We know that seasonal changes in the amount of daylight we get can have a significant impact on us – such as Seasonal Affective Disorder (SAD), for example. But now, scientists have been able to look at these effects right down to the level of brain neurons.

In a new study on mice, neurons in the suprachiasmatic nucleus (SCN) – the brain's 24-hour time tracker tucked inside the hypothalamus – were observed coordinating with each other to adapt to different lengths of daylight, with changes in individual cells as well as the network as a whole.

Both the mix and the expression of key neurotransmitters were altered in response to the amount of light each day.

We already know that shifts in the SCN can affect the workings of the paraventricular nucleus (PVN), the brain region also inside the hypothalamus which helps manage stress, metabolism, the immune system, biological growth and more.

Now, researchers have a molecular link between daylight and our behavior.

Light and neuron illustration
Illustration showing how light affects neural activity. (National Institute of General Medical Sciences)

"We revealed novel molecular adaptations of the SCN-PVN network in response to day length in adjusting hypothalamic function and daily behavior," says neuroscientist Alessandra Porcu from the University of California San Diego.

In both mice and humans, the SCN is part of the brain's timekeeping mechanisms, in charge of the physical, mental, and behavioral circadian rhythms that follow a 24-hour pattern. The SCN is controlled by special photosensitive cells in the retina, passing on information about available light and the length of each day.

What's not clear – and what this study offers a major insight into – is how the small group of 20,000 or so neurons in the SCN react in response to the data that's coming in about day length. Knowing more about this could be useful in treating health problems like SAD, as well as other conditions where light is used as a treatment option.

The researchers were able to identify changes in the neurotransmitters neuromedin S (NMS) and vasoactive intestinal polypeptide (VIP) in the mice, which could then be manipulated to alter network activity in the PVN.

In other words, we're getting closer to being able to manage our reaction to more or less daylight.

"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 hypothalamic PVN network," says neuroscientist Davide Dulcis from the University of California San Diego.

This research is still at an early stage – although there are strong similarities between the mouse brain and the human brain, which makes mice suitable test subjects, it remains to be seen if human neurons function in exactly the same way.

But building on previous research, the findings have the potential to give us new ways of treating neural disorders using light therapy. The team suggests that the mechanism they've discovered might also influence our 'memory' of how much daylight to expect as the seasons change.

The study is an example of how scientists are able to dig deeper, to the level of molecular mechanisms, using discoveries that have already been made. One of the next steps will be to see if the same mechanisms are at work in the human brain.

"The multi-synaptic neurotransmitter switching we showed in this study might provide the anatomical/functional link mediating the seasonal changes in mood and the effects of light therapy," says Porcu.

The research has been published in Science Advances.