Serotonergic dysfunction: brain imaging and behavioral correlates

Imaging receptor changes in human drug abusers

This chapter will review the literature on differences in the brain chemistry of alcohol- and drug-dependent individuals compared to healthy controls as measured with positron emission tomography and single photon emission computed tomography. Specifically, alterations in dopamine, serotonin, opioid, and GABA systems in cocaine, alcohol, nicotine, and heroin dependence have been examined. These neurochemical systems are integrated and play significant roles in a final common pathway mediating addiction in the brain. One recurrent finding is that dopaminergic dysfunction is prevalent in both alcohol and drug dependent populations, and specifically there is a lower availability of dopamine type 2/3 receptors in cocaine-, alcohol-, nicotine-, and heroin-dependent individuals compared to healthy controls. The development of novel radiotracers that target additional receptor systems will further our understanding of the neurochemical basis of addiction.

Multifinality in the development of personality disorders: a Biology x Sex x Environment interaction model of antisocial and borderline traits

Although antisocial personality disorder (ASPD) is more common among males and borderline PD (BPD) is more common among females, some authors have suggested that the two disorders reflect multifinal outcomes of a single etiology. This assertion is based on several overlapping symptoms and features, including trait impulsivity, emotional lability, high rates of depression and suicide, and a high likelihood of childhood abuse and/or neglect. Furthermore, rates of ASPD are elevated in the first degree relatives of those with BPD, and concurrent comorbidity rates for the two disorders are high. In this article, we present a common model of antisocial and borderline personality development. We begin by reviewing issues and problems with diagnosing and studying PDs in children and adolescents. Next, we discuss dopaminergic and serotonergic mechanisms of trait impulsivity as predisposing vulnerabilities to ASPD and BPD. Finally, we extend shared risk models for ASPD and BPD by specifying genetic loci that may confer differential vulnerability to impulsive aggression and mood dysregulation among males and impulsive self-injury and mood dysregulation among females. Although the precise mechanisms of these sex-moderated genetic vulnerabilities remain poorly understood, they appear to interact with environmental risk factors including adverse rearing environments to potentiate the development of ASPD and BPD.

Susceptibility locus in neurokinin-1 receptor gene associated with alcohol dependence

Substance P (SP), a neurotransmitter in stress pathways, exerts its effects mainly through the neurokinin-1 receptor (NK1R). Genetic and pharmacological studies show that binding of ligands to NK1R decreases anxiety-related behaviors, and therefore, self-administration of alcohol in mice and craving for alcohol in humans. As genetic variants may result in differential expression of the receptor through various molecular mechanisms, we examined whether allelic variations in the NK1R gene are associated with alcohol dependence (AD) by genotyping 11 single-nucleotide polymorphisms (SNPs) across NK1R in alcoholic (n=271) and healthy control (n=337) participants of Caucasian descent. The AD was diagnosed using the Structured Clinical Interview for Diagnostic and Statistical Manual of Mental Disorders, fourth edition. Associations of the SNPs with AD were assessed at both the individual SNP and haplotype levels. We found that genotype and allele frequencies of rs6715729, a synonymous SNP in exon 1, differed significantly in alcoholics and in controls (p=0.0006; OR (odds ratio)=6.13; 95% CI=4.06, 9.23). Haplotype analyses indicated two risk haplotypes for AD in the 5' end of the gene, formed by the three-SNP combinations rs6715729-rs735668-rs6741029. Taken together, we conclude that polymorphisms of NK1R are significantly associated with the development of AD in Caucasian individuals. Additional studies are needed to replicate these results in other samples and to elucidate the mechanism(s) by which these polymorphisms affect NK1R function in the brain.

The 5-HTTLPR polymorphism modulates the association of serious life events (SLE) and impulsivity in patients with Borderline Personality Disorder

BACKGROUND

Impulsivity belongs to the key features of Borderline Personality Disorder (BPD). It has been linked to altered serotoninergic neurotransmission and, genetically, to an over-representation of the short (S) allele of the serotonin transporter promoter-linked polymorphic region polymorphism (5-HTTLPR). On the other hand, serious life events (SLE) are of major importance in the development of BPD. However, the inter-relations between SLEs, impulsivity, and 5-HTTLPR are not understood.

