Modulation of calcium wave propagation in the dendrites and to the soma of rat hippocampal pyramidal neurons

Differential behaviors of calcium-induced calcium release in one dimensional dendrite by Nernst-Planck equation, cable model and pure diffusion model

The source and dynamics of calcium is the key factor that regulates dendritic integration. Apart from the voltage-gated and ligand-gated calcium influx, an important source of calcium is from inner store of endoplasmic reticulum with a regenerative process of calcium-induced calcium release (CICR). To trigger this process, inositol 1,4,5-trisphosphate (IP3) and calcium are needed to satisfy certain requirements. The aim of our paper is to investigate how the CICR depends on the dynamics of membrane potential. We utilize one dimensional dendritic model to calculate membrane potential by Nernst-Planck Equation (NPE) and cable model and Pure Diffusion (PD) model, computational simulations are carried out to inject the calcium influx by synaptic stimulation and to predict subsequent CICR and calcium wave propagation. Our results demonstrate that CICR initiation and calcium wave propagation have much difference between electro-diffusion process of NPE and cable model. We find that cable model has lower threshold of IP3 stimulation to trigger CICR but is more difficult for calcium propagation than NPE, PD model requires even higher threshold of IP3 to initiate CICR process and calcium duration is shorter than NPE; the regenerative calcium wave propagates with faster speed in NPE than that in cable model and in PD model. Our work addresses the important role of electro-diffusion dynamics of charged ions in regulating CICR process in dendritic structure; and provides theoretical predictions for neurological process which requires sustaining calcium for downstream signaling processes.

Real-time imaging of Arc/Arg3.1 transcription ex vivo reveals input-specific immediate early gene dynamics

The ability of neurons to process and store salient environmental features underlies information processing in the brain. Long-term information storage requires synaptic plasticity and regulation of gene expression. While distinct patterns of activity have been linked to synaptic plasticity, their impact on immediate early gene (IEG) expression remains poorly understood. The activity regulated cytoskeleton associated (Arc) gene has received wide attention as an IEG critical for long-term synaptic plasticity and memory. Yet, to date, the transcriptional dynamics of Arc in response to compartment and input-specific activity is unclear. By developing a knock-in mouse to fluorescently tag Arc alleles, we studied real-time transcription dynamics after stimulation of dentate granule cells (GCs) in acute hippocampal slices. To our surprise, we found that Arc transcription displayed distinct temporal kinetics depending on the activation of excitatory inputs that convey functionally distinct information, i.e., medial and lateral perforant paths (MPP and LPP, respectively). Moreover, the transcriptional dynamics of Arc after synaptic stimulation was similar to direct activation of GCs, although the contribution of ionotropic glutamate receptors, L-type voltage-gated calcium channel, and the endoplasmic reticulum (ER) differed. Specifically, we observed an ER-mediated synapse-to-nucleus signal that supported elevations in nuclear calcium and, thereby, rapid induction of Arc transcription following MPP stimulation. By delving into the complex excitation-transcription coupling for Arc, our findings highlight how different synaptic inputs may encode information by modulating transcription dynamics of an IEG linked to learning and memory.

RyR-mediated Ca2+ release elicited by neuronal activity induces nuclear Ca2+ signals, CREB phosphorylation, and Npas4/RyR2 expression

The expression of several hippocampal genes implicated in learning and memory processes requires that Ca²⁺ signals generated in dendritic spines, dendrites, or the soma in response to neuronal stimulation reach the nucleus. The diffusion of Ca²⁺ in the cytoplasm is highly restricted, so neurons must use other mechanisms to propagate Ca²⁺ signals to the nucleus. Here, we present evidence showing that Ca²⁺ release mediated by the ryanodine receptor (RyR) channel type-2 isoform (RyR2) contributes to the generation of nuclear Ca²⁺ signals induced by gabazine (GBZ) addition, glutamate uncaging in the dendrites, or high-frequency field stimulation of primary hippocampal neurons. Additionally, GBZ treatment significantly increased cyclic adenosine monophosphate response element binding protein (CREB) phosphorylation-a key event in synaptic plasticity and hippocampal memory-and enhanced the expression of Neuronal Per Arnt Sim domain protein 4 (Npas4) and RyR2, two central regulators of these processes. Suppression of RyR-mediated Ca²⁺ release with ryanodine significantly reduced the increase in CREB phosphorylation and the enhanced Npas4 and RyR2 expression induced by GBZ. We propose that RyR-mediated Ca²⁺ release induced by neuronal activity, through its contribution to the sequential generation of nuclear Ca²⁺ signals, CREB phosphorylation, Npas4, and RyR2 up-regulation, plays a central role in hippocampal synaptic plasticity and memory processes.

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.

Mapping Phosphodiesterase 4D (PDE4D) in Macaque Dorsolateral Prefrontal Cortex: Postsynaptic Compartmentalization in Layer III Pyramidal Cell Circuits

cAMP signaling has powerful, negative effects on cognitive functions of the primate dorsolateral prefrontal cortex (dlPFC), opening potassium channels to reduce firing and impair working memory, and increasing tau phosphorylation in aging neurons. This contrasts with cAMP actions in classic circuits, where it enhances plasticity and transmitter release. PDE4 isozymes regulate cAMP actions, and thus have been a focus of research and drug discovery. Previous work has focused on the localization of PDE4A and PDE4B in dlPFC, but PDE4D is also of great interest, as it is the predominant PDE4 isoform in primate association cortex, and PDE4D expression decreases with aging in human dlPFC. Here we used laser-capture microdissection transcriptomics and found that PDE4D message is enriched in pyramidal cells compared to GABAergic PV-interneurons in layer III of the human dlPFC. A parallel study in rhesus macaques using high-spatial resolution immunoelectron microscopy revealed the ultrastructural locations of PDE4D in primate dlPFC with clarity not possible in human post-mortem tissue. PDE4D was especially prominent in dendrites associated with microtubules, mitochondria, and likely smooth endoplasmic reticulum (SER). There was substantial postsynaptic labeling in dendritic spines, associated with the SER spine-apparatus near glutamatergic-like axospinous synapses, but sparse labeling in axon terminals. We also observed dense PDE4D labeling perisynaptically in astroglial leaflets ensheathing glutamatergic connections. These data suggest that PDE4D is strategically positioned to regulate cAMP signaling in dlPFC glutamatergic synapses and circuits, especially in postsynaptic compartments where it is localized to influence cAMP actions on intracellular trafficking, mitochondrial physiology, and internal calcium release.

