Recruitment of N-Type Ca(2+) channels during LTP enhances low release efficacy of hippocampal CA1 perforant path synapses

Cholecystokinin facilitates the formation of long-term heterosynaptic plasticity in the distal subiculum

It has been well established that Cornu Ammonis-(CA1) and subiculum (SUB) serve as the major output components of the entorhinal-hippocampal circuitry. Nevertheless, how the neuromodulators regulate the neurocircuitry in hippocampal formation has remained elusive. Cholecystokinin (CCK), is the most abundant neuropeptide in the central nervous system, which broadly regulates the animal's physiological status at multiple levels, including neuroplasticity and its behavioral consequences. Here, we uncover that exogenous CCK potentiates the excitatory synaptic transmission in the CA1-SUB projections via CCK-B receptor. Dual-color light theta burst stimulation elicits heterosynaptic long-term potentiation in distal SUB region. Light activation of medial entorhinal cortex (MEC) derived CCK-positive neurons triggers the CCK release in the SUB. Neuronal activities of SUB-projecting MECCCK neurons are necessary for conveying and processing of navigation-related information. In conclusion, our findings prove a crucial role of CCK in regulating neurobiological functions in the SUB region.

Impact of Omega-3 on Endocannabinoid System Expression and Function, Enhancing Cognition and Behavior in Male Mice

Background/Objectives: Omega-3 long-chain polyunsaturated fatty acids (PUFAs) support brain cell membrane integrity and help mitigate synaptic plasticity deficits. The endocannabinoid system (ECS) is integral to synaptic plasticity and regulates various brain functions. While PUFAs influence the ECS, the effects of omega-3 on the ECS, cognition, and behavior in a healthy brain remain unclear. Methods and Results: Here, we demonstrate that hippocampal synaptosomes from male mice fed an omega-3-rich diet exhibit increased levels of cannabinoid CB1 receptors (~30%), phospholipase C β1 (PLCβ1, ~30%), monoacylglycerol lipase (MAGL, ~30%), and cannabinoid receptor-interacting protein 1a (Crip1a, ~60%). Conversely, these synaptosomes show decreased levels of diacylglycerol lipase α (DAGLα, ~40%), synaptosomal-associated protein 25kDa (SNAP-25, ~30%), and postsynaptic density protein 95 (PSD-95, ~40%). Omega-3 intake also reduces Gαo and Gαi3 levels, though receptor-stimulated [35S]GTPγS binding remains unaffected. Stimulation of the medial perforant path (MPP) induced long-term potentiation (LTP) in omega-3-fed mice. This LTP was dependent on group I metabotropic glutamate receptors (mGluR), 2 arachidonoylglycerol (2-AG), CB1 receptors, N-type Ca2+ channels, and actin filaments. Behaviorally, omega-3-fed mice displayed reduced exploratory behavior and significantly improved object discrimination in the novel object recognition test (NORT). They also spent more time in open arms and exhibited reduced freezing time in the elevated plus maze (EPM), indicative of reduced anxiety-like behavior. Conclusions: Our findings suggest that omega-3 leverages the ECS to enhance brain function under normal conditions.

Rescue of synaptosomal glutamate release defects in tau transgenic mice by the tau aggregation inhibitor hydromethylthionine

Glutamatergic neurotransmission, important for learning and memory, is disrupted in different ways in patients with Alzheimer's disease (AD) and frontotemporal dementia (FTD) tauopathies. We have previously reported that two tau transgenic mouse models, L1 and L66, produce different phenotypes resembling AD and FTD, respectively. The AD-like L1 model expresses the truncated core aggregation domain of the AD paired helical filament (PHF) form of tau (tau296-390) whereas the FTD-like L66 model expresses full-length tau carrying two mutations at P301S/G335D. We have used synaptosomes isolated from these mice to investigate K+-evoked glutamate release and, if abnormal, to determine responsiveness to hydromethylthionine, a tau aggregation inhibitor previously shown to reduce tau pathology in these models. We report that the transgenes in these two mouse lines cause opposite abnormalities in glutamate release. Over-expression of the core tau unit in L1 produces a significant reduction in glutamate release and a loss of Ca2+-dependency compared with wild-type control mice. Full-length mutant tau produces an increase in glutamate release that retains normal Ca2+-dependency. Chronic pre-treatment with hydromethylthionine normalises both reduced (L1) and excessive glutamate (L66) and restores normal Ca2+-dependency in L1 mice. This implies that both patterns of impairment are the result of tau aggregation, but that the direction and Ca2+-dependency of the abnormality is determined by expression of the disease-specific transgene. Our results lead to the conclusion that the tauopathies need not be considered a single entity in terms of the downstream effects of pathological aggregation of tau protein. In this case, directionally opposite abnormalities in glutamate release resulting from different types of tau aggregation in the two mouse models can be corrected by hydromethylthionine. This may help to explain the activity of hydromethylthionine on cognitive decline and brain atrophy in both AD and behavioural-variant FTD.

Entorhinohippocampal cholecystokinin modulates spatial learning by facilitating neuroplasticity of hippocampal CA3-CA1 synapses

The hippocampus is broadly impacted by neuromodulations. However, how neuropeptides shape the function of the hippocampus and the related spatial learning and memory remains unclear. Here, we discover the crucial role of cholecystokinin (CCK) in heterosynaptic neuromodulation from the medial entorhinal cortex (MEC) to the hippocampus. Systematic knockout of the CCK gene impairs CA3-CA1 LTP and space-related performance. The MEC provides most of the CCK-positive neurons projecting to the hippocampal region, which potentiates CA3-CA1 long-term plasticity heterosynaptically in a frequency- and NMDA receptor (NMDAR)-dependent manner. Selective inhibition of MEC CCKergic neurons or downregulation of their CCK mRNA levels also impairs CA3-CA1 LTP formation and animals' performance in the water maze. This excitatory extrahippocampal projection releases CCK upon high-frequency excitation and is active during animal exploration. Our results reveal the critical role of entorhinal CCKergic projections in bridging intra- and extrahippocampal circuitry at electrophysiological and behavioral levels.

Leptin regulation of synaptic function at hippocampal TA-CA1 and SC-CA1 synapses

Increasing evidence indicates that the metabolic hormone, leptin markedly influences the functioning of the hippocampus. In particular, exposure to leptin results in persistent changes in synaptic efficacy at both temporoammonic (TA) and Schaffer Collateral (SC) inputs to hippocampal CA1 neurons. The ability of leptin to regulate TA-CA1 and SC-CA1 synapses has important functional implications, as both synaptic connections play important roles in hippocampal-dependent learning and memory. Here we review the modulatory actions of the hormone leptin at these hippocampal CA1 synapses and explore the impact on learning and memory processes.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Separation of presynaptic Cav2 and Cav1 channel function in synaptic vesicle exo- and endocytosis by the membrane anchored Ca2+ pump PMCA

