The effects of glucocorticoids on the availability of L-tryptophan and tyrosine in the plasma of depressed patients

The effect of inflammation, SARS-CoV-2 infection, age and mental health on serotonin, and kynurenine and catecholamine pathway metabolites

BACKGROUND

A high prevalence of mental disorders following COVID-19 has been described. It is therefore essential to elucidate underlying biological mechanisms linking SARS-CoV-2 infection and mental health. The kynurenine and catecholamine metabolic pathways are modulated by inflammation and can affect systemic levels of serotonin and dopamine. Their activity may hence link physical disorders with mental health. We investigated factors that affect kynurenine and catecholamine pathway activity in SARS-CoV-2 infection and recovery.

METHODS

The cross-sectional SIMMUN (n = 165) and longitudinal INCOV cohort (n = 167, Su et al. 2022) were analyzed. Demographic and clinical characteristic, inflammatory markers, SARS-CoV-2 infection, symptoms of depression and anxiety (HADS), and mental stress (PSS-4) served as explanatory variables. Blood serotonin and markers of kynurenine (kynurenine/tryptophan ratio), and catecholamine pathway activity (dopamine 3-O-sulfate, phenylalanine/tyrosine ratio) were modeled by multi-parameter linear regression.

RESULTS

In the SIMMUN cohort, the inflammatory marker neopterin (β = 0.47 [95% CI: 0.34-0.61]), SARS-CoV-2-positivity (0.42 [0.16-0.68]), mental stress (0.18 [0.055-0.31]), and age (0.26 [0.12-0.39]) were positively associated with the kynurenine/tryptophan ratio. The phenylalanine/tyrosine ratio was lower in SARS-CoV-2-positive than uninfected participants (-0.38 [-0.68 to -0.08]). In the INCOV cohort, markers of inflammation were associated with lower serotonin (IL6: -0.22 [-0.38 to -0.053]) and dopamine 3-O-sulfate levels (interferon-gamma: -0.15 [-0.26 to -0.036]). Serotonin (0.76 [0.34-1.2]) and dopamine 3-O-sulfate levels (0.63 [0.28-0.99]) were higher during recovery than in acute SARS-CoV-2 infection.

CONCLUSION

SARS-CoV-2 infection, inflammation, age and mental stress are key independent predictors of kynurenine pathway activity, which may influence serotonin availability. The catecholamine pathway was also affected in SARS-CoV-2 infection. Altered activity of these pathways may contribute to impaired mental health following COVID-19.

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.

The new '5-HT' hypothesis of depression: cell-mediated immune activation induces indoleamine 2,3-dioxygenase, which leads to lower plasma tryptophan and an increased synthesis of detrimental tryptophan catabolites (TRYCATs), both of which contribute to the onset of depression

This paper reviews the body of evidence that not only tryptophan and consequent 5-HT depletion, but also induction of indoleamine 2,3-dioxygenase (IDO) and the detrimental effects of tryptophan catabolites (TRYCATs) play a role in the pathophysiology of depression. IDO is induced by interferon (IFN)γ, interleukin-6 and tumor necrosis factor-α, lipopolysaccharides and oxidative stress, factors that play a role in the pathophysiology of depression. TRYCATs, like kynurenine and quinolinic acid, are depressogenic and anxiogenic; activate oxidative pathways; cause mitochondrial dysfunctions; and have neuroexcitatory and neurotoxic effects that may lead to neurodegeneration. The TRYCAT pathway is also activated following induction of tryptophan 2,3-dioxygenase (TDO) by glucocorticoids, which are elevated in depression. There is evidence that activation of IDO reduces plasma tryptophan and increases TRYCAT synthesis in depressive states and that TDO activation may play a role as well. The development of depressive symptoms during IFNα-based immunotherapy is strongly associated with IDO activation, increased production of detrimental TRYCATs and lowered levels of tryptophan. Women show greater IDO activation and TRYCAT production following immune challenge than men. In the early puerperium, IDO activation and TRYCAT production are associated with the development of affective symptoms. Clinical depression is accompanied by lowered levels of neuroprotective TRYCATs or increased levels or neurotoxic TRYCATs, and lowered plasma tryptophan, which is associated with indices of immune activation and glucocorticoid hypersecretion. Lowered tryptophan and increased TRYCATs induce T cell unresponsiveness and therefore may exert a negative feedback on the primary inflammatory response in depression. It is concluded that activation of the TRYCAT pathway by IDO and TDO may be associated with the development of depressive symptoms through tryptophan depletion and the detrimental effects of TRYCATs. Therefore, the TRYCAT pathway should be a new drug target in depression. Direct inhibitors of IDO are less likely to be useful drugs than agents, such as kynurenine hydroxylase inhibitors; drugs which block the primary immune response; compounds that increase the protective effects of kynurenic acid; and specific antioxidants that target IDO activation, the immune and oxidative pathways, and 5-HT as well.

