Changes in morphine-induced activation of cerebral Na+,K+-ATPase during morphine tolerance: Biochemical and behavioral consequences

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There is ample evidence of the biological changes produced by the sustained activation of opioid receptors. We evaluated the adaptive changes of cerebral Na+,K+-ATPase in response to the sustained administration of morphine (minipumps, 45 mg/kg/day, 6 days) in CD-1 mice and the functional role of these changes in opioid antinociception. The antinociceptive effect of morphine as determined with tail-flick tests was reduced in morphine-tolerant mice. There were no significant changes in the density of high-affinity Na+,K+-ATPase α subunits labeled with [3H]ouabain in forebrain membranes from morphine-tolerant compared to those of morphine-naive animals. Western blot analysis showed that there were no significant differences between groups in the changes in relative abundance of α1 and α3 subunits of Na+,K+-ATPase in the spinal cord or forebrain. However, the morphine-induced stimulation of Na+,K+-ATPase activity was significantly lower in brain synaptosomes from morphine-tolerant mice (EC50 = 1.79 ± 0.10 μM) than in synaptosomes from morphine-naive mice (EC50 = 0.69 ± 0.12 μM). Furthermore, adaptive alterations in the time-course of basal Na+,K+-ATPase activity were observed after sustained morphine treatment, with a change from a bi-exponential decay model (morphine-naive mice) to a mono-exponential model (morphine-tolerant mice). In behavioral studies the antinociceptive effects of morphine (s.c.) in the tail-flick test were dose-dependently antagonized by ouabain (1 and 10 ng/mouse, i.c.v.) in morphine-naive mice, but not in morphine-tolerant mice. These findings suggest that during morphine tolerance, adaptive cellular changes take place in cerebral Na+,K+-ATPase activity which are of functional relevance for morphine-induced antinociception.

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