Intracellular injections of permanent tracers in the fixed slice: a comparison of HRP and biocytin

Here we describe a method for intracellularly injecting mixtures of the fluorescent dye Lucifer Yellow and the permanent tracers HRP or biocytin into aldehyde-fixed slices of the dorsal lateral geniculate nucleus in young postnatal cats. Lucifer Yellow was used for visual control in the injection procedure and the inclusion of HRP or biocytin allowed the subsequent use of simple histochemical processing to give a permanent record of the injected cells. Both tracer mixtures revealed the dendritic morphology of injected cells. However, HRP was found to be superior to biocytin, in that dendrites were better defined and fine details of cellular morphology such as spines were consistently revealed. Using this technique we were able to demonstrate that the dendritic morphology of geniculate cells is much more mature between birth and 2 weeks than was thought from previous studies using Golgi methods.