5-HTT Knockout Research Articles

Male and female stress in serotonin transporter knockout mice

Serotonin (5-HT) is an important modulator of many physiological, behavioural and developmental processes and it plays an important role in stress coping reactions. Anxiety disorders and depression are stress-related disorders and they are associated with malfunction of the serotonin system, in which the serotonin transporter (5-HTT) plays an important role. 5-Htt knockout (KO) mice represent an artificially hyperserotonergic environment, show an increased anxiety-like behaviour and seem to be a good model to investigate the role of the serotonin system concerning stress reactions and anxiety disorders. As synaptic vesicle proteins (SVPs) seem to be involved in stress reactions we studied the effect of acute immobilization stress on SVP expression in the 5-Htt KO mouse model as well as the expression of the two immediate early genes (IEGs) FBJ osteosarcoma oncogene (c-Fos) and fos-like antigen 2 (Fra-2). Based on gender- and genotype-dependent differences in corticosterone levels we looked for expression differences in the brain by performing a quantitative real time-PCR study using primer pairs specific for the SVPs Synaptotagmin (Syt) I, Syt IV and Syntaxin (Stx) 1a and for the IEGs c-Fos and Fra-2 in five different brain regions in 5-Htt KO and wildtype mice. We found mainly gender-dependent differences but less stress effects on the expression of SVPs. Regarding the expression of IEGs we found stress-, gender- and genotype-dependent differences mainly in hypothalamus.

Genotype-dependent and stress-related morphological changes of neurons in the brain of wildtype and serotonin transporter knockout mice

Genetic variations of the serotonin transporter (5-HTT) and stressful life events have been implicated in the processing of fear and anxiety. 5-HTT knockout mice display an anxious phenotype and exaggerated adrenomedullary stress responses and are a common animal model for stress-related disorders. Stress exposure has been shown to cause neuroadaptive mechanisms including structural changes. We subjected male wildtype (WT) and 5-HTT deficient mice to a resident-intruder-paradigm which semi-chronically stresses the animals due to a loser experience, and subsequently analysed morphological changes of neurons in Golgi-Cox-stained sections of limbic brain areas in stressed and unstressed animals of both genotypes using a computer based microscopy system. While no differences concerning dendritic lengths, branching patterns and spine density were found in hippocampus and cingulate cortex, significantly lower spine number and shorter apical dendrites were detected in neurons of the infralimbic cortex of non-stressed 5-HTT deficient mice compared to non-stressed WT animals. Furthermore, infralimbic neurons of stressed WT mice displayed longer dendrites compared to non-stressed WT mice. Further analyses are presently being carried out on amygdaloid nuclei. These results indicate that, although in this model drastic alterations of neuronal morphology are absent, subtle changes are found in specific brain areas which may represent neural substrates underlying behavioural phenomena.

Gene expression in lungs of mice lacking the 5-hydroxytryptamine transporter gene

BackgroundWhile modulation of the serotonin transporter (5HTT) has shown to be a risk factor for pulmonary arterial hypertension for almost 40 years, there is a lack of in vivo data about the broad molecular effects of pulmonary inhibition of 5HTT. Previous studies have suggested effects on inflammation, proliferation, and vasoconstriction. The goal of this study was to determine which of these were supported by alterations in gene expression in serotonin transporter knockout mice.MethodsEight week old normoxic mice with a 5-HTT knock-out (5HTT-/-) and their heterozygote(5HTT+/-) or wild-type(5HTT+/+) littermates had right ventricular systolic pressure(RVSP) assessed, lungs collected for RNA, pooled, and used in duplicate in Affymetrix array analysis. Representative genes were confirmed by quantitative RT-PCR and western blot.ResultsRVSP was normal in all groups. Only 124 genes were reliably changed between 5HTT-/- and 5HTT+/+ mice. More than half of these were either involved in inflammatory response or muscle function and organization; in addition, some matrix, heme oxygenase, developmental, and energy metabolism genes showed altered expression. Quantitative RT-PCR for examples from each major group confirmed changes seen by array, with an intermediate level in 5HTT +/- mice.ConclusionThese results for the first time show the in vivo effects of 5HTT knockout in lungs, and show that many of the downstream mechanisms suggested by cell culture and ex vivo experiments are also operational in vivo. This suggests that the effect of 5HTT on pulmonary vascular function arises from its impact on several systems, including vasoreactivity, proliferation, and immune function.

