Brain serotonin2 receptors in major depression: a positron emission tomography study

BACKGROUND: Postmortem and brain imaging studies that measured brain serotinin(2) (5-HT(2)) receptors in major depression reported an increase, decrease, and no change compared with controls. In this study, we assessed brain 5-HT(2) receptors in 20 depressed patients (mean +/- SD age, 40.1 +/- 9.5 years; range, 22-60 years) and 20 healthy controls similar in age (37.2 +/- 12.6 years; range, 19-59 years) using positron emission tomography and setoperone labeled with fluorine 18 ([(18)F]setoperone). METHODS: Patients with DSM-IV major depression and healthy controls underwent scanning with [(18)F]setoperone. All study subjects were drug free for at least 2 weeks. The 5-HT(2) binding images were created using region-to-cerebellum ratios. The differences in 5-HT(2) receptor binding potential between the two groups were determined with statistical parametric mapping software and region of interest analysis. RESULTS: There was a significant negative correlation between 5-HT(2) receptor binding potential and age in both patients and controls, and the magnitude of this correlation was similar in both groups. Both statistical parametric mapping and region of interest analyses showed that, compared with healthy controls, depressed patients had significantly lower 5-HT(2) receptor binding potential in frontal, temporal, parietal, and occipital cortical regions. Statistical parametric mapping analysis showed that the mean decrease in 5-HT(2) receptor binding potential for the entire cluster in these regions was 22%, and it ranged from 22% to 27% for local maxima within the clusters of significant voxels. CONCLUSION: This study suggests that brain 5-HT(2) receptors are decreased in patients with major depression.     

The effect of paroxetine on 5-HT(2A) receptors in depression: an [(18)F]setoperone PET imaging study

OBJECTIVE: In the cortex of animals, serotonin (5-HT) levels increase after several weeks of treatment with selective serotonin reuptake inhibitors (SSRIs). Studies using an intrasubject design to examine the effects of SSRI treatment on 5-HT(2A) receptors in the cortex of drug-free depressed patients are needed. In theory, agonist stimulation of 5-HT(2A) receptors could be relevant to SSRI treatment by promoting neuronal growth and survival as well as direct elevation of mood. The objective of this study was to evaluate the effect of 6 weeks of paroxetine treatment on 5-HT(2A) receptors in depressed patients. METHOD: After a medication-free period of at least 3 months, 19 depressed patients were treated for 6 weeks with paroxetine, 20 mg/day. The authors used [(18)F]setoperone and positron emission tomography to assess 5-HT(2A) receptor binding potential in the patients before and after treatment and in 19 age-matched healthy subjects. RESULTS: 5-HT(2A) binding potential declined with age in all cortical regions in the depressed and healthy subjects. There was a significant interaction between age and treatment effect on 5-HT(2A) binding potential in all cortical regions. Subjects aged 20 to 30 years had a 10% decrease in 5-HT(2A) binding potential after treatment, whereas subjects aged 30 to 40 had no change. No regional differences in 5-HT(2A) binding potential between depressed and healthy subjects were found. CONCLUSIONS: 5-HT(2A) receptors down-regulate in young depressed subjects after treatment with paroxetine, but this down-regulation attenuates with age. This suggests that over 6 weeks paroxetine treatment increases 5-HT agonism on 5-HT(2A) receptors in the cortex of young patients with depression.     

Decrease in brain serotonin 2 receptor binding in patients with major depression following desipramine treatment: a positron emission tomography study with fluorine-18-labeled setoperone.

BACKGROUND: The neuroreceptor changes involved in therapeutic efficacy of various antidepressants remain unclear. Preclinical studies have shown that long-term administration of various antidepressants causes down-regulation of brain serotonin 2 (5-HT2) receptors in rodents, but it is unknown if similar changes occur following antidepressant treatment in depressed patients. Our purpose, therefore, was to assess the effects of treatment with desipramine hydrochloride on brain 5-HT2 receptors in depressed patients using positron emission tomography (PET) and fluorine-18 (18F)-labeled setoperone. METHODS: Eleven patients who met DSM-IV criteria for major depression as determined by a structured clinical interview for DSM-III-R diagnosis and suitable for treatment with desipramine were recruited. Ten patients underwent a PET scan before and another after 3 to 4 weeks of treatment with desipramine. RESULTS: Eight of the 10 patients responded to desipramine treatment as indicated by more than 50% decrease in Hamilton Depression Rating Scale scores. Depressed patients showed a significant decrease in 5-HT2 receptor binding as measured by setoperone binding in frontal, temporal, parietal, and occipital cortical regions following desipramine treatment. The decrease in 5-HT2 receptor binding was observed bilaterally and was particularly prominent in frontal cortex. CONCLUSIONS: Depressed patients showed a significant reduction in available 5-HT2 receptors in the brain following desipramine treatment, but it is unknown if this change in 5-HT2 receptors is due to clinical improvement or an effect of desipramine that is unrelated to clinical status.     

