St John’s Wort is known to be beneficial at treating mood disorders, particularly mild to moderate depression. Evidence suggests that St John’s Wort acts as an antidepressant by increasing brain levels of serotonin. Some of this effects may be due to the flavonoid content of the plant. In one study researchers administered St John’s Wort with either 6 or 50 % flavonoids to rats and assessed the impact on brain chemistry. Both types of St John’s Wort were standardised for 0.3 % hypericin. Both types of St John’s Wort extract increased brain levels of serotonin in the cortex, and this effect was also seen with the antidepressant drug fluoxetine. However, only the extract containing 50 % flavonoids also increased serotonin levels in the diencephalon and the brainstem. Fluoxetine also increased serotonin in the diencephalon but not the brainstem. Therefore, the flavonoid content of the St John’s Wort may have a significant impact on the overall changes to brain levels of serotonin.
Potentially the most important constituent of St John’s Wort is the naphthodianthrone hypericin. However, evidence suggests that the flavonoid content might also be important. St John’s Wort with a high flavonoid content may result in increased serotonin levels in a larger portion of the brain when compared to hypericin alone.
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