Functional interplay between NMDA receptors, SK channels and voltage-gated Ca2+ channels regulates synaptic excitability in the medial prefrontal cortex

GluN2A- and GluN2B-containing pre-synaptic N-methyl-d-aspartate receptors differentially regulate action potential-evoked Ca2+ influx via modulation of SK channels

N-methyl-d-aspartate receptors (NMDARs) play a pivotal role in synaptic plasticity. While the functional role of post-synaptic NMDARs is well established, pre-synaptic NMDAR (pre-NMDAR) function is largely unexplored. Different pre-NMDAR subunit populations are documented at synapses, suggesting that subunit composition influences neuronal transmission. Here, we used electrophysiological recordings at Schaffer collateral-CA1 synapses partnered with Ca2+ imaging and glutamate uncaging at boutons of CA3 pyramidal neurones to reveal two populations of pre-NMDARs that contain either the GluN2A or GluN2B subunit. Activation of the GluN2B population decreases action potential-evoked Ca2+ influx via modulation of small-conductance Ca2+-activated K+ channels, while activation of the GluN2A population does the opposite. Critically, the level of functional expression of the subunits is subject to homeostatic regulation, bidirectionally affecting short-term facilitation, thus providing a capacity for a fine adjustment of information transfer. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

D-serine reconstitutes synaptic and intrinsic inhibitory control of pyramidal neurons in a neurodevelopmental mouse model for schizophrenia

The hypothesis of N-methyl-D-aspartate receptor (NMDAR) dysfunction for cognitive impairment in schizophrenia constitutes the theoretical basis for the translational application of NMDAR co-agonist D-serine or its analogs. However, the cellular mechanism underlying the therapeutic effect of D-serine remains unclear. In this study, we utilize a mouse neurodevelopmental model for schizophrenia that mimics prenatal pathogenesis and exhibits hypoexcitability of parvalbumin-positive (PV) neurons, as well as PV-preferential NMDAR dysfunction. We find that D-serine restores excitation/inhibition balance by reconstituting both synaptic and intrinsic inhibitory control of cingulate pyramidal neurons through facilitating PV excitability and activating small-conductance Ca2+-activated K+ (SK) channels in pyramidal neurons, respectively. Either amplifying inhibitory drive via directly strengthening PV neuron activity or inhibiting pyramidal excitability via activating SK channels is sufficient to improve cognitive function in this model. These findings unveil a dual mechanism for how D-serine improves cognitive function in this model.

GluN2D subunit-containing NMDA receptors regulate reticular thalamic neuron function and seizure susceptibility

Thalamic regulation of cortical function is important for several behavioral aspects including attention and sensorimotor control. This region has also been studied for its involvement in seizure activity. Among the NMDA receptor subunits GluN2C and GluN2D are particularly enriched in several thalamic nuclei including nucleus reticularis of the thalamus (nRT). We have previously found that GluN2C deletion does not have a strong influence on the basal excitability and burst firing characteristics of reticular thalamus neurons. Here we find that GluN2D ablation leads to reduced depolarization-induced spike frequency and reduced hyperpolarization-induced rebound burst firing in nRT neurons. Furthermore, reduced inhibitory neurotransmission was observed in the ventrobasal thalamus (VB). A model with preferential downregulation of GluN2D from parvalbumin (PV)-positive neurons was generated. Conditional deletion of GluN2D from PV neurons led to a decrease in excitability and burst firing. In addition, reduced excitability and burst firing was observed in the VB neurons together with reduced inhibitory neurotransmission. Finally, young mice with GluN2D downregulation in PV neurons showed significant resistance to pentylenetetrazol-induced seizure and differences in sensitivity to isoflurane anesthesia but were normal in other behaviors. Conditional deletion of GluN2D from PV neurons also affected expression of other GluN2 subunits and GABA receptor in the nRT. Together, these results identify a unique role of GluN2D-containing receptors in the regulation of thalamic circuitry and seizure susceptibility which is relevant to mutations in GRIN2D gene found to be associated with pediatric epilepsy.

Antidepressant activity of pharmacological and genetic deactivation of the small-conductance calcium-activated potassium channel subtype-3

RATIONALE

The voltage-insensitive, small-conductance calcium-activated potassium (SK) channel is a key regulator of neuronal depolarization and is implicated in the pathophysiology of depressive disorders.

OBJECTIVE

We ascertained whether the SK channel is impaired in the chronic unpredictable stress (CUS) model and whether it can serve as a molecular target of antidepressant action.

METHODS

We assessed the depressive-like behavioral phenotype of CUS-exposed rats and performed post-mortem SK channel binding and activity-dependent zif268 mRNA analyses on their brains. To begin an assessment of SK channel subtypes involved, we examined the effects of genetic and pharmacological inhibition of the SK3 channel using conditional knockout mice and selective SK3 channel negative allosteric modulators (NAMs).

RESULTS

We found that [¹²⁵I]apamin binding to SK channels is increased in the prefrontal cortex and decreased in the hippocampus, an effect that was associated with reciprocal levels of zif268 mRNA transcripts indicating abnormal regional cell activity in this model. We found that genetic and pharmacological manipulations significantly decreased immobility in the forced swim test without altering general locomotor activity, a hallmark of antidepressant-like activity.

CONCLUSIONS

Taken together, these findings link depression-related neural and behavioral pathophysiology with abnormal SK channel functioning and suggest that this can be reversed by the selective inhibition of SK3 channels.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Complicity of α-synuclein oligomer and calcium dyshomeostasis in selective neuronal vulnerability in Lewy body disease

α-Synuclein oligomers and Ca²⁺ dyshomeostasis have been thoroughly investigated with respect to the pathogenesis of Lewy body disease (LBD). In LBD, α-synuclein oligomers exhibit a neuron-specific cytoplasmic distribution. Highly active neurons and neurons with a high Ca²⁺ burden are prone to damage in LBD. The neuronal vulnerability may be determined by transneuronal axonal transmission of the pathological processes; however, this hypothesis seems inconsistent with pathological findings that neurons anatomically connected to LBD-vulnerable neurons, such as neurons in the ventral tegmentum, are spared in LBD. This review focuses on and discusses the crucial roles played by α-synuclein oligomers and Ca²⁺ dyshomeostasis in early intraneural pathophysiology in LBD-vulnerable neurons. A challenging view is proposed on the synergy between retrograde transport of α-synuclein and vesicular Ca release, whereby neuronal vulnerability is propagated backward along repeatedly activated signaling pathway.

The role of intrinsic excitability in the evolution of memory: Significance in memory allocation, consolidation, and updating

Memory is a dynamic process that is continuously regulated by both synaptic and intrinsic neural mechanisms. While numerous studies have shown that synaptic plasticity is important in various types and phases of learning and memory, neuronal intrinsic excitability has received relatively less attention, especially regarding the dynamic nature of memory. In this review, we present evidence demonstrating the importance of intrinsic excitability in memory allocation, consolidation, and updating. We also consider the intricate interaction between intrinsic excitability and synaptic plasticity in shaping memory, supporting both memory stability and flexibility.

Inherent Motor Impulsivity Associates with Specific Gene Targets in the Rat Medial Prefrontal Cortex

High impulsivity characterizes a myriad of neuropsychiatric diseases, and identifying targets for neuropharmacological intervention to reduce impulsivity could reveal transdiagnostic treatment strategies. Motor impulsivity (impulsive action) reflects in part the failure of "top-down" executive control by the medial prefrontal cortex (mPFC). The present study profiled the complete set of mRNA molecules expressed from genes (transcriptome) in the mPFC of male, outbred rats stably expressing high (HI) or low (LI) motor impulsivity based upon premature responses in the 1-choice serial reaction time (1-CSRT) task. RNA-sequencing identified expression of 18 genes that was higher in the mPFC of HI vs. LI rats. Functional gene enrichment revealed that biological processes related to calcium homeostasis and G protein-coupled receptor (GPCR) signaling pathways, particularly glutamatergic, were overrepresented in the mPFC of HI vs. LI rats. Transcription factor enrichment identified mothers against decapentaplegic homolog 4 (SMAD4) and RE1 silencing transcription factor (REST) as overrepresented in the mPFC of HI rats relative to LI rats, while in silico analysis predicted a conserved SMAD binding site within the voltage-gated calcium channel subunit alpha1 E (CACNA1E) promoter region. qRT-PCR analyses confirmed that mRNA expression of CACNA1E, as well as expression of leucyl and cystinyl aminopeptidase (LNPEP), were higher in the mPFC of HI vs. LI rats. These outcomes establish a transcriptomic landscape in the mPFC that is related to individual differences in motor impulsivity and propose novel gene targets for future impulsivity research.

