PHA-L analysis of projections from the nucleus reticularis pontis oralis, the pedunculopontine tegmental nucleus and the median raphe nucleus in the rat: Implications for the modulation of the hippocampal EEG

Projections of the nucleus reticularis pontis oralis (RPO), the pedunculopontine tegmental nucleus (PPT) and the median raphe nucleus (MR) were examined using the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L). The RPO projected strongly throughout the reticular core, to the PPT and to the intralaminar thalamic nuclei. Light projections were observed in the posterior hypothalamus but not the supramammillary nucleus. The pattern of labeling suggests that the RPO is arranged topographically in longitudinal columns. The PPT projected to the RPO, MR, medial thalamic and intralaminar nuclei, the supramammillary nucleus and septum. The MR was found to project heavily to the supramammillary nucleus, intralaminar thalamic nuclei, the septum, the hippocampus and several cortical areas. The results are discussed in terms of the modulation of hippocampal EEG and a model is introduced emphasizing interactions among ascending hippocampal EEG synchronizing and desynchronizing systems.