Letter: Factors influencing effect of hydrocortisone on rat brain tryptophan metabolism

Neurochemical Anatomy of Cushing's Syndrome

The neurochemical anatomy underlying Cushing's syndrome is examined for regional brain metabolism as well as neurotransmitter levels and receptor binding of biogenic amines and amino acids. Preliminary studies generally indicate that glucose uptake, blood flow, and activation on fMRI scans decreased in neocortical areas and increased in subcortical areas of patients with Cushing's syndrome or disease. Glucocorticoid-mediated increases in hippocampal metabolism occurred despite in vitro evidence of glucocorticoid-induced decreases in glucose uptake or consumption, indicating that in vivo increases are the result of indirect, compensatory, or preliminary responses. In animal studies, glucocorticoid administration decreased 5HT levels and 5HT1A receptor binding in several brain regions while adrenalectomy increased such binding. Region-specific effects were also obtained in regard to the dopaminergic system, with predominant actions of glucocorticoid-induced potentiation of reuptake blockers and releasing agents. More in-depth neuroanatomical analyses are warranted of these and amino acid-related neurotransmission.

Time-dependent effects of L-tryptophan administration on urinary excretion of L-tryptophan metabolites

We have previously reported that dietary supplementation with up to 5.0 g/d of L-tryptophan (L-Trp) for 21 d has no adverse effects, judging from the levels of general blood variables, in healthy women. We performed a randomized, double-blind, placebo-controlled, crossover intervention study in 17 apparently healthy Japanese women. The subjects were randomly assigned to receive a placebo (0 g/d) or 1.0, 2.0, 3.0, 4.0, or 5.0 g/d of L-Trp for 21 d each with a 5-wk washout period between trials. We examined the 24-h urine profiles on days -1 (1 d before starting L-Trp), 7, 14, and 21 to determine whether administration of L-Trp at doses of up to 5.0 g/d affects time-dependent urinary excretion of L-Trp or its metabolites in healthy women. The urinary excretion of L-Trp, kynurenic acid, 3-hydroxykynurenine, xanthurenic acid, 3-hydroxyanthranilic acid, quinolinic acid, N(1)-methylnicotinamide, N(1)-methyl-2-pyridone-5-carboxamide, and N(1)-methyl-4-pyridone-3-carboxamide increased in an L-Trp dose-dependent manner on day 7. The amount of urinary excretion of these compounds was unchanged on days 14 and 21. The urinary excretion of serotonin, 5-hydroxyindole-3-acetic acid, 2-oxoadipic acid, and nicotinamide was unaffected by L-Trp at any of the doses tested. L-Trp doses had weak effects on the urinary excretion of kynurenine and anthranilic acid. Based on these findings, we conclude that there are no time-dependent effects of L-Trp administration in urinary excretion of L-Trp metabolites. Additionally, L-Trp and its metabolites do not accumulate in the body.

Inhibition of stress-induced hepatic tryptophan 2,3-dioxygenase exhibits antidepressant activity in an animal model of depressive behaviour

The role of hepatic tryptophan 2,3 dioxygenase (TDO) was assessed in the provocation of stress-induced depression-related behaviour in the rat. TDO drives tryptophan metabolism via the kynurenine pathway (KP) and leads to the production of neuroactive metabolites including kynurenine. A single 2 h period of restraint stress in adult male Sprague-Dawley rats provoked an increase in circulating concentrations of the glucocorticoid corticosterone and induction of hepatic TDO expression and activity. Repeated exposure to stress (10 d of 2 h restraint each day) provoked an increase in immobility in the forced swimming test (FST) indicative of depression-related behaviour. Immobility was accompanied by an increase in the circulating corticosterone concentrations, expression and activity of hepatic TDO and increase in the expression of TDO in the cerebral cortex. Increased TDO activity was associated with raised circulating kynurenine concentrations and a reduction in circulating tryptophan concentrations indicative of KP activation. Co-treatment with the TDO inhibitor allopurinol (20 mg/kg, i.p.), attenuated the chronic stress-related increase in immobility in the FST and the accompanying increase in circulating kynurenine concentrations. These findings indicate that stress-induced corticosterone and consequent activation of hepatic TDO, tryptophan metabolism and production of kynurenine provoke a depression-related behavioural phenotype. Inhibition of stress-related hepatic TDO activity promotes antidepressant activity. TDO may therefore represent a promising target for the treatment of depression associated with stress-related disorders in which there is evidence for KP activation.

