SK channels and NMDA receptors form a Ca2+-mediated feedback loop in dendritic spines

NMDA receptor expression and activity in osteoarthritic human articular chondrocytes

OBJECTIVE

Classical neuronal signalling molecules such as substance P and glutamate have been identified in cartilage and have roles in regulation of chondrocyte function. This study looks at expression and activity of the ionotropic glutamate NMDA (N-methyl-D-aspartic acid) receptor (NMDAR) in human osteoarthritic (OA) chondrocytes.

METHOD

Chondrocytes were obtained from human knee joint arthroplasty specimens. NMDAR subunits and PSD-95 (postsynaptic density protein 95) expression were analysed by reverse transcription-polymerase chain reaction and Western blotting. Activity of NMDAR was assayed by radioactive calcium(45) uptake and changes in membrane potential in the presence and absence of NMDA and NMDAR antagonists and blockade of cell membrane ion channels.

RESULTS

NMDAR 1, 2A, 2B and PSD-95 were detected in human OA chondrocytes whereas NR2B was absent from normal chondrocytes. NMDA induced calcium flux into OA chondrocytes and cell membrane depolarisation. These responses were blocked by NMDAR antagonists, removal of extracellular calcium, inhibition of nNOS (neuronal nitric oxide synthase) activity and uncoupling of NMDAR from PSD-95. Blockade of sodium channels by tetrodotoxin resulted in NMDA-induced membrane hyperpolarisation which was, in turn inhibited by apamin, a blocker of SK channels. NMDA-induced changes in cell membrane potential were not affected by l-type and stretch activated calcium channel inhibitors.

CONCLUSIONS

Human OA and normal articular chondrocytes differ in the expression of NMDAR subunits. In OA chondrocytes NMDAR signalling requires extracellular calcium, association with PSD-95, and nNOS activity. Downstream signalling results in activation of tetrodotoxin sensitive sodium channels and SK channels, a response that differs from that of normal chondrocytes suggesting altered activity of NMDAR in OA.

Modulation of SK channel trafficking by beta adrenoceptors enhances excitatory synaptic transmission and plasticity in the amygdala

Emotionally arousing events are particularly well remembered. This effect is known to result from the release of stress hormones and activation of beta adrenoceptors in the amygdala. However, the underlying cellular mechanisms are not understood. Small conductance calcium-activated potassium (SK) channels are present at glutamatergic synapses where they limit synaptic transmission and plasticity. Here, we show that beta adrenoceptor activation regulates synaptic SK channels in lateral amygdala pyramidal neurons, through activation of protein kinase A. We show that SK channels are constitutively recycled from the postsynaptic membrane and that activation of beta adrenoceptors removes SK channels from excitatory synapses. This results in enhanced synaptic transmission and plasticity. Our findings demonstrate a novel mechanism by which beta adrenoceptors control synaptic transmission and plasticity, through regulation of SK channel trafficking, and suggest that modulation of synaptic SK channels may contribute to beta adrenoceptor-mediated potentiation of emotional memories.

Control of K(Ca) channels by calcium nano/microdomains

Transient elevations in cytoplasmic Ca(2+) trigger a multitude of Ca(2+)-dependent processes in CNS neurons and many other cell types. The specificity, speed, and reliability of these processes is achieved and ensured by tightly restricting Ca(2+) signals to very local spatiotemporal domains, "Ca(2+) nano- and microdomains," that are centered around Ca(2+)-permeable channels. This arrangement requires that the Ca(2+)-dependent effectors reside within these spatial boundaries where the properties of the Ca(2+) domain and the Ca(2+) sensor of the effector determine the channel-effector activity. We use Ca(2+)-activated K(+) channels (K(Ca)) with either micromolar (BK(Ca) channels) or submicromolar (SK(Ca) channels) affinity for Ca(2+) ions to provide distance constraints for Ca(2+)-effector coupling in local Ca(2+) domains and review their significance for the cell physiology of K(Ca) channels in the CNS. The results may serve as a model for other processes operated by local Ca(2+) domains.

Neurotransmitter modulation of small-conductance Ca2+-activated K+ channels by regulation of Ca2+ gating

Small-conductance Ca2+-activated K+ (SK) channels are widely expressed in neuronal tissues where they underlie post-spike hyperpolarizations, regulate spike-frequency adaptation, and shape synaptic responses. SK channels constitutively interact with calmodulin (CaM), which serves as Ca2+ sensor, and with protein kinase CK2 and protein phosphatase 2A, which modulate their Ca2+ gating. By recording coupled activities of Ca2+ and SK2 channels, we showed that SK2 channels can be inhibited by neurotransmitters independently of changes in the activity of the priming Ca2+ channels. This inhibition involvesSK2-associated CK2 and results from a 3-fold reduction in the Ca2+ sensitivity of channel gating. CK2phosphorylated SK2-bound CaM but not KCNQ2-bound CaM, thereby selectively regulating SK2 channels. We extended these observations to sensory neurons by showing that noradrenaline inhibits SK current and increases neuronal excitability in aCK2-dependent fashion. Hence, neurotransmitter-initiated signaling cascades can dynamically regulate Ca2+ sensitivity of SK channels and directly influence somatic excitability.

SK (KCa2) channels do not control somatic excitability in CA1 pyramidal neurons but can be activated by dendritic excitatory synapses and regulate their impact

Calcium-activated K(+) channels of the K(Ca)2 type (SK channels) are prominently expressed in the mammalian brain, including hippocampus. These channels are thought to underlie neuronal excitability control and have been implicated in plasticity, memory, and neural disease. Contrary to previous reports, we found that somatic spike-evoked medium afterhyperpolarizations (mAHPs) and corresponding excitability control were not caused by SK channels but mainly by Kv7/KCNQ/M channels in CA1 hippocampal pyramidal neurons. Thus apparently, these SK channels are hardly activated by somatic Na(+) spikes. To further test this conclusion, we used sharp electrode, whole cell, and perforated-patch recordings from rat CA1 pyramidal neurons. We found that SK channel blockers consistently failed to suppress mAHPs under a range of experimental conditions: mAHPs following single spikes or spike trains, at -60 or -80 mV, at 20-30 degrees C, in low or elevated extracellular [K(+)], or spike trains triggered by synaptic stimulation after blocking N-methyl-d-aspartic acid receptors (NMDARs). Nevertheless, we found that SK channels in these cells were readily activated by artificially enhanced Ca(2+) spikes, and an SK channel opener (1-ethyl-2-benzimidazolinone) enhanced somatic AHPs following Na(+) spikes, thus reducing excitability. In contrast to CA1 pyramidal cells, bursting pyramidal cells in the subiculum showed a Na(+) spike-evoked mAHP that was reduced by apamin, indicating cell-type-dependent differences in mAHP mechanisms. Testing for other SK channel functions in CA1, we found that field excitatory postsynaptic potentials mediated by NMDARs were enhanced by apamin, supporting the idea that dendritic SK channels are activated by NMDAR-dependent calcium influx. We conclude that SK channels in rat CA1 pyramidal cells can be activated by NMDAR-mediated synaptic input and cause feedback regulation of synaptic efficacy but are normally not appreciably activated by somatic Na(+) spikes in this cell type.

