The role of light signaling on astrocytic morphological plasticity in the adult male rat suprachiasmatic nucleus

The body’s master clock lies at the base of the hypothalamus immediately above the optic chiasm. Because of the intimate connection to the optic chiasm, this hypothalamic nucleus was named the suprachiasmatic nucleus (SCN). Environmental light signaling conveys time of day and seasonal changes to the SCN via the retinohypothalamic tract (RHT). The SCN then relays this information to the brain and the rest of the body through synaptic signaling and indirectly through circadian regulation of hormonal signaling. The SCN is unique in that in the absence of external signaling, circadian rhythms will persist. This is accomplished through a transcription-translation feedback loop consisting of both positive and negative transcription factors. The interactions of the players within this loop create a near 24 hour rhythm. Although research within the SCN has focused primarily on neuronal signaling astrocytes comprise nearly a third of the total number of cells within the nucleus based on stereological analysis. Moreover, the astrocyte cytoskeletal marker, glial fibrillary acidic protein (GFAP), is expressed in much higher levels compared to other local hypothalamic regions containing neuronal fibers. GFAP allows for the rough estimation of the overall astrocyte cell shape and despite a lack of in vivo polymerization dynamics, in vitro GFAP filaments have been shown to be dynamically regulated by phosphorylation by known kinases. Additionally, the synaptic signals encoding light information, glutamate and pituitary adenylate cyclase activating peptide (PACAP), have been shown to bind to receptors that activate kinases responsible for GFAP phosphorylation. Based on this work, we hypothesized that the RHT regulates astrocytic cytoskeletal dynamics within the SCN of the male rat. To show this we established that GFAP immunofluorescence is significantly different between early day and early night within the SCN. We then showed that this observable difference is likely due to a shift in GFAP polymerization state from filaments into soluble monomers. Second, we clearly show that this polymerization shift is regulated by the optic nerve and not a circadian phenomenon. We further establish that long term enucleation decreases the overall GFAP levels relative to other local hypothalamic regions, suggesting that the higher levels of GFAP within the SCN is regulated by the optic nerve. Lastly, in order to establish a model system to study effects RHT signals have on SCN astrocytes we characterized the polymerization state of GFAP within the brain slice. Moreover, we studied the effects of glutamate and PACAP on the brain slice. In conclusion, we have determined that signals from the RHT drive the observed levels of GFAP as well as the polymerization state of the GFAP cytoskeleton in the adult male rat SCN