Activation kinetics of glutamate receptor channels from wild-type Drosophila muscle.

Outside-out patches from wild-type Drosophila larval muscle were exposed briefly to L-Glutamate (Glu) using a piezo-driven application system. Glu in concentrations of 0.1 to 30 mM was applied and the responses to repeated applications of a given concentration were averaged. The peak current, î, and the current rise time, tr, from 0.1 î to 0.9 î were determined from the averages. Half-maximum activation of the channels was reached with approximately 2 mM Glu. î increased proportional to the power n = 3. 5 to n = 5.8 (average of four experiments, n = 4.4) for Glu concentrations between 0.3 and 0.5 mM. tr increased from approximately 0.2 ms at 10 mM Glu to a value of approximately 3.5 ms at 0.2 mM Glu. A linear reaction scheme with five binding steps preceding the channel-opening conformational change is proposed as the kinetic mechanism of channel activation and investigated in computer simulations. A set of rate constants assuming the same affinity for each binding site is found to describe the data better than one assuming positive cooperativity. The results are very similar to those for Glu-gated channels of crayfish and locust muscle, which is evidence for a common kinetic mechanism of these channels.