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Circadian rhythms in mammals are driven by a clock located in the suprachiasmatic nucleus of the hypothalamus (SCN). The transmission of information about the photic environment to the circadian clock involves a complex array of neurotransmitters, receptors, and second messenger systems. Light pulses given during early subjective night cause phase delays of circadian rhythms of behavior in mice and hamsters. Identification of novel light induced genes in the SCN of hamsters and mice was done using a combination of laser capture microscopy and expression microarray analysis. While the hamsters yielded no novel genes, a total of 12 genes were identified in mice as significantly upregulated, with 4 novel light induced genes, Egr2, Pim3, Gadd45b, and Tiparp. Next, comparative genomic analysis was performed to ask whether all of the light-inducible genes in the SCN contained conserved CRE elements in their promoters. The genes identified as being light-induced share the common feature of CRE elements within their promoter regions that are conserved across multiple mammalian orders. The time course of light-induced genes was examined to determine whether all genes followed the same expression pattern. Three patterns of mRNA expression were observed; (1) high induction at the first time point and decrease to baseline levels by the last time point, as exemplified by Fos, (2) low induction of mRNA at the first time point, followed by an increase in expression up to the last time point as seen in Per2, and (3) quick induction of expression at the first time point that remains elevated throughout the course of the study, as seen in Rrad. Due to the high light-induced expression level of Rrad two hours after the end of photic stimulation, an attempt to localize Rrad was made using in situ hybridization and the mRNA was sequenced. A meta-analysis comparing previously published microarray studies in the SCN of mice and rats was performed to compare neurotransmitter and neuropeptide systems in the SCN. Taken together, these data expand our current knowledge of the transcriptome of the SCN and its response to photic stimulation.

The suprachiasmatic nucleus of the hypothalamus (SCN) contains a circadian pacemaker that drives daily physiological and behavioral rhythms including reproductive cycles. The exact mechanisms by which gonadal hormones regulate the SCN are not completely understood, though there are several hypotheses about possible pathways linking circadian rhythms and the estrous cycle. This thesis examines these links by asking whether estrous cycles alter the response of the circadian clock to photic input, whether there are estrogen receptors expressed in the hamster SCN and if there are sex differences in expression, and whether acute administration of estradiol into the SCN region can alter circadian phase. CONCLUSIONS: The results generated for this thesis indicate that locomotor activity rhythms vary across the estrous in Syrian hamsters and that the response of the circadian clock to light varies by sex and by estrous cycle phase. Estrogen receptor (ER)-β messenger RNA was detected in the SCN using laser capture microdissection and real-time PCR, and this is the first time this combination of techniques was successfully applied in hamsters. In addition, despite the sex differences in behavior there were no differences in the expression of ER-β between males and females. Finally, estradiol did not shift rhythms in intact, mature male Syrian hamsters. These data demonstrate the importance of investigating circadian rhythmicity in both sexes for a complete understanding of clock mechanisms.