Influence of l-3-methoxytyrosine on monoamine synthesis in rat brain

Summary Rats were injected i.p. with l-3-methoxytyrosine, 100 or 300 mg/kg. One h later brain, liver, heart and blood plama were analyzed for catecholamines and their precursors. In brain Dopa as well as dopamine and noradrenaline levels were unchanged, while demethylation of l-3-methoxytyrosine might have occurred in peripheral organs since Dopa levels in liver and dopamine in heart were elevated. 3-Methoxytyramine could not be detected in brain and liver after treatment with l-3-methoxytyrosine.Monoamine synthesis in vivo was measured in whole brain by determining the accumulation of Dopa and 5-hydroxytryptophan 30 min after the i.p. injection of NSD 1015 (3-hydroxybenzylhydrazine HCl, 100 mg/kg), an inhibitor of the aromatic amino acid decarboxylase. l-3-Methoxytyrosine attenuated the formation of Dopa and 5-hydroxytryptophan by about 25% in brain tissue. The effect was paralleled by a decrease in the brain concentration of tryptophan.     

Brain biochemical and behavioral changes in 4 weeks old rats after neonatal oxygen deprivation.

The acquisition of a conditioned avoidance response (CAR) and the effects on monoamine neurotransmittor synthesis was investigated in 28 days old rats after neonatal oxygen deprivation (100% N₂ for 20 min or 6% O₂-94% N₂ for 4.5 hr). The rats subjected to 6% O₂-94% N₂ for 4.5 hr at 1 day of age were markedly inferior in the CAR acquisition than the control group. The animals subjected to 20 min neonatal anoxia did not differ in avoidance responding compared to controls. Tyrosine hydroxylase and tryptophan hydroxylase activity was studied in vivo in whole brain at 28 days of age by measuring the accumulation of dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) respectively after inhibition of aromatic L-amino acid decarboxylase with NSD 1015. An inhibition of tyrosine hydroxylase and tryptophan hydroxylase activity was found after neonatal exposure to 6% O₂-94% N₂ for 4.5 hr while the rats exposed to 100% N₂ for 20 min did not differ from controls. The precursor amino acids tyrosine and tryptophan and the levels of the endogenous monoamines dopamine (DA), noradrenaline (NA) and 5-hydroxytryptamine (5-HT) did not differ from controls in any of the groups. It is suggested that the observed behavioral deficits after prolonged hypoxia may be due to an impaired development of central catecholamine mechanisms.     

Ethanol-induced increase in brain concentrations of administered neutral amino acids

Summary Ethanol 2 g kg^–1 i.p. to rat increased the concentrations in the brain of administered large neutral amino acids (tyrosine, tryptophan, 5-hydroxytryptophan and -methyldopa). We have previously found a similar effect of ethanol on administered l-Dopa, resulting in increased brain/plasma ratios of dopa. Since large neutral amino acids are known to compete with each other for the carrier-mediated transport into the brain we suggest that the increased concentrations of the administered amino acids in the brain are at least partly the consequence of the ethanol-induced decrease in plasma amino acids observed previously.     

Simultaneous measurement of tyrosine and tryptophan hydroxylase activities in brain in Vivo using an inhibitor of the aromatic amino acid decarboxylase

Summary DOPA and 5-HTP accumulated in vivo in rat brain after decarboxylase inhibition with NSD 1015 (3-hydroxybenzylhydrazine). This accumulation was linear for the first 30 min and occurred in several brain regions over a wide range of NSD 1015 doses. After a peripheral decarboxylase inhibitor much less, if any, DOPA or 5-HTP accumulated in the brain. The accumulation of DOPA was prevented by H 44/68 (methylester of -methyl para-tyrosine), a tyrosine hydroxylase inhibitor. DOPA, which accumulated before H 44/68 was given, appeared stable for at least 20 min. There were no significant changes in the levels of NA, DA, 5-HT or tryptophan shortly after NSD 1015 administration, but a rise in tyrosine was noted. Increased brain tyrosine after l-tyrosine administration did not alter the DOPA accumulation, however. These data as well as the distribution of the accumulated amino acids suggest that the accumulation of DOPA and 5-HTP after decarboxylase inhibition occurs intraneuronally, that the decarboxylase enzyme is completely inhibited, and that the accumulated products are not appreciably metabolized or transported from the region studied. Amine synthesis rates and rate constants were calculated from the data and compare well with similar values determined by other methods. Thus this accumulation appears to be a reliable measure of the in vivo hydroxylation of tyrosine and tryptophan.     

