Anxiety is associated with reduced central serotonin transporter availability in unmedicated patients with unipolar major depression: a [11C]DASB PET study

Ubiquitous extremely low frequency electromagnetic fields induces anxiety-like behavior: mechanistic perspectives

Anxiety is an adaptive condition characterized by heightened uneasiness, which in the long term can cause complications such as reducing the quality of life and problems related to the mental and physical health. Concerns have been raised regarding the potential dangers of extremely low frequency electromagnetic fields (ELF-EMF) ranging from 3 to 3000 Hz, which are omnipresent in our daily lives and there have been studies about the anxiogenic effects of these fields. Studies conducted in this specific area has revealed that ELF-EMF can have an impact on various brain regions, such as the hippocampus. In conclusion, studies have shown that ELF-EMF can interfere with hippocampus-prefrontal cortex pathway, inducing anxiety behavior. Also, ELF-EMF may initiate anxiety behavior by generating oxidative stress in hypothalamus and hippocampus. Moreover, ELF-EMF may induce anxiety behavior by reducing hippocampus neuroplasticity and increasing the NMDA2A receptor expression in the hippocampus. Furthermore, supplementation with antioxidants could serve as an effective protective measure against the adverse effects of FLF-FMF in relation to anxiety behavior.

The role of serotonin in depression-A historical roundup and future directions

Depression is one of the most common psychiatric disorders worldwide, affecting approximately 280 million people, with probably much higher unrecorded cases. Depression is associated with symptoms such as anhedonia, feelings of hopelessness, sleep disturbances, and even suicidal thoughts. Tragically, more than 700 000 people commit suicide each year. Although depression has been studied for many decades, the exact mechanisms that lead to depression are still unknown, and available treatments only help a fraction of patients. In the late 1960s, the serotonin hypothesis was published, suggesting that serotonin is the key player in depressive disorders. However, this hypothesis is being increasingly doubted as there is evidence for the influence of other neurotransmitters, such as noradrenaline, glutamate, and dopamine, as well as larger systemic causes such as altered activity in the limbic network or inflammatory processes. In this narrative review, we aim to contribute to the ongoing debate on the involvement of serotonin in depression. We will review the evolution of antidepressant treatments, systemic research on depression over the years, and future research applications that will help to bridge the gap between systemic research and neurotransmitter dynamics using biosensors. These new tools in combination with systemic applications, will in the future provide a deeper understanding of the serotonergic dynamics in depression.

Progress in the application of molecular imaging in psychiatric disorders

Psychiatric disorders have always attracted a lot of attention from researchers due to the difficulties in their diagnoses and treatments. Molecular imaging, as an emerging technology, has played an important role in the researchers of various diseases. In recent years, molecular imaging techniques including magnetic resonance spectroscopy, nuclear medicine imaging, and fluorescence imaging have been widely used in the study of psychiatric disorders. This review will briefly summarize the progression of molecular imaging in psychiatric disorders.

In vivo serotonin transporter and 1A receptor binding potential and ecological momentary assessment (EMA) of stress in major depression and suicidal behavior

We examined relationships between the serotonin system and stress in major depression and suicidal behavior. Twenty-five medication-free depressed participants (13 suicide attempters) underwent same-day [11C]DASB and [11C]CUMI-101 positron emission tomography (PET) imaging. Binding potential (BPND) to the serotonin transporter (5-HTT) and serotonin 1A (5-HT1A) receptor, respectively, was quantified using the NRU 5-HT atlas, reflecting distinct spatial distributions of multiple serotonin targets. Ecological momentary assessment (EMA) measured current stress over one week proximal to imaging. EMA stress did not differ between attempters and non-attempters. In all depressed participants, 5-HTT and 5-HT1A BPND were unrelated to EMA stress. There were region-specific effects of 5-HTT (p=0.002) and 5-HT1A BPND (p=0.03) in attempters vs. nonattempters. In attempters, region-specific associations between 5-HTT (p=0.03) and 5-HT1A (p=0.005) BPND and EMA stress emerged. While no post-hoc 5-HTT BPND correlations were significant, 5-HT1A BPND correlated positively with EMA stress in attempters in 9/10 regions (p-values<0.007), including the entire cortex except the largely occipital region 5. Brodmann-based regional analyses found diminished effects for 5-HTT and subcortically localized positive corrrelations between 5-HT1A and EMA stress, in attempters only. Given comparable depression severity and childhood and current stress between attempters and nonattempters, lower 5-HTT binding in attempters vs. nonattempters may suggest a biological risk marker. Localized lower 5-HTT and widespread higher 5-HT1A binding with stress among attempters specifically may suggest that a serotonergic phenotype might be a key determinant of risk or resiliency for suicidal behavior.

Association between Dairy Consumption and Psychological Symptoms: Evidence from a Cross-Sectional Study of College Students in the Yangtze River Delta Region of China

BACKGROUND

Assessing the dairy consumption and psychological symptoms of Chinese college students as a reference for the mental health of Chinese college students.

METHODS

A three-stage stratified whole-group sampling method was used to investigate dairy consumption and psychological symptoms among 5904 (2554 male students, accounting for 43.3% of the sample) college students in the Yangtze River Delta region. The mean age of the subjects was 20.13 ± 1.24 years. Psychological symptoms were surveyed using the Brief Questionnaire for the Assessment of Adolescent Mental Health. The detection rates of emotional problems, behavioral symptoms, social adaptation difficulties and psychological symptoms among college students with different dairy consumption habits were analyzed using chi-square tests. The association between dairy consumption and psychological symptoms was assessed using a logistic regression model.

RESULTS

College students from the "Yangtze River Delta" region of China participated in the study, of which 1022 (17.31%) had psychological symptoms. The proportions of participants with dairy consumption of ≤2 times/week, 3-5 times/week, and ≥6 times/week were 25.68%, 42.09%, and 32.23%, respectively. Using dairy consumption ≥6 times/week as a reference, multifactor logistic regression analysis showed that college students with dairy consumption ≤2 times/week (OR = 1.42, 95% CI: 1.18, 1.71) were at higher risk of psychological symptoms (p < 0.001).

CONCLUSION

During the COVID-19 pandemic, Chinese college students with lower dairy consumption exhibited higher detection rates of psychological symptoms. Dairy consumption was negatively associated with the occurrence of psychological symptoms. Our study provides a basis for mental health education and increasing knowledge about nutrition among Chinese college students.

Serotonin transporter binding in major depressive disorder: impact of serotonin system anatomy

Serotonin transporter (5-HTT) binding deficits are reported in major depressive disorder (MDD). However, most studies have not considered serotonin system anatomy when parcellating brain regions of interest (ROIs). We now investigate 5-HTT binding in MDD in two novel ways: (1) use of a 5-HTT tract-based analysis examining binding along serotonergic axons; and (2) using the Copenhagen University Hospital Neurobiology Research Unit (NRU) 5-HT Atlas, based on brain-wide binding patterns of multiple serotonin receptor types. [11C]DASB 5-HTT PET scans were obtained in 60 unmedicated participants with MDD in a current depressive episode and 31 healthy volunteers (HVs). Binding potential (BPP) was quantified with empirical Bayesian estimation in graphical analysis (EBEGA). Within the [11C]DASB tract, the MDD group showed significantly lower BPP compared with HVs (p = 0.02). This BPP diagnosis difference also significantly varied by tract location (p = 0.02), with the strongest MDD binding deficit most proximal to brainstem raphe nuclei. NRU 5-HT Atlas ROIs showed a BPP diagnosis difference that varied by region (p < 0.001). BPP was lower in MDD in 3/10 regions (p-values < 0.05). Neither [11C]DASB tract or NRU 5-HT Atlas BPP correlated with depression severity, suicidal ideation, suicide attempt history, or antidepressant medication exposure. Future studies are needed to determine the causes of this deficit in 5-HTT binding being more pronounced in proximal axon segments and in only a subset of ROIs for the pathogenesis of MDD. Such regional specificity may have implications for targeting antidepressant treatment, and may extend to other serotonin-related disorders.

Concurrent anxiety in patients with major depression and cerebral serotonin 4 receptor binding. A NeuroPharm-1 study

Concurrent anxiety is frequent in major depressive disorder and a shared pathophysiological mechanism between anxiety and other depressive symptoms is plausible. The serotonin 4 receptor (5-HT₄R) has been implicated in both depression and anxiety. This is the first study to investigate the association between the cerebral 5-HT₄R binding and anxiety in patients with depression before and after antidepressant treatment and the association to treatment response. Ninety-one drug-free patients with depression were positron emission tomography scanned with the 5-HT₄R ligand [¹¹C]-SB207145. Depression severity and concurrent anxiety was measured at baseline and throughout 8 weeks of antidepressant treatment. Anxiety measures included four domains: anxiety/somatization factor score; Generalized Anxiety Disorder 10-items (GAD-10) score; anxiety/somatization factor score ≥7 (anxious depression) and syndromal anxious depression. Forty patients were rescanned at week 8. At baseline, we found a negative association between global 5-HT₄R binding and both GAD-10 score (p < 0.01) and anxiety/somatization factor score (p = 0.06). Further, remitters had a higher baseline anxiety/somatization factor score compared with non-responders (p = 0.04). At rescan, patients with syndromal anxious depression had a greater change in binding relative to patients with non-syndromal depression (p = 0.04). Concurrent anxiety in patients with depression measured by GAD-10 score and anxiety/somatization factor score is negatively associated with cerebral 5-HT₄R binding. A lower binding may represent a subtype with reduced natural resilience against anxiety in a depressed state, and concurrent anxiety may influence the effect on the 5-HT₄R from serotonergic antidepressants. The 5-HT₄R is a promising neuroreceptor for further understanding the underpinnings of concurrent anxiety in patients with depression.

