The norepinephrine transporter in physiology and disease

Tributyltin exposure influences predatory behavior, neurotransmitter content and receptor expression in Sebastiscus marmoratus

Tributyltin (TBT) is a ubiquitous marine contaminant due to its extensive use as a biocide, fungicide and antifouling agent. However, the neurotoxic effect of TBT has not been extensively studied, especially in marine fish. This study was conducted to investigate the effects of TBT (10, 100 and 1000 ng/L) on the predatory behavior of Sebastiscus marmoratus and to look into the mechanism involved. The results showed that TBT exposure depressed predatory activity after 50 days exposure. Dopamine levels in the fish brains increased in a dose-dependent manner, while 5-hydroxytryptamine and norepinephrine levels decreased significantly in the TBT exposure group compared to the control. The mRNA levels of dopamine receptors, which have functions such as cognition, motor activity, motivation and reward, mood, attention and learning, were significantly down-regulated by TBT exposure. Although the levels of amino acid neurotransmitters, including glutamate, did not show marked alteration, the expression of the glutamatergic signaling pathway such as N-methyl-D-aspartate receptors, a-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor, calmodulin, Ca(2+)/calmodulin-dependent protein kinases-II and cyclic adenosine monophosphate responsive element binding protein, was significantly reduced by TBT exposure, which indicated that central nerve activities were in a state of depression, thus affecting the predatory activities of the fish.

Investigation of the correlation between norepinephrine transporter gene polymorphisms and essential hypertension

Essential hypertension (EH) is an etiological risk factor that poses a serious threat to human health. The aim of this study therefore was to investigate the correlation between the two SNPs (rs1805067 and rs2397771) of the norepinephrine (NE) transporter gene in the sympathetic nervous system and essential hypertension (EH). A total of 96 hypertensive patients (EH group) and 96 normal subjects (control group) were selected by epidemiological investigation and the sequences at rs1805067 and rs2397771 in the two groups were investigated using polymerase chain reaction (PCR) and Sanger sequencing. In the EH group, systolic and diastolic blood pressure, body mass index (BMI), levels of creatinine, low-density lipoprotein cholesterol (LDL-C), triglycerides (TG), and the percentage of drinkers were found to be higher compared with those of the control group (P<0.05). The results of the analysis of the allele frequencies of rs1805067 (A/G) revealed that there was no difference between the two groups (P>0.05). The analysis results of the allele frequencies of rs2397771 (C/G) in the EH group revealed that G accounted for 48.96% and C for 51.04% of the patients, while in the control group, these values were 64.58 and 35.42%, respectively. These results indicated a significant difference between the two groups (P<0.05). In the population studied, the onset of EH may be correlated with BMI, TG, LDL-C and high-density lipoprotein cholesterol (HDL-C). EH may be related to the rs2397771 (C/G) C allele polymorphism of the NE transporter gene but not to rs1805067 (A/G).

Characterization of [³H]CFT binding to the norepinephrine transporter suggests that binding of CFT and nisoxetine is not mutually exclusive

The norepinephrine transporter (NET) is an important target for a wide variety of antidepressants and psychostimulants. Despite its prominence as a drug target, there is only one radioligand in use for NET competitive binding assays, [(3)H]nisoxetine. However, traditional [(3)H]nisoxetine binding protocols often give an underestimation for the affinity of certain classes of NET ligands, particularly cocaine and other tropanes. Here, we explore the feasibility of using the phenyltropane [(3)H]CFT for labeling human NET (hNET) in heterologous cell-based binding studies. Assays were optimized for time and protein content and specific, one-site binding was observed. Potencies of tested NET ligands for inhibition of [(3)H]CFT binding to whole cells (at physiological [Na(+)] and 25°C) were similar to potencies observed in the [(3)H]NE uptake assay. Inhibition constants (K(i)) for binding assays were highly correlated with uptake inhibition constants for all compounds tested (R(2)=0.99, p<0.0001). Cell-free membrane preparations did not display the same pharmacological profile. Under conditions routinely used for measuring [(3)H]nisoxetine binding to membrane preparations (4°C for 3h, [Na(+)] at 295 mM), the potency of nisoxetine and desipramine in inhibiting [(3)H]CFT binding became greater than that measured in a functional assay of [(3)H]NE uptake at physiological [Na(+)]. However, the opposite was true for CFT and cocaine. Interestingly, while investigating [(3)H]CFT as a potential NET radioligand, we uncovered evidence suggesting that CFT and nisoxetine are not mutually exclusive in binding to the NET. Dixon plots of the interaction between nisoxetine and CFT in inhibition of [(3)H]dopamine uptake by the NET indicate that the two compounds can simultaneously bind to the transporter.

Influence and interaction of genetic polymorphisms in catecholamine neurotransmitter systems and early life stress on antidepressant drug response

BACKGROUND

Catecholamine neurotransmission plays an important role in major depression. Variation in genes implicated in the synthesis and signal transduction of catecholamines (norepinephrine and dopamine) may interact with environmental factors to affect the outcome of antidepressant treatment. We aimed to determine how a range of polymorphisms in noradrenergic and dopaminergic genes influence this response to treatment and how they interact with childhood trauma and recent life stress in a Chinese sample.

