A new cradle of brain cell formation has been discovered in the adult amygdala, a region that gives memories their emotional bite.
The discovery could lead to a deeper understanding of post-traumatic stress disorder, anxiety, and phobias – and also to better treatments.
The formation of freshly minted neurons – the cells that send and receive signals in the brain – was long thought to be non-existent in adults. But that dogma was overturned in the early 1990s with the discovery of neuron-forming stem cells in the adult hippocampus, a seahorse-shaped region that acts as the brain’s memory hub. The smell-processing olfactory bulb – in rodents, at least – also churns out baby neurons throughout life.
Now, the almond-shaped amygdala can be added to this select club.
“Our discovery has enormous implications for understanding the amygdala’s role in regulating fear and fearful memories,” says Pankaj Sah of the Queensland Brain Institute at the University of Queensland, one of the study’s lead authors.
The amygdala plays an important role in imbuing memories with emotion. This helps us to learn from our experiences, but the process can go awry in post-traumatic stress disorder, anxiety, or phobias. In these conditions, fear can overwhelm, thanks to an overzealous amygdala.
Researchers have had inklings that the amygdala can make new neurons. But the new work, published in Molecular Psychiatry, is the first to clearly show that stem cells forming in the mouse amygdala grow into bona fide, fully functioning neurons, “which is actually pretty revolutionary,” says Jee Kim from the Florey Institute of Neuroscience & Mental Health at the University of Melbourne, who was not involved in the study.
The discovery is likely to hold true for the similarly wired human amygdala, says Dhanisha Jhaveri of the Queensland Brain Institute, who co-led the study.
The amygdala only contains a small number of these newborn cells – fewer than in the hippocampus. But “even small numbers could have a big impact,” says Jhaveri.
That’s because the newly formed cells are interneurons, cells that dampen down activity in neural circuits. In the amygdala, these cells help to keep emotions like fear in check.
Now that the fledgling neurons have been discovered, the researchers are working to nut out the pulleys and levers that control their formation. In the hippocampus, exercise and antidepressant drugs can spur new cell formation, whereas stress and inflammation stymy the process.
Similar triggers could affect neurogenesis in the amygdala, too, says Jhaveri. Understanding these triggers, and identifying drugs that specifically encourage new cell formation in the amygdala could ultimately lead to better treatments for anxiety-related disorders.
The finding could also help to explain why exposure therapy, designed to gradually dampen a person’s response to a phobia or traumatic memory, works, according to Kim. “These neurons may be important in how we learn to reduce our fear,” she says.
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