L-Tryptophan: Basic Metabolic Functions, Behavioral Research and Therapeutic Indications

An essential component of the human diet, L-tryptophan is critical in a number of metabolic functions and has been widely used in numerous research and clinical trials. This review provides a brief overview of the role of L-tryptophan in protein synthesis and a number of other metabolic functions. With emphasis on L-tryptophan's role in synthesis of brain serotonin, details are provided on the research uses of L-tryptophan, particularly L-tryptophan depletion, and on clinical trials that have been conducted using L-tryptophan supplementation. The ability to change the rates of serotonin synthesis in the brain by manipulating concentrations of serum tryptophan is the foundation of much research. As the sole precursor of serotonin, experimental research has shown that L-tryptophan's role in brain serotonin synthesis is an important factor involved in mood, behavior, and cognition. Furthermore, clinical trials have provided some initial evidence of L-tryptophan's efficacy for treatment of psychiatric disorders, particularly when used in combination with other therapeutic agents.

A link between stress and depression: shifts in the balance between the kynurenine and serotonin pathways of tryptophan metabolism and the etiology and pathophysiology of depression

Alteration of tryptophan (TRP) metabolism elicited by proinflammatory cytokines has gained attention as a new concept to explain the etiological and pathophysiological mechanisms of major depression. The kynurenine (KYN) pathway, which is initiated by indoleamine 2,3-dioxygenase (IDO), is the main TRP metabolic pathway. It shares TRP with the serotonin (5-HT) pathway. Proinflammatory cytokines induce IDO under stress, promote the KYN pathway, deprive the 5-HT pathway of TRP, and reduce 5-HT synthesis. The resultant decrease in 5-HT production may relate to the monoamine hypothesis of major depression. Furthermore, metabolites of the KYN pathway have neurotoxic/neuroprotective activities; 3-hydroxykynurenine and quinolinic acid are neurotoxic, whereas kynurenic acid is neuroprotective. The hippocampal atrophy that appears in chronic depression may be associated with imbalances in neurotoxic/neuroprotective activities. Because proinflammatory cytokines also activate the hypothalamo-pituitary-adrenal (HPA) axis, these imbalances may inhibit the hippocampal negative feedback system. Thus, changes in the TRP metabolism may also relate to the HPA axis-hyperactivity hypothesis of major depression. In this article, we review the changes in TRP metabolism by proinflammatory cytokines under stress, which is assumed to be a risk factor for major depression, and the relationship between physiological risk factors for major depression and proinflammatory cytokines.

Implantation of a slow release corticosterone pellet induces long-term alterations in serotonergic neurochemistry in the rat brain

Many studies point to an involvement of deficits in the serotonergic nervous system and hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis function with depression. Indeed early life stress, involving HPA axis activation, may predispose susceptible individuals to develop depression in later life. This study investigates the effects of elevating the neuroendocrine stress hormone, corticosterone, for 1 week in adolescent rats on markers of serotonergic neurone function at adulthood. Slow release corticosterone pellets were implanted for 7 days and various serotonergic parameters, as well as plasma corticosterone levels, were measured on day 7 or on day 28 (21 days following removal of the pellet). The corticosterone implant attenuated weight gain and reduced adrenal weights compared to that in control rats implanted with a cholesterol pellet. After 7 days, with the implant still in place, the diurnal variation in plasma corticosterone was reduced so that the level was approximately at that of the evening peak throughout the day. Twenty-one days after removal of the implant, the diurnal variation in plasma corticosterone returned. Corticosterone treatment decreased [3H] 8-hydroxy-2-(di-n-propylamino)tetralin binding to the 5-hydroxytryptamine1A receptor in the cortex but not in the hippocampus. Corticosterone treatment also enhanced the circadian rhythm observed in 5-hydroxyindoleacetic acid level and the ratio of 5-hydroxyindoleacetic acid to the 5-hydroxytryptamine in the frontal cortex. Despite corticosterone pellet removal 21 days earlier, there was a persistent decrease in whole body and adrenal weight, cortical 5-hydroxytryptamine1A receptor binding and an alteration in the diurnal variation in the 5-hydroxytryptamine "turnover" in the frontal cortex.

