(R)-KETAMINE AMELIORATES DEPRESSION-LIKE BEHAVIORS AND COGNITIVE IMPAIRMENT VIA ACTIVATION OF THE ANTERIOR INSULAR CORTEX IN SOCIAL ISOLATION-REARED MICE
Authors: Yokoyama *Rei , Ago Yukio , Igarashi Hisato , Higuchi Momoko , Tanuma Masato , Shimazaki Yuto , Seiriki Kaoru , Hayashida Misuzu , Yamaguchi Shun , Tanaka Hirokazu , Nakazawa Takanobu , Hashimoto Kenji , Kasai Atsushi , Hashimoto Hitoshi 
Published: February 2025

Abstract

Background
A number of clinical studies have demonstrated that subanesthetic doses of (R,S)-ketamine and (S)- ketamine have significant therapeutic effects on depression, including treatment-resistant depression (TRD). (S)-ketamine has been approved for the treatment of TRD by FDA. Additionally, preclinical studies have shown that (R)-ketamine exerts more potent antidepressant effects than (S)-ketamine and is also effective for cognitive dysfunction associated with depression. Although several mechanisms underlying the antidepressant effects of (R,S)-ketamine and (S)-ketamine have been proposed, the mechanism of action of (R)-ketamine remains unclear.

Aims & Objectives
In this study, we aimed to identify brain regions involved in the differential effects of the ketamine enantiomers, (R)- and (S)-ketamine, in a mouse model of depression induced by post-weaning social isolation.

Method
All animal studies were approved by the Animal Care and Use Committee of the Graduate School of Pharmaceutical Sciences, Osaka University. To visualize the neural activity, Arc-dVenus reporter mice were used. For social isolation rearing, the mice were individually isolated starting at 3 weeks of age and kept in isolation for at least 6 weeks.

Results
Post-weaning social isolation increased the immobility time of male C57BL6/J mice in the forced swim test (FST), and (R)-ketamine showed antidepressant-like effects at lower doses than (S)-ketamine in this model. To identify several candidates of brain areas, we used brain-wide mapping of immediate early gene expression in social isolation-reared Arc-dVenus mice and found that both (R)-ketamine and (S)- ketamine increased neuronal activity in cortical and subcortical regions. Then, the machine learning classifiers identified several candidates of brain areas including the anterior insular cortex (aIC) that may contribute to the antidepressant-like effect of (R)-ketamine. Furthermore, we demonstrated that a temporary suppression of aIC activity by the Gi-DREADD system blocked the antidepressant-like effect of (R)-ketamine, but not of (S)-ketamine, in the FST. To clarify the roles of aIC activation in the effects of (R)-ketamine on social cognition, we further analyzed aIC calcium activity by using fiber photometry before and after administration of (R)-ketamine and (S)-ketamine in the three-chamber test. Isolation- reared mice treated with saline exhibited a similar aIC activity pattern after contact with an object and mouse. (R)-ketamine, but not (S)-ketamine, caused a significant increase in aIC activity after contact with a mouse compared with the activity after contact with an object. Moreover, we performed five- trial social memory test and found that (R)-ketamine rapidly promotes social memory formation to the same intruder mouse in isolation-reared mice. This effect of (R)-ketamine was abolished under inhibition of aIC activity by Gi-DREADD.

Discussion & Conclusion
These results indicated that activation of the aIC contributes to the effects of (R)-ketamine on depression-like behaviors and social cognitive deficits. Our findings also provide the evidence that (R)- ketamine has different mechanism of action from (S)-ketamine and therapeutic potential for social cognitive deficits in depression.