Increase In Input Resistance Research Articles

Medial prefrontal cortex pyramidal neurons exhibit functional defects during early stage of Alzheimer’s disease in 3xTg‐AD mice

The brain regions that show early increase of beta-amyloid (Aβ) in Alzheimer's disease (AD) overlap with the default mode network (DMN) (Palmqvist, S. et. al., 2017), which comprises, but is not limited to, the medial prefrontal cortex (mPFC), a key regulator of neurocognition (Zott, B. et al., 2018). DMN is active during mental processes that don't require attention, but it is deactivated when undertaking neurocognitive tasks (Zott, B. et al., 2018). However, such DMN deactivation is impaired in AD, often preceding formation of the Aβ plaque and onset of clinical symptoms (Chhatwal, J. P. et al., 2013; Sheline, Y. I. et al., 2010). Previous studies suggest that neuronal hyperexcitability is responsible for increased DMN vulnerability to Aβ deposition (Bero, A. W. et al., 2011; Li, X. et al., 2013). But whether and how mPFC neuronal function are altered in the early stages of AD is unknown and has not been studied.In this study, we used young 3xTg-AD mice (3-5 months of age) to mimic the early stages of AD before the formation of cortical Aβ plaques. Patch-clamp electrophysiology was applied to assess functional activity of mPFC pyramidal neurons in brain slices. Action potentials were evoked by depolarizing currents and spontaneous excitatory postsynaptic currents (sEPSCs) were assessed in Mg2+ -free bath with holding potential of -70mV. Ca2+ spikes (indicating Ca2+ influx through voltage-gated Ca2+ channels) were evoked by membrane depolarization with blockade of voltage-gated Na+ /K+ channels.We found that mPFC pyramidal neurons in young 3xTg-AD mice showed significantly increased firing, associated with depolarized resting membrane potential, decreased rheobase, increased input resistance, and reduced firing threshold. We also found that the frequency of sEPSCs was significantly increased. Meanwhile, Ca2+ spikes showed a trend of increase in their duration/area (a larger sample size is needed to confirm this finding) in mPFC neurons.Collectively, these novel findings demonstrate that the neuronal excitability and excitatory neurotransmission are aberrantly increased in the young mPFC during the early stages of AD preceding Aβ plaque formation. Whether such mPFC neuronal dysfunction could contribute to Aβ plaque accumulation is currently unknown and requires further investigation.

Sphingosine-1-phosphate receptor 1 agonist SEW2871 alters membrane properties of late-firing somatostatin expressing neurons in the central lateral amygdala

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid that mediates a wide spectrum of biological processes including apoptosis, immune response and inflammation. Here, we sought to understand how S1P signaling affects neuronal excitability in the central amygdala (CeA), which is a brain region associated with fear learning, aversive memory, and the affective dimension of pain. Because the G-protein coupled S1P receptor 1 (S1PR1) has been shown to be the primary mediator of S1P signaling, we utilized S1PR1 agonist SEW2871 and S1PR1 antagonist NIBR to determine a potential role of S1PR1 in altering the cellular physiology of neurons in the lateral division of the CeA (CeL) that share the neuronal lineage marker somatostatin (Sst). CeL-Sst neurons play a critical role in expression of conditioned fear and pain modulation. Here we used transgenic breeding strategies to identify fluorescently labeled CeL-Sst neurons for electrophysiological recordings. Using principal component analysis, we identified two primary subtypes of Sst neurons within the CeL in both male and female mice. We denoted the two types regular-firing (type A) and late-firing (type B) CeL-Sst neurons. In response to SEW2871 application, Type A neurons exhibited increased input resistance, while type B neurons displayed a depolarized resting membrane potential and voltage threshold, increased current threshold, and decreased voltage height. NIBR application had no effect on CeL Sst neurons, indicating the absence of tonic S1P-induced S1PR1. Our findings reveal subtypes of Sst neurons within the CeL that are uniquely affected by S1PR1 activation, which may have implications for how S1P alters supraspinal circuits.

