Dopamine transporter gene may be associated with bipolar disorder and its personality traits

Dysregulation of oxytocin and dopamine in the corticostriatal circuitry in bipolar II disorder

The oxytocin (OXT) and dopamine systems synergistically facilitate striatal reactivity. Abnormal striatal activation has repeatedly been observed in patients with bipolar disorder (BD); however, such abnormality remains unclear in BD II. Here we aimed to investigate whether the corticostriatal connectivity was altered and the possible relationships among corticostriatal connectivity, OXT, and dopamine systems in BD II. Twenty-five BD II patients, as defined by the DSM-V, and 29 healthy controls (HC) were enrolled in this study. Plasma OXT was measured and striatal dopamine transporter (DAT) availability was assessed using [^99mTc]TRODAT-1 single-photon emission computed tomography (SPECT). Brain network functional connectivity (FC) was measured during the resting-state using functional magnetic resonance imaging, and the dorsal caudate (DC) was selected as the seed region. The results showed that the OXT level was significantly lower in the BD II patients, while the striatal DAT availability was not significantly different between the BD II and HC groups. The BD II patients exhibited significantly lower FC between the DC and the executive control network (dorsolateral prefrontal, anterior cingulate cortex, and posterior parietal cortex) as compared with the HC. Only observed in HC, the DC-posterior parietal cortex FC was negatively correlated with the OXT level and striatal DAT availability. Our findings in the HC support a model in which the OXT and dopamine systems act in tandem to regulate corticostriatal circuitry, while the synergistic interaction was perturbed in BD II. Taken together, these results implied a maladaptive neuroplasticity in BD II.

Short-active photoperiod gestation induces psychiatry-relevant behavior in healthy mice but a resiliency to such effects are seen in mice with reduced dopamine transporter expression

A higher incidence of multiple psychiatric disorders occurs in people born in late winter/early spring. Reduced light exposure/activity level impacts adult rodent behavior and neural mechanisms, yet few studies have investigated such light exposure on gestating fetuses. A dysfunctional dopamine system is implicated in most psychiatric disorders, and genetic polymorphisms reducing expression of the dopamine transporter (DAT) are associated with some conditions. Furthermore, adult mice with reduced DAT expression (DAT-HT) were hypersensitive to short active (SA; 19:5 L:D) photoperiod exposure versus their wildtype (WT) littermates. Effects of SA photoperiod exposure during gestation in these mice have not been examined. We confirmed adult females exhibit a heightened corticosterone response when in SA photoperiod. We then tested DAT-HT mice and WT littermates in psychiatry-relevant behavioral tests after SA or normal active (NA; 12:12 L:D) photoperiod exposure during gestation and early life. SA-born WT mice exhibited sensorimotor gating deficits (males), increased reward preference, less immobility, open arm avoidance (females), less motivation to obtain a reward, and reversal learning deficits, vs. NA-born WT mice. DAT-HT mice were largely resilient to these effects, however. Future studies will determine the mechanism(s) by which SA photoperiod exposure influences brain development to predispose toward emergence of psychiatry-relevant behaviors.

Whole-brain mapping of socially isolated zebrafish reveals that lonely fish are not loners

The zebrafish was used to assess the impact of social isolation on behaviour and brain function. As in humans and other social species, early social deprivation reduced social preference in juvenile zebrafish. Whole-brain functional maps of anti-social isolated (lonely) fish were distinct from anti-social (loner) fish found in the normal population. These isolation-induced activity changes revealed profound disruption of neural activity in brain areas linked to social behaviour, social cue processing, and anxiety/stress. Several of the affected regions are modulated by serotonin, and we found that social preference in isolated fish could be rescued by acutely reducing serotonin levels.

Socialising is good for people’s mental health and wellbeing. The connections and relationships that we form can make us more resilient and healthier. Researchers also know that prolonged periods of social isolation, and feeling lonely, can be detrimental to our health, especially in early childhood. The paradox is that loneliness often results in an even lower desire for social contact, leading to further isolation. But not everyone craves social contact. Some people prefer to be alone and feel more comfortable avoiding social interaction.

Zebrafish display the same social preferences. This, along with their transparent brains, makes them a useful model to study the links between social behaviour and brain activity. Like humans, zebrafish are social animals, with most fish taking a strong liking to social interactions by the time they are a few weeks old. A small number of ‘loner’ fish, however, prefer to avoid interacting with their siblings or tank mates. And so, if loneliness quells the desire for more social contact, the question becomes, does isolation turn otherwise social fish into loners?

