Nerve growth factor receptor TrkA is down-regulated during postnatal development by a subset of dorsal root ganglion neurons

Identification of Neuronal Cells in Sciatic Nerves of Adult Rats

Prior research generally confirms that there are no neuronal cell bodies in the adult sciatic nerve. However, we occasionally find some neuronal cells in adult rat sciatic nerves, either intact or crush-injured. By whole-mount staining and optical imaging of the hyalinized sciatic nerves for Stmn2 (a specific marker for neuronal cells), we found those neuronal cells with irregular distribution in the sciatic nerves in both crushed model and normal rats. We investigated the identity of those cells and established a cultured sciatic nerve model. Immunohistochemistry evidence both in vivo and in vitro illustrated that some of those cells are mature neurons in sciatic nerves. With single-cell sequencing of neuronal cells in adeno-associated virus (AAV)-infected sciatic nerves, we identified that some of those cells are a kind of neuronal stem-like cells. Then we constructed a Nestin-CreERT 2 rat line and traced those cells with fluorescence labeling which was induced by tamoxifen. Interesting, we proved that neuronal stem-like cells could proliferate by combination of EdU incorporation with staining in the sciatic nerves of transgenic rats. Together, the discovery of neuronal cells in adult sciatic nerves will make us aware of the distribution of neurons in the peripheral nervous system. Especially our data suggest that neuronal stem-like cells could proliferate in the sciatic nerves of adult rats.

Influencers in the Somatosensory System: Extrinsic Control of Sensory Neuron Phenotypes

Somatosensory neurons in dorsal root ganglia (DRG) comprise several main subclasses: high threshold nociceptors/thermoceptors, high- and low-threshold mechanoreceptors, and proprioceptors. Recent years have seen an explosion in the identification of molecules that underlie the functional diversity of these sensory modalities. They also have begun to reveal the developmental mechanisms that channel the emergence of this subtype diversity, solidifying the importance of peripheral instructive signals. Somatic sensory neurons collectively serve numerous essential physiological and protective roles, and as such, an increased understanding of the processes that underlie the specialization of these sensory subtypes is not only biologically interesting but also clinically relevant.

The development of somatosensory neurons: Insights into pain and itch

Primary nociceptors are a heterogeneous class of peripheral somatosensory neurons, responsible for detecting noxious, pruriceptive, and thermal stimuli. These neurons are further divided into several molecularly defined subtypes that correlate with their functional sensory modalities and morphological features. During development, all nociceptors arise from a common pool of embryonic precursors, and then segregate progressively into their mature specialized phenotypes. In this review, we summarize the intrinsic transcriptional programs and extrinsic trophic factor signaling mechanisms that interact to control nociceptor diversification. We also discuss how recent transcriptome profiling studies have significantly advanced the field of sensory neuron development.

Nerve growth factor antibody for the treatment of osteoarthritis pain and chronic low-back pain: mechanism of action in the context of efficacy and safety

Chronic pain continues to be a significant global burden despite the availability of a variety of nonpharmacologic and pharmacologic treatment options. Thus, there is a need for new analgesics with novel mechanisms of action. In this regard, antibodies directed against nerve growth factor (NGF-Abs) are a new class of agents in development for the treatment of chronic pain conditions such as osteoarthritis and chronic low-back pain. This comprehensive narrative review summarizes evidence supporting pronociceptive functions for NGF that include contributing to peripheral and central sensitization through tropomyosin receptor kinase A activation and stimulation of local neuronal sprouting. The potential role of NGF in osteoarthritis and chronic low-back pain signaling is also examined to provide a mechanistic basis for the observed efficacy of NGF-Abs in clinical trials of these particular pain states. Finally, the safety profile of NGF-Abs in terms of common adverse events, joint safety, and nerve structure/function is discussed.

Functional Anatomy of the Sensory Nervous System: Updates From the Neuroscience Bench

The simple tripartite classification of sensory neurons as A-beta, A-delta, and C fibers fails to convey the complexity of the neurons that encode stimuli as diverse as the texture of a surface, the location of a pinprick, or the direction of hair movement as a breeze moves across the skin. It has also proven to be inadequate when investigating the molecular mechanisms underlying pain, which can encompass any combination of chemical, tactile, and thermal modalities. Beginning with a brief overview of visceral and sensory neuroanatomy, this review expands upon sensory innervation of the skin as a prime example of the heterogeneity and complexity of the somatosensory nervous system. Neuroscientists have characterized defining features of over 15 subtypes of sensory neurons that innervate the skin of the mouse. This has enabled the study of cell-specific mechanisms of pain, which suggests that diverse sensory neuron subtypes may have distinct susceptibilities to toxic injury and different roles in pathologic mechanisms underlying altered sensation. Leveraging this growing body of knowledge for preclinical trials and models of neurotoxicity can vastly improve our understanding of peripheral nervous system dysfunction, advancing the fields of toxicologic pathology and neuropathology alike.

Dependence of Neuroprosthetic Stimulation on the Sensory Modality of the Trigeminal Neurons Following Nerve Injury. Implications in the Design of Future Sensory Neuroprostheses for Correct Perception and Modulation of Neuropathic Pain

Amputation of a sensory peripheral nerve induces severe anatomical and functional changes along the afferent pathway as well as perception alterations and neuropathic pain. In previous studies we showed that electrical stimulation applied to a transected infraorbital nerve protects the somatosensory cortex from the above-mentioned sensory deprivation-related changes. In the present study we focus on the initial tract of the somatosensory pathway and we investigate the way weak electrical stimulation modulates the neuroprotective-neuroregenerative and functional processes of trigeminal ganglia primary sensory neurons by studying the expression of neurotrophins (NTFs) and Glia-Derived Neurotrophic Factors (GDNFs) receptors. Neurostimulation was applied to the proximal stump of a transected left infraorbitary nerve using a neuroprosthetic micro-device 12 h/day for 4 weeks in freely behaving rats. Neurons were studied by in situ hybridization and immunohistochemistry against RET (proto-oncogene tyrosine kinase "rearranged during transfection"), tropomyosin-related kinases (TrkA, TrkB, TrkC) receptors and IB4 (Isolectin B4 from Griffonia simplicifolia). Intra-group (left vs. right ganglia) and inter-group comparisons (between Control, Axotomization and Stimulation-after-axotomization groups) were performed using the mean percentage change of the number of positive cells per section [100∗(left-right)/right)]. Intra-group differences were studied by paired t-tests. For inter-group comparisons ANOVA test followed by post hoc LSD test (when P < 0.05) were used. Significance level (α) was set to 0.05 in all cases. Results showed that (i) neurostimulation has heterogeneous effects on primary nociceptive and mechanoceptive/proprioceptive neurons; (ii) neurostimulation affects RET-expressing small and large neurons which include thermo-nociceptors and mechanoceptors, as well as on the IB4- and TrkB-positive populations, which mainly correspond to non-peptidergic thermo-nociceptive cells and mechanoceptors respectively. Our results suggest (i) electrical stimulation differentially affects modality-specific primary sensory neurons (ii) artificial input mainly acts on specific nociceptive and mechanoceptive neurons (iii) neuroprosthetic stimulation could be used to modulate peripheral nerve injuries-induced neuropathic pain. These could have important functional implications in both, the design of effective clinical neurostimulation-based protocols and the development of neuroprosthetic devices, controlling primary sensory neurons through selective neurostimulation.

