Nerve growth factor regulates expression of neuropeptide genes in adult sensory neurons

Differential regulation of TRP channels in a rat model of neuropathic pain

Neuropathic pain is a chronic disease resulting from dysfunction of the nervous system often due to peripheral nerve injury. Hypersensitivity to sensory stimuli (mechanical, thermal or chemical) is a common source of pain in patients and ion channels involved in detecting these stimuli are possible candidates for inducing and/or maintaining the pain. Transient receptor potential (TRP) channels expressed on nociceptors respond to different sensory stimuli and a few of them have been studied previously in the models of neuropathic pain. Using real-time PCR for quantification of all known TRP channels we identified several TRP channels, which have not been associated with nociception or neuropathic pain before, to be expressed in the DRG and to be differentially regulated after spared nerve injury (SNI). Of all TRP channel members, TRPML3 showed the most dramatic change in animals exhibiting neuropathic pain behaviour compared to control animals. In situ hybridisation showed a widespread increase of expression in neurons of small, medium and large cell sizes, indicating expression in multiple subtypes. Co-localisation of TRPML3 with CGRP, NF200 and IB4 staining confirmed a broad subtype distribution. Expression studies during development showed that TRPML3 is an embryonic channel that is induced upon nerve injury in three different nerve injury models investigated. Thus, the current results link for the first time a re-expression of TRPML3 with the development of neuropathic pain conditions. In addition, decreased mRNA levels after SNI were seen for TRPM6, TRPM8, TRPV1, TRPA1, TRPC3, TRPC4 and TRPC5.

Atropine-enhanced, antigen challenge-induced airway hyperreactivity in guinea pigs is mediated by eosinophils and nerve growth factor

Although anticholinergic therapy inhibits bronchoconstriction in asthmatic patients and antigen-challenged animals, administration of atropine 1 h before antigen challenge significantly potentiates airway hyperreactivity and eosinophil activation measured 24 h later. This potentiation in airway hyperreactivity is related to increased eosinophil activation and is mediated at the level of the airway nerves. Since eosinophils produce nerve growth factor (NGF), which is known to play a role in antigen-induced airway hyperreactivity, we tested whether NGF mediates atropine-enhanced, antigen challenge-induced hyperreactivity. Antibody to NGF (Ab NGF) was administered to sensitized guinea pigs with and without atropine pretreatment (1 mg/kg iv) 1 h before challenge. At 24 h after challenge, animals were anesthetized, vagotomized, paralyzed, and ventilated. Electrical stimulation of both vagus nerves caused bronchoconstriction that was increased in challenged animals. Atropine pretreatment potentiated antigen challenge-induced hyperreactivity. Ab NGF did not affect eosinophils or inflammatory cells in any group, nor did it prevent hyperreactivity in challenged animals that were not pretreated with atropine. However, Ab NGF did prevent atropine-enhanced, antigen challenge-induced hyperreactivity and eosinophil activation (assessed by immunohistochemistry). This effect was specific to NGF, since animals given control IgG remained hyperreactive. These data suggest that anticholinergic therapy amplifies eosinophil interactions with airway nerves via NGF. Therefore, therapeutic strategies that target both eosinophil activation and NGF-mediated inflammatory processes in allergic asthma are likely to be beneficial.

Emerging peripheral receptor targets for deep-tissue craniofacial pain therapies

While effective therapies are available for some types of craniofacial pain, treatments for deep-tissue craniofacial pain such as temporomandibular disorders are less efficacious. Several ion channels and receptors which are prominent in craniofacial nociceptive mechanisms have been identified on trigeminal primary afferent neurons. Many of these receptors and channels exhibit unusual distributions compared with extracranial regions. For example, expression of the ATP receptor P2X(3) is strongly implicated in nociception and is more abundant on trigeminal primary afferent neurons than analogous extracranial neurons, making them potentially productive targets specifically for craniofacial pain therapies. The initial part of this review therefore focuses on P2X(3) as a potential therapeutic target to treat deep-tissue craniofacial pain. In the trigeminal ganglion, P2X(3) receptors are often co-expressed with the nociceptive neuropeptides CGRP and SP. Therefore, we discuss the role of CGRP and SP in deep-tissue craniofacial pain and suggest that neuropeptide antagonists, which have shown promise for the treatment of migraine, may have wider therapeutic potential, including the treatment of deep-tissue craniofacial pain. P2X(3), TRPV1, and ASIC3 are often co-expressed in trigeminal neurons, implying the formation of functional complexes that allow craniofacial nociceptive neurons to respond synergistically to altered ATP and pH in pain. Future therapeutics for craniofacial pain thus might be more efficacious if targeted at combinations of P2X(3), CGRP, TRPV1, and ASIC3.

Up-regulation of TNF-alpha in neurons of dorsal root ganglia and spinal cord during coronary artery occlusion in rats

Knowledge about the physiologic and pathophysiologic roles of tumor necrosis factor-alpha (TNF-alpha) in acute myocardial ischemia/infarction is still very limited. Evidence implies that TNF-alpha is involved in neural activity including nociception in peripheral and central nervous system. Current study was designed to examine the association of change in TNF-alpha and its mRNA in upper thoracic dorsal root ganglia and spinal cord (T1-T5) during acute myocardial ischemia/infarction induced by coronary artery occlusion (CAO) in rats. The experiment was performed using immunohistochemistry, enzyme immunoassay, in situ hybridization and real time reverse transcription-polymerase chain reaction techniques. At 0.5h, 1h, 3h and 6h of acute myocardial ischemia/infarction, TNF-alpha was mainly up-regulated in a sub-population of small and medium neurons and satellite cells in the dorsal root ganglia (DRG) and spinal neurons, mainly in laminae I, II and V, VI of the spinal dorsal horn of upper thoracic segments. The up-regulation of TNF-alpha mRNA was observed at 30min of CAO, which was statistically significant, compared with the control and the sham surgery groups (P<0.01). The TNF-alpha mRNA was located in the satellite cells and afferent neurons of the DRG and spinal neurons, located mainly in laminae II-VI. The findings indicate an association of up-regulation of TNF-alpha in DRG and spinal cord with acute myocardial ischemia/infarction, suggesting that TNF-alpha may be associated with the nociception initiated by acute myocardial ischemia/infarction, while the pathophysiological role needs to be studied.

Interleukin-1alpha regulates substance P expression and release in adult sensory neurons

Nerve injury frequently results in development of chronic, dysesthetic pain and allodynia (painful sensation in response to benign stimulation). Following nerve injury, spinal cord glia become activated and secrete a number of inflammatory cytokines, including interleukin-1 (IL-1), which exists as two genetically distinct proteins, IL-1alpha and IL-1beta. To investigate whether neuropeptide expression could be altered by exposure to these cytokines, dorsal root neurons from mature rats were grown in culture and substance P (SP) expression was analyzed. IL-1alpha and IL-1beta both increased neuronal content of SP. Interestingly, IL-1alpha was significantly more efficient than IL-1beta in inducing SP expression. Cultured neurons exposed to either cytokine secreted substantially more SP with capsaicin stimulation than did control cultures, supporting a physiologic role for these inflammatory cytokines after nerve injury. However, when IL-1beta was added in combination with IL-1alpha to cultured neurons, the amount of SP expressed was significantly lower than that induced by IL-1alpha alone. Evidence is presented that both cytokines alter SP expression via the IL-1 receptor, and that the signaling pathway involves nerve growth factor (NGF) expression and transcription. In summary, IL-1alpha was significantly more efficient than IL-1beta at up-regulating SP expression than IL-1beta. Taken together, these observations suggest an important role for IL-1alpha in the events following nerve injury.

