Journal of Neuroscience, Vol 10, 2059-2065, Copyright © 1990 by Society for Neuroscience
Terminal sprouting is not responsible for enhanced transmitter release at disused neuromuscular junctions of the rat
T Tsujimoto, M Umemiya and M Kuno
Department of Physiology, Kyoto University Faculty of Medicine, Japan.
Chronic block of nerve-muscle activity is known to induce sprouting of
motor nerve terminals and to enhance transmitter release at the
neuromuscular junction. Increased transmitter release has been assumed to
be a physiological correlate of disuse-induced sprouting of nerve
terminals. We examined this assumption in the rat extensor digitorum longus
muscle following chronic conduction block of the sciatic nerve with TTX.
The minimal period of nerve block required for the expression of terminal
sprouting was 3 d, whereas transmitter release, measured by the quantal
analysis of end-plate potentials, was already enhanced within 24 hr of
nerve block. Following 6 d of nerve block, sprouting was observed in about
35% of the motor nerve terminals examined. Under this condition, the total
length of individual terminals was significantly greater in the terminals
with sprouts than those without sprouts. However, enhancement of
transmitter release occurred uniformly at these junctions regardless of the
presence or absence of terminal sprouts. Also, transmitter release enhanced
by nerve block for 2 d remained elevated for at least 4 d even after
resumption of nerve activity without the formation of terminal sprouts. It
is concluded that terminal sprouting and increased transmitter release
induced in disused neuromuscular junctions are not causally related and
that the signals for inducing these 2 events are at least quantitatively
different.
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