Non-Peptidergic Nociceptive Neurons Are Essential for Mechanical Inflammatory Hypersensitivity in Mice

Small nerve fibers that bind the isolectin B4 (IB4⁺ C-fibers) are a subpopulation of primary afferent neurons that are involved in nociceptive sensory transduction and do not express the neuropeptides substance P and calcitonin-gene related peptide (CGRP). Several studies have attempted to elucidate the functional role of IB4⁺-nociceptors in different models of pain. However, a functional characterization of the non-peptidergic nociceptors in mediating mechanical inflammatory hypersensitivity in mice is still lacking. To this end, in the present study, the neurotoxin IB4-Saporin (IB4-Sap) was employed to ablate non-peptidergic C-fibers. Firstly, we showed that intrathecal (i.t.) administration of IB4-Sap in mice depleted non-peptidergic C-fibers, since it decreased the expression of purinoceptor 3 (P2X₃) and transient receptor potential cation channel subfamily V member 1 (TRPV1) in the dorsal root ganglia (DRGs) as well as IB4 labelling in the spinal cord. Non-peptidergic C-fibers depletion did not alter the mechanical nociceptive threshold, but it inhibited the mechanical inflammatory hypersensitivity induced by glial cell-derived neurotrophic factor (GDNF), but not nerve growth factor (NGF). Depletion of non-peptidergic C-fibers abrogated mechanical inflammatory hypersensitivity induced by carrageenan. Finally, it was found that the inflammatory mediators PGE₂ and epinephrine produced a mechanical inflammatory hypersensitivity that was also blocked by depletion of non-peptidergic C-fibers. These data suggest that IB4-positive nociceptive nerve fibers are not involved in normal mechanical nociception but are sensitised by inflammatory stimuli and play a crucial role in mediating mechanical inflammatory hypersensitivity.

Mechanisms Underlying Bone and Joint Pain

PURPOSE OF REVIEW

The goal of this review is to provide a broad overview of the current understanding of mechanisms underlying bone and joint pain.

RECENT FINDINGS

Bone or joint pathology is generally accompanied by local release of pro-inflammatory cytokines, growth factors, and neurotransmitters that activate and sensitize sensory nerves resulting in an amplified pain signal. Modulation of the pain signal within the spinal cord and brain that result in net increased facilitation is proposed to contribute to the development of chronic pain. Great strides have been made in our understanding of mechanisms underlying bone and joint pain that will guide development of improved therapeutic options for these patients. Continued research is required for improved understanding of mechanistic differences driving different components of bone and/or joint pain such as movement related pain compared to persistent background pain. Advances will guide development of more individualized and comprehensive therapeutic options.

Local NGF and GDNF levels modulate morphology and function of porcine DRG neurites, In Vitro

Nerve terminals of primary sensory neurons are influenced by their environment through target derived trophic factors, like nerve growth factor (NGF) or glial cell line-derived neurotrophic factor (GDNF). In mice, subpopulations of DRG neurons express receptors either for NGF or GDNF and therefore differentially respond to these neurotrophic factors. We probed neurite endings from porcine DRG neurons cultured in either NGF or GDNF and examined their shape, elongation and stimulus-evoked CGRP release. A compartmentalized culture system was employed allowing spatial separation of outgrown neurites from their somata and use of different growth factors in the compartments. We show that neurites of GDNF cultured somata extend into lateral compartments without added growth factor, unlike neurites of NGF cultured ones. Neurites of NGF cultured somata extend not only into NGF- but also into GDNF-containing compartments. GDNF at the site of terminals of NGF responsive somata led to a strong neurite arborization and formation of large growth cones, compared to neurites in medium with NGF. Functionally, we could detect evoked CGRP release from as few as 7 outgrown neurites per compartment and calculated release per mm neurite length. CGRP release was detected both in neurites from NGF and GDNF cultured somata, suggesting that also the latter ones are peptidergic in pig. When neurites of NGF cultured somata were grown in GDNF, capsaicin evoked a lower CGRP release than high potassium, compared to those grown in NGF. Our experiments demonstrate that the compartmented culture chamber can be a suitable model to assess neurite properties from trophic factor specific primary sensory neurons. With this model, insights into mechanisms of gain or loss of function of specific nociceptive neurites may be achieved.

Functional and Molecular Characterization of Mechanoinsensitive "Silent" Nociceptors

Mechanical and thermal hyperalgesia (pain hypersensitivity) are cardinal signs of inflammation. Although the mechanism underlying thermal hyperalgesia is well understood, the cellular and molecular basis of mechanical hyperalgesia is poorly described. Here, we have identified a subset of peptidergic C-fiber nociceptors that are insensitive to noxious mechanical stimuli under normal conditions but become sensitized to such stimuli when exposed to the inflammatory mediator nerve growth factor (NGF). Strikingly, NGF did not affect mechanosensitivity of other nociceptors. We show that these mechanoinsensitive "silent" nociceptors are characterized by the expression of the nicotinic acetylcholine receptor subunit alpha-3 (CHRNA3) and that the mechanically gated ion channel PIEZO2 mediates NGF-induced mechanosensitivity in these neurons. Retrograde tracing revealed that CHRNA3⁺ nociceptors account for ∼50% of all peptidergic nociceptive afferents innervating visceral organs and deep somatic tissues. Hence, our data suggest that NGF-induced "un-silencing" of CHRNA3⁺ nociceptors significantly contributes to the development of mechanical hyperalgesia during inflammation.

The histology, physiology, neurochemistry and circuitry of the substantia gelatinosa Rolandi (lamina II) in mammalian spinal cord

The substantia gelatinosa Rolandi (SGR) was first described about two centuries ago. In the following decades an enormous amount of information has permitted us to understand - at least in part - its role in the initial processing of pain and itch. Here, I will first provide a comprehensive picture of the histology, physiology, and neurochemistry of the normal SGR. Then, I will analytically discuss the SGR circuits that have been directly demonstrated or deductively envisaged in the course of the intensive research on this area of the spinal cord, with particular emphasis on the pathways connecting the primary afferent fibers and the intrinsic neurons. The perspective existence of neurochemically-defined sets of primary afferent neurons giving rise to these circuits will be also discussed, with the proposition that a cross-talk between different subsets of peptidergic fibers may be the structural and functional substrate of additional gating mechanisms in SGR. Finally, I highlight the role played by slow acting high molecular weight modulators in these gating mechanisms.

Increased Expression of Transcription Factor SRY-box-Containing Gene 11 (Sox11) Enhances Neurite Growth by Regulating Neurotrophic Factor Responsiveness

The peripherally projecting axons of dorsal root ganglion (DRG) neurons readily regenerate after damage while their centrally projecting branches do not regenerate to the same degree after injury. One important reason for this inconsistency is the lack of pro-regeneration gene expression that occurs in DRG neurons after central injury relative to peripheral damage. The transcription factor SRY-box-containing gene 11 (Sox11) may be a crucial player in the regenerative capacity of axons as previous evidence has shown that it is highly upregulated after peripheral axon damage but not after central injury. Studies have also shown that overexpression or inhibition of Sox11 after peripheral nerve damage can promote or block axon regeneration, respectively. To further understand the mechanisms of how Sox11 regulates axon growth, we artificially overexpressed Sox11 in DRG neurons in vitro to determine if increased levels of this transcription factor could enhance neurite growth. We found that Sox11 overexpression significantly enhanced neurite branching in vitro, and specifically induced the expression of glial cell line-derived neurotrophic factor (GDNF) family receptors, GFRα1 and GFRα3. The upregulation of these receptors by Sox11 overproduction altered the neurite growth patterns of DRG neurons alone and in response to growth factors GDNF and artemin; ligands for GFRα1 and GFRα3, respectively. These data support the role of Sox11 to promote neurite growth by altering responsiveness of neurotrophic factors and may provide mechanistic insight as to why peripheral axons of sensory neurons readily regenerate after injury, but the central projections do not have an extensive regenerative capacity.

