Department of Psychiatry, Hadrian Clinic, Newcastle General Hospital, Newcastle upon Tyne
Department of Psychiatry, St Nicholas Hospital, Newcastle upon Tyne
Stanley Foundation Bipolar Research Centre, Department of Neuroscience and Psychiatry, University of Newcastle upon Tyne, Newcastle upon Tyne
Correspondence: Professor Allan Young, Stanley Foundation Bipolar Research Centre, Department of Neuroscience and Psychiatry, University of Newcastle upon Tyne, Leazes Wing, The Royal Victoria Infirmary, Queen Victoria Road, Newcastle upon Tyne NEI 4LP, UK. Tel: 01912275272; fax: 01912275108; e-mail: A.H.Young{at}ncl.ac.uk
Declaration of interest Funded by the Theodore and Vada Stanley Foundation.
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ABSTRACT |
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Aims To examine the effects of acute tryptophan depletion on mood and suicidal ideation in bipolar patients who were symptomatically stable on lithium.
Method Nineteen subjects satisfying DSMIV criteria for bipolar I disorder participated in a within-subject, double-blind, placebo-controlled random-order crossover study. Symptoms were evaluated following acute tryptophan depletion, which was induced by a 100 g amino acid drink following an overnight fast.
Results Plasma tryptophan fell significantly after the depleting drink, but not after the control drink (P < 0.05, paired t-test, mean reduction 83%). No significant changes in mood or suicidality scores were recorded after acute tryptophan depletion.
Conclusions Acute tryptophan depletion does not reverse lithium's effects on mood and suicidality in bipolar disorder.
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INTRODUCTION |
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METHOD |
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Experimental design
The experiment was a within-subject, double-blind, placebo-controlled,
counter-balanced crossover design study with at least 7 days' washout between
experimental sessions. In the case of female subjects, the experimental
sessions took place in the follicular phase of the menstrual cycle with only
one procedure per cycle. Subjects presented following an overnight fast to the
Department of Psychiatry at 08.30 h and underwent baseline assessments. The
experimental drink was then administered. Four hours after consumption of the
drink, mood and suicidality were rated. Following the experimental session,
subjects returned home and underwent a further assessment of mood and
suicidality, which was carried out the following day at 28 hours after
consumption of the drink.
Mood rating scales
Objective mood ratings
The Hamilton Rating Scale for Depression, a 21-item standardised scale for
the measurement of the severity of depressive symptoms, was used
(Hamilton, 1960), with
subjects rated at baseline, 4 hours and 28 hours.
The Young Mania Rating Scale, an 11-item measure of the severity of manic symptoms developed by Young et al (1978), was used. Subjects were interviewed and rated at baseline, 4 hours and 28 hours.
Subjective mood ratings
The Internal State Scale, a self-report instrument for the simultaneous
assessment of the severity of depressive and manic symptoms, was used
(Bauer et al, 1991).
Visual analogue line format was used for the 17 items, with the anchor points
coding for severity and frequency of symptoms. Items are grouped in scoring to
four subscales: Activation, Well-being, Perceived Conflict and Depression
Index. Subjects were asked to make a vertical mark at the appropriate point on
the line which corresponded with the way they felt at the time of rating and
this point was measured from the left-hand anchor point as the zero.
The Carroll Bipolar Visual Analogue Scale consists of a 23-item scale based on the CarrollKlein model of manicdepressive illness (Carroll, 1991). Each item was presented as a visual analogue scale on a 100-mm long line with appropriate anchor statements describing the manic and depressive extremes of each item. The midpoint denotes the euthymic state and a score of zero, with the extreme depressive response scoring -50 and the extreme manic response scoring +50. Of the 23 items, seven items each represented consummatory reward, central pain and psychomotor regulation, and two items represented incentive reward (major components of the CarrollKlein model). Subjects were asked to make a vertical mark at the appropriate point on the line which corresponded with the way they felt at the time of rating and this point was measured from the zero midpoint.
The Beck Depression Inventory, a 21-item assisted self-rating inventory used to assess the severity of depressive states, was used (Beck et al, 1961). Each item was graded on a 4-point scale of 0-3, with the total score being used to assess the degree of depression (the higher the score the worse the depression). The total score was used in the analysis.
The Profile of Mood States is a multiple affective adjective checklist with 67 items rated by the subject on a 5-point scale (0-4 inclusively), developed by McNair et al (1992). Subjects were asked to rate how they felt right now at baseline and at 4 and 28 hours post-drink by reading the adjective list and assigning the relevant score. The six factor scores TensionAnxiety, DepressionDejection, AngerHostility, VigourActivity, FatigueInertia and ConfusionBewilderment were then calculated and the composite score for total mood generated.
The Hourly Visual Analogue Rating, a further visual analogue scale of five 100-mm lines marked with very and not at all at respective anchor points for each of five categories (Sadness, Anxiety, Irritability, Difficulty concentrating and Energy) was used. Subjects rated themselves prior to ingestion of the drink, at hourly intervals throughout the experimental session and during the following day.