Serotonin-immune interactions in elderly volunteers and in patients with Alzheimer's disease (DAT): lower plasma tryptophan availability to the brain in the elderly and increased serum interleukin-6 in DAT

The aims of this study were to examine the plasma availability of tryptophan, the precursor of 5-hydroxytryptamine (5-HT), and serum cytokines, such as interleukin-6 (IL-6) and IL-8, in normal elderly volunteers and in patients with Alzheimer's disease (DAT). Elderly normal volunteers (mean age = 78.3 +/- 5.7 years) had a significantly lower tryptophan/competing amino acids (valine + leucine + isoleucine + phenylalanine + tyrosine) ratio than younger subjects (mean age = 32.9 +/- 8.1 years). In normal volunteers, there were significant and inverse relationships between age and either plasma tryptophan or the tryptophan/competing amino acids ratio, and between the availability of tryptophan to the brain and serum IL-6 or IL-8. DAT patients had significantly higher serum IL-6, but not IL-8, than age-matched normal volunteers. There were no significant differences in the availability of tryptophan to the brain between DAT patients and age-matched normal volunteers. The results suggest that: 1) in normal humans, the availability of plasma tryptophan to the brain decreases with age, and with activation of the immune system; and 2) increased production of IL-6 may play a role in the pathogenesis of DAT.

Serotonin-immune interactions in detoxified chronic alcoholic patients without apparent liver disease: activation of the inflammatory response system and lower plasma total tryptophan

The aims of the present study were to examine (1) the inflammatory response system (IRS), through measurements of serum interleukin-6 (IL-6), soluble IL-6 receptor (sIL-6R), sgp130 (the soluble form of the IL-6 transducer signal protein), CC16 (Clara Cell protein; an endogenous anti-cytokine), IL-1R antagonist (IL-1RA), IL-8 and sCD14; and (2) the availability of plasma total tryptophan to the brain in chronic alcoholic patients without apparent liver disease (AWLD). Detoxified AWLD patients had significantly lower plasma tryptophan and serum CC16 and significantly higher serum IL-1RA and IL-8 concentrations than normal volunteers. There were significant correlations between the availability of tryptophan to the brain and serum IL-6, IL-8 and IL-1RA (all negative) and CC16 (positive). The results suggest that (1) there is, in detoxified AWLD patients, an activation of the monocytic arm of cell-mediated immunity and a lowered anti-inflammatory capacity of the serum; and that (2) lower availability of plasma tryptophan to the brain in detoxified AWLD patients is related to activation of the IRS. Lower CC16 may be one factor predisposing chronic alcoholic patients toward infectious disorders.

Serum levels of excitatory amino acids, serine, glycine, histidine, threonine, taurine, alanine and arginine in treatment-resistant depression: modulation by treatment with antidepressants and prediction of clinical responsivity

Previous research has revealed that major depression is accompanied by disorders in excitatory amino acids, e.g. glutamate and aspartate, and alterations in serum levels of other amino acids, e.g. serine, glycine and taurine. The aim of the present study was to examine serum levels of aspartate, asparagine, glutamate, glutamine, serine, glycine, threonine, histidine, alanine, taurine and arginine in major depression patients with treatment-resistant depression (TRD). No significant differences in the serum concentrations of any of the above amino acids could be found between patients with and without TRD and normal controls. Non-responders to treatment with antidepressants during a period of 5 weeks were characterized by significantly lower serum levels of aspartate, asparagine, serine, threonine and taurine. A 5-week period of treatment with antidepressants significantly reduced the serum levels of aspartate, glutamate and taurine, and significantly increased the serum concentrations of glutamine. The results suggest that alterations in serum levels of aspartate, asparagine, serine, threonine and taurine may predict the subsequent response to treatment with antidepressants, and that the latter may modulate serum levels of excitatory amino acids and taurine.

