Abstract.
Morphine-induced place preference was demonstrated recently in wild-type mice, whereas this conditioned behaviour was not observed in µ-opioid receptor-deficient mice. In the present study, we investigated locomotor effects of subcutaneously (s.c.) injected morphine as well as intracerebroventricular (i.c.v.) morphine self-administration in µ-opioid receptor-knockout mice.
After s.c. morphine injection, locomotor activity significantly increased in wild-type animals. As expected, in the self-administration test the rate of self-administration constantly increased in wild-type mice reflecting reward effects of morphine. This increase was independent of locomotor/motor activity. In contrast, self-administration rates and locomotor/motor activity significantly decreased in the receptor-deficient animals. It was shown that this aversive effect might partly be due to κ-opioid receptor interaction.
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Becker, A., Grecksch, G., Brödemann, R. et al. Morphine self-administration in µ-opioid receptor-deficient mice. Naunyn-Schmied Arch Pharmacol 361, 584–589 (2000). https://doi.org/10.1007/s002100000244
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DOI: https://doi.org/10.1007/s002100000244