The effect of aging on efferent nerve fibers regeneration in mice

Schwann Cells as Orchestrators of Nerve Repair: Implications for Tissue Regeneration and Pathologies

Peripheral nerves exist in a stable state in adulthood providing a rapid bidirectional signaling system to control tissue structure and function. However, following injury, peripheral nerves can regenerate much more effectively than those of the central nervous system (CNS). This multicellular process is coordinated by peripheral glia, in particular Schwann cells, which have multiple roles in stimulating and nurturing the regrowth of damaged axons back to their targets. Aside from the repair of damaged nerves themselves, nerve regenerative processes have been linked to the repair of other tissues and de novo innervation appears important in establishing an environment conducive for the development and spread of tumors. In contrast, defects in these processes are linked to neuropathies, aging, and pain. In this review, we focus on the role of peripheral glia, especially Schwann cells, in multiple aspects of nerve regeneration and discuss how these findings may be relevant for pathologies associated with these processes.

Peripheral nerve injuries in children-prevalence, mechanisms and concomitant injuries: a major trauma center's experience

BACKGROUND

Peripheral nerve injuries are severe conditions with potential lifelong impairment, which is especially meaningful for the pediatric population. Knowledge on prevalence, injury mechanisms and concomitant injuries is, therefore, of utmost importance to increase clinician awareness and enable early diagnosis and treatment. As current literature on pediatric nerve lesions and concomitant injuries is scarce, we aimed to analyze all details of our patient population.

METHODS

A total of 110 667 patients treated at our level 1 trauma center from 2012 to 2021 were evaluated for pediatric peripheral nerve injuries, causes, concomitant injuries and assessed for lesion classification (in continuity, partial lesion, dissection) and further relevant intraoperative findings.

RESULTS

We found 5026 patients of all ages with peripheral nerve lesions, whereof 288 were pediatric, resulting in a prevalence of 5.7% of pediatric patients with nerve injuries. Mean age was 12.4 ± 4.6 years. Most common lesions were digital nerves (48.2%), followed by median (14.9%), ulnar (14.6%), radial (8.8%), peroneal nerve (5.2%) and brachial plexus injuries (2.1%). Of all pediatric nerve injuries, 3.8% were iatrogenic, only 30.2% had preserved continuity and 47.3% a concomitant vessel injury. Fractures were accompanied in 22.6%.

DISCUSSION

We observed that a large proportion of injures had complete transections, often accompanied by concomitant vessel injuries especially in distally located injuries, highlighting the importance of early surgical exploration. Radial, ulnar and lower extremity nerve injuries were often associated with fractures. Early surgical nerve repair is key to improve motor and sensory outcomes. Knowledge on mechanisms and concomitant injuries facilitates timely diagnosis and treatment, thereby potentially preventing lifelong impairment.

Regulation of UNC-40/DCC and UNC-6/Netrin by DAF-16 promotes functional rewiring of the injured axon

The adult nervous system has a limited capacity to regenerate after accidental damage. Post-injury functional restoration requires proper targeting of the injured axon to its postsynaptic cell. Although the initial response to axonal injury has been studied in great detail, it is rather unclear what controls the re-establishment of a functional connection. Using the posterior lateral microtubule neuron in Caenorhabditis elegans, we found that after axotomy, the regrowth from the proximal stump towards the ventral side and accumulation of presynaptic machinery along the ventral nerve cord correlated to the functional recovery. We found that the loss of insulin receptor DAF-2 promoted 'ventral targeting' in a DAF-16-dependent manner. We further showed that coordinated activities of DAF-16 in neuron and muscle promoted 'ventral targeting'. In response to axotomy, expression of the Netrin receptor UNC-40 was upregulated in the injured neuron in a DAF-16-dependent manner. In contrast, the DAF-2-DAF-16 axis contributed to the age-related decline in Netrin expression in muscle. Therefore, our study revealed an important role for insulin signaling in regulating the axon guidance molecules during the functional rewiring process.

Revitalizing the common peroneal function index for assessing functional recovery following nerve injury

BACKGROUND AND AIMS

Peripheral nerve injury is common with poor functional recovery and consequent high personal and societal costs. Sciatic nerve transection and assessment of recovery using sciatic functional index (SFI) are widely used. SFI is biologically limited as axonal misdirection of axons supplying flexors and extensors in the hindlimb, after nerve injury can lead to synkinetic innervation and function which does not correspond to the degree of axonal regeneration.

METHODS

We reevaluated the use of traditional metrics such as print length (PL), toe spread (TS), and intermediate toe spread (ITS) as well as hock angle at mid-swing as approaches for determining recovery. We used two alternative approaches in discrete cohorts of rats following common peroneal crush injury, transection with repair and critical gap, using transection with ligation as a negative control. We compared walking track analysis (print) with digital capture and kinematics.

RESULTS

PL, TS, and ITS varied as expected after injury. The traditional functional index for common peroneal injury using inked prints failed to describe recovery and we derived new indices to describe recovery (all R²  > 0.88, p < .0001) although pre-injury PFI was never attained by any of the models. Kinematic analysis identified hock angle at mid-swing as a useful predictor of recovery (p < .0001).

INTERPRETATION

Using complementary approaches.

Macrophages and Associated Ligands in the Aged Injured Nerve: A Defective Dynamic That Contributes to Reduced Axonal Regrowth

The regenerative capacity of injured peripheral nerves is diminished with aging. To identify factors that contribute to this impairment, we compared the immune cell response in young vs. aged animals following nerve injury. First, we confirmed that macrophage accumulation is delayed in aged injured nerves which is due to defects in monocyte migration as a result of defects in site-specific recruitment signals in the aged nerve. Interestingly, impairment in both macrophage accumulation and functional recovery could be overcome by transplanting bone marrow from aged animals into young mice. That is, upon exposure to a youthful environment, monocytes/macrophages originating from the aged bone marrow behaved similarly to young cells. Transcriptional profiling of aged macrophages following nerve injury revealed that both pro- and anti-inflammatory genes were largely downregulated in aged compared to young macrophages. One ligand of particular interest was macrophage-associated secreted protein (MCP1), which exhibited a potent role in regulating aged axonal regrowth in vitro. Given that macrophage-derived MCP1 is significantly diminished in the aged injured nerve, our data suggest that age-associated defects in MCP1 signaling could contribute to the regenerative deficits that occur in the aged nervous system.