Tryptophan: the key to boosting brain serotonin synthesis in depressive illness

It has been proposed that focusing on brain serotonin synthesis can advance antidepressant drug development. Biochemical aspects of the serotonin deficiency in major depressive disorder (MDD) are discussed here in detail. The deficiency is caused by a decreased availability of the serotonin precursor tryptophan (Trp) to the brain. This decrease is caused by accelerated Trp degradation, most likely induced by enhancement of the hepatic enzyme tryptophan 2,3-dioxygenase (TDO) by glucocorticoids and/or catecholamines. Induction of the extrahepatic Trp-degrading enzyme indolylamine 2,3-dioxygenase (IDO) by the modest immune activation in MDD has not been demonstrated and, if it occurs, is unlikely to make a significant contribution. Liver TDO appears to be a target of many antidepressants, the mood stabilisers Li(+) and carbamazepine and possibly other adjuncts to antidepressant therapy. The poor, variable and modest antidepressant efficacy of Trp is due to accelerated hepatic Trp degradation, and efficacy can be restored or enhanced by combination with antidepressants or other existing or new TDO inhibitors. Enhancing Trp availability to the brain is thus the key to normalisation of serotonin synthesis and could form the basis for future antidepressant drug development.

Tryptophan metabolism and brain function: focus on kynurenine and other indole metabolites

The synthesis of NAD (or NADP) from tryptophan involves a series of enzymes and the formation of a number of intermediates which are collectively called 'kynurenines.' In the late 1970s and early 1980s, it became clear that intraventricular administration of several 'kynurenines' could cause convulsions and that one of the 'kynurenines,' quinolinic acid, was an agonist of a sub-population of NMDA receptors and caused excitotoxic neuronal death. A related metabolite, kynurenic acid, could, on the other hand, reduce excitotoxin-induced neuronal death by antagonising ionotropic glutamate receptors. Since then, modifications in quinolinic and kynurenic acid synthesis have been proposed as a pathogenetic mechanism in Huntington's chorea and epilepsy. It was subsequently shown that a robust activation of the kynurenine pathway and a large accumulation of quinolinic acid in the central nervous system occurred in several inflammatory neurological disorders. More recently, it has been shown that 3OH-kynurenine or 3OH-anthranilic acid, two other kynurenine metabolites, may cause either apoptotic or necrotic neuronal death in cultures and that inhibitors of kynurenine hydroxylase may reduce neuronal death in in vitro and in vivo models of brain ischaemia or excitotoxicity. Finally, it has been reported that indole metabolites, indirectly linked to the kynurenine pathway, are able to modify neuronal function and animal behaviour by interacting with voltage-dependent Na+ channels. Oxindole, one of these metabolites, has sedative and anticonvulsant properties and accumulates in the blood and brain when liver function is impaired. In conclusion, a number of metabolites affecting brain function originate from tryptophan metabolism. Selective inhibitors of their forming enzymes may be useful to understand their role in physiology or as therapeutic agents in pathology.

Regulation of 5-HT receptors by corticosteroids: where do we stand?

Twenty five years ago, experimental procedures such as adrenalectomy and corticosteroid administration (to intact rats) allowed the recognition of direct and indirect controls of central 5-HT synthesis rate by corticosteroids. These effects indicated that the activity of the hypothalamo-pituitary-adrenal (HPA) axis, whether under basal conditions or during stress, is endowed with a modulatory action upon serotonergic neurons. Nowadays, in situ hybridisation, in vitro autoradiography, and radioligand binding on the one hand, and electrophysiological, behavioural, and neuroendocrinological responses on the other hand, are tools that allow the analysis of direct corticosteroid effects upon 5-HT receptors. Among the dozen of 5-HT receptors identified so far, four receptors (namely the 5-HT1A, 5-HT1B, 5-HT2A, and 5-HT2C receptors)--and the 5-HT uptake system--have been the focus of studies aimed at detecting corticosteroid modulatory effects. The results that are reviewed herein indicate that hippocampal 5-HT1A receptors are under the tonic inhibitory control of corticosterone. This control is directly exerted at the level of the 5-HT1A receptor gene, essentially through mineralocorticoid receptors; as well, electrophysiological findings bring support for an additional modulation of hippocampal 5-HT1A receptor-mediated functions by indirect (ie 5-HT1A receptor gene-independent) genomic actions of corticosteroids. In keeping with the respective effects of stressful stimuli and psychotropic drugs upon the HPA axis and central serotonergic systems, it is likely that these corticosteroid-5-HT1A receptor interactions in the hippocampus have consequences in the pathophysiology of mood disorders. However, because the data regarding a corticosteroid control of other 5-HT receptors are either scarce and contradictory (eg 5-HT1B, 5-HT2A, 5-HT2C receptors and 5-HT uptake systems) or lacking, it is at the present time unknown whether corticosteroids exert other effects on 5-HT receptor-mediated functions, including those related to homeostasis.

