Voltage-gated sodium channels (VGSCs) are essential for action potential generation. Furthermore, drugs that directly target VGSCs are widely used to treat common diseases, such as pain, mood disorders, muscle spasms, seizures, and cardiac arrhythmias. However, side effects arise because of the widespread distribution of VGSCs and cross-sensitivity of the various VGSC subtypes to blockers. In addition, these drugs are not completely effective, underlining a substantial need for new drugs that target VGSCs. This has motivated us to identify and characterize new mechanisms by which VGSC function can be regulated. Regulation of voltage- gated ion channel function is an important pathway by which neuronal signaling and brain function is regulated, and G-protein coupled receptors (GPCRs) form a major element of the endogenous transduction mechanisms by which this occurs. However, unlike other ion channels, VGSCs have been assumed to be relatively insensitive to modulation by GPCR signaling. We have recently identified a pathway that is modulated by agents known to interact with the CaSR (calcium-sensing receptor). This pathway is widespread, present in the vast majority of neocortical neurons, and strong enough to completely and reversibly block VGSC currents when maximally stimulated. This novel, dynamic signaling pathway is positioned to substantially modulate neuronal excitability and brain function. Detailed knowledge about the underlying mechanisms is crucial to understand its many effects. The objectives of this proposal are to determine how CaSR modulators regulate VGSCs. Using a combination of electrophysiology and unbiased biochemical approaches we will identify the receptors mediating the inhibition of VGSC currents, measure the relative sensitivity to block of different VGSC isoforms, and determine if the pathway differentially regulates action potentials at nerve terminals and soma. These specific aims will test the hypothesis that CaSR modulators actions via VGSCs represent important new pathways for modulating neuronal excitability. We are ideally suited to perform this project because of our preliminary data and expertise. Our rationale is that the identification and characterization of a novel and prevalent receptor(s) and downstream pathway will facilitate our understanding of a prevalent and potentially powerful neurobiological signaling pathway. Successful completion of these specific aims will characterize new drug targets and eventually will lead to new therapeutics to improve control of pain, seizures, muscle spasm, and arrhythmias.