Excitability Of Cortical Neurons Research Articles

DV21 decreases excitability of cortical pyramidal neurons and acts in epilepsy

Epilepsy is one of the most common neurological disorders and the administration of antiepileptic drugs (AEDs) is the most common treatment. Although there are more than 15 AEDs available, a third of epilepsy patients remain refractory to available drugs, so novel effective drugs are needed. Here, we found that DV21, which is a natural triterpenoid compound extracted from plants of the Asclepiadaceae family, significantly decreased the incidence and stages of seizures in three classical drug-induced acute seizure models in C57BL/6 mice. Furthermore, we also found that the antiepileptic effect of DV21 might be partly mediated through reducing the excitability of cortical pyramidal neurons by increasing M current, which are low-threshold non-inactivating voltage-gated potassium currents. Moreover, the application of XE991, an inhibitor of M current, could block most the antiepileptic effect of DV21. Taken together, our results indicated that DV21 might be a novel leading compound for the treatment of epilepsy.

Homeostatic Changes in GABA and Glutamate Receptors on Excitatory Cortical Neurons during Sleep Deprivation and Recovery.

Neuronal activity is regulated in a homeostatic manner through changes in inhibitory GABA and excitatory glutamate (Glu) AMPA (A) receptors (GluARs). Using immunofluorescent staining, we examined whether calcium/calmodulin-dependent protein kinase IIα (CaMKIIα)-labeled (+) excitatory neurons in the barrel cortex undergo such homeostatic regulation following enforced waking with associated cortical activation during the day when mice normally sleep the majority of the time. Sleep deprived mice were prevented from falling asleep by unilateral whisker stimulation and sleep recovery (SR) mice allowed to sleep freely following deprivation. In parallel with changes in c-Fos reflecting changes in activity, (β2-3 subunits of) GABAA Rs were increased on the membrane of CaMKIIα+ neurons with enforced waking and returned to baseline levels with SR in barrel cortex on sides both contra- and ipsilateral to the whisker stimulation. The GABAAR increase was correlated with increased gamma electroencephalographic (EEG) activity across conditions. On the other hand, (GluA1 subunits of) AMPA Rs were progressively removed from the membrane of CaMKIIα+ neurons by (Rab5+) early endosomes during enforced waking and returned to the membrane by (Rab11+) recycling endosomes during SR. The internalization of the GluA1Rs paralleled the expression of Arc, which mediates homeostatic regulation of AMPA receptors through an endocytic pathway. The reciprocal changes in GluA1Rs relative to GABAARs suggest homeostatic down-scaling during enforced waking and sensory stimulation and restorative up-scaling during recovery sleep. Such homeostatic changes with sleep-wake states and their associated cortical activities could stabilize excitability and activity in excitatory cortical neurons.

In Utero Electroporation Approaches to Study the Excitability of Neuronal Subpopulations and Single-cell Connectivity.

The nervous system is composed of an enormous range of distinct neuronal types. These neuronal subpopulations are characterized by, among other features, their distinct dendritic morphologies, their specific patterns of axonal connectivity, and their selective firing responses. The molecular and cellular mechanisms responsible for these aspects of differentiation during development are still poorly understood. Here, we describe combined protocols for labeling and characterizing the structural connectivity and excitability of cortical neurons. Modification of the in utero electroporation (IUE) protocol allows the labeling of a sparse population of neurons. This, in turn, enables the identification and tracking of the dendrites and axons of individual neurons, the precise characterization of the laminar location of axonal projections, and morphometric analysis. IUE can also be used to investigate changes in the excitability of wild-type (WT) or genetically modified neurons by combining it with whole-cell recording from acute slices of electroporated brains. These two techniques contribute to a better understanding of the coupling of structural and functional connectivity and of the molecular mechanisms controlling neuronal diversity during development. These developmental processes have important implications on axonal wiring, the functional diversity of neurons, and the biology of cognitive disorders.

