Effects of Intravenous (IV) Citalopram Hydrochloride During Transcranial Magnetic Stimulation in Major Depressive Disorder (MDD)
Effects of Intravenous (IV) Citalopram Hydrochloride During Transcranial Magnetic Stimulation in Major Depressive Disorder (MDD)
This study will recruit 30 subjects diagnosed with Major Depressive Disorder (MDD). Subjects
 will be recieve one infusion treatment of citalopram or placebo and 10 treatments of a form
 of transcranial magnetic stimulation, theta burst stimulation (TBS). Subjects will also
 undergo brain scans, quantitative electroencephalography (qEEG) brain activity recordings,
 and mood surveys. Study activities will be performed over the course of 4 weeks.
Repetitive Transcranial Magnetic Stimulation (rTMS) is an increasingly common treatment for
 Major Depressive Disorder (MDD). rTMS applied to dorsolateral prefrontal cortex (DLPFC), the
 most common treatment target, appears to change neuronal excitability in this region. rTMS
 also changes function of brain circuits connected to DLPFC. This application proposes an
 innovative approach to elucidating the mechanism of action (MOA) underlying these circuit
 changes, using MDD as a significant translational model.

 One form of rTMS, theta burst stimulation (TBS), has particularly strong effects on cortical
 excitability: intermittent pulsing over left DLPFC (iTBS) increases excitability, while
 continuous pulsing over right DLPFC (cTBS) reduces excitability. TBS applied to DLPFC alters
 functional connectivity with the anterior cingulate, medial frontal, and orbitofrontal
 cortices (ACC, MFC, and OFC); however, the MOA underlying these changes in connectivity is
 incompletely understood. Pilot data will be obtained for a larger grant application that will
 test the hypothesis that changes in local excitability underlie the changes in functional
 connectivity and the therapeutic efficacy of TBS for MDD.

 TBS modulation of cortical excitability may be modulated in turn by the serotonergic (5HT)
 neurotransmitter system, which is also a key target of classical antidepressant medications.
 The investigators will use the 5HT transport inhibitor citalopram (CIT), a widely-used
 clinical antidepressant agent, to investigate the serotonergic modulation of functional
 connectivity and neurophysiologic measures of excitability.

 Intravenous citalopram hydrochloride has been available for prescription to patients with
 treatment-refractory major depression and anxiety disorders in most of continental Europe for
 more than 30 years. In the U.S., it has been used extensively as an investigational drug to
 study human neurochemistry and in clinical trials for depressive disorders. An IND for the
 University of Pittsburgh has utilized this compound safely as a research tool for more than
 10 years (Smith et al., 2009).

 This double-blinded study will recruit 30 subjects diagnosed with Major Depressive Disorder
 (MDD). Subjects will be randomized to acute (single-dose) citalopram (CIT) 40 mg iv treatment
 or placebo, counterbalanced and combined with one of two forms of unblinded Transcranial
 Magnetic Stimulation, i.e., either intermittent Theta Burst Stimulation or continuous Theta
 Burst Stimulation. Assessments for this study include brain scans, qEEG recordings, and
 cognitive and mood scales. One citalopram/placebo infusion and 10 TBS treatments will be
 administered over the course of approximately two weeks.

 Thirty subjects ages 21-55 with a DSM-V diagnosis of MDD will be enrolled in a two-week
 treatment study. After a baseline diagnostic assessment, all subjects will undergo
 pretreatment assessment of brain functional connectivity (qEEG and fMRI) and structural
 connectivity (DTI). They then will be randomized 1:1 to treatment with an intravenous dose of
 CIT or placebo (PBO), followed by 1:1 randomized assignment to two weeks (10 sessions) of
 treatment with either iTBS to left or cTBS to right DLPFC (four treatment conditions). The
 end of Week 2 will constitute the primary endpoint.

