Estimation of quantal parameters at the calyx of Held synapse

Making quantal analysis more convenient, fast, and accurate: user-friendly software QUANTAN

Quantal analysis of synaptic transmission is an important tool for understanding the mechanisms of synaptic plasticity and synaptic regulation. Although several custom-made and commercial algorithms have been created for the analysis of spontaneous synaptic activity, software for the analysis of action potential evoked release remains very limited. The present paper describes a user-friendly software package QUANTAN which has been created to analyze electrical recordings of postsynaptic responses. The program package is written using Borland C++ under Windows platform. QUANTAN employs and compares several algorithms to extract the average quantal content of synaptic responses, including direct quantal counts, the analysis of synaptic amplitudes, and the analysis of integrated current traces. The integration of several methods in one user-friendly program package makes quantal analysis of action potential evoked release more reliable and accurate. To evaluate the variability in quantal content, QUANTAN performs deconvolution of the distributions of amplitudes or areas of synaptic responses employing a ridge regression method. Other capabilities of QUANTAN include the analysis of the time-course and stationarity of quantal release. In summary, QUANTAN uses digital records of synaptic responses as an input and computes the distribution of quantal content and synaptic parameters. QUANTAN is freely available to other scholars over the internet.

Potentiation phase of spike timing-dependent neuromodulation by a serotonergic interneuron involves an increase in the fraction of transmitter release

In the mollusk, Tritonia diomedea, the serotonergic dorsal swim interneuron (DSI) produces spike timing-dependent neuromodulation (STDN) of the synaptic output of ventral swim interneuron B (VSI) resulting in a biphasic, bidirectional change of synaptic strength characterized by a rapid heterosynaptic potentiation followed by a more prolonged heterosynaptic depression. This study examined the mechanism underlying the potentiation phase of STDN. In the presence of 4-aminopyridine, which blocks the depression phase and enhances transmitter release from VSI, rapidly stimulating VSI led to a steady-state level of transmitter depletion during which potentiation by DSI or serotonin (5-HT) was eliminated. Cumulative plots of excitatory postsynaptic currents were used to estimate changes in the size and replenishment rate of the readily releasable pool (RRP) and the fraction of release. 5-HT application increased transmitter release without altering replenishment rate. The magnitude of 5-HT-evoked potentiation correlated with the increase in the fraction of release. A phenomenological model of the synapse further supported the hypothesis that 5-HT-induced potentiation was caused by an increase in the fraction of release and correctly predicted no change in frequency facilitation. A dynamic version of the model correctly predicted the effect of DSI stimulation under a variety of conditions. Finally, depletion of internal Ca(2+) stores with cyclopiazonic acid showed that Ca(2+) from internal stores is necessary for the 5-HT-induced potentiation. The data indicate that 5-HT released from DSI increases the fraction of the RRP discharged during VSI action potentials using a mechanism that involves Ca(2+) extrusion from internal stores, resulting in time- and state-dependent neuromodulation.

Analysis of quantal size of voltage responses to retinal stimulation in the accessory optic system

In the intact vertebrate central nervous system, the quantal nature of synaptic transmission is difficult to measure because the postsynaptic sites may be distributed along a tortuous dendritic tree that cannot be readily clamped spatially to a uniform potential. Titrating the intact brain's extracellular concentration of calcium ions is also challenging because of its strong buffering mechanisms. In this study, using a whole brain with eye attached preparation, quantal neurotransmission was examined in the turtle brainstem in vitro, by recording from accessory optic system neurons that receive direct input from visually responsive retinal ganglion cells. Unitary EPSPs, evoked by microstimulation of a single ganglion cell, were measured during whole cell current-clamp recordings. In this preparation, the neurons exhibit direction-selectivity, despite the hypoxic conditions. Bath application of cadmium to reduce calcium influx also reduced evoked EPSP amplitudes to that of the spontaneous synaptic events. Statistical analyses indicated that these evoked response amplitudes could be well fitted to a Poisson distribution for most brainstem neurons. Therefore, the spontaneous miniature excitatory synaptic events of approximately 1 mV, as also observed during spike blockade of the retina [Kogo, N., Ariel, M., 1997. Membrane properties and monosynaptic retinal excitation of neurons in the turtle accessory optic system. Journal of Neurophysiology 78, 614-627], are likely responses to the neurotransmitter of single vesicles release by retinal axon terminals.

