MEASURING COMPLEX ADMITTANCE AND RECEPTOR CURRENT BY SINGLE ELECTRODEVOLTAGE-CLAMP

Studying the membrane properties of small excitable cells like sensory receptors in situ is often difficult. Two new techniques are describe d here which utilize white noise during single-electrode voltage-clamp . Cells are impaled with a single microelectrode and voltage-clamped t o a given holding potential, using a time-sharing technique. The first method, based on modulating the voltage command with repeated sequenc es of a pseudorandom stimuli, allows measurements of cell conductance (complex admittance) in the frequency domain. The second method is des igned to characterize the dynamics of the receptor current in the freq uency domain. In both cases, R1-6 type blowfly photoreceptors were use d as experimental models. The photoreceptor was first light-adapted to a steady light background and then clamped to the resulting potential . A pseudorandomly modulated light stimulus was then superimposed on t he steady light background and the resulting receptor current was reco rded. The frequency response was then calculated from the light modula tion and the receptor current via fast Fourier transform (FFT). By usi ng intracellularly applied ion channel blockers, the effects of active and passive membrane properties in modulating the transmitted signals could also be studied.