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Abstract
Nociceptin/orphanin FQ (N/OFQ) is the endogenous
ligand for the N/OFQ receptor (NOP), whose characteristics
in the dog are unknown. We therefore compared
[3H]N/OFQ binding in dog and rat brain membranes.
Radioligand saturation/competition studies
with these membranes and leucyl-[3H]N/OFQ(1–
17)OH or the novel radioligand [3H]N/OFQ(1–13)NH2
were performed to determine receptor density and ligand
affinity. The density of classic opioid receptors
was determined by using [3H]diprenorphine. Leucyl-
[3H]N/OFQ(1–17)OH binding was concentration dependent
and saturable in dog (maximum binding capacity
[Bmax], 28.7 2.8 fmol/mg of protein;
equilibrium dissociation constant as negative log [pKd],
10.27 0.11) and rat (Bmax, 137.0 12.9 fmol/mg of
protein; pKd, 10.410.05). In comparison, the Bmax and
pKd of [3H]diprenorphine were, respectively, 77.75.3
fmol/mg of protein and 9.74 0.09 in dog and 79.1
18.2 fmol/mg of protein and 9.51 0.04 in rat. In dog,
[3H]N/OFQ(1–13)NH2 binding to NOP receptors was
also saturable (Bmax, 23.7 2.0 fmol/mg of protein;
pKd, 10.16 0.12). In both species, leucyl-[3H]N/
OFQ(1–17)OH was displaced by various NOP ligands.
Dynorphin A, N/OFQ(1–5)NH2, and nocistatin were
essentially inactive. There was a significant positive
correlation (r2 0.95; P 0.0001) between pKi values
(an estimate of affinity) obtained in displacement studies
in rat and dog. We have demonstrated a low density
of NOP receptors, measured with two radioligands, in
dog, and these receptors display a high degree of pharmacological
similarity with those natively expressed in
the rat
