“…The self-organization of the brain serotonergic matrix remains an unsolved problem in neuroscience. This matrix, or a meshwork of axons (fibers), is present in virtually all brain regions, likely across the entire vertebrate clade ( Wolters et al, 1985 ; Challet et al, 1991 , 1996 ; Stuesse et al, 1991 ; Metzger et al, 2002 ; López and González, 2014 ; Janušonis, 2018 ; Awasthi et al, 2021 ; Bhat and Ganesh, 2023 ). It appears to be fundamentally associated with neuroplasticity ( Lesch and Waider, 2012 ; Teissier et al, 2017 ; Daws et al, 2022 ), raising new questions ( Vargas et al, 2023 ), and is remarkable in its ability to robustly regenerate in the adult mammalian brain ( Jin et al, 2016 ; Cooke et al, 2022 ).…”