Evidence that endogenous beta nerve growth factor is responsible for the collateral sprouting, but not the regeneration, of nociceptive axons in adult rats

Neuroanatomy of the Cutaneous Nervous System Regarding Wound Healing

Wound healing is an important topic in modern medicine across many disciplines. Healing of all cutaneous wounds, whether accidentally sustained or intentionally created, requires the common yet complex set of interactions between the immune, circulatory, nervous, endocrine, and integumentary systems. Deficits in any of these systems or the molecular factors that mediate their communications can contribute to impaired healing of cutaneous wounds. While the stages of wound repair, angiogenesis, growth factors, and cytokines involved have been extensively studied, the role of the cutaneous nervous system in wound healing has not been well outlined. We have provided a basic overview of cutaneous innervation and wound repair for the dermatologic surgeon by outlining the normal cutaneous nervous anatomy and function and discussing the most important neuropeptides that mediate the wound healing process.

Vascular smooth muscle cell-derived nerve growth factor regulates sympathetic collateral branching to innervate blood vessels in embryonic skin

Blood vessels serve as intermediate conduits for the extension of sympathetic axons towards target tissues, while also acting as crucial targets for their homeostatic processes encompassing the regulation of temperature, blood pressure, and oxygen availability. How sympathetic axons innervate not only blood vessels but also a wide array of target tissues is not clear. Here we show that in embryonic skin, after the establishment of co-branching between sensory nerves and blood vessels, sympathetic axons invade the skin alongside these sensory nerves and extend their branches towards these blood vessels covered by vascular smooth muscle cells (VSMCs). Our mosaic labeling technique for sympathetic axons shows that collateral branching predominantly mediates the innervation of VSMC-covered blood vessels by sympathetic axons. The expression of nerve growth factor (NGF), previously known to induce collateral axon branching in culture, can be detected in the vascular smooth muscle cell (VSMC)-covered blood vessels, as well as sensory nerves. Indeed, VSMC-specific Ngf knockout leads to a significant decrease of collateral branching of sympathetic axons innervating VSMC-covered blood vessels. These data suggest that VSMC-derived NGF serves as an inductive signal for collateral branching of sympathetic axons innervating blood vessels in the embryonic skin.

Skin Reinnervation by Collateral Sprouting Following Spared Nerve Injury in Mice

Following peripheral nerve injury, denervated tissues can be reinnervated via regeneration of injured neurons or collateral sprouting of neighboring uninjured afferents into denervated territory. While there has been substantial focus on mechanisms underlying regeneration, collateral sprouting has received less attention. Here, we used immunohistochemistry and genetic neuronal labeling to define the subtype specificity of sprouting-mediated reinnervation of plantar hindpaw skin in the mouse spared nerve injury (SNI) model, in which productive regeneration cannot occur. Following initial loss of cutaneous afferents in the tibial nerve territory, we observed progressive centripetal reinnervation by multiple subtypes of neighboring uninjured fibers into denervated glabrous and hairy plantar skin of male mice. In addition to dermal reinnervation, CGRP-expressing peptidergic fibers slowly but continuously repopulated denervated epidermis, Interestingly, GFRα2-expressing nonpeptidergic fibers exhibited a transient burst of epidermal reinnervation, followed by a trend towards regression. Presumptive sympathetic nerve fibers also sprouted into denervated territory, as did a population of myelinated TrkC lineage fibers, though the latter did so inefficiently. Conversely, rapidly adapting Aβ fiber and C fiber low threshold mechanoreceptor (LTMR) subtypes failed to exhibit convincing sprouting up to 8 weeks after nerve injury in males or females. Optogenetics and behavioral assays in male mice further demonstrated the functionality of collaterally sprouted fibers in hairy plantar skin with restoration of punctate mechanosensation without hypersensitivity. Our findings advance understanding of differential collateral sprouting among sensory neuron subpopulations and may guide strategies to promote the progression of sensory recovery or limit maladaptive sensory phenomena after peripheral nerve injury.

SKIN REINNERVATION BY COLLATERAL SPROUTING FOLLOWING SPARED NERVE INJURY IN MICE

Following peripheral nerve injury, denervated tissues can be reinnervated via regeneration of injured neurons or via collateral sprouting of neighboring uninjured afferents into the denervated territory. While there has been substantial focus on mechanisms underlying regeneration, collateral sprouting has received relatively less attention. In this study, we used immunohistochemistry and genetic neuronal labeling to define the subtype specificity of sprouting-mediated reinnervation of plantar hind paw skin in the mouse spared nerve injury (SNI) model, in which productive regeneration cannot occur. Following an initial loss of cutaneous afferents in the tibial nerve territory, we observed progressive centripetal reinnervation by multiple subtypes of neighboring uninjured fibers into denervated glabrous and hairy plantar skin. In addition to dermal reinnervation, CGRP-expressing peptidergic fibers slowly but continuously repopulated the denervated epidermis, Interestingly, GFRα2-expressing nonpeptidergic fibers exhibited a transient burst of epidermal reinnervation, followed by trend towards regression. Presumptive sympathetic nerve fibers also sprouted into the denervated territory, as did a population of myelinated TrkC lineage fibers, though the latter did so less efficiently. Conversely, rapidly adapting Aβ fiber and C fiber low threshold mechanoreceptor (LTMR) subtypes failed to exhibit convincing collateral sprouting up to 8 weeks after nerve injury. Optogenetics and behavioral assays further demonstrated the functionality of collaterally sprouted fibers in hairy plantar skin with restoration of punctate mechanosensation without hypersensitivity. Our findings advance understanding of differential collateral sprouting among sensory neuron subpopulations and may guide strategies to promote the progression of sensory recovery or limit maladaptive sensory phenomena after peripheral nerve injury.

Significance Statement

Following nerve injury, whereas one mechanism for tissue reinnervation is regeneration of injured neurons, another, less well studied mechanism is collateral sprouting of nearby uninjured neurons. In this study, we examined collateral sprouting in denervated mouse skin and showed that it involves some, but not all neuronal subtypes. Despite such heterogeneity, a significant degree of restoration of punctate mechanical sensitivity is achieved. These findings highlight the diversity of collateral sprouting among peripheral neuron subtypes and reveal important differences between pre- and post-denervation skin that might be appealing targets for therapeutic correction to enhance functional recovery from denervation and prevent unwanted sensory phenomena such as pain or numbness.

Standardized Quantitative Sensory Testing to Assess Insufficient Recovery of Touch Discrimination in Free Flap Surgery

BACKGROUND

With major advances in microsurgical techniques, free tissue transfer has become a widely adopted approach to treat complex soft-tissue defects. However, sensory recovery is poor, leaving the anesthetic skin prone to injuries.

METHODS

Twenty-eight patients with 22 anterior lateral thigh flaps and six latissimus dorsi flaps on their extremities participated in the study. Quantitative sensory testing and two-point discrimination was performed in three test areas and one control on the contralateral unaffected extremity. Physical disability, mental health, quality of life, and characteristics of pain were assessed by the painDetect, Disabilities of the Arm, Shoulder, and Hand, Lower Extremity Functional Scale, and 12-Item Short Form questionnaires, respectively.

