Postsynaptic Calcium Research Articles

Timing Is Everything, Even for Cholinergic Control

Synaptic plasticity is widely considered to be a cellular mechanism underlying learning and memory. In this issue of Neuron, Gu and Yakel show that the precise timing of a single cholinergic pulse of activity can determine whether plasticity will occur at a glutamatergic synapse and confer long-term potentiation versus depression.

Regulation of Long-Term Plasticity Induction by the Channel and C-Terminal Domains of GluN2 Subunits

Conventional long-term potentiation (LTP) and long-term depression (LTD) are induced by different patterns of synaptic stimulation, but both forms of synaptic modification require calcium influx through NMDA receptors (NMDARs). A prevailing model (the "calcium hypothesis") suggests that high postsynaptic calcium elevation results in LTP, whereas moderate elevations give rise to LTD. Recently, additional evidence has come to suggest that differential activation of NMDAR subunits also factors in determining which type of plasticity is induced. While the growing amount of data suggest that activation of NMDARs containing specific GluN2 subunits plays an important role in the induction of plasticity, it remains less clear which subunit is tied to which form of plasticity. Additionally, it remains to be determined which properties of the subunits confer upon them the ability to differentially induce long-term plasticity. This review highlights recent studies suggesting differential roles for the subunits, as well as findings that begin to shed light on how two similar subunits may be linked to the induction of opposing forms of plasticity.

Dendritic EGFP‐STIM1 activation after type I metabotropic glutamate and muscarinic acetylcholine receptor stimulation in hippocampal neuron

Several signaling pathways in neurons engage the endoplasmic reticulum (ER) calcium store by triggering calcium release. After release, ER calcium levels must be restored. In many non-neuronal cell types, this is mediated by store-operated calcium entry (SOCE), a cellular homeostatic mechanism that activates specialized store-operated calcium channels (SOC). Although much evidence supports the existence of SOCE in neurons, its importance has been difficult to determine because of the abundance of calcium channels expressed and the lack of SOC-specific pharmacological agents. We have explored the function of the SOCE-inducing protein STIM1 in neurons. In EGFP-STIM1-expressing hippocampal neurons, the sarco- and endoplasmic reticulum calcium ATPase (SERCA) inhibitor thapsigargin caused rapid aggregation (i.e., activation) of STIM1 in soma and dendrites. Upon STIM1 activation by thapsigargin, a dramatic reduction in STIM1 mobility was detected by fluorescence recovery after photobleaching (FRAP). By triggering release of ER calcium with 3,5-dihydroxyphenylglycine (DHPG) or carbachol (Cch), agonists of type I metabotropic glutamate receptors (mGluR) and muscarinic acetylcholine receptors (mAChR), respectively, STIM1 was activated, and calcium entry (likely to represent SOCE) occurred in dendrites. It is therefore possible that neuronal SOCE is activated by physiological stimuli, some of which may alter the postsynaptic calcium signaling properties.

Role of presynaptic and postsynaptic IP3‐dependent intracellular calcium release in long‐term potentiation in sympathetic ganglion of the rat

In the rat superior cervical ganglion, a form of long term potentiation (LTP) can be elicited by a brief high frequency stimuli applied to the preganglionic nerve. Cumulative evidence shows that a transient increase in cytoplasmic Ca²+ concentration is essential for the generation of the ganglionic LTP. Calcium influx and calcium release from intracellular calcium stores contribute to LTP. However, the differential role of presynaptic and postsynaptic calcium signaling has not been established. Herein, by using heparin, a membrane-impermeant inositol trisphosphate receptor (IP3R) blocker, we explored the contribution of presynaptic and postsynaptic IP3-sensitive calcium stores to the ganglionic LTP. The LTP was produced by a conditioning train of 40 Hz for 3 s. We analyzed the effects of heparin on the posttetanic potentiation: PTP magnitude and PTP time constant, and on two parameters that describe the LTP: LTP decay time (elapsed time required by the potentiated response to fall to 20% above the basal value) and LTP extent (the integral of the potentiated response). Heparin (100 and 200 μg/ml) was loaded in the preganglionic, the postganglionic, or in both nerves. We found that in all tested conditions heparin significantly decreased LTP but practically did not affect PTP. The preganglionic and postganglionic inhibitory effects of heparin were not additive. De-N-sulfated heparin, an ineffective IP3R blocker, had no effect on LTP, but abolished the heparin blocking effect. Data suggest that presynaptic and postsynaptic IP3-dependent intracellular calcium release equally contribute to ganglionic LTP, supporting our proposal of a trans-synaptic mechanism for LTP.

