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Nearly every substance that helps run and control the brain has turned up in the gut, Dr. Gershon said. Major neurotransmitters like serotonin, dopamine, glutamate, norepinephrine and nitric oxide are there. Two dozen small brain proteins, called neuropeptides, are in the gut, as are major cells of the immune system. Enkephalins, one class of the body's natural opiates, are in the gut. And in a finding that stumps researchers, the gut is a rich source of benzodiazepines -- the family of psychoactive chemicals that includes such ever popular drugs as Valium and Xanax. In evolutionary terms, it makes sense that the body has two brains, said Dr. David Wingate, a professor of gastrointestinal science at the University of London and a consultant at Royal London Hospital. The first nervous systems were in tubular animals that stuck to rocks and waited for food to pass by, Dr. Wingate said. The limbic system is often referred to as the "reptile brain."