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Orexin Influences Pupil Size

For the first time, researchers have worked out the mechanism behind the way the brain regulates pupil size.

Until now, the main determinant of pupil size alongside light was considered to be noradrenaline – known as a stress hormone – and its receptor system.

But now, scientists from the Swiss public research university ETH Zurich have shown that instead, the neurotransmitter orexin, also known as hypocretin, may be behind all these changes through cells called orexin neurons.

These cells extend from the hypothalamus, the tiny structure deep inside our brains that channels our body’s hormone responses to the rest of the organ.

In mice studies, the scientists found that when the orexin neurons in the brain were stimulated, pupils expanded. When the orexin system was switched off, the pupils remained constricted.

“The effect was so strong, we knew immediately that we were onto something important,” neuroscientist Nikola Grujic said.1

In their experiments, the researchers also established a dose-dependent link between nerve cell activity and pupil diameter, with the pupils showing precisely how active the orexin neurons in the hypothalamus are.

The research was published in Nature Neuroscience.2

1. ETH Zurich, Orexin influences pupil size (media release), available at: ethz.ch/en/news-and-events/eth-news/news/2023/07/orexin-influences-pupil-size.html [accessed 24 July 2023].
2. Grujic, N., Tesmer, A., Bracey, E. et al. Control and coding of pupil size by hypothalamic orexin neurons. Nat Neurosci 26, 1160–1164 (2023). doi.org/10.1038/s41593-023-01365-w.