G-protein activation revealed by [35S]-GTPγS binding assay is involved on the antidepressant-like effect of Hypericum caprifoliatum and Hypericum polyanthemum cyclohexane extracts

Previous studies by us demonstrated the antidepressant-like and antinociceptive effects of lipophilic extracts and dimeric acyl-phloroglucinols from species of the genus Hypericum native to Southern Brazil. Uliginosin B and HC1 (an enriched phloroglucinol fraction from Hypericum caprifoliatum) are able to inhibit monoamine synaptosomal uptake without binding to the monoaminergic sites on neuronal transporters, unlike classical antidepressants. The current study aimed at investigating the action of H. caprifoliatum Cham. & Schltdl. and Hypericum polyanthemum Klotzsch ex Reichardt, Hypericaceae, cyclohexane extracts and their main component, HC1 and uliginosin B, on G protein coupled receptors by using the [35S]-guanosine-5′-O-(3-thio)triphosphate ([35S]-GTPγS) binding assay, which reveals the G protein activity. The antidepressant-like effect of acute (one or three treatments within 24 h) and repeated (five days with and without a three day wash-out) treatments with the cyclohexane extracts was evaluated using the rat forced swimming test. The [35S]-GTPγS binding to monoamines and opioid receptors stimulated by agonists was performed ex vivo in brain membranes of rats acutely or repeatedly treated with the cyclohexane extracts. The effect of HC1 and Uliginosin B on [35S]-GTPγS binding assay was performed by direct incubation with brain membranes in the absence of agonists. Their antidepressant-like effect was evaluated through the mice forced swimming test. The extracts, HC1 and Uliginosin B showed antidepressant-like effect in the forced swimming test. The acute treatments with extracts increased the [35S]-GTPγS binding stimulated by the monoamines, while after five days of treatment the [35S]-GTPγS binding was reduced even after three day wash-out. These effects are not due to HC1 or Uliginosin B interaction with the receptors, since direct incubation with these phloroglucinols did not affect [35S]-GTPγS binding to membranes. Our findings indicate that H. caprifoliatum and H. polyanthemum extracts bring about adaptive changes in monoamine receptors, which reinforces their antidepressant-like profile.

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