Collateral projections from the supramammillary nucleus to the medial septum and hippocampus.

Previous reports have shown that the supramammillary nucleus projects to the medial septum and to the hippocampus, and specifically to the dentate gyrus and the CA2/CA3a region of the hippocampus. The aim of the present study was to examine collateral projections from the supramammillary nucleus to the septum and hippocampus. The fluorescent retrograde tracers, Fluororuby and Fluorogold, were injected into regions of the septum and hippocampus, respectively, and the supramammillary nucleus was examined for the presence of single- and double-labeled neurons. The main findings were: 1) pronounced numbers of single-labeled cells (about 40-60/section) were present in the supramammillary nucleus following retrograde tracer injections in either the septum or hippocampus; 2) single and double retrogradely labeled neurons were intermingled within the supramammillary nucleus and mainly localized to the lateral two-thirds of the supramammillary nucleus; 3) approximately 5-10% of supramammillary cells were double-labeled, ipsilaterally, and 2-4%, contralaterally, with injections in medial or lateral parts of the medial septum and the dentate gyrus of the hippocampus; and 4) approximately 3-5% of supramammillary cells were double-labeled, ipsilaterally, and 1-2%, contralaterally, with injections in the medial septum and CA2/CA3a of the dorsal hippocampus. Cells of the supramammillary nucleus have been shown to fire rhythmically in bursts synchronous with the hippocampal theta rhythm and have been implicated in the generation of the theta rhythm. The supramammillary cells that we identified with collateral projections to the septum and hippocampus may be directly involved in generation of the theta rhythm.