Mesembryanthemum tortuosum L. alkaloids modify anxiety-like behaviour in a zebrafish model

ETHNOPHARMACOLOGICAL RELEVANCE

Mesembryanthemum tortuosum L. (previously known as Sceletium tortuosum (L.) N.E. Br.) is indigenous to South Africa and traditionally used to alleviate anxiety, stress and depression. Mesembrine and its alkaloid analogues such as mesembrenone, mesembrenol and mesembranol have been identified as the key compounds responsible for the reported effects on the central nervous system.

AIM OF THE STUDY

To investigate M. tortuosum alkaloids for possible anxiolytic-like effects in the 5-dpf in vivo zebrafish model by assessing thigmotaxis and locomotor activity.

MATERIALS AND METHODS

Locomotor activity and reverse-thigmotaxis, recognised anxiety-related behaviours in 5-days post fertilization zebrafish larvae, were analysed under simulated stressful conditions of alternating light-dark challenges. Cheminformatics screening and molecular docking were also performed to rationalize the inhibitory activity of the alkaloids on the serotonin reuptake transporter, the accepted primary mechanism of action of selective serotonin reuptake inhibitors. Mesembrine has been reported to have inhibitory effects on serotonin reuptake, with consequential anti-depressant and anxiolytic effects.

RESULTS

All four alkaloids assessed decreased the anxiety-related behaviour of zebrafish larvae exposed to the light-dark challenge. Significant increases in the percentage of time spent in the central arena during the dark phase were also observed when larvae were exposed to the pure alkaloids (mesembrenone, mesembrenol, mesembrine and mesembrenol) compared to the control. However, mesembrenone and mesembranol demonstrated a greater anxiolytic-like effect than the other alkaloids. In addition to favourable pharmacokinetic and physicochemical properties revealed via in silico predictions, high-affinity interactions characterized the binding of the alkaloids with the serotonin transporter.

CONCLUSIONS

M. tortuosum alkaloids demonstrated an anxiolytic-like effect in zebrafish larvae providing evidence for its traditional and modern day use as an anxiolytic.

Antidepressant-like Activity of Patchouli Oil var. Tapak Tuan (Pogostemon cablin Benth) via Elevated Dopamine Level: A Study Using Rat Model

Essential oils are gaining popularity for their use in treating depression, including that extracted from patchouli leaves and stems (Pogostemon cablin). Herein, we used patchouli oil (PO) containing a high amount of patchouli alcohol derived from P. cablin var. Tapak Tuan. The aim of this study was to investigate the antidepressant potential of PO, with a variety of patchouli alcohol concentrations obtained from a separation process using vacuum distillation with different temperature ranges. The initial patchouli oil (iPO) was traditionally distilled by a local farmer and further distilled using a rotary evaporator at temperature ranges of 115−160 °C (POF-1); 120−160 °C (POF-2), and 125−160 °C (POF-3), resulting in products with different patchouli alcohol concentrations. POF-3, with the highest patchouli alcohol content of 60.66% (based on gas chromatography-mass spectrometry), was used for cooling crystallization, resulting in 100% patchouli alcohol crystal (pPA). A tail suspension test (TST) was performed on a rat model to screen the antidepressant potential of iPO and its derivatives. The TST results revealed that POF-3 had the best antidepressant-like effect and was second only to the fluoxetine-based antidepressant, Kalxetin®, where both groups had significant reductions of immobility time post-treatment (p < 0.0001). Other than patchouli alcohol, POF-3 also contained ledol and trans-geraniol, which have been reported for their antidepressant-related activities. Brain dopamine levels increased significantly in the group treated with POF-3 (p < 0.05 as compared with the control group), suggesting its primary anti-depressant mechanism. These findings suggest the potential of vacuum-distilled patchouli oil in reducing depression via dopamine elevation.

Serotonin Receptor and Transporter Endocytosis Is an Important Factor in the Cellular Basis of Depression and Anxiety

Depression and anxiety are common, debilitating psychiatric conditions affecting millions of people throughout the world. Current treatments revolve around selective serotonin reuptake inhibitors (SSRIs), yet these drugs are only moderately effective at relieving depression. Moreover, up to 30% of sufferers are SSRI non-responders. Endocytosis, the process by which plasma membrane and extracellular constituents are internalized into the cell, plays a central role in the regulation of serotonin (5-hydroxytryptophan, 5-HT) signaling, SSRI function and depression and anxiety pathogenesis. Despite their therapeutic potential, surprisingly little is known about the endocytosis of the serotonin receptors (5-HT receptors) or the serotonin transporter (SERT). A subset of 5-HT receptors are endocytosed by clathrin-mediated endocytosis following serotonin binding, while for the majority of 5-HT receptors the endocytic regulation is not known. SERT internalizes serotonin from the extracellular space into the cell to limit the availability of serotonin for receptor binding and signaling. Endocytosis of SERT reduces serotonin uptake, facilitating serotonin signaling. SSRIs predominantly inhibit SERT, preventing serotonin uptake to enhance 5-HT receptor signaling, while hallucinogenic compounds directly activate specific 5-HT receptors, altering their interaction with endocytic adaptor proteins to induce alternate signaling outcomes. Further, multiple polymorphisms and transcriptional/proteomic alterations have been linked to depression, anxiety, and SSRI non-response. In this review, we detail the endocytic regulation of 5-HT receptors and SERT and outline how SSRIs and hallucinogenic compounds modulate serotonin signaling through endocytosis. Finally, we will examine the deregulated proteomes in depression and anxiety and link these with 5-HT receptor and SERT endocytosis. Ultimately, in attempting to integrate the current studies on the cellular biology of depression and anxiety, we propose that endocytosis is an important factor in the cellular basis of depression and anxiety. We will highlight how a thorough understanding 5-HT receptor and SERT endocytosis is integral to understanding the biological basis of depression and anxiety, and to facilitate the development of a next generation of specific, efficacious antidepressant treatments.

Relationships between Dairy and Calcium Intake and Mental Health Measures of Higher Education Students in the United States: Outcomes from Moderation Analyses

Background: The prevalence of mental health concerns among university students in the United States (U.S.) continues to increase, while current treatments, including medication and counseling, present shortcomings. Higher dairy and calcium intakes are associated with protective effects on mental health; however, previous studies have focused on investigating singular relationships between dairy and calcium intakes and mental health measures. A more complex exploration of these relationships is warranted to better examine whether increasing dairy and calcium intakes could serve as an intervention to improve mental health. The present study sought to further characterize the relationships between dairy and calcium intake, perceived stress, and a variety of mental health measures using linear regression and moderation analyses. Methods: The present cross-sectional study involved students studying at three large U.S. universities, and data collection occurred from April to May 2020 when students were learning remotely due to the COVID-19 pandemic. An online survey comprising validated tools was distributed among students to assess dairy and calcium intake, perceived stress, anxiety, negative and positive moods, rumination, and resilience, sleep quality and duration, dietary risk, and physical activity. Results: A total of 1233 students completed the study. Higher dairy and calcium intake was coincident with lower perceived stress and higher positive mood scores, while higher calcium intake was also coincident with lower anxiety, rumination, and higher resilience scores. Additionally, as calcium intake increased, the relationship between perceived stress and anxiety and the relationship between perceived stress and negative mood weakened. Dairy intake did not have this effect. Conclusions: Based on the results, and considering that calcium is a shortfall nutrient, universities should consider initiating programs and public health campaigns to promote dairy and calcium intake among this population.

Pro-Inflammatory Cytokines: Potential Links between the Endocannabinoid System and the Kynurenine Pathway in Depression

Substance use/abuse is one of the main causes of depressive symptoms. Cannabis and synthetic cannabinoids in particular gained significant popularity in the past years. There is an increasing amount of clinical data associating such compounds with the inflammatory component of depression, indicated by the up-regulation of pro-inflammatory cytokines. Pro-inflammatory cytokines are also well-known to regulate the enzymes of the kynurenine pathway (KP), which is responsible for metabolizing tryptophan, a precursor in serotonin synthesis. Enhanced pro-inflammatory cytokine levels may over-activate the KP, leading to tryptophan depletion and reduced serotonin levels, which can subsequently precipitate depressive symptoms. Therefore, such mechanism might represent a possible link between the endocannabinoid system (ECS) and the KP in depression, via the inflammatory and dysregulated serotonergic component of the disorder. This review will summarize the data regarding those natural and synthetic cannabinoids that increase pro-inflammatory cytokines. Furthermore, the data on such cytokines associated with KP activation will be further reviewed accordingly. The interaction of the ECS and the KP has been postulated and demonstrated in some studies previously. This review will further contribute to this yet less explored connection and propose the KP to be the missing link between cannabinoid-induced inflammation and depressive symptoms.

