Sensitivity and kinetics of mouse rod flash responses determined in vivo from paired-flash electroretinograms

1. Electroretinograms (ERGs) were recorded corneally from. C57BL/8J mice using a paired-flash procedure in which a brief test flash at time zero was followed at time t(probe) by a bright probe flash of fixed strength, and in which the probe response amplitude was determined at time t = t(probe) + 6 ms. Probe responses obtained in a series of paired-flash trials were analysed to derive A(t), a family of amplitudes that putatively represents the massed response of the rod photoreceptors to the test flash. A central aim was to obtain a mathematical description of the normalized derived response A (t)/A(mo) as a function of I-test, the test flash strength. 2. With fixed tprobe (80 less than or equal to t(probe) less than or equal to 1200 ms), A(t)/A(mo) was described by the saturating exponential function [1 - exp(-k(t)I(test))], where k(t) is a time-dependent sensitivity parameter. For t = 86 ms, a time near the peak of A(t), k(86) was 7.0 +/- 1.2 (scotopic ed s m(-2))(-1) (mean +/- S.D.; n = 4). 3. A(t)/A(mo) data were analysed in relation to the equation below, a time-generalized form of the above exponential function in which (k(86)I(test)) is replaced by the product [k(86)I(test)u(t)], and where u(t) is independent of the test flash strength. The function u(t) was modelled as the product of a scaling factor gamma, an activation term {1 - exp[-alpha(t - t(d))(2)]}, and a decay term exp(- t/tau(omega)): A(t)A(mo) = 1 - exp[-k(86)I(test)u(t)]; u(t) = gamma{1 - exp[-alpha(t - t(d))(2)]}exp(-t/tau(omega)), where t(d) is a brief delay, tau(omega) is an exponential time constant, and a characterizes the acceleration of the activation term. For I-test up to similar to 2.57 scotopic ed s m(-2), the overall time course of A(t) was well described by the above equation with gamma = 2.21, t(d) = 3.1 ms, tau(omega) = 132 ms and alpha = 2.32 x 10(-4) ms(-2). An approximate halving of alpha improved the fit of the above equation to ERG a-wave and A(t)/A(mo) data obtained at t about 0-20 ms. 4. Kinetic and sensitivity properties of A(t) suggest that it approximates the in vivo massed photocurrent response of the rods to a test flash, and imply that u(t) in the above equation is the approximate kinetic description of a unit, i.e. single photon, response.