A study of synaptic transmission and AMPA receptor channel kinetics
Hiroshi Kojima and Chloe Okuno
Laboratory for Cellular and Molecular Physiology, Department of Mechanical (Intelligent) Information Systems, Tamagawa University, 6-1-1 Tamagawa-Gakuen, Machida, Tokyo 194-8610, Japan
Excitatory postsynaptic currents (EPSCs) were recorded by whole-cell patch clamp method from cultured mouse hippocampal cells in order to characterize the excitatory postsynaptic transmission mediated with AMPA receptors. The kinetics of AMPA receptor channel was investigated by analyzing the shapes of spontaneous EPSCs. Furthermore, the current responses from the soma obtained by activation of AMPA receptor a small membrane area of dendritic trees with IR and UV laser uncaging of MNI-caged glutamate (detailed experimental method was reported elsewhere). The kinetics of the spontaneous EPSCs was compared with those of the laser evoked responses. The range of the peak amplitudes of the recorded EPSCs were between 10 to 100 pA at a holding potential of -80 mV. The elimination of these currents responses with 4 μ CNQX indicated that they are mediated with AMPA receptor channels. The application of TTX (1 μM) abolished the most of the currents greater than 50 pA, which suggested that these currents were evoked by spontaneous action potentials arrived at the presynaptic terminal that contacted onto the recording cells. 2 μM bicuculline was added to the bath solution to eliminate the possible inhibitory postsynaptic currents (IPSCs). In order to characterize the kinetics properties of AMPA receptor channels, the time course of synaptic current was examined. The decay phase of the current was well approximated with a single exponential function and the histogram of the time constants of the exponentials was asymmetrical showing a tail of longer time constant side. The histogram of the peak amplitude of the spontaneous miniature synaptic currents was also asymmetrical. The responses by laser-photolysis were also analyzed by the same method. It was suggested form the present experiments that the kinetics properties of AMPA receptor channels proposed by other physiological experiments was reconfirmed by analyzing the current responses evoked with laser photolysis.