The formation of synapses in reinnervated mammalian striated muscle

Reinnervation of Paralyzed Limb Muscle by Nerve-Muscle-Endplate Grafting Technique

BACKGROUND

We have developed a novel reinnervation technique called nerve-muscle-endplate grafting in the native motor zone (NMEG-NMZ). However, it remains unknown whether the NMEG-NMZ is effective for limb reinnervation.

OBJECTIVE

To evaluate the efficacy of the NMEG-NMZ in limb muscle reinnervation.

METHODS

Forty-five adult rats were divided into 3 groups: NMEG, end-to-end anastomosis (EEA, technique control), and denervation control (DC). The left tibialis anterior muscle was denervated by resecting its nerve. For NMEG-NMZ, the denervated tibialis anterior was reinnervated by transferring a NMEG pedicle from the lateral gastrocnemius muscle. Three months after surgery, static toe spread analysis was performed for all rats and muscle force was measured for the rats treated with NMEG and EEA. Muscle weight, myofiber morphology, regenerated axons, and reinnervated motor endplates in the treated muscles were also quantified and compared with those in the DC group.

RESULTS

NMEG-NMZ technique resulted in better muscle force recovery (79% of the control) compared with EEA (51% of the control, P = .048). Toe spread analysis in NMEG-NMZ reinnervated muscles showed static sciatic index = -16.8, whereas -41.4 in EEA, P < .0001). The average weight of the NMEG-NMZ reinnervated muscles (86%) was greater than those of the EEA treated (71%) and DC (26%) muscles (all P < .0001). The mean count of the regenerated axons in the muscles with NMEG-NMZ was 76% of the control, which was larger than that in the muscles with EEA (46%), P < .0001.

CONCLUSION

NMEG-NMZ technique has unique advantages and is superior to EEA for muscle reinnervation and functional recovery.

Limb Muscle Reinnervation with the Nerve-Muscle-Endplate Grafting Technique: An Anatomical Feasibility Study

BACKGROUND

Peroneal nerve injuries results in tibialis anterior (TA) muscle paralysis. TA paralysis could cause "foot drop," a disabling condition that can make walking difficult. As current treatment methods result in poor functional recovery, novel treatment approaches need to be studied. The aim of this study was to explore anatomical feasibility of limb reinnervation with our recently developed nerve-muscle-endplate grafting (NMEG) in the native motor zone (NMZ).

METHODS

As the NMEG-NMZ technique involves in nerves and motor endplates (MEPs), the nerve supply patterns and locations of the MEP bands within the gastrocnemius (GM) and TA muscles of rats were investigated using Sihler's stain and whole-mount acetylcholinesterase (AChE) staining, respectively. Five adult rats underwent TA nerve transaction. The denervated TA was reinnervated by transferring an NMEG pedicle from the ipsilateral lateral GM. At the end of a 3-month recovery period, maximal muscle force was measured to document functional recovery.

RESULTS

The results showed that the TA was innervated by the deep peroneal nerve. A single MEP band was located obliquely in the middle of the TA. The GM was composed of two neuromuscular compartments, lateral (GM-l) and medial (GM-m), each of which was innervated by a separate nerve branch derived from the tibial nerve and had a vertically positioned MEP band. The locations of MEP bands in the GM and TA muscles and nerve supply patterns demonstrated that an NMEG pedicle can be harvested from the GM-l and implanted into the NMZ within the TA muscle. The NMEG-NMZ pilot study showed that this technique resulted in optimal muscle force recovery.

CONCLUSION

NMEG-NMZ surgery is feasible for limb reinnervation. Specifically, the denervated TA caused by peroneal nerve injuries can be reinnervated with a NMEG from the GM-l.

Nerve growth factor and basic fibroblast growth factor promote reinnervation by nerve-muscle-endplate grafting

INTRODUCTION

This study was designed to test whether exogenous application of nerve growth factor (NGF) and basic fibroblast growth factor (FGF-2) to muscles reinnervated with nerve-muscle-endplate band grafting (NMEG) could promote specific outcomes.

METHODS

The right sternomastoid muscle in adult rats was experimentally denervated and immediately reinnervated by implanting an NMEG pedicle from the ipsilateral sternohyoid muscle. A fibrin sealant containing NGF and FGF-2 was focally applied to the implantation site. Maximal tetanic force, muscle weight, regenerated axons, and motor endplates were analyzed 3 months after treatment.

RESULTS

Mean tetanic force, wet muscle weight, and number of regenerated axons in the treated muscles were 91%, 92%, and 84% of the contralateral controls, respectively. The majority of endplates (86%) in the treated muscles were reinnervated by regenerated axons.

DISCUSSION

Focal administration of NGF and FGF-2 promotes efficacy of the NMEG technique. Muscle Nerve 57: 449-459, 2018.

Reinnervation of denervated muscle by implantation of nerve-muscle-endplate band graft to the native motor zone of the target muscle

INTRODUCTION

Motor endplate reinnervation is critical for restoring motor function of the denervated muscle. We developed a novel surgical technique called nerve-muscle-endplate band grafting (NMEG) for muscle reinnervation.

METHODS

Experimentally denervated sternomastoid muscle in the rat was reinnervated by transferring a NMEG from the ipsilateral sternohyoid muscle to the native motor zone (NMZ) of the target muscle. A NMEG pedicle contained a block of muscle (~ 6 × 6 × 3 mm), a nerve branch with axon terminals, and a motor endplate band with numerous neuromuscular junctions. At 3 months after surgery, maximal tetanic muscle force measurement, muscle mass and myofiber morphology, motoneurons, regenerated axons, and axon-endplate connections of the muscles were analyzed.

RESULTS

The mean force of the reinnervated muscles was 82% of the contralateral controls. The average weight of the treated muscles was 89% of the controls. The reinnervated muscles exhibited extensive axonal regeneration. Specifically, the mean count of the regenerated axons in the reinnervated muscles reached up to 76.8% of the controls. The majority (80%) of the denervated endplates in the target muscle regained motor innervation.

CONCLUSIONS

The NMZ of the denervated muscle is an ideal site for NMEG implantation and for the development of new microsurgical and therapeutic strategies to achieve sufficient axonal regeneration, rapid endplate reinnervation, and optimal functional recovery. NMEG-NMZ technique may become a useful tool in the treatment of muscle paralysis caused by peripheral nerve injuries in certain clinical situations.

Comparison of muscle force after immediate and delayed reinnervation using nerve-muscle-endplate band grafting

BACKGROUND

Because of poor functional outcomes of currently used reinnervation methods, we developed novel treatment strategy for the restoration of paralyzed muscles-the nerve-muscle-endplate band grafting (NMEG) technique. The graft was obtained from the sternohyoid muscle (donor) and implanted into the ipsilateral paralyzed sternomastoid (SM) muscle (recipient).

METHODS

Rats were subjected to immediate or delayed (1 or 3 mo) reinnervation of the experimentally paralyzed SM muscles using the NMEG technique or the conventionally used nerve end-to-end anastomosis. The SM muscle at the opposite side served as a normal control.

RESULTS

NMEG produced better recovery of muscle force as compared with end-to-end anastomosis. A larger force produced by NMEG was most evident for small stimulation currents.

CONCLUSIONS

The NMEG technique holds great potential for successful muscle reinnervation. We hypothesize that even better muscle reinnervation and functional recovery could be achieved with further improvement of the environment that favors axon-end plate connections and accelerates axonal growth and sprouting.

Nerve-muscle-endplate band grafting: a new technique for muscle reinnervation

BACKGROUND

Because currently existing reinnervation methods result in poor functional recovery, there is a great need to develop new treatment strategies.

OBJECTIVE

To investigate the efficacy of our recently developed nerve-muscle-endplate band grafting (NMEG) technique for muscle reinnervation.

