Analyses of Glutamate Current Fluctuations and Glutamate Single Channel Currents of Cultured Brain Neurons

Glutamate is an excitatory neurotransmitter which activates glutamate ion channels located on the membrane of neurons. We have investigated kinetics of the ion channels activated by application of glutamate or its agonist N-methyl-D-asparate (NMDA) in cultured rat brain neurons. Methods of recording from the neurons are the whole cell clamp to make clear characteristics of the glutamate channels in the whole cell by analyzing the membrane current fluctuation, and the patch clamp to record directly the gatings of one or a few channels and to study their statistical characteristics in detail. Power spectrum density (PWS) of current fluctuations is obtained by FFT. The mean open time and the mean conductance of the channels are estimated by fitting PWS to the theoretical PWS curve (Lorentzian) by computer calculations or eye inspection. PWS fits, in most cases, a sum of two Lorentzian functions for glutamate current, whereas that of NMDA current fits a single Lorentzian with few exceptions. The fluctuated currents are depressed by Mg ions in the saline in negative membrane potential region. Fluctuation analysis shows that the opening probability decreases by the presence of Mg ions. The histograms of the conductans and the open time of NMDA channel obtained from patch clamp data. The mean open time estimated from the FFT anaiysis is 6 msec, which is close to the value (9 msec) estimated from the assumption that the mean open time obtained from the patch clamp recording has an exponential distribution.