Modulation of signal processing by ion channels in rod photoreceptors

Rod photoreceptors perform the task of night vision. Ion channels in the inner segments of rods play a great role in the signal processing at this first level of vision. Among complicated ionic mechanisms, non-inactivating potassium Kx channels and L-type Ca2+ channels are the most two prominent types of ion channels. The dynamics of light response is mainly determined by Kx channels especially for dim light conditions, manifested by such features as high-pass filtering and calcium spikes. The L-type Ca 2+ channels function as the voltage dependent actuator governing synaptic transmission from rods to the second order cells. Modulation of these ion channels is a way that can modulate physiological functions of rods. Zinc slowed down the kinetics of the IKx current and TEA, a conventional K+ channel blocker, inhibited the maximum conductance of Kx channels, as indicated by patch-clamp data. Experimental and theoretical studies showed that TEA attenuated the high-pass filtering whereas Zn2+ enhanced it. TEA also induced an action potential like spikes, closely related to the calcium dynamics in rods. Based on the modulated parameters of Kx channels, the mechanisms of above phenomena were revealed by the results from experiments, computational models and linearization circuit models. For Ca2+ channels, nitric oxide (NO), another physiological modulator, was selected to investigate possible role of Ca 2+ channel modulation. Two isomers of S-nitrosocysteine (SNC), i.e., D-SNC and L-SNC, were used as NO donors to compare their modulations on Ca 2+ channels. By using patch clamp and calcium fluorescence imaging techniques, D-SNC was found to exert inhibition effects in contrast to the facilitation effects by L-SNC. The differential modulation effects by D-SNC and L-SNC strongly suggested that NO modulation of Ca2+ channels can be carried out through diverse mechanisms. When the NO modulation of Ca 2+ channels in rods were compared with cones by looking at both presynaptic and postsynaptic cells, it was found out that Ca2+ channels in rods and cones were reciprocally modulated by L-SNC, suggesting a potential role of NO modulation in light adaptation.