Molecular Mechanisms of Non-ionotropic NMDA Receptor Signaling in Dendritic Spine Shrinkage

Non-ionotropic voltage-gated calcium channel signaling

Voltage-gated calcium channels (VGCCs) are the major conduits for calcium ions (Ca2+) within excitable cells. Recent studies have highlighted the non-ionotropic functionality of VGCCs, revealing their capacity to activate intracellular pathways independently of ion flow. This non-ionotropic signaling mode plays a pivotal role in excitation-coupling processes, including gene transcription through excitation-transcription (ET), synaptic transmission via excitation-secretion (ES), and cardiac contraction through excitation-contraction (EC). However, it is noteworthy that these excitation-coupling processes require extracellular calcium (Ca2+) and Ca2+ occupancy of the channel ion pore. Analogous to the "non-canonical" characterization of the non-ionotropic signaling exhibited by the N-methyl-D-aspartate receptor (NMDA), which requires extracellular Ca2+ without the influx of ions, VGCC activation requires depolarization-triggered conformational change(s) concomitant with Ca2+ binding to the open channel. Here, we discuss the contributions of VGCCs to ES, ET, and EC coupling as Ca2+ binding macromolecules that transduces external stimuli to intracellular input prior to elevating intracellular Ca2+. We emphasize the recognition of calcium ion occupancy within the open ion-pore and its contribution to the excitation coupling processes that precede the influx of calcium. The non-ionotropic activation of VGCCs, triggered by the upstroke of an action potential, provides a conceptual framework to elucidate the mechanistic aspects underlying the microseconds nature of synaptic transmission, cardiac contractility, and the rapid induction of first-wave genes.

Piezo1 ion channels are capable of conformational signaling

Piezo1 is a mechanically activated ion channel that senses forces with short latency and high sensitivity. Piezos undergo large conformational changes, induce far-reaching deformation onto the membrane, and modulate the function of two-pore potassium (K2P) channels. Taken together, this led us to hypothesize that Piezos may be able to signal their conformational state to other nearby proteins. Here, we use chemical control to acutely restrict Piezo1 conformational flexibility and show that Piezo1 conformational changes, but not ion permeation through them, are required for modulating the K2P channel K2P2.1 (TREK1). Super-resolution imaging and stochastic simulations further reveal that both channels do not co-localize, which implies that modulation is not mediated through direct binding interactions; however, at high Piezo1 densities, most TREK1 channels are within the predicted Piezo1 membrane footprint, suggesting that the footprint may underlie conformational signaling. We speculate that physiological roles originally attributed to Piezo1 ionotropic function could, alternatively, involve conformational signaling.

L-type Ca2+ channel activation of STIM1-Orai1 signaling remodels the dendritic spine ER to maintain long-term structural plasticity

Learning and memory require coordinated structural and functional plasticity at neuronal glutamatergic synapses located on dendritic spines. Here, we investigated how the endoplasmic reticulum (ER) controls postsynaptic Ca2+ signaling and long-term potentiation of dendritic spine size, i.e., sLTP that accompanies functional strengthening of glutamatergic synaptic transmission. In most ER-containing (ER+) spines, high-frequency optical glutamate uncaging (HFGU) induced long-lasting sLTP that was accompanied by a persistent increase in spine ER content downstream of a signaling cascade engaged by N-methyl-D-aspartate receptors (NMDARs), L-type Ca2+ channels (LTCCs), and Orai1 channels, the latter being activated by stromal interaction molecule 1 (STIM1) in response to ER Ca2+ release. In contrast, HFGU stimulation of ER-lacking (ER-) spines expressed only transient sLTP and exhibited weaker Ca2+ signals noticeably lacking Orai1 and ER contributions. Consistent with spine ER regulating structural metaplasticity, delivery of a second stimulus to ER- spines induced ER recruitment along with persistent sLTP, whereas ER+ spines showed no additional increases in size or ER content in response to sequential stimulation. Surprisingly, the physical interaction between STIM1 and Orai1 induced by ER Ca2+ release, but not the resulting Ca2+ entry through Orai1 channels, proved necessary for the persistent increases in both spine size and ER content required for expression of long-lasting late sLTP.

APP fragment controls both ionotropic and non-ionotropic signaling of NMDA receptors

NMDA receptors (NMDARs) are ionotropic receptors crucial for brain information processing. Yet, evidence also supports an ion-flux-independent signaling mode mediating synaptic long-term depression (LTD) and spine shrinkage. Here, we identify AETA (Aη), an amyloid-β precursor protein (APP) cleavage product, as an NMDAR modulator with the unique dual regulatory capacity to impact both signaling modes. AETA inhibits ionotropic NMDAR activity by competing with the co-agonist and induces an intracellular conformational modification of GluN1 subunits. This favors non-ionotropic NMDAR signaling leading to enhanced LTD and favors spine shrinkage. Endogenously, AETA production is increased by in vivo chemogenetically induced neuronal activity. Genetic deletion of AETA production alters NMDAR transmission and prevents LTD, phenotypes rescued by acute exogenous AETA application. This genetic deletion also impairs contextual fear memory. Our findings demonstrate AETA-dependent NMDAR activation (ADNA), characterizing AETA as a unique type of endogenous NMDAR modulator that exerts bidirectional control over NMDAR signaling and associated information processing.

