Separate cortical and hippocampal cell populations target the rat nucleus reuniens and mammillary bodies

Abstract

Nucleus reuniens receives dense projections from both the hippocampus and the frontal cortices. Reflecting these connections, this nucleus is thought to enable executive functions, including those involving spatial learning. The mammillary bodies, which also support spatial learning, again receive dense hippocampal inputs, as well as lighter projections from medial frontal areas. The present study, therefore, compared the sources of these inputs to nucleus reuniens and the mammillary bodies. Retrograde tracer injections in rats showed how these two diencephalic sites receive projections from separate cell populations, often from adjacent layers in the same cortical areas. In the subiculum, which projects strongly to both sites, the mammillary body inputs originate from a homogenous pyramidal cell population in more superficial levels, while the cells that target nucleus reuniens most often originate from cells positioned at a deeper level. In these deeper levels, a more morphologically diverse set of subiculum cells contributes to the thalamic projection, especially at septal levels. While both diencephalic sites also receive medial frontal inputs, those to nucleus reuniens are especially dense. The densest inputs to the mammillary bodies appear to arise from the dorsal peduncular cortex, where the cells are mostly separate from deeper neurons that project to nucleus reuniens. Again, in those other cortical regions that innervate both nucleus reuniens and the mammillary bodies, there was no evidence of collateral projections. The findings support the notion that these diencephalic nuclei represent components of distinct, but complementary, systems that support different aspects of cognition.

Prefrontal Pathways Provide Top-Down Control of Memory for Sequences of Events

“…Only a few cells were found in anterior cingulate cortex. mPFC→PER projecting cells were observed throughout layers II/III and V, with the highest density in layer II/III (Figure 1Fii; dPL, t (3) = 3.654, p = 0.035; IL, t (3) = 3.849, p = 0.031), consistent with previous literature (Hwang et al, 2018; Mathiasen et al, 2019). We did not find labeled cells in layer I.…”

The Anatomical Boundary of the Rat Claustrum

“…A total of 43 male Lister Hooded rats (Envigo, United Kingdom) with pre-procedural weights of between 230–350 g were used in the study. In 6 animals, retrograde tracer injections targeted nucleus reuniens (RE) and/or the rhomboid (Rh) nucleus of the midline thalamus and in one of these cases, a further retrograde tracer injection targeted the mediodorsal (MD), centromedial (CM) and paraventricular thalamic nuclei (some of these cases used in Mathiasen et al, 2019; Table 1). In 2 of the animals with RE/Rh injections the injection site also included a portion of the centromedial thalamic nucleus.…”

The anatomical boundary of the rat claustrum

“…(RE) and/or the rhomboid (Rh) nucleus of the midline thalamus and in one of these cases, a further 13 retrograde tracer injection targeted the mediodorsal (MD), centromedial (CM) and paraventricular 14 thalamic nuclei (some of these cases used in Mathiasen et al, 2019; Table 1). In 2 of the animals 15 with RE/Rh injections the injection site also included a portion of the centromedial thalamic nucleus.…”

“…Based on their recent guidance and clarification on the issue of how to consider the claustrum in 13 relation to the endopiriform nuclei, we follow the classification of Smith and colleagues (2018) and 14 consider a claustrum-endopiriform complex comprising the claustrum proper and the dorsal 15 endopiriform nucleus (DEn), with the claustrum proper comprising dorsal (dCLA) and ventral (vCLA) 16 subdivisions. In addition to atlas-based (Paxinos and Watson, 2005) definitions of the insular/orbital 17 region, Nissl stained sections were used to delineate the border between the insular and orbital 18…”

“…Expression of all three markers delineated a nuclear boundary that extended considerably (~500 µm) 13 beyond the anterior horn of the neostriatum. Cortical retrograde and anterograde neuronal tracer 14 injections, respectively, revealed distributions of cortically-projecting claustral neurons and cortical 15 efferent inputs to the claustrum that overlapped with the gene marker-derived claustrum boundary. 16 Finally, retrograde tracer injections centred in nucleus reuniens, whilst including the rhomboid, 17 mediodorsal and centromedial nuclei, revealed that insular cortico-thalamic projections 18 encapsulated a claustral area, with strongly diminished cell label, that corresponded to the 19 claustrum.…”