Optical induction of plasticity at single synapses reveals input-specific accumulation of alphaCaMKII

Long-term potentiation (LTP), a form of synaptic plasticity, is a primary experimental model for understanding learning and memory formation. Here, we use light-activated channelrhodopsin-2 (ChR2) as a tool to study the molecular events that occur in dendritic spines of CA1 pyramidal cells during LTP induction. Two-photon uncaging of MNI-glutamate allowed us to selectively activate excitatory synapses on optically identified spines while ChR2 provided independent control of postsynaptic depolarization by blue light. Pairing of these optical stimuli induced lasting increase of spine volume and triggered translocation of αCaMKII to the stimulated spines. No changes in αCaMKII concentration or cytoplasmic volume were observed in neighboring spines on the same dendrite, providing evidence that αCaMKII accumulation at postsynaptic sites is a synapse-specific memory trace of coincident activity.     

Astrocyte-secreted IL-33 mediates homeostatic synaptic plasticity in the adult hippocampus

Hippocampal synaptic plasticity is important for learning and memory formation. Homeostatic synaptic plasticity is a specific form of synaptic plasticity that is induced upon prolonged changes in neuronal activity to maintain network homeostasis. While astrocytes are important regulators of synaptic transmission and plasticity, it is largely unclear how they interact with neurons to regulate synaptic plasticity at the circuit level. Here, we show that neuronal activity blockade selectively increases the expression and secretion of IL-33 (interleukin-33) by astrocytes in the hippocampal cornu ammonis 1 (CA1) subregion. This IL-33 stimulates an increase in excitatory synapses and neurotransmission through the activation of neuronal IL-33 receptor complex and synaptic recruitment of the scaffold protein PSD-95. We found that acute administration of tetrodotoxin in hippocampal slices or inhibition of hippocampal CA1 excitatory neurons by optogenetic manipulation increases IL-33 expression in CA1 astrocytes. Furthermore, IL-33 administration in vivo promotes the formation of functional excitatory synapses in hippocampal CA1 neurons, whereas conditional knockout of IL-33 in CA1 astrocytes decreases the number of excitatory synapses therein. Importantly, blockade of IL-33 and its receptor signaling in vivo by intracerebroventricular administration of its decoy receptor inhibits homeostatic synaptic plasticity in CA1 pyramidal neurons and impairs spatial memory formation in mice. These results collectively reveal an important role of astrocytic IL-33 in mediating the negative-feedback signaling mechanism in homeostatic synaptic plasticity, providing insights into how astrocytes maintain hippocampal network homeostasis.

