Depression and stress-related disorders are leading causes of death worldwide. Standard treatments elevating serotonin or noradrenaline levels are not sufficiently effective and cause adverse side effects. A connection between dopamine pathways and stress-related disorders has been suggested. Compounds derived from herbal medicine could be a promising alternative. We examined the neuroprotective effects of ursolic acid (UA) by focusing on dopamine signalling.
Trolox equivalent capacity assay was used to determine the antioxidant activities of UA in vitro. C. elegans N2 wildtype and dopamine receptor-knockout mutants (dop1-deficient RB665 and dop3-deficient LX703 strains) were used as in vivo models. H2DCFDA and acute juglone assays were applied to determine the antioxidant activity in dependency of dopamine pathways in vivo. Stress was assessed by heat and acute osmotic stress assays. The influence of UA on overall survival was analyzed by a life span assay. The dop1 and dop3 mRNA expression was determined by real time RT-PCR. We also examined the binding affinity of UA towards C. elegans Dop1 and Dop3 receptors as well as human dopamine receptors D1 and D3 by molecular docking.
Antioxidant activity assays showed that UA exerts strong antioxidant activity. UA increased resistance towards oxidative, osmotic and heat stress. Additionally, UA increased life span of nematodes. Moreover, dop1 and dop3 gene expression was significantly enhanced upon UA treatment. Docking analysis revealed stronger binding affinity of UA to C. elegans and human dopamine receptors than the natural ligand, dopamine. Binding to Dop1 was stronger than to Dop3.
UA reduced stress-dependent ROS generation and acted through Dop1 and to a lesser extent through Dop3 to reduce stress and prolong life span in C. elegans. These results indicate that UA could be a promising lead compound for the development of new antidepressant medications.