Physiopharmacological interactions between stress hormones and central serotonergic systems

The present review tries to delineate some mechanisms through which the sympathetic nervous system (SNS) and the hypothalamo-pituitary-adrenal (HPA) interact with central serotonergic systems. The recent progress in 5-hydroxytryptamine (5-HT) receptor pharmacology has helped to define the means by which central serotonergic activity may alter the respective activities of the SNS (sympathetic nerves and adrenomedulla) and of the HPA axis. These pharmacological findings have also helped to characterize the differential effects of central 5-HT upon different branches of the SNS and the numerous sites at which 5-HT exerts stimulatory influences upon the HPA axis. Although relevant to stress-related neuroendocrinology, the extent to which these interactions are involved in the antidepressant/anxiolytic properties of some serotonergic agents still remains to be clarified. Beside these findings, there is also abundant evidence for a tight control of central serotonergic systems by stress hormones. Activation of the SNS increases, by numerous means, central availability of tryptophan, whereas glucocorticoids exert differential actions upon the intra- and the extraneuronal regulation of 5-HT function. Actually, a significant number of these mechanisms is involved in the maintenance of homeostasis during stressful events, thereby conferring to these mechanisms a key role in adaptation processes.

Effects of the haem precursor 5-aminolaevulinate on tryptophan metabolism and disposition in the rat

5-Aminolaevulinate administration to rats inhibits cerebral 5-hydroxytryptamine synthesis by decreasing tryptophan availability to the brain secondarily to activation of hepatic tryptophan pyrrolase. The results show that tryptophan metabolism and disposition can be influenced by changes in liver haem concentration, and are discussed briefly in relation to mood disorders in the hepatic porphyrias.

Diabetes-induced changes in monoamine concentrations of rat hypothalamic nuclei

Streptozotocin-induced diabetes produced marked alterations of monoamine concentrations in several hypothalamic nuclei of male and female rats. Norepinephrine (NE) concentrations were significantly elevated in the median eminence (ME), supraoptic nucleus (SON) and periventricular nucleus (PEVN) in both sexes of diabetic rats. NE concentrations in the suprachiasmatic nucleus (SCN) and ventromedial nucleus (VMN) of male and female diabetic animals remained unaltered. Serotonin (5-HT) concentrations were increased in PEVN of male and female diabetic rats. No significant changes in hypothalamic dopamine (DA) concentrations were observed. Insulin treatment reversed the diabetes-related changes in monoamine concentrations in most of the nuclei. The significance of these biochemical changes relative to the endocrine and behavioral abnormalities in diabetes is discussed.

Psychotic and nonpsychotic depressions: I. Comparison of plasma catecholamines and cortisol measures

Unconjugated plasma catecholamines and cortisol were measured before and after a 1 mg dose of dexamethasone in 22 medication-free depressed patients and 6 healthy, medication-free control subjects. Plasma dopamine (DA) levels in the psychotically depressed subgroup (n = 4) were significantly higher both before and after dexamethasone than those in the nonpsychotic depressed group and higher before dexamethasone than in the control group. Similarly, the psychotically depressed group exhibited significantly higher cortisol levels both before and after dexamethasone than the nonpsychotic depressed group or the control group. In contrast, the psychotically depressed group had significantly lower postdexamethasone plasma norepinephrine levels compared to the nonpsychotic depressed group. In both patients and controls, plasma DA was significantly higher after dexamethasone administration than before, but the magnitude of the increase was 10 times greater in controls than in patients.

The antiglucocorticoid RU486 inhibits the ethanol-induced increase of tryptophan oxygenase

The effect of the glucocorticoid-antagonist RU486 (Roussel-Uclaf, France) on the increased activity of hepatic tryptophan oxygenase (TO) after administration of corticosterone and ethanol in rats was studied. RU486 (40 mg/kg per os) inhibited completely the effect of corticosterone (5-15 mg/kg injected intraperitoneally) on TO. Ethanol (4 g/kg) given intraperitoneally is followed by peak corticosterone concentrations comparable to those seen after the administration of 5-10 mg exogenous corticosterone, and increased the TO activity 3-fold 4 h after the injection. RU486 inhibited completely the ethanol-induced increase of TO, indicating that this increase is mediated by corticosterone.