Synaptic plasticity, the ability of neurons to alter the structure and strength of synapses, is important for the refinement of neuronal circuits in response to sensory experience during development (1) as well as learning and memory formation in adults (2, 3). To maintain the stability of neuronal network activity, the synaptic strength of neurons is modified through a negative-feedback mechanism termed homeostatic synaptic plasticity (4–6). Specifically, inhibiting neuronal activity in cultured neuronal cells or hippocampal slices by pharmacological administration of the sodium channel blocker tetrodotoxin (TTX) increases the strength of excitatory synapses to rebalance network activity (5–7).The hippocampus, which comprises the cornu ammonis 1 (CA1), CA2, CA3, and dentate gyrus subregions, is important for memory storage and retrieval. In particular, the CA1 subregion constitutes the primary output of the hippocampus, which is thought to be essential for most hippocampus-dependent memories (8, 9). Moreover, experience-driven synaptic changes in the CA1 microcircuitry impact how information is integrated (10, 11). Accordingly, the induction and expression of synaptic plasticity at hippocampal CA1 excitatory synapses are critically dependent on the structural remodeling and composition of synapses as well as functional modifications of pre- and postsynaptic proteins and neurotransmitter receptors (4–6, 12). As such, structural plasticity is a major regulatory mechanism of homeostatic synaptic plasticity in the hippocampal CA1 region. While most excitatory synapses are located at dendritic spines, morphological changes of dendritic spines likely participate in compensatory adaptations of hippocampal network activity and are therefore involved in learning, memory formation (13), and memory extinction (14).The efficacy of synaptic transmission and the wiring of neuronal circuitry are regulated not only by bidirectional communication between pre- and postsynaptic neurons, but also through the interactions between neurons and their associated glial cells (15–17). Astrocytes, as the most abundant type of glia in the central nervous system, actively regulate synapse formation, function, and maintenance during development and in the adult brain (18–20). However, the molecular basis of astrocyte–neuron communication in synaptic plasticity is largely unknown. Nevertheless, one of the mechanisms by which astrocytes regulate synapses is by secreting factors (21–25); the most well-characterized one is TNFα. Notably, pharmacologically induced deprivation of neuronal activity increases TNFα release from astrocytes, which modulates homeostatic plasticity in both excitatory and inhibitory neurons through regulation of neuronal glutamate and GABA receptor trafficking (24, 26). Further in vivo studies on germline knockout mice support the roles of astrocyte-secreted TNFα in homeostatic adaptations of cortical circuitry during sensory deprivation (27, 28). Another cytokine interleukin-33 (IL-33) is secreted by astrocytes to regulate synapse development in spinal cord and thalamus (29). Nevertheless, it remains largely unknown how astrocytes respond to changes in neuronal activity to regulate homeostatic synaptic plasticity in the hippocampus as well as learning and memory formation.In this study, we identified IL-33 as an astrocyte-secreted factor which mediates homeostatic synaptic plasticity in the CA1 subregion of adult hippocampus. Pharmacological blockade of neuronal activity or in vivo optogenetic inhibition of CA1 pyramidal neurons stimulates a local increase in the expression and release of IL-33 from the astrocytes. In turn, this astrocyte-secreted IL-33 and its ST2/IL-1RAcP receptor complex mediate the increase of excitatory synapses and neurotransmission in homeostatic synaptic plasticity. Two-photon imaging of CA1 pyramidal neurons in vivo reveals that IL-33 promotes dendritic spine formation through the synaptic recruitment of postsynaptic scaffolding protein PSD-95. Importantly, conditional knockout of IL-33 in astrocytes decreases excitatory synapses in the CA1 subregion, and inhibition of IL-33/ST2 signaling in adult mice abolishes the homeostatic synaptic plasticity in CA1 pyramidal neurons, resulting in impaired spatial memory formation. Hence, our findings collectively show that astrocyte-secreted IL-33 plays an important role in homeostatic synaptic plasticity in the adult hippocampus and spatial memory formation.     

Impaired long-term potentiation induction in dentate gyrus of calretinin-deficient mice

Calretinin (Cr) is a Ca²⁺ binding protein present in various populations of neurons distributed in the central and peripheral nervous systems. We have generated Cr-deficient (Cr^−/−) mice by gene targeting and have investigated the associated phenotype. Cr^−/− mice were viable, and a large number of morphological, biochemical, and behavioral parameters were found unaffected. In the normal mouse hippocampus, Cr is expressed in a widely distributed subset of GABAergic interneurons and in hilar mossy cells of the dentate gyrus. Because both types of cells are part of local pathways innervating dentate granule cells and/or pyramidal neurons, we have explored in Cr^−/− mice the synaptic transmission between the perforant pathway and granule cells and at the Schaffer commissural input to CA1 pyramidal neurons. Cr^−/− mice showed no alteration in basal synaptic transmission, but long-term potentiation (LTP) was impaired in the dentate gyrus. Normal LTP could be restored in the presence of the GABA_A receptor antagonist bicuculline, suggesting that in Cr^−/− dentate gyrus an excess of γ-aminobutyric acid (GABA) release interferes with LTP induction. Synaptic transmission and LTP were normal in CA1 area, which contains only few Cr-positive GABAergic interneurons. Cr^−/− mice performed normally in spatial memory task. These results suggest that expression of Cr contributes to the control of synaptic plasticity in mouse dentate gyrus by indirectly regulating the activity of GABAergic interneurons, and that Cr^−/− mice represent a useful tool to understand the role of dentate LTP in learning and memory.     

