Spatial organization of direct hippocampal field CA1 axonal projections to the rest of the cerebral cortex

The spatial distribution of axonal projections descending from rat field CA1 to thalamus and hypothalamus was analyzed previously with the PHAL method [Cenquizca, L.A., Swanson, L.W. 2006. An analysis of direct hippocampal cortical field CA1 axonal projections to diencephalon in the rat. J Comp Neurol 497:101–114.]. The same experimental material was used here to define the topography of field CA1 association projections to other cerebral cortical areas. First, the results confirm and extend known intrahippocampal formation inputs to dentate gyrus, subiculum, presubiculum, parasubiculum, and entorhinal area, which are arranged generally along the formation's transverse axis and dominated by the subicular projection—by far the densest established by field CA1 anywhere in the brain. And second, field CA1 innervates a virtually complete ring of extrahippocampal formation cortex via three routes. A dorsal pathway from the dorsal third of field CA1 innervates moderately the retrosplenial area; a moderately strong ventral pathway from the ventral two thirds of field CA1 passing through the longitudinal association bundle sends offshoots to visual, auditory, somatosensory, gustatory, main and accessory olfactory, and visceral areas—as well as the basolateral amygdalar complex and the agranular insular and orbital areas; and a cortical–subcortical–cortical pathway through the fornix from the whole longitudinal extent of field CA1 innervates rather strongly a rostral region that includes the tenia tecta along with the anterior cingulate, prelimbic, infralimbic, and orbital areas. The functional consequences of long-term potentiation in field CA1 projection neurons remain to be explored.