Ethanol increases rat liver tryptophan oxygenase: evidence for corticosterone mediation

Acute administration of ethanol (4.0 g/kg) intragastrically or intraperitoneally induced rat liver tryptophan oxygenase (TO) activity 3-4 fold 4-5 hr later. Ethanol administration increased the concentration of plasma free tryptophan and free fatty acids. Pretreatment with a beta-receptor blocker, propranolol, modified the latter responses without affecting the TO induction due to ethanol. The rise of the level of plasma free tryptophan due to ethanol was too small to influence TO activity. Liver tryptophan concentration and TO half-life was unchanged after ethanol administration. Ethanol administration increased the concentration of plasma corticosterone sufficiently to increase TO activity. Pretreatment with a glucocorticoid antagonist blocked this TO response to ethanol. The increased TO activities found after ethanol or corticosterone treatment were influenced in the same manner and to the same extent by cycloheximide. Taken together it is concluded that ethanol induces TO through a rise of glucocorticoid hormones and not by a tryptophan-linked mechanism.

Ethanol-induced increase in liver tryptophan oxygenase activity in the starved rat: evidence against tryptophan mediation

A single oral dose of ethanol (4.0 g/kg) increased the activity of liver tryptophan oxygenase in starved male rats. The peak increase of 340% for the total activity and 400% for the holoenzyme activity occurred 6 hr after ethanol administration. At or after these peaks, the levels of tryptophan in plasma and brain but not in liver, decreased significantly. Plasma total tryptophan and brain tryptophan started to decrease significantly as early as 0.5-1.0 hr after the ethanol treatment, while the activity of liver tryptophan oxygenase was still at the control level. These findings suggest that not all the changes in tissue tryptophan concentrations seen after acute ethanol treatment are caused by increased liver tryptophan oxygenase activity. Prior to the increase in liver tryptophan oxygenase activity, an increase of 104 and 50% in plasma corticosterone and free tryptophan, respectively, were seen 15 min after ethanol treatment. However, the increase in liver tryptophan at this time appeared to be small (13%) and statistically insignificant. With tryptophan treatment, the initial peak levels of liver tryptophan and plasma free tryptophan required to stimulate an increase in tryptophan oxygenase activity were 170 times higher than those caused by ethanol. It was therefore concluded that increases in plasma and liver tryptophan after acute ethanol ingestion, probably mediated by the lipolytic action of ethanol, are too small to cause the increase in liver tryptophan oxygenase activity seen after ethanol administration. However, experiments with different corticosterone doses showed that ethanol-induced increases in plasma corticosterone concentrations are high enough to cause an increase in liver tryptophan oxygenase activity.

Cushing's syndrome and depression--a prospective study of 26 patients

Of 26 patients with active Cushing's syndrome assessed before and at three and 12 months after treatment, 21 had pituitary-dependent disease. Median urinary free cortisol values (per 24 hours) were 680, 180 and 200 nmol at zero, three and 12 months (normal less than 270 nmol), with significant improvement (P less than 0.001) at three and 12 months. Depression on the Hamilton rating scale was significantly less at three months (P less than 0.01) and at 12 months (P less than 0.001). We have already demonstrated that some patients with Cushing's syndrome have PSE diagnoses of depression and are more depressed than patients with other pituitary tumours. This is the firmest evidence to date that when Cushing's syndrome occurs it commonly causes depressive illness.

Mechanism of decline in rat brain 5-hydroxytryptamine after induction of liver tryptophan pyrrolase by hydrocortisone: roles of tryptophan catabolism and kynurenine synthesis

