Reduced emotional and corticosterone responses to stress in μ-opioid receptor knockout mice

Soichiro Ide, Ichiro Sora, Kazutaka Ikeda, Masabumi Minami, George R. Uhl, Kumatoshi Ishihara

Research output: Contribution to journalArticlepeer-review

Abstract

The detailed mechanisms of emotional modulation in the nervous system by opioids remain to be elucidated, although the opioid system is well known to play important roles in the mechanisms of analgesia and drug dependence. In the present study, we conducted behavioral tests of anxiety and depression and measured corticosterone concentrations in both male and female μ-opioid receptor knockout (MOP-KO) mice to reveal the involvement of μ-opioid receptors in stress-induced emotional responses. MOP-KO mice entered more and spent more time in the open arms of the elevated plus maze compared with wild-type mice. MOP-KO mice also displayed significantly decreased immobility in a 15 min tail-suspension test compared with wild-type mice. Similarly, MOP-KO mice exhibited significantly decreased immobility on days 2, 3, and 4 in a 6 min forced swim test conducted for 5 consecutive days. The increase in plasma corticosterone concentration induced by tail-suspension, repeated forced swim, or restraint stress was reduced in MOP-KO mice compared with wild-type mice. Corticosterone levels were not different between wild-type and MOP-KO mice before stress exposure. In contrast, although female mice tended to exhibit fewer anxiety-like responses in the tail-suspension test in both genotypes, no significant gender differences were observed in stress-induced emotional responses. These results suggest that MOPs play an important facilitatory role in emotional responses to stress, including anxiety- and depression-like behavior and corticosterone levels.

Original languageEnglish (US)
Pages (from-to)241-247
Number of pages7
JournalNeuropharmacology
Volume58
Issue number1
DOIs
StatePublished - Jan 2010
Externally publishedYes

Keywords

  • μ-Opioid receptor
  • Anxiety
  • Corticosterone
  • Depression
  • Knockout mouse
  • Stress

ASJC Scopus subject areas

  • Cellular and Molecular Neuroscience
  • Pharmacology

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