Characteristics of tryptophan accumulation by isolated rat forebrain synaptosomes

Zebrafish disease models in drug discovery: from preclinical modelling to clinical trials

Numerous drug treatments that have recently entered the clinic or clinical trials have their genesis in zebrafish. Zebrafish are well established for their contribution to developmental biology and have now emerged as a powerful preclinical model for human disease, as their disease characteristics, aetiology and progression, and molecular mechanisms are clinically relevant and highly conserved. Zebrafish respond to small molecules and drug treatments at physiologically relevant dose ranges and, when combined with cell-specific or tissue-specific reporters and gene editing technologies, drug activity can be studied at single-cell resolution within the complexity of a whole animal, across tissues and over an extended timescale. These features enable high-throughput and high-content phenotypic drug screening, repurposing of available drugs for personalized and compassionate use, and even the development of new drug classes. Often, drugs and drug leads explored in zebrafish have an inter-organ mechanism of action and would otherwise not be identified through targeted screening approaches. Here, we discuss how zebrafish is an important model for drug discovery, the process of how these discoveries emerge and future opportunities for maximizing zebrafish potential in medical discoveries.

Interference with cellular energy metabolism reduces kynurenic acid formation in rat brain slices: reversal by lactate and pyruvate

This study was designed to investigate the role of cellular energy metabolism in the de novo formation of the endogenous excitatory amino acid receptor antagonist, kynurenic acid. Using rat cortical tissue slices, the roles of glucose transport, glycolysis, tricarboxylic acid cycle intermediates and oxidative phosphorylation were studied. Inhibition of glucose utilization resulted in quantitatively similar decreases in kynurenine uptake, kynurenic acid production and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction, a marker of mitochondrial activity. The end product of glycolysis, pyruvate, as well as lactate, attenuated all three deficits. Pyruvate also significantly increased kynurenic acid formation in normal brain slices without affecting kynurenine uptake. Oxaloacetate and alpha-ketoglutarate (tricarboxylic acid cycle intermediates) were the only compounds tested which were capable of duplicating the effects of pyruvate, indicating that 2-oxoacids can stimulate kynurenic acid synthesis by acting as aminoacceptors in the enzymatic transamination of kynurenine. When the mitochondrial electron transport chain was blocked by specific inhibitors, coincubation with succinate restored the rate of MTT formazan formation to normal (except in the case of 3-nitropropionic acid), yet failed to prevent the resulting reduction in kynurenic acid synthesis. Conversely, pyruvate increased kynurenic acid production in the presence of all inhibitors (except cyanide), but did not attenuate the reduction in kynurenine uptake and MTT formazan formation. Taken together, these results demonstrate that interference with cellular energy metabolism causes mechanistically diverse, pronounced reductions in the cerebral neosynthesis of kynurenic acid, and that 2-oxoacids and lactate can effectively reverse most of these detrimental effects.

Substantial regional and hemispheric differences in brain nitric oxide synthase (NOS) inhibition following intracerebroventricular administration of N omega-nitro-L-arginine (L-NA) and its methyl ester (L-NAME)

Nitric oxide synthase (NOS) enzyme activity was determined in a comprehensive selection of regions of the rat brain. The effects of lateral ventricular administration of N omega-nitro-L-arginine (L-NA, 30 micrograms) and its methyl ester (L-NAME, 3-100 micrograms) on NOS activity were examined in the ipsilateral and contralateral areas of 4 of these brain regions and in the cerebellum. NOS activity was determined using a new and rapid ex vivo assay method which ensures minimal dissociation of the enzyme-inhibitor complex. Following infusion of L-NAME, NOS activity was rapidly and dose-dependently inhibited in all brain regions studied (cerebral cortex, striatum, hippocampus, cerebellum and thalamus). However, NOS activity of brain regions within the contralateral hemisphere was inhibited significantly less than in ipsilateral regions, with the exception of the thalamus. The degree of NOS inhibition varied markedly between brain regions within each hemisphere and correlated with their ventricular proximity to the site of NOS inhibitor administration. Therefore, NOS in the thalamus was inhibited most effectively and NOS in the cerebral cortex the least. Within the cerebral cortex further regional differences could be observed, with NOS in the frontal/parietal areas inhibited more effectively than NOS in the temporal/occipital areas. Maximal inhibition of NOS was sustained for approx 6 hr after administration of 30 and 100 micrograms L-NAME. No inhibition of NOS was observed 24 hr after administration. Lateral ventricular administration of the metabolite and active moiety of L-NAME, L-NA, resulted in a similar degree of inhibition and time of inhibitory onset. In contrast, when L-NAME was administered i.p., a significant delay in the onset of NOS inhibition was observed in the above brain regions compared to L-NA. However, no regional or hemispheric differences in NOS inhibition were detected following peripheral administration of these inhibitors. These results indicate that central administration of NOS inhibitors yields a complex pattern of NOS inhibition and that data obtained on brain physiology following the i.c.v. administration of NOS inhibitors, or for that matter any other CNS effector, should therefore be interpreted with extreme caution.

