Hz Trains Research Articles

Repetitive Contractions at Short and Long Lengths: Do Not Subtract Passive Force!

Several reports have compared the consequence of repetitive contractions at long and short lengths, with the goal of gaining an understanding of factors causing muscle fatigue: metabolic vs ion distribution. This is traditionally done calculating active force as peak force - passive force. Alternatively, it has recently been shown that during contraction of whole muscle, fascicle length shortens, and it would be more appropriate to subtract the passive force associated with the fascicle length at the peak of the contraction. These two approaches will give different results, for contractions at long length. Contractions of the rat medial gastrocnemius muscle were obtained at 0.3Hz (trains of 50 Hz, 400 ms duration) at a length either 3.6mm shorter or 3.6mm longer than the reference length. During the repetitive contractions, muscle length was changed periodically to the other length to observe 1-2 contractions, then returned to the test length. Initial active force was 2.53±0.4 (mean ± SD) and 6.26±1.2 N at short and long lengths respectively. Active force at the long length would be similar to that of the short length if active force was calculated in the traditional manner. Active force decreased to 1.90±0.5 and 1.8±1.0 N at the short and long lengths respectively. During repetitive contractions at the short length, active force was 3.6±1.1 N, when measured at the long length. During repetitive contractions at the long length, active force was 0.67±0.4 N, when measured at the short length. Clearly, the long length resulted in substantially greater fatigue than the short length. There would be no explanation for this if active force was calculated in the traditional manner. The higher real active force and therefore metabolic demand of contractions at the long length can explain the greater fatigue.

Local Detection Of Intracellular Reactive Oxygen Species In Single Intact Contracting Skeletal Muscle Fibers

Skeletal muscles produce low levels of reactive oxygen species (ROS) under resting conditions while during contractile activity the rate of ROS production increases. Low levels of ROS production may act to stimulate adaptive responses in skeletal muscle, while increased ROS dependent oxidation at sarcoplasmic reticulum, myofilaments and other EC coupling components likely lead to decrements in contractile function. Multiple sites exist for ROS production, including mitochondria and NADPH oxidase; however, the contribution of each of these and the factors that regulate the increased production of ROS during contractile activity remains to be determined. Here we use repetitive field stimulation of single FDB myofibers as a model of ROS secondary to repetitive activity. In FDB's loaded with the cytosolic, non-specific ROS probe DCFH, we have imposed intermittent (0.5 Hz) trains (150msec, 0.5msec sq. pulse @, 50Hz) of tetanic stimulation to establish a reliable in vitro model for activity dependent ROS production. In recent studies with this model, we have begun to explore site dependent generation of ROS with a redox sensitive variant of green fluorescent protein (roGFP) that is targeted to the mitochondria (mito-roGFP). Following cDNA electroporation in vivo, expression of mito-roGFP, and FDB isolation, we report evidence of the fidelity and specificity of this probe in mitochondria and the activity dependent mitochondrial redox status during our stimulation paradigm.

The Absence of Utrophin Does Not Further the Impairment of Ca2+ Release Displayed by mdx Muscle

The double mutant utr-/-/mdx mouse has been postulated to be a better model of Duchenne Muscular Dystrophy than mdx mouse because it displays a progression of pathological features comparable to that in humans. We previously demonstrated that FDB fibers from mdx mice show limitations in action potential (AP) elicited Ca2+ release. Here we investigated the properties of the APs and the ability to release Ca2+ (30 mM [EGTA], 20°C) in response to single and trains of APs (20 pulses, at 33Hz and 100Hz) in FDB fibers isolated from control, mdx and utr-/-/mdx mice. Single APs of normal amplitude but longer duration were recorded in utr-/-/mdx fibers, whereas the amplitude of the Ca2+ release was ∼37% smaller than in normal fibers, but comparable with that found in mdx fibers. Fibers from the three strains sustained trains of Ca release at 33 Hz in which the amplitude of individual Ca2+ release transients decayed exponentially towards a sustained release amplitude with two time constants (τ1=10ms, τ2=200ms). In response to 100Hz trains, the amplitude of Ca2+ release in normal fibers decayed still with a double exponential (τ1=3.5ms, τ2=41ms) in which the amplitude of the 2nd and 20th transient along the train were ∼50% and ∼35% that of the first one, respectively. In contrast, fibers isolated from both mdx and utr-/-/mdx mice could be divided in two groups according to their tetanic response to 100Hz trains: approximately 75% of mdx and 33% of utr-/-/mdx fibers showed a behavior similar to that observed in normal fibers; in the remaining 25% of mdx and 67% of utr-/-/mdx fibers, respectively, the amplitude of the 2nd Ca2+ release transient was ∼20% of the first one, and this amplitude was sustained throughout the train.