METHODS

Twenty-five adult rats were used. Sternohyoid (SH) and sternomastoid (SM) muscles served as donor and recipient muscle, respectively. Neural organization of the SH and SM muscles and surgical feasibility of the NMEG technique were determined. An NMEG contained a muscle block, a nerve branch with nerve terminals, and a motor endplate band with numerous neuromuscular junctions. After a 3-month recovery period, the degree of functional recovery was evaluated with a maximal tetanic force measurement. Retrograde horseradish peroxidase tracing was used to track the origin of the motor innervation of the reinnervated muscles. The reinnervated muscles were examined morphohistologically and immunohistochemically to assess the extent of axonal regeneration.

RESULTS

Nerve supply patterns and locations of the motor endplate bands in the SH and SM muscles were documented. The results demonstrated that the reinnervated SM muscles gained motor control from the SH motoneurons. The NMEG technique yielded extensive axonal regeneration and significant recovery of SM muscle force-generating capacity (67% of control). The mean wet weight of the NMEG-reinnervated muscles (87% of control) was greater than that of the denervated SM muscles (36% of control).

CONCLUSION

The NMEG technique resulted in successful muscle reinnervation and functional recovery. This technique holds promise in the treatment of muscle paralysis.

Fatigue and abnormal neuromuscular transmission in Kennedy's disease

We describe a patient with Kennedy's disease (X-linked bulbospinal neuronopathy) who experienced leg muscle fatigue with long-distance running. The patient also reported muscle twitching involving the face and extremities and long-standing muscle cramps. Aside from mild facial and tongue weakness (and fasciculations), his examination was normal, including completely preserved muscle strength in the extremities. Electrodiagnostic evaluation revealed evidence for a chronic motor axonopathy/neuronopathy and abnormal sensory nerve action potentials. In addition, repetitive nerve stimulation studies were normal, but neuromuscular jitter tested in the same muscle was markedly abnormal. The normal clinical strength and repetitive nerve stimulation studies in a muscle showing markedly increased neuromuscular jitter suggested a mechanism for this patient's symptoms of muscle fatigue, related to failure of neuromuscular transmission at a critical number of endplates during extremes of physical activity.

Reinnervation of muscles after transection of the sciatic nerve in adult rats

Functional recovery after transection of the sciatic nerve in adult rats is poor, probably because of abnormalities in reinnervation. Denervation and reinnervation patterns were studied morphologically in the lateral gastrocnemius (LGC), tibialis anterior (TA), and soleus (SOL) muscles for 21 weeks after nerve transection (motor endplates by acetylcholinesterase staining; nerves by silver impregnation). Motor endplates in the TA showed improving morphology with age, and, at 21 weeks, three-quarters of these were normal. Poorest recovery was observed in the SOL, as, at 21 weeks, only one-third of the motor endplates had a normal morphology. Polyneuronal innervation initially was more pronounced in the SOL, but, at 21 weeks, 10% of the motor endplates in all three muscles were still polyneuronally innervated. Our results indicate important differences in the reinnervation of these three hindleg muscles, and, even at 5 months, abnormalities were still present. These factors may in part explain the abnormal locomotion in rats as well as the limited recovery of function observed clinically in humans after nerve transection.

Synapse formation molecules in muscle and autonomic ganglia: the dual constraint hypothesis

In 1970 it was thought that if the motor-nerve supply to a muscle was interrupted and then allowed to regenerate into the muscle, motor-synaptic terminals most often formed presynaptic specializations at random positions over the surface of the constituent muscle fibres, so that the original spatial pattern of synapses was not restored. However, in the early 1970s a systematic series of experiments were carried out showing that if injury to muscles was avoided then either reinnervation or cross-reinnervation reconstituted the pattern of synapses on the muscle fibres according to an analysis using the combined techniques of electrophysiology, electronmicroscopy and histology on the muscles. It was thus shown that motor-synaptic terminals are uniquely restored to their original synaptic positions. This led to the concept of the synaptic site, defined as that region on a muscle fibre that contains molecules for triggering synaptic terminal formation. However, nerves in developing muscles were found to form connections at random positions on the surface of the very short muscle cells, indicating that these molecules are not generated by the muscle but imprinted by the nerves themselves; growth in length of the cells on either side of the imprint creates the mature synaptic site in the approximate middle of the muscle fibres. This process is accompanied at first by the differentiation of an excess number of terminals at the synaptic site, and then the elimination of all but one of the terminals. In the succeeding 25 years, identification of the synaptic site molecules has been a major task of molecular neurobiology. This review presents an historical account of the developments this century of the idea that synaptic-site formation molecules exist in muscle. The properties that these molecules must possess if they are to guide the differentiation and elimination of synaptic terminals is considered in the context of a quantitative model of this process termed the dual-constraint hypothesis. It is suggested that the molecules agrin, ARIA, MuSK and S-laminin have suitable properties according to the dual-constraint hypothesis to subserve this purpose. The extent to which there is evidence for similar molecules at neuronal synapses such as those in autonomic ganglia is also considered.

Use of dexamethasone with TTX block of nerve conduction shows that muscle membrane properties are fully controlled by evoked activity

This paper provides further evidence that motorneurons control extrajunctional properties of skeletal muscles through the activity evoked in the muscle fibres. The experiments compare the amount of action potential resistance to tetrodotoxin (TTX resistance) in denervated soleus muscle with that in soleus whose nerve was crushed and then allowed to regenerate in the presence of a block of the sciatic impulse conduction. Measurements were taken after about 2-3 weeks to allow full reinnervation and recovery of trophic regulation by the nerve. Blocking sciatic impulse conduction with TTX solutions containing low doses of the anti-inflammatory drug dexamethasone induced values of extrajunctional TTX resistance identical to those caused by denervation. In contrast lower levels of TTX resistance were obtained with dexamethasone-free solutions or when the drug was administered through the systemic path rather than topically applied to the nerve. These results indicate that physiological neural regulatory signals other than activity do not participate to the regulation of extrajunctional properties of skeletal muscles. Furthermore the low levels of TTX resistance measured with dexamethasone-free blocks confirm our previous experiments indicating that reported differences between denervation and pure inactivity are attributable to incomplete suppression of nerve impulse conduction.

Cluster of wound botulism in California: clinical, electrophysiologic, and pathologic study

Over a period of 15 months we have seen 6 patients with long-standing history of subcutaneous heroin injections who experienced acute blurred vision, dysphagia, dysarthria, and generalized weakness. Decreased or absent deep tendon reflexes, pupillary abnormalities, incremental responses to fast repetitive nerve stimulation, and positive serology for Clostridia botulinum toxin A were found, but not in all cases. Muscle biopsies showed variable signs of neurogenic atrophy. In vitro electrophysiology studies revealed decreased end-plate potentials quantal content, confirming the presynaptic nature of the disorder. Mechanical ventilation was required in 5 patients. Half of the patients were treated with polyvalent antitoxiin. Prognosis was favorable, though recovery was slow. In conclusion, acute bulbar weakness with visual symptoms in patients with subcutaneous heroin abuse strongly suggets the possibility of wound botulism. High diagnostic suspicion combined with histology and in vitro electrophysiology confirmation of presynaptic failure, especially in seronegative cases, may significantly improve morbidity.

Novel modulatory effect of L-type calcium channels at newly formed neuromuscular junctions

This study aimed to examine changes of presynaptic voltage-sensitive calcium channel (VSCC) subtypes during synapse formation and regeneration in relation to transmitter release at the neuromuscular junction (NMJ). Synaptic potentials were recorded from developing rat NMJs and from regenerating mouse and frog NMJs. As in normal adult NMJs, evoked transmitter release was reduced by an N-type VSCC blocker in the frog and by a P/Q-type VSCC blocker in the mammal at immature NMJs; however, various L-type VSCC blockers, both dihydropyridine and nondihydropyridine antagonists, increased evoked but not spontaneous release in a dose-dependent manner at newly formed NMJs. This presynaptic potentiation disappeared as NMJs matured. A rapid intracellular Ca2+ buffer, bis(O-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid-AM, prevented the potentiation effect of nifedipine, but a slow Ca2+ buffer, EGTA-AM, did not. Thus, the potentiation effect of L-type blockers requires Ca2+ transients. Pretreatment with Ca2(+)-activated K+ channel blockers, iberiotoxin or charybdotoxin, did not prevent potentiation by nifedipine at regenerating frog NMJs. Thus, Ca(2+)-activated K+ channels were not likely involved in this potentiation. In contrast, no additional potentiation by nifedipine was seen in muscles pretreated with pertussis toxin (PTX), a G-protein blocker, which by itself enhances evoked transmitter release at regenerating frog NMJs. These results suggest the existence of multiple subtypes of VSCCs at newly formed motor nerve terminals. In addition to the normal N- or P/Q-type VSCCs that mediate transmitter release, L-type VSCCs may play a novel modulatory role in evoked transmitter release by activating a mechanism linked to PTX-sensitive G-proteins during synapse maturation.