The neurobiology of tryptophan depletion in depression: effects of intravenous tryptophan infusion

BACKGROUND

Previous work has suggested that acute depletion of the serotonin (5-HT) precursor tryptophan (TRP) causes transient compensatory changes in the 5-HT system that might be exploited for their antidepressant effects. In this study, neuroendocrine and mood responses to intravenous (i.v.) infusion of TRP were examined in order to evaluate central 5-HT function in depressed patients undergoing acute TRP depletion.

METHODS

Thirty-eight drug-free patients with DSM-III-R major depression participated. Each patient underwent two randomized, double-blind TRP depletion tests, one sham and one active. At the estimated time of maximum TRP depletion, each patient received an i.v. infusion of TRP 100 mg/kg. Blood was obtained for serum cortisol, prolactin, and growth hormone. Mood was assessed using standardized rating scales.

RESULTS

The cortisol response to i.v. TRP was significantly greater during TRP depletion than during sham depletion. Depressive symptoms showed a tendency to decrease after i.v. TRP following active, but not sham, TRP depletion.

CONCLUSIONS

These findings are consistent with the present hypothesis and previous evidence that acute TRP depletion in drug-free depressed patients induces compensatory upregulation of postsynaptic 5-HT receptors. These changes are insufficient to serve as a means of effecting clinical improvement, but suggest that the antidepressant properties of rapid, marked manipulations of 5-HT function warrant further study.

Hypothalamic-pituitary-adrenal axis function and platelet serotonin concentrations in depressed patients

Plasma cortisol and platelet serotonin (5-hydroxytryptamine, 5-HT) concentrations were determined in 39 male psychotic and 39 male non-psychotic depressed inpatients, and in 69 male healthy control subjects. Psychotic or non-psychotic depressed patients had higher predexamethasone plasma cortisol levels than found in the control group. After the dexamethasone suppression test (DST), psychotic and non-psychotic depressed patients were subdivided into suppressors and non-suppressors. Psychotic and non-psychotic patients had significantly different platelet 5-HT concentrations among themselves and compared with the control group. However, there was no significant correlation between plasma cortisol levels and platelet 5-HT concentrations. Dexamethasone administration did not affect platelet 5-HT concentrations within subtypes of depressed patients. Abnormal cortisol suppression after the DST occurred more frequently in psychotic than in non-psychotic patients. Platelet 5-HT and plasma cortisol concentrations were decreased in patients with pronounced suicidal behaviour. Our results suggest that plasma cortisol and platelet 5-HT concentrations might serve as independent biological markers for different subtypes of depression.

Serotonin-immune interactions in major depression: lower serum tryptophan as a marker of an immune-inflammatory response

Serum total tryptophan and the five competing amino acids (CAA), i.e., valine, leucine, tyrosine, phenylalanine, and isoleucine were determined in 35 major depressed subjects of whom 27 with treatment resistant depression (TRD), and 15 normal controls. Twenty-five of the depressed subjects had repeated measurements of the amino acids both before and after antidepressive treatment. The following immune-inflammatory variables were assayed in the above subjects: serum zinc (Zn), total serum protein (TSP), albumin (Alb), transferrin (Tf), iron (Fe), high-density lipoprotein cholesterol (HDL-C), number of peripheral blood leukocytes, and the CD4+/CD8+ T cell (T-helper/T-suppressor) ratio. Serum tryptophan and the tryptophan/CAA ratio were significantly lower in major depressed subjects than in normal controls. The tryptophan/CAA ratio was significantly lower in patients with TRD than in patients without TRD and normal controls. There were no significant alterations in any of the amino acids upon successful therapy. There were significant correlations between serum tryptophan and serum Zn, TSP, Alb, Tf, Fe, and HDL-C (all positive), and number of leukocytes and the CD4+/CD8+ T-cell ratio (all negative). The tryptophan/CAA ratio was significantly and negatively related to the number of leukocytes and the CD4+/CD8+ T-cell ratio. The results suggest that (a) TRD is characterized by lower availability of serum tryptophan; (b) the availability of tryptophan may remain decreased despite clinical recovery; and (c) the lower availability of tryptophan is probably a marker of the immune-inflammatory response during major depression.