alpha-Methyldopa metabolism in central serotonergic nerve terminals: effects on serotonin levels, synthesis and release

The direct effects of in vivo methyldopa administration on serotonin (5-HT) neurochemistry was investigated. Specifically the ability of methyldopa to alter nerve terminal-associated 5-HT synthesis, storage and release and the possibility that 5-HT nerve terminals accumulate methyldopamine (the product of decarboxylation of methyldopa) was investigated. Synaptosomes isolated from rats given 200 mg/kg of methyldopa (calculated as the free amino acid) 2 h prior to killing exhibited a 25% reduction in intrasynaptosomal 5-HT and a 15% reduction in 5-HT synthesis when compared to synaptosomes from saline-treated animals. In addition a 15% reduction in synaptosomal tryptophan levels was observed. Despite these changes there was no apparent decrease in basal or depolarization-induced 5-HT release from synaptosomes of methyldopa-treated rats. The presence of methyldopamine within 5-HT-containing synaptosomes was confirmed by demonstrating that p-chloroamphetamine, a selective 5-HT releasing agent, could release both methyldopamine and 5-HT from synaptosomes and that this release could be selectively antagonised by fluoxetine, a selective 5-HT uptake inhibitor. The significance of these data with respect to the involvement of 5-HT neurons in the hypotensive action of methyldopa is discussed.

Circadian variation in the uptake of tryptophan by cortical synaptosomes of the rat brain

The kinetics of the high affinity uptake system for L-tryptophan (L-Try) have been measured over 24 hr in cortical synaptosome preparations of rat brain. Both the Km and Vmax of the uptake process showed a statistically significant 24 hr variation. The highest Km value, 6.71 X 10(-5) M, was measured at the beginning of the light phase and the lowest value, 4.23 X 10(-5) M, 6 hr into the dark phase. Vmax was highest at the end of the dark phase (10.43 nmol/mg/5 min) and lowest (4.80 nmol/mg/5 min) 3 hr into the dark phase. In contrast, there was no variation over 24 hr in the Vmax/Km ratio. These results suggest that the high affinity uptake process serves to ensure a constant rate of L-tryptophan entry into the neuron in the face of circadian or ultradian variations in extracellular concentration of tryptophan.

Synthesis and high affinity uptake of serotonin and dopamine by human Y79 retinoblastoma cells

Human Y79 retinoblastoma cells are capable of synthesizing the putative retinal neurotransmitters dopamine and serotonin. Separation of the catecholamines and indolamines by high performance liquid chromatography combined with electrochemical detection showed that the cells readily convert tyrosine to 3,4-dihydroxyphenylalanine (DOPA) and, to a lesser extent, dopamine. When DOPA was added, a large quantity of dopamine was produced, as well as norepinephrine, epinephrine, and 3,4-dihydroxyphenylacetic acid. Exogenous tryptophan added to the cells was partially converted to 5-hydroxytryptophan and serotonin. A larger quantity of serotonin was produced when 5-hydroxytryptophan was added. Y79 cells have a high- and low-affinity uptake system for dopamine and serotonin. The K'm and V'max for the high-affinity uptake of dopamine and serotonin are 2.34 +/- 0.64 and 3.63 +/- 1.15 microM and 4.77 +/- 1.12 and 3.20 +/- 1.20 pmol min-1 mg protein-1, respectively. These kinetic parameters are similar to those reported for other retinal preparations where dopamine and serotonin have been suggested to function as neurotransmitters. Tyrosine and tryptophan, the physiologic precursors of dopamine and serotonin, respectively, and phenylalanine are also taken up by high- and low-affinity transport systems. The kinetic parameters for their high-affinity uptake systems are all very similar, suggesting that they may be taken up by the same transporter. These studies show that a tumor cell line derived from the human retina synthesizes dopamine and serotonin and has high-affinity uptake systems for these compounds and their precursors.(ABSTRACT TRUNCATED AT 250 WORDS)

Transport of the aromatic amino acids into isolated rat liver cells. Properties of uptake by two distinct systems