Neocortex saves energy by reducing coding precision during food scarcity

SummaryInformation processing is energetically expensive. In the mammalian brain, it is unclear how information coding and energy use are regulated during food scarcity. Using whole-cell recordings and two-photon imaging in layer 2/3 mouse visual cortex, we found that food restriction reduced AMPA receptor conductance, reducing synaptic ATP use by 29%. Neuronal excitability was nonetheless preserved by a compensatory increase in input resistance and a depolarized resting potential. Consequently, neurons spiked at similar rates as controls but spent less ATP on underlying excitatory currents. This energy-saving strategy had a cost because it amplified the variability of visually-evoked subthreshold responses, leading to a 32% broadening of orientation tuning and impaired fine visual discrimination. This reduction in coding precision was associated with reduced levels of the fat mass-regulated hormone leptin and was restored by exogenous leptin supplementation. Our findings reveal that metabolic state dynamically regulates the energy spent on coding precision in neocortex.

Effects of Cooling Temperatures via Thermal K2P Channels on Regeneration of High-Frequency Action Potentials at Nodes of Ranvier of Rat Aβ-Afferent Nerves.

Temperature-sensitive two-pore domain potassium channels (thermal K2P) are recently shown to cluster at nodes of Ranvier (NRs) and play a key role in action potential (AP) regeneration and conduction on Aβ-afferent nerves. Cooling temperatures affect AP regeneration and conduction on Aβ-afferent nerves but the underlying mechanisms are not completely understood. Here, we have performed patch-clamp recordings directly at the NR in an ex vivo trigeminal nerve preparation. We have characterized the effects of cooling temperatures on intrinsic electrophysiological properties and AP regeneration at the NR on rat Aβ-afferent nerves, and determined whether and how thermal K2P channels may be involved in the effects of cooling temperatures. We show that cooling temperatures from 35°C to 15°C decrease outward leak currents, increase input resistance, depolarize resting membrane potential (RMP), broaden AP width and increase latency of AP threshold at the NR. We further demonstrate that cooling temperatures impair regeneration of high-frequency AP trains at the NR. The effects of cooling temperatures on the intrinsic electrophysiological properties and regeneration of high-frequency AP trains at the NR can be partially reversed by BL-1249 (BL), arachidonic acid (AA), and intra-axonal protons, three thermal K2P activators, indicating the involvement of thermal K2P channels. Moreover, we show that at cooling temperatures there are interplays among thermal K2P channels, RMPs, and voltage-gated Na+ channels, which together limit regeneration of high-frequency AP trains at the NR. Our findings demonstrate a new role of thermal K2P channels in temperature-dependent conduction of high-frequency sensory signals.

Acute neuroinflammation increases excitability of prefrontal parvalbumin interneurons and their functional recruitment during novel object recognition

There is an emerging body of literature suggesting that unlike the chronic neuroinflammatory response, acute neuroinflammation is self-regulated and is beneficial for central nervous system homeostasis and cognitive integrity. However, the neurophysiological alterations upon acute neuroinflammation and their implications on cognitive function remain poorly understood. In the present study, we reliably established a mouse model of acute and self-limiting neuroinflammation by administering a single intraperitoneal injection of low-dose lipopolysaccharide, which induced reversible sickness behavior and increased pro-inflammatory cytokine expression in the medial prefrontal cortex (mPFC). During acute neuroinflammation, fast-spiking parvalbumin-expressing interneurons (PV interneurons) in the mPFC exhibited a hyperexcitable phenotype exemplified by increased input resistance, decreased rheobase current, and a higher frequency of action potentials. Furthermore, PV interneurons in the prelimbic subregion of the mPFC were excessively recruited into circuits supporting novel object recognition memory, which remained intact after acute neuroinflammation. Together, our findings suggest that alterations in PV neuronal excitability resulting from acute neuroinflammation may mediate neuronal recruitment and confer a beneficial outcome on functional integrity of NOR circuit in the mPFC.

