Localizing quantal currents along frog neuromuscular junctions

Advances in colonic motor complexes in mice

The primary functions of the gastrointestinal (GI) tract are to absorb nutrients, water, and electrolytes that are essential for life. This is accompanied by the capability of the GI tract to mix ingested content to maximize absorption and effectively excrete waste material. There have been major advances in understanding intrinsic neural mechanisms involved in GI motility. This review highlights major advances over the past few decades in our understanding of colonic motor complexes (CMCs), the major intrinsic neural patterns that control GI motility. CMCs are generated by rhythmic coordinated firing of large populations of myenteric neurons. Initially, it was thought that serotonin release from the mucosa was required for CMC generation. However, careful experiments have now shown that neither the mucosa nor endogenous serotonin are required, although, evidence suggests enteroendocrine (EC) cells modulate CMCs. The frequency and extent of propagation of CMCs are highly dependent on mechanical stimuli (circumferential stretch). In summary, the isolated mouse colon emerges as a good model to investigate intrinsic mechanisms underlying colonic motility and provides an excellent preparation to explore potential therapeutic agents on colonic motility, in a highly controlled in vitro environment. In addition, during CMCs, the mouse colon facilitates investigations into the emergence of dynamic assemblies of extensive neural networks, applicable to the nervous system of different organisms.

Leaky synapses: regulation of spontaneous neurotransmission in central synapses

The mechanisms underlying spontaneous neurotransmitter release are not well understood. Under physiological as well as pathophysiological circumstances, spontaneous fusion events can set the concentration of ambient levels of neurotransmitter within the synaptic cleft and in the extracellular milieu. In the brain, unregulated release of excitatory neurotransmitters, exacerbated during pathological conditions such as stroke, can lead to neuronal damage and death. In addition, recent findings suggest that under physiological circumstances spontaneous release events can trigger postsynaptic signaling events independent of evoked neurotransmitter release. Therefore, elucidation of mechanisms underlying spontaneous neurotransmission may help us better understand the functional significance of this form of release and provide tools for its selective manipulation. For instance, our recent investigations indicate that the level of cholesterol in the synapse plays a critical role in limiting spontaneous synaptic vesicle fusion. Therefore, alterations in synaptic cholesterol metabolism can be a critical determinant of glutamatergic neurotransmission at rest. This article aims to provide a closer look into our current understanding of the mechanisms underlying spontaneous neurotransmission and the signaling triggered by these unitary release events.

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.

An electrically coupled network of skeletal muscle in zebrafish distributes synaptic current

Fast and slow skeletal muscle types are readily distinguished in larval zebrafish on the basis of differences in location and orientation. Additionally, both muscle types are compact, rendering them amenable to in vivo patch clamp study of synaptic function. Slow muscle mediates rhythmic swimming, but it does so purely through synaptic drive, as these cells are unable to generate action potentials. Our patch clamp recordings from muscle pairs of zebrafish reveal a network of electrical coupling in slow muscle that allows sharing of synaptic current within and between segmental boundaries of the tail. The synaptic current exhibits slow kinetics (tau(decay) approximately 4 ms), which further facilitates passage through the low pass filter, a consequence of the electrically coupled network. In contrast to slow muscle, fast skeletal muscle generates action potentials to mediate the initial rapid component of the escape response. The combination of very weak electrical coupling and synaptic kinetics (tau(decay) <1 ms) too fast for the network low pass filter minimizes intercellular sharing of synaptic current in fast muscle. These differences between muscle types provide insights into the physiological role(s) of electrical coupling in skeletal muscle. First, intrasegmental coupling among slow muscle cells allows effective transfer of synaptic currents within tail segments, thereby minimizing differences in synaptic depolarization. Second, a fixed intersegmental delay in synaptic current transit, resulting from the low pass filter properties of the slow muscle network, helps coordinate the rostral-caudal wave of contraction.