RESULTS

Somatosensory profiles of all flaps were characterized by an overall loss of nerve function. Small-fiber function was mostly recovered, whereas large-fiber function, and thus touch discrimination, was severely impaired. Mechanical detection thresholds improved over time and from center to the periphery. Reported pain was mild to moderate and correlated with decreased physical function.

CONCLUSIONS

Standardized quantitative sensory testing provides a useful tool kit to assess the sensory regeneration after surgical treatment of soft-tissue defects. After free tissue transfer, small-fiber function recovers with nerve ingrowth in a centripetal direction from the flap margins to the center, likely by way of collateral axonal sprouting from the undamaged nerves surrounding the flap. Myelinated fibers recover slowly and inefficiently.

CLINICAL QUESTION/LEVEL OF EVIDENCE

Therapeutic, IV.

Trigeminal Sensory Supply Is Essential for Motor Recovery after Facial Nerve Injury

Recovery of mimic function after facial nerve transection is poor. The successful regrowth of regenerating motor nerve fibers to reinnervate their targets is compromised by (i) poor axonal navigation and excessive collateral branching, (ii) abnormal exchange of nerve impulses between adjacent regrowing axons, namely axonal crosstalk, and (iii) insufficient synaptic input to the axotomized facial motoneurons. As a result, axotomized motoneurons become hyperexcitable but unable to discharge. We review our findings, which have addressed the poor return of mimic function after facial nerve injuries, by testing the hypothesized detrimental component, and we propose that intensifying the trigeminal sensory input to axotomized and electrophysiologically silent facial motoneurons improves the specificity of the reinnervation of appropriate targets. We compared behavioral, functional, and morphological parameters after single reconstructive surgery of the facial nerve (or its buccal branch) with those obtained after identical facial nerve surgery, but combined with direct or indirect stimulation of the ipsilateral infraorbital nerve. We found that both methods of trigeminal sensory stimulation, i.e., stimulation of the vibrissal hairs and manual stimulation of the whisker pad, were beneficial for the outcome through improvement of the quality of target reinnervation and recovery of vibrissal motor performance.

Metformin Promotes Axonal Regeneration and Functional Recovery in Diabetic Rat Model of Sciatic Nerve Transection Injury

In our previous study, metformin was able to promote nerve regeneration after sciatic nerve crushing in rats under diabetic conditions. However, a crush injury also has a strong ability to spontaneously recover. Therefore, in our present study, a model of transection injury of the sciatic nerve in diabetic rats was utilized to detect whether metformin could still promote nerve regeneration. Diabetes was induced via an injection of 50 mg/kg of streptozotocin in rats. After transection injury of the sciatic nerve, the rats were randomly divided into a high-dose metformin group (500 mg/kg/d), mid-dose metformin group (200 mg/kg/d), low-dose metformin group (30 mg/kg/d) and control group (normal saline). The metformin or normal saline was intraperitoneally injected for 4 weeks. Then, behavioral, electrophysiological and morphometric analyses were performed. The results showed that metformin could significantly promote functional restoration and axonal regeneration of the sciatic nerve after transection injury under diabetic conditions. Furthermore, high doses and middle doses of metformin presented more of this ability than a low dose of metformin. In conclusion, metformin is able to accelerate sciatic nerve repair after transection injury under diabetic conditions, showing the therapeutic potential of metformin in the management of nerve injuries during diabetes mellitus.

In vivo confocal microscopic study of corneal innervation in Sjögren's Syndrome with or without small fiber neuropathy

PURPOSE

To study changes in the subbasal nerve plexus by In vivo confocal microscopy (IVCM) in Sjögren's Syndrome (SS) with or without associated Small Fiber Neuropathy (SFN), in order to prevent diagnostic delay.

METHODS

Seventy-one patients with SS, including 19 with associated SFN, 20 healthy volunteers and 20 patients with Meibomian gland dysfunction (MGD) were included in this retrospective case-control study. IVCM was used to investigate subbasal nerve plexus density and morphology.

RESULTS

Corneal sensitivity as evaluated with the Cochet-Bonnet aesthesiometer was significantly reduced in the SS group versus the control group (P = 0.026) and the MGD group (P = 0.037). The number of inflammatory cells was significantly increased in the SS group to 86.2 ± 82.1 cells/mm² compared to the control group (P < 0.001). The density of the subbasal nerve plexus was significantly reduced to 16.7 ± 6.5 mm/mm² in the SS group compared to the control group (P < 0.005) and the MGD group (P = 0.042). The tortuosity of the nerves in the SS group was significantly increased compared to the control group (P < 0.001) and the MGD group (P = 0.025). The average number of subbasal nerve plexus neuromas was significantly increased in the SS group compared to the control group (P = 0.001), with a significant increase in the average number of neuromas in SS patients with associated SFN compared to SS patients without SFN (P = 0.008).

CONCLUSION

IVCM can be useful to detect corneal nerve changes in SS patients and may allow earlier diagnosis of the disease and to consider new therapeutic approaches.

Sex-Related Pain Behavioral Differences following Unilateral NGF Injections in a Rat Model of Low Back Pain

Low back pain (LBP) is a globally prevalent and costly societal problem with multifactorial etiologies and incompletely understood pathophysiological mechanisms. To address such shortcomings regarding the role of neurotrophins in the underlying mechanisms of pain, an LBP model was developed in rats involving two unilateral intramuscular injections of nerve growth factor (NGF) into deep trunk muscles. To date, behavioral investigations of this NGF-LBP model have been limited, especially as it pertains to female pain behaviors. This study compared mechanical sensitivity to noxious (hyperalgesia) and non-noxious (hypersensitivity) stimuli in control and NGF-injected male and female rats through pain resolution. Although the baseline testing revealed no differences between males and females, NGF-injected females demonstrated prolonged ipsilateral deep trunk mechanical hyperalgesia that resolved seven days later than males. Moreover, females showed bilateral trunk mechanical sensitivity to noxious and non-noxious stimuli compared to only ipsilateral behaviors in males. Sex differences were also observed in the severity of behavioral responses, with females displaying greater mean differences from baseline at several timepoints. Overall, these NGF-LBP behavioral findings mirror some of the sex differences reported in the clinical presentation of LBP and accentuate the translatability of this NGF-LBP model. Future studies using this LBP-NGF model could help to elucidate the neurobiological mechanisms responsible for the development, severity, and/or resolution of muscular LBP as well as to provide insights into the processes governing the transition from acute to chronic LBP.