Functional Consequences of Calcium-Dependent Synapse-to-Nucleus Communication: Focus on Transcription-Dependent Metabolic Plasticity

In the nervous system, calcium signals play a major role in the conversion of synaptic stimuli into transcriptional responses. Signal-regulated gene transcription is fundamental for a range of long-lasting adaptive brain functions that include learning and memory, structural plasticity of neurites and synapses, acquired neuroprotection, chronic pain, and addiction. In this review, we summarize the diverse mechanisms governing calcium-dependent transcriptional regulation associated with central nervous system plasticity. We focus on recent advances in the field of synapse-to-nucleus communication that include studies of the signal-regulated transcriptome in human neurons, identification of novel regulatory mechanisms such as activity-induced DNA double-strand breaks, and the identification of novel forms of activity- and transcription-dependent adaptations, in particular, metabolic plasticity. We summarize the reciprocal interactions between different kinds of neuroadaptations and highlight the emerging role of activity-regulated epigenetic modifiers in gating the inducibility of signal-regulated genes.

The neuronal stimulation-transcription coupling map

Neurons transcribe different genes in response to different extracellular stimuli, and these genes regulate neuronal plasticity. Thus, understanding how different stimuli regulate different stimulus-dependent gene modules would deepen our understanding of plasticity. To systematically dissect the coupling between stimulation and transcription, we propose creating a 'stimulation-transcription coupling map' that describes the transcription response to each possible extracellular stimulus. While we are currently far from having a complete map, recent genomic experiments have begun to facilitate its creation. Here, we describe the current state of the stimulation-transcription coupling map as well as the transcriptional regulation that enables this coupling.

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.

Stores, Channels, Glue, and Trees: Active Glial and Active Dendritic Physiology

Glial cells and neuronal dendrites were historically assumed to be passive structures that play only supportive physiological roles, with no active contribution to information processing in the central nervous system. Research spanning the past few decades has clearly established this assumption to be far from physiological realities. Whereas the discovery of active channel conductances and their localized plasticity was the turning point for dendritic structures, the demonstration that glial cells release transmitter molecules and communicate across the neuroglia syncytium through calcium wave propagation constituted path-breaking discoveries for glial cell physiology. An additional commonality between these two structures is the ability of calcium stores within their endoplasmic reticulum (ER) to support active propagation of calcium waves, which play crucial roles in the spatiotemporal integration of information within and across cells. Although there have been several demonstrations of regulatory roles of glial cells and dendritic structures in achieving common physiological goals such as information propagation and adaptability through plasticity, studies assessing physiological interactions between these two active structures have been few and far. This lacuna is especially striking given the strong connectivity that is known to exist between these two structures through several complex and tightly intercoupled mechanisms that also recruit their respective ER structures. In this review, we present brief overviews of the parallel literatures on active dendrites and active glial physiology and make a strong case for future studies to directly assess the strong interactions between these two structures in regulating physiology and pathophysiology of the brain.

mGluR5-dependent modulation of dendritic excitability in CA1 pyramidal neurons mediated by enhancement of persistent Na+ currents

KEY POINTS

High-frequency stimulation (HFS) of the Schaffer collateral pathway activates metabotropic glutamate receptor 5 (mGluR5) signalling in the proximal apical dendrites of CA1 pyramidal neurons. The synaptic activation of mGluR5-mediated calcium signalling causes a significant increase in persistent sodium current (INa,P ) in the dendrites. Increased INa,P by HFS underlies potentiation of synaptic inputs at both the proximal and distal dendrite, leading to an enhanced probability of action potential firing associated with decreased action potential thresholds. Therefore, HFS-induced activation of intracellular mGluR5 serves an important role as an instructive signal for potentiation of upcoming inputs by increasing dendritic excitability.

ABSTRACT

Dendritic Na+ channels in pyramidal neurons are known to amplify synaptic signals, thereby facilitating action potential (AP) generation. However, the mechanisms that modulate dendritic Na+ channels have remained largely uncharacterized. Here, we report a new form of short-term plasticity in which proximal excitatory synaptic inputs to hippocampal CA1 pyramidal neurons transiently elevate dendritic excitability. High-frequency stimulations (HFS) to the Schaffer collateral (SC) pathway activate mGluR5-dependent Ca2+ signalling in the apical dendrites, which, with calmodulin, upregulates specifically Nav1.6 channel-mediated persistent Na+ currents (INa,P ) in the dendrites. This HFS-induced increase in dendritic INa,P results in transient increases in the amplitude of excitatory postsynaptic potentials induced by both proximal SC and distal perforant path stimulation, leading to the enhanced probability of AP firing associated with decreased AP thresholds. Taken together, our study identifies dendritic INa,P as a novel target for mediating activity-dependent modulation of dendritic integration and neuronal output.

Muscarinic Long-Term Enhancement of Tonic and Phasic GABAA Inhibition in Rat CA1 Pyramidal Neurons

Acetylcholine (ACh) regulates network operation in the hippocampus by controlling excitation and inhibition in rat CA1 pyramidal neurons (PCs), the latter through gamma-aminobutyric acid type-A receptors (GABA_ARs). Although, the enhancing effects of ACh on GABA_ARs have been reported (Dominguez et al., 2014, 2015), its role in regulating tonic GABA_A inhibition has not been explored in depth. Therefore, we aimed at determining the effects of the activation of ACh receptors on responses mediated by synaptic and extrasynaptic GABA_ARs. Here, we show that under blockade of ionotropic glutamate receptors ACh, acting through muscarinic type 1 receptors, paired with post-synaptic depolarization induced a long-term enhancement of tonic GABA_A currents (tGABA_A) and puff-evoked GABA_A currents (pGABA_A). ACh combined with depolarization also potentiated IPSCs (i.e., phasic inhibition) in the same PCs, without signs of interactions of synaptic responses with pGABA_A and tGABA_A, suggesting the contribution of two different GABA_A receptor pools. The long-term enhancement of GABA_A currents and IPSCs reduced the excitability of PCs, possibly regulating plasticity and learning in behaving animals.

Calcium, Reactive Oxygen Species, and Synaptic Plasticity

In this review article, we address how activity-dependent Ca2+ signaling is crucial for hippocampal synaptic/structural plasticity and discuss how changes in neuronal oxidative state affect Ca2+ signaling and synaptic plasticity. We also analyze current evidence indicating that oxidative stress and abnormal Ca2+ signaling contribute to age-related synaptic plasticity deterioration.

Ryanodine-receptor-driven intracellular calcium dynamics underlying spatial association of synaptic plasticity

Synaptic modifications induced at one synapse are accompanied by hetero-synaptic changes at neighboring sites. In addition, it is suggested that the mechanism of spatial association of synaptic plasticity is based on intracellular calcium signaling that is mainly regulated by two types of receptors of endoplasmic reticulum calcium store: the ryanodine receptor (RyR) and the inositol triphosphate receptor (IP3R). However, it is not clear how these types of receptors regulate intracellular calcium flux and contribute to the outcome of calcium-dependent synaptic change. To understand the relation between the synaptic association and store-regulated calcium dynamics, we focused on the function of RyR calcium regulation and simulated its behavior by using a computational neuron model. As a result, we observed that RyR-regulated calcium release depended on spike timings of pre- and postsynaptic neurons. From the induction site of calcium release, the chain activation of RyRs occurred, and spike-like calcium increase propagated along the dendrite. For calcium signaling, the propagated calcium increase did not tend to attenuate; these characteristics came from an all-or-none behavior of RyR-sensitive calcium store. Considering the role of calcium dependent synaptic plasticity, the results suggest that RyR-regulated calcium propagation induces a similar change at the synapses. However, according to the dependence of RyR calcium regulation on the model parameters, whether the chain activation of RyRs occurred, sensitively depended on spatial expression of RyR and nominal fluctuation of calcium flux. Therefore, calcium regulation of RyR helps initiate rather than relay calcium propagation.

