Pharmacological characterization of repeated corticosterone injection-induced depression model in rats.

Dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis has been reported in patients with psychotic major depression (PMD), and a higher rate of cortisol hypersecretion is observed in PMD than in nonpsychotic patients. Approximately 19% of patients who meet the criteria for major depressive disorder (MDD) have psychotic features. Accumulated studies have indicated that repeated corticosterone (CORT) injections induce depressive behavioral and neurochemical manifestations in rodents. However, the pharmacological characterization of this model has not been fully established. In the present study, we investigated the pharmacological characteristics of this model. Rats received CORT injections (20 mg/kg, subcutaneously), once per day for 21 consecutive days prior to a behavioral test. The rats were then tested for depressive behavior using a forced swimming test. The repeated CORT injections increased the immobility time in the forced swimming test, indicating an increase in depressive-like behavior. An acute treatment with a glucocorticoid receptor antagonist, mifepristone, counteracted the depressive-like behavior. In contrast, an acute treatment with a selective serotonin reuptake inhibitor (SSRI), fluvoxamine, and a tricyclic antidepressant (TCA), imipramine, did not have any effect on this condition, while a combination of fluvoxamine and risperidone exerted an antidepressant-like effect. This observation is of interest in the light of the clinical findings that a combination of antidepressants and antipsychotics, but not SSRIs and TCAs, is effective for the treatment of patients with PMD. Based on previous findings and the present results, this model could be used as an animal model of PMD and may be useful for evaluating the antidepressant-like potential of compounds targeting the HPA axis.