Serotonin transporter availability increases in patients recovering from a depressive episode

Molecular imaging studies have shown low cerebral concentration of serotonin transporter in patients suffering from depression, compared to healthy control subjects. Whether or not this difference also is present before disease onset and after remission (i.e. a trait), or only at the time of the depressive episode (i.e. a state) remains to be explored. We examined 17 patients with major depressive disorder with positron emission tomography using [¹¹C]MADAM, a radioligand that binds to the serotonin transporter, before and after treatment with internet-based cognitive behavioral therapy. In all, 17 matched healthy control subjects were examined once. Cerebellum was used as reference to calculate the binding potential. Differences before and after treatment, as well as between patients and controls, were assessed in a composite cerebral region and in the median raphe nuclei. All image analyses and confirmatory statistical tests were preregistered. Depression severity decreased following treatment (p < 0.001). [¹¹C]MADAM binding in patients increased in the composite region after treatment (p = 0.01), while no change was observed in the median raphe (p = 0.51). No significant difference between patients at baseline and healthy controls were observed in the composite region (p = 0.97) or the median raphe (p = 0.95). Our main finding was that patients suffering from a depressive episode show an overall increase in cerebral serotonin transporter availability as symptoms are alleviated. Our results suggest that previously reported cross-sectional molecular imaging findings of the serotonin transporter in depression most likely reflect the depressive state, rather than a permanent trait. The finding adds new information on the pathophysiology of major depressive disorder.

Anatomical and neurochemical organization of the serotonergic system in the mammalian brain and in particular the involvement of the dorsal raphe nucleus in relation to neurological diseases

The brainstem is a neglected brain area in neurodegenerative diseases, including Alzheimer's and Parkinson's disease, frontotemporal lobar degeneration and autonomic dysfunction. In Depression, several observations have been made in relation to changes in one particular the Dorsal Raphe Nucleus (DRN) which also points toward as key area in various age-related and neurodevelopmental diseases. The DRN is further thought to be related to stress regulated processes and cognitive events. It is involved in neurodegeneration, e.g., amyloid plaques, neurofibrillary tangles, and impaired synaptic transmission in Alzheimer's disease as shown in our autopsy findings. The DRN is a phylogenetically old brain area, with projections that reach out to a large number of regions and nuclei of the central nervous system, particularly in the forebrain. These ascending projections contain multiple neurotransmitters. One of the main reasons for the past and current interest in the DRN is its involvement in depression, and its main transmitter serotonin. The DRN also points toward the increased importance and focus of the brainstem as key area in various age-related and neurodevelopmental diseases. This review describes the morphology, ascending projections and the complex neurotransmitter nature of the DRN, stressing its role as a key research target into the neural bases of depression.

The neurobiology of human aggressive behavior: Neuroimaging, genetic, and neurochemical aspects

In modern societies, there is a strive to improve the quality of life related to risk of crimes which inevitably requires a better understanding of brain determinants and mediators of aggression. Neurobiology provides powerful tools to achieve this end. Pre-clinical and clinical studies show that changes in regional volumes, metabolism-function and connectivity within specific neural networks are related to aggression. Subregions of prefrontal cortex, insula, amygdala, basal ganglia and hippocampus play a major role within these circuits and have been consistently implicated in biology of aggression. Genetic variations in proteins regulating the synthesis, degradation, and transport of serotonin and dopamine as well as their signal transduction have been found to mediate behavioral variability observed in aggression. Gene-gene and gene-environment interactions represent additional important risk factors for aggressiveness. Considering the social burden of pathological forms of aggression, more basic and translational studies should be conducted to accelerate applications to clinical practice, justice courts, and policy making.

Pharmacogenetics of anxiety disorders

Anxiety disorders are common and disabling conditions the treatment of which remains a challenge. While different groups of medication are available for their treatment, a substantial proportion of patients remain refractory to pharmacotherapy. The reason for this variation in the individual response to treatment has yet to be understood; however genetic factors have been shown to play an important role. Up to now there have been limited publications about pharmacogenetics of anxiety disorders, compared to studies in depression. Published studies are focused on pharmacogenetics of antidepressants rather than being disease specific. This review summarizes pharmacogenetic findings related to the anxiolytic treatment response and their possible functional mechanisms. This inevitably focuses on genes involved in the pharmacodynamics of the medications used, along with some genes implicated in the disease process, as well as briefly mentioning genetic factors associated with psychotherapeutic response.

Pet ownership and symptoms of depression: A prospective study of older adults

BACKGROUND

This paper aims to examine associations between pet ownership and symptoms of depression in a large, population-based sample of older adults. Specifically, we tested whether: (i) people who report more depressive symptoms are more likely to own a pet; (ii) pet ownership protects against an increase in depressive symptoms over time; (iii) associations differ by symptom type.

METHODS

Data were drawn from the English Longitudinal Study of Ageing, a longitudinal panel study of men and women aged 50 and older (n = 7,617, 52.5% female). Pet ownership (dog/cat/other/none) was self-reported in 2010/11. Depressive symptoms were assessed in 2010/11 and 2016/17 using the 8-item centre for Epidemiologic Studies Depression (CES-D) scale. We analysed total CES-D score and derived symptom subscales (depressed mood, anhedonia, somatic symptoms) in relation to pet ownership, adjusting for sociodemographic and health-related covariates.

RESULTS

A one-symptom increase in total CES-D score was associated with 7% increased odds of dog ownership (OR=1.07, 95% CI 1.03-1.11). Significant associations were observed between each subset of depressive symptoms and dog ownership, with models run on z-scores showing a slightly stronger association for symptoms of depressed mood (OR=1.13, 95% CI 1.06-1.21) compared with anhedonia (OR=1.10, 95% CI 1.04-1.17) or somatic symptoms (OR=1.10, 95% CI 1.03-1.18). Prospectively, no significant associations were found.

LIMITATIONS

Self-reported data; small sample size for some pet categories.

CONCLUSION

Amongst older adults in England, those with more depressive symptoms are more likely to own a dog, but pet ownership is not significantly associated with change in depressive symptoms over time.

Smoking moderates association of 5-HTTLPR and in vivo availability of serotonin transporters

Although preclinical studies clearly indicate an effect of 5-HTTLPR genotype on 5-HT transporter (5-HTT) expression, studies in humans provided inconclusive results, hypothetically due to environmental factors and differences in individual behavior. For example, nicotine and other constituents of tobacco smoke elevate serotonin (5-HT) levels in the brain and may thereby cause homeostatic adaptations in 5-HTT availability that moderate effects of 5-HTTLPR genotype. To test whether 5-HTT availability in the midbrain is affected by smoking status and 5-HTTLPR genotype, we pooled data from prior studies on in vivo 5-HTT availability (BP_ND) measured with positron emission tomography (PET) and [¹¹C]DASB. In total, we reanalyzed 5-HTT availability in 116 subjects using ANCOVA statistics. ROI analysis revealed that current smokers and non-smokers do not differ in midbrain BP_ND. Interestingly, smoking status significantly interacted with 5-HTTLPR genotype: active smoking was associated with reduced 5-HTT availability only in LL subjects but not in carriers of the S-allele. From the perspective of genotype effects, non-smokers showed the expected association with 5-HTTLPR, i.e. higher 5-HTT availability in LL subjects compared to carriers of the S-allele, whereas this pattern was actually reversed for active smokers. Our study indicates that smoking status moderates the association of 5-HTTLPR genotype and 5-HTT expression, which may help to explain inconsistent findings in previous studies. Regarding the mechanism, we suggest that smoking may induce epigenetic processes such as methylation of SLC6A4, which can differ depending on its genetic constitution.

Brain serotonin transporter binding, plasma arachidonic acid and depression severity: A positron emission tomography study of major depression

BACKGROUND

Serotonin transporter (5-HTT) binding and polyunsaturated fatty acids (PUFAs) are implicated in major depressive disorder (MDD). Links between the two systems in animal models have not been investigated in humans.

METHODS

Using positron emission tomography (PET) and [¹¹C]DASB, we studied relationships between 5-HTT binding potential and plasma levels of PUFAs docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), and arachidonic acid (AA) in medication-free MDD patients (n = 21). PUFAs were quantified using transesterification and gas chromatography. Binding potential BP_P, and alternative outcome measures BP_F and BP_ND, were determined for [¹¹C]DASB in six a priori brain regions of interest (ROIs) using likelihood estimation in graphical analysis (LEGA) to calculate radioligand total distribution volume (V_T), and a validated hybrid deconvolution approach (HYDECA) that estimates radioligand non-displaceable distribution volume (V_ND) without a reference region. Linear mixed models used PUFA levels as predictors and binding potential measures as outcomes across the specified ROIs; age and sex as fixed effects; and subject as random effect to account for across-region binding correlations. As nonlinear relationships were observed, a quadratic term was added to final models.

