Action Potential Overshoot Research Articles

Action potentials of the superior cervical ganglion neurons in the rats in diabetes mellitus

It’s well known that sympathetic and sensory neurons are affected in the early stages of diabetes mellitus (DM). However, the functional disorders that occur in neurons of the superior cervical ganglion (SCG) under the conditions of DM remain insufficiently studied. Therefore, the aim of this study was to evaluate the effect of streptozotocin-induced diabetes mellitus (DM) of the rats on action potentials (AP) recorded in the superior cervical ganglion`s (SCG) neurons. Rats with blood sugar level more than 30 mM were taken into experiment. The SCG of healthy control rats (n=12), rats at week 4 (n=9), and rats at week 12 after streptozotocin injection (n=9) were studied in vitro. AP of the SCG neurons were registered by the microelectrode technique. Neurons of the SCG were stimulated directly with 150 ms depolarizing current in pulse of 100 pA. The AP parameters of 36 SCG neurons of control rats were alternately compared with the corresponding AP parameters of 22 neurons of rats at week 4 and 30 SCG neurons of rats at week 12 after streptozotocin injection. The data obtained demonstrate that the AP amplitude and overshoot of AP, maximum rise and fall rates, and afterhyperpolarization amplitude significantly decreased at 12 weeks after DM induction. At the same time, the rheobase value significantly increased, this may indicates decreasing of the neurons plasma membrane excitability. Only the AP maximum rate of fall decreased statistically significant at week 4, the maximum rate of rise had an insignificant tendency to decrease at that time. However, the resting membrane potential and excitation threshold didn’t change even at 12 weeks after the injection. Thus, functional disorders of rat SCG neurons were appeared at a quite late stage of DM. The differences in AP parameters may result from neurons’ membrane ionic conductivity alterations, decreasing of its excitability and reducing ion channels efficiency in later stages of DM. This suggests that SCG is an important target of pathophysiological disorders caused by DM.

Aging-associated weakening of the action potential in fast-spiking interneurons in the human neocortex

Aging is associated with the slowdown of neuronal processing and cognitive performance in the brain; however, the exact cellular mechanisms behind this deterioration in humans are poorly elucidated. Recordings in human acute brain slices prepared from tissue resected during brain surgery enable the investigation of neuronal changes with age. Although neocortical fast-spiking cells are widely implicated in neuronal network activities underlying cognitive processes, they are vulnerable to neurodegeneration. Herein, we analyzed the electrical properties of 147 fast-spiking interneurons in neocortex samples resected in brain surgery from 106 patients aged 11–84 years. By studying the electrophysiological features of action potentials and passive membrane properties, we report that action potential overshoot significantly decreases and spike half-width increases with age. Moreover, the action potential maximum-rise speed (but not the repolarization speed or the afterhyperpolarization amplitude) significantly changed with age, suggesting a particular weakening of the sodium channel current generated in the soma. Cell passive membrane properties measured as the input resistance, membrane time constant, and cell capacitance remained unaffected by senescence. Thus, we conclude that the action potential in fast-spiking interneurons shows a significant weakening in the human neocortex with age. This may contribute to the deterioration of cortical functions by aging.

Popeye domain containing proteins modulate the voltage-gated cardiac sodium channel Nav1.5

Popeye domain containing (POPDC) proteins are predominantly expressed in the heart and skeletal muscle, modulating the K2P potassium channel TREK-1 in a cAMP-dependent manner. POPDC1 and POPDC2 variants cause cardiac conduction disorders with or without muscular dystrophy. Searching for POPDC2-modulated ion channels using a functional co-expression screen in Xenopus oocytes, we found POPDC proteins to modulate the cardiac sodium channel Nav1.5. POPDC proteins downregulate Nav1.5 currents in a cAMP-dependent manner by reducing the surface expression of the channel. POPDC2 and Nav1.5 are both expressed in different regions of the murine heart and consistently POPDC2 co-immunoprecipitates with Nav1.5 from native cardiac tissue. Strikingly, the knock-down of popdc2 in embryonic zebrafish caused an increased upstroke velocity and overshoot of cardiac action potentials. The POPDC modulation of Nav1.5 provides a new mechanism to regulate cardiac sodium channel densities under sympathetic stimulation, which is likely to have a functional impact on cardiac physiology and inherited arrhythmias.