Protein trafficking from synapse to nucleus in control of activity-dependent gene expression

Long-lasting changes in neuronal excitability require activity-dependent gene expression and therefore the transduction of synaptic signals to the nucleus. Synaptic activity is rapidly relayed to the nucleus by membrane depolarization and the propagation of Ca(2+)-waves. However, it is unlikely that Ca(2+)-transients alone can explain the specific genomic response to the plethora of extracellular stimuli that control gene expression. In recent years a steadily growing number of studies report the transport of proteins from synapse to nucleus. Potential mechanisms for active retrograde transport and nuclear targets for these proteins have been identified and recent reports assigned first functions to this type of long-distance signaling. In this review we will discuss how the dissociation of synapto-nuclear protein messenger from synaptic and extrasynaptic sites, their transport, nuclear import and the subsequent genomic response relate to the prevailing concept behind this signaling mechanism, the encoding of signals at their site of origin and their decoding in the nucleus.

Dendritic spine heterogeneity and calcium dynamics in basolateral amygdala principal neurons

Glutamatergic synapses on pyramidal neurons are formed on dendritic spines where glutamate activates ionotropic receptors, and calcium influx via N-methyl-d-aspartate receptors leads to a localized rise in spine calcium that is critical for the induction of synaptic plasticity. In the basolateral amygdala, activation of metabotropic receptors is also required for synaptic plasticity and amygdala-dependent learning. Here, using acute brain slices from rats, we show that, in basolateral amygdala principal neurons, high-frequency synaptic stimulation activates metabotropic glutamate receptors and raises spine calcium by releasing calcium from inositol trisphosphate-sensitive calcium stores. This spine calcium release is unevenly distributed, being present in proximal spines, but largely absent in more distal spines. Activation of metabotropic receptors also generated calcium waves that differentially invaded spines as they propagated toward the soma. Dendritic wave invasion was dependent on diffusional coupling between the spine and parent dendrite which was determined by spine neck length, with waves preferentially invading spines with short necks. Spine calcium is a critical trigger for the induction of synaptic plasticity, and our findings suggest that calcium release from inositol trisphosphate-sensitive calcium stores may modulate homosynaptic plasticity through store-release in the spine head, and heterosynaptic plasticity of unstimulated inputs via dendritic calcium wave invasion of the spine head.

Dendritic calcium nonlinearities switch the direction of synaptic plasticity in fast-spiking interneurons

Postsynaptic calcium (Ca2+) nonlinearities allow neuronal coincidence detection and site-specific plasticity. Whether such events exist in dendrites of interneurons and play a role in regulation of synaptic efficacy remains unknown. Here, we used a combination of whole-cell patch-clamp recordings and two-photon Ca2+ imaging to reveal Ca2+ nonlinearities associated with synaptic integration in dendrites of mouse hippocampal CA1 fast-spiking interneurons. Local stimulation of distal dendritic branches within stratum oriens/alveus elicited fast Ca2+ transients, which showed a steep sigmoidal relationship to stimulus intensity. Supralinear Ca2+ events required Ca2+ entry through AMPA receptors with a subsequent Ca2+ release from internal stores. To investigate the functional significance of supralinear Ca2+ signals, we examined activity-dependent fluctuations in transmission efficacy triggered by Ca2+ signals of different amplitudes at excitatory synapses of interneurons. Subthreshold theta-burst stimulation (TBS) produced small amplitude postsynaptic Ca2+ transients and triggered long-term potentiation. In contrast, the suprathreshold TBS, which was associated with the generation of supralinear Ca2+ events, triggered long-term depression. Blocking group I/II metabotropic glutamate receptors (mGluRs) during suprathreshold TBS resulted in a slight reduction of supralinear Ca2+ events and induction of short-term depression. In contrast, blocking internal stores and supralinear Ca2+ signals during suprathreshold TBS switched the direction of plasticity from depression back to potentiation. These data reveal a novel type of supralinear Ca2+ events at synapses lacking the GluA2 AMPA subtype of glutamate receptors and demonstrate a general mechanism by which Ca2+ -permeable AMPA receptors, together with internal stores and mGluRs, control the direction of plasticity at interneuron excitatory synapses.

Calcium channelopathies and Alzheimer's disease: insight into therapeutic success and failures

Calcium ions are versatile and universal biological signaling factors that regulate numerous cellular processes ranging from cell fertilization, to neuronal plasticity that underlies learning and memory, to cell death. For these functions to be properly executed, calcium signaling requires precise regulation, and failure of this regulation may tip the scales from a signal for life to a signal for death. Disruptions in calcium channel function can generate complex multi-system disorders collectively referred to as "calciumopathies" that can target essentially any cell type or organ. In this review, we focus on the multifaceted involvement of calcium signaling in the pathophysiology of Alzheimer's disease (AD), and summarize the various therapeutic options currently available to combat this disease. Detailing the series of disappointing AD clinical trial results on cognitive outcomes, we emphasize the urgency to design alternative therapeutic strategies if synaptic and memory functions are to be preserved. One such approach is to target early calcium channelopathies centrally linked to AD pathogenesis.

Nuclear calcium signalling in the regulation of brain function

Synaptic activity initiates biochemical processes that have various outcomes, including the formation of memories, increases in neuronal survival and the development of chronic pain and addiction. Virtually all activity-induced, long-lasting adaptations of brain functions require a dialogue between synapses and the nucleus that results in changes in gene expression. Calcium signals that are induced by synaptic activity and propagate into the nucleus are a major route for synapse-to-nucleus communication. Recent findings indicate that diverse forms of neuroadaptation require calcium transients in the nucleus to switch on the necessary genomic programme. Deficits in nuclear calcium signalling as a result of a reduction in synaptic activity or increased extrasynaptic NMDA receptor signalling may underlie the aetiologies of various diseases, including neurodegeneration and cognitive dysfunction.