Resveratrol protects bupivacaine-induced neuro-apoptosis in dorsal root ganglion neurons via activation on tropomyosin receptor kinase A

BACKGROUND

General anesthesia in spinal cord may lead to unexpected but irreversible neurotoxicity. We investigated whether resveratrol (RSV) may protect bupivacaine (BUP)-induced neuro-apoptosis in spinal cord dorsal root ganglia (DRG).

METHODS

Mouse DRG cells were cultured in vitro, pre-treated with RSV and then 5 mM BUP. A concentration-dependent effect of RSV on reducing BUP-induced apoptosis of DRG neurons (DRGNs) was evaluated using a TUNEL assay. QRT-PCR and western blot assays were also conducted to evaluate gene and protein expressions of tropomyosin receptor kinase A/B/C (TrkA/B/C) and activated (phosphorylated) Trk receptors, phospho-TrkA/B/C. In addition, a functional TrkA blocking antibody MNAC13 was applied in DRG culture to further measure the functional role of Trk receptor in RSV-initiated apoptotic protection on BUP-damaged DRGNs.

RESULTS

BUP promoted significant apoptosis in DRG. RSV exhibited protective effects against BUP-induced neuro-apoptosis in a concentration-dependent manner. qRT-PCR and western blot showed that RSV did not alter TrkA/B/C gene or protein expression, but significantly upregulated phospho-TrkA. Conversely, application of MNAC13 decreased phospho-TrkA and reversed RSV-initiated neuro-protection on BUP-induced DRGN apoptosis.

CONCLUSION

Resveratrol may protect anesthesia-induced DRG neuro-apoptosis, and activation of TrkA signaling pathway may be the underlying mechanism in this process.

Neurotrophic Factors NGF, GDNF and NTN Selectively Modulate HSV1 and HSV2 Lytic Infection and Reactivation in Primary Adult Sensory and Autonomic Neurons

Herpes simplex viruses (HSV1 and HSV2) establish latency in peripheral ganglia after ocular or genital infection, and can reactivate to produce different patterns and frequencies of recurrent disease. Previous studies showed that nerve growth factor (NGF) maintains HSV1 latency in embryonic sympathetic and sensory neurons. However, adult sensory neurons are no longer dependent on NGF for survival, some populations cease expression of NGF receptors postnatally, and the viruses preferentially establish latency in different populations of sensory neurons responsive to other neurotrophic factors (NTFs). Thus, NGF may not maintain latency in adult sensory neurons. To identify NTFs important for maintaining HSV1 and HSV2 latency in adult neurons, we investigated acute and latently-infected primary adult sensory trigeminal (TG) and sympathetic superior cervical ganglia (SCG) after NTF removal. NGF and glial cell line-derived neurotrophic factor (GDNF) deprivation induced HSV1 reactivation in adult sympathetic neurons. In adult sensory neurons, however, neurturin (NTN) and GDNF deprivation induced HSV1 and HSV2 reactivation, respectively, while NGF deprivation had no effects. Furthermore, HSV1 and HSV2 preferentially reactivated from neurons expressing GFRα2 and GFRα1, the high affinity receptors for NTN and GDNF, respectively. Thus, NTN and GDNF play a critical role in selective maintenance of HSV1 and HSV2 latency in primary adult sensory neurons.

NGF-TrkA Signaling by Sensory Nerves Coordinates the Vascularization and Ossification of Developing Endochondral Bone

Developing tissues dictate the amount and type of innervation they require by secreting neurotrophins, which promote neuronal survival by activating distinct tyrosine kinase receptors. Here, we show that nerve growth factor (NGF) signaling through neurotrophic tyrosine kinase receptor type 1 (TrkA) directs innervation of the developing mouse femur to promote vascularization and osteoprogenitor lineage progression. At the start of primary ossification, TrkA-positive axons were observed at perichondrial bone surfaces, coincident with NGF expression in cells adjacent to centers of incipient ossification. Inactivation of TrkA signaling during embryogenesis in TrkA(F592A) mice impaired innervation, delayed vascular invasion of the primary and secondary ossification centers, decreased numbers of Osx-expressing osteoprogenitors, and decreased femoral length and volume. These same phenotypic abnormalities were observed in mice following tamoxifen-induced disruption of NGF in Col2-expressing perichondrial osteochondral progenitors. We conclude that NGF serves as a skeletal neurotrophin to promote sensory innervation of developing long bones, a process critical for normal primary and secondary ossification.

Can Diabetic Neuropathy Be Modeled In Vitro?

Diabetic neuropathy is a common secondary complication of diabetes that impacts on patient's health and well-being. Distal axon degeneration is a key feature of diabetic neuropathy, but the pathological changes which underlie axonal die-back are incompletely understood; despite decades of research a treatment has not yet been identified. Basic research must focus on understanding the complex mechanisms underlying changes that occur in the nervous system during diabetes. To this end, tissue culture techniques are invaluable as they enable researchers to examine the intricate mechanistic responses of cells to high glucose or other factors in order to better understand the pathogenesis of nerve dysfunction. This chapter describes the use of in vitro models to study a wide range of specific cellular effects pertaining to diabetic neuropathy including apoptosis, neurite outgrowth, neurodegeneration, activity, and bioenergetics. We consider problems associated with in vitro modeling and future refinement such as use of induced pluripotent stem cells and microfluidic technology.

Mas-Related G Protein-Coupled Receptors Offer Potential New Targets for Pain Therapy

The founding member of the Mas-related G-protein-coupled receptor (Mrgpr) family was discovered in 1986. Since then, many more members of this receptor family have been identified in multiple species, and their physiologic functions have been investigated widely. Because they are expressed exclusively in small-diameter primary sensory neurons, the roles of Mrgpr proteins in pain and itch have been best studied. This review will focus specifically on the current knowledge of their roles in pathological pain and the potential development of new pharmacotherapies targeted at some Mrgprs for the treatment of chronic pain. We will also discuss the limitations and future scope of this receptor family in pain treatment.

Extrinsic and intrinsic signals converge on the Runx1/CBFβ transcription factor for nonpeptidergic nociceptor maturation

The generation of diverse neuronal subtypes involves specification of neural progenitors and, subsequently, postmitotic neuronal differentiation, a relatively poorly understood process. Here, we describe a mechanism whereby the neurotrophic factor NGF and the transcription factor Runx1 coordinate postmitotic differentiation of nonpeptidergic nociceptors, a major nociceptor subtype. We show that the integrity of a Runx1/CBFβ holocomplex is crucial for NGF-dependent nonpeptidergic nociceptor maturation. NGF signals through the ERK/MAPK pathway to promote expression of Cbfb but not Runx1 prior to maturation of nonpeptidergic nociceptors. In contrast, transcriptional initiation of Runx1 in nonpeptidergic nociceptor precursors is dependent on the homeodomain transcription factor Islet1, which is largely dispensable for Cbfb expression. Thus, an NGF/TrkA-MAPK-CBFβ pathway converges with Islet1-Runx1 signaling to promote Runx1/CBFβ holocomplex formation and nonpeptidergic nociceptor maturation. Convergence of extrinsic and intrinsic signals to control heterodimeric transcription factor complex formation provides a robust mechanism for postmitotic neuronal subtype specification.

