Biomedical subjects

M A Fahim

Publications and source records attributed to M A Fahim.

16 recordsLinked to original sources

PubMed1992

Effects of ionophore X-537A on spontaneous transmitter release and the ultrastructure of locust neuromuscular junctions.

The spontaneous quantal release of neurotransmitter and the fine structure of a glutamatergic synapse has been examined in the presence of ionophore X-537A. Bath applications of X-537A to extensor tibiae nerve-muscle preparations of locust, Schistocerca gregaria, increased the frequency of miniature excitatory post-synaptic potentials (min. e.p.s.p.'s). This action was completely reversible, if preparations were exposed to ionophore for less than 60 min. Application of ionophore for longer times, i.e., longer than 60 min., transiently elevated min. e.p.s.p. frequency to greater than 100/s. Following this period of high activity, miniature frequency declined to 0.4/s and were mostly of "giant" miniature potentials type. The frequency and amplitude of these "giant" miniature potentials remained unchanged after subsequent washing with standard saline. Exposure of nerve terminals to ionophore for 60 min. produced no ultrastructure changes. Longer ionophore treatments, however, led to depletion of synaptic vesicles, damaged mitochondria and disintegration of microtubules and neurofilaments within nerve terminals, suggesting irreversible changes at the locust neuromuscular junction.

Animals↗

PubMed1992

An analysis of miniature synaptic events recorded from locust muscle--variations in amplitude and time courses.

A study of miniature post-synaptic potentials (min. e.p.s.p.'s) in metathoracic extensor tibiae muscle fibres at neuromuscular junctions of adult locusts (Schistocerca gregaria) has been undertaken. Extracellular min. e.p.s.p.'s recorded from single junctional sites and their "marked" intracellular min. e.p.s.p. counterparts varied both in time course and amplitude and at many sites a small proportion of abnormal miniatures were observed, i.e., of large amplitude and/or of long duration. Positive correlations between the rise times and 1/2-decay times of the extracellular min. e.p.s.p.'s and between rise times and amplitudes and 1/2-decay times and amplitudes of "marked" intracellular events were found. Comparison of results obtained from differently innervated muscle fibres demonstrates that the occurrence of abnormal miniatures is independent of the type of innervation i.e., "fast" or "slow" excitatory motoneurons. Mechanisms that lead to the occurrence of abnormal miniatures are discussed in relation to the presence of large vesicles in the terminals of the excitatory motoneurons but the occurrence of these events could equally well be explained by the "post-synaptic saturation hypothesis".

Animals↗

PubMed1991

Depolarization reverses age-related decrease of spontaneous transmitter release.

The effect of increasing extracellular Ca concentration on spontaneous transmitter release was studied at soleus nerve terminals of young (10 mo) and old (24 mo) C57BL/6J mice depolarized by high extracellular K concentration ([K]o). By using intracellular recording, miniature end-plate potentials (MEPPs) were first recorded in a normal [K]o Krebs solution. Subsequently, MEPPs were recorded in high [K]o Krebs solutions with four different Ca concentrations: Ca-free/ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid and 0.5, 1.5, and 2.5 mM Ca. In both the normal [K]o Krebs and the Ca-free-high [K]o Krebs solutions, MEPP frequency was lower at old than at young nerve terminals. In the three high [K]o Krebs solutions with Ca, MEPP frequency was progressively higher at old than at young nerve terminals with higher Ca concentrations. Periodic oscillations were observed in MEPP frequency of depolarized nerve terminals. The period of oscillation was inversely proportional to spontaneous transmitter release. These results demonstrate that when the nerve terminal is depolarized, permeability of the terminal membrane to Ca increases because of opening of voltage-dependent Ca channels. In the present study resting MEPP frequency was lower at old than at young terminals. On depolarization, MEPP frequency became higher at old than at young terminals. The study demonstrates that voltage-dependent Ca entry increases during aging at the soleus nerve terminal.

Aging↗

PubMed1990

Effects of chronic corticosterone treatment on the morphology of rat neuromuscular junctions.

