Long-term potentiation in cultures of single hippocampal granule cells: a presynaptic form of plasticity

Structure, biophysics, and circuit function of a "giant" cortical presynaptic terminal

The hippocampal mossy fiber synapse, formed between axons of dentate gyrus granule cells and dendrites of CA3 pyramidal neurons, is a key synapse in the trisynaptic circuitry of the hippocampus. Because of its comparatively large size, this synapse is accessible to direct presynaptic recording, allowing a rigorous investigation of the biophysical mechanisms of synaptic transmission and plasticity. Furthermore, because of its placement in the very center of the hippocampal memory circuit, this synapse seems to be critically involved in several higher network functions, such as learning, memory, pattern separation, and pattern completion. Recent work based on new technologies in both nanoanatomy and nanophysiology, including presynaptic patch-clamp recording, paired recording, super-resolution light microscopy, and freeze-fracture and "flash-and-freeze" electron microscopy, has provided new insights into the structure, biophysics, and network function of this intriguing synapse. This brings us one step closer to answering a fundamental question in neuroscience: how basic synaptic properties shape higher network computations.

Mechanistic insights into cAMP-mediated presynaptic potentiation at hippocampal mossy fiber synapses

Presynaptic plasticity is an activity-dependent change in the neurotransmitter release and plays a key role in dynamic modulation of synaptic strength. Particularly, presynaptic potentiation mediated by cyclic adenosine monophosphate (cAMP) is widely seen across the animals and thought to contribute to learning and memory. Hippocampal mossy fiber-CA3 pyramidal cell synapses have been used as a model because of robust presynaptic potentiation in short- and long-term forms. Moreover, direct presynaptic recordings from large mossy fiber terminals allow one to dissect the potentiation mechanisms. Recently, super-resolution microscopy and flash-and-freeze electron microscopy have revealed the localizations of release site molecules and synaptic vesicles during the potentiation at a nanoscale, identifying the molecular mechanisms of the potentiation. Incorporating these growing knowledges, we try to present plausible mechanisms underlying the cAMP-mediated presynaptic potentiation.

Recruitment of release sites underlies chemical presynaptic potentiation at hippocampal mossy fiber boutons

Synaptic plasticity is a cellular model for learning and memory. However, the expression mechanisms underlying presynaptic forms of plasticity are not well understood. Here, we investigate functional and structural correlates of presynaptic potentiation at large hippocampal mossy fiber boutons induced by the adenylyl cyclase activator forskolin. We performed 2-photon imaging of the genetically encoded glutamate sensor iGluu that revealed an increase in the surface area used for glutamate release at potentiated terminals. Time-gated stimulated emission depletion microscopy revealed no change in the coupling distance between P/Q-type calcium channels and release sites mapped by Munc13-1 cluster position. Finally, by high-pressure freezing and transmission electron microscopy analysis, we found a fast remodeling of synaptic ultrastructure at potentiated boutons: Synaptic vesicles dispersed in the terminal and accumulated at the active zones, while active zone density and synaptic complexity increased. We suggest that these rapid and early structural rearrangements might enable long-term increase in synaptic strength.

SCAMP5 mediates activity-dependent enhancement of NHE6 recruitment to synaptic vesicles during synaptic plasticity

Na+(K+)/H+ exchanger 6 (NHE6) on synaptic vesicle (SV) is critical for the presynaptic regulation of quantal size at the glutamatergic synapses by converting the chemical gradient (ΔpH) into membrane potential (Δψ) across the SV membrane. We recently found that NHE6 directly interacts with secretory carrier membrane protein 5 (SCAMP5), and SCAMP5-dependent recruitment of NHE6 to SVs controls the strength of synaptic transmission by modulation of quantal size of glutamate release at rest. It is, however, unknown whether NHE6 recruitment by SCAMP5 plays a role during synaptic plasticity. Here, we found that the number of NHE6-positive presynaptic boutons was significantly increased by the chemical long-term potentiation (cLTP). Since cLTP involves new synapse formation, our results indicated that NHE6 was recruited not only to the existing presynaptic boutons but also to the newly formed presynaptic boutons. Knock down of SCAMP5 completely abrogated the enhancement of NHE6 recruitment by cLTP. Interestingly, despite an increase in the number of NHE6-positive boutons by cLTP, the quantal size of glutamate release at the presynaptic terminals remained unaltered. Together with our recent results, our findings indicate that SCAMP5-dependent recruitment of NHE6 plays a critical role in manifesting presynaptic efficacy not only at rest but also during synaptic plasticity. Since both are autism candidate genes, reduced presynaptic efficacy by interfering with their interaction may underlie the molecular mechanism of synaptic dysfunction observed in autism.

De novo proteomic methods for examining the molecular mechanisms underpinning long-term memory

Memory formation is a fundamental function of the nervous system that enables the experience-based adaptation of behaviour. The formation, recall and updating of long-term memory (LTM) requires new protein synthesis through its direct involvement in neuronal processes, such as long-term potentiation (LTP), long-term depression (LTD) and synaptic scaling. We discuss the advantages and limitations of several emerging techniques which enable the tagging of newly synthesised proteins, including stable isotope labelling with amino acids in cell culture (SILAC), puromycin labelling, and non-canonical amino acid (NCAA) labelling. We further present how these methods allow for the identification and visualisation of proteins which are newly synthesised during different stages of memory formation. These emerging techniques will continue to expand our understanding of how memories are formed, consolidated and retrieved.

Ultrastructural Imaging of Activity-Dependent Synaptic Membrane-Trafficking Events in Cultured Brain Slices

Electron microscopy can resolve synapse ultrastructure with nanometer precision, but the capture of time-resolved, activity-dependent synaptic membrane-trafficking events has remained challenging, particularly in functionally distinct synapses in a tissue context. We present a method that combines optogenetic stimulation-coupled cryofixation ("flash-and-freeze") and electron microscopy to visualize membrane trafficking events and synapse-state-specific changes in presynaptic vesicle organization with high spatiotemporal resolution in synapses of cultured mouse brain tissue. With our experimental workflow, electrophysiological and "flash-and-freeze" electron microscopy experiments can be performed under identical conditions in artificial cerebrospinal fluid alone, without the addition of external cryoprotectants, which are otherwise needed to allow adequate tissue preservation upon freezing. Using this approach, we reveal depletion of docked vesicles and resolve compensatory membrane recycling events at individual presynaptic active zones at hippocampal mossy fiber synapses upon sustained stimulation.

Graphene Oxide Ameliorates the Cognitive Impairment Through Inhibiting PI3K/Akt/mTOR Pathway to Induce Autophagy in AD Mouse Model

Alzheimer's disease (AD) is a neurodegenerative disease of the central nervous system characterised by cognitive impairment. Its major pathological feature is the deposition of β-amyloid (Aβ) peptide, which triggers a series of pathological cascades. Autophagy is a main pathway to eliminate abnormal aggregated proteins, and increasing autophagy represents a plausible treatment strategy against relative overproduction of neurotoxic Aβ. Graphene oxide (GO) is an emerging carbon-based nanomaterial. As a derivative of graphene with neuroprotective effects, it can effectively increase the clearance of abnormally aggregated protein. In this article, we investigated the protective function of GO in an AD mouse model. GO (30 mg/kg, intraperitoneal) was administered for 2 weeks. The results of the Morris water maze test and the novel object recognition test suggested that GO ameliorated learning and memory impairments in 5xFAD mice. The long-term potentiation and depotentiation from the perforant path to the dentate gyrus in the hippocampus were increased with GO treatment in 5xFAD mice. Furthermore, GO upregulated the expression of synapse-related proteins and increased the cell density in the hippocampus. Our results showed that GO up-regulated LC3II/LC3I and Beclin-1 and decreased p62 protein levels in 5xFAD mice. In addition, GO downregulated the PI3K/Akt/mTOR signalling pathway to induce autophagy. These results have revealed the protective potential of GO in AD.

