Journal of Neuroscience, Vol 8, 1313-1325, Copyright © 1988 by Society for Neuroscience
Synaptic transmission mediated by single club endings on the goldfish Mauthner cell. II. Plasticity of excitatory postsynaptic potentials
JW Lin and DS Faber
Department of Physiology, State University of New York, Buffalo 14214.
Simultaneous pre- and postsynaptic intracellular recordings were used to
analyze the properties of chemically mediated synaptic transmission between
single club endings of eighth nerve afferents and the goldfish Mauthner
(M-) cell lateral dendrite. The EPSPs exhibited pronounced facilitation
when the presynaptic fiber fired high-frequency bursts of 2 or 3 impulses
at intervals of 2-4 msec. The amplitudes of the EPSPs evoked by the second
and third presynaptic impulses of a burst were, on average, 99 and 108%
larger than that evoked by the first impulse. A cross-correlation analysis
showed that the amplitudes of the control and facilitated EPSPs fluctuated
independently, indicating that the facilitation was mediated by a
presynaptic mechanism. This conclusion was supported by a comparison of the
coefficient of variation for the control and facilitated EPSPs, on the
basis of a binomial release model. In addition, the value of binomial n,
the number of presynaptic release units, was not changed during
facilitation. The origin of EPSP fluctuations was analyzed by examining the
correlation between the amplitudes of EPSPs and those of the electrotonic
coupling potentials associated with them. The absence of correlation
between the 2 variables suggested that the fluctuations of EPSPs were not
due to a variable presynaptic impulse invasion. The EPSP fluctuations were
further analyzed by assuming that the facilitation was associated with an
increase in the probability (p) of transmitter release and that the release
process followed simple binomial statistics. The binomial variables thus
calculated were n = 6-11, p = 0.29-0.44, and q = 31-61 microV, values
comparable to the estimates for other CNS synapses. More importantly, these
parameters provided satisfactory fits to the amplitude histograms of the
control and facilitated EPSPs. The number of release units, n, was smaller
than, but in a range similar to, the number of active zones identified in
the freeze-fracture study of the club endings (Kohno and Noguchi, 1986).
This correlation is consistent with the notion that active zones are the
structural correlates of quantal release units. In the preceding paper, it
was shown that impulses in a majority of club endings electrotonically
coupled to the M-cell do not produce a detectable chemically mediated EPSP,
although the contacts have the morphological correlates of chemical
synapses. In an attempt to activate these "silent" connections, 2
approaches were used. First, the burst-firing paradigm, which could
effectively facilitate EPSPs already present, failed to reveal any EPSPs at
the silent junctions.(ABSTRACT TRUNCATED AT 400 WORDS)
Previous Article | Next Article