P193 Effect of transcranial direct current stimulation (tDCS) on sleep-dependent memory consolidation in healthy young subjects

Transcranial direct current stimulation (tDCS) is a method to stimulate the brain in a noninvasive manner leading either to an increased (anodal stimulation) or a decreased (cathodal stimulation) excitability of cortical neurons. Slow oscillations with a frequency below 1 Hz are assumed to be associated with sleep-dependent consolidation of declarative memories by exerting a temporal synchronizing influence on hippocampal reactivations and thalamic spindle activity. Many studies have revealed that offline consolidation during sleep varies with age showing better performances in the young compared to the elderly. A study in young subjects provided evidence of a beneficial impact of an anodal slow oscillation tDCS (0.75 Hz) during early SWS-rich on declarative memory but not on procedural memory. These results could not be confirmed in a study on elderly subjects (Eggert et al., 2013). The present study aims to replicate the results, which have been observed in younger subjects (Marshal et al., 2006). To be able to test an effect of 0.65 with a power of 0.8 and a double-sided alpha 2 ) is triggered after the first 8 consecutive epochs of NREM sleep 2 immediately before the first occurrence of slow wave sleep. The total 25 ′ 8 ″ stimulation period is divided into five 5 ′ 16 ″ intervals, each of them followed by 1 min break without stimulation. To test for a stimulation-dependent effect on offline consolidation during sleep a declarative memory task (word pair test) and a procedural memory task (finger-tapping) have to be completed before sleep and after awakening. Furthermore, sleep stages were scored and EEG power and spindle densities were computed. Statistical analysis was based upon comparisons between the tDCS and the sham stimulation condition. Currently 18 subjects (10 males and 8 females) are already enrolled. First results will be presented at the meeting.

P122 Contribution of integrated somatosensory and auditory inputs to the cortical response evoked by transcranial magnetic stimulation: A sham TMS-EEG study

Transcranial Magnetic Stimulation (TMS) can effectively stimulate non-invasively the human cortex. The TMS-evoked cortical response can be recorded with electroencephalography (EEG). However, TMS also stimulates our senses by stimulating peripheral trigeminal nerve fibers and creating a loud click. This implies that the TMS-evoked EEG response not only reflects neural activity induced by transcranial excitations of neurons but also neural activity due to somatosensory and auditory stimulation. To characterize the contribution of multisensory peripheral stimulation to TMS-evoked cortical potentials (TEPs), we recorded the evoked EEG response caused by a somato-auditory sham condition which mimicked real TMS. In 20 healthy individuals, TMS was delivered with a figure-of-eight coil over two target sites (posterior parietal cortex and superior frontal gyrus) using two different coil orientations, perpendicular or parallel to sulcus orientation. The sham condition comprised of simultaneous somatosensory and auditory stimulation over the same hotspots as for real TMS. Somatosensory stimulation was achieved via cutaneous electrical stimulation of the scalp, while the TMS coil was used for auditory stimulation ensuring no electric field was induced in the brain by physically separating the coil from the scalp. EEG was acquired with a 61-channel TMS-compatible EEG system. While the early cortical potentials evoked by real or sham TMS differed, TEPs were closely matched in terms of shape and spatial distribution for late components of the evoked EEG responses 70–200 ms after the TMS pulse (see Fig. 1 Download : Download high-res image (666KB) Download : Download full-size image ). This was also the case for the N100 which has been commonly attributed to TMS-induced cortical inhibition. The resemblance of the EEG responses evoked by real and sham TMS challenges the notion that TEPs are mainly reflecting transcranial excitation of cortical neurons. This work has been funded by the Novo Nordisk Foundation Interdisciplinary Synergy Program 2014 [“Biophysically adjusted state-informed cortex stimulation (BASICS); Grant No. NNF14OC0011413].

P243 Investigating the mechanisms of repetitive transcranial magnetic stimulation using mouse motor learning paradigms and in vivo 2 photon imaging