SynaptoPAC, an optogenetic tool for induction of presynaptic plasticity

Optogenetic manipulations have transformed neuroscience in recent years. While sophisticated tools now exist for controlling the firing patterns of neurons, it remains challenging to optogenetically define the plasticity state of individual synapses. A variety of synapses in the mammalian brain express presynaptic long-term potentiation (LTP) upon elevation of presynaptic cyclic adenosine monophosphate (cAMP), but the molecular expression mechanisms as well as the impact of presynaptic LTP on network activity and behavior are not fully understood. In order to establish optogenetic control of presynaptic cAMP levels and thereby presynaptic potentiation, we developed synaptoPAC, a presynaptically targeted version of the photoactivated adenylyl cyclase bPAC. In cultures of hippocampal granule cells of Wistar rats, activation of synaptoPAC with blue light increased action potential-evoked transmission, an effect not seen in hippocampal cultures of non-granule cells. In acute brain slices of C57BL/6N mice, synaptoPAC activation immediately triggered a strong presynaptic potentiation at mossy fiber synapses in CA3, but not at Schaffer collateral synapses in CA1. Following light-triggered potentiation, mossy fiber transmission decreased within 20 min, but remained enhanced still after 30 min. The optogenetic potentiation altered the short-term plasticity dynamics of release, reminiscent of presynaptic LTP. Our work establishes synaptoPAC as an optogenetic tool that enables acute light-controlled potentiation of transmitter release at specific synapses in the brain, facilitating studies of the role of presynaptic potentiation in network function and animal behavior in an unprecedented manner. Read the Editorial Highlight for this article on page 270.

Autaptic Cultures: Methods and Applications

Neurons typically form daisy chains of synaptic connections with other neurons, but they can also form synapses with themselves. Although such self-synapses, or autapses, are comparatively rare in vivo, they are surprisingly common in dissociated neuronal cultures. At first glance, autapses in culture seem like a mere curiosity. However, by providing a simple model system in which a single recording electrode gives simultaneous access to the pre- and postsynaptic compartments, autaptic cultures have proven to be invaluable in facilitating important and elegant experiments in the area of synaptic neuroscience. Here, I provide detailed protocols for preparing and recording from autaptic cultures (also called micro-island or microdot cultures). Variations on the basic procedure are presented, as well as practical tips for optimizing the outcomes. I also illustrate the utility of autaptic cultures by reviewing the types of experiments that have used them over the past three decades. These examples serve to highlight the power and elegance of this simple model system, and will hopefully inspire new experiments for the interrogation of synaptic function.

Cerebellar Kainate Receptor-Mediated Facilitation of Glutamate Release Requires Ca2+-Calmodulin and PKA

We elucidated the mechanisms underlying the kainate receptor (KAR)-mediated facilitatory modulation of synaptic transmission in the cerebellum. In cerebellar slices, KA (3 μM) increased the amplitude of evoked excitatory postsynaptic currents (eEPSCs) at synapses between axon terminals of parallel fibers (PF) and Purkinje neurons. KA-mediated facilitation was antagonized by NBQX under condition where AMPA receptors were previously antagonized. Inhibition of protein kinase A (PKA) suppressed the effect of KA on glutamate release, which was also obviated by the prior stimulation of adenylyl cyclase (AC). KAR-mediated facilitation of synaptic transmission was prevented by blocking Ca²⁺ permeant KARs using philanthotoxin. Furthermore, depletion of intracellular Ca²⁺ stores by thapsigargin, or inhibition of Ca²⁺-induced Ca²⁺-release by ryanodine, abrogated the synaptic facilitation by KA. Thus, the KA-mediated modulation was conditional on extracellular Ca²⁺ entry through Ca²⁺-permeable KARs, as well as and mobilization of Ca²⁺ from intracellular stores. Finally, KAR-mediated facilitation was sensitive to calmodulin inhibitors, W-7 and calmidazolium, indicating that the increased cytosolic [Ca²⁺] sustaining KAR-mediated facilitation of synaptic transmission operates through a downstream Ca²⁺/calmodulin coupling. We conclude that, at cerebellar parallel fiber-Purkinje cell synapses, presynaptic KARs mediate glutamate release facilitation, and thereby enhance synaptic transmission through Ca²⁺-calmodulin dependent activation of adenylyl cyclase/cAMP/protein kinase A signaling.

The Impact of Synaptic Zn2+ Dynamics on Cognition and Its Decline

The basal levels of extracellular Zn²⁺ are in the range of low nanomolar concentrations and less attention has been paid to Zn²⁺, compared to Ca²⁺, for synaptic activity. However, extracellular Zn²⁺ is necessary for synaptic activity. The basal levels of extracellular zinc are age-dependently increased in the rat hippocampus, implying that the basal levels of extracellular Zn²⁺ are also increased age-dependently and that extracellular Zn²⁺ dynamics are linked with age-related cognitive function and dysfunction. In the hippocampus, the influx of extracellular Zn²⁺ into postsynaptic neurons, which is often linked with Zn²⁺ release from neuron terminals, is critical for cognitive activity via long-term potentiation (LTP). In contrast, the excess influx of extracellular Zn²⁺ into postsynaptic neurons induces cognitive decline. Interestingly, the excess influx of extracellular Zn²⁺ more readily occurs in aged dentate granule cells and intracellular Zn²⁺-buffering, which is assessed with ZnAF-2DA, is weakened in the aged dentate granule cells. Characteristics (easiness) of extracellular Zn²⁺ influx seem to be linked with the weakened intracellular Zn²⁺-buffering in the aged dentate gyrus. This paper deals with the impact of synaptic Zn²⁺ signaling on cognition and its decline in comparison with synaptic Ca²⁺ signaling.

The Role of mGlu Receptors in Hippocampal Plasticity Deficits in Neurological and Psychiatric Disorders: Implications for Allosteric Modulators as Novel Therapeutic Strategies

Long-term potentiation (LTP) and long-term depression (LTD) are two distinct forms of synaptic plasticity that have been extensively characterized at the Schaffer collateral-CA1 (SCCA1) synapse and the mossy fiber (MF)-CA3 synapse within the hippocampus, and are postulated to be the molecular underpinning for several cognitive functions. Deficits in LTP and LTD have been implicated in the pathophysiology of several neurological and psychiatric disorders. Therefore, there has been a large effort focused on developing an understanding of the mechanisms underlying these forms of plasticity and novel therapeutic strategies that improve or rescue these plasticity deficits. Among many other targets, the metabotropic glutamate (mGlu) receptors show promise as novel therapeutic candidates for the treatment of these disorders. Among the eight distinct mGlu receptor subtypes (mGlu1-8), the mGlu1,2,3,5,7 subtypes are expressed throughout the hippocampus and have been shown to play important roles in the regulation of synaptic plasticity in this brain area. However, development of therapeutic agents that target these mGlu receptors has been hampered by a lack of subtype-selective compounds. Recently, discovery of allosteric modulators of mGlu receptors has provided novel ligands that are highly selective for individual mGlu receptor subtypes. The mGlu receptors modulate the multiple forms of synaptic plasticity at both SC-CA1 and MF synapses and allosteric modulators of mGlu receptors have emerged as potential therapeutic agents that may rescue plasticity deficits and improve cognitive function in patients suffering from multiple neurological and psychiatric disorders.

