Mice study casts light on depression
Fluctuations in light levels, due to the long winter nights, shift work or long-haul travel for example, can negatively impact our mood, ability to think and even our health. But because of the complex relationship between light, body clocks and our sleep patterns, scientists have found it difficult to unpick the exact role of light in this problem. Now, new research in mice has shown that light can affect mood and memory independently of sleep and body clocks, and has identified the light-sensitive cells responsible.
The mice were exposed to 3.5 hours of light followed by 3.5 hours of dark, instead of the normal 12-hour light and 12-hour dark cycle. This didn’t affect their sleep patterns or internal body clock. A hormone called corticosterone, representing the body clock with its natural rhythm cycling from high to low levels throughout the 24 hour day, was not affected. However, the overall level of corticosterone was raised, which in people is linked to depression.
Depression-like behavior was measured in two ways. Depressed mice have a decreased appetite for sugary solutions and also give up trying to find an escape route sooner when placed in water. Mice exposed to the abnormal light cycle had a depressed appetite and spent less time swimming when placed in water than those kept on a normal 24 hours light cycle.
Despite a normal sleep cycle and hormone rhythm, the mice showed depression-like behaviours and impaired memory, suggesting that these symptoms are not side-effects of sleep or body clock problems but rather directly linked to light-exposure.
To determine which light sensitive cells in the eye are responsible for the effect of light on mood, the experiment was repeated with mice genetically engineered to lack different eye cells. Rods and cones, the two main types of light sensitive cells were found not to be important as the mice’s behaviour was still affected by the light. The removal of a third class of cells, called photosensitive ganglion cells, did however block the effect on mood and memory.
This study has helped scientists understand how light can have such profound psychological effects. The mice may also be used by researchers as an experimental model of depression for the testing of anti-depressant therapies.