Analysis of calcium dynamics for dim-light responses in rod and cone photoreceptors

Rod and cone photoreceptors in the retina of vertebrate eyes are fundamental sensory neurons underlying vision. They use a sophisticated signal transduction pathway consisting of a series of biochemical processes to convert the absorption of light into an electrical current response. Several of these processes are modulated by feedback that depends on the intracellular Ca2+ concentration. In this work we use a representative phototransduction model to study how changing the Ca2+ kinetics by fast buffering affects sensitivity and dynamics of the light response in mouse rod and cone photoreceptors. We derive analytic results for dim-light stimulations that provide quantitative and conceptual insight. We show that flash responses are monophasic with low buffering, and the change in the Ca2+ concentration occurs in proportion to the current. If the amount of fast buffering is increased, the Ca2+ kinetics becomes slowed down and delayed with respect to the current, and biphasic responses emerge (damped oscillations). This shows that a biphasic response is not necessarily a manifestation of slow buffering reactions. A phase space analysis shows that the emergence of biphasic responses depends on the ratio between the effective rate ca that controls the Ca2+ kinetics, and the dark turnover rate of cyclic GMP {beta}d. We further investigate how the light response is altered by modifying the extracellular Ca2+ concentration. In summary, we provide a comprehensive quantitative analysis that precisely links the dynamics of Ca2+ concentration to the observed current response.