Synaptic vesicle (SV) release, recycling, and plastic changes of release probability co-occur side by side within nerve terminals and rely on local Ca²⁺ signals with different temporal and spatial profiles. The mechanisms that guarantee separate regulation of these vital presynaptic functions during action potential (AP)-triggered presynaptic Ca²⁺ entry remain unclear. Combining Drosophila genetics with electrophysiology and imaging reveals the localization of two different voltage-gated calcium channels at the presynaptic terminals of glutamatergic neuromuscular synapses (the Drosophila Ca_v2 homolog, Dmca1A or cacophony, and the Ca_v1 homolog, Dmca1D) but with spatial and functional separation. Ca_v2 within active zones is required for AP-triggered neurotransmitter release. By contrast, Ca_v1 localizes predominantly around active zones and contributes substantially to AP-evoked Ca²⁺ influx but has a small impact on release. Instead, L-type calcium currents through Ca_v1 fine-tune short-term plasticity and facilitate SV recycling. Separate control of SV exo- and endocytosis by AP-triggered presynaptic Ca²⁺ influx through different channels demands efficient measures to protect the neurotransmitter release machinery against Ca_v1-mediated Ca²⁺ influx. We show that the plasma membrane Ca²⁺ ATPase (PMCA) resides in between active zones and isolates Ca_v2-triggered release from Ca_v1-mediated dynamic regulation of recycling and short-term plasticity, two processes which Ca_v2 may also contribute to. As L-type Ca_v1 channels also localize next to PQ-type Ca_v2 channels within axon terminals of some central mammalian synapses, we propose that Ca_v2, Ca_v1, and PMCA act as a conserved functional triad that enables separate control of SV release and recycling rates in presynaptic terminals.

Frequency-dependent layer-specific differences in short-term synaptic plasticity in the dorsal and ventral CA1 hippocampal field

Information from the entorhinal cortex arrives to the hippocampal CA1 microcircuit directly through the temporoammonic path (TA) that terminates in the stratum lacunosum-moleculare (SLM), and indirectly through Schaffer collateral pathway (SC) that terminates in the stratum radiatum (SR). By virtue of this input convergence, CA1 circuitry may act to compare and integrate incoming cortical information. Although a remarkable dorsal-ventral difference in short-term plasticity (STP) has been recently described at SC-CA1 synapses, the corresponding properties at TA-CA1 synapses have not been examined. Here, we report that stimulation of TA in the dorsal hippocampus produces significant facilitation of all conditioned responses evoked by 1-30 Hz, peaking at 20-30 Hz, and significant depression of steady-state responses to 50-100 Hz. Dorsal SC-CA1 synapses display a similar pattern of responses, yet, facilitation peaked at 10 Hz and depression (at 75-100 Hz) is weaker. Strikingly, stimulation of TA in the ventral hippocampus produces facilitation of steady-state responses to 1-30 Hz and highly contrasts with the depression of SC-CA1 synapses. Steady-state responses to 40-100 Hz in the ventral hippocampus depress in both layers similarly. High-frequency TA input (40-100 Hz) to the dorsal hippocampus depresses more in proximal than in distal SLM, while low-frequency (1-3 Hz) TA input to the ventral hippocampus facilitates more in distal than in proximal SLM. The present evidence suggests that direct and indirect entorhinal cortical inputs across the septotemporal extent of hippocampal CA1 field display frequency selectivity both in the radial and transverse axes, and that a rapid information processing may take place through direct ventral hippocampal CA1-EC circuit interactions independently of trisynaptic circuit.

Distance-dependent regulation of NMDAR nanoscale organization along hippocampal neuron dendrites

Hippocampal pyramidal neurons are characterized by a unique arborization subdivided in segregated dendritic domains receiving distinct excitatory synaptic inputs with specific properties and plasticity rules that shape their respective contributions to synaptic integration and action potential firing. Although the basal regulation and plastic range of proximal and distal synapses are known to be different, the composition and nanoscale organization of key synaptic proteins at these inputs remains largely elusive. Here we used superresolution imaging and single nanoparticle tracking in rat hippocampal neurons to unveil the nanoscale topography of native GluN2A- and GluN2B-NMDA receptors (NMDARs)-which play key roles in the use-dependent adaptation of glutamatergic synapses-along the dendritic arbor. We report significant changes in the nanoscale organization of GluN2B-NMDARs between proximal and distal dendritic segments, whereas the topography of GluN2A-NMDARs remains similar along the dendritic tree. Remarkably, the nanoscale organization of GluN2B-NMDARs at proximal segments depends on their interaction with calcium/calmodulin-dependent protein kinase II (CaMKII), which is not the case at distal segments. Collectively, our data reveal that the nanoscale organization of NMDARs changes along dendritic segments in a subtype-specific manner and is shaped by the interplay with CaMKII at proximal dendritic segments, shedding light on our understanding of the functional diversity of hippocampal glutamatergic synapses.

Rhythmic light flicker rescues hippocampal low gamma and protects ischemic neurons by enhancing presynaptic plasticity

The complex relationship between specific hippocampal oscillation frequency deficit and cognitive dysfunction in the ischemic brain is unclear. Here, using a mouse two-vessel occlusion (2VO) cerebral ischemia model, we show that visual stimulation with a 40 Hz light flicker drove hippocampal CA1 slow gamma and restored 2VO-induced reduction in CA1 slow gamma power and theta-low gamma phase-amplitude coupling, but not those of the high gamma. Low gamma frequency lights at 30 Hz, 40 Hz, and 50 Hz, but not 10 Hz, 80 Hz, and arrhythmic frequency light, were protective against degenerating CA1 neurons after 2VO, demonstrating the importance of slow gamma in cognitive functions after cerebral ischemia. Mechanistically, 40 Hz light flicker enhanced RGS12-regulated CA3-CA1 presynaptic N-type calcium channel-dependent short-term synaptic plasticity and associated postsynaptic long term potentiation (LTP) after 2VO. These results support a causal relationship between CA1 slow gamma and cognitive dysfunctions in the ischemic brain.

Proximodistal Heterogeneity in Learning-promoted Pathway-specific Plasticity at Dorsal CA1 Synapses

Contextual learning requires the delivery of AMPA receptors to CA1 synapses in the dorsal hippocampus. However, proximodistal heterogeneity of pathway-specific plasticity remains unclear. Here, we examined the proximodistal heterogeneity in learning-induced plasticity at the CA1 synapses with inputs from the entorhinal cortex layer III (ECIII) or from CA3. We subjected male rats to an inhibitory avoidance task and prepared acute hippocampal slices for whole-cell patch clamp experiments, where we stimulated ECIII-CA1 or CA3-CA1 input fibers to analyze evoked excitatory postsynaptic currents (EPSCs). Compared to untrained controls, trained rats exhibited higher AMPA/NMDA current ratios at CA3-CA1 synapses of proximal and intermediate, but not distal CA1 neurons, which suggested that region-specific plasticity occurred after learning. Moreover, trained rats exhibited higher AMPA/NMDA current ratios at ECIII-CA1 synapses of intermediate and distal, but not proximal CA1 neurons. These findings suggested the presence of proximodistal heterogeneity in pathway-specific postsynaptic plasticity. Regarding presynaptic plasticity, training slightly, but significantly increased the paired-pulse ratios of CA3-CA1 synapses of proximal and intermediate, but not distal CA1 neurons. Moreover, trained rats exhibited higher paired-pulse ratios at ECIII-CA1 synapses of intermediate and distal, but not proximal CA1 neurons, which suggested region-specific presynaptic plasticity. Finally, learning was clearly prevented by the bilateral microinjection of a plasticity blocker in the proximal or intermediate, but not distal CA1 subfields, which suggested functional heterogeneity along the proximodistal axis. Understanding region- and pathway-specific plasticity at dorsal CA1 synapses could aid in controlling encoded memory.