The transport of the aromatic amino acids into isolated rat liver cells was studied. There was a rapid and substantial binding of the aromatic amino acids, L-alanine and L-leucine to the plasma membrane. This has important consequences for the determination of rates of transport and intracellular concentrations of the amino acids. Inhibition studies with a variety of substrates of various transport systems gave results consistent with aromatic amino acid transport being catalysed by two systems: a 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid (BCH)-insensitive aromatic D- and L-amino acid-specific system, and the L-type system (BCH-sensitive). The BCH-insensitive component of transport was Na+-independent and facilitated non-concentrative transport of the aromatic amino acids; it was unaffected by culture of liver cells for 24 h, by 48 h starvation, dexamethasone phosphate or glucagon. Kinetic properties of the BCH-inhibitable component were similar to those previously reported for the L2-system in liver cells. The BCH-insensitive component was a comparatively low-Km low-Vmax. transport system that we suggest is similar to the T-transport system previously seen only in human red blood cells. The results are discussed with reference to the importance of the T- and L-systems in the control of aromatic L-amino acid degradation in the liver.

Uptake and release of tryptophan and serotonin: an HPLC method to study the flux of endogenous 5-hydroxyindoles through synaptosomes

An HPLC assay with fluorometric detection has been developed that is sensitive enough to measure simultaneously endogenous levels of tryptophan, serotonin (5-hydroxytryptamine, or 5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) inside synaptosomes as well as that released into the incubation medium. Using this assay, we have observed that tryptophan is rapidly taken up by synaptosomes and turned over to 5-HIAA without a concurrent release of 5-HT. Exogenous 5-HT is also rapidly taken up, and, within 20-30 min, 80% of the 5-HT is deaminated. Veratridine induces release of both tryptophan and 5-HT from synaptosomes. Changes in the disposition of exogenous tryptophan or 5-HT can be completely accounted for by uptake or by stoichiometric changes in metabolites. This assay method should be valuable in the study of 5-HT pools and in the determination of from which pool 5-HT release occurs.

Inhibition of monoamine oxidase selectively in brain monoamine nerves using the bioprecursor (E)-beta-fluoromethylene-m-tyrosine (MDL 72394), a substrate for aromatic L-amino acid decarboxylase

(E)-beta-Fluoromethylene-m-tyrosine (FMMT) is a dual-enzyme-activated inhibitor of monoamine oxidase (MAO). The compound is not an inhibitor per se but is decarboxylated by aromatic L-amino acid decarboxylase (AADC) to yield a potent enzyme-activated irreversible inhibitor of MAO, (E)-beta-fluoromethylene-m-tyramine, which shows some selectivity for inhibition of MAO type A. Decarboxylation of FMMT was demonstrated in vitro using hog kidney AADC and in vivo in rats by the ability of alpha-monofluoromethyldopa (MFMD), a potent inhibitor of AADC, to prevent MAO inhibition produced by FMMT. In isolated synaptosomes, FMMT was decarboxylated by AADC, and, furthermore, the compound was actively transported into these isolated nerve endings. An active transport into the CNS has also been demonstrated in vivo by performing competition experiments with leucine. To demonstrate that FMMT is preferentially decarboxylated within monoamine nerves of the CNS, the nigrostriatal 3,4-dihydroxyphenylethylamine (dopamine) pathway of rats was unilaterally lesioned with 6-hydroxydopamine or infused with MFMD. Under these conditions, MAO inhibition produced by orally administered FMMT in the striatum ipsilateral to the lesion or infusion was markedly attenuated. Combination of FMMT with an inhibitor of extracerebral AADC, such as carbidopa, protected peripheral organs against the MAO inhibitory effects and concomitantly enhanced MAO inhibition in the CNS. Such combinations had a greatly reduced propensity to augment the cardiovascular effects of intraduodenally administered tyramine, when compared with FMMT given alone or with clorgyline, a selective inhibitor of MAO type A. The results obtained with FMMT suggest the possibility of achieving selective inhibition of MAO within monoamine nerves of the CNS and, further, suggest that combination of FMMT with an inhibitor of extracerebral AADC will reduce the propensity of this inhibitor to produce adverse interactions with tyramine.

