ORGANIZATION OF CONNECTIONS BETWEEN THE THALAMIC RETICULAR AND THE ANTERIOR THALAMIC NUCLEI IN THE RAT

The thalamic reticular nucleus (TRN) receives topographically organize d input from specific sensory nuclei such as the lateral geniculate nu cleus. The present study shows this in the rat. However, the pattern o f thalamic connections to the limbic reticular sector is unknown. Inje cting biocytin into the ventral parts of anteroventral and anteromedia l nuclei labeled neurons and axons in the rostral TRN. Filled axon col laterals and their terminals occupied a rectangular sheet in a plane c lose to the horizontal, and were confined to the inner zone (the media l portion) of the limbic TRN. Retrogradely filled cells were in the mi ddle of the rostral pole in the same horizontal plane, receiving synap ses from surrounding labeled boutons. In electron micrographs, thalami c terminals were found to contain round, densely packed synaptic vesic les and formed asymmetrical synapses onto reticular somata and dendrit ic profiles. Displacing the injection site along the dorso-ventral and rostro-caudal axis in the anterior nuclei produced corresponding shif ts of antero- and retrograde labeling within the inner reticular zone. Projections from the dorsal portions of the anterior nuclei did not f ollow this pattern. Axons from the anterodorsal nucleus occupied the r ostralmost tip of both inner and outer zones of the dorsal limbic sect or. In accordance with earlier reports, the limbic sector was found to represent several dorsal thalamic nuclei parallel to each other medio -laterally. A topography is described for the limbic reticulo-thalamic connections, suggesting that the rostral TRN is able to influence cir cumscribed areas of the limbic thalamus. (C) 1995 Wiley-Liss, Inc.