Duration Of Afterhyperpolarization Research Articles

Slack K+ channels limit kainic acid-induced seizure severity in mice by modulating neuronal excitability and firing

Mutations of the Na+-activated K+ channel Slack (KCNT1) are associated with terrible epilepsy syndromes that already begin in infancy. Here we report increased severity of acute kainic acid-induced seizures in adult and juvenile Slack knockout mice (Slack−/−) in vivo. Fittingly, we find exacerbation of cell death following kainic acid exposure in organotypic hippocampal slices as well as dissociated hippocampal cultures from Slack−/− in vitro. Furthermore, in cultured Slack−/− neurons, kainic acid-triggered Ca2+ influx and K+ efflux as well as depolarization-induced tetrodotoxin-sensitive inward currents are higher compared to the respective controls. This apparent changes in ion homeostasis could possibly explain altered action potential kinetics of Slack−/− neurons: steeper rise slope, decreased threshold, and duration of afterhyperpolarization, which ultimately lead to higher action potential frequencies during kainic acid application or injection of depolarizing currents. Based on our data, we propose Slack as crucial gatekeeper of neuronal excitability to acutely limit seizure severity.

Diabetes mellitus differently affects electrical membrane properties of vagal afferent neurons of rats.

To study whether diabetes mellitus (DM) would cause electrophysiological alterations in nodose ganglion (NG) neurons, we used patch clamp and intracellular recording for voltage and current clamp configuration, respectively, on cell bodies of NG from rats with DM. Intracellular microelectrodes recording, according to the waveform of the first derivative of the action potential, revealed three neuronal groups (A0 , Ainf , and Cinf ), which were differently affected. Diabetes only depolarized the resting potential of A0 (from -55 to -44 mV) and Cinf (from -49 to -45 mV) somas. In Ainf neurons, diabetes increased action potential and the after-hyperpolarization durations (from 1.9 and 18 to 2.3 and 32 ms, respectively) and reduced dV/dtdesc (from -63 to -52 V s-1 ). Diabetes reduced the action potential amplitude while increasing the after-hyperpolarization amplitude of Cinf neurons (from 83 and -14 mV to 75 and -16 mV, respectively). Using whole cell patch clamp recording, we observed that diabetes produced an increase in peak amplitude of sodium current density (from -68 to -176 pA pF-1 ) and displacement of steady-state inactivation to more negative values of transmembrane potential only in a group of neurons from diabetic animals (DB2). In the other group (DB1), diabetes did not change this parameter (-58 pA pF-1 ). This change in sodium current did not cause an increase in membrane excitability, probably explainable by the alterations in sodium current kinetics, which are also induced by diabetes. Our data demonstrate that diabetes differently affects membrane properties of different nodose neuron subpopulations, which likely have pathophysiological implications for diabetes mellitus.

Inhibitory interneurons show early dysfunction in a SOD1 mouse model of amyotrophic lateral sclerosis.

Few studies in amyotrophic lateral sclerosis (ALS) measure effects of the disease on inhibitory interneurons synapsing onto motoneurons (MNs). However, inhibitory interneurons could contribute to dysfunction, particularly if altered before MN neuropathology, and establish a long-term imbalance of inhibition/excitation. We directly assessed excitability and morphology of glycinergic (GlyT2 expressing) ventral lumbar interneurons from SOD1G93AGlyT2eGFP (SOD1) and wild-type GlyT2eGFP (WT) mice on postnatal days 6-10. Patch clamp revealed dampened excitability in SOD1 interneurons, including depolarized persistent inward currents (PICs), increased voltage and current threshold for firing action potentials, along with a marginal decrease in afterhyperpolarization duration. Primary neurites of ventral SOD1 inhibitory interneurons were larger in volume and surface area than WT. GlyT2 interneurons were then divided into threesubgroups based on location: (1) interneurons within 100μm of the ventral white matter, where Renshaw cells (RCs) are located, (2) interneurons interspersed with MNs in lamina IX, and (3) interneurons in the intermediate ventral area including laminae VII and VIII. Ventral interneurons in the RC area were the most profoundly affected, exhibiting more depolarized PICs and larger primary neurites. Interneurons in lamina IX had depolarized PIC onset. In lamina VII-VIII, interneurons were least affected. In summary, inhibitory interneurons show very early region-specific perturbations poised to impact excitatory/inhibitory balance of MNs, modify motor output and provide early biomarkers of ALS. Therapeutics like riluzole that universally reduce CNS excitability could exacerbate the inhibitory dysfunction described here. KEY POINTS: Spinal inhibitory interneurons could contribute to amyotrophic lateral sclerosis (ALS) pathology, but their excitability has never been directly measured. We studied the excitability and morphology of glycinergic interneurons in early postnatal transgenic mice (SOD1G93A GlyT2eGFP). Interneurons were less excitable and had marginally smaller somas but larger primary neurites in SOD1mice. GlyT2 interneurons were analysed according to their localization within the ventral spinal cord. Interestingly, the greatest differences were observed in the most ventrally located interneurons. We conclude that inhibitory interneurons show presymptomatic changes that may contribute to excitatory/inhibitory imbalance in ALS.

