Inhibition of L-type Ca2+ channels by curcumin requires a novel protein kinase-theta isoform in rat hippocampal neurons

Curcumin, a major active compound of Curcuma longa, has been reported to have potent neuroprotective activities. However to date, the relevant mechanisms still remain unclear. In this study, we report that curcumin selectively inhibits L-type Ca2+ channel currents in cultured rat hippocampal neurons. Whole-cell currents were recorded using 10 mM barium as a charge carrier. Curcumin reversibly inhibited high-voltage-gated Ca2+ channel (HVGCC) currents (IBa) in a concentration-dependent manner but had no apparent effects on the cells treated with nifedipine, a specific L-type Ca2+ channel blocker. Curcumin did not markedly affect the activation of L-type Ca2+ channels while significantly shifted the inactivation curve in the hyperpolarizing direction. Pretreatment of cells with the classical and novel PKC antagonists GF109203X and calphostin C completely abolished curcumin-induced IBa inhibition, whereas the classical PKC antagonist Gö6976 or inhibition of PKA activity elicited no such effects. Moreover, the curcumin-induced IBa response was abolished by intracellular application of the PKC-θ inhibitory peptide PKC-θ-IP or by siRNA knockdown of PKC-θ in cultured rat hippocampal neurons. In these neurons, novel isoforms of PKC including delta (PKC-δ), epsilon (PKC-ɛ) and theta (PKC-θ), but not eta (PKC-η), were endogenously expressed. Taken together, these results suggest that curcumin selectively inhibits IBavia a novel PKC-θ-dependent pathway, which could contribute to its neuroprotective effects in rat hippocampal neurons.