d-Serine Inhibits Non-ionotropic NMDA Receptor Signaling

NMDA-type glutamate receptors (NMDARs) are widely recognized as master regulators of synaptic plasticity, most notably for driving long-term changes in synapse size and strength that support learning. NMDARs are unique among neurotransmitter receptors in that they require binding of both neurotransmitter (glutamate) and co-agonist (e.g., d-serine) to open the receptor channel, which leads to the influx of calcium ions that drive synaptic plasticity. Over the past decade, evidence has accumulated that NMDARs also support synaptic plasticity via ion flux-independent (non-ionotropic) signaling upon the binding of glutamate in the absence of co-agonist, although conflicting results have led to significant controversy. Here, we hypothesized that a major source of contradictory results might be attributed to variable occupancy of the co-agonist binding site under different experimental conditions. To test this hypothesis, we manipulated co-agonist availability in acute hippocampal slices from mice of both sexes. We found that enzymatic scavenging of endogenous co-agonists enhanced the magnitude of long-term depression (LTD) induced by non-ionotropic NMDAR signaling in the presence of the NMDAR pore blocker MK801. Conversely, a saturating concentration of d-serine completely inhibited LTD and spine shrinkage induced by glutamate binding in the presence of MK801 or Mg2+ Using a Förster resonance energy transfer (FRET)-based assay in cultured neurons, we further found that d-serine completely blocked NMDA-induced conformational movements of the GluN1 cytoplasmic domains in the presence of MK801. Our results support a model in which d-serine availability serves to modulate NMDAR signaling and synaptic plasticity even when the NMDAR is blocked by magnesium.

D-Serine inhibits non-ionotropic NMDA receptor signaling

NMDA-type glutamate receptors (NMDARs) are widely recognized as master regulators of synaptic plasticity, most notably for driving long-term changes in synapse size and strength that support learning. NMDARs are unique among neurotransmitter receptors in that they require binding of both neurotransmitter (glutamate) and co-agonist (e.g. d -serine) to open the receptor channel, which leads to the influx of calcium ions that drive synaptic plasticity. Over the past decade, evidence has accumulated that NMDARs also support synaptic plasticity via ion flux-independent (non-ionotropic) signaling upon the binding of glutamate in the absence of co-agonist, although conflicting results have led to significant controversy. Here, we hypothesized that a major source of contradictory results can be attributed to variable occupancy of the co-agonist binding site under different experimental conditions. To test this hypothesis, we manipulated co-agonist availability in acute hippocampal slices from mice of both sexes. We found that enzymatic scavenging of endogenous co-agonists enhanced the magnitude of LTD induced by non-ionotropic NMDAR signaling in the presence of the NMDAR pore blocker, MK801. Conversely, a saturating concentration of d -serine completely inhibited both LTD and spine shrinkage induced by glutamate binding in the presence of MK801. Using a FRET-based assay in cultured neurons, we further found that d -serine completely blocked NMDA-induced conformational movements of the GluN1 cytoplasmic domains in the presence of MK801. Our results support a model in which d -serine inhibits ion flux-independent NMDAR signaling and plasticity, and thus d -serine availability could serve to modulate NMDAR signaling even when the NMDAR is blocked by magnesium.

Significance Statement

NMDARs are glutamate-gated cation channels that are key regulators of neurodevelopment and synaptic plasticity and unique in their requirement for binding of a co-agonist (e.g. d -serine) in order for the channel to open. NMDARs have been found to drive synaptic plasticity via non-ionotropic (ion flux-independent) signaling upon the binding of glutamate in the absence of co-agonist, though conflicting results have led to controversy. Here, we found that d -serine inhibits non-ionotropic NMDAR-mediated LTD and LTD-associated spine shrinkage. Thus, a major source of the contradictory findings might be attributed to experimental variability in d -serine availability. In addition, the developmental regulation of d -serine levels suggests a role for non-ionotropic NMDAR plasticity during critical periods of plasticity.

Piezo1 ion channels are capable of conformational signaling

Piezo1 is a mechanically activated ion channel that senses forces with short latency and high sensitivity. Piezos undergo large conformational changes, induce far-reaching deformation onto the membrane, and modulate the function of two-pore potassium (K2P) channels. Taken together, this led us to hypothesize that Piezos may be able to signal their conformational state to other nearby proteins. Here, we use chemical control to acutely restrict Piezo1 conformational flexibility and show that Piezo1 conformational changes, but not ion permeation through it, are required for modulating the K2P channel TREK1. Super-resolution imaging and stochastic simulations further reveal that both channels do not co-localize, which implies that modulation is not mediated through direct binding interactions; however, at high Piezo1 densities, most TREK1 channels are within the predicted Piezo1 membrane footprint, suggesting the footprint may underlie conformational signaling. We speculate that physiological roles originally attributed to Piezo1 ionotropic function could, alternatively, involve conformational signaling.

Two Signaling Modes Are Better than One: Flux-Independent Signaling by Ionotropic Glutamate Receptors Is Coming of Age

Glutamate is the major excitatory neurotransmitter in the central nervous system. Glutamatergic transmission can be mediated by ionotropic glutamate receptors (iGluRs), which mediate rapid synaptic depolarization that can be associated with Ca2+ entry and activity-dependent change in the strength of synaptic transmission, as well as by metabotropic glutamate receptors (mGluRs), which mediate slower postsynaptic responses through the recruitment of second messenger systems. A wealth of evidence reported over the last three decades has shown that this dogmatic subdivision between iGluRs and mGluRs may not reflect the actual physiological signaling mode of the iGluRs, i.e., α-amino-3-hydroxy-5-methyl-4-isoxasolepropionic acid (AMPA) receptors (AMPAR), kainate receptors (KARs), and N-methyl-D-aspartate (NMDA) receptors (NMDARs). Herein, we review the evidence available supporting the notion that the canonical iGluRs can recruit flux-independent signaling pathways not only in neurons, but also in brain astrocytes and cerebrovascular endothelial cells. Understanding the signaling versatility of iGluRs can exert a profound impact on our understanding of glutamatergic synapses. Furthermore, it may shed light on novel neuroprotective strategies against brain disorders.

Extrasynaptic distribution of NMDA receptors in cochlear inner hair cell afferent signaling complex

OBJECTIVE

The distribution and role of NMDA receptors is unclear in the afferent signaling complex of the cochlea. The present study aimed to examine the distribution of NMDA receptors in cochlear afferent signaling complex of the adult mouse, and their relationship with ribbon synapses of inner hair cells (IHCs) and GABAergic efferent terminals of the lateral olivocochlear (LOC).

