Morus nigra leaves extract revokes the depressive-like behavior, oxidative stress, and hippocampal damage induced by corticosterone: a pivotal role of the phenolic syringic acid

The pathophysiology of depression includes glucocorticoids excess, glutamatergic excitotoxicity, and oxidative stress impairment. Previous study demonstrated Morus nigra L. leaves extract and syringic acid (4-hydroxy-3,5-dimethoxybenzoic acid), its major phenolic compound, administered orally for 7 days, decreased the immobility time in the tail suspension test, without locomotor alteration. Therefore, the purpose of this study was to investigate the antidepressant-like effects, antioxidant effects, and neuroprotective effects of M. nigra leaves extract and syringic acid in an animal model of depression induced by corticosterone. Herein, corticosterone administered in male Swiss mice, 60-90 days of age, at 20 mg/kg, once a day, for 21 days, was effective to induce depressive-like phenotype. This alteration was accompanied by the increase of oxidative stress markers (lipid peroxidation, nitrite, and protein carbonyl) and the decrease in nonprotein thiols level, besides impairment in the hippocampus. Conversely, the treatment with M. nigra leaves extract (10 mg/kg), syringic acid (1 mg/kg), or fluoxetine (10 mg/kg), administered once a day for the last 7 days of the corticosterone treatment, was able to abolish the behavioral alterations elicited by corticosterone, reinforcing evidence of the M. nigra leaves extract and syringic acid having antidepressant-like effect. Both treatments also exerted antioxidant property in the mice's brain, reducing the amount of oxidative stress and abolishing the corticosterone-induced damage in the hippocampal slices. In addition, the treatments protected the hippocampus against the damage induced by the association between corticosterone administration and glutamate excess. In conclusion, M. nigra leaves extract and syringic acid revoke depressive-like behavior induced by corticosterone via inhibition of oxidative stress and hippocampal damage.