Figure 7. Model of hyperforin action on monoamine uptake.

(a, b) Relative changes of the SBFI fluorescence ratio (F₃₄₀/F₃₈₀), representing intracellular Na⁺ changes (a), and the BCECF fluorescence ratio (F₄₉₀/F₄₅₀), representing intracellular pH changes (b), in HEK cells during the application (see arrowhead) of 30 μM (a) and 10 μM (b) hyperforin in the presence and absence of Na⁺ (replaced by NMDG⁺). In (a) 0.3% DMSO was applied as control. In (b) control experiments without application of hyperforin are shown in faint colors. Data represent means ± S.E.M. of the indicated number (n) of experiments including x cells (n/x). (c) Mechanism of monoamine uptake (left side of the model): Cellular (synaptic) uptake of monoamines (MA⁺) is arranged by the plasma membrane monoamine-sodium symporter (MAT; (1)). The negative membrane potential and low intracellular Na⁺ concentration provide the driving force for cellular Na⁺ and MA⁺ uptake. Vesicular monoamine uptake is arranged by the vesicular monoamine-proton antiporter (VMAT; (2)), moving MA⁺ into the vesicle in exchange for two protons. The huge H⁺ gradient and the vesicular membrane potential, both established by the vesicular proton pump (H⁺-ATPase) drives H⁺ out of the vesicle and thereby promotes vesicular MA⁺ uptake. Effects of hyperforin on monoamine uptake (right side of the model): The protonophore action of hyperforin moves H⁺ into the cell (synapse) due to the negative membrane potential (3). In addition, hyperforin moves H⁺ out of the vesicles, driven by the huge H⁺ gradient and positive vesicular membrane potential (4). The dissipation of the vesicular H⁺ gradient ruins the driving force for the vesicular VMAT-dependent MA⁺ uptake (2). The cytosolic acidification, mediated by the hyperforin-dependent H⁺ influx and H⁺ release from vesicles, increases the activity of the plasma membrane sodium-proton exchanger (NHE; (5)), which is driven by the Na⁺ gradient and the negative membrane potential, and moves one H⁺ out of the cell in exchange of one Na⁺ into the cell. This results in an increase of cytosolic Na⁺ concentration, reducing the driving force for the MAT-dependent cellular MA⁺ uptake (1).