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Table 2 Competition affinities for different opioid receptors and ligands.

From: In Vivo Regulation of the μ Opioid Receptor: Role of the Endogenous Opioid Agents

Receptor Ligand K (mean ± SEM) (nmol/L) % Displacement
dre-µ Met-ENK 684 ± 15 73.97 ± 1.29
  Leu-ENK 1,317 ± 166 58.51 ± 2.68
  β-END 186 ± 25 88.45 ± 2.03
  MEGY 204 ± 53 78.98 ± 3.73
  (d-Ala2)-MEGY 744 ± 27 66.72 ± 0.81
  (d-Ala2, Val5)-MEGY 3,645 ± 621 43.16 ± 3.39
  Morphine 187 ± 94 100
  Naloxone 10.65 ± 1.53 100
   Ki (nmol/L) Reference
dre-δ1 β-END 36.6 29
  MEGY 427 21
  Morphine 22 29
dre-δ2 Met-ENK 45 30
  Leu-ENK 175
  MEGY 146 21
  Morphine 1,400 30
rn-µa Met-ENK 1.80 5
  Leu-ENK 6.19
  MERF 0.37
  Morphine 7.48
rn-δb Met-ENK 0.45
  Leu-ENK 0.37
  MERF 0.57
  Morphine 302
tg-µc Met-ENK 70.7 32
  Leu-ENK 117
  β-END 55.9
tg-δc Met-ENK 24.9
  Leu-ENK 198
  β-END 284
  1. This table summarizes the results obtained in competition binding assays (Ki and % displacement) using (3H)-diprenorphine on membrane homogenates from HEK293 cells that stably express the µ opioid receptor from zebrafish. The results of other competition binding studies for the zebrafish δ opioid receptors dre-δ1 and dre-δ2, prototypical mammalian δ and µ receptors rn-oprm1 (rn, Rattus norvegicus (rat)) and rn-oprm1 and the amphibian µ and δ receptors tg-oprm1 and tg-oprd1 (tg, Taricha granulosa (newt)) have also been included for comparison.
  2. aIn this case, the radioligand used was the µ-selective peptidic analog (3H)-DAMGO and not (3H)-diprenorphine.
  3. bIn this case, the radioligand used was the δ-selective peptidic analog (3H)-DPDPE and not (3H)-diprenorphine.
  4. cIn this case, the radioligand used was the nonspecific antagonist (3H)-naloxone and not (3H)-diprenorphine.