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Abnormal activation of the kynurenine and serotonin pathways of tryptophan metabolism is linked to a host of neuropsychiatric disorders. Concurrently, noninvasive brain stimulation (NIBS) techniques demonstrate high therapeutic efficacy across neuropsychiatric disorders, with indications for modulated neuroplasticity underlying such effects. We therefore conducted a scoping review with meta-analysis of eligible studies, conforming with the PRISMA statement, by searching the PubMed and Web of Science databases for clinical and preclinical studies that report the effects of NIBS on biomarkers of tryptophan metabolism. NIBS techniques reviewed were electroconvulsive therapy (ECT), transcranial magnetic stimulation (TMS), and transcranial direct current stimulation (tDCS). Of the 564 search results, 65 studies were included with publications dating back to 1971 until 2022. The Robust Bayesian Meta-Analysis on clinical studies and qualitative analysis identified general null effects by NIBS on biomarkers of tryptophan metabolism, but moderate evidence for TMS effects on elevating serum serotonin levels. We cannot interpret this as evidence for or against the effects of NIBS on these biomarkers, as there exists several confounding methodological differences in this literature. Future controlled studies are needed to elucidate the effects of NIBS on biomarkers of tryptophan metabolism, an under-investigated question with substantial implications to clinical research and practice.
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; Lin, Tim T Z 1 ; Kan, Rebecca L D 1 ; Zhang, Bella B B 1 ; Suk Yu Yau 2 ; Kranz, Georg S 3
1 Department of Rehabilitation Sciences, The Hong Kong Polytechnic University, Hong Kong SAR, China
2 Department of Rehabilitation Sciences, The Hong Kong Polytechnic University, Hong Kong SAR, China; Mental Health Research Center (MHRC), The Hong Kong Polytechnic University, Hong Kong SAR, China
3 Department of Rehabilitation Sciences, The Hong Kong Polytechnic University, Hong Kong SAR, China; Mental Health Research Center (MHRC), The Hong Kong Polytechnic University, Hong Kong SAR, China; Department of Psychiatry and Psychotherapy, Comprehensive Center for Clinical Neurosciences and Mental Health, Medical University of Vienna, 1090 Vienna, Austria; The State Key Laboratory of Brain and Cognitive Sciences, The University of Hong Kong, Hong Kong SAR, China