Molecular machines integrate coincident synaptic signals

Downstream Allosteric Modulation of NMDA Receptors by 3-Benzazepine Derivatives

N-Methyl-D-aspartate receptors (NMDARs) composed of different splice variants display distinct pH sensitivities and are crucial for learning and memory, as well as for inflammatory or injury processes. Dysregulation of the NMDAR has been linked to diseases like Parkinson's, Alzheimer's, schizophrenia, and drug addiction. The development of selective receptor modulators, therefore, constitutes a promising approach for numerous therapeutical applications. Here, we identified (R)-OF-NB1 as a promising splice variant selective NMDAR antagonist. We investigated the interaction of (R)-OF-NB1 and NMDAR from a biochemical, bioinformatical, and electrophysiological perspective to characterize the downstream allosteric modulation of NMDAR by 3-benzazepine derivatives. The allosteric modulatory pathway starts at the ifenprodil binding pocket in the amino terminal domain and immobilizes the connecting α5-helix to the ligand binding domain, resulting in inhibition. In contrast, the exon 5 splice variant GluN1-1b elevates the NMDARs flexibility and promotes the open state of its ligand binding domain.

Endogenous μ-opioid-Neuropeptide Y Y1 receptor synergy silences chronic postoperative pain in mice

Tissue injury creates a delicate balance between latent pain sensitization (LS) and compensatory endogenous analgesia. Inhibitory G-protein-coupled receptor (GPCR) interactions that oppose LS, including μ-opioid receptor (MOR) or neuropeptide Y Y1 receptor (Y1R) activity, persist in the spinal cord dorsal horn (DH) for months, even after the resolution of normal pain thresholds. Here, we demonstrate that following recovery from surgical incision, a potent endogenous analgesic synergy between MOR and Y1R activity persists within DH interneurons to reduce the intensity and duration of latent postoperative hypersensitivity and ongoing pain. Failure of such endogenous GPCR signaling to maintain LS in remission may underlie the transition from acute to chronic pain states.

Effects of developmental alcohol exposure on cortical multisensory integration

Fetal alcohol spectrum disorder (FASD) is one of the most common causes of mental disabilities in the world with a prevalence of 1%-6% of all births. Sensory processing deficits and cognitive problems are a major feature in this condition. Because developmental alcohol exposure can impair neuronal plasticity, and neuronal plasticity is crucial for the establishment of neuronal circuits in sensory areas, we predicted that exposure to alcohol during the third trimester equivalent of human gestation would disrupt the development of multisensory integration (MSI) in the rostral portion of the posterior parietal cortex (PPr), an integrative visual-tactile area. We conducted in vivo electrophysiology in 17 ferrets from four groups (saline/alcohol; infancy/adolescence). A total of 1157 neurons were recorded after visual, tactile and combined visual-tactile stimulation. A multisensory (MS) enhancement or suppression is characterized by a significantly increased or decreased number of elicited spikes after combined visual-tactile stimulation compared to the strongest unimodal (visual or tactile) response. At the neuronal level, those in infant animals were more prone to show MS suppression whereas adolescents were more prone to show MS enhancement. Although alcohol-treated animals showed similar developmental changes between infancy and adolescence, they always 'lagged behind' controls showing more MS suppression and less enhancement. Our findings suggest that alcohol exposure during the last months of human gestation would stunt the development of MSI, which could underlie sensory problems seen in FASD.

The endocytosis, trafficking, sorting and signaling of neurotrophic receptors

Neurotrophins are soluble factors secreted by neurons themselves as well as by post-synaptic target tissues. Neurotrophic signaling regulates several processes such as neurite growth, neuronal survival and synaptogenesis. In order to signal, neurotrophins bind to their receptors, the tropomyosin receptor tyrosine kinase (Trk), which causes internalization of the ligand-receptor complex. Subsequently, this complex is routed into the endosomal system from where Trks can start their downstream signaling. Depending on their endosomal localization, co-receptors involved, but also due to the expression patterns of adaptor proteins, Trks regulate a variety of mechanisms. In this chapter, I provide an overview of the endocytosis, trafficking, sorting and signaling of neurotrophic receptors.

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.

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.

Gαi1 inhibition mechanism of ATP-bound adenylyl cyclase type 5

Conversion of adenosine triphosphate (ATP) to the second messenger cyclic adenosine monophosphate (cAMP) is an essential reaction mechanism that takes place in eukaryotes, triggering a variety of signal transduction pathways. ATP conversion is catalyzed by the enzyme adenylyl cyclase (AC), which can be regulated by binding inhibitory, Gα_i, and stimulatory, Gα_s subunits. In the past twenty years, several crystal structures of AC in isolated form and complexed to Gα_s subunits have been resolved. Nevertheless, the molecular basis of the inhibition mechanism of AC, induced by Gα_i, is still far from being fully understood. Here, classical molecular dynamics simulations of the isolated holo AC protein type 5 and the holo binary complex AC5:Gα_i have been analyzed to investigate the conformational impact of Gα_i association on ATP-bound AC5. The results show that Gα_i appears to inhibit the activity of AC5 by preventing the formation of a reactive ATP conformation.