Cellular and System Biology of Memory: Timing, Molecules, and Beyond

The storage of information in the mammalian nervous systems is dependent on a delicate balance between change and stability of neuronal networks. The induction and maintenance of processes that lead to changes in synaptic strength to a multistep process which can lead to long-lasting changes, which starts and ends with a highly choreographed and perfectly timed dance of molecules in different cell types of the central nervous system. This is accompanied by synchronization of specific networks, resulting in the generation of characteristic "macroscopic" rhythmic electrical fields, whose characteristic frequencies correspond to certain activity and information-processing states of the brain. Molecular events and macroscopic fields influence each other reciprocally. We review here cellular processes of synaptic plasticity, particularly functional and structural changes, and focus on timing events that are important for the initial memory acquisition, as well as mechanisms of short- and long-term memory storage. Then, we cover the importance of epigenetic events on the long-time range. Furthermore, we consider how brain rhythms at the network level participate in processes of information storage and by what means they participating in it. Finally, we examine memory consolidation at the system level during processes of sleep.

Presynaptic D1 heteroreceptors and mGlu autoreceptors act at individual cortical release sites to modify glutamate release

The aim of this work was to study release of glutamic acid (GLU) from one-axon terminal or bouton at-a-time using cortical neurons grown in vitro to study the effect of presynaptic auto- and heteroreceptor stimulation. Neurons were infected with release reporters SypHx2 or iGluSnFR at 7 or 3 days-in-vitro (DIV) respectively. At 13-15 DIV single synaptic boutons were identified from images obtained from a confocal scanning microscope before and after field electrical stimulation. We further stimulated release by raising intracellular levels of cAMP with forskolin (10µM). Forskolin-mediated effects were dependent on protein kinase A (PKA) and did not result from an increase in endocytosis, but rather from an increase in the size of the vesicle readily releasable pool. Once iGluSnFR was confirmed as more sensitive than SypHx2, it was used to study the participation of presynaptic auto- and heteroreceptors on GLU release. Although most receptor agonizts (carbamylcholine, nicotine, dopamine D2, BDNF) did not affect electrically stimulated GLU release, a significant increase was observed in the presence of metabotropic D1/D5 heteroreceptor agonist (SKF38393 10µM) that was reversed by PKA inhibitors. Interestingly, stimulation of group II metabotropic mGLU2/3 autoreceptors (LY379268 50nM) induced a decrease in GLU release that was reversed by the specific mGLU2/3 receptor antagonist (LY341495 1µM) and also by PKA inhibitors (KT5720 200nM and PKI14-22 400nM). These changes in release probability at individual release sites suggest another level of control of the distribution of transmitter substances in cortical tissue.

Extracellular proteolysis in structural and functional plasticity of mossy fiber synapses in hippocampus

Brain is continuously altered in response to experience and environmental changes. One of the underlying mechanisms is synaptic plasticity, which is manifested by modification of synapse structure and function. It is becoming clear that regulated extracellular proteolysis plays a pivotal role in the structural and functional remodeling of synapses during brain development, learning and memory formation. Clearly, plasticity mechanisms may substantially differ between projections. Mossy fiber synapses onto CA3 pyramidal cells display several unique functional features, including pronounced short-term facilitation, a presynaptically expressed long-term potentiation (LTP) that is independent of NMDAR activation, and NMDA-dependent metaplasticity. Moreover, structural plasticity at mossy fiber synapses ranges from the reorganization of projection topology after hippocampus-dependent learning, through intrinsically different dynamic properties of synaptic boutons to pre- and postsynaptic structural changes accompanying LTP induction. Although concomitant functional and structural plasticity in this pathway strongly suggests a role of extracellular proteolysis, its impact only starts to be investigated in this projection. In the present report, we review the role of extracellular proteolysis in various aspects of synaptic plasticity in hippocampal mossy fiber synapses. A growing body of evidence demonstrates that among perisynaptic proteases, tissue plasminogen activator (tPA)/plasmin system, β-site amyloid precursor protein-cleaving enzyme 1 (BACE1) and metalloproteinases play a crucial role in shaping plastic changes in this projection. We discuss recent advances and emerging hypotheses on the roles of proteases in mechanisms underlying mossy fiber target specific synaptic plasticity and memory formation.

Regulation of density of functional presynaptic terminals by local energy supply

Background

The density of functional synapses is an important parameter in determining the efficacy of synaptic transmission. However, how functional presynaptic terminal density is regulated under natural physiological conditions is still poorly understood.

Results

We studied the factors controlling the density of presynaptic functional terminals at single dendritic branches of hippocampal neurons and found that elevation of intracellular Mg²⁺ concentration was effective in increasing the density of functional terminals. Interestingly, the upregulation was not due to synaptogenesis, but to the conversion of a considerable proportion of presynaptic terminals from nonfunctional to functional. Mechanistic studies revealed that the nonfunctional terminals had inadequate Ca²⁺-sensitivity-related proteins, resulting in very low Ca²⁺ sensitivity within their vesicle release machinery. We identified energy-dependent axonal transport as a primary factor controlling the amount of Ca²⁺-sensitivity-related proteins in terminals. The elevation of intracellular Mg²⁺ enhanced local energy supply and promoted the increase of Ca²⁺-sensitivity-related proteins in terminals, leading to increased functional terminal density.

Conclusions

Our study suggests that local energy supply plays a critical role in controlling the density of functional presynaptic terminals, demonstrating the link between energy supply and efficacy of synaptic transmission.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-015-0132-z) contains supplementary material, which is available to authorized users.

Induction of motor associative plasticity in the posterior parietal cortex-primary motor network

There is anatomical and functional connectivity between the primary motor cortex (M1) and posterior parietal cortex (PPC) that plays a role in sensorimotor integration. In this study, we applied corticocortical paired-associative stimuli to ipsilateral PPC and M1 (parietal ccPAS) in healthy right-handed subjects to test if this procedure could modulate M1 excitability and PPC-M1 connectivity. One hundred and eighty paired transcranial magnetic stimuli to the PPC and M1 at an interstimulus interval (ISI) of 8 ms were delivered at 0.2 Hz. We found that parietal ccPAS in the left hemisphere increased the excitability of conditioned left M1 assessed by motor evoked potentials (MEPs) and the input-output curve. Motor behavior assessed by the Purdue pegboard task was unchanged compared with controls. At baseline, conditioning stimuli over the left PPC potentiated MEPs from left M1 when ISI was 8 ms. This interaction significantly attenuated at 60 min after left parietal ccPAS. Additional experiments showed that parietal ccPAS induced plasticity was timing-dependent, was absent if ISI was 100 ms, and could also be seen in the right hemisphere. Our results suggest that parietal ccPAS can modulate M1 excitability and PPC-M1 connectivity and is a new approach to modify motor excitability and sensorimotor interaction.

