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.     

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.     

Distribution of excitatory junction potential amplitude in cat cerebral arteries: examination of its quantal nature and modulation by opiates

We used scorpion venom to release small amounts of an excitatory neurotransmitter from adventitial nerves in cat left anterior descending cerebral artery. We used glass microelectrodes to measure and record postsynaptic electrical events of minimal amplitude. These events were similar to postsynaptic spontaneous and electrically evoked excitatory junction potentials (ejp's) seen in skeletal muscle. We performed a frequency analysis of the ejp amplitudes to determine if they fit a unimodal or multimodal distribution. We also investigated the effects of phentolamine, norepinephrine, hydromorphone, and morphine on ejp amplitude and frequency in the artery. Statistical analysis of the ejp frequency and amplitude revealed a multipeaked distribution with decreasing peaks. These results were similar to the distribution reported for acetylcholine release in skeletal muscle. The ejps were inhibited by phentolamine, which suggested that these events were adrenergically mediated. Norepinephrine and the opiates, hydromorphone and morphine, reduced the frequency and amplitude of the ejp's. The vessels also constricted to increasing doses of norepinephrine both under control conditions and in the presence of opiate. These results suggest that norepinephrine blocks the ejp's by a feedback mechanism at the presynaptic membrane and that endorphins and/or enkephalins, also acting at this presynaptic site, may modulate neurotransmission in the cerebral circulation.     

Numerical reconstruction of the quantal event at nicotinic synapses.

To test our present quantitative knowledge of nicotinic transmission, we reconstruct the postsynaptic conductance change that results after a presynaptic nerve terminal liberates a quantum of acetylcholine (ACh) into the synaptic cleft. The theory assumes that ACh appears suddenly in the cleft and that is subsequent fate is determined by radial diffusion, by enzymatic hydrolysis, and by binding to receptors. Each receptor has one channel and two ACh binding sites; the channel opens when both sites are occupied and the rate-limiting step id the binding and dissociation of the second ACh molecule. The calculations reproduce the experimentally measured growth phase (200 microseconds), peak number of open channels (2,000), and exponential decay phase. The time constant of the decay phase exceeds the channel duration by approximately equal to 20%. The normal event is highly localized: at the peak, two-thirds of the open channels are within an area of 0.15 micrometer 2. This represents 75% of the available channels within this area. The model also simulates voltage and temperature dependence and effects of inactivating esterase and receptors. The calculations show that in the absence of esterase, transmitter is buffered by binding to receptors and the postsynaptic response can be potentiated.     

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.     

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.     

Plasticity of GABAergic synapses in the neonatal rat hippocampus

While the development and plasticity of excitatory synaptic connections have been studied into detail, little is known about the development of inhibitory synapses. As proposed for excitatory synapses, recent studies have indicated that activity-dependent forms of synaptic plasticity, such as long-term potentiation and long-term depression, may play a role in the establishment of functional inhibitory synaptic connections. Here, I review these different forms of plasticity and focus on their possible role in the developing neuronal network.     

Quantification of the synapses in the hippocampus of aged rats

The synapses in the stratum lacunosum-molecular (str. L-M) of CA1 hippocampal field in 3-month old and 24-month old rats were examined using quantitative ultrastructural methods. No significant difference in the density of synapses and postsynaptic dendritic spines was found between the two age groups. The area of presynaptic terminals and postsynaptic dendritic spines was decreased slightly but significantly in the group of aged as compared to that in the group of young-mature rats. The vesicle number per presynaptic terminal, per area of presynaptic terminals and per volume of neuropil was not changed while the vesicle number per area of synaptic contact zones (SCZ) was increased in the group of aged rats. The mean length, total length and total surface of SCZ were diminished in the group of aged as compared to those in the group of young-mature rats. The same width of the str.radiatum and str.L-M in the two age groups showed that there was no any shrinkage of the neuropil in aged rats. The quantitative alterations in the synapses were accompanied by an increased number of dense and lamellar bodies in presynaptic terminals as well as with a presence of hypertrophic astroglial processes.     

N-cofilin can compensate for the loss of ADF in excitatory synapses

Actin plays important roles in a number of synaptic processes, including synaptic vesicle organization and exocytosis, mobility of postsynaptic receptors, and synaptic plasticity. However, little is known about the mechanisms that control actin at synapses. Actin dynamics crucially depend on LIM kinase 1 (LIMK1) that controls the activity of the actin depolymerizing proteins of the ADF/cofilin family. While analyses of mouse mutants revealed the importance of LIMK1 for both pre- and postsynaptic mechanisms, the ADF/cofilin family member n-cofilin appears to be relevant merely for postsynaptic plasticity, and not for presynaptic physiology. By means of immunogold electron microscopy and immunocytochemistry, we here demonstrate the presence of ADF (actin depolymerizing factor), a close homolog of n-cofilin, in excitatory synapses, where it is particularly enriched in presynaptic terminals. Surprisingly, genetic ablation of ADF in mice had no adverse effects on synapse structure or density as assessed by electron microscopy and by the morphological analysis of Golgi-stained hippocampal pyramidal cells. Moreover, a series of electrophysiological recordings in acute hippocampal slices revealed that presynaptic recruitment and exocytosis of synaptic vesicles as well as postsynaptic plasticity were unchanged in ADF mutant mice. The lack of synaptic defects may be explained by the elevated n-cofilin levels observed in synaptic structures of ADF mutants. Indeed, synaptic actin regulation was impaired in compound mutants lacking both ADF and n-cofilin, but not in ADF single mutants. From our results we conclude that n-cofilin can compensate for the loss of ADF in excitatory synapses. Further, our data suggest that ADF and n-cofilin cooperate in controlling synaptic actin content.     

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.