A STUDY OF PROPOSED DETERMINANTS OF BRAIN TRYPTOPHAN CONCENTRATION IN RATS AFTER PORTOCAVAL ANASTOMOSIS OR SHAM OPERATION

Liver dysfunction was produced in rats by surgical portocaval anastomosis (PCA), and the time-course of changes in brain tryptophan and 5-HT metabolism studied in relation to plasma changes possibly influencing brain tryptophan concentration. Brain tryptophan and 5-hydroxyindolylacetic acid (5-HIAA) levels were increased greatly and maximally on the day after PCA and remained high. 5-HT changes were less marked but had a similar time-course. Plasma total tryptophan was little changed but plasma free tryptophan was raised. The latter change showed a similar time-course to that of brain tryptophan but was not large enough to account completely for it. Sham operation was followed by significant but transient increases in plasma free tryptophan, brain tryptophan and 5-HIAA but these were much smaller than after PCA. Brain tryptophan did not correlate with plasma total tryptophan either in control or PCA rats but it correlated significantly with plasma free tryptophan in both groups. However brain levels were much higher in PCA rats than in controls with similar plasma free tryptophan levels at all times from the first day after operation. The increase of brain tryptophan in anastomosed rats not accounted for by plasma free tryptophan was explained neither by insulin changes nor by an increase of the insulin/glucagon ratio nor by changes in plasma concentrations of those amino acids which compete with tryptophan for entry into brain. The results therefore indicate an unknown influence on brain tryptophan concentration in PCA rats. As tyrosine changes in brain and plasma after PCA were very similar to those of tryptophan this influence may not be specific to tryptophan. Results suggest that under the conditions used brain tryptophan concentrations of both PCA and control rats are more influenced by changes of plasma free tryptophan concentration than by changes of plasma concentrations of competing amino acids.     

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.     

Total and free plasma concentrations of progesterone, cortisol and oestradiol-17 beta during pregnancy, parturition and early lactation in sows

The total (bound plus free) concentrations of progesterone, 20 alpha-dihydroprogesterone, oestradiol-17 beta and cortisol were determined in the plasma of sows at three stages during pregnancy and more intensively from 5 days pre-partum to 5 days post-partum. The free fractions of progesterone, oestradiol-17 beta and cortisol were measured in the same samples by a rate dialysis method. Up to day 110 of gestation, the amounts of free hormone in plasma did not fluctuate independently of their total concentrations. During farrowing, the total and free concentrations of progesterone and cortisol varied independently of each other, whereas total and free oestradiol-17 beta declined simultaneously. The initiation of parturition was associated with a decrease in circulating total progesterone, and was accentuated by a decrease in the free fraction (P less than 0.005) so that its active free concentration was only 20% of its day 1 pre-partum value. Total and free cortisol concentrations rose rapidly during labour so that at 12-18 h after birth of the first piglet 30% of that cortisol in maternal plasma was free hormone.     

Indolic Substances in Plasma, Cerebrospinal Fluid, and Frontal Cortex of Human Subjects Infused with Saline or Tryptophan

Psychiatric patients undergoing the psychosurgical operation of stereotactic subcaudate tractotomy were infused intravenously with either saline or L-tryptophan (15 mg/kg/h). Plasma, lumbar cerebrospinal fluid (CSF), ventricular CSF and a specimen of frontal cortex were collected. The relationships of plasma concentrations of substances claimed to influence brain tryptophan concentration (total tryptophan, free tryptophan, large neutral amino acids) with the concentration of tryptophan in the cortex and CSF were investigated. Tryptophan infusion resulted in plasma tryptophan values comparable to those found after oral doses used in treating depression or insomnia, and about sixfold increases of tryptophan in the cerebral cortex. Increased brain 5-hydroxytryptamine synthesis was indicated by significant rises of CSF 5-hydroxyindoleacetic acid. The concentration of plasma free tryptophan was a better predictor than plasma total tryptophan of cortex tryptophan concentration. As all correlation coefficients of plasma versus brain or plasma versus ventricular CSF tryptophan concentrations were decreased when allowance was made for differences of concentration of large neutral amino acids, the results suggest that the role of these substances within their physiological range as inhibitors of tryptophan transport to the brain may previously have been overemphasised.     

