"In vitro" study of the mode of action of antidepressants in cell culture models : comparison of the effects of "Hypericum perforatum L." extracts and classical synthetic compounds on the [beta]-adrenergic signal pathway

The clinical effectiveness of the plant extract of Hypericum perforatum L. in treating mild to moderate depression is well established. The extract shows a more favourable side effect profile than other antidepressant drugs, like tricyclics and selective serotonin reuptake inhibitors. Until now, the mode of action of antidepressant drugs is not well understood and it is possible that immediate and adaptive effects contribute to their effectiveness of treating depression. Reuptake inhibition of neurotransmitters, an immediate effect of antidepressant drugs, occurs after drug exposure in minutes to hours. Adaptive effects, like changes in the number of neurotransmitter receptors, phsopholipid (PL) accumulation and alterations of cell membrane properties, need a prolonged, chronic exposure of several days to weeks to develop.
In the present work, influences of chronic exposure of Hypericum perforatum extracts to rat C6 glioblastoma cells (C6 cells) and human skin fibroblasts were investigated. Changes on the cellular level including cell shape, morphology and proliferation rate and on the β-adrenergic signal pathway including membrane properties, number of β-adrenoceptor and accumulation of the second messenger cAMP were monitored. These effects were compared with those of the selective serotonin reuptake inhibitor (SSRI) fluoxetine and the tricyclic antidepressant desipramine (DMI).
Chronic exposure of C6 cells and fibroblasts to a Hypericum extract, fluoxetine or DMI led to a change in the cell morphology. The cell body broadened and vesicular inclusions were observed. These changes were more pronounced, especially in C6 cells, after chronic exposure to fluoxetine or DMI. The effects were not due to cell hypertrophy since the DNA/protein ration remained constant under each treatment condition. In contrast to fluoxetine and DMI, Hypericum extract exposed cells showed a distinct cell growth inhibition.
Chronic exposure of fibroblasts and C6 cells to the Hypericum extract led to change in membrane properties. Fibroblasts exposed to the plant extract showed an accumulation of the total amount of PL, as observed after exposure to fluoxetine or DMI. Moreover, the membrane fluidity of C6 cells and fibroblasts was increased after Hypericum extract treatment, whereas exposure to DMI or fluoxetine did not influence the membrane fluidity.
Complex changes in the relative PL content of the cellular and plasma membrane composition were observed after chronic exposure of C6 cells to Hypericum extract or DM. The most pronounced and consistent effect was the increase of phosphatidylinositol after both treatments.
The β-adrenoceptor number and its signal pathway were affected after chronic exposure of C6 cells to Hypericum extract, fluoxetine or DMI. All treatment conditions resulted in a down-regulation of the β-adrenoceptor number at varying degrees. The cAMP accumulation ofter chronic exposure to all drugs was reduced.
Our results showed that the in vitro effects of the Hypericum extract were compareble to those of the well-established tricyclic antidepressant DMI and the SSRI antidepressant fluoxetine and they might, therefore, share a common mode of action. Hyperforin, a constituent of the Hypericum perforatum extract, ahs gained more attention in recent years. It has been shown that mainly hyperforin is responsible for interactions with co-administered drugs that may lead to severe conditions. Thus, using a hyperforin-"free" and hyperforin-rich fraction of the Hypericum extract, the sifgnificance of hyperforin on the in vitro antidepressant effects in C6 cells was investigated. Down-regulation of the β-adrenoceptor number could be observed in C6 cells after chronic exposure to the hyperforin-"free" fraction. This reduction was comparable to that obtained with the Hypericum extract. Contrarily, chronic exposure of C6 cells to a hyperforin-rich fraction did not reduce the β-adrenoceptor number. Interestingly, exposure of C6 cells to both fractions inhibited the cAMP accumulation comparable to that level observed after exposure to Hypericum extract.
It was shown that a hyperforin-"free" fraction induced changes on the β-adrenoceptor number and on the cAMP accumulation similar to a Hypericum extract, the tricyclic DMI and the SSRI fluoxetine. Consequently, it is tempting to point out that a hyperforin-free Hypericum extract might possess the same potential as a hydroalcoholic Hypericum extract in treating mild to moderate depression, but without inducing metabolic interactions with co-administered drugs.