Seasonal changes in light – longer days in summer, shorter days in winter – have long been associated with human behaviors, affecting everything from sleep and eating habits to brain and hormonal activity. Seasonal affective disorder (SAD) is a prime example: a type of depression linked to reduced exposure to natural sunlight, usually occurring during winter months and more often at higher latitudes when daylight hours are the shortest.
Light therapy has been shown to be an effective remedy for treating SAD, as well as conditions such as non-seasonal major depression, postpartum depression and bipolar disorder, but how seasonal changes in day length and light exposure to light affects and alters the brain at the cellular and circuitry level. kept scientists largely in the dark.
In a new study, published on September 2, 2022 in Scientists progressresearchers at the University of California, San Diego School of Medicine used a mouse model to illuminate a process in which affected neurons alter the expression of neurotransmitters in response to day-length stimuli, triggering associated behavioral changes.
The work was led by the study’s lead author, Davide Dulcis, PhD, an associate professor in the Department of Psychiatry at UC San Diego School of Medicine and a member of the Center for Circadian Biology at UC San Diego.
Nestled in the hypothalamus of the human brain is a small structure called the suprachiasmatic nucleus (SCN), each made up of approximately 20,000 neurons. (The average human brain contains about 86 billion neurons and 85 billion non-neuronal cells.)
The SCN is the body’s timekeeper, regulating most circadian rhythms – physical, mental and behavioral changes that follow a 24-hour cycle and affect everything from metabolism and body temperature to when hormones are released. The SCN works based on the supply of specialized light-sensitive cells in the retina, which communicate changes in light and day length to our body.
In the new study, Dulcis and colleagues describe how SCN neurons coordinate to adapt to different lengths of daylight, changing at the cellular and network levels. Specifically, they found that in mice, whose brains function similarly to humans, neurons change their mix and expression of key neurotransmitters which, in turn, alter brain activity and day-to-day behaviors. later.
Seasonal changes in light exposure have also been shown to alter the number of neurons expressing neurotransmitters in the paraventricular nucleus (PVN), a region of the brain that plays a critical role in controlling stress, metabolism, growth, reproduction, immunity and other autonomic functions. .
“The most impressive new finding from this study is that we discovered how to artificially manipulate the activity of specific SCN neurons and successfully induce dopamine expression in the hypothalamic PVN network,” Dulcis said.
“We revealed novel molecular adaptations of the SCN-PVN network in response to daylength in adjusting hypothalamic function and daily behavior,” added first author Alexandra Porcu, PhD, a member of Dulcis’ lab. “The multi-synaptic neurotransmitter switching we have shown in this study may provide the anatomical/functional link mediating seasonal changes in mood and the effects of light therapy.”
The authors suggest their findings provide a new mechanism for how the brain adapts to seasonal changes in light exposure. And because adaptation occurs within neurons exclusively located in the SCN, the latter represents a promising target for new treatments for disorders associated with seasonal changes in light exposure.
Co-authors include: Anna Nilsson, Sathwik Booreddy, Samuel A. Barnes, and David K. Welsh, all at UC San Diego.