Pre-synaptic kainate receptor-mediated facilitation of glutamate release involves PKA and Ca(2+) -calmodulin at thalamocortical synapses

We have investigated the mechanisms underlying the facilitatory modulation mediated by kainate receptor (KAR) activation in the cortex, using isolated nerve terminals (synaptosomes) and slice preparations. In cortical nerve terminals, kainate (KA, 100 μM) produced an increase in 4-aminopyridine (4-AP)-evoked glutamate release. In thalamocortical slices, KA (1 μM) produced an increase in the amplitude of evoked excitatory post-synaptic currents (eEPSCs) at synapses established between thalamic axon terminals from the ventrobasal nucleus onto stellate neurons of L4 of the somatosensory cortex. In both, synaptosomes and slices, the effect of KA was antagonized by 6-cyano-7-nitroquinoxaline-2,3-dione, and persisted after pre-treatment with a cocktail of antagonists of other receptors whose activation could potentially have produced facilitation of release indirectly. Mechanistically, the observed effects of KA appear to be congruent in synaptosomal and slice preparations. Thus, the facilitation by KA of synaptosomal glutamate release and thalamocortical synaptic transmission were suppressed by the inhibition of protein kinase A and occluded by the stimulation of adenylyl cyclase. Dissecting this G-protein-independent regulation further in thalamocortical slices, the KAR-mediated facilitation of synaptic transmission was found to be sensitive to the block of Ca(2+) permeant KARs by philanthotoxin. Intriguingly, the synaptic facilitation was abrogated by depletion of intracellular Ca(2+) stores by thapsigargin, or inhibition of Ca(2+) -induced Ca(2+) -release by ryanodine. Thus, the KA-mediated modulation was contingent on both Ca(2+) entry through Ca(2+) -permeable KARs and liberation of intracellular Ca(2+) stores. Finally, sensitivity to W-7 indicated that the increased cytosolic [Ca(2+) ] underpinning KAR-mediated regulation of synaptic transmission at thalamocortical synapses, requires downstream activation of calmodulin. We conclude that neocortical pre-synaptic KARs mediate the facilitation of glutamate release and synaptic transmission by a Ca(2+) -calmodulin dependent activation of an adenylyl cyclase/cAMP/protein kinase A signalling cascade, independent of G-protein involvement.

Differential requirement for protein synthesis in presynaptic unmuting and muting in hippocampal glutamate terminals

Synaptic function and plasticity are crucial for information processing within the nervous system. In glutamatergic hippocampal neurons, presynaptic function is silenced, or muted, after strong or prolonged depolarization. This muting is neuroprotective, but the underlying mechanisms responsible for muting and its reversal, unmuting, remain to be clarified. Using cultured rat hippocampal neurons, we found that muting induction did not require protein synthesis; however, slow forms of unmuting that depend on protein kinase A (PKA), including reversal of depolarization-induced muting and forskolin-induced unmuting of basally mute synapses, required protein synthesis. In contrast, fast unmuting of basally mute synapses by phorbol esters was protein synthesis-independent. Further studies of recovery from depolarization-induced muting revealed that protein levels of Rim1 and Munc13-1, which mediate vesicle priming, correlated with the functional status of presynaptic terminals. Additionally, this form of unmuting was prevented by both transcription and translation inhibitors, so proteins are likely synthesized de novo after removal of depolarization. Phosphorylated cyclic adenosine monophosphate response element-binding protein (pCREB), a nuclear transcription factor, was elevated after recovery from depolarization-induced muting, consistent with a model in which PKA-dependent mechanisms, possibly including pCREB-activated transcription, mediate slow unmuting. In summary, we found that protein synthesis was required for slower, PKA-dependent unmuting of presynaptic terminals, but it was not required for muting or a fast form of unmuting. These results clarify some of the molecular mechanisms responsible for synaptic plasticity in hippocampal neurons and emphasize the multiple mechanisms by which presynaptic function is modulated.

Astrocyte-derived thrombospondins mediate the development of hippocampal presynaptic plasticity in vitro

Astrocytes contribute to many neuronal functions, including synaptogenesis, but their role in the development of synaptic plasticity remains unclear. Presynaptic muting of hippocampal glutamatergic terminals defends against excitotoxicity. Here we studied the role of astrocytes in the development of presynaptic muting at glutamatergic synapses in rat hippocampal neurons. We found that astrocytes were critical for the development of depolarization-dependent and G(i/o)-dependent presynaptic muting. The ability of cAMP analogues to modulate presynaptic function was also impaired by astrocyte deficiency. Although astrocyte deprivation resulted in postsynaptic glutamate receptor deficits, this effect appeared independent of astrocytes' role in presynaptic muting. Muting was restored with chronic, but not acute, treatment with astrocyte-conditioned medium, indicating that a soluble factor is permissive for muting. Astrocyte-derived thrombospondins (TSPs) are likely responsible because TSP1 mimicked the effect of conditioned medium, and gabapentin, a high-affinity antagonist of TSP binding to the α2δ-1 calcium channel subunit, mimicked astrocyte deprivation. We found evidence that protein kinase A activity is abnormal in astrocyte-deprived neurons but restored by TSP1, so protein kinase A dysfunction may provide a mechanism by which muting is disrupted during astrocyte deficiency. In summary our results suggest an important role for astrocyte-derived TSPs, acting through α2δ-1, in maturation of a potentially important form of presynaptic plasticity.

Dendritic position is a major determinant of presynaptic strength

Different regulatory principles influence synaptic coupling between neurons, including positional principles. In dendrites of pyramidal neurons, postsynaptic sensitivity depends on synapse location, with distal synapses having the highest gain. In this paper, we investigate whether similar rules exist for presynaptic terminals in mixed networks of pyramidal and dentate gyrus (DG) neurons. Unexpectedly, distal synapses had the lowest staining intensities for vesicular proteins vGlut, vGAT, Synaptotagmin, and VAMP and for many nonvesicular proteins, including Bassoon, Munc18, and Syntaxin. Concomitantly, distal synapses displayed less vesicle release upon stimulation. This dependence of presynaptic strength on dendritic position persisted after chronically blocking action potential firing and postsynaptic receptors but was markedly reduced on DG dendrites compared with pyramidal dendrites. These data reveal a novel rule, independent of neuronal activity, which regulates presynaptic strength according to dendritic position, with the strongest terminals closest to the soma. This gradient is opposite to postsynaptic gradients observed in pyramidal dendrites, and different cell types apply this rule to a different extent.

Presynaptic LTP and LTD of excitatory and inhibitory synapses

Ubiquitous forms of long-term potentiation (LTP) and depression (LTD) are caused by enduring increases or decreases in neurotransmitter release. Such forms or presynaptic plasticity are equally observed at excitatory and inhibitory synapses and the list of locations expressing presynaptic LTP and LTD continues to grow. In addition to the mechanistically distinct forms of postsynaptic plasticity, presynaptic plasticity offers a powerful means to modify neural circuits. A wide range of induction mechanisms has been identified, some of which occur entirely in the presynaptic terminal, whereas others require retrograde signaling from the postsynaptic to presynaptic terminals. In spite of this diversity of induction mechanisms, some common induction rules can be identified across synapses. Although the precise molecular mechanism underlying long-term changes in transmitter release in most cases remains unclear, increasing evidence indicates that presynaptic LTP and LTD can occur in vivo and likely mediate some forms of learning.

Vesicular zinc promotes presynaptic and inhibits postsynaptic long-term potentiation of mossy fiber-CA3 synapse

The presence of zinc in glutamatergic synaptic vesicles of excitatory neurons of mammalian cerebral cortex suggests that zinc might regulate plasticity of synapses formed by these neurons. Long-term potentiation (LTP) is a form of synaptic plasticity that may underlie learning and memory. We tested the hypothesis that zinc within vesicles of mossy fibers (mf) contributes to mf-LTP, a classical form of presynaptic LTP. We synthesized an extracellular zinc chelator with selectivity and kinetic properties suitable for study of the large transient of zinc in the synaptic cleft induced by mf stimulation. We found that vesicular zinc is required for presynaptic mf-LTP. Unexpectedly, vesicular zinc also inhibits a form of postsynaptic mf-LTP. Because the mf-CA3 synapse provides a major source of excitatory input to the hippocampus, regulating its efficacy by these dual actions, vesicular zinc is critical to proper function of hippocampal circuitry in health and disease.

