Presynaptic NMDARs in the hippocampus facilitate transmitter release at theta frequency

GluN2A- and GluN2B-containing pre-synaptic N-methyl-d-aspartate receptors differentially regulate action potential-evoked Ca2+ influx via modulation of SK channels

N-methyl-d-aspartate receptors (NMDARs) play a pivotal role in synaptic plasticity. While the functional role of post-synaptic NMDARs is well established, pre-synaptic NMDAR (pre-NMDAR) function is largely unexplored. Different pre-NMDAR subunit populations are documented at synapses, suggesting that subunit composition influences neuronal transmission. Here, we used electrophysiological recordings at Schaffer collateral-CA1 synapses partnered with Ca2+ imaging and glutamate uncaging at boutons of CA3 pyramidal neurones to reveal two populations of pre-NMDARs that contain either the GluN2A or GluN2B subunit. Activation of the GluN2B population decreases action potential-evoked Ca2+ influx via modulation of small-conductance Ca2+-activated K+ channels, while activation of the GluN2A population does the opposite. Critically, the level of functional expression of the subunits is subject to homeostatic regulation, bidirectionally affecting short-term facilitation, thus providing a capacity for a fine adjustment of information transfer. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Regulation by Presynaptic NMDA Receptors of Non-Linear Postsynaptic Summation of the Cortical Input to CA1 Pyramidal Neurons

Entorhinal cortex (EC) LIII and LII glutamatergic neurons make monosynaptic connections onto distal apical dendrites of CA1 and CA2 pyramidal neurons (PNs) in the hippocampus, respectively, through perforant path (PP) projections. We previously reported that a brief train of PP stimuli evokes strong supralinear temporal summation of excitatory postsynaptic response (EPSP) in CA1 PNs that requires NMDAR activation, with relatively little summation in CA2 PNs in mice of either sex. Here we provide evidence from combined immunogold electron microscopy, cell-type specific genetic deletion and pharmacology that the NMDARs required for supralinear temporal summation of the CA1 PP EPSP are presynaptic, located in the PP terminals. Moreover, we found that the number of NMDARs in PP terminals innervating CA1 PNs is significantly greater than that found in PP terminals innervating CA2 PNs, providing a potential explanation for the difference in temporal summation in these two classes of hippocampal PNs.

Presynaptic glutamate receptors in nociception

Chronic pain is a frequent, distressing and poorly understood health problem. Plasticity of synaptic transmission in the nociceptive pathways after inflammation or injury is assumed to be an important cellular basis for chronic, pathological pain. Glutamate serves as the main excitatory neurotransmitter at key synapses in the somatosensory nociceptive pathways, in which it acts on both ionotropic and metabotropic glutamate receptors. Although conventionally postsynaptic, compelling anatomical and physiological evidence demonstrates the presence of presynaptic glutamate receptors in the nociceptive pathways. Presynaptic glutamate receptors play crucial roles in nociceptive synaptic transmission and plasticity. They modulate presynaptic neurotransmitter release and synaptic plasticity, which in turn regulates pain sensitization. In this review, we summarize the latest understanding of the expression of presynaptic glutamate receptors in the nociceptive pathways, and how they contribute to nociceptive information processing and pain hypersensitivity associated with inflammation / injury. We uncover the cellular and molecular mechanisms of presynaptic glutamate receptors in shaping synaptic transmission and plasticity to mediate pain chronicity, which may provide therapeutic approaches for treatment of chronic pain.

Metamodulation of presynaptic NMDA receptors: New perspectives for pharmacological interventions

Metamodulation shifted the scenario of the central neuromodulation from a simplified unimodal model to a multimodal one. It involves different receptors/membrane proteins physically associated or merely colocalized that act in concert to control the neuronal functions influencing each other. Defects or maladaptation of metamodulation would subserve neuropsychiatric disorders or even synaptic adaptations relevant to drug dependence. Therefore, this "vulnerability" represents a main issue to be deeply analyzed to predict its aetiopathogenesis, but also to propose targeted pharmaceutical interventions. The review focusses on presynaptic release-regulating NMDA receptors and on some of the mechanisms of their metamodulation described in the literature. Attention is paid to the interactors, including both ionotropic and metabotropic receptors, transporters and intracellular proteins, which metamodulate their responsiveness in physiological conditions but also undergo adaptation that are relevant to neurological dysfunctions. All these structures are attracting more and more the interest as promising druggable targets for the treatment of NMDAR-related central diseases: these substances would not exert on-off control of the colocalized NMDA receptors (as usually observed with NMDAR full agonists/antagonists), but rather modulate their functions, with the promise of limiting side effects that would favor their translation from preclinic to clinic.

Loss of neuron network coherence induced by virus-infected astrocytes: a model study

Coherent activations of brain neuron networks underlie many physiological functions associated with various behavioral states. These synchronous fluctuations in the electrical activity of the brain are also referred to as brain rhythms. At the cellular level, rhythmicity can be induced by various mechanisms of intrinsic oscillations in neurons or the network circulation of excitation between synaptically coupled neurons. One specific mechanism concerns the activity of brain astrocytes that accompany neurons and can coherently modulate synaptic contacts of neighboring neurons, synchronizing their activity. Recent studies have shown that coronavirus infection (Covid-19), which enters the central nervous system and infects astrocytes, can cause various metabolic disorders. Specifically, Covid-19 can depress the synthesis of astrocytic glutamate and gamma-aminobutyric acid. It is also known that in the post-Covid state, patients may suffer from symptoms of anxiety and impaired cognitive functions. We propose a mathematical model of a spiking neuron network accompanied by astrocytes capable of generating quasi-synchronous rhythmic bursting discharges. The model predicts that if the release of glutamate is depressed, normal burst rhythmicity will suffer dramatically. Interestingly, in some cases, the failure of network coherence may be intermittent, with intervals of normal rhythmicity, or the synchronization can disappear.

The Pathological Effects of Circulating Hydrophobic Bile Acids in Alzheimer's Disease

Recent clinical studies have revealed that the serum levels of toxic hydrophobic bile acids (deoxy cholic acid, lithocholic acid [LCA], and glycoursodeoxycholic acid) are significantly higher in patients with Alzheimer's disease (AD) and amnestic mild cognitive impairment (aMCI) when compared to control subjects. The elevated serum bile acids may be the result of hepatic peroxisomal dysfunction. Circulating hydrophobic bile acids are able to disrupt the blood-brain barrier and promote the formation of amyloid-β plaques through enhancing the oxidation of docosahexaenoic acid. Hydrophobic bile acid may find their ways into the neurons via the apical sodium-dependent bile acid transporter. It has been shown that hydrophobic bile acids impose their pathological effects by activating farnesoid X receptor and suppressing bile acid synthesis in the brain, blocking NMDA receptors, lowering brain oxysterol levels, and interfering with 17β-estradiol actions such as LCA by binding to E2 receptors (molecular modelling data exclusive to this paper). Hydrophobic bile acids may interfere with the sonic hedgehog signaling through alteration of cell membrane rafts and reducing brain 24(S)-hydroxycholesterol. This article will 1) analyze the pathological roles of circulating hydrophobic bile acids in the brain, 2) propose therapeutic approaches, and 3) conclude that consideration be given to reducing/monitoring toxic bile acid levels in patients with AD or aMCI, prior/in combination with other treatments.