Effects of clofibrate on plasma tryptophan, growth hormone, and prolactin and on brain tryptophan and serotonin in prepubertal rats

The effects of clofibrate administration (200 mg/kg, po) on somatic growth, plasma levels of lipids, tryptophan, growth hormone (GH), and prolactin (PRL), as well as on brain concentrations of tryptophan and 5-hydroxytryptamine (5-HT) were studied in prepubertal male rats. The drug did not significantly alter ponderal growth, but an appreciable reduction of tail length was observed in rats treated for 30 days. Triglyceride concentrations in plasma showed a 43% diminution after 30 days of treatment, whereas free fatty acid (FFA) levels were not modified. Clofibrate administration for 7, 15, or 30 days caused a fall in total tryptophan and a significant increase of the free fraction in plasma with no change in brain tryptophan levels. Brain 5-HT was generally unaffected but a marked elevation of this parameter was noted in rats treated for 15 days. Plasma GH and PRL concentrations remained unaltered. It may be concluded from these findings that the slight reduction of somatic growth, the diminution of triglycerides, and the increase of free tryptophan in plasma, induced by chronic clofibrate treatment, are not associated with variations in brain tryptophan and 5-HT levels or with modifications of plasma GH and PRL titers.     

Effects of Albumin, Amino Acids and Clofibrate on the Uptake of Tryptophan by the Rat Brain

Abstract: The influences of total tryptophan concentration, albumin binding and amino acid competition on the rate of tryptophan influx into rat brain were compared using a single-pass injection technique with tritiated water as a freely diffusible reference. Omission of 3% bovine albumin from a bolus containing tryptophan in Krebs–Ringer bicarbonate buffer injected into the carotid artery increased non-albumin bound (free) tryptophan concentration threefold but tryptophan uptake by only 35% and 30% into forebrain and hypothalamus, respectively. However, tryptophan uptake from injected rat plasma was more markedly elevated when free tryptophan concentration was raised. Thus, when free tryptophan was doubled, but total tryptophan unchanged, by in vitro addition of clofibrate to a plasma bolus, uptake was increased by 53% and 28% into forebrain and hypothalamus respectively. When clofibrate was injected in vivo so that plasma total tryptophan concentration was decreased by 45% but neither free tryptophan nor competing amino acid concentrations were altered, then uptake from a bolus of the rat's own plasma was unchanged. Addition of competing amino acids at physiological concentrations to tryptophan in Krebs-Ringer buffer significantly reduced tryptophan influx into both brain regions, but did not increase the effect of albumin binding. The results indicate that tryptophan uptake into rat forebrain is substantially influenced by albumin binding and competition from other amino acids, but that hypothalamic uptake is less influenced by these factors.     

Injected tryptophan increases brain but not plasma tryptophan levels more in ethanol treated rats

In previous studies, long term treatment with ethanol has been shown to enhance brain 5-hydroxytryptamine 5-(HT) metabolism by increasing the activity of the regulatory enzyme tryptophan hydroxylase and or availability of circulating tryptophan secondarily to an inhibition of hepatic tryptophan pyrrolase. In the present study ethanol treatment given for two weeks decreased hepatic apo-tryptophan pyrrolase but not total tryptophan pyrrolase activity in rats. Tryptophan levels in plasma and brain did not increase significantly. But there was a marked increase of 5-HT but not 5-hydroxyindoleacetic acid (5-HIAA) concentration in brain, suggesting a possible increase in the activity of tryptophan hydroxylase. The effect of a tryptophan load on brain 5-HT metabolism was therefore compared in controls and ethanol treated rats. One hour after tryptophan injection (50 mg/kg i.p.) plasma concentrations of total and free tryptophan were identical in controls and ethanol treated rats, but the increases of brain tryptophan 5-HT and 5-HIAA were considerably greater in the latter group. The results are consistent with long term ethanol treatment enhancing brain serotonin metabolism and show that brain uptake/utilization of exogenous tryptophan is increased in ethanol treated rats and may be useful to understand the role and possible mechanism of tryptophan/serotonin involvement in mood regulation.     

