It is not some secret that stress can be bad for animal brains, and social stress itself can lead to a build up of oxidative stress. This imbalance of free radicals and antioxidants damages the cells and can cause a wide range of issues from cancer to neurological disorders to arthritis.
In order to understand what turns stress from external social pressures into oxidative stress, scientists are searching for clues with a very social and hierarchical fish species called cichlids. The stress from their social rankings appears to raise levels of oxidative stress in the brains of low-status cichlids, though researchers caution not to compare this to human beings’ experience just yet. The findings are detailed in a study published November 26 in the journal Frontiers in Behavioral Neuroscience.
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“We found that low rank was generally linked to higher levels of oxidative stress in the brain,” study co-author and Central Michigan University biologist Peter Dijkstra said in a statement. “The patterns of oxidative stress in the brain are likely influenced by social experience modulating cellular processes in the brain.”
There are more than 1,3000 species of cichlids. These primarily freshwater fishes are native to tropical America, mainland Africa, Madagascar, and southern Asia and are popular in home aquariums. Male cichlids have a strict and highly visible social hierarchy. The dominant males tend to be larger, more aggressive, and more colorful than subordinate males. Their physiology and social lives make the species a promising proxy for studying the impact of social stress on oxidative stress levels in the brain.
In the study, a team of scientists set up 15 large tanks divided in two. They placed six females and two males into each experimental compartment. Each group of fish was allowed to interact freely with one another, while also visually interacting with the neighboring group that they could see in transparent tanks from short distances in the lab. The team also added half of a special aquarium flowerpot to each compartment. This gave the dominant male a territory to defend. In all but two groups, the cichlids quickly developed a hierarchy. That pecking order remained stable over the weeks of the experiment.
The scientists then took blood samples from the male fish, dissected their brains, and measured the size of their gonads relative to their body. In cichlids, the larger gonads indicate higher levels of androgens. Importantly, they measured markers of oxidative stress and antioxidant capacity in each division of the brain. These antioxidants can counteract the effects of oxidative stress and they calculated the oxidant capacity and oxidative DNA damage present in each brain section.
The low-status fish had higher levels of oxidative stress and lower antioxidant capacity than the more high-status fish. While the higher-status fish had higher indicators for oxidative stress in their blood, their brains seemed to be better protected from it. Indicators of reproductive activity–which are associated with high status in these fish–were also linked to higher levels of antioxidants and lower levels of oxidative stress in the brain, keeping it shielded.
The team believes that this could be linked to their hormone levels, which can be neuroprotective. The low-status fish also had smaller gonads with lower hormone levels that could have an impact on keeping their brains protected from the effects of oxidative stress.
However, they found that the effects of status on oxidative stress to be a bit complicated. When the team looked at markers of oxidative DNA damage and antioxidant capacity in different brain divisions, the various markers were impacted by social status in different ways.
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“It’s important to note that the patterns detected relative to oxidative stress in the brain may not be ‘bad’ for the animal,” said Dijkstra. “Maybe in the future we will study the fitness consequences of oxidative stress in the brain. But showing that patterns of oxidative stress are linked to social stress is important, and may provide important insights into mechanisms by which social stressors promote oxidative stress and disease in the brain.”
Dijkstra also stressed not to make too many comparisons of our society to the cichlids. Their hierarchy is merely a model system for understanding how social stress impacts the brain in this one species. More targeted studies will also be needed to understand the impact between social stress and oxidative stress on specific regions of the brain.
“I think we just uncovered some interesting patterns across different divisions of the brain,” said Dijkstra. “The next step is to understand the regulation of oxidative stress better and how social stress influences this. This requires more rigorous experimental studies.”
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