Calcium Dynamics and Synaptic Plasticity

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

(2R,6R)-hydroxynorketamine rapidly potentiates hippocampal glutamatergic transmission through a synapse-specific presynaptic mechanism

Preclinical studies indicate that (2R,6R)-hydroxynorketamine (HNK) retains the rapid and sustained antidepressant-like actions of ketamine, but is spared its dissociative-like properties and abuse potential. While (2R,6R)-HNK is thought to exert its antidepressant-like effects by potentiating α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-mediated synaptic transmission, it is unknown how it exerts this effect. The acute synaptic effects of (2R,6R)-HNK were examined by recording field excitatory postsynaptic potentials (fEPSPs) and miniature excitatory postsynaptic currents (mEPSCs) in rat hippocampal slices. (2R,6R)-HNK bath application caused a rapid and persistent potentiation of AMPAR-mediated Schaffer collateral (SC)-CA1 fEPSPs in slices derived from male and female rats. The (2R,6R)-HNK-induced potentiation occurred independent of N-methyl-D-aspartate receptor (NMDAR) activity, was accompanied by a concentration-dependent decrease in paired pulse ratios, and was occluded by raising glutamate release probability. In additon, in the presence of tetrodotoxin, (2R,6R)-HNK increased the frequency, but not amplitude, of mEPSC events, confirming a presynaptic site of action that is independent of glutamatergic network disinhibition. A dual extracellular recording configuration revealed that the presynaptic effects of (2R,6R)-HNK were synapse-selective, occurring in CA1-projecting SC terminals, but not in CA1-projecting temporoammonic terminals. Overall, we found that (2R,6R)-HNK enhances excitatory synaptic transmission in the hippocampus through a concentration-dependent, NMDAR-independent, and synapse-selective increase in glutamate release probability with no direct actions on AMPAR function. These findings provide novel insight regarding (2R,6R)-HNK's acute mechanism of action, and may inform novel antidepressant drug mechanisms that could yield superior efficacy, safety, and tolerability.

Modulation of Ion Channels in the Axon: Mechanisms and Function

The axon is responsible for integrating synaptic signals, generating action potentials (APs), propagating those APs to downstream synapses and converting them into patterns of neurotransmitter vesicle release. This process is mediated by a rich assortment of voltage-gated ion channels whose function can be affected on short and long time scales by activity. Moreover, neuromodulators control the activity of these proteins through G-protein coupled receptor signaling cascades. Here, we review cellular mechanisms and signaling pathways involved in axonal ion channel modulation and examine how changes to ion channel function affect AP initiation, AP propagation, and the release of neurotransmitter. We then examine how these mechanisms could modulate synaptic function by focusing on three key features of synaptic information transmission: synaptic strength, synaptic variability, and short-term plasticity. Viewing these cellular mechanisms of neuromodulation from a functional perspective may assist in extending these findings to theories of neural circuit function and its neuromodulation.

Leptin Regulation of Synaptic Function at Hippocampal TA-CA1 and SC-CA1 Synapses: Implications for Health and Disease

Growing evidence indicates that the endocrine hormone leptin regulates hippocampal synaptic function in addition to its established role as a hypothalamic satiety signal. Indeed, numerous studies show that leptin facilitates the cellular events that underlie hippocampal learning and memory including activity-dependent synaptic plasticity and glutamate receptor trafficking, indicating that leptin may be a potential cognitive enhancer. Although there has been extensive investigation into the modulatory role of leptin at hippocampal Schaffer collateral (SC)-CA1 synapses, recent evidence indicates that leptin also potently regulates excitatory synaptic transmission at the anatomically distinct temporoammonic (TA) input to hippocampal CA1 neurons. The cellular mechanisms underlying activity-dependent synaptic plasticity at TA-CA1 synapses differ from those at SC-CA1 synapses and the TA input is implicated in spatial and episodic memory formation. Furthermore, the TA input is an early target for neurodegeneration in Alzheimer's disease (AD) and aberrant leptin function is linked to AD. Here, we review the evidence that leptin regulates hippocampal synaptic function at both SC- and TA-CA1 synapses and discuss the consequences for neurodegenerative disorders like AD.

Early life stress impairs fear memory and synaptic plasticity; a potential role for GluN2B

Programming of the brain by early life stress has been associated with alterations in structure and function of the dorsal hippocampus. Yet, the underlying molecular mechanisms remain largely elusive. In this study, we examined the effects of early life stress (ELS) - by housing mouse dams with limited nesting and bedding material from postnatal days 2-9 and examined in 6 month old offspring; 1) auditory fear conditioning, 2) expression of the hippocampal N-methyl-d-aspartate receptor (NMDA-R) subunits 2A and 2B (GluN2A, GluN2B), and expression of PSD-95 and synaptophysin, and 3) short- and long-term (LTP) synaptic plasticity. Given its critical role in NMDA receptor function and synaptic plasticity, we further examined the role of GluN2B in effects of ELS on synaptic plasticity and fear memory formation. We demonstrate that ELS impaired fear memory in 6 month old mice and decreased hippocampal LTP as well as the paired-pulse ratio (PPR). ELS also reduced hippocampal GluN2B expression. Interestingly, pharmacological blockade of GluN2B with the selective antagonist Ro25 6981 was less effective to reduce synaptic plasticity in ELS mice, and was also ineffective to impair memory retrieval in ELS mice. These studies suggest that ELS reduces hippocampal synaptic plasticity and fear memory formation and hampers GluN2B receptor function. As such, GluN2B may provide an important target for future strategies to prevent lasting ELS effects on cognition.

Homeostatic Presynaptic Plasticity Is Specifically Regulated by P/Q-type Ca2+ Channels at Mammalian Hippocampal Synapses

Voltage-dependent Ca²⁺ channels (VGCC) represent the principal source of Ca²⁺ ions driving evoked neurotransmitter release at presynaptic boutons. In mammals, presynaptic Ca²⁺ influx is mediated mainly via P/Q-type and N-type VGCC, which differ in their properties. Changes in their relative contributions tune neurotransmission both during development and in Hebbian plasticity. However, whether this represents a functional motif also present in other forms of activity-dependent regulation is unknown. Here, we study the role of VGCC in homeostatic plasticity (HSP) in mammalian hippocampal neurons using optical techniques. We find that changes in evoked Ca²⁺ currents specifically through P/Q-type, but not N-type, VGCC mediate bidirectional homeostatic regulation of both neurotransmitter release efficacy and the size of the major synaptic vesicle pools. Selective dependence of HSP on P/Q-type VGCC in mammalian terminals has important implications for phenotypes associated with P/Q-type channelopathies, including migraine and epilepsy.