Ionic and neuromodulatory regulation of burst discharge controls frequency tuning

Sensory neurons encode natural stimuli by changes in firing rate or by generating specific firing patterns, such as bursts. Many neural computations rely on the fact that neurons can be tuned to specific stimulus frequencies. It is thus important to understand the mechanisms underlying frequency tuning. In the electrosensory system of the weakly electric fish, Apteronotus leptorhynchus, the primary processing of behaviourally relevant sensory signals occurs in pyramidal neurons of the electrosensory lateral line lobe (ELL). These cells encode low frequency prey stimuli with bursts of spikes and high frequency communication signals with single spikes. We describe here how bursting in pyramidal neurons can be regulated by intrinsic conductances in a cell subtype specific fashion across the sensory maps found within the ELL, thereby regulating their frequency tuning. Further, the neuromodulatory regulation of such conductances within individual cells and the consequences to frequency tuning are highlighted. Such alterations in the tuning of the pyramidal neurons may allow weakly electric fish to preferentially select for certain stimuli under various behaviourally relevant circumstances.

Dendritic excitability and synaptic plasticity

Most synaptic inputs are made onto the dendritic tree. Recent work has shown that dendrites play an active role in transforming synaptic input into neuronal output and in defining the relationships between active synapses. In this review, we discuss how these dendritic properties influence the rules governing the induction of synaptic plasticity. We argue that the location of synapses in the dendritic tree, and the type of dendritic excitability associated with each synapse, play decisive roles in determining the plastic properties of that synapse. Furthermore, since the electrical properties of the dendritic tree are not static, but can be altered by neuromodulators and by synaptic activity itself, we discuss how learning rules may be dynamically shaped by tuning dendritic function. We conclude by describing how this reciprocal relationship between plasticity of dendritic excitability and synaptic plasticity has changed our view of information processing and memory storage in neuronal networks.

Ca2+ and synaptic plasticity

The induction and maintenance of synaptic plasticity is well established to be a Ca2+-dependent process. The use of fluorescent imaging to monitor changes [Ca2+]i in neurones has revealed a diverse array of signaling patterns across the different compartments of the cell. The Ca2+ signals within these compartments are generated by voltage or ligand-gated Ca2+ influx, and release from intracellular stores. The changes in [Ca2+]i are directly linked to the activity of the neurone, thus a neurone's input and output is translated into a dynamic Ca2+ code. Despite considerable progress in measuring this code much still remains to be determined in order to understand how the code is interpreted by the Ca2+ sensors that underlie the induction of compartment-specific plastic changes.

Active dendrites: colorful wings of the mysterious butterflies

Santiago Ramón y Cajal had referred to neurons as the 'mysterious butterflies of the soul.' Wings of these butterflies--their dendrites--were traditionally considered as passive integrators of synaptic information. Owing to a growing body of experimental evidence, it is now widely accepted that these wings are colorful, endowed with a plethora of active conductances, with each family of these butterflies made of distinct hues and shades. Furthermore, rapidly evolving recent literature also provides direct and indirect demonstrations for activity-dependent plasticity of these active conductances, pointing toward chameleonic adaptability in these hues. These experimental findings firmly establish the immense computational power of a single neuron, and thus constitute a turning point toward the understanding of various aspects of neuronal information processing. In this brief historical perspective, we track important milestones in the chameleonic transmogrification of these mysterious butterflies.

Potassium channels: newly found players in synaptic plasticity

One of the major issues for modern neuroscience research concerns the molecular and cellular mechanisms that underlie the acquisition, storage, and recollection of memories by the brain. Regulation of the strength of individual synaptic inputs (synaptic plasticity) has, for decades, been the front-running candidate mechanism for cellular information storage, with some direct supporting evidence recently obtained. Research into the molecular mechanisms responsible for changing synaptic strength has, to date, primarily focused on trafficking and properties of the neurotransmitter receptors themselves (AMPARs and NMDARs). However, recent evidence indicates that, subsequent to receptor activation, synaptic inputs are subject to regulation by synaptically located K+ channels. It is therefore critical to understand the biophysical properties and subcellular localization (density and distribution) of these channels and how their properties are modulated. Here we will review recent findings showing that two different classes of K+ channels (A-type and small conductance, Ca2+ -activated K+ channels), beyond their traditional role in regulating action potential firing, contribute to the regulation of synaptic strength in the hippocampus. In addition, we discuss how modulation of these channels' properties and expression might contribute to synaptic plasticity.

Contextual memory deficits observed in mice overexpressing small conductance Ca2+-activated K+ type 2 (KCa2.2, SK2) channels are caused by an encoding deficit

Hippocampal-dependent synaptic plasticity and memory are modulated by apamin-sensitive small conductance Ca2+-activated K+ (SK) channels. Transgenic mice overexpressing SK2 channels (SK2+/T mice) exhibit marked deficits in hippocampal memory and synaptic plasticity, as previously reported. Here, we examined whether SK2 overexpression affects the encoding or retention of contextual memory. Compared with wild-type littermates, SK2+/T mice exhibited significantly less context-dependent freezing 10 min and 24 h after conditioning. Interestingly, this contextual memory impairment was eliminated if SK2+/T mice were permitted longer pre-exposure to the conditioning chamber. These data support converging evidence that SK2 channels restrict the encoding of hippocampal memory.

Differential distribution of SK channel subtypes in the brain of the weakly electric fish Apteronotus leptorhynchus

Calcium signals in vertebrate neurons can induce hyperpolarizing membrane responses through the activation of Ca(2+)-activated potassium channels. Of these, small conductance (SK) channels regulate neuronal responses through the generation of the medium after-hyperpolarization (mAHP). We have previously shown that an SK channel (AptSK2) contributes to signal processing in the electrosensory system of Apteronotus leptorhynchus. It was shown that for pyramidal neurons in the electrosensory lateral line lobe (ELL), AptSK2 expression selectively decreases responses to low-frequency signals. The localization of all the SK subunits throughout the brain of Apteronotus then became of substantial interest. We have now cloned two additional SK channel subunits from Apteronotus and determined the expression patterns of all three AptSK subunits throughout the brain. In situ hybridization experiments have revealed that, as in mammalian systems, the AptSK1 and 2 channels showed a partially overlapping expression pattern, whereas the AptSK3 channel was expressed in different brain areas. The AptSK1 and 2 channels were the primary subunits found in the major electrosensory processing areas. Immunohistochemistry further revealed distinct compartmentalization of the AptSK1 and 2 channels in the ELL. AptSK1 was localized to the apical dendrites of pyramidal neurons, whereas AptSK2 channels are primarily somatic. The distinct expression patterns of all three AptSK channels may reflect subtype-specific contributions to neuronal function, and the high homology between subtypes from a number of species suggests that the functional roles for each channel subtype are conserved from early vertebrate evolution.

