Modifications of retinal afferent activity induce changes in astroglial plasticity in the hamster circadian clock

The circadian clock, located in the suprachiasmatic nucleus (SCN) of the hy pothalamus in mammals, exhibits astroglial plasticity indicated by GFAP exp ression over the 24-h period. In this study, we evaluated the role of neuro nal retinal input in the observed changes. Modifications of retinal input, either by rearing animals under darkness (DD) or under constant light (LL), or by suppressing afferent input (bilateral enucleation), induced drastic changes in astroglial plasticity. In enucleated animals, a dramatic decreas e in GFAP expression was evident in the area of the SCN deprived of retinal projections, whereas persistence of a rhythmic variation was in those area s still exhibiting GFAP expression. By contrast, no changes in astrocytic p lasticity were detected in hamsters maintained under LL. These data suggest two fundamental roles for astrocytes within the SCN: (1) to regulate and m ediate glutamate released by retinal terminals throughout the neuronal netw ork to facilitate photic signal transmission; (2) to contribute to synchron ization between suprachiasmatic neurons. GLIA 34:88-100, 2001. (C) 2001 Wil ey-Liss, Inc.