Poster B86, Sunday, March 25, 8:00-10:00 am, Exhibit Hall C
In vivo manganese tract tracing of macaque saccadic eye movement circuitry: a comparison with diffusion tensor imaging
David J Schaeffer1, Kevin Johnston1, Joseph S Gati1, Ravi S Menon1, Stefan Everling1; 1Robarts Research Institute, University of Western Ontario, London, Ontario, Canada
The saccadic eye movement system has emerged as a valuable model in the neurosciences for studying neural circuitry related to flexible control of behavior. Although connections of the saccade circuit are well documented via histochemical tracers, these methods require fixed tissue and thus cannot provide longitudinal assessments of connectivity. To circumvent this, diffusion tensor imaging (DTI) is often used as a proxy for connectivity in vivo, allowing researchers to trace connections longitudinally and noninvasively. As an indirect measure of connectivity, however, DTI is limited by the ability to accurately estimate connections based on water diffusion. Here, we demonstrate the use of ionized manganese as an MRI-based in vivo labelling technique that allows for direct tract tracing without the need for fixed tissue. Manganese (Mn2+) is a strong paramagnetic contrast agent used for MRI. Mn2+ is taken up by excitable cells via voltage-gated calcium channels and transsynaptically propagates, resulting in anterograde-like tracing that is MRI visible. Here, we directly injected MnCl2 into a key saccadic node (frontal eye fields; FEF) of two rhesus macaques and collected ultra-high field MRI data (7T; T1, DTI). The results demonstrate that MnCl2-traced FEF connections parallel those established by histochemical tracing (albeit at a lower spatial resolution) and suggest that DTI underestimates FEF connectivity, with false negatives across the network, likely due to crossing fibers and small tract size. These results highlight the lack of DTI sensitivity for tracing FEF fibers, but also suggest MnCl2-based tracing as a powerful alternative for assessing these connections in vivo.
Topic Area: NEUROANATOMY