1 Two mechanisms have been proposed to explain the decline in brain tryptophan and 5-hydroxytryptamine (5-HT) after administration of hydrocortisone and the subsequent induction of liver pyrrolase. These are depletion of tryptophan by high rates of tryptophan catabolism and inhibition of tryptophan uptake by elevated levels of the tryptophan catabolite, kynurenine.2 The increase in plasma kynurenine after hydrocortisone injection (25 mg/kg) was small, and kynurenine, at a concentration ten fold greater, did not inhibit tryptophan uptake by brain as measured by the Oldendorf technique. Thus, inhibition of tryptophan uptake by kynurenine is not an important mechanism in the control of brain tryptophan and 5-HT.3 The decline in brain tryptophan after hydrocortisone was comparable to that seen in other tissues, which comprise more than half of the body weight of a rat.4 The total decline in free tryptophan stores in whole animals treated with hydrocortisone was estimated to be about 450 mug. This amount of tryptophan would be catabolized by tryptophan pyrrolase in about 20 min, when the enzyme is induced, according to an earlier estimate of the rate of tryptophan catabolism in vivo.5 Tryptophan pyrrolase activity remains high for much longer than 20 min, suggesting that there is net protein catabolism, which releases tryptophan and prevents non-protein tryptophan levels falling very far.6 These results demonstrate that the decline in brain tryptophan and 5-HT after hydrocortisone is caused by depletion of tryptophan stores due to the high activity of tryptophan pyrrolase. However, our data suggest that this effect is diminished by release of tryptophan from proteins. Thus, peripheral protein metabolism may be an important factor in the control of brain tryptophan levels and 5-HT synthesis.

Possible involvement of the enhanced tryptophan pyrrolase activity in the corticosterone- and starvation-induced increases in concentrations of nicotinamide-adenine dinucleotides (phosphates) in rat liver

1. Deoxycorticosterone, which does not enhance tryptophan pyrrolase activity, also fails to alter the concentrations of the NAD(P) couples in livers of fed rats, whereas corticosterone increases both pyrrolase activity and dinucleotide concentrations. 2. Starvation of rats increases serum corticosterone concentration, lipolysis, tryptophan availability to the liver, tryptophan pyrrolase activity and liver [NADP(H)]. Glucose prevents all these changes. 3. The beta-adrenoceptor-blocking agent propranolol prevents the starvation-induced lipolysis and the consequent increase in tryptophan availability to the liver, but does not influence the increase in serum corticosterone concentration, liver pyrrolase activity and [NADP(H)]. 4. Actinomycin D, which prevents the starvation-induced increases in liver pyrrolase activity and [NADP(H)], does not affect those in serum corticosterone concentration and tryptophan availability to the liver. 5. Allopurinol, which blocks the starvation-induced enhancement of pyrrolase activity, also abolishes the increases in liver [NADP(H)], but not those in serum corticosterone concentration or tryptophan availability to the liver. 6. It is suggested that liver tryptophan pyrrolase activity plays an important role in NAD+ synthesis from tryptophan in the rat.

The role of liver tryptophan pyrrolase in the opposite effects of chronic administration and subsequent withdrawal of drugs of dependence on rat brain tryptophan metabolism

1. Chronic administration of morphine, nicotine or phenobarbitone has previously been shown to inhibit rat liver tryptophan pyrrolase activity by increasing hepatic [NADPH], whereas subsequent withdrawal enhances pyrrolase activity by a hormonal-type mechanism. 2. It is now shown that this enhancement is associated with an increase in the concentration of serum corticosterone. 3. Chronic administration of the above drugs enhances, whereas subsequent withdrawal inhibits, brain 5-hydroxytryptamine synthesis. Under both conditions, tryptophan availability to the brain is altered in the appropriate direction. 4. The chronic drug-induced enhancement of brain tryptophan metabolism is reversed by phenazine methosulphate, whereas the withdrawal-induced inhibition is prevented by nicotinamide. 5. The chronic morphine-induced changes in liver [NADPH], pyrrolase activity, tryptophan availability to the brain and brain 5-hydroxytryptamine synthesis are all reversed by the opiate antagonist naloxone. 6. It is suggested that the opposite effects on brain tryptophan metabolism of chronic administration and subsequent withdrawal of the above drugs of dependence are mediated by the changes in liver tryptophan pyrrolase activity. 6. Similar conclusions based on similar findings have previously been made in relation to chronic administration and subsequent withdrawal of ethanol. These findings with all four drugs are briefly discussed in relation to previous work and the mechanism(s) of drug dependence.

