Cobalt iontophoresis techniques for tracing afferent and efferent connections in the vertebrate CNS.

In recent years several dye and cobalt iontophoresis techniques have been successfully used by invertebrate neurophysiologists for the localization of neuron somata and their processes. The cobalt iontophoresis technique has now been extended for use in the tracing of nerve fiber pathways and the localization of neuron somata in vertebrates. The brain and spinal cord of an animal are removed following perfusion with saline, and placed in a dish of cold saline. A suction electrode, filled with 300 mM cobalt chloride, is then placed over the cut end of the nerve trunk. Cobalt ions are then iontophoresed (by means of a voltage divider) within the nerve fibers, along their course. Following iontophoresis, the brain is bathed in an ammonium sulfide solution to precipitate the cobalt as black cobalt sulfide. The brain is then processed for histological procedures. A wide variety of vertebrates has been used, including amphibians, reptiles, aves and mammals, with uniform success. The cobalt iontophoresis technique presently in use has a wide range of applicability for neuroanatomical studies.