Abstract
Purpose::
To determine the time course of the light-evoked decrease in photoreceptor metabolism in the parafoveal monkey retina.
Methods::
Transient PO2 changes at the onset of light were recorded by using double-barreled oxygen electrodes in the parafovea in five monkeys (Rhesus and Cynomolgus) anesthetized with 70% N2O/30% O2 and 1-2% isoflurane. To measure the light-induced PO2 changes, the oxygen electrode was positioned in the inner segment layer, where the PO2 reached the minimum under dark adaptation. Baseline PO2 was recorded for 30 seconds in darkness (Pmin) before a light stimulus was given, and a transient PO2 response was recorded for 2 minutes at different illuminations of diffuse white light (0 to 5 log unit attenuation from the maximum). Pmax and a time constant Tau were obtained by fitting a single exponential equation to the PO2 responses: PO2 (t)= (Pmax-Pmin)(1-exp((t-td)/Tau)) + Pmin, where td is a delay. The maximum light evoked PO2 change in each response was defined as ΔPO2 = Pmax-Pmin.
Results::
At the onset of light, the PO2 increased at all illuminations in all five monkeys. The ΔPO2 generally increased with increasing illumination over 3 to 4 log units, but decreased slightly in 3 monkeys at the highest illumination. The average maximum ΔPO2 was 17±11 mm Hg (n=5). The time constant Tau tended to decrease slightly with increasing illumination, but this was not significant. The time constant was not normally distributed, and the median value of Tau was 26.84 sec.
Conclusions::
A first order exponential provided good fits to the transient PO2 responses in the inner segment layer in the parafoveal monkey retina. As found previously in cat and toad retina, light decreases photoreceptor oxidative metabolism by an amount that depends on illumination. Because O2 diffusion is relatively fast over the short distance from the choroid to the inner segments, the time constant of the PO2 change is the same as the time constant of the underlying change in oxidative metabolism. However, the illumination appears to have little effect on the time constant.
Keywords: retina • electrophysiology: non-clinical