Competition between calcium-activated K+ channels determines cholinergic action on firing properties of basolateral amygdala projection neurons

Acetylcholine (ACh) is an important modulator of learning, memory, and synaptic plasticity in the basolateral amygdala (BLA) and other brain regions. Activation of muscarinic acetylcholine receptors (mAChRs) suppresses a variety of potassium currents, including sI(AHP), the calcium-activated potassium conductance primarily responsible for the slow afterhyperpolarization (AHP) that follows a train of action potentials. Muscarinic stimulation also produces inositol 1,4,5-trisphosphate (IP(3)), releasing calcium from intracellular stores. Here, we show using whole-cell patch-clamp recordings and high-speed fluorescence imaging that focal application of mAChR agonists evokes large rises in cytosolic calcium in the soma and proximal dendrites in rat BLA projection neurons that are often associated with activation of an outward current that hyperpolarizes the cell. This hyperpolarization results from activation of small conductance calcium-activated potassium (SK) channels, secondary to the release of calcium from intracellular stores. Unlike bath application of cholinergic agonists, which always suppressed the AHP, focal application of ACh often evoked a paradoxical enhancement of the AHP and spike-frequency adaptation. This enhancement was correlated with amplification of the action potential-evoked calcium response and resulted from the activation of SK channels. When SK channels were blocked, cholinergic stimulation always reduced the AHP and spike-frequency adaptation. Conversely, suppression of the sI(AHP) by the beta-adrenoreceptor agonist, isoprenaline, potentiated the cholinergic enhancement of the AHP. These results suggest that competition between cholinergic suppression of the sI(AHP) and cholinergic activation of the SK channels shapes the AHP and spike-frequency adaptation.

SK2 channel plasticity contributes to LTP at Schaffer collateral-CA1 synapses

Long-term potentiation (LTP) of synaptic strength at Schaffer collateral synapses has largely been attributed to changes in the number and biophysical properties of AMPA receptors (AMPARs). Small-conductance Ca(2+)-activated K(+) channels (SK2 channels) are functionally coupled with NMDA receptors (NMDARs) in CA1 spines such that their activity modulates the shape of excitatory postsynaptic potentials (EPSPs) and increases the threshold for induction of LTP. Here we show that LTP induction in mouse hippocampus abolishes SK2 channel activity in the potentiated synapses. This effect is due to SK2 channel internalization from the postsynaptic density (PSD) into the spine. Blocking PKA or cell dialysis with a peptide representing the C-terminal domain of SK2 that contains three known PKA phosphorylation sites blocks the internalization of SK2 channels after LTP induction. Thus the increase in AMPARs and the decrease in SK2 channels combine to produce the increased EPSP underlying LTP.

Multi-photon excitation imaging of dynamic processes in living cells and tissues

Over the past decade, two-photon microscopy has successfully made the transition from the laser laboratory into a true biological research setting. This has been due in part to the recent development of turnkey ultrafast laser systems required for two-photon microscopy, allowing ease of use in nonspecialist laboratories. The advantages of two-photon microscopy over conventional optical sectioning techniques are for greater imaging depths and reduced overall phototoxicity, as such enabling noninvasive intra-vital imaging of cellular and subcellular processes. Greater understanding of these advantages has allowed this technique to be more effectively utilized in a biological research setting. This review will cover the recent widespread uses of two-photon microscopy and highlight the wide range of physiological studies enabled in fields such as neurosciences, developmental biology, immunology, cancer biology, and endocrinology.

Regulation of synaptic signalling by postsynaptic, non-glutamate receptor ion channels

Activation of glutamatergic synapses onto pyramidal neurons produces a synaptic depolarization as well as a buildup of intracellular calcium (Ca(2+)). The synaptic depolarization propagates through the dendritic arbor and can be detected at the soma with a recording electrode. Current influx through AMPA-type glutamate receptors (AMPARs) provides the depolarizing drive, and the amplitudes of synaptic potentials are generally thought to reflect the number and properties of these receptors at each synapse. In contrast, synaptically evoked Ca(2+) transients are limited to the spine containing the active synapse and result primarily from Ca(2+) influx through NMDA-type glutamate receptors (NMDARs). Here we review recent studies that reveal that both synaptic depolarizations and spine head Ca(2+) transients are strongly regulated by the activity of postsynaptic, non-glutamate receptor ion channels. In hippocampal pyramidal neurons, voltage- and Ca(2+)-gated ion channels located in dendritic spines open as downstream consequences of glutamate receptor activation and act within a complex signalling loop that feeds back to regulate synaptic signals. Dynamic regulation of these ion channels offers a powerful mechanism of synaptic plasticity that is independent of direct modulation of glutamate receptors.

Voltage and calcium transients in basal dendrites of the rat prefrontal cortex

Higher cortical functions (perception, cognition, learning and memory) are in large part based on the integration of electrical and calcium signals that takes place in thin dendritic branches of neocortical pyramidal cells (synaptic integration). The mechanisms underlying the synaptic integration in thin basal dendrites are largely unexplored. We use a recently developed technique, multisite voltage-calcium imaging, to compare voltage and calcium transients from multiple locations along individual dendritic branches. Our results reveal characteristic electrical transients (plateau potentials) that trigger and shape dendritic calcium dynamics and calcium distribution during suprathreshold glutamatergic synaptic input. We regularly observed three classes of voltage-calcium interactions occurring simultaneously in three different zones of the same dendritic branch: (1) proximal to the input site, (2) at the input site, and (3) distal to the input site. One hundred micrometers away from the synaptic input site, both proximally and distally, dendritic calcium transients are in tight temporal correlation with the dendritic plateau potential. However, on the same dendrite, at the location of excitatory input, calcium transients outlast local dendritic plateau potentials by severalfold. These Ca2+ plateaus (duration 0.5-2 s) are spatially restricted to the synaptic input site, where they cause a brief down-regulation of dendritic excitability. Ca2+ plateaus are not mediated by Ca2+ release from intracellular stores, but rather by an NMDA-dependent small-amplitude depolarization, which persists after the collapse of the dendritic plateau potential. These unique features of dendritic voltage and calcium distributions may provide distinct zones for simultaneous long-term (bidirectional) modulation of synaptic contacts along the same basal branch.

Functions of SK channels in central neurons

1. SK channels are small-conductance calcium-activated potassium channels that are widely expressed in neurons. The traditional view of the functional role of SK channels is in mediating one component of the after-hyperpolarization that follows action potentials. Calcium influx via voltage-gated calcium channels active during action potentials opens SK channels and the resultant hyperpolarization lowers the firing frequency of action potentials in many neurons. 2. Recent advances have shown that, in addition to controlling action potential firing frequency, SK channels are also important in regulating dendritic excitability, synaptic transmission and synaptic plasticity. 3. In accordance with their role in modulating synaptic plasticity, SK channels are also important in regulating several learning and memory tasks and may also play a role in a number of neurological disorders. 4. The present review discusses recent findings on the role of SK channels in central neurons.

