Efferent Connections of the Anteromedial Nucleus of the Thalamus of the Rat

The projections from the anteromedial nucleus of the thalamus (AM) were investigated using anterograde and retrograde tracing techniques. AM projects to nearly the entire rostrocaudal extent of limbic cortex and to visual cortex. Anteriorly, AM projects to medial orbital, frontal polar, precentral agranular, and infraradiata cortices. Posteriorly, AM projects to retrosplenial granular, entorhinal, perirhinal and presubicular cortices, and to the subiculum. Further, AM projects to visual cortical area 18b, and to the lateral and basolateral nuclei of the amygdala. AM projections are topographically organized, i.e., projections to different cortical areas arise from distinct parts of AM. The neurons projecting to rostral infraradiata cortex (IRalpha) are more caudally located in AM than the neurons projecting to caudal infraradiata cortex (IRbeta). The neuronal cell bodies that project to the terminal field in area 18b are located primarily in ventral and lateral parts of AM, whereas neurons projecting to perirhinal cortex and amygdala are more medially located in AM. Injections into the most caudal, medial part of AM (i.e., the interanteromedial [IAM] nucleus) label terminals in the rostral precentral agranular, caudal IRbeta, and caudal perirhinal cortices. Whereas most AM axons terminate in layers I and V-VI, exceptions to this pattern include area 18b (axons and terminals in layers I and IV-V), the retrosplenial granular cortex (axons and terminals in layers I and V), and the presubicular, perirhinal, and entorhinal cortices (axons and terminals predominantly in layer V). Together, these findings suggest that AM influences a widespread area of limbic cortex.