Localization and dynamic regulation of biogenic amine transporters in the mammalian central nervous system

Expression Level Changes in Serotonin Transporter are Associated with Food Deprivation in the Pond Snail Lymnaea stagnalis

In the pond snail Lymnaea stagnalis, serotonin (5-HT) plays an important role in feeding behavior and its associated learning (e.g., conditioned taste aversion: CTA). The 5-HT content in the central nervous system (CNS) fluctuates with changes in the nutritional status, but it is also expected to be influenced by changes in the serotonin transporter (SERT) expression level. In the present study, we identified SERT in Lymnaea and observed its localization in 5-HTergic neurons, including the cerebral giant cells (CGCs) in the cerebral ganglia and the pedal A cluster neurons and right and left pedal dorsal 1 neurons in the pedal ganglia by in situ hybridization. Real-time PCR revealed that the SERT mRNA expression level was lower under severe food deprivation than under mild food deprivation in the whole CNS as well as in a single CGC. These results inversely correlated with previous data that the 5-HT content in the CNS was higher in the severely food-deprived state than in the mildly food-deprived state. Furthermore, in single CGCs, we observed that the 5-HT level was significantly increased in the severely food-deprived state compared with the mildly food-deprived state. Our present findings suggest that changes in the SERT expression level associated with food deprivation may affect 5-HT signaling, probably contributing to learning and memory mechanisms in Lymnaea.

Dopamine Transporter Deficient Rodents: Perspectives and Limitations for Neuroscience

The key element of dopamine (DA) neurotransmission is undoubtedly DA transporter (DAT), a transmembrane protein responsible for the synaptic reuptake of the mediator. Changes in DAT's function can be a key mechanism of pathological conditions associated with hyperdopaminergia. The first strain of gene-modified rodents with a lack of DAT were created more than 25 years ago. Such animals are characterized by increased levels of striatal DA, resulting in locomotor hyperactivity, increased levels of motor stereotypes, cognitive deficits, and other behavioral abnormalities. The administration of dopaminergic and pharmacological agents affecting other neurotransmitter systems can mitigate those abnormalities. The main purpose of this review is to systematize and analyze (1) known data on the consequences of changes in DAT expression in experimental animals, (2) results of pharmacological studies in these animals, and (3) to estimate the validity of animals lacking DAT as models for discovering new treatments of DA-related disorders.

From Neurotransmitters to Networks: Transcending Organisational Hierarchies with Molecular-informed Functional Imaging

The human brain exhibits complex interactions across micro, meso-, and macro-scale organisational principles. Recent synergistic multi-modal approaches have begun to link micro-scale information to systems level dynamics, transcending organisational hierarchies and offering novel perspectives into the brain's function and dysfunction. Specifically, the distribution of micro-scale properties (such as receptor density or gene expression) can be mapped onto macro-scale measures from functional MRI to provide novel neurobiological insights. Methodological approaches to enrich functional imaging analyses with molecular information are rapidly evolving, with several streams of research having developed relatively independently, each offering unique potential to explore the trans-hierarchical functioning of the brain. Here, we address the three principal streams of research - spatial correlation, molecular-enriched network, and in-silico whole brain modelling analyses - to provide a critical overview of the different sources of molecular information, how this information can be utilised within analyses of fMRI data, the merits and pitfalls of each methodology, and, through the use of key examples, highlight their promise to shed new light on key domains of neuroscientific inquiry.

Dopamine, Immunity, and Disease

The neurotransmitter dopamine is a key factor in central nervous system (CNS) function, regulating many processes including reward, movement, and cognition. Dopamine also regulates critical functions in peripheral organs, such as blood pressure, renal activity, and intestinal motility. Beyond these functions, a growing body of evidence indicates that dopamine is an important immunoregulatory factor. Most types of immune cells express dopamine receptors and other dopaminergic proteins, and many immune cells take up, produce, store, and/or release dopamine, suggesting that dopaminergic immunomodulation is important for immune function. Targeting these pathways could be a promising avenue for the treatment of inflammation and disease, but despite increasing research in this area, data on the specific effects of dopamine on many immune cells and disease processes remain inconsistent and poorly understood. Therefore, this review integrates the current knowledge of the role of dopamine in immune cell function and inflammatory signaling across systems. We also discuss the current understanding of dopaminergic regulation of immune signaling in the CNS and peripheral tissues, highlighting the role of dopaminergic immunomodulation in diseases such as Parkinson's disease, several neuropsychiatric conditions, neurologic human immunodeficiency virus, inflammatory bowel disease, rheumatoid arthritis, and others. Careful consideration is given to the influence of experimental design on results, and we note a number of areas in need of further research. Overall, this review integrates our knowledge of dopaminergic immunology at the cellular, tissue, and disease level and prompts the development of therapeutics and strategies targeted toward ameliorating disease through dopaminergic regulation of immunity. SIGNIFICANCE STATEMENT: Canonically, dopamine is recognized as a neurotransmitter involved in the regulation of movement, cognition, and reward. However, dopamine also acts as an immune modulator in the central nervous system and periphery. This review comprehensively assesses the current knowledge of dopaminergic immunomodulation and the role of dopamine in disease pathogenesis at the cellular and tissue level. This will provide broad access to this information across fields, identify areas in need of further investigation, and drive the development of dopaminergic therapeutic strategies.

Correspondence between gene expression and neurotransmitter receptor and transporter density in the human brain

Neurotransmitter receptors modulate signaling between neurons. Thus, neurotransmitter receptors and transporters play a key role in shaping brain function. Due to the lack of comprehensive neurotransmitter receptor/transporter density datasets, microarray gene expression measuring mRNA transcripts is often used as a proxy for receptor densities. In the present report, we comprehensively test the spatial correlation between gene expression and protein density for a total of 27 neurotransmitter receptors, receptor binding-sites, and transporters across 9 different neurotransmitter systems, using both PET and autoradiography radioligand-based imaging modalities. We find poor spatial correspondences between gene expression and density for all neurotransmitter receptors and transporters except four single-protein metabotropic receptors (5-HT_1A, CB₁, D₂, and MOR). These expression-density associations are related to gene differential stability and can vary between cortical and subcortical structures. Altogether, we recommend using direct measures of receptor and transporter density when relating neurotransmitter systems to brain structure and function.

DAT and TH expression marks human Parkinson's disease in peripheral immune cells

Parkinson's disease (PD) is marked by a loss of dopamine neurons, decreased dopamine transporter (DAT) and tyrosine hydroxylase (TH) expression. However, this validation approach cannot be used for diagnostic, drug effectiveness or investigational purposes in human patients because midbrain tissue is accessible postmortem. PD pathology affects both the central nervous and peripheral immune systems. Therefore, we immunophenotyped blood samples of PD patients for the presence of myeloid derived suppressor cells (MDSCs) and discovered that DAT+/TH+ monocytic MDSCs, but not granulocytic MDSCs are increased, suggesting a targeted immune response to PD. Because in peripheral immune cells DAT activity underlies an immune suppressive mechanism, we investigated whether expression levels of DAT and TH in the peripheral immune cells marks PD. We found drug naïve PD patients exhibit differential DAT+/TH+ expression in peripheral blood mononuclear cells (PBMCs) compared to aged/sex matched healthy subjects. While total PBMCs are not different between the groups, the percentage of DAT+/TH+ PBMCs was significantly higher in drug naïve PD patients compared to healthy controls irrespective of age, gender, disease duration, disease severity or treatment type. Importantly, treatment for PD negatively modulates DAT+/TH+ expressing PBMCs. Neither total nor the percentage of DAT+/TH+ PBMCs were altered in the Alzheimer's disease cohort. The mechanistic underpinning of this discovery in human PD was revealed when these findings were recapitulated in animal models of PD. The reverse translational experimental strategy revealed that alterations in dopaminergic markers in peripheral immune cells are due to the disease associated changes in the CNS. Our study demonstrates that the dopaminergic machinery on peripheral immune cells displays an association with human PD, with exciting implications in facilitating diagnosis and investigation of human PD pathophysiology.

