Habitual exercise enhances neuromuscular transmission efficacy of rat soleus muscle in situ

Activation of reverse Na+-Ca2+ exchanger by skeletal Na+ channel isoform increases excitation-contraction coupling efficiency in rabbit cardiomyocytes

Our prior work has shown that Na⁺ current (I_Na) affects sarcoplasmic reticular (SR) Ca²⁺ release by activating early reverse of the Na⁺-Ca²⁺ exchanger (NCX). The resulting Ca²⁺ entry primes the dyadic cleft, which appears to increase Ca²⁺ channel coupling fidelity. It has been shown that the skeletal isoform of the voltage-gated Na⁺ channel (Na_v1.4) is the main tetrodotoxin (TTX)-sensitive Na_v isoform expressed in adult rabbit ventricular cardiomyocytes. Here, I tested the hypothesis that it is also the principal isoform involved in the priming mechanism. Action potentials (APs) were evoked in isolated rabbit ventricular cells loaded with fluo-4, and simultaneously recorded Ca²⁺ transients before and after the application of either relatively low doses of TTX (100 nM), the specific Na_v1.4 inhibitor μ-Conotoxin GIIIB or the specific Na_v1.1 inhibitor ICA 121430. Although APs changes after the application of each drug reflected the relative abundance of each isoform, the effects of TTX and GIIIB on SR Ca²⁺ release (measured as the transient maximum upstroke velocity) were no different. Furthermore, this reduction in SR Ca²⁺ release was comparable with the value that we obtained previously when total I_Na was inactivated with a ramp applied under voltage clamp. Finally, SR Ca²⁺ release was unaltered by the same ramp in the presence of TTX or GIIB. In contrast, application of ICA had no effect of SR Ca²⁺ release. These results suggest that Na_v1.4 is the main Na_v isoform involved in regulating the efficiency of excitation-contraction coupling in rabbit cardiomyocytes by priming the junction via activation of reverse-mode NCX.NEW & NOTEWORTHY A number of studies suggest that the Na⁺-Ca²⁺ exchanger (NCX) activated by Na⁺ currents is involved in the process of excitation-contraction (EC) coupling in cardiac ventricular myocytes. Although insufficient to trigger sarcoplasmic Ca²⁺ release alone, the Ca²⁺ entering through reverse NCX during an action potential can prime the dyadic cleft and increase the Ca²⁺ current coupling fidelity. Using specific Na⁺ inhibitors in this study, we show that in rabbit ventricular cells the skeletal Na⁺ channel isoform (Na_v1.4) is the main isoform responsible for this priming. Our study provides insights into a mechanism that may have an increased relevance where EC coupling is remodeled.

Effects of moderate- and high-intensity chronic exercise on brain-derived neurotrophic factor expression in fast and slow muscles

INTRODUCTION

Brain-derived neurotrophic factor (BDNF) protein expression is sensitive to cellular activity. In the sedentary state, BDNF expression is affected by the muscle phenotype.

METHODS

Eighteen Wistar rats were divided into the following 3 groups: sedentary (S); moderate-intensity training (MIT); and high-intensity training (HIT). The training protocol lasted 8 weeks. Forty-eight hours after training, total RNA and protein levels in the soleus and plantaris muscles were obtained.

RESULTS

In the plantaris, the BDNF protein level was lower in the HIT than in the S group (P < 0.05). A similar effect was found in the soleus (without significant difference). In the soleus, higher Bdnf mRNA levels were found in the HIT group (P < 0.001 vs. S and MIT groups). In the plantaris muscle, similar Bdnf mRNA levels were found in all groups.

CONCLUSIONS

These results indicate that high-intensity chronic exercise reduces BDNF protein level in fast muscles and increases Bdnf mRNA levels in slow muscles.

Morphological and functional remodelling of the neuromuscular junction by skeletal muscle PGC-1α

The neuromuscular junction (NMJ) exhibits high morphological and functional plasticity. In the mature muscle, the relative levels of physical activity are the major determinants of NMJ function. Classically, motor neuron-mediated activation patterns of skeletal muscle have been thought of as the major drivers of NMJ plasticity and the ensuing fibre-type determination in muscle. Here we use muscle-specific transgenic animals for the peroxisome proliferator-activated receptor γ co-activator 1α (PGC-1α) as a genetic model for trained mice to elucidate the contribution of skeletal muscle to activity-induced adaptation of the NMJ. We find that muscle-specific expression of PGC-1α promotes a remodelling of the NMJ, even in the absence of increased physical activity. Importantly, these plastic changes are not restricted to post-synaptic structures, but extended to modulation of presynaptic cell morphology and function. Therefore, our data indicate that skeletal muscle significantly contributes to the adaptation of the NMJ subsequent to physical activity.