Mathematical relationships between spinal motoneuron properties.

Our understanding of the behaviour of spinal alpha-motoneurons (MNs) in mammals partly relies on our knowledge of the relationships between MN membrane properties, such as MN size, resistance, rheobase, capacitance, time constant, axonal conduction velocity, and afterhyperpolarization duration. We reprocessed the data from 40 experimental studies in adult cat, rat, and mouse MN preparations to empirically derive a set of quantitative mathematical relationships between these MN electrophysiological and anatomical properties. This validated mathematical framework, which supports past findings that the MN membrane properties are all related to each other and clarifies the nature of their associations, is besides consistent with the Henneman's size principle and Rall's cable theory. The derived mathematical relationships provide a convenient tool for neuroscientists and experimenters to complete experimental datasets, explore the relationships between pairs of MN properties never concurrently observed in previous experiments, or investigate inter-mammalian-species variations in MN membrane properties. Using this mathematical framework, modellers can build profiles of inter-consistent MN-specific properties to scale pools of MN models, with consequences on the accuracy and the interpretability of the simulations.

First-recruited motor units adopt a faster phenotype in amyotrophic lateral sclerosis.

Amyotrophic lateral sclerosis (ALS) is an incurable neurodegenerative disorder of motor neurons, carrying a short survival. High-density motor unit recordings permit analysis of motor unit size (amplitude) and firing behaviour (afterhyperpolarization duration and muscle fibre conduction velocity). Serial recordings from biceps brachii indicated that motor units fired faster and with greater amplitude as disease progressed. First-recruited motor units in the latter stages of ALS developed characteristics akin to fast-twitch motor units, possibly as a compensatory mechanism for the selective loss of this motor unit subset. This process may become maladaptive, highlighting a novel therapeutic target to reduce motor unit vulnerability. Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder with a median survival of 3 years. We employed serial high-density surface electromyography (HDSEMG) to characterize voluntary and ectopic patterns of motor unit (MU) firing at different stages of disease. By distinguishing MU subtypes with variable vulnerability to disease, we aimed to evaluate compensatory neuronal adaptations that accompany disease progression. Twenty patients with ALS and five patients with benign fasciculation syndrome (BFS) underwent 1-7 assessments each. HDSEMG measurements comprised 30min of resting muscle and 1min of light voluntary activity from biceps brachii bilaterally. MU decomposition was performed by the progressive FastICA peel-off technique. Inter-spike interval, firing pattern, MU potential area, afterhyperpolarization duration and muscle fibre conduction velocity were determined. In total, 373 MUs (ALS=287; BFS=86) were identified from 182 recordings. Weak ALS muscles demonstrated a lower mean inter-spike interval (82.7ms) than strong ALS muscles (96.0ms; P=0.00919) and BFS muscles (95.3ms; P=0.0039). Mean MU potential area (area under the curve: 487.5 vs. 98.7μV ms; P<0.0001) and muscle fibre conduction velocity (6.2 vs. 5.1m/s; P=0.0292) were greater in weak ALS muscles than in BFS muscles. Purely fasciculating MUs had a greater mean MU potential area than MUs also under voluntary command (area under the curve: 679.6 vs. 232.4 μV ms; P=0.00144). These results suggest that first-recruited MUs develop a faster phenotype in the latter stages of ALS, likely driven by the preferential loss of vulnerable fast-twitch MUs. Inhibition of this potentially maladaptive phenotypic drift may protect the longevity of the MU pool, stimulating a novel therapeutic avenue.

