SYNAPTIC ORGANIZATION OF RETROHIPPOCAMPAL PATHWAYS

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The objective of this research is to study the synaptic organization of efferent and intrinsic projections of the subicular complex in adult rats. Differences of synaptic organization within the sub-regions of the subicular complex (prosubiculum, subiculum proper, presubiculum, and parasubiculum) will be studied in order to define physiological differences related to the marked cytological differences in these sub-regions. Interdisciplinary neurophysiological, intracellular HRP tracing, and immunocytochemical techniques will be used together, and will provide information that none of the techniques could provide alone. The efferent projections from the subicular complex to the entorhinal cortex and amygdala will be examined, as well as circuitry intrinsic to these areas. The pathways will be studied by intracellular recording; intracellular injection of HRP to identify neurons and trace their axonal projections; and glutamic acid decarboxylase (GAD) immunocytochemistry to localize the somata and synaptic terminals of candidate inhibitory interneurons and their relationship to principal neurons. The proposed studies will (1) establish the entorhinal and amygdala targets of subicular efferent axons and the physiological action of subicular efferents on these targets, (2) provide quantitative information about differences in the degree of axonal branching to target areas of cells within different sub-regions of the subicular complex (by the use of intracellular tracing techniques), (3) indicate the localization and physiological properties of efferent cells within specific sub-regions of the subicular complex, and (4) clarify the synaptic relationship between principal neurons and GABAergic inhibitory interneurons. These studies will fill significant gaps in our knowledge about the synaptic organization of the subicular complex, a structure that is now widely recognized as the major source of hippocampal formation efferents.