Familial risk for major depression is associated with lower striatal 5-HT₄ receptor binding

Chronic, combinatorial targeting of NMDARs and 5-HT4Rs exerts extended behavioral effects against stress-induced perseverative behavior and hyponeophagia

Serotonin (5-HT) receptors and N-methyl-D-aspartate receptors (NMDARs) have both been implicated in stress-induced psychiatric disorders. However, there is a paucity of studies evaluating the effectiveness of novel combinatorial pharmacological treatments to treat stress-related disorders. Here, we evaluated whether administration of combinatorial (R,S)-ketamine, an NMDAR antagonist and Food and Drug Administration (FDA)-approved anesthetic, and prucalopride, a 5-HT type IV receptor (5-HT4R) agonist and FDA-approved drug for chronic idiopathic constipation (CIC), would have additional effects when administered after stress. A single injection of saline (Sal), (R,S)-ketamine (K), prucalopride (P), or a combined dose of (R,S)-ketamine and prucalopride (K + P) was administered for 1x, 2x, or 7x per week for 2 weeks after either contextual fear conditioning (CFC), learned helplessness (LH), stress enhanced fear learning (SEFL), or chronic corticosterone (CORT) stress in both sexes. Drug efficacy was assayed using assays to measure fear, behavioral despair, perseverative, and/or hyponeophagia. Combinatorial drug administration was also tested using intranasal delivery. We found that combinatorial K + P exerted additional effects, compared to either drug alone, in reducing a variety of stress-induced behaviors in both sexes. Moreover, intranasal dosing was also effective. Our results indicate that chronic administration of K + P has extended benefits for combating stress-induced pathophysiology. Our findings provide strong evidence that future clinical studies using this chronic treatment strategy may prove advantageous in decreasing a broad range of stress-induced psychiatric disorders.

Effect of Antidepressant Treatment on 5-HT4 Receptor Binding and Associations With Clinical Outcomes and Verbal Memory in Major Depressive Disorder

BACKGROUND

Brain serotonin 4 receptor (5-HT4R) levels are lower in untreated patients with major depressive disorder (MDD) and are linked to verbal memory. Here, we investigated the relationship between 5-HT4R levels, clinical outcomes, and cognitive function in patients with MDD who initiated selective serotonin reuptake inhibitor drug treatment.

METHODS

Ninety patients with moderate to severe depression underwent molecular brain imaging to measure 5-HT4R binding prior to antidepressant treatment with escitalopram. Pretreatment 5-HT4R binding was assessed for its ability to predict treatment outcome at weeks 4, 8, or 12. In 40 patients who were rescanned 8 weeks posttreatment, change in cerebral 5-HT4R binding was correlated with change in verbal memory and with change in depressive symptoms, as evaluated by the 6-item Hamilton Depression Rating Scale.

RESULTS

After 8 weeks of serotonergic intervention, neostriatal 5-HT4R binding was reduced by 9%. Global change in 5-HT4R binding from baseline was associated with verbal memory outcomes, but not with overall clinical depressive symptom outcomes. Pretreatment 5-HT4R binding did not predict clinical recovery status at week 8 and was not associated with change in the 6-item Hamilton Depression Rating Scale scores.

CONCLUSIONS

In patients with moderate to severe MDD, treatment with selective serotonin reuptake inhibitors downregulated neostriatal 5-HT4R levels, which is consistent with the notion that the drugs increase cerebral extracellular serotonin. The less global brain 5-HT4R levels were downregulated after selective serotonin reuptake inhibitors, the more verbal memory improved, highlighting the potential importance of 5-HT4R as a treatment target in MDD. The findings offer insights into mechanisms that underlie antidepressant effects and point to new directions for precision medicine treatments for MDD.

Changes in hippocampal volume, 5-HT4 receptor binding, and verbal memory over the course of antidepressant treatment in major depressive disorder

Serotonin reuptake inhibitors have been reported to increase hippocampal volume and improve memory function in patients with Major depressive disorder (MDD). The postsynaptic 5-HT4 receptor (5-HT4R) is involved in hippocampal development, familial risk for depression and depressive pathology. In an open-label trial with 91 patients (72% female, mean 27.2 years) with MDD, we investigated the relation between changes in hippocampal volume, 5-HT4R, and verbal memory during 12 weeks treatment with 10-20 mg escitalopram. Depression severity, verbal memory, MRI-determined hippocampus volume and PET-determined 5-HT4R were measured pretreatment. Forty-three patients were rescanned at week 8. HAMD17 was reassessed at week 8 and together with verbal memory at week 12. We used mixed-effects models and linear regressions. We estimated a 27 mm3 (p = 0.086) reduction in mean hippocampus volume over the course of eight weeks. In patients clinically responding to treatment, we estimated a 45 mm3 reduction (p = 0.019), 8 mm3 increase in non-responders (p = 0.78), and a 52 mm3 group difference (p = 0.12). Hippocampal 5-HT4 receptor binding before treatment and at week eight was negatively associated with hippocampal volume in females, regardless of treatment response (p-values≤0.006). However, no clear evidence for an association in males or sex interaction could be established (p-values≥0.16). Although the hippocampus volume did not increase with treatment, we found a decrease in clinically responsive patients. Our findings suggest an association between 5-HT4R signalling and changes in hippocampal volume in females with MDD during antidepressant treatment, highlighting the need for further investigation into the role of serotonergic mechanisms in hippocampal plasticity.

