Dopamine genes (DRD2/ANKK1-TaqA1 and DRD4-7R) and executive function: their interaction with obesity

A meta-analysis of the relationship between brain dopamine receptors and obesity: a matter of changes in behavior rather than food addiction?

Addiction to a wide range of substances of abuse has been suggested to reflect a ‘Reward Deficiency Syndrome'. That is, drugs are said to stimulate the reward mechanisms so intensely that, to compensate, the population of dopamine D₂ receptors (DD2R) declines. The result is that an increased intake is necessary to experience the same degree of reward. Without an additional intake, cravings and withdrawal symptoms result. A suggestion is that food addiction, in a similar manner to drugs of abuse, decrease DD2R. The role of DD2R in obesity was therefore examined by examining the association between body mass index (BMI) and the Taq1A polymorphism, as the A1 allele is associated with a 30–40% lower number of DD2R, and is a risk factor for drug addiction. If a lower density of DD2R is indicative of physical addiction, it was argued that if food addiction occurs, those with the A1 allele should have a higher BMI. A systematic review found 33 studies that compared the BMI of those who did and did not have the A1 allele. A meta-analysis of the studies compared those with (A1/A1 and A1/A2) or without (A2/A2) the A1 allele; no difference in BMI was found (standardized mean difference 0.004 (s.e. 0.021), variance 0.000, Z=0.196, P<0.845). It was concluded that there was no support for a reward deficiency theory of food addiction. In contrast, there are several reports that those with the A1 allele are less able to benefit from an intervention that aimed to reduce weight, possibly a reflection of increased impulsivity.

Effect of obesity on cognition in adults with and without a mood disorder: study design and methods

INTRODUCTION

Obesity is a common medical illness that is increasingly recognised as conferring risk of decline in cognitive performance, independent of other comorbid medical conditions. Individuals with mood disorders (bipolar disorder (BD) or major depressive disorder (MDD)) display an increased prevalence of both obesity and risk factors for cardiovascular diseases. Moreover, BD and MDD are associated with impairment in cognitive functioning across multiple domains. The independent contribution of obesity to cognitive decline in this population has not been explored. This study examines the impact of obesity on cognition by comparing neuropsychological performance in obese individuals, with or without a mood disorder before and after undergoing bariatric surgery.

METHODS AND ANALYSIS

This study compares measures of declarative memory, executive functioning and attention in obese individuals (body mass index >35 kg/m(2)) with BD or MDD, and 2 control populations (obese individuals without a psychiatric illness and healthy non-obese controls) prior to and following bariatric surgery. Participants (ages 18-60) receive a psychiatric diagnosis via the Structured Clinical Interview for the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV; SCID). Mood ratings, physical measurements, nutritional and health questionnaires are also administered. A standardised battery of neuropsychological tests aimed at establishing performance in areas of declarative memory, executive functioning and attention are administered. Warrington's Recognition Memory Task (RMT) and an N-Back Task are performed in a 3 T functional MRI to investigate patterns of neural activation during cognitive performance. Additionally, anatomical MRI data are obtained to investigate potential changes in neural structures. Baseline data will be analysed for between-group differences and later compared with postsurgical data to investigate cognitive change.

ETHICS AND DISSEMINATION

This study has been approved by the Hamilton Integrated Research Ethics Board (09-3254). Results will be available in peer-reviewed scientific publications and scientific meetings presentations, and released in lay form to media.

Functional network centrality in obesity: A resting-state and task fMRI study

Obesity is associated with structural and functional alterations in brain areas that are often functionally distinct and anatomically distant. This suggests that obesity is associated with differences in functional connectivity of regions distributed across the brain. However, studies addressing whole brain functional connectivity in obesity remain scarce. Here, we compared voxel-wise degree centrality and eigenvector centrality between participants with obesity (n=20) and normal-weight controls (n=21). We analyzed resting state and task-related fMRI data acquired from the same individuals. Relative to normal-weight controls, participants with obesity exhibited reduced degree centrality in the right middle frontal gyrus in the resting-state condition. During the task fMRI condition, obese participants exhibited less degree centrality in the left middle frontal gyrus and the lateral occipital cortex along with reduced eigenvector centrality in the lateral occipital cortex and occipital pole. Our results highlight the central role of the middle frontal gyrus in the pathophysiology of obesity, a structure involved in several brain circuits signaling attention, executive functions and motor functions. Additionally, our analysis suggests the existence of task-dependent reduced centrality in occipital areas; regions with a role in perceptual processes and that are profoundly modulated by attention.

Affected connectivity organization of the reward system structure in obesity

With the prevalence of obesity rapidly increasing worldwide, understanding the processes leading to excessive eating behavior becomes increasingly important. Considering the widely recognized crucial role of reward processes in food intake, we examined the white matter wiring and integrity of the anatomical reward network in obesity. Anatomical wiring of the reward network was reconstructed derived from diffusion weighted imaging in 31 obese participants and 32 normal-weight participants. Network wiring was compared in terms of the white matter volume as well as in terms of white matter microstructure, revealing lower number of streamlines and lower fiber integrity within the reward network in obese subjects. Specifically, the orbitofrontal cortex and striatum nuclei including accumbens, caudate and putamen showed lower strength and network clustering in the obesity group as compared to healthy controls. Our results provide evidence for obesity-related disruptions of global and local anatomical connectivity of the reward circuitry in regions that are key in the reinforcing mechanisms of eating-behavior processes.