This study examined the effects of chronic corticosterone (CORT) treatment on the morphology of two physiologically different muscles. Nerve terminals from the slow twitch soleus (SOL) and fast twitch extensor digitorum longus (EDL) of Fischer 344 rats were stained using the zinc iodide osmium (ZIO) technique. Nerve terminal area, perimeter, and longitudinal extent length were measured using computer-aided morphometry. Slow and fast muscle fibers from animals which received 5-10 mg CORT per day for 3 months were atrophied compared with controls. Treated animals failed to gain weight during the study, while controls gained 37%. Adrenal weights in treated animals were 30% less than controls, after correction for body weight. Morphological parameters for SOL nerve terminals were generally larger in the CORT group, while EDL nerve terminals from the CORT group did not differ significantly from controls. Soleus nerve terminal area was 43% greater, perimeter 14% longer, and longitudinal extent length 18% longer than the control nerve terminals. This study demonstrates a greater effect of CORT treatment on slow twitch muscle than has been demonstrated in previous studies. Changes in the nerve terminal morphology of the SOL were also greater than in previous studies and suggest that a functional adaptation or remodelling may occur following CORT treatment to maintain the neuromuscular interface during the enhanced catabolic effects of the steroid. These steroid-induced stress changes are similar in some respects to those observed in aging and disuse studies of the vertebrate neuromuscular junction. This suggests that glucocorticoid hormones may play an etiological role in the homeostasis of the neuromuscular junction in response to various stimuli.

Animals↗

PubMed1990

Aging increases calcium influx at motor nerve terminal.

To determine whether increased transmitter release from soleus nerve terminals of old C57BL/6J mice is caused by an altered Ca2+ regulation, the time course of post-tetanic potentiation of miniature endplate potential (MEPP) frequency was used as an indicator of the kinetics of Ca2+ metabolism in young (10 months) and old (24 months) mice. Post-tetanic potentiation properties were studied in either (1) 0.2 mM Ca2+, 5.0 mM Mg2+ Krebs; or (2) Ca2(+)-free/EGTA Krebs to eliminate Ca2+ influx, and thereby isolated Ca2+ buffering. In the 0.2 mM Ca2+ Krebs, the time constants of decay of augmentation (TA) and potentiation (TP) were longer in old (TA = 10.3 +/- 1.0 sec, TP = 195.3 +/- 5.4 sec) than in young (TA = 7.0 +/- 0.7 sec, TP = 78.8 +/- 6.6 sec) nerve terminals. Evoked transmitter release was measured in 0.4 mM Ca2+, 2.75 mM Mg2+ Krebs. Quantal content of the endplate potential was positively correlated with TA (r = 0.95) and with TP (r = 0.98). In the Ca2(+)-free/EGTA Krebs, there was no difference in post-tetanic potentiation properties between young and old terminals. These results suggest that Ca2+ influx into the soleus nerve terminal increases with aging. This may explain, at least in part, the increased quantal content observed at old terminals.

Aging↗

PubMed1989

Rapid neuromuscular remodeling following limb immobilization.

The effect of immobilization on endplate morphology of the rat soleus muscles was studied qualitatively and quantitatively. The endplate was visualized by light microscopic zinc iodide osmium (ZIO) staining and by electron microscopy. The soleus muscle was immobilized by pinning of ankle and knee joints at right angles for 5 days. Immobilized muscles were then compared to the contralateral side and to normal litter mates. After 5 days of partial disuse, muscle fibers atrophied and nerve terminal area increased in ZIO-determined measurements. Neuromuscular junctions (NMJs) of disuse muscle fibers visualized by electron microscopy exhibited greater amounts of degeneration than either contralateral or control NMJs. Degeneration consisted of nerve terminal disruption, exposed junctional folds, and postsynaptic areas which contained little or no postjunctional folds. Regeneration also occurred in the same NMJs, consisting of small terminals associated with large expansion of junctional folds, several small terminals occurring within the same primary synaptic cleft, and several axons wrapped by the same Schwann cell. These observations demonstrate, for the first time, that partial disuse for only 5 days produces muscle atrophy as well as denervation-like changes at the NMJ, which leads to terminal sprouting within the endplate area and remodeling.

