Intrinsic Neuronal Excitability Research Articles

The novel persistent sodium current inhibitor PRAX-562 has potent anticonvulsant activity with improved protective index relative to standard of care sodium channel blockers.

ObjectiveThis study investigates the effects of PRAX‐562 on sodium current (INa), intrinsic neuronal excitability, and protection from evoked seizures to determine whether a preferential persistent INa inhibitor would exhibit improved preclinical efficacy and tolerability compared to two standard voltage‐gated sodium channel (NaV) blockers.MethodsInhibition of INa was characterized using patch clamp analysis. The effect on intrinsic excitability was measured using evoked action potentials recorded from hippocampal CA1 pyramidal neurons in mouse brain slices. Anticonvulsant activity was evaluated using the maximal electroshock seizure (MES) model, and tolerability was assessed by measuring spontaneous locomotor activity (sLMA).ResultsPRAX‐562 potently and preferentially inhibited persistent INa induced by ATX‐II or the SCN8A mutation N1768D (half‐maximal inhibitory concentration [IC50] = 141 and 75 nmol·L–1, respectively) relative to peak INa tonic/resting block (60× preference). PRAX‐562 also exhibited potent use‐dependent block (31× preference to tonic block). This profile is considerably different from standard NaV blockers, including carbamazepine (CBZ; persistent INa IC50 = 77 500 nmol·L–1, preference ratios of 30× [tonic block], less use‐dependent block observed at various frequencies). In contrast to CBZ, PRAX‐562 reduced neuronal intrinsic excitability with only a minor reduction in action potential amplitude. PRAX‐562 (10 mg/kg po) completely prevented evoked seizures without affecting sLMA (MES unbound brain half‐maximal efficacious concentration = 4.3 nmol·L–1, sLMA half‐maximal tolerated concentration = 69.7 nmol·L–1, protective index [PI] = 16×). In contrast, CBZ and lamotrigine (LTG) had PIs of approximately 5.5×, with significant overlap between doses that were anticonvulsant and that reduced locomotor activity.SignificancePRAX‐562 demonstrated robust preclinical anticonvulsant activity similar to CBZ but improved compared to LTG. PRAX‐562 exhibited significantly improved preclinical tolerability compared with standard NaV blockers (CBZ and LTG), potentially due to the preference for persistent INa. Preferential targeting of persistent INa may represent a differentiated therapeutic option for diseases of hyperexcitability, where standard NaV blockers have demonstrated efficacy but poor tolerability.

Inhibition of intracellular proton-sensitive Ca2+-permeable TRPV3 channels protects against ischemic brain injury

Ischemic brain stroke is pathologically characterized by tissue acidosis, sustained calcium entry and progressive cell death. Previous studies focusing on antagonizing N-methyl-d-aspartate (NMDA) receptors have failed to translate any clinical benefits, suggesting a non-NMDA mechanism involved in the sustained injury after stroke. Here, we report that inhibition of intracellular proton-sensitive Ca2+-permeable transient receptor potential vanilloid 3 (TRPV3) channel protects against cerebral ischemia/reperfusion (I/R) injury. TRPV3 expression is upregulated in mice subjected to cerebral I/R injury. Silencing of TRPV3 reduces intrinsic neuronal excitability, excitatory synaptic transmissions, and also attenuates cerebral I/R injury in mouse model of transient middle cerebral artery occlusion (tMCAO). Conversely, overexpressing or re-expressing TRPV3 increases neuronal excitability, excitatory synaptic transmissions and aggravates cerebral I/R injury. Furthermore, specific inhibition of TRPV3 by natural forsythoside B decreases neural excitability and attenuates cerebral I/R injury. Taken together, our findings for the first time reveal a causative role of neuronal TRPV3 channel in progressive cell death after stroke, and blocking overactive TRPV3 channel may provide therapeutic potential for ischemic brain injury.

