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Presynaptic Inhibition via a Phospholipase C- and Phosphatidylinositol Bisphosphate-Dependent Regulation of Neuronal Ca²⁺ Channels

Department of Pharmacology, Center of Biomolecular Medicine and Pharmacology, Medical University of Vienna, Vienna, Austria

Presynaptic inhibition of transmitter release is commonly mediated by a direct interaction between G protein subunits and voltage-activated Ca²⁺ channels. To search for an alternative pathway, the mechanisms by which presynaptic bradykinin receptors mediate an inhibition of noradrenaline release from rat superior cervical ganglion neurons were investigated. The peptide reduced noradrenaline release triggered by K⁺-depolarization but not that evoked by ATP, with Ca²⁺ channels being blocked by Cd²⁺. Bradykinin also reduced Ca²⁺ current amplitudes measured at neuronal somata, and this effect was pertussis toxin-insensitive, voltage-independent, and developed slowly within 1 min. The inhibition of Ca²⁺ currents was abolished by a phospholipase C inhibitor, but it was not altered by a phospholipase A₂ inhibitor, by the depletion of intracellular Ca²⁺ stores, or by the inactivation of protein kinase C or Rho proteins. In whole-cell recordings, the reduction of Ca²⁺ currents was irreversible but became reversible when 4 mM ATP or 0.2 mM dioctanoyl phosphatidylinositol-4,5-bisphosphate was included in the pipette solution. In contrast, the effect of bradykinin was entirely reversible in perforated-patch recordings but became irreversible when the resynthesis of phosphatidylinositol-4,5-bisphosphate was blocked. Thus, the inhibition of Ca²⁺ currents by bradykinin involved a consumption of phosphatidylinositol-4,5-bisphosphate by phospholipase C but no downstream effectors of this enzyme. The reduction of noradrenaline release by bradykinin was also abolished by the inhibition of phospholipase C or of the resynthesis of phosphatidylinositol-4,5-bisphosphate. These results show that the presynaptic inhibition was mediated by a closure of voltage-gated Ca²⁺ channels through depletion of membrane phosphatidylinositol bisphosphates via phospholipase C.

Address correspondence to: Dr. Stefan Boehm, Department of Pharmacology, Center of Biomolecular, Medicine and Pharmacology, Medical University of Vienna, Waehringerstrasse 13a, A-1090 Vienna, Austria. E-mail: Stefan.Boehm{at}meduniwien.ac.at

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