Chronic stimulation of the spinal cord in developing chick embryo causes the differentiation of multiple clusters of acetylcholine receptor in the posterior latissimus dorsi muscle

Acetylcholine receptor formation in mouse-chick chimera

This study investigated possible interactions between motoneurons and somitic-derived muscle cells in the formation of neuromuscular synapses in the myotome. The peculiarities of the neuromuscular synaptic pattern in chick and mouse embryos provided a model for studying the achievement of synaptogenesis between chick motoneurons and mouse muscle cells. In chick embryo, initial AChR clustering occurs well before innervation of the myotome, whereas in mouse embryo nerve axons invade the myotome extensively before the appearance of AChR clusters. Our approach was to replace somites from a chick host embryo with those derived from mouse donor embryos. We show that muscle cells from mouse myotome can differentiate in the chick embryo environment and form neuromuscular contacts with chick motor axons. Host axons invaded in ovo differentiating mouse myotome at a time when they had not yet reached the host myotome. This particular ingrowth of motor nerves was attributable to the mouse transplant since use of a quail somite did not produce the same effect as the mouse somite, which suggests that developing mouse muscles specifically modify the time course of chick axogenesis. The synaptic areas formed between chick motor axons and mouse myotubes developed according to the mouse pattern. Both the timing of their appearance and their morphology correlated perfectly with events in mouse synaptogenesis. These results indicate the important role played by postsynaptic membrane in controlling the first steps of AChR formation.

Interactions between spinal cord stimulation and activity blockade in the regulation of synaptogenesis and motoneuron survival in the chick embryo

The present study investigated the effects of spinal cord stimulation, neuromuscular blockade, or a combination of the two on neuromuscular development both during and after the period of naturally occurring motoneuron death in the chick embryo. Electrical stimulation of the spinal cord was without effect on motoneuron survival, synaptogenesis, or muscle properties. By contrast, activity blockade rescued motoneurons from cell death and altered synaptogenesis. A combination of spinal cord stimulation and activity blockade resulted in a marked increase in motoneuron death, and also altered synaptogenesis similar to that seen with activity blockade alone. Perturbation of normal nerve-muscle interactions by activity blockade may increase the vulnerability of developing motoneurons to excessive excitatory afferent input (spinal cord stimulation) resulting in excitotoxic-induced cell death.

In vitro regulation of the innervation pattern of quail muscle fibers by quail and mouse neurons

Myoblasts from rudiments of slow and fast muscle, anterior latissimus dorsi (ALD) and posterior latissimus dorsi (PLD) respectively, of 9-day-old quail embryos were cultured in vitro for a period of up to 60 days in order to give rise to well-differentiated muscle fibres. These fibres were innervated by neurons from either quail or mouse embryo spinal cord and their innervation pattern was examined by the visualization of acetylcholine receptors (ACh-R) and of acetylcholinesterase (ACh-E) activity at the neuromuscular contacts. In the culture system used, quail neurons always innervated muscle fibres at several sites and only when a fast-type activity was imposed on these neurons did a reduction in the number of the previously established neuromuscular contacts take place. In contrast, in the muscle fibres innervated by mouse neurons, a spontaneous reduction in the number of the previously established neuromuscular contacts occurred but this spontaneous reduction depended upon the level of differentiation reached by the muscle fibres in vitro. In the cultures of muscle fibres previously innervated by mouse neurons, the addition of quail neurons did not provoke any modification in the initial innervation pattern, and no quail ACh-R cluster was observed. In contrast, in the muscle fibres previously innervated by quail neurons, the mouse neurons contacted these fibres, resulting in a decrease in the number of quail ACh-R clusters. These results emphasize the part played by neurons in the establishment of the innervation pattern when muscle fibres have reached a high level of differentiation. In vitro, the slow and fast characteristics of the muscle fibres do not influence this pattern.

Elimination of distributed synaptic acetylcholine receptor clusters on developing avian fast-twitch muscle fibres accompanies loss of polyneuronal innervation

