Efferent connections of the anteromedial nucleus of the thalamus of the rat

The projections from the anteromedial nucleus of the thalamus (AM) were inv estigated using anterograde and retrograde tracing techniques. AM projects to nearly the entire rostrocaudal extent of limbic cortex and to visual cor tex. Anteriorly, AM projects to medial orbital, frontal polar, precentral a granular, and infraradiata cortices. Posteriorly, AM projects to retrosplen ial granular, entorhinal, perirhinal and presubicular cortices, and to the subiculum. Further, AM projects to visual cortical area 18b, and to the lat eral and basolateral nuclei of the amygdala. AM projections are topographic ally organized, i.e., projections to different cortical areas arise from di stinct parts of AM. The neurons projecting to rostral infraradiata cortex ( IR alpha) are more caudally located in AM than the neurons projecting to ca udal infraradiata cortex (IR beta). The neuronal cell bodies that project t o the terminal field in area 18b are located primarily in ventral and later al parts of AM, whereas neurons projecting to perirhinal cortex and amygdal a are more medially located in AM. Injections into the most caudal, medial part of AM (i.e., the interanteromedial [IAM] nucleus) label terminals in t he rostral precentral agranular, caudal IR beta, and caudal perirhinal cort ices. Whereas most AM axons terminate in layers I and V-VI, exceptions to t his pattern include area 18b (axons and terminals in layers I and IV-V), th e retrosplenial granular cortex (axons and terminals in layers I and V), an d the presubicular, perirhinal, and entorhinal cortices (axons;md terminals predominantly in layer V). Together, these findings suggest that AM influe nces a widespread area of limbic cortex. (C) 1999 Elsevier Science B.V. All rights reserved.