[Central and peripheral mu-opioid systems in the mechanisms of emotional stress]

The aim of the work was to study effect of peripheral administration of mu-opioid receptor ligands unable to penetrate through the hematoencephalic barrier on the measures of emotionality in rats and on the release of beta-endorphins from nerve endings of the anterior cingulate cortex during emotional stress. The mu-opioid receptor agonist loperimide mostly acted as an anxiolytic whereas the receptor antagonist methylnaloxon showed depressive activity. Lifetime microdialysis and subsequent immunoenzyme assay demonstrated that intraventricular infusion of loperamide and methylnaloxon decreased and increased respectively the surge of beta-endorphin into the intercellular space. Immobilization-induced emotional stress insignificantly increased the beta-endorphin level in the cingulate cortex. Peripheral administration of loperamide but not methylnaloxon markedly increased the release of the neuropeptide during stress. These findings confirm the hypothesis of the authors about reciprocal interaction of central and peripheral components of the endogenous opioid system and explain the mechanism of antistress action of loperamide.