UC Santa Barbara

All neural processes in the human brain take place in a dense matrix of thin fibers (axons) that release serotonin, an ancient neurotransmitter that supports the plasticity of neural tissue and has profound implications for mental health. The self-organization of the serotonergic matrix is not well understood, despite recent advances in experimental and theoretical approaches. Previous work in our laboratory has demonstrated that individual serotonergic axons produce highly stochastic trajectories, fundamental to the construction of regional fiber densities, but further advances in predictive computer simulations require more accurate experimental information. However, visualizing this dynamic behavior in vivo is currently extremely difficult. Recent technological advances (e.g., digital holotomography, three-dimensional (3D)-cell culture systems, microphysiological systems) have changed the way complex neuronal systems can be studied and offer promising alternatives to in vivo approaches. This research examined single serotonergic axons in culture systems (co-cultures and mono-cultures), by using a set of complementary high-resolution methods: confocal microscopy, holotomography (refractive index-based live imaging), and super-resolution (STED) microscopy. Several 3D-hydrogel systems were additionally tested. This research shows that serotonergic axon walks in neural tissue may strongly reflect the stochastic geometry of the tissue and it also provides new insights into the morphology and branching properties of serotonergic axons. The proposed experimental platform can support next-generation analyses of the serotonergic matrix, including seamless integration with computational approaches.