METHODS

In a sample of 308 Chinese Han patients with major depressive disorder, 13 single nucleotide polymorphisms (SNPs) in coding regions of six genes (MAOA, SLC6A2, TH, COMT, DRD2, DRD3) with minor allele frequencies >5% were successfully genotyped from an initial series of 35 SNPs in 11 candidate genes associated with catecholamine neurotransmission. The responses to 6 weeks' treatment with antidepressant drugs was determined by changes in the 17-item Hamilton Depression Rating Scale (HAMD-17) score, and previous stressful events were evaluated by the Life Events Scale (LES) and Childhood Trauma Questionnaire-Short Form (CTQ-SF). Single SNP and haplotype associations with treatment response were analysed by UNPHASED 3.0.13, gene-gene interactions were analysed by generalized multifactor dimensionality reduction (GMDR) and gene-environment interactions by logistic regression.

RESULTS

A haplotype in MAOA (rs1137070 and rs6323) was significantly associated with antidepressant response in the total group, the nonSSRI subgroup and the female subgroup. Two haplotypes in COMT (involving rs4633, rs4818 and rs769224) were significantly associated with antidepressant response in the nonSSRI subgroup. The SLC6A2 SNPs interacted with childhood trauma to influence antidepressant response.

CONCLUSIONS

A haplotype in MAOA and two haplotypes in COMT are found to be associated with antidepressant treatment response in this sample. Stressors in early life may interact with polymorphisms in SLC6A2 to influence response to antidepressant treatment.

The ugly side of amphetamines: short- and long-term toxicity of 3,4-methylenedioxymethamphetamine (MDMA, 'Ecstasy'), methamphetamine and D-amphetamine

Amphetamine ('Speed'), methamphetamine ('Ice') and its congener 3,4-methylenedioxymethamphetamine (MDMA; 'Ecstasy') are illicit drugs abused worldwide for their euphoric and stimulant effects. Despite compelling evidence for chronic MDMA neurotoxicity in animal models, the physiological consequences of such toxicity in humans remain unclear. In addition, distinct differences in the metabolism and pharmacokinetics of MDMA between species and different strains of animals prevent the rationalisation of realistic human dose paradigms in animal studies. Here, we attempt to review amphetamine toxicity and in particular MDMA toxicity in the pathogenesis of exemplary human pathologies, independently of confounding environmental factors such as poly-drug use and drug purity.

The expression level of ecto-NTP diphosphohydrolase1/CD39 modulates exocytotic and ischemic release of neurotransmitters in a cellular model of sympathetic neurons

Once released, norepinephrine is removed from cardiac synapses via reuptake into sympathetic nerves, whereas transmitter ATP is catabolized by ecto-NTP diphosphohydrolase 1 (E-NTPDase1)/CD39, an ecto-ATPase. Because ATP is known to modulate neurotransmitter release at prejunctional sites, we questioned whether this action may be ultimately controlled by the expression of E-NTPDase1/CD39 at sympathetic nerve terminals. Accordingly, we silenced E-NTPDase1/CD39 expression in nerve growth factor-differentiated PC12 cells, a cellular model of sympathetic neuron, in which dopamine is the predominant catecholamine. We report that E-NTPDase1/CD39 deletion markedly increases depolarization-induced exocytosis of ATP and dopamine and increases ATP-induced dopamine release. Moreover, overexpression of E-NTPDase1/CD39 resulted in enhanced removal of exogenous ATP, a marked decrease in exocytosis of ATP and dopamine, and a large decrease in ATP-induced dopamine release. Administration of a recombinant form of E-NTPDase1/CD39 reproduced the effects of E-NTPDase1/CD39 overexpression. Exposure of PC12 cells to simulated ischemia elicited a release of ATP and dopamine that was markedly increased in E-NTPDase1/CD39-silenced cells and decreased in E-NTPDase1/CD39-overexpressing cells. Therefore, transmitter ATP acts in an autocrine manner to promote its own release and that of dopamine, an action that is controlled by the level of E-NTPDase1/CD39 expression. Because ATP availability greatly increases in myocardial ischemia, recombinant E-NTPDase1/CD39 therapeutically used may offer a novel approach to reduce cardiac dysfunctions caused by excessive catecholamine release.

Regulation of monoamine transporters: Role of transporter phosphorylation

Presynaptic biogenic amine transporters mediate reuptake of released amines from the synapse, thus regulating serotonin, dopamine and norepinephrine neurotransmission. Medications utilized in the treatment of depression, attention deficit-hyperactivity disorder and other psychiatric disorders possess high affinity for amine transporters. In addition, amine transporters are targets for psychostimulants. Altered expression of biogenic amine transporters has long been implicated in several psychiatric and degenerative disorders. Therefore, appropriate regulation and maintenance of biogenic amine transporter activity is critical for the maintenance of normal amine homoeostasis. Accumulating evidence suggests that cellular protein kinases and phosphatases regulate amine transporter expression, activity, trafficking and degradation. Amine transporters are phosphoproteins that undergo dynamic control under the influence of various kinase and phosphatase activities. This review presents a brief overview of the role of amine transporter phosphorylation in the regulation of amine transport in the normal and diseased brain. Understanding the molecular mechanisms by which phosphorylation events affect amine transporter activity is essential for understanding the contribution of transporter phosphorylation to the regulation of monoamine neurotransmission and for identifying potential new targets for the treatment of various brain diseases.

