Distribution of acetylcholinesterase in the hippocampal region of the rabbit: II. Subiculum and hippocampus.

Abstract

The distribution of acetylcholinesterase (AChE) has been examined in two areas of the hippocampal region of the adult rabbit, viz., the subiculum and the hippocampus. AChE was demonstrated histochemically by using a modification of the Koelle copper thiocholine method. Moderate amounts of AChE were observed in the subiculum, whereas the hippocampus had a high content of this enzyme. In each area, the AChE staining displayed a distinctly stratified pattern which has been compared in detail with the fields and layers defined on the basis of cyto- and fibro-architectonics. Most of the enzyme activity was confined to the neuropil, but a considerable number of nerve cell bodies were moderately or intensely stained in both the subiculum and the hippocampus. In the subiculum, the plexiform layer showed a complex distribution of AChE, displaying four horizontal, differently stained subzones. In the cell layer, the staining was most intense in two narrow bands, one being immediately beneath the plexiform layer and the other bordering directly on the white matter. The remaining major part of the subicular cell layer generally had a low AChE content. In regio superior of the hippocampus, intense staining was observed in the deep part of the molecular layer, in a narrow suprapyramidal zone of the stratum radiatum, and in the stratum oriens. In regio inferior, very high AChE activity was present in the molecular layer, in two narrow bands bordering the mossy fiber layer superficially and at depth, and in the stratum oriens. The pyramidal cell bodies and the mossy fibers were unstained. The distribution of AChE in the rabbit was compared with that in the rat and guinea pig; the latter two have been reported. The staining patterns of all three species share many conspicuous histochemical features, though notable species-specific traits do exist. Detailed consideration is given to possible structural correlates of the AChE observed in the two areas, in particular the relation to fiber systems known from either normal material or experimental investigations. A considerable portion of the enzyme is probably contained in afferents of septal origin, but it seems very likely that some of the AChE is associated with other fiber systems, the identity of which are unknown at present. A possible role of some of the AChE observed in the hippocampal region in the hydrolysis of substance P and enkephalin is discussed briefly.