Somatospiny neurons in the rat lateral septal area are synaptic targets of hippocamposeptal fibers: a combined EM/Golgi and degeneration study.

The mediolateral part of the lateral septal area (LSA) is a common target of hippocamposeptal afferents, neuropeptide containing, catecholaminergic, cholinergic, and GABAergic pericellular baskets of different origins. This specific innervation pattern as well as electrophysiological data concerning this area suggest a convergent input from different sources to particular LSA neuron populations. Light and electron microscopy combined with Golgi impregnation and acute anterograde degeneration techniques following transection of the fimbria-fornix were employed to determine whether LSA neurons with hippocampal input have any characteristic and distinctive morphological signs. About 20% of all Golgi impregnated LSA neurons were found to have somatic spines. All of these somatospiny neurons are synaptic targets of hippocamposeptal fibers. The degenerated hippocamposeptal boutons establish asymmetric synaptic contacts on their soma, somatic and dendritic spines, and on dendritic shafts. Somatospiny neurons located in the most medial and dorsal parts of the LSA seem to project toward the medial septum while all of the others appear to send descending fibers to ventral areas. Somatospiny neuron axons occasionally give out recurrent collaterals. Quantitative analysis on the spatial distribution of the somatospiny neurons revealed that practically all of them are encountered in the mediolateral division of the LSA. This area includes the lateral part of the intermediolateral septal nucleus and adjacent lateral portions of the dorsolateral and the ventrolateral septal nuclei.