A study in male and female 5-HT transporter knockout rats: an animal model for anxiety and depression disorders

Boron-Doped Diamond Microelectrodes Reveal Reduced Serotonin Uptake Rates in Lymphocytes from Adult Rhesus Monkeys Carrying the Short Allele of the 5-HTTLPR

Uptake resolved by high-speed chronoamperometry on a second-by-second basis has revealed important differences in brain serotonin transporter function associated with genetic variability. Here, we use chronoamperometry to investigate variations in serotonin transport in primary lymphocytes associated with the rhesus serotonin transporter gene-linked polymorphism (rh5-HTTLPR), a promoter polymorphism whose orthologs occur only in higher order primates including humans. Serotonin clearance by lymphocytes is Na(+)-dependent and inhibited by the serotonin-selective reuptake inhibitor paroxetine (Paxil®), indicative of active uptake by serotonin transporters. Moreover, reductions in serotonin uptake rates are evident in lymphocytes from monkeys with one or two copies of the short 's' allele of the rh5-HTTLPR (s/s<s/l<l/l). These findings illustrate that rh5-HTTLPR-related alterations in serotonin uptake are present during adulthood in peripheral blood cells natively expressing serotonin transporters. Moreover, they suggest that lymphocytes can be used as peripheral biomarkers for investigating genetic or pharmacologic alterations in serotonin transporter function. Use of boron-doped diamond microelectrodes for measuring serotonin uptake, in contrast to carbon fiber microelectrodes used previously in the brain, enabled these high-sensitivity and high-resolution measurements. Boron-doped diamond microelectrodes show excellent signal-to-noise and signal-to-background ratios due mainly to low background currents and are highly resistant to fouling when exposed to lymphocytes or high concentrations of serotonin.

Rat genomics applied to psychiatric research

Psychiatric diseases are very debilitating and some of them highly prevalent (e.g., depression or anxiety). The rat remains one model of choice in this discipline to investigate the neural mechanisms underlying normal and pathological traits. Genomic tools are now applied to identify genes involved in psychiatric illnesses and also to provide new biomarkers for diagnostic and prognosis, new targets for treatment and more generally to better understand the functioning of the brain. In this report, we will review rat models, behavioral approaches used to model psychiatry-related traits and the major studies published in the field including genetic mapping of quantitative trait loci (QTL), transcriptomics, proteomics and transgenic models.

Serotonin transporter deficiency increases abdominal fat in female, but not male rats

Depression and abdominal obesity often co-occur, predominantly in women, and are associated with an increased risk for the development of glucose intolerance and subsequently type 2 diabetes. The underlying mechanisms are poorly understood. We found that female, but not male, depression-prone serotonin transporter knockout (SERT(-/-)) rats had a strong increase (54%) in abdominal fat, whereas no increases in plasma concentrations of glucose and insulin were observed. Surprisingly, application of a high-fat, high-sucrose (HFHS)-choice diet, which results in increased abdominal fat deposition and increased plasma glucose levels in wild-type rats, did not result in elevated plasma glucose levels in female SERT(-/-) rats. Our results show that serotonin transporter deficiency affects abdominal fat deposition in a sex-dependent way, but protects against rises in glucose levels, and thereby potentially glucose intolerance. The increased abdominal fat formation could result from serotonin-mediated developmental changes and provides heuristic value for understanding the effects of the depression-associated serotonin transporter promoter polymorphism in humans.

Blockade of dopamine, but not noradrenaline, transporters produces hyperthermia in rats that lack serotonin transporters

To investigate whether life-long disturbed serotonin neurotransmission may result in adaptive changes of dopaminergic and noradrenergic systems, effects of drugs on stress-induced hyperthermia were studied in serotonin transporter knockout rats. The noradrenalin transporter blocker atomoxetine was more effective in reducing stress-induced hyperthermia, induced by an injection, in serotonin transporter (SERT) knockout (SERT(-/-)) rats compared to SERT(+/+) rats. The dopamine transporter blocker GBR12909 increased the core body temperature in SERT(-/-) rats, and had no effect on the SERT(+/+) rats. Finally, the noradrenalin transporter together with dopamine transporter blocker bupropion was more effective in decreasing the stress of an injection in SERT(-/-) rats than in SERT(+/+) rats. These data suggest that the sensitivity of dopamine and noradrenalin receptors is changed in serotonin transporter knockout rats. The lack of the serotonin transporter in SERT(-/-) rats might reflect humans with a life-long disturbed serotonin system, making this rat a good model to study possible changes in dopaminergic and noradrenergic systems in psychiatric disorders.