A comparison between exocytic control mechanisms in adrenal chromaffin cells and a glutamatergic synapse

It has been known since the work of Katz and collaborators in the early 1950s that an increase in intracellular Ca(++) concentration ([Ca(++)]) is the immediate trigger for neurotransmitter release. Later work has shown that, next to Ca(++), many other signaling pathways, particularly via cyclic AMP, modulate the release of both neurotransmitters and hormones. However, regulated secretion is a multistep process and the signaling mechanisms involved act at many stages. Biochemical and traditional electrophysiological techniques very often cannot distinguish whether a change in secretion is caused by regulation of ion channels, vesicle trafficking, or the exocytic process itself. My laboratory has made an effort to dissect the stimulus secretion pathway by developing assays in chromaffin cells (for catecholamine release) and at a glutamatergic central nervous synapse (the calyx of Held, a component of the auditory pathway), which permit the study of secretion in single cells under voltage clamp conditions. This enables us to clearly distinguish between consequences of changes in electrical signaling, from those regarding the process of vesicle recruitment or the process of exocytosis.

VGLUTs: 'exciting' times for glutamatergic research?

Glutamate is the principal excitatory neurotransmitter in the mammalian central nervous system (CNS). Glutamate is first synthesized in the cytoplasm of presynaptic terminals before being loaded into synaptic vesicles, which fuse with the plasma membrane, releasing their contents, in response to neuronal activity. The important process of synaptic vesicle loading is mediated by a transport protein, collectively known as vesicular glutamate transporter (VGLUT). Controlling the activity of these transporters could potentially modulate the efficacy of glutamatergic neurotransmission. In recent years, three isoforms of mammalian VGLUTs have been cloned and molecularly characterized in detail. Probing these three VGLUTs has been proven to be the most reliable way of visualizing sites of glutamate release in the mammalian CNS. Immunohistochemical studies on VGLUTs suggest that glutamatergic neurons are categorized into subgroups depending on which VGLUT isoform they contain. Recent studies on VGLUT1-deficient mice have led various models to be postulated concerning the possible roles of VGLUTs in synaptic physiology, such as presynaptic regulation of quantal size and activity-dependent short-term plasticity.

Convergence of auditory-nerve fiber projections onto globular bushy cells

Globular bushy cells are a key element of brainstem circuits that mediate the early stages of sound localization. Many of their physiological properties have been attributed to convergence of inputs from the auditory nerve, many of which are large with complex geometry, but the number of these terminals contacting individual cells has not been measured directly. Herein we report, using cats as the experimental model, that this number ranged greatly (9-69) across a population of 12 cells, but over one-half of the cells (seven of 12) received between 15 and 23 inputs. In addition, we provide the first measurements of cell body surface area, which also varies considerably within this population and is uncorrelated with convergence. For one cell, we were able to document axonal structure over a distance greater than 100 microm, between the soma and the location where the axon expanded to its characteristic large diameter. These data were combined with accumulated physiological information on vesicle release, receptor kinetics and voltage-gated ionic conductances, and incorporated into computational models for four cells that are representative of the structural variation within our sample population. This predictive model reveals that basic physiological features, such as precise first spike latencies and peristimulus time histogram shapes, including primary-like with notch and onset-L, can be generated in these cells without including inhibitory inputs. However, phase-locking is not significantly enhanced over auditory-nerve fibers. These combined anatomical and computational approaches reveal additional parameters, such as active zone density, nerve terminal size, numbers and sources of inhibitory inputs and their activity patterns, that must be determined and incorporated into next-generation models to understand the physiology of globular bushy cells.

