Endogenous nerve growth factor is required for regulation of the low affinity neurotrophin receptor (p75) in sympathetic but not sensory ganglia

Role of Nerve Growth Factor in Orofacial Pain

Some chronic pain conditions in the orofacial region are common and the mechanisms underlying orofacial pain are unresolved. Nerve growth factor (NGF) is a member of a family of neurotrophins and regulates the growth, maintenance and development of neurons. Increasing evidence suggests that NGF plays a crucial role in the generation of pain and hyperalgesia in different pain states. This review investigates the role of NGF in orofacial pain and their underlying cellular mechanisms, which may provide essential guidance to drug-discovery programmes. A systemic literature search was conducted in Pubmed focusing on NGF and orofacial pain. Articles were reviewed, and those discussing in vitro studies, animal evidence, clinical course, and possible mechanisms were summarized. We found a hyperalgesic effect of NGF in peripheral sensitization in orofacial pain models. We also summarize the current knowledge regarding NGF-dependent pain mechanism, which is initiated by retrograde transport of the ligand-receptor complex, ensuing transcriptional regulation of many important nociceptor genes involved in nociceptive processing. Phase III trials suggest that anti-NGF drug is endorsed with anti-inflammatory and pain-relieving effects with good tolerance in a variety of pain conditions, including pain associated with osteoarthritis and chronic lower back pain. Based on the data reviewed herein, NGF is believed to be an important hyperalgesic mediator in orofacial pain. The identification of underlying mechanisms and pathways of orofacial pain opens new frontiers for pain management.

Knockdown of Fidgetin Improves Regeneration of Injured Axons by a Microtubule-Based Mechanism

Fidgetin is a microtubule-severing protein that pares back the labile domains of microtubules in the axon. Experimental depletion of fidgetin results in elongation of the labile domains of microtubules and faster axonal growth. To test whether fidgetin knockdown assists axonal regeneration, we plated dissociated adult rat DRGs transduced using AAV5-shRNA-fidgetin on a laminin substrate with spots of aggrecan, a growth-inhibitory chondroitin sulfate proteoglycan. This cell culture assay mimics the glial scar formed after CNS injury. Aggrecan is more concentrated at the edge of the spot, such that axons growing from within the spot toward the edge encounter a concentration gradient that causes growth cones to become dystrophic and axons to retract or curve back on themselves. Fidgetin knockdown resulted in faster-growing axons on both laminin and aggrecan and enhanced crossing of axons from laminin onto aggrecan. Strikingly, axons from within the spot grew more avidly against the inhibitory aggrecan concentration gradient to cross onto laminin, without retracting or curving back. We also tested whether depleting fidgetin improves axonal regeneration in vivo after a dorsal root crush in adult female rats. Whereas control DRG neurons failed to extend axons across the dorsal root entry zone after injury, DRG neurons in which fidgetin was knocked down displayed enhanced regeneration of axons across the dorsal root entry zone into the spinal cord. Collectively, these results establish fidgetin as a novel therapeutic target to augment nerve regeneration and provide a workflow template by which microtubule-related targets can be compared in the future.SIGNIFICANCE STATEMENT Here we establish a workflow template from cell culture to animals in which microtubule-based treatments can be tested and compared with one another for their effectiveness in augmenting regeneration of injured axons relevant to spinal cord injury. The present work uses a viral transduction approach to knock down fidgetin from rat neurons, which coaxes nerve regeneration by elevating microtubule mass in their axons. Unlike previous strategies using microtubule-stabilizing drugs, fidgetin knockdown adds microtubule mass that is labile (rather than stable), thereby better recapitulating the growth status of a developing axon.

Peripheral sympathetic mechanisms in orofacial pain

Sympathetic nervous system (SNS) is a part of the autonomic nervous system which involuntarily regulates internal body functions. It appears to modulate the processing of nociceptive information. Many orofacial pain conditions involve inflammation of orofacial tissues and/or injury of nerve, some of which might be attributed to SNS. Thus, the aim of this review was to bring together the data available regarding the peripheral sympathetic mechanisms involved in orofacial pain. A clearer understanding of SNS–sensory interactions in orofacial pain may provide a basis for novel therapeutic strategies for conditions that respond poorly to conventional treatments.