Uptake of inorganic mercury by human locus ceruleus and corticomotor neurons: implications for amyotrophic lateral sclerosis

Background

Environmental toxins are suspected to play a role in the pathogenesis of amyotrophic lateral sclerosis (ALS). In an attempt to determine which pathways these toxins can use to enter motor neurons we compared the distribution of mercury in the CNS of a human and of mice that had been exposed to inorganic mercury.

Results

In the human who had been exposed to metallic mercury, mercury was seen predominantly in the locus ceruleus and corticomotor neurons, as well as in scattered glial cells. In mice that had been exposed to mercury vapor or mercuric chloride, mercury was present in lower motor neurons in the spinal cord and brain stem.

Conclusions

In humans, inorganic mercury can be taken up predominantly by corticomotor neurons, possibly when the locus ceruleus is upregulated by stress. This toxin uptake into corticomotor neurons is in accord with the hypothesis that ALS originates in these upper motor neurons. In mice, inorganic mercury is taken up predominantly by lower motor neurons. The routes toxins use to enter motor neurons depends on the nature of the toxin, the duration of exposure, and possibly the amount of stress (for upper motor neuron uptake) and exercise (for lower motor neuron uptake) at the time of toxin exposure.

Aging influences adaptations of the neuromuscular junction to endurance training

This investigation sought to determine if aging affected adaptations of the neuromuscular junction (NMJ) to exercise training. Twenty young adult (8 months) and 20 aged (24 months) rats were assigned to either a program of treadmill exercise, or sedentary conditions. Following the 10-week experimental period, rats were euthanized, and soleus and plantaris muscles were removed and frozen. Longitudinal sections of the muscles were fluorescently stained to visualize pre-synaptic nerve terminals and post-synaptic endplates on both slow- and fast-twitch fibers. Images were collected with confocal microscopy and quantified. Muscle cross-sections were histochemically stained to assess muscle fiber profiles (size and fiber type). Our analysis of NMJs revealed a high degree of specificity and sensitivity to aging, exercise training, and their interaction. In the soleus, slow-twitch NMJs demonstrated significant (P ≤ 0.05) training-induced adaptations in young adult, but not aged rats. In the fast-twitch NMJs of the soleus, aging, but not training, was associated with remodeling. In the plantaris, aging, but not training, remodeled the predominant fast-twitch NMJs, but only pre-synaptically. In contrast, the slow-twitch NMJs of the plantaris displayed morphologic adaptations to both aging and exercise in pre- and post-synaptic components. Muscle fiber profiles indicated that changes in NMJ size were unrelated to adaptations of their fibers. Our data show that aging interferes with the ability of NMJs to adapt to exercise training. Results also reveal complexity in the coordination of synaptic responses among different muscles, and different fiber types within muscles, in their adaptation to aging and exercise training.

Rat alpha- and gamma-motoneuron soma size and succinate dehydrogenase activity are independent of neuromuscular activity level

The chronic level of neuromuscular activity, that is, activation and loading, strongly influences the morphological, metabolic, phenotypic, and physiological properties of skeletal muscles. The effects on the innervating motoneurons, however, are less established. We determined and compared the effects of 30 days of decreased activity (induced by a complete mid-thoracic spinal cord transection, ST) or near inactivity (induced by spinal cord isolation, SI) on the soma size and succinate dehydrogenase (SDH) activity of motoneurons innervating a predominantly slow ankle extensor (soleus) and a predominantly fast ankle flexor (tibialis anterior) muscle of adult rats. Soleus and tibialis anterior motoneuron pools were labeled retrogradely using nuclear yellow. The alpha- and gamma-motoneurons were classified based on soma size. Mean number of labeled motoneurons, and mean soma size and SDH activity for both alpha- and gamma-motoneurons were similar in control, ST, and SI rats. Compared to previous reports showing significant decreases in muscle fiber size and adaptations toward a "faster" metabolic profile following ST and SI, the results indicate that, unlike the muscles they innervate, the motoneurons are relatively unresponsive to chronic reductions in neuromuscular activity. The implication of these results is that mean size and SDH activity are independent of the number of action potentials generated by both alpha- and gamma-motoneurons and that even the absence of afferent input to the spinal cord has no influence on size and oxidative metabolic potential of the motoneuron soma.

