Noise analysis predicts at least four states for channels closed by glutamate

For ion channels that are opened by neurotransmitters, analysis of current noise has given valuable information on the kinetics of synaptic channel gating. In depolarizing bipolar cells of the vertebrate retina, we have recently characterized a synaptic current for which the neurotransmitter glutamate closes channels, and for which the channel open probability is low even in the absence of glutamate. We present here predictions for the current noise spectrum expected for various models of glutamate's action on the ion channels. Comparison of these theoretical predictions with experimental data allows us to rule out several simple kinetic schemes for the action of glutamate, and to conclude that the channels closed by glutamate must be able to exist in at least four different states.