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P/Q-type VGCC regulate homeostatic synaptic plasticity (HSP) in mammals

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Changes in synaptic vesicle pool sizes during HSP are also mediated via P/Q-type VGCC

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Expression of HSP is independent of N-type VGCC regulation

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P/Q-type regulation of HSP may explain phenotypes of P/Q-type channelopathies

Long-Term Plasticity of Neurotransmitter Release: Emerging Mechanisms and Contributions to Brain Function and Disease

Long-lasting changes of brain function in response to experience rely on diverse forms of activity-dependent synaptic plasticity. Chief among them are long-term potentiation and long-term depression of neurotransmitter release, which are widely expressed by excitatory and inhibitory synapses throughout the central nervous system and can dynamically regulate information flow in neural circuits. This review article explores recent advances in presynaptic long-term plasticity mechanisms and contributions to circuit function. Growing evidence indicates that presynaptic plasticity may involve structural changes, presynaptic protein synthesis, and transsynaptic signaling. Presynaptic long-term plasticity can alter the short-term dynamics of neurotransmitter release, thereby contributing to circuit computations such as novelty detection, modifications of the excitatory/inhibitory balance, and sensory adaptation. In addition, presynaptic long-term plasticity underlies forms of learning and its dysregulation participates in several neuropsychiatric conditions, including schizophrenia, autism, intellectual disabilities, neurodegenerative diseases, and drug abuse.

Calcium dependence of spontaneous neurotransmitter release

Spontaneous release of neurotransmitters is regulated by extracellular [Ca2+ ] and intracellular [Ca2+ ]. Curiously, some of the mechanisms of Ca2+ signaling at central synapses are different at excitatory and inhibitory synapses. While the stochastic activity of voltage-activated Ca2+ channels triggers a majority of spontaneous release at inhibitory synapses, this is not the case at excitatory nerve terminals. Ca2+ release from intracellular stores regulates spontaneous release at excitatory and inhibitory terminals, as do agonists of the Ca2+ -sensing receptor. Molecular machinery triggering spontaneous vesicle fusion may differ from that underlying evoked release and may be one of the sources of heterogeneity in release mechanisms.

Hippocampal area CA2: properties and contribution to hippocampal function

This review focuses on area CA2 of the hippocampus, as recent results have revealed the unique properties and surprising role of this region in encoding social, temporal and contextual aspects of memory. Originally identified and described by Lorente de No, in 1934, this region of the hippocampus has unique intra-and extra-hippocampal connectivity, sending and receiving input to septal and hypothalamic regions. Recent in vivo studies have indicated that CA2 pyramidal neurons encode spatial information during immobility and play an important role in the generation of sharp-wave ripples. Furthermore, CA2 neurons act to control overall excitability in the hippocampal network and have been found to be consistently altered in psychiatric diseases, indicating that normal function of this region is necessary for normal cognition. With its unique role, area CA2 has a unique molecular profile, interneuron density and composition. Furthermore, this region has an unusual manifestation of synaptic plasticity that does not occur post-synaptically at pyramidal neuron dendrities but through the local network of inhibitory neurons. While much progress has recently been made in understanding the large contribution of area CA2 to social memory formation, much still needs to be learned.

Syringaresinol suppresses excitatory synaptic transmission and picrotoxin-induced epileptic activity in the hippocampus through presynaptic mechanisms

Many neuromodulating drugs acting on the nervous system originate from botanical sources. These plant-derived substances modulate the activity of receptors, ion channels, or transporters in neurons. Their properties make the substances useful for medicine and research. Here, we show that the plant lignan (+)-syringaresinol (SYR) suppresses excitatory synaptic transmission via presynaptic modulation. Bath application of SYR rapidly reduced the slopes of the field excitatory postsynaptic potentials (fEPSPs) at the hippocampal Schaffer collateral (SC)-CA1 synapse in a dose-dependent manner. SYR preferentially affected excitatory synapses, while inhibitory synaptic transmission remained unchanged. SYR had no effect on the conductance or the desensitization of AMPARs but increased the paired-pulse ratios of synaptic responses at short (20-200 ms) inter-stimulus intervals. These presynaptic changes were accompanied by a reduction of the readily releasable pool size. Pretreatment of hippocampal slices with the G_i/o protein inhibitor N-ethylmaleimide (NEM) abolished the effect of SYR on excitatory synaptic transmission, while the application of SYR significantly decreased Ca²⁺ currents and hyperpolarized the resting membrane potentials of hippocampal neurons. In addition, SYR suppressed picrotoxin-induced epileptiform activity in hippocampal slices. Overall, our study identifies SYR as a new neuromodulating agent and suggests that SYR suppresses excitatory synaptic transmission by modulating presynaptic transmitter release.

Degeneracy in the regulation of short-term plasticity and synaptic filtering by presynaptic mechanisms

KEY POINTS

We develop a new biophysically rooted, physiologically constrained conductance-based synaptic model to mechanistically account for short-term facilitation and depression, respectively through residual calcium and transmitter depletion kinetics. We address the specific question of how presynaptic components (including voltage-gated ion channels, pumps, buffers and release-handling mechanisms) and interactions among them define synaptic filtering and short-term plasticity profiles. Employing global sensitivity analyses (GSAs), we show that near-identical synaptic filters and short-term plasticity profiles could emerge from disparate presynaptic parametric combinations with weak pairwise correlations. Using virtual knockout models, a technique to address the question of channel-specific contributions within the GSA framework, we unveil the differential and variable impact of each ion channel on synaptic physiology. Our conclusions strengthen the argument that parametric and interactional complexity in biological systems should not be viewed from the limited curse-of-dimensionality standpoint, but from the evolutionarily advantageous perspective of providing functional robustness through degeneracy.

ABSTRACT

Information processing in neurons is known to emerge as a gestalt of pre- and post-synaptic filtering. However, the impact of presynaptic mechanisms on synaptic filters has not been quantitatively assessed. Here, we developed a biophysically rooted, conductance-based model synapse that was endowed with six different voltage-gated ion channels, calcium pumps, calcium buffer and neurotransmitter-replenishment mechanisms in the presynaptic terminal. We tuned our model to match the short-term plasticity profile and band-pass structure of Schaffer collateral synapses, and performed sensitivity analyses to demonstrate that presynaptic voltage-gated ion channels regulated synaptic filters through changes in excitability and associated calcium influx. These sensitivity analyses also revealed that calcium- and release-control mechanisms were effective regulators of synaptic filters, but accomplished this without changes in terminal excitability or calcium influx. Next, to perform global sensitivity analysis, we generated 7000 randomized models spanning 15 presynaptic parameters, and computed eight different physiological measurements in each of these models. We validated these models by applying experimentally obtained bounds on their measurements, and found 104 (∼1.5%) models to match the validation criteria for all eight measurements. Analysing these valid models, we demonstrate that analogous synaptic filters emerge from disparate combinations of presynaptic parameters exhibiting weak pairwise correlations. Finally, using virtual knockout models, we establish the variable and differential impact of different presynaptic channels on synaptic filters, underlining the critical importance of interactions among different presynaptic components in defining synaptic physiology. Our results have significant implications for protein-localization strategies required for physiological robustness and for degeneracy in long-term synaptic plasticity profiles.