Animals↗

PubMed1989

Proximodistal gradients of the postjunctional folds at the frog neuromuscular junction: a scanning electron microscopic study.

Frog endplates were studied with the scanning electron microscope following the removal of the presynaptic terminal by collagenase and acid treatments. Endplates had 2-14 branches of primary cleft. The longest branches were parallel to the muscle fiber. Short branches oblique or perpendicular to the muscle fiber were also present near the central region of the endplates. The openings of postjunctional folds in the primary cleft were clearly visible at the bottom of the primary cleft and could be counted and measured. The longest primary cleft branches of each endplate were divided into segments of 20 microns (length corrected for shrinkage). The number of postjunctional folds per micrometer of primary cleft, the average postjunctional fold length (i.e. across the primary cleft) and the total postjunctional fold's length per micrometer of primary cleft were evaluated for each 20-microns segment of primary cleft. Negative proximodistal gradients were observed for these three parameters for the long branches of primary cleft, i.e. values were higher in the proximal region (near the motor axon) than in the distal region. These postsynaptic gradients probably reflect similar or smaller proximodistal presynaptic gradients for the active zones along the nerve.

Animals↗

PubMed1989

Nerve terminal morphology in C57BL/6NNia mice at different ages.

The mammalian neuromuscular junction (NMJ) appears to undergo a dynamic remodeling process with advancing age. Previous studies suggest that the NMJ becomes progressively larger with age, to a greater degree in fast-twitch muscles than slow-twitch muscles. To better understand this process, quantitative morphometry was conducted on soleus and extensor digitorum longus (EDL) nerve terminals from C57BL/6NNia mice aged 4, 8, 10, 12, 18, 22, 24, and 32 months. Computer-aided morphometry of camera lucida drawings of aging nerve terminals indicated significant differences in the size and complexity of NMJs in both muscles between different age groups; however, there were no significant net trends with age. The magnitude and direction of the morphometric changes depended on the particular ages compared, and occurred to a greater degree in the EDL. This study is the first comprehensive morphometric study of C57BL/6NNia nerve terminals at so many ages. The results are similar to those obtained for the Fischer 344 rat, and support the hypothesis that nerve terminal remodeling is a dynamic process, not a progressive elaboration.

Aging↗

PubMed1988

Scanning electron microscopic study of the neuromuscular junction of dystrophic mice.

The structure of the end-plate regions of normal and dystrophic 3-month-old mice were studied by scanning electron microscopy after the presynaptic terminals were removed by hydrochloric acid treatment. Quantitative analysis revealed that the end-plate area correlated positively with the muscle fiber diameter in both the normal and dystrophic animals. However, the motor end-plate area was significantly smaller in the dystrophic mice. The total length of the primary cleft of an end-plate correlated positively with the end-plate area and with the muscle fiber diameter in both normal and dystrophic mice. However, the total length of the primary cleft of an end-plate was significantly shorter in dystrophic mice, especially in large-diameter muscle fibers. Finally, the end-plate of dystrophic mice was characterized by shorter primary clefts with less branching points. These changes of several morphometric characteristics of the postsynaptic membrane suggest that the functional denervation of the mouse dystrophic neuromuscular junction has a postsynaptic origin.

Animals↗

PubMed1988

Endurance exercise alters the morphology of fast- and slow-twitch rat neuromuscular junctions.