RESULTS

AA predicted both 5-HTT BP_P and depression severity nonlinearly, described by an inverted U-shaped curve. 5-HTT binding potential mediated the relationship between AA and depression severity.

LIMITATIONS

Given the small sample and multiple comparisons, results require replication.

CONCLUSIONS

Our findings suggest that AA status may impact depression pathophysiology through effects on serotonin transport. Future studies should examine whether these relationships explain therapeutic effects of PUFAs in the treatment of MDD.

Cortical and striatal serotonin transporter binding in a genetic rat model of depression and in response to electroconvulsive stimuli

Depression is a debilitating mental illness and two thirds of patients respond insufficiently to conventional antidepressants. Electroconvulsive therapy (ECT) remains the most effective treatment to alleviate drug-refractory depression, however the neurobiological mechanisms are mostly unknown. The serotonergic system plays an important role in depression and alterations in the serotonin transporter (SERT) are seen both in depression and response to antidepressant pharmacotherapies. The first aim of this study was to investigate SERT density in a genetic rat model of depression, Flinders Sensitive Line (FSL), compared to control Flinders Resistant Line (FRL) and Sprague-Dawley (SD) rats. The second aim was to investigate SERT density in response to electroconvulsive stimuli (ECS), an animal model of ECT. Female rats of each strain were treated with ECS or sham (ear-clip placement with no current) for 10 days before brains were removed, frozen and cut into 20 µm thick sections. SERT density was measured in striatal and cortical regions by quantitative in vitro autoradiography using the SERT-radioligand, [³H]-DASB. Higher SERT density was observed in FSL rats compared to SD rats by 36-48% in motor cortex and striatum under sham conditions. In response to ECS, SD rats displayed a significant effect of treatment, whereas no changes were observed in FRL and FSL rats. Increased SERT binding in FSL rats compared to SD supports a dysfunction of the serotonergic system in depression. The increased SERT density after ECS, seen in SD rats but not FSL rats, suggests a different mechanism of action between depressive-like rats and controls.

Trait Openness and serotonin 2A receptors in healthy volunteers: A positron emission tomography study

Recent research found lasting increases in personality trait Openness in healthy individuals and patients after administration of the serotonin 2A receptor (5-HT_2A R) agonist psilocybin. However, no studies have investigated whether 5-HT_2A R availability as imaged using positron emission tomography (PET) is associated with this trait. In 159 healthy individuals (53 females), the association between 5-HT_2A R binding in neocortex imaged with [¹⁸ F]altanserin or [¹¹ C]Cimbi-36 PET and personality trait Openness was investigated using linear regression models. In these models the influence of sex on the association was also investigated. Trait Openness was assessed with the NEO Personality Inventory-Revised. No significant associations between neocortical 5-HT_2A R binding and trait Openness were found for [¹⁸ F]altanserin (p = 0.5) or [¹¹ C]Cimbi-36 (p = 0.8). Pooling the data in a combined model did not substantially change our results (p = 0.4). No significant interactions with sex were found (p > 0.35). Our results indicate that differences in 5-HT_2A R availability are not related to variations in trait Openness in healthy individuals. Although stimulation of the 5-HT_2A R with compounds such as psilocybin may contribute to long-term changes in trait Openness, there is no evidence in favor of an association between 5-HT_2A R and trait Openness.

Computational psychopharmacology: a translational and pragmatic approach

RATIONALE

Psychopharmacology needs novel quantitative measures and theoretical approaches based on computational modelling that can be used to help translate behavioural findings from experimental animals to humans, including patients with neuropsychiatric disorders.

OBJECTIVES

This brief review exemplifies this approach when applied to recent published studies of the effects of manipulating central dopaminergic and serotoninergic systems in rodents and marmoset monkeys, and possible comparisons with healthy human volunteers receiving systemic agents or patients with depression and schizophrenia.

METHODS

Behavioural effects of central depletions of dopamine or serotonin in monkeys in probabilistic learning paradigms are characterised further by computational modelling methods and related to rodent and human data.

RESULTS

Several examples are provided of the power of computational modelling to derive new measures and reappraise conventional explanations of regional neurotransmitter depletion and other drug effects, whilst enhancing construct validation in patient groups. Specifically, effects are shown on such parameters as 'stimulus stickiness' and 'side stickiness', which occur over and above effects on standard parameters of reinforcement learning, reminiscent of some early innovations in data analysis in psychopharmacology.

CONCLUSIONS

Computational modelling provides a useful methodology for further detailed analysis of behavioural mechanisms that are affected by pharmacological manipulations across species and will aid the translation of experimental findings to understand the therapeutic effects of medications in neuropsychiatric disorders, as well as facilitating future drug discovery.

Lifetime major depression and grey-matter volume

BACKGROUND

There is evidence of structural brain alterations in major depressive disorder (MDD), but little is known about how these alterations might be affected by age at onset or genetic vulnerability. This study examines whether lifetime episodes of MDD are associated with specific alterations in grey-matter volume, and whether those alterations vary according to sex or serotonin transporter-linked promoter region (5-HTTLPR) genotype (LL, SL or SS).

METHODS

We used structural MRI to acquire anatomic scans from 610 community-dwelling participants. We derived quantitative regional estimates of grey-matter volume in 16 subregions using FreeSurfer software. We diagnosed MDD according to DSM-IV criteria. We adjusted analyses for age, sex, total brain volume, education level, head injury and comorbidities.

RESULTS

Lifetime MDD was associated with a smaller insula, thalamus, ventral diencephalon, pallidum and nucleus accumbens and with a larger pericalcarine region in both men and women. These associations remained after adjustment for false discovery rate. Lifetime MDD was also associated with a smaller caudate nucleus and amygdala in men and with a larger rostral anterior cingulate cortex in women. Late-onset first episodes of MDD (after age 50 years) were associated with a larger rostral anterior cingulate cortex and lingual and pericalcarine regions; early-onset MDD was associated with a smaller ventral diencephalon and nucleus accumbens. Some associations differed according to 5-HTTLPR genotype: the thalamus was smaller in participants with MDD and the LL genotype; pericalcarine and lingual volumes were higher in those with the SL genotype.

LIMITATIONS

This study was limited by its cross-sectional design.

CONCLUSION

Major depressive disorder was associated with persistent volume reductions in the deep nuclei and insula and with enlargements in visual cortex subregions; alterations varied according to age of onset and genotype.

Gut microbiome and depression: what we know and what we need to know

Gut microbiome diversity has been strongly associated with mood-relating behaviours, including major depressive disorder (MDD). This association stems from the recently characterised bi-directional communication system between the gut and the brain, mediated by neuroimmune, neuroendocrine and sensory neural pathways. While the link between gut microbiome and depression is well supported by research, a major question needing to be addressed is the causality in the connection between the two, which will support the understanding of the role that the gut microbiota play in depression. In this article, we address this question by examining a theoretical ‘chronology’, reviewing the evidence supporting two possible sequences of events. First, we discuss that alterations in the gut microbiota populations of specific species might contribute to depression, and secondly, that depressive states might induce modification of specific gut microbiota species and eventually contribute to more severe depression. The feasibility of both sequences is supported by pre-clinical trials. For instance, research in rodents has shown an onset of depressive behaviour following faecal transplantations from patients with MDD. On the other hand, mental induction of stress and depressive behaviour in rodents resulted in reduced gut microbiota richness and diversity. Synthesis of these chronology dynamics raises important research directions to further understand the role that gut microbiota play in mood-relating behaviours, which holds substantial potential clinical outcomes for persons who experience MDD or related depressive disorders.

Brain neuroreceptor density and personality traits: towards dimensional biomarkers for psychiatric disorders

Positron emission tomography has, for 30 years, been used in numerous case-control studies searching for hypothesized differences in the density of neuroreceptor or transporter proteins in psychiatric disorders such as schizophrenia and depression. In most cases, the results have not been conclusive. One reason could be the sizeable interindividual variability in biochemical markers, which in twin studies have shown to emanate from both environmental and genetic factors, leading to low statistical power for the detection of group effects. On the other hand, the same interindividual variability has served as an opportunity for correlative studies on the biological underpinning of behaviour. Using this approach, a series of studies has linked markers for the dopamine and serotonin system to personality traits associated with psychiatric conditions. Based on increasing evidence for the view that many psychopathological states represent extremes of a continuum rather than distinct categories, this research strategy may lead to new biological insights about the vulnerability to and pathophysiology of major psychiatric disorders.This article is part of the theme issue 'Diverse perspectives on diversity: multi-disciplinary approaches to taxonomies of individual differences'.

Serotonin transporter binding and anxiety symptoms in Parkinson's disease

BACKGROUND

Anxiety is a common neuropsychiatric symptom in Parkinson's disease (PD), yet the neural mechanisms have been scarcely investigated. Disturbances in dopaminergic and serotonergic signalling may play a role in its pathophysiology. ¹²³I-N-ω-fluoropropyl-2β-carbomethoxy-3β-(4-iodophenyl)nortropane (¹²³I-FP-CIT) is a single-photon emission CT radiotracer, and its binding in striatal and extrastriatal subcortical brain areas represents predominant binding to the presynaptic dopamine transporter (DAT) and the serotonin transporter (SERT), respectively. Availability of DAT and SERT may thus provide an in vivo measure for the integrity of both dopamine and serotonin neurons.