METHODS

Immunofluorescence staining in combination with confocal microscopy was used to investigate the distribution of glutamatergic NMDA and AMPA receptors in afferent terminals of SGNs, and their relationship with ribbon synapses of IHCs and GABAergic efferent terminals of LOC.

RESULTS

Terminals with AMPA receptors along with Ribbons of IHC formed afferent synapses in the basal pole of IHCs, and those with NMDA receptors were mainly distributed longitudinally in the IHCs nuclei region. Significant difference was found in the distribution of NMDA and AMPA receptors in IHC afferent signaling complex (P<0.05). Some GABAergic terminals colocalized with NMDA receptors at the IHC nucleus region (P>0.05).

CONCLUSION

There is significant difference in the distribution of NMDA and AMPA receptors in cochlear afferent signaling complex. NMDA receptors are present in the extra-synaptic region of ribbon synapses of IHCs, and they are related to GABA efferent terminals of the afferent signaling complex.

Metabotropic NMDA Receptor Signaling Contributes to Sex Differences in Synaptic Plasticity and Episodic Memory

Men generally outperform women on encoding spatial components of episodic memory whereas the reverse holds for semantic elements. Here we show that female mice outperform males on tests for non-spatial aspects of episodic memory ("what", "when"), suggesting that the human findings are influenced by neurobiological factors common to mammals. Analysis of hippocampal synaptic plasticity mechanisms and encoding revealed unprecedented, sex-specific contributions of non-classical metabotropic NMDA receptor (NMDAR) functions. While both sexes used non-ionic NMDAR signaling to trigger actin polymerization needed to consolidate long-term potentiation (LTP), NMDAR GluN2B subunit antagonism blocked these effects in males only and had the corresponding sex-specific effect on episodic memory. Conversely, blocking estrogen receptor alpha eliminated metabotropic stabilization of LTP and episodic memory in females only. The results show that sex differences in metabotropic signaling critical for enduring synaptic plasticity in hippocampus have significant consequences for encoding episodic memories.

Insights into early pathogenesis of sporadic Alzheimer's disease: role of oxidative stress and loss of synaptic proteins

Oxidative stress, induced by impaired insulin signaling in the brain contributes to cognitive loss in sporadic Alzheimer's disease (sAD). This study evaluated early hippocampal oxidative stress, pre- and post-synaptic proteins in intraperitoneal (IP) and intracerebroventricular (ICV) streptozotocin (STZ) models of impaired insulin signaling. Adult male Wistar rats were injected with STZ, IP, or ICV, and sacrificed 1-, 3-, or 6-weeks post injection. Rat's cognitive behavior was assessed using Morris water maze (MWM) tests at weeks 3 and 6. Hippocampal synaptosomal fractions were examined for oxidative stress markers and presynaptic [synapsin I, synaptophysin, growth-associated protein-43 (GAP-43), synaptosomal-associated protein-25 (SNAP-25)] and postsynaptic [drebrin, synapse-associated protein-97 (SAP-97), postsynaptic density protein-95 (PSD-95)] proteins. IP-STZ and ICV-STZ treatment impaired rat's cognition, decreased the levels of reduced glutathione (GSH) and increased the levels of thiobarbituric acid reactive species (TBARS) in a time dependent manner. In addition, it reduced the expression of pre- and post-synaptic proteins in the hippocampus. The decline in cognition is significantly correlated with the reduction in synaptic proteins in the hippocampus. In conclusion, impaired insulin signaling in the brain is deleterious in causing early synaptosomal oxidative damage and synaptic loss that exacerbates with time and correlates with cognitive impairments. Our data implicates oxidative stress and synaptic protein loss as an early feature of sAD and provides insights into early biochemical and behavioral changes during disease progression.

Therapeutic potential of N-methyl-D-aspartate receptor modulators in psychiatry

N-methyl-D-aspartate (NMDA) receptors mediate a slow component of excitatory synaptic transmission, are widely distributed throughout the central nervous system, and regulate synaptic plasticity. NMDA receptor modulators have long been considered as potential treatments for psychiatric disorders including depression and schizophrenia, neurodevelopmental disorders such as Rett Syndrome, and neurodegenerative conditions such as Alzheimer's disease. New interest in NMDA receptors as therapeutic targets has been spurred by the findings that certain inhibitors of NMDA receptors produce surprisingly rapid and robust antidepressant activity by a novel mechanism, the induction of changes in the brain that well outlast the presence of drug in the body. These findings are driving research into an entirely new paradigm for using NMDA receptor antagonists in a host of related conditions. At the same time positive allosteric modulators of NMDA receptors are being pursued for enhancing synaptic function in diseases that feature NMDA receptor hypofunction. While there is great promise, developing the therapeutic potential of NMDA receptor modulators must also navigate the potential significant risks posed by the use of such agents. We review here the emerging pharmacology of agents that target different NMDA receptor subtypes, offering new avenues for capturing the therapeutic potential of targeting this important receptor class.

Excitation-transcription coupling, neuronal gene expression and synaptic plasticity

Excitation-transcription coupling (E-TC) links synaptic and cellular activity to nuclear gene transcription. It is generally accepted that E-TC makes a crucial contribution to learning and memory through its role in underpinning long-lasting synaptic enhancement in late-phase long-term potentiation and has more recently been linked to late-phase long-term depression: both processes require de novo gene transcription, mRNA translation and protein synthesis. E-TC begins with the activation of glutamate-gated N-methyl-D-aspartate-type receptors and voltage-gated L-type Ca2+ channels at the membrane and culminates in the activation of transcription factors in the nucleus. These receptors and ion channels mediate E-TC through mechanisms that include long-range signalling from the synapse to the nucleus and local interactions within dendritic spines, among other possibilities. Growing experimental evidence links these E-TC mechanisms to late-phase long-term potentiation and learning and memory. These advances in our understanding of the molecular mechanisms of E-TC mean that future efforts can focus on understanding its mesoscale functions and how it regulates neuronal network activity and behaviour in physiological and pathological conditions.

