Calcium: amplitude, duration, or location?

Competitive processes shape multi-synapse plasticity along dendritic segments

Neurons receive thousands of inputs onto their dendritic arbour, where individual synapses undergo activity-dependent plasticity. Long-lasting changes in postsynaptic strengths correlate with changes in spine head volume. The magnitude and direction of such structural plasticity - potentiation (sLTP) and depression (sLTD) - depend upon the number and spatial distribution of stimulated synapses. However, how neurons allocate resources to implement synaptic strength changes across space and time amongst neighbouring synapses remains unclear. Here we combined experimental and modelling approaches to explore the elementary processes underlying multi-spine plasticity. We used glutamate uncaging to induce sLTP at varying number of synapses sharing the same dendritic branch, and we built a model incorporating a dual role Ca2+-dependent component that induces spine growth or shrinkage. Our results suggest that competition among spines for molecular resources is a key driver of multi-spine plasticity and that spatial distance between simultaneously stimulated spines impacts the resulting spine dynamics.

Synaptic Plasticity in the Injured Brain Depends on the Temporal Pattern of Stimulation

Neurostimulation protocols are increasingly used as therapeutic interventions, including for brain injury. In addition to the direct activation of neurons, these stimulation protocols are also likely to have downstream effects on those neurons' synaptic outputs. It is well known that alterations in the strength of synaptic connections (long-term potentiation, LTP; long-term depression, LTD) are sensitive to the frequency of stimulation used for induction; however, little is known about the contribution of the temporal pattern of stimulation to the downstream synaptic plasticity that may be induced by neurostimulation in the injured brain. We explored interactions of the temporal pattern and frequency of neurostimulation in the normal cerebral cortex and after mild traumatic brain injury (mTBI), to inform therapies to strengthen or weaken neural circuits in injured brains, as well as to better understand the role of these factors in normal brain plasticity. Whole-cell (WC) patch-clamp recordings of evoked postsynaptic potentials in individual neurons, as well as field potential (FP) recordings, were made from layer 2/3 of visual cortex in response to stimulation of layer 4, in acute slices from control (naive), sham operated, and mTBI rats. We compared synaptic plasticity induced by different stimulation protocols, each consisting of a specific frequency (1 Hz, 10 Hz, or 100 Hz), continuity (continuous or discontinuous), and temporal pattern (perfectly regular, slightly irregular, or highly irregular). At the individual neuron level, dramatic differences in plasticity outcome occurred when the highly irregular stimulation protocol was used at 1 Hz or 10 Hz, producing an overall LTD in controls and shams, but a robust overall LTP after mTBI. Consistent with the individual neuron results, the plasticity outcomes for simultaneous FP recordings were similar, indicative of our results generalizing to a larger scale synaptic network than can be sampled by individual WC recordings alone. In addition to the differences in plasticity outcome between control (naive or sham) and injured brains, the dynamics of the changes in synaptic responses that developed during stimulation were predictive of the final plasticity outcome. Our results demonstrate that the temporal pattern of stimulation plays a role in the polarity and magnitude of synaptic plasticity induced in the cerebral cortex while highlighting differences between normal and injured brain responses. Moreover, these results may be useful for optimization of neurostimulation therapies to treat mTBI and other brain disorders, in addition to providing new insights into downstream plasticity signaling mechanisms in the normal brain.

Nanofluidic logic with mechano-ionic memristive switches

Neuromorphic systems are typically based on nanoscale electronic devices, but nature relies on ions for energy-efficient information processing. Nanofluidic memristive devices could thus potentially be used to construct electrolytic computers that mimic the brain down to its basic principles of operation. Here we report a nanofluidic device that is designed for circuit-scale in-memory processing. The device, which is fabricated using a scalable process, combines single-digit nanometric confinement and large entrance asymmetry and operates on the second timescale with a conductance ratio in the range of 9 to 60. In operando optical microscopy shows that the memory capabilities are due to the reversible formation of liquid blisters that modulate the conductance of the device. We use these mechano-ionic memristive switches to assemble logic circuits composed of two interactive devices and an ohmic resistor.

Altered neuronal group 1 metabotropic glutamate receptor- and endoplasmic reticulum-mediated Ca2+ signaling in two rodent models of Alzheimer's disease

Calcium mobilization from the endoplasmic reticulum (ER) induced by, for example, IP3 receptor (IP3R) stimulation, and its subsequent crosstalk with extracellular Ca2+ influx mediated through voltage-gated calcium channels (VGCCs) and neuronal store-operated calcium entry (nSOCE), is essential for normal neuronal signaling and cellular homeostasis. However, several studies suggest that chronic calcium dysregulation may play a key role in the onset and/or progression of neurodegenerative conditions, particularly Alzheimer's disease (AD). Here, using early postnatal hippocampal tissue from two transgenic murine models of AD, we provide further evidence that not only are crucial calcium signaling pathways dysregulated, but also that such dysregulation occurs at very early stages of development. Utilizing epifluorescence calcium imaging, we investigated ER-, nSOCE- and VGCC-mediated calcium signaling in cultured primary hippocampal neurons from two transgenic rodent models of AD: 3xTg-AD mice (PS1M146V/APPSWE/TauP301L) and TgF344-AD rats (APPSWE/PS1ΔE9) between 2 and 9 days old. Our results reveal that, in comparison to control hippocampal neurons, those from 3xTg-AD mice possessed significantly greater basal ER calcium levels, as measured by larger responses to I-mGluR-mediated ER Ca2+ mobilization (amplitude; 4 (0-19) vs 21(12-36) a.u., non-Tg vs 3xTg-AD; median difference (95 % Cl) = 14 a.u. (11-18); p = 0.004)) but reduced nSOCE (15 (4-22) vs 8(5-11) a.u., non-Tg vs 3xTg-AD; median difference (95 % Cl) = -7 a.u. (-3- -10 a.u.); p < 0.0001). Furthermore, unlike non-Tg neurons, where depolarization enhanced the amplitude, duration and area under the curve (A.U.C.) of I-mGluR-evoked ER-mediated calcium signals when compared with basal conditions, this was not apparent in 3xTg-AD neurons. Whilst the amplitude of depolarization-enhanced I-mGluR-evoked ER-mediated calcium signals from both non-Tg F344 and TgF344-AD neurons was significantly enhanced relative to basal conditions, the A.U.C. and duration of responses were enhanced significantly upon depolarization in non-Tg F344, but not in TgF344-AD, neurons. Overall, the nature of basal I-mGluR-mediated calcium responses did not differ significantly between non-Tg F344 and TgF344-AD neurons. In summary, our results characterizing ER- and nSOCE-mediated calcium signaling in neurons demonstrate that ER Ca2+ dyshomeostasis is an early and potentially pathogenic event in familial AD.

Heterosynaptic plasticity-induced modulation of synapses

Plasticity is a common feature of synapses that is stated in different ways and occurs through several mechanisms. The regular action of the brain needs to be balanced in several neuronal and synaptic features, one of which is synaptic plasticity. The different homeostatic processes, including the balance between excitation/inhibition or homeostasis of synaptic weights at the single-neuron level, may obtain this. Homosynaptic Hebbian-type plasticity causes associative alterations of synapses. Both homosynaptic and heterosynaptic plasticity characterize the corresponding aspects of adjustable synapses, and both are essential for the regular action of neural systems and their plastic synapses.In this review, we will compare homo- and heterosynaptic plasticity and the main factors affecting the direction of plastic changes. This review paper will also discuss the diverse functions of the different kinds of heterosynaptic plasticity and their properties. We argue that a complementary system of heterosynaptic plasticity demonstrates an essential cellular constituent for homeostatic modulation of synaptic weights and neuronal activity.

Making time and space for calcium control of neuron activity

Calcium directly controls or indirectly regulates numerous functions that are critical for neuronal network activity. Intracellular calcium concentration is tightly regulated by numerous molecular mechanisms because spatial domains and temporal dynamics (not just peak amplitude) are critical for calcium control of synaptic plasticity and ion channel activation, which in turn determine neuron spiking activity. The computational models investigating calcium control are valuable because experiments achieving high spatial and temporal resolution simultaneously are technically unfeasible. Simulations of calcium nanodomains reveal that specific calcium sources can couple to specific calcium targets, providing a mechanism to determine the direction of synaptic plasticity. Cooperativity of calcium domains opposes specificity, suggesting that the dendritic branch might be the preferred computational unit of the neuron.

