No correlation between body mass index and striatal dopamine transporter availability in healthy volunteers using SPECT and [123I]PE2I

Authors


Correspondence: Gerda Thomsen (gerda@nru.dk)

Abstract

Objective

Dopamine plays an important role in both the rewarding and conditioning effects of food. These effects involve mesolimbic, mesocortical, and nigrostriatal pathways. In humans, the most consistent finding has been reduced striatal dopamine D2/3 receptor availability. In striatum, dopamine is inactivated by reuptake via the dopamine transporter (DAT). The aim of the study was to test the hypothesis of lower DAT availability in obese healthy subjects using a selective DAT radiotracer in a sample of subjects with a wide range of BMI values.

Design and Methods

Thirty-three healthy subjects with a mean age of 48.4 ± 13.3 (range, 21-71) years and a mean BMI of 29.6 ± 7.8 kg/m2 (range, 21.0-49.5) were included in the study. We used [123I]PE2I and SPECT to measure DAT availability.

Results

Using multiple linear regression analyses with striatal DAT as the dependent variable and BMI, age and gender as predictors was performed. We found no correlation between BMI and striatal DAT availability in striatum (P = 0.99), caudate nucleus (P = 0.61), and putamen (P = 0.30). Furthermore, we found no group difference between obese/severely obese (BMI > 30 kg/m2) and normal weight controls (BMI ≤ 25 kg/m2).

Conclusions

We did not find any correlation between BMI and DAT availability in healthy volunteers.

Introduction

The prevalence of obesity is increasing in the western world and obesity is one of the major and fastest growing public health problems. Globally, approximately 1.6 billion adults weigh too much, and WHO has estimated that by 2015, this number will be 2.3 billion (WHO 2006). The regulation of eating behavior is complex [1, 2], and the neurobiological mechanisms behind overeating and obesity are only poorly understood [3].

As in other disorders of hedonic excess (e.g., drug addiction), the dopamine (DA) system seems to be implicated in regulating food intake. Transgenic dopamine-deficient mice become hypoactive and hypophagic and die of starvation [4]. Restoration of dopamine production in striatum restores feeding behavior [4]. In vivo microdialysis studies in rodents have shown that striatal dopamine is increased during feeding and the increase is related to the rewarding properties of the food [5]. In obese rats, both striatal dopamine D2 receptors [6] and striatal dopamine transporter (DAT) [7, 8] are down-regulated. The rodent finding of feeding induced dopamine release has been replicated in a human PET-study [9], and a PET-study in humans has suggested that the more anticipation of food intake increases dopamine release [10]. However, we do not know whether dopamine release is different in obese subjects compared to that in lean subjects. The finding of a decreased striatal D2 receptor density in obese rats has been replicated in three human PET and SPECT studies involving overweight individuals [11]. So far, two studies examined DAT binding in relation to body weight: Chen et al. demonstrated a negative correlation between BMI and striatal DAT measured with [99mTc]TRODAT-1 SPECT in 50 subjects (BMI: 18.7-30.6 kg/m2) [14]. Koskela et al. could, however, not demonstrate a significant difference in striatal DAT availability between cotwins with high BMI and cotwins with lower BMI [15].

The aim of this study was to test the hypothesis of lower striatal DAT in obese subjects using the highly selective DAT radiotracer [123I]PE2I in a sample of 33 subjects with a wide range of BMI values.

Methods

Subjects

Thirty-three healthy subjects (16 females) (all Caucasian) were included in the study. The mean age was 49.8 years (SD ± 13.3 years; range, 21-71 years), the mean body weight was 90 kg (SD ± 28.2 kg; range, 54-164 kg), and the mean BMI was 29.6 kg/m2 (SD ± 7.9 kg/m2; range, 21-49 kg/m2). The subjects were divided into three groups; normal-weighted (BMI ≤ 25 kg/m2, N = 12), over-weighted (25 < BMI< 30 kg/m2, N = 9), and obese/severely obese (BMI 30 kg/m2 < 35 kg/m2 N = 3 obese) and (BMI 35 kg/m2 N = 9 severely obese). The mean BMI in the normal-weight group was 22.7 kg/m2 (SD ± 1.4 kg/m2; range, 21.0-24.5 kg/m2), the mean BMI in the overweight group was 26.9 kg/m2 (SD ± 1.6 kg/m2; range, 25.3-29.7 kg/m2), and the mean BMI in the obese/severely obese group was 38.5 kg/m2 (SD ± 5.7 kg/m2; range, 30.9-49.5 kg/m2).

