Dieting decreases plasma tryptophan and increases the prolactin response to d-fenfluramine in women but not men

Sex Differences in Tryptophan Metabolism: A Systematic Review Focused on Neuropsychiatric Disorders

Tryptophan (Tryp) is an essential amino acid and the precursor of several neuroactive compounds within the central nervous system (CNS). Tryp metabolism, the common denominator linking serotonin (5-HT) dysfunctions and neuroinflammation, is involved in several neuropsychiatric conditions, including neurological, neurodevelopmental, neurodegenerative, and psychiatric diseases. Interestingly, most of those conditions occur and progress in a sex-specific manner. Here, we explore the most relevant observations about the influence of biological sex on Tryp metabolism and its possible relation to neuropsychiatric diseases. Consistent evidence suggests that women have a higher susceptibility than men to suffer serotoninergic alterations due to changes in the levels of its precursor Tryp. Indeed, female sex bias in neuropsychiatric diseases is involved in a reduced availability of this amino acid pool and 5-HT synthesis. These changes in Tryp metabolism could lead to sexual dimorphism on the prevalence and severity of some neuropsychiatric disorders. This review identifies gaps in the current state of the art, thus suggesting future research directions. Specifically, there is a need for further research on the impact of diet and sex steroids, both involved in this molecular mechanism as they have been poorly addressed for this topic.

Diet Versus Exercise in Weight Loss and Maintenance: Focus on Tryptophan

An association between mood disturbance, the inability to lose or to stop gaining weight, and a craving for carbohydrates is manifested by many people who are overweight or are becoming so. In a recent study, we observed that low-calorie weight loss diet lowered not only levels of leptin but also levels of essential amino acid tryptophan (TRP) significantly. The disturbed metabolism of TRP might affect biosynthesis of serotonin and could thereby increase the susceptibility for mood disturbances and carbohydrate craving, increasing the cessation probability of weight reduction programs. Alternatively, moderate physical exercise – a potent stimulus to modulate (reduce/normalize) proinflammatory cytokines, which may affect TRP levels – could be helpful in improving mood status and preventing uncontrolled weight gain. In contrast, excessive physical exercise may induce breakdown of TRP when proinflammatory cascades together with TRP-degrading enzyme indoleamine 2,3-dioxygenase-1 are stimulated, which may lead to neuropsychiatric symptoms such as fatigue and low mood.

Aldosterone Signals the Onset of Depressive Behaviour in a Female Rat Model of Depression along with SSRI Treatment Resistance

Depression is a serious condition that occurs more frequently in women and is often associated with treatment resistance. The main hypotheses of this study are that (a) aldosterone is an early marker of depression onset and (b) a tryptophan (TRP) depletion model of depression previously validated in male rats is treatment resistant in females. To explore possible underlying mechanisms, we have focused on factors shown to be altered in patients with treatment-resistant depression. Female Sprague-Dawley rats were treated with a control or low-TRP-containing diet for various time periods up to 21 days. The results show that aldosterone secretion increased after 4 days of TRP depletion and prior to corticosterone. Optimal effects of TRP depletion occurred at 14 days. In addition to neurochemical and behavioural changes observed previously in males, TRP depletion in females was associated with a significant decline in serum magnesium concentrations, increased serum interleukin-6, enhanced gene expression of orexin A in the frontal cortex and induced a rise in N-methyl-D-aspartate (NMDA) receptor Bmax in the amygdala. Depression-like behaviour, NMDA receptor upregulation, enhancement of the kynurenine-to-kynurenic acid ratio and magnesium were resistant to paroxetine treatment (10 mg/kg/day in drinking water for 14 days). In conclusion, aldosterone may represent an important early marker for the onset of depression-like behaviour. With respect to treatment resistance, the underlying mechanisms may involve pro-inflammatory cytokines, the kynurenine pathway, magnesium, glutamate neurotransmission and the orexin pathway. This model of treatment-resistant depression may be useful for the future development of new compounds with novel antidepressant properties.