Binge-like ethanol exposure during the brain growth spurt disrupts the function of retrosplenial cortex-projecting anterior thalamic neurons in adolescent mice

Ethanol (EtOH) exposure during late pregnancy leads to enduring impairments in learning and memory that may stem from damage to components of the posterior limbic memory system, including the retrosplenial cortex (RSC) and anterior thalamic nuclei (ATN). In rodents, binge-like EtOH exposure during the first week of life (equivalent to the third trimester of human pregnancy) triggers apoptosis in these brain regions. We hypothesized that this effect induces long-lasting alterations in the function of RSC-projecting ATN neurons. To test this hypothesis, vesicular GABA transporter-Venus mice (expressing fluorescently tagged GABAergic interneurons) were subjected to binge-like EtOH vapor exposure on postnatal day (P) 7. This paradigm activated caspase 3 in the anterodorsal (AD), anteroventral (AV), and reticular thalamic nuclei at P7 but did not reduce neuronal density in these areas at P60-70. At P40-60, we injected red retrobeads into the RSC and performed patch-clamp slice electrophysiological recordings from retrogradely labeled neurons in the AD and AV nuclei 3–4 days later. We found significant effects of treatment on instantaneous action potential (AP) frequency and AP overshoot, as well as sex × treatment interactions for AP threshold and overshoot in AD neurons. A sex × treatment interaction was detected for AP number in AV neurons. EtOH exposure also reduced the frequency and amplitude of spontaneous excitatory postsynaptic currents and increased the charge transfer of spontaneous inhibitory postsynaptic currents. These results highlight a novel cellular mechanism that could contribute to the lasting learning and memory deficits associated with developmental EtOH exposure.

Ca2+ spark latency and control of intrinsic Ca2+ release dyssynchrony in rat cardiac ventricular muscle cells

Cardiac excitation-contraction coupling (ECC) depends on Ca2+ release from intracellular stores via ryanodine receptors (RyRs) triggered by L-type Ca2+ channels (LCCs). Uncertain numbers of RyRs and LCCs form ‘couplons’ whose activation produces Ca2+ sparks, which summate to form a cell-wide Ca2+ transient that switches on contraction. Voltage (Vm) changes during the action potential (AP) and stochasticity in channel gating should create variability in Ca2+ spark timing, but Ca2+ transient wavefronts have remarkable uniformity. To examine how this is achieved, we measured the Vm-dependence of evoked Ca2+ spark probability (Pspark) and latency over a wide voltage range in rat ventricular cells. With depolarising steps, Ca2+ spark latency showed a U-shaped Vm-dependence, while repolarising steps from 50 mV produced Ca2+ spark latencies that increased monotonically with Vm. A computer model based on reported channel gating and geometry reproduced our experimental data and revealed a likely RyR:LCC stoichiometry of ∼ 5:1 for the Ca2+ spark initiating complex (IC). Using the experimental AP waveform, the model revealed a high coupling fidelity (Pcpl ∼ 0.5) between each LCC opening and IC activation. The presence of ∼ 4 ICs per couplon reduced Ca2+ spark latency and increased Pspark to match experimental data. Variability in AP release timing is less than that seen with voltage steps because the AP overshoot and later repolarization decrease Pspark due to effects on LCC flux and LCC deactivation respectively. This work provides a framework for explaining the Vm- and time-dependence of Pspark, and indicates how ion channel dispersion in disease can contribute to dyssynchrony in Ca2+ release.

Characterization of atrial fibrillation linked mutations using the Nav1.5 knock-out of atrial cardiomyocytes derived from iPSCs.

