The Gq-GPCR Pathway Evokes Tightly Controlled TRPV1 Activation

Abstract

The detection and propagation of a pain signal must be coupled with the regulation and control of such a signal. TRPV1 is well established as a major pain receptor, capable of detecting a multitude of stimuli, including endogenous inflammatory mediators such as arachidonic acid and anandamide, in addition to exogenous molecules such as the spicy chemical capsaicin. However, endogenous inflammatory mediators evoke noticeably weaker channel activation in comparison to exogenous molecules. Here, we employed pharmacological and molecular biology tools to dissect the regulated activation of TRPV1 in the inflammatory response via the Gq/GPCR pathway. By employing specific techniques such as perforated-patch recordings, the DREADD system, and calcium imaging of both neurons and a heterologous system, we worked to preserve the native intracellular cascades which are essential for Gq-mediated TRPV1 activation. By doing so, we found that two downstream pathways converge on TRPV1 to evoke a tightly regulated response. The first, the production of endo-vanilloids by lipoxygenases, are essential in producing the ligand that activates TRPV1 through the intracellularly located vanilloid binding site. The second, channel sensitization through PKC phosphorylation, is necessary for the metabolites to successfully activate the channel. We propose that through this dual cascade, a tightly regulated channel activation is achieved. Functionally, this regulation prevents the depolarization block associated with exogenous activation of TRPV1 by capsaicin, and ensures prolonged neuronal firing, as is found during nociceptor activation by inflammatory mediators. Overall, we propose that the requirement of multiple signalling events maintains a subdued TRPV1 activation in order to evoke regulated neuronal response during inflammation.