In this study, the DAT availability in the VST, caudate nucleus, and putamen did not differ with the SLC6A3 genotype (rs2652511; rs2937639). However, changes in DAT availability after glucose loading were affected by the SLC6A3 genotype (rs2652511; rs2937639). In all participants with rs2652511 CT/rs2937639 AG, striatal DAT availability was increased after glucose loading.
Eating behavior is regulated by both the homeostatic and hedonic systems of the brain (van Galen et al., 2018). The hypothalamus plays a central role in maintaining the physiologic requirements of the body, while the striatum is the major organ that regulates eating behavior through the reward system (Khanh et al., 2014). Among neurotransmitters, dopamine plays a particularly important role in reward processing (Wang et al., 2011). Unlike the dopamine receptor, the DAT, which actively translocates dopamine from the extracellular space into presynaptic neurons (Vaughan and Foster, 2013) was not thought to be involved in the neurobiology underlying obesity in humans (Thomsen et al., 2013) as there was no significant correlation between DAT availability and body mass index (BMI)(Thomsen et al., 2013; van de Giessen et al., 2013; Nam et al., 2018). We have previously highlighted the role of DAT (Pak et al., 2020). In a previous study, substantial increases in DAT availability were observed at least 18% after glucose loading, although the paired t-test of DAT availability between placebo and glucose loading did not find a significant difference (Pak et al., 2020). In addition, BMI was negatively correlated with DAT availability after glucose loading (Pak et al., 2020). In this regard, participants with a lower BMI may have 1) a higher clearance of synaptic dopamine, and 2) a lower endogenous concentration of dopamine due to higher DAT availability, leading to the stop of eating behavior (Pak et al., 2020). According to Jones et al., insulin activates the PI3K/Akt signaling pathway, enhancing the surface expression of striatal DAT in animal studies (Jones et al., 2017). However, not all participants showed an increase in striatal BPND after glucose loading equally, leading to the insignificant results in paired t-tests (Pak et al., 2020). Therefore, we hypothesized that SLC6A3 gene polymorphisms may affect the changes in DAT availability after glucose loading.
Dysfunction of the DAT has been linked to neuropsychiatric disorders, such as attention-deficit/hyperactivity disorder (Roessner et al., 2010), bipolar disorder (Mick et al., 2008), and alcoholism (Du et al., 2011). Additionally, SLC6A3 gene polymorphisms have been shown to be associated with the expression of striatal DAT (van de Giessen et al., 2009). Among polymorphisms of the SLC6A3 gene, most previous studies have focused on the 40-base-pair VNTR in the 3′ untranslated region of the SLC6A3 gene. According to a meta-analysis by Faraone et al., the 9R allele of the VNTR is associated with increased DAT availability, and the VNTR has an effect on DAT availability (Faraone et al., 2014). In a study by van de Giessen et al. (van de Giessen et al., 2009), 58.4% (45/77) of the participants had a VNTR of 10R/10R, while this was present in 81.6% (31/38) of participants included in the current study. Ethnic differences may explain this difference in the proportion of the 10R/10R VNTR in each study group. To rule out the effect of the VNTR on DAT availability, only the 31 participants with the 10R/10R VNTR were included in this study. Previously, only two studies investigated the effects of SLC6A3 gene polymorphisms other than the VNTR on DAT availability (Drgon et al., 2006; van de Giessen et al., 2009). Drgon et al. screened and identified SNPs of rs2652511 and rs2937639 at the 5′ end of the SLC6A3 gene (Drgon et al., 2006). rs2652511 is located in the 5′ flanking sequences of the SLC6A3 gene, while rs2937639 is located in intron 1 of the SLC6A3 gene (van de Giessen et al., 2009). Participants with rs2652511 C/rs2937639 G had more frequent in highly expressed DAT availability with 11C-cocaine PET (Drgon et al., 2006). However, in a study by van de Giessen et al., neither rs2652511 nor rs2937639 was associated with striatal DAT availability as measured using 123I-β-CIT SPECT (van de Giessen et al., 2009), consistent with the findings of the present study. As we discovered the changes in DAT after glucose loading in humans (Pak et al., 2020), we investigated the association between SLC6A3 genotypes (rs2652511; rs2937639) and DAT availability after glucose loading. However, the SLC6A3 genotype did not have an effect on DAT availability after glucose loading. Consistent with two previous studies, rs2652511 and rs2937639 were inherited as fixed allele combinations in 29 of 31 participants (93.5%); either rs2652511 CT/rs2937639 AG (n = 6) or rs2652511 TT/rs2937639 AA (n = 23). In all participants with rs2652511 CT/rs2937639 AG, DAT availability in the dorsal striatum (caudate nucleus and putamen) increased after glucose loading, whereas they were increased in 47.8% (caudate nucleus) and 56.5% (putamen) of participants with rs2652511 TT/rs2937639 AA. Therefore, SLC6A3 gene polymorphisms (rs2652511; rs2937639) may affect changes in DAT availability after glucose loading according to this preliminary study. However, the changes in DAT availability in the VST were not affected by the SLC6A3 genotype. The VST and dorsal striatum (caudate nucleus and putamen) are known to have distinct roles. The VST plays a major role in processing reward cues and in the motivation to seek rewards (Caravaggio et al., 2015), while the dorsal striatum (caudate nucleus and putamen) is involved in non-hedonic food motivations of caloric requirements for survival. Therefore, region-dependent DAT regulatory mechanisms (ventral vs. dorsal striatum) might exist in the response to glucose loading reflecting eating behavior.
This study has some limitations. A small number of participants were included in the study. To validate this preliminary study, further studies with larger numbers of participants are needed. To consider the effect of rs2652511 and rs2937639 genotypes, individuals with a VNTR other than 10R/10R were excluded from this study. In addition, a screening procedure might be needed to investigate other candidates of SLC6A3 gene polymorphisms that have an effect on DAT availability.
In conclusion, we have highlighted that the SLC6A3 gene polymorphism is associated with the changes in DAT availability after glucose loading. However, DAT availability after glucose or placebo loading in the VST, caudate nucleus, and putamen did not differ according to the SLC6A3 genotype. Further studies with a larger number of participants are needed to validate this finding.