Neuropharmacology of St. John's Wort (Hypericum)

To review preclinical information related to possible antidepressant mechanism(s) of action of St. John's wort in order to address the issue of whether its purported clinical effectiveness has a rational pharmacologic basis.

Primary and review articles were identified by a MEDLINE search (1966-January 1998) and through secondary sources. Many of the original German articles had English abstracts, but where necessary, German articles were translated into English. The results of a new screen of hypericin activity at receptor and uptake sites are summarized.

All of the articles identified from the data sources were evaluated and all information deemed relevant was included in this review.

The neuropharmacology of St. John's wort has been examined in only a few studies. A mechanism similar to that of the synthetic antidepressants, such as the selective serotonin-reuptake inhibitors or monoamine oxidase (MAO) inhibitors, might play a role, but other mechanisms are possible.

Hypericum extracts have only weak activity in assays related to mechanisms of the synthetic antidepressants, that is, inhibition of MAO, catechol O-methyltransferase, or serotonin reuptake. It has been postulated that the clinical efficacy of St. John's wort could be attributable to the combined contribution of several mechanisms, each one too weak by itself to account for the overall effect. The recent demonstration of a significant affinity of hypericin for sigma receptors presents new possibilities for consideration.