Stimulating β-adrenergic receptors promotes synaptic potentiation by switching CaMKII movement from LTD to LTP mode

Learning, memory, and cognition are thought to require synaptic plasticity, specifically including hippocampal long-term potentiation and depression (LTP and LTD). LTP versus LTD is induced by high-frequency stimulation versus low-frequency, but stimulating β-adrenergic receptors (βARs) enables LTP induction also by low-frequency stimulation (1 Hz) or theta frequencies (∼5 Hz) that do not cause plasticity by themselves. In contrast to high-frequency stimulation-LTP, such βAR-LTP requires Ca2+-flux through L-type voltage-gated Ca2+-channels, not N-methyl-D-aspartate-type glutamate receptors. Surprisingly, we found that βAR-LTP still required a nonionotropic scaffolding function of the N-methyl-D-aspartate-type glutamate receptor: the stimulus-induced binding of the Ca2+/calmodulin-dependent protein kinase II (CaMKII) to its GluN2B subunit that mediates CaMKII movement to excitatory synapses. In hippocampal neurons, β-adrenergic stimulation with isoproterenol (Iso) transformed LTD-type CaMKII movement to LTP-type movement, resulting in CaMKII movement to excitatory instead of inhibitory synapses. Additionally, Iso enabled induction of a major cell-biological feature of LTP in response to LTD stimuli: increased surface expression of GluA1 fused with super-ecliptic pHluorein. Like for βAR-LTP in hippocampal slices, the Iso effects on CaMKII movement and surface expression of GluA1 fused with super-ecliptic pHluorein involved L-type Ca2+-channels and specifically required β2-ARs. Taken together, these results indicate that Iso transforms LTD stimuli to LTP signals by switching CaMKII movement and GluN2B binding to LTP mode.

NMDA receptor functions in health and disease: Old actor, new dimensions

N-Methyl-D-aspartate ionotropic glutamate receptors (NMDARs) play key roles in synaptogenesis, synaptic maturation, long-term plasticity, neuronal network activity, and cognition. Mirroring this wide range of instrumental functions, abnormalities in NMDAR-mediated signaling have been associated with numerous neurological and psychiatric disorders. Thus, identifying the molecular mechanisms underpinning the physiological and pathological contributions of NMDAR has been a major area of investigation. Over the past decades, a large body of literature has flourished, revealing that the physiology of ionotropic glutamate receptors cannot be restricted to fluxing ions, and involves additional facets controlling synaptic transmissions in health and disease. Here, we review newly discovered dimensions of postsynaptic NMDAR signaling supporting neural plasticity and cognition, such as the nanoscale organization of NMDAR complexes, their activity-dependent redistributions, and non-ionotropic signaling capacities. We also discuss how dysregulations of these processes may directly contribute to NMDAR-dysfunction-related brain diseases.

Functional crosstalk of the glycine transporter GlyT1 and NMDA receptors

NMDA-type glutamate receptors (NMDARs) constitute one of the main glutamate (Glu) targets in the central nervous system and are involved in synaptic plasticity, which is the molecular substrate of learning and memory. Hypofunction of NMDARs has been associated with schizophrenia, while overstimulation causes neuronal death in neurodegenerative diseases or in stroke. The function of NMDARs requires coincidental binding of Glu along with other cellular signals such as neuronal depolarization, and the presence of other endogenous ligands that modulate their activity by allosterism. Among these allosteric modulators are zinc, protons and Gly, which is an obligatory co-agonist. These characteristics differentiate NMDARs from other receptors, and their structural bases have begun to be established in recent years. In this review we focus on the crosstalk between Glu and glycine (Gly), whose concentration in the NMDAR microenvironment is maintained by various Gly transporters that remove or release it into the medium in a regulated manner. The GlyT1 transporter is particularly involved in this task, and has become a target of great interest for the treatment of schizophrenia since its inhibition leads to an increase in synaptic Gly levels that enhances the activity of NMDARs. However, the only drug that has completed phase III clinical trials did not yield the expected results. Notwithstanding, there are additional drugs that continue to be investigated, and it is hoped that knowledge gained from the recently published 3D structure of GlyT1 may allow the rational design of more effective new drugs.

NMDA Receptor and Its Emerging Role in Cancer

Glutamate is a key player in excitatory neurotransmission in the central nervous system (CNS). The N-methyl-D-aspartate receptor (NMDAR) is a glutamate-gated ion channel which presents several unique features and is involved in various physiological and pathological neuronal processes. Thanks to great efforts in neuroscience, its structure and the molecular mechanisms controlling its localization and functional regulation in neuronal cells are well known. The signaling mediated by NMDAR in neurons is very complex as it depends on its localization, composition, Ca²⁺ influx, and ion flow-independent conformational changes. Moreover, NMDA receptors are highly diffusive in the plasma membrane of neurons, where they form heterocomplexes with other membrane receptors and scaffold proteins which determine the receptor function and activation of downstream signaling. Interestingly, a recent paper demonstrates that NMDAR signaling is involved in epithelial cell competition, an evolutionary conserved cell fitness process influencing cancer initiation and progress. The idea that NMDAR signaling is limited to CNS has been challenged in the past two decades. A large body of evidence suggests that NMDAR is expressed in cancer cells outside the CNS and can respond to the autocrine/paracrine release of glutamate. In this review, we survey research on NMDAR signaling and regulation in neurons that can help illuminate its role in tumor biology. Finally, we will discuss existing data on the role of the glutamine/glutamate metabolism, the anticancer action of NMDAR antagonists in experimental models, NMDAR synaptic signaling in tumors, and clinical evidence in human cancer.