GluN2B inhibition rescues impaired potentiation and epileptogenicity at associational-commissural CA3 synapses in a model of anti-NMDAR encephalitis

Anti-N-methyl-d-aspartate receptor (anti-NMDAR) encephalitis is an autoimmune epilepsy associated with memory deficits. Research has demonstrated that anti-NMDAR inhibit long-term potentiation, and, at the same time, lead to disinhibition in the form of epileptiform afterpotentials in the potentiated state. While both effects may give rise to the key symptoms of the disease, the molecular basis of being simultaneously inhibitory and disinhibitory is difficult to explain. Here, we explored a possible involvement of the GluN2B subunit. To this aim, we injected cerebrospinal fluid from anti-NMDAR encephalitis patients into the rat hippocampus and prepared brain slices for in vitro field potential recordings. Associational-commissural-fiber-CA3 synapses from anti-NMDAR-treated animals showed increased field potential amplitudes with concomitantly enhanced paired-pulse ratios as compared to control tissue. GluN2B inhibition by Ro25-6981 mimicked these effects in controls but had no effect in anti-NMDAR tissues indicating a presynaptic and occluding effect of anti-NMDAR. We then induced potentiation of associational-commissural-fiber-CA3 synapses, and confirmed that slices from anti-NMDAR-treated animals showed reduced potentiation and pronounced epileptiform afterpotentials. Intriguingly, both effects were absent when Ro25-6981 was added in vitro before inducing potentiation. These results indicate that GluN2B-containing NMDARs, partially expressed presynaptically, show differential sensitivity to anti-NMDAR, and that altered GluN2B function is particularly apparent in the potentiated state rather than under baseline conditions. Since GluN2B inhibition rescued the effects of anti-NMDAR in the potentiated state, this opens the possibility that at least a subgroup of patients could benefit from a GluN2B antagonist.

GRIN2B-related neurodevelopmental disorder: current understanding of pathophysiological mechanisms

The GRIN2B-related neurodevelopmental disorder is a rare disease caused by mutations in the GRIN2B gene, which encodes the GluN2B subunit of NMDA receptors. Most individuals with GRIN2B-related neurodevelopmental disorder present with intellectual disability and developmental delay. Motor impairments, autism spectrum disorder, and epilepsy are also common. A large number of pathogenic de novo mutations have been identified in GRIN2B. However, it is not yet known how these variants lead to the clinical symptoms of the disease. Recent research has begun to address this issue. Here, we describe key experimental approaches that have been used to better understand the pathophysiology of this disease. We discuss the impact of several distinct pathogenic GRIN2B variants on NMDA receptor properties. We then critically review pivotal studies examining the synaptic and neurodevelopmental phenotypes observed when disease-associated GluN2B variants are expressed in neurons. These data provide compelling evidence that various GluN2B mutants interfere with neuronal differentiation, dendrite morphogenesis, synaptogenesis, and synaptic plasticity. Finally, we identify important open questions and considerations for future studies aimed at understanding this complex disease. Together, the existing data provide insight into the pathophysiological mechanisms that underlie GRIN2B-related neurodevelopmental disorder and emphasize the importance of comparing the effects of individual, disease-associated variants. Understanding the molecular, cellular and circuit phenotypes produced by a wide range of GRIN2B variants should lead to the identification of core neurodevelopmental phenotypes that characterize the disease and lead to its symptoms. This information could help guide the development and application of effective therapeutic strategies for treating individuals with GRIN2B-related neurodevelopmental disorder.

Presynaptic NMDARs on spinal nociceptor terminals state-dependently modulate synaptic transmission and pain

Postsynaptic NMDARs at spinal synapses are required for postsynaptic long-term potentiation and chronic pain. However, how presynaptic NMDARs (PreNMDARs) in spinal nociceptor terminals control presynaptic plasticity and pain hypersensitivity has remained unclear. Here we report that PreNMDARs in spinal nociceptor terminals modulate synaptic transmission in a nociceptive tone-dependent manner. PreNMDARs depresses presynaptic transmission in basal state, while paradoxically causing presynaptic potentiation upon injury. This state-dependent modulation is dependent on Ca²⁺ influx via PreNMDARs. Small conductance Ca²⁺-activated K⁺ (SK) channels are responsible for PreNMDARs-mediated synaptic depression. Rather, tissue inflammation induces PreNMDARs-PKG-I-dependent BDNF secretion from spinal nociceptor terminals, leading to SK channels downregulation, which in turn converts presynaptic depression to potentiation. Our findings shed light on the state-dependent characteristics of PreNMDARs in spinal nociceptor terminals on modulating nociceptive transmission and revealed a mechanism underlying state-dependent transition. Moreover, we identify PreNMDARs in spinal nociceptor terminals as key constituents of activity-dependent pain sensitization.

Postsynaptic NMDARs at spinal synapses are required for postsynaptic long-term potentiation and chronic pain. Here, the authors show that also presynaptic NMDARs in spinal nociceptor terminals modulate synaptic transmission in a nociceptive tone-dependent manner.

Low-intensity blast induces acute glutamatergic hyperexcitability in mouse hippocampus leading to long-term learning deficits and altered expression of proteins involved in synaptic plasticity and serine protease inhibitors

Neurocognitive consequences of blast-induced traumatic brain injury (bTBI) pose significant concerns for military service members and veterans with the majority of "invisible injury." However, the underlying mechanism of such mild bTBI by low-intensity blast (LIB) exposure for long-term cognitive and mental deficits remains elusive. Our previous studies have shown that mice exposed to LIB result in nanoscale ultrastructural abnormalities in the absence of gross or apparent cellular damage in the brain. Here we tested the hypothesis that glutamatergic hyperexcitability may contribute to long-term learning deficits. Using brain slice electrophysiological recordings, we found an increase in averaged frequencies with a burst pattern of miniature excitatory postsynaptic currents (mEPSCs) in hippocampal CA3 neurons in LIB-exposed mice at 1- and 7-days post injury, which was blocked by a specific NMDA receptor antagonist AP5. In addition, cognitive function assessed at 3-months post LIB exposure by automated home-cage monitoring showed deficits in dynamic patterns of discrimination learning and cognitive flexibility in LIB-exposed mice. Collected hippocampal tissue was further processed for quantitative global-proteomic analysis. Advanced data-independent acquisition for quantitative tandem mass spectrometry analysis identified altered expression of proteins involved in synaptic plasticity and serine protease inhibitors in LIB-exposed mice. Some were correlated with the ability of discrimination learning and cognitive flexibility. These findings show that acute glutamatergic hyperexcitability in the hippocampus induced by LIB may contribute to long-term cognitive dysfunction and protein alterations. Studies using this military-relevant mouse model of mild bTBI provide valuable insights into developing a potential therapeutic strategy to ameliorate hyperexcitability-modulated LIB injuries.