Metabotropic Modulation of Potassium Channels During Synaptic Plasticity

Besides their primary function mediating the repolarization phase of action potentials, potassium channels exquisitely and ubiquitously regulate the resting membrane potential of neurons and therefore have a key role establishing their intrinsic excitability. This group of proteins is composed of a very diverse collection of voltage-dependent and -independent ion channels, whose specific distribution is finely tuned at the level of the synapse. Both at the presynaptic and postsynaptic membranes, different types of potassium channels are subjected to modulation by second messenger signaling cascades triggered by metabotropic receptors, which in this way serve as a link between neurotransmitter actions and changes in the neuron membrane excitability. On the one hand, by regulating the resting membrane potential of the postsynaptic membrane, potassium channels appear to be critical towards setting the threshold for the induction of long-term potentiation and depression. On the other hand, these channels maintain the presynaptic membrane potential under control, therefore influencing the probability of neurotransmitter release underlying different forms of short-term plasticity. In the present review, we examine in detail the role of metabotropic receptors translating their activation by different neurotransmitters into a final effect modulating several types of potassium channels. Furthermore, we evaluate the consequences that this interplay has on the induction and maintenance of different forms of synaptic plasticity.

Modification of Synaptic-Input Clustering by Intrinsic Excitability Plasticity on Cerebellar Purkinje Cell Dendrites

The role of dendrites in the integration of widespread synaptic activity has been studied in experiments and theories (Johnston et al., 1996; Magee, 2007). However, whether the conduction of synaptic currents from dendrites to the soma depends on excitability of those dendritic branches is unclear. How modulation of the branch excitability affects the conduction of synaptic inputs and their selection on dendrites is also elusive. Here, I performed simultaneous voltage-clamp recordings from the soma and dendrites of single cerebellar Purkinje neurons in male Sprague-Dawley rats and analyzed the relationship between spontaneous EPSCs on both sides. I found that EPSCs on distal dendrites have a salient discordance in amplitude compared with those on the soma. Furthermore, individual ratios of the EPSC concurrently recorded on the soma and dendrites were not unique, but discrete, suggesting the occurrence of various attenuations in different paths of dendritic branches to the soma. The obtained data and simulations indicate several distinct groups (4.5 ± 0.3, n = 22 somatodendritic recordings) of co-occurred synaptic inputs in Purkinje cell dendrites. This clustering of synaptic currents was suggested to emerge at farther distances than the secondary bifurcations. Finally, ratios of the co-EPSCs were uniformly distributed after either intrinsic plasticity induction or SK-channel blockade. Overall, results suggest that in Purkinje cells the excitability along the dendrite processes modulates the conduction of EPSCs and makes active inputs heterogeneous through SK channel activity, intrinsic plasticity, and dendritic branching. These properties of dendrites may confer branch-specific computational power to neurons.SIGNIFICANCE STATEMENT I have previously studied the "non-synaptic" plasticity of the intrinsic excitability in the cerebellar Purkinje cells (Belmeguenai et al., 2010), and branch-specific increase of intrinsic excitability of the dendrites (Ohtsuki et al., 2012b; Ohtsuki and Hansel, 2018) through the downregulation of SK (small conductance Ca2+-activated K+) channels. In this study, I show that a dendritic filtering of synaptic electroconductivity is heterogeneous among the branches on distal dendrites and that the increase in the dendritic excitability accompanied with the intrinsic plasticity alters a state with the heterogeneity to a globally excitable state in Purkinje neurons. My findings propose a new learning model relying on the intrinsic excitability plasticity of the dendritic branch fields.

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

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

A Key Role for Prefrontocortical Small Conductance Calcium-Activated Potassium Channels in Stress Adaptation and Rapid Antidepressant Response

The muscarinic acetylcholine receptor antagonist scopolamine elicits rapid antidepressant activity, but its underlying mechanism is not fully understood. In a chronic stress model, a single low-dose administration of scopolamine reversed depressive-like reactivity. This antidepressant-like effect was mediated via a muscarinic M1 receptor–SKC pathway because it was mimicked by intra-medial prefrontal cortex (intra-mPFC) infusions of scopolamine, of the M1 antagonist pirenzepine or of the SKC antagonist apamin, but not by the selective serotonin reuptake inhibitor (SSRI) antidepressant fluoxetine. Extracellular and whole-cell recordings revealed that scopolamine and ketamine attenuate the SKC-mediated action potential hyperpolarization current and rapidly enhance mPFC neuronal excitability within the therapeutically relevant time window. The SKC agonist 1-EBIO abrogated scopolamine-induced antidepressant activity at a dose that completely suppressed burst firing activity. Scopolamine also induced a slow-onset activation of raphe serotonergic neurons, which in turn was dependent on mPFC-induced neuroplasticity or excitatory input, since mPFC transection abolished this effect. These early behavioral and mPFC activational effects of scopolamine did not appear to depend on prefrontocortical brain-derived neurotrophic factor and serotonin-1A activity, classically linked to SSRIs, and suggest a novel mechanism associated with antidepressant response onset through SKC-mediated regulation of activity-dependent plasticity.

Modulation of burst firing of neurons in nucleus reticularis of the thalamus by GluN2C-containing NMDA receptors

The GluN2C subunit of the NMDA receptor is enriched in the neurons in nucleus reticularis of the thalamus (nRT), but its role in regulating their function is not well understood. We found that deletion of GluN2C subunit did not affect spike frequency in response to depolarizing current injection or hyperpolarization-induced rebound burst firing of nRT neurons. D-cycloserine or CIQ (GluN2C/GluN2D positive allosteric modulator) did not affect the depolarization-induced spike frequency in nRT neurons. A newly identified highly potent and efficacious co-agonist of GluN1/GluN2C NMDA receptors, AICP, was found to reduce the spike frequency and burst firing of nRT neurons in wildtype but not GluN2C knockout. This effect was potentially due to facilitation of GluN2C-containing receptors because inhibition of NMDA receptors by AP5 did not affect spike frequency in nRT neurons. We evaluated the effect of intracerebroventricular injection of AICP. AICP did not affect basal locomotion or prepulse inhibition but facilitated MK-801-induced hyperlocomotion. This effect was observed in wildtype but not in GluN2C knockout mice demonstrating that AICP produces GluN2C-selective effects in vivo Using a chemogenetic approach we examined the role of nRT in this behavioral effect. Gq or Gi coupled DREADDs were selectively expressed in nRT neurons using cre-dependent viral vectors and PV-Cre mouse line. We found that similar to AICP effect, activation of Gq but not Gi coupled DREADD facilitated MK-801-induced hyperlocomotion. Together, these results identify a unique role of GluN2C-containing receptors in the regulation of nRT neurons and suggest GluN2C-selective in vivo targeting of NMDA receptors by AICP. SIGNIFICANCE STATEMENT: The nucleus reticularis of the thalamus composed of GABAergic neurons is termed as guardian of the gateway and is an important regulator of corticothalamic communication which may be impaired in autism, non-convulsive seizures and other conditions. We found that strong facilitation of tonic activity of GluN2C subtype of NMDA receptors using AICP, a newly identified glycine-site agonist of NMDA receptors, modulates the function of reticular thalamus neurons. AICP was also able to produce GluN2C-dependent behavioral effects in vivo. Together, these finding identify a novel mechanism and a pharmacological tool to modulate activity of reticular thalamic neurons in disease states.

Small conductance calcium activated potassium (SK) channel dependent and independent effects of riluzole on neuropathic pain-related amygdala activity and behaviors in rats

BACKGROUND AND PURPOSE

Chronic neuropathic pain is an important healthcare issue with significant emotional components. The amygdala is a brain region involved in pain and emotional-affective states and disorders. The central amygdala output nucleus (CeA) contains small-conductance calcium-activated potassium (SK) channels that can control neuronal activity. A clinically available therapeutic, riluzole can activate SK channels and may have antinociceptive effects through a supraspinal action. We tested the hypothesis that riluzole inhibits neuropathic pain behaviors by inhibiting pain-related changes in CeA neurons, in part at least through SK channel activation.

EXPERIMENTAL APPROACH

Brain slice physiology and behavioral assays were done in adult Sprague Dawley rats. Audible and ultrasonic vocalizations and von Frey thresholds were measured in sham and neuropathic rats 4 weeks after left L5 spinal nerve ligation (SNL model). Whole cell patch-clamp recordings of regular firing CeA neurons in brain slices were used to measure synaptic transmission and neuronal excitability.