HCN1 channels reduce the rate of exocytosis from a subset of cortical synaptic terminals

The hyperpolarization-activated cyclic nucleotide-gated (HCN1) channels are predominantly located in pyramidal cell dendrites within the cortex. Recent evidence suggests these channels also exist pre-synaptically in a subset of synaptic terminals within the mature entorhinal cortex (EC). Inhibition of pre-synaptic HCN channels enhances miniature excitatory post-synaptic currents (mEPSCs) onto EC layer III pyramidal neurons, suggesting that these channels decrease the release of the neurotransmitter, glutamate. Thus, do pre-synaptic HCN channels alter the rate of synaptic vesicle exocytosis and thereby enhance neurotransmitter release? To address this, we imaged the release of FM1-43, a dye that is incorporated into synaptic vesicles, from EC synaptic terminals using two photon microscopy in slices obtained from forebrain specific HCN1 deficient mice, global HCN1 knockouts and their wildtype littermates. This coupled with electrophysiology and pharmacology showed that HCN1 channels restrict the rate of exocytosis from a subset of cortical synaptic terminals within the EC and in this way, constrain non-action potential-dependent and action potential-dependent spontaneous release as well as synchronous, evoked release. Since HCN1 channels also affect post-synaptic potential kinetics and integration, our results indicate that there are diverse ways by which HCN1 channels influence synaptic strength and plasticity.

Persistent Associative Plasticity at an Identified Synapse Underlying Classical Conditioning Becomes Labile with Short-Term Homosynaptic Activation

Synapses express different forms of plasticity that contribute to different forms of memory, and both memory and plasticity can become labile after reactivation. We previously reported that a persistent form of nonassociative long-term facilitation (PNA-LTF) of the sensorimotor synapses in Aplysia californica, a cellular analog of long-term sensitization, became labile with short-term heterosynaptic reactivation and reversed when the reactivation was followed by incubation with the protein synthesis inhibitor rapamycin. Here we examined the reciprocal impact of different forms of short-term plasticity (reactivations) on a persistent form of associative long-term facilitation (PA-LTF), a cellular analog of classical conditioning, which was expressed at Aplysia sensorimotor synapses when a tetanic stimulation of the sensory neurons was paired with a brief application of serotonin on 2 consecutive days. The expression of short-term homosynaptic plasticity [post-tetanic potentiation or homosynaptic depression (HSD)], or short-term heterosynaptic plasticity [serotonin-induced facilitation or neuropeptide Phe-Met-Arg-Phe-NH2 (FMRFa)-induced depression], at synapses expressing PA-LTF did not affect the maintenance of PA-LTF. The kinetics of HSD was attenuated at synapses expressing PA-LTF, which required activation of protein kinase C (PKC). Both PA-LTF and the attenuated kinetics of HSD were reversed by either a transient blockade of PKC activity or a homosynaptic, but not heterosynaptic, reactivation when paired with rapamycin. These results indicate that two different forms of persistent synaptic plasticity, PA-LTF and PNA-LTF, expressed at the same synapse become labile when reactivated by different stimuli.

SIGNIFICANCE STATEMENT

Activity-dependent changes in neural circuits mediate long-term memories. Some forms of long-term memories become labile and can be reversed with specific types of reactivations, but the mechanism is complex. At the cellular level, reactivations that induce a reversal of memory must evoke changes in neural circuits underlying the memory. What types of reactivations induce a labile state at neural connections that lead to reversal of different types of memory? We find that a critical neural connection in Aplysia, which is modified with different stimuli that mediate different types of memory, becomes labile with different types of reactivations. These results provide insights for developing strategies in alleviating maladaptive memories accompanying anxiety disorders.

Long-term Potentiation at Temporoammonic Path-CA1 Synapses in Freely Moving Rats

Hippocampal area CA1 receives direct entorhinal layer III input via the temporoammonic path (TAP) and recent studies implicate TAP-CA1 synapses are important for some aspects of hippocampal memory function. Nonetheless, as few studies have examined TAP-CA1 synaptic plasticity in vivo, the induction and longevity of TAP-CA1 long-term potentiation (LTP) has not been fully characterized. We analyzed CA1 responses following stimulation of the medial aspect of the angular bundle and investigated LTP at medial temporoammonic path (mTAP)-CA1 synapses in freely moving rats. We demonstrate monosynaptic mTAP-CA1 responses can be isolated in vivo as evidenced by observations of independent current sinks in the stratum lacunosum moleculare of both areas CA1 and CA3 following angular bundle stimulation. Contrasting prior indications that TAP input rarely elicits CA1 discharge, we observed mTAP-CA1 responses that appeared to contain putative population spikes in 40% of our behaving animals. Theta burst high frequency stimulation of mTAP afferents resulted in an input specific and N-methyl-D-aspartate (NMDA) receptor-dependent LTP of mTAP-CA1 responses in behaving animals. LTP of mTAP-CA1 responses decayed as a function of two exponential decay curves with time constants (τ) of 2.7 and 148 days to decay 63.2% of maximal LTP. In contrast, mTAP-CA1 population spike potentiation longevity demonstrated a τ of 9.6 days. To our knowledge, these studies provide the first description of mTAP-CA1 LTP longevity in vivo. These data indicate TAP input to area CA1 is a physiologically relevant afferent system that displays robust synaptic plasticity.

Dendritic sodium spikes are required for long-term potentiation at distal synapses on hippocampal pyramidal neurons

Dendritic integration of synaptic inputs mediates rapid neural computation as well as longer-lasting plasticity. Several channel types can mediate dendritically initiated spikes (dSpikes), which may impact information processing and storage across multiple timescales; however, the roles of different channels in the rapid vs long-term effects of dSpikes are unknown. We show here that dSpikes mediated by Nav channels (blocked by a low concentration of TTX) are required for long-term potentiation (LTP) in the distal apical dendrites of hippocampal pyramidal neurons. Furthermore, imaging, simulations, and buffering experiments all support a model whereby fast Nav channel-mediated dSpikes (Na-dSpikes) contribute to LTP induction by promoting large, transient, localized increases in intracellular calcium concentration near the calcium-conducting pores of NMDAR and L-type Cav channels. Thus, in addition to contributing to rapid neural processing, Na-dSpikes are likely to contribute to memory formation via their role in long-lasting synaptic plasticity.

PKC enhances the capacity for secretion by rapidly recruiting covert voltage-gated Ca2+ channels to the membrane

It is unknown whether neurons can dynamically control the capacity for secretion by promptly changing the number of plasma membrane voltage-gated Ca(2+) channels. To address this, we studied peptide release from the bag cell neurons of Aplysia californica, which initiate reproduction by secreting hormone during an afterdischarge. This burst engages protein kinase C (PKC) to trigger the insertion of a covert Ca(2+) channel, Apl Cav2, alongside a basal channel, Apl Cav1. The significance of Apl Cav2 recruitment to secretion remains undetermined; therefore, we used capacitance tracking to assay secretion, along with Ca(2+) imaging and Ca(2+) current measurements, from cultured bag cell neurons under whole-cell voltage-clamp. Activating PKC with the phorbol ester, PMA, enhanced Ca(2+) entry, and potentiated stimulus-evoked secretion. This relied on channel insertion, as it was occluded by preventing Apl Cav2 engagement with prior whole-cell dialysis or the cytoskeletal toxin, latrunculin B. Channel insertion reduced the stimulus duration and/or frequency required to initiate secretion and strengthened excitation-secretion coupling, indicating that Apl Cav2 accesses peptide release more readily than Apl Cav1. The coupling of Apl Cav2 to secretion also changed with behavioral state, as Apl Cav2 failed to evoke secretion in silent neurons from reproductively inactive animals. Finally, PKC also acted secondarily to enhance prolonged exocytosis triggered by mitochondrial Ca(2+) release. Collectively, our results suggest that bag cell neurons dynamically elevate Ca(2+) channel abundance in the membrane to ensure adequate secretion during the afterdischarge.

