Thalamic innervation of striatal and subthalamic neurons projecting to the rat entopeduncular nucleus.

The present study analyses the anatomical arrangement of the projections linking the Wistar rat parafascicular thalamic nucleus (PF) and basal ganglia structures, such as the striatum and the subthalamic nucleus (STN), by using neuroanatomical tract-tracing techniques. Both the thalamostriatal and the striato-entopeduncular projections were topographically organized, and several areas of overlap between identified circuits were noticed, sustaining the existence of up to three separated channels within the Nauta-Mehler loop. Thalamic afferents arising from dorsolateral PF territories are in register with striatofugal neurons located in dorsolateral striatal areas, which in turn project to dorsolateral regions of the entopeduncular nucleus (ENT). Medial ENT regions are innervated by striatal neurons located within medial striatal territories, these neurons being the target for thalamic afferents coming from medial PF areas. Finally, afferents from neurons located in ventrolateral PF areas approached striatal neurons in ventral and lateral striatal territories, which in turn project towards ventral and lateral ENT regions. Efferent STN neurons projecting to ENT were found to be the apparent postsynaptic target for thalamo-subthalamic axons. The thalamo-subthalamic projection was also topographically organized. Medial, central and lateral STN territories are innervated by thalamic neurons located within medial, ventrolateral and dorsolateral PF areas, respectively. Thus, each individual PF subregion projects in a segregated fashion to specific parts of the striato-entopeduncular and subthalamo-entopeduncular systems. These circuits enabled the caudal intralaminar nuclei to modulate basal ganglia output.