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A polymorphism in the gene encoding the serotonin (5-HT) transporter (5-HTT) has been shown to moderate the response to CO

Obsessive-compulsive disorder (OCD) is a common neuropsychiatric disease affecting about 2% of the general population. It is characterized by persistent intrusive thoughts and repetitive ritualized behaviors. While gene variations, malfunction of cortico-striato-thalamo-cortical (CSTC) circuits, and dysregulated synaptic transmission have been implicated in the pathogenesis of OCD, the underlying mechanisms remain largely unknown. Here we show that OCD-like behavior in mice is caused by deficiency of SPRED2, a protein expressed in various brain regions and a potent inhibitor of Ras/ERK-MAPK signaling. Excessive self-grooming, reflecting OCD-like behavior in rodents, resulted in facial skin lesions in SPRED2 knockout (KO) mice. This was alleviated by treatment with the selective serotonin reuptake inhibitor fluoxetine. In addition to the previously suggested involvement of cortico-striatal circuits, electrophysiological measurements revealed altered transmission at thalamo-amygdala synapses and morphological differences in lateral amygdala neurons of SPRED2 KO mice. Changes in synaptic function were accompanied by dysregulated expression of various pre- and postsynaptic proteins in the amygdala. This was a result of altered gene transcription and triggered upstream by upregulated tropomyosin receptor kinase B (TrkB)/ERK-MAPK signaling in the amygdala of SPRED2 KO mice. Pathway overactivation was mediated by increased activity of TrkB, Ras, and ERK as a specific result of SPRED2 deficiency and not elicited by elevated brain-derived neurotrophic factor levels. Using the MEK inhibitor selumetinib, we suppressed TrkB/ERK-MAPK pathway activity in vivo and reduced OCD-like grooming in SPRED2 KO mice. Altogether, this study identifies SPRED2 as a promising new regulator, TrkB/ERK-MAPK signaling as a novel mediating mechanism, and thalamo-amygdala synapses as critical circuitry involved in the pathogenesis of OCD.

Anxiety disorders result from a complex interplay of genetic and environmental factors such as stress. On the level of cellular signaling, regulator of G protein signaling 2 (Rgs2) has been implicated in human and rodent anxiety. However, there is limited knowledge about the role of Rgs2 in fear learning and reactivity to stress. In this study, Rgs2

As soon as in the 1960's, the role of serotonin (5-Hydroxytryptamin, 5-HT) in psychiatric disorders was realized, which was further substantiated by several lines of evidence amounting to a huge body of knowledge. The indolamine 5-HT belongs to the class of monoamine transmitters and can be found in the serotonergic neurons of the raphe nuclei in the brain stem. In the periphery, it is mainly present in the gastrointestinal system and the pineal gland. 5-HT is implicated in a variety of cognitive, emotional and vegetative behaviors, as well as in the regulation of circadian rhythms. Apart from its role as a neurotransmitter, it has an important function in prenatal development, where its expression pattern is tightly regulated, and in adult neurogenesis. The numerous effects of 5-HT are mediated by specific pre- and postsynaptic receptors, whose localization and functions are further described here. The serotonin transporter (SERT), which accomplishes the re-uptake of 5-HT into the neuron following its release in the synaptic cleft, not only has an important role in the termination of serotonergic neurotransmission but is also an important drug target for antidepressant compounds. In this part of the review, the neurobiological underpinnings of 5-HT synthesis, metabolism, and neurotransmission as well as the corresponding physiological consequences are summarized, while in the second part, an overview on clinical findings is provided and critically discussed.

Several lines of evidence implicate a dysregulation of the serotonin (5-HT) system in emotional behavior and stress, and point to its relevance for the etiology and pathogenesis of various neuropsychiatric disorders. This is evidenced by behavioral pharmacology as well as genetic studies, yet the impact of genetic variation within the 5-HT system on human disorders remains controversial. The generation of tissue-specific and inducible knockout mice lacking genes belonging to the 5-HT system further established the importance of the 5-HT system for neuronal development and the regulation of emotions. This part of the review provides a summary and critical discussion of genetic, neurobiological and pharmacological studies along with recent clinical research. Together, these data underscore the complex effects of 5-HT on human behavior and psychiatric disorders. Epigenetic mechanisms add to the complexity of the 5-HT system and will be increasingly studied in the coming years. Thus, the serotonergic system still remains in the centre of current hypotheses regarding the pathogenesis of disorders with the shared feature of emotional dysregulation.

Summary Serotonin (5-hydroxytryptamine, 5-HT) is one of the most extensively studied neurotransmitters of the central nervous system. It also has been identified in constituents of the immune system. Therefore serotonin has been suggested to serve as a mediator of bidirectional interactions between the nervous system and the immune system. We investigated this interaction in experimental autoimmune encephalomyelitis (EAE), a well-defined animal model of autoimmune disease of the central nervous system (CNS) mimicking features of the human disease multiple sclerosis. EAE was induced by immunization with the autoantigens myelin basic protein (MBP) or the immunodominant peptide of myelin oligodendrocyte glycoprotein (MOG) spanning amino acids 35–55 (MOGp 35–55). We studied EAE in knockout (KO) mice lacking the 5-HT transporter (5-HTT) on a C57.BL/6 background, in comparison with wild-type C57.BL/6 animals. After immunization with MOGp 35–55, or with rat MBP, the disease courses of the 5-HTT knockout mice were attenuated as compared to wildtype control mice. This difference was more pronounced in female animals. To dissect potential immune mechanisms underlying this phenomenon, histological studies of the CNS and cytokine measurements in mononuclear cells from the spleens of 5-HTT KO mice and wild-type controls were performed. We found a reduction of the inflammatory infiltrate in the CNS and of the neuroantigen-specific production of IFN-γ in splenocytes, again accompanied by a gender difference. These findings suggest a potential role of extracellular 5-HT homeostasis in the fine-tuning of neuroantigen-specific immune responses.

