Brain Tryptophan Concentrations Research Articles

The role of free serum tryptophan in the biphasic effect of acute ethanol administration on the concentrations of rat brain tryptophan, 5-hydroxytryptamine and 5-hydroxyindol-3-ylacetic acid.

1. Acute administration of ethanol exerts a biphasic effect on the concentrations of rat brain tryptophan, 5-hydroxytryptamine and 5-hydroxyindol-3-ylacetic acid. Both effects are associated with corresponding changes in the availability of circulating free tryptophan. 2. The initial increases in the above concentrations are prevented by ergotamine, are unaltered by allopurinol and are potentiated by theophylline, whereas the later decreases are prevented by both ergotamine and allopurinol. 3. It is suggested that the initial enhancement by ethanol of brain tryptophan metabolism is caused by catecholamine-mediated lipolysis followed by displacement of protein-bound serum tryptophan, whereas the activation of liver tryptophaan pyrrolase, which is produced by the same mechanism, leads to the later decreases in the brain concentrations of tryptophan and its metabolites. 4. The initial effects of ethanol can be reproduced by an equicaloric dose of sucrose, and a comparison of the two treatments alone could therefore be misleading. 5. The effects of ethanol on liver and brain tryptophan metabolism have also been examined in mice, and a comparison of the results with those previously reported suggests that the ethanol effects are strain-dependent.

Effects of altered brain 5-hydroxytryptaminergic activity on brain tryptophan, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid

The effects of procedures known to alter 5-hydroxytryptaminergic activity (raphe stimulation, raphe lesions and exposure to elevated temperature) on brain tryptophan, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid concentrations were investigated. Electrical stimulation of the dorsal raphe nucleus of unanaesthetised unrestrained rats for 1 hr, using implanted electrodes, significantly increased 5-hydroxyindoleacetic acid concentrations in the hypothalamus, striatum, midbrain and cortex but not in the hippocampus. The level of 5-hydroxytryptamine fell proportionately with the rise of 5-hydroxyindoleacetic acid in the midbrain but did not alter significantly in any other region. Tryptophan concentration was not significantly altered except in the hypothalamus where it increased moderately. Rats kept at 40°C for 60 min had significantly increased plasma and brain tryptophan and brain 5-hydroxyindoleacetic acid. Lysergic acid diethylamide prevented only the increase in 5-hydroxyindoleacetic acid. Electrolytic lesions in the dorsal and median raphe nuclei markedly lowered brain 5-hydroxytryptamine and 5-hydroxyindoleacetic acid but had no effect on brain or plasma tryptophan. Therefore, evidence is against a major role for brain tryptophan changes in the regulation of 5-hydroxytryptamine synthesis following alteration of 5-hydroxytryptaminergic activity. Other possible factors involved in the relationship of 5-hydroxytryptamine metabolism to neuronal activity are discussed.

Effect of dietary protein and calorie deficiency on tryptophan levels in the developing rat brain.

The pattern of development of brain tryptophan in the rat was studied in the progeny of mothers fed a 7.5% protein diet ad lib., a 20% protein diet ad lib. and those fed a 20% protein diet pair-fed with mothers who received the 7.5% protein. The pattern of development was similar in all three groups. Starting with a high brain tryptophan content at birth, all animals showed a progressive reduction during the next 3 weeks. However, tryptophan levels at birth were several fold higher in the brains of pups born to mothers receiving either the low protein diet fed ad lib. or those born to mothers who received the 20% protein diet in restricted amounts. From the 14th day after birth, tryptophan concentration of brain in undernourished pups was significantly lower until the 35th day. The implications of this finding are discussed.

Control of brain tryptophan concentration in rats on a high fat diet.

SYNTHESIS in the brain of the neurotransmitter 5-hydroxytryptamine (5-HT) is influenced by the concentration of its precursor tryptophan1,2. Much attention has therefore been paid to the determinants of brain tryptophan concentration. Evidence suggests that plasma free tryptophan is an important influence on brain tryptophan3,4 and that both concentrations increase when plasma unesterified fatty acids (UFA) rise3,5,6 and release plasma tryptophan from albumin7,8. Other results, however, disagree. Thus feeding rats for 2 h with high fat diets apparently led to large increases in serum UFA and free tryptophan but not brain tryptophan9,10,11. We now present evidence suggesting that the latter findings may have been due to in vitro lipolysis during either the preparation of serum or its dialysis at 37 °C for 3.5 h to separate free tryptophan9. Here we describe work in which precautions were taken to avoid in vitro changes and which is consistent with a relationship between plasma free tryptophan and brain tryptophan concentrations.

