Long-term stress, like the kind that occurs when someone cares for a chronically ill parent or spouse, can impair short-term memory.
Researchers believe that this occurs because constant stress affects an area of the brain necessary for learning and memory, and the underlying mechanisms for this may also be to blame in more serious ailments, such as post-traumatic stress disorder (PTSD) and severe depression.
Rockefeller University scientists are inching closer to understanding exactly what these mechanisms are, and in a paper published this month in PNAS they expose one piece of the puzzle: An animal’s response to stress is at least partially dependent on an enzyme called tissue plasminogen activator, or tPA.
The hippocampus is a structure in the brain that’s responsible for episodic and contextual memory, those recollections that
involve people, places, and events. But two collaborating Rockefeller labs have found that chronic stress alters the neurons in an animal’s hippocampus - a finding that begins to explain the molecular mechanisms underlying stress-related memory impairment. Research by Sidney Strickland, head of the Laboratory of Neurobiology and Genetics, Bruce McEwen, Alfred E. Mirsky Professor and head of the Harold and Margaret Milliken Hatch Laboratory of Neuroendocrinology, and their colleagues, shows that daily stress causes those hippocampal neurons to experience a decrease in the number of both their dendritic spines and NMDA receptors, two structures that play an important role in memory and dictate the strength of neuronal connection.
Two research associates in Strickland’s lab, Robert Pawlak and Jerry Melchor, placed unstressed mice in a water maze - a test designed to assess how well they could locate a hidden platform, and how long it took them to learn and remember its location. They then compared their performance to mice in which they had induced chronic stress by subjecting them to six hours of restricted motion every day for three weeks.
They found that the stressed mice were much slower to learn where the platform was located, but once they had figured it out they were just as good at remembering the location as the unstressed mice. “Stress fuzzes your brain because it slows down the cognitive process - the animal still gets to the same point if you train it long enough, but it’s slower,” McEwen says.