Perineural Capsaicin Treatment Inhibits Collateral Sprouting of Intact Cutaneous Nociceptive Afferents

Perineural treatment of peripheral nerves with capsaicin produces a long-lasting selective regional thermo- and chemo-analgesia and elimination of the neurogenic inflammatory response involving degeneration of nociceptive afferent fibers. In this study, we examined longitudinal changes in mustard oil–induced sensory neurogenic vasodilatation and plasma extravasation following perineural capsaicin treatment of the rat saphenous nerve utilizing scanning laser Doppler imaging and vascular labeling with colloidal silver. Capsaicin treatment resulted in a marked decrease in mustard oil–induced vasodilatation in the skin area served by the saphenous nerve. Repeated imaging of the vasodilatatory response showed no recovery for at least 7 weeks. However, following transection and ligation of the capsaicin-treated saphenous nerve, a substantial recovery of the vasodilatatory response was observed, suggesting a reinnervation of the chemodenervated skin area by collateral sprouting of neighboring intact sciatic nerve afferents. Elimination of the recovered vascular reaction by capsaicin treatment of the sciatic nerve supported this conclusion. Similar results have been obtained by using the vascular labeling technique. These findings indicate an inhibitory effect of persisting cutaneous nerve fibers on the collateral sprouting of intact nerve fibers into the chemodenervated skin area. These observations may bear implications for the development of sensory disturbances following peripheral nerve injuries.

Sensory Reinnervation With Subcutaneously Embedded Innervated Flaps: An Experimental Study in Rats

BACKGROUND

In breast reconstruction, both aesthetic outcomes and sensory function are important for postoperative quality of life. Innervated flaps are useful in reconstruction after conventional mastectomy (CM), which leaves a large portion of the skin paddle exposed on the body surface. However, whether they are also useful in skin-sparing mastectomy (SSM) and nipple-sparing mastectomy (NSM) remains unclear. This study aimed to examine the usefulness of innervated flaps in restoring sensation after SSM and NSM using a rat model.

METHODS

Dorsal cutaneous nerves of rats were entirely eliminated except for the medial branch of the dorsal cutaneous nerve of thoracic segment 13, resulting in an innervated field surrounded by a denervated field. The innervated field was elevated as an innervated island flap and then subcutaneously embedded, with the skin paddle deepithelialized entirely (NSM group, n = 5) or except at the center (SSM group, n = 6). In the control model (CM group, n = 5), the flap was sutured back into its original position. Postoperative changes in the mechanonociceptive field were evaluated using the cutaneous trunci muscle reflex test. Immunohistochemical evaluation of regenerated nerves in the new mechanonociceptive field was performed at postoperative week 12.

RESULTS

In the SSM and CM groups, the mechanonociceptive field expanded around the skin paddle. In the NSM group, a new mechanonociceptive field appeared at postoperative week 4 and expanded thereafter. Areas of the mechanonociceptive field at postoperative week 12 did not differ significantly between the SSM and NSM groups, but were significantly smaller compared with the CM group and comparable to original flap areas. Histologically, S100- and PGP9.5-positive nerve fibers were observed in the dermis of the new mechanonociceptive field and subcutaneous flap tissue.

CONCLUSIONS

Subcutaneously embedded innervated island flaps induced nerve regeneration and sensory reinnervation of the denervated skin, suggesting that innervated flaps may also be useful in reconstruction post-SSM/NSM.

Cytoskeleton and Membrane Organization at Axon Branches

Axon branching is a critical process ensuring a high degree of interconnectivity for neural network formation. As branching occurs at sites distant from the soma, it is necessary that axons have a local system to dynamically control and regulate axonal growth. This machinery depends on the orchestration of cellular functions such as cytoskeleton, subcellular transport, energy production, protein- and membrane synthesis that are adapted for branch formation. Compared to the axon shaft, branching sites show a distinct and dynamic arrangement of cytoskeleton components, endoplasmic reticulum and mitochondria. This review discusses the regulation of axon branching in the context of cytoskeleton and membrane remodeling.

Targeted sensory reinnervation by direct neurotization of skin: An experimental study in rats

BACKGROUND

No effective methods currently exist for breast neurotization in implant-based breast reconstruction. Here, we focused on direct neurotization (DN), in which axons regenerating from nerve stumps are directed to the mastectomy flap and aimed to assess whether DN can generate a new mechano-nociceptive field using a rat model of back skin sensory denervation.

METHODS

Dorsal cutaneous nerves (DCNs) of rats were exposed and transected, leaving only the left medial branch of the DCN of thoracic segment 13 (mDCN-T13) intact. This procedure resulted in an isolated innervated field surrounded by a denervated field. The mDCN-T13 was transected, and the proximal nerve stump was sutured to the subdermis (DN subdermal group, n = 6) or dermis (DN dermal group, n = 5) of a different region of the denervated field. In the Crush group (n = 5), the intact mDCN-T13 was only crushed. We evaluated the generation of a new mechano-nociceptive field over time using the cutaneous trunci muscle (CTM) reflex test and histomorphometrically evaluated regenerating nerves in the reinnervated region.

RESULTS

In the DN groups, the CTM reflex appeared in the DN area after postoperative week 4. The new mechano-nociceptive field gradually expanded afterwards, and by postoperative week 12, the area was substantially larger than the original region innervated by the mDCN-T13 in the DN dermal group, although not as large as that in the Crush group. In histomorphometric evaluations, many S100-positive myelinated fibers were observed in the dermis of the reinnervated area for all groups.

CONCLUSION

In targeted sensory reinnervation, DN of the skin is revolutionary in that it allows a new innervated area to be generated at a desired location regardless of whether a distal nerve stump is available. DN may present an effective approach for breast neurotization in breast reconstruction after mastectomy, particularly for procedures that cannot use sensate flaps such as implant-based breast reconstruction.

Systemic hypoxia mimicry enhances axonal regeneration and functional recovery following peripheral nerve injury

Despite the ability of peripheral nerves to regenerate after injury, failure occurs due to an inability of supporting cells to maintain growth, resulting in long-term consequences such as sensorimotor dysfunction and neuropathic pain. Here, we investigate the potential of engaging the cellular adaptive response to hypoxia, via inhibiting its negative regulators, to enhance the regenerative process. Under normoxic conditions, prolyl hydroxylase domain (PHD) proteins 1, 2, and 3 hydroxylate the key metabolic regulator hypoxia inducible factor 1α (HIF1α), marking it for subsequent proteasomal degradation. We inhibited PHD protein function systemically via either individual genetic deletion or pharmacological pan-PHD inhibition using dimethyloxalylglycine (DMOG). We show enhanced axonal regeneration after sciatic nerve crush injury in PHD1^-/- mice, PHD3^-/- mice, and in DMOG-treated mice, and in PHD1^-/- and DMOG-treated mice a reduction in hypersensitivity to cooling after permanent sciatic ligation. Electromyographically, PHD1^-/- and PHD3^-/- mice showed an increased CMAP amplitude one-month post-injury, probably due to protection against denervation induced muscle atrophy, while DMOG-treated and PHD2^+/- mice showed reduced latencies, indicating improved motor axon function. DMOG treatment did not affect the growth of dorsal root ganglion neurites in vitro, suggesting a lack of direct effects of DMOG on axonal regrowth. Enhanced regeneration in vivo was concurrent with an increase in macrophage density, and a shift in macrophage polarization state ratios (from M1-like toward M2-like) in DMOG-treated animals. These results indicate PHD proteins as a novel therapeutic target to improve regenerative and functional outcomes after peripheral nerve injury without manipulating molecular O₂.

