Changes in the neurotransmitter system that occur due to stress prevent the brain from forming the correct memory of life’s troubles.
Memories awaken in us under the influence of various stimuli: we either want to remember something ourselves, or we are prompted to do so by something from the outside – a sound, a smell, a view outside the window, someone’s words. The external stimulus must be related to what we remember, and it acts on special engram neurons. When the brain remembers something, some neurons become like keepers of the keys to a specific fragment of memory – when they sense a certain signal, they activate this fragment. These neurons are called engram neurons.
On the other hand, memory strongly depends on the mental state; it works differently when we are calm or happy, or, conversely, when we are stressed, when something worries or frightens us. In a recent article in Cell describes how stress affects the appearance of engram cells. They put stress on experimental mice by locking them in a narrow pipe for half an hour – the mice felt that they could neither get out nor turn around, and began to get very worried; as a result, their levels of the stress hormone corticosterone increased. Some mice were simply injected with a dose of corticosterone, which tells the organs and tissues that something bad has happened, and they need to devote all their strength to dealing with this bad thing.
The stressed and normal mice were then placed in a cage where they heard a sound for 30 seconds, after which nothing happened. Then there was a break, and in the same cage another sound was heard, with a higher frequency – it also lasted 30 seconds, but after it the mice were given a weak shock to the legs. The mice had to remember the connection between a certain sound and an unpleasant event, and in ordinary mice this is what happened: when they were put in another cage and the “electric” sound was turned on again, they froze in place, demonstrating the usual stress reaction of rodents. At the same time, they ignored the first sound, which was not associated with any troubles.
But mice that had previously experienced stress in a narrow pipe (or that were simply injected with corticosterone) did not distinguish between both sounds – they were afraid of both the “electric” sound and the neutral one. Their memory of the unpleasant event turned out to be nonspecific, and this nonspecificity arose due to too many engram cells that the brain allocated for memorization. Engram neurons can be identified by the special activity that occurs in them at the moment of memorization, and then when they sense a certain signal prompting them to include an episode from the memory. In normal mice, the activity of potential engram cells in the memory center was inhibited by neurons in the amygdala, or amygdala, one of whose functions is to work with emotions. Amygdala neurons released a lot of the neurotransmitter GABA (gamma-aminobutyric acid), which reduced the activity of memory neurons. But in mice after stress, the level of the inhibitory neurotransmitter GABA was lower, and therefore too many memory neurons remained active when it was necessary to remember the circumstances of the unpleasant experience with an electric shock. Too many potential engram cells meant that many different stimulus signals would be associated with an unpleasant experience – even those that were not related to this experience. Stress spoiled memory by fixing unnecessary connections between phenomena.
The inhibitory neurotransmitter GABA became insufficient because in the stressed brain there became a lot of other neurotransmitters from the group of endocannabinoids: they influenced the neurons of the amygdala, and they ceased to suppress the activity of potential engram cells. By suppressing the production of endocannabinoids or preventing them from interacting with receptors on neurons, stress-induced memory erosion can be prevented, but this must be done precisely at the moment when the memory is being formed.
Most likely, this mechanism is also related to human post-traumatic syndrome – with it, the feeling of danger turns out to be tied to non-dangerous stimuli. It is possible that the new results will help develop some effective drugs against post-traumatic syndrome. It can be added that stress affects memory not only at the time of its formation. We once wrote that prolonged psychological stress, acting through the immune system, can literally erase a ready-made memory. True, in such cases the question arises of how much stress harms memory, and how experimental stress in mice can be compared with the stress that a person faces in everyday life or in some exceptional circumstances of life.
Source: www.nkj.ru