BNST GluN2D-Containing NMDA Receptors Influence Anxiety- and Depressive-like Behaviors and ModulateCell-Specific Excitatory/Inhibitory Synaptic Balance

Excitatory signaling mediated by NMDARs has been shown to regulate mood disorders. However, current treatments targeting NMDAR subtypes have shown limited success in treating patients, highlighting a need for alternative therapeutic targets. Here, we identify a role for GluN2D-containing NMDARs in modulating emotional behaviors and neural activity in the bed nucleus of the stria terminalis (BNST). Using a GluN2D KO mouse line (GluN2D-/-), we assessed behavioral phenotypes across tasks modeling emotional behavior. We then used a combination of ex vivo electrophysiology and in vivo fiber photometry to assess changes in BNST plasticity, cell-specific physiology, and cellular activity profiles. GluN2D-/- male mice exhibit evidence of exacerbated negative emotional behavior, and a deficit in BNST synaptic potentiation. We also found that GluN2D is functionally expressed on corticotropin-releasing factor (CRF)-positive BNST cells implicated in driving negative emotional states, and recordings in mice of both sexes revealed increased excitatory and reduced inhibitory drive onto GluN2D-/- BNST-CRF cells ex vivo and increased activity in vivo Using a GluN2D conditional KO line (GluN2Dflx/flx) to selectively delete the subunit from the BNST, we find that BNST-GluN2Dflx/flx male mice exhibit increased depressive-like behaviors, as well as altered NMDAR function and increased excitatory drive onto BNST-CRF neurons. Together, this study supports a role for GluN2D-NMDARs in regulating emotional behavior through their influence on excitatory signaling in a region-specific manner, and suggests that these NMDARs may serve as a novel target for selectively modulating glutamate signaling in stress-responsive structures and cell populations.SIGNIFICANCE STATEMENT Excitatory signaling mediated through NMDARs plays an important role in shaping emotional behavior; however, the receptor subtypes/brain regions through which this occurs are poorly understood. Here, we demonstrate that loss of GluN2D-containing NMDARs produces an increase in anxiety- and depressive-like behaviors in mice, deficits in BNST synaptic potentiation, and increased activity in BNST-CRF neurons known to drive negative emotional behavior. Further, we determine that deleting GluN2D in the BNST leads to increased depressive-like behaviors and increased excitatory drive onto BNST-CRF cells. Collectively, these results demonstrate a role for GluN2D-NMDARs in regulating the activity of stress-responsive structures and neuronal populations in the adult brain, suggesting them as a potential target for treating negative emotional states in mood-related disorders.

Association of Both Inhibitory and Stimulatory Gα Subunits Implies Adenylyl Cyclase 5 Deactivation

Adenylyl cyclase (AC) generates cyclic AMP required for a variety of cellular functions, and its regulation plays a major role in cellular signal transduction in eukaryotes and prokaryotes. All membrane-bound AC isoforms in eukaryotes can be activated by stimulatory G-proteins, but only AC1, AC5, and AC6 can be both stimulated and inhibited by active Gα subunits, Gα_s and Gα_i, respectively. In principle, these Gα_i-sensitive AC isoforms could form both binary and ternary complexes with Gα subunits due to the noncompetitive association of inhibitory and stimulatory Gα. However, the formation and possible catalytic activity of a putative ternary complex have not yet been experimentally confirmed due to its proposed short-lived nature. Here, the catalytic activity of such a ternary complex consisting of apo AC5, stimulatory Gα_olf, and inhibitory Gα_i1 is investigated via classical molecular dynamics simulations. Trajectories of inhibited and stimulated binary complexes, AC5:Gα_i1 and AC5:Gα_olf, respectively, as well as Gα-free AC5 were also obtained to compare the sampled AC5 conformation in the ternary complex to those sampled under different Gα conditions. This comparison suggests that association of both Gα subunits results in an AC5 conformation similar to that sampled by the AC5:Gα_i1 complex, indicating that the ternary complex mainly samples an inactive conformation.

Regulation of adenylyl cyclase 5 in striatal neurons confers the ability to detect coincident neuromodulatory signals

Long-term potentiation and depression of synaptic activity in response to stimuli is a key factor in reinforcement learning. Strengthening of the corticostriatal synapses depends on the second messenger cAMP, whose synthesis is catalysed by the enzyme adenylyl cyclase 5 (AC5), which is itself regulated by the stimulatory Gαolf and inhibitory Gαi proteins. AC isoforms have been suggested to act as coincidence detectors, promoting cellular responses only when convergent regulatory signals occur close in time. However, the mechanism for this is currently unclear, and seems to lie in their diverse regulation patterns. Despite attempts to isolate the ternary complex, it is not known if Gαolf and Gαi can bind to AC5 simultaneously, nor what activity the complex would have. Using protein structure-based molecular dynamics simulations, we show that this complex is stable and inactive. These simulations, along with Brownian dynamics simulations to estimate protein association rates constants, constrain a kinetic model that shows that the presence of this ternary inactive complex is crucial for AC5's ability to detect coincident signals, producing a synergistic increase in cAMP. These results reveal some of the prerequisites for corticostriatal synaptic plasticity, and explain recent experimental data on cAMP concentrations following receptor activation. Moreover, they provide insights into the regulatory mechanisms that control signal processing by different AC isoforms.

Sensory Axon Growth Requires Spatiotemporal Integration of CaSR and TrkB Signaling

