A Brownian model of glutamate diffusion in excitatory synapses of hippocampus

We simulated the diffusion of glutamate, following the release of a single vesicle from a pre-synaptic terminal, in the synaptic cleft by using a Brownian diffusion model based on Langevin equations. The synaptic concentration time course and the time course of quantal excitatory post-synaptic current have been analyzed. The results showed that they depend on the number of receptors located at post-synaptic membrane. Their time course are dependent both on the total number of the post-synaptic receptors and on the eccentricity of the pre-synaptic glutamate vesicle.     

Temperature dependence of vesicular dynamics at excitatory synapses of rat hippocampus

How vesicular dynamics parameters depend on temperature and how temperature affects the parameter change during prolonged high frequency stimulation was determined by fitting a model of vesicular storage and release to the amplitudes of the excitatory post-synaptic currents (EPSC) recorded from CA1 neurons in rat hippocampal slices. The temperature ranged from low (13 °C) to higher and more physiological temperature (34 °C). Fitting the model of vesicular storage and release to the EPSC amplitudes during a single pair of brief high–low frequency stimulation trains yields the estimates of all parameters of the vesicular dynamics, and with good precision. Both fractional release and replenishment rate decrease as the temperature rises. Change of the underlying ‘basic’ parameters (release coupling, replenishment coupling and readily releasable pool size), which the model-fitting also yields is complex. The replenishment coupling between the readily releasable pool (RRP) and resting pool increases with temperature (which renders the replenishment rate higher), but this is more than counterbalanced by greater RRP size (which renders the replenishment rate lower). Finally, during long, high frequency patterned stimulation that leads to significant synaptic depression, the replenishment rate decreases markedly and rapidly at low temperatures (<22 °C), but at high temperatures (>28 °C) the replenishment rate rises with stimulation, making synapses better able to maintain synaptic efficacy.     

Synaptic activity slows vesicular replenishment at excitatory synapses of rat hippocampus

Short-term synaptic depression mainly reflects the depletion of the readily releasable pool (RRP) of quanta. Its dynamics, and especially the replenishment rate of the RRP, are still not well characterized in spite of decades of investigation. Main reason is that the vesicular storage and release system is treated as time-independent. If it is time-dependent all parameters thus estimated become problematic. Indeed the reports about how prolonged stimulation affects the dynamics are contradictory. To study this, we used patterned stimulation on the Schaeffer collateral fiber pathway and model-fitting of the excitatory post-synaptic currents (EPSC) recorded from CA1 neurons in rat hippocampal slices. The parameters of a vesicular storage and release model with two pools were estimated by minimizing the squared difference between the ESPC amplitudes and simulated model output. This yields the ‘basic’ parameters (release coupling, replenishment coupling and RRP size) that underlie the ‘derived’ and commonly used parameters (fractional release and replenishment rate). The fractional release increases when [Ca⁺⁺]_o is raised, whereas the replenishment rate is [Ca⁺⁺]_o independent. Fractional release rises because release coupling increases, and the RRP becomes less able to contain quanta. During prolonged stimulation, the fractional release remains generally unaltered, whereas the replenishment rate decreases down to ~10 % of its initial value with a decay time of ~15 s, and this decrease in the replenishment rate significantly contributes to synaptic depression. In conclusion, the fractional release is [Ca⁺⁺]_o-dependent and stimulation-independent, whereas the replenishment rate is [Ca⁺⁺]_o-independent and stimulation-dependent.     