The serotonin transporter (5-HTT) regulates serotonergic neurotransmission via clearance of extracellular serotonin. Abnormalities in 5-HTT expression or function are found in mood and anxiety disorders, and the 5-HTT is a major target for antidepressants and anxiolytics. The 5-HTT is further implicated in the pathophysiology of these disorders by evidence that genetic variation in the promoter region of the HTT (SLC6A4) is associated with individual differences in anxiety and neural responses to fear. To further evaluate the role of the 5-HTT in anxiety, we employed a mouse model in which the 5-HTT gene (htt) was constitutively inactivated. 5-HTT -/- mice were characterized for anxiety-related behaviors using a battery of tests (elevated plus maze, light--dark exploration test, emergence test, and open field test). Male and female 5-HTT -/- mice showed robust phenotypic abnormalities as compared to +/+ littermates, suggestive of increased anxiety-like behavior and inhibited exploratory locomotion. The selective 5-HT(1A) receptor antagonist, WAY 100635 (0.05-0.3 mg/kg), produced a significant anxiolytic-like effect in the elevated plus maze in 5-HTT -/- mice, but not +/+ controls. The present findings demonstrate abnormal behavioral phenotypes in 5-HTT null mutant mice in tests for anxiety-like and exploratory behavior, and suggest a role for the 5-HT(1A) receptor in mediating these abnormalities. 5-HTT null mutant mice provide a model to investigate the role of the 5-HTT in mood and anxiety disorders.

The 5-hydroxytryptamine (5-HT)1A receptor has been the focus of considerable research effort over a decade. However, the definitive classification of this receptor and the full characterization of its pharmacology is still in progress. On the one hand, the selective serotonin-1A receptor partial agonist anxiolytics were developed first and represented a new class of pharmacological agents that have demonstrated efficacy in the treatment of generalized anxiety disorder (GAD). These compounds (e.g. buspirone, gepirone, tandospirone or ipsapirone) offered a completely different pharmacologic approach to this disorder from previous medications as benzodiazepines or alcohol. On the other hand, beta-adrenoceptor blockers (e.g. [-]-pindolol, [-]-propranolol) have been shown to have antagonistic properties on the 5-HT1A receptor side. Preliminary results suggest that beta-adrenoceptor blockers may be useful as adjunctive medication in the treatment of depression by augmenting the antidepressant efficacy of selective serotonin reuptake inhibitors. As the beta-adrenoceptor blockers show also affinities to other 5-HT receptors than the 5-HT1A receptor the recently discovered more selective 5-HT1A antagonists became of interest. The pharmacological properties and potential therapeutic utility of these novel compounds will also be discussed.

The U-373 MG glioblastoma and the IMR-32 neuroblastoma cell lines were found to express the dopamine (DA) and vesicular monoamine transporters, using reverse transcriptase-polymerase chain reaction (RT-PCR). To further characterize the DA transporter, [3H]GBR-12935 binding and [3H]DA uptake studies were performed. Specific binding of [3H]GBR-12935 to U-373 MG and IMR-32 cells is saturable as saturation experiments indicated. Scatchard analysis revealed two binding sites on U-373 MG as well as on IMR-32 cells. The high-affinity sites exhibited a KD of 2.95 and 0.42 nM and a Bmax of 6.4 and 0.83 fmol/mg protein for U-373 MG and IMR-32 cells, respectively. The low-affinity sites exhibited a KD of 144 and 251 nM and a Bmax of 37.5 and 119 fmol/mg protein for the same cells, respectively. The high-affinity binding of both types of cells probably represents the "classic" DA uptake site identified in other studies from human and rat striatal membranes or synaptosomes, while the low-affinity binding may represent a mazindol-insensitive binding site (the "piperazine acceptor site"). [3H]DA uptake was 0.55 +/- 0.16 and 1.08 +/- 0.33 pmol/mg protein for U-373 MG and IMR-32 cells, respectively. Since the DA transporter has been implicated as an important site for drugs and toxins, the above-mentioned cell lines may be a useful tool in the study of the mechanism of action of DA transporter modulating substances.

As soon as in the 1960's, the role of serotonin (5-Hydroxytryptamin, 5-HT) in psychiatric disorders was realized, which was further substantiated by several lines of evidence amounting to a huge body of knowledge. The indolamine 5-HT belongs to the class of monoamine transmitters and can be found in the serotonergic neurons of the raphe nuclei in the brain stem. In the periphery, it is mainly present in the gastrointestinal system and the pineal gland. 5-HT is implicated in a variety of cognitive, emotional and vegetative behaviors, as well as in the regulation of circadian rhythms. Apart from its role as a neurotransmitter, it has an important function in prenatal development, where its expression pattern is tightly regulated, and in adult neurogenesis. The numerous effects of 5-HT are mediated by specific pre- and postsynaptic receptors, whose localization and functions are further described here. The serotonin transporter (SERT), which accomplishes the re-uptake of 5-HT into the neuron following its release in the synaptic cleft, not only has an important role in the termination of serotonergic neurotransmission but is also an important drug target for antidepressant compounds. In this part of the review, the neurobiological underpinnings of 5-HT synthesis, metabolism, and neurotransmission as well as the corresponding physiological consequences are summarized, while in the second part, an overview on clinical findings is provided and critically discussed.

