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.

Parvalbumin-containing cells of the angular portion of the vertical limb terminate on calbindin-immunoreactive neurons located at the border between the lateral and medial septum of the rat.

In the septal complex, both parvalbumin and calbindin neurons cocontain GABA. In the same area, a large number of GABA-GABA synaptic connections can be observed. In order to further characterize their neurochemical nature, as well as the extrinsic and/or intrinsic origin of these GABA terminals, the following experiments were performed: (1) correlated light- and electron-microscopic double immunostaining for calbindin and parvalbumin on septal sections of control rats: (2) light microscopic parvalbumin immunostaining of septal sections after surgical isolation (5 days) of the septum from its telencephalic or (3) hypothalamic afferents; and (4) parvalbumin immunostaining of sections prepared from the entire brain 2 days following horseradish peroxidase injection into the border between the lateral and medial septum. The results demonstrated that: (1) in a well-circumscribed, vertically longitudinal area located between the lateral and medial septum, 0.1-0.6 mm anterior to the bregma, a group of calbindin-containing, nonsomatospiny neurons are surrounded by parvalbumin-immunoreactive baskets; (2) these basket-forming axon terminals establish symmetric synaptic contacts with their targets; and (3) their cells of origin are not in the medial septum, but in the angular portion of the vertical limb. These observations indicate that a portion of the septal complex GABA-GABA synaptic connections represent functional interaction between two different types of GABAergic neurons. The presynaptic GABAergic neurons contain parvalbumin, and the postsynaptic GABAergic cells are immunoreactive for calbindin. Furthermore, a population of the medial septum/diagonal band parvalbumin neurons project only to the hippocampus, while others, which may also send axons to the hippocampus, terminate on lateral septum calbindin cells as well.

Postsynaptic targets of GABAergic hippocampal neurons in the medial septum-diagonal band of broca complex.

The termination pattern of hippocamposeptal nonpyramidal cells was investigated by injecting Phaseolus vulgaris leucoagglutinin (PHAL) into stratum oriens of the CA1 region. Electron microscopic analysis showed that the majority of the anterogradely labeled boutons formed symmetric synapses with dendrites and occasionally with cell bodies located in the medial septum-diagonal band of Broca complex. We have demonstrated with postembedding GABA immunocytochemistry that the majority of PHAL-labeled axon terminals were GABAergic. The neurochemical character of the postsynaptic target cells was also investigated using immunocytochemical double staining. Our data showed that the majority of the labeled hippocamposeptal axons innervated parvalbumin-immunoreactive cells representing GABAergic projection neurons, and a smaller number of contacts were found on ChAT-positive neurons. Septohippocampal neurons identified by retrograde HRP transport were also shown to reactive direct hippocamposeptal input. According to recent results, the lateral septum is unlikely to relay the hippocampal feedback to the medial septum; therefore, the direct hippocampal projection to the medial septum, arising from GABAergic nonpyramidal cells, seems to be the only feedback pathway to the area containing septohippocampal neurons. A novel circuit diagram, based on our recent morphological-immunocytochemical findings, is shown for the synaptic organization of the septo-hippocampo-septal loop. We suggest that the GABAergic hippocamposeptal feedback controls the activity of septal (mostly GABAergic) projection neurons as a function of hippocampal synchrony. The newly discovered reciprocal interactions may give a better insight into septohippocampal physiology.