Aberrant maturation and connectivity of prefrontal cortex in schizophrenia-contribution of NMDA receptor development and hypofunction

The neurobiology of schizophrenia involves multiple facets of pathophysiology, ranging from its genetic basis over changes in neurochemistry and neurophysiology, to the systemic level of neural circuits. Although the precise mechanisms associated with the neuropathophysiology remain elusive, one essential aspect is the aberrant maturation and connectivity of the prefrontal cortex that leads to complex symptoms in various stages of the disease. Here, we focus on how early developmental dysfunction, especially N-methyl-D-aspartate receptor (NMDAR) development and hypofunction, may lead to the dysfunction of both local circuitry within the prefrontal cortex and its long-range connectivity. More specifically, we will focus on an "all roads lead to Rome" hypothesis, i.e., how NMDAR hypofunction during development acts as a convergence point and leads to local gamma-aminobutyric acid (GABA) deficits and input-output dysconnectivity in the prefrontal cortex, which eventually induce cognitive and social deficits. Many outstanding questions and hypothetical mechanisms are listed for future investigations of this intriguing hypothesis that may lead to a better understanding of the aberrant maturation and connectivity associated with the prefrontal cortex.

Multiple roles of GluN2D-containing NMDA receptors in short-term potentiation and long-term potentiation in mouse hippocampal slices

The GluN2 subunits of N-methyl-d-aspartate receptors (NMDARs) are key drivers of synaptic plasticity in the brain, where the particular GluN2 composition endows the NMDAR complex with distinct pharmacological and physiological properties. Compared to GluN2A and GluN2B subunits, far less is known about the role of the GluN2D subunit in synaptic plasticity. In this study, we have used a GluN2C/2D selective competitive antagonist, UBP145, in combination with a GluN2D global knockout (GluN2D KO) mouse line to study the contribution of GluN2D-containing NMDARs to short-term potentiation (STP) and long-term potentiation (LTP) in the CA1 region of mouse hippocampal slices. We made several distinct observations: First, GluN2D KO mice have higher levels of LTP compared to wild-type (WT) mice, an effect that was occluded by blockade of GABA receptor-mediated inhibition or by using a strong LTP induction protocol. Second, UBP145 partially inhibited LTP in WT but not GluN2D KO mice. Third, UBP145 inhibited a component of STP, termed STP2, in WT but not GluN2D KO mice. Taken together, these findings suggest an involvement for GluN2D-containing NMDARs in both STP and LTP in mouse hippocampus.

Glutamate receptors and synaptic plasticity: The impact of Evans and Watkins

On the occasion of the 40 year anniversary of the hugely impactful review by Richard (Dick) Evans and Jeff Watkins, we describe how their work has impacted the field of synaptic plasticity. We describe their influence in each of the major glutamate receptor subtypes: AMPARs, NMDARs, KARs and mGluRs. Particular emphasis is placed on how their work impacted our own studies in the hippocampus. For example, we describe how the tools and regulators that they identified for studying NMDARs (e.g., NMDA, D-AP5 and Mg²⁺) led to the understanding of the molecular basis of the induction of LTP. We also describe how other tools that they introduced (e.g., (1S,3R)-ACPD and MCPG) helped lead to the concept of metaplasticity.

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.

Frequency-Dependent Synaptic Dynamics Differentially Tune CA1 and CA2 Pyramidal Neuron Responses to Cortical Input

Entorhinal cortex neurons make monosynaptic connections onto distal apical dendrites of CA1 and CA2 pyramidal neurons through the perforant path (PP) projection. Previous studies show that differences in dendritic properties and synaptic input density enable the PP inputs to produce a much stronger excitation of CA2 compared with CA1 pyramidal neurons. Here, using mice of both sexes, we report that the difference in PP efficacy varies substantially as a function of presynaptic firing rate. Although a single PP stimulus evokes a 5- to 6-fold greater EPSP in CA2 compared with CA1, a brief high-frequency train of PP stimuli evokes a strongly facilitating postsynaptic response in CA1, with relatively little change in CA2. Furthermore, we demonstrate that blockade of NMDARs significantly reduces strong temporal summation in CA1 but has little impact on that in CA2. As a result of the differences in the frequency- and NMDAR-dependent temporal summation, naturalistic patterns of presynaptic activity evoke CA1 and CA2 responses with distinct dynamics, differentially tuning CA1 and CA2 responses to bursts of presynaptic firing versus single presynaptic spikes, respectively.SIGNIFICANCE STATEMENT Recent studies have demonstrated that abundant entorhinal cortical innervation and efficient dendritic propagation enable hippocampal CA2 pyramidal neurons to produce robust excitation evoked by single cortical stimuli, compared with CA1. Here we uncovered, unexpectedly, that the difference in efficacy of cortical excitation varies substantially as a function of presynaptic firing rate. A burst of stimuli evokes a strongly facilitating response in CA1, but not in CA2. As a result, the postsynaptic response of CA1 and CA2 to presynaptic naturalistic firing displays contrasting temporal dynamics, which depends on the activation of NMDARs. Thus, whereas CA2 responds to single stimuli, CA1 is selectively recruited by bursts of cortical input.

NMDARs in granule cells contribute to parallel fiber-Purkinje cell synaptic plasticity and motor learning

Long-term synaptic plasticity is believed to be the cellular substrate of learning and memory. Synaptic plasticity rules are defined by the specific complement of receptors at the synapse and the associated downstream signaling mechanisms. In young rodents, at the cerebellar synapse between granule cells (GC) and Purkinje cells (PC), bidirectional plasticity is shaped by the balance between transcellular nitric oxide (NO) driven by presynaptic N-methyl-D-aspartate receptor (NMDAR) activation and postsynaptic calcium dynamics. However, the role and the location of NMDAR activation in these pathways is still debated in mature animals. Here, we show in adult rodents that NMDARs are present and functional in presynaptic terminals where their activation triggers NO signaling. In addition, we find that selective genetic deletion of presynaptic, but not postsynaptic, NMDARs prevents synaptic plasticity at parallel fiber-PC (PF-PC) synapses. Consistent with this finding, the selective deletion of GC NMDARs affects adaptation of the vestibulo-ocular reflex. Thus, NMDARs presynaptic to PCs are required for bidirectional synaptic plasticity and cerebellar motor learning.

Alpha-Synuclein as a Prominent Actor in the Inflammatory Synaptopathy of Parkinson's Disease

Parkinson’s disease (PD) is considered the most common disorder of synucleinopathy, which is characterised by intracellular inclusions of aggregated and misfolded α-synuclein (α-syn) protein in various brain regions, and the loss of dopaminergic neurons. During the early prodromal phase of PD, synaptic alterations happen before cell death, which is linked to the synaptic accumulation of toxic α-syn specifically in the presynaptic terminals, affecting neurotransmitter release. The oligomers and protofibrils of α-syn are the most toxic species, and their overexpression impairs the distribution and activation of synaptic proteins, such as the SNARE complex, preventing neurotransmitter exocytosis and neuronal synaptic communication. In the last few years, the role of the immune system in PD has been increasingly considered. Microglial and astrocyte activation, the gene expression of proinflammatory factors, and the infiltration of immune cells from the periphery to the central nervous system (CNS) represent the main features of the inflammatory response. One of the actors of these processes is α-syn accumulation. In light of this, here, we provide a systematic review of PD-related α-syn and inflammation inter-players.