Dopamine-dependent corticostriatal synaptic filtering regulates sensorimotor behavior

Modulation of corticostriatal synaptic activity by dopamine is required for normal sensorimotor behaviors. After loss of nigrostriatal dopamine axons in Parkinson's disease, l-3,4-dihydroxyphenlalanine (l-DOPA) and dopamine D2-like receptor agonists are used as replacement therapy, although these drugs also trigger sensitized sensorimotor responses including dyskinesias and impulse control disorders. In mice, we lesioned dopamine projections to the left dorsal striatum and assayed unilateral sensorimotor deficits with the corridor test as well as presynaptic corticostriatal activity with the synaptic vesicle probe, FM1-43. Sham-lesioned mice acquired food equivalently on both sides, while D2 receptor activation filtered the less active corticostriatal terminals, a response that required coincident co-activation of mGlu-R5 metabotropic glutamate and CB1 endocannabinoid receptors. Lesioned mice did not acquire food from their right, but overused that side following treatment with l-DOPA. Synaptic filtering on the lesioned side was abolished by either l-DOPA or a D2 receptor agonist, but when combined with a CB1 receptor antagonist, l-DOPA or D2 agonists normalized both synaptic filtering and behavior. Thus, high-pass filtering of corticostriatal synapses by the coordinated activation of D2, mGlu-R5, and CB1 receptors is required for normal sensorimotor response to environmental cues.

Bassoon specifically controls presynaptic P/Q-type Ca(2+) channels via RIM-binding protein

Voltage-dependent Ca(2+) channels (CaVs) represent the principal source of Ca(2+) ions that trigger evoked neurotransmitter release from presynaptic boutons. Ca(2+) influx is mediated mainly via CaV2.1 (P/Q-type) and CaV2.2 (N-type) channels, which differ in their properties. Their relative contribution to synaptic transmission changes during development and tunes neurotransmission during synaptic plasticity. The mechanism of differential recruitment of CaV2.1 and CaV2.2 to release sites is largely unknown. Here, we show that the presynaptic scaffolding protein Bassoon localizes specifically CaV2.1 to active zones via molecular interaction with the RIM-binding proteins (RBPs). A genetic deletion of Bassoon or an acute interference with Bassoon-RBP interaction reduces synaptic abundance of CaV2.1, weakens P/Q-type Ca(2+) current-driven synaptic transmission, and results in higher relative contribution of neurotransmission dependent on CaV2.2. These data establish Bassoon as a major regulator of the molecular composition of the presynaptic neurotransmitter release sites.

Lack of the presynaptic RhoGAP protein oligophrenin1 leads to cognitive disabilities through dysregulation of the cAMP/PKA signalling pathway

Loss-of-function mutations in the gene encoding for the RhoGAP protein of oligophrenin-1 (OPHN1) lead to cognitive disabilities (CDs) in humans, yet the underlying mechanisms are not known. Here, we show that in mice constitutive lack of Ophn1 is associated with dysregulation of the cyclic adenosine monophosphate/phosphate kinase A (cAMP/PKA) signalling pathway in a brain-area-specific manner. Consistent with a key role of cAMP/PKA signalling in regulating presynaptic function and plasticity, we found that PKA-dependent presynaptic plasticity was completely abolished in affected brain regions, including hippocampus and amygdala. At the behavioural level, lack of OPHN1 resulted in hippocampus- and amygdala-related learning disabilities which could be fully rescued by the ROCK/PKA kinase inhibitor fasudil. Together, our data identify OPHN1 as a key regulator of presynaptic function and suggest that, in addition to reported postsynaptic deficits, loss of presynaptic plasticity contributes to the pathophysiology of CDs.

The expression of long-term potentiation: reconciling the preists and the postivists

Long-term potentiation (LTP) of excitatory synaptic transmission in the hippocampus has been investigated in great detail over the past 40 years. Where and how LTP is actually expressed, however, remain controversial issues. Considerable evidence has been offered to support both pre- and postsynaptic contributions to LTP expression. Though it is widely held that postsynaptic expression mechanisms are the primary contributors to LTP expression, evidence for that conclusion is amenable to alternative explanations. Here, we briefly review some key contributions to the 'locus' debate and describe data that support a dominant role for presynaptic mechanisms. Recognition of the state-dependency of expression mechanisms, and consideration of the consequences of the spatial relationship between postsynaptic glutamate receptors and presynaptic vesicular release sites, lead to a model that may reconcile views from both sides of the synapse.

Ionotropic glutamate receptors and voltage-gated Ca²⁺ channels in long-term potentiation of spinal dorsal horn synapses and pain hypersensitivity

Over the last twenty years of research on cellular mechanisms of pain hypersensitivity, long-term potentiation (LTP) of synaptic transmission in the spinal cord dorsal horn (DH) has emerged as an important contributor to pain pathology. Mechanisms that underlie LTP of spinal DH neurons include changes in the numbers, activity, and properties of ionotropic glutamate receptors (AMPA and NMDA receptors) and of voltage-gated Ca²⁺ channels. Here, we review the roles and mechanisms of these channels in the induction and expression of spinal DH LTP, and we present this within the framework of the anatomical organization and synaptic circuitry of the spinal DH. Moreover, we compare synaptic plasticity in the spinal DH with classical LTP described for hippocampal synapses.

Nitric oxide signaling exerts bidirectional effects on plasticity inductions in amygdala

It has been well known that long-term potentiation (LTP) of synaptic transmission in the lateral nucleus of the amygdala (LA) constitutes an essential cellular mechanism contributing to encoding of conditioned fear. Nitric oxide (NO), produced by activation of the postsynaptic N-methyl-D-aspartate receptors (NMDAR) in thalamic input to the LA, has been thought to promote LTP, contributing to the establishment of conditioned fear. However, it is not known whether and how NO, released from cortical input to the LA, plays the role on the plasticity induction and fear memory. Here we report that the diffusion of NO, released in response to activation of presynaptic NMDAR on cortical afferent fibers in the LA, could suppress heterosynaptically a form of presynaptic kainate receptor (KAR) dependent LTP (pre-LTP) in thalamic input, which was induced by low-frequency presynaptic stimuli without postsynaptic depolarization. We also confirmed that NO, produced by activation of postsynaptic NMDAR in thalamic input, can promote postsynaptic NMDAR-dependent LTP (post-LTP), which was induced by pairing protocol. These LTPs were occluded following fear conditioning, indicating that they could contribute to encoding of conditioned fear memory. However, their time courses are different; Post-LTP was more rapidly formed than pre-LTP in the course of fear conditioning. NO, produced by activation of presynaptic NMDAR in cortical input and postsynaptic NMDAR in thalamic input, may control conditioned fear by suppressing pre-LTP and promoting post-LTP, respectively, in thalamic input to the LA.

Dominance of P/Q-type calcium channels in depolarization-induced presynaptic FM dye release in cultured hippocampal neurons

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We analyzed depolarization-induced synaptic FM dye release in hippocampal neurons.

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We pharmacologically isolated the contribution of voltage-gated Ca²⁺ channels.