Serotonin, gasping, autoresuscitation, and SIDS—a contrarian view

in their study in the Journal of Applied Physiology , Erikson and Sposato ([4][1]) induced anoxic apnea in neonatal Pet -1 knockout (KO) and wild-type mice at postnatal day (P) 4.5 and P9 and examined the subsequent quality of gasping and autoresuscitation while the animals breathed room air. Pet-1

Chronic Reductions in Serotonin Transporter Function Prevent 5-HT1B-Induced Behavioral Effects in Mice

Obsessive-compulsive disorder (OCD) is characterized by intrusive thoughts, images, or impulses and/or repetitive stereotypical behavior. Obsessive-compulsive disorder patients exhibit reduced prepulse inhibition (PPI) and symptom exacerbation after challenge with 5-HT1B receptor agonists. Recently, gain-of-function alleles of the serotonin transporter (5-HTT) have been associated with OCD. We tested the hypothesis that reducing 5-HTT function chronically, either genetically or via serotonin reuptake inhibitor (SRI) treatment, attenuates PPI deficits and perseverative hyperlocomotion induced by 5-HT1B agonists in mice. Mice received subchronic or chronic pretreatment with the SRI fluoxetine and acute treatment with RU24969 (5-HT1A/1B agonist) or 8-OH-DPAT (5-HT1A agonist) and were assessed for PPI, locomotor activity, and spatial patterns of locomotion. The same measures were evaluated in 5-HTT wild-type (WT), heterozygous (HT), and knockout (KO) mice after RU24969 treatment. The effects of WAY100635 (5-HTA antagonist) or GR127935 (5-HT1B/D antagonist) pretreatment on RU24969-induced effects were evaluated. Finally, 5-HT1B binding and functional coupling were assessed in 5-HTT-WT, -HT, and -KO mice, and normal fluoxetine-treated mice. Chronic, but not subchronic, fluoxetine treatment prevented RU24969-induced PPI deficits and perseverative hyperlocomotion. These RU24969-induced effects were mediated via 5-HT1B and not 5-HT1A receptors. 5-HTT-KO mice showed no effects of RU24969, and 5-HTT-HT mice exhibited intermediate phenotypes. 5-HT1B binding and functional coupling were reduced in the globus pallidus and substantia nigra of 5-HTT-KO mice. Our results demonstrate that chronic, but not subchronic, fluoxetine treatment and 5-HTT knockout robustly attenuate 5-HT1B agonist-induced PPI deficits and perseverative hyperlocomotion. These results may have implications for the etiology and treatment of OCD.

Serotonin transporter knockout mice have a reduced ventilatory response to hypercapnia (predominantly in males) but not to hypoxia

Medullary serotonergic (5-HT) neurons are implicated in central chemoreception and 5-HT abnormalities are present in many cases of the sudden infant death syndrome (SIDS). Mice with a targeted disruption of the serotonin transporter (5-HTT) develop in the presence of excess 5-HT in brain extracellular fluid (ECF). As adults they exhibit reduced 5-HT neuron activity and 5-HT1A receptor binding with varying changes in postsynaptic 5-HT receptor function. They exhibit behavioural phenotypes (anxiety, reduced aggression) but little is known about their control of breathing. We show that conscious adult male and female 5-HTT knockout mice breathing air at room temperature have a higher resting (.)VO2, breathing frequency and (.)VE but a normal body temperature and (.)VE/ (.)VO2 ratio (the ventilatory equivalent) compared to wild-type (WT) controls. In hypercapnia, there is a reduced ventilatory response (expressed as the (.)VE/ (.)VO2 ratio) that is much more prominent in males (-68%) than females (-22%). In hypoxia, both males and females exhibit a higher (.)VE, (.)VO2 and body temperature but their (.)VE/ (.)VO2 ratio is normal. We conclude that 5-HTT knockout mice have a diminished function of the medullary 5-HT system, which is manifest most remarkably in a substantial loss of CO2 sensitivity predominantly in males. This finding supports the importance of medullary 5-HT neurons in central chemoreception. Females either rely less on 5-HT neurons in chemoreception or adapt more readily to the loss of 5-HT function. This genetic model allows examination of the role of excess 5-HT in ECF in the development of the control of breathing and central chemoreception, which may be pertinent to SIDS.

