Tetrahydrobiopterin synthesis rate and turnover time in neuronal cultures from embryonic rat mesencephalon and hypothalamus

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.

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.

Tetrahydrobiopterin levels regulate endothelial cell proliferation

Vascular abnormalities, including altered angiogenesis, are major factors contributing to the morbidity and mortality of diabetes. We hypothesized that impaired angiogenesis in diabetes results from decreased tetrahydrobiopterin (BH4)-dependent synthesis of nitric oxide (NO) by endothelial cells (EC). To test this hypothesis, we utilized EC from spontaneously diabetic BB (BBd) and nondiabetes-prone BB (BBn) rats to investigate the link between BH4 and EC proliferation. There were significant decreases in the proliferation rate and expression of proliferating cell nuclear antigen in BBd versus BBn EC, with no evidence of apoptosis in either group. Sepiapterin (a precursor of BH4 via the salvage pathway) increased BH4 synthesis and enhanced proliferation of BBd EC. The stimulating effect of sepiapterin on EC proliferation was attenuated by NG-monomethyl-L-arginine, a NO synthase inhibitor. Reducing BH4 concentrations in BBn EC caused a decrease in proliferation, which was attenuated by a long-acting NO donor. Our results suggest that BH4 levels regulate proliferation of normal EC and that a BH4 deficiency impairs NO-dependent proliferation of BBd EC.

GTP cyclohydrolase I feedback regulatory protein is expressed in serotonin neurons and regulates tetrahydrobiopterin biosynthesis

Tetrahydrobiopterin, the coenzyme required for hydroxylation of phenylalanine, tyrosine, and tryptophan, regulates its own synthesis through feedback inhibition of GTP cyclohydrolase I (GTPCH) mediated by a regulatory subunit, the GTP cyclohydrolase feedback regulatory protein (GFRP). In the liver, L-phenylalanine specifically stimulates tetrahydrobiopterin synthesis by displacing tetrahydrobiopterin from the GTPCH-GFRP complex. To explore the role of this regulatory system in rat brain, we examined the localization of GFRP mRNA using double-label in situ hybridization. GFRP mRNA expression was abundant in serotonin neurons of the dorsal raphe nucleus but was undetectable in dopamine neurons of the midbrain or norepinephrine neurons of the locus coeruleus. Simultaneous nuclease protection assays for GFRP and GTPCH mRNAs showed that GFRP mRNA is most abundant within the brainstem and that the ratio of GFRP to GTPCH mRNA is much higher than in the ventral midbrain. Two species of GFRP mRNA differing by approximately 20 nucleotides in length were detected in brainstem but not in other tissues, with the longer, more abundant form being common to other brain regions. It is interesting that the pineal and adrenal glands did not contain detectable levels of GFRP mRNA, although GTPCH mRNA was abundant in both. Primary neuronal cultures were used to examine the role of GFRP-mediated regulation of GTPCH on tetrahydrobiopterin synthesis within brainstem serotonin neurons and midbrain dopamine neurons. L-Phenylalanine increased tetrahydrobiopterin levels in serotonin neurons to a maximum of twofold in a concentration-dependent manner, whereas D-phenylalanine and L-tryptophan were without effect. In contrast, tetrahydrobiopterin levels within cultured dopamine neurons were not altered by L-phenylalanine. The time course of this effect was very rapid, with a maximal response observed within 60 min. Inhibitors of tetrahydrobiopterin biosynthesis prevented the L-phenylalanine-induced increase in tetrahydrobiopterin levels. 7,8-Dihydroneopterin, a reduced pteridine capable of inhibiting GTPCH in a GFRP-dependent manner, decreased tetrahydrobiopterin levels in cultures of both serotonin and dopamine neurons. This inhibition was reversed by L-phenylalanine in serotonin but not in dopamine neurons. Our data suggest that GTPCH activity within serotonin neurons is under a tonic inhibitory tone mediated by GFRP and that tetrahydrobiopterin levels are maintained by the balance of intracellular concentrations of tetrahydrobiopterin and L-phenylalanine. In contrast, although tetrahydrobiopterin biosynthesis within dopamine neurons is also feedback-regulated, L-phenylalanine plays no role, and therefore tetrahydrobiopterin may have a direct effect on GTPCH activity.