Here, Tunbak et al. use zebrafish to study how social isolation changes brain activity and behaviour. Social fish were isolated from others in the tank for a few days. These so-called ‘lonely fish’ were then allowed back in contact with the other fish. This revealed that, after isolation, previously social fish did avoid interacting with others.

With this experimental set-up, Tunbak et al. also compared the brains of lonely and loner fish. When fish that prefer social interaction were deprived of social contact, they had increased activity in areas of the brain related to stress and anxiety. These lonely fish became anxious and very sensitive to stimuli; and their brain activity suggested that social interaction became overwhelming rather than rewarding. Positively, the lonely fish quickly recovered their normal, social behaviour when given a drug that reduces anxiety.

This work provides a glimpse into how human behaviour could be affected by lengthy periods in isolation. These results suggest that humans could feel anxious upon returning to normal life after spending a long time alone. Moreover, the findings show the impact that social interaction and isolation can have on the young, developing brain.

A Meta-analysis of the Association Between SLC6A3 Gene Polymorphisms and Schizophrenia

The dopamine transporter is coded by the SLC6A3 gene and plays an important role in regulation of the neurotransmitter dopamine. To detect the association between the SLC6A3 gene and the risk of schizophrenia, 31 case-control articles were included in this meta-analysis. There were 23 studies with 40 bp VNTR (3246 cases and 3639 controls), 4 studies with rs40184 (2020 cases and 1674 controls), rs6347 (1317 cases and 1917 controls), rs403636 (2045 cases and 1704 controls), and rs2975226 (849 cases and 904 controls); and 3 studies with rs12516948 (1920 cases and 1569 controls), rs27072 (984 cases and 1015 controls), rs6869645 (1142 cases and 1082 controls), rs37022 (1168 cases and 1091 controls), rs464049 (1169cases and 1096 controls), rs2652511 (707 cases and 714 controls), and rs3756450 (1176 cases and 1096 controls). Pooled, subgroup, and sensitivity analyses were performed, and the results were visualized by forest and funnel plots. In the dominant genetic model, the genotype AA+AT of rs2975226 in the Indian population (P_z = 0, odds ratio [OR] = 3.245, 95% confidence interval [CI] = 1.806-5.831), TT of rs464049 (P_z = 0.002, OR = 1.389, 95% CI = 1.129-1.708), and TT of rs3756450 (P_z = 0.014, OR = 1.251, 95% CI = 1.047-1.495) might be risk factors for schizophrenia. Additionally, no other single nucleotide polymorphisms were observed. These results indicate that more functional studies are warranted.

Presence of recombination hotspots throughout SLC6A3

The human dopamine transporter gene SLC6A3 is involved in substance use disorders (SUDs) among many other common neuropsychiatric illnesses but allelic association results including those with its classic genetic markers 3'VNTR or Int8VNTR remain mixed and unexplainable. To better understand the genetics for reproducible association signals, we report the presence of recombination hotspots based on sequencing of the entire 5' promoter regions in two small SUDs cohorts, 30 African Americans (AAs) and 30 European Americans (EAs). Recombination rate was the highest near the transcription start site (TSS) in both cohorts. In addition, each cohort carried 57 different promoter haplotypes out of 60 and no haplotypes were shared between the two ethnicities. A quarter of the haplotypes evolved in an ethnicity-specific manner. Finally, analysis of five hundred subjects of European ancestry, from the 1000 Genome Project, confirmed the promoter recombination hotspots and also revealed several additional ones in non-coding regions only. These findings provide an explanation for the mixed results as well as guidance for selection of effective markers to be used in next generation association validation (NGAV), facilitating the delineation of pathogenic variation in this critical neuropsychiatric gene.

The dopamine transporter role in psychiatric phenotypes

The dopamine transporter (DAT) is one of the most relevant and investigated neurotransmitter transporters. DAT is a plasma membrane protein which plays a homeostatic role, controlling both extracellular and intracellular concentrations of dopamine (DA). Since unbalanced DA levels are known to be involved in numerous mental disorders, a wealth of investigations has provided valuable insights concerning DAT role into normal brain functioning and pathological processes. Briefly, this extensive but non-systematic review discusses what is recently known about the role of SLC6A3 gene which encodes the dopamine transporter in psychiatric phenotypes. DAT protein, SLC6A3 gene, animal models, neuropsychology, and neuroimaging investigations are also concisely discussed. To conclude, current challenges are reviewed in order to provide perspectives for future studies.