Insulin reveals Akt signaling as a novel regulator of norepinephrine transporter trafficking and norepinephrine homeostasis

Noradrenergic signaling in the CNS plays an essential role in circuits involving attention, mood, memory, and stress as well as providing pivotal support for autonomic function in the peripheral nervous system. The high-affinity norepinephrine (NE) transporter (NET) is the primary mechanism by which noradrenergic synaptic transmission is terminated. Data indicate that NET function is regulated by insulin, a hormone critical for the regulation of metabolism. Given the high comorbidity of metabolic disorders such as diabetes and obesity with mental disorders such as depression and schizophrenia, we sought to determine how insulin signaling regulates NET function and thus noradrenergic homeostasis. Here, we show that acute insulin treatment, through the downstream kinase protein kinase B (Akt), significantly decreases NET surface expression in mouse hippocampal slices and superior cervical ganglion neuron boutons (sites of synaptic NE release). In vivo manipulation of insulin/Akt signaling, with streptozotocin, a drug that induces a type 1-like diabetic state in mice, also results in aberrant NET function and NE homeostasis. Notably, we also demonstrate that Akt inhibition or stimulation, independent of insulin, is capable of altering NET surface availability. These data suggest that aberrant states of Akt signaling such as in diabetes and obesity have the potential to alter NET function and noradrenergic tone in the brain. Furthermore, they provide one potential molecular mechanism by which Akt, a candidate gene for mood disorders such as schizophrenia and depression, can impact brain monoamine homeostasis.

Prefrontal cortex and drug abuse vulnerability: translation to prevention and treatment interventions

Vulnerability to drug abuse is related to both reward seeking and impulsivity, two constructs thought to have a biological basis in the prefrontal cortex (PFC). This review addresses similarities and differences in neuroanatomy, neurochemistry and behavior associated with PFC function in rodents and humans. Emphasis is placed on monoamine and amino acid neurotransmitter systems located in anatomically distinct subregions: medial prefrontal cortex (mPFC); lateral prefrontal cortex (lPFC); anterior cingulate cortex (ACC); and orbitofrontal cortex (OFC). While there are complex interconnections and overlapping functions among these regions, each is thought to be involved in various functions related to health-related risk behaviors and drug abuse vulnerability. Among the various functions implicated, evidence suggests that mPFC is involved in reward processing, attention and drug reinstatement; lPFC is involved in decision-making, behavioral inhibition and attentional gating; ACC is involved in attention, emotional processing and self-monitoring; and OFC is involved in behavioral inhibition, signaling of expected outcomes and reward/punishment sensitivity. Individual differences (e.g., age and sex) influence functioning of these regions, which, in turn, impacts drug abuse vulnerability. Implications for the development of drug abuse prevention and treatment strategies aimed at engaging PFC inhibitory processes that may reduce risk-related behaviors are discussed, including the design of effective public service announcements, cognitive exercises, physical activity, direct current stimulation, feedback control training and pharmacotherapies. A major challenge in drug abuse prevention and treatment rests with improving intervention strategies aimed at strengthening PFC inhibitory systems among at-risk individuals.

Depression and obesity: do shared genes explain the relationship?

BACKGROUND

Studies have found a modest association between depression and obesity, especially in women. Given the substantial genetic contribution to both depression and obesity, we sought to determine whether shared genetic influences are responsible for the association between these two conditions.

METHODS

Data were obtained from 712 monozygotic and 281 dizygotic female twin pairs who are members of the community-based University of Washington Twin Registry. The presence of depression was determined by self-report of doctor-diagnosed depression. Obesity was defined as body mass index of > or =30 kg/m(2), based on self-reported height and weight. Generalized estimating regression models were used to assess the age-adjusted association between depression and obesity. Univariate and bivariate structural equation models estimated the components of variance attributable to genetic and environmental influences.

RESULTS

We found a modest phenotypic association between depression and obesity (odds ratio=1.6, 95% confidence interval=1.2-2.1). Additive genetic effects contributed substantially to depression (57%) and obesity (81%). The best-fitting bivariate model indicated that 12% of the genetic component of depression is shared with obesity.

CONCLUSIONS

The association between depression and obesity in women may be in part due to shared genetic risk for both conditions. Future studies should examine the genetic, environmental, social, and cultural mechanisms underlying the relationship between this association.