Dopamine modulates synaptic plasticity in dendrites of rat and human dentate granule cells

The mechanisms underlying memory formation in the hippocampal network remain a major unanswered aspect of neuroscience. Although high-frequency activity appears essential for plasticity, salience for memory formation is also provided by activity in ventral tegmental area (VTA) dopamine projections. Here, we report that activation of dopamine D1 receptors in dentate granule cells (DGCs) can preferentially increase dendritic excitability to both high-frequency afferent activity and high-frequency trains of backpropagating action potentials. Using whole-cell patch clamp recordings, calcium imaging, and neuropeptide Y to inhibit postsynaptic calcium influx, we found that activation of dendritic voltage-dependent calcium channels (VDCCs) is essential for dopamine-induced long-term potentiation (LTP), both in rat and human dentate gyrus (DG). Moreover, we demonstrate previously unreported spike-timing-dependent plasticity in the human hippocampus. These results suggest that when dopamine is released in the dentate gyrus with concurrent high-frequency activity there is an increased probability that synapses will be strengthened and reward-associated spatial memories will be formed.

NMDA-Mediated Ca2+Influx Drives Aberrant Ryanodine Receptor Activation in Dendrites of Young Alzheimer's Disease Mice

Deficits in synaptic function, particularly through NMDA receptors (NMDARs), are linked to late-stage cognitive impairments in Alzheimer's disease (AD). At earlier disease stages, however, there is evidence for altered endoplasmic reticulum (ER) calcium signaling in human cases and in neurons from AD mouse models. Despite the fundamental importance of calcium to synaptic function, neither the extent of ER calcium dysregulation in dendrites nor its interaction with synaptic function in AD pathophysiology is known. Identifying the mechanisms underlying early synaptic calcium dysregulation in AD pathogenesis is likely a key component to understanding, and thereby preventing, the synapse loss and downstream cognitive impairments. Using two-photon calcium imaging, flash photolysis of caged glutamate, and patch-clamp electrophysiology in cortical brain slices, we examined interactions between synaptically and ER-evoked calcium release at glutamatergic synapses in young AD transgenic mice. We found increased ryanodine receptor-evoked calcium signals within dendritic spine heads, dendritic processes, and the soma of pyramidal neurons from 3xTg-AD and TAS/TPM AD mice relative to NonTg controls. In addition, synaptically evoked postsynaptic calcium responses were larger in the AD strains, as were calcium signals generated from NMDAR activation. However, calcium responses triggered by back-propagating action potentials were not different. Concurrent activation of ryanodine receptors (RyRs) with either synaptic or NMDAR stimulation generated a supra-additive calcium response in the AD strains, suggesting an aberrant calcium-induced calcium release (CICR) effect within spines and dendrites. We propose that presenilin-linked disruptions in RyR signaling and subsequent CICR via NMDAR-mediated calcium influx alters synaptic function and serves as an early pathogenic factor in AD.

