Low dopamine striatal D2 receptors are associated with prefrontal metabolism in obese subjects: Possible contributing factors

Abstract

Dopamine's role in inhibitory control is well recognized and its disruption may contribute to behavioral disorders of discontrol such as obesity. However, the mechanism by which impaired dopamine neurotransmission interferes with inhibitory control is poorly understood. We had previously documented a reduction in dopamine D2 receptors in morbidly obese subjects. To assess if the reductions in dopamine D2 receptors were associated with activity in prefrontal brain regions implicated in inhibitory control we assessed the relationship between dopamine D2 receptor availability in striatum with brain glucose metabolism (marker of brain function) in ten morbidly obese subjects (BMI>40 kg/m 2 ) and compared it to that in twelve non-obese controls. PET was used with [ 11 C]raclopride to assess D2 receptors and with [ 18 F] FDG to assess regional brain glucose metabolism. In obese subjects striatal D2 receptor availability was lower than controls and was positively correlated with metabolism in dorsolateral prefrontal, medial orbitofrontal, anterior cingulate gyrus and somatosensory cortices. In controls correlations with prefrontal metabolism were not significant but comparisons with those in obese subjects were not significant, which does not permit to ascribe the associations as unique to obesity. The associations between striatal D2 receptors and prefrontal metabolism in obese subjects suggest that decreases in striatal D2 receptors could contribute to overeating via their modulation of striatal prefrontal pathways, which participate in inhibitory control and salience attribution. The association between striatal D2 receptors and metabolism in somatosensory cortices (regions that process palatability) could underlie one of the mechanisms through which dopamine regulates the reinforcing properties of food. KeywordsOrbitofrontal cortex; Cingulate gyrus; Dorsolateral prefrontal; Dopamine transporters; Raclopride, PET The increase in obesity and associated metabolic diseases seen over the past decade has raised concern that if not controlled this may become the number one preventable public health threat for the 21st century (Sturm, 2002 (Berthoud, 2007). The extent to which individuals differ in their ability to inhibit these responses and control how much they eat is likely to modulate their risk for overeating in our current food rich environments (Berthoud, 2007).We had shown that in healthy individuals D2 receptor availability in the striatum modulated eating behavioral patterns (Volkow et al., 2003). Specifically the tendency to eat when exposed to negative emotions was negatively correlated with D2 receptor availability (the lower the D2 receptors the higher the likelihood that an individual would eat if emotionally stressed). In addition, in a different study, we showed that morbidly obese subjects (BMI>40) had lower than normal D2 receptor availability and these reductions were proportional to their BMI (Wang et al., 2001). These findings led us to postulate that low D2 receptor availability could put an...

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