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: The same (M1) class of mRGC is responsible for routing rod and cone input to both the circadian clock and that portion of the pretectum responsible for the PLR (the oliviary pretectal nuclei shell; Baver et al., 2008; Güler et al., 2008). To establish whether cones contribute to circadian photoentrainment, we set out to determine whether that response showed an equivalent red shift in sensitivity in Opn1mwR mice. To this end, we constructed irradiance response curves for phase shifts in the free-running locomotor activity rhythm in Opn1mwR mice elicited by 15 min 500 or 650 nm stimuli presented in the early subjective night (CT16). As expected, 500 nm stimuli induced marked phase delays whose magnitude was irradiance dependent (Figure 3). Strikingly however, Opn1mwR mice showed very poor sensitivity to 650 nm. Significant phase delays were elicited only by bright (>1013 photons/cm2/s) stimuli, representing an increase in threshold irradiance of at least 1000× compared to 500 nm. This is much greater than that predicted for a response driven by red cones (Figure 3). Surprisingly, therefore, it seems that cones do not make a significant contribution to this assay of circadian photoentrainment even when presented with stimuli that lie within the photopic range.
Affiliation: Faculty of Life Sciences, AV Hill Building, University of Manchester, Manchester M13 9PT, UK.