Tetrahydrobiopterin turnover in cultured rat sympathetic neurons: developmental profile, pharmacologic sensitivity, and relationship to norepinephrine synthesis

Tetrahydrobiopterin activates brown adipose tissue and regulates systemic energy metabolism

Brown adipose tissue (BAT) is a central organ that acts to increase energy expenditure; its regulatory factors could be clinically useful in the treatment of obesity. Tetrahydrobiopterin (BH4) is an essential cofactor of tyrosine hydroxylase and nitric oxide synthase (NOS). Although BH4 regulates the known regulatory factors of BAT, such as noradrenaline (NA) and NO, participation of BH4 in BAT function remains unclear. In the present study, we investigate the role of BH4 in the regulation of BAT. Hph-1 mice, a mouse model of BH4 deficiency, exhibit obesity, adiposity, glucose intolerance, insulin resistance, and impaired BAT function. Impaired BAT function was ameliorated together with systemic metabolic disturbances by BAT transplantation from BH4-sufficient mice (control mice) into BH4-deficient mice, strongly suggesting that BH4-induced BAT has a critical role in the regulation of systemic energy metabolism. Both NA derived from the sympathetic nerve and NO derived from endothelial NOS in the blood vessels participate in the regulation of BH4. In addition, a direct effect of BH4 in the stimulation of brown adipocytes via NO is implicated. Taken together, BH4 activates BAT and regulates systemic energy metabolism; this suggests an approach for metabolic disorders, such as obesity and diabetes.

The neurobiology of tetrahydrobiopterin biosynthesis: a model for regulation of GTP cyclohydrolase I gene transcription within nigrostriatal dopamine neurons

Within the brain, the reduced pteridine cofactor 6R-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4) is absolutely required for the synthesis of the monoamine (MA) neurotransmitters dopamine (DA), norepinephrine, epinephrine (E), and serotonin (5-HT), the novel gaseous neurotransmitter nitric oxide and the production of yet to be identified 1-O-alkylglycerol-derived lipids. GTP cyclohydrolase I (GTPCH) catalyzes the first and limiting step in the BH4 biosynthetic pathway, which is now thought to involve up to eight different proteins supporting six alternate de novo and two alternate salvage pathways. Gene expression analysis across different regions of the human brain shows the abundance of transcripts coding for all eight of these proteins to be highly correlated with each other and to be enriched within human MA neurons. The potential for multiple routes for BH4 synthesis therefore exists within the human brain. GTPCH expression is particularly heterogeneous across different populations of human and rodent MA-containing neurons, with low expression levels and therefore BH4 being a characteristic of nigrostriatal DA (NSDA) neurons. Basic knowledge of how GCH1 gene transcription is controlled within NSDA neurons may explain the distinctive susceptibility of these neurons to human genetic mutations that result in BH4 deficiency. A model for cyclic adenosine monophosphate-dependent GCH1 transcription is described that involves a unique combination of DNA regulatory sequences and transcription factors. This model proposes that low levels of GCH1 transcription within NSDA neurons are driven by their distinctive physiology, suggesting that pharmacological manipulation of GCH1 gene transcription can be used to modify BH4 levels and therefore DA synthesis in the basal ganglia.

Synthesis and recycling of tetrahydrobiopterin in endothelial function and vascular disease

Nitric oxide, generated by the nitric oxide synthase (NOS) enzymes, plays pivotal roles in cardiovascular homeostasis and in the pathogenesis of cardiovascular disease. The NOS cofactor, tetrahydrobiopterin (BH4), is an important regulator of NOS function, since BH4 is required to maintain enzymatic coupling of L-arginine oxidation, to produce NO. Loss or oxidation of BH4 to 7,8-dihydrobiopterin (BH2) is associated with NOS uncoupling, resulting in the production of superoxide rather than NO. In addition to key roles in folate metabolism, dihydrofolate reductase (DHFR) can 'recycle' BH2, and thus regenerate BH4. It is therefore likely that net BH4 cellular bioavailability reflects the balance between de novo BH4 synthesis, loss of BH4 by oxidation to BH2, and the regeneration of BH4 by DHFR. Recent studies have implicated BH4 recycling in the direct regulation of eNOS uncoupling, showing that inhibition of BH4 recycling using DHFR-specific siRNA and methotrexate treatment leads to eNOS uncoupling in endothelial cells and the hph-1 mouse model of BH4 deficiency, even in the absence of oxidative stress. These studies indicate that not only BH4 level, but the recycling pathways regulating BH4 bioavailability represent potential therapeutic targets and will be discussed in this review.

