Reduction of high-affinity β2-adrenergic receptor binding by hyperforin and hyperoside on rat C6 glioblastoma cells measured by fluorescence correlation spectroscopy

beta-Adrenergic receptors (beta-AR) are potential targets for antidepressants. Desensitization and downregulation of beta-AR are discussed as possible modes of action for antidepressants. We have investigated the effects of hyperforin and hyperoside, compounds with potentially antidepressant activity from St. John's Wort, on the binding behavior and dynamics of beta(2)-AR in living rat C6 glioblastoma cells, compared to desipramine (desmethylimipramine; DMI) by means of fluorescence correlation spectroscopy (FCS) and fluorescence microscopy. FCS-binding studies with the fluorescently labeled ligand Alexa532-noradrenaline (Alexa532-NA) binding to beta(2)-AR of C6 cells showed a significant reduction in total beta(2)-AR binding after preincubation with hyperforin and hyperoside for 3 days, respectively, which was also found for DMI. This was mainly observed in high-affinity receptor-ligand complexes with hindered lateral mobility (D-2 = 1.1 (+/- 0.4) mu m(2)/s) in the biomembrane. However, internalization of beta(2)-AR was found neither in z-scans of these C6 cells nor in HEK 293 cells stably transfected with GFP-tagged beta(2)-adrenergic receptors (beta(2)AR-GFP) after incubation up to 6 days with either DMI, hyperforin, or hyperoside. Thus, under these conditions reduction of beta(2)-AR binding was not mediated by receptor internalization. Additionally, preincubation of C6 cells with DMI, hyperforin, and hyperoside led to a loss of second messenger cAMP after beta(2)-adrenergic stimulating conditions with terbutaline. Our current results indicate that hyperforin and hyperoside from St. John's Wort, as well as DMI, reduce beta(2)-adrenergic sensitivity in C6 cells, emphazising the potential usefulness of St. John's Wort dry extracts in clinical treatment of depressive symptoms.