Symptoms of depression and anxiety in anorexia nervosa: links with plasma tryptophan and serotonin metabolism

Depressive, anxiety and obsessive symptoms frequently co-occur with anorexia nervosa (AN). The relationship between these clinical manifestations and the biological changes caused by starvation is not well understood. It has been hypothesised that reduced availability of tryptophan (TRP) could reduce serotonin activity and thus trigger these comorbid symptoms. The aim of this study, during re-feeding in individuals with AN, was to analyse covariations across measures of nutritional status, depressive and anxiety symptoms, and peripheral serotonin markers. Depressive and anxiety symptoms, nutritional status and serotonin markers--whole blood serotonin content, plasma TRP and the ratio between TRP and large neutral amino acids--were assessed for 42 AN participants at admission to inpatient treatment and after re-feeding. Biological measures were compared to those obtained in 42 non-eating disordered subjects. For those with AN, psychological, nutritional and biological parameters improved significantly during hospitalisation. Levels of serotonin markers were significantly lower in the AN group compared to the control group, at admission and at discharge. Increase in the TRP/LNAA ratio was correlated with a decrease in depressive symptoms. In addition, there was a positive correlation between serotonin levels and symptoms of both anxiety and depression at discharge. We speculate that enhanced TRP availability during re-feeding, as a result of the increase in the TRP/LNAA ratio, could restore serotonin neurotransmission and lead to a decrease in depressive symptoms. The association between serotonin and anxiety and depressive symptoms would be consistent with numerous observations indicating abnormal functioning of the serotoninergic system in AN.

Lateral hypothalamic serotonin is not stimulated during central leptin hypophagia

Whether leptin targets the hypothalamic serotonergic system to inhibit food intake is not established. We examined the effect of a short-term i.c.v. leptin treatment on serotonin microdialysate levels in rat lateral hypothalamus. Adipose tissue gene expression was also evaluated. Male rats received four daily injections of leptin (5 μg) or vehicle (with pair-feeding to leptin-induced intake) and a fifth injection during collection of LH microdialysates. We found that serotonin and 5-HIAA levels were not affected by the leptin pre-treatment, as basal levels were similar between the leptin and the pair-fed group. These levels remained unaltered after the acute leptin injection. For gene expression studies, rats were pre-treated with five daily injections of either leptin (5 μg) or vehicle (with either pair-feeding or ad libitum intake). mRNA levels of resistin, adiponectin, lipoprotein lipase, and PPAR-gamma were unaltered by either leptin or pair-feeding. Leptin gene expression was significantly reduced by leptin but not by pair-feeding, in both the retroperitoneal (-74%) and the epididymal (-99%) depots while no differences were observed in the subcutaneous depot. The observations confirmed the absence of an acute stimulatory effect of central leptin on serotonin release in the lateral hypothalamus and showed that the pre-treatment with leptin failed to modify this pattern. This indicates that components of the serotonergic system are probably not directly affected by leptin. Additionally, the central effect of leptin was able to downregulate its own adipose tissue gene expression in a depot-specific manner while other adipokine genes were not affected.

Sub-chronic dietary tryptophan depletion--an animal model of depression with improved face and good construct validity

Sub-chronic tryptophan depletion (SCTD) is proposed as an animal model for depression. Aims were to test the hypothesis and optimise the time of SCTD-induced depression-related behaviour and associated biochemical changes. Sprague Dawley rats were treated with a low tryptophan (TRP) containing diet for 0, 7 or 14 days. Peripheral and central neurochemical markers were measured. SCTD-induced depression-related behaviour was assessed by the forced swim test (FST). Model sensitivity to antidepressants was tested by concomitant treatment with paroxetine. SCTD-induced significant reductions in weight gain and measures of peripheral and central TRP. Corticosterone, aldosterone and kynurenine (K), increased whilst kynurenic acid (KA), an NMDA antagonist decreased. 5-HT(2) receptor binding Bmax was enhanced but was reversed by paroxetine. Corticosterone and aldosterone were significantly negatively-correlated to weight gain. SCTD increased floating time and reduced swimming time in the FST but were reversed by paroxetine. Aldosterone was increased at 7 and 14 days, whereas other changes maximised at 14 days. Aldosterone may be an early marker or causal link for depression development. Increased corticosterone and brain tissue 5-HT-receptor density may be correlates of depressive behaviour. Consequential increases in NMDA signalling through increased K/KA ratios suggest the model may be useful for testing novel antidepressants.

Plasma free tryptophan revisited: what you need to know and do before measuring it

Plasma free tryptophan (Trp) is an important peripheral parameter widely used by psychopharmacologists to assess Trp entry into the brain for cerebral serotonin synthesis, although, along with total Trp, it can give much more information on Trp metabolism and disposition. Plasma free Trp is, however, a labile parameter easily influenced by a great many modulators, including fasting, food intake, many prescribed and over the counter medications, consumption of alcoholic and of common hot beverages, illicit drug use, some hormones, exercise and mild stressors. Interpretation of changes in plasma free Trp requires appropriate preparation of ultrafiltrates from freshly isolated plasma or serum, accurate analytical methodology and awareness of the multitude of physiological and pharmacological modulators of its concentration. This article highlights these points and makes recommendations aimed at avoiding pitfalls in studies involving this parameter.