Adrenergic nonspecific potentiation of yohimbine toxicity in mice by antidepressants and related drugs and antiyohimbine action of antiadrenergic and serotonergic drugs

Thirteen imipramine-like antidepressants and three anticholinergics potentiated yohimbine toxicity in mice. The strongest potentiation was observed after the nonantidepressant adrenergic compound AW 151129. Monomethyl derivatives were significantly stronger than their parent dimethyl compounds. Quaternary imipramine and amitriptyline, amphetamine, and inhibitors of monoamine oxidase were inactive. Pretreatment with an inhibitor of tyrosine hydroxylase (-methyl-p-tyrosine) or the -adrenergic-blocking drug pindolol diminished lethality in mice treated with a combination of desmethylimipramine (AW 151129) and yohimbine. The inhibitor of brain serotonin synthesis p-chlorophenylalanine and a depletor of serotonin and catecholamine synthesis, reserpine, enhanced yohimbine toxicity whereas serotonin and its precursors tryptophan and 5-hydroxytryptophan pro-longed survival time. Thus, a test of yohimbine potentiation toxicity in mice reveals an adrenergic component of pharmacological activity of antidepressants. The predictive value of this test is limited similarly to antihypothermic tests of antagonism to reserpine and the related drugs apomorphine and 5-hydroxytryptophan.     

Neurotensin and its amide analogue [Gln4]-neurotensin: effects on brain monoamine turnover

Summary Intracerebroventricularly administered neurotensin and [Gln⁴]-neurotensin (50–200 g) increased the formation of Dopa in different brain regions of rats after inhibition of the aromatic l-amino acid decarboxylase. For both neuropeptides these increases were dose dependent (20–150%). In the corpus striatum [Gln⁴]-neurotensin was twice as active as neurotensin and it tended to be more active also in other brain regions. The brain tyrosine concentrations were also increased. [Gln⁴]-neurotensin (100–200 g) following inhibition of the aromatic l-amino acid decarboxylase, increased the accumulation of 5-hydroxytryptophan in all brain regions by 30–60%. In contrast, neurotensin was completely inactive. In both cases the brain tryptophan concentrations were increased. Both neurotensin and [Gln⁴]-neurotensin also accelerated the disappearance of dopamine, noradrenaline and 5-hydroxytryptamine after inhibition of monoamine synthesis. These results show an increased brain monoamine turnover induced by both neuropeptides.     

The effect of tryptophan and a tryptophan/5-hydroxytryptophan combination on indoles in the brains of rats fed a tryptophan deficient diet

Rats maintained on a tryptophan deficient diet had reduced brain and serum tryptophan and brain 5-hydroxy-indolacetic acid levels compared to controls. 5-Hydroxytryptophan and l-tryptophan administered to these deficient rats in a combination (595) produced a greater elevation of indolamines and tryptophan in the brain than either amino acid alone. In rats maintained on a normal diet the urinary output of 3-hydroxykynurenine was considerably reduced by treatment with the combination of amino acids as com pared to tryptophan treatment. 5-Hydroxytryptophan reduced the induction of kynurenine synthesis in the liver produced by tryptophan, implying that it is capable of inhibiting the enzyme tryptophan pyrrolase in vivo. It is suggested that the possession by 5-hydroxytryptophan of tryptophan pyrrolase inhibitory properties may make the administration of the combination a better treatment of depressed patients exhibiting an indolamine deficit than either amino acid alone.     