Neural circuit development involves the coordinated growth and guidance of axons. During this process, axons encounter many different cues, but how these cues are integrated and translated into growth is poorly understood. In this study, we report that receptor signaling does not follow a linear path but changes dependent on developmental stage and coreceptors involved. Using developing chicken embryos of both sexes, our data show that calcium-sensing receptor (CaSR), a G-protein-coupled receptor important for regulating calcium homeostasis, regulates neurite growth in two distinct ways. First, when signaling in isolation, CaSR promotes growth through the PI3-kinase-Akt pathway. At later developmental stages, CaSR enhances tropomyosin receptor kinase B (TrkB)/BDNF-mediated neurite growth. This enhancement is facilitated through a switch in the signaling cascade downstream of CaSR (i.e., from the PI3-kinase-Akt pathway to activation of GSK3α Tyr279). TrkB and CaSR colocalize within late endosomes, cotraffic and coactivate GSK3, which serves as a shared signaling node for both receptors. Our study provides evidence that two unrelated receptors can integrate their individual signaling cascades toward a nonadditive effect and thus control neurite growth during development.SIGNIFICANCE STATEMENT This work highlights the effect of receptor coactivation and signal integration in a developmental setting. During embryonic development, neurites grow toward their targets guided by cues in the extracellular environment. These cues are sensed by receptors at the surface that trigger intracellular signaling events modulating the cytoskeleton. Emerging evidence suggests that the effects of guidance cues are diversified, therefore expanding the number of responses. Here, we show that two unrelated receptors can change the downstream signaling cascade and regulate neuronal growth through a shared signaling node. In addition to unraveling a novel signaling pathway in neurite growth, this research stresses the importance of receptor coactivation and signal integration during development of the nervous system.

Lateral Habenular Burst Firing as a Target of the Rapid Antidepressant Effects of Ketamine

The revolutionary discovery of the rapid antidepressant ketamine has been a milestone in psychiatry field in the last half century. Unlike conventional antidepressants that often take weeks to months to show efficacy, ketamine causes rapid antidepressant effects, emerging as early as within 1h after administration. However, how ketamine improves mood symptoms so quickly has remained elusive. Here, we first introduce the historical background of ketamine as a rapid antidepressant. We then discuss current hypotheses underlying ketamine's rapid antidepressant effects, with a focus on our latest discovery that ketamine silences NMDAR-dependent burst firing in the 'anti-reward center', the lateral habenula. While ketamine may act on many brain regions, we argue that its rapid antidepressant effects are critically dependent on ketamine's action in the lateral habenula, with this brain region acting as a primary site of action (or one among a few primary nodes). This molecular-, cellular-, and circuit-based mechanism advances our understanding of the etiology of depression and suggests a new conceptual framework for the rapid antidepressant effects of ketamine.

Structure, function, and allosteric modulation of NMDA receptors

Hansen et al. review recent structural data that have provided insight into the function and allosteric modulation of NMDA receptors.

NMDA-type glutamate receptors are ligand-gated ion channels that mediate a Ca²⁺-permeable component of excitatory neurotransmission in the central nervous system (CNS). They are expressed throughout the CNS and play key physiological roles in synaptic function, such as synaptic plasticity, learning, and memory. NMDA receptors are also implicated in the pathophysiology of several CNS disorders and more recently have been identified as a locus for disease-associated genomic variation. NMDA receptors exist as a diverse array of subtypes formed by variation in assembly of seven subunits (GluN1, GluN2A-D, and GluN3A-B) into tetrameric receptor complexes. These NMDA receptor subtypes show unique structural features that account for their distinct functional and pharmacological properties allowing precise tuning of their physiological roles. Here, we review the relationship between NMDA receptor structure and function with an emphasis on emerging atomic resolution structures, which begin to explain unique features of this receptor.

Molecular Mechanisms of Anesthetic Neurotoxicity: A Review of the Current Literature

Data from epidemiologic studies and animal models have raised a concern that exposure to anesthetic agents during early postnatal life may cause lasting impairments in cognitive function. It is hypothesized that this is due to disruptions in brain development, but the mechanism underlying this toxic effect remains unknown. Ongoing research, particularly in rodents, has begun to address this question. In this review we examine currently postulated molecular mechanisms of anesthetic toxicity in the developing brain, including effects on cell death pathways, growth factor signaling systems, NMDA and GABA receptors, mitochondria, and epigenetic factors. The level of evidence for each putative mechanism is critically evaluated, and we attempt to draw connections between them where it is possible to do so. Although there are many promising avenues of research, at this time no consensus can be reached as to a definitive mechanism of injury.

NMDA Receptors in the Central Nervous System

NMDA-type glutamate receptors are ligand-gated ion channels that mediate a major component of excitatory neurotransmission in the central nervous system (CNS). They are widely distributed at all stages of development and are critically involved in normal brain functions, including neuronal development and synaptic plasticity. NMDA receptors are also implicated in the pathophysiology of numerous neurological and psychiatric disorders, such as ischemic stroke, traumatic brain injury, Alzheimer's disease, epilepsy, mood disorders, and schizophrenia. For these reasons, NMDA receptors have been intensively studied in the past several decades to elucidate their physiological roles and to advance them as therapeutic targets. Seven NMDA receptor subunits exist that assemble into a diverse array of tetrameric receptor complexes, which are differently regulated, have distinct regional and developmental expression, and possess a wide range of functional and pharmacological properties. The diversity in subunit composition creates NMDA receptor subtypes with distinct physiological roles across neuronal cell types and brain regions, and enables precise tuning of synaptic transmission. Here, we will review the relationship between NMDA receptor structure and function, the diversity and significance of NMDA receptor subtypes in the CNS, as well as principles and rules by which NMDA receptors operate in the CNS under normal and pathological conditions.

Cooperation and crosstalk in axon guidance cue integration: Additivity, synergy, and fine-tuning in combinatorial signaling

Neural circuit development involves the coordinated growth and guidance of axons to their targets. Following the identification of many guidance cue molecules, recent experiments have focused on the interactions of their signaling cascades, which can be generally classified as additive or non-additive depending on the signal convergence point. While additive (parallel) signaling suggests limited molecular interaction between the pathways, non-additive signaling involves crosstalk between pathways and includes more complex synergistic, hierarchical, and permissive guidance cue relationships. Here the authors have attempted to classify recent studies that describe axon guidance signal integration according to these divisions. They also discuss the mechanistic implications of such interactions, as well as general ideas relating signal integration to the generation of diversity of axon guidance responses. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 891-904, 2017.

