Single intralaminar thalamic neurons project to cerebral cortex, striatum and nucleus reticularis thalami. A retrograde anatomical tracing study in the rat

The hypothesis of triple axonal branching to the cortex, striatum and nucleus reticularis thalami (RT) of the forebrain projecting thalamic intralaminar neurons (TIN) was studied by retrograde axonal transport of horseradish peroxidase (HRP), iron-dextran complex and [3H]wheat germ agglutinin [3H]WGA. The best combination of tracers for this purpose was demonstrated to be: HRP-pellet implantation in the rostral cortex, iron-dextran injections into the striatum and [3H]WGA injections into the rostral RT. Prussian blue labeled neurons were observed in the ipsilateral TIN, substantia nigra, mediodorsal nucleus, medial part of the ventral anterior-ventral lateral complex, anterior medial and anterior ventral nuclei. HRP labeled neurons were observed in ipsilateral ventral nuclei, mediodorsal nucleus and the TIN. Radiolabeled neurons were located only in the TIN. HRP-Prussian blue labeled neurons (cortex-striatum branched neurons) were scattered in the TIN. Prussian blue radiolabeled neurons (striatum-RT branched neurons) could be observed in the TIN, as well as a few HRP radiolabeled neurons (cortex-RT branched neurons). Triply labeled neurons were scattered throughout the TIN until the rostral part of the centre median nucleus. These results demonstrate the existence of triple axonal branching on TIN efferent axons directed to the cerebral cortex, striatum and RT. The RT directed branch provides an anatomical basis to describe an intrathalamic regulatory loop well suited to control ascending messages arising from the TIN.