The role of intracellular calcium stores in synaptic plasticity and memory consolidation

Memory processing requires tightly controlled signalling cascades, many of which are dependent upon intracellular calcium (Ca(2+)). Despite this, most work investigating calcium signalling in memory formation has focused on plasma membrane channels and extracellular sources of Ca(2+). The intracellular Ca(2+) release channels, ryanodine receptors (RyRs) and inositol (1,4,5)-trisphosphate receptors (IP3Rs) have a significant capacity to regulate intracellular Ca(2+) signalling. Evidence at both cellular and behavioural levels implicates both RyRs and IP3Rs in synaptic plasticity and memory formation. Pharmacobehavioural experiments using young chicks trained on a single-trial discrimination avoidance task have been particularly useful by demonstrating that RyRs and IP3Rs have distinct roles in memory formation. RyR-dependent Ca(2+) release appears to aid the consolidation of labile memory into a persistent long-term memory trace. In contrast, IP3Rs are required during long-term memory. This review discusses various functions for RyRs and IP3Rs in memory processing, including neuro- and glio-transmitter release, dendritic spine remodelling, facilitating vasodilation, and the regulation of gene transcription and dendritic excitability. Altered Ca(2+) release from intracellular stores also has significant implications for neurodegenerative conditions.

Cytosolic [Ca2+] regulation of InsP3-evoked puffs

InsP3-mediated puffs are fundamental building blocks of cellular Ca2+ signalling, and arise through the concerted opening of clustered InsP3Rs (InsP3 receptors) co-ordinated via Ca2+-induced Ca2+ release. Although the Ca2+ dependency of InsP3Rs has been extensively studied at the single channel level, little is known as to how changes in basal cytosolic [Ca2+] would alter the dynamics of InsP3-evoked Ca2+ signals in intact cells. To explore this question, we expressed Ca2+-permeable channels (nicotinic acetylcholine receptors) in the plasma membrane of voltage-clamped Xenopus oocytes to regulate cytosolic [Ca2+] by changing the electrochemical gradient for extracellular Ca2+ entry, and imaged Ca2+ liberation evoked by photolysis of caged InsP3. Elevation of basal cytosolic [Ca2+] strongly increased the amplitude and shortened the latency of global Ca2+ waves. In oocytes loaded with EGTA to localize Ca2+ signals, the number of sites at which puffs were observed and the frequency and latency of puffs were strongly dependent on cytosolic [Ca2+], whereas puff amplitudes were only weakly affected. The results of the present study indicate that basal cytosolic [Ca2+] strongly affects the triggering of puffs, but has less of an effect on puffs once they have been initiated.

Extending the IP3 receptor model to include competition with partial agonists

The inositol 1,4,5-trisphosphate (IP(3)) receptor is a Ca(2+) channel located in the endoplasmic reticulum and is regulated by IP(3) and Ca(2+). This channel is critical to calcium signaling in cell types as varied as neurons and pancreatic beta cells to mast cells. De Young and Keizer (1992) created an eight-state, nine-variable model of the IP(3) receptor. In their model, they accounted for three binding sites, a site for IP(3), activating Ca(2+), and deactivating Ca(2+). The receptor is only open if IP(3) and activating Ca(2+) is bound. Li and Rinzel followed up this paper in 1994 by introducing a reduction that made it into a two variable system. A recent publication by Rossi et al. (2009) studied the effect of introducing IP(3)-like molecules, referred to as partial agonists (PA), into the cell to determine the structure-function relationship between IP(3) and its receptor. Initial results suggest a competitive model, where IP(3) and PA fight for the same binding site. We extend the original eight-state model to a 12-state model in order to illustrate this competition, and perform a similar reduction to that of Li and Rinzel in the first modeling study we are aware of considering PA effect on an IP(3) receptor. Using this reduction we solve for the equilibrium open probability for calcium release in the model. We replicate graphs provided by the Rossi paper, and find that optimizing the subunit affinities for IP(3) and PA yields a good fit to the data. We plug our extended reduced model into a full cell model, in order to analyze the effects PA have on whole cell properties specifically the propagation of calcium waves in two dimensions. We conclude that PA creates qualitatively different calcium dynamics than would simply reducing IP(3), but that effectively PA can act as an IP(3) knockdown.

Early presynaptic and postsynaptic calcium signaling abnormalities mask underlying synaptic depression in presymptomatic Alzheimer's disease mice

Alzheimer's disease (AD)-linked presenilin (PS) mutations result in pronounced endoplasmic reticulum calcium disruptions that occur before detectable histopathology and cognitive deficits. More subtly, these early AD-linked calcium alterations also reset neurophysiological homeostasis, such that calcium-dependent presynaptic and postsynaptic signaling appear functionally normal yet are actually operating under aberrant calcium signaling systems. In these 3xTg-AD mouse brains, upregulated ryanodine receptor (RyR) activity is associated with a shift toward synaptic depression, likely through a reduction in presynaptic vesicle stores and increased postsynaptic outward currents through small-conductance calcium-activated potassium SK2 channels. The deviant RyR-calcium involvement in the 3xTg-AD mice also compensates for an intrinsic predisposition for hippocampal long-term depression (LTD) and reduced long-term potentiation (LTP). In this study, we detail the impact of disrupted RyR-mediated calcium stores on synaptic transmission properties, LTD, and calcium-activated membrane channels of hippocampal CA1 pyramidal neurons in presymptomatic 3xTg-AD mice. Using electrophysiological recordings in young 3xTg-AD and nontransgenic (NonTg) hippocampal slices, we show that increased RyR-evoked calcium release in 3xTg-AD mice "normalizes" an altered synaptic transmission system operating under a shifted homeostatic state that is not present in NonTg mice. In the process, we uncover compensatory signaling mechanisms recruited early in the disease process that counterbalance the disrupted RyR-calcium dynamics, namely increases in presynaptic spontaneous vesicle release, altered probability of vesicle release, and upregulated postsynaptic SK channel activity. Because AD is increasingly recognized as a "synaptic disease," calcium-mediated signaling alterations may serve as a proximal trigger for the synaptic degradation driving the cognitive loss in AD.

Putative roles for phospholipase Cη enzymes in neuronal Ca2+ signal modulation

The most recently identified PLC (phospholipase C) enzymes belong to the PLCη family. Their unique Ca2+-sensitivity and their specific appearance in neurons have attracted great attention since their discovery; however, their physiological role(s) in neurons are still yet to be established. PLCη enzymes are expressed in the neocortex, hippocampus and cerebellum. PLCη2 is also expressed at high levels in pituitary gland, pineal gland and in the retina. Driven by the specific localization of PLCη enzymes in different brain areas, in the present paper, we discuss the roles that they may play in neural processes, including differentiation, memory formation, circadian rhythm regulation, neurotransmitter/hormone release and the pathogenesis of neurodegenerative disorders associated with aberrant Ca2+ signalling, such as Alzheimer's disease.

Calcium signaling in dendritic spines

Calcium (Ca(2+)) is a ubiquitous signaling molecule that accumulates in the cytoplasm in response to diverse classes of stimuli and, in turn, regulates many aspects of cell function. In neurons, Ca(2+) influx in response to action potentials or synaptic stimulation triggers neurotransmitter release, modulates ion channels, induces synaptic plasticity, and activates transcription. In this article, we discuss the factors that regulate Ca(2+) signaling in mammalian neurons with a particular focus on Ca(2+) signaling within dendritic spines. This includes consideration of the routes of entry and exit of Ca(2+), the cellular mechanisms that establish the temporal and spatial profile of Ca(2+) signaling, and the biophysical criteria that determine which downstream signals are activated when Ca(2+) accumulates in a spine. Furthermore, we also briefly discuss the technical advances that made possible the quantitative study of Ca(2+) signaling in dendritic spines.

