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Vol. 281, Issue 1, 123-128, 1997
Departments of
Pharmacology (W.J., C.C.) and
Anesthesiology
(G.W.T.), University of Washington, Seattle, Washington
We investigated whether chronic, in vivo administration
of U50,488H, a kappa-1 opioid agonist, caused the
development of tolerance to both the electrophysiological effects of
applied kappa opioids and endogenously released
dynorphins. In hippocampal slices from drug-naive guinea pigs,
application of U69,593, a kappa-1 agonist, produced a
concentration-dependent inhibition (EC50 = 20 nM) of the
amplitude of the granule cell population response in the dentate gyrus.
In slices from chronically U50,488H-treated animals, the concentration-response curve for U69,593 was shifted 3-fold to the
right (EC50 = 59 nM), with a significant decrease in the
maximal effect of U69,593. We also found that the effects of
endogenously released dynorphins were significantly attenuated by
chronic U50,488H treatment. There was no cross-tolerance between
kappa and mu opioid receptor agonists as
measured with the in vitro electrophysiological assay,
and the noncompetitive N-methyl-D-aspartate receptor
antagonist MK801 did not prevent the development of tolerance to either
the electrophysiological effects or the hypothermic effects of
kappa opioids. Our study demonstrates that
receptor-selective tolerance to the kappa opioid actions
in the guinea pig hippocampus does develop after chronic U50,488H
treatment; but, unlike the mechanisms reported to underlie tolerance to
kappa opioid analgesia, the inhibitory effects in the
hippocampus did not depend on activation of
N-methyl-D-aspartate receptors.
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