Functional characterization of GABAergic pallidopallidal and striatopallidal synapses in the rat globus pallidus in vitro

As a central integrator of basal ganglia function, the external segment of the globus pallidus (GP) plays a critical role in the control of voluntary movement. Driven by intrinsic mechanisms and excitatory glutamatergic inputs from the subthalamic nucleus, GP neurons receive GABAergic inhibitory input from the striatum (Str-GP) and from local collaterals of neighbouring pallidal neurons (GP-GP). Here we provide electrophysiological evidence for functional differences between these two inhibitory inputs. The basic synaptic characteristics of GP-GP and Str-GP GABAergic synapses were studied using whole-cell recordings with paired-pulse and train stimulation protocols and variance-mean (VM) analysis. We found (i) IPSC kinetics are consistent with local collaterals innervating the soma and proximal dendrites of GP neurons whereas striatal inputs innervate more distal regions. (ii) Compared to GP-GP synapses Str-GP synapses have a greater paired-pulse ratio, indicative of a lower probability of release. This was confirmed using VM analysis. (iii) In response to 20 and 50 Hz train stimulation, GP-GP synapses are weakly facilitatory in 1 mM external calcium and depressant in 2.4 mM calcium. This is in contrast to Str-GP synapses which display facilitation under both conditions. This is the first quantitative study comparing the properties of GP-GP and Str-GP synapses. The results are consistent with the differential location of these inhibitory synapses and subtle differences in their release probability which underpin stable GP-GP responses and robust short-term facilitation of Str-GP responses. These fundamental differences may provide the physiological basis for functional specialization.

Repetitive transcranial magnetic stimulation in the treatment of epilepsia partialis continua

Repetitive transcranial magnetic stimulation (rTMS) is a technique for noninvasive focal brain stimulation by which small intracranial electrical currents are generated by a fluctuating extracranial magnetic field. In clinical epilepsy, rTMS has been applied most often interictally to reduce seizure frequency. Less often, rTMS has been used to terminate ongoing seizures, as in instances of epilepsia partialis continua (EPC). Whether ictal rTMS is effective and safe in the treatment of EPC has not been extensively studied. Here, we describe our recent experience with rTMS in the treatment of EPC, as an early step toward evaluating the safety and efficacy of rTMS in the treatment of intractable ongoing focal seizures. Seven patients with EPC of mixed etiologies were treated with rTMS applied over the seizure. rTMS was delivered in high-frequency (20-100 Hz) bursts or as prolonged low-frequency (1 Hz) trains. The EEG was recorded for three of the seven patients. rTMS resulted in a brief (20-30 min) pause in seizures in three of seven patients and a lasting (>or=1 day) pause in two of seven. A literature search identified six additional reports of EPC treated with rTMS where seizures were suppressed in three of six. Seizures were not exacerbated by rTMS in any patient. Generally mild side effects included transient head and limb pain, and limb stiffening during high-frequency rTMS trains. Our clinical observations in a small number of patients suggest that rTMS may be safe and effective in suppressing ongoing seizures associated with EPC. However, a controlled trial is needed to assess the safety and anticonvulsive efficacy of rTMS in the treatment of EPC.

