Organization of the septal region in the rat brain: a Golgi/EM study of lateral septal neurons.

The combined Golgi/electron microscope (EM) technique was used to analyze the fine structure and synaptic organization of the various types of neurons in the rat lateral septum (LS), i.e., in the dorsolateral (LSd), intermediolateral (LSi), and ventrolateral (LSv) nuclei of the septal complex. Two characteristic cell types were observed in the LSd: type I with thick, short dendrites densely covered with short spines, and type II with longer and thinner dendrites exhibiting fewer but longer spines. This latter type was by far the most frequently impregnated cell type in the LSd and was also present in the LSi. Synaptic contacts on spines of either cell type were asymmetric; the majority of the presynaptic boutons contained clear round synaptic vesicles. Occasionally terminals were found that contained both clear and dense-core vesicles. Typical fusiform neurons with a low number of spines and rather long dendrites, sometimes invading other LS nuclei, were found in the LSi. The LSv contained numerous small neurons with small dendritic fields. A relatively large number of terminals with dense-core vesicles were found to establish synaptic contacts with identified LSv neurons. The morphological heterogeneity of LS neurons is discussed with regard to other studies on afferent and efferent fiber systems as well as immunohistochemical studies of this particular region of the septal complex.