Estrogen and female reproductive tract innervation: cellular and molecular mechanisms of autonomic neuroplasticity

The female reproductive tract undergoes remarkable functional and structural changes associated with cycling, conception and pregnancy, and it is likely advantageous to both individual and species to alter relationships between reproductive tissues and innervation. For several decades, it has been appreciated that the mammalian uterus undergoes massive sympathetic axon depletion in late pregnancy, possibly representing an adaptation to promote smooth muscle quiescence and sustained blood flow. Innervation to other structures such as cervix and vagina also undergo pregnancy-related changes in innervation that may facilitate parturition. These tissues provide highly tractable models for examining cellular and molecular mechanisms underlying peripheral nervous system plasticity. Studies show that estrogen elicits rapid degeneration of sympathetic terminal axons in myometrium, which regenerate under low-estrogen conditions. Degeneration is mediated by the target tissue: under estrogen's influence, the myometrium produces proteins repulsive to sympathetic axons including BDNF, neurotrimin, semaphorins, and pro-NGF, and extracellular matrix components are remodeled. Interestingly, nerve depletion does not involve diminished levels of classical sympathetic neurotrophins that promote axon growth. Estrogen also affects sympathetic neuron neurotrophin receptor expression in ways that appear to favor pro-degenerative effects of the target tissue. In contrast to the uterus, estrogen depletes vaginal autonomic and nociceptive axons, with the latter driven in part by estrogen-induced suppression of BMP4 synthesis. These findings illustrate that hormonally mediated physiological plasticity is a highly complex phenomenon involving multiple, predominantly repulsive target-derived factors acting in concert to achieve rapid and selective reductions in innervation.

Extracellular signals regulating sympathetic neuron survival and target innervation during development

The comparative ease with which paravertebral sympathetic neurons are studied in vitro and in vivo at stages throughout their development has facilitated major advances in our understanding of several key aspects of neuronal development. Detailed anatomical descriptions of the in vivo development of these neurons, studies of the effects of various extracellular signalling molecules on these neurons in vitro and analysis of the sympathetic phenotype of relevant transgenic mice have provided an in-depth understanding of how different extracellular signals orchestrate sequential steps in the establishment and refinement of sympathetic innervation. In this review, I will document the roles of neurotrophic factors, cytokines and other extracellular signals in regulating sympathetic neuron survival and target innervation at sequential stages of development.

Activating transcription factor 3 induction in sympathetic neurons after axotomy: response to decreased neurotrophin availability

Activating transcription factor 3 (ATF3) is induced in a high proportion of axotomized sensory and motor neurons after sciatic nerve transection. In the present study, we looked at the expression of this factor in the superior cervical ganglion (SCG) after axotomy and after other manipulations that induce certain aspects of the cell body response to axotomy. Sympathetic ganglia from intact rats and mice exhibit only a very occasional neuronal nucleus with activating transcription factor 3-like immunoreactivity (ATF3-IR); however, as early as 6 h and as late as 3 weeks postaxotomy, many of the neurons showed intense ATF3-IR. A second population of cells had smaller and generally less intensely stained nuclei, and at least some of these cells were satellite cells. Lesions distal to the SCG induced by administration of 6-hydroxydopamine or unilateral removal of the salivary glands produced increases in ATF3-IR similar to those seen after proximal axotomy, indicating that this response is not strictly dependent on the distance of the lesion from the cell body. Two proposed signals for triggering ATF3 expression were examined: reduction in nerve growth factor (NGF) availability and induction of the cytokine leukemia inhibitory factor (LIF). While administration of an antiserum raised against NGF to intact animals induced ATF3-IR, induction of ATF3-IR after axotomy was not reduced in LIF null mutant mice. Since axotomy, 6-hydroxydopamine, and sialectomy are known to decrease the concentration of NGF in the SCG, our data suggest that these decreases in NGF lead to increases in ATF3-IR. Furthermore, since the number of neurons in the SCG expressing ATF3-IR was greater after axotomy than after antiserum against NGF treatment, this raises the possibility that decreased NGF is not the only process regulating ATF3 expression after axotomy.

Reduction in nerve growth factor availability leads to a conditioning lesion-like effect in sympathetic neurons

Axotomized peripheral neurons are capable of regeneration, and the rate of regeneration can be enhanced by a conditioning lesion (i.e., a lesion prior to the lesion after which neurite outgrowth is measured). A possible signal that could trigger the conditioning lesion effect is the reduction in availability of a target-derived factor resulting from the disconnection of a neuron from its target tissue. We tested this hypothesis with respect to nerve growth factor (NGF) and sympathetic neurons by administering an antiserum to NGF to adult mice for 7 days prior to explantation or dissociation of the superior cervical ganglion (SCG) and subsequently measuring neurite outgrowth. The antiserum treatment dramatically lowered the concentration of NGF in the SCG and increased the rate of neurite outgrowth in both explants and cell cultures. The increase in neurite outgrowth was similar in magnitude to that seen after a conditioning lesion. To determine if exogenous NGF could block the effect of a conditioning lesion, mice were injected with NGF or cytochrome C immediately prior to unilateral axotomy of the SCG, and for 7 days thereafter. A conditioning lesion effect of similar magnitude was seen in NGF-treated and control animals. While NGF treatment increased NGF levels in the contralateral control ganglion, it did not significantly elevate levels in the axotomized ganglion. The results suggest that the decreased availability of NGF after axotomy is a sufficient stimulus to induce the conditioning lesion effect in sympathetic neurons. While NGF administration did not prevent the conditioning lesion effect, this may be due to the markedly decreased ability of sympathetic neurons to accumulate the growth factor after axotomy.