Motor unit recruitment patterns during reflex compensation of muscle yield investigated by computer simulations

 An important function of the stretch reflex in the soleus muscle in the decerebrate cat preparation is to compensate for the tendency of muscle suddenly to yield during ramp increases in length. As the level of background (i.e. pre-stretch) force increases, there is a systematic change in the curvature of the force trajectory during this reflex compensation, from concave to convex with respect to increasing force. The hypothesis that this change in curvature was due to background force-dependent changes in the recruitment pattern of motor units was investigated with a combined computer simulation/ experimental technique. The simulation consisted of 20 model motor units for the soleus muscle, each based on a distributed moment muscle model. The timing of recruitment of the motor units was optimized to allow the simulation outputs to fit a set of experimental data records on the reflex response to stretch initiated at five different levels of pre-stretch force. The resulting recruitment patterns showed that a tendency for recruitment to be concentrated progressively in the early portion of the stretch as pre-stretch force increased could account for the changes in reflex force curvature. These results are consistent with the skewed distribution of intrinsic electrical thresholds of motoneurons, in which low-threshold units are much more frequent than high-threshold ones. Therefore the changes in recruitment pattern and reflex force curvature may be due primarily to the intrinsic properties of motoneurons. Received: 18 September 1995/Accepted in revised form: 21 May 1996     

Effect of filaments within the synaptic cleft on the response of excitatory synapses simulated by computer experiments

Mathematical models of the excitatory synapse are furnishing valuable information about the synaptic response. Based on Brownian-diffusion of glutamate molecules, a synapse model was utilized to investigate the synaptic response on a femto-second time scale by the use of a parallel computer. In particular, the presence of fibrils crossing the synaptic cleft was simulated, which could have a role in shaping the brain activity. To this aim the model of synapse was modified by considering trans-synaptic filaments with diameters ranging from 7 nm to 3 nm, disposed on a grid with spacing of 14 nm or 8 nm. The simulation demonstrated that the presence of filaments induced an increase in the synaptic response, most likely linked to an increment in the probability of encounter between glutamate molecules and receptors. The increase was small - from 5 to 20%, but metabolic and functional considerations provide substantive hints about the importance of these small changes for brain activity. Moreover, it was shown that the presence of filaments made more stable the response of the synapse to random variations of pre-synaptic elements. Originated by these computational results, some inferences about the biological bases of mind diseases such as autism, mental retardation and schizophrenia, are reported in the Discussion.     

Different types of rectification at electrical synapses made by a single crayfish neurone investigated experimentally and by computer simulation

Summary The rectification properties of electrical synapses made by the segmental giant (SG) neurone of crayfish (Pacifastacus leniusculus) were investigated. The SG acts as an interneurone, transmitting information from the giant command fibres (GFs) to the abdominal fast flexor (FF) motoneurones. The GF-SG (input) synapses are inwardly-rectifying electrical synapses, while the SG-FF (output) synapses are outwardly rectifying electrical synapses. This implies that a single neurone can make gap junction hemichannels with different rectification properties.The coupling coefficient of these synapses is dependent upon transjunctional potential. There is a standing gradient in resting potential between the GFs, SG and FFs, with the GFs the most hyperpolarized, and the FFs the most depolarized. The gradient thus biases each synapse into the low-conductance state under resting conditions.There is functional double rectification between the bilateral pairs of SGs within a single segment, such that depolarizing membrane potential changes of either SG pass to the other SG with less attenuation than do hyperpolarizing potential changes. Computer simulation suggests that this may result from coupling through the intermediary FF neurones.Abbreviations l left - r right - FF fast flexor motoneurone - GF giant fibre - LG lateral giant interneurone - MG medial giant interneurone - MoG motor giant motoneurone - R root, e.g. 1R1 is the first root on the left side - SG Segmental giant neurone     

Potential causes of linkage disequilibrium in a European maize breeding program investigated with computer simulations

Knowledge about the forces generating and conserving linkage disequilibrium (LD) is important for drawing conclusions about the prospects and limitations of association mapping. The objectives of our research were to examine the importance of (1) selection, (2) mutation, and (3) genetic drift for generating LD in a typical maize breeding program. We conducted computer simulations based on genotypic data of Central European maize open-pollinated varieties which have played an important role as founders of the European flint heterotic group. The breeding scheme and the dimensioning underlying our simulations reflect essentially the maize breeding program of the University of Hohenheim. Results suggested that in a plant breeding program of the examined dimension and breeding scheme, genetic drift and selection are major forces generating LD. The currently used population-based association mapping tests do not explicitly correct for LD caused by these two forces. Therefore, increased type I error rates are expected if these tests are applied to plant breeding populations. As a consequence, we recommend to use family-based association tests for association mapping approaches in plant breeding populations.     

