Subset of neurons characterized by the presence of NADPH‐diaphorase in human substantia innominata

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Abstract
The substantia innominata encompasses an area of the basal forebrain that is ventral to the lenticular nucleus and anterior commissure, medial to the claustrum and external capsule, and lateral to the hypothalamus. The nucleus basalis of Meynert consists primarily of large acetylcholinesterase (AchE)‐positive neurons embedded within the substantia innominata. Damage to these neurons may be important in the pathogenesis of cortical dysfunction in Alzheimer's disease. In order to characterize other neuronal elements in the substantia innominata and their relationship to the nucleus basalis, we chose to study a biochemically distinct neuronal subset containing the enzyme nicotinamide adenine dinucleotide phosphate diaphorase (NADPH‐d). The substantia innominata was blocked from six normal brains obtained postmortem and fixed in neutral‐buffered formalin at 4°C for 48 hours. Free‐floating 50‐μm sections from several levels were stained for NADPH‐d or AchE activities. Selected sections were double stained for NADPH‐d and AchE. NADPH‐d activity was present in a network of pleomorphic neurons that extended through all levels of the substantia innominata and into the striatum and amygdala. NADPH‐d neurons were particularly numerous at the level of the anterior commisure and were closely associated with the cholinergic neurons of the nucleus basalis. They were not seen in the ventral pallidum, or the vertical limb of the diagonal band of Broca or in the islands of Calleja. The cell bodies of NADPH‐d neurons were quite varied in shape, ranging from ovoid to fusiform, and about half the cells were bipolar. Where neuronal density was high, their dendrites formed an interlacing pattern. NADPH‐d‐positive fibres were seen coursing through the external capsule, hypothalamus, and amygdala. This novel set of neurons in the substantia innominata may be part of a more extensive network that interacts with the magnocellular basal fore‐brain system at the level of the nucleus basalis. Whether other neurotransmitters are present within these neurons and whether NADPH‐d neurons are involved in Alzheimer's disease remain to be elucidated.