Metabolism of an oral tryptophan load. I: Effects of dose and pretreatment with tryptophan

1 The metabolism of three oral doses of L-tryptophan (50, 25 and 10 mg/kg) in healthy young males has been investigated. 2 There was a linear relationship between both peak and area under curve of the total plasma tryptophan concentrations whilst the relationship between these parameters and plasma free tryptophan was hyperbolic. 3 Before the tryptophan load about 85% of plasma tryptophan was bound to albumin. As plasma tryptophan concentrations increased there was a hyperbolic increase in free tryptophan. Scatchard analysis revealed 1.4 binding sites/molecule albumin with a dissociation constant (Kd) of 57.9 microM. Following administration of L-tryptophan (50 mg/kg) twice daily for 7 days there was no alteration in the number of binding sites but the dissociation constant (Kd) had decreased to 30.9 microM. 4 L-Tryptophan (50 mg/kg twice daily for 7 days) markedly increased both basal plasma total and free tryptophan. However following a further load the total tryptophan curve was comparable to that seen after acute administration. The plasma free tryptophan curve was lowered relative to that seen after an acute dose. 5 Increasing the tryptophan dose shortened the plasma half-life and decreased the volume of distribution and the rate of clearance. Longer term tryptophan administration had no significant effect on plasma half-life or volume of distribution but did decrease the rate of plasma clearance. 6 The plasma kynurenine concentration increased with increasing tryptophan dose and basal concentrations increased markedly after longer term tryptophan administration. 7 Tryptophan administration either acutely or chronically produced little change in urinary tryptophan or 5-hydroxyindole acetic acid excretion. Urinary kynurenine and indole acetic acid excretion increased with increasing doses of tryptophan. 8 Data are discussed in relation to the administration of L-tryptophan for the treatment of depression.

Inhibition of rat brain tryptophan metabolism by ethanol withdrawal and possible involvement of the enhanced liver tryptophan pyrrolase activity

1. Chronic ethanol administration to rats was previously shown to enhance brain 5-hydroxytryptamine synthesis by increasing the availability of circulating tryptophan to the brain secondarily to the NAD(P)H-mediated inhibition of liver tryptophan pyrrolase activity. 2. At 24h after ethanol withdrawal, all the above effects were observed because liver [NAD(P)H] was still increased. By contrast, all aspects of liver and brain tryptophan metabolism were normal at 12 days after withdrawal. 3. At 7--9 days after withdrawal, brain 5-hydroxytryptamine synthesis was decreased, as was tryptophan availability to the brain. Liver tryptophan pyrrolase activity at these time-intervals was maximally enhanced. 4. Administration of nicotinamide during the withdrawal phase not only abolished the withdrawal-induced enhancement of tryptophan pyrrolase activity on day 8, but also maintained the inhibition previously caused by ethanol. Under these conditions, the withdrawal-induced decreases in brain 5-hydroxytryptamine synthesis and tryptophan availability to the brain were abolished, and both functions were enhanced. Nicotinamide alone exerted similar effects in control rats. 5. It is suggested that ethanol withdrawal inhibits brain 5-hydroxytryptamine synthesis by decreasing tryptophan availability to the brain secondarily to the enhanced liver tryptophan pyrrolase activity. 6. The results are discussed in relation to the possible involvement of 5-hydroxytryptamine in dependence on ethanol and other drugs.

Cushing's syndrome, tryptophan and depression

Fifteen patients with active Cushing's syndrome have been compared with 15 other patients who had been treated successfully for Cushing's syndrome and with 13 patients with other pituitary tumours. Depression was the main psychiatric diagnosis made by the CATEGO programme after Present State Examinations. Patients with active Cushing's syndrome were significantly more depressed (Hamilton Rating Scores), than were the other patients. Compared with the control patients, those with active Cushing's syndrome had slightly lower plasma concentrations of total tryptophan, though the concentrations of freely diffusible tryptophan were not significantly changed.

Tryptophan disposition in psychiatric patients before and after stress

Non-esterified fatty acid and total and free tryptophan were determined in the plasma of psychiatric patients unselected with respect to psychiatric diagnosis and in the plasma of normal subjects before and after physiological and psychiatric tests. Retarded patients had significantly low total and free tryptophan values which correlated negatively with agitation. Total tryptophan fell significantly after testing in the non-retarded subjects. The only biochemical abnormality significantly associated with a diagnosis of primary depression was the rise of plasma non-esterified fatty acid after testing. Thus, tryptophan abnormalities were associated more with psychiatric rating scores than with diagnoses.