Novel modulatory effect of L-type calcium channels at newly formed neuromuscular junctions

This study aimed to examine changes of presynaptic voltage-sensitive calcium channel (VSCC) subtypes during synapse formation and regeneration in relation to transmitter release at the neuromuscular junction (NMJ). Synaptic potentials were recorded from developing rat NMJs and from regenerating mouse and frog NMJs. As in normal adult NMJs, evoked transmitter release was reduced by an N-type VSCC blocker in the frog and by a P/Q-type VSCC blocker in the mammal at immature NMJs; however, various L-type VSCC blockers, both dihydropyridine and nondihydropyridine antagonists, increased evoked but not spontaneous release in a dose-dependent manner at newly formed NMJs. This presynaptic potentiation disappeared as NMJs matured. A rapid intracellular Ca2+ buffer, bis(O-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid-AM, prevented the potentiation effect of nifedipine, but a slow Ca2+ buffer, EGTA-AM, did not. Thus, the potentiation effect of L-type blockers requires Ca2+ transients. Pretreatment with Ca2(+)-activated K+ channel blockers, iberiotoxin or charybdotoxin, did not prevent potentiation by nifedipine at regenerating frog NMJs. Thus, Ca(2+)-activated K+ channels were not likely involved in this potentiation. In contrast, no additional potentiation by nifedipine was seen in muscles pretreated with pertussis toxin (PTX), a G-protein blocker, which by itself enhances evoked transmitter release at regenerating frog NMJs. These results suggest the existence of multiple subtypes of VSCCs at newly formed motor nerve terminals. In addition to the normal N- or P/Q-type VSCCs that mediate transmitter release, L-type VSCCs may play a novel modulatory role in evoked transmitter release by activating a mechanism linked to PTX-sensitive G-proteins during synapse maturation.

Calcium channels involved in synaptic transmission at the mature and regenerating mouse neuromuscular junction

1. The involvement of the different types of voltage-dependent calcium channels (VDCCs) in synaptic transmission at the mature and newly formed mammalian neuromuscular junction was studied by evaluating the effects of L-, P/Q- and N-type VDCC antagonists on transmitter release in normal and reinnervating levator auris preparations of adult mice. 2. Nerve-evoked transmitter release was blocked by omega-agatoxin IVA (omega-AgaIVA), a P/Q-type VDCC blocker, both in normal and reinnervating endplates (100 nM omega-AgaIVA caused > 90% inhibition). The N-type VDCC antagonist omega-conotoxin GVIA (omega-CgTX; 1 and 5 microM), as occurs in normal preparations, did not significantly affect this type of release during reinnervation. Nitrendipine (1-10 microM), an L-type VDCC blocker, strongly antagonized release in reinnervating muscles (approximately 40-69% blockade) and lacked any effect in normal preparations. 3. In reinnervating muscles, spontaneous release was not dependent on Ca2+ entry through either P- or L-type VDCCs. Neither 100 nM omega-AgaIVA nor 10 microM nitrendipine affected the miniature endplate potential (MEPP) frequency or amplitude. 4. At the newly formed endplates, K(+)-evoked release was dependent on Ca2+ entry through VDCCs of the P-type family (100 nM omega-AgaIVA reduced approximately 70% of the K(+)-evoked MEPP frequency). L-type VDCCs were found not to participate in this type of release (10 microM nitrendipine lacked any effect). 5. In reinnervating muscles, the L-type VDCC blocker, nitrendipine (10 microM), provoked a significant increase (approximately 25%) in the latency of the evoked endplate potential (EPP). This drug also caused an increase (approximately 0.3 ms) in the latency of the presynaptic currents. The P/Q- and Ny-type VDCC blockers did not affect the latency of the EPP. 6. These results show that at newly formed mouse neuromuscular junctions, as occurs in mature preparations, VDCCs of the P-type family play a prominent role in evoked transmitter release whereas N-type channels are not involved in this process. In addition, signal conduction and transmitter release become highly sensitive to nitrendipine during reinnervation. This suggests that L-type VDCCs may play a role in synaptic transmission at the immature mammalian neuromuscular junction.

Electrical stimulation of denervated muscle: is it worthwhile?

Research conducted over the past 25 years has demonstrated that muscle activity, not neurotrophic substances, is the most important factor in the regulation of specific physiological and biochemical properties of muscle fibers. Application of this knowledge has led to considerable experimentation with chronic electrical stimulation as a possible clinical tool for the treatment of denervated muscles. Evidence accumulated from animal studies has indicated that direct electrical stimulation of denervated muscles can to a large extent substitute for innervation and preserve or restore the normal properties of the muscles. Appropriate stimulation parameters were critical for a successful intervention, and the best results were obtained when the stimulation pattern resembled the firing pattern of the normal motoneuron. Thus, fast muscles required intermittent, brief, high frequency stimulation and slow muscles needed continuous, low frequency stimulation. For human denervated muscles, critical questions still remain to be resolved before electrical stimulation will yield the optimum benefit. Research must be performed in human subjects to define the appropriate stimulation parameters the stimulation current, and the type and placement of electrodes.

The origin of Gaussian distributions of synaptic potentials

Spontaneous synaptic potentials were identified at the motor endplate 40 years ago. These were shown to possess amplitudes that could be described by a Gaussian distribution as could the amplitudes of evoked synaptic potentials under conditions of very low probability for secretion. As these Gaussians were identical, the idea of a unit or quantum of transmission was conceived. The failure to obtain similar Gaussian distributions for both spontaneous and low-probability evoked potentials during development of endplates indicated that a unit of transmission was not operating. However both the spontaneous and very low-probability evoked potentials could each be described by mixtures of Gaussians indicating a subunit of transmission might be operative. There are no ganglionic or central synapses at which comparisons have been made between spontaneous and low-probability evoked potentials that show each can be described by a Gaussian distribution, let alone that these are the same indicating a unit of transmission as originally conceived. There is some evidence that mixtures of Gaussians can be used to describe both spontaneous and very low-probability evoked synaptic potential amplitudes, opening up the possibility for a subunit of transmission at these synapses. The vesicle hypothesis, that the quantum of transmission at the endplate is due to the exocytosis of the contents of a synaptic vesicle, was also enunciated nearly 40 years ago. The existence of subunits of transmission has required reconsideration of this hypothesis. Three alternatives are considered: in one, the calcium-transient hypothesis, the subunit of secretion is due to the release of calcium from one of several calcium stores in the nerve terminal, so that several subunits are released when a number of these calcium stores are engaged in a regenerative response to the terminal action potential; a second alternative, the mediatophore hypothesis, is that a subunit of secretion occurs when a single transmitter transport protein channels transmitter across the terminal membrane, several such mediatophore proteins acting in concert then give multiple subunit release; finally, there is the vesicle fusion-pore hypothesis, in which individual transient openings of a fusion-pore channel joining a synaptic vesicle to the terminal membrane are responsible for secretion of a transmitter subunit, with multiple transients giving several subunits. Perhaps we will have distinguished between these possibilities before the quantal hypothesis is 50 years old.

