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

John R. Hetling * and David R. Pepperberg

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.

With fixed t_probe (80 t_probe 1200 ms), A(t)/A_mo was described by the saturating exponential function [1 - exp(-k_tI_test)], where k_t is a time-dependent sensitivity parameter. For t = 86 ms, a time near the peak of A(t), k₈₆ was 7·0 ± 1·2 (scotopic cd s m^-2)^-1 (mean ± s.d.; n = 4).

A(t)/A_mo data were analysed in relation to the equation below, a time-generalized form of the above exponential function in which (k₈₆I_test) is replaced by the product [k₈₆I_testu(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 , an activation term 1 - exp[-(t - t_d)²]}, and a decay term
exp(-t/):
A(t)/A_mo = 1 - exp[-k₈₆I_testu(t)]; u(t) = {1 - exp[-(t - t_d)²]}exp(-t/)
where t_d is a brief delay, is an exponential time constant, and 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 = 2·21, t_d = 3·1 ms, = 132 ms and = 2·32 × 10^-4 ms^-2. An approximate halving of improved the fit of the above equation to ERG a-wave and A(t)/A_mo data obtained at t about 0-20 ms.

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.

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