Molecular characterization of the serotonergic transporter from the cestode Echinococcus granulosus: pharmacology and potential role in the nervous system

Echinococcus granulosus, the etiological agent of human cystic echinococcosis (formerly known as hydatid disease), represents a serious worldwide public health problem with limited treatment options. The essential role played by the neuromuscular system in parasite survival and the relevance of serotonin (5-HT) in parasite movement and development make the serotonergic system an attractive source of drug targets. In this study, we cloned and sequenced a cDNA coding for the serotonin transporter from E. granulosus (EgSERT). Bioinformatic analyses suggest that EgSERT has twelve transmembrane domains with highly conserved ligand and ionic binding sites but a less conserved allosteric site compared with the human orthologue (HsSERT). Modeling studies also suggest a good degree of conservation of the overall structure compared with HsSERT. Functional and pharmacological studies performed on the cloned EgSERT confirm that this protein is indeed a serotonin transporter. EgSERT is specific for 5-HT and does not transport other neurotransmitters. Typical monoamine transport inhibitors also displayed inhibitory activities towards EgSERT, but with lower affinity than for the human SERT (HsSERT), suggesting a high divergence of the cestode transporter compared with HsSERT. In situ hybridization studies performed in the larval protoscolex stage suggest that EgSERT is located in discrete regions that are compatible with the major ganglia of the serotonergic nervous system. The pharmacological properties, the amino acidic substitutions at important functional regions compared with the HsSERT, and the putative role of EgSERT in the nervous system suggest that it could be an important target for pharmacological intervention.

[11C]dihydrotetrabenazine Positron Emission Tomography in Manganese-Exposed Workers

OBJECTIVE

To understand the neurotoxic effects of manganese (Mn) exposure on monoaminergic function, utilizing [C]dihydrotetrabenazine (DTBZ) positron emission tomography (PET) to measure vesicular monoamine transporter 2 (VMAT2).

METHODS

Basal ganglia and thalamic DTBZ binding potentials (BPND) were calculated on 56 PETs from 41 Mn-exposed workers. Associations between cumulative Mn exposure, regional BPND, and parkinsonism were examined by mixed linear regression.

RESULTS

Thalamic DTBZ BPND was inversely associated with exposure in workers with less than 3 mg Mn/m-yrs, but subsequently remained stable. Pallidal DTBZ binding increased in workers with less than 2 mg Mn/m-yrs of exposure, but decreased thereafter. Thalamic DTBZ binding was inversely associated with parkinsonism (P = 0.003).

CONCLUSION

Mn-dose-dependent associations with thalamic and pallidal DTBZ binding indicate direct effects on monoaminergic VMAT2. Thalamic DTBZ binding was also associated with parkinsonism, suggesting potential as an early biomarker of Mn neurotoxicity.

Genetics of human startle reactivity: A systematic review to acquire targets for an anxiety endophenotype

OBJECTIVES

Startle response is an objective physiological measure integral to the human defense system and a promising target for endophenotype investigations of anxiety. Given the alterations in startle reactivity observed among anxiety and related disorders, we searched for genetic variants associated with startle reactivity as they may be further involved in pathological anxiety risk.

METHODS

A systematic literature review was performed to identify genetic variants associated with startle reactivity in humans, specifically baseline and fear- or anxiety-potentiated startle.

RESULTS

The polymorphisms Val66Met (rs6265) from brain-derived neurotrophic factor (BDNF), Val158Met (rs4680) from catechol-O-methyltransferase (COMT), and the serotonin transporter-linked polymorphic region (5-HTTLPR) from the serotonin transporter gene (SLC6A4) were most commonly studied in human startle. In addition, several other genetic variants have also been identified as potential candidates that warrant further research, especially given their novelty in in the context of anxiety.

CONCLUSIONS

Similar to psychiatric genetic studies, the studies on startle reactivity primarily focus on candidate genes and are plagued by non-replication. Startle reactivity is a promising endophenotype that requires concerted efforts to collect uniformly assessed, large, well-powered samples and hypothesis-free genome-wide strategies. To further support startle as an endophenotype for anxiety, this review suggests advanced genetic strategies for startle research.

Molar activity of [18F]FP-(+)-DTBZ radiopharmaceutical: Determination and its effect on quantitative analysis of VMAT2 autoradiography

[¹⁸F]fluoropropyl-(+)-dihydrotetrabenazine ([¹⁸F]FP-(+)-DTBZ) is a rising positron tracer for imaging vesicular monoamine transporter II (VMAT2) in the central nervous system. The present work was to develop a novel chromatographic method capable of the molar activity (A_m) determination of [¹⁸F]FP-(+)-DTBZ. As a complement work of the A_m measurement, we also investigated the effect of A_m on the quantitative analysis of VMAT2 autoradiography with [¹⁸F]FP-(+)-DTBZ. The A_m determination was performed by high performance liquid chromatography (HPLC) using the non-radioactive standard (FP-(+)-DTBZ) for calibration plot of peak area against concentration. Based on this correlation, the A_m of [¹⁸F]FP-(+)-DTBZ was calculated and corrected to the end of synthesis. In the quantitative analysis of in vitro VMAT2 autoradiography, the striatum radioactivity uptake together with the uptake ratio of striatum versus cortex reduced along with the decrease of A_m and the increase of the FP-(+)-DTBZ content. Therefore, the A_m and the corresponding FP-(+)-DTBZ content have a significant effect on the quantitative analysis of VMAT2 autoradiography using [¹⁸F]FP-(+)-DTBZ.

Toxicological Study on Chiral Fluoxetine Exposure to Adult Zebrafish (Danio rerio): Enantioselective and Sexual Mechanism on Disruption of the Brain Serotonergic System

The increasing number of people with depression worldwide has led to concerns regarding antidepressant contamination in aquatic environments, which could have the risk of negative effects on aquatic organisms. Chirality increases its toxicity potentials. Accordingly, we investigated the negative effects of racemic (rac-), R-, and S-FX at environmental levels (100 ng/L) on the brain serotonergic system in zebrafish (Danio rerio) for 42 days. Additionally, we measured the whole-body concentrations of FX and norfluoxetine (NFX). We found that S-FX exposure disrupted the brain serotonergic system more severely than rac- and R-FX exposure. The mechanism underlying this disruption induced by S-FX was sex-specific, with female zebrafish showing disruption of the serotonin (5-HT) release process but male zebrafish showing disruption of the 5-HT synthesis process. In addition, enantioselective enrichment and biotransformation (R-FX to R-NFX and S-FX to S-NFX) occurred in zebrafish. Sex-specific accumulation was also observed, with higher concentrations in females. Our study provides evidence for enantiomer- and sex-specific effects of FX exposure at biologically relevant concentrations. More broadly, our study demonstrated that SSRI antidepressants, such as FX, can affect aquatic life by causing important shifts in not only their active sites of the serotonin transporter.