Membrane transport of sepiapterin and dihydrobiopterin by equilibrative nucleoside transporters: a plausible gateway for the salvage pathway of tetrahydrobiopterin biosynthesis

Tetrahydrobiopterin (BH(4)) is synthesized de novo in particular cells, but in the case of a systemic or local BH(4) deficiency, BH(4) supplementation therapy is applied. BH(4)-responsive PKU has also been effectively treated with BH(4) supplementation. However, the rapid clearance of the supplemented BH(4) has prevented the therapy from being widely accepted. Deposition of BH(4) after supplementation involves oxidation of BH(4) to dihydrobiopterin (BH(2)) and subsequent conversion to BH(4) by the salvage pathway. This pathway is known to be almost ubiquitous in the body. However, the mechanism for the redistribution and exclusion of BH(4) across the plasma membrane remains unclear. The aim of this work was to search for the key transporter of the uptake precursor of the salvage pathway. Based on the observed sensitivity of pterin transport to nitrobenzylthioinosine (NBMPR), we examined the ability of ENT1 and ENT2, representative equilibrative nucleoside transporters, to transport sepiapterin (SP), BH(2) or BH(4) using HeLa cell and Xenopus oocyte expression systems. hENT2 was capable of transporting the pterins with an efficiency of SP>BH(2)>BH(4). hENT1 could also transport the pterins but less efficiently. Non-transfected HeLa cells and rat aortic endothelial cells were able to incorporate the pterins and accumulate BH(4) via uptake that is likely mediated by ENT2 (SP>BH(2)>BH(4)). When exogenous BH(2) was given to mice, it was efficiently converted to BH(4) and its tissue deposition was similar to that of sepiapterin as reported (Sawabe et al., 2004). BH(4) deposition after BH(2) administration was influenced by prior treatment with NBMPR, suggesting that the distribution of the administered BH(2) was largely mediated by ENT2, although urinary excretion appeared to be managed by other mechanisms. The molecular basis of the transport of SP, BH(2), and BH(4) across the plasma membrane has now been described for the first time: ENT2 is a transporter of these pterins and is a plausible gateway to the salvage pathway of BH(4) biosynthesis, at least under conditions of exogenous pterin supplementation. The significance of the gateway was discussed in terms of BH(2) uptake for BH(4) accumulation and the release for modifying the intracellular BH(2)/BH(4) ratio.

Cellular uptake of sepiapterin and push-pull accumulation of tetrahydrobiopterin

Cellular uptake of sepiapterin resulted in an efficient accumulation of tetrahydrobiopterin. Tetrahydrobiopterin is much less permeable across the cell membrane than sepiapterin or dihydrobiopterin, the precursors of the tetrahydrobiopterin-salvage pathway. The uptake of sepiapterin by the cell was examined under metabolic arrest with N-acetylserotonin, an inhibitor of sepiapterin reductase. The release profile of previously accumulated sepiapterin was also analyzed. Two routes were clearly distinguishable, namely rapid and slow. Both were apparently bi-directional and equilibrating in type. Each route was connected to non-mixable pools somehow separated in the cell. The rapid process was too fast to analyze by the current methods of cell handling. The slower process was associated with conversion of sepiapterin to tetrahydrobiopterin in the absence of N-acetylserotonin, suggesting that this route opens into the cytosolic compartment where use of the salvage pathway was strongly driven by sepiapterin reductase and dihydrofolate reductase with a supply of NADPH which favors tetrahydrobiopterin accumulation. Consequently, sepiapterin was enforcedly taken up by the cell where it accumulated tetrahydrobiopterin in the cytosol in continuous manner.

Mechanisms for the role of tetrahydrobiopterin in endothelial function and vascular disease

NO produced by eNOS (endothelial nitric oxide synthase) is a key mediator of vascular homoeostasis. NO bioavailability is reduced early in vascular disease states, such as hypercholesterolaemia, diabetes and hypertension, and throughout the progression of atherosclerosis. This is a result of both reduced NO synthesis and increased NO consumption by reactive oxygen species. eNOS enzymatic activity appears to be determined by the availability of its cofactor BH4 (tetrahydrobiopterin). When BH4 levels are adequate, eNOS produces NO; when BH4 levels are limiting, eNOS becomes enzymatically uncoupled and generates superoxide, contributing to vascular oxidative stress and endothelial dysfunction. BH4 bioavailability is determined by a balance of enzymatic de novo synthesis and recycling, versus oxidative degradation in dysfunctional endothelium. Augmenting vascular BH4 levels by pharmacological supplementation, by enhancing the rate of de novo biosynthesis or by measures to reduce BH4 oxidation have been shown in experimental studies to enhance NO bioavailability. Thus BH4 represents a potential therapeutic target for preserving eNOS function in vascular disease.

