Mice’s individuality is influenced by their relations

Inserm/Latron Patrice, 2009, Inserm images

Individuality exists in all animals, and a number of factors shape it over time. For mice, one of those factors is the social environment, as researchers at CNRS, INSERM and Sorbonne Université have just shown. In this species, some stable character traits may even be inscribed in an individual’s neuron activity and change when the group’s composition changes. These results are published on August 6, 2018 in Nature Communication.

Individuality is not exclusive to humans. Though this idea was previously rebutted by biologists, today it is accepted that individuality is found in all animal species. It is defined as all the behavior differences between individuals of a single species that are relatively stable over time. Though the process called individuation is supported by genetic and development components, researchers have just demonstrated in mice that the social environment and activity of some neurons also participate in determining the emergence of distinct individuals.

To reach this conclusion, teams at Laboratoire Neuroscience Paris-Seine (CNRS/INSERM/Sorbonne Université), Laboratoire Adaptation Biologique et Vieillissement (CNRS/Sorbonne Université)1 and at Sorbonne Université’s Institut de la Longévité at Hôpital Charles Foix (AP-HP) studied the life of mice living in “Souris City” (Souris is the French word for mouse), an innovative experimental device having two common living spaces for the animals, and an option to make them take a test one by one, without human intervention. It is by means of this test that the researchers have identified different “personalities” among the mice. This device includes a T-shaped maze where the mice had to choose one of the two arms, where one led to normal water and the other led to sweetened water. These two positions were alternated regularly. When faced with this problem, two radically different strategies emerged: some mice varied their choice very often, others hardly ever.

The first thing that the authors observed was that the type of behavior adopted by each individual was correlated to the activity of dopamine-producing neurons, which are especially involved in decision making. For example, the mice that alternated the most had lower dopaminergic activity. Therefore, one can say that mice have biologically inscribed individuality.

To understand the role of mice’s social environment on how these different individualities develop, the researchers continued their experiments by changing the composition of the groups in Souris City. They grouped the individuals who adopted the same test strategy, those who rarely alternated on one side, and those who alternated often on the other. Surprise! After a few weeks, roles had redistributed in both groups! Some mice who had rarely varied their choice had become the more adventurous ones in their new group, and vice versa.

What was even more surprising, this behavior change was correlated with a change in dopamine pathway activity in the mice.

These results suggest that decision-making mechanisms, behavioral repertoires and activity levels in the nervous systems of each individual are far from being set and adapt according to the social structure in which the animals evolve.

The fact that the social environment contributes to differences between the individuals has implications for sociology, psychology, biology and medicine. Social factors also participate in the development of psychiatric pathologies such as addiction. This is a field that the researchers are going to investigate, studying the influence of social environment on vulnerability to drugs.

1 These laboratories are members of the Institut de Biologie Paris-Seine

Alcohol Consumption and Risk of Dementia

The conclusions of a new study coordinated by Inserm show that excessive long-term alcohol consumption is linked to an increased risk of dementia. The results also suggest an increased risk among those who abstain, even if the underlying mechanisms in the two groups are very likely to differ. With those consuming to excess, the cases of hospitalization for chronic alcohol-related disease were linked to a four times higher risk of dementia, while in the abstainers this risk is only 1.5 times higher and can be explained in part by an increased risk of cardiometabolic disease. These results were published in British Medical Journal based on data from the British Whitehall II cohort.

With the increase in life expectancy, the number of people affected by dementia is expected to triple by 2050. However, not all of its risk factors have been identified. That is why a team of Inserm researchers based in France and the UK undertook to study the link between alcohol consumption in midlife (between the ages of 40 and 60) and the risk of developing dementia in the 23 years that followed. They also examined whether cardiometabolic diseases (which include stroke, coronary heart disease and diabetes) had any effect on this link.

Their study reveals that those who abstain from drinking alcohol or those who consume more than 14 units of alcohol per week between the ages of 40 and 60 present an increased risk of developing dementia with aging.

The results they obtained are based on 9,087 British civil servants aged between 35 and 55 in 1985 who participated in Whitehall II, a study which examined the consequences of social, economic, biological and lifestyle factors on long-term health. At regular intervals between 1985 and 1993, the participants (average age 50) were evaluated for their alcohol consumption and alcohol dependence.

Hospital admissions for alcohol-related chronic diseases and dementia cases from 1991, as well as cardiometabolic diseases were identified from hospitalization records.

Out of the 9,087 participants, 397 cases of dementia were recorded over an average follow-up period of 23 years. The average age when dementia was diagnosed was 76 years.

After taking into account sociodemographic data, lifestyle and health-related factors which could have affected the results, the researchers discovered that abstinence or the consumption of more than 14 units of alcohol (112 g of alcohol) per week were linked to a higher risk of dementia compared with a weekly consumption of between 1 and 14 units of alcohol. Among those consuming more than 14 units of alcohol per week, each additional 7 units consumed per week was linked to a 17 % increase in dementia risk.

Cases of hospitalization for alcohol-related chronic diseases were linked to a four times higher risk of dementia.

While these findings show that both abstinence and excessive alcohol consumption are linked to an increased risk of dementia, the researchers agree than the underlying mechanisms in each of the two groups are very likely to differ.

In the abstainers, the researchers show that part of the additional dementia risk was linked to a higher risk of cardiometabolic disease. However, other health factors, alcohol consumption earlier in life, other unmeasured sociodemographic characteristics could also explain the excess risk of dementia observed in the abstainers.

For Séverine Sabia, Inserm researcher and principal author of this research: “these findings support the data according to which excessive alcohol consumption is a risk factor for dementia, and they encourage the recommendation of lower consumption thresholds to promote better cognitive aging. In no way should the results observed in the abstainers encourage people who currently do not drink to start doing so because, according to French Public Health Agency figures, alcohol consumption is responsible in France for 49,000 deaths from cancer, cirrhosis, psychosis and alcohol dependence”.

In France, the public health recommendations in terms of “acceptable risk” are 10 glasses per week and no more than two glasses per day on the basis that one glass in France corresponds to 10 g of alcohol, i.e. 100 g per week. These recommendations are now the same for both men and women.

Finally, given that this is an observational study, it is not possible to draw definitive conclusions as to a causal relationship.