Reduced nicotinamide adenine dinucleotide phosphate‐diaphorase/nitric oxide synthase profiles in the human hippocampal formation and perirhinal cortex

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Abstract
Nicotinamide adenine dinucleotide phosphate‐diaphorase (NADPH‐d)‐stained profiles were evaluated throughout the human hippocampal formation (i. e., dentate gyrus, Ammon's horn, subicular complex, entorhinal cortex) and perirhinal cortex. NADPH‐d staining revealed pleomorphic cells, fibers, and blood vessels. Within the entorhinal and the perirhinal cortices, darkly stained (type 1) NADPH‐d pyramidal, fusiform, bipolar, and multipolar neurons with extensive dendrites were scattered mainly within deep layers and subjacent white matter. Moderately stained (type 2) NADPH‐d round or oval neurons were seen mainly in layers II and III of the entorhinal and perirhinal cortices, in the dentate gyrus polymorphic layer, in the CA fields stratum pyramidal and radiatum, and in the subicular complex. The distribution of type 2 cells was more abundant in the perirhinal cortex compared to the hippocampal formation. Lightly stained (type 3) NADPH‐d pyramidal and oval neurons were distributed in CA4, the entorhinal cortex medial subfields, and the amygdalohippocampal transition area. Sections concurrently stained for NADPH‐d and nitric oxide synthase (NOS) revealed that all type 1 neurons coexpressed NOS, whereas types 2 and 3 were NOS immunonegative. NADPH‐d fibers were heterogeneously distributed within the different regions examined and were frequently in close apposition to reactive blood vessels. The greatest concentration of fibers was in layers III and V–VI of the entorhinal and perirhinal cortices, dentate gyrus polymorphic and molecular layers, and CA1 and CA4. A band of fibers coursing within CA1 divided into dorsal and ventral bundles to reach the presubiculum and entorhinal cortex, respectively. Although the distribution of NADPH‐d fibers was conserved across all ages examined (28–98 years), we observed an increase in the density of fiber staining in the aged cases. These results may be relevant to our understanding of selective vulnerability of neuronal systems within the human hippocampal formation in aging and in neurodegenerative diseases.