 High-density qEEG will be recorded throughout the initial CIT-TBS treatment session using
 TMS-compatible qEEG. These recordings will be used to assess changes in excitability with
 CIT/PBO and iTBS/cTBS treatment (i.e., TMS evoked Local Mean Field Power, or LMFP). The
 investigators will determine whether changes in cortical excitability in DLPFC are modulated
 by 5HT neurotransmission.

 Next, changes in neuroplasticity in limbic circuits involving DLPFC will be examined, from
 pretreatment baseline to after the first treatment, and from baseline to the primary
 endpoint, using functional connectivity measures. qEEG will be used to measure changes in
 signal synchronization and information flow (i.e., lagged coherence, Granger Causality), as
 well as location and spread of current sources (LORETA source localization). fMRI will be
 used to examine resting state network function (BOLD signal). The investigators will test
 whether changes in excitability measured in the first treatment are correlated with changes
 in connectivity with the first treatment, as well as over the full course of treatment.
 Correlation would suggest that excitability may exert a metaplastic effect on functional
 connectivity in limbic circuits involving DLPFC.

 Finally, the investigators will examine symptom change at the primary endpoint, and determine
 whether excitability and connectivity changes correlate with symptom improvement. Integrated
 examination of changes in excitability, immediate and longer-term connectivity, and symptom
 improvement, will elucidate the MOA of TBS and could lead to strategies to enhance treatment
 outcomes.

 Clinical diagnosis and assessment. Thirty subjects with a DSM-V diagnosis of MDD established
 using the Mini International Neuropsychiatric Interview (MINI, version 7.0)
 (http://www.medical-outcomes.com/index/mini7fororganizations) will be enrolled, with all
 subjects having depressive symptoms of at least moderate severity as indicated by a 17-item
 Hamilton Depression Rating Scale [Ham-D17] score > 17 (23 below) will be enrolled. Subjects
 will undergo clinical assessment using methods similar to those employed in our previous
 treatment studies using TMS (Leuchter et al., 2015). Subjects will have failed to enter
 remission with at least two prior antidepressant medications (Sackeim et al., 1990; Vasavada
 et al., 2016) and must have been free of any medications known to significantly affect brain
 function for at least ten days prior to enrollment (except fluoxetine, which will require a
 five-week washout) (Vasavada et al., 2016). Subjects will be excluded if they meet DSM-V
 criteria for any other current primary Axis I mood, anxiety, or psychotic disorder,
 depression secondary to a general medical condition, or substance-induced illness. Subjects
 also will be excluded if they have current suicidal intent or plan, a history of substance
 abuse or dependence within the past six months (except nicotine and caffeine), Bipolar
 Disorder or psychotic disorder (lifetime), eating disorder (current or within the past year),
 Obsessive Compulsive Disorder (lifetime), Post-Traumatic Stress Disorder (PTSD, current or
 within the past year), medical or neurologic illness that would contraindicate administration
 of study interventions or complicate interpretation of study results, or have an implanted
 medical device or metal in their body that would contraindicate an MRI or TMS treatment
 (Leuchter et al., 2015). Subjects also will be excluded if they have had prior history with
 IV citalopram. Women who are currently pregnant, not using a medically acceptable means of
 birth control, or are breastfeeding will be excluded. A urine drug screen will be performed,
 and subjects with a positive screen for illicit substances will be excluded.

 Mood symptoms will be examined after the first, second, and 10th treatment sessions using the
 Clinician Global Impressions-Severity of Illness (CGI-S) and Improvement (CGI-I) (Cohen et
 al., 2014), and the subject-rated Inventory of Depressive Symptomatology-Self Report (IDS-SR)
 (Connolly et al., 2012), with symptoms of suicidal ideation assessed using the Columbia-
 Suicide Severity Rating Scale (C-SSRS) (Cornwell et al., 2012). Assessments at time of entry
 and exit from the study will include symptom ratings with the Ham-D17 and quality of life and
 functional status ratings with the Quality of Life Inventory (QOLI) (Dandash et al., 2015).
 Treatment response at the primary endpoint is defined as a 50% or greater improvement from
 baseline on the Ham-D17 and remission as a final Ham-D17 score < 7.

