Reserpine- and tetrabenazine-sensitive transport of (3)H-histamine by the neuronal isoform of the vesicular monoamine transporter

Neurochemical mechanisms of deep brain stimulation for depression in animal models

Deep brain stimulation (DBS) has emerged as a neuromodulation therapy for treatment-resistant depression, but its actual efficacy and mechanisms of action are still unclear. Changes in neurochemical transmission are important mechanisms of antidepressant therapies. Here, we review the preclinical DBS literature reporting behavioural and neurochemical data associated with its antidepressant-like effects. The most commonly studied target in preclinical models was the ventromedial prefrontal cortex (vmPFC). In rodents, DBS delivered to this target induced serotonin (5-HT) release and increased 5-HT1_B receptor expression. The antidepressant-like effects of vmPFC DBS seemed to be independent of the serotonin transporter and potentially mediated by the direct modulation of prefrontal projections to the raphe. Adenosinergic and glutamatergic transmission might have also play a role. Medial forebrain bundle (MFB) DBS increased dopamine levels and reduced D2 receptor expression, whereas nucleus accumbens (NAcc), and lateral habenula (LHb) stimulation increased catecholamine levels in different brain regions. In rodents, subthalamic nucleus (STN) DBS induced robust depression-like responses associated with a reduction in serotonergic transmission, as revealed by a decrease in serotonin release. Some of these effects seemed to be mediated by 5HT1_A receptors. In conclusion, the antidepressant-like effects of DBS in preclinical models have been well documented in multiple targets. Though variable mechanisms have been proposed, DBS-induced acute and long-term changes in neurochemical substrates seem to play an important role in the antidepressant-like effects of this therapy.

Tetrabenazine Facilitates Exocytosis by Enhancing Calcium-Induced Calcium Release through Ryanodine Receptors

Vesicular monoamine transporter-2 is expressed in the presynaptic secretory vesicles membrane in the brain. Its blockade by tetrabenazine (TBZ) causes depletion of dopamine at striatal basal ganglia; this is the mechanism underlying its long-standing use in the treatment of Huntington's disease. In the frame of a project aimed at investigating the kinetics of exocytosis from vesicles with partial emptying of their neurotransmitter, we unexpectedly found that TBZ facilitates exocytosis; thus, we decided to characterize such effect. We used bovine chromaffin cells (BCCs) challenged with repeated pulses of high K⁺ Upon repeated K⁺ pulsing, the exocytotic catecholamine release responses were gradually decaying. However, when cells were exposed to TBZ, responses were mildly augmented and decay rate delayed. Facilitation of exocytosis was not due to Ca²⁺ entry blockade through voltage-activated calcium channels (VACCs) because, in fact, TBZ mildly blocked the whole-cell Ca²⁺ current. However, TBZ mimicked the facilitatory effects of exocytosis elicited by BayK8644 (L-subtype VACC agonist), an effect blocked by nifedipine (VACC antagonist). On the basis that TBZ augmented the secretory responses to caffeine (but not those of histamine), we monitored its effects on cytosolic Ca²⁺ elevations ([Ca²⁺]_c) triggered by caffeine or histamine. While the responses to caffeine were augmented twice by TBZ, those of histamine were unaffected; the same happened in rat cortical neurons. Hence, we hypothesize that TBZ facilitates exocytosis by increasing Ca²⁺ release through the endoplasmic reticulum ryanodine receptor channel (RyR). Confirming this hypothesis are docking results, showing an interaction of TBZ with RyRs. This is consonant with the existence of a healthy Ca²⁺-induced-Ca²⁺-release mechanism in BCCs. SIGNIFICANCE STATEMENT: A novel mechanism of action for tetrabenazine (TBZ), a drug used in the therapy of Huntington's disease (HD), is described here. Such mechanism consists of facilitation by combining TBZ with the ryanodine receptor of the endoplasmic reticulum, thereby increasing Ca²⁺-induced Ca²⁺ release. This novel mechanism should be taken into account when considering the efficacy and/or safety of TBZ in the treatment of chorea associated with HD and other disorders. Additionally, it could be of interest in the development of novel medicines to treat these pathological conditions.

Merkel Cells Activate Sensory Neural Pathways through Adrenergic Synapses

Epithelial-neuronal signaling is essential for sensory encoding in touch, itch, and nociception; however, little is known about the release mechanisms and neurotransmitter receptors through which skin cells govern neuronal excitability. Merkel cells are mechanosensory epidermal cells that have long been proposed to activate neuronal afferents through chemical synaptic transmission. We employed a set of classical criteria for chemical neurotransmission as a framework to test this hypothesis. RNA sequencing of adult mouse Merkel cells demonstrated that they express presynaptic molecules and biosynthetic machinery for adrenergic transmission. Moreover, live-cell imaging directly demonstrated that Merkel cells mediate activity- and VMAT-dependent release of fluorescent catecholamine neurotransmitter analogs. Touch-evoked firing in Merkel-cell afferents was inhibited either by pre-synaptic silencing of SNARE-mediated vesicle release from Merkel cells or by neuronal deletion of β₂-adrenergic receptors. Together, these results identify both pre- and postsynaptic mechanisms through which Merkel cells excite mechanosensory afferents to encode gentle touch. VIDEO ABSTRACT.