The relevance of both plasma free tryptophan and insulin to rat brain tryptophan concentration.

The relevance of both plasma free tryptophan and insulin to rat brain tryptophan concentration.

Inhibition by albumin of tryptophan uptake by rat brain.

CHANGES in the tryptophan content of the brain influence the rate of synthesis of brain 5-hydroxytryptamine (5-HT). Therefore, factors that affect the delivery of tryptophan to the brain may affect its 5-HT content. Tryptophan in plasma can be displaced from its binding site on albumin by non-esterified fatty acids (NEFA)1 and by certain drugs2. Although in some circumstances changes in non-albumin bound (NAB) plasma tryptophan, brought about by drugs and changes in plasma NEFA, parallel those of brain tryptophan2,3, this relationship does not apply in all conditions. For example, after administration of glucose plasma NEFA decline, as does NAB tryptophan, but the brain tryptophan content increases4. Thus, it is uncertain whether binding of tryptophan to albumin is important in controlling brain tryptophan and hence brain 5-HT. To resolve this controversy we looked at the influence of albumin on the uptake of tryptophan by brain in vivo. We found that albumin inhibits tryptophan uptake, and that diffusion, a process that will depend only on the NAB plasma tryptophan concentration, has a small but significant role in the uptake of tryptophan by brain.

In vivo and in vitro activation of soluble tryptophan hydroxylase from rat brainstem.

TRYPTOPHAN hydroxylation is the rate-limiting step in the biosynthesis of 5-hydroxytryptamine (5-HT) in central serotonergic neurones. In physiological states, tryptophan hydroxylase (EC 1.14.16.4), the key enzyme, is not saturated by its substrate1. Thus, three factors which modulate the concentration of tryptophan in brain—the levels of free tryptophan in plasma2,3, the ratio of total tryptophan levels to those of other neutral amino acids4, and the uptake process for tryptophan in brain tissues5,6—are important in the control of 5-HT synthesis. On the basis of indirect evidence7,8, it was suggested that tryptophan hydroxylase could also play a regulatory role. Among the mechanisms possibly involved, rapid changes in the enzyme affinity for its substrate and/or its cofactor may take place. Indeed, such changes are responsible for the activation of tyrosine hydroxylase seen in dopaminergic terminals after blockade of dopaminergic receptors by neuroleptics9. We used methiothepin(1-[10,11-dihydro-8-(methylthio)-dibenzo (b,f) thiepin-10-yl]-4-methylpiperazine maleate), a potent blocker of central serotonergic receptors10, to look for a similar activation of tryptophan hydroxylase. Our results indicate that the affinity of this enzyme for its substrate is markedly enhanced in methiothepin-treated tissue.

Quipazine: Its effects on rat brain 5-hydroxytryptamine metabolism, monoamine oxidase activity and behaviour

Quipazine (2-(l-piperazinyl) quinoline maleate) administration (25 mg/kg) to rats produced behavioural changes, including hyperactivity, identical to those seen following tranylcypromine and l-tryptophan. This suggests that quipazine is a 5-hydroxytryptamine agonist and this view is strengthened by the observation that when quipazine was given to rats pretreated with p-chlorophenylalanine. the hyperactivity was enhanced. α-Methyl- p-tyrosine pretreatment diminished the behavioural responses and previous experiments have suggested that dopamine is involved in the behavioural responses to increased 5-hydroxytryptamine receptor stimulation. Quipazine did not alter the concentration of brain tryptophan but caused a small rise of brain 5-hydroxytryptamine. Oxidation of 5-hydroxytryptamine and dopamine by monoamine oxidase (MAO) was partially inhibited during this period. Tryptamine oxidation, however, was unaffected. In vitro studies demonstrated that quipazine inhibited MAO activity reversibly and competitively. Dopamine and 5-hydroxytryptamine oxidation were more readily inhibited than tryptamine and kynuramine, which suggests that quipazine is predominantly a type A MAO inhibitor. Quipazine-induced hyperactivity was enhanced by administration of deprenil, a type B MAO inhibitor, but not by the type A MAO inhibitor clorgyline. Quipazine plus deprenil inhibited oxidation of both 5-hydroxytryptamine (a type A substrate) and phenylethylamine (a type B substrate). Since dopamine is metabolised by both forms of the enzyme, one interpretation of these results is that when MAO activity is inhibited in dopaminergic neurones the hyperactivity following 5-hydroxytryptamine receptor stimulation is enhanced. We conclude that quipazine is a 5-hydroxytryptamine agonist with weak reversible MAO inhibitory action.