None of the subjects had any history of physical, neurological, or psychiatric disorders, and they all claimed to be alcohol- and drug free. Neurological examination (included Unified Parkinson Disease Rating Scale and Mini Mental State Examination) and routine blood tests were normal in all subjects (including P-Glucose and HbA1c).

The healthy volunteers were recruited through advertising on the Internet and in newspapers, and they all gave informed written consent. The study was performed in accordance with the ethical standards of the Declaration of Helsinki and was approved by the ethical committee of Copenhagen Capital Region (Protocol number: H-1 2010-109 and Protocol H-B-2008-024).

Tracer injection

123I-labeled-N-(3-iodoprop-2E-enyl)-2-β-carbomethoxy-3β-(4-methylphenyl) nortropane ([123I]PE2I) (MAP Medical Technologies) is a highly selective DAT ligand with fast kinetics and high target-to-background ratio in striatum. To block thyroidal uptake of free radioiodine, all subjects received 200 mg potassium perchloride intravenously 30 min before [123I]PE2I injection. [123I]PE2I was administrated as a bolus injection followed by a constant infusion for 3 h. The size of the bolus was worth 2.7 h of constant infusion [16]. An average intravenous bolus of 77 MBq (range, 65-92 MBq) of [123I]PE2I was administered.

Imaging procedures

SPECT imaging was performed with a triple-head IRIX camera (Philips Medical, Cleveland, USA) fitted with low-energy, general-all-purpose, parallel-holed collimators (spatial resolution 8.5 mm at 10 cm). Mean radius of rotation was 13.9 cm. Each camera covered 120° of the circular orbit. The total scanning time was 60 min. Scans were performed in continuous mode. Reconstruction of the images was performed with a MATLAB 6.5 (MathWorks) based program in 128×128 matrices (2.33 mm pixels and identical slice thickness) using standard filtered back projection with a low pass fourth-order Butterworth filter at 0.3 Nyquist (=0.64 c−1). The imaging energy window was positioned at 143-175 keV. High-energy photons of 123-I penetrated through the lead of the collimator, and Compton scatter in the scintillation crystal caused erroneous counts in the imaging energy window. A second energy window positioned at 184-216 keV was used to correct for these down-scattered photons in the imaging window. Before reconstruction, the projection images of the second energy window were subtracted from the imaging energy window with a weight of 1.1.

Binding potentials (BPND)

[123I]PE2I BPND was used as a measure of DAT availability. [123I]PE2I BPND was calculated as the ratio at steady-state of the concentration of specifically bound [123I]PE2I (concentration of [123I]PE2I in a volume of interest (VOI) minus concentration of [123I]PE2I in a reference region) to the concentration of [123I]PE2I in a reference region. Tracer steady-state was attained from 120 to 180 min after [123I]PE2I injection and cerebellum was used as the reference tissue devoid of DAT [16].

For all subjects, regional BPND values were calculated in striatum, caudate nucleus, and putamen using an in-house developed algorithm (DATquan). DATquan offers a fast, accurate, and highly reproducible method for semiautomatic VOI delineation and [123I]PE2I BPND calculation using a template-based approach [17].

Statistical analysis

The association between DAT availability (dependent variable) and BMI in the three VOIs was analyzed in a linear regression model with adjustment for age and gender.

For comparisons between the normal-weighted, overweighted, and obese/severely obese groups, ANCOVA analyses were done with striatal BPND as dependent variable, BMI group as independent variable, and age as a covariate. P < 0.05 (two-tailed) was considered statistically significant. Unless otherwise stated, all values were expressed as mean ± SD. We used SPSS software (IBM) version 19 for statistical analysis.