Spinal Dopaminergic Mechanisms Regulating the Micturition Reflex in Male Rats with Complete Spinal Cord Injury

Traumatic spinal cord injury (SCI) often causes micturition dysfunction. We recently discovered a low level of spinally-derived dopamine (DA) that regulates recovered bladder and sphincter reflexes in SCI female rats. Considering substantial sexual dimorphic features in the lower urinary tract, it is unknown if the DA-ergic mechanisms act in the male. Histological analysis showed a similar distribution of tyrosine hydroxylase (TH)⁺ neurons in the lower cord of male rats and the number increased following thoracic SCI. Subsequently, focal electrical stimulation in slices obtained from L6/S1 spinal segments of SCI rats elicited detectable DA release with fast scan cyclic voltammetry. Using bladder cystometrogram and external urethral sphincter (EUS) electromyography in SCI male rats, intravenous (i.v.) administration of SCH 23390, a D₁-like receptor (DR₁) antagonist, induced significantly increased tonic EUS activity and a trend of increased residual volume, whereas activation of these receptors with SKF 38393 did not influence the reflex. Meanwhile, blocking spinal D₂-like receptors (DR₂) with remoxipride had no effect but stimulating these receptors with quinpirole elicited EUS bursting to increase voiding volume. Further, intrathecal delivery of SCH 23390 and quinpirole resulted in similar responses to those with i.v. delivery, respectively, which indicates the central action regardless of delivery route. In addition, metabolic cage assays showed that quinpirole increased the voiding frequency and total voiding volume in spontaneous micturition. Collectively, spinal DA-ergic machinery regulates recovered micturition reflex following SCI in male rats; spinal DR₁ tonically suppress tonic EUS activity to enable voiding and activation of DR₂ facilitates voiding.

General Overview of Organic Cation Transporters in Brain

Inhibitors of Na⁺/Cl^- dependent high affinity transporters for norepinephrine (NE), serotonin (5-HT), and/or dopamine (DA) represent frequently used drugs for treatment of psychological disorders such as depression, anxiety, obsessive-compulsive disorder, attention deficit hyperactivity disorder, and addiction. These transporters remove NE, 5-HT, and/or DA after neuronal excitation from the interstitial space close to the synapses. Thereby they terminate transmission and modulate neuronal behavioral circuits. Therapeutic failure and undesired central nervous system side effects of these drugs have been partially assigned to neurotransmitter removal by low affinity transport. Cloning and functional characterization of the polyspecific organic cation transporters OCT1 (SLC22A1), OCT2 (SLC22A2), OCT3 (SLC22A3) and the plasma membrane monoamine transporter PMAT (SLC29A4) revealed that every single transporter mediates low affinity uptake of NE, 5-HT, and DA. Whereas the organic transporters are all located in the blood brain barrier, OCT2, OCT3, and PMAT are expressed in neurons or in neurons and astrocytes within brain areas that are involved in behavioral regulation. Areas of expression include the dorsal raphe, medullary motoric nuclei, hypothalamic nuclei, and/or the nucleus accumbens. Current knowledge of the transport of monoamine neurotransmitters by the organic cation transporters, their interactions with psychotropic drugs, and their locations in the brain is reported in detail. In addition, animal experiments including behavior tests in wildtype and knockout animals are reported in which the impact of OCT2, OCT3, and/or PMAT on regulation of salt intake, depression, mood control, locomotion, and/or stress effect on addiction is suggested.

The Role of Estrogen Receptors and Their Signaling across Psychiatric Disorders

Increasing evidence suggests estrogen and estrogen signaling pathway disturbances across psychiatric disorders. Estrogens are not only crucial in sexual maturation and reproduction but are also highly involved in a wide range of brain functions, such as cognition, memory, neurodevelopment, and neuroplasticity. To add more, the recent findings of its neuroprotective and anti-inflammatory effects have grown interested in investigating its potential therapeutic use to psychiatric disorders. In this review, we analyze the emerging literature on estrogen receptors and psychiatric disorders in cellular, preclinical, and clinical studies. Specifically, we discuss the contribution of estrogen receptor and estrogen signaling to cognition and neuroprotection via mediating multiple neural systems, such as dopaminergic, serotonergic, and glutamatergic systems. Then, we assess their disruptions and their potential implications for pathophysiologies in psychiatric disorders. Further, in this review, current treatment strategies involving estrogen and estrogen signaling are evaluated to suggest a future direction in identifying novel treatment strategies in psychiatric disorders.

Chronic restraint stress induces serotonin transporter expression in the rat adrenal glands

Chronic restraint stress (CRS) magnifies restraint-induced corticosterone secretion through a mechanism involving increased adrenocortical 5-HT content and turnover. We analysed the impact of CRS on serotonin transporter (SERT) expression and distribution in rat adrenal glands. Male Wistar rats were submitted to CRS (20 min/day) or undisturbed control conditions for 14 days. Exposure to CRS induced a remarkable increase in SERT-like immunoreactivity in the adrenal cortex, which closely matched that of chromogranin A immunostaining, along with a significant increase in SERT protein and mRNA levels in whole adrenals as determined by immunohistochemistry, Western blot and RT-PCR assays, respectively; all these CRS-induced changes occurred almost exclusively in left adrenals. Closely similar results were obtained in animals that received a 14-day chronic corticosterone treatment. These results unravel an interesting association between chronic stress exposure and SERT expression in adrenocortical chromogranin A-positive cells, which seems to be a glucocorticoid-dependent phenomenon.

The New Frontier in Oxytocin Physiology: The Oxytonic Contraction

Oxytocin (Oxt) is a nine amino acid peptide important in energy regulation and is essential to stress-related disorders. Specifically, low Oxt levels are associated with obesity in human subjects and diet-induced or genetically modified animal models. The striking evidence that Oxt is linked to energy regulation is that Oxt- and oxytocin receptor (Oxtr)-deficient mice show a phenotype characterized by late onset obesity. Oxt-/- or Oxtr-/- develop weight gain without increasing food intake, suggesting that a lack of Oxt reduce metabolic rate. Oxt is differentially expressed in skeletal muscle exerting a protective effect toward the slow-twitch muscle after cold stress challenge in mice. We hypothesized that Oxt potentiates the slow-twitch muscle as it does with the uterus, triggering "the oxytonic contractions". Physiologically, this is important to augment muscle strength in fight/flight response and is consistent with the augmented energetic need at time of labor and for the protection of the offspring when Oxt secretion spikes. The normophagic obesity of Oxt-/- or Oxtr-/- mice could have been caused by decreased skeletal muscle tonicity which drove the metabolic phenotype. In this review, we summarized our findings together with the recent literature on this fascinating subjects in a "new oxytonic perspective" over the physicology of Oxt.