Atrial fibrillation (AF) is the most common cardiac arrhythmia diagnosed and affects more than 33 million on people word wide. AF increases the risk of heart failure and myocardial infarction. Some studies carried out on HEK293 cells and murine models have shown that SCN5A mutations are linked to AF. However, the mechanism by which NaV1.5 mutations generate cardiac arrhythmias is still to be elucidated. Human induced pluripotent stem cells (hiPSCs) hold great promise to model NaV1.5-linked AF mutations and recapitulate the functional changes. Previously, our laboratory established an NaV1.5 knock-out (KO) iPSC line with CRISPR/Cas9 technology. In this study, atrial cardiomyocytes (iPSC-aCMs) were differentiated from the Control and NaV1.5 KO iPSC lines. We aim to characterise the atrial statue of the iPSC-aCMs, and then, use the model to express two AF Nav1.5-linked mutations (K1493R and M1875T). To verify that NaV1.5 protein expression was suppressed, immunocytofluorescence and Western blot analysis were performed. Action potentials (APs) recordings in current-clamp mode showed that APs from the NaV1.5 KO iPSC-aCMs exhibited a significant decrease of the AP overshoot and the upstroke velocity. Voltage-clamp analysis in NaV1.5/K1493R-transfected KO iPSC-aCMs showed a shift of the steady-state activation toward hyperpolarized voltage while the NaV1.5/M1875T expression provoked an increase of the Na+ current density and a shift of the steady-state inactivation toward depolarized voltage. These alterations increase the window current which indicate a channel gain-of-function. The AP recordings showed a decrease of the depolarization threshold corresponding to a cellular excitability increase. 61 and 80 % of NaV1.5/K1493R and /M1875T-transfected iPSC-aCMs respectively, exhibited arrhythmic events such as early and delayed afterdepolarizations (EADs and DADs). Our results confirm that the gain-of-function by shifting of the activation and inactivation properties is the cause of AF.

Another Unique Feature of the Nociceptor-specific Sodium Channel NaV1.9

The voltage-gated Na+ channel NaV1.9 remains an important pain target because of its preferential expression in nociceptors, its involvement in rodent models of nociception, and the association between gain- or loss-of-function NaV1.9 mutations with the presence or absence of pain in humans. At room temperature, NaV1.9 has a low threshold for activation, carrying a persistent current that may influence the resting membrane potential, action potential threshold, and burst firing. We recently demonstrated that in the absence of extracellular Na+, NaV1.9 may also contribute to a transient outward current. Here, we used whole cell patch clamp recordings of human and rat dorsal root ganglion neurons to further characterize this outward current. We found that this current had pharmacological and biophysical properties consistent with a role for NaV1.9: insensitivity to K+, Na+, Cl-, and TRP channel blockers, sensitivity to internally-administered Na+ channel blocker QX-314, and an NaV1.9-like voltage-dependence of inactivation, steady-state inactivation, recovery from inactivation. Interestingly, in contrast to TTX- and A-803467-sensitive currents, the transient outward current persisted in the absence of intracellular Na+, suggesting that NaV1.9 is less selective than other voltage-gated Na+ channels in human and rat sensory neurons. Consistent with this suggestion, the reversal potential for persistent (NaV1.9) inward currents was left shifted relative to that of TTX- and A-803467-sensitive currents. Furthermore, the outward current persisted in the presence of even relatively large intracellular cations (Cs+ and TEA+). Our results suggest NaV1.9 is a relatively non-selective ion channel. Together, these observations have at least two important implications. First, NaV1.9-mediated currents may contaminate voltage-clamp recordings of voltage-gated K+, Ca2+, and Cl- channels in sensory neurons. Second, NaV1.9 may have a more significant impact on the action potential waveform than previously appreciated, including the attenuating the action potential overshoot. Grant support from NIH R01NS122784, NIH 4UH3TR003090.

