A brain chemical linked to pleasure and depression may also trigger fear, according to a new study. Researchers say this may explain why the neurotransmitter dopamine, known to cause addictive behavior, may also play a role in anxiety disorders.

"Showing that dopamine can enhance both approach and avoidance behaviors is an important finding," says Howard Fields, a neurobiologist at the University of California, San Francisco. Approach behavior describes what someone attracted to an object does to obtain it. Fields says the finding reveals a new potential target for treating puzzling neurological disorders such as schizophrenia.

Scientists have long suspected that dopamine was linked to dread as well as delight. To confirm their suspicions, University of Michigan at Ann Arbor researchers studied what happens to rats when the neurotransmitter is blocked from reaching the rear portion of the nucleus accumbens, a brain region where dopamine is produced and reward-seeking activities (such as eating and other urges) as well as emotions including fear are processed.

Their findings, published in the Journal of Neurology: the animals remained calm even when scientists also removed a fear-controlling brain chemical (glutamate), which ordinarily would have sent them into a tizzy. This suggests that too much dopamine in the rear of the nucleus accumbens (linked to dread) may at least be partly responsible for the paranoia that many schizophrenia patients experience, study co-author Kent Berridge says.

"Some researchers have thought that dopamine may drive paranoia in schizophrenics," he adds. "The results are consistent with that idea."