The present investigation studied the effects of an endurance exercise program on the morphology of nerve terminals of phasic extensor digitorum longus (EDL) and tonic soleus (SOL) muscles from young mature female Sprague-Dawley rats. The animals were treadmill exercised for 30 consecutive days for 1 h/day at velocities up to 30 m/min. The area, perimeter, nerve longitudinal extent length, and fiber diameter of camera lucida drawings of zinc iodide osmium-stained nerve terminals were analyzed with a computerized morphometry system. Additionally, the number of nerve terminal branches and percentage of end-plates with sprouts were determined. Nerve terminals from exercised EDL and SOL had significantly greater areas than those from corresponding controls, with no changes in the perimeters. The magnitude of the changes were greater in the EDL. The exercised EDL nerve terminals were also significantly longer than controls and had more branches and sprouts. No training differences in fiber diameter were observed for either muscle. These data confirm previous suggestions that physical activity or exercise levels affect the morphology of the neuromuscular junction.

Adaptation, Physiological↗

PubMed1987

Effects of endurance exercise on the morphology of mouse neuromuscular junctions during ageing.

It has been suggested that age changes in the morphology of the neuromuscular junction (NMJ) may reflect altered physical activity levels rather than the unique effects of ageing. Additionally, previous studies have indicated that the structure of the NMJ may be modulated with exercise. To investigate these questions, quantitative morphometry was determined on soleus and extensor digitorum longus (EDL) nerve terminals stained with zinc iodide-osmium from C57BL/6NNia mice under control and endurance exercised conditions at 12, 18 and 24 months of age. As previously observed, the area, perimeter, extent length and branch number of nerve terminals increased with age in both soleus and EDL. The changes were similar between the muscle types, although the changes were more pronounced in the phasic EDL. In 12-month-old animals, 2 months of endurance exercise resulted in significantly larger nerve terminals in both soleus and EDL, suggesting a functional adaptation. Exercised 18- and 24-month-old nerve terminals were smaller than corresponding controls, which indicated that exercise minimized or prevented further age-related nerve terminal elaboration. At all ages the exercised nerve terminals comprised a more homogeneous population than corresponding controls, which indicates that uniform physical activity can modulate NMJ morphometry. The magnitude of the changes suggests that subtle alterations in normal cage activity with advancing age do not have a significant effect on the morphology of nerve terminals. However, the morphology of the NMJ does change significantly in response to physical exercise training.

Aging↗

PubMed1986

Remodelling of the neuromuscular junction after subtotal disuse.

Within 5 days after reduction of activity of the rat soleus muscle, there was remodelling of the neuromuscular junction. Post-synaptic clefts were shorter, wider, and flatter, whereas nerve terminals showed sprouting and longitudinal distortion. The results, which differ from those found after denervation or total disuse, reveal the plasticity of the mature neuromuscular synapse.

Animals↗

PubMed1985

AVEC-DIC and electron microscopic analyses of axonally transported particles in cold-blocked squid giant axons.

Anterogradely and retrogradely transported membranous organelles were analysed separately by focally cooling axons (cold-blocking) for 2-4 h. Video-enhanced differential interference contrast light microscopy (AVEC-DIC) and dark field light microscopy showed that particles accumulated in large numbers on both the anterograde and the retrograde sides of the cold-block and that the accumulated particles resumed their transport when the preparation was rewarmed to 18 degrees C. The particles accumulated in files on both sides of the cold-block suggesting that particles move along linear pathways in the axoplasm. Comparisons of the results obtained by AVEC-DIC light microscopy with those obtained by electron microscopy indicate that the AVEC-DIC method is capable of detecting all of the different types of rapidly transported membranous organelles, including the smallest (35-80 nm) vesicles that move anterogradely. Electron microscopic analyses of the transported particles demonstrate that the anterogradely transported organelles are structurally distinct from those that are transported retrogradely. The anterogradely transported particles consisted of normal mitochondria and small (35-80 nm) tubulovesicular profiles. By contrast, the retrogradely transported particles were 150 nm or larger and they often contained complex membranous inclusions. The largest retrogradely transported particles appeared to be degenerating mitochondria. The results are consistent with the hypothesis that the direction of organelle movement is related to the physiological state of the organelle. That is, organelles containing newly synthesized membrane components move primarily anterogradely and organelles that contain transformed and degraded membrane components move retrogradely.

Animals↗

PubMed1985

Progression of age changes in mature mouse motor nerve terminals and its relation to locomotor activity.

