Early Function of Specialized Neurons Marks ‘First Light’ in Retinal Maturation

June 16, 2005

Researchers have discovered that a set of light-responsive retinal cells that form connections to the circadian clock are functional very early in development, from the day of birth. Although the cells are sensitive to light, they do not participate in image formation, a process that matures later on.

The work has been reported in the journal Current Biology by Sumathi Sekaran, of Imperial College London, and colleagues there and at John Hopkins University and the University of Manchester.

The visual system is known to be composed of the classical image-forming pathway, which involves the function of rod and cone photoreceptors, as well as the more recently discovered non-image-forming pathway, which involves specialized neurons called intrinsically photoreceptive retinal ganglion cells (ipRGCs). Although extensive research has characterized the delayed functional maturation of rod and cone photoreception, information pertaining to the development of the ipRGCs has been lacking. It was known, however, that a photopigment present in mature ipRGCs, retinal melanopsin, is expressed long before the classical rod and cone photopigments.

The new research directly studied the functional development of the ipRGCs in mice via a range of approaches, including the examination of melanopsin expression, the physiological recording of ganglion-cell light responses, and the measuring of functional outputs of these cells to higher brain regions. The researchers found that, quite remarkably, the melanopsin-expressing ganglion cells are present in abundance and act as functional photoreceptors from the day of birth, when it has been widely assumed the mouse retina lacks photodetection. At the time of birth, a significant percentage of cells in the retinal ganglion-cell layer express melanopsin and respond to light.

At this early age, these cells provide input conveying light conditions to the suprachiasmatic nucleus, the site of the central circadian pacemaker. The findings show that the non-image-forming pathway is functional at birth, long before development of photosensitivity of the mainstream image-forming visual pathway. It also has implications to the effect of light on early retinal development.

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