Effects of precursors on brain neurotransmitter synthesis and brain functions

This paper reviews factors which influence the levels of aminergic transmitters in the brain. In particular precursor availability to the brain influences the rates of synthesis of serotonin, the catecholamines, and acetylcholine by brain neurons. The diet readily influences brain neurotransmitter formation via this mechanism. At present, the importance of this relationship to body regulation is not well understood. Nonetheless, precursors (tryptophan, tyrosine, choline, and lecithin) have begun to find uses as therapeutic agents in the treatment of disease states involving diminished transmitter formation and release. Hopefully, these compounds will find a wide range of uses, as they lack many of the side effects that accompany the use of drugs.     

Acute tyrosine depletion and alcohol ingestion in healthy women

BACKGROUND: Recently we reported that, in vervet monkeys, ingestion of an amino acid mixture deficient in the catecholamine precursors, phenylalanine and tyrosine, produced a decrease in alcohol self-administration. We now report the results of a similar study in humans. METHODS: Three groups of healthy female social drinkers were administered a nutritionally balanced amino acid mixture (B, n = 13), a mixture deficient in the serotonin precursor, tryptophan (Trp-free, n = 14), or a phenylalanine/tyrosine deficient mixture (Phe/Tyr-free, n = 12). Six hours after administration of the amino acid mixture, alcohol ingestion was measured during a free-choice "Taste Test." RESULTS: Compared to the B mixture, Phe/Tyr-free, but not Trp-free, significantly decreased the ingestion of alcohol [p < 0.02]. Neither Phe/Tyr-free nor Trp-free significantly decreased orange juice ingestion or the self-reported "Liking" of either substance. Some subjects experienced transient nausea and/or regurgitated the amino acid mixtures, but excluding these subjects did not change the results. CONCLUSIONS: The results suggest that (a) Phe/Tyr-free may be a suitable method for investigating the role of catecholamines in the self-administration and subjective effects of alcohol, (b) acutely decreased catecholamine neurotransmission might disrupt mechanisms mediating alcohol self-administration, and (c) acutely decreased serotonin neurotransmission seems not to alter alcohol self-administration.     

Treatment of brain disease with dietary precursors of neurotransmitters.

A growing number of brain diseases are characterized by decreased levels of one or more of the neurotransmitters. Recent experimental evidence indicates that nutritional factors strongly influence the regulation of two of these neurotransmitters, serotonin and acetylcholine. As a result, attempts are now being made to treat diseases associated with low levels of serotonin or acetylcholine by administering their dietary precursors, tryptophan and choline, respectively. This treatment may increase the amount of the deficient neurotransmitter at synapses and produce clinical benefit. Such efforts to elevate brain neurotransmitter levels with a naturally occurring precursor represent a new approach in medical therapeutics and will probably continue. We review the scientific basis for such treatment and show that brain levels of serotonin and acetylcholine depend upon the amounts of tryptophan and choline available to the brain; these, in turn, fluctuate according to dietary factors.     

Pharmacologic inhibition of L-tyrosine degradation ameliorates cerebral dopamine deficiency in murine phenylketonuria (PKU)

Monoamine neurotransmitter deficiency has been implicated in the etiology of neuropsychiatric symptoms associated with chronic hyperphenylalaninemia in phenylketonuria (PKU). Two proposed explanations for neurotransmitter deficiency in PKU include first, that chronically elevated blood L-phenylalanine (Phe) inhibits the transport of L-tyrosine (Tyr) and L-tryptophan (Trp), the substrates for dopamine and serotonin synthesis respectively, into brain. In the second hypothesis, elevated Phe competitively inhibits brain tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH) activities, the rate limiting steps in dopamine and serotonin synthesis. Dietary supplementation with large neutral amino acids (LNAA) including Tyr and Trp has been recommended for individuals with chronically elevated blood Phe in an attempt to restore amino acid and monoamine homeostasis in brain. As a potential alternative treatment approach, we demonstrate that pharmacologic inhibition of Tyr degradation through oral administration of nitisinone (NTBC) yielded sustained increases in blood and brain Tyr, decreased blood and brain Phe, and consequently increased dopamine synthesis in a murine model of PKU. Our results suggest that Phe-mediated inhibition of TH activity is the likely mechanism of impaired dopamine synthesis in PKU. Pharmacologic inhibition of Tyr degradation may be a promising adjunct therapy for CNS monoamine neurotransmitter deficiency in hyperphenylalaninemic individuals with PKU.     