Recent findings on the pathogenesis of bronchial asthma. Part I. Asthma as a neurohumoral disorder, a pathological vago-vagal axon reflex

The novel data on the pathogenesis of asthma are summarized in this three-part review. Its immunological background is well established but it is more than an immunological disorder. Multiple lines indicate that both peripheral and central neural mechanisms are also involved in the pathogenesis of asthma. In the present first part of the review asthma is described as vago-vagal axon reflex brought about by multiple positive feed-back mechanisms, receptor upregulation, wind-up, phenotypic switch and formation of a pathological conditioned reflex. In the coming second part the main dispositional (mostly hormonal) and external contributing factors are reviewed, while the third part deals with the role of inheritance, i.e., with gene alleles leading to enhanced production of mediators of asthma.

The role of neuro-immune cross-talk in the regulation of inflammation and remodelling in asthma

Despite recent advances in the development of anti-asthmatic medication, asthma continues to be a major health problem worldwide. The symptoms of asthmatic patients include wheezing, chest tightness, cough and shortness of breath, which, together with airway hyperresponiveness, previously have been attributed to a dysfunction of airway nerves. However, research in the last two decades identified Th2-sensitization and the subsequent allergic reaction to innocuous environmental antigens as a basic immunological mechanism leading to chronic airway inflammation. Recent evidence suggests that the development of allergic asthma is influenced by events and circumstances in early childhood and even in utero. Allergen, ozone or stress exposure, as well as RSV infection in early life could be able to induce irreversible changes in the developing epithelial-mesenchymal trophic unit of the airways. The co-existence of chronic inflammation and neural dysfunction have recently drawn attention to the involvement of interaction pathways between the nervous and the immune system in the airways. Intensive basic research has accumulated morphological as well as functional evidence for the interaction between nerves and immune cells. Neuropeptides and neurotrophins have come into focus of attention as the key mediators of neuro-immune interactions, which lead to the development of several pharmacological compounds specifically targeting these molecules. This review will integrate our current knowledge on the involvement of neuro-immune pathways in asthma on the cellular and molecular level. It will summarize the results of pharmacological studies addressing the potential of neuropeptides and neurotrophins as novel therapeutic targets in asthma.

Contribution of nerve growth factor to augmented TRPV1 responses of muscle sensory neurons by femoral artery occlusion

In rats, hindlimb muscle ischemia induced by femoral artery occlusion augments the sympathetic nervous response to stimulation of transient receptor potential vanilloid type 1 (TRPV1) by injection of capsaicin into the arterial blood supply of the hindlimb muscles. The enhanced sympathetic response is due to alterations in TRPV1 receptor expression and its responsiveness in sensory neurons. The underlying mechanism by which TRPV1 receptor responses are increased after muscle vascular insufficiency/ischemia is unclear. In this report we tested the hypothesis that muscle ischemia elevates nerve growth factor (NGF) levels in primary afferent neurons, thereby increasing TRPV1 responsiveness. Muscle vascular insufficiency induced by the femoral artery ligation significantly increased NGF in the dorsal root ganglion (DRG) compared with sham controls. Furthermore, when NGF was infused in the hindlimb muscles of healthy rats (72 h using an osmotic minipump), the magnitude of the DRG neuron response to capsaicin was augmented (5.4 +/- 0.54 nA with NGF infusion vs. 3.0 +/- 0.17 nA in control; P < 0.05). With the addition of NGF in the culture dish containing the DRG neurons, the magnitude of the DRG neuron response to capsaicin was greater (6.4 +/- 0.27 nA; P < 0.05 vs. control) than that seen in control (2.9 +/- 0.16 nA). Note that this NGF effect was seen in isolectin B(4)-negative DRG neurons, a group of thin fiber nerves that contain neuropeptides and depend on NGF for survival. These data suggest that NGF affects a selective subpopulation of the afferent neurons in mediating augmented TRPV1 responses after femoral artery occlusion.

Cardiovascular actions of neurotrophins

Neurotrophins were christened in consideration of their actions on the nervous system and, for a long time, they were the exclusive interest of neuroscientists. However, more recently, this family of proteins has been shown to possess essential cardiovascular functions. During cardiovascular development, neurotrophins and their receptors are essential factors in the formation of the heart and critical regulator of vascular development. Postnatally, neurotrophins control the survival of endothelial cells, vascular smooth muscle cells, and cardiomyocytes and regulate angiogenesis and vasculogenesis, by autocrine and paracrine mechanisms. Recent studies suggest the capacity of neurotrophins, via their tropomyosin-kinase receptors, to promote therapeutic neovascularization in animal models of hindlimb ischemia. Conversely, the neurotrophin low-affinity p75(NTR) receptor induces apoptosis of endothelial cells and vascular smooth muscle cells and impairs angiogenesis. Finally, nerve growth factor looks particularly promising in treating microvascular complications of diabetes or reducing cardiomyocyte apoptosis in the infarcted heart. These seminal discoveries have fuelled basic and translational research and thus opened a new field of investigation in cardiovascular medicine and therapeutics. Here, we review recent progress on the molecular signaling and roles played by neurotrophins in cardiovascular development, function, and pathology, and we discuss therapeutic potential of strategies based on neurotrophin manipulation.

The p75 receptor is associated with inflammatory thermal hypersensitivity

Inflammatory pain, characterized by a decrease in the nociceptive threshold, arises through the actions of inflammatory mediators, and one of the key molecules is nerve growth factor (NGF). Here we report that the administration of neutralizing antibody to the neurotrophin receptor p75 (p75(NTR)) blocks hyperalgesia, which develops with complete Freund's adjuvant (CFA)-induced inflammation or with an intraplantar injection of NGF. Although CFA injection results in the up-regulation of calcitonin gene-related peptide (CGRP) levels in the primary sensory neurons, blocking p75(NTR) abolishes this effect. We further demonstrate that pro-NGF is the predominant ligand of p75(NTR) in vivo. Plasmin treatment, which is intended to decompose pro-NGF, ameliorates CFA-induced hyperalgesia. In addition, an intraplantar injection of pro-NGF induces hyperalgesia. These data together suggest that pro-NGF, as well as mature NGF, binding to p75(NTR) plays an important role in inflammation-induced hyperalgesia. Interference in the binding may provide a therapeutic approach for the treatment of inflammatory pain.

Future treatment strategies for neuropathic pain

The prevalence of people suffering from chronic pain is extremely high and pain affects millions of people worldwide. As such, persistent pain represents a major health problem and an unmet clinical need. The reason for the high incidence of chronic pain patients is in a large part due to a paucity of effective pain control. An important reason for poor pain control is undoubtedly a deficit in our understanding of the underlying causes of chronic pain and as a consequence our arsenal of analgesic therapies is limited. However, there is considerable hope for the development of new classes of analgesic drugs by targeting novel processes contributing to clinically relevant pain. In this chapter we highlight a number of molecular species which are potential therapeutic targets for future neuropathic pain treatments. In particular, the roles of voltage-gated ion channels, neuroinflammation, protein kinases and neurotrophins are discussed in relation to the generation of neuropathic pain and how by targeting these molecules it may be possible to provide better pain control than is currently available.