The Influence of Neuron-Extrinsic Factors and Aging on Injury Progression and Axonal Repair in the Central Nervous System

In the aging western population, the average age of incidence for spinal cord injury (SCI) has increased, as has the length of survival of SCI patients. This places great importance on understanding SCI in middle-aged and aging patients. Axon regeneration after injury is an area of study that has received substantial attention and made important experimental progress, however, our understanding of how aging affects this process, and any therapeutic effort to modulate repair, is incomplete. The growth and regeneration of axons is mediated by both neuron intrinsic and extrinsic factors. In this review we explore some of the key extrinsic influences on axon regeneration in the literature, focusing on inflammation and astrogliosis, other cellular responses, components of the extracellular matrix, and myelin proteins. We will describe how each element supports the contention that axonal growth after injury in the central nervous system shows an age-dependent decline, and how this may affect outcomes after a SCI.

A Surgical Mouse Model for Advancing Laryngeal Nerve Regeneration Strategies

Iatrogenic recurrent laryngeal nerve (RLN) injury is a morbid complication of anterior neck surgical procedures. Existing treatments are predominantly symptomatic, ranging from behavioral therapy to a variety of surgical approaches. Though laryngeal reinnervation strategies often provide muscle tone to the paralyzed vocal fold (VF), which may improve outcomes, there is no clinical intervention that reliably restores true physiologic VF movement. Moreover, existing interventions neglect the full cascade of molecular events that affect the entire neuromuscular pathway after RLN injury, including the intrinsic laryngeal muscles, synaptic connections within the central nervous system, and laryngeal nerve anastomoses. Systematic investigations of this pathway are essential to develop better RLN regenerative strategies. Our aim was to develop a translational mouse model for this purpose, which will permit longitudinal investigations of the pathophysiology of iatrogenic RLN injury and potential therapeutic interventions. C57BL/6J mice were divided into four surgical transection groups (unilateral RLN, n = 10; bilateral RLN, n = 2; unilateral SLN, n = 10; bilateral SLN, n = 10) and a sham surgical group (n = 10). Miniaturized transoral laryngoscopy was used to assess VF mobility over time, and swallowing was assessed using serial videofluoroscopy. Histological assays were conducted 3 months post-surgery for anatomical investigation of the larynx and laryngeal nerves. Eight additional mice underwent unilateral RLN crush injury, half of which received intraoperative vagal nerve stimulation (iVNS). These 8 mice underwent weekly transoral laryngoscopy to investigate VF recovery patterns. Unilateral RLN injury resulted in chronic VF immobility but only acute dysphagia. Bilateral RLN injury caused intraoperative asphyxiation and death. VF mobility was unaffected by SLN transection (unilateral or bilateral), and dysphagia (transient) was evident only after bilateral SLN transection. The sham surgery group retained normal VF mobility and swallow function. Mice that underwent RLN crush injury and iVNS treatment demonstrated accelerated and improved VF recovery. We successfully developed a mouse model of iatrogenic RLN injury with impaired VF mobility and swallowing function that can serve as a clinically relevant platform to develop translational neuroregenerative strategies for RLN injury.

DLK regulates a distinctive transcriptional regeneration program after peripheral nerve injury

Following damage to a peripheral nerve, injury signaling pathways converge in the cell body to generate transcriptional changes that support axon regeneration. Here, we demonstrate that dual leucine zipper kinase (DLK), a central regulator of injury responses including axon regeneration and neuronal apoptosis, is required for the induction of the pro-regenerative transcriptional program in response to peripheral nerve injury. Using a sensory neuron-conditional DLK knockout mouse model, we show a time course for the dependency of gene expression changes on the DLK pathway after sciatic nerve injury. Gene ontology analysis reveals that DLK-dependent gene sets are enriched for specific functional annotations such as ion transport and immune response. A series of comparative analyses shows that the DLK-dependent transcriptional program is distinct from that promoted by the importin-dependent retrograde signaling pathway, while it is partially shared between PNS and CNS injury responses. We suggest that DLK-dependency might provide a selective filter for regeneration-associated genes among the injury-responsive transcriptome.

Comprehensive approach to reestablishing form and function after radical parotidectomy

INTRODUCTION

The reconstructive goals following radical parotidectomy include restoration of symmetry, reanimation of the face, and reestablishment of oral competence. We present our experience utilizing the anterolateral thigh (ALT) free flap, orthodromic temporalis tendon transfer (OTTT), and facial nerve cable grafting to reestablish form and function.

MATERIAL AND METHODS

From 2010 to 2016, 17 patients underwent radical parotidectomy followed by immediate reconstruction. An ALT was harvested to accommodate the volume and skin defect. Additional fascia lata and motor nerve to vastus lateralis (MNVL) were obtained. Anastomosis of the ALT to recipient vessels was performed, most commonly using the facial artery and internal jugular vein. OTTT was performed by securing the medial tendon of the temporalis to orbicularis oris through a nasolabial incision. Fascia lata was tunneled through the lower lip, then secured laterally to the temporalis tendon. The MNVL was cable grafted from either the proximal facial nerve or masseteric nerve to the distal facial nerve branches. ALT fascia was suspended to the superficial muscular aponeurotic system.

RESULTS

Average follow up was 19 months. Only one patient failed to achieve symmetry attributed to dehiscence of OTTT. All patients achieved oral competence and dynamic smile with OTTT activation. Facial nerve recovery was seen in 8 patients. 5 reached a House Brackman Score of 3. Two donor site seromas and two wound infections occurred.

CONCLUSION

Simultaneous ALT, OTTT, and facial nerve cable grafting provides early reestablishment of facial symmetry, facial reanimation, and oral competence with minimal morbidity.

Advances and Limitations of Current Epigenetic Studies Investigating Mammalian Axonal Regeneration

Axonal regeneration relies on the expression of regenerative associated genes within a coordinated transcriptional programme, which is finely tuned as a result of the activation of several regenerative signalling pathways. In mammals, this chain of events occurs in neurons following peripheral axonal injury, however it fails upon axonal injury in the central nervous system, such as in the spinal cord and the brain. Accumulating evidence has been suggesting that epigenetic control is a key factor to initiate and sustain the regenerative transcriptional response and that it might contribute to regenerative success versus failure. This review will discuss experimental evidence so far showing a role for epigenetic regulation in models of peripheral and central nervous system axonal injury. It will also propose future directions to fill key knowledge gaps and to test whether epigenetic control might indeed discriminate between regenerative success and failure.

Wound healing, cellular regeneration and plasticity: the elegans way

Regeneration and wound healing are complex processes that allow organs and tissues to regain their integrity and functionality after injury. Wound healing, a key property of epithelia, involves tissue closure that in some cases leads to scar formation. Regeneration, a process rather limited in mammals, is the capacity to regrow (parts of) an organ or a tissue, after damage or amputation. What are the properties of organs and the features of tissue permitting functional regrowth and repair? What are the cellular and molecular mechanisms underlying these processes? These questions are crucial both in fundamental and applied contexts, with important medical implications. The mechanisms and cells underlying tissue repair have thus been the focus of intense investigation. The last decades have seen rapid progress in the domain and new models emerging. Here, we review the fundamental advances and the perspectives that the use of C. elegans as a model have brought to the mechanisms of wound healing and cellular plasticity, axon regeneration and transdifferentiation in vivo.

