Effects of stimulation on the multiquantal spontaneous synaptic potentials in guinea pig hypogastric ganglia

Depolarization-induced Ca2+ entry preferentially evokes release of large quanta in the developing Xenopus neuromuscular junction

The amplitude histogram of spontaneously occurring miniature synaptic currents (mSCs) is skewed positively at developing Xenopus neuromuscular synapses formed in culture. To test whether the quantal size of nerve-evoked quanta (eSCs) distributes similarly, we compared the amplitude histogram of single quantum eSCs in low external Ca(2+) with that of mSCs and found that nerve stimulation preferentially released large quanta. Depolarization of presynaptic terminals by elevating [K(+)] in the external solution or by direct injection of current through a patch pipette increased the mSC frequency and preferentially, but not exclusively, evoked the release of large quanta, resulting in a second broad peak in the amplitude histogram. Formation of the second peak under these conditions was blocked by the N-type Ca(2+) channel blocker, ω-conotoxin GVIA. In contrast, when the mSC frequency was elevated by thapsigargin- or caffeine-induced mobilization of internal Ca(2+), formation of the second peak did not occur. We conclude that the second peak in the amplitude histogram is generated by Ca(2+) influx through N-type Ca(2+) channels, causing a local elevation of internal Ca(2+). The mSC amplitude in the positively skewed portion of the histogram varied over a wide range. A competitive blocker of acetylcholine (ACh) receptors, d-tubocurarine, reduced the amplitude of smaller mSCs in this range relatively more than that of larger mSCs, suggesting that this variation in the mSC amplitude is due to variable amounts of ACh released from synaptic vesicles. We suggest that Ca(2+) influx through N-type Ca(2+) channels preferentially induces release of vesicles with large ACh content.

Synaptic Transmission at Single Boutons in Sympathetic Ganglia

Synaptic transmission has traditionally been studied at the level of the entire nerve terminal rather than at one of its constituent boutons. Autonomic ganglia provide preparations for recording from individual boutons, as well as for determining the calcium transients necessary for transmitter release at these boutons. The results suggest a new paradigm for synaptic transmission.

Quantal secretion from visualized boutons on rat pelvic ganglion neurones

Synaptic transmission from single preganglionic hypogastric nerves innervating monopolar pelvic ganglion neurones has been studied with intracellular electrodes to record transmission from all the boutons and with extracellular electrodes placed over boutons visualized with DiOC2(5) in order to record transmission from selected boutons. Intracellular electrodes revealed spontaneous excitatory postsynaptic potentials (EPSPs) with amplitude histograms that show increasing numbers of large EPSPs as the external calcium ([Ca2+]o) was increased from 0.15 to 1.0 mM. These histograms were in general well fitted by a Poisson mixture of gamma distributions. Extracellular electrodes placed over visualized boutons revealed evoked excitatory postsynaptic potentials (extracellular EPSPs) with amplitude histograms that were best described by single gamma distributions in most cases in low [Ca2+]o (less than 0.5 mM). The standard deviation of these gammas was not much larger than that of the electrical noise. In a minority of extracellular recordings the amplitude histogram of evoked extracellular EPSPs was best described by a gamma distribution in which the standard deviation was much greater than that of the noise. Confocal microscopy of boutons orthogradely labelled with dextran-rhodamine showed that about 30% of these formed closely apposing pairs on the surface of the neurones. These observations are discussed in terms of the hypothesis that multiquantal release at boutons occurs as a consequence of the coupled secretion from closely apposed boutons.

Probabilistic secretion of quanta: spontaneous release at active zones of varicosities, boutons, and endplates

The amplitude-frequency histogram of spontaneous miniature endplate potentials follows a Gaussian distribution at mature endplates. This distribution gives the mean and variance of the quantum of transmitter. According to the vesicle hypothesis, this quantum is due to exocytosis of the contents of a single synaptic vesicle. Multimodal amplitude-frequency histograms are observed in varying degrees at developing endplates and at peripheral and central synapses, each of which has a specific active zone structure. These multimodal histograms may be due to the near synchronous exocytosis of more than one vesicle. In the present work, a theoretical treatment is given of the rise of intraterminal calcium after the stochastic opening of a calcium channel within a particular active zone geometry. The stochastic interaction of this calcium with the vesicle-associated proteins involved in exocytosis is then used to calculate the probability of quantal secretions from one or several vesicles at each active zone type. It is shown that this procedure can account for multiquantal spontaneous release that may occur at varicosities and boutons, compared with that at the active zones of motor nerve terminals.

The origin of Gaussian distributions of synaptic potentials

Spontaneous synaptic potentials were identified at the motor endplate 40 years ago. These were shown to possess amplitudes that could be described by a Gaussian distribution as could the amplitudes of evoked synaptic potentials under conditions of very low probability for secretion. As these Gaussians were identical, the idea of a unit or quantum of transmission was conceived. The failure to obtain similar Gaussian distributions for both spontaneous and low-probability evoked potentials during development of endplates indicated that a unit of transmission was not operating. However both the spontaneous and very low-probability evoked potentials could each be described by mixtures of Gaussians indicating a subunit of transmission might be operative. There are no ganglionic or central synapses at which comparisons have been made between spontaneous and low-probability evoked potentials that show each can be described by a Gaussian distribution, let alone that these are the same indicating a unit of transmission as originally conceived. There is some evidence that mixtures of Gaussians can be used to describe both spontaneous and very low-probability evoked synaptic potential amplitudes, opening up the possibility for a subunit of transmission at these synapses. The vesicle hypothesis, that the quantum of transmission at the endplate is due to the exocytosis of the contents of a synaptic vesicle, was also enunciated nearly 40 years ago. The existence of subunits of transmission has required reconsideration of this hypothesis. Three alternatives are considered: in one, the calcium-transient hypothesis, the subunit of secretion is due to the release of calcium from one of several calcium stores in the nerve terminal, so that several subunits are released when a number of these calcium stores are engaged in a regenerative response to the terminal action potential; a second alternative, the mediatophore hypothesis, is that a subunit of secretion occurs when a single transmitter transport protein channels transmitter across the terminal membrane, several such mediatophore proteins acting in concert then give multiple subunit release; finally, there is the vesicle fusion-pore hypothesis, in which individual transient openings of a fusion-pore channel joining a synaptic vesicle to the terminal membrane are responsible for secretion of a transmitter subunit, with multiple transients giving several subunits. Perhaps we will have distinguished between these possibilities before the quantal hypothesis is 50 years old.