Astrocyte and neuronal Panx1 support long-term reference memory in mice

Pannexin 1 (Panx1) are ubiquitously expressed proteins that form plasma membrane channels permeable to anions and moderate sized signaling molecules (e.g., ATP, glutamate). In the nervous system, activation of Panx1 channels have been extensively shown to contribute to distinct neurological disorders (epilepsy, chronic pain, migraine, neuroAIDS, etc.) but knowledge of extent to which these channels have a physiological role remains restricted to three studies supporting their involvement in hippocampus dependent learning. Given that Panx1 channels may provide an important mechanism for activity-dependent neuron-glia interaction, we used Panx1 transgenic mice with global and cell-type specific deletions of Panx1 to interrogate their participation in working and reference memory. Using the 8-arm radial maze, we show that long-term spatial reference memory, but not spatial working memory, is deficient in Panx1-null mice and that both astrocyte and neuronal Panx1 contribute to the consolidation of long-term spatial memory. Field potential recordings in hippocampal slices of Panx1-null mice revealed an attenuation of both long-term potentiation (LTP) of synaptic strength and long-term depression (LTD) at Schaffer collateral - CA1 synapses without alterations basal synaptic transmission or pre-synaptic paired-pulse facilitation. Our results implicate both neuronal and astrocyte Panx1 channels as critical players for the development and maintenance of long-term spatial reference memory in mice.

Unraveling the mysteries of dendritic spine dynamics: Five key principles shaping memory and cognition

Recent research extends our understanding of brain processes beyond just action potentials and chemical transmissions within neural circuits, emphasizing the mechanical forces generated by excitatory synapses on dendritic spines to modulate presynaptic function. From in vivo and in vitro studies, we outline five central principles of synaptic mechanics in brain function: P1: Stability - Underpinning the integral relationship between the structure and function of the spine synapses. P2: Extrinsic dynamics - Highlighting synapse-selective structural plasticity which plays a crucial role in Hebbian associative learning, distinct from pathway-selective long-term potentiation (LTP) and depression (LTD). P3: Neuromodulation - Analyzing the role of G-protein-coupled receptors, particularly dopamine receptors, in time-sensitive modulation of associative learning frameworks such as Pavlovian classical conditioning and Thorndike's reinforcement learning (RL). P4: Instability - Addressing the intrinsic dynamics crucial to memory management during continual learning, spotlighting their role in "spine dysgenesis" associated with mental disorders. P5: Mechanics - Exploring how synaptic mechanics influence both sides of synapses to establish structural traces of short- and long-term memory, thereby aiding the integration of mental functions. We also delve into the historical background and foresee impending challenges.

Astrocyte and Neuronal Panx1 Support Long-Term Reference Memory in Mice

Pannexin 1 (Panx1) is an ubiquitously expressed protein that forms plasma membrane channels permeable to anions and moderate-sized signaling molecules (e.g., ATP, glutamate). In the nervous system, activation of Panx1 channels has been extensively shown to contribute to distinct neurological disorders (epilepsy, chronic pain, migraine, neuroAIDS, etc.), but knowledge of the extent to which these channels have a physiological role remains restricted to three studies supporting their involvement in hippocampus dependent learning. Given that Panx1 channels may provide an important mechanism for activity-dependent neuron-glia interaction, we used Panx1 transgenic mice with global and cell-type specific deletions of Panx1 to interrogate their participation in working and reference memory. Using the eight-arm radial maze, we show that long-term spatial reference memory, but not spatial working memory, is deficient in Panx1-null mice and that both astrocyte and neuronal Panx1 contribute to the consolidation of long-term spatial memory. Field potential recordings in hippocampal slices of Panx1-null mice revealed an attenuation of both long-term potentiation (LTP) of synaptic strength and long-term depression (LTD) at Schaffer collateral-CA1 synapses without alterations of basal synaptic transmission or pre-synaptic paired-pulse facilitation. Our results implicate both neuronal and astrocyte Panx1 channels as critical players for the development and maintenance of long-term spatial reference memory in mice.

Targeting NMDA Receptors in Emotional Disorders: Their Role in Neuroprotection

Excitatory glutamatergic neurotransmission mediated through N-methyl-D-Aspartate (NMDA) receptors (NMDARs) is essential for synaptic plasticity and neuronal survival. While under pathological states, abnormal NMDAR activation is involved in the occurrence and development of psychiatric disorders, which suggests a directional modulation of NMDAR activity that contributes to the remission and treatment of psychiatric disorders. This review thus focuses on the involvement of NMDARs in the pathophysiological processes of psychiatric mood disorders and analyzes the neuroprotective mechanisms of NMDARs. Firstly, we introduce NMDAR-mediated neural signaling pathways in brain function and mood regulation as well as the pathophysiological mechanisms of NMDARs in emotion-related mental disorders such as anxiety and depression. Then, we provide an in-depth summary of current NMDAR modulators that have the potential to be developed into clinical drugs and their pharmacological research achievements in the treatment of anxiety and depression. Based on these findings, drug-targeting for NMDARs might open up novel territory for the development of therapeutic agents for refractory anxiety and depression.

Revisiting the molecular basis of synaptic transmission

Measurements of the time elapsed during synaptic transmission has shown that synaptic vesicle (SV) fusion lags behind Ca²⁺-influx by approximately 60 microseconds (µsec). The conventional model cannot explain this extreme rapidity of the release event. Synaptic transmission occurs at the active zone (AZ), which comprises of two pools of SV, non-releasable "tethered" vesicles, and a readily-releasable pool of channel-associated Ca²⁺-primed vesicles, "RRP". A recent TIRF study at cerebellar-mossy fiber-terminal, showed that subsequent to an action potential, newly "tethered" vesicles, became fusion-competent in a Ca²⁺-dependent manner, 300-400 milliseconds after tethering, but were not fused. This time resolution may correspond to priming of tethered vesicles through Ca²⁺-binding to Syt1/Munc13-1/complexin. It confirms that Ca²⁺-priming and Ca²⁺-influx-independent fusion, are two distinct events. Notably, we have established that Ca²⁺ channel signals evoked-release in an ion flux-independent manner, demonstrated by Ca²⁺-impermeable channel, or a Ca²⁺ channel in which Ca²⁺ is replaced by impermeable La³⁺. Thus, conformational changes in a channel coupled to RRP appear to directly activate the release machinery and account for a µsec Ca²⁺-influx-independent vesicle fusion. Rapid vesicle fusion driven by non-ionotropic channel signaling strengthens a conformational-coupling mechanism of synaptic transmission, and contributes to better understanding of neuronal communication vital for brain function.