Left–right dissociation of hippocampal memory processes in mice

Left–right asymmetries have likely evolved to make optimal use of bilaterian nervous systems; however, little is known about the synaptic and circuit mechanisms that support divergence of function between equivalent structures in each hemisphere. Here we examined whether lateralized hippocampal memory processing is present in mice, where hemispheric asymmetry at the CA3–CA1 pyramidal neuron synapse has recently been demonstrated, with different spine morphology, glutamate receptor content, and synaptic plasticity, depending on whether afferents originate in the left or right CA3. To address this question, we used optogenetics to acutely silence CA3 pyramidal neurons in either the left or right dorsal hippocampus while mice performed hippocampus-dependent memory tasks. We found that unilateral silencing of either the left or right CA3 was sufficient to impair short-term memory. However, a striking asymmetry emerged in long-term memory, wherein only left CA3 silencing impaired performance on an associative spatial long-term memory task, whereas right CA3 silencing had no effect. To explore whether synaptic properties intrinsic to the hippocampus might contribute to this left–right behavioral asymmetry, we investigated the expression of hippocampal long-term potentiation. Following the induction of long-term potentiation by high-frequency electrical stimulation, synapses between CA3 and CA1 pyramidal neurons were strengthened only when presynaptic input originated in the left CA3, confirming an asymmetry in synaptic properties. The dissociation of hippocampal long-term memory function between hemispheres suggests that memory is routed via distinct left–right pathways within the mouse hippocampus, and provides a promising approach to help elucidate the synaptic basis of long-term memory.Unilateral specializations may facilitate greater processing power in bilateral brain structures by using the available neuronal circuitry more effectively. Nevertheless, the nature of the mechanisms that can act within the confines of duplicate neural structures to support different cognitive functions in each hemisphere remains elusive.The hippocampus is essential for certain forms of learning and memory, both in humans (1) and in rodents (2, 3), and also plays an important role in navigation (4). The left and right mammalian hippocampi comprise the same anatomical areas and directional connectivity, and yet in the human hippocampus, task-related activity may be localized to only one hemisphere (5). This lateralization may enable the left and right hippocampus to support complementary functions in human episodic memory, with left hippocampal activity associated with an egocentric, sequential representation of space but greater activity in the right hippocampus when an allocentric representation is used (6). It has been suggested that human hippocampal asymmetry is primarily dictated by external asymmetry—namely, the left hemispheric involvement in language processing and the stronger contribution of the right hemisphere to visuospatial attention (7), supported by observations of left hippocampal dominance when semantic information is most task-relevant, compared with right hippocampal dominance when spatial information becomes more pertinent (8). However, a seminal discovery in the mouse brain suggests that left–right asymmetry may actually be a fundamental property of the mammalian hippocampus: it was found that the postsynaptic spine morphology and receptor distribution in CA1 pyramidal neurons is determined by whether the presynaptic input originates in the left or right CA3 (9, 10). Specifically, apical CA1 postsynaptic spines receiving input from the left CA3 are primarily thin and rich in GluN2B subunit-containing NMDA receptors (NMDARs); in contrast, there is a higher proportion of mushroom-shaped spines receiving right CA3 projection, and these larger spines have a lower density of GluN2B subunit-containing NMDARs (9, 10). Interestingly, synaptic plasticity also shows hemispheric asymmetry: irrespective of the hemispheric location of the CA1 neuron, GluN2B NMDAR-requiring spike timing-dependent long-term potentiation (LTP) was induced at synapses where presynaptic input originates in the left CA3, but not in the right CA3 (11).These left–right synaptic differences raise the question as to whether memory processing in mice, as in humans, might differ between the left and right hippocampus. Therefore, in this study, we asked whether acutely inactivating one part of the asymmetric CA3–CA1 network unilaterally would affect learning and memory differentially between hemispheres. To test this, we silenced excitatory cells of CA3 in either the left or the right hippocampus, and consequently also both their ipsilateral and contralateral projections to CA1, using the light-sensitive chloride pump halorhodopsin (eNpHR3.0) coexpressed with enhanced YFP (eYFP) (12).     

Developmental switch in the kinase dependency of long-term potentiation depends on expression of GluA4 subunit-containing AMPA receptors

