Recovery of Learning and Memory Deficits Despite Persistent Pyknosis of the Hippocampal Pyramidal Neurons of Adult Hydrocephalic Mice.

Background: The hippocampal alterations resulting from hydrocephalus are associated with various cognitive dysfunctions. Reduced learning and memory are early functional deficits that recover with time in experimental hydrocephalus. This study investigated the recovery processes of learning and memory loss in relation to the morphology of hippocampal pyramidal neurons and the degree of expansion of the ventricles.

Materials and methods: Hydrocephalus was induced in adult mice by intracisternal injection of sterile kaolin while controls received sham operation. Neurobehavioral tests for memory and learning were conducted, after which the animals were sacrificed in batches: 7 (acute) and 28 (subacute) days postinduction. After sacrifice, mice were categorized into mild and moderate hydrocephalus, and their fixed brain samples were processed for hematoxylin, eosin, and Nissl stains.

Results: In moderate acute hydrocephalus, the indices of learning and memory were reduced escape latency (67.20 ± 12.83 s), number of platform crossing (4.000 ± 1.658), duration in platform quadrant (4.000 ± 1.658), and percent of total investigation (44.857% ± 3.981%) but not in the subacute stage. Pyknotic indices (PI) were significantly higher in the cornu ammonis (CA)1 and 3 regions in all hydrocephalic groups than in controls. However, within groups, PI was significantly higher only in the CA1 of moderate acute (28.149% ± 1.875%) compared to moderate subacute hydrocephalic group (12.903% ± 3.23%).

Conclusion: Hydrocephalus caused cellular injury to the hippocampus associated with spatial learning and memory deficits. However, these functional deficits were partially reversed in moderate subacute hydrocephalus despite the persistence of the structural alterations in the CA1 and CA3 subregions.