Differential potentiation of early and late components evoked in olfactory cortex by stimulation of cortical association fibers

The present study examined in detail the development and decay of potentiation induced in vivo by repeated high-frequency stimulation of cortical association fibers (AF) in piriform cortex (PC). Male Long–Evans rats with chronically-implanted stimulating and recording electrodes were administered potentiating AF stimulation (thirty 10-pulse 100-Hz trains) on 8 consecutive days, followed by a ninth administration after an 8-day layoff. The time course of potentiation was monitored by local field potentials evoked in the PC and olfactory bulb (OB) by 0.1 Hz single-pulse AF test stimulation before, during, and following each potentiating treatment. AF test stimulation evoked two distinct components in the PC, an early component (EC) and a late component (LC). High-frequency AF stimulation produced potentiation of each component, but with very different characteristics. EC potentiation consisted of a brief augmentation during each bout of potentiating stimulation that persisted < 2 min after the last high-frequency train and showed no cumulative effects following repeated induction across days. In contrast, LC potentiation developed gradually, requiring several daily potentiation treatments to reach maximum amplitude, and decayed more slowly each time it was induced. Furthermore, LC potentiation persisted in latent form for at least 8 days following its apparent decay and could be reinstated by repeated test stimulation that was without effect at the beginning of the experiment. Potentiation in the OB resembled LC potentiation in its characteristics, but with less latent potentiation. These results indicate that the potentiation reported here is distinctly different from the long-term potentiation previously demonstrated in vitro in the PC, and suggest that this potentiation represents an increase in excitability within the cortical association fiber system that can be stored in latent form and retrieved at a later time. These characteristics make this potentiation a suitable candidate for participation in long-term functional changes within olfactory cortex.

Short-Term Synaptic Depression and Recovery at the Mature Mammalian Endbulb of Held Synapse in Mice

The endbulb of Held synapses between the auditory nerve fibers (ANF) and cochlear nucleus bushy neurons convey fine temporal information embedded in the incoming acoustic signal. The dynamics of synaptic depression and recovery is a key in regulating synaptic transmission at the endbulb synapse. We studied short-term synaptic depression and recovery in mature (P22-38) CBA mice with stimulation rates that were comparable to sound-driven activities recorded in vivo. Synaptic depression in mature mice is less severe ( approximately 40% at 100 Hz) than reported for immature animals and the depression is predominately due to depletion of releasable vesicles. Recovery from depression depends on the rate of activity and accumulation of intracellular Ca2+ at the presynaptic terminal. With a regular stimulus train at 100 Hz in 2 mM external [Ca2+], the recovery from depletion was slow (tauslow, approximately 2 s). In contrast, a fast (taufast, approximately 25 ms), Ca2+-dependent recovery followed by a slower recovery (tauslow, approximately 2 s) was seen when stimulus rates or external [Ca2+] increased. In normal [Ca2+], recovery from a 100-Hz Poisson-like train is rapid, suggesting that Poisson-like trains produce a higher internal [Ca2+] than regular trains. Moreover, the fast recovery was slowed by approximately twofold in the presence of calmidazolium, a Ca2+/calmodulin inhibitor. Our results suggest that endbulb synapses from high spontaneous firing rate auditory nerve fibers normally operate in a depressed state. The accelerated synaptic recovery during high rates of activity is likely to ensure that reliable synaptic transmission can be achieved at the endbulb synapse.

Transgenic Mice Lacking NMDAR-Dependent LTD Exhibit Deficits in Behavioral Flexibility

While most studies have focused on the role of long-term potentiation in behavior, far less is known about the role of long-term depression (LTD). To examine the potential involvement of LTD in learning and memory, we generated transgenic mice that express a fragment of the SV40 small t antigen known to inhibit protein phosphatase 2A (PP2A). Small t antigen expression blocked both stimulus-induced and chemically induced NMDAR-dependent LTD at Schaffer collateral synapses but did not affect potentiation, depotentiation, or mGluR-dependent LTD. This physiological phenotype was associated with deficits in behavioral flexibility in both the Morris water maze and a delayed nonmatch to place T-maze task, suggesting that NMDAR-dependent LTD is required for behavioral flexibility and may act by weakening previously encoded memory traces when new information is learned.