Several lines of evidence implicate a dysregulation of the serotonin (5-HT) system in emotional behavior and stress, and point to its relevance for the etiology and pathogenesis of various neuropsychiatric disorders. This is evidenced by behavioral pharmacology as well as genetic studies, yet the impact of genetic variation within the 5-HT system on human disorders remains controversial. The generation of tissue-specific and inducible knockout mice lacking genes belonging to the 5-HT system further established the importance of the 5-HT system for neuronal development and the regulation of emotions. This part of the review provides a summary and critical discussion of genetic, neurobiological and pharmacological studies along with recent clinical research. Together, these data underscore the complex effects of 5-HT on human behavior and psychiatric disorders. Epigenetic mechanisms add to the complexity of the 5-HT system and will be increasingly studied in the coming years. Thus, the serotonergic system still remains in the centre of current hypotheses regarding the pathogenesis of disorders with the shared feature of emotional dysregulation.

Serotonin (5-HT) and the serotonergic system have recently been indicated as modulators of adult hippocampal neurogenesis. In this study, we evaluated the role of 5-HT on the functional features in neurospheres derived from adult neural stem cells (ANSC). We cultured neurospheres derived from mouse hippocampus in serum-free medium containing epidermal (EGF) and type-2 fibroblast growth factor (FGF2). Under these conditions ANSC expressed both isoforms of tryptophane-hydroxylase (TPH) and produced 5-HT. Blocking TPH function by para-chlorophenylalanine (PCPA) reduced ANSC proliferation, which was rescued by exogenous 5-HT. 5-HT action on ANSC was mediated predominantly by the serotonin receptor subtype 5-HT1A and, to a lesser extent, through the 5-HT2C (receptor) subtype, as shown by selectively antagonizing these receptors. Finally, we documented a 5-HT-induced increase of ANSC migration activity. In summary, we demonstrated a powerful serotonergic impact on ANSC functional features, which was mainly mediated by 5-HT1A receptors.

Signal-transducing G proteins are central to the coordination of receptor-effector communication. We have explored the effects of long-term fluoxetine administration of G alpha s, G alpha i1, G alpha i2, G alpha o, G alpha q and G alpha 12 mRNA expression in various rat brain regions using reverse transcriptase-polymerase chain reaction (RT-PCR)-mediated cross-species partial cDNA cloning. Northern blot analysis, and RNase protection assay techniques. Fluoxetine decreased G alpha s mRNA in midbrain, while mRNA expression of the novel G protein alpha subunits, G alpha q and G alpha 12, was increased in neostriatum and frontal cortex. We conclude that in addition to post-translational modification, regulation of G protein function by antidepressant drugs may occur at the level of gene expression.

Understanding the pathway for amyloid precursor protein (APP) catabolism has become an important line of investigation. APP is a ubiquitous membrane bound protein that is rapidly cleaved at the membrane, yielding a secreted protein identical to protease nexin II and an internalized 11.5 kDa 100 residue C terminal derivative (CTD). The levels of CTDs in a variety of cell lines have been examined and were found to differ. Cell types associated with the pathology of Alzheimer's disease (AD), such as olfactory neuroblasts (ON) and cortical vascular endothelial cells, have higher levels of CTDs than lymphoblasts and melanoma cells. The mechanism of CTD catabolism appears to involve the lysosome because blockade of lysosomal but not endosomal or mitochondrial function results in increased levels of CTDs. Under these conditions, production of larger, amyloidogenic CTDs is also seen. In cells possessing higher levels of CTDs we find that the mechanism for production of amyloidogenic CTDs may involve the internalization of intact full-length APP. Thus, inhibition of the lysosomal system appears capable of generating amyloidogenic peptides. The amount of amyloidogenic peptides appears to vary among cell lines. Such variation may shed light on why amyloid accumulates around specific cell types such as vascular endothelial cells, neurons, and glia. Finally, disfunction of the lysosomal system may play a role in the pathogenesis of Alzheimer's disease. © 1992 Wiley-Liss, Inc.

Attention-deficit hyperactivity disorder (ADHD) is defined as a clinically heterogeneous neurodevelopmental syndrome with the contribution of numerous genetic and environmental risk factors. The goal of interdisciplinary and translational neurobiological research is to clarify the interdependent relationship between molecular mechanisms and structural-functional substrates in the pathogenesis of ADHD and its significance to the disorder's long-term course. Work on ADHD-specific molecular genetic and developmental biological essentials of brain function and on the structural-functional basis of behavior holds the promise of developing predictors and differential strategies for effective therapy of severe and chronic courses of ADHD. To define evolutionary conserved ADHD-relevant principles of structure and function of the brain and behavior typical to the syndrome, an integrated approach in the elucidation of specific neuro- and psychobiological mechanisms and thus systemic pathophysiology of ADHD is crucial. Regarding compromised neurodevelopment, pathophysiological models of ADHD, particularly its syndromal and comorbid dimensions, therefore require the combination of molecular genetic, neuroimaging, neuropsychology, behavioral, and psychosocial strategies to explain complete causal chains.

