Post-mortem studies of the brains of Alzheimer’s patients have shown that intraneuronal neurofibrillary changes and neurofibrillary tangles in the entorhinal cortex (EC) occur at a very early stage of the disease. These changes appear to correlate with the memory deficits present during the early progression of Alzheimer’s disease, suggesting that these cognitive dysfunctions might be caused by disturbed hippocampal function due to degeneration of much of the architecture of the EC. Lesioning the EC in rats is considered as an animal model of this aspect of the early phase of Alzheimer’s disease.

A series of nine experiments was performed in which the effects of putative cognition enhancers on the performance of rats with bilateral lesions of the EC were assessed in the Morris water escape task. Only the sham-lesioned and the ibotenic acid (IBO)-lesioned groups were used for the present inter-experiment comparison. All animals received bilateral stereotaxically guided operations of the EC. The sham-lesioned rats were injected with vehicle only, whereas the EC-lesioned rats were injected with IBO. The coordinates are depicted in Table 1.

Table 1. Stereotaxic coordinates used to induce entorhinal cortex lesions. The coordinates are with respect to the intra-aural line, the incisor bar was set at 3.3 mm above intra-aural line, and the syringe was lowered under an angle of 15° (mediolateral orientation). Per injection site, 0.5 µl vehicle or ibotenic acid (10 mg/ml) was applied through a Hamilton syringe.

After a recovery period of about 1 week, the rats were tested in the Morris water escape task with four trials per session on five successive days. Observations were performed using the automatic video tracking system EthoVision. Lesioning of the EC induced learning deficits in all experiments. However, there were considerable differences between experiments with respect to the degree and time-course of the impairments. A certain degree of heterogeneity was also observed for the probe trial. All but one sham-lesioned group showed a strong bias for the previous training quadrant, whereas no such bias was apparent in the EC-lesioned rats.

Comparison of the Morris water escape performance shows that there exists a variability between shipments of rats, which cannot be neglected. All data were collected by the automatic video tracking system EthoVision. We have previously found in control experiments, where EthoVision and a manual scoring method were used in parallel, that the video tracking system yields highly reliable results [1].

Considering the inter-experiment variability, which we found in the present comparison study, it is of utmost importance to control and reduce intra-experiment variability. Reliable video-tracking systems such as EthoVision provide adequate tools to reach this goal. Although very reliable data can also be collected with simple manual scoring techniques, video tracking systems permit an in-depth analysis of rat behavior in the water tank. Search strategies, for example, can only be identified if the X-Y-coordinates of the actual position of the animal in the pool for each sample are stored.

References

1. Eijkenboom, M.; van der Staay, F.J. (1998). Effects of brain lesions on Morris water maze performance in rats: testing the reliability of an automatic video tracking system. This volume.