Contributions of NaV1.8 and NaV1.9 to excitability in human induced pluripotent stem-cell derived somatosensory neurons

The inhibition of voltage-gated sodium (NaV) channels in somatosensory neurons presents a promising novel modality for the treatment of pain. However, the precise contribution of these channels to neuronal excitability, the cellular correlate of pain, is unknown; previous studies using genetic knockout models or pharmacologic block of NaV channels have identified general roles for distinct sodium channel isoforms, but have never quantified their exact contributions to these processes. To address this deficit, we have utilized dynamic clamp electrophysiology to precisely tune in varying levels of NaV1.8 and NaV1.9 currents into induced pluripotent stem cell-derived sensory neurons (iPSC-SNs), allowing us to quantify how graded changes in these currents affect different parameters of neuronal excitability and electrogenesis. We quantify and report direct relationships between NaV1.8 current density and action potential half-width, overshoot, and repetitive firing. We additionally quantify the effect varying NaV1.9 current densities have on neuronal membrane potential and rheobase. Furthermore, we examined the simultaneous interplay between NaV1.8 and NaV1.9 on neuronal excitability. Finally, we show that minor biophysical changes in the gating of NaV1.8 can render human iPSC-SNs hyperexcitable, in a first-of-its-kind investigation of a gain-of-function NaV1.8 mutation in a human neuronal background.

Angiotensin 1-7 prevents the excessive force loss resulting from 14- and 28-day denervation in mouse EDL and soleus muscle.

Denervation leads to muscle atrophy, which is described as muscle mass and force loss, the latter exceeding expectation from mass loss. The objective of this study was to determine the efficiency of angiotensin (Ang) 1-7 at reducing muscle atrophy in mouse extensor digitorum longus (EDL) and soleus following 14- and 28-d denervation periods. Some denervated mice were treated with Ang 1-7 or diminazene aceturate (DIZE), an ACE2 activator, to increase Ang 1-7 levels. Ang 1-7/DIZE treatment had little effect on muscle mass loss and fiber cross-sectional area reduction. Ang 1-7 and DIZE fully prevented the loss of tetanic force normalized to cross-sectional area and accentuated the increase in twitch force in denervated muscle. However, they did not prevent the shift of the force-frequency relationship toward lower stimulation frequencies. The Ang 1-7/DIZE effects on twitch and tetanic force were completely blocked by A779, a MasR antagonist, and were not observed in MasR-/- muscles. Ang 1-7 reduced the extent of membrane depolarization, fully prevented the loss of membrane excitability, and maintained the action potential overshoot in denervated muscles. Ang 1-7 had no effect on the changes in α-actin, myosin, or MuRF-1, atrogin-1 protein content or the content of total or phosphorylated Akt, S6, and 4EPB. This is the first study that provides evidence that Ang 1-7 maintains normal muscle function in terms of maximum force and membrane excitability during 14- and 28-d periods after denervation.

Age-related reductions in the excitability of phasic dorsal root ganglion neurons innervating the urinary bladder in female rats

Previous studies have revealed an impairment in bladder sensory transduction in aged animals. To examine the contributions of electrical property changes of bladder primary afferents to this impairment, we compared the electrical properties of dorsal root ganglion (DRG) neurons innervating the bladder among young (3 months), middle-aged (12 months), and old (24 months) female rats. The DRG neurons were labeled using axonal tracing techniques. Whole-cell current-clamp recordings of small and medium-sized neurons were performed to assess their passive and active properties. Two patterns of firing were identified based on responses to super-threshold stimuli (1.5, 2.0, 2.5, and 3.0 × rheobase): tonic neurons fired more action potentials (APs), whereas phasic neurons fired only one AP at the onset of stimulus. Tonic neurons were smaller and had a slower rate of AP rise, longer AP duration, more depolarized voltage threshold, and greater rheobase than phasic neurons. In phasic neurons, there was an age-associated increase in voltage threshold and an increase of rheobase (P < 0.05), suggesting an age-related decrease in excitability. In addition, both middle-aged and old rats had longer AP durations and slower rates of AP rise than young rats (P < 0.05). In tonic neurons, old rats had a greater AP overshoot and greater rate of AP rise, but no age-associated changes were identified in any other electrical properties. Our results suggest that the electrical properties of tonic and phasic bladder afferents are differentially altered with aging. A decrease in excitability may contribute to age-related reductions in bladder sensory function.