Neuromuscular transmission in amyotrophic lateral sclerosis

The functional and structural characteristics of the neuromuscular junction were studied in anconeus muscle biopsies of 10 patients with amyotrophic lateral sclerosis (ALS). Intracellular recordings revealed decreased amplitudes of miniature endplate potentials (MEPPs). The MEPP frequencies were highly variable in ALS patients but the average MEPP frequency was not different from that of control patients. The mean quantal content of endplate potentials (m), the mean quanta available for immediate release (n), and the mean quantal stores (N) were all decreased. In contrast, the mean probability of quantal release (p) was normal and the mean probability of quantal store release (P) was surprisingly high at the majority of ALS endplates. Histologic evidence of denervation and small or absent nerve terminals were observed in all ALS patients. These functional and structural abnormalities of the neuromuscular junction may explain the fatigability and the electromyographic evidence of impaired neuromuscular transmission often encountered in ALS patients.

Electrophysiological responses in the rat tail artery during reinnervation following lesions of the sympathetic supply

1. Responses to perivascular stimuli have been recorded with intracellular microelectrodes from the smooth muscle of isolated segments of the main caudal artery of rats at various times between 7 and 128 days after all four collector nerve trunks had been lesioned near the base of the tail at 21 days of age. 2. In proximal segments (< 40 mm distal to the lesions), excitatory junction potentials (EJPs) and neurogenic alpha-depolarizations (NADs) evoked by stimuli presented via a proximally located suction electrode were similar to those in the same segments of unoperated control animals of the same age. Supramaximal EJPs in these segments decreased in amplitude with age. 3. Stimuli just supramaximal for EJPs in innervated preparations failed to evoke responses in segments farther than 30-40 mm distal to the lesions at any time after the nerves had been cut and 1 cm excised. Higher voltages evoked slow depolarizing potentials (SDPs) which were of longer time course than EJPs. Similar responses occurred in segments over 60 mm distal to the lesions at 20-50 days after the nerves had been frozen, and in all segments sampled over 100 mm distal to nerve lesions. 4. Spontaneous transient depolarizations (STDs) were recorded at all depths of the media in denervated segments. These occurred at frequencies similar to those of spontaneous events (including attenuated spontaneous EJPs) in innervated segments. 5. The earliest signs of reinnervation (24-42 days after freeze lesions) consisted of very small amplitude EJPs of normal time course which facilitated markedly during a short train of stimuli (5-10 Hz); these were followed by NADs which were large relative to the amplitudes of the EJPs. Less commonly, small focal EJPs of brief time course (resembling spontaneous EJPs in superficial cells of innervated arteries) were evoked in very restricted regions of the vessel wall. 6. At later times (57-128 days postoperative), six of eight segments located 40-70 mm distal to freeze lesions showed EJPs of nearly control amplitude, but NADs that were larger than in equivalent segments from control animals. In the remaining two cases, reinnervation at this level was similar to that seen at the earliest postoperative times. High stimulus voltages prolonged the decay of EJPs in both control and reinnervated arteries. 7. Sensitivity to exogenous noradrenaline, assessed in terms of membrane depolarization, was increased in both denervated and reinnervated segments. 8. Catecholamine fluorescence disappeared from the arteries at a distance greater than 30-40 mm distal to the site of the nerve lesions.(ABSTRACT TRUNCATED AT 400 WORDS)

Characteristics of synaptic transmission in reinnervating rat skeletal muscle

Synaptic transmission, and its sensitivity to the effects of 3,4-diaminopyridine (3,4-DAP) and the phosphatase 2,3-butanedione monoxime (BDM), was examined for "crushed fiber" preparations of rat extensor digitorum longus muscle undergoing reinnervation after nerve crush. While mean quantal content (m) of endplate potentials (EPPs) was low early during reinnervation (10-24 days after nerve crush), elevation of temperature or extracellular calcium concentration restored m toward normal. However, m achieved control values for reinnervating preparations exposed to 3,4-DAP. 3,4-DAP also activated quiescent motor nerve terminals: after exposure to this drug, synaptic transmission was detected as early as 8 days after nerve crush. BDM too activated quiescent regenerating motor nerve terminals and increased m to normal. It also prolonged EPP and endplate current decay, suggesting a pre-synaptic effect on the synchrony of transmitter release and/or a post-synaptic effect on the open time of acetylcholine-gated endplate channels. While the effects of temperature, extracellular calcium, 3,4-DAP, and BDM suggest that regenerating nerve terminals can mobilise a reserve of quanta, this reserve is abnormally low, since hemicholinium-3 caused rapid rundown of EPP amplitude at repetitively stimulated regenerating endplates.

Selective culture of mitotically active human Schwann cells from adult sural nerves

We devised a simple method to isolate mitotically active human Schwann cells from sural nerve biopsy specimens and expand the population in culture. Nerve fascicles were treated with cholera toxin for 7 days in culture before dissociation, which increased the cell yield at least twenty-five-fold over immediated tissue dissociation. Digesting the tissue completely with enzymes in serum-containing medium resulted in the highest cell viability, and released 2 to 6 x 10(4) cells/mg of tissue. Seeding the cells on a poly-L-lysine substrate in a small volume of serum-free medium optimized the plating efficiency. Although Schwann cells comprised 90% of the initial culture population, their numbers declined over time due to a faster mitotic rate of the fibroblasts in the presence of cholera toxin alone. However, treating the cultures with a combination of cholera toxin and forskolin, which act synergistically to elevate cyclic AMP levels, inhibited fibroblast growth without causing Schwann cell toxicity. Adding glial growth factor to the adenyl cyclase activators maximized Schwann cell proliferation, and the population rapidly and selectively expanded. Therefore, it should be possible to generate large numbers of Schwann cells from diseased nerves to study defects in cell function or from normal nerves to study the effects of Schwann cell grafts on neuronal regeneration.

Neuromuscular transmission as a function of motor unit size in patients with prior poliomyelitis

We studied neuromuscular transmission in 16 patients with prior poliomyelitis by measuring single fiber electromyographic (SFEMG) jitter. This was compared with 3 indirect methods of assessing reinnervation: SFEMG fiber density, macro EMG, and the presence of fiber type grouping on muscle biopsy. In patients with acute poliomyelitis before the age of 10, there was a positive correlation between the extent of neuromuscular transmission impairment, demonstrated by increased SFEMG jitter, and the enlargement of the motor unit, as indicated by increased fiber density, increased macro EMG signals, and fiber type grouping on muscle biopsy. However, there was no correlation between any of these parameters and the presence or absence of new symptoms of weakness. These findings suggest that impaired neuromuscular transmission is most common in patients with prior poliomyelitis whose motor units have been maximally enlarged by axonal sprouting, but is independent of the presence or absence of new symptoms of weakness.

Neuromuscular transmission at newly formed neuromuscular junctions in the regenerating soleus muscle of the rat

1. A study of neuromuscular function in regenerating skeletal muscle fibres in the rat soleus muscle has been made. The muscle fibres were damaged in vivo by the injection of the myotoxic venom component notexin, and then allowed to regenerate spontaneously. 2. Regenerating muscle fibres generated action potentials and contracted following direct intracellular stimulation as early as 4 days after the injection of notexin. 3. Miniature endplate potentials (MEPPs) were recorded at a minority of synapses at 3 days, and from all synapses by 5 days. The mean amplitude of MEPPs in a given fibre was directly proportional to muscle fibre input resistance. 4. Spontaneous transmitter release in the regenerating fibres was relatively insensitive to changes in [K+]0 but the effect of Ruthenium Red on spontaneous release was similar in the regenerating and control muscle fibres. 5. Functional innervation, defined as the ability to generate an indirect action potential, was restored in 97% of fibres by 10 days. The generation of an action potential was always associated with a twitch of the muscle fibre. Those fibres that were unable to generate an action potential usually exhibited a low membrane potential (ca -50 mV). These fibres could generate action potentials if they were hyperpolarized using an intracellular current-passing microelectrode. 6. The quantal content of endplate potentials (EPPs) was estimated from the mean EPP and mean MEPP amplitudes in cut muscle fibre preparations. These estimates suggested that quantal content was low at the earliest stages of regeneration, but increased as the muscle fibres matured and became normal at 10-21 days. 7. During repetitive stimulation at 30 Hz there was a fall in the amplitude of EPPs of 40-45%. The fall was similar in regenerating and control fibres. Conducting synapses never exhibited failure to generate an EPP during the period of high-frequency stimulation.