Rab11 supports amphetamine-stimulated norepinephrine transporter trafficking

The norepinephrine transporter (NET) is a presynaptic plasma membrane protein that mediates reuptake of synaptically released norepinephrine. NET is also a major target for medications used for the treatment of depression, attention deficit/hyperactivity disorder, narcolepsy, and obesity. NET is regulated by numerous mechanisms, including catalytic activation and membrane trafficking. Amphetamine (AMPH), a psychostimulant and NET substrate, has also been shown to induce NET trafficking. However, neither the molecular basis nor the nature of the relevant membrane compartments of AMPH-modulated NET trafficking has been defined. Indeed, direct visualization of drug-modulated NET trafficking in neurons has yet to be demonstrated. In this study, we used a recently developed NET antibody and the presence of large presynaptic boutons in sympathetic neurons to examine basal and AMPH-modulated NET trafficking. Specifically, we establish a role for Rab11 in AMPH-induced NET trafficking. First, we found that, in cortical slices, AMPH induces a reduction in surface NET. Next, we observed AMPH-induced accumulation and colocalization of NET with Rab11a and Rab4 in presynaptic boutons of cultured neurons. Using tagged proteins, we demonstrated that NET and a truncated Rab11 effector (FIP2DeltaC2) do not redistribute in synchrony, whereas NET and wild-type Rab11a do. Analysis of various Rab11a/b mutants further demonstrates that Rab11 regulates NET trafficking. Expression of the truncated Rab11a effector (FIP2DeltaC2) attenuates endogenous Rab11 function and prevented AMPH-induced NET internalization as does GDP-locked Rab4 S22N. Our data demonstrate that AMPH leads to an increase of NET in endosomes of single boutons and varicosities in a Rab11-dependent manner.

Interaction of biogenic amine hormones with acetaldehyde

Dopamine (DA), epinephrine (EP), serotonin (5-HT), norepinephrine (NE), and histamine (H) are each hormones derived from aromatic or basic amino acids. Each contains primary or secondary amines, and all, except H, contain hydroxyl groups. These functional groups are capable of reacting with acetaldehyde (AcH), the primary product of ethanol metabolism. In this study, it is shown that 5-min preincubations of the amines with AcH affect a reduction in the prolongation of activated partial thromboplastin time (aPTT) which is caused by AcH. The data suggest that these hormones may form Schiff bases, hemiacetals, and acetals, thereby detoxifying the AcH. It is also suggested that physiological effects of these hormones may be lowered as a consequence of interaction with acetaldehyde in alcoholics. In the absence of AcH, 0.91 mM DA, 5-HT, and EP each exhibited a small statistically significant procoagulant effect on the aPTT assay with values of 30.8 +/- 0.3 s (P = 0.03), 28.5 +/- 1.0 s (P = 0.006), and 31.3 +/- 0.2 s (P = 0.001), respectively, relative to the controls of 32.6 +/- 0.6 s, 32.9 +/- 0.6 s, and 33.0 +/- 0.3 s. Values for 0.91 mM NE and 9.1 mM H were 33.5 +/- 0.8 s (P = 0.14) and 36.0 +/- 0.8 s (P = 0.14), respectively, relative to the controls (32.4 +/- 0.2 s and 34.2 +/- 0.4 s), and exerted a nonsignificant effect upon aPTT.

Membrane-Bound Catechol-O-Methyl Transferase in Cortical Neurons and Glial Cells is Intracellularly Oriented

Catechol-O-methyl transferase (COMT) is involved in the inactivation of dopamine in brain regions in which the dopamine transporter (DAT1) is sparsely expressed. The membrane-bound isoform of COMT (MB-COMT) is the predominantly expressed form in the mammalian central nervous system (CNS). It has been a matter of debate whether in neural cells of the CNS the enzymatic domain of MB-COMT is oriented toward the cytoplasmic or the extracellular compartment. Here we used live immunocytochemistry on cultured neocortical neurons and glial cells to investigate the expression and membrane orientation of native COMT and of transfected MB-COMT fused to green fluorescent protein (GFP). After live staining, COMT immunoreactivity was reliably detected in both neurons and glial cells after permeabilization, but not on unpermeabilized cells. Similarly, autofluorescence of COMT-GFP fusion protein and antibody fluorescence showed overlap only in permeabilized neurons. Our data provide converging evidence for an intracellular membrane orientation of MB-COMT in neurons and glial cells, suggesting the presence of a DAT1-independent postsynaptic uptake mechanism for dopamine, prior to its degradation via COMT.

Interaction of the human plasma membrane monoamine transporter (hPMAT) with antidepressants and antipsychotics

Monoamine neurotransmission is efficiently terminated through synaptic reuptake of released neurotransmitters by high-affinity Na(+)- and Cl(-)-dependent neuronal monoamine transporters of the SLC6A family located in the plasma membrane of presynaptic nerve terminals. Recently, a low-affinity, high-capacity Na(+)- and Cl(-)-independent plasma membrane monoamine transporter (PMAT) belonging to the SLC29 solute carrier family has been cloned. PMAT was shown to transport monoamine neurotransmitters as well as organic cations such as 1-phenyl-4-methyl-pyridinium (MPP(+)). Thus, the PMAT which is highly expressed in the human brain may be involved in the modulation of central monoaminergic neurotransmission and it may be a target for drugs used to treat depression and schizophrenia, i.e., dysregulations of the monoamine homeostasis in the central nervous system (CNS). Therefore, we examined in transfected cells the influence on [(3)H]-MPP(+) transport by the human PMAT (hPMAT) of nine monoamine transport inhibiting antidepressants (ADs) belonging to pharmacologically diverse classes (imipramine, desipramine, amitriptyline, bupropion, fluoxetine, sertraline, paroxetine, reboxetine, and venlafaxine), of the atypical ADs tianeptine and trimipramine and of five antipsychotics (levomepromazine, haloperidol, clozapine, olanzapine, and risperidone). All examined drugs inhibited the hPMAT; however, half-maximum inhibition (IC(50)) was observed at concentrations which were much higher than reported clinical plasma concentrations of these drugs. Thus, inhibition of the hPMAT by these CNS drugs may not (or only marginally) contribute to their therapeutic effects.

