Dopamine release during human emotional processing

Abstract

Involvement of dopamine neurotransmission in human emotional processing is unclear but animal studies have indicated that it is critical for processing of fear response. In this experiment we examined dopaminergic involvement in the processing of human emotions. We used a novel dynamic molecular imaging technique to detect and map dopamine released during presentation of emotional stimuli. The technique exploited the competition between endogenously released dopamine and its ligand for receptor occupancy and involved dynamic voxel-wise measurement of the rate at which a dopamine receptor ligand ((18)F-Fallypride) was displaced from receptor sites during emotional processing. An increase in the rate indicated dopamine release. We found that the rate of ligand displacement increased significantly in the left amygdala, left medial temporal lobe (MTL) and left inferior frontal gyrus. The results provide the first direct evidence of dopaminergic modulation of human emotional processing and suggest that the modulation occurs at multiple levels of processing. This finding indicates that the neurocognitive models of human emotion should take into account dopaminergic effects, and that, there is a need to investigate whether manipulation of the dopaminergic system could be an alternate strategy for treatment of conditions in which emotional processing is impaired.

FIGURE 1

Dopamine was released in the amygdala during emotional processing. The figure shows t-maps of the rate of ligand (¹⁸F-Fallypride) displacement before and after task initiation. The maps were superimposed on the mean PET images and represent changes across volunteers. The time-activity curve shows the concentration history (circles) and least square fits (solid line) for the ligand in the activated region of a volunteer. There was a significant increase in the rate of ligand displacement after task was initiated (vertical line). The ligand concentration is expressed as kBq/cc.

FIGURE 2

The figure (t-map) shows the left medial temporal lobe (MTL) area where dopamine was released during emotional processing. The time-activity curve depicts the concentration history of the ligand in the activated MTL region, which included the hippocampus and parahippocampus. The ligand concentration (expressed as kBq/cc) reduced significantly in this area after task initiation (vertical line).

FIGURE 3

The areas of the inferior frontal gyrus where dopamine was released during emotional processing. The time activity curve suggests significant decrease in the ligand concentration (expressed as kBq/cc) after task initiation (vertical line).

FIGURE 4

There was no change in the rate of ligand displacement after task initiation in the cerebellum, which was used as a reference region because of paucity of dopamine receptors in this area. The curve indicates that nonspecific binding of the ligand was not affected by the task. The ligand concentration is expressed as kBq/cc.