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85% of synapses utilize N- and P/Q-type channels, 15% only P/Q-type channels.

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In both groups of synapses release kinetics are determined by P/Q-type channels.

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We propose a more direct coupling of P/Q-type channels to synaptic release.

Neurotransmitter release probability is related by high power to the local concentration of calcium in presynaptic terminals, which in turn is controlled by voltage-gated calcium channels. P/Q- and N-type channels trigger synaptic transmission in the majority of neurons of the central nervous system. However, whether and under which conditions both channel types act cooperatively or independently is still insufficiently understood. Previous studies suggested either a dominance of N- or P/Q-type channels, or a synergistic action of both channels, depending on the experimental paradigms. Thus, to provide insight into the properties of neurotransmitter release in cultured mouse hippocampal neurons, we used quantitative analysis of FM dye release from presynaptic boutons induced by high potassium membrane depolarization. Increasing extracellular potassium concentrations revealed a sigmoid dependence of FM dye release to the stimulation strength. Individual and combined application of the P/Q- and N-type channel-specific blockers ω-agatoxin-IVA and ω-conotoxin-GVIA, respectively, allowed us to specifically isolate the contribution of both channel types to release triggered with 40 mM KCl. Analysis of the release kinetics and the fractional release amplitude demonstrate that, whereas in only 15% of the synapses release depended exclusively on P/Q-type channels, the majority of synapses (85%) contained both N- and P/Q-type channels. Nevertheless, the kinetics of FM dye release in synapses containing both channel types was determined by the P/Q-type channels. Together, our data suggest a more direct coupling of P/Q-type channels to synaptic release compared to N-type channels, which may explain the high prevalence of neurological P/Q-type channelopathies.

The temporoammonic input to the hippocampal CA1 region displays distinctly different synaptic plasticity compared to the Schaffer collateral input in vivo: significance for synaptic information processing

In terms of its sub-regional differentiation, the hippocampal CA1 region receives cortical information directly via the perforant (temporoammonic) path (pp-CA1 synapse) and indirectly via the tri-synaptic pathway where the last relay station is the Schaffer collateral-CA1 synapse (Sc-CA1 synapse). Research to date on pp-CA1 synapses has been conducted predominantly in vitro and never in awake animals, but these studies hint that information processing at this synapse might be distinct to processing at the Sc-CA1 synapse. Here, we characterized synaptic properties and synaptic plasticity at the pp-CA1 synapse of freely behaving adult rats. We observed that field excitatory postsynaptic potentials at the pp-CA1 synapse have longer onset latencies and a shorter time-to-peak compared to the Sc-CA1 synapse. LTP (>24 h) was successfully evoked by tetanic afferent stimulation of pp-CA1 synapses. Low frequency stimulation evoked synaptic depression at Sc-CA1 synapses, but did not elicit LTD at pp-CA1 synapses unless the Schaffer collateral afferents to the CA1 region had been severed. Paired-pulse responses also showed significant differences. Our data suggest that synaptic plasticity at the pp-CA1 synapse is distinct from the Sc-CA1 synapse and that this may reflect its specific role in hippocampal information processing.

Neuroligin1 drives synaptic and behavioral maturation through intracellular interactions

In vitro studies suggest that the intracellular C terminus of Neuroligin1 (NL1) could play a central role in the maturation of excitatory synapses. However, it is unknown how this activity affects synapses in vivo, and whether it may impact the development of complex behaviors. To determine how NL1 influences the state of glutamatergic synapses in vivo, we compared the synaptic and behavioral phenotypes of mice overexpressing a full-length version of NL1 (NL1FL) with mice overexpressing a version missing part of the intracellular domain (NL1ΔC). We show that overexpression of full-length NL1 yielded an increase in the proportion of synapses with mature characteristics and impaired learning and flexibility. In contrast, the overexpression of NL1ΔC increased the number of excitatory postsynaptic structures and led to enhanced flexibility in mnemonic and social behaviors. Transient overexpression of NL1FL revealed that elevated levels are not necessary to maintain synaptic and behavioral states altered earlier in development. In contrast, overexpression of NL1FL in the fully mature adult was able to impair normal learning behavior after 1 month of expression. These results provide the first evidence that NL1 significantly impacts key developmental processes that permanently shape circuit function and behavior, as well as the function of fully developed neural circuits. Overall, these manipulations of NL1 function illuminate the significance of NL1 intracellular signaling in vivo, and enhance our understanding of the factors that gate the maturation of glutamatergic synapses and complex behavior. This has significant implications for our ability to address disorders such as autism spectrum disorders.

Long-term fluoxetine treatment induces input-specific LTP and LTD impairment and structural plasticity in the CA1 hippocampal subfield

Antidepressant drugs are usually administered for several weeks for the treatment of major depressive disorder. However, they are also prescribed in several additional psychiatric conditions as well as during long-term maintenance treatments. Antidepressants induce adaptive changes in several forebrain structures which include modifications at glutamatergic synapses. We recently found that repetitive administration of the selective serotonin reuptake inhibitor (SSRI) fluoxetine to naïve adult male rats induced an increase of mature, mushroom-type dendritic spines in several forebrain regions. This was associated with an increase of GluA2-containing α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors (AMPA-Rs) in telencephalic postsynaptic densities. To unravel the functional significance of such a synaptic re-arrangement, we focused on glutamate neurotransmission in the hippocampus. We evaluated the effect of four weeks of 0.7 mg/kg fluoxetine on long-term potentiation (LTP) and long-term depression (LTD) in the CA1 hippocampal subfield. Recordings in hippocampal slices revealed profound deficits in LTP and LTD at Schaffer collateral-CA1 synapses associated to increased spine density and enhanced presence of mushroom-type spines, as revealed by Golgi staining. However, the same treatment had neither an effect on spine morphology, nor on LTP and LTD at perforant path-CA1 synapses. Cobalt staining and immunohistochemical experiments revealed decreased AMPA-R Ca(2+) permeability in the stratum radiatum (s.r.) together with increased GluA2-containing Ca(2+) impermeable AMPA-Rs. Therefore, 4 weeks of fluoxetine treatment promoted structural and functional adaptations in CA1 neurons in a pathway-specific manner that were selectively associated with impairment of activity-dependent plasticity at Schaffer collateral-CA1 synapses.

Expression of NMDA receptor-dependent LTP in the hippocampus: bridging the divide

A consensus has famously yet to emerge on the locus and mechanisms underlying the expression of the canonical NMDA receptor-dependent form of LTP. An objective assessment of the evidence leads us to conclude that both presynaptic and postsynaptic expression mechanisms contribute to this type of synaptic plasticity.