Introduction Modulation of cortical plasticity with repetitive transcranial magnetic stimulation (rTMS) has become a popular method of neuromodulation in both clinical and non-clinical populations. In particular, complex pattern rTMS such as intermittent theta burst stimulation (iTBS) has been shown to induce long lasting effects. However, the biological mechanisms underpinning rTMS induced plasticity remains poorly understood. Objectives Rodent models offer the potential to investigate not only the behavioural, but also the structural and molecular mechanisms induced with stimulation. This study examined changes in skilled motor behaviour and structural reorganisation at the synaptic level following iTBS to the motor cortex. Methods and materials We use a rodent-specific TMS circular coil (8 mm outer diameter) to deliver iTBS (600 pulses) over the motor cortex of awake adult male mice. For motor learning paradigms, C57Bl6/J mice receive daily iTBS or sham immediately prior to ( n = 16 per group) or after ( n = 8 per group) skilled pellet-reaching training for 10 days. In a separate group; Thy1-GFPM mice ( n = 5) underwent cranial window insertion overlying the right motor cortex to enable visualisation of excitatory cortical neurons in the upper layers of the motor cortex. Images of synaptic structures were collected at regular intervals before and after iTBS and analysed for alterations in connectivity resulting from stimulation. Results Priming iTBS increased skill accuracy by 8.5% ± 3.4 ( p = 0.02) but did not alter the rate of learning ( p = 0.24). Preliminary analysis shows that consolidation iTBS did not alter skill accuracy ( p = 0.25) or the rate of learning ( p = 0.12). A single session of iTBS decreased dendritic spine density at 24 h (−7% p = 0.015) and 48 h (−9% p = 0.001) post-stimulation which returned to baseline levels by 7 days post-stimulation ( p = 0.2). Conclusions These results show that rTMS induces both behavioural and structural plasticity in the motor cortex. Priming iTBS had a non-homeostatic interaction with motor behaviour whilst iTBS alone transiently decreased dendritic spine density. These result aid in the understanding of rTMS induced plasticity mechanisms which is essential in the optimisation of rTMS to treat neurological disease and disorders.

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.

Optogenetic Restoration of Disrupted Slow Oscillations Halts Amyloid Deposition and Restores Calcium Homeostasis in an Animal Model of Alzheimer's Disease.

Slow oscillations are important for consolidation of memory during sleep, and Alzheimer’s disease (AD) patients experience memory disturbances. Thus, we examined slow oscillation activity in an animal model of AD. APP mice exhibit aberrant slow oscillation activity. Aberrant inhibitory activity within the cortical circuit was responsible for slow oscillation dysfunction, since topical application of GABA restored slow oscillations in APP mice. In addition, light activation of channelrhodopsin-2 (ChR2) expressed in excitatory cortical neurons restored slow oscillations by synchronizing neuronal activity. Driving slow oscillation activity with ChR2 halted amyloid plaque deposition and prevented calcium overload associated with this pathology. Thus, targeting slow oscillatory activity in AD patients might prevent neurodegenerative phenotypes and slow disease progression.

CDYL Deficiency Disrupts Neuronal Migration and Increases Susceptibility to Epilepsy

During brain development, the correct migration of newborn neurons is one of the determinants of circuit formation, and neuronal migration defects may lead to neurological and psychiatric disorders. The molecular mechanisms underlying neuronal migration and related disorders are poorly understood. Here, we report that Chromodomain Y-like (CDYL) is critical for neuronal migration in mice. Knocking down CDYL caused neuronal migration defects and disrupted both mobility and multipolar-to-bipolar transition of migrating neurons. We find that CDYL regulates neuronal migration by transcriptionally repressing RhoA. In addition, CDYL deficiency increased the excitability of cortical pyramidal neurons and the susceptibility of mice to convulsant-induced seizures. These results demonstrate that CDYL is a regulator of neuronal migration and shed light on the pathogenesis of seizure-related neurodevelopmental disorders.

Dual face of axonal inhibitory inputs in the modulation of neuronal excitability in cortical pyramidal neurons.

Limited by the tiny structure of axons, the effects of these axonal hyperpolarizing inputs on neuronal activity have not been directly elucidated. Here, we imitated these processes by simultaneously recording the activities of the somas and proximal axons of cortical pyramidal neurons. We found that spikes and subthreshold potentials propagate between somas and axons with high fidelity. Furthermore, inhibitory inputs on axons have opposite effects on neuronal activity according to their temporal integration with upstream signals. Concurrent with somatic depolarization, inhibitory inputs on axons decrease neuronal excitability and impede spike generation. In addition, following action potentials, inhibitory inputs on an axon increase neuronal spike capacity and improve spike precision. These results indicate that inhibitory inputs on proximal axons have dual regulatory functions in neuronal activity (suppression or facilitation) according to neuronal network patterns.

Differential Inputs to the Perisomatic and Distal-Dendritic Compartments of VIP-Positive Neurons in Layer 2/3 of the Mouse Barrel Cortex.

