Expression of the Per1 gene in the hamster: Brain atlas and circadian characteristics in the suprachiasmatic nucleus

Recent progress in study on the molecular component of mammalian clocks has claimed that mammals and Drosophila share the similar fundamental clock oscillating system. In the present study, we investigated expression of Per 1, the first gene of the mammalian homolog of the Drosophila clock gene period, in the hamster brain, and we also examined its circadian expression pattern in the mammalian clock center, the suprachiasmatic nucleus (SCN). In situ hybridization using isotope-labeled cRNA probes revealed a wide and region-specific distribution of Per 1 in the hamster brain and spinal cord. High levels of Per 1 were found in the internal granular layer of the granular cells of the olfactory bulb, anterior olfactory nuclei, tenia tecta, olfactory tubercle, piriform cortex, suprachiasmatic nucleus, and gyrus dentatus of hippocampus. Moderate levels of expression were detected in many brain regions including the granular layer of the cerebellum, anterior paraventricular thalamic nucleus, caudate-putamen, inferior colliculus, pontine nuclei, inferior olive, and nucleus of the solitary tract. We examined the circadian profile of hamster Per 1 mRNA in the SCN in constant darkness and found that Per 1 expression showed a peak at subjective day (circadian time [CT] 4) and formed a trough at subjective night (CT16-CT20). A brief exposure of light at CT16 could acutely induce large quantities of Per 1 mRNA in the hamster SCN, except for its dorsomedial subdivision. These findings suggest that the characteristics of Per 1 gene expression in the mammalian circadian center (showing a peak in the daytime and a trough in the nighttime and a rapid inducibility by light) are common among mammalian species. Lastly, in hamster brain, Per 1 gene is also inducible in extra-SCN brain nuclei, since light at night also elicited Per 1 mRNA in neurons of the hypothalamic paraventricular nucleus.