Altered expression and modulation of activity-regulated cytoskeletal associated protein (Arc) in serotonin transporter knockout rats

A gene variant in the human serotonin transporter (SERT) can increase the vulnerability to mood disorders. SERT knockout animals show similarities to the human condition and represent an important tool to investigate the mechanisms underlying the pathologic condition in humans. Along this line of thinking, we used SERT KO rats (SERT(+/-) and SERT(-/-)) to investigate abnormalities in the expression and function of the activity-regulated gene Arc (Activity-regulated cytoskeletal associated protein) and the early inducible gene Zif-268, (zinc finger binding protein clone 268), which are important players in neuronal plasticity. We found lower basal Arc mRNA levels in hippocampus and prefrontal cortex of mutant rats in comparison with wild-type animals. Moreover SERT mutant rats show altered stress responsiveness. Indeed an acute swim stress significantly up-regulated the levels of Arc mRNA in hippocampus and prefrontal cortex, as well as of Zif-268 in frontal cortex, only in SERT(+/-) and SERT(-/-) rats. These alterations may be associated to behavioral traits linked to SERT and may contribute to the neuroplastic and morphological changes observed in depression.

Chronic citalopram administration causes a sustained suppression of serotonin synthesis in the mouse forebrain

BACKGROUND

Serotonin (5-HT) is a neurotransmitter with important roles in the regulation of neurobehavioral processes, particularly those regulating affect in humans. Drugs that potentiate serotonergic neurotransmission by selectively inhibiting the reuptake of serotonin (SSRIs) are widely used for the treatment of psychiatric disorders. Although the regulation of serotonin synthesis may be an factor in SSRI efficacy, the effect of chronic SSRI administration on 5-HT synthesis is not well understood. Here, we describe effects of chronic administration of the SSRI citalopram (CIT) on 5-HT synthesis and content in the mouse forebrain.

METHODOLOGY/PRINCIPAL FINDINGS

Citalopram was administered continuously to adult male C57BL/6J mice via osmotic minipump for 2 days, 14 days or 28 days. Plasma citalopram levels were found to be within the clinical range. 5-HT synthesis was assessed using the decarboxylase inhibition method. Citalopram administration caused a suppression of 5-HT synthesis at all time points. CIT treatment also caused a reduction in forebrain 5-HIAA content. Following chronic CIT treatment, forebrain 5-HT stores were more sensitive to the depleting effects of acute decarboxylase inhibition.

CONCLUSIONS/SIGNIFICANCE

Taken together, these results demonstrate that chronic citalopram administration causes a sustained suppression of serotonin synthesis in the mouse forebrain. Furthermore, our results indicate that chronic 5-HT reuptake inhibition renders 5-HT brain stores more sensitive to alterations in serotonin synthesis. These results suggest that the regulation of 5-HT synthesis warrants consideration in efforts to develop novel antidepressant strategies.

Conserved role for the serotonin transporter gene in rat and mouse neurobehavioral endophenotypes

The serotonin transporter knockout (SERT(-/-)) mouse, generated in 1998, was followed by the SERT(-/-) rat, developed in 2006. The availability of SERT(-/-) rodents creates the unique possibility to study the conservation of gene function across species. Here we summarize SERT(-/-) mouse and rat data, and discuss species (dis)similarities in neurobehavioral endophenotypes. Both SERT(-/-) rodent models show a disturbed serotonergic system, altered nociception, higher anxiety, decreased social behavior, as well as increased negative emotionality, behavioral inhibition and decision making. Used to model a wide range of psychiatric disorders, SERT(-/-) rodents may be particularly valuable in research on neurodevelopmental disorders such as depression, anxiety, and possibly autism. We conclude that SERT function is conserved across mice and rats and that their behavioral profile arises from common neurodevelopmental alterations. Because mice and rats have species-specific characteristics that confer differential research advantages, a comparison of the two models has heuristic value in understanding the mechanisms and behavioral outcome of SERT genetic variation in humans.

