Research and clinical application of transcranial magnetic stimulation (TMS) for adolescents with major depressive disorder (MDD) has advanced slowly. Significant gaps persist in our understanding of optimized, age-specific protocols and dosing strategies. This study aimed to compare the clinical effects of 1 Hz versus 10 Hz TMS regimens and examine a biomarker-informed treatment approach with glutamatergic intracortical facilitation (ICF). Participants with moderate-to-severe symptoms of MDD were randomized to 30 sessions of left prefrontal 1 Hz or 10 Hz TMS, stratified by baseline ICF measures. The primary clinical outcome measure was the Children's Depression Rating Scale, Revised (CDRS-R). The CDRS-R and ICF biomarker were collected weekly. Forty-one participants received either 1 Hz (n = 22) or 10 Hz (n = 19) TMS treatments. CDRS-R scores improved compared to baseline in both 1 Hz and 10 Hz groups. For participants with low ICF at baseline, the overall least squares means of CDRS-R scores over the 6-week trial showed that depressive symptom severity was lower for the group treated with 1 Hz TMS than for those who received 10 Hz TMS. There were no significant changes in weekly ICF measurements across the 6 weeks of TMS treatment. Low ICF may reflect optimal glutamatergic N-methyl-d-aspartate (NMDA) receptor activity that facilitates the therapeutic effect of 1 Hz TMS through long-term depression-like mechanisms on synaptic plasticity. The stability of ICF suggests that it is a tonic, trait-like measure of NMDA receptor-mediated neurotransmission, with potential utility to inform parameter selection for therapeutic TMS in adolescents with MDD.
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