Results

Table 1 shows the characteristics of our three groups of subjects. The age of the overweight group was significantly higher than the age of the normal weight group (P = 0.036, Student's t-test) and of the group of obese/severely obese subjects (P = 0.004, Student's t-test). Gender was similarly distributed in the three groups. As shown in Table 1, there was no significant difference in the BPND values of the normal weight group compared to the obese/severely obese group (striatum, P = 0.83; caudate nucleus, P = 0.91; and putamen, P = 0.57). We found no group difference in BPND values in striatum (P = 0.43), caudate nucleus (P = 0.36), and putamen (P = 0.41) (ANCOVA).

Table 1. Characteristics and BPND values of study participants
 Normal weight (BMI ≤ 25 kg/m2)Overweight (25 < BMI < 30 kg/m2)Obese/severely obese (BMI ≥ 30 kg/m2)
Number of subjects12912
Number of females646
Age
Mean ± SD48.0 ± 13.959.8 ± 8.244.3 ± 12.3
Range28–6946–7129–59
BPND striatum
Mean ± SD4.0 ± 0.63.4 ± 0.73.9 ± 0.9
Range2.9–5.22.1–4.42.8–5.7
BPND caudate
Mean ± SD5.0 ± 1.04.1 ± 0.85.0 ± 1.2
Range3.3–7.22.9–5.23.6–7.6
BPND putamen
Mean ± SD4.3 ± 0.63.7 ± 0.74.0 ± 1.0
Range3.1–5.12.3–4.72.3–5.7

Figure 1 shows striatal DAT availability and BMI with 95% pointwise confidence limits. Multiple linear regression analyses using DAT availability as the dependent variable and BMI, age, and gender as predictors did not show statistical significant correlations in striatum (P = 0.99), caudate nucleus (P = 0.61), and putamen (P = 0.30). A statistically significant effect of age on BMI was not demonstrated (striatum, P = 0.25; caudate nucleus, P = 0.32; and putamen, P = 0.12). The striatal age decline was 5.4% per decade in line with previous studies of the effect of age on DAT availability [18]. However, in the putamen only a trend toward a correlation between DAT availability and age was demonstrated (P = 0.09). There was no statistically significant correlation between DAT availability and gender in striatum (P = 0.18), caudate nucleus (P = 0.27), and putamen (P = 0.11).

Figure 1.

Striatal DAT availability as function of BMI with 95% pointwise confidence limits with age and gender as covariate.

Discussion

Overweight and obesity is a health threat of increasing concern, and understanding the neurobiology behind obesity is instrumental to the development of effective treatment regimes.

Dopamine plays an important role in mediating both the rewarding and conditioning effects of food and drugs (recently reviewed by Ref. [3]). This effect involves both mesolimbic, mesocortical, and nigrostriatal pathways and both dopamine D1 receptors and dopamine D2 receptors [19]. The most effective mechanism to control the efficacy of striatal dopaminergic transmission is through the regulation of extracellular DA dynamics via selective dopamine reuptake by the presynaptic DAT. In this SPECT study, we did not find any statistically significant effect of BMI on striatal DAT availability as measured with [123I]PE2I in a sample of 33 healthy people with BMI ranging from 21 to 49 kg/m2. Furthermore, we did not find any group differences between obese/severely obese (BMI > 30 kg/m2) compared to normal weight healthy controls (BMI ≤ 25 kg/m2).

