Ca2+ Imaging of CNS Axons in Culture Indicates Reliable Coupling between Single Action Potentials and Distal Functional Release Sites

Mackenzie, Paul J.; Umemiya, Masashi; Murphy, Timothy H.

doi:10.1016/s0896-6273(00)80098-7

Cited by 93 publications

(81 citation statements)

References 63 publications

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“…There are many reasons to doubt that this requirement is met. These include (i) failures to stimulate the axon, (ii) all-or-none Ca 2ϩ signals in boutons (35), (iii) changes in Ca 2ϩ amplification in the terminal by internal release (36,37), and perhaps most importantly, (iv) stochastic variation in the number of Ca 2ϩ channels that open (4,38).…”

Section: Discussionmentioning

confidence: 99%

The high variance of AMPA receptor- and NMDA receptor-mediated responses at single hippocampal synapses: Evidence for multiquantal release

Conti

Lisman

2003

Proc. Natl. Acad. Sci. U.S.A.

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Most of our knowledge about transmission at central synapses has been obtained by studying populations of synapses, but some important properties of synapses can be determined only by studying them individually. An important issue is whether a presynaptic action potential causes, at most, a single vesicle to be released, or whether multiquantal transmission is possible. Previous work in the CA1 region has shown that the response to stimulation of a single axon can be highly variable, apparently because it is composed of a variable number of quantal elements (Ϸ5 pA in amplitude). These quantal events have a low coefficient of variation (CV). Because the number of synaptic contacts involved is not known, the response could be because of uniquantal transmission at a varying number of synapses, or to multliquantal transmission at a single synapse. The former predicts that the CV at individual synapses should be small. We have used optical methods to measure the N-methyl-D-aspartate receptormediated Ca 2؉ elevation at single active synapses. Our main finding is that the amplitude of nonfailure responses could be highly variable, having a CV as large as 0.63. In one fortuitous experiment, the optically studied synapse was the only active synapse, and we could therefore measure both its N-methyl-D-aspartate (NMDA) receptorand ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated signals. At this synapse, both signals varied over a 10-fold range and were highly correlated. These results strongly suggest that transmission at single CA1 synapses can be multiquantal. Furthermore, the individual quantal response is very far from saturation, allowing the effective summation of many quanta. The existence of multiquantal release has important implications for defining synaptic strength and understanding the mechanisms of synaptic plasticity.A lthough there has been rapid progress in understanding the properties of central synapses, most experiments have been done on populations of synapses. Such studies can answer many types of questions, but provide little direct evidence regarding the stochastic properties of individual synapses. Recently, individual central synapses have begun to be studied (reviewed in ref. 1). An important question relates to the number of vesicles that are released and the interaction between them. There are indications that transmission can be uniquantal at some synapses, but multiquantal at others (2-4).The most extensively studied central synapse is the Schaffer collateral synapse onto CA1 hippocampal pyramidal cells. However, even for this synapse, there remain substantial questions about the properties of quantal transmission. A minimal stimulation method has been extensively used to study the ''unitary'' responses generated by single axonal inputs. There are now reports from seven independent laboratories indicating that the amplitude distribution of the excitatory postsynaptic potential (EPSP) can have evenly spaced peaks (5-12), the signature of quantal transmission. Stati...

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Section: Discussionmentioning

confidence: 99%

The high variance of AMPA receptor- and NMDA receptor-mediated responses at single hippocampal synapses: Evidence for multiquantal release

Conti

Lisman

2003

Proc. Natl. Acad. Sci. U.S.A.

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“…Imaging of neuronal Ca 2ϩ transients was performed with wide-field microscopy using a Zeiss Axioskop FS microscope (Zeiss, Oberkochen, Germany) equipped with an intensified CCD camera (Stanford Photonics) as described previously (Mackenzie et al, 1996), using a 60ϫ water immersion 0.9 numerical aperture objective (Olympus, Tokyo, Japan) for optical recordings. The camera acquired data at 15 frames per second (pixel size ϭ 0.2 m), and the images were captured to a PC using a frame grabber (EPIX, Buffalo Grove, IL).…”

Section: Methodsmentioning

confidence: 99%

Optical Postsynaptic Measurement of Vesicle Release Rates for Hippocampal Synapses Undergoing Asynchronous Release during Train Stimulation

Otsu

Murphy²

2004

J. Neurosci.

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Developing hippocampal neurons in microisland culture were found to undergo rapid depression of excitatory synaptic activity caused by consumption of their readily releasable pool (RRP) of vesicles in response to 20 Hz trains of stimulation. Associated with depression was a switch to an asynchronous release mode that maintained transmission at a high steady-state rate equivalent to ϳ2.1 RRPs per second. We have applied postsynaptic Ca 2ϩ imaging to directly monitor these asynchronous release events to estimate both the steady rate of transmitter release and the number of quanta within the RRP at individual hippocampal synapses. Based on the frequency of asynchronous release measured at individual synapses postsynaptically using Ca 2ϩ imaging (5-17 sec after train stimulation) and with knowledge of the time course by which asynchronous release rates decay, we estimate that individual hippocampal synapses exhibit (in response to train stimulation) peak release rates of up to 21 quanta per second from an RRP that contains, on average, 10 quanta. Use-dependent block of evoked synaptic activity by MK-801 [(ϩ)-5-methyl-10,11-dihydro-5H-dibenzo [a,d]cyclohepten-5,10-imine maleate] confirmed that synapses undergoing asynchronous release are not significantly different from the general population with regard to their composition of NMDA receptor and/or release probability. Given that high-frequency trains deplete the synapse of readily releasable quanta (and that these release rates can only be maintained for a few seconds), these high rates of asynchronous release likely reflect refilling of vesicles from a reserve pool and not necessarily the continuous action of a relatively slow clathrin-and endosomedependent process.

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“…Propagation failures have been induced in only 30% of cases 66 , showing that propagation is generally reliable in hippocampal axons [98][99][100] . I A -dependent conduction failures occur at some axon collaterals but not at others 66 .…”

Section: Box 1 | Theory Of Geometrical Constraints On Axonal Propagationmentioning

confidence: 99%

Information processing in the axon

2004

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Ca2+ Imaging of CNS Axons in Culture Indicates Reliable Coupling between Single Action Potentials and Distal Functional Release Sites

Cited by 93 publications

References 63 publications

The high variance of AMPA receptor- and NMDA receptor-mediated responses at single hippocampal synapses: Evidence for multiquantal release

The high variance of AMPA receptor- and NMDA receptor-mediated responses at single hippocampal synapses: Evidence for multiquantal release

Optical Postsynaptic Measurement of Vesicle Release Rates for Hippocampal Synapses Undergoing Asynchronous Release during Train Stimulation

Information processing in the axon

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