Argyrophilic dark neurons represent various states of neuronal damage in brain insults: some come to die and others survive.

Argyrophilic dark neurons (DNs) reflect the early histopathological state of neurons following various brain insults. We examined the fate of DNs, about to either die or recover, following two types (heavy and light damage) of brain insult. Wistar rats were injected ibotenic acid unilaterally into the hippocampal CA1 region (ibotenic acid [IA] injection) or were forced to swim (SWIM). Argyrophil III (DNs)-, activated caspase-3 immuno-, TUNEL- and hematoxylin-eosin (H-E)-staining and ultrastructural examinations were then performed. One to three hours after IA injection, typical DNs (argyrophilic both in somata and dendrites) with corkscrew-like dendrites were densely packed in the pyramidal cell layer of hippocampal CA1 around the injection site. After 12-24 h, DNs were argyrophilic only in the somata and proximal dendrites but absent in distal dendrites in the CA1 region. However, at this time typical DNs were found in remote areas. At 3 h, caspase-3 activation was detected at the injection site, which increased to a peak level after 12 h. Three to 7 days after injection, TUNEL positive cells were detected in the CA1 pyramidal cell layer. Immediately following SWIM, "brown" rather than "dark" neurons were detected in the various areas and most frequently in the CA1 pyramidal cell layer. No typical DNs were detected over the first 3 days. Some activation of caspase-3 was detected in a few CA3 pyramidal cells but no TUNEL-positive cells were detected. Ultrastructural examination revealed a diffuse distribution of aggregated silver particles in the dendrites and cytoplasm of pyramidal cells at the sites of IA injection. After SWIM, silver particles were detected mainly on mitochondria of affected cells. These data suggest that DNs provide a measure of neuronal damage: typically dark neurons with broad damage to the cytoskeleton of dendrites would die, while non-typical brown neurons, that may have a disturbance in mitochondria, predominantly survive.