Red blood cell L-tryptophan uptake in depression: kinetic analysis in untreated depressed patients and healthy volunteers

Kinetic parameters (Vmax and K(m)) of L-tryptophan (TRP) uptake into red blood cells (RBC) were measured in 72 drug-free depressed inpatients and 35 healthy volunteers. Mean Vmax and K(m) values were not significantly different between patients and volunteers. The frequency distributions of Vmax values, however, largely differed in the two groups: Vmax values were homogeneous in the volunteers while they were widely scattered in the depressed patients. Only 15 out of the 72 depressed patients (21%) had Vmax values within 1 SD from the mean control value. Forty-four percent of the patients (n = 32) had Vmax values above the control mean + 1 SD in 11 patients and above the control mean + 2 SD in 21 patients. Thirty-five depressed patients (n = 25) had Vmax values below the control mean - 1 SD in 8 patients and below the control mean - 2 SD in 17 patients. High and low K(m) values were observed in combination with high and low Vmax values. The alterations in kinetic parameters were neither associated with severity of depression nor with a specific diagnostic subtype of depression. The data show abnormalities in RBC L-TRP uptake in most depressed patients that likely reflect a disturbance in peripheral availability of TRP on which central serotonin synthesis closely depends.

Evidence for an immune response in major depression: a review and hypothesis

1. This paper reviews recent findings on cellular and humoral immunity and inflammatory markers in depression. 2. It is shown that major depression may be accompanied by systemic immune activation or an inflammatory response with involvement of phagocytic (monocytes, neutrophils) cells, T cell activation, B cell proliferation, an "acute" phase response with increased plasma levels of positive and decreased levels of negative acute phase proteins, higher autoantibody (antinuclear, antiphospholipid) titers, increased prostaglandin secretion, disorders in exopeptidase enzymes, such as dipeptidyl peptidase IV, and increased production of interleukin (IL)-1 beta and IL-6 by peripheral blood mononuclear cells. 3. It is hypothesized that increased monocytic production of interleukins (Il-1 beta and Il-6) in severe depression may constitute key phenomena underlying the various aspects of the immune and "acute" phase response, while contributing to hypothalamic-pituitary-adrenal-axis hyperactivity, disorders in serotonin metabolism, and to the vegetative symptoms (i.e. the sickness behavior) of severe depression.

Relationships between lower plasma L-tryptophan levels and immune-inflammatory variables in depression

Despite much research, the pathophysiology underlying lower L-tryptophan (L-TRP) availability in major depression has remained elusive. The present study investigates whether lower L-TRP availability in major depression is related to immune activation which may occur in that illness and is known to modulate L-TRP metabolism. Toward this end, the authors have measured the following in depressed patients and normal control subjects: plasma levels of L-TRP, and the competing amino acids (CAA) valine, leucine, isoleucine, tyrosine, and phenylalanine, together with indices of immune function such as haptoglobin (Hp) and transferrin (Tf) plasma levels, dipeptidyl peptidase IV (DPP IV) serum activity, and mitogen-induced culture supernatant interleukin-6 (Il-6) production. Both plasma levels of L-TRP and the L-TRP/CAA ratio were significantly lower in major depressed subjects as compared with healthy control subjects. There were significant correlations between plasma L-TRP levels, on the one hand, and Tf plasma levels, DPP IV activity (both positive), Il-6 production, and Hp plasma levels (both negative), on the other. Up to 63.7% of the variance in L-TRP plasma concentrations could be explained by DPP IV, Hp, Il-6 values, and gender. Up to 50% of the variance in the L-TRP/CAA ratio could be explained by Hp values (negative correlation) and gender. It is hypothesized that lower plasma L-TRP availability in major depression may be related to the immune response in that illness.

Can disulfiram potentiate the dexamethasone suppression test in depressive patients?

Transient DST non-suppression during disulfiram-treatment was fortuitously observed in a depressed patient. A disulfiram-potentiated DST (DSTd) was constructed by giving 800 mg disulfiram on the dexamethasone and postdexamethasone days. DST and DSTd were compared intraindividually in 21 patients and 14 controls. Disulfiram-potentiation was significant in constantly depressed patients, and in questionable controls due to possible subclinical depression or somatic factors which may cause 'false-positive' results. It did not occur in depressed patients during marked improvement or long-term recovery, and not in accurately screened controls. DSTd was significantly correlated with depression ratings, whereas DST was not.