Activity-dependent regulation of synaptic strength and neuronal excitability in central auditory pathways

Neural activity plays an important role in regulating synaptic strength and neuronal membrane properties. Attempts to establish guiding rules for activity-dependent neuronal changes have led to such concepts as homeostasis of cellular activity and Hebbian reinforcement of synaptic strength. However, it is clear that there are diverse effects resulting from activity changes, and that these changes depend on the experimental preparation, and the developmental stage of the neural circuits under study. In addition, most experimental evidence on activity-dependent regulation comes from reduced preparations such as neuronal cultures. This review highlights recent results from studies of the intact mammalian auditory system, where changes in activity have been shown to produce alterations in synaptic and membrane properties at the level of individual neurons, and changes in network properties, including the formation of tonotopic maps.

Synaptic transmission at retinal ribbon synapses

The molecular organization of ribbon synapses in photoreceptors and ON bipolar cells is reviewed in relation to the process of neurotransmitter release. The interactions between ribbon synapse-associated proteins, synaptic vesicle fusion machinery and the voltage-gated calcium channels that gate transmitter release at ribbon synapses are discussed in relation to the process of synaptic vesicle exocytosis. We describe structural and mechanistic specializations that permit the ON bipolar cell to release transmitter at a much higher rate than the photoreceptor does, under in vivo conditions. We also consider the modulation of exocytosis at photoreceptor synapses, with an emphasis on the regulation of calcium channels.

A comparison of release kinetics and glutamate receptor properties in shaping rod-cone differences in EPSC kinetics in the salamander retina

Synaptic transmission from cones is faster than transmission from rods. Using paired simultaneous recordings from photoreceptors and second-order neurones in the salamander retina, we studied the contributions of rod-cone differences in glutamate receptor properties and synaptic release rates to shaping postsynaptic responses. Depolarizing steps evoked sustained calcium currents in rods and cones that in turn produced transient excitatory postsynaptic currents (EPSCs) in horizontal and OFF bipolar cells. Cone-driven EPSCs rose and decayed faster than rod-driven EPSCs, even when comparing inputs from a rod and cone onto the same postsynaptic neurone. Thus, rod-cone differences in EPSCs reflect properties of individual rod and cone synapses. Experiments with selective AMPA and KA agonists and antagonists showed that rods and cones both contact pharmacologically similar AMPA receptors. Spontaneous miniature EPSCs (mEPSCs) exhibited unimodal distributions of amplitude and half-amplitude time width and there were no rod-cone differences in mEPSC properties. To examine how release kinetics shape the EPSC, we convolved mEPSC waveforms with empirically determined release rate functions for rods and cones. The predicted EPSC waveform closely matched the actual EPSC evoked by cones, supporting a quantal release model at the photoreceptor synapse. Convolution with the rod release function also produced a good match in rod-driven cells, although the actual EPSC was often somewhat slower than the predicted EPSC, a discrepancy partly explained by rod-rod coupling. Rod-cone differences in the rates of exocytosis are thus a major factor in producing faster cone-driven responses in second-order retinal neurones.