Activation of D1/D5 receptors ameliorates decreased intrinsic excitability of hippocampal neurons induced by neonatal blockade of N‐methyl‐d‐aspartate receptors

Dysregulation of dopaminergic transmission combined with transient hypofunction of N-methyl-d-aspartate receptors (NMDARs) is a key mechanism that may underlie cognitive symptoms of schizophrenia. Therefore, we aimed to identify electrophysiologic alterations in animals neonatally treated with the NMDA receptor antagonist, MK-801, or with saline solution. Patch-clamp whole-cell recordings from MK-801-treated animals revealed altered passive and active electrophysiologic properties compared with CA1 pyramidal cells from saline-treated animals, including up-regulation of the K+ inward-rectifier conductance and fast-inactivating and slow/non-inactivating K+ currents. Up-regulation of these membrane ionic currents reduced the overall excitability and altered the firing properties of CA1 pyramidal cells. We also explored the capability of cells treated with MK-801 to express intrinsic excitability potentiation, a non-synaptic form of hippocampal plasticity associated with cognition and memory formation. CA1 pyramidal cells from animals treated with MK-801 were unable to convey intrinsic excitability potentiation and had blunted synaptic potentiation. Furthermore, MK-801-treated animals also exhibited reduced cognitive performance in the Barnes maze task. Notably, activation of D1/D5 receptors with SKF-38,393 partially restored electrophysiologic alterations caused by neonatal treatment with MK-801. Our results offer a molecular and mechanistic explanation based on dysregulation of glutamatergic transmission, in addition to dopaminergic transmission, that may contribute to the understanding of the cognitive deterioration associated with schizophrenia.

Microglia-triggered hypoexcitability plasticity of pyramidal neurons in the rat medial prefrontal cortex

Lipopolysaccharide (LPS), an outer component of Gram-negative bacteria, induces a strong response of innate immunity via microglia, which triggers a modulation of the intrinsic excitability of neurons. However, it is unclear whether the modulation of neurophysiological properties is similar among neurons. Here, we found the hypoexcitability of layer 5 (L5) pyramidal neurons after exposure to LPS in the medial prefrontal cortex (mPFC) of juvenile rats. We recorded the firing frequency of L5 pyramidal neurons long-lastingly under in vitro whole-cell patch-clamp, and we found a reduction of the firing frequency after applying LPS. A decrease in the intrinsic excitability against LPS-exposure was also found in L2/3 pyramidal neurons but not in fast-spiking interneurons. The decrease in the excitability by immune-activation was underlain by increased activity of small-conductance Ca2+-activated K+ channels (SK channels) in the pyramidal neurons and tumor necrosis factor (TNF)-α released from microglia. We revealed that the reduction of the firing frequency of L5 pyramidal neurons was dependent on intraneuronal Ca2+ and PP2B. These results suggest the hypoexcitability of pyramidal neurons caused by the upregulation of SK channels via Ca2+-dependent phosphatase during acute inflammation in the mPFC. Such a mechanism is in contrast to that of cerebellar Purkinje cells, in which immune activation induces hyperexcitability via downregulation of SK channels. Further, a decrease in the frequency of spontaneous inhibitory synaptic transmission reflected network hypoactivity. Therefore, our results suggest that the directionality of the intrinsic plasticity by microglia is not consistent, depending on the brain region and the cell type.

Cranial irradiation impairs intrinsic excitability and synaptic plasticity of hippocampal CA1 pyramidal neurons with implications for cognitive function.