AAV-compatible optogenetic tools for activating endogenous calcium channels in vivo

Calcium ions (Ca2+) play pivotal roles in regulating diverse brain functions, including cognition, emotion, locomotion, and learning and memory. These functions are intricately regulated by a variety of Ca2+-dependent cellular processes, encompassing synaptic plasticity, neuro/gliotransmitter release, and gene expression. In our previous work, we developed 'monster OptoSTIM1' (monSTIM1), an improved OptoSTIM1 that selectively activates Ca2+-release-activated Ca2+ (CRAC) channels in the plasma membrane through blue light, allowing precise control over intracellular Ca2+ signaling and specific brain functions. However, the large size of the coding sequence of monSTIM1 poses a limitation for its widespread use, as it exceeds the packaging capacity of adeno-associated virus (AAV). To address this constraint, we have introduced monSTIM1 variants with reduced coding sequence sizes and established AAV-based systems for expressing them in neurons and glial cells in the mouse brain. Upon expression by AAVs, these monSTIM1 variants significantly increased the expression levels of cFos in neurons and astrocytes in the hippocampal CA1 region following non-invasive light illumination. The use of monSTIM1 variants offers a promising avenue for investigating the spatiotemporal roles of Ca2+-mediated cellular activities in various brain functions. Furthermore, this toolkit holds potential as a therapeutic strategy for addressing brain disorders associated with aberrant Ca2+ signaling.

Asymmetric Voltage Attenuation in Dendrites Can Enable Hierarchical Heterosynaptic Plasticity

Long-term synaptic plasticity is mediated via cytosolic calcium concentrations ([Ca2+]). Using a synaptic model that implements calcium-based long-term plasticity via two sources of Ca2+ - NMDA receptors and voltage-gated calcium channels (VGCCs) - we show in dendritic cable simulations that the interplay between these two calcium sources can result in a diverse array of heterosynaptic effects. When spatially clustered synaptic input produces a local NMDA spike, the resulting dendritic depolarization can activate VGCCs at nonactivated spines, resulting in heterosynaptic plasticity. NMDA spike activation at a given dendritic location will tend to depolarize dendritic regions that are located distally to the input site more than dendritic sites that are proximal to it. This asymmetry can produce a hierarchical effect in branching dendrites, where an NMDA spike at a proximal branch can induce heterosynaptic plasticity primarily at branches that are distal to it. We also explored how simultaneously activated synaptic clusters located at different dendritic locations synergistically affect the plasticity at the active synapses, as well as the heterosynaptic plasticity of an inactive synapse "sandwiched" between them. We conclude that the inherent electrical asymmetry of dendritic trees enables sophisticated schemes for spatially targeted supervision of heterosynaptic plasticity.

Pharmacological Potential of 3-Benzazepines in NMDAR-Linked Pathophysiological Processes

The number of N-Methyl-D-aspartate receptor (NMDAR) linked neurodegenerative diseases such as Alzheimer's disease and dementia is constantly increasing. This is partly due to demographic change and presents new challenges to societies. To date, there are no effective treatment options. Current medications are nonselective and can lead to unwanted side effects in patients. A promising therapeutic approach is the targeted inhibition of NMDARs in the brain. NMDARs containing different subunits and splice variants display different physiological properties and play a crucial role in learning and memory, as well as in inflammatory or injury processes. They become overactivated during the course of the disease, leading to nerve cell death. Until now, there has been a lack of understanding of the general functions of the receptor and the mechanism of inhibition, which need to be understood in order to develop inhibitors. Ideal compounds should be highly targeted and even splice-variant-selective. However, a potent and splice-variant-selective NMDAR-targeting drug has yet to be developed. Recently developed 3-benzazepines are promising inhibitors for further drug development. The NMDAR splice variants GluN1-1b-4b carry a 21-amino-acid-long, flexible exon 5. Exon 5 lowers the NMDAR's sensitivity to allosteric modulators by probably acting as an NMDAR modulator itself. The role of exon 5 in NMDAR modulation is still poorly understood. In this review, we summarize the structure and pharmacological relevance of tetrahydro-3-benzazepines.

Growing dendrites enhance a neuron's computational power and memory capacity

Neocortical pyramidal neurons have many dendrites, and such dendrites are capable of, in isolation of one-another, generating a neuronal spike. It is also now understood that there is a large amount of dendritic growth during the first years of a humans life, arguably a period of prodigious learning. These observations inspire the construction of a local, stochastic algorithm based on an earlier stochastic, homeostatic, Hebbian developmental theory. Here we investigate the neurocomputational advantages and limits on this novel algorithm that combines dendritogenesis with supervised adaptive synaptogenesis. Neurons created with this algorithm have enhanced memory capacity, can avoid catastrophic interference (forgetting), and have the ability to unmix mixture distributions. In particular, individual dendrites develop within each class, in an unsupervised manner, to become feature-clusters that correspond to the mixing elements of class-conditional mixture distribution. Error-free classification is demonstrated with input perturbations up to 40%. Although discriminative problems are used to understand the capabilities of the stochastic algorithm and the neuronal connectivity it produces, the algorithm is in the generative class, it thus seems ideal for decisions that require generalization, i.e., extrapolation beyond previous learning.

The Therapeutic Potential of Non-Invasive and Invasive Cerebellar Stimulation Techniques in Hereditary Ataxias

The degenerative ataxias comprise a heterogeneous group of inherited and acquired disorders that are characterized by a progressive cerebellar syndrome, frequently in combination with one or more extracerebellar signs. Specific disease-modifying interventions are currently not available for many of these rare conditions, which underscores the necessity of finding effective symptomatic therapies. During the past five to ten years, an increasing number of randomized controlled trials have been conducted examining the potential of different non-invasive brain stimulation techniques to induce symptomatic improvement. In addition, a few smaller studies have explored deep brain stimulation (DBS) of the dentate nucleus as an invasive means to directly modulate cerebellar output, thereby aiming to alleviate ataxia severity. In this paper, we comprehensively review the clinical and neurophysiological effects of transcranial direct current stimulation (tDCS), repetitive transcranial magnetic stimulation (rTMS), and dentate nucleus DBS in patients with hereditary ataxias, as well as the presumed underlying mechanisms at the cellular and network level and perspectives for future research.

Target cell-specific plasticity rules of NMDA receptor-mediated synaptic transmission in the hippocampus

Long-term potentiation and depression of NMDA receptor-mediated synaptic transmission (NMDAR LTP/LTD) can significantly impact synapse function and information transfer in several brain areas. However, the mechanisms that determine the direction of NMDAR plasticity are poorly understood. Here, using physiologically relevant patterns of presynaptic and postsynaptic burst activities, whole-cell patch clamp recordings, 2-photon laser calcium imaging in acute rat hippocampal slices and immunoelectron microscopy, we tested whether distinct calcium dynamics and group I metabotropic glutamate receptor (I-mGluR) subtypes control the sign of NMDAR plasticity. We found that postsynaptic calcium transients (CaTs) in response to hippocampal MF stimulation were significantly larger during the induction of NMDAR-LTP compared to NMDAR-LTD at the MF-to-CA3 pyramidal cell (MF-CA3) synapse. This difference was abolished by pharmacological blockade of mGluR5 and was significantly reduced by depletion of intracellular calcium stores, whereas blocking mGluR1 had no effect on these CaTs. In addition, we discovered that MF to hilar mossy cell (MF-MC) synapses, which share several structural and functional commonalities with MF-CA3 synapses, also undergoes NMDAR plasticity. To our surprise, however, we found that the postsynaptic distribution of I-mGluR subtypes at these two synapses differ, and the same induction protocol that induces NMDAR-LTD at MF-CA3 synapses, only triggered NMDAR-LTP at MF-MC synapses, despite a comparable calcium dynamics. Thus, postsynaptic calcium dynamics alone cannot predict the sign of NMDAR plasticity, indicating that both postsynaptic calcium rise and the relative contribution of I-mGluR subtypes likely determine the learning rules of NMDAR plasticity.