The regulation of quantal size

Quantal size can be altered experimentally by numerous treatments that seem to lack any common thread. The observations may seem haphazard and senseless unless clear distinctions are made from the outset. Some treatments shift the size of the entire population of quanta. These quanta are released by nerve stimulation. Other treatments add quanta of abnormal size or shape--monstrosities--to the population (4.0). Usually, perhaps even invariably, the monstrosities are not released by nerve stimulation. 6.1. POPULATION SIZE INCREASES. 6.1.1. Quantal size must be regulated. The size of the entire quantal population can be experimentally shifted to a larger size, with the mean rising two- or even four-fold. Before these observations, it was reasonable to suppose that quantal size was relatively fixed, with little room for maneuver. A logical picture is that synaptic vesicles have a maximum transmitter capacity, and usually they are filled to the brim. This picture is wrong. The quantity of transmitter packaged in the quantum must be regulated by the neuron, so depending on circumstances, quantal size can be increased or decreased. Figure 18 makes the case for regulation more strongly than words. We are beginning to identify some of the signals for up and down regulation, and the first steps have been made in discovering the signal transduction pathways, but we are far from a true understanding. This is hardly surprising, because our information about how transmitter molecules are assembled into quantal packages is still imperfect. Until we understand the engine, it may be difficult to picture the accelerator or the brake. 6.1.2. Signals that up regulate size. Stimulation of the presynaptic neuron increases quantal size at the NMJ, at synapses in autonomic ganglia and in hippocampus. The stimulus parameters necessary to elicit the quantal size increase have not been explored sufficiently in any of these cases, and all deserve further investigation. At both frog and mouse NMJs quantal size is roughly doubled following exposure to hypertonic solutions, which elevate the rate of spontaneous quantal release. This discovery, coupled with the increases caused by tetanic stimulation, suggested that the signal for up regulation is a period of greatly enhanced quantal output. The size increase takes about 15 min in hypertonic solution in mouse and about 60 min in frog. Highly hypertonic solutions do not increase the rate of quantal release in frog; they also do not increase quantal size. This supported the idea that quantal release rate is the signal for up regulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Spontaneous activity at long-term silenced synapses in rat muscle

1. The impulse activity in the sciatic nerve of rats was blocked for 30-59 days by a chronic infusion of tetrodotoxin (TTX) into a cuff around the nerve from an external mechanical pump. After this treatment the extensor digitorum longus muscle was isolated and the electrical activity at the endplates was recorded by intracellular electrodes. Endplates in the paralysed muscles were still functional as the muscle contracted briskly upon stimulation of the nerve distal to the block. 2. The spontaneous miniature endplate potentials (MEPPs) differed from those in normal muscles in having a more variable amplitude; 24% of the events were more than twice as large as the modal amplitude. In 30% of the fibres the largest of these 'giant' MEPPs (GMEPPs) triggered muscle action potentials. The amplitude distributions often had suggestive peaks indicating that the GMEPPs might consist of multiple quanta of the same size as those constituting the nerve impulse-evoked endplate potentials (EPPs). 3. The GMEPPs were more prolonged, but were similar in shape to MEPPs from normal muscles, with a smooth, relatively fast rising phase and a more prolonged decay. The mean time-to-peak was higher than for impulse-evoked EPPs of the same size, suggesting that the spontaneous release was more distant or less synchronized than after a nerve impulse. The half-decay time of the GMEPPs showed no large increase with increasing amplitude suggesting that the release of transmitter was not focal. The half-decay times were, however, longer than for impulse-evoked EPPs of the same size, suggesting that the spontaneous release might be less distributed than impulse-evoked release. 4. GMEPPs were not influenced by TTX, they were larger than the impulse-evoked EPPs in solutions containing high Mg2+ and low Ca2+, and they were not increased by high extracellular Ca2+ concentrations. Thus, the GMEPPs were not caused by spontaneous action potentials and probably not by Ca2+ influx. 5. In most cases the frequency of large and small MEPPs in paralysed muscles was influenced in the same way as those in normal muscles. It was increased by an increase in the extracellular K+ concentration or osmolarity, and reduced by a decrease in Ca2+ or an increase in Mg2+. The frequency was increased by Ruthenium Red. Also, like MEPPs in normal muscles, the frequency of small MEPPs in paralysed muscles was increased by increasing the extracellular Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)

Tetrahydroaminoacridine (tacrine) stimulates neurosecretion at mammalian motor endplates

1. Tacrine (20 microM) induced, like 4-aminoquinoline (4-AQ, 200 microM), the appearance of a population of miniature endplate potentials (m.e.p.ps) with more than twice the normal amplitude or time-to-peak. The times-to-peak of nerve impulse-evoked endplate potentials were not similarly affected. 2. Cholinesterase inhibition by edrophonium (25 microM) did not prevent tacrine or 4-AQ from inducing this population of m.e.p.ps. 3. Nerve-muscle preparations in which the normal calcium-sensitive quantal release of acetylcholine had been blocked by botulinum neurotoxin type A also responded to tacrine by an increase in the frequency of giant or slow m.e.p.ps. 4. Reduction of the temperature from 30 degrees to 14 degrees C reduced the frequency of giant or slow m.e.p.ps induced either by tacrine or by 4-AQ. A similar effect was obtained by colchicine (5 mM). This supports the idea that proximo-distal axonal transport is required for the secretory activity. 5. The neurosecretion evoked by tacrine could explain the therapeutic effects of the drug claimed in the treatment of Alzheimer's type of dementia.

Repeated, in vivo observation of frog neuromuscular junctions: remodelling involves concurrent growth and retraction

The fluorescent dye 4-(4-diethylaminostyryl)-N-methylpyridinium iodide was used as a vital stain to study remodelling of motor nerve terminals in sartorius muscles of living frogs (Rana pipiens). Identified terminals were observed twice in vivo at intervals of 87-192 days. After the second observation, muscles were fixed and stained with the nitroblue tetrazolium method for nerve terminals and with cholinesterase stain. Observations were made of 243 junctions in 26 frogs. Most nerve terminals grew during the observation interval, with an average increase in total terminal length of 29%. This growth involved substantial remodelling. Within single junctions, the change in size was the net result of differing degrees of growth or shrinkage in individual nerve terminal branches. At least one new terminal branch appeared in 25% of the junctions. Terminal retraction was also common, with branch shortening seen in 60% of junctions and the complete disappearance of a branch in 12%. In one case the original axonal input retracted completely and the junction was partially reinnervated by a terminal sprout from a junction on an adjacent fibre. Some discrepancies between histological and in vivo observations of remodelling were noted. These observations confirm that frog neuromuscular junctions are highly dynamic synapses, subject to profound structural remodelling throughout adult life.

Motor nerve terminal restoration after focal destruction in young and old mice

Regeneration of soleus motor nerve terminals after focal destruction by black widow spider venom (BWSV) or its active factor alpha-latrotoxin (LTx) was compared in young and old CBF-1 mice. The object was to determine whether previously reported delayed regeneration after nerve injury in old rodents was due to altered removal of debris, or delay or aberrancy in structural or functional restoration of the neuromuscular junction. In addition, the use of a new fluorescent technique permitted for the first time quantitation of the accuracy of early nerve terminal regeneration in mammalian muscle. Immunohistochemical and electron micrographic studies showed no age difference in destruction of terminals and removal of debris 2 days after toxin application. The indirect twitch and structural reinnervation (measured with flourescent techniques) returned to an equal extent in young and old mice beginning at 3 days after LTx treatment. BWSV (as opposed to LTx) delayed regeneration 1 day in young but not in old mice. On the first day of reinnervation, there was perisynaptic outgrowth in both young and old mice, although in the latter, there was a higher incidence of aberrant outgrowth. The relation between return of twitch strength and recovery of nerve terminal area (measured in teased zinc iodide-stained preparations) showed no age dependency. We conclude that factors cited to explain altered reactive sprouting in the aging CNS do not apply to regeneration of peripheral motor nerve terminals. However, it is possible that the aberrant regrowth observed at the neuromuscular junction in old mice will pertain to the aging CNS. Altered axonal rather than nerve terminal regeneration is the likely source of delayed peripheral nerve regeneration in old animals.

