Enhancement of Melanopsin-Mediated Photoreception by Cis- and Trans- Retinal

Abstract

Purpose: : To determine the chromophore preference of melanopsin. Melanopsin, the photopigment in instrinsically photosensitive retinal ganglion cells (ipRGCs), is proposed to be a bistable pigment. Bistable pigments can regenerate chromophore autonomously. If melanopsin is a bistable pigment then it should be capable of generating cis retinal from trans retinal and continue to respond to light. If this is the case then no significant difference should be seen in the preference for cis vs. all-trans retinal in melanopsin’s light response.

Methods: : Retinas were isolated from P8-P10 mice. Isolated retina were placed ganglion side down on a multi electrode array (MEA) for extracellular recording of activity. Retina were perfused with carbogen-bubbled Ames buffer with nicotinic and glutamatergic blockers (10 nm epibatidine, 50 [[Unsupported Character - Symbol Font µ]]m CNQX, 50 [[Unsupported Character - Symbol Font µ]]m AP-4). Recordings were made from ipRGCs under illumination with a xenon light source filtered through a 480 nm bandpass filter. Baseline irradiance responses were determined at 3.98 x 10¹³, 3.98 x 10¹², and 3.98 x 10¹¹ photon cm^-2 s^-1. Retinas were then bathed in either 9-cis retinal or all-trans-retinal and irradiance responses reassessed.

Results: : Both 9-cis retinal and all-trans retinal increased baseline irradiance responses. Responses of ipRGCs were divided into low-sensitivity and high sensitivity responses. After treatment with 9-cis retinal and under illumination with 3.98 x 10¹³ photon cm^-2 s^-1, high-sensitivity responses increased 1.5 while the low-sensitivity group response increased by 5 fold. After treatment with 9-cis retinal and under illumination with 3.98 x 10¹² photon cm^-2 s^-1, high-sensitivity responses increased 1.5 while the low-sensitivity group response increased by 10 fold. In comparison treatment with all-trans retinal and illumination with 3.98 x 10¹³ photon cm^-2 s^-1, caused an increases of 1.5 fold in the high-sensitivity responses and an increase of 6 fold in the low-sensitivity group. At lower illumination 3.98 x 10¹² photon cm^-2 s^-1, treatment with all-trans increased the high-sensitivity responses increased 6 fold while the low-sensitivity group response increased by 25 fold

Conclusions: : Both cis- and trans-retinal augment the light response of ipRGCs. This is consistent either with melanopsin being a bistable pigment, or with the presence of a retinal isomerase in the sensory retina.

Keywords: retina: proximal (bipolar, amacrine, and ganglion cells) • circadian rhythms • electrophysiology: non-clinical