Interplay between neuromodulator‐induced switching of short‐term plasticity at sensorimotor synapses in the neonatal rat spinal cord

In the present study, we investigated the modulation of short-term depression (STD) at synapses between sensory afferents and rat motoneurons by serotonin, dopamine and noradrenaline. STD was elicited with trains of 15 stimuli at 1, 5 and 10 Hz and investigated using whole-cell voltage-clamp recordings from identified motoneurons in the neonatal rat spinal cord in vitro. STD was differentially modulated by the amines. Dopamine was effective at all stimulation frequencies, whereas serotonin affected STD only during 5 and 10 Hz stimulus trains and noradrenaline during 1 and 5 Hz trains. Dopamine and serotonin homogenized the degree of depression observed with the different stimulation modalities, in contrast to noradrenaline, which amplified the rate differences. The different modulatory profiles observed with the amines were partly due to GABAergic interneuron activity. In the presence of GABA(A) and GABA(B) receptor antagonists, the rate and/or kinetics of STD did not vary with the stimulation frequency in contrast to the control condition, and noradrenaline failed to alter either synaptic amplitude or STD, suggesting indirect actions. Dopamine and serotonin strongly decreased STD and converted depression to facilitation at 5 and 10 Hz during the blockade of the GABAergic receptors in 50% of the neurons tested. Altogether, these results show that STD expressed at sensorimotor synapses in the neonatal rat not only is a function of the frequency of afferent firing but also closely depends on the neuromodulatory state of these connections, with a major contribution from GABAergic transmission.

Centrally mediated sensory decline induced by differential C-fiber stimulation

It is increasingly recognized that pain-induced plasticity may not only provoke sensory gain (hyperalgesia), but also sensory decline, i.e. hypoesthesia and hypoalgesia. We investigated perceptual changes by conditioning electrical stimulation of peptidergic C-nociceptors differing in stimulation frequencies and duty cycles at the left forearm. Four noxious electrical stimulation paradigms ( Stim1: 0.5 Hz, continuously; Stim2: 20 Hz, continuously; Stim3: 1 s 20 Hz train, 1 s break; Stim4: 1 s 20 Hz train, 2 s break) were applied. Stim1 led to mechanical hyperalgesia and hypoesthesia. In contrast, Stim2 generated both hypoalgesia and hypoesthesia, which was not blocked by an anaesthetic ring around the stimulated skin area and markedly exceeded the primary stimulation site, providing evidence for a centrally mediated mechanism. Finally, when electrical high frequency trains were applied with two different duty cycles (i.e. Stim3 and 4), both stimulation paradigms produced hypoesthesia. However, only high frequency trains interrupted by the shorter inter train interval led to hypoalgesia. In contrast, high frequency trains interrupted by the longer inter train interval produced significant mechanical hyperalgesia. In summary, we describe here that depending on the applied frequencies and duty cycles, either sensory gain (i.e. hyperalgesia) or sensory decline (i.e. hypoesthesia and hypoalgesia) can be induced. Sensory decline was found to be centrally mediated. Underlying mechanisms may include differential recruitment of inhibitory and facilitating gain control systems leading to homo- and heterosynaptic inhibition or facilitation at the level of the spinal cord or interference of noxious input with tactile processing in the cortex.

External anal sphincter fatigue is not improved by N‐acetylcysteine in an animal model

Oxidative stress is associated with skeletal muscle fatigue. This study tests the hypotheses that N-acetylcysteine (NAC) reduces fatigue and accelerates recovery of the rat external anal sphincter (EAS). Fifteen female Wistar rats were killed humanely. The EAS was mounted as a ring preparation and electrically stimulated with 50 Hz trains of 200 ms in duration every 4 s for three and a half minutes. Three groups were analysed: a control group (n = 5), a group pretreated with NAC (10(-4) mol L(-1); n = 5) and a group pretreated with NAC (10(-3) mol L(-1); n = 5). A novel fatigue index was formulated and was compared to a conventional method of expressing fatigue. There was no significant difference at concentrations of NAC (10(-4) mol L(-1); P > 0.05). At high concentrations of NAC (10(-3) mol L(-1)) there was a significant depression in peak twitch amplitude before fatigue (P = 0.04). N-acetylcysteine in both concentrations used, did not alter fatigue or recovery of the rat EAS. There was a significant positive correlation between the two methods of expressing fatigue but the conventional method produced a higher fatigue index (22.4% on average). N-acetylcysteine does not ameliorate fatigue or accelerate recovery of the EAS and may not be a useful medical therapy for faecal incontinence.

