Feeling stressed? Researchers at IU are studying how stress reshapes the brain

This post is part 1 of a two part series. Check out part 2 here.

An illustration of the brain surrounded by a pattern of multicolored squares

Imagine it’s 9:45 am. You have a meeting across town in 15 minutes and you just realized that you overslept your alarm! You throw on some clothes, grab a cup of yesterday’s coffee, and rush out the door, only to realize that your car has a flat tire…

Feeling stressed? Anyone who has experienced a situation like that knows what stress feels like. But, while stressful experiences aren’t pleasant, we typically find ways to deal with them. We solve whatever issues that have come up, find ways to relax, and move on. But what happens when severely stressful circumstances hit? Or when stressful experiences are unrelenting?

Stress research has shown that high levels of stress can permanently change our behavior. For example, individuals who have suffered long-term financial hardship or emotional abuse are more likely to develop depression and anxiety disorders later in life, and even a single traumatic experience can result in post-traumatic stress disorder (PTSD). The severe, long-term effects of stress have led neuroscientists to believe that stress “programs” the brain to function differently later on.

Dr. Cara Wellman and other members of Indiana University’s Neurobiology of Stress Lab are studying how, exactly, this happens. Their research focuses on individual brain cells, or neurons. Neurons have long, finger-like projections that look like tree branches extending from the body of the cell. These “branches” are called dendrites, and they reach out to receive signals from other cells. In this way, when one neuron is activated it can activate other neurons, propagating a signal through a network of neurons.

As an example of how this works, we can refer to our earlier anecdote. Imagine that you’re now rushing to class, assignment in hand, and you pass a cherry tree that is swarming with bees. One of the bees follows you, and you begin the embarrassing ritual of spinning around and swatting at the bee while simultaneously trying to run away. All of this is coordinated by neurons in various regions of your brain. The visual image of the bee and the sound of its buzz activate neurons in sensory brain regions, which activate other neurons that coordinate a fearful response.

An important feature of dendrites is that they can change in size and shape. One neuron’s dendrites can “reach out” to another neuron, and the two become more strongly connected. In our example, neurons that are activated by the sound of a bee could connect with neurons that carry information about the cherry tree. Later, the mere sight of the cherry tree or sound of a bee buzzing by could activate the network and trigger a memory of the past experience – learning has occurred. This is a simplified model of how memory formation works, but it demonstrates the basic idea: neurons that are activated during an experience create networks that govern future responses.

A figure with three panels, from left to right titled "Before Learning," "During Learning," and "After Learning." The first panel, "Before Learning," reads: “Many of the connections between neurons in brain regions that support learning are unstable. They will become stronger when signals from the environment require them to communicate with one another and coordinate a behavioral response.” Underneath the text in that panel, three groups of neurons are shown in grey. The second panel, “During Learning,” reads: Signals from the environment causes neurotransmitter release in sensory brain regions. This initiates a cascade of signals – sensory neurons activate downstream neurons that integrate information, which activate other neurons to control the behavioral response.” Underneath the text in that panel, the same three groups of neurons are shown, but one neuron in each group is shown in black. Small circles representing neurotransmitters are depicted between each black neuron. A flow diagram displays the following text next to each group of neurotransmitters: “‘Input’ neurons receive signals about an event,” “Signals converge on ‘integration’ neurons,” “Integrated signal is relayed to ‘output’ neurons,” and “‘Output’ neurons trigger a behavioral response.” The third panel, “After Learning,” reads: Neurons that were activated during learning change in shape to form a neural network. Signals that activate this network will trigger the behavioral response more efficiently, as the neural circuit connecting the signal to the response is already in place.” Underneath the text in that panel, the same black neurons are shown, but have additional projections (dendrites) so that neurons in each group contact those in neighboring groups.
During learning, neurons that signal to one another become more strongly connected. This forms a neural network that links different components of an experience together, and connects the experience to a behavioral response.

We can now consider the effects of stress on this process. Imagine that the bee situation becomes more stressful… Every time you walk by the tree you are chased by angry bees, and often stung. Not a great way to start your day. Eventually, you become nervous as you approach the tree, even before you see or hear a bee. Hearing even one bee buzz by might also cause you to become more worried and anxious than you would have before you had those experiences. Obviously, stress has changed your behavior, but how?

Researchers in IU’s Neurobiology of Stress Lab believe that stress changes the neurons that support learning, and could therefore change how we respond to and learn about stressful situations. For example, they have shown that stressed rats have shorter dendrites in the medial prefrontal cortex, a brain region that dampens emotional and fearful responses. If dendrites in this region become shorter in response to stress, a stressed individual may be less able to control emotional responses to stressful situations. This could have relevance for patients suffering from stress-induced mood and anxiety disorders. For example, patients with PTSD show more stress-induced activation of areas that are involved in fear responding. Better understanding the effects of stress on neurons in these brain regions could be key to understanding how stress-induced psychological disorders arise and how they may be treated.

Read on to Part 2 here.

Edited by Anna Jessee and Karna Desai

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