Specific stimulation of PV+ neurons at early stage ameliorates prefrontal ischemia-induced spatial working memory impairment

Prefrontal ischemia can cause impairments in learning and memory, executive functions and cognitive flexibility. However, the related cellular mechanisms at the early stage are still elusive. The present study used ischemic stroke in medial prefrontal cortex and systemically investigated the electrophysiological changes of the parvalbumin (PV+) interneurons 12 h post ischemia. We found that Ih and the related voltage sags in PV+ interneurons are downregulated post ischemia, which correlates with hyperpolarization of the membrane potentials and increased input resistance in these interneurons. Consistent with the suppression of Ih, postischemic PV+ interneurons exhibited a reduction in excitability and exerted a less inhibitory control over the neighboring pyramidal excitatory neurons. Moreover, we found that specifically chemogenetic activation of PV+ neurons at early stage ameliorated prefrontal ischemia-induced spatial working memory dysfunction in T-maze without effects on the locomotor coordination and balance. In contrast, suppression of PV+ neurons by blockade of Ih leaded to further aggravate the damage of spatial memory. These findings indicate that dysfunctional Ih in the PV+ neuron postischemia induces the imbalance of excitation and inhibition, which might represent a novel mechanism underlying the prefrontal ischemia-induced cognitive impairment.

Genetically engineered MAPT 10+16 mutation causes pathophysiological excitability of human iPSC-derived neurons related to 4R tau-induced dementia

Human iPSC lines represent a powerful translational model of tauopathies. We have recently described a pathophysiological phenotype of neuronal excitability of human cells derived from the patients with familial frontotemporal dementia and parkinsonism (FTDP-17) caused by the MAPT 10+16 splice-site mutation. This mutation leads to the increased splicing of 4R tau isoforms. However, the role of different isoforms of tau protein in initiating neuronal dementia-related dysfunction, and the causality between the MAPT 10+16 mutation and altered neuronal activity have remained unclear. Here, we employed genetically engineered cells, in which the IVS10+16 mutation was introduced into healthy donor iPSCs to increase the expression of 4R tau isoform in exon 10, aiming to explore key physiological traits of iPSC-derived MAPT IVS10+16 neurons using patch-clamp electrophysiology and multiphoton fluorescent imaging techniques. We found that during late in vitro neurogenesis (from ~180 to 230 days) iPSC-derived cortical neurons of the control group (parental wild-type tau) exhibited membrane properties compatible with “mature” neurons. In contrast, MAPT IVS10+16 neurons displayed impaired excitability, as reflected by a depolarized resting membrane potential, an increased input resistance, and reduced voltage-gated Na+- and K+-channel-mediated currents. The mutation changed the channel properties of fast-inactivating Nav and decreased the Nav1.6 protein level. MAPT IVS10+16 neurons exhibited reduced firing accompanied by a changed action potential waveform and severely disturbed intracellular Ca2+ dynamics, both in the soma and dendrites, upon neuronal depolarization. These results unveil a causal link between the MAPT 10+16 mutation, hence overproduction of 4R tau, and a dysfunction of human cells, identifying a biophysical basis of changed neuronal activity in 4R tau-triggered dementia. Our study lends further support to using iPSC lines as a suitable platform for modelling tau-induced human neuropathology in vitro.

Sex Differences in The Regulation of Neuron Excitability within Layer VI of The Mouse Entorhinal Cortex

The entorhinal cortex (EC) comprises part of the hippocampal formation and plays an important role to facilitate cognitive processes including learning and memory. The EC may be divided into six cellular layers that have unique neuron populations and communication targets. In particular, the sixth EC layer (ECVI) receives afferent signals from the hippocampus and contains glutamatergic pyramidal neurons that transmit efferent signals to other regions of the brain. Although several studies have characterized basic electrophysiological properties of neurons within the more superficial layers of the EC, detailed characteristics of pyramidal neurons within ECVI are not well understood. We recently identified a profound sex difference in the excitability of ECVI neurons during early postnatal life, in that neurons from female mice are more excitable than neurons from male mice. While this sex difference correlates with a smaller afterhyperpolarization of medium duration (mAHP) in female ECVI neurons, the precise mechanisms have not been determined. This study aims to determine the contribution of small-conductance calcium-activated potassium (SK) channels toward sex differences in ECVI neuron excitability in young postnatal (P5-10) and adult (P80-120) mice. Contrary to our previous finding in young postnatal mice, we observed no baseline sex differences in adult mice for passive electrophysiological properties such as resting membrane potential and membrane resistance, or for active electrophysiological properties such as input-output responses or mAHP magnitude. The SK channel blocker apamin increased input resistance and decreased rheobase in both sexes. However, the ability of apamin to decrease mAHP magnitude and increase input-output excitability responses was far greater in male than in females. Ongoing experiments aim to determine the effect of SK channel blockade on neuronal excitability in young postnatal mice. These novel findings demonstrate that SK channels play a greater role in regulating ECVI neuron excitability in male mice than in female mice. This knowledge advances our understanding of neuron function within the hippocampal formation and mechanisms underlying potential sex differences in hippocampal-dependent cognitive functions.