Electrophysiological properties of maxillary trigeminal Aβ-afferent neurons of rats.

Aβ-afferents in maxillary or V2 trigeminal ganglion (TG) neurons are somatosensory neurons that may be involved in both non-nociceptive and nociceptive functions in orofacial regions. However, electrophysiological properties of these V2 trigeminal Aβ-afferent neurons have not been well characterized so far. Here, we used rat ex vivo trigeminal nerve preparations and applied patch-clamp recordings to large-sized V2 TG neurons to characterize their electrophysiological properties. All the cells recorded had afferent conduction velocities in the range of Aβ-afferent conduction speeds. However, these V2 trigeminal Aβ-afferent neurons displayed different action potential (AP) properties. APs showed fast kinetics in some cells but slow kinetics with shoulders in repolarization phases in other cells. Based on the derivatives of voltages in AP repolarization with time (dV/dt), we classified V2 trigeminal Aβ-afferent neurons into four types: type I, type II, type IIIa and type IIIb. Type I V2 trigeminal Aβ-afferent neurons had the largest dV/dt of repolarization, the fastest AP conduction velocities, the shortest AP and afterhyperpolarization (AHP) durations, and the highest AP success rates. In contrast, type IIIb V2 trigeminal Aβ-afferent neurons had the smallest dV/dt of AP repolarization, the slowest AP conduction velocities, the longest AP and AHP durations, and the lowest AP success rates. The type IIIb cells also had significantly lower voltage-activated K+ currents. For type II and type IIIa V2 trigeminal Aβ-afferent neurons, AP parameters were in the range between those of type I and type IIIb V2 trigeminal Aβ-afferent neurons. Our electrophysiological classification of V2 trigeminal Aβ-afferent neurons may be useful in future to study their non-nociceptive and nociceptive functions in orofacial regions.

Modeling bursting in neuronal networks using facilitation-depression and afterhyperpolarization

In the absence of inhibition, excitatory neuronal networks can alternate between bursts and interburst intervals (IBI), with heterogeneous length distributions. As this dynamic remains unclear, especially the durations of each epoch, we develop here a bursting model based on synaptic depression and facilitation that also accounts for afterhyperpolarization (AHP), which is a key component of IBI. The framework is a novel stochastic three dimensional dynamical system perturbed by noise: numerical simulations can reproduce a succession of bursts and interbursts. Each phase corresponds to an exploration of a fraction of the phase-space, which contains three critical points (one attractor and two saddles) separated by a two-dimensional stable manifold Σ. We show here that bursting is defined by long deterministic excursions away from the attractor, while IBI corresponds to escape induced by random fluctuations. We show that the variability in the burst durations, depends on the distribution of exit points located on Σ that we compute using WKB and the method of characteristics. Finally, to better characterize the role of several parameters such as the network connectivity or the AHP time scale, we compute analytically the mean burst and AHP durations in a linear approximation. To conclude the distribution of bursting and IBI could result from synaptic dynamics modulated by AHP.

Spinal motoneurones are intrinsically more responsive in the adult G93A SOD1mouse model of amyotrophic lateral sclerosis.

Although in vitro recordings using neonatal preparations from mouse models of amyotrophic lateral sclerosis (ALS) suggest increased motoneurone excitability, in vivo recordings in adult ALS mouse models have been conflicting. In adult G93A SOD1models, spinal motoneurones have previously been shown to have deficits in repetitive firing, in contrast to the G127X SOD1mouse model. Our in vivo intracellular recordings in barbiturate-anaesthetized adult male G93A SOD1mice reveal that the incidence of failure to fire with current injection was equally low in control and ALS mice (∼2%). We show that failure to fire repetitively can be a consequence of experimental protocol and should not be used alone to classify otherwise normal motoneurones as hypo-excitable. Motoneurones in the G93A SOD1mice showed an increased response to inputs, with lower rheobase, higher input-output gains and increased activation of persistent inward currents. In vitro studies from transgenic amyotrophic lateral sclerosis models have suggested an increased excitability of spinal motoneurones. However, in vivo intracellular recordings from adult amyotrophic lateral sclerosis mice models have produced conflicting findings. Previous investigations using barbiturate anaesthetized G93A SOD1mice have suggested that some motoneurones are hypo-excitable, defined by deficits in repetitive firing. Our own previous recordings in G127X SOD1mice using different anaesthesia, however, showed no repetitive firing deficits and increased persistent inward currents at symptom onset. These discrepancies may be a result of differences between models, symptomatic stage, anaesthesia or technical differences. To investigate this, we repeated our original experiments, but in adult male G93A SOD1mice, at both presymptomatic and symptomatic stages, under barbiturate anaesthesia. In vivo intracellular recordings from antidromically identified spinal motoneurones revealed that the incidence of failure to fire with current injection was equally low in control and G93A SOD1mice (∼2%). Motoneurones in G93A SOD1mice fired significantly more spontaneous action potentials. Rheobase was significantly lower and the input resistance and input-output gain were significantly higher in both presymptomatic and symptomatic G93A SOD1mice. This was despite a significant increase in the duration of the post-spike after-hyperpolarization in both presymptomatic and symptomatic G93A SOD1mice. Finally, evidence of increased activation of persistent inward currents was seen in both presymptomatic and symptomatic G93A SOD1mice. Our results do not confirm previous reports of hypo-excitability of spinal motoneurones in the G93A SOD1mouse and demonstrate that the motoneurones show an increased response to inputs.

