Period Gene Expression in the Brain of a Dual-Phasing Rodent, the Octodon degus

Aug 10, 2013Journal of biological rhythms

Daily cycle gene activity in the brain of a rodent with two active phases

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Abstract

Per1 and Per2 gene expression exhibits distinct patterns in the brains of different chronotypes of Octodon degus.

In the suprachiasmatic nucleus (SCN), Per1 and Per2 levels were consistently higher during the light phase (ZT4) across all chronotypes.
Outside the SCN, hippocampal Per1 expression in entrained nocturnal degus was higher during the dark phase (ZT16), indicating a phase shift compared to SCN expression.
Masked nocturnal degus showed a similar trend in Per1 expression, but the day-night difference was not statistically significant.
Diurnal degus did not exhibit daily differences in Per1 expression in the hippocampus.
Per2 levels in the hippocampus and specific cortices were in phase with activity rhythms, with diurnal degus peaking during the light phase and nocturnal degus peaking during the dark phase.
The findings suggest that mechanisms affecting activity-phase preference may differ between masked and entrained nocturnal degus.

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Clock gene expression is not only confined to the master circadian clock in the suprachiasmatic nucleus (SCN) but is also found in many other brain regions. The phase relationship between SCN and extra-SCN oscillators may contribute to known differences in chronotypes. The Octodon degus is a diurnal rodent that can shift its activity-phase preference from diurnal to nocturnal when running wheels become available. To understand better the relationship between brain clock gene activity and chronotype, we studied the day-night expression of the Period genes, Per1 and Per2, in the SCN and extra-SCN brain areas in diurnal and nocturnal degus. Since negative masking to light and entrainment to the dark phase are involved in the nocturnalism of this species, we also compare, for the first time, Per expression between entrained (EN) and masked nocturnal (MN) degus. The brains of diurnal, MN, and EN degus housed with wheels were collected during the light (ZT4) and dark (ZT16) phases. Per1 and Per2 mRNA levels were analyzed by in situ hybridization. Within the SCN, signals for Per1 and Per2 were higher at ZT4 irrespective of chronotype. However, outside of the SCN, Per1 expression in the hippocampus of EN degus was out of phase (higher values at ZT16) with SCN values. Although a similar trend was seen in MN animals, this day-night difference in Per1 expression was not significant. Interestingly, daily differences in Per1 expression were not seen in the hippocampus of diurnal degus. For other putative brain areas analyzed (cortices, striatum, arcuate, ventromedial hypothalamus), no differences in Per1 levels were found between chronotypes. Both in diurnal and nocturnal degus, Per2 levels in the hippocampus and in the cingulate and piriform cortices were in phase with their activity rhythms. Thus, diurnal degus showed higher Per2 levels at ZT4, whereas in both types of nocturnal degus, Per2 expression was reversed, peaking at ZT16. Together, the present study supports the hypothesis that the mechanisms underlying activity-phase preference in diurnal and nocturnal mammals reside downstream from the SCN, but our data also indicate that there are fundamental differences between nocturnal masked and entrained degus.

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Period Gene Expression in the Brain of a Dual-Phasing Rodent, the Octodon degus

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