Longitudinal Study of Functional Reinnervation of the Denervated Skin by Collateral Sprouting of Peptidergic Nociceptive Nerves Utilizing Laser Doppler Imaging

Restitution of cutaneous sensory function is accomplished by neural regenerative processes of distinct mechanisms following peripheral nerve lesions. Although methods available for the study of functional cutaneous nerve regeneration are specific and accurate, they are unsuitable for the longitudinal follow-up of the temporal and spatial aspects of the reinnervation process. Therefore, the aim of this study was to develop a new, non-invasive approach for the longitudinal examination of cutaneous nerve regeneration utilizing the determination of changes in the sensory neurogenic vasodilatatory response, a salient feature of calcitonin gene-related peptide-containing nociceptive afferent nerves, with scanning laser Doppler flowmetry. Scanning laser Doppler imaging was applied to measure the intensity and spatial extent of sensory neurogenic vasodilatation elicited by the application of mustard oil onto the dorsal skin of the rat hindpaw. Mustard oil induced reproducible and uniform increases in skin perfusion reaching maximum values at 2-4 min after application whereafter the blood flow gradually returned to control level after about 8-10 min. Transection and ligation of the saphenous nerve largely eliminated the vasodilatatory response in the medial aspect of the dorsal skin of the hindpaw. In the 2 nd to 4 th weeks after injury, the mustard oil-induced vasodilatatory reaction gradually recovered. Since regeneration of the saphenous nerve was prevented, the recovery of the vasodilatatory response may be accounted for by the collateral sprouting of neighboring intact sciatic afferent nerve fibers. This was supported by the elimination of the vasodilatatory response in both the saphenous and sciatic innervation territories following local treatment of the sciatic nerve with capsaicin to defunctionalize nociceptive afferent fibers. The present findings demonstrate that this novel technique utilizing scanning laser Doppler flowmetry to quantitatively measure cutaneous sensory neurogenic vasodilatation, a vascular response mediated by peptidergic nociceptive nerves, is a reliable non-invasive approach for the longitudinal study of nerve regeneration in the skin.

Neurotrophins induce fission of mitochondria along embryonic sensory axons

Neurotrophins are growth factors that have a multitude of roles in the nervous system. We report that neurotrophins induce the fission of mitochondria along embryonic chick sensory axons driven by combined PI3K and Mek-Erk signaling. Following an initial burst of fission, a new steady state of neurotrophin-dependent mitochondria length is established. Mek-Erk controls the activity of the fission mediator Drp1 GTPase, while PI3K may contribute to the actin-dependent aspect of fission. Drp1-mediated fission is required for nerve growth factor (NGF)-induced collateral branching in vitro and expression of dominant negative Drp1 impairs the branching of axons in the developing spinal cord in vivo. Fission is also required for NGF-induced mitochondria-dependent intra-axonal translation of the actin regulatory protein cortactin, a previously determined component of NGF-induced branching. Collectively, these observations unveil a novel biological function of neurotrophins; the regulation of mitochondrial fission and steady state mitochondrial length and density in axons.

Facial Sensory Restoration After Trigeminal Sensory Rhizotomy by Collateral Sprouting From the Occipital Nerves

BACKGROUND AND IMPORTANCE

Lesioning procedures are effective for trigeminal neuralgia (TN), but late pain recurrence associated with sensory recovery is common. We report a case of recurrence of type 1A TN and recovery of facial sensory function after trigeminal rhizotomy associated with collateral sprouting from upper cervical spinal nerves.

CLINICAL PRESENTATION

A 41-yr-old woman presented 2 yr after open left trigeminal sensory rhizotomy for TN with pain-free anesthesia in the entire left trigeminal nerve distribution. Over 18 mo, she developed gradual recovery of facial sensation migrating anteromedially from the occipital region, eventually extending to the midpupillary line across the distribution of all trigeminal nerve branches. She reported recurrence of her triggered lancinating TN pain isolated to the area of recovered sensation with no pain in anesthetic areas. Nerve ultrasound demonstrated enlargement of ipsilateral greater and lesser occipital nerves, and occipital nerve block restored facial anesthesia and resolved her pain, indicating that recovered facial sensation was provided exclusively by the upper cervical spinal nerves. She underwent C2/C3 ganglionectomy, and ganglia were observed to be hypertrophic. Postoperatively, trigeminal anesthesia was restored with complete resolution of pain that persisted at 12-mo follow-up.

CONCLUSION

This is the first documented case of a spinal nerve innervating a cranial dermatome by collateral sprouting after cranial nerve injury. The fact that typical TN pain can occur even when sensation is mediated by spinal nerves suggests that the disorder can be centrally mediated and late failure after lesioning procedures may result from maladaptive reinnervation.

The Role of BDNF in Peripheral Nerve Regeneration: Activity-Dependent Treatments and Val66Met

Despite the ability of peripheral nerves to spontaneously regenerate after injury, recovery is generally very poor. The neurotrophins have emerged as an important modulator of axon regeneration, particularly brain derived neurotrophic factor (BDNF). BDNF regulation and signaling, as well as its role in activity-dependent treatments including electrical stimulation, exercise, and optogenetic stimulation are discussed here. The importance of a single nucleotide polymorphism in the BDNF gene, Val66Met, which is present in 30% of the human population and may hinder the efficacy of these treatments in enhancing regeneration after injury is considered. Preliminary data are presented on the effectiveness of one such activity-dependent treatment, electrical stimulation, in enhancing axon regeneration in mice expressing the met allele of the Val66Met polymorphism.

Macrophage biology in the peripheral nervous system after injury

Neuroinflammation has positive and negative effects. This review focuses on the roles of macrophage in the PNS. Transection of PNS axons leads to degeneration and clearance of the distal nerve and to changes in the region of the axotomized cell bodies. In both locations, resident and infiltrating macrophages are found. Macrophages enter these areas in response to expression of the chemokine CCL2 acting on the macrophage receptor CCR2. In the distal nerve, macrophages and other phagocytes are involved in clearance of axonal debris, which removes molecules that inhibit nerve regeneration. In the cell body region, macrophage trigger the conditioning lesion response, a process in which neurons increase their regeneration after a prior lesion. In mice in which the genes for CCL2 or CCR2 are deleted, neither macrophage infiltration nor the conditioning lesion response occurs in dorsal root ganglia (DRG). Macrophages exist in different phenotypes depending on their environment. These phenotypes have different effects on axonal clearance and neurite outgrowth. The mechanism by which macrophages affect neuronal cell bodies is still under study. Overexpression of CCL2 in DRG in uninjured animals leads to macrophage accumulation in the ganglia and to an increase in the growth potential of DRG neurons. This increased growth requires activation of neuronal STAT3. In contrast, in acute demyelinating neuropathies, macrophages are involved in stripping myelin from peripheral axons. The molecular mechanisms that trigger macrophage action after trauma and in autoimmune disease are receiving increased attention and should lead to avenues to promote regeneration and protect axonal integrity.