The behaviour of sensory ganglia in relation to their environment

The nodose ganglion was removed from rats and transplanted into the denervated tongue and into the denervated or innervated sternocleidomastoid muscle. In another procedure the ganglion was sutured to the distal hypoglossal or sciatic nerve. The ganglia failed to develop motor end-plates on denervated muscle. They did, however, induce formation of taste buds on the lingual epithelium. The regenerated nerve fibres ramified extensively within the connective tissue, invading perivascular tissue and lingual epithelium. At a cellular level some ganglion cells were covered with a meshwork of axonal material while others were found outside the confines of the implanted ganglion. The latter cell group are thought to achieve their location by a process of migration.

Scanning electron microscopic study of denervated and reinnervated neuromuscular junction

Morphological changes of the subneural apparatus (SNA) during denervation and reinnervation are demonstrated using the hamster peroneus longus muscle. One week after nerve section, synaptic grooves (SGs) were more shallow, with lowered sarcoplasmic ridges. After 4 weeks, the whole SNA area was elevated above the sarcolemma as a flat plate, which persisted as a fusiform bulge for more than 8 weeks. There was a rapid and progressive decrease in the number of junctional folds (JFs) and slit-to-pit transformation of their openings. There was no trace of the SNA after 16 weeks. By 4 weeks after nerve suture, SGs were flattened but did not form the plate-like elevation. Many shallow pit-like JFs still persisted. At 10 weeks, deep SGs started to be restored with an increased number of deep slit-like JFs. The SNA recovered its normal structural organization by the week 20. However, a few pit-like vestigial JFs were present outside the groove.

Migration of Schwann cells and axons into developing chick forelimb muscles following removal of either the neural tube or the neural crest

A study has been made of the effects of neural crest and neural tube removal at the brachial level on the migration of Schwann cells and axons into the flexor digitorum profundus (fdp) and flexor carpi ulnaris (fcu) muscles of the avian forelimb. The identification of Schwann cells was based on the assumption that antibody HNK-1 uniquely labels these cells at the growing end of limb nerves. Myotubes and nerves were identified by using antibodies to myosin and to neurofilament protein, respectively. The removal of neural crest cells at stage 13 gave a complete Schwann cell-free embryo at the brachial level. Motor axons only grew to the base of the forelimb, forming a rudimentary plexus by stage 27, and failed to penetrate the limb. Removal of the neural tube at stage 13 did not prevent sensory axons from forming a plexus at the base of the limb; these axons subsequently developed into the brachialis longus inferior (bli n) and superior (bls n) nerves. By stage 27 the bli n had branched into the interosseus nerve (in n) and the medial-ulnar nerve (m-u n) trunks. However, unlike the result in control embryos, no nerves were detected amongst the developing fdp and fcu muscles, thus indicating that sensory axons do not grow into the muscles in the absence of motor axons. In contrast, Schwann cells were observed amongst the myotubes at the level of the in n and m-u nerve trunks. The present observations show that motor axons do not enter the limb bud and innervate limb muscles in the absence of Schwann cells. Furthermore, in the absence of motor axons (neural-tube-removed embryos) sensory axons still enter the limb (behind migrating Schwann cells) but fail to innervate developing muscles even though Schwann cells are present among the developing myotubes.

Distribution and role in regeneration of N-CAM in the basal laminae of muscle and Schwann cells

The neural cell adhesion molecule (N-CAM) is a membrane glycoprotein involved in neuron-neuron and neuron-muscle adhesion. It can be synthesized in various forms by both nerve and muscle and it becomes concentrated at the motor endplate. Biochemical analysis of a frog muscle extract enriched in basal lamina revealed the presence of a polydisperse, polysialylated form of N-CAM with an average Mr of approximately 160,000 as determined by SDS-PAGE, which was converted to a form of 125,000 Mr by treatment with neuraminidase. To define further the role of N-CAM in neuromuscular junction organization, we studied the distribution of N-CAM in an in vivo preparation of frog basal lamina sheaths obtained by inducing the degeneration of both nerve and muscle fibers. Immunoreactive material could be readily detected by anti-N-CAM antibodies in such basal lamina sheaths. Ultrastructural analysis using immunogold techniques revealed N-CAM in close association with the basal lamina sheaths, present in dense accumulation at places that presumably correspond to synaptic regions. N-CAM epitopes were also associated with collagen fibrils in the extracellular matrix. The ability of anti-N-CAM antibodies to perturb nerve regeneration and reinnervation of the remaining basal lamina sheaths was then examined. In control animals, myelinating Schwann cells wrapped around the regenerated axon and reinnervation occurred only at the old synaptic areas; new contacts between nerve and basal lamina had a terminal Schwann cell capping the nerve terminal. In the presence of anti-N-CAM antibodies, three major abnormalities were observed in the regeneration and reinnervation processes: (a) regenerated axons in nerve trunks that had grown back into the old Schwann cell basal lamina were rarely associated with myelinating Schwann cell processes, (b) ectopic synapses were often present, and (c) many of the axon terminals lacked a terminal Schwann cell capping the nerve-basal lamina contact area. These results suggest that N-CAM may play an important role not only in the determination of synaptic areas but also in Schwann cell-axon interactions during nerve regeneration.

Spontaneous transmitter release at the neuromuscular junction

The classical studies of Katz and co-workers have shown that nerve impulses release quanta of acetylcholine at the neuromuscular junction. This release is regulated by presynaptic calcium and accounts for the trans-synaptic transmission of nerve impulses. In resting conditions it gives rise to small spontaneous potentials, i.e. miniature endplate potentials. In addition these investigators described a spontaneous molecular leakage of acetylcholine from the motor nerve. I have studied a third type of acetylcholine release. It is a spontaneous intermittent secretion of acetylcholine which postsynaptically causes large, generally slow rising potentials. This release is unaffected by presynaptic calcium and is therefore not influenced by nerve activity. The acetylcholine responsible for these potentials comes from the same pool of transmitter as that liberated by nerve impulses. The observation that the release is blocked by drugs that prevent the accumulation of acetylcholine into synaptic vesicles indicates that the secretion originates from clusters of vesicles or large vesicle-like structures in the nerve terminal. This type of release is present at a low frequency at normal neuromuscular junctions. It is markedly accelerated whenever the calcium-dependent quantal release of acetylcholine is blocked or impaired. The drug 4-aminoquinoline selectively stimulates this release. I speculate that this type of transmitter secretion is important for the development of synaptic connexions.

Changes in the distribution of synapses during growth: a quantitative morphological study of the neuromuscular system of fishes

The distribution of synaptic sites on multiply innervated muscle fibres was analysed in four teleost fish species (zebrafish, trout, goldfish and stickleback), using acetylcholinesterase histochemistry. Fishes were chosen for this study rather than other vertebrates because of their long period of growth and continuous increase of muscle fibre size. We found that length and diameter of the fibres increase linearly with fish length but that the distance between synaptic sites increases only as the square root of the fish length and of muscle fibre size. This is explained functionally in connection with the increase of the space constant of a muscle fibre that is expected to accompany the increase of its diameter. We suggest that the change in the synaptic distribution is caused by factors associated with the increasingly wider spread of postsynaptic potentials along the growing fibres, as the intersynaptic distance was found to correlate more strongly with fibre size than with other factors, such as age, speed of growth and genetical background.