Plasticity in rat uterine sympathetic nerves: the role of TrkA and p75 nerve growth factor receptors

Uterine sympathetic innervation undergoes profound remodelling in response to physiological and experimental changes in the circulating levels of sex hormones. It is not known, however, whether this plasticity results from changes in the innervating neurons, the neuritogenic properties of the target tissue or both. Using densitometric immunohistochemistry, we analysed the effects of prepubertal chronic oestrogen treatment (three subcutaneous injections of 20 microg of beta-oestradiol 17-cypionate on days 25, 27 and 29 after birth), natural peripubertal transition and late pregnancy (19-20 days post coitum) on the levels of TrkA and p75 nerve growth factor receptors in uterine-projecting sympathetic neurons of the thoraco-lumbar paravertebral sympathetic chain (T7-L2) identified using the retrograde tracer Fluorogold. For comparative purposes, levels of TrkA and p75 were assessed in the superior cervical ganglion (SCG) following prepubertal chronic oestrogen treatment. These studies showed that the vast majority of uterine-projecting neurons expressed both TrkA and p75. Both prepubertal chronic oestrogen treatment and the peripubertal transition increased the ratio p75 to TrkA in uterine-projecting neurons, whereas pregnancy elicited the opposite effect. Prepubertal chronic oestrogen treatment had no effects on levels of TrkA or p75 in sympathetic neurons of the SCG. Taken together, our data suggest that neurotrophin receptor-mediated events may contribute to regulate sex hormone-induced plasticity in uterine sympathetic nerves, and are in line with the idea that, in vivo, plasticity in uterine nerves involves changes in both the target and the innervating neurons.

Hyperinnervation of mesenteric arteries in spontaneously hypertensive rats by sympathetic but not primary afferent axons

Hypertrophy of the perivascular plexus is thought to play a role in the development of hypertension in spontaneously hypertensive rats (SHR). However, it is not known whether the sympathetic varicosities are more numerous or larger, or form more neurovascular junctions. Further, a parallel hypertrophy of primary afferent terminals around the vessels might modulate any effects of hypertrophied sympathetic terminals. We have investigated the perivascular plexus around second-order mesenteric arteries of SHR and Wistar-Kyoto (WKY) rats by electron microscopy. Noradrenergic terminals were identified by the presence of small granular vesicles after chromaffin fixation, and substance P (SP+) afferent axons were identified by immunohistochemistry. The numbers of noradrenergic axon and varicosity profiles were higher (48 and 25%, respectively) in SHR than in WKY rats, and the majority lay closer to the medio-adventitial border. In contrast, there was no difference in the numbers of SP+ axons. Sympathetic and SP+ varicosities were indistinguishable in size, shape, vesicle content and mitochondrion content between each other and between the strains. However, both the number of neuromuscular junctions and the proportion of varicosities that formed them in SHR arteries were more than double those in WKY vessels. The data clearly show that hyperinnervation in SHR is specific for noradrenergic axons.

The influences of p75 neurotrophin receptor and brain-derived neurotrophic factor in the sympathetic innervation of target tissues during murine postnatal development

Post-ganglionic sympathetic neurons express the p75 neurotrophin receptor (p75NTR) and brain-derived neurotrophic factor (BDNF), which together have been implicated in controlling the degree of efferent innervation of peripheral organs [Kohn, J., Aloyz, R.S., Toma, J.G., Haak-Frendscho, M., Miller, F.D. 1999. Functionally Antagonistic Interactions between the TrkA and p75 Neurotrophin Receptors Regulate Sympathetic Neuron Growth and Target Innervation. J. Neurosci. 19, 5393-5408]. To examine this concept further, we developed null mutant mice lacking both p75NTR and BDNF, and assessed whether the loss of this receptor-ligand interaction negatively impacts the degree of sympathetic innervation to various target tissues. Between postnatal days 10 and 14, hearts, urinary bladders, kidneys, and submandibular salivary glands were isolated from p75(-/-)/BDNF-/-, p75-/-, BDNF-/-, and wild type siblings. Sympathetic axons were visualized using tyrosine hydroxylase (TH) immunohistochemistry, and TH protein levels were quantified by immunoblotting. Concerning the sympathetic innervation of the heart, urinary bladder and kidneys, no differences were seen in single and double null mutant mice, as compared with their wild type siblings. Sympathetic innervation of the submandibular salivary gland was, however, increased in both p75-/- and p75(-/-)/BDNF-/- mice over control mice. These results reveal that an absence of p75NTR and/or BDNF expression does not perturb the degree of sympathetic innervation of many peripheral tissues during postnatal development, and that a lack of p75NTR expression may actually enhance the density of these efferent fibers in other target tissues, such as the salivary glands.

