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

Whole-cell, patch-clamp recordings from acutely isolated cerebellar Purkinj e neurons demonstrate a two-stage modulation of P-type high-voltage-activat ed (HVA) Ca2+ current by a constituent of St. John's wort, hyperforin (0.04 -0.8 mu M). The first stage of modulation was voltage dependent and reversi ble. It comprised slow-down of the activation kinetics and a shift in the v oltage dependence of P-current to more negative voltages. Hyperforin (0.8 m u M) 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 wit hin the period of study. Neither form of modulation was abolished by intrac ellular guanosine 5'-O-(2-thiodiphosphate) (GDP beta S) or guanosine 5'-O-( 3-thiotriphosphate) (GTP gamma S) or by strong depolarising pre-pulses, ind icating that modulation via guanine nucleotide-binding proteins (G proteins ) is not involved in the observed phenomenon. Calmidazolium (0.5 mu M), an antagonist of the intracellular Ca2+-binding protein calmodulin significant ly inhibited the hyperforin-induced shift of the I/V curve maximum and the slowdown of the activation kinetics. It did not, however, affect the delaye d inhibition of P-current, indicating that the two stages of modulation are mediated by separate mechanisms.