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.

Blockade of tyrosine hydroxylase activity in the median eminence partially reverses the long day-induced inhibition of pulsatile LH secretion in the ewe

The photoperiod-induced stimulation of LH secretion is associated with a decrease in dopamine content, as well as in the activity of its rate limiting enzyme, tyrosine hydroxylase (TH), in the median eminence (ME) of the ewe. We therefore hypothesize that ME-TH activity can constitute a limiting factor of photoperiod-induced inhibition of LH pulsatile secretion. To test this hypothesis, we studied whether the inhibition of ME-TH activity can reverse the long day-induced inhibition of LH. Using microdialysis, a 3 mM solution of alpha methyl-p-tyrosine (alpha MPT; a competitive inhibitor of TH), was administered in the ME of ovariectomized ewes bearing a 0.5 cm oestradiol implant at the beginning of a LD-induced inhibition of LH secretion. The vehicle solution was infused for 4 h followed by a 3 mM alpha MPT solution infused for an additional 4 h. LH pulsatile secretory patterns within the same animal were compared between the control period and the alpha MPT period. alpha MPT infusion in the ME was associated with an increase in LH pulse frequency whereas it did not affect prolactin secretion. In conclusion, our results suggest that the inhibition of TH activity in the ME causes a stimulation of LH secretion in long-day inhibited ewes.

Tetrahydrobiopterin biosynthesis in C6 glioma cells: induction of GTP cyclohydrolase I gene expression by lipopolysaccharide and cytokine treatment

The possibility that 5,6,7,8-tetrahydrobiopterin (BH4) biosynthesis is stimulated in glial cells by treatment with lipopolysaccharide (LPS) and tumor necrosis factor (TNF-alpha) was examined in the astrocyte-derived C6 glioma cell line. Under basal culture conditions BH4 levels were found to be at the limit of detection. Concurrent treatment with 10 micrograms/ml LPS and 50 ng/ml TNF-alpha caused a time-dependent 13-fold increase in the levels of BH4. This treatment paradigm also induced nitric oxide synthase activity, as evidenced by increased levels of nitrite, an oxidized metabolite of NO, in the culture medium. LPS and TNF-alpha treatment led to a 25-fold increase in GTPCH enzyme activity, the first and rate-limiting enzyme in BH4 synthesis, and a corresponding 23-fold increase in GTPCH protein levels. Northern blot analysis showed that increased levels of GTPCH mRNA preceded changes in GTPCH protein, GTPCH enzyme activity and BH4 levels and reached a maximal of 44-fold that was sustained for at least 48 h. These results demonstrate that LPS and TNF-alpha stimulate de-novo BH4 biosynthesis and suggest that C6 cells offer a model system for studying the molecular events that control the induction of GTPCH gene expression and BH4 synthesis in glial cells.

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.

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.

Coordinate regulation of tetrahydrobiopterin turnover and phenylalanine hydroxylase activity in rat liver cells