Effects of dopaminergic modulation on the integrative properties of the ventral striatal medium spiny neuron

Dopaminergic modulation produces a variety of functional changes in the principal cell of the striatum, the medium spiny neuron (MSN). Using a 189-compartment computational model of a ventral striatal MSN, we simulated whole cell D1- and D2-receptor-mediated modulation of both intrinsic (sodium, calcium, and potassium) and synaptic currents (AMPA and NMDA). Dopamine (DA) modulations in the model were based on a review of published experiments in both ventral and dorsal striatum. To objectively assess the net effects of DA modulation, we combined reported individual channel modulations into either D1- or D2-receptor modulation conditions and studied them separately. Contrary to previous suggestions, we found that D1 modulation had no effect on MSN nonlinearity and could not induce bistability. In agreement with previous suggestions, we found that dopaminergic modulation leads to changes in input filtering and neuronal excitability. Importantly, the changes in neuronal excitability agree with the classical model of basal ganglia function. We also found that DA modulation can alter the integration time window of the MSN. Interestingly, the effects of DA modulation of synaptic properties opposed the effects of DA modulation of intrinsic properties, with the synaptic modulations generally dominating the net effect. We interpret this lack of synergy to suggest that the regulation of whole cell integrative properties is not the primary functional purpose of DA. We suggest that D1 modulation might instead primarily regulate calcium influx to dendritic spines through NMDA and L-type calcium channels, by both direct and indirect mechanisms.

SK channels provide a novel mechanism for the control of frequency tuning in electrosensory neurons

One important characteristic of sensory input is frequency, with sensory neurons often tuned to narrow stimulus frequency ranges. Although vital for many neural computations, the cellular basis of such frequency tuning remains mostly unknown. In the electrosensory system of Apteronotus leptorhynchus, the primary processing of important environmental and communication signals occurs in pyramidal neurons of the electrosensory lateral line lobe. Spike trains transmitted by these cells can encode low-frequency prey stimuli with bursts of spikes and high-frequency communication signals with single spikes. Here, we demonstrate that the selective expression of SK2 channels in a subset of pyramidal neurons reduces their response to low-frequency stimuli by opposing their burst responses. Apamin block of the SK2 current in this subset of cells induced bursting and increased their response to low-frequency inputs. SK channel expression thus provides an intrinsic mechanism that predisposes a neuron to respond to higher frequencies and thus specific, behaviorally relevant stimuli.

Regulation of dendritic excitability by activity-dependent trafficking of the A-type K+ channel subunit Kv4.2 in hippocampal neurons

Voltage-gated A-type K+ channel Kv4.2 subunits are highly expressed in the dendrites of hippocampal CA1 neurons. However, little is known about the subcellular distribution and trafficking of Kv4.2-containing channels. Here we provide evidence for activity-dependent trafficking of Kv4.2 in hippocampal spines and dendrites. Live imaging and electrophysiological recordings showed that Kv4.2 internalization is induced rapidly upon glutamate receptor stimulation. Kv4.2 internalization was clathrin mediated and required NMDA receptor activation and Ca2+ influx. In dissociated hippocampal neurons, mEPSC amplitude depended on functional Kv4.2 expression level and was enhanced by stimuli that induced Kv4.2 internalization. Long-term potentiation (LTP) induced by brief glycine application resulted in synaptic insertion of GluR1-containing AMPA receptors along with Kv4.2 internalization. We also found evidence of Kv4.2 internalization upon synaptically evoked LTP in CA1 neurons of hippocampal slice cultures. These results present an additional mechanism for synaptic integration and plasticity through the activity-dependent regulation of Kv4.2 channel surface expression.

Metaplastic effect of apamin on LTP and paired-pulse facilitation

In area CA1 of hippocampal slices, a single 1-sec train of 100-Hz stimulation generally triggers a short-lasting long-term potentiation (S-LTP) of 1-2 h. Here, we found that when such a train was applied 45 min after application of the small conductance Ca(2+)-activated K(+ )(SK) channel blocker apamin, it induced a long-lasting LTP (L-LTP) of several hours, instead of an S-LTP. Apamin-induced SK channel blockage is known to resist washing. Nevertheless, the aforementioned effect is not a mere delayed effect; it is metaplastic. Indeed, when a single train was delivered to the Schaffer's collaterals during apamin application, it induced an S-LTP, like in the control situation. At the moment of this LTP induction (15th min of apamin application), the SK channel blockage was nevertheless complete. Indeed, at that time, under the influence of apamin, the amplitude of the series of field excitatory postsynaptic potentials (fEPSPs) triggered by a stimulation train was increased. We found that the metaplastic effect of apamin on LTP was crucially dependent on the NO-synthase pathway, whereas the efficacy of the NMDA receptors was not modified at the time of its occurrence. We also found that apamin produced an increase in paired-pulse facilitation not during, but after, the application of the drug. Finally, we found that the induction of each of these two metaplastic phenomena was mediated by NMDA receptors. A speculative unitary hypothesis to explain these phenomena is proposed.

Alterations in intrinsic neuronal excitability during normal aging

Normal aging subjects, including humans, have difficulty learning hippocampus-dependent tasks. For example, at least 50% of normal aging rabbits and rats fail to meet a learning criterion in trace eyeblink conditioning. Many factors may contribute to this age-related learning impairment. An important cause is the reduced intrinsic excitability observed in hippocampal pyramidal neurons from normal aging subjects, as reflected by an enlarged postburst afterhyperpolarization (AHP) and an increased spike-frequency adaptation (accommodation). In this review, we will focus on the alterations in the AHP and accommodation during learning and normal aging. We propose that age-related increases in the postburst AHP and accommodation in hippocampal pyramidal neurons play an integral role in the learning impairment observed in normal aging subjects.

Formation of cellular projections in neural progenitor cells depends on SK3 channel activity

Ion channels are potent modulators for developmental processes in progenitor cells. In a screening approach for different ion channels in neural progenitor cells (NPCs) we observed a 1-ethyl-2-benzimidazolinone (1-EBIO) activated inward current, which could be blocked by scyllatoxin (ScTX, IC50=2+/- 0.3 nmol/L). This initial evidence for the expression of the small conductance Ca2+ activated K+-channel SK3 was confirmed by the detection of SK3 transcripts and protein in NPCs. Interestingly, SK3 proteins were highly expressed in non-differentiated NPCs with a focused localization in lamellipodia as well as filopodial structures. The activation of SK3 channels using 1-EBIO lead to an immediate filopodial sprouting and the translocation of the protein into these novel filopodial protrusions. Both effects could be prevented by the pre-incubation of NPCs with ScTX. Our study gives first evidence that the formation and prolongation of filopodia in NPCs is, at least in part, effectively induced and regulated by SK3 channels.

