DEPOLARIZATION INCREASES THE SINGLE-CHANNEL CONDUCTANCE AND THE OPEN PROBABILITY OF CRAYFISH GLUTAMATE CHANNELS

We have studied the voltage sensitivity of glutamate receptors in outs ide-out patches taken from crayfish muscles. We found that single-chan nel conductance, measured directly at the single-channel level, increa ses as depolarization rises. At holding potentials from -90 mV to simi lar to 20 mV, the conductance is 109 pS. At holding potentials positiv e to 20 mV, the conductance is 213 pS. This increase in single-channel conductance was also observed in cell-attached patches. In addition, desensitization, rise time, and the dose-response curve were all affec ted by depolarization. To further clarify these multifaceted effects, we evaluated the kinetic properties of single-channel activity recorde d from cell-attached patches in hyperpolarization (membrane potential around -75 mV) and depolarization (membrane potential similar to 105 m V), We found that the glutamate dissociation rate constant (k_) was af fected most significantly by membrane potential; it declined 6.5-fold under depolarization. The rate constant of channel closing (k(c)) was also significantly affected; it declined 1.8-fold. The rate constant o f channel opening (k(o)) declined only 1.2-fold. The possible physiolo gical significance of the depolarization-mediated changes in the above rate constants is discussed.