How do neurons manage to encode the memory of a lived event and integrate it into a life course? A study, published on November 6 in the journal Naturebrings new pieces to the brain palace of memory, this “sentinel of the spirit”according to William Shakespeare (1564-1616).
A team at Mount Sinai School of Medicine in New York wanted to understand how a memory could be integrated into recent life experiences. In adult mice, the authors followed the evolution of the memory of a stressful event: a small electric shock in a paw, which the rodent learns to associate with a specific spatial environment. For example, the animal does not receive any shocks when placed in a blue triangular room, but only when placed in a red rectangular room.
The researchers mapped the activated neurons at two time points: first, while the mice were learning about these negative experiences, and second, while they were resting, motionless, a few minutes later (“quiet awakening” duration periods). To do this, they used calcium imaging, which consists of injecting a virus into the brain, causing the neurons to produce a certain protein: a probe. which changes fluorescence depending on the concentrations of calcium ions in these cells. However, these concentrations increase when a neuron is activated. Therefore, the measurement of fluorescence changes allows the activity of each of them to be quantified.
Consolidate a memory
The authors focused on neurons in the hippocampus, this pillar of memory encoding. This brain structure is, in particular, a defender of comparisons about life experiences: it creates links between the sensory and the “where”, the “what” and the “when” of the events experienced..
First observation: a few minutes after each experience, while the mouse is in a period of quiet wakefulness, its brain “replays” this experience. Clearly, the neural circuit activated during this experience, in the hippocampus, is then reactivated. A phenomenon, in fact, already known.
“In 1989 and then in 1994, we discovered that neurons that encode a recent memory can be reactivated during sleep in animals”indicates Raphaël Brito, postdoctoral researcher in neuroscience at the Collège de France (CNRS, Inserm). So, “ in 2006 and in 2007 we realized that these neurons could also be reactivated during periods of wakefulness, at the time of memory acquisition and shortly after.adds Céline Drieu, a postdoctoral researcher at Johns Hopkins University in Baltimore, Maryland.
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