Effects of exercise training on α-motoneurons

Evidence is presented that one locus of adaptation in the “neural adaptations to training” is at the level of the α-motoneurons. With increased voluntary activity, these neurons show evidence of dendrite restructuring, increased protein synthesis, increased axon transport of proteins, enhanced neuromuscular transmission dynamics, and changes in electrophysiological properties. The latter include hyperpolarization of the resting membrane potential and voltage threshold, increased rate of action potential development, and increased amplitude of the afterhyperpolarization following the action potential. Many of these changes demonstrate intensity-related adaptations and are in the opposite direction under conditions in which chronic activity is reduced. A five-compartment model of rat motoneurons that innervate fast and slow muscle fibers (termed “fast” and “slow” motoneurons in this paper), including 10 active ion conductances, was used to attempt to reproduce exercise training-induced adaptations in electrophysiological properties. The results suggest that adaptations in α-motoneurons with exercise training may involve alterations in ion conductances, which may, in turn, include changes in the gene expression of the ion channel subunits, which underlie these conductances. Interestingly, the acute neuromodulatory effects of monoamines on motoneuron properties, which would be a factor during acute exercise as these monoaminergic systems are activated, appear to be in the opposite direction to changes measured in endurance-trained motoneurons that are at rest. It may be that regular increases in motoneuronal excitability during exercise via these monoaminergic systems in fact render the motoneurons less excitable when at rest. More research is required to establish the relationships between exercise training, resting and exercise motoneuron excitability, ion channel modulation, and the effects of neuromodulators.

Motoneurones "learn" and "forget" physical activity

In spite of our knowledge of activity related adaptations in supraspinal neurones and skeletal muscles, very little is known concerning adaptations in alpha-motoneurones to alterations in chronic activity levels. Recent evidence shows that the biophysical properties of alpha-motoneurones are plastic and adapt to both increases and decreases in chronic activation. The nature of the adaptations--in resting membrane potential, spike threshold, afterhyper-polarization amplitude,and rate of depolarization during spike generation--point to involvement of density, type, location, and/or metabolic modulation of ion conductance channels in the motoneuronal membrane. These changes will have significant effects on how motoneurones respond when activated during the generation of movements, and on the effort required to sustain activation during prolonged exercise. Since the adaptations most likely involve structural changes in the motoneurones and changes in protein synthesis, and change the output response of the cells to input, they are considered to be learning responses. Future research directions for examining this issue are outlined.

Resting Membrane Potential of Rat Plantaris Muscle Fibers After Prolonged Indirect Stimulation in Situ: Effect of Glucose Infusion

The purpose of this study was to determine whether glucose infusion during prolonged indirect muscle stimulation (50 Hz for 200 ms every 2.7 s at 5 V) would have an effect on resting membrane potential (BMP). The BMP measured at Min 1 in the recovery period following stimulation of the rat plantaris muscle for 60 min in situ was significantly decreased in control rats, but was back to baseline values within 2 min. When glucose was infused ([glucose] ∼10 mM), no change was observed in RMP, and muscle fatigue and the reduction in M-wave peak-to-peak amplitude were both attenuated. However, muscle force and the electrical properties of the membrane were deteriorated both in rats infused with glucose and in control rats at Min 2 during the recovery period, at a time when RMP was not modified. These observations suggest that the effect of increased circulating glucose on fatigue-associated reductions in muscle fiber RMP seems to be modest and short-lived. Therefore, the attenuating effect of elevated glucose on muscle fatigue responses could be through mechanisms other than those associated with maintenance of RMP during fatigue. Key words: Na+/K+ pump, muscle fatigue, muscle force, M-wave, contractility

Effect of rat soleus muscle overload on neuromuscular transmission efficacy during continuous and intermittent activation

Increased neuromuscular activity is known to provoke morphological and functional adaptations at the neuromuscular synapse. Most of these changes have been documented following endurance exercise training programmes. In this study, the effect of rat soleus muscle overload produced by tenotomy plus voluntary wheel-cage activity on neuromuscular transmission efficacy was investigated. The overload protocol increased miniature endplate potential (MEPP) and endplate potential (EPP) amplitudes by 17 and 19%, respectively (both P < 0.01), and increased MEPP frequency by 86% (P < 0.01). EPP amplitude rundown during continuous trains of activation was attenuated by approximately 10% in the overloaded group (P < 0.01). Also, during intermittent activation, the overload protocol attenuated EPP amplitude rundown, mainly by enhancing EPP amplitude recovery by approximately 10% during the quiescent periods (P < 0.01). Although the present results show that both the degree and direction of adaptation are similar to what has been observed at rat soleus neuromuscular junctions following an endurance training protocol, there are important nuances between the results, suggesting different mechanisms through which these changes may occur.