Endogenous co-agonists of the NMDA receptor modulate contextual fear in trace conditioning

We have used mutant mice to probe the roles of the endogenous co-agonists of the NMDA receptor (NMDAR), D-serine and glycine, in fear learning and memory. Serine racemase knockout (SR-/-) mice have less than 15% of wild type forebrain levels of D-serine, whereas glycine transporter 1 heterozygous knockout (GlyT1+/-) mice have elevated synaptic glycine. While cued fear was normal in both delay and trace conditioned mice of both mutant genotypes, contextual fear was affected in trace conditioned subjects: SR-/- mice showed decreased contextual freezing, whereas GlyT1+/- mice showed elevated contextual freezing. These results indicate that endogenous co-agonists of the NMDAR modulate the conditioning of contextual fear responses, particularly in trace conditioning. They further suggest that endogenous glycine can compensate for the D-serine deficiency in cued and contextual fear following delay conditioning.

Astrocytes Assemble Thalamocortical Synapses by Bridging NRX1α and NL1 via Hevin

Proper establishment of synapses is critical for constructing functional circuits. Interactions between presynaptic neurexins and postsynaptic neuroligins coordinate the formation of synaptic adhesions. An isoform code determines the direct interactions of neurexins and neuroligins across the synapse. However, whether extracellular linker proteins can expand such a code is unknown. Using a combination of in vitro and in vivo approaches, we found that hevin, an astrocyte-secreted synaptogenic protein, assembles glutamatergic synapses by bridging neurexin-1alpha and neuroligin-1B, two isoforms that do not interact with each other. Bridging of neurexin-1alpha and neuroligin-1B via hevin is critical for the formation and plasticity of thalamocortical connections in the developing visual cortex. These results show that astrocytes promote the formation of synapses by modulating neurexin/neuroligin adhesions through hevin secretion. Our findings also provide an important mechanistic insight into how mutations in these genes may lead to circuit dysfunction in diseases such as autism.

NMDA Receptors Containing the GluN2D Subunit Control Neuronal Function in the Subthalamic Nucleus

The GluN2D subunit of the NMDA receptor is prominently expressed in the basal ganglia and associated brainstem nuclei, including the subthalamic nucleus (STN), globus pallidus, striatum, and substantia nigra. However, little is known about how GluN2D-containing NMDA receptors contribute to synaptic activity in these regions. Using Western blotting of STN tissue punches, we demonstrated that GluN2D is expressed in the rat STN throughout development [age postnatal day 7 (P7)-P60] and in the adult (age P120). Immunoelectron microscopy of the adult rat brain showed that GluN2D is predominantly expressed in dendrites, unmyelinated axons, and axon terminals within the STN. Using subunit-selective allosteric modulators of NMDA receptors (TCN-201, ifenprodil, CIQ, and DQP-1105), we provide evidence that receptors containing the GluN2B and GluN2D subunits mediate responses to exogenously applied NMDA and glycine, as well as synaptic NMDA receptor activation in the STN of rat brain slices. EPSCs in the STN were mediated primarily by AMPA and NMDA receptors and GluN2D-containing NMDA receptors controlled the slow deactivation time course of EPSCs in the STN. In vivo recordings from the STN of anesthetized adult rats demonstrated that the spike firing rate was increased by the GluN2C/D potentiator CIQ and decreased by the GluN2C/D antagonist DQP-1105, suggesting that NMDA receptor activity can influence STN output. These data indicate that the GluN2B and GluN2D NMDA receptor subunits contribute to synaptic activity in the STN and may represent potential therapeutic targets for modulating subthalamic neuron activity in neurological disorders such as Parkinson's disease.

Agonist binding to the NMDA receptor drives movement of its cytoplasmic domain without ion flow

The NMDA receptor (R) plays important roles in brain physiology and pathology as an ion channel. Here we examine the ion flow-independent coupling of agonist to the NMDAR cytoplasmic domain (cd). We measure FRET between fluorescently tagged cytoplasmic domains of GluN1 subunits of NMDARs expressed in neurons. Different neuronal compartments display varying levels of FRET, consistent with different NMDARcd conformations. Agonist binding drives a rapid and transient ion flow-independent reduction in FRET between GluN1 subunits within individual NMDARs. Intracellular infusion of an antibody targeting the GluN1 cytoplasmic domain blocks agonist-driven FRET changes in the absence of ion flow, supporting agonist-driven movement of the NMDARcd. These studies indicate that extracellular ligand binding to the NMDAR can transmit conformational information into the cell in the absence of ion flow.

The impact of NMDA receptor hypofunction on GABAergic neurons in the pathophysiology of schizophrenia

While the dopamine hypothesis has dominated schizophrenia research for several decades, more recent studies have highlighted the role of fast synaptic transmitters and their receptors in schizophrenia etiology. Here we review evidence that schizophrenia is associated with a reduction in N-methyl-d-aspartate receptor (NMDAR) function. By highlighting postmortem, neuroimaging and electrophysiological studies, we provide evidence for preferential disruption of GABAergic circuits in the context of NMDAR hypo-activity states. The functional relationship between NMDARs and GABAergic neurons is realized at the molecular, cellular, microcircuit and systems levels. A synthesis of findings across these levels explains how NMDA-mediated inhibitory dysfunction may lead to aberrant interactions among brain regions, accounting for key clinical features of schizophrenia. This synthesis of schizophrenia unifies observations from diverse fields and may help chart pathways for developing novel diagnostics and therapeutics.

