A new method for application of horseradish peroxidase into a restricted area of the brain.

A new method for horseradish peroxidase (HRP) application into the very restricted area of the brain was developed. Crystalline HRP was packed into the tips of fine micropipettes having tip size of about 50 micrometer in outer diameter. The HRP micropipettes were stereotaxically implanted into the globus pallidus of rats. After survival period of 6 to 48 hours, brain was excised and HRP distribution was investigated following conventional procedures. The diffusion area of HRP at the application site was restricted within the limits of 200--250 micrometer in diameter completely inside the globus pallidus. It was known that the slowly solubilized crystalline HRP was taken up by the nerve terminals in the globus pallidus, and transported retrogradely to the cell bodies in the pars compacta of substantia nigra, indicating the existence of nigro-pallidal projections.