Radiation therapy is a standard treatment for head and neck tumors. However, patients often exhibit cognitive impairments following radiation therapy. Previous studies have revealed that hippocampal dysfunction, specifically abnormal hippocampal neurogenesis or neuroinflammation, plays a key role in radiation-induced cognitive impairment. However, the long-term effects of radiation with respect to the electrophysiological adaptation of hippocampal neurons remain poorly characterized. We found that mice exhibited cognitive impairment 3 months after undergoing 10 minutes of cranial irradiation at a dose rate of 3 Gy/min. Furthermore, we observed a remarkable reduction in spike firing and excitatory synaptic input, as well as greatly enhanced inhibitory inputs, in hippocampal CA1 pyramidal neurons. Corresponding to the electrophysiological adaptation, we found reduced expression of synaptic plasticity marker VGLUT1 and increased expression of VGAT. Furthermore, in irradiated mice, long-term potentiation in the hippocampus was weakened and GluR1 expression was inhibited. These findings suggest that radiation can impair intrinsic excitability and synaptic plasticity in hippocampal CA1 pyramidal neurons.

Homeostatic regulation of neuronal excitability by probiotics in male germ-free mice.

Emerging evidence indicates that probiotics can influence the gut-brain axis to ameliorate somatic and behavioral symptoms associated with brain disorders. However, whether probiotics have effects on the electrophysiological activities of individual neurons in the brain has not been evaluated at a single-neuron resolution, and whether the neuronal effects of probiotics depend on the gut microbiome status have yet to be tested. Thus, we conducted whole-cell patch-clamp recording-assisted electrophysiological characterizations of the neuronal effects of probiotics in male germ-free (GF) mice with and without gut microbiome colonization. Two weeks of treatment with probiotics (Lactobacillus rhamnosus and Bifidobacterium animalis) significantly and selectively increased the intrinsic excitability of hippocampal CA1 pyramidal neurons, whereas reconstituting gut microbiota in GF mice reversed the effects of the probiotics leading to a decreased intrinsic excitability in hippocampal neurons. This bidirectional modulation of neuronal excitability by probiotics was observed in hippocampal neurons with corresponding basal membrane property and action potential waveform changes. However, unlike the hippocampus, the amygdala excitatory neurons did not show any electrophysiological changes to the probiotic treatment in either GF or conventionalized GF mice. Our findings demonstrate for the first time how probiotic treatment can have a significant influence on the electrophysiological properties of neurons, bidirectionally modulating their intrinsic excitability in a gut microbiota and brain area-specific manner.

Differential alterations of entorhinal versus CA3 impact on ventral CA1 pyramidal neurons in 5xFAD mice.

Most hippocampal physiologic studies in Alzheimer disease (AD) have focused on memory circuit dysfunction in dorsal CA1. Ventral CA1 (vCA1) has received little attention, despite evidence supporting its increased susceptibility and its role in mood and affective symptoms, which play a prominent role in AD. We aimed to study the effect of amyloid pathology on circuit dysfunction in vCA1 using 5xFAD mice. We evaluated intrinsic excitability of vCA1 pyramidal neurons (PNs) as well as the impact of two distinct inputs onto their apical dendrite: entorhinal cortex (EC) input onto the distal compartment and CA3 inputs onto the proximal compartment. Acute ventral hippocampal slices were prepared from 12-14 month old 5xFAD and age-matched wild type (WT) mice. Whole cell patch clamp recordings were performed on vCA1 PNs. EC and CA3 input responses were elicited with extracellular stimulation of stratum lacunosum moleculare and stratum radiatum. We also addressed presynaptic function (paired pulse ratio), dendritic excitability (decay time constant of synaptic response), and intrinsic excitability (resting membrane potential, input resistance, sag ratio, firing rate). The EC stimulus-response curve reduced in 5xFAD vs. WT mice (n=6-10 PNs; p=0.0096) with a peak reduction of ∼33% (1.5mV vs. 1.0mV). In these same cells, the CA3 stimulus-response curve was also reduced (p<0.0001) but to a much larger extent of ∼67% at the peak (4.3mV vs. 1.4mV). The paired pulse ratio and response decay time constant were not affected with either input. Lastly, there were no statistically significant changes in intrinsic excitability measures. In 5xFAD mice, vCA1 PN circuit dysfunction is dominated by synaptic not intrinsic excitability changes. These changes are not in presynaptic function or postsynaptic ion currents, suggesting glutamate receptor changes or spine loss as alternative mechanisms. Moreover, these alterations are much more evident in CA3 versus EC input responses, perhaps driven by the inherent differences of the proximal and distal dendritic compartment. As such, with amyloidosis, vCA1 PN output shifts from being heavily dependent on CA3 input to being nearly equivalently dependent on EC input. These changes are important for understanding the circuit basis of mood and affect dysfunction in AD.

