Neuroscience and Psychiatry Unit, University of Manchester, Manchester
Institute of Psychiatry, London
Correspondence: Dr Paul Strickland, Neuroscience and Psychiatry Unit, Stopford Building, University of Manchester, Oxford Road, Manchester M13 9PT. Tel: 0161 275 7427; fax: 0161 275 7429; e-mail: P.Strickland{at}man.ac.uk
Declaration of interest Wellcome Trust funding.
See editorial, pp.
99100, this issue.
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ABSTRACT |
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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 hypothalamicpituitaryadrenal axis is sensitive to social stress but does not mediate vulnerability to depression. Exaggerated 5-HT2 receptor responsiveness to stress may play a role in the evolution of depression.
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INTRODUCTION |
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METHOD |
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Interviews and assessments
The women were interviewed twice in their homes. A psychiatrist (P.L.S.)
conducted the first interview. The subjects were given a complete description
of the study, written informed consent was obtained, demographic details were
collected and a Schedules for Clinical Assessment in Neuropsychiatry (SCAN)
diagnostic interview (World Health
Organization, 1992) was conducted. Four tubes were left for saliva
collection over two days at 09.00 and 23.00 h during the follicular phase of
the menstrual cycle (day 0-12) or on pill-free days for women taking the oral
contraceptive pill.
At the second interview, within 2 weeks of the first, the Life Events and Difficulties Schedule (LEDS; Brown & Harris, 1989) for the previous 6 months and the Self Evaluation and Social Support Scales (SESS; Brown et al, 1990) were administered by social researchers (C.P., J.D.). These interviews were used to determine vulnerability to depression and current depression.
Serotonin neurotransmission
Serotonin function was assessed by measuring prolactin responses to
dexfenfluramine challenge in subgroups (20 depressed, 63 vulnerable and 61
non-vulnerable) during the follicular phase of the menstrual cycle (days 0-12
of cycle, similar test-timing for all groups) or during the
contraceptive-pill-free days. The subjects were drug-free and fasted from
23.00 h. Dexfenfluramine (30 mg, oral) and placebo tests were separated by at
least 2 days and the order was randomised and double blind. Baseline blood
samples were taken an hour before (09.30 h) and at the time of drug/placebo
administration (10.30 h). Hourly samples were taken thereafter. Baseline
assays were l-tryptophan, branched-chain amino acids, cortisol and
prolactin. Prolactin, fenfluramine and norfenfluramine were assayed in
subsequent samples.
Assays
Salivary cortisol was measured by competitive radioimmunoassay
(Kane, 1979). Inter-assay
precision was <13% at all levels. Intra-assay precision was <6% across
the whole assay range. Plasma-free tryptophan, branched-chain amino acids,
fenfluramine and norfenfluramine concentrations were determined by a
fluorimetric method (Franklin et
al, 1995).
Social adversity
Life events were categorised into severe and non-severe and into recent
(within the past month) and non-recent (>1 month, <6 months) using LEDS
criteria.
Vulnerability and depression
Vulnerability was defined according to Brown et al
(1990) as the presence of two
or more vulnerability factors, at least one being environmental. Environmental
factors are negative evaluation of core relationship (partner or child),
social isolation and chronic difficulty. Psychological factors are low
self-esteem, subclinical depression and parental separation for more than 1
year before the age of 16 years. Women assessed as being vulnerable to
depression have a 25-30% risk of having a new onset of depression in the
following year (Brown et al,
1990). Depression was defined using ICD-10 criteria
(World Health Organization,
1993): depressive episode F32.0-32.2 or recurrent depressive
disorder F33.0-33.2. Subsidiary analyses used DSM-IV criteria
(American Psychiatric Association,
1994): major depression without psychotic symptoms
296.2-296.3.
Statistical analysis
Average morning and evening salivary cortisol and diurnal change were
analysed. Cortisol concentrations at 23.00 h were not normally distributed and
vulnerability and life event groups were compared using 2 and
MannWhitney U tests. Wilcoxon paired rank tests were used for
within-subject comparisons.
Dexfenfluramine challenge was assessed using analysis of covariance of all prolactin data, with a repeated measures factor for sample time and factors for drug/placebo and vulnerability group, and using area under the prolactin curve (AUC) on the fenfluramine day minus AUC on the placebo day (placebo-controlled AUC, PC-AUC). The PC-AUC was normally distributed and therefore analysed by analysis of covariance. The following potential covariates were measured: AUC of fenfluramine and norfenfluramine concentrations; serum cortisol, tryptophan and branched-chain amino acid concentrations; the ratio of tryptophan to branched-chain amino acids; and weight change over the past month (categories were: none; <2 kg; a loss of >2 kg; a gain of >2 kg). The Statistical Package for the Social Sciences, version 7.5 (SPSS, 1997) was used.
