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: To investigate the photoreceptors regulating the clock in more detail, we turned to a different assay of circadian photoentrainment. When exposed to constant light, the period (τ) of mouse circadian rhythms lengthens according to Aschoff's rule (Daan and Pittendrigh, 1976). To determine the photoreceptors driving this response, we described its irradiance dependence in Opn1mwR mice exposed to either continuous mid- (498 nm) or long- (644 nm) wavelength light. Both wavelengths effectively lengthened τ (Figure 4). However, there was a marked decrease in sensitivity to the longer wavelength. Matching the irradiance response relationships for the relative sensitivity of red cones revealed that there was no irradiance at which τ was defined by cone photoreception (Figure 4D; F-test statistic; slopes at 498 nm and 644 nm similar, p > 0.05; intercepts significantly different [p < 0.001] when corrected for red cone spectral sensitivity). The curves were more equivalent when normalized for melanopsin sensitivity, but responses were clearly enhanced at 650 nm compared to that predicted for melanopsin, especially at lower irradiances (Figure 4C; F-test comparing intercepts, p < 0.001). By contrast, the curves became superimposed when corrected for rod sensitivity (Figure 4E; F-test comparing intercepts, p > 0.05), confirming that rod activity dominates this assay of photoentrainment, at least at low-moderate irradiances. The threshold for τ lengthening was around that reported for scotopic vision in mice (Nathan et al., 2006; Sampath et al., 2005), suggesting that mRGCs receive input from the highest-sensitivity rod pathways.
Affiliation: Faculty of Life Sciences, AV Hill Building, University of Manchester, Manchester M13 9PT, UK.