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Vol. 299, Issue 3, 1120-1125, December 2001
Department of Anesthesiology, Medical College of Wisconsin,
Milwaukee, Wisconsin
Development of tolerance in mice pretreated intracerebroventricularly
with µ-opioid receptor agonist endomorphin-1, endomorphin-2, or
[D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin
(DAMGO) was compared between endomorphin-1- and endomorphin-2-induced antinociception with the tail-flick test. A 2-h pretreatment with endomorphin-1 (30 nmol) produced a 3-fold shift to the right in the
dose-response curve for endomorphin-1. Similarly, a 1-h pretreatment with endomorphin-2 (70 nmol) caused a 3.9-fold shift to the right for
endomorphin-2. In cross-tolerance experiments, pretreatment with
endomorphin-2 (70 nmol) caused a 2.3-fold shift of the dose-response curve for endomorphin-1, whereas pretreatment with endomorphin-1 (30 nmol) caused no change of the endomorphin-2 dose-response curve. Thus,
mice acutely tolerant to endomorphin-1 were not cross-tolerant to
endomorphin-2, although mice made tolerant to endomorphin-2 were
partially cross-tolerant to endomorphin-1; an asymmetric cross-tolerance occurred. Pretreatment with DAMGO 3 h before
intracerebroventricular injection of endomorphin-1, endomorphin-2, or
DAMGO attenuated markedly the antinociception induced by endomorphin-1
and DAMGO but not endomorphin-2. It is proposed that two separate
subtypes of µ-opioid receptors are involved in antinociceptive
effects induced by endomorphin-1 and endomorphin-2. One subtype of
opioid µ-receptors is stimulated by DAMGO, endomorphin-1, and
endomorphin-2, and another subtype of µ-opioidreceptors is
stimulated solely by endomorphin-2.
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