Aromatic amino acids in weight-recovered females with anorexia nervosa

OBJECTIVE

Most previous studies investigating amino acid levels in anorexia nervosa (AN) have focused on acutely underweight patients. The present study assessed the availability of aromatic amino acids in the plasma of weight-recovered outpatients with AN (recAN) in comparison to acutely underweight AN patients (acAN) and healthy control woman (HCW).

METHOD

Plasma tryptophan (TRP), tyrosine (TYR), and phenylalanine (PHEN) as well as leptin concentration were determined in 32 recAN, 32 acAN, and 32 HCW.

RESULTS

Both recAN and acAN patients showed significantly lower levels of TRP and PHEN when compared to HCW. TYR was reduced in acAN patients only.

DISCUSSION

Normal weight and normal leptin levels but lower availability of TRP and PHEN in recAN patients might indicate that outside a tightly controlled setting these patients still engage in abnormal eating patterns. Reduced peripheral availability of these precursor amino acids could impact on 5-HT and catecholamine functioning in the brain.

A history of caloric restriction induces neurochemical and behavioral changes in rats consistent with models of depression

A history of dieting is common in individuals suffering from eating disorders for which depression and mood disturbances are also comorbid. We investigated the effect of a history of caloric restriction (HCR) in rats that involved cyclic food restriction and refeeding with varying levels of access to palatable food (PF) on: 1) responses to the SSRI, fluoxetine; 2) monoamine levels in brain regions central to the control of feeding, reward, and mood regulation; and 3) behavioral tests of anxiety and depression. HCR coupled with intermittent but not daily access to PF exaggerated rats' anorectic response to fluoxetine (p<0.05); was associated with a significant 71% and 58% reduction of 5-HT and dopamine, respectively, in the medial prefrontal cortex; and induced behaviors consistent with models of depression. HCR, irrespective of access to PF, abolished the strong association between 5-HT and dopamine turnover in the nucleus accumbens in control rats (r=0.71 vs. -0.06, p<0.01). Access to PF, irrespective of HCR, reduced hypothalamic dopamine. Together, these findings suggest that a history of frequent food restriction-induced weight fluctuation imposes neurochemical changes that negatively impact feeding and mood regulation.

A history of human-like dieting alters serotonergic control of feeding and neurochemical balance in a rat model of binge-eating

OBJECTIVE

This study replicated a model of stress-induced binge-eating in rats with a history of caloric restriction (HCR), tested their response to SSRI (fluoxetine) treatment, and explored changes in brain monoamine levels.

METHOD

Young female rats with no-HCR/no-Stress, no-HCR/Stress, HCR/no-Stress, and HCR+Stress (binge-eating) were treated with fluoxetine. Post-mortem levels of serotonin, dopamine, and metabolites were assessed from brain regions key to feeding and reward.

RESULTS

A 3 mg/kg dose of fluoxetine without effect in the no-HCR groups suppressed intake of HCR groups, normalizing the binge-eating of HCR/Stress rats. No differences in monoamines were detected in the hypothalamus or tegmentum but a strong positive relationship between accumbens serotonin and dopamine turnover in no-HCR rats was absent in rats with HCR.

CONCLUSION

Despite lack of hunger, a history of human-like dieting alters serotonin function in ways suggesting consequences not only to feeding but also control of reward and mood that are dependent on dopamine/serotonin interactions.

Are there gender differences in core symptoms, temperament, and short-term prospective outcome in anorexia nervosa?

OBJECTIVE

The objective of this study was to compare symptoms, premorbid personality phenotypes, and short-term outcome between males and females with anorexia nervosa.

METHOD

Symptom and personality ratings were obtained at the time of hospital admission, and outcome was assessed at discharge and again 1 year later.

RESULTS

Gender effects were negligible at admission, with the exception of greater weight concern among females. Lifetime anxiety disorders and personality traits implicated in liability for anorexia nervosa were common among patients of both genders. Females had greater persistence of symptom morbidity over the 1-year follow-up.

CONCLUSION

Although gender has little effect on the clinical features of anorexia nervosa, the illness runs a more protracted early course in females. Possible mechanisms underlying greater persistence of morbidity in females include sexual dimorphisms in brain neurotransmission, gender differences in attitudes regarding ideal body weight, and anxiety-related personality phenotypes associated with anorexia nervosa.

