A suprachiasmatic-independent circadian clock(s) in the habenula is affected by Per gene mutations and housing light conditions in mice

For many years, the suprachiasmatic nucleus (SCN) was considered as the unique circadian pacemaker in the mammalian brain. Currently, it is known that other brain areas are able to oscillate in a circadian manner. However, many of them are dependent on, or synchronized by, the SCN. The Habenula (Hb), localized in the epithalamus, is a key nucleus for the regulation of monoamine activity (dopamine, serotonin) and presents circadianfeatures; nonetheless, the clock properties of the Hb are not fully described. Here, we report, first, circadian expression of clock genes in the lateral habenula (LHb) under constant darkness (DD) condition in wild-type mice which is disturbed in double Per1(-/-)-Per2(Brdm1) clock-mutant mice. Second, using Per2::luciferase transgenic mice, we observed a self-sustained oscillatory ability (PER2::LUCIFERASE bioluminescence rhythmicity) in the rostral and caudal part of the Hb of arrhythmic SCN-ablated animals. Finally, in Per2::luciferase mice exposed to different lighting conditions (light-dark, constant darkness or constant light), the period or amplitude of PER2 oscillations, in both the rostral and caudal Hb, were similar. However, under DD condition or from SCN-lesioned mice, these two Hb regions were out of phase, suggesting an uncoupling of two putative Hb oscillators. Altogether, these results suggest that an autonomous clock in the rostral and caudal part of the Hb requires integrity of circadian genes to tick, and light information or SCN innervation to keep synchrony. The relevance of the Hb timing might reside in the regulation of circadian functions linked to motivational (reward) and emotional (mood) processes.