Cortico-amygdala-striatal circuits are organized as hierarchical subsystems through the primate amygdala

Abstract

The prefrontal and insula cortex, amygdala, and striatum are key regions for emotional processing, yet the amygdala's role as an interface between the cortex and striatum is not well understood. In the nonhuman primate (Macaque fascicularis), we analyzed a collection of bidirectional tracer injections in the amygdala to understand how cortical inputs and striatal outputs are organized to form integrated cortico-amygdala-striatal circuits. Overall, diverse prefrontal and insular cortical regions projected to the basal and accessory basal nuclei of the amygdala. In turn, these amygdala regions projected to widespread striatal domains extending well beyond the classic ventral striatum. Analysis of the cases in aggregate revealed a topographic colocalization of cortical inputs and striatal outputs in the amygdala that was additionally distinguished by cortical cytoarchitecture. Specifically, the degree of cortical laminar differentiation of the cortical inputs predicted amygdalostriatal targets, and distinguished three main cortico-amygdala-striatal circuits. These three circuits were categorized as "primitive," "intermediate," and "developed," respectively, to emphasize the relative phylogenetic and ontogenetic features of the cortical inputs. Within the amygdala, these circuits appeared arranged in a pyramidal-like fashion, with the primitive circuit found in all examined subregions, and subsequent circuits hierarchically layered in discrete amygdala subregions. This arrangement suggests a stepwise integration of the functions of these circuits across amygdala subregions, providing a potential mechanism through which internal emotional states are managed with external social and sensory information toward emotionally informed complex behaviors.

Figure 1.

Schematic of injection sites in the macaque amygdala. A, Coronal section of the macaque amygdala stained with AChE. Scale bar, 1 mm. Amygdala nuclei and subnuclei display differential AChE immunoreactivity. B, Schematic of injection sites in the accessory basal (green) and basal nuclei (yellow) at approximately the same level as A. C, Schematic of injection sites in the caudal amygdala. D, Coronal section of the caudal amygdala at approximately the same level as C stained with AChE. Scale bar, 1 mm. E, Photomicrograph of J15LY injection into the centromedial Bpc taken with dark-field microscopy. Scale bar, 0.5 mm. Arrow points to the center of the injection site. F, AChE-stained section adjacent to E. Scale bar, 0.5 mm. G, Photomicrograph of J20FS injection encompassing both ABmc and ABpc taken with dark-field microscopy. Scale bar, 0.5 mm. Arrow points to the center of the injection site. H, AChE-stained section adjacent to G. Scale bar, 0.5 mm. AHA, amygdalohippocampal area; Bi, basal nucleus, intermediate subdivision; CeL, central nucleus, lateral core; CeM, central nucleus, medial subdivision; H, hippocampus; L, lateral nucleus; M, medial nucleus; OT, optic tract; P, putamen; PAC, periamygdaloid cortex; V, ventricle.

Figure 2.

Cortical and striatal subregions. A, A 3D view of macaque brain showing mPFC, OFC, and rostral aspect of the insula on the caudal orbital surface. Subdivisions are those of Carmichael and Price, 1994. B, Insula within the Sylvian fissure. C, NeuN staining demonstrates the differing cytoarchitectonics of cortical subdivisions. Granular cortex has a well defined granular layer IV (area 10m, 20×). Scale bar, 50 μm. D, Dysgranular cortex has a less distinct layer IV (area 14r, 20×). Scale bar, 50 μm. E, Agranular cortex has no layer IV (area 25, 20×). Scale bar, 50 μm. Area 25 additionally has less differentiation of other layers. F, Coronal section of rostral striatum (stained with CaBP). Scale bar, 1 mm. The shell (s) of the ventral striatum is relatively devoid of CaBP-IR. G, Elements of the caudoventral striatum include caudoventral putamen (P), the Astr, and the IPAC (data not shown; stained with CaBP). Scale bar, 1 mm. The latter are also CaBP-poor regions. H, Coronal section of the genu of the caudate nucleus, C(g) (stained with CaBP). Scale bar, 1 mm. C, caudate nucleus; IC, internal capsule.

