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M. H. Do, S. H. Kang, H. Zhong, H.-W. Liao, D. E. Bergles, K.-W. Yau; Melanopsin Signaling: Large Single-Photon Response and Sparse Pigment. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5834.
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The intrinsically photosensitive retinal ganglion cells (ipRGCs; Berson et al., 2002) in the mammalian eye express the photopigment, melanopsin (Provencio et al., 1999), and drive non-image-forming visual functions such as circadian photoentrainment and the pupillary light reflex (reviewed by Berson, 2007). Consistent with the role of melanopsin in these behaviours, the ipRGCs are much less sensitive to light than rods and cones (Berson et al., 2002; Dacey et al., 2005; Tu et al., 2005). We seek to address the factors underlying this low sensitivity.
We recorded from mouse ipRGCs in the flat-mount retina or after acute dissociation. Flash responses were recorded under voltage clamp in the perforated-patch configuration. The response to a single, photoactivated melanopsin molecule (single-photon response) was obtained by fluctuation analysis and by direct measurement, in order to estimate the gain of phototransduction. From the incident light intensity required to elicit one single-photon response, the effective density of melanopsin on the plasma membrane was calculated.
Despite their overall low sensitivity, the single-photon response of ipRGCs (~1.5 pA at 35° C) was 3-fold larger than that of rods and 100-fold larger than that of cones. This unitary response was extremely slow, with a highly asymmetrical waveform (fast rise and slow decay) describable by the convolution of two single-exponential decays. This waveform is much simpler than the rod response, indicating fundamental differences in the transduction mechanism. To trigger one single-photon response, ~ 5 × 105 photons µm-2 at 480 nm ( max for melanopsin and ipRGCs) were required, giving an effective melanopsin density on the plasma membrane of ~5 µm-2, or 10,000-fold lower than that of visual pigments on the disc membranes of rods and cones.
The predominant factor for low ipRGC sensitivity appears to be a very low probability of photon capture. A low pigment density helps prevent attenuation of light reaching the rods and cones situated behind the ipRGCs. At the same time, the effect of absorbed light is increased by the large size of the single-photon response, and by the temporal summation afforded by its slow kinetics.
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