Fatty acid and tryptophan changes on disturbing groups of rats and caging them singly

The effects of disturbing groups of 24 hr fasted rats on plasma unesterified fatty acid (UFA) and tryptophan concentrations and brain tryptophan concentrations were investigated. Removing rats from cages rapidly increased plasma UFA and corticosterone and decreased plasma and whole blood tryptophan of cage mates. The disturbance also appeared to influence biochemical values of rats in other cages within the same chamber. Effects specific to individual cages were also suggested. In subsequent experiments 24 fasting rats caged together were rapidly transferred to 24 separate cages and killed at intervals. Plasma UFA rose to a maximum by 12 min and then fell toward initial values. Plasma total tryptophan concurrently fell then rose. Its percentage in the free (ultrafilterable) state, and in some experiments the absolute values of free tryptophan rose then fell. When the latter rise was marked then brain tryptophan and the 5-HT metabolite 5-hydroxyindoleacetic acid rose. Tyrosine changes were negligible. Thus altered brain tryptophan level and 5-HT metabolism may be associated with plasma tryptophan changes caused by brief environmental disturbance.

The acute effects of ethanol on liver and brain tryptophan metabolism

1. The effects of acute ethanol administration of liver and brain tryptophan metabolism are reviewed. 2. Ethanol enhances the activity of rat liver tryptophan pyrrolase by increasing the availability of circulating free tryptophan to the liver by catecholamine-mediated lipolysis followed by displacement of protein-bound serum tryptophan. 3. The response of the mouse liver enzyme to ethanol is strain-dependent. Ethanol activates the enzyme in CBA/CA but not in C57/BL mice. 4. Ethanol exerts a biphasic effect on the concentrations of rat brain tryptophan, 5-hydroxytryptamine and 5-hydroxyindol-3-ylacetic acid. 5. Both aspects of this biphasic effect are associated with an altered availability of circulating free tryptophan. 6. The initial enhancement by ethanol of brain tryptophan metabolism may be due to the above-mentioned lipolytic mechanism, whereas the subsequent decrease in brain indoles may be caused by the enhanced tryptophan pyrrolase activity. 7. Brain tryptophan metabolism is decreased by ethanol in CBA/CA whereas no change is observed in that in C57/BL mice. 8. These results are discussed in relation to previous work on the acute effects of ethanol on rat and mouse brain 5-hydroxytryptamine metabolism.

Effect of aminophylline on tryptophan and other aromatic amino acids in plasma, brain and other tissues and on brain 5-hydroxytryptamine metabolism

1 Aminophylline and other methylxanthines increase brain tryptophan and hence 5-hydroxytryptamine turnover. The mechanism of this effect of aminophylline was investigated. 2 At lower doses (greater than 100 mg/kg i.p.) the brain tryptophan increase could be explained by the lipolytic action of the drug, i.e. increased plasma unesterified fatty acid freeing plasma tryptophan from protein binding so that it became available to the brain. 3 Plasma unesterified fatty acid did not increase when aminophylline (109 mg/kg i.p.) was given to nicotinamide-treated rats but as both plasma total and free tryptophan rose, a tryptophan increase in the brain still occurred. 4 The rise in brain tryptophan concentration following the injection of a higher dose of the drug (150 mg/kg i.p.) could no longer be explained by a rise of plasma free tryptophan as the ratio of brain tryptophan to plasma free tryptophan rose considerably. Plasma total tryptophan fell and the plasma insulin concentration rose. 5 The increase of brain tryptophan concentration after injection of 150 mg/kg aminophylline appeared specific for this amino acid as brain tyrosine and phenyllanine did not increase. However as their plasma concentrations fell the brain/plasma ratio for all three amino acids rose. 6 The higher dose of aminophylline increased the muscle concentration of tryptophan but that of tyrosine fell and that of phenylalanine remained unaltered. The liver concentrations were not affected. 7 The aminophylline-induced increase of the ratio of brain tryptophan of plasma free tryptophan no longer occurred when the drug was given to animals injected with the beta-adrenoreceptor blocking agent propranolol or the diabetogenic agent streptozotocin. 8 The changes in brain tryptophan upon aminophylline injection may be explained by (a) increased availability of plasma tryptophan to the brain due to increased lipolysis and (b) increased effectiveness of uptake of tryptophan by the brain due to increased insulin secretion.

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.