Understanding calcium waves and sparks in central neurons

All cells use changes in intracellular calcium concentration ([Ca(2+)](i)) to regulate cell signalling events. In neurons, with their elaborate dendritic and axonal arborizations, there are clear examples of both localized and widespread Ca(2+) signals. [Ca(2+)](i) changes that are generated by Ca(2+) entry through voltage- and ligand-gated channels are the best characterized. In addition, the release of Ca(2+) from intracellular stores can result in increased [Ca(2+)](i); the signals that trigger this release have been less well-studied, in part because they are not usually associated with specific changes in membrane potential. However, recent experiments have revealed dramatic widespread Ca(2+) waves and localized spark-like events, particularly in dendrites. Here we review emerging data on the nature of these signals and their functions.

Calcium signaling in synapse-to-nucleus communication

Changes in the intracellular concentration of calcium ions in neurons are involved in neurite growth, development, and remodeling, regulation of neuronal excitability, increases and decreases in the strength of synaptic connections, and the activation of survival and programmed cell death pathways. An important aspect of the signals that trigger these processes is that they are frequently initiated in the form of glutamatergic neurotransmission within dendritic trees, while their completion involves specific changes in the patterns of genes expressed within neuronal nuclei. Accordingly, two prominent aims of research concerned with calcium signaling in neurons are determination of the mechanisms governing information conveyance between synapse and nucleus, and discovery of the rules dictating translation of specific patterns of inputs into appropriate and specific transcriptional responses. In this article, we present an overview of the avenues by which glutamatergic excitation of dendrites may be communicated to the neuronal nucleus and the primary calcium-dependent signaling pathways by which synaptic activity can invoke changes in neuronal gene expression programs.

Early calcium dysregulation in Alzheimer's disease: setting the stage for synaptic dysfunction

Alzheimer's disease (AD) is an irreversible and progressive neurodegenerative disorder with no known cure or clear understanding of the mechanisms involved in the disease process. Amyloid plaques, neurofibrillary tangles and neuronal loss, though characteristic of AD, are late stage markers whose impact on the most devastating aspect of AD, namely memory loss and cognitive deficits, are still unclear. Recent studies demonstrate that structural and functional breakdown of synapses may be the underlying factor in AD-linked cognitive decline. One common element that presents with several features of AD is disrupted neuronal calcium signaling. Increased intracellular calcium levels are functionally linked to presenilin mutations, ApoE4 expression, amyloid plaques, tau tangles and synaptic dysfunction. In this review, we discuss the role of AD-linked calcium signaling alterations in neurons and how this may be linked to synaptic dysfunctions at both early and late stages of the disease.

Endoplasmic reticulum Ca(2+) handling in excitable cells in health and disease

The endoplasmic reticulum (ER) is a morphologically and functionally diverse organelle capable of integrating multiple extracellular and internal signals and generating adaptive cellular responses. It plays fundamental roles in protein synthesis and folding and in cellular responses to metabolic and proteotoxic stress. In addition, the ER stores and releases Ca(2+) in sophisticated scenarios that regulate a range of processes in excitable cells throughout the body, including muscle contraction and relaxation, endocrine regulation of metabolism, learning and memory, and cell death. One or more Ca(2+) ATPases and two types of ER membrane Ca(2+) channels (inositol trisphosphate and ryanodine receptors) are the major proteins involved in ER Ca(2+) uptake and release, respectively. There are also direct and indirect interactions of ER Ca(2+) stores with plasma membrane and mitochondrial Ca(2+)-regulating systems. Pharmacological agents that selectively modify ER Ca(2+) release or uptake have enabled studies that revealed many different physiological roles for ER Ca(2+) signaling. Several inherited diseases are caused by mutations in ER Ca(2+)-regulating proteins, and perturbed ER Ca(2+) homeostasis is implicated in a range of acquired disorders. Preclinical investigations suggest a therapeutic potential for use of agents that target ER Ca(2+) handling systems of excitable cells in disorders ranging from cardiac arrhythmias and skeletal muscle myopathies to Alzheimer disease.

Metabotropic glutamate receptors regulate hippocampal CA1 pyramidal neuron excitability via Ca²⁺ wave-dependent activation of SK and TRPC channels

Group I metabotropic glutamate receptors (mGluRs) play an essential role in cognitive function. Their activation results in a wide array of cellular and molecular responses that are mediated by multiple signalling cascades. In this study, we focused on Group I mGluR activation of IP3R-mediated intracellular Ca2+ waves and their role in activating Ca2+-dependent ion channels in CA1 pyramidal neurons. Using whole-cell patch-clamp recordings and high-speed Ca2+ fluorescence imaging in acute hippocampal brain slices, we show that synaptic and pharmacological stimulation of mGluRs triggers intracellular Ca2+ waves and a biphasic electrical response composed of a transient Ca2+-dependent SK channel-mediated hyperpolarization and a TRPC-mediated sustained depolarization. The generation and magnitude of the SK channel-mediated hyperpolarization depended solely on the rise in intracellular Ca2+ concentration ([Ca2+]i), whereas the TRPC channel-mediated depolarization required both a small rise in [Ca2+]i and mGluR activation. Furthermore, the TRPC-mediated current was suppressed by forskolin-induced rises in cAMP. We also show that SK- and TRPC-mediated currents robustly modulate pyramidal neuron excitability by decreasing and increasing their firing frequency, respectively. These findings provide additional evidence that mGluR-mediated synaptic transmission makes an important contribution to regulating the output of hippocampal neurons through intracellular Ca2+ wave activation of SK and TRPC channels. cAMP provides an additional level of regulation by modulating TRPC-mediated sustained depolarization that we propose to be important for stabilizing periods of sustained firing.

The role of glutamate receptors in traumatic brain injury: implications for postsynaptic density in pathophysiology

Traumatic brain injury (TBI) is the major cause of death and disability, and the incidence of TBI continues to increase rapidly. In recent years, increasing attention has been paid to an important structure at the postsynaptic membrane: the postsynaptic density (PSD). Glutamate receptors, as major components of the PSD, are highly responsive to alterations in the glutamate concentration at excitatory synapses and activate intracellular signal transduction via calcium and other second messengers following TBI. PSD scaffold proteins (PSD-95, Homer, and Shank), which anchor glutamate receptors and form a network structure, also have potential effects on these downstream signaling pathways. The changes in the function and structure of these major PSD proteins are also induced by TBI, indicating that there is a more complicated mechanism associated with PSD proteins in the pathophysiological process of TBI.