 Treatment procedures Citalopram (CIT) and placebo (PBO) infusion. CIT and saline PBO will be
 administered intravenously using established clinical procedures. A single dose of citalopram
 40 mg in 60 cc normal saline, or saline PBO, will be delivered intravenously under
 double-blind conditions via pump over a 40-minute period. Blood will be drawn at the
 conclusion of infusion to obtain a plasma sample for CIT levels. Subjects will be fasting
 after midnight or for a minimum of 8 hours prior to undergoing infusion. Vital sign
 monitoring will include blood pressure, pulse oximetry, and respiratory rate recording every
 3 minutes and a continuous cardiac rhythm strip. A change of greater than 25% increase in
 heart rate or blood pressure from baseline or absolute increase in heart rate above 120 bpm
 or systolic blood pressure ≥ 180 mm Hg or diastolic blood pressure ≥ 105 mm Hg sustained from
 more than 2 minutes will prompt the immediate discontinuation of double-blind infusion and
 evaluation for intervention. Similarly, a drop of O2 saturation via pulse oximetry to less
 than 92% will prompt evaluation of subject, initiate use of nasal cannula O2 at 2 liters/min
 or rate necessary to return O2 saturation to baseline at room air or greater than 95%. A
 physician investigator will administer the CIT or PBO infusion, perform subject assessment
 and direct nursing staff. Mental status monitoring will also occur during double-blind
 infusion to assess for any untoward behavioral or psychological effects. Subjects will be
 instructed not to drive for up to 24 hours after infusion and will need to be driven to and
 from UCLA for the procedure.

 Theta burst transcranial magnetic stimulation (TBS). Stimulation will be performed using a
 Magstim Rapid2 biphasic stimulator with a figure-8 coil (14 cm width) and 2 T peak field
 strength. Stimulation percentages are expressed as a proportion of this individual unit's
 Maximum Stimulator Output (MSO). This unit can generate the theta-burst stimulation patterns
 at intensities of 45% MSO or below, well within range of most participant's individual motor
 threshold (MT). To determine MT, the coil will be held mediolaterally over the region of the
 left motor cortex with the handle pointing backwards and 45° from sagittal midline. This
 technique induces current roughly perpendicular to the central sulcus. The right first dorsal
 interosseus muscle (FDI) will be monitored with surface electromyography (5000 Hz). TMS
 pulses will be delivered in a grid at suprathreshold intensities in order to identify the
 location which produces the largest, most consistent motor evoked potential (MEP) recorded
 from the FDI. Intensities at this hotspot will then be lowered 1% with each stimulation. The
 lowest intensity stimulation that produces peak-to-peak MEP amplitudes >100 μV on at least 5
 of 10 trials under conditions of gentle activation of the FDI is defined as the active motor
 threshold (AMT). TBS intensity is then set at 120% AMT.

 TBS consists of three TMS pulses given at 50 Hz, with this triplet repeated at a frequency of
 5 Hz (every 200 ms) (Huang et al., 2005). 1800 pulses of cTBS will be delivered to the right
 DLPFC, and an equal number to left DLPFC following the iTBS paradigm of a 2 s train of
 repeated every 10 s. This number of pulses has been shown to have antidepressant efficacy
 after two weeks of treatment (Li et al., 2014). Left and right DLPFC will be targeted using
 the F3 and F4 EEG electrode locations, respectively. This method for magnet placement bears a
 close relationship to placing the magnet over radiographically defined Broadmann areas (BAs)
 in DLPFC (i.e., BA46) (Ahdab et al., 2010; Fitzgerald et al., 2009). This approach, widely
 used in clinical practice, will allow us to easily relate the findings of this project to
 clinical use. The investigators recognize that this probabilistic method for defining DLPFC
 may result in some variability of magnet placement relative to underlying neuroanatomy. Given
 interindividual variability in sulcal and gyral anatomy, however, it may not be possible to
 reliably identify a specific BA in all subjects, and there is no "gold-standard" method for
 standardizing neuroanatomic targeting across subjects. The investigators will have digitized
 surface electrode locations from all subjects, and these locations will be fused with the
 structural MRI images obtained from each subject at baseline and after the 10th iTBS
 treatment. This will allow us to utilize neuroanatomic coil placement data as a post-hoc
 covariate in data analyses (see MRI-EEG image integration below).

