Direct projections from the anterior thalamic nuclei to the retrohippocampal region in the rat.

The present study examined the areal and laminar distribution of direct projections from the anterior thalamic nuclei to the retrohippocampal region in the rat, with anterograde transport of Phaseolus vulgaris-leucoagglutinin. The anteromedial nucleus (AM) projects to the temporal subiculum, medial entorhinal area, perirhinal area, and caudomedial part of the lateral entorhinal area. The interanteromedial nucleus (IAM) projects to the perirhinal area and the caudolateral part of the lateral entorhinal area. Furthermore, both the AM and IAM project to the temporal area 2, occipital area 1, and lateral occipital area 2. The projections from the AM and IAM to these retrohippocampal and neocortical regions terminate mainly in deep layers. The anteroventral nucleus (AV) projects to the subicular complex with a complex topographic organization. The most rostral part of the AV projects to layers I and III of the ventral presubiculum, the pyramidal cell layer of the temporal subiculum, and deep layers of the parasubiculum and medial entorhinal area. At the midrostrocaudal level of the AV, the lateral and the dorsal quadrants of the AV project, respectively, to layers I and III and to layers I and IV-VI of the ventral presubiculum, whereas the ventral and the medial quadrants project, respectively, to layers I and III and to layers I and IV-VI of the dorsal presubiculum. Furthermore, the lateral and dorsal quadrants project to the pyramidal cell layer of the temporal subiculum, whereas the ventral and medial quadrants project more septally. At the caudal third level of the AV, the dorsolateral part projects to layers I and III of the presubiculum with a patchy pattern and to the pyramidal cell layer of the septal subiculum. The anterodorsal nucleus projects mainly to deep layers of the presubiculum, parasubiculum, and entorhinal area. The results show that each subdivision of the anterior thalamic nuclei projects to a distinct field in the retrohippocampal region. This suggests that each of these projections may have a distinct modulatory influence upon the activity of retrohippocampal neurons that play important roles in limbic functioning such as memory and learning processes.