Common and Divergent Mechanisms in Developmental Neuronal Remodeling and Dying Back Neurodegeneration

Cell death is an inherent process that is required for the proper wiring of the nervous system. Studies over the last four decades have shown that, in a parallel developmental pathway, axons and dendrites are eliminated without the death of the neuron. This developmentally regulated 'axonal death' results in neuronal remodeling, which is an essential mechanism to sculpt neuronal networks in both vertebrates and invertebrates. Studies across various organisms have demonstrated that a conserved strategy in the formation of adult neuronal circuitry often involves generating too many connections, most of which are later eliminated with high temporal and spatial resolution. Can neuronal remodeling be regarded as developmentally and spatially regulated neurodegeneration? It has been previously speculated that injury-induced degeneration (Wallerian degeneration) shares some molecular features with 'dying back' neurodegenerative diseases. In this opinion piece, we examine the similarities and differences between the mechanisms regulating neuronal remodeling and those being perturbed in dying back neurodegenerative diseases. We focus primarily on amyotrophic lateral sclerosis and peripheral neuropathies and highlight possible shared pathways and mechanisms. While mechanistic data are only just beginning to emerge, and despite the inherent differences between disease-oriented and developmental processes, we believe that some of the similarities between these developmental and disease-initiated degeneration processes warrant closer collaborations and crosstalk between these different fields.

Neuroprotective Effects of Exercise Treatments After Injury: The Dual Role of Neurotrophic Factors

BACKGROUND

Shared connections between physical activity and neuroprotection have been studied for decades, but the mechanisms underlying this effect of specific exercise were only recently brought to light. Several evidences suggest that physical activity may be a reasonable and beneficial method to improve functional recovery in both peripheral and central nerve injuries and to delay functional decay in neurodegenerative diseases. In addition to improving cardiac and immune functions, physical activity may represent a multifunctional approach not only to improve cardiocirculatory and immune functions, but potentially modulating trophic factors signaling and, in turn, neuronal function and structure at times that may be critical for neurodegeneration and regeneration.

METHODS

Research content related to the effects of physical activity and specific exercise programs in normal and injured nervous system have been reviewed.

RESULTS

Sustained exercise, particularly if applied at moderate intensity and early after injury, exerts anti-inflammatory and pro-regenerative effects, and may boost cognitive and motor functions in aging and neurological disorders. However, newest studies show that exercise modalities can differently affect the production and function of brain-derived neurotrophic factor and other neurotrophins involved in the generation of neuropathic conditions. These findings suggest the possibility that new exercise strategies can be directed to nerve injuries with therapeutical benefits.

CONCLUSION

Considering the growing burden of illness worldwide, understanding of how modulation of neurotrophic factors contributes to exercise-induced neuroprotection and regeneration after peripheral nerve and spinal cord injuries is a relevant topic for research, and represents the beginning of a new non-pharmacological therapeutic approach for better rehabilitation of neural disorders.

Sensory Neurodegeneration in Diabetes: Beyond Glucotoxicity

Diabetic polyneuropathy in humans is of gradual, sometimes insidious onset, and is more likely to occur if glucose control is poor. Arguments that the disorder arises chiefly from glucose toxicity however ignore the greater complexity of a unique neurodegenerative disorder. For example, sensory neurons regularly thrive in media with levels of glucose at or exceeding those of poorly controlled diabetic persons. Also, all of the linkages between hyperglycemia and neuropathy develop in the setting of altered insulin availability or sensitivity. Insulin itself is recognized as a potent growth, or trophic factor for adult sensory neurons. Low doses of insulin, insufficient to alter blood glucose levels, reverse features of diabetic neurodegeneration in animal models. Insulin resistance, as occurs in diabetic adipose tissue, liver, and muscle, also develops in sensory neurons, offering a mechanism for neurodegeneration in the setting of normal or elevated insulin levels. Other interventions that "shore up" sensory neurons prevent features of diabetic polyneuropathy from developing despite persistent hyperglycemia. More recently evidence has emerged that a series of subtle molecular changes in sensory neurons can be linked to neurodegeneration including epigenetic changes in the control of gene expression. Understanding the new complexity of sensory neuron degeneration may give rise to therapeutic strategies that have a higher chance of success in the clinical trial arena.

Mitochondrial dysfunction in diabetic neuropathy: a series of unfortunate metabolic events

Diabetic neuropathy is a dying back neurodegenerative disease of the peripheral nervous system where mitochondrial dysfunction has been implicated as an etiological factor. Diabetes (type 1 or type 2) invokes an elevation of intracellular glucose concentration simultaneously with impaired growth factor support by insulin, and this dual alteration triggers a maladaptation in metabolism of adult sensory neurons. The energy sensing pathway comprising the AMP-activated protein kinase (AMPK)/sirtuin (SIRT)/peroxisome proliferator-activated receptor-γ coactivator α (PGC-1α) signaling axis is the target of these damaging changes in nutrient levels, e.g., induction of nutrient stress, and loss of insulin-dependent growth factor support and instigates an aberrant metabolic phenotype characterized by a suppression of mitochondrial oxidative phosphorylation and shift to anaerobic glycolysis. There is discussion of how this loss of mitochondrial function and transition to overreliance on glycolysis contributes to the diminishment of collateral sprouting and axon regeneration in diabetic neuropathy in the context of the highly energy-consuming nerve growth cone.

Transcriptional changes in sensory ganglia associated with primary afferent axon collateral sprouting in spared dermatome model

Primary afferent collateral sprouting is a process whereby non-injured primary afferent neurons respond to some stimulus and extend new branches from existing axons. Neurons of both the central and peripheral nervous systems undergo this process, which contributes to both adaptive and maladaptive plasticity (e.g., [1], [2], [3], [4], [5], [6], [7], [8], [9]). In the model used here (the “spared dermatome” model), the intact sensory neurons respond to the denervation of adjacent areas of skin by sprouting new axon branches into that adjacent denervated territory. Investigations of gene expression changes associated with collateral sprouting can provide a better understanding of the molecular mechanisms controlling this process. Consequently, it can be used to develop treatments to promote functional recovery for spinal cord injury and other similar conditions. This report includes raw gene expression data files from microarray experiments in order to study the gene regulation in spared sensory ganglia in the initiation (7 days) and maintenance (14 days) phases of the spared dermatome model relative to intact (“naïve”) sensory ganglia. Data has been deposited into GEO (GSE72551).

The neuroimmunology of degeneration and regeneration in the peripheral nervous system

Peripheral nerves regenerate following injury due to the effective activation of the intrinsic growth capacity of the neurons and the formation of a permissive pathway for outgrowth due to Wallerian degeneration (WD). WD and subsequent regeneration are significantly influenced by various immune cells and the cytokines they secrete. Although macrophages have long been known to play a vital role in the degenerative process, recent work has pointed to their importance in influencing the regenerative capacity of peripheral neurons. In this review, we focus on the various immune cells, cytokines, and chemokines that make regeneration possible in the peripheral nervous system, with specific attention placed on the role macrophages play in this process.