Reduced d-serine levels drive enhanced non-ionotropic NMDA receptor signaling and destabilization of dendritic spines in a mouse model for studying schizophrenia

Schizophrenia is a psychiatric disorder that affects over 20 million people globally. Notably, schizophrenia is associated with decreased density of dendritic spines and decreased levels of d-serine, a co-agonist required for opening of the N-methyl-d-aspartate receptor (NMDAR). We hypothesized that lowered d-serine levels associated with schizophrenia would enhance ion flux-independent signaling by the NMDAR, driving destabilization and loss of dendritic spines. We tested our hypothesis using the serine racemase knockout (SRKO) mouse model, which lacks the enzyme for d-serine production. We show that activity-dependent spine growth is impaired in SRKO mice, but can be acutely rescued by exogenous d-serine. Moreover, we find a significant bias of synaptic plasticity toward spine shrinkage in the SRKO mice as compared to wild-type littermates. Notably, we demonstrate that enhanced ion flux-independent signaling through the NMDAR contributes to this bias toward spine destabilization, which is exacerbated by an increase in synaptic NMDARs in hippocampal synapses of SRKO mice. Our results support a model in which lowered d-serine levels associated with schizophrenia enhance ion flux-independent NMDAR signaling and bias toward spine shrinkage and destabilization.

The Intriguing Contribution of Hippocampal Long-Term Depression to Spatial Learning and Long-Term Memory

Long-term potentiation (LTP) and long-term depression (LTD) comprise the principal cellular mechanisms that fulfill established criteria for the physiological correlates of learning and memory. Traditionally LTP, that increases synaptic weights, has been ascribed a prominent role in learning and memory whereas LTD, that decreases them, has often been relegated to the category of "counterpart to LTP" that serves to prevent saturation of synapses. In contradiction of these assumptions, studies over the last several years have provided functional evidence for distinct roles of LTD in specific aspects of hippocampus-dependent associative learning and information encoding. Furthermore, evidence of the experience-dependent "pruning" of excitatory synapses, the majority of which are located on dendritic spines, by means of LTD has been provided. In addition, reports exist of the temporal and physical restriction of LTP in dendritic compartments by means of LTD. Here, we discuss the role of LTD and LTP in experience-dependent information encoding based on empirical evidence derived from conjoint behavioral and electrophysiological studies conducted in behaving rodents. We pinpoint the close interrelation between structural modifications of dendritic spines and the occurrence of LTP and LTD. We report on findings that support that whereas LTP serves to acquire the general scheme of a spatial representation, LTD enables retention of content details. We argue that LTD contributes to learning by engaging in a functional interplay with LTP, rather than serving as its simple counterpart, or negator. We propose that similar spatial experiences that share elements of neuronal representations can be modified by means of LTD to enable pattern separation. Therewith, LTD plays a crucial role in the disambiguation of similar spatial representations and the prevention of generalization.

Treatment of Cerebral Ischemia Through NMDA Receptors: Metabotropic Signaling and Future Directions

Excessive activation of N-methyl-d-aspartic acid (NMDA) receptors after cerebral ischemia is a key cause of ischemic injury. For a long time, it was generally accepted that calcium influx is a necessary condition for ischemic injury mediated by NMDA receptors. However, recent studies have shown that NMDA receptor signaling, independent of ion flow, plays an important role in the regulation of ischemic brain injury. The purpose of this review is to better understand the roles of metabotropic NMDA receptor signaling in cerebral ischemia and to discuss the research and development directions of NMDA receptor antagonists against cerebral ischemia. This mini review provides a discussion on how metabotropic transduction is mediated by the NMDA receptor, related signaling molecules, and roles of metabotropic NMDA receptor signaling in cerebral ischemia. In view of the important roles of metabotropic signaling in cerebral ischemia, NMDA receptor antagonists, such as GluN2B-selective antagonists, which can effectively block both pro-death metabotropic and pro-death ionotropic signaling, may have better application prospects.

Ion flux-independent NMDA receptor signaling

NMDA receptors play vital roles in a broad array of essential brain functions, from synaptic transmission and plasticity to learning and memory. Historically, the fundamental roles of NMDARs were attributed to their specialized properties of ion flux. More recently, it has become clear that NMDARs also signal in an ion flux-independent manner. Here, we review these non-ionotropic NMDAR signaling mechanisms that have been reported to contribute to a broad array of neuronal functions and dysfunctions including synaptic transmission and plasticity, cell death and survival, and neurological disorders.

Behind the scenes: Are latent memories supported by calcium independent plasticity?

N-methyl-D-aspartate receptors (NMDARs) can be considered to be the de facto "plasticity" receptors in the brain due to their central role in the activity-dependent modification of neuronal morphology and synaptic transmission. Since the 1980s, research on NMDARs has focused on the second messenger properties of calcium and the downstream signaling pathways that mediate alterations in neural form and function. Recently, NMDARs were shown to drive activity-dependent synaptic plasticity without calcium influx. How this "nonionotropic" plasticity occurs in vitro is becoming clearer, but research on its involvement in behavior and cognition is in its infancy. There is a partial overlap in the downstream signaling molecules that are involved in ionotropic and nonionotropic NMDAR-dependent plasticity. Given this, and prior studies of the cognitive impacts of ionotropic NMDAR plasticity, a preliminary model explaining how NMDAR nonionotropic plasticity affects learning and memory can be established. We hypothesize that nonionotropic NMDAR plasticity takes part in latent memory encoding in immature rodents through nonassociative depression of synaptic efficacy, and possibly shrinking of dendritic spines. Further, the late postnatal alteration in NMDAR composition in the hippocampus appears to reduce nonionotropic signaling and remove a restriction on memory retrieval. This framework substantially alters the canonical model of NMDAR involvement in spatial cognition and hippocampal maturation and provides novel and exciting inroads for future studies.

