Activation and Quenching of Human Rod Phototransduction—An Electroretinographic Study

Abstract

Purpose : To produce a simplified numeric model consistent with known properties of both activation and quenching of the photoresponse in isolated rods. Routinely, an exponential of the form f=α.(1-exp(-(τ.I.(t-t_d)^s-1)/2)), with amplitude α, time-constant τ (scaled by intensity I), irreducible delay t_d introduced by the amplifier and other processes, and exponent s-1, is fit to the early period of the flash electroretinogram (ERG). Notably, s (an integer) represents the 3 integrating steps in the rod amplification cascade (rhodopsin isomerization, transducin activation, and cGMP hydrolysis). If similar kinetics describe quenching, then a difference of exponentials (DoE) with common α and t_d parameters, respective time constants for activation (A) and quenching (Q), and appropriate values of s (s_A, s_Q) should approximate the whole timecourse of the rod photoresponse.

Methods : In 24 healthy eyes of respective subjects, the amplitude (a_max) of the response to a rod-saturating (20 cd.s.m^-2) probe flash (PF), 10 ms after presentation, was taken as proportional to the maximal rod dark current. Then, the timecourse of the response to a 0.17 cd.s.m^-2 conditioning flash (CF) was determined. Next, the CF and PF were presented in tandem, separated by 10 predetermined inter-stimulus intervals from 0.01 to 1.4 s. In these double-flash ERGs, the CF response, as recorded alone, served as the baseline for measuring the amplitude (a_sat,t) of the PF response at each inter-stimulus interval (ISI). The proportion of the rod response suppressed by the CF at each ISI was taken as SF_t=1-a_sat,t/a_max. Finally, the values of SF_t in all subjects were ensemble fit to a DoE where s_A, s_Q, and t_d were shared among subjects while α, A, and Q were free to vary.

Results : The optimal values of s_A and s_Q were 3 and 2, respectively; t_d was 3.1 ms. With these parameters fixed, α was 0.79±0.10, A was 10,000±5,100 m².cd^-1.s^-3, and Q was 7.4±2.4 m².cd^-1.s^-2 (mean±SD), for respective coefficients of variance 0.13, 0.49, and 0.33. The overall r² was 0.90.

Conclusions : A method, including a DoE model with just three free parameters (α, A, Q), that robustly captures the magnitude and time-constants of the complete rod response, was produced. In contrast to activation, apparently only two integrating steps (likely inactivation of transducin and rhodopsin) quench the photoresponse.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.