Recalling happier memories can reverse depression
From the 17 June 2015 MIT news release
MIT neuroscientists have shown that they can cure the symptoms of depression in mice by artificially reactivating happy memories that were formed before the onset of depression.
The findings, described in the June 18 issue of Nature, offer a possible explanation for the success of psychotherapies in which depression patients are encouraged to recall pleasant experiences. They also suggest new ways to treat depression by manipulating the brain cells where memories are stored. The researchers believe this kind of targeted approach could have fewer side effects than most existing antidepressant drugs, which bathe the entire brain.
“Once you identify specific sites in the memory circuit which are not functioning well, or whose boosting will bring a beneficial consequence, there is a possibility of inventing new medical technology where the improvement will be targeted to the specific part of the circuit, rather than administering a drug and letting that drug function everywhere in the brain,” says Susumu Tonegawa, the Picower Professor of Biology and Neuroscience, director of the RIKEN-MIT Center for Neural Circuit Genetics at MIT’s Picower Institute for Learning and Memory, and senior author of the paper.
In 2012, Tonegawa, former MIT postdoc Xu Liu, Ramirez, and colleagues first reported that they could label and reactivate clusters of brain cells that store specific memories, which they called engrams. More recently, they showed that they could plant false memories, and that they couldswitch the emotional associations of a particular memory from positive to negative, and vice versa.
In their new study, the researchers sought to discover if their ability to reactivate existing memories could be exploited to treat depression.
To do this, the researchers first exposed mice to a pleasurable experience. In this case, all of the mice were male and the pleasurable experience consisted of spending time with female mice. During this time, cells in the hippocampus that encode the memory engram were labeled with a light-sensitive protein that activates the neuron in response to blue light.
After the positive memory was formed, the researchers induced depression-like symptoms in the mice by exposing them to chronic stress. These mice show symptoms that mimic those of human sufferers of depression, such as giving up easily when faced with a difficult situation and failing to take pleasure in activities that are normally enjoyable.
However, when the mice were placed in situations designed to test for those symptoms, the researchers found that they could dramatically improve the symptoms by reactivating the neurons that stored the memory of a past enjoyable experience. Those mice began to behave just like mice that had never been depressed — but only for as long as the pleasant memory stayed activated.
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