N-acetylcysteine amide ameliorates mitochondrial dysfunction and reduces oxidative stress in hiPSC-derived dopaminergic neurons with POLG mutation

The inability to reliably replicate mitochondrial DNA (mtDNA) by mitochondrial DNA polymerase gamma (POLG) leads to a subset of common mitochondrial diseases associated with neuronal death and depletion of neuronal mtDNA. Defining disease mechanisms in neurons remains difficult due to the limited access to human tissue. Using human induced pluripotent stem cells (hiPSCs), we generated functional dopaminergic (DA) neurons showing positive expression of dopaminergic markers TH and DAT, mature neuronal marker MAP2 and functional synaptic markers synaptophysin and PSD-95. These DA neurons were electrophysiologically characterized, and exhibited inward Na + currents, overshooting action potentials and spontaneous postsynaptic currents (sPSCs). POLG patient-specific DA neurons (POLG-DA neurons) manifested a phenotype that replicated the molecular and biochemical changes found in patient post-mortem brain samples namely loss of complex I and depletion of mtDNA. Compared to disease-free hiPSC-derived DA neurons, POLG-DA neurons exhibited loss of mitochondrial membrane potential, loss of complex I and loss of mtDNA and TFAM expression. POLG driven mitochondrial dysfunction also led to neuronal ROS overproduction and increased cellular senescence. This deficit was selectively rescued by treatment with N-acetylcysteine amide (NACA). In conclusion, our study illustrates the promise of hiPSC technology for assessing pathogenetic mechanisms associated with POLG disease, and that NACA can be a promising potential therapy for mitochondrial diseases such as those caused by POLG mutation.

Selective inhibition of electrical conduction within the pulmonary veins by \u03b11-adrenergic receptors activation in the Rat

Pulmonary veins (PV) are involved in the pathophysiology of paroxysmal atrial fibrillation. In the rat, left atrium (LA) and PV cardiomyocytes have different reactions to α1-adrenergic receptor activation. In freely beating atria-PV preparations, we found that electrical field potential (EFP) originated from the sino-atrial node propagated through the LA and the PV. The α1-adrenergic receptor agonist cirazoline induced a progressive loss of EFP conduction in the PV whereas it was maintained in the LA. This could be reproduced in preparations electrically paced at 5 Hz in LA. During pacing at 10 Hz in the PV where high firing rate ectopic foci can occur, cirazoline stopped EFP conduction from the PV to the LA, which allowed the sino-atrial node to resume its pace-making function. Loss of conduction in the PV was associated with depolarization of the diastolic membrane potential of PV cardiomyocytes. Adenosine, which reversed the cirazoline-induced depolarization of the diastolic membrane potential of PV cardiomyocytes, restored full over-shooting action potentials and EFP conduction in the PV. In conclusion, selective activation of α1-adrenergic receptors results in the abolition of electrical conduction within the PV. These results highlight a potentially novel pharmacological approach to treat paroxysmal atrial fibrillation by targeting directly the PV myocardium.

Enriched Environment Shortens the Duration of Action Potentials in Cerebellar Granule Cells.

Environmental enrichment for rodents is known to enhance motor performance. Structural and molecular changes have been reported to be coupled with an enriched environment, but functional alterations of single neurons remain elusive. Here, we compared mice raised under control conditions and an enriched environment. We tested the motor performance on a rotarod and subsequently performed whole-cell patch-clamp recordings in cerebellar slices focusing on granule cells of lobule IX, which is known to receive vestibular input. Mice raised in an enriched environment were able to remain on an accelerating rotarod for a longer period of time. Electrophysiological analyses revealed normal passive properties of granule cells and a functional adaptation to the enriched environment, manifested in faster action potentials (APs) with a higher depolarized voltage threshold and larger AP overshoot. Furthermore, the maximal firing frequency of APs was higher in mice raised in an enriched environment. These data show that enriched environment causes specific alterations in the biophysical properties of neurons. Furthermore, we speculate that the ability of cerebellar granule cells to generate higher firing frequencies improves motor performance.