It has been shown that dopamine (DA) influences performance on neurocognitive tests, which are thought to rely on prefrontal activity. The purpose of this study was to explore the effects of gene polymorphisms related to DA activity, namely the D4 DA receptor (DRD4) gene exon III polymorphisms, on prefrontal cortex (PFC) activation. In this study we measured the brain oxygenation of the PFC during an n-back task with near-infrared spectroscopy (NIRS). We investigated 40 young healthy subjects, 12 of which carried the DRD4 exon III 7-repeat allele (group 7). These were compared with subjects without a 7-repeat allele (n=28, group 4). Additionally, we compared good and bad performers with respect to brain activation. As expected, we found significant increases in the concentration of oxygenated haemoglobin [O2Hb] during the 1-back and 2-back condition compared with baseline, and a corresponding significant decrease of deoxyhaemoglobin concentration. As a main result of this study we also found an interaction effect between task condition and DRD4 genotype with higher increases of [O2Hb] during the 2-back version compared with the 1-back version for the subjects of the 7-repeat allele group only. The same effect was seen as a statistical trend, when we compared bad performers with good performers. Therefore, we interpret the effects of the 7-repeat allele group of DRD4 as a sign of ineffective brain activity, perhaps even as a sign of prefrontal noise.

Animal models have not only become an essential tool for investigating the neurobiological function of genes that are involved in the etiopathogenesis of human behavioral and psychiatric disorders but are also fundamental in the development novel therapeutic strategies. As an example, inactivation of the serotonin (5HT) transporter (5Htt, Slc6a4) gene in mice expanded our view of adaptive 5HT uptake regulation and maintenance of 5HT homeostasis in the developing human brain and molecular processes underlying anxiety-related traits, as well as affective spectrum disorders including depression. 5Htt-deficient mice have been employed as a model complementary to direct studies of genetically complex traits and disorders, with important findings in biochemical, morphological, behavioral, and pharmacological areas. Based on growing evidence for a critical role of the 5HTT in the integration of synaptic connections in the rodent, nonhuman primate, and human brain during critical periods of development and adult life, more in-depth knowledge of the molecular mechanisms implicated in these fine-tuning processes is currently evolving. Moreover, demonstration of a joint influence of the 5HTT variation and environmental sources during early brain development advanced our understanding of the mechanism of genexgene and genexenvironment interactions in the developmental neurobiology of anxiety and depression. Lastly, imaging techniques, which become increasingly elaborate in displaying the genomic influence on brain system activation in response to environmental cues, have provided the means to bridge the gap between small effects of 5HTT variation and complex behavior, as well as psychopathological dimensions. The combination of elaborate genetic, epigenetic, imaging, and behavioral analyses will continue to generate new insight into 5HTT's role as a master control gene of emotion regulation.

Animal models have not only become an essential tool for investigating the neurobiological function of genes that are involved in the etiopathogenesis of human behavioral and psychiatric disorders but are also fundamental in the development novel therapeutic strategies. As an example, inactivation of the serotonin (5HT) transporter (5Htt, Slc6a4) gene in mice expanded our view of adaptive 5HT uptake regulation and maintenance of 5HT homeostasis in the developing human brain and molecular processes underlying anxiety-related traits, as well as affective spectrum disorders including depression. 5Htt-deficient mice have been employed as a model complementary to direct studies of genetically complex traits and disorders, with important findings in biochemical, morphological, behavioral, and pharmacological areas. Based on growing evidence for a critical role of the 5HTT in the integration of synaptic connections in the rodent, nonhuman primate, and human brain during critical periods of development and adult life, more in-depth knowledge of the molecular mechanisms implicated in these fine-tuning processes is currently evolving. Moreover, demonstration of a joint influence of the 5HTT variation and environmental sources during early brain development advanced our understanding of the mechanism of genexgene and genexenvironment interactions in the developmental neurobiology of anxiety and depression. Lastly, imaging techniques, which become increasingly elaborate in displaying the genomic influence on brain system activation in response to environmental cues, have provided the means to bridge the gap between small effects of 5HTT variation and complex behavior, as well as psychopathological dimensions. The combination of elaborate genetic, epigenetic, imaging, and behavioral analyses will continue to generate new insight into 5HTT's role as a master control gene of emotion regulation.

The relative contribution of genetic and environmental factors in the configuration of behavioral differences is among the most prolonged and contentious controversies in intellectual history. Although current views emphasize the joint influence of genes and environmental sources during early brain development, the physiological complexities of multiple gene-gene and gene-environment interactions in the developmental neurobiology of fear and anxiety remain elusive. Variation in genes coding for proteins that control serotonin (5-hydroxytryptamine, 5-HT) system development and plasticity, establish 5-HT neuron identity, and modulate 5-HT receptor-mediated signal transduction as well as cellular pathways have been implicated in the genetics of anxiety and related disorders. This review selects anxiety and avoidance as paradigmatic traits and behaviors, and it focuses on mouse models that have been modified by deletion of genes coding for key players of serotonergic neurotransmission. In particular, pertinent approaches regarding phenotypic changes in mice bearing inactivation mutations of 5-HT receptors, 5-HT transporter, and monoamine oxidase A and other genes related to 5-HT signaling will be discussed and major findings highlighted.

