ABSTRACT

There is growing clinical evidence that many psychiatric illnesses have overlapping genetic mechanisms. Understanding these mechanisms is important to the improvement of psychiatric treatment and preventions of the disorders, and animal genetic models continue to be a critical avenue of research towards these ends. As serotonin is a key neurotransmitter with important roles in normal behavioral processes and has been implicated in the pathogenesis of psychopathological conditions such as depression, anxiety, and addiction, it is a prime target for investigation in behavioral neurogenetics. The serotonin transporter (SERT) is a key brain protein that regulates the amount of serotonin that can activate the receptor. It is becoming evident that SERT interacts with brain-derived neurotrophic factor (BDNF), an important modulator of dopaminergic, cholinergic, and serotonergic neurons, which has been linked to memory function, activity, eating behavior, depression, and anxiety. The pivotal roles played by these two brain molecules have resulted in the development of numerous mutant animal models that have reduced function of SERT, BDNF, or both. Interestingly, SERT × BDNF mutant mice show numerous different behavioral phenotypes that are distinct from either SERT mutants or BDNF mutants alone, displaying phenotypes that are highly relevant to human clinical scenarios and bringing them added validity. This chapter will provide data from numerous experiments utilizing these rodent models and will explain their relevance and validity for research into the genetics of neuropsychiatric disorders.

ABSTRACT

Many antidepressants are believed to relieve depressed mood and excessive anxiety by inhibiting the reuptake of serotonin so as to cause increases in extracellular serotonin. This homeostatic alteration is thought to underlie further adaptive processes – which have not been fully clarified – that together constitute the cellular mechanisms of current antidepressant therapy. Here, we review the literature on presynaptic adaptive responses to chronic antidepressant treatment, focusing on alterations in serotonin transporter (SERT) expression, extracellular and intracellular serotonin levels, and serotonergic innervation. We contrast this with studies on constitutive loss of SERT gene expression. A partial genetic reduction in SERT expression results in modest increases in extracellular serotonin, while the total absence of SERT is associated with substantial increases in extracellular serotonin, decreases in intracellular serotonin, and a reduction in serotonin immunopositive cell bodies and axons in the dorsal raphe and hippocampus, respectively. Adaptive changes in SERT protein levels and extracellular and intracellular serotonin concentrations following many different regimens of chronic antidepressant administration were found to be more variable, often falling in between those resulting from partial and complete genetic ablation of SERT. This might reflect incomplete pharmacologic inhibition of SERT and the wide variety of drug administration paradigms utilized. The microdialysis literature, in particular, suggests that it is difficult to conclude that chronic antidepressant treatment reliably causes elevated extracellular serotonin.

ABSTRACT

The serotonin system plays a key modulatory role in central nervous system processes that appear to be dysregulated in psychiatric disorders. Specifically, the serotonin transporter (SERT) is thought to be critical to many aspects of emotional dysregulation and has been a successful target for medications that treat several psychiatric disorders. Here, we narrowly focused on two psychiatric conditions; anxiety and depression, for which mice with SERT genetic manipulations have provided insight. Specifically, we suggest that dissecting syndromes according to a trait and state perspective may help us understand the complex and at times contradictory rodent results. The most compelling reason for this approach is provided by human studies, in which increased trait-neuroticism and stress-mediated vulnerability to develop depression were reported for subjects carrying the 5-HTTLPR s/s allele of the SERT gene, and thus placing the contribution of SERT to mood disorders in a gene × environment and trait/state context. Accordingly, current behavioral results in SERT knock-out (KO) mice are consistent with both increased trait and state anxiety-like behaviors, while evidence in support of a trait-based model of depression in SERT KO mice are inconsistent and mostly based on tests with limited relevance to human depression. However, comorbid symptoms associated with a wider definition of depression, such as altered gastrointestinal functions, lower pain threshold, and greater sensitivity to stress, have been reported in SERT KO mice, suggesting the presence of a pro-depressive state resulting from low SERT.

