Hippocampofugal gamma-aminobutyric acid (GABA)-containing neuron system in the rat: a study using a double-labeling method that combines retrograde tracing and immunocytochemistry.

Using a double-labeling method that combines retrograde tracing and immunocytochemistry, we demonstrated the presence of a hippocampofugal gamma-aminobutyric acid (GABA)-like immunoreactive (GABA-LI) neuron system in the rat. As a preliminary experiment, knife-cut of the dorsal fornix was made and it resulted in a remarkable accumulation of GABA-LI fibers caudal to the section. This suggested the presence of a hippocampofugal GABAergic projection. The hypothesis was confirmed by the double-labeling method: injection of the biotinylated wheat germ agglutinin (B-WGA) into the septum resulted in labeling numerous neurons in the hippocampus and some of them also showed GABA-like immunoreactivity by the simultaneous immunostaining with the antiserum against GABA.