METHODS

We studied the association between anxiety symptoms, measured with an affective subscale of the Beck Anxiety Inventory, and (extra)striatal ¹²³I-FP-CIT binding in 127 non-demented patients with PD with a median disease duration of 2.55 (IQR 2.90) years. We conducted the analyses on patients currently on or not on dopamine replacement therapy (DRT).

RESULTS

Severity of anxiety symptoms showed a significant negative association with ¹²³I-FP-CIT binding ratios in the right thalamus (β=-0.203, p=0.019; ΔR²=0.040) (multiple testing p_corr <0.020). In the subgroup of patients not on DRT (n=81), we found a significant negative association between anxiety and thalamic ¹²³I-FP-CIT binding ratios bilaterally (right: β=-0.349, p=0.001, ΔR²=0.119; left: β=-0.269, p=0.017, ΔR²=0.071) (p_corr <0.020).

CONCLUSION

This study shows that higher levels of anxiety in patients with PD are associated with lower thalamic ¹²³I-FP-CIT binding, pointing towards a contribution of serotonergic degeneration to anxiety symptoms in PD.

A positron emission tomography study of the serotonergic system in relation to anxiety in depression

Symptoms of anxiety are highly comorbid with major depressive disorder (MDD) and are known to alter the course of the disease. To help elucidate the biological underpinnings of these prevalent disorders, we previously examined the relationship between components of anxiety (somatic, psychic and motoric) and serotonin 1A receptor (5-HT_1A) binding in MDD and found that higher psychic and lower somatic anxiety was associated with greater 5-HT_1A binding. In this work, we sought to examine the correlation between these anxiety symptom dimensions and 5-HTT binding. Positron emission tomography with [¹¹C]-3-amino-4-(3-dimethylamino-methylphenylsulfanyl)-benzonitrile ([¹¹C]DASB) and a metabolite-corrected arterial input function were used to estimate regional 5-HTT binding in 55 subjects with MDD and anxiety symptoms. Somatic anxiety was negatively correlated with 5-HTT binding in the thalamus (β=-.33, p=.025), amygdala (β=-.31, p=.007) and midbrain (β=-.72, p<.001). Psychic anxiety was positively correlated with 5-HTT binding in midbrain only (β=.46, p=.0025). To relate to our previous study, correlation between 5-HT_1A and 5-HTT binding was examined, and none was found. We also examined how much of the variance in anxiety symptom dimensions could be explained by both 5-HTT and 5-HT_1A binding. The developed model was able to explain 68% (p<.001), 38% (p=.012) and 32% (p=.038) of the total variance in somatic, psychic, and motoric anxiety, respectively. Results indicate the tight coupling between the serotonergic system and anxiety components, which may be confounded when using aggregate anxiety measures. Uncovering serotonin's role in anxiety and depression in this way may give way to a new generation of therapeutics and treatment strategies.

Decreased Pretreatment Amygdalae Serotonin Transporter Binding in Unipolar Depression Remitters: A Prospective PET Study

Major depressive disorder (MDD) is a debilitating condition that affects over 14 million Americans. Remission occurs only in a minority of individuals after first-line antidepressant treatment (∼35%); predictors of treatment outcome are therefore needed. Using PET imaging with a radiotracer specific for the serotonin transporter (5-HTT), ¹¹C-McN5652, we found that patients with MDD who did not achieve remission after 12 mo of naturalistic treatment had lower pretreatment midbrain and amygdala binding than healthy volunteers. Here, using a superior 5-HTT tracer, ¹¹C-DASB, we repeated this study with a prospective design with 8 wk of standardized treatment with escitalopram. As this same cohort also underwent ¹¹C-WAY100635 scans (serotonin-1A receptor [5-HT_1A]), we examined whether using both pretreatment 5-HTT and 5-HT_1A binding could improve prediction of posttreatment remission status. Methods: Thirty-one healthy controls (Hamilton Depression Rating Scale-24 item [HDRS-24] = 1.7) and 26 medication-free patients with MDD (HDRS-24 = 24.8) underwent PET scanning using ¹¹C-DASB. MDD subjects then received 8 wk of standardized pharmacotherapy with escitalopram. The relationship between pretreatment binding and posttreatment clinical status was examined. Arterial blood samples were collected to calculate the metabolite-corrected arterial input function. The outcome measure was V_T/f_P (V_T is volume of distribution in region of interest, f_P is free fraction in plasma). Remission was defined as a posttreatment depression score of less than 10 as well as 50% or more reduction in the score from baseline, resulting in 14 nonremitters (HDRS-24 = 17.6) and 12 remitters (HDRS-24 = 5.3). Results: A linear mixed-effects model comparing group differences in the a priori regions of interest (amygdala and midbrain) revealed a significant difference in amygdala binding between controls and remitters (P = 0.03, unadjusted), where remitters had an 11% lower amygdala binding than controls. Differences in amygdala binding between remitters and nonremitters approached significance (P = 0.06). No additional differences were found between any groups (all P > 0.05). Additionally, we found no relationship between pretreatment amygdala binding and posttreatment depression score, and were unable to predict posttreatment depression severity using both pretreatment 5-HTT (in the amygdala) and 5-HT_1A binding (in the raphe). Conclusion: These results suggest 5-HTT amygdala binding should be examined further, in conjunction with other measures, as a potential biomarker for remission after standardized escitalopram treatment.

No evidence for a role of the serotonin 4 receptor in five-factor personality traits: A positron emission tomography brain study

Serotonin (5-HT) brain architecture appears to be implicated in normal personality traits as supported by genetic associations and studies using molecular brain imaging. However, so far, no studies have addressed potential contributions to variation in normal personality traits from in vivo serotonin 4 receptor (5-HT₄R) brain availability, which has recently become possible to image with Positron Emission Tomography (PET). This is particularly relevant since availability of 5-HT₄R has been shown to adapt to synaptic levels of 5-HT and thus offers information about serotonergic tone in the healthy brain. In 69 healthy participants (18 females), the associations between personality traits assessed with the five-factor NEO Personality Inventory-Revised (NEO PI-R) and regional cerebral 5-HT₄R binding in neocortex, amygdala, hippocampus, and anterior cingulate cortex (ACC) were investigated using linear regression models. The associations between each of the five personality traits and a latent variable construct of global 5-HT₄R levels were also evaluated using latent variable structural equation models. We found no significant associations between the five NEO personality traits and regional 5-HT₄R binding (all p-values > .17) or the latent construct of global 5-HT₄R levels (all p-values > .37). Our findings indicate that NEO personality traits and 5-HT₄R are not related in healthy participants. Under the assumption that global 5-HT₄R levels index 5-HT tone, our data also suggest that 5-HT tone per se is not directly implicated in normal personality traits.

Neurobiology of Chronic Stress-Related Psychiatric Disorders: Evidence from Molecular Imaging Studies

Chronic stress accounts for billions of dollars of economic loss annually in the United States alone, and is recognized as a major source of disability and mortality worldwide. Robust evidence suggests that chronic stress plays a significant role in the onset of severe and impairing psychiatric conditions, including major depressive disorder, bipolar disorder, and posttraumatic stress disorder. Application of molecular imaging techniques such as positron emission tomography and single photon emission computed tomography in recent years has begun to provide insight into the molecular mechanisms by which chronic stress confers risk for these disorders. The present paper provides a comprehensive review and synthesis of all positron emission tomography and single photon emission computed tomography imaging publications focused on the examination of molecular targets in individuals with major depressive disorder, posttraumatic stress disorder, or bipolar disorder to date. Critical discussion of discrepant findings and broad strengths and weaknesses of the current body of literature is provided. Recommended future directions for the field of molecular imaging to further elucidate the neurobiological substrates of chronic stress-related disorders are also discussed. This article is part of the inaugural issue for the journal focused on various aspects of chronic stress.

Different patterns of 5-HT receptor and transporter dysfunction in neuropsychiatric disorders--a comparative analysis of in vivo imaging findings

Impairment of serotonin receptor and transporter function is increasingly recognized to play a major role in the pathophysiology of neuropsychiatric diseases including anxiety disorder (AD), major depressive disorder (MDD), bipolar disorder (BD) and schizophrenia (SZ). We conducted a PubMed search, which provided a total of 136 in vivo studies with PET and SPECT, in which 5-HT synthesis, 5-HT transporter binding, 5-HT1 receptor binding or 5-HT2 receptor binding in patients with the primary diagnosis of acute AD, MDD, BD or SZ was compared to healthy individuals. A retrospective analysis revealed that AD, MDD, BD and SZ differed as to affected brain region(s), affected synaptic constituent(s) and extent as well as direction of dysfunction in terms of either sensitization or desensitization of transporter and receptor binding sites.