Estradiol increases Pet-1 and serotonin transporter mRNA in the midbrain raphe nuclei of ovariectomized rats

Previous research has shown that estradiol increases the anorexia associated with serotonin (5-HT) neurotransmission. To examine further the putative relationship between estradiol and 5-HT, we investigated whether estradiol increases the expression of Pet-1 and the 5-HT transporter (5-HTT), two genes implicated in the development and regulation of the 5-HT system. Ovariectomized (OVX) rats (n=5-6/group) were treated with 0, 2, or 10 microg estradiol benzoate (EB) in sesame oil on 2 consecutive days. Food intake and body weight were recorded 2 days later when EB-treated rats typically display signs of behavioral estrus (e.g., reduced feeding). Following the collection of behavioral data, rats were perfused, brains were removed, and coronal sections were cut through the midbrain raphe nuclei. Pet-1 and 5-HTT mRNA levels were quantified throughout the dorsal and median raphe nuclei (DRN and MRN) by conducting in situ hybridization on free-floating tissue sections using (35)S-labeled cDNA probes. As expected, EB treatment decreased food intake and body weight on the day that modeled estrus. At this same time, EB treatment increased Pet-1 and 5-HTT mRNA levels within the DRN and MRN. We conclude that a physiologically relevant regimen of estradiol treatment in OVX rats increases Pet-1 and 5-HTT mRNA levels in the midbrain raphe nuclei at a time when the anorexigenic effect of estradiol is apparent. Further studies are required to determine whether the increased expression of Pet-1 and 5-HTT mRNA plays a causal role in the anorexigenic effect of estradiol.

Long-lasting behavioural and molecular alterations induced by early postnatal fluoxetine exposure are restored by chronic fluoxetine treatment in adult mice

There is evidence that antidepressant drug treatment during a critical period of postnatal development renders mice susceptible to depression- and anxiety-related behaviour in adulthood. The mechanism of how early antidepressant treatment brings about long-term effects in emotional behaviour is not yet understood, but neurotrophins, particularly brain-derived neurotrophic factor (BDNF), have been implicated in this context. We examined the long-term effects of a transient early postnatal fluoxetine treatment on depression- and anxiety-related behaviours as well as gene expression of BDNF and its receptor TrkB in C57BL/6J mice. Treatment with fluoxetine between postnatal days P4 and P21 resulted in a significant loss of body weight and long-lasting behavioural inhibition in adult mice in response to stressful events such as the light-dark or open field tests. Postnatal fluoxetine exposure also decreased behavioural despair in the forced swim test. Both body weight and behavioural alterations were restored by chronic fluoxetine treatment in adulthood. The behavioral alterations were accompanied by changes in hippocampal BDNF mRNA. Specifically, we show that early-life fluoxetine exposure resulted in the long-term upregulation of BDNF expression in adult mice. However, chromatin immunoprecipitation studies did not reveal any changes in the acetylation or trimethylation of histone H3 at the BDNF promoters. Our experiments show that behavioural and molecular changes induced by early postnatal fluoxetine administration are reversed by chronic fluoxetine treatment of adult mice to control levels.

A trans-dimensional approach to the behavioral aspects of depression

Depression, a complex mood disorder, displays high comorbidity with anxiety and cognitive disorders. To establish the extent of inter-dependence between these behavioral domains, we here undertook a systematic analysis to establish interactions between mood [assessed with the forced-swimming (FST) and sucrose consumption tests (SCT)], anxiety [elevated-plus maze (EPM) and novelty suppressed feeding (NSF) tests] and cognition (spatial memory and behavioral flexibility tests) in rats exposed to unpredictable chronic-mild-stress (uCMS). Expectedly, uCMS induced depressive-like behavior, a hyperanxious phenotype and cognitive impairment; with the exception of the measure of anxiety in the EPM, these effects were attenuated by antidepressants (imipramine, fluoxetine). Measures of mood by the FST and SCT were strongly correlated, whereas no significant correlations were found between the different measures of anxiety (EPM and NSF); likewise, measures of cognition by spatial memory and behavioral flexibility tests were poorly correlated. Inter-domain analysis revealed significant correlations between mood (FST and SCT) and anxiety-like behavior (NSF, but not EPM). Furthermore, significant correlations were found between cognitive performance (reverse learning task) and mood (FST and SCT) and anxiety-like behavior (NSF). These results demonstrate interactions between different behavioral domains that crosscut the disciplines of psychiatry and neurology.

