Tracing neuronal connections in postmortem human hippocampal complex with the carbocyanine dye DiI

This report describes the ability of the carbocyanine dye DiI to trace hippocampal complex connections in a paraformaldehyde immersion-fixed human postmortem brain. Six months after the placement of DiI crystals into the hilus of the dentate gyrus, the CA1 hippocampal subfield and the lateral entorhinal cortex, 50-microns thick, vibratome cut sections were examined using an epifluorescence microscope with a rhodamine filter. In association with DiI-labeled granule, pyramidal and multipolar type neurons, we observed dendrites containing dendritic spines and axons. DiI-labeled fibers were observed coursing within classically described hippocampal pathways for at least 8 mm distal to the injection site. Photoconversion of diaminobenzidine (DAB)-treated DiI sections produced a stable record of labeled profiles. These findings indicate that DiI is a useful method for investigating intrinsic local circuit connections in normal aldehyde-fixed postmortem human brain and suggests that DiI could be a powerful tool to examine altered neural connectivity in humans with neurological disease.