Cone Cycle in the Mammalian Retina

Abstract

Purpose:: A novel retinoid cycle has recently been characterized in the cone-dominated avian retina. Light exposure induces an accumulation of 11-cis retinyl esters in the retina and all-trans retinyl esters in the RPE, while subsequent dark adaptation returns esters to baseline level. This change in the levels of retinyl ester takes place concurrently with an inverse change in the level of 11-cis retinal. The purpose of this study is to demonstrate a similar cone cycle in the cone-dominated mammalian retina.

Methods:: Experiments were carried out with animals from TLS Research and from the University of Texas Permian Basin. After overnight dark adaptation, animals were exposed to 2000 Lux (from cool white fluorescent tubes) for different durations. Retina and RPE were separated before extraction and analysis of retinyl esters and retinal by HPLC.

Results:: Dark-adapted retina contained 0.08 nmol 11-cis and 0.06 nmol all-trans retinyl ester per retina, while dark adapted RPE contained 0.08 nmol 11-cis and 0.08 nmol all-trans retinyl ester per RPE. Light exposure for 6 hrs resulted in a 2-fold increase in both 11-cis (to 0.17nmol/retina) and all-trans retinyl esters (to 0.12nmol/retina) in the retina as well as in the RPE (11-cis retinyl ester: 0.14 nmol/PRE and all-trans 0.18 nmol/RPE). Subsequent dark adaptation of 11-cis and all-trans retinyl ester return to baseline levels within 5 min. Concurrent to the changes of retinyl ester, corresponding inverse changes in 11-cis retinal were observed. Dark-adapted retina contained 0.15 nmol 11-cis retinal per retina and this level decreased 2 fold to 0.08 nmol/retina in 5 min. Subsequent dark adaptation returns the level of 11-cis retinal to 0.19 nmol/retina in 5 min.

Conclusions:: Similar to the cone-dominated avian retina, the cone-dominated mammalian retina responded to light and dark-adaptation by reciprocal changes in the levels of 11-cis retinal and retinyl esters. Furthermore, light exposure also resulted in the accumulation of retinyl esters in the RPE of the mammalian eye. Moreover, there is a 6 hr delay in the light-induced accumulation of retinyl ester in the mammalian retina, suggesting that the net rate of ester accumulation may be time-dependent. Overall, our data provide strong evidence that the cone-dominated mammalian retina also possesses a light-driven cone visual cycle, similar to that observed in the cone-dominated avian retina (Trevino, et al. 2005). Details of the mammalian cone cycle remain to be studied.

Keywords: photoreceptors • retina • retinoids/retinoid binding proteins