Raising the speed limit--fast Ca(2+) handling in dendritic spines

Autonomous CaMKII mediates both LTP and LTD using a mechanism for differential substrate site selection

Traditionally, hippocampal long-term potentiation (LTP) of synaptic strength requires Ca(2+)/calmodulin (CaM)-dependent protein kinase II (CaMKII) and other kinases, whereas long-term depression (LTD) requires phosphatases. Here, we found that LTD also requires CaMKII and its phospho-T286-induced "autonomous" (Ca(2+)-independent) activity. However, whereas LTP is known to induce phosphorylation of the AMPA-type glutamate receptor (AMPAR) subunit GluA1 at S831, LTD instead induced CaMKII-mediated phosphorylation at S567, a site known to reduce synaptic GluA1 localization. GluA1 S831 phosphorylation by "autonomous" CaMKII was further stimulated by Ca(2+)/CaM, as expected for traditional substrates. By contrast, GluA1 S567 represents a distinct substrate class that is unaffected by such stimulation. This differential regulation caused GluA1 S831 to be favored by LTP-type stimuli (strong but brief), whereas GluA1 S567 was favored by LTD-type stimuli (weak but prolonged). Thus, requirement of autonomous CaMKII in opposing forms of plasticity involves distinct substrate classes that are differentially regulated to enable stimulus-dependent substrate-site preference.

A mathematical model for astrocytes mediated LTP at single hippocampal synapses

Many contemporary studies have shown that astrocytes play a significant role in modulating both short and long form of synaptic plasticity. There are very few experimental models which elucidate the role of astrocyte over Long-term Potentiation (LTP). Recently, Perea and Araque (Science 317:1083-1086, 2007) demonstrated a role of astrocytes in induction of LTP at single hippocampal synapses. They suggested a purely pre-synaptic basis for induction of this N-methyl-D-Aspartate (NMDA) Receptor-independent LTP. Also, the mechanisms underlying this pre-synaptic induction were not investigated. Here, in this article, we propose a mathematical model for astrocyte modulated LTP which successfully imitates the experimental findings of Perea and Araque (Science 317:1083-1086, 2007). Our study suggests the role of retrograde messengers, possibly Nitric Oxide (NO), for this pre-synaptically modulated LTP.

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.

Experimental and computational aspects of signaling mechanisms of spike-timing-dependent plasticity

STDP (spike-timing-dependent synaptic plasticity) is thought to be a synaptic learning rule that embeds spike-timing information into a specific pattern of synaptic strengths in neuronal circuits, resulting in a memory. STDP consists of bidirectional long-term changes in synaptic strengths. This process includes long-term potentiation and long-term depression, which are dependent on the timing of presynaptic and postsynaptic spikings. In this review, we focus on computational aspects of signaling mechanisms that induce and maintain STDP as a key step toward the definition of a general synaptic learning rule. In addition, we discuss the temporal and spatial aspects of STDP, and the requirement of a homeostatic mechanism of STDP in vivo.

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.

Distributed Network Actions by Nicotine Increase the Threshold for Spike-Timing-Dependent Plasticity in Prefrontal Cortex

Nicotine enhances attention and working memory by activating nicotinic acetylcholine receptors (nAChRs). The prefrontal cortex (PFC) is critical for these cognitive functions and is also rich in nAChR expression. Specific cellular and synaptic mechanisms underlying nicotine's effects on cognition remain elusive. Here we show that nicotine exposure increases the threshold for synaptic spike-timing-dependent potentiation (STDP) in layer V pyramidal neurons of the mouse PFC. During coincident presynaptic and postsynaptic activity, nicotine reduces dendritic calcium signals associated with action potential propagation by enhancing GABAergic transmission. This results from a series of presynaptic actions involving different PFC interneurons and multiple nAChR subtypes. Pharmacological block of nAChRs or GABA(A) receptors prevented nicotine's actions and restored STDP, as did increasing dendritic calcium signals with stronger postsynaptic activity. Thus, by activating nAChRs distributed throughout the PFC neuronal network, nicotine affects PFC information processing and storage by increasing the amount of postsynaptic activity necessary to induce STDP.

Local signaling with molecular diffusion as a decoder of Ca2+ signals in synaptic plasticity

Synaptic plasticity is induced by the influx of calcium ions (Ca²⁺) through N-methyl-D-aspartate receptors (NMDARs), and the direction and strength of the response depend on the frequency of the synaptic inputs. Recent studies have shown that the direction of synaptic plasticity is also governed by two distinct NMDAR subtypes (NR1/NR2A, NR1/NR2B). How are the different types of regulation (frequency-dependent and receptor-specific) processed simultaneously? To clarify the molecular basis of this dual dependence of synaptic plasticity, we have developed a mathematical model of spatial Ca²⁺ signaling in a dendritic spine. Our simulations revealed that calmodulin (CaM) activation in the vicinity of NMDARs is strongly affected by the diffusion coefficient of CaM itself, and that this ‘local CaM diffusion system' works as a dual decoder of both the frequency of Ca²⁺ influxes and their postsynaptic current shapes, generated by two NMDAR subtypes, implying that spatial factors may underlie the complicated regulation scheme of synaptic plasticity.

