Relation of spatial learning of rats in the Morris water maze task to the number of viable CA1 neurons following four-vessel occlusion.

Male Wistar rats were tested in the Morris water maze task 1 week after 6, 9, or 12 min of transient global ischemia. The 9-min and 12-min ischemia groups were significantly impaired in the acquisition and the reversal experiment. A systematic counting of CA1 neurons in the whole hippocampal formation revealed a unilateral number of CA1 neurons of 286,000 in the sham group, of which 2/3 were located in the dorsal hippocampus. The ischemia groups showed a significant decline in the number of dorsal CA1 neurons, whereas only the 12-min ischemia group showed a significant but minor decline (10%-15%) in the number of ventral CA1 neurons. A correlation analysis showed that the escape distance declined with increasing number of viable CA1 neurons, but poor correlation coefficients were obtained. Thus, some of the ischemic rats with even very few viable CA1 neurons in the dorsal hippocampus were capable of performing this spatial learning task at sham-group level.