Characteristic of Dopamine-Producing System and Dopamine Receptors in the Suprachiasmatic Nucleus in Rats in Ontogenesis

One of the features of the developing suprachiasmatic nucleus (SCN), the "biological clock" of the body, is the early expression of dopamine (DA) receptors in the absence of dopaminergic neurons as a source of DA. Only recently we showed that DA in SCN is synthesized together by nerve fibers containing only tyrosine hydroxylase (TH) and neurons containing only aromatic L-amino acid decarboxylase (AADC). This study was aimed to assess specific characteristics of the phenotype of TH-fibers in ontogenesis. For this purpose, PCR and immunohistochemical analysis of the expression of genes and proteins such as TH, AADC, vesicular monoamine transporter (VMAT), and receptors for DA (D1, D2) was performed. We have detected numerous TH-immunoreactive fibers in SCN of young and adult rats. VMAT was observed in some of them, which suggests vesicular storage of L-DOPA. Considering the key role of TH-fibers in cooperative synthesis of DA, we assumed the presence of their dopamine regulation. Using double immunolabeling, we showed that D1 and D2 are present in TH-fibers in adult rats, and only D1 in young rats. According to PCR, D1 and D2 are also expressed in neurons of SCN in adult rats and only D1 in young rats. Thus, it was shown for the first time that VMAT and D1 are coexpressed in TH-fibers synthesizing L-DOPA in SCN in young and adult rats, and also D2 receptors in adult rats, which suggests vesicular storage and dopamine regulation of L-DOPA secretion, respectively.

Amygdala 5-HTT Gene Network Moderates the Effects of Postnatal Adversity on Attention Problems: Anatomo-Functional Correlation and Epigenetic Changes

Variations in serotoninergic signaling have been related to behavioral outcomes. Alterations in the genome, such as DNA methylation and histone modifications, are affected by serotonin neurotransmission. The amygdala is an important brain region involved in emotional responses and impulsivity, which receives serotoninergic input. In addition, studies suggest that the serotonin transporter gene network may interact with the environment and influence the risk for psychiatric disorders. We propose to investigate whether/how interactions between the exposure to early life adversity and serotonin transporter gene network in the amygdala associate with behavioral disorders. We constructed a co-expression-based polygenic risk score (ePRS) reflecting variations in the function of the serotonin transporter gene network in the amygdala and investigated its interaction with postnatal adversity on attention problems in two independent cohorts from Canada and Singapore. We also described how interactions between ePRS-5-HTT and postnatal adversity exposure predict brain gray matter density and variation in DNA methylation across the genome. We observed that the expression-based polygenic risk score, reflecting the function of the amygdala 5-HTT gene network, interacts with postnatal adversity, to predict attention and hyperactivity problems across both cohorts. Also, both postnatal adversity score and amygdala ePRS-5-HTT score, as well as their interaction, were observed to be associated with variation in DNA methylation across the genome. Variations in gray matter density in brain regions linked to attentional processes were also correlated to our ePRS score. These results confirm that the amygdala 5-HTT gene network is strongly associated with ADHD-related behaviors, brain cortical density, and epigenetic changes in the context of adversity in young children.

Expression of the noradrenaline transporter in the peripheral nervous system

The noradrenaline transporter (NAT) transfers noradrenaline released into the synaptic cleft back into the presynaptic terminal, thus terminating neurotransmission. Although the distribution of NAT within the central nervous system has been well-characterized, less is known about its distribution elsewhere in the peripheral nervous system and in organs such as the skin. To address this in the present study, NAT expression was investigated using immunohistochemistry in the hind paw skin and more proximally in the sciatic nerve, dorsal root ganglia and spinal cord of five male Wistar rats. It was hypothesised that NAT would be expressed exclusively on nerve fibres labelled by dopamine beta hydroxylase (DβH), an enzyme involved in the conversion of dopamine to noradrenaline. NAT co-localised with DβH in neurons in the spinal cord, dorsal root ganglia and sciatic nerve. Unexpectedly, however, NAT-like immunoreactivity was not observed in DβH immuno-reactive fibres that innervated dermal blood vessels, suggesting that a mechanism other than presynaptic re-uptake of noradrenaline through NAT regulates transmission at neurovascular junctions in the skin. Furthermore, a novel association between NAT-like immunoreactivity and the myelin marker myelin basic protein (MBP) was identified in peripheral nerves. Specifically, NAT and MBP appeared to congregate around primary afferent nerve fibres labelled by neurofilament 200, a marker of neurons with medium- and large-diameter axons. NAT-like immunoreactivity was also detected in cultured Schwann cells immunohistochemically and at the mRNA level. Together, these findings imply a hitherto unrecognised role of Schwann cells in clearance of noradrenaline in the peripheral nervous system.

para-Trifluoromethyl-methcathinone is an allosteric modulator of the serotonin transporter

The transporters for dopamine (DAT) and serotonin (SERT) are important targets in the treatment of psychiatric disorders including major depression, anxiety and attention-deficit hyperactivity disorder. Drugs acting at these transporters can act as inhibitors or as releasers. In addition, it has been recently appreciated that some compounds are less efficacious releasers than amphetamine. Thus, they are classified as partial releasers. Compounds can act on both SERT and DAT or display exquisite selectivity for either SERT or DAT, but the structural basis for selectivity is poorly understood. The trifluoromethyl-substitution of methcathinone in the para-position has been shown to dramatically shift the selectivity of methcathinone (MCAT) towards SERT. Here, we examined MCAT, para-trifluoromethyl-methcathinone (pCF₃MCAT) and other analogues to understand (i) the determinants of selectivity and (ii) the effects of the para-CF₃-substitution of MCAT on the transport cycle. We systematically tested different para-substituted MCATs by biochemical, computational and electrophysiological approaches: addition of the pCF₃group, but not of other substituents with larger van der Waal's volume, lipophilicity or polarity, converted the DAT-selective MCAT into a SERT-selective partial releaser. Electrophysiological and superfusion experiments, together with kinetic modelling, showed that pCF₃MCAT, but not MCAT, trapped a fraction of SERTs in an inactive state by occupying the S2-site. These findings define a new mechanism of action for partial releasers, which is distinct from the other two known binding modes underlying partial release. Our observations highlight the fact that the substrate permeation pathway of monoamine transporters supports multiple binding modes, which can be exploited for drug design. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Effects of Estrogens on Central Nervous System Neurotransmission: Implications for Sex Differences in Mental Disorders

Nearly one of every five US individuals aged 12 years old or older lives with certain types of mental disorders. Men are more likely to use various types of substances, while women tend to be more susceptible to mood disorders, addiction, and eating disorders, all of which are risks associated with suicidal attempts. Fundamental sex differences exist in multiple aspects of the functions and activities of neurotransmitter-mediated neural circuits in the central nervous system (CNS). Dysregulation of these neural circuits leads to various types of mental disorders. The potential mechanisms of sex differences in the CNS neural circuitry regulating mood, reward, and motivation are only beginning to be understood, although they have been largely attributed to the effects of sex hormones on CNS neurotransmission pathways. Understanding this topic is important for developing prevention and treatment of mental disorders that should be tailored differently for men and women. Studies using animal models have provided important insights into pathogenesis, mechanisms, and new therapeutic approaches of human diseases, but some concerns remain to be addressed. The purpose of this chapter is to integrate human and animal studies involving the effects of the sex hormones, estrogens, on CNS neurotransmission, reward processing, and associated mental disorders. We provide an overview of existing evidence for the physiological, behavioral, cellular, and molecular actions of estrogens in the context of controlling neurotransmission in the CNS circuits regulating mood, reward, and motivation and discuss related pathology that leads to mental disorders.

