A Neurotomographic Approach for Mesoscale Mapping of Neural Circuits

Abstract

Background Brain regions integrate neural information arriving from several convergent projection sources. At the mesoscale level, neural projections can potentially span both hemispheres and extend along the entire rostrocaudal axis, which complicates efforts to map their full extent. To address this issue, we describe a novel method for mapping such mesoscale connectivity in vivo and ex vivo. Our ‘neurotomographic’ approach utilizes micro-computed tomography (micro-CT) to image the spatial distribution of neural tracers bound to high Z-elements, e.g, gold. Methods In this study, we conjugated colloidal gold to a retrograde tracer wheat-germ agglutinin apo-horseradish peroxidase (WGA-HRP) and then stereotactically injected the gold-bound tracer (WAHG) into the mouse forebrain. Micro-CT was then used to image the brain in vivo and ex vivo, followed by three-dimensional reconstruction of tracer distribution. We then validated our approach by histologically processing the brains using silver enhancement to label gold particles; this enabled a direct comparison of histological labeling with the neurotomographic images. Results We found that micro-CT imaging could reveal the major spatial distributions of the gold-bound tracer, which was consistent across in vivo and ex vivo imaging conditions. Moreover, the neurotomographically determined patterns corresponded with the labeling observed in histologically processed tissue, with the major sites of labeling reliably detected in reconstructed neurotomographic images. Conclusions Overall, our findings demonstrate a potential novel method for non-destructive, three-dimensional mapping of neural tracers in vivo. This novel approach can potentially guide targeted multi-site recordings, enable validation of injection site placement, and facilitate rapid longitudinal connectomic analyses in vivo.