Acute tryptophan depletion. Part I: rationale and methodology

OBJECTIVE

Acute tryptophan depletion (ATD) is an experimental technique that has been widely used over the last decade to investigate the role of serotonin (5-HT) in a variety of disorders. This review, the first of two articles, describes the rationale behind this technique and provides detail on how it is applied in research settings.

METHOD

The authors outline the development of this technique with reference to the seminal literature and more recent findings from neuroimaging and neuroendocrine studies. This is supplemented by the authors' clinical experience of over 5 years of continuous experimental work with this paradigm in over 50 subjects.

RESULTS

Acute tryptophan depletion is a method that significantly reduces central 5-HT in human subjects. Non-serotonergic explanations of the effects of ATD have not been confirmed, supporting the specificity of this method.

CONCLUSIONS

The ATD technique is a valid method of manipulating central 5-HT levels. The second article in this series will review the application of ATD in depression, anxiety and other psychiatric conditions.

Eating disorders: clinical features and pathophysiology

Anorexia nervosa (AN) and bulimia nervosa (BN) are disorders of eating and weight-related behavior that together afflict some 1-3% of women in the United States. One of the remarkable features about each of the eating disorders is how persistent the disordered eating behavior becomes once it has begun. Substantial psychological, social, and physiological disturbances are associated with eating disorders, and it has been very difficult to disentangle those factors that may result from the disturbed behavior from the factors that may have predisposed individuals to, or precipitated the development of, the disorder. This article will briefly review the definitions, phenomenology, and identified risk factors for development of each of the major eating disorders. Pathophysiology will be discussed, with a particular focus on candidate factors that might sustain disordered eating behavior, as informed by clinical and basic science research. Future research directions will be suggested.

Eating disorders and aggressiveness among adolescents

OBJECTIVE

To explore the links between propensity to aggression and eating disorders in a sample of school-aged adolescents in a northeastern area of Italy.

METHOD

In a mixed male-female sample of 1000 adolescents (10% of the district's population aged 15-19 years) we administered the Eating Attitudes Test, the Bulimic Investigatory Test of Edinburgh and the Body Attitudes Test, as measures of abnormal eating attitudes and behaviours, and the Aggression Questionnaire, as a measure of the propensity to aggression.

RESULTS

Females scored significantly higher than males at all eating disorders inventories (P < 0.0001). Males scored higher than females at the Aggression Questionnaire. In both genders, there was a positive correlation between scores at any of the eating disorders inventories and those at the Aggression Questionnaire (P < 0.0001).

CONCLUSION

Overtly expressed aggression might have a negative impact on the course of eating disorders and on the compliance with treatment, also enhancing the risk of suicide.

Eating disorders

Eating disorders rank among the most debilitating psychiatric disturbances that affect young women. Knowledge has increased in recent years about the two major eating disorders, anorexia nervosa (AN) and bulimia nervosa (BN); however, much remains unknown. This review article will provide an overview of the epidemiology, proposed risk factors and clinical features of AN, and BN, as well as current recommendations for evaluation and treatment of these disorders.

Anxiolytic effects of acute tryptophan depletion in anorexia nervosa

OBJECTIVE

Recent studies have raised the question as to whether a dysregulation of the neurotransmitter serotonin may contribute to the alterations in mood seen in anorexia nervosa (AN). People with AN tend to be anxious, obsessional, perfectionistic, and harm avoidant. These traits are premorbid and persist after recovery. It has been suggested that increased activity of brain serotonin systems could contribute to this pathologic condition. Dieting in AN, which serves to reduce plasma levels of tryptophan (TRP), may serve to reduce symptoms of dysphoric mood.

METHOD

Fourteen women currently symptomatic with AN (ILL AN), 14 women recovered from AN (REC AN), and 15 healthy control women (CW) underwent acute tryptophan depletion (ATD). Measures of psychological state were self-assessed at baseline and hourly after ATD to determine whether ATD would reduce negative mood.

RESULTS

ILL AN and REC AN had significantly higher mean baseline TRP/LNAA (tryptophan/large neutral amino acids) ratios compared with CW. In contrast to placebo, the ATD challenge demonstrated a significantly greater reduction in the TRP/LNAA ratio for ILL AN (-95%) and REC AN (-84%) compared with CW (-70 %). Both the ILL AN and REC AN had a significant reduction in anxiety on the ATD day compared with the placebo day.

DISCUSSION

These data demonstrate that a dietary-induced reduction of TRP, the precursor of serotonin, is associated with decreased anxiety in people with AN. Restricting dietary intake may represent a mechanism through which individuals with AN modulate a dysphoric mood.