KEY RESULTS

In brain slices, riluzole increased the SK channel-mediated afterhyperpolarization and synaptic inhibition, but inhibited neuronal excitability through an SK channel independent action. SNL rats had increased vocalizations and decreased withdrawal thresholds compared to sham rats, and intra-CeA administration of riluzole inhibited vocalizations and depression-like behaviors but did not affect withdrawal thresholds. Systemic riluzole administration also inhibited these changes, demonstrating the clinical utility of this strategy. SK channel blockade in the CeA attenuated the inhibitory effects of systemic riluzole on vocalizations, confirming SK channel involvement in these effects.

CONCLUSIONS AND IMPLICATIONS

The results suggest that riluzole has beneficial effects on neuropathic pain behaviors through SK channel dependent and independent mechanisms in the amygdala.

Ionic Mechanism Underlying Rebound Depolarization in Medial Prefrontal Cortex Pyramidal Neurons

Rebound depolarization (RD) occurs after membrane hyperpolarization and converts an arriving inhibitory signal into cell excitation. The purpose of our study was to clarify the ionic mechanism of RD in synaptically isolated layer V medial prefrontal cortex (mPFC) pyramidal neurons in slices obtained from 58- to 62-day-old male rats. The RD was evoked after a step hyperpolarization below -80 mV, longer than 150 ms in 192 of 211 (91%) tested neurons. The amplitude of RD was 30.6 ± 1.2 mV above the resting membrane potential (-67.9 ± 0.95 mV), and it lasted a few 100 ms (n = 192). RD could be observed only after preventing BK channel activation, which was attained either by using paxilline, by removal of Ca⁺⁺ from the extra- or intracellular solution, by blockade of Ca⁺⁺ channels or during protein kinase C (PKC) activation. RD was resistant to tetrodotoxin (TTX) and was abolished after the removal of Na⁺ from the extracellular solution or application of an anti-Nav1.9 antibody to the cell interior. We conclude that two membrane currents are concomitantly activated after the step hyperpolarization in the tested neurons: a. a low-threshold, TTX-resistant, Na⁺ current that evokes RD; and b. an outward K⁺ current through BK channels that opposes Na⁺-dependent depolarization. The obtained results also suggest that a. low-level Ca⁺⁺ in the external medium attained upon intense neuronal activity may facilitate the formation of RD and seizures; and b. RD can be evoked during the activation of PKC, which is an effector of a number of transduction pathways.

Pleiotropic effects of schizophrenia-associated genetic variants in neuron firing and cardiac pacemaking revealed by computational modeling

Schizophrenia patients have an increased risk of cardiac dysfunction. A possible factor underlying this comorbidity are the common variants in the large set of genes that have recently been discovered in genome-wide association studies (GWASs) as risk genes of schizophrenia. Many of these genes control the cell electrogenesis and calcium homeostasis. We applied biophysically detailed models of layer V pyramidal cells and sinoatrial node cells to study the contribution of schizophrenia-associated genes on cellular excitability. By including data from functional genomics literature to simulate the effects of common variants of these genes, we showed that variants of voltage-gated Na⁺ channel or hyperpolarization-activated cation channel-encoding genes cause qualitatively similar effects on layer V pyramidal cell and sinoatrial node cell excitability. By contrast, variants of Ca²⁺ channel or transporter-encoding genes mostly have opposite effects on cellular excitability in the two cell types. We also show that the variants may crucially affect the propagation of the cardiac action potential in the sinus node. These results may help explain some of the cardiac comorbidity in schizophrenia, and may facilitate generation of effective antipsychotic medications without cardiac side-effects such as arrhythmia.

SK Channels Regulate Resting Properties and Signaling Reliability of a Developing Fast-Spiking Neuron

Reliable and precise signal transmission is essential in circuits of the auditory brainstem to encode timing with submillisecond accuracy. Globular bushy cells reliably and faithfully transfer spike signals to the principal neurons of the medial nucleus of the trapezoid body (MNTB) through the giant glutamatergic synapse, the calyx of Held. Thus, the MNTB works as a relay nucleus that preserves the temporal pattern of firing at high frequency. Using whole-cell patch-clamp recordings, we observed a K⁺ conductance mediated by small-conductance calcium-activated potassium (SK) channels in the MNTB neurons from rats of either sex. SK channels were activated by intracellular Ca²⁺ sparks and mediated spontaneous transient outward currents in developing MNTB neurons. SK channels were also activated by Ca²⁺ influx through voltage-gated Ca²⁺ channels and synaptically activated NMDA receptors. Blocking SK channels with apamin depolarized the resting membrane potential, reduced resting conductance, and affected the responsiveness of MNTB neurons to signal inputs. Moreover, SK channels were activated by action potentials and affected the spike afterhyperpolarization. Blocking SK channels disrupted the one-to-one signal transmission from presynaptic calyces to postsynaptic MNTB neurons and induced extra postsynaptic action potentials in response to presynaptic firing. These data reveal that SK channels play crucial roles in regulating the resting properties and maintaining reliable signal transmission of MNTB neurons.SIGNIFICANCE STATEMENT Reliable and precise signal transmission is required in auditory brainstem circuits to localize the sound source. The calyx of Held synapse in the mammalian medial nucleus of the trapezoid body (MNTB) plays an important role in sound localization. We investigated the potassium channels that shape the reliability of signal transfer across the calyceal synapse and observed a potassium conductance mediated by small-conductance calcium-activated potassium (SK) channels in rat MNTB principal neurons. We found that SK channels are tonically activated and contribute to the resting membrane properties of MNTB neurons. Interestingly, SK channels are transiently activated by calcium sparks and calcium influx during action potentials and control the one-to-one signal transmission from presynaptic calyces to postsynaptic MNTB neurons.

Pharmacological modulation of the voltage-gated neuronal Kv7/KCNQ/M-channel alters the intrinsic excitability and synaptic responses of pyramidal neurons in rat prefrontal cortex slices

The prefrontal cortex (PFC) critical for higher cognition is implicated in neuropsychiatric diseases, such as Alzheimer's disease, depression and schizophrenia. The voltage-activated Kv7/KCNQ/M-channel or M-current modulates the neuronal excitability that defines the fundamental mechanism of brain function. However, whether M-current functions to regulate the excitability of PFC neurons remains elusive. In this study, we recorded the native M-current from PFC layer V pyramidal neurons in rat brain slices and showed that it modulated the intrinsic excitability and synaptic responses of PFC pyramidal neurons. Application of a specific M-channel blocker XE991 (40 μmol/L) or opener retigabine (10 μmol/L) resulted in inhibition or activation of M-current, respectively. In the current-clamp recordings, inhibition of M-current was evidenced by the increased average spike frequency and the reduced first inter-spike interval (ISI), spike onset latency and fast afterhyperpolarization (fAHP), whereas activation of M-current caused opposite responses. Furthermore, inhibition of M-current significantly increased the amplitude of excitatory postsynaptic potentials (EPSPs) and depolarized the resting membrane potential (RMP) without affecting the miniature EPSC (mEPSC) frequency. These data demonstrate that voltage-gated neuronal Kv7/KCNQ/M-current modulates the excitability and synaptic transmission of PFC neurons, suggesting that pharmacological modulation of M-current in the PFC may exert beneficial effects on cognitive deficits implicated in the pathophysiology of neuropsychiatric disorders.