Is elevated noradrenaline an aetiological factor in a number of diseases?

1 Here I put forth the hypothesis that noradrenaline (NA), which is a signalling molecule in the brain and sympathetic nervous system (SNS), is an aetiological factor in a number of diseases. 2 In a previous paper (Fitzgerald, Int. J. Cancer, 124, 2009, 257), I examined evidence that elevated NA is a factor in various types of cancer. Here I extend the argument to several other diseases, including diabetes mellitus, open-angle glaucoma, osteoarthritis and rheumatoid arthritis and asthma. 3 The principal hypothesis is that, largely as a result of genetics, elevated noradrenergic tone in the SNS predisposes a large number of individuals to a broad range of diseases. 4 For each of the above five diseases, I briefly examine the following four lines of evidence to assess the hypothesis: i) whether pharmacological studies in rodents that manipulate NA levels or receptors affect these diseases; ii) whether pharmacological manipulation of NA in humans affects these diseases; iii) whether bipolar disorder, excessive body weight, and hypertension, which may all three involve elevated NA, tend to be comorbid with these diseases and iv) whether psychological stressors tend to cause or exacerbate these conditions, since psychological stress is associated with increased release of NA. 5 The four lines of evidence tend to support the hypothesis.

Interference of the noradrenergic neurotoxin DSP4 with neuronal and nonneuronal monoamine transporters

The haloalkylamine DSP4 (N[-2-chloroethyl]-N-ethyl-2-bromobenzylamine) is a noradrenergic neurotoxin, which is used for the chemical denervation of noradrenergic neurons, and it has been proposed to be a selective substrate for the neuronal, Na(+)- and Cl(-)-dependent noradrenaline transporter (NAT). In the present study, we investigated whether DSP4 not only interacts with the human NAT (hNAT) but also with other neuronal monoamine transporters such as the transporters for dopamine (hDAT) and serotonin (hSERT) or with nonneuronal (Na(+)-independent) monoamine transporters also known as organic cation transporters (OCTs), such as hOCT(1), hOCT(2), and hOCT(3). Using human embryonic kidney HEK293 cells heterologously expressing the corresponding transporter, we show that DSP4 irreversibly inhibits the hNAT, hDAT, hSERT, and hOCT(3). However, this inhibition includes a reversible component at the hDAT, hSERT, and hOCT(3) but not at the hNAT. The inhibitory potency of DSP4 at the neuronal transporters was highest at the hNAT (IC(50) about 5 microM), and it was about five and 40 times lower at the hSERT and hDAT, respectively. DSP4 inhibited all three hOCTs with high potency (IC(50) about 1 microM) but in a completely reversible manner at hOCT(1) and hOCT(2). Cytotoxicity by 24-h exposure of hNAT- or hOCT-expressing cells to low DSP4 concentrations (<10 microM) could be observed only in hNAT-expressing cells. Thus, DSP4's high-affinity uptake through the NAT together with its completely irreversible mode of interaction with the NAT may contribute to its selectivity as noradrenergic neurotoxin.

Association of major depression with rare functional variants in norepinephrine transporter and serotonin1A receptor genes

Dysregulations of central noradrenergic and serotonergic neurotransmission have been suggested to contribute to the pathogenesis of neuropsychiatric disorders such as depression. The norepinephrine transporter (NET; SLC6A2) and the serotonin (5-HT)(1A) receptor (5-HT(1A) receptor; HTR1A) play an important role in central nervous monoaminergic homeostasis. As shown previously, variations in the human NET and 5-HT(1A) receptor genes can alter noradrenergic and serotonergic signaling in the brain: a single nucleotide polymorphism (SNP) in the coding region of the NET gene resulting in a F528C substitution increased plasma membrane expression of this NET variant, and a SNP in the human 5-HT(1A) receptor gene leading to the R219L receptor variant almost abolished cellular signal transduction subsequent to receptor activation. The present study aimed at investigating whether these NET and 5-HT(1A) receptor variants are associated with major depression (MD). The sample comprised 426 patients suffering from unipolar MD as well as 643 healthy control subjects for the variants of the 5-HT(1A) receptor and the NET. Both SNPs were shown to be associated with MD. In conclusion, our results favor the hypothesis that monoaminergic neurotransmission in general and the F528C NET and R219L 5-HT(1A) receptor variants in particular are involved in the pathogenesis of depression.