The AMPA-receptor subunit GluA4 is expressed transiently in CA1 pyramidal neurons at the time synaptic connectivity is forming, but its physiological significance is unknown. Here we show that GluA4 expression is sufficient to alter the signaling requirements of long-term potentiation (LTP) and can fully explain the switch in the LTP kinase dependency from PKA to Ca2⁺/calmodulin-dependent protein kinase II during synapse maturation. At immature synapses, activation of PKA leads to a robust potentiation of AMPA-receptor function via the mobilization of GluA4. Analysis of GluA4-deficient mice indicates that this mechanism is critical for neonatal PKA-dependent LTP. Furthermore, lentiviral expression of GluA4 in CA1 neurons conferred a PKA-dependent synaptic potentiation and LTP regardless of the developmental stage. Thus, GluA4 defines the signaling requirements for LTP and silent synapse activation during a critical period of synapse development.Activity-dependent plasticity at immature glutamatergic synapses is thought to underlie fine tuning of the synaptic circuitry and optimize the network for its adult functions. The synaptic mechanisms of plasticity at immature contacts differ from those in the adult because of developmental alterations in the expression of several molecules that are critical in mediating and modulating synaptic transmission. For example, in area CA1 of the hippocampus, the signaling cascades necessary for long-term potentiation (LTP) are altered during the first weeks of postnatal life, corresponding to the time of formation and maturation of glutamatergic synapses. In the neonate, LTP is dependent mainly on the activation of PKA, but later in development LTP requires the activation of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) together with other kinases (1, 2). In parallel, expression of the AMPA-receptor subunit GluA4 in the hippocampal pyramidal neurons is strongly down-regulated and replaced by other subunits, including GluA1 (3, 4).Both GluA4 and GluA1 and a splice variant of GluA2, GluA2_L, contain a long intracellular C-terminal domain (CTD) that is thought to be involved in activity-dependent synaptic incorporation of AMPA receptors (5–8, but also see ref. 9). Spontaneous synaptic activity and consequent activity-dependent PKA phosphorylation is sufficient to drive recombinant GluA4, but not GluA1, into synapses (4, 10), suggesting that the switch in the subunit composition of AMPA receptors may explain some of the developmental changes in the mechanisms of LTP. However, the exact role of the developmentally restricted expression of GluA4 in synaptic transmission and plasticity remains unknown.Here we show that GluA4 expression is sufficient to alter the signaling mechanisms underlying LTP and to confer PKA-dependent postsynaptic potentiation. Thus, the expression of GluA4 can explain fully the developmental switch in the LTP kinase dependency in CA1 pyramidal neurons.     

AMPA receptors are exocytosed in stimulated spines and adjacent dendrites in a Ras-ERK–dependent manner during long-term potentiation

The exocytosis of AMPA receptors is a key step in long-term potentiation (LTP), yet the timing and location of exocytosis and the signaling pathways involved in exocytosis during synaptic plasticity are not fully understood. Here we combine two-photon uncaging with two-photon imaging of a fluorescent label of surface AMPA receptors to monitor individual AMPA receptor exocytosis events near spines undergoing LTP. AMPA receptors that reached the stimulated spine came from a combination of preexisting surface receptors (70–90%) and newly exocytosed receptors (10–30%). We observed exocytosis in both the dendrite and spine under basal conditions. The rate of AMPA receptor exocytosis increased ∼5-fold during LTP induction and decayed to the basal level within ∼1 min, both in the stimulated spine and in the dendrite within ∼3 μm of the stimulated spine. AMPA receptors inserted in the spine were trapped in the spine in an activity-dependent manner. The activity-dependent exocytosis required the Ras-ERK pathway, but not CaMKII. Thus, diffusive Ras-ERK signaling presumably serves as an important means for signaling from synapses to dendritic shafts to recruit AMPA receptors into synapses during LTP.     

Rapid enhancement of two-step wiring plasticity by estrogen and NMDA receptor activity

Cortical information storage requires combined changes in connectivity and synaptic strength between neurons, but the signaling mechanisms underlying this two-step wiring plasticity are unknown. Because acute 17β-estradiol (E2) modulates cortical memory, we examined its effects on spine morphogenesis, AMPA receptor trafficking, and GTPase signaling in cortical neurons. Acute E2 application resulted in a rapid, transient increase in spine density, accompanied by temporary formation of silent synapses through reduced surface GluR1. These rapid effects of E2 were dependent on a Rap/AF-6/ERK1/2 pathway. Intriguingly, NMDA receptor (NMDAR) activation after E2 treatment potentiated silent synapses and elevated spine density for as long as 24 h. Hence, we show that E2 transiently increases neuronal connectivity by inducing dynamic nascent spines that “sample” the surrounding neuropil and that subsequent NMDAR activity is sufficient to stabilize or “hold” E2-mediated effects. This work describes a form of two-step wiring plasticity relevant for cortical memory and identifies targets that may facilitate recovery from brain injuries.     

cAMP/PKA signaling and RIM1alpha mediate presynaptic LTP in the lateral amygdala

NMDA receptor-dependent long-term potentiation (LTP) of glutamatergic synaptic transmission in sensory pathways from auditory thalamus or cortex to the lateral amygdala (LA) underlies the acquisition of auditory fear conditioning. Whereas the mechanisms of postsynaptic LTP at thalamo–LA synapses are well understood, much less is known about the sequence of events mediating presynaptic NMDA receptor-dependent LTP at cortico–LA synapses. Here, we show that presynaptic cortico–LA LTP can be entirely accounted for by a persistent increase in the vesicular release probability. At the molecular level, we found that signaling via the cAMP/PKA pathway is necessary and sufficient for LTP induction. Moreover, by using mice lacking the active-zone protein and PKA target RIM1α (RIM1α^−/−), we demonstrate that RIM1α is required for both chemically and synaptically induced presynaptic LTP. Further analysis of cortico–LA synaptic transmission in RIM1α^−/− mice revealed a deficit in Ca²⁺-release coupling leading to a lower baseline release probability. Our results reveal the molecular mechanisms underlying the induction of presynaptic LTP at cortico–LA synapses and indicate that RIM1α-dependent LTP may involve changes in Ca²⁺-release coupling.     