Peripheral neuron plasticity is enhanced by brief electrical stimulation and overrides attenuated regrowth in experimental diabetes

Peripheral nerve regrowth is less robust than commonly assumed, particularly when it accompanies common clinical scenarios such as diabetes mellitus. Brief extracellular electrical stimulation (ES) facilitates the regeneration of peripheral nerves in part through early activation of the conditioning injury response and BDNF. Here, we explored intrinsic neuronal responses to ES to identify whether ES might impact experimental diabetes, where regeneration is attenuated. ES altered several regeneration related molecules including rises in tubulin, Shh (Sonic hedgehog) and GAP43 mRNAs. ES was associated with rises in neuronal intracellular calcium but its strict linkage to regrowth was not confirmed. In contrast, we identified PI3K-PTEN involvement, an association previously linked to diabetic regenerative impairment. Following ES there were declines in PTEN protein and mRNA both in vitro and in vivo and a PI3K inhibitor blocked its action. In vitro, isolated diabetic neurons were capable of mounting robust responsiveness to ES. In vivo, ES improved electrophysiological and behavioral indices of nerve regrowth in a chronic diabetic model of mice with pre-existing neuropathy. Regrowth of myelinated axons and reinnervation of the epidermis were greater following ES than sham stimulation. Taken together, these findings identify a role for ES in supporting regeneration during the challenges of diabetes mellitus.

Neurotrophin selectivity in organizing topographic regeneration of nociceptive afferents

Neurotrophins represent some of the best candidates to enhance regeneration. In the current study, we investigated the effects of artemin, a member of the glial derived neurotrophic factor (GDNF) family, on sensory axon regeneration following a lumbar dorsal root injury and compared these effects with that observed after either NGF or GDNF expression in the rat spinal cord. Unlike previously published data, artemin failed to induce regeneration of large-diameter myelinated sensory afferents when expressed within either the spinal cord or DRG. However, artemin or NGF induced regeneration of calcitonin gene related peptide positive (CGRP(+)) axons only when expressed within the spinal cord. Accordingly, artemin or NGF enhanced recovery of only nociceptive behavior and showed a cFos distribution similar to the topography of regenerating axons. Artemin and GDNF signaling requires binding to different co-receptors (GFRα3 or GFRα1, respectively) prior to binding to the signaling receptor, cRet. Approximately 70% of DRG neurons express cRet, but only 35% express either co-receptor. To enhance artemin-induced regeneration, we co-expressed artemin with either GFRα3 or GDNF. Co-expression of artemin and GFRα3 only slightly enhanced regeneration of IB4(+) non-peptidergic nociceptive axons, but not myelinated axons. Interestingly, this co-expression also disrupted the ability of artemin to produce topographic targeting and lead to significant increases in cFos immunoreactivity within the deep dorsal laminae. This study failed to demonstrate artemin-induced regeneration of myelinated axons, even with co-expression of GFRα3, which only promoted mistargeted regeneration.

Role of neurotrophin in the taste system following gustatory nerve injury

Taste system is a perfect system to study degeneration and regeneration after nerve injury because the taste system is highly plastic and the regeneration is robust. Besides, degeneration and regeneration can be easily measured since taste buds arise in discrete locations, and nerves that innervate them can be accurately quantified. Neurotrophins are a family of proteins that regulate neural survival, function, and plasticity after nerve injury. Recent studies have shown that neurotrophins play an important role in the developmental and mature taste system, indicating neurtrophin might also regulate taste system following gustatory nerve injury. This review will summarize how taste system degenerates and regenerates after gustatory nerve cut and conclude potential roles of neurotrophin in regulating the process.

Molecular basis of intervertebral disc degeneration and herniations: what are the important translational questions?

BACKGROUND

Intervertebral disc degeneration is a common condition with few inexpensive and effective modes of treatment, but current investigations seek to clarify the underlying process and offer new treatment options. It will be important for physicians to understand the molecular basis for the pathology and how it translates to developing clinical treatments for disc degeneration. In this review, we sought to summarize for clinicians what is known about the molecular processes that causes disc degeneration.

RESULTS

A healthy disc requires maintenance of a homeostatic environment, and when disrupted, a catabolic cascade of events occurs on a molecular level resulting in upregulation of proinflammatory cytokines, increased degradative enzymes, and a loss of matrix proteins. This promotes degenerative changes and occasional neurovascular ingrowth potentially contributing to the development of pain. Research demonstrates the molecular changes underlying the harmful effects of aging, smoking, and obesity seen clinically while demonstrating the variable influence of exercise. Finally, oral medications, supplements, biologic treatments, gene therapy, and stem cells hold great promise but require cautious application until their safety profiles are better outlined.

CONCLUSIONS

Intervertebral disc degeneration occurs where there is a loss of homeostatic balance with a predominantly catabolic metabolic profile. A basic understanding of the molecular changes occurring in the degenerating disc is important for practicing clinicians because it may help them to inform patients to alter lifestyle choices, identify beneficial or harmful supplements, or offer new biologic, genetic, or stem cell therapies.

Calcium signalling in sensory neurones and peripheral glia in the context of diabetic neuropathies

Peripheral sensory nervous system is comprised of neurones with their axons and neuroglia that includes satellite glial cells in sensory ganglia, myelinating, non-myelinating and perisynaptic Schwann cells. Pathogenesis of peripheral diabetic polyneuropathies is associated with aberrant function of both neurones and glia. Deregulated Ca(2+) homoeostasis and aberrant Ca(2+) signalling in neuronal and glial elements contributes to many forms of neuropathology and is fundamental to neurodegenerative diseases. In diabetes both neurones and glia experience metabolic stress and mitochondrial dysfunction which lead to deregulation of Ca(2+) homeostasis and Ca(2+) signalling, which in their turn lead to pathological cellular reactions contributing to development of diabetic neuropathies. Molecular cascades responsible for Ca(2+) homeostasis and signalling, therefore, can be regarded as potential therapeutic targets.

Sustained delivery of VEGF maintains innervation and promotes reperfusion in ischemic skeletal muscles via NGF/GDNF signaling

Tissue reinnervation following trauma, disease, or transplantation often presents a significant challenge. Here, we show that the delivery of vascular endothelial growth factor (VEGF) from alginate hydrogels ameliorates loss of skeletal muscle innervation after ischemic injury by promoting both maintenance and regrowth of damaged axons in mice. Nerve growth factor (NGF) and glial-derived neurotrophic factor (GDNF) mediated VEGF-induced axonal regeneration, and the expression of both is induced by VEGF presentation. Using both in vitro and in vivo modeling approaches, we demonstrate that the activity of NGF and GDNF regulates VEGF-driven angiogenesis, controlling endothelial cell sprouting and blood vessel maturation. Altogether, these studies produce evidence of new mechanisms of VEGF action, further broaden the understanding of the roles of NGF and GDNF in angiogenesis and axonal regeneration, and suggest approaches to improve axonal and ischemic tissue repair therapies.

Signaling pathways that regulate axon regeneration

Neurons in the mammalian central nervous system (CNS) cannot regenerate axons after injury. in contrast, neurons in the mammalian peripheral nervous system and in some non-mammalian models, such as C. elegans and Drosophila, are able to regrow axons. Understanding the molecular mechanisms by which these neurons support axon regeneration will help us find ways to enhance mammalian CNS axon regeneration. Here, recent studies in which signaling pathways regulating naturally-occurring axon regeneration that have been identified are reviewed, focusing on how these pathways control gene expression and growth-cone function during axon regeneration.

