Converging Prefronto-Insula-Amygdala Pathways in Negative Emotion Regulation in Marmoset Monkeys

Abstract

Background: Impaired regulation of emotional responses to potential threat is a core feature of affective disorders. However, while the subcortical circuitry responsible for processing and expression of fear has been well characterized, the top-down control of this circuitry is less well understood. Our recent studies demonstrated that heightened emotionality, as measured both physiologically and behaviorally, during conditioned fear and innate/social threat was induced, independently, by excitotoxic lesions of either the anterior orbitofrontal cortex (antOFC) or ventrolateral prefrontal cortex (vlPFC). An important outstanding question is whether the antOFC and vlPFC act on common or distinct downstream targets to regulate negative emotion.

Methods: The question was addressed by combining localized excitotoxic lesions in the PFC of a nonhuman primate and functional neuroimaging ([¹⁸F]fluorodeoxyglucose positron emission tomography) with a fear-regulating extinction paradigm. Marmoset monkeys with unilateral lesions of either the antOFC or vlPFC were scanned immediately following exposure to a fearful or safe context, and differences in [¹⁸F]fluorodeoxyglucose uptake were evaluated.

Results: [¹⁸F]fluorodeoxyglucose uptake in the insula and amygdala of the intact hemisphere was significantly increased in response to the fearful context compared with the safe context. Such discrimination between the two contexts was not reflected in the activity of the insula-amygdala of the antOFC or vlPFC-lesioned hemisphere. Instead, uptake was at an intermediate level in both contexts.

Conclusions: These findings demonstrate that the distinct control functions of the antOFC and vlPFC converge on the same downstream targets to promote emotion regulation, taking us closer to a mechanistic understanding of different forms of anxiety.

Keywords: Amygdala; Emotion regulation; Insula; Orbitofrontal cortex; Prefrontal cortex; Ventrolateral prefrontal cortex.

Figure 1

(A) Order of the experimental procedures. (B) Schematic diagram of the test apparatus. Light was provided by two LED light tubes, lined along the ceiling edges. Three cameras mounted on the inside walls of the chamber recorded the behavior of the subject. The dotted line on the floor of the carrying box represents the imaginary line between the zones near to and far from the snake. The model snake resembled a coiled cobra with its head raised (27 cm in height) and was dark brownish in color with black stripes. (C) Sequence of stimulus presentations for the fear and safety conditions. [¹⁸F]FDG, [¹⁸F]fluorodeoxyglucose; MRI, magnetic resonance imaging; PET, positron emission tomography.

Figure 2

(A–C) Schematics of coronal sections through the frontal lobe (anterior–posterior) of the left hemisphere of the marmoset brain. (A) The dark and pale shading represents the anterior orbitofrontal cortex (antOFC) and ventrolateral prefrontal cortex (vlPFC) areas targeted for the excitotoxic lesion, respectively. The numbers indicate anterior–posterior coordinates from the interaural line. Numerical designations reflect cytoarchitechtonic regions identified within the PFC, redrawn from Burman and Rosa . (B, C) The pale to dark shading reflects the area of cell loss in 1 to 7 animals with excitotoxic lesions of the antOFC (B) and 1 to 4 animals with excitotoxic lesions of the vlPFC (C). For illustrative purposes, those in which lesions were placed in the right hemisphere are shown on the left. All animals in the antOFC-lesioned group sustained significant neuronal cell loss throughout area 11 and anteromedial area 13. In the vlPFC-lesioned group, all animals sustained marked neuronal loss throughout the anterior sector of area 12, with more varied neuronal loss in more caudal sectors. (D, E) Photomicrographs of a representative coronal section through the PFC showing the lesion extent (dotted line) for the antOFC (D) and vlPFC (E).

Figure 3

(A) Proportion of time spent in either the near (striped bar) or far (dotted bar) sector of the test box during the fear induction and safety conditions for the anterior orbitofrontal cortex (antOFC)-lesioned (black background bar) and ventrolateral prefrontal cortex (vlPFC)-lesioned (gray background bar) groups. (B) The same pattern was present in the 3 telemetry-implanted animals. Error bars indicate SE. (C) Accompanying systolic blood pressure (BP) is shown for these additional 3 animals, with increases occurring during the fear sessions compared with safety. Mean systolic BP (in millimeters of mercury [mmHg]) is plotted for the total duration (30 minutes) of the first fear session (blue), the safety session (red), and the second fear session (green). The data are presented after being smoothed using a Savitzky–Golay filter, which increases the signal-to-noise ratio for ease of visualization (bold line). The raw systolic BP trace is also presented (faint line). Shading represents ± SEM. The significance indicates the difference of the BP between the safety and the mean of two fear conditions for each phase: **p < .01, ***p < .001. Additional analyses (Supplemental Results) revealed that while there was a small but significant decline between the first and second fear conditions, importantly, both conditions were independently significantly different from the safety condition.

Figure 4

(A, B) Differences in fluorodeoxyglucose (FDG) uptake between fear and safety from the voxelwise analysis showing coronal sections (top–bottom: anterior–posterior) (A) and sagittal sections (top–bottom: medial–lateral) (B) from the intact side of the brain. For brain images with a left-sided lesion, the intact side is depicted on the left for illustrative purposes only. Numbers indicate the distance in millimeters from the interaural line for the coronal sections and the midline for the sagittal sections. Colored areas indicate regions with significantly greater fluorodeoxyglucose uptake (red: p < .005; green: p < .001) during the fear induction condition compared with the safety condition. The dotted white circle indicates the amygdala region of interest, based on the marmoset brain atlas . Line graphs show the mean of the standardized fluorodeoxyglucose uptake scores from the significant cluster (p < .001). (C-i) The same cluster masked by the amygdala region of interest (C-ii) and the remaining insula cluster (C-iii) across the three conditions. Standardized scores for the amygdala and remaining insula clusters were calculated using the mean and SD from the significant cluster. Error bars indicate SE.

Figure 5

Graphs showing the fluorodeoxyglucose uptake in response to the fear-inducing condition (averaged across fear 1 and fear 2) proportional to the safety condition, for the p < .001 cluster identified by the voxel-based analysis (A), the cluster masked with the amygdala region of interest (B), and the remaining cluster encompassing the insula (C) for both intact and lesioned hemispheres. While the intact hemisphere exhibited a marked change between the fear-inducing and safety conditions, such a differential response was absent in the lesioned hemisphere. No significant difference between the anterior orbitofrontal cortex (OFC)-lesioned group (black bar) and the ventrolateral prefrontal cortex (vlPFC)-lesioned group (gray bar) was found. Error bars indicate SE. **p < .01, †p = .058.