Paired corticomotoneuronal stimulation of the preactivated ankle dorsiflexor: an open-label study of magnetic and electrical painless protocols

Paired corticomotoneuronal stimulation (or electrical PCMS: ePCMS) is the repetitive pairing of an electrical stimulus to a nerve with a transcranial magnetic stimulation of the primary motor cortex (TMS-of-M1) to noninvasively influence spinal plasticity. We compared ePCMS with the new painless PCMS protocol pairing a magnetic stimulus to the nerve with TMS-of-M1 (mPCMS) in the preactivated tibial anterior muscle (TA). Sixteen healthy adults participated in two sessions (mPCMS, ePCMS), each with 180 pairs of [low-intensity TMS-of-M1 + nerve stimulation] at 0.2 Hz. TA motor-evoked potentials (MEP) to single-pulse TMS at pre-PCMS, immediately and 30 min after PCMS, were cluster-analyzed to discriminate responders and non-responders. Paired-pulse TMS-of-M1 and F-waves were also tested and BDNF polymorphism influence was explored. Both PCMS protocols significantly increased MEP amplitudes (n = 9 responders each), but the time-course differed with mPCMS inducing larger MEP increase over time. The number of BDNF-methionine carriers tended to be larger than Val66Val in mPCMS and the reverse in ePCMS, thus warranting further investigations. The MEP changes of the preactivated TA likely occurred at the pre-motoneuronal level and larger mPCMS after-effects over time may be related to the afferents recruited. mPCMS seems relevant to be tested in future studies as a painless noninvasive approach to induce sustained pre-motoneuronal plasticity in spinal cord injury.

Adaptive control of synaptic plasticity integrates micro- and macroscopic network function

Synaptic plasticity configures interactions between neurons and is therefore likely to be a primary driver of behavioral learning and development. How this microscopic-macroscopic interaction occurs is poorly understood, as researchers frequently examine models within particular ranges of abstraction and scale. Computational neuroscience and machine learning models offer theoretically powerful analyses of plasticity in neural networks, but results are often siloed and only coarsely linked to biology. In this review, we examine connections between these areas, asking how network computations change as a function of diverse features of plasticity and vice versa. We review how plasticity can be controlled at synapses by calcium dynamics and neuromodulatory signals, the manifestation of these changes in networks, and their impacts in specialized circuits. We conclude that metaplasticity-defined broadly as the adaptive control of plasticity-forges connections across scales by governing what groups of synapses can and can't learn about, when, and to what ends. The metaplasticity we discuss acts by co-opting Hebbian mechanisms, shifting network properties, and routing activity within and across brain systems. Asking how these operations can go awry should also be useful for understanding pathology, which we address in the context of autism, schizophrenia and Parkinson's disease.

Recording plasticity in neuronal activity in the rodent intrinsic cardiac nervous system using calcium imaging techniques

The intrinsic cardiac nervous system (ICNS) is composed of interconnected clusters of neurons called ganglionated plexi (GP) which play a major role in controlling heart rate and rhythm. The function of these neurons is particularly important due to their involvement in cardiac arrhythmias such as atrial fibrillation (AF), and previous work has shown that plasticity in GP neural networks could underpin aberrant activity patterns that drive AF. As research in this field increases, developing new techniques to visualize the complex interactions and plasticity in this GP network is essential. In this study we have developed a calcium imaging method enabling the simultaneous recording of plasticity in neuronal activity from multiple neurons in intact atrial GP networks. Calcium imaging was performed with Cal-520 AM labeling in aged spontaneously hypertensive rats (SHRs), which display both spontaneous and induced AF, and age-matched Wistar Kyoto (WKY) controls to determine the relationship between chronic hypertension, arrhythmia and GP calcium dynamics. Our data show that SHR GPs have significantly larger calcium responses to cholinergic stimulation compared to WKY controls, as determined by both higher amplitude and longer duration calcium responses. Responses were significantly but not fully blocked by hexamethonium, indicating multiple cholinergic receptor subtypes are involved in the calcium response. Given that SHRs are susceptible to cardiac arrhythmias, our data provide evidence for a potential link between arrhythmia and plasticity in calcium dynamics that occur not only in cardiomyocytes but also in the GP neurons of the heart.

Synaptic Plasticity 101: The Story of the AMPA Receptor for the Brain Stimulation Practitioner

The fields of Neurobiology and Neuromodulation have never been closer. Consequently, the phrase "synaptic plasticity" has become very familiar to non-basic scientists, without actually being very familiar. We present the "Story of the AMPA receptor," an easy-to-understand "10,000 ft" narrative overview of synaptic plasticity, oriented toward the brain stimulation clinician or scientist without basic science training. Neuromodulation is unparalleled in its capacity to both modulate and probe plasticity, yet many are not comfortable with their grasp of the topic. Here, we describe the seminal discoveries that defined the canonical mechanisms of long-term potentiation (LTP), long-term depression (LTD), and homeostatic plasticity. We then provide a conceptual framework for how plasticity at the synapse is accomplished, describing the functional roles of N-methyl-d-aspartate (NMDA) receptors and calcium, their effect on calmodulin, phosphatases (ie, calcineurin), kinases (ie, calcium/calmodulin-dependent protein kinase [CaMKII]), and structural "scaffolding" proteins (ie, post-synaptic density protein [PSD-95]). Ultimately, we describe how these affect the α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor. More specifically, AMPA receptor delivery to (LTP induction), removal from (LTD), or recycling within (LTP maintenance) the synapse is determined by the status of phosphorylation and protein binding at specific sites on the tails of AMPA receptor subunits: GluA1 and GluA2. Finally, we relate these to transcranial magnetic stimulation (TMS) treatment, highlighting evidences for LTP as the basis of high-frequency TMS therapy, and briefly touch on the role of plasticity for other brain stimulation modalities. In summary, we present Synaptic Plasticity 101 as a singular introductory reference for those less familiar with the mechanisms of synaptic plasticity.

Synaptic Plasticity Is Predicted by Spatiotemporal Firing Rate Patterns and Robust to In Vivo-like Variability

Synaptic plasticity, the experience-induced change in connections between neurons, underlies learning and memory in the brain. Most of our understanding of synaptic plasticity derives from in vitro experiments with precisely repeated stimulus patterns; however, neurons exhibit significant variability in vivo during repeated experiences. Further, the spatial pattern of synaptic inputs to the dendritic tree influences synaptic plasticity, yet is not considered in most synaptic plasticity rules. Here, we investigate how spatiotemporal synaptic input patterns produce plasticity with in vivo-like conditions using a data-driven computational model with a plasticity rule based on calcium dynamics. Using in vivo spike train recordings as inputs to different size clusters of spines, we show that plasticity is strongly robust to trial-to-trial variability of spike timing. In addition, we derive general synaptic plasticity rules describing how spatiotemporal patterns of synaptic inputs control the magnitude and direction of plasticity. Synapses that strongly potentiated have greater firing rates and calcium concentration later in the trial, whereas strongly depressing synapses have hiring firing rates early in the trial. The neighboring synaptic activity influences the direction and magnitude of synaptic plasticity, with small clusters of spines producing the greatest increase in synaptic strength. Together, our results reveal that calcium dynamics can unify diverse plasticity rules and reveal how spatiotemporal firing rate patterns control synaptic plasticity.

