Dual projection from the medial septum to the supramammillary nucleus in the rat.

The supramammillary nucleus, collecting information about the physiological state of the animal, innervates medial septal neurons that are involved in the generation of hippocampal theta activity. Here we demonstrate that septal neurons located in an area bordering the medial and lateral septal nucleus project back to the supramammillary nucleus, and most of these cells contain calretinin, calbindin or both. GABA-immunoreactive boutons of these neurons (60%) form symmetrical synapses, whereas the remaining GABA-negative terminals form asymmetrical synapses (40%) with their supramammillary targets. We hypothesize that the septosupramammillary feedback, because of the specific location of its parent cells, carries information about the activity of theta generator cells in the medial septum and supramammillary nucleus, as well as about the resulting theta activity in the hippocampus.