Neuronal functions of FOXO/DAF-16

The FOXO family of transcription factors plays a conserved role in longevity and tissue homeostasis across species. In the mammalian nervous system, emerging evidence has implicated FOXOs in cognitive performance, stem cell maintenance, regeneration, and protection against stress. Much of what we know about neuronal functions of FOXO emerged from recent studies in C. elegans. Similar to mammalian FOXO, the worm FOXO ortholog, called DAF-16, regulates learning and memory, regeneration, and stress resistance in neurons. Here, we discuss the current state of our knowledge of FOXO’s functions in neurons in mammals and invertebrates, and highlight areas where our understanding is limited. Defining the function of FOXO factors in the healthy, aged, and diseased brain may have important implications for improving healthspan and treating neurodegenerative disease.

The age factor in axonal repair after spinal cord injury: A focus on neuron-intrinsic mechanisms

Age is an important consideration for recovery and repair after spinal cord injury. Spinal cord injury is increasingly affecting the middle-aged and aging populations. Despite rapid progress in research to promote axonal regeneration and repair, our understanding of how age can modulate this repair is rather limited. In this review, we discuss the literature supporting the notion of an age-dependent decline in axonal growth after central nervous system (CNS) injury. While both neuron-intrinsic and extrinsic factors are involved in the control of axon growth after injury, here we focus on possible intrinsic mechanisms for this age-dependent decline.

Axon regeneration in C. elegans: Worming our way to mechanisms of axon regeneration

How axons repair themselves after injury is a fundamental question in neurobiology. With its conserved genome, relatively simple nervous system, and transparent body, C. elegans has recently emerged as a productive model to uncover the cellular mechanisms that regulate and execute axon regeneration. In this review, we discuss the strengths and weaknesses of the C. elegans model of regeneration. We explore the technical advances that enable the use of C. elegans for in vivo regeneration studies, review findings in C. elegans that have contributed to our understanding of the regeneration response across species, discuss the potential of C. elegans research to provide insight into mechanisms that function in the injured mammalian nervous system, and present potential future directions of axon regeneration research using C. elegans.

Effects of age and insulin-like growth factor-1 on rat neurotrophin receptor expression after nerve injury

INTRODUCTION

Neurotrophin receptors, such as p75(NTR) , direct neuronal response to injury. Insulin-like growth factor-1 receptor (IGF-1R) mediates the increase in p75(NTR) during aging. The aim of this study was to examine the effect of aging and insulin-like growth factor-1 (IGF-1) treatment on recovery after peripheral nerve injury.

METHODS

Young and aged rats underwent tibial nerve transection with either local saline or IGF-1 treatment. Neurotrophin receptor mRNA and protein expression were quantified.

RESULTS

Aged rats expressed elevated baseline IGF-1R (34% higher, P = 0.01) and p75(NTR) (68% higher, P < 0.01) compared with young rats. Post-injury, aged animals expressed significantly higher p75(NTR) levels (68.5% above baseline at 4 weeks). IGF-1 treatment suppressed p75(NTR) gene expression at 4 weeks (17.2% above baseline, P = 0.002) post-injury.

CONCLUSIONS

Local IGF-1 treatment reverses age-related declines in recovery after peripheral nerve injuries by suppressing p75(NTR) upregulation and pro-apoptotic complexes. IGF-1 may be considered a viable adjuvant therapy to current treatment modalities. Muscle Nerve 54: 769-775, 2016.

The effect of lengthening contractions on neuromuscular junction structure in adult and old mice

Skeletal muscles of old mice demonstrate a profound inability to regenerate fully following damage. Such a failure could be catastrophic to older individuals where muscle loss is already evident. Degeneration and regeneration of muscle fibres following contraction-induced injury in adult and old mice are well characterised, but little is known about the accompanying changes in motor neurons and neuromuscular junctions (NMJs) following this form of injury although defective re-innervation of muscle following contraction-induced damage has been proposed to play a role in sarcopenia. This study visualised and quantified structural changes to motor neurons and NMJs in Extensor digitorum longus (EDL) muscles of adult and old Thy1-YFP transgenic mice during regeneration following contraction-induced muscle damage. Data demonstrated that the damaging contraction protocol resulted in substantial initial disruption to NMJs in muscles of adult mice, which was reversed entirely within 28 days following damage. In contrast, in quiescent muscles of old mice, ∼15 % of muscle fibres were denervated and ∼80 % of NMJs showed disruption. This proportion of denervated and partially denervated fibres remained unchanged following recovery from contraction-induced damage in muscles of old mice although ∼25 % of muscle fibres were completely lost by 28 days post-contractions. Thus, in old mice, the failure to restore full muscle force generation that occurs following damage does not appear to be due to any further deficit in the percentage of disrupted NMJs, but appears to be due, at least in part, to the complete loss of muscle fibres following damage.

Impaired regeneration in aged nerves: Clearing out the old to make way for the new

Although many observational studies have shown that peripheral nerve regeneration is impaired with aging, underlying cellular and molecular mechanisms have remained obscure until recently. A series of recent genetic, live imaging and heterochronic parabiosis experiments are providing new insights into the underlying mechanisms of reduced regenerative capacity with aging. These studies show that Schwann cells pose a primary impediment to axon regeneration in older animals as they fail to support regenerating axons, while the contribution from macrophages remains an unresolved issue. Neurons do not appear to have an intrinsic defect of axonal elongation with aging but are impaired when they encounter an inhibitory environment, suggesting that therapeutic approaches to improve intrinsic neuronal regeneration capacity across inhibitory environments, as it is being done in central nervous system regeneration, can improve peripheral nerve regeneration as well. As in many aspects of neuroscience therapeutics development, a combinatorial approach may yield the best outcomes for nerve regeneration in aged individuals.

