Evidence for a motor nerve growth factor

Electrically Stimulated Tunable Drug Delivery From Polypyrrole-Coated Polyvinylidene Fluoride

Electrical stimulus-responsive drug delivery from conducting polymers such as polypyrrole (PPy) has been limited by lack of versatile polymerization techniques and limitations in drug-loading strategies. In the present study, we report an in-situ chemical polymerization technique for incorporation of biotin, as the doping agent, to establish electrosensitive drug release from PPy-coated substrates. Aligned electrospun polyvinylidene fluoride (PVDF) fibers were used as a substrate for the PPy-coating and basic fibroblast growth factor and nerve growth factor were the model growth factors demonstrated for potential applications in musculoskeletal tissue regeneration. It was observed that 18-h of continuous polymerization produced an optimal coating of PPy on the surface of the PVDF electrospun fibers with significantly increased hydrophilicity and no substantial changes observed in fiber orientation or individual fiber thickness. This PPy-PVDF system was used as the platform for loading the aforementioned growth factors, using streptavidin as the drug-complex carrier. The release profile of incorporated biotinylated growth factors exhibited electrosensitive release behavior while the PPy-PVDF complex proved stable for a period of 14 days and suitable as a stimulus responsive drug delivery depot. Critically, the growth factors retained bioactivity after release. In conclusion, the present study established a systematic methodology to prepare PPy coated systems with electrosensitive drug release capabilities which can potentially be used to encourage targeted tissue regeneration and other biomedical applications.

The role of muscle in the development and differentiation of spinal motoneurons: in vitro studies

Results of in vivo experiments suggest that muscle cells, and probably other cell types, produce factors upon which motoneurons depend for survival and normal development. Most attempts to characterize such factors have used cultures of enriched or identified motoneurons, and have studied effects of muscle-derived substances on survival, neurite outgrowth and acetylcholine synthesis. Results from different laboratories vary widely, both in terms of the estimated abundance of motoneurons as a fraction of total dissociated spinal cord and in terms of the molecular weight estimates for factors tentatively proposed as candidate motoneuron growth factors. Nevertheless, there are several independent reports of 40-55 kDa species affecting each of the three parameters of spinal neuron development. We have begun to characterize one of these, partially purified from extracts of denervated muscle on the basis of its neurite-promoting activity for a subpopulation of 4.5-day embryonic chicken spinal neurons. Comparison between the factors under study in different systems, and confirmation of their importance in vivo, await the preparation of specific blocking antibodies.

Pathophysiology of peripheral nerve injury: a brief review

Clinicians caring for patients with brachial plexus and other nerve injuries must possess a clear understanding of the peripheral nervous system's response to trauma. In this article, the authors briefly review peripheral nerve injury (PNI) types, discuss the common injury classification schemes, and describe the dynamic processes of degeneration and reinnervation that characterize the PNI response.

Peripheral nerve injury and repair

Peripheral nerve injuries are common, and there is no easily available formula for successful treatment. Incomplete injuries are most frequent. Seddon classified nerve injuries into three categories: neurapraxia, axonotmesis, and neurotmesis. After complete axonal transection, the neuron undergoes a number of degenerative processes, followed by attempts at regeneration. A distal growth cone seeks out connections with the degenerated distal fiber. The current surgical standard is epineurial repair with nylon suture. To span gaps that primary repair cannot bridge without excessive tension, nerve-cable interfascicular auto-grafts are employed. Unfortunately, results of nerve repair to date have been no better than fair, with only 50% of patients regaining useful function. There is much ongoing research regarding pharmacologic agents, immune system modulators, enhancing factors, and entubulation chambers. Clinically applicable developments from these investigations will continue to improve the results of treatment of nerve injuries.

Effects of nerve growth factor on crushed sciatic nerve regeneration in rats

The effects of nerve growth factor (NGF) on crushed sciatic nerve regeneration were studied in 30 rats, with 60 bilateral nerves. The nerves were crushed at a site 6 mm distal to the sciatic notch by the standard technique and 3 mm wide crush injuries were created. Then 2.1 microliters of normal saline in the control groups and an equal volume of NGF solution (containing 1 microgram of NGF) in the NGF-treated groups was injected into the crush sites and followed for 12, 28, and 56 days, respectively. At the end of the observation, electrophysiological evaluation was carried out; then samples 10 mm distal to the crush site were removed and prepared for histological and morphometric studies. Evoked muscle action potential (MAP) was recorded in 50% of the NGF-treated group at 12 days but not in the control group; the difference was statistically significant (P < 0.05). The motor nerve conduction velocity (MNCV) was increased in NGF-treated groups compared with control groups at 28 and 56 days (P < 0.05). Morphometrically, significantly more regenerated myelinated fibers (RMFs) were seen at 12 days, and larger diameter RMFs were found at 12, 28, and 56 days in NGF-treated groups than in control groups. These results indicate that topically applied NGF stimulates nerve regeneration and promotes function recovery in crushed rat sciatic nerves.