ABSTRACT

Serotonin transporter (SERT, 5-HTT) plays an important role in the regulation of emotional states. It is a target for the most widely used class of antidepressants, selective serotonin reuptake inhibitors (SSRIs), and is also related to a genetic factor underlying the pathogenesis of affective disorders. Humans with lower SERT expression genotypes show a higher neuroticism score and are more sensitive to stress, suggesting that low SERT expression during development may be a trigger for affective disorders. On the other hand, repeated administration of SSRIs reduces the stress response and treats affective disorders. These observations suggest that disruption of SERT function early in life and in adulthood produces different phenotypes. Thus, understanding the cellular and molecular mechanisms underlying these phenotypes will help us to understand the pathogenesis of affective disorders and develop better therapeutic approaches for their treatment. Animal models with altered SERT function provide useful tools for the studies concerning this purpose. This chapter is intended to overview current available data concerning the cellular and molecular alterations in the models in which SERT functions are disrupted during different developmental stages. We will focus on a comparison between constitutive SERT knock-out mice and repeated administration of SSRIs in adulthood. Furthermore, studies concerning the prenatal administration of SSRIs and genomic manipulation of SERT expression in adulthood are also discussed.

INTRODUCTION

The serotonin (5-HT) transporter (SERT, 5-HTT) functions as a 5-HT reuptake site to take extracellular 5-HT back into the nerve terminals and, therefore, terminates the action of 5-HT. Thus, the function of SERT is critical for controlling 5-HT activity, which plays an important role in emotional regulation.

ABSTRACT

From invertebrates to humans, serotonin (5-HT) exerts structural effects, especially during development. The 5-HT transporter (SERT) directly regulates these effects by maintaining extracellular 5-HT concentrations within a physiological range and possibly by modulating the intracellular redox state of the cell. This chapter addresses 5-HT trophic effects on developing neural and non-neural mammalian cells, and summarizes SERT roles in 5HT-mediated structural effects from basic neurodevelopment to human teratology.

INTRODUCTION

The neurotransmitter serotonin (5-HT) is known to influence behavioral, autonomic, and cognitive functions, including learning and memory, sleep, temperature regulation, appetite, and mood. 5HT also plays a major role in human disorders such as anxiety, fear, depression, obsessive compulsive behavior, autism, and aggression. In addition to triggering a wide variety of electrophysiological effects, 5-HT also exerts important developmental roles in neural and non-neural tissues from early embryogenesis. In many regions of the central nervous system (CNS), this dual “functional” and “structural” involvement is interestingly paralleled at the histological and molecular levels by classical synaptic neurotransmission co-existing with paracrine mechanisms typical of “volume” or “mass” transmission. Indeed, many serotoninergic presynaptic terminals are not in direct proximity to postsynaptic elements. Many 5-HT receptors display CNS distributions necessarily implying the existence of abundant extrasynaptic binding sites, and the 5-HT transporter (SERT) is distributed along 5-HT axonal membranes mostly at extrasynaptic, non-junctional sites.

ABSTRACT

Depression is an etiologically and clinically heterogeneous syndrome frequently co-occurring in a wide spectrum of psychiatric disorders. Characterized by a wide range of symptoms that reflect alterations in cognitive, emotional, and psychomotor processes, this syndrome is moderately to highly heritable, and caused by interaction of genes and adverse life events. Differentiation of risk-related psychobiological and neuropsychological markers is essential for the dissection of the complex genetic susceptibility to depression and comorbid disorders. A brain serotonin (5-HT) system dysfunction is thought to be involved in the pathogenesis of depression by modulating cognitive dysfunction, stress response, neuroadaptive processes, and resulting pervasive emotional disturbance. A regulatory variation in the gene encoding the 5-HT transporter (5-HTT), the master controller in the fine-tuning of 5-HT signaling, is not only associated with anxiety-related traits, but also modifies the risk of developing disorders of emotion regulation. Yet the neural and molecular mechanisms underlying gene × environment interaction are poorly understood. This paper investigates innate variability of brain 5-HTT function from an interdisciplinary perspective blending behavioral genetics and cognitive neuroscience. Following an appraisal of imaging neural correlates of genomic variation and epigenetic mechanisms as a strategy for psychiatric disorder risk assessment, future challenges for biosocial sciences in the perspective of the complex genetic architecture of emotional behavior and social interaction in non-human primates and humans are defined.