A typical N-terminal extensions confer novel regulatory properties on GTP cyclohydrolase isoforms in Drosophila melanogaster

The cofactor tetrahydrobiopterin plays critical roles in the modulation of the signaling molecules dopamine, serotonin, and nitric oxide. Deficits in cofactor synthesis have been associated with several human hereditary diseases. Responsibility for the regulation of cofactor pools resides with the first enzyme in its biosynthetic pathway, GTP cyclohydrolase I. Because organisms must be able to rapidly respond to environmental and developmental cues to adjust output of these signaling molecules, complex regulatory mechanisms are vital for signal modulation. Mammalian GTP cyclohydrolase is subject to end-product inhibition via an associated regulatory protein and to positive regulation via phosphorylation, although target residues are unknown. GTP cyclohydrolase is composed of a highly conserved homodecameric catalytic core and non-conserved N-terminal domains proposed to be regulatory sites. We demonstrate for the first time in any organism that the N-terminal arms of the protein serve regulatory functions. We identify two different modes of regulation of the enzyme mediated through the N-terminal domains. The first is end-product feedback inhibition, catalytically similar to that of the mammalian enzyme, except that feedback inhibition by the cofactor requires sequences in the N-terminal arms rather than a separate regulatory protein. The second is a novel inhibitory interaction between the N-terminal arms and the active sites, which can be alleviated through the phosphorylation of serine residues within the N termini. Both mechanisms allow for acute and highly responsive regulation of cofactor production as required by downstream signaling pathways.

Delivery of exogenous tetrahydrobiopterin (BH4) to cells of target organs: role of salvage pathway and uptake of its precursor in effective elevation of tissue BH4

Cells in target organs such as liver do not generally incorporate tetrahydrobiopterin (BH4) in its fully reduced form. Instead, they transiently take up BH4 from the extracellular fluid, instantaneously oxidize it and then expel virtually all of it. However, a small but stable accumulation of BH4 was observed after BH4 administration to the cell cultures. This accumulation was inhibited by methotrexate, an inhibitor of dihydrofolate reductase, a phenomenon that was first suggested based on results of in vitro studies which used established cell lines such as RBL2H3 and PC12. These cells also take up dihydrobiopterin (BH2) and reduce it to enzymically active BH4. Their ability to accumulate usable BH4 upon BH4 administration was attributed to the incorporation of BH2, which in typical experiments was produced by the cells as well as by auto-oxidation of BH4. Most cells of the various cell lines so far examined behaved similarly in culture. Our in vivo work with individual mice demonstrated that administration of sepiapterin, BH2, and BH4 was comparably effective in raising BH4 levels in target organs. BH4 accumulation in various tissues after supplementation with BH4, BH2 or sepiapterin was also inhibited by methotrexate, as in the case of our cell culture system. It was concluded that the elevation in BH4 by supplementation was mainly through a "salvage pathway" that included BH2 as the key intermediate in the production of BH4 through the action of dihydrofolate reductase.

Nitric oxide production and regulation of neuronal NOS in tyrosine hydroxylase containing neurons

CAD cells are a murine CNS catecholaminergic (tyrosine hydroxylase-positive; TH+) neuronal cell line that undergoes morphological differentiation to resemble CNS catecholaminergic neurons upon serum deprivation. We show here that CAD cells also express neuronal nitric oxide synthase (nNOS) mRNA and protein and produce readily measurable levels of NO. Since both NO and catecholamines (L-DOPA; dopamine; norepinephrine) are redox active molecules, their production within the same cell may affect the cell's vulnerability to insult. Thus, we examined the regulation of NO production by CAD cells and the effect of NO on cell survival. NO is generated in a dose-dependent fashion by treatment with agents (ionomycin; A23817; KCl) known to increase calcium entry across the cell membrane. The NO level can be increased further by pretreatment with sepiapterin, a membrane permeable precursor for BH4 synthesis, suggesting that the BH4 levels or access required for nNOS activation is limited in CAD cells. Reducing mitochondrial Ca2+ uptake using ruthenium red (RuR) increased ionomycin-mediated NO production over ionomycin alone and indicates a critical role for mitochondria in nNOS regulation. Cell death was significantly increased by ionomycin treatment alone or in conjunction with reduced mitochondrial Ca2+ uptake. However, NO was not the primary mediator of cell death since NOS inhibitors rescued only less than 10% of the cells. These data suggest that endogenous NO production by nNOS is not a major factor in CAD cell death under these conditions.