Frequency of long allele in serotonin transporter gene is increased in depressed suicide victims.

BACKGROUND: There is evidence indicating that serotonin uptake and density of 5-HT2A receptors are altered in brain regions of depressed suicide victims and in platelets of depressed suicidal subjects. The present investigation tested the hypothesis that these changes in the serotonergic system in depressed suicide victims are trait rather than state markers and associated with a polymorphism in respective candidate genes. METHODS: Two polymorphic variants (102T/C polymorphism and His452Tyr functional polymorphism) of the 5-HT2A receptor gene and a functional polymorphism in the 5' regulatory region of the 5-HT transporter gene, have been determined in genomic DNA obtained from postmortem brain samples of 24 depressed suicide victims and 31 control subjects of the same ethnic background. In a subset of subjects, density (Bmax) of 5-HT uptake sites (labeled with 3H-paroxetine) and of 5-HT2A receptors (labeled with 3H-ketanserin) was also determined in prefrontal cortex samples. RESULTS: The major finding of this study was a significantly higher frequency of the 5-HT transporter gene long (L) allele (chi 2 = 3.9, df = 1; p = .048) in depressed suicides. No significant differences between suicides and controls were observed for the 102T/C polymorphism and His452Tyr polymorphism of 5-HT2A receptor gene. The density of 3H-paroxetine binding sites tended to be higher in subjects expressing the short (S) allele of 5-HT transporter gene. Furthermore, there was a significant difference in serotonin transporter binding sites between the genotype S/S and combined genotypes S/L and L/L. CONCLUSIONS: Our finding provides the first evidence suggesting that a functional polymorphism in the regulatory region of serotonin transporter gene may be associated with suicide in depressed subjects.     

Imaging serotonergic neurotransmission in depression: hippocampal pathophysiology may mirror global brain alterations.

The recent development of [carbonyl-(11)C]WAY-100635 for serotonin (5-HT)(1A) and [(18)F]setoperone and [(18)F]altanserin for 5-HT(2A) positron emission tomography receptor imaging has allowed studies of 5-HT neurotransmission in depressive disorders. The hippocampus is likely to be an important brain structure in the pathophysiology of depression because it may mediate both cognitive deficits and hypercortisolemia found in this disorder. Decreased 5-HT(1A) binding was reported in the medial temporal cortex, which receives dense 5-HT innervation, and also throughout neocortical regions. Because the 5-HT(1A) antagonist pindolol may hasten antidepressant effects of selective serotonin reuptake inhibitor medications, its receptor occupancy has been measured in both presynaptic and postsynaptic sites. The results are controversial but suggest that pindolol has preferential occupancy of somatodendritic autoreceptors in the raphe. The results of 5-HT(2A) receptors are mixed, with one showing a significant decrease in the right orbitoinsular cortex and three not detecting a significant change. The disparate findings in patients with depression almost certainly reflect the heterogeneity of the disorder, and we highlight the utility of the hippocampus as a useful target region not only to compare depressed subjects with healthy subjects but also to correlate findings with cognitive function and activity of the limbic-hypothalamic-pituitary axis system.     