ErbB4 in parvalbumin-positive interneurons is critical for neuregulin 1 regulation of long-term potentiation

Neuregulin 1 (NRG1) is a trophic factor that acts by stimulating ErbB receptor tyrosine kinases and has been implicated in neural development and synaptic plasticity. In this study, we investigated mechanisms of its suppression of long-term potentiation (LTP) in the hippocampus. We found that NRG1 did not alter glutamatergic transmission at SC-CA1 synapses but increased the GABA(A) receptor-mediated synaptic currents in CA1 pyramidal cells via a presynaptic mechanism. Inhibition of GABA(A) receptors blocked the suppressing effect of NRG1 on LTP and prevented ecto-ErbB4 from enhancing LTP, implicating a role of GABAergic transmission. To test this hypothesis further, we generated parvalbumin (PV)-Cre;ErbB4(-/-) mice in which ErbB4, an NRG1 receptor in the brain, is ablated specifically in PV-positive interneurons. NRG1 was no longer able to increase inhibitory postsynaptic currents and to suppress LTP in PV-Cre;ErbB4(-/-) hippocampus. Accordingly, contextual fear conditioning, a hippocampus-dependent test, was impaired in PV-Cre;ErbB4(-/-) mice. In contrast, ablation of ErbB4 in pyramidal neurons had no effect on NRG1 regulation of hippocampal LTP or contextual fear conditioning. These results demonstrate a critical role of ErbB4 in PV-positive interneurons but not in pyramidal neurons in synaptic plasticity and support a working model that NRG1 suppresses LTP by enhancing GABA release. Considering that NRG1 and ErbB4 are susceptibility genes of schizophrenia, these observations contribute to a better understanding of how abnormal NRG1/ErbB4 signaling may be involved in the pathogenesis of schizophrenia.     

Involvement of Notch signaling in hippocampal synaptic plasticity

During development of the nervous system, the fate of stem cells is regulated by a cell surface receptor called Notch. Notch is also present in the adult mammalian brain; however, because Notch null mice die during embryonic development, it has proven difficult to determine the functions of Notch. Here, we used Notch antisense transgenic mice that develop and reproduce normally, but exhibit reduced levels of Notch, to demonstrate a role for Notch signaling in synaptic plasticity. Mice with reduced Notch levels exhibit impaired long-term potentiation (LTP) at hippocampal CA1 synapses. A Notch ligand enhances LTP in normal mice and corrects the defect in LTP in Notch antisense transgenic mice. Levels of basal and stimulation-induced NF-kappa B activity were significantly decreased in mice with reduced Notch levels. These findings suggest an important role for Notch signaling in a form of synaptic plasticity known to be associated with learning and memory processes.     

Very short-term plasticity in hippocampal synapses

Hippocampal pyramidal neurons often fire in bursts of action potentials with short interspike intervals (2–10 msec). These high-frequency bursts may play a critical role in the functional behavior of hippocampal neurons, but synaptic plasticity at such short times has not been carefully studied. To study synaptic modulation at very short time intervals, we applied pairs of stimuli with interpulse intervals ranging from 7 to 50 msec to CA1 synapses isolated by the method of minimal stimulation in hippocampal slices. We have identified three components of short-term paired-pulse modulation, including (i) a form of synaptic depression manifested after a prior exocytotic event, (ii) a form of synaptic depression that does not depend on a prior exocytotic event and that we postulate is based on inactivation of presynaptic N-type Ca²⁺ channels, and (iii) a dependence of paired-pulse facilitation on the exocytotic history of the synapse.     

