Long-lasting depression and the depletion hypothesis at crayfish neuromuscular junctions

Abstract

1. Synaptic depression was studied at the neuromuscular junctions of the crayfish giant motoneurone onto the abdominal fast flexor muscles. The kinetics of depression were compared quantitatively to predictions of the depletion hypothesis of synaptic depression. 2. Synaptic transmission was depressed about 71% following a single motoneurone impulse. Transmission recovered to normal along an exponential time course with an average time constant of 285 s (Fig. 1). 3. This recovery curve was used to predict the depression that repeated responses would be expected to suffer. Even after correction for a superimposed facilitation (Fig. 2) and non-linear postsynaptic summation, the response to repeated stimulation at frequencies at or above once per minute were less depressed than expected (Figs. 3, 4). Increasing the frequency above this rate did not enhance depression, contrary to predictions. The discrepancy was not due to a mobilization of transmitter into a releasable store (Fig. 5). 4. The depression caused by each stimulus in a train was much less than that following a single impulse. This contradicts the depletion hypothesis and suggests that depression is accompanied by a large change in the fraction of the releasable transmitter store liberated by an impulse, with very little accompanying depletion. 5. When transmitter release is reduced in high magnesium solution, there is no change in the depression observed to repeated stimulation, contrary to the depletion hypothesis (Fig. 6). 6. This discrepancy could be due to an effect of magnesium on mobilization. However, no difference was found in the slow recovery rates following an impulse in different magnesium concentrations (Fig. 7). 7. It is concluded that the depletion hypothesis is untenable as a basis for depression at this neuromuscular junction.