Inhibition of striatal tyrosine hydroxylase by low concentrations of apomorphine

Summary The inhibitory effect of apomorphine on tyrosine hydroxylase (TH) was tested using enzyme preparations from rat striatum, neuroblastoma clone N1E-115 and pheochromocytoma clone PC-12. When the striatal enzyme preparation was incubated at pH 7.2 with (6R,S)-l-erythro-5,6,7,8-tetrahydrobiopterin (BH₄) as cofactor (100–1,000 mol/l), the IC₅₀ for apomorphine was found to be in the 0.1–1 mol/l range depending on the BH₄-concentration used. Changing the incubation medium to pH 6.0 yielded an IC₅₀ of about 2.5 mol/l (BH₄=100 mol/l) Apomorphine was even less effective when 2-amino-4-hydroxy-6,7-dimethyl-5,6,7,8-tetrahydroteridine (100 mol/l) was used as cofactor (IC₅₀10 mol/l). Similar results were obtained with the enzyme preparations of the two cell clones.These experiments show that, even in low concentrations, apomorphine inhibits TH directly, provided more physiological test conditions are used. The relevance of these results for the autoreceptor-mediated mechanism of the apomorphine action on catecholamine synthesis is discussed.Abbreviations used BH₄ (6R,S)-l-erythro-5,6,7,8-tetrahydrobiopterin - DMPH₄ 2-amino-4-hydroxy-6,7-dimethyl-5,6,7,8-tetrahydropteridine - DOPA 3,4-dihydroxyphenylalanine - ELCD electrochemical detection - HPLC high performance liquid chromatography - NEAA non essential amino acid concentrate - NSD 1015 3-hydroxy-benzyl-hydrazine - TH tyrosine hydroxylase     

Stimulatory effects of brain natriuretic peptide on cyclic GMP accumulation and tyrosine hydroxylase activity in cultured bovine adrenal medullary cells

Summary We studied the effect of brain natriuretic peptide (BNP) on the accumulation of cyclic GMP and the phosphorylation and activity of tyrosine hydroxylase, compared with that of atrial natriuretic peptide (ANP), in cultured bovine adrenal medullary cells. 1. BNP as well as ANP increased cellular cyclic GMP accumulation in a concentration-dependent manner (10–1000 nmol/1). BNP (1 mol/1) and ANP (1 mol/1) produced a 60-fold and 30-fold increase in cyclic GMP accumulation, respectively. 2. The stimulatory effects of BNP and ANP on cyclic GMP accumulation were observed even when Ca²⁺ or Na⁺ was removed from the incubation medium. 3. 12-O-Tetradecanoylphorbol 13-acetate (TPA), an activator of protein kinase C, inhibited the stimulatory effect of BNP on cyclic GMP accumulation in a concentration-dependent manner (1–100 nmol/1). Furthermore, the BNP-induced accumulation of cyclic GMP was attenuated by forskolin (1 mol/1), an activator of adenylate cyclase. 4. BNP (1 mol/1) and ANP (1 mol/1) caused a significant increase in phosphorylation and activity of tyrosine hydroxylase in the cells. 5. In digitonin-permeabilized cells, cyclic GMP (1–100 mol/1) activated tyrosine hydroxylase in the presence of ATP and Mg²⁺.These results suggest that BNP stimulates the accumulation of cyclic GMP in a manner similar to that of ANP. The increased accumulation of cyclic GMP by these peptides may be negatively modulated by protein kinase C and cyclic AMP and may cause the phosphorylation and activation of tyrosine hydroxylase-in cultured bovine adrenal medullary cells.     

Direct inhibition of tyrosine hydroxylase from PC-12 cells by catechol derivatives

Summary Several drugs with a catechol moiety were studied for their potency to inhibit tyrosine hydroxylase (TH) from PC-12 cells in vitro. When the natural compounds tested were compared, dopamine, norepinephrine and 2(3,4-dihydroxyphenyl)-ethanol (DOPET) were most effective (IC₅₀ between 1.4 and 3.6 M with 0.5 M 6(R,S)-l-erythro-5,6,7,8-tetrahydrobiopterin as cofactor). 3,4-Dihydroxyphenylalanine (DOPA; IC₅₀: 35 M) and 3,4-dihydroxyphenylacetic acid (DOPAC; IC₅₀: 180 M were less potent inhibitors. Among the synthetic drugs possessing catechol moiety, isoproterenol, (±)-2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene (6,7-ADTN) and (±)-2-dimethylamino-6,7-dihydroxy-tetrahydronaphthalene (TL-99) had the same inhibitory effects as the natural catecholamines (IC₅₀ between 1.6 and 3.9 M), whereas the apomorphine derivatives and 2,3,4,5-tetrahydro-1-phenyl-1 H-3-benzazepine-7,8-diol (SKF 38393) were even more potent (IC₅₀: 0.5–0.8 M).These results demonstrate that natural catechols and certain drugs (e.g. 6,7-ADTN, TL-99, SKF 38393) are more effective direct blockers of tyrosine hydroxylase than generally assumed provided appropriate assay conditions are used. In the case of dopamine and norepinephrine, these findings suggest a reevaluation of their role for feedback control of tyrosine hydroxylase in vivo.Abbreviations 6,7-ADTN (±)-2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene - BH₄ 6(R,S)-l-erythro-5,6,7,8-tetrahydrobiopterin - DMPH₄ 2-amino-4-hydroxy-6,7-dimethyl-tetrahydropteridine - DOPA 3,4-dihydroxyphenylalanine - DOPAC 3,4-dihydroxyphenylacetic acid - DOPET 2(3,4-dihydroxyphenyl)ethanol - ELCD electrochemical detection - 6-MPH₄ 6-methyl-5,6,7,8-tetrahydropteridine - NSD 1015 3-hydroxy-benzyl-hydrazine - SKF 38393 2,3,4,5-tetrahydro-1-phenyl-1 H-3-benzazepine-7,8-diol - TH tyrosine hydroxylase - TL-99 (±)-2-dimethylamino-6,7-dihydroxy-tetrahydronaphthalene     