Imaging Dopamine and Serotonin Systems on MPTP Monkeys: A Longitudinal PET Investigation of Compensatory Mechanisms

It is now widely accepted that compensatory mechanisms are involved during the early phase of Parkinson's disease (PD) to delay the expression of motor symptoms. However, the neurochemical mechanisms underlying this presymptomatic period are still unclear. Here, we measured in vivo longitudinal changes of both the dopaminergic and serotonergic systems in seven asymptomatic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-intoxicated monkeys (when motor symptoms are less apparent) using PET. We used the progressively MPTP-intoxicated monkey model that expresses recovery from motor symptoms to study the changes in dopamine synthesis ([(18)F]DOPA), dopamine D2/D3 receptors ([(11)C]raclopride), and serotonin transporter (11)C-N,N-dimethyl-2-(-2-amino-4-cyanophenylthio) benzylamine ([(11)C]DASB) and serotonin 1A receptor ([(18)F]MPPF) levels between four different states (baseline, early symptomatic, full symptomatic and recovered). During the early symptomatic state, we observed increases of [(18)F]DOPA uptake in the anterior putamen, [(11)C]raclopride binding in the posterior striatum, and 2'-methoxyphenyl-(N-2'-pyridinyl)-p-[(18)F]fluoro-benzamidoethylpiperazine [(18)F]MPPF uptake in the orbitofrontal cortex and dorsal ACC. After recovery from motor symptoms, the results mainly showed decreased [(11)C]raclopride binding in the anterior striatum and limbic ACC. In addition, our findings supported the importance of pallidal dopaminergic neurotransmission in both the early compensatory mechanisms and the functional recovery mechanisms, with reduced aromatic L-amino acid decarboxylase (AAAD) activity closely related to the appearance or perseveration of motor symptoms. In parallel, this study provides preliminary evidence of the role of the serotonergic system in compensatory mechanisms. Nonetheless, future studies are needed to determine whether there are changes in SERT availability in the early symptomatic state and if [(18)F]MPPF PET imaging might be a promising biomarker of early degenerative changes in PD.

SIGNIFICANCE STATEMENT

The present research provides evidence of the potential of combining a multitracer PET imaging technique and a longitudinal protocol applied on a progressively 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-intoxicated monkey model to further elucidate the nature of the compensatory mechanisms involved in the preclinical period of Parkinson's disease (PD). In particular, by investigating the dopaminergic and serotonergic changes both presynaptically and postsynaptically at four different motor states (baseline, early symptomatic, full symptomatic, and recovered), this study has allowed us to identify putative biomarkers for future therapeutic interventions to prevent and/or delay disease expression. For example, our findings suggest that the external pallidum could be a new target for cell-based therapies to reduce PD symptoms.

Baseline symptom severity predicts serotonin transporter change during psychotherapy in patients with major depression

AIMS

The role of the serotonin transporter (SERT) in the pathophysiology of depression is unclear and only a few follow-up studies exist. Our aim was to measure changes in SERT availability during psychodynamic psychotherapy in patients with major depression over a follow-up time of 12 or 18 months.

METHODS

The patients were studied with iodine-123 labelled 2β-carbomethoxy-3β-(4-iodophenyl) serial single-photon emission tomography imaging and clinical rating scales of symptoms.

RESULTS

Changes in SERT availability had no correlation with the change of symptoms, but the change of SERT availability during psychotherapy in the midbrain was predicted by the baseline severity of the clinical symptoms measured by the Symptom Checklist Depression Scale and the Symptom Checklist Global Severity Index. With cut-off values applied, it was found that SERT availabilities increased in patients with high baseline symptoms, and decreased in patients with low baseline symptoms.

CONCLUSIONS

Together with our earlier finding of decreased SERT in patients with depression, these results indicate a state-dependent and possibly a compensatory role of decreased SERT availability in depression.

The serotonin transporter in depression: Meta-analysis of in vivo and post mortem findings and implications for understanding and treating depression

BACKGROUND

Altered serotonin transporter levels have been reported in blood and brain of patients with major depressive disorders. However, the strength and consistency of the evidence for altered serotonin transporter availability in major depressive disorder is not clear.

METHODS

To address this, a comprehensive meta-analysis was conducted of all available in vivo neuroimaging and post mortem studies reporting serotonin transporter availability in patients with depression compared with healthy controls.

RESULTS

The final sample consisted of fifty (n=27 in vivo and n=25 post mortem) studies including 877 patients with depression (mean age: 42.9 years) and 968 healthy controls (mean age: 42.7 years). In vivo neuroimaging studies indicated reduced serotonin transporter binding in the striatum (g=-0.39, p=0.01), the amygdala (g=-0.37, p=0.01) and the brainstem (g=-0.31, p=0.01), including the midbrain (g=-0.27, p=0.02), but no significant alteration in the thalamus or the hippocampus. The post mortem findings indicated no significant change in serotonin transporter binding in depression in the brainstem (p=0.64), the frontal cortex (p=0.75) and the hippocampus (p=0.32, corrected for publication bias). Although there were too few studies for a meta-analysis, the post mortem studies in the amygdala and striatum showed reduced SERT binding in MDD in absolute terms, consistent with the imaging findings.

LIMITATIONS

A number of potential factors might have biased the results of the present meta-analysis such as the imaging modality (post mortem or in vivo neuroimaging), partial volume effects, susceptibility of some radiotracers to synaptic serotonin levels or binding to other monoamine transporters.

CONCLUSIONS

The results indicate that serotonin transporter availability in depressed patients is reduced in key regions of the limbic system. This provides direct support for the serotonin hypothesis of depression, and underlines the importance of the serotonin transporter as a target of pharmacological treatments.

Suicidal ideation modulates the reduction in serotonin transporter availability in male military conscripts with major depression: A 4-[18F]-ADAM PET study

OBJECTIVES

Suicide is an important issue in the military service, since it can influence military morale and create dangerous situations for other personnel. The serotonin transporter (SERT) has been suggested to be involved in the pathophysiology of depression and suicidal behaviours. The aims of this study were to examine whether the brain SERT availability differs between military conscripts with depression and control subjects, and whether suicidal ideation is correlated with SERT availability.

METHODS

We used N,N-dimethyl-2-(2-amino-4-[(18)F]-fluorophenylthio)benzylamine (4-[(18)F]-ADAM) as a radioligand for positron emission tomography (PET) imaging. All participants completed the Hamilton Depression Rating Scale and Beck Scale for Suicide Ideation (BSS) prior to PET imaging.

RESULTS

The effect of major depression and BSS scores had an interaction on SERT availability. After adjusting for the BSS score, subjects with depression had lower SERT availability than control subjects (F1,17 = 23.85, P < 0.001). A positive correlation between SERT availability and BSS scores was observed in the depression group (F1,8 = 30.67, P = 0.001). The status of depression and intensity of suicidal ideation exert opposite effects on SERT availability.

CONCLUSIONS

The extent of suicidal ideation may moderate the reduction effect in SERT binding observed in major depression in male military conscripts.

The serotonin transporter in psychiatric disorders: insights from PET imaging

Over the past 20 years, psychotropics affecting the serotonergic system have been used extensively in the treatment of psychiatric disorders. Molecular imaging, in particular PET, has allowed for elucidation of the essential contribution of the serotonin transporter to the pathophysiology of various psychiatric disorders and their treatment. We review studies that use PET to measure cerebral serotonin transporter activity in psychiatric disorders, focusing on major depressive disorder and antidepressant treatment. We also discuss opportunities and limitations in the application of this neuroimaging method in clinical practice. Although results from individual studies diverge, meta-analysis indicates a trend towards reduced serotonin transporter availability in patients with major depressive disorder. Inconsistencies in results might suggest symptom heterogeneity in major depressive disorder and might therefore be relevant for stratification of patients into clinical subsets. PET has enabled the elucidation of mechanisms of response to selective serotonin reuptake inhibitors (SSRIs) and hence provides a basis for rational pharmacological treatment of major depressive disorder. Such imaging studies have also suggested that the pattern of serotonin transporter binding before treatment might predict response to antidepressant treatment, which could potentially be clinically useful in the future. Additionally, this Review discusses PET studies investigating the serotonin transporter in anxiety, obsessive-compulsive disorder, and eating disorders. Few studies have shown changes in serotonin transporter activity in schizophrenia and attention deficit hyperactivity disorder. By showing the scarcity of data in these psychiatric disorders, we highlight the potential for further investigation in this field.

ITI-007 demonstrates brain occupancy at serotonin 5-HT₂A and dopamine D₂ receptors and serotonin transporters using positron emission tomography in healthy volunteers

RATIONALE

Central modulation of serotonin and dopamine underlies efficacy for a variety of psychiatric therapeutics. ITI-007 is an investigational new drug in development for treatment of schizophrenia, mood disorders, and other neuropsychiatric disorders.

OBJECTIVES

The purpose of this study was to determine brain occupancy of ITI-007 at serotonin 5-HT2A receptors, dopamine D2 receptors, and serotonin transporters using positron emission tomography (PET) in 16 healthy volunteers.

METHODS

Carbon-11-MDL100907, carbon-11-raclopride, and carbon-11-3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile) (carbon-11-DASB) were used as the radiotracers for imaging 5-HT2A receptors, D2 receptors, and serotonin transporters, respectively. Brain regions of interest were outlined using magnetic resonance tomography (MRT) with cerebellum as the reference region. Binding potentials were estimated by fitting a simplified reference tissue model to the measured tissue-time activity curves. Target occupancy was expressed as percent change in the binding potentials before and after ITI-007 administration.

