Evaluating Apelin as a Potential Biomarker in Major Depressive Disorder: Its Correlation with Clinical Symptomatology

To date, only a limited number of studies have investigated the potential effects of apelin on mood regulation and emotional behavior. Therefore, this study investigated apelin's role in major depressive disorder (MDD) by comparing the serum and plasma apelin concentrations between 30 patients with MDD and 30 healthy controls (HCs), and the correlated serum and plasma apelin levels and the severity of depressive symptoms using the Montgomery-Åsberg Depression Rating Scale (MADRS). Blood samples were collected following 12 h of fasting, and the apelin levels were measured using an ELISA kit. The serum apelin concentrations showed no significant difference between the MDD and HC groups, while the plasma apelin levels were significantly lower in the MDD group (<i>p</i> = 0.002). Among the patients with MDD, a positive moderate correlation was observed between the total MADRS scores and plasma apelin levels (r = 0.439), with statistical significance (<i>p</i> < 0.05). Additionally, significant positive correlations (<i>p</i> < 0.05) were found between both the serum and plasma apelin levels and the MADRS subscales 5 (reduced appetite) and 6 (concentration difficulties). These preliminary findings, although not definitive, suggest that apelin profiles may help to identify distinct subgroups within MDD patients, warranting further investigation into the different apelin isoforms and their associations in different populations of MDD patients.