Abstract
There is abundant evidence for formation of G protein-coupled receptor heteromers in heterologous expression systems, but little is known of the function of heteromers in native systems. Heteromers of δ and κ opioid receptors (DOR-KOR heteromers) have been identified in native systems. We previously reported that activation of DOR-KOR heteromers expressed by rat pain-sensing neurons (nociceptors) produces robust, peripherally mediated antinociception. Moreover, DOR agonist potency and efficacy is regulated by KOR antagonists via allosteric interactions within the DOR-KOR heteromer in a ligand-dependent manner. Here we assessed the reciprocal regulation of KOR agonist function by DOR antagonists in adult rat nociceptors in culture and in a behavioral assay of nociception. Naltrindole enhanced the potency of the KOR agonist 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1-phenyl-2-pyrrolidin-1-ylethyl]acetamide (ICI-199441) 10- to 20-fold, but did not alter responses to 2-(3,4-dichlorophenyl)-N-methyl-N-[(1R,2R)-2-pyrrolidin-1-ylcyclohexyl]acetamide (U50488). By contrast, the potency of U50488 was enhanced 20-fold by 7-benzylidenenaltrexone. The efficacy of 6ʹ-guanidinonaltrindole (6ʹ-GNTI) to inhibit nociceptors was blocked by small interfering RNA knockdown of DOR or KOR. Replacing 6ʹ-GNTI occupancy of DOR with either naltrindole or 7-benzylidenenaltrexone abolished 6ʹ-GNTI efficacy. Further, peptides derived from DOR transmembrane segment 1 fused to the cell membrane–penetrating HIV transactivator of transcription peptide also blocked 6ʹ-GNTI–mediated responses ex vivo and in vivo, suggesting that 6ʹ-GNTI efficacy in nociceptors is due to its positive allosteric regulation of KOR via occupancy of DOR in a DOR-KOR heteromer. Together, these results provide evidence for the existence of functional DOR-KOR heteromers in rat peripheral sensory neurons and that reciprocal, ligand-dependent allosteric interactions occur between the DOR and KOR protomers.
Footnotes
- Received July 18, 2017.
- Accepted February 1, 2018.
This work was supported by U.S. Public Health Service grants from the National Institutes of Health National Institute of General Medical Sciences [Grant R01 GM 106035] and National Institute on Drug Abuse [Grant R21 DA 037572] (to W.P.C and K.A.B.); the National Institute of Dental and Craniofacial Research (COSTAR) [Training Grant T32 DE14318] (to B.A.J.); and the National Institute of Neurological Disorders and Stroke [Training Grant T32 NS082145] (to M.M.P.).
↵This article has supplemental material available at molpharm.aspetjournals.org.
- Copyright © 2018 by The American Society for Pharmacology and Experimental Therapeutics
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