Rac-mediated Stimulation of Phospholipase Cγ2 Amplifies B Cell Receptor-induced Calcium Signaling

The Rho GTPase Rac is crucially involved in controlling multiple B cell functions, including those regulated by the B cell receptor (BCR) through increased cytosolic Ca(2+). The underlying molecular mechanisms and their relevance to the functions of intact B cells have thus far remained unknown. We have previously shown that the activity of phospholipase Cγ2 (PLCγ2), a key constituent of the BCR signalosome, is stimulated by activated Rac through direct protein-protein interaction. Here, we use a Rac-resistant mutant of PLCγ2 to functionally reconstitute cultured PLCγ2-deficient DT40 B cells and to examine the effects of the Rac-PLCγ2 interaction on BCR-mediated changes of intracellular Ca(2+) and regulation of Ca(2+)-regulated and nuclear-factor-of-activated-T-cell-regulated gene transcription at the level of single, intact B cells. The results show that the functional Rac-PLCγ2 interaction causes marked increases in the following: (i) sensitivity of B cells to BCR ligation; (ii) BCR-mediated Ca(2+) release from intracellular stores; (iii) Ca(2+) entry from the extracellular compartment; and (iv) nuclear translocation of the Ca(2+)-regulated nuclear factor of activated T cells. Hence, Rac-mediated stimulation of PLCγ2 activity serves to amplify B cell receptor-induced Ca(2+) signaling.

TSPAN7: A new player in excitatory synapse maturation and function

Tetraspanins regulate the signaling, trafficking and biosynthetic processing of associated proteins, and may link the extracellular domain of α-chain integrins with intracellular signaling molecules, including PI4K and PKC, both of which regulate cytoskeletal architecture. We showed that TSPAN7, a member of tetraspannin-family, promotes filopodia and dendritic spine formation in cultured hippocampal neurons, and is required for spine stability and normal synaptic transmission. TSPAN7 directly interacts with the PDZ domain of protein interacting with C kinase 1 (PICK1), and associates with AMPAR subunit GluA2 and β1-integrin. TSPAN7 regulates PICK1 and GluA2/3 association, and AMPA receptor trafficking. These findings identify TSPAN7 as a key player in the morphological and functional maturation of glutamatergic synapses.

Activity-controlled proteolytic cleavage at the synapse

Activity-controlled enzymatic cleavage of proteins on the surface of synaptic membranes or in the synaptic or perisynaptic interstitial compartment represents a direct way to regulate synaptic structure, function, and number. Extracellular proteolysis at synapses was initially understood to be plasticity enabling by freeing synapses from the constraints provided by the extracellular matrix. However, recent observations indicate that at least part of the extracellular protein cleavage results in activation of previously cryptic functions that regulate adaptive changes of synapses and neuronal circuits. Here, we focus on peptidases with distinct localization and function at synapses combined with regulation by neuronal and synaptic activity, and evaluate their function in the context of developmental and/or adult synaptic plasticity.

Bipolar disorder: involvement of signaling cascades and AMPA receptor trafficking at synapses

There is increasing evidence that severe mood disorders are associated with impairment of structural plasticity and cellular resilience. Cumulative data demonstrate that mood stabilizers regulate intracellular signaling cascades, including protein kinase C (PKC), PKA, mitogen-activated protein (MAP) kinase, glycogen synthase kinase 3-beta (GSK3-beta) and intracellular calcium, which are signaling pathways that regulate synaptic plasticity. In this context, it is noteworthy that a growing body of data indicates that the glutamatergic system, has a major role in neuronal plasticity and cellular resilience, might be involved in the pathophysiology and treatment of mood disorders. AMPA glutamate-receptor trafficking is important in synaptic plasticity and might play crucial roles in maintaining critical neuronal circuits associated with mood. Two clinically effective, structurally dissimilar, antimanic agents, lithium and valproate (VPA), down-regulate synaptic expression of AMPA receptor subunit GluR1 in hippocampus in chronically treated rats. This reduction in synaptic GluR1 by lithium and VPA is due to attenuated phosphorylation of GluR1 at a specific PKA site (residue 845 of GluR1), which is crucial for AMPA receptor insertion. By contrast,imipramine, which can provoke mania, increases synaptic expression of GluR1 in the hippocampus in vivo. Furthermore, there is ample evidence from preclinical and clinical research that the glutamatergic system is involved in the pathophysiology of mood disorders and that many of the somatic treatments used for mood disorders including antidepressants, mood stabilizers, atypical antipsychotic drugs and electroconvulsive therapy have both direct and indirect effects on the glutamatergic system. Given these findings, further research with medications that specifically affect the glutamatergic system is warranted. Recent studies in our lab have shown that riluzole, a FDA approved medicine that regulates the glutamatergic system, shows antidepressant efficacy in unipolar and bipolar depression. These studies indicate that regulation of glutamate-mediated synaptic plasticity might play a role in the treatment of mood disorders, and raise new avenues for novel therapies for this devastating illness.

Enhancing synaptic plasticity and cellular resilience to develop novel, improved treatments for mood disorders

There is mounting evidence that recurrent mood disorders - once considered “good prognosis diseases”- are, in fact, often very severe and life-threatening illnesses. Furthermore, although mood disorders have traditionally been conceptualized as neurochemical disorders, there is now evidence from a variety of sources demonstrating regional reductions in central nervous system (CNS) volume, as well as reductions in the numbers and/or sizes ofglia and neurons in discrete brain areas. Although the precise cellular mechanisms underlying these morphometric changes remain to be fully elucidated, the data suggest that mood disorders are associated with impairments of synaptic plasticity and cellular resilience. In this context, it is noteworthy that there is increasing preclinical evidence that antidepressants regulate the function of the glutamatergic system. Moreover, although clearly preliminary, the available clinical data suggest that attenuation of N-methyl-D-aspartate (NMDA) function has antidepressant effects. Recent preclinical and clinical studies have shown that signaling pathways involved in regulating cell survival and cell death are long-term targets for the actions of antidepressant agents. Antidepressants and mood stabilizers indirectly regulate a number of factors involved in cell survival pathways, including cyclic adenosine monophosphate (cAMP) response element binding protein (CREB), brain-derived neurotrophic factor (BDNF), the antiapoptotic protein bcl-2, and mitogen-activated protein (MAP) kinases, and may thus bring about some of their delayed long-term beneficial effects via underappreciated neurotrophic effects. There is much promise for the future development of treatments that more directly target molecules in critical CNS signaling pathways regulating synaptic plasticity and cellular resilience. These will represent improved long-term treatments for mood disorders.