Response of tyrosine hydroxylase and GTP cyclohydrolase I gene expression to estrogen in brain catecholaminergic regions varies with mode of administration

The effect of different dose, mode and duration of estradiol administration was examined in the different brain catecholaminergic areas in ovariectomized (OVX) female rats. We determined changes in mRNA levels of tyrosine hydroxylase (TH), rate-limiting enzyme in catecholamine (CA) biosynthesis of GTP cyclohydrolase I (GTPCH), rate-limiting enzyme in biosynthesis as well as of tetrahydrobiopterin (BH4), and concentration of BH4, which is an essential cofactor for TH, tryptophan hydroxylase and nitric oxide synthase. Short-term administration of estradiol benzoate (EB) by five injections of 15 or 40 microg/kg 12 h apart led to increase in TH and GTPCH mRNA levels in dopaminergic and noradrenergic cell bodies of the ventral tegmental area (VTA), substantia nigra (SN), locus coeruleus (LC) and the nucleus of solitary tract (NTS) depending on dose of administration. Estrogen-elicited alterations in BH4 concentrations were mostly correlated with changes in GTPCH mRNA levels, except in SN. Long-term administration of estradiol by injections (EB: 25 microg/kg, 16 injections 26 h apart; 50 microg/kg, 16 injections 48 h apart) or pellets (0.1 mg 17 beta-estradiol, 14 days) were not very effective in modulating mRNA levels for both genes in most locations except the NTS. Long-term injections of EB elevated GTPCH mRNA levels throughout the NTS and in microvessels. Administration of estradiol by pellets led to decline of TH mRNA in rostral-medial and elevation in caudal parts of the NTS. Thus, estradiol has a complex and differential effect on TH and GTPCH gene expression in a tissue specific manner and depends on the mode of administration.

Potential mechanisms responsible for chlorotriazine-induced alterations in catecholamines in pheochromocytoma (PC12) cells

Chlorotriazines interact with undifferentiated PC12 cells in vitro to modulate catecholamine synthesis and release, but the mechanism(s) responsible for this effect had not been determined. In this study we evaluated the effect of atrazine, simazine and cyanazine on the protein expression of the enzymes responsible for the synthesis of dopamine [tyrosine hydroxylase (TH)] and norepinephrine [dopamine-beta-hydroxylase (DbetaH)]. We also examined the possible intracellular pathway associated with chlorotriazine-induced changes in catecholamine synthesis and release. Incubating PC12 cells in the presence of 100 microM atrazine and simazine decreased intracellular dopamine (DA), norepinephrine (NE) concentration and NE release, and the protein expression of TH (approximately 20%) and DbetaH (approximately 50 and 25%, respectively) after 12-24 h exposure. In contrast, cyanazine (100 microM) stimulated intracellular and released NE concentration, and the protein expression of TH (approximately 20%) and DbetaH (approximately 225%) after 12-36 h exposure. Simultaneous exposure to the essential TH co-factors (iron and tetrahydrobiopterine) was ineffective in altering cellular DA. Agents known to enhance TH and DbetaH transcription, phosphorylation or activity (e.g., 8-bromo cAMP, forskolin or dexamethasone) reversed the inhibitory effects of atrazine and simazine on the NE. Again, in contrast to atrazine and simazine, cyanazine attenuated catecholamine-depleting effect of alpha-Methyl-p-tyrosine (alphaMpT) on NE. Both DA and NE synthesis can be altered by the chlorotriazines and suggest these occur via an alteration of the synthetic enzymes TH and DbetaH.

Developmental regulation of neurotransmitter phenotype through tetrahydrobiopterin

During development, sympathetic neurons innervating rodent sweat glands undergo a target-induced change in neurotransmitter phenotype from noradrenergic to cholinergic. Although the sweat gland innervation in the adult mouse is cholinergic and catecholamines are absent, these neurons continue to express tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine synthesis. The developmental suppression of noradrenergic function in these mouse sympathetic neurons is not well understood. We investigated whether the downregulation of the enzyme aromatic l-amino acid decarboxylase (AADC) or the TH cofactor tetrahydrobiopterin (BH4) could account for the loss of catecholamines in these neurons. AADC levels did not decrease during development, and adult cholinergic sympathetic neurons were strongly immunoreactive for AADC. In contrast, BH4 levels dropped significantly in murine sweat gland-containing footpads during the time period when the gland innervation was switching from making norepinephrine to acetylcholine. Immunoreactivity for the rate-limiting BH4 synthetic enzyme GTP cyclohydrolase (GCH) became undetectable in the sweat gland neurons during this phenotypic conversion, suggesting that sweat glands reduce BH4 levels by suppressing GCH expression during development. Furthermore, extracts from sweat gland-containing footpads suppressed BH4 in cultured mouse sympathetic neurons, and addition of the BH4 precursor sepiapterin rescued catecholamine production in neurons treated with footpad extracts. Together, these results suggest that the mouse sweat gland-derived cholinergic differentiation factor functionally suppresses the noradrenergic phenotype during development by inhibiting production of the TH cofactor, BH4. These data also indicate that GCH expression, which is often coordinately regulated with TH expression, can be controlled independently of TH during development.