Graphene oxide supported p(AAm)/PANI cryogel polymer composites and its potential sensor application to paraquat, glyphospahte, nitrophenol, methylene blue, methyl orange

Ischemia-reperfusion injury is the major complication of abdominal aortic surgery, and it mainly affects the lower extremities and remote organs. In the present study, the electrophysiological alterations in diaphragm that underlie the post-operative respiratory dysfunction were investigated.Wistar Albino rats were randomly divided into two groups: SHAM (only laparotomy was performed) and IR (abdominal aorta was clamped for 30 min and reperfused for 2 h). Following the operational period diaphragm muscles were isolated and electrophysiological experiments were carried out in-vitro. 3 nM Ryanodine application, Na+ and K+ current blockage (0.3 mM 4-Aminopyridine and 127 mM N-methyl-d-glukamine) experiments were also conducted to further reveal any alterations.Twitch and tetanic force were decreased significantly. Action potential overshoot, amplitude and area were increased while diaphragm muscle cells were found to be hyperpolarized significantly.Mechanical alterations were shown to be caused by deterioration of Ca++ homeostasis. At resting state, a decrease in persistent Na+ current was found. The reshaping of action potential, on the other hand, was shown to be due to altered kinetics of Na+ channels and delayed activation of voltage dependent K+ channels.

Abstract 11: Lamin A/C Mutations Epigenetically Dysregulate Scn5a Gene Expression, Perturbing Action Potential Properties in IPSC-derived Cardiomyocytes

Mutations of the LMNA gene, encoding the nuclear lamina proteins Lamin A/C, are a common cause of dilated cardiomyopathy, typically manifesting in association with cardiac conduction defects. LaminA/C regulate various nuclear activities, including maintenance of the nuclear structure, gene transcription and chromatin organization. Most studies on the consequences of Lamin A/C defects were conducted on fibroblasts, while studies on human cardiomyocytes (CMs) are scarce. We therefore generated a cardiac model of laminopathy obtained by differentiation of CMs from induced pluripotent stem cells (iPSCs) of patients carrying the K219T Lamin A/C mutation. In vitro, these cells recapitulate the morphological features of dilated cardiomyopathy, specifically sarcomeric disorganization and increased size. Using this model, we performed a comprehensive analysis of the electrophysiological properties of LMNA-CMs both at single cell level and in a multi-cellular setting. Using patch-clamp technique, results revealed significant changes in maximal upstroke velocity (dV/dt max ), action potential amplitude (APA) and overshoot (OV) in LMNA-CMs compared to those obtained from family-matched healthy controls (CNTR); these defects were associated with a reduction of the peak sodium currents and a diminished conduction velocity, measured in strands of electrically-coupled CMs. Biochemical studies showed a significant reduction of both the sodium channel Nav1.5 protein and its transcript in LMNA-CMs, accompanied by an increased binding of LaminA/C to the promoter of its coding gene, SCN5A. Binding of the Polycomb group protein SUZ12 and of the H3K27me3 histone repressive mark was also increased. Consistently, 3D-FISH experiments also indicated a preferential localization of SCN5A genomic loci at the nuclear periphery in LMNA-CMs. As a whole, our findings support a model in which mutated Lamin A/C perturb SCN5A gene expression by favouring PRC2 (Polycomb Repressive Complex 2) binding to its promoter, leading to decreased sodium current peak and slower conduction velocity. This mechanism may eventually sustain the conduction abnormalities inevitably occurring in patients with LMNA-cardiomyopathy.

Action potentials and ion conductances in wild-type and CALHM1-knockout type II taste cells.

