Signaling characteristics and functional regulation of delta opioid-kappa opioid receptor (DOP-KOP) heteromers in peripheral sensory neurons

Abstract

Receptor heteromers often display distinct pharmacological and functional properties compared to the individual receptor constituents. In this study, we compared the properties of the DOP-KOP heteromer agonist, 6'-guanidinonaltrindole (6'-GNTI), with agonists for DOP ([D-Pen2,5]enkephalin [DPDPE]) and KOP (U50488) in peripheral sensory neurons in culture and in vivo. In primary cultures, all three agonists inhibited PGE 2 -stimulated cAMP accumulation as well as activated extracellular signal-regulated kinase ½ (ERK) with similar efficacy. ERK activation by U50488 was Gi-protein mediated but that by DPDPE or 6'-GNTI was Gi-protein independent (i.e., pertussis toxin insensitive). Brief pretreatment with DPDPE or U50488 resulted in loss of cAMP signaling, however, no desensitization occurred with 6'-GNTI pretreatment. In vivo, following intraplantar injection, all three agonists reduced thermal nociception. The dose-response curves for DPDPE and 6'-GNTI were monotonic whereas the curve for U50488 was an inverted U-shape. Inhibition of ERK blocked the downward phase and shifted the curve for U50488 to the right. Following intraplantar injection of carrageenan, antinociceptive responses to either DPDPE or U50488 were transient but could be prolonged with inhibitors of 12/15-lipoxgenases (LOX). By contrast, responsiveness to 6'-GNTI remained for a prolonged time in the absence of LOX inhibitors. Further, pretreatment with the 12/15-LOX metabolites, 12-and 15hydroxyeicosatetraenoic acid, abolished responses to U50488 and DPDPE but had no effect on 6'-GNTI-mediated responses either in cultures or in vivo. Overall, these results suggest that DOP-KOP heteromers exhibit unique signaling and functional regulation in peripheral sensory neurons and may be a promising therapeutic target for the treatment of pain.

Characterization of a Potential KOR/DOR Dual Agonist with No Apparent Abuse Liability via a Complementary Structure–Activity Relationship Study on Nalfurafine Analogues

“…Besides the likely lower abuse liability to be developed, another rationale to study KOR/DOR dual agonists as analgesics is the observation that the co-administration of a KOR agonist and a DOR agonist can elicit a synergistic response of antinociception. , Although the existence of a KOR/DOR heterodimer is still debatable, growing in vivo evidence indicates KOR/DOR co-localization and cross-talking both centrally and peripherally. − …”

GPCR heteromers: An overview of their classification, function and physiological relevance

“…These studies were the first strong evidence describing a heteromer-selective ligand, highlighting the potential for the therapeutic targeting of heteromers. Indeed, 6’-GNTI was recently shown to produce prolonged anti-nociception compared to single receptor δ or κ receptor agonists in rats ( 108 ), giving further support for 6’-GNTI and specific targeting of the δ-κ heteromer as a promising avenue for the development of novel pain treatments.…”

Strategies for DevelopingκOpioid Receptor Agonists for the Treatment of Pain with Fewer Side Effects

“…Furthermore, in striatal neurons, 6'-GNTI induced the phosphorylation of Akt, but not ERK1/2, compared to the traditional agonist U69,593 which activated both kinases (Schmid et al, 2013). It appears that the delta opioid receptor (DOPr) has a role in the function of 6'-GNTI, either due to actions on KOPr/DOPr heterodimers or convergence of down-stream signalling pathways (Waldhoer et al, 2005;Jacobs et al, 2019). 6'-GNTI has antinociceptive effects in the radiant-heat tail-withdrawal assay in male mice (Waldhoer et al, 2005) and did not display CPA (Zangrandi et al, 2016).…”