By conferring allele-specific transcriptional activity on the monoamine oxidase A (MAOA) gene in humans, length variation of a repetitive sequence [(variable number of tandem repeat (VNTR)] in the MAOA promoter influences a constellation of personality traits related to aggressive and antisocial behavior and increases the risk of neurodevelopmental and psychiatric disorders. Here, we have analyzed the presence and variability of this MAOA promoter repeat in several species of nonhuman primates. Sequence analysis of MAOA's transcriptional control region revealed the presence of the VNTR in chimpanzee (Pan troglodytes), bonobo (Pan paniscus), gorilla (Gorilla gorilla), orangutan (Pongo pygmaeus), rhesus macaque (Macaca mulatta) and Gelada baboon (Theropithecus gelada). The majority of P. troglodytes and P. paniscus showed a single repeat with a sequence identical to the VNTR sequence in humans. In contrast, analyses of the remaining species revealed shorter sequences similar to the first 18 bp of human VNTR. Compared with other nonhuman primates, the VNTR sequence of M. mulatta showed the highest length variability with allele frequencies of 35, 25 and 40% for the five, six and seven repeat variants, respectively. The extent of variability of the MAOA promoter repeat in both rhesus monkeys and humans supports the notion that there may be a relationship between functional MAOA expression and aggression-related traits in humans and rhesus macaque populations.

The relative contribution of genetic and environmental factors in the configuration of behavioral differences is among the most prolonged and contentious controversies in intellectual history. Although current views emphasize the joint influence of genes and environmental sources during early brain development, the physiological complexities of multiple gene-gene and gene-environment interactions in the developmental neurobiology of fear and anxiety remain elusive. Variation in genes coding for proteins that control serotonin (5-hydroxytryptamine, 5-HT) system development and plasticity, establish 5-HT neuron identity, and modulate 5-HT receptor-mediated signal transduction as well as cellular pathways have been implicated in the genetics of anxiety and related disorders. This review selects anxiety and avoidance as paradigmatic traits and behaviors, and it focuses on mouse models that have been modified by deletion of genes coding for key players of serotonergic neurotransmission. In particular, pertinent approaches regarding phenotypic changes in mice bearing inactivation mutations of 5-HT receptors, 5-HT transporter, and monoamine oxidase A and other genes related to 5-HT signaling will be discussed and major findings highlighted.

Variation in the serotonin transporter gene-linked polymorphic region (5-HTTLPR) has been associated with anxiety and harm avoidance and is weakly associated with a number of neuropsychiatric disorders, including Type II alcoholism, which has a high rate of comorbidity with antisocial personality disorder. Studies have also demonstrated interactions between 5-HTLPR variation and environmental stress on the incidence of depression. As in humans, there is a serotonin transporter gene promoter length polymorphism in rhesus macaques that produces similar decreases in transcriptional efficiency. Macaques with histories of early-life stress have been shown to exhibit impulsive aggression, incompetent social behavior and increased behavioral and endocrine responsivity to stress. In this paper, we review studies performed previously in our lab and present preliminary data examining interactions between early rearing and serotonin transporter gene promoter variation on the incidences of play behavior and aggression in infant rhesus macaques. The data presented here highlight the importance of considering gene-environment interactions when studying childhood risk factors for aggression, anxiety and related neuropsychiatric disorders and support the use of the nonhuman primate for studing gene by environment interactions in behavioral research.

Current evidence indicates that virtually all neuropsychiatric disorders, like many other common medical disorders, are genetically complex, with combined influences from multiple interacting genes, as well as from the environment. However, additive or epistatic gene interactions have proved quite difficult to detect and evaluate in human studies. Mouse phenotypes, including behaviors and drug responses, can provide relevant models for human disorders. Studies of gene-gene interactions in mice could thus help efforts to understand the molecular genetic bases of complex human disorders. The serotonin transporter (SERT, 5-HTT, SLC6A4) provides a relevant model for studying such interactions for several reasons: human variants in SERT have been associated with several neuropsychiatric and other medical disorders and quantitative traits; SERT blockers are effective treatments for a number of neuropsychiatric disorders; there is a good initial understanding of the phenotypic features of heterozygous and homozygous SERT knockout mice; and there is an expanding understanding of the interactions between variations in SERT expression and variations in the expression of a number of other genes of interest for neuropsychiatry and neuropharmacology. This paper provides examples of experimentally-obtained interactions between quantitative variations in SERT gene expression and variations in the expression of five other mouse genes: DAT, NET, MAOA, 5-HT(1B) and BDNF. In humans, all six of these genes possess polymorphisms that have been independently investigated as candidates for neuropsychiatric and other disorders in a total of500 reports. In the experimental studies in mice reviewed here, gene-gene interactions resulted in either synergistic, antagonistic (including 'rescue' or 'complementation') or more complex, quantitative alterations. These were identified in comparisons of the behavioral, physiological and neurochemical phenotypes of wildtype mice vs. mice with single allele or single gene targeted disruptions and mice with partial or complete disruptions of multiple genes. Several of the descriptive phenotypes could be best understood on the basis of intermediate, quantitative alterations such as brain serotonin differences. We discuss the ways in which these interactions could provide models for studies of gene-gene interactions in complex human neuropsychiatric and other disorders to which SERT may contribute, including developmental disorders, obesity, polysubstance abuse and others.