Homeostatic regulation of axonal Kv1.1 channels accounts for both synaptic and intrinsic modifications in the hippocampal CA3 circuit

Homeostatic plasticity of intrinsic excitability goes hand in hand with homeostatic plasticity of synaptic transmission. However, the mechanisms linking the two forms of homeostatic regulation have not been identified so far. Using electrophysiological, imaging, and immunohistochemical techniques, we show here that blockade of excitatory synaptic receptors for 2 to 3 d induces an up-regulation of both synaptic transmission at CA3-CA3 connections and intrinsic excitability of CA3 pyramidal neurons. Intrinsic plasticity was found to be mediated by a reduction of Kv1.1 channel density at the axon initial segment. In activity-deprived circuits, CA3-CA3 synapses were found to express a high release probability, an insensitivity to dendrotoxin, and a lack of depolarization-induced presynaptic facilitation, indicating a reduction in presynaptic Kv1.1 function. Further support for the down-regulation of axonal Kv1.1 channels in activity-deprived neurons was the broadening of action potentials measured in the axon. We conclude that regulation of the axonal Kv1.1 channel constitutes a major mechanism linking intrinsic excitability and synaptic strength that accounts for the functional synergy existing between homeostatic regulation of intrinsic excitability and synaptic transmission.

Contralateral S1 function is involved in electroacupuncture treatment-mediated recovery after focal unilateral M1 infarction.

Acupuncture at acupoints Baihui (GV20) and Dazhui (GV14) has been shown to promote functional recovery after stroke. However, the contribution of the contralateral primary sensory cortex (S1) to recovery remains unclear. In this study, unilateral local ischemic infarction of the primary motor cortex (M1) was induced by photothrombosis in a mouse model. Electroacupuncture (EA) was subsequently performed at acupoints GV20 and GV14 and neuronal activity and functional connectivity of contralateral S1 and M1 were detected using in vivo and in vitro electrophysiological recording techniques. Our results showed that blood perfusion and neuronal interaction between contralateral M1 and S1 is impaired after unilateral M1 infarction. Intrinsic neuronal excitability and activity were also disturbed, which was rescued by EA. Furthermore, the effectiveness of EA treatment was inhibited after virus-mediated neuronal ablation of the contralateral S1. We conclude that neuronal activity of the contralateral S1 is important for EA-mediated recovery after focal M1 infarction. Our study provides insight into how the S1-M1 circuit might be involved in the mechanism of EA treatment of unilateral cerebral infarction. The animal experiments were approved by the Committee for Care and Use of Research Animals of Guangzhou University of Chinese Medicine (approval No. 20200407009) April 7, 2020.

Ghrelin signaling in dCA1 suppresses neuronal excitability and impairs memory acquisition via PI3K/Akt/GSK-3β cascades

Ghrelin is a circulating peptide hormone that promotes feeding and regulates metabolism in humans and rodents. The action of ghrelin is mediated by the growth hormone secretagogue receptor type 1a (GHSR-1a) that is widely distributed in the brain, including the hippocampus. Studies have demonstrated the critical role of hippocampal ghrelin/GHS-R1a signaling in synaptic physiology and memory. However, those findings are controversial, and the mechanism underlying ghrelin modulation of learning and memory is uncertain. Here, we report that micro-infusion of ghrelin in the CA1 region of the dorsal hippocampus during training specifically impairs memory acquisition. The activation of GHS-R1a and the subsequent PI3K/Akt/GSK3β signaling cascades are involved in this process. Moreover, we report that bath application of ghrelin suppresses the intrinsic excitability of dCA1 pyramidal neurons through activating GHS-R1a, and PI3K inhibitor LY294002 blocks ghrelin's effect. However, LY294002 fails to rescue ghrelin-induced LTP impairment. Our findings support an adverse effect of ghrelin-dependent activation of GHS-R1a on memory acquisition, and suggest that PI3K/Akt/GSK3β signaling-dependent repression of neuronal intrinsic excitability is an important novel mechanism underlying memory inhibition of ghrelin in the hippocampus.This article is part of the special Issue on ‘Stress, Addiction and Plasticity'.

