The results were striking, said Mickal Degoulet, a postdoctoral researcher in Morikawa's lab. The isolated rats were much quicker to form a preference for the small, distinctive box in which they received amphetamine or alcohol than were the never-isolated control group. Nearly all the isolated rats showed a preference after just one exposure to either drug. The control rats only became conditioned after repeated exposures.
Morikawa said that this kind of preference for the environmental context in which the reward was received provides researchers with a more useful way of understanding addiction than seeing it as a desire for more of the addictive substance.
"When you drink or take addictive drugs, that triggers the release of dopamine," he said. "People commonly think of dopamine as a happy transmitter or a pleasure transmitter, which may or may not be true, but it is becoming increasingly clear that it is also a learning transmitter. It strengthens those synapses that are active when dopamine is released. It tells our brain that what we're doing at that moment is rewarding and thus worth repeating."
In an important sense, says Morikawa, you don't become addicted to the experience of pleasure or relief but to the constellation of environmental, behavioral and physiological cues that are reinforced when the substance triggers the release of dopamine in the brain.
Morikawa and Whitaker have also been able to document these changes at the neuronal level. Social isolation primes dopamine neurons in the rats' brain to quickly learn to generate spikes in response to inputs from other brain areas. So dopamine neurons will learn to respond to the context more quickly.
If the control, group-housed rats are given enough repeated exposure to amphetamine, they eventually achieve the same degree of addiction as the socially isolated rats. Even from this point of comparable addiction, however, there are d
|Contact: Daniel Oppenheimer|
University of Texas at Austin