Evaluation of VNTR polymorphisms of dopamine transporter gene and the risk of bipolar disorder in Zahedan, southeast Iran

The exact role of dopamine transporter gene (DAT1) in the pathogenesis of bipolar disorder type 1 (BD) is not understood. In the present study, we aimed to evaluate the possible association between 30, 40 and 63 bp variable number tandem repeat (VNTR) polymorphisms of DAT1 gene and the risk of type 1 (BD) in a sample of Iranian population. This case-control study was performed on 152 BD patients and 153 psychiatrically healthy subjects. Genotyping of the variant was done by polymerase chain reaction method. Totally, the findings did not support an association between DAT1 VNTR polymorphisms and the risk of BD in a sample of southeast Iranian population.

The SLC6A3 gene possibly affects susceptibility to late-onset alcohol dependence but not specific personality traits in a Han Chinese population

Dopaminergic dysfunction has an important role in the pathoetiology of alcohol dependence (AD). The purpose of this study was to determine whether the solute carrier family 6 member 3 (SLC6A3) gene (also known as the dopamine transporter DAT gene) was associated with AD, and whether variants in the SLC6A3 locus were associated with specific personality traits in patients with AD. Sixteen polymorphisms in SLC6A3 were analyzed using 637 patients with AD and 523 healthy controls. To reduce clinical heterogeneity, patients were classified into two subgroups: early-onset AD (EOAD) and late-onset AD (LOAD). The Tridimensional Personality Questionnaire was used to assess the personality traits novelty seeking (NS) and harm avoidance (HA) in the patients with AD. Using allele frequency and genotype distribution comparisons and logistic regression analysis, we found evidence of association between rs6350 and AD (P < 0.05). Following subgroup analysis, we confirmed evidence of an association in patients with LOAD (P = 0.003), but not in patients with EOAD. Heterozygous carriers of the A allele have a nearly 3 times greater risk to develop LOAD compared to individuals who do not have an A allele. Although we found that patients with AD had higher NS and HA scores compared to controls (P < 0.001), we did not find evidence of association between SLC6A3 polymorphisms and either NS or HA in patients with AD using linear regression analysis. The findings from our study indicate that the SLC6A3 gene may have a role in susceptibility to late-onset AD in the Han Chinese population.

Research Domain Criteria versus DSM V: How does this debate affect attempts to model corticostriatal dysfunction in animals?

For decades, the nosology of mental illness has been based largely upon the descriptions in the Diagnostic and Statistical Manual of the American Psychiatric Association (DSM). A recent challenge to the DSM approach to psychiatric nosology from the National Institute on Mental Health (USA) defines Research Domain Criteria (RDoC) as an alternative. For RDoC, psychiatric illnesses are not defined as discrete categories, but instead as specific behavioral dysfunctions irrespective of DSM diagnostic categories. This approach was driven by two primary weaknesses noted in the DSM: (1) the same symptoms occur in very different disease states; and (2) DSM criteria lack grounding in the underlying biological causes of mental illness. RDoC intends to ground psychiatric nosology in those underlying mechanisms. This review addresses the suitability of RDoC vs. DSM from the view of modeling mental illness in animals. A consideration of all types of psychiatric dysfunction is beyond the scope of this review, which will focus on models of conditions associated with frontostriatal dysfunction.

A meta-analysis of temperament and character dimensions in patients with mood disorders: Comparison to healthy controls and unaffected siblings

BACKGROUND

Cloninger's psychobiological model of personality has been extensively applied to subjects affected by mood disorders (MOOD). However, most studies are widely heterogeneous in terms of sample size, methods of assessment, and selection of participants.

METHODS

We conducted a systematic review of literature and a random effects meta-analysis of studies comparing at least two of the following groups: (a) adults with a primary MOOD diagnosis (Bipolar Disorder (BP) or major depressive disorder (MDD)), (b) their unaffected siblings (SIB) or (c) healthy subjects (HS), and reporting quantitative results from the Tridimensional Personality Questionnaire (TPQ) or the Temperament and Character Inventory (TCI). Subgroup, sensitivity and meta-regression analyses were also conducted.

RESULTS

High Harm Avoidance and low Self-Directedness were consistently associated with MOOD and SIB samples. BP was characterized by higher scores in Novelty Seeking and Self-Transcendence than HS, SIB and MDD. Age seemed to have a negative effect on Novelty Seeking and a positive effect on Harm Avoidance, Cooperativeness and Self-Transcendence. An euthymic mood state was associated with reduced Harm Avoidance, but increased Reward Dependence, Self-Directedness and Cooperativeness.

LIMITATIONS

The quality of the included studies varied and was relatively low. Moreover, publication bias and heterogeneity in the distribution of effect sizes may also have limited our results.

CONCLUSION

High Harm Avoidance and Low Self-Directedness may be trait markers for MOOD in general, while high Novelty Seeking and high Self-Transcendence may be specific to BP. Future studies are needed to disentangle the state-trait effect of each personality dimension.