Reduced Synaptic Transmission and Intrinsic Excitability of a Subtype of Pyramidal Neurons in the Medial Prefrontal Cortex in a Mouse Model of Alzheimer's Disease.

Abnormal morphology and function of neurons in the prefrontal cortex (PFC) are associated with cognitive deficits in rodent models of Alzheimer's disease (AD), particularly in cortical layer-5 pyramidal neurons that integrate inputs from different sources and project outputs to cortical or subcortical structures. Pyramidal neurons in layer-5 of the PFC can be classified as two subtypes depending on the inducibility of prominent hyperpolarization-activated cation currents (h-current). However, the differences in the neurophysiological alterations between these two subtypes in rodent models of AD remain poorly understood. To investigate the neurophysiological alterations between two subtypes of pyramidal neurons in hAPP-J20 mice, a transgenic model for early onset AD. The synaptic transmission and intrinsic excitability of pyramidal neurons were investigated using whole-cell patch recordings. The morphological complexity of pyramidal neurons was detected by biocytin labelling and subsequent Sholl analysis. We found reduced synaptic transmission and intrinsic excitability of the prominent h-current (PH) cells but not the non-PH cells in hAPP-J20 mice. Furthermore, the function of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels which mediated h-current was disrupted in the PH cells of hAPP-J20 mice. Sholl analysis revealed that PH cells had less dendritic intersections in hAPP-J20 mice comparing to control mice, implying that a lower morphological complexity might contribute to the reduced neuronal activity. These results suggest that the PH cells in the medial PFC may be more vulnerable to degeneration in hAPP-J20 mice and play a sustainable role in frontal dysfunction in AD.

Developmental Regulation of Homeostatic Plasticity in Mouse Primary Visual Cortex.

Homeostatic plasticity maintains network stability by adjusting excitation, inhibition, or the intrinsic excitability of neurons, but the developmental regulation and coordination of these distinct forms of homeostatic plasticity remains poorly understood. A major contributor to this information gap is the lack of a uniform paradigm for chronically manipulating activity at different developmental stages. To overcome this limitation, we used designer receptors exclusively activated by designer drugs (DREADDs) to directly suppress neuronal activity in layer2/3 (L2/3) of mouse primary visual cortex of either sex at two important developmental timepoints: the classic visual system critical period [CP; postnatal day 24 (P24) to P29], and adulthood (P45 to P55). We show that 24 h of DREADD-mediated activity suppression simultaneously induces excitatory synaptic scaling up and intrinsic homeostatic plasticity in L2/3 pyramidal neurons during the CP, consistent with previous observations using prolonged visual deprivation. Importantly, manipulations known to block these forms of homeostatic plasticity when induced pharmacologically or via visual deprivation also prevented DREADD-induced homeostatic plasticity. We next used the same paradigm to suppress activity in adult animals. Surprisingly, while excitatory synaptic scaling persisted into adulthood, intrinsic homeostatic plasticity was completely absent. Finally, we found that homeostatic changes in quantal inhibitory input onto L2/3 pyramidal neurons were absent during the CP but were present in adults. Thus, the same population of neurons can express distinct sets of homeostatic plasticity mechanisms at different development stages. Our findings suggest that homeostatic forms of plasticity can be recruited in a modular manner according to the evolving needs of a developing neural circuit.SIGNIFICANCE STATEMENT Developing brain circuits are subject to dramatic changes in inputs that could destabilize activity if left uncompensated. This compensation is achieved through a set of homeostatic plasticity mechanisms that provide slow, negative feedback adjustments to excitability. Given that circuits are subject to very different destabilizing forces during distinct developmental stages, the forms of homeostatic plasticity present in the network must be tuned to these evolving needs. Here we developed a method to induce comparable homeostatic compensation during distinct developmental windows and found that neurons in the juvenile and mature brain engage strikingly different forms of homeostatic plasticity. Thus, homeostatic mechanisms can be recruited in a modular manner according to the developmental needs of the circuit.

