Acceleration of axonal outgrowth in rat sciatic nerve at one week after axotomy

Brief Electrical Stimulation Promotes Recovery after Surgical Repair of Injured Peripheral Nerves

Injured peripheral nerves regenerate their axons in contrast to those in the central nervous system. Yet, functional recovery after surgical repair is often disappointing. The basis for poor recovery is progressive deterioration with time and distance of the growth capacity of the neurons that lose their contact with targets (chronic axotomy) and the growth support of the chronically denervated Schwann cells (SC) in the distal nerve stumps. Nonetheless, chronically denervated atrophic muscle retains the capacity for reinnervation. Declining electrical activity of motoneurons accompanies the progressive fall in axotomized neuronal and denervated SC expression of regeneration-associated-genes and declining regenerative success. Reduced motoneuronal activity is due to the withdrawal of synaptic contacts from the soma. Exogenous neurotrophic factors that promote nerve regeneration can replace the endogenous factors whose expression declines with time. But the profuse axonal outgrowth they provoke and the difficulties in their delivery hinder their efficacy. Brief (1 h) low-frequency (20 Hz) electrical stimulation (ES) proximal to the injury site promotes the expression of endogenous growth factors and, in turn, dramatically accelerates axon outgrowth and target reinnervation. The latter ES effect has been demonstrated in both rats and humans. A conditioning ES of intact nerve days prior to nerve injury increases axonal outgrowth and regeneration rate. Thereby, this form of ES is amenable for nerve transfer surgeries and end-to-side neurorrhaphies. However, additional surgery for applying the required electrodes may be a hurdle. ES is applicable in all surgeries with excellent outcomes.

An In Vitro Model for Conditioning Lesion Effect

Axons of a peripheral nerve grow faster after an axotomy if it attains a prior injury a few days earlier. This is called conditioning lesion effect (CLE) and very much valued since it may provide new insights into neuron biology and axonal regeneration. There are established in vivo experimental paradigms to study CLE, however, there is a need to have an in vitro conditioning technique where CLE occurs in a maximally controlled environment. Mouse primary sensory neurons were isolated from lumbar 4-5 dorsal root ganglia and incubated at 37 °C on a silicon-coated watch glass that prevents cell attachment. After this conditioning period they were transferred to laminin coated culture dishes. Similar cultures were set up with freshly isolated neurons from control animals and from the animals that received a sciatic nerve cut 3 days earlier. All preparations were placed on a live cell imaging microscopy providing physiological conditions and photographed for 48 h. Axonal regeneration and neuronal survival was assessed. During the conditioning incubation period neurons remained in suspended aggregates and did not grow axons. The regeneration rate of the in vitro conditioned neurons was much higher than the in vivo conditioned and control preparations during the first day of normal incubation. However, higher regeneration rates were compromised by progressive substantial neuronal death in both types of conditioned cultures but not in the control preparations. By using neutralizing antibodies, we demonstrated that activity of endogenous leukemia inhibitory factor is essential for induction of CLE in this model.

Crush injury to motor nerves in the G93A transgenic mouse model of amyotrophic lateral sclerosis promotes muscle reinnervation and survival of functionally intact nerve-muscle contacts

Selective survival of small motor nerve fibers and their neuromuscular contacts in the SOD1^G93A transgenic mouse model of amyotrophic lateral sclerosis (ALS) suggests that smaller regenerated nerve fibers are more able to sustain reformed nerve-muscle connections as functionally intact motor units (MUs). The sciatic nerve was crushed unilaterally in SOD1^G93A transgenic mice at 40 days of age and contractile forces of reinnervated muscles and their MUs were recorded at 90 days in order to determine the capacities of the nerves to regenerate and to form and retain functional neuromuscular connections. Reduced MU numbers in fast-twitch tibialis anterior, extensor digitorum longus and medial gastrocnemius muscles and the lesser reductions in slow-twitch soleus muscle of SOD1^G93A transgenic mice were reversed in reinnervated muscles: there were more reinnervated MUs and their contractile forces and the muscle forces and weights increased. In line with the contrasting ability of only small not large nerve fibers to sprout to form enlarged MUs in the SOD1^G93A transgenic mouse, the smaller regenerating nerve fibers formed enlarged MUs that were better able to survive. Because nerve fibers with and without muscle contacts were severed by the sciatic nerve crush injury, the conditioning lesion is untenable as the explanation for improved maintenance of reinnervated neuromuscular junctions. Elevated neurotrophic factor expression in axotomized motoneurons and/or denervated Schwann cells and the synapse withdrawal from axotomized motoneurons are other factors that, in addition to reduced size of nerve fibers reinnervating muscles, may account for increased survival and size of reinnervated MUs in ALS.

Neurite growth acceleration of adult Dorsal Root Ganglion neurons illuminated by low-level Light Emitting Diode light at 645 nm

The effect of a 645 nm Light Emitting Diode (LED) light irradiation on the neurite growth velocity of adult Dorsal Root Ganglion (DRG) neurons with peripheral axon injury 4-10 days before plating and without previous injury was investigated. The real amount of light reaching the neurons was calculated by taking into account the optical characteristics of the light source and of media in the light path. The knowledge of these parameters is essential to be able to compare results of the literature and a way to reduce inconsistencies. We found that 4 min irradiation of a mean irradiance of 11.3 mW/cm(2) (corresponding to an actual irradiance reaching the neurons of 83 mW/cm(2)) induced a 1.6-fold neurite growth acceleration on non-injured neurons and on axotomized neurons. Although the axotomized neurons were naturally already in a rapid regeneration process, an enhancement was found to occur while irradiating with the LED light, which may be promising for therapy applications. Dorsal Root Ganglion neurons (A) without previous injury and (B) subjected to a conditioning injury.