Meal composition and plasma amino acid ratios: effect of various proteins or carbohydrates, and of various protein concentrations

We examined the effects of meals containing various proteins and carbohydrates, and of those containing various proportions of protein (0% to 20% of a meal, by weight) or of carbohydrate (0% to 75%), on plasma levels of certain large neutral amino acids (LNAA) in rats previously fasted for 19 hours. We also calculated the plasma tryptophan ratios (the ratio of the plasma tryptophan concentration to the summed concentrations of the other large neutral amino acids) and other plasma amino acid ratios. (The plasma tryptophan ratio has been shown to determine brain tryptophan levels and, thereby, to affect the synthesis and release of the neurotransmitter serotonin). A meal containing 70% to 75% of an insulin-secreting carbohydrate (dextrose or dextrin) increased plasma insulin levels and the tryptophan ratio; those containing 0% or 25% carbohydrate failed to do so. Addition of as little as 5% casein to a 70% carbohydrate meal fully blocked the increase in the plasma tryptophan ratio without affecting the secretion of insulin--probably by contributing much larger quantities of the other LNAA than of tryptophan to the blood. Dietary proteins differed in their ability to suppress the carbohydrate-induced rise in the plasma tryptophan ratio. Addition of 10% casein, peanut meal, or gelatin fully blocked this increase, but lactalbumin failed to do so, and egg white did so only partially. (Consumption of the 10% gelatin meal also produced a major reduction in the plasma tyrosine ratio, and may thereby have affected brain tyrosine levels and catecholamine synthesis).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of tyrosine and tryptophan ingestion on plasma catecholamine and 3,4-dihydroxyphenylacetic acid concentrations

The effect of oral tyrosine or tryptophan ingestion on plasma concentrations of norepinephrine (NE), dopamine (DA), epinephrine (EPI), and 3,4-dihydroxyphenylacetic acid (DOPAC) was investigated in a double blind, placebo-controlled study in fasted men. Tyrosine ingestion induced within 45 min a significant but short-lasting (approximately 30 min) increase in plasma concentrations of NE, EPI, and DA and a coincident decrease in plasma DOPAC levels. Ingestion of tryptophan or lactose placebo did not after plasma DA, EPI, NE, or DOPAC levels. Since plasma catecholamines derive from peripheral sources, while circulating DOPAC may reflect both brain and peripheral DA turnover, these results suggest that the oral ingestion of tyrosine can exert acute effects on catecholamine systems within and outside the brain.     

Plasma precursor amino acids of central nervous system monoamines in children with coeliac disease.

Some children with coeliac disease show behavioural disorders such as depression and other signs which have been correlated with reduced central monoamine metabolism. We have therefore investigated the brain availability of the monoamine precursors tryptophan and tyrosine in 15 untreated children with coeliac disease and 12 treated children with coeliac disease as well as in 12 control children. Significantly decreased plasma concentrations of tryptophan were found in untreated children (mean (SD) 13 (4) mumols/l, p less than 0.001) compared with treated children (31 (13) mumols/l), and in both groups of coeliac children when compared with control children (81 (22) mumols/l). A significantly lower ratio of plasma tryptophan to large neutral amino acids (tyrosine, valine, isoleucine, leucine, and phenylalanine) was also observed, which could indicate impaired brain availability of tryptophan in coeliac children and was more pronounced in untreated children. The impaired availability of tryptophan could produce decreased central serotonin synthesis and in turn behaviour disorders in children with coeliac disease.     