NMDA receptor function, memory, and brain aging

An increasing level of N-methyl-D-aspartate (NMDA) receptor hypofunction within the brain is associated with memory and learning impairments, with psychosis, and ultimately with excitotoxic brain injury. As the brain ages, the NMDA receptor system becomes progressively hypofunctional, contributing to decreases in memory and learning performance. In those individuals destined to develop Alzheimer's disease, other abnormalities (eg, amyloidopathy and oxidative stress) interact to increase the NMDA receptor hypofunction (NRHypo) burden. In these vulnerable individuals, the brain then enters into a severe and persistent NRHypo state, which can lead to widespread neurodegeneration with accompanying mental symptoms and further cognitive deterioration. If the hypotheses described herein prove correct, treatment implications may be considerable. Pharmacological methods for preventing the overstimulation of vulnerable corticolimbic pyramidal neurons developed in an animal model may be applicable to the prevention and treatment of Alzheimer's disease.

Spine calcium transients induced by synaptically-evoked action potentials can predict synapse location and establish synaptic democracy

CA1 pyramidal neurons receive hundreds of synaptic inputs at different distances from the soma. Distance-dependent synaptic scaling enables distal and proximal synapses to influence the somatic membrane equally, a phenomenon called “synaptic democracy”. How this is established is unclear. The backpropagating action potential (BAP) is hypothesised to provide distance-dependent information to synapses, allowing synaptic strengths to scale accordingly. Experimental measurements show that a BAP evoked by current injection at the soma causes calcium currents in the apical shaft whose amplitudes decay with distance from the soma. However, in vivo action potentials are not induced by somatic current injection but by synaptic inputs along the dendrites, which creates a different excitable state of the dendrites. Due to technical limitations, it is not possible to study experimentally whether distance information can also be provided by synaptically-evoked BAPs. Therefore we adapted a realistic morphological and electrophysiological model to measure BAP-induced voltage and calcium signals in spines after Schaffer collateral synapse stimulation. We show that peak calcium concentration is highly correlated with soma-synapse distance under a number of physiologically-realistic suprathreshold stimulation regimes and for a range of dendritic morphologies. Peak calcium levels also predicted the attenuation of the EPSP across the dendritic tree. Furthermore, we show that peak calcium can be used to set up a synaptic democracy in a homeostatic manner, whereby synapses regulate their synaptic strength on the basis of the difference between peak calcium and a uniform target value. We conclude that information derived from synaptically-generated BAPs can indicate synapse location and can subsequently be utilised to implement a synaptic democracy.

Neurons receive information from other neurons via hundreds of contacts (synapses) spread across their dendritic branches. Input signals from synapses propagate along a dendrite to the cell body (soma), where the neuron decides whether or not to produce an action potential. Signals that travel further decay more. Were all synapses equally strong, a synapse far from the soma would have less influence on the decision than a synapse close by. However, neurons in the hippocampus, which are involved in learning and memory, have synapses far from the soma that are stronger than those close by, so that all synapses have an equal voice (“synaptic democracy”). But how can a synapse “know” how far it is from the soma? Using a computational model of a hippocampal neuron, we show that the action potential, which propagates from the soma back into the dendrites, contains information with which synapses can estimate their somatic distance. Specifically, the calcium concentration at the synapse, which is modulated by the backpropagating action potential, decreases with distance from the soma. We show that when the strength of a synapse is adapted in a self-organising manner based on calcium concentration, synaptic democracy is obtained.

Ret is a multifunctional coreceptor that integrates diffusible- and contact-axon guidance signals

Growing axons encounter multiple guidance cues, but it is unclear how separate signals are resolved and integrated into coherent instructions for growth cone navigation. We report that glycosylphosphatidylinositol (GPI)-anchored ephrin-As function as "reverse" signaling receptors for motor axons when contacted by transmembrane EphAs present in the dorsal limb. Ephrin-A receptors are thought to depend on transmembrane coreceptors for transmitting signals intracellularly. We show that the receptor tyrosine kinase Ret is required for motor axon attraction mediated by ephrin-A reverse signaling. Ret also mediates GPI-anchored GFRα1 signaling in response to GDNF, a diffusible chemoattractant in the limb, indicating that Ret is a multifunctional coreceptor for guidance molecules. Axons respond synergistically to coactivation by GDNF and EphA ligands, and these cooperative interactions are gated by GFRα1 levels. Our studies uncover a hierarchical GPI-receptor signaling network that is constructed from combinatorial components and integrated through Ret using ligand coincidence detection.

