G Protein-Coupled Receptor Endosomal Signaling and Regulation of Neuronal Excitability and Stress Responses: Signaling Options and Lessons From the PAC1 Receptor.

Our understanding of G protein coupled receptor (GPCR) mechanisms and functions have evolved considerably. Among the many conceptual realignments, GPCRs can exist in an ensemble of active microstates that have the potential to differentially engage specific downstream signaling events. Furthermore, among GPCR dynamics, GPCR internalization and vesicular trafficking are no longer solely mechanisms for desensitization, but now appreciated to form intricate endosomal signaling complexes that can potentially target second messengers to intracellular compartments with high temporal and spatial resolution. The PACAPergic system is important in the maintenance of physiological homeostasis in the central and peripheral nervous systems and activation of the PACAP-selective PAC1 receptor can generate differential but coordinate plasma membrane and endosomal signals for cellular responses. The integration of these signals can modulate PACAP-induced changes in ionic conductances that gate neuronal excitability. PACAP/PAC1 receptor generation of endosomal ERK signals participate in chronic pain and anxiety-like responses which can be attenuated with endocytosis inhibitors. From the abilities of ligands to stabilize the different GPCR microstates for biased downstream signaling, the development of biased PAC1 receptor agonists and antagonists may provide opportunities to dissociate the homeostatic regulatory signals of PACAP from the maladaptive effects. In particular, the development of biased antagonists to PAC1 receptor-mediated endosomal signaling may offer therapeutic options for chronic pain and stress-related disorders. J. Cell. Physiol. 232: 698-706, 2017.