Mossy fiber-evoked subthreshold responses induce timing-dependent plasticity at hippocampal CA3 recurrent synapses

Dentate granule cells exhibit exceptionally low levels of activity and rarely elicit action potentials in targeted CA3 pyramidal cells. It is thus unclear how such weak input from the granule cells sustains adequate levels of synaptic plasticity in the targeted CA3 network. We report that subthreshold potentials evoked by mossy fibers are sufficient to induce synaptic plasticity between CA3 pyramidal cells, thereby complementing the sparse action potential discharge. Repetitive pairing of a CA3–CA3 recurrent synaptic response with a subsequent subthreshold mossy fiber response induced long-term potentiation at CA3 recurrent synapses in rat hippocampus in vitro. Reversing the timing of the inputs induced long-term depression. The underlying mechanism depends on a passively conducted giant excitatory postsynaptic potential evoked by a mossy fiber that enhances NMDA receptor-mediated current at active CA3 recurrent synapses by relieving magnesium block. The resulting NMDA spike generates a supralinear depolarization that contributes to synaptic plasticity in hippocampal neuronal ensembles implicated in memory.The CA3 area of the hippocampus exhibits a distinctive, highly recurrent circuitry proposed to support autoassociative memory representation (1, 2). This prediction has been confirmed by experimental work demonstrating the pattern completion capabilities of CA3 networks (3), as well as their roles in the spatial tuning of CA1 pyramidal cells, in one-trial contextual learning (4) and in certain forms of memory consolidation (5). CA3 pyramidal cells receive, via the mossy fibers, information processed by granule cells important for both pattern separation (6, 7) and pattern completion functions (7). The faithful transmission of mossy fiber input appears to be ensured by giant synapses composed of presynaptic boutons with up to 45 release sites (8) that target massive spines, the thorny excrescences, on the apical dendrite of CA3 pyramidal cells. Thus, the mossy fiber synapse is often referred to as a detonator synapse (9). In fact, mossy fiber signaling is more compatible with a gatekeeper function than a high-throughput data relay. Although high-frequency bursts of action potentials in a hippocampal granule cell can discharge a targeted CA3 pyramidal cell, the majority of responses evoked by granule cells in CA3 pyramidal cells do not attain the firing threshold (10). Nevertheless, mossy fibers generate powerful signals evoking subthreshold responses that are much larger than typical synaptic events in the brain, with excitatory postsynaptic potentials (EPSPs) and excitatory postsynaptic currents (EPSCs) reaching amplitudes of 10 mV and 1 nA, respectively (11). Here we examined in rat slice cultures how EPSPs generated at mossy fiber synapses are processed in CA3 pyramidal cell dendrites, and evaluated whether subthreshold synaptic responses evoked by mossy fiber stimulation can act as instructive signals to induce plasticity at the pyramidal cell synapses forming the CA3 recurrent network.     

PirB regulates a structural substrate for cortical plasticity

Experience-driven circuit changes underlie learning and memory. Monocular deprivation (MD) engages synaptic mechanisms of ocular dominance (OD) plasticity and generates robust increases in dendritic spine density on L5 pyramidal neurons. Here we show that the paired immunoglobulin-like receptor B (PirB) negatively regulates spine density, as well as the threshold for adult OD plasticity. In PirB^−/− mice, spine density and stability are significantly greater than WT, associated with higher-frequency miniature synaptic currents, larger long-term potentiation, and deficient long-term depression. Although MD generates the expected increase in spine density in WT, in PirB^−/− this increase is occluded. In adult PirB^−/−, OD plasticity is larger and more rapid than in WT, consistent with the maintenance of elevated spine density. Thus, PirB normally regulates spine and excitatory synapse density and consequently the threshold for new learning throughout life.Experience generates both functional and structural changes in neural circuits. The learning process is robust at younger ages during developmental critical periods and continues, albeit at a lower level, into adulthood and old age (1–3). For example, young barn owls exposed to horizontally shifting prismatic spectacles can adapt readily to altered visual input, but adult owls cannot. The experience in the young owls results in a rearranged audiovisual map in tectum that is accompanied by ectopic axonal projections (1). Experience-dependent structural changes have also been observed in the mammalian cerebral cortex. Enriched sensory experience or motor learning are both associated with an increase in dendritic spine density, and a morphological shift from immature thin spines to mushroom spines which harbor larger postsynaptic densities (PSDs) and stronger synapses (4–7). On the flip side, bilateral sensory deprivation induces spine loss (8, 9). Abnormal sensory experience also results in structural modification of inhibitory synapses and circuitry that is temporally and spatially coordinated with changes in excitatory synapses on dendritic spines (10–13).These experience-driven spine changes are thought to involve synaptic mechanisms of long-term potentiation (LTP) and long-term depression (LTD). In hippocampal slices, induction of LTP causes new spines to emerge, as well as spine head enlargement on existing spines (14–16); induction of LTD results in rapid spine regression (14, 17). Importantly, the emergence or regression of spines starts soon after the induction of LTP or LTD, suggesting that these structural changes underlie the persistent expression of long-term plasticity (14, 17).Little is known about molecular mechanisms that restrict experience-dependent plasticity at circuit and synaptic levels and connect it to spine stability. Paired Ig-like receptor B (PirB), a receptor expressed in cortical pyramidal neurons, is known to limit ocular dominance (OD) plasticity both during the critical period and in adulthood (18). PirB binds major histocompatibility class I (MHCI) ligands, whose expression is regulated by visual experience and neural activity (19–21) and thus could act as a key link connecting functional to structural plasticity. If so, mice lacking PirB might be expected to have altered synaptic plasticity rules on the one hand and changes in the density and stability of dendritic spines on the other.     