A Modeling and Analysis Study Reveals That CaMKII in Synaptic Plasticity Is a Dominant Affecter in CaM Systems in a T286 Phosphorylation-Dependent Manner

NMDAR-dependent synaptic plasticity in the hippocampus consists of two opposing forces: long-term potentiation (LTP), which strengthens synapses and long-term depression (LTD), which weakens synapses. LTP and LTD are associated with memory formation and loss, respectively. Synaptic plasticity is controlled at a molecular level by Ca2+-mediated protein signaling. Here, Ca2+ binds the protein, calmodulin (CaM), which modulates synaptic plasticity in both directions. This is because Ca2+-bound CaM activates both LTD-and LTP-inducing proteins. Understanding how CaM responds to Ca2+ signaling and how this translates into synaptic plasticity is therefore important to understanding synaptic plasticity induction. In this paper, CaM activation by Ca2+ and calmodulin binding to downstream proteins was mathematically modeled using differential equations. Simulations were monitored with and without theoretical knockouts and, global sensitivity analyses were performed to determine how Ca2+/CaM signaling occurred at various Ca2+ signals when CaM levels were limiting. At elevated stimulations, the total CaM pool rapidly bound to its protein binding targets which regulate both LTP and LTD. This was followed by CaM becoming redistributed from low-affinity to high-affinity binding targets. Specifically, CaM was redistributed away from LTD-inducing proteins to bind the high-affinity LTP-inducing protein, calmodulin-dependent kinase II (CaMKII). In this way, CaMKII acted as a dominant affecter and repressed activation of opposing CaM-binding protein targets. The model thereby showed a novel form of CaM signaling by which the two opposing pathways crosstalk indirectly. The model also found that CaMKII can repress cAMP production by repressing CaM-regulated proteins, which catalyze cAMP production. The model also found that at low Ca2+ stimulation levels, typical of LTD induction, CaM signaling was unstable and is therefore unlikely to alone be enough to induce synaptic depression. Overall, this paper demonstrates how limiting levels of CaM may be a fundamental aspect of Ca2+ regulated signaling which allows crosstalk among proteins without requiring directly interaction.

Periodicity Pitch Perception Part III: Sensibility and Pachinko Volatility

Neuromorphic computer models are used to explain sensory perceptions. Auditory models generate cochleagrams, which resemble the spike distributions in the auditory nerve. Neuron ensembles along the auditory pathway transform sensory inputs step by step and at the end pitch is represented in auditory categorical spaces. In two previous articles in the series on periodicity pitch perception an extended auditory model had been successfully used for explaining periodicity pitch proved for various musical instrument generated tones and sung vowels. In this third part in the series the focus is on octopus cells as they are central sensitivity elements in auditory cognition processes. A powerful numerical model had been devised, in which auditory nerve fibers (ANFs) spike events are the inputs, triggering the impulse responses of the octopus cells. Efficient algorithms are developed and demonstrated to explain the behavior of octopus cells with a focus on a simple event-based hardware implementation of a layer of octopus neurons. The main finding is, that an octopus' cell model in a local receptive field fine-tunes to a specific trajectory by a spike-timing-dependent plasticity (STDP) learning rule with synaptic pre-activation and the dendritic back-propagating signal as post condition. Successful learning explains away the teacher and there is thus no need for a temporally precise control of plasticity that distinguishes between learning and retrieval phases. Pitch learning is cascaded: At first octopus cells respond individually by self-adjustment to specific trajectories in their local receptive fields, then unions of octopus cells are collectively learned for pitch discrimination. Pitch estimation by inter-spike intervals is shown exemplary using two input scenarios: a simple sinus tone and a sung vowel. The model evaluation indicates an improvement in pitch estimation on a fixed time-scale.

Behind the scenes: Are latent memories supported by calcium independent plasticity?

N-methyl-D-aspartate receptors (NMDARs) can be considered to be the de facto "plasticity" receptors in the brain due to their central role in the activity-dependent modification of neuronal morphology and synaptic transmission. Since the 1980s, research on NMDARs has focused on the second messenger properties of calcium and the downstream signaling pathways that mediate alterations in neural form and function. Recently, NMDARs were shown to drive activity-dependent synaptic plasticity without calcium influx. How this "nonionotropic" plasticity occurs in vitro is becoming clearer, but research on its involvement in behavior and cognition is in its infancy. There is a partial overlap in the downstream signaling molecules that are involved in ionotropic and nonionotropic NMDAR-dependent plasticity. Given this, and prior studies of the cognitive impacts of ionotropic NMDAR plasticity, a preliminary model explaining how NMDAR nonionotropic plasticity affects learning and memory can be established. We hypothesize that nonionotropic NMDAR plasticity takes part in latent memory encoding in immature rodents through nonassociative depression of synaptic efficacy, and possibly shrinking of dendritic spines. Further, the late postnatal alteration in NMDAR composition in the hippocampus appears to reduce nonionotropic signaling and remove a restriction on memory retrieval. This framework substantially alters the canonical model of NMDAR involvement in spatial cognition and hippocampal maturation and provides novel and exciting inroads for future studies.

Synaptic protein interaction networks encode experience by assuming stimulus-specific and brain-region-specific states

A core network of widely expressed proteins within the glutamatergic post-synapse mediates activity-dependent synaptic plasticity throughout the brain, but the specific proteomic composition of synapses differs between brain regions. Here, we address the question, how does proteomic composition affect activity-dependent protein-protein interaction networks (PINs) downstream of synaptic activity? Using quantitative multiplex co-immunoprecipitation, we compare the PIN response of in vivo or ex vivo neurons derived from different brain regions to activation by different agonists or different forms of eyeblink conditioning. We report that PINs discriminate between incoming stimuli using differential kinetics of overlapping and non-overlapping PIN parameters. Further, these "molecular logic rules" differ by brain region. We conclude that although the PIN of the glutamatergic post-synapse is expressed widely throughout the brain, its activity-dependent dynamics show remarkable stimulus-specific and brain-region-specific diversity. This diversity may help explain the challenges in developing molecule-specific drug therapies for neurological disorders.

Brain is modulated by neuronal plasticity during postnatal development

Neuroplasticity is referred to the ability of the nervous system to change its structure or functions as a result of former stimuli. It is a plausible mechanism underlying a dynamic brain through adaptation processes of neural structure and activity patterns. Nevertheless, it is still unclear how the plastic neural systems achieve and maintain their equilibrium. Additionally, the alterations of balanced brain dynamics under different plasticity rules have not been explored either. Therefore, the present article primarily aims to review recent research studies regarding homosynaptic and heterosynaptic neuroplasticity characterized by the manipulation of excitatory and inhibitory synaptic inputs. Moreover, it attempts to understand different mechanisms related to the main forms of synaptic plasticity at the excitatory and inhibitory synapses during the brain development processes. Hence, this study comprised surveying those articles published since 1988 and available through PubMed, Google Scholar and science direct databases on a keyword-based search paradigm. All in all, the study results presented extensive and corroborative pieces of evidence for the main types of plasticity, including the long-term potentiation (LTP) and long-term depression (LTD) of the excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs).

Super-resolution microscopy: a closer look at synaptic dysfunction in Alzheimer disease

The synapse has emerged as a critical neuronal structure in the degenerative process of Alzheimer disease (AD), in which the pathogenic signals of two key players - amyloid-β (Aβ) and tau - converge, thereby causing synaptic dysfunction and cognitive deficits. The synapse presents a dynamic, confined microenvironment in which to explore how key molecules travel, localize, interact and assume different levels of organizational complexity, thereby affecting neuronal function. However, owing to their small size and the diffraction-limited resolution of conventional light microscopic approaches, investigating synaptic structure and dynamics has been challenging. Super-resolution microscopy (SRM) techniques have overcome the resolution barrier and are revolutionizing our quantitative understanding of biological systems in unprecedented spatio-temporal detail. Here we review critical new insights provided by SRM into the molecular architecture and dynamic organization of the synapse and, in particular, the interactions between Aβ and tau in this compartment. We further highlight how SRM can transform our understanding of the molecular pathological mechanisms that underlie AD. The application of SRM for understanding the roles of synapses in AD pathology will provide a stepping stone towards a broader understanding of dysfunction in other subcellular compartments and at cellular and circuit levels in this disease.

Glucocorticoid modulation of synaptic plasticity in the human temporal cortex of epilepsy patients: Does chronic stress contribute to memory impairment?

