Circadian clock proteins in mood regulation

Abstract

Mood regulation is known to be affected by the change of seasons. Recent research findings have suggested that mood regulation may be influenced by the function of circadian clocks. In addition, the activity of brown adipocytes has been hypothesized to contribute to mood regulation. Here, the overarching link to mood disorders might be the circadian clock protein nuclear receptor subfamily 1, group D, member 1. There are seasonal variations in mood and behavior, including those in sleep duration, social activity, mood, appetite, weight, and energy levels that tend to reoccur year after year (1). These variations are common and present as a continuum: from individuals not having any marked variation, some having experienced the variations as a problem, and to those having the seasonal pattern of mood disorder known as seasonal affective disorder (2). The current diagnostic criteria for seasonal affective disorder require that an episode of either recurrent major depressive disorder or bipolar disorder routinely occurs during a particular period of the year (3). Typically, in individuals with the winter type of seasonal affective disorder, or winter depression, the shortage of light during the autumn routinely induces depressive episodes, whereas the abundance of light during the spring routinely leads to remission. The treatment of choice for winter depression is bright light therapy, in which timed and repeated light exposures in the morning are administered via the eyes during the winter. Concerning seasons and mood disorders, the clinical picture is more complex than these routine seasonal variations, as it has been noticed for centuries that a depressive episode tends to deepen and becomes life-threatening in the spring, but not to an equal extent in any other time of the year (4). There is no clear explanation to this phenomenon. A clue to understanding it may lie in reactions of the body to changes in daylight and ambient temperature during spring. Here, the circadian clocks that anticipate and try to adapt the body to the changes are in a key position. The function of brown fat being inducible in adult humans is another target to which attention is worth paying. In the following, I present my perspective on the circadian clocks and brown fat in mood disorders.