Before the Rods and Cones

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The rods and cones in the mouse retina, which are necessary for image formation, become responsive to light on the tenth day after birth (P10). However, another class of photoreceptors that express the photopigment melanopsin and can detect brightness--the intrinsically photosensitive retinal ganglion cells (ipRGCs)--has recently been identified, and the time course of their development is unknown. By assaying the responses of retinae loaded with a fluorescent calcium indicator, Sekaran et al. determined that about 5.4% of cells in the ganglion-cell layer responded to 470 nm light at P4 to P5, whereas about 13.7% responded at P0 to P1. The response to light was not affected by pharmacological blockade of glutamate receptors but was absent in retinae from mice that lacked melanopsin. The fraction of light-responsive cells at birth and at P4 to P5 was greater than that previously found in adults, and these differences in the fraction of responsive cells did not depend on coupling through gap junctions. Immunohistochemical analysis indicated that the density of melanopsin-expressing cells was lower at P14 and in adults than earlier in development, peaking at about P4 to P5. ipRGCs project to the suprachiasmatic nucleus of the hypothalamus (SCN, the location of the master circadian clock), and immunohistochemical analysis of light-induced Fos expression in the SCN of wild-type mice and mice lacking melanopsin indicated that functional connections from ipRGCs to the SCN were present at P0. Thus, the authors conclude that the ability to detect light substantially predates the ability to form images. S. Sekaran, D. Lupi, S. L. Jones, C. J. Sheely, S. Hattar, K.-W. Yau, R. J. Lucas, R. G. Foster, M. W. Hankins, Melanopsin-dependent photoreception provides earliest light detection in the mammalian retina. Current Biol. 15 , 1099-1107 (2005). [PubMed]