Morphology and intrinsic excitability of regenerating sensory and motor neurons grown on a line micropattern

Axonal regeneration is one of the greatest challenges in severe injuries of peripheral nerve. To provide the bridge needed for regeneration, biological or synthetic tubular nerve constructs with aligned architecture have been developed. A key point for improving axonal regeneration is assessing the effects of substrate geometry on neuronal behavior. In the present study, we used an extracellular matrix-micropatterned substrate comprising 3 µm wide lines aimed to physically mimic the in vivo longitudinal axonal growth of mice peripheral sensory and motor neurons. Adult sensory neurons or embryonic motoneurons were seeded and processed for morphological and electrical activity analyses after two days in vitro. We show that micropattern-guided sensory neurons grow one or two axons without secondary branching. Motoneurons polarity was kept on micropattern with a long axon and small dendrites. The micro-patterned substrate maintains the growth promoting effects of conditioning injury and demonstrates, for the first time, that neurite initiation and extension could be differentially regulated by conditioning injury among DRG sensory neuron subpopulations. The micro-patterned substrate impacts the excitability of sensory neurons and promotes the apparition of firing action potentials characteristic for a subclass of mechanosensitive neurons. The line pattern is quite relevant for assessing the regenerative and developmental growth of sensory and motoneurons and offers a unique model for the analysis of the impact of geometry on the expression and the activity of mechanosensitive channels in DRG sensory neurons.

Motor neuron target selectivity and survival after prolonged axotomy

PURPOSE

After a cut peripheral nerve is repaired, motor neurons usually regenerate across the lesion site, however they often enter an inappropriate Schwann cell tube and may be directed to an inappropriate target organ such as skin, resulting in continued loss of function. In fact, only about 10% of adults who receive a peripheral nerve repair display full functional recovery. The reasons for this are many and complex, however one aspect is whether the motor neuron has undergone a prolonged period of axotomy prior to nerve repair. Previous studies have suggested a deleterious effect of prolonged axotomy.

METHODS

We examined the influence of prolonged axotomy on target selectivity using a cross-reinnervation model of rat obturator motor neurons regrowing into the distal femoral nerve, with its normal bifurcating pathways to muscle and skin.

RESULTS

Surprisingly, we found that a prolonged period of axotomy resulted in an increase in motor neuron regeneration accuracy. In addition, we found that regeneration accuracy could be increased even further by a simple surgical manipulation of the distal terminal nerve pathway to skin.

CONCLUSIONS

These results suggest that under certain conditions prolonged axotomy may not be detrimental to the final accuracy of motor neuron regeneration and highlight that a simple manipulation of terminal nerve pathways may be one approach to increase such regeneration accuracy.

Calcium-activated chloride current expression in axotomized sensory neurons: what for?

Calcium-activated chloride currents (CaCCs) are activated by an increase in intracellular calcium concentration. Peripheral nerve injury induces the expression of CaCCs in a subset of adult sensory neurons in primary culture including mechano- and proprioceptors, though not nociceptors. Functional screenings of potential candidate genes established that Best1 is a molecular determinant for CaCC expression among axotomized sensory neurons, while Tmem16a is acutely activated by inflammatory mediators in nociceptors. In nociceptors, such CaCCs are preferentially activated under receptor-induced calcium mobilization contributing to cell excitability and pain. In axotomized mechano- and proprioceptors, CaCC activation does not promote electrical activity and prevents firing, a finding consistent with electrical silencing for growth competence of adult sensory neurons. In favor of a role in the process of neurite growth, CaCC expression is temporally correlated to neurons displaying a regenerative mode of growth. This perspective focuses on the molecular identity and role of CaCC in axotomized sensory neurons and the future directions to decipher the cellular mechanisms regulating CaCC during neurite (re)growth.

Reduced BACE1 activity enhances clearance of myelin debris and regeneration of axons in the injured peripheral nervous system

β-Site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) is an aspartyl protease best known for its role in generating the amyloid-β peptides that are present in plaques of Alzheimer's disease. BACE1 has been an attractive target for drug development. In cultured embryonic neurons, BACE1-cleaved N-terminal APP is further processed to generate a fragment that can trigger axonal degeneration, suggesting a vital role for BACE1 in axonal health. In addition, BACE1 cleaves neuregulin 1 type III, a protein critical for myelination of peripheral axons by Schwann cells during development. Here, we asked whether axonal degeneration or axonal regeneration in adult nerves might be affected by inhibition or elimination of BACE1. We report that BACE1 knock-out and wild-type nerves degenerated at a similar rate after axotomy and to a similar extent in the experimental neuropathies produced by administration of paclitaxel and acrylamide. These data indicate N-APP is not the sole culprit in axonal degeneration in adult nerves. Unexpectedly, however, we observed that BACE1 knock-out mice had markedly enhanced clearance of axonal and myelin debris from degenerated fibers, accelerated axonal regeneration, and earlier reinnervation of neuromuscular junctions, compared with littermate controls. These observations were reproduced in part by pharmacological inhibition of BACE1. These data suggest BACE1 inhibition as a therapeutic approach to accelerate regeneration and recovery after peripheral nerve damage.

Best1 is a gene regulated by nerve injury and required for Ca2+-activated Cl- current expression in axotomized sensory neurons

We investigated the molecular determinants of Ca(2+)-activated chloride current (CaCC) expressed in adult sensory neurons after a nerve injury. Dorsal root ganglia express the transcripts of three gene families known to induce CaCCs in heterologous systems: bestrophin, tweety, and TMEM16. We found with quantitative transcriptional analysis and in situ hybridization that nerve injury induced upregulation of solely bestrophin-1 transcripts in sensory neurons. Gene screening with RNA interference in single neurons demonstrated that mouse Best1 is required for the expression of CaCC in injured sensory neurons. Transfecting injured sensory neurons with bestrophin-1 mutants inhibited endogenous CaCC. Exogenous expression of the fusion protein green fluorescent protein-Bestrophin-1 in naive neurons demonstrated a plasma membrane localization of the protein that generates a CaCC with biophysical and pharmacological properties similar to endogenous CaCC. Our data suggest that Best1 belongs to a group of genes upregulated by nerve injury and supports functional CaCC expression in injured sensory neurons.

Preserved Regeneration and Functional Recovery of the Injured Recurrent Laryngeal Nerve after Secondary Surgical Repair in Adult Rats

Objectives:

Transection of the recurrent laryngeal nerve (RLN) is accompanied by poor functional recovery, despite primary repair, because of regeneration difficulties. Nimodipine can promote regeneration, but it is not yet clear whether preoperative treatment is necessary. It is also not clear whether surgical repair following RLN injury may be performed in a second procedure, with preserved regeneration. This study investigated the time window for secondary surgical repair of the transected RLN and the need for preoperative administration of nimodipine.

Methods:

In adult rats, the left RLN was transected and repaired at time intervals up to 3 weeks after transection, in combination with nimodipine treatment starting either before or after the operation. Regeneration and neuromuscular recovery were assessed by electrophysiology, retrograde tracing, and immunohistochemistry.