OBJECTIVE

Memory impairment is common in patients with temporal lobe epilepsy and seriously affects life quality. Chronic stress is a recognized cofactor in epilepsy and can also impair memory function. Furthermore, increased cortisol levels have been reported in epilepsy patients. Animal models have suggested that aggravating effects of stress on memory and synaptic plasticity were mediated via glucocorticoids. The aim of this study was, therefore, to investigate the effect of glucocorticoid receptor (GR) modulation on synaptic plasticity in the human cortex of epilepsy patients.

METHODS

We performed field potential recordings in acute slices from the temporal neocortex of patients who underwent surgery for drug-resistant temporal lobe epilepsy. Synaptic plasticity was investigated by a theta-burst stimulation (TBS) protocol for induction of long-term potentiation (LTP) in the presence of GR modulators.

RESULTS

LTP was impaired in temporal cortex from epilepsy patients. Pretreatment of the slices with the GR antagonist mifepristone (RU486) improved LTP induction, suggesting that LTP impairment was due to baseline GR activation in the human cortex. The highly potent GR agonist dexamethasone additionally weakened synaptic strength in an activity-dependent manner when applied after TBS.

SIGNIFICANCE

Our results show a direct negative glucocorticoid effect on synaptic potentiation in the human cortex and imply chronic activation of GRs. Chronic stress may therefore contribute to memory impairment in patients with temporal lobe epilepsy. Furthermore, the activity-dependent acute inhibitory effect of dexamethasone suggests a mechanism of synaptic downscaling by which postictally increased cortisol levels may prevent pathologic plasticity upon seizures.

Mechanisms of endocannabinoid control of synaptic plasticity

The endogenous cannabinoid transmitter system regulates synaptic transmission throughout the nervous system. Unlike conventional transmitters, specific stimuli induce synthesis of endocannabinoids (eCBs) in the postsynaptic neuron, and these travel backwards to modulate presynaptic inputs. In doing so, eCBs can induce short-term changes in synaptic strength and longer-term plasticity. While this eCB regulation is near ubiquitous, it displays major regional and synapse specific variations with different synapse specific forms of short-versus long-term plasticity throughout the brain. These differences are due to the plethora of pre- and postsynaptic mechanisms which have been implicated in eCB signalling, the intricacies of which are only just being realised. In this review, we shall describe the current understanding and highlight new advances in this area, with a focus on the retrograde action of eCBs at CB1 receptors (CB₁Rs).

Pitfalls of NMDA Receptor Modulation by Neuroactive Steroids. The Effect of Positive and Negative Modulation of NMDA Receptors in an Animal Model of Schizophrenia

Evidence from clinical and preclinical studies implicates dysfunction of N-methyl-D-aspartate receptors (NMDARs) in schizophrenia progression and symptoms. We investigated the antipsychotic effect of two neuroactive steroids in an animal model of schizophrenia induced by systemic application of MK-801. The neuroactive steroids differ in their mechanism of action at NMDARs. MS-249 is positive, while PA-Glu is a negative allosteric NMDAR modulator. We hypothesized that the positive NMDA receptor modulator would attenuate deficits caused by MK-801 co-application more effectively than PA-Glu. The rats were tested in a battery of tests assessing spontaneous locomotion, anxiety and cognition. Contrary to our expectations, PA-Glu exhibited a superior antipsychotic effect to MS-249. The performance of MS-249-treated rats in cognitive tests differed depending on the level of stress the rats were exposed to during test sessions. In particular, with the increasing severity of stress exposure, the performance of animals worsened. Our results demonstrate that enhancement of NMDAR function may result in unspecific behavioral responses. Positive NMDAR modulation can influence other neurobiological processes besides memory formation, such as anxiety and response to stress.

The role of Bcl-2 proteins in modulating neuronal Ca2+ signaling in health and in Alzheimer's disease

The family of B-cell lymphoma-2 (Bcl-2) proteins exerts key functions in cellular health. Bcl-2 primarily acts in mitochondria where it controls the initiation of apoptosis. However, during the last decades, it has become clear that this family of proteins is also involved in controlling intracellular Ca2+ signaling, a critical process for the function of most cell types, including neurons. Several anti- and pro-apoptotic Bcl-2 family members are expressed in neurons and impact neuronal function. Importantly, expression levels of neuronal Bcl-2 proteins are affected by age. In this review, we focus on the emerging roles of Bcl-2 proteins in neuronal cells. Specifically, we discuss how their dysregulation contributes to the onset, development, and progression of neurodegeneration in the context of Alzheimer's disease (AD). Aberrant Ca2+ signaling plays an important role in the pathogenesis of AD, and we propose that dysregulation of the Bcl-2-Ca2+ signaling axis may contribute to the progression of AD and that herein, Bcl-2 may constitute a potential therapeutic target for the treatment of AD.

Stochastic reaction-diffusion modeling of calcium dynamics in 3D dendritic spines of Purkinje cells

Calcium (Ca²⁺) is a second messenger assumed to control changes in synaptic strength in the form of both long-term depression and long-term potentiation at Purkinje cell dendritic spine synapses via inositol trisphosphate (IP₃)-induced Ca²⁺ release. These Ca²⁺ transients happen in response to stimuli from parallel fibers (PFs) from granule cells and climbing fibers (CFs) from the inferior olivary nucleus. These events occur at low numbers of free Ca²⁺, requiring stochastic single-particle methods when modeling them. We use the stochastic particle simulation program MCell to simulate Ca²⁺ transients within a three-dimensional Purkinje cell dendritic spine. The model spine includes the endoplasmic reticulum, several Ca²⁺ transporters, and endogenous buffer molecules. Our simulations successfully reproduce properties of Ca²⁺ transients in different dynamical situations. We test two different models of the IP₃ receptor (IP₃R). The model with nonlinear concentration response of binding of activating Ca²⁺ reproduces experimental results better than the model with linear response because of the filtering of noise. Our results also suggest that Ca²⁺-dependent inhibition of the IP₃R needs to be slow to reproduce experimental results. Simulations suggest the experimentally observed optimal timing window of CF stimuli arises from the relative timing of CF influx of Ca²⁺ and IP₃ production sensitizing IP₃R for Ca²⁺-induced Ca²⁺ release. We also model ataxia, a loss of fine motor control assumed to be the result of malfunctioning information transmission at the granule to Purkinje cell synapse, resulting in a decrease or loss of Ca²⁺ transients. Finally, we propose possible ways of recovering Ca²⁺ transients under ataxia.

Unifying Long-Term Plasticity Rules for Excitatory Synapses by Modeling Dendrites of Cortical Pyramidal Neurons

A large number of experiments have indicated that precise spike times, firing rates, and synapse locations crucially determine the dynamics of long-term plasticity induction in excitatory synapses. However, it remains unknown how plasticity mechanisms of synapses distributed along dendritic trees cooperate to produce the wide spectrum of outcomes for various plasticity protocols. Here, we propose a four-pathway plasticity framework that is well grounded in experimental evidence and apply it to a biophysically realistic cortical pyramidal neuron model. We show in computer simulations that several seemingly contradictory experimental landmark studies are consistent with one unifying set of mechanisms when considering the effects of signal propagation in dendritic trees with respect to synapse location. Our model identifies specific spatiotemporal contributions of dendritic and axo-somatic spikes as well as of subthreshold activation of synaptic clusters, providing a unified parsimonious explanation not only for rate and timing dependence but also for location dependence of synaptic changes.

•

A phenomenological synaptic plasticity rule is applied to a pyramidal neuron model

•

Model reproduces rate-, timing-, and location-dependent plasticity results

•

Active dendrites allow plasticity via dendritic spikes and subthreshold events

•

Cooperative plasticity exists across the dendritic tree and within single branches

A unified computational model for cortical post-synaptic plasticity

Signalling pathways leading to post-synaptic plasticity have been examined in many types of experimental studies, but a unified picture on how multiple biochemical pathways collectively shape neocortical plasticity is missing. We built a biochemically detailed model of post-synaptic plasticity describing CaMKII, PKA, and PKC pathways and their contribution to synaptic potentiation or depression. We developed a statistical AMPA-receptor-tetramer model, which permits the estimation of the AMPA-receptor-mediated maximal synaptic conductance based on numbers of GluR1s and GluR2s predicted by the biochemical signalling model. We show that our model reproduces neuromodulator-gated spike-timing-dependent plasticity as observed in the visual cortex and can be fit to data from many cortical areas, uncovering the biochemical contributions of the pathways pinpointed by the underlying experimental studies. Our model explains the dependence of different forms of plasticity on the availability of different proteins and can be used for the study of mental disorder-associated impairments of cortical plasticity.

