Linear system analysis of ion channel modulation in rod photoreceptors under dim light conditions

Rod photoreceptors perform the task of night vision. The dynamics of dim light response is mainly determined by non-inactivating potassium Kx channels in the inner segment, manifested by such a feature as high-pass filtering. The L-type Ca²⁺ channels function as the voltage dependent actuator governing synaptic transmission from rods to the second order cells. Modulation of these ion channels can affect physiological features of rods. We introduced here an analytical approach to construct a linear circuit model of the rod membrane to study the effects of modulation of ion channels on rod function. The effects of tetraethylammonium (TEA) and Zn²⁺ were analyzed in frequency domain. The obtained results were consistent with the computational and experimental studies, which showed that TEA attenuated the high-pass filtering whereas Zn²⁺ enhanced it. The role of Ca²⁺ channels in the dynamics of light response was also investigated to show that the activation gate exhibited a larger impact on the overall frequency response than the inactivation gate. Linearization of ion channels for small perturbations provides an attractive alternative method to quantitatively evaluate the physiological significance of the effects of the modulation of ion channels. The resultant linear system may help reveal important features of neurons which can hardly be obtained otherwise.