ABNORMAL DISTRIBUTION OF ACETYLCHOLINESTERASE ACTIVITY IN THE HIPPOCAMPAL-FORMATION OF THE DREHER MUTANT MOUSE

The distribution of acetylcholinesterase(AChE) in the hippocampal formation of the dreher mutant mouse was studied by comparing homozygous mutant (dr(sst-J)/dr(sst-J)) with littermate control (+/? or +/+). In the control mice, AChE activity was most intense in the inner one-third of the stratum oriens and lacnosum of the hippocampus, and in the inner one-fifth of the molecular layer of the dentate gyrus. In contrast, in homozygous dreher mice, AChE activity in area CA3c of the hippocampus was not restricted to the stratum oriens, and extended upward into the infrapyramidal and suprapyramidal mossy fiber layers, the lower part of the stratum radiatum, the pyramidal cell layer, and downward toward the alveus. In addition, the distribution of AChE activity was modified by accompanying with ectopic pyramidal cells or with disruption of the pyramidal cell layer. AChE activity in the dentate gyrus of the dreher mouse was not confined to the inner one-fifth of the molecular layer. These findings indicated that the cholinergic input to the hippocampal formation is not normal in the dreher mutant mouse. Since the areas of AChE activity correspond to the presence of ectopic pyramidal cells in the dreher mouse, incoming cholinergic fibers may form synapses with these ectopic cells and with the dendrites of normal pyramidal cells that extend into the expanded area of AChE activity.