Afferent Input Modulates Neurotrophins and Synaptic Plasticity in the Spinal Cord

The effects of eliminating or decreasing neuromuscular activity on the expression of neurotrophins and associated molecules in the spinal cord and subsequent effects on spinal cord plasticity were determined. Spinal cord isolation (SI), which eliminates any supraspinal and peripheral monosynaptic input to the lumbar region but maintains the motoneuron-muscle connectivity, decreased the levels of brain-derived neurotrophic factor (BDNF) and neurotrophin 3 (NT-3) mRNA and protein in the isolated segments. Synapsin I, an important mediator for the effects of BDNF on synaptic plasticity, also was lower in the lumbar region of SI rats. In contrast, the levels of BDNF, synapsin, and growth-associated protein (GAP-43) were increased in the cervical spinal cord enlargement rostral to the isolated region, most likely reflecting an increased use of the forelimbs in the SI rats. GAP-43 levels were also increased in the lumbar spinal cord region, probably associated with compensatory mechanisms related to the deafferentation. In a separate set of experiments, the soleus muscle was paralyzed unilaterally via intramuscular botulinum toxin type A (BTX-A) injection to determine the effects of reducing the propioceptive input, of this normally highly active muscle on neurotrophin expression in the spinal cord. BDNF and synapsin I mRNAs were lower and NT-3 levels were higher in the lumbar hemicord ipsilateral to the BTX-A injection. Combined, these results indicate that the level of supraspinal and muscle afferent input plays an important role in modulating the levels of BDNF and NT-3 in the spinal cord.

Enhanced neuromuscular transmission efficacy in overloaded rat plantaris muscle

To better understand the effect of muscle hypertrophy on the physiological properties of transmitter release, we investigated neuromuscular transmission (NMT) efficacy in overloaded rat plantaris muscle in situ. In the overload group, following bilateral tenotomy of plantaris synergists, rats were confined to wheel-cages. Age-matched rats in the control group were confined to plastic cages. During the terminal experiment, muscle action potentials were blocked using micro-conotoxin, and full-sized endplate potentials (EPPs) were recorded at 25, 50, and 75 HZ to determine their amplitude rundown. Quantal contents for the control and overload groups were 37.0 and 74.3, respectively (P <0.01). There was a significant group difference in EPP amplitude rundown at all frequencies examined, with increased rundown occurring in the overload group (P < 0.01). Cumulative quantal release was 139% and 153% higher in the overload group at 25 and 50 HZ, respectively (P < 0.05). Together, these data suggest the safety factor for NMT is increased by neuromuscular overload. Furthermore, these findings support and supplement previously reported activity-dependent improvements in NMT efficacy that are probably mediated via presynaptic adaptations.

Effect of load during electrical stimulation training in spinal cord injury

Electrical stimulation training is known to alter skeletal muscle characteristics after a spinal cord injury, but the effect of load on optimizing the training protocol has not been fully investigated. This study investigated two electrical-stimulation training regimes with different loads on intramuscular parameters of the paralyzed lower limbs. Six paraplegic individuals with a spinal cord injury underwent electrical stimulation training (45 min daily for 3 days per week for 10 weeks). One leg was trained statically with load, and the contralateral leg was trained dynamically with minimal load. Isometric force assessed with 35-HZ stimuli increased significantly in both legs from baseline, with the static-trained leg also being significantly higher than the dynamic-trained leg. The vastus lateralis muscle of the statically trained leg showed a significant increase in type I fibers, fiber cross-sectional area, capillary-to-fiber ratio, and citrate synthase activity when compared to both baseline and the dynamically trained leg. Relative oxygenation of the vastus lateralis muscle as determined by near infrared spectroscopy was also significantly greater after static training. This study indicates that the load that is applied to paralyzed muscle during an electrical stimulation training program is an important factor in determining the amount of muscle adaptation that can be achieved.

