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 a l:ute experiments on urethane-anesthetized rats in order to evaluate t he hypothesis that the posterior hypothalamic and supramammillary nucl ei comprise a critical part of the ascending brainstem pathway for pro ducing synchronous hippocampal formation (HPC) field activity (theta). Given confirmation of this hypothesis a second objective was to deter mine the nature of the contribution made by this midline posterior hyp othalamic 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 effi cacy of and recovery from procaine inactivation of the PH was quantita tively analyzed either by electrical stimulation of the nucleus pontis oralis (PO) (two experiments]) or the PH (four experiments). The resu lts are summarized under the following three headings: 1) The first is the effect of procaine inactivation of the PH on HPC theta elicited c audal 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 the ta produced by electrical stimulation of the PO) was totally abolished for a minimum 10-min period. HPC theta induced rostral to the PH by t he intrahippocampal infusion of carbachol was unaffected, while HPC th eta induced by infusions of carbachol into either the medial septum (M S) or PH was reduced in amplitude with no effect on frequency. 2) Next are comparisons of pre- and post-PH procaine trials of electrical sti mulation of the PO and PH. In all experiments, regardless of the anato mical locus or technique used to induce HPC theta, pre- and post-PH pr ocaine comparisons of the PO and PH stimulation trials revealed that f requency modulation of HPC theta recovered significantly more slowly t han 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 carb achol into the HPC, MS, and PH. In addition, the post-carbachol HPC th eta frequencies induced by electrical stimulation were significantly h igher than those produced in the pre-carbachol conditions. The data su pported the following conclusions: 2) The PH region (posterior hypotha lamic and supramammillary nuclei) comprises a critical part of the asc ending brainstem pathway For producing HPC theta field activity. Input s ascending from the brainstem caudal to the PH contribute primarily t o the frequency of HPC theta and secondarily to the amplitude of HPC t heta;:!) a portion of the neural components of the ascending brainstem pathway, up to and including tile HPC, are cholinergic, cholinoceptiv e, or both. (c) 1994 Wiley-Liss, Inc.