Miniature IPSCs in hippocampal granule cells are triggered by voltage-gated Ca2+ channels via microdomain coupling

The coupling between presynaptic Ca(2+) channels and Ca(2+) sensors of exocytosis is a key determinant of synaptic transmission. Evoked release from parvalbumin (PV)-expressing interneurons is triggered by nanodomain coupling of P/Q-type Ca(2+) channels, whereas release from cholecystokinin (CCK)-containing interneurons is generated by microdomain coupling of N-type channels. Nanodomain coupling has several functional advantages, including speed and efficacy of transmission. One potential disadvantage is that stochastic opening of presynaptic Ca(2+) channels may trigger spontaneous transmitter release. We addressed this possibility in rat hippocampal granule cells, which receive converging inputs from different inhibitory sources. Both reduction of extracellular Ca(2+) concentration and the unselective Ca(2+) channel blocker Cd(2+) reduced the frequency of miniature IPSCs (mIPSCs) in granule cells by ∼50%, suggesting that the opening of presynaptic Ca(2+) channels contributes to spontaneous release. Application of the selective P/Q-type Ca(2+) channel blocker ω-agatoxin IVa had no detectable effects, whereas both the N-type blocker ω-conotoxin GVIa and the L-type blocker nimodipine reduced mIPSC frequency. Furthermore, both the fast Ca(2+) chelator BAPTA-AM and the slow chelator EGTA-AM reduced the mIPSC frequency, suggesting that Ca(2+)-dependent spontaneous release is triggered by microdomain rather than nanodomain coupling. The CB(1) receptor agonist WIN 55212-2 also decreased spontaneous release; this effect was occluded by prior application of ω-conotoxin GVIa, suggesting that a major fraction of Ca(2+)-dependent spontaneous release was generated at the terminals of CCK-expressing interneurons. Tonic inhibition generated by spontaneous opening of presynaptic N- and L-type Ca(2+) channels may be important for hippocampal information processing.

CLC-3 chloride channels moderate long-term potentiation at Schaffer collateral-CA1 synapses

The chloride channel CLC-3 is expressed in the brain on synaptic vesicles and postsynaptic membranes. Although CLC-3 is broadly expressed throughout the brain, the CLC-3 knockout mouse shows complete, selective postnatal neurodegeneration of the hippocampus, suggesting a crucial role for the channel in maintaining normal brain function. CLC-3 channels are functionally linked to NMDA receptors in the hippocampus; NMDA receptor-dependent Ca(2+) entry, activation of Ca(2+)/calmodulin kinase II and subsequent gating of CLC-3 link the channels via a Ca(2+)-mediated feedback loop. We demonstrate that loss of CLC-3 at mature synapses increases long-term potentiation from 135 ± 4% in the wild-type slice preparation to 154 ± 7% above baseline (P < 0.001) in the knockout; therefore, the contribution of CLC-3 is to reduce synaptic potentiation by ∼40%. Using a decoy peptide representing the Ca(2+)/calmodulin kinase II phosphorylation site on CLC-3, we show that phosphorylation of CLC-3 is required for its regulatory function in long-term potentiation. CLC-3 is also expressed on synaptic vesicles; however, our data suggest functionally separable pre- and postsynaptic roles. Thus, CLC-3 confers Cl(-) sensitivity to excitatory synapses, controls the magnitude of long-term potentiation and may provide a protective limit on Ca(2+) influx.

Regulation of N-type voltage-gated calcium channels and presynaptic function by cyclin-dependent kinase 5

N-type voltage-gated calcium channels localize to presynaptic nerve terminals and mediate key events including synaptogenesis and neurotransmission. While several kinases have been implicated in the modulation of calcium channels, their impact on presynaptic functions remains unclear. Here we report that the N-type calcium channel is a substrate for cyclin-dependent kinase 5 (Cdk5). The pore-forming α(1) subunit of the N-type calcium channel is phosphorylated in the C-terminal domain, and phosphorylation results in enhanced calcium influx due to increased channel open probability. Phosphorylation of the N-type calcium channel by Cdk5 facilitates neurotransmitter release and alters presynaptic plasticity by increasing the number of docked vesicles at the synaptic cleft. These effects are mediated by an altered interaction between N-type calcium channels and RIM1, which tethers presynaptic calcium channels to the active zone. Collectively, our results highlight a molecular mechanism by which N-type calcium channels are regulated by Cdk5 to affect presynaptic function.

The pharmacology of neuroplasticity induced by non-invasive brain stimulation: building models for the clinical use of CNS active drugs

The term neuroplasticity encompasses structural and functional modifications of neuronal connectivity. Abnormal neuroplasticity is involved in various neuropsychiatric diseases, such as dystonia, epilepsy, migraine, Alzheimer's disease, fronto-temporal degeneration, schizophrenia, and post cerebral stroke. Drugs affecting neuroplasticity are increasingly used as therapeutics in these conditions. Neuroplasticity was first discovered and explored in animal experimentation. However, non-invasive brain stimulation (NIBS) has enabled researchers recently to induce and study similar processes in the intact human brain. Plasticity induced by NIBS can be modulated by pharmacological interventions, targeting ion channels, or neurotransmitters. Importantly, abnormalities of plasticity as studied by NIBS are directly related to clinical symptoms in neuropsychiatric diseases. Therefore, a core theme of this review is the hypothesis that NIBS-induced plasticity can explore and potentially predict the therapeutic efficacy of CNS-acting drugs in neuropsychiatric diseases. We will (a) review the basics of neuroplasticity, as explored in animal experimentation, and relate these to our knowledge about neuroplasticity induced in humans by NIBS techniques. We will then (b) discuss pharmacological modulation of plasticity in animals and humans. Finally, we will (c) review abnormalities of plasticity in neuropsychiatric diseases, and discuss how the combination of NIBS with pharmacological intervention may improve our understanding of the pathophysiology of abnormal plasticity in these diseases and their purposeful pharmacological treatment.

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.

Long-lasting potentiation of hippocampal synaptic transmission by direct cortical input is mediated via endocannabinoids

Hippocampal CA1 pyramidal neurons receive sensory inputs from the entorhinal cortex directly through the perforant path (PP) and indirectly through Schaffer collaterals (SC). Direct cortical inputs to CA1 pyramidal neurons through the PP provide instructive signals for hippocampal long-term synaptic plasticity. However, the molecules conveying synaptic signalling in this new form of heterosynaptic plasticity remain unclear. Endocannabinoids, important endogenous signalling mediators, modulate synaptic efficacy primarily through inhibition of GABAergic or glutamatergic synaptic transmission via presynaptically expressed CB1 receptors. Here, we report that pairing of direct and indirect cortical inputs to CA1 pyramidal neurons resulted in a long-lasting potentiation of synaptic responses at SC synapses, but not at the PP. The pairing-potentiated synaptic transmission at the SC was accompanied by a reduced ratio of paired-pulse facilitation (PPR). Enhanced synaptic response at the SC by pairing of PP–SC stimuli is Ca(2+) dependent and requires the presence of functional GABAergic and glutamatergic synaptic transmissions and activation of group I metabotropic glutamate receptors. Pharmacological inhibition or genetic deletion of the CB1 receptor eliminated the pairing-induced long-term synaptic plasticity and decreased PPR at the SC. The potentiation induced by pairing of PP–SC stimuli primarily is the glutamatergic synaptic transmission. While the pairing-induced long-lasting potentiation of synaptic response was blocked by inhibitors for diacylglycerol lipase (DGL), which biosynthesizes 2-AG, inhibition of monoacylglycerol lipase (MAGL), which metabolizes 2-AG, facilitated the potentiation at SC synapses by pairing of weak PP–SC stimuli. Our results suggest that 2-AG functions as a signalling mediator tuning synaptic efficacy at the proximal synapses of hippocampal CA1 pyramidal neurons while direct and indirect cortical inputs to the same neurons are spatiotemporally primed.