Injected tryptophan increases brain but not plasma tryptophan levels more in ethanol treated rats

In previous studies, long term treatment with ethanol has been shown to enhance brain 5-hydroxytryptamine 5-(HT) metabolism by increasing the activity of the regulatory enzyme tryptophan hydroxylase and or availability of circulating tryptophan secondarily to an inhibition of hepatic tryptophan pyrrolase. In the present study ethanol treatment given for two weeks decreased hepatic apo-tryptophan pyrrolase but not total tryptophan pyrrolase activity in rats. Tryptophan levels in plasma and brain did not increase significantly. But there was a marked increase of 5-HT but not 5-hydroxyindoleacetic acid (5-HIAA) concentration in brain, suggesting a possible increase in the activity of tryptophan hydroxylase. The effect of a tryptophan load on brain 5-HT metabolism was therefore compared in controls and ethanol treated rats. One hour after tryptophan injection (50 mg/kg i.p.) plasma concentrations of total and free tryptophan were identical in controls and ethanol treated rats, but the increases of brain tryptophan 5-HT and 5-HIAA were considerably greater in the latter group. The results are consistent with long term ethanol treatment enhancing brain serotonin metabolism and show that brain uptake/utilization of exogenous tryptophan is increased in ethanol treated rats and may be useful to understand the role and possible mechanism of tryptophan/serotonin involvement in mood regulation.     

Potential role for nonesterified fatty acids in beta-adrenoceptor-induced increases in brain tryptophan

We tested the hypothesis that beta2- and beta3-adrenergic receptor-mediated increases in brain tryptophan are due to the liberation of fatty acids, which in turn displace tryptophan from its albumin-binding site and thus facilitate its entry into the brain. Male CD-1 mice were injected with subtype-selective beta-adrenergic agonists 1h before brain samples were collected for analysis of tryptophan content by HPLC with electrochemical detection, and blood samples were collected for analysis of total and free tryptophan and nonesterified fatty acid (NEFA) concentrations. The beta2-selective agonist, clenbuterol (0.1 mg/kg), increased concentrations of tryptophan in all brain regions studied and decreased plasma total tryptophan, but had no effect on plasma free tryptophan or NEFAs. The beta3-selective agonists, BRL 37344 (0.2 mg/kg) or CL 316243 (0.01 mg/kg), increased brain tryptophan, plasma NEFAs and free tryptophan. Pretreatment with nicotinic acid (500 mg/kg), an inhibitor of lipolysis, almost completely prevented the increase in plasma free tryptophan and NEFAs, and attenuated the increase in brain tryptophan induced by CL 316243. These results suggest that beta2- and beta3-adrenergic agonists increase brain tryptophan by a mechanism other than the liberation of NEFAs. Nonetheless, beta3-adrenergic agonists appear to increase brain tryptophan by a mechanism that may depend partially on elevations of plasma NEFAs.     

Immobilization Decreases Amino Acid Concentrations in Plasma but Maintains or Increases Them in Brain

Immobilization for 2 h significantly decreased plasma concentrations of 13 of 16 amino acids assayed, including the transmitter amine precursors tyrosine and total tryptophan. The level of plasma free tryptophan, however, was increased. Despite the reduced plasma levels, corresponding brain concentrations of many large neutral amino acids (LNAAs) were increased (tryptophan, phenylalanine, valine, leucine, and isoleucine). Brain concentrations of tyrosine and the other amino acids measured were unaltered. The results for the LNAAs were not explained by calculated brain influx rates. Therefore, altered influx kinetics or perhaps altered brain protein metabolism or efflux may be responsible. Comparison of calculated brain influxes and brain concentrations of LNAAs suggests that the rise in level of plasma free tryptophan during immobilization is not responsible for the increase in level of brain tryptophan and that the mechanism responsible for the maintenance of or increase in brain concentrations of the other LNAAs is probably involved. Maintenance of brain concentrations of basic amino acids is explicable by reduced competition for brain uptake.     