HID-1 controls formation of large dense core vesicles by influencing cargo sorting and trans-Golgi network acidification

The peripheral membrane protein HID-1 localizes to the trans-Golgi network, where it contributes to the formation of large dense core vesicles of neuroendocrine cells by influencing cargo sorting and trans-Golgi network acidification.

Large dense core vesicles (LDCVs) mediate the regulated release of neuropeptides and peptide hormones. They form at the trans-Golgi network (TGN), where their soluble content aggregates to form a dense core, but the mechanisms controlling biogenesis are still not completely understood. Recent studies have implicated the peripheral membrane protein HID-1 in neuropeptide sorting and insulin secretion. Using CRISPR/Cas9, we generated HID-1 KO rat neuroendocrine cells, and we show that the absence of HID-1 results in specific defects in peptide hormone and monoamine storage and regulated secretion. Loss of HID-1 causes a reduction in the number of LDCVs and affects their morphology and biochemical properties, due to impaired cargo sorting and dense core formation. HID-1 KO cells also exhibit defects in TGN acidification together with mislocalization of the Golgi-enriched vacuolar H⁺-ATPase subunit isoform a2. We propose that HID-1 influences early steps in LDCV formation by controlling dense core formation at the TGN.

Forgotten but not gone: new developments in the understanding and treatment of tardive dyskinesia

The broad use of atypical antipsychotics was expected to dramatically reduce the prevalence and incidence of tardive dyskinesia (TD), but data show that TD remains an important challenge due the persistent nature of its symptoms and resistance to numerous treatment modalities, including antipsychotic discontinuation. Recent insights on genetic risk factors and new concepts surrounding pathophysiology have spurred interest in the possibility of targeted treatment for TD. As will be reviewed in this article, the number of evidence-based strategies for TD treatment is small: only clonazepam, amantadine, ginkgo biloba extract, and the vesicular monoamine transporter 2 (VMAT2) inhibitor tetrabenazine have compelling data. Using new insights into the metabolism of tetrabenazine and the properties of its active metabolites, 2 modifications of tetrabenazine have been synthesized to improve the kinetic profile, and are currently involved in double-blind placebo controlled studies aimed at U.S. Food and Drug Administration (FDA) regulatory approval. The possible availability of these new agents, deuterated tetrabenazine and valbenazine, significantly widens the range of treatment choices for patients with TD. For clinicians with patients at risk for TD due to dopamine antagonist exposure, experience has shown that the problem of TD will be an ongoing issue in modern psychiatry, and that an appreciation of new developments in the pathophysiology of, risk factors for, and treatment of TD is crucial to managing this condition.

Immunochemical localization of vesicular monoamine transporter 2 (VMAT2) in mouse brain

Vesicular monoamine transporter 2 (VMAT2, SLC18A2) is a transmembrane transporter protein that packages dopamine, serotonin, norepinephrine, and histamine into vesicles in preparation for neurotransmitter release from the presynaptic neuron. VMAT2 function and related vesicle dynamics have been linked to susceptibility to oxidative stress, exogenous toxicants, and Parkinson's disease. To address a recent depletion of commonly used antibodies to VMAT2, we generated and characterized a novel rabbit polyclonal antibody generated against a 19 amino acid epitope corresponding to an antigenic sequence within the C-terminal tail of mouse VMAT2. We used genetic models of altered VMAT2 expression to demonstrate that the antibody specifically recognizes VMAT2 and localizes to synaptic vesicles. Furthermore, immunohistochemical labeling using this VMAT2 antibody produces immunoreactivity that is consistent with expected VMAT2 regional distribution. We show the distribution of VMAT2 in monoaminergic brain regions of mouse brain, notably the midbrain, striatum, olfactory tubercle, dopaminergic paraventricular nuclei, tuberomammillary nucleus, raphe nucleus, and locus coeruleus. Normal neurotransmitter vesicle dynamics are critical for proper health and functioning of the nervous system, and this well-characterized VMAT2 antibody will be a useful tool in studying neurodegenerative and neuropsychiatric conditions characterized by vesicular dysfunction.

The expression and function of histamine H₃ receptors in pancreatic beta cells

BACKGROUND AND PURPOSE

Histamine and its receptors in the CNS play important roles in energy homeostasis. Here, we have investigated the expression and role of histamine receptors in pancreatic beta cells, which secrete insulin.

EXPERIMENTAL APPROACH

The expression of histamine receptors in pancreatic beta cells was examined by RT-PCR, Western blotting and immunostaining. Insulin secretion assay, ATP measurement and calcium imaging studies were performed to determine the function and signalling pathway of histamine H₃ receptors in glucose-induced insulin secretion (GIIS) from MIN6 cells, a mouse pancreatic beta cell line. The function and signalling pathway of H₃ receptors in MIN6 cell proliferation were examined using pharmacological assay and Western blotting.

KEY RESULTS

Histamine H₃ receptors were expressed in pancreatic beta cells. A selective H₃ receptor agonist, imetit, and a selective inverse H₃ receptor agonist, JNJ-5207852, had inhibitory and facilitatory effects, respectively, on GIIS in MIN6 cells. Neither imetit nor JNJ-5207852 altered intracellular ATP concentration, or intracellular calcium concentration stimulated by glucose and KCl, indicating that GIIS signalling was affected by H3 receptor signalling downstream of the increase in intracellular calcium concentration. Moreover, imetit attenuated bromodeoxyuridine incorporation in MIN6 cells. The phosphorylation of cAMP response element-binding protein (CREB), which facilitated beta cell proliferation, was inhibited, though not significantly, by imetit, indicating that activated H₃ receptors inhibited MIN6 cell proliferation, possibly by decreasing CREB phosphorylation.