Nuclear calcium sensors reveal that repetition of trains of synaptic stimuli boosts nuclear calcium signaling in CA1 pyramidal neurons

Nuclear calcium is a key signal in the dialogue between synapse and nucleus that controls the genomic responses required for persistent adaptations, including memory and acquired neuroprotection. The amplitude and duration of nuclear calcium transients specify activity-induced transcriptional changes. However, the precise relationship between synaptic input and nuclear calcium output is unknown. Here, we used stereotaxic delivery to the rat brain of recombinant adeno-associated viruses encoding nuclear-targeted calcium sensors to assess nuclear calcium transients in CA1 pyramidal neurons after stimulation of the Schaffer collaterals. We show that in acute hippocampal slices, a burst of synaptic activity elicits a nuclear calcium signal with a regenerative component at above-threshold stimulation intensities. Using classical stimulation paradigms (i.e., high-frequency stimulation (HFS) and θ burst stimulation (TBS)) to induce early LTP (E-LTP) and transcription-dependent late LTP (L-LTP), we found that the magnitude of nuclear calcium signals and the number of action potentials activated by synaptic stimulation trains are greatly amplified by their repetition. Nuclear calcium signals and action potential generation were reduced by blockade of either NMDA receptors or L-type voltage-gated calcium channels, but not by procedures that lead to internal calcium store depletion or by blockade of metabotropic glutamate receptors. These findings identify a repetition-induced switch in nuclear calcium signaling that correlates with the transition from E-LTP to L-LTP, and may explain why the transcription-dependent phase of L-LTP is not induced by a single HFS or TBS but requires repeated trains of activity. Recombinant, nuclear-targeted indicators may prove useful for further analysis of nuclear calcium signaling in vivo.

IP(3) mobilization and diffusion determine the timing window of Ca(2+) release by synaptic stimulation and a spike in rat CA1 pyramidal cells

Synaptically activated calcium release from internal stores in CA1 pyramidal neurons is generated via metabotropic glutamate receptors by mobilizing IP(3). Ca(2+) release spreads as a large amplitude wave in a restricted region of the apical dendrites of these cells. These Ca(2+) waves have been shown to induce certain forms of synaptic potentiation and have been hypothesized to affect other forms of plasticity. Pairing a single backpropagating action potential (bAP) with repetitive synaptic stimulation evokes Ca(2+) release when synaptic stimulation alone is subthreshold for generating release. We examined the timing window for this synergistic effect under conditions favoring Ca(2+) release. The window, measured from the end of the train, lasted 250-500 ms depending on the duration of stimulation tetanus. The window appears to correspond to the time when both IP(3) concentration and [Ca(2+)](i) are elevated at the site of the IP(3) receptor. Detailed analysis of the mechanisms determining the duration of the window, including experiments using different forms of caged IP(3) instead of synaptic stimulation, suggest that the most significant processes are the time for IP(3) to diffuse away from the site of generation and the time course of IP(3) production initiated by activation of mGluRs. IP(3) breakdown, desensitization of the IP(3) receptor, and the kinetics of IP(3) unbinding from the receptor may affect the duration of the window but are less significant. The timing window is short but does not appear to be short enough to suggest that this form of coincidence detection contributes to conventional spike timing-dependent synaptic plasticity in these cells.

Learning and aging related changes in intrinsic neuronal excitability

A goal of many laboratories that study aging is to find a key cellular change(s) that can be manipulated and restored to a young-like state, and thus, reverse the age-related cognitive deficits. We have chosen to focus our efforts on the alteration of intrinsic excitability (as reflected by the postburst afterhyperpolarization, AHP) during the learning process in hippocampal pyramidal neurons. We have consistently found that the postburst AHP is significantly reduced in hippocampal pyramidal neurons from young adults that have successfully learned a hippocampus-dependent task. In the context of aging, the baseline intrinsic excitability of hippocampal neurons is decreased and therefore cognitive learning is impaired. In aging animals that are able to learn, neuron changes in excitability similar to those seen in young neurons during learning occur. Our challenge, then, is to understand how and why excitability changes occur in neurons from aging brains and cause age-associated learning impairments. After understanding the changes, we should be able to formulate strategies for reversing them, thus making old neurons function more as they did when they were young. Such a reversal should rescue the age-related cognitive deficits.

Synaptic activity induces dramatic changes in the geometry of the cell nucleus: interplay between nuclear structure, histone H3 phosphorylation, and nuclear calcium signaling

Synaptic activity initiates many adaptive responses in neurons. Here we report a novel form of structural plasticity in dissociated hippocampal cultures and slice preparations. Using a recently developed algorithm for three-dimensional image reconstruction and quantitative measurements of cell organelles, we found that many nuclei from hippocampal neurons are highly infolded and form unequally sized nuclear compartments. Nuclear infoldings are dynamic structures, which can radically transform the geometry of the nucleus in response to neuronal activity. Action potential bursting causing synaptic NMDA receptor activation dramatically increases the number of infolded nuclei via a process that requires the ERK-MAP kinase pathway and new protein synthesis. In contrast, death-signaling pathways triggered by extrasynaptic NMDA receptors cause a rapid loss of nuclear infoldings. Compared with near-spherical nuclei, infolded nuclei have a larger surface and increased nuclear pore complex immunoreactivity. Nuclear calcium signals evoked by cytosolic calcium transients are larger in small nuclear compartments than in the large compartments of the same nucleus; moreover, small compartments are more efficient in temporally resolving calcium signals induced by trains of action potentials in the theta frequency range (5 Hz). Synaptic activity-induced phosphorylation of histone H3 on serine 10 was more robust in neurons with infolded nuclei compared with neurons with near-spherical nuclei, suggesting a functional link between nuclear geometry and transcriptional regulation. The translation of synaptic activity-induced signaling events into changes in nuclear geometry facilitates the relay of calcium signals to the nucleus, may lead to the formation of nuclear signaling microdomains, and could enhance signal-regulated transcription.

Deviant ryanodine receptor-mediated calcium release resets synaptic homeostasis in presymptomatic 3xTg-AD mice

Presenilin mutations result in exaggerated endoplasmic reticulum (ER) calcium release in cellular and animal models of Alzheimer's disease (AD). In this study, we examined whether dysregulated ER calcium release in young 3xTg-AD neurons alters synaptic transmission and plasticity mechanisms before the onset of histopathology and cognitive deficits. Using electrophysiological recordings and two-photon calcium imaging in young (6-8 weeks old) 3xTg-AD and non-transgenic (NonTg) hippocampal slices, we show a marked increase in ryanodine receptor (RyR)-evoked calcium release within synapse-dense regions of CA1 pyramidal neurons. In addition, we uncovered a deviant contribution of presynaptic and postsynaptic ryanodine receptor-sensitive calcium stores to synaptic transmission and plasticity in 3xTg-AD mice that is not present in NonTg mice. As a possible underlying mechanism, the RyR2 isoform was found to be selectively increased more than fivefold in the hippocampus of 3xTg-AD mice relative to the NonTg controls. These novel findings demonstrate that 3xTg-AD CA1 neurons at presymptomatic ages operate under an aberrant, yet seemingly functional, calcium signaling and synaptic transmission system long before AD histopathology onset. These early signaling alterations may underlie the later synaptic breakdown and cognitive deficits characteristic of later stage AD.

