Stabilization Of Synaptic Connections Research Articles

Depolarization-induced bursts of miniature synaptic currents in individual synapses of developing cerebellum.

In central synapses, spontaneous transmitter release observed in the absence of action potential firing is often considered as a random process lacking time or space specificity. However, when studying miniature glutamatergic currents at cerebellar synapses between parallel fibers and molecular layer interneurons, we found that these currents were sometimes organized in bursts of events occurring at high frequency (about 30 Hz). Bursts displayed homogeneous quantal size amplitudes. Furthermore, in the presence of the desensitization inhibitor cyclothiazide, successive events within a burst displayed quantal amplitude occlusion. Based on these findings, we conclude that bursts originate in individual synapses. Bursts were enhanced by increasing either the external potassium concentration or the external calcium concentration, and they were strongly inhibited when blocking voltage-gated calcium channels by cadmium. Bursts were prevalent in elevated potassium concentration during the formation of the molecular layer but were infrequent later in development. Since postsynaptic AMPA receptors are largely calcium permeant in developing parallel fiber-interneuron synapses, we propose that bursts involve presynaptic calcium transients implicating presynaptic voltage-gated calcium channels, together with postsynaptic calcium transients implicating postsynaptic AMPA receptors. These simultaneous pre- and postsynaptic calcium transients may contribute to the formation and/or stabilization of synaptic connections.

Computational Studies of the Molecular Mechanisms Responsible for Ca2+ Permeation and Mg2+ Block of NMDA Receptors

Glutamate receptors are membrane proteins activated by the neurotransmitter glutamate that mediate fast synaptic excitation in the mammalian brain. NMDA receptors constitute a glutamate receptor subfamily specifically activated by N-methyl-d-aspartate. Due to their high Ca2+ permeability and voltage-dependent channel block by Mg2+, NMDA receptors play a central role in development through stabilization of synaptic connections, as well as in learning and memory by mediating many forms of synaptic plasticity. The mechanisms by which the ion channel of NMDA receptors selects Ca2+ for permeation over all other physiological ions, while binding Mg2+ and restricting its permeation, are not well understood. We hypothesize that the slightly different radii and electronic properties of Mg2+ and Ca2+ ions result in drastically different free energy barriers for transition of the ions from a binding site in the selectivity filter toward the intracellular solution. We are applying quantitative theoretical “bottom up” approaches to this complex system by combining methods of computational chemistry, molecular mechanics (MM), and bioinformatics. The structure of the NMDA receptor channel is constructed and refined using experimental information, homology modeling and extensive molecular dynamics simulations. We are performing quantum chemical calculations to determine the energy of the transition state of model ligand exchange reactions that mimic divalent ion transition from the selectivity filter of NMDA receptors to water. Quantum calculations are used to parameterize a polarizable molecular mechanics force field for these divalent ion interactions with organic ligands with the further goal to perform MM simulations. Umbrella sampling and thermodynamic integration simulations are used to compute free energies of transfer of divalent ions between water and a model of the NMDA ion selectivity filter as well as free energy barriers for these transitions.

Molecular Mechanisms of Ca 2+ Selectivity and Mg 2+ Block of NMDA Receptors

Glutamate receptors are membrane proteins activated by the neurotransmitter glutamate that mediate synaptic excitation in the mammalian brain. NMDA receptors constitute a glutamate receptor subfamily specifically activated by N-methyl-d-aspartate. Due to their high Ca2+ permeability and voltage-dependent channel block by Mg2+, NMDA receptors play a central role in development through stabilization of synaptic connections, as well as in learning and memory by mediating many forms of synaptic plasticity. The mechanisms by which the ion channel of NMDA receptors selects Ca2+ for permeation over all other physiological ions, while binding and restricting Mg2+ permeation, are not well understood. We hypothesize that the slightly different radii of Mg2+ and Ca2+ ions result in drastically different free energy barriers for movement of the ions from a binding site in the selectivity filter to water. We applied quantitative theoretical “bottom up” approaches to this complex system by combining methods of computational chemistry, molecular mechanics, and bioinformatics. We performed high accuracy quantum chemical (QC) calculations to determine the energy of the transition state of the ligand exchange reaction that occurs when divalent ions transition from the selectivity filter of NMDA receptors to water. The results of QC calculations are used to parameterize accurately the polarizable molecular mechanics force fields for divalent ion-ligand interactions. Molecular dynamics simulations with polarizable and non-polarizable force fields are used to predict quantitatively ion interactions with the heterogeneous environment of the pore. The structure of the NMDA receptor channel is constructed and refined using a combination of experimental studies, homology modeling, molecular dynamics, and an optimized structure of a Mg2+ - Asn ligand complex. A homology model of NMDA receptors is refined through extensive equilibration and all-atom molecular dynamics simulations on the multi-nanosecond time-scale to acquire a stable model structure.