The recurrent network composed of excitatory and inhibitory neurons is fundamental to neocortical function. Inhibitory neurons in the mammalian neocortex are molecularly diverse, and individual cell types play unique functional roles in the neocortical microcircuit. Recently, vasoactive intestinal polypeptide-positive (VIP+) neurons, comprising a subclass of inhibitory neurons, have attracted particular attention because they can disinhibit pyramidal cells through inhibition of other types of inhibitory neurons, such as parvalbumin- (PV+) and somatostatin-positive (SOM+) inhibitory neurons, promoting sensory information processing. Although VIP+ neurons have been reported to receive synaptic inputs from PV+ and SOM+ inhibitory neurons as well as from cortical and thalamic excitatory neurons, the somatodendritic localization of these synaptic inputs has yet to be elucidated at subcellular spatial resolution. In the present study, we visualized the somatodendritic membranes of layer (L) 2/3 VIP+ neurons by injecting a newly developed adeno-associated virus (AAV) vector into the barrel cortex of VIP-Cre knock-in mice, and we determined the extensive ramification of VIP+ neuron dendrites in the vertical orientation. After immunohistochemical labeling of presynaptic boutons and postsynaptic structures, confocal laser scanning microscopy revealed that the synaptic contacts were unevenly distributed throughout the perisomatic (<100 μm from the somata) and distal-dendritic compartments (≥100 μm) of VIP+ neurons. Both corticocortical and thalamocortical excitatory neurons preferentially targeted the distal-dendritic compartment of VIP+ neurons. On the other hand, SOM+ and PV+ inhibitory neurons preferentially targeted the distal-dendritic and perisomatic compartments of VIP+ neurons, respectively. Notably, VIP+ neurons had few reciprocal connections. These observations suggest different inhibitory effects of SOM+ and PV+ neuronal inputs on VIP+ neuron activity; inhibitory inputs from SOM+ neurons likely modulate excitatory inputs locally in dendrites, while PV+ neurons could efficiently interfere with action potential generation through innervation of the perisomatic domain of VIP+ neurons. The present study, which shows a precise configuration of site-specific inputs, provides a structural basis for the integration mechanism of synaptic inputs to VIP+ neurons.

Establishment of high reciprocal connectivity between clonal cortical neurons is regulated by the Dnmt3b DNA methyltransferase and clustered protocadherins.

BackgroundThe specificity of synaptic connections is fundamental for proper neural circuit function. Specific neuronal connections that underlie information processing in the sensory cortex are initially established without sensory experiences to a considerable extent, and then the connections are individually refined through sensory experiences. Excitatory neurons arising from the same single progenitor cell are preferentially connected in the postnatal cortex, suggesting that cell lineage contributes to the initial wiring of neurons. However, the postnatal developmental process of lineage-dependent connection specificity is not known, nor how clonal neurons, which are derived from the same neural stem cell, are stamped with the identity of their common neural stem cell and guided to form synaptic connections.ResultsWe show that cortical excitatory neurons that arise from the same neural stem cell and reside within the same layer preferentially establish reciprocal synaptic connections in the mouse barrel cortex. We observed a transient increase in synaptic connections between clonal but not nonclonal neuron pairs during postnatal development, followed by selective stabilization of the reciprocal connections between clonal neuron pairs. Furthermore, we demonstrate that selective stabilization of the reciprocal connections between clonal neuron pairs is impaired by the deficiency of DNA methyltransferase 3b (Dnmt3b), which determines DNA-methylation patterns of genes in stem cells during early corticogenesis. Dnmt3b regulates the postnatal expression of clustered protocadherin (cPcdh) isoforms, a family of adhesion molecules. We found that cPcdh deficiency in clonal neuron pairs impairs the whole process of the formation and stabilization of connections to establish lineage-specific connection reciprocity.ConclusionsOur results demonstrate that local, reciprocal neural connections are selectively formed and retained between clonal neurons in layer 4 of the barrel cortex during postnatal development, and that Dnmt3b and cPcdhs are required for the establishment of lineage-specific reciprocal connections. These findings indicate that lineage-specific connection reciprocity is predetermined by Dnmt3b during embryonic development, and that the cPcdhs contribute to postnatal cortical neuron identification to guide lineage-dependent synaptic connections in the neocortex.Electronic supplementary materialThe online version of this article (doi:10.1186/s12915-016-0326-6) contains supplementary material, which is available to authorized users.

Postnatal Administration of Dizocilpine Inhibits Neuronal Excitability in PFC and Induces Social Deficits Detected by MiceProfiler.