A Neurotrophin Signaling Cascade Coordinates Sympathetic Neuron Development through Differential Control of TrkA Trafficking and Retrograde Signaling

A fundamental question in developmental biology is how a limited number of growth factors and their cognate receptors coordinate the formation of tissues and organs endowed with enormous morphological complexity. We report that the related neurotrophins NGF and NT-3, acting through a common receptor, TrkA, are required for sequential stages of sympathetic axon growth and, thus, innervation of target fields. Yet, while NGF supports TrkA internalization and retrograde signaling from distal axons to cell bodies to promote neuronal survival, NT-3 cannot. Interestingly, final target-derived NGF promotes expression of the p75 neurotrophin receptor, in turn causing a reduction in the sensitivity of axons to intermediate target-derived NT-3. We propose that a hierarchical neurotrophin signaling cascade coordinates sequential stages of sympathetic axon growth, innervation of targets, and survival in a manner dependent on the differential control of TrkA internalization, trafficking, and retrograde axonal signaling.

Inflammation in sympathetic ganglia proximal to sciatic nerve transection in rats

Comparison was made between recruitment of T-lymphocytes and macrophages into lumbar sympathetic ganglia (SGs) and dorsal root ganglia (DRGs) following sciatic nerve transection in rats. In both control and lesioned SGs, resident (ED2+) macrophages expressed less major histocompatibility complex class II (MHC II), but MHC II+ macrophage density was higher, than in equivalent DRGs. The influx of T-cells was larger and the influx and activation of macrophages were more sustained in SGs than in DRGs. Only two of the five subtypes of macrophage that invade lesioned DRGs were recruited to SGs. While some MHC II+ cells phagocytosed dead sympathetic neurones, most phagocytes in SGs lacked a macrophage marker. The different patterns of response between ganglia may provide clues about macrophage involvement in neuronal death and hyperexcitability after peripheral nerve lesions.

Sympathetic neurons synthesize and secrete pro-nerve growth factor protein

Postmitotic sympathetic neuronal survival is dependent upon nerve growth factor (NGF) provided by peripheral targets, and this dependency serves as a central tenet of the neurotrophic hypothesis. In some other systems, NGF has been shown to play an autocrine role, although the pervasiveness and significance of this phenomenon within the nervous system remain unclear. We show here that rat sympathetic neurons synthesize and secrete NGF. NGF mRNA is expressed in nearly half of superior cervical ganglion sympathetic neurons at embryonic day 17, rising to over 90% in the early postnatal period, and declining in the adult. Neuronal immunoreactivity is reduced when retrograde transport is interrupted by axotomy, but persists in a subpopulation of neurons despite diminished mRNA expression, suggesting that intrinsic protein synthesis occurs. Cultured neonatal neurons express NGF mRNA, which is maintained even when they are undergoing apoptosis. To determine which NGF isoforms are secreted, we performed metabolic labeling and immunoprecipitation of NGF-immunoreactive proteins synthesized by cultured NGF-dependent and -independent neurons. Conditioned medium contained high molecular weight NGF precursor proteins, which varied depending upon the state of NGF dependence. Mature NGF was undetectable by these methods. High molecular weight NGF isoforms were also detected in ganglion homogenates, and persisted at diminished levels following axotomy. We conclude that sympathetic neurons express NGF mRNA, and synthesize and secrete pro-NGF protein. These findings suggest that a potential NGF-sympathetic neuron autocrine loop may exist in this prototypic target-dependent system, but that the secreted forms of this neurotrophin apparently do not support neuronal survival.

Rapid effects of neurotrophic factors on calcium homeostasis in rat visceral afferent neurons

Neurotrophic factors maintain and modulate neuron function in adults. We tested the hypothesis that neurotrophic factors rapidly alter intracellular calcium concentrations, thereby affecting neuron excitability. The majority of rat nodose neurons express TrkA, TrkB and TrkC receptor after 1 day in culture. Addition of nerve growth factor, brain derived neurotrophic factor or glial derived neurotrophic factor increased cytosolic calcium in about one third of the neurons within less than 10 min. This increase was due to calcium release from intracellular stores and could be blocked by the tyrosine kinase inhibitor K252a. The rapid effect of neurotrophic factors suggests a role of these molecules in the early response after inflammation as potential mediators for sensitization of afferent neurons.