Purine nucleosides in neuroregeneration and neuroprotection

In the present review, we stress the importance of the purine nucleosides, adenosine and guanosine, in protecting the nervous system, both centrally and peripherally, via activation of their receptors and intracellular signalling mechanisms. A most novel part of the review focus on the mechanisms of neuronal regeneration that are targeted by nucleosides, including a recently identified action of adenosine on axonal growth and microtubule dynamics. Discussion on the role of the purine nucleosides transversally with the most established neurotrophic factors, e.g. brain derived neurotrophic factor (BDNF), glial derived neurotrophic factor (GDNF), is also focused considering the intimate relationship between some adenosine receptors, as is the case of the A2A receptors, and receptors for neurotrophins. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Motor axon regeneration and muscle reinnervation in young adult and aged animals

Injuries to peripheral nerves can cause paralysis and sensory disturbances, but such functional impairments are often short lived because of efficient regeneration of damaged axons. The time required for functional recovery, however, increases with advancing age (Verdú et al., 2000; Kawabuchi et al., 2011). Incomplete or delayed recovery after peripheral nerve damage is a major health concern in the aging population because it can severely restrict a person's mobility and independence. A variety of possible causes have been suggested to explain why nervous systems in aged individuals recover more slowly from nerve damage. Potential causes include age-related declines in the regenerative potential of peripheral axons and decreases in the supply or responsivity to trophic and/or tropic factors. However, there have been few direct analyses of age-related axon regeneration. Our aim here was to observe axons directly in young and old mice as they regenerate and ultimately reoccupy denervated neuromuscular synaptic sites to learn what changes in this process are age related. We find that damaged nerves in aged animals clear debris more slowly than nerves in young animals and that the greater number of obstructions regenerating axons encounter in the endoneurial tubes of old animals give rise to slower regeneration. Surprisingly, however, axons from aged animals regenerate quickly when not confronted by debris and reoccupy neuromuscular junction sites efficiently. These results imply that facilitating clearance of axon debris might be a good target for the treatment of nerve injury in the aged.

Insulin/IGF1 signaling inhibits age-dependent axon regeneration

The ability of injured axons to regenerate declines with age, yet the mechanisms that regulate axon regeneration in response to age are not known. Here we show that axon regeneration in aging C. elegans motor neurons is inhibited by the conserved insulin/IGF1 receptor DAF-2. DAF-2's function in regeneration is mediated by intrinsic neuronal activity of the forkhead transcription factor DAF-16/FOXO. DAF-16 regulates regeneration independently of lifespan, indicating that neuronal aging is an intrinsic, neuron-specific, and genetically regulated process. In addition, we found that DAF-18/PTEN inhibits regeneration independently of age and FOXO signaling via the TOR pathway. Finally, DLK-1, a conserved regulator of regeneration, is downregulated by insulin/IGF1 signaling, bound by DAF-16 in neurons, and required for both DAF-16- and DAF-18-mediated regeneration. Together, our data establish that insulin signaling specifically inhibits regeneration in aging adult neurons and that this mechanism is independent of PTEN and TOR.

The effects of age and ganglioside composition on the rate of motor nerve terminal regeneration following antibody-mediated injury in mice

Gangliosides are glycosphingolipids highly enriched in neural plasma membranes, where they mediate a diverse range of functions and can act as targets for auto-antibodies present in human immune-mediated neuropathy sera. The ensuing autoimmune injury results in axonal and motor nerve terminal (mNT) degeneration. Both aging and ganglioside-deficiency have been linked to impaired axonal regeneration. To assess the effects of age and ganglioside expression on mNT regeneration in an autoimmune injury paradigm, anti-ganglioside antibodies and complement were applied to young adult and aged mice wildtype (WT) mice, mice deficient in either b- and c-series (GD3sKO) or mice deficient in all complex gangliosides (GM2sKO). The extent of mNT injury and regeneration was assessed immediately or after 5 days, respectively. Depending on ganglioside expression and antibody-specificity, either a selective mNT injury or a combined injury of mNTs and neuromuscular glial cells was elicited. Immediately after induction of the injury, between 1.5% and 11.8% of neuromuscular junctions (NMJs) in the young adult groups exhibited healthy mNTs. Five days later, most NMJs, regardless of age and strain, had recovered their mNTs. No significant differences could be observed between young and aged WT and GM2sKO mice; aged GD3sKO showed a mildly impaired rate of mNT regeneration when compared with their younger counterparts. Comparable rates were observed between all strains in the young and the aged mice. In summary, the rate of mNT regeneration following anti-ganglioside antibody and complement-mediated injury does not differ majorly between young adult and aged mice irrespective of the expression of particular gangliosides.

Age affects reciprocal cellular interactions in neuromuscular synapses following peripheral nerve injury

Studies of the influence of age on regeneration and reinnervation in the peripheral nervous system (PNS) and neuromuscular junction (NMJ) are reviewed, with a particular focus on aged and denervated skeletal muscles. The morphological and functional features of incomplete regeneration and reinnervation are compared between adult and aged animals. In addition, some possible mechanisms of the age-related defects will be discussed. Increased fragmentation or damage in individual components of the NMJ (terminal Schwann cells (TSCs), axon terminals and acetylcholine receptor sites occurs during muscle reinnervation following PNS injury in the aged animals. The capacity to produce ultraterminal sprouting or multiple innervation secondary to PNS injury is maintained, but not the capacity to eliminate such anomalous axonal profiles. The frequency and accuracy of reoccupation of the synaptic sites by TSCs and axon terminals are impaired. Thus, despite the capability of extending neural processes, the rate at which regenerating nerve fibers grow, mature and precisely appose the postsynaptic muscle fiber is impaired, resulting in the failure of re-establishment of the normal single motor innervation in the NMJ. A complex set of cellular interactions in the NMJ are known to participate in the neurotrophism and neurotrophism to support growth of the regenerating and sprouting axons and their pathfinding to direct the target muscle fiber. Besides the capability of α-motoneurons, signaling originating from the TSCs and muscle may be impaired during aging.

Effects of topiramate on mouse eccrine sweat gland responsiveness to heat exposure

Young mice (2 weeks old) were given topiramate daily for 1 month, and sudomotor function was evaluated utilizing impression mould techniques to determine the number of sweat glands reactive to heat exposure and sweat output per gland on the plantar surface of mice hind-paws. Immunohistochemical quantitation of protein gene product 9.5, choline acetyltransferase and tyrosine hydroxylase in footpads was determined after topiramate treatment. While a 25% decrease in the number of secreting sweat glands and a 42% decline in sweat output per gland were observed following topiramate treatment, no significant differences were noted in sudomotor innervation, expressed as length of choline acetyltransferase, tyrosine hydroxylase and protein gene product 9.5 immunoreactive nerve profiles in single secretory coils or in sweat gland sizes within the secretory coil area. Long-term topiramate stimulation resulted in a reduction in the number of reactive sweat glands, without changes in sweat gland innervation, suggesting that the diminished responsiveness of the glands to heat exposure induced by topiramate might have resulted from a decrease in the intrinsic regulatory activity of sweat glands, as opposed to the loss of periglandular neurotransmitters or the impairment of the structure of the glands.