Mood, emotion and cognition are modulated by serotonergic neurotransmission, while the physiological function of serotonergic synapses depends on serotonin reuptake, which is mediated by the serotonin transporter (5-HTT). Allelic variation of 5-HTT expression in humans is caused by a functional gene-promoter polymorphism with two predominant variant alleles, which are associated with variations in anxiety measures as previously reported. Here we report that administration of dexamethasone, a potent glucocorticosteroid hormone, results in an increase in 5-HTT expression in immortalized human B-lymphoblastoid cells, which express the human 5-HTT. Functional reporter gene assays as well as 5-HT uptake and inhibitor binding measures revealed a genotype-dependent dose-response to glucocorticosteroid administration, which was antagonized by RU 38486, a non-specific glucocorticosteroid hormone antagonist. The allele-specific differences after administration of dexamethasone depended on the repetitive GC-rich sequence located approximately 1.4 kb upstream of the 5-HTT gene transcription site because of absence of a significant steroid effect after transfecting a deletional mutant reporter gene construct, which lacks this repetitive promoter sequence. Our findings may contribute to explain the vulnerability to stress-related disorders in susceptible individuals, in whom further clinical studies should follow up on these in vitro findings.

The intensity dependence of the auditory evoked potential (AEP) has been suggested as an indicator of central serotonergic function, a strong intensity dependence presumably reflecting low serotonergic activity. As individual differences in serotonergic neurotransmission can be accounted for in part by genetic variation in genes of the serotonergic pathway, we investigated whether a functional polymorphism in the promoter region of the serotonin transporter gene (5-HTTLPR) is associated with the AEP intensity dependence. Because dopaminergic influences on the intensity dependence have also been reported, we furthermore explored the role of a functional polymorphism in the dopamine D4 receptor gene (DRD4 exon III) in the modulation of the AEP intensity dependence. AEPs to tones of six intensity levels were recorded from 60 healthy young individuals, and N1/P2 linear as well as median slopes at central electrode sites were computed as measures of the AEP intensity dependence. Analyses of variance showed that there was a significant effect of the 5-HTTLPR on the AEP intensity dependence. Individuals with the ll genotype exhibited a stronger intensity dependence compared to individuals with the ls genotype. This effect was even more pronounced when DRD4 exon III was considered in the analyses. In conclusion, these findings provide further evidence for a role of serotonergic neurotransmission in the modulation of the AEP intensity dependence. The results also point to possible dopaminergic influences on the AEP intensity dependence.

The serotonin (5HT) transporter (5HTT) regulates serotonergic neurotransmission by mediating the reuptake of 5HT from the synaptic cleft. Although lacking the high affinity and selectivity of the 5HTT, the brain expresses a large number of other transporters, including the polyspecific organic cation transporters (OCTs). OCT1 and OCT3, members of the potential-sensitive organic cation transporter gene family, physiologically transport a wide spectrum of organic cations. In addition, both transporters mediate low-affinity 5HT transport and, therefore, may participate in the clearance of excessive 5HT. Because concentrations of extracellular 5HT are increased in the brain of 5HTT-deficient mice, they are a model for investigating the role of OCTs in 5HT system homeostasis. Here, we analyzed OCT1 and OCT3 gene expression in the brain of 5HTT knockout mice by semiquantitative competitive polymerase chain reaction and in situ hybridization. We demonstrate that, in 5HTT-deficient mice, OCT3 mRNA concentrations were significantly increased in the hippocampus, but not in other brain regions, including cortex, striatum, cerebellum, and brainstem. In contrast, no difference in OCT1 expression was detected between 5HTT knockout and control mice. Up-regulation of OCT3 expression and enhanced low-affinity 5HT uptake may limit the adverse effects of elevated extracellular 5HT and may play a critical role in maintaining 5HT-dependent functions of the hippocampus in the absence of 5HTT.

The role of the neurokinine 1 receptors (NK1R) and its endogenous ligand, the neuropeptide substance P (SP) in the pathophysiology and treatment response of anxiety and depression has been controversial. In a new study, however, mice with a targeted inactivation of the NK1R show a phenotype that is associated with anxiety-related behaviors and stress responses. Since this behavioral phenotype was associated with an increase in serotonergic function, NK1R knockout mice represent a systematic model for advanced investigations of SP/NK1R and serotonin system interaction in anxiety responses. The availability of an increasing number of knockout mouse anxiety models as well as integration of emerging tools and technologies for genetic analysis will provide the groundwork for the genetic dissection of anxiety and related disorders.

We report here that a truncated 5-HTT protein is produced in the neurons of the raphe, in serotonin transporter (5-HTT) knockout (KO) mice. The 5-HTT gene has exon 2 deleted and we found that one main transcript, shortened by 450 bp, is produced in these KO mice. The mutated 5-HTT protein is only recognized by antibodies against the C-terminal portion of 5-HTT. This protein is not functional as there is no high-affinity serotonin uptake in 5-HTT KO mice, in adults or during development. Conversely, low-affinity serotonin uptake was detected in vitro, and in dopaminergic neurons of the substantia nigra in vivo. The truncated 5-HTT, recognized by antibodies to the C-terminus, is present exclusively in the somatodendritic compartment of the raphe neurons instead of being exported to axons. As shown with confocal and electron microscopy, the truncated 5-HTT does not reach the plasma membrane and is essentially retained in the endoplasmic reticulum. However, this does not seem to trigger refolding or degradation responses, as no upregulation of the chaperone BiP or of the degradation signal ubiquitin was detected. Last, as observed in heterozygous mice, the presence of the truncated 5-HTT protein, although produced in large quantities, does not disturb the normal trafficking of the wild-type protein. This study therefore validates the 5-HTT KO model despite the occurrence of an incomplete translation, and brings novel information on the in vivo 5-HT uptake and cellular processing of an abnormal 5-HTT protein.

