Nitrous Oxide Alters Functional Connectivity in Medial Limbic Structures in Treatment-Resistant Major Depression.

While nitrous oxide (NO) has demonstrated antidepressant properties in treatment-resistant major depression (TRD), little is known about neural mechanisms mediating these effects. Employing serial resting-state functional magnetic resonance imaging (rs-fMRI), we compared spatiotemporal effects of inhaled NO on brain functional connectivity in TRD patients (n=14) and non-depressed healthy controls (n=16, CNTL). Participants received sequential, one-hour inhalations of either 50% NO/oxygen or air/oxygen (placebo), with sessions separated by at least one month in random cross-over order. BOLD-contrast rs-fMRI scans were acquired at three time points: pre-inhalation, 2 hours post-inhalation, and 24 hours post-inhalation. For the rs-fMRI functional connectivity analyses, five seeds in medial limbic structures targeted cortical networks implicated in major depression - the salience, anterior and posterior default mode, reward, and cingulo-opercular networks - and a nexus in the dorsal paracingulate region previously identified in MDD ("dorsal nexus"). Depression, dissociation, and psychosis assessments were made before and after inhalations. In TRD patients, functional connectivity was reduced in all seeded networks and the voxel-wise global analysis after NO exposure. NO progressively connectivity in patients with TRD but connectivity in healthy controls. In TRD patients, each seeded network demonstrated post-inhalation functional connectivity reductions in the dorsal paracingulate gyrus ("dorsal nexus"). This study further elucidates neural mechanisms underlying the antidepressant properties of NO, supporting the notion that NO specifically alters mood-associated brain regions in the depressed brain state by reducing functional connectivity within these brain networks. The trial was registered at ClinicalTrials.gov (NCT02994433).