Synaptic Mechanisms of Intermittent Theta Burst Stimulation for Major Depressive Disorder

Many people with depression do not get better with standard treatments like medications or talk therapy. Transcranial magnetic stimulation (TMS) is a non-invasive brain stimulation treatment that uses magnetic pulses to stimulate areas of the brain involved in depression. One form of TMS called intermittent theta burst stimulation (iTBS) is FDA-cleared for depression and takes only 3 minutes to deliver. However, about one-third of patients do not respond to iTBS, and another one-third do not reach full remission. Improving iTBS requires a better understanding of how it works in the brain.

iTBS is thought to work by strengthening connections between brain cells, a process called synaptic plasticity. This process depends on a type of brain receptor called the NMDA receptor. Most of what researchers know about how iTBS affects these connections comes from studies of healthy people. It is not known whether iTBS works the same way in the prefrontal cortex - the brain region targeted during depression treatment - or in people who actually have depression.

This study has two phases.

In Phase 1, both healthy volunteers and people with depression will complete 4 research visits to test how iTBS changes brain activity in the prefrontal cortex and whether medications that increase or decrease NMDA receptor activity change those effects. Each visit involves active or sham (inactive) iTBS combined with one of three study medications: a placebo (inactive pill), d-cycloserine (a medication that increases NMDA receptor activity), or dextromethorphan (a medication that decreases NMDA receptor activity). Brain activity is measured before and after each TMS session using electroencephalography (EEG), a painless test that records electrical signals from the scalp through a cap placed on the head. All participants also complete a brain MRI before beginning study visits for targeting purposes.

In Phase 2, participants with depression will be offered a standard clinical course of 30 daily iTBS sessions (Monday through Friday over 6 weeks). Each session is combined with one blinded study medication (placebo, d-cycloserine, or dextromethorphan) taken daily. Brain activity measurements and standard depression and anxiety questionnaires are collected weekly throughout this phase to track how the brain changes over the course of treatment and whether those changes relate to improvements in symptoms.

Together, the two phases of this study aim to identify the brain mechanism by which iTBS works in people with depression. This knowledge could lead to more effective TMS treatments for people who have not responded to medications or other therapies.