Cutaneous Aβ-Non-nociceptive, but Not C-Nociceptive, Dorsal Root Ganglion Neurons Exhibit Spontaneous Activity in the Streptozotocin Rat Model of Painful Diabetic Neuropathy in vivo.

Diabetic peripheral neuropathic pain (DPNP) is the most devastating complication of diabetes mellitus. Unfortunately, successful therapy for DPNP remains a challenge because its pathogenesis is still elusive. However, DPNP is believed to be due partly to abnormal hyperexcitability of dorsal root ganglion (DRG) neurons, but the relative contributions of specific functional subtypes remain largely unknown. Here, using the strepotozotocin (STZ) rat model of DPNP induced by a STZ injection (60 mg/kg, i.p), and intracellular recordings of action potentials (APs) from DRG neurons in anesthetized rats, we examined electrophysiological changes in C-and Aβ-nociceptive and Aβ-low threshold mechanoreceptive (LTM) neurons that may contribute to DPNP. Compared with control, we found in STZ-rats with established pain hypersensitivity (5 weeks post-STZ) several significant changes including: (a) A 23% increase in the incidence of spontaneous activity (SA) in Aβ-LTMs (but not C-mechanosensitive nociceptors) that may cause dysesthesias/paresthesia suffered by DPNP patients, (b) membrane hyperpolarization and a ∼85% reduction in SA rate in Aβ-LTMs by Kv7 channel activation with retigabine (6 mg/kg, i.v.) suggesting that Kv7/M channels may be involved in mechanisms of SA generation in Aβ-LTMs, (c) decreases in AP duration and in duration and amplitude of afterhyperpolarization (AHP) in C-and/or Aβ-nociceptors. These faster AP and AHP kinetics may lead to repetitive firing and an increase in afferent input to the CNS and thereby contribute to DPNP development, and (d) a decrease in the electrical thresholds of Aβ-nociceptors that may contribute to their sensitization, and thus to the resulting hypersensitivity associated with DPNP.

Spreading Depolarization Facilitates the Transition of Neuronal Burst Firing from Interictal to Ictal State

The transition of neuronal burst firing from the interictal to ictal state contributes to seizure initiation in human temporal lobe epilepsy. The low-Mg2+ model of seizure is characterized by initial spontaneous interictal bursting events, which later developed into ictaform discharges. Both experimental and clinical studies point to a complex link between spreading depolarization (SD) and epileptiform field potentials (EFP), including SD-induced epileptic seizures. To investigate the mechanism of SD and EFP interactions, the effect of SD on the transition of interictal to ictal state in low-Mg2+ model of seizure was studied in the rat hippocampus in vitro. After the appearance of interictal activities, SD was elicited by local application of KCl. SD significantly increased the amplitude and duration of action potentials and after-hyperpolarization, and hyperpolarized the membrane potential. Furthermore, SD significantly increased the duration of interictal activities and the threshold potentials of interictal activities. In addition, SD significantly accelerated the transition from interictal to ictal state compared to the control tissues. Ictal activities after induction of SD exhibited a significantly longer duration. This study revealed that SD accelerates interictal-to-ictal transitions and facilitates development of ictaform discharges, possibly via the enhancement of neural synchronization, and points to the potential role of SD in seizure initiation.

