Convergent vasopressinergic and hippocampal input onto somatospiny neurons of the rat lateral septal area

Electron microscopic immunocytochemistry was combined with acute anterograde axon degeneration, following transection of the fimbria-fornix, to describe the innervation of somatospiny neurons by vasopressin-immunoreactive and degenerated hippocamposeptal axon terminals in the rat lateral septal area. Vasopressin-immunopositive boutons characterized by symmetric synaptic membrane specializations, and the degenerated hippocamposeptal axon terminals which form asymmetric synaptic contacts, frequently terminate on the same dendritic and somatic profiles, and particularly on the somata of somatospiny neurons. Although hippocamposeptal fibers predominantly form axospinous synapses in the lateral septal area, they terminate mainly on the dendritic shafts and soma of the vasopressin-receptive neurons. Of 720 vasopressin-immunoreactive terminals in the mediolateral part of the lateral septal area, 80% form synaptic contacts with dendritic shafts: 50% on small (distal) dendritic profiles and 30% on large (proximal) dendrites. Synaptic contacts between vasopressin-immunoreactive terminals and dendritic spines were not observed. The remaining 20% of immunoreactive boutons formed axosomatic synaptic contacts with a total of 58 neurons; 31% of these neurons exhibited somatic spines in the plane of the section analysed. Previous studies have demonstrated that in the lateral septal area vasopressin modulates the action of the excitatory amino acid-containing hippocamposeptal fibers, and also plays a role in the maintenance of long term potentiation evoked by fimbria-fornix stimulation. The convergent vasopressinergic and hippocampal input onto the same somatospiny neurons of the lateral septal area suggests that these neurons are targets of these physiological actions.