Changes in the distribution of large acetylcholine receptor clusters (AChR-Cs) on developing fast-twitch fibres of the chicken posterior latissimus dorsi (PLD) muscle have been studied during the period of loss of polyneuronal innervation using fluorescein-conjugated alpha-bungarotoxin. Embryonic muscles were ultrasonically dissociated into single fibre fragments and presumptive fast-twitch fibres were distinguished from the minority of slow-type fibres in the PLD by immunofluorescence using an antibody against slow-type myosin. Whereas mature PLD muscle fibres are focally innervated, at embryonic day 11 (E11) many fibre fragments from the PLD displayed two or more large (longer than 2 micron) AChR-Cs. Double labelling with anti-neurofilament antibody suggested that most of these AChR-Cs (82 +/- 2%) were associated with neuromuscular contacts. There was a progressive decline in the number of large (synaptic) AChR-Cs per 1000 micron of fibre, from 3.2 +/- 0.5 at E11 to 0.4 +/- 0.1 at E18. No further decline occurred between E18 and one week post-hatch. Primary generation muscle cells identified at E11 and E16 by tritiated thymidine labelling showed a decline in the number of large AChR-Cs per 1000 micron proportional to that seen in the fibre population as a whole, suggesting that distributed synaptic AChR-Cs are eliminated from individual fibres as they mature. When embryos were treated with d-tubocurarine starting at E6 the loss of distributed AChR-Cs from fast-type PLD fibres between E11 and E14 did not occur, suggesting that neuromuscular activity may play an important role in establishing the focal synaptic site AChR-C.

Effects of low and high frequency patterns of stimulation on contractile properties, enzyme activities and myosin light chain accumulation in slow and fast denervated muscles of the chicken

The effects of denervation and direct stimulation in fast and slow latissimus dorsii muscles were investigated in chicken. In slow ALD muscle, denervation resulted in an incompleteness of the relaxation, a decrease in MDH and CPK activities and an increase in fast myosin light chains (MLC) accumulation. Direct stimulation at either fast or slow rhythm prevented the effects of denervation on relaxation and CPK activity but was ineffective on MDH activity and fast MLC accumulation. Moreover, direct stimulation of denervated ALD caused rhythm-dependent change in tetanic contraction. In fast PLD muscle, the main changes in muscle properties following denervation were a slowing down of the time course of the twitch and an incompleteness of the relaxation, a decrease in LDH and CPK activities and in LC3F accumulation. Stimulation at a high frequency partly prevented the effects of denervation and resulted in a large accumulation of LC3F, while a low frequency stimulation did not restore the twitch time to peak, increased MDH activity and induced synthesis of slow MLC. This study emphasizes the role of muscle activity and its pattern in some properties of slow and fast chicken muscles following denervation.

An electrophysiological study of skeletal muscle fibres in the 'muscular dysgenesis' mutation of the mouse

Experiments were performed on muscles of 18-19 day mice fetuses affected with muscular dysgenesis (mdg). Action potentials generated by electrical stimulation or potassium depolarization failed to trigger muscle contraction in mdg muscle fibres. By contrast, muscle contraction could be obtained by caffeine (15 mM) and, to a lesser degree, by nerve stimulation. We conclude that a defect in excitation-contraction (E-C) coupling is the cause of muscle paralysis. An early after potential (EAP) was present in the decay phase of the action potential and a potential 'creep' occurred in response to hyperpolarizing current pulses which can be taken as evidence for the presence of T-tubules in mdg muscle fibres. Data obtained from square pulse analysis and EAP measurements indicate larger input impedance and membrane time constant in mdg as compared to controls, which contrasts with similar surface membrane time constant (as estimated from the foot of the action potential) in both types of muscle. The excitability of the T-tubule system was tested by recording action potentials at early stages of TTX (5 X 10(-7) M) perfusion or washout in mdg and control muscles. In both cases, the action potentials decreased in amplitude and rate of rise and displayed two peaks, the second of which was suppressed by detubulation using the formamide treatment. This indicates action potential generation in the T-tubule membrane of mdg muscles. In all the impaled muscle fibers, nerve stimulation evoked epps which were accompanied by a weak local contraction in relation with Ca2+ influx through postsynaptic channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of spinal cord stimulation on the metabolism of developing latissimus dorsii muscles in chick embryo

Influence of chronic spinal cord stimulation upon some characteristic enzyme activities of energy metabolism was investigated in slow anterior (ALD) and fast posterior (PLD) latissimus dorsii muscles of the chick embryo. During embryonic life, oxidative metabolism (as evaluated by the activity of malate dehydrogenase (MDH] represents the main energetic pathway in both slow and fast muscles. At the end of embryonic life, an increase in anaerobic (as evaluated by the activity of lactate dehydrogenase (LDH] and creatine phosphokinase (CPK) activities occurs in PLD muscle. Chronic spinal cord stimulation at a low frequency was performed from the 10th day to the 16th day of embryonic development. In ALD, the enzyme activities were unaffected, while in PLD a concomitant decrease in LDH and CPK activities was observed.