Morphological differentiation of human muscle cocultured with mouse spinal cord

Human muscle fibres have been cocultured with sections of embryonic mouse spinal cord for periods of up to 2 months. The muscle fibres regenerated to form a bundle of myotubes, a proportion of which developed cross-striations and contractions. This proportion was variable between biopsies, and morphological differentiation was not as successful as when mouse muscle and mouse nerve were cultured together. Regeneration and morphological differentiation were unaffected by storing samples in liquid nitrogen, and were not improved by the presence of original synaptic areas in the explanted bundle or by alterations in the growth media. These involved changing the levels of serum and embryo extract, using different sources of serum, and the incorporation of additives in the medium. A comparison of the growth characteristics of samples of muscle from 30 patients (including some control samples) indicated that although muscle from younger patients (less than 14 years) regenerated more quickly, the myotubes did not have better differentiation. It also indicated that the growth characteristics of regenerated myotubes from diseased and normal muscle were indistinguishable within the 4-8 weeks observation period. Muscle from patients with Duchenne muscular dystrophy regenerated and differentiated less well than would be expected from age-matched controls, but this was not thought to reflect an intrinsic abnormality in the regenerative capacity of the muscle.

Growth of axons into developing muscles of the chick forelimb is preceded by cells that stain with Schwann cell antibodies

A study has been made of the development of limb and muscle nerves in relation to the first appearance of Schwann cells in the flexor digitorum profundus (fdp) and flexor carpi ulnaris (fcu) muscles of the avian forelimb. Schwann cells were identified by immunofluorescent techniques with antibodies to the glycoprotein HNK-1. Myotubes and nerves were identified by using antibodies to myosin and to neurofilament, respectively. At stage 24/25 the brachialis longus inferior (Bli n) and superior (Bls n) nerve trunks within proximal regions of the forelimb were surrounded by Schwann cells. These cells extended in a column for a distance of approximately 100 microns beyond the growing ends of nerves. At stage 26 both interosseus nerve (in n) and the medial-ulnar nerve (m-u n) had formed from the Bli n; each of these branches was surrounded by Schwann cells, which again extended approximately 100 microns beyond the growing ends of the nerves. By stage 26/27 the fdp and fcu muscles were clearly delineated by groups of myotubes. No nerves were detected within these groups; however, Schwann cells were observed between the myotubes. At stage 27 axons had left the in n and m-u n and grown into the fdp and fcu muscles, respectively. These axons were surrounded by Schwann cells. The present observations show that Schwann cells are located ahead of the main limb and muscle nerves as they grow into the fdp and fcu muscles of the limb. It is possible that these Schwann cells play a role in guiding nerves to their correct muscles in the developing chick forelimb.

Late reinnervation of the rat soleus muscle is differentially suppressed by chronic stimulation and by ectopic innervation

Soleus (SOL) and extensor digitorum longus (EDL) muscles in adult rats were kept denervated for 2 months by four repeated freezes at 2-week intervals of the sciatic nerve. Reinnervation was studied in the absence or presence of chronic muscle stimulation, starting 1 month before reinnervation began. In addition, reinnervation was studied in SOL muscles where a previously transplanted fibular (FIB) nerve had formed ectopic neuromuscular junctions outside the original endplate area. After repeated freezes only, reinnervation was complete judged by tension measurements and histochemical examinations in SOL (n = 7) and EDL (n = 8) muscles. In directly stimulated muscles reinnervation was incomplete, and the force tensions evoked from indirect stimulation was on average 87 (n = 5) and 82% (n = 5) of direct muscle stimulation in SOL and EDL muscles, respectively. Of ectopically innervated SOL muscle fibres, only 26% became reinnervated in 12 muscles. Denervation and reinnervation increased the number of muscle fibres in stimulated (n = 4) and unstimulated (n = 5) EDL muscles by 18 and 15%, respectively. In stimulated (n = 4) and unstimulated (n = 7) SOL muscles, on the other hand, the number of muscle fibres remained normal. The stronger suppression of reinnervation in ectopically reinnervated compared to chronically stimulated SOL fibres indicates that reinnervation can also be suppressed by activity independent influences from the foreign nerve.

Calcium-insensitive miniature endplate potentials at the neuromuscular junction

Several different types of acetylcholine secretion have been shown to coexist at the neuromuscular junction along with the Ca2+-dependent quantal release producing miniature endplate potentials (mepps) and endplate potentials. One of these, the Ca2+-insensitive, slow-rising mepps (slow mepps), is present in normal untreated muscles but is most prominent in many conditions where the Ca2+-dependent quantal release mechanism is not functioning properly. Slow mepps occur at a frequency of less than 0.1 Hz in normal muscles, with large variability between fibres and muscles, and can reach frequencies of 1-2 Hz in several pathological conditions. The potentials are also highly variable in size and shape, being generally of high amplitude (0.1-15 mV) and prolonged time course (1-15 ms rise time). Most importantly, slow mepps are not affected by procedures which increase the intraterminal Ca2+ concentration, including nerve stimulation, thus being unable to contribute to the function of synaptic transmission. The cellular source of the Ca2+-insensitive mepps has been determined to be the nerve terminal and not the Schwann cells or nerve sprouts. The release process producing slow mepps is generally insensitive to many drugs, ions, and procedures, stimulation being observed with vinblastine, cytochalasin B, and caffeine. Depression of this secretion is effected by uncouplers of oxidative phosphorylation and by a drug (AH5183) which inhibits the vesicular active acetylcholine transport system. It is concluded that the slow mepps are due to an exocytic fusion of unique synaptic vesicles with the plasma membrane near the active zones, in a process insensitive to many intracellular ions and regulators. Since slow mepps are prominent in many pathological conditions of nerve and muscle, it is speculated that they play some role in the recovery or development of synaptic function.

Altered expression of neuronal cell adhesion molecules induced by nerve injury and repair

Peripheral nerve injury results in short-term and long-term changes in both neurons and glia. In the present study, immunohistological and immunoblot analyses were used to examine the expression of the neural cell adhesion molecule (N-CAM) and the neuron-glia cell adhesion molecule (Ng-CAM) within different parts of a functionally linked neuromuscular system extending from skeletal muscle to the spinal cord after peripheral nerve injury. Histological samples were taken from 3 to 150 d after crushing or transecting the sciatic nerve in adult chickens and mice. In unperturbed tissues, both N-CAM and Ng-CAM were found on nonmyelinated axons, and to a lesser extent on Schwann cells and myelinated axons. Only N-CAM was found on muscles. After denervation, the following changes were observed: The amount of N-CAM in muscle fibers increased transiently on the surface and in the cytoplasm, and in interstitial spaces between fibers. Restoration of normal N-CAM levels in muscle was dependent on reinnervation; in a chronically denervated state, N-CAM levels remained high. After crushing or cutting the nerve, the amount of both CAMs increased in the area surrounding the lesion, and the predominant form of N-CAM changed from a discrete Mr 140,000 component to the polydisperse high molecular weight embryonic form. Anti-N-CAM antibodies stained neurites, Schwann cells, and the perineurium of the regenerating sciatic nerve. Anti-Ng-CAM antibodies labeled neurites, Schwann cells and the endoneurial tubes in the distal stump. Changes in CAM distribution were observed in dorsal root ganglia and in the spinal cord only after the nerve was cut. The fibers within affected dorsal root ganglia were more intensely labeled for both CAMs, and the motor neurons in the ventral horn of the spinal cord of the affected segments were stained more intensely in a ring pattern by anti-N-CAM and anti-Ng-CAM than their counterparts on the side contralateral to the lesion. Taken together with the previous studies (Rieger, F., M. Grumet, and G. M. Edelman, J. Cell Biol. 101:285-293), these data suggest that local signals between neurons and glia may regulate CAM expression in the spinal cord and nerve during regeneration, and that activity may regulate N-CAM expression in muscle. Correlations of the present observations are made here with established events of nerve degeneration and suggest a number of roles for the CAMs in regenerative events.(ABSTRACT TRUNCATED AT 400 WORDS)