Development of the pteridine pathway in the zebrafish, Danio rerio

In the zebrafish, the peripheral neurons and the pigment cells are derived from the neural crest and share the pteridine pathway, which leads either to the cofactor tetrahydrobiopterin or to xanthophore pigments. The components of the pteridine pattern were identified as tetrahydrobiopterin, sepiapterin, 7-oxobiopterin, isoxanthopterin, and 2,4,7-trioxopteridine. The expression of GTP cyclohydrolase I activity during the first 24-h postfertilization, followed by 6-pyruvoyl-5,6,7,8-tetrahydropterin synthase and sepiapterin reductase, suggest an early supply of tetrahydrobiopterin for neurotransmitter synthesis in the neurons and for tyrosine supply in the melanophores. At 48-h postfertilization, sepiapterin formation branches off the de novo pathway of tetrahydrobiopterin synthesis. Sepiapterin, via 7,8-dihydrobiopterin and biopterin, serves as a precursor for the formation of 7-oxobiopterin, which may be further catabolized to isoxanthopterin and 2,4,7-trioxopteridine. Neither 7, 8-dihydrobiopterin nor biopterin is a substrate for xanthine oxidoreductase. In contrast, both of these compounds are oxidized at C-7 by a xanthine oxidase variant form, which is inactivated by KCN, but is insensitive to allopurinol. The oxidase and the dehydrogenase form of xanthine oxidoreductase as well as the xanthine oxidase variant have specific developmental patterns. It follows that GTP cyclohydrolase I, the formation of sepiapterin, and the xanthine oxidoreductase family control the pteridine pathway in the zebrafish.

Regulation of pteridine-requiring enzymes by the cofactor tetrahydrobiopterin

Tetrahydrobiopterin (BH4) is synthesized from guanosine triphosphate (GTP) by GTP cyclohydrolase I (GCH), 6-pyruvoyltetrahydropterin synthase (PTS), and sepiapterin reductase (SPD). GCH is the rate-limiting enzyme. BH4 is a cofactor for three pteridine-requiring monooxygenases that hydroxylate aromatic L-amino acids, i.e., tyrosine hydroxylase (TH), tryptophan hydroxylase (TPH), and phenylalanine hydroxylase (PAH), as well as for nitric oxide synthase (NOS). The intracellular concentrations of BH4, which are mainly determined by GCH activity, may regulate the activity of TH (an enzyme-synthesizing catecholamines from tyrosine), TPH (an enzyme-synthesizing serotonin and melatonin from tryptophan), PAH (an enzyme required for complete degradation of phenylalanine to tyrosine, finally to CO2 + H2O), and also the activity of NOS (an enzyme forming NO from arginine), Dominantly inherited hereditary progressive dystonia (HPD), also termed DOPA-responsive dystonia (DRD) or Segawa's disease, is a dopamine deficiency in the nigrostriatal dopamine neurons, and is caused by mutations of one allele of the GCH gene. GCH activity and BH4 concentrations in HPD/DRD are estimated to be 2-20% of the normal value. By contrast, recessively inherited GCH deficiency is caused by mutations of both alleles of the GCH gene, and the GCH activity and BH4 concentrations are undetectable. The phenotypes of recessive GCH deficiency are severe and complex, such as hyperphenylalaninemia, muscle hypotonia, epilepsy, and fever episode, and may be caused by deficiencies of various neurotransmitters, including dopamine, norepinephrine, serotonin, and NO. The biosynthesis of dopamine, norepinephrine, epinephrine, serotonin, melatonin, and probably NO by individual pteridine-requiring enzymes may be differentially regulated by the intracellular concentration of BH4, which is mainly determined by GCH activity. Dopamine biosynthesis in different groups of dopamine neurons may be differentially regulated by TH activity, depending on intracellular BH4 concentrations and GCH activity. The nigrostriatal dopamine neurons may be most susceptible to a partial decrease in BH4, causing dopamine deficiency in the striatum and the HPD/DRD phenotype.

