THE MIDLINE POSTERIOR HYPOTHALAMIC REGION COMPRISES A CRITICAL PART OF THE ASCENDING BRAIN-STEM HIPPOCAMPAL SYNCHRONIZING PATHWAY

Electrical stimulation and microinfusion techniques were utilized in al:ute experiments on urethane-anesthetized rats in order to evaluate the hypothesis that the posterior hypothalamic and supramammillary nuclei comprise a critical part of the ascending brainstem pathway for producing synchronous hippocampal formation (HPC) field activity (theta). Given confirmation of this hypothesis a second objective was to determine the nature of the contribution made by this midline posterior hypothalamic region (PH) to the frequency and amplitude components of HPC theta field activity. The cholinergic nature of the ascending pathway was also examined. Reversible inactivation of the PH was achieved by microinfusion of the local anesthetic procaine hydrochloride. The efficacy of and recovery from procaine inactivation of the PH was quantitatively analyzed either by electrical stimulation of the nucleus pontis oralis (PO) (two experiments) or the PH (four experiments). The results are summarized under the following three headings: 1) The first is the effect of procaine inactivation of the PH on HPC theta elicited caudal to, at the level of, or rostral to the PH. Ah HPC theta induced caudal to the PH (spontaneous theta, tail pinch-induced theta, and theta produced by electrical stimulation of the PO) was totally abolished for a minimum 10-min period. HPC theta induced rostral to the PH by the intrahippocampal infusion of carbachol was unaffected, while HPC theta induced by infusions of carbachol into either the medial septum (MS) or PH was reduced in amplitude with no effect on frequency. 2) Next are comparisons of pre- and post-PH procaine trials of electrical stimulation of the PO and PH. In all experiments, regardless of the anatomical locus or technique used to induce HPC theta, pre- and post-PH procaine comparisons of the PO and PH stimulation trials revealed that frequency modulation of HPC theta recovered significantly more slowly than amplitude. 3) Last is the effect of electrical stimulation of the PO and PH on HPC theta induced by carbachol infusions at the level of the HPC, MS, or PH. Tn all experiments, electrical stimulation of both the PO and PH, at appropriate intensities, resulted in increasing HPC theta frequencies above the frequency induced by the infusion of carbachol into the HPC, MS, and PH. In addition, the post-carbachol HPC theta frequencies induced by electrical stimulation were significantly higher than those produced in the pre-carbachol conditions. The data supported the following conclusions: 2) The PH region (posterior hypothalamic and supramammillary nuclei) comprises a critical part of the ascending brainstem pathway For producing HPC theta field activity. Inputs ascending from the brainstem caudal to the PH contribute primarily to the frequency of HPC theta and secondarily to the amplitude of HPC theta;:!) a portion of the neural components of the ascending brainstem pathway, up to and including the HPC, are cholinergic, cholinoceptive, or both. (c) 1994 Wiley-Liss, Inc.