Peripheral mechanisms of pain and analgesia

This review summarizes recent findings on peripheral mechanisms underlying the generation and inhibition of pain. The focus is on events occurring in peripheral injured tissues that lead to the sensitization and excitation of primary afferent neurons, and on the modulation of such mechanisms. Primary afferent neurons are of particular interest from a therapeutic perspective because they are the initial generator of noxious impulses traveling towards relay stations in the spinal cord and the brain. Thus, if one finds ways to inhibit the sensitization and/or excitation of peripheral sensory neurons, subsequent central events such as wind-up, sensitization and plasticity may be prevented. Most importantly, if agents are found that selectively modulate primary afferent function and do not cross the blood-brain-barrier, centrally mediated untoward side effects of conventional analgesics (e.g. opioids, anticonvulsants) may be avoided. This article begins with the peripheral actions of opioids, turns to a discussion of the effects of adrenergic co-adjuvants, and then moves on to a discussion of pro-inflammatory mechanisms focusing on TRP channels and nerve growth factor, their signaling pathways and arising therapeutic perspectives.

Airway nerves and dyspnea associated with inflammatory airway disease

The neurobiology of dyspnea is varied and complex, but there is little doubt that vagal nerves within the airways are capable of causing or modulating some dyspneic sensations, especially those associated with inflammatory airway diseases. A major contributor to the dyspnea associated with inflammatory airway disease is explained by airway narrowing and increases in the resistance to airflow. The autonomic (parasympathetic) airway nerves directly contribute to this by regulating bronchial smooth muscle tone and mucus secretion. In addition, a component of the information reaching the brainstem via airway mechanosensing and nociceptive afferent nerves likely contributes to the overall sensations of breathing. The airway narrowing can lead to activation of low threshold mechanosensitive stretch receptors, and vagal and spinal C-fibers as well as some rapidly adapting stretch receptor in the airways that are directly activated by various aspects of the inflammatory response. Inflammatory mediators can induce long lasting changes in afferent nerve activity by modulating the expression of key genes. The net effect of the increase in afferent traffic to the brainstem modulates synaptic efficacy at the second-order neurons via various mechanisms collectively referred to as central sensitization. Many studies have shown that stimuli that activate bronchopulmonary afferent nerves can lead to dyspnea in healthy subjects. A logical extension of the basic research on inflammation and sensory nerve function is that the role of vagal sensory nerve in causing or shaping dyspneic sensations will be exaggerated in those suffering from inflammatory airway disease.

Rodent sensory neuron culture and analysis

Sensory neurons have proven very useful for analysis of neuronal differentiation in vivo and in vitro. Their utility for in vitro work is based on the fact that sensory neurons are relatively easy to isolate in large numbers and are amenable to manipulations in culture. Lumbar ganglia are usually used because their location in the caudal nervous system means they are the least differentiated at any developmental stage, allowing the analysis of relatively undifferentiated cells. Rodent sensory ganglia from embryonic to adult stages can be dissected effectively and maintained in serum-free medium or in coculture with other cells or factors. This unit describes generation of embryonic rat lumbar dorsal root ganglia (DRG) cultures, which form an important model system for investigating the cellular and molecular mechanisms that regulate neuronal differentiation. Adult DRG can also be successfully cultured, with a few modifications of the general protocol.

Calcitonin gene-related peptide release from intact isolated dorsal root and trigeminal ganglia

Neuropeptides like calcitonin gene-related peptide (CGRP) and substance P are found in significant proportions of primary afferent neurons. Release of these neuropeptides as well as prostaglandin E(2) is an approved index for the activation of these primary afferents. Previous studies have used cultures of enzyme-treated and mechanically dissociated primary afferent neurons, fresh tissue slices or cubes. In the present study we demonstrate CGRP and prostaglandin E(2) release from intact isolated dorsal root and trigeminal ganglia. Stimulation with noxious heat, low pH, inflammatory mediators and high potassium concentration increased CGRP release. In conclusion, neuropeptide release from intact isolated ganglia is a reliable method to study the responsiveness of sensory neurons in situ in comparison with neuronal cell cultures.

Mechanisms in prostatitis/chronic pelvic pain syndrome

PURPOSE

We reviewed the current literature on mechanisms involved in the pathogenesis of prostatitis/chronic pelvic pain syndrome (CPPS).

MATERIALS AND METHODS

A literature review for the years 1966 to 2003 was performed using the MEDLINE database of the United States National Library of Medicine.

RESULTS

National Institutes of Health categories I and II prostatitis result from identifiable prostatic infections, whereas patients with category IV are asymptomatic. The majority of symptomatic cases are category III or chronic prostatitis (CP)/CPPS. The etiology of CP/CPPS is unknown. The traditional marker of inflammation, namely white blood cells in prostatic fluids, does not correlate with the predominant symptom of pelvic pain. An imbalance toward increased proinflammatory and decreased anti-inflammatory cytokines has been implicated and a few studies have shown some correlation of this with pelvic pain. The imbalance in some men may result from polymorphisms at the cytokine loci. An autoimmune process may be involved and experimental evidence indicates that this can be under hormonal influence. Recent findings include possible defects in the androgen receptor. The prostate may not even be the source of the symptoms. Pelvic pain also correlates with the neurotrophin nerve growth factor implicated in neurogenic inflammation and central sensitization. Finally, psychological stress may produce measurable biochemical changes and influence the other processes. The role of normal prostatic bacterial flora in inciting the inflammatory response has also been reconsidered.

CONCLUSIONS

The symptoms of CP/CPPS appear to result from an interplay between psychological factors and dysfunction in the immune, neurological and endocrine systems.

Revisiting the Koebner phenomenon: role of NGF and its receptor system in the pathogenesis of psoriasis

Nerve growth factor (NGF) influences the key pathological events of psoriasis: keratinocyte proliferation, angiogenesis, and T-cell activation. We have systematically examined the kinetics of NGF expression, keratinocyte proliferation, and migration of T lymphocytes in the epidermis in Koebner-induced developing psoriatic plaques. In skin traumatized by the tape-stripping method (n = 12), a marked up-regulation of NGF in Koebner-positive lesions (n = 7) was observed 24 hours after trauma. Synthesis of NGF reached its maximum level in the 2nd week. Furthermore, cultured keratinocytes from nonlesional skin of psoriasis patients produced 10 times higher levels of NGF compared with keratinocytes from healthy individuals. To substantiate the in vivo effect of NGF secreted by keratinocytes in psoriatic plaques, we studied psoriatic plaques and normal human skin in a SCID-human skin xenograft model. The transplanted psoriatic plaques demonstrated marked proliferation of NGF-R (p75)-positive nerve fibers compared with only a few nerves in the transplanted normal human skin. Our results demonstrate that 1) in a developing psoriatic lesion, up-regulation of NGF together with keratinocyte proliferation are early events and precede epidermotropism of T lymphocytes; 2) keratinocytes in patients with psoriasis are primed to produce elevated levels of NGF; and 3) NGF synthesized by these keratinocytes is functionally active.

Acute myocardial ischemia up-regulates nociceptin/orphanin FQ in dorsal root ganglion and spinal cord of rats

Nociceptin/orphanin FQ (N/OFQ) possesses modulatory effects on somatic noxious signals in spinal cord, while the potential role in visceral nociception remains elusive. We designed this study to investigate the hypothesis that cardiac nociceptive signals from acute ischemic myocardium to the spinal cord are transmitted or modulated by mechanisms including N/OFQ. We examined the changes of N/OFQ and its mRNA in the dorsal root ganglia and spinal cord of upper thoracic segments innervating the heart of rats. Thoracic epidural anesthesia was performed to confirm neural mechanism underlying the changes. We observed that selective coronary artery occlusion significantly up-regulated N/OFQ and ppN/OFQ mRNA in the dorsal root ganglia and spinal cord. Thoracic epidural anesthesia abolished the changes in the expression of N/OFQ and its mRNA. The observations indicate that cardiac noxious neural afferent drive is responsible for the up-regulation of N/OFQ in the primary afferent neurons and intrinsic spinal neurons.

