RESUMO
OBJECTIVE: This study was carried out to delineate differences between major depressive disorder (MDD) and healthy controls in dynorphin and kappa opioid receptor (KOR) levels in association with changes in the ß-endorphin - mu opioid receptor (MOR) and immune-inflammatory system. METHODS: The present study examines dynorphin, KOR, ß-endorphin, MOR, interleukin (IL)-6 and IL-10 in 60 drug-free male participants with MDD and 30 age-matched healthy males. RESULTS: Serum dynorphin, KOR, ß-endorphin and MOR are significantly higher in MDD as compared to controls. The increases in the dynorphin/KOR system and ß-endorphin/MOR system are significantly intercorrelated and are both strongly associated with increased IL-6 and IL-10 levels. Dynorphin, ß-endorphin, KOR and both cytokines showed a good diagnostic performance for MDD versus controls with a bootstrapped (n = 2000) area under the receiver operating curve of 0.972. The dynorphin/KOR system is significantly decreased in depression with comorbid nicotine dependence. CONCLUSION: Our findings suggest that, in MDD, immune activation is associated with a simultaneous activation of dynorphin/KOR and ß-endorphin/MOR signaling and that these opioid systems may participate in the pathophysiology of depression by (a) exerting immune-regulatory activities attenuating the primary immune response and (b) modulating reward responses and mood as well as emotional and behavioural responses to stress.