Results:

Similar (whether 0, 2, or 7 days) regenerative results were obtained when the RLN was repaired up to 1 week after injury, given nimodipine administration, whereas fewer motor neurons managed to regenerate after nerve repair at 3 weeks after the initial transection. No beneficial effect was detected from preoperative nimodipine administration.

Conclusions:

Provided that nimodipine is administered, surgical reconstruction of the RLN can be performed within 1 week after the initial nerve trauma, with preserved neuromuscular function. Nimodipine may be administered at the time of RLN transection injury.

Electrical stimulation of intact peripheral sensory axons in rats promotes outgrowth of their central projections

A lesion of a peripheral nerve before a second injury (conditioning lesion, CL), enhances peripheral and central regeneration of dorsal root ganglion (DRG) axons. This effect is mediated by elevated neuronal cAMP. Here we wanted to investigate whether electrical stimulation (ES) of an intact nerve, which has been shown to accelerate peripheral axon outgrowth, is also effective in promoting axon regeneration of injured DRG axons in vitro and of the central DRG axons in vivo and, whether this effect is mediated by elevation of cAMP. For the in vitro assay, the intact sciatic nerve of adult rats was stimulated at 20 Hz for 1 h, 7 days before harvest and primary culture of DRG neurons on a growth permissive substrate. In the in vivo study, the central axons of the lumbosacral DRGs were cut in the Th8 dorsal column, and the sciatic nerve was either cut or left intact, and subjected to 1 h ES at 20 Hz or 200 Hz. In vitro, ES increased neurite outgrowth 4-fold as compared to non-stimulated DRG neurons. In vivo, ES at 20 Hz significantly increased axon outgrowth into the central lesion site as compared to the Sham control. The 20 Hz ES was as effective as the CL in increasing axon outgrowth into the lesion site but not in promoting axonal elongation even though 20 Hz ES increased intracellular cAMP levels in DRG neurons as effectively as the CL. Thus elevation of cAMP may account for the central axonal outgrowth after ES and a CL.

NKCC1 phosphorylation stimulates neurite growth of injured adult sensory neurons

Peripheral nerve section promotes regenerative, elongated neuritic growth of adult sensory neurons. Although the role of chloride homeostasis, through the regulation of ionotropic GABA receptors, in the growth status of immature neurons in the CNS begins to emerge, nothing is known of its role in the regenerative growth of injured adult neurons. To analyze the intracellular Cl- variation after a sciatic nerve section in vivo, gramicidin perforated-patch recordings were used to study muscimol-induced currents in mice dorsal root ganglion neurons isolated from control and axotomized neurons. We show that the reversal potential of muscimol-induced current, E(GABA-A), was shifted toward depolarized potentials in axotomized neurons. This was attributable to Cl- influx because removal of extracellular Cl- prevented this shift. Application of bumetanide, an inhibitor of NKCC1 cotransporter and E(GABA-A) recordings in sensory neurons from NKCC1-/- mice, identified NKCC1 as being responsible for the increase in intracellular Cl- in axotomized neurons. In addition, we demonstrate with a phospho-NKCC1 antibody that nerve injury induces an increase in the phosphorylated form of NKCC1 in dorsal root ganglia that could account for intracellular Cl- accumulation. Time-lapse recordings of the neuritic growth of axotomized neurons show a faster growth velocity compared with control. Bumetanide, the intrathecal injection of NKCC1 small interfering RNA, and the use of NKCC1-/- mice demonstrated that NKCC1 is involved in determining the velocity of elongated growth of axotomized neurons. Our results clearly show that NKCC1-induced increase in intracellular chloride concentration is a major event accompanying peripheral nerve regeneration.

Neuroregeneration in Composite Tissue Allografts: Effect of Low-Dose FK506 and Mycophenolate Mofetil Immunotherapy

BACKGROUND

The immunosuppressant FK506 has been reported to increase the rate of peripheral nerve regeneration in nerve crush injury and nerve allograft models. The purpose of this study was to determine whether low doses of FK506 and mycophenolate mofetil had a neuroregenerative effect in revascularized peripheral nerve allografts in a rat hind limb transplantation model.

METHODS

Wistar Furth rat recipients received limbs from syngeneic Wistar Furth donors (group 1, n = 4) or from allogeneic August X Copenhagen Irish rat donors (group 2, n = 6). Wistar Furth recipients received limbs from August X Copenhagen Irish donors and were treated with FK506/mycophenolate mofetil for 5 months (group 3, n = 7). At the end of the follow-up period, histomorphometric analysis of sciatic and tibial nerves from transplanted and intact hind limbs was conducted. Sciatic and tibial nerves were examined at the level of coaptation and near the neuromuscular junction, respectively.

RESULTS

Transplanted limbs in groups 1 and 3 completed the study without rejection, while the limbs in group 2 were rejected within a few days. Sciatic and tibial nerve analysis in groups 1 and 3 limbs showed myelinated axons of various diameters but in significantly fewer numbers than in nontransplanted contralateral nerves. The number and size of myelinated axons of transplanted nerves at corresponding levels were not significantly different between syngeneic and allogeneic (FK506/mycophenolate mofetil-treated) transplants.

CONCLUSIONS

The authors conclude that long-term neuroregeneration of revascularized peripheral nerves using low-dose FK506/mycophenolate mofetil was similar to that of syngeneic transplants. The occurrence of acute rejection episodes with low-dose FK506/mycophenolate mofetil did not appear to benefit nor impair neuroregeneration.

Conditioning lesions enhance axonal regeneration of descending brain neurons in spinal-cord-transected larval lamprey

In larval lamprey, with increasing recovery times after a transection of the rostral spinal cord, there is a gradual recovery of locomotor behavior, and descending brain neurons regenerate their axons for progressively greater distances below the transection site. In the present study, spinal cord "conditioning lesions" (i.e., transections) were performed in the spinal cord at 30% body length (BL; normalized distance from the head) or 50% BL. After various "lesion delay times" (D), a more proximal spinal cord "test lesion" (i.e., transection) was performed at 10% BL, and then, after various recovery times (R), horseradish peroxidase was applied to the spinal cord at 20% BL to determine the extent of axonal regeneration of descending brain neurons. Conditioning lesions at 30% BL, lesion delay times of 2 weeks, and recovery times of 4 weeks (D-R = 2-4 group) resulted in a significant enhancement of axonal regeneration for the total numbers of descending brain neurons as well as neurons in certain brain cell groups compared to control animals without conditioning lesions. Experiments with hemiconditioning lesions, which reduce interanimal variability, confirmed that conditioning lesions do significantly enhance axonal regeneration and indicate that axotomy rather than diffusible factors released at the injury site is primarily involved in this enhancement. Results from the present study suggest that conditioning lesions "prime" descending brain neurons via cell body responses and enhance subsequent axonal regeneration, probably by reducing the initial delay and/or increasing the initial rate of axonal outgrowth.