Fluoxetine modulates the transcription of genes involved in serotonin, dopamine and adrenergic signalling in zebrafish embryos

Neurotransmitters pathways in fish and mammals are phylogenetically conserved. Therefore, the environmental presence of psychopharmaceuticals, such as fluoxetine (FLU), are likely to interact with fish serotonergic, dopaminergic and adrenergic systems, affecting their response and associated biological functions. Hence, the present work aimed at evaluating the effects of FLU in the transcription of genes involved in serotonin, dopamine and adrenergic transporters and receptors signalling in early stages of Danio rerio development. Embryos (1 hpf) were exposed for 80 h to different concentrations of FLU (0.0015, 0.05, 0.1, 0.5 and 0.8 μM) and mRNA levels of sert, 5-ht1a, 5-ht2c, dat, drd1b, drd2b, net, adra2a, adra2b, adra2c, vmat and mao were evaluated. A sensorimotor reflex assay was also performed demonstrating a significant decrease in tail reflex at 0.1 and 0.5 μM. The transcription levels of serotonergic and dopaminergic transporters (sert and dat) and vmat were down-regulated at environmentally relevant concentration (0.0015 μM). Receptors 5-ht2c, drd2b adra2b and adra2c mRNA levels also displayed a down regulation pattern after FLU exposure. In conclusion, this study demonstrated the interaction of FLU with the neurotransmission system at environmentally relevant concentrations by changing transcription patterns. Therefore, given the importance of these signalling pathways it is possible that their disruption can ultimately disturb the escape behaviour and biological functions in fish. Hence, evaluating the presence of this psychopharmaceutical in the aquatic environment should be implemented in future monitoring programmes.

A High-Resolution In Vivo Atlas of the Human Brain's Serotonin System

The serotonin (5-hydroxytryptamine, 5-HT) system modulates many important brain functions and is critically involved in many neuropsychiatric disorders. Here, we present a high-resolution, multidimensional, in vivo atlas of four of the human brain's 5-HT receptors (5-HT_1A, 5-HT_1B, 5-HT_2A, and 5-HT₄) and the 5-HT transporter (5-HTT). The atlas is created from molecular and structural high-resolution neuroimaging data consisting of positron emission tomography (PET) and magnetic resonance imaging (MRI) scans acquired in a total of 210 healthy individuals. Comparison of the regional PET binding measures with postmortem human brain autoradiography outcomes showed a high correlation for the five 5-HT targets and this enabled us to transform the atlas to represent protein densities (in picomoles per milliliter). We also assessed the regional association between protein concentration and mRNA expression in the human brain by comparing the 5-HT density across the atlas with data from the Allen Human Brain atlas and identified receptor- and transporter-specific associations that show the regional relation between the two measures. Together, these data provide unparalleled insight into the serotonin system of the human brain.

SIGNIFICANCE STATEMENT

We present a high-resolution positron emission tomography (PET)- and magnetic resonance imaging-based human brain atlas of important serotonin receptors and the transporter. The regional PET-derived binding measures correlate strongly with the corresponding autoradiography protein levels. The strong correlation enables the transformation of the PET-derived human brain atlas into a protein density map of the serotonin (5-hydroxytryptamine, 5-HT) system. Next, we compared the regional receptor/transporter protein densities with mRNA levels and uncovered unique associations between protein expression and density at high detail. This new in vivo neuroimaging atlas of the 5-HT system not only provides insight in the human brain's regional protein synthesis, transport, and density, but also represents a valuable source of information for the neuroscience community as a comparative instrument to assess brain disorders.

Metabolic profiling of tyrosine, tryptophan, and glutamate in human urine using gas chromatography-tandem mass spectrometry combined with single SPE cleanup

The tyrosine, tryptophan, and glutamate metabolic pathways play key roles on pathological state of neuronal functions and the change of their levels in biological systems reflects the progress degree of neuronal diseases. Comprehensive profiling of these metabolites is important to find new biomarkers for diagnosis or prognosis of various neuronal diseases. However, the overall profiling analysis of various neurochemicals in biological sample is confronted with several limitations due to their low concentration and physicochemical properties and the coexistence of matrices. We developed an efficient and feasible method using gas chromatography-tandem mass spectrometry (GC-MS/MS). Wide-bore mixed cation exchange (MCX) SPE process enables a rapid and effective cleanup of 20 neurochemicals even including acidic and basic neurochemicals in a single SPE cartridge by using different composition of eluents. Selective derivatization of various types of metabolites was applied to achieve highly chromatographic separation and sensitive mass detection. Appropriate selection of precursor and product transition ions used in multiple reaction-monitoring (MRM) mode based on the MS/MS fragmentations of the derivatized neurochemicals could be significantly minimized the matrix effects and enhanced the reliability of quantification results. The developed method was validated in terms of linearity, limits of detection, precision, accuracy, and matrix effects. The intra- and inter-assay analytical variations were less than 10%. The overall linearity for all of the targets was excellent (R²≥0.996). The detection limits ranged between 0.38 and 8.13ng/mL for the acidic neurochemicals and between 0.02 and 11.1ng/mL for the basic neurochemicals. The developed protocol will be expected to be a promising tool for the understanding of the pathological state and diagnosis of various neuronal diseases.

Electrophysiological Actions of Synthetic Cathinones on Monoamine Transporters

Products containing psychoactive synthetic cathinones, such as mephedrone and 3,4-methylenedioxypyrovalerone (MDPV) are prevalent in our society. Synthetic cathinones are structurally similar to methamphetamine, and numerous synthetics have biological activity at dopamine, serotonin, and norepinephrine transporters. Importantly, monoamine transporters co-transport sodium ions along with their substrate, and movement of substrates and ions through the transporter can generate measurable ionic currents. Here we review how electrophysiological information has enabled us to determine how synthetic cathinones affect transporter-mediated currents in cells that express these transporters. Specifically, drugs that act as transporter substrates induce inward depolarizing currents when cells are held near their resting membrane potential, whereas drugs that act as transporter blockers induce apparent outward currents by blocking an inherent inward leak current. We have employed the two-electrode voltage-clamp technique in Xenopus laevis oocytes overexpressing monoamine transporters to determine whether synthetic cathinones found in the so-called bath salts products behave as blockers or substrates. We also examined the structure-activity relationships for synthetic cathinone analogs related to the widely abused compound MDPV, a common constituent in "bath salts" possessing potent actions at the dopamine transporter.

The Ups and Downs of Neurotransmitter Transporters

Plasma membrane neurotransmitter transporters are a family of integral membrane proteins, found on both neurons and glia, that have the capacity to influence neuronal signaling through a number of mechanisms including transmitter reuptake and ionic flux. Clinically, these proteins are of interest because their dysfunction is associated with several neurological and psychiatric disorders, and because they are the targets of many drugs of abuse and therapy. In this review, the authors focus on one of the more recent, fascinating discoveries about neurotransmitter transporters; namely, that transporter function is regulated by altering the number of transporters on the cell surface. These data suggest that transporter expression is in continual flux and that transporters respond to their environment in an effort to maintain baseline transmitter levels in the brain. The authors examine the mechanisms underlying changes in transporter number, discuss clinical disorders that are correlated with transporter expression, and suggest that controlling transporter redistribution may be a future therapeutic strategy for disorders related to abnormal transmitter levels.

The antidepressant drug paroxetine as a new lead candidate in schistosome drug discovery

Paroxetine is a new anti-schistosomal lead with potent activity against schistosomula and adult life stages of S. mansoni.