Potentiation of Electrical and Chemical Synaptic Transmission Mediated by Endocannabinoids

Endocannabinoids are well established as inhibitors of chemical synaptic transmission via presynaptic activation of the cannabinoid type 1 receptor (CB1R). Contrasting this notion, we show that dendritic release of endocannabinoids mediates potentiation of synaptic transmission at mixed (electrical and chemical) synaptic contacts on the goldfish Mauthner cell. Remarkably, the observed enhancement was not restricted to the glutamatergic component of the synaptic response but also included a parallel increase in electrical transmission. This effect involved the activation of CB1 receptors and was indirectly mediated via the release of dopamine from nearby varicosities, which in turn led to potentiation of the synaptic response via a cAMP-dependent protein kinase-mediated postsynaptic mechanism. Thus, endocannabinoid release can potentiate synaptic transmission, and its functional roles include the regulation of gap junction-mediated electrical synapses. Similar interactions between endocannabinoid and dopaminergic systems may be widespread and potentially relevant for the motor and rewarding effects of cannabis derivatives.

Neural substrates of awakening probed with optogenetic control of hypocretin neurons

The neural underpinnings of sleep involve interactions between sleep-promoting areas such as the anterior hypothalamus, and arousal systems located in the posterior hypothalamus, the basal forebrain and the brainstem. Hypocretin (Hcrt, also known as orexin)-producing neurons in the lateral hypothalamus are important for arousal stability, and loss of Hcrt function has been linked to narcolepsy. However, it is unknown whether electrical activity arising from Hcrt neurons is sufficient to drive awakening from sleep states or is simply correlated with it. Here we directly probed the impact of Hcrt neuron activity on sleep state transitions with in vivo neural photostimulation, genetically targeting channelrhodopsin-2 to Hcrt cells and using an optical fibre to deliver light deep in the brain, directly into the lateral hypothalamus, of freely moving mice. We found that direct, selective, optogenetic photostimulation of Hcrt neurons increased the probability of transition to wakefulness from either slow wave sleep or rapid eye movement sleep. Notably, photostimulation using 5-30 Hz light pulse trains reduced latency to wakefulness, whereas 1 Hz trains did not. This study establishes a causal relationship between frequency-dependent activity of a genetically defined neural cell type and a specific mammalian behaviour central to clinical conditions and neurobehavioural physiology.

Offsetting of aberrations associated with seizure proneness in rat hippocampus area CA1 by theta pulse stimulation–induced activity pattern

Epileptiform activity induces long term aberrations in hippocampal network functions. This study was conducted in pentylenetetrazol (PTZ) -kindled rats to examine offsetting of aberrations associated with seizure proneness in hippocampus area CA1 by theta pulse stimulation (TPS: 5 Hz trains for 3 min) –induced activity pattern. In hippocampal slices from both control and kindled rats, the field excitatory postsynaptic potentials (fEPSP) and population spikes (PS) were simultaneously recorded through electrodes in the apical dendrites and stratum pyramidale, respectively. The following changes in kindled vs. control slices were observed. The fEPSP needed to be greater to produce the PS recorded in the cell body layer. The fEPSP was reduced by paired stimuli whereas the PS amplitude was increased. TPS selectively depressed the PS in a lasting fashion, and shifted the fEPSP slope and the PS amplitude relation toward what was observed in controls. Both the fEPSP and PS were increased by paired stimuli at 60 min after TPS application. The lasting depressive effect of TPS on the PS amplitude was converted into facilitation by adenosine A1 receptor antagonist 8-cyclopentyl-1, 3-dipropylxanthine (CPX). Potentiation of the PS amplitude by TPS in the presence of CPX was blocked by an N-methyl- d-aspartate receptor antagonist AP5. We hypothesize that the extracellular adenosine spillover, acting through adenosine A1 receptors, during TPS-induced activity pattern could trigger a homeostatic process for correcting network imbalances caused by epileptiform activity.