\u03b1-Synuclein oligomers mediate the aberrant form of spike-induced calcium release from IP3 receptor

Emerging evidence implicates α-synuclein oligomers as potential culprits in the pathogenesis of Lewy body disease (LBD). Soluble oligomeric α-synuclein accumulation in cytoplasm is believed to modify neuronal activities and intraneural Ca2+ dynamics, which augment the metabolic burden in central neurons vulnerable to LBD, although this hypothesis remains to be fully tested. We evaluated how intracellular α-synuclein oligomers affect the neuronal excitabilities and Ca2+ dynamics of pyramidal neurons in neocortical slices from mice. Intracellular application of α-synuclein containing stable higher-order oligomers (αSNo) significantly reduced spike frequency during current injection, elongated the duration of spike afterhyperpolarization (AHP), and enlarged AHP current charge in comparison with that of α-synuclein without higher-order oligomers. This αSNo-mediated alteration was triggered by spike-induced Ca2+ release from inositol trisphosphate receptors (IP3R) functionally coupled with L-type Ca2+ channels and SK-type K+ channels. Further electrophysiological and immunochemical observations revealed that α-synuclein oligomers greater than 100 kDa were directly associated with calcium-binding protein 1, which is responsible for regulating IP3R gating. They also block Ca2+-dependent inactivation of IP3R, and trigger Ca2+-induced Ca2+ release from IP3R during multiple spikes. This aberrant machinery may result in intraneural Ca2+ dyshomeostasis and may be the molecular basis for the vulnerability of neurons in LBD brains.

PyMUS: Python-Based Simulation Software for Virtual Experiments on Motor Unit System.

We constructed a physiologically plausible computationally efficient model of a motor unit and developed simulation software that allows for integrative investigations of the input–output processing in the motor unit system. The model motor unit was first built by coupling the motoneuron model and muscle unit model to a simplified axon model. To build the motoneuron model, we used a recently reported two-compartment modeling approach that accurately captures the key cell-type-related electrical properties under both passive conditions (somatic input resistance, membrane time constant, and signal attenuation properties between the soma and the dendrites) and active conditions (rheobase current and afterhyperpolarization duration at the soma and plateau behavior at the dendrites). To construct the muscle unit, we used a recently developed muscle modeling approach that reflects the experimentally identified dependencies of muscle activation dynamics on isometric, isokinetic and dynamic variation in muscle length over a full range of stimulation frequencies. Then, we designed the simulation software based on the object-oriented programing paradigm and developed the software using open-source Python language to be fully operational using graphical user interfaces. Using the developed software, separate simulations could be performed for a single motoneuron, muscle unit and motor unit under a wide range of experimental input protocols, and a hierarchical analysis could be performed from a single channel to the entire system behavior. Our model motor unit and simulation software may represent efficient tools not only for researchers studying the neural control of force production from a cellular perspective but also for instructors and students in motor physiology classroom settings.

Bilateral changes in afterhyperpolarization duration of spinal motoneurones in post-stroke patients.

This paper extends the observations presented in the previously published work on the afterhyperpolarization (AHP) duration changes in motoneurones (MNs) on the paretic (more affected) side of 11 post-stroke patients by the same analysis on the non-paretic (less-affected) side. The estimated AHP duration for patients’ MNs supplying more-affected muscles was significantly longer than control values and the elongation decreased with patient age and disorder duration. For MNs supplying less-affected muscles, dependency of AHP duration on age was closer to the control data, but the scatter was substantially bigger. However, the AHP duration estimate of less-affected MNs tended to be longer than that of controls in the short time elapsed since the stroke, and shorter than controls in the long time. Our results thus suggest that the spinal MNs on both sides respond to the cerebral stroke rapidly with prolongation of AHP duration, which tends to normalize with time, in line with functional recovery. This suggestion is in concert with the published research on post-stroke changes in brain hemispheres. To our knowledge, these dependencies have never been investigated before. Since the number of our data was limited, the observed trends should be verified in a larger sample of patients and such a verification could take into account the suggestions for data analysis that we provide in this paper. Our data are in line with the earlier published research on MN firing characteristics post-stroke and support the conclusion that the MUs of the muscles at the non-paretic side are also affected and cannot be considered a suitable control for the MUs on the paretic side.

