Effects of oral administration of extracts of Hypericum perforatum (St John's wort) on brain serotonin transporter, serotonin uptake and behaviour in mice

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Abstract
The pharmacological effects of extracts of Hypericum perforatum (St John's wort) were characterized in‐vitro and ex‐vivo, in relation to its behavioural effects. In in‐vitro experiments, St John's wort inhibited brain synaptosomal [3H]serotonin uptake in mice with little effect on specific [3H]paroxetine binding. For selective serotonin‐reuptake inhibitors (SSRIs), the IC50 value for [3H]serotonin uptake (molar concentration of unlabelled drug necessary to displace 50% of specific uptake) correlated well with the inhibition constant Ki value for [3H]paroxetine binding in mouse brain. Oral administration of St John's wort (900 mg kg−1), paroxetine (1 mg kg−1) and sertraline (10 mg kg−1) brought about significant increases in the Km value for [3H]serotonin uptake into brain synaptosomes 4 h later, and only SSRIs suppressed specific [3H]paroxetine binding in mouse brain. St John's wort and SSRIs significantly inhibited marble‐burying behaviour in mice and the time‐course of attenuation of this behaviour by St John's wort was similar to that of [3H]serotonin uptake inhibition. In the forced swimming test, St John's wort, but not SSRIs, suppressed the immobility time of mice after oral administration. These results provide the first in‐vivo evidence to suggest that the mode of antidepressant action of St John's wort differs from that of SSRIs. Thus, this study may have a significant impact on phytotherapy with St John's wort.