Neuronal Plasticity: Neuronal Organization is Associated with Neurological Disorders

Stimuli from stressful events, attention in the classroom, and many other experiences affect the functionality of the brain by changing the structure or reorganizing the connections between neurons and their communication. Modification of the synaptic transmission is a vital mechanism for generating neural activity via internal or external stimuli. Neuronal plasticity is an important driving force in neuroscience research, as it is the basic process underlying learning and memory and is involved in many other functions including brain development and homeostasis, sensorial training, and recovery from brain injury. Indeed, neuronal plasticity has been explored in numerous studies, but it is still not clear how neuronal plasticity affects the physiology and morphology of the brain. Thus, unraveling the molecular mechanisms of neuronal plasticity is essential for understanding the operation of brain functions. In this timeline review, we discuss the molecular mechanisms underlying different forms of synaptic plasticity and their association with neurodegenerative/neurological disorders as a consequence of alterations in neuronal plasticity.

The Relationship Between Glutamate Dynamics and Activity-Dependent Synaptic Plasticity

The spatiotemporal dynamics of excitatory neurotransmission must be tightly regulated to achieve efficient synaptic communication. By limiting spillover, glutamate transporters are believed to prevent excessive activation of extrasynaptically located receptors that can impair synaptic plasticity. While glutamate transporter expression is reduced in numerous neurodegenerative diseases, the contributions of transporter dysfunction to disease pathophysiology remain ambiguous as the fundamental relationship between glutamate dynamics and plasticity, and the mechanisms linking these two phenomena, remain poorly understood. Here, we combined electrophysiology and real-time high-speed imaging of extracellular glutamate transients during LTP induction and characterized the sensitivity of the relationship between glutamate dynamics during theta burst stimulation (TBS) and the resulting magnitude of LTP consolidation, both in control conditions and following selective and nonselective glutamate transporter blockade. Glutamate clearance times were negatively correlated with LTP magnitude following nonselective glutamate transporter inhibition but not following selective blockade of a majority of GLT-1, the brain's most abundant glutamate transporter. Although glutamate transporter inhibition reduced the postsynaptic population response to TBS, calcium responses to TBS were greatly exaggerated. The source of excess calcium was dependent on NMDARs, L-type VGCCs, GluA2-lacking AMPARs, and internal calcium stores. Surprisingly, inhibition of L-type VGCCs, but not GluA2-lacking AMPARs or ryanodine receptors, was required to restore robust LTP. In all, these data provide a detailed understanding of the relationship between glutamate dynamics and plasticity and uncover important mechanisms by which poor glutamate uptake can negatively impact LTP consolidation.SIGNIFICANCE STATEMENT Specific patterns of neural activity can promote long-term changes in the strength of synaptic connections through a phenomenon known as synaptic plasticity. Synaptic plasticity is well accepted to represent the cellular mechanisms underlying learning and memory, and many forms of plasticity are initiated by the excitatory neurotransmitter glutamate. While essential for rapid cellular communication in the brain, excessive levels of extracellular glutamate can negatively impact brain function. In this study, we demonstrate that pharmacological manipulations that increase the availability of extracellular glutamate during neural activity can have profoundly negative consequences on synaptic plasticity. We identify mechanisms through which excess glutamate can negatively influence synaptic plasticity, and we discuss the relevance of these findings to neurodegenerative diseases and in the aging brain.

Calcium Dynamics and Synaptic Plasticity

Synaptic plasticity is a fundamental property of neurons referring to the activity-dependent changes in the strength and efficacy of synaptic transmission at preexisting synapses. Such changes can last from milliseconds to hours, days, or even longer and are involved in learning and memory as well as in development and response of the brain to injuries. Several types of synaptic plasticity have been described across neuronal types, brain regions, and species, but all of them share in one way or another capital importance of Ca²⁺-mediated processes. In this chapter, we will focus on the Ca²⁺-dependent events necessary for the induction and expression of multiple forms of synaptic plasticity.

Computational Modeling of Genetic Contributions to Excitability and Neural Coding in Layer V Pyramidal Cells: Applications to Schizophrenia Pathology

Pyramidal cells in layer V of the neocortex are one of the most widely studied neuron types in the mammalian brain. Due to their role as integrators of feedforward and cortical feedback inputs, they are well-positioned to contribute to the symptoms and pathology in mental disorders-such as schizophrenia-that are characterized by a mismatch between the internal perception and external inputs. In this modeling study, we analyze the input/output properties of layer V pyramidal cells and their sensitivity to modeled genetic variants in schizophrenia-associated genes. We show that the excitability of layer V pyramidal cells and the way they integrate inputs in space and time are altered by many types of variants in ion-channel and Ca²⁺ transporter-encoding genes that have been identified as risk genes by recent genome-wide association studies. We also show that the variability in the output patterns of spiking and Ca²⁺ transients in layer V pyramidal cells is altered by these model variants. Importantly, we show that many of the predicted effects are robust to noise and qualitatively similar across different computational models of layer V pyramidal cells. Our modeling framework reveals several aspects of single-neuron excitability that can be linked to known schizophrenia-related phenotypes and existing hypotheses on disease mechanisms. In particular, our models predict that single-cell steady-state firing rate is positively correlated with the coding capacity of the neuron and negatively correlated with the amplitude of a prepulse-mediated adaptation and sensitivity to coincidence of stimuli in the apical dendrite and the perisomatic region of a layer V pyramidal cell. These results help to uncover the voltage-gated ion-channel and Ca²⁺ transporter-associated genetic underpinnings of schizophrenia phenotypes and biomarkers.

Developmental Aspects of Glucose and Calcium Availability on the Persistence of Memory Function Over the Lifespan

An important aspect concerning the underlying nature of memory function is an understanding of how memories are acquired and lost. The stability, and ultimate demise, of memory over the lifespan of an organism remains a critical topic in determining the neurobiological mechanisms that mediate memory representations. This has important implications for the elucidation and treatment of neurodegenerative diseases such as Alzheimer's disease (AD). One important question in the context of preserving functional plasticity over the lifespan is the determination of the neurobiological structural and functional changes that contribute to the formation of memory during the juvenile time frame that might provide protection against later memory dysfunction by promoting the establishment of redundant neural pathways. The main question being, if memory formation during the juvenile period does strengthen and preserve memory stability over the lifespan, what are the neurobiological structural or functional substrates that mediate this effect? One neural attribute whose function may be altered with early life experience and provide a mechanism to preserve memory through the lifespan is glucose transport-linked calcium (Ca2+) buffering. Because peak increases in glucose utilization overlap with a timeframe during which spatial training can enhance later memory processing, it might be the case that learning-associated changes in glucose utilization would provide an important neural functional change to preserve memory function throughout the lifespan. The glucose transporters are proteins that are reduced in AD pathology and there is evidence that glucose reductions can impair Ca2+ buffering. In the absence of an appropriate supply of ATP, provided via glucose transport and glycolysis, Ca2+ levels can rise leading to neural vulnerability with ensuing pathological outcomes. In this review, we explore the hypothesis that enhancing glucose utilization with spatial training during the preadolescent period will provide a functional enhancement that regulates glucose-dependent Ca2+ signaling during aging or neurodegeneration and provide essential neural resources to preserve functional plasticity and memory function.