Relationship between rat brain and cisternal CSF tryptophan concentrations.

Tryptophan was measured in the cisternal CSF and brains of rats. In untreated rats there was a significant but not very close correlation between the tryptophan concentration in these two compartments. Factors that change the brain tryptophan concentration such as starvation, glucose feeding, and lithium treatment affected the CSF tryptophan in the same way as the brain tryptophan. Diurnal changes were parallel for brain and CSF. When we take into account our knowledge of the disposition of tryptophan in human CSF, these data suggest that measurement of lumbar CSF tryptophan in man may be a useful approach to the study of human brain tryptophan. However, because the correlation between brain and CSF is not very close, measurements on CSF tryptophan would be more meaningful in groups of patients than in individuals.

Parallel variation of ventricular CSF tryptophan and free serum tryptophan in man.

Tryptophan was measured in the ventricular CSE and serum and the neutral amino acids leucine, isoleucine, valine, phenylalanine, and tyrosine were measured in the serum of two cases with ventricular drains. Samples were taken every two hours for 24 hours in one case and for 16 hours in the other. The CSF tryptophan was correlated significantly with the free--that is, non-albumin-bound--serum tryptophan but not with the total serum tryptophan. CSF tryptophan was not correlated significantly with the ratio of free serum tryptophan to the sum of the neutral amino acids. These data suggest that, in man, brain tryptophan concentrations are influenced by the free and not the total serum tryptophan and that physiological variations of the neutral amino acids do not appreciably influence the concentration of brain trytophan.

Thyroid state and brain monoamine metabolism.

The rates at which rat brain synthesizes catecholamines and serotonin were estimated by measuring the accumulation of DOPA and 5-hydroxytryptophan (5-HTP) 45 min after ip administration of the decarboxylase inhibitor RO4-4602 (800 mg/kg BW). Following thyroparathy-roidectomy, hypothyroid rats showed a decreased accumulation of both precursor amino acids. On the other hand, hyperthyroidism (caused by administering 15 mug T4/100 g BW for 25 days) accelerated the accumulation of catecholamines and serotonin. The accumulation of 5-HTP correlated closely with brain tryptophan concentration in all treatment groups; DOPA accumulation, however, did not similarly correspond to brain tyrosine levels.

Effects of Skim Milk, Whole Milk and Light Cream on Serum Tryptophan Binding and Brain Tryptophan Concentrations in Rats

One hour after rats fasted for 13 hours consume a single meal of skim milk, whole milk, or light cream, serum nonesterified fatty acid (NEFA) concentrations changed in direct proportion to the fat content of the diet: serum NEFA levels decreased in rats ingesting skim milk (0.09% fat) to 34% of fasting control values; in animals consuming whole milk (3.59% fat), levels dropped to 50% of fasting levels; these levels did not change significantly, however, in rats consuming light cream (18.26% fat). The percentages of total serum tryptophan not associated with albumin in rats ingesting skim milk, whole milk, or light cream were 22.2, 26.0, and 42.5%, respectively. These variations in serum free tryptophan were not accompanied by significant differences in brain tryptophan among the three treatment groups. These results thus confirm that, with a natural food source, (a) postprandial serum NEFA levels reflect the total fat content of the diet; (b) serum free tryptophan concentration shift proportionately with serum NEFA; and (c) serum free tryptophan concentrations do not reliably predict brain tryptophan levels.

