Modulation of G[subscript q] - Protein - Coupled Inositol Trisphosphate and Ca[superscript 2+] Signaling by the Membrane Potential

G[subscript q]-protein-coupled receptors (G[subscript q]PCRs) are widely distributed in the CNS and play fundamental roles in a variety of neuronal processes. Their activation results in phosphatidylinositol 4,5-bisphosphate (PIP[subscript 2]) hydrolysis and Ca[superscript 2+] release from intracellular stores via the phospholipase C (PLC)–inositol 1,4,5-trisphosphate (IP[subscript 3]) signaling pathway. Because early G[subscript q]PCR signaling events occur at the plasma membrane of neurons, they might be influenced by changes in membrane potential. In this study, we use combined patch-clamp and imaging methods to investigate whether membrane potential changes can modulate G[subscript q]PCR signaling in neurons. Our results demonstrate that G[subscript q]PCR signaling in the human neuronal cell line SH-SY5Y and in rat cerebellar granule neurons is directly sensitive to changes in membrane potential, even in the absence of extracellular Ca[superscript 2+]. Depolarization has a bidirectional effect on G[subscript q]PCR signaling, potentiating thapsigargin-sensitive Ca[superscript 2+] responses to muscarinic receptor activation but attenuating those mediated by bradykinin receptors. The depolarization-evoked potentiation of the muscarinic signaling is graded, bipolar, non-inactivating, and with no apparent upper limit, ruling out traditional voltage-gated ion channels as the primary voltage sensors. Flash photolysis of caged IP[subscript 3]/GPIP[subscript 2] (glycerophosphoryl-myo-inositol 4,5-bisphosphate) places the voltage sensor before the level of the Ca[superscript 2+] store, and measurements using the fluorescent bioprobe eGFP–PH[subscript PLCδ] (enhanced green fluorescent protein–pleckstrin homology domain–PLCδ) directly demonstrate that voltage affects muscarinic signaling at the level of the IP[subscript 3] production pathway. The sensitivity of G[subscript q]PCR IP[subscript 3] signaling in neurons to voltage itself may represent a fundamental mechanism by which ionotropic signals can shape metabotropic receptor activity in neurons and influence processes such as synaptic plasticity in which the detection of coincident signals is crucial.