The role of Plasma Membrane Calcium ATPases (PMCAs) in neurodegenerative disorders

Selective degeneration of differentiated neurons in the brain is the unifying feature of neurodegenerative disorders such as Parkinson's disease (PD) or Alzheimer's disease (AD). A broad spectrum of evidence indicates that initially subtle, but temporally early calcium dysregulation may be central to the selective neuronal vulnerability observed in these slowly progressing, chronic disorders. Moreover, it has long been evident that excitotoxicity and its major toxic effector mechanism, neuronal calcium overload, play a decisive role in the propagation of secondary neuronal death after acute brain injury from trauma or ischemia. Under physiological conditions, neuronal calcium homeostasis is maintained by a fine-tuned interplay between calcium influx and releasing mechanisms (Ca²⁺-channels), and calcium efflux mechanisms (Ca²⁺-pumps and -exchangers). Central functional components of the calcium efflux machinery are the Plasma Membrane Calcium ATPases (PMCAs), which represent high-affinity calcium pumps responsible for the ATP-dependent removal of calcium out of the cytosol. Beyond a growing body of experimental evidence, it is their high expression level, their independence of secondary ions or membrane potential, their profound redox regulation and autoregulation, their postsynaptic localization in close proximity to the primary mediators of pathological calcium influx, i.e. NMDA receptors, as well as evolutionary considerations which all suggest a pivotal role of the PMCAs in the etiology of neurodegeneration and make them equally challenging and alluring candidates for drug development. This review aims to summarize the recent literature on the role of PMCAs in the pathogenesis of neurodegenerative disorders.

From membrane receptors to protein synthesis and actin cytoskeleton: Mechanisms underlying long lasting forms of synaptic plasticity

Synaptic plasticity, the activity dependent change in synaptic strength, forms the molecular foundation of learning and memory. Synaptic plasticity includes structural changes, with spines changing their size to accomodate insertion and removal of postynaptic receptors, which are correlated with functional changes. Of particular relevance for memory storage are the long lasting forms of synaptic plasticity which are protein synthesis dependent. Due to the importance of spine structural plasticity and protein synthesis, this review focuses on the signaling pathways that connect synaptic stimulation with regulation of protein synthesis and remodeling of the actin cytoskeleton. We also review computational models that implement novel aspects of molecular signaling in synaptic plasticity, such as the role of neuromodulators and spatial microdomains, as well as highlight the need for computational models that connect activation of memory kinases with spine actin dynamics.

An integrated microfluidic/microelectrode array for the study of activity-dependent intracellular dynamics in neuronal networks

In the central nervous system, neurons are organized in specific neural networks with distinct electrical patterns, input integration capacities, and intracellular dynamics. In order to better understand how neurons process information, it is crucial to keep the complex organization of brain circuits. However, performing subcellular investigations with high spatial and temporal resolution in vivo is technically challenging, especially in fine structures, such as axonal projections. Here, we present an on-a-chip system that combines a microfluidic platform with a dedicated matrix of electrodes to study activity-dependent dynamics in the physiological context of brain circuits. Because this system is compatible with high-resolution video-microscopy, it is possible to simultaneously record intracellular dynamics and electrical activity in presynaptic axonal projections and in their postsynaptic neuronal targets. Similarly, specific patterns of electrical activity can be applied to both compartments in order to investigate how intrinsic and network activities influence intracellular dynamics. The fluidic isolation of each compartment further allows the selective application of drugs at identified sites to study activity-dependent synaptic transmission. This integrated microfluidic/microelectrode array (microMEA) platform is a valuable tool for studying various intracellular and synaptic dynamics in response to neuronal activity in a physiologically relevant context that resembles in vivo brain circuits.

Inhibition enhances spatially-specific calcium encoding of synaptic input patterns in a biologically constrained model

Synaptic plasticity, which underlies learning and memory, depends on calcium elevation in neurons, but the precise relationship between calcium and spatiotemporal patterns of synaptic inputs is unclear. Here, we develop a biologically realistic computational model of striatal spiny projection neurons with sophisticated calcium dynamics, based on data from rodents of both sexes, to investigate how spatiotemporally clustered and distributed excitatory and inhibitory inputs affect spine calcium. We demonstrate that coordinated excitatory synaptic inputs evoke enhanced calcium elevation specific to stimulated spines, with lower but physiologically relevant calcium elevation in nearby non-stimulated spines. Results further show a novel and important function of inhibition-to enhance the difference in calcium between stimulated and non-stimulated spines. These findings suggest that spine calcium dynamics encode synaptic input patterns and may serve as a signal for both stimulus-specific potentiation and heterosynaptic depression, maintaining balanced activity in a dendritic branch while inducing pattern-specific plasticity.

Metaplasticity at the addicted tetrapartite synapse: A common denominator of drug induced adaptations and potential treatment target for addiction

In light of the current worldwide addiction epidemic, the need for successful therapies is more urgent than ever. Although we made substantial progress in our basic understanding of addiction, reliable therapies are lacking. Since 40-60% of patients treated for substance use disorder return to active substance use within a year following treatment discharge, alleviating the vulnerability to relapse is regarded as the most promising avenue for addiction therapy. Preclinical addiction research often focuses on maladaptive synaptic plasticity within the reward pathway. However, drug induced neuroadaptations do not only lead to a strengthening of distinct drug associated cues and drug conditioned behaviors, but also seem to increase plasticity thresholds for environmental stimuli that are not associated with the drug. This form of higher order plasticity, or synaptic metaplasticity, is not expressed as a change in the efficacy of synaptic transmission but as a change in the direction or degree of plasticity induced by a distinct stimulation pattern. Experimental addiction research has demonstrated metaplasticity after exposure to multiple classes of addictive drugs. In this review we will focus on the concept of synaptic metaplasticity in the context of preclinical addiction research. We will take a closer look at the tetrapartite glutamatergic synapse and outline forms of metaplasticity that have been described at the addicted synapse. Finally we will discuss the different potential avenues for pharmacotherapies that target glutamatergic synaptic plasticity and metaplasticity. Here we will argue that aberrant metaplasticity renders the reward seeking circuitry more rigid and hence less able to adapt to changing environmental contingencies. An understanding of the molecular mechanisms that underlie this metaplasticity is crucial for the development of new strategies for addiction therapy. The correction of drug-induced metaplasticity could be used to support behavioral and pharmacotherapies for the treatment of addiction.

A Glutamatergic Spine Model to Enable Multi-Scale Modeling of Nonlinear Calcium Dynamics

In synapses, calcium is required for modulating synaptic transmission, plasticity, synaptogenesis, and synaptic pruning. The regulation of calcium dynamics within neurons involves cellular mechanisms such as synaptically activated channels and pumps, calcium buffers, and calcium sequestrating organelles. Many experimental studies tend to focus on only one or a small number of these mechanisms, as technical limitations make it difficult to observe all features at once. Computational modeling enables incorporation of many of these properties together, allowing for more complete and integrated studies. However, the scale of existing detailed models is often limited to synaptic and dendritic compartments as the computational burden rapidly increases when these models are integrated in cellular or network level simulations. In this article we present a computational model of calcium dynamics at the postsynaptic spine of a CA1 pyramidal neuron, as well as a methodology that enables its implementation in multi-scale, large-scale simulations. We first present a mechanistic model that includes individually validated models of various components involved in the regulation of calcium at the spine. We validated our mechanistic model by comparing simulated calcium levels to experimental data found in the literature. We performed additional simulations with the mechanistic model to determine how the simulated calcium activity varies with respect to presynaptic-postsynaptic stimulation intervals and spine distance from the soma. We then developed an input-output (IO) model that complements the mechanistic calcium model and provide a computationally efficient representation for use in larger scale modeling studies; we show the performance of the IO model compared to the mechanistic model in terms of accuracy and speed. The models presented here help achieve two objectives. First, the mechanistic model provides a comprehensive platform to describe spine calcium dynamics based on individual contributing factors. Second, the IO model is trained on the main dynamical features of the mechanistic model and enables nonlinear spine calcium modeling on the cell and network level simulation scales. Utilizing both model representations provide a multi-level perspective on calcium dynamics, originating from the molecular interactions at spines and propagating the effects to higher levels of activity involved in network behavior.