The prolactin response to fenfluramine in depression: effects of melancholia and baseline cortisol

Depression may be associated with a hypofunction of central serotonergic systems. The prolactin response to fenfluramine, an indicator of serotonergic activity, has been reported to be blunted in depressed patients compared to controls. It has also been suggested that blunting is more likely in melancholic depression. Baseline cortisol, prolactin and tryptophan availability have also been suggested to affect this response. Forty-eight men and 61 women with a major depressive episode, and who were drug free, and 20 healthy control men underwent clinical evaluation and fenfluramine challenge with dl-fenfluramine 1 mg/kg. When baseline variables were covaried, there was no difference in prolactin response to fenfluramine between males with depression and age-matched controls. Amongst all the depressed patients, body mass index showed a significant association with prolactin response to fenfluramine. There was an interaction between baseline cortisol and DSM-III-R melancholic subtype of depression whereby non-melancholic patients appeared more likely to increase prolactin response to fenfluramine in response to higher cortisol levels. Prolactin response to fenfluramine was not blunted in major depression and there was no difference between melancholic and non-melancholic depression. However, the relationship between prolactin response to fenfluramine and baseline cortisol levels appeared to differ between these two subtypes of depression.

Bio-social origins of depression in the community. Interactions between social adversity, cortisol and serotonin neurotransmission

BACKGROUND

Social adversity may be a risk factor for depression, by increasing cortisol secretion, which impairs serotonin (5-HT) neurotransmission.

AIMS

To examine this causal pathway in a community setting.

METHOD

Women who were currently ICD-10 depressed (n=94), vulnerable to depression but not depressed (n=166) and non-vulnerable controls (n=177) were recruited. We assessed social adversity and vulnerability (Life Events and Difficulties Schedule; Self Evaluation and Social Support Scales) and psychiatric state (Schedules for Clinical Assessment in Neuropsychiatry). Salivary cortisol concentrations were measured at 09.00 and 23.00 h. Serotonin function was assessed using prolactin responses to dexfenfluramine.

RESULTS

Cortisol concentrations were not increased in the depressed or vulnerable. Morning salivary and serum cortisol were reduced in depression. Evening cortisol was increased after recent life events. Life events and depression were associated with increased prolactin responses.

CONCLUSIONS

The hypothalamic-pituitary-adrenal axis is sensitive to social stress but does not mediate vulnerability to depression. Exaggerated 5-HT(2) receptor responsiveness to stress may play a role in the evolution of depression.

Gender differences in the efficacy of fluoxetine and maprotiline in depressed patients: a double-blind trial of antidepressants with serotonergic or norepinephrinergic reuptake inhibition profile

BACKGROUND

Depression has emerged as a contrastive area of gender differences in psychiatry, as epidemiological data has consistently shown depression is twice as common in women as men. The pharmacodynamic effect of antidepressants may also show gender differences, as suggested by reports of better response of young women to non-tricyclic antidepressants.

METHODS

The antidepressive effect of an SSRI (fluoxetine) and a tetracyclic antidepressant with selective norepinephrine reuptake inhibitory effect (maprotiline) was compared in a 6-week, double-blind trial of 105 depressed patients.

RESULTS

No significant difference was observed in the change of HAMD17 total score from baseline to week 6 between fluoxetine- and maprotiline-treated patients. A significant difference was observed in females (fluoxetine, -17.8; maprotiline, -13.9; P=0.017) between treatment groups, but not in males. Amongst females, the difference was significant in women aged <44 years (fluoxetine, -18.4; maprotiline, -12.9; P=0.023) but not > or =44 years.

CONCLUSIONS

Females in their reproductive period are more responsive to SSRI (fluoxetine) than norepinephrinergic tetracyclic antidepressant (maprotiline) treatment. Normal cyclical ovulation, and estrogen release may have a clinically relevant pharmacodynamic interaction with serotonergic antidepressants.

Anorexia and bulimia nervosa

Anorexia nervosa (AN) and bulimia nervosa (BN) are disorders characterized by abnormal patterns of weight regulation and eating behavior and by disturbances in attitudes and perceptions toward weight and body shape. Etiologic research has indicated substantial genetic influence on these disorders, suggesting significant biological contributions to their development. Obsessional, perfectionistic, and anxious personality styles may be premorbid traits that contribute to this pathogenesis. Studies of neuroendocrine, neuropeptide, and neurotransmitter functioning in patients with AN and BN indicate that disturbances of these systems may contribute to the maintenance as well as the etiology of these sometimes fatal disorders. The efficacy of psychological treatments and pharmacotherapy has been more clearly established for BN than for AN.