Journey to the skin: Somatosensory peripheral axon guidance and morphogenesis

The peripheral axons of vertebrate tactile somatosensory neurons travel long distances from ganglia just outside the central nervous system to the skin. Once in the skin these axons form elaborate terminals whose organization must be regionally patterned to detect and accurately localize different kinds of touch stimuli. This review describes key studies that identified choice points for somatosensory axon growth cones and the extrinsic molecular cues that function at each of those steps. While much has been learned in the past 20 years about the guidance of these axons, there is still much to be learned about how the peripheral axons of different kinds of somatosensory neurons adopt different trajectories and form specific terminal structures.

Differential effects of activity dependent treatments on axonal regeneration and neuropathic pain after peripheral nerve injury

Activity treatments are useful strategies to increase axonal regeneration and functional recovery after nerve lesions. They are thought to benefit neuropathy by enhancing neurotrophic factor expression. Nevertheless the effects on sensory function are still unclear. Since neurotrophic factors also play a fundamental role in peripheral and central sensitization, we studied the effects of acute electrical stimulation and early treadmill exercise on nerve regeneration and on neuropathic pain, and the relation with the expression of neurotrophins. After sciatic nerve section and suture repair, rats were subjected to electrical stimulation (ES) for 4h after injury, forced treadmill running (TR) for 5 days, or both treatments combined. Sciatic nerve section induced hyperalgesia in the medial area of the plantar skin in the injured paw. TR and ES differently but positively reduced adjacent neuropathic pain before and after sciatic reinnervation. ES enhanced motor and sensory reinnervation, and combination with TR induced strong agonistic effects in relieving pain. The differential effects of these activity treatments were related to changes in neurotrophic factor mRNA levels in sensory and motor neurons. ES speeded up expression of BDNF and GDNF in DRG, and of BDNF and NT3 in the ventral horn. TR reduced the levels of pro-nociceptive factors such as BDNF, NGF and GDNF in DRG. Combination of ES and TR induced intermediate levels suggesting an optimal balancing of treatment effects.

Novel systems for in vivo monitoring and microenvironmental investigations of diabetic neuropathy in a murine model

Peripheral neuropathy is a devastating complication of diabetes conferring vast morbidity and mortality. Despite prolonged efforts to elucidate the mechanisms underlying diabetic related neuropathic phenomena and develop effective therapies, current treatment is for the most part glycemic control and symptomatic care. This is partially due to the intricate pathophysiology of diabetic neuropathy and the scarcity of valid experimental models. The aim of the study was to establish novel systems enabling monitoring and dissection of significant processes in the development of diabetic neuropathy. In a non-invasive in vivo model, two-photon microscopy is applied to evaluate mechanoreceptors (Meissner corpuscles) within an intact footpad of transgenic mice expressing a fluorescent neuronal tracer. By applying this advanced technology, which couples potent tissue penetration with superb resolution, we documented qualitative and quantitative diabetes-specific alterations in these sensory structures. Detection of such changes previously required laborious invasive histopathological techniques. In parallel, we present an ex vivo system that mimics the native microenvironment of the nerve ending via a unique co-culture of primary sensory neurons and thin skin slices. In conjunction with innovative high-throughput digital axonal measurements and computerized quantification tools, this method enables an unbiased exploration of neuronal autonomous and non-autonomous malfunctions. Using this setup we demonstrate that while the diabetic nerve retains a near-normal growth and regeneration capacities, the diabetic skin exhibits a decreased ability to support axonal outgrowth. Thus, an early target organ failure rather than intrinsic neuronal failure may initiate the neuropathy. Overall, the illustrated experimental platforms may greatly facilitate the holistic investigation of diabetic neuropathy.

Specificity of peripheral nerve regeneration: interactions at the axon level

Peripheral nerves injuries result in paralysis, anesthesia and lack of autonomic control of the affected body areas. After injury, axons distal to the lesion are disconnected from the neuronal body and degenerate, leading to denervation of the peripheral organs. Wallerian degeneration creates a microenvironment distal to the injury site that supports axonal regrowth, while the neuron body changes in phenotype to promote axonal regeneration. The significance of axonal regeneration is to replace the degenerated distal nerve segment, and achieve reinnervation of target organs and restitution of their functions. However, axonal regeneration does not always allows for adequate functional recovery, so that after a peripheral nerve injury, patients do not recover normal motor control and fine sensibility. The lack of specificity of nerve regeneration, in terms of motor and sensory axons regrowth, pathfinding and target reinnervation, is one the main shortcomings for recovery. Key factors for successful axonal regeneration include the intrinsic changes that neurons suffer to switch their transmitter state to a pro-regenerative state and the environment that the axons find distal to the lesion site. The molecular mechanisms implicated in axonal regeneration and pathfinding after injury are complex, and take into account the cross-talk between axons and glial cells, neurotrophic factors, extracellular matrix molecules and their receptors. The aim of this review is to look at those interactions, trying to understand if some of these molecular factors are specific for motor and sensory neuron growth, and provide the basic knowledge for potential strategies to enhance and guide axonal regeneration and reinnervation of adequate target organs.

The role of aberrant mitochondrial bioenergetics in diabetic neuropathy

Diabetic neuropathy is a neurological complication of diabetes that causes significant morbidity and, because of the obesity-driven rise in incidence of type 2 diabetes, is becoming a major international health problem. Mitochondrial phenotype is abnormal in sensory neurons in diabetes and may contribute to the etiology of diabetic neuropathy where a distal dying-back neurodegenerative process is a key component contributing to fiber loss. This review summarizes the major features of mitochondrial dysfunction in neurons and Schwann cells in human diabetic patients and in experimental animal models (primarily exhibiting type 1 diabetes). This article attempts to relate these findings to the development of critical neuropathological hallmarks of the disease. Recent work reveals that hyperglycemia in diabetes triggers nutrient excess in neurons that, in turn, mediates a phenotypic change in mitochondrial biology through alteration of the AMP-activated protein kinase (AMPK)/peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) signaling axis. This vital energy sensing metabolic pathway modulates mitochondrial function, biogenesis and regeneration. The bioenergetic phenotype of mitochondria in diabetic neurons is aberrant due to deleterious alterations in expression and activity of respiratory chain components as a direct consequence of abnormal AMPK/PGC-1α signaling. Utilization of innovative respirometry equipment to analyze mitochondrial function of cultured adult sensory neurons from diabetic rodents shows that the outcome for cellular bioenergetics is a reduced adaptability to fluctuations in ATP demand. The diabetes-induced maladaptive process is hypothesized to result in exhaustion of the ATP supply in the distal nerve compartment and induction of nerve fiber dissolution. The role of mitochondrial dysfunction in the etiology of diabetic neuropathy is compared with other types of neuropathy with a distal dying-back pathology such as Friedreich ataxia, Charcot-Marie-Tooth disease type 2 and human immunodeficiency virus-associated distal-symmetric neuropathy.

