Takahiro Masuda,1,2 Takeshi Inoue,1 Yan An,1 Naoki Takamura,1,3 Shin Nakagawa,1 Yuji Kitaichi,1 Tsukasa Koyama,1 Ichiro Kusumi1 1Department of Psychiatry, Hokkaido University Graduate School of Medicine, Sapporo Japan; 2Medical Affairs, Dainippon Sumitomo Pharma, Co, Ltd, Tokyo, Japan; 3Regenerative and Cellular Medicine Office, Dainippon Sumitomo Pharma, Co, Ltd, Osaka, Japan Background: Mirtazapine, a noradrenergic and specific serotonergic antidepressant, which blocks the α2-adrenergic autoreceptors and heteroreceptors, has shown anxiolytic properties in clinical trials and preclinical animal experiments. The addition of mirtazapine to selective serotonin reuptake inhibitors (SSRIs) is clinically suggested to be more effective for anxiety disorders. In this study, we examined the combined effects of mirtazapine and citalopram, an SSRI, on the freezing behavior of rats, which was induced by contextual conditioned fear as an index of anxiety or fear. Methods: Male Sprague Dawley rats individually received footshocks in a shock chamber, and 24 hours later, they were given citalopram and/or mirtazapine injections. One hour after citalopram and 30 minutes after mirtazapine administration, freezing behavior was analyzed in the same shock chamber without shocks. Results: Mirtazapine decreased freezing in a dose-dependent manner, which is consistent with a previous report; it also enhanced an anxiolytic-like effect at a high dose (30 mg/kg) of citalopram. Because mirtazapine blocks α2-adrenoreceptors, the combined effect of atipamezole, a selective α2 receptor antagonist, with citalopram was also examined. Similar to mirtazapine, atipamezole reduced freezing dose-dependently, but the enhancement of citalopram's effects by atipamezole was not clear when compared with mirtazapine. Conclusion: The present findings suggest that mirtazapine has an anxiolytic-like effect and may enhance the anxiolytic-like effect of SSRIs, but this enhancement may not be explained by its anti-α2 property alone. Keywords: anxiety, conditioned fear, selective serotonin reuptake inhibitor, mirtazapine, α2-adrenoreceptor