[Cellular and molecular effects of the antidepressant hyperforin on brain cells: Review of the literature].

BACKGROUND

Hypericum perforatum is, with Ginkgo biloba, one of the most frequently prescribed medicinal plants in the world. Its popular name, St. John's wort (SJW), is due to the fact that its flowers, yellow, are gathered around the feast of St. John the Baptist (24th June) whereas "wort" is an old English word for plant. Of interest, SJW possesses antidepressant actions and is currently used to alleviate symptoms of mild to moderate depression. Nearly two dozens of bioactive compounds have been isolated from SJW. Hypericin, originally described as a monoamine oxidase inhibitor type A, was thought to be responsible for the antidepressant properties of SJW extracts. However, subsequent studies could not confirm this observation and hyperforin, a phloroglucinol derivative, was shown to display antidepressive properties. Indeed, the efficiency of the extracts of SJW has been reported to be dependent on the concentration of hyperforin. However, its effects on brain cells and on the mechanisms underlying its putative clinical antidepressant effect remain poorly characterized.

OBJECTIVE

The aim of this review article is to propose an overview of the recent scientific publications that have provided new and relevant insights into the neurobiological actions of hyperforin.

RESULTS

Hyperforin has been described as an inhibitor of the reuptake of many neurotransmitters such as dopamine, norepinephrine, serotonin or glutamate. It is thus a potent modulator of synaptic transmission. In addition, it blocks the activity of many receptors such as gamma-aminobutyric acid (GABA) and N-Methyl-D-aspartate (NMDA) receptors. More recently, hyperforin has been shown to activate TRPC6, a Ca(2+)-conducting channel of the plasma membrane, which is the only channel opened by this molecule. Interestingly, the other transient receptor potential channels of C type (TRPC) isoforms (TRPC1, TRPC3, TRPC4, TRPC5 and TRPC7) are insensitive to hyperforin. Due to this specific property, it is now used as a convenient pharmacological tool to investigate the functions of endogenous TRPC6 channels in various cell types. Chronically applied to neuronal cell line PC12, hyperforin promotes the extension of neurites via a mechanism implying TRPC6 channels. It is also known to trigger an intracellular signalling pathway that involves the cAMP-dependent protein kinase A and the transcription factor cyclic adenosine monophosphate response element binding protein (CREB). This leads to an up-regulation of the expression of the brain-derived neurotrophic factor (BDNF) receptor neurotrophic tyrosine kinase (TrkB) and TRPC6. This hyperforin-dependent cascade is controlled by Ca(2+) ions and occurs specifically in the cortex but not in the hippocampus. One key aspect of the cellular responses induced by hyperforin is its impact on the homeostasis of several cations (Na(+), Ca(2+), Zn(2+) and H(+)). In vitro experiments demonstrated that hyperforin, which changes the fluidity of membranes, elevates the intracellular concentration of these elements by promoting their influx and/or their release from internal compartments.

CONCLUSIONS

The phloroglucinol derivative hyperforin is an important bioactive molecule of Hypericum perforatum exhibiting antidepressive properties. Although it inhibits the reuptake of many neurotransmitters, hyperforin is in fact a multi-target drug influencing the cellular homeostatic mechanisms of Ca(2+), Zn(2+), H(+) and Na(+) due to its effects on their influx and/or release from internal stores. In addition, hyperforin is a potent modulator of mitochondrial functions. In spite of recent progress in the characterization of the cellular hyperforin responses, it remains unclear what pharmacological aspects of hyperforin functions are relevant in vivo.