Clinical studies of 5-HT function using i.v. L-tryptophan

Mood and behavior regulation: interaction of lithium and dopaminergic system

Lithium is one of the most effect mood-stabilizing drugs prescribed especially for bipolar disorder. Lithium has wide range effects on different molecular factors and neural transmission including dopaminergic signaling. On the other hand, mesolimbic and mesocortical dopaminergic signaling is significantly involved in the pathophysiology of neuropsychiatric disorders. This review article aims to study lithium therapeutic mechanisms, dopaminergic signaling, and the interaction of lithium and dopamine. We concluded that acute and chronic lithium treatments often reduce dopamine synthesis and level in the brain. However, some studies have reported conflicting results following lithium treatment, especially chronic treatment. The dosage, duration, and type of lithium administration, and the brain region selected for measuring dopamine level were not significant differences in different chronic treatments used in previous studies. It was suggested that lithium has various mechanisms affecting dopaminergic signaling and mood, and that many molecular factors can be involved, including brain-derived neurotrophic factor (BDNF), cAMP response element-binding protein (CREB), β-catenin, protein kinase B (Akt), and glycogen synthase kinase-3 beta (GSK-3β). Thus, molecular effects of lithium can be the most important mechanisms of lithium that also alter neural transmissions including dopaminergic signaling in mesolimbic and mesocortical pathways.

Concentrations of various tryptophan metabolites are higher in patients with diabetes mellitus than in healthy aged male adults

Tryptophan metabolites in plasma samples from 20 male subjects with type 2 diabetes mellitus (T2DM) and 20 nondiabetic reference males were analyzed by ultra high performance liquid chromatography. Tryptophan levels in the diabetic subjects were significantly lower than those in nondiabetic subjects. The concentrations of 5-hydroxytryptophan, 5-hydroxyindoleacetic acid, kynurenic acid, 3-hydroxykynurenine, 3-hydroxyanthranilic acid, and xanthurenic acid were found to be higher in the diabetic patients. When the diabetic patients were divided into higher- and lower-tryptophan groups, the concentrations of 5-hydroxytryptophan, indole-3-acetic acid, kynurenine, 5-hydroxykynurenine, and kynurenic acid were found to be higher in the diabetic patients with higher tryptophan levels. However, diabetic patients with lower plasma tryptophan levels had higher levels of 5-hydroxyindoleacetic acid than the patients with higher tryptophan levels. These results suggest that tryptophan was metabolized more in T2DM patients than in nondiabetic subjects. In the kynurenine pathway, the degradation of tryptophan seems to be accelerated in patients with higher plasma levels of tryptophan than in patients with lower levels of tryptophan. In the serotonin pathway, when the level of tryptophan is low, the conversion of serotonin to 5-hydroxyindoleacetic acid appears to be accelerated. In conclusion, our results suggest that T2DM patients may be exposed to stress constantly.

Neuroendocrine and subjective responses to pharmacological challenge with citalopram: a controlled study in male and female ecstasy/MDMA users

Despite evidence that +/-3,4-methylenedioxymethamphetamine (MDMA; 'ecstasy') causes persistent alterations to the serotonergic system of animals, evidence for long-term neurological effects of ecstasy/MDMA in humans remains equivocal. The current study assessed serotonin functioning of nine male and 11 female recreational ecstasy polydrug users by measuring neuroendocrine (prolactin, cortisol) responses to pharmacological challenge with the selective serotonin reuptake inhibitor citalopram, compared with nine male and five female cannabis polydrug users and 11 male and 11 female non-drug using controls. A single-blind, randomised, placebo-controlled design was used. Subjective responses, other substance use, mood, personality traits and demographic variables were measured to control for potentially confounding variables. There were no significant differences between ecstasy polydrug users, cannabis polydrug users and non-drug using controls in neuroendocrine or subjective responses to serotonergic challenge, and there were no sex by drug group interactions. There was no relationship between extent of ecstasy use and neuroendocrine functioning, alone or in combination with potential confounding variables. Subjective responses to the pharmacological challenge (nausea, tremor, dry mouth), novelty seeking and lifetime dose of alcohol were the only variables that contributed to one or more of the neuroendocrine outcome variables. These data do not support the premise that recreational ecstasy/MDMA use results in measurable impairment of serotonergic control of endocrine activity.

