Daily variations of various parameters of serotonin metabolism in the rat brain. I. Circadian variations of tryptophan-5-hydroxylase in the raphe nuclei and the striatum.

Daily changes in tryptophan-5-hydroxylase (TrH) activity have been studied in six different 5-HT-containing cell groups (B3, B4, B5, B6, B7, B8,) in the brain stem of rats maintained in a regular cycle of 12 h of light and 12 h of darkness. Significant circadian alternations of TrH activity were observed in most of the raphe nuclei tested, although the changes were not identical from one structure to another. The striatum showed daily variations in TrH activity in opposite phase to the nucleus raphe dorsalis which projects specific terminals into this area. Monoamine oxidase (MAO) activity was simultaneously estimated in these nuclei but did not exhibit and significant rhythm, suggesting a specific regulation of TrH. Total protein levels were also subject to daily changes.