Structural determinants of specificity and regulation of activity in the allosteric loop network of human KLK8/neuropsin

Human KLK8/neuropsin, a kallikrein-related serine peptidase, is mostly expressed in skin and the hippocampus regions of the brain, where it regulates memory formation by synaptic remodeling. Substrate profiles of recombinant KLK8 were analyzed with positional scanning using fluorogenic tetrapeptides and the proteomic PICS approach, which revealed the prime side specificity. Enzyme kinetics with optimized substrates showed stimulation by Ca²⁺ and inhibition by Zn²⁺, which are physiological regulators. Crystal structures of KLK8 with a ligand-free active site and with the inhibitor leupeptin explain the subsite specificity and display Ca²⁺ bound to the 75-loop. The variants D70K and H99A confirmed the antagonistic role of the cation binding sites. Molecular docking and dynamics calculations provided insights in substrate binding and the dual regulation of activity by Ca²⁺ and Zn²⁺, which are important in neuron and skin physiology. Both cations participate in the allosteric surface loop network present in related serine proteases. A comparison of the positional scanning data with substrates from brain suggests an adaptive recognition by KLK8, based on the tertiary structures of its targets. These combined findings provide a comprehensive picture of the molecular mechanisms underlying the enzyme activity of KLK8.

Modulation of Spike-Timing Dependent Plasticity: Towards the Inclusion of a Third Factor in Computational Models

In spike-timing dependent plasticity (STDP) change in synaptic strength depends on the timing of pre- vs. postsynaptic spiking activity. Since STDP is in compliance with Hebb's postulate, it is considered one of the major mechanisms of memory storage and recall. STDP comprises a system of two coincidence detectors with N-methyl-D-aspartate receptor (NMDAR) activation often posited as one of the main components. Numerous studies have unveiled a third component of this coincidence detection system, namely neuromodulation and glia activity shaping STDP. Even though dopaminergic control of STDP has most often been reported, acetylcholine, noradrenaline, nitric oxide (NO), brain-derived neurotrophic factor (BDNF) or gamma-aminobutyric acid (GABA) also has been shown to effectively modulate STDP. Furthermore, it has been demonstrated that astrocytes, via the release or uptake of glutamate, gate STDP expression. At the most fundamental level, the timing properties of STDP are expected to depend on the spatiotemporal dynamics of the underlying signaling pathways. However in most cases, due to technical limitations experiments grant only indirect access to these pathways. Computational models carefully constrained by experiments, allow for a better qualitative understanding of the molecular basis of STDP and its regulation by neuromodulators. Recently, computational models of calcium dynamics and signaling pathway molecules have started to explore STDP emergence in ex and in vivo-like conditions. These models are expected to reproduce better at least part of the complex modulation of STDP as an emergent property of the underlying molecular pathways. Elucidation of the mechanisms underlying STDP modulation and its consequences on network dynamics is of critical importance and will allow better understanding of the major mechanisms of memory storage and recall both in health and disease.

Competitive tuning: Competition's role in setting the frequency-dependence of Ca2+-dependent proteins

A number of neurological disorders arise from perturbations in biochemical signaling and protein complex formation within neurons. Normally, proteins form networks that when activated produce persistent changes in a synapse’s molecular composition. In hippocampal neurons, calcium ion (Ca²⁺) flux through N-methyl-D-aspartate (NMDA) receptors activates Ca²⁺/calmodulin signal transduction networks that either increase or decrease the strength of the neuronal synapse, phenomena known as long-term potentiation (LTP) or long-term depression (LTD), respectively. The calcium-sensor calmodulin (CaM) acts as a common activator of the networks responsible for both LTP and LTD. This is possible, in part, because CaM binding proteins are “tuned” to different Ca²⁺ flux signals by their unique binding and activation dynamics. Computational modeling is used to describe the binding and activation dynamics of Ca²⁺/CaM signal transduction and can be used to guide focused experimental studies. Although CaM binds over 100 proteins, practical limitations cause many models to include only one or two CaM-activated proteins. In this work, we view Ca²⁺/CaM as a limiting resource in the signal transduction pathway owing to its low abundance relative to its binding partners. With this view, we investigate the effect of competitive binding on the dynamics of CaM binding partner activation. Using an explicit model of Ca²⁺, CaM, and seven highly-expressed hippocampal CaM binding proteins, we find that competition for CaM binding serves as a tuning mechanism: the presence of competitors shifts and sharpens the Ca²⁺ frequency-dependence of CaM binding proteins. Notably, we find that simulated competition may be sufficient to recreate the in vivo frequency dependence of the CaM-dependent phosphatase calcineurin. Additionally, competition alone (without feedback mechanisms or spatial parameters) could replicate counter-intuitive experimental observations of decreased activation of Ca²⁺/CaM-dependent protein kinase II in knockout models of neurogranin. We conclude that competitive tuning could be an important dynamic process underlying synaptic plasticity.

Learning and memory formation are likely associated with dynamic fluctuations in the connective strength of neuronal synapses. These fluctuations, called synaptic plasticity, are regulated by calcium ion (Ca²⁺) influx through ion channels localized to the post-synaptic membrane. Within the post-synapse, the dominant Ca²⁺ sensor protein, calmodulin (CaM), may activate a variety of downstream binding partners, each contributing to synaptic plasticity outcomes. The conditions at which certain binding partners most strongly activate are increasingly studied using computational models. Nearly all computational studies describe these binding partners in combinations of only one or two CaM binding proteins. In contrast, we combine seven well-studied CaM binding partners into a single model wherein they simultaneously compete for access to CaM. Our dynamic model suggests that competition narrows the window of conditions for optimal activation of some binding partners, mimicking the Ca²⁺-frequency dependence of some proteins in vivo. Further characterization of CaM-dependent signaling dynamics in neuronal synapses may benefit our understanding of learning and memory formation. Furthermore, we propose that competitive binding may be another framework, alongside feedback and feed-forward loops, signaling motifs, and spatial localization, that can be applied to other signal transduction networks, particularly second messenger cascades, to explain the dynamical behavior of protein activation.

Diversity of Evoked Astrocyte Ca2+ Dynamics Quantified through Experimental Measurements and Mathematical Modeling

Astrocytes are a major cell type in the mammalian brain. They are not electrically excitable, but generate prominent Ca²⁺ signals related to a wide variety of critical functions. The mechanisms driving these Ca²⁺ events remain incompletely understood. In this study, we integrate Ca²⁺ imaging, quantitative data analysis, and mechanistic computational modeling to study the spatial and temporal heterogeneity of cortical astrocyte Ca²⁺ transients evoked by focal application of ATP in mouse brain slices. Based on experimental results, we tune a single-compartment mathematical model of IP₃-dependent Ca²⁺ responses in astrocytes and use that model to study response heterogeneity. Using information from the experimental data and the underlying bifurcation structure of our mathematical model, we categorize all astrocyte Ca²⁺ responses into four general types based on their temporal characteristics: Single-Peak, Multi-Peak, Plateau, and Long-Lasting responses. We find that the distribution of experimentally-recorded response types depends on the location within an astrocyte, with somatic responses dominated by Single-Peak (SP) responses and large and small processes generating more Multi-Peak responses. On the other hand, response kinetics differ more between cells and trials than with location within a given astrocyte. We use the computational model to elucidate possible sources of Ca²⁺ response variability: (1) temporal dynamics of IP₃, and (2) relative flux rates through Ca²⁺ channels and pumps. Our model also predicts the effects of blocking Ca²⁺ channels/pumps; for example, blocking store-operated Ca²⁺ (SOC) channels in the model eliminates Plateau and Long-Lasting responses (consistent with previous experimental observations). Finally, we propose that observed differences in response type distributions between astrocyte somas and processes can be attributed to systematic differences in IP₃ rise durations and Ca²⁺ flux rates.

Memory Takes Time

Memory is an adaptation to particular temporal properties of past events, such as the frequency of occurrence of a stimulus or the coincidence of multiple stimuli. In neurons, this adaptation can be understood in terms of a hierarchical system of molecular and cellular time windows, which collectively retain information from the past. We propose that this system makes various timescales of past experience simultaneously available for future adjustment of behavior. More generally, we propose that the ability to detect and respond to temporally structured information underlies the nervous system's capacity to encode and store a memory at molecular, cellular, synaptic, and circuit levels.