METHOD

159 BPD patients from Germany were included in this study. Impulsivity was assessed by the Barratt Impulsiveness Scale (BIS). We analysed (1) the effects of SLEs on impulsivity; and (2) modulating effects of the 5-HTTLPR polymorphism on the effects of SLEs on impulsivity.

RESULTS

Regression analyses confirmed a decreasing effect of childhood sexual abuse, the cumulative SLE-related reactions and the impairment by SLEs on BIS sum score. Regarding BIS sum score, all SLEs except for rape were associated with a decrease of impulsivity in SS/SL carriers and an increase of BIS sum score in LL carriers.

CONCLUSIONS

This study analyzing a specific gene x environment interaction in BPD patients suggests an interaction between SLEs and the 5-HTTLPR S/L polymorphism in the development of impulsivity in BPD patients. Clinical and research implications are discussed.

Characterization of a functional polymorphism in the 3' UTR of SLC6A4 and its association with drinking intensity

BACKGROUND

The propensity for severe drinking is hypothesized to be regulated by differential expression of serotonin transporter gene (SLC6A4) in the human brain. The SLC6A4 promoter region 5-HTTLPR has been examined previously as a candidate polymorphic variant associated with severe drinking. In this study, we investigated whether other SLC6A4 single nucleotide polymorphisms (SNPs) are associated with drinking intensity among treatment-seeking alcoholics and whether these polymorphic variants result in differential SLC6A4 expression levels.

METHODS

We analyzed associations of drinking intensity in 275 (78.5% male) treatment-seeking alcoholics of Caucasian and Hispanic origin, with 6 SLC6A4 polymorphisms. Next, to examine the functionality of the SNP that showed a significant association with drinking intensity, we transfected the 2 alleles of rs1042173 into HeLa cell cultures and measured serotonin transporter mRNA and protein expression levels by using qRT-PCR and western blotting techniques.

RESULTS

One of the 6 polymorphisms we examined, rs1042173 in the 3' untranslated region (3'-UTR) of SLC6A4, showed a significant association with drinking intensity. The G allele carriers for rs1042173 were associated with significantly lower drinking intensity (p = 0.0034) compared to T-allele homozygotes. In HeLa cell cultures, the cells transfected with G allele showed a significantly higher mRNA and protein levels than the T allele-transfected cells.

CONCLUSION

These findings suggest that the allelic variations of rs1042173 affect drinking intensity in alcoholics possibly by altering serotonin transporter expression levels. This provides additional support to the hypothesis that SLC6A4 polymorphisms play an important role in regulating propensity for severe drinking.

The role of duloxetine in the treatment of anxiety disorders

Anxiety disorders (ADs) are the most common type of psychiatric disorders, with a mean incidence of 18.1% and a lifetime prevalence of 28.8%. Pharmacologic options studied for treating ADs may include benzodiazepines, tricyclic antidepressants (TCA), selective serotonin reuptake inhibitors (SSRIs), noradrenergic and specific serotonergic drug (NaSSA) and dual-reuptake inhibitors of serotonin and norepinephrine (SNRIs). In this context, the development of SNRIs (venlafaxine and duloxetine) has been particularly useful. As a dual-acting intervention that targets two neurotransmitter systems, these medications would appePar promising for the treatment of ADs. The purpose of this review was to elucidate current facts and views about the role of duloxetine in the treatment of ADs. In February 2007, duloxetine was approved by FDA for the treatment of generalized anxiety disorder (GAD). The results of trials evaluating the use duloxetine in the treatment of GAD are supportive on its efficacy even if further studies on long-term use are needed. Apart from some interesting case reports, no large studies are, to date, present in literature about duloxetine and other ADs such as panic disorder, social anxiety disorder, obsessive-compulsive disorder and post-traumatic stress disorder. Therefore, the clinical efficacy and the relative good tolerability of duloxetine may be further investigated to widen the therapeutic spectrum of ADs.