Effects of a 5-HT4 receptor antagonist in the caudate nucleus on the performance of macaques in a delayed reward task

Temporal discounting, in which the recipient of a reward perceives the value of that reward to decrease with delay in its receipt, is associated with impulsivity and psychiatric disorders such as depression. Here, we investigate the role of the serotonin 5-HT4 receptor (5-HT4R) in modulating temporal discounting in the macaque dorsal caudate nucleus (dCDh), the neurons of which have been shown to represent temporally discounted value. We first mapped the 5-HT4R distribution in macaque brains using positron emission tomography (PET) imaging and confirmed dense expression of 5-HT4R in the dCDh. We then examined the effects of a specific 5-HT4R antagonist infused into the dCDh. Blockade of 5-HT4R significantly increased error rates in a goal-directed delayed reward task, indicating an increase in the rate of temporal discounting. This increase was specific to the 5-HT4R blockade because saline controls showed no such effect. The results demonstrate that 5-HT4Rs in the dCDh are involved in reward-evaluation processes, particularly in the context of delay discounting, and suggest that serotonergic transmission via 5-HT4R may be a key component in the neural mechanisms underlying impulsive decisions, potentially contributing to depressive symptoms.

No association between peripheral serotonin-gene-related DNA methylation and brain serotonin neurotransmission in the healthy and depressed state

BACKGROUND

Methylation of serotonin-related genes has been proposed as a plausible gene-by-environment link which may mediate environmental stress, depressive and anxiety symptoms. DNA methylation is often measured in blood cells, but little is known about the association between this peripheral epigenetic modification and brain serotonergic architecture. Here, we evaluated the association between whole-blood-derived methylation of four CpG sites in the serotonin transporter (SLC6A4) and six CpG sites of the tryptophan hydroxylase 2 (TPH2) gene and in-vivo brain levels of serotonin transporter (5-HTT) and serotonin 4 receptor (5-HT4) in a cohort of healthy individuals (N = 254) and, for 5-HT4, in a cohort of unmedicated patients with depression (N = 90). To do so, we quantified SLC6A4/TPH2 methylation using bisulfite pyrosequencing and estimated brain 5-HT4 and 5-HTT levels using positron emission tomography. In addition, we explored the association between SLC6A4 and TPH2 methylation and measures of early life and recent stress, depressive and anxiety symptoms on 297 healthy individuals.

RESULTS

We found no statistically significant association between peripheral DNA methylation and brain markers of serotonergic neurotransmission in patients with depression or in healthy individuals. In addition, although SLC6A4 CpG2 (chr17:30,236,083) methylation was marginally associated with the parental bonding inventory overprotection score in the healthy cohort, statistical significance did not remain after accounting for blood cell heterogeneity.

CONCLUSIONS

We suggest that findings on peripheral DNA methylation in the context of brain serotonin-related features should be interpreted with caution. More studies are needed to rule out a role of SLC6A4 and TPH2 methylation as biomarkers for environmental stress, depressive or anxiety symptoms.

The Serotonin 4 Receptor Subtype: A Target of Particular Interest, Especially for Brain Disorders

In recent years, particular attention has been paid to the serotonin 4 receptor, which is well expressed in the brain, but also peripherally in various organs. The cerebral distribution of this receptor is well conserved across species, with high densities in the basal ganglia, where they are expressed by GABAergic neurons. The 5-HT4 receptor is also present in the cerebral cortex, hippocampus, and amygdala, where they are carried by glutamatergic or cholinergic neurons. Outside the central nervous system, the 5-HT4 receptor is notably expressed in the gastrointestinal tract. The wide distribution of the 5-HT4 receptor undoubtedly contributes to its involvement in a plethora of functions. In addition, the modulation of this receptor influences the release of serotonin, but also the release of other neurotransmitters such as acetylcholine and dopamine. This is a considerable asset, as the modulation of the 5-HT4 receptor can therefore play a direct or indirect beneficial role in various disorders. One of the main advantages of this receptor is that it mediates a much faster antidepressant and anxiolytic action than classical selective serotonin reuptake inhibitors. Another major benefit of the 5-HT4 receptor is that its activation enhances cognitive performance, probably via the release of acetylcholine. The expression of the 5-HT4 receptor is also altered in various eating disorders, and its activation by the 5-HT4 agonist negatively regulates food intake. Additionally, although the cerebral expression of this receptor is modified in certain movement-related disorders, it is still yet to be determined whether this receptor plays a key role in their pathophysiology. Finally, there is no longer any need to demonstrate the value of 5-HT4 receptor agonists in the pharmacological management of gastrointestinal disorders.

Functional Dimerization of Serotonin Receptors: Role in Health and Depressive Disorders

Understanding the neurobiological underpinnings of depressive disorder constitutes a pressing challenge in the fields of psychiatry and neurobiology. Depression represents one of the most prevalent forms of mental and behavioral disorders globally. Alterations in dimerization capacity can influence the functional characteristics of serotonin receptors and may constitute a contributing factor to the onset of depressive disorders. The objective of this review is to consolidate the current understanding of interactions within the 5-HT receptor family and between 5-HT receptors and members of other receptor families. Furthermore, it aims to elucidate the role of such complexes in depressive disorders and delineate the mechanisms through which antidepressants exert their effects.

Stress Hormone Dynamics Are Coupled to Brain Serotonin 4 Receptor Availability in Unmedicated Patients With Major Depressive Disorder: A NeuroPharm Study

BACKGROUND

A prominent finding in major depressive disorder (MDD) is distorted stress hormone dynamics, which is regulated by serotonergic brain signaling. An interesting feature of the cerebral serotonin system is the serotonin 4 receptor (5-HT4R), which is lower in depressed relative to healthy individuals and also has been highlighted as a promising novel antidepressant target. Here, we test the novel hypothesis that brain 5-HT4R availability in untreated patients with MDD is correlated with cortisol dynamics, indexed by the cortisol awakening response (CAR). Further, we evaluate if CAR changes with antidepressant treatment, including a selective serotonin reuptake inhibitor, and if pretreatment CAR can predict treatment outcome.