Increased intrinsic excitability and decreased synaptic inhibition in aged somatosensory cortex pyramidal neurons

Sensorimotor performance declines during advanced age, partially due to deficits in somatosensory acuity. Cortical receptive field expansion contributes to somatosensory deficits, suggesting increased excitability or decreased inhibition in primary somatosensory cortex (S1) pyramidal neurons. To ascertain changes in excitability and inhibition, we measured both properties in neurons from vibrissal S1 in brain slices from young and aged mice. Because adapting and non-adapting neurons—the principal pyramidal types in layer 5 (L5)—differ in intrinsic properties and inhibitory inputs, we determined age-dependent changes according to neuron type. We found an age-dependent increase in intrinsic excitability in adapting neurons, caused by a decrease in action potential threshold. Surprisingly, in non-adapting neurons we found both an increase in excitability caused by increased input resistance, and a decrease in synaptic inhibition. Spike frequency adaptation, already small in non-adapting neurons, was further reduced by aging, whereas sag, a manifestation of Ih, was increased. Therefore, aging caused both decreased inhibition and increased intrinsic excitability, but these effects were specific to pyramidal neuron type.

Mechanisms associated with the antidepressant-like effects of L-655,708.

Previous research has demonstrated that selective modulation of hippocampal transmission by systemic administration of an α5-GABAA receptor negative allosteric modulator, L-655,708, reproduces the sustained antidepressant-like (AD-like) effect of R,S-ketamine in the absence of any psychotomimetic or abuse-related effects. Pharmacological, electrophysiological (whole-cell patch clamp), and behavioral approaches were used to examine the mechanisms by which L-655,708 produces plasticity within the hippocampus that accounts for its sustained AD-like effect in rats. Inhibitors of either transcription or translation prevented the sustained AD-like effect of L-655,708. Unlike R,S-ketamine, L-655,708 did not cause an increase in the phosphorylation of the receptor for BDNF, TrkB, in the ventral hippocampus (vHipp) 30 or 60 min after its administration nor did administration of the TrkB inhibitor, K252a, directly into the vHipp, block the sustained AD-like effect of L-655,708. Similar to previous results with R,S-ketamine, administration of L-655,709 increased levels of GluA1 in the mPFC and, blockade of such receptors by direct administration of NBQX into the mPFC blocked the sustained AD-like effect of L-655,708. Patch-clamp recordings of ventral CA1 pyramidal cells 24 h after a single systemic administration of L-655,708 revealed a significant increase in input resistance, which resulted in an approximately two-fold increase in action potential frequency. These experiments indicate that the sustained AD-like effects of L-655,708 require protein synthesis and plasticity of GluA1 glutamate receptors in the mPFC. The drug also caused changes in GABAA receptor gating properties in the vHipp with resultant changes in ventral CA1 that indirectly increases neuronal excitability. Such effects likely contribute to its sustained AD-like activity.

BRAFV600E expression in neural progenitors results in a hyperexcitable phenotype in neocortical pyramidal neurons.