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.

Nerve growth factor enhances neurokinin A-induced airway responses and exhaled nitric oxide via a histamine-dependent mechanism

The neurotrophin nerve growth factor (NGF) is elevated in serum and locally in the lung in asthmatics and has been suggested to evoke airway hyperresponsiveness. The aim of this study was to explore mechanisms behind NGF-evoked changes in airway responsiveness. We studied if NGF could evoke increased airway responsiveness to tachykinins, such as neurokinin A (NKA), in a similar way as for histamine and, if so, whether an NGF-evoked increase in NKA airway responsiveness could involve a histamine receptor-dependent mechanism. Contractile responses to cumulative doses of histamine or NKA were studied in guinea-pig tracheal rings in vitro in organ baths. Furthermore, insufflation pressure (IP), pulmonary resistance, lung compliance and exhaled NO (FeNO) were measured in vivo in anaesthetised guinea-pigs challenged with histamine or NKA. NGF pre-treatment in vitro increased the contractile response evoked by histamine, but not by NKA, in tracheal rings. NGF pre-treatment in vivo increased IP, pulmonary resistance and levels of FeNO, and further decreased lung compliance, upon histamine and NKA challenge. The NGF-evoked enhancement of IP, pulmonary resistance, lung compliance as well as FeNO in response to NKA was reversed by the histamine receptor antagonist mepyramine. We suggest that NGF can induce an increase in tachykinin-evoked airway responses and NO formation via a histamine receptor-dependent pathway. This points to an important role for the mast cell in neurotrophin-evoked airway hyperresponsiveness and changes in exhaled NO.

Neurotrophins modulate monocyte chemotaxis without affecting macrophage function

Neurotrophins nerve growth factor (NGF), brain-derived growth factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4 (NT-4) and their high-affinity tyrosine protein kinase receptor (Trk) family, TrkA, TrkB, TrkC, and low-affinity p75(NTR) receptor, are key molecules implicated in the development of the central nervous system. Increasing evidence suggests that they also have physiological and pathological roles outside the nervous system. In this study we examined the expression of neurotrophins and their receptors in human activated macrophages and to what extent neurotrophins themselves modulate macrophage activation, in a model of primary adult monocyte-derived macrophage. Our data indicate that macrophages express neurotrophin and neurotrophin receptor genes differentially, and respond to cell stimulation by specific inductions. Neurotrophins did not modify the antigen-presenting capacities of macrophages or their production of proinflammatory cytokines, but somehow skewed their activation phenotype. In contrast, NGF clearly increased CXCR-4 expression in macrophage and their chemotactic response to low CXCL-12 concentration. The differential effect of specific macrophage stimuli on neurotrophin expression, in particular NGF and NT-3, and the specific enhancement of CXCR-4 expression suggest that neurotrophins might participate in tissue-healing mechanisms that should be investigated further in vivo.

Regulatory effect of nerve growth factor on release of substance P in cultured dorsal root ganglion neurons of rat

OBJECTIVE

To investigate the regulatory effects of nerve growth factor (NGF) on basal and capsaicin-induced release of neuropeptide substance P (SP) in primary cultured embryonic rat dorsal root ganglion (DRG) neurons.

METHODS

DRGs were dissected from 15-day-old embryonic Wistar rats. DRG neurons were dissociated and cultured, and then exposed to different concentrations of NGF (10 ng/mL, 30 ng/mL, or 100 ng/mL) for 72 h. The neurons cultured in media without NGF served as control. RT-PCR were used for detecting the mRNAs of SP and vanilloid receptor 1 (VR1) in the DRG neurons. The SP basal and capsaicin (100 nmol/L)-induced release in the culture were measured by radioimmunoassay (RIA).

RESULTS

SP mRNA and VR1 mRNA expression increased in primary cultured DRG neurons in a dose-dependent manner of NGF. Both basal release and capsaicin-evoked release of SP increased in NGF-treated DRG neurons compared with in control group. The capsaicin-evoked release of SP also increased in a dose-dependent manner of NGF.

CONCLUSION

NGF may promote both basal release and capsaicin-evoked release of SP. NGF might increase the sensitivity of nociceptors by increasing the SP mRNA or VR1 mRNA.

Neuritin mediates nerve growth factor-induced axonal regeneration and is deficient in experimental diabetic neuropathy

OBJECTIVE

Axonal regeneration is defective in both experimental and clinical diabetic neuropathy, contributing to loss of axonal extremities and neuronal dysfunction. The mechanisms behind this failure are not fully understood; however, a deficit in neurotrophic support and signaling has been implicated.

RESEARCH DESIGN AND METHODS

We investigated the expression of neuritin (also known as candidate plasticity gene 15, cpg15) in the sensory nervous system of control rats and rats with streptozotocin (STZ)-induced diabetes using microarray PCR, Western blotting, and immunocytochemical analysis. The functional role of neuritin in sensory neurons in vitro was assessed using silencing RNA.

RESULTS

Neuritin was expressed by a population of small-diameter neurons in the dorsal root ganglia (DRG) and was anterogradely and retrogradely transported along the sciatic nerve in vivo. Nerve growth factor (NGF) treatment induced an increase in the transcription and translation of neuritin in sensory neurons in vitro. This increase was both time and dose dependent and occurred via mitogen-activated protein kinase or phosphatidylinositol-3 kinase activation. Inhibition of neuritin using silencing RNA abolished NGF-mediated neurite outgrowth, demonstrating the crucial role played by neuritin in mediating regeneration. Neuritin levels were reduced in both the DRG and sciatic nerve of rats with 12 weeks of STZ-induced diabetes, and these deficits were reversed in vivo by treatment with NGF.

CONCLUSIONS

Manipulation of neuritin levels in diabetes may therefore provide a potential target for therapeutic intervention in the management of neuropathy.

Substance P and its receptors in bone metabolism

Accumulating evidence on bone physiopathology has indicated that the skeleton contains numerous nerve fibers and its metabolism is regulated by the nervous system. Until now, more than 10 neuropeptides have been identified in bone. Substance P (SP) is a neuropeptide released from axons of sensory neurons, belongs to the tachykinin family and plays important roles in many physiological and pathological processes by acting as a neurotransmitter, neuromodulator, or trophic factor. It activates signal transduction cascades by acting on the neurokinin-1 receptor (NK(1)-R). Previous studies have confirmed that the SP-immunoreactive (IR) axons innervate bone and adjacent tissues, and that their density varies depending on the regions and physiological or pathological conditions. Over the past few decades, it has been found that SP takes part in the stimulation of bone resorption, and its receptors have been demonstrated to be located in osteoclasts. Notably, in studies of skeletal ontogeny, SP-IR axons have been shown to appear at an early stage, mostly coinciding with the sequence of long bone mineralization. These findings, together with data obtained from chemically or surgically targeted nerve deletions, strongly suggest that SP is a potent regulator of skeletal physiology. The specific distribution of SP-IR nerve fibers, the different amount of SP within regions, and the various levels of expression of NK(1)-R in targeted cells presumably related to and participate in bone metabolism. It can be predicted that the indirect roles of SP through other cytokines are as important as its direct roles in bone metabolism. This new regulating pathway of bone metabolism would have enormous implications in skeletal physiology and the relevant research might present curative potentials to a spectrum of bone diseases.

