Dorsal and ventral parts of thalamic nucleus submedius project to different areas of rat orbitofrontal cortex: A single neuron-tracing study using virus vectors.

The rodent orbitofrontal cortex is involved in a variety of cognitive and behavioral functions that require thalamic input to be successfully expressed. Although the thalamic nucleus submedius (Sm) is a major source of afferents to the orbitofrontal cortex, thalamocortical projection from the Sm has not been fully elucidated. In the present study, we first divided the rat Sm into dorsal and ventral parts according to the distribution of vesicular glutamate transporter 2-immunoreactive varicosities, which were somatosensory afferents from the brain stem. Subsequently we investigated dendritic and axonal arborizations of individual dorsal and ventral Sm neurons by visualizing the processes with Sindbis virus vectors expressing membrane-targeted fluorescent proteins. The number of dendritic processes of ventral Sm neurons was greater than that of dorsal Sm neurons. In the cerebral cortex, all the reconstructed Sm neurons sent axons primarily to layers 2-5. Interestingly, dorsal Sm neurons formed a single axon arbor exclusively within the ventrolateral orbital area, whereas ventral Sm neurons made two axon arbors in the lateral orbital and ventral orbital areas simultaneously. The spread of each axon arbor was 500-1000 µm in diameter in the direction tangential to the cortical surface. These results indicate that the dorsal and ventral Sm comprise two distinct thalamocortical pathways. The dorsal Sm pathway relay somatosensory information to the ventrolateral orbital area and may be involved in emotional and aversive aspects of nociceptive information processing, whereas the ventral Sm pathway seems to co-activate distant orbitofrontal cortical areas, and may link their functions under certain circumstances.