Serotonin, dieting, and bulimia nervosa

Dieting is a common behaviour which may trigger eating disorders such as bulimia nervosa in predisposed subjects. We found that in healthy women moderate dieting for 3 weeks lowered plasma concentrations of the 5-HT precursor, L-tryptophan (TRP) and impaired brain 5-HT neurotransmission as judged by 5-HT neuroendocrine tests. In recovered female subjects with a history of bulimia nervosa we found that TRP depletion produced by an amino acid mixture lacking TRP caused a temporary return of depressive symptoms together with concerns about weight and shape and fear of loss of control of eating. Taken together the data suggest that dieting-induced decreases in TRP availability may trigger the development of bulimia nervosa is susceptible individuals.

Effects of acute tryptophan depletion on mood in bulimia nervosa

BACKGROUND

The present study investigated the role of serotonin in the pathophysiology of bulimia nervosa (BN) by studying the affective and appetitive responses of women ill with BN to an acute tryptophan depletion (ATD) paradigm.

METHODS

Twenty-two women with BN and 16 healthy control women (CW) were studied on 2 separate days during the follicular stage of the menstrual cycle. Participants drank a control mix of essential amino acids (100 g + 4.6 g tryptophan) on one day and a tryptophan deficient (100 g - 4.6 g tryptophan) mixture (ATD) on the other in a double-blind fashion. Mood/appetite ratings and blood samples were taken at baseline and at intervals up to 420 minutes. Participants were then presented with an array of foods and were allowed to binge and vomit if they desired.

RESULTS

CW and BN women had a similar and significant reduction in plasma tryptophan levels and the tryptophan: LNAA ratio after ATD. After ATD, the BN women had a significantly greater increase in peak (minus baseline) depression, mood lability, sadness and desire to binge compared to the CW. BN subjects and CW had similar peak changes in mood after the control amino acid mixture. BN subjects and CW consumed similar amounts of food after the two amino acid treatments.

CONCLUSIONS

Women with BN seem more vulnerable to the mood lowering effects of ATD, suggesting they have altered modulation of central 5-HT neuronal systems.

Impaired regulation of brain serotonin function during dieting in women recovered from depression

BACKGROUND

Amino acid mixtures that lower brain availability of the serotonin (5-HT) precursor tryptophan produce acute depressive relapse in women with a history of major depression. Dieting also lowers brain tryptophan availability, but its effects on brain 5-HT function in recovered depressed women have not been studied.

AIM

To test the hypothesis that women with a history of major depression would show impaired regulation of brain 5-HT function during a period of dieting-induced tryptophan depletion.

METHOD

Women with and without a history of major depression were placed on a daily 1000 kcal (approximately 4200 kj) diet for three weeks. Before the diet and in the final week we measured fasting plasma tryptophan levels and the prolactin response to an intravenous tryptophan challenge.

RESULTS

Dieting lowered plasma tryptophan levels equivalently in women with and without a history of depression. In women without a history of depression, dieting also increased the prolactin response to tryptophan. This increase did not occur in women with a history of depression.

CONCLUSIONS

Women with a history of depression showed impaired regulation of brain 5-HT function in response to dieting.

Twin studies and the etiology of eating disorders

OBJECTIVE

To evaluate the major twin studies of eating disorders in order to consider the empirical standing of the suggestion that these disorders, notably bulimia nervosa, are largely genetically determined.

METHOD

Examination of the relevant twin studies and the methods used.

RESULTS

The findings of the twin studies are inconsistent and difficult to interpret, with estimates for the heritability of liability to bulimia nervosa ranging from 0% to 83%, and from 0% to 70% for anorexia nervosa. Methodological reasons for this variability include issues of definition of phenotype, diagnostic reliability, violation of the equal environments assumption, and small sample sizes. Many of these issues also apply to twin studies of other psychiatric disorders.

CONCLUSIONS

The heritability of eating disorders remains unknown. A broad view on the etiology of eating disorders should be maintained, with a focus on environmental mechanisms and gene-environment interactions, as well as continuing genetic studies.