Activin acts with nerve growth factor to regulate calcitonin gene-related peptide mRNA in sensory neurons

Calcitonin gene-related peptide (CGRP) increases in sensory neurons after inflammation and plays an important role in abnormal pain responses, but how this neuropeptide is regulated is not well understood. Both activin A and nerve growth factor (NGF) increase in skin after inflammation and induce CGRP in neurons in vivo and in vitro. This study was designed to understand how neurons integrate these two signals to regulate the neuropeptide important for inflammatory pain. In adult dorsal root ganglion neurons, NGF but not activin alone produced a dose-dependent increase in CGRP mRNA. When added together with NGF, activin synergistically increased CGRP mRNA, indicating that sensory neurons combine these signals. Studies were then designed to learn if that combination occurred at a common receptor or shared intracellular signals. Studies with activin IB receptor or tyrosine receptor kinase A inhibitors suggested that each ligand required its cognate receptor to stimulate the neuropeptide. Further, activin did not augment NGF-initiated intracellular mitogen-activated protein kinase signals but instead stimulated Smad phosphorylation, suggesting these ligands initiated parallel signals in the cytoplasm. Activin synergy required several NGF intracellular signals to be present. Because activin did not further stimulate, but did require NGF intracellular signals, it appears that activin and NGF converge not in receptor or cytoplasmic signals, but in transcriptional mechanisms to regulate CGRP in rat sensory neurons after inflammation.

Neutralization of nerve growth factor induces plasticity of ATP-sensitive P2X3 receptors of nociceptive trigeminal ganglion neurons

The molecular mechanisms of migraine pain are incompletely understood, although migraine mediators such as NGF and calcitonin gene-related peptide (CGRP) are believed to play an algogenic role. Although NGF block is proposed as a novel analgesic approach, its consequences on nociceptive purinergic P2X receptors of trigeminal ganglion neurons remain unknown. We investigated whether neutralizing NGF might change the function of P2X3 receptors natively coexpressed with NGF receptors on cultured mouse trigeminal neurons. Treatment with an NGF antibody (24 h) decreased P2X3 receptor-mediated currents and Ca2+ transients, an effect opposite to exogenously applied NGF. Recovery from receptor desensitization was delayed by anti-NGF treatment without changing desensitization onset. NGF neutralization was associated with decreased threonine phosphorylation of P2X3 subunits, presumably accounting for their reduced responses and slower recovery. Anti-NGF treatment could also increase the residual current typical of heteromeric P2X2/3 receptors, consistent with enhanced membrane location of P2X2 subunits. This possibility was confirmed with cross-linking and immunoprecipitation studies. NGF neutralization also led to increased P2X2e splicing variant at mRNA and membrane protein levels. These data suggest that NGF controlled plasticity of P2X3 subunits and their membrane assembly with P2X2 subunits. Despite anti-NGF treatment, CGRP could still enhance P2X3 receptor activity, indicating separate NGF- or CGRP-mediated mechanisms to upregulate P2X3 receptors. In an in vivo model of mouse trigeminal pain, anti-NGF pretreatment suppressed responses evoked by P2X3 receptor activation. Our findings outline the important contribution by NGF signaling to nociception of trigeminal sensory neurons, which could be counteracted by anti-NGF pretreatment.

Neurotrophin activation of NFAT-dependent transcription contributes to the regulation of pro-nociceptive genes

Nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) play key roles in the development of inflammation-induced hyperalgesia by triggering the expression of pro-nociceptive genes within primary afferent and spinal neurons. However, the mechanisms by which neurotrophins elicit gene expression remain largely unknown. Recently, neurotrophins have been shown to activate members of the calcineurin (CaN)-regulated, nuclear factor of activated T-cells (NFATc) family of transcription factors within brain. Thus, we hypothesized that NFATc transcription factors couple neurotrophin signaling to gene expression within primary afferent and spinal neurons. In situ hybridization revealed NFATc4 mRNA within the dorsal root ganglion and spinal cord. In cultured dorsal root ganglion cells, NGF triggered NFAT-dependent transcription in a CaN-sensitive manner. Further, increased BDNF expression following NGF treatment relied on CaN, thereby suggesting that NGF regulates BDNF transcription via activation of NFATc4. Within cultured spinal cells, BDNF also activated CaN-dependent, NFAT-regulated gene expression. Interestingly, BDNF stimulation increased the expression of the pro-nociceptive genes cyclooxygenase-2, neurokinin-1 receptor, inositol trisphosphates receptor type 1, and BDNF itself, through both NFAT-dependent and NFAT-independent transcriptional mechanisms. Our results suggest that regulation of pro-nociceptive genes through activation of NFAT-dependent transcription is one mechanism by which NGF and BDNF signaling contributes to the development of persistent pain states.

Phosphorylation of extracellular signal-regulated kinases in urinary bladder in rats with cyclophosphamide-induced cystitis

Phosphorylated ERK expression has been demonstrated in the central and peripheral nervous system after various stimuli, including visceral stimulation. Changes in the activation (i.e., phosphorylation) of extracellular signal-regulated kinases (pERK) were examined in the urinary bladder after 4 h (acute), 48 h (intermediate), or chronic (10 day) cyclophosphamide (CYP) treatment. CYP-induced cystitis significantly ( P ≤ 0.01) increased pERK expression in the urinary bladder with intermediate (48 h) and chronic CYP treatment. Immunohistochemistry for pERK immunoreactivity revealed little pERK-IR in control or acute (4 h) CYP-treated rat urinary bladders. However, pERK expression was significantly ( P ≤ 0.01) upregulated in the urothelium after 48 h or chronic CYP treatment. Whole mount preparations of urothelium/lamina propria or detrusor smooth muscle from control (noninflamed) rats showed no pERK-IR in PGP9.5-labeled nerve fibers in the suburothelial plexus. However, with CYP-treatment (48 h, chronic), a few pERK-IR nerve fibers in the suburothelial plexus of whole mount preparations of bladder and at the serosal edge of urinary bladder sections were observed. pERK-IR cells expressing the CD86 antigen were also observed in urinary bladder from CYP-treated rats (48 h, chronic). Treatment with the upstream inhibitor of ERK phosphorylation, U0126, significantly ( P ≤ 0.01) increased bladder capacity in CYP-treated rats (48 h). These studies suggest that therapies targeted at pERK pathways may improve urinary bladder function in CYP-treated rats.

Antibodies to nerve growth factor reverse established tactile allodynia in rodent models of neuropathic pain without tolerance

A considerable body of evidence implicates endogenous nerve growth factor (NGF) in conditions in which pain is a prominent feature, including neuropathic pain. However, previous studies of NGF antagonism in animal models of neuropathic pain have examined only the prevention of hyperalgesia and allodynia after injury, whereas the more relevant issue is whether treatment can provide relief of established pain, particularly without tolerance. In the current work, we studied the effects of potent, neutralizing anti-NGF antibodies on the reversal of tactile allodynia and thermal hyperalgesia in established models of neuropathic and inflammatory pain in rats and mice. In the complete Freund's adjuvant-induced hind-paw inflammation, spinal nerve ligation and streptozotocin-induced neuropathic pain models, a single intraperitoneal injection of a polyclonal anti-NGF antibody reversed established tactile allodynia from approximately day 3 to day 7 after treatment. Effects on thermal hyperalgesia were variable with a significant effect observed only in the spinal nerve ligation model. In the mouse chronic constriction injury (CCI) model, a mouse monoclonal anti-NGF antibody reversed tactile allodynia when administered 2 weeks after surgery. Repeated administration of this antibody to CCI mice for 3 weeks produced a sustained reversal (days 4 to 21) of tactile allodynia that returned 5 days after the end of dosing. In conclusion, NGF seems to play a critical role in models of established neuropathic and inflammatory pain in both rats and mice, with no development of tolerance to antagonism. Antagonists of NGF, such as fully human monoclonal anti-NGF antibodies, may have therapeutic utility in analogous human pain conditions.

