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

1 Electroretinograms (ERGs) were recorded corneally from C57BL/6J 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 t probe (80 ≤ t probe ≤ 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 cd 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 γ, an activation term 1 ‐ exp[‐α( t ‐ t d ) 2 ]}, and a decay term exp(‐ t /τ ω ): where t d is a brief delay, τ ω is an exponential time constant, and α characterizes the acceleration of the activation term. For I test up to ∼2·57 scotopic cd s m −2 , the overall time course of A ( t ) was well described by the above equation with γ= 2·21, t d = 3·1 ms, τ ω = 132 ms and α= 2·32 × 10 −4 ms −2 . An approximate halving of α 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.