Synaptic activation and membrane potential changes modulate the frequency of spontaneous elementary Ca2+ release events in the dendrites of pyramidal neurons

In most neurons postsynaptic [Ca(2+)](i) changes result from synaptic activation opening voltage gated channels, ligand gated channels, or mobilizing Ca(2+) release from intracellular stores. In addition to these changes that result directly from stimulation we found that in pyramidal cells there are spontaneous, rapid, Ca(2+) release events, predominantly, but not exclusively localized at dendritic branch points. They are clearest on the main apical dendrite but also have been detected in the finer branches and in the soma. Typically they have a spatial extent at initiation of approximately 2 microm, a rise time of <15 ms, duration <100 ms, and amplitudes of 10-70% of that generated by a backpropagating action potential at the same location. These events are not caused by background electrical or synaptic activity. However, their rate can be increased by repetitive synaptic stimulation at moderate frequencies, mainly through metabotropic glutamate receptor mobilization of IP(3). In addition, their frequency can be modulated by changes in membrane potential in the subthreshold range, predominantly by affecting Ca(2+) entry through L-type channels. They resemble the elementary events ("sparks" and "puffs") mediated by IP(3) receptors and ryanodine receptors that have been described primarily in non-neuronal preparations. These spontaneous Ca(2+) release events may be the fundamental units underlying some postsynaptic signaling cascades in mature neurons.

2-Aminoethoxydiphenyl-borate (2-APB) increases excitability in pyramidal neurons

Calcium ions (Ca(2+)) released from inositol trisphosphate (IP(3))-sensitive intracellular stores may participate in both the transient and extended regulation of neuronal excitability in neocortical and hippocampal pyramidal neurons. IP(3) receptor (IP(3)R) antagonists represent an important tool for dissociating these consequences of IP(3) generation and IP(3)R-dependent internal Ca(2+) release from the effects of other, concurrently stimulated second messenger signaling cascades and Ca(2+) sources. In this study, we have described the actions of the IP(3)R and store-operated Ca(2+) channel antagonist, 2-aminoethoxydiphenyl-borate (2-APB), on internal Ca(2+) release and plasma membrane excitability in neocortical and hippocampal pyramidal neurons. Specifically, we found that a dose of 2-APB (100 microM) sufficient for attenuating or blocking IP(3)-mediated internal Ca(2+) release also raised pyramidal neuron excitability. The 2-APB-dependent increase in excitability reversed upon washout and was characterized by an increase in input resistance, a decrease in the delay to action potential onset, an increase in the width of action potentials, a decrease in the magnitude of afterhyperpolarizations (AHPs), and an increase in the magnitude of post-spike afterdepolarizations (ADPs). From these observations, we conclude that 2-APB potently and reversibly increases neuronal excitability, likely via the inhibition of voltage- and Ca(2+)-dependent potassium (K(+)) conductances.

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.

Initiation and propagation of a neuronal intracellular calcium wave

The ability to image calcium movement within individual neurons inspires questions of functionality including whether calcium entry into the nucleus is related to genetic regulation for phenomena such as long term potentiation. Calcium waves have been initiated in hippocampal pyramidal cells with glutmatergic signals both in the presence and absence of back propagating action potentials (BPAPs). The dendritic sites of initiation of these calcium waves within about 100 microm of the soma are thought to be localized near oblique junctions. Stimulation of synapses on oblique dendrites leads to production of inositol 1,4,5-trisphosphate (IP(3)) which diffuses to the apical dendrite igniting awaiting IP(3) receptors (IP(3)Rs) and initiating and propagating catalytic calcium release from the endoplasmic reticulum. We construct a reduced mathematical system which accounts for calcium wave initiation and propagation due to elevated IP(3). Inhomogeneity in IP(3) distribution is responsible for calcium wave initiation versus subthreshold or spatially uniform suprathreshold activation. However, the likelihood that a calcium wave is initiated does not necessarily increase with more calcium entering from BPAPs. For low transient synaptic stimuli, timing between IP(3) generation and BPAPs is critical for calcium wave initiation. We also show that inhomogeneity in IP(3)R density can account for calcium wave directionality. Simulating somatic muscarinic receptor production of IP(3), we can account for the critical difference between calcium wave entry into the soma and failure to do so.

Mitochondrial Ca2+ signalling in hippocampal neurons

High-resolution fluorescent imaging of mitochondrial-targeted probes was used to examine the ability of mitochondria to decode complex spatial and temporal Ca2+ signals evoked in synaptically active networks of hippocampal neurons. Green-to-red photoconversion of the mitochondrial-targeted probe, mito-Kaede, demonstrated that mitochondria were present as discrete organelles 2-6 microm in length. Real-time imaging of mitochondrial-targeted ratiometric pericam (2 mtRP) visualised rapid, repetitive, transient mitochondrial Ca2+ fluxes in response to periods of synaptic activation. Mitochondrial Ca2+ fluxes within cellular compartments were dependent on the extent of synaptic recruitment, but independent of cross-talk with the endoplasmic reticulum or the presence of an interconnected mitochondrial network. Mitochondria in dendritic regions demonstrated a greater sensitivity to synaptic activation compared with somatic mitochondria. Temporal decoding of synaptic signals was rate-limited by the activity of the mitochondrial Na+/Ca2+ exchanger. Spatial regulation of mitochondrial Ca2+ uptake was determined by the magnitude of the cytosolic Ca2+ rise in each cellular compartment.

Priming of intracellular calcium stores in rat CA1 pyramidal neurons

Repetitive synaptic stimulation evokes large amplitude Ca(2+) release waves from internal stores in many kinds of pyramidal neurons. The waves result from mGluR mobilization of IP(3) leading to Ca(2+)-induced Ca(2+) release. In most experiments in slices, regenerative Ca(2+) release can be evoked for only a few trials. We examined the conditions required for consistent release from the internal stores in hippocampal CA1 pyramidal neurons. We found that priming with action potentials evoked at 0.5-1 Hz for intervals as short as 15 s were sufficient to fill the stores, while sustained subthreshold depolarization or subthreshold synaptic stimulation lasting from 15 s to 2 min was less effective. A single episode of priming was effective for about 2-3 min. Ca(2+) waves could also be evoked by uncaging IP(3) with a UV flash in the dendrites. Priming was necessary to evoke these waves repetitively; 7-10 spikes in 15 s were again effective for this protocol, indicating that priming acts to refill the stores and not at a site upstream to the production of IP(3). These results suggest that normal spiking activity of pyramidal neurons in vivo should be sufficient to maintain their internal stores in a primed state ready to release Ca(2+) in response to an appropriate physiological stimulus. This may be a novel form of synaptic plasticity where a cell's capacity to release Ca(2+) is modulated by its average firing frequency.

