Here's a diagram of how sunlight affects the SCN and helps control circadian rhythms

How changes in the length of the day alter the brain and subsequent behavior

To summarize: Neurons in the suprachiasmatic nucleus coordinate to adapt to different lengths of sunlight, changing at the cellular and network levels. Neurons change in the mixing and expression of dopamine, which alters brain activity and subsequent daily behavior.

resource: UC San Diego

Seasonal changes in lighting — longer days in summer and shorter winters — have long been linked to human behavior, affecting everything from sleep and eating patterns to brain and hormonal activity.

Seasonal affective disorder (SAD) is a prime example: a type of depression associated with reduced natural sunlight exposure that usually occurs in winter and is more common in high latitudes where daylight hours are shortest.

Bright light therapy has been shown to be an effective treatment for SAD as well as conditions such as non-seasonal major depressive disorder, postpartum depression and bipolar disorder, but how seasonal changes in day length and light exposure affect and change at the cellular and circuit levels The brain has kept scientists largely in the dark.

In a new study published September 2, 2022, scientific progressUsing a mouse model, researchers at UC San Diego School of Medicine elucidate a process in which affected neurons switch neurotransmitter expression in response to day-length stimuli, triggering associated behavioral changes.

The work was led by senior study author Davide Dulcis, Ph.D., associate professor in the Department of Psychiatry at the UC San Diego School of Medicine and member of the UC San Diego Center for Circadian Biology.

Hidden within the hypothalamus of the human brain is a small structure called the suprachiasmatic nucleus (SCN), each consisting of about 20,000 neurons. (The average human brain contains about 86 billion neurons and another 85 billion non-neuronal cells.)

The SCN is the body’s timekeeper, regulating most circadian rhythms — changes in the body, mind, and behavior that follow a 24-hour cycle that affect everything from metabolism and body temperature to the timing of hormone release.

The SCN operates based on input from specialized light-sensitive cells in the retina that communicate changes in light and day length to our bodies.

In the new study, Dulcis and colleagues describe how SCN neurons coordinate with each other to adapt to different lengths of sunlight, changing at the cellular and network levels. Specifically, they found that in mice whose brains functioned similarly to humans, the combination of neurons and the expression of key neurotransmitters changed, which in turn altered brain activity and subsequent day-to-day behavior.

In this schematic, sunlight prompts signals from neurons in the suprachiasmatic nucleus (the brain’s master clock), which in turn coordinate the circadian clock that regulates function throughout the body and corresponding behavior.Image credit: National Institute of General Medical Sciences

Seasonal changes in illumination have also been shown to alter the number of neurotransmitter-expressing neurons in the paraventricular nucleus (PVN), a neurotransmitter that plays an important role in the control of stress, metabolism, growth, reproduction, immunity, and other autonomic functions. brain regions.

“The most impressive new finding in this study is that we discovered how to artificially manipulate the activity of specific SCN neurons and successfully induce dopamine expression within the PVN network of the hypothalamus,” Dulcis said.

“We reveal new molecular adaptations of the SCN-PVN network in regulating hypothalamic function and everyday behavior,” added first author Alexandra Porca, Ph.D., a member of the Dulcis lab.

“The polysynaptic neurotransmitter switching we demonstrate in this study may provide an anatomical/functional link that modulates seasonal changes in mood and the effects of phototherapy.”

The authors believe their findings provide a new mechanism to explain how the brain adapts to seasonal changes in lighting. And because adaptation occurs only in neurons located in the SCN, the latter represents a promising target for new therapies for diseases associated with seasonal changes in illumination.

About this Neuroscience Research News

author: Scott Rafe
resource: UC San Diego
touch: Scott La Fee – UC San Diego
picture: The image is credited to the National Institute of General Medical Sciences

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Original research: Open access.
“Seasonal variation in day length induces polysynaptic neurotransmitter switching to modulate hypothalamic network activity and behavior” by Alessandra Porcu et al. scientific progress


Seasonal changes in day length induce polysynaptic neurotransmitter switching to regulate hypothalamic network activity and behavior

Seasonal changes in day length (photoperiod) affect many physiological functions. The suprachiasmatic nucleus (SCN)-paraventricular nucleus (PVN) axis plays a key role in processing photoperiod-related information.

Seasonal changes in SCN and PVN neurotransmitter expression have been observed in human and animal models. However, the molecular mechanism by which the SCN-PVN network responds to photoperiod changes remains unclear.

Here, we show in mice that neuregulin S (NMS) and vasoactive intestinal polypeptide (VIP) neurons in the SCN display photoperiod-induced neurotransmitter plasticity.

In vivo recordings of calcium dynamics show that NMS neurons alter PVN network activity in response to winter-like photoperiods. Chronic manipulation of NMS neurons is sufficient to induce neurotransmitter switching in PVN neurons and affect motor activity.

Our findings reveal a previously unknown molecular adaptation of the SCN-PVN network to modulate hypothalamic function to day length in response to seasonality and that NMS neurons influence behavior through coordinated polysynaptic neurotransmitter switching.

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