5-HTTLPR and gender moderate changes in negative affect responses to tryptophan infusion

Expression of the serotonin transporter is affected by the genotype of the 5-HTTLPR (short and long forms) as well as the genotype of the SNP rs25531 within this region. Based on the combined genotypes for these polymorphisms, we designated each allele as a high or low expressing allele according to established expression levels-resulting in HiHi, HiLo, & LoLo genotype groups for analysis. We evaluated effects of gender and the promoter genotype on induction of negative affect by intravenous infusion of L: -tryptophan (TRP). The protocol consisted of a day-1 sham saline infusion and a day-2 active TRP infusion. Models assessed 5-HTTLPR composite genotype and gender as predictors of change in ratings of negative emotion during TRP infusion. During sham infusion there were no significant changes from baseline in mood ratings. During TRP infusion all negative affect ratings increased significantly from baseline (P's < .02). The genotype x gender interaction was a significant predictor of depression-dejection (P = .013), and trended towards predicting anger-hostility (P = .084). Males in the HiHi group had greater increases in negative affect during infusion, compared to all groups except LoLo females, who also showed increased negative affect.

Lithium addition in antidepressant-resistant depression: effects on platelet 5-HT, plasma 5-HT and plasma 5-HIAA concentration

The efficacy of the addition of lithium to an established course of antidepressant treatment can be explained by a synergistic effect of the two drugs on central 5-HT neurotransmission. In the present study we investigated the effect of lithium addition on the 5-HT concentration in plasma and platelets and the concentration of 5-HIAA. Thirty-nine depressed inpatients who fulfilled the DSM-IV criteria for major depressive disorder and who did not respond to monotherapy with either imipramine or fluvoxamine participated in this study. Concentration of 5-HT in both plasma and platelets did not change significantly during lithium addition. The 5-HT ratio (plasma concentration/platelet concentration) shows a small non-significant increase after 3 weeks lithium addition. The mean concentration of 5-HIAA shows a significant increase during lithium addition; with no difference between the imipramine and the fluvoxamine sample. The increments in 5-HIAA concentration during lithium addition are indicative of an increased 5-HT turnover.

Fünfundzwanzig Jahre Lithiumaugmentation

Twenty-five years ago the research group of the Canadian psychiatrist de Montigny reported treating antidepressant-refractory depressive patients successfully by adding lithium to their antidepressant. The report, published in 1981 as an open-label uncontrolled observation of only eight patients, falls short of today's methodological standards, but the treatment method, subsequently known as lithium augmentation, nonetheless was to change profoundly the pharmacological strategies for depressive disorders. The story of its development is remarkable, starting with a strictly theoretical idea conceived by Montigny and his colleagues after animal experiments in the 1970s had revealed that pretreatment with an antidepressant over several weeks led to sensitization of central nervous serotonin receptors. The team postulated that the proserotonergic characteristics of lithium, which had been systematically used as a psychotropic drug since 1949, could thus be used specifically to stimulate these receptors. Lithium augmentation demonstrated its effectiveness in the 1980s and 1990s, first in open-label and later in randomized and placebo-controlled studies. In the late 1990s studies aimed at optimizing its clinical application indicated that lithium augmentation must be administered for at least 2 weeks, with lithium serum levels within the range established for prophylactic treatment and assuming patient response, and that the combination of lithium and antidepressant must be continued as a maintenance therapy for 6 to 12 months. Research has yet to clarify how lithium augmentation actually works. Current results show that in addition to the idea postulated by Montigny, lithium could also have an activating effect on the cortisol axis. Thanks to the sound body of evidence which has accrued in the meantime, lithium augmentation is recommended in most guidelines and treatment algorithms as a main strategy for patients who do not respond to antidepressant monotherapy.

Lithium augmentation therapy in refractory depression: clinical evidence and neurobiological mechanisms

OBJECTIVE

This systematic review examines the evidence and discusses the clinical relevance of lithium augmentation as a treatment strategy for refractory major depressive episodes. It also examines hypotheses on the mode of action of lithium augmentation, with a focus on serotonin (5-HT) and neuroendocrine systems, and proposes recommendations for future research.

METHOD

We searched the Medline computer database and the Cochrane Library for relevant original studies published in English from January 1966 to February 2003. The key words were as follows: lithium, augmentation strategies, lithium augmentation, major depression, refractory depression, treatment-resistant depression, neuroendocrinology, and serotonin.