Effects of matrix metalloproteinase inhibition on short- and long-term plasticity of schaffer collateral/CA1 synapses

It is increasingly evident that matrix metalloproteinases (MMPs), a family of zinc containing extracellular endopeptidases, participate in processes supporting hippocampal synaptic plasticity. The purpose of this study was to further the understanding of MMPs involvement in hippocampal plasticity. Acute hippocampal slices, generated from 20- to 30-day-old male Sprague-Dawley rats, were subjected to various electrophysiologic stimulatory paradigms to produce either short-term or long-term modifications to synaptic efficacy. Slices exposed to broad-spectrum MMP inhibitor, FN-439, exhibited impairments in paired-pulse facilitation, theta-burst facilitation, and long-term depression. Additionally, we observed that MMP inhibition impaired both the induction and stability of long-term potentiation (LTP). Furthermore, evidence indicated that the effect of MMP inhibition on LTP maintenance is dependent upon integrin-directed adhesion, whereas the effects of MMP inhibition on LTP induction are independent of integrin-directed adhesion. Together, these data support a generalized role for MMPs in short-term and long-term hippocampal plasticity and indicate that MMPs are a necessary facet of integrin-mediated cell adhesion supporting LTP stabilization.

Ca(2+) signaling in dendritic spines

Recent studies have revealed that Ca(2+) signals evoked by action potentials or by synaptic activity within individual dendritic spines are regulated at multiple levels. Ca(2+) influx through glutamate receptors and voltage-sensitive Ca(2+) channels located on spines depends on the channel subunit composition, the activity of kinases and phosphatases, the local membrane potential and past patterns of activity. Furthermore, sources of spine Ca(2+) interact nonlinearly such that activation of one Ca(2+) channel can enhance or depress the activity of others. These studies have revealed that each spine is a complex and partitioned Ca(2+) signaling domain capable of autonomously regulating the electrical and biochemical consequences of synaptic activity.

Organization and regulation of small conductance Ca2+-activated K+ channel multiprotein complexes

Small conductance Ca2+-activated K+ channels (SK channels) are complexes of four alpha pore-forming subunits each bound by calmodulin (CaM) that mediate Ca2+ gating. Proteomic analysis indicated that SK2 channels also bind protein kinase CK2 (CK2) and protein phosphatase 2A (PP2A). Coexpression of SK2 with the CaM phosphorylation surrogate CaM(T80D) suggested that the apparent Ca2+ sensitivity of SK2 channels is reduced by CK2 phosphorylation of SK2-bound CaM. By using 4,5,6,7-tetrabromo-2-azabenzimidazole, a CK2-specific inhibitor, we confirmed that SK2 channels coassemble with CK2. PP2A also binds to SK2 channels and counterbalances the effects of CK2, as shown by coexpression of a dominant-negative mutant PP2A as well as a mutant SK2 channel no longer able to bind PP2A. In vitro binding studies have revealed interactions between the N and C termini of the channel subunits as well as interactions among CK2 alpha and beta subunits, PP2A, and distinct domains of the channel. In the channel complex, lysine residue 121 within the N-terminal domain of the channel activates SK2-bound CK2, and phosphorylation of CaM is state dependent, occurring only when the channels are closed. The effects of CK2 and PP2A indicate that native SK2 channels are multiprotein complexes that contain constitutively associated CaM, both subunits of CK2, and at least two different subunits of PP2A. The results also show that the Ca2+ sensitivity of SK2 channels is regulated in a dynamic manner, directly through CK2 and PP2A, and indirectly by Ca2+ itself via the state dependence of CaM phosphorylation by CK2.

Anomalous diffusion in Purkinje cell dendrites caused by spines

We combined local photolysis of caged compounds with fluorescence imaging to visualize molecular diffusion within dendrites of cerebellar Purkinje cells. Diffusion of a volume marker, fluorescein dextran, within spiny dendrites was remarkably slow in comparison to its diffusion in smooth dendrites. Computer simulations indicate that this retardation is due to a transient trapping of molecules within dendritic spines, yielding anomalous diffusion. We considered the influence of spine trapping on the diffusion of calcium ions (Ca(2+)) and inositol-1,4,5-triphospate (IP(3)), two synaptic second messengers. Diffusion of IP(3) was strongly influenced by the presence of dendritic spines, while Ca(2+) was removed so rapidly that it could not diffuse far enough to be trapped. We conclude that an important function of dendritic spines may be to trap chemical signals and thereby create slowed anomalous diffusion within dendrites.

The role of synaptic ion channels in synaptic plasticity

The nervous system receives a large amount of information about the environment through elaborate sensory routes. Processing and integration of these wide-ranging inputs often results in long-term behavioural alterations as a result of past experiences. These relatively permanent changes in behaviour are manifestations of the capacity of the nervous system for learning and memory. At the cellular level, synaptic plasticity is one of the mechanisms underlying this process. Repeated neural activity generates physiological changes in the nervous system that ultimately modulate neuronal communication through synaptic transmission. Recent studies implicate both presynaptic and postsynaptic ion channels in the process of synapse strength modulation. Here, we review the role of synaptic ion channels in learning and memory, and discuss the implications and significance of these findings towards deciphering the molecular biology of learning and memory.

BK potassium channels facilitate high-frequency firing and cause early spike frequency adaptation in rat CA1 hippocampal pyramidal cells

Neuronal potassium (K(+)) channels are usually regarded as largely inhibitory, i.e. reducing excitability. Here we show that BK-type calcium-activated K(+) channels enhance high-frequency firing and cause early spike frequency adaptation in neurons. By combining slice electrophysiology and computational modelling, we investigated functions of BK channels in regulation of high-frequency firing in rat CA1 pyramidal cells. Blockade of BK channels by iberiotoxin (IbTX) selectively reduced the initial discharge frequency in response to strong depolarizing current injections, thus reducing the early spike frequency adaptation. IbTX also blocked the fast afterhyperpolarization (fAHP), slowed spike rise and decay, and elevated the spike threshold. Simulations with a computational model of a CA1 pyramidal cell confirmed that the BK channel-mediated rapid spike repolarization and fAHP limits activation of slower K(+) channels (in particular the delayed rectifier potassium current (I(DR))) and Na(+) channel inactivation, whereas M-, sAHP- or SK-channels seem not to be important for the early facilitating effect. Since the BK current rapidly inactivates, its facilitating effect diminishes during the initial discharge, thus producing early spike frequency adaptation by an unconventional mechanism. This mechanism is highly frequency dependent. Thus, IbTX had virtually no effect at spike frequencies < 40 Hz. Furthermore, extracellular field recordings demonstrated (and model simulations supported) that BK channels contribute importantly to high-frequency burst firing in response to excitatory synaptic input to distal dendrites. These results strongly support the idea that BK channels play an important role for early high-frequency, rapidly adapting firing in hippocampal pyramidal neurons, thus promoting the type of bursting that is characteristic of these cells in vivo, during behaviour.