Axotomy-induced expression of calcium-activated chloride current in subpopulations of mouse dorsal root ganglion neurons

Whole cell patch-clamp recordings of calcium-activated chloride current [ICl(Ca)] were made from adult sensory neurons of naive and axotomized mouse L4-L6 lumbar dorsal root ganglia after 1 day of culture in vitro. A basal ICl(Ca) was specifically expressed in a subset of naive medium-diameter neurons (30-40 microm). Prior nerve injury, induced by sciatic nerve transection 5 days before experiments, increased both ICl(Ca) amplitude and its expression in medium-diameter neurons. Moreover, nerve injury also induced ICl(Ca) expression in a new subpopulation of neurons, the large-diameter neurons (40-50 microm). Small-diameter neurons (inferior to 30 microm) never expressed ICl(Ca). Regulated ICl(Ca) expression was strongly correlated with injury-induced regenerative growth of sensory neurons in vitro and nerve regeneration in vivo. Cell culture on a substrate not permissive for growth, D,L-polyornithine, prevented both elongation growth and ICl(Ca) expression in axotomized neurons. Regenerative growth and the induction of ICl(Ca) expression take place 2 days after injury, peak after 5 days of conditioning in vivo, slowly declining thereafter to control values. The selective expression of ICl(Ca) within medium- and large-diameter neurons conditioned for rapid, efficient growth suggests that these channels play a specific role in postinjury behavior of sensory neuron subpopulations such as neuropathic pain and/or axonal regeneration.

Age, diet and injury affect the survival of facial motoneurons

Using the model of facial nerve avulsion, we have compared the effects of injury, age and diet on motoneuronal survival. One to four weeks after nerve avulsion, 50-75% motoneuron loss was quantified in ad libitum-fed rats aged 7 days (neonate), 6 months (adult) and 24 months (aging) at the time of injury. Evidence of apoptosis was found for neonatal rats at 3 days post-injury, but not for neonates examined 7 days or adult or aging rats examined 1 month after injury. Non-operated, ad libitum-fed rats showed no significant loss of facial motoneurons by 24 months. Surprisingly, non-operated rats whose food intake was restricted to 15 g standard rat chow per day from the age of 6 months lost 50% of their motoneurons by 24 months. Facial nerve avulsion of 24-month-old rats raised on this restricted diet did not result in any additional loss of motoneurons one month after injury. These results challenge the common view that aging results in neuronal loss and that dietary restriction is universally beneficial.

Sciatic nerve axotomy in aged rats: response of motoneurons and the effect of GM1 ganglioside treatment

The number, size, and staining intensity of choline acetyltransferase (ChAT)-immunopositive cells in the retrodorsal lateral nucleus (RDLN) of the spinal cord were studied in young (3-5 months old) and aged (22-24 months old) rats following left sciatic nerve distal transection (axotomy) and treatment with GM1 ganglioside. The cell size and the ChAT immunostaining density were decreased in the RDLN of non-manipulated as well as in the contralateral intact side of axotomized aged rats. Axotomy had no effect on the number of RDLN motoneurons in both aged and young rats. In the young rats, there was a decrease in the size of motoneurons 7 days post-axotomy and a partial spontaneous recovery occurred by 21 days. Axotomy did not reduce further the size of aged motoneurons, however. The ChAT staining intensity of the axotomized RDLN declined in both age groups after 7 days, and there was spontaneous near normal recovery by 21 days. In the aged rats, GM1 administration for 7 days corrected the cell size and ChAT immunoreactivity of the contralateral intact RDLN. With regard to axotomized RDLN neurons, 7 days of GM1 restored the cell size but not the ChAT immunostaining in young animals. The same treatment schedule, however, corrected both cell size and staining in aged rats. Administration of GM1 for 21 days had no further effect on the morphometric parameters of the axotomized motoneurons in aged rats, but slightly enhanced the recovery of ChAT immunostaining in young rats. Thus, it appears that GM1 facilitates the phenotypic recovery of RDLN motoneurons during aging and after axotomy.

Involvement of alpha7beta1 integrin in the conditioning-lesion effect on sensory axon regeneration

Conditioning lesions of peripheral nerves improve axonal regeneration after injury and involve changes in expression of proteins required for axonal growth. Integrin alpha7beta1 expression in motor and sensory neurons increases following nerve lesions and motor axon regeneration is impaired in alpha7 integrin KO mice (J. Neurosci. 20, 1822-1830). To investigate the role of alpha7beta1 integrin in sensory axon regeneration, dorsal root ganglia of adult mice were cultured in gels of laminin-rich extracellular matrix (Matrigel) or collagen. Normal dorsal root ganglia in Matrigel or collagen supplemented with laminin showed spontaneous axonal outgrowth, which was greatly increased in conditioned preparations, but only in the presence of laminin. Conditioned dorsal root ganglia from normal mice cultured with a blocking antibody to beta1 integrin and from alpha7 integrin KO mice showed reduced axonal growth in both Matrigel- and laminin-supplemented collagen gels. Enhanced axonal regeneration after conditioning lesions therefore involves increased responsiveness to laminin and integrin alpha7beta1 expression.

Calcium/calmodulin-dependent protein kinase IIbeta isoform is expressed in motor neurons during axon outgrowth and is part of slow axonal transport

Previously, we identified calcium/calmodulin-dependent protein kinase IIbeta (CaMKIIbeta) mRNA in spinal motor neurons with 372 bp inserted in what corresponds to the "association" domain of the protein. This was interesting because known additions and deletions to CaMKIIbeta mRNA are usually less than 100 bp in size and found in the "variable" region. Changes in the association domain of CaMKIIbeta could influence substrate specificity, activity or intracellular targeting. We show that three variations of this insert are found in CNS neurons or sciatic motor neurons of Sprague-Dawley rats. We used PCR and nucleic acid sequencing to identify inserts of 114, 243, or 372 bases. We also show that addition of the 372 bases is associated with outgrowth of the axon (the standard CaMKIIbeta downregulates when axon outgrowth occurs). Radiolabeling, immunoblots, and 2D PAGE identified this larger CaMKIIbeta as part of the group of soluble proteins moving at the slowest rate of axonal transport (SCa) in sciatic motor neurons (similar1 mm/day). This group is composed mainly of structural proteins (e.g., tubulin) used to assemble the cytoskeleton of regrowing axons.

