Effects of anterior or dorsomedial thalamic ibotenic lesions on learning and memory in rats

Sprague-Dawley rats were used to study the effects of ibotenic acid lesions of the anterior (A.Th.) and the dorsomedial (MD) thalamic nuclei on learning and memory. Memory was assessed by employing a temporal alternation task in a straight alley with varying intertrial intervals. In addition, spatial orientation and response flexibility were evaluated on a radial maze and on a spatial reversal task (SSDR). The results indicated that MD rats required more trials to learn the temporal alternation task and exhibited impaired performance compared to A.Th. and control groups at the shortest delay (15 s). In contrast, compared to the control group, A.Th. subjects which required less trials to master the task and exhibited normal performance at the 15-s delay were impaired when the intertrial interval was increased to 45 s. Whatever the lesion, no impairments were found in the SSDR or the radial maze while only MD lesions were found to result in a night hyperactivity associated with greater food and water consumptions. These findings indicate that A.Th. and MD are differentially involved in learning and memory processes. It is suggested that the MD is mostly involved in registering new information while the A.Th. plays a role in the maintenance of information over time.