"MEMORY BYTES" — MOLECULAR MATCH FORCaMKIIPHOSPHORYLATION ENCODING OF MICROTUBULE LATTICES

Learning, memory and long-term potentiation (LTP) are supported by factors including post-synaptic calcium ion flux activating and transforming the hexagonal calcium-calmodulin kinase II (CaMKII) holoenzyme. Upon calcium-induced activation, up to six kinase domains extend upward, and up to six kinase domains extend downward from the CaMKII association domain, the fully activated holoenzyme resembling a robotic insect 20 nanometers in length. Each extended kinase domain can be phosphorylated, and able to phosphorylate other proteins, thus potentially further encoding synaptic information at intraneuronal molecular sites for memory storage, processing and distribution. Candidate sites for phosphorylation-encoded molecular memory include microtubules, cylindrical lattice polymers of the protein tubulin. Using molecular modeling, we find spatial dimensions and geometry of the six extended CaMKII kinase domains can precisely match those of microtubule hexagonal lattice neighborhoods (both A- and B-lattices), and show two feasible phosphorylation mechanisms. In one, phosphorylation sites (e.g., valine 208) on a CaMKII extended kinase domain interact with serine 444 on a C-terminal "tail" of tubulin. In the second, the CaMKII kinase domain unfurls, enabling phosphorylation sites to contact threonine and serine sites on the tubulin surface. We suggest sets of six CaMKII kinase domains phosphorylate hexagonal microtubule lattice neighborhoods collectively, e.g., conveying synaptic information as ordered arrays of six "bits", and thus a "byte", with (minimally) 2⁶ (64) possible bit states per CaMKII-microtubule interaction. We model two levels of interaction between CaMKII and microtubules, suggesting a testable framework for molecular memory encoding.

GABA inhibition modulates NMDA-R mediated spike timing dependent plasticity (STDP) in a biophysical model

Spike timing dependent plasticity (STDP) has been demonstrated in various neural systems of many animals. It has been shown that STDP depends on the target and the location of the synapse and is dynamically regulated by the activity of adjacent synapses, the presence of postsynaptic calcium, presynaptic GABA inhibition or the action of neuromodulators. Recent experimental evidence has reported that the profile of STDP in the CA1 pyramidal neuron can be classified into two types depending on its dendritic location: (1) A symmetric STDP profile in the proximal to the soma dendrites, and (2) an asymmetric one in the distal dendrites. Bicuculline application revealed that GABA(A) is responsible for the symmetry of the STDP curve. We investigate via computer simulations how GABA(A) shapes the STDP profile in the CA1 pyramidal neuron dendrites when it is driven by excitatory spike pairs (doublets). The model constructed uses calcium as the postsynaptic signaling agent for STDP and is shown to be consistent with classical long-term potentiation (LTP) and long-term depression (LTD) induced by several doublet stimulation paradigms in the absence of inhibition. Overall, simulation results provide computational evidence for the first time that the switch between the symmetrical and the asymmetrical STDP operational modes is indeed due to GABA inhibition. Furthermore, gamma frequency inhibition and not theta one is responsible for the transition from asymmetry-to-symmetry. The resulted symmetrical STDP profile is centered at +10 ms with two distinct LTD tails at -10 and +40 ms. Finally, the asymmetry-to-symmetry transition is strongly dependent on the strength (conductance) of inhibition and its relative onset with respect to pre- and postsynaptic spike stimulation.

Deactivation of L-type Ca Current by Inhibition Controls LTP at Excitatory Synapses in the Cerebellar Nuclei

Long-term potentiation (LTP) of mossy fiber EPSCs in the cerebellar nuclei is controlled by synaptic inhibition from Purkinje neurons. EPSCs are potentiated by a sequence of excitation, inhibition, and disinhibition, raising the question of how these stimuli interact to induce plasticity. Here, we find that synaptic excitation, inhibition, and disinhibition couple to different calcium-dependent signaling pathways. In LTP induction protocols, constitutively active calcineurin can replace synaptic excitation, and constitutively active alpha-CaMKII can replace calcium influx associated with resumption of spiking upon disinhibition. Additionally, nimodipine can replace hyperpolarization, indicating that inhibition of firing decreases Ca influx through L-type Ca channels, providing a necessary signal for LTP. Together, these data suggest that potentiation develops after a calcineurin priming signal combines with an alpha-CaMKII triggering signal if and only if L-type Ca current is reduced. Thus, hyperpolarization induced by synaptic inhibition actively controls excitatory synaptic plasticity in the cerebellar nuclei.

