DIRECT PROJECTIONS FROM THE ANTERIOR THALAMIC NUCLEI TO THE RETROHIPPOCAMPAL REGION IN THE RAT

The present study examined the areal and laminar distribution of direc t projections from the anterior thalamic nuclei to the retrohippocampa l region in the rat, with anterograde transport of Phaseolus vulgaris- leucoagglutinin. The anteromedial nucleus (AM) projects to the tempora l subiculum, medial entorhinal area, perirhinal area, and caudomedial part of the lateral entorhinal area. The interanteromedial nucleus (IA M) projects to the perirhinal area and the caudolateral part of the la teral 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 LAM to these retrohippocampal and neocorti cal regions terminate mainly in deep layers. The anteroventral nucleus (AV) projects. to the subicular complex with a complex topographic or ganization. The most rostral part of the AV projects to layers I and I II of the ventral presubiculum, the pyramidal cell layer of the tempor al subiculum, and deep layers of the parasubiculum and medial entorhin al area. At the midrostrocaudal level of the AV, the lateral and the d orsal quadrants of the AV project, respectively, to layers I and III a nd to layers I and IV-VI of the ventral presubiculum, whereas the vent ral and the medial quadrants project, respectively, to layers I and II I and to layers I and IV-VI of the dorsal presubiculum. Furthermore, t he lateral and dorsal quadrants project to the pyramidal cell layer of the temporal subiculum, whereas the ventral and medial quadrants proj ect more septally. At the caudal third level of the AV, the dorsolater al 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 presubiculu m, parasubiculum, and entorhinal area. The results show that each subd ivision of the anterior thalamic nuclei projects to a distinct field i n the retrohippocampal region. This suggests that each of these projec tions may have a distinct modulatory influence upon the activity of re trohippocampal neurons that play important roles in limbic functioning such as memory and learning processes. (C) 1993 Wiley-Liss, Inc.