Dopamine is produced in the rat spinal cord and regulates micturition reflex after spinal cord injury

Dopamine (DA) neurons in the mammalian central nervous system are thought to be restricted to the brain. DA-mediated regulation of urinary activity is considered to occur through an interaction between midbrain DA neurons and the pontine micturition center. Here we show that DA is produced in the rat spinal cord and modulates the bladder reflex. We observed numerous tyrosine hydroxylase (TH)⁺ neurons in the autonomic nuclei and superficial dorsal horn in L6-S3 spinal segments. These neurons are dopamine-β-hydroxylase (DBH)^- and some contain detectable dopamine decarboxylase (DDC), suggesting their capacity to produce DA. Interestingly, following a complete thoracic spinal cord injury (SCI) to interrupt supraspinal projections, more TH⁺ neurons emerged in the lumbosacral spinal cord, coincident with a sustained, low level of DA expression there and a partially recovered micturition reflex. Non-selective blockade of spinal DA receptors reduced bladder activity whereas activation of spinal D₂-like receptors increased bladder activity and facilitated voiding. Additionally, depletion of lumbosacral TH⁺ neurons with 6-hydroxydopamine (6-OHDA) decreased bladder non-voiding contractions and voiding efficiency. Furthermore, injecting the transsynaptic neuronal tracer pseudorabies virus (PRV) into the bladder detrusor labeled TH⁺ cells in the lumbosacral cord, confirming their involvement in spinal micturition reflex circuits. These results illustrate that DA is synthesized in the rat spinal cord; plasticity of lumbosacral TH⁺ neurons following SCI may contribute to DA expression and modulate the spinal bladder reflex. Thus, spinally-derived DA and receptors could be a novel therapeutic target to improve micturition recovery after SCI.

Branch specific and spike-order specific action potential invasion in basal, oblique, and apical dendrites of cortical pyramidal neurons

In neocortical pyramidal neurons, action potentials (APs) propagate from the axon into the dendritic tree to influence distal synapses. Traditionally, AP backpropagation was studied in the thick apical trunk. Here, we used the principles of optical imaging developed by Cohen to investigate AP invasion into thin dendritic branches (basal, oblique, and tuft) of prefrontal cortical L5 pyramidal neurons. Multisite optical recordings from neighboring dendrites revealed a clear dichotomy between two seemingly equal dendritic branches belonging to the same cell ("sister branches"). We documented the variable efficacy of AP invasion in basal and oblique branches by revealing their AP voltage waveforms. Using fast multisite calcium imaging, we found that trains of APs are filtered differently between two apical tuft branches. Although one dendritic branch passes all spikes in an AP train, another branch belonging to the same neuron, same cortical layer, and same path distance from the cell body, experiences only one spike. Our data indicate that the vast differences in dendritic voltage and calcium transients, detected in dendrites of pyramidal neurons, arise from a nonuniform distribution of A-type [Formula: see text] conductance, an aggregate number of branch points in the path of the AP propagation and minute differences in dendritic diameter.

Treadmill running restores MDMA-mediated hyperthermia prevented by inhibition of the dorsomedial hypothalamus

The contribution of exercise to hyperthermia mediated by MDMA is not known. We recently showed that inhibiting the dorsomedial hypothalamus (DMH) attenuated spontaneous locomotion and hyperthermia and prevented deaths in rats given MDMA in a warm environment. The goal of this study was to confirm that restoring locomotion through a treadmill would reverse these effects thereby confirming that locomotion mediated by the DMH contributes to MDMA-mediated hyperthermia. Rats were randomized to receive bilateral microinjections, into the region of the DMH, of muscimol (80pmol/100nl) or artificial CSF followed by a systemic dose of either MDMA (7.5mg/kg, i.v.) or saline. Immediately after the systemic injection, rats were placed on a motorized treadmill maintained at 32°C. Rats were exercised at a fixed speed (10m/min) until their core temperature reached 41°C. Our results showed that a fixed exercise load abolished the decreases in temperature and mortality, seen previously with inhibition of the DMH in freely moving rats. Therefore, locomotion mediated by neurons in the DMH is critical to the development of hyperthermia from MDMA.

Serotonin and stress coping

Coping is the necessary outcome of any stressful situation and the major determinant of stress resilience. Coping strategies can be divided into two broad categories, based on the presence (active) or absence (passive) of attempts to act upon the stressor. The role of brain serotonin (5-hydroxytryptamine, 5-HT) in coping behavior that is emerging from studies in animals and humans is the subject of this article. We have focused attention on studies that consider the coping behavior exhibited when the individual is faced with a new stressful experience. Coping styles characterize different species with different evolutionary histories, from fishes to mammals, and evidence shows that serotonin transmission in the central nervous system, with differences in transporter, receptor types and hormone or neurotransmitter influences is critical in determining coping behavior. Moreover, a major role of environmental challenges throughout the lifespan affects brain systems that control coping outcomes through 5-HT transmission. In particular early experiences, for their long-term effects in adulthood, and social experiences throughout the life span, for the effects on serotonin functioning, received attention in preclinical research because of their parallelism in humans and animals. Based on growing evidence pointing to a medial prefrontal cortex-amygdala system in mediating adaptive and maladaptive stress responses, we propose a brain circuit in which serotonin neurons in the dorsal raphe depending on the CRF (corticotropin releasing factor) regulatory action engage a prefrontal cortical-amygdala pathway through 5-HT1A receptors, GABA and Glutamate to moderate coping behavior.

Monoamine transporter inhibitors and substrates as treatments for stimulant abuse

The acute and chronic effects of abused psychostimulants on monoamine transporters and associated neurobiology have encouraged development of candidate medications that target these transporters. Monoamine transporters, in general, and dopamine transporters, in particular, are critical molecular targets that mediate abuse-related effects of psychostimulants such as cocaine and amphetamine. Moreover, chronic administration of psychostimulants can cause enduring changes in neurobiology reflected in dysregulation of monoamine neurochemistry and behavior. The current review will evaluate evidence for the efficacy of monoamine transporter inhibitors and substrates to reduce abuse-related effects of stimulants in preclinical assays of stimulant self-administration, drug discrimination, and reinstatement. In considering deployment of monoamine transport inhibitors and substrates as agonist-type medications to treat stimulant abuse, the safety and abuse liability of the medications are an obvious concern, and this will also be addressed. Future directions in drug discovery should identify novel medications that retain efficacy to decrease stimulant use but possess lower abuse liability and evaluate the degree to which efficacious medications can attenuate or reverse neurobiological effects of chronic stimulant use.

Modeling of the hypothalamic-pituitary-adrenal axis-mediated interaction between the serotonin regulation pathway and the stress response using a Boolean approximation: a novel study of depression

Major depressive disorder (MDD) is a multifactorial disorder known to be influenced by both genetic and environmental factors. MDD presents a heritability of 37%, and a genetic contribution has also been observed in studies of family members of individuals with MDD that imply that the probability of suffering the disorder is approximately three times higher if a first-degree family member is affected. Childhood maltreatment and stressful life events (SLEs) have been established as critical environmental factors that profoundly influence the onset of MDD. The serotonin pathway has been a strong candidate for genetic studies, but it only explains a small proportion of the heritability of the disorder, which implies the involvement of other pathways. The serotonin (5-HT) pathway interacts with the stress response pathway in a manner mediated by the hypothalamic-pituitary-adrenal (HPA) axis. To analyze the interaction between the pathways, we propose the use of a synchronous Boolean network (SBN) approximation. The principal aim of this work was to model the interaction between these pathways, taking into consideration the presence of selective serotonin reuptake inhibitors (SSRIs), in order to observe how the pathways interact and to examine if the system is stable. Additionally, we wanted to study which genes or metabolites have the greatest impact on model stability when knocked out in silico. We observed that the biological model generated predicts steady states (attractors) for each of the different runs performed, thereby proving that the system is stable. These attractors changed in shape, especially when anti-depressive drugs were also included in the simulation. This work also predicted that the genes with the greatest impact on model stability were those involved in the neurotrophin pathway, such as CREB, BDNF (which has been associated with major depressive disorder in a variety of studies) and TRkB, followed by genes and metabolites related to 5-HT synthesis.

