The 5-HT deficiency theory of depression: perspectives from a naturalistic 5-HT deficiency model, the tryptophan hydroxylase 2Arg439His knockin mouse

MDGA1-deficiency attenuates prepulse inhibition with alterations of dopamine and serotonin metabolism: An ex vivo HPLC-ECD analysis

MDGA1 (MAM domain-containing glycosylphosphatidylinositol anchor) has recently been linked to schizophrenia and bipolar disorder. Dysregulation of dopamine (DA) and serotonin (5-HT) systems has long been associated with schizophrenia and other neuropsychiatric disorders. Here, we measured prepulse inhibition (PPI) of the startle response and ex vivo tissue content of monoamines and their metabolites in the frontal cortex, striatum and hippocampus of Mdga1 homozygous (Mdga1-KO), Mdga1 heterozygous (Mdga1-HT) and wild-type (WT) male mice. We found that Mdga1-KO mice exhibited statistically significant impairment of PPI, and had higher levels of homovanillic acid in all three brain regions studied compared with Mdga1-HT and WT mice (P < 0.05), while levels of norepinephrine, DA and its metabolites 3,4-dihydroxyphenylacetic acid and 3-methoxytyramine remained unchanged. Mdga1-KO mice also had a lower 5-hydroxyindoleacetic acid level in the striatum (P < 0.05) compared with WT mice. 5-HT levels remained unchanged with the exception of a significant increase in the level in the cortex. These data are the first evidence suggesting that MDGA1 deficiency leads to a pronounced deficit in PPI and plays an important role in perturbation of DA and 5-HT metabolism in mouse brain; such changes may contribute to a range of neuropsychiatric disorders.

Naringenin protects AlCl3/D-galactose induced neurotoxicity in rat model of AD via attenuation of acetylcholinesterase levels and inhibition of oxidative stress

Currently prescribed medications for the treatment of Alzheimer's disease (AD) that are based on acetylcholinesterase inhibition only offer symptomatic relief but do not provide protection against neurodegeneration. There appear to be an intense need for the development of therapeutic strategies that not only improve brain functions but also prevent neurodegeneration. The oxidative stress is one of the main causative factors of AD. Various antioxidants are being investigated to prevent neurodegeneration in AD. The objective of this study was to investigate the neuroprotective effects of naringenin (NAR) against AlCl3+D-gal induced AD-like symptoms in an animal model. Rats were orally pre-treated with NAR (50 mg/kg) for two weeks and then exposed to AlCl3+D-gal (150 mg/kg + 300 mg/kg) intraperitoneally for one week to develop AD-like symptoms. The standard drug, donepezil (DPZ) was used as a stimulator of cholinergic activity. Our results showed that NAR pre-treatment significantly protected AD-like behavioral disturbances in rats. In DPZ group, rats showed improved cognitive and cholinergic functions but the neuropsychiatric functions were not completely improved and showed marked histopathological alterations. However, NAR not only prevented AlCl3+D-gal induced AD-like symptoms but also significantly prevented neuropsychiatric dysfunctions in rats. Results of present study suggest that NAR may play a role in enhancing neuroprotective and cognition functions and it can potentially be considered as a neuroprotective compound for therapeutic management of AD in the future.

Correlations of serum leptin and leptin resistance with depression and anxiety in patients with type 2 diabetes

AIM

The associations of serum leptin/soluble leptin receptor (sObR) and leptin resistance with symptoms of depression and anxiety were investigated in patients with type 2 diabetes (T2D).

METHODS

We report the results of two cross-sectional studies, performed 2 years apart, that included 216 and 237 T2D patients, respectively. Symptoms of depression and anxiety were assessed with specific questionnaires (Patient Health Questionnaire-9, Center for Epidemiologic Studies Depression Scale, and Generalized Anxiety Disorder-7, respectively). Laboratory data (including leptin and sObR) were collected, and free leptin index (FLI), as an estimate of leptin resistance, was calculated. One hundred forty patients had laboratory data available on both occasions, and were evaluated longitudinally. Simple and multiple correlations between depression/anxiety and parameters of interest were performed.

RESULTS

In both studies, serum leptin levels were higher, whereas resting energy expenditure/leptin ratios were lower in T2D patients with depressive and moderate-severe anxiety symptoms. In the second study, patients with depressive symptoms had higher FLI and lower sObR levels, while those with moderate-severe anxiety only had higher FLI. Depression scores correlated with serum leptin (r = 0.29, [95%CI: 0.14-0.42]; r = 0.32, [95%CI: 0.18-0.45]) and FLI (r = 0.30, [95%CI: 0.15-0.43]; r = 0.32, [95%CI: 0.17-0.45]; P < 0.0001 for all). Multiple regression analyses identified leptin (β = 0.167; t ratio = 1.98) and FLI (β = 2.935, t ratio = 2.44) (P < 0.05 for both) as variables that significantly contributed to depressive symptoms. Depressive symptoms were present in significantly more patients with leptin levels in the highest versus the lowest quartiles on both evaluations (odds ratio: 5.98, 95%CI [1.76-20.32], P < 0.01).