Presynaptic NMDA receptors facilitate short-term plasticity and BDNF release at hippocampal mossy fiber synapses

Neurotransmitter release is a highly controlled process by which synapses can critically regulate information transfer within neural circuits. While presynaptic receptors - typically activated by neurotransmitters and modulated by neuromodulators - provide a powerful way of fine-tuning synaptic function, their contribution to activity-dependent changes in transmitter release remains poorly understood. Here, we report that presynaptic NMDA receptors (preNMDARs) at mossy fiber boutons in the rodent hippocampus can be activated by physiologically relevant patterns of activity and selectively enhance short-term synaptic plasticity at mossy fiber inputs onto CA3 pyramidal cells and mossy cells, but not onto inhibitory interneurons. Moreover, preNMDARs facilitate brain-derived neurotrophic factor release and contribute to presynaptic calcium rise. Taken together, our results indicate that by increasing presynaptic calcium, preNMDARs fine-tune mossy fiber neurotransmission and can control information transfer during dentate granule cell burst activity that normally occur in vivo.

Neuropathic pain: Spotlighting anatomy, experimental models, mechanisms, and therapeutic aspects

The International Association for the Study of Pain defines neuropathic pain as "pain arising as a direct consequence of a lesion or disease affecting the somatosensory system". The associated changes can be observed in the peripheral as well as the central nervous system. The available literature discusses a wide variety of causes as predisposing for the development and amplification of neuropathic pain. Further, key interactions within sensory pathways have been discovered, but no common molecular mechanism leading to neuropathic pain has been identified until now. In the first part of this review, the pain mediating lateral spinothalamic tract is described. Different in vivo models are presented that allow studying trauma-, chemotherapy-, virus-, and diabetes-induced neuropathic pain in rodents. We furthermore discuss approaches to assess neuropathic pain in these models. Second, the current knowledge about cellular and molecular mechanisms suggested to underlie the development of neuropathic pain is presented and discussed. A summary of established therapies that are already applied in the clinic and novel, promising approaches closes the paper. In conclusion, the established animal models are able to emulate the diversity of neuropathic pain observed in the clinics. However, the assessment of neuropathic pain in the presented in vivo models should be improved. The determination of common molecular markers with suitable in vitro models would simplify the assessment of neuropathic pain in vivo. This would furthermore provide insights into common molecular mechanisms of the disease and establish a basis to search for satisfying therapeutic approaches.

Tyrosine phosphatase PTP1B impairs presynaptic NMDA receptor-mediated plasticity in a mouse model of Alzheimer's disease

Mutations in the beta-amyloid protein (APP) cause familial Alzheimer's disease. In hAPP-J20 mice expressing mutant APP, pharmacological inhibition or genetic ablation of the tyrosine phosphatase PTP1B prevents CA3 hippocampus neuron loss and cognitive decline. However, how targeting PTP1B affects the cellular mechanisms underlying these cognitive deficits remains unknown. Changes in synaptic strength at the hippocampus can affect information processing for learning and memory. While prior studies have focused on post-synaptic mechanisms to account for synaptic deficits in Alzheimer's disease models, presynaptic mechanisms may also be affected. Here, using whole cell patch-clamp recording, coefficient of variation (CV) analysis suggested a profound presynaptic deficit in long-term potentiation (LTP) of CA3:CA1 synapses in hAPP-J20 mice. While the membrane-impermeable ionotropic NMDA receptor (NMDAR) blocker norketamine in the post-synaptic recording electrode had no effect on LTP, additional bath application of the ionotropic NMDAR blockers MK801 could replicate the deficit in LTP in wild type mice. In contrast to LTP, the paired-pulse ratio and short-term facilitation (STF) were aberrantly increased in hAPP-J20 mice. These synaptic deficits in hAPP-J20 mice were associated with reduced phosphorylation of NMDAR GluN2B and the synaptic vesicle recycling protein NSF (N-ethylmaleimide sensitive factor). Phosphorylation of both proteins, together with synaptic plasticity and cognitive function, were restored by PTP1B ablation or inhibition by the PTP1B-selective inhibitor Trodusquemine. Taken together, our results indicate that PTP1B impairs presynaptic NMDAR-mediated synaptic plasticity required for spatial learning in a mouse model of Alzheimer's disease. Since Trodusquemine has undergone phase 1/2 clinical trials to treat obesity, it could be repurposed to treat Alzheimer's disease.

Presynaptic release-regulating NMDA receptors in isolated nerve terminals: A narrative review

The existence of presynaptic, release‐regulating NMDA receptors in the CNS has been long matter of discussion. Most of the reviews dedicated to support this conclusion have preferentially focussed on the results from electrophysiological studies, paying little or no attention to the data obtained with purified synaptosomes, even though this experimental approach has been recognized as providing reliable information concerning the presence and the role of presynaptic release‐regulating receptors in the CNS. To fill the gap, this review is dedicated to summarising the results from studies with synaptosomes published during the last 40 years, which support the existence of auto and hetero NMDA receptors controlling the release of transmitters such as glutamate, GABA, dopamine, noradrenaline, 5‐HT, acetylcholine and peptides, in the CNS of mammals. The review also deals with the results from immunochemical studies in isolated nerve endings that confirm the functional observations.

Presynaptic NMDARs cooperate with local spikes toward GABA release from the reciprocal olfactory bulb granule cell spine

In the rodent olfactory bulb the smooth dendrites of the principal glutamatergic mitral cells (MCs) form reciprocal dendrodendritic synapses with large spines on GABAergic granule cells (GC), where unitary release of glutamate can trigger postsynaptic local activation of voltage-gated Na+-channels (Navs), that is a spine spike. Can such single MC input evoke reciprocal release? We find that unitary-like activation via two-photon uncaging of glutamate causes GC spines to release GABA both synchronously and asynchronously onto MC dendrites. This release indeed requires activation of Navs and high-voltage-activated Ca2+-channels (HVACCs), but also of NMDA receptors (NMDAR). Simulations show temporally overlapping HVACC- and NMDAR-mediated Ca2+-currents during the spine spike, and ultrastructural data prove NMDAR presence within the GABAergic presynapse. This cooperative action of presynaptic NMDARs allows to implement synapse-specific, activity-dependent lateral inhibition, and thus could provide an efficient solution to combinatorial percept synthesis in a sensory system with many receptor channels.