Short-Term Epileptiform Activity Potentiates Excitatory Synapses but Does Not Affect Intrinsic Membrane Properties of Pyramidal Neurons in the Rat Hippocampus In Vitro.

Even brief epileptic seizures can lead to activity-dependent structural remodeling of neural circuitry. Animal models show that the functional plasticity of synapses and changes in the intrinsic excitability of neurons can be crucial for epileptogenesis. However, the exact mechanisms underlying epileptogenesis remain unclear. We induced epileptiform activity in rat hippocampal slices for 15 min using a 4-aminopyridine (4-AP) in vitro model and observed hippocampal hyperexcitability for at least 1 h. We tested several possible mechanisms of this hyperexcitability, including changes in intrinsic membrane properties of neurons and presynaptic and postsynaptic alterations. Neither input resistance nor other essential biophysical properties of hippocampal CA1 pyramidal neurons were affected by epileptiform activity. The glutamate release probability also remained unchanged, as the frequency of miniature EPSCs and the paired amplitude ratio of evoked responses did not change after epileptiform activity. However, we found an increase in the AMPA/NMDA ratio, suggesting alterations in the properties of postsynaptic glutamatergic receptors. Thus, the increase in excitability of hippocampal neural networks is realized through postsynaptic mechanisms. In contrast, the intrinsic membrane properties of neurons and the probability of glutamate release from presynaptic terminals are not affected in a 4-AP model.

Biophysical models of intrinsic homeostasis: Firing rates and beyond.

In view of ever-changing conditions both in the external world and in intrinsic brain states, maintaining the robustness of computations poses a challenge, adequate solutions to which we are only beginning to understand. At the level of cell-intrinsic properties, biophysical models of neurons permit one to identify relevant physiological substrates that can serve as regulators of neuronal excitability and to test how feedback loops can stabilize crucial variables such as long-term calcium levels and firing rates. Mathematical theory has also revealed a rich set of complementary computational properties arising from distinct cellular dynamics and even shaping processing at the network level. Here, we provide an overview over recently explored homeostatic mechanisms derived from biophysical models and hypothesize how multiple dynamical characteristics of cells, including their intrinsic neuronal excitability classes, can be stably controlled.

Electrophysiological and Morphological Features of Rebound Depolarization Characterized Interneurons in Rat Superficial Spinal Dorsal Horn.

