Until about five years ago, the very idea that peptide
hormones might be made anywhere in the brain besides
the hypothalamus was astounding. But 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 DNA's (cDNA's) that can serve as
molecular probes to seek out the messenger RNA's
(mRNA's) of the peptide hormones. The brain cells
containing these mRNA's can then be isolated and their
mRNA's decoded to determine just what their protein
products are and how closely the products resemble the
true peptide hormones.