Presynaptically silent synapses: dormancy and awakening of presynaptic vesicle release

Synapses represent the main junctures of communication between neurons in the nervous system. In many neurotransmitter systems, a fraction of presynaptic terminals fails to release vesicles in response to action potential stimulation and strong calcium influx. These silent presynaptic terminals exhibit a reversible functional dormancy beyond low vesicle release probability, and dormancy status may have important implications in neural function. Recent advances have implicated presynaptic proteins interacting with vesicles downstream of cAMP and protein kinase A signaling cascades in modulating the number of these mute presynaptic terminals, and dormancy induction may represent a homeostatic neuroprotective mechanism active during pathological insults involving excitotoxicity. Interestingly, dormancy reversal may also be induced during Hebbian plasticity. Here, details of synaptic dormancy, recent insights into the molecular signaling cascades involved, and potential clinical and mechanistic implications of this form of synaptic plasticity are described.

Long-term potentiation in cultured hippocampal neurons

Studies performed on low-density primary neuronal cultures have enabled dissection of molecular and cellular changes during N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP). Various electrophysiological and chemical induction protocols were developed for the persistent enhancement of excitatory synaptic transmission in hippocampal neuronal cultures. The characterisation of these plasticity models confirmed that they share many key properties with the LTP of CA1 neurons, extensively studied in hippocampal slices using electrophysiological techniques. For example, LTP in dissociated hippocampal neuronal cultures is also dependent on Ca(2+) influx through post-synaptic NMDA receptors, subsequent activation and autophosphorylation of the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and an increase in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor insertion at the post-synaptic membrane. The availability of models of LTP in cultured hippocampal neurons significantly facilitated the monitoring of changes in endogenous postsynaptic receptor proteins and the investigation of the associated signalling mechanisms that underlie LTP. A central feature of LTP of excitatory synapses is the recruitment of AMPA receptors at the postsynaptic site. Results from the use of cell culture-based models started to establish the mechanism by which synaptic input controls a neuron's ability to modify its synapses in LTP. This review focuses on key features of various LTP induction protocols in dissociated hippocampal neuronal cultures and the applications of these plasticity models for the investigation of activity-induced changes in native AMPA receptors.

Rapid active zone remodeling during synaptic plasticity

How can synapses change the amount of neurotransmitter released during synaptic plasticity? Although release in general is intensely investigated, its determinants during plasticity are still poorly understood. As a model for plastic strengthening of synaptic release, we here use the well-established presynaptic homeostatic compensation during interference with postsynaptic glutamate receptors at the Drosophila neuromuscular junction. Combining short-term plasticity analysis, cumulative EPSC analysis, fluctuation analysis, and quantal short-term plasticity modeling, we found an increase in the number of release-ready vesicles during presynaptic strengthening. High-resolution light microscopy revealed an increase in the amount of the active zone protein Bruchpilot and an enlargement of the presynaptic cytomatrix structure. Furthermore, these functional and structural alterations of the active zone were not only observed after lifelong but already after minutes of presynaptic strengthening. Our results demonstrate that presynaptic plasticity can induce active zone remodeling, which regulates the number of release-ready vesicles within minutes.

Calcium-independent inhibitory G-protein signaling induces persistent presynaptic muting of hippocampal synapses

Adaptive forms of synaptic plasticity that reduce excitatory synaptic transmission in response to prolonged increases in neuronal activity may prevent runaway positive feedback in neuronal circuits. In hippocampal neurons, for example, glutamatergic presynaptic terminals are selectively silenced, creating "mute" synapses, after periods of increased neuronal activity or sustained depolarization. Previous work suggests that cAMP-dependent and proteasome-dependent mechanisms participate in silencing induction by depolarization, but upstream activators are unknown. We, therefore, tested the role of calcium and G-protein signaling in silencing induction in cultured hippocampal neurons. We found that silencing induction by depolarization was not dependent on rises in intracellular calcium, from either extracellular or intracellular sources. Silencing was, however, pertussis toxin sensitive, which suggests that inhibitory G-proteins are recruited. Surprisingly, blocking four common inhibitory G-protein-coupled receptors (GPCRs) (adenosine A(1) receptors, GABA(B) receptors, metabotropic glutamate receptors, and CB(1) cannabinoid receptors) and one ionotropic receptor with metabotropic properties (kainate receptors) failed to prevent depolarization-induced silencing. Activating a subset of these GPCRs (A(1) and GABA(B)) with agonist application induced silencing, however, which supports the hypothesis that G-protein activation is a critical step in silencing. Overall, our results suggest that depolarization activates silencing through an atypical GPCR or through receptor-independent G-protein activation. GPCR agonist-induced silencing exhibited dependence on the ubiquitin-proteasome system, as was shown previously for depolarization-induced silencing, implicating the degradation of vital synaptic proteins in silencing by GPCR activation. These data suggest that presynaptic muting in hippocampal neurons uses a G-protein-dependent but calcium-independent mechanism to depress presynaptic vesicle release.

Activation of silent and weak synapses by cAMP-dependent protein kinase in cultured cerebellar granule neurons

Presynaptic long term potentiation of synaptic transmission activates silent synapses and potentiates existing active synapses. We sought to visualise these two processes by studying the cAMP-dependent protein kinase (PKA) potentiation of presynaptic vesicle cycling in cultured cerebellar granule neurons.Using FM dyes to label the pool of recycling synaptic vesicles,we found that trains of electrical stimulation which do not potentiate already active synapses are sufficient to rapidly activate a discrete population comprising silent and very low activity synapses. Silent synapse activation required PKA activity and conversely, active synapses could be silenced by PKA inhibition. Surprisingly, the recycling pool of synaptic vesicles in recently activated synapses was larger than in already active synapses and equivalent to synapses treated with forskolin. Imaging of synaptic vesicle cycling and cytosolic Ca(2+) in individual nerve terminals confirmed that silent synapses have evoked Ca(2+) transients comparable to those of active synapses. Furthermore, across populations of active synapses, changes in Ca(2+) influx did not correlate with changes in the size of the pool of recycling synaptic vesicles. Finally, we found that stimulation of synapsin phosphorylation, but not RIM1α, by PKA was frequency dependent and long lasting. These data are consistent with the idea that PKA regulates synaptic vesicle recycling downstream of Ca(2+) influx and that this pathway is highly active in recently activated synapses.

Structural and functional plasticity of the cytoplasmic active zone

The presynaptic active zone (AZ) membrane is the site where vesicle fusion mediates information transfer between connected neurons. Reaching into the cytoplasm, an electron-dense cytomatrix (CAZ) is found to decorate the AZ membranes. CAZ architectures are meant not only to regulate the synaptic vesicle exocycle/endocycle, but also to structurally stabilize the presynaptic site. The CAZ is composed of a set of large scaffold proteins, many of which are evolutionarily conserved. Recently, several signaling factors controlling the developmental assembly of CAZs were found by unbiased genetics in Drosophila and Caenorhabditis elegans. At the same time, post-translational modification of CAZ proteins was implicated in changing the strength of mammalian brain synapses. Studying how processes of structural and functional CAZ plasticity get integrated within circuit remodeling remains an important challenge.