RESULTS

Oral ITI-007 (10-40 mg) was safe and well tolerated. ITI-007 rapidly entered the brain with long-lasting and dose-related occupancy. ITI-007 (10 mg) demonstrated high occupancy (>80 %) of cortical 5-HT2A receptors and low occupancy of striatal D2 receptors (~12 %). D2 receptor occupancy increased with dose and significantly correlated with plasma concentrations (r (2) = 0.68, p = 0.002). ITI-007 (40 mg) resulted in peak occupancy up to 39 % of striatal D2 receptors and 33 % of striatal serotonin transporters.

CONCLUSIONS

The results provide evidence for a central mechanism of action via dopaminergic and serotonergic pathways for ITI-007 in living human brain and valuable information to aid dose selection for future clinical trials.

Long-term consequences of perinatal and adolescent cannabinoid exposure on neural and psychological processes

Marihuana is the most widely consumed illicit drug, even among adolescents and pregnant women. Given the critical developmental processes that occur in the adolescent and fetal nervous system, marihuana consumption during these stages may have permanent consequences on several brain functions in later adult life. Here, we review what is currently known about the long-term consequences of perinatal and adolescent cannabinoid exposure. The most consistent findings point to long-term impairments in cognitive function that are associated with structural alterations and disturbed synaptic plasticity. In addition, several neurochemical modifications are also evident after prenatal or adolescent cannabinoid exposure, especially in the endocannabinoid, glutamatergic, dopaminergic and opioidergic systems. Important sexual dimorphisms are also evident in terms of the long-lasting effects of cannabinoid consumption during pregnancy and adolescence, and cannabinoids possibly have a protective effect in adolescents who have suffered traumatic life challenges, such as maternal separation or intense stress. Finally, we suggest some future research directions that may encourage further advances in this exciting field.

High fat diet decreases beneficial effects of estrogen on serotonin-related gene expression in marmosets

The administration of estradiol-17β (E) to animal models after loss of ovarian steroid production has many beneficial effects on neural functions, particularly in the serotonin system in nonhuman primates (NHPs). E also has anorexic effects, although the mechanism of action is not well defined. In the US, obesity has reached epidemic proportions, and blame is partially directed at the Western style diet, which is high in fat and sugar. This study examined the interaction of E and diet in surgically menopausal nonhuman primates with a 2×2 block design. Marmosets (Callithrix jacchus; n=4/group) were placed on control-low fat diet (LFD; 14%kcal from fat) or high fat diet (HFD; 28%kcal from fat) 1month prior to ovariectomy (Ovx). Empty (placebo) or E-filled Silastic capsules were implanted immediately following Ovx surgery. Treatments extended 6months. The established groups were: placebo+LFD, E+LFD, placebo+HFD, or E+HFD. At necropsy, the brain was flushed with saline and harvested. The midbrain was dissected and a small block containing the dorsal raphe nucleus was processed for qRT-PCR using Evagreen (Biotinum). Genes previously found to impact serotonin neural functions were examined. Results were compared with 2-way ANOVA followed by Bonferroni post-hoc tests or Cohen's D analysis. There was a significant effect of treatment on tryptophan hydroxylase 2 (TPH2) across the groups (p=0.019). E stimulated TPH2 expression and HFD prevented E-stimulated TPH2 expression (p<0.01). Treatment differentially affected monoamine oxidase B (MAO-B) across the groups (p=0.05). E increased MAO-B with LFD, and this stimulatory effect was prevented by HFD (p<0.05). There was a significant difference between treatments in corticotrophin releasing factor-receptor 2 (CRF-R2) expression (p=0.012). E increased CRF-R2 and this stimulatory effect was blocked by HFD (p<0.01). Regardless of diet, E increased Fev mRNA (p=0.028) and decreased CRF-receptor 1 (CRF-R1) mRNA (p=0.04). HFD suppressed urocortin 1 (UCN1; stresscopin) expression (p=0.045) but E treatment had no effect. Monoamine oxidase A (MAO-A) was different due to treatment across the groups (p=0.028). MAO-A was increased in the E+HFD group (p<0.01) whereas previous studies showed E suppressed MAO-A in macaques. The serotonin reuptake transporter (SERT), the serotonin 1A receptor (5HT1A), estrogen receptor beta (ERβ) and progestin receptor (PR) expressions were not different between groups. Estrogen receptor alpha (ERα) was undetectable. In summary, the data indicate that important actions of hormone therapy in the serotonin system may be lost in the context of a HFD.

Disproportionate Reduction of Serotonin Transporter May Predict the Response and Adherence to Antidepressants in Patients with Major Depressive Disorder: A Positron Emission Tomography Study with 4-[18F]-ADAM

BACKGROUND

Many lines of evidence suggest the role of serotonin transporter (SERT)-mediated reuptake of serotonin in the pathophysiology and treatment of major depressive disorder (MDD). This study aimed to examine whether the pretreatment of SERT binding potential or SERT binding ratio between terminal projection regions relative to the midbrain raphe nuclei was associated with treatment outcomes to SERT-targeted antidepressants.

METHODS

We recruited 39 antidepressant-naïve patients with MDD and 39 heathy controls. Positron emission tomography with N,N-dimethyl-2-(2-amino-4-[(18)F]fluorophenylthio)benzylamine (4-[(18)F]-ADAM) was used to measure in vivo SERT availability prior to antidepressant treatment. The 21-item Hamilton Depression Rating Scale (HDRS) was use to assess the severity of depression from baseline to week 6. All the patients with MDD had HDRS scores of 18 or more.

RESULTS

Pretreatment SERT binding in the thalamus and striatum positively correlated with an early reduction in HDRS scores at week 3. Nonresponders and dropout patients showed a proportionate reduction in SERT binding in the terminal projection regions and midbrain compared to healthy controls. In contrast, a disproportionate reduction in SERT binding in the terminal projection regions relative to midbrain was observed in responders.

CONCLUSIONS

The results of this study suggested that a disproportionate reduction in SERT binding between terminal projection regions and midbrain may predict better treatment outcomes in patients with MDD.

Incongruent reduction of serotonin transporter associated with suicide attempts in patients with major depressive disorder: a positron emission tomography study with 4-[18F]-ADAM

BACKGROUND

Much evidence supports the role of the serotonin transporter (SERT) in the pathophysiology and pharmacotherapy of major depressive disorder (MDD) and suicidal behaviors.

METHODS

In this study, we recruited 17 antidepressant-naïve patients with MDD and 17 age- and gender-matched healthy controls. SERT availability was measured in vivo with N,N-dimethyl-2-(2-amino-4-[(18)F]fluorophenylthio)benzylamine (4-[(18)F]-ADAM) positron emission tomography (PET) imaging. The 21-item Hamilton Depression Rating Scale (HDRS) and Beck Scale for Suicide Ideation were used to assess the severity of depression and the intent of suicide ideation prior to PET imaging. All subjects with MDD were in a current state of depression with HDRS scores ≧18. Subjects who attempted suicide within two weeks of the study onset were recruited in the depressed suicidal group (n = 8). Subjects with MDD who denied any prior suicide attempt were recruited into the depressed non-suicidal group (n = 9).

RESULTS

A significant reduction of SERT availability in the midbrain, thalamus, and striatum was noted in the MDD group relative to the control group (Bonferroni-adjusted p-value < 0.05). Moreover, this effect was more pronounced in the depressed suicidal group compared to the control group (Bonferroni-adjusted p-value < 0.01). Relative to both the depressed non-suicidal and control groups, the depressed suicidal group showed an increased prefrontal cortex (PFC)/midbrain SERT binding ratio (Bonferroni-adjusted p-value < 0.01).

CONCLUSIONS

This study suggests an incongruent reduction of PFC SERT binding relative to the midbrain might discriminate between depressed suicide attempters and non-attempters in patients with MDD and may be involved in the pathophysiology of suicide behaviors.

Dimensional psychiatry: reward dysfunction and depressive mood across psychiatric disorders

RATIONALE

A dimensional approach in psychiatry aims to identify core mechanisms of mental disorders across nosological boundaries.

OBJECTIVES

We compared anticipation of reward between major psychiatric disorders, and investigated whether reward anticipation is impaired in several mental disorders and whether there is a common psychopathological correlate (negative mood) of such an impairment.

METHODS

We used functional magnetic resonance imaging (fMRI) and a monetary incentive delay (MID) task to study the functional correlates of reward anticipation across major psychiatric disorders in 184 subjects, with the diagnoses of alcohol dependence (n = 26), schizophrenia (n = 44), major depressive disorder (MDD, n = 24), bipolar disorder (acute manic episode, n = 13), attention deficit/hyperactivity disorder (ADHD, n = 23), and healthy controls (n = 54). Subjects' individual Beck Depression Inventory-and State-Trait Anxiety Inventory-scores were correlated with clusters showing significant activation during reward anticipation.