A suicidality rating was generated by combining the individual items in the Beck Depression Inventory and the Hamilton Rating Scale for Depression. Suicidality was evaluated at baseline and at 4 hours and 28 hours after the drink.
Drink composition
Both drinks were of identical composition, with the exception of the
addition of 2.3 g tryptophan to the control drink. A 100 g amino acid drink
was used containing L-alanine 5.5 g, L-arginine 4.9 g, L-cysteine 2.7 g,
L-glycine 3.2 g, L-histidine 3.2 g, L-isoleucine 8 g, L-leucine 13.5 g,
L-lysine monohydrochloride 11 g, L-methionine 3 g, L-phenylalanine 5.7 g,
L-proline 12.2 g, L-serine 6.9 g, L-threonine 6.5 g. L-tyrosine 6.9 g,
L-valine 8.9 g. This was mixed in 300 ml water, flavoured with blackcurrant
and sweetened.
Biochemical measures
On four separate occasions, 20 ml venous blood was taken during each
experimental session. A sample was taken before the amino acid drink (at time
zero), before symptomatic evaluation (after 240 minutes), after symptomatic
evaluation (after 330 minutes) and the following day (after 28 hours). Plasma
was immediately separated by refrigerated centrifugation and a sample for free
tryptophan was further centrifuged using an ultrafiltrate tube. All samples
were stored at -20°C until assay. Plasma total and free tryptophan were
determined by high-pressure liquid chromatography by the method of Marshall
et al (1987). Serum
lithium was determined using atomic absorption spectrophotometry on the
remainder of the sample in the local biochemistry laboratory.
Statistical analysis
SPSS for Windows, Release 7 (SPSS,
1998) was used for statistical analysis. The
KolmogorovSmirnov test and, where necessary, non-parametric statistical
methods were used. Biochemical and mood rating scale scores were analysed
using repeated measures analysis of variance (ANOVA) with drink condition
(tryptophan depletion or placebo) and time as within-subject variables and
order as a between-subjects variable. Reported P values were
corrected using the HuynhFeldt correction factor when the sphericity
assumption was not met. For clarity, uncorrected degrees of freedom are
reported. Psychological variables were analysed using post-hoc paired
t-tests (two-tailed). These data are quoted as means (s.e.m.) with
95% confidence intervals.
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RESULTS |
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Biochemistry
Tryptophan levels
Tryptophan depletion was generally well tolerated in those who remained in
the study. There was a significant effect of depleting drink compared to
control drink on both free and total tryptophan concentration, with 83%
depletion of free tryptophan (paired t-test, P=0.016) and
84% depletion of total tryptophan (paired t-test, P=0.001;
see Fig. 1).
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Serum lithium levels
Serum lithium fell during the test day and this reached statistical
significance (F3,42=27.99, P<0.001; see
Fig. 2). However, there was no
significant difference between the two groups (depletion/non-depletion) at any
of the time points (paired t-test, P>0.2) and no
significant difference between levels prior to testing within each trial
(paired t-test, P>0.6).
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Mood rating scales
Objective mood ratings
On the Hamilton Rating Scale for Depression the mean baseline score did not
differ between the two conditions (placebo=2; active=2). There was no effect
of depletion (F1,14=0.2, P=0.665) or time
(F2,28=0.39, P=0.684) and no interaction between
drink and time (F2,28=0.04, P=0.956).
On the Young Mania Rating Scale, the mean baseline score also did not differ between the two conditions (placebo=<1, active=<1). There was no effect of depletion (F1,14=2.95, P=0.108) or time (F2,28=2.63, P=0.09) and no interaction between drink and time (F2,28=0.11, P=0.892).
Subjective mood ratings
On the Internal State Scale there was a significant effect of drink on the
depressive index sub-scale, with subjects receiving the balanced drink
reporting decrease in the depressive index (less depressed) rating at 4 hours
but not at 28 hours (F1,14=4.72, P=0.048),
whereas the depleting drink caused no change in mood. There was no other
significant effect of the drink on any other sub-scale (see
Table 2).
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On the Carroll Bipolar Visual Analogue Scale the baseline scores for the individual items (consummatory reward, central pain, psychomotor regulation and incentive reward) were not significantly different between the two conditions. There was no effect of depletion or time and no interaction between drink and time for all the individual items (see Table 2).
On the Beck Depression Inventory there was no significant difference between baseline scores for depleted (mean=3.7) and placebo (mean=3.8) and no effect of depletion or time and no interaction between drink and time for this item (see Table 2).
On the Profile of Mood States there was no significant difference at baseline between the two conditions on both the total score and the individual sub-scales (see Table 2). The depleting drink did not alter the total score or the individual sub-scales significantly. There was a significant decrease in the total mood score (F2,28=3.31, P=0.05), and the AngerHostility score (F2,28=4.28), P=0.024) with both depleting and balanced drinks.