Protein secretion from mouse skeletal muscle: coupling of increased exocytotic and endocytotic activities in denervated muscle

The secretion of proteins labelled by incorporation of radioactive amino acids was studied in innervated and 10 to 13-day-denervated mouse skeletal muscle. The secretion of 3H-leucine-labelled proteins, expressed per mg muscle wet weight, increased after denervation, and the kinetics of the secretory process was also altered in denervated muscle. Separation of secreted 35S-methionine-labelled proteins by sodium dodecyl sulphate polyacrylamide gel electrophoresis followed by autoradiography revealed some denervation-induced alterations in the pattern of secreted proteins. The secretion from both innervated and denervated muscle was highly temperature sensitive and was reversibly inhibited by brefeldin A, a drug that blocks forward membrane transport from the endoplasmic reticulum/Golgi apparatus. This drug was also found to inhibit the uptake of fluorescein isothiocyanate-labelled dextran in denervated muscle but had no effect on the endocytotic activity of innervated muscle. This lends support to the hypothesis that the increased endocytotic activity in denervated muscle is coupled to a high secretory activity.

The quest for better recovery from peripheral nerve injury: current status of nerve regeneration research

The author reviews relevant nerve regeneration research in the past half-century to give the reader familiarity with the background of current research efforts. Recent research has been aided by newer knowledge of the biology of nerve regeneration. Early efforts to improve nerve regeneration centered on improvements in technical or surgical repairs, with only modest gains. Although current technical improvements with lasers or fibrin glue to repair nerves may show promise, the approach of deMedinaceli et al. combines several new ideas with some improvement experimentally. With the discovery of nerve growth factors and a host of newer nerve growth promoting factors, the biochemical arena has explored. Special tubes to repair nerves enable easy manipulation of the environment to study the effects of various factors on nerve regeneration. Silastic and bioresorbable tubes show the most promise to enhance nerve regeneration by tubulization. Because of the explosion of knowledge and high levels of activity of research, it is apparent that further improvements of nerve regeneration are on the horizon.

Clinical status of motoneuron disease does not correlate with serum neurotoxicity on cultured neurons

To test the hypothesis that a serum factor may be toxic to motoneurons in Motoneuron disease (MND), we evaluated neuronal survival in dissociated cultures from chick ciliary ganglia neurons in which MND or control sera were applied. No neurotoxic activity was found when comparing neuronal survival at 1 and 3 days in MND and control group. To examine the influence of the clinical MND status on neuronal survival, we used the following parameters: age at onset, sex, months from onset of disease to sera sampling, MND form, rhythm of progression, and scoring with Appel scale. No effect was noted for any of these factors. Sera from the same patient obtained at different clinical stages did not allow any conclusion of a fixed behavior for the same patient. Our findings argue against the existence of a cytotoxic factor in sera from MND patients.

Muscle denervation increases thyrotropin-releasing hormone (TRH) biosynthesis in the rat medullary raphe

To determine whether thyrotropin-releasing hormone (TRH) could exert a trophic role in ventral horn motor neurons, we examined the effect of muscle denervation with botulinum toxin A on TRH mRNA in the rat medullary raphe by in situ hybridization histochemistry. Compared to controls, denervated rats showed a significant increase in the number and silver grain density of hybridized medullary raphe neurons. Increased proTRH gene expression in the medullary raphe in response to motor unit perturbation indicates that TRH may be trophic to lower motor neurons.

AAEM minimonograph #14: The influence of temperature in clinical neurophysiology

AAEM MINIMONOGRAPH # 14 Temperature affects biologic and neurophysiologic processes and is, therefore, always well controlled in in vitro experiments. Its role is equally important in the clinical laboratory but has often been neglected. Lower temperature cause slower nerve conduction velocities (NCVs), and increased amplitudes of muscle and nerve potentials. Fibrillations may disappear, and muscle contraction will be slower and weaker. Neuromuscular transmission improves. Somatosensory evoked potentials (SEPs) are similarly vulnerable in the peripheral segments, or with changes in central temperature. As a result, abnormalities are artificially created or existing defects are not detected, resulting in false or missed diagnoses. Control of temperature, albeit somewhat time consuming, will result in greater diagnostic accuracy.

