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New neurons in the adult brain help us adapt

neurons

The discovery that the human brain continues to produce new neurons in adulthood challenged a major dogma in the field of neuroscience, but the role of these neurons in behavior and cognition is still not clear. In a review article researchers synthesize the vast literature on this topic, reviewing environmental factors that influence the birth of new neurons in the adult hippocampus, a region of the brain that plays an important role in memory and learning.

The authors discuss how the birth of such neurons may help animals and humans adapt to their current environment and circumstances in a complex and changing world. They advocate for testing these ideas using naturalistic designs, such as allowing laboratory rodents to live in more natural social burrow settings and observing how circumstances such as social status influence the rate at which new neurons are born.

“New neurons may serve as a means to fine-tune the hippocampus to the predicted environment,” Opendak says.

“In particular, seeking out rewarding experiences or avoiding stressful experiences may help each individual optimize his or her own brain. However, more naturalistic experimental conditions may be a necessary step toward understanding the adaptive significance of neurons born in the adult brain.”

In recent years, it has become increasingly clear that environmental influences have a profound effect on the adult brain in a wide range of mammalian species. Stressful experiences, such as restraint, social defeat, exposure to predator odors, inescapable foot shock, and sleep deprivation, have been shown to decrease the number of new neurons in the hippocampus. By contrast, more rewarding experiences, such as physical exercise and mating, tend to increase the production of new neurons in the hippocampus.

The birth of new neurons in adulthood may have important behavioral and cognitive consequences. Stress-induced suppression of adult neurogenesis has been associated with impaired performance on hippocampus-dependent cognitive tasks, such as spatial navigation learning and object memory. Stressful experiences have also been shown to increase anxiety-like behaviors that are associated with the hippocampus. In contrast, rewarding experiences are associated with reduced anxiety-like behavior and improved performance on cognitive tasks involving the hippocampus.

The team proposed that stress-induced decreases in new neuron formation might improve the chances of survival by increasing anxiety and inhibiting exploration, thereby prioritizing safety and avoidant behavior at the expense of performing optimally on cognitive tasks. On the other hand, reward-induced increases in new neuron number may reduce anxiety and facilitate exploration and learning, leading to greater reproductive success.

“Because the past is often the best predictor of the future, a stress-modeled brain may facilitate adaptive responses to life in a stressful environment, whereas a reward-modeled brain may do the same but for life in a low-stress, high-reward environment,” says Elizabeth Gould , a professor of psychology and neuroscience at Princeton University.

However, when aversive experiences far outnumber rewarding ones in both quantity and intensity, the system may reach a breaking point and produce a maladaptive outcome. For example, repeated stress produces continued reduction in the birth of new neurons, and ultimately the emergence of heightened anxiety and depressive-like symptoms.

“Such a scenario could represent processes that are engaged under pathological conditions and may be somewhat akin to what humans experience when exposed to repeated traumatic stress,” Maya Opendak says.

Because many studies that investigate adult neurogenesis use controlled laboratory conditions, the relevance of the findings to real-world circumstances remains unclear. The use of a visible burrow system–a structure consisting of tubes, chambers, and an open field–has allowed researchers to recreate the conditions that allow for the production of dominance hierarchies that rats naturally form in the wild, replicating the stressors, rewards, and cognitive processes that accompany this social lifestyle.

“This more realistic setting has revealed individual differences in adult neurogenesis, with more new neurons produced in dominant versus subordinate male rats,” Gould says.

“Taking findings from laboratory animals to the next level by exploring complex social interactions in settings that maximize individual variability, a hallmark of the human experience, is likely to be especially illuminating.”

Sources:
Opendak, M., & Gould, E. (2015). Adult neurogenesis: a substrate for experience-dependent change Trends in Cognitive Sciences DOI: 10.1016/j.tics.2015.01.001

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2 responses

  1. Generally, it has been said that the adult brain could modify the shape of the neurons, but the brain could not develop new ones. However, recent research has found some exceptions when doing testing in animals like songbirds and rodents, then comparing them to human brains. To elaborate, researchers focused on the development of new neurons in the olfactory bulb in the brains of songbirds. In the colder months, this area loses neurons, but regains them during mating season that follows in the spring due to increased activity in that portion of the brain. In addition, new neuron formations have been found in the hippocampus of mammals, which is responsible for memory. The formation of new neurons allows the hippocampus to pick up on new tasks. In addition, it has been found that by blocking the formation of new neurons, the memory becomes impaired.
    On the other hand, different results have been found in humans by researchers who used Carbon-14 to measure the amount of new neurons in the brain. For example, it has been shown that the C-14 found in the cerebral cortex of a human brain relates back to the C-14 concentration from the time that person was born. This indicates that under normal circumstances, the cerebral cortex forms little to no new neurons after a person is born. Furthermore, the hippocampus of the human brain indicates that we make less than 2 percent of new neurons per year in that section. Additionally, when researchers focused on the neurons in the heart, the data they gathered was that the neurons were the same neurons that developed during fetal growth. However, if you were to examine the C-14 found in a person’s skin cells, it would be of the same concentration of C-14 for that year, which is the exception that has been found for neural growth in the human body.
    My personal take on this blog topic is that the human brain may be able to produce new neurons; however, where and how many neurons can be produced into adulthood is the standing question. That being said, I do believe environmental factors, such as stress, play a huge role in the development of neurons in the hippocampus.

    February 23, 2015 at 5:27 pm

    • Wow, that was a super thorough comment! Thank you for taking time to type all of that, seriously that was awesome.

      I agree stress and environmental factors probably play a big role in the creation of neurons in the brain, but when you think about it really it isn’t that odd of an idea. Our brain has to be able to change and adapt to be as so-called “plastic” as it is.

      Of course, things like alzheimer’s and dementia would suggest that — at least in the very elderly — new neuron formation is very limited (given the limited results they found, as you pointed out).

      Although for those who experience head trauma war veterans, this new growth — even if it is limited — could be good news from a recovery standpoint given the amount of damage IED’s and explosions in general can cause to the brain.

      February 23, 2015 at 6:25 pm

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