A comparison of active zone structure in frog neuromuscular junctions from two fast muscles with different synaptic efficacy

To search for ultrastructural correlates of differences in synaptic safety factor and neurotransmitter release, neuromuscular junctions from the cutaneous pectoris and cutaneous dorsi muscles of the grass frog Rana pipiens were freeze fractured. Synaptic efficacy in these muscles was determined by the extent to which isometric twitch tension could be blocked by lowering [Ca2+] in the bathing solution. We found that junctions in the cutaneous pectoris were significantly more effective than those of the cutaneous dorsi. Morphometric analysis of 16 junctions from each type of muscle showed significant differences in some aspects of active zone structure. Cutaneous pectoris terminals had longer active zone segments and active zones spaced more closely together. This resulted in 20% more active zone length per unit terminal length in the cutaneous pectoris. Cutaneous dorsi terminals had active zones that were more often segmented into two or more sections at a single junctional fold. Mean active zone length per junctional fold and the number of active zone particles per micrometre of active zone length were not significantly different. As a result of the somewhat larger terminal width in the cutaneous dorsi, the percentage of terminal width occupied by active zone was greater in the cutaneous pectoris. As an attempt to indirectly estimate active zone spacing with the light microscope, we applied rhodamine-conjugated alpha bungarotoxin to neuromuscular junctions from the cutaneous pectoris and cutaneous dorsi. No significant difference in the spacing of fluorescently labelled acetylcholine receptor bands was found between the two types of junctions. Our results indicated that the greater active zone length per unit terminal length in the cutaneous pectoris was associated with its increased synaptic efficacy. In addition the continuity and particle organization of active zones may have contributed to the observed differences in synaptic safety factor at frog neuromuscular junctions.

The clustering of acetylcholine receptors and formation of neuromuscular junctions in regenerating mammalian muscle grafts

The present investigation was undertaken to study the relationship between acetylcholine receptor (AchR) clustering and endplate formation within regenerating skeletal muscle grafts. Silver staining of nerves was combined with rhodamine-alpha-bungarotoxin labeling of AchR clusters in heterotopic grafts of the rat soleus muscle. Two major graft procedures were used: whole muscle grafts and grafts which lacked the zone of original motor endplates (MEP-less grafts). These categories were subdivided into standard grafts, where subsequent innervation was allowed, and noninnervated grafts, which were experimentally deprived of innervation. Grafting brought about the death and removal of muscle fibers, followed by regeneration of myotubes within surviving basal lamina sheaths. A transient population of small extra-junctional AchR clusters spontaneously appears shortly after myotube formation in all four muscle graft types. Early myotubes of whole muscle grafts (both innervated and standard grafts, prior to the time of innervation) also develop presumptive secondary synaptic clefts and large, organized aggregations of AchRs at original synaptic sites. At later times, nerves regenerating into standard whole muscle and MEP-less grafts lead to the formation of numerous ectopic endplates. In whole muscle grafts, endplates may also form at original synaptic sites. Functional graft innervation is achieved in whole muscle and MEP-less grafts as early as 20 days postgrafting. The results of this study support the existence of still-unknown factors associated with the original synaptic site which can direct postsynaptic differentiation independent of innervation. They also demonstrate that functional endplates may form in mammalian muscle grafts at both original synaptic sites and ectopic locations, thus indicating that the zone of original synaptic sites is not necessary for the establishment of numerous functional and morphologically well-differentiated endplates.

Miniature end-plate potentials in rat skeletal muscle poisoned with botulinum toxin

Spontaneous transmitter release, recorded as miniature end-plate potentials (m.e.p.p.s), was studied in rat extensor digitorum longus (e.d.l.) and soleus muscles partially or completely paralysed by botulinum toxin type A (BoTx). Normal unpoisoned muscles were examined for comparison. Analysis of m.e.p.p.s in both normal and BoTx-poisoned muscles confirmed the presence of two populations of potentials. One population, which comprised about 96% of the m.e.p.p.s recorded at non-poisoned end-plates, was characterized by a uniform time course and a mean time-to-peak of 0.5-0.7 ms. These potentials had a shape and time-to-peak similar to that of quantal end-plate potentials (e.p.p.s) evoked by nerve stimuli. These were designated 'fast m.e.p.p.s'. The other population of m.e.p.p.s was characterized by a slower, more variable rise-time, the time-to-peak exceeding 1.1 ms, and generally a larger amplitude. These were designated 'slow m.e.p.p.s'. In both partial and complete paralysis by BoTx the frequency of fast m.e.p.p.s was reduced by more than 90% and the reduction lasted several weeks. After 6-10 days of poisoning the frequency of slow m.e.p.p.s gradually increased. The highest frequency of slow m.e.p.p.s (0.4 Hz) was recorded in the partially paralysed soleus muscle, the frequency being about ten times that at unpoisoned end-plates. In both partially paralysed muscles slow m.e.p.p. frequency returned towards normal 28 days after poisoning. A significant correlation (r = 0.67) was observed between the quantal content of e.p.p.s and the frequency of fast m.e.p.p.s in partially paralysed e.d.l. muscles. No significant correlation was observed between quantal content of e.p.p.s and the frequency of slow m.e.p.p.s. To further study if muscle activity influenced the appearance of slow m.e.p.p.s, partially paralysed soleus muscles were directly stimulated in vivo during the first 11-13 days following BoTx poisoning, using a stimulation pattern which inhibits nerve terminal sprouting and the appearance of denervation changes. This procedure did not alter the frequency of slow m.e.p.p.s as compared to unstimulated poisoned controls. It is concluded that enhancement of slow m.e.p.p. frequency in muscles poisoned with BoTx is related to the blockade of evoked Ca2+-dependent quantal transmitter release. However, additional factors influence this type of spontaneous and Ca2+-insensitive release of acetylcholine since there is a great variability between fibres and a time lag between the disappearance of fast m.e.p.p.s and the activation of slow m.e.p.p. frequency.(ABSTRACT TRUNCATED AT 400 WORDS)

Interaction of inactivity and nerve breakdown products in the origin of acute denervation changes in rat skeletal muscle

The action of nerve breakdown products on innervated fibres of soleus and extensor digitorum longus muscles was investigated with the following procedures: partial denervation, sensory or sympathetic denervation, section of a previously transplanted foreign nerve. Each procedure was performed either in isolation or combined with chronic muscle inactivity obtained by blocking impulse conduction along the sciatic nerve. Silastic cuffs containing tetrodotoxin (TTX) and sodium chloride were utilized for the block. Partial denervation induced extrajunctional sensitivity to acetylcholine (ACh) and resistance to tetrodotoxin not only in the denervated but also in the innervated fibres. The effects in the innervated fibres were equal in magnitude to those in the denervated fibres, provided they were paralysed. The onset of the membrane changes was synchronous in the two classes of fibres and their amount correlated with the extent of partial denervation. If the innervated fibres were normally active, the membrane changes were still detectable, but considerably smaller than in the denervated fibres. Sensory denervation (removal of dorsal root ganglia L4 and L5) was followed by the development of moderate ACh supersensitivity and TTX resistance in chronically paralysed muscles. Furthermore, section of radicular nerves (total denervation, i.e. efferent plus afferent) induced larger membrane changes than those observed following section of ventral roots alone (efferent denervation). Sympathetic denervation was ineffective even when associated with chronic muscle paralysis. Section of a previously transplanted mixed nerve (superficial fibular) was ineffective if the soleus muscle was normally active, while it induced marked extrajunctional ACh sensitivity and TTX resistance when combined with chronic paralysis of the muscle. Section of a transplanted sensory nerve (sural) also induced extrajunctional membrane changes in paralysed soleus muscles, but their magnitude was much smaller than after section of mixed nerves. We conclude that products of nerve destruction, especially those of motor axons, induce membrane changes of striking magnitude when potentiated by muscle inactivity. Such an action may also explain the greater efficacy of denervation vs. pure inactivity, at least at early times after their onset.