Role of NCAM in Emotion and Learning

NCAM is an abundant cell adhesion molecule known to be important during development. Together with its posttranslational modification consisting of the addition of the polysaccharide polysialic acid (PSA), NCAM has been classically implicated in the regulation - among other developmental functions - of neurite outgrowth and stabilization of synaptic connections. A large body of work has also demonstrated that NCAM is required in the adult brain for different behavioral functions. In this review, we focus on those studies that have shown a role of NCAM and PSA-NCAM in the regulation of emotional responses and in the learning and memory processes.

Synaptic destabilization by neuronal Nogo-A

Formation and maintenance of a neuronal network is based on a balance between plasticity and stability of synaptic connections. Several molecules have been found to regulate the maintenance of excitatory synapses but nothing is known about the molecular mechanisms involved in synaptic stabilization versus disassembly at inhibitory synapses. Here, we demonstrate that Nogo-A, which is well known to be present in myelin and inhibit growth in the adult CNS, is present in inhibitory presynaptic terminals in cerebellar Purkinje cells at the time of Purkinje cell-Deep Cerebellar Nuclei (DCN) inhibitory synapse formation and is then downregulated during synapse maturation. We addressed the role of neuronal Nogo-A in synapse maturation by generating several mouse lines overexpressing Nogo-A, starting at postnatal ages and throughout adult life, specifically in cerebellar Purkinje cells and their terminals. The overexpression of Nogo-A induced a progressive disassembly, retraction and loss of the inhibitory Purkinje cell terminals. This led to deficits in motor learning and coordination in the transgenic mice. Prior to synapse disassembly, the overexpression of neuronal Nogo-A led to the downregulation of the synaptic scaffold proteins spectrin, spectrin-E and beta-catenin in the postsynaptic neurons. Our data suggest that neuronal Nogo-A might play a role in the maintenance of inhibitory synapses by modulating the expression of synaptic anchoring molecules.

Control of spontaneous activity during development

Electrical activity participates in the development of the nervous system and comes in two general forms. Use-dependent or experience-driven activity occurs relatively late in development, and is important in events of terminal nervous system differentiation, such as stabilization of synaptic connections. Earlier in development, activity is spontaneous, occurring independently of normal sensory input and motor output. Spontaneous activity participates in many of the initial events of axon outgrowth, pruning of synaptic connections, and maturation of neuronal signaling properties. Despite its importance, the genesis of spontaneous activity is poorly understood. What is clear is that spontaneous activity must be regulated by the patterns with which voltage- and ligand-gated ion channels develop in individual neurons. This review explores how that regulation most likely occurs.

ON and OFF field potentials in the rat superior colliculus during development

The present investigation is aimed at characterizing the development of ON and OFF visually evoked responses in the rat superior colliculus from postnatal day 13 (P13) to postnatal day 25. Depth profiles of field potentials reveal that ON and OFF long latency biphasic field potentials are already present when collicular cells are first responsive to light (P13). There is an inversion in the polarity of these responses as the electrode penetrates the collicular layers, suggesting a synaptic organization similar to the one found in adult animals. At P15, OFF field potentials begin to exhibit oscillatory activity. Local cobalt injections within the superior colliculus abolishes these OFF oscillations, suggesting a postsynaptic origin. Fast Fourier transform (FFT) analysis of the OFF field potentials demonstrates that oscillatory activity increases in frequency during development. This increase is thought to reflect the myelination and stabilization of synaptic connections that occur during this period. To our knowledge, this is the first report of OFF oscillatory responses in the superior colliculus.

