Effects of repeated drug administration on behaviors in normal mice and fluoxetine efficacy in chronic unpredictable mild stress mice

Mental disorders are characterized by high incidence and high recurrence rates, and only part of patients responded to drug medication. In this case, substantial preclinical investigations are needed. Most antipsychotics taken daily orally in clinics are administered through injection, oral gavage, or minipum implant in rodents, which may induce stress and affect the results of behavioral tests. How drug administrations on behaviors and drug efficacy remains an unsolved problem. In this study, we compared the intraperitoneal injection (IP), intragastric administration (IG), and tail vein injection (TVI) on behaviors, as well as the difference between administration-induced stress and chronic unpredictable mild stress (CUMS). Next, we studied the effects of IG on CUMS model and drug efficacy. We found that IP, IG, and TVI, especially IG, induced a behavior-like phenotype of depression and anxiety, which we call the "CUMS-like behaviors". However, such behaviors were not equal to depression. When treated CUMS mice with saline by gavage, they didn't show any aggravated phenotype compared with CUMS alone. We observed that fluoxetine by intraperitoneal injection was more effective than intragastric administration. Our study confirmed that repeated administrations lead to CUMS-like behaviors. Although these behaviors are not depression, they have adverse effects on drug efficacy.