Free and total plasma tryptophan in endogenous depression

Mood effects of the amino acids tryptophan and tyrosine: 'Food for Thought' III

OBJECTIVE

Reflecting increased scientific interest in any nutritional contribution to the onset and treatment of mood disorders, we overview research into two neurotransmitter precursors - the amino acids tryptophan and tyrosine - particularly examining whether any deficiency increases risk to depression and whether those amino acids have any antidepressant properties.

METHOD

The theoretical relevance of the two amino acids was overviewed by considering published risk and intervention studies, technical papers and reviews.

RESULTS

There is some limited evidence, suggesting that depressed patients, especially those with a melancholic depression, have decreased tryptophan levels. Whether such findings reflect a causal contribution or are a consequence of a depressed state remains an open question. There is a small database supporting tryptophan preparations as benefitting depressed mood states. There is no clear evidence as to whether tyrosine deficiency contributes to depression, while the only randomized double-blind study examining tyrosine supplementation did not show antidepressant benefit.

CONCLUSION

Acute tryptophan depletion continues to provide a research tool for investigating the relevance of serotonin to depression onset. There is limited evidence that tryptophan loading is effective as a treatment for depression through its action of increasing serotonin production. Most clinical studies are dated, involve small sample sizes and/or were not placebo controlled. The development of the new serotonin reuptake inhibitor drugs seemingly signalled an end to pursuing such means of promoting increased serotonin as a treatment for depression. The evidence for tyrosine loading promoting catecholamine production as a possible treatment for depression appears even less promising, and depletion studies less informative.

Specificity of the acute tryptophan and tyrosine plus phenylalanine depletion and loading tests I. Review of biochemical aspects and poor specificity of current amino Acid formulations

The acute tryptophan or tyrosine plus phenylalanine depletion and loading tests are powerful tools for studying the roles of serotonin, dopamine and noradrenaline in normal subjects and those with behavioural disorders. The current amino acid formulations for these tests, however, are associated with undesirable decreases in ratios of tryptophan or tyrosine plus phenylalanine to competing amino acids resulting in loss of specificity. This could confound biochemical and behavioural findings. Compositions of current formulations are reviewed, the biochemical principles underpinning the tests are revisited and examples of unintended changes in the above ratios and their impact on monoamine function and behaviour will be demonstrated from data in the literature. The presence of excessive amounts of the 3 branched-chain amino acids Leu, Ile and Val is responsible for these unintended decreases and the consequent loss of specificity. Strategies for enhancing the specificity of the different formulations are proposed.

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.

Interferon, tryptophan and depression

Depression is a frequent comorbid disorder of many inflammatory diseases and it is suggested that brain inflammatory processes have a pathogenic role in mood dysregulation. Several immunocompromised patients have been treated with cytokines and long-term treatments have resulted in a variety of neuropsychiatric side-effects. The objective of the study was to present evidence for an association between the induction of neuropsychiatric side-effects during treatment with interferon-α (IFN-α) and changes in serotonergic and immunological parameters. Moreover, the use of IFN-α-induced depression as a paradigm for research into the pathophysiology of depressive disorders in general will be discussed. This literature review focused on the relationships between tryptophan, serotonin, cytokines and depression associated with interferon treatment. Immunotherapy with IFN-α influences several immunological and serotonergic parameters, and induces in most patients neurovegetative, somatic and depressive symptoms. Literature findings indicate that the development of depressive symptoms in patients undergoing cytokine therapy are secondary to cytokine induction and could be mediated by a reduced availability of tryptophan to the brain, resulting ultimately in decreased serotonergic activity. Changes in the metabolism of tryptophan and consequently of serotonin may play a role in the pathophysiology of interferon-induced depression. Studies on interferon-induced neuropsychiatric side-effects may be a promising research paradigm and shed light on the role of immunological and serotonergic factors in the pathogenesis of depressive disorders in general. However, first the appropriate symptomatology of the interferon-induced depressive states has to be documented.

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.