Intrasomatic injection of radioactive precursors for studying transmitter synthesis in identified neurons of Aplysia californica

We introduced radioactive precursors directly into identified neurons of Aplysia californica. [(3)H]-Choline and L-[(3)H]tryptophan were injected with pressure into nerve cell bodies to study synthesis of acetylcholine and serotonin. We confirmed the cholinergic nature of R2, L10, and L11, identified neurons of the abdominal ganglion. Cells in the LD cluster (which contains motor neurons to the heart and gill) also converted most of the injected choline into acetylcholine. Neurons in the RB cluster (which contains an excitatory motor neuron to the heart) and the two metacerebral cells of the cerebral ganglion converted injected tryptophan to serotonin. No cell studied could convert both choline to acetylcholine and tryptophan to serotonin. Pressure permits rapid injection of precursors, from small amounts to amounts large enough to saturate intracellular synthetic pathways. In contrast to the results with injection, we found far less synthesis of acetylcholine and serotonin in identified nerve cell bodies when ganglia were incubated in the presence of the radioactive precursors.     

Influence of experimental diabetes on brain levels of monoamine neurotransmitters and their precursor amino acids during tryptophan loading

Summary The variations in serum and brain concentrations of the large neutral amino acids and the simultaneous changes in brain levels of monoamine neurotransmitters have been studied in normal and streptozotocin-diabetic rats after tryptophan loading. An impaired acute accumulation of tryptophan and serotonin in the brain of diabetic ras was observed, concomitantly with a much faster disappearance of the administered tryptophan from the bloodstream in these animals than in controls. Following the tryptophan load, transient differences in the brain levels of catecholamine neurotransmitters became also apparent between the two groups of rats in correlation with differences in the brain uptake and levels of tyrosine. In diabetic animals, the basal brain concentrations of serotonin and dopamine were normal and those of norepinephrine were increased. Since uptake of the precursors tryptophan and tyrosine from the blood is chronically reduced, it is likely that long-term adjustments of neurotransmitter metabolism occur in the diabetic brain. Supported by grants n ^o 83.02727.56 and n ^o 84.02413.56 of theConsiglio Nazionale delle Ricerche, Rome (Progetto Finalizzato ‘Medicina Preventiva e Riabilitativa’, Sottoprogetto ‘Malattie Degenerative’, Linea ‘Complicanze del Diabete’).     

Effect of various oral glucose doses on plasma neutral amino acid levels

Six healthy, nonobese, fasting subjects each received, on different days 0, 6, 12.5, 25, or 50 g of glucose (Glucola) in a total volume of 100 ml. Blood was taken at intervals and assayed for plasma levels of the branched-chain amino acids (valine, isoleucine and leucine); the other major large neutral amino acids (LNAA) (methionine, phenylalanine, tyrosine and tryptophan); and, in some cases, insulin and glucose. Insulin levels were significantly elevated 30 min after consumption of 12.5, 25, or 50 g of glucose, and were higher after the 50 g dose than after 12.5 g. Changes in plasma glucose concentrations were small and did not correlate with glucose dose. Mean percent reductions of LNAA tended to exhibit dose-dependence, most clearly observed after 120 min. In some subjects as little as 6 g of glucose transiently decreased LNAA concentrations. Branched-chain amino acids were most sensitive, decreasing by 35%-41% after 50 g of glucose. Plasma tryptophan concentrations fell only by 23%, hence the ratio of plasma tryptophan to other plasma LNAA (which affects brain serotonin synthesis) increased significantly.     