Airway hyper-responsiveness in allergic asthma in guinea-pigs is mediated by nerve growth factor via the induction of substance P: a potential role for trkA

BACKGROUND

The neurotrophin nerve growth factor (NGF) has been implicated as a mediator in allergic asthma. Direct evidence that inhibition of NGF-induced activation of neurotrophin receptors leads to improvement of airway symptoms is lacking. We therefore studied the effects of inhibitors of NGF signal transduction on the development of airway hyper-responsiveness (AHR) and pulmonary inflammation in a guinea-pig model for allergic asthma.

METHODS

Airway responsiveness to the contractile agonist histamine was measured in vivo in guinea-pigs that were sensitized and challenged with ovalbumin (OVA). Inflammatory cell influx and NGF levels were determined in bronchoalveolar lavage fluid (BALF). Substance P, a key mediator of inflammation, was measured in lung tissue by radioimmunoassay, while substance P immunoreactive neurons in nodose ganglia were measured by immunohistochemistry.

RESULTS

OVA challenge induced an AHR after 24 h in OVA-sensitized guinea-pigs. This coincided with an increase in the amount of NGF in BALF. Simultaneously, an increase in the percentage of substance P immunoreactive neurons in the nodose ganglia and an increase in the amount of substance P in lung tissue were found. We used tyrosine kinase inhibitors to block the signal transduction of the high-affinity NGF receptor, tyrosine kinase A (trkA). Treatment with the tyrosine kinase inhibitors (K252a or tyrphostin AG879) both inhibited the development of AHR, and prevented the increase in substance P in the nodose ganglia and lung tissue completely whereas both inhibitors had no effect on baseline airway resistance. Neither treatment with K252a or tyrphostin AG879 changed the influx of inflammatory cells in the BALF due to allergen challenge.

CONCLUSIONS

We conclude that substance P plays a role in the induction of AHR in our model for allergic asthma which is most likely mediated by NGF. As both tyrosine kinase inhibitors AG879 and K252a show a similar inhibitory effect on airway function after allergen challenge, although both tyrosine kinase inhibitors exhibit different non-specific inhibitory effects on targets other than trkA tyrosine kinases, it is likely that the induction of substance P derived from sensory nerves is mediated by NGF via its high-affinity receptor trkA.

The role of neuropeptides in psoriasis

The pathogenesis of psoriasis is incompletely understood but cutaneous neurogenic inflammation is probably involved. This involvement is suggested by a number of clinical and histological observations. Reports about the distribution of cutaneous nerves and the quantification of nerve growth factor and neuropeptides, including calcitonin gene-related peptide and vasoactive intestinal peptide, in lesional and nonlesional psoriatic skin suggest that sensory neuropeptides contribute to the development of psoriasis. This review summarizes what is known about the role of neurogenic markers in psoriasis.

Dynamic changes in nerve growth factor and substance P in the murine hair cycle induced by depilation

Increasing evidence suggests that various neurotrophins and neuropeptides play an important role in the progression of hair follicle cycling. Among them, nerve growth factor (NGF) and substance P (SP) have attracted special interest recently. However, the interaction between these factors during hair cycling has not yet been systematically studied. We therefore investigated the mutual relationships between NGF and SP and the mechanism by which the anagen stage of the hair cycle is initiated. Fluctuations in numbers of SP-positive nerve fibers and variations in amounts of SP, NGF, and another neurotrophic factor, glial cell-derived neurotrophic factor, in skin in the C57BL/6 mouse depilation-induced hair cycle model, together with the spatiotemporal expression patterns of each of these factors, were followed simultaneously by enzyme-linked immunosorbent assay and immunohistochemistry. The main finding was that a surge in NGF expression and a rapid increase in NGF content in skin is an initial event within 1 day after depilation, followed by elevation of SP content and numbers of SP-containing fibers 2 days after the increase in NGF. Our findings suggest that a rapid and abundant increase in NGF plays a key role in the induction and progression of anagen hair cycling through keratinocyte growth promotion. NGF may also induce plastic changes such as sprouting and hyperplasia in dermal nerve fibers and enhance their SP production. Elevated levels of SP in skin may additionally contribute to the progression of consecutive anagen hair cycles.

Adrenomedullin facilitates reinnervation of phenol-injured perivascular nerves in the rat mesenteric resistance artery

Our previous report showed that innervation of calcitonin gene-related peptide (CGRP)- and neuropeptide Y (NPY)-containing nerves in rat mesenteric resistance arteries was markedly reduced by topical application of phenol, and that nerve growth factor (NGF) facilitates the reinnervation of both nerves. We also demonstrated that a CGRP superfamily peptide, adrenomedullin, is distributed in perivascular nerves of rat mesenteric resistance arteries. In the present study, we investigated the influence of adrenomedullin on the reinnervation of mesenteric perivascular nerves following topical phenol treatment. Under pentobarbital-Na anesthesia, 8-week-old Wistar rats underwent in vivo topical application of phenol (10% phenol in 90% ethanol) to the superior mesenteric artery proximal to the bifurcation of the abdominal aorta. After the treatment, the animals were subjected to immunohistochemistry of the third branch of small arteries proximal to the intestine and to vascular responsiveness testing on day 7. Topical phenol treatment caused marked reduction of the density of NPY-like immunoreactive (LI)- and CGRP-LI nerve fibers in the arteries. Adrenomedullin (360 or 1000 ng/h) or NGF (250 ng/h), which was administered intraperitoneally for 7 days using an osmotic mini-pump immediately after topical phenol treatment, significantly increased the density of CGRP-LI- and NPY-LI nerve fibers compared with saline. Treatment with adrenomedullin (1000 ng/h) or NGF restored adrenergic nerve-mediated vasoconstriction and CGRP nerve-mediated vasodilation in the perfused mesenteric artery treated topically with phenol. These results suggest that adrenomedullin, like NGF, has a facilitatory effect on the reinnervation of perivascular nerves.

Coronary artery occlusion alters expression of substance P and its mRNA in spinal dorsal horn in rats

The painful sensation during acute myocardial ischemia or infarction is a common symptom and results from neural activity in humans. Little is known about the role of neuropeptides in this effect of myocardial ischemia. The aim of the current study was to investigate the role of substance P in mediating the noxious neural signals in spinal cord in acute myocardial ischemia by exploring the change in substance P and its mRNA in thoracic dorsal root ganglia and spinal dorsal horn (T1-T5) after coronary artery occlusion. The experiment was performed with immunohistochemistry, enzyme immunoassay and real time reverse transcription-polymerase chain reaction techniques on rats' hearts. In acute myocardial ischemia (<6 h), substance P and preprotachykinin mRNA were up-regulated in the neurons of the dorsal root ganglia and spinal dorsal horn. The increase in the density of immunoreactive material was mainly observed in small-diameter neurons of the dorsal root ganglia and the superficial laminae (I and II) of the spinal cord. The increase in the expressions was statistically significant compared with the control and the sham surgery groups (P<0.05). The results suggest that substance P is involved in the mediation of the noxious neural signals of acute myocardial ischemia in spinal cord. The pathophysiological role and significance need to be investigated.

