller than that in crushed rats (P<0.01). All these findings indicate that inflammation in peripheral target tissue suppresses GAP-43 expression in DRG and eventually suppresses functional and morphological recovery of the crushed nerve.

Spinal and peripheral mechanisms of cannabinoid antinociception: behavioral, neurophysiological and neuroanatomical perspectives

A large body of literature indicates that cannabinoids suppress behavioral responses to acute and persistent noxious stimulation. This review examines behavioral, neurophysiological and neuroanatomical evidence supporting a role for cannabinoids in suppressing nociceptive transmission at spinal and peripheral levels. The development of subtype-selective competitive antagonists and high-affinity agonists provides the pharmacological tools required to study cannabinoid antinociceptive mechanisms. These studies provide insight into the functional roles of cannabinoid receptor subtypes, CB1 and CB2, in cannabinoid antinociceptive mechanisms as revealed in animal models of acute and persistent (somatic inflammatory, visceral inflammatory, neuropathic) pain. Localization studies employing receptor binding and quantitative autoradiography, immunocytochemistry and in situ hybridization are reviewed to examine the distribution of cannabinoid receptors at these levels and provide a neuroanatomical framework with which to understand the roles of endogenous cannabinoids in sensory processing.

Activation of p38 MAP kinase in the rat dorsal root ganglia and spinal cord following peripheral inflammation and nerve injury

The intrathecal administration of p38 MAP kinase (p38) inhibitor has been shown to reduce hyperalgesia. In the present study, we investigated the activation of p38 in the rat dorsal root ganglion (DRG) and spinal cord following peripheral tissue inflammation and nerve injury immunohistochemically. Peripheral inflammation and chronic constriction injury (CCI) of the sciatic nerve induced a significant increase in the percentage of phosphorylated (P-) p38-immunoreactive (IR) neurons, primarily small sized ones in bilateral DRGs. In contrast, following axotomy, a significant decrease in the percentage of IR neurons was observed in ipsilateral DRGs. In addition, a marked increase was observed in the number of P-p38-IR microglia in the ipsilateral laminae I-IV and IX of the spinal cord following peripheral inflammation, CCI or axotomy. These findings suggest that p38 may play an important role in hyperalgesia and the activation of the spinal microglia.

Treatment of the chronic inflammation in peripheral target tissue improves the crushed nerve recovery in the rat: histopathological assessment of the nerve recovery

An experimental study was performed to investigate the influence of subsidence of chronic inflammation in peripheral target tissue on the recovery of crushed nerve. Seventy-eight male Wistar rats weighing 300-370 g were used. The sciatic nerve was operatively crushed unilaterally with an aneurysm clip (250 gf) applied for 5 min. Chronic inflammation, localized to the ankle, was induced by intra-articular injection of complete Freund's adjuvant 1 week preoperatively. Prednisolone farnesylate (PNF-21) 1.4% gel was applied on the ankle as an anti-inflammatory agent for consecutive days after the operation. The animals were divided into five groups as follows: crush injury with ipsilateral arthritis (CIA); crush injury with ipsilateral arthritis and PNF-21 gel applied on the ipsilateral ankle (CIA + IPNF); crush injury with ipsilateral arthritis and PNF-21 gel applied on the contralateral ankle (CIA + CPNF); crush injury with contralateral arthritis (CCA); crush injury without arthritis (C). Specimens for histopathological examination were taken from the nerve at a site 5 mm distal to the crush lesion at 4 weeks postoperatively. The average axon diameter was significantly larger in the CIA + IPNF group than in the CIA group (p < 0.01). No significant difference was observed between the CIA + CPNF group and the CIA group. In conclusion, chronic inflammation in peripheral target tissue suppresses recovery of the crushed nerve, and subsidence of this chronic inflammation improves this suppression histopathologically.

p38 MAPK activation by NGF in primary sensory neurons after inflammation increases TRPV1 levels and maintains heat hyperalgesia

Peripheral inflammation induces p38 MAPK activation in the soma of C fiber nociceptors in the dorsal root ganglion (DRG) after 24 hr. Inflammation also increases protein, but not mRNA levels, of the heat-gated ion channel TRPV1 (VR1) in these cells, which is then transported to peripheral but not central C fiber terminals. Inhibiting p38 activation in the DRG reduces the increase in TRPV1 in the DRG and inflamed skin and diminishes inflammation-induced heat hypersensitivity without affecting inflammatory swelling or basal pain sensitivity. p38 activation in the DRG is secondary to peripheral production of NGF during inflammation and is required for NGF-induced increases in TRPV1. The activation of p38 in the DRG following retrograde NGF transport, by increasing TRPV1 levels in nociceptor peripheral terminals in a transcription-independent fashion, contributes to the maintenance of inflammatory heat hypersensitivity.

Influence of chronic inflammation in peripheral target tissue on recovery of crushed nerve injury

An experimental study was performed to investigate the influence of chronic inflammation in peripheral target tissue on recovery of the sciatic nerve after crush injury. Thirty-four male Wistar rats, weighing 300-370 g were used. The sciatic nerve was crushed unilaterally with an aneurysm clip (250 gf; holding force; 5 min). One week before the operation, chronic inflammation, localized in the tibiotarsal joint of one hind limb, was produced by the intraarticular injection of complete Freund's adjuvant. The animals were divided into five groups, as follows: CIA (crush injury with ipsilateral arthritis), CCA (crush injury with contralateral arthritis), C (crush injury without arthritis), A (sham operation and ipsilateral arthritis), and S (sham operation without arthritis). Specimens for histological examination were taken from the nerve at a site 5 mm distal to the crush injury 4 weeks postoperatively. Histological study showed that the diameters of the axons in group CIA were significantly smaller than those in group CCA and those in group C. No significant differences were observed between group CCA and group C. In conclusion, peripheral nerve recovery after crush injury was suppressed by chronic inflammation in peripheral target tissue.

Tachykinins as modulators of the micturition reflex in the central and peripheral nervous system

In the normal urinary bladder, tachykinins (TKs) are expressed in a population of bladder nociceptors that is sensitive to the excitatory and desensitizing effects of capsaicin (i.e., capsaicin-sensitive primary afferent neurons (CSPANs)). Several endobiotics or xenobiotics excite CSPANs and release TKs and other mediators at both the peripheral and spinal cord level. The peripheral release of TKs determines a set of responses (known as neurogenic inflammation) that includes vasodilatation, plasma protein extravasation, smooth muscle contraction and stimulation of afferent nerves. Following chronic inflammation, both immune cells and capsaicin-resistant sensory neurons can de novo express TKs: whether these pools of TKs are releasable and contribute to inflammatory processes is presently unsettled. At the spinal cord level, the release of TKs contributes in determining an altered pattern of vesicourethral reflexes in response to nociceptive stimulation of the bladder by conveying: (a) the afferent transmission to supraspinal sites, and (b) descending or sensory inputs to the sacral parasympathetic nucleus (SPN). Recent evidence also attribute a synergetic role of TKs in the supraspinal modulation of the sensory arm of the micturition reflex. The overall available information suggests that TK receptor antagonists may affect bladder motility/reflexes which occur during different pathological states, while having little influence on the normal motor bladder function.