EVIDENCE FOR DIFFERENTIAL CONTROL OF POSTERIOR HYPOTHALAMIC, SUPRAMAMMILLARY, AND MEDIAL MAMMILLARY THETA-RELATED CELLULAR DISCHARGE BY ASCENDING AND DESCENDING PATHWAYS

Single-unit discharge was recorded from cells in the posterior hypotha lamic nucleus (PH), supramammillary nucleus (SuM), and medial mammilla ry nucleus (MM) during hippocampal theta (theta) elicited by stimulati on of the reticular nucleus pontis oralis (RPO). In agreement with pre viously published work, theta-related cells in the PH (12 cells) were all classified as tonic theta-ON (increased tonic discharge rate durin g hippocampal theta), whereas those in the SuM (9 cells) and MM (15 ce lls) were all classified as phasic theta-ON (rhythmic discharge, in ph ase with ongoing theta). The effect of RPO stimulation on cell dischar ge was tested after hippocampal theta was abolished by infusion of pro caine into the medial septum/vertical limb of the diagonal band. The R PO-elicited discharge patterns of all PH tonic theta-ON cells and all SuM phasic theta-ON cells survived septal procaine infusion. Further, the discharge rate oi PH cells and the frequency of burst discharge of SuM cells during RPO stimulation both increased after the infusion. I n contrast, septal procaine infusion abolished the RPO-elicited rhythm ic discharge pattern in MM phasic theta-ON cells and attenuated their discharge rates. These results indicate that the PH and SuM form parts of an ascending system mediating hippocampal theta, whereas the MM re ceives (and perhaps relays to other parts of the limbic system) rhythm ic input descending from the septo-hippocampal system. In addition, PH and SuM receive descending inputs that limit the discharge rates of t heir theta-related cells during hippocampal theta.