Evidence for Differential Control of Posterior Hypothalamic, Supramammillary, and Medial Mammillary Theta-Related Cellular Discharge by Ascending and Descending Pathways

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Abstract
Single-unit discharge was recorded from cells in the posterior hypothalamic nucleus (PH), supramammillary nucleus (SuM), and medial mammillary nucleus (MM) during hippocampal theta (θ) elicited by stimulation of the reticular nucleus pontis oralis (RPO). In agreement with previously published work, θ-related cells in the PH (12 cells) were all classified as tonic θ-ON (increased tonic discharge rate during hippocampal θ), whereas those in the SuM (9 cells) and MM (15 cells) were all classified as phasic θ-ON (rhythmic discharge, in phase with ongoing θ). The effect of RPO stimulation on cell discharge was tested after hippocampal θ was abolished by infusion of procaine into the medial septum/vertical limb of the diagonal band. The RPO-elicited discharge patterns of all PH tonic θ-ON cells and all SuM phasic θ-ON cells survived septal procaine infusion. Further, the discharge rate of PH cells and the frequency of burst discharge of SuM cells during RPO stimulation both increased after the infusion. In contrast, septal procaine infusion abolished the RPO-elicited rhythmic discharge pattern in MM phasic θ-ON cells and attenuated their discharge rates. These results indicate that the PH and SuM form parts of an ascending system mediating hippocampal θ, whereas the MM receives (and perhaps relays to other parts of the limbic system) rhythmic input descending from the septo-hippocampal system. In addition, PH and SuM receive descending inputs that limit the discharge rates of their θ-related cells during hippocampal θ.