Anatomical, electrophysiological and pharmacological studies of ascending brainstem hippocampal synchronizing pathways.

The present review has provided evidence that very potent ascending brainstem hippocampal synchronizing pathways originate in the rostral pons region (RPO and PPT), and ascend to and synapse with several midline caudal diencephalic nuclei (posterior hypothalamic and supramammillary) which send projections to the medial septal region (MS/vDBB). The medial septal region in turn is a critical nodal point, sending projections to limbic structures such as the hippocampal formation, cingulate cortex, and entorhinal cortex. The pontine and diencephalic nuclei appear to play a critical role in determining the translation of increasing levels of activation into moment to moment changes in the frequency of hippocampal theta field and theta-related cellular discharges, relayed to the MS/vDBB nuclei. The MS/vDBB nuclei appear to play a critical role in translating increasing levels of ascending synchronizing activation into moment to moment changes in the amplitude of hippocampal theta field activity and the accompanying rate and pattern of phasic theta-ON cells. The MS/vDBB carries out this role through a balance of activity in the septohippocampal cholinergic and GABA-ergic projections. Cholinergic projections provide the afferent excitatory drive for hippocampal theta-ON cells and the GABA-ergic projections act to reduce the overall level of inhibition by inhibiting hippocampal GABA-ergic interneurons (theta-OFF cells). Both activities must be present for the generation of hippocampal theta and theta-related cellular activities. The balance between the cholinergic and GABA-ergic projections may determine whether hippocampal synchrony (theta) or asynchrony (LIA, large amplitude irregular activity) occurs. These same ascending pathways influence the electrophysiological and pharmacological properties of the neocortex as well. The functional significance of the ascending brainstem synchronizing pathways is the generalized regulation of activities in these cortical structures as they relate to sensorimotor behavior.