Daily variation in the distribution of glycogen phosphorylase in the suprachiasmatic nucleus of Syrian hamsters

Dynamic changes in astrocytic processes in the Syrian hamster suprachiasmatic nucleus (SCN) have been reported with maximal process extension in the light phase and maximal process retraction in the dark phase of a daily light:dark cycle. In the present study, we asked whether dynamic changes occur in the distribution of an astrocytic metabolic marker, glycogen phosphorylase (GP), using a histochemical assay to reveal the distribution of both active and total GP, in the hamster SCN. Changes in glial acidic fibrillary protein (GFAP) immunoreactivity also were assessed using a relative optical density measure (ROD). We observed changes in the localization and distribution of GP both in the SCN and in the paraventricular nucleus of the hypothalamus (PVN) as a function of time of day. In the light phase, there were concentrated, large, dot‐like deposits of GP throughout the SCN and PVN on an empty background. In the dark phase, diffuse, small, granular particles were seen throughout both nuclei. Selectively, in the dark‐phase SCN, these granular particles formed a rim of intense GP reactivity on the lateral, ventral, posterior, and medial borders. Significantly higher levels of GP reactivity were seen in anterior sections of the medial optic chiasm in the light phase. GFAP‐immunoreactive astrocytic processes had higher ROD levels in the dark phase. In conclusion, the astrocytic metabolic marker, GP, exhibits a significant daily variation in localization in both the SCN and the PVN that correlates with dynamic changes in the distribution of astrocytic processes in the SCN. Increased GP activity also occurs in astrocytes among optic fibers subjacent to the SCN during light input. J. Comp. Neurol. 435:249–258, 2001. © 2001 Wiley‐Liss, Inc.