Using positron emission scanning and brain MRI, neuroanatomical correlates of hunger and satiety have been investigated. Importantly, analyses of this collected data has indicated that the left dorsolateral prefrontal cortex, an area of the brain important in reward processing, may be a satiety center. Neuronal activation in the left dorsolateral prefrontal cortex following a meal is consistently lower in this area in obese versus lean individuals, in both men and women. Moreover, we demonstrated that in subjects with Prader-Willi there is actual deactivation of this same area. Prader-Willi patients have uncontrolled hunger due to failure of meals to suppress satiety, so this deactivation (as opposed to only lesser activation in obese individuals) is further evidence of an important role for the prefrontal cortex in appetite control. We then conducted a parallel design study of active anodal stimulation to the left dorsolateral prefrontal cortex versus sham in obese individuals. We again measured ad libitum energy intake, but also had a follow-up 4 week outpatient follow-up in which tDCS was continued three times per week. In addition to measuring ad libitum energy intake, we also measured hunger scores and conducted post meal snack food tests. In the parallel design study, we were not able to replicate the lower ad libitum energy intake or weight loss in the active treatment group, but we found a steady decrease in hunger and urge to eat scores and lower intake of snack foods after 4 weeks of treatment. These results indicate it may take weeks for the effect of tDCS to be manifest. We are beginning a longer 9 week outpatient study which will include functional MRI of the brain prior to and following the first and final tDCS session to investigate the acute and chronic effects of tDCS on the pre-frontal cortex and other brain regions. Previous studies investigating the association of gray matter density with adiposity have not differentiated between fat mass (FM, adipose tissue only) and fat free mass (FFM), and both increase with increasing adiposity. We found that fat free mass indexed to height (FFMI) was associated with reduced gray matter volume (GMV) in the bilateral temporal medial and inferior gyri, the bilateral ventromedial prefrontal cortex extending to the anterior cingulate, and the bilateral orbitofrontal cortex with extension to the insula on the left. Similar overlapping associations were seen with fat mass indexed to height (FMI). Percent body fat was associated only with reduced GMV in left temporal lobe and left cerebellum. Most importantly, in models adjusting for both FFM and percent body fat or FFM and FM, only FFM remained associated with the above brain regions. These regions are part of important brain networks which monitor reward related behavior and are involved in homeostatic regulation. We have followed up this analysis by investigating FFM as a determinant of cerebral blood flow (rCBF) in individuals who had whole brain PET scans. We have demonstrated that FFM is associated with rCBF in specific mid-brain regions which connect the hypothalamus and higher brain regions. Moreover there is substantial overlap between these associations between FFM and hunger scores in this study. Further analyses indicated that the rCBF in these midbrain regions mediated the affect of FFM on hunger scores. Our results indicate that differences in brain regions with increased adiposity are due to the associated increases in fat free rather than fat mass, and that there are regions that mediate the associated between FFM and hunger.
