Depressed? Just turn the lights on!

Seasonal affective disorder: The neural connections that link your emotions with the seasons.

After a recent vacation to a sun-filled place, I suffered a great deal of difficulty adjusting to the Muenster summer, which usually consists of an unexpected day of warm weather and bright blue skies followed by several painfully chilly, rainy and almost autumn-like days. Annoyed by the perennial obstruction of the sun by grey clouds, I looked up Pubmed to see if there had been any research done on the subject of post-holiday blues, which I seemed to be suffering from. Although I did not come across any research on the mood-altering nature of vacations, I did find a little bit more about ‘seasonal affective disorder’ (SAD).

Sunlight is known to have profound effects on an individual’s psychological well-being. Seasonal changes, usually the decreased availability of sunlight in the autumn/winter, can cause anxiety and depression of varying severity in people. Recurrent depressive episodes with the change of season mark SAD - a disorder surprisingly not uncommon. As a person from a sub-tropical region, my encounter with winter each year always mounts to a psychological warfare that ends in a strong desire to hibernate.

A disturbance of the circadian rhythm has been attributed to the mood-altering effects of sunlight. Although the symptoms of SAD ease with the change in weather, therapy involving exposure to light is commonly used to treat depression. In some instances, a circadian rhythm-independent depression can also occur. But how does light influence depression in such cases? By subjecting diurnal grass rats to (low intensity) lighting conditions, similar to winter, researchers have now confirmed a correlation between depressive behaviour and orexinergic signaling.

Retinal cells relay light information to the hypothalamus, and this connection controls varied bodily functions in accordance with the light intensity. Orexin is a neurotransmitter with a well-known role in wakefulness, appetite and mood, and is an established stimulator of dopaminergic neurons - which has led to its implication in schizophrenia and drug/food-seeking behaviour. The orexin-producing neurons are in fact located within the hypothalamus in a region that receives direct and indirect retinal inputs.

It was previously shown in diurnal grass rats that light actually stimulates the activity of orexin neurons. Extended work has revealed further that in grass rats - exposed to low intensity lights and exhibiting SAD-like symptoms - a marked decrease in the orexin (A) immunoreactive neurons occurs. Blocking the orexin receptors, and consequently the orexinergic signaling pathway, with selective antagonists also results in depression-like behaviour in the animals. Thus light and emotion appear to be connected by the orexin system. However, the precise (region of the brain) influenced by the light-dependent orexin release is yet to be determined.  

Interestingly, in a completely unrelated work, an elevation of skin β-endorphins was observed in UV-exposed rodents, which in turn has suggested an addictive sun-seeking behaviour. So remember, sunlight is good, maybe even addictive, but always remember to carry a sunscreen! 

Cover image: Flickr

References

1. Deats et al., Attenuated orexinergic signaling underlies depression-like responses induced by daytime light deficiency, Neuroscience (2014)|doi: 10.1016/j.neuroscience.2014.04.069. 

2. Adidharma et al., Orexinergic signaling mediates light-induced neuronal activation in the dorsal raphe nucleus, Neuroscience (2012)|doi: 10.1016/j.neuroscience.2012.06.020

3. Sakurai,T.,  The neural circuit of orexin (hypocretin): maintaining sleep and wakefulness, Nature Reviews Neuroscience (2007)|doi:10.1038/nrn2092

4. Borglandand Labouèbe, Orexin/hypocretin in psychiatric disorders: present state of knowledge and future potential, Neuropsychopharmacology (2010) | doi:10.1038/npp.2009.119

5. Fell et al., Skin β-Endorphin Mediates Addiction to UV Light, Cell (2014)| doi: 10.1016/j.cell.2014.04.032

6. Purves et al., Neuroscience (2nd edition), Sunderland (MA): Sinauer Associates; 2001. http://www.ncbi.nlm.nih.gov/books/NBK11145/

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