Taste bud type II cells fire action potentials in response to tastants, triggering nonvesicular ATP release to gustatory neurons via voltage-gated CALHM1-associated ion channels. Whereas CALHM1 regulates mouse cortical neuron excitability, its roles in regulating type II cell excitability are unknown. In this study, we compared membrane conductances and action potentials in single identified TRPM5-GFP-expressing circumvallate papillae type II cells acutely isolated from wild-type (WT) and Calhm1 knockout (KO) mice. The activation kinetics of large voltage-gated outward currents were accelerated in cells from Calhm1 KO mice, and their associated nonselective tail currents, previously shown to be highly correlated with ATP release, were completely absent in Calhm1 KO cells, suggesting that CALHM1 contributes to all of these currents. Calhm1 deletion did not significantly alter resting membrane potential or input resistance, the amplitudes and kinetics of Na+ currents either estimated from action potentials or recorded from steady-state voltage pulses, or action potential threshold, overshoot peak, afterhyperpolarization, and firing frequency. However, Calhm1 deletion reduced the half-widths of action potentials and accelerated the deactivation kinetics of transient outward currents, suggesting that the CALHM1-associated conductance becomes activated during the repolarization phase of action potentials.NEW & NOTEWORTHY CALHM1 is an essential ion channel component of the ATP neurotransmitter release mechanism in type II taste bud cells. Its contribution to type II cell resting membrane properties and excitability is unknown. Nonselective voltage-gated currents, previously associated with ATP release, were absent in cells lacking CALHM1. Calhm1 deletion was without effects on resting membrane properties or voltage-gated Na+ and K+ channels but contributed modestly to the kinetics of action potentials.

Atrial Fibrillation in Hypertensive Heart Disease is Associated with Distinct Patterns of Electrical Remodeling in the Left and Right Atria

Atrial fibrillation (AF) is a major complication in hypertensive heart disease and has been linked to alterations in angiotensin II (AngII) signaling; however, the mechanisms underlying the increase in susceptibility to AF in the presence of AngII are poorly understood. The goal of this study was to characterize changes in atrial electrophysiology in mice treated chronically with AngII. AngII treated mice developed elevated blood pressure and cardiac hypertrophy in comparison to saline treated controls. In intracardiac programmed stimulation studies, 50% of AngII treated mice (n=10) were induced into AF vs. 0% of saline treated mice (P<0.05; n=9). P wave duration, a measure of atrial conduction, was prolonged in AngII treated mice (P<0.001; n=12 mice). Electrical remodeling following AngII treatment was investigated by patch-clamping atrial myocytes. In right atrial myocytes, AngII prolonged action potential (AP) duration compared to saline (P<0.05; n=9-11 myocytes) without changes in AP upstroke velocity (Vmax; P=0.66) or overshoot (OS; P=0.53). Strikingly, in left atrial myocytes AngII treatment resulted in greater AP prolongation (P<0.001; n=12-14 myocytes) and reductions (P<0.05) in AP Vmax and AP overshoot vs. saline controls. Voltage clamp studies demonstrate that in right atrial myocytes (n=21-26) the transient outward potassium current (Ito) was reduced (P<0.05) following AngII treatment. Similarly, 4-aminopyradine (4-AP; 100 µM) reduced right atrial K+ currents (P<0.05) indicating a reduction in the ultra-rapid delayed rectifier K+ current (IKur) after AngII treatment. In left atrial myocytes, (n=11-13) Ito and 4-AP sensitivity were both reduced (P<0.05) following AngII treatment. The reduction in Ito was larger (P<0.05) in left vs. right atrial myocytes. Our study demonstrates that chronic AngII treatment creates a substrate for AF in association with distinct electrophysiological alterations in right and left atrial myocytes.