Imaging and analysis of evoked excitatory-postsynaptic-calcium-transients by individual presynaptic-boutons of cultured Aplysia sensorimotor synapse

The use of the sensory–motor (SN-MN) synapse of the Aplysia gill withdrawal reflex has contributed immensely to the understanding of synaptic transmission, learning and memory acquisition processes. Whereas the majority of the studies focused on analysis of the presynaptic mechanisms, recent studies indicated that as in mammalian synapses, long term potentiation (LTP) formed by Aplysia SN-MN synapse depends on elevation of the postsynaptic free intracellular calcium concentration ([Ca2+]i). Consistently, injection of the fast calcium chelator BAPTA to the MN prevents the formation of serotonin-induced LTP. Nevertheless, currently there are no published reports that directly examine and document whether evoked synaptic transmission is associated with transient increase in the postsynaptic [Ca2+]i. In the present study we imaged, for the first time, alterations in the postsynaptic [Ca2+]i in response to presynaptic stimulation and analyzed the underlying mechanisms.Using live imaging of the postsynaptic [Ca2+]i while monitoring the EPSP, we found that evoked transmitter release generates excitatory postsynaptic calcium concentration transients (EPSCaTs) by two mechanisms: (a) activation of DNQX-sensitive postsynaptic receptors-gated calcium influx and (b) calcium influx through nitrendipine-sensitive voltage-gated calcium channels (VGCCs). Concomitant confocal imaging of presynaptic boutons and EPSCaTs revealed that approximately 86% of the presynaptic boutons are associated with functional synapses.

Muscarinic receptor activation modulates the excitability of hilar mossy cells through the induction of an afterdepolarization

In the present study we used electrophysiological techniques in an in vitro preparation of the rat dentate gyrus to examine the effect of muscarinic acetylcholine receptor activation on the intrinsic excitability of hilar neurons. We found that bath application of muscarine caused a direct depolarization in approximately 80% of mossy cells tested, and also produced a clear afterdepolarization (ADP) in nearly 100% of trials. The ADP observed in hilar mossy cells is produced by the opening of a Na + permeant and yet largely TTX insensitive ion channel. It requires an increase in postsynaptic calcium for activation, and is blocked by flufenamic acid, an antagonist of a previously identified calcium activated non-selective cation channel ( I CAN). Further, we demonstrate that induction of an ADP in current clamp causes release of cannabinoids, and subsequent depression of GABAergic transmission that is comparable to that produced in the same cells by a more conventional 5 s depolarization in voltage clamp. By contrast, other types of hilar neurons were less strongly depolarized by bath application of muscarinic agonists, and uniformly lacked a similar muscarinic ADP. Overall, the data presented here extend our understanding of the specific mechanisms through which muscarinic agonists are likely to modulate neuronal excitability in the hilar network, and further reveal a mechanism that could plausibly promote endocannabinoid mediated signaling in vivo.

Calcium homeostasis of acutely denervated and lesioned dentate gyrus in organotypic entorhino-hippocampal co-cultures

Denervation of neurons, e.g. upon traumatic injury or neuronal degeneration, induces transneuronal degenerative events, such as spine loss, dendritic pruning, and even cell loss. We studied one possible mechanism proposed to trigger such events, i.e. excess glutamate release from severed axons conveyed transsynaptically via postsynaptic calcium influx. Using 2-photon microscopical calcium imaging in organotypic entorhino-hippocampal co-cultures, we show that acute transection of the perforant path elicits two independent effects on calcium homeostasis in the dentate gyrus: a brief, short-latency elevation of postsynaptic calcium levels in denervated granule cells, which can be blocked by preincubation with tetrodotoxin, and a long-latency astroglial calcium wave, not blocked by tetrodotoxin and propagating slowly through the hippocampus. While neuronal calcium elevations upon axonal transection placed remote from the target area were similar to those elicited by brief trains of electrical stimulation of the perforant path, large-scale calcium signals were observed upon lesions placed close to or within the dendritic field of granule cells. Concordantly, induction of c-fos in denervated neurons coincided spatially with cell populations showing prolonged calcium elevations upon concomitant dendritic damage. Since denervation of dentate granule cells by remote transection of the perforant path induces transsynaptic dendritic reorganization in the utilized organotypic cultures, a generalized breakdown of the cellular calcium homeostasis is unlikely to underlie these transneuronal changes.