Tryptophan metabolism in the isolated perfused liver of the rat: effects of tryptophan concentration, hydrocortisone and allopurinol on tryptophan pyrrolase activity and kynurenine formation

1 The effect of tryptophan concentration on the rate of kynurenine appearance and tryptophan disappearance in the medium perfused through the isolated liver of the rat has been investigated. The effect of pretreatment of the rat with hydrocortisone or allopurinol was also examined, together with the effects of these treatments on liver tryptophan pyrrolase activity measured in vitro at the beginning and end of perfusion. 2 Hydrocortisone (5 mg/kg) injection 3 h before perfusion resulted in a four-fold increase in kynurenine production by the liver during perfusion with a medium containing either 0.1 mmol/1 or 1.0 mmol/1 tryptophan. Injection of allopurinol (20 mg/kg) together with hydrocortisone and addition of allopurinol (4 mg/100 ml) to the medium abolished the hydrocortisone-induced rise of kynurenine in the 0.1 mmol/tryptophan medium but not the 1.0 mmol/1 tryptophan medium. 3 Injection of cycloheximide (30 mg/kg) with hydrocortisone (5 mg/kg) 3 h before perfusion inhibited the hydrocortisone-induced rise of kynurenine production and the increase in pyrrolase activity measured in vitro both before and at the end of perfusion with 1.0 mmol/1 tryptophan. This last result suggests that protein synthesis is involved not only in hydrocortisone induction of pyrrolase but also in substrate induction. 4 Kynurenine production in the 1.0 mmol/1 tryptophan medium was less in both saline- and hydrocortisone-treated older rats (335-450 g) compared to younger rats (180-220 g). In agreement with a previous study, pyrrolase activity in vitro was also lower in both saline- and hydrocortisone- treated older rats at the beginning of the perfusion although activity had risen equally in both young and older rats at the end of perfusion. 5 There was little correlation between the rate of tryptophan disappearance from the medium and the activity of tryptophan pyrrolase either as measured in vitro or as indicated by the rate of kynurenine production. 6 In general, the production of kynurenine in the medium at the end of the 60 min perfusion was indicative of in vitro pyrrolase activity at the start of the perfusion. 7 It is concluded that while in vitro pyrrolase assay does not give a quantitative index of kynurenne production, it does provide a qualitative index. Furthermore, if kynurenine production in the isolated perfused liver of the rat is indicative of in vivo pyrrolase activity, then hydrocortisone must induce pyrrolase activity in vivo.

Effects of monoamine oxidase inhibition by clorgyline, deprenil or tranylcypromine on 5-hydroxytryptamine concentrations in rat brain and hyperactivity following subsequent tryptophan administration

1 The effect of various doses of tranylcypromine on the degree of inhibition of rat brain monoamine oxidase (MAO) using 5-hydroxytryptamine (5-HT), dopamine and phenylethylamine as substrates has been examined 120 min after injection of the inhibitor. The concentration of brain 5-HT was also examined both after tranylcypromine alone and also when L-tryptophan (100 mg/kg) had been given 30 min after the tranylcypromine. 2 All doses of tranylcypromine greater than 2.5 mg/kg totally inhibited MAO oxidation of 5-HT, phenylethylamine and dopamine as measured in vitro and produced a similar rise of brain 5-HT in vivo. When tryptophan was also given, there was a further rise of brain 5-HT, which was comparable after all doses of tranylcypromine above 2.5 mg/kg and the characteristic syndrome of hyperactivity made is appearance. 3 Clorgyline (a "Type A" MAO inhibitor), in doses up to 10 mg/kg, did not totally inhibit MAO activity towards phenylethylamine although it did inhibit 5-HT oxidation by 100%. Deprenil (a "Type B" MAO inhibitor) at doses up to 10 mg/kg did not fully inhibit 5-HT oxidation although phenylethylamine oxidation was inhibited almost completely. Administration of either compound alone did not produce as great an accumulation of brain 5-HT as that seen after tranylcypromine (2.5 mg/kg) and subsequent administration of tryptophan did not cause hyperactivity or the rise of brain 5-HT seen after tranylcypromine (2.5 mg/kg) plus tryptophan. 4 Administration of clorgyline plus deprenil (2.5 mg/kg of each) almost totally inhibited oxidation of both 5-HT and phenylethylamine; subsequent tryptophan administration resulted in a rise of brain 5-HT nearly as great as that seen following tranylcypromine (2.5 mg/kg) plus tryptophan and the animals became hyperactive. 5 No evidence was found pointing to the formation of any other 5-substituted indole in the brain following tranylcypromine plus L-tryptophan administration as suggested by others. 6 It is concluded that while 5-HT may normally be metabolized in the brain by "Tye A" MAO in vivo, when this form is inhibited, 5-HT can still be metabolized by "Type B" enzyme. It is only when both forms are almost totally inhibited that the largest rise of brain 5-HT is seen and subsequent tryptophan administration produces the hyperactivity syndrome.