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Until about five years ago, the very idea that peptide hormones might be made anywhere in the brain besides the hypothalamus was astounding. Peptide hormones, scientists thought, were made by endocrine glands and the hypothalamus was thought to be the brains only endocrine gland. What is more, because peptide hormones cannot cross the blood-brain barrier, researchers believed that they never got to any part of the brain other than the hypothalamus, where they were simply produced and then released into the bloodstream. But these beliefs about peptide hormones were questioned as laboratory after laboratory found that antiserums to peptide hormones, when injected into the brain, bind in places other than the hypothalamus, indicating that either the hormones or substances that cross-react with the antiserums are present. The immunological method of detecting peptide hormones by means of antiserums, however, is imprecise. Cross-reactions are possible and this method cannot determine whether the substances detected by the antiserums really are the hormones, or merely close relatives. Furthermore, this method cannot be used to determine the location in the body where the detected substances are actually produced. New techniques of molecular biology, however, provide a way to answer these questions. It is possible to make specific complementary DNAs that can serve as molecular probes seek out the messenger RNAs of the peptide hormones. If brain cells are making the hormones, the cells will contain these mRNAs. If the products the brain cells make resemble the hormones but are not identical to them, then the c DNAs should still bind to these mRNAs, but should not bind as tightly as they would to m RNAs for the true hormones. The cells containing these mRNAs can then be isolated and their mRNAs decoded to determine just what their protein products are and how closely the products resemble the true peptide hormones. | 