Astrocyte-mediated switch in spike timing-dependent plasticity during hippocampal development

Presynaptic spike timing-dependent long-term depression (t-LTD) at hippocampal CA3-CA1 synapses is evident until the 3^rd postnatal week in mice, disappearing during the 4^th week. At more mature stages, we found that the protocol that induced t-LTD induced t-LTP. We characterized this form of t-LTP and the mechanisms involved in its induction, as well as that driving this switch from t-LTD to t-LTP. We found that this t-LTP is expressed presynaptically at CA3-CA1 synapses, as witnessed by coefficient of variation, number of failures, paired-pulse ratio and miniature responses analysis. Additionally, this form of presynaptic t-LTP does not require NMDARs but the activation of mGluRs and the entry of Ca²⁺ into the postsynaptic neuron through L-type voltage-dependent Ca²⁺ channels and the release of Ca²⁺ from intracellular stores. Nitric oxide is also required as a messenger from the postsynaptic neuron. Crucially, the release of adenosine and glutamate by astrocytes is required for t-LTP induction and for the switch from t-LTD to t-LTP. Thus, we have discovered a developmental switch of synaptic transmission from t-LTD to t-LTP at hippocampal CA3-CA1 synapses in which astrocytes play a central role and revealed a form of presynaptic LTP and the rules for its induction.

Presynaptic spike timing-dependent long-term depression at hippocampal CA3-CA1 synapses is evident until the third postnatal week in mice. The authors show that maturation beyond four weeks is associated with a switch to long-term potentiation in which astrocytes play a central role.

Adenosine Receptor-Mediated Developmental Loss of Spike Timing-Dependent Depression in the Hippocampus

Critical periods of synaptic plasticity facilitate the reordering and refining of neural connections during development, allowing the definitive synaptic circuits responsible for correct adult physiology to be established. Presynaptic spike timing-dependent long-term depression (t-LTD) exists in the hippocampus, which depends on the activation of NMDARs and that probably fulfills a role in synaptic refinement. This t-LTD is present until the third postnatal week in mice, disappearing in the fourth week of postnatal development. We were interested in the mechanisms underlying this maturation related loss of t-LTD and we found that at CA3–CA1 synapses, presynaptic NMDA receptors (pre-NMDARs) are tonically active between P13 and P21, mediating an increase in glutamate release during this critical period of plasticity. Conversely, at the end of this critical period (P22–P30) and coinciding with the loss of t-LTD, these pre-NMDARs are no longer tonically active. Using immunogold electron microscopy, we demonstrated the existence of pre-NMDARs at Schaffer collateral synaptic boutons, where a decrease in the number of pre-NMDARs during development coincides with the loss of both tonic pre-NMDAR activation and t-LTD. Interestingly, this t-LTD can be completely recovered by antagonizing adenosine type 1 receptors (A₁R), which also recovers the tonic activation of pre-NMDARs at P22–P30. By contrast, the induction of t-LTD was prevented at P13–P21 by an agonist of A₁R, as was tonic pre-NMDAR activation. Furthermore, we found that the adenosine that mediated the loss of t-LTD during the fourth week of development is supplied by astrocytes. These results provide direct evidence for the mechanism that closes the window of plasticity associated with t-LTD, revealing novel events probably involved in synaptic remodeling during development.

Calcineurin Inhibition Causes α2δ-1-Mediated Tonic Activation of Synaptic NMDA Receptors and Pain Hypersensitivity

Calcineurin inhibitors, such as tacrolimus (FK506) and cyclosporine, are widely used as standard immunosuppressants in organ transplantation recipients. However, these drugs can cause severe pain in patients, commonly referred to as calcineurin inhibitor-induced pain syndrome (CIPS). Although calcineurin inhibition increases NMDAR activity in the spinal cord, the underlying mechanism remains enigmatic. Using an animal model of CIPS, we found that systemic administration of FK506 in male and female mice significantly increased the amount of α2δ-1-GluN1 complexes in the spinal cord and the level of α2δ-1-bound GluN1 proteins in spinal synaptosomes. Treatment with FK506 significantly increased the frequency of mEPSCs and the amplitudes of monosynaptic EPSCs evoked from the dorsal root and puff NMDAR currents in spinal dorsal horn neurons. Inhibiting α2δ-1 with gabapentin or disrupting the α2δ-1-NMDAR interaction with α2δ-1Tat peptide completely reversed the effects of FK506. In α2δ-1 gene KO mice, treatment with FK506 failed to increase the frequency of NMDAR-mediated mEPSCs and the amplitudes of evoked EPSCs and puff NMDAR currents in spinal dorsal horn neurons. Furthermore, systemic administration of gabapentin or intrathecal injection of α2δ-1Tat peptide reversed thermal and mechanical hypersensitivity in FK506-treated mice. In addition, genetically deleting GluN1 in dorsal root ganglion neurons or α2δ-1 genetic KO similarly attenuated FK506-induced thermal and mechanical hypersensitivity. Together, our findings indicate that α2δ-1-bound NMDARs mediate calcineurin inhibitor-induced tonic activation of presynaptic and postsynaptic NMDARs at the spinal cord level and that presynaptic NMDARs play a prominent role in the development of CIPS.SIGNIFICANCE STATEMENT Calcineurin inhibitors are immunosuppressants used to prevent rejection of transplanted organs and tissues. However, these drugs can cause severe, unexplained pain. We showed that calcineurin inhibition enhances physical interaction between α2δ-1 and NMDARs and their synaptic trafficking in the spinal cord. α2δ-1 is essential for calcineurin inhibitor-induced aberrant activation of presynaptic and postsynaptic NMDARs in the spinal cord. Furthermore, inhibiting α2δ-1 or disrupting α2δ-1-NMDAR interaction reduces calcineurin inhibitor-induced pain hypersensitivity. Eliminating NMDARs in primary sensory neurons or α2δ-1 KO also attenuates calcineurin inhibitor-induced pain hypersensitivity. This new information extends our mechanistic understanding of the role of endogenous calcineurin in regulating synaptic plasticity and nociceptive transmission and suggests new strategies for treating this painful condition.

Synaptic GluN2A-Containing NMDA Receptors: From Physiology to Pathological Synaptic Plasticity

N-Methyl-d-Aspartate Receptors (NMDARs) are ionotropic glutamate-gated receptors. NMDARs are tetramers composed by several homologous subunits of GluN1-, GluN2-, or GluN3-type, leading to the existence in the central nervous system of a high variety of receptor subtypes with different pharmacological and signaling properties. NMDAR subunit composition is strictly regulated during development and by activity-dependent synaptic plasticity. Given the differences between GluN2 regulatory subunits of NMDAR in several functions, here we will focus on the synaptic pool of NMDARs containing the GluN2A subunit, addressing its role in both physiology and pathological synaptic plasticity as well as the contribution in these events of different types of GluN2A-interacting proteins.

A BDNF-Mediated Push-Pull Plasticity Mechanism for Synaptic Clustering

During development, activity-dependent synaptic plasticity refines neuronal networks with high precision. For example, spontaneous activity helps sorting synaptic inputs with similar activity patterns into clusters to enhance neuronal computations in the mature brain. Here, we show that TrkB activation and postsynaptic brain-derived neurotrophic factor (BDNF) are required for synaptic clustering in developing hippocampal neurons. Moreover, BDNF and TrkB modulate transmission at synapses depending on their clustering state, indicating that endogenous BDNF/TrkB signaling stabilizes locally synchronized synapses. Together with our previous data on proBDNF/p75^NTR signaling, these findings suggest a push-pull plasticity mechanism for synaptic clustering: BDNF stabilizes clustered synapses while proBDNF downregulates out-of-sync synapses. This idea is supported by our observation that synaptic clustering requires matrix-metalloproteinase-9 activity, a proBDNF-to-BDNF converting enzyme. Finally, NMDA receptor activation mediates out-of-sync depression upstream of proBDNF signaling. Together, these data delineate an efficient plasticity mechanism where proBDNF and mature BDNF establish synaptic clustering through antagonistic modulation of synaptic transmission.

