Distribution of acetylcholinesterase in the hippocampal region of the rabbit: III. The dentate area.

The distribution of acetylcholinesterase (AChE) in the dentate area, a part of the hippocampal region, of the adult rabbit was described. A modification of the Koelle copper thiocholine method was used for the histochemical demonstration of AChE. The dentate area contained high amounts of this enzyme, distributed in a well-defined stratified pattern. Thus, in the molecular layer seven distinct and differently stained layers were observed at most septotemporal levels. The granular cell bodies were entirely devoid of AChE, but stained precipitate occurred between the cell bodies, in particular in the superficial half of the granular cell layer. In the hilus, five layers of alternating stronger and weaker activity were recognizable. In the molecular layer and the granular cell layer, almost all activity was restricted to the neuropil, whereas a great number of intensely stained cell bodies were observed in the hilus. Criteria for the delimitation of the dentate area, as defined by Blackstad, are discussed in view of the concepts of Cajal and Lorente de Nó and in relation to more recent hodological and histochemical data. The results obtained in the present report compare well with the concept of a layered rabbit hilus, the cytoarchitecture of this being representative of one group of animals including the guinea pig, monkey, and humans and differing from the nonlayered hilus found in, for example, the rat and mouse. The distribution of AChE in the rabbit was compared with that in the rat and guinea pig, described previously. Very striking differences in the staining pattern of the molecular layer were observed, whereas the granular cell layer had a virtually identical appearance. The stratified pattern observed in the rabbit hilus corresponds to the distribution profile of the enzyme in the guinea pig, but contrasts with the rather diffuse distribution in the rat. This variation in the staining pattern of the hilus, however, mainly reflects the differences in cytomorphology between the rabbit and guinea pig on the one hand and the rat on the other, rather than being representative of a true species difference. The possible correlation of the AChE observed in the rabbit dentate area with cyto- and fibroarchitecture, in particular the terminal fields of fiber systems known from experimental investigations, is discussed in detail. The possibility that some of the AChE observed in the hippocampal region could be involved in the hydrolysis of a number of neuropeptides, in particular substance P and enkephalin, is considered.