The lateral thoracic nerve and the cutaneous maximus muscle--a novel in vivo model system for nerve degeneration and regeneration studies

We report a novel in vivo mouse model system to study regeneration of injured motor nerve and spatiotemporal pattern of denervation in experimental nerve diseases. The lateral thoracic nerve (LTN), as a pure motor nerve, innervates the cutaneous maximus muscle (CMM) by some of the shortest and the longest motor nerve fibers in the mouse body. Its branches and nerve terminals can be imaged in whole mount preparations. Here we describe the branching pattern of the LTN and its innervation of the CMM, and characterize degeneration and regeneration over time after a LTN crush by morphological and electrophysiological analyses. We demonstrate the utility of this model in a well-established neurotoxicity paradigm and in a genetic disease model of the peripheral neuropathy. Furthermore, this system enables punch biopsies that allow repeated and multi-location examinations for LTN regeneration and CMM reinnervation over time. The presence of the LTN and the CMM in a variety of species and its easy accessibility suggests that this in vivo model system offers considerable promise for future nerve degeneration and regeneration research.

gp130 cytokines are positive signals triggering changes in gene expression and axon outgrowth in peripheral neurons following injury

Adult peripheral neurons, in contrast to adult central neurons, are capable of regeneration after axonal damage. Much attention has focused on the changes that accompany this regeneration in two places, the distal nerve segment (where phagocytosis of axonal debris, changes in the surface properties of Schwann cells, and induction of growth factors and cytokines occur) and the neuronal cell body (where dramatic changes in cell morphology and gene expression occur). The changes in the axotomized cell body are often referred to as the "cell body response." The focus of the current review is a family of cytokines, the glycoprotein 130 (gp130) cytokines, which produce their actions through a common gp130 signaling receptor and which function as injury signals for axotomized peripheral neurons, triggering changes in gene expression and in neurite outgrowth. These cytokines play important roles in the responses of sympathetic, sensory, and motor neurons to injury. The best studied of these cytokines in this context are leukemia inhibitory factor (LIF) and interleukin (IL)-6, but experiments with conditional gp130 knockout animals suggest that other members of this family, not yet determined, are also involved. The primary gp130 signaling pathway shown to be involved is the activation of Janus kinase (JAK) and the transcription factors Signal Transducers and Activators of Transcription (STAT), though other downstream pathways such as mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) may also play a role. gp130 signaling may involve paracrine, retrograde, and autocrine actions of these cytokines. Recent studies suggest that manipulation of this cytokine system can also stimulate regeneration by injured central neurons.

Transsynaptic activity-dependent regulation of axon branching and neurotrophin expression in vivo

The two major classes of activity-dependent neuroplasticity predict different consequences of activity alteration on circuit response. Hebbian plasticity (positive feedback) posits that alteration of neuronal activity causes a parallel response within a circuit. In contrast, homeostatic plasticity (negative feedback) predicts that altering neuronal activity results in compensatory responses within a circuit. The relative roles of these modes of plasticity in vivo are unclear, since neuronal circuits are difficult to manipulate in the intact organism. In this study, we tested the in vivo effects of activity deprivation in the superior cervical ganglion-pineal circuit of adult rats, which can be noninvasively silenced by exposing animals to constant light. We demonstrated that total deprivation of sympathetic activity markedly decreased the presence of axonal proteins in the pineal and reduced the density and thickness of sympathetic axonal arbors. In addition, we demonstrated that sympathetic inactivity eliminated pineal function and markedly decreased pineal expression of neurotrophins. Administration of β-adrenergic agonist restored the expression of presynaptic and postsynaptic proteins. Furthermore, compensatory axonal growth through collateral sprouting, normally seen following unilateral denervation of the pineal, was profoundly impaired in the absence of neural activity. Thus, these data suggest that sympathetic axonal terminals are maintained by neural activity that induces neurotrophins, which may act through a retrograde mechanism to preserve the integrity of axonal arbors via a positive feedback loop. Conversely, by using Hebbian-like neuroplasticity, silent yet intact circuits enter a hibernation mode marked by reduction of presynaptic axonal structures and dramatically reduced postsynaptic expression of neurotrophins.

Wallerian degeneration: gaining perspective on inflammatory events after peripheral nerve injury

In this review, we first provide a brief historical perspective, discussing how peripheral nerve injury (PNI) may have caused World War I. We then consider the initiation, progression, and resolution of the cellular inflammatory response after PNI, before comparing the PNI inflammatory response with that induced by spinal cord injury (SCI).In contrast with central nervous system (CNS) axons, those in the periphery have the remarkable ability to regenerate after injury. Nevertheless, peripheral nervous system (PNS) axon regrowth is hampered by nerve gaps created by injury. In addition, the growth-supportive milieu of PNS axons is not sustained over time, precluding long-distance regeneration. Therefore, studying PNI could be instructive for both improving PNS regeneration and recovery after CNS injury. In addition to requiring a robust regenerative response from the injured neuron itself, successful axon regeneration is dependent on the coordinated efforts of non-neuronal cells which release extracellular matrix molecules, cytokines, and growth factors that support axon regrowth. The inflammatory response is initiated by axonal disintegration in the distal nerve stump: this causes blood-nerve barrier permeabilization and activates nearby Schwann cells and resident macrophages via receptors sensitive to tissue damage. Denervated Schwann cells respond to injury by shedding myelin, proliferating, phagocytosing debris, and releasing cytokines that recruit blood-borne monocytes/macrophages. Macrophages take over the bulk of phagocytosis within days of PNI, before exiting the nerve by the circulation once remyelination has occurred. The efficacy of the PNS inflammatory response (although transient) stands in stark contrast with that of the CNS, where the response of nearby cells is associated with inhibitory scar formation, quiescence, and degeneration/apoptosis. Rather than efficiently removing debris before resolving the inflammatory response as in other tissues, macrophages infiltrating the CNS exacerbate cell death and damage by releasing toxic pro-inflammatory mediators over an extended period of time. Future research will help determine how to manipulate PNS and CNS inflammatory responses in order to improve tissue repair and functional recovery.

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.

Non-invasive stimulation of the vibrissal pad improves recovery of whisking function after simultaneous lesion of the facial and infraorbital nerves in rats

We have recently shown that manual stimulation of target muscles promotes functional recovery after transection and surgical repair to pure motor nerves (facial: whisking and blink reflex; hypoglossal: tongue position). However, following facial nerve repair, manual stimulation is detrimental if sensory afferent input is eliminated by, e.g., infraorbital nerve extirpation. To further understand the interplay between sensory input and motor recovery, we performed simultaneous cut-and-suture lesions on both the facial and the infraorbital nerves and examined whether stimulation of the sensory afferents from the vibrissae by a forced use would improve motor recovery. The efficacy of 3 treatment paradigms was assessed: removal of the contralateral vibrissae to ensure a maximal use of the ipsilateral ones (vibrissal stimulation; Group 2), manual stimulation of the ipsilateral vibrissal muscles (Group 3), and vibrissal stimulation followed by manual stimulation (Group 4). Data were compared to controls which underwent surgery but did not receive any treatment (Group 1). Four months after surgery, all three treatments significantly improved the amplitude of vibrissal whisking to 30° versus 11° in the controls of Group 1. The three treatments also reduced the degree of polyneuronal innervation of target muscle fibers to 37% versus 58% in Group 1. These findings indicate that forced vibrissal use and manual stimulation, either alone or sequentially, reduce target muscle polyinnervation and improve recovery of whisking function when both the sensory and the motor components of the trigemino-facial system regenerate.