Wheel-running exercise alters rat diaphragm action potentials and their regulation by K+ channels

Endurance exercise modifies regulatory systems that control skeletal muscle Na+ and K+ fluxes, in particular Na+-K+-ATPase-mediated transport of these ions. Na+ and K+ ion channels also play important roles in the regulation of ionic movements, specifically mediating Na+ influx and K+ efflux that occur during contractions resulting from action potential depolarization and repolarization. Whether exercise alters skeletal muscle electrophysiological properties controlled by these ion channels is unclear. The present study tested the hypothesis that endurance exercise modifies diaphragm action potential properties. Exercised rats spent 8 wk with free access to running wheels, and they were compared with sedentary rats living in conventional rodent housing. Diaphragm muscle was subsequently removed under anesthesia and studied in vitro. Resting membrane potential was not affected by endurance exercise. Muscle from exercised rats had a slower rate of action potential repolarization than that of sedentary animals (P = 0.0098), whereas rate of depolarization was similar in the two groups. The K+ channel blocker 3,4-diaminopyridine slowed action potential repolarization and increased action potential area of both exercised and sedentary muscle. However, these effects were significantly smaller in diaphragm from exercised than sedentary rats. These data indicate that voluntary running slows diaphragm action potential repolarization, most likely by modulating K+ channel number or function.

Slowing of rat diaphragm action potential depolarization by endurance treadmill training

This study tested the hypothesis that the action potential properties of the diaphragm muscle are altered by endurance exercise treadmill training. Rats underwent treadmill running or sham training for 8 weeks, and intracellular electrophysiological recordings were subsequently performed in vitro. Diaphragm resting membrane potential was not altered by training. The maximal rate of action potential depolarization was reduced significantly by exercise training, from 551+/-16 to 445+/-15 mV/ms (P<0.00002). In contrast the rate of action potential repolarization was not significantly different between the two groups (P=0.25). Action potential height was significantly higher in control compared with trained muscle (84.5+/-1.0 vs. 78.4+/-1.2 mV, P<0.0005). The combination of slowed action depolarization and decreased peak action potential height resulted in no net change in action potential area. Thus treadmill running endurance exercise training slows rat diaphragm action potential depolarization but not repolarization, suggestive of altered Na+ but not K+ channel function.

Endurance training alters the biophysical properties of hindlimb motoneurons in rats

The purpose of the study was to determine the effect of daily endurance treadmill training (2 h/day, 30 m/min) on motoneuron biophysical properties. Electrophysiological properties of tibial motoneurons were measured in situ in anesthetized (ketamine/xylazine) control and trained rats using sharp glass microelectrodes. Motoneurons from trained rats had significantly hyperpolarized resting membrane potentials and spike trigger levels, and faster antidromic spike rise-times. "Fast" motoneurons (after-hyperpolarization half-decay time <20 ms) in trained rats also had a significantly larger mean cell capacitance than those in control rats, suggesting that they were larger, although this had no effect on indices of excitability (rheobase, cell input resistance). Motoneurons are thus targets for activity-induced adaptations, which may have clinical significance for the role of physical activity as a therapeutic modality in cases of neurological deficit. The specific adaptations noted, which reflect alterations in ionic conductances, may serve to offset decreases in membrane excitability that occur during sustained excitation.

Incomplete recovery of endplate potential amplitude while intermittently activating rat soleus neuromuscular junctions in situ

Studies dealing with neuromuscular transmission efficacy typically employ continuous patterns of activation to demonstrate decrements in endplate potential (epp) amplitude. Recent evidence from rat diaphragm muscle has shown that including periods of quiescence to the stimulation protocol allows epp amplitude to recover between series of contractions. Whether similar recovery occurs in rat hindlimb muscle is unknown. In this study, we have measured declines in epp amplitude in rat soleus muscle during trains of stimulation evoked either continuously (10 s) or intermittently (400 ms repeated every second), using an in situ approach. As in diaphragm, we found that rest periods within the intermittent trains significantly improved neuromuscular transmission efficacy. However, unlike the diaphragm, epp amplitude recovery was incomplete even by the second train in the intermittent protocols, recovery being frequency-dependent and ranging from 40 to 50%. The results suggest that the kinetics of epp amplitude rundown and recovery may be muscle-specific, and should be considered when evaluating situations in which neuromuscular transmission efficacy may be altered.