Comparison of buspirone with diazepam and fluvoxamine on aversive classical conditioning in humans

The effects of buspirone, fluvoxamine and diazepam were investigated, using healthy volunteers, in an aversive conditioning paradigm, a putative model for conditioned anxiety. The main prediction was that buspirone, an anxiolytic agent which reduces activity in serotonin (5-hydroxytryptophan, 5-HT) neurones, would attenuate aversively conditioned skin conductance responses. Skin conductance responses were recorded to 10 neutral tones (habituation phase). Tone 11 was immediately followed by a 1-s 90-dB aversive white noise (unconditioned stimulus). The conditioning trial reinstated responding to a second presentation of the tones (extinction phase). Skin conductance response amplitude, inter-response level and spontaneous fluctuations were recorded. There were five treatment groups comprising five men and five women. One control group took placebo, another control group received nothing; there was no effect of placebo on any measure. Diazepam (2 mg, p.o.), a positive comparator, markedly reduced the amplitude of skin conductance responses at all phases of the experiment, but only in women. Buspirone (5 mg, p.o.) had the predicted effect of accelerating extinction but also of unexpectedly accelerated habituation of skin conductance responses. There was a trend to reduce spontaneous fluctuations and no effect on skin conductance level. The effects of buspirone were thus specific to responses to stimuli. Fluvoxamine (25 mg, p.o.) had similar effects to buspirone and diazepam in women. An action common to buspirone, fluvoxamine and diazepam, which may account for their shared effect on conditioned autonomic responses, is the suppression of neural activity in the dorsal raphe nucleus. It is argued that enhanced habituation must involve a different mechanism, such as enhanced 5-HT1A function in the terminal fields of the median raphe nucleus.

Estrogen-serotonin interactions: implications for affective regulation

A burgeoning literature documents the convergence of reproductive endocrine and central serotonergic systems in the regulation of a variety of behaviors. This review will focus on one element of this interaction, the modulation of serotonergic function by estrogen. After describing the manifold neuroregulatory effects of gonadal steroids, we summarize the effects of estrogen on central serotonin systems in animals and humans as inferred from studies demonstrating the impact of gender, estrus (or menstrual) cycle, or hormone manipulation. Finally, we summarize the putative roles of estrogen and serotonin in two reproductive-endocrine-related mood disorders: premenstrual syndrome and perimenopausal depression.

Serotonin neuronal function and selective serotonin reuptake inhibitor treatment in anorexia and bulimia nervosa

Anorexia nervosa (AN) and bulimia nervosa (BN) are disorders characterized by aberrant patterns of feeding behavior and weight regulation, and disturbances in attitudes toward weight and shape and the perception of body shape. Emerging data support the possibility that substantial biologic and genetic vulnerabilities contribute to the pathogenesis of AN and BN. Multiple neuroendocrine and neurotransmitter abnormalities have been documented in AN and BN, but for the most part, these disturbances are state-related and tend to normalize after symptom remission and weight restoration; however, elevated concentrations of 5-hydroxyindoleacetic acid in the cerebrospinal fluid after recovery suggest that altered serotonin activity in AN and BN is a trait-related characteristic. Elevated serotonin activity is consistent with behaviors found after recovery from AN and BN, such as obsessionality with symmetry and exactness, harm avoidance, perfectionism, and behavioral over control. In BN, serotonergic modulating antidepressant medications suppress symptoms independently of their antidepressant effects. Selective serotonin reuptake inhibitors (SSRIs) are not useful when AN subjects are malnourished and under-weight; however, when given after weight restoration, fluoxetine may significantly reduce the extremely high rate of relapse normally seen in AN. Nonresponse to SSRI medication in ill AN subjects could be a consequence of an inadequate supply of nutrients, which are essential to normal serotonin synthesis and function. These data raise the possibility that a disturbance of serotonin activity may create a vulnerability for the expression of a cluster of symptoms that are common to both AN and BN and that nutritional factors may affect SSRI response in depression, obsessive-compulsive disorder, or other conditions characterized by disturbances in serotonergic pathways.

Prolactin response to d-fenfluramine is blunted in people with anorexia nervosa

BACKGROUND

Several studies have explored serotonin (5-HT) transmission in people with anorexia nervosa, but their results have been inconsistent.

METHOD

According to a double-blind placebo-controlled design, plasma prolactin response to the specific serotonergic probed d-fenfluramine was investigated in 10 under weight and two normal-weight women with anorexia, and in 12 age-matched healthy females. Eating-related psychopathology, depressive and obsessive--compulsive symptoms, and aggressiveness were measured by appropriate rating scales.

RESULTS

Compared with healthy control subjects, the women with anorexia showed reduced baseline prolactin and oestrogen levels and increased basal cortisol concentrations. The prolactin response to d-fenfluramine was blunted and did not correlate with psychopathological measures.

CONCLUSIONS

These results support a dysfunction of 5-HT transmission in anorexia nervosa. This dysfunction does not seem to be related to concomitant depressive or obsessive--compulsive symptoms or to the level of aggressiveness of the patients.

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.

