For instance, Merck and Pfizer spent years trying to develop drugs that worked on a mysterious brain chemical called substance P.

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When they then gave the animals a compound that blocked substance P, they found that it also blocked the distress response.

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Blocking substance P triggers a molecular chain reaction that boosts levels of the neurotransmitter serotonin, apparently much more selectively than Prozac does.

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Since substance P is activated in response to noxious stimuli, scientists long thought that it might be a central messenger for pain.

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But drugs that block substance P proved mostly ineffective as painkillers.

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Merck faces heavy competition in almost every area of brain research, especially from Pfizer, which has been working on substance P blockers for depression and on new, safer antianxiety drugs.

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Collaborating with colleagues in the U.S. in the mid-1990s, the Terlings Park researchers were the first to show that substance P might play a role in depression, when they noticed that guinea pigs cried out in alarm when compounds similar to substance P were injected into their brains.

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