Effects of neurotrophic stress on maternal metabolism and binding of plasma cortisol in late pregnant guinea-pigs and their fetuses

The maternal metabolic clearance rate (MCR) and the binding of cortisol in the plasma of fetus and mother were estimated 8 days before term in conscious pregnant guinea-pigs, control or subjected to a neurotrophic stress (they were immobilized for 3 h in a dark room in front of an intermittent luminous flash). The maternal MCR of cortisol dropped during pregnancy related to an increase in corticosteroid binding globulin (CBG) and the stress mimicked these changes with a decrease in MCR and a rise in CBG. The increase in cortisol concentrations occurring in the fetal plasma after maternal stress could be related to the increase in free cortisol in the stressed mother. Nevertheless, although the fetal CBG did not rise after maternal stress, the increase in free cortisol in the fetal plasma remained moderate, due to the numerous free sites of CBG and albumin for cortisol.     

The metabolism of L-tryptophan by isolated rat liver cells. Effect of albumin binding and amino acid competition on oxidatin of tryptophan by tryptophan 2,3-dioxygenase.

1. Novel methods, using L-[ring-2-14C]tryptophan, are described for the measurement of tryptophan 2,3-dioxygenase activity and tryptophan accumulation in isolated rat liver cells. 2. The effects of bovine serum albumin, non-esterified fatty acids and neutral amino acids on tryptophan oxidation by hepatocytes and on the partition of tryptophan between free and albumin-bound forms were investigated. 3. Oxidation of physiological concentrations (0.1 mM) of tryptophan was inhibited by approx. 50% in the presence of 2% (w/v) bovine serum albumin; no effects were found at tryptophan concentrations of 0.5 mM and above. 4. Increases in free tryptophan concentrations produced by displacement of 0.1 mM-tryptophan from albumin-binding sites by palmitate resulted in increased flux through tryptophan dioxygenase. 5. Addition of a mixture of neutral amino acids, at plasma concentrations, to hepatocyte incubations had no effect on the rate of tryptophan oxidation. 6. It is concluded that alterations in free tryptophan concentrations consequent to changes in albumin binding may be an important factor in regulating tryptophan uptake and catabolism by the liver. The results are briefly discussed with reference to possible consequences on brain tryptophan metabolism.     

Effect of food restriction on serotonin metabolism in rat brain

The brain concentration of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) increased in rats maintained on restricted volume of low-protein or normal-protein diet, whereas these two agents decreased in rats fed low-protein diet ad libitum. In these two food-restricted groups brain 5-HT and 5-HIAA concentrations were not correlated with brain tryptophan hydroxylase activity, but the concentrations correlated closely with cerebral tryptophan concentrations. The cerebral tryptophan concentration in the two food-restricted groups was not consistent with the total or free tryptophan concentration in plasma. In these restricted rats cerebral tryptophan concentration was elevated, and, unlike the plasma tryptophan, it showed no diurnal variation. These results suggested that tryptophan uptake into the brain from plasma was enhanced by limiting food volume intake. Tryptophan uptake was increased by glucagon injection without changing the plasma tryptophan level, but injection of hydrocortisone or insulin had little or no effect on tryptophan concentration in either the plasma or brain.d-Glucose injection elevated plasma tryptophan concentration but decreased brain tryptophan concentration.     

REGIONAL BRAIN STUDY OF INDOLEAMINE METABOLISM IN THE RAT IN ACUTE HEPATIC FAILURE

Abstract— Tryptophan, 5-hydroxytryptamine and 5-hydroindoleacetic acid were found to be greatly increased in various parts of the brains of rats in acute hepatic failure following two stage hepatic devascularization. However, the increases in 5-hydroxytryptamine and 5-hydroxyindoleacetic acid varied by region and are not explicable solely in terms of increased concentrations of tryptophan. The results are discussed in terms of differences in the regional metabolism of 5-hydroxyindoleamines. Plasma free fatty acids, albumin, total tryptophan and free tryptophan were measured in plasma in hopes of elucidating the mechanism responsible for the cerebral elevation of tryptophan. Increased plasma free tryptophan appears sufficient to explain the rapid increase in brain tryptophan. The relationship between these results and recent observations in hepatic encephalopathy is discussed.     

