neuroanatomical investigation of the median raphe nucleus, supramammillary nucleus, and nucleus reuniens: Possible implications for the modulation of the hippocampal EEG

The theta rhythm of the hippocampus, present in area CA1 of Ammon's horn and the dentate gyrus, is thought to serve a role in short-term memory processing. Rhythmically bursting cells of the medial septum projecting to the hippocampus are responsible for pacing theta. Further anatomical investigation of limbic-related circuitry in the rat, particularly connections of the medial septum and hippocampus, will lead to a better understanding of the pathways which influence the hippocampal EEG. Our first study examined single and collateral projections from the supramammillary nucleus to the medial septum and hippocampus, employing the retrograde fluorescent tracers FluoroGold and FluoroRuby. The supramammillary nucleus neurons we identified with collateral projections to the medial septum and hippocampus may be directly involved in generation of the theta rhythm. The second study examined single and collateral projections from the median raphe nucleus to the medial septum and hippocampus, employing the retrograde tracers FluoroGold and FluoroRuby. It has been proposed that the median raphe nucleus serves a direct role in desynchronization of the hippocampal EEG, or blockade of theta. The median raphe nucleus neurons we identified with collateral projections to the medial septum and hippocampus may be directly involved in the termination of theta, in turn modulating hippocampal memory processing. The third study examined afferent projections to the nucleus reuniens of the thalamus, employing the retrograde tracer FluoroGold, combined with a primary-antibody immunohistochemical procedure, in order to identify FluoroGold labeled cells by means of bright-field microscopy. RE afferents originate from widespread regions of the brain, providing multi-sensory and limbic input to RE. The fourth study examined efferents of the nucleus reuniens, employing the anterograde tracer PHA-L (Phaseolus vulgaris-leucoagglutinin). RE efferents terminated largely in regions of the telencephalon, and may influence working memory and sensorimotor systems. Investigations have linked the theta rhythm of the hippocampus to memory processing. The activity of the supramammillary nucleus, median raphe nucleus, and nucleus reuniens may influence the hippocampal EEG, particularly theta, and hence hippocampal mnemonic processing, by means of the pathways described in this dissertation.