Postsynaptic organization and regulation of excitatory synapses

Dynamic regulation of synaptic efficacy is one of the mechanisms thought to underlie learning and memory. Many of the observed changes in efficacy, such as long-term potentiation and long-term depression, result from the functional alteration of excitatory neurotransmission mediated by postsynaptic glutamate receptors. These changes may result from the modulation of the receptors themselves and from regulation of protein networks associated with glutamate receptors. Understanding the interactions in this synaptic complex will yield invaluable insight into the molecular basis of synaptic function. This review focuses on the molecular organization of excitatory synapses and the processes involved in the dynamic regulation of glutamate receptors.     

Selective depression of low-release probability excitatory synapses by sodium channel blockers

Sodium channels (NaChs) play a central role in action potential generation and are uniquely poised to influence the efficacy of transmitter release. We evaluated the effect of partial NaCh blockade on two aspects of synaptic efficacy First, we evaluated whether NaCh blockade accounts for the ability of certain drugs to selectively depress glutamate release. Second, we evaluated the contribution of NaChs to intraneuronal variability in glutamate release probability (p(r)). The antiglutamate drug riluzole nearly completely depresses glutamate excitatory postsynaptic currents (EPSCs) at concentrations that barely affect GABAergic inhibitory postsynaptic currents (IPSCs). NaCh inhibition explains the selective depression. Unlike other presynaptic depressants, partial NaCh blockade increases paired-pulse EPSC depression. This result is explained by selective depression of low-p(r) synapses. We conclude that local variations in the action potential contribute to p(r) variability among excitatory synapses.     

Dioxygen affinity in heme proteins investigated by computer simulation

We present an investigation of the molecular basis of the modulation of oxygen affinity in heme proteins using computer simulation. QM-MM calculations are applied to explore distal and proximal effects on O(2) binding to the heme, while classical molecular dynamics simulations are employed to investigate ligand migration across the polypeptide to the active site. Trends in binding energies and in the kinetic constants are illustrated through a number of selected examples highlighting the virtues and the limitations of the applied methodologies. These examples cover a wide range of O(2)-affinities, and include: the truncated-N and truncated-O hemoglobins from Mycobacterium tuberculosis, the mammalian muscular O(2) storage protein: myoglobin, the hemoglobin from the parasitic nematode Ascaris lumbricoides, the oxygen transporter in the root of leguminous plants: leghemoglobin, the Cerebratulus lacteus nerve tissue hemoglobin, and the Alcaligenes xyloxidans cytochrome c'.     

Allostery in G protein-coupled receptors investigated by molecular dynamics simulations

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Differences in the cytoskeleton in inhibitory and excitatory synapses

The cytoskeleton of afferent chemical synapses, with various ultrastructure of contact zones, was examined in the Mauthner cells of the goldfish. The synapses with combined active zones and desmosome-like specialized contacts possessed a well developed cytoskeleton consisting of filaments and microtubules oriented towards the synaptic apposition. Regular arrays of synaptic vesicles oriented in the same direction were observed beyond and near the active zones. The cytoskeleton of the synapses lacking desmosome-like formations was diffusely organized throughout the boutons. The distribution of vesicles in the vicinity of active zones was also not ordered. The role of cytoskeleton in organization of the two morphologically distinct synapses is discussed. A special function of cytoskeleton as an intermediary between synaptoplasm and membrane is regarded as a necessary basis for plasticity of excitatory rather than inhibitory synapses.     

