A report in the November Cell Metabolism reveals powerful effects of light on the adrenal glands, a finding that might explain the broad benefits of bright light therapy for a variety of conditions, including sleep and depressive disorders, according to researchers.
The body's two adrenal glands sit atop each kidney, where they secrete hormones that regulate stress response and metabolism.
The researchers found in mice that light sparks a cascade of gene activity in the adrenal gland through its effects on the suprachiasmatic nucleus (SCN). Located in the brain region called the hypothalamus, the SCN is the seat of the circadian clock, the body's internal clock that regulates the roughly 24-hour cycle of biological processes.
Moreover, the researchers report, the gene expression changes accompany a massive surge of the steroid hormone corticosterone in the animals' blood and brain. That hormonal response increased with light intensity, they found.
Glucocorticoids--including cortisone in humans and corticosterone in mice--play many roles throughout the body, including metabolic response to starvation, antiinflammatory immune response, and the timing of circadian rhythms in peripheral organs. Therefore, light-induced secretion of glucocorticoids may play a key role in physiological changes in the body and the brain evoked by light, reported study author Hitoshi Okamura of Kobe University Graduate School of Medicine in Japan
First introduced in the early 1980s for the treatment of seasonal affective disorder, bright light therapy has been applied to many sleep disorders, including jet lag syndrome and shift work sleep disorder, the reseachers said. Shift work sleep disorder, which affects people who frequently rotate shifts or work at night, is often accompanied by metabolic symptoms, including hypertension, cancer, and diabetes.
"In these patients, light therapy improves not only psychiatric status, but also disordered hormones and metabolisms," Okamura said. "However, effects of light had only been established on melatonin, and the remaining powerful and broad effects of light on body metabolism and hormones remained to be clarified."
The researchers examined the activity of the clock gene Per1 in the organs of living animals. The team found that nighttime light exposure induced Per1 expression in the adrenal gland. Further analysis of the gland revealed numerous changes in the activity of almost 200 genes, followed by the delayed release of corticosterone.