Schizophrenia is a devastating mental disease with social deficit as its core component of negative symptoms, which could be induced in rodents by dizocilpine (MK-801), a noncompetitive NMDA receptor antagonist. NMDA receptors are highly expressed during the postnatal period. However, less attention has been paid to the effects of postnatal MK-801 administration on social interaction. In this study, we evaluated the effects of postnatal administration of MK-801 on social interaction and explored the possible mechanisms. Postnatal day-7 mice were intraperitoneally injected with MK-801 twice daily for 5days, and their social interaction repertoire was monitored by a computerized video in the 10th week. The contact event, relative position event, stop-state, and dynamic event were analyzed with MiceProfiler automatic idTracker system. The results showed that MK-801 reduced the number of the contact events, relative position events, and stop-states, while increased the number and duration of dynamic events. These changes implied that MK-801-injected mice had indifference and lower motivation in social interaction and could be a useful model for studies on the social deficit of schizophrenia. The prefrontal cortex is the key region for social interaction behaviors. Slice patch clamp was performed to analyze the cellular excitability of prefrontal cortical neurons after postnatal treatment with MK-801 in mice. The results demonstrated that MK-801 injection reduced the frequency and amplitude of action potentials, but increased the frequency of miniature inhibitory postsynaptic currents. These data illustrated that the excitability of neurons in the prefrontal cortex was inhibited. Finally, immunoblotting data demonstrated that MK-801 significantly decreased the levels of sirtuin 1 (SIRT1) and phosphorylated protein kinase B (p-PKB) in the prefrontal cortex (both P<0.05). Taken together, our results indicated that administration of MK-801 to postnatal mice induces social interaction deficits possibly due to inhibiting the neuronal excitability and decreasing the levels of SIRT1 and p-PKB in the prefrontal cortex.

MEF2C regulates cortical inhibitory and excitatory synapses and behaviors relevant to neurodevelopmental disorders.

Numerous genetic variants associated with MEF2C are linked to autism, intellectual disability (ID) and schizophrenia (SCZ) - a heterogeneous collection of neurodevelopmental disorders with unclear pathophysiology. MEF2C is highly expressed in developing cortical excitatory neurons, but its role in their development remains unclear. We show here that conditional embryonic deletion of Mef2c in cortical and hippocampal excitatory neurons (Emx1-lineage) produces a dramatic reduction in cortical network activity in vivo, due in part to a dramatic increase in inhibitory and a decrease in excitatory synaptic transmission. In addition, we find that MEF2C regulates E/I synapse density predominantly as a cell-autonomous, transcriptional repressor. Analysis of differential gene expression in Mef2c mutant cortex identified a significant overlap with numerous synapse- and autism-linked genes, and the Mef2c mutant mice displayed numerous behaviors reminiscent of autism, ID and SCZ, suggesting that perturbing MEF2C function in neocortex can produce autistic- and ID-like behaviors in mice.

A Postsynaptic Role for Short-Term Neuronal Facilitation in Dendritic Spines.

Synaptic plasticity is a fundamental component of information processing in the brain. Presynaptic facilitation in response to repetitive stimuli, often referred to as paired-pulse facilitation (PPF), is a dominant form of short-term synaptic plasticity. Recently, an additional cellular mechanism for short-term facilitation, short-term postsynaptic plasticity (STPP), has been proposed. While a dendritic mechanism was described in hippocampus, its expression has not yet been demonstrated at the levels of the spine. Furthermore, it is unknown whether the mechanism can be expressed in other brain regions, such as sensory cortex. Here, we demonstrated that a postsynaptic response can be facilitated by prior spine excitation in both hippocampal and cortical neurons, using 3D digital holography and two-photon calcium imaging. The coordinated action of pre- and post-synaptic plasticity may provide a more thorough account of information processing in the brain.

Effects of Ethanol on Cellular Composition and Network Excitability of Human Pluripotent Stem Cell‐Derived Neurons