Comparison of Biodegradable Conduits within Aged Rat Sciatic Nerve Defects

BACKGROUND

Considering that little is known about the peripheral nerve regenerative capacity of elderly patients, the authors studied nerve regenerative capacity in aged rats and compared the effect of three synthetic nerve guides with different material characteristics and porosity. The authors hypothesized that the use of a biodegradable composite nerve guide (CultiGuides) would promote nerve regeneration and functional recovery in a manner similar to treatment with autografts or U.S. Food and Drug Administration-approved polyglycolic acid Neurotubes in an aged rat sciatic nerve defect model.

METHODS

Aged Sprague-Dawley rats (11 months old) underwent a 1-cm sciatic nerve resection in the right leg [group 1, control (contralateral leg samples), n = 10; group 2, negative (nerve gap defect), n = 6; group 3, autograft, n = 10; group 4, polycaprolactone, n = 10; group 5, CultiGuides, n = 10; and group 6, Neurotube, n = 10].

RESULTS

After 12 weeks, the negative group did not demonstrate any nerve regeneration. In the regenerated and distal nerve, all treated groups had increased myelinated areas compared with the negative control. In the regenerated nerve, there was a significant increase in myelination in the Neurotube group compared with the polycaprolactone group (p < 0.001). However, in the distal nerve, there were no differences among the treatment groups. Walking track analyses and gastrocnemius muscle weight ratios were not different among treatment groups 3 through 6.

CONCLUSIONS

The results showed differences in myelination; Neurotubes promoted the highest degree of myelination (p < 0.001) as compared with all groups. The authors found no improvement in function of the repaired nerve as measured by percentage of autotomy, the sciatic function index, and gastrocnemius muscle weight. No group was able to recover function in this aged model.

Metabosensitive Afferent Fiber Responses after Peripheral Nerve Injury and Transplantation of an Acellular Muscle Graft in Association with Schwann Cells

Studies dedicated to the repair of peripheral nerve focused almost exclusively on motor or mechanosensitive fiber regeneration. Poor attention has been paid to the metabosensitive fibers from group III and IV (also called ergoreceptor). Previously, we demonstrated that the metabosensitive response from the tibialis anterior muscle was partially restored when the transected nerve was immediately sutured. In the present study, we assessed motor and metabosensitive responses of the regenerated axons in a rat model in which 1 cm segment of the peroneal nerve was removed and immediately replaced by an autologous nerve graft or an acellular muscle graft. Four groups of animals were included: control animals (C, no graft), transected animals grafted with either an autologous nerve graft (Gold Standard-GS) or an acellular muscle filled with Schwann Cells (MSC) or Culture Medium (MCM). We observed that (1) the tibialis anterior muscle was atrophied in GS, M(SC) and M(CM) groups, with no significant difference between grafted groups; (2) the contractile properties of the reinnervated muscles after nerve stimulation were similar in all groups; (3) the metabosensitive afferent responses to electrically induced fatigue was smaller in M(SC) and MCM groups; and (4) the metabosensitive afferent responses to two chemical agents (KCl and lactic acid) was decreased in GS, M(SC) and M(CM) groups. Altogether, these data indicate a motor axonal regeneration and an immature metabosensitive afferent fiber regrowth through acellular muscle grafts. Similarities between the two groups grafted with acellular muscles suggest that, in our conditions, implanted Schwann cells do not improve nerve regeneration. Future studies could include engineered conduits that mimic as closely as possible the internal organization of uninjured nerve.

Influence of Aging on Regeneration in End-to-Side Neurorrhaphy

Aging profoundly affects the structural and functional characteristics of the peripheral nervous system. Although several experiments have investigated the effect of aging on nerve regeneration after crush and transection nerve injuries, little is known about the influence of age on end-to-side nerve repairs. It was hypothesized that decreased terminal and collateral sprouting in older animals would be associated with less robust regeneration through end-to-side nerve repairs. In this study, 27 Lewis rats underwent end-to-side repair at ages 2 weeks, 3 months, or 1 year. Histomorphometric assessments at 12 weeks demonstrated increased fiber width, percent neural tissue, and neural density in animals undergoing nerve reconstruction at the age of 2 weeks (P < 0.05). A trend toward further decline in regeneration was noted at ages 1 year versus 3 months. After end-to-side nerve repair, younger animals exhibit a more robust regenerative response, consistent with prior experience in other nerve injury models.

Growth-associated gene and protein expression in the region of axonal sprouting in the aged brain after stroke

Aged individuals exhibit reduced functional recovery after stroke. We examined the expression profile in aged animals of a recently identified group of growth-associated genes that underlies post-stroke axonal sprouting in the young adult. Basal levels of most growth-promoting genes are higher in aged cortex compared with young adult, and are further induced after stroke. Compared with the young adult, these genes are induced at later time points after stroke. For growth-inhibitory molecules, myelin-associated glycoprotein and ephrin A5 are uniquely induced in the aged brain; chondroitin sulfate proteoglycans and oligodendrocyte myelin glycoprotein are induced at earlier time points; and Nogo-A, semaphorin IIIa and NG2 decline in aged vs. young adult after stroke. The aged brain does not simply have a reduction in growth-associated molecules after stroke, but a completely unique molecular profile of post-stroke axonal sprouting.

Recovery of neurophysiological features with time after rat sciatic nerve repair: a magneto-neurographic study

Experimental assessment of peripheral nerve regeneration in rats by electrophysiology is controversial due to low reproducibility of electrophysiological indicators and diminished quantitative evaluation in conventional experimental set-ups. Magnetoneurography (MNG) counteracts these drawbacks by magnetically recording electrophysiological signals ex vivo, thereby providing accurate and quantitative data. In 50 rats, sciatic nerve transection was followed by direct repair. MNG outcome parameters, footprints [static toe spread factor (TSF); function] and muscle weight (MW) were studied for their recovery pattern from 2 to 24 weeks. By using MNG, we showed that the regeneration process still continues when functional recovery (static TSF) becomes stagnant. With regression analysis, MNG parameters amplitude, amplitude area and conduction velocity (CV) demonstrated moderate significant correlation with MW, whereas CV was not significantly associated with static TSF. No significant association exists between MW and static TSF. A Kaplan-Meier survival curve revealed that autotomy/contracture of rat hind paws was not related to decreased MNG outcome values. In conclusion, this study highlights and discusses the dissimilarities between direct (MNG) and indirect (static TSF and MW) assessment techniques of the regeneration process. We emphasise the significance of MNG as a direct derivative of axon regeneration in experimental rat studies. Additionally, we stress the must for right-left ratios, as neurophysiological indicators vary with age, and we confute possible bias in footprint analysis caused by exclusion of autotomy/contracture animals.