Increased 5-HT(2A) receptor binding in euthymic, medication-free patients recovered from depression: a positron emission study with [(11)C]MDL 100,907

OBJECTIVE: A previous positron emission tomography (PET) study reported increased serotonin 5-HT(2A) receptor binding in unmedicated depressed patients with high scores on the Dysfunctional Attitudes Scale. The purpose of the present study was to use the highly selective 5-HT(2A) receptor ligand [(11)C]MDL 100,907 in a PET imaging paradigm to assess 1) 5-HT(2A) receptor binding potential in euthymic subjects with a history of recurrent depression and 2) the relationship between receptor binding and scores on the Dysfunctional Attitudes Scale. METHOD: Cortical 5-HT(2A) receptor binding was measured in 20 unmedicated, fully recovered unipolar depressed patients and 20 age- and gender-matched comparison subjects. Regional estimates of binding potential were obtained using a reversible plasma input function compartmental model and the cerebellum as a reference region to estimate the free and non-specifically bound [(11)C]MDL 100,907 in brain tissue. RESULTS: Relative to the comparison subjects, the recovered depressed patients demonstrated significantly higher 5-HT(2A) receptor binding potential in the frontal cortex (mean increase: 19%), parietal cortex (mean increase: 25%), and occipital cortex (mean increase: 19%). 5-HT(2A) receptor binding potential correlated negatively with age in both patients and comparison subjects and positively with the Dysfunctional Attitudes Scale in the recovered patients. CONCLUSIONS: These findings should be considered preliminary but suggest that recovered subjects with a history of recurrent major depression have elevated binding potential of cortical 5-HT(2A) receptors. The correlation of increased 5-HT(2A) receptor binding potential with increased scores on Dysfunctional Attitudes Scale supports earlier work suggesting that increased 5-HT(2A) receptor availability characterizes a group of depressed patients with high levels of dysfunctional attitudes.     

Dysfunctional attitudes and 5-HT2 receptors during depression and self-harm

OBJECTIVE: Dysfunctional attitudes are negatively biased assumptions and beliefs regarding oneself, the world, and the future. In healthy subjects, increasing serotonin (5-HT) agonism with a single dose of d-fenfluramine lowered dysfunctional attitudes. To investigate whether the converse, a low level of 5-HT agonism, could account for the higher levels of dysfunctional attitudes observed in patients with major depression or with self-injurious behavior, cortex 5-HT(2) receptor binding potential and dysfunctional attitudes were measured in patients with major depressive disorder, patients with a history of self-injurious behavior, and healthy comparison subjects (5-HT(2) receptor density increases during 5-HT depletion). METHOD: Twenty-nine healthy subjects were recruited to evaluate the effect of d-fenfluramine or of clonidine (control condition) on dysfunctional attitudes. Dysfunctional attitudes were assessed with the Dysfunctional Attitude Scale 1 hour before and 1 hour after drug administration. In a second experiment, dysfunctional attitudes and 5-HT(2) binding potential were measured in 22 patients with a major depressive episode secondary to major depressive disorder, 18 patients with a history of self-injurious behavior occurring outside of a depressive episode, and another 29 age-matched healthy subjects. Cortex 5-HT(2) binding potential was measured with [(18)F]setoperone positron emission tomography. RESULTS: In the first experiment, dysfunctional attitudes decreased after administration of d-fenfluramine. In the second experiment, in the depressed group, dysfunctional attitudes were positively associated with cortex 5-HT(2) binding potential, especially in Brodmann's area 9 (after adjustment for age). Depressed subjects with extremely dysfunctional attitudes had higher 5-HT(2) binding potential, compared to healthy subjects, particularly in Brodmann's area 9. CONCLUSIONS: Low levels of 5-HT agonism in the brain cortex may explain the severely pessimistic, dysfunctional attitudes associated with major depression.     

Brain 5-HT2 receptor binding sites in depressed suicide victims

5-HT2 receptor binding sites were measured (by saturation binding of [3H]ketanserin) in brain tissue obtained at postmortem from 19 suicide victims with definite evidence of depression and 19 sex and age-matched control subjects. Five of the suicide victims were receiving antidepressant drugs prior to death; 13 suicide victims had not been prescribed antidepressant or other psychoactive drugs recently and none were found in their blood at postmortem. The number of serotonin-2 (5-HT2) binding sites in frontal, temporal and occipital cortex and amygdala did not differ significantly between the depressed suicide victims and controls, either in the total suicide group or in the antidepressant drug-free suicides. The number of 5-HT2 binding sites in the hippocampus did not differ from controls in the total suicide group but was significantly lower (by 23%) in the antidepressant-free suicide group. The affinity of [3H]ketanserin binding did not differ from controls in the antidepressant-free suicides but was lower (increased Kd) in those subjects receiving antidepressant drugs. No correlation was found between the time of death and storage of tissue or the duration of tissue storage prior to assay and the number or affinity of 5-HT2 binding sites. A significant negative correlation was found between age of the subject and the number of 5-HT2 binding sites in the frontal and occipital cortex. The present study of suicide victims with definite evidence of depression do not confirm previous studies of increased numbers of 5-HT2 binding sites in suicide victims and suggest that these previous findings may be related to factors other than depression.     