Knockout of the norepinephrine transporter and pharmacologically diverse antidepressants prevent behavioral and brain neurotrophin alterations in two chronic stress models of depression

Diverse factors such as changes in neurotrophins and brain plasticity have been proposed to be involved in the actions of antidepressant drugs (ADs). However, in mouse models of depression based on chronic stress, it is still unclear whether simultaneous changes in behavior and neurotrophin expression occur and whether these changes can be corrected or prevented comparably by chronic administration of ADs or genetic manipulations that produce antidepressant-like effects such as the knockout of the norepinephrine transporter (NET) gene. Here we show that chronic restraint or social defeat stress induce comparable effects on behavior and changes in the expression of neurotrophins in depression-related brain regions. Chronic stress caused down-regulation of BDNF, nerve growth factor, and neurotrophin-3 in hippocampus and cerebral cortex and up-regulation of these targets in striatal regions. In wild-type mice, these effects could be prevented by concomitant chronic administration of five pharmacologically diverse ADs. In contrast, NET knock out (NETKO) mice were resistant to stress-induced depressive-like changes in behavior and brain neurotrophin expression. Thus, the resistance of the NETKO mice to the stress-induced depression-associated behaviors and biochemical changes highlight the importance of noradrenergic pathways in the maintenance of mood. In addition, these mice represent a useful model to study depression-resistant behaviors, and they might help to provide deeper insights into the identification of downstream targets involved in the mechanisms of antidepressants.

Brain derived neurotrophic factor and neurotrophin-4 employ different intracellular pathways to modulate norepinephrine uptake and release in rat hypothalamus

Classical actions of the neurotrophin family are related to cellular survival and differentiation. Moreover, acute effects of neurotrophins have been reported. Although neurotrophins effects on synaptic transmission at central nervous system level have been largely studied, acute effects of neurotrophins on hypothalamic noradrenergic transmission are still poorly understood. Thus, we have studied the effects of the neurotrophin family members nerve growth factor (NGF), brain derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4) on norepinephrine (NE) neuronal uptake and its evoked release, as well as the receptor and the intracellular pathways involved in these processes in rat hypothalamus. Present results indicate that BDNF increased NE uptake and decreased its evoked release through a mechanism that involve Trk B receptor and phospholipase C. Moreover, NT-4, also through the Trk B receptor, decreased NE uptake and its evoked release by activating phosphatidylinositol 3-OH-kinase. These effects were observed in whole hypothalamus as well as in the anterior hypothalamic zone. On the other hand, NGF did not modify noradrenergic transmission. In conclusion, we showed for the first time that BDNF and NT-4 activate two different intracellular signalling pathways through a Trk B receptor dependent mechanism. Furthermore, present findings support the hypothesis that BDNF and NT-4 acutely applied, could be considered as modulators of noradrenergic transmission and thus may regulate hypothalamic physiological as well as pathophysiological responses.

Subcellular localization of the antidepressant-sensitive norepinephrine transporter

BACKGROUND

Reuptake of synaptic norepinephrine (NE) via the antidepressant-sensitive NE transporter (NET) supports efficient noradrenergic signaling and presynaptic NE homeostasis. Limited, and somewhat contradictory, information currently describes the axonal transport and localization of NET in neurons.

RESULTS

We elucidate NET localization in brain and superior cervical ganglion (SCG) neurons, aided by a new NET monoclonal antibody, subcellular immunoisolation techniques and quantitative immunofluorescence approaches. We present evidence that axonal NET extensively colocalizes with syntaxin 1A, and to a limited degree with SCAMP2 and synaptophysin. Intracellular NET in SCG axons and boutons also quantitatively segregates from the vesicular monoamine transporter 2 (VMAT2), findings corroborated by organelle isolation studies. At the surface of SCG boutons, NET resides in both lipid raft and non-lipid raft subdomains and colocalizes with syntaxin 1A.

CONCLUSION

Our findings support the hypothesis that SCG NET is segregated prior to transport from the cell body from proteins comprising large dense core vesicles. Once localized to presynaptic boutons, NET does not recycle via VMAT2-positive, small dense core vesicles. Finally, once NET reaches presynaptic plasma membranes, the transporter localizes to syntaxin 1A-rich plasma membrane domains, with a portion found in cholera toxin-demarcated lipid rafts. Our findings indicate that activity-dependent insertion of NET into the SCG plasma membrane derives from vesicles distinct from those that deliver NE. Moreover, NET is localized in presynaptic membranes in a manner that can take advantage of regulatory processes targeting lipid raft subdomains.

Norepinephrine and stimulant addiction

No pharmacotherapies are approved for stimulant use disorders, which are an important public health problem. Stimulants increase synaptic levels of the monoamines dopamine (DA), serotonin and norepinephrine (NE). Stimulant reward is attributable mostly to increased DA in the reward circuitry, although DA stimulation alone cannot explain the rewarding effects of stimulants. The noradrenergic system, which uses NE as the main chemical messenger, serves multiple brain functions including arousal, attention, mood, learning, memory and stress response. In pre-clinical models of addiction, NE is critically involved in mediating stimulant effects including sensitization, drug discrimination and reinstatement of drug seeking. In clinical studies, adrenergic blockers have shown promise as treatments for cocaine abuse and dependence, especially in patients experiencing severe withdrawal symptoms. Disulfiram, which blocks NE synthesis, increased the number of cocaine-negative urines in five randomized clinical trials. Lofexidine, an alpha(2)-adrenergic agonist, reduces the craving induced by stress and drug cues in drug users. In addition, the NE transporter (NET) inhibitor atomoxetine attenuates some of d-amphetamine's subjective and physiological effects in humans. These findings warrant further studies evaluating noradrenergic medications as treatments for stimulant addiction.