Plastic modifications induced by object recognition memory processing

Long-term potentiation (LTP) phenomenon is widely accepted as a cellular model of memory consolidation. Object recognition (OR) is a particularly useful way of studying declarative memory in rodents because it makes use of their innate preference for novel over familiar objects. In this study, mice had electrodes implanted in the hippocampal Schaffer collaterals–pyramidal CA1 pathway and were trained for OR. Field EPSPs evoked at the CA3-CA1 synapse were recorded at the moment of training and at different times thereafter. LTP-like synaptic enhancement was found 6 h posttraining. A testing session was conducted 24 h after training, in the presence of one familiar and one novel object. Hippocampal synaptic facilitation was observed during exploration of familiar and novel objects. A short depotentiation period was observed early after the test and was followed by a later phase of synaptic efficacy enhancement. Here, we show that OR memory consolidation is accompanied by transient potentiation in the hippocampal CA3-CA1 synapses, while reconsolidation of this memory requires a short-lasting phase of depotentiation that could account for its well described vulnerability. The late synaptic enhancement phase, on the other hand, would be a consequence of memory restabilization.     

The brain-derived neurotrophic factor enhances synthesis of Arc in synaptoneurosomes

Protein synthesis in neurons is essential for the consolidation of memory and for the stabilization of activity-dependent forms of synaptic plasticity such as long-term potentiation (LTP). Activity-dependent translation of dendritically localized mRNAs has been proposed to be a critical source of new proteins necessary for synaptic change. mRNA for the activity-regulated cytoskeletal protein, Arc, is transcribed during LTP and learning, and disruption of its translation gives rise to deficits in both. We have found that selective translation of Arc in a synaptoneurosomal preparation is induced by the brain-derived neurotrophic factor, a neurotrophin that is released during high-frequency stimulation patterns used to elicit LTP. This effect involves signaling through the TrkB receptor and is blocked by the N-methyl-d-aspartate-type glutamate receptor antagonist, MK801. The results suggest there is a synergy between neurotrophic and ionotropic mechanisms that may influence the specificity and duration of changes in synaptic efficacy at glutamatergic synapses.     

Kalirin and Trio proteins serve critical roles in excitatory synaptic transmission and LTP

