Afferent Projections to the Paratenial Nucleus of the Dorsal Midline Thalamus

The dorsal midline thalamus (DMT) is composed of the paraventricular (PV) and paratenial (PT) nuclei. While the anatomical and functional properties of PV are well-established, PT has remarkably received very little attention—even though the efferent projections of PV and PT are very similar. Using a combination of retrograde tracing and immunohistochemistry, we examined the anatomical inputs to PT and compared them with those to the anterior and posterior PV and to the anterodorsal nucleus of the thalamus. In addition, we examined orexinergic and serotonergic afferents to the PT, comparing them with those to other thalamic nuclei. We found that PT and PV receive input from a common set of structures, including the orbitomedial prefrontal cortex, nuclei of the diagonal band, septum, subiculum of the hippocampus, bed nucleus of the stria terminalis, hypothalamus, reticular nucleus of the thalamus, dorsal raphe nucleus, and periaqueductal gray. However, the pattern and density of these various afferents to PT and PV significantly differed. For instance, PT received much stronger inputs from the orbitofrontal cortex, while PV received stronger projections from the subiculum of the hippocampus and more widespread input from the hypothalamus and the brainstem. By comparison, afferents to AD differed from PT (and PV), as AD received substantial input from the retrosplenial and anterior cingulate cortices, and uniquely from the lateral mammillary nucleus. Further, orexinergic (ORX) and serotonergic (5-HT) fibers distributed at best modestly to PT, which contrasted with quite dense ORX and 5-HT innervation of PV. The present findings, essentially representing the first comprehensive examination of afferent projections to PT, show that the inputs to PT mainly arise from limbic forebrain structures—with pronounced projections from the orbitofrontal cortex, nuclei of the diagonal band, and the reticular nucleus of the thalamus. The functional properties of PT partially overlap with those of PV, but as described herein PT also participates in unique affective, cognitive, and motivational behaviors.