Abstract
Background: Melanopsin-expressing intrinsically photosensitive Retinal Ganglion Cells (ipRGCs) signal non-imaging forming effects of environmental light for circadian phoentrainment, the pupil light reflex, and mood regulation. In seasonal affective disorder, ipRGC dysfunction is thought to cause abberant transmission of the external illumination for photoentrainment. It is not known if patients with non-seasonal depression have abnormal melanospin mediated signaling and/or irregular environmental light exposure.Methods: Twenty-one adults who live in a sub-tropical region, including eight patients with non-seasonal depression and thirteen age-matched healthy controls were recruited. The Mini International Neuropsychiatry Interview diagnosed the presence of a major depressive disorder. Light exposure was determined using actigraphy over a 2 week period. The melanopsin mediated post-illumination pupil response (PIPR) and outer retinal inputs to ipRGCs (transient pupil response and maximum pupil constriction amplitude) were measured in response to 1 s, short and long wavelength light with high and low melanopsin excitation.Results: The mean daylight exposure as a function of clock hours and total light exposure duration (mins) to illumination levels commonly recommended for depression therapy were not significantly different between groups. Out of 84 pupil measurements (42 each in the depression and control groups), the melanopsin-mediated PIPR amplitude, transient pupil response, and pupil constriction amplitude were not significantly different between groups.Conclusions: This report provides initial evidence of normal melanopsin function and environmental light exposures in patients with pre-dominately mid and moderate non-seasonal depression in a subtropical location in the southern hemisphere.
Highlights
The discovery of projections of melanopsin-expressing intrinsically photosensitive Retinal Ganglion Cells to brain mood centers [1] has redefined understanding of light-mediated mood regulation [2, 3]
In major depressive disorder (MDD) patients and healthy controls in the northern hemisphere, the post-illumination pupil response (PIPR) amplitude is less pronounced in winter compared to the summer months with longer daylight hours [14] and irrespective of season, the transient pupil response to dim light is impaired in MDD, which may reflect dysfunctional outer retinal inputs to intrinsically photosensitive Retinal Ganglion Cells (ipRGCs) [14]
This study objectively determined the mean daylight and daily hourly light exposure and duration of exposure to illumination levels recommended for light therapy in depression [15] in a group of patients with non-seasonal depression and healthy participants, and quantified melanopsin function with an optimized pupillometric paradigm that is robust to the presence of subtle melanopsin defects at early stages of retinal disease [16, 17]
Summary
The discovery of projections of melanopsin-expressing intrinsically photosensitive Retinal Ganglion Cells (ipRGCs) to brain mood centers [1] has redefined understanding of light-mediated mood regulation [2, 3]. These inner retinal cells have major roles in non-image-forming functions including entrainment of the body clock to a ∼24 h day-night circadian rhythm [4], regulating the Melanopsin, Light, and Depression pupil light response [5, 6] as well as in image forming brightness perception, temporal, and color processing in humans [7,8,9]. IpRGC dysfunction is thought to cause abberant transmission of the external illumination for photoentrainment It is not known if patients with non-seasonal depression have abnormal melanospin mediated signaling and/or irregular environmental light exposure
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