Rapid activation of dormant presynaptic terminals by phorbol esters

Presynaptic stimulation stochastically recruits transmission according to the release probability (P(r)) of synapses. The majority of central synapses have relatively low P(r), which includes synapses that are completely quiescent presynaptically. The presence of presynaptically dormant versus active terminals presumably increases synaptic malleability when conditions demand synaptic strengthening or weakening, perhaps by triggering second messenger signals. However, whether modulator-mediated potentiation involves recruitment of transmission from dormant terminals remains unclear. Here, by combining electrophysiological and fluorescence imaging approaches, we uncovered rapid presynaptic awakening by select synaptic modulators. A phorbol ester phorbol 12,13-dibutyrate (PDBu) (a diacylglycerol analog), but not forskolin (an adenylyl cyclase activator) or elevated extracellular calcium, recruited neurotransmission from presynaptically dormant synapses. This effect was not dependent on protein kinase C activation. After PDBu-induced awakening, these previously dormant terminals had a synaptic P(r) spectrum similar to basally active synapses naive to PDBu treatment. Dormant terminals did not seem to have properties of nascent or immature synapses, judged by NR2B NMDAR (NMDA receptor) receptor subunit contribution after PDBu-stimulated awakening. Strikingly, synapses rendered inactive by prolonged depolarization, unlike basally dormant synapses, were not awakened by PDBu. These results suggest that the initial release competence of synapses can dictate the acute response to second messenger modulation, and the results suggest multiple pathways to presynaptic dormancy and awakening.

Autaptic cultures of single hippocampal granule cells of mice and rats

When a single neuron is grown on a small island of glial cells, the neuron forms synapses onto itself. The so-called autaptic culture systems have proven extremely valuable in elucidating basic mechanisms of synaptic transmission, as they allow application of technical approaches that cannot be used in slice preparations. However, this method has been almost exclusively used for pyramidal cells and interneurons. In this study, we generated autaptic cultures from granule cells isolated from the dentate gyrus of rodent hippocampi. Our subsequent morphological and functional characterisation of these cells confirms that this culture model is suitable for investigating basic mechanisms of granule cell synaptic transmission. Importantly, the autosynaptic connectivity allows recordings of pure mossy fibre miniature EPSCs, which are not possible in slice preparations. Further, by fast application of hypertonic sucrose solutions it is possible to directly measure the readily releasable pool and to calculate the probability of vesicular release.

Glutamate plasticity in the drunken amygdala: the making of an anxious synapse

Plasticity at glutamatergic synapses is believed to be the cellular correlate of learning and memory. Classic fear conditioning, for example, is dependent upon NMDA-type glutamate receptor activation in the lateral/basolateral amygdala followed by increased synaptic expression of AMPA-type glutamate receptors. This review provides an extensive comparison between the initiation and expression of glutamatergic plasticity during learning/memory and glutamatergic alterations associated with chronic ethanol exposure and withdrawal. The parallels between these neuro-adaptive processes suggest that long-term ethanol exposure might "chemically condition" amygdala-dependent fear/anxiety via the increased function of pre- and post-synaptic glutamate signaling.

Autocrine activation of neuronal NMDA receptors by aspartate mediates dopamine- and cAMP-induced CREB-dependent gene transcription

cAMP can stimulate the transcription of many activity-dependent genes via activation of the transcription factor, cAMP response element-binding protein (CREB). However, in mouse cortical neuron cultures, prior to synaptogenesis, neither cAMP nor dopamine, which acts via cAMP, stimulated CREB-dependent gene transcription when NR2B-containing NMDA receptors (NMDARs) were blocked. Stimulation of transcription by cAMP was potentiated by inhibitors of excitatory amino acid uptake, suggesting a role for extracellular glutamate or aspartate in cAMP-induced transcription. Aspartate was identified as the extracellular messenger: enzymatic scavenging of l-aspartate, but not glutamate, blocked stimulation of CREB-dependent gene transcription by cAMP; moreover, cAMP induced aspartate but not glutamate release. Together, these results suggest that cAMP acts via an autocrine or paracrine pathway to release aspartate, which activates NR2B-containing NMDARs, leading to Ca(2+) entry and activation of transcription. This cAMP/aspartate/NMDAR signaling pathway may mediate the effects of transmitters such as dopamine on axon growth and synaptogenesis in developing neurons or on synaptic plasticity in mature neural networks.

beta-Phorbol ester-induced enhancement of exocytosis in large mossy fiber boutons of mouse hippocampus

beta-Phorbol esters (BPE), synthetic analogues of diacylglycerol (DAG), induce the potentiation of transmission in many kinds of synapses through activating the C(1) domain-containing receptors. However, their effects on synaptic vesicle exocytosis have not yet been investigated. Here, we evaluated the vesicular exocytosis directly from individual large mossy fiber boutons (LMFBs) in hippocampal slices from transgenic mice that selectively express synaptopHluorin (SpH). We found that the activity-dependent increment of SpH fluorescence (DeltaSpH) was enhanced by 4beta-phorbol 12,13-diacetate (PDAc), one of the BPEs, without influencing the recycled component of SpH. These PDAc effects on DeltaSpH were almost completely inhibited by staurosporine, a non-selective antagonist of protein kinases. However, intermittent synaptic transmission was still potentiated through a staurosporine-resistant mechanism. The staurosporine-sensitive cascade may facilitate the vesicle replenishment, thus maintaining the fidelity of transmission at a high level during repetitive firing of the presynaptic neuron.

Regulation of brain-derived neurotrophic factor-mediated transcription of the immediate early gene Arc by intracellular calcium and calmodulin

The induction of the immediate early gene Arc is strongly implicated in synaptic plasticity. Although the role of ERK has been demonstrated, the regulation of Arc expression is largely unknown. In this study, we investigated the major signaling pathways underlying brain-derived neurotrophic factor (BDNF)-mediated Arc transcription in cultured cortical neurons. The BDNF-stimulated Arc transcription was regulated solely by the Ras-Raf-MAPK signaling through ERK, but not by phosphoinositide 3-kinase (PI3K) and PLC-gamma activities. Although it was demonstrated that BDNF might promote calcium entry through calcium channels and NMDA receptors, chelating extracellular calcium with EGTA failed to block Arc transcription. In contrast, chelating intracellular calcium ([Ca(2+)](i)) by BAPTA-AM abolished BDNF-mediated Arc up-regulation. Surprisingly, BAPTA-AM did not block ERK activation, indicating that [Ca(2+)](i) and Ras-Raf-MAPK are not coupled, and the activation of ERK alone is not sufficient to up-regulate Arc transcription. Moreover, we found that inhibition of calmodulin (CaM) by W13 blocked both Arc transcription and ERK activation, revealing a Ca(2+)-independent function of CaM. These data suggested novel functions of [Ca(2+)](i) and CaM in BDNF signaling. Comparison of the Arc transcription profiles between Ca(2+)-stimulated and BDNF-stimulated neurons demonstrated that the regulatory mechanisms were distinctively tailored to the complex features of neuronal activity. Specifically, PI3K and CaM-dependent protein kinase (CaMK) activity were required for Ca(2+)-stimulated Arc transcription through regulating ERK signaling. Such cross-talks between PI3K, CaMK, and ERK was absent in BDNF-stimulated neurons.

Role of rut adenylyl cyclase in the ensemble regulation of presynaptic terminal excitability: reduced synaptic strength and precision in a Drosophila memory mutant

Although modulation of presynaptic terminal excitability can profoundly affect transmission efficacy, how excitability along axonal terminal branches is regulated requires further investigations. We performed focal patch recording in Drosophila larval neuromuscular junctions (NMJs) to monitor the activity of individual synaptic boutons along the presynaptic terminal. Analysis of the learning mutant rutabaga (rut) suggests a tight regulation of presynaptic terminal excitability by rut adenylyl cyclase (AC) that is responsible for Ca2+/calmodulin-dependent cAMP synthesis. Focal excitatory junctional currents (ejcs) demonstrated that disrupted cAMP metabolism in rut mutant boutons leads to decreased transmitter release, coupled with temporal dispersion and amplitude fluctuation of ejcs during repetitive activity. Strikingly, rut motor terminals displayed greatly increased variability among corresponding terminal branches of identified NMJs in different preparations. However, boutons throughout single terminal branches were relatively uniform in either WT or rut mutant larvae. The use of electrotonic depolarization to directly evoke transmitter release from axonal terminals revealed that variability in neurotransmission originated from varying degrees of weakened excitability in rut terminals. Pharmacological treatments and axonal action potential recordings raised the possibility that defective rut AC resulted in reduced Ca2+ currents in the nerve terminal. Thus, our data indicate that rut AC not only affects transmitter release machinery, but also plays a previously unsuspected role in local excitability control, both contributing to transmission level and precision along the entire axonal terminal.

