Distribution of acetylcholinesterase in the hippocampal region of the rabbit: I. Entorhinal area, parasubiculum, and presubiculum

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Abstract
The distribution of acetylcholinesterase (AChE) was examined in three areas of the hippocampal region of adult rabbit, viz., the entorhinal area, the parasubiculum, and the presubiculum. AChE was demonstrated histochemically according to a modification of the Koelle copper thiocholine method. In each of the examined areas the pattern of AChE was distinctly stratified and corresponded extensively to fields and layers defined on the basis of cyto- and fibro-architectonics. The enzyme activity was mainly present in the neuropil, but in addition, moderately to weakly stained nerve cell bodies were seen scattered in the entorhinal area and the presubiculum. Only a very small minority of the total number of neuronal somata showed an intense staining reaction for AChE. Two subfields were discernible in the entorhinal area, these being termed pars medialis and pars lateralis. Pars medialis showed a distinctly stratified enzyme distribution, whereas stratification was less conspicuous in pars lateralis, especially at basal levels. The deep cortical layers (IV–VI) showed a similar distribution of AChE in all three areas—a medium-stained layer IV, a weakly stained layer V, and a weakly to moderately stained layer VI. However, at more dorsal horizontal levels of pars medialis, heavily stained patches occurred in layer IV. In the entorhinal area, the superficial cortical layers (I–III) contained most enzyme activity in the superficial two-thirds of layer I, the interstices between the stellate cell bodies in layer II, and the superficial part of layer III. In the parasubiculum, layers I–III formed a wedge-shaped field with a very high content of AChE. The presubiculum was well stained in layers I and III, whereas the densely packed cell bodies of layer II were unstained. The distribution of AChE in the rabbit was compared with that in the rat and guinea pig, previously described. The staining pattern is fundamentally similar in all three species, but certain clear differences exist. Possible structural correlates to the AChE observed in the three areas are considered in detail, especially the relation to the distribution of afferent systems known from experimental studies in other animal species. Although a substantial amount of the enzyme is probably associated with the septohippocampal projection, at present it cannot be excluded that other fiber systems may account for some of the observed AChE.