Persistent reduction in brain serotonin1A receptor binding in recovered depressed men measured by positron emission tomography with [11C]WAY-100635

Positron emission tomography (PET) studies with the selective 5-HT(1A) receptor ligand, [(11)C]WAY-100635, have indicated that the binding potential (BP) of brain 5-HT(1A) receptors is lowered in unmedicated subjects with acute major depression. However, it is unclear if these changes persist after recovery from depression. To resolve this issue, we used [(11)C]WAY-100635 in conjunction with PET imaging to compare 5-HT(1A) BP in 18 healthy controls and 14 male subjects with recurrent major depression who were clinically recovered and free of antidepressant medication. BP values, derived from a reference tissue model, were analysed by region of interest and statistical parametric mapping. Both analyses showed a widespread and substantial (17%) decrease in 5-HT(1A) receptor BP in cortical areas in the recovered depressed subjects. In contrast, 5-HT(1A) BP in the raphe nuclei did not distinguish depressed subjects from controls. Our results suggest a persistent dysfunction in cortical 5-HT(1A) BP as measured by [(11)C]WAY-100635 in recovered depressed men. Lowered 5-HT(1A) receptor binding availability could represent a trait abnormality that confers vulnerability to recurrent major depression.     

Functional imaging, serotonin and the suicidal brain

The involvement of the serotonergic system in the pathophysiology of suicidal behaviour has been established through indirect and direct research on serotonin and its metabolites and on serotonin transporters and receptors. Indirect research results include a reduced 5-HIAA in cerebrospinal fluid in violent suicide attempters and a blunted increase in prolactin after a fenfluramine challenge. Direct post-mortem research demonstrated an increase in 5-HT2A receptors. Direct in vivo functional imaging with PET or SPECT demonstrated a reduction in 5-HT2A binding index in suicide attempts in anxious and depressed suicide attempters and an increase in 5-HT2A binding in impulsive suicide attempters. These results are in keeping with 5-HT2A binding studies in depressed patients and impulsive animal research. Interestingly, both an increase and a decrease in 5-HT2A binding index seem to normalize with SSRI treatment.     

Serotonin receptor subtype and p11 mRNA expression in stress-relevant brain regions of suicide and control subjects

OBJECTIVE: Studies comparing people suffering from depression who committed suicide with control subjects have yielded inconsistent results regarding serotonin (5-HT) involvement in pathology, possibly owing to procedural factors. Our objective was to investigate which 5-HT receptor subtypes might be associated with depression and suicide, whether receptor differences vary across brain regions and whether they are moderated by sex. METHODS: We assessed messenger ribonucleic acid (mRNA) expression of several 5-HT receptor subtypes and that of p11, a protein involved in the functional expression of 5-HT(1B), in several stress-relevant brain regions. Tissue was obtained soon after death, and RNA integrity and pH was confirmed to be appropriate. Brain tissue from suicide subjects suffering from depression and from control subjects who had died from other causes (10 men and 10 women in each condition) was obtained within 6.5 hours postmortem. Quantitative polymerase chain reaction analyses determined mRNA expression of 5-HT receptor subtypes and p11 within the frontopolar cortex, orbitofrontal cortex, hippocampus, amygdala and paraventricular nucleus. The 5-HT transporter (5-HTT) was also assessed in the raphe nucleus. RESULTS: Differences of 5-HT(1A), 5-HT(1B) and p11 mRNA expression between people who committed suicide and control subjects were relatively widespread, whereas 5-HT(2A) and 5-HT(2C) variations were restricted to the frontopolar cortex and amygdala. Within the dorsal raphe nucleus, neither 5-HT(1A) nor 5-HTT mRNA expression differed between those who committed suicide and control subjects. CONCLUSION: Several 5-HT receptor subtypes are associated with depression and suicide, but these receptor differences vary across brain regions and are moderated by sex.     