The effect of the dopamine agonist pergolide on tyrosine hydroxylase activity in rat striatal and limbic miniprisms in vitro: A model for the dopamine autoreceptor?

Summary A method for the assay of tyrosine hydroxylase activity in rat striatal and limbic (nucleus accumbens + olfactory tubercle) brain miniprisms is described. The dopamine agonists apomorphine (1 mol/l) and pergolide (0.01–100 mol/l) inhibited the tyrosine hydroxylase activity in both regions. The inhibition produced by 1 mol/l pergolide was antagonised partially in striatal miniprisms and completely in limbic miniprisms by 1 mol/l haloperidol. The dopamine D₂-selective antagonist raclopride, at concentrations up to 300 nmol/l, did not antagonise the inhibition produced by pergolide in striatal miniprisms, but appeared partially to antagonise the inhibition in limbic miniprisms. It is concluded that whilst pergolide potently inhibits tyrosine hydroxylase activity in striatal and limbic miniprisms, the inhibition is of doubtful value as a predictive model of dopamine autoreceptor function in striatal miniprisms, but may be useful when limbic miniprisms are used.     

Inhibition of rat brain tryptophan hydroxylation with p-chloroamphetamine.

p-Chloroamphetamine (PCA) at high concentrations (0.0025 to 0.02 M) inhibited rat brainstem tryptophan hydroxylase and, to a lesser extent, rat corpus striatum tyrosine hydroxylase. Hog kidney aromatic l-amino acid decarboxylase was not affected. Inhibition of tryptophan hydroxylase by p-chloroamphetamine was competitive with tryptophan and noncompetitive with 6,7-dimethyl-5,6,7,8-tetrahydropterine (DMPH₄). In rats given p-chloroamphetamine, brainstem tryptophan-hydroxylating activity was only slightly reduced, whereas striatal tyrosine-hydroxylating activity was not altered. Synthesis rates of brain serotonin and dopamine in rats treated with p-chloroamphetamine, estimated by measuring the accumulation of 5-hydroxytryptophan and dopa, respectively, after blockade of decarboxylase, were both decreased. In view of the high concentrations of p-chloroamphetamine required to inhibit tryptophan and tyrosine hydroxylases in vitro, mechanisms other than inhibition of enzyme by PCA might be responsible for the decelerated synthesis of serotonin and dopamine in brain (e.g. blockade of monoamine re-uptake).     

Susceptibility of chick brain L-glutamic acid decarboxylase and other neurotransmitter enzymes to hyperbaric oxygen in vitro

Homogenates from chick whole brain were exposed to oxygen at 30 psi gauge pressure for 20 min at 25°. Controls were similarly incubated, but under a continuous flow of nitrogen at ambient pressure. The activities of the following neurotransmitter enzymes were then assayed in the homogenates: l-glutamic acid decarboxylase (GAD), 4-aminobutyrate-α-oxoglutarate aminotransferase (GABA-T), choline acetyltransferase (ChAc), acetylcholinesterase (AChE), tyrosine hydroxylase, dihydroxyphenylalanine decarboxylase (dopa decarboxylase), tryptophan hydroxylase, 5-hydroxytryptophan decarboxylase (5-HTP decarboxylase), monoamine oxidase (MAO) and catechol-O-methyl transferase (COMT). The activity of each of the biosynthetic enzymes, except that of the tyrosine and tryptophan hydroxylases, was inhibited by the high pressure oxygen (OHP), with that of GAD displaying the largest percentage decrease (GAD, 51 per cent; Dopa decarboxylase, 12 per cent; 5-HTP decarboxylase, 10 per cent; ChAc, 7 per cent inhibition). The four degradative enzymes were not inhibited by OHP. In a second experiment, homogenates were exposed to oxygen at 90 psi gauge pressure. Again GAD showed the greatest decrease in activity (GAD, 66 per cent; Dopa decarboxylase, 18 per cent; 5-HTP decarboxylase, 24 per cent; and ChAc, 12 per cent inhibition). The relatively high sensitivity of GAD to inhibition by OHP was in harmony with the hypothesis that a deranged γ-aminobutyric acid metabolism plays a major role in the etiology of OHP-induced seizures.     