MGluR-mediated calcium waves that invade the soma regulate firing in layer V medial prefrontal cortical pyramidal neurons

Factors that influence the activity of prefrontal cortex (PFC) pyramidal neurons are likely to play an important role in working memory function. One such factor may be the release of Ca2+ from intracellular stores. Here we investigate the hypothesis that metabotropic glutamate receptors (mGluRs)-mediated waves of internally released Ca2+ can regulate the intrinsic excitability and firing patterns of PFC pyramidal neurons. Synaptic or focal pharmacological activation of mGluRs triggered Ca2+ waves in the dendrites and somata of layer V medial PFC pyramidal neurons. These Ca2+ waves often evoked a transient SK-mediated hyperpolarization followed by a prolonged depolarization that respectively decreased and increased neuronal excitability. Generation of the hyperpolarization depended on whether the Ca2+ wave invaded or came near to the soma. The depolarization also depended on the extent of Ca2+ wave propagation. We tested factors that influence the propagation of Ca2+ waves into the soma. Stimulating more synapses, increasing inositol trisphosphate concentration near the soma, and priming with physiological trains of action potentials all enhanced the amplitude and likelihood of evoking somatic Ca2+ waves. These results suggest that mGluR-mediated Ca2+ waves may regulate firing patterns of PFC pyramidal neurons engaged by working memory, particularly under conditions that favor the propagation of Ca2+ waves into the soma.

Differential regulation of action potential- and metabotropic glutamate receptor-induced Ca2+ signals by inositol 1,4,5-trisphosphate in dopaminergic neurons

Ca2+ signals associated with action potentials (APs) and metabotropic glutamate receptor (mGluR) activation exert distinct influences on neuronal activity and synaptic plasticity. However, it is not clear how these two types of Ca2+ signals are differentially regulated by neurotransmitter inputs in a single neuron. We investigated this issue in dopaminergic neurons of the ventral midbrain using brain slices. Intracellular Ca2+ was assessed by measuring Ca2+-sensitive K+ currents or imaging the fluorescence of Ca2+ indicator dyes. Tonic activation of metabotropic neurotransmitter receptors (mGluRs, alpha1 adrenergic receptors, and muscarinic acetylcholine receptors), attained by superfusion of agonists or weak, sustained (approximately 1 s) synaptic stimulation, augmented AP-induced Ca2+ transients. In contrast, Ca2+ signals elicited by strong, transient (50-200 ms) activation of mGluRs with aspartate iontophoresis were suppressed by superfusion of agonists. These opposing effects on Ca2+ signals were both mediated by an increase in intracellular inositol 1,4,5-trisphosphate (IP3) levels, because they were blocked by heparin, an IP3 receptor antagonist, and reproduced by photolytic application of IP3. Evoking APs repetitively at low frequency (2 Hz) caused inactivation of IP3 receptors and abolished IP3 facilitation of single AP-induced Ca2+ signals, whereas facilitation of Ca2+ signals triggered by bursts of APs (five at 20 Hz) was attenuated by less than half. We further obtained evidence suggesting that the psychostimulant amphetamine may augment burst-induced Ca2+ signals via both depression of basal firing and production of IP3. We propose that intracellular IP3 tone provides a mechanism to selectively amplify burst-induced Ca2+ signals in dopaminergic neurons.

Distribution of IP3-mediated calcium responses and their role in nuclear signalling in rat basolateral amygdala neurons

Metabotropic receptor activation is important for learning, memory and synaptic plasticity in the amygdala and other brain regions. Synaptic stimulation of metabotropic receptors in basolateral amygdala (BLA) projection neurons evokes a focal rise in free Ca(2+) in the dendrites that propagate as waves into the soma and nucleus. These Ca(2+) waves initiate in the proximal dendrites and show limited propagation centrifugally away from the soma. In other cell types, Ca(2+) waves have been shown to be mediated by either metabotropic glutamate receptor (mGluR) or muscarinic receptor (mAChR) activation. Here we show that mGluRs and mAChRs act cooperatively to release Ca(2+) from inositol 1,4,5-trisphosphate (IP(3))-sensitive intracellular Ca(2+) stores. Whereas action potentials (APs) alone were relatively ineffective in raising nuclear Ca(2+), their pairing with metabotropic receptor activation evoked an IP(3)-receptor-mediated Ca(2+)-induced Ca(2+) release, raising nuclear Ca(2+) into the micromolar range. Metabotropic-receptor-mediated Ca(2+)-store release was highly compartmentalized. When coupled with metabotropic receptor stimulation, large robust Ca(2+) rises and AP-induced amplification were observed in the soma, nucleus and sparsely spiny dendritic segments with metabotropic stimulation. In contrast, no significant amplification of the Ca(2+) transient was detected in spine-dense high-order dendritic segments. Ca(2+) rises evoked by photolytic uncaging of IP(3) showed the same distribution, suggesting that IP(3)-sensitive Ca(2+) stores are preferentially located in the soma and proximal dendrites. This distribution of metabotropic-mediated store release suggests that the neuromodulatory role of metabotropic receptor stimulation in BLA-dependent learning may result from enhanced nuclear signalling.

Calcium-triggered exit of F-actin and IP(3) 3-kinase A from dendritic spines is rapid and reversible

The structure of the actin cytoskeleton in dendritic spines is thought to underlie some forms of synaptic plasticity. We have used fixed and live-cell imaging in rat primary hippocampal cultures to characterize the synaptic dynamics of the F-actin binding protein inositol trisphosphate 3-kinase A (IP3K), which is localized in the spines of pyramidal neurons derived from the CA1 region. IP3K was intensely concentrated as puncta in spine heads when Ca(2+) influx was low, but rapidly and reversibly redistributed to a striated morphology in the main dendrite when Ca(2+) influx was high. Glutamate stimulated the exit of IP3K from spines within 10 s, and re-entry following blockage of Ca(2+) influx commenced within a minute; IP3K appeared to remain associated with F-actin throughout this process. Ca(2+)-triggered F-actin relocalization occurred in about 90% of the cells expressing IP3K endogenously, and was modulated by the synaptic activity of the cultures, suggesting that it is a physiological process. F-actin relocalization was blocked by cytochalasins, jasplakinolide and by the over-expression of actin fused to green fluorescent protein. We also used deconvolution microscopy to visualize the relationship between F-actin and endoplasmic reticulum inside dendritic spines, revealing a delicate microorganization of IP3K near the Ca(2+) stores. We conclude that Ca(2+) influx into the spines of CA1 pyramidal neurons triggers the rapid and reversible retraction of F-actin from the dendritic spine head. This process contributes to changes in spine F-actin shape and content during synaptic activity, and might also regulate spine IP3 signals.