Journal of Neuroscience, Vol 12, 454-466, Copyright © 1992 by Society for Neuroscience
Autoradiographic localization of voltage-dependent sodium channels on the mouse neuromuscular junction using 125I-alpha scorpion toxin. II. Sodium distribution on postsynaptic membranes
JL Boudier, T Le Treut and E Jover
Laboratoire de Biologie Cellulaire-Histologie, U.A. CNRS 1179, Faculte de Medecine Nord de Marseille, France.
A radioiodinated alpha-scorpion toxin (toxin II from Androctonus australis
Hector) (alpha ScTx) was used as a probe for EM autoradiography to study
the distribution of voltage-dependent sodium channels (Na+ channel) on the
postsynaptic side of the mouse neuromuscular junction. Silver grain
distribution was analyzed by the cross-fire method to assess the relative
Na+ channel density in each membrane domain measured by stereology. This
analysis showed that the maximum Na+ channel density was located on the
edge of the synaptic gutter, where it reached about twice the mean density
in the postsynaptic fold membrane. Na+ channel densities have been
calculated using ACh receptor (AChR) density in fold crests as reference.
Sodium channel density on the edge of the synaptic gutter was estimated at
about 5000/microns 2. Sodium channel distribution in the postsynaptic folds
was compared to AChR distribution using density distribution analysis
(Fertuck and Salpeter, 1976). The results confirmed that, as already
observed by immunogold labeling (Flucher and Daniels, 1989), there are no
Na+ channels on fold crests. Na+ channels are located in the rest of the
fold membrane (bottom) and may be distributed according to two possible
models. In the first, density would be uniformly high, although lower than
on the gutter edge. In the second, density would decrease from the crest
border, where the value was that of the gutter edge, to the fold end, where
the value would be 50% lower. Based on the latter model, which was the
"best-fit model," we propose that the postsynaptic membrane includes two
domains. The first is the fold crest, which contains almost exclusively
AChRs. This domain is devoted to reception-transduction of the chemical
signal. The second includes both the fold bottom membrane and the
perisynaptic membrane. Sodium channel density is highest along the crest
border and decreases moving away. Its functions are the integration of
postsynaptic potentials and generation-conduction of the muscle action
potential.
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