Regional brain monoamines and their metabolites after portacaval shunting

Disturbances in brain monoamine neurotransmitter metabolism have been implicated in the development of hepatic encephalopathy produced by portacaval shunting or liver disease. We have measured the content of serotonin, norepinephrine and dopamine, as well as their metabolites 5-hydroxyindoleacetic acid, dihydroxyphenylacetic acid and homovanillic acid in nine selected brain areas of rats with portacaval shunts and sham-operated control rats. All substances were measured in single samples using high performance liquid chromatography with electrochemical detection, after a simple extraction procedure. In shunted rats serotonin content was 26% higher in the raphe nuclei area, and 5-hydroxyindoleacetic acid throughout the brain (by 51 to 137%), suggesting increased serotonin turnover. Norepinephrine content was higher by 26% in the frontal cortex. Dopamine content was unaffected; however its metabolites were higher in a few areas including the caudate and ventral tegmentum. Brain content of the monoamine precursor amino acids tryptophan, tyrosine and phenylalanine was higher throughout the brain in the shunted rats. The results suggest that serotonin metabolism is altered throughout the brain after portacaval shunting, which could be related to some of the characteristic behavioral abnormalities found in this condition. Catecholamine metabolism appears to be more selectively and less extensively affected.     

Relations between dietary choline or lecithin intake, serum choline levels, and various metabolic indices

Choline administration increases blood choline, brain choline, and brain acetylcholine levels in rats. It also increases blood choline levels in humans and appears to be a useful treatment for some patients with tardive dyskinesia, a brain disease probably associated with deficient cholinergic tone. In order to characterize other possible metabolic and hormonal effects of choline-containing compounds, we measured changes in serum choline, glucose, insulin, cortisol, prolactin, cholesterol, and triglyceride levels resulting from ingestion of low- or high-choline meals in 16 normal human subjects. After the consumption of a single meal containing 3 g choline chloride, serum choline rose by 86% (p < 0.01), attaining peak values after 30 min. When the same subjects ate a meal containing an equivalent amount of choline in the form of lecithin, serum choline levels rose by 33% after 30 min, and continued to rise for at least 12 hr, to 265% over control values (p < 0.001). Serum choline concentrations were related to the amount of choline in the diet: they did not vary significantly during 24-hr periods when the subjects consumed a low-choline diet for two consecutive days, but rose substantially (p < 0.01) after each high-choline meal. Serum glucose, insulin, cortisol, and prolactin levels were not significantly modified by choline or lecithin ingestion. Lecithin consumption increased serum triglyceride levels and lowered serum cholesterol concentration.     

Reduction in brain serotonin synthesis rate in streptozotocin-diabetic rats

The rate of serotonin synthesis in brain was determined in streptozotocin-diabetic and normal rats using two methods. Both the rate of 5-hydroxytryptophan accumulation after aromatic amino acid decarboxylase inhibition, and the decline rate of 5-hydroxyindole acetic acid after pargyline treatment were significantly reduced in diabetic rats. The reduced rate of synthesis may be a direct result of significantly lowered brain tryptophan levels in diabetic rats.     

Synapse formation and cognitive brain development: effect of docosahexaenoic acid and other dietary constituents

The brain is unusual among organs in that the rates of many of its characteristic enzymatic reactions are controlled by the local concentrations of their substrates, which also happen to be nutrients that cross the blood-brain barrier. Thus, for example, brain levels of tryptophan, tyrosine, or choline can control the rates at which neurons synthesize serotonin, dopamine, or acetylcholine, respectively. The rates at which brain cells produce membrane phospholipids such as phosphatidylcholine (PC) are also under such control, both in adult animals and, especially, during early development. If pregnant rats are fed the 3 dietary constituents needed for PC synthesis- docosahexaenoic acid, uridine, and choline-starting 10 days before parturition and continuing for 20 days during nursing, brain levels of PC, and of the other membrane phosphatides (per cell or per mg protein), are increased by 50% or more. In adult animals, this treatment is also known to increase synaptic proteins (eg, synapsin-l, syntaxin-3, GluR-l, PSD-95) but not ubiquitous proteins like beta-tubulin and to increase (by 30% or more) the number of dendritic spines on hippocampal neurons. Docosahexaenoic acid currently is widely used, in human infants, to diminish the negative effects of prematurity on cognitive development. Moreover, docosahexaenoic acid, uridine (as uridine monophosphate), and choline are all found in mother's milk, and included in most infant formulas. It is proposed that these substances are part of a regulatory mechanism through which plasma composition influences brain development.     