CONCLUSIONS AND IMPLICATIONS

Histamine H₃ receptors were expressed in mouse beta cells and could play a role in insulin secretion and, possibly, beta cell proliferation.

Vesicular neurotransmitter transporters: an approach for studying transporters with purified proteins

Vesicular storage and subsequent release of neurotransmitters are the key processes of chemical signal transmission. In this process, vesicular neurotransmitter transporters are responsible for loading the signaling molecules. The use of a "clean biochemical" approach with purified, recombinant transporters has helped in the identification of novel vesicular neurotransmitter transporters and in the analysis of the control of signal transmission.

Localization and expression of VMAT2 aross mammalian species: a translational guide for its visualization and targeting in health and disease

VMAT2 is the vesicular monoamine transporter that allows DA, NE, Epi, His, and 5-HT uptake into neurons and endocrine cells. A second isoform, VMAT1, has similar structure and function, but does not recognize histamine as a substrate. VMAT1 is absent from neurons, and its major function appears to be in endocrine cells, that is, enterochromaffin cells, which scavenge 5-HT, but not histamine, from dietary sources. This chapter provides an update on the neuroanatomical distribution of VMAT2 across various mammalian species, including human, primate, pig, rat, and mouse. When necessary, VMAT1 expression is provided as a contrast. The main purpose of this chapter is to allow clinicians, in particular endocrinologists and diagnosing neuroradiologists and neuropathologists, an acquaintanceship with the possibilities for VMAT2 as a target for in vivo imaging, and drug development, based on this updated information.

Tetrabenazine for the treatment of chorea and other hyperkinetic movement disorders

Tetrabenazine (TBZ; Xenazine) is a potent, selective, reversible depletor of monoamines from nerve terminals. TBZ inhibits the vesicular monoamine transporter type 2 which, in humans, is expressed nearly exclusively in the brain. TBZ is rapidly metabolized in the liver by carbonyl reductase to stereoisomers of hydrotetrabenazine, some of which are potent inhibitors of vesicular monoamine transporter type 2. Initially developed in the 1950s for schizophrenia, since the 1970s several publications have reported on the efficacy of TBZ in the treatment of various hyperkinetic movement disorders. Although quite effective in controlling the involuntary movements, there were considerable inter-individual differences in the optimal dose, defined as the dose judged by the investigator to provide the greatest efficacy with minimal or tolerable adverse events. This variability is in part owing to differences in severity and mechanism of the target symptoms and to variable activity of the enzyme carbonyl reductase that metabolizes TBZ to its active metabolites. Dose-limiting adverse events, consisting mainly of sedation, parkinsonism, akathisia and depression, are usually rapidly reversible upon dosage reduction. In addition to its established antichorea efficacy in Huntington's disease, the drug has been reported to also be effective in a variety of other hyperkinetic movement disorders, including tardive dyskinesia and tics associated with Tourette's syndrome.

Tetrabenazine: for chorea associated with Huntington's disease

Oral tetrabenazine is currently the only drug approved by the US FDA for the treatment of chorea associated with Huntington's disease (HD). Although the precise antichorea mechanism of action is unknown, it most likely involves reversible depletion of monoamines, particularly dopamine, from presynaptic terminals via inhibition of human vesicular monoamine transporter type 2. In a 12-week, double-blind, placebo-controlled trial conducted in the US in patients with HD, oral tetrabenazine (≤100 mg/day; n = 54) was significantly (p = 0.0001) more efficacious than placebo (n = 30) at improving adjusted mean Unified HD Rating Scale (UHDRS) total maximum chorea scores (reduced from baseline by 5 vs 1.5) [primary endpoint]. After 12 weeks, improvements in UHDRS total maximum chorea scores of >3 were achieved by significantly (p < 0.0001) more patients in the tetrabenazine group than in the placebo group. The antichorea efficacy of tetrabenazine was maintained in an 80-week extension study (n = 75), with the adjusted mean UHDRS total maximum chorea score significantly (p < 0.001) reduced from baseline (score of 14.9) by 4.6 points (primary outcome). In the 12-week trial and 80-week extension study, treatment-emergent adverse events in the tetrabenazine group mainly occurred during the dosage-titration phase, a period during which the dosage was individually optimized. Most of these events were mild to moderate and were manageable with dosage adjustments or discontinuation of study drug.

RNAi screen identifies a role for adaptor protein AP-3 in sorting to the regulated secretory pathway

AP-3 concentrates proteins within large dense-core vesicles to promote regulated exocytosis.

The regulated release of proteins depends on their inclusion within large dense-core vesicles (LDCVs) capable of regulated exocytosis. LDCVs form at the trans-Golgi network (TGN), but the mechanism for protein sorting to this regulated secretory pathway (RSP) and the cytosolic machinery involved in this process have remained poorly understood. Using an RNA interference screen in Drosophila melanogaster S2 cells, we now identify a small number of genes, including several subunits of the heterotetrameric adaptor protein AP-3, which are required for sorting to the RSP. In mammalian neuroendocrine cells, loss of AP-3 dysregulates exocytosis due to a primary defect in LDCV formation. Previous work implicated AP-3 in the endocytic pathway, but we find that AP-3 promotes sorting to the RSP within the biosynthetic pathway at the level of the TGN. Although vesicles with a dense core still form in the absence of AP-3, they contain substantially less synaptotagmin 1, indicating that AP-3 concentrates the proteins required for regulated exocytosis.