DOI:http://dx.doi.org/10.7554/eLife.10874.001

Animals detect and respond to their environment using their sensory nervous system, which forms through a complex, multi-step process. A precursor nerve cell’s fate is set early in its development, and determines the different nerve types it can become. As development progresses, sensory nerve cells develop further into distinct subtypes that perform particular tasks, such as responding to touch or pain.

Nociceptors are the specialised sensory nerves that respond to potentially harmful stimuli. They form two distinct subtypes: peptidergic nerves detect potentially dangerous temperatures, whereas non-peptidergic nerves detect potentially dangerous mechanical sensations. Both subtypes originate from the same precursor nerve cell and both initially depend on an external molecule called NGF for their development and survival. During their development, non-peptidergic neurons stop responding to NGF and start producing a protein called Runx1, considered to be the ‘master regulator’ of non-peptidergic nerve cell development. Runx1 works by forming a complex with another protein called CBFbβ, and this complex activates a program of gene expression that is specific to non-peptidergic nerves. However it was unclear how an external signal, like NGF, can coordinate with or influence a nerve cell’s internal genetic program during the nerve’s development. It was also not known whether NGF and Runx1 interact with each other.

By studying non-peptidergic nerve cell development in mice that lack NGF, Runx1 and other associated proteins, Huang et al. have now established the sequence of events that regulate the development of this nerve cell subtype. Two signalling pathways converge to switch on non-peptidergic nerve cell development. An NGF-driven signalling pathway activates the production of CBFβ, while another protein binds to the Runx1 gene to switch it on. This leads to the production of the Runx1 and CBFβ proteins that complex together to activate the non-peptidergic neuronal genetic program.

These findings demonstrate how two different mechanisms converge to produce the component parts of a complex, which then activates a genetic program that drives the development of a particular neuronal subtype. Whether this mechanism is involved in determining the fate of other cell types remains a question for future work.

DOI:http://dx.doi.org/10.7554/eLife.10874.002

Mouse DRG Cell Line with Properties of Nociceptors

In vitro cell lines from DRG neurons aid drug discovery because they can be used for early stage, high-throughput screens for drugs targeting pain pathways, with minimal dependence on animals. We have established a conditionally immortal DRG cell line from the Immortomouse. Using immunocytochemistry, RT-PCR and calcium microfluorimetry, we demonstrate that the cell line MED17.11 expresses markers of cells committed to the sensory neuron lineage. Within a few hours under differentiating conditions, MED17.11 cells extend processes and following seven days of differentiation, express markers of more mature DRG neurons, such as NaV1.7 and Piezo2. However, at least at this time-point, the nociceptive marker NaV1.8 is not expressed, but the cells respond to compounds known to excite nociceptors, including the TRPV1 agonist capsaicin, the purinergic receptor agonist ATP and the voltage gated sodium channel agonist, veratridine. Robust calcium transients are observed in the presence of the inflammatory mediators bradykinin, histamine and norepinephrine. MED17.11 cells have the potential to replace or reduce the use of primary DRG culture in sensory, pain and developmental research by providing a simple model to study acute nociception, neurite outgrowth and the developmental specification of DRG neurons.

Development of nNOS-positive neurons in the rat sensory ganglia after capsaicin treatment

To gain a better understanding of the neuroplasticity of afferent neurons during postnatal ontogenesis, the distribution of neuronal nitric oxide synthase (nNOS) immunoreactivity was studied in the nodose ganglion (NG) and Th2 and L4 dorsal root ganglia (DRG) from vehicle-treated and capsaicin-treated female Wistar rats at different ages (10-day-old, 20-day-old, 30-day-old, and two-month-old). The percentage of nNOS-immunoreactive (IR) neurons decreased after capsaicin treatment in all studied ganglia in first 20 days of life, from 55.4% to 36.9% in the Th2 DRG, from 54.6% to 26.1% in the L4 DRG and from 37.1% to 15.0% in the NG. However, in the NG, the proportion of nNOS-IR neurons increased after day 20, from 11.8% to 23.9%. In the sensory ganglia of all studied rats, a high proportion of nNOS-IR neurons bound isolectin B4. Approximately 90% of the sensory nNOS-IR neurons bound to IB4 in the DRG and approximately 80% in the NG in capsaicin-treated and vehicle-treated rats. In 10-day-old rats, a large number of nNOS-IR neurons also expressed TrkA, and the proportion of nNOS(+)/TrkA(+) neurons was larger in the capsaicin-treated rats compared with the vehicle-treated animals. During development, the percentage of nNOS(+)/TrkA(+) cells decreased in the first month of life in both groups. The information provided here will also serve as a basis for future studies investigating mechanisms of sensory neuron development.

Molecular control of the neural crest and peripheral nervous system development

A transient and unique population of multipotent stem cells, known as neural crest cells (NCCs), generate a bewildering array of cell types during vertebrate development. An attractive model among developmental biologists, the study of NCC biology has provided a wealth of knowledge regarding the cellular and molecular mechanisms important for embryogenesis. Studies in numerous species have defined how distinct phases of NCC specification, proliferation, migration, and survival contribute to the formation of multiple functionally distinct organ systems. NCC contributions to the peripheral nervous system (PNS) are well known. Critical developmental processes have been defined that provide outstanding models for understanding how extracellular stimuli, cell-cell interactions, and transcriptional networks cooperate to direct cellular diversification and PNS morphogenesis. Dissecting the complex extracellular and intracellular mechanisms that mediate the formation of the PNS from NCCs may have important therapeutic implications for neurocristopathies, neuropathies, and certain forms of cancer.

Developmental regulation of trigeminal TRPA1 by the cornea

PURPOSE

The cornea is densely innervated with nociceptive nerves that detect deleterious stimuli at the ocular surface and transduce these stimuli as sensations of pain. Thus, nociception is a major factor involved in preventing damage to corneal tissues. One class of molecules that is thought to be involved in detecting such stimuli is the transient receptor potential (TRP) family of ion channels. However, little is known about the acquisition of these channels during corneal development. Therefore, the present study examined the developmental acquisition of these receptors and elucidated certain parameters involved in this acquisition.

METHODS

Quantitative RT-PCR was used to measure the expression of genes including TRPA and Ret in vivo. In vitro cocultures between cornea and the ophthalmic lobe of the trigeminal ganglion were used to test interactions between nerves and corneas along with recombinant proteins.

RESULTS

TRPA1 mRNA showed a progressive temporal increase in the ophthalmic lobe of the trigeminal ganglion in vivo during embryonic development. In vitro, TRPA1 expression was significantly increased in the ganglion when cocultured with cornea, compared to ganglia cultured alone. Similarly, the addition of exogenous neurotrophin-3 (NT3) protein to cultured ganglia increased the expression of TRPA1 more than 100-fold. Addition of NT3 and neurturin synergistically increased TRPA1 expression in embryonic day (E)8 ganglia, but this effect was lost at E12. At E8, Ret+ nonpeptidergic neurons are specified in the trigeminal ganglion.