METHODS

Sixty-six patients (76% women) with a moderate to severe depressive episode underwent positron emission tomography imaging with [11C]SB207145 for quantification of brain 5-HT4R binding using BPND as outcome. Serial home sampling of saliva in the first hour from awakening was performed to assess CAR before and after 8 weeks of antidepressant treatment. Treatment outcome was measured by change in Hamilton Depression Rating Scale 6 items.

RESULTS

In the unmedicated depressed state, prefrontal and anterior cingulate cortices 5-HT4R binding was positively associated with CAR. CAR remained unaltered after 8 weeks of antidepressant treatment, and pretreatment CAR did not significantly predict treatment outcome.

CONCLUSIONS

Our findings highlight a link between serotonergic disturbances in MDD and cortisol dynamics, which likely is involved in disease and treatment mechanisms. Further, our data support 5-HT4R agonism as a promising precision target in patients with MDD and disturbed stress hormone dynamics.

Sexual health and serotonin 4 receptor brain binding in unmedicated patients with depression-a NeuroPharm study

Sexual dysfunction is prominent in Major Depressive Disorder (MDD) and affects women with depression more than men. Patients with MDD relative to healthy controls have lower brain levels of the serotonin 4 receptor (5-HT₄R), which is expressed with high density in the striatum, i.e. a key hub of the reward system. Reduced sexual desire is putatively related to disturbed reward processing and may index anhedonia in MDD. Here, we aim to illuminate plausible underlying neurobiology of sexual dysfunction in unmedicated patients with MDD. We map associations between 5-HT₄R binding, as imaged with [¹¹C]SB207145 PET, in the striatum, and self-reported sexual function. We also evaluate if pre-treatment sexual desire score predicts 8-week treatment outcome in women. From the NeuroPharm study, we include 85 untreated MDD patients (71% women) who underwent eight weeks of antidepressant drug treatment. In the mixed sex group, we find no difference in 5-HT₄R binding between patients with sexual dysfunction vs normal sexual function. However, in women we find lower 5-HT₄R binding in the sexual dysfunctional group compared to women with normal sexual function (β = -0.36, 95%CI[-0.62:-0.09], p = 0.009) as well as a positive association between sexual desire and 5-HT₄R binding (β = 0.07, 95%CI [0.02:0.13], p = 0.012). Sexual desire at baseline do not predict treatment outcome (ROC curve AUC = 52%[36%:67%]) in women. Taken together, we find evidence for a positive association between sexual desire and striatal 5-HT₄R availability in women with depression. Interestingly, this raises the question if direct 5-HT₄R agonism can target reduced sexual desire or anhedonia in MDD.

Latest updates on the serotonergic system in depression and anxiety

Psychiatric disorders are among the leading causes of global health burden, with depression and anxiety being the most disabling subtypes. The two common disorders, depression and anxiety, usually coexist and are pathologically polygenic with complicated etiologies. Current drug-based therapies include selective serotonin reuptake inhibitors, serotonin and norepinephrine reuptake inhibitors, and 5-hydroxytryptamine partial agonists. However, these modalities share common limitations, such as slow onset and low efficacy, which is why potential mechanistic insights for new drug targets are needed. In this review, we summarize recent advances in brain localization, pathology, and therapeutic mechanisms of the serotonergic system in depression and anxiety.

Serotonin 4 Receptor Brain Binding in Major Depressive Disorder and Association With Memory Dysfunction

Importance

The cerebral serotonin 4 (5-HT4) receptor is a promising novel target for treatment of major depressive disorder (MDD), and pharmacological stimulation of the 5-HT4 receptor has been associated with improved learning and memory in healthy individuals.

Objective

To map the neurobiological signatures of patients with untreated MDD compared with healthy controls and to examine the association between cerebral 5-HT4 receptor binding and cognitive functions in the depressed state.

Design, Setting, and Participants

This case-control study used baseline data from the NeuroPharm clinical depression trial in Denmark. Adult participants included antidepressant-free outpatients with a current moderate to severe depressive episode and healthy controls. All participants completed positron emission tomography (PET) scanning with [11C]SB207145 for quantification of brain 5-HT4 receptor binding, but only the patients underwent cognitive testing. Data analyses were performed from January 21, 2020, to April 22, 2022.

Main Outcomes and Measures

The main study outcome was the group difference in cerebral 5-HT4 receptor binding between patients with MDD and healthy controls. In addition, the association between 5-HT4 receptor binding and verbal memory performance in the patient group was tested. Other cognitive domains (working memory, reaction time, emotion recognition bias, and negative social emotions) were assessed as secondary outcomes.

Results

A total of 90 patients with untreated MDD (mean [SD] age, 27.1 [8.2] years; 64 women [71.1%]) and 91 healthy controls (mean [SD] age, 27.1 [8.0] years; 55 women [60.4%]) were included in the analysis. Patients with current MDD had significantly lower cerebral 5-HT4 receptor binding than healthy controls (-7.0%; 95% CI, -11.2 to -2.7; P = .002). In patients with MDD, there was a correlation between cerebral 5-HT4 receptor binding and verbal memory (r = 0.29; P = .02).

Conclusions and Relevance

Results of this study show that cerebral 5-HT4 receptor binding was lower in patients with MDD than in healthy controls and that the memory dysfunction in patients with MDD was associated with lower cerebral 5-HT4 receptor binding. The cerebral 5-HT4 receptor is a promising treatment target for memory dysfunction in patients with MDD.