Effect of activity on the selective stabilization of the motor innervation of fast muscle posterior latissimus dorsi from chick embryo

The role of neuromuscular activity in the maturation of the motor innervation was investigated in the fast focally innervated posterior latissimus dorsi (PLD) muscle of the chick embryo. The axonal supply in the PLD motor nerve, and the focal multiple innervation of the endplates were described on days 15 and 16 of embryonic life in normal and experimental embryos. In the first series of experiments, chick embryos were paralyzed by repeated injections between days 4 and 10 in ovo of the curare-like agent, flaxedil. Twice more axons in the PLD motor nerve and about twice more nerve terminal profiles at the endplates in the PLD muscles were found in paralyzed than in control embryos. In a second series of experiments, electrodes were implanted around the spinal cord of 7-day-old embryos and electric pulses delivered at 0.5 Hz frequency from day 10 to days 15-16 of incubation. At day 15.5, no change was observed in the axonal supply in the PLD motor nerve of stimulated embryos, while a two-fold decrease was observed in the number of motor nerve terminal profiles per endplate in the corresponding PLD muscle. The statistical distribution of the number of motor nerve terminal profiles per endplate was described from complete semi-serial sections in the PLD muscle from normal, paralyzed and stimulated chick embryos. In these three cases, the distribution of supernumerary nerve terminal profiles followed a Poisson law after one nerve ending had been subtracted from the number of nerve endings counted per endplate.

Effects of percutaneous electrical stimulation on functional ability, plasma creatine kinase, and pectoralis musculature of normal and genetically dystrophic chickens

The breast musculature of genetically dystrophic Line 413 and genetically related normal Line 412 chickens were treated in three separate trials with high-frequency electrical stimulation (ES). Beginning on days 7 or 14 ex ovo, each bird received three ES treatments per week. Each stimulation cycle repeated five times per day consisted of 15 s "on" followed by 50 s "off". In the third trial only, the birds were additionally treated beginning day 3 ex ovo with either leucine (100 mg/kg) or the proteinase inhibitor Ep475 (10 mg/kg). ES significantly delayed the onset of righting disability in the dystrophic chickens. However, this improvement was temporary and could be masked by single treatments of either leucine or Ep475. Plasma creatine kinase activities were increased generally in both the stimulated normal and dystrophic birds. In two trials ES increased the relative muscle mass, and in one trial increased protein. ES had little effect on normal muscle mass or protein. However, ES treatment together with either leucine or Ep475 appeared to improve both normal and dystrophic muscle mass and protein. Furthermore ES decreased dystrophic muscle calcium but not acetylcholinesterase activity. On the other hand, ES had no effect on the total normal muscle calcium but increased normal acetylcholinesterase values. In both normal and dystrophic muscle samples, ES treatment in combination with leucine appeared to increase the mean muscle fiber diameters and number of myonuclei, and in the case of the dystrophic muscle, appeared to decrease the relative proportion of vacuolated, degenerating, and intensely oxidative histochemical fibers. In general, stimulation (especially in combination with leucine) appears to alter in varying degrees the phenotypic expression of the muscle disease exhibited in the dystrophic chicken.

Spatial distribution and size of acetylcholine receptor clusters determined by motor nerves in developing chick muscles

The size and distribution of acetylcholine receptor clusters (AChR-C) on normal and aneural developing muscle fibres of the chick wing were studied by labelling AChR with fluorescent conjugates of alpha-bungarotoxin (alpha-BGT). AChR-C of a size typical of initial synaptic contacts (5 micron long) were present at 7 days incubation, shortly after the appearance of nerves, and were grouped in bands corresponding to muscle nerve branches. A regular distribution of large (approximately equal to 5 micron) AChR-C separated by 100-200 micron had developed by 10-14 days in the slow-tonic anterior latissimus dorsi and ulnimetacarpalis dorsalis muscles. The role of motor innervation in the formation of AChR-C was assessed by removing the brachial neural tube at 2 days incubation in order to prevent nerves entering the wing. Neural-tube removal prevented the appearance of the large AChR-C normally associated with the early synaptic contacts. Small AChR-C (less than 2 micron long) appeared in aneural muscles, but these were not grouped into bands characteristic of the large AChR-C in normal muscles. The results suggest that the formation of junctional AChR-C is dependent on nerves.