Bridging behavior and physiology: ion-channel perspective on mushroom body-dependent olfactory learning and memory in Drosophila

An important body of evidence documents the differential expression of ion channels in brains, suggesting they are essential to endow particular brain structures with specific physiological properties. Because of their role in correlating inputs and outputs in neurons, modulation of voltage-dependent ion channels (VDICs) can profoundly change neuronal network dynamics and performance, and may represent a fundamental mechanism for behavioral plasticity, one that has received less attention in learning and memory studies. Revisiting three paradigmatic mutations altering olfactory learning and memory in Drosophila (dunce, leonardo, amnesiac) a link was established between each mutation and the operation of VDICs in Kenyon cells, the intrinsic neurons of the mushroom bodies (MBs). In Drosophila, MBs are essential to the emergence of olfactory associative learning and retention. Abnormal ion channel operation might underlie failures in neuronal physiology, and be crucial to understand the abnormal associative learning and retention phenotypes the mutants display. We also discuss the only case in which a mutation in an ion channel gene (shaker) has been directly linked to olfactory learning deficits. We analyze such evidence in light of recent discoveries indicating an unusual ion current profile in shaker mutant MB intrinsic neurons. We anticipate that further studies of acquisition and retention mutants will further confirm a link between such mutations and malfunction of specific ion channel mechanisms in brain structures implicated in learning and memory.

SK3 trafficking in hippocampal cells: the role of different molecular domains

The regulative steps that control trafficking of ion channels are fundamental determinants of their qualitative and quantitative expression on the cell membrane. In this work the trafficking of the small conductance calcium-activated potassium channel, SK3 was studied in neurons in order to identify relevant molecular domains involved in this process. Hippocampal cell cultures were transfected with fusion proteins of green fluorescent protein (GFP) and different SK3 subunit truncations. The differential distribution of the mutants was analyzed by confocal microscopy and compared to the localization of the control fusion protein with full length SK3. The transport of chimeric proteins was quantified from fluorescence images by developing a morphometric analytical method. We found that the full length SK3 was distributed in cell body, axon and dendrites, whereas the deleted forms GFPDelta578-736 (deletion of the entire C-terminal domain), GFPDeltaCaMBD (deletion of the calmodulin-binding site) and GFPDeltaN (deletion of the N-terminal domain) were not transported into cell processes but accumulated in the cell body. The GFPDelta640-736 (deletion of the distal C-terminal domain) showed a distribution similar to control. The quantification and statistical analysis confirmed the differences in distribution across the three groups. In conclusion, the current work provides evidence for a fundamental role of the N-terminal domain and the calmodulin binding domain in SK3 trafficking in neurons.

Hyperexcitability of distal dendrites in hippocampal pyramidal cells after chronic partial deafferentation

Traumatic injury to the CNS results in chronic partial deafferentation of subsets of surviving neurons. Such injuries are often followed by a delayed but long-lasting period of aberrant hyperexcitability. The cellular mechanisms underlying this delayed hyperexcitability are poorly understood. We developed an in vitro model of deafferentation and reactive hyperexcitability using organotypic hippocampal slice cultures to study the underlying cellular mechanisms. One week after transection of the Schaffer collateral and temporoammonic afferents to CA1 neurons, brief tetanic stimulation of the residual excitatory synapses produced abnormally prolonged depolarizations, compared with responses in normally innervated neurons. Responses to weak stimulation, in contrast, were unaffected after deafferentation. Direct stimulation of distal apical dendrites using focal photolysis of caged glutamate triggered abnormally prolonged plateau potentials in the deafferented neurons when strong stimulation was given, but responses to weak stimulation were not different from controls. An identical phenotype was produced by chronic "chemical deafferentation" with glutamate receptor antagonists. Responses to strong synaptic and photolytic stimulation were selectively prolonged by small-conductance (SK-type) calcium-activated potassium channel blockers in normally innervated cells but not after deafferentation. No significant changes in SK2 mRNA or protein levels, GABAergic inhibition, glutamate receptor function, input resistance, or action potential parameters were observed after chronic deafferentation. We suggest that a posttranslational downregulation of SK channel function in thin distal dendrites is a significant contributor to deafferentation-induced reactive hyperexcitability.

Nonlinear Regulation of Unitary Synaptic Signals by CaV2.3 Voltage-Sensitive Calcium Channels Located in Dendritic Spines

The roles of voltage-sensitive sodium (Na) and calcium (Ca) channels located on dendrites and spines in regulating synaptic signals are largely unknown. Here we use 2-photon glutamate uncaging to stimulate individual spines while monitoring uncaging-evoked excitatory postsynaptic potentials (uEPSPs) and Ca transients. We find that, in CA1 pyramidal neurons in acute mouse hippocampal slices, CaV(2.3) voltage-sensitive Ca channels (VSCCs) are found selectively on spines and act locally to dampen uncaging-evoked Ca transients and somatic potentials. These effects are mediated by a regulatory loop that requires opening of CaV(2.3) channels, voltage-gated Na channels, small conductance Ca-activated potassium (SK) channels, and NMDA receptors. Ca influx through CaV(2.3) VSCCs selectively activates SK channels, revealing the presence of functional Ca microdomains within the spine. Our results suggest that synaptic strength can be modulated by mechanisms that regulate voltage-gated conductances within the spine but do not alter the properties or numbers of synaptic glutamate receptors.

The sigma-1 receptor modulates NMDA receptor synaptic transmission and plasticity via SK channels in rat hippocampus

The sigma receptor (sigmaR), once considered a subtype of the opioid receptor, is now described as a distinct pharmacological entity. Modulation of N-methyl-D-aspartate receptor (NMDAR) functions by sigmaR-1 ligands is well documented; however, its mechanism is not fully understood. Using patch-clamp whole-cell recordings in CA1 pyramidal cells of rat hippocampus and (+)pentazocine, a high-affinity sigmaR-1 agonist, we found that sigmaR-1 activation potentiates NMDAR responses and long-term potentiation (LTP) by preventing a small conductance Ca2+-activated K+ current (SK channels), known to shunt NMDAR responses, to open. Therefore, the block of SK channels and the resulting increased Ca2+ influx through the NMDAR enhances NMDAR responses and LTP. These results emphasize the importance of the sigmaR-1 as postsynaptic regulator of synaptic transmission.