Extending the bandwidth of long‐term plasticity at the cerebellar input stage

Extending the bandwidth of long‐term plasticity at the cerebellar input stage

NMDAR-mediated calcium transients elicited by glutamate co-release at developing inhibitory synapses

Before hearing onset, the topographic organization of the inhibitory sound localization pathway from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO) is refined by means of synaptic silencing and strengthening. During this refinement period MNTB-LSO synapses not only release GABA and glycine but also release glutamate. This co-released glutamate can elicit postsynaptic currents that are predominantly mediated by NMDA receptors (NMDARs). To gain a better understanding of how glutamate contributes to synaptic signaling at developing MNTB-LSO inhibitory synapses, we investigated to what degree and under what conditions NMDARs contribute to postsynaptic calcium responses. Our results demonstrate that MNTB-LSO synapses can elicit compartmentalized calcium responses along aspiny LSO dendrites. These responses are significantly attenuated by the NMDAR antagonist APV. APV, however, had no effect on somatically recorded electrical postsynaptic responses, indicating little, if any, contribution of NMDARs to spike generation. NMDAR-mediated calcium responses were decreased when increasing extracellular magnesium concentrations to physiological levels indicating that MNTB-LSO synapses activate magnesium sensitive NMDAR on immature LSO dendrites. In Fura-2 AM loaded neurons, blocking GABAA and glycine receptors increased NMDAR contribution to somatic calcium responses suggesting that GABA and glycine, perhaps by shunting backpropagating action potentials, decrease the level of NMDAR activation under strong stimulus conditions.

A Model of Spike-Timing-Dependent Plasticity

It has been shown that postsynaptic calcium dynamics plays an important role in spike timing-dependent plasticity (STDP), a process in which changes in synaptic strength are determined by the relative timing of pre- and postsynaptic activity. It has been suggested STDP involves a postsynaptic chemical network with stable states corresponding to long term potentiation (LTP) and long term depression (LTD). It is believed that the switching of this network between these states is driven by the postsynaptic Ca2+ concentration, but the manner in which the Ca2+ dynamics causes the switching to depend on the relative timing of pre- and postsynaptic activity remains unclear. We describe a model of STDP that combines the chemical network model of Pi and Lisman (2008), with a model of Ca2+ dynamics that builds on the work of Shouval and coworkers (2002). Following Shouval and coworkers, we assume that a portion of the influx of postsynaptic Ca2+ is controlled by NMDA receptors that allow an inward Ca2+ current in response to both glutamate binding and a back propagating action potential (BPAP). To this we add a contribution from voltage dependent calcium channels (VDCCs). We show that this model is able to reproduce the observed time dependence of STDP when a single presynaptic pulse is either followed or preceded by a single BPAP. The behavior of the model with triplet pulse protocols, e.g., two presynaptic pulses separated by a BPAP, or two BPAPs separated by a presynaptic pulse, and the incorporation of more sophisticated models of AMPA trafficking are also described.Pi, H. J. and Lisman, J. E. (2008) J. Neurosci 28: 13132-13138.Shouval, H. Z., Bear, M. F., and Cooper, L. N. (2002) Proc. Natl. Acad. Sci. USA 99: 10831-10836.

Modeling spike timing dependent plasticity with a retrograde transmitter

Introduction Spike timing dependent plasticity (STDP) is a special form of plasticity where the timing of preand postsynaptic events determine if the synapse undergoes LTP or LTD. A popular way to model STDP is to use the derivative of the postsynaptic potential or Calcium concentration and correlate this with the NMDA activation [1]. This approach leads automatically to the well-known biphasic STDP curve. Common to all models is that at the end the actual computation is performed postsynaptically for both LTP and LTD.