Interneuronal signalling is involved in induction of collateral sprouting of nociceptive axons

Collateral sprouting of cutaneous nociceptive axons into the adjacent denervated skin critically depends on the nerve growth factor, presumably originating from the degenerated neural pathways and denervated skin. We hypothesised that the degenerated neural pathways are necessary, but not sufficient, to induce collateral sprouting of nociceptive axons, and, in addition, that the interaction between the injured and non-injured neurones within a dorsal root ganglion can trigger sprouting of nociceptive axons also in the absence of the denervated skin. End-to-side nerve anastomosis, made in female Wistar rats by suturing the end of an excised peroneal nerve segment to the side of the intact sural nerve, was used as a model for sprouting which allowed us to study the putative induction mechanisms separately. If the nerves adjacent to the sural nerve were transected concomitantly with the coaptation of the end-to-side anastomosis, robust nociceptive axon sprouting into the anastomosed nerve segment was observed by the nerve pinch test and counting of myelinated axons. Collateral sprouting did not occur, however, either if the cells in the anastomosed nerve segment were killed by freezing and thawing, or if the adjacent nerves had not been injured. However, if the ipsilateral dorsal cutaneous nerves, having their neurones in the same dorsal root ganglia as the sural nerve, were transected, but no other nerves were injured, then the sural nerve axons sprouted in abundance through the anastomosis even in the absence of denervated skin around the sural nerve terminals. From these results we suggest that cells (probably proliferating Schwann cells) in the degenerated neural pathways are necessary but not sufficient to induce collateral sprouting of nociceptive axons, and that interactions between the injured and non-injured neurones within the dorsal root ganglion (i.e. direct or indirect interneuronal signalling) are important in this regard.

Nerve growth factor antiserum induces axotomy-like changes in neuropeptide expression in intact sympathetic and sensory neurons

Axonal transection of adult sympathetic and sensory neurons leads to a decrease in their content of target-derived nerve growth factor (NGF) and to dramatic changes in the expression of several neuropeptides and enzymes involved in transmitter biosynthesis. For example, axotomy of sympathetic neurons in the superior cervical ganglion (SCG) dramatically increases levels of galanin, vasoactive intestinal peptide (VIP), and substance P and their respective mRNAs and decreases mRNA levels for neuropeptide Y (NPY) and tyrosine hydroxylase (TH). Axotomy of sensory neurons in lumbar dorsal root ganglia (DRG) increases protein and mRNA levels for galanin and VIP and decreases levels for substance P and calcitonin gene-related peptide (CGRP). To assess whether reduction in the availability of endogenous NGF might play an important role in triggering these changes, we injected nonoperated animals with an antiserum against NGF (alphaNGF). alphaNGF increased levels of peptide and mRNA for galanin and VIP in neurons in both the SCG and DRG. NPY protein and mRNA were decreased in the SCG, but levels of TH protein and mRNA remained unchanged. In sensory neurons the levels of SP and CGRP protein decreased after alphaNGF treatment. These data suggest that the reduction in levels of NGF in sympathetic and sensory neurons after axotomy is partly responsible for the subsequent changes in neuropeptide expression. Thus, the peptide phenotype of these axotomized neurons is regulated both by the induction of an "injury factor," leukemia inhibitory factor, as shown previously, and by the reduction in a target-derived growth factor.

Levels of nerve growth factor and neurotrophin-3 are affected differentially by the presence of p75 in sympathetic neurons in vivo

The development and survival of sympathetic neurons is critically dependent on the related neurotrophic factors nerve growth factor (NGF) and neurotrophin-3 (NT3), the actions of which must be executed appropriately despite spatial and temporal overlaps in their activities. The tyrosine receptor kinases, trkA and trkC, are the cognate receptors for NGF and NT3, respectively. The p75 neurotrophin receptor has been implicated in neurotrophin binding and signaling for both NGF and NT3. In this study, the authors used mice that overexpressed NGF (NGF-OE) or NT3 (NT3-OE) in skin and mice that lacked p75 (p75(-/-)) to understand the dynamics of sympathetic neuron response to each neurotrophin and to address the role of p75. NGF and NT3 were measured in sympathetic ganglia and skin (a major target of sympathetic neurons) by using the enzyme-linked immunosorbent assay (ELISA) technique. A three- to four-fold increase in skin NT3 was seen in both NT3-OE and p75(-/-) mice. Moreover, both mouse lines exhibited a three-fold increase in ganglionic NT3. However, the increase in ganglionic NT3 was accompanied by a decrease in ganglionic NGF in p75(-/-) mice but not in NT3-OE mice. This indicated that p75 plays an important role in determining the level of NGF within sympathetic neurons. In NGF-OE mice, the overexpression of NGF was correlated with increased ganglionic NGF and increased ganglionic expression of p75 mRNA. In addition, in NGF-OE mice, ganglionic trkC expression was decreased, as was the amount of NT3 present within sympathetic ganglia. These results indicate that the level of p75 is integral in determining the level of sympathetic NGF and that NGF competes with NT3 by increasing the expression of p75 and decreasing the expression of trkC.