Silent synapses and the emergence of a postsynaptic mechanism for LTP

Silent synapses abound in the young brain, representing an early step in the pathway of experience-dependent synaptic development. Discovered amidst the debate over whether long-term potentiation reflects a presynaptic or a postsynaptic modification, silent synapses--which in the hippocampal CA1 subfield are characterized by the presence of NMDA receptors but not AMPA receptors--have stirred some mechanistic controversy of their own. Out of this literature has emerged a model for synapse unsilencing that highlights the central role for postsynaptic AMPA-receptor trafficking in the expression of excitatory synaptic plasticity.

A specific role for Ca2+-dependent adenylyl cyclases in recovery from adaptive presynaptic silencing

Glutamate generates fast postsynaptic depolarization throughout the CNS. The positive-feedback nature of glutamate signaling likely necessitates flexible adaptive mechanisms that help prevent runaway excitation. We have previously explored presynaptic adaptive silencing, a form of synaptic plasticity produced by ongoing neuronal activity and by strong depolarization. Unsilencing mechanisms that maintain active synapses and restore normal function after adaptation are also important, but mechanisms underlying such presynaptic reactivation remain unexplored. Here we investigate the involvement of the cAMP pathway in the basal balance between silenced and active synapses, as well as the recovery of baseline function after depolarization-induced presynaptic silencing. Activation of the cAMP pathway activates synapses that are silent at rest, and pharmacological inhibition of cAMP signaling silences basally active synapses. Adenylyl cyclase (AC) 1 and AC8, the major Ca2+-sensitive AC isoforms, are not crucial for the baseline balance between silent and active synapses. In cells from mice doubly deficient in AC1 and AC8, the baseline percentage of active synapses was only modestly reduced compared with wild-type synapses, and forskolin unsilencing was similar in the two genotypes. Nevertheless, after strong presynaptic silencing, recovery of normal function was strongly inhibited in AC1/AC8-deficient synapses. The entire recovery phenotype of the double null was reproduced in AC8-deficient but not AC1-deficient cells. We conclude that, under normal conditions, redundant cyclase activity maintains the balance between presynaptically silent and active synapses, but AC8 plays a particularly important role in rapidly resetting the balance of active to silent synapses after adaptation to strong activity.

Preparation and maintenance of single-cell micro-island cultures of basal forebrain neurons

Micro-island cultures provide a simplified system for studying the expression of cellular phenotype, excitability, synapse formation and pre- and postsynaptic regulatory mechanisms without the usual problems that arise from complex interactions between large numbers of other cells. The technique relies on the ability to constrain the attachment and growth of either single or small groups of neurons to discrete (20-500 microm) 'islands' of cell-permissive substrate applied over a nonadherent background layer. Constrained in this way, neurons form large numbers of conventional synaptic and/or autaptic contacts that can be easily visualized, making them ideally suited for studying synaptic physiology using electrophysiological and/or high-resolution optical imaging techniques. The protocol described here requires approximately 2 h for preparation of the culture dishes and a further 3-4 h for isolation and plating out the cells. Once established, the cultures can be maintained for prolonged periods (>6 weeks) permitting manipulations to be made to their local environment and the effects on individually identified cells to be repeatedly monitored.

Differential modulation of short-term synaptic dynamics by long-term potentiation at mouse hippocampal mossy fibre synapses

Synapses continuously experience short- and long-lasting activity-dependent changes in synaptic strength. Long-term plasticity refers to persistent alterations in synaptic efficacy, whereas short-term plasticity (STP) reflects the instantaneous and reversible modulation of synaptic strength in response to varying presynaptic stimuli. The hippocampal mossy fibre synapse onto CA3 pyramidal cells is known to exhibit both a presynaptic, NMDA receptor-independent form of long-term potentiation (LTP) and a pronounced form of STP. A detailed description of their exact interdependence is, however, lacking. Here, using electrophysiological and computational techniques, we have developed a descriptive model of transmission dynamics to quantify plasticity at the mossy fibre synapse. STP at this synapse is best described by two facilitatory processes acting on time-scales of a few hundred milliseconds and about 10 s. We find that these distinct types of facilitation are differentially influenced by LTP such that the impact of the fast process is weakened as compared to that of the slow process. This attenuation is reflected by a selective decrease of not only the amplitude but also the time constant of the fast facilitation. We henceforth argue that LTP, involving a modulation of parameters determining both amplitude and time course of STP, serves as a mechanism to adapt the mossy fibre synapse to its temporal input.

Synaptic vesicle dynamics in the mossy fiber-CA3 presynaptic terminals of mouse hippocampus

The mossy fiber (MF)-CA3 synapse in the hippocampus is unique in the CNS because of its wide dynamic range of transmitter release during short- and long-term plasticity. The presynaptic mechanisms underlying the fidelity of transmission were investigated for the MF-CA3 synapses. The relative size of readily releasable pool (RRP) of vesicles was estimated by counting the number of docked vesicles at an active zone (AZ) on the transmission electron microscopy (TEM) image. The size of the releasable pool and the exo-endocytosis kinetics were directly measured from individual large MF boutons in hippocampal slices of transgenic mice that selectively express synaptopHluorin (SpH), a pH-sensitive GFP fused to the lumenal aspect of one of the vesicular membrane proteins, VAMP-2, in these boutons. Here we found (1) there are distinct two vesicle pools, the resting pool which is resistant to exocytosis, and the releasable pool, (2) the initially docked vesicles are easily depleted and the RRP is maintained by refilling from the reserve subpopulation of releasable pool ("reserve" releasable pool), and (3) the contribution of rapid reuse of recycled vesicles is relatively small. Therefore, the fidelity of transmission is suggested to be ensured by the rapid refilling rate of RRP.

Differential control of two forms of glutamate release by group III metabotropic glutamate receptors at rat entorhinal synapses

Neurotransmitter release at CNS synapses occurs via both action potential-dependent and independent mechanisms, and it has generally been accepted that these two forms of release are regulated in parallel. We examined the effects of activation of group III metabotropic glutamate receptors (mGluRs) on stimulus-evoked and spontaneous glutamate release onto entorhinal cortical neurones in rats, and found a differential regulation of action potential-dependent and independent forms of release. Activation of presynaptic mGluRs depressed the amplitude of stimulus-evoked excitatory postsynaptic currents, but concurrently enhanced the frequency of spontaneous excitatory currents. Moreover, these differential effects on glutamate release were mediated by pharmacologically separable mechanisms. Application of the specific activator of adenylyl cyclase, forskolin, mimicked the effect of mGluR activation on spontaneous, but not evoked release, and inhibition of adenylyl cyclase with 9-tetrahydro-2-furanyl)-9H-purin-6-amine (SQ22536) blocked mGluR-mediated enhancement of spontaneous release, but not depression of evoked release. Occlusion studies with calcium channel blockers suggested that the group III mGluRs might depress evoked release through inhibition of both N and P/Q, but not R-type calcium channels. We suggest that the concurrent depression of action potential-evoked, and enhancement of action potential-independent glutamate release operate through discrete second messenger/effector systems at excitatory entorhinal terminals in rat brain.

