Localization of neuronal and endothelial nitric oxide synthase isoforms in human hippocampus

The aim of the study was to use immunohistochemistry to identify, in the hippocampal region of human brain. the distribution of neuronal and endothelial isoforms of the enzyme nitric oxide synthase. Numerous pyramidal neurons and small, presumed GABAergic interneurons throughout the pyramidal cell layer of CA1-CA3 exhibited neuronal nitric oxide synthase-like immunoreactivity. Comparable immunopositive cells were seen in the granule cell and polymorphic layers of the dentate gyrus and in the stratum oriens. A dense plexus of immunopositive fibres was seen in the granule cell layer of the dentate gyrus. In contrast, endothelial nitric oxide synthase-like immunoreactivity was localized specifically, and with a pronounced punctate distribution, to the cell bodies of CA1 pyramidal neurons. The endothelial isoform was also present in blood vessels and in cells which resembled astroglia. These latter cells had a similar appearance and distribution to astroglia identified by their positive reaction to glial fibrillary acidic protein. The most frequently used method for identifying nitric oxide synthase-containing cells in brain, the NADPH-diaphorase reaction, was also applied to hippocampal sections. Only occasional NADPH-diaphorase-positive cells were seen in the hippocampus where, in contrast to their nitric oxide synthase-like immunoreactivity, the pyramidal cells did not stain for NADPH-diaphorase. Similarly, only occasional NADPH-diaphorase-reactive varicose axons were found in the hippocampus in these experiments. This study is the first to identify mostly separate populations of cells containing neuronal and endothelial nitric oxide synthase isoforms in human hippocampus. The data show that NADPH-diaphorase histochemistry, which is frequently used to show the presence of nitric oxide synthase, greatly underestimates the potential for hippocampal cells to produce nitric oxide. The fact that human hippocampus has a great many nitric oxide synthase-containing cells implies that nitric oxide has a role in human hippocampal functions although, at the present time, these actions are not clear. Whether those stimuli known to produce nitric oxide, such as activation of glutamate N-methyl-D-aspartate receptors, cause both enzyme isoforms in CA1 pyramidal cells to produce nitric oxide remains to be determined.