Camphor elicits epileptiform discharges in snail neurons: The role of ion channels modulation

Generalized convulsion is one of the most prominent manifestations of camphor toxicity, but the mechanism underlying this effect has not been elucidated. Here, we examined the excitatory and epileptogenic action of camphor in snail neurons and studied the cellular and molecular mechanisms that are involved to these effects. The spontaneous activities of neurons from subesophageal ganglia of snail Caucasotachea atrolabiata were recorded using single-electrode current clamp under control condition and after exposure to different concentrations of camphor and ion channel blockers. Under control condition, the studied neurons showed regularly spaced spontaneous action potentials. Exposure to low concentration of camphor (0.25mM) reduced the duration of afterhyperpolarization and disrupted the spontaneous rhythmic activity, which was evidenced by an increase in the coefficient of variation of interspike intervals. The medium concentration of camphor (0.5mM) induced more disruption in the precision of spontaneous action potential and increased the frequency of firing along with a reduction of action potential falling slope and afterhyperpolarization. Neurons showed paroxysmal depolarization shift and burst firing after exposure to camphor at high concentration (1.5mM). We found that the blockade of K+ channels and upregulation of Ca2+ inward currents is essential for camphor-induced epileptiform activity, but the Na+ currents and ion channel phosphorylation with protein kinases A and C are not required. This work provided novel evidence at cellular and subcellular level that the modulation of ion channels, especially direct inhibition of K+ channels, is mechanistically involved to proconvulsive action of camphor.

Expression and function of a CP339,818-sensitive K⁺ current in a subpopulation of putative nociceptive neurons from adult mouse trigeminal ganglia.

Trigeminal ganglion (TG) neurons are functionally and morphologically heterogeneous, and the molecular basis of this heterogeneity is still not fully understood. Here we describe experiments showing that a subpopulation of neurons expresses a delayed-rectifying K(+) current (IDRK) with a characteristically high (nanomolar) sensitivity to the dihydroquinoline CP339,818 (CP). Although submicromolar CP has previously been shown to selectively block Kv1.3 and Kv1.4 channels, the CP-sensitive IDRK found in TG neurons could not be associated with either of these two K(+) channels. It could neither be associated with Kv2.1 channels homomeric or heteromerically associated with the Kv9.2, Kv9.3, or Kv6.4 subunits, whose block by CP, tested using two-electrode voltage-clamp recordings from Xenopus oocytes, resulted in the low micromolar range, nor to the Kv7 subfamily, given the lack of blocking efficacy of 3 μM XE991. Within the group of multiple-firing neurons considered in this study, the CP-sensitive IDRK was preferentially expressed in a subpopulation showing several nociceptive markers, such as small membrane capacitance, sensitivity to capsaicin, and slow afterhyperpolarization (AHP); in these neurons the CP-sensitive IDRK controls the membrane resting potential, the firing frequency, and the AHP duration. A biophysical study of the CP-sensitive IDRK indicated the presence of two kinetically distinct components: a fast deactivating component having a relatively depolarized steady-state inactivation (IDRKf) and a slow deactivating component with a more hyperpolarized V1/2 for steady-state inactivation (IDRKs).

Electrophysiological characteristics of IB4-negative TRPV1-expressing muscle afferent DRG neurons

Muscle afferent neurons that express transient receptor potential vanilloid type I (TRPV1) are responsible for muscle pain associated with tissue acidosis. We have previously found that TRPV1 of isolectin B4 (IB4)-negative muscle nociceptors plays an important role in the acid-induced hyperalgesic priming and the development of chronic hyperalgesia in a mouse model of fibromyalgia. To understand the electrophysiological properties of the TRPV1-expressing muscle afferent neurons, we used whole-cell patch clamp recording to study the acid responsiveness and action potential (AP) configuration of capsaicin-sensitive neurons innervating to gastrocnemius muscle. Here we showed that IB4-negative TRPV1-expressing muscle afferent neurons are heterogeneous in terms of cell size, resting membrane potential, AP configuration, tetrodotoxin (TTX)-resistance, and acid-induced current (Iacid), as well as capsaicin-induced current (Icap). TRPV1-expressing neurons were all acid-sensitive and could be divided into two acid-sensitive groups depending on an acid-induced sustained current (type I) or an acid-induced biphasic ASIC3-like current (type II). Type I TRPV1-expressing neurons were distinguishable from type II TRPV1-expressing neurons in AP overshoot, after-hyperpolarization duration, and all Iacid parameters, but not in AP threshold, TTX-resistance, resting membrane potential, and Icap parameters. These differential biophysical properties of TRPV1-expressing neurons might partially annotate their different roles involved in the development and maintenance of chronic muscle pain.