ABSTRACT

By selective breeding for the extreme values of platelet serotonin level (PSL), two sublines of rats with constitutional hyperserotonemia/hyposerotonemia were developed. The velocity of platelet serotonin uptake (PSU), the main determinant of PSL, was used as a further, more specific selection criterion. Directed breeding for its extremes resulted in two sublines of rats with constitutional upregulation/downregulation of platelet 5HT transporter activity, and showed consequent alterations of entire 5HT homeostasis. These sublines, termed Wistar–Zagreb 5HT (WZ-5HT) rats, constitute a genetic rodent model described in this chapter. Besides changes in peripheral 5HT homeostasis, high-5HT and low-5HT sublines of WZ-5HT rats also demonstrate changes in central serotonergic mechanisms. Under physiological conditions, neurochemical differences in the 5HT system between sublines were almost undetectable, but they became evident upon specific pharmacologic challenge as shown by brain microdialysis study. Differential behavioral phenotypes of 5HT sublines in response to various environmental challenges provide further evidence for differences in their brain functioning. Thus, high-5HT rats exhibit enhanced anxiety-like behaviors while depressive-like behavior and higher alcohol intake co-occur in low-5HT rats. Observed functional and behavioral differences between sublines of WZ-5HT rats strongly indicate that brain serotonergic activity was increased in rats from the high-5HT subline as compared to low-5HT rats. The WZ-5HT rat model may represent an integrative model for serotonin and serotonin transporter research, incorporating changes at the genomic/genetic and phenotypic (neurodevelopmental, structural, biochemical, behavioral, etc.) levels, and encompassing both central and peripheral 5HT functioning.

ABSTRACT

Numerous studies provide persuasive evidence that a polymorphism in the serotonin promoter, 5-HTTLPR, interacts with environmental risk factors to produce heightened rates of depression, anxiety, antisocial and borderline personality disorders, and substance abuse in adults and adolescents. Investigations with the rhesus monkey have demonstrated similar gene–environment (G×E) interactions on both behavioral and biological outcomes. In this chapter, we review the history of primate models in serotonin transporter (5-HTT) research. Work with non-human primates has noted associations between behavioral differences and variation in serotonin metabolism (5-hydroxy-indole acetic acid, 5-HIAA). Investigations in several non-human primate species have also indicated that manipulation of early experience results in changes in behavior along with alterations in serotonergic functioning. These lines of research have contributed to the discovery of short (s) and long (l) forms within the serotonin promoter (rh5-HTTLPR) in the rhesus monkey. Researchers have since documented associations between the l/s or s/s genotypes and reduced cognitive flexibility, greater impulsivity, and anxious-like behavior, as well as higher rates of alcohol consumption. Furthermore, multiple G×E interactions have been documented for levels of 5-HIAA, hypothalamic–pituitary–adrenocortical (HPA) axis activity, alcohol consumption, rates of behavioral pathology, social play, aggression, and infant temperament. In most cases, these interactions were due to worse outcomes in l/s subjects that had been subjected to early maternal deprivation. This program of research demonstrates that l/s monkeys are more vulnerable to the effects of early-life stress, whereas l/l monkeys are more resilient.

ABSTRACT

This chapter dicusses the most recent data on the serotonin transporter knock-out rat, a unique rat model that has been generated by target-selected N-ethyl-N-nitrosourea (ENU) driven mutagenesis. The knock-out rat is the result of a premature stopcodon in the serotonin transporter gene, and the absence of the serotonin transporter has been confirmed at mRNA, protein, and functional levels. The serotonin transporter (SERT) plays a crucial role in serotonin reuptake and its absence has a huge effect on serotonin neurotransmission – exemplified by increased extracellular serotonin levels, reduced serotonin tissue/platelet/blood levels, and reduced evoked serotonin release – yet the animals appear normal and do not differ from wildtype littermates in respect to breeding and health. Behavioral phenotypes are only apparent when the animals are exposed to certain stimuli. For instance, the serotonin transporter knock-out rat displays increased stress sensitivity in a variety of anxiety- and depression-like tests, such as the elevated plus maze test and the forced swim test. Also remarkable, while general activity is not changed, the knock-out rats show a “neurotic-like” exploratory pattern. In line with the serotonin hypothesis of impulsivity, which argues that there is an inverse relationship between the two, serotonin transporter knock-out rats show reduced motor impulsivity in the five-choice serial reaction time task, and a reduction in social interaction during play and aggressive encounters. Interestingly, abdominal fat seems to be increased in the knock-out rat, despite normal body weight. Pharmacological compounds also elicit genotype-dependent responses in the knock-out rats.