Quantal potential fields around individual active zones of amphibian motor-nerve terminals

The release of a quantum from a nerve terminal is accompanied by the flow of extracellular current, which creates a field around the site of transmitter action. We provide a solution for the extent of this field for the case of a quantum released from a site on an amphibian motor-nerve terminal branch onto the receptor patch of a muscle fiber and compare this with measurements of the field using three extracellular electrodes. Numerical solution of the equations for the quantal potential field in cylindrical coordinates show that the density of the field at the peak of the quantal current gives rise to a peak extracellular potential, which declines approximately as the inverse of the distance from the source at distances greater than about 4 microm from the source along the length of the fiber. The peak extracellular potential declines to 20% of its initial value in a distance of about 6 microm, both along the length of the fiber and in the circumferential direction around the fiber. Simultaneous recordings of quantal potential fields, made with three electrodes placed in a line at right angles to an FM1-43 visualized branch, gave determinations of the field strengths in accord with the numerical solutions. In addition, the three electrodes were placed so as to straddle the visualized release sites of a branch. The positions of these sites were correctly predicted on the basis of the theory and independently ascertained by FM1-43 staining of the sites. It is concluded that quantal potential fields at the neuromuscular junction that can be measured with available recording techniques are restricted to regions within about 10 microm of the release site.

Spatial variability in release at the frog neuromuscular junction measured with FM1-43

We quantified the spatial variability in release properties at different synaptic vesicle clusters in frog motor nerve terminals, using a combination of fluorescence and electron microscopy. Individual synaptic vesicle clusters labeled with FM1-43 varied more than 10-fold in initial intensity (integrated FM1-43 fluorescence) and in absolute rate of dye loss during tetanic electrical nerve stimulation. Most of this variability arose because large vesicle clusters spanned more than one presynaptic active zone (inferred from postsynaptic acetylcholine receptor stripes labeled with rhodamine-conjugated alpha-bungarotoxin); when the rate of dye loss was normalized to the length of receptor stripe covered, variability from spot to spot was greatly reduced. In addition, electron microscopic measurements showed that large vesicle clusters (i.e., those spanning multiple active zones) were also thicker, and the increased depth of vesicles led to increased total spot fluorescence without a corresponding increase in the rate of dye loss during stimulation. These results did not reveal the presence of "hot zones" of secretory activity.Key words: synaptic transmission, exocytosis, synaptic vesicles, neuromuscular junction.

Quantal unit populations at the Drosophila larval neuromuscular junction

Focal extracellular recording at visualized boutons of the Drosophila larval neuromuscular junction was used to determine frequency and time course of the spontaneously occurring quantal events. When simultaneous intracellular recordings from the innervated muscle cell were made, more than one class of quantal event occurred at some of the individual boutons. "True" signals (arising at the bouton within the focal macropatch electrode) were often contaminated by additional signals generated outside the lumen of the focal electrode. Inclusion of these contaminating signals gave spuriously low values for relative amplitude, and spuriously high values for spontaneous quantal emission, for the synapses within the focal electrode. The contaminating signals, which appeared to be conducted along the subsynaptic reticulum surrounding the nerve terminals, generally were characterized by relatively small extracellular signals associated with normal intracellular events in the muscle fiber. From plots of simultaneous extracellular and intracellular recordings, the individual data points were classified according to the angles they subtended with the x axis (extracellular signal axis). Statistical procedures were developed to separate the true signals and contaminants with a high level of confidence. Populations of quantal events were found to be well described by Gaussian mixtures of two or three components, one of which could be characterized as the true signal population. Separation of signals from contaminants provides a basis for improving the estimates of quantal size and spontaneous frequency for the synapses sampled by the focal extracellular electrode.

Vesicle-associated proteins and quantal release at single active zones of amphibian (Bufo marinus) motor-nerve terminals

A study was made to determine the disposition of vesicle-associated proteins (syntaxin, SV2, SNAP-25) and calcium channels with respect to the spatial extent of spontaneous and evoked quantal release within regions of amphibian motor-nerve terminal branches delineated by FM1-43 stained vesicle clusters (blobs). Discrete concentrations of vesicles revealed approximately 2 microm apart along the length of terminal branches through FM1-43 staining were identical in size and spacing to those identified along terminal branches with SV2 antibody (AbSV2). Fluorescent antibodies to syntaxin 1 (AbS), SNAP-25 (AbS25) and the calcium channel alpha1B subunit (Abalpha1B) were found in relatively high concentrations coincident with the AbSV2 blobs. Three extracellular recording electrodes were placed in the vicinity of individual FM1-43 blobs, and an algorithm was used to determine the spatial origin of miniature endplate potentials (MEPPs) and EPPs together with their relative amplitudes. MEPPs and EPPs originated throughout the region stained by FM1-43 but not elsewhere; amplitude-frequency distributions of MEPPs and EPPs were similar for all FM1-43 blobs with average coefficients of variation of no less than 0.28. A linear relationship existed between the size of an FM1-43 blob, measured as the integrated extent of FM1-43 staining of a blob, and the frequency of MEPPs as well as the probability of EPPs from the blob. There was a proximo-distal gradient in the size of FM1-43 blobs along the length of single terminal branches, suggesting a gradient in release probability along the branches. The frequency distribution of the distances between blobs was approximately Gaussian, whereas the frequency distribution of the size of blobs was highly skewed and was best fitted with a gamma distribution. It is concluded that there are correlations among the extent of labeling of SNAP-25, syntaxin and calcium channels at a release site, the store of vesicles to be found there, and the probability of spontaneous and evoked quantal release.

