Tryptophan hydroxylase in discrete brain nuclei: comparison of activity in vitro and in vivo.

The increase of 5-hydroxytryptophan after treatment with the decarboxylase inhibitor Ro 4/4602 was 30 times greater in the raphe nuclei (which contain serotonergic cell bodies) than the hippocampus or caudate (which contain serotonergic terminals). Pretreatment with chlorimipramine reduced the 5-hydroxytryptophan accumulation in the median but not dorsal raphe. The large amount of tryptophan hydroxylase in cell bodies appears to be active in vivo. The serotonin rapidly turning over there may have a functional role.