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Annette E Allen, Riccardo Storchi, Timothy Brown, Robert J Lucas; Influences of melanopsin on cone visual pathways. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5767.
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© ARVO (1962-2015); The Authors (2016-present)
Intrinsically photosensitive retinal ganglion cells (ipRGCs), which express the photopigment melanopsin, are known to efficiently encode ambient light levels in order to drive many neurophysiological responses to light, such as modulating pupil size and synchronising circadian clocks to the light-dark cycle. In addition to their central projections, there is evidence that ipRGCs signal to neighbouring cells within the retina via dendritic glutamate release and gap junctions. ipRGCs thus have both the sensory capabilities and intra-retinal connections to provide an independent mechanism for adjusting retinal circuits according to ambient illumination.
To determine whether ipRGCs do indeed perform such a function, we presented cone-activating stimuli to the mouse eye against backgrounds that were equiluminant for cones but differed in their relative activation of melanopsin. Responses were recorded concurrently at two physiological levels; in the retina, via the electroretinogram (ERG), and centrally, by measuring the light-evoked change in firing rate within the dorsal Lateral Geniculate Nucleus (dLGN).
A direct comparison of the ERG response under different backgrounds revealed that the amplitude of the cone-driven ERG was modulated according to the level of melanopsin activation. Changes were also detected downstream in the dLGN, where the amplitude and timing of cone-driven responses were also significantly affected by melanopsin activity. These differences were absent in mice lacking melanopsin.
These data indicate that the irradiance-coding functions of ipRGCs are employed to adjust the retinal circuitry according to ambient light levels and that this impacts the quality of visual information available to the brain.
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