Spike Frequency Adaptation in Neurons of the Central Nervous System

Neuronal firing patterns and frequencies determine the nature of encoded information of the neurons. Here we discuss the molecular identity and cellular mechanisms of spike-frequency adaptation in central nervous system (CNS) neurons. Calcium-activated potassium (K_Ca) channels such as BK_Ca and SK_Ca channels have long been known to be important mediators of spike adaptation via generation of a large afterhyperpolarization when neurons are hyper-activated. However, it has been shown that a strong hyperpolarization via these K_Ca channels would cease action potential generation rather than reducing the frequency of spike generation. In some types of neurons, the strong hyperpolarization is followed by oscillatory activity in these neurons. Recently, spike-frequency adaptation in thalamocortical (TC) and CA1 hippocampal neurons is shown to be mediated by the Ca²⁺-activated Cl- channel (CACC), anoctamin-2 (ANO2). Knockdown of ANO2 in these neurons results in significantly reduced spike-frequency adaptation accompanied by increased number of spikes without shifting the firing mode, which suggests that ANO2 mediates a genuine form of spike adaptation, finely tuning the frequency of spikes in these neurons. Based on the finding of a broad expression of this new class of CACC in the brain, it can be proposed that the ANO2-mediated spike-frequency adaptation may be a general mechanism to control information transmission in the CNS neurons.

mTORC1 Is a Local, Postsynaptic Voltage Sensor Regulated by Positive and Negative Feedback Pathways

The mammalian/mechanistic target of rapamycin complex 1 (mTORC1) serves as a regulator of mRNA translation. Recent studies suggest that mTORC1 may also serve as a local, voltage sensor in the postsynaptic region of neurons. Considering biochemical, bioinformatics and imaging data, we hypothesize that the activity state of mTORC1 dynamically regulates local membrane potential by promoting and repressing protein synthesis of select mRNAs. Our hypothesis suggests that mTORC1 uses positive and negative feedback pathways, in a branch-specific manner, to maintain neuronal excitability within an optimal range. In some dendritic branches, mTORC1 activity oscillates between the "On" and "Off" states. We define this as negative feedback. In contrast, positive feedback is defined as the pathway that leads to a prolonged depolarized or hyperpolarized resting membrane potential, whereby mTORC1 activity is constitutively on or off, respectively. We propose that inactivation of mTORC1 increases the expression of voltage-gated potassium alpha (Kv1.1 and 1.2) and beta (Kvβ2) subunits, ensuring that the membrane resets to its resting membrane potential after experiencing increased synaptic activity. In turn, reduced mTORC1 activity increases the protein expression of syntaxin-1A and promotes the surface expression of the ionotropic glutamate receptor N-methyl-D-aspartate (NMDA)-type subunit 1 (GluN1) that facilitates increased calcium entry to turn mTORC1 back on. Under conditions such as learning and memory, mTORC1 activity is required to be high for longer periods of time. Thus, the arm of the pathway that promotes syntaxin-1A and Kv1 protein synthesis will be repressed. Moreover, dendritic branches that have low mTORC1 activity with increased Kv expression would balance dendrites with constitutively high mTORC1 activity, allowing for the neuron to maintain its overall activity level within an ideal operating range. Finally, such a model suggests that recruitment of more positive feedback dendritic branches within a neuron is likely to lead to neurodegenerative disorders.

Small conductance Ca2+-activated K+ channels in the plasma membrane, mitochondria and the ER: Pharmacology and implications in neuronal diseases

Ca²⁺-activated K⁺ (K_Ca) channels regulate after-hyperpolarization in many types of neurons in the central and peripheral nervous system. Small conductance Ca²⁺-activated K⁺ (K_Ca2/SK) channels, a subfamily of K_Ca channels, are widely expressed in the nervous system, and in the cardiovascular system. Voltage-independent SK channels are activated by alterations in intracellular Ca²⁺ ([Ca²⁺]_i) which facilitates the opening of these channels through binding of Ca²⁺ to calmodulin that is constitutively bound to the SK2 C-terminus. In neurons, SK channels regulate synaptic plasticity and [Ca²⁺]_i homeostasis, and a number of recent studies elaborated on the emerging neuroprotective potential of SK channel activation in conditions of excitotoxicity and cerebral ischemia, as well as endoplasmic reticulum (ER) stress and oxidative cell death. Recently, SK channels were discovered in the inner mitochondrial membrane and in the membrane of the endoplasmic reticulum which sheds new light on the underlying molecular mechanisms and pathways involved in SK channel-mediated protective effects. In this review, we will discuss the protective properties of pharmacological SK channel modulation with particular emphasis on intracellularly located SK channels as potential therapeutic targets in paradigms of neuronal dysfunction.

'Tagging' along memories in aging: Synaptic tagging and capture mechanisms in the aged hippocampus

Aging is accompanied by a general decline in the physiological functions of the body with the deteriorating organ systems. Brain is no exception to this and deficits in cognitive functions are quite common in advanced aging. Though a variety of age-related alterations are observed in the structure and function throughout the brain, certain regions show selective vulnerability. Medial temporal lobe, especially the hippocampus, is one such preferentially vulnerable region and is a crucial structure involved in the learning and long-term memory functions. Hippocampal synaptic plasticity, such as long-term potentiation (LTP) and depression (LTD), are candidate cellular correlates of learning and memory and alterations in these properties have been well documented in aging. A related phenomenon called synaptic tagging and capture (STC) has been proposed as a mechanism for cellular memory consolidation and to account for temporal association of memories. Mounting evidences from behavioral settings suggest that STC could be a physiological phenomenon. In this article, we review the recent data concerning STC and provide a framework for how alterations in STC-related mechanisms could contribute to the age-associated memory impairments. The enormity of impairment in learning and memory functions demands an understanding of age-associated memory deficits at the fundamental level given its impact in the everyday tasks, thereby in the quality of life. Such an understanding is also crucial for designing interventions and preventive measures for successful brain aging.

SK channel subtypes enable parallel optimized coding of behaviorally relevant stimulus attributes: A review

Ion channels play essential roles toward determining how neurons respond to sensory input to mediate perception and behavior. Small conductance calcium-activated potassium (SK) channels are found ubiquitously throughout the brain and have been extensively characterized both molecularly and physiologically in terms of structure and function. It is clear that SK channels are key determinants of neural excitability as they mediate important neuronal response properties such as spike frequency adaptation. However, the functional roles of the different known SK channel subtypes are not well understood. Here we review recent evidence from the electrosensory system of weakly electric fish suggesting that the function of different SK channel subtypes is to optimize the processing of independent but behaviorally relevant stimulus attributes. Indeed, natural sensory stimuli frequently consist of a fast time-varying waveform (i.e., the carrier) whose amplitude (i.e., the envelope) varies slowly and independently. We first review evidence showing how somatic SK2 channels mediate tuning and responses to carrier waveforms. We then review evidence showing how dendritic SK1 channels instead determine tuning and optimize responses to envelope waveforms based on their statistics as found in the organism's natural environment in an independent fashion. The high degree of functional homology between SK channels in electric fish and their mammalian orthologs, as well as the many important parallels between the electrosensory system and the mammalian visual, auditory, and vestibular systems, suggest that these functional roles are conserved across systems and species.

Dynamic DNA methylation regulates neuronal intrinsic membrane excitability

Epigenetic modifications, such as DNA cytosine methylation, contribute to the mechanisms underlying learning and memory by coordinating adaptive gene expression and neuronal plasticity. Transcription-dependent plasticity regulated by DNA methylation includes synaptic plasticity and homeostatic synaptic scaling. Memory-related plasticity also includes alterations in intrinsic membrane excitability mediated by changes in the abundance or activity of ion channels in the plasma membrane, which sets the threshold for action potential generation. We found that prolonged inhibition of DNA methyltransferase (DNMT) activity increased intrinsic membrane excitability of cultured cortical pyramidal neurons. Knockdown of the cytosine demethylase TET1 or inhibition of RNA polymerase blocked the increased membrane excitability caused by DNMT inhibition, suggesting that this effect was mediated by subsequent cytosine demethylation and de novo transcription. Prolonged DNMT inhibition blunted the medium component of the after-hyperpolarization potential, an effect that would increase neuronal excitability, and was associated with reduced expression of the genes encoding small-conductance Ca(2+)-activated K(+) (SK) channels. Furthermore, the specific SK channel blocker apamin increased neuronal excitability but was ineffective after DNMT inhibition. Our results suggested that DNMT inhibition enables transcriptional changes that culminate in decreased expression of SK channel-encoding genes and decreased activity of SK channels, thus providing a mechanism for the regulation of neuronal intrinsic membrane excitability by dynamic DNA cytosine methylation. This study has implications for human neurological and psychiatric diseases associated with dysregulated intrinsic excitability.

Oxidation of ion channels in the aging nervous system

Ion channels are integral membrane proteins that allow passive diffusion of ions across membranes. In neurons and in other excitable cells, the harmonious coordination between the numerous types of ion channels shape and propagate electrical signals. Increased accumulation of reactive oxidative species (ROS), and subsequent oxidation of proteins, including ion channels, is a hallmark feature of aging and may contribute to cell failure as a result. In this review we discuss the effects of ROS on three major types of ion channels of the central nervous system, namely the potassium (K(+)), calcium (Ca(2+)) and sodium (Na(+)) channels. We examine two general mechanisms through which ROS affect ion channels: via direct oxidation of specific residues and via indirect interference of pathways that regulate the channels. The overall status of the present studies indicates that the interaction of ion channels with ROS is multimodal and pervasive in the central nervous system and likely constitutes a general mechanism of aging susceptibility.

