A Simplified Model of Activation and Deactivation of Human Rod Phototransduction—An Electroretinographic Study

Abstract

Purpose To test the hypothesis that a simple model having properties consistent with activation and deactivation in the rod approximates the whole time course of the photoresponse. Methods Routinely, an exponential of the form f = α ·(1 – exp(–( τ ·( t – t eff ) s –1 ))), with amplitude α , rate constant τ (often scaled by intensity), irreducible delay t eff , and time exponent s –1, is fit to the early period of the flash electroretinogram. Notably, s (an integer) represents the three integrating stages in the rod amplification cascade (rhodopsin isomerization, transducin activation, and cGMP hydrolysis). The time course of the photoresponse to a 0.17 cd·s·m −2 conditioning flash (CF) was determined in 21 healthy eyes by presenting the CF plus a bright probe flash (PF) in tandem, separated by interstimulus intervals (ISIs) of 0.01 to 1.4 seconds, and calculating the proportion of the PF a -wave suppressed by the CF at each ISI. To test if similar kinetics describe deactivation, difference of exponential (DoE) functions with common α and t eff parameters, respective rate constants for the initiation ( I ) and quenching ( Q ) phases of the response, and specified values of s ( s I , s Q ), were compared to the photoresponse time course. Results As hypothesized, the optimal values of s I and s Q were 3 and 2, respectively. Mean ± SD α was 0.80 ± 0.066, I was 7700 ± 2400 m 2 ·cd –1 ·s –3 , and Q was 1.4 ± 0.47 s –1 . Overall, r 2 was 0.93. Conclusions A method, including a DoE model with just three free parameters ( α , I , Q ), that robustly captures the magnitude and time-constants of the complete rod response, was produced. Only two steps integrate to quench the rod photoresponse.