Motor neuron disease (amyotrophic lateral sclerosis)

Amyotrophic lateral sclerosis is an insidiously developing, adult-onset, progressive anterior horn cell degeneration with associated degeneration of descending motor pathways. It has been recognized as an important clinical syndrome since the middle of the 19th century. Despite increasing clinical and research interest in this condition, its cause remains obscure, even in the broadest terms. Epidemiologic characteristics of the disease have been interpreted as evidence of both genetic and environmental causes. A major change in the view of this disease is the widely developing perception that it is a disease of elderly persons more than of middle-aged adults as was previously taught. Etiologic hypotheses encompass a broad range of postulated pathophysiologic mechanisms, and we review these in detail. The clinical limits of the disease can now be better defined by using modern diagnostic techniques. Although interest in supportive symptomatic therapy is growing, no intervention has yet been shown to modify the biologically determined motor system degeneration.

Enhancement of peripheral nerve regeneration

Numerous factors external to the nerve cell can support and enhance nerve regeneration after injury. The definition of these factors and the elucidation of their mechanisms of action are the central goals of much contemporary neurobiologic research. This research will hopefully lead to the discovery of factors that will prove to be therapeutically beneficial for patients with either peripheral nervous system (PNS) injury or central nervous system (CNS) injury. This article reviews the biology of the regeneration response of the nerve to injury and discusses many of the factors that enhance nerve growth. Finally, the nerve guide or nerve regeneration chamber model for the evaluation of putative nerve regeneration enhancing agents in vivo is also discussed.

Rapid neural regulation of muscle urokinase-like plasminogen activator as defined by nerve crush

Muscle plasminogen activators (PAs), such as urokinase-like PA and, to a lesser extent, tissue PA, increase dramatically after denervation induced by axotomy. The PA/plasmin system has also been implicated in degradation of specific components of the muscle fiber basement membrane after local activation of plasminogen. These results suggest that neural regulation of muscle extracellular matrix metabolism accompanies or precedes regeneration after injury and is mediated by activation of PAs. In the present study, we have used nerve crush to explore the neural regulation of muscle PA activities directly after subtotal axon interruption and during the process of reinnervation. Muscle contraction after nerve stimulation and estimation of choline acetyltransferase activity were used to monitor reinnervation. Within 24 hr of nerve crush, muscle urokinase (but not tissue PA) activity rose in soluble and membrane-bound muscle fractions, as shown by an amidolytic assay and a fibrin zymography. Membrane-bound activity was 5-fold higher than cytosol activity, but there was no shift between cellular compartments during the time course of denervation. Coincident with the return of choline acetyltransferase activity and muscle contractility, muscle urokinase returned almost to baseline levels. These results show tight regulation of muscle urokinase levels by some neural influence.

Age-related increase in soluble and cell surface-associated neurite-outgrowth factors from rat muscle

While the number of nerve terminals per endplate decreases with age in the rat extensor digitorum longus (EDL) muscle, the number of endplates exhibiting ultraterminal sprouting, characteristic of denervation, increases. To determine if these changes associated with aging are accompanied by alterations in the production of muscle-derived neurite-outgrowth factors, we examined the effects of soluble and cell surface-associated components from innervated and denervated 10- and 25-month rat EDL muscles on a motoneuron-enriched fraction of embryonic chick spinal cord cells in vitro. Cells were cultured for 72 h with muscle extract or on muscle cross-sections. While soluble components of the extract affected initiation of neurite outgrowth, muscle cell surface-associated molecules influenced neurite elongation. Both muscle extract and muscle cross-sections from 10-month denervated animals were more effective in promoting neurite outgrowth than 10-month innervated muscle. There was no difference between 25-month innervated and 25-month denervated muscle. However, 25-month innervated and denervated muscles were significantly more effective in promoting neurite outgrowth than 10-month innervated muscle, but not different from 10-month denervated muscle. These results suggest that an age-related increase in muscle-derived soluble and cell surface-associated neurite-outgrowth factors may contribute to denervation-like morphological changes associated with aging at the neuromuscular junction.

High endocytotic and lysosomal activities in segments of rat myotubes differentiated in vitro

Endocytosis and the lysosome system have been studied in rat myotubes differentiated in vitro. Horseradish peroxidase was used as marker for endocytosis and was found to accumulate unevenly in the myotubes. Small segments of myotubes display very high endocytotic activity. Similar segments contained numerous lysosomes, as seen by the accumulation of neutral red or histochemical staining for acid phosphatase. The segments also contained accumulations of acetylcholine receptors as determined by binding of tetramethyl rhodamine-labelled alpha-bungarotoxin. Unstained segments in living cultures could be recognized by phase-contrast microscopy since they often appeared somewhat dilated and were not as well spread on the culture surface as the main parts of the myotubes. Ultrastructurally, the segments contained an intensely proliferating tubular system in communication with the extracellular space, which therefore probably represents the developing transverse tubular system. The segments also contained endocytosed marker within large phagosomes. Contractile filaments occurred in the segments but were frequently less well-organized than in other parts of the myotubes. The described characteristics of the segments in rat myotubes differentiated in vitro bear resemblance to some of the characteristics of the denervated endplate region of adult muscle.

