The purpose of the first experiment with corticosterone (CORT) injection in rats was to define the pharmacological characteristics of repeated CORT injections, their neurochemical manifestation, and their influence on depressive behavior. The purpose of the second experiment was to define the correlation between behavior and depression through animal models and identify potential neurobiological mechanisms to apply the findings to therapy in the future. The overall goals of both experiments were to identify the potential neurobiological mechanisms behind depression through animal studies. If the CORT hypersecretion can induce depressive behavior, then antidepressants and antipsychotics in combination can be more effective for patients with psychotic major depression. On the other hand, if neurobiological mechanisms in the social defeat animal model can predict depressive behavior, then the application of animal models can be helpful in therapy for depression.
The last experiment presents a manipulation check of the impact of social stress on the development of depression. Through the application of the social defeat stress paradigm, the experiment aimed to induce behavior similar to depression in rats. Furthermore, as a simulation measure of the social defeat procedure in the experiment, the scientists transferred rats from their home cages for introduction and physical confrontation with resident rats. The social defeat procedure in the experiment was repeated on a regular basis. After the first week of the experiment, the rats were subjected to different tests, including the sucrose preference test, open field test, and forced swim test. The study defined that rats exposed to regular social defeat procedures in the experiment had a lower sucrose preference level than the control group rats. Moreover, the study determined alterations in metabolites of rats exposed to the social defeat procedure, particularly the lipid metabolism, amino acids, and energy metabolism, were perturbed. Thus, the experiment suggests that social defeat causes neurobiological changes, which cause behavior similar to depression in animals.