Altered 5-HT2A and 5-HT4 postsynaptic receptors and their intracellular signalling systems IP3 and cAMP in brains from depressed violent suicide victims

Serotonin 5-HT2A and 5-HT4 binding parameters and their second messengers 1,4,5-inositol triphosphate (IP3) and cyclic adenosyl monophosphate (cAMP) were studied in the frontal cortex, hippocampus, caudate nucleus and amygdala of 19 control subjects and 19 antidepressant-free, violent suicide victims. A significantly higher number of 5-HT4 receptors and higher second messenger cAMP concentrations were found in the frontal cortex and caudate nucleus of the depressed suicide victims as compared with the control group. Furthermore, significantly increased 5-HT2A binding sites and IP3 concentrations were noted in the caudate nucleus of the suicide victims, together with a significantly reduced number of 5-HT2A binding sites, higher binding affinity and increased IP3 concentrations in the hippocampus. No significant alterations in 5-HT4 and cAMP or in 5-HT2A and IP3 concentrations were observed in the amygdala. The caudate nucleus of depressed suicide victims seems to be the brain region with the highest alteration of the serotonergic system, and hence with the most diagnostic sensitivity. Further studies on suicidality and depression should focus on the functionality of the caudate nucleus.     

5-Hydroxytryptamine 1A receptors, major depression, and suicidal behavior.

BACKGROUND: Several lines of evidence suggest a clear relationship between serotonin (5-hydroxytryptamine, 5-HT) hypoactivity and suicidal behavior across several psychiatric diagnoses. Few data are available, however, regarding the possible specific role of 5-HT1A receptors in the biology of suicidality. Therefore, the aim of our study was to use a neuroendocrine strategy to test the hypothesis of a role for 5-HT1A receptors in the biology of suicidal behavior. METHODS: Hormonal (adrenocorticotropic hormone [ACTH], cortisol, prolactin [PRL]) and temperature responses after administration of flesinoxan, a highly potent and selective 5-HT1A receptor full agonist, were assessed in 40 inpatients with major depression, divided into two subgroups (20 suicide attempters and 20 nonattempters), compared with 20 normal control subjects matched for gender and age. RESULTS: Compared with nonattempters, suicide attempters exhibited significantly lower PRL (p = .01), cortisol (p = .014), and temperature (p = .0002) responses. Prolactin (p = .007), cortisol (p = .04), and temperature (p = .00003) responses were also decreased in suicide attempters compared with normal control subjects. In contrast, we did not observe any significant differences in hormonal or temperature responses to flesinoxan between depressed patients without a history of suicide attempt and normal control subjects. CONCLUSIONS: The present study tends to confirm the role of 5-HT and more specifically 5-HT1A receptors in the biology of suicidal behavior in major depression.     

Higher postmortem prefrontal 5-HT2A receptor binding correlates with lifetime aggression in suicide.

BACKGROUND: In vivo studies find altered serotonin function associated with aggressive and suicidal behaviors. Postmortem studies also reveal serotonergic alterations in suicide subjects but have not reported on the relationship between aggression and the serotonin system. We measured 5-hydroxytryptamine 2A (5-HT(2A)) receptor binding in prefrontal cortex of suicide and nonsuicide subjects and explored the relationship between 5-HT(2A) receptor binding, lifetime aggression, and suicide. METHODS: The 5-HT(2A) receptor binding in coronal sections of prefrontal cortex was quantified by autoradiography with [(3)H] ketanserin in 37 suicide subjects and 73 nonsuicide subjects. The relationship between [(3)H] ketanserin binding and lifetime aggression, rated on the Brown-Goodwin Aggression History Scale, was assessed controlling for age and sex. RESULTS: In suicide subjects, lifetime aggression scores correlated positively with [(3)H] ketanserin binding in all prefrontal Brodmann areas examined, after adjusting for age and sex. This was not the case in nonsuicide subjects. We found no significant differences in aggression scores or [(3)H] ketanserin binding between the suicide subjects and nonsuicide subjects. CONCLUSIONS: The relationship between aggression and 5-HT(2A) receptor binding in suicide subjects, but not in nonsuicide subjects, may reflect differences in the regulation of the 5-HT(2A) receptor related to suicidal behavior and perhaps other proaggressive changes in brains of suicide subjects.     

