An 1-hour session of mixed oxygen-air gas(inspired oxygen concentration ~50%)

Participants will receive resting-state functional magnetic resonance imaging (MRI). Scans will be performed on a 3.0-T Siemens Magnetom Skyra scanner (Siemens Healthineers, Erlangen, Germany). During scanning, all participants were instructed to remain motionless, and to think of nothing in particular but to not fall asleep.

Change in Functional connectivity of the brain networks between baseline and after the inhaled Nitrous oxide 24hours as assessed by measure of connectivity in multimodal MRI.

Peripheral blood sample will be collected and the concentration of IL-6, TNF-α, and CRP will be assessed.

The objective of investigator is to characterize gut microbiome in patients with TRD. To explore the specific diversity of gut microbiome.

The stool samples were collected in the collection kits at the baseline, and were frozen at -80 °C. After the process of DNA Purification, 16S rRNA Gene Amplification and Illumina MiSeq sequencing, the study finally find the abundance and diversity of microbiota.

Trail Making Test (TMT) is a cognitive test designed to assess scanning, visuomotor tracking, executive function, and cognitive flexibility. It consists of two parts, A and B: the patient must draw lines to connect consecutively numbered circles (part A) and then connect consecutively numbered and lettered circles alternating between the two sequences (part B). The time taken to complete the two parts is recorded. Part A assesses cognitive processing speed. The lower the score the faster the processing speed.

Stroop Colour Naming Test (STROOP) is a cognitive test designed to assess the ability to inhibit a prepotent response to reading words while performing a task that requires attention control. It comprises two sheets with 50 words on each, and each word is the name of a colour. On the first sheet, the Congruent STROOP Sheet, the word and ink colour match; on the Incongruent STROOP Sheet, the word and ink colour do not match. For each sheet, the patient has 4 minutes to name the ink colour of each word. When the patient finishes the sheet, or once 4 minutes is up, the clinician notes the time taken and counts the number of correct and incorrect responses. The scale ranges from 0-100, the higher score the greater the cognitive flexibility.

Score range 0-12 for each of for dimensions: Digit span forward, digit span backward, block span forward, block span backward. Higher scores indicate better cognitive performance.

Digit Symbol Substitution Test (DSST) is a cognitive test designed to assess psychomotor speed of performance requiring visual perception, spatial decision-making, and motor skills. It consists of 133 digits and requires the patient to substitute each digit with a simple symbol in a 90-second period. Each correct symbol is counted, and the total score ranges from 0 (less than normal functioning) to 133 (greater than normal functioning)." as a description of DSST.

The Snaith-Hamilton Pleasure Scale (SHAPS) assesses the anhedonia symptoms of MDD patients, with higher score indicating more severe anhedonia.

Evaluating the patient's pleasure experience and choosing the degree of agreement with the happy response in some pleasant situations, and each item was rated on a 4-point scale."1 = Strongly agree, 2 = agree, 3 = disagree and 4 = strongly disagree" This scale evaluates the status of patients in the recent period of time, The total score of the scale is the sum of the scores of 14 items, with a total score of 14-56. The higher the total score, the more serious the anhedonia.

Assessed via the Hamilton Anxiety Rating Scale (HAM-A). The HAM-A score ranges from 0 (no present anxiety) to 56 (maximum anxiety score).

The CADSS is a 23-item self-report scale for the assessment of dissociative states. It is a reliable, valid self-report instrument. The severity of each dissociative symptom ranges from 0 (not present) to 4 (extreme). The total score is calculated by summing across items, with a total possible range of 0-92. The CADSS was administered right before inhaled, and 2hours and 24hours after the inhaled. The timeframe is "at this moment".

Change From Baseline in Clinician-Administered Dissociative States Scale (CADSS) Score.

The scale contains 19 items rated on a scale from 0 to 2, allowing score range from 0 to 38, score over 4 (first five items) will be considered inclusion criteria. Higher values indicate worsening or presence of suicidal ideation.

Item 4 and 5 at the end of the last week was assessed. If the answer to both questions was "no", the patient was judged to have no suicidal ideation.If either answer is "weak" or "moderate to strong," it is considered suicidal ideation.The intensity of suicidal ideation was judged according to the mean value of items 1-5 in the scale. The higher the score, the greater the intensity of suicidal ideation.The risk of suicide was assessed according to items 6-19 of the suicide ideation scale.The total score was calculated by [(the sum of item 6-19 scores -9)/33]*100. The higher the score, the greater the risk of suicide.