Mirror trends of plasticity and stability indicators in primate prefrontal cortex

Abstract

Research on plasticity markers in the cerebral cortex has largely focused on their timing of expression and role in shaping circuits during critical and normal periods. By contrast, little attention has been focused on the spatial dimension of plasticity-stability across cortical areas. The rationale for this analysis is based on the systematic variation in cortical structure that parallels functional specialization and raises the possibility of varying levels of plasticity. Here, we investigated in adult rhesus monkeys the expression of markers related to synaptic plasticity or stability in prefrontal limbic and eulaminate areas that vary in laminar structure. Our findings revealed that limbic areas are impoverished in three markers of stability: intracortical myelin, the lectin Wisteria floribunda agglutinin, which labels perineuronal nets, and parvalbumin, which is expressed in a class of strong inhibitory neurons. By contrast, prefrontal limbic areas were enriched in the enzyme calcium/calmodulin-dependent protein kinase II (CaMKII), known to enhance plasticity. Eulaminate areas have more elaborate laminar architecture than limbic areas and showed the opposite trend: they were enriched in markers of stability and had lower expression of the plasticity-related marker CaMKII. The expression of glial fibrillary acidic protein (GFAP), a marker of activated astrocytes, was also higher in limbic areas, suggesting that cellular stress correlates with the rate of circuit reshaping. Elevated markers of plasticity may endow limbic areas with flexibility necessary for learning and memory within an affective context, but may also render them vulnerable to abnormal structural changes, as seen in neurologic and psychiatric diseases.

Keywords: eulaminate; limbic; macaque monkey; plasticity; selective vulnerability.

Figure 1

Limbic and eulaminate areas of the monkey prefrontal cortex differ in laminar structure. A, B, Maps of monkey prefrontal cortex (Barbas & Pandya, 1989). A, Medial surface; B, Lateral surface. The maps show areas with the lowest (black) and highest (lightest grey) laminar elaboration. C–F, Photomicrographs of areas 25, 32, 10m, and 46d stained with Nissl. C, D, Areas 25 and 32 have a rudimentary layer IV (dysgranular). Layer I is thick and shows poor delimitation with layer II. Deep layers V–VI are more prominent than superficial layers II–III. E, Eulaminate (I) area 10m has six layers. F, Layer IV in eulaminate (II) area 46d is better developed than in area 10m. Layer I is thinner than in areas 25 and 32 and is delineated from layer II. Superficial layers II–III are denser than in limbic areas 25 and 32. Abbreviations: MPAll, medial periallocortex; WM, white matter. Arabic numerals show cortical areas according to Barbas and Pandya (1989). Roman numerals indicate cortical layers. Calibration bar in F applies to C–F.

Figure 2

Distribution of parvalbumin positive (PV+) neurons in limbic and eulaminate areas of the monkey prefrontal cortex. A–D, Photomicrographs of areas 25, 32, 10m, and 46d. A, In area 25, PV+ neurons are sparse across layers and form a thin band in layer V. B, PV+ neurons are more evenly distributed within layers in area 32 than in area 25. C, D, Eulaminate areas 10m and 46d have more PV+ neurons than areas 25 and 32 with a dense band in layers IV and V. E, PV+ neuron density is higher in eulaminate areas 10m and 46 than in areas 25 and 32 across layers. F, PV+ neuron density is higher in eulaminate areas 10m and 46 than in areas 25 and 32 in the superficial layers. G, PV+ neuron density is higher in eulaminate areas 10m and 46 than in areas 25 and 32 in the deep layers (IV–VI for limbic, V–VI for eulaminate). H, Layer IV, which is distinct in areas 10m and 46d, has the highest PV+ neuron density. I, The proportion of PV+ neurons for the entire neuron population is higher in eulaminate areas 10m and 46d than in limbic areas 25 and 32 across layers. J, The proportion of PV+ neurons for the neuron population in superficial layers is higher in eulaminate areas 10m and 46d than in limbic areas 25 and 32. K, The proportion of PV+ neurons for the neuron population in the deep layers is higher in eulaminate areas 10m and 46d than in limbic areas 25 and 32 (layers IV–VI for limbic, V–VI for eulaminate). L, The proportion of PV+ neurons for the neuron population in layer IV is high in areas 10m and 46d. WM, white matter. Roman numerals indicate cortical layers. Asterisks in E, F, I–K indicate significant differences between pairs of areas, as determined by post hoc analysis (Bonferroni method) conducted after one-way ANOVA. Scatter plots in E–L represent individual cases denoted by different symbols; greyscale horizontal bars represent case averages; vertical lines on bars show the standard error. Calibration bar in D applies to A–D.

