Evoked quantal currents at neuromuscular junctions of wild type Drosophila larvae

Evoked excitatory postsynaptic currents (EPSC) were recorded with an extracellular macropatch electrode from glutamatergic neuromuscular junctions of Drosophila larvae. At 20 degrees C quantal current amplitude was about -400 pA and the 10-90% rise time was slightly below 0.2 ms for the fastest rising events and on average 0.3+/-0.1 ms in our best recordings. The quantal currents often had 'shoulders' but decayed approximately monoexponentially from half amplitude. The time constant of the exponential fit varied with mean values ranging from 2.5 ms to 7.7 ms in 13 experiments and an average value of 4.4+/-1.6 ms. Comparison of these results with data obtained earlier with outside-out patches of larval muscle membrane (Heckmann, M. and Dudel, J., Biophys. J., 72 (1997) 2160-2169) leads us to conclude that glutamate has to reach a saturating peak concentration of at least 10 mM in the synaptic cleft to allow the observed short quantal current rise times. To account for the time course of the quantal current decay one has to assume that the glutamate concentration in the synaptic cleft remains in the millimolar range for more than a millisecond and that the time course of the decay of the quantal currents is in part due to desensitization of the postsynaptic receptor channels.