Nerve sprouting induced by a piece of peripheral nerve placed over a normally innervated frog muscle

1. The effects of a segment of peripheral nerve on the innervation of a skeletal muscle were investigated in the cutaneous pectoris muscle of the frog Rana esculenta. An explant of the sciatic nerve was placed in an aneural region of the muscle at a distance of 1-3 mm from the zone of neuromuscular junctions. The muscles were examined morphologically and electrophysiologically at different post-operative times. 2. After the first month nerve sprouts were seen arising from both nerve terminals and intramuscular nodes of Ranvier. Both the number of sites of sprouting and the relative distance to the explant tip increased with time (up to 5 months), suggesting spread of a nerve sprouting-promoting stimulus. 3. Most neurites resulting from sprouting were seen growing towards the nerve explant, in the vicinity of which active neurite proliferation occurred. Some of them entered the explant as observed in semi-thin and thin sections which revealed the presence of both myelinated and unmyelinated nerve fibres. 4. Electrical stimulation of the nerve explant in preparations autografted for more than 2 months resulted in the contraction of bundles of muscle fibres. Endplate potentials of similar amplitudes were recorded intracellularly in some muscle fibres of such preparations when stimulating either the nerve explant or the cutaneous pectoris nerve. When two stimuli were paired by gradually reducing the interval of time between them, the second response was gradually facilitated. This confirmed that nerve fibres stimulated through the explant corresponded to new neuritic processes resulting from motor nerve sprouting. 5. Pieces of perineural tissue and segments of peripheral nerve killed by alcohol treatment or by repeated freezing and thawing were used as controls. They did not induce any nerve sprouting. 6. The results indicate that cells surviving in a segment of peripheral nerve trunk actively induce intact axons to sprout both from their terminals and intramuscular nodes of Ranvier; moreover these cells promote attraction and proliferation of growing neurites. The possibility of release of a diffusible factor by glial cells is discussed.

Motor nerve terminal morphologic plasticity induced by small changes in the locomotor activity of the adult rat

The possible existence of plastic changes in the branching pattern and the complexity of silver-impregnated motor nerve terminals was studied in the extensor digitorum longus muscle of adult rats housed in metabolic cages or trained to walk. Results indicate a precise inverse relation between the amount of locomotor activity and the complexity finally attained by the motor nerve terminals.

Differential effects of nerve, muscle, and fat tissue on regenerating nerve fibers in vivo

Axonal regeneration through silicone tubes was studied using distal nerve stumps, denervated, preatrophied muscle tissue, as well as fat tissue as a target. During the first stage of regeneration, i.e., within 2-3 weeks after surgery, a thin, filamentous structure consisting of fibrin and connective tissue was seen bridging the gap in all systems. Thereafter, this cord obviously served as a guideline for the outgrowth of increasing numbers of axons into distal nerve stumps as well as into muscle tissue, but not into fat tissue. These findings confirm that preatrophied muscle tissue has a similar "neurotrophic" effect on regenerating nerve fibers as distal nerve stumps. The ineffectivity of fat tissue in promoting nerve fiber regeneration could be attributed either to the absence of "neurotrophic factors" or even to an inhibitory effect.

Early terminal and nodal sprouting of motor axons after botulinum toxin

Axonal sprouting in distal motor axons was studied in an attempt to answer two questions: (a) is the cell body required for early axonal sprouting?, and (b) do nodal, as well as terminal, axonal sprouts arise after muscle inactivity not caused by nerve injury? Botulinum toxin (BT) was used to induce axonal sprouting without nerve trauma. Mice were injected in the right calf with a sublethal dose of BT and the soleus muscle examined ultrastructurally at times varying from 3 h to 5 days post-injection. Terminal axonal sprouts were seen 2 days after injection, and based on the time taken for BT to act and the growth rate of sprouts, axons were calculated to sprout within 24 h of muscle inactivity. This short time suggests that early axonal regrowth is initiated and controlled at the distal axon. Sprouts were seen arising from the intramuscular nodes of Ranvier from 2 days after BT injection. Unlike the terminal sprouts which elongated over time, the nodal sprouts remained short and confined by the basal lamina overlying the node, probably because without structural denervation there were no empty perineural sheaths to act as pathways to the motor endplates. The finding of terminal and nodal sprouts after botulinum toxin supports the hypothesis that muscle inactivity gives rise to a single growth factor for both terminal and nodal sprouting.

