The supramammillary nucleus innervates cholinergic and GABAergic neurons in the medial septum-diagonal band of Broca complex.

In the present study, the connectivity between two subcortical nuclei involved in hippocampal theta activity, the supramammillary nucleus and the medial septum-diagonal band of Broca complex, was examined. Targets of the supramammillary afferents in the medial septum-diagonal band of Broca complex were identified by combining anterograde transport of Phaseolus vulgaris leucoagglutinin with immunostaining for putative postsynaptic neurons, i.e. for parvalbumin and choline acetyltransferase that are known to label the GABAergic and cholinergic neurons, respectively, of the medial septum-diagonal band of Broca complex. Double retrograde transport experiments using the tracers horseradish peroxidase and wheat germ agglutinin-conjugated colloidal gold were employed to identify supramammillary neurons that project both to the hippocampus and the medial septum-diagonal band of Broca complex. Phaseolus vulgaris leucoagglutinin injections into the supramammillary nucleus of the rat resulted in dense fibre and terminal labelling in the medial septum-diagonal band of Broca complex. Labelled terminals formed asymmetric synapses mainly on distal dendrites of medial septal neurons. Proximal dendrites and somata were rarely contacted. The supramammillary afferents showed no target selectivity for a particular cell type; they innervated both cholinergic and GABAergic cells. Occasionally, perisomatic, basket-like terminals of supramammillary origin were found around parvalbumin-containing neurons. Double-retrograde experiments revealed that at least 25% of the supramammillo-hippocampal cells also projected to the medial septum-diagonal band of Broca. These data suggest that the nucleus, known to modulate the hippocampal electrical activity directly by the supramammillo-hippocampal pathway, also has the potential for an indirect action via the innervation of both the GABAergic and cholinergic septohippocampal neurons. This dual modulation may originate, at least in part, from the same population of supramammillary neurons.

Distribution of calretinin-containing neurons relative to other neurochemically identified cell types in the medial septum of the rat.

The topographic distribution of calretinin-immunoreactive neurons was studied in the medial septum diagonal band of Broca complex of the rat, in relation to the localization of other neurochemically identified cell groups containing choline acetyltransferase, parvalbumin or calbindin D28k. Double-labelling experiments revealed that these four antigen-containing cells formed distinct dorsoventrally running lamellae overlayed on top of each other similar to onion leaves. There was only a slight overlapping of the various cell groups. None of the four antigens were co-localized in the same cells. The lamella occupied by calretinin-positive neurons is situated at the border of the medial septum and the intermediolateral septal nucleus, and shows some overlap with the area occupied by cholinergic neurons. Retrograde transport of horseradish peroxidase from the hippocampus combined with immunostaining for calretinin revealed that calretinin-containing neurons do not participate in the septohippocampal projection. The lack of projection to the amygdala was also confirmed. Thus, calretinin-containing neurons represent a distinct cell group in the medial septal region, which either projects to subcortical areas, or may function as interneurons relaying hippocampal feedback to the medial septal projection neurons.