In vivo activation of the SK channel in the spinal cord reduces the NMDA receptor antagonist dose needed to produce antinociception in an inflammatory pain model

N-methyl-D-aspartate receptor (NMDAR) antagonists have been shown to reduce mechanical hypersensitivity in animal models of inflammatory pain. However, their clinical use is associated with significant dose-limiting side effects. Small-conductance Ca-activated K channels (SK) have been shown to modulate NMDAR activity in the brain. We demonstrate that in vivo activation of SK channels in the spinal cord can alleviate mechanical hypersensitivity in a rat model of inflammatory pain. Intrathecal (i.t.) administration of the SK channel activator, 6,7-dichloro-1H-indole-2,3-dione 3-oxime (NS309), attenuates complete Freund adjuvant (CFA)-induced mechanical hypersensitivity in a dose-dependent manner. Postsynaptic expression of the SK channel subunit, SK3, and apamin-sensitive SK channel-mediated currents recorded from superficial laminae are significantly reduced in the dorsal horn (DH) after CFA. Complete Freund adjuvant-induced decrease in SK-mediated currents can be reversed in vitro by bath application of NS309. In addition, immunostaining for the SK3 subunit indicates that SK3-containing channels within DH neurons can have both somatic and dendritic localization. Double immunostaining shows coexpression of SK3 and NMDAR subunit, NR1, compatible with functional interaction. Moreover, we demonstrate that i.t. coadministration of NS309 with an NMDAR antagonist reduces the dose of NMDAR antagonist, DL-2-amino-5-phosphonopentanoic acid (DL-AP5), required to produce antinociceptive effects in the CFA model. This reduction could attenuate the unwanted side effects associated with NMDAR antagonists, giving this combination potential clinical implications.

Small-conductance calcium-activated potassium (SK) channels in the amygdala mediate pain-inhibiting effects of clinically available riluzole in a rat model of arthritis pain

BACKGROUND

Arthritis pain is an important healthcare issue with significant emotional and affective consequences. Here we focus on potentially beneficial effects of activating small-conductance calcium-activated potassium (SK) channels in the amygdala, a brain center of emotions that plays an important role in central pain modulation and processing. SK channels have been reported to regulate neuronal activity in the central amygdala (CeA, output nucleus). We tested the effects of riluzole, a clinically available drug for the treatment of amyotrophic lateral sclerosis, for the following reasons. Actions of riluzole include activation of SK channels. Evidence in the literature suggests that riluzole may have antinociceptive effects through an action in the brain but not the spinal cord. Mechanism and site of action of riluzole remain to be determined. Here we tested the hypothesis that riluzole inhibits pain behaviors by acting on SK channels in the CeA in an arthritis pain model.

RESULTS

Systemic (intraperitoneal) application of riluzole (8 mg/kg) inhibited audible (nocifensive response) and ultrasonic (averse affective response) vocalizations of adult rats with arthritis (5 h postinduction of a kaolin-carrageenan monoarthritis in the knee) but did not affect spinal withdrawal thresholds, which is consistent with a supraspinal action. Stereotaxic administration of riluzole into the CeA by microdialysis (1 mM, concentration in the microdialysis fiber, 15 min) also inhibited vocalizations, confirming the CeA as a site of action of riluzole. Stereotaxic administration of a selective SK channel blocker (apamin, 1 µM, concentration in the microdialysis fiber, 15 min) into the CeA had no effect by itself but inhibited the effect of systemic riluzole on vocalizations. Off-site administration of apamin into the basolateral amygdala (BLA) as a placement control or stereotaxic application of a selective blocker of large-conductance calcium-activated potassium (BK) channels (charybdotoxin, 1 µM, concentration in the microdialysis fiber, 15 min) into the CeA did not affect the inhibitory effects of systemically applied riluzole.

CONCLUSIONS

The results suggest that riluzole can inhibit supraspinally organized pain behaviors in an arthritis model by activating SK, but not BK, channels in the amygdala (CeA but not BLA).

NR2B subunit in the prefrontal cortex: A double-edged sword for working memory function and psychiatric disorders

The prefrontal cortex (PFC) is a brain region featured with working memory function. The exact mechanism of how working memory operates within the PFC circuitry is unknown, but persistent neuronal firing recorded from prefrontal neurons during a working memory task is proposed to be the neural correlate of this mnemonic encoding. The PFC appears to be specialized for sustaining persistent firing, with N-methyl-D-aspartate (NMDA) receptors, especially slow-decay NR2B subunits, playing an essential role in the maintenance of sustained activity and normal working memory function. However, the NR2B subunit serves as a double-edged sword for PFC function. Because of its slow kinetics, NR2B endows the PFC with not only "neural psychic" properties, but also susceptibilities for neuroexcitotoxicity and psychiatric disorders. This review aims to clarify the interplay among working memory, the PFC, and NMDA receptors; demonstrate the importance of NR2B in the maintenance of persistent activity; understand the risks and vulnerabilities of how NR2B is related to the development of neuropsychiatric disorders; identify gaps that currently exist in our understanding of these processes; and provide insights regarding future directions that may clarify these issues. We conclude that the PFC is a specialized brain region with distinct delayed maturation, unique neuronal circuitry, and characteristic NMDA receptor function. The unique properties and development of NMDA receptors, especially enrichment of NR2B subunits, endow the PFC with not only the capability to generate sustained activity for working memory, but also serves as a major vulnerability to environmental insults and risk factors for psychiatric disorders.

Developmental mapping of small-conductance calcium-activated potassium channel expression in the rat nervous system

Early electrical activity and calcium influx regulate crucial aspects of neuronal development. Small-conductance calcium-activated potassium (SK) channels regulate action potential firing and shape calcium influx through feedback regulation in mature neurons. These functions, observed in the adult nervous system, make them ideal candidates to regulate activity-and calcium-dependent processes in neurodevelopment. However, to date little is known about the onset of expression and regions expressing SK channel subunits in the embryonic and postnatal development of the central nervous system (CNS). To allow studies on the contribution of SK channels to different phases of development of single neurons and networks, we have performed a detailed in situ hybridization mapping study, providing comprehensive distribution profiles of all three SK subunits (SK1, SK2, and SK3) in the rat CNS during embryonic and postnatal development. SK channel transcripts are expressed at early stages of prenatal CNS development. The three SK channel subunits display different developmental expression gradients in distinct CNS regions, with time points of expression and up-or downregulation that can be associated with a range of diverse developmental events. Their early expression in embryonic development suggests an involvement of SK channels in the regulation of developmental processes. Additionally, this study shows how the postnatal ontogenetic patterns lead to the adult expression map for each SK channel subunit and how their coexpression in the same regions or neurons varies throughout development. J. Comp. Neurol. 522:1072–1101, 2014. © 2013 Wiley Periodicals, Inc.

A synaptic mechanism for network synchrony

Within neural networks, synchronization of activity is dependent upon the synaptic connectivity of embedded microcircuits and the intrinsic membrane properties of their constituent neurons. Synaptic integration, dendritic Ca²⁺ signaling, and non-linear interactions are crucial cellular attributes that dictate single neuron computation, but their roles promoting synchrony and the generation of network oscillations are not well understood, especially within the context of a defined behavior. In this regard, the lamprey spinal central pattern generator (CPG) stands out as a well-characterized, conserved vertebrate model of a neural network (Smith et al., 2013a), which produces synchronized oscillations in which neural elements from the systems to cellular level that control rhythmic locomotion have been determined. We review the current evidence for the synaptic basis of oscillation generation with a particular emphasis on the linkage between synaptic communication and its cellular coupling to membrane processes that control oscillatory behavior of neurons within the locomotor network. We seek to relate dendritic function found in many vertebrate systems to the accessible lamprey central nervous system in which the relationship between neural network activity and behavior is well understood. This enables us to address how Ca²⁺ signaling in spinal neuron dendrites orchestrate oscillations that drive network behavior.

Location matters: somatic and dendritic SK channels answer to distinct calcium signals

Voltage-dependent calcium channels (VDCCs) couple neuronal activity to diverse intracellular signals with exquisite spatiotemporal specificity. Using calcium imaging and electrophysiology, Jones and Stuart (J Neurosci 33: 19396-19405, 2013) examined the intimate relationship between distinct types of VDCCs and small-conductance calcium-activated potassium (SK) channels that contribute to the compartmentalized control of excitability in the soma and dendrites of cortical pyramidal neurons. Here we discuss the importance of calcium domains for signal specificity, explore the possible functions and mechanisms for local control of SK channels, and highlight technical considerations for the optical detection of calcium signals.