CONCLUSIONS

Corneal-derived factors increase TRPA1 expression in trigeminal nonpeptidergic neurons during their embryonic specification.

Functional distinction between NGF-mediated plasticity and regeneration of nociceptive axons within the spinal cord

Successful regeneration after injury requires either the direct reformation of the circuit or the formation of a bridge circuit to provide partial functional return through a more indirect route. Presently, little is known about the specificity of how regenerating axons reconnect or reconstruct functional circuits. We have established an in vivo Dorsal root entry zone (DREZ) model, which in the presence of Nerve Growth Factor (NGF), shows very robust regeneration of peptidergic nociceptive axons, but not other sensory axons. Expression of NGF in normal, non-injured animals leads to robust sprouting of only the peptidergic nociceptive axons. Interestingly, NGF-induced sprouting of these axons leads to severe chronic pain, whereas, regeneration leads to protective-like pain without chronic pain. Using this model we set out to compare differences in behavioral outcomes and circuit features between these two groups. In this study, we examined pre-synaptic and post-synaptic markers to evaluate the relationship between synaptic connections and behavioral responses. NGF-induced sprouting of calcitonin gene-related peptide (CGRP) axons resulted in a significant redistribution of synapses and cFos expression into the deeper dorsal horn. Regeneration of only the CGRP axons showed a general reduction in synapses and cFos expression within laminae I and II; however, inflammation of the hindpaw induced peripheral sensitization. These data show that although NGF-induced sprouting of peptidergic axons induces robust chronic pain and cFos expression throughout the entire dorsal horn, regeneration of the same axons resulted in normal protective pain with a synaptic and cFos distribution similar, albeit significantly less than that shown by the sprouting of CGRP axons.

Nogo receptor homolog NgR2 expressed in sensory DRG neurons controls epidermal innervation by interaction with Versican

Primary sensory afferents of the dorsal root ganglion (DRG) that innervate the skin detect a wide range of stimuli, such as touch, temperature, pain, and itch. Different functional classes of nociceptors project their axons to distinct target zones within the developing skin, but the molecular mechanisms that regulate target innervation are less clear. Here we report that the Nogo66 receptor homolog NgR2 is essential for proper cutaneous innervation. NgR2(-/-) mice display increased density of nonpeptidergic nociceptors in the footpad and exhibit enhanced sensitivity to mechanical force and innocuous cold temperatures. These sensory deficits are not associated with any abnormality in morphology or density of DRG neurons. However, deletion of NgR2 renders nociceptive nonpeptidergic sensory neurons insensitive to the outgrowth repulsive activity of skin-derived Versican. Biochemical evidence shows that NgR2 specifically interacts with the G3 domain of Versican. The data suggest that Versican/NgR2 signaling at the dermo-epidermal junction acts in vivo as a local suppressor of axonal plasticity to control proper density of epidermal sensory fiber innervation. Our findings not only reveal the existence of a novel and unsuspected mechanism regulating epidermal target innervation, but also provide the first evidence for a physiological role of NgR2 in the peripheral nervous system.

Organization of sensory input to the nociceptive-specific cutaneous trunk muscle reflex in rat, an effective experimental system for examining nociception and plasticity

Detailed characterization of neural circuitries furthers our understanding of how nervous systems perform specific functions and allows the use of those systems to test hypotheses. We have characterized the sensory input to the cutaneous trunk muscle (CTM; also cutaneus trunci [rat] or cutaneus maximus [mouse]) reflex (CTMR), which manifests as a puckering of the dorsal thoracolumbar skin and is selectively driven by noxious stimuli. CTM electromyography and neurogram recordings in naïve rats revealed that CTMR responses were elicited by natural stimuli and electrical stimulation of all segments from C4 to L6, a much greater extent of segmental drive to the CTMR than previously described. Stimulation of some subcutaneous paraspinal tissue can also elicit this reflex. Using a selective neurotoxin, we also demonstrate differential drive of the CTMR by trkA-expressing and nonexpressing small-diameter afferents. These observations highlight aspects of the organization of the CTMR system that make it attractive for studies of nociception and anesthesiology and plasticity of primary afferents, motoneurons, and the propriospinal system. We use the CTMR system to demonstrate qualitatively and quantitatively that experimental pharmacological treatments can be compared with controls applied either to the contralateral side or to another segment, with the remaining segments providing controls for systemic or other treatment effects. These data indicate the potential for using the CTMR system as both an invasive and a noninvasive quantitative assessment tool providing improved statistical power and reduced animal use.

Developmental co-expression and functional redundancy of tyrosine phosphatases with neurotrophin receptors in developing sensory neurons

Receptor-type protein tyrosine phosphatases (RPTPs) have been implicated as direct or indirect regulators of neurotrophin receptors (TRKs). It remains less clear if and how such RPTPs might regulate TRK proteins in vivo during development. Here we present a comparative expression profile of RPTP genes and Trk genes during early stages of murine, dorsal root ganglion maturation. We find little if any specific, temporal mRNA co-regulation between individual RPTP and Ntrk genes between E12.5 and E14.5. Moreover, a double fluorescent in-situ hybridization and immunofluorescence study of seven Rptp genes with Ntrks revealed widespread co-expression of RPTPs in individual neurons, but no tight correlation with Trk expression profiles. No Rptp is expressed in 100% of Ntrk1-expressing neurons, whereas at least 6 RPTPs are expressed in 100% of Ntrk2- and Ntrk3-expressing neurons. An exception is Ptpro, which showed very selective expression. Short hairpin RNA suppression of Ptprf, Ptprs or Ptpro in primary, E13.5 DRG neurons did not alter TRK signalling. We therefore propose that TRK signalling may not be simply dependent on rate-limiting regulation by individual RPTP subtypes during sensory neuron development. Instead, TRK signalling has the potential to be buffered by concurrent inputs from several RPTPs in individual neurons.

Neurotrophin signalling and transcription programmes interactions in the development of somatosensory neurons

Somatosensory neurons of the dorsal root ganglia are generated from multipotent neural crest cells by a process of progressive specification and differentiation. Intrinsic transcription programmes active in somatosensory neuron progenitors and early post-mitotic neurons drive the cell-type expression of neurotrophin receptors. In turn, signalling by members of the neurotrophin family controls expression of transcription factors that regulate neuronal sub-type specification. This chapter explores the mechanisms by which this crosstalk between neurotrophin signalling and transcription programmes generates the diverse functional sub-types of somatosensory neurons found in the mature animal.

Target-derived neurotrophins coordinate transcription and transport of bclw to prevent axonal degeneration

Establishment of neuronal circuitry depends on both formation and refinement of neural connections. During this process, target-derived neurotrophins regulate both transcription and translation to enable selective axon survival or elimination. However, it is not known whether retrograde signaling pathways that control transcription are coordinated with neurotrophin-regulated actions that transpire in the axon. Here we report that target-derived neurotrophins coordinate transcription of the antiapoptotic gene bclw with transport of bclw mRNA to the axon, and thereby prevent axonal degeneration in rat and mouse sensory neurons. We show that neurotrophin stimulation of nerve terminals elicits new bclw transcripts that are immediately transported to the axons and translated into protein. Bclw interacts with Bax and suppresses the caspase6 apoptotic cascade that fosters axonal degeneration. The scope of bclw regulation at the levels of transcription, transport, and translation provides a mechanism whereby sustained neurotrophin stimulation can be integrated over time, so that axonal survival is restricted to neurons connected within a stable circuit.