Effects of endogenous neurotrophins on sympathetic sprouting in the dorsal root ganglia and allodynia following spinal nerve injury

Peripheral nerve injury is often complicated by a chronic pain syndrome that is difficult to treat. In animal models of peripheral nerve injury, sympathetic nerve terminals in the dorsal root ganglia (DRG) sprout to form baskets around large diameter neurons, an anatomical change that has been implicated in the induction of neuropathic pain. In the present study, we have investigated whether neurotrophins derived from peripheral sources play any roles in sympathetic sprouting and neuropathic pain in a rat model of peripheral nerve injury. After transection of the left lumbar (L) 5 spinal nerve, antisera specific to neurotrophins were injected intraperitoneally twice a week for 2 weeks. The foot withdrawal response to von Frey hairs was examined on days 1, 3, 7, 10, and 14 postlesion. After completion of behavioral tests, sympathetic sprouting in DRG was examined by tyrosine hydroxylase (TH) immunohistochemistry. The number of TH-immunoreactive (ir) fibers and baskets around large neurons within the lesioned DRG was dramatically increased in the rats treated with control normal sheep serum. Antisera specific to nerve growth factor (NGF), neurotrophin-3 (NT3), and brain-derived neurotrophic factor (BDNF) significantly reduced the sympathetic sprouting and the formation of baskets. L5 spinal nerve lesion induced a significant increase in foot withdrawal responses to von Frey hair stimuli, which was attenuated by treatment of antisera to neurotrophins with a different time sequential. The effect of BDNF antiserum occurred earlier and lasted longer than those of NGF and NT3 antisera. These results implicate that peripherally derived neurotrophins are involved in the induction of sympathetic sprouting and neuropathic pain following peripheral nerve injury.

Dental injury models: experimental tools for understanding neuroinflammatory interactions and polymodal nociceptor functions

Recent research has shown that peripheral mechanisms of pain are much more complex than previously thought, and they differ for acutely injured normal tissues compared with chronic inflammation or neuropathic (nerve injury) pain. The purpose of the present review is to describe uses of dental injury models as experimental tools for understanding the normal functions of polymodal nociceptive nerves in healthy tissues, their neuroinflammatory interactions, and their roles in healing. A brief review of normal dental innervation and its interactions with healthy pulp tissue will be presented first, as a framework for understanding the changes that occur after injury. Then, the different types of dental injury that allow gradation of the extent of tissue damage will be described, along with the degree and duration of inflammation, the types of reactions in the trigeminal ganglion and brainstem, and the type of healing. The dental injury models have some unique features compared with neuroinflammation paradigms that affect other peripheral tissues such as skin, viscera, and joints. Peripheral inflammation models can all be contrasted to nerve injury studies that produce a different kind of neuroplasticity and neuropathic pain. Each of these models provides different insights about the normal and pathologic functions of peripheral nerve fibers and their effects on tissue homeostasis, inflammation, and wound healing. The physical confinement of dental pulp and its innervation within the tooth, the high incidence of polymodal A-delta and C-fibers in pulp and dentin, and the somatotopic organization of the trigeminal ganglion provide some special advantages for experimental design when dental injury models are used for the study of neuroinflammatory interactions.

BDNF is involved in sympathetic sprouting in the dorsal root ganglia following peripheral nerve injury in rats

Peripheral nerve injury results in sympathetic sprouting around large diameter sensory neurons in the dorsal root ganglia (DRG). The mechanism underlying this pathological phenomenon is not known. Brain-derived neurotrophic factor (BDNF) is up-regulated in large sensory neurons and ensheathing satellite cells following a sciatic nerve injury. In the present study, we investigated the effects of BDNF on the sympathetic sprouting in the DRG, by delivering BDNF antibody or antisense oligodeoxynucleotide to injured DRGs, or by delivering exogenous BDNF to intact DRGs. The sheep antibody to BDNF, characterized by bioassays and dot blots, specifically reacted with BDNF but not other neurotrophins. Noradrenergic fibers were visualized by immunostaining of tyrosine hydroxylase (TH) and quantified by an NIH Imaging program. Two weeks following L5 spinal nerve lesion, a dramatic increase in TH-immunoreactive (-ir) fibres was observed in both ipsi- and contralateral DRGs in normal sheep IgG treated rats. BDNF antibody significantly reduced the sprouting of sympathetic nerves in both ipsi- and contra-lateral DRGs by 67% and 42% respectively. BDNF antisense oligodeoxynucleotide, by inhibiting BDNF synthesis in DRGs, also significantly suppressed the sprouting by 67% and 60% respectively in the ipsi- and contra-lateral DRGs. Delivery of exogenous BDNF into an intact L5 DRGs resulted in an increase in the sprouting by 4.2-fold. Our results clearly indicate that BDNF, synthesized in and secreted from the DRGs, is involved in the sympathetic sprouting in the DRG following the peripheral nerve injury.