Selective Alterations of Cerebrospinal Fluid Amino Acids in Dogs with Congenital Portosystemic Shunts

Numerous studies suggest that modifications in concentrations of both excitatory and inhibitory amino acids are implicated in the pathophysiology of portal-systemic encephalopathy (PSE), a neuropsychiatric disorder associated with chronic liver disease in humans. In this study, amino acid levels were measured by High Performance Liquid Chromatography (HPLC) in Cerebrospinal Fluid (CSF) of 10 dogs (age range: 3 mo.- 3 yr 4 mo.) exhibiting a congenital portal-systemic shunt, either intra or extra-hepatic, and 8 age-matched control dogs who showed no signs of hepatic or neurologic disorders. Dogs with congenital shunts manifested signs of encephalopathy such as disorientation, head pressing, vocalization, depression, seizures and coma. CSF from dogs with congenital shunts contained significantly increased amounts of glutamate (2 to 3-fold increase, p<0.01), glutamine (6-fold increase, p<0.05) and aromatic amino acids (phenylalanine, tyrosine and tryptophan) compared to CSF of control dogs. Concentrations of GABA and branched chain amino acids (valine, leucine, isoleucine) were within normal limits. Modifications of brain glutamate (an excitatory amino acid) as well as tryptophan (the precursor of serotonin) could contribute to the neurological syndrome characteristic of congenital PSE in dogs.     

Trans-synaptic induction of adrenomedullary tyrosine hydroxylase activity by choline: evidence that choline administration can increase cholinergic transmission.

Twenty-four hours after rats receive choline chloride (20 mmol/kg, by stomach tube) the activity of tyrosine hydroxylase [tyrosine 3-monooxygenase; L-tyrosine, tetrahydropteridine:oxygen oxidoreductase (3-hydroxylating), EC 1.14.16.2] increases by 31% within adrenomedullary chromaffin cells. This treatment also causes major elevations in the levels of choline and acetylcholine within the adrenal gland; however, acetylcholine levels return to normal by 16 hr after the choline is given. The daily administration of 10 or 20 mmol/kg of choline for 4 days elevates adrenal tyrosine hydroxylase activity by 29% or 51%, respectively. Such increases in tyrosine hydroxylase activity are not observed in animals given ammonium chloride, another basic chloride-containing compound, by stomach tube or in animals treated with cycloheximide, an inhibitor of adrenal protein synthesis. They are also absent in denervated adrenals. These observations demonstrate that the increase in presynaptic acetylcholine levels produced by giving animals the neurotransmitter's precursor (choline) can be associated with parallel changes in the transmission of signals across cholinergic synapses, probably because more of the transmitter is released per nerve impulse.     

THE EFFECTS OF RESERPINE, α-METHYLTYROSINE, AND L-3,4-DIHYDROXYPHENYLALANINE ON BRAIN TYROSINE TRANSAMINASE

The effects of a variety of pharmacological and physiological manipulations on the activity of hepatic tyrosine transminase have been extensively investigated. Brain tyrosine transaminase, however, has received only limited attention. Since tyrosine transaminase may be important in the regulation of catecholamine biosynthesis in the brain, the effects of agents affecting catecholamine storage and synthesis on brain tyrosine transamination were investigated. Transamination in the 12,000 x g fraction was measured by a radioactive procedure. alpha-Methyltyrosine and reserpine, agents which deplete brain catecholamines, decreased tyrosine transaminase activity. Administration of the catecholamine precursor, L-3,4-dihydroxyphenylalanine to the reserpine-and alpha-methyltyrosine-treated rats elevated the tyrosine transaminase activity to normal. The possible implications of these findings in the regulation of the biosynthesis of the catecholamines are discussed.     