In vivo evidence for a greater brain tryptophan hydroxylase capacity in female than in male rats

Summary Previous studies have revealed that brain levels of tryptophan, 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) are moderately higher in female than in male rats. Since tryptophan hydroxylase is only about half saturated with substrate, the greater availability of precursor in female rats might contribute to their higher 5-hydroxyindole levels. The present investigation was aimed at clarifying whether there is a sex difference in central tryptophan hydroxylase capacity. Hence, both sexes received a high dose of l-tryptophan (400 mg/kg), which resulted in a tenfold increase in brain tryptophan concentrations and presumably a virtual saturation of tryptophan hydroxylase. Following such treatment, 5-hydroxytryptophan (5-HTP) levels, measured after l-amino acid decarboxylase inhibition, were compared in males and females. Both in saline-and l-tryptophan-treated rats, 5-HTP levels were generally higher in females. In another group of animals, receiving 400 mg/kg of l-tryptophan as sole treatment, 5-HT and 5-HIAA concentrations were measured. As in the case of 5-HTP, the higher 5-HT and 5-HIAA levels observed in females persisted after l-tryptophan treatment. The present data suggest that brain tryptophan hydroxylase activity is greater in females; this sex difference probably contributes to the higher 5-HT and 5-HIAA levels in females.Send offprint requests to M. Carlsson at the above address     

Veratridine-induced phosphorylation and activation of tyrosine hydroxylase,and synthesis of catecholamines in cultured bovine adrenal medullary cells

Summary The mechanism of the synthesis of catecholamines by veratridine was studied in cultured bovine adrenal medullary cells. (1) Veratridine increased the phosphorylation and activity of tyrosine hydroxylase as well as the synthesis of [¹⁴C]catecholamines from [¹⁴C]tyrosine, all of which were inhibited by tetrodotoxin. Veratridine-induced activation of tyrosine hydroxylase and synthesis of [¹⁴C]catecholamines were reduced in 20 mmol/l extracellular Na⁺ or in Ca²⁺-free medium. (2) 12-O-Tetradecanoylphorbol-13-acetate (TPA), an activator of protein kinase C, increased the synthesis of [¹⁴C]catecholamines. In the presence of TPA, veratridine did not produce any additional increase in [¹⁴C]catecholamine synthesis. In protein kinase C-deficient cells which were prepared by pretreatment with 1 ol/1 TPA for 24 h, TPA failed to increase [¹⁴C]catecholamine synthesis and veratridine-induced [¹⁴C]catecholamine synthesis was suppressed by 50%. (3) Polymyxin B, an inhibitor of protein kinase C and N-(6-aminohexyl)-5-chloro-l-naphthalenesulfonamide (W-7), an inhibitor of calmodulin, inhibited veratridine-stimulated synthesis of [¹⁴C]catecholamines as well as veratridine-induced influx of ²²Na⁺ and ⁴⁵Ca²⁺ with similar potencies. (4) In digitonin-permeabilized cells, polymyxin B attenuated the activation of tyrosine hydroxylase caused by Ca²⁺. These results suggest that veratridine-induced synthesis of catecholamines and activation of tyrosine hydroxylase were mediated by Ca²⁺-dependent phosphorylation of this enzyme, and protein kinase C may be responsible, at least in part, for this process. Send offprint requests to Y. Uezono at the above address     

Intranigral application of substance P antagonists prevents the haloperidol-induced activation of striatal tyrosine hydroxylase