Somatic mutations have emerged as the likely cause of focal epilepsies associated with developmental malformations and epilepsy-associated glioneuronal tumors (GNT). Somatic BRAFV600E mutations in particular have been detected in the majority of low-grade neuroepithelial tumors (LNETS) and in neurons in focal cortical dysplasias adjacent to epilepsy-associated tumors. Furthermore, conditional expression of an activating BRAF mutation in neocortex causes seizures in mice. In this study we characterized the cellular electrophysiology of layer 2/3 neocortical pyramidal neurons induced to express BRAFV600E from neural progenitor stages. In utero electroporation of a piggyBac transposase plasmid system was used to introduce transgenes expressing BRAF wild type (BRAFwt), BRAFV600E, and/or enhanced green fluorescent protein (eGFP) and monomeric red fluorescent protein (mRFP) into radial glia progenitors in mouse embryonic cortex. Whole cell patch-clamp recordings of pyramidal neurons in slices prepared from both juvenile and adult mice showed that BRAFV600E resulted in neurons with a distinct hyperexcitable phenotype characterized by depolarized resting membrane potentials, increased input resistances, lowered action potential (AP) thresholds, and increased AP firing frequencies. Some of the BRAFV600E-expressing neurons normally destined for upper cortical layers by their birthdate were stalled in their migration and occupied lower cortical layers. BRAFV600E-expressing neurons also displayed increased hyperpolarization-induced inward currents (Ih) and decreased sustained potassium currents. Neurons adjacent to BRAFV600E transgene-expressing neurons, and neurons with TSC1 genetically deleted by CRISPR or those induced to carry PIK3CAE545K transgenes, did not show an excitability phenotype similar to that of BRAFV600E-expressing neurons. Together, these results indicate that BRAFV600E leads to a distinct hyperexcitable neuronal phenotype.NEW & NOTEWORTHY This study is the first to report the cell autonomous effects of BRAFV600E mutations on the intrinsic neuronal excitability. We show that BRAFV600E alters multiple electrophysiological parameters in neocortical neurons. Similar excitability changes did not occur in cells neighboring BRAFV600E-expressing neurons, after overexpression of wild-type BRAF transgenes, or after introduction of mutations affecting the mammalian target of rapamycin (mTOR) or the catalytic subunit of phosphoinositide 3-kinase (PIK3CA). We conclude that BRAFV600E causes a distinct, cell autonomous, highly excitable neuronal phenotype when introduced somatically into neocortical neuronal progenitors.

Multimodal Analysis of STRADA Function in Brain Development.

mTORopathies are a heterogeneous group of neurological disorders characterized by malformations of cortical development (MCD), enhanced cellular mechanistic target of rapamycin (mTOR) signaling, and epilepsy that results from mutations in mTOR pathway regulatory genes. Homozygous mutations (del exon 9–13) in the pseudokinase STE20-related kinase adaptor alpha (STRAD-α; STRADA), an mTOR modulator, are associated with Pretzel Syndrome (PS), a neurodevelopmental disorder within the Old Order Mennonite Community characterized by megalencephaly, intellectual disability, and intractable epilepsy. To study the cellular mechanisms of STRADA loss, we generated CRISPR-edited Strada mouse N2a cells, a germline mouse Strada knockout (KO−/−) strain, and induced pluripotent stem cell (iPSC)-derived neurons from PS individuals harboring the STRADA founder mutation. Strada KO in vitro leads to enhanced mTOR signaling and iPSC-derived neurons from PS individuals exhibit enhanced cell size and mTOR signaling activation, as well as subtle alterations in electrical firing properties e.g., increased input resistance, a more depolarized resting membrane potential, and decreased threshold for action potential (AP) generation. Strada−/− mice exhibit high rates of perinatal mortality and out of more than 100 litters yielding both WT and heterozygous pups, only eight Strada−/− animals survived past P5. Strada−/− mice are hypotonic and tremulous. Histopathological examination (n = 5 mice) revealed normal gross brain organization and lamination but all had ventriculomegaly. Ectopic neurons were seen in all five Strada−/− brains within the subcortical white matter mirroring what is observed in human PS brain tissue. These distinct experimental platforms demonstrate that STRADA modulates mTOR signaling and is a key regulator of cell size, neuronal excitability, and cortical lamination.

