Simultaneous labelling of two different central nervous system pathways with horseradish peroxidase and cobalt in Gnathonemus petersii and Rana esculenta.

A new and simple simultaneous labelling procedure is described that gives the possibility of studying interneuronal connections and territorial overlap of different pathways. This method uses horseradish peroxidase (HRP) labelling in combination with the cobalt-filling technique. The blue colour of the HRP reaction end-product formed from 0-tolidine (Neuroscience, 4 (1979) 1805-1852) and the dark brown of cobalt-filled neurons and their processes allow easy distinction of the two different labels. Since both markers can be transported anterogradely and retrogradely by axons, this combination of the two techniques seems to be a very useful tool for neuroanatomical research. We have tested this method in a weakly electric fish, Gnathonemus petersii, and in frog, Rana esculenta.