(FORTUNE Magazine) – There's money in the brain's complexity. Twenty or so new biotech companies and giants like Eli Lilly and Hoffmann-La Roche are trying to cash in on neuroscience. By FORTUNE's estimate, the small fry have raised about $250 million from venture capitalists and Wall Street since 1985. Between 1983 and 1988, big corporations more than doubled spending on neuroscience research, to nearly $800 million a year. As the brain's complexities become better understood, opportunities multiply for drugs that are precision-engineered to treat specific disorders: -- Depression. The biggest winner so far is Eli Lilly, whose antidepressant Prozac will account for an estimated $775 million in sales this year. Earlier antidepressants, known as tricyclics, affected a number of chemicals in the brain but caused disturbances in heart rhythm, blurred vision, wooziness, and other side effects. Lilly scientists designed Prozac to maintain a healthy level of a single chemical, serotonin, which plays a key role in emotions and moods. This rifle shot approach limits side effects while improving the patient's sense of well-being. -- Food cravings. In 1983, MIT biochemists Richard and Judith Wurtman discovered a key link between food and moods. They proved that people who crave carbohydrates -- he calls them ''the pasta eaters, the bagel eaters, the croissant eaters'' -- do so because carbohydrates boost serotonin levels in the brain. To control this craving without inducing depression, Richard Wurtman applied an experimental drug called Dexfenfluramine. In 1984, MIT licensed it to a privately owned company in France; this year the drug's sales in Europe are expected to exceed $90 million. The Wurtmans plan to seek FDA approval to introduce the drug in the U.S. through Interneuron Pharmaceuticals, a private company they founded two years ago. -- Alzheimer's disease. Warner-Lambert hopes to win FDA approval soon for tacrine, a memory enhancer that may help victims of Alzheimer's maintain some of their cognitive abilities. Meanwhile Eli Lilly and Athena Neurosciences, a privately held four-year-old company in South San Francisco, are probing the causes of Alzheimer's in an ambitious joint venture employing more than 50 scientists. -- Anxiety. Neurogen, a two-year-old public company in Branford, Connecticut, is designing a drug to modulate the uptake of GABA, a brain chemical related to anxiety. GABA binds itself to neurons at submicroscopic docking ports known as receptors. Using genetic engineering techniques, Neurogen has isolated the receptors in a test tube; now it is engineering a drug for a precise fit that will supplement GABA with minimum side effects. The new drug is being tested on rats; Neurogen expects clinical trials on humans to begin in 1992. -- Migraine. Glaxo Inc., an American subsidiary of Britain's Glaxo Holdings PLC, has applied for FDA approval of a drug called sumatriptan to combat migraine. The illness is thought to result from the swelling of small blood vessels around the brain; sumatriptan works by causing them to constrict. Other major disorders, such as stroke and Parkinson's disease, are the focus of a dozen or more startup companies, from Neurex of Menlo Park, California, to Cephalon of West Chester, Pennsylvania. Regeneron, of Tarrytown, New York, is working on antiparalysis drugs that will help victims of spinal injuries by stimulating the regeneration of nerve cells. Alkermes, of Cambridge, Massachusetts, is seeking a way to get medication past the blood-brain barrier, a sheath of cells that protects the brain from many foreign substances. If it can be outsmarted, many more drugs that look promising in the lab will find their way into the market.