Summary The effect of intranigral injections of stable substance P analogs with antagonist properties, namely [D-Pro², D-Trp^7,9]-substance P and [D-Pro², D-Phe⁷, D-Trp⁹]-substance P, on the activity of the nigrostriatal dopaminergic pathway was studied. Dopaminergic neural activity was evaluated by measuring changes in striatal tyrosine hydroxylase activity in response to systemic treatment with haloperidol. When injected into the substantia nigra of normal rats, neither of the two substance P analogs influenced the V _max of striatal tyrosine hydroxylase or the affinity of the enzyme for its pteridine cofactor. However, when applied into the substantia nigra 10 min before systemic haloperidol, 20 g of either substance P analog was able to prevent the haloperidol-induced activation of striatal tyrosine hydroxylase. The effect was not altered by concurrent microinjection of the GABA-antagonist bicuculline. These results suggests that nigral substance P plays an important role in the activation of nigrostriatal dopamine neurons produced by haloperidol. Thus, both substance P and GABA may reciprocally and independently regulate the activity of nigrostriatal dopamine neurons in response to changes in dopamine receptor activity.     

Evidence for selective inhibition of limbic forebrain dopamine synthesis by 8-OH-DPAT in the rat

Summary Regional dopamine synthesis in the rat striatum was estimated by measuring DOPA accumulation following inhibition of cerebral aromatic l-amino acid decarboxylase by means of NSD-1015, 100 mg kg^–1 intraperitoneally. In animals treated with reserpine, 5 mg kg^–1 subcutaneously –18 h, there was a statistically significant increase in DOPA accumulation in the nucleus accumbens, the ventro-medial neostriatum, the dorso-lateral neostriatum and in the posterior limb of the neostriatum. This increase in DOPA accumulation was antagonized dose-dependently in the nucleus accumbens and ventro-medial neostriatum, but not in the other two regions, by treatment with the 5-HT_1A receptor agonist 8-OH-DPAT, 0.15–2.4 mol kg^–1, whereas the partial dopamine D₂ receptor agonist (–)3-PPP, 2.5–10.0 mol kg^–1, or the full dopamine D₂ receptor agonist quinpirole, 0.05–0.8 mol kg^–, antagonized the reserpine-induced increase in DOPA accumulation uniformly in all four regions of the striatum. The suppression of DOPA accumulation by 8-OH-DPAT in reserpine-treated animals, was completely antagonized by raclopride, 1 mol kg^–1, but not by (–)pindolol, 8 mol kg^–1. The accumulation of 5-HTP in all regions of the striatum as well as in the neocortex following decarboxylase inhibition and reserpine pretreatment, was also inhibited by 8-OH-DPAT, and this inhibition was unaffected by treatment with raclopride or (–)pindolol. It is concluded that 8-OH-DPAT, in addition to general effects on forebrain 5-hydroxytryptamine synthesis, selectively affects limbic forebrain dopamine synthesis. This latter effect is probably due to direct stimulation of dopamine autoreceptors, since it was obtained in reserpine-treated rats, and was completely antagonized by raclopride, but not (–)pindolol.Presented in part at the 7th General Meeting of the European Society for Neurochemistry, June 12–17th 1988, Göteborg, Sweden Send offprint requests to S. Ahlenius at the above address     

Tyrosine and tryptophan hydroxylases as therapeutic targets in human disease

Introduction: The ancient and ubiquitous monoamine signalling molecules serotonin, dopamine, norepinephrine, and epinephrine are involved in multiple physiological functions. The aromatic amino acid hydroxylases tyrosine hydroxylase (TH), tryptophan hydroxylase 1 (TPH1), and tryptophan hydroxylase 2 (TPH2) catalyse the rate-limiting steps in the biosynthesis of these monoamines. Genetic variants of TH, TPH1, and TPH2 genes are associated with neuropsychiatric disorders. The interest in these enzymes as therapeutic targets is increasing as new roles of these monoamines have been discovered, not only in brain function and disease, but also in development, cardiovascular function, energy and bone homeostasis, gastrointestinal motility, hemostasis, and liver function.

Areas covered: Physiological roles of TH, TPH1, and TPH2. Enzyme structures, catalytic and regulatory mechanisms, animal models, and associated diseases. Interactions with inhibitors, pharmacological chaperones, and regulatory proteins relevant for drug development.

Expert opinion: Established inhibitors of these enzymes mainly target their amino acid substrate binding site, while tetrahydrobiopterin analogues, iron chelators, and allosteric ligands are less studied. New insights into monoamine biology and 3D-structural information and new computational/experimental tools have triggered the development of a new generation of more selective inhibitors and pharmacological chaperones. The enzyme complexes with their regulatory 14–3–3 proteins are also emerging as therapeutic targets.     