Experimental Verification of the Effects on Normal Domestic Cats by Feeding Prescription Diet for Decreasing Stress

The objective of this study was to evaluate the effects of diet on the feline stress response by measuring plasma and urinary cortisol. A study diet was developed with a unique combination of nutrients that supports the management of stressful situations. The specific formulation of the diet included alpha-casozepine, which is believed to have an anxiolytic effect, and tryptophan supplementation. Tryptophan is the precursor for the synthesis of the neurotransmitter serotonin. Twenty-one indoor cats were fed with the study diet (n = 10) or a control diet (n = 11) for 8 weeks, after which physiological responses were evaluated. The study diet significantly increased the ratio of plasma tryptophan to large neutral amino acids and decreased urinary cortisol concentrations after being consumed daily for 8 weeks, but there was no effect on plasma cortisol levels following a stressful event (veterinary examination and blood draw). Further studies, such as behavioral analyses, are needed to clarify the effects of the study diet.     

The effect of marathon racing on serum concentrations of selected hormones in health and disabled individuals

Serum concentrations of the selected hormones were assayed in students who did not train long-distance races and in trained disabled individuals who passed the distance on a wheel chairs. Assays were carried out both prior to and after Marathon race. An exercise significantly increased serum concentrations of the following hormones in both groups: cortisol, ACTH, aldosteron, and plasma renin activity. An increase in total thyroxine, free thyroxine and decrease in LH were noted in disabled individuals. Increased concentrations of ACTH, free and total thyroxine, decreased concentrations of cortisol, LH, plasma renin activity, and aldosteron were found in disabled individuals who passed the distance of Marathon race on the wheel chairs comparing with those seen in the students.     

Variables Influencing the Effect of a Meal on Brain Tryptophan

Previous work from our laboratory points to plasma free tryptophan being a useful predictor of brain tryptophan concentration in many circumstances. Other work, in particular various studies on the acute effects of food intake, has emphasized the roles of plasma total tryptophan and of plasma large neutral amino acids that compete with tryptophan for transport to the brain. We have now studied associations between the above variables under different dietary conditions. Rats were allowed to feed for restricted periods during a 12-h light-12-h dark cycle. In the first study, rats were given access to a carbohydrate diet for 2 h midway through the light cycle and following an 18-h fast. The resultant rise of brain tryptophan was explicable largely by the associated fall in large neutral amino acids. In a second study, rats were adapted to a regimen whereby they were allowed access to the standard laboratory diet for 4 h during the dark cycle for 3 weeks. A postprandial decrease in brain tryptophan was associated with a fall in free tryptophan and of its ratio to competing amino acids. The brain change could be attributed neither to changes in plasma total tryptophan (which increased) nor to changes of its ratio to the competers (which remained unchanged). Results as a whole are thus consistent with changes of plasma free tryptophan and large neutral amino acid concentrations affecting brain tryptophan concentration under different dietary circumstances. It is suggested that these influences serve to maintain brain tryptophan when dietary supplies are defective.     

Antepartum Depression Severity is Increased During Seasonally Longer Nights: Relationship to Melatonin and Cortisol Timing and Quantity