 Neurophysiologic recording and analyses. EEG recording. Data will be recorded using the "eego
 mylab" TMS-compatible EEG system at a sampling rate of 1000 Hz (Advanced Neuro Technology
 [ANT]; Enschede, Netherlands). Electrodes will be applied using the 64-electrode "WaveGuard"
 system with sintered Ag/AgCl electrodes mounted in an elastic cap and positioned according to
 the Extended 10-20 System. The material and shape of the electrodes prevents current loops is
 designed for minimal DC shifts and optimal stability of the incoming signal during TMS. The
 cap utilizes active shielding of each lead to limit electric noise. Data are recorded using
 full-band EEG DC amplifiers that return to physiologic baseline signal level within 10 ms
 after the end of the TMS pulse. Filters will not be applied during data acquisition, and
 recording will be performed using a common average reference with impedance kept below 5 kΩ.
 EOG will be recorded by placing two electrodes above and below the left eye. EEG will be
 processed off-line in BrainVisionAnalyzer2 (BVA2) (BrainProducts GmbH; Gilching, Germany)
 with a digital band-pass filter (Butterworth zero-phase shift, 0.5-70 Hz, 12 dB/oct; plus 60
 Hz notch) before segmenting into 2 s epochs (100 ms pre-stimulus period), detrending, and
 baseline correction. Data initially will be processed using semi-automated artifact rejection
 methods previously described including ±100 μV peak-to-peak voltage step gradient or
 persistent low activity (Leuchter et al., 2012), followed by visual inspection by two
 independent technicians to eliminate data contaminated by muscle, head, or eye movement
 artifacts. Adaptive Mixture ICA (AMICA) also will be used to separate out non-brain source
 processes including eye blinks and saccades, scalp muscle, electrocardiographic artifact, and
 line noise, increasing signal-to-noise (artifact) ratio and increasing reliability of high
 frequency (beta and gamma) frequency analyses as well as brain source localization
 (Bigdely-Shamlo et al., 2013; Delorme et al., 2011).

 Excitability measures. Excitability will be measured using the Local Mean Field Power (LMFP)
 and Global Mean Field Power (GMFP) methods. Oscillatory voltage amplitude is the principal
 measurement that directly reflects cortical excitability, and therefore changes in EEG field
 power are used as the principal indicator of changes in excitability. LMFP can be used as a
 measure of excitability at the site of TMS stimulation in any region of cortex; GMFP is a
 measure of global excitability that has been used to study a number of non-invasive
 neuromodulation treatments (Casarotto et al., 2013; Chung et al., 2015; Huber et al., 2008;
 Pellicciari et al., 2013; Romero Lauro et al., 2014). After administration of a local
 stimulus, a focal change in excitability may come to elicit a global change; as a result,
 GMFP can be used to interpret LMFP and determine whether a local change in excitability
 remains focal, or becomes part of a global change in excitability. For the excitability
 determinations, single TMS pulses will delivered to left or right DLPFC (F3 or F4 electrode
 location, coinciding with the treatment site) 10 minutes before and after treatment sessions.
 Given the number of TBS pulses delivered in the protocol, changes in excitability can be
 expected to remain stable for at least one hour following TBS treatment (Huang et al., 2005).
 Pulses will be administered at a frequency of 0.25 Hz so that they are sufficiently
 infrequent so as not affect excitability. During excitability determination, subjects will
 wear earplugs and a masking noise will be played to reproduce the TMS "click" in time-varying
 frequency components in order to suppress auditory evoked potentials. TEPs will be computed
 by averaging valid artifact-free single trials, filtering between 2 and 40 Hz, and performing
 baseline correction before and after the TMS pulse. LMFP will be calculated from the
 amplitudes of the TEPs averaged from F3 or F4 and their respective four surrounding
 electrodes, while GMFP will be calculated from all 64 electrodes. GMFP and LMFP will be
 calculated in 30 ms time windows following the TMS pulse ranging from the end of the pulse to
 400 ms (i.e., 0-30 ms, 30-60 ms, 60-90 ms, etc.) so that early and later changes in
 excitability can be detected. Early time points (< 90 ms) will be used to assess LMFP, and
 later time points will be used to assess GMFP and spread altered excitability to other
 cortical regions.

