Distribution of serotonin and synthesizing enzymes in discrete areas of the brain.

With the use of sensitive radio-enzymatic microtechniques, serotonin and its synthesizing enzymes, tryptophan hydroxylase and L-amino acid decarboxylase, can be quantitatively detected in discrete regions of the rat brain. A microdissecting technique allows us to study their localization in specific brain areas and nuclei. Serotonin and its related enzymes are concentrated in the raphe nuclei. Relatively high serotonin concentrations are also present in areas and nuclei of the hypothalamus, limbic system, and brain stem, as well as in the circumventricular organs and the pituitary gland. The localization of its synthesizing enzymes in the same regions suggests that the formation of serotonin actually takes place in these areas. Catecholamines are also present in the raphe nuclei of the rat. Changes in catecholamine metabolism result in changes in serotonin levels in specific brain areas. These results suggest that serotonin may be involved in the regulation of a number of autonomic functions, and in neuroendocrine control mechanisms. The neuroanatomical and biochemical relations between the catecholaminergic and serotonergic system in the brain support the hypothesis of a physiological interaction of these two systems in specific, localized brain structures.