Modulating Action of Hyperforin on the P-Type Calcium Channels in the Membranes of Rat Cerebellar Purkinje Neurons

A modulating action of hyperforin (an active compound of the extract from Hypericum perforatum) on a high-threshold component of the calcium current, sensitive to application of 100 nM -Aga-IVA toxin and identified as P current, was studied on freshly isolated Purkinje neurons with the use of a patch-clamp technique in the whole-cell configuration. It was shown that extracellular application of 0.8 M hyperforin caused a shift of the current-voltage (I-V) relationship of P current by -(8 ± 2) mV, slowdown of the activation kinetics, and a decrease in the amplitude of this current. The shift of the I-V relationship and slowdown of activation kinetics developed for less than 10 sec, while the P-current amplitude decreased for a much longer time (several minutes) and depended on the intracellular concentration of Ca²⁺ ions. -Aga-IVA toxin at the concentration of 100 nM completely blocked the recorded inward current in the presence of 0.8 M hyperforin. In experiments with intracellular perfusion of Purkinje neurons, we found that interaction of hyperforin with its binding site occurs at the external side of the cell membrane. The study of the mechanisms involved in the hyperforin-induced P-current modulation revealed that 1 mM GTPS (activating G_Ts proteins, as well as activating or blocking G_Ms proteins) or 1-2 mM GDPS (blocking G_Ts and G_Ms proteins) in the intracellular solution did not affect the hyperforin-induced modulation of P current. Hyperforin-induced Ca²⁺-independent shift of the I-V relationship and slowdown of the activation kinetics of P current were abolished in the presence of 0.5 M calmidazolium in the extracellular medium.