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

The neuromuscular junction: anatomical features and adaptations to various forms of increased, or decreased neuromuscular activity

The neuromuscular junction (NMJ) allows communication between motor neurons and muscle fibers. During development, marked morphological changes occur as the functional NMJ is formed. During the postnatal period of rapid growth and muscle enlargement, endplate size concurrently increases. Even beyond this period of pronounced plasticity, the NMJ undergoes subtle morphological remodeling--expansion and retraction--although its overall dimensions remain stable. This natural, continual NMJ remodeling is amplified with alterations in neuromuscular activity. Increased activity, presented by exercise training, typically results in expansion of NMJ size. Disuse, brought about by neurotoxins, denervation, or spaceflight, also elicits substantial reconfiguring of the endplate.

Changes in the neuromuscular synapse induced by an antibody against gangliosides

In this study, we used a monoclonal IgM antibody from a patient with a pure motor chronic demyelinating polyneuropathy, which binds specifically to the complex gangliosides GM(2), GalNAc-GD(1a), and GalNAc-GM(1b), which appear to have a common epitope of -[GalNAcbeta1-4Gal(3-2alphaNeuAc)beta1]. This was done for the following reasons: (1) to localize these gangliosides in specific cellular components of the neuromuscular junction (NMJ), and (2) to describe the anti-ganglioside antibody-induced structural and functional changes in the NMJs to gain insight into the role of gangliosides in the synaptic function. Using immunofluorescence techniques, we found that these gangliosides are located only in the presynaptic component of the motor end-plates, both in nerve terminals and in Schwann cells. After 2 weeks of continued passive transfer of the IgM monoclonal antibody over the mouse levator auris longus muscle, electromyography showed an axonal or NMJ disorder. Morphology showed important nerve terminal growth and retraction changes. Using intracellular recording electrophysiology, we found neurotransmitter release alterations, including quantal content reduction and an immature expression of voltage-dependent calcium channels similar to what occurred during NMJ development and regeneration. These changes were complement independent. The results showed that these gangliosides were involved in the reciprocal Schwann cell-nerve terminal interactions, including structural stability and neurotransmission.

Role and expression of thrombin receptor PAR-1 in muscle cells and neuromuscular junctions during the synapse elimination period in the neonatal rat

A role for thrombin and its receptor (ThR) during mammalian skeletal muscle cell differentiation and neuromuscular junction (NMJ) formation has been suggested. Previously, we found that the synapse elimination process in the neonatal rat muscle was accelerated by thrombin and blocked by hirudin, its specific inhibitor (Lanuza et al. [2001] J. Neurosci. Res. 63:330-340). To test whether this process resulted from a signal transduction cascade initiated by activation of ThR, in particular PAR-1, we applied to the levator auris longus (LAL) muscle of newborn rats two synthetic peptides (SFLL and FSLL). SFLL is a potent specific agonist for activation of PAR-1, whereas FSLL is an inactive peptide. We have demonstrated that the activation of PAR-1 by SFLL produced acceleration of the presynaptic loss of connections and the postsynaptic maturation of NMJs. Moreover, Western blot analysis showed that PAR-1 was present in the skeletal muscle, and by immunohistochemistry we detected PAR-1 in muscle fibers concentrated in the synaptic area but also in satellite cells. Several lines of evidence suggested that PAR-1 is localized in the postsynaptic membrane: PAR-1 immunofluorescence was concentrated at denervated synaptic sites and was present in the myotube membrane in vitro in the absence of neurons and in dissociated single muscle fibers from which nerve terminals and Schwann cells had been removed. Taken together, these results indicate that thrombin mediates certain stages of activity-dependent synapse elimination in the skeletal muscle and does so through its action on the thrombin receptor PAR-1 localized, at least in part, on the postsynaptic membrane.

GDNF is regulated in an activity-dependent manner in rat skeletal muscle

Glial cell line-derived neurotrophic factor (GDNF) is produced by skeletal muscle and affects peripheral motor neurons. Elevated expression of GDNF in skeletal muscle leads to hyperinnervation of neuromuscular junctions, whereas postnatal administration of GDNF causes synaptic remodeling at the neuromuscular junction. Studies have demonstrated that altered physical activity causes changes in the neuromuscular junction. However, the role played by GDNF in this process in not known. The objective of this study was to determine whether changes in neuromuscular activity cause altered GDNF content in rat skeletal muscle. Following 4 weeks of walk-training on a treadmill, or 2 weeks of hindlimb unloading, soleus, gastrocnemius, and pectoralis major were removed and analyzed for GDNF content by enzyme-linked immunosorbant assay. Results indicated that walk-training is associated with increased GDNF content. Skeletal muscle from hindlimb-unloaded animals showed a decrease in GDNF in soleus and gastrocnemius, and an increase in pectoralis major. The altered production of GDNF may be responsible for activity-dependent remodeling of the neuromuscular junction and may aid in recovery from injury and disease.

