When the battery of your mobile drops below a certain percentage, the battery saving mode is automatically activated. The screen brightness is reduced, the phone runs a little slower, and you will probably miss notifications from emails or WhatsApp. Thus, the mobile manage to hold out a little longer before finding a Plug. But does the same thing happen in our brain? A study in mice might indicate yes.
Although it constitutes less than 2% of our body mass, the brain consumes up to 20% of the energy we ingest. If there is a part of the body where a power saving mode makes sense Energyit is in the brain. Indeed, when we are hungry it is more difficult for us to concentrate, the decisions we make are more vulnerable to the unconscious prejudices that we have and, in short, our attention is directed towards anything that allows us to obtain food as quickly as possible. Several experiments, both in mice and in humans, confirm this.
These are the short-term effects of hunger. But if the lack of food extends over time, how does the brain adapt? The one in flies disables a process dedicated to forming long-term memories, apparently as a survival mechanism. And now we know that mice also have a strategy to save energy that affects your visual system. Mice forced to lose 15% of their usual weight through dietary restriction spend less energy on visual processing than those fed normally. Saving power has consequences, and negatively affects the accuracy with which the mice see.
Mice on a diet
in the experimentthe dietary intake of a group of mice male for three weeks, until they lost 15% of their usual weight. Meanwhile, another group of mice was eating normally. But, to prevent starvation from affecting the outcome of the study, the mice were fed just before the vision tests. First, they were shown images of black bars with different orientations. Since neurons in the primary visual cortex have a preferred orientation to which they respond with an electrical impulse, these images were used to compare electrical signals from mice with and without food restriction.
The result surprised the research team: on the one hand, it was clear that the neurons of the food-restricted mice used less of a compound (adenosine triphosphate, or ATP) necessary for their functioning. That is, they consumed less energy. However, the neurons communicated at the same rate in both groups of mice, since they sent out the same number of impulses. There had to be a mechanism to compensate for the lack of ATP to preserve the number of impulses. They searched and found not one, but two complementary processes. The neurons became more excitable, and also increased their background electrical activity so that it was easier to reach the threshold that triggers an impulse.
But in exchange for maintaining the impulses, the food-restricted mice lost accuracy. The neurons did not distinguish their preferred orientation as clearly, but also sent impulses when seeing bars in orientations close to the preferred one. Moreover, this loss of precision was not just an internal curiosity, but also affected the behavior of the mice. To verify this, the research team designed a water maze for mice. It was a chamber with two corridors, each marked with an image of black bars on a white background with different orientations. One of the corridors had a hidden platform that the mice could use to climb out of the water.
Both the food-restricted and normal-fed mice learned to associate the starting platform with a certain orientation of the bars. But then the difficulty of the task was increased to compare the visual accuracy of each group of mice. Progressively, the orientation of the bars corresponding to the wrong corridor was changed to make it more and more similar to that of the bars corresponding to the platform corridor. As expected, both groups of mice failed more the more similar the orientation of some bars was to others. However, the food-restricted mice showed particular difficulty when the bars differed by an angle of 10º or less.
This result suggests that, in the absence of food, the brain deactivates functions that are less critical for survival. But, in a plot twist, it turns out that the loss of accuracy is very easy to recover, even without regaining weight. In the experiment, a single dose of the hormone that regulates hunger and body energy (called leptin) was enough for the mice to regain their ability to distinguish the orientation of the bars with normal precision. Just as a lack of leptin drives you to want to eat, the new experiment suggests that low levels of this hormone signal the brain to conserve energy.
The consequences of malnutrition
If the conclusions of this experiment are also confirmed in humans, they could help to understand the consequences of malnutrition or even some types of diets, and to find out the role of hormones in our perception of the world. What is clear is that the brain, at least in male mice, does not need to become hungry in order to withdraw and save energy.. It is not clear whether the same process will occur in female mice, as their fat levels and hormones behave differently than males. But in any case, the conclusion of this study has a critical consequence for many of the previous experiments on neurons in mice.
A very common strategy to motivate mice to collaborate in this type of experiment is to restrict their food for a few weeks before starting. This way they can be rewarded with food when they perform the corresponding tasks. But now we know that this dietary restriction may have affected precisely the neural capacity of the mice in question. Changes in ATP and the compensation mechanism may be relevant in learning and memory processes, so it will be important to take into account the conclusions of this experiment to design future studies. Perhaps mice are more perceptive than we thought.
DO NOT SCREW IT:
- Although this experiment has focused on visual perception, it could be that the brain’s energy-saving mechanism also affects other senses, such as smell. In fact, the research team considers it likely that a similar effect occurs, to a variable degree, in other areas of the cerebral cortex.
- Padamsey, Zahid et al. “Neocortex Saves Energy By Reducing Coding Precision During Food Scarcity.” neuron, vol 110, no. 2, 2022, p. 280-296.e10. Elsevier B.V.https://doi.org/10.1016/j.neuron.2021.10.024.
- LaBar, Kevin S. et al. “Hunger Selectively Modulates Corticolimbic Activation To Food Stimuli In Humans.”. Behavioral Neuroscience, vol 115, no. 2, 2001, p. 493-500. American Psychological Association (APA)https://doi.org/10.1037/0735-7044.115.2.493.
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