On the Hourly Visual Analogue Rating, tryptophan depletion produced no significant effect on any of the measures. There was a significant effect of time on both Anxiety (F7,98=2.25, P=0.038) and Sadness (F7,98=3.5, P=0.002). Patients were less anxious and less sad at the end of each testing day irrespective of the drink taken.
On the Suicidality Rating, there was no effect of depletion on this composite score (F1,14=0.27, P=0.61) and no change over time (F2,28=1.56, P=0.229) or any drink by time interaction (F2,28=0.48, P=0.062).
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DISCUSSION |
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Acute tryptophan depletion has been performed using a variety of experimental protocols in previous studies (for review see Reilly et al, 1997) and produces a depressive mood shift in patients recovered from unipolar depression (Smith et al, 1997). Our study, in common with this latter study, used a well-established paradigm and produced a large drop in plasma tryptophan, and thus brain 5-HT levels. This study did not employ a low tryptophan diet preceding depletion, but rather used an overnight fast. Low tryptophan diet is used to minimise diet-induced fluctuations in baseline tryptophan levels (Reilly et al, 1997). However, in this study, baseline free tryptophan levels did not show a difference.
5-HT and lithium
Despite our findings, the pharmacological locus of lithium's effects in
bipolar disorder may still be within the brain 5-HT system. Serotonergic
neurons arise from the dorsal and median raphe nuclei in the brain-stem and
project widely throughout the brain and spinal cord. Multiple 5-HT receptors
have been described, with the 5-HT1A and 5-HT2 receptor
subtypes being most clearly implicated in mood disorders
(Cowen, 1996). Importantly,
these receptors appear to play different roles in mood disorders and lithium
has distinct effects on each receptor subtype. The 5-HT1A receptor
has been implicated in the mechanism of action of antidepressants, in the
neurobiology of unipolar disorder and in augmentation of antidepressants by
lithium (Cowen et al,
1991; Cowen, 1996).
Previous studies have found that lithium enhances the neuro-endocrine response
to the 5-HT precursor L-tryptophan, an effect that is mediated by the
5-HT1A receptor (McCance et
al, 1989; Smith et
al, 1991). Studies in depressed patients suggest that the
pharmacological basis of lithium's augmentation of antidepressant action in
unipolar disorder is by an action on 5-HT1A-mediated
neurotransmission (Cowen et al,
1991). The effect of lithium on
5-HT1A-receptor-mediated neurotransmission is probably dependent on
5-HT availability, as lithium has no effect on 5-HT1A receptor
sensitivity (Walsh et al,
1991). Acute tryptophan depletion has been shown to reverse the
action of antidepressants (Delgado et
al, 1990), suggesting that this procedure alters
neurotransmission through the 5-HT1A receptor subtype. Our findings
suggest that the beneficial actions of lithium on mood and suicidal ideation
in bipolar disorder are not mediated by an effect on 5-HT1A
receptor neurotransmission and are not acutely dependent on 5-HT
availability.
The molecular effects of lithium on 5-HT2 receptor numbers and linked second messenger systems are complex (Moorman & Leslie, 1998), but lithium has been shown to increase slow-wave sleep in normal volunteers, an effect mediated by 5-HT2 receptor function (Friston et al, 1989). D-fenfluramine, the 5-HT-releasing agent, appears to act via 5-HT2 postsynaptic receptors, as judged in serotonin neuroendocrine tests (Cowen, 1993). Lithium does not alter this neuroendocrine response to the 5-HT (Power et al, 1993), suggesting that the effect of lithium on 5-HT2-receptor-mediated neurotransmission is not dependent on 5-HT availability but is a direct effect on the receptor or post-receptor mechanisms.
Lithium prevents suicide in bipolar affective disorder (Tondo et al, 1997a,b) and may differ from other mood stabilisers, such as carbamazapine, in this property (Thies-Flechtner et al, 1996). The 5-HT2 receptor subtype has also been implicated in mood disorders and in suicide (Ferrier & Perry, 1992) and this receptor has been postulated to be involved in the beneficial effects of lithium on suicidality (Goodwin & Ghaemi, 1998). In keeping with these observations, our results do not support a mechanism of action acutely dependent on central 5-HT availability acting through post-synaptic 5-HT1A receptors, but support the alternative proposition that the effects of lithium are upon 5-HT2 receptor numbers and related cellular signalling mechanisms. Future studies of lithium's mechanism of action on mood and suicidal ideation in bipolar disorder should therefore focus on its effects on the 5-HT2 receptor.
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Clinical Implications and Limitations |
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LIMITATIONS
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ACKNOWLEDGMENTS |
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Received for publication November 30, 1999. Revision received May 15, 1999. Accepted for publication May 19, 2000.