Adult neurogenesis in serotonin transporter deficient mice

Serotonin (5-HT) is a regulator of morphogenetic activities during early brain development and neurogenesis, including cell proliferation, migration, differentiation, and synaptogenesis. The 5-HT transporter (5-HTT, SLC6A4) mediates high-affinity reuptake of 5-HT into presynaptic terminals and thereby fine-tunes serotonergic neurotransmission. Inactivation of the 5-HTT gene in mice reduces 5-HT clearance resulting in persistently increased concentrations of synaptic 5-HT. In the present study, we investigated the effects of elevated 5-HT levels on adult neurogenesis in the hippocampus of 5-HTT deficient mice, including stem cell proliferation, survival, and differentiation. Using an in vivo approach, we showed an increase in proliferative capacity of hippocampal adult neural stem cells in aged 5-HTT knockout mice (approximately 14.5 months) compared to wildtype controls. In contrast, in vivo and additional in vitro analyses of younger adult 5-HTT knockout mice (approximately 7 weeks and approximately 3.0 months) did not reveal significant changes in proliferation of neural stem cells or survival of newborn cells. We showed that the cellular fate of newly generated cells in 5-HTT knockout mice is not different with respect to the total number and percentage of neurons or glial cells from wildtype controls. Our findings indicate that elevated synaptic 5-HT concentration throughout early development and later life of 5-HTT deficient mice does not induce adult neurogenesis in adult mice, but that elevated 5-HT levels in aged mice influence stem cell proliferation.

Impaired Stress-Coping and Fear Extinction and Abnormal Corticolimbic Morphology in Serotonin Transporter Knock-Out Mice

A lesser-expressing form of the human 5-HT transporter (5-HTT) gene has been associated with increased fear and anxiety and vulnerability to the effects of stress. These phenotypic abnormalities are linked to functional and anatomical disturbances in a neural pathway connecting the prefrontal cortex (PFC) and amygdala. Likewise, rodent and nonhuman primate studies indicate a major role for PFC and amygdala in the mediation of fear- and stress-related behaviors. We used a 5-HTT knock-out (KO) mouse to examine the effects of genetically driven loss of 5-HTT function for the following: (1) depression-related behavior in response to repeated stress, and pavlovian fear conditioning, extinction, and extinction recall; and (2) dendritic morphology and spine density of Golgi-stained pyramidal neurons in the infralimbic cortex (IL) and the basolateral amygdala (BLA). 5-HTT KO mice exhibited increased depressive-like immobility after repeated exposure to forced swim stress, compared with wild-type (WT) controls. Whereas fear conditioning and fear extinction was normal, 5-HTT KO mice exhibited a significant deficit in extinction recall. The apical dendritic branches of IL pyramidal neurons in 5-HTT KO mice were significantly increased in length relative to WT mice. Pyramidal neurons in BLA had normal dendritic morphology but significantly greater spine density in 5-HT KO mice compared with WT mice. Together, the present findings demonstrate a specific phenotypic profile of fear- and stress-related deficits in 5-HTT KO mice, accompanied by morphological abnormalities in two key neural loci. These data provide insight into the behavioral sequelae of loss of 5-HTT gene function and identify potential neural substrates underlying these phenotypes.

Effects of Mild Early Life Stress on Abnormal Emotion-related Behaviors in 5-HTT Knockout Mice

A low-expressing polymorphic variant of the serotonin transporter (5-HTT) gene has been associated with emotional disorders in humans and non-human primates following exposure to early life trauma. 5-HTT gene knockout (KO) mice exhibit increased anxiety- and depression-related behaviors, and provide a model to study interactions between 5-HTT gene variation and early life stress. The present study assessed the effects of postnatal footshock stress on the development of emotion-related behaviors in 5-HTT KO mice. Results showed that 5-HTT KO mice displayed a profile of suppressed exploratory behavior and increased anxiety-like behavior in the light/dark, elevated plus-maze and open field tests, as well as increased depression-related behavior in the forced swim test following repeated exposure to the test. Postnatal exposure to footshock stress did not affect emotion-related behaviors in non-mutant C57BL/6J mice or modify phenotypic abnormalities in 5-HTT KO. Data provide further evidence of emotional abnormalities following genetic disruption of the 5-HTT.