Inotropic effects of the K+ channel blocker 3,4-diaminopyridine on fatigued diaphragm muscle

K(+) channels play important roles in skeletal muscle contraction by regulating action potential duration. Blocking these channels, for example with 3,4-diaminopyridine (DAP), augments muscle force considerably, and these force increases are maintained well during fatigue-inducing contractions. The present study tested the hypothesis that K(+) channel blockade also improves force of previously fatigued muscle. Rat diaphragm underwent fatigue-inducing stimulation in vitro with four different stimulation protocols consisting of 20 Hz vs. 50 Hz trains and 1 min vs. 4 min stimulation durations. DAP administered at the onset of the recovery period produced significant force increases irrespective of the amount of antecedent force loss. These force gains considerably exceeded those resulting from normal force recovery in untreated muscle. Furthermore contraction time was prolonged by DAP in all cases, and half-relaxation time was prolonged by DAP in most cases. Several differences were found compared with previous studies of DAP in fresh muscle, including smaller magnitude and slower time course of force increases. Intracellular electrophysiological recordings found smaller effects of DAP on action potential overshoot and time-depolarization integral in previously stimulated compared with fresh muscle. These data indicate that K(+) channel blockade does indeed increase force of fatigued diaphragm, but to an attenuated extent relative to its effects on non-fatigued muscle, which can be explained on the basis of electrophysiological findings. Nonetheless DAP-induced force increases were usually sufficient to restore force to values present prior to the onset of fatigue-inducing stimulation.

Production of defects in ZBLAN, ZBLAN:Tm 3+ and ZBLAN:Cr 3+ glasses by ultra-short pulses laser interaction

In this work we have studied pure and thulium- and chromium-doped ZBLAN glasses irradiated by ultra-short laser pulses. A Ti:sapphire CPA system was used, producing a 500 Hz train of pulses, centered at 830 nm, with 375 μJ of energy and 50 fs of duration (FWHM). The beam was focused by a 20 mm lens, producing a converging beam with a waist of 12 μm. The absorption spectra before and after laser irradiation were obtained showing production of color centers in pure, thulium-doped and chromium-doped ZBLAN glasses. A damage threshold of 9.56 T W/cm 2 was determined for ZBLAN.

A comparative study of the effects of repetitive paired transcranial magnetic stimulation on motor cortical excitability

Objectives Various methods of application of repetitive transcranial magnetic stimulation (TMS) have been evaluated for their potential capacity to alter motor cortical excitability. Initial research suggests that the repetitive application of paired TMS pulses (repetitive paired pulse TMS (rppTMS)) may have greater effects on cortical excitability, perhaps through the facilitation of I-wave interaction. We aimed to compare the post-train effects of 15 min trains of rppTMS to investigate the potential therapeutic application of this technique as well as to compare it to a standard high frequency repetitive TMS paradigm. Methods Ten normal subjects received three 15 min sessions of rppTMS, 5 Hz high frequency rTMS and sham TMS in randomised order. rppTMS consisted of a single train of 180 pulse pairs (0.2 Hz, 1.5 ms inter-stimulus interval, supra-threshold intensity) administered over 15 min. The rTMS condition involved 750 pulses provided in 5 s 5 Hz trains with a 25 s inter-train interval at 90% of the RMT. Motor evoked potential size and cortical silent period duration were assessed before and after each session. Results There were no significant changes in cortical excitability produced by any of the stimulation conditions. Five hertz rTMS produced an increase in cortical silent period duration ( p = 0.004) which was not affected by rppTMS. Conclusions Fifteen minutes trains of 1.5 ms rppTMS do not substantially increase post train cortical excitability. Repetitive brief trains of 5 Hz rTMS also do not alter excitability but appear to effect cortical inhibition.

Prolonged Exposure to Levetiracetam Reveals a Presynaptic Effect on Neurotransmission

The antiepileptic drug levetiracetam (LEV) is an enigma. Despite the fact that it specifically binds to the presynaptic vesicle protein, SV2A, no satisfactory mechanism of action has yet been identified. Using a combination of electrophysiological and cellular imaging techniques, we carefully tested the hypothesis that LEV directly interferes with neurotransmitter release. We measured extracellular evoked responses in the CA1 region of rat hippocampal slices after paired pulse stimulation and after application of up to 10 pulses applied at 5-80 Hz. In parallel experiments, we used quantitative 2-photon microscopy and the fluorescent vesicular marker FM1-43 to measure the effect of repetitive stimulation on presynaptic vesicle release. Acute exposure to LEV (100 microM) had no effect on paired pulse synaptic responses. However, when slices were exposed to LEV for 3 h, there was a significant alteration in paired pulse responses and a more striking reduction in late synaptic potentials delivered in an 80 Hz train. LEV significantly reduced the rate of vesicle release assessed by FM1-43 destaining during 1 Hz stimulation. LEV is unique among currently available antiepileptics, because it directly inhibits presynaptic neurotransmitter release in a use-dependent fashion. While there are alternate explanations for this observation, it is plausible that LEV exerts its effect by binding to a protein selectively expressed in presynaptic nerve terminals.