Disturbances in dexamethasone suppression test and lower availability of L-tryptophan and tyrosine in early puerperium and in women under contraceptive therapy

This study investigates the function of the hypothalamic-pituitary-adrenal (HPA)-axis and the availability of L-tryptophan and tyrosine to the brain in postpartum women and in women taking long-term oral contraceptives. To this end, we have measured the following parameters in 50 women (i.e. 9 normal controls, 10 women taking oral contraceptives, and 31 postpartum females): plasma cortisol, L-tryptophan, tyrosine and the amino acids (CAA) known to compete with them for transport through the blood-brain barrier. We have determined the effects of 1 mg of dexamethasone on the above-mentioned biological markers in postpartum females. Plasma cortisol and tyrosine were significantly higher and lower, respectively, in puerperium and in women under contraceptive therapy as opposed to normal controls. L-Tryptophan was significantly lower in postpartum females, whilst the L-tryptophan/CAA ratio did not differ across the three study groups. Postpartum females revealed a significant negative relationship between the availability of L-tryptophan to the brain and postpartum mood, as measured by Zung's Depression and Anxiety Scales and State Anxiety Inventory. Dexamethasone had a significant suppressive effect on L-tryptophan/CAA and tyrosine/CAA ratios, with cortisol nonsuppression appearing in 82% of the postpartum females.

A study on the blunted natural killer cell activity in severely depressed patients

Recently, some investigators have established a blunted natural killer cell activity (NKCA) in severely depressed patients. In order to replicate these findings NKC cytotoxicity assays--on fresh cell suspensions in human plasma and fetal calf serum--were performed in healthy controls and depressed inpatients. Instead of the commonly used 51Cr-release assay we have used a fluorescent NKC cytotoxicity assay, which allows a greater sensitivity. We observed a significantly blunted NKCA in melancholic patients as compared with healthy controls and minor depressives, whilst simple major depressives exhibited an intermediate position. NKC cytotoxicity assays in fetal calf serum were significantly and negatively correlated with the severity of illness. We were unable to establish any relationship between NKCA and measures of hypothalamic-pituitary-adrenal-axis function, such as baseline, postdexamethasone plasma cortisol and 24 hr urinary cortisol secretion. In addition, we did not find any effects of dexamethasone administration (1 mg orally) on NKCA.

Decreased plasma tryptophan concentration in major depression: relationship to melancholia and weight loss

Plasma total tryptophan (TRP) concentration was significantly lower in 31 patients with major depression compared to a healthy control group. The ratio of plasma TRP concentration to that of other branch chain amino acids (the TRP:BCAA ratio) was also decreased. Further analysis revealed that the decrease in plasma TRP and TRP:BCAA ratio was most apparent in patients with major depression and melancholia. Overall, women but not men had significantly decreased plasma tryptophan concentrations, perhaps because of a contributory effect of weight loss; this latter effect, however, could not be distinguished clearly from a diagnosis of melancholia. Our data suggest that in some depressed patients, reductions in plasma tryptophan availability may contribute to abnormalities in brain 5-hydroxytryptamine function.

The decreased availability of L-tryptophan in depressed females: clinical and biological correlates

1. The plasma levels of L-tryptophan (L-TRP) and the sum of five competing amino acids (CAA) namely tyrosine, phenylalanine, valine, leucine, isoleucine, were determined in 79 depressed females categorized according to the DSM-III. 2. In these patients the authors measured several parameters known to affect the availability of the above amino acids, i.e. triidothyronine (FT3) and thyroxine (FT4), vanilylmandelic acid (VMA), noradrenaline and adrenaline in 24 hr urine, the sex hormonal and nutritional state. 3. The 1 mg dexamethasone suppression test was performed and the pre and postdexamethasone cortisol and adrenocorticotropic hormone (ACTH) levels were determined at 8 a.m. 4. L-TRP and the ratio L-TRP/CAA were significantly lower in severely depressed females (296.X3, 296.X4) as compared with minor (300.40, 309.00) and simple major depressives (296.X2). The ratio L-TRP/CAA performed well as a clinical tool separating melancholic from minor depression. 5. FT3, FT4, VMA and noradrenaline were significantly increased in the severely depressed females, but these data did not correlate with the availability of L-TRP. Neither baseline cortisol nor the sex hormonal, nor the nutritional state related to the L-TRP data. The ratio L-TRP/CAA was significantly and negatively correlated with the postdexamethasone cortisol and ACTH values.