TOPOGRAPHIC ORGANIZATION OF PROJECTIONS FROM THE THALAMIC RETICULAR NUCLEUS TO THE ANTERIOR THALAMIC NUCLEI IN THE RAT

We have investigated connections between the thalamic reticular nucleu s (TRN) and the anterior thalamic nuclei (ATN) in the rat, following i njections of horseradish peroxidase (HRP) into subnuclei of the ATN an d different regions of the rostral TRN. Three nonoverlapping groups of neurons in the dorsal part of the ipsilateral rostral TRN project to, and receive reciprocal projections from, specific subnuclei of the AT N. A vertical sheet of neurons in the most dorsal part of the rostral TRN projects to the dorsal half of the posterior subdivision of the an teroventral thalamic nucleus (AVp), the dorsal region of the medial su bdivision of the anteroventral thalamic nucleus (AVm), and the dorsola teral part of the rostral anterodorsal thalamic nucleus (AD). Immediat ely ventral to this part of TRN, but still within its dorsal portion, are a lateral cluster of neurons and a medially located vertical sheet of neurons. The lateral cluster projects to the ventral part of AVp a nd to the dorsomedial part of rostral AD. The medial sheet projects to the ventral part of AVm, the ventral part of rostral AD, and to the c audal portions of both AV and AD. There appears to be no input to the anteromedial thalamic nucleus (AM) from the TRN. These findings shed n ew light on the anatomy of the rostral TRN, the ATN, and the connectio ns between the two, and are relevant to emerging hypotheses about the functional organization of the TRN and reticulo-thalamic projections.