RESULTS

During reward anticipation, we observed significant group differences in ventral striatal (VS) activation: patients with schizophrenia, alcohol dependence, and major depression showed significantly less ventral striatal activation compared to healthy controls. Depressive symptoms correlated with dysfunction in reward anticipation regardless of diagnostic entity. There was no significant correlation between anxiety symptoms and VS functional activation.

CONCLUSION

Our findings demonstrate a neurobiological dysfunction related to reward prediction that transcended disorder categories and was related to measures of depressed mood. The findings underline the potential of a dimensional approach in psychiatry and strengthen the hypothesis that neurobiological research in psychiatric disorders can be targeted at core mechanisms that are likely to be implicated in a range of clinical entities.

Molecular imaging as a guide for the treatment of central nervous system disorders

Molecular imaging techniques have a number of advantages for research into the pathophysiology and treatment of central nervous system (CNS) disorders. Firstly, they provide a noninvasive means of characterizing physiological processes in the living brain, enabling molecular alterations to be linked to clinical changes. Secondly, the pathophysiological target in a given CNS disorder can be measured in animal models and in experimental human models in the same way, which enables translational research. Moreover, as molecular imaging facilitates the detection of functional change which precedes gross pathology, it is particularly useful for the early diagnosis and treatment of CNS disorders.

This review considers the application of molecular imaging to CNS disorders focusing on its potential to inform the development and evaluation of treatments. We focus on schizophrenia, Parkinson's disease, depression, and dementia as major CNS disorders. We also review the potential of molecular imaging to guide new drug development for CNS disorders.

Molecular imaging for depressive disorders

Molecular imaging is the visualization, characterization, and measurement of biologic processes at the molecular and cellular levels in humans and other living systems. Molecular imaging techniques such as MR spectroscopy and PET have been used to explore the molecular pathophysiology of depression and assess treatment responses. MR spectroscopy is a noninvasive technique that assesses the levels of biochemical metabolites in the brain, while PET uses radioligands injected in the bloodstream that have high binding affinity for target molecules. MR spectroscopy findings suggest a role for glutamate/glutamine and gamma-aminobutyric acid in depression. PET has generally failed to find a correlation between radioligand binding potential and depression severity or treatment response, though it may offer promise in distinguishing responders and nonresponders to treatment. A major challenge for both modalities is that depression is a heterogeneous, multifactorial disorder, while MR spectroscopy and PET are limited to examining a few metabolites or a single radioligand at a time. This difference makes a comprehensive evaluation of neurochemical changes in the brain difficult.

Positron emission tomography molecular imaging in late-life depression

Molecular imaging represents a bridge between basic and clinical neuroscience observations and provides many opportunities for translation and identifying mechanisms that may inform prevention and intervention strategies in late-life depression (LLD). Substantial advances in instrumentation and radiotracer chemistry have resulted in improved sensitivity and spatial resolution and the ability to study in vivo an increasing number of neurotransmitters, neuromodulators, and, importantly, neuropathological processes. Molecular brain imaging studies in LLD will be reviewed, with a primary focus on positron emission tomography. Future directions for the field of molecular imaging in LLD will be discussed, including integrating molecular imaging with genetic, neuropsychiatric, and cognitive outcomes and multimodality neuroimaging.

Glia and immune cell signaling in bipolar disorder: insights from neuropharmacology and molecular imaging to clinical application

Bipolar disorder (BD) is a debilitating mental illness characterized by severe fluctuations in mood, sleep, energy and executive functioning. Pharmacological studies of selective serotonin reuptake inhibitors and the monoamine system have helped us to clinically understand bipolar depression. Mood stabilizers such as lithium and valproic acid, the first-line treatments for bipolar mania and depression, inhibit glycogen synthase kinase-3 beta (GSK-3β) and regulate the Wnt pathway. Recent investigations suggest that microglia, the resident immune cells of the brain, provide a physiological link between the serotonin system and the GSK-3β/Wnt pathway through neuroinflammation. We review the pharmacological, translational and brain imaging studies that support a role for microglia in regulating neurotransmitter synthesis and immune cell activation. These investigations provide a model for microglia involvement in the pathophysiology and phenotype of BD that may translate into improved therapies.

Differences in neurobiological pathways of four "clinical content" subtypes of depression

Although often considered as a mental disorder, depression is best described as a behavioral-neurobiological phenomenon. In addition, although usually reported as a unitary diagnosis, major depressive episode is composed of a range of different symptoms that can occur in nearly 1500 possible combinations to fulfill the required diagnostic criterion. To investigate and describe the underlying behavioral and neurobiological substrates of these symptoms, they were clustered into "clinical content" subtypes of depression according to their predominant common behavioral characteristics. These subtypes were then found to possess different neurobiological pathways that argue for different treatment approaches.

Positron emission tomography quantification of serotonin transporter in suicide attempters with major depressive disorder

BACKGROUND

Several lines of evidence implicate abnormal serotonergic function in suicidal behavior and completed suicide, including low serotonin transporter binding in postmortem studies of completed suicide. We have also reported low in vivo serotonin transporter binding in major depressive disorder (MDD) during a major depressive episode using positron emission tomography (PET) with [(11)C]McN5652. We quantified regional brain serotonin transporter binding in vivo in depressed suicide attempters, depressed nonattempters, and healthy controls using PET and a superior radiotracer, [(11)C]DASB.

METHODS

Fifty-one subjects with DSM-IV current MDD, 15 of whom were past suicide attempters, and 32 healthy control subjects underwent PET scanning with [(11)C]DASB to quantify in vivo regional brain serotonin transporter binding. Metabolite-corrected arterial input functions and plasma free-fraction were acquired to improve quantification.

RESULTS

Depressed suicide attempters had lower serotonin transporter binding in midbrain compared with depressed nonattempters (p = .031) and control subjects (p = .0093). There was no difference in serotonin transporter binding comparing all depressed subjects with healthy control subjects considering six a priori regions of interest simultaneously (p = .41).

CONCLUSIONS

Low midbrain serotonin transporter binding appears to be related to the pathophysiology of suicidal behavior rather than of major depressive disorder. This is consistent with postmortem work showing low midbrain serotonin transporter binding capacity in depressed suicides and may partially explain discrepant in vivo findings quantifying serotonin transporter in depression. Future studies should investigate midbrain serotonin transporter binding as a predictor of suicidal behavior in MDD and determine the cause of low binding.

Estradiol and brain serotonin reuptake transporter in long-term ovariectomized parkinsonian monkeys

This study evaluated the effect of a one month 17β-estradiol treatment on brain serotonin (5-HT) reuptake transporter (SERT) in long-term ovariectomized (OVX) female monkeys (Macaca fascicularis) bearing a unilateral lesion with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) injected directly into the left substantia nigra modeling Parkinson disease (PD). Ovariectomy and MPTP lesion were performed four years before the estrogen treatment to model postmenopausal PD patients. SERT was measured by autoradiography using the radioligand [(3)H]Citalopram. Specific binding to SERT decreased in anterior cerebral cortex, nucleus accumbens, caudate nucleus and putamen on the lesioned side of 17β-estradiol and vehicle-treated monkeys compared to the intact side. In caudate nucleus and putamen the lesioned-induced decrease of [(3)H]Citalopram specific binding was more extensive in anterior and middle than posterior parts. [(3)H]Citalopram specific binding was increased in the cortex anterior cingulate gyrus of monkeys treated with 17β-estradiol in both brain hemispheres and was unchanged in the other brain regions investigated including the raphe nucleus. Positive correlations between [(3)H]Citalopram specific binding and 5-HT as well as 5-HIAA concentrations (reported previously) were obtained in the caudate nucleus and putamen and a negative correlation between SERT binding and 5-HIAA/5-HT concentration ratio suggesting MPTP lesion-induced 5-HT neuronal loss and lower 5-HT neurotransmission controlling and decreasing SERT for homeostasis. 17β-estradiol treatment initiated four years after ovariectomy of monkeys modeling hormonal conditions of post-menopause shows that SERT still displays some responsiveness to estrogens as observed in the anterior cingulate cortex. These results support a role of estrogens in 5-HT activity in PD.

Predicting adverse drug reaction profiles by integrating protein interaction networks with drug structures

The prediction of adverse drug reactions (ADRs) has become increasingly important, due to the rising concern on serious ADRs that can cause drugs to fail to reach or stay in the market. We proposed a framework for predicting ADR profiles by integrating protein-protein interaction (PPI) networks with drug structures. We compared ADR prediction performances over 18 ADR categories through four feature groups-only drug targets, drug targets with PPI networks, drug structures, and drug targets with PPI networks plus drug structures. The results showed that the integration of PPI networks and drug structures can significantly improve the ADR prediction performance. The median AUC values for the four groups were 0.59, 0.61, 0.65, and 0.70. We used the protein features in the best two models, "Cardiac disorders" (median-AUC: 0.82) and "Psychiatric disorders" (median-AUC: 0.76), to build ADR-specific PPI networks with literature supports. For validation, we examined 30 drugs withdrawn from the U.S. market to see if our approach can predict their ADR profiles and explain why they were withdrawn. Except for three drugs having ADRs in the categories we did not predict, 25 out of 27 withdrawn drugs (92.6%) having severe ADRs were successfully predicted by our approach.