Enhancement of Potency and Efficacy of NADA by PKC-Mediated Phosphorylation of Vanilloid Receptor

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Abstract
The search for an endogenous ligand for the vanilloid receptor (VR or TRPV1) has led to the identification of N-arachidonyl dopamine (NADA). This study investigates the role of protein kinase C (PKC)-mediated phosphorylation on NADA-induced membrane currents in Xenopus oocytes heterologously expressing TRPV1 and in dorsal root ganglion (DRG) neurons. In basal state, current induced by 10 μM NADA is 5-10% of the current induced by 1 μM capsaicin or protons at pH 5. However, PKC activator, phorbol 12,13-dibutyrate (PDBu) strongly potentiated (∼15-fold) the NADA-induced current. Repeated application of NADA at short intervals potentiated its own response approximately fivefold in a PKC-dependent manner. PKC inhibitor, bisindolylmaleimide (BIM, 500 nM), a mutant TRPV1 (S800A/S502A), and maximal activation of PKC abolished the potentiation induced by repeated application of NADA. As a further confirmation that NADA could stimulate PKC, pretreatment with NADA potentiated the response of protons at pH 5 (∼20 fold), which was dramatically reduced in the mutant TRPV1. In DRG neurons, capsaicin (100 nM) induced a ∼15 mV depolarization and initiated a train of action potentials compared with 1 μM NADA that produced a ∼5 mV response. Pretreatment with PDBu induced significantly larger depolarization and potentiated NADA-induced current. Furthermore, exposure of NADA to the intracellular surface of the membrane-induced larger currents suggesting inaccessibility to the intracellular binding site might contribute to its weaker action. These results indicate that NADA is a potent agonist of VR when the receptor is in the PKC-mediated phosphorylation state.