8 Hippocampal region

The anatomy of the hippocampal region has been studied intensely in rodents since Cajal (1911) and his pupil, Lorente de N6 (1934), carried out their classic slUdies using the Golgi method. In recent years, the cytoarchitecture and connections of the hippocampal region have been further elucidated by the work of Blackstad (1956, 1958), Raisman et al. (1965, 1966) and Hjorth-Simonsen (1972, 1973) using lesion methods. Meibach and Siegal (1975, 1977a, b), Swanson and Cowan (1975, 1976, 1977) and Steward (1976) used physiologic transport methods to study hippocampal connections. Finally, Swanson et al. (1981) used refinements of axonal tracing techniques and double-labeling methods to add to the literature on hippocampal anatomy. These studies, and many others, have contributed to make the rat hippocampal region one of the best known neural structures. The hippocampal region occupies most of the ventroposterior and ventro­ lateral walls of the cerebral cortex. All the major components of the hippo­ campal region can be viewed simultaneously in sections taken in the horizontal plane. Fig. I is a Nissl stained section which corresponds closely to Plate 60 of the stereotaxic atlas of Paxinos and Watson (1982). In proceeding from the rhinal fissure (RF) to the medial cortical edge, one finds six distinct structures (separated by dashed lines): entorhinal cortex (EntL, EntI, EntM, area 28), parasubiculum (PaS, area 49), presubiculum (PrS, area 27), subiculum proper (S), fields CA1-CA3 in Ammon's horn (Amn), and dentate gyrus (DG). The hippocampus (Ammon's horn and the dentate gyrus) extends anteriorly beneath the corpus callosum to the posterior level of the septal nuclei. A rudiment of the hippocampal region extends in a small dorsomedial ridge above the corpus callosum (indusium griseum) into the anteromedial septal region (dorsal tenia tecta). The rudiment continues anteroventrally to mingle with medial parts of the anterior olfactory nucleus (ventral tenia tecta). In this chapter, I will discuss