An autism-associated mutation in GluN2B prevents NMDA receptor trafficking and interferes with dendrite growth

Autism spectrum disorders (ASDs) are neurodevelopmental disorders with multiple genetic associations. Analysis of de novo mutations identified GRIN2B, which encodes the GluN2B subunit of NMDA receptors, as a gene linked to ASDs with high probability. However, the mechanisms by which GRIN2B mutations contribute to ASD pathophysiology are not understood. Here, we investigated the cellular phenotypes induced by a human mutation that is predicted to truncate GluN2B within the extracellular loop. This mutation abolished NMDA-dependent Ca²⁺ influx. Mutant GluN2B co-assembled with GluN1 but was not trafficked to the cell surface or dendrites. When mutant GluN2B was expressed in developing cortical neurons, dendrites appeared underdeveloped, with shorter and fewer branches, while spine density was unaffected. Mutant dendritic arbors were often dysmorphic, displaying abnormal filopodial-like structures. Interestingly, dendrite maldevelopment appeared when mutant GluN2B was expressed on a wild-type background, reflecting the disease given that individuals are heterozygous for GRIN2B mutations. Restoring the fourth transmembrane domain and cytoplasmic tail did not rescue the phenotypes. Finally, abnormal development was not accompanied by reduced mTOR signaling. These data suggest that mutations in GluN2B contribute to ASD pathogenesis by disrupting dendrite development.

Persistent memories of long-term potentiation and the N-methyl-d-aspartate receptor

In this article, we describe our involvement in the early days of research into long-term potentiation. We start with a description of the early experiments conducted in Oslo and London where long-term potentiation was first characterised. We discuss the ways in which the molecular pharmacology of glutamate receptors control the induction and expression of long-term potentiation and its counterpart, long-term depression. We then go on to summarise the extraordinary advances in understanding the cellular mechanisms of synaptic plasticity that have taken place in the subsequent half century. Finally, the increasing evidence that impaired long-term potentiation is a core feature of many brain disorders (LToPathies) is addressed by way of a few selected examples.

Presynaptic NMDA receptors control nociceptive transmission at the spinal cord level in neuropathic pain

Chronic neuropathic pain is a debilitating condition that remains challenging to treat. Glutamate N-methyl-D-aspartate receptor (NMDAR) antagonists have been used to treat neuropathic pain, but the exact sites of their actions have been unclear until recently. Although conventionally postsynaptic, NMDARs are also expressed presynaptically, particularly at the central terminals of primary sensory neurons, in the spinal dorsal horn. However, presynaptic NMDARs in the spinal cord are normally quiescent and are not actively involved in physiological nociceptive transmission. In this review, we describe the emerging role of presynaptic NMDARs at the spinal cord level in chronic neuropathic pain and the implications of molecular mechanisms for more effective treatment. Recent studies indicate that presynaptic NMDAR activity at the spinal cord level is increased in several neuropathic pain conditions but not in chronic inflammatory pain. Increased presynaptic NMDAR activity can potentiate glutamate release from primary afferent terminals to spinal dorsal horn neurons, which is crucial for the synaptic plasticity associated with neuropathic pain caused by traumatic nerve injury and chemotherapy-induced peripheral neuropathy. Furthermore, α2δ-1, previously considered a calcium channel subunit, can directly interact with NMDARs through its C-terminus to increase presynaptic NMDAR activity by facilitating synaptic trafficking of α2δ-1-NMDAR complexes in neuropathic pain caused by chemotherapeutic agents and peripheral nerve injury. Targeting α2δ-1-bound NMDARs with gabapentinoids or α2δ-1 C-terminus peptides can attenuate nociceptive drive form primary sensory nerves to dorsal horn neurons in neuropathic pain.

Multiplex imaging relates quantal glutamate release to presynaptic Ca2+ homeostasis at multiple synapses in situ

Information processing by brain circuits depends on Ca2+-dependent, stochastic release of the excitatory neurotransmitter glutamate. Whilst optical glutamate sensors have enabled detection of synaptic discharges, understanding presynaptic machinery requires simultaneous readout of glutamate release and nanomolar presynaptic Ca2+ in situ. Here, we find that the fluorescence lifetime of the red-shifted Ca2+ indicator Cal-590 is Ca2+-sensitive in the nanomolar range, and employ it in combination with green glutamate sensors to relate quantal neurotransmission to presynaptic Ca2+ kinetics. Multiplexed imaging of individual and multiple synapses in identified axonal circuits reveals that glutamate release efficacy, but not its short-term plasticity, varies with time-dependent fluctuations in presynaptic resting Ca2+ or spike-evoked Ca2+ entry. Within individual presynaptic boutons, we find no nanoscopic co-localisation of evoked presynaptic Ca2+ entry with the prevalent glutamate release site, suggesting loose coupling between the two. The approach enables a better understanding of release machinery at central synapses.

The Role of NMDA Receptors in Alzheimer's Disease

In Alzheimer’s disease (AD), early synaptic dysfunction is associated with the increased oligomeric amyloid-beta peptide, which causes NMDAR-dependent synaptic depression and spine elimination. Memantine, low-affinity NMDAR channel blocker, has been used in the treatment of moderate to severe AD. However, clear evidence is still deficient in demonstrating the underlying mechanisms and a relationship between NMDARs dysfunction and AD. This review focuses on not only changes in expression of different NMDAR subunits, but also some unconventional modes of NMDAR action.

Increased α2δ-1-NMDA receptor coupling potentiates glutamatergic input to spinal dorsal horn neurons in chemotherapy-induced neuropathic pain

Painful peripheral neuropathy is a severe and difficult-to-treat neurological complication associated with cancer chemotherapy. Although chemotherapeutic drugs such as paclitaxel are known to cause tonic activation of presynaptic NMDA receptors (NMDARs) to potentiate nociceptive input, the molecular mechanism involved in this effect is unclear. α2δ-1, commonly known as a voltage-activated calcium channel subunit, is a newly discovered NMDAR-interacting protein and plays a critical role in NMDAR-mediated synaptic plasticity. Here we show that paclitaxel treatment in rats increases the α2δ-1 expression level in the dorsal root ganglion and spinal cord and the mRNA levels of GluN1, GluN2A, and GluN2B in the spinal cord. Paclitaxel treatment also potentiates the α2δ-1-NMDAR interaction and synaptic trafficking in the spinal cord. Strikingly, inhibiting α2δ-1 trafficking with pregabalin, disrupting the α2δ-1-NMDAR interaction with an α2δ-1 C-terminus-interfering peptide, or α2δ-1 genetic ablation fully reverses paclitaxel treatment-induced presynaptic NMDAR-mediated glutamate release from primary afferent terminals to spinal dorsal horn neurons. In addition, intrathecal injection of pregabalin or α2δ-1 C-terminus-interfering peptide and α2δ-1 knockout in mice markedly attenuate paclitaxel-induced pain hypersensitivity. Our findings indicate that α2δ-1 is required for paclitaxel-induced tonic activation of presynaptic NMDARs at the spinal cord level. Targeting α2δ-1-bound NMDARs, not the physiological α2δ-1-free NMDARs, may be a new strategy for treating chemotherapy-induced neuropathic pain. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Short-term hypoxia induces bidirectional pathological long-term plasticity of neurotransmission in visual retinocollicular pathway