Substantia gelatinosa (SG) neurons, which are located in the spinal dorsal horn (lamina II), have been identified as the “central gate” for the transmission and modulation of nociceptive information. Rebound depolarization (RD), a biophysical property mediated by membrane hyperpolarization that is frequently recorded in the central nervous system, contributes to shaping neuronal intrinsic excitability and, in turn, contributes to neuronal output and network function. However, the electrophysiological and morphological properties of SG neurons exhibiting RD remain unclarified. In this study, whole-cell patch-clamp recordings were performed on SG neurons from parasagittal spinal cord slices. RD was detected in 44.44% (84 out of 189) of the SG neurons recorded. We found that RD-expressing neurons had more depolarized resting membrane potentials, more hyperpolarized action potential (AP) thresholds, higher AP amplitudes, shorter AP durations, and higher spike frequencies in response to depolarizing current injection than neurons without RD. Based on their firing patterns and morphological characteristics, we propose that most of the SG neurons with RD mainly displayed tonic firing (69.05%) and corresponded to islet cell morphology (58.82%). Meanwhile, subthreshold currents, including the hyperpolarization-activated cation current (Ih) and T-type calcium current (IT), were identified in SG neurons with RD. Blockage of Ih delayed the onset of the first spike in RD, while abolishment of IT significantly blunted the amplitude of RD. Regarding synaptic inputs, SG neurons with RD showed lower frequencies in both spontaneous and miniature excitatory synaptic currents. Furthermore, RD-expressing neurons received either Aδ- or C-afferent-mediated monosynaptic and polysynaptic inputs. However, RD-lacking neurons received afferents from monosynaptic and polysynaptic Aδ fibers and predominantly polysynaptic C-fibers. These findings demonstrate that SG neurons with RD have a specific cell-type distribution, and may differentially process somatosensory information compared to those without RD.

Early life stress induces anxiety-like behavior during adulthood through dysregulation of neuronal plasticity in the basolateral amygdala

AimsEarly life stress (ELS) increases the risk of psychiatric diseases such as anxiety disorders and depression in later life. Hyperactivation of the basolateral amygdala (BLA) neurons plays a pivotal role in the pathogenesis of stress-related diseases. However, the functional roles of BLA neurons in ELS-induced anxiety disorders are not completely understood. Main methodsMice were subjected to maternal separation (MS) during postnatal days 3 to 21 to mimic ELS. Anxiety-like behavior was tested by open field test (OFT), elevated plus maze (EPM), and novelty suppressed feeding (NSF). Then, c-fos expression, a proxy for neuronal activity, was evaluated by immunofluorescence. Finally, synaptic transmission and intrinsic excitability were measured by whole-cell patch-clamp recordings. Key findingsMS significantly increased anxiety-like behavior in adulthood, as indicated by less time spent in the center area of the OFT, less time spent in and fewer entries to the open arms of the EPM, and increased latency to feed in NSF. Mechanistically, MS increased the expression of c-fos in BLA. MS enhanced the excitatory, but not inhibitory, synaptic transmission onto BLA projection neurons (PNs), which was caused by enhanced presynaptic glutamate release. Moreover, MS also markedly increased the intrinsic neuronal excitability of BLA PNs, probably due to the reduced medium afterhyperpolarization (mAHP) in BLA PNs. SignificanceOur results suggest that the changes of neuronal activity and synaptic transmission in the BLA PNs may play a crucial role in ELS-induced anxiety-like behavior, and these findings provide new insights into the pathological mechanisms of stress-related anxiety disorders.

Plasticity of intrinsic excitability across the estrous cycle in hypothalamic CRH neurons

Stress responses are highly plastic and vary across physiological states. The female estrous cycle is associated with a number of physiological changes including changes in stress responses, however, the mechanisms driving these changes are poorly understood. Corticotropin-releasing hormone (CRH) neurons are the primary neural population controlling the hypothalamic–pituitary–adrenal (HPA) axis and stress-evoked corticosterone secretion. Here we show that CRH neuron intrinsic excitability is regulated over the estrous cycle with a peak in proestrus and a nadir in estrus. Fast inactivating voltage-gated potassium channel (IA) currents showed the opposite relationship, with current density being lowest in proestrus compared to other cycle stages. Blocking IA currents equalized excitability across cycle stages revealing a role for IA in mediating plasticity in stress circuit function over the female estrous cycle.

Reciprocal regulation of spontaneous synaptic vesicle fusion by Fragile X mental retardation protein and group I metabotropic glutamate receptors.