Aging, peripheral nerve injury and nociception: effects of the antioxidant 16-desmethyltirilazad

Peripheral neuropathies increase with aging, and reactive oxygen species contribute to the symptomatology of neuropathic pain disorders. In this study, we examined age-related differences in the therapeutic efficacy of pre- or post-treatments of the amino-steroidal antioxidant 16-desmethyltirilazad in delaying the onset and/or limiting the duration of tactile-evoked allodynia following the induction of peripheral mononeuropathies in rats. Two different models of nerve injury were utilized to induce allodynia in young and aged rats: (1) the chronic constriction injury (CCI) model of Bennett and Xie [Bennett GJ, Xie Y-K. A peripheral mononeuropathy in rat that produces disorders of pain sensation like those seen in man. Pain 1988;33:87-107]; (2) the partial sciatic nerve ligation (PSNL) model of Seltzer et al. [Seltzer Z, Dubner R, Shir YA. Novel behavioral model of neuropathic pain disorders produced in rats by partial sciatic nerve injury. Pain 1990;43:205-18]. Calibrated von Frey filaments were used to examine changes in paw withdrawal threshold values. The results demonstrated that pre-treating young and aged rats with 16-desmethyltirilazad prior to the induction of peripheral mononeuropathies prevented the onset of neuropathic pain. However, once post-operative tactile allodynia was established, post-treatment therapy was ineffective at reversing the symptoms. These findings support the mediatory role of reactive oxygen species in neuropathic pain disorders, and suggest that the antiallodynic efficacy of antioxidant intervention is dependent on the time course of administration.

Peripheral and Spinal Motor Reorganization after Nerve Injury and Repair

Functional recovery after peripheral nerve injury depends on the amount as well as on the accuracy of reinnervation by regenerative axons. In this study, the rat sciatic nerve was subjected to crush injury or complete transection repaired by either (1) straight nerve suture, (2) crossed nerve suture of tibial and peroneal fascicles, or (3) silicone tubulization leaving a gap of 4 mm. The compound muscle action potentials (CMAP) of gastrocnemius, tibialis anterior and plantar muscles were recorded 90 days post operation to assess functional reinnervation and Fast Blue, Fluoro Gold and DiI were applied to the nerve branches projecting into these muscles to quantify morphological reinnervation. The CMAP amplitude achieved in gastrocnemius, tibialis anterior and plantar muscles was higher after nerve crush (86%, 82%, 65% of control) than after any surgical nerve repair (straight suture: 49%, 53%, 32%; crossed suture: 56%, 50%, 31%; silicone tube: 42%, 44%, 25%). The total number of labeled motoneurons, however, did not significantly differ between groups (control: 1238 +/- 82, crush: 1048 +/- 49, straight suture: 1175 +/- 106, crossed suture: 1085 +/- 84, silicone tube: 1250 +/- 182). The volume occupied by labeled motoneurons within the spinal cord was larger after surgical nerve repair than in crush or normal control animals, and fewer neurons showed abnormal multiple projections after crush (2.5%) or straight suture (2.2%) than following crossed suture (5%) or silicone tube (6%). In conclusion, nerve repair with a silicone tube leaving a short gap does not increase accuracy of reinnervation.

Age differences in morphological patterns of axonal sprouting and multiple innervation of neuromuscular junctions during muscle reinnervation following nerve crush injury

During the first 4-20 weeks after sciatic nerve crushing injury regrowing axons return to the neuromuscular junction and its reformation is in progress. During this time period age differences in patterns of axonal reinnervation from Wistar rats, with special reference to multiple axonal innervation and sprouting, was morphologically investigated using a neuronal marker (protein gene product 9.5). In young (4 months old) and aged (24 months old) animals, terminal outgrowth at the junction consisted of offshoots extending out from the junctional zone (extraterminal sprouts), and an extraterminal sprout extending to an adjacent endplate (endplate-to-endplate connections). Endplate-to-endplate connections and a nodal sprout served as partners of multiple axonal innervation. Large and complex junctions were formed by multiple innervation and elaboration of terminal branching. The most obvious changes in aged animals were as follows. (1) There were consistently more frequent numbers of extraterminal sprouting, endplate-to-endplate connections, and multiple innervation. The rates of process extension in extraterminal sprouting, however, displayed a significant drop at 4 and 8 weeks post-crush. (2) Late in reinnervation (12, 20 weeks), persistent aberrant changes in axonal reinnervation were more frequently observed, such as clumping of poorly organized nerve bundles, aggregates of multiple extensions, and poorly developed endplate-to-endplate connections, along with disorderly development of nerve terminals. Thus, age affects the reinnervating and sprouting capabilities of axons giving rise to persistent compensatory (though impaired) growth, extension, and branching in the formation of motor pathways during muscle reinnervation and endplate regeneration. The spatiotemporal relationship of these axonal changes to that of the postsynaptic receptor region is discussed.

Bimodal dose-dependence of FK506 on the rate of axonal regeneration in mouse peripheral nerve

FK506 has been shown to enhance the rate of axonal regeneration after peripheral nerve lesions. However, quite variable doses of FK506 have been used in different animal studies. We examined the dose-dependence of FK506 on the rate of axonal regeneration after crush lesion of the mouse sciatic nerve. Mice received daily subcutaneous injections of FK506 at 0.2, 0.5, 1, 2, 5, or 10 mg/kg for 7 days after lesioning. A control group was injected with saline. The distance that regenerative axons advanced from the crush site was measured by the pinch test at 2, 4, and 7 days. Regenerating axons reached greater mean distances in all FK506-treated groups compared to the control group. The fastest regeneration rate was found at 5 mg/kg (12% increase over controls), although the 0.2 and 2 mg/kg doses achieved similar regeneration rates. In contrast, intermediate doses (0.5 and 1 mg/kg) and a higher dose (10 mg/kg) were not different from controls. Calcitonin gene-related peptide immunohistochemical labeling of regenerating axons yielded similar results to those found with the pinch test. Based on our finding of a double peak in the dose-response for FK506, it is hypothesized that at least two mechanisms of action (perhaps corresponding to distinct functional binding sites) are evoked at different concentrations of the drug to accelerate nerve regeneration. These results have clinical implications for the pharmacological treatment of nerve injuries while avoiding immunosuppressive effects and for the design of related drugs with more specific activities.