Nerve regeneration enhancement by tourniquet

The use of tourniquet compression as a non-invasive method to enhance axonal regeneration was assessed in the rat sciatic nerve. One hind limb of the rat was subjected to compression by a tourniquet set at 300 mmHg for 30 or 120 min followed by bilateral test crush lesions performed either directly or after a conditioning interval of 3 or 6 days, with the non-compressed side serving as a control. Axonal regeneration distances were evaluated after 3 days by the pinch reflex test. We found that compression caused an increased outgrowth length of sensory axons compared to the controls. The effect was most obvious after 120 min of compression with a conditioning interval of 6 days. Tourniquet compression has a conditioning lesion effect on peripheral nerve and may enhance nerve regeneration.

Cytoplasmic dynein conversion at a crush injury in rat peripheral axons

Cytoplasmic dynein is a motor for retrograde axonal transport for movement of membranous organelles toward the neuronal cell body. However, cytoplasmic dynein is synthesized in the cell body and conveyed along the axon to nerve terminals. To characterize the axonal transport of cytoplasmic dynein in relation to synaptic vesicles and other membrane compartments, immunocytochemical and cytofluorimetric scanning analyses of crush-operated rat sciatic nerves were performed. Distal to the crush, the kinetics of dynein accumulation were consistent with its role in the retrograde transport of membranous organelles. During the initial 3 hr after crush, only small amounts of dynein-immunoreactive material accumulated proximal to the crush. This is consistent with metabolic labeling studies showing that most of the dynein moving in the anterograde direction is in the slow component of axonal transport. Thereafter, the rate of proximal accumulation of dynein increased, and by 8 hr postcrush a large amount of dynein immunoreactivity was observed. This accelerated accumulation may be due to recruitment of dynein from slow component b onto organelles proximal to the crush. Double labeling demonstrated that dynein immunoreactivity colocalized with synaptophysin, a transmembrane protein found in small, clear synaptic vesicles. In contrast, dynein immunoreactivity did not colocalize well with calcitonin gene-related peptide (CGRP), a peptide matrix marker for some large dense-cored vesicles. Finally, dynein immunoreactivity colocalized with the anterograde transport motor kinesin both proximal and distal to a crush, suggesting that kinesin may carry some dynein-containing membrane compartments during fast anterograde axonal transport.

Respective roles of inflammation and axonal breakdown in the regulation of peripheral nerve hemopexin: an analysis in rats and in C57BL/Wlds mice

We have previously demonstrated that one of the peripheral nerve responses to injury is the overexpression of hemopexin (HPX). Here, we demonstrate that Wallerian degeneration is required for this response, since HPX does not increase in C57BL/Wlds mice, which display a severely impaired Wallerian degeneration. We also show that HPX synthesis is dramatically increased in macrophages during their activation or after IL-6 stimulation. However, IL-6-driven HPX overexpression occurs in vivo and in vitro in the absence of substantial macrophage invasion. We conclude that, after nerve injury, HPX overexpression occurs first in Schwann cells as a result of axotomy and is subsequently regulated by inflammation. Furthermore, our results and those already described suggest that IL-6, synthesized by the various cell types producing HPX, control nerve HPX expression via paracrine and autocrine mechanisms.

Up-regulated uridine kinase gene identified by RLCS in the ventral horn after crush injury to rat sciatic nerves

Rat sciatic nerve crush injury is one of the models commonly employed for studying the mechanisms of nerve regeneration. In this study, we analyzed the temporal change of gene expression after injury in this model, to elucidate the molecular mechanisms involved in nerve regeneration. First, a cDNA analysis method, Restriction Landmark cDNA Scanning (RLCS), was applied to cells in the ventral horn of the spinal cord during a 7-day period after the crush injury. A total of 1991 cDNA species were detected as spots on gels, and 37 of these were shown to change after the injury. Temporally changed patterns were classified into three categories: the continuously up-regulated type (10 species), the transiently up-regulated type (22 species), and the down-regulated type (5 species). These complex patterns of gene expression demonstrated after the injury suggest that precise regulation in molecular pathways is required for accomplishing nerve regeneration. Secondly, the rat homologue of uridine kinase gene was identified as one of the up-regulated genes. Northern blot analysis on rat ventral horn tissue and brain revealed that the UK gene had three transcripts with different sizes (4.3, 1. 4, and 1.35 kb, respectively). All of the transcripts, especially the 4.3 kb one, were up-regulated mainly in a bimodal fashion during the 28-day period after the injury. The RLCS method that we employed in the present study shows promise as a means to fully analyze molecular changes in nerve regeneration in detail.

Peripheral but not central axotomy promotes axonal outgrowth and induces alterations in neuropeptide synthesis in the nodose ganglion of the rat

We investigated the effects of central and peripheral axotomy of the sensory neurons in the nodose ganglion on neurite outgrowth and neuropeptide expression. Axonal outgrowth was studied in ganglia subjected to a conditioning lesion of the vagus nerve 6 days prior to in vitro explantation. In such cultures, a conditioning effect, i. e. a shorter initial delay and faster axonal outgrowth, was observed after peripheral axotomy, while central axotomy had no effect. Neuropeptide expression was measured by immunocytochemistry 3 days after axotomy. Peripheral axotomy induced an increase in the number of neurons expressing the C-terminal flanking peptide of neuropeptide Y (C-PON), galanin (GAL) and vasoactive intestinal peptide (VIP). In contrast, central axotomy did not affect neuropeptide expression. These results suggest that both axonal outgrowth and expression of neuropeptides in the sensory neurons of the nodose ganglion could be regulated by the contact of the cells with their peripheral, but not their central targets.