Increase of Input Resistance of a Normal-Mode Helical Antenna (NMHA) in Human Body Application

In recent years, the development of healthcare monitoring devices requires high performance and compact in-body sensor antennas. A normal-mode helical antenna (NMHA) is one of the most suitable candidates that meets the criteria, especially with the ability to achieve high efficiency when the antenna structure is in self-resonant mode. It was reported that when the antenna was placed in a human body, the antenna efficiency was decreased due to the increase of its input resistance (Rin). However, the reason for Rin increase was not clarified. In this paper, the increase of Rin is ensured through experiments and the physical reasons are validated through electromagnetic simulations. In the simulation, the Rin is calculated by placing the NMHA inside a human’s stomach, skin and fat. The dependency of Rin to conductivity (σ) is significant. Through current distribution calculation, it is verified that the reason of the increase in Rin is due to the decrease of antenna current. The effects of Rin to bandwidth (BW) and electrical field are also numerically clarified. Furthermore, by using the fabricated human body phantom, the measured Rin and bandwidth are also obtained. From the good agreement between the measured and simulated results, the condition of Rin increment is clarified.

Intramuscular Botulinum toxin A injections induce central changes to axon initial segments and cholinergic boutons on spinal motoneurones in rats

Intramuscular injections of botulinum toxin block pre-synaptic cholinergic release at neuromuscular junctions producing a temporary paralysis of affected motor units. There is increasing evidence, however, that the effects are not restricted to the periphery and can alter the central excitability of the motoneurones at the spinal level. This includes increases in input resistance, decreases in rheobase currents for action potentials and prolongations of the post-spike after-hyperpolarization. The aim of our experiments was to investigate possible anatomical explanations for these changes. Unilateral injections of Botulinum toxin A mixed with a tracer were made into the gastrocnemius muscle of adult rats and contralateral tracer only injections provided controls. Immunohistochemistry for Ankyrin G and the vesicular acetylcholine transporter labelled axon initial segments and cholinergic C-boutons on traced motoneurones at 2 weeks post-injection. Soma size was not affected by the toxin; however, axon initial segments were 5.1% longer and 13.6% further from the soma which could explain reductions in rheobase. Finally, there was a reduction in surface area (18.6%) and volume (12.8%) but not frequency of C-boutons on treated motoneurones potentially explaining prolongations of the after-hyperpolarization. Botulinum Toxin A therefore affects central anatomical structures controlling or modulating motoneurone excitability explaining previously observed excitability changes.

Alterations of Electrophysiological Properties and Ion Channel Expression in Prefrontal Cortex of a Mouse Model of Schizophrenia.

Maternal immune activation (MIA) and juvenile social isolation (SI) are two most prevalent and widely accepted environmental insults that could increase the propensity of psychiatric illnesses. Using a two-hit mouse model, we examined the impact of the combination of these two factors on animal behaviors, neuronal excitability and expressions of voltage-gated sodium (Nav) and small conductance calcium-activated potassium (SK) channels in the prefrontal cortex (PFC). We found that MIA-SI induced a number of schizophrenia-related behavioral deficits. Patch clamp recordings revealed alterations in electrophysiological properties of PFC layer-5 pyramidal cells, including hyperpolarized resting membrane potential (RMP), increased input resistance and enhanced medium after-hyperpolarization (mAHP). MIA-SI also increased the ratio of the maximal slope of somatodendritic potential to the peak slope of action potential upstroke, indicating a change in perisomatic Nav availability. Consistently, MIA-SI significantly increased the expression level of Nav1.2 and SK3 channels that contribute to the somatodendritic potential and the mAHP, respectively. Together, these changes may alter neuronal signaling in the PFC and behavioral states, representing a molecular imprint of environmental insults associated with neuropsychiatric illnesses.

Three-week treadmill training changes the electrophysiological properties of spinal interneurons in the mice.