Development and regional expression of chicken neuroleukin (glucose-6-phosphate isomerase) messenger RNA

Neuroleukin (NLK) is a protein identical with the glycolytic enzyme glucose-6-phosphate isomerase (GPI) that has been reported to support the survival of a subpopulation of neurons in embryonic dorsal root ganglia and spinal cord neurons in culture. In this report we have studied the developmental expression of NLK mRNA in the chick embryo in order to evaluate its possible role as a neurotrophic factor. The chicken gene encoding NLK was isolated by cross-hybridization to a mouse NLK cDNA clone. A DNA fragment from the chicken NLK gene with a 90% nucleotide sequence homology to mouse NLK cDNA encoding amino acids 310-355 was then used as a hybridization probe in a series of RNA-blots. In the entire embryo NLK mRNA was found already at embryonic day 3.5 (E3.5) and the level of expression was significantly decreased between E3.5 and hatching. Roughly similar levels of NLK mRNA were found in all tissues of the E8 embryo analyzed with the exception of the brain, which contained only low levels. When the developmental expression was analyzed in different tissues separately, NLK mRNA expression was found to decrease during development in the heart and bursa of Fabricius, whereas the level of mRNA in the brain showed a large increase shortly after hatching. The spinal cord and the pectoral and femoral muscles all showed high levels of NLK mRNA throughout development. In the adult chick, the highest levels of NLK mRNA were found in the muscle, brain, and kidney, where the NLK mRNA was estimated to account for approximately 0.1% of the total mRNA in these tissues. A widespread expression of NLK mRNA was observed in the adult brain with approximately similar levels in all brain regions tested. Similar results were also obtained when NLK mRNA expression was analyzed in adult rats. Our results show that developmental expression of the NLK gene is independently regulated in different tissues. The widespread and abundant expression of both the avian and rodent NLK gene is in accordance with its newly discovered identity as a glycolytic enzyme. Consequently, the developmental and adult pattern of NLK mRNA expression does not favour a specific trophic role for this protein in accordance with other known neurotrophic factors.

Regeneration of muscle axons in the frog is directed by diffusible factors from denervated muscle and nerve tubes

In the frog, peripheral muscle axons regenerate after a lesion to reinnervate the original synaptic sites on muscle fibers. Previous experiments in the frog have shown that satellite cells of the nerve tube direct the outgrowth of regenerating muscle axons over distances of many millimeters. In the present experiments, denervated muscle was used as a target for regenerating muscle axons. Muscle and satellite cells of the nerve tube also were placed in filters to determine if their influence on axonal outgrowth was exerted by diffusible factors. Filters were used with a pore size of 0.22 micron. With this pore size, target cells were isolated from physical contact with the surrounding cells; yet an exchange of fluids--and therefore of molecules released by the target cells--could occur across the filter. In the presence of denervated muscle or satellite cells of the nerve tube in filters, regenerating axons turn and grow toward the target cells. This influence on the direction of axonal outgrowth was produced over distances of 6 mm by muscles and 4 mm by cells of the nerve tubes. This directed outgrowth is in marked contrast to the random pattern of outgrowth in the absence of the targets. The present findings set the stage for tissue culture experiments in which the phenomena observed in vivo can be analyzed in terms of mechanisms. The present finding that denervated muscle attracts regenerating axons means that sufficient material may be available for the characterization and isolation of the relevant molecules.

Calcium dependent modulation of fast axonal transport

The highly differentiated structure of the neuron poses special problems for the intracellular movement of molecules throughout the cell. Molecular transport distances from the synthesizing neuron cell body along the axon (which has no substantial synthetic capabilities) to the axon terminal are very great. The transported substances, transport support structures, translocator motors, and control elements are currently the focus of intense research. Interruption of this flow of molecules could have disastrous effects upon the cell and ultimately the organism resulting in neuropathological conditions. Calcium plays a critical role in modulating fast-axonal transport (FAT) speeds. Before discussing the effect of calcium on FAT, we summarize our broad perspective on the role of axonal transport in neurologic disease.