Ethanol‐Related Behaviors in Serotonin Transporter Knockout Mice

Increasing evidence supports a role for 5-hydroxytryptamine (5-HT) and the 5-HT transporter (5-HTT) in modulating the neural and behavioral actions of ethanol (EtOH) and other drugs of abuse. We used a 5-HTT knockout (KO) mouse model to further study this relationship. 5-Hydroxytryptamine transporter KO mice were tested for the sedative/hypnotic, hypothermia-inducing, motor-incoordinating (via accelerating rotarod), and depression-related (via tail suspension test) effects of acute EtOH administration. Reward-related effects of EtOH were assessed in 5-HTT KO mice using the conditioned place preference (CPP) paradigm. 5-Hydroxytryptamine transporter KO mice were tested for voluntary consumption of EtOH in a modified 2-bottle choice test that measured the temporal organization of drinking over the circadian cycle via "lickometers." Replicating previous findings, 5-HTT KO mice exhibited significantly increased sensitivity to EtOH-induced sedation/hypnosis relative to wild-type controls. Additionally, 5-HTT KO mice showed motor-coordination deficits at baseline and in response to EtOH. Hypothermic, pro-depressive-like, and reward-related effects of EtOH were no different across genotypes. Gross EtOH consumption was modestly reduced in 5-HTT KO mice, due to significantly lesser consumption during the peak period of drinking in the early dark phase. Data extend the finding that loss of 5-HTT gene function alters certain neural and behavioral effects of EtOH, with implications for better understanding the pathophysiology and treatment of alcoholism.

Altered Neocortical Cell Density and Layer Thickness in Serotonin Transporter Knockout Mice: A Quantitation Study

The neurotransmitter serotonin (5-HT) plays morphogenetic roles during development, and their alteration could contribute to autism pathogenesis in humans. To further characterize 5-HT's contributions to neocortical development, we assessed the thickness and neuronal cell density of various cerebral cortical areas in serotonin transporter (5-HTT) knockout (ko) mice, characterized by elevated extracellular 5-HT levels. The thickness of layer IV is decreased in 5-HTT ko mice compared with wild-type (wt) mice. The overall effect on cortical thickness, however, depends on the genetic background of the mice. Overall cortical thickness is decreased in many cortical areas of 5-HTT ko mice with a mixed c129-CD1-C57BL/6J background. Instead, 5-HTT ko mice backcrossed into the C57BL/6J background display increases in supragranular and infragranular layers, which compensate entirely for decreased layer IV thickness, resulting in unchanged or even enhanced cortical thickness. Moreover, significant increases in neuronal cell density are found in 5-HTT ko mice with a C57BL/6J background (wt:hz:ko ratio = 1.00:1.04:1.17) but not in the mixed c129-CD1-C57BL/6J 5-HTT ko animals. These results provide evidence of 5-HTT gene effects on neocortical morphology in epistatic interaction with genetic variants at other loci and may model the effect of functional 5-HTT gene variants on neocortical development in autism.

Unaltered susceptibility to scrapie in serotonin transporter deficient mice

The serotonergic system has been hypothesized to play an important role in prion diseases. Specifically, hyperactivity of the serotonergic system in prion diseases is suggested by an increase in the turnover rate of the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) in human and experimental prion diseases. The 5-HT transporter (5-HTT) determines the duration of serotonergic neurotransmission by way of reuptake of 5-HT from the extracellular space. 5-HTT availability is reduced in brains of patients with the human prion disease familial fatal insomnia. To further clarify a possible role of the 5-HTT in prion diseases we investigated whether mice lacking the 5-HTT display an altered susceptibility to experimental scrapie infection. Surprisingly, 5-HTT knockout mice developed mouse scrapie in a time course similar to wildtype control mice with accumulation of the pathological prion protein, PrP Sc and with typical pathological hallmarks of the disease. These findings argue against a major role of the 5-HTT in the pathogenesis of prion diseases in mice.