Selective phosphorylation modulates the PIP2 sensitivity of the CaM-SK channel complex

Phosphatidylinositol bisphosphate (PIP2) regulates the activities of many membrane proteins, including ion channels, through direct interactions. However, the affinity of PIP2 is so high for some channel proteins that its physiological role as a modulator has been questioned. Here we show that PIP2 is a key cofactor for activation of small conductance Ca2+-activated potassium channels (SKs) by Ca(2+)-bound calmodulin (CaM). Removal of the endogenous PIP2 inhibits SKs. The PIP2-binding site resides at the interface of CaM and the SK C terminus. We further demonstrate that the affinity of PIP2 for its target proteins can be regulated by cellular signaling. Phosphorylation of CaM T79, located adjacent to the PIP2-binding site, by casein kinase 2 reduces the affinity of PIP2 for the CaM-SK channel complex by altering the dynamic interactions among amino acid residues surrounding the PIP2-binding site. This effect of CaM phosphorylation promotes greater channel inhibition by G protein-mediated hydrolysis of PIP2.

Age-dependent sensitivity to glucocorticoids in the developing mouse basolateral nucleus of the amygdala

Experiences of severe trauma during childhood are thought to be risk factors for developing mental disorders, such as anxiety and mood disorders, later in life. Correspondingly, exposure of rodents to early-life stress has been shown to affect neuronal circuitry and emotional behavior in adulthood, indicating a significant impact of stress on brain development. One current hypothesis proposes that the developing central nervous system is more sensitive to environmental influences, such as stress, than the adult. To test this hypothesis, we compared long-lasting effects of systemic corticosterone (CORT) administrations in two distinct early developmental periods. Mice exposed to early-neonatal CORT treatment on postnatal days (PD) 2-4 exhibited strongly enhanced excitability of neurons of the basolateral nucleus of the amygdala (BLA) in early adolescence and displayed impaired extinction of contextually conditioned fear memory, a type of behavior in which the BLA plays an important role. Furthermore, gene-expression of NMDA receptor subunits as well as calcium-activated K(+)-channels was reduced in the amygdala. In contrast, exposure to the same CORT concentrations in a late-neonatal period (PD17-19) did not significantly affect BLA electrophysiology or extinction learning in adolescence. These results suggest age-dependent consequences of neonatal CORT exposure in amygdala neurons and provide evidence for a detrimental influence of early-neonatal stress on adolescent fear-memory processing.

SK channels modulate the excitability and firing precision of projection neurons in the robust nucleus of the arcopallium in adult male zebra finches

OBJECTIVE

Motor control is encoded by neuronal activity. Small conductance Ca(2+)-activated K(+) channels (SK channels) maintain the regularity and precision of firing by contributing to the afterhyperpolarization (AHP) of the action potential in mammals. However, it is not clear how SK channels regulate the output of the vocal motor system in songbirds. The premotor robust nucleus of the arcopallium (RA) in the zebra finch is responsible for the output of song information. The temporal pattern of spike bursts in RA projection neurons is associated with the timing of the acoustic features of birdsong.

METHODS

The firing properties of RA projection neurons were analyzed using patch clamp whole-cell and cell-attached recording techniques.

RESULTS

SK channel blockade by apamin decreased the AHP amplitude and increased the evoked firing rate in RA projection neurons. It also caused reductions in the regularity and precision of firing. RA projection neurons displayed regular spontaneous action potentials, while apamin caused irregular spontaneous firing but had no effect on the firing rate. In the absence of synaptic inputs, RA projection neurons still had spontaneous firing, and apamin had an evident effect on the firing rate, but caused no significant change in the firing regularity, compared with apamin application in the presence of synaptic inputs.

CONCLUSION

SK channels contribute to the maintenance of firing regularity in RA projection neurons which requires synaptic activity, and consequently ensures the precision of song encoding.

Synaptic excitation is regulated by the postsynaptic dSK channel at the Drosophila larval NMJ

In the mammalian central nervous system, the postsynaptic small-conductance Ca(2+)-dependent K(+) (SK) channel has been shown to reduce postsynaptic depolarization and limit Ca(2+) influx through N-methyl-d-aspartate receptors. To examine further the role of the postsynaptic SK channel in synaptic transmission, we studied its action at the Drosophila larval neuromuscular junction (NMJ). Repetitive synaptic stimulation produced an increase in postsynaptic membrane conductance leading to depression of excitatory postsynaptic potential amplitude and hyperpolarization of the resting membrane potential (RMP). This reduction in synaptic excitation was due to the postsynaptic Drosophila SK (dSK) channel; synaptic depression, increased membrane conductance and RMP hyperpolarization were reduced in dSK mutants or after expressing a Ca(2+) buffer in the muscle. Ca(2+) entering at the postsynaptic membrane was sufficient to activate dSK channels based upon studies in which the muscle membrane was voltage clamped to prevent opening voltage-dependent Ca(2+) channels. Increasing external Ca(2+) produced an increase in resting membrane conductance and RMP that was not seen in dSK mutants or after adding the glutamate-receptor blocker philanthotoxin. Thus it appeared that dSK channels were also activated by spontaneous transmitter release and played a role in setting membrane conductance and RMP. In mammals, dephosphorylation by protein phosphatase 2A (PP2A) increased the Ca(2+) sensitivity of the SK channel; PP2A appeared to increase the sensitivity of the dSK channel since PP2A inhibitors reduced activation of the dSK channel by evoked synaptic activity or increased external Ca(2+). It is proposed that spontaneous and evoked transmitter release activate the postsynaptic dSK channel to limit synaptic excitation and stabilize synapses.

Modulating fear extinction memory by manipulating SK potassium channels in the infralimbic cortex

Fear extinction correlates with increased infralimbic (IL) neuronal excitability. Since small conductance Ca²⁺-dependent K⁺ (SK) channels modulate neuronal excitability and certain types of learning and memory, pharmacological modulation of SK channels could be used to regulate IL excitability and fear extinction. To test this, we first determined the effect of blocking SK channels with apamin on the intrinsic excitability of IL pyramidal neurons in brain slices. In whole-cell patch-clamp recordings, apamin increased the number of spikes evoked by a depolarizing current pulse, increased the firing frequency, and reduced the fast afterhyperpolarizing potential (fAHP) indicating that blockade of SK channels could be used to enhance the intrinsic excitability of IL neurons. Next, we assessed whether SK channels in IL regulate extinction of conditioned fear by infusing apamin into IL of fear conditioned rats prior to extinction training. Apamin infusion did not affect conditioned freezing at the beginning of the extinction session or within-session extinction. However, the following day, apamin-infused rats showed significantly less conditioned freezing. To further examine the importance of SK channels in IL in fear extinction, we assessed the effect of the SK channel activator DCEBIO on IL neuronal excitability and fear extinction. Activation of SK channels with DCEBIO decreased the number of evoked spikes, reduced the firing frequency, and enhanced the fAHP of IL neurons. Infusion of DCEBIO into IL prior to fear extinction impaired recall of fear extinction without affecting acquisition of extinction. Taken together, these findings suggest that SK channels are involved in regulating IL excitability and extinction-induced plasticity. Therefore, SK channels are a potential target for the development of new pharmacological treatments to facilitate extinction in patients suffering from anxiety disorders.

Distinct Ca2+ sources in dendritic spines of hippocampal CA1 neurons couple to SK and Kv4 channels

Small conductance Ca(2+)-activated K(+) (SK) channels and voltage-gated A-type Kv4 channels shape dendritic excitatory postsynaptic potentials (EPSPs) in hippocampal CA1 pyramidal neurons. Synaptically evoked Ca(2+) influx through N-methyl-D-aspartate receptors (NMDARs) activates spine SK channels, reducing EPSPs and the associated spine head Ca(2+) transient. However, results using glutamate uncaging implicated Ca(2+) influx through SNX-482-sensitive (SNX-sensitive) Cav2.3 (R-type) Ca(2+) channels as the Ca(2+) source for SK channel activation. The present findings show that, using Schaffer collateral stimulation, the effects of SNX and apamin are not mutually exclusive and SNX increases EPSPs independent of SK channel activity. Dialysis with 1,2-bis(o-aminophenoxy)ethane-N'N'N'-tetraacetic acid (BAPTA), application of 4-Aminopyridine (4-AP), expression of a Kv4.2 dominant negative subunit, and dialysis with a KChIPs antibody occluded the SNX-induced increase of EPSPs. The results suggest two distinct Ca(2+) signaling pathways within dendritic spines that link Ca(2+) influx through NMDARs to SK channels and Ca(2+) influx through R-type Ca(2+) channels to Kv4.2-containing channels.