Brain neurons partly expressing dopaminergic phenotype: location, development, functional significance, and regulation

In addition to catecholaminergic neurons possessing all the enzymes of catecholamine synthesis and the specific membrane transporters, neurons partly expressing the catecholaminergic phenotype have been found a quarter of a century ago. Most of them express individual enzymes of dopamine (DA) synthesis, tyrosine hydroxylase (TH), or aromatic l-amino acid decarboxylase (AADC), lacking the DA membrane transporter and the vesicular monoamine transporter, type 2. These so-called monoenzymatic neurons are widely distributed throughout the brain in ontogenesis and adulthood being in some brain regions even more numerous than dopaminergic (DA-ergic) neurons. Individual enzymes of DA synthesis are expressed in these neurons continuously or transiently in norm and pathology. It has been proven that monoenzymatic TH neurons and AADC neurons are capable of producing DA in cooperation. It means that l-3,4-dihydroxyphenylalanine (l-DOPA) synthesized from l-tyrosine in monoenzymatic TH neurons is transported to monoenzymatic AADC neurons for DA synthesis. Such cooperative synthesis of DA is considered as a compensatory reaction under a failure of DA-ergic neurons, for example, in neurodegenerative diseases like hyperprolactinemia and Parkinson's disease. Moreover, l-DOPA, produced in monoenzymatic TH neurons, is assumed to play a role of a neurotransmitter or neuromodulator affecting the target neurons via catecholamine receptors. Thus, numerous widespread neurons expressing individual complementary enzymes of DA synthesis serve to produce DA in cooperation that is a compensatory reaction at failure of DA-ergic neurons.

Chronic social defeat up-regulates expression of the serotonin transporter in rat dorsal raphe nucleus and projection regions in a glucocorticoid-dependent manner

Chronic stress and dysfunction of the serotonergic system in the brain have been considered two of the major risks for development of depression. In this study, adult Fischer 344 rats were subjected to a regimen of chronic social defeat (CSD). To mimic stressful conditions, some rats were not exposed to CSD, but instead treated with corticosterone (CORT) in oral solution while maintained in their home cage. Protein levels of the serotonin transporter (SERT) in the dorsal raphe nucleus (DRN), hippocampus, frontal cortex, and amygdala were examined by Western blotting or immunofluorescence staining. The results showed that CSD up-regulated SERT protein levels in the DRN, hippocampus, frontal cortex, and amygdala regions. This up-regulation was abolished or prevented by adrenalectomy, or treatment with antagonists of corticosteroid receptors mifepristone and spironolactone, alone or in combination. Similarly, up-regulated SERT protein levels in these brain regions were also observed in rats treated with oral CORT ingestion, which was analogously prevented by treatment with mifepristone and spironolactone. Furthermore, both CSD- and CORT-induced up-regulation of SERT protein levels in the DRN and three brain regions were attenuated by simultaneous treatment with fluoxetine, an antidepressant that specifically inhibits serotonin reuptake. The results indicate that up-regulation in SERT protein levels in the DRN and forebrain limbic structures caused by CSD regimen was mainly motivated by CORT through corticosteroid receptors. The present findings demonstrate that chronic stress is closely correlated with the serotonergic system by acting on the regulation of the SERT expression in the DRN and its projection regions, which may contribute to the development of depression.

Repeated exposure to the herbicide atrazine alters locomotor activity and the nigrostriatal dopaminergic system of the albino rat

Atrazine (ATR) is used as a pre- and post-emergent herbicide; although banned in several countries of the European Community, it is still used extensively around the world. A recent study in rats has shown that chronic, daily exposure to 10 mg ATR/kg BW causes hyperactivity, disrupts motor coordination and learning of behavioral tasks, and decreases dopamine levels in the brain. In order to evaluate the short-term effect of ATR exposure on locomotor activity, monoamine markers, and antioxidants, adult male Sprague-Dawley rats received six IP injections of 100 mg ATR/kg BW or vehicle over two weeks. After every ATR injection we found hypoactivity that lasted up to five days, and it was accompanied by reductions in levels of striatal DA, DOPAC, and HVA without any alteration in the striatal expression of the mRNAs for Mn-SOD, Trx-1, DAR-D(1), or DAR-D(2). In contrast, in the nucleus accumbens no changes in monoamine markers were observed, and a down-regulation of Trx-1 expression was detected shortly after the ATR treatment. Moreover, in the ventral midbrain, we found that ATR induced a down-regulation of mRNA for Th and DAT, but it increased VMAT2 mRNA expression. Decreases of monoamine levels and of locomotor activity disappeared three months after ATR treatment; however, an amphetamine challenge (1 mg/kg) given two months after the ATR treatment resulted in a significant stimulation in the exposed group, revealing hidden effects of ATR on dopaminergic systems. These results indicate that ATR exposure differentially modifies the dopaminergic systems, and these modifications may underlie the behavioral changes observed.

Development of the serotonergic cells in murine raphe nuclei and their relations with rhombomeric domains

The raphe nuclei represent the origin of central serotonergic projections. The literature distinguishes seven nuclei grouped into rostral and caudal clusters relative to the pons. The boundaries of these nuclei have not been defined precisely enough, particularly with regard to developmental units, notably hindbrain rhombomeres. We hold that a developmental point of view considering rhombomeres may explain observed differences in connectivity and function. There are twelve rhombomeres characterized by particular genetic profiles, and each develops between one and four distinct serotonergic populations. We have studied the distribution of the conventional seven raphe nuclei among these twelve units. To this aim, we correlated 5-HT-immunoreacted neurons with rhombomeric boundary landmarks in sagittal mouse brain sections at different developmental stages. Furthermore, we performed a partial genoarchitectonic analysis of the developing raphe nuclei, mapping all known serotonergic differentiation markers, and compared these results, jointly with others found in the literature, with our map of serotonin-containing populations, in order to examine regional variations in correspondence. Examples of regionally selective gene patterns were identified. As a result, we produced a rhombomeric classification of some 45 serotonergic populations, and suggested a corresponding modified terminology. Only a minor rostral part of the dorsal raphe nucleus lies in the midbrain. Some serotonergic neurons were found in rhombomere 4, contrary to the conventional assumption that it lacks such neurons. We expect that our reclassification of raphe nuclei may be useful for causal analysis of their differential molecular specification, as well as for studies of differential connectivity and function.