Receptor trafficking and the plasticity of excitatory synapses

Newly discovered features of the trafficking of AMPA receptors to and from the postsynaptic membrane of excitatory synapses are now bringing the mechanisms of synaptic plasticity into focus. Recent advances, including the existence of slots, anchors, transport factors and pathways for activity-dependent control, have elucidated the role of the individual AMPA receptor subunits and their binding partners. The latest views describe how subunit type dictates the assembly of heteromeric receptors, and how these heteromers interact with the receptor trafficking machinery and synaptic anchorage factors. Moreover, phosphorylation may play an important role in receptor transport and synaptic turnover.     

Neuroenergetics and the kinetic design of excitatory synapses

Why is the characteristic timescale of neural information processing in the millisecond range, corresponding to a 'clock speed' of about 1 kHz, whereas the clock speed of modern computers is about 3 GHz? Here we investigate how the brain's energy supply limits the maximum rate at which the brain can compute, and how the molecular components of excitatory synapses have evolved properties that are matched to the information processing they perform.     

Nonhomogeneous excitatory synapses of a crab stomach muscle

Neuromuscular synapses of pyloric muscle P1 in the blue crab Callinectes sapidus were examined using electrophysiological and electron microscopic methods. The muscle is innervated by a single excitatory axon of the stomatogastric ganglion. Excitatory postsynaptic potentials show striking facilitation at very low frequencies of stimulation, indicating very slow decay of the facilitation process after a single nerve impulse. Quantal content of transmitter release at a low frequency of stimulation averaged 1.5. Evidence was obtained that not all synapses on a muscle fiber are equivalent. This was particularly evident at the morphological level in serially sectioned nerve terminals. On each nerve terminal examined, a wide range of synapse sizes was found. Synaptic contact areas ranged from less than 0.5 micron2 to almost 10 micron2; the latter value is large compared with those obtained for other crustacean neuromuscular synapses. Most of the smaller synapses lacked the presynaptic dense bodies which are putative release sites for the transmitter substance. The larger synapses all had presynaptic dense bodies, and some showed evidence of splitting apart into smaller subunits. It is postulated that about half the morphologically identified synapses are relatively inactive.     

Novel protein kinase A-dependent long-term depression of excitatory synapses

Dopamine neurons of the ventral tegmental area (VTA) are critically involved in processing novel and rewarding information, and mediate the addictive properties of many drugs of abuse. Excitatory synapses on these neurons, like those in other brain regions, exhibit long-term depression (LTD). Amphetamine or dopamine block LTD at VTA synapses, indicating that both pathological and local physiological stimuli regulate LTD. Here we show that in common with other forms of LTD, VTA LTD results from a selective decrease in AMPA receptor function accompanied by a decrease in cell surface AMPA receptors. However, unlike the case for any previously described form of LTD, activation of cyclic AMP-dependent protein kinase (PKA) is necessary and sufficient to trigger LTD at synapses on VTA dopamine neurons.     

Activity-dependent validation of excitatory versus inhibitory synapses by neuroligin-1 versus neuroligin-2

Neuroligins enhance synapse formation in vitro, but surprisingly are not required for the generation of synapses in vivo. We now show that in cultured neurons, neuroligin-1 overexpression increases excitatory, but not inhibitory, synaptic responses, and potentiates synaptic NMDAR/AMPAR ratios. In contrast, neuroligin-2 overexpression increases inhibitory, but not excitatory, synaptic responses. Accordingly, deletion of neuroligin-1 in knockout mice selectively decreases the NMDAR/AMPAR ratio, whereas deletion of neuroligin-2 selectively decreases inhibitory synaptic responses. Strikingly, chronic inhibition of NMDARs or CaM-Kinase II, which signals downstream of NMDARs, suppresses the synapse-boosting activity of neuroligin-1, whereas chronic inhibition of general synaptic activity suppresses the synapse-boosting activity of neuroligin-2. Taken together, these data indicate that neuroligins do not establish, but specify and validate, synapses via an activity-dependent mechanism, with different neuroligins acting on distinct types of synapses. This hypothesis reconciles the overexpression and knockout phenotypes and suggests that neuroligins contribute to the use-dependent formation of neural circuits.