The entorhinal cortex (EC) has been regarded as a transition between the hippocampus and neocortex, providing the major source of afferent input to the hippocampus, a structure that is critically involved in spatial discrimination learning. We assessed the severity and duration of EC lesions, induced by different doses of the excitotoxin ibotenic acid (IBO), and the reliability of a video-tracking system, EthoVision, as a tool to measure Morris water escape behavior.

Male Møllegaard Wistar rats were randomly assigned to one of four experimental conditions (n=15 per group): bilateral sham or EC-lesion, induced by three stereotaxically guided injections of vehicle (0.9% NaCl), or of 1 mg/ml, 3 mg/ml, or 10 mg/ml IBO. The excitotoxin or vehicle was injected bilaterally in an application volume of 0.5 µl per site. Behavioral testing took place in a Morris water maze with a diameter of approximately 150 cm. Testing started 10-14 days after surgery and was conducted in ten daily sessions with a 9 day resting period after the fifth day of acquisition. During each daily sessions the animals received four trials. After the fourth trial of the fifth and the tenth session, an additional trial was given as a probe trial in order to assess the spatial bias for the training quadrant. Performance on the Morris water maze task was registered automatically using the video tracking system EthoVision; in parallel, a manual scoring technique was applied.

In order to validate the EthoVision system, product-moment correlation coefficients were calculated across the sixty animals per day between the two scoring methods for escape latencies and distances traveled to find the platform. The correlation analysis revealed that EthoVision produced highly reliable results (see Table 1).

Table 1. Product-moment correlation coefficients calculated per session across all rats (n=60) per session between distances traveled and the platform escape latencies, scored either automatically by EthoVision, or manually, by an experienced experimenter. The lowest and highest correlation coefficient is shown.

The rats lesioned by 10 mg/ml IBO displayed retarded acquisition of the Morris water escape task during the first five daily sessions than the other three groups. However, this deficit was no longer apparent during the second series of five sessions, and all groups showed a strong bias for the training quadrant during the probe trials on the fifth and tenth day of training. This indicates that all groups had reached a similar performance level by the end of the fifth session. In the present study, entorhinal cortex lesions by IBO induced a dose-related, transient deficit in spatial orientation learning.