The regulatory role of NO-PKG in the cerebellar long-term depression

Introduction Long-term depression (LTD) is a persistent decrease in the efficacy of synaptic transmission that results from the removal of AMPA receptors (AMPARs) from the postsynaptic cellular membrane [1]. In Purkinje cells, LTD can be induced by increasing the postsynaptic calcium concentration ([Ca2+]) using flash photolysis of caged Ca2+ [2], which indicates that other second messengers are not fundamental to the occurrence of LTD. However, recent experimental data pointed out that nitric oxide (NO)cGMP-dependent protein kinase (PKG) pathway can act upstream of Ca2+-signals to regulate Ca2+-induced LTD To gain insights on the biochemical mechanisms involved in this process, we built a computational model to simulate the cerebellar LTD.

Plasticity of neuronal excitability in vivo

Plasticity of neuronal excitability <i>in vivo</i>

Neuroendocrine Proopiomelanocortin Neurons Are Excited by Hypocretin/Orexin

Hypocretin/orexin, produced by a group of neurons in the lateral hypothalamus/perifornical area, enhances cognitive arousal and also may play a crucial role in modulating the neuroendocrine system. How hypocretin modulates the endocrine system remains an open question. Hypocretin cells innervate the mediobasal hypothalamus where they can potentially influence the activity of specific cell populations within the arcuate nucleus. Here, we examine whether hypocretin modulates the median eminence-projecting proopiomelanocortin (POMC) neurons identified by selective green fluorescent protein expression and antidromic stimulation or retrograde Evans blue dye tracing in transgenic mice. We find that POMC neurons, in general, and, in addition, those that project their axons to the median eminence, were robustly activated by hypocretin in a dose-dependent manner. These excitatory actions included a threefold increase in spike frequency and direct membrane depolarization of up to 22 mV (mean, 17.9+/-7.2 mV). Direct postsynaptic depolarization was decreased at more positive membrane potentials, inhibited by the sodium-calcium exchanger antagonist KB-R7943, and reduced by lowering the bath temperature, or by buffering the postsynaptic calcium with BAPTA, suggesting that the primary mechanism for hypocretin-mediated excitation is the activation of the sodium-calcium exchanger. Hypocretin also enhanced excitatory inputs to POMC cells via a presynaptic mechanism and indirectly increased the release of GABA onto these cells in a spike-dependent manner. However, these synaptic actions were not necessary to cause postsynaptic membrane depolarization and spiking. Thus, in contrast to previous suggestions that hypocretin inhibited POMC cells, our results demonstrate robust direct excitation of POMC neurons by hypocretin.

Increased Expression of Ryanodine Receptors and the Iron Transporter DMT1 in Hippocampal Neurons by Brain Derived Neurotrophic Factor (BDNF), NMDA or Spatial Memory Training

Ryanodine receptors (RyR) mediate skeletal and cardiac muscle contraction and amplify via CICR postsynaptic calcium signals generated by activity-dependent calcium influx through NMDA receptors (NMDAR) in hippocampal glutamatergic synapses. We have recently shown that reactive oxygen species (ROS) and iron, which as shown here promotes ROS generation in neurons, stimulate RyR-mediated calcium release when added to primary hippocampal neurons. Here, we report that 5 min incubation of hippocampal cells in primary culture with NMDA (50 μM), induced RyR-mediated calcium signals that were inhibited by pre-incubation with the iron chelator desferroxamine. Incubation with NMDA also enhanced >2-fold the expression (measured 24 h later) of the iron transporter DMT1 (IRE form), while incubation with BDNF (50 ng/ml) increased >5-fold RyR expression. Additionally, we investigated if spatial memory training of male rats in a Morris water maze affected RyR and DMT1 expression. The hippocampus was dissected 6 h after the last behavioral task (5d, 2d rest, 1d platform free) and samples from tissue were prepared for Western blot and RT-PCR experiments. We found that spatial memory training increased the mRNA and protein expression of DMT1, RyR2 and RyR3. Our results confirm enhanced RyR2 expression following spatial memory training and correlate for the first time enhanced in vivo expression of the iron transporter DMT1 and RyR3 with spatial memory acquisition/consolidation. We propose that iron-induced ROS production stimulates the emergence of RyR-mediated intracellular calcium signals that promote RyR and DMT1 expression during the spatial memory task.FONDECYT (PostDoc) 3070035, CEMC-FONDAP 15010006, Millennium P05-001F, FONDECYT 1060177.