Changes in motoneuron afterhyperpolarization duration in stroke survivors.

Hemispheric brain injury resulting from a stroke is often accompanied by muscle weakness in limbs contralateral to the lesion. In the present study, we investigated whether weakness in contralesional hand muscle in stroke survivors is partially attributable to alterations in motor unit activation, including alterations in firing rate modulation range. The afterhyperpolarization (AHP) potential of a motoneuron is a primary determinant of motoneuron firing rate. We examined differences in AHP duration in motoneurons innervating paretic and less impaired (contralateral) limb muscles of hemiparetic stroke survivors as well as in control subjects. A novel surface EMG (sEMG) electrode was used to record motor units from the first dorsal interosseous muscle. The sEMG data were subsequently decomposed to derive single-motor unit events, which were then utilized to produce interval (ISI) histograms of the motoneuron discharges. A modified version of interval death rate (IDR) analysis was used to estimate AHP duration. Results from data analyses performed on both arms of 11 stroke subjects and in 7 age-matched control subjects suggest that AHP duration is significantly longer for motor units innervating paretic muscle compared with units in contralateral muscles and in units of intact subjects. These results were supported by a coefficient of variation (CV) analysis showing that paretic motor unit discharges have a lower CV than either contralateral or control units. This study suggests that after stroke biophysical changes occur at the motoneuron level, potentially contributing to lower firing rates and potentially leading to less efficient force production in paretic muscles.

Afterhyperpolarization of human motoneurons firing double and triple discharges.

During isometric voluntary contractions of a healthy human muscle, motoneurons (MNs) fire usually with low mean rates, rarely exceeding 25/s (e.g., Garland and Griffin, 1999). However, there are MNs, which sometimes fire double discharges (doublets) with interspike interval (ISI) of few ms. This is observed seldom in normal MNs (e.g., Denslow, 1948; Kudina, 1974; Bawa and Calancie, 1983) but does so more often in neuromuscular disorders (Partanen, 1978; Kostera-Pruszczyk et al., 2002; Piotrkiewicz et al., 2008), where it is considered to be an early sign of MN dysfunction (Rowinska-Marcinska et al., 1999). Our special attention is paid to the so-called “true” doublets recorded in some MNs under conditions of constant synaptic drive, as opposed to those recorded during rapid contractions (Bawa and Calancie, 1983; Kudina and Andreeva, 2013a). Initial doublets, observed during repetitive movements such as locomotion (Zajac and Young, 1980; Hennig and Lomo, 1987) or respiration (Kirkwood and Munson, 1996), where the synaptic drive undergoes periodic changes, also cannot be classified as “true” doublets. For the sake of clarity, the adjective “true” will be omitted in the further text. It has been observed that not each human MN is capable of firing doublets (Kudina, 1974; Bawa and Calancie, 1983). By the analogy to animal studies, it was hypothesized that doublets may be generated only in these MNs, which possess the delayed depolarization (DD, Granit et al., 1963; Kernell, 1964; Calvin, 1974) with a prominent hump that may spontaneously cross the firing threshold and evoke an extra spike (Nelson and Burke, 1967; Calvin, 1973). This hypothesis was verified by Kudina and Churikova (1990), who tested the changes in excitability of human motoneurons within ISI by studying the responses of single MNs to the stimulation of Ia afferents. In MNs capable of firing doublets they revealed the period of increased excitability during the first 15 ms after regular discharge, which corresponded well to the duration of DD observed in animal experiments (Granit et al., 1963; Kernell, 1964; Nelson and Burke, 1967; Calvin, 1973). Nowadays, DD is widely accepted as the mechanism responsible for doublet generation in human MNs (e.g., Kudina, 1974; Bawa and Calancie, 1983; Kudina and Churikova, 1990; Garland and Griffin, 1999). However, it can explain only the origin of occasional doublets, interspersed in regular rhythmic MN activity. Repetitive doublets, i.e., series of doublet-postdoublet ISIs, require some additional mechanism, which would support the DD hump, shown to disappear during MN rhythmic firing (Granit et al., 1963; Calvin and Loeser, 1975). It was recently hypothesized that repetitive doublet firing may be related to plateau potentials (Kudina and Andreeva, 2010, 2013a). It is still under debate, whether DD is related to other characteristics of a MN, measurable in human experiments. Kernell (1964) noted that DD with a definite hump had a tendency to be more common in MNs with short afterhyperpolarization (AHP), although it could occur with an AHP of any duration. Also in humans, MNs capable of firing doublets were more often found in the fast muscles (Bawa and Calancie, 1983; Kudina and Alexeeva, 1992) than in the slower ones (Andreassen and Rosenfalck, 1980; Kudina and Andreeva, 2013b). For the long time it was believed that the MNs supplying one of the most often investigated slow human muscle, the soleus, are devoid of DD. However, it has been recently shown that also MNs supplying human soleus may fire doublets (Piotrkiewicz et al., 2013), which seems to question Kernell's finding. In the present paper we will compare the estimates of AHP duration of MNs capable and not capable of firing double discharges and comment on this controversy. We will also propose a hypothesis on the AHP-related mechanism underlying firing of triplets, which can be observed in some doublet firing MNs.

