Watermaze performance after middle cerebral artery occlusion in the rat: the role of sensorimotor versus memory impairments

In rodent stroke models, investigation of deficits in spatial memory using the Morris watermaze may be confounded by coexisting sensory or motor impairments. To target memory specifically, we devised a watermaze protocol to minimize the impact of sensory and motor impairments in female Lister-hooded rats exposed to proximal electrocoagulation of the middle cerebral artery (MCAO). Rats were trained in a reference-memory task comprising 4 trials/day; trial 1 being a probe trial (platform absent for the first 60 seconds). Training ended once animals reached a strict criterion based on the probe-trial performance. Memory retention was tested 1, 7, and 28 days later. The MCAO did not affect the number of days to reach criterion during acquisition or the time spent in target quadrant during retention testing, compared with sham or unoperated rats. However, MCAO rats showed slightly poorer accuracy in crossing the platform location and increased thigmotactic swimming compared with controls. Our results show that spatial memory deficits are minimal in this rodent stroke model, and suggest that previously published watermaze impairments are attributable to sensory and motor deficits but not memory deficits. We recommend using probe trials and training to a predetermined performance criterion in future studies assessing watermaze memory deficits in rodent stroke models.