Hyperforin modulates gating of P-type Ca²⁺ current in cerebellar Purkinje neurons

Abstract.

Whole-cell, patch-clamp recordings from acutely isolated cerebellar Purkinje neurons demonstrate a two-stage modulation of P-type high-voltage-activated (HVA) Ca²⁺ current by a constituent of St. John's wort, hyperforin (0.04–0.8 µM). The first stage of modulation was voltage dependent and reversible. It comprised slow-down of the activation kinetics and a shift in the voltage dependence of P-current to more negative voltages. Hyperforin (0.8 µM) shifted the maximum of the current/voltage (I/V) relationship by –8±2 mV. The second, voltage-independent stage of modulation was manifested as a slowly developing inhibition of P-current that could not be reversed within the period of study. Neither form of modulation was abolished by intracellular guanosine 5′-O-(2-thiodiphosphate) (GDPβS) or guanosine 5′-O-(3-thiotriphosphate) (GTPγS) or by strong depolarising pre-pulses, indicating that modulation via guanine nucleotide-binding proteins (G proteins) is not involved in the observed phenomenon. Calmidazolium (0.5 µM), an antagonist of the intracellular Ca²⁺-binding protein calmodulin significantly inhibited the hyperforin-induced shift of the I/V curve maximum and the slow-down of the activation kinetics. It did not, however, affect the delayed inhibition of P-current, indicating that the two stages of modulation are mediated by separate mechanisms.