 Regional measures of neurophysiologic activity. Current source density (CSD) in gray matter
 structures will be calculated using the eLORETA (exact Low Resolution Brain Electromagnetic
 TomogrAphy) method (http://www.uzh.ch/keyinst/loreta.htm) (Lehmann et al., 2014). eLORETA
 computes local neurophysiologic activity (Current Source Density, or CSD) as a linear
 weighted sum of scalp electrical potentials. CSD measurements using eLORETA have been
 validated as cortical sources of neurophysiologic activity, and obviate the the ambiguity of
 source localization and the reference-dependence that are inherent in scalp EEG measurements
 (Lehmann et al., 2014; Pascual-Marqui et al., 2011). The method identifies the smoothest
 possible distribution of sources in a three-shell spherical head model, consisting of CSD at
 each of 6239 cortical voxels (plus hippocampus and amygdala) (5 mm spatial resolution) in
 Montreal Neurological Institute (MNI) space with electrode coordinates assigned according to
 cross-registration between spherical and realistic head geometry (Towle et al., 1993).
 Reported Brodmann areas utilize MNI space corrected to coincide with Talairach space (Brett
 et al., 2002). Regions of interest (ROIs) will be created for subregions of the ACC, OFC,
 hippocampus, insula, and other cortical ROIs in the limbic system based upon our prior work
 and the broader literature (Arns et al., 2015; Korb et al., 2008; Korb et al., 2009; Korb et
 al., 2011; Whitton et al., 2016).

 Neurophysiologic connectivity measures. The investigators will use one primary
 neurophysiologic measure and several complementary techniques as secondary (exploratory)
 measures in order to examine baseline and changes in neurophysiologic connectivity. Our
 primary connectivity measure based upon CSD data will be examined using "lagged" coherence
 (omitting zero phase angle) based on intracortical source modeling techniques (Lehmann et
 al., 2014; Pasi et al., 1989). The investigators will identify cortical ROIs throughout the
 PFC, hippocampus, and amygdala based upon BAs and subcortical probabilistic atlases, and will
 apply the eLORETA option "all nearest voxels" to assigned voxels to the ROIs. The
 investigators will use the ROIs identified by eLORETA as showing high activity to seed the
 connectivity analyses; The investigators anticipate that the primary ROIs of interest will be
 those with strong connections to DLPFC in the limbic system, including OFC, MFC, ACC,
 hippocampus, and insula. Lagged coherence represents one of the two main components of
 neurophysiologic connectivity: instantaneous and lagged. The lagged component is mediated by
 physiological time delays, with the instantaneous contribution to the connectivity is
 eliminated. This method selectively retains connectivity due to physiologic processes (for
 any non-zero, measurable time delay) that is not confounded by low resolution and volume
 conduction effects (Lehmann et al., 2014).