There is indirect evidence for morphologic 'turnover' of motor nerve terminals, but the consequences in terms of changing nerve terminal structure throughout mature lifetime are unknown. Therefore, quantitative morphometry of nerve terminals stained with zinc iodide-osmium was carried out in soleus and extensor digitorum longus (EDL) muscles of CBF-1 mice at various ages from 5 to 32 months. Because of previous questions as to the role of disuse in producing age changes in nerve terminal structure, locomotor activity was recorded for an average of 20 days continuously in mice spanning the same ages. The length, area, perimeter and tortuosity of nerve terminals increased either early in maturity (soleus) or progressively with age (EDL). Another prominent change, i.e. increased numbers of nerve terminal 'regions' (branches or boutons that are spatially separate or only connected by fine nerve filaments) per junction, only appeared late in life (at or after 25 months). This regionalization was characteristic of all terminals and involved a redistribution rather than an accretion of nerve terminal area. None of the morphological changes with age sufficiently account for previously reported physiologic findings in the same muscles. Locomotor activity (including peaks and troughs of daily activity and circadian rhythm) was not significantly altered over the period of mature lifetime in which nerve terminals were remodelling; therefore, disuse was not a factor in this process. However, slightly retarded and smaller age changes in soleus than in EDL nerve terminals may reflect a modifying effect of activity. It is inferred that two major processes account for morphologic alterations in nerve terminals of mature mice: changes that are simple continuations of normal development and late changes that reflect newly arising age-dependent extrinsic or intrinsic factors.

Age Factors↗

PubMedDate not supplied

Fixation effects on synaptic vesicle density in neuromuscular junctions of young and old mice.

It was previously reported that in soleus neuromuscular junctions of old mice, synaptic vesicle density was decreased while transmitter release was increased (compared to results in young mice). In the present study, two hypotheses that might resolve this disparity were tested. The first was that the density of readily releasable vesicles close to the preterminal membrane, rather than those in the whole terminal, would correlate with the physiological results. This hypothesis was excluded because both vesicle density in the 200 nm region just within the presynaptic terminal membrane, and total vesicle density were similarly reduced in old soleus junctions. The second hypothesis was that more transmitter was released during fixation at old than at young neuromuscular junctions, leading to an age-related depletion of vesicles. This was tested by counting vesicles in muscles fixed after transmission block was attained in Krebs solution lacking calcium, and by direct recording of quantal release during conventional fixation. This second hypothesis was excluded: in neuromuscular junctions exposed to zero-calcium Krebs solution before fixation, the age-related reduction in vesicle density was still present, and intracellular recording revealed only a slight increase in quantal transmitter release during fixation. Therefore, as discussed, other mechanisms must be considered.

Aging↗

PubMedDate not supplied

Effect of exercise on physiological age-related change at mouse neuromuscular junctions.

To determine the effect of endurance exercise on physiological age-related change at the mouse neuromuscular junction (NMJ), synaptic function was studied for extensor digitorum longus (EDL) and soleus muscles of three C57BL/6J mouse groups, 1) young adult control (YC: 10 months), 2) old control (OC: 20 months), and 3) old mice which exercised (OE: 20 months) since young-adulthood. Electrophysiological properties were studied with intracellular recording techniques. Safety margin was studied by measuring indirect isometric twitch tension in different calcium concentrations. With sedentary aging, EDL and soleus quantal contents increased. Following aging combined with 10 months of exercise, the EDL quantal contents in OE and YC animals were similar. In contrast, soleus quantal content was greater in OE than in YC animals. Determined safety margins were OC greater than YC = OE for EDL, and OC = YC = OE for soleus. This is the first study to indicate that physiological age-related changes at NMJs of EDL and soleus muscles are affected differently by endurance exercise. Exercise prevented all physiological age-related changes in EDL NMJs but not in soleus NMJs, this suggests that EDL changes are associated with inactivity during aging, while soleus changes are "fundamental" age changes.

Aging↗