Association between the loudness dependence of auditory evoked potential, serotonergic neurotransmission and treatment outcome in patients with depression

Previous studies have suggested that the loudness dependence of auditory evoked potential (LDAEP) is associated with the effectiveness of antidepressant treatment in patients with major depressive disorders (MDD). Furthermore, both LDAEP and the cerebral serotonin 4 receptor (5-HT₄R) density is inversely related to brain serotonin levels. We included 84 patients with MDD and 22 healthy controls to examined the association between LDAEP and treatment response and its association with cerebral 5-HT₄R density. Participants underwent both EEG and 5-HT₄R neuroimaging with [¹¹C]SB207145 PET. Thirty-nine patients with MDD were re-examined after 8 weeks of treatment with selective serotonin reuptake inhibitors/serotonin noradrenaline reuptake inhibitor (SSRI/SNRI). We found that the cortical source of LDAEP was higher in untreated patients with MDD compared to healthy controls (p=0.03). Prior to SSRI/SNRI treatment, subsequent treatment responders had a negative association between LDAEP and depressive symptoms and a positive association between scalp LDAEP and symptom improvement at week 8. This was not found in source LDAEP. In healthy controls, we found a positive correlation between both scalp and source LDAEP and cerebral 5-HT₄R binding but that was not observed in patients with MDD. We did not see any changes in scalp and source LDAEP in response to SSRI/SNRI treatment. These results support a theoretical framework where both LDAEP and cerebral 5-HT₄R are indices of cerebral 5-HT levels in healthy individuals while this association seems to be disrupted in MDD. The combination of the two biomarkers may be useful for stratifying patients with MDD. Clinical Trials Registration:https://clinicaltrials.gov/ct2/show/NCT02869035?draw=1Registration number: NCT0286903.

Beyond monoamines: I. Novel targets and radiotracers for Positron emission tomography imaging in psychiatric disorders

With the emergence of positron emission tomography (PET) in the late 1970s, psychiatry had access to a tool capable of non-invasive assessment of human brain function. Early applications in psychiatry focused on identifying characteristic brain blood flow and metabolic derangements using radiotracers such as [¹⁵ O]H₂ O and [¹⁸ F]FDG. Despite the success of these techniques, it became apparent that more specific probes were needed to understand the neurochemical bases of psychiatric disorders. The first neurochemical PET imaging probes targeted sites of action of neuroleptic (dopamine D₂ receptors) and psychoactive (serotonin receptors) drugs. Based on the centrality of monoamine dysfunction in psychiatric disorders and the measured success of monoamine-enhancing drugs in treating them, the next 30 years witnessed the development of an armamentarium of PET radiopharmaceuticals and imaging methodologies for studying monoamines. Continued development of monoamine-enhancing drugs over this time however was less successful, realizing only modest gains in efficacy and tolerability. As patent protection for many widely prescribed and profitable psychiatric drugs lapsed, drug development pipelines shifted away from monoamines in search of novel targets with the promises of improved efficacy, or abandoned altogether. Over this period, PET radiopharmaceutical development activities closely paralleled drug development priorities resulting in the development of new PET imaging agents for non-monoamine targets. Part one of this review will briefly survey novel PET imaging targets with relevance to the field of psychiatry, which include the metabotropic glutamate receptor type 5 (mGluR5), purinergic P₂ X₇ receptor, type 1 cannabinoid receptor (CB₁ ), phosphodiesterase 10A (PDE10A), and describe radiotracers developed for these and other targets that have matured to human subject investigations. Current limitations of the targets and techniques will also be discussed.

The Roles of Serotonin in Neuropsychiatric Disorders

The serotonergic system extends throughout the central nervous system (CNS) and the gastrointestinal (GI) tract. In the CNS, serotonin (5-HT, 5-hydroxytryptamine) modulates a broad spectrum of functions, including mood, cognition, anxiety, learning, memory, reward processing, and sleep. These processes are mediated through 5-HT binding to 5-HT receptors (5-HTRs), are classified into seven distinct groups. Deficits in the serotonergic system can result in various pathological conditions, particularly depression, schizophrenia, mood disorders, and autism. In this review, we outlined the complexity of serotonergic modulation of physiologic and pathologic processes. Moreover, we provided experimental and clinical evidence of 5-HT's involvement in neuropsychiatric disorders and discussed the molecular mechanisms that underlie these illnesses and contribute to the new therapies.

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.

The GG genotype of the serotonin 4 receptor genetic polymorphism, rs1345697, is associated with lower remission rates after antidepressant treatment: Findings from the METADAP cohort

BACKGROUND

Pharmacological studies have yielded valuable insights into the role of the serotonin 4 receptor (HTR4) in major depressive episodes (MDE) and response to antidepressant drugs (AD). A genetic association has been shown between HTR4 and susceptibility to mood disorders. Our study aims at assessing the association between the HTR4 genetic polymorphism, rs1345697, and improvement in depressive symptoms and remission after antidepressant treatment in MDE patients.

METHODS

492 depressed patients from the METADAP cohort were treated prospectively for 6 months with ADs. The clinical outcomes according to HTR4 rs1345697 were compared after 1 (M1), 3 (M3), and 6 (M6) months of treatment. Mixed-effects logistic regression and adjusted linear models assessed the association between rs1345697 and 17-item Hamilton Depression Rating Scale (HDRS) score improvement and response/remission.