The development of cholinergic neurons

Motoneuron precursors acquire some principles of their spatial organization early in their cell lineage, probably at the blastula stage. A predisposition to the cholinergic phenotype in motoneurons and some neural crest cells is detectable at the gastrula to neurula stages. Cholinergic expression is evident upon cessation of cell division. Cholinergic neurons can synthesize ACh during their migration and release ACh from their growth cones prior to target contact or synapse formation. Neurons of different cell lineages can express the cholinergic phenotype, suggesting the importance of secondary induction. Early cholinergic commitment can be modified or reversed until later in development when it is amplified during interaction with target. Motoneurons extend their axons and actively sort out in response to local environmental cues to make highly specific connections with appropriate muscles. The essential elements of the matching mechanism are not species-specific. A certain degree of topographic matching is present throughout the nervous system. In dissociated cell culture, most topographic specificity is lost due to disruption of local environmental cues. Functional cholinergic transmission occurs within minutes of contact between the growth cone and a receptive target. These early contacts contain a few clear vesicles but lack typical ultrastructural specializations and are physiologically immature. An initial stabilization of the nerve terminal with a postsynaptic AChR cluster is not prevented by blocking ACh synthesis, electrical activity, or ACh receptors, but AChR clusters are not induced by non-cholinergic neurons. After initial synaptic contact, there is increasing deposition of presynaptic active zones and synaptic vesicles, extracellular basal lamina and AChE, and postjunctional ridges over a period of days to weeks. There is a concomitant increase in m.e.p.p. frequency, mean quantal content, metabolic stabilization of AChRs, and maturation of single channel properties. At the onset of synaptic transmission, cell death begins to reduce the innervating population of neurons by about half over a period of several days. If target tissue is removed, almost all neurons die. If competing neurons are removed or additional target is provided, cell death is reduced in the remaining population. Pre- or postsynaptic blockade of neuromuscular transmission postpones cell death until function returns.(ABSTRACT TRUNCATED AT 400 WORDS)

Spontaneous reinnervation of a free muscle flap

A latissimus dorsi muscle flap was used to repair a severe traumatic avulsion defect of the dorsum of the foot in a 3-year-old girl. The severed peroneal nerve apparently regenerated across a large gap and spontaneously reinnervated the denervated muscle flap. This resulted in a functional flap as demonstrated clinically and electromyographically. Surgical methods of muscle reinnervation and the influence of neurotrophic factors are discussed.

Schwann cell-conditioned medium supports neurite outgrowth and survival of spinal cord neurons in culture

The effect of Schwann cell-conditioned medium (SCM) on the development in vitro of spinal cord neurons was studied. Spinal cord neurons from 18-day-old rat embryos were cultured in serum-free conditioned medium obtained from confluent rat Schwann cells. In cultures fed SCM, the cells developed typical neuronal morphology and were identified by indirect immunofluorescence using a monoclonal antibody to neurofilament protein. SCM stimulated neurite outgrowth and supported survival of spinal cord neurons. Preliminary characterization suggests that the neurotrophic factor in SCM appears to be a protein with a molecular weight greater than 8000 daltons.

Control of number and distribution of synapses during ectopic synapse formation in adult rat soleus muscles

Changes in the number and distribution of synaptic inputs and acetylcholine receptor clusters were studied during the formation of ectopic nerve-muscle junctions between the transplanted fibular nerve and the denervated soleus muscle of adult rats. The tibial nerve was cut 3 weeks after implanting the fibular nerve. New sites of transmission were first detected 3 days after the cut. These sites were located electrophysiologically, marked by dye and found to coincide with clusters of acetylcholine receptors. There were no ectopic clusters away from fibular nerve sprouts and no clusters on muscles which had not been denervated. Three days after cutting the tibial nerve, the acetylcholine receptor clusters, and probably also the sites of transmission, were randomly distributed along individual muscle fibres. Six days after the cut, the clusters continued to be randomly distributed whereas the synaptic inputs were either close together (within 300 microns) or more than 600 microns apart. Two weeks later the spatial distributions of both clusters and inputs were similar with peaks around 100-300 microns, 1200-1400 microns and 2000-2600 microns. No ectopic clusters were closer than 0.5 mm to the original endplate. We conclude that nerve-muscle contacts and associated acetylcholine receptor clusters initially form at random. One or a few of these contacts develop further and, as a result, the surrounding regions undergo changes that prevent the contacts initially formed there from being maintained. Apparently, in this preparation, approximately 1.5 mm length of fibre is needed to support the maturation and maintenance of each ectopic endplate (mean length 111 micron).