 Secondary measures also will examine connectivity and cross-frequency coupling in the
 electrode space using the Source Information Flow Toolbox (SIFT) (Delorme et al., 2011). This
 method uses time-varying (adaptive) multivariate autoregressive modeling to detect and
 measure fluctuations in effective connectivity between sources of neural activity. this
 method is complementary to the lagged coherence method in using advanced multivariate
 measures of directed information flow (e.g., Granger Causality Modeling and Directed Transfer
 Function modeling). SIFT functions will be used to examine changes in effective connectivity
 among the different experimental conditions and between treatment groups, using
 bootstrap/resampling techniques to correct for multiple comparisons. 'Network Projection',
 which is an extension of Measure Projection (Bigdely-Shamlo et al., 2013) will be performed
 using pairwise connectivity to measure input, which is computed by blurring the dipole
 locations and loading them with connectivity measures. The brain space will then be segmented
 into 88 pre-defined anatomical ROIs provided by automated anatomical labeling (AAL) obtained
 from WFU Pick Atlas (which is a SPM plugin). At this point, all the subjects have 88x88
 matrix of connectivity values, therefore every subject has 7744 (88x88=7744) possible
 combinations of directed causal flows. Diagnostic condition differences will be computed and
 bootstrap statistics will be performed for each condition to control for type 1 error.

 MRI scanning and analyses. Structural and functional MRIs including DTI will be acquired
 using a head-only Siemens Prisma-FIT 3 Tesla scanner at the UCLA Ahmanson-Lovelace Brain
 Mapping Center. This scanner is the latest Siemens 3T product modeled after the custom made
 Skyra system originally developed for the Human Connectome Project (HCP). The investigators
 will utilize the optimized HCP protocols (http://www.humanconnectome.org/) to ensure that the
 connectivity data are collected and processed with state-of-the-art methods that will be
 interpretable in the context of other ongoing NIH research. Total scanning time will about
 one hour.

 Structural MRI (sMRI). A whole brain structural scan will be obtained for registration with
 EEG data and evaluating morphology (MPRAGE, TR=2300 ms, TE=3 ms, FOV=256 mm, 208 slices, .8 x
 .8 x .8 mm voxels). The standardized HCP sMRI protocols include both 3D T1- and T2-weighted
 (T1w and T2w) high resolution scans. Analyzed together, these sequences improve the automated
 extraction of brain features, though both T1 and T2 scans may also be examined separately.

 Diffusion tensor MRI (DTI). The acquisition of DTI data will be used to measure the
 structural connectivity of white matter. DTI will be acquired with two diffusion-weighted
 gradients of b=0 and 1000 s/mm2 in 98 orthogonal directions (TE = 89.2 ms, TR = 3222 ms,
 FOV=210mm, 92 slices, 1.5 x 1.5 x 1.5 mm voxels). The HCP also uses HARDI (High Angular
 Resolution Diffusion Imaging) that provides greater precision for mapping the direction of
 fibers in regions where fibers are crossing and allows for probabilistic tractography to
 estimate fiber trajectories from ROIs defined with other imaging modalities (sMRI, fMRI, or
 EEG). These same acquisition protocols will be used for this project.

 Resting-state fMRI (fMRI). fMRI will be used to characterize resting state network function
 in a manner complementary to EEG. The investigators will use an echo-planar sequence (72
 axial slices per volume, 104×90 matrix [2.0×2.0×2.0mm3], FOV=208mm, TE = 33.1ms, TR = 720ms,
 FA = 52°, 420 volumes) to gauge resting-state BOLD signals. The HCP fMRI multiband EPI
 sequence offers excellent spatial and temporal resolution and contain anterior-posterior and
 posterior-anterior phase encoding to correct for anatomic distortions.

 MRI image processing. The investigators already have installed the HCP pipeline developed by
 the Washington University group and have processed HCP-compliant data collected at UCLA using
 this pipeline, achieving high quality tissue segmentation on the T1w MRI, white matter
 tractography using HARDI, and resting brain networks based on fMRI. Each pipeline is
 described separately below.

 Structural MRI. Three HCP pipelines are applied to sMRI (T1w & T2w) data. The PreFreeSurfer
 pipeline registers T1w and T2w images, performs bias-field correction, and normalizes these
 images to MNI space. The FreeSurfer pipeline recreates FreeSurfer's reconall pipeline, which
 segments tissue, reconstructs cortical surfaces, and aligns images to standard volume and
 surface atlases.

