Ligand requirements for involvement of PKCε in synergistic analgesic interactions between spinal μ and δ opioid receptors.
OBJECTIVE
We recently found that PKCε was required for spinal analgesic synergy between two GPCRs, δ opioid receptors and α2 A adrenoceptors, co-located in the same cellular subpopulation. We sought to determine if co-delivery of μ and δ opioid receptor agonists would similarly result in synergy requiring PKCε.


METHODS
Combinations of μ and δ opioid receptor agonists were co-administered intrathecally by direct lumbar puncture to PKCε-wild-type (PKCε-WT) and -knockout (PKCε-KO) mice. Antinociception was assessed using the hot-water tail-flick assay. Drug interactions were evaluated by isobolographic analysis.


RESULTS
All agonists produced comparable antinociception in both PKCε-WT and PKCε-KO mice. Of 19 agonist combinations that produced analgesic synergy, only 3 required PKCε for a synergistic interaction. In these three combinations, one of the agonists was morphine, although not all combinations involving morphine required PKCε. Morphine + deltorphin II and morphine + deltorphin I required PKCε for synergy, whereas a similar combination, morphine + deltorphin, did not. Additionally, morphine + oxymorphindole required PKCε for synergy, whereas a similar combination, morphine + oxycodindole, did not.


CONCLUSIONS
We discovered biased agonism for a specific signalling pathway at the level of spinally co-delivered opioid agonists. As the bias is only revealed by an appropriate ligand combination and cannot be accounted for by a single drug, it is likely that the receptors these agonists act on are interacting with each other. Our results support the existence of μ and δ opioid receptor heteromers at the spinal level in vivo.


BACKGROUND
This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.
Department of Neuroscience, Medical School, University of Minnesota, Minneapolis, MN, USA.
D J
DJ
Schuster
M D
MD
Metcalf
K F
KF
Kitto
R O
RO
Messing
C A
CA
Fairbanks
G L
GL
Wilcox
D000701:Analgesics, Opioid / Q000494:pharmacology* / Q000627:therapeutic use*; D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects; D004359:Drug Therapy, Combination; D005260:Female; D006358:Hot Temperature; D008024:Ligands; D008297:Male; D018345:Mice, Knockout; D009020:Morphine / Q000494:pharmacology / Q000627:therapeutic use; D009025:Morpholines / Q000494:pharmacology / Q000627:therapeutic use; D009842:Oligopeptides / Q000494:pharmacology / Q000627:therapeutic use; D010146:Pain / Q000188:drug therapy* / Q000378:metabolism; D051744:Protein Kinase C-epsilon / Q000378:metabolism*; D055503:Protein Multimerization; D017465:Receptors, Opioid, delta / Q000819:agonists / Q000378:metabolism*; D017450:Receptors, Opioid, mu / Q000819:agonists / Q000378:metabolism*; D013116:Spinal Cord / Q000378:metabolism
D000701:Analgesics, Opioid; D008024:Ligands; D009025:Morpholines; D009842:Oligopeptides; D017465:Receptors, Opioid, delta; D017450:Receptors, Opioid, mu; C070418:oxymorphindole; C059567:deltorphin; C060208:deltorphin I, Ala(2)-; C060209:deltorphin II, Ala(2)-; D009020:Morphine; D051744:Protein Kinase C-epsilon
PKCε; biased agonism; heterodimer; heteromer; intrathecal; spinal; synergy; δ opioid receptor; μ opioid receptor