An amino acid outside the pore region influences apamin sensitivity in small conductance Ca2+-activated K+ channels

Small conductance calcium-activated potassium channels (SK, K(Ca)) are a family of voltage-independent K+ channels with a distinct physiology and pharmacology. The bee venom toxin apamin inhibits exclusively the three cloned SK channel subtypes (SK1, SK2, and SK3) with different affinity, highest for SK2, lowest for SK1, and intermediate for SK3 channels. The high selectivity of apamin made it a valuable tool to study the molecular makeup and function of native SK channels. Three amino acids located in the outer vestibule of the pore are of particular importance for the different apamin sensitivities of SK channels. Chimeric SK1 channels, enabling the homomeric expression of the rat SK1 (rSK1) subunit and containing the core domain (S1-S6) of rSK1, are apamin-insensitive. By contrast, channels formed by the human orthologue human SK1 (hSK1) are sensitive to apamin. This finding hinted at the involvement of regions beyond the pore as determinants of apamin sensitivity, because hSK1 and rSK1 have an identical amino acid sequence in the pore region. Here we investigated which parts of the channels outside the pore region are important for apamin sensitivity by constructing chimeras between apamin-insensitive and -sensitive SK channel subunits and by introducing point mutations. We demonstrate that a single amino acid situated in the extracellular loop between the transmembrane segments S3 and S4 has a major impact on apamin sensitivity. Our findings enabled us to convert the hSK1 channel into a channel that was as sensitive for apamin as SK2, the SK channel with the highest sensitivity.

Molecular coupling of a Ca2+-activated K+ channel to L-type Ca2+ channels via alpha-actinin2

Cytoskeletal proteins are known to sculpt the structural architecture of cells. However, their role as bridges linking the functional crosstalk of different ion channels is unknown. Here, we demonstrate that a small conductance Ca(2+)-activated K(+) channels (SK2 channel), present in a variety of cells, where they integrate changes in intracellular Ca(2+) concentration [Ca(2+)(i)] with changes in K(+) conductance and membrane potential, associate with L-type Ca(2+) channels; Ca(v)1.3 and Ca(v)1.2 through a physical bridge, alpha-actinin2 in cardiac myocytes. SK2 channels do not physically interact with L-type Ca(2+) channels, instead, the 2 channels colocalize via their interaction with alpha-actinin2 cytoskeletal protein. The association of SK2 channel with alpha-actinin2 localizes the channel to the entry of external Ca(2+) source, which regulate the channel function. Furthermore, we demonstrated that the functions of SK2 channels in atrial myocytes are critically dependent on the normal expression of Ca(v)1.3 Ca(2+) channels. Null deletion of Ca(v)1.3 channel results in abnormal function of SK2 channel and prolongation of repolarization and atrial arrhythmias. Our study provides insight into the molecular mechanisms of the coupling of SK2 channel with voltage-gated Ca(2+) channel, and represents the first report linking the coupling of 2 different types of ion channels via cytoskeletal proteins.

CLC-3 Channels Modulate Excitatory Synaptic Transmission in Hippocampal Neurons

It is well established that ligand-gated chloride flux across the plasma membrane modulates neuronal excitability. We find that a voltage-dependent Cl(-) conductance increases neuronal excitability in immature rodents as well, enhancing the time course of NMDA receptor-mediated miniature excitatory postsynaptic potentials (mEPSPs). This Cl(-) conductance is activated by CaMKII, is electrophysiologically identical to the CaMKII-activated CLC-3 conductance in nonneuronal cells, and is absent in clc-3(-/-) mice. Systematically decreasing [Cl(-)](i) to mimic postnatal [Cl(-)](i) regulation progressively decreases the amplitude and decay time constant of spontaneous mEPSPs. This Cl(-)-dependent change in synaptic strength is absent in clc-3(-/-) mice. Using surface biotinylation, immunohistochemistry, electron microscopy, and coimmunoprecipitation studies, we find that CLC-3 channels are localized on the plasma membrane, at postsynaptic sites, and in association with NMDA receptors. This is the first demonstration that a voltage-dependent chloride conductance modulates neuronal excitability. By increasing postsynaptic potentials in a Cl(-) dependent fashion, CLC-3 channels regulate neuronal excitability postsynaptically in immature neurons.

Learning, aging and intrinsic neuronal plasticity

In vitro experiments indicate that intrinsic neuronal excitability, as evidenced by changes in the post-burst afterhyperpolarization (AHP) and spike-frequency accommodation, is altered during learning and normal aging in the brain. Here we review these studies, highlighting two consistent findings: (i) that AHP and accommodation are reduced in pyramidal neurons from animals that have learned a task; and (ii) that AHP and accommodation are enhanced in pyramidal neurons from aging subjects, a cellular change that might contribute to age-related learning impairments. Findings from in vivo single-neuron recording studies complement the in vitro data. From these consistently reproduced findings, we propose that the intrinsic AHP level might determine the degree of synaptic plasticity and learning. Furthermore, it seems that reductions in the AHP must occur before learning if young and aging subjects are to learn a task successfully.

Inhibitory Gating Modulation of Small Conductance Ca2+-Activated K+Channels by the Synthetic Compound (R)-N-(Benzimidazol-2-yl)-1,2,3,4-tetrahydro-1-naphtylamine (NS8593) Reduces Afterhyperpolarizing Current in Hippocampal CA1 Neurons

SK channels are small conductance Ca(2+)-activated K(+) channels important for the control of neuronal excitability, the fine tuning of firing patterns, and the regulation of synaptic mechanisms. The classic SK channel pharmacology has largely focused on the peptide apamin, which acts extracellularly by a pore-blocking mechanism. 1-Ethyl-2-benzimidazolinone (1-EBIO) and 6,7-dichloro-1H-indole-2,3-dione 3-oxime (NS309) have been identified as positive gating modulators that increase the apparent Ca(2+) sensitivity of SK channels. In the present study, we describe inhibitory gating modulation as a novel principle for selective inhibition of SK channels. In whole-cell patch-clamp experiments, the compound (R)-N-(benzimidazol-2-yl)-1,2,3,4-tetrahydro-1-naphtylamine (NS8593) reversibly inhibited recombinant SK3-mediated currents (human SK3 and rat SK3) with potencies around 100 nM. However, in contrast to known pore blockers, NS8593 did not inhibit (125)I-apamin binding. Using excised patches, it was demonstrated that NS8593 decreased the Ca(2+) sensitivity by shifting the activation curve for Ca(2+) to the right, only slightly affecting the maximal Ca(2+)-activated SK current. NS8593 inhibited all the SK1-3 subtypes Ca(2+)-dependently (K(d) = 0.42, 0.60, and 0.73 microM, respectively, at 0.5 microM Ca(2+)), whereas the compound did not affect the Ca(2+)-activated K(+) channels of intermediate and large conductance (hIK and hBK channels, respectively). The site of action was accessible from both sides of the membrane, and the NS8593-mediated inhibition was prevented in the presence of a high concentration of the positive modulator NS309. NS8593 was further tested on mouse CA1 neurons in hippocampal slices and shown to inhibit the apaminand tubocurarine-sensitive SK-mediated afterhyperpolarizing current, at a concentration of 3 microM.