Dental pulp and gingivomucosa in rats are innervated by two morphologically and neurochemically different populations of nociceptors

OBJECTIVES

Difference in phenotypes of sensory neurons innervating dental pulp or gingivomucosa may be responsible for intense pain sensations in pulpitis in contrast to relatively painless chronic periodontitis. Therefore, we classified these neurons according to their size and two neurochemical characteristics of nociceptors, their TrkA expression and isolectin IB4 binding.

DESIGN

In rats (n=6) fluorescent tracers Fluorogold and TrueBlue were simultaneously applied into the standard-sized tooth cavity and nearby gingival sulcus, respectively. After the fluorescence on paraffin trigeminal ganglia (TG) sections was identified and photographed, immunohistochemistry for TrkA expression and IB4 binding was performed on the same sections.

RESULTS

The average sizes of TG neurons projecting to the gingivomucosa and dental pulp were 894±441μm(2) and 1012±381μm(2), respectively. The proportions of small-sized gingival and pulpal neurons were 14% and 5%, respectively (p<0.05). The proportions of TrkA-positive neurons among all gingival or pulpal neurons were 76% and 86%, respectively (p<0.05). Among all gingival or pulpal neurons the proportions of IB4-positive neurons were 46% and 3% (p<0.001), respectively, and the majority of them were small-medium sized.

CONCLUSIONS

Dental pulp and gingivomucosa are richly innervated by nociceptive TrkA-expressing neurons. However, while great majority of pulpal neurons are larger NGF-dependent A-fibre nociceptors without affinity to bind IB4, almost half of the gingival neurons are smaller IB4 binding C-fibre nociceptors. The difference in phenotype of sensory neurons might partially explain the different sensitivity of both tissues during normal and pathological conditions.

Signals and switches in Mammalian neural crest cell differentiation

Neural crest cells (NCCs) comprise a multipotent, migratory cell population that generates a diverse array of cell and tissue types during vertebrate development. These include cartilage and bone, tendons, and connective tissue, as well as neurons, glia, melanocytes, and endocrine and adipose cells; this remarkable lineage potential persists into adult life. Taken together with a limited capacity for self-renewal, neural crest cells bear the hallmarks of stem and progenitor cells and are considered to be synonymous with vertebrate evolution. The neural crest has provided a system for exploring the mechanisms that govern developmental processes such as morphogenetic induction, cell migration, and fate determination. Today, much of the focus on neural crest cells revolves around their stem cell-like characteristics and potential for use in regenerative medicine. A thorough understanding of the signals and switches that govern mammalian neural crest patterning is central to potential therapeutic application of these cells and better appreciation of the role that neural crest cells play in vertebrate evolution, development, and disease.

Optimality and saturation in axonal chemotaxis

Chemotaxis (detecting and following chemical gradients) plays a crucial role in the function of many biological systems. In particular, gradient following by neuronal growth cones is important for the correct wiring of the nervous system. There is increasing interest in the constraints that determine how small chemotacting devices respond to gradients, but little quantitative information is available in this regard for neuronal growth cones. Mortimer et al. ( 2009 ) and Mortimer, Dayan, Burrage, and Goodhill ( 2011 ) proposed a Bayesian ideal observer model that predicts chemotactic performance for shallow gradients. Here we investigated two important aspects of this model. First, we found by numerical simulation that although the analytical predictions of the model assume shallow gradients, these predictions remain remarkably robust to large deviations in gradient steepness. Second, we found experimentally that the chemotactic response increased linearly with gradient steepness for very shallow gradients as predicted by the model; however, the response saturated for steeper gradients. This saturation could be reproduced in simulations of a growth rate modulation response mechanism. Together these results illuminate the domain of validity of the Bayesian model and give further insight into the biological mechanisms of axonal chemotaxis.

The Transcriptional Response of Neurotrophins and Their Tyrosine Kinase Receptors in Lumbar Sensorimotor Circuits to Spinal Cord Contusion is Affected by Injury Severity and Survival Time

Traumatic spinal cord injury (SCI) results in changes to the anatomical, neurochemical, and physiological properties of cells in the central and peripheral nervous system. Neurotrophins, acting by binding to their cognate Trk receptors on target cell membranes, contribute to modulation of anatomical, neurochemical, and physiological properties of neurons in sensorimotor circuits in both the intact and injured spinal cord. Neurotrophin signaling is associated with many post-SCI changes including maladaptive plasticity leading to pain and autonomic dysreflexia, but also therapeutic approaches such as training-induced locomotor improvement. Here we characterize expression of mRNA for neurotrophins and Trk receptors in lumbar dorsal root ganglia (DRG) and spinal cord after two different severities of mid-thoracic injury and at 6 and 12 weeks post-SCI. There was complex regulation that differed with tissue, injury severity, and survival time, including reversals of regulation between 6 and 12 weeks, and the data suggest that natural regulation of neurotrophins in the spinal cord may continue for months after birth. Our assessments determined that a coordination of gene expression emerged at the 12-week post-SCI time point and bioinformatic analyses address possible mechanisms. These data can inform studies meant to determine the role of the neurotrophin signaling system in post-SCI function and plasticity, and studies using this signaling system as a therapeutic approach.

Nav1.8 expression is not restricted to nociceptors in mouse peripheral nervous system

A vast diversity of salient cues is sensed by numerous classes of primary sensory neurons, defined by specific neuropeptides, ion channels, or cytoskeletal proteins. Recent evidence has demonstrated a correlation between the expression of some of these molecular markers and transmission of signals related to distinct sensory modalities (eg, heat, cold, pressure). Voltage-gated sodium channel Na(v)1.8 has been reported to be preferentially expressed in small-diameter unmyelinated sensory afferents specialized for the detection of noxious stimuli (nociceptors), and Na(v)1.8-Cre mice have been widely used to investigate gene function in nociceptors. However, the identity of neurons in which Cre-mediated recombination occurs in these animals has not been resolved, and whether expression of Na(v)1.8 in these neurons is dynamic during development is not known, rendering interpretation of conditional knockout mouse phenotypes problematic. Here, we used genetics, immunohistochemistry, electrophysiology, and calcium imaging to precisely characterize the expression of Na(v)1.8 in the peripheral nervous system. We demonstrate that 75% of dorsal root ganglion (DRG) neurons express Na(v)1.8-Cre, including >90% of neurons expressing markers of nociceptors and, unexpectedly, a large population (∼40%) of neurons with myelinated A fibers. Furthermore, analysis of DRG neurons' central and peripheral projections revealed that Na(v)1.8-Cre is not restricted to nociceptors but is also expressed by at least 2 types of low-threshold mechanoreceptors essential for touch sensation, including those with C and Aβ fibers. Our results indicate that Na(v)1.8 underlies electrical activity of sensory neurons subserving multiple functional modalities, and call for cautious interpretation of the phenotypes of Na(v)1.8-Cre-driven conditional knockout mice.

Specification of neural crest into sensory neuron and melanocyte lineages

Elucidating the mechanisms by which multipotent cells differentiate into distinct lineages is a common theme underlying developmental biology investigations. Progress has been made in understanding some of the essential factors and pathways involved in the specification of different lineages from the neural crest. These include gene regulatory networks involving transcription factor hierarchies and input from signaling pathways mediated from environmental cues. In this review, we examine the mechanisms for two lineages that are derived from the neural crest, peripheral sensory neurons and melanocytes. Insights into the specification of these cell types may reveal common themes in the specification processes that occur throughout development.

