Daily Immediate Early Gene Expression in the Suprachiasmatic Nucleus of Male and Female Octodon degus

Abstract

Diurnal and nocturnal species have different patterns of general activity, sleep-wake rhythms, and endocrine rhythms, but the mechanisms underlying these differences are not clear. Here, we tested the hypothesis that rhythms in immediate early gene (IEG) products in the suprachiasmatic nucleus (SCN) of a diurnal rodent (Octodon degus) reflect their diurnal chronotype. We also compared male and female degus with respect to temporal patterns of expression of one of these gene products, Fos-related Antigen (FRA). Animals were killed across the light:dark cycle, brains were collected, and sections through the SCN were stained for FRA (females and males) or c-Fos and Calbindin (males only) using immunocytochemistry. An additional set of females placed in constant darkness was pulsed with light (or not) during the subjective day or night. Labeled cells in the SCN were counted in all animals. In males, c-Fos/FRA positive (+) cell counts in the dorsomedial SCN were low after lights-on and peaked around the time of lights-off (ZT 13, ZT 0=lights-on). Effect size analyses indicated that females have a 24 h rhythm in FRA expression that is shifted relative to that of males. Light pulses in the subjective day and night produced area-specific changes in IEG expression in females that differs from that reported for males. Overall, these studies reveal the pattern of immediate early gene expression in the SCN of degus is not the same in males and females, and that it differs from the one seen in other species, both nocturnal and diurnal.

• Octodon degus
• Suprachiasmatic nucleus
• Immediate early gene
• Light pulse
• diurnal

Acknowledgments

The authors would like to thank Bethany Nestor, Shilpa Reddy, Joshua Krier, Theresa McElhinny, Anna Baumgras, and Dr. Russell Van Horn for help with immunocytochemistry and quantification. We would also like to thank three anonymous reviewers for their thoughtful comments on an earlier draft of the paper.