Association between olfactory function and inhibition of emotional competing distractors in major depressive disorder

Abstract

We aimed to investigate the changes of olfaction of major depressive disorder (MDD) before and after medical treatment, and to preliminarily scrutinize the association between the olfactory function and the severity of depressive symptoms, response inhibition, and emotional responding. Forty-eight medicine-naïve MDD patients plus 33 healthy controls (HC) matched on gender, ages, and level of education, were recruited in the test group. The Chinese Smell Identification Test (CSIT), Self-reported Olfactory Scale (SROS), 17-item Hamilton Depression Rating Scale (HAMD-17), Hamilton Anxiety Rating Scale (HAMA), and mean reaction time/accuracy rate (ΔMRt) of emotional Stroop test were measured. The patients were assessed before the treatment (baseline) and 3 months after the treatment (follow-up). The data at the baseline level were measured then associated using multiple linear regression stepwise analysis. The MDD patients had lower scores of the CSIT and SROS and longer ΔMRt at baseline level compared to Hc while the ΔMRt of MDD patients remained longer after 3-month treatment (p's < 0.05). At the baseline level, the regression equation including age and ΔMRt of negative word-color congruent (NEG-C), was finally observed as follows: y(CSIT) = 10.676-0.063 × 1-0.002 × 2, [x 1 = the age(y), x 2 = the NEG-C (ms)]. The olfactory function of MDD appears to be correlated negatively with the age and the ΔMRT of negative stimuli before treatment. After the remission of MDD, the olfactory dysfunction was improved, which might be regarded as a responding phenotype of brain function of MDD rather than the emotional responding. Major depressive disorder (MDD) and olfaction disorder (OD) are both chronic diseases that affect human being's life quality 1. A systematic review on ten studies chosen for using the Sniffin' Sticks Test and the 40-item Smell Identification Test assessed to reveal the relationship between depression and OD. Results show that both of the olfactory threshold, olfactory discrimination, and olfactory identification were influenced by depression 2. Furthermore, symptoms of depression worsen with the severity of olfactory loss 3. It was demonstrated that olfactory bulbectomy, as a well-known method, could induce an animal model of depression in animal experiment 4. In depression-related brain areas including hippocampus, frontal cortex and hypothalamus, the turnover of serotonin and dopamine decreases in the olfactory bulbectomized rats 5. Similarly, in human studies, Cory et al. 6 found that patients with depression have decreased activities in the left medial orbitofrontal cortex (OFC) accompanied with the defect of the olfactory identification. However, Zucco et al. 7 argued that the changes mentioned above occur in patients with the MDD only, not with the mild or moderate depressive disorder. In the neural circuits involved in emotional processing 8 , these key structures such as amygdala, hippocampus, anterior cingulate cortex(ACC) and OFC, are the imperative functional regions of the human olfactory map...

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