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D. R. Copenhagen, V. Wu, T. Porco, R. C. Renteria, R. Van Gelder, J. Johnson; Melanopsin-dependent Light Avoidance in Neonatal Mice. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3053.
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Functional rod and cone visual signaling begins approximately 11 days after birth in mice. Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) are present at birth. At this age these neurons are photosensitive and light can induce early gene expression in the suprachiasmatic nucleus, a target area for ipRGCs. While ipRGCs can control pupillary responses and circadian photoentrainment in adults, no light-induced physiological function is established for these neurons in younger neonates. We tested the hypothesis that ipRGCs mediate light-induced behavior in mice before the onset of rod and cone visual signaling.
We tested neonatal mice between the ages of P6 and P9 for phototactic behavior. An animal’s position in a cylindrical chamber was recorded under infrared illumination. Orientation of the pup as a function of time was quantified. Behavior in darkness was compared to that under conditions when a bright light was directed initially towards the face of the animal. Wild type and melanopsin knockout (opn4 -/-) mice were tested.
Pups turned away from the light, a behavior termed negative phototaxis. Quantitatively, pups spent significantly less total time faced in their original orientation when stimulated with light than in darkness. The latency to the first turn away from their original position was also significantly faster when mice were stimulated with light. No phototactic behavior could be discerned in melanopsin knockout mice.
These findings indicate that light avoidance is evident in neonatal pups before the onset of visual signaling from rods and cones. Furthermore, the absence of negative phototaxis in melanopsin knockout mice argues strongly that ipRGCs mediate this behavior.
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