Architectonic distribution of the serotonin transporter within the orbitofrontal cortex of the vervet monkey

Abstract

To elucidate the organization of the serotoninergic innervation within the orbitofrontal cortex (OFC), serotonin transporter (SERT) density was quantified by autoradiography using [(3)H]cyanoimipramine binding. In six adult vervet monkeys, 15 architectonic areas were delineated according to cytoarchitectonic (Nissl), myeloarchitectonic (Gallyas) and chemoarchitectonic (acetylcholinesterase) criteria to assess SERT distribution at two levels of organization: cortical area and cortical type. For cortical type, the 15 areas were evenly divided into three different categories primarily based upon the degree of granularization of layer IV: agranular, dysgranular, and granular. Within agranular and dysgranular, but not granular cortical types, SERT density was area-specific and progressively decreased in a medial to lateral gradient. Across cortical types, SERT density decreased in a caudal to rostral gradient: agranular>dysgranular>granular. A similar caudal to rostral gradient was seen when serotonin content was measured (using high performance liquid chromatography) in areas representative of each cortical type. Collectively, these results suggest that the serotoninergic innervation is organized according to both cortical type and area, and is thus structured to differentially modulate information processing within the OFC.

Figure 1

Schematic map of the monkey orbitofrontal cortex with the 15 architectonic areas analyzed in this study (Price et al., 1996), permission pending from Elsevier Science). Shading denotes the cortical type of each area. Iapm, agranular insula posteromedian; Iam, agranular insula medial; Iai, agranular insula intermediate; Ial, agranular insula lateral; AON, anterior olfactory nucleus; PrCO, precentral operculum; G, gustatory cortex.

Figure 2

Nissl stained sections from cortical areas of the vervet monkey demonstrating structural differences between cortical types: agranular (A), dysgranular (B), and granular (C). From A to C, there is an increase in laminar differentiation as well as a progressive increase in the density of granule cells particularly in layer IV, but also in layer II. In addition to these structural features used to delineate cortical type, each area is also distinguished by unique cytoarchitectural features. The agranular area Iam (A) is characterized by indistinct lamination, particularly with respect to the boundaries between layers III and V as well as V and VI. The dysgranular area 13l (B) has large, darkly staining pyramidal cells in the deep portion of layer III. Additionally, layer V is sublaminated into cell-dense inner and outer laminae that bracket a cell-sparse middle lamina. In the granular area 10o (C), layer V is thinner and the sublamina are less distinct.

Figure 3

Comparison of ventral surfaces of the orbitofrontal cortex in A) Vervet monkey (Chlorocebus aethiops) and B) Rhesus monkey (Macaca mulatta; (Chiavaras and Petrides, 2000), permission pending from Academic Press). MOS, medial orbital sulcus; LOS, lateral orbital sulcus; Olf tract, olfactory tract.

Figure 4

Left column: representative histological sections (A,C,E) used in the delineation of architectonic areas and corresponding autoradiographs of [³H]cyanoimipramine binding (B,D,F). White lines denote architectonic boundaries. Right column: low magnification images of the coronal sections pictured in the left column, box denotes high magnification portion. Scale bar for all images is 5 mm. A). Acetylcholinesterase stain, Iam and Iai are distinguished by the narrower band of densely stained fibers in layer V of Iai, which is distinguished from Ial primarily by the decrease in staining density in Ial. B). [³H]Cyanoimipramine binding is area specific in this plane of the agranular insula. Across all animals (n =6), binding in Iam was 1.3 fold greater than in Iai and 1.8 fold greater than in Ial. C). Myelin (Gallyas) stain, 13m and 13l are distinguished by the more darkly stained outer band of Baillarger in 13l; 12m and 13l can be delineated by the less dense outer band of Baillarger in 12m as well as the presence of a prominent internal band of Baillarger. D). [³H]Cyanoimipramine binding in areas 13m, 13l, and 12m. In the full sample, binding in 13m and 13l was ∼1.35 fold greater than binding in 12m. E). Nissl stain, 13L and 12o are distinguished primarily by the lack of a sublaminated layer V in area 12o. F). [³H]Cyanoimipramine binding in areas 13m, 13l, and 12o. In the full sample, binding in 13m and 13l was ∼1.25 fold greater than binding in 12o.

Figure 5

[³H]Succinimidyl propionate binding in coronal sections before (A), and after (B) delipidation. Scale bar = 5 mm.

Figure 6

Saturation binding of [³H]cyanoimipramine (K_d = 0.11 ± 0.02 nM) determined in cortical sections.

Figure 7

A) Coronal autoradiograms of [³H]cyanoimipramine binding in the prefrontal cortex of the vervet monkey. Sections are arranged in a rostral to caudal manner with the most rostral section located in the upper left corner and the most caudal section located in the lower right corner. Sections are not spaced at regular intervals. The scale bar denotes density of binding (fmol [³H]cyanoimipramine/mg protein). B) Sketches of the autoradiograms shown in A. The 15 orbitofrontal areas for which [³H]cyanoimipramine binding was quantified are shown in bold with architectonic boundaries delineated. Scale bar = 5mm.

Figure 8A

[³H]Cyanoimipramine binding (mean, SEM) in the orbitofrontal cortex as a function of cortical type. Significance of planned contrasts denoted by *** p < 0.001. Each cortical type category consisted of five areas: granular (10o, 11m, 11l, 12l, 12m), dysgranular (13b, 13m, 13l, 12o, 12r), and agranular (13b, Iapm, Iam, Iai, Ial). (See Figure 2 for a map of these areas). 8B. Serotonin Content in orbitofrontal cortical areas as a function of cortical type; *** p < .001, ** p < 0.01, * p < 0.05.

Figure 9

[³H]Cyanoimpramine binding (mean, SEM) within cortical type. A) Agranular areas; B) Dysgranular areas; C) Granular areas; D) Architectonic map with shading corresponding to the respective graphs; *** p < 0.001, ** p < 0.01.

Figure 10

Summary map of [³H]cyanoimpramine binding density in the orbitofrontal cortex as a function of architectonic area.