Enhancement of hippocampal pyramidal cell excitability by the novel selective slow-afterhyperpolarization channel blocker 3-(triphenylmethylaminomethyl)pyridine (UCL2077)

The slow afterhyperpolarization (sAHP) in hippocampal neurons has been implicated in learning and memory. However, its precise role in cell excitability and central nervous system function has not been explicitly tested for 2 reasons: 1) there are, at present, no selective inhibitors that effectively reduce the underlying current in vivo or in intact in vitro tissue preparations, and 2) although it is known that a small conductance K(+) channel that activates after a rise in [Ca(2+)](i) underlies the sAHP, the exact molecular identity remains unknown. We show that 3-(triphenylmethylaminomethyl)pyridine (UCL2077), a novel compound, suppressed the sAHP present in hippocampal neurons in culture (IC(50) = 0.5 microM) and in the slice preparation (IC(50) approximately 10 microM). UCL2077 was selective, having minimal effects on Ca(2+) channels, action potentials, input resistance and the medium afterhyperpolarization. UCL2077 also had little effect on heterologously expressed small conductance Ca(2+)-activated K(+) (SK) channels. Moreover, UCL2077 and apamin, a selective SK channel blocker, affected spike firing in hippocampal neurons in different ways. These results provide further evidence that SK channels are unlikely to underlie the sAHP. This study also demonstrates that UCL2077, the most potent, selective sAHP blocker described so far, is a useful pharmacological tool for exploring the role of sAHP channels in the regulation of cell excitability in intact tissue preparations and, potentially, in vivo.

NMDA receptor function: subunit composition versus spatial distribution

NMDA receptors (NMDARs) play a pivotal role in the regulation of neuronal communication and synaptic function in the central nervous system. The subunit composition and compartmental localization of NMDARs in neurons affect channel activity and downstream signaling. This review discusses the distinct NMDAR subtypes and their function at synaptic, perisynaptic, and extrasynaptic sites of excitatory and inhibitory neurons. Many neurons express more than one of the modulatory NR2 subunits that participate in the formation of di- and/or triheteromeric channel assemblies (e.g., NR1/NR2A, NR1/NR2B, and/or NR1/NR2A/NR2B). Depending on the subunit composition and presence or absence of intracellular binding partners along the postsynaptic membrane, these NMDAR subtypes are allocated to distinct synaptic inputs converging onto a neuron or are distributed differentially among synaptic or extrasynaptic sites. These sites can carry NR2A and NR2B subunits, supporting the hypothesis that the spatial distribution of scaffolding and signaling complexes critically determines the full spectrum of NMDAR signaling.

Differential effects of alphaCaMKII mutation on hippocampal learning and changes in intrinsic neuronal excitability

Alpha-calcium/calmodulin-dependent kinase II (alphaCaMKII) is central to synaptic plasticity but it remains unclear whether this kinase contributes to neuronal excitability changes, which are a cellular correlate of learning. Using knock-in mice with a targeted T286A mutation that prevents the autophosphorylation of alphaCaMKII (alphaCaMKII(T286A)), we studied the role of alphaCaMKII signaling in regulating hippocampal neuronal excitability during hippocampus-dependent spatial learning in the Morris water maze. Wild-type control mice showed increased excitability of CA1 pyramidal neurons, as assessed by a reduction in the postburst afterhyperpolarization (AHP), after spatial training in the water maze. Importantly, wild-type mice did not show AHP changes when they were exposed to the water maze without the escape platform and swam the same amount of time as the trained mice (swim controls), thus manifesting learning-specific increases in hippocampal CA1 excitability associated with spatial training. Meanwhile, alphaCaMKII(T286A) mice showed impairments in spatial learning but exhibited reduced levels of AHP that were similar to wild-type controls after water-maze training. Notably, both trained and swim-control groups of alphaCaMKII(T286A) mutants showed similar increased excitability, indicating that swimming by itself is enough to induce changes in excitability in the absence of normal alphaCaMKII function. This result demonstrates dissociation of alphaCaMKII-independent changes in intrinsic neuron excitability from learning and synaptic plasticity mechanisms, suggesting that increases in excitability per se are not perfectly correlated with learning. Our findings suggest that alphaCaMKII signaling may function to suppress learning-unrelated changes during training, thereby allowing hippocampal CA1 neurons to increase their excitability appropriately for encoding spatial memories.

Variability, compensation and homeostasis in neuron and network function

Neurons in most animals live a very long time relative to the half-lives of all of the proteins that govern excitability and synaptic transmission. Consequently, homeostatic mechanisms are necessary to ensure stable neuronal and network function over an animal's lifetime. To understand how these homeostatic mechanisms might function, it is crucial to understand how tightly regulated synaptic and intrinsic properties must be for adequate network performance, and the extent to which compensatory mechanisms allow for multiple solutions to the production of similar behaviour. Here, we use examples from theoretical and experimental studies of invertebrates and vertebrates to explore several issues relevant to understanding the precision of tuning of synaptic and intrinsic currents for the operation of functional neuronal circuits.

Functional uncoupling between Ca2+ release and afterhyperpolarization in mutant hippocampal neurons lacking junctophilins

Junctional membrane complexes (JMCs) composed of the plasma membrane and endoplasmic/sarcoplasmic reticulum seem to be a structural platform for channel crosstalk. Junctophilins (JPs) contribute to JMC formation by spanning the sarcoplasmic reticulum membrane and binding with the plasma membrane in muscle cells. In this article, we report that mutant JP double-knockout (JP-DKO) mice lacking neural JP subtypes exhibited an irregular hindlimb reflex and impaired memory. Electrophysiological experiments indicated that the activation of small-conductance Ca(2+)-activated K(+) channels responsible for afterhyperpolarization in hippocampal neurons requires endoplasmic reticulum Ca(2+) release through ryanodine receptors, triggered by NMDA receptor-mediated Ca(2+) influx. We propose that in JP-DKO neurons lacking afterhyperpolarization, the functional communications between NMDA receptors, ryanodine receptors, and small-conductance Ca(2+)-activated K(+) channels are disconnected because of JMC disassembly. Moreover, JP-DKO neurons showed an impaired long-term potentiation and hyperactivation of Ca(2+)/calmodulin-dependent protein kinase II. Therefore, JPs seem to have an essential role in neural excitability fundamental to plasticity and integrated functions.

Ketamine appears associated with better word recall than etomidate after a course of 6 electroconvulsive therapies

Ten patients treated with electroconvulsive therapy (ECT) for depressive illness received anesthesia with either etomidate or ketamine. Three patients received both etomidate and ketamine anesthesia for ECT during separate episodes of depression. Patients anesthetized with ketamine for ECT had significantly less impairment of short-term memory function than did patients who received ECT with etomidate anesthesia. All patients who received both anesthetics for ECT during 2 different episodes had less memory loss during ECT with ketamine than with etomidate. These results show the importance of studying the effects of all anesthetic agents used during ECT on cognitive functions. The results imply that the effect of ECT on memory may be largely caused by effects mediated by glutamate at N-methyl-d-aspartate receptors and suggest that N-methyl-d-aspartate antagonists may offer protection from memory dysfunction during ECT.