Protective actions of the vesicular monoamine transporter 2 (VMAT2) in monoaminergic neurons

Vesicular monoamine transporters (VMATs) are responsible for the packaging of neurotransmitters such as dopamine, serotonin, norepinephrine, and epinephrine into synaptic vesicles. These proteins evolved from precursors in the major facilitator superfamily of transporters and are among the members of the toxin extruding antiporter family. While the primary function of VMATs is to sequester neurotransmitters within vesicles, they can also translocate toxicants away from cytosolic sites of action. In the case of dopamine, this dual role of VMAT2 is combined-dopamine is more readily oxidized in the cytosol where it can cause oxidative stress so packaging into vesicles serves two purposes: neurotransmission and neuroprotection. Furthermore, the deleterious effects of exogenous toxicants on dopamine neurons, such as MPTP, can be attenuated by VMAT2 activity. The active metabolite of MPTP can be kept within vesicles and prevented from disrupting mitochondrial function thereby sparing the dopamine neuron. The highly addictive drug methamphetamine is also neurotoxic to dopamine neurons by using dopamine itself to destroy the axon terminals. Methamphetamine interferes with vesicular sequestration and increases the production of dopamine, escalating the amount in the cytosol and leading to oxidative damage of terminal components. Vesicular transport seems to resist this process by sequestering much of the excess dopamine, which is illustrated by the enhanced methamphetamine neurotoxicity in VMAT2-deficient mice. It is increasingly evident that VMAT2 provides neuroprotection from both endogenous and exogenous toxicants and that while VMAT2 has been adapted by eukaryotes for synaptic transmission, it is derived from phylogenetically ancient proteins that originally evolved for the purpose of cellular protection.

Recommedition for Apllication and Determination Tumor Markers of Neuroendocrine System

Neuroendokrini tumori vode poreklo iz neuroendokrinih ćelija, koji pod uslovima specifične stimulacije sekretuju hormone, regulišući niz različitih funkcija u organizmu. U radu je opisana grupa neuroendokrinih tumora koja se sastoji od: karcinoida endokrinih tumora pankreasa, neuroblastoma, medularnog karcinoma tiroidee i feohromocitoma. Biće diskutovani tumori koji potiču iz endokrinih žlezda, kao i oni koji vode poreklo iz takozvanog »difuznog« neuroendokrinog ćelijskog sistema gastrointestinalnog trakta. Radi se o najvećem endokrinom organu u organizmu. Oko 50 različitih neuroendokrinih ćelijskih tipova gastrointestinalnog trakta je indentifikovano. Važan dijagnostički segment u tretmanu ovih tumora, je određivanje biogenih amina i peptida, čija je aktivnost izmenjena kod pojave neoplazmi. Širok je spektar kliničkih simptoma kod pojave ove vrste tumora, a svi su posledica sposobnosti tumora da sekretuju povećane količine peptinih hormona i biogenih amina. Glavni ciljevi ovog teksta su procena važnosti ove grupe tumorskih markera, kao i predlozi za njihovo koriŠćenje u našim uslovima.

Comparative sequence analysis and tissue localization of members of the SLC6 family of transporters in adult Drosophila melanogaster

The SLC6 family comprises proteins that move extracellular neurotransmitters, amino acids and osmolytes across the plasma membrane into the cytosol. In mammals, deletion of SLC6 family members has dramatic physiologic consequences, but in the model organism Drosophila melanogaster, little is known about this family of proteins. Therefore, in this study we carried out an initial analysis of 21 known or putative SLC6 family members from the Drosophila genome. Protein sequences from these genes segregated into either well-defined subfamilies, including the novel insect amino acid transporter subfamily, or into a group of weakly related sequences not affiliated with a recognized subfamily. Reverse transcription-polymerase chain reaction analysis and in situ hybridization showed that seven of these genes are expressed in the CNS. In situ hybridization revealed that two previously cloned SLC6 members, the serotonin and dopamine transporters, were localized to presumptive presynaptic neurons that previously immunolabelled for these transmitters. RNA for CG1732 (the putative GABA transporter) and CG15088 (a member of the novel insect amino acid transporter family) was localized in cells likely to be subtypes of glia, while RNA for CG5226, CG10804 (both members of the orphan neurotransmitter transporter subfamily) and CG5549 (a putative glycine transporter) were expressed broadly throughout the cellular cortex of the CNS. Eight of the 21 sequences were localized outside the CNS in the alimentary canal, Malpighian tubules and reproductive organs. Localization for six sequences was not found or not attempted in the adult fly. We used the Drosophila ortholog of the mammalian vesicular monoamine transporter 2, CG33528, to independently identify monoaminergic neurons in the adult fly. RNA for CG33528 was detected in a limited number of cells in the central brain and in a beaded stripe at the base of the photoreceptors in the position of glia, but not in the photoreceptors themselves. The SLC6 localization observations in conjunction with likely substrates based on phylogenetic inferences are a first step in defining the role of Na/Cl-dependent transporters in Drosophila physiology.