Platelet serotonin-2 receptor binding sites in depression and suicide

In order to examine the role of serotonin-2 (5HT2) receptors in depression and suicide, we determined 5HT2 receptors using 125I-lysergic acid diethylamide (LSD) as the binding ligand in platelets obtained from 20 normal control and 23 drug-free depressed patients. Our results indicate significantly increased 125I-LSD binding sites (Bmax) in the platelets of depressed patients compared with normal control subjects. We also observed that a subgroup of depressed patients with a recent history of suicide attempts or suicidal ideation had significantly higher 5HT2 binding sites as compared with nonsuicidal depressed patients and normal controls. There were no significant differences in the apparent dissociation constant (Kd) values in the platelets of depressed patients compared with normal control subjects. To examine if the baseline 5HT2 receptors are related to either the severity of illness or treatment response, we determined the relationships of the baseline Bmax and Kd with baseline Hamilton Depression Rating Scale (HDRS) and Brief Psychiatric Rating Scale (BPRS) scores and change in scores after treatment. We found no significant correlation between baseline Bmax and Kd with the baseline HDRS or BPRS scores or change in these scores after psychoactive drug treatment. These results thus indicate increased platelet 5HT2 receptors in depression, but much more so in depressed patients with suicidal ideation or attempts.     

Brain 5-HT uptake sites, labelled with [3H]paroxetine, in antidepressant-free depressed suicides

Brain serotonin (5-HT) uptake sites were quantitated, by saturation binding of [3H]paroxetine, in 10 brain regions from 22 suicide victims and 20 control subjects. Suicide victims were restricted to those subjects in whom a firm retrospective diagnosis of depression was established and who had not recently been prescribed antidepressant drugs. The Kd and Bmax of [3H]paroxetine did not differ significantly between controls and depressed suicides in any of the brain regions. In putamen, Bmax values of suicides who died non-violently were lower than controls, whereas those who died by violent methods did not differ from controls. No significant differences between violent or non-violent suicides and their matched controls were found in other brain areas. These results offer little support for the view that suicide/depression is associated with an abnormality in 5-HT uptake.     

Increased cerebral serotonin-2A receptor binding in depressed patients with myocardial infarction

Serotonin (5-HT) has been implicated in the pathophysiology of depression. It is not known whether depression in post-myocardial infarction (MI) patients is also serotonin-mediated. In somatically healthy depressed persons, increased brain 5-HT(2A) receptor binding has been reported in some studies. In animal studies, decreased serotonin activity was found after induction of MI. In the present study, it was hypothesized that depressed post-MI patients would exhibit increased brain 5-HT(2A) receptor binding compared with non-depressed post-MI patients. Single photon emission computed tomography (SPECT) with the radioligand 123I-5-I-R91150, a 5-HT(2A) receptor antagonist, was used to study 5-HT(2A) receptor binding. SPECT scans were performed in nine depressed post-MI patients, 10 non-depressed post-MI patients and 10 healthy control subjects. Results were analysed using statistical parametric mapping. Depressed post-MI patients showed increased 5-HT(2A) receptor binding compared with non-depressed post-MI patients, and MI patients showed decreased 5-HT(2A) receptor binding compared with control persons. Both post-MI depression and MI seem to be associated with changes in 5-HT(2A) receptor binding.     

Brain 5-HT1A receptor binding in chronic fatigue syndrome measured using positron emission tomography and [11C]WAY-100635.

BACKGROUND: Research from neuroendocrine challenge and other indirect studies has suggested increased central 5-HT function in chronic fatigue syndrome (CFS) and increased 5-HT1A receptor sensitivity. We assessed brain 5-HT1A receptor binding potential directly using the specific radioligand [11C]WAY-100635 and positron emission tomography (PET). METHODS: We selected 10 patients from a tertiary referral clinic who fulfilled the CDC consensus criteria for CFS. To assemble a homogenous group and avoid confounding effects, we enrolled only subjects who were completely medication-free and did not have current comorbid psychiatric illness. We also scanned 10 healthy control subjects. RESULTS: There was a widespread reduction in 5-HT1A receptor binding potential in CFS relative to control subjects. This was particularly marked in the hippocampus bilaterally, where a 23% reduction was observed. CONCLUSIONS: There is evidence of decreased 5-HT1A receptor number or affinity in CFS. This may be a primary feature of CFS, related to the underlying pathophysiology, or a finding secondary to other processes, such as previous depression, other biological changes or the behavioral consequences of CFS.