Fast axonal transport alterations in amyotrophic lateral sclerosis (ALS) and in parathyroid hormone (PTH)-treated axons

Video-enhanced contrast techniques have been used to study fast axonal transport of organelles in diseased and normal human axons. A broad perspective on the importance of axonal transport in the pathogenesis of human neurological disorders is presented and problems in dealing with human nerve summarized. Results from analysis of organelle traffic in axons from motor nerve in patients with amyotrophic lateral sclerosis (ALS) show: 1) higher mean speed of anterograde organelles, 2) lower mean speed of retrograde organelles, and 3) lower retrograde organelle traffic density. Hyperparathyroidism, another human clinical syndrome, can mimic ALS. The effect of parathyroid hormone (PTH) on axons in vitro is to increase the mean speed of both anterograde and retrograde organelle traffic. The dose response curve and time course of the PTH effect are delineated. Dihydropyridine calcium channel antagonists block the PTH effect, implicating extracellular calcium in the alteration of organelle traffic speed. The results are discussed in relation to neuronal function and the regulation of fast axonal transport.

Neurite outgrowth on cryostat sections of innervated and denervated skeletal muscle

To localize factors that guide axons reinnervating skeletal muscle, we cultured ciliary ganglion neurons on cryostat sections of innervated and denervated adult muscle. Neurons extended neurites on sections of muscle (and several other tissues), generally in close apposition to sectioned cell surfaces. Average neurite length was greater on sections of denervated than on sections of innervated muscle, supporting the existence of functionally important differences between innervated and denervated muscle fiber surfaces. Furthermore, outgrowth was greater on sections of denervated muscle cut from endplate-rich regions than on sections from endplate-free regions, suggesting that a neurite outgrowth-promoting factor is concentrated near synapses. Finally, 80% of the neurites that contacted original synaptic sites (which are known to be preferentially reinnervated by regenerating axons in vivo) terminated precisely at those contacts, thereby demonstrating a specific response to components concentrated at endplates. Together, these results support the hypothesis that denervated muscles use cell surface (membrane and matrix) molecules to inform regenerating axons of their state of innervation and proximity to synaptic sites.

Muscle-derived factors that support survival and promote fiber outgrowth from embryonic chick spinal motor neurons in culture

The purposes of the experiments reported is to provide an unambiguous demonstration that embryonic skeletal muscle contains factors that act directly on embryonic spinal motor neurons both to support their survival and to stimulate the outgrowth of neurites. Cells of lumbar and brachial ventral spinal cords from 6-day-old chick embryos were separated by centrifugation in a two-step metrizamide gradient, and a motor neuron enriched fraction was obtained. Motor neurons were identified by retrogradely labeling with rhodamine isothiocyanate, and were enriched fourfold in the motor neuron fraction relative to unfractionated cells. In culture, the isolated motor neurons died within 3-4 days unless they were supplemented with embryonic chick skeletal muscle extract. Two functionally distinct entities separable by ammonium sulfate precipitation were responsible for the effects of muscle extracts on motor neurons. The 0-25% ammonium sulfate precipitate contained molecules that alone had no effect on neuronal survival but when bound to polyornithine-coated culture substrata, stimulated neurite outgrowth and potentiated the survival activity present in muscle. Most of this activity was due to a laminin-like molecule being immunoprecipitated with antisera against laminin, and immunoblotting demonstrated the presence of both the A and B chains of laminin. A long-term survival activity resided in the 25-70% ammonium sulfate fraction, and its apparent total and specific activities were strongly dependent on the culture substrate. In contrast to the motor neurons, the cells from the other metrizamide fraction (including neuronal cells) could be kept in culture for a prolonged time without addition of exogenous factor(s).

Sera from patients with motor neuron disease and associated paraproteinaemia fail to inhibit experimentally induced sprouting of motor nerve terminals

IgG kappa paraproteinaemia was discovered in one patient with amyotrophic lateral sclerosis and one with chronic spinal muscular atrophy. Serum from these patients was injected into mouse muscles paralysed by botulinum toxin which is known to induce sprouting from motor nerve terminals. Daily injection of serum for 7 days failed to inhibit terminal sprouting. It is concluded that the paraproteins did not recognise a growth factor postulated as being implicated in the pathogenesis of motor neuron disease.

Suppression of terminal axonal sprouting at the neuromuscular junction by monoclonal antibodies against a muscle-derived antigen of 56,000 daltons

After the partial denervation or paralysis of a muscle, the remaining motor axon terminals may sprout fine, neuritic processes (terminal sprouts) which escape the endplate region of the neuromuscular junction. We previously identified a muscle-derived, protein antigen of 56,000 daltons (56 kD) which plays a necessary role in terminal sprouting. A panel of monoclonal antibodies have been produced against the 56-kD antigen, some of which also partially suppress motor axon terminal sprouting. These monoclonal antibodies define at least two different epitopes upon the surface of the antigen, one of which is necessary for it to effect its biological role in vivo.