Membrane recycling due to low and high rates of nerve stimulation at release sites in the amphibian (Bufo marinus) neuromuscular junction

The activity-dependent labelling of motor nerve terminals with the dye FM1-43 has been used to estimate the relative levels of membrane recycling (due to synaptic vesicle exocytosis and recovery) at release sites in response to 1,200 nerve stimulations delivered at either low (0.5 Hz) or high (30 Hz) frequency. Dye in terminals appears as fluorescent spots distributed along the terminal branches; each spot is thought to be a cluster of labelled vesicles associated with a release site. Relative fluorescence in spots was quantified from images obtained with a confocal microscope. Spot intensities varied widely within branches following labelling at both frequencies, but the distribution was highly skewed towards lower intensities at low frequency stimulation; at high frequency, more spots had stronger fluorescence. Both weak and strongly stained spots were uniformly distributed along the length of terminal branches after low frequency stimulation; however, there was a gradual decline in all spot intensities towards the distal end of branches loaded with dye at high frequency stimulation. Antibody staining for synaptic vesicles was, on average, uniformly distributed along the branches. The increase in number of more strongly FM1-43-labelled spots in terminal branches stimulated at high compared with low frequency suggests that more release sites are active at high rates of nerve stimulation. This "recruitment" of release sites at high frequency stimulation occurs mostly in the proximal half of terminal branches and is not related to the abundance of synaptic vesicles in the terminal.

Disinhibition during myoelectric complexes in the mouse colon

Intracellular microelectrodes were used to record electrically evoked inhibitory junction potentials (IJPs) and electrotonic potentials during spontaneous cyclical depolarisations (myoelectric complexes, MCs) in the circular muscle layer of mouse colon in vitro. In the presence of nifedipine (1-2 microM) and atropine (1 microM), MCs were recorded every 264 +/- 18 s. Between MCs, single electrical stimuli (15 V, 0.6 ms, every 8 s) elicited IJPs whose amplitudes remained constant. In comparison, during the depolarising phase of MCs, the mean IJP amplitude was reduced by 61 +/- 7%, while during the late plateau and early repolarising phase of MCs, IJP amplitude was increased (up to 20%). NG-nitro-L-arginine (NOLA, 100 microM) abolished the repolarisation phase between MCs, so that the circular muscle remained depolarised and the amplitude of MCs was reduced by 73 +/- 6%. However, the amplitude of evoked IJPs was unaffected, as was the decrease in their amplitude during the depolarising phase of the residual MCs. In the presence of NOLA (100 microM), the further addition of apamin (250 nM) reduced the amplitude of evoked IJPs by approximately half. However, the amplitudes of NOLA- and apamin-resistant IJPs were also attenuated by 82 +/- 5% during the depolarising phase of residual MCs (amplitude: 1.9 +/- 1 mV). However, during this phase, the amplitude of an electrotonic potential (evoked by extracellular current application) was not attenuated. Addition of hexamethonium (500 microM), or tetrodotoxin (TTX) (1.6 microM) to solutions containing NOLA and apamin were without effect on membrane potential, but the residual MCs and the cyclical attenuation in IJP amplitude were abolished. During the intervals between MCs, membrane potential is maintained under tonic inhibition, via spontaneous release of inhibitory neurotransmitter(s), predominantly through nitrergic mechanisms. The cyclical attenuation in the amplitude of the non-nitrergic IJP does not arise from cyclical postjunctional changes in membrane resistance or potential. Moreover, the generation of the depolarising phase of MCs involves the simultaneous suppression of both nitrergic and non-nitrergic inhibitory neurotransmission. It is suggested that MCs arise from presynaptic suppression of ongoing inhibitory neurotransmitter release.