This work had two purposes: (i) to determine in vivo whether liver phenylalanine hydroxylase (PAH) is regulated by its substrates phenylalanine and tetrahydrobiopterin (BH4) as studies with purified enzyme suggest and (ii) to investigate in vivo the relationship between PAH activity and BH4 turnover. We found there are two BH4 pools in hepatocytes, one that is metabolically available (free BH4) and one that is not (bound BH4). Bound BH4 appears bound to PAH; the PAH-BH4 complex has much less catalytic activity and is less readily phenylalanine activated than uncomplexed enzyme. Interconversion of activated and unactivated PAH and bound and free BH4 is driven by phenylalanine; and free BH4 concentration is determined by the state of activation and activity of PAH. In hepatocytes, BH4 and PAH (subunit) concentrations are equal, all intracellular BH4 appears to be available to PAH, and free BH4 turns over rapidly (t1/2 approximately 1 hr). There is no evidence for feedback inhibition of BH4 synthesis; the BH4 synthetic rate appears high when free BH4 concentration is high and low when free BH4 is low. The data provide support in vivo that phenylalanine and BH4 are positive and negative regulators of the activity and activation state of PAH in the proposed manner; they also imply that regulation of BH4 turnover and PAH activity are linked processes, which are both controlled by phenylalanine concentration.

Hereditary progressive dystonia with marked diurnal fluctuation caused by mutations in the GTP cyclohydrolase I gene

Hereditary progressive dystonia with marked diurnal fluctuation (HPD) (also known as dopa responsive dystonia) is a dystonia with onset in childhood that shows a marked response without any side effects to levodopa. Recently the gene for dopa responsive dystonia (DRD) was mapped to chromosome 14q. Here we report that GTP cyclohydrolase I is mapped to 14q22.1-q22.2. The identification of four independent mutations of the gene for GTP cyclohydrolase I in patients with HPD, as well as a marked decrease in the enzyme's activity in mononuclear blood cells, confirms that the GTP cyclohydrolase I gene is a causative gene for HPD/DRD. This is the first report of a causative gene for the inherited dystonias.

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.

Dopamine neuron membrane physiology: characterization of the transient outward current (IA) and demonstration of a common signal transduction pathway for IA and IK

Dopamine neurons derived from the mesencephalon of embryonic rats were maintained in primary culture, identified and studied with whole-cell patch recording techniques. These neurons demonstrated a rapidly activating and inactivating voltage-dependent outward current which required the presence of K+ ions. This current was termed IA because of its transient nature. It was elicited by step depolarizations from holding potentials more negative than -50 mV and exhibited steady-state inactivation at a membrane potential more positive than -40 mV and half-maximal inactivation observed at -65 mV. This current rapidly achieved peak activation in less than 8 msec and decayed with a time constant (tau) of 58 +/- 5 msec. This current was observed in the presence of tetraethylammonium but was readily blocked by 4-aminopyridine (2-4 mM). This current was also observed to be modulated by stimulation of D2 dopamine receptors (DA autoreceptors) located on the dopamine neurons. Thus, both DA and the D2 receptor agonist quinpirole enhanced the peak IA observed, while the partial D1 receptor agonist SKF 38393 was without effect. The enhancement of IA was confirmed to be due to the activation of D2 receptors as the effects of either DA or quinpirole were blocked by the D2 receptor antagonists eticlopride and sulpiride, but not by the D1 receptor antagonist SCH 23390. Since we have previously demonstrated that the IK present in these cells is also enhanced by D2 receptor stimulation, we investigated the signal transduction pathways involved in coupling DA autoreceptors to both IA and IK. The response of both these potassium currents to DA autoreceptor stimulation was completely abolished by the preincubation of cultures with pertussis toxin, indicating the possible involvement of the G proteins Gi and G(o). In an attempt to further characterize which G protein may be involved, additional experiments were performed. The ability of DA autoreceptor stimulation to augment both currents was also blocked completely when G protein activation was prevented by the intracellular application of GDP beta S (100 microM). In contrast, irreversible activation of G proteins by intracellular application of the nonhydrolyzable GTP analog GTP gamma S (100 microM) mimicked the effects of DA autoreceptor stimulation on both IA and IK. In addition, the intracellular application of a polyclonal antibody that was selective for the alpha-subunit of G(o) completely abolished the DA autoreceptor modulation of both currents while preimmune serum was without effect.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

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.

