Light-dependent effects on mood: Mechanistic insights from animal models

Light is an important environmental stimulus that exerts a powerful influence on physiology and behavior across multiple timescales. Organisms have adapted to respond to the predictable 24-h light/dark cycle imposed by the solar day using this light information to appropriately time physiological and behavioral functions while acute changes in the light environment provide important salient cues to induce rapid responses. Variations in the light environment caused by seasonal changes in daylength as well as those prevalent in modern day life (artificial lighting, transmeridian travel) have made the light environment more irregular and unpredictable. Alterations in the regular timing of light input can have dramatic physiological and behavioral effects including a significant impact on mental health and increased prevalence of mood disorders. While the relationship between light and mood has been well established, the neuronal mechanisms underlying this relationship have remained unclear. Animal models paired with advanced technology have allowed scientists to perform detailed studies about light- dependent effects on mood-related behaviors that are not possible in human subjects. The contributions of these studies have provided novel insight into the features of light information (e.g., timing, wavelength, etc.) that are responsible for observed changes in mood-related behaviors while uncovering the brain regions, neurons, and molecules involved. In this chapter, we discuss the advancements made in deciphering neuronal mechanisms mediating light-dependent effects on mood using animal models.