An experiment on small rodents clarified the mechanism by which changes in ambient light
– for example by alternating continuously short cycles of light and darkness of the same duration – can lead to mood disorders, until the depression, and learning deficits: there are specific retinal cells that mediate this influence. It is therefore a direct interaction, which does not involve neither sleep nor circadian rhythms.
The light can affect the learning capacity and disturb the mood by acting on specific retinal cells, the ganglion cells. It was demonstrated by Tara LeGates of the Department of biology at Johns Hopkins University in Baltimore, Maryland, and her colleagues of a wide international collaboration, that at this proposal signed an article in the magazine “Nature”. It is a fact that the variations of light can negatively affect mood and cognitive functions: for example, the small length of day during the winter, particularly in northern countries, can lead to depressive syndromes. It is also experimentally verified that study in daylight conditions is more profitable than studying at night with a lamp.
However, up to now the neuronal circuits through which this influence of lighting conditions is exerted were not known, nor was it clear the function of such circuits. The prevailing hypothesis was that changes in the sun exposure provoked an alteration to the circadian rhythms (the processes that control our “biological clock” with 24-hour cycle). Another indirect factor about brain function, so far considered crucial, was sleep deprivation. To test these hypotheses, LeGates and colleagues subjected laboratory mice to an aberrant light/dark cycle defined ultradiano (3.5 hours of light followed by 3.5 of darkness, so avoiding to make exposure to light or vice versa to darkness prevail) controlling that it had no effect either on the rhythm of sleep or on circadian rhythms. The isolation of several factors involved made it possible to demonstrate that light directly rules the behaviours linked to mood and cognitive functions.
The animals showed an increase in depressive-like behaviours and clear learning deficits. In particular, the results of the test show that mood problems precede those of learning, since the administration of antidepressants like desipramine and fluoxetine allowed to re-establish the correct learning rhythms and sleep in mice exposed to the aberrant light/dark cycle. With a second test, researchers have identified the neural pathway that mediates this process: they are photosensitive retinals ganglion cells. Mice lacking these specific retinal cells, in fact, have not expressed any learning deficits or alterations in mood.