Glucocorticoid regulation of motoneuronal parameters in rats with spinal cord injury

1. Glucocorticoids exert beneficial effects after acute CNS injury in humans and experimental animals. To elucidate potential mechanisms of glucocorticoid action in the lesioned spinal cord, we have studied if treatment with dexamethasone (DEX) modulated the neurotrophin binding receptor p75 (p75NTR) and choline acetyltransferase (ChAT), a marker of neuronal functional viability. 2. Rats with a sham operation or with spinal cord transection at the thoracic level received vehicle or DEX several times postlesion and were sacrificed 48 hr after surgery. The lumbar region caudal to the lesion was processed for p75NTR and ChAT immunoreactivity (IR) using quantitative densitometric analysis. 3. We observed that p75NTR-IR was absent from ventral horn motoneurons of sham-operated rats, in contrast to strong staining of neuronal perikaryon in TRX rats. Administration of DEX to TRX rats had no effect on the number of neuronal cell bodies expressing p75NTR-IR but significantly increased the number and length of immunostained neuronal processes. 4. Furthermore, spinal cord transection reduced ChAT immunostaining of motoneurons by 50%, whereas DEX treatment reverted this pattern to cells with a strong immunoreaction intensity in perikaryon and cell processes. 5. It is hypothesized that increased expression of p75NTR in cell processes and of ChAT in motoneurons may be part of a mechanism by which glucocorticoids afford neuroprotection, in addition to their known antiinflammatory effects.

Neuroreceptor mRNA expression in the rat mesenteric artery develops independently of innervation

Studies in autonomic targets have shown that nerves may be required for the development and maintenance of postsynaptic receptor populations. We have examined this relationship in the rat mesenteric artery, assessing mRNA expression levels for a range of neuroreceptors after neonatal sympathectomy, using 6-hydroxydopamine or antisera directed against nerve growth factor, and sensory denervation, using capsaicin. Total RNA was extracted from 28 day old rats and reverse transcription-polymerase chain reaction was performed, using primers specific for the alpha1(A,B,D)- and alpha2(A,B,C)-adrenergic, neurokinin (NKI-NK3), muscarinic (M1-M5) and P2X purinergic (P2x1-7) receptor families. Results showed no decreases in mRNA expression of any of the specific receptor subtypes after either sympathetic or sensory denervation. Small increases in mRNA expression were detected following sensory denervation for some of the receptor subtypes. We conclude that neither sympathetic nor sensory nerves are mandatory for the expression of mRNA of a range of neuroreceptors in the mesenteric vascular bed of the rat.

Sympathetic and sensory axons invade the brains of nerve growth factor transgenic mice in the absence of p75NTR expression

Collateral sprouting, a nerve growth factor (NGF)-mediated growth response of undamaged peripheral axons, can be divided into reparative and aberrant axonal growth. We have previously shown that aberrant growth occurs in transgenic mice overexpressing NGF centrally under the control of the glial fibrillary acidic protein promoter. Both sympathetic and sensory fibers, stained immunohistochemically for tyrosine hydroxylase and calcitonin gene-related peptide, respectively, invade the cerebellum of postnatal transgenic mice, whereas no such axons are seen in age-matched wild-type cerebellum. Recent examination of mice possessing a null mutation for p75NTR has suggested that axon growth may be influenced by the functional expression of this receptor. To address the potential role of p75NTR in axon growth, we have generated a new line of hybrid mice overexpressing NGF but lacking functional p75NTR expression. Postnatal (day 14) hybrid cerebellum possessed fewer aberrant sensory and sympathetic fibers compared to their age-matched transgenic counterparts. By adulthood, however, hybrid cerebellum displayed a robust plexus of axons stained immunohistochemically for calcitonin gene-related peptide and tyrosine hydroxylase. No neuronal or nonneuronal localization of p75NTR-immunoreactive elements was observed in postnatal and adult hybrid cerebellum. Interestingly, sympathetic axons within the hybrid cerebellum displayed a markedly reduced axon density and staining intensity for NGF, suggesting a possible alteration in axonal sequestration of NGF. These results show that p75NTR is not vital for new growth of NGF-sensitive sympathetic and sensory axons and that immunohistochemical detection of NGF at sympathetic axons requires the functional expression of p75NTR.

