Decreased cerebrospinal fluid concentration of free phenylacetic acid in depressive illness.

Cerebrospinal fluid free phenylacetic acid concentration in a series of depressive patients was significantly lower than values in control subjects. This acid derives from phenylethylamine and the findings may reflect a decrease in its brain formation. Such a deficit may be related to other recent observations of a decrease in urinary output of the major metabolites of the "trace amines", octopamine and tyramine: phenylethylamine is thought to be the precursor of these "trace amines".