Effects of feeding conditions on sensitivity to tubocurarine of nerve-muscle preparations from the mouse

1. The effects of feeding conditions on the sensitivity to the effect of tubocurarine (dTc) in vitro were compared among various nerve-muscle preparations from mice. The mice were fed under conditions that restricted or compelled their movement for 64 days and controls were fed conventionally. 2. The sensitivity to the effect of dTc differed considerably among preparations. It was much higher in the sciatic nerve-extensor digitorum longus muscle (EDL), moderately higher in the sciatic nerve-soleus muscle (SOL) and lower in the phrenic nerve-diaphragm (DPH) in control mice. 3. The order of the sensitivity was not altered by either type of conditioning. Constant restriction of movement or compelled movement did not modify the sensitivity of DPH to the effect of dTc in vitro. 4. Compulsion facilitated the sensitivity in both SOL and EDL. Restriction selectively increased the sensitivity of EDL. Both types of conditioning selectively and significantly reduced twitch development in EDL. 5. These results indicate that the sensitivity to dTc of neuromuscular transmission reflects constant states of motor activity.

The neuromuscular junction. Muscle fibre type differences, plasticity and adaptability to increased and decreased activity

The neuromuscular junction (NMJ) of adult mammalian muscle is the site of the transduction of electrical stimuli, generated by the nervous system, to the underlying muscle fibres, resulting in muscle action. It has been demonstrated that, in some ways, the morphology of the NMJ is specific to muscle fibre type. It is also known that while the structure of the NMJ generally remains stable in young, healthy adults, a subtle form of remodelling continuously occurs at this synapse. The morphology and physiology of the NMJ have been shown to adapt to both increased, and decreased use. Indeed, morphological changes of the NMJ are associated with functional alterations in neuromuscular transmission. Increased activity of the myoneural synapse results in adaptations that enhance neuromuscular transmission and, thus, muscle performance. Similarly to increased usage, decreased neuromuscular activity results in structural alterations of the NMJ. However, unlike those responses observed with enhanced activity, decreased recruitment of the myoneural synapse can impair neuromuscular transmission and muscle performance. Thus, the NMJ demonstrates both anatomical and physiological adaptations following substantial changes in its pattern of activity. These NMJ adaptations can affect the functional capacity of skeletal muscle in vivo.

Changing the light intensity of the visual environment results in large differences in numbers of synapses and in photoreceptor size in the retina of the young adult rat

A quantitative light- and electron-microscopic study has been made of the retinae of rats which were exposed to different lighting conditions for between one and 15 weeks in young adulthood, having been reared in identical conditions during development. The width of the inner and outer segments of the photoreceptors and the width of the outer plexiform layer varied inversely with the light intensity under diurnal lighting conditions of 10 h light/14 h dark. Linear regression analysis showed that the widths were inversely related to the fourth root of the light intensity as measured in lux. Both central and peripheral areas of retina showed a similar change. No change was seen in the widths of the inner plexiform layer, or of the inner and outer nuclear cell layers. Nor was there a difference in the packing density or size of the nuclei in the nuclear cell layers. The number of ribbon synapses in the outer plexiform layer also varied inversely with the intensity of diurnal light. Linear regression analysis showed that the number of synapses was inversely correlated with the fourth root of the light intensity and was positively correlated with the width of the outer plexiform layer. The number of ribbon synapses was increased by up to two and a half times in constant darkness compared to diurnal light of 35 lux. The increase was present but not maximal after one week of exposure. The length of synaptic ribbons was unchanged. The nerve terminals forming such synapses were increased in size but not in number. After one week, there was little or no additional change in the retinal widths and number of synaptic ribbons with time. However, there was a progressive increase with time in nerve terminal size (two-fold in area) in constant darkness. There was some evidence of a slight decrease in nerve terminal number and increase in size of retinal nuclei with age. It is concluded that the adult retina responds to a different lighting environment by a relatively rapid change in the size of photoreceptor segments, by a progressive and large change in number of ribbon synapses and by a slower progressive and large change in the size of photoreceptor nerve terminals. The response is quantitatively determined by the strength of the stimulus but not in a linear fashion. These results are compared with the effects of environmental stimulation of other areas of the nervous system.