Unfaithful neurotransmitter transporters: focus on serotonin uptake and implications for antidepressant efficacy

Biogenic amine transporters for serotonin, norepinephrine and dopamine (SERT, NET and DAT respectively), are the key players terminating transmission of these amines in the central nervous system by their high-affinity uptake. They are also major targets for many antidepressant drugs. Interestingly however, drugs targeted to a specific transporter do not appear to be as clinically efficacious as those that block two or all three of these transporters. A growing body of literature, reviewed here, supports the idea that promiscuity among these transporters (the uptake of multiple amines in addition to their "native" transmitter) may account for improved therapeutic effects of dual and triple uptake blockers. However, even these drugs do not provide effective treatment outcomes for all individuals. An emerging literature suggests that "non-traditional" transporters such as organic cation transporters (OCT) and the plasma membrane monoamine transporter (PMAT) may contribute to the less than hoped for efficacy of currently prescribed uptake inhibitors. OCT and PMAT are capable of clearing biogenic amines from extracellular fluid and may serve to buffer the effects of frontline antidepressants, such as selective serotonin reuptake inhibitors. In addition, polymorphisms that occur in the genes encoding the transporters can lead to variation in transporter expression and function (e.g. the serotonin transporter linked polymorphic region; 5-HTTLPR) and can have profound effects on treatment outcome. This may be accounted for, in part, by compensatory adaptations in other transporters. This review synthesizes the existing literature, focusing on serotonin to illustrate and revive a model for the rationale design of improved antidepressants.

Regulation of the neuronal norepinephrine transporter by endothelin-1 and -3 in the rat anterior and posterior hypothalamus

We previously reported that endothelin-1 and endothelin-3 modulate norepinephrine neuronal release and tyrosine hydroxylase activity and expression in the hypothalamus. In the present study we sought to establish the role of endothelin-1 and -3 in the regulation of norepinephrine uptake in the anterior and posterior hypothalamus. Results showed that in the anterior hypothalamus endothelin-3 increased neuronal norepinephrine uptake whereas endothelin-1 decreased it. Conversely, in the posterior hypothalamic region both endothelins diminished the neuronal uptake of the amine. Endothelins response was concentration dependent and maintained at all studied times. Endothelins also modified the kinetic and internalization of the NE neuronal transporter. In the anterior hypothalamic region endothelin-3 increased the V(max) and the B(max) whereas endothelin-1 decreased them. However, in the posterior hypothalamic region both endothelins diminished the V(max) as well as B(max). Neither endothelin-1 nor endothelin-3 modified neuronal norepinephrine transporter K(d) in the studied hypothalamic regions. These findings support that in the posterior hypothalamic region both endothelins diminished neuronal norepinephrine transporter activity by reducing the amine transporter expression on the plasmatic membrane. Conversely, in the anterior hypothalamic region endothelin-3 enhanced neuronal norepinephrine transporter activity by increasing the expression of the transporter on the presynaptic membrane, whereas endothelin-1 induced the opposite effect. Present results permit us to conclude that both endothelins play an important role in the regulation of norepinephrine neurotransmission at the presynaptic nerve endings in the hypothalamus.

N-Ethylmaleimide differentially inhibits substrate uptake by and ligand binding to the noradrenaline transporter

Using transfected HEK293 cells that express the human (h) noradrenaline transporter (hNAT), we show differential inhibitory effects of the thiol reagent N-ethylmaleimide (NEM) on [(3)H]NA uptake and [(3)H]nisoxetine binding. Irreversible inhibition of uptake by NEM was complete, faster, and occurred at lower concentrations. Furthermore, hNAT ligands (substrates and inhibitors) prevented NEM-induced inhibition of binding but not that of uptake, indicating different underlying mechanisms of inhibition. NEM-induced uptake inhibition was not primarily due to inhibition of the Na(+)/K(+)-ATPase since ouabain caused only partial inhibition. For the first time, we show that NEM at low concentrations causes a rapid and complete depletion of cellular adenosine triphosphate (ATP) not only in HEK293 cells but also in several other eukaryotic cell lines. Thus, while high NEM concentrations alkylate the NAT protein in a ligand-protectable manner, low concentrations inhibit substrate uptake through a loss of the Na(+) and K(+) gradient as a driving force by depleting cellular ATP.

Regulated interactions of the norepineprhine transporter by the actin and microtubule cytoskeletons

One role of the actin cytoskeleton is to maintain the structural morphology and activity of the pre-synaptic terminal. We sought to determine if the actin cytoskeleton plays a role in regulating interactions between the norepinephrine transporter (NET) and alpha-Synuclein (alpha-Syn), two proteins expressed in the pre-synaptic terminal. In cells transfected with either 0.5 microg/mL or 3 microg/mL of alpha-Syn and 1 microg/mL of NET DNA, treatment with cytochalasin D, an actin depolymerizing agent, caused a dose-dependent decrease and increase, respectively, in [3H]-NE uptake. Protein interactions between NET, beta-actin, and alpha-Syn were modified, along with levels of surface transporters. Treatment of primary brainstem neurons and frontal cortex synaptosomes with cytochalasin D caused a 115% and 28% increase, respectively, in NET activity. Depolymerization of both actin and microtubules did not alter NET activity in cells with 0.5 microg/mL alpha-Syn, but caused an increase in [3H]-NE uptake in cells transfected with 3 microg/mL of alpha-Syn and primary neurons. This is the first direct demonstration of NET activity being regulated via actin and modulated by interactions with alpha-Syn.