Molecular interactions underlying the specification of sensory neurons

Sensory neurons of the dorsal root ganglion (DRG) respond to many different kinds of stimulus. The ability to discriminate between the diverse types of sensation is reflected by the existence of functionally and morphologically specialized sensory neurons. This neuronal diversity is created in a step-wise process extending well into postnatal life. Here, we review the hierarchical organization and the molecular process involving interactions between environmental growth factors, used and reused in different developmental contexts in self-reinforcing and cross-inhibitory mechanisms, and intrinsic gene programs that underlie the progressive diversification of sensory progenitors into specialized neurons. The recent advance in knowledge of sensory neuron specification may provide mechanistic principles that could extend to other parts of the nervous system.

TRPV1 and TRPA1 function and modulation are target tissue dependent

The nerve growth factor (NGF) and glial cell line-derived neurotrophic factor (GDNF) families of growth factors regulate the sensitivity of sensory neurons. The ion channels transient receptor potential vanilloid 1 (TRPV1) and transient receptor potential channel, subfamily A, member 1 (TRPA1), are necessary for development of inflammatory hypersensitivity and are functionally potentiated by growth factors. We have shown previously that inflamed skin exhibits rapid increases in artemin mRNA with slower, smaller increases in NGF mRNA. Here, using mice, we show that, in inflamed colon, mRNA for both growth factors increased with a pattern distinct from that seen in skin. Differences were also seen in the pattern of TRPV1 and TRPA1 mRNA expression in DRG innervating inflamed skin and colon. Growth factors potentiated capsaicin (a specific TRPV1 agonist) and mustard oil (a specific TRPA1 agonist) behavioral responses in vivo, raising the question as to how these growth factors affect individual afferents. Because individual tissues are innervated by afferents with unique properties, we investigated modulation of TRPV1 and TRPA1 in identified afferents projecting to muscle, skin, and colon. Muscle and colon afferents are twice as likely as skin afferents to express functional TRPV1 and TRPA1. TRPV1 and TRPA1 responses were potentiated by growth factors in all afferent types, but compared with skin afferents, muscle afferents were twice as likely to exhibit NGF-induced potentiation and one-half as likely to exhibit artemin-induced potentiation of TRPV1. Furthermore, skin afferents showed no GDNF-induced potentiation of TRPA1, but 43% of muscle and 38% of colon afferents exhibited GDNF-induced potentiation. These results show that interpretation of afferent homeostatic mechanisms must incorporate properties that are specific to the target tissue.

Differential gene expression of neonatal and adult DRG neurons correlates with the differential sensitization of TRPV1 responses to nerve growth factor

Cultures of neonatal and adult dorsal root ganglion (DRG) neurons are commonly used in in vitro models to study the ion channels and signaling events associated with peripheral sensation under various conditions. Differential responsiveness between neonatal and adult DRG neurons to physiological or pathological stimuli suggests potential differences in their gene expression profiles. We performed a microarray analysis of cultured adult and neonatal rat DRG neurons, which revealed distinct gene expression profiles especially of ion channels and signaling molecules at the genomic level. For example, Ca(2+)-stimulated adenylyl cyclase (AC) isoforms AC3 and AC8, PKCδ and CaMKIIα, the voltage-gated sodium channel β1 and β4, and potassium channels K(v)1.1, K(v)3.2, K(v)4.1, K(v)9.1, K(v)9.3, K(ir)3.4, K(ir)7.1, K(2P)1.1/TWIK-1 had significantly higher mRNA expression in adult rat DRG neurons, while Ca(2+)-inhibited AC5 and AC6, sodium channel Na(v)1.3 α subunit, potassium channels K(ir)6.1, K(2P)10.1/TREK-2, calcium channel Ca(v)2.2 α1 subunit, and its auxiliary subunits β1 and β3 were conversely down regulated in adult neurons. Importantly, higher adult neuron expression of ERK1/2, PI3K/P110α, but not of TRPV1 and TrkA, was found and confirmed by PCR and western blot. These latter findings are consistent with the key role of ERK and PI3K signaling in sensitization of TRPV1 by NGF and may explain our previously published observation that adult, but not neonatal, rat DRG neurons are sensitized by NGF.

Neonatal tissue damage facilitates nociceptive synaptic input to the developing superficial dorsal horn via NGF-dependent mechanisms

Tissue injury during a critical period of early life can facilitate spontaneous glutamatergic transmission within developing pain circuits in the superficial dorsal horn (SDH) of the spinal cord. However, the extent to which neonatal tissue damage strengthens nociceptive synaptic input to specific subpopulations of SDH neurons, as well as the mechanisms underlying this distinct form of synaptic plasticity, remains unclear. Here we use in vitro whole-cell patch clamp recordings from rodent spinal cord slices to demonstrate that neonatal surgical injury selectively potentiates high-threshold primary afferent input to immature lamina II neurons. In addition, the increase in the frequency of miniature excitatory postsynaptic currents after hindpaw incision was prevented by neonatal capsaicin treatment, suggesting that early tissue injury enhances glutamate release from nociceptive synapses. This occurs in a widespread manner within the developing SDH, as incision elevated miniature excitatory postsynaptic current frequency in both GABAergic and presumed glutamatergic lamina II neurons of Gad-GFP transgenic mice. The administration of exogenous nerve growth factor into the rat hindpaw mimicked the effects of early tissue damage on excitatory synaptic function, while blocking trkA receptors in vivo abolished the changes in both spontaneous and primary afferent-evoked glutamatergic transmission following incision. These findings illustrate that neonatal tissue damage can alter the gain of developing pain pathways by activating nerve growth factor-dependent signaling cascades, which modify synaptic efficacy at the first site of nociceptive processing within the central nervous system.

Schwann cell p75NTR prevents spontaneous sensory reinnervation of the adult spinal cord

Schwann cells are attractive candidates for repair of the injured spinal cord. Transplanted Schwann cells are permissive to regeneration, but their ability to promote regeneration into distal spinal cord remains weak despite their production of growth-promoting neurotrophins. Schwann cell activation such as that which accompanies peripheral nerve injury results in massive upregulation of the p75(NTR) pan-neurotrophin-receptor. Here we test the hypothesis that this p75(NTR) upregulation following dorsal root injury limits availability of endogenous neurotrophin to axons and restricts regeneration of injured axons into the spinal cord. We injured dorsal roots (fourth cervical to second thoracic) in mice lacking the neurotrophin-binding domain of p75(NTR) and in wild-type littermates. Axonal regeneration was assessed by selective tracing of neurotrophin-responsive and non-responsive dorsal root ganglion neurons. Functional reinnervation of the spinal cord was assessed in behavioural experiments and via Fos immunohistochemistry following formalin injection into the forepaw. We also measured levels of nerve growth factor and neurotrophin-3 following nerve injury in knockout and wild-type mice, and used Trk-Fc receptor chimeras to block nerve growth factor and neurotrophin-3 signalling in dorsal root ganglion/Schwann cell co-cultures and following dorsal root injury in vivo. The roles of neuronal and glial p75(NTR) were assessed in transplant experiments in vivo and in co-cultures. We found that nerve growth factor and neurotrophin-3-responsive axons regenerated into the spinal cord of p75(NTR) knockout mice where they made functional connections with dorsal horn neurons. Despite equivalent levels of nerve growth factor and neurotrophin-3 in wild-type and knockout mice, successful regeneration in knockouts was neurotrophin-dependent. Transplantation of p75(-/-) neurons into a wild-type environment, p75(-/-) peripheral nerve grafts into the injured p75(+/+) spinal cord, and dissociated sensory neuron/Schwann cell co-cultures showed that the absence of p75(NTR) from glia, not from neurons, promotes regeneration. These findings indicate that Schwann cell p75(NTR) restricts neurotrophin availability to the extent that it prevents spontaneous sensory axon regeneration into the spinal cord. The implication is that inactivating p75(NTR) in Schwann (or olfactory ensheathing) cells may enable axons to grow beyond transplants, improving the outcome of spinal cord injury.