Using the paired patch-clamp technique, we studied the effects of short-term hypoxia on retinocollicular synaptic transmission in an originally-developed coculture of dissociated retinal cells and superficial superior colliculus (SSC) neurons. Pharmacologically isolated N-methyl-D-aspartate receptor (NMDA)-, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA)- and gamma-aminobutyric acid receptor (GABA_A)-mediated postsynaptic currents (PSCs) were evoked in SSC neurons by generation action potentials in presynaptic retinal ganglion cells. Spontaneous and miniature PSCs were recorded in SSC neurons in the absence of presynaptic stimulation. Short-term (up to 5 min) hypoxia induced long-term potentiation of NMDA transmission, long-term depression of GABA_A neurotransmission and temporary suppression of AMPA transmission. Also, we observed hypoxia-induced reduction of voltage-dependent magnesium blockade of evoked NMDA response. Evoked, spontaneous and miniature postsynaptic currents were analyzed in terms of a binomial model. This analysis revealed that hypoxia acts mainly presynaptically on excitatory neurotransmission and both pre‒ and postsynaptically on inhibitory retinocollicular transmission. Thus, we showed for the first time hypoxia-induced bidirectional long-term plasticity of the retinocollicular synaptic transmission. The results obtained reflect the electrophysiological basis of hypoxia-involved pathological lesion of the retinocollicular pathway.

Persistent but Labile Synaptic Plasticity at Excitatory Synapses

Short-term synaptic plasticity contributes to many computations in the brain and allows synapses to keep a finite record of recent activity. Here we have investigated the mechanisms underlying an intriguing form of short-term plasticity termed labile LTP, at hippocampal and PFC synapses in male rats and male and female mice. In the hippocampus, labile LTP is triggered by high-frequency activation of presynaptic axons and is rapidly discharged with further activation of those axons. However, if the synapses are quiescent, they remain potentiated until further presynaptic activation. To distinguish labile LTP from NMDAR-dependent forms of potentiation, we blocked NMDARs in all experiments. Labile LTP was synapse-specific and was accompanied by a decreased paired pulse ratio, consistent with an increased release probability. Presynaptic Ca²⁺ and protein kinase activation during the tetanus appeared to be required for its initiation. Labile LTP was not reversed by a PKC inhibitor and did not require either RIM1α or synaptotagmin-7, proteins implicated in other forms of presynaptic short-term plasticity. Similar NMDAR-independent potentiation could be elicited at synapses in mPFC. Labile LTP allows for rapid information storage that is erased under controlled circumstances and could have a role in a variety of hippocampal and prefrontal cortical computations related to short-term memory.SIGNIFICANCE STATEMENT Changes in synaptic strength are thought to represent information storage relevant to particular nervous system tasks. A single synapse can exhibit multiple overlapping forms of plasticity that shape information transfer from presynaptic to postsynaptic neurons. Here we investigate the mechanisms underlying labile LTP, an NMDAR-independent form of plasticity induced at hippocampal synapses. The potentiation is maintained for long periods as long as the synapses are infrequently active, but with regular activation, the synapses are depotentiated. Similar NMDAR-independent potentiation can also be induced at L2/3-to-L5 synapses in mPFC. Labile LTP requires a rise in presynaptic Ca²⁺ and protein kinase activation but is unaffected in RIM1α or synaptotagmin-7 mutant mice. Labile LTP may contribute to short-term or working memory in hippocampus and mPFC.

A Presynaptic Glutamate Receptor Subunit Confers Robustness to Neurotransmission and Homeostatic Potentiation

Homeostatic signaling systems are thought to interface with other forms of plasticity to ensure flexible yet stable levels of neurotransmission. The role of neurotransmitter receptors in this process, beyond mediating neurotransmission itself, is not known. Through a forward genetic screen, we have identified the Drosophila kainate-type ionotropic glutamate receptor subunit DKaiR1D to be required for the retrograde, homeostatic potentiation of synaptic strength. DKaiR1D is necessary in presynaptic motor neurons, localized near active zones, and confers robustness to the calcium sensitivity of baseline synaptic transmission. Acute pharmacological blockade of DKaiR1D disrupts homeostatic plasticity, indicating that this receptor is required for the expression of this process, distinct from developmental roles. Finally, we demonstrate that calcium permeability through DKaiR1D is necessary for baseline synaptic transmission, but not for homeostatic signaling. We propose that DKaiR1D is a glutamate autoreceptor that promotes robustness to synaptic strength and plasticity with active zone specificity.

The interplay of BDNF-TrkB with NMDA receptor in propofol-induced cognition dysfunction : Mechanism for the effects of propofol on cognitive function

Background

The aim of the present study was to verify whether propofol impaired learning and memory through the interplay of N-methyl-D-aspartate (NMDA) receptor with brain-derived neurotrophic factor (BDNF)-tyrosine kinase B (TrkB) signaling pathway.

Methods

120 Sprague-Dawley (SD) rats were randomly assigned into eight groups. Experimental drugs including saline, intralipid, propofol, N-methyl-D-aspartate (NMDA), 7,8-dihydroxyflavone (7,8-DHF), K252a and MK-801. Spatial learning and memory of rats were tested by the Morris water maze (MWM) test. The mRNA and protein expression were determined by immunohistochemistry, RT-PCR and western blot. Finally, hippocampus cells proliferation and apoptosis were examined by PCNA immunohistochemistry and TUNEL respectively.

Results

The memory and learning was diminished in the propofol exposure group, however, the impaired memory and learning of rats were improved with the addition of NMDA and 7,8-DHF, while the improvement of memory and learning of rats were reversed with the addition of K252a and MK-801. In addition, the mRNA and protein expression levels and hippocampus cells proliferation were the same trend with the results of the MWM test, while apoptosis in hippocampus was reversed.

Conclusion

The propofol can impair memory and learning of rats and induce cognition dysfunction through the interplay of NMDA receptor and BDNF-TrkB-CREB signaling pathway.

Discovering long-term potentiation (LTP) - recollections and reflections on what came after

Chance events led me to a lifelong career in scientific research. They paved the way for being the first to see long-term potentiation of synaptic efficiency (LTP) in Per Andersen's laboratory in Oslo in 1966. Here I describe my way to this discovery and the experiments with Tim Bliss in 1968-1969 that led to Bliss and Lømo, 1973. Surprisingly, we later failed to reproduce these results. I discuss possible reasons for this failure, which made us both leave LTP research, in my case for good, in Tim's case for several years. After 30 years of work in a different field, I renewed my interest in the hippocampus and LTP in the early 2000s and published, for the first time, results that I had obtained 40 years earlier. Here I present my take on how interest in and research on LTP evolved after the early years. This includes a discussion of the functions of hippocampus as seen in those early days, the case of patient H.M., Donald Hebb's place in the story, the search for 'memory molecules' such as PKMζ, and the primary site for LTP expression (pre- and/or post-synaptic?). Throughout, I reflect on my life in science, how science is done and what drives it. The reflections are quite personal and I admit to mixed feelings about broadcasting them.