Serotonergic genes and amygdala activity in response to negative affective facial stimuli in Korean women

Serotonergic genes have been implicated in mood disorders, alcoholism and certain personality traits. We investigated the possible relationship between several polymorphisms in the serotonin (5-HT) system and amygdala responses to negative facial stimuli in Korean women using functional magnetic resonance imaging. All participants were genotyped with regard to the following polymorphisms: the serotonin transporter-gene-linked polymorphic region (5-HTTLPR), tryptophan hydroxylase 2 (TPH2) G(-703)T, 5-HT(1A) C(-1019)G and 5-HT(2A) single nucleotide polymorphism (SNP) rs6311. We found increased activations in response to angry facial stimuli in the bilateral amygdala of subjects with the long allele of 5-HTTLPR compared with those with two copies of the short allele. Higher activations in response to sad facial stimuli were found in the bilateral amygdala of subjects with the T/T genotype of 5-HT(2A) SNP rs6311, compared with C allele carriers, and in subjects with the G/G genotype of TPH2 G(-703)T, compared with those with T/T and G/T genotypes. Our results for individuals from an Asian population countered a previous finding for a Caucasian population and identified the moderating role of genetic background in the relationships between these serotonergic gene polymorphisms and amygdala function elicited by negative emotional stimuli.

Depression like characteristics of 5HTTLPR polymorphism and temperament in excessive internet users

INTRODUCTION

Excessive internet use (EIU) has been reported to be comorbid with depression and the manifestation of its symptoms. This study examines the characteristics of excessive internet users that are similar to those of patients with depressive disorders in terms of serotonin transporter gene expression and harm avoidance.

METHODS

91 male adolescents with EIU and 75 healthy comparison subjects were recruited. Between group comparisons were made on genetic polymorphisms of the serotonin transport gene and with respect to novelty seeking and harm avoidance (HA) of Cloninger's Temperament Character Inventory.

RESULTS

The homozygous short allelic variant of the serotonin transporter gene (SS-5HTTLPR) is more frequent in the EIU group (chi(2)=4.38, df=1, p<0.05). The HA and Beck Depression Inventory (BDI) scores were significantly higher in the EIU group than in the healthy comparison group (t=7.03, df=164, p<0.01; t=2.12, df=164, p=0.04). EIU subjects expressing SS-5HTTLPR also showed higher HA (HA1, HA2, HA4, and total HA) and Young's internet addiction scale scores than EIU subjects expressing the other serotonin transporter gene allele variants (t=2.47, df=89, p=0.01; t=2.33, df=89, p=0.02; t=2.17, df=89, p=0.03; t=2.25, df=89, p=0.03; t=2.93, df=89, p<0.01 respectively).

CONCLUSIONS

The EIU group had higher SS-5HTTLPR frequencies, harm avoidance, and BDI scores. SS-5HTTLPR expression was closely related to harm avoidance in EIU. The results of this study suggest that EIU subjects may have genetic and personality traits similar to depressed patients.

Ethanol-BDNF interactions: still more questions than answers

Brain-derived neurotrophic factor (BDNF) has emerged as a regulator of development, plasticity and, recently, addiction. Decreased neurotrophic activity may be involved in ethanol-induced neurodegeneration in the adult brain and in the etiology of alcohol-related neurodevelopmental disorders. This can occur through decreased expression of BDNF or through inability of the receptor to transduce signals in the presence of ethanol. In contrast, recent studies implicate region-specific up-regulation of BDNF and associated signaling pathways in anxiety, addiction and homeostasis after ethanol exposure. Anxiety and depression are precipitating factors for substance abuse and these disorders also involve region-specific changes in BDNF in both pathogenesis and response to pharmacotherapy. Polymorphisms in the genes coding for BDNF and its receptor TrkB are linked to affective, substance abuse and appetitive disorders and therefore may play a role in the development of alcoholism. This review summarizes historical and pre-clinical data on BDNF and TrkB as it relates to ethanol toxicity and addiction. Many unresolved questions about region-specific changes in BDNF expression and the precise role of BDNF in neuropsychiatric disorders and addiction remain to be elucidated. Resolution of these questions will require significant integration of the literature on addiction and comorbid psychiatric disorders that contribute to the development of alcoholism.

