Terminal sprouting is not responsible for enhanced transmitter release at disused neuromuscular junctions of the rat

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.