TRPV1 Enhances CCK Signaling in Vagal Afferent Neurons

Abstract

Subcellular G-protein coupled receptor (GPCR) regulation of membrane coupled ion channels remains incompletely understood. The gut peptide cholecystokinin (CCK) is released during feeding and promotes satiation by increasing excitation of vagal afferent neurons innervating the upper gastrointestinal tract. While the effects of CCK have been studied for decades, specific receptor signaling and coupling to membrane ion channels remain uncharacterized. Vagal afferent neurons express the GPCR CCK1 receptors at the peripheral terminals, in the neuronal cell bodies contained in the nodose ganglion, and at the central terminals in the nucleus of the solitary tract (NTS). The multimodal ion channel transient receptor potential vanilloid subtype 1 (TRPV1) is also expressed throughout vagal afferent neurons and contributes to many forms of signaling, including quantal forms of excitatory glutamate release onto NTS neurons. Previous findings detail that CCK1 receptors couple through a ruthenium red-sensitive pathway and that CCK preferentially activates TRPV1-containing afferents. Together, these findings suggest that CCK1 receptors may directly couple with TRPV1 ion channels. We tested this hypothesis using selective pharmacology, molecular biology, and functional measurements in clonal cells expressing the cognate proteins, dissociated nodose ganglion neurons, and brain slice electrophysiology in the NTS. We found that the presence of TRPV1 was predictive of large and prolonged CCK responses compared to TRPV1-lacking neurons and that removal or augmentation of TRPV1 respectively decreased or increased CCK-driven responses. Further, disruption of inositol binding using a rapamycin inducible system diminished the ability of CCK1 receptors to couple to TRPV1. Together, these results suggest that CCK1 receptor signaling recruits TRPV1 and enhances the efficacy of CCK induced activation of vagal afferent neurons.