Acceleration of peripheral nerve regeneration after crush injury in rat

It the sciatic nerve of a rat is crushed in the thigh, axons from the proximal side of the crush will regenerate so that the toe-spreading reflex becomes observable again after 10.4 +/- 1.7 (mean +/- S.D.) days. If the nerve is electrically stimulated for 0.25-1.0 h at the crush site, just after the crush occurs, the toe-spreading reflex first becomes observable 4.14 +/- 1.6 (mean +/- S.D.) days after the crush. Stimulation is most effective if delivered immediately after the crush but can be delayed up to an hour and still cause significantly faster regeneration. This phenomenon could be useful in clinical management of crushed peripheral nerves.

Evolution of axonal swellings in cats intoxicated with beta,beta'-iminodipropionitrile (IDPN). An electrophysiological and morphological study

beta,beta'-Iminodipropionitrile (IDPN) causes formation of axonal swellings in the proximal internodes of spinal motor axons. The swellings enlarge and demyelinate with the progression of the neuropathy. The correlation between axonal swellings and electrophysiologic function of motoneurons was examined in cats 2 to 35 days after initial administration of IDPN. Morphologic changes in intraspinal motor axons, occasionally observed 2 days after the first injection, became progressively more evident at later times, with enlargement at the first internode in some axons and appearance of fusiform or balloon-like axonal swellings. At 7 days axonal swellings were infrequently observed and the main structural feature was a reduction in myelin thickness in affected nerve fibers. Despite scant histopathologic changes, motoneuron action potential discharge at this time was significantly altered in latency to onset of spike and rate of rise. Abnormal motoneuron firing patterns were observed at this time. As the neuropathy progressed, both the frequency of occurrence and the size of axonal swellings increased markedly but at no time was there morphologic evidence of chromatolysis. At 14 and 35 days, (after two or five IDPN injections) action potential discharge became further altered in latency to spike onset, rate of rise, initial segment conduction time and somal-dendritic threshold. The incidence of repetitive firing increased and axonal conduction block was observed in several motoneuron recordings. The electrophysiologic changes closely resemble those reported in chromatolytic motoneurons after axotomy. The axonal swellings induced by IDPN may produce an axotomy-like condition which becomes more prominent as the neuropathy progresses but without morphologic evidence of chromatolysis.

Inhibition of terminal axonal sprouting by serum from patients with amyotrophic lateral sclerosis

To investigate the pathogenesis of amyotrophic lateral sclerosis, we compared the effect of serum from patients with this disease on the regenerative sprouting of terminal axons in botulinum-treated mouse gluteus muscle with the effects of serum from controls and from patients with diabetic peripheral neuropathy. Serum from 9 of 19 patients with the sporadic form of amyotrophic lateral sclerosis and from 2 of 6 patients with the familial form caused a reduction in the proportion of sprouting terminal axons, as compared with that found in muscles treated with serum from controls or diabetic patients. Immunoglobulin from patients with amyotrophic lateral sclerosis, when tested on immunoblots, recognized a 56-kilodalton protein secreted by denervated rat diaphragm muscle; rabbit antiserum raised against this protein also suppressed terminal axonal sprouting. Thus, we have detected an antibody in the serum of patients with amyotrophic lateral sclerosis that inhibits sprouting of neurons and subsequent reinnervation of skeletal muscle. Whether this antibody is of primary pathogenic importance or represents a secondary response to neuromuscular destruction is not known. In either case, serum from patients with amyotrophic lateral sclerosis may provide reagents for studies of the trophic communications between muscle and motor neurons.

Nerve growth and ectopic synapse formation induced by muscle damage in the frog

The effect of muscle damage on nerve growth and ectopic synapse formation in the frog was investigated by bisecting the cutaneus pectoris (c.p.) muscle following implantation of the hypoglossal nerve. Axons from the stump of the implanted nerve grew considerable distances towards the ends of the cut muscle fibres where they formed synapses. In some preparations, c.p. was only partially cut and in these muscles axonal growth from the implanted nerve occurred across intact fibres to reach the cut ends in the damaged area, suggesting the local release of a growth-stimulating factor. Stimulation of the hypoglossal nerve elicited contraction of the end-plate-free half of c.p. The characteristics of these ectopic synapses were similar to those found in a previous study when the hypoglossal nerve innervated the original end-plates after denervation of c.p. The mean quantal content of the end-plate potentials (e.p.p.s) was abnormally low, with long latencies between nerve stimulation and the onset of the e.p.p.s and frequent occurrence of multiphasic e.p.p.s in response to single nerve stimulation.