The innervation of skeletal muscles in chickens curarized during early development

Chicken embryos were treated with partially paralysing doses of d-tubocurarine (dtc) from embryonic (E) days 6 to 10. The pattern of innervation of the lateral gastrocnemius (GL) muscle was examined both morphologically and physiologically just before hatching on day E20 or E21. There was a 70% increase in number of surviving motor neurons in the lateral motor column and a 50% increase in the number of myelinated axons in the nerve to GL. The GL muscle was significantly atrophic, with an average weight of 40% of normal. The atrophy was largely due to the reduced size of the muscle fibres. The mean size of the motor units was essentially unchanged or perhaps slightly increased. There was a striking increase in the level of polyneuronal innervation of the muscle fibres, both in terms of number of synaptic sites per fibre and number of axons innervating each site. Spontaneous miniature endplate potentials (mepps) indicated focal innervation of the fibres in the normal muscle. Most fibres in the dtc-treated muscles had mepps of widely varying time courses, and there was no simple relation between amplitude and rise time. Many of the slow mepps were not represented in the endplate potentials evoked by nerve stimulation. The quantal content of the endplate potential (epp) was generally increased in the dtc-treated muscles. The findings are discussed in terms of a retrograde signal from muscle to nerve and its dependence on muscle activity.

Appearance and distribution "in situ" of nicotinic acetylcholine receptors in cervical myotomes of young chick embryos. Radioautographic studies by light and electron microscopy

Localization of the acetylcholine (nicotinic) receptor sites was investigated in the developing cervical myotomes of the early chick embryo by radioautography at the light and electron microscope level, using 125I-alpha-bungarotoxin. The presence of cholinergic receptor sites was detected in situ as early as 60 hours of incubation (stage 17); their relative density increased in the myotome during the differentiation of the somite. Specific labeling of these receptor sites was detected in the myotomal tissue but not in the notochord, spinal cord or periaxial mesenchyme. The distribution of the receptor sites was uniform in the myotome at 3 days in ovo. An anterior-posterior asymmetry of the density appeared at 4 days in ovo and developed up to the 6th day. The highest density of these toxin-binding receptor sites was observed near the spinal motor nerve bundle as revealed by silver staining. These observations, made in situ, are discussed with respect to the possible neurotrophic or physical effects of the early motor innervation.

Innervation of avian latissimus dorsi muscles and axonal outgrowth pattern in the posterior latissimus dorsi motor nerve during embryonic development

The distribution of the innervation to the anterior latissimus dorsi (ALD) and posterior latissimus dorsi (PLD) muscles of the chicken are described on the day of hatching and 6 weeks later using electron microscopy. In the ALD muscle, there are 5,000 muscle fibres and 374,000 endplates supplied by about 169 skeletomotor axons; in the PLD muscle, there are 12,000 focally innervated muscle fibers supplied by about 20 skeletomotor axons. On the cell surface of the muscle fibers the mean total subsynaptic area contacted by each motor axon is comparable in the ALD and PLD muscles. The growth pattern of the axons in the PLD motor nerve was described from the ninth day in ovo up to 6 weeks after hatching. The axons arrive in the PLD muscle in two successive waves: first, the large somatic axons which are already present before the ninth day in ovo and second, the small autonomic axons which continue to accumulate until hatching. The total number of somatic axons decreases from the ninth day until the hatching day when it reaches its definitive value. This decrease takes place during a period when the numbers of myofibers and of endplates dramatically increase, and it coincides with the axonal segregation by the Schwann cells. The myelination of the axons starts on the 15th day in ovo and is essentially complete upon hatching. Despite the decreasing number of somatic axons in the PLD nerve, the decrease in number of nerve endings per PLD endplate and the increasing number of PLD endplates per PLD muscle, it was found that between the 16th day in ovo and 6 weeks after hatching the mean number of axonal branches per PLD motor axon does not decrease.

Morphological and histochemical differentiation of intrafusal fibres in the posterior latissimus dorsi muscle of the developing chick

Morphological and histochemical differentiation of neuromuscular spindles was studied in the posterior latissimus dorsi (PLD) of the chick during embryonic and post-hatching development. A rapid increase in the number of spindles takes place between the 13th and 15th of embryonic life. By the 15th day in ovo, the spindle capsule appears filled with numerous contiguous cells. Large sensory endings and small primitive motor endings are observed on intrafusal fibres. Ultrastructural observations of the nerve supply of the spindles confirm that each developing spindle receives one thick Ia axon with one to three thin gamma axons. The intracapsular space differentiates by the 17th day of embryonic development. All intrafusal fibres are morphologically of the nuclear-chain type, while two fibre types are distinguished as early as the 14th day of embryonic life, when myofibrillar ATPase activity is demonstrated after acid preincubation. These two histochemical types of intrafusal fibres are also described in the adult. The relation between these two histochemical types and different functional activity of intrafusal fibres is suggested.