It was shown in previous studies that endurance training enhanced excitability of rat spinal motoneurons. However, the influence of the training on the spinal interneurons remains unclear. In this study, we investigated the training effects on spinal interneurons in dorsal and ventromedial area in mice (P42-P50). The electrophysiological properties of the interneurons were recorded from spinal cord slices (T13-L6) by whole-cell patch-clamp recording. The interneurons could be classified into three types based on their response to step currents: single spike (type 1), phasic firing (type 2), and tonic firing (type 3) in both control and trained mice. Interneurons collected from control mice possessed rheobase of 11.3 ± 6.0 pA and voltage threshold (Vth) of -37.3 ± 4.7mV. Treadmill training reduced the rheobase by 4.8 ± 1.5 pA and Vth by 3.1 ± 1.2mV (P < 0.05). Furthermore, the training effects were dependent on the distribution and types of the interneurons. Treadmill training hyperpolarized Vth and decreased rheobase in ventromedial interneurons, while the significant change was observed only in the action potation height of the interneurons in dorsal horn. Treadmill training also hyperpolarized Vth and increased input resistance in type 3 interneurons, but none of these changes was shown in type 1 and 2 interneurons. Bath application of 5-HT (10-20μM) increased the neuronal excitability in both control and trained mice. Serotonin had similar effect on membrane properties of the interneurons collected from both groups. This study suggested that treadmill training increased excitability of spinal interneurons of the mice and thus would make the spinal motor system easier to generate locomotion.

Introduction of Tau Oligomers into Cortical Neurons Alters Action Potential Dynamics and Disrupts Synaptic Transmission and Plasticity.

Tau is a highly soluble microtubule-associated protein that acts within neurons to modify microtubule stability. However, abnormally phosphorylated tau dissociates from microtubules to form oligomers and fibrils which associate in the somatodendritic compartment. Although tau can form neurofibrillary tangles (NFTs), it is the soluble oligomers that appear to be the toxic species. There is, however, relatively little quantitative information on the concentration-dependent and time-dependent actions of soluble tau oligomers (oTau) on the electrophysiological and synaptic properties of neurons. Here, whole-cell patch clamp recording was used to introduce known concentrations of oligomeric full-length tau-441 into mouse hippocampal CA1 pyramidal and neocortical Layer V thick-tufted pyramidal cells. oTau increased input resistance, reduced action potential amplitude and slowed action potential rise and decay kinetics. oTau injected into presynaptic neurons induced the run-down of unitary EPSPs which was associated with increased short-term depression. In contrast, introduction of oTau into postsynaptic neurons had no effect on basal synaptic transmission, but markedly impaired the induction of long-term potentiation (LTP). Consistent with its effects on synaptic transmission and plasticity, oTau puncta could be observed in the soma, axon and in the distal dendrites of injected neurons.

Early Life Inflammation Increases CA1 Pyramidal Neuron Excitability in a Sex and Age Dependent Manner through a Chloride Homeostasis Disruption.

Early life, systemic inflammation causes long-lasting changes in behavior. To unmask possible mechanisms associated with this phenomenon, we asked whether the intrinsic membrane properties in hippocampal neurons were altered as a consequence of early life inflammation. C57BL/6 mice were bred in-house and both male and female pups from multiple litters were injected with lipopolysaccharide (LPS; 100 μg/kg, i.p.) or vehicle at postnatal day (P)14, and kept until adolescence (P35-P45) or adulthood (P60-P70), when brain slices were prepared for whole-cell and perforated-patch recordings from CA1 hippocampal pyramidal neurons. In neurons of adult male mice pretreated with LPS, the number of action potentials elicited by depolarizing current pulses was significantly increased compared with control neurons, concomitant with increased input resistance, and a lower action potential threshold. Although these changes were not associated with changes in relevant sodium channel expression or differences in capacitance or dendritic architecture, they were linked to a mechanism involving intracellular chloride overload, revealed through a depolarized GABA reversal potential and increased expression of the chloride transporter, NKCC1. In contrast, no significant changes were observed in neurons of adult female mice pretreated with LPS, nor in adolescent mice of either sex. These data uncover a potential mechanism involving neonatal inflammation-induced plasticity in chloride homeostasis, which may contribute to early life inflammation-induced behavioral alterations.SIGNIFICANCE STATEMENT Early life inflammation results in long-lasting changes in many aspects of adult physiology. In this paper we reveal that a brief exposure to early life peripheral inflammation with LPS increases excitability in hippocampal neurons in a sex- and age-dependent manner through a chloride homeostasis disruption. As this hyperexcitability was only seen in adult males, and not in adult females or adolescent animals of either sex, it raises the possibility of a hormonal interaction with early life inflammation. Furthermore, as neonatal inflammation is a normal feature of early life in most animals, as well as humans, these findings may be very important for the development of animal models of disease that more appropriately resemble the human condition.

