Structural and functional deficits of the hippocampus in hydrocephalic rats: the role of age at onset and duration of disease

We studied the pyramidal neurons of the hippocampus in neonatal, juvenile and adult rats with hydrocephalus of varying durations and related these changes to their learning and memory. Hydrocephalus was induced in 7-day-old neonates, 4-week-old juvenile and 16-week-old adult Albino rats by intra-cisternal injection 0.02-0.04 ml of 150mg/ml of kaolin in sterile water (150 mg/ml). We studied escape latency and platform crossings with the Morris water maze prior to animal sacrifice at 2 and 4 weeks post induction in neonates and at 4 and 8 weeks post induction in juveniles and adults. We examined pyramidal neurons with cresyl violet and modified Golgi stain and analyzed behavioural scores and pyknotic indices. Statistical significance was determined at p<0.05. The basal dendrites of the pyramidal neurons were reduced in the hydrocephalic groups. In the CA1, the pyknotic index was significantly increased in both groups of hydrocephalic neonates but only in hydrocephalic juveniles and adults sacrificed at 4 weeks. In the CA3 it was increased in hydrocephalic neonates sacrificed at 2 weeks and hydrocephalic juveniles sacrificed at 4 weeks. The escape latency was greater and the number of platform crossing was lower in the hydrocephalic rats than in their age matched controls. Pyramidal neurons were morphologically altered in hydrocephalus, in association with changes in spatial learning and memory. The CA1 region in young animals was particularly vulnerable. Functional recovery occurs with time and to a greater extent in older animals.