The brain constantly generates new neurons, but new research finds that it does so in a region known as the amygdala.
Neurogenesis is the name given to the process by which the adult mammalian brain generates new neurons.
It is a known fact that adult brains produce new cells during learning, neurogenesis being crucial to the brain's cognitive plasticity in humans and other mammals. In fact, it is estimated that the adult human brain produces 700 new neurons every day.
Such cells, however, are normally born in the hippocampus, which is a brain region known to be involved in forming and storing new memories.
However, researchers from the University of Queensland in St. Lucia, Australia, have discovered that the adult mammalian brain can generate neurons in another region: the brain area known as the amygdala.
The amygdala is sometimes referred to as the brain's "fear hub" because it is an area responsible for triggering our natural "fight-or-flight" response in dangerous situations.
First study author Dr. Dhanisha Jhaveri, of the University of Queensland's Queensland Brain Institute, explains the role of the amygdala in "fear learning" or "fear conditioning" - that is, the neurological process by which mammals associate a conditioning stimulus with a frightening experience, such as receiving an electric shock.
"Fear learning," says Dr. Jhaveri, "leads to the classic flight-or-fight response - increased heart rate, dry mouth, sweaty palms - but the amygdala also plays a role in producing feelings of dread and despair, in the case of phobias or PTSD [post-traumatic stress disorder], for example."
The findings were published in the journal Molecular Psychiatry.
Finding new neurons in adult mice
Dr. Jhaveri and team used a neurosphere assay - which is a standard technique widely used to study neurogenesis and to model neural development - to study neurogenesis in adult mice.
Cultivating neurospheres in vitro allows neural stem cells to propagate and generate progenitor cells, recreating the natural stages of brain development.
Neurospheres are the in vitro cultured "version" of neural stem cells, which are the brain's natural, undifferentiated cells that typically go on to develop and take the specific form of neurons.
Using neurosphere assays, the researchers found a small number of precursor cells in the basolateral amygdala of the adult mouse's brain. They then confirmed that these cells go on to develop into "mature and functional interneurons that persist in the [basolateral amygdala] for at least 8 weeks after birth."
Precursor cells are "more" differentiated stem cells; unlike pluripotent stem cells, precursor cells are already "committed" to transforming into a specific type of cell.
Additionally, the researchers investigated whether or not contextual fear learning has any effect on these neurons by conditioning the mice and then dissecting their brains.
Dr. Jhaveri and colleagues found that it did not: fear conditioning did not increase the number of neurospheres in the basolateral amygdala.
"These results demonstrate that neurogenic precursor cells are present in the adult [basolateral amygdala], and generate functional interneurons, but also show that their activity is not regulated by an amygdala-dependent learning paradigm," say the researchers.
Corresponding author Prof. Pankaj Sah says that the findings mark a shift in our understanding of the brain's regenerative abilities. He says, "While it was previously known that new neurons are produced in the adult brain, excitingly this is the first time that new cells have been discovered in the amygdala."
"Our discovery has enormous implications for understanding the amygdala's role in regulating fear and fearful memories," he adds.
Prof. Sah also mentions that neurogenesis was first discovered by Queensland Brain Institute founding director Prof. Perry Bartlett. As Prof. Sah explains, "[His] discovery overturned the belief at the time that the adult brain was fixed and unable to change."
"We have now found stem cells in the amygdala in adult mice, which suggests that neurogenesis occurs in both the hippocampus and the amygdala. The discovery deepens our understanding of brain plasticity and provides the framework for understanding the functional contribution of new neurons in the amygdala."
Prof. Pankaj Sah