# G Protein‐Coupled Receptor Kinase 5 (GRK5) Modulates Nociceptin/Orphanin FQ Opioid (NOP) Receptor Desensitization in Rat Sympathetic Neurons

## TL;DR

This study shows that GRK5, a specific kinase, is crucial for desensitizing NOP opioid receptors in rat sympathetic neurons, affecting calcium channel activity.

## Contribution

The first report showing GRK5 regulates NOP receptor desensitization linked to CaV2.2 channels in sympathetic neurons.

## Key findings

- GRK5 silencing prevents full desensitization of Ca2+ currents in NOP receptors.
- GRK2 silencing alone does not alter NOP receptor desensitization.
- GRK5 modulates NOP receptor coupling to CaV2.2 channels in rat SG neurons.

## Abstract

Stimulation of nociceptin/orphanin FQ peptide (NOP) opioid receptors by the endogenous ligand nociceptin (Noc) leads to voltage‐gated Ca2+ channel inhibition or G protein inwardly rectifying K+ channel activation. One mechanism of G protein‐coupled receptor (GPCR) desensitization occurs when G protein‐coupled receptor kinases (GRK) phosphorylate the agonist‐bound receptors. In the continued presence of an agonist, Gβγ recruits GRK to the plasma membrane where GPCR are then phosphorylated by GRK. The purpose of this study was to identify the GRK subtype responsible for desensitization of the Noc‐mediated Ca2+ current inhibition in rat stellate ganglion (SG) neurons. We observed that GRK2 and GRK5 are expressed in SG neurons. Further, silencing either GRK subtype alone or together employing siRNA did not overtly alter their Noc pharmacological profile. We assessed NOP receptor desensitization employing a protocol where the peak Ca2+ current inhibition was measured during intermittent application of high Noc concentrations in the continued presence of the IC50 Noc concentration. With this approach, we observed complete Ca2+ current desensitization in neurons transfected with either scrambled or GRK2 siRNA following exposure to high Noc concentrations. On the other hand, full desensitization of the Ca2+ currents was not observed in neurons in which GRK5 was silenced alone or with GRK2. That is, coupling of NOP receptors with Ca2+ channels was still observed following application of high Noc concentration. These results suggest that GRK5 plays a key role in the mechanism that mediates NOP receptor desensitization in SG neurons.

NOP opioid receptors are expressed in sympathetic stellate ganglion (SG) neurons. We examined the role of G protein‐coupled receptor kinase (GRK) proteins in the nociceptin‐mediated NOP receptor desensitization of Ca2+ currents in rat SG neurons. Silencing of GRK2 or GRK5 individually or together demonstrated that Ca2+ current desensitization was modulated by GRK5. This is the first report to show that the membrane‐associated GRK5 isoform regulates the desensitization pathway that couples NOP opioid receptors with CaV2.2 channels.