Unconventional NMDA Receptor Signaling

In the classical view, NMDA receptors (NMDARs) are stably expressed at the postsynaptic membrane, where they act via Ca²⁺ to signal coincidence detection in Hebbian plasticity. More recently, it has been established that NMDAR-mediated transmission can be dynamically regulated by neural activity. In addition, NMDARs have been found presynaptically, where they cannot act as conventional coincidence detectors. Unexpectedly, NMDARs have also been shown to signal metabotropically, without the need for Ca²⁺ This review highlights novel findings concerning these unconventional modes of NMDAR action.

Glutamate is required for depression but not potentiation of long-term presynaptic function

Hebbian plasticity is thought to require glutamate signalling. We show this is not the case for hippocampal presynaptic long-term potentiation (LTP_pre), which is expressed as an increase in transmitter release probability (P_r). We find that LTP_pre can be induced by pairing pre- and postsynaptic spiking in the absence of glutamate signalling. LTP_pre induction involves a non-canonical mechanism of retrograde nitric oxide signalling, which is triggered by Ca²⁺ influx from L-type voltage-gated Ca²⁺ channels, not postsynaptic NMDA receptors (NMDARs), and does not require glutamate release. When glutamate release occurs, it decreases P_r by activating presynaptic NMDARs, and promotes presynaptic long-term depression. Net changes in P_r, therefore, depend on two opposing factors: (1) Hebbian activity, which increases P_r, and (2) glutamate release, which decreases P_r. Accordingly, release failures during Hebbian activity promote LTP_pre induction. Our findings reveal a novel framework of presynaptic plasticity that radically differs from traditional models of postsynaptic plasticity.

Neurons communicate with one another at junctions called synapses. One neuron at the synapse releases a chemical substance called a neurotransmitter, which binds to and activates the other neuron. The release of neurotransmitter thus enables the electrical activity of one cell to influence the electrical activity of another. The efficiency of this communication can change over time, as is thought to occur during learning. If the neurons on both sides of a synapse are repeatedly active at the same time, the ability of the neurons to transmit electrical signals to each other increases.

One way that communication between neurons can become more efficient is if the first neuron becomes more likely to release neurotransmitter. Most synapses in the brain release a neurotransmitter called glutamate, and most types of learning involve changes in the efficiency of communication at glutamatergic synapses. But glutamate release is unreliable. Active glutamatergic neurons fail to release glutamate about 80% of the time. If glutamate has a key role in learning, how does the brain learn efficiently when glutamate release is so unlikely?

To find out, Padamsey et al. studied glutamatergic synapses in slices of tissue from mouse and rat brains. When both neurons at a synapse were repeatedly active at the same time, the first neuron would sometimes become more likely to release glutamate. But this only happened at synapses in which the first neuron usually failed to release glutamate in the first place. This suggests that communication failures help to drive change at synapses. When two neurons that are often active at the same time do not communicate efficiently, this failure triggers molecular changes that make future communication more reliable.

Previous results have shown that synapses can change when glutamate release occurs. The current results show that they can also change when it does not. This means that the brain can continue to learn despite frequent communication failures between neurons. Many neurological disorders, including Alzheimer’s disease, show altered glutamate signalling at synapses. Padamsey et al. hope that a better understanding of this process will lead to new therapies for these disorders.

NMDA Receptor Expression in the Thalamus of the Stargazer Model of Absence Epilepsy

In the stargazer mouse model of absence epilepsy, altered corticothalamic excitation of reticular thalamic nucleus (RTN) neurons has been suggested to contribute to abnormal synchronicity in the corticothalamic-thalamocortical circuit, leading to spike-wave discharges, the hallmark of absence seizures. AMPA receptor expression and function are decreased in stargazer RTN, due to a mutation of AMPAR auxiliary subunit stargazin. It is unresolved and debated, however, if decreased excitation of RTN is compatible with epileptogenesis. We tested the hypothesis that relative NMDAR expression may be increased in RTN and/or thalamic synapses in stargazers using Western blot on dissected thalamic nuclei and biochemically isolated synapses, as well as immunogold cytochemistry in RTN. Expression of main NMDAR subunits was variable in stargazer RTN and relay thalamus; however, mean expression values were not statistically significantly different compared to controls. Furthermore, no systematic changes in synaptic NMDAR levels could be detected in stargazer thalamus. In contrast, AMPAR subunits were markedly decreased in both nucleus-specific and synaptic preparations. Thus, defective AMPAR trafficking in stargazer thalamus does not appear to lead to a ubiquitous compensatory increase in total and synaptic NMDAR expression, suggesting that elevated NMDAR function is not mediated by changes in protein expression in stargazer mice.

Modulation of Synaptic Plasticity in the Cortex Needs to Understand All the Players

The prefrontal cortex (PFC) is involved in cognitive tasks such as working memory, decision making, risk assessment and regulation of attention. These functions performed by the PFC are supposed to rely on rhythmic electrical activity generated by neuronal network oscillations determined by a precise balance between excitation and inhibition balance (E/I balance) resulting from the coordinated activities of recurrent excitation and feedback and feedforward inhibition. Functional alterations in PFC functions have been associated with cognitive deficits in several pathologies such as major depression, anxiety and schizophrenia. These pathological situations are correlated with alterations of different neurotransmitter systems (i.e., serotonin (5-HT), dopamine (DA), acetylcholine…) that result in alterations of the E/I balance. The aim of this review article is to cover the basic aspects of the regulation of the E/I balance as well as to highlight the importance of the complementarity role of several neurotransmitters in the modulation of the plasticity of excitatory and inhibitory synapses. We illustrate our purpose by recent findings that demonstrate that 5-HT and DA cooperate to regulate the plasticity of excitatory and inhibitory synapses targeting layer 5 pyramidal neurons (L5PyNs) of the PFC and to fine tune the E/I balance. Using a method based on the decomposition of the synaptic conductance into its excitatory and inhibitory components, we show that concomitant activation of D1-like receptors (D1Rs) and 5-HT_1ARs, through a modulation of NMDA receptors, favors long term potentiation (LTP) of both excitation and inhibition and consequently does not modify the E/I balance. We also demonstrate that activation of D2-receptors requires functional 5-HT_1ARs to shift the E-I balance towards more inhibition and to favor long term depression (LTD) of excitatory synapses through the activation of glycogen synthase kinase 3β (GSK3β). This cooperation between different neurotransmitters is particularly relevant in view of pathological situations in which alterations of one neurotransmitter system will also have consequences on the regulation of synaptic efficacy by other neurotransmitters. This opens up new perspectives in the development of therapeutic strategies for the pharmacological treatment of neuronal disorders.