The etiology of late-onset Alzheimer's disease (AD) and idiopathic Parkinson's disease (PD) is not known. In both disorders there is an extensive degeneration of serotonergic neurons, with corresponding losses of the serotonin (5HT) transporter (5HTT), which is responsible for the reuptake of 5HT from the synaptic cleft. An increasing body of evidence indicates that allelic variation of the 5HTT gene promoter (5HTT gene-linked polymorphic region, 5HTTLPR) determines high or low 5HT uptake in normal human brain. Association studies show that the low-activity allele of the 5HTTLPR is a risk factor for late-onset AD. In PD, the 5HTTLPR influences the risk of developing depression, a common symptom in PD patients. A compromised serotonergic system thus plays an important role in the pathophysiology of both AD and PD.

The expression of aggressiveness, which constitutes many facets of behavior, is influenced by a complex interaction of biologic, psychologic, and social variables. Even though individual differences in impulsivity and the behavioral consequences, such as aggression, addiction, and suicidality, are substantially heritable, they ultimately result from an interplay between genetic variations and environmental factors. While formation and integration of multiple neural networks is dependent on the actions of neurotransmitters, such as serotonin (5HT), converging lines of evidence indicate that genetically determined variability in serotonergic gene expression influences complex traits including that of inappropriately aggressive behavior. This contribution reviews studies of major gene effects in inbred and knockout strains of mice with increased aggression-related behavior and discusses the relevance of several serotonergic gene variations in humans which include high aggressiveness as part of the phenotype. Although special emphasis is given to the molecular psychobiology of 5HT in aggression-related behavior in rodents, nonhuman primates, and humans, relevant conceptual and methodological issues in the search for candidate genes for impulsivity and aggressiveness and for the development of mouse models of aggressive and antisocial behavior in humans are also considered.

The tryptamine-derived dopaminergic neurotoxin 1-trichloromethyl-1,2,3,4-tetrahydro-beta-carboline ('TaClo'), which was found to occur in humans after intake of the hypnotic chloral hydrate, was also shown to strongly disturb serotonergic cells. Incubation experiments using the human serotonergic cell line JAR clearly revealed TaClo to significantly reduce serotonin (5-HT) uptake (IC(50) = 59 microM) and to induce a distinct loss of cellular viability at increasing TaClo concentrations. In contrast to well-known serotonergic neurotoxins such as amphetamines, however, TaClo toxicity is not mediated by the 5-HT transporter (5-HTT). In the presence of the specific 5-HTT inhibitor imipramine, the uptake of TaClo into JAR cells was not reduced, hinting at an exclusively passive penetration of this highly lipophilic beta-carboline through cell membranes. Similar toxic effects towards JAR cells were also observed for the 5-HT-related TaClo analog 6-hydroxy-1-trichloromethyl-1,2,3,4-tetrahydro-beta-carboline ('6-OH-TaClo') (IC50 = 26 gM). The dopamine-derived alkaloid-type heterocycle 6,7-dihydroxy-1-trichloromethyl-1,2,3,4-tetrahydroisoquinoline ('DaClo'), by contrast, was found to be less toxic, showing only a weak inhibitory activity (IC50 = 260 microM) on 5-HT uptake. The pronounced toxicitiy of TaClo and 6-OH-TaClo against serotonergic cells became also evident from morphological findings: Dose-dependently, the survival of JAR cells was significantly impaired, while human dopaminergic IMR-32 cells were only moderately affected at similar toxin concentrations.

The serotonin transporter (5-HTT) regulates serotonergic neurotransmission and is thought to influence emotion. A 5-HTT-linked polymorphic region (5-HTTLPR) has two common variants, short (s) and long (l). We previously found population and within-family associations between the lower-expressing s allele and neuroticism, a trait related to anxiety, hostility, and depression, on a standard measure (the NEO Personality Inventory, Revised [NEO-PI-R]) in a primarily male population (n=505), and that the s allele was dominant. We investigated this association in a new sample (n=397, 84% female, primarily sib-pairs). The results robustly replicated the 5-HTTLPR neuroticism association, and the dominance of the s allele. Combined data from the two studies (n=902) showed a highly significant association between the s allele and higher NEO Neuroticism both across individuals and within families. Association between genotype and a related measure, Anxiety on the 16PF inventory, was replicated in the new population and within families in the combined sample. Association to another trait, estimated TPQ Harm Avoidance, was not replicated in the new sample but found only within the combined sibship group. Another association found in our original study, between the s allele and lower scores on NEO-PI-R Agreeableness, was also replicated and was more robust in the current and the combined samples. Associations between the functional 5-HTTLPR polymorphism were similar in women and men. These results help to define specific personality features reproducibly associated with 5-HTTLPR genotype. Such associations were strongest for traits defined by the NEO, enhancing the attractiveness of the five-factor personality model in genetic research on complex behavioral dimensions. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:202-216, 2000. Published 2000 Wiley-Liss, Inc.

