Replaying positive emotional memories in the minds of mice reverses depression symptoms, scientists have discovered.
Depression, which can affect up to one person in five at any one time, can be paralysing. Sufferers become demotivated, lose the ability to find experiences pleasurable and often lack the perspective needed to put themselves on the path to recovery.
Now new light has been shed on the disabling effects of depression by a US research team at the Masachussetts Institute of Technology led by Nobel prizewinner Susumu Tonegawa. The team used mice that had been genetically engineered to produce a light-sensitive molecule in nerve cells that were forming new memories. This process is controlled by withholding a drug that works like a chemical switch to activate or deactivate the system.
One group of mice were given a rewarding experience involving interacting with a female mouse when the drug was withheld, while a second group continued as they had previously and a third group were subjected to a stressful experience. All of the animals then underwent a ten day period of chronic stress resulting in them displaying signs of depression.
The researchers then implanted tiny fibre optics into the memory-related hippocampal areas in the animals' brains, with the effect that any nerve cells that were now producing the light-sensitive molecule would be activated. This had the effect of triggering the "playback" of memories stored previously of either the pleasant, neutral or stressful experiences in each of the different mouse groups.
Five days of plesant memory playback, the team found, reversed the symptoms of pleasure loss and depression in the stressed mice. Animals in the neutral or negative experience group showed no such improvements.
The team, who have published the work this week in Nature, speculate that the recall of the positive memories triggered by the light stimulus in turn activated a network of brain systems including the pleasure centres. This, they suggest, has a replenishing and restoratative effect on mood. Similarly, in the animals replaying positive thoughts, the rate of production of new nerve cells in the hippocampus, which deteriorates in depressed individuals, became higher.
Together, these findings suggest that the healthy brain relies on memories of previous happy and pleasant situations to remember its way to contentment. Perhaps, then, this process is disrupted in depression leading sufferers to forget how to be happy.