Induction of input-specific spine shrinkage on dendrites of rodent hippocampal CA1 neurons using two-photon glutamate uncaging

Shrinkage and loss of dendritic spines are vital components of the neuronal plasticity that supports learning. To investigate the mechanisms of spine shrinkage and loss, Oh and colleagues established a two-photon glutamate uncaging protocol that reliably induces input-specific spine shrinkage on dendrites of rodent hippocampal CA1 pyramidal neurons. Here, we provide a detailed description of that protocol and also an optimized version that can be used to induce input- and synapse-specific shrinkage of dendritic spines at physiological Ca²⁺ levels. For complete details on the use and execution of this protocol, please refer to Oh et al. (2013), Stein et al. (2015), Stein et al. (2020), and Stein et al. (2021).

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.

Imaging neuronal protein signaling dynamics in vivo

The activity patterns of Individual neurons are highly coordinated and synchronized within neuronal circuits in the brain, much like individual orchestra tools playing together to achieve harmony. Inside neurons, complex protein signaling cascades provide the molecular notes and instructions to each neuron. However, until recently, the dynamic nature of intracellular protein signaling in the intact brain has been eluded. In this review, we focus on recent advancements and the development of approaches to study neuronal signaling dynamics in vivo. We will discuss approaches for the implementation of biosensors for monitoring of protein signaling activities at the levels of individual synapses, dendritic branches, cell-wide neuromodulation, and transcription in the nucleus. Future improvement in these methods and their utilization will undoubtedly yield new insights regarding the intricate link between functional and molecular neuronal dynamics and how they underlie animal's behavior.

Latent Sex Differences in CaMKII-nNOS Signaling That Underlie Antidepressant-Like Effects of Yueju-Ganmaidazao Decoction in the Hippocampus

Previous studies have demonstrated that Yueju-Ganmaidazao (YG) decoction induces rapid antidepressant-like effects, and the antidepressant response is mostly dependent on the suppression of nitric oxide-cyclic guanosine monophosphate signaling in male mice. This study aimed to investigate the sex difference mediated by calcium/calmodulin-dependent protein kinase II (CaMKII)-neuronal nitric oxide synthase (nNOS) signaling involved in the antidepressant-like effect of YG in mice. We found that the immobility times in the tail suspension test (TST) were found to be decreased after the single injection of YG in male and female mice with the same dosage. Additionally, chronic administration for 4 days of subthreshold dosage of YG and escitalopram (ES) also significantly decreased the immobility time in mice of both sexes. Chronic subthreshold dosage of YG and ES in LPS-treated mice and in chronic unpredictable stress (CUS) mice both decreased the immobility time, which was increased by stress. Meanwhile, in CUS-treated mice, sucrose preference test, forced swimming test, and open field test were applied to further confirm the antidepressant-like effects of YG and ES. Moreover, CUS significantly decreased the expression of nNOS and CaMKII, and both YG and ES could enhance the expression in the hippocampus of female mice, which was opposite to that in male mice, while endothelial nitric oxide synthase expression was not affected by stress or drug treatment neither in male mice nor in female mice. Finally, subthreshold dosage of YG combined with 7-nitroindazole (nNOS inhibitor) induced the antidepressant-like effects both in female and in male mice, while the single use of YG or 7-NI did not display any effect. However, pretreatment with KN-93 (CaMKII inhibitor) only blocked the antidepressant-like effect of high-dosage YG in female mice. Meanwhile, in CUS mice, chronic stress caused NR1 overexpression and inhibited cAMP response element binding protein action, which were both reversed by YG and ES in male and female mice, implying that YG and ES produced the same antidepressant-like effect in mice of both sexes. The study revealed that chronic treatment with a subthreshold dose of YG also produced antidepressant-like effects in female mice, and these effects depended on the regulation of the CaMKII-nNOS signaling pathway.

Spine dynamics in the brain, mental disorders and artificial neural networks

In the brain, most synapses are formed on minute protrusions known as dendritic spines. Unlike their artificial intelligence counterparts, spines are not merely tuneable memory elements: they also embody algorithms that implement the brain's ability to learn from experience and cope with new challenges. Importantly, they exhibit structural dynamics that depend on activity, excitatory input and inhibitory input (synaptic plasticity or 'extrinsic' dynamics) and dynamics independent of activity ('intrinsic' dynamics), both of which are subject to neuromodulatory influences and reinforcers such as dopamine. Here we succinctly review extrinsic and intrinsic dynamics, compare these with parallels in machine learning where they exist, describe the importance of intrinsic dynamics for memory management and adaptation, and speculate on how disruption of extrinsic and intrinsic dynamics may give rise to mental disorders. Throughout, we also highlight algorithmic features of spine dynamics that may be relevant to future artificial intelligence developments.

Non-ionotropic NMDA receptor signaling gates bidirectional structural plasticity of dendritic spines

Experience-dependent refinement of neuronal connections is critically important for brain development and learning. Here, we show that ion-flow-independent NMDA receptor (NMDAR) signaling is required for the long-term dendritic spine growth that is a vital component of brain circuit plasticity. We find that inhibition of p38 mitogen-activated protein kinase (p38 MAPK), which is downstream of non-ionotropic NMDAR signaling in long-term depression (LTD) and spine shrinkage, blocks long-term potentiation (LTP)-induced spine growth but not LTP. We hypothesize that non-ionotropic NMDAR signaling drives the cytoskeletal changes that support bidirectional spine structural plasticity. Indeed, we find that key signaling components downstream of non-ionotropic NMDAR function in LTD-induced spine shrinkage are also necessary for LTP-induced spine growth. Furthermore, NMDAR conformational signaling with coincident Ca²⁺ influx is sufficient to drive CaMKII-dependent long-term spine growth, even when Ca²⁺ is artificially driven through voltage-gated Ca²⁺ channels. Our results support a model in which non-ionotropic NMDAR signaling gates the bidirectional spine structural changes vital for brain plasticity.

