This study aims at investigating the persistence of antidepressant effect of Nitrous Oxide
(N2O) for Treatment-Resistant Depression(TRD). The investigators also aim to assess the
effect of N2O on the electroencephalograph, multimodal magnetic resonance imaging(MRI), blood
cytokines, feces bacteria flora and neuropsychological performance in patients with TRD. The
investigators further aim to identify the predictors of N2O's antidepressant effeect using
the above techniques.