New Neurons Born in Adult Rat Cortex

NIH News
FOR IMMEDIATE RELEASE
Thursday, February 3, 2005

Recent evidence suggesting that antidepressants may act by
triggering the birth of new neurons in the adult
hippocampus,* the brain's memory hub, has heightened
interest in such adult neurogenesis and raised the
question: Could new neurons also be sprouting up in the
parts of the adult brain involved in the thinking and mood
disturbances of depression and anxiety?

Now, scientists at the National Institute of Health's (NIH)
National Institute of Mental Health (NIMH) have found newly
born neurons that communicate via the chemical messenger
GABA (gamma-aminobutyric acid) in adult rat cortex, seat of
higher order "executive" functions, and in the striatum,
site of habits, reward and motor skill learning. In the
cortex, the new neurons appear to arise from previously
unknown precursor cells native to the area, rather than
from cells migrating in from another area. NIMH's Drs.
Heather Cameron, Alexandre Dayer, and colleagues, report on
their findings in the January 31, 2005 "Journal of Cell
Biology".

Their discovery adds to the scientific debate over adult
neurogenesis, which has potential implications for
understanding a variety of brain disorders, possibly
including Alzheimer's and schizophrenia. While most
researchers agree that new neurons are generated in the
adult hippocampus and olfactory bulb, the existence of
adult neurogenesis in other brain regions remains
controversial.

The NIMH team used many more markers than previous studies
to track newborn neurons as they matured and to identify
the type of neurotransmitters they secreted. The markers
exploited antibody affinities for specific proteins to tag
particular cell types with telltale color codes, visible on
brain slices under fluorescence with a laser-powered
microscope.

The researchers found that the cortex and striatum were
giving birth to new, widely scattered small cells, called
interneurons, that make and secrete GABA, a
neurotransmitter that dampens neuronal activity. The new
interneurons closely resembled those seen in the
hippocampus and olfactory bulb and seemed to arise at
similar rates. Interneurons are thought to play a role in
regulating larger types of neurons that make long-distance
connections between brain regions and predominate in these
areas.

The NIMH team was surprised to find that the new cortex
interneurons appeared to arise from a previously unknown
class of local precursor cells rather than from cells that
migrate into the area from the subventricular zone, where
other neurons - including those seen in the striatum and
olfactory bulb - originate during adulthood. However,
during development, both the cortex and striatum precursors
likely stem from common ancestor cells that somehow retain
their ability to divide and generate new GABA interneurons,
propose the researchers.

"Since antidepressants increase neurogenesis in the adult
hippocampus, they might have similar effects in the cortex,
the region probably responsible for mood dysregulation in
depression," suggested Cameron. "But answers to such
questions about regulation and possible functions of the
new neurons must await results of future studies."

Also participating the project were Kathryn Cleaver and
Thamara Abouantoun of the NIMH Unit on Neuroplasticity. Dr.
Dayer's work was supported by the Swiss National Fund.