Rapid repletion of brain serotonin in malnourished corn-fed rats following L-tryptophan injection

The concentration of tryptophan in serum, and the levels of tryptophan, serotonin (5-HT), and 5-hydroxyindole-acetic acid (5-HIAA) in brain are substantially reduced in rats that consume for 6 weeks a diet in which corn is the only source of protein. Single injections of L-tryptophan (25, 50, or 100 mg/kg) cause dose-related increases in brain tryptophan, 5-HT, and 5-HIAA in corn-fed animals. At each dose, brain tryptophan content rises to a proportionately greater extent in corn-fed rats than in well-nourished controls, even though serum tryptophan concentrations attain higher levels in controls. This difference may reflect the greatly reduced serum concentrations in corn-fed rats of other large neutral amino acids that compete with tryptophan for uptake into the brain (tyrosine, phenylalanine, leucine, isoleucine, and valine). However, the substantial decrease in serum albumin levels also diminishes the binding of tryptophan to serum albumin; thus it is not yet possible to state which of these changes is responsible for the much greater increments in brain tryptophan observed in corn-fed rats after tryptophan injection. The fact that tryptophan administration rapidly restores brain 5-hydroxyindole levels in corn-fed animals suggests that the reductions in 5-HT and 5-HIAA levels associated with this type of malnutrition may be largely caused by inadequate availability of substrate.

Tryptophan and tyrosine disposition and brain tryptophan metabolism in acute carbon tetrachloride poisoning

Forty-eight hours after injecting rats with carbon tetrachloride there were large increases of plasma non-esterifed fatty acid and free tryptophan concentrations. Total plasma tryptophan was not significantly altered. Brain, liver and muscle tryptophan concentrations increased significantly with percentage changes in the order: brain > liver > muscle > kidney. The increased brain tryptophan was associated with a small but significant increase of 5-hydroxytrptamine and a larger increase of its metabolite 5-hydroxyindolylacetic acid. Tyrosine concentrations increased significantly in plasma, brain, liver, muscle and kidney with percentage changes in the same order as those of tryptophan. Food intake was decreased and this correlated significantly with brain tryptophan and 5-hydroxyindolylacetic acid concentrations. Control rats kept for 48 hr on restricted intake did not show the above brain changes though plasma non-esterified fatty acid concentration was increased as in the drug treated rats. Results are discussed in relation to previous findings in acute liver failure and to the possible roles of brain tryptophan and 5-hydroxytryptamine in liver failure and appetite control.

Unchanged sensitivity to electric shock in L-tryptophan treated rats

Injections of L-tryptophan (50, 100 and 200 mg/kg) i.p. in rats resulted in dose dependent increases in brain concentrations of tryptophan, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid. Sensitivity to electric shock as measured by flinch and jump thresholds was not changed by L-tryptophan 100 and 200 mg/kg. The results indicate that increased 5-HT turnover does not influence sensitivity to electric shock.

Liver and brain tryptophan metabolism following hydrocortisone administration to rats and gerbils.

1 Liver tryptophan pyrrolase activity is low in the mongolian gerbil (Meriones unguiculatus) and is not induced by hydrocortisone (5 mg/kg). In contrast, there is measurable activity in the rat liver and this is induced by hydrocortisone. In vivo measurements confirmed the absence of induction in gerbils but suggested that they were able to metabolize tryptophan. However no detectable pyrrolase activity was found in any other tissues either before or after hydrocortisone. 2 In agreement with previous observations hydrocortisone decreased rat brain 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) 6 h after administration. Brain tryptophan concentrations were also decreased at this time. In contrast, hydrocortisone did not alter gerbil brain 5-HT, 5-HIAA or trytophan. alpha-Methyltryptophan activated hepatic tryptophan pyrrolase and decreased brain 5-HT and 5-HIAA in both animals. 3 Results suggest that the decrease in rat brain 5-HT and 5-HIAA following hydrocortisone may be associated with the rise in liver tryptophan pyrrolase and that the brain amine changes are mediated through the decrease in brain tryptophan concentration.

Relevance of free tryptophan in serum to tissue tryptophan concentrations

The consumption of a carbohydrate diet by fasted rats is followed by major decreases in serum nonesterified fatty acids (NEFA) and nonalbumin-bound tryptophan (unbound tryptophan), but by increases in serum total tryptophan and brain tryptophan; the tryptophan concentrations of liver and small intestine are unchanged, while that of skeletal muscle falls slightly. The addition of 15% or 30% fat to a protein-carbohydrate diet results in dose-related increases in serum NEFA and serum unbound tryptophan, but no significant changes in serum total tryptophan or brain tryptophan. The observation that diet-induced changes in serum unbound tryptophan does not correlate with brain tryptophan concentrations is independent of the method used to separate free from albumin-bound serum tryptophan. These studies confirm that, in the rat, a major physiologic regulator of the extent to which serum tryptophan binds to albumin is the concentration of NEFA in serum. These studies also provide additional evidence that the concentration of tryptophan in the brain is not necessarily determined by the size of the unbound pool of tryptophan in blood as measured in serum.