Different calcium sources control somatic versus dendritic SK channel activation during action potentials

Small-conductance calcium-activated potassium (SK) channels play an important role in regulating neuronal excitability. While SK channels at the soma have long been known to contribute to the medium afterhyperpolarization (mAHP), recent evidence indicates they also regulate NMDA receptor activation in dendritic spines. Here we investigate the activation of SK channels in spines and dendrites of rat cortical pyramidal neurons during action potentials (APs), and compare this to SK channel activation at the soma. Using confocal calcium imaging, we demonstrate that the inhibition of SK channels with apamin results in a location-dependent increase in calcium influx into dendrites and spines during backpropagating APs (average increase, ~40%). This effect was occluded by block of R-type voltage-dependent calcium channels (VDCCs), but not by inhibition of N- or P/Q-type VDCCs, or block of calcium release from intracellular stores. During these experiments, we noticed that the calcium indicator (Oregon Green BAPTA-1) blocked the mAHP. Subsequent experiments using low concentrations of EGTA (1 mm) produced the same result, suggesting that somatic SK channels are not tightly colocalized with their calcium source. Consistent with this idea, all known subtypes of VDCCs except R-type were calcium sources for the apamin-sensitive mAHP at the soma. We conclude that SK channels in spines and dendrites of cortical pyramidal neurons regulate calcium influx during backpropagating APs in a distance-dependent manner, and are tightly coupled to R-type VDCCs. In contrast, SK channels activated by APs at the soma of these neurons are weakly coupled to a variety of VDCCs.

Impact of subthreshold membrane potential on synaptic responses at dendritic spines of layer 5 pyramidal neurons in the prefrontal cortex

Glutamatergic inputs onto cortical pyramidal neurons are received and initially processed at dendritic spines. AMPA and NMDA receptors generate both synaptic potentials and calcium (Ca) signals in the spine head. These responses can in turn activate a variety of Ca, sodium (Na), and potassium (K) channels at spines. In principle, the roles of these receptors and channels can be strongly regulated by the subthreshold membrane potential. However, the impact of different receptors and channels has usually been studied at the level of dendrites. Much less is known about their influence at spines, where synaptic transmission and plasticity primarily occur. Here we examine single-spine responses in the basal dendrites of layer 5 pyramidal neurons in the mouse prefrontal cortex. Using two-photon microscopy and two-photon uncaging, we first show that synaptic potentials and Ca signals differ at resting and near-threshold potentials. We then determine how subthreshold depolarizations alter the contributions of AMPA and NMDA receptors to synaptic responses. We show that voltage-sensitive Ca channels enhance synaptic Ca signals but fail to engage small-conductance Ca-activated K (SK) channels, which require greater numbers of inputs. Finally, we establish how the subthreshold membrane potential controls the ability of voltage-sensitive Na channels and K channels to influence synaptic responses. Our findings reveal how subthreshold depolarizations promote electrical and biochemical signaling at dendritic spines by regulating the contributions of multiple glutamate receptors and ion channels.

Molecular influences on working memory circuits in dorsolateral prefrontal cortex

The working memory circuits of the primate dorsolateral prefrontal cortex (dlPFC) are modulated in a unique manner, often opposite to the molecular mechanisms needed for long-term memory consolidation. Working memory, our "mental sketch pad" is an ephemeral process, whereby transient, mental representations form the foundation for abstract thought. The microcircuits that generate mental representations are found in deep layer III of the dlPFC, where pyramidal cells excite each other to keep information "in mind" through NMDA receptor synapses on spines. The catecholaminergic and cholinergic arousal systems have rapid and flexible influences on the strength of these connections, thus allowing coordination between arousal and cognitive states. These modulators can rapidly weaken connectivity, for example, as occurs during uncontrollable stress, via feedforward calcium-cAMP signaling opening potassium (K(+)) channels near synapses on spines. Lower levels of calcium-cAMP-K(+) channel signaling provide negative feedback within recurrent excitatory circuits, and help to gate inputs to shape the contents of working memory. There are also explicit mechanisms to inhibit calcium-cAMP signaling and strengthen connectivity, for example, postsynaptic α2A-adrenoceptors on spines. This work has led to the development of the α2A agonist, guanfacine, for the treatment of a variety of dlPFC disorders. In mental illness, there are a variety of genetic insults to the molecules that normally serve to inhibit calcium-cAMP signaling in spines, thus explaining why so many genetic insults can lead to the same phenotype of impaired dlPFC cognitive function. Thus, the molecular mechanisms that provide mental flexibility may also confer vulnerability when dysregulated in cognitive disorders.

Neurochemical mechanisms of alcohol withdrawal

Alcohol dependence encompasses a serious medical and societal problem that constitutes a major public health concern. A serious consequence of dependence is the emergence of symptoms associated with the alcohol withdrawal syndrome when drinking is abruptly terminated or substantially reduced. Clinical features of alcohol withdrawal include signs of central nervous system hyperexcitability, heightened autonomic nervous system activation, and a constellation of symptoms contributing to psychologic discomfort and negative affect. The development of alcohol dependence is a complex and dynamic process that ultimately reflects a maladaptive neurophysiologic state. Perturbations in a wide range of neurochemical systems, including glutamate, γ-aminobutyric acid, monoamines, a host of neuropeptide systems, and various ion channels produced by the chronic presence of alcohol ultimately compromise the functional integrity of the brain. These neuroadaptations not only underlie the emergence and expression of many alcohol withdrawal symptoms, but also contribute to enhanced relapse vulnerability as well as perpetuation of uncontrolled excessive drinking. This chapter highlights the hallmark features of the alcohol withdrawal syndrome, and describes neuroadaptations in a wide array of neurotransmitter and neuromodulator systems (amino acid and monoamine neurotransmitter, neuropeptide systems, and various ion channels) as they relate to the expression of various signs and symptoms of alcohol withdrawal, as well as their relationship to the significant clinical problem of relapse and uncontrolled dangerous drinking.

Dendritic SK channels convert NMDA-R-dependent LTD to burst timing-dependent plasticity

Feedback and descending projections from higher to lower brain centers play a prominent role in all vertebrate sensory systems. Feedback might be optimized for the specific sensory processing tasks in their target brain centers, but it has been difficult to connect the properties of feedback synapses to sensory tasks. Here, we use the electrosensory system of a gymnotiform fish ( Apteronotus leptorhynchus) to address this problem. Cerebellar feedback to pyramidal cells in the first central electrosensory processing region, the electrosensory lateral line lobe (ELL), is critical for canceling spatially and temporally redundant electrosensory input. The ELL contains four electrosensory maps, and we have previously analyzed the synaptic and network bases of the redundancy reduction mechanism in a map (centrolateral segment; CLS) believed to guide electrolocation behavior. In the CLS, only long-term depression was induced by pairing feedback presynaptic and pyramidal cell postsynaptic bursts. In this paper, we turn to an ELL map (lateral segment; LS) known to encode electrocommunication signals. We find remarkable differences in synaptic plasticity of the morphologically identical cerebellar feedback input to the LS. In the LS, pyramidal cell SK channels permit long-term potentiation (LTP) of feedback synapses when pre- and postsynaptic bursts occur at the same time. We hypothesize that LTP in this map is required for enhancing the encoding of weak electrocommunication signals. We conclude that feedback inputs that appear morphologically identical in sensory maps dedicated to different tasks, nevertheless display different synaptic plasticity rules contributing to differential sensory processing in these maps.

Signal processing by T-type calcium channel interactions in the cerebellum

T-type calcium channels of the Cav3 family are unique among voltage-gated calcium channels due to their low activation voltage, rapid inactivation, and small single channel conductance. These special properties allow Cav3 calcium channels to regulate neuronal processing in the subthreshold voltage range. Here, we review two different subthreshold ion channel interactions involving Cav3 channels and explore the ability of these interactions to expand the functional roles of Cav3 channels. In cerebellar Purkinje cells, Cav3 and intermediate conductance calcium-activated potassium (IKCa) channels form a novel complex which creates a low voltage-activated, transient outward current capable of suppressing temporal summation of excitatory postsynaptic potentials (EPSPs). In large diameter neurons of the deep cerebellar nuclei, Cav3-mediated calcium current (I T) and hyperpolarization-activated cation current (I H) are activated during trains of inhibitory postsynaptic potentials. These currents have distinct, and yet synergistic, roles in the subthreshold domain with I T generating a rebound burst and I H controlling first spike latency and rebound spike precision. However, by shortening the membrane time constant the membrane returns towards resting value at a faster rate, allowing I H to increase the efficacy of I T and increase the range of burst frequencies that can be generated. The net effect of Cav3 channels thus depends on the channels with which they are paired. When expressed in a complex with a KCa channel, Cav3 channels reduce excitability when processing excitatory inputs. If functionally coupled with an HCN channel, the depolarizing effect of Cav3 channels is accentuated, allowing for efficient inversion of inhibitory inputs to generate a rebound burst output. Therefore, signal processing relies not only on the activity of individual subtypes of channels but also on complex interactions between ion channels whether based on a physical complex or by indirect effects on membrane properties.

