Light adaptation characteristics of melanopsin.

Following photopigment bleaching, the rhodopsin and cone-opsins show a characteristic exponential regeneration in the dark with a photocycle dependent on the retinal pigment epithelium. Melanopsin pigment regeneration in animal models requires different pathways to rods and cones. To quantify melanopsin-mediated light adaptation in humans, we first estimated its photopigment regeneration kinetics through the photo-bleach recovery of the intrinsic melanopsin pupil light response (PLR). An intense broadband light (~120,000 Td) bleached 43% of melanopsin compared to 86% of the cone-opsins. Recovery from a 43% bleach was 3.4X slower for the melanopsin than cone-opsin. Post-bleach melanopsin regeneration followed an exponential growth with a 2.5 min time-constant (τ) that required 11.2 min for complete recovery; the half-bleaching level (Ip) was ~ 4.47 log melanopic Td (16.10 log melanopsin effective photons.cm-2.s-1; 8.25 log photoisomerisations.photoreceptor-1.s-1). The effect on the cone-directed PLR of the level of the melanopsin excitation during continuous light adaptation was then determined. We observed that cone-directed pupil constriction amplitudes increased by ~ 10% when adapting lights had a higher melanopic excitation but the same mean photometric luminance. Our findings suggest that melanopsin light adaptation enhances cone signalling along the non-visual retina-brain axis. Parameters τ and Ip will allow estimation of the level of melanopsin bleaching in any light units; the data have implications for quantifying the relative contributions of putative melanopsin pathways to regulate the post-bleach photopigment regeneration and adaptation.

Ocular morbidity among street children in Kathmandu Valley.

PURPOSE: Prevalence of ocular morbidity among street children is largely unknown. The present study was carried out to determine the prevalence of ocular morbidity among street children in the Kathmandu Valley. METHODS: A cross-sectional study consisting of an eye examination program among 569 street children aged younger than 18 years was conducted from March 2013 to February 2014. Children were included from 11 safe houses of 6 non-governmental organizations and an independent eye camp. Eye examination included visual acuity testing, anterior segment and posterior segment examination, retinoscopy and refraction, cover test, convergence, accommodation and color vision tests. Chi-square test was used to analyze the association of ocular morbidity with age, sex and living conditions. RESULTS: The majority of children (43.8%) were in the age group of 12-15 years, and the male to female ratio was 3.9:1. Uncorrected and best-corrected visual acuity ≥6/9 in at least one eye was found in 89.8% and 98.4% of children, respectively. Total ocular morbidity was observed at 31.6%. The most common types of ocular morbidity were conjunctivitis (11.0%) and refractive error (11.6%). Ocular morbidity was more common in children over 15 years of age (40.9%; p < 0.01; odds ratio 1.8). CONCLUSIONS: Ocular infection and refractive error represent the most common ocular morbidities in street children in the Kathmandu Valley.