RESULTS

Over the 6 months of treatment, mixed-effects regressions showed lower improvements in HDRS scores (Coefficient=1.52; Confident Interval (CI) 95% [0.37-2.67]; p = 0.009) and lower remission rates (Odds Ratio=2.0; CI95% [1.0-4.1]; p = 0.05) in GG homozygous patients as compared to allele A carriers.

LIMITATIONS

The major limitations of our study are the uncertainty of the rs1345697 effect on HTR4 function, the substantial drop-out rate, and the fact that analysis is not based on randomization between polymorphism groups.

CONCLUSIONS

In our study, patients who were homozygous carriers of the variant G of the HTR4 rs1345697 had lower depressive symptoms improvement and 2-fold lower remission rates after antidepressant treatment as compared to allele A carriers. Randomization study should be done to confirm these results.

Astroglial Serotonin Receptors as the Central Target of Classic Antidepressants

Major depressive disorder (MDD) presents multiple clinical phenotypes and has complex underlying pathological mechanisms. Existing theories cannot completely explain the pathophysiological mechanism(s) of MDD, while the pharmacology of current antidepressants is far from being fully understood. Astrocytes, the homeostatic and defensive cells of the central nervous system, contribute to shaping behaviors, and regulating mood and emotions. A detailed introduction on the role of astrocytes in depressive disorders is thus required, to which this chapter is dedicated. We also focus on the interactions between classic antidepressants and serotonin receptors, overview the role of astrocytes in the pharmacological mechanisms of various antidepressants, and present astrocytes as targets for the treatment of bipolar disorder. We provide a foundation of knowledge on the role of astrocytes in depressive disorders and astroglial 5-HT_2B receptors as targets for selective serotonin reuptake inhibitors in vivo and in vitro.

Cold-Blooded and on Purpose: A Review of the Biology of Proactive Aggression

Proactive aggression (PA) is a planned and unprovoked form of aggression that is most often enacted for personal gain or in anticipation of a reward. Frequently described as "cold-blooded" or goal oriented, PA is thought to be associated with low autonomic arousal. With this view in mind, we performed a scoping review of the biological correlates of PA and identified 74 relevant articles. Physiological findings indicated a robust association between PA and reduced resting heart rate, and to a lesser extent a relationship between PA and decreased heart rate and skin conductance reactivity, perhaps indicating dampened sympathetic function. The twin literature identified PA as a heritable trait, but little evidence implicates specific genes in the pathogenesis of PA. Neuroimaging studies of PA pinpoint impaired amygdala function in the assessment and conditioning of aversive stimuli, which may influence the establishment of behavioral patterns. Nodes of the default mode network were identified as possible neural correlates of PA, suggesting that altered function of this network may be involved in the genesis of PA. Given the overlap of PA with reactive aggression and the overall behavioral complexity of PA, it is clear that multiple endophenotypes of PA exist. This comprehensive review surveys the most salient neurobiologically informed research on PA.

Activation and Blockade of Serotonin-4 Receptors in the Lateral Habenula Produce Antidepressant Effects in the Hemiparkinsonian Rat

BACKGROUND

The 5-hydroxytryptamine (5-HT) neurotransmitter system and lateral habenula (LHb) are involved in the regulation of depression, while the mechanisms remain to be clarified.

OBJECTIVES

The effects and possible mecha-nism underlying activation or blockade of 5-HT4 receptors (5-HT4Rs) in the LHb in depression were investigated by behavioral and neurochemical methods based on a Parkinson's disease (PD) rat model.

METHOD

6-Hydroxydopamine (6-OHDA) was injected unilaterally into the substantia nigra pars compacta to establish the PD rat model. The depressive-like behaviors were measured by the forced swimming test (FST) and sucrose preference test (SPT). The concentrations of dopamine (DA), noradrenaline (NA) and 5-HT in the related brain regions were measured by a neurochemical method.

RESULTS

The 6-OHDA lesions increased the immobility time in the FST and decreased the sucrose consumption in the SPT, suggesting the induction of depressive-like behaviors. Intra-LHb injection of BIMU-8 (5-HT4R agonist) or GR113808 (5-HT4R antagonist) produced antidepressant effects in the lesioned rats. Intra-LHb injection of BIMU-8 significantly increased the DA levels in the medial prefrontal cortex (mPFC) and ventral hippocampus (vHip), increased the 5-HT level in the mPFC and decreased the NA level in the vHip only in the lesioned rats, while intra-LHb injection of GR113808 changed DA, NA and 5-HT levels in the mPFC, LHb and vHip in both sham and the lesioned rats.

CONCLUSIONS

All these results suggest that activation or blockade of the LHb 5-HT4Rs produce antidepressant effects in the 6-OHDA-lesioned rats, which are related to the changes of monoamines in the limbic and limbic-related regions.

Translating the promise of 5HT4 receptor agonists for the treatment of depression

Animal experimental studies suggest that 5-HT4 receptor activation holds promise as a novel target for the treatment of depression and cognitive impairment. 5-HT4 receptors are post-synaptic receptors that are located in striatal and limbic areas known to be involved in cognition and mood. Consistent with this, 5-HT4 receptor agonists produce rapid antidepressant effects in a number of animal models of depression, and pro-cognitive effects in tasks of learning and memory. These effects are accompanied by molecular changes, such as the increased expression of neuroplasticity-related proteins that are typical of clinically useful antidepressant drugs. Intriguingly, these antidepressant-like effects have a fast onset of their action, raising the possibility that 5-HT4 receptor agonists may be a particularly useful augmentation strategy in the early stages of SSRI treatment. Until recently, the translation of these effects to humans has been challenging. Here, we review the evidence from animal studies that the 5-HT4 receptor is a promising target for the treatment of depression and cognitive disorders, and outline a potential pathway for the efficient and cost-effective translation of these effects into humans and, ultimately, to the clinic.