Mechanism of the medium-duration afterhyperpolarization in rat serotonergic neurons

Most serotonergic neurons display a prominent medium-duration afterhyperpolarization (mAHP), which is mediated by small-conductance Ca(2+) -activated K(+) (SK) channels. Recent ex vivo and in vivo experiments have suggested that SK channel blockade increases the firing rate and/or bursting in these neurons. The purpose of this study was therefore to characterize the source of Ca(2+) which activates the mAHP channels in serotonergic neurons. In voltage-clamp experiments, an outward current was recorded at -60 mV after a depolarizing pulse to +100 mV. A supramaximal concentration of the SK channel blockers apamin or (-)-bicuculline methiodide blocked this outward current. This current was also sensitive to the broad Ca(2+) channel blocker Co(2+) and was partially blocked by both ω-conotoxin and mibefradil, which are blockers of N-type and T-type Ca(2+) channels, respectively. Neither blockers of other voltage-gated Ca(2+) channels nor DBHQ, an inhibitor of Ca(2+)-induced Ca(2+) release, had any effect on the SK current. In current-clamp experiments, mAHPs following action potentials were only blocked by ω-conotoxin and were unaffected by mibefradil. This was observed in slices from both juvenile and adult rats. Finally, when these neurons were induced to fire in an in vivo-like pacemaker rate, only ω-conotoxin was able to increase their firing rate (by ~30%), an effect identical to the one previously reported for apamin. Our results demonstrate that N-type Ca(2+) channels are the only source of Ca(2+) which activates the SK channels underlying the mAHP. T-type Ca(2+) channels may also activate SK channels under different circumstances.

Properties of BK-type Ca(+) (+)-dependent K(+) channel currents in medial prefrontal cortex pyramidal neurons in rats of different ages

The medial prefrontal cortex (PFC) is involved in cognitive functions, which undergo profound changes during adolescence. This alteration of the PFC function derives from neuron activity, which, in turn, may depend on age-dependent properties and the expression of neuronal ion channels. BK-type channels are involved in controlling both the Ca(+) (+) ion concentration in the cell interior and cell excitability. The purpose of this study was to test the properties of BK currents in the medial PFC pyramidal neurons of young (18- to 22-day-old), adolescent (38- to 42-day-old), and adult (60- to 65-day-old) rats. Whole-cell currents evoked by depolarizing voltage steps were recorded from dispersed medial PFC pyramidal neurons. A selective BK channel blocker - paxilline (10 μM) - irreversibly decreased the non-inactivating K(+) current in neurons that were isolated from the young and adult rats. This current was not significantly affected by paxilline in the neurons obtained from adolescent rats. The properties of single-channel K(+) currents were recorded from the soma of dispersed medial PFC pyramidal neurons in the cell-attached configuration. Of the K(+) channel currents that were recorded, ~90% were BK and leak channel currents. The BK-type channel currents were dependent on the Ca(+) (+) concentration and the voltage and were inhibited by paxilline. The biophysical properties of the BK channel currents did not differ among the pyramidal neurons isolated from young, adolescent, and adult rats. Among all of the recorded K(+) channel currents, 38.9, 12.7, and 21.1% were BK-type channel currents in the neurons isolated from the young, adolescent, and adult rats, respectively. Furthermore, application of paxilline effectively prolonged the half-width of the action potential in pyramidal neurons in slices isolated from young and adult rats but not in neurons isolated from adolescent rats. We conclude that the availability of BK channel currents decreases in medial PFC pyramidal neurons of adolescent rats compared with those in the neurons of young and adult rats while their properties did not change across ages.

Loss of NR1 subunit of NMDARs in primary sensory neurons leads to hyperexcitability and pain hypersensitivity: involvement of Ca(2+)-activated small conductance potassium channels

It is well established that activation of NMDARs plays an essential role in spinal cord synaptic plasticity (i.e., central sensitization) and pain hypersensitivity after tissue injury. Despite prominent expression of NMDARs in DRG primary sensory neurons, the unique role of peripheral NMDARs in regulating intrinsic neuronal excitability and pain sensitivity is not well understood, in part due to the lack of selective molecular tools. To address this problem, we used Advillin-Cre driver to delete the NR1 subunit of NMDARs selectively in DRG neurons. In NR1 conditional knock-out (NR1-cKO) mice, NR1 expression is absent in DRG neurons but remains normal in spinal cord neurons; NMDA-induced currents are also eliminated in DRG neurons of these mice. Surprisingly, NR1-cKO mice displayed mechanical and thermal hypersensitivity compared with wild-type littermates. NR1-deficient DRG neurons show increased excitability, as indicated by increased frequency of action potentials, and enhanced excitatory synaptic transmission in spinal cord slices, as indicated by increased frequency of miniature EPSCs. This hyperexcitability can be reproduced by the NMDAR antagonist APV and by Ca(2+)-activated slow conductance K(+) (SK) channel blocker apamin. Furthermore, NR1-positive DRG neurons coexpress SK1/SK2 and apamin-sensitive afterhyperpolarization currents are elevated by NMDA and suppressed by APV in these neurons. Our findings reveal the hitherto unsuspected role of NMDARs in controlling the intrinsic excitability of primary sensory neurons possibly via Ca(2+)-activated SK channels. Our results also call attention to potential opposing effects of NMDAR antagonists as a treatment for pain and other neurological disorders.

Induction and modulation of persistent activity in a layer V PFC microcircuit model

Working memory refers to the temporary storage of information and is strongly associated with the prefrontal cortex (PFC). Persistent activity of cortical neurons, namely the activity that persists beyond the stimulus presentation, is considered the cellular correlate of working memory. Although past studies suggested that this type of activity is characteristic of large scale networks, recent experimental evidence imply that small, tightly interconnected clusters of neurons in the cortex may support similar functionalities. However, very little is known about the biophysical mechanisms giving rise to persistent activity in small-sized microcircuits in the PFC. Here, we present a detailed biophysically—yet morphologically simplified—microcircuit model of layer V PFC neurons that incorporates connectivity constraints and is validated against a multitude of experimental data. We show that (a) a small-sized network can exhibit persistent activity under realistic stimulus conditions. (b) Its emergence depends strongly on the interplay of dADP, NMDA, and GABA_B currents. (c) Although increases in stimulus duration increase the probability of persistent activity induction, variability in the stimulus firing frequency does not consistently influence it. (d) Modulation of ionic conductances (I_h, I_D, I_sAHP, I_caL, I_caN, I_caR) differentially controls persistent activity properties in a location dependent manner. These findings suggest that modulation of the microcircuit's firing characteristics is achieved primarily through changes in its intrinsic mechanism makeup, supporting the hypothesis of multiple bi-stable units in the PFC. Overall, the model generates a number of experimentally testable predictions that may lead to a better understanding of the biophysical mechanisms of persistent activity induction and modulation in the PFC.

From lamprey to salamander: an exploratory modeling study on the architecture of the spinal locomotor networks in the salamander

The evolutionary transition from water to land required new locomotor modes and corresponding adjustments of the spinal "central pattern generators" for locomotion. Salamanders resemble the first terrestrial tetrapods and represent a key animal for the study of these changes. Based on recent physiological data from salamanders, and previous work on the swimming, limbless lamprey, we present a model of the basic oscillatory network in the salamander spinal cord, the spinal segment. Model neurons are of the Hodgkin-Huxley type. Spinal hemisegments contain sparsely connected excitatory and inhibitory neuron populations, and are coupled to a contralateral hemisegment. The model yields a large range of experimental findings, especially the NMDA-induced oscillations observed in isolated axial hemisegments and segments of the salamander Pleurodeles waltlii. The model reproduces most of the effects of the blockade of AMPA synapses, glycinergic synapses, calcium-activated potassium current, persistent sodium current, and [Formula: see text]-current. Driving segments with a population of brainstem neurons yields fast oscillations in the in vivo swimming frequency range. A minimal modification to the conductances involved in burst-termination yields the slower stepping frequency range. Slow oscillators can impose their frequency on fast oscillators, as is likely the case during gait transitions from swimming to stepping. Our study shows that a lamprey-like network can potentially serve as a building block of axial and limb oscillators for swimming and stepping in salamanders.