A commercially available extract of the aerial parts of Hypericum perforatum, LI 160,
showed pronounced activity in selected animal bioassays. These include the forced
swimming test (FST) and the tail suspension test, used to determine antidepressant
activity, and tests indicating activity on the central nervous system, such as body
temperature and ketamine induced sleeping time. The counteracting effects of drugs
known to interfere with the central dopaminergic system strongly suggested that dopamine
mediated activity is important for the activity of the extract. Dose-response experiments
of the total extract and of fractions rich in flavonoids and naphthodianthrones produced
inverted U-shaped dose response curves.