Serotonin (5-HT), which acts at multiple pre- and postsynaptic receptor sites, fulfills the criteria for a neurotransmitter and modulator of synaptic signal transduction in many functional systems of the brain. As a consequence of this enormous versatility, brainstem 5-HT systems affect many diverse functions, such as mood, cognition, appetite and satiation, sleep, motor behavior, neuroendocrine and circadian rhythms, social and reproductive behavior. By determining the magnitude and duration of postsynaptic receptor-mediated signaling, carrier-facilitated 5-HT transport into and release from the presynaptic neuron plays a key role in the spatiotemporal fine-tuning of 5-HT neurotransmission (Fig. 1). Recent advances have resulted from the molecular and functional characterization of the serotonin transporter (5-HTT), from pharmacological and neurochemical studies relating the actions of psychoactive drugs (e.g., tricyclics, selective 5-HT reuptake inhibitors, psychostimulants) to discrete effects on 5-HT uptake and release, and from the molecular dissection of the complex changes in functional expression as a consequence of altered gene transcription. On the basis of its molecular structure and anatomical distribution, substrate specificity, electrophysiological properties, and drug-binding profiles the 5-HTT has been placed in the extended gene family of Na+/C1--dependent cell surface transport proteins.

The aim of the present study was to determine whether alterations in 5-hydroxytryptamine (5-HT)(1A) receptors would be found in knockout mice lacking the serotonin transporter (5-HTT). Hypothermic and neuroendocrine responses to the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT) were used to examine the function of 5-HT(1A) receptors. Initial studies evaluated the dose-response and time course of 8-OH-DPAT-induced hypothermia and hormone secretion in normal CD-1 mice (the background strain of the 5-HTT knockout mice). 8-OH-DPAT dose-dependently produced hypothermic responses that peaked at 20 min postinjection. 8-OH-DPAT-induced hypothermia was blocked by the 5-HT(1A) antagonist WAY-100635. 8-OH-DPAT dose-dependently increased the concentrations of plasma oxytocin, corticotropin, and corticosterone. In the 5-HTT knockout (-/-) mice, the hypothermic response to 8-OH-DPAT (0.1 mg/kg s.c.) was completely abolished. Furthermore, 5-HTT-/- mice had significantly attenuated plasma oxytocin and corticosterone responses to 8-OH-DPAT. No significant changes in the hypothermic or hormonal responses to 8-OH-DPAT were observed in heterozygous (5-HTT+/-) mice. [(3)H]8-OH-DPAT- and [(125)I]MPPI [4-(2'-methoxyphenyl)-1-[2'-[N-(2"-pyridinyl)-iodobenzamido]ethyl] pip erazine]-binding sites in the hypothalamus and [(125)I]MPPI-binding sites in the dorsal raphe were significantly decreased in 5-HTT-/- mice. The results indicate that lack of the 5-HTT is associated with a functional desensitization of 5-HT(1A) receptor responses to 8-OH-DPAT, which may be a consequence, at least in part, of the decrease in density of 5-HT(1A) receptors in the hypothalamus and dorsal raphe of 5-HTT-/- mice.

A possible dysregulation of dopaminergic neurotransmission has been implicated in the aetiology of schizophrenic psychoses, in particular of paranoid-hallucinatory states, and of the manic episodes of bipolar affective disorder. In the present study we analysed allelic and genotypic variations of a recently described functional deletion/insertion variant (-141C Ins/Del) in the 5' flanking region of the human dopamine D2 receptor gene. We investigated a total of 620 unrelated individuals, comprising 260 schizophrenic patients, 70 patients with bipolar affective disorder, and 290 population controls. Analysis of the -141C Ins/Del variant revealed that the schizophrenic, bipolar affective and control groups did not differ significantly regarding genotype frequencies and allele frequencies. No evidence of an allelic association with either a family history of schizophrenic psychosis or a diagnosis of schizophrenia of the paranoid type (according to ICD 10) was found. Our findings indicate that the -141C Del variant in the 5' flanking region of the human dopamine D2 receptor gene is unlikely to play a substantial role in genetic predisposition to major psychiatric disorders in Caucasians.

The present study tested the hypothesis that the short, low activity variant of a biallelic polymorphism in the 5' regulatory region of the human serotonin transporter (5-HTT) gene confers susceptibility to severe alcohol dependence marked by severe withdrawal symptoms. Applying a phenotype-genotype strategy, our population-based association analysis included 216 German controls and an extreme sample of 103 severely affected alcoholics who were selected from 315 German alcohol-dependent subjects by a history of alcohol withdrawal seizure or delirium. The frequency of the short allele (S) was significantly increased in the severely affected alcoholics, compared with that in the controls (X2 = 3.87, df = 1, nominal p = 0.049). The post-hoc exploration indicated that this allelic association resulted exclusively from a significant excess of the S/S genotype in the severely affected alcoholics (p = 0.035), suggesting a recessively acting effect. Consistently, we found a weak but significant correlation (p = 0.013) between the frequency of the S/S genotype and severity of withdrawal symptoms (WDS): no WDS [18.3%, odds ratio (OR) = 1.16], vegetative WDS only (21.8%, OR = 1.44), and severe WDS with either withdrawal seizure only or delirium only (25.0%, OR = 1.69), and both withdrawal seizure and delirium (30.8%, OR = 2.30). Further studies are required to test whether the tentative genotype-phenotype relationship occurred by chance or reflects a real genotypic association between a recessively modifying effect of the short variant of the functional 5-HTT promoter polymorphism and alcohol withdrawal vulnerability.