Spreading depression triggers ictaform activity in partially disinhibited neuronal tissues

There is unequivocal electrophysiological evidence that spreading depression (SD) can trigger epileptiform field potentials. In vitro experiments on human brain tissues indicated that γ-aminobutyric acid (GABA)-mediated inhibition prevented this process. Intra- and extracellular recordings of bioelectrical activities were performed in the rodent neocortex, hippocampus and amygdala after perfusion of low concentrations of the GABAA antagonist bicuculline and induction of SD by KCl application. Induction of SD in combined amygdala–hippocampus–cortex slices pre-treated with low concentration of bicuculline triggered epileptiform burst discharges in cortical as well as subcortical brain structures. Propagation of SD significantly depolarized the membrane, decreased the amplitude and duration of action potentials (APs) and after-hyperpolarization as well as the neuronal membrane input resistance and the amplitude of threshold potentials. Ten to twenty minutes after induction of SD, the pattern of APs changed from regular firing to a series of APs riding on an underlying paroxysmal depolarization shift before the appearance of typical ictaform activities. Changes of characteristic features of APs occurred after SD persisted during the appearance of epileptiform activities. These results indicate that SD increases neuronal excitability and facilitates synchronization of neuronal discharges in the presence of partial disinhibition of neuronal tissues. Our findings might explain the occurrence of seizures in neurological disorders with partial impairment of inhibitory tone, such as brain ischemia and epilepsy.

Kv2 dysfunction after peripheral axotomy enhances sensory neuron responsiveness to sustained input

Peripheral nerve injuries caused by trauma are associated with increased sensory neuron excitability and debilitating chronic pain symptoms. Axotomy-induced alterations in the function of ion channels are thought to largely underlie the pathophysiology of these phenotypes. Here, we characterise the mRNA distribution of Kv2 family members in rat dorsal root ganglia (DRG) and describe a link between Kv2 function and modulation of sensory neuron excitability. Kv2.1 and Kv2.2 were amply expressed in cells of all sizes, being particularly abundant in medium-large neurons also immunoreactive for neurofilament-200. Peripheral axotomy led to a rapid, robust and long-lasting transcriptional Kv2 downregulation in the DRG, correlated with the onset of mechanical and thermal hypersensitivity. The consequences of Kv2 loss-of-function were subsequently investigated in myelinated neurons using intracellular recordings on ex vivo DRG preparations. In naïve neurons, pharmacological Kv2.1/Kv2.2 inhibition by stromatoxin-1 (ScTx) resulted in shortening of action potential (AP) after-hyperpolarization (AHP). In contrast, ScTx application on axotomized neurons did not alter AHP duration, consistent with the injury-induced Kv2 downregulation. In accordance with a shortened AHP, ScTx treatment also reduced the refractory period and improved AP conduction to the cell soma during high frequency stimulation. These results suggest that Kv2 downregulation following traumatic nerve lesion facilitates greater fidelity of repetitive firing during prolonged input and thus normal Kv2 function is postulated to limit neuronal excitability. In summary, we have profiled Kv2 expression in sensory neurons and provide evidence for the contribution of Kv2 dysfunction in the generation of hyperexcitable phenotypes encountered in chronic pain states.