Figure 3.

Distribution of retrogradely labeled cells in the PFC and insula (left), and anterogradely labeled fibers in the striatum (right) resulting from an injection in the Bmc (J12FR). This injection site received a broad distribution of cortical inputs and striatal outputs. Retrogradely labeled cells were found in subdivisions of the mPFC, OFC, and agranular and dysgranular insula (rostral to caudal is displayed top to bottom). Anterogradely labeled fibers were distributed in the shell and core of the rostroventral striatum, as well as in the caudal ventral striatum, and caudal caudate nucleus body, tail, and genu. C, caudate nucleus; G, gustatory cortex; IC, internal capsule; OT, optic tract; P, putamen; s, shell; V, ventricle.

Figure 4.

Photos of retrogradely labeled cells and anterogradely labeled fibers. A, Coronal section of area 25 showing retrogradely labeled cells stained for LY tracer following an injection into the rostral ABmc (J8LY). Most retrogradely labeled cells were found in layer V-VI, with some cells found in layer II–III (20×). Scale bar, 50 μm. B, Coronal section showing anterogradely labeled fibers stained for LY tracer in the IPAC following an injection into the ventromedial Bpc (J20LY) (4×). Scale bar, 0.5 mm. See Figure 5 for accompanying schematic. C, Coronal section showing dense anterogradely labeled fibers stained for FR tracer in the genu of the caudate nucleus following an injection into the Bmc (J12FR) (4×). Scale bar, 0.5 mm. See Figure 3 for accompanying schematic.

Figure 5.

Distribution of retrogradely labeled cells in the insula cortex (left), and anterogradely labeled fibers (right) in the striatum resulting from an injection in the rostromedial Bpc (J20LY). This region of the amygdala received input from all subdivisions of the insula (rostral to caudal is displayed top to bottom). Except for a few retrogradely labeled cells in area 14c, no retrogradely labeled cells were found in any other PFC region. Insert photo shows cells labeled with LY tracer in Ial <20×, bright-field microscopy. Scale bar, 50 μm. Striatal output targets involved the rostromedial shell (s) and core, lateral IPAC, rostroventral caudate body, and caudoventral putamen. AC, anterior commissure; C, caudate nucleus, G, gustatory cortex; GPe, globus pallidus, external segment; GPi, globus pallidus, internal segment; IC, internal capsule; OT, optic tract; P, putamen; V, ventricle.

Figure 6.

Distribution of retrogradely labeled cells in the agranular insula and caudal mPFC (left), and anterogradely labeled fibers in the rostroventral striatum (right) resulting from an injection in the caudomedial Bpc (J15FS). Retrogradely labeled cells were limited to agranular insula and areas 25, 32, and 14c (rostral to caudal is displayed top to bottom). Anterogradely labeled fibers were limited to only the very dorsal section of the dorsal shell. This was also found in the neighboring injection (J14FS). Insert shows FS-labeled fibers in a coronal section of the dorsal shell (s) (10×, dark-field microscopy). Scale bar, 1 mm. C, caudate nucleus; G, gustatory cortex; IC, internal capsule; OT, optic tract; P, putamen; V, ventricle.

Figure 7.

Distribution of retrogradely labeled cells in the mPFC and insula (left), and anterogradely labeled fibers in the striatum (right) resulting from an injection in the central-medial Bpc (J15LY). Retrogradely labeled cells were found in agranular and dysgranular subdivisions of the mPFC and insula (rostral to caudal is displayed top to bottom). Anterogradely labeled fibers were found in the rostroventral shell (s), lateral IPAC and Astr, and caudoventral putamen. AC, anterior commissure; C, caudate nucleus; GP, globus pallidus; GPe, globus pallidus, external segment; GPi, globus pallidus, internal segment; IC, internal capsule; OT, optic tract; P, putamen.

Figure 8.