Absence of reuptake of serotonin influences susceptibility to clinical autoimmune disease and neuroantigen-specific interferon-gamma production in mouse EAE.

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-gamma 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.

Female, but not male, serotonin reuptake transporter (5-HTT) knockout mice exhibit bladder instability

Correlations exist between the incidence of depression, irritable bowel syndrome (IBS) and overactive bladder [Masand, P.S., Kaplan, D.S., Gupta, S., Bhandary, A.N., Nasra, G.S., Kline, M.D., Margo, K.L., 1995. Major depression and irritable bowel syndrome: is there a relationship? J. Clin. Psychiatry 56, 363–367.; Cukier, J.M., Cortina-Borja, M., Brading, A.F., 1997. A case-control study to examine any association between idiopathic detrusor instability and gastrointestinal tract disorder, and between irritable bowel syndrome and urinary tract disorder. Br. J. Urol. 79, 865–878.; Monga, A.K., Marrero, J.M., Stanton, S.L., Lemieux, M.C., Maxwell, J.D., 1997. Is there an irritable bladder in the irritable bowel syndrome? Br. J. Obstet. Gynaecol. 104, 1409–1412.; Zorn, B.H., Montgomery, H., Pieper, K., Gray, M., Steers, W.D., 1999. Urinary incontinence and depression. J. Urol. 162, 82–84.]. Furthermore, alterations in serotonergic neurotransmission may play a common role in the etiology of these disorders. Serotonin reuptake transporter knockout mice (5-HTT -/-) display phenotypes consistent with clinical features of mood and bowel disorders including anxiety and abnormal gastrointestinal motility [Holmes, A., Murphy, D.L., Crawley, J.N., 2003. Abnormal behavioral phenotypes of serotonin transporter knockout mice: parallels with human anxiety and depression. Biol. Psychiatry 54, 953–959.]. In the present study, we evaluated bladder function in 5-HTT -/- mice. We have found that female 5-HTT -/- mice exhibit bladder dysfunction, characterized by significant increases in the frequency of spontaneous non-voiding bladder contractions and decreases in void volume compared to control female mice. These differences were not observed in male knockout mice. These studies provide significant supporting data for a mechanistic link between alterations in 5-HT, depression, IBS and overactive bladder in women.

An organic cation transporter capable of transporting serotonin is upregulated in serotonin transporter-deficient mice

The serotonin (5HT) transporter (5HTT) regulates serotonergic neurotransmission by mediating the reuptake of 5HT from the synaptic cleft. The organic cation transporter (OCT) gene family physiologically transports a wide spectrum of organic cations. Although lacking the high affinity and selectivity of the 5HTT, OCT1 and OCT3 mediate low-affinity 5HT transport and therefore may participate in the clearance of excessive 5HT. Since concentrations of extracellular 5HT are increased in the brain of 5HTT-deficient mice, we investigated the role of OCTs in 5HT system homeostasis. We analyzed OCT1 and OCT3 gene expression in the brain of 5HTT knockout mice by semi-quantitative competitive PCR and in situ hybridization. We demonstrate that in 5HTT-deficient mice OCT3 mRNA concentrations were significantly increased in the hippocampus. In contrast, OCT1 expression was unchanged. Upregulation 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.

P.1.155 Remission of early responders in patients with major depression treated with mirtazapine

The interaction of early life stress (ELS) and the serotonin transporter (5-HTT) gene-linked polymorphic region (5-HTTLPR) has been associated with increased risk to develop depression in later life. We have used the maternal separation paradigm as a model for ELS exposure in homozygous and heterozygous 5-HTT knockout rats and measured urocortin 1 (Ucn1) mRNA and/or protein levels, Ucn1 DNA methylation, as well as 5-HT innervation in the centrally projecting Edinger–Westphal (EWcp) and dorsal raphe (DR) nuclei, both implicated in the regulation of stress response. We found that ELS and 5-HTT genotype increased the number of 5-HT neurons in specific DR subdivisions, and that 5-HTT knockout rats showed decreased 5-HT innervation of EWcp-Ucn1 neurons. Furthermore, ELS was associated with increased DNA methylation of the promoter region of the Ucn1 gene and increased expression of 5-HT receptor 1A in the EWcp. In contrast, 5-HTT deficiency was associated with site-specific alterations in DNA methylation of the Ucn1 promoter, and heterozygous 5-HTT knockout rats showed decreased expression of CRF receptor 1 in the EWcp. Together, our findings extend the existing literature on the relationship between EWcp-Ucn1 and DR-5-HT neurons. These observations will further our understanding on their potential contribution to mediate affect as a function of ELS interacting with 5-HTTLPR.