Oral contraceptives and the serotonin 4 receptor: a molecular brain imaging study in healthy women

OBJECTIVE

Sex steroid hormones potently shape brain functions, including those critical to maintain mental health such as serotonin signaling. Use of oral contraceptives (OCs) profoundly changes endogenous sex steroid hormone levels and dynamics. Recent register-based studies show that starting an OC is associated with increased risk of developing depression. Here, we investigate whether use of OCs in healthy women is associated with a marker of the serotonin system in terms of serotonin 4 receptor (5-HT4R) brain imaging.

METHODS

[11C]SB207145-PET imaging data on 53 healthy women, of whom 16 used OCs, were available from the Cimbi database. We evaluated global effects of OC use on 5-HT4R binding in a latent variable model based on 5-HT4R binding across cortical and subcortical regions.

RESULTS

We demonstrate that OC users have 9-12% lower global brain 5-HT4R binding potential compared to non-users. Univariate region-based analyses (pallidostriatum, caudate, hippocampus, amygdala, anterior cingulate cortex, and neocortex) supported the global effect of OC use with the largest difference present in the hippocampus (-12.8% (95% CI [-21.0; -3.9], Pcorrected = 0.03).

CONCLUSION

We show that women who use OCs have markedly lower brain 5-HT4R binding relative to non-users, which constitutes a plausible molecular link between OC use and increased risk of depressive episodes. We propose that this reflects a reduced 5-HT4R gene expression, possibly related to a blunted ovarian hormone state among OC users.

The effect of 5-HT4 serotonin receptors in the CA3 hippocampal region on D-AP5-induced anxiolytic-like effects: Isobolographic analyses

Increasing evidence shows the close relationship between hippocampal glutamatergic and serotonergic systems through the modulation of behavioral responses. This study aimed to investigate the possible involvement of 5-HT₄ receptors in the CA3 hippocampal region in anxiolytic-like effects induced by D-AP5 (a competitive antagonist of the glutamate NMDA [N-Methyl-D-aspartate] receptor). Male Wistar rats were placed in the elevated plus maze (EPM) apparatus that is used to assess anxiety-related behaviors, and the percentages of open arm time (%OAT) and open arm entries (%OAE) which are associated with anxiety-related behaviors were measured. The close arm entries (CAE) which is correlated with locomotor activity was also evaluated. The results showed that, intra-CA3 injection of D-AP5 (0.4 μg/rat), RS67333 (1.2 μg/rat; a 5-HT₄ receptor agonist), and RS23597-190 (1.2 μg/rat; a 5-HT₄ receptor antagonist) increased %OAT and %OAE, indicating the anxiolytic-like effect of these drugs. Also, only RS23597-190 (1.2 μg/rat) decreased CAE. Intra-CA3 injection of sub-threshold dose of RS67333 (0.012 μg/rat) or RS23597-190 (0.012 μg/rat), 5 min before the injection of D-AP5 (0.2 μg/rat) increased %OAT, indicating potentiating the anxiolytic-like effect of D-AP5. The isobolographic analyses also showed the additive or synergistic anxiolytic-like effect of intra-CA3 co-administration of D-AP5 with RS67333 or RS23597-190, respectively. In conclusion, CA3 5-HT₄ receptors are involved in D-AP5-induced anxiolytic-like behaviors in rats.

Central 5-HT receptors and their function; present and future

Since our review of central 5-HT receptors and their function twenty years ago, no new 5-HT receptor has been discovered and there is little evidence that this situation will change in the near future. Nevertheless, over this time significant progress has been made in our understanding of the properties of these receptors and in the clinical translation of this information, and some of these developments are highlighted herein. Such highlights include extensive mapping of 5-HT receptors in both animal and human brain, culminating in readily-accessible brain atlases of 5-HT receptor distribution, as well as emerging data on how 5-HT receptors are distributed within complex neural circuits. Also, a range of important pharmacological and genetic tools have been developed that allow selective 5-HT receptor manipulation, in cells through to whole organism models. Moreover, unexpected complexity in 5-HT receptor function has been identified including agonist-dependent signalling that goes beyond the pharmacology of canonical 5-HT receptor signalling pathways set down in the 1980s and 1990s. This new knowledge of 5-HT signalling has been extended by the discovery of combined signalling of 5-HT and co-released neurotransmitters, especially glutamate. Another important advance has been the progression of a large number of 5-HT ligands through to experimental medicine studies and clinical trials, and some such agents have already become prescribed therapeutic drugs. Much more needs to be discovered and understood by 5-HT neuropharmacologists, not least how the diverse signalling effects of so many 5-HT receptor types interact with complex neural circuits to generate neurophysiological changes which ultimately lead to altered cognitions and behaviour. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Therapeutic potential of serotonin 4 receptor for chronic depression and its associated comorbidity in the gut