Platelet serotonin and plasma tryptophan decreases in endogenous depression. Clinical, therapeutic, and biological correlations

Platelet serotonin (5-HT) and plasma tryptophan (free and total) levels were measured in 25 unmedicated depressed patients and in 25 age- and sex-matched healthy controls. The same parameters were determined in the patients after 3 weeks and 2 months of imipramine treatment. Comparisons between patients and control values showed a significant decrease in total plasma tryptophan and platelet 5-HT in unmedicated patients. During treatment, the clinical condition of the patients improved, while plasma tryptophan and platelet 5-HT values normalized after 3 weeks and 2 months, respectively. Clinical status, plasma tryptophan, platelet 5-HT, as well as other biological parameters determined concurrently in the patients, such as platelet monoamine oxidase (MAO), 5-HT uptake, and imipramine binding were compared in search of significant correlations: neither the individual values of any of them nor the magnitude of their changes at any given stage or interval of the study, respectively, were found significantly correlated. These results suggest that a series of 5-HT-related biological parameters are altered in endogenous depression and tend to normalize with imipramine treatment leading to clinical recovery. Within individuals, those parameters are not correlated, suggesting that both the effect of the drug and clinical improvement affect them separately.

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.

Decreased plasma tryptophan levels in major depression

Total plasma tryptophan was significantly lower in 12 depressed patients than in 12 normal controls. The ratio of total tryptophan to neutral amino acids was also decreased in the depressed patients suggesting that tryptophan availability to the brain might be reduced. There was no correlation between basal tryptophan concentrations in the depressed subjects and the subsequent neuroendocrine response to intravenous tryptophan.

Tryptophan and tyrosine ratios to neutral amino acids in endogenous depression. Relation to antidepressant response to amitriptyline and lithium + L-tryptophan

The plasma ratios of tryptophan and tyrosine to those amino acids that compete with them during transport across the blood-brain barrier have been determined in depressed patients before and after treatment for four weeks with amitriptyline or lithium + L-tryptophan. There was no relation between the absolute plasma concentrations of free or total tryptophan or tyrosine and the clinical response to amitriptyline. There was also no relation between pre-treatment ratio of plasma tyrosine to competing amino acids and response to amitriptyline, but depressives with subnormal tryptophan ratio improved significantly more than patients with supernormal tryptophan ratio with comparable serum drug levels. The therapeutic response to lithium + L-tryptophan was predicted neither by the absolute plasma concentrations of free or total tryptophan or tyrosine nor by the tyrosine ratio, but there was also a trend towards greater improvement in patients with subnormal compared with supernormal tryptophan ratio. The results suggest that the pre-treatment plasma ratio of tryptophan to competing amino acids is a useful predictor of clinical response to amitriptyline. The possible mode of action of amitriptyline and lithium + L-tryptophan is briefly discussed.

Lithium-induced changes in plasma amino acid levels during treatment of affective disorders

Plasma amino acid (AA) profiles were determined in patients with bipolar affective disorders before and during treatment with lithium (Li) carbonate. After 90 days of Li intake, there were moderately (20-60%) but significantly decreased plasma levels of isoleucine, leucine, and valine. After 150 days of Li intake, the plasma lysine was also significantly decreased (12%). No significant changes in plasma levels were noted for any of the other 17 AAs that were studied. Since isoleucine, leucine, and valine are important competitors of tryptophan uptake at the blood-brain barrier, these observations suggest a possible mechanism for the therapeutic activity of Li, i.e., the enhancement of brain serotonin levels.

Tryptophan tolerance and metabolism in endogenous depression

The concentration of free and total tryptophan and kynurenine in plasma from 49 female depressives and 26 female controls was measured following oral loading with L-tryptophan, 100 mg/kg body weight. There was no significant difference between five depressives and six controls in the area under curve for free or total tryptophan or kynurenine in plasma. The peak concentration of kynurenine occurred 4 h after loading and in correlated significantly with the area under curve for kynurenine. There was no significant correlation between the L-tryptophan dose (g) and the plasma concentration of kynurenine a 4 h in the 49 depressives or 26 controls. The mean plasma levels of tryptophan and kynurenine at 4 h in the depressives were not significantly different from control levels. There was no clear relationship between the plasma levels of tryptophan or kynurenine at 4 h and the therapeutic response in 13 depressives treated with L-tryptophan for 15 days. It is concluded that the absorption, the plasma clearance, and the degradation to kynurenine of loading doses of L-tryptophan are normal in depressed patients. Results furthermore suggest that the plasma levels of tryptophan and kynurenine at 4 h are poor predictors of the response to L-tryptophan treatment in depressives.

