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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Growth hormone is a naturally occurring substance secreted by the brain that helps guide growth in childhood and maintain muscle mass during adulthood.

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Why would such an oddball substance be found in the brain at all?

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They work by inhibiting prostaglandin, a hormone-like substance that plays a part in signalling pain to the brain.

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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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New Scientist magazine reports that the researchers, led by Mark Tuszynski, of the University of California, San Diego, believe a substance called nerve growth factor (NGF) can prevent the death of cells in the brain known as cholinergenic neurons, and stimulate new connections between the cells.

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