Histidine decarboxylase, DOPA decarboxylase, and vesicular monoamine transporter 2 expression in neuroendocrine tumors: immunohistochemical study and gene expression analysis

Histidine decarboxylase (HDC) and vesicular monoamine transporter 2 (v-MAT2) are involved in the biosynthesis and storage of histamine. DOPA decarboxylase (DDC) is involved in the biosynthesis of a variety of amines and shares a high degree of homology with HDC. HDC and v-MAT2 immunoreactivities (IR) have recently been detected in well-differentiated neuroendocrine tumors (WDNETs) and poorly differentiated neuroendocrine carcinomas (PDNECs) of various sites and have been proposed as general endocrine markers. We evaluated HDC and v-MAT2 IR in a series of 117 WDNETs and PDNECs from different sites. Western blotting analysis was performed to verify the specificity of anti-DDC and anti-HDC antibodies. Real-time RT-PCR was performed using specific probes for HDC and DDC on 42 cases, examined also for DDC IR. HDC and v-MAT2 IR were observed in the majority of WDNETs and PDNECs of all sites and HDC-IR cases were always also DDC-IR. In contrast, high levels of HDC mRNA were detected only in the gastroenteropancreatic WDNETs, which did not show increased DDC mRNA levels. On the other hand, bronchial carcinoids and lung PDNECs showed high DDC mRNA levels, but nearly undetectable HDC mRNA levels. Western blotting analysis showed a cross-reaction between anti-HDC and anti-DDC antibodies. HDC should not be considered as a general endocrine marker and HDC IR in bronchial carcinoids and PDNECs of the lung can probably be attributed to a cross-reaction with DDC.

Vesicular monoamine transporter 2 (VMAT2) expression in hematopoietic cells and in patients with systemic mastocytosis

Uptake of monoamines into secretory granules is mediated by the vesicular monoamine transporters VMAT1 and VMAT2. In this study, we analyzed their expression in inflammatory and hematopoietic cells and in patients suffering from systemic mastocytosis (SM) and chronic myelogenous leukemia (CML). Normal human and monkey tissue specimens and tissues from patients suffering from SM and CML were analyzed by means of immunohistochemistry, radioactive in situ hybridization, real time RT-PCR, double fluorescence confocal laser scanning microscopy, and immunoelectron microscopy. In normal tissue specimens, VMAT2, but not VMAT1, was expressed in mast cells, megakaryocytes, thrombocytes, basophil granulocytes, and cutaneous Langerhans cells. Further hematopoietic and lymphoid cells showed no expression of VMATs. VMAT2 was expressed in all types of SM, as indicated by coexpression with the mast cell marker tryptase. In CML, VMAT2 expression was retained in neoplastic megakaryocytes and basophil granulocytes. In conclusion, the identification of VMAT2 in mast cells, megakaryocytes, thrombocytes, basophil granulocytes, and cutaneous Langerhans cells provides evidence that these cells possess molecular mechanisms for monoamine storage and handling. VMAT2 identifies normal and neoplastic mast cells, megakaryocytes, and basophil granulocytes and may therefore become a valuable tool for the diagnosis of mastocytosis and malignant systemic diseases involving megakaryocytes and basophil granulocytes.

The vesicular amine transporter family (SLC18): amine/proton antiporters required for vesicular accumulation and regulated exocytotic secretion of monoamines and acetylcholine

The vesicular amine transporters (VATs) are expressed as integral proteins of the lipid bilayer membrane of secretory vesicles in neuronal and endocrine cells. Their function is to allow the transport of acetylcholine (by the vesicular acetylcholine transporter VAChT; SLC18A3) and biogenic amines (by the vesicular monoamine transporters VMAT1 and VMAT2; SLC18A1 and SLC18A2) into secretory vesicles, which then discharge them into the extracellular space by exocytosis. Transport of positively charged amines by members of the SLC18 family in all cases utilizes an electrochemical gradient across the vesicular membrane established by proton pumping into the vesicle via a vacuolar ATPase; the amine is accumulated in the vesicle at the expense of the proton gradient, at a ratio of one translocated amine per two translocated protons. The members of the SLC18 family have become important histochemical markers for chemical coding in neuroendocrine tissues and cells. The structural basis of their remarkable ability to transport positively charged amines against a very large concentration gradient, as well as potential disease association with impaired transporter function and expression, are under intense investigation.

Measurement of hypocretin/orexin content in the mouse brain using an enzyme immunoassay: the effect of circadian time, age and genetic background

The hypocretins (1 and 2) have emerged as key regulators of sleep and wakefulness. We developed a high-throughput enzyme immunoassay (EIA) to measure total brain hypocretin levels from large numbers of mice. Hypocretin levels were not altered by circadian time or age. However, significant differences in one or both hypocretin peptides were observed between different mouse strains. We studied hypocretin levels in knockout and transgenic mouse models with obesity, circadian gene mutations or monoaminergic defects. Compared to controls, only histamine receptor knockouts had lower hypocretin levels. This was most pronounced in H1 receptor knockouts suggesting the existence of a positive feedback loop between hypocretin and histaminergic neurons.

Human megakaryocytes cultured in vitro accumulate serotonin but not meta-iodobenzylguanidine whereas platelets concentrate both

OBJECTIVE

Thrombocytopenia is the major toxicity of radio-iodinated meta-iodobenzylguanidine (MIBG) therapy in patients with recurrent neuroblastoma. MIBG is taken up in platelets via the serotonin transporter. Given the delayed appearance and long duration of the thrombocytopenia, it seems likely that the precursor megakaryocytes are the primary targets of [131I]MIBG radiotoxicity.