The effects of dieting on plasma tryptophan concentration and food intake in healthy women

Although many people diet, relatively few dieters are successful in maintaining weight loss. The extent to which dieting behavior might dampen satiety responses normally mediated by the neurotransmitter serotonin remains uncertain. This study tested the hypothesis that dieting behavior decreases the availability of plasma tryptophan (TRP) and the ratio of TRP to other branched-chain amino acids (BCAA) that compete for entry into the central nervous system (CNS). This effect could diminish the CNS concentration of TRP, the amino acid precursor for serotonin synthesis, thus interfering with serotonin-mediated influences on food intake. Using a fixed-order design, 15 healthy, normal-weight women were studied longitudinally during an ad lib dietary intake phase and subsequent reduced-calorie diet phase. Physiological and behavioral measures were collected at baseline, at the end of the ad lib-intake phase, and at the end of the 4-week study diet phase. Food intake was measured by a single-item test meal. Plasma TRP and TRP: sigma BCAA significantly decreased following the study diet compared to baseline (p < 0.05). Change in TRP and TRP: sigma BCAA significantly correlated with decrease in body weight (p < 0.01). No significant relationship was observed between postdiet change in TRP or TRP: sigma BCAA ratio and postdiet change in test meal food intake, with covariation for weight loss. The observed decreases in plasma TRP and TRP: sigma BCAA extend previous reports suggesting that dieting behavior may diminish central serotonin function through a reduction in precursor availability.

Adolescent dieting: healthy weight control or borderline eating disorder?

Dieting in adolescent girls is ubiquitous but its health significance is uncertain. On the one hand it might be seen as promoting healthy weight control and on the other it might be considered as a risk factor for eating disorders. Dieting levels were systematically assessed in a representative group of 2525 Australian teenagers and classified using item response theory. In this group, 38% of girls and 12% of boys were categorised as intermediate dieters; 7% of girls and 1% of boys fell into a group of extreme dieters. Body mass carried a strong positive association with intermediate dieting. Most female dieters, nevertheless, fell within a normal weight range. Psychiatric morbidity was the clearest factor associated with extreme dieting and 62% of extreme dieters reported high levels of depression and anxiety. Extreme dieting might reasonably be viewed as lying on a spectrum with clinical eating disorders. Most dieting is unjustified on the grounds of appropriate weight control and appears to reflect a widespread striving of teenage girls towards body shapes at the lower end of age-adjusted norms.

Headache induced by serotonergic agonists--a key to the interpretation of migraine pathogenesis?

Serotonergic agonists such as m-chlorophenylpiperazine (m-CPP) and fenfluramine may induce migraine attacks. This has led to opposing theories concerning the role of 5-hydroxytryptamine (5HT) in triggering migraine attacks; is there hyperfunction or hypofunction of the central serotonergic system. Our review of the literature strongly suggests that m-CPP and fenfluramine provoke migraine attacks by stimulating, directly or indirectly, the 5HT2C/5HT2B receptors, although there is no total agreement with this interpretation. Central 5HT hypersensitivity in migraine patients, probably due to 5HT neuronal depletion, is proposed on the basis of review of electrophysiological tests and neuroendocrine challenge paradigms.

5-HT neuroendocrine function in major depression: prolactin and cortisol responses to D-fenfluramine

We studied the prolactin and cortisol responses to the 5-HT releasing agent, D-fenfluramine in 31 drug-free depressed patients and 29 healthy controls, using a double-blind, placebo-controlled design. There was no difference in either endocrine response between depressives and controls. Examining the sexes separately, or restricting comparisons to patients with melancholic depression, did not lead to any differences between the groups. Our findings suggest that in contrast to other 5-HT neuroendocrine probes such as L-tryptophan and clomipramine, the prolactin response to fenfluramine are not consistently blunted in depressed patients.

Moderate dieting causes 5-HT2C receptor supersensitivity

Dieting is a widespread behaviour in developed countries, which in predisposed individuals can lead to the development of clinical eating disorders such as bulimia nervosa and anorexia nervosa. We studied the effect of moderate dieting in healthy women on the prolactin response to the serotonin (5-HT) receptor agonist, m-chlorophenylpiperazine (mCPP), a measure of the sensitivity of post-synaptic 5-HT2C receptors. Dieting significantly increased the prolactin response to mCPP and lowered plasma concentrations of the 5-HT precursor, tryptophan. We propose that dieting in women is associated with the development of functional supersensitivity of 5-HT2C receptors, probably in response to lowered levels of brain 5-HT. Alterations in brain 5-HT neurotransmission could play a part in dieting-induced dysregulation of eating and the development of clinical eating disorders in predisposed individuals.