Effects of taurine,homotaurine and GABA on hypothalamic and striatal dopamine metabolism

Summary To elucidate the effects of taurine on hypothalamic and striatal dopaminergic neurotransmission we compared its effects to those of -aminobutyric acid (GABA) and homotaurine (a GABA_A-receptor agonist) on hypothalamic and striatal concentrations of dopamine (DA) and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and, in the case of striatum, 3-methoxytyramine (3-MT) in rats. In addition, hypothalamic and striatal 5-hydroxytryptamine (5-HT) und 5-hydroxyindoleacetic acid, hypothalamic noradrenaline (NA) and 3-methoxy-4-hydroxyphenylglycol sulfate, and pituitary DA concentrations were also measured. The amino acids were injected into the lateral brain ventricles of conscious male rats in doses of 10 and 36 mol/rat, and rat were sacrificed 15 and 60 min later, respectively.Homotaurine (by 11010) but not the other two amino acids elevated striatal DA, whereas hypothalamic DA was increased by both taurine (36%) and homotaurine (31%). All three amino acids at 36 mol elevated striatal DOPAC, homotaurine (51%) more than taurine (31%) or GABA (30%), and hypothalamic DOPAC, both taurine (102%) and homotaurine (82%) clearly more than GABA (34%). Neither striatal nor hypothalamic HVA was altered by any of the amino acids. At 10 mol the amino acids decreased striatal 3-MT by about 40%. At 36 mol taurine and homotaurine reduced 3-MT by about 70%, whereas increasing the dose of GABA did not further reduce 3-MT. Both taurine and homotaurine at 36 mol decreased hypothalamic NA content. Neither hypothalamic nor striatal 5-HT metabolism was altered. In the neurointermediate lobe of the pituitary gland taurine at 10 mol but not at 36 mol slightly (20%) increased DA.These results show that taurine and homotaurine clearly alter the hypothalamic DA metabolism, and give further support to the suggestion that taurine and homotaurine similarly to GABA reduce the release of DA in the striatum. Homotaurine seems to alter the striatal DA metabolism more effectively than taurine or GABA. On the other hand, hypothalamic DA seems to be equally sensitive to taurine and homotaurine, and is clearly less sensitive to GABA. The pronounced elevation of hypothalamic DA and DOPAC after taurine and homotaurine could be related to changes in hypothalamic NA. Send offprint requests to L. Ahtee at the above address     

AMINONITRILES AND AMINOTHIOAMIDES RELATED TO NATURAL AMINO ACIDS

N-p-Methoxybenzyloxycarbonyl and N-tert.-butyloxycarbonyl amino acid amides related to a series of natural amino acids were dehydrated to the corresponding Meoz- and Boc-α-aminonitriles. Deprotection of the latter derivatives afforded α-aminonitriles related to alanine, tyrosine, phenylalanine, dihydrophenylalanine, histidine, Dopa, ornithine, asparagine and glutamine. Thioamidation with H₂S/NH₃ or H₂S/NEt₃ in general converted the protected amino nitriles to Meoz- and Boc-α-aminothioamides. When deprotected these furnished the α-aminothioamides corresponding to alanine, tyrosine, phenylalanine, dihydrophenylalanine and histidine. For dehydration and thioamidation of histidine and Dopa, Nα-Boc-^im trityl-histidine and N-Boc-0, 0′-diacetyldihydroxyphenylalanine were useful. Dopa was obtained as the free and Boc-thiohydrazide. Also prepared were Nα,ω -diMeoz-ornithine DCHA, Meoz-2,5-dihydrophenylalanine DCHA and N,0-diMeoz-tyrosine as starting materials and N,0-dicarbobenzyloxycarbonyltyrosinamide, N,0-diZ-tyrosine nitrile and Z-β-cyano-β-alaninamide as model compounds. During deprotection of Meoz-alanine thioamide, transfer of an anisyl group from the N-Meoz protecting group to sulfur took place as a side reaction that yielded alanine p-methoxybenzyl β-imidothiolic ester. This study provides two new series of amino acid analogs with potential antimetabolite activity. Also suitable for incorporation into peptide analogs, these afford approaches to relating structure and conformation to activity in biologically active peptides.