MATERIALS AND METHODS

We investigated MIBG and serotonin uptake in cultured human megakaryocytes grown in vitro from CD34(+) cells obtained from bone marrow.

RESULTS

With radio-iodinated MIBG, cell-associated radioactivity was negligible, even after prolonged incubations for up to 16 hours. In contrast, after 4 or 16 hours with 10(-8) M [3H]serotonin, 6% or 14% of the added substrate was accumulated in the megakaryocytes. This uptake approached saturation above 10(-7) M and was reduced greater than 90% by coincubation by imipramine. This indicates specific uptake, which was confirmed by fluvoxamine and citalopram. The serotonin reuptake inhibitors fluvoxamine (0.3 nM) and citalopram (1 nM) effectively reduced serotonin uptake to 44% +/- 3% and 30% +/- 9% of the controls, respectively.

CONCLUSIONS

Megakaryocytes efficiently retain serotonin in storage granules, as concluded from the consistent reductive effect of tetrabenazine on uptake, retention, and localization (micro-autoradiographic) of serotonin. Thus, serotonin, but not MIBG, is taken up by cultured megakaryocytes.

Repetitive administration of tetrabenazine induces irreversible changes in locomotion and morphology of the substantia nigra in rats

We investigated the effects of a 7 day repetitive administration of tetrabenazine (TBZ), which depletes monoamines, on both locomotor behavior and histomorphometrial findings of substantia nigra in rats. These results were compared with the effects of a single dose of TBZ, which is a common paradigm in animal models of depression. A single dose of TBZ causes reversible decrease of voluntary movement and no histological changes. In contrast, as for repetitive administration, this experiment demonstrated irreversible and significant decrease in spontaneous locomotion, as well as histological changes in the neurons of the substantia nigra pars compacta. These results have led us to propose that prolonged TBZ administration could provide a novel and useful model for the behavioral characteristics and anatomical pathology of Parkinson's disease as one of the oxidative stress models induced by abnormal dopamine metabolism.

Gastrin induces expression and promoter activity of the vesicular monoamine transporter subtype 2

Gastric enterochromaffin-like cells produce histamine in response to the antral hormone gastrin and accumulate the biogenic amine in secretory organelles via vesicular monoamine transporter subtype 2. The putative effects of gastrin on vesicular monoamine transporter subtype 2 expression and promoter activity are poorly understood. In the present study we used highly enriched rat enterochromaffin-like cells (purity, >90%) and rat pheochromocytoma cells stably transfected with a gastrin/cholecystokinin B receptor to investigate the expression and transcriptional regulation of vesicular monoamine transporter subtype 2. Stimulation of vesicular monoamine transporter subtype 2 mRNA and protein expression was observed in isolated enterochromaffin-like cells after 3- to 7-h incubation with gastrin (10(-7) M), forskolin (10(-5) M), or ionomycin (10(-5) M). Deletion analysis of the rat vesicular monoamine transporter subtype 2 promoter defined the minimal promoter sequence necessary for full basal activity as a -121 bp segment upstream of exon 1 containing two Sp1 sites (-97 to -88 bp and -68 to -59 bp) and a cAMP-responsive element (-44 to -35 bp). Gastrin (10(-7) M) stimulated extracellular signal related kinase1/2 phosphorylation, activated Sp1 and cAMP-responsive element-binding protein, and further induced activity of the complete rat vesicular monoamine transporter subtype 2 promoter (-800 bp) in gastrin/cholecystokinin B receptor cells. The -121-bp fragment was able to confer full gastrin responsiveness, and site-directed mutagenesis of the Sp1 and cAMP-responsive element motifs demonstrated their crucial importance for basal and inducible activities. Comparison of promoter activity of histidine decarboxylase, chromogranin A, or vesicular monoamine transporter subtype 2 in transfected cell lines revealed significant differences in basal and gastrin-stimulated activities. Our current study provides the first evidence that gastrin directly stimulates the expression and promoter activity of vesicular monoamine transporter subtype 2. Sp1 and cAMP-responsive element-binding protein recognition motifs located within 121 bp upstream of exon 1 appear to be indispensable for full basal and inducible promoter activities. Diverging effects of gastrin on histidine decarboxylase, chromogranin A, and vesicular monoamine transporter subtype 2 promoter may account for the coordinated synthesis and storage of histamine in this neuroendocrine cell type.

Identified neurons in C. elegans coexpress vesicular transporters for acetylcholine and monoamines

We have identified four neurons (VC4, VC5, HSNL, HSNR) in Caenorhabditis elegans adult hermaphrodites that express both the vesicular acetylcholine transporter and the vesicular monoamine transporter. All four of these cells are motor neurons that innervate the egg-laying muscles of the vulva. In addition, they all express choline acetyltransferase, the synthetic enzyme for acetylcholine. The distributions of the vesicular acetylcholine transporter and the vesicular monoamine transporter are not identical within the individual cells. In mutants deficient for either of these transporters, there is no apparent compensatory change in the expression of the remaining transporter. This is the first report of neurons that express two different vesicular neurotransmitter transporters in vivo.