Nociceptor subpopulations involved in hyperalgesic priming

We have previously developed a model in the rat for the transition from acute to chronic pain, hyperalgesic priming, in which a long-lasting neuroplastic change in signaling pathways mediates a prolongation of proinflammatory cytokine-induced nociceptor sensitization and mechanical hyperalgesia, induced at the site of a previous inflammatory insult. Induction of priming is mediated by activation of protein kinase C(epsilon) (PKC(epsilon)) in the peripheral terminal of the primary afferent nociceptor. Given that hyperalgesic mediator-induced PKC(epsilon) translocation occurs in isolectin B4 (IB4)(+)-nonpeptidergic but not in receptor tyrosine kinase (TrkA)(+)-peptidergic nociceptors, we tested the hypothesis that hyperalgesic priming was restricted to the IB4(+) subpopulation of nociceptors. After recovery from nerve growth factor (NGF)- and GDNF-induced hyperalgesia, a proinflammatory cytokine, prostaglandin E(2) (PGE(2)) induced, PKC(epsilon)-dependent, markedly prolonged hyperalgesia, two features that define the development of the primed state. Thus, hyperalgesic priming occurs in both the IB4(+)-nonpeptidergic and TrkA(+)-peptidergic subpopulations of nociceptive afferents. Of note, however, while attenuation of PKC(epsilon) prevented NGF-induced priming, the hyperalgesia induced by NGF is PKC(epsilon) independent. We propose that separate intracellular pools of PKC(epsilon), in the peripheral terminals of nociceptors, mediate nociceptor sensitization and the induction of hyperalgesic priming.

Dynamic expression of the TRPM subgroup of ion channels in developing mouse sensory neurons

Despite the significance of transient receptor potential (TRP) channels in sensory physiology, little is known of the expression and developmental regulation of the TRPM (melastatin) subgroup in sensory neurons. In order to find out if the eight TRPM subgroup members (TRPM1-TRPM8) have a possible role in the sensory nervous system, we characterized the developmental regulation of their expression in mouse dorsal root ganglion (DRG) from embryonic (E) day 12 to adulthood. Transcripts for all channels except for TRPM1 were detected in lumbar and thoracic DRG and in nodose ganglion (NG) with distinguishable expression patterns from E12 until adult. For most channels, the expression increased from E14 to adult with the exception of TRPM5, which displayed transient high levels during embryonic and early postnatal stages. Cellular localization of TRPM8 mRNA was found only in a limited subset of very small diameter neurons distinct in size from other populations. These neurons did not bind isolectin B4 (IB4) and expressed neither the neuropeptide calcitonin gene-related peptide (CGRP) nor neurofilament (NF)200. This suggests that TRPM8(+) thermoreceptive sensory neurons fall into a separate group of very small sized neurons distinct from peptidergic and IB4(+) subtypes of sensory neurons. Our results, showing the expression and dynamic regulation of TRPM channels during development, indicate that many TRPM subfamily members could participate during nervous system development and in the adult by determining distinct physiological properties of sensory neurons.

ALK (Anaplastic Lymphoma Kinase) expression in DRG neurons and its involvement in neuron-Schwann cells interaction

Anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase (RTK) transiently expressed in specific regions of the central and peripheral nervous systems. In this study, we focused on the rat developing dorsal root ganglion (DRG). This ganglion is composed of heterogeneous sensory neurons characterized by the expression of RTK for neurotrophic factors, such as the nerve growth factor receptor TrkA or the glial-derived neurotrophic factor family receptor Ret, which are specifically detected in nociceptive neurons. In DRG, ALK expression reached a maximum around birth. We showed that ALK is specifically present in a subtype of neurons during DRG development, and that the majority of these neurons co-expressed TrkA and Ret. Interestingly, we identified only one form (220 kDa) of ALK in DRG neurons both in vivo and in vitro. On the opposite, in transfected cells as well as in brain extracts, ALK was identified as two forms (220 and 140 kDa). The DRG is composed of neurons and glial cells, principally satellite Schwann cells. Thus, we hypothesized that the presence of satellite Schwann cells was involved in the absence of truncated ALK. Using two different cell types, HEK293 cells stably expressing ALK, and MSC80 cells, a previously described Schwann cell line, we showed that a factor secreted by the Schwann cells is likely involved in the absence of ALK cleavage. All these data hence open new perspectives concerning the role of ALK in the specification of nociceptive DRG neurons and in the neurons-Schwann cells interaction.

Distinct subsets of unmyelinated primary sensory fibers mediate behavioral responses to noxious thermal and mechanical stimuli

Behavioral responses to painful stimuli require peripheral sensory neurons called nociceptors. Electrophysiological studies show that most C-fiber nociceptors are polymodal (i.e., respond to multiple noxious stimulus modalities, such as mechanical and thermal); nevertheless, these stimuli are perceived as distinct. Therefore, it is believed that discrimination among these modalities only occurs at spinal or supraspinal levels of processing. Here, we provide evidence to the contrary. Genetic ablation in adulthood of unmyelinated sensory neurons expressing the G protein-coupled receptor Mrgprd reduces behavioral sensitivity to noxious mechanical stimuli but not to heat or cold stimuli. Conversely, pharmacological ablation of the central branches of TRPV1(+) nociceptors, which constitute a nonoverlapping population, selectively abolishes noxious heat pain sensitivity. Combined elimination of both populations yielded an additive phenotype with no additional behavioral deficits, ruling out a redundant contribution of these populations to heat and mechanical pain sensitivity. This double-dissociation suggests that the brain can distinguish different noxious stimulus modalities from the earliest stages of sensory processing.

BDNF exerts contrasting effects on peripheral myelination of NGF-dependent and BDNF-dependent DRG neurons

Although brain-derived neurotrophic factor (BDNF) has been shown to promote peripheral myelination during development and remyelination after injury, the precise mechanisms mediating this effect remain unknown. Here, we determine that BDNF promotes myelination of nerve growth factor-dependent neurons, an effect dependent on neuronal expression of the p75 neurotrophin receptor, whereas BDNF inhibits myelination of BDNF-dependent neurons via the full-length TrkB receptor. Thus, BDNF exerts contrasting effects on Schwann cell myelination, depending on the complement of BDNF receptors that are expressed by different subpopulations of dorsal root ganglion neurons. These results demonstrate that BDNF exerts contrasting modulatory roles in peripheral nervous system myelination, and that its mechanism of action is acutely regulated and specifically targeted to neurons.

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

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