The role of brain dopamine in the hyperactivity syndrome produced by increased 5-hydroxytryptamine synthesis in rats

The syndrome of hyperactivity produced in rats by the administration of tranylcypromine (20 mg kg ) followed 30 min later by tryptophan was inhibited by α-methyl- p-tyrosine (α-MPT), a tyrosine hydroxylase inhibitor given in two doses 18 and 16 hr previously. While brain tryptophan and 5-hydroxytryptamine (5-HT) concentrations rose to levels observed in rats not pretreated with α-MPT, noradrenaline and dopamine concentrations were depleted by 75%. The administration of l-DOPA (150 mg kg ) 1 hr after the second dose of α-MPT almost restored the brain dopamine concentrations, did not restore brain noradrenaline concentrations but did cause return of the hyperactivity produced by tranylcypromine and tryptophan. The dopamine β-hydroxylase inhibitor disulfiram (400 mg kg ) given 6 hr before tranylcypromine and l-tryptophan depleted brain noradrenaline by 75% but had no effect on brain dopamine and did not inhibit hyperactivity. The β-MPT analogue, α-methyl- m-tyrosine (α-MMT), which does not inhibit catecholamine synthesis did not inhibit the hyperactivity. The hyperactivity response seen after administration of tranylcypromine (20 mg kg ) and 5 methoxy N, N-dimethyltryptamine which probably acts as a 5-HT analogue stimulating postsynaptic 5-HT receptor sites was completely inhibited by α-MPT pretreatment. Results are tentatively interpreted as indicating that at some point between the postsynaptic 5-HT receptor sites initiating the production of the hyperactivity syndrome and the mechanisms responsible for the expression of the syndrome, lies a system of dopaminergic neurones. Activity of these neurones depends on adequate dopamine concentrations and depletion breaks the neuronal sequences necessary for the behavioural expression of 5-HT receptor site stimulation.

Involvement of the liver in the regulation of tryptophan availability. Possible role in the responses of liver and brain to starvation

The concentrations of free and total (free plus albumin bound) tryptophan were measured in plasma of blood taken from the portal vein, hepatic vein and abdominal aorta of male rats, fed, and starved for one and three days. Liver and brain tryptophan concentrations were measured in similar groups of rats. On starvation, there was an increase in arterial plasma free tryptophan concentration which took place peripherally and was paralleled by an increase in brain tryptophan. In both the fed and starved rats, the portal vein concentrations of free tryptophan were high and as the blood flowed through the liver they were reduced to relatively low levels not directly related to the arterial values. All these changes were due to alterations in degree of binding of tryptophan to plasma albumin. The measurements of plasma total tryptophan concentrations showed that postabsorptively and during starvation there was a net uptake of tryptophan by the peripheral tissues (which included brain), but no overall fall in plasma concentration. At the same time, there was a net release from the liver, and to a lesser extent from the portal-drained tissues. The released tryptophan largely entered the albumin bound plasma pool. Accompanying the hepatic output was a fall in tryptophan concentration in the liver which was apparently caused by altered cell membrane transport. The results suggest (1) that the liver protects the brain from the high free tryptophan level in portal blood, (2) that the availability of tryptophan to the brain is maintained postabsorptively and during starvation by hepatic output into the albumin bound pool and (3) that this release of tryptophan from the liver and the fall in intracellular tryptophan concentration are initiated by altered membrane transport. The pattern of changes is consistent with a role for tryptophan in the mediation of changes in liver protein synthesis and gluconeogenesis and cerebral serotonin turnover on starvation.

Effect of clofibrate on free and total tryptophan in serum and brain tryptophan metabolism

The influence of clofibrate (CPIB) on free and total tryptophan concentrations in rat serum and on brain concentrations of tryptophan, 5-hydroxytryptamine (5HT), and 5-hydroxyindolacetic acid (5HIAA) has been established. A single dose of CPIB administered orally caused a pronounced fall of total tryptophan, and a significant increase of free tryptophan in serum. Under the same experimental condition increases of brain tryptophan and 5HIAA concentrations were observed, with no change in brain 5HT. Similar modifications were observed after repeated administrations of the drug. From these results it may be inferred that CPIB increases brain 5HT synthesis by displacement of protein bound tryptophan in serum and thus by increasing in brain the availability of the substrate to tryptophan-hydroxylase.