The molecular mechanism underlying long-term potentiation (LTP) is critical for understanding learning and memory. CaMKII, a key kinase involved in LTP, is both necessary and sufficient for LTP induction. However, how CaMKII gives rise to LTP is currently unknown. Recent studies suggest that Rho GTPases are necessary for LTP. Rho GTPases are activated by Rho guanine exchange factors (RhoGEFs), but the RhoGEF(s) required for LTP also remain unknown. Here, using a combination of molecular, electrophysiological, and imaging techniques, we show that the RhoGEF Kalirin and its paralog Trio play critical and redundant roles in excitatory synapse structure and function. Furthermore, we show that CaMKII phosphorylation of Kalirin is sufficient to enhance synaptic AMPA receptor expression, and that preventing CaMKII signaling through Kalirin and Trio prevents LTP induction. Thus, our data identify Kalirin and Trio as the elusive targets of CaMKII phosphorylation responsible for AMPA receptor up-regulation during LTP.One of the most remarkable properties of the brain is its ability to store vast amounts of information. It is now widely accepted that this storage involves the rapid enhancement of synaptic strength, which can persist for prolonged periods. This phenomenon, known as long-term potentiation (LTP), has been observed at numerous glutamatergic excitatory synapses throughout the brain. At hippocampal CA1 synapses, LTP is expressed as a rapid increase in the number of postsynaptic AMPA-type glutamate receptors (AMPARs) following the coincident activation of presynaptic and postsynaptic neurons (1–4). This form of LTP is dependent on the activation of NMDA-type glutamate receptors (NMDARs), which transiently elevate spine calcium. This calcium influx activates calcium-calmodulin–dependent protein kinase II (CaMKII). Although CaMKII activation has been shown to be both necessary and sufficient for LTP (5), the critical downstream targets of CaMKII have yet to be identified.One possible target of CaMKII during LTP is the family of neuronal Rho guanine nucleotide exchange factors (RhoGEFs). RhoGEFs catalyze GDP/GTP exchange on small Rho guanine nucleotide-binding proteins (Rho GTPases), which in turn regulate the actin cytoskeleton. Previous studies have shown that the Rho GTPase Rac1 regulates synaptic AMPAR expression (6), and that the Rho GTPases Cdc42 and RhoA are required for LTP and the structural enlargement of spines that accompanies LTP (i.e., sLTP) (7, 8); however, which RhoGEFs are responsible for synaptic Rho GTPase activation and whether RhoGEF regulation is involved in the changes in synaptic function that occur during LTP remain unknown.Most studies reported to date have focused on the RhoGEF Kalirin. Alternative splicing of a single Kalirin gene results in the expression of several Kalirin isoforms. Previous work has shown that the Kalirin isoform Kalirin-7 is enriched in spines, is involved in synaptic maintenance, and is phosphorylated by CaMKII, and that Kalirin-7 overexpression (OE) in dissociated cortical neurons results in increased spine size (9). Such data support a role for Kalirin proteins in the structural changes in spines that accompany LTP; however, LTP in the hippocampus is largely normal in Kalirin KO mice, in which all Kalirin proteins resulting from the single Kalirin gene have been eliminated (10), and thus Kalirin proteins cannot be solely responsible for LTP. One possible explanation is that Kalirin supports LTP along with a functionally redundant, as-yet unidentified RhoGEF protein.Here we used molecular, imaging, and electrophysiological approaches to evaluate the contributions of RhoGEFs to excitatory synapse structure, function, and plasticity in hippocampal CA1 neurons. Our findings demonstrate, for the first time to our knowledge, that the Kalirin paralog Trio plays an important role in postsynaptic function, and that Trio and Kalirin serve critical and functionally redundant roles in supporting excitatory synapse structure and function in CA1 pyramidal cells of the hippocampus. We also report that although inhibiting Kalirin function alone has no effect on LTP, simultaneously inhibiting CaMKII signaling through Kalirin and Trio eliminates LTP induction. Furthermore, phosphorylation of Kalirin-7 by CaMKII is sufficient to enhance synaptic AMPAR-mediated synaptic transmission. Taken together, our data strongly suggest that NMDAR-mediated activation of CaMKII induces functional LTP through phosphorylation of Kalirin and Trio, which then give rise to the synaptic changes underlying synaptic AMPAR up-regulation.     

ERK activation in axonal varicosities modulates presynaptic plasticity in the CA3 region of the hippocampus through synapsin I

Activity-dependent changes in the strength of synaptic connections in the hippocampus are central for cognitive processes such as learning and memory storage. In this study, we reveal an activity-dependent presynaptic mechanism that is related to the modulation of synaptic plasticity. In acute mouse hippocampal slices, high-frequency stimulation (HFS) of the mossy fiber (MF)-CA3 pathway induced a strong and transient activation of extracellular-regulated kinase (ERK) in MF giant presynaptic terminals. Remarkably, pharmacological blockade of ERK disclosed a negative role of this kinase in the regulation of a presynaptic form of plasticity at MF-CA3 contacts. This ERK-mediated inhibition of post-tetanic enhancement (PTE) of MF-CA3 synapses was both frequency- and pathway-specific and was observed only with HFS at 50 Hz. Importantly, blockade of ERK was virtually ineffective on PTE of MF-CA3 synapses in mice lacking synapsin I, 1 of the major presynaptic ERK substrates, and triple knockout mice lacking all synapsin isoforms displayed PTE kinetics resembling that of wild-type mice under ERK inhibition. These findings reveal a form of short-term synaptic plasticity that depends on ERK and is finely tuned by the firing frequency of presynaptic neurons. Our results also demonstrate that presynaptic activation of the ERK signaling pathway plays part in the activity-dependent modulation of synaptic vesicle mobilization and transmitter release.