Connectivity of neurons in identified auditory circuits studied with transport of dextran and microspheres plus intracellular injection of Lucifer Yellow

The components of a neural circuit are usually distinguished in separate experiments to identify long connections, presynaptic, and postsynaptic components. We describe a procedure to visualize these components in the same experiment. Neurons in the inferior colliculus the axons of which project to the medial geniculate body were identified by retrograde transport of latex microspheres, while their innervation from the cochlear nucleus was simultaneously visualized by anterograde transport of dextrans. In aldehyde-fixed slices, the microsphere-labeled neurons near dextran-labeled axons were injected with biotinylated Lucifer Yellow. Subsequent avidin-biotin histochemistry allowed permanent visualization. The specific neurons involved in this circuit and the axonal contacts they received were easily visualized with the light microscope. This method allows the study of complex innervation patterns in the mammalian central nervous system.