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

We have examined the turnover of 5,6,7,8-tetrahydrobiopterin (BH4) and the effect of decreasing BH4 levels on in situ tyrosine hydroxylase (TH) activity and norepinephrine (NE) content in a homogeneous population of NE-containing neurons derived from the superior cervical ganglion (SCG) of the neonatal rat and maintained in tissue culture. Initial studies indicated that the level of BH4 within SCG cultures increased fourfold between 5 and 37 days in vitro (DIV). This increase in BH4 levels was determined to result from an increase in the rate of BH4 biosynthesis without a change in the rate of degradation. Regardless of culture age, the BH4 content of SCG neurons was observed to turn over with a half-life of approximately 2.5 h. BH4 synthesis by SCG neurons was found to be five times more sensitive to inhibition by 2,4-diamino-6-hydroxypyrimidine (DAHP) and 25 times less sensitive to inhibition by N-acetylserotonin than was previously reported for CNS neurons in culture. Under basal conditions, the rates of in situ TH activity and BH4 biosynthesis were similar. In response to inhibition of BH4 biosynthesis by DAHP and a 90-95% decrease in BH4 levels, in situ TH activity declined by 75%. NE levels declined by 30% following a 24-h period of inhibition of BH4 synthesis. After 2 days of BH4 synthesis inhibition, the level of NE was decreased by 47%. On treatment days 3 and 4, the decline in NE content plateaued at 24% of control levels.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel action of 6R-L-erythro-5,6,7,8-tetrahydrobiopterin, a cofactor for hydroxylases of phenylalanine, tyrosine and tryptophan: enhancement of acetylcholine release in vivo in the rat hippocampus

Recently, we have reported that 6R-L-erythro-tetrahydrobiopterin (6R-BH4), a natural cofactor for hydroxylases of phenylalanine, tyrosine and tryptophan, has a dopamine releasing action in the rat striatum independent of its cofactor role. Here we studied the effects of 6R-BH4 on acetylcholine (ACh) release in the rat hippocampus using brain microdialysis. Intracerebroventricular injection of 6R-BH4 dose-dependently increased extracellular ACh levels monitored by brain microdialysis. Biopterin, an oxidized form of 6R-BH4, and 6S-BH4, an unnatural diastereoisomer of 6R-BH4, had little effect on the ACh levels. The 6R-BH4-induced increase in ACh levels was eliminated after inhibition of voltage-dependent Na+ channels by tetrodotoxin, but not after depletion of catecholamines by reserpine. These results show that 6R-BH4 has direct ACh releasing action in vivo.

Alterations in cell tetrahydrobiopterin levels may regulate queuine hypomodification of tRNA during differentiation of murine erythroleukemia cells

The base at the first anticodon ("wobble") position of certain eukaryotic tRNA species is either guanine or the hypermodified base queuine. These tRNA species are synthesized with guanine in the wobble position (tRNAG); this guanine can then be replaced with queuine by the action of the enzyme tRNA-guanine ribosyltransferase. In the present report, we show that tRNAG levels increased in response to the induction of erythroid differentiation of murine erythroleukemia (MEL) cells. We also found that tRNA-guanine ribosyltransferase was significantly inhibited by tetrahydrobiopterin. MEL cells showed a transient threefold increase in tetrahydrobiopterin levels 6 to 12 h after exposure of the cells to inducers such as DMSO or tetramethylurea. The increase in tetrahydrobiopterin preceded the increase in tRNAG which in turn preceded the appearance of phenotypic changes characteristic of differentiation. By contrast, a mutant MEL cell line unable to differentiate in response to inducers showed no change in the level of tetrahydrobiopterin or of tRNAG upon exposure to DMSO. N-acetylserotonin, a well-characterized inhibitor of tetrahydrobiopterin synthesis, prevented the DMSO-mediated increase in tetrahydrobiopterin in normal MEL cells. N-acetylserotonin also inhibited the increase in tRNAG levels and the appearance of phenotypic differentiation in these cells.