Singapore: Working with mice, Johns Hopkins researchers say they have figured out how stem cells found in a part of the brain responsible for learning, memory and mood regulation decide to remain dormant or create new brain cells. Apparently, the stem cells "listen in" on the chemical communication among nearby neurons to get an idea of what is stressing the system and when they need to act.

The researchers say understanding this process of chemical signaling may shed light on how the brain reacts to its environment and how current antidepressants work, because in animals these drugs have been shown to increase the number of brain cells. The findings were reported on July 29 in the advance online publication of Nature.

"What we learned is that brain stem cells don't communicate in the official way that neurons do, through synapses or by directly signaling each other," says Dr Hongjun Song, professor of neurology and director of Johns Hopkins Medicine's Institute for Cell Engineering's Stem Cell Program. "Synapses, like cell phones, allow nerve cells to talk with each other. Stem cells don't have synapses, but our experiments show they indirectly hear the neurons talking to each other; it's like listening to someone near you talking on a phone."

The "indirect talk" that the stem cells detect comprises chemical messaging fueled by the output of neurotransmitters that leak from neuronal synapses, the structures at the ends of brain cells that facilitate communication. These neurotransmitters, released from one neuron and detected by a another one, trigger receiving neurons to change their electrical charges, which either causes the neuron to fire off an electrical pulse propagating communication or to settle down, squelching further messages.

To find out which neurotransmitter brain stem cells can detect, the researchers took mouse brain tissue, attached electrodes to the stem cells and measured any change in electrical charge after the addition of certain neurotransmitters. When they treated the stem cells with the neurotransmitter GABA - a known signal-inhibiting product the stem cells' electrical charges changed, suggesting that the stem cells can detect GABA messages.