The control of 5-hydroxytryptamine and dopamine synthesis in the brain: A theoretical approach

Summary The transport of the eight amino acids (phenylalanine, tyrosine, tryptophan, valine, leucine, isoleucine, histidine and methionine) using the large neutral amino acid transporter of the blood-brain barrier (BBB) has been calculated using published kinetic data. The fate of the amino acids has been followed from blood to interstitial space, to cell and through metabolism which included, for tyrosine and tryptophan, the hydroxylases. The system was analysed in terms of flux control coefficients. Since the summation theorem did not hold, the system clearly behaved as a non-homogeneous system. At physiological levels of these eight amino acids, the largest contribution to the control of the flux of tyrosine is given by the hydroxylase step, followed by the diffusional component of the transport across the BBB. For tryptophan it is the hydroxylase step, followed by the carrier-mediated transport across the BBB. For the other amino acids it is the metabolism, followed by the diffusional component of the BBB transport.These parameters for tyrosine and tryptophan were determined at increased levels of blood phenylalanine, tyrosine or histidine. The flux through tryptophan hydroxylase can be affected by high blood levels of tyrosine and histidine to values also observed in hyperphenylalaninaemia. Since hypertyrosinaemia (type II) and hyperhistidinaemia are not associated with mental retardation, it is concluded that interference with transport across the BBB of tyrosine and tryptophan, as well as the flux through tryptophan hydroxylase leading to the synthesis of 5-hydroxytryptamine, do not contribute to the cause of permanent brain dysfunction in hyperphenylalaninaemia.It can be calculated that addition of tyrosine to the diet to raise the blood tyrosine level in phenylketonuria patients may have a beneficial effect for the synthesis of neurotransmitters derived from tyrosine.     

The effect of plasma valine,isoleucine and leucine on the control of the flux through tyrosine- and tryptophan-hydroxylase in the brain

Summary The effect of increasing blood levels of valine, isoleucine and leucine on the fluxes through tyrosine and tryptophan hydroxylase in brain has been calculated and analysed in terms of flux control coefficients. It is concluded that any beneficial effect of increasing the concentration of these amino acids in phenylketonuria patients is not due to a decrease in brain phenylalanine or an improved neurotransmitter synthesis through tyrosine or tryptophan hydroxylase.     

Tryptophan administration induces oxidative stress in brain cortex of rats

Despite the significant brain abnormalities, the neurotoxic mechanisms of brain injury in hypertryptophanemia are virtually unknown. In this work, we determined the thiobarbituric acid-reactive substances, 2',7'-dihydrodichlorofluorescein oxidation, reduced glutathione and the activities of catalase, superoxide dismutase and glutathione peroxidase in cerebral cortex from rats loaded with L-tryptophan. High L-tryptophan concentrations, similar to those found in hypertryptophanemic patients were induced by three subcutaneous injections of saline-buffered tryptophan (2 micromol/g body weight) to 30-day-old Wistar rats. The parameters were assessed 1 h after the last injection. It was observed that tryptophan significantly increased thiobarbituric acid-reactive substances, 2',7'-dihydrodichlorofluorescein oxidation and reduced glutathione, whereas it reduced catalase activity. Pre-treatment with taurine (1.6 micromol/g of body weight), or alpha-tocopherol plus ascorbic acid (40 and 100 microg/g body weight, respectively) prevented those effects of tryptophan, reinforcing the hypothesis that tryptophan induces oxidative stress in brain cortex of the rats. Therefore, these findings also occur in human hypertryptophanemia or in other neurodegenerative diseases in which tryptophan accumulates, then oxidative stress may be involved in the mechanisms leading to the brain injury observed in patients affected by these disorders.