Electrophysiological alterations in diaphragm muscle caused by abdominal ischemia-reperfusion

Ischemia-reperfusion injury is the major complication of abdominal aortic surgery, and it mainly affects the lower extremities and remote organs. In the present study, the electrophysiological alterations in diaphragm that underlie the post-operative respiratory dysfunction were investigated.Wistar Albino rats were randomly divided into two groups: SHAM (only laparotomy was performed) and IR (abdominal aorta was clamped for 30min and reperfused for 2h). Following the operational period diaphragm muscles were isolated and electrophysiological experiments were carried out in-vitro. 3nM Ryanodine application, Na+ and K+ current blockage (0.3mM 4-Aminopyridine and 127mM N-methyl-d-glukamine) experiments were also conducted to further reveal any alterations.Twitch and tetanic force were decreased significantly. Action potential overshoot, amplitude and area were increased while diaphragm muscle cells were found to be hyperpolarized significantly.Mechanical alterations were shown to be caused by deterioration of Ca++ homeostasis. At resting state, a decrease in persistent Na+ current was found. The reshaping of action potential, on the other hand, was shown to be due to altered kinetics of Na+ channels and delayed activation of voltage dependent K+ channels.

Low Somatic Sodium Conductance Enhances Action Potential Precision in Time-Coding Auditory Neurons.

Proper hearing depends on analyzing temporal aspects of sounds with high precision. Auditory neurons that specialize in precise temporal information have a suite of unusual intrinsic properties, including nonovershooting action potentials and few sodium channels in the soma. However, it was not clear how low sodium channel availability in the soma influenced the temporal precision of action potentials initiated in the axon initial segment. We studied this using dynamic clamp to mimic sodium channels in the soma, which yielded normal, overshooting action potentials. Increasing somatic sodium conductance had major negative consequences: synaptic activity evoked action potentials with lower fidelity, and the precision of coincidence detection was degraded. Thus, low somatic sodium channel availability appears to enhance fidelity and temporal precision.

Knockout of Cyclophilin-D Provides Partial Amelioration of Intrinsic and Synaptic Properties Altered by Mild Traumatic Brain Injury.

Mitochondria are central to cell survival and Ca2+ homeostasis due to their intracellular buffering capabilities. Mitochondrial dysfunction resulting in mitochondrial permeability transition pore (mPTP) opening has been reported after mild traumatic brain injury (mTBI). Cyclosporine A provides protection against the mPTP opening through its interaction with cyclophilin-D (CypD). A recent study has found that the extent of axonal injury after mTBI was diminished in neocortex in cyclophilin-D knockout (CypDKO) mice. Here we tested whether this CypDKO could also provide protection from the increased intrinsic and synaptic neuronal excitability previously described after mTBI in a mild central fluid percussion injury mice model. CypDKO mice were crossed with mice expressing yellow fluorescent protein (YFP) in layer V pyramidal neurons in neocortex to create CypDKO/YFP-H mice. Whole cell patch clamp recordings from axotomized (AX) and intact (IN) YFP+ layer V pyramidal neurons were made 1 and 2 days after sham or mTBI in slices from CypDKO/YFP-H mice. Both excitatory post synaptic currents (EPSCs) recorded in voltage clamp and intrinsic cellular properties, including action potential (AP), afterhyperpolarization (AHP), and depolarizing after potential (DAP) characteristics recorded in current clamp were evaluated. There was no significant difference between sham and mTBI for either spontaneous or miniature EPSC frequency, suggesting that CypDKO ameliorates excitatory synaptic abnormalities. There was a partial amelioration of intrinsic properties altered by mTBI. Alleviated were the increased slope of the AP frequency vs. injected current plot, the increased AP, AHP and DAP amplitudes. Other properties that saw a reversal that became significant in the opposite direction include the current rheobase and AP overshoot. The AP threshold remained depolarized and the input resistance remained increased in mTBI compared to sham. Additional altered properties suggest that the CypDKO likely has a direct effect on membrane properties, rather than producing a selective reduction of the effects of mTBI. These results suggest that inhibiting CypD after TBI is an effective strategy to reduce synaptic hyperexcitation, making it a continued target for potential treatment of network abnormalities.