A multifaceted architectural framework of the mouse claustrum complex

Abstract

Accurate anatomical characterizations are necessary to investigate neural circuitry on a fine scale, but for the rodent claustrum complex (CC) this has yet to be fully accomplished. The CC is generally considered to comprise two major subdivisions, the claustrum (CL) and the dorsal endopiriform nucleus (DEn), but regional boundaries to these areas are highly debated. To address this, we conducted a multifaceted analysis of fiber- and cyto-architecture, genetic marker expression, and connectivity using mice of both sexes, to create a comprehensive guide for identifying and delineating borders to the CC. We identified four distinct subregions within the CC, subdividing both the CL and the DEn into two. Additionally, we conducted brain-wide tracing of inputs to the entire CC using a transgenic mouse line. Immunohistochemical staining against myelin basic protein (MBP), parvalbumin (PV), and calbindin (CB) revealed intricate fiber-architectural patterns enabling precise delineations of the CC and its subregions. Myelinated fibers were abundant in dorsal parts of the CL but absent in ventral parts, while parvalbumin labelled fibers occupied the entire CL. Calbindin staining revealed a central gap within the CL, which was also visible at levels anterior to the striatum. Furthermore, cells in the CL projecting to the retrosplenial-cortex were located within the myelin sparse area. By combining our own experimental data with digitally available datasets of gene expression and input connectivity, we could demonstrate that the proposed delineation scheme allows anchoring of datasets from different origins to a common reference framework.

Single neurons and networks in the claustrum integrate input from widespread cortical sources

“…The RSP is uniquely positioned for use in this technique as it does not receive inputs from structures around the CLA but receives specifically dense innervation from the CLA itself. We found that CLA RSP neurons span the rostrocaudal axis of the CLA and align with previously identified markers of the CLA “core” (34, 38, 39, 53). Retrograde tracing from RSP offered a simple method for accurately assessing the anatomical and physiological features CLA neurons in later experiments.…”

“…S1). Further, this strategy consistently demonstrated that retrosplenial-projecting CLA (CLA RSP ) neurons could be found along the whole CLA length, as defined by other sources (33)(34)(35)(36). Moreover, the level of overlap between CLA neurons projecting to different regions of RSP varied depending on the injection site (Fig.…”

“…A detailed description of rabies tracing procedures and methods can be found in (34). Briefly, EnvA-pseudotyped, G-protein deleted CVS-N2c recombinant rabies virus expressing tdTomato (RABV-tdTomato) was used in these experiments.…”

“…We next compared known markers of the CLA against the labeling of CLA RSP neurons using immunohistochemistry to obtain a better understanding of the intraclaustral localization of CLA RSP neurons (20,34,(37)(38)(39) (n = 3 mice; Fig. 1f, Supp.…”

Temporal origin of mouse claustrum and development of its cortical projections

“…While the latter is not easily identified with classical histological stains in the mouse brain, it has been reported that the claustrum is surrounded by densely myelinated fibers ( Wang et al. 2017 , 2022 ; Grimstvedt et al. 2022 ).…”