Chemical neuroanatomy of the vesicular amine transporters

Acetylcholine, catecholamines, serotonin, and histamine are classical neurotransmitters. These small molecules also play important roles in the endocrine and immune/inflammatory systems. Serotonin secreted from enterochromaffin cells of the gut epithelium regulates gut motility; histamine secreted from basophils and mast cells is a major regulator of vascular permeability and skin inflammatory responses; epinephrine is a classical hormone released from the adrenal medulla. Each of these molecules is released from neural, endocrine, or immune/inflammatory cells only in response to specific physiological stimuli. Regulated secretion is possible because amines are stored in secretory vesicles and released via a stimulus-dependent exocytotic event. Amine storage-at concentrations orders of magnitude higher than in the cytoplasm-is accomplished in turn by specific secretory vesicle transporters that recognize the amines and move them from the cytosol into the vesicle. Immunohistochemical visualization of specific vesicular amine transporters (VATs) in neuronal, endocrine, and inflammatory cells provides important new information about how amine-handling cell phenotypes arise during development and how vesicular transport is regulated during homeostatic response events. Comparison of the chemical neuroanatomy of VATs and amine biosynthetic enzymes has also revealed cell groups that express vesicular transporters but not enzymes for monoamine synthesis, and vice versa: their function and regulation is a new topic of investigation in mammalian neurobiology. The chemical neuroanatomy of the vesicular amine transporters is reviewed here. These and similar data emerging from the study of the localization of the recently characterized vesicular inhibitory and excitatory amino acid transporters will contribute to understanding chemically coded synaptic circuitry in the brain, and amine-handling neuroendocrine and immune/inflammatory cell regulation.

Differential quantal release of histamine and 5-hydroxytryptamine from mast cells of vesicular monoamine transporter 2 knockout mice

The recent availability of mice lacking the neuronal form of the vesicular monoamine transporter 2 (VMAT2) affords the opportunity to study its roles in storage and release. Carbon fiber microelectrodes were used to measure individual secretory events of histamine and 5-hydroxytryptamine (5-HT) from VMAT2-expressing mast cells as a model system for quantal release. VMAT2 is indispensable for monoamine storage because mast cells from homozygous (VMAT2(-/-)) mice, while undergoing granule-cell fusion, do not release monoamines. Cells from heterozygous animals (VMAT2(+/-)) secrete lower amounts of monoamine per granule than cells from wild-type controls. Investigation of corelease of histamine and 5-HT from granules in VMAT2(+/-) cells revealed 5-HT quantal size was reduced more than that of histamine. Thus, although vesicular transport is the limiting factor determining quantal size of 5-HT and histamine release, intragranular association with the heparin matrix also plays a significant role.

Histamine depletion does not affect pancreastatin secretion from isolated rat stomach ECL cells

ECL cells co-secrete histamine and pancreastatin, a chromogranin A-derived peptide, in response to gastrin. The aim of the study was to explore possible ways to deplete ECL cells of histamine without affecting pancreastatin and to examine how histamine depletion affects pancreastatin secretion. Isolated rat stomach ECL cells (80-85% purity), prepared by counter-flow elutriation, were cultured for 48 h in the presence of alpha-fluoromethylhistidine (histidine decarboxylase inhibitor), bafilomycin A(1) (inhibitor of vacuolar-type proton-translocating ATPase) or reserpine (inhibitor of vesicular monoamine transporter). At this stage, the cells were challenged with 10 nM (EC(100)) gastrin-17 for 30 min. Histamine and pancreastatin were determined by radioimmunoassay. Maximally effective concentrations of alpha-fluoromethylhistidine, bafilomycin A(1) and reserpine were found to lower ECL-cell histamine (by 60%, 78% and 80%, respectively) without affecting pancreastatin. Basal histamine secretion was reduced in a dose-dependent manner by all three drugs. Gastrin-evoked histamine secretion was reduced greatly by the three agents, while pancreastatin secretion was unaffected. The results show that histamine can be depleted not only by inhibiting its formation (alpha-fluoromethylhistidine), but also (and more effectively) by inhibiting histamine vesicular uptake, directly (reserpine) or indirectly (bafilomycin A(1)). The results also indicate that although histamine is co-stored with pancreastatin, it is not required for either storage or secretion of pancreastatin.

The cat-1 gene of Caenorhabditis elegans encodes a vesicular monoamine transporter required for specific monoamine-dependent behaviors

We have identified the Caenorhabditis elegans homolog of the mammalian vesicular monoamine transporters (VMATs); it is 47% identical to human VMAT1 and 49% identical to human VMAT2. C. elegans VMAT is associated with synaptic vesicles in approximately 25 neurons, including all of the cells reported to contain dopamine and serotonin, plus a few others. When C. elegans VMAT is expressed in mammalian cells, it has serotonin and dopamine transport activity; norepinephrine, tyramine, octopamine, and histamine also have high affinity for the transporter. The pharmacological profile of C. elegans VMAT is closer to mammalian VMAT2 than VMAT1. The C. elegans VMAT gene is cat-1; cat-1 knock-outs are totally deficient for VMAT immunostaining and for dopamine-mediated sensory behaviors, yet they are viable and grow relatively well. The cat-1 mutant phenotypes can be rescued by C. elegans VMAT constructs and also (at least partially) by human VMAT1 or VMAT2 transgenes. It therefore appears that the function of amine neurotransmitters can be completely dependent on their loading into synaptic vesicles.