 DTI. The Diffusion Preprocessing pipeline performs standard preprocessing steps using FSL,
 including equalizing intensity of b0s across runs, removing EPI and eddy-current distortions,
 correcting for motion and gradient nonlinearities, and registration to T1w images. The
 investigators perform postprocessing using BrainSuite and its BrainSuite Diffusion Pipeline
 (BDP) (http://brainsuite.org), which enables us to compute deterministic tractography using
 the ODF data to generate whole-brain diffusion tracts. The investigators will then compute
 the average fractional anisotropy (FA), mean diffusivity (MD), radial diffusivity (RD), and
 number of tracts along the white matter pathways connecting the anatomical ROIs (specific
 circuits associated with depression, including tracts originating from the subgenual ACC and
 DLPFC).

 Resting state fMRI. Preprocessing protocols for functional data include two pipelines. The
 first, fMRIVolume, corrects images for spatial distortions and motion, registers to T1w
 images, corrects for subject motion, among other standard steps. The fMRISurface pipeline
 aligns the output of fMRIVolume to cortical surfaces, including mapping onto a "grayordinate"
 system to facilitate comparison and analyses across subjects. The investigators perform
 resting state fMRI postprocessing using a novel method that is sensitive to spatial
 differences between subjects. For graph theoretic analysis of brain networks, The
 investigators will use independent components computed by ICA as well as structural ROIs
 based on the Destrieux atlas as nodes, and the connectional relationships between each pair
 of components will be evaluated by partial correlation analysis. The brain networks will be
 characterized by network parameters computed from their adjacency matrixes to quantitatively
 describe topology of networks, including 1) node degree, degree distribution and
 assortativity; 2) clustering coefficient and motifs; 3) path length and efficiency; 4)
 connection density or cost; 5) hubs, centrality and robustness; and 5) modularity.

 MRI-EEG image integration. In order to identify the location of the F3 electrode scalp site
 at which TBS is being delivered into the MRI space, as well as fully integrate the EEG and
 MRI datasets, The investigators will employ the Advanced Neuro Technology Xensor 3D electrode
 digitizer system (ANT Neuro; Enschede, Netherlands). This system uses a digitizing wand with
 reflective markers and an infrared camera to identify the 3D locations of all 64 EEG
 electrodes and three major fiducials (nasion, left and right preauricular points) for each
 participant in less than 10 minutes. The 3D digitized locations then are coregistered to the
 Montreal Neurological Institute (MNI-Colin27) template brain (Montreal Neurological
 Institute, Montreal, Canada. URL www.bic.mni.mcgill.ca/brainweb). Electrode locations can be
 used in the Visor 2.0 neuronavigation system (ANT Neuro; Enschede, Netherlands) to visualize
 the stimulation target and associated scalp site that was actually used during TBS in each
 individual. After loading each subject's raw MRI images into the Visor software, the
 relationship of the F3 stimulation site to the DLPFC target, as well as any individual
 electrode to underlying neuroanatomy, can be identified using the MNI coordinates.
Inclusion Criteria:

 -  21-55 years of age. MDD currently depressed subjects will meet DSM-V criteria for MDD
 based on the Mini-International Neuropsychiatric Interview (MINI)
 (http://www.medicaloutcomes.com/index/mini7fororganizations) with a 17-item Hamilton
 Depression Rating Scale (HamD17) (Hamilton, 1960) score > 17.

 -  Subjects must have failed to enter remission with at least two prior antidepressant
 medications in the current episode (Vasavada et al., 2016)

 -  Must have been free of any medications known to significantly affect brain function
 for at least ten days prior to enrollment (except fluoxetine, which will require a
 five-week washout).

 Exclusion Criteria:

 -  No unstable medical illness that would prevent completion of participation in the
 trial (determined as needed from physical examination, ECG, laboratory safety tests,
 as well as a review of systems).

