Experiments with swimming mice have provided researchers with a big break in identifying the region of the brain that's primed to respond to seasonal light cycle changes such as those that drive seasonal affective disorder (SAD).
The role of the neurotransmitter, serotonin, in SAD has been debated for a long time. Logically, there would seem to be a link because high concentrations are associated with feeling good emotions while low levels are found in people suffering with depression. Researchers at the Vanderbilt University, USA decided to focus on a small region in the mid-brain called the dorsal raphe nucleus because, in humans and mice, it contains high numbers of specialised neurons that control serotonin levels.
Previous research showed that SAD sufferers were more likely to have been born in the winter.
Since exposure to daylight controls circadian and seasonal rhythms and is also known to play a role in SAD, the mice were split into three groups, the only difference being born and raised under different lighting conditions:
- Summertime light cycle with 16 hrs light / 8 hrs dark
- Transitional (like spring and autumn) cycle with 12 hrs light / 12 hrs dark
- Wintertime light cycle with 8hrs light / 16 hrs dark.
The scientists then conducted swim tests to assess how ‘depressed’ the groups of mice were; depressed mice spend more time passively floating in water than swimming to get out. In the swim tests, summertime mice showed much less depressive-type behaviour than either transitional or winter mice.
Vanderbilt's Stevenson Chair in Biological Sciences, Douglas McMahon, who supervised the study said, “Before, we thought serotonin was probably involved. Now we know that serotonergic neurons are definitely involved."
And when they looked at neurological brain chemistry, they discovered that the serotonin neurons were firing faster in the summer-born mice and they also had higher levels of serotonin. These effects persisted for several months even after summer-born mice were switched to a winter light cycle.
"This showed that early life seasonal photoperiods can have enduring effects on the serotonin neurons," said McMahon. A previous study at the University showed a correlation with season of birth in people with SAD.