From behavioral context to receptors: serotonergic modulatory pathways in the IC

In addition to ascending, descending, and lateral auditory projections, inputs extrinsic to the auditory system also influence neural processing in the inferior colliculus (IC). These types of inputs often have an important role in signaling salient factors such as behavioral context or internal state. One route for such extrinsic information is through centralized neuromodulatory networks like the serotonergic system. Serotonergic inputs to the IC originate from centralized raphe nuclei, release serotonin in the IC, and activate serotonin receptors expressed by auditory neurons. Different types of serotonin receptors act as parallel pathways regulating specific features of circuitry within the IC. This results from variation in subcellular localizations and effector pathways of different receptors, which consequently influence auditory responses in distinct ways. Serotonin receptors may regulate GABAergic inhibition, influence response gain, alter spike timing, or have effects that are dependent on the level of activity. Serotonin receptor types additionally interact in nonadditive ways to produce distinct combinatorial effects. This array of effects of serotonin is likely to depend on behavioral context, since the levels of serotonin in the IC transiently increase during behavioral events including stressful situations and social interaction. These studies support a broad model of serotonin receptors as a link between behavioral context and reconfiguration of circuitry in the IC, and the resulting possibility that plasticity at the level of specific receptor types could alter the relationship between context and circuit function.

Acupuncture stimulation at HT7 alleviates depression-induced behavioral changes via regulation of the serotonin system in the prefrontal cortex of maternally-separated rat pups

A possible application of acupuncture in alleviating depression-like behavioral changes and regulating serotonin signaling in the prefrontal cortex (PFC) of maternally-separated rat pups was investigated in this study. On postnatal day 15, rat pups were maternally-separated and received acupuncture stimulation at acupoint HT7 or ST36 once a day for 7 days. On postnatal day 21, the tail suspension test was performed and the PFC was harvested. Tissue levels of serotonin (5-HT) and 5-hydroxyindole-3-acetic acid (5-HIAA) were then measured by high-performance liquid chromatography and expression of serotonin transporter (5-HTT) and brain-derived neurotrophic factor (BDNF) were assessed by western blotting. Levels of 5-HT and 5-HIAA were not significantly changed, but the 5-HIAA/5-HT ratio was significantly increased by maternal separation. The immobility time of maternally-separated rat pups was increased, and increased 5-HTT expression and reduced BDNF level were observed in the PFC. But acupuncture stimulation at HT7 alleviated the behavioral change and regulated the changes of 5-HIAA/5-HT ratio, 5-HTT, and BDNF. In conclusion, acupuncture stimulation at HT7 can relieve maternal separation-induced changes, and we propose that regulation of the 5-HIAA/5-HT ratio and of 5-HTT expression by acupuncture stimulation are important acupuncture-induced benefits in this animal model of depression.

PET imaging a MPTP-induced mouse model of Parkinson's disease using the fluoropropyl-dihydrotetrabenazine analog [18F]-DTBZ (AV-133)

Parkinson's disease (PD) is characterized by the loss of dopamine-producing neurons in the nigrostriatal system. Numerous researchers in the past have attempted to track the progression of dopaminergic depletion in PD. We applied a quantitative non-invasive PET imaging technique to follow this degeneration process in an MPTP-induced mouse model of PD. The VMAT2 ligand (18)F-DTBZ (AV-133) was used as a radioactive tracer in our imaging experiments to monitor the changes of the dopaminergic system. Intraperitoneal administrations of MPTP (a neurotoxin) were delivered to mice at regular intervals to induce lesions consistent with PD. Our results indicate a significant decline in the levels of striatal dopamine and its metabolites (DOPAC and HVA) following MPTP treatment as determined by HPLC method. Images obtained by positron emission tomography revealed uptake of (18)F-DTBZ analog in the mouse striatum. However, reduction in radioligand binding was evident in the striatum of MPTP lesioned animals as compared with the control group. Immunohistochemical analysis further confirmed PET imaging results and indicated the progressive loss of dopaminergic neurons in treated animals compared with the control counterparts. In conclusion, our findings suggest that MPTP induced PD in mouse model is appropriate to follow the degeneration of dopaminergic system and that (18)F-DTBZ analog is a potentially sensitive radiotracer that can used to diagnose changes associated with PD by PET imaging modality.

4-(4-(dimethylamino)phenyl)-1-methylpyridinium (APP+) is a fluorescent substrate for the human serotonin transporter

Monoamine transporters terminate synaptic neurotransmission and are molecular targets for antidepressants and psychostimulants. Fluorescent reporters can monitor real-time transport and are amenable for high-throughput screening. However, until now, their use has mostly been successful to study the catecholamine transporters but not the serotonin (5HT) transporter. Here, we use fluorescence microscopy, electrophysiology, pharmacology, and molecular modeling to compare fluorescent analogs of 1-methyl-4-phenylpyridinium (MPP(+)) as reporters for the human serotonin transporter (hSERT) in single cells. The fluorescent substrate 4-(4-(dimethylamino)phenyl)-1-methylpyridinium (APP(+)) exhibits superior fluorescence uptake in hSERT-expressing HEK293 cells than other MPP(+) analogs tested. APP(+) uptake is Na(+)- and Cl(-)-dependent, displaced by 5HT, and inhibited by fluoxetine, suggesting APP(+) specifically monitors hSERT activity. ASP(+), which was previously used to study catecholamine transporters, is 10 times less potent than APP(+) at inhibiting 5HT uptake and has minimal hSERT-mediated uptake. Furthermore, in hSERT-expressing oocytes voltage-clamped to -60 mV, APP(+) induced fluoxetine-sensitive hSERT-mediated inward currents, indicating APP(+) is a substrate, whereas ASP(+) induced hSERT-mediated outward currents and counteracted 5HT-induced hSERT currents, indicating ASP(+) possesses activity as an inhibitor. Extra-precise ligand receptor docking of APP(+) and ASP(+) in an hSERT homology model showed both ASP(+) and APP(+) docked favorably within the active region; accordingly, comparable concentrations are required to elicit their opposite electrophysiological responses. We conclude APP(+) is better suited than ASP(+) to study hSERT transport fluorometrically.

[18F]CFT synthesis and binding to monoamine transporters in rats

BACKGROUND

We present the electrophilic synthesis of [18F]2β-carbomethoxy-3β-(4-fluoro)tropane [[18F]CFT] and the pharmacological specificity and selectivity of [18F]CFT for monoamine transporters in the brain and peripheral organs of rats. The human radiation dose is extrapolated from the animal data.

METHODS

[18F]CFT was synthesized by electrophilic fluorination of a stannylated precursor by using post-target-produced [18F]F2 as a fluorinating agent. The ex vivo 18F-activity biodistribution of [18F]CFT in the brain of rats was studied by autoradiography. The binding of [18F]CFT to the monoamine transporters was studied using in vivo blocking experiments with dopamine transporter [DAT], norepinephrine transporter [NET], or serotonin transporter [SERT] inhibitors. In vivo animal positron emission tomography was used as a comparative method to determine tracer kinetics. Human radiation dose was assessed using OLINDA software.

RESULTS

The radiochemical yield of [18F]CFT from the initial [18F]F-, decay corrected to the end of bombardment, was 3.2 ± 1.0%. The specific activity [SA] was 14.5 ± 3.4 GBq/μmol, decay corrected to the end of synthesis. Radiochemical purity exceeded 99%. DAT-specific binding was found in the striatum, locus coeruleus, and pancreas. NET-specific binding was found in the locus coeruleus. SERT-specific binding was not found in any of the studied organs. Effective dose equivalent [EDE] estimated for the standard human model was 12.8 μSv/MBq. Effective dose [ED] was 9.17 μSv/MBq.

CONCLUSIONS

Post-target-produced high-SA [18F]F2 was used to incorporate18F directly into the phenyl ring of [18F]CFT. The final product had high radiochemical and chemical purities and a high SA for DAT and NET studies in vivo. In periphery, [18F]CFT showed a specific uptake in the pancreas. EDE and ED corresponded well with other18F-radioligands.