Prenatal alcohol exposure (PAE) in animal models results in excitatory-inhibitory (E/I) imbalance in neocortex due to alterations in the GABAergic interneuron (IN) differentiation and migration. Thus, E/I imbalance is a potential cause for intellectual disability in individuals with fetal alcohol spectrum disorder (FASD), but whether ethanol (EtOH) changes glutamatergic and GABAergic IN specification during human development remains unknown. Here, we created a human cellular model of PAE/FASD and tested the hypothesis that EtOH exposure during differentiation of human pluripotent stem cell-derived neurons (hPSNs) would cause the aberrant production of glutamatergic and GABAergic neurons, resulting in E/I imbalance. We applied 50mM EtOH daily to differentiating hPSNs for 50days to model chronic first-trimester exposure. We used quantitative polymerase chain reaction, immunocytochemical, and electrophysiological analysis to examine the effects of EtOH on hPSN specification and functional E/I balance. We found that EtOH did not alter neural induction nor general forebrain patterning and had no effect on the expression of markers of excitatory cortical pyramidal neurons. In contrast, our data revealed highly significant changes to levels of transcripts involved with IN precursor development (e.g., GSX2, DLX1/2/5/6, NR2F2) as well as mature IN specification (e.g., SST, NPY). Interestingly, EtOH did not affect the number of GABAergic neurons generated nor the frequency or amplitude of miniature excitatory and inhibitory postsynaptic currents. Similar to invivo rodent studies, EtOH significantly and specifically altered the expression of genes involved with IN specification from hPSNs, but did not cause imbalances of synaptic excitation-inhibition. Thus, our findings corroborate previous studies pointing to aberrant neuronal differentiation as an underlying mechanism of intellectual disability in FASD. However, in contrast to rodent binge models, our chronic exposure model suggests possible compensatory mechanisms that may cause more subtle defects of network processing rather than gross alterations in total E/I balance.

Repetitive Transcranial Magnetic Stimulation: A Call for Better Data.

Repetitive Transcranial Magnetic Stimulation (rTMS) is increasingly used to treat stroke, Parkinson's disease and depression (Fregni et al., 2005; Loo and Mitchell, 2005; Hallet, 2007; O'Reardon et al., 2007; Ridding and Rothwell, 2007). rTMS uses bursts of magnetic pulses to change the excitability and connection strengths of cortical neurons. However, the evidence to inform clinical application is highly inconsistent (Thut and Pascual-Leone, 2010; Hamada et al., 2013) and substantially based on trial and error. Systematic theory is lacking. Typically, in rTMS research, measurements of motor-evoked potential (MEP) are made, often in terms of the strength of the MEP and the length of the cortical silent period that follows. However, the MEP is probably a poor and certainly an indirect measure of changes in the brain (Nicolo et al., 2015), clouding our understanding of rTMS mechanisms. In practice, therefore, particular amplitudes and timing of pulses in an rTMS sequence are selected because they show promise in small subsets of people. However, even basics such as the sign of any change in the outcome measure (e.g., does the MEP increase or decrease?) is debated. Many results show a wide spread in responses. It has become common to talk about “responders” and “non-responders” although evidence for a binary distinction in these two groups is lacking—in reality there is usually a continuum of response often including potentiation in some and depression in others (Nettekoven et al., 2015). Moreover, Heroux et al. (2015) provide evidence that the irreproducibility of results may be due to small sample sizes, unscientific screening of subjects and data, and selective reporting of results. In rTMS a regular train of pulses is applied. There is considerable variation in possible stimulation sequences, leading to many parameters that could potentially affect the results. Pulses are applied at a particular amplitude (normally recorded as percent motor threshold, (%RMT), or sometimes percent machine output, themselves imprecise measures), at a given rate (pulses per second, or hertz, Hz), until a particular number of pulses have been applied. There are three numerical parameters here. Additionally, one can consider variation in coil shape, orientation and place of application. Different waveforms for the magnetic pulse are also available. Thus, for an ostensibly straightforward pulse sequence such as non-bursting rTMS, the parameter space is considerable. Fitzgerald et al. (2006) carried out a comprehensive review of rTMS effects. They concluded that low frequency (0.9–5 Hz) stimulation generally results in a decreased MEP, while high frequency stimulation (5–20 Hz) results in an increased MEP. Very low frequency stimulation gave no effect. The “ 5 Hz, potentiation” mantra is now well established in rTMS literature. More recently, bursting protocols have become popular. The quadripulse protocol (Tsutsumi et al., 2014) is one example—four pulses are applied in quick succession, with this pattern repeated at regular time intervals. Theta-burst stimulation (TBS) is another; bursts of pulses are applied at theta-band (4–8 Hz) frequency. A continuous theta-burst stimulation (cTBS) protocol adds two more parameters, the number of pulses in a burst and the burst frequency. Intermittent theta-burst stimulation (iTBS) protocols require a further two parameters. Here, the bursting protocol is applied for a given time (denoted ON time, often 2 s), then removed for a given time (denoted OFF time, often 8 s) before being active for another 2 s period, etc. Thus, the parameter space for describing intermittent bursting pulse sequences becomes vast. Experiments performed to date do not come close to spanning it.