Activity‐dependent changes in temporal components of neurotransmission at the juvenile mouse calyx of Held synapse

The temporal fidelity of synaptic transmission is constrained by the reproducibility of time delays such as axonal conduction delay and synaptic delay, but very little is known about the modulation of these distinct components. In particular, synaptic delay is not generally considered to be modifiable under physiological conditions. Using simultaneous paired patch-clamp recordings from pre- and postsynaptic elements of the calyx of Held synapse, in juvenile mouse auditory brainstem slices, we show here that synaptic activity (20-200 Hz) leads to activity-dependent increases in synaptic delay and its variance as well as desynchronization of evoked responses. Such changes were most robust at 200 Hz in 2 mM extracellular Ca(2+) ([Ca(2+)](o)), and could be attenuated by lowering [Ca(2+)](o) to 1 mM, increasing temperature to 35 degrees C, or application of the GABA(B)R agonist baclofen, which inhibits presynaptic Ca(2+) currents (I(Ca)). Conduction delay also exhibited slight activity-dependent prolongation, but this prolongation was only sensitive to temperature, and not to [Ca(2+)](o) or baclofen. Direct voltage-clamp recordings of I(Ca) evoked by repeated action potential train template (200 Hz) revealed little jitter in the timing and kinetics of I(Ca) under various conditions, suggesting that increases in synaptic delay and its variance occur downstream of Ca(2+) entry. Loading the Ca(2+) chelator EGTA-AM into terminals reduced the progression rate, the extent of activity-dependent increases in various delay components, and their variance, implying that residual Ca(2+) accumulation in the presynaptic nerve terminal induces these changes. Finally, by applying a test pulse at different intervals following a 200 Hz train (150 ms), we demonstrated that prolongation in the various delay components reverses in parallel with recovery in synaptic strength. These observations suggest that a depletion of the readily releasable pool of SVs during high-frequency activity may downregulate not only synaptic strength but also decrease the temporal fidelity of neurotransmission at this and other central synapses.

Turning on the central contribution to contractions evoked by neuromuscular electrical stimulation

Neuromuscular electrical stimulation can generate contractions through peripheral and central mechanisms. Direct activation of motor axons (peripheral mechanism) recruits motor units in an unnatural order, with fatigable muscle fibers often activated early in contractions. The activation of sensory axons can produce contractions through a central mechanism, providing excitatory synaptic input to spinal neurons that recruit motor units in the natural order. Presently, we quantified the effect of stimulation frequency (10-100 Hz), duration (0.25-2 s of high-frequency bursts, or 20 s of constant-frequency stimulation), and intensity [1-5% maximal voluntary contraction (MVC) torque generated by a brief 100-Hz train] on the torque generated centrally. Electrical stimulation (1-ms pulses) was delivered over the triceps surae in eight subjects, and plantar flexion torque was recorded. Stimulation frequency, duration, and intensity all influenced the magnitude of the central contribution to torque. Central torque did not develop at frequencies < or = 20 Hz, and it was maximal at frequencies > or = 80 Hz. Increasing the duration of high-frequency stimulation increased the central contribution to torque, as central torque developed over 11 s. Central torque was greatest at a relatively low contraction intensity. The largest amount of central torque was produced by a 20-s, 100-Hz train (10.7 +/- 5.5 %MVC) and by repeated 2-s bursts of 80- or 100-Hz stimulation (9.2 +/- 4.8 and 10.2 +/- 8.1% MVC, respectively). Therefore, central torque was maximized by applying high-frequency, long-duration stimulation while avoiding antidromic block by stimulating at a relatively low intensity. If, as hypothesized, the central mechanism primarily activates fatigue-resistant muscle fibers, generating muscle contractions through this pathway may improve rehabilitation applications.