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RESULTS |
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Cortisol concentrations
Salivary cortisol concentrations showed diurnal variation
(Table 2 and
Fig. 1a). Although separated by
several weeks, morning cortisol concentrations from the salivary and
fenfluramine studies were correlated significantly (r=0.31,
n=160, P<0.001).
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Subjects meeting the criteria for ICD-10 depression did not have increased salivary cortisol concentrations either in the evening or morning (Fig. 1a and Table 2). Indeed, there were reductions in the 09.00 h salivary cortisol and diurnal change in the depressed subjects of borderline statistical significance by two-tailed tests (Table 2). Similarly, average morning serum cortisol (from those who had dexfenfluramine tests) was reduced in depressed subjects (mean=319.9 and s.e.=125.7 nmol/l, n=24) compared with non-depressed subjects (mean=403.2 and s.e.=190.4 nmol/l, n=135, P=0.04, one-way analysis of variance).
Severe recent life events were associated with increased 23.00 h salivary
cortisol concentrations (Fig.
1b; P<0.002, 2 test), whereas recent
non-severe events and severe events occurring more than a month previously
were not. This association between life events and high evening salivary
cortisol concentration remained unchanged when depressed patients were
excluded. The association with life events was no more (or less) apparent in
the depressed group.
The vulnerable, non-depressed group showed no evidence of increased cortisol concentrations. Almost all the currently depressed women fulfilled LEDS/SESS vulnerability criteria, largely owing to psychosocial difficulties and poor relationships. In a subsidiary analysis we investigated whether the hypothalamic-pituitary-adrenal (HPA) axis becomes responsive to these vulnerability factors in depression. Although those with current depression as a group did not have greater cortisol concentration than controls, dividing the depressed group into high- and low-difficulty groups by median difficulty score revealed that, in depression, chronic stress is associated with increased cortisol secretion (Fig. 1c). This was not seen in the non-depressed.
Dexfenfluramine challenge tests
Prolactin responses to dexfenfluramine were greater in depressed subjects
than in vulnerable or non-vulnerable subjects by repeated-measures analysis
(Fig. 2a; group x drug
x sample: F=2.7; 12,846; P=0.002) and by PC-AUC, with
dexfenfluramine AUC and average l-tryptophan concentrations as
covariates. Baseline cortisol and branched-chain amino acid concentrations
were not significant covariates. Exploration of the effects of life events on
5-HT function was hampered by small subgroups, but fenfluramine responses were
greater in the depressed with recent life events; in the absence of life
events, prolactin responses tended to be reduced
(Fig. 2b; life event x
group interaction: F=5.7; 2,137; P=0.004).
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Tryptophan concentrations were reduced in those with recent severe life events compared with those with no recent severe events (Table 3). No reduction was found in other categories of life event severity or recency. Depression (ICD-10 or DSM-IV) was not associated with low tryptophan concentration. However, in the depressed subjects there were significant increases in branched-chain amino acid concentrations and this resulted in statistically significant reductions in the ratio of tryptophan to branched-chain amino acids (Table 3). None of the measures of tryptophan availability was a significant covariate in the analysis of covariance.
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Self-rated weight loss did not influence the concentration of tryptophan or branched-chain amino acids. Fenfluramine responses were greater in the subgroup reporting mild degrees of weight loss (<2 kg) compared with the no-weight-loss group and the other groups; however, those with more severe degrees of weight loss and those with weight gain had entirely normal responses (data not shown).
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DISCUSSION |
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Hypothalamicpituitaryadrenal axis function and
depression
Increased secretion of cortisol in depression is widely held to be central
to the pathogenesis of symptoms and to be caused by psychosocial stress
(Stokes et al, 1984;
Dinan, 1994;
Nemeroff, 1996). This
hypothesis has been difficult to reconcile with the low prevalence of
hypercortisolaemia in milder depressive illnesses in which social stress plays
an important aetiological role. In our study, cortisol was responsive to
recent psychosocial stress but was not raised in depression. The majority of
the cases of depression were of mild severity, but even in more severe cases
and those fulfilling DSM-IV criteria (n=48;
Table 1) there was no trend for
increased cortisol concentrations. Indeed, we found evidence that morning
cortisol in serum was reduced in depressed subjects and the same trend was
seen in salivary cortisol. Reduced 09.00 h salivary cortisol has been reported
in chronic fatigue syndrome (Strickland
et al, 1998), in some studies of post-traumatic stress
disorder and after traumatic events
(Yehuda et al, 1995).
These disorders are associated with anxiety and in the present study the
currently depressed subjects with comorbid anxiety diagnoses had lower morning
serum cortisol concentrations (data not shown).