Plasticity of local GABAergic interneurons drives olfactory habituation

Despite its ubiquity and significance, behavioral habituation is poorly understood in terms of the underlying neural circuit mechanisms. Here, we present evidence that habituation arises from potentiation of inhibitory transmission within a circuit motif commonly repeated in the nervous system. In Drosophila, prior odorant exposure results in a selective reduction of response to this odorant. Both short-term (STH) and long-term (LTH) forms of olfactory habituation require function of the rutabaga-encoded adenylate cyclase in multiglomerular local interneurons (LNs) that mediate GABAergic inhibition in the antennal lobe; LTH additionally requires function of the cAMP response element-binding protein (CREB2) transcription factor in LNs. The odorant selectivity of STH and LTH is mirrored by requirement for NMDA receptors and GABA(A) receptors in odorant-selective, glomerulus-specific projection neurons(PNs). The need for the vesicular glutamate transporter in LNs indicates that a subset of these GABAergic neurons also releases glutamate. LTH is associated with a reduction of odorant-evoked calcium fluxes in PNs as well as growth of the respective odorant-responsive glomeruli. These cellular changes use similar mechanisms to those required for behavioral habituation. Taken together with the observation that enhancement of GABAergic transmission is sufficient to attenuate olfactory behavior, these data indicate that habituation arises from glomerulus-selective potentiation of inhibitory synapses in the antennal lobe. We suggest that similar circuit mechanisms may operate in other species and sensory systems.

Fetal alcohol spectrum disorders and abnormal neuronal plasticity

The ingestion of alcohol during pregnancy can result in a group of neurobehavioral abnormalities collectively known as fetal alcohol spectrum disorders (FASD). During the past decade, studies using animal models indicated that early alcohol exposure can dramatically affect neuronal plasticity, an essential property of the central nervous system responsible for the normal wiring of the brain and involved in processes such as learning and memory. The abnormalities in neuronal plasticity caused by alcohol can explain many of the neurobehavioral deficits observed in FASD. Conversely, improving neuronal plasticity may have important therapeutic benefits. In this review, the author discuss the mechanisms that lead to these abnormalities and comment on recent pharmacological approaches that have been showing promising results in improving neuronal plasticity in FASD.

Central synaptic mechanisms underlie short-term olfactory habituation in Drosophila larvae

Naive Drosophila larvae show vigorous chemotaxis toward many odorants including ethyl acetate (EA). Chemotaxis toward EA is substantially reduced after a 5-min pre-exposure to the odorant and recovers with a half-time of ∼20 min. An analogous behavioral decrement can be induced without odorant-receptor activation through channelrhodopsin-based, direct photoexcitation of odorant sensory neurons (OSNs). The neural mechanism of short-term habituation (STH) requires the (1) rutabaga adenylate cyclase; (2) transmitter release from predominantly GABAergic local interneurons (LNs); (3) GABA-A receptor function in projection neurons (PNs) that receive excitatory inputs from OSNs; and (4) NMDA-receptor function in PNs. These features of STH cannot be explained by simple sensory adaptation and, instead, point to plasticity of olfactory synapses in the antennal lobe as the underlying mechanism. Our observations suggest a model in which NMDAR-dependent depression of the OSN-PN synapse and/or NMDAR-dependent facilitation of inhibitory transmission from LNs to PNs contributes substantially to short-term habituation.

Activators of G-protein signaling 3: a drug addiction molecular gateway

Drug addiction is marked by continued drug-seeking behavior despite deleterious consequences and a heightened propensity to relapse not withstanding long, drug-free periods. The enduring nature of addiction has been hypothesized to arise from perturbations in intracellular signaling, gene expression, and brain circuitry induced by substance abuse. Ameliorating some of these aberrations should abate behavioral and neurochemical markers associated with an 'addiction phenotype'. This review summarizes data showing that protein expression and signaling through the nonreceptor activator of G-protein signaling 3 (AGS3) are altered by commonly abused substances in rat and in in-vitro addiction models. AGS3 structure and function are unrelated to the more broadly studied regulator of G-protein signaling family. Thus, the unique role of AGS3 is the focus of this review. Intriguingly, AGS3 protein changes persist into drug abstinence. Accordingly, studies probing the role of AGS3 in the neurochemistry of drug-seeking behavior and relapse are studied in detail. To illuminate this study, AGS3 structure, cellular localization, and function are covered so that an idealized AGS3-targeted pharmacotherapy can be proposed.

Beyond the wiring diagram: signalling through complex neuromodulator networks

During the computations performed by the nervous system, its 'wiring diagram'--the map of its neurons and synaptic connections--is dynamically modified and supplemented by multiple actions of neuromodulators that can be so complex that they can be thought of as constituting a biochemical network that combines with the neuronal network to perform the computation. Thus, the neuronal wiring diagram alone is not sufficient to specify, and permit us to understand, the computation that underlies behaviour. Here I review how such modulatory networks operate, the problems that their existence poses for the experimental study and conceptual understanding of the computations performed by the nervous system, and how these problems may perhaps be solved and the computations understood by considering the structural and functional 'logic' of the modulatory networks.

AA/12-Lipoxygenase Signaling Contributes to Inhibitory Learning in Hermissenda Type B Photoreceptors

Conditioned inhibition (CI) is a major category of associative learning that occurs when an organism learns that one stimulus predicts the absence of another. In addition to being important in its own right, CI is interesting because its occurrence implies that the organism has formed an association between stimuli that are non-coincident. In contrast to other categories of associative learning that are dependent upon temporal contiguity (pairings) of stimuli, the neurobiology of CI is virtually unexplored. We have previously described a simple form of CI learning in Hermissenda, whereby animals' phototactic behavior is increased by repeated exposures to explicitly unpaired (EU) presentations of light and rotation. EU conditioning also produces characteristic reductions in the excitability and light response, and increases several somatic K(+) currents in Type B photoreceptors. Type B photoreceptors are a major site of plasticity for classical conditioning in Hermissenda. Because arachidonic acid (AA) and/or its metabolites open diverse K(+) channels in many cell types, we examined the potential contribution of AA to CI. Our results indicate that AA contributes to one of the major effects of EU-conditioning on Type B photoreceptors: decreases in light-evoked spike activity. We find that AA increases the transient (I(A)) somatic K(+) current in Type B photoreceptors, further mimicking CI training. In addition, our results indicate that metabolism of AA by a 12-lipoxygenase enzyme is critical for these effects of AA, and further that 12-lipoxygenase metabolites are apparently generated during CI training.

