Differences in the effects of mu- and delta-opioid receptor antagonists upon plasma corticosterone levels in stressed mice.

The effects of intraperitoneal injections of two opioid antagonists on plasma corticosterone levels in stressed mice was investigated. The effects of the mu-opioid receptor antagonist naloxone was dependent on the dose used. Low doses (0.5 and 1 mg/kg) potentiated the elevation of plasma corticosterone observed in saline treated or ether stressed mice, whereas a higher dose (10 mg/kg) produced the opposite effect. The delta-opioid receptor antagonist ICI 154,129 had no significant effect on corticosterone levels. The findings suggest that mu- rather than delta-opioid receptors are important in modifying corticosterone responses to stress and raise the question as to specificity of naloxone at doses above 1 mg/kg.