Set-shifting ability across the spectrum of eating disorders and in overweight and obesity: a systematic review and meta-analysis

BACKGROUND

In this meta-analysis we review the findings from neuropsychological studies on set-shifting in people with eating disorders (EDs) or overweight/obesity.

METHOD

Four databases (PubMed, PsycINFO, PSYNDEX and Web of Science) were searched for eligible studies. Effect sizes (ESs) were pooled using random-effects models. Moderator analyses were conducted for ED and overweight/obese subgroups, adult/adolescent samples and measures of set-shifting.

RESULTS

Sixty-four studies with a total of 1825 ED patients [1394 anorexia nervosa (AN), 376 bulimia nervosa (BN) and 55 binge eating disorder (BED)] and 10 studies with a total of 449 overweight/obese individuals were included. The meta-analysis revealed a small to medium ES for inefficient set-shifting across all three ED diagnoses (Hedges' g = -0.45). Subgroup analyses yielded small to medium ESs for each ED subtype (g = -0.44 for AN, -0.53 for BED, -0.50 for BN), which did not differ significantly. There was a medium ES for restricting type AN (ANR; g = -0.51) but no significant ES for binge/purge type AN (AN/BP; g = -0.18). A medium ES was found across obesity studies (g = -0.61). The ES across overweight studies was not significant (g = -0.07). Adult samples did not differ from adolescent samples in either ED or overweight/obesity studies. The different set-shifting measures were associated with largely varying ESs.

CONCLUSIONS

The meta-analysis provides strong support that inefficient set-shifting is a salient neuropsychological phenomenon across ED subtypes and obesity, but is less prominent in AN/BP and overweight. Compulsivity seems to be a common underlying factor supporting a dimensional and transdiagnostic conceptualization of EDs and obesity.

Hormonal and neural mechanisms of food reward, eating behaviour and obesity

With rising rates of obesity, research continues to explore the contributions of homeostatic and hedonic mechanisms related to eating behaviour. In this Review, we synthesize the existing information on select biological mechanisms associated with reward-related food intake, dealing primarily with consumption of highly palatable foods. In addition to their established functions in normal feeding, three primary peripheral hormones (leptin, ghrelin and insulin) play important parts in food reward. Studies in laboratory animals and humans also show relationships between hyperphagia or obesity and neural pathways involved in reward. These findings have prompted questions regarding the possibility of addictive-like aspects in food consumption. Further exploration of this topic may help to explain aberrant eating patterns, such as binge eating, and provide insight into the current rates of overweight and obesity.

The effect of the ANKK1/DRD2 Taq1A polymorphism on weight changes of dopaminergic treatment in prolactinomas

Treatment with dopamine agonists in patients with prolactinomas has been associated with weight loss in short term studies. However, long-term studies on weight changes are lacking. Taq1A is a restriction fragment length polymorphism considered as a gene marker for the DRD2 gene. The presence of at least one A1 allele is linked to reduced brain dopaminergic activity due to reduced receptor binding and lower density of the dopamine 2 receptor. We aimed at testing the hypothesis that the dopaminergic treatment in prolactinoma patients leads to sustained weight loss and that the presence of diminished weight loss response under dopamine agonists is associated with the minor A1 allele of Taq1A.We included n = 44 patients (17 male and 27 female, 26 macroadenomas and 18 microadenomas) with prolactinomas treated with dopamine agonists. Outcome measures were weight and body mass index (BMI) change under dopaminergic treatment after 2 years with regard to Taq1A status and sex. We observed that the dopaminergic treatment leads to a significant mean weight loss of 3.1 ± 6.25 kg after 2 years. Regarding Taq1A polymorphisms, 21 patients were carriers of at least one A1 allele and 23 patients had a genotype of A2/A2. However, the presence of the A1 allele was neither associated with the mean BMI at baseline nor with an altered weight loss response under dopamine agonist therapy. Our results implicate that the dopaminergic treatment leads to a sustained weight loss in patients with prolactinomas after 2 years. However, there was no association to the A1 allele of Taq1A, observation that needs to be analysed in larger cohorts.

The interaction effect between BDNF val66met polymorphism and obesity on executive functions and frontal structure

The prevalence of obesity is increasing worldwide. Previous research has shown a relationship between obesity and both executive functioning alterations and frontal cortex volume reductions. The Brain Derived Neurotrophic Factor val66met polymorphism, involved in eating behavior, has also been associated with executive functions and prefrontal cortex volume, but to date it has not been studied in relation to obesity. Our aim is to elucidate whether the interaction between the Brain Derived Neurotrophic Factor val66met polymorphism and obesity status influences executive performance and frontal-subcortical brain structure. Sixty-one volunteers, 34 obese and 27 controls, age range 12-40, participated in the study. Participants were assigned to one of two genotype groups (met allele carriers, n = 16, or non-carriers, n = 45). Neuropsychological assessment comprised the Trail Making Test, the Stroop Test and the Wisconsin Card Sorting Test, all tasks that require response inhibition and cognitive flexibility. Subjects underwent magnetic resonance imaging in a Siemens TIM TRIO 3T scanner and images were analyzed using the FreeSurfer software. Analyses of covariance controlling for age and intelligence showed an effect of the obesity-by-genotype interaction on perseverative responses on the Wisconsin Card Sorting Test as well as on precentral and caudal middle frontal cortical thickness: obese met allele carriers showed more perseverations on the Wisconsin Card Sorting Test and lower frontal thickness than obese non-carriers and controls. In conclusion, the Brain Derived Neurotrophic Factor may play an important role in executive functioning and frontal brain structure in obesity.