Potentials and Limitations of Noise Analysis of Light-induced Conductance Changes in Photoreceptors

Abstract

The potentials and limitations of noise analysis applied to light-induced currents in photoreceptors are discussed in this paper. Two limiting cases are presented for which noise analysis may yield some reliable information on the underlying mechanisms. The “bump approach” is based upon the assumption that the “elementary events” underlying the noisy current are the quantum bumps which obey Poisson statistics. From the experimentally observed power spectrum of the noisy current one then draws conclusions for the properties of the single bumps, including their variation under different conditions of light adaptation. The method is usually extended to values of the stimulating light intensity for which single bumps are no longer directly observable due to light adaptation.

On the other hand, the “steady-state approach” is based upon the assumption that the “elementary events” underlying the noisy current are the incoherent opening and closing events of individual ionic channels. This approach assumes a light-dependent, stationary but fluctuating intracellular level of some transmitter molecules which cause the opening (or closing) of the channels. From the experimentally observed power spectrum one then draws conclusions for the kinetics of the single channels and its control by the transmitter.

For each of the two approaches, a detailed model is presented and discussed. Qualitatively, one would expect the bump approach to be appropriate for “low” intensities of the stimulating light and the steady-state approach for “high” intensities. The crucial question, however, as to whether there is a sharp border line between the two approaches, an overlapping region of simultaneous applicability or a gap where both approaches fail, cannot be answered rigorously by the method of noise analysis itself since it is an indirect method which may continue to give answers even if the underlying assumptions become unrealistic on physical or physiological grounds. For any particular case of noise analysis applied to photoreceptors, we would like to evaluate the experimental data by means of both the bump and the steady-state approach and to discuss and compare the conclusions very carefully.