Double transduction with GTP cyclohydrolase I and tyrosine hydroxylase is necessary for spontaneous synthesis of L-DOPA by primary fibroblasts

Gene transfer of tyrosine hydroxylase (TH) in animal models of Parkinson's disease (PD), using either genetically modified cells or recombinant virus vectors, has produced partial restoration of behavioral and biochemical deficits. The limited success of this approach may be related to the availability of the cofactor, tetrahydrobiopterin (BH4), because neither the dopamine-depleted striatum nor the cells used for gene transfer possess a sufficient amount of BH4 to support TH activity. To determine the role of BH4 in gene therapy, fibroblast cells transduced with the gene for TH were additionally modified with the gene for GTP cyclohydrolase l; an enzyme critical for BH4 synthesis. In contrast to cells transduced with only TH, doubly transduced fibroblasts spontaneously produced both BH4 and 3, 4-dihydroxy-L-phenylalanine. To examine further the importance of GTP cyclohydrolase I in gene therapy for PD, in vivo micro-dialysis was used to assess the biochemical changes in the dopamine-denervated striatum containing grafts of genetically modified fibroblasts. Only denervated striata grafted with fibro-blasts possessing both TH and GTP cyclohydrolase I genes displayed biochemical restoration. However, no significant differences from controls were observed in apomorphine-induced rotation. This is partly attributable to a limited duration of gene expression in vivo. These differences between fibroblasts transduced with TH alone and those additionally modified with the GTP cyclohydrolase I gene indicate that BH4 is critical for biochemical restoration in a rat model of PD and that GTP cyclohydrolase I is sufficient for production of BH4.

Tetrahydrobiopterin biosynthesis in the rat brain: heterogeneity of GTP cyclohydrolase I mRNA expression in monoamine-containing neurons

GTP cyclohydrolase I is the first and rate-limiting enzyme in the biosynthesis of tetrahydrobiopterin. A quantitative in situ hybridization technique was used to study the expression of GTP cyclohydrolase I mRNA in the rat brain at the cellular level. Coronal sections between the diencephalon and myelencephalon were exposed to a 35S-labelled antisense GTP cyclohydrolase I cRNA probe. Sections serial to these were hybridized with a 35S-labelled antisense cRNA probe complementary to tyrosine hydroxylase mRNA. Tyrosine hydroxylase and GTP cyclohydrolase I mRNAs were found to colocalize within catecholamine neurons located throughout the brain. The overall distribution of neurons expressing GTP cyclohydrolase I mRNA was observed to correspond exactly to the known distribution of the dopamine, norepinephrine/epinephrine and serotonin-containing cell groups. Overall, a 30-fold range of GTP cyclohydrolase I mRNA expression was observed, with the transcript being significantly more abundant in serotonin than in dopamine or norepinephrine/epinephrine neurons. Comparisons across serotonin cell groups indicated that neurons of the median raphe nucleus, caudal linear nucleus raphe (B8) and the dorsal raphe (B6/B7) expressed the highest levels of GTP cyclohydrolase I mRNA. Comparisons across dopamine cell groups indicated that the transcript was more abundant in neurons of the ventral tegmental area (A10) than in neurons of the substantia nigra pars compacta (A9) and that both A9 and A10 dopamine neurons exhibited higher levels of expression than the DA neurons of the hypothalamus (A11-A14). Norepinephrine neurons of the locus coeruleus (A6) and subcoeruleus (A6v) exhibited significantly higher levels of GTP cyclohydrolase I mRNA than did neurons in other norepinephrine (A1 and A2) or epinephrine (C1 and C2) cell groups. GTP cyclohydrolase I mRNA could not be detected unequivocally in neurons known to contain nitric oxide synthase. Heterogeneity in the level of expression of GTP cyclohydrolase I mRNA by monoamine-containing neurons may play an important role in determining steady state levels of tetrahydrobiopterin and, ultimately, the regulation of monoamine biosynthesis.