Strategies for the development of drugs for pharmacoresistant epilepsies.

Presently, most strategies for development of antiepileptic drugs (AEDs) center around seizure models that are known to respond to presently marketed AEDs. These strategies do not take into account that epilepsy can be a progressive disease. Moreover, region-specific aspects of epileptogenesis are rarely considered when new AEDs are developed. Seizures in the temporal lobe are often difficult to treat. Animal studies on various seizure models in the hippocampus and the entorhinal cortex (EC) suggest that these structures do not a priori produce seizures that are difficult to treat. However, seizure-like events in the EC tend to progress to a state of status epilepticus-like activity that cannot be suppressed by presently marketed AEDs. Loss of gamma-aminobutyric acid (GABA)ergic neurotransmission and increased excitatory synaptic coupling seem to cooperate for induction of this state. Epilepsy induced alterations in the interaction between the EC and the hippocampus may lead to alterations that facilitate precipitation of seizures. Because of the recurrent interaction between the hippocampus and the EC, these seizures may reach an intensity that is no longer controllable by presently available AEDs. Ontogenetic alterations of the circuitry between the EC and the hippocampus, seizure-induced stabilization of synaptic connections overexpressed during ontogenesis, seizure-induced lesions and subsequent rearrangements of internal cell properties, and synaptic arrangements and kindling-like alterations of nerve cell and glial behavior may all be involved in the generation of a neuronal aggregate whose balance between inhibitory and excitatory processes becomes readily disturbed. Strategies for the development of AEDs treating such seizures should suppress hyperactivity and prevent progression of epileptogenesis. AEDs directed against seizures may be effective if they can be given in sufficient concentrations to suppress very intense local seizures.

Activity-dependent synaptic competition in vitro: heterosynaptic suppression of developing synapses.

The development and stability of synaptic connections in the nervous system are influenced by the pattern of electrical activity and the competitive interaction between the adjacent nerve terminals. To investigate this influence, a culture system of nerve and muscle cells has been developed in which a single embryonic muscle cell is coinnervated by two spinal neurons. The effect of electrical activity on the synaptic efficacy was examined after repetitive electrical stimulation was applied to one or both neurons. Brief tetanic stimulation of one neuron resulted in immediate functional suppression of the synapse made by the unstimulated neuron innervating the same muscle cell. This heterosynaptic suppression was largely absent when the tetanic stimulation was applied concurrently to both neurons. This result demonstrates that activity-dependent synaptic competition can be studied in vitro at a cellular level.

The formation of cooperative cell assemblies in the visual cortex

ABSTRACT During a critical period of postnatal development of the mammalian visual cortex, synaptic connections are susceptible to use-dependent modifications. Synaptic connections strengthen if pre- and postsynaptic elements are active simultaneously and postsynaptic depolarization is sufficient to allow for the activation of N-methyl-D-aspartate (NMDA)-receptor-gated conductances. By contrast, synaptic gain decreases if postsynaptic activation exceeds a critical threshold and presynaptic afferents are not capable of activating NMDA- receptor-dependent conductances. These processes lead to selective stabilization of connections between neuronal elements which often exhibit correlated activity and thus modify connectivity according to functional criteria, it is suggested that such experience-dependent selection of circuits serves different purposes at different levels of visual processing. At the input stage to the striate cortex it contributes to optimize the match between the representations of the two eyes. At a later stage of processing it participates in the development of selective connections between cortical columns and thereby serves to establish neuronal representations for frequently occurring constellations of features. Use-dependent changes of synaptic gain can also be induced in the mature visual cortex. These modifications follow the same rules as those occurring during early development and appear to depend on similar molecular mechanisms. However, in the adult the changes of synaptic gain do not seem to be followed by major rearrangements of connectivity. This suggests developmental alterations in mechanisms responsible for growth, removal and stabilization of synaptic connections. Actually, many of the cellular mechanisms thought to be involved in use-dependent synaptic plasticity change during development but it is still unclear which of them are responsible for the definitive stabilization of functionally confirmed pathways.