Cortical hierarchy governs rat claustrocortical circuit organization

Abstract

The claustrum is a telencephalic gray matter structure with various proposed functions, including sensory integration and attentional allocation. Underlying these concepts is the reciprocal connectivity of the claustrum with most, if not all, areas of the cortex. What remains to be elucidated to inform functional hypotheses further is whether a pattern exists in the strength of connectivity between a given cortical area and the claustrum. To this end, we performed a series of retrograde neuronal tract tracer injections into rat cortical areas along the cortical processing hierarchy, from primary sensory and motor to frontal cortices. We observed that the number of claustrocortical projections increased as a function of processing hierarchy; claustrum neurons projecting to primary sensory cortices were scant and restricted in distribution across the claustrum, whereas neurons projecting to the cingulate cortex were densely packed and more evenly distributed throughout the claustrum. This connectivity pattern suggests that the claustrum may preferentially subserve executive functions orchestrated by the cingulate cortex. J. Comp. Neurol. 525:1347-1362, 2017. © 2016 Wiley Periodicals, Inc.

Keywords: AB_10000344; AB_10000345; AB_10013220; AB_2313584; AB_2314408; AB_2314412; AB_2337244; AB_2340428; AB_2340433; AB_2340602; AB_2340806; AB_2341099; AB_477329; RGD_737903; anterograde; cingulate cortex; claustrum; connectivity; nif-0000-00110; retrograde.

Figure 1

Retrograde tract tracer deposits into motor and primary sensory cortices results in a restricted pattern of labeling within the claustrum. Black shading indicates tracer deposit core; dark gray shading indicates the penumbra of less dense labeling surrounding the core of the tracer deposit; light gray shading indicates the location of white matter structures. Dark gray in the claustrum reconstructions indicates white matter of the external capsule; light gray indicates the body of the claustrum as defined by parvalbumin (PV) immunoreactivity; black dots represent retrogradely labeled cell bodies. Claustral reconstructions proceed anterior to posterior, from left to right, as represented diagrammatically in the top row of A, with black boxes indicating approximate location of claustrum reconstructions. Photomicrographs of representative PV immunoreactivity, defining the boundaries of the claustrum, are given in the bottom row of A. Reconstruction of tracer deposit site, approximate anterior/posterior level of tracer deposit, and resultant labeling in and around the claustrum is shown for primary motor cortex (M1; B), posterior M1/secondary motor cortex (M2; C), primary visual cortex (V1; D), secondary visual cortex (V2, E), forelimb region of the primary somatosensory cortex (S1; F), and primary auditory cortex (A1; G). A/P, anterior/posterior; EC, external capsule; CL, claustrum; CTX, cortex. Scale bars=200 μm.

Figure 2

Retrograde tract tracer deposits into lateral parietal association cortex and medial prefrontal regions result in labeling restricted to dorsal and ventral portions of the claustrum, respectively. Reconstruction of retrograde labeling in the claustrum and surrounding insular cortex following deposits into lateral parietal association cortex (A), prelimbic part of the medial prefrontal cortex (B), and infralimbic cortex of the medial prefrontal cortex (C). A/P, anterior/posterior.

Figure 3

Retrograde tract tracer deposits into different aspects of the cingulate cortex reveal a much wider pattern of labeling in the claustrum than in other sensory, motor, and association cortices. Reconstruction of claustral and insular cortical labeling following deposit into the pregenual anterior cingulate cortex (ACC; A), a more posterior level of pregenual ACC (B), middle cingulate cortex (C), posterior cingulate cortex (D), retrosplenial dysgranular cingulate cortex (E), and a more posterior aspect of retrosplenial dysgranular cortex (F). A/P, anterior/posterior.

Figure 4

Photomicrographs illustrating minimal double labeling in the claustrum after dual retrograde tract tracer deposits into posterior cingulate and ACC. Tracer deposits of cholera toxin B subunit conjugated to AlexaFluor-488 (CTb-488) into the right ACC (A) and CTb conjugated to AlexaFluor-555 into the right posterior cingulate cortex with invasion into the retrosplenial dysgranular cortex (B). C: CTb-555 (red), CTb-488 (green) and double-labeled CTb-555+CTb488 (yellow) neurons from the right claustrum. D: Boxed area from C showing an example of a double-labeled cell (arrowhead). A/P, anterior/posterior. See Supporting Figure 1 for magenta-green copy. Scale bars=1mm in A,B; 150 μm in C; 50μm in D.

Figure 5

Photomicrographs illustrating minimal double labeling in the claustrum after dual retrograde tract tracer deposits into ACC and M1. Tracer deposits of cholera toxin B subunit conjugated to AlexaFluor-488 (CTb-488) into the right ACC (A) and CTb conjugated to AlexaFluor-555 into M1 (B). C: CTb-555 (red), CTb-488 (green), and double-labeled CTb-555+CTb488 (yellow) neurons from the right claustrum. D: Boxed area from C showing examples of double-labeled cells (arrowheads). A/P, anterior/posterior. See Supporting Figure 2 for magenta-green copy. Scale bars=1mm in A,B; 150 μm in C; 50μm in D.

Figure 6

Dual retrograde tract tracer deposits of the cortex reveal few double-labeled cells in the claustrum. A: Top: Reconstructions of a cholera toxin B subunit (CTb) deposit into the ACC and FluoroGold (FG) deposit of M1. Bottom: Retrograde labeling in the area of the claustrum. Few double CTb-+FG-labeled cells were seen (stars). B: Top: Reconstructions of a CTb deposit into the ACC and an FG deposit into a region that straddles the posterior cingulate and retrosplenial dysgranular cortex. Bottom: Again, few double-labeled cells were seen. A/P, anterior/posterior.

Figure 7

Anterograde/retrograde tract tracers deposited contralaterally into the cingulate cortex reveals a unilateral convergence of fibers and cell bodies in the claustrum. Injection sites for the anterograde tracer deposit of BDA into the left cingulate (A) and retrograde tracer deposit of CTb-488 into the right cingulate (B) slightly caudal to the genu of the corpus callosum. Photomicrographs illustrating a section of the left (C) and right (D) claustrum lying roughly equidistant between the genu of the corpus callosum and the decussation of the anterior commissure. BDA-ir (red) fibers and CTb-488-labeled (green) cell bodies were seen predominantly in the right claustrum (D), indicating projections from the cingulate to the claustrum originate primarily from the contralateral hemisphere, whereas projections from the claustrum to the cingulate originate primarily from the ipsilateral hemisphere. A/P, anterior/posterior. See Supporting Figure 3 for magenta-green copy. Scale bars=1mm in A,B; 150 μm in C,D.

Figure 8

Cortical hierarchy determines the distribution of claustrum projection neurons. Claustral projections to sensory and motor cortices are sparse and exhibit a restricted distribution within the claustrum, whereas projections to prelimbic cortex (area 32) are numerous and exhibit a more widespread distribution. Claustral projections to all aspects of cingulate cortices are numerous and distributed throughout the extent of the claustrum.