Current research suggests that mood varies from season to season in some individuals, in conjunction with light-modulated alterations in chronobiologic indices such as melatonin and cortisol. The primary aim of this study was to evaluate the effects of seasonal variations in darkness on mood in depressed antepartum women, and to determine the relationship of seasonal mood variations to contemporaneous blood melatonin and cortisol measures; a secondary aim was to evaluate the influence of seasonal factors on measures of melancholic versus atypical depressive symptoms. We obtained measures of mood and overnight concentrations of plasma melatonin and serum cortisol in 19 depressed patients (DP) and 12 healthy control (HC) antepartum women, during on-going seasonal variations in daylight/darkness, in a cross-sectional design. Analyses of variance showed that in DP, but not HC, Hamilton Depression Rating Scale (HRSD) scores were significantly higher in women tested during seasonally longer versus shorter nights. This exacerbation of depressive symptoms occurred when the dim light melatonin onset, the melatonin synthesis offset, and the time of maximum cortisol secretion (acrophase) were phase-advanced (temporally shifted earlier), and melatonin quantity was reduced, in DP but not HC. Serum cortisol increased across gestational weeks in both the HC and DP groups, which did not differ significantly in cortisol concentration. Nevertheless, serum cortisol concentration correlated positively with HRSD score in DP but not HC; notably, HC showed neither significant mood changes nor altered melatonin and cortisol timing or quantity in association with seasonal variations. These findings suggest that depression severity during pregnancy may become elevated in association with seasonally related phase advances in melatonin and cortisol timing and reduced melatonin quantity that occur in DP, but not HC. Thus, women who experience antepartum depression may be more susceptible than their nondepressed counterparts to phase alterations in melatonin and cortisol timing during seasonally longer nights. Interventions that phase delay melatonin and/or cortisol timing—for example, increased exposure to bright evening light—might serve as an effective intervention for antepartum depressions whose severity is increased during seasonally longer nights.     

Effect of Age on Variables Influencing the Supply of Tryptophan to the Brain

Abstract: The relations of plasma concentrations of substances claimed to influence brain tryptophan concentration (total tryptophan, free tryptophan, large neutral amino acids) with the concentrations of tryptophan, 5-hydroxytryptamine (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) in the forebrain were investigated in rats of different ages (from 8 days to 16 months after birth). In brain, tryptophan fell by 46%, whereas 5-HT rose by 20% between 8 and 40/42 days after birth. Thereafter, the levels of both tryptophan and 5-HT remained essentially constant. Brain 5-HIAA showed a more complex pattern, rising by 63% between 8 and 19 days, falling between 19 and 40/42 days, and then gradually rising until values at 16 months were significantly higher than those at 40/42 days. In plasma, the concentrations of free fatty acids, free and total tryptophan, and large neutral amino acids all decreased between 8 and 19 days and thereafter either remained constant or increased slowly, the exception being total tryptophan values, which showed large increases between 28/30 and 60/70 days. Also, the unidirectional uptake of tryptophan from blood to brain was determined using a carotid artery injection technique. Uptake values obtained using a tracer concentration of tryptophan in the injection solution decreased progressively with age. Kinetic analysis of the data in terms of the Michaelis-Menten equation for carrier-mediated transport indicated significantly lower values for V_max and K_D (a component for nonsaturable transport) in 6-month-old rats as compared to 19-day-old suckling rats, whereas K_m values were the same at both ages. Detailed analysis of these results indicated that the age-related changes in brain tryptophan were largely explicable in terms of plasma free tryptophan in association with blood-brain transport characteristics; moderate differences in concentration of amino acids competing for transport were without apparent effect between 19 days and 16 months. The larger differences between 8 and 19 days after birth could be important.     

Response of plasma and gastrointestinal melatonin,plasma cortisol and activity rhythms of European sea bass (<Emphasis Type="Italic">Dicentrarchus labrax</Emphasis>) to dietary supplementation with tryptophan and melatonin

Melatonin is an effective antioxidant, immunostimulant, gonadal maturating regulator and antistress indoleamine that may be potentially useful for fish farmers. We have explored two possible ways of increasing plasma melatonin levels through the diet: direct melatonin supplementation (ME diet) and supplementation with the melatonin precursor tryptophan (TRP diet). To this end, a group of sea bass was fed a commercial diet (STD diet) at a regular time for 16 days, after which plasma, intestine, and bile samples were taken at four different time points: 120 min before, and 15, 180 and 480 min after feeding. Locomotor activity, intestinal and biliary melatonin, and plasma melatonin, serotonin and cortisol levels were measured. This same sampling process and analyses were also carried out after feeding sea bass TRP diet or ME diet for 1 week. Our results show that melatonin, but not tryptophan supplementation of the diet increases plasma, intestine and bile levels of melatonin. Plasma serotonin levels, on the other hand, were increased by dietary tryptophan, but not by melatonin, confirming the availability of supplemented tryptophan for serotonin synthesis. Both treatments were equally effective in reducing the high cortisol levels observed with the STD diet.