Abstract
Purpose :
Histidine has been shown to double the peak amplitude of aspartate-isolated a-wave responses in dark adapted skate retinas. This study was designed to examine effects of histidine on the ability of photoreceptor incremental sensitivity to remain Weber-Fechner-like as adapting background intensities increase several log units above the intensity at which dark-adapted photoreceptor responses saturate.
Methods :
The skate eyecup 50mM aspartate-isolated a-wave remains Weber-Fechner-like as adapting intensities increase over 6 log units, providing an all-rod retina ideally suited to study effects of adding 100µM L-histidine. Electroretinogram (ERG) a-wave responses to 200msec flashes were recorded with a DC-coupled differential preamplifier input to an EPC9 patch clamp amplifier and recorded using HEKA PatchMaster software. The incremental sensitivity threshold criterion was 3µV.
Results :
Despite histidine's doubling the peak amplitude of the aspartate-isolated a-wave (see red vs. green curves below), its incremental sensitivity curve remained Weber-Fechner-like. Its slope changed only slightly to 0.91 in histidine+aspartate vs. 1.0 in aspartate Ringer alone observed in the classic studies using this preparation conducted by Dowling and Ripps in 1972. Importantly, however, the curve shifted left by 1.5 log units with histidine in the superfusate. While this indicated increased incremental sensitivity to dim light flashes in dark-adapted retina, it also meant a decrease of 1.3 log in incremental sensitivity in the presence of steady background light. Therefore, incremental sensitivity in presence of background illumination was 20 times greater in aspartate alone than in histidine+aspartate.
Conclusions :
Considering these results in terms of the decrement concept of Dowling and Ripps leads to the conclusion that there is a dark decrement of 50% in rod photoreceptor response revealed by addition of histidine. It appears that histidine is revealing process(es) which reduce dark sensitivity of photoreceptors to enhance incremental sensitivity under conditions of ambient illumination common to the natural environment. Knowledge of the histidine-sensitive process(es) involved may prove useful in clinical diagnosis and treatment. It is also likely to provide insight into such anomalies of response dynamics as surround enhancement and delay in recovery of incremental sensitivity after turning on modulated spots.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.