 -  Clinically significant physical abnormalities as indicated by physical examination,
 hematological laboratory assay, or urinalysis, defined as:

 hematology and chemistry laboratory tests that are within normal (+/- 10%) limits with the
 following exceptions: a) liver function tests (total bilirubin, ALT, AST, and alkaline
 phosphatase) < 3 x the upper limit of normal, and b) kidney function tests (creatinine and
 BUN) < 2 x the upper limit of normal;

 -  A screening ECG that demonstrates anything other than normal sinus rhythm, normal
 conduction, and no clinically significant arrhythmias

 -  History of epilepsy, seizures, or severe head trauma;

 -  Resting vital signs on any study visit outside of acceptable parameters (i.e., pulse
 of 60-100 bpm, blood pressures of 90-150 mm Hg systolic, 50- 90 mm Hg diastolic);

 -  Any indication of suicidal ideation (e.g. as assessed by the suicidality question on
 the HamD17or the Columbia Suicide Severity Rating Scale.

 -  Baseline QT prolongation (QTc> 450 ms): Given that citalopram has been found to be
 associated with a dose-dependent risk of ECG QT interval prolongation, in order to
 avoid the potential risk of causing ventricular arrhythmias including Torsades de
 Pointes, we will exclude participants from the study who exhibit baseline QTc
 prolongation.

 -  For women of childbearing age, a positive urine pregnancy test, as well as women who
 are currently breastfeeding or not using a medically acceptable method of birth
 control

 -  Presence of any implanted medical device or metal in the body that would render it
 unsafe to perform TMS or an MRI.

 -  Axis I: the presence of any other primary mood, anxiety, or psychotic disorder,
 depression secondary to a general medical condition, or substance- induced illness.
 Subjects also will be excluded if they have current suicidal intent or plan, a history
 of substance abuse or dependence within the past six months (except nicotine and
 caffeine), Bipolar Disorder or psychotic disorder (lifetime), eating disorder (current
 or within the past year), Obsessive Compulsive Disorder (lifetime), Post-Traumatic
 Stress Disorder (PTSD, current or within the past year);

 -  Axis III: active medical illness known to significantly affect brain function or that
 could be etiologically related to the ongoing depression (e.g., untreated
 hypothyroidism);

 -  Current treatment with a medication known to affect brain function. This would include
 both psychiatric and centrally-acting neurological agents.

 The investigators have chosen to exclude these subjects because current medication could
 affect measures of brain function as well as introduce an uncontrolled treatment effect
 into the study. Prospective subjects who are currently taking psychiatric medications will
 also be excluded as the risk of antidepressant discontinuation outweighs the potential
 benefit of study participation. A history of prior treatment with IV CIT. We have chosen to
 exclude subjects who have received this treatment because they may have a degree of
 treatment resistance that would make it less likely for them to respond to treatment in the
 current protocol. Additionally, if they previously have received CIT the PBO treatment
 blind in the current protocol may not be effective;

 -  Current treatment with a medication known to affect brain function. We have chosen to
 exclude these subjects because current medication could affect measures of brain
 function as well as introduce an uncontrolled treatment effect into the study. These
 medications include: antidepressants, barbiturates, anticonvulsants/mood stabilizers,
 benzodiazepines, anticholinergics, herbal preparations, antipsychotics, muscle
 relaxants, antimigraine, psychostimulants, anti-Parkinsonian medications, sedating
 antihistamines, corticosteroids (oral; topical preparations OK), Zyban (bupropion for
 use in smoking cessation);

 -  Treatment with any of the following medications within the last 30 days prior to
 randomization: antidepressants, anticonvulsants, hypnotics, antipsychotics,
 psychomotor stimulants, anti-anxiety agents, or cimetidine;

 -  Current illicit drug use. We will perform urine toxicology screens at baseline;

 -  History of stroke, skull fracture, brain surgery, or transient ischemic attacks, or
 other brain disease that could affect results;

 -  For women of childbearing age, a positive urine pregnancy test, as well as women who
 are currently breastfeeding or not using a medically acceptable method of birth
 control;

 -  History of allergic reaction or intolerance to citalopram (any formulation); and,

 -  History of ECT within the past six months, or history of failure to benefit from prior
 TMS treatment of MDD.