Distribution of cholinergic neurotransmitter enzymes in the hippocampus and the dentate gyrus of the adult and the developing mouse

The distribution of the cholinergic neutrotransmitter enzyme, acetylcholinesterase, in the hippocampus and the dentate gyrus of the adult and the developing mouse was examined using a histochemical procedure. The pattern of acetylcholinesterase staining in the hippocampal region of the adult mouse closely resembled that reported for the rat by other investigators. Enzyme activity was present predominantly in the neuropil, where it was concentrated in the supra- and the infrapyramidal zones of the hippocampus as well as in the supragranular region and the hilus of the dentate gyrus. In contrast to the adult pattern, during the first week of postnatal development acetyleholinesterase activity appeared to be largely intracellular. Strong staining was observed in the cytoplasm of scattered neurons throughout the neuropil laminae, particularly in the hilus of the dentate gyrus. During the succeeding weeks, the characteristic neuropil reaction developed in a slow and progressive manner. reaching the adult pattern by the end of the third postnatal week. Between the third and the fifth weeks, there was a substantial increase in the staining intensity of the enzyme. As a result of the increased neuropil reaction, acetylcholinesterase-positive cells became less conspicuous after the second postnatal week. The progressive acquisition of staining for acetylcholinesterase in the neuropil of the hippocampus and the dentate gyrus of the mouse during the early postnatal period compared well with the proposed model of development of septohippocampal connections in the rat.The histochemical distribution of choline acetyltransferase in the hippocampus and the dentate gyrus of the adult mouse was also examined. The reaction was largely intracellular, in the cytoplasm of the pyramidal and the granule cells. Neuropil staining was confined to the mossy fibers and their terminals. This distribution profile is in conflict with the localization of this enzyme in the hippocampal region established by other investigators on the basis of microdissection and assay. The significance of the results of choline acetyltransferase histochemistry in relation to methodological problems is discussed.