Newly Formed Neurons Help Brain “Catalog Memories In Time”

The human brain generates 1,400 new neurons per day, but their function had until now remained unknown. Kaonos/Shutterstock
Ben Taub 11/03/2016, 16:11

Sometimes it can feel every day is Groundhog Day: you wake up, go to work, see the same people as the day before, and come home again. And yet, you experience each day as a completely new event, fully aware that you are living it for the first time. Exactly how the brain distinguishes between apparently similar contexts without mixing them up has perplexed scientists for some time, but new evidence suggests that “newborn” brain cells may hold the answer.

The vast majority of brain cells – or neurons – are formed before birth and do not divide or regenerate at any point during a person’s lifetime. However, a small subpopulation of cells located in a tiny brain region called the dentate gyrus are able to do so, producing new cells via a process known as neurogenesis. Yet while the human brain produces around 1,400 of these so-called adult born granule cells (abGCs) per day, the function of these young neurons had until now remained completely unknown.

To investigate this, researchers from Columbia University and the Zuckerman Institute used 2-photon calcium imaging to monitor and compare the activity of newly-formed and mature neurons in the dentate gyrus of mice as they encountered certain stimuli. The study, published in the journal Neuron, is the first to monitor abGCs in live animals.

During the experiment, mice were placed on treadmills that were lined with a range of multisensory cues, such as textured materials, lights and smells. Results showed that abGCs less than six weeks old were significantly more active than mature neurons as the mice encountered these stimuli, suggesting that they may be have been actively encoding memories of the sensory experience.

In contrast, mature neurons appeared to be less sensitive to these multisensory inputs, instead becoming stimulated only by major changes in spatial arrangements.

Speaking to IFLScience, study coauthor Mazen Kheirbek explained that “unlike the mature neurons, the younger neurons seem to be very sensitive to the changes in the stimuli around them, so we think that they are much better at taking in novel information.”

Even when every day feels the same, we know it is not. Rawpixel.com/Shutterstock

To test this hypothesis, the researchers genetically engineered mice to carry light-sensitive genes that can control the firing of abGCs – a method known as optogenetics. These mice were repeatedly placed in a chamber and given an electric shock to the foot, until they learned to associate the environment with the shock, causing them to automatically freeze in fear every time they entered the room.

Using flashing lights to inhibit their abGCs, researchers then placed the mice in a similar but slightly different room, in which they did not receive a shock. While “normal” mice were able to tell the two chambers apart and therefore only exhibited the conditioned fear response in the shock room, those with silenced abGCs displayed this freezing reaction in both rooms, suggesting an inability to distinguish between the two settings.

As such, the study authors conclude that the sensitivity of newborn neurons to multisensory cues enables the brain to distinguish between highly similar yet novel contexts – a phenomenon known as pattern separation.

This research could lead to the development of new treatments for mental disorders such as post-traumatic stress disorder, which occurs due to “a deficit in the ability to catalog memories in time or distinguish a new experience from a previous traumatic experience.”

Accordingly, Kheirbek says researchers’ “long-term goal is to stimulate the activity of these young neurons so that we can treat different cognitive disorders, especially those involving deficits in the ability to distinguish between something new versus something in the past.”

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