The latest estimates from world health organization suggest that more than 450 million people are suffering from depression and other psychiatric conditions. Of these, 50-60% have been reported to have progression of gut diseases. In the last two decades, researchers introduced incipient physiological roles for serotonin (5-HT) receptors (5-HTRs), suggesting their importance as a potential pharmacological target in various psychiatric and gut diseases. A growing body of evidence suggests that 5-HT systems affect the brain-gut axis in depressive patients, which leads to gut comorbidity. Recently, preclinical trials of 5-HT4R agonists and antagonists were promising as antipsychotic and prokinetic agents. In the current review, we address the possible pharmacological role and contribution of 5-HT4R in the pathophysiology of chronic depression and associated gut abnormalities. Physiologically, during depression episodes, centers of the sympathetic and parasympathetic nervous system couple together with neuroendocrine systems to alter the function of hypothalamic-pituitary-adrenal (HPA) axis and enteric nervous system (ENS), which in turn leads to onset of gastrointestinal tract (GIT) disorders. Consecutively, the ENS governs a broad spectrum of physiological activities of gut, such as visceral pain and motility. During the stages of emotional stress, hyperactivity of the HPA axis alters the ENS response to physiological and noxious stimuli. Consecutively, stress-induced flare, swelling, hyperalgesia and altered reflexes in gut eventually lead to GIT disorders. In summary, the current review provides prospective information about the role and mechanism of 5-HT4R-based therapeutics for the treatment of depressive disorder and possible consequences for the gut via brain-gut axis interactions. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Associations Among Monoamine Neurotransmitter Pathways, Personality Traits, and Major Depressive Disorder

Major depressive disorder (MDD) is a complex psychiatric disease requiring multidisciplinary approaches to identify specific risk factors and establish more efficacious treatment strategies. Although the etiology and pathophysiology of MDD are not clear until these days, it is acknowledged that they are almost certainly multifactorial and comprehensive. Monoamine neurotransmitter system dysfunction and specific personality traits are independent risk factors for depression and suicide. These factors also demonstrate complex interactions that influence MDD pathogenesis and symptom expression. In this review, we assess these relationships with the aim of providing a reference for the development of precision medicine.

Alterations of Expression of the Serotonin 5-HT4 Receptor in Brain Disorders

The serotonin 4 receptor, 5-HT₄R, represents one of seven different serotonin receptor families and is implicated in a variety of physiological functions and their pathophysiological variants, such as mood and depression or anxiety, food intake and obesity or anorexia, or memory and memory loss in Alzheimer's disease. Its central nervous system expression pattern in the forebrain, in particular in caudate putamen, the hippocampus and to lesser extent in the cortex, predispose it for a role in executive function and reward-related actions. In rodents, regional overexpression or knockdown in the prefrontal cortex or the nucleus accumbens of 5-HT₄R was shown to impact mood and depression-like phenotypes, food intake and hypophagia; however, whether expression changes are causally involved in the etiology of such disorders is not clear. In this context, more data are emerging, especially based on PET technology and the use of ligand tracers that demonstrate altered 5-HT₄R expression in brain disorders in humans, confirming data stemming from post-mortem tissue and preclinical animal models. In this review, we would like to present the current knowledge of 5-HT₄R expression in brain regions relevant to mood/depression, reward and executive function with a focus on 5-HT₄R expression changes in brain disorders or caused by drug treatment, at both the transcript and protein levels.

Limbic Intrinsic Connectivity in Depressed and High-Risk Youth

OBJECTIVE

Depression runs in families and has been associated with dysfunctional limbic connectivity. Whether aberrant limbic connectivity is a risk factor for or a consequence of depression is unclear. To examine this question, we compared resting state functional connectivity (RSFC) in youth with depressive disorders (DEP), healthy offspring of parents with depression (DEP-risk), and healthy comparison (HC) youth.

METHOD

Magnetic resonance imaging at rest was acquired from 119 youth, aged 8 to 17 years (DEP, n = 41, DEP-risk, n = 39, and HC, n = 39) and analyzed using seed-based RSFC in bilateral amygdala and nucleus accumbens (NAcc), covarying for age, IQ, and sex.

RESULTS

We found distinct risk- and disorder-specific patterns of RSFC across groups. DEP-risk and DEP youth shared reduced negative amygdala-right frontal cortex RSFC and reduced positive amygdala-lingual gyrus RSFC compared to HC youth (p < .001). DEP-risk youth had weaker negative amygdala-precuneus RSFC compared to DEP and HC youth (p < .001), suggesting a resilience marker for depression. In contrast, DEP youth had increased positive NAcc-left frontal cortex RSFC and reduced positive NAcc-insula RSFC compared to DEP-risk and HC youth (p < .001), suggestive of disorder-specific features of depression. Greater depression severity was correlated with disorder-specific amygdala and NAcc RSFC (p < .05).

CONCLUSION

RSFC in the amygdala and NAcc may represent selective disorder- and risk-specific markers in youth with, and at familial risk for, depression. Longitudinal studies are needed to determine whether these patterns predict long-term clinical outcomes.

CK2 regulates 5-HT4 receptor signaling and modulates depressive-like behavior

The serotonergic neurotransmitter system has been widely implicated in the pathophysiology of mood-related disorders such as anxiety and major depressive disorder (MDD). The onset of therapeutic efficacy of traditional antidepressants is delayed by several weeks. The 5-HT₄ receptor has emerged as a new therapeutic target since agonists of this receptor induce rapid antidepressant-like responses in rodents. Here we show that the 5-HT₄ receptor is regulated by CK2, at transcriptional and post-transcriptional levels. We present evidence, in two different CK2α knockout mouse lines, that this regulation is region-specific, with the 5-HT₄ receptor upregulated in prefrontal cortex (PFC) but not striatum or hippocampus where CK2α is also ablated. 5-HT₄ receptor signaling is enhanced in vitro, as evidenced by enhanced cAMP production or receptor plasma membrane localization in the presence of CK2 inhibitor or shRNA targeting CK2α. In vivo, 5-HT₄ receptor signaling is also upregulated since ERK activation is elevated and sensitive to the inverse agonist, GR113808 in the PFC of CK2α KO mice. Behaviorally, KO mice as well as mice with AAV-mediated deletion of CK2α in the PFC show a robust 'anti-depressed-like' phenotype and display an enhanced response to antidepressant treatment when tested in paradigms for mood and anxiety. Importantly, it is sufficient to overexpress the 5-HT₄ receptor in the mPFC to generate mice with a similar 'anti-depressed-like' phenotype. Our findings identify the mPFC as the region that mediates the effect of enhanced 5-HT₄ receptor activity and CK2 as modulator of 5-HT4 receptor levels in this brain region that regulates mood-related phenotypes.

