Profound but transient deficits in learning and memory after global ischemia using a novel water maze test

The pyramidal CA1 neurons of the hippocampus are critically involved in spatial learning and memory. These neurons are especially vulnerable to cerebral ischemia, but in spite of this, it has been consistently difficult to show any learning and memory deficits in two-vessel occlusion models of global ischemia. Transient global ischemia was induced in adult male rats under general anaesthesia administered by artificial respiration to prevent respiratory arrest. Systemic blood pressure was reduced to below 50 mmHg by instant adjustments of the halothane concentration, before and during bilateral occlusion of the carotid arteries. Cerebral blood flow was monitored by laser-Doppler flowmetry. Dying neurons were detected by TUNEL at 14 days after ischemia and surviving neurons by NeuN at 14 and 125 days after ischemia. Learning and memory was assessed in a novel water maze with three successive left-right choices. Transient global ischemia produced a profound and selective degeneration of CA1 neurons at 14 days after ischemia. This degeneration was associated with severe impairments in learning at 13 days after ischemia and in memory, as tested 24 h afterwards. At 125 days after ischemia, there was no significant learning and memory impairment, whereas the number of CA1 neurons was increased. These results show that transient global ischemia induced by two-vessel occlusion may lead to severe, but transient, impairments in learning and memory using a novel water maze, and that restored learning and memory is associated with an increased number of CA1 neurons.