Epilepsy-Induced Reduction in HCN Channel Expression Contributes to an Increased Excitability in Dorsal, But Not Ventral, Hippocampal CA1 Neurons.

CA1 neurons in epileptic animals are vulnerable to selective changes in ion channel expression, called acquired channelopathies, which can increase the excitability of a neuron. Under normal conditions there is a gradient of ion channel expression and intrinsic excitability along the longitudinal, dorsoventral axis of hippocampal area CA1 of the rodent. Many of these channels, including M-channels, GIRK channels and HCN channels, all have dorsoventral expression gradients that might be altered in rodent models of epilepsy. Here, we show that the excitability of dorsal, but not ventral CA1 neurons, had an increased firing rate, reduced interspike interval (ISI) and increased input resistance in a status epilepticus (SE) model of temporal lobe epilepsy (TLE). As a result, the excitability of CA1 neurons became uniform across the dorsoventral axis of the rat hippocampus post-SE. Using current clamp recordings with pharmacology and immunohistochemistry, we demonstrate that the expression of HCN channels was downregulated in the dorsal CA1 region post-SE, while the expression of M and GIRK channels were unchanged. We did not find this acquired channelopathy in ventral CA1 neurons post-SE. Our results suggest that the excitability of dorsal CA1 neurons post-SE increase to resemble the intrinsic properties of ventral CA1 neurons, which likely makes the hippocampal circuit more permissible to seizures, and contributes to the cognitive impairments associated with chronic epilepsy.

The Alteration of Intrinsic Excitability and Synaptic Transmission in Lumbar Spinal Motor Neurons and Interneurons of Severe Spinal Muscular Atrophy Mice.

Spinal muscular atrophy (SMA) is the leading genetic cause of death in infants. Studies with mouse models have demonstrated increased excitability and loss of afferent proprioceptive synapses on motor neurons (MNs). To further understand functional changes in the motor neural network occurring in SMA, we studied the intrinsic excitability and synaptic transmission of both MNs and interneurons (INs) from ventral horn in the lumbar spinal cord in the survival motor neuron (SMN)Δ7 mouse model. We found significant differences in the membrane properties of MNs in SMA mice compared to littermate controls, including hyperpolarized resting membrane potential, increased input resistance and decreased membrane capacitance. Action potential (AP) properties in MNs from SMA mice were also different from controls, including decreased rheobase current, increased amplitude and an increased afterdepolarization (ADP) potential. The relationship between AP firing frequency and injected current was reduced in MNs, as was the threshold current, while the percentage of MNs showing long-lasting potentiation (LLP) in the intrinsic excitability was higher in SMA mice. INs showed a high rate of spontaneous firing, and those from SMA mice fired at higher frequency. INs from SMA mice showed little difference in their input-output relationship, threshold current, and plasticity in intrinsic excitability. The changes observed in both passive membrane and AP properties suggest greater overall excitability in both MNs and INs in SMA mice, with MNs showing more differences. There were also changes of synaptic currents in SMA mice. The average charge transfer per post-synaptic current of spontaneous excitatory and inhibitory synaptic currents (sEPSCs/sIPSCs) were lower in SMA MNs, while in INs sIPSC frequency was higher. Strikingly in light of the known loss of excitatory synapses on MNs, there was no difference in sEPSC frequency in MNs from SMA mice compared to controls. For miniature synaptic currents, mEPSC frequency was higher in SMA MNs, while for SMA INs, both mEPSC and mIPSC frequencies were higher. In SMA-affected mice we observed alterations of intrinsic and synaptic properties in both MNs and INs in the spinal motor network that may contribute to the pathophysiology, or alternatively, may be a compensatory response to preserve network function.