Nerve growth factor secretion in cultured enteric glia cells is modulated by proinflammatory cytokines

The enteric nervous system is composed of neurones and glial cells. These enteric glia cells (EGC) appear to be essential for the maintenance of gut homeostasis and mucosal integrity. Neurotrophin nerve growth factor (NGF) also plays an important role for the gut integrity by regulating sensory and inflammatory processes in the intestines. Here, we demonstrate EGCs as one source of NGF and show increased levels of NGF mRNA/protein and tropomyosin receptor kinase A (TrkA) mRNA in cultured EGCs upon stimulation with proinflammatory cytokines and lipopolysaccharides. NGF is continuously secreted from cultured EGCs and proinflammatory cytokines and lipopolysaccharides stimulate the secretion of this neurotrophin in a time- and dose- dependent manner, whereas interleukin-4 had no effect on NGF expression. Furthermore, NGF secretion was sustained for more than 12 h after withdrawal of the proinflammatory cytokines, suggesting the involvement of transcriptional and/or translational processes. Thus, the release of proinflammatory cytokines can increase NGF secretion by EGCs and leads to a higher expression of TrkA in EGCs. NGF, in turn, can increase visceral sensitivity and, on the other hand, appears to improve gut inflammation. Therefore, NGF secreting EGCs may play a key role in modulating visceral sensitivity and might be involved in inflammatory processes of the gut.

Activation of sensory neurons contributes to reduce spinal cord injury in rats

We previously demonstrated that activation of sensory neurons increases endothelial prostaglandin I(2) (PGI(2)) production by releasing calcitonin gene-related peptide (CGRP). Since PGI(2) reduces post-traumatic spinal cord injury (SCI) by inhibiting tumor necrosis factor (TNF) production, activation of sensory neurons in the spinal cord tissue may ameliorate spinal cord injury. This study examines these possibilities using rat models of compression trauma-induced SCI. Both SB366791, a specific vanilloid receptor antagonist, and CGRP (8-37), a CGRP receptor antagonist, significantly inhibited trauma-induced increases in spinal cord tissue 6-keto-PGF(1alpha) levels. SB366791, CGRP (8-37) and indomethacin (IM) enhanced increases in spinal cord tissue TNF levels at 2h after trauma and exacerbated motor disturbances. Administration of CGRP significantly reduced motor disturbances and inhibited increases in spinal cord tissue TNF levels through enhancement of increases in tissue levels of 6-keto-PGF(1alpha). These observations strongly suggest that activation of sensory neurons might ameliorate compression trauma-induced SCI, inhibiting TNF production through enhancement of endothelial PGI(2) production. Thus, although the spinal cord sensory neurons function as nociceptive neurons, they could also be critically involved in the cytoprotective system that attenuates SCI development and, thus, pharmacological stimulation of spinal cord sensory neurons might contribute to reduce spinal cord injury.

Expression of neurotrophic factors in neonatal rats after peripheral inflammation

Neonatal peripheral inflammatory insult might result in the alteration of neuronal development in the nociceptive circuit. During early postnatal period, neurotrophins play important roles in neural development and sensory nerve innervation in the central and peripheral nervous systems. In this study, we investigated mRNA expression for neurotrophic factors and their receptors in the dorsal root ganglia of rat pups during postnatal life after peripheral inflammation induced by injection of complete Freund's adjuvant (CFA) into hind paw on postnatal day 1. Our results showed that mRNA expression levels of alpha-calcitonin gene-related peptides, tropomyosin-related kinase-A (trkA), p75 neurotrophin receptor (p75(NTR)), and brain-derived neurotrophic factor (BDNF) elevated significantly after CFA treatment. Such an increase began 1 day after CFA treatment and lasted 2 to 3 days for trkA, p75(NTR), and BDNF. In contrast, there was no change in mRNA expression levels for neurotrophin-4/5, beta-nerve growth factor (beta-NGF), trkB, glial cell line-derived neurotrophin factor, and receptor protein tyrosine kinase protein. Our study demonstrated that neonatal peripheral inflammatory insult might result in molecular changes of neurotrophic factors, particularly in NGF receptors and BDNF, in the process of neuronal development and plasticity in primary afferents during early neonatal period.

PERSPECTIVE

Neonatal peripheral inflammation model has been used for the exploration of neuropathic pain mechanism for years. This work provided further detailed information about possible neurotransmitters and peptides involved in this process. This might also lead to future clinical application.

The role of activin in neuropeptide induction and pain sensation

Signals from target tissues play critical roles in the functional differentiation of neuronal cells, and in their subsequent adaptations to peripheral changes in the adult. Sensory neurons in the dorsal root ganglia (DRG) provide an excellent model system for the study of signals that regulate the development of neuronal diversity. DRG have been well characterized and contain both neurons that convey information from muscles about limb position, as well as other neurons that provide sensations from skin about pain information. Sensory neurons involved in pain sensation can be distinguished physiologically and antigenically, and one hallmark characteristic is that these neurons contain neuropeptides important for their functions. The transforming growth factor (TGF) beta family member activin A has recently been implicated in neural development and response to injury. During sensory neuron development, peripheral target tissues containing activin or activin itself can regulate pain neuropeptide expression. Long after development has ceased, skin target tissues retain the capacity to signal neurons about changes or injury, to functionally refine synapses. This review focuses on the role of activin as a target-derived differentiative factor in neural development that has additional roles in response to cutaneous injuries in the adult.

Multiple role of histamine H1-receptor-PKC-MAPK signalling pathway in histamine-stimulated nerve growth factor synthesis and secretion

Histamine is a potent stimulator of nerve growth factor (NGF) production in the central nerve system and in the periphery as well. In this review, the biochemical mechanisms of histamine-stimulated NGF synthesis and secretion, and interactions between histamine, interleukin-1beta, and interleukin-6 are discussed. The main signalling pathway, involved in the stimulation of NGF production by histamine, includes activation of histamine H(1)-receptor, stimulation of Ca(2+)-dependent protein kinase C and mitogen-activated protein kinase. The same signalling pathway is involved in the interactions between histamine, interleukin-1beta, and interleukin-6, where NGF secretion is amplified. Whereas histamine and interleukin-1beta cause additive stimulatory effect on NGF secretion, interaction between histamine and interleukin-6 causes a long-term synergism. Thus, activation of histamine H(1)-receptor-protein kinase C-mitogen-activated protein kinase signalling pathway plays a crucial role not only in the direct stimulation of NGF secretion by histamine, but also in the indirect stimulation via different types of interactions between histamine, interleukin-1beta, and interleukin-6, which may have important therapeutic implications in modulation of NGF production.

Nerve growth factor, glial cell line-derived neurotrophic factor and neurturin prevent semaphorin 3A-mediated growth cone collapse in adult sensory neurons

Developmentally, semaphorin 3A (sema3A) is an important chemorepellent that guides centrally projecting axons of dorsal root ganglion (DRG) neurons. Sema3A-mediated growth cone collapse can be prevented by cyclic GMP (cGMP) and nerve growth factor (NGF) in embryonic neurons. Sema3A may also play a role in directing regrowth of injured axons in adults, and interactions with neurotrophic factors near the injury site may determine the extent and targeting of both regenerative and aberrant growth. The aim of this study was to determine whether NGF, glial cell line-derived neurotrophic factor (GDNF) and neurturin (NTN) modulate sema3A-mediated growth cone collapse in cultured adult rat DRG neurons. Sema3A caused a significant increase in growth cone collapse, which was completely prevented by prior treatment with NGF, GDNF or NTN. Immunocytochemical experiments showed that sema3A-sensitive neurons were heterogeneous in their expression of neurotrophic factor receptors and responses to neurotrophic factors, raising the possibility of novel, convergent signaling mechanisms between these substances. Increasing cGMP levels caused growth cone collapse, whereas sema3A-mediated collapse was prevented by inhibition of guanylate cyclase or by increasing cyclic AMP levels. In conclusion, sema3A signaling pathways in adult neurons differ to those described in embryonic neurons. Three different neurotrophic factors each completely prevent sema3A-mediated collapse, raising the possibility of novel converging signaling pathways. These studies also show that there is considerable potential for neurotrophic factors to regulate sema3A actions in the adult nervous system. This may provide insights into the mechanisms underling misdirected growth and targeting of sensory fibers within the spinal cord after injury, that is thought to contribute to development of autonomic dysreflexia and neuropathic pain.