Genetic approaches to modeling anxiety in animals

Anxiety disorders are a growing health problem world-wide. However, the causative factors, etiology, and underlying mechanisms of anxiety disorders, as for most psychiatric disorders, remain relatively poorly understood. The current status of clinical research indicates that anxiety traits and anxiety disorder in man have a genetic component, and therefore genetic modeling in animals is a logical approach to gain a greater insight into their neurobiology. However, it is also clear that the nature of these genetic contributions is highly complex. Moreover, the success of this approach is largely contingent upon the utility of available behavioral paradigms for modeling anxiety-related behaviors in mice. Animal genetic models provide a unique and comprehensive methodological tool to aid discovery into the etiology, neurobiology, and ultimately, the therapy of human anxiety disorders. The approach, however, is challenged with a number of complexities. In particular, the heterogeneous nature of anxiety disorders in man coupled with the associated multifaceted and descriptive diagnostic criteria, create challenges in both animal modeling and in clinical research. In this article, we describe some of the powerful modem genetic techniques that are uniquely amenable to the laboratory mouse and thus provide a strategy for approaching some of these challenges. Moreover, we focus on recent advances which have highlighted the relative contribution of genetic modeling in animals to the understanding of underlying neurobiology and genetic basis of anxiety disorders.

Pitfalls in the interpretation of genetic and pharmacological effects on anxiety-like behaviour in rodents

Over the last 15 years, genetically modified mice have added important data to our knowledge on psychiatric diseases including anxiety. This has produced many behavioural publications, partially by non-behaviourists, in which differences between mutants and normal wild-type animals were described. The popularity of these novel tools allowing the study of new mechanisms also, however, led to observations that could not be confirmed. This review attempts to summarize various factors that can lead to difficult and partially incorrect interpretation of data collected in anxiety-related paradigms. These pitfalls are explained by using virtual data. Our analysis illustrates that determining anxiety in rodents is more complicated than measuring a single parameter in a particular paradigm. It is important to use proper controls such as additional measures in the same or other procedures, as well as a conservative estimation of the chance of finding an actual effect. In this way, it is possible to enhance confidence in the findings. Alternative explanations for findings, like side effects or main effects in a different domain, such as cognition, should always be taken into account. Finally, several examples from the literature are presented as illustrations of the theoretical issues discussed. We believe that considering the pitfalls presented here will help researchers to design optimized experiments that can be more readily interpreted and replicated across laboratories.

How the serotonin story is being rewritten by new gene-based discoveries principally related to SLC6A4, the serotonin transporter gene, which functions to influence all cellular serotonin systems

Discovered and crystallized over sixty years ago, serotonin's important functions in the brain and body were identified over the ensuing years by neurochemical, physiological and pharmacological investigations. This 2008 M. Rapport Memorial Serotonin Review focuses on some of the most recent discoveries involving serotonin that are based on genetic methodologies. These include examples of the consequences that result from direct serotonergic gene manipulation (gene deletion or overexpression) in mice and other species; an evaluation of some phenotypes related to functional human serotonergic gene variants, particularly in SLC6A4, the serotonin transporter gene; and finally, a consideration of the pharmacogenomics of serotonergic drugs with respect to both their therapeutic actions and side effects. The serotonin transporter (SERT) has been the most comprehensively studied of the serotonin system molecular components, and will be the primary focus of this review. We provide in-depth examples of gene-based discoveries primarily related to SLC6A4 that have clarified serotonin's many important homeostatic functions in humans, non-human primates, mice and other species.

Acute tryptophan depletion dose dependently impairs object memory in serotonin transporter knockout rats

RATIONALE

Acute tryptophan depletion (ATD) transiently lowers central serotonin levels and can induce depressive mood states and cognitive defects. Previous studies have shown that ATD impairs object recognition in rats.