Desvenlafaxine succinate identifies novel antagonist binding determinants in the human norepinephrine transporter

Desvenlafaxine succinate (DVS) is a recently introduced antagonist of the human norepinephrine and serotonin transporters (hNET and hSERT, respectively), currently in clinical development for use in the treatment of major depressive disorder and vasomotor symptoms associated with menopause. Initial evaluation of the pharmacological properties of DVS (J Pharmacol Exp Ther 318:657-665, 2006) revealed significantly reduced potency for the hNET expressed in membranes compared with whole cells when competing for [(3)H]nisoxetine (NIS) binding. Using hNET in transfected human embryonic kidney-293 cells, this difference in potency for DVS at sites labeled by [(3)H]NIS was found to distinguish DVS, the DVS analog rac-(1-[1-(3-chloro-phenyl)-2-(4-methylpiperazin-1-yl)-ethyl]cyclohexanol (WY-46824), methylphenidate, and the cocaine analog 3beta-(4-iodophenyl)tropane-2beta-carboxylic acid methyl ester (RTI-55) from other hNET antagonists, such as NIS, mazindol, tricyclic antidepressants, and cocaine. These differences seem not to arise from preparation-specific perturbations of ligand intrinsic affinity or antagonist-specific surface trafficking but rather from protein conformational alterations that perturb the relationships between distinct hNET binding sites. In an initial search for molecular features that differentially define antagonist binding determinants, we document that Val148 in hNET transmembrane domain 3 selectively disrupts NIS binding but not that of DVS.

Molecular cloning and functional expression of the murine noradrenaline transporter

The cDNA of the murine noradrenaline transporter (mNAT) was cloned from the RNA of the placenta of a C57BL/6 mouse. The cloned mNAT differs from a previously published sequence in two amino acids within the C-terminal region. A cDNA obtained from an inbred mouse strain showed a further amino acid exchange (Ile(505)Val) within the fifth intracellular loop. The pharmacological properties of both, the wild-type mNAT and the variant (mNAT-I(505)V), were studied in human embryonic kidney HEK293 cells transfected with the corresponding cDNA. The kinetic constants for transport (K (m), V (max)) of [(3)H]noradrenaline ([(3)H]-NA) and binding (K (D), B (max)) of the selective NAT inhibitor [(3)H]nisoxetine were not different between the two isoforms; the mean kinetic constants amounted to about 4 microM and 120pmol/mg protein for K (m) and V (max) and 6nM and 18pmol/mg protein for K (D) and B (max), respectively. [(3)H]-NA transport by both isoforms showed the typical properties of an NAT because it was dependent on sodium and chloride and inhibited with almost identical K (i) values by various NAT substrates and inhibitors. The only significant pharmacological difference identified between the two mNAT isoforms was an about threefold higher affinity for cocaine of the very rare mNAT-I(505)V variant.

Regulation of the norepinephrine transporter by alpha-synuclein-mediated interactions with microtubules

alpha-Synuclein (alpha-Syn) regulates catecholaminergic neurotransmission. We demonstrate that alpha-Syn regulates the activity and surface expression of the norepinephrine transporter (NET), depending on its expression levels. In cells co-transfected with NET and low amounts of alpha-Syn, NET activity and cell surface expression were increased and protein interactions with alpha-Syn decreased, compared with cells transfected with NET alone. Converse effects were observed at higher levels of alpha-Syn expression. Treatment with nocodazole and other microtubule (MT) destabilizers abolished the expression-dependent bimodal regulation of NET by alpha-Syn. At low alpha-Syn levels, nocodazole had no effect on NET surface expression or protein interactions, while inducing increases in these measures at higher levels. Cells that were transfected with NET alone displayed no sensitivity to nocodazole, indicating that alpha-Syn expression was necessary for the MT-dependent changes in NET activity. MT destabilizers also caused a significant increase in [(3)H]-NE uptake in brainstem primary neurons and synaptosomes from the frontal cortex, but not striatal synaptosomes. These findings suggest that the surface localization and activity of NET is modulated by alpha-Syn in a manner that is both dependent on interactions with the MT cytoskeleton and varies across brain regions.

The functional impact of SLC6 transporter genetic variation

Solute carrier 6 (SLC6) is a gene family of ion-coupled plasma membrane cotransporters, including transporters of neurotransmitters, amino acids, and osmolytes that mediate the movement of their substrates into cells to facilitate or regulate synaptic transmission, neurotransmitter recycling, metabolic function, and fluid homeostasis. Polymorphisms in transporter genes may influence expression and activity of transporters and contribute to behavior, traits, and disease. Determining the relationship between the monoamine transporters and complex psychiatric disorders has been a particular challenge that is being met by evolving approaches. Elucidating the functional consequences of and interactions among polymorphic sites is advancing our understanding of this relationship. Examining the influence of environmental influences, especially early-life events, has helped bridge the gap between genotype and phenotype. Refining phenotypes, through assessment of endophenotypes, specific behavioral tasks, medication response, and brain network properties has also improved detection of the impact of genetic variation on complex behavior and disease.