ABSTRACT

In recent years, gene knock-out studies have greatly expanded understanding of the molecular basis of drug reward and drug addiction. One of the consequences of these studies has been to produce a more pluralistic view of the underlying neurochemical mechanisms that mediate drug reward after the development of a strongly dopamine-centered view in the 1980s. This is not to say that dopamine does not have a central role in drug reward and drug addiction, but rather a fuller examination of these mechanisms involves the complex neurocircuitry of which dopamine systems are a part. This view is not new, but has been expressed from a variety of perspectives. Gene knock-out studies have indicated a particular approach to examining the nature of interactions between different parts of this circuitry. This chapter will focus on the role of serotonin, and in particular the serotonin transporter (SERT), in drug reward. This more pluralistic perspective became apparent in gene knock-out studies of the rewarding effects of drugs of abuse which demonstrated that deletion of the dopamine transporter (DAT) did not eliminate the rewarding effects of cocaine, and subsequent findings that implicated a critical role of SERT in a variety of circumstances. These studies also validated the central role of dopamine in drug reward, and consequently the role of SERT must be considered largely from the point of view of interactions with dopamine systems.

ABSTRACT

The serotonin transporter (SERT), a membrane protein responsible for the reuptake of extracellular serotonin, is a prominent target of antidepressants. Moreover, a polymorphism of this gene that decreases serotonin uptake has been linked to depression. However, the role of SERT in depression is poorly understood. Several functional impairments, notably in behavior, sleep, and response to stress, are consistently found in animal models of depression, but consistent correlation with serotonergic dysfunction has not been demonstrated. Nevertheless, in certain genetic backgrounds, the same impairments are also found in mutant rodents in which serotonin transport has been abolished. These impairments are also observed in adult rodents after a transient disruption of serotonin transport during the first postnatal month. Conversely, they may be prevented in mutant rodents by normalizing serotonergic transmission postnatally. Therefore, the function of the serotonin transporter during postnatal development is critical for the proper maturation of brain circuits, while susceptibility to depression caused by reduced serotonin transporter function may be determined, in part, during development.

INTRODUCTION

Depression is one of the most common psychiatric disorders in developed countries. This disease affects mood, psychomotor activity, neurovegetative functions, and cognition (Fava and Kendler,2000). Estimates indicate a lifetime prevalence up to 20% for major depression (Blazer, 2000; Fava and Kendler, 2000; Kornstein et al., 2000), and the likelihood of experiencing this disorder is twice as high in women as in men (Kornstein et al., 2000). Depression can be a lifelong episodic disorder with multiple recurrences.

The serotonin transporter (SERT) has gained research popularity due to its prominent role in normal and aberrant brain processes. This key brain protein reuptakes serotonin from the synaptic cleft into presynaptic neurons, thereby modulating serotonergic neurotransmission. An entire class of psychotropic drugs, the serotonin reuptake inhibitors (SRIs), is dedicated to the action of this single protein. The fact that selective SRIs are becoming the world's most prescribed psychotropic medication emphasizes the utmost importance of SERT research for clinical psychiatry. The growing body of knowledge on SERT's role in the brain also emphasizes the need for experimental models of SERT function. Collectively, this has stimulated the compilation of this book, the aim of which is to provide a comprehensive update spanning the breadth of SERT research from animal models to their clinical parallels.

Although the exact functional mechanisms of SERT are not yet fully elucidated, it is thought to contain 12 hydrophobic transmembrane domains and to bind Na+, Cl−, and serotonin simultaneously. This results in a conformational change in the molecule, forming a barrier against the exterior of the cell, and opens the protein inwardly to the cytoplasmic membrane. The serotonin then disassociates from SERT, and the transporter returns to its original conformation receptive to extracellular serotonin once again. This process is the main mechanism of serotonin modulation in the brain, and the dysregulation of this system can affect brain and behavior markedly.