Distribution of acetylcholinesterase in the hippocampal region of the mouse: I. Entorhinal area, parasubiculum, retrosplenial area, and Presubiculum

The distribution of acetylcholinesterase (AChE) was examined in the multilayered posterior part of the hippocampal region of the adult mouse ( Mus musculus domesticus ), namely, the entorhinal area, the parasubiculum, the presubiculum, and those parts of the retrosplenial cortex that extend into the posterior hippocampal region (area retrosplenialis 29d and 29e). A modification of the Koelle copper thiocholine method was employed for the histochemical demonstration of AChE. The AChE staining resulted in a distinctly stratified pattern, which has been compared in detail with the fields and layers defined by cyto‐ and fibro‐architecture. Most of the enzyme activity was located in the neuropil, but both moderately and intensely stained nerve cell bodies were observed too. In the entorhinal area two main subfields were identified, which have been designated pars medialis and pars lateralis. In pars medialis, the superficial two thirds of layer I, the interstices between the stellate cell bodies, in layer II, and layers IV and VI showed moderate to high content of AChE, whereas layer V and, especially, layer III were poor in enzyme activity. A particular feature was the occurrence of cone‐shaped, darkly stained areas within layer II and, occasionally, the deep part of layer I. The staining of pars lateralis differed in several respects from that of pars medialis, the most prominent feature being a less conspicuous stratification. In addition, intensely stained somata occurred more frequently than in pars medialis, although they still constituted only a very small minority of the total number of nerve cell bodies. In the parasubiculum, a clear cytoarchitectural subdivision into a posterolateral parasubiculum a and an anteromedial parasubiculum b was observed. These subfields showed, however, only minor differences in AChE staining, Thus, in both subfields, layers I and IV stained intensely, whereas layers II and III showed moderate to intense staining. Layers V and VI did not differ in appearance from the corresponding layers of the entorhinal area. The retrosplenial areas 29d and 29e appeared very light in the AChE pattern, area 29e being the better stained. The presubiculum was very rich in AChE, with layers I, III and IV being particularly intensely stained. The small nerve cell bodies of layer II were unstained, whereas the intervening neuropil was intensely stained. The distribution of AChE in the mouse was compared with that in the rat, guinea pig, and rabbit, described previously. The staining pattern is largely similar in all four species, but striking species‐specific differences do exist. Possible structural correlates of the AChE observed in the four cortical fields are discussed. In the mouse, septohippocampal afferents appear associated with most of the entorhinal and parasubiculum AChE, but only to a lesser degree to the AChE of area retrosplenialis 29e and the presubiculum. A possible role of some of the AChE observed in the hippocampal region in the hydrolysis of a number of neuropeptides, in particular substance P and enkephalin, is considered.