Editore attuale:
BLACKWELL PUBLISHING LTD, 9600 GARSINGTON RD, OXFORD, ENGLAND, OXON, OX4 2DG
precedente:
Cambridge University Press / New York:40 West 20th Street:New York, NY 10011:(800)872-7423, (212)924-3900, EMAIL: [email protected], INTERNET: http://www.journals.cambridge.org, Fax: (212)691-3239
Doi
Abstract
1. The mechanism of transmitter release at the cytoneural junction of the frog posterior
canal was investigated by recording intracellularly subthreshold postsynaptic potentials
(EPSPs), and performing a statistical analysis of time intervals and peak amplitudes. In
single units EPSPs display highly variable size, so it is not clear whether they are
generated by the release of single quanta of transmitter and whether large ones represent
giant events, multiquantal events, or the random summation of independent unitary
events.
2. In units with low resting EPSP rates, peak amplitudes and time intervals between
EPSPs were measured directly. Peak amplitude histograms were continuous, unimodal
and well fitted by log normal distributions. Time-interval histograms were well described
by single exponentials.
3. At high EPSP rates (either at rest or during experimental treatments), where single
events overlapped extensively, peak amplitude histograms were skewed markedly
towards high values. Under these conditions, the EPSP waveform was estimated by
autoregressive fit to the autocorrelation of the recorded signal. The fit was used to build a
Wiener filter, for sharpening the original signal, before computing time-interval and
peak amplitude histograms. This yielded consistent log normal peak amplitude
distributions with no 'excess' skewness, similar to those obtained with low resting rates.
4. After sharpening by the Wiener filter, shoulders or small second peaks in amplitude
distributions were observed only at the highest EPSP rates (> 300 s1). The number of
'multiquantal' events was reduced by Wiener filtering, and was in general consistent
with the expectation that more than one independent event occurred within the
duration of the single event. This suggests that the events are uniquantal, random and
independent, i.e. miniature EPSPs (mEPSPs).
5. In general, peak amplitude distributions obtained with modified external Ca21
concentration ([Ca2+]0) and/or during mechanical stimulation or under efferent activation
were not significantly altered with respect to those obtained in the same units at rest.
Time-interval histograms were generally mono-exponential at rest as well as during
mechanical or efferent stimulation, and irrespective of [Ca2+]0. Resting mEPSP rate was
slightly increased by elevated [Ca2+]O and reduced by low [Ca21].. The increase in mEPSP
rate produced by mechanical excitation was depressed by both high and low [Ca21].,
whereas both conditions enhanced mechanical inhibition. Efferent inhibition was little
affected. High [Ca2+]0 hastened adaptation during efferent facilitation. Low [Ca2+]0
reduced peak response during facilitation, but suppressed its waning.
6. In the presence of ATP a consistent though transient increase in resting mEPSP rate was
observed in about 50% of units. ATP effect was absent in all fibres where efferent
stimulation produced inhibition and present in all fibres under facilitatory efferent
control. In these fibres, efferent facilitation, measured after the effect of ATP had
vanished, was reduced with respect to facilitation in control solution. The effects of ATP
were mimicked by its analogue adenosine-5'-0-3-thiotriphosphate (ATP-y-S)
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