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-td)s-1)/2)), with amplitude α, time-constant τ (scaled by intensity I), irreducible delay td 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 td parameters, respective time constants for activation (A) and quenching (Q), and appropriate values of s (sA, sQ) should approximate the whole timecourse of the rod photoresponse.
Methods :
In 24 healthy eyes of respective subjects, the amplitude (amax) 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 (asat,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 SFt=1-asat,t/amax. Finally, the values of SFt in all subjects were ensemble fit to a DoE where sA, sQ, and td were shared among subjects while α, A, and Q were free to vary.
Results :
The optimal values of sA and sQ were 3 and 2, respectively; td 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.