OBJECTIVES

As individual differences exist in central serotonin neurotransmission, the impact of ATD may vary accordingly. In this experiment, we investigated the hypothesis that male serotonin transporter knockout (SERT(-/-)), rats marked by a lower SERT function, are more vulnerable to the effects of ATD in an object recognition task than male wildtype (SERT(+/+)) and heterozygous (SERT(+/-)) rats.

MATERIALS AND METHODS

Twelve male SERT(+/+), SERT(+/-), and SERT(-/-) rats were treated with standard dose and low-dose ATD using a gelatine-based protein-carbohydrate mixture lacking tryptophan. In the control treatment, L: -tryptophan was added to the mixture. Four hours after treatment, the rats were subjected to the object recognition task. In addition, the effects of ATD on plasma amino acid concentrations were measured, and concentrations of 5-HT and 5-HIAA were measured in the frontal cortex and hippocampus of these rats.

RESULTS

Plasma TRP levels and central 5-HT and 5-HIAA levels were decreased in all genotypes after ATD, but effects were stronger in SERT(-/-) rats. The standard dose of ATD impaired object recognition in all genotypes. SERT(-/-) and SERT(+/-) rats were more vulnerable to low dose of ATD in the object recognition task compared to SERT(+/+) rats.

CONCLUSIONS

These results indicate a greater sensitivity to ATD in SERT(-/-) and SERT(+/-) rats, which may be related to stronger central depletion effects in these rats.

Role of serotonin in obsessive–compulsive disorder

Obsessive–compulsive disorder (OCD) is a psychiatric disorder consisting of obsessions and compulsions. Over the past two decades, it has been suggested that OCD might be related to the functioning of brain serotonin systems, mainly because of the antiobsessional efficacy of selective serotonin-reuptake inhibitors (SSRIs). Although the efficacy of SSRIs suggests a role of the serotonergic system in OCD, the exact function of serotonin is still unclear. Is the serotonergic system implicated in the pathophysiology of OCD, or is it implicated in the treatment effect in OCD? Do SSRIs compensate for a fundamental abnormality of the serotonergic system, or do SSRIs modulate an intact serotonergic system to compensate for another neurotransmitter mechanism? This review summarizes evidence supporting a role for the serotonin transporter and serotonin receptor subtypes in the pathophysiology and treatment of OCD.

Stress-induced hyperthermia and basal body temperature are mediated by different 5-HT(1A) receptor populations: a study in SERT knockout rats

Disturbances in the serotonergic system are implicated in many central nervous system disorders. The serotonin transporter (SERT) regulates the serotonin homeostasis in the synapse. We recently developed a rat which lacks the serotonin transporter (SERT(-/-)). It is likely that adaptive changes take place at the level of pre- and postsynaptic 5-HT receptors. Because autonomic responses are often used to measure 5-HT(1A) receptor function, we analysed these responses by examining the effects of a 5-HT(1A) receptor agonist and antagonist under in vivo conditions in the SERT(-/-) rat. Moreover, we studied the effect of a mild stressor on the body temperature (stress-induced hyperthermia) because of the known involvement of 5-HT(1A) receptors in this phenomenon. Results show that core body temperature did not differ between genotypes under basal, non-stressed conditions. Compared to SERT(+/+) rats, stress-induced hyperthermia was reduced in SERT(-/-) rats. The 5-HT(1A) receptor agonist [R(+)-N-(2[4-(2,3-dihydro-2-2-hydroxy-methyl-1,4-benzodioxin-5-yl)-1-piperazininyl]ethyl)-4-fluorobenzoamide HCl (flesinoxan) reduced stress-induced hyperthermia in both genotypes. The flesinoxan-induced hypothermia in SERT(+/+) rats was blocked by the 5-HT(1A) receptor antagonist [N-(2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)-N-(2-pyridinyl) cyclohexane carboxamide 3HCl (WAY100635). Moreover, WAY100635-induced hyperthermia in SERT(-/-), but not in SERT(+/+) rats. In SERT(-/-) rats, WAY100635 completely blocked the flesinoxan-induced reduction of stress-induced hyperthermia. Interestingly, flesinoxan-induced hypothermia was absent in SERT(-/-) rats. It is concluded that the SERT knockout rat reveals that 5-HT(1A) receptors modulating stress-induced hyperthermia belong to a population of receptors that differs from that involved in hypothermia.