RESULTS

Of 27 prospective clinical studies published since 1981, 10 were double-blind, placebo-controlled trials, 4 were randomized comparator trials, and 13 were open-label trials. Five of 9 acute-phase placebo-controlled trials demonstrated that lithium augmentation had substantial efficacy. In the acute-treatment trials, the average response rate in the lithium group was 45%, and in the placebo group, 18% (P < 0.001). One placebo-controlled trial showed the efficacy of lithium augmentation in the continuation-phase treatment. Summarizing the open and controlled data, approximately 50% of patients responded to lithium augmentation within 2 to 6 weeks. Animal studies offer robust evidence that lithium augmentation increases 5-HT neurotransmission, possibly by a synergistic action of lithium and the antidepressant on brain 5-HT pathways.

CONCLUSIONS

Augmentation of antidepressants with lithium is the best-documented augmentation therapy in the treatment of refractory depression. Emerging data from animal studies suggest that the 5-HTergic system is involved in the augmentatory effect of lithium.

Changes in quantitatively assessed tremor during treatment of major depression with lithium augmented by paroxetine or amitriptyline

Tremor is a relatively frequent side effect of lithium and of antidepressants with serotonergic properties. It can be expected that combinations of lithium (which is itself serotonergic) with such antidepressants will enhance not only efficacy, but also the incidence of side effects, including tremor. To quantitatively monitor the effect of antidepressant augmentation of ongoing lithium therapy on tremor, lithium-maintained patients with a breakthrough episode of major depression were randomly assigned under double-blind conditions to receive paroxetine 20 mg/day (N = 14) or amitriptyline 75 mg/day (N = 17). The initial dosages could be increased after 2 weeks to 40 mg/day and 150 mg/day, respectively, and the patients were treated for 6 weeks. Tremor activity was assessed weekly, quantitatively by accelerometry and qualitatively with the Dosage Record and Treatment Emergent Symptom Scale. Statistical analysis detected no significant difference between the treatment groups with respect to changes in mean tremor activity relative to baseline. However, analysis of the pooled data showed that tremor increased significantly during the course of combined lithium and antidepressant therapy, with the greatest increments occurring independent of dosage approximately 3 weeks after initiation of combination treatment. Although the mean tremor activity subsided toward the end of treatment, tremor activity on the whole was still significantly greater after 6 weeks of combined lithium and antidepressant treatment than at the start of combination therapy. Increased tremor was not associated with decreased medication compliance, and no patient discontinued treatment because of increased tremor. Tremor frequency was not affected by the study treatments.

(+/-)3,4-Methylenedioxymethamphetamine ('Ecstasy')-induced serotonin neurotoxicity: clinical studies

(+/-)3,4-Methylenedioxymethamphetamine (MDMA, 'Ecstasy') is a brain serotonergic neurotoxin in experimental animals, including nonhuman primates. It is also an increasingly popular recreational drug of abuse, and doses of MDMA that are used recreationally overlap with those that produce serotonin (5-HT) neurotoxicity in animals. Studies in human MDMA users probing for evidence of brain serotonergic neurotoxicity indicate that some MDMA users may incur MDMA-related 5-HT neural injury and, possibly, functional sequelae. In particular, MDMA users have selective decrements in cerebrospinal fluid 5-hydroxyindoleacetic acid and brain 5-HT transporters, similar to nonhuman primates with documented MDMA-induced neurotoxicity. Functional abnormalities seen in MDMA users that may be related to 5- HT injury include cognitive deficits, altered sleep architecture, altered neuroendocrine function, altered behavioral responses to 5-HT selective drugs, and increased impulsivity. Additional studies in animals, as well as longitudinal and epidemiological studies in MDMA users, are required to confirm and extend the present data, and to determine whether MDMA users are at increased risk for developing neuropsychiatric illness as they age.

Serotonergic and kynurenic pathways in rats exposed to foot shock

Electric foot shock was applied to rats and levels of tryptophan and its metabolites were measured in the plasma, central nervous system and peripheral tissues. Metabolites of tryptophan are the results of the enhancement of serotonergic and kynurenine pathways. Plasma levels of tryptophan increased significantly immediately after the foot shock and returned to normal values within 24 h. Tryptophan levels also increased in all the brain areas immediately after stress application and returned to normal values within 24 h. Foot shock elevated the levels of kynurenine in the plasma, liver, kidney and every parts of the brain. 3-Hydroxykynurenine and kynurenic acid levels were increased in the brain. The present observations suggest that stress activates not only serotonergic pathway but also kynurenine pathway in the central nervous system and periphery. Some metabolites of kynurenine pathway, such as 3-hydroxykynurenine, are neurotoxic while other metabolite, such as kynurenic acid, may be neuroprotective. Increase in serotonin level in the hypothalamus and midbrain stabilises emotion and prevents mood disorders. Therefore, some brain dysfunction resulting from stress may be prevented by the metabolites of tryptophan. The balance of these functions may be important in the maintenance of nerve integrity under stress conditions.

