Thalamo-insular cortex connections in the rat and human.

The insular cortex (ICx) has a role in large a variety of functions. Thalamus plays an important role in modulating cortical functions. The present study aims to show thalamic-ICx connections using the fluoro-gold (FG) tracing method in rats and diffusion tensoring-based tractography (DTI) in humans. Wistar albino rats were pressure injected with the FG tracer into the anterior and posterior ICx. The DTI data were obtained from the Human Connectome Project database. Our findings showed that the thalamic-ICx connections were strictly ipsilateral in the rat, however, bilateral connections were present in humans. The anterior ICx was connected to the paraventricular, centromedial, paracentral, centrolateral, ventral posteromedial, and medial geniculate thalamic nuclei. The posterior ICx was connected to the centromedian, parafasicular, renuence, lateral, posterior, ventral posteromedial, and medial geniculate thalamic nuclei. The DTI in humans corresponded with the results of the experimental study on rats. The results of the current study may provide an understanding of how thalamic nuclei may contribute to higher-order ICx functions. The ipsilateral connections in the rat and bilateral in humans may provide insights into anatomical evolution and functional differences of the ICx circuit in humans and rats. Further, stimulation of the thalamus can be a potential target for treating or modulating ICx functions such as anxiety, depression, and certain chronic pain conditions.