Morphine antinociception is enhanced in mdr1a gene-deficient mice.
OBJECTIVE
Previous studies have suggested that P-glycoprotein (P-gp) modulates opioid antinociception for selected mu-and delta-agonists. This study was undertaken to assess morphine antinociception in mice lacking the mdr1a gene for expression of P-gp in the CNS.


METHODS
Morphine (n = 4-5/group) was administered as a single s.c. dose to mdr1a(-/-) mice (3-5 mg/kg) or wild-type FVB controls (8-10 mg/kg). Tail-flick response to radiant heat, expressed as percent of maximum response (%MPR), was used to determine the antinociceptive effect of morphine. Concentrations in serum, brain tissue, and spinal cord samples obtained immediately after the tail-flick test were determined by HPLC with fluorescence detection. Parallel experiments with R(+)-verapamil, a chemical inhibitor of P-gp, also were performed to further investigate the effect of P-gp on morphine-associated antinociception.


RESULTS
Morphine-associated antinociception was increased significantly in the mdr1a(-/-) mice. The ED50 for morphine was > 2-fold lower in mdr1a(-/-) (3.8+/-0.2 mg/kg) compared to FVB (8.8+/-0.2 mg/kg) mice. However, the EC50 derived from the brain tissue was similar between the two mouse strains (295 ng/g vs. 371 ng/g). Pretreatment with R(+)-verapamil produced changes similar to those observed in gene-deficient mice. P-gp does not appear to affect morphine distribution between spinal cord and blood, as the spinal cord:serum morphine concentration ratio was similar between gene-deficient and wild-type mice (0.47+/-0.03 vs. 0.56+/-0.04, p>0.05).


CONCLUSIONS
The results of this study are consistent with the hypothesis that P-gp attenuates the antinociceptive action of morphine by limiting the brain:blood partitioning of the opioid.
Zong
J
J
Division of Drug Delivery and Disposition, School of Pharmacy, University of North Carolina at Chapel Hill 27599-7360, USA.
Pollack
G M
GM
D018435:ATP Binding Cassette Transporter, Subfamily B / Q000235:genetics / Q000502:physiology*; D018528:ATP-Binding Cassette Transporters / Q000235:genetics / Q000502:physiology*; D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D018432:Drug Resistance, Multiple; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D009020:Morphine / Q000494:pharmacology*; D014700:Verapamil / Q000494:pharmacology
D018435:ATP Binding Cassette Transporter, Subfamily B; D018528:ATP-Binding Cassette Transporters; D000701:Analgesics, Opioid; D009020:Morphine; C095122:multidrug resistance protein 3; D014700:Verapamil