The art of forgetting: microglia eats memories away in mice

As important as remembering, it is forgetting. As Sherlock Holmes said the brain is like a room that you furnish with memories but where space is limited and unnecesary information should be erased. Much on how this erasing works is unkown, but a new study points out to a certain immune cell population in the brain, the microglia, to be key in pruning away memories in mice.

Microglia are a type of glial cells in the central nervous system (CNS). This “supporting” cells are the macrophages of the brain. And as such are the first immune barrier in the nervous system. It is known that during maturation of the CNS, many synapses -contacts between neurons- are pruned, that is, lost. And microglial cells are involved in this synaptic erasing. Therefore, the hypothesis was if they would also be involved in general memory loss, since they are also known to paly a role in neurodegenerative diseases.

To test it, the researchers delivered a foot shock to mice in a particular cage. After 5 days, the mice still remembered that the cage was no good and froze. However, 35 days after they were starting to forget what happened in the cage. Their memories were being erased.

Then to prove microglial activity in this memory loss, they used a drug to get rid of them and saw that those mice with fewer of those cells took longer to forget, and therefore froze for more time when placed back in the shock-cage. The same was true when microglial cells were incapacitated to eat the synaptic connections among neurons. These results point to a microglia-mediated forgetting in these mice.

Another experiment where the neurons coding this fearful experience were labelled and inhibited to fire away their signals, showed that this inhibition of activity made them more susceptible for microglial synaptic disruption. This result would point to the observation that memories that are not exercised will often be forgotten more easily.

So, microglia are involved in forgetting in mice. At least fearful memories in the hippocampus. Since this brain region is where early memories are stored before being moved to other brain regions for long-term storage, it is not known whether this also takes place elsewhere in the brain And also it does not answer why some old memories barely used are not deleted as easily.