The spatiotemporal relationship among Schwann cells, axons and postsynaptic acetylcholine receptor regions during muscle reinnervation in aged rats

To morphologically define the aging-related features during muscle reinnervation the spatiotemporal relationships among the major components of the neuromuscular junctions (NMJs) were investigated. A total of 64 rats, 30 adults (4 months old) and 34 aged adults (24 months old), were used. Between 1 and 12 weeks after sciatic nerve-crushing injury, cryosections of skeletal muscle were single or double labeled for S100, a marker of Schwann cells (SCs), for protein gene product 9.5, a neuronal marker, and for alpha-bungarotoxin (alpha-BT), a marker of the acetylcholine receptor site (AChR site), and then observed by confocal laser microscopy. The most obvious age changes were noted: (1) the regenerating SCs and axons were delayed in their arrival at the NMJ, (2) the dimensions of terminal SCs and AChR sites displayed a drastic and long-lasting drop (for terminal SCs, during 1-8 weeks; for AChR sites, during 1-12 weeks); (3) the degree of spatial overlap between AChR sites and terminal SCs was markedly low until 8 weeks post-crush; (4) damage and poor formation in the SCs, terminal axons and AChR sites, together with poor process extension from the terminal SC or terminal axon, were pronounced; (5) persistent aberrant changes, such as multiple innervation and terminal axon sprouting, together with poorly formed collateral innervation, nerve bundles, and NMJs, more frequently occurred in the later reinnervation period. Thus, with aging, regeneration is impaired during the period in which regenerating SC strands and axons extend into NMJs and the subsequent establishment of nerve-muscle contact is in progress. A complex set of morphological abnormalities between or among the TSCs, terminal axons, and AChR sites may be important in slowing of regeneration and reinnervation in aged motor endplates.

Specific force deficit in skeletal muscles of old rats is partially explained by the existence of denervated muscle fibers

We tested the hypothesis that denervated muscle fibers account for part of the specific force (sF(o)) deficit observed in muscles from old adult (OA) mammals. Whole muscle force (F(o)) was quantified for extensor digitorum longus (EDL) muscles of OA and young adult (YA) rats. EDL muscle sF(o) was calculated by dividing F(o) by either total muscle fiber cross-sectional area (CSA) or by innervated fiber CSA. Innervated fiber CSA was estimated from EDL muscle cross sections labeled for neural cell adhesion molecules, whose presence is a marker for muscle fiber denervation. EDL muscles from OA rats contained significantly more denervated fibers than muscles from YA rats (5.6% vs 1.1% of total CSA). When compared with YA muscle, OA muscle demonstrated deficits of 34.1% for F(o), 28.3% for sF(o), and 24.9% for sF(o) calculated by using innervated CSA as the denominator. Denervated muscle fibers accounted for 11.3% of the specific force difference between normal YA and OA skeletal muscle. Other mechanisms in addition to denervation account for the majority of the sF(o) deficit with aging.

Capsaicin-sensitive afferents are involved in signalling transneuronal effects between cutaneous sensory nerves

The aim of the present study was to investigate changes in contralateral nerves associated with peripheral nerve injuries. Transection and subsequent regeneration of the saphenous nerve on one side caused a suppression of the ability of the contralateral saphenous nerve to produce a neurogenic plasma extravasation response. This effect was transient, and was first evident two weeks after injury, reaching its maximum at four weeks, but was no longer detectable at eight weeks. This change was paralleled by a decrease in the content of substance P, a neuropeptide involved in neurogenic plasma extravasation, in the contralateral nerve. The neurotoxin capsaicin was used to deplete the nerve of a subclass of C-fibres, namely the polymodal nociceptor afferents. Pretreatment of the nerve to be lesioned with capsaicin was sufficient to significantly attenuate the changes in the plasma extravasation response and substance P content observed on the contralateral side. The effectiveness of the capsaicin treatment was confirmed by histological examination. These results strongly suggest that changes observed at a site distant from the location of the nerve injury are dependent on the integrity of capsaicin-sensitive C-fibre afferents within the injured nerve. Furthermore, given that the contralateral nerve has commonly been used as the control for an injury conducted on the homologous nerve or muscle on the opposite side of the body, the underlying assumption being that the contralateral nerve remained unchanged, the present findings emphasize the need for separate groups of control animals which have undergone no surgical procedures.

Aging in peripheral nerves: regulation of myelin protein genes by steroid hormones

The process of aging deeply influences morphological and functional parameters of the peripheral nerves. Interestingly, recent observations performed in our laboratory on the rat sciatic nerves have indicated that the deterioration of myelin occurring in the peripheral nerves during aging may be explained by the fall of the messenger levels of the major peripheral myelin proteins (glycoprotein Po, myelin basic protein and peripheral myelin protein 22). At least in the case of the Po, the low levels of its messengers and of the protein itself found in aged animals are increased by the treatment with a physiological progesterone derivative like dihydroprogesterone. It has also been found that in normal adult male rats the levels of the messengers for Po in the sciatic nerve are increased by progesterone, dihydroprogesterone and tetrahydroprogesterone; surprisingly, the gene expression of peripheral myelin protein 22 is stimulated only by tetrahydroprogesterone. These observations have been confirmed in parallel studies performed on Schwann cell cultures. Since tetrahydroprogesterone does not bind to the progesterone receptor but is a ligand for the GABAA receptor, the hypothesis has been put forward that part of the steroidal effects reported might occur not through the classical progesterone receptor, but rather via an interaction with the GABAA receptor. In other experiments it has been found that the gene expression of Po may be decreased by orchidectomy and restored by treatment with the androgen dihydrotestosterone. Altogether, these observations suggest the future use of physiological and/ or synthetic steroid hormones as a possible therapeutic approach for some pathological situations occurring in peripheral nerves during aging and demyelinating diseases.

Aging and neuropathic pain

Although chronic neuropathic pain disorders are more prevalent in the senescent population, little is known about how the aging process alters the thermal hyperalgesic sensitivity to peripheral nerve injury. In this study, neuropathic pain was induced in young, mature and aged FBNF1 hybrid rats via unilateral ligation of the left sciatic nerve. The extent to which the aging process affects the thermal hyperalgesic responsiveness of these animals was investigated. The results demonstrate that the aging process differentially alters nociceptive processing.

Facial nerve rehabilitation after radical parotidectomy

OBJECTIVE

Examine functional outcomes in patients undergoing radical parotidectomy and facial nerve grafting. Identify factors that may affect rehabilitation in these patients.

STUDY DESIGN

Retrospective chart review and photographic analyses of 12 patients undergoing radical parotidectomy with interposition nerve grafts for facial nerve reconstruction.

METHODS

Data obtained for each patient regarding age, sex, histology of parotid neoplasm, cable graft source, administration of postoperative radiotherapy, and treatment for eye rehabilitation. Functional outcomes were assessed with the House-Brackmann grading system at 6 months, 1 year, and 2 years after surgery.

RESULTS

All nerve grafts were harvested from cervical plexus sensory nerves with microscopic epineural repair performed for all neurorrhaphies. Overall, 9 of 12 patients achieved a grade III 2 years after surgery. All patients under age 30 obtained a grade III. Of the seven patients receiving postoperative radiation, five achieved a grade III. Older patients often required surgical procedures to facilitate eye closure.