Effects of age on serum tryptophan and urine indican in adults given a tryptophan load test

An oral load of L-tryptophan (490 mumol/kg) was administered to 25 men and 25 women between the ages of 30 and 80 years. Blood samples were drawn before the load and at 2-h intervals for 6 h after the load. Urine samples were collected for 5 days. Fasting serum tryptophan levels averaged 79.5 mumol/l (+/- 12.9 s.d.). Peak serum tryptophan levels (911-1002 mumol/l) occurred 2 h after the load. Urinary indican excretion on the day of the load averaged 6.4 mumol/24h/kg of body weight (+/- 2.8 s.d.). The concentration of tryptophan in serum, the amount of indican excreted in urine, and the indican:creatinine ratio in urine depended on the age of the subjects. The findings are discussed in relation to previous reports on effects of age on tryptophan pharmacokinetics.

Plasma tryptophan binding to albumin in unipolar depressives

The equilibrium binding constants of the interaction of tryptophan with albumin have been determined in plasma samples from both controls and patients suffering from affective disorders. The data corresponded with literature values for both the albumin concentration in plasma and the equilibrium constant for tryptophan binding to albumin. No differences in the equilibrium constants were detected between patients and controls but a small increase in the albumin concentration was noted in the depressives. It was agreed that this rise would be inadequate to lead to a significant effect on free tryptophan levels. Non-esterified fatty acid analysis indicated no differences between patient groups.

Evaluation of the relative potency of individual competing amino acids to tryptophan transport in endogenously depressed patients

There is evidence that the plasma ratio of tryptophan to five competing amino acids may predict the response of depressed patients to treatment with L-tryptophan. The relative potency of the individual competitors had been evaluated retrospectively. Phenylalanine consistently reduced, and isoleucine slightly reduced the predictive value of the plasma tryptophan ratio. The ratio of tryptophan to the sum of valine, leucine, and tyrosine was identified as most predictive for the therapeutic response. The L-tryptophan responders showed a normal plasma total tryptophan concentration as did the nonresponders, whereas the concentration of the three competitors was significantly elevated. It is concluded that while the plasma ratio of tryptophan to the sum of valine, leucine, and tyrosine is a useful predictor of the course of depressives on L-tryptophan, it does not definitely separate out the L-tryptophan responders from the control subjects.

Tryptophan, cortisol and puerperal mood

Plasma cortisol, free and total tryptophan were determined in 71 subjects on 8 occasions between 36 weeks gestation and 6 weeks post-partum. Affect was measured by rating scales and clinical interview. Twenty-eight subjects were judged to have experienced post-partum 'blues'. Seasonal variation occurred in the incidence of 'blues' and in cortisol and free tryptophan levels. Puerperally-depressed mood was correlated with high cortisol at 38 weeks irrespective of season. Free tryptophan was reduced in 'blues' subjects but only at the time of year when free tryptophan was normally high. Total tryptophan was low antenatally; a rapid rise on days 1 and 2 post-partum was superimposed on a slower return to normal. This initial peak was clearly absent in 37 per cent of subjects. Its absence was significantly related to occurrence of post-partum 'blues' and of complaints of depression in the ensuing 6 months. This finding is discussed in relation to the possible occurrence of an occult disturbance of tryptophan handling in subjects susceptible to depression.

Relationship between plasma ratio of tryptophan to competing amino acids and the response to L-tryptophan treatment in endogenously depressed patients

The ratio of the plasma of total tryptophan to those amino acids that compete with tryptophan during transport into the brain was determined in 60 control subjects and 87 patients suffering from endogenous depression, all females. The plasma ratio in the control subjects showed a significant negative correlation with age. There was no significant difference in the distribution of the biochemical data between the control subjects and the depressed patients. There was a significant higher proportion of bipolar depressed subjects compared to unipolar depressives and patients of uncertain polarity who showed a plasma ratio in the lower normal range. Thirty-two patients were subsequently treated with L-tryptophan. In the patients who showed a particularly low plasma ratio of tryptophan to competing amino acids a remission frequency of 80% was observed on day 14. The efficacy of L-tryptophan in the patients who showed a plasma ratio within the upper normal range was extremely poor. The results suggest that the ratio in the plasma of tryptophan to competing amino acids is a useful predictor of the course of treatment of depressed subjects with L-tryptophan.