Vesicular amine transporter expression and isoform selection in developing brain, peripheral nervous system and gut

The vesicular monoamine transporters VMAT1 and VMAT2 are essential components of monoaminergic neurons and endocrine cells whose expression in development may provide insight into lineage pathways for chemical coding in the diffuse neuroendocrine system. Thus, the brain is a compartment in which only monoaminergic neurons are generated, the gut epithelium generates only endocrine monoamine-containing cells, and the neural crest produces both autonomic monoaminergic neurons and endocrine/paracrine monoaminergic cells. Selection of either the VMAT1 or VMAT2 isoform was examined in these three compartments during development. In the central nervous system VMAT2, but not VMAT1, was expressed in neuroepithelial cells by embryonic day 12 (E12), and all major monoaminergic cell groups by E14. Thalamocortical and hypothalamic neurons that do not express VMAT2 in adulthood were transiently VMAT2-positive from E16 to postnatal day 6 (P6). EC cells of the gut expressed exclusively VMAT1 from E19 on, while histamine-containing enterochromaffin-like (ECL) cells of the stomach expressed only VMAT2 by E19 and throughout postnatal development. VMAT2 and the vesicular acetylcholine transporter VAChT were co-expressed in early development of the primary sympathetic chain as well as in the cranial parasympathetic ganglia. VAChT was progressively restricted to a small population of VMAT2-negative post-ganglionic neurons in the adult sympathetic chain, while VMAT2 expression persisted in sympathetic principal ganglion and SIF cells but was eventually extinguished in cranial parasympathetic ganglia. VMAT1 was co-expressed with VAChT and VMAT2 mRNA in the primary sympathetic chain on E12, but progressively restricted to small intensely fluorescent (SIF) and chromaffin cells thereafter. Thus, expression of the vesicular amine transporters appropriate for chemical coding of brain neurons and gut endocrine cells are pre-determined developmentally. In contrast, the neural crest-derived sympathoadrenal and neural crest-derived parasympathetic cell groups examined here initially co-express two or more vesicular amine transporters, followed by extinction of the inappropriate transporter(s) later in development. Some neural crest-derived neuroendocrine cell populations continue to express both isoforms of VMAT even in adulthood. Lineage distinctions in ontogeny of vesicular amine transporter expression in brain, gut and autonomic nervous system make it likely that the same genes are regulated differently in the autonomic nervous system compared to brain and gut.

Vesicular neurotransmitter transporters. Potential sites for the regulation of synaptic function

Neurotransmission depends on the regulated release of chemical transmitter molecules. This requires the packaging of these substances into the specialized secretory vesicles of neurons and neuroendocrine cells, a process mediated by specific vesicular transporters. The family of genes encoding the vesicular transporters for biogenic amines and acetylcholine have recently been cloned. Direct comparison of their transport characteristics and pharmacology provides information about vesicular transport bioenergetics, substrate feature recognition by each transporter, and the role of vesicular amine storage in the mechanism of action of psychopharmacologic and neurotoxic agents. Regulation of vesicular transport activity may affect levels of neurotransmitter available for neurosecretion and be an important site for the regulation of synaptic function. Gene knockout studies have determined vesicular transport function is critical for survival and have enabled further evaluation of the role of vesicular neurotransmitter transporters in behavior and neurotoxicity. Molecular analysis is beginning to reveal the sites involved in vesicular transporter function and the sites that determine substrate specificity. In addition, the molecular basis for the selective targeting of these transporters to specific vesicle populations and the biogenesis of monoaminergic and cholinergic synaptic vesicles are areas of research that are currently being explored. This information provides new insights into the pharmacology and physiology of biogenic amine and acetylcholine vesicular storage in cardiovascular, endocrine, and central nervous system function and has important implications for neurodegenerative disease.

Distinct pharmacological properties and distribution in neurons and endocrine cells of two isoforms of the human vesicular monoamine transporter

A second isoform of the human vesicular monoamine transporter (hVMAT) has been cloned from a pheochromocytoma cDNA library. The contribution of the two transporter isoforms to monoamine storage in human neuroendocrine tissues was examined with isoform-specific polyclonal antibodies against hVMAT1 and hVMAT2. Central, peripheral, and enteric neurons express only VMAT2. VMAT1 is expressed exclusively in neuroendocrine, including chromaffin and enterochromaffin, cells. VMAT1 and VMAT2 are coexpressed in all chromaffin cells of the adrenal medulla. VMAT2 alone is expressed in histamine-storing enterochromaffin-like cells of the oxyntic mucosa of the stomach. The transport characteristics and pharmacology of each VMAT isoform have been directly compared after expression in digitonin-permeabilized fibroblastic (CV-1) cells, providing information about substrate feature recognition by each transporter and the role of vesicular monoamine storage in the mechanism of action of psychopharmacologic and neurotoxic agents in human. Serotonin has a similar affinity for both transporters. Catecholamines exhibit a 3-fold higher affinity, and histamine exhibits a 30-fold higher affinity, for VMAT2. Reserpine and ketanserin are slightly more potent inhibitors of VMAT2-mediated transport than of VMAT1-mediated transport, whereas tetrabenazine binds to and inhibits only VMAT2. N-methyl-4-phenylpyridinium, phenylethylamine, amphetamine, and methylenedioxymethamphetamine are all more potent inhibitors of VMAT2 than of VMAT1, whereas fenfluramine is a more potent inhibitor of VMAT1-mediated monamine transport than of VMAT2-mediated monoamine transport. The unique distributions of hVMAT1 and hVMAT2 provide new markers for multiple neuroendocrine lineages, and examination of their transport properties provides mechanistic insights into the pharmacology and physiology of amine storage in cardiovascular, endocrine, and central nervous system function.