Long-term citalopram maintenance in mice: selective reduction of alcohol-heightened aggression

BACKGROUND

Selective serotonin reuptake inhibitors (SSRIs) alleviate many affective disturbances in human clinical populations and are used in animal models to study the influence of serotonin (5-HT) on aggressive behavior and impulsivity.

OBJECTIVE

We hypothesized that long-term SSRI treatment may reduce aggressive behavior escalated by alcohol consumption in mice. Therefore, aggression was tested in male CFW mice to determine whether repeated citalopram (CIT) administration reduces alcohol-heightened aggression.

MATERIALS AND METHODS

Resident male mice self-administered alcohol by performing an operant response on a panel placed in their home cage that delivered a 6% alcohol solution. Mice repeatedly confronted an intruder 15 min after self-administration of either 1 g/kg alcohol (EtOH) or water (H(2)O). Aggressive behaviors were higher in most mice when tests occurred after EtOH intake relative to H(2)O. Once baseline aggression was established, animals were injected (i.p.) twice daily with 10 mg/kg CIT or saline (SAL) for 32 days. Every 4 days throughout the CIT treatment period, aggressive encounters occurred 6 h after CIT injections, with testing conditions alternating between EtOH and H(2)O intake.

RESULTS

Aggression was only modestly affected by CIT in the first 2 weeks of treatment. However, by day 17 of CIT treatment, alcohol-heightened aggressive behavior was abolished, while baseline aggression remained stable. These data lend support for the role of the 5-HT transporter in the control of alcohol-related aggressive behavior, and the time course of effects suggests that a change in density of 5HT(1A) autoreceptors is necessary before antidepressant drugs produce beneficial outcomes.

Effects of chronic alcohol drinking on receptor-binding, internalization, and degradation of human immunodeficiency virus 1 envelope protein gp120 in hepatocytes

Although alcohol drinking increases susceptibility to human immunodeficiency virus (HIV) infection, possible mechanisms underlying the effects of alcohol are not yet known. Since the HIV envelope protein gp120 plays a key role in progression of HIV infection, the aim of the present study was to evaluate the toxicity and degradation of gp120 in hepatocytes isolated from liver of alcohol-preferring rats drinking either 15% ethanol in water or pure water for 70 days. The hypothesis was that alcohol drinking augmented the toxicity, but suppressed degradation of gp120. Hepatocytes from water-drinking rats (C-cells) or ethanol-drinking rats (Et-cells) were treated with laptacystin, anti-CD4 antibodies, CCR5 antagonist, or mannose, followed by [(125)I]gp120 or native gp120. At predetermined intervals, control (C) and ethanol exposed (Et) cells were analyzed for toxicity and degradation of gp120. In C-cells, [(125)I]gp120 binding and internalization peaked within 5-45 min and remained elevated for up to 10h and then decreased gradually. In Et-cells, [(125)I]gp120 binding peaked comparably to C-cells, but the binding remained to the peak level throughout the experimental period. C-cells exhibited the lysosomal/ubiquitin-mediated degradation of intracellular gp120, resulting in released gp120 fragments into the incubation medium that suppressed gp120-CD4 binding, improved cell viability, and inhibited gp120-induced apoptosis. Ethanol drinking suppressed gp120 degradation in and release of gp120 fragments from hepatocytes. The incubation medium of Et-cells did not suppress gp120-CD4 binding or the gp120-mediated apoptosis in hepatocytes. Thus, chronic alcohol drinking augmented the adverse effects of gp120 possibly by suppressing its degradation in hepatocytes. The present observation also suggests that a number of CCR5 or ubiquitin-based therapeutic drugs may not be effective in suppressing HIV infection in alcohol-drinking subjects.