Neurotrophin-3 attenuates galanin expression in the chronic constriction injury model of neuropathic pain

We have recently shown that exogenous neurotrophin-3 (NT-3) acts antagonistically to nerve growth factor (NGF) in regulation of nociceptor phenotype in intact neurons and suppresses thermal hyperalgesia and expression of molecules complicit in this behavioral response induced by chronic constriction injury (CCI) of the sciatic nerve. The present study examines whether there is a global influence of NT-3 in mitigating alterations in peptide and NGF receptor expression; molecules believed to also contribute to CCI-associated pain. Thus, the influence of NT-3 on phenotypic changes in dorsal root ganglion (DRG) neurons in rats coincident with CCI was examined using in situ hybridization. Seven days following injury, the incidence of expression of the neuropeptides galanin and pituitary adenylate cyclase-activating polypeptide (PACAP) was increased in L5 sensory neurons ipsilateral to the injury from 12% to 60% and 16% to 37% respectively, in addition to an increased level of expression. In contrast, there was no consistent significant change in tropomyosin-related kinase A (trkA) expression following CCI. Intrathecal infusion of NT-3 globally mitigated both the increased incidence and elevated levels of galanin messenger RNA (mRNA) expression observed following CCI, reducing the former from 60% to 39%. NT-3 infusion resulted in a limited reduction in the incidence and level of neuronal PACAP in medium to large size, but not small size, DRG neurons. NT-3 had no significant net effect on CCI-induced alterations in trkA mRNA expression.

Plasticity of peripheral mechanisms of cough

The cough reflex pathway is characterized by a remarkable plasticity often resulting in a persistent and uncontrollable urge to cough during airway inflammation. In many instances cough becomes up regulated to the extent that ceases to fulfill its defensive role in protecting the airways. The exact mechanisms underlying this plasticity are unknown and likely involves a variety of factors influencing the function of the peripheral and central nervous system. This review outlines the evidence of increased cough sensitivity during airway disease. This is followed by a discussion of the peripheral mechanisms involved including the potential role of inflammatory mediators, neutrophins and changes in the airway mucosal structure. A greater understanding of the mechanisms leading to enhanced cough should lead to the development of more effective therapeutic strategies.

The long-term exposure of rat cultured dorsal root ganglion cells to bradykinin induced the release of prostaglandin E2 by the activation of cyclooxygenase-2

The effects of long-term exposure of primary cultured rat dorsal root ganglion (DRG) cells to bradykinin (BK), compared to short-term exposure, were investigated to establish whether BK could induce prostaglandin E2 (PGE2) release from DRG cells. Short-term exposure (30 min) resulted in a small but significant amount of PGE2 release which was mainly inhibited by a selective COX-1 inhibitor, SC-560 but only partially by a selective COX-2 inhibitor, NS-398, and did not induce COX-2 protein as determined by Western blotting. In contrast, long-term exposure (3 h) induced a large amount of PGE2 release, which was completely abolished by indomethacin or NS-398. The level of COX-2 mRNA began to be detected by ribonuclease protection assay after 30 min of 100 nM BK exposure, maintained maximal expression for 1 h, and subsequently declined to the basal level. The level of COX-2 protein was expressed to follow the time course of COX-2 mRNA induction by BK in a delayed but similar kinetic manner. The expression of COX-2 induced by BK in DRG cells was inhibited by a BK B2 receptor antagonist, HOE140, but not a B1 receptor antagonist, Lys-des-Arg9, (Leu8)-BK. Thus, BK has been shown to induce COX-2 protein by B2 receptor, which may cause prostanoid generation in rat DRG cells, which may play an important role in the pathogenesis of inflammatory pain and hyperalgesia around the primary sensory neurons.

Post-traumatic application of brain-derived neurotrophic factor and glia-derived neurotrophic factor on the rat spinal cord enhances neuroprotection and improves motor function

We examined the potential efficacy of brain-derived neurotrophic factor (BDNF) and glial-derived neurotrophic factor (GDNF) applied over traumatized spinal cord, alone or in combination, for attenuating motor dysfunction, blood-spinal cord barrier (BSCB) breakdown, edema formation, and cell injury in a rat model. Under Equithesin anesthesia, spinal cord injury (SCI) was performed by making a unilateral incision into the right dorsal horn of the T10-11 segment. The rats were allowed to survive 5 hours after trauma. The BDNF or GDNF was applied (0.1 to 1 microg/10 microl in phosphate buffer saline) 30, 60, or 90 minutes after SCI. Topical application of BDNF or GDNF 30 minutes after SCI in high concentration (0.5 microg and 1 microg) significantly improved motor function and reduced BSCB breakdown, edema formation, and cell injury at 5 hours. These beneficial effects of neurotrophins were markedly absent when administered separately either 60 or 90 minutes after injury. However, combined application of BDNF and GDNF at 60 or 90 minutes after SCI resulted in a significant reduction in motor dysfunction and spinal cord pathology. These novel observations suggest that neurotrophins in combination have potential therapeutic value for the treatment of SCI in clinical situations.

Chronic nerve growth factor administration increases the peripheral exocytotic activity of capsaicin-sensitive cutaneous neurons

Nerve growth factor (NGF) plays an important role in inflammation and pain and has been suggested to regulate the responsiveness and sensitivity of nociceptive fibers. However, no study has evaluated whether chronic NGF alters the exocytotic capacity of peripheral terminals of peptidergic fibers. To test this hypothesis, rats were injected subcutaneously every other day with either murine recombinant NGF (mNGF; 1.0 mg/kg) or vehicle for 7 days; or mNGF (0.1 mg/kg), mNGF (1 mg/kg) or vehicle every other day for 13 days. Treatment of rats with NGF over a 13-day period produced a significant increase in capsaicin-evoked iCGRP release from isolated biopsies of hindpaw skin, as assessed by in vitro superfusion and RIA. This effect was dose-dependent and exhibited a temporal requirement, because the enhancement was only observed after 13 days of treatment and was not evident after 7 days of treatment. This NGF enhancement of capsaicin-evoked iCGRP release was not due solely to increases in peripheral iCGRP content since only the 1mg/kg dose of NGF elevated cutaneous pools of iCGRP, whereas both doses significantly increased capsaicin-evoked peptide release. Moreover, NGF also enhanced capsaicin-evoked thermal hyperalgesia under similar dose- and time-related conditions. Collectively, the chronic administration of NGF not only increases capsaicin-evoked hyperalgesia, but also significantly primes peripheral fibers to enhanced peptidergic exocytosis following activation of the capsaicin receptor. Collectively, these data are consistent with the hypothesis that persistently elevated NGF levels may contribute to enhanced neurogenic regulation of inflammatory and wound healing processes in injured tissue.