Effects of conditioned running on plasma, liver and brain tryptophan and on brain 5-hydroxytryptamine metabolism of the rat

An investigation was made into the effects of conditioned running (1 h and 2 h at 20 m min-1), which accelerates lipolysis, on the concentrations of tryptophan (Trp) in plasma, liver and brain and on 5-hydroxytrptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels in brain. Running caused time-dependent increases in plasma free Trp and brain Trp of the rat, leading to increased brain 5-HT turnover as revealed by higher amounts of its metabolite, 5-HIAA. The ratio of brain Trp to plasma free Trp was decreased after 2 h of running. Liver Trp content rose only after 3 h of running, while liver unesterified fatty acid (UFA) concentrations remained unmodified. A comparison between food deprivation and running (both of which promote lipolysis) was performed. Running for 2 h affected to the same extent plasma Trp disposition when compared with 24 h food deprivation. Nevertheless, the ratio of brain Trp to plasma free Trp was decreased in the food-deprived rats, when compared to the runners. Nicotinic acid, which inhibits fat catabolism, completely abolished the plasma UFA increase induced by 1 h of running. The drug did not affect plasma free Trp, brain Trp, 5-HT or 5-HIAA but enhanced plasma total Trp level. Naloxone, an opiate antagonist, which decreased running-induced lipolysis, did not alter plasma Trp disposition. Desipramine, an antidepressant compound, affected only peripheral Trp concentrations of the runners. Plasma free and total Trp concentrations were increased in desipramine-treated runners, compared with saline-treated runners. In addition, desipramine increased the ratio of brain Trp to plasma free Trp of the runners. Brain 5-HT and 5-HIAA were increased in both desipramine-treated controls and runners. 9 The results suggest that running, which like food deprivatiQn accelerates lipolysis, increases brain Trp content and then 5-HT turnover. Comparison of these two physiological situations suggests that effectiveness of brain Trp entry is much more altered by fasting.

The role of tryptophan 2,3-dioxygenase in the hormonal control of tryptophan metabolism in isolated rat liver cells. Effects of glucocorticoids and experimental diabetes

The metabolism of L-tryptophan by isolated liver cells prepared from control, adrenalectomized, glucocorticoid-treated, acute-diabetic, chronic-diabetic and insulin-treated chronic-diabetic rats was studied. Liver cells from adrenalectomized rats metabolized tryptophan at rates comparable with the minimum diurnal rates of controls, but different from rates determined for cells from control rats 4h later. Administration of dexamethasone phosphate increased the activity of tryptophan 2,3-dioxygenase (EC 1.13.11.11) 7-8-fold, and the flux through the kynurenine pathway 3-4-fold, in cells from both control and adrenalectomized rats. Increases in flux through kynureninase (EC 3.7.1.3) and to acetyl-CoA can be explained in terms of increased substrate supply from tryptophan 2,3-dioxygenase. The metabolism of tryptophan was increased 3-fold in liver cells isolated from acutely (3 days) diabetic rats, with a 7-8-fold increase in the maximal activity of tryptophan 2,3-dioxygenase. The oxidation of tryptophan to CO2 and metabolites of the glutarate pathway increased 4-5-fold, consistent with an increase in picolinate carboxylase (EC 4.1.1.45) activity. Liver cells isolated from chronic (10 days) diabetic rats metabolized tryptophan at rates comparable with those of cells from acutely diabetic rats, but with a 50% decrease in the activity of tryptophan 2,3-dioxygenase. The proportion of flux from tryptophan 2,3-dioxygenase to acetyl-CoA, however, was increased by 50%; this was indicative of further increases in the activity of picolinate carboxylase. Administration of insulin partially reversed the effects of chronic diabetes on the activity of tryptophan 2,3-dioxygenase and flux through the kynurenine pathway, but had no effect on the increased activity of picolinate carboxylase. The role of tryptophan 2,3-dioxygenase in regulating the blood tryptophan concentration is discussed with reference to its sensitivity to the above conditions.

Tryptophan uptake and hydroxylation in rat forebrain synaptosomes

Tryptophan uptake, hydroxylation, and decarboxylation in isolated synaptosomes were studied to assess how their properties may determine the rate of serotonin synthesis in the presynaptic nerve terminals of the brain. Simultaneous measurements of the rates of uptake, hydroxylation, and decarboxylation in the presence and absence of various inhibitors showed that tryptophan hydroxylase is rate-limiting for serotonin synthesis in this model system. There was significant direct decarboxylation of tryptophan to tryptamine. Measurement of tryptophan hydroxylase flux with varying internal concentrations of tryptophan allowed the determination of the Km of tryptophan hydroxylase in synaptosomes for tryptophan of 120 +/- 15 microM. Depolarisation of synaptosomes with veratridine caused both a reduction in the internal tryptophan concentration and an apparent activation of tryptophan hydroxylase. This activation did not occur in the absence of Ca2+ or in the presence of trifluoperazine. Synaptosomal serotonin synthesis and brain stem-soluble tryptophan hydroxylase were inhibited by low concentrations of noradrenaline or dopamine. Dibutyryl cyclic AMP, glucagon, insulin, and vasopressin were observed to have no effect on tryptophan uptake or hydroxylation in synaptosomes.