Exaggerated effect of fluvoxamine in heterozygote serotonin transporter knockout mice.

Clearance rates for serotonin (5-HT) in heterozygote (+/-) and homozygote (-/-) serotonin transporter (5-HTT) knockout (KO) mice have not been determined in vivo. Moreover, the effect of selective serotonin reuptake inhibitors (SSRIs) on 5-HT clearance in these mice has not been examined. In this study, the rate of clearance of exogenously applied 5-HT was measured in the CA3 region of the hippocampus of anesthetized mice using high-speed chronoamperometry. Compared with wild-type mice, the maximal rate of 5-HT clearance from extracellular fluid (ECF) was decreased in heterozygotes and more markedly so in KO mice. Heterozygote mice were more sensitive to the 5-HT uptake inhibitor, fluvoxamine, resulting in longer clearance times for 5-HT than in wild-type mice; as expected, the KO mice were completely unresponsive to fluvoxamine. There were no associated changes in norepinephrine transporter density, nor was there an effect of the norepinephrine uptake inhibitor, desipramine, on 5-HT clearance in any genotype. Thus, adaptive changes in the norepinephrine transport system do not occur in the CA3 region of hippocampus as a consequence of 5-HTT KO. These data highlight the potential of the heterozygote 5-HTT mutant mice to model the dynamic in vivo consequences of the human 5-HTT polymorphism.

Reduced programmed cell death in brains of serotonin transporter knockout mice.

Serotonin (5-HT) is known to reduce apoptosis and in rodent models of brain ischemia. Modulation of programmed cell death during neural development was assessed in early postnatal brains of serotonin transporter (5-HTT) knockout mice, characterized by elevated extracellular 5-HT levels. The number of apoptotic cells visualized at postnatal day-1 (P1) by ISEL+ or TUNEL staining was significantly reduced in the striatum, thalamus/hypothalamus, cerebral cortex and hippocampus of 5-HTT knockout mice, compared to wild type and heterozygote mice, with differences displaying an increasing fronto-caudal gradient and regional specificity. These findings underscore 5-HT roles in the regulation of programmed cell death during brain development, and spur interest into pharmacological interventions aimed at relieving pathological apoptosis by potentiating serotoninergic neurotransmission.

Brain region-specific alterations of 5-HT2A and 5-HT2C receptors in serotonin transporter knockout mice.

The aim of the present studies was to determine the effects of reduced or absent serotonin (5-HT) transporters (5-HTTs) on 5-HT2A and 5-HT2C receptors. The density of 5-HT2C receptors was significantly increased in the amygdala and choroid plexus of 5-HTT knockout mice. On the other hand, the density of 5-HT2A receptors was significantly increased in the hypothalamus and septum, but reduced in the striatum, of 5-HTT knockout mice. However, 5-HT2A mRNA was not changed in any brain region measured. 5-HT2C mRNA was significantly reduced in the choroid plexus and lateral habenula nucleus of these mice. The function of 5-HT2A receptors was evaluated by hormonal responses to (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). Oxytocin, but not adrenocorticotrophic hormone or corticosterone, responses to DOI were significantly greater in 5-HTT knockout mice. In addition, Gq and G11 proteins were not significantly changed in any brain region measured. The present results suggest that the constitutive alteration in the function of 5-HTTs changes the density of 5-HT2A and 5-HT2C receptors in a brain region-specific manner. These changes may not be mediated by alterations in their gene expression or in the level of Gq/11 proteins. The alterations in these receptors may be related to the altered behaviors of 5-HTT knockout mice.

Organic cation transporter capable of transporting serotonin is up-regulated in serotonin transporter-deficient mice.

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.