Brain cells can adopt a new chemical code in response to cues from the outside world, scientists working with tadpoles at the University of California, San Diego report in the journal Nature this week.
The discovery opens the possibility that brain chemistry could be selectively altered by stimulating specific circuits to remedy low levels of neural chemicals that underlie some human ailments.
Dark tadpoles don pale camouflage when exposed to bright light. The researchers have now identified cells in the tadpole brain that respond to illumination by making dopamine, a chemical message, or neurotransmitter, recognized by the system that controls pigmentation.
"We used to think activity turned a switch to specify which transmitters a neuron would use only in early development," said co-author Davide Dulcis, a postdoctoral fellow in neurobiology who designed and conducted the experiments. "But this is happening after hatching."
The cells, found in a cluster called the suprachiasmatic nucleus, connect to a gland that releases a hormone that disperses pigments to darken skin. Dopamine squelches hormone release leaving pigments tightly packed in skin cells and the tadpoles nearly transparent.
"The behavior meets an ecological need." Dulcis said. "Pale tadpoles are difficult for predators to see in a bright environment, so the faster the tadpoles change their pigmentation, the better they are able to survive."
Cells in the core of the cluster always make dopamine, but a ring of surrounding cells normally don't, even though they are connected to the gland.
Bright light alters this pattern, however. After just two hours, cells in the surrounding ring show signs of making the new neurotransmitter. Because they are already hooked up to the hormone-producing target, illumination can result in noticeably paler tadpoles in as little as ten minutes.
"The new dopamine neurons are not simply activated at random," said co-author Nicholas Spitzer, a professor of neurobiology who leads the research group. "It's as if they are a kind of national guard, waiting in reserve to be called out. There's a pool of neurons waiting for the right sensory stimulus to be called into action and to adopt a new transmitter."