The transcription factor NeuroD2 coordinates synaptic innervation and cell intrinsic properties to control excitability of cortical pyramidal neurons.

Synaptic excitation and inhibition must be properly balanced in individual neurons and neuronal networks to allow proper brain function. Disrupting this balance may lead to autism spectral disorders and epilepsy. We show the basic helix-loop-helix transcription factor NeuroD2 promotes inhibitory synaptic drive but also decreases cell-intrinsic neuronal excitability of cortical pyramidal neurons both in vitro and in vivo. We identify two genes potentially downstream of NeuroD2-mediated transcription that regulate these parameters: gastrin-releasing peptide and the small conductance, calcium-activated potassium channel, SK2. Our results reveal an important function for NeuroD2 in balancing synaptic neurotransmission and intrinsic excitability. Our results offer insight into how synaptic innervation and intrinsic excitability are coordinated during cortical development. Synaptic excitation and inhibition must be properly balanced in individual neurons and neuronal networks for proper brain function. Disruption of this balance during development may lead to autism spectral disorders and epilepsy. Synaptic excitation is counterbalanced by synaptic inhibition but also by attenuation of cell-intrinsic neuronal excitability. To maintain proper excitation levels during development, neurons must sense activity over time and regulate the expression of genes that control these parameters. While this is a critical process, little is known about the transcription factors involved in coordinating gene expression to control excitatory/inhibitory synaptic balance. We show here that the basic helix-loop-helix transcription factor NeuroD2 promotes inhibitory synaptic drive but also decreases cell-intrinsic neuronal excitability of cortical pyramidal neurons both in vitro and in vivo as shown by ex vivo analysis of a NeuroD2 knockout mouse. Using microarray analysis and comparing wild-type and NeuroD2 knockout cortical networks, we identified two potential gene targets of NeuroD2 that contribute to these processes: gastrin-releasing peptide (GRP) and the small conductance, calcium-activated potassium channel, SK2. We found that the GRP receptor antagonist RC-3059 and the SK2 specific blocker apamin partially reversed the effects of increased NeuroD2 expression on inhibitory synaptic drive and action potential repolarization, respectively. Our results reveal an important function for NeuroD2 in balancing synaptic neurotransmission and intrinsic excitability and offer insight into how these processes are coordinated during cortical development.

Somatostatin and Somatostatin-Containing Neurons in Shaping Neuronal Activity and Plasticity.

Since its discovery over four decades ago, somatostatin (SOM) receives growing scientific and clinical interest. Being localized in the nervous system in a subset of interneurons somatostatin acts as a neurotransmitter or neuromodulator and its role in the fine-tuning of neuronal activity and involvement in synaptic plasticity and memory formation are widely recognized in the recent literature. Combining transgenic animals with electrophysiological, anatomical and molecular methods allowed to characterize several subpopulations of somatostatin-containing interneurons possessing specific anatomical and physiological features engaged in controlling the output of cortical excitatory neurons. Special characteristic and connectivity of somatostatin-containing neurons set them up as significant players in shaping activity and plasticity of the nervous system. However, somatostatin is not just a marker of particular interneuronal subpopulation. Somatostatin itself acts pre- and postsynaptically, modulating excitability and neuronal responses. In the present review, we combine the knowledge regarding somatostatin and somatostatin-containing interneurons, trying to incorporate it into the current view concerning the role of the somatostatinergic system in cortical plasticity.

Rats with Malformations of Cortical Development Exhibit Decreased Length of AIS and Hypersensitivity to Pilocarpine-Induced Status Epilepticus.

Malformations of cortical development (MCD) are critical brain development disorders associated with varied abnormalities in both anatomic structures and neural functioning. It is also a very common etiology to the epilepsy, in which the alteration on excitability of cortical neurons is hypothesized as one of important causes to the epileptic seizures. Due to the key role in regulating neuron firing properties, the plasticity of axon initial segment (AIS) was investigated in present study to further determine the relation between MCD and epilepsy. Our results showed a prolonged decrease in the length of AIS occurred in MCD animal models. Besides, the AIS was also found greatly shortened in MCD models during the acute, but not chronic phase of status epileptics compared with intact controls. Our findings of identification of AIS plasticity in MCD animal models and its hypersensitivity to status epilepsy are significant in furthering our understanding of the pathophysiological mechanisms involved in this disorder.