The organization of projections from the anterior thalamic nuclei to the cingulate cortex was analyzed in the rat by the anterograde transport of Phaseolus vulgaris-leucoagglutinin. The rostral part of the anteromedial nucleus projects to layers I, V and VI of the anterior cingulate areas 1 and 2, layers I and III of the ventral orbital area, layers I, V and VI of area 29D of the retrosplenial area, and layers I and V of the caudal part of the retrosplenial granular and agranular areas. In contrast, the caudal part of the anteromedial nucleus projects to layer V of the frontal area 2, and layers I and V of the rostral part of the retrosplenial granular and agranular areas. The interanteromedial nucleus projects to layers I, III and V of the frontal area 2, layer V of the agranular insular area, and layers I, V and VI of area 29D. The anteroventral nucleus projects to layers I and IV of the retrosplenial granular area, whereas the anterodorsal nucleus projects to layers I, III and IV of the same area. Projections from the anteroventral and anterodorsal nuclei were, furthermore, organized such that their ventral parts project to the rostral part of the retrosplenial granular area, whereas their dorsal parts project to the more caudal part. The results suggest that the anterior thalamic nuclei project to more widespread areas and laminae of the cingulate cortex than was previously assumed. The projections are organized such that the anteromedial and interanteromedial nuclei project to layer I and the deep layers of the anterior cingulate and retrosplenial cortex, whereas the anteroventral and anterodorsal nuclei project to the superficial layers of the retrosplenial cortex. These thalamocortical projections may play important roles in behavioral learning such as discriminative avoidance behavior.

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.

Subcortical projections to the anterior thalamic nuclei were studied in the rat, with special reference to projections from the mammillary nuclei, by retrograde and anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase. The medial mammillary nucleus (MM) projects predominantly ipsilaterally to the entire anterior thalamic nuclei, whereas the lateral mammillary nucleus projects bilaterally to the anterodorsal nucleus (AD) of the anterior thalamic nuclei. A topographic relationship was recognized between the MM and the anterior thalamic nuclei. The dorsal region of the pars mediana of the MM projects to the interanteromedial nucleus (IAM), whereas the ventral region projects to the rostral part of the anteromedial nucleus (AM). The dorsal and the ventral regions of the pars medialis project to the dorsomedial part of the AM at its caudal and rostral levels, respectively. The dorsomedial region of the pars lateralis projects to the ventral AM. The ventrolateral region of the pars lateralis projects to the ventral part of the anteroventral nucleus (AV) in such a manner that rostral cells project rostrally and caudal cells project caudally. The pars basalis projects predominantly ipsilaterally to the dorsolateral AV and bilaterally to the AD. The rostrolateral region of the pars posterior projects to the lateral AV, whereas the medial and the caudal regions of the pars posterior project to the dorsomedial AV. The rostrodorsal part of the nucleus reticularis thalami was found to project to the anterior thalamic nuclei; cells located rostrally in this part project to the IAM and AM, whereas cells located caudodorsally project to the AV and AD. The laterodorsal tegmental nucleus projects predominantly ipsilaterally to the AV, especially to its dorsolateral part. The present study demonstrates that subdivisions of the subcortical structures are connected to the subnuclei of the anterior thalamic nuclei, with a clear-cut topography arranged in the dorsoventral and the rostrocaudal dimensions.

The cells of origin and projection fields of the descending afferents to the mammillary nuclei were studied in the rat with retrograde and anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase. The subiculum projects bilaterally to the entire medial mammillary nucleus (MM) in a topographic fashion along the two axes: 1) the proximal part of the subiculum along the presubiculo-CA1 axis projects to the caudal and lateral regions of the MM whereas the more distal part of the subiculum projects to the medial region; 2) the septal part of the subiculum projects to the caudodorsal region of the MM whereas the more temporal part projects progressively to the more rostroventral regions. The ventral subiculum also projects ipsilaterally to the ventral and lateral margin of the lateral mammillary nucleus (LM). The presubiculum projects bilaterally to the dorsolateral region of the pars posterior of the MM and ipsilaterally to the LM. The infra-limbic cortex projects bilaterally to the rostrodorsal region of the MM, whereas the retrosplenial cortex (areas 29a and 29b) projects bilaterally to the medial region at the midrostrocaudal and middorsoventral levels of the MM. The nucleus of the diagonal band projects bilaterally to the caudomedial region of the MM, whereas the lateral septal nucleus projects bilaterally to the pars mediana and the mammillary fiber capsule. A part of the anterior hypothalamic area ventromedial to the fornix projects predominantly ipsilaterally to the rostroventral part of the MM, whereas other basal forebrain regions such as the bed nucleus of the stria terminalis, the medial preoptic and anterior hypothalamic areas, and the area of the tuber cinereum send fibers predominantly ipsilaterally to the mammillary fiber capsule. The results reveal a complex organization of the descending projections to the mammillary nuclei, which may reflect the complex functions of these nuclei within the limbic circuitry.

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