Dendritic spine geometry: functional implication and regulation

Dendritic spines are tiny protrusions on dendritic shafts where most excitatory synapses are located. Recent advances in imaging technologies have given us great insight into the function of spines as biochemical compartments. Here we review recent evidence suggesting that the geometry of dendritic spines controls postsynaptic calcium signaling and is bidirectionally regulated during synaptic plasticity.

Spine-neck geometry determines NMDA receptor-dependent Ca2+ signaling in dendrites

Increases in cytosolic Ca2+ concentration ([Ca2+]i) mediated by NMDA-sensitive glutamate receptors (NMDARs) are important for synaptic plasticity. We studied a wide variety of dendritic spines on rat CA1 pyramidal neurons in acute hippocampal slices. Two-photon uncaging and Ca2+ imaging revealed that NMDAR-mediated currents increased with spine-head volume and that even the smallest spines contained a significant number of NMDARs. The fate of Ca2+ that entered spine heads through NMDARs was governed by the shape (length and radius) of the spine neck. Larger spines had necks that permitted greater efflux of Ca2+ into the dendritic shaft, whereas smaller spines manifested a larger increase in [Ca2+]i within the spine compartment as a result of a smaller Ca2+ flux through the neck. Spine-neck geometry is thus an important determinant of spine Ca2+ signaling, allowing small spines to be the preferential sites for isolated induction of long-term potentiation.

The kinetic profile of intracellular calcium predicts long-term potentiation and long-term depression

Efficiency of synaptic transmission within the neocortex is regulated throughout life by experience and activity. Periods of correlated or uncorrelated presynaptic and postsynaptic activity lead to enduring changes in synaptic efficiency [long-term potentiation (LTP) and long-term depression (LTD), respectively]. The initial plasticity triggering event is thought to be a precipitous rise in postsynaptic intracellular calcium, with higher levels inducing LTP and more moderate levels inducing LTD. We used a pairing protocol in visual cortical brain slices from young guinea pigs with whole-cell recording and calcium imaging to compare the kinetic profiles of calcium signals generated in response to individual pairings along with the cumulative calcium wave and plasticity outcome. The identical pairing protocol applied to layer 2/3 pyramidal neurons results in different plasticity outcomes between cells. These differences are not attributable to variations in the conditioning protocol, cellular properties, inter-animal variability, animal age, differences in spike timing between the synaptic response and spikes, washout of plasticity factors, recruitment of inhibition, or activation of different afferents. The different plasticity outcomes are reliably predicted by individual intracellular calcium transients in the dendrites after the first few pairings. In addition to the differences in the individual calcium transients, the cumulative calcium wave that spreads to the soma also has a different profile for cells that undergo LTP versus LTD. We conclude that there are biological differences between like-type cells in the dendritic calcium signals generated by coincident synaptic input and spiking that determine the sign of the plasticity response after brief associations.

Electrophysiological classes of neocortical neurons

Neocortical network behavior and neocortical function emerge from synaptic interactions among neurons with specific electrophysiological and morphological characteristics. The intrinsic electrophysiological properties of neurons define their firing patterns and their input-output functions with critical consequences for their functional properties within the network. Understanding the role played by the active non-linear properties caused by ionic conductances distributed in the soma and the dendrites is a critical step towards understanding cortical function. Here I present a brief description of electrophysiological and morphological characteristics of neocortical cells that allow their classification in categories. I review some examples of differences in functional properties among different electrophysiological cell classes in the visual cortex, as well as the role played by specific ionic conductances in defining firing and accommodation properties of neocortical neurons.

Long-lasting alterations in neuronal calcium homeostasis in an in vitro model of stroke-induced epilepsy

Altered calcium homeostatic mechanisms have been implicated in the development of acquired epilepsy in numerous models. Stroke is one of the leading brain injuries that cause acquired epilepsy, yet little is known concerning the molecular mechanisms underlying stroke-induced epileptogenesis. Recently an in vitro model of stroke-induced epilepsy was developed and characterized as a powerful tool to study the pathophysiology of injury and stroke-induced epileptogenesis. Using this glutamate injury-induced epileptogenesis model, we have investigated the role of altered calcium homeostatic mechanisms in the development and maintenance of stroke-induced epilepsy. Epileptic neurons manifested elevated intracellular calcium levels compared to control neurons independent of neuronal activity and seizure discharge for the remainder of the life of the neurons in culture. In addition, epileptic neurons were found to have alterations in the ability to reduce intracellular calcium levels following a calcium load. These long-term epileptic changes in calcium homeostasis were dependent on calcium during the initial glutamate injury. The data demonstrate that significant alterations in calcium homeostatic mechanisms occur in association with stroke-induced epilepsy and suggest that these changes may play a role in both the induction and maintenance of the epileptic phenotype in this model.

Can the human brain do quantum computing?

The electrical membrane properties have been the key issues in the understanding of the cerebral physiology for more than almost two centuries. But, molecular neurobiology has now discovered that biochemical transactions play an important role in neuronal computations. Quantum computing (QC) is becoming a reality both from the theoretical point of view as well as from practical applications. Quantum mechanics is the most accurate description at atomic level and it lies behind all chemistry that provides the basis for biology ... maybe the magic of entanglement is also crucial for life. The purpose of the present paper is to discuss the dendrite spine as a quantum computing device, taking into account what is known about the physiology of the glutamate receptors and the cascade of biochemical transactions triggered by the glutamate binding to these receptors.