Efficacy of delayed or discontinuous FK506 administrations on nerve regeneration in the rat sciatic nerve crush model: lack of evidence for a conditioning lesion-like effect

We examined whether the nerve regenerative property of FK506 exhibits a 'window-of-opportunity' corresponding to the time of injury for maximal efficacy in the sciatic nerve crush model. FK506 (5 mg/kg, s.c.) was administered over the 18-day period of study according to three dosage regiments: delayed (days 9-17), discontinuous (days 0-8) and continuous (days 0-17) administrations. Quantitation of axonal calibers and the extend of myelination in the soleus nerve at 18 days demonstrated that both delayed and discontinuous administrations were equally effective, arguing against a 'window-of-opportunity' for FK506 nerve regenerative effect. However, both protocols were less effective than continuous administration indicating that the compound needs to be given during the entire regenerative period to elicit maximal efficacy.

The effect of age and injury on the expression of inducible nitric oxide synthase in facial motor neurons in F344 rats

Nitric oxide has been implicated in both normal neuronal aging as well as nerve repair events because of its known roles in synaptic plasticity, synaptogenesis and neuroplathologic processes. In this study, we have determined the effect of aging, by comparing brainstem facial motor neurons (FMNs) as well as blood vessels, from adult F344 rats to those in old animals. Inducible nitric oxide synthase (iNOS) expression was determined both by immunohistochemistry using an antibody to iNOS on tissue sections and slot blots. In adult rats, iNOS expression was detectable only in FMNs and not in blood vessels. In old rats, there were robust levels of iNOS protein in blood vessels, while iNOS protein was not detectable in FMNs from old rats. There was also a 12-fold increase in iNOS expression in isolated blood vessels from old rats compared to vessels from adult animals. To determine the effect of injury on iNOS expression, the facial nerve was transected and immunocytochemistry performed as above. After nerve transection in adult rats, iNOS was demonstrable only in blood vessels after 1 day, but by 7 days iNOS protein immunoreactivity was robust in FMNs. In old animals, iNOS protein expression was observed only in FMNs at 1 day, but by 7 days after injury, protein immunoreactivity was localized to the blood vessels. These data suggest that aging and injury differentially affect the expression of iNOS and that the up-regulation of iNOS may be important for the availability of nitric oxide in the aged or injured nervous system.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Lumbar motor neuron size and number is affected by age in male F344 rats

Motor neurons in the spinal cord of old rats appear similar in size but less numerous compared with those in mature rats; they also contain a large amount of lipofuscin, the lipid peroxidation by-product whose function is largely unknown. The object of this study was to morphometrically characterize motor neurons found in the L4/L5 lumbar spinal cord of mature (6-month) and old (22-month) rats. Paraformaldehyde-fixed, lumbar spinal cords from six rats at each age were embedded in paraffin, sectioned at 6 microm and stained with 0.1% toluidine blue. The nucleolar diameter and area from a minimum of 34 motor neurons per spinal cord were measured. Motor neuron number was calculated using Abercrombie's (Abercrombie, 1946) formula after correcting for tissue shrinkage. Motor neuron number was decreased with age while the neuronal area increased with age. Nucleolar diameter also increased in old rats. Frequency distributions of motor neuron area revealed unimodal distributions of motor neurons rats of both ages. We suggest that larger nucleolar diameter reflects more metabolically active neurons in old rats while larger neuron area is a reflection of the presence of lipofuscin in old motor neurons.

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.

Oral administration of a nonimmunosuppressant FKBP-12 ligand speeds nerve regeneration

We recently showed that s.c. injections of a nonimmunosuppressant FK506 binding protein-12 (FKBP-12) ligand (V-10,367) accelerates nerve regeneration in the rat sciatic nerve crush model. Here we examined the oral efficacy of this compound for speeding nerve regeneration. Rats receiving V-10,367 (5, 15 or 50 mg/kg/day) by oral gavage all demonstrated an increase in nerve regeneration compared to vehicle-treated controls. Functional recovery was observed earliest and axonal calibers of regenerating axons in the soleus nerve were largest in the 15 mg/kg group, mean axonal areas being increased by 66% compared to controls. Orally active nonimmunosuppressant FKBP-12 ligands may be useful for the treatment of human peripheral nerve disorders.

Mechanisms of enhancement of neurite regeneration in vitro following a conditioning sciatic nerve lesion

To examine the mechanisms responsible for the more rapid nerve regeneration observed after a previous (conditioning) nerve injury, adult rats were subjected to a midthigh sciatic nerve transection by using one of three protocols designed to facilitate or restrict nerve regeneration: 1) ligation, in which transected axons were prevented from regenerating; 2) cut, in which transected axons were permitted to extend into peripheral target tissue but were separated from the denervated peripheral nerve stump; and 3) crush, in which axons could regenerate normally through the denervated distal nerve tract. The affected dorsal root ganglia (DRG) were subsequently removed, dissociated, and cultured for up to 3 days, and the timing of neurite initiation, rate of outgrowth, and arborization pattern of previously injured neurons were compared with control DRG. Our results indicate that conditioning lesions have at least four distinct and differentially regulated effects on neuronal morphogenesis: 1) conditioning lesions promote earlier neurite initiation, 2) prior nerve injury decreases the ability of neurons to extend long neurites following a second axotomy, 3) exposure to the environment of a denervated peripheral nerve stimulates greater initial rates of neurite outgrowth, and 4) conditioning lesions reduces initial neuritic branching frequency, resulting in straighter neurites whose growth cones extend further distances from their cell bodies. The primary effect of all conditioning lesions on cultured DRG neurons appeared to be to advance the timing of morphogenesis, resulting in conditioning-lesioned neurons that exhibited characteristics consistent with control neurons that had been cultured for an additional day or more. A secondary effect of conditioning lesions on neurite outgrowth rates was dependent on the local environment of the axons prior to culturing.

Changes in expression and localization of hemopexin and its transcripts in injured nervous system: a comparison of central and peripheral tissues

The recent demonstration of hemopexin synthesis in the adult rat sciatic nerve and its accumulation after injury has raised the question of the possible role of this acute phase protein during the process of nerve repair. To gain insight into its function, we have compared the distribution of both hemopexin and its messenger RNA in the peripheral and the central nervous systems. We find that hemopexin is present in all types of peripheral nerves and ganglia, confined to the extracellular matrix and basement membranes of the endoneurium, blood vessels and connective tissues. After injury, hemopexin messenger RNA is overexpressed by Schwann cells, fibroblasts and invading macrophages. The content in hemopexin protein increases in all nerves studied, without changes in localization. Therefore, hemopexin does not appear to be associated with the fate of myelin or with the regeneration of a particular type of nerve fibre. In the central nervous system, hemopexin messenger RNA cannot be detected and the protein is only found in basement membranes of the vascular system (capillaries, meninges and choroid plexus). Furthermore, hemopexin and its messenger RNA remain absent from the distal part of the injured optic nerves. Our results further support the idea that hemopexin plays specific roles during nerve repair, and that it may be associated with the endoneurial extracellular matrix.