Figure 3

Perineuronal net (PNN) label by the lectin Wisteria floribunda agglutinin (WFA) in limbic and eulaminate areas of the monkey prefrontal cortex. A–D, Photomicrographs of areas 25, 32, 10m, and 46d stained for WFA. A, area 25 shows scant label in layers II–III and a band of WFA staining in the neuropil of layer V; black arrow points at PNN in a pyramidal neuron. B, C, Areas 32 and 10m show more label for WFA in layers II–III and IV–V than area 25. D, Area 46d shows the highest label for WFA across layers II–VI compared to other areas. E, The mean gray level index through the depth of the cortex shows higher levels of WFA staining in area 46d. F, WFA content increases towards the middle layers in the four areas, shown along the course from the surface of the cortex (left) to the edge of the white matter (right); the highest density is found consistently in area 46d and the lowest in area 25. WM, white matter. Roman numerals indicate cortical layers. Scatter plots in E represent individual cases denoted by different symbols; greyscale horizontal bars represent case averages; vertical lines on bars show the standard error. Calibration bar in D applies to A–D.

Figure 4

Myelin content in limbic and eulaminate areas of the monkey prefrontal cortex. A–D, Photomicrographs of areas 25, 32, 10m, and 46d stained with the Gallyas technique for myelin. A, B, In areas 25 and 32 the content of intracortical myelin is lower than in eulaminate areas. C, D, Area 10m and area 46d show progressive increase of intracortical myelin. E, The mean gray level index of myelin through the depth of the cortex also shows this trend. F, Myelin content increases towards the white matter in the four areas, and is higher in the middle-deep bins of areas 46d and 10m than in areas 25 and 32. WM, white matter. Roman numerals indicate cortical layers. Scatter plots in E represent individual cases denoted by different symbols; greyscale horizontal bars represent case averages; vertical lines on bars show the standard error. Calibration bar in D applies to A–D.

Figure 5

Expression of the alpha subunit of the calcium/calmodulin-dependent protein kinase II (αCaMKII) in limbic and eulaminate areas of the rhesus monkey prefrontal cortex. A–D, Photomicrographs of areas 25, 32, 10m, and 46d stained for αCaMKII. A, Area 25 shows high neuropil expression of αCaMKII across layers (dark brown). B, C, In area 32 and in area 10m, αCaMKII expression is lower than in area 25. D, Area 46d shows lower expression of αCaMKII than areas 25, 32, and 10m across layers except in layer I. E, The mean gray level index of αCaMKII also shows the trend of A–D. F, αCaMKII expression decreases towards the white matter in the four areas. WM, white matter. Roman numerals indicate cortical layers. Asterisks in E indicate significant differences between pairs of areas, as determined by post hoc analysis (Bonferroni method) conducted after one-way ANOVA. Scatter plots in E represent individual cases denoted by different symbols; greyscale horizontal bars represent case averages; vertical lines on bars show the standard error. Calibration bar in D applies to A–D.

Figure 6

Expression of glial fibrillary acidic protein (GFAP) in limbic and eulaminate prefrontal areas in rhesus monkeys. A–D, Photomicrographs of areas 25, 32, 10m, and 46d stained using immunohistochemistry for GFAP (gold label). A, Area 25 shows dense GFAP labeling across layers. B, GFAP labeling is also dense in area 32 but the middle layers show moderate labeling (less gold labeling). C, D, The middle layers in eulaminate areas 10m and 46d show light labeling of GFAP with dense expression in layers I, II, and VI. E, The mean gray level index of GFAP through the depth of the cortex also shows this trend. F, GFAP expression decreases from layer I and the white matter towards the middle part of the cortex in the four areas, a pattern that is more pronounced in eulaminate areas 10m and 46d. WM, white matter. Roman numerals indicate cortical layers. Asterisks in E indicate significant differences between pairs of areas, as determined by post hoc analysis (Bonferroni method) conducted after one-way ANOVA. Scatter plots in E represent individual cases denoted by different symbols; greyscale horizontal bars represent case averages; vertical lines on bars show the standard error. Calibration bar in D applies to A–D.

Figure 7

Nonmetric multidimensional scaling (NMDS) diagram shows separation between limbic areas 25 and 32 (left) and eulaminate areas 10m and 46d (right) with a negligible level of stress (~6×10⁻¹⁷), indicating that the 2-dimensional space accurately reproduced the differences among areas.

Figure 8

Markers of plasticity in the cortex parallel laminar differentiation. A, Cartoon depicts expression of factors that limit synaptic plasticity, which are higher in eulaminate than in limbic areas. B, Conversely, expression of αCaMKII, known to enhance synaptic plasticity, is higher in limbic areas; GFAP expression, a marker of cellular stress, is also higher in limbic areas than in eulaminate areas. C, The distribution of these markers suggests that cortical plasticity and stability change systematically with laminar differentiation as shown in the cartoon of cellular density across areas.