Development of a robust central auditory synapse in congenital deafness

Within the medial nucleus of the trapezoid body (MNTB) in the auditory brain stem, there is a large central synapse known as the calyx of Held, which mediates high-fidelity glutamatergic transmission. We investigated the effects of congenital deafness on the development of pre- and postsynaptic parameters of synaptic strength at the calyx of Held. Whole cell recordings of evoked excitatory postsynaptic currents (EPSCs) and immunohistochemistry of GluR1-4 subunits were performed using brain stem slices from congenitally deaf or hearing mice at postnatal days P5 and P12. In both hearing and deaf mice there was a similar developmental decrease in the NMDA component of the evoked EPSC. There was a concurrent increase in release probability and number of release sites, contributing to a fivefold increase in evoked AMPA-mediated EPSC amplitude. The increase in release probability is opposite to that found in previous studies at the calyx of Held in the rat. There was also a seven- to eightfold increase in the size of the readily releasable pool of vesicles and a decrease in tetanic depression. The postsynaptic glutamate receptor subunits were similarly developmentally regulated and unaffected by deafness. GluR1 and 4 dominated at both ages. There was a decrease in expression of GluR1-3 from P5 to P12 and a shift from GluR2 to GluR3, indicating that AMPA receptor complexes at P12 are predominantly calcium-permeable. These results demonstrate that early development at this robust synapse proceeds normally with congenital deafness, suggesting that auditory nerve activity does not affect the development of synaptic strength at the calyx of Held.

Synaptic vesicle pools

Communication between cells reaches its highest degree of specialization at chemical synapses. Some synapses talk in a 'whisper'; others 'shout'. The 'louder' the synapse, the more synaptic vesicles are needed to maintain effective transmission, ranging from a few hundred (whisperers) to nearly a million (shouters). These vesicles reside in different 'pools', which have been given a bewildering array of names. In this review, we focus on five tissue preparations in which synaptic vesicle pools have been identified and thoroughly characterized. We argue that, in each preparation, each vesicle can be assigned to one of three distinct pools.

Cortical injury affects short-term plasticity of evoked excitatory synaptic currents

The hypothesis that plastic changes in the efficacy of excitatory neurotransmission occur in areas of chronic cortical injury was tested by assessing short-term plasticity of evoked excitatory synaptic currents (EPSCs) in neurons of partially isolated neocortical islands (undercut cortex). Whole cell recordings were obtained from layer V pyramidal neurons of sensorimotor cortical slices prepared from P36-P43 control and undercut rats. AMPA/kainate receptor-mediated EPSCs elicited by stimuli delivered at 40 to 66.7 Hz exhibited more paired-pulse depression (PPD) in undercut cortex than control, the time constant of depression evoked by trains of 20- to 66.7-Hz stimuli was faster, and the steady-state amplitude of EPSCs reached after five to seven EPSCs was lower. An antagonist of the glutamate autoreceptor, group II mGluR, increased the steady-state amplitude of EPSCs from undercut but not control cortex, suggesting that activation of presynaptic receptors by released glutamate is more prominent in undercut cortex. In contrast, the GABA(B) receptor antagonist (2S)-3-[[(1S)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl](phenylmethyl)phosphinic acid had no effect. Increasing [Ca(2+)](o) from 2 to 4 mM increased PPD, with a smaller effect in neurons of the undercut. The I-V relationship of AMPA/kainate receptor-mediated EPSCs was close to linear in both control and undercut neurons, and spermine had no significant effect on the EPSCs, suggesting that decreases in postsynaptic glutamate receptors containing the GluR2 subunit were not involved in the alterations in short-term plasticity. Results are compatible with an increase in the probability of transmitter release at excitatory synapses in undercut cortex due to functional changes in presynaptic terminals.

Synaptic vesicle pools at the frog neuromuscular junction

We have characterized the morphological and functional properties of the readily releasable pool (RRP) and the reserve pool of synaptic vesicles in frog motor nerve terminals using fluorescence microscopy, electron microscopy, and electrophysiology. At rest, about 20% of vesicles reside in the RRP, which is depleted in about 10 s by high-frequency nerve stimulation (30 Hz); the RRP refills in about 1 min, and surprisingly, refilling occurs almost entirely by recycling, not mobilization from the reserve pool. The reserve pool is depleted during 30 Hz stimulation with a time constant of about 40 s, and it refills slowly (half-time about 8 min) as nascent vesicles bud from randomly distributed cisternae and surface membrane infoldings and enter vesicle clusters spaced at regular intervals along the terminal. Transmitter output during low-frequency stimulation (2-5 Hz) is maintained entirely by RRP recycling; few if any vesicles are mobilized from the reserve pool.