Selective serotonin reuptake inhibitors and neuroendocrine function

Selective serotonin (5-HT) reuptake inhibitors (SSRIs) are effective drugs for the treatment of several neuropsychiatric disorders associated with reduced serotonergic function. Serotonergic neurons play an important role in the regulation of neuroendocrine function. This review will discuss the acute and chronic effects of SSRIs on neuroendocrine function. Acute administration of SSRIs increases the secretion of several hormones, but chronic treatment with SSRIs does not alter basal blood levels of hormones. However, adaptive changes are induced by long-term treatment with SSRIs in serotonergic, noradrenergic and peptidergic neural function. These adaptive changes, particularly in the function of specific post-synaptic receptor systems, can be examined from altered adrenocorticotrophic hormone (ACTH), cortisol, oxytocin, vasopressin, prolactin, growth hormone (GH) and renin responses to challenges with specific agonists. Neuroendocrine challenge tests both in experimental animals and in humans indicate that chronic SSRIs produce an increase in serotonergic terminal function, accompanied by desensitization of post-synaptic 5-HT1A receptor-mediated ACTH, cortisol, GH and oxytocin responses, and by supersensitivity of post-synaptic 5-HT2A (and/or 5-HT2C) receptor-mediated secretion of hormones. Chronic exposure to SSRIs does not alter the neuroendocrine stress-response and produces inconsistent changes in alpha2 adrenoceptor-mediated GH secretion. Overall, the effects of SSRIs on neuroendocrine function are dependent on adaptive changes in specific neurotransmitter systems that regulate the secretion of specific hormones.

A review of physiological and metabolic effects of essential amino acids

The authors review ten essential amino acids with regard to their metabolic, physiologic and therapeutic effects throughout the human body. Physical properties of these biologically active compounds are discussed as a foundation for their diverse roles in special nitrogen containing products, neurotransmitters, and as alternative energy sources. Both normal and abnormal amino acid metabolism are considered in the areas of digestion, elimination of metabolic products, metabolic intermediates, and defects in these systems. Recent developments in therapeutic applications are further examined for clinical utility and as an economical alternative to traditional clinical treatment modalities.

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.

Effects of meta-chlorophenylpiperazine on neuroendocrine and behavioral responses in male schizophrenic patients and normal volunteers

Functional alterations in the central serotonergic system, including presynaptic and postsynaptic function, have been reported in schizophrenia. Recently, there have been conflicting reports that the increase in plasma cortisol or prolactin concentrations induced by meta-chlorophenylpiperazine (mCPP) was significantly blunted in schizophrenic patients compared with normal volunteers. Studies of the behavioral effects of mCPP, a serotonin (5-HT) receptor partial agonist with high affinity for 5-HT1C binding sites, have also yielded conflicting results in schizophrenic patients. The purpose of this study was to examine plasma levels of prolactin and cortisol, body temperature, and behavioral responses to mCPP and placebo in a single-blind study in 25 schizophrenic and 15 normal men. No differences either between schizophrenic patients and normal volunteers or between paranoid and undifferentiated/residual subtypes of schizophrenia were found in mCPP-induced prolactin, cortisol, or temperature responses. Schizophrenic patients and normal volunteers reported significant increases in feeling calm and feeling strange of comparable magnitude following mCPP. No significant differences between normal volunteers and schizophrenic patients were found in post-mCPP behavioral ratings, such as anxiety, irritability, depression, restlessness, or arousal.

Serotonergic function during lithium augmentation of refractory depression

Serotonergic mechanisms have been implicated in the pathophysiology of depression and in the neuropharmacology of antidepressant treatment. One measure of central serotonergic function is the prolactin (PRL) response to IV L-tryptophan (L-TRP). We used the L-TRP test to assess the role of serotonin in the mechanism of action of lithium augmentation in refractory major depression. Twenty-six patients with antidepressant-refractory major depression each received three L-TRP tests (after 2 weeks of placebo, after 4 weeks of active primary antidepressant, and after 1 week of lithium augmentation). Primary antidepressant treatment did not increase the PRL response, but lithium augmentation resulted in a statistically significant increase in PRL response as compared to both placebo pretreatment (P less than 0.04) and antidepressant treatment alone (P less than 0.025). This study supports a role for serotonergic mechanisms in the action of lithium augmentation.