CONCLUSIONS

Facial nerve rehabilitation after radical parotidectomy can be successfully achieved with cervical plexus interposition nerve grafts. Postoperative radiotherapy did not appear to affect return of function, and younger patients consistently achieved good functional outcomes after nerve grafting. Older patients frequently require surgical procedures for eye rehabilitation after radical parotidectomy.

Changes in cholinergic responses of sweat glands during denervation and reinnervation

Functional sudomotor responses have been studied in sweat glands reinnervated after sciatic nerve crush and partially denervated by cisplatin intoxication in the mouse. The sudomotor function mediated by the sciatic nerve was evaluated by silicone imprints on the plantar surface of the hindpaws. Five days after nerve crush, completely denervated sweat glands became unresponsive to cholinergic stimulation with pilocarpine. During the following weeks, the number of reinnervated, reactive sweat glands increased progressively to reach a maximum of 89% of preoperative control counts by 40 days after nerve crush. At this time, the mean volume of sweat secreted per gland was normal, but reinnervated glands showed a secretory activity abnormally sustained over time after pilocarpine stimulation and, on the other hand, had an increased resistance to the inhibition of secretion induced by atropine. The effects of cisplatin administration on sudomotor function were investigated in two groups of mice, one treated with high doses of cisplatin (10 mg/kg/week for 4 weeks) and another treated with low doses of cisplatin (5 mg/kg/week for 8 weeks). Cisplatin intoxication produced abnormal sudomotor responses indicative of denervation from cumulative doses of 10 mg/kg. The first abnormality found was a partial resistance of sweat glands to atropine, followed by a decrease in the sweat output per gland and finally a decline in the number of sweat glands activated by pilocarpine. These abnormalities in the sudomotor responses were more pronounced in mice treated with a high dose than in those with a lower dose regime.

Acceleration of axonal outgrowth in motor axons from mature and old F344 rats after a conditioning lesion

The conditioning lesion paradigm has proven to be a very useful model in which to examine the mechanisms of axonal outgrowth after injury. In the present study, we have used the conditioning lesion model to examine the ability of motor axons from mature (6-8 months) and old (22-24 months) Fischer 344 rats to form new axonal sprouts. We show that after a single lesion (sham-conditioned axons followed by a testing lesion), axonal outgrowth rates are slower at earlier vs longer postlesion times in mature rats: between 4 and 8 days postlesion, outgrowth rates are 2.4 +/- 0.4 mm/day, whereas between 8 and 11 days postlesion outgrowth rates are 4.6 +/- 0.7 mm/day. After a single lesion in the old rat, at early postlesion times, the axonal outgrowth rate is 1.9 +/- 0.4 mm/day but with increasing time after injury, outgrowth rates slow down to 1.1 +/- 0.8 mm/day. In conditioned motor axons from mature rats, outgrowth rates increase from 3.1 +/- 0.4 mm/day at early postlesion times to 5.2 +/- 0.6 mm/day at longer postlesion times. An even more dramatic increase in outgrowth rate is seen in conditioned axons from old rats: 2.4 +/- 0. 4 mm/day at early postlesion times to 6.3 +/- 1.0 mm/day at later times after lesion. There is no change in the initial delay before sprouting under any condition. These data support the hypothesis that axons from old animals can be stimulated to repair themselves at rates comparable to those seen in younger animals and suggest that there may be an absolute maximum outgrowth rate attainable by newly forming axon sprouts.

Decreased receptivity of pathway connective tissue to sympathetic nerve ingrowth in the developing rat

Sympathetic axons can form atypical pathways to denervated orbital targets in neonatal rats but not in rats aged 30 or more days. The objective of this study was to determine if connective tissue pathways that carry sympathetic nerves lose their ability to sustain axonal sprouting during the early postnatal period. Regions of periorbital sheath known to contain large numbers of sympathetic axons that travel to distal orbital targets were excised from rats (sympathectomized 3 days previously) on postnatal days 6-7, 14-15, 30-31, and 48-49 and placed in anterior chambers of adult host rats. Tissues were removed 3, 6, or 10 days post-transplant and sympathetic ingrowth was analyzed by catecholamine histofluorescence in whole-mount or cryosectioned specimens. Connective tissue transplants from 6-15-day-old donors showed significant fiber ingrowth by 3 days in oculo, and innervation was maximal by 6 days. In contrast, sprouting into 30-49-day-old tissue was significantly slower, with most transplants lacking fibers at 3 days, and with small numbers of short fibers present at 6 days. We conclude that maturational changes occur in periorbital connective tissue pathways in the early postnatal period which make them less receptive to ingrowth by sympathetic nerves. The findings that connective tissue pathways are better substrates for sympathetic sprouting in the neonatal rat supports the view that developmental changes in these tissues are likely to contribute to the impaired reinnervation of orbital targets by contralateral neurons in juvenile and adult rats.

Axonal regeneration of sensory nerves is delayed by continuous intrathecal infusion of nerve growth factor

While it is well established that nerve growth factor is growth promoting for sensory neurons in culture, it is unclear whether it serves such a function in vivo. In fact, our previous studies led to the hypothesis that nerve growth factor could actually impair axonal regeneration by reducing the neuronal cell body response to injury. In the present study, the consequence of continuous intrathecal infusion of nerve growth factor on regeneration of sensory neurons was examined in rats given a bilateral sciatic nerve crush. Rats received nerve growth factor (125 ng/h) as a continuous infusion into the subarachnoid space of the lumbar spinal cord via an osmotic minipump (Alzet); controls received cytochrome C. At seven or 10 days, the pump was removed and L4 or L5 dorsal root ganglion exposed and injected with 50 microCi of (3H)leucine. Animals were killed 24 h later, the sciatic nerves removed, cut into 3 mm segments and the radioactivity in each segment determined by liquid scintillation spectrophotometry. Maximal regeneration distances (determined from the front of the resultant transport curves) were similarly reduced (by approximately 6 mm) in nerve growth factor-infused compared to cytochrome C-infused rats. Thus, regeneration rates (determined between eight and 11 days) were unaltered by nerve growth factor infusion; regeneration rates from cytochrome C-infused and nerve growth factor-infused animals were 2.8 mm/day and 3.1 mm/day, respectively. However, nerve growth factor significantly (P < 0.005) increased the delay to onset for regeneration by two days. Taken together, the present study demonstrates that nerve growth factor delays the onset of regeneration without affecting the rate of regeneration. The results implicate the involvement of at least two signals in the regulation of axonal regeneration in dorsal root ganglion neurons. It is suggested that the loss of nerve growth factor serves as an early, induction signal regulating the onset of regeneration and that a second, unidentified signal independently serves to maintain regeneration.