Fragile X mental retardation protein (FMRP) is a neuronal protein mediating multiple functions, with its absence resulting in one of the most common monogenic causes of autism, Fragile X syndrome (FXS). Analyses of FXS pathophysiology have identified a range of aberrations in synaptic signaling pathways and plasticity associated with group I metabotropic glutamate (mGlu) receptors. These studies, however, have mostly focused on the post-synaptic functions of FMRP and mGlu receptor activation, and relatively little is known about their presynaptic effects. Neurotransmitter release is mediated via multiple forms of synaptic vesicle (SV) fusion, each of which contributes to specific neuronal functions. The impacts of mGlu receptor activation and loss of FMRP on these SV fusion events remain unexplored. Here we combined electrophysiological and fluorescence imaging analyses on primary hippocampal cultures prepared from an Fmr1 knockout (KO) rat model. Compared to wild-type (WT) hippocampal neurons, KO neurons displayed an increase in the frequency of spontaneous excitatory post-synaptic currents (sEPSCs), as well as spontaneous SV fusion events. Pharmacological activation of mGlu receptors in WT neurons caused a similar increase in spontaneous SV fusion and sEPSC frequency. Notably, this increase in SV fusion was not observed when spontaneous activity was blocked using the sodium channel antagonist tetrodotoxin. Importantly, the effect of mGlu receptor activation on spontaneous SV fusion was occluded in Fmr1 KO neurons. Together, our results reveal that FMRP represses spontaneous presynaptic SV fusion, whereas mGlu receptor activation increases this event. This reciprocal control appears to be mediated via their regulation of intrinsic neuronal excitability.

Severe deficiency of the voltage-gated sodium channel NaV1.2 elevates neuronal excitability in adult mice.

SUMMARYScn2a encodes the voltage-gated sodium channel NaV1.2, a main mediator of neuronal action potential firing. The current paradigm suggests that NaV1.2 gain-of-function variants enhance neuronal excitability, resulting in epilepsy, whereas NaV1.2 deficiency impairs neuronal excitability, contributing to autism. However, this paradigm does not explain why ~20%–30% of individuals with NaV1.2 deficiency still develop seizures. Here, we report the counterintuitive finding that severe NaV1.2 deficiency results in increased neuronal excitability. Using a NaV1.2-deficient mouse model, we show enhanced intrinsic excitability of principal neurons in the prefrontal cortex and striatum, brain regions known to be involved in Scn2a-related seizures. This increased excitability is autonomous and reversible by genetic restoration of Scn2a expression in adult mice. RNA sequencing reveals downregulation of multiple potassium channels, including KV1.1. Correspondingly, KV channel openers alleviate the hyperexcitability of NaV1.2-deficient neurons. This unexpected neuronal hyperexcitability may serve as a cellular basis underlying NaV1.2 deficiency-related seizures.

Single Prolonged Stress Reduces Intrinsic Excitability and Excitatory Synaptic Drive Onto Pyramidal Neurons in the Infralimbic Prefrontal Cortex of Adult Male Rats.

Post-traumatic stress disorder (PTSD) is a chronic, debilitating mental illness marked by abnormal fear responses and deficits in extinction of fear memories. The pathophysiology of PTSD is linked to decreased activation of the ventromedial prefrontal cortex (vmPFC). This study aims to investigate underlying functional changes in synaptic drive and intrinsic excitability of pyramidal neurons in the rodent homolog of the vmPFC, the infralimbic cortex (IL), following exposure to single prolonged stress (SPS), a paradigm that mimics core symptoms of PTSD in rats. Rats were exposed to SPS and allowed 1 week of recovery, following which brain slices containing the PFC were prepared for whole-cell patch clamp recordings from layer V pyramidal neurons in the IL. Our results indicate that SPS reduces spontaneous excitatory synaptic drive to pyramidal neurons. In addition, SPS decreases the intrinsic membrane excitability of IL PFC pyramidal cells, as indicated by an increase in rheobase, decrease in input resistance, hyperpolarization of resting membrane potential, and a reduction in repetitive firing rate. Our results suggest that SPS causes a lasting reduction in PFC activity, supporting a body of evidence linking traumatic stress with prefrontal hypoactivity.