Depression is a common issue but there is no marker of response to an antidepressant treatment.The measurement of variation of the cortical excitability in responders to a selective serotonin reuptake inhibitor (SSRI) (compared to no-responders) had never been done before. In the study of Robol et al. (2004) concerning the acute effects of citalopram on markers of cortical excitability, the authors have pointed out an increase on the cortical silent period (CSP) after administration of citalopram (2.5 hours). The investigators hypothesize that this effect remains later and that the diminution of cortical excitability could be a biomarker of antidepressant response. In this case, they expect that the variation of CSP between day 1 and day 28 is higher in responders to a SSRI (citalopram) compared to non-responders. the investigators lead a pilot, prospective, multicentric study in drug-naive patients to compare the variation of the markers of cortical excitability (the CSP but to the resting motor threshold RMT, the motor evoked potential MEP, the intra-cortical inhibition ICI and the intra-cortical facilitation ICF) between day 1 and day 28 in responders to citalopram, compared to non-responders.

Inclusion criteria:

• major depressive episode (according to DSM 5 criteria). Severity of depressive episode assessed by Hamilton Depressive Rating Scale 21 items (HAMD-21) : score > 15 (significant impairment), low suicide risk (score < 2 on suicide item).
• Drug-naive patient (or antidepressant stopped for more than 3 months).
• Patient covered by security social system.
• Patient who is able to read and understand the information paper. Patient who is able to sign the consent.
• For women of childbearing age, effective method of contraception (estrogen-progestin contraceptive or intra-uterine device or tubal ligation) for more than 1 month (negative pregnancy test).

Exclusion criteria:

• Coprescription of psychoactive or neurological drugs known to alter cortical excitability
• Other psychiatric disorders (psychotic disorders, eating disorders).
• Change of antidepressive drug during the study.
• Abuse or addiction at other substances than nicotine or caffeine.
• Unsteady consumption of nicotine or caffeine.
• Dermatologic disease, dementia, medical history of seizure or epilepsy, brain tumor, metallic biomedical implants in brain.
• Ongoing treatment by magnetic or electric stimulation (for example : transcutaneous nerve stimulation or spinal cord stimulation).
• Women of childbearing age without effective contraception, pregnant or breastfeeding.
• Patient who was already included in a clinical trial within 30 days before the inclusion visit.
• Patient deprived of liberty and under guardianship.