Structural plasticity of dendritic spines is a critical step in the remodeling of brain circuits during learning. Stein et al. demonstrate a vital role for ion-flux-independent NMDAR signaling in plasticity-associated dendritic spine growth, supporting a model in which non-ionotropic NMDAR signaling primes the spine actin cytoskeleton for bidirectional structural plasticity.

NMDA receptors in axons: there's no coincidence

In the textbook view, N-methyl-d-aspartate (NMDA) receptors are postsynaptically located detectors of coincident activity in Hebbian learning. However, controversial presynaptically located NMDA receptors (preNMDARs) have for decades been repeatedly reported in the literature. These preNMDARs have typically been implicated in the regulation of short-term and long-term plasticity, but precisely how they signal and what their functional roles are have been poorly understood. The functional roles of preNMDARs across several brain regions and different forms of plasticity can differ vastly, with recent discoveries showing key involvement of unusual subunit composition. Increasing evidence shows preNMDAR can signal through both ionotropic action by fluxing calcium and in metabotropic mode even in the presence of magnesium blockade. We argue that these unusual properties may explain why controversy has surrounded this receptor type. In addition, the expression of preNMDARs at some synapse types but not others can underlie synapse-type-specific plasticity. Last but not least, preNMDARs are emerging therapeutic targets in disease states such as neuropathic pain. We conclude that axonally located preNMDARs are required for specific purposes and do not end up there by accident.

GluN2B but Not GluN2A for Basal Dendritic Growth of Cortical Pyramidal Neurons

NMDA receptors are important players for neuronal differentiation. We previously reported that antagonizing NMDA receptors with APV blocked the growth-promoting effects evoked by the overexpression of specific calcium-permeable or flip-spliced AMPA receptor subunits and of type I transmembrane AMPA receptor regulatory proteins which both exclusively modify apical dendritic length and branching of cortical pyramidal neurons. These findings led us to characterize the role of GluN2B and GluN2A for dendritogenesis using organotypic cultures of rat visual cortex. Antagonizing GluN2B with ifenprodil and Ro25-6981 strongly impaired basal dendritic growth of supra- and infragranular pyramidal cells at DIV 5–10, but no longer at DIV 15–20. Growth recovered after washout, and protein blots revealed an increase of synaptic GluN2B-containing receptors as indicated by a enhanced phosphorylation of the tyrosine 1472 residue. Antagonizing GluN2A with TCN201 and NVP-AAM077 was ineffective at both ages. Dendrite growth of non-pyramidal interneurons was not altered. We attempted to overexpress GluN2A and GluN2B. However, although the constructs delivered currents in HEK cells, there were neither effects on dendrite morphology nor an enhanced sensitivity to NMDA. Further, co-expressing GluN1-1a and GluN2B did not alter dendritic growth. Visualization of overexpressed, tagged GluN2 proteins was successful after immunofluorescence for the tag which delivered rather weak staining in HEK cells as well as in neurons. This suggested that the level of overexpression is too weak to modify dendrite growth. In summary, endogenous GluN2B, but not GluN2A is important for pyramidal cell basal dendritic growth during an early postnatal time window.

Functions of p38 MAP Kinases in the Central Nervous System

Mitogen-activated protein (MAP) kinases are a central component in signaling networks in a multitude of mammalian cell types. This review covers recent advances on specific functions of p38 MAP kinases in cells of the central nervous system. Unique and specific functions of the four mammalian p38 kinases are found in all major cell types in the brain. Mechanisms of p38 activation and downstream phosphorylation substrates in these different contexts are outlined and how they contribute to functions of p38 in physiological and under disease conditions. Results in different model organisms demonstrated that p38 kinases are involved in cognitive functions, including functions related to anxiety, addiction behavior, neurotoxicity, neurodegeneration, and decision making. Finally, the role of p38 kinases in psychiatric and neurological conditions and the current progress on therapeutic inhibitors targeting p38 kinases are covered and implicate p38 kinases in a multitude of CNS-related physiological and disease states.

Superoxide and Non-ionotropic Signaling in Neuronal Excitotoxicity

Excitotoxicity is classically attributed to Ca²⁺ influx through NMDA receptors (NMDAr), leading to production of nitric oxide by neuronal nitric oxide synthase and superoxide by mitochondria, which react to form highly cytotoxic peroxynitrite. More recent observations warrant revision of the classic view and help to explain some otherwise puzzling aspects of excitotoxic cell injury. Studies using pharmacological and genetic approaches show that superoxide produced by NMDAr activation originates primarily from NADPH oxidase rather than from mitochondria. As NADPH oxidase is localized to the plasma membrane, this also provides an explanation for the extracellular release of superoxide and cell-to-cell “spread” of excitotoxic injury observed in vitro and in vivo. The signaling pathway linking NMDAr to NADPH oxidase involves Ca²⁺ influx, phosphoinositol-3-kinase, and protein kinase Cζ, and interventions at any of these steps can prevent superoxide production and excitotoxic injury. Ca²⁺ influx specifically through NMDAr is normally required to induce excitotoxicity, through a mechanism presumed to involve privileged Ca²⁺ access to local signaling domains. However, experiments using selective blockade of the NMDAr ion channel and artificial reconstitution of Ca²⁺ by other routes indicate that the special effects of NMDAr activation are attributable instead to concurrent non-ionotropic NMDAr signaling by agonist binding to NMDAr. The non-ionotropic signaling driving NADPH oxidase activation is mediated in part by phosphoinositol-3-kinase binding to the C-terminal domain of GluN2B receptor subunits. These more recently identified aspects of excitotoxicity expand our appreciation of the complexity of excitotoxic processes and suggest novel approaches for limiting neuronal injury.