Selective pharmacological inhibition of distinct nitric oxide synthase isoforms

Nitric oxide (NO) is produced in physiological and pathophysiological conditions by three distinct isoforms of NO synthase (NOS): endothelial NOS (ecNOS), inducible NOS (iNOS), and brain NOS (bNOS). Selective inhibition of iNOS may be beneficial in various forms of shock and inflammation, whereas inhibition of bNOS may protect against neuroinjury. This article surveys the enzymatic mechanism of NO production, lists the strategies and pharmacological tools for selective inhibition of distinct NOS isoforms, and considers the side-effects of the various approaches. Selective inhibition of NOS isoforms is achieved by: (a) targeting the differential co-factor (calmodulin or tetrahydrobiopterin) requirement of various NOS isoforms, and NOS; (b) targeting the differential substrate requirements of cells expressing various isoforms of NOS (L-arginine uptake blockers or arginase); (c) the use of pharmacological agents that are selectively taken up by cells expressing various isoforms of NOS (7-nitroindazole); or (d) developing pharmacological NOS inhibitors with isoform specificity. The amino acid-based NOS inhibitor, NG-nitro-L-arginine, shows a preference for ecNOS and bNOS over iNOS, whereas L-N6-(1-iminoethyl)lysine is selective for iNOS over bNOS. Certain non-amino acid-based small molecules, such as aminoguanidine and certain S-alkylated isothioureas, also express selectivity towards iNOS and have anti-inflammatory and anti-shock properties. 7-nitroindazole, a bNOS-selective inhibitor, protects in central nervous system injury. Clearly, there are a number of distinct approaches that are worthy of further research efforts in order to achieve even more selective targeting of various NOS isoforms

Regulation of GTP cyclohydrolase I gene expression and tetrahydrobiopterin content by nerve growth factor in cultures of superior cervical ganglia

Monolayer cultures of superior cervical ganglia free of support cells were maintained in the presence of 100 ng/ml 7S-NGF for 4 days. The concentration of NGF was then changed to between 50 and 400 ng/ml and cultures continued for an additional 7 days. Tetrahydrobiopterin (BH4) content, GTP cyclohydrolase (GTPCH) enzyme activity and mRNA levels were then determined. All three of these measures were found to be elevated between 2- to 4-fold by treatment with increasing concentrations of NGF. Tyrosine hydroxylase (TH) enzyme activity and mRNA levels were increased from 8 to 13-fold by these same treatments. These results indicate that the content of BH4 within sympathetic neurons can be regulated by NGF receptor-mediated changes in GTPCH gene expression. Moreover, concomitant increases in TH enzyme activity and BH4 content demonstrate a coordinated regulation by NGF of this enzyme and its essential cofactor.

Conditions required for the measurement of nitric oxide synthase activity in a myenteric plexus/smooth muscle preparation from the rat ileum

Nitric oxide synthase (NOS) activity was measured, by the conversion of arginine to citrulline, in a preparation from the rat ileum consisting of the myenteric plexus and smooth muscle layers. A variety of incubating media were used in order to establish the optimal conditions required for the assay. NOS activity was present in the soluble fraction and was Ca(2+)- and calmodulin-dependent, characteristic of neuronal NOS. Exogenous Ca2+ was required for activity to be detectable but NOS activity progressively decreased with Ca2+ concentrations above 1.25 mM. Activity varied with arginine concentration, reaching saturation at 6 microM, and required the addition of the co-substrate NADPH. Endogenous levels of co-factors in the crude soluble fraction were not sufficient to maintain NOS activity. Omission of flavin adenine dinucleotide and tetrahydrobiopterin from the incubation medium reduced activity by 90%, and both co-factors had to be present for maximal activity to occur. These results emphasize the need to control assay conditions when measuring NOS activity in crude preparations from peripheral tissue.

A comparison of the developing dopamine neuron phenotype in cultures of embryonic rat mesencephalon and hypothalamus