Calcium/calmodulin-dependent protein kinase II expression in motor neurons: effect of axotomy

Although Ca2+/calmodulin-dependent (CaM) protein kinase II isoforms are present in the nervous system in high amounts, many aspects of in vivo expression, localization, and function remain unexplored. During development, CaM kinase IIalpha and IIbeta are differentially expressed. Here, we examined CaM kinase II isoforms in Sprague-Dawley rat sciatic motor neurons before and after axotomy. We cut the L4-5 spinal nerves unilaterally and exposed the proximal nerve stumps to a fluoroprobe, to retrogradely label the neurons of origin. Anti-CaM kinase IIbeta antibody showed immunoreactivity in motor neurons, which decreased to low levels by 4 days after axotomy. We found a similar response by in situ hybridization with riboprobes. The decrease in expression of mRNA and protein was confined to fluorescent motor neurons. For CaM kinase IIalpha, in situ hybridization showed that the mRNA was in sciatic motor neurons, with a density unaffected by axotomy. However, these neurons were also enlarged, suggesting an up-regulation of expression. Northern blots confirmed an mRNA increase. We were unable to find CaM kinase IIalpha immunoreactivity before or after axotomy in sciatic motor neuron cell bodies, suggesting that CaM kinase IIalpha is in the axons or dendrites, or otherwise unavailable to the antibody. Using rats with crush lesions, we radiolabeled axonal proteins being synthesized in the cell body and used two-dimensional polyacrylamide gel electrophoresis with Western blots to identify CaM kinase IIalpha as a component of slow axonal transport. This differential regulation and expression of kinase isoforms suggests separate and unique intracellular roles. Because we find CaM kinase IIbeta down-regulates during axonal regrowth, its role in these neurons may be related to synaptic transmission. CaM kinase IIalpha appears to support axonal regrowth.

A nonimmunosuppressant FKBP-12 ligand increases nerve regeneration

The immunosuppressant drugs FK506 and cyclosporin A inhibit T-cell proliferation via a common mechanism: calcineurin inhibition following binding to their respective binding proteins, the peptidyl prolyl isomerases FKBP-12 and cyclophilin A. In contrast, FK506, but not cyclosporin A, accelerates nerve regeneration. In the present study, we show that the potent FKBP-12 inhibitor V-10,367, which lacks the structural components of FK506 required for calcineurin inhibition, increases neurite outgrowth in SH-SY5Y neuroblastoma cells and speeds nerve regeneration in the rat sciatic nerve crush model. In SH-SY5Y cells, V-10,367 increased the lengths of neurite processes in a concentration-dependent (between 1 and 10 nM) fashion over time (up to 168 h). Daily subcutaneous injections of V-10,367 accelerated the onset of clinical signs of functional recovery in the hind feet compared to vehicle-treated control animals. Interdigit distances (between the first and fifth digits) measured on foot prints obtained during walking showed an increase in toe spread in V-10,367-treated rats compared to vehicle-treated controls. Electron microscopy demonstrated larger regenerating axons distal to the crush site in the sciatic nerve from V-10,367-treated rats. Quantitation of axonal areas in the soleus nerve revealed a shift to larger axonal calibers in V-10,367-treated rats (400 or 200 mg/kg/day); mean axonal areas were increased by 52 and 59%, respectively, compared to vehicle-treated controls. FKBP-12 ligands lacking calcineurin inhibitory activity represent a new class of potential drugs for the treatment of human peripheral nerve disorders.

Axonal regrowth upregulates beta-actin and Jun D mRNA expression

When quiescent cells are perturbed, mRNAs encoding proteins that regulate gene transcription and the cell cycle are expressed at higher level. Jun and Fos are examples of proteins that mediate mitogenic signals and influence differentiation. In neurons, axon interruption (axotomy) increases the content of actin, tubulin, Jun D, and c-Jun proteins in association with increases in actin mRNA levels. Jun D protein binds to gene promoter regions, and its expression has been linked to several aspects of cell differentiation. Because Jun D and beta-actin messages have been described as "constitutive" in expression, we wanted to know whether these messages were responsive to axotomizing lesions of the sciatic motor nerve. We crushed the right sciatic nerve in Sprague-Dawley rats and extracted mRNA from the half spinal cord that serves each leg. At 4 days, Northern blots showed a 2.3-fold increase in beta-actin mRNA and a 2.5-fold increase in Jun D mRNA in the right hemicord. In situ hybridization showed either an undiminished or increased concentration of both mRNAs in motor neurons ipsilateral to the lesion at 4 days, even though many had enlarged two-to threefold. By introducing Fluoro-Ruby at the axotomy site, we were able to show that only the axotomized neurons have enlarged. We conclude that aspects of axonal regeneration resemble the embryonic program for neuronal differentiation and are reinitiated by axotomy.

Priming events and retrograde injury signals. A new perspective on the cellular and molecular biology of nerve regeneration

Successful axon regeneration requires that signals from the site of injury reach the nucleus to elicit changes in transcription. In spite of their obvious importance, relatively few of these signals have been identified. Recent work on regeneration in the marine mollusk Aplysia californica has provided several insights into the molecular events that occur in neurons after axon injury. Based on these findings, we propose a model in which axon regeneration is viewed as the culmination of a series of temporally distinct but overlapping phases. Within each phase, specific signals enter the nucleus to prime the cell for the arrival of subsequent signals. The first phase begins with the arrival of injury-induced action potentials, which act via calcium and cAMP to turn on genes used in the early stages of repair. In the next phase, MAP-kinases and other intrinsic constituents activated at the injury site are retrogradely transported through the axon to the nucleus, informing the nucleus of the severity of the axonal injury, reinforcing the earlier events, and triggering additional changes. The third phase is characterized by the arrival of signals that originate from extrinsic growth factors and cytokines released by cells at the site of injury. In the last phase, signals from target-derived growth factors arrive in the cell soma to stop growth. Because many of these events appear to be universal, this framework may be useful in studies of nerve repair in both invertebrates and vertebrates.

