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

Dynamic changes in astrocytic processes in the Syrian hamster suprachiasmat ic nucleus (SCN) have been reported with maximal process extension in the l ight phase and maximal process retraction in the dark phase of a daily ligh t:dark cycle. In the present study, we asked whether dynamic changes occur in the distribution of an astrocytic metabolic marker, glycogen phosphoryla se (GP), using a histochemical assay to reveal the distribution of both act ive and total GP, in the hamster SCN. Changes in glial acidic fibrillary pr otein (GFAP) immunoreactivity also were assessed using a relative optical d ensity measure (ROD). We observed changes in the localization and distribut ion of GP both in the SCN and in the paraventricular nucleus of the hypotha lamus (PVN) as a function of time of day. In the light phase, there were co ncentrated, large, dot-like deposits of GP throughout the SCN and PVN on an empty background. In the dark phase, diffuse, small, granular particles we re seen throughout both nuclei. Selectively, in the dark-phase SCN, these g ranular particles formed a rim of intense GP reactivity on the lateral, ven tral, posterior, and medial borders. Significantly higher levels of GP reac tivity were seen in anterior sections of the medial optic chiasm in the lig ht phase. GFAP-immunoreactive astrocytic processes had higher ROD levels in the dark phase. In conclusion, the astrocytic metabolic marker, GP, exhibi ts a significant daily variation in localization in both the SCN and the PV N that correlates with dynamic changes in the distribution of astrocytic pr ocesses in the SCN. Increased GP activity also occurs in astrocytes among o ptic fibers subjacent to the SCN during light input. (C) 2001 Wiley-Liss, I nc.