Inhibition of 5-HT neuron activity and induction of depressive-like behavior by high-frequency stimulation of the subthalamic nucleus

Bilateral, high-frequency stimulation (HFS) of the subthalamic nucleus (STN) is the surgical therapy of choice for movement disability in advanced Parkinson's disease (PD), but this procedure evokes debilitating psychiatric effects, including depressed mood, of unknown neural origin. Here, we report the unexpected finding that HFS of the STN inhibits midbrain 5-hydroxytryptamine (5-HT) neurons to evoke depression-related behavioral changes. We found that bilateral HFS of the STN consistently inhibited (40-50%) the firing rate of 5-HT neurons in the dorsal raphe nucleus of the rat, but not neighboring non-5-HT neurons. This effect was apparent at clinically relevant stimulation parameters (> or =100 Hz, > or =30 microA), was not elicited by HFS of either neighboring or remote structures to the STN, and was still present in rat models of PD. We also found that bilateral HFS of the STN evoked clear-cut, depressive-like behavior in a widely used experimental paradigm of depression (forced swim test), and this effect was also observed in a PD model. Importantly, the depressive-like behavior elicited by HFS of the STN was reversed by a selective 5-HT-enhancing antidepressant, thereby linking the behavioral change to decreased 5-HT neuronal activity. Overall, these findings link reduced 5-HT function to the psychiatric effects of HFS of the STN observed in PD patients and provide a rational basis for their clinical management. More generally, the powerful interaction between the STN and 5-HT system uncovered here offers insights into the high level of comorbidity of basal ganglia disease and mood disorder.

Cell-type specific induction of tryptophan hydroxylase-2 transcription by calcium mobilization

Alterations in brain serotonin levels are implicated in major depression and are regulated by tryptophan hydroxylase-2 (TPH2). To study its regulation, we measured TPH2 RNA by quantitative RT-PCR in differentiated serotonergic rat raphe RN46A and GH4C1 pituitary cells, which express TPH2. Upon calcium mobilization using KCl (40 mmol/L), TPH2 RNA was rapidly (1 h) and strongly (> 10-fold) induced in differentiated RN46A cells, but not in GH4C1 cells. This effect was blocked by actinomycin D, implicating transcriptional activation. Similarly, calcium ionophore ionomycin induced TPH2 RNA by threefold in RN46A cells. To address the promoter sites involved, the transcription start site was identified and a series of TATA-containing TPH2 promoter-luciferase constructs were analyzed. In differentiated RN46A cells, the TPH2 promoter was induced 2.5-fold by ionomycin, similar to its action on TPH2 RNA. By contrast, ionomycin had no effect on TPH2 promoter activity in GH4C1 cells or TPH2-negative L6 myoblasts. Ionomycin sensitivity was localized to within 88 bp of the start site, containing putative CCATT-enhancer binding protein element, activator protein-1 and -2 (AP-1, AP-2) elements. These results are the first to identify calcium-mediated regulation of the proximal TPH2 promoter as critical for cell-specific TPH2 expression.

Genes, brain, and behavior: bridging disciplines

With excitement surrounding the publication of the human genome, scientists have set out to uncover the functions of specific genes. This special issue on Genes, Brain, and Behavior attempts to present research strategies that connect major avenues of genetic research across disciplines. For example, anatomical information provided by brain imaging can serve as a convenient link between anatomical abnormalities seen in knockout/transgenic mouse models and abnormal patterns of brain activity seen in certain patient populations. Identifying genetic risk factors for disorders with carefully designed cognitive assays is another strategy that has gained increasing attention. These approaches are being combined with behavioral studies of mouse models of gene function. Alone, each of these approaches provides limited information on gene function in complex human behavior, but together, they are forming bridges between animal models and human psychiatric disorders.