High brain serotonin levels in migraine between attacks: A 5-HT4 receptor binding PET study

Migraine has been hypothesized to be a syndrome of chronic low serotonin (5-HT) levels, but investigations of brain 5-HT levels have given equivocal results. Here, we used positron emission tomography (PET) imaging of the 5-HT4 receptor as a proxy for brain 5-HT levels. Given that the 5-HT4 receptor is inversely related to brain 5-HT levels, we hypothesized that between attacks migraine patients would have higher 5-HT4 receptor binding compared to controls. Eighteen migraine patients without aura (migraine free >48 h), and 16 age- and sex-matched controls underwent PET scans after injection of [11C]SB207145, a specific 5-HT4 receptor radioligand. An investigator blinded to group calculated a neocortical mean [11C]SB207145 binding potential (BPND). Three migraine patients reported a migraine attack within 48 h after the scan and were excluded from the primary analysis. Comparing 15 migraine patients and 16 controls, we found that migraine patients have significantly lower neocortical 5-HT4 receptor binding than controls (0.60 ± 0.09 vs. 0.67 ± 0.05, p = .024), corrected for 5-HTTLPR genotype, sex and age. We found no association between 5-HT4 receptor binding and attack frequency, years with migraine or time since last migraine attack. Our finding of lower 5-HT4 receptor binding in migraine patients is suggestive of higher brain 5-HT levels. This is in contrast with the current belief that migraine is associated with low brain 5-HT levels. High brain 5-HT levels may represent a trait of the migraine brain or it could be a consequence of migraine attacks.

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.

The role of 5-HT receptors in depression

Depression is a polygenic and highly complex psychiatric disorder that remains a major burden on society. Antidepressants, such as selective serotonin reuptake inhibitors (SSRIs), are some of the most commonly prescribed drugs worldwide. In this review, we will discuss the evidence that links serotonin and serotonin receptors to the etiology of depression and the mechanisms underlying response to antidepressant treatment. We will then revisit the role of serotonin in three distinct hypotheses that have been proposed over the last several decades to explain the pathophysiology of depression: the monoamine, neurotrophic, and neurogenic hypotheses. Finally, we will discuss how recent studies into serotonin receptors have implicated specific neural circuitry in mediating the antidepressant response, with a focus being placed on the hippocampus.

Brain serotonin 4 receptor binding is inversely associated with verbal memory recall

BACKGROUND

We have previously identified an inverse relationship between cerebral serotonin 4 receptor (5-HT ₄R) binding and nonaffective episodic memory in healthy individuals. Here, we investigate in a novel sample if the association is related to affective components of memory, by examining the association between cerebral 5-HT ₄R binding and affective verbal memory recall.

METHODS

Twenty-four healthy volunteers were scanned with the 5-HT ₄R radioligand [¹¹C]SB207145 and positron emission tomography, and were tested with the Verbal Affective Memory Test-24. The association between 5-HT ₄R binding and affective verbal memory was evaluated using a linear latent variable structural equation model.

RESULTS

We observed a significant inverse association across all regions between 5-HT ₄R binding and affective verbal memory performances for positive (p = 5.5 × 10^-4) and neutral (p = .004) word recall, and an inverse but nonsignificant association for negative (p = .07) word recall. Differences in the associations with 5-HT ₄R binding between word categories (i.e., positive, negative, and neutral) did not reach statistical significance.

CONCLUSION

Our findings replicate our previous observation of a negative association between 5-HT ₄R binding and memory performance in an independent cohort and provide novel evidence linking 5-HT ₄R binding, as a biomarker for synaptic 5-HT levels, to the mnestic processing of positive and neutral word stimuli in healthy humans.

Type A monoamine oxidase and serotonin are coordinately involved in depressive disorders: from neurotransmitter imbalance to impaired neurogenesis

Type A monoamine oxidase (MAOA) catabolizes monoamine transmitters, serotonin, norepinephrine and dopamine, and plays a major role in the onset, progression and therapy of neuropsychiatric disorders. In depressive disorders, increase in MAOA expression and decrease in brain levels of serotonin and norepinephrine are proposed as the major pathogenic factors. The functional polymorphism of MAOA gene and genes in serotonin signal pathway are associated with depression. This review presents recent advance in studies on the role of MAOA in major depressive disorder and related emotional disorders. MAOA and serotonin regulate the prenatal development and postnatal maintenance of brain architecture and neurocircuit, as shown by MAOA-deficient humans and MAO knockout animal models. Impaired neurogenesis in the mature hippocampus has been proposed as "adult neurogenesis" hypothesis of depression. MAOA modulates the sensitivity to stress in the stages of brain development and maturation, and the interaction of gene-environmental factors in the early stage regulates the onset of depressive behaviors in adulthood. Vice versa environmental factors affect MAOA expression by epigenetic regulation. MAO inhibitors not only restore compromised neurotransmitters, but also protect neurons from cell death in depression through induction of anti-apoptotic Bcl-2 and prosurvival neurotrophic factors, especially brain-derived neurotrophic factor, the deficiency of which is detected in depression. This review discusses novel role of MAOA and serotonin in the pathogenesis and therapy of depressive disorders.

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.