Development of the dopamine (DA) neuron phenotype was monitored in cultures of embryonic rat mesencephalon (MES) and hypothalamus (HYP) maintained for 1 to 21 days in vitro (DIV) in the absence of glial support cells. Cell counts following immunohistochemistry for tyrosine hydroxylase (TH) demonstrated that the number of DA neurons declined by 85% in MES cultures yet increased 5-fold in cultures of HYP, so that by 21 DIV equal numbers of DA neurons were present in these culture systems. After 21 DIV MES DA neurons exhibited a multipolar morphology, with numerous branching processes. HYP DA neurons were primarily fusiform in shape with fewer processes and process branch points. Double-label immunohistochemistry for TH and microtubule-associated protein 2 identified the majority of TH-positive processes in either culture system as dendrites. Individual MES but not HYP DA neurons were also found to generate axons. Western analysis showed that between 1 and 21 DIV the concentration of TH protein increased 2-fold in MES and 4-fold in HYP cultures. After 21 DIV the concentration of TH protein in MES cultures was twice that found in cultures of HYP. In the period between 1 and 21 DIV levels of tetrahydrobiopterin (BH4) increased by 6-fold in MES and 20-fold in HYP cultures. After 21 DIV BH4 content was 3-fold higher in HYP than in MES cultures. The abundance of the mRNA encoding for GTP cyclohydrolase I, the rate-limiting enzyme in BH4 biosynthesis, was similar in MES and HYP cultures despite this difference in BH4 levels. In contrast, TH mRNA was 4-fold more abundant in MES than in HYP cultures. Treatment of MES cultures with the DA neuron toxin 1-methyl-4-phenylpyridinium decreased DA cell numbers, TH protein content and BH4 levels, demonstrating that BH4 is localized primarily to DA neurons. Similar treatment of HYP cultures did not effect any of these parameters. Steady-state levels of DA and the rate of DA synthesis were both 3-fold higher in MES than in HYP cultures. A 95% decline in BH4 content produced by inhibiting BH4 biosynthesis resulted in 64% and 84% declines in the rate of MES and HYP DA synthesis, respectively. Overall, these observations indicate that, with the exception of the capacity to synthesize DA, DA neurons in MES and HYP cultures share few common properties.

Tetrahydrobiopterin cofactor biosynthesis: GTP cyclohydrolase I mRNA expression in rat brain and superior cervical ganglia

GTP cyclohydrolase I (GTPCH) is the rate-limiting enzyme in the biosynthesis of tetrahydrobiopterin, the reduced pteridine cofactor required for catecholamine (CA), indoleamine, and nitric oxide biosynthesis. We have used the reverse transcription-polymerase chain reaction technique, based on the published cDNA sequence for rat liver GTPCH, to clone a portion of the GTPCH transcript from rat adrenal gland mRNA and have used this clone for the analysis of GTPCH mRNA in brain and other tissues of the rat by northern blot, nuclease protection assay, and in situ hybridization. Two GTPCH mRNA transcripts of 1.2 and 3.8 kb in length were detected by northern blot, with the 1.2-kb form predominating in the liver and the 3.8-kb form in the pineal gland, adrenal gland, brainstem, and hypothalamic neurons maintained in culture. In situ hybridization studies localized GTPCH mRNA to CA-containing perikarya in the locus ceruleus, ventral tegmental area, and substantia nigra, pars compacta. Levels of GTPCH mRNA in central and peripheral catecholamine neurons determined by nuclease protection assay were increased twofold 24 h after a single injection of the CA-depleting drug reserpine; both the 1.2- and 3.8-kb transcripts were increased in the adrenal gland. Low levels of GTPCH mRNA were also detected by nuclease protection assay in the striatum, hippocampus, and cerebellum, brain regions that do not contain monoaminergic perikarya.

Substance P gene expression in sympathetic neurons is regulated by neuron/support cell interaction

In agreement with previous findings, the presence of support cells was found to increase the level of preprotachykinin (i.e. substance P-encoding) mRNA in cultures of sympathetic neurons. Treatment of neuron-only cultures, which did not express detectable levels of preprotachykinin mRNA, with conditioned medium from support cell-only cultures, also increased the level of preprotachykinin mRNA. This elevation in substance P gene expression was reflected in a 2-fold increase in the number of substance P-like immunoreactive neurons. In contrast, treatment of neuron-only cultures with conditioned medium from co-cultures of sympathetic neurons and support cells did not increase the level of preprotachykinin mRNA or the number of neurons containing substance P-like immunoreactivity. These observations suggest that while support cells release a soluble factor(s) capable of inducing substance P expression in sympathetic neurons, the production or action of this factor(s) is inhibited by the interaction between support cells and sympathetic neurons. Thus, by interacting with non-neuronal cells in their environment, sympathetic neurons appear to play an active role in determining which neurotransmitter phenotype they express.

Differential metabolism of tetrahydrobiopterin in monoamine neurons: a hypothesis based upon clinical and basic research

This chapter has attempted to describe and integrate some of the clinical and basic research that support our hypothesis that the metabolism of BH4 is normally heterogeneous across different populations of monoamine-containing neurons. Based upon this hypothesis, there may now be reason to support the idea that certain neuropsychiatric illnesses, which are though to be the result (at least in part) of altered monoamine metabolism, might find their roots in an abnormal metabolism of BH4 within specific monoaminergic cell groups. Such a specific dysfunction might not be apparent in the rest of the brain or peripheral nervous system, thereby being difficult to detect. Perhaps the application of molecular biological techniques to studies of BH4 metabolism in man will shed new light on these problems.