Distinct contributions of rod, cone, and melanopsin photoreceptors to encoding irradiance.
Bottom Line: These photoreceptors define circadian responses at very dim "scotopic" light levels but also at irradiances at which pattern vision relies heavily on cones.By contrast, cone input to irradiance responses dissipates following light adaptation to the extent that these receptors make a very limited contribution to circadian and pupillary light responses under these conditions.Our data provide new insight into retinal circuitry upstream of mRGCs and optimal stimuli for eliciting irradiance responses.
Affiliation: Faculty of Life Sciences, AV Hill Building, University of Manchester, Manchester M13 9PT, UK.Show MeSH
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Mentions: We first explored the effects of prior treatment with 644 nm on subsequent cone-dependent pupillary responses in Opn1mwR mice. We found that 5 min of exposure to 1013 photons/cm2/s of 644 nm light greatly reduced pupil responses to a subsequent 650 nm test stimulus of equivalent irradiance (Figure 5A). Higher test irradiances drove larger constrictions, but the response magnitude was always substantially smaller than that elicited by an equivalent stimulus under dark-adapted conditions (Figure 5A). More extensive light exposure (15 min 1.2 × 1015 photons/cm2/s 644 nm) rendered the pupil entirely refractory to irradiances (1014 photons/cm2/s 650 nm) that drove large constrictions when dark-adapted (Figure 5B). To trace the kinetics of this effect, we assessed responses to the 650 nm test stimulus after 15 s, 30 s, 1 min, and 15 min exposure to 644 nm at 1.2 × 1015 photons/cm2/s. A significant decrease in response amplitude was observed at all exposure times bar the shortest, but only after 15 min were the animals entirely refractory to the 650 nm test pulse (Figure 5C). We traced dark adaptation as defined by the recovery of responses to the test stimulus following exposure to 15 min of 1.2 × 1015 photons/cm2/s 644 nm. This was also slow, with full recovery taking up to 1 hr (Figure 5D). Importantly, this did not reflect a general decrease in pupillary responsiveness because constrictions to 500 nm test stimuli survived even the brightest 644 nm pretreatment (Figure 5B). These data suggest that prior light exposure over timescales ranging from tens of seconds to tens of minutes reduces the ability of cones to regulate pupil size.
Affiliation: Faculty of Life Sciences, AV Hill Building, University of Manchester, Manchester M13 9PT, UK.