Bad timing is depressing: Disrupting the brain's internal clock causes depressive-like behavior in mice

"Our data show that perturbing circadian rhythms in otherwise totally undisturbed animals is enough to cause behaviors similar to human depression," commented first author Dr. Dominic Landgraf of the University of California, San Diego.

Inherent circadian clocks help us function throughout the day, by telling us when to sleep, wake and eat, as well as by synchronizing our bodily processes. "It is perhaps not surprising that disruptions of our natural synchronization can have heavy impacts on our physical and mental health," Dr. Landgraf added.

However, until now researchers did not know if disturbed circadian rhythms were a cause or consequence of mood disorders. In the new study, a team led by David K. Welsh has shown for the first time a causal relationship between functioning circadian clocks and mood regulation.

The researchers developed a new genetic mouse model by suppressing Bmal1, one of the master genes that drives circadian rhythms, in the suprachiasmatic nucleus (SCN), which serves as the brain's central clock regulator. Diminished Bmal1 expression reduced the strength of the clock signals produced by the SCN by about 80%. Targeting this particular brain region allowed the researchers to focus on the specific effects of the SCN circadian rhythms, and to avoid alterations in other brain regions that have confounded previous studies.

In behavioral tests, mice with reduced circadian rhythms, relative to control mice, were less motivated to escape an uncomfortable situation, which is commonly interpreted as despair or hopelessness in the animal. The mice also showed increased aversion to brightly lit areas, considered to be an indicator of anxiety-like behavior.

In addition to the altered behavior, mice with reduced circadian rhythms gained more weight than normal mice, even though they consumed the same amount of food. This finding suggests that disrupted SCN circadian rhythms could lead to metabolic abnormalities observed in many depressed patients.

Importantly, the findings show that even though the SCN does not directly regulate mood, alterations to circadian rhythms in the SCN are sufficient to cause depression- and anxiety-like behaviors in mice.

"We have long known that disruptions in circadian rhythms may contribute to depression, particularly in people at risk for major depression or bipolar disorder," said Dr. John Krystal, Editor of Biological Psychiatry. "This new study provides additional evidence implicating the Bmal1 gene in the relationship between these circadian rhythms and mood."

According to Dr. Landgraf, the results are an important step toward developing new depression treatments that directly target the circadian clock in humans.