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Vol. 284, Issue 3, 1095-1103, March 1998
Leiden/Amsterdam Center for Drug Research, Division of
Pharmacology, University of Leiden, Sylvius Laboratory, P.O. Box
9503, 2300 RA Leiden, The Netherlands
The purpose of our investigation was to characterize the relationships
between the pharmacodynamics of synthetic opioids in vivo
and the interaction at the mu-opioid receptor. The
pharmacokinetics and pharmacodynamics were determined in
vivo after a single i.v. infusion of 3.14 mg/kg alfentanil (A),
0.15 mg/kg fentanyl (F) or 0.030 mg/kg sufentanil (S) in rats.
Amplitudes in the 0.5 to 4.5 Hz frequency band of the
electroencephalogram (EEG) was used as pharmacodynamic endpoint. The
EEG effect intensity was related to the (free) concentration in blood
(A) or in a hypothetical effect compartment (F, S) on basis of the
sigmoidal Emax pharmacodynamic model. The interaction at
the mu-opioid receptor was determined in vitro on
basis of the displacement of [3H]-naloxone binding in
washed rat brain membranes. The value of the sodium shift was used as a
measure of in vitro intrinsic efficacy. For the EEG effect
the in vivo potencies based on free drug concentrations (EC50,u) were 4.62 ± 0.66 ng/ml (A), 0.69 ± 0.05 ng/ml (F) and 0.29 ± 0.06 ng/ml (S). In the receptor binding
studies the affinities at the mu-opioid receptor
(KI) were 47.4 ± 6.6 nM (A), 8.6 ± 4.1 nM (F) and 2.8 ± 0.2 nM (S). For each opioid the ratio
between EC50,u and KI was the same
with a value of 0.23-0.25, indicating the existence of receptor
reserve for the EEG effect. The intrinsic activity (Emax)
of the three opioids in vivo was similar with values of
111 ± 10 µV (A), 89 ± 11 µV (F) and 104 ± 4 µV
(S). However, the values of the sodium shift varied between 2.8 (S) and
19.1 (A). Further analysis of the in vivo pharmacodynamic data on basis of an operational model of agonism provided evidence for
a large receptor reserve, which explains why compounds with different
values of the sodium shift all behave as full agonists in
vivo.
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