Assembly of microfilaments and microtubules from axonally transported actin and tubulin after axotomy

The slow component (SC) of axonal transport conveys structural proteins, regulatory proteins, and glycolytic enzymes toward the axon tip at 1-6 mm/day. Following axon interruption (axotomy), the rate of outgrowth corresponds to the rate of SCb-the fastest subcomponent of SC. Both axonal outgrowth and SCb accelerate 20-25% after axotomy. Tubulin and actin are the major proteins being carried by SCb. To further characterize the acceleration of SCb, we measured the equilibrium between subunits and polymers for both actin and tubulin. We radiolabeled newly synthesized proteins in rat motor neurons by microinjecting [35S]methionine into the spinal cord 7 days after crushing the sciatic nerve (85 mm from the spinal cord). Nerves were removed 7 days later for homogenization in polymer-stabilizing buffer (PSB) and centrifugation, followed by SDS-PAGE of supernatants (S) and pellets (P). We removed beta-tubulin, actin, and the medium-weight neurofilament protein (NF-M) from each gel by using the fluorogram as a template. After solubilizing gel segments for liquid scintillation spectrometry, we expressed counts as a polymerization ratio: P/[S+P]. In the nerve segments that contained radiolabeled Scb proteins, located 24-36 mm from the spinal cord, axotomy increased the polymerization ratio of SCb actin from 0.23 to 0.36 (P < 0.05) but had no effect on SCb beta-tubulin. In a separate experiment, we added 12 microM taxol to PSB to stabilize newly assembled microtubules. Adding taxol did not alter the polymerization ratio for SCb beta-tubulin in sham-axotomized nerves but aid increase the ratio in axotomized nerves, from 0.44 to 0.63 (P < 0.05); polymerization ratios for SCb actin were unaffected. We conclude that the assembly of microfilaments and microtubules increases to provide cytoskeletal elements for axon sprouts. The resulting loss of actin and tubulin subunits may play a role in the acceleration of SCb.

Acute treatment with pulsed electromagnetic fields and its effect on fast axonal transport in normal and regenerating nerve

The mechanism whereby low-frequency electromagnetic fields accelerate axonal regrowth and regeneration of peripheral nerve after crush lesion is not known. One candidate is an alteration in axonal transport. In this study we exposed unoperated rats for 15 min/day, and rats that had undergone a crush lesion of the sciatic nerve, for 1 hr/day for 2 days, to 2-Hz pulsed electromagnetic fields. To label fast transported proteins, [3H]-proline was microinjected into the spinal cord, and the sciatic nerves were removed 2, 3.5, and 5 hr later. The rates of fast axonal transport were obtained for animals in all groups by counting sequential 2-mm segments of nerves. The following transport rates were found: in unoperated normal sciatic nerve not exposed to PEMF, 373 +/- 14 mm/day; in unoperated normal nerve exposed to PEMF, 383 +/- 14 mm/day; in sham crush nerves not exposed to PEMF, 379 +/- 19 mm/day; in sham crush nerve exposed to PEMF, 385 +/- 17 mm/day; in crushed nerves not exposed to PEMF, 393 +/- 16 mm/day. and in crushed nerves exposed to PEMF, 392 +/- 15 mm/day. The results of these experiments indicate that 1) a crush injury to the sciatic nerve does not alter the rate of fast axonal transport, and 2) low-frequency pulsed electromagnetic fields do not alter fast axonal transport rates in operated (crush) or unoperated sciatic nerves.

Fast axonal transport rates are unchanged in 6- and 24-month F344 rats

The maximum rate of fast axonal transport in motor axons at 6 and 24 months was measured in F344 rats. Tritiated proline was injected near sciatic motoneurons and rats were killed after 2-5 h. Nerves were processed for liquid scintillation spectroscopy and fast transport rates calculated. The rates, in 6- and 24-month rats, were 373 +/- 12 mm/day and 368 +/- 10 mm/day, respectively. Thus, the maximum fast transport rate is unchanged with age in F344 rats.

Direct measurement of fast axonal transport rates in corticospinal axons of the adult rat

The bi-directional movement of proteins from the soma to the axon terminal is called axonal transport. Fast anterograde transport moves organelles and membrane-bound proteins distally. Fast transport rates were measured in corticospinal tract axons of male Sprague-Dawley rats by microinjection of tritiated proline into the sensorimotor cortex. Animals were killed after 3-5 h and the tract cut into 1 mm segments. A bimodal wave of radiolabeled proteins was evident, with the first peak at the spino-medullary junction and the second peak in cervical spinal segments. The fast transport rate was calculated at the leading edge of the distal wave, and found to be 303 +/- 44 mm/day.

Repair of the central nervous system: lessons from lesions in leeches

In contrast to the limited repair observed in the mammalian central nervous system (CNS), injured neurons in the leech reliably regenerate synapses and restore function with remarkable accuracy at the level of individual neurons. New and recent results reveal important roles for microglial cells and extracellular matrix components, including laminin, in repair. Tissue culture experiments have permitted isolation of neurons and manipulation of their environment, providing insights into the influence of substrate, electrical activity, and other cells, including microglia, on axon growth and synapse formation. The results account for distinctive features of successful repair in the adult leech, where axonal sprouting and target selection can be influenced by unequal competition between neurons. Differences between the formation of connections during embryonic development and repair in the adult include dissimilarities in the roles of glia and microglia in adults and embryos, suggesting that axon growth during regeneration in the CNS is not simply a recapitulation of processes observed during embryonic development. It may be possible in the future to improve mammalian CNS regeneration by recruiting cells whose counterparts in the leech have been identified as instrumental in repair.

RNA transcription in axotomized dorsal root ganglion neurons

Levels of RNA transcription were examined in L5 sensory ganglion neurons after unilateral crush injuries of the sciatic nerve using an in situ autoradiographic technique. Neuronal transcription increased in a biphasic pattern on the lesioned side within the first two weeks after injury. The timing and pattern of increases in transcription corresponded to previously reported increases RNA synthesis. The data demonstrate a dynamic regulation of RNA transcription in regenerating neuronal populations. This technique may prove to be useful in the study of factors controlling transcriptional activity in injured neurons.