Sex-dependent effects of tamoxifen on mouse behavior.

This study aimed to investigate the sex-dependent behavioral effects of tamoxifen, commonly used to induce Cre recombination in transgenic systems, across three developmental stages (adult, adolescent, and neonatal) and two CreERT2 mouse lines (CaMKIIα-CreERT2 and Aldh1l1-CreERT2). Both male and female mice, including wild-type C57BL/6J and CreERT2 transgenic lines, were subjected to tamoxifen treatment followed by behavioral tests assessing locomotion, anxiety-like behavior (open field test and elevated plus maze), social interaction, recognition memory (new object recognition), sucrose preference, and depression-like behavior (forced swimming test) at least 4 weeks post-treatment. We found that adult tamoxifen treatment increased depression-like behavior specifically in males, while adolescent treatment increased sucrose preference only in males, yet impaired recognition memory and increased depression-like behavior in both sexes. Neonatal treatment caused pervasive impairments, reducing locomotion and increasing anxiety- and depression-like behavior in both sexes, while enhancing social interaction only in males. Furthermore, effects differed in adult CaMKIIα-CreERT2 and Aldh1l1-CreERT2 mice. In the former, tamoxifen treatment impaired locomotion, increased anxiety-like behavior, and reduced recognition memory specifically in females, while increasing sucrose preference in males. In the latter, tamoxifen treatment impaired recognition memory in both sexes but increased sucrose preference only in males. These results demonstrate that tamoxifen alone induced long-lasting, sex-specific behavioral alterations dependent on developmental exposure. Furthermore, interactions of tamoxifen with CreERT2 expression introduced additional complexity, potentially confounding genetic studies. These findings emphasize the necessity of including appropriate controls (vehicle, tamoxifen-only, Cre-only) and both sexes in studies using the tamoxifen-inducible CreERT2-loxP system.