Rat mature sympathetic neurones derive neurotrophin 3 from peripheral effector tissues

In a previous study we have demonstrated that endogenous neurotrophin 3 (NT3) is required for the survival of most sympathetic neurones in postnatal rats. However, the mechanisms underlying the action of NT3 on sympathetic neurones is not known. Neither is it understood whether NT3 is retrogradely transported from peripheral tissues or acts locally in an autocrine fashion. In the present study, NT3-mRNA was quantified in sympathetic effector tissues and NT3 protein was localized in intact and lesioned sympathetic nerves in rats. NT3-mRNA is expressed and developmentally regulated in many effector tissues including mesenteric arteries, salivary gland, heart and kidney but hardly detectable in the superior cervical ganglia of adult animals. The majority of sympathetic neurones express immunoreactivity for TrkA and TrkC in both neonatal and adult rats. Sympathetic somata are normally immunoreactive for NT3, but the immunoreactivity is abolished by systemic administration of NT3 antibodies or axotomy of postganglionic neurones, suggesting an accumulation of NT3 from extraneuronal sources. Furthermore, the detection of NT3-immunoreactivity in the internal carotid nerve as early as 3 h following a compression and only on the distal side indicates endogenous NT3 is retrogradely transported by sympathetic neurones. These studies provide evidence indicating that NT3, like nerve growth factor, is an effector tissue-derived neurotrophic factor for sympathetic neurones both during development and in the adult animal. Thus, we have provided a clear example that one type of neurone derives, through a retrograde transport mechanism, two neurotrophic factors simultaneously from its peripheral effector tissues.

Neurotrophic factors are required by mature sympathetic neurons for survival, transmission and connectivity

1. Two neuronal growth factors, nerve growth factor (NGF) and neurotrophin 3 (NT3), have been studied for their action on the developing and mature sympathetic nervous system. 2. Antibodies to each factor have proved useful as reagents for the detection and quantification of NGF and NT3. They have also proved valuable in uncovering the functional roles of each factor by their ability to neutralize the endogenous molecules. 3. Nerve growth factor acts on postnatal neurons to control neurotransmission, connectivity and survival. Like NGF, NT3 is synthesized by effector tissues and is retrogradely transported by post-ganglionic neurons to prevent cell death. However, the two factors have been shown to have quite distinct functions in mature neurons, indicating the existence of different signalling pathways. This differential action extends to secondary influences on satellite glia. 4. Pathological consequences result from excessive growth factor synthesis leading, in the hypertensive rat, to hyperinnervation and elevated blood pressure. Satellite glial cell synthesis of the factors and their receptors following peripheral nerve damage appears to be responsible for the establishment of inappropriate neuronal connections between sympathetic nerve terminals and sensory somata. 5. It is concluded that these potent factors control, by both coincident and independent mechanisms, sympathetic neuronal function throughout the life of the animal.

Differential expression of the p75 nerve growth factor receptor in glia and neurons of the rat dorsal root ganglia after peripheral nerve transection

Sympathetic nerve terminals on blood vessels within the dorsal root ganglia sprout after sciatic nerve lesions in the rat. The mechanism underlying this phenomenon is not clear, but might be predicted to involve nerve growth factor or its homologs because these factors are known to trigger collateral sprouting of undamaged sympathetic noradrenergic terminals. We have found that sciatic nerve lesions lead to a decreased expression of neuronal p75, the low-affinity receptor for the neurotrophins, but an increased expression of glial p75 in ipsilateral dorsal root ganglia. Intriguingly, the increased expression of p75 was found primarily in association with glia surrounding large-diameter neurons, which are those associated with the noradrenergic sprouts. A smaller but significant glial response was also found in contralateral ganglia. The glial response in ipsilateral ganglia could be mimicked by ventral, but not dorsal, root transection. The dorsal root lesion-induced glial responses in contralateral ganglia were greater than those induced by ventral root or sciatic nerve lesions. Combined lesions of dorsal root and either ventral root or sciatic nerve did not prevent the glial responses of ipsilateral ganglia, suggesting that a peripheral signal is involved. Colocalization studies indicate that tyrosine hydroxylase-immunoreactive nerve sprouts were associated with p75-immunoreactive glial cells. Thus, increased glial synthesis of p75 might provide an explanation for the abnormal growth of sympathetic fibers in dorsal root ganglia after peripheral nerve injury.