Composite of cortical inputs to the amygdala. Amygdala injections are color coded according to similarities in cortical labeling (see Results). A, Rostral granular mPFC and OFC regions projected only to the dorsal Bmc and ABmc. B, OFC subregions projected only to the Bmc and ABmc. More rostral aspects of mPFC areas 32 and 24 projected to the dorsal (purple) and centromedial (pink) amygdala. C, Subgenual mPFC areas 32 and 25 projected to all examined amygdala subregions except rostromedial Bpc. OFC sent projections only to Bmc and ABmc. D, mPFC area 25 projected to all examined amygdala subregions except rostromedial Bpc. Area 24 projected to Bmc, ABmc, and centromedial Bpc (pink). Agranular insula subregions Iai and Ial projected broadly to all examined amygdala subregions. E, Anterior insula projected broadly to all examined amygdala subregions, in particular to rostromedial Bpc (orange). F, Agranular insula within the Sylvian fissure also projected broadly, while dysgranular and granular insula had more restricted projections. See Table 1 for details.

Figure 9.

Composite of amygdala outputs to the striatum. Amygdala subdivisions are color coded according to similarity in striatal projections (see Results). A, The dorsomedial shell received amygdala inputs mostly from the caudomedial Bpc (blue). The ventral and ventrolateral subdivisions of the rostral shell, and core received amygdala inputs from all other amygdala subregions. B, Lateral IPAC, part of the caudal shell, received inputs from the dorsal Bmc and ABmc (purple), and ventromedial Bpc (orange and pink). At this level, the ventromedial caudate nucleus body also received input from the ventromedial (orange and pink) and dorsal (purple) amygdala. C, Lateral Astr, part of the caudal shell, received inputs from the Bmc and ABmc (purple) and the centromedial Bpc (pink). The caudoventral putamen received inputs from the Bmc, ABmc (purple), and centromedial (pink) and rostromedial (orange) Bpc. At this level, the dorsolateral caudate body received input from the Bmc and ABmc. D, The caudate tail (D) received input from the Bmc and ABmc. E, The caudate genu also received inputs only from Bmc and ABmc. AC, anterior commissure; C, caudate nucleus; C(g), genu of the caudate nucleus; C(t), tail of the caudate nucleus; GPe, globus pallidus, external segment; GPi, globus pallidus, internal segment; IC, internal capsule; IPAC (Lat), interstitial nucleus of the posterior limb of the anterior commissure, lateral subdivision; P, putamen; V, ventricle.

Figure 10.

Cortical (A) and striatal (C) components of the three main cortico–amygdala–striatal circuits, and their overlap in the basal and accessory basal nuclei (B). A, The cortical inputs of the primitive circuit (in blue) arose from mPFC areas 25 and 32, and agranular insula, the most poorly differentiated cortices. The intermediate circuit (in green) had cortical inputs from mPFC areas 24 and 14, and dysgranular and granular insula. The cortical inputs of the developed circuit (in red) arose from OFC and mPFC area 10m, relatively well differentiated cortices. B, In the amygdala, the three circuits were hierarchically layered in a pyramidal-like fashion. The primitive circuit (blue) influenced the entire B and ABmc, serving as a foundational circuit. C, The striatal outputs of the primitive circuit (blue) were rostral, ventral striatal subregions, which were influenced by all amygdala subregions. The intermediate circuit (green) had additional outputs to the caudoventral striatum, specifically lateral IPAC and Astr, rostral ventromedial caudate body, and caudoventral putamen. The striatal outputs of the developed circuit (red) included the dorsolateral and caudate nucleus body, the genu, and tail, which were influenced by the dorsal amygdala only. Bi, basal nucleus, intermediate subdivision; C, caudate nucleus; CeL, central nucleus, lateral core; CeM, central nucleus, medial subdivision; C(g), genus of the caudate nucleus; C(t), tail of the caudate nucleus; GP, globus pallidus; GPe, globus pallidus, external segment; GPi, globus pallidus, internal segment; Hipp, hippocampus; IC, internal capsule; IPAC (Lat), interstitial nucleus of the posterior limb of the anterior commissure, lateral subdivision; L, lateral nucleus; OT, optical tract; P, putamen; V, ventricle.