Activation of NMDA receptors is necessary for the recovery of cortical binocularity

Classic experiments have indicated that monocular deprivation (MD) for a few days during a critical period of development results in a decrease in the strength of connections mediating responses to the deprived eye, leading to a dramatic breakdown of cortical neuron binocularity. Despite the substantial functional change in the visual cortex, recovery from the effects of MD can be obtained if binocular vision is promptly restored. While great efforts have been made to elucidate the mechanisms regulating loss of deprived eye function, the mechanisms that underlie the recovery of cortical binocularity are poorly understood. Here, we examined whether activation of the N-methyl-d-aspartate receptor (NMDAR) is required for the recovery of cortical binocularity by pharmacologically blocking the NMDAR using d,l-2-amino-5-phosphonopentanoic (APV). Ferrets (n = 10) were monocularly deprived for 6 days, and osmotic minipumps, filled with APV (5.6 mg/ml) or saline, were surgically implanted into the primary visual cortex. One day after surgery, the deprived eye was reopened, and the animals were allowed 24 h of binocular vision. Extracellular recordings showed that intracortical infusion of the NMDAR antagonist, APV, prevented recovery of cortical binocularity while preserving neuronal responsiveness. These findings provide an important new insight for a specific role of NMDARs in the recovery of cortical binocularity from the effects of MD.

Forgetting is regulated through Rac activity in Drosophila

Initially acquired memory dissipates rapidly if not consolidated. Such memory decay is thought to result either from the inherently labile nature of newly acquired memories or from interference by subsequently attained information. Here we report that a small G protein Rac-dependent forgetting mechanism contributes to both passive memory decay and interference-induced forgetting in Drosophila. Inhibition of Rac activity leads to slower decay of early memory, extending it from a few hours to more than one day, and to blockade of interference-induced forgetting. Conversely, elevated Rac activity in mushroom body neurons accelerates memory decay. This forgetting mechanism does not affect memory acquisition and is independent of Rutabaga adenylyl cyclase-mediated memory formation mechanisms. Endogenous Rac activation is evoked on different time scales during gradual memory loss in passive decay and during acute memory removal in reversal learning. We suggest that Rac's role in actin cytoskeleton remodeling may contribute to memory erasure.

Critical period plasticity matches binocular orientation preference in the visual cortex

Changes of ocular dominance in the visual cortex can be induced by visual manipulations during a critical period in early life. However, the role of critical period plasticity in normal development is unknown. Here we show that at the onset of this time window, the preferred orientations of individual cortical cells in the mouse are mismatched through the two eyes and the mismatch decreases and reaches adult levels by the end of the period. Deprivation of visual experience during this period irreversibly blocks the binocular matching of orientation preference, but has no effect in adulthood. The critical period of binocular matching can be delayed by long-term visual deprivation from birth, like that of ocular dominance plasticity. These results demonstrate that activity-dependent changes induced by normal visual experience during the well-studied critical period serve to match eye-specific inputs in the cortex, thus revealing a physiological role for critical period plasticity during normal development.

Phosphodiesterase inhibition increases CREB phosphorylation and restores orientation selectivity in a model of fetal alcohol spectrum disorders

Background

Fetal alcohol spectrum disorders (FASD) are the leading cause of mental retardation in the western world and children with FASD present altered somatosensory, auditory and visual processing. There is growing evidence that some of these sensory processing problems may be related to altered cortical maps caused by impaired developmental neuronal plasticity.

Methodology/Principal Findings

Here we show that the primary visual cortex of ferrets exposed to alcohol during the third trimester equivalent of human gestation have decreased CREB phosphorylation and poor orientation selectivity revealed by western blotting, optical imaging of intrinsic signals and single-unit extracellular recording techniques. Treating animals several days after the period of alcohol exposure with a phosphodiesterase type 1 inhibitor (Vinpocetine) increased CREB phosphorylation and restored orientation selectivity columns and neuronal orientation tuning.

Conclusions/Significance

These findings suggest that CREB function is important for the maturation of orientation selectivity and that plasticity enhancement by vinpocetine may play a role in the treatment of sensory problems in FASD.

Mice with genetically altered glutamate receptors as models of schizophrenia: a comprehensive review

Recent clinical evidence for the effectiveness of new antipsychotic drugs that specifically target glutamate receptors has rekindled interest in the glutamatergic system regarding pathophysiology and treatment of schizophrenia. The glutamatergic hypothesis of schizophrenia was triggered by the clinical/behavioural observation that NMDA receptor antagonists can induce psychosis in humans and abnormal behaviour with schizophrenia-like symptoms in animals. Initial models focused on NMDA receptor hypofunction as a potential pathogenetic mechanism. More recent genetic and pharmacological studies revealed that malfunction of other components of the glutamatergic system might also be relevant in explaining specific symptoms of this complex disease. Here, we review mutant mouse models with relevance for schizophrenia. These rodent models, in which specific glutamate receptor subtypes or various components of their intracellular transduction machinery are genetically altered, permit a detailed dissection of the contribution of different components of the glutamate system in inducing schizophrenia-like behaviours. They may provide insight into the pathophysiology of schizophrenia and prove useful in the development of new therapeutics.