Deficits in synaptic transmission and learning in amyloid precursor protein (APP) transgenic mice require C-terminal cleavage of APP

Synaptic dysfunction has been shown to be one of the earliest correlates of disease progression in animal models of Alzheimer's disease. Amyloid-beta protein (Abeta) is thought to play an important role in disease-related synaptic dysfunction, but the mechanism by which Abeta leads to synaptic dysfunction is not understood. Here we describe evidence that cleavage of APP in the C terminus may be necessary for the deficits present in APP transgenic mice. In APP transgenic mice with a mutated cleavage site at amino acid 664, normal synaptic transmission, synaptic plasticity, and learning were maintained despite the presence of elevated levels of APP, Abeta42, and even plaque accumulation. These results indicate that cleavage of APP may play a critical role in the development of synaptic and behavioral dysfunction in APP transgenic mice.

Mossy fiber synaptic transmission: communication from the dentate gyrus to area CA3

Communication between the dentate gyrus (DG) and area CA3 of the hippocampus proper is transmitted via axons of granule cells--the mossy fiber (MF) pathway. In this review we discuss and compare the properties of transmitter release from the MFs onto pyramidal neurons and interneurons. An examination of the anatomical connectivity from DG to CA3 reveals a surprising interplay between excitation and inhibition for this circuit. In this respect it is particularly relevant that the major targets of the MFs are interneurons and that the consequence of MF input into CA3 may be inhibitory or excitatory, conditionally dependent on the frequency of input and modulatory regulation. This is further complicated by the properties of transmitter release from the MFs where a large number of co-localized transmitters, including GABAergic inhibitory transmitter release, and the effects of presynaptic modulation finely tune transmitter release. A picture emerges that extends beyond the hypothesis that the MFs are simply "detonators" of CA3 pyramidal neurons; the properties of synaptic information flow from the DG have more subtle and complex influences on the CA3 network.

Kainate Receptor–Mediated Inhibition of Glutamate Release Involves Protein Kinase A in the Mouse Hippocampus

The mechanisms involved in the inhibition of glutamate release mediated by the activation of presynaptic kainate receptors (KARs) at the hippocampal mossy fiber–CA3 synapse are not well understood. We have observed a long-lasting inhibition of CA3 evoked excitatory postsynaptic currents (eEPSCs) after a brief application of kainate (KA) at concentrations ranging from 0.3 to 10 μM. The inhibition outlasted the change in holding current caused by the activation of ionotropic KARs in CA3 pyramidal cells, indicating that this action is not contingent on the opening of the receptor channels. The inhibition of the eEPSCs by KA was prevented by G protein and protein kinase A (PKA) inhibitors and was enhanced after stimulation of the adenylyl cyclase (AC) with forskolin. We conclude that KARs present at mossy fiber terminals mediate the inhibition of glutamate release through a metabotropic mechanism that involves the activation of an AC-second messenger cAMP-PKA signaling cascade.

Timing and efficacy of transmitter release at mossy fiber synapses in the hippocampal network

It is widely accepted that the hippocampus plays a major role in learning and memory. The mossy fiber synapse between granule cells in the dentate gyrus and pyramidal neurons in the CA3 region is a key component of the hippocampal trisynaptic circuit. Recent work, partially based on direct presynaptic patch-clamp recordings from hippocampal mossy fiber boutons, sheds light on the mechanisms of synaptic transmission and plasticity at mossy fiber synapses. A high Na(+) channel density in mossy fiber boutons leads to a large amplitude of the presynaptic action potential. Together with the fast gating of presynaptic Ca(2+) channels, this generates a large and brief presynaptic Ca(2+) influx, which can trigger transmitter release with high efficiency and temporal precision. The large number of release sites, the large size of the releasable pool of vesicles, and the huge extent of presynaptic plasticity confer unique strength to this synapse, suggesting a large impact onto the CA3 pyramidal cell network under specific behavioral conditions. The characteristic properties of the hippocampal mossy fiber synapse may be important for pattern separation and information storage in the dentate gyrus-CA3 cell network.

Synaptic plasticity at hippocampal mossy fibre synapses

The dentate gyrus provides the main input to the hippocampus. Information reaches the CA3 region through mossy fibre synapses made by dentate granule cell axons. Synaptic plasticity at the mossy fibre-pyramidal cell synapse is unusual for several reasons, including low basal release probability, pronounced frequency facilitation and a lack of N-methyl-D-aspartate receptor involvement in long-term potentiation. In the past few years, some of the mechanisms underlying the peculiar features of mossy fibre synapses have been elucidated. Here we describe recent work from several laboratories on the various forms of synaptic plasticity at hippocampal mossy fibre synapses. We conclude that these contacts have just begun to reveal their many secrets.

Gene targeting of presynaptic proteins in synaptic plasticity and memory: across the great divide

The past few decades have seen an explosion in our understanding of the molecular basis of learning and memory. The majority of these studies in mammals focused on post-synaptic signal transduction cascades involved in post-synaptic long-lasting plasticity. Until recently, relatively little work examined the role of presynaptic proteins in learning and memory in complex systems. The synaptic cleft figuratively represents a "great divide" between our knowledge of post- versus presynaptic involvement in learning and memory. While great strides have been made in our understanding of presynaptic proteins, we know very little of how presynaptically expressed forms of short- and long-term plasticity participate in information processing and storage. The paucity of cognitive behavioral research in the area of presynaptic proteins, however, is in stark contrast to the plethora of information concerning presynaptic protein involvement in neurotransmitter release, in modulation of release, and in both short- and long-term forms of presynaptic plasticity. It is now of great interest to begin to link the extensive literature on presynaptic proteins and presynaptic plasticity to cognitive behavior. In the future there is great promise with these approaches for identifying new targets in the treatment of cognitive disorders. This review article briefly surveys current knowledge on the role of presynaptic proteins in learning and memory in mammals and suggests future directions in learning and memory research on the presynaptic rim of the "great divide."

A similar impairment in CA3 mossy fibre LTP in the R6/2 mouse model of Huntington's disease and in the complexin II knockout mouse

Complexin II is reduced in Huntington's disease (HD) patients and in the R6/2 mouse model of HD. Mice lacking complexin II (Cplx2-/- mice) show selective cognitive deficits that reflect those seen in R6/2 mice. To determine whether or not there is a common mechanism that might underlie the cognitive deficits, long-term potentiation (LTP) was examined in the CA3 region of hippocampal slices from R6/2 mice and Cplx2-/- mice. While associational/commissural (A/C) LTP was not significantly different, mossy fibre (MF) LTP was significantly reduced in slices from R6/2 mice and Cplx2-/- mice compared with wild-type (WT) and Cplx2+/+ control mice. MF field excitatory postsynaptic potentials (fEPSPs) in response to paired stimuli were not significantly different between control mice and R6/2 or Cplx2-/- mice, suggesting that MF basal glutamate release is unaffected. Forskolin (30 microm) caused an increase in glutamate release at MF synapses in slices from R6/2 mice and from Cplx2-/- mice that was not significantly different from that seen in control mice, indicating that the capacity for increased glutamate release is not diminished. Thus, R6/2 mice and Cplx2-/- mice have a common selective impairment of MF LTP in the CA3 region. Together, these data suggest that complexin II is required for MF LTP, and that depletion of complexin II causes a selective impairment in MF LTP in the CA3 region. This impairment in MF LTP could contribute to spatial learning deficits observed in R6/2 and Cplx2-/- mice.