Hypothalamicpituitaryadrenal axis function and
psychosocial stress
In the present study high ratings of psychosocial difficulty were
associated with increased evening cortisol concentration, but only in the
depressed group (Fig. 1c). This
raises the possibility that some cases of community depression may involve a
primary dysregulation of the HPA axis, which results in exaggerated cortisol
responses to persistent adversity. Further analysis may reveal whether the
dysregulation arises from early life experiences or from constitutional
factors, and whether there is some symptomatic or personality correlate. A
primary dysregulation of the HPA axis may account for the many reports of
increased cortisol in in-patients with current depression, with the stress of
admission interacting with a sensitised HPA axis
(Maes et al, 1994). Indeed, a number of studies suggest that admission is an important influence
on the prevalence of HPA axis dysregulation in depression. Our results
contrast with a community-based study that found increased evening salivary
cortisol in children and adolescents during depression
(Goodyer et al,
1996). The explanation may lie in differences in age, gender,
social class and, possibly, severity of the two samples. Our finding that life
events are associated with greater salivary cortisol concentrations was not
reported in two community-based high-risk studies, but both studies
specifically excluded currently depressed individuals, who would have had high
rates of life events, and they were smaller studies (Goodyear et al,
2000; Harris et al,
2000).
Serotonin function in depression: dexfenfluramine challenge
tests
Unexpectedly, we found that prolactin responses to dexfenfluramine were
increased in the currently depressed individuals, especially those with recent
life events, although the latter observation is based on small numbers. This
result contrasts with three previous studies of psychiatric inpatients that
have reported attenuated prolactin responses to d/l- or
d-fenfluramine (O'Keane &
Dinan, 1991; Lichtenberg
et al, 1992; Siever
et al, 1984). A further study reported decreased
responses in an endogenous subgroup but the difference was not statistically
significant when low baseline prolactin levels were covaried
(Mitchell & Smythe, 1990). Individuals with endogenous depression had smaller responses than patients
with milder depression in one study
(Lopez-Ibor et al,
1988) but in another the reverse was observed
(Maes et al, 1989)
and neither had healthy control groups. In a large study of primary care
attenders with depression no change was found
(Park et al, 1996).
In these studies control groups have not been defined beyond matching on basic
variables and by exclusion criteria. Our finding of increased prolactin
responses is based on two exceptionally large control groups drawn from the
same locality and socio-economic grouping as the depressed group, one of which
also controlled for psychosocial vulnerability. Furthermore, the finding
survives correction for all known influences on prolactin responses to
fenfluramine: placebo challenge, baseline cortisol and prolactin, menstrual
phase, drug and metabolite levels, tryptophan availability and weight
loss.
One possible explanation for increased prolactin responses to dexfenfluramine is that weight loss and/or reduced tryptophan availability in the depressed group caused reduced 5-HT release and a secondary adaptive up-regulation of 5-HT receptor responsivity. This mechanism has been suggested for increased responses following dieting (Walsh et al, 1995; Cowen et al, 1996). In depressed patients, the ratio of tryptophan to branched-chain amino acids, which compete with each other for transport into the brain, was indeed reduced by 20% but this was not a significant covariate of responses. Furthermore, subjects with self-rated weight loss of > 2 kg showed no trend to increased dexfenfluramine responsiveness and there was no relationship between weight loss and measures of tryptophan availability. These considerations suggest that 5-HT receptor upregulation may not adequately explain the findings.
Serotonin receptor subtypes: role of 5-HT2 receptors in
depression
There is evidence that prolactin responses to dexfenfluramine are mediated
through 5-HT2c receptors
(Goodall et al,
1993). Thus, a more straightforward explanation for increased
prolactin responses is that they are due to increased neurotransmission
through 5-HT synapses with 5-HT2c receptors in untreated
mild-moderate depression in the community. In more advanced or serious
depressive illness, perhaps in combination with the effects of hospital
admission, responses may become attenuated, as suggested by some fenfluramine
studies discussed above. Our finding is in keeping with the theory that
5-HT2 systems are activated by adversity and mediate the anxiety
component of depression (Deakin,
1988). Together with our previous evidence of attenuated
5-HT1A-mediated tryptophan responses, the results are compatible
with the theory that depression involves an imbalance between excessive
5-HT2C and reduced 5-HT1A functioning
(Deakin, 1988;
Deakin & Graeff, 1991).
Activation of 5-HT systems in response to stress
We have shown that psychosocial stresses activate the HPA axis but that
this is not the mechanism by which psychosocial stress causes depression.
Depression occurs in the absence of sustained hypercortisolaemia. ubjects with
depression had reduced morning cortisol, which may be related to coexistent
anxiety. Central 5-HT2 neurotransmission is enhanced in depression
and is responsive to life events. This is compatible with the idea that this
5-HT system is concerned with central responses to adversity. Life events and
depression influence peripheral amino acid metabolism. More research is needed
to determine whether this affects central 5-HT function and vulnerability to
depression.
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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 December 21, 2000. Revision received July 2, 2001. Accepted for publication October 10, 2001.
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