To study key neurons in our brain, which are used for learning, researchers have genetically modified mice. This modification accidentally cut off their ability to learn by “zombifying” these key neurons. This chance is not without utility for scientific advances.
These are neurons that no longer function normally, while remaining active and alive: a study published on April 2, 2024 highlights the discovery of “zombie neurons” in the brain. And contrary to what this expression – used by the authors – may suggest, there is nothing dangerous about these neurons. It’s even quite the opposite.
It is at the level of the cerebellum (cerebellum, the “little brain”), which is found in vertebrates, that this discovery is located. This is a defining area; it coordinates our movements and our balance, as well as our learning. To achieve this, the cerebellum constantly receives error signals and modifies our behavioral response by iterating over these very specific signals.
We do not know precisely what form these signals take in the cerebellum or how they are processed. This was precisely the aim of this study, based on mice, whose brains have many similarities with ours. To do this, they used a technique called optogenetics: it involves activating specific neurons using light signals captured by the eyes. In this case, the challenge was to activate the “climbing fibers” which serve as entry points for signals received by the cerebellum.
“ Climbing fibers normally respond to sensory stimuli, such as a puff of air into the eye », Explain the researchers. “ By precisely activating these fibers optogenetically, we were able to make the mouse believe that it had received a puff of air, when this was not the case. » Over the course of stimulation of the climbing fibers via this visual cue, the mice learned to blink in response to this stimulation, even if it did not exist. There was no real breath of air here. “ This proves that these fibers are sufficient to drive this type of associative learning. »
Zombified neurons that teach us a lot
The first discovery of the study is therefore to prove the usefulness of climbing fibers in associative learning. But a second, unforeseen discovery slipped into their work.
To mobilize optogenetics, the researchers genetically modified the mice by injecting, into their climbing fibers, a protein reactive to light, called Channelrhodopsin-2 (ChR2 for short). But these genetically modified mice were no longer able to respond normally to normal signals such as a real puff of air.
“ The most compelling evidence to date that climbing fiber signals are essential for associative learning »
The climbing fibers were accidentally cut off from the rest of the circuit, even though they continued to function. In short, they existed, they were active, but no longer served any purpose. “ It turned out that introducing ChR2 into climbing fibers changed their natural properties, preventing them from responding appropriately to standard sensory stimuli such as puffs of air. », Explain the researchers. Result ? This had the effect of “ completely block animals’ ability to learn » (apart from optogenetic stimulation).
For their authors, the very fact that a modification of the climbing fibers interrupts the natural capacity for learning represents ” the most compelling evidence to date that climbing fiber signals are essential for associative learning (…)”. They now want to understand why and why the addition of the ChR2 protein causes these neurons to zombify — because this could provide a better understanding of their fundamental functioning.
If you liked this article, you will like the following: don’t miss them by subscribing to Numerama on Google News.