Our study do not support the findings of Chen et al. reporting a negative correlation between BMI and striatal DAT measured with [99mTc]TRODAT-1 SPECT in 50 subjects. Their finding was highly significant but a R2 = 0.20 suggests that the fraction of the total variance of BMI that is explained by the model is limited [14]. Our study differs from the study of Chen et al. in several ways: Chen et al. only included 11 overweight subjects (BMI, 26-31 kg/m2) out of a total of 50 subjects. In our study, we included 21 overweight subjects (BMI, 25-49 kg/m2) out of a total of 33 subjects. It is unknown whether differences in race and cultural background between the two studies are of importance. There are also differences in the use of radiotracer for imaging of DAT. [123I]PE2I is more selective for DAT [20] than [99mTc]TRODAT-1, which also binds significantly to the serotonin transporter (SERT) [21, 22]. This is potentially important since striatal and thalamic SERT availability recently was shown to be negatively correlated to BMI in 60 subjects [23]. In addition, the ratio of [99mTc]TRODAT-1 specifically bound radioligand to that of nondisplaceable radioligand in tissue is low (=1.98) [24] compared to a value of 4.3-5.6 for [123I]PE2I [16, 25], which will affect the statistical quality of the data.

How does an unchanged DAT availability in people with high BMI compared to those with a normal BMI contribute to our understanding of the neurochemistry underlying obesity? We know that the presence of DAT on the pre-synaptic nerve cells' surface is actively regulated by ligands interacting with the DAT including DA itself [26] but also by appetite-related hormones like insulin [27]. Thus, at the time of measurement, the DAT availability will reflect sum of both genetic factors and environmental factors common or unique to the individual studied; factors that may counterbalance each other. Koskela et al. elegantly addressed this question and found no difference in DAT availability in monozygotic twin pairs (n = 15) with different BMI 24.5 ± 3.1 kg/m2 versus 26.8 ± 3.6 kg/m2 [15]. However, the difference in BMI between twin siblings probably was too small to definitely exclude the involvement of DAT in obesity beyond what is inherited.

The alternative hypothesis is that the DAT is not involved in the neurobiology underlying obesity in humans. In line with the finding in the present study consistent changes in DAT availability in smokers have not been demonstrated [28], and recently, we failed to demonstrate significant changes in the DAT availability in a European multicenter study of 26 active smokers (the effect of both constitutive features and chronic exposure to nicotine) and 48 ex-smokers (the effect of constitutive features) compared to 65 nonsmokers [31]. In contrast to the findings in nicotine abuse, several studies have demonstrated a reduced DAT availability in subjects both actively abusing methamphetamine and in subjects detoxified for months [32, 33]. However, reductions in DAT availability is believed to reflect a direct neurotoxic effects of methamphetamine on striatal dopamine nerve terminals mediated by DAT [34]. It is surprising that changes in DAT availability is not more apparent in substance abuse and obesity considering the importance of DAT in regulating dopaminergic neurotransmission and in particular in activating the striatal high affinity D2-like receptors [35]. The striatal D2 receptor is the binding site most consistently demonstrated to be reduced in molecular imaging studies of various forms of substance abuse [36] and obesity [11].

Molecular imaging studies using PET and SPECT have contributed importantly to the demonstration of shared neurochemical pathways of substance abuse and obesity, especially, with respect to the dopamine-based reward circuitry. Molecular imaging studies addressing D1-like and D2-like receptors in areas outside striatum involved in addiction are warranted but hampered by methodological problems related to the properties of the available radioligands and imaging areas with a low density of binding sites. Other neurotransmitters than dopamine (cannabinoids, opioids, GABA, glutamate, and acetylcholine) and neuropeptides play important roles in reward and addiction and future molecular imaging studies will shed further light on the common pathways of drug abuse and obesity but importantly also demonstrate unique differences in the addicted brain related to the unique differences in reward, conditioning/memory, executive control, and motivation [40] of the different disorders.

Conclusion

In our group of 33 subjects with BMI ranging from 21 to 49.5 kg/m2, we could not demonstrate a statistically significant correlation between DAT and BMI, as measured with [123I]PE2I SPECT. Thus, our study does not support that DAT is altered in obesity. Clinically, the results imply that [123I]PE2I SPECT images can be interpreted without taking BMI into account.

Acknowledgments

The authors thank Glenna Skouboe and Svitlana Olsen for expert technical assistance. This study was supported by the Copenhagen Hospital Corporation, the Toyota Foundation and Lundbeck Foundation. The study was performed in accordance with the ethical standards of the Declaration of Helsinki and was approved by the ethical committee of Copenhagen and Frederiksberg (K).

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