Organization of cortical afferents to the rostral, limbic sector of the rat thalamic reticular nucleus.

The organization of limbic cortical afferents to the thalamic reticular nucleus (TRN) is described. Wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP), biocytin, neurobiotin, or fluorescent dextrans was delivered into the rat cingulate, retrosplenial, and, for comparison, somatosensory cortices. In other species a slab-like arrangement of cortical terminals has been described for sensory TRN sectors. Here this is seen in the rat somatosensory sector. Terminals from limbic cortices did not cluster into slabs but were found to fill the entire thickness of distinct rostral TRN regions. The cingulate and retrosplenial recipient TRN regions overlap, as do the projections from these cortical areas to anterior thalamic nuclei. Retrosplenial fibres contacted the dorsal and rostral TRN, which is known to be connected to the retrosplenial-recipient anteroventral, anterodorsal, and laterodorsal thalamic nuclei. Cingulate terminals occupied more ventral regions of the rostral TRN. This area is connected to thalamic nuclei also innervated by the cingulate cortex: the mediodorsal and anteromedial nuclei. A loose, but clear, topography could be defined for the cingulate-reticular pathway: rostrocaudal and mediolateral directions in the cortex are represented by ventrodorsal and rostrocaudal directions in the TRN, respectively. This organization of limbic corticoreticular pathway corresponds to the arrangement of limbic corticothalamic connections. The ultrastructure of the limbic cortical axon terminals was similar to that of the cortical boutons (D-type) described previously. The labelled terminals formed asymmetrical synapses onto dendritic profiles of reticular neurons. These findings, together with data in the literature, show significant morphological and connectional differences within the TRN that imply functional heterogeneities.