CONCLUSION

Depressive and moderate-severe anxiety symptoms were associated with high leptin concentrations and leptin resistance in T2D patients.

Slow-release delivery enhances the pharmacological properties of oral 5-hydroxytryptophan: mouse proof-of-concept

5-hydroxytryptophan (5-HTP) has shown therapeutic promise in a range of human CNS disorders. But native 5-HTP immediate release (IR) is poorly druggable, as rapid absorption causes rapid onset of adverse events, and rapid elimination causes fluctuating exposure. Recently, we reported that 5-HTP delivered as slow-release (SR) in mice augmented the brain pro-serotonergic effect of selective serotonin reuptake inhibitors (SSRIs), without the usual adverse events associated with 5-HTP IR. However, our previous study entailed translational limitations, in terms of route, dose, and duration. Here we modeled oral 5-HTP SR in mice by administering 5-HTP via the food. We modeled oral SSRI treatment via fluoxetine in the water, in a regimen recapitulating clinical pharmacokinetics and pharmacodynamics. 5-HTP SR produced plasma 5-HTP levels well within the range enhancing brain 5-HT function in humans. 5-HTP SR robustly increased brain 5-HT synthesis and levels. When administered with an SSRI, 5-HTP SR enhanced 5-HT-sensitive behaviors and neurotrophic mRNA expression. 5-HTP SR's pro-serotonergic effects were stronger in mice with endogenous brain 5-HT deficiency. In a comprehensive screen, 5-HTP SR was devoid of overt toxicological effects. The present preclinical data, appreciated in the context of published 5-HTP clinical data, suggest that 5-HTP SR could represent a new therapeutic approach to the plethora of CNS disorders potentially treatable with a pro-serotonergic drug. 5-HTP SR might in particular be therapeutically relevant when brain 5-HT deficiency is pathogenic and as an adjunctive augmentation therapy to SSRI therapy.

Role of Estradiol in the Expression of Genes Involved in Serotonin Neurotransmission: Implications for Female Depression

BACKGROUND

In women, changes in estrogen levels may increase the incidence and/or symptomatology of depression and affect the response to antidepressant treatments. Estrogen therapy in females may provide some mood benefits as a single treatment or might augment clinical response to antidepressants that inhibit serotonin reuptake.

OBJECTIVE

We analyzed the mechanisms of estradiol action involved in the regulation of gene expression that modulates serotonin neurotransmission implicated in depression.

METHOD

Publications were identified by a literature search on PubMed.

RESULTS

The participation of estradiol in depression may include regulation of the expression of tryptophan hydroxylase-2, monoamine oxidase A and B, serotonin transporter and serotonin-1A receptor. This effect is mediated by estradiol binding to intracellular estrogen receptor that interacts with estrogen response elements in the promoter sequences of tryptophan hydroxylase-2, serotonin transporter and monoamine oxidase-B. In addition to directly binding deoxyribonucleic acid, estrogen receptor can tether to other transcription factors, including activator protein 1, specificity protein 1, CCAAT/enhancer binding protein β and nuclear factor kappa B to regulate gene promoters that lack estrogen response elements, such as monoamine oxidase-A and serotonin 1A receptor.

CONCLUSION

Estradiol increases tryptophan hydroxylase-2 and serotonin transporter expression and decreases the expression of serotonin 1A receptor and monoamine oxidase A and B through the interaction with its intracellular receptors. The understanding of molecular mechanisms of estradiol regulation on the protein expression that modulates serotonin neurotransmission will be helpful for the development of new and more effective treatment for women with depression.

Effects of Serotonin and Slow-Release 5-Hydroxytryptophan on Gastrointestinal Motility in a Mouse Model of Depression

BACKGROUND & AIMS

Mood disorders and constipation are often comorbid, yet their shared etiologies have rarely been explored. The neurotransmitter serotonin (5-HT) regulates central nervous system and enteric nervous system (ENS) development and long-term functions, including gastrointestinal (GI) motility and mood. Therefore, defects in neuron production of 5-HT might result in brain and intestinal dysfunction. Tryptophan hydroxylase 2 (TPH2) is the rate-limiting enzyme in 5-HT biosynthesis. A variant of TPH2 that encodes the R441H substitution (TPH2-R441H) was identified in individuals with severe depression. We studied mice with an analogous mutation (TPH2-R439H), which results in a 60%-80% decrease in levels of 5-HT in the central nervous system and behaviors associated with depression in humans. Feeding chow that contains 5-HTP slow release (5-HTP SR) to TPH2-R439H mice restores levels of 5-HT in the central nervous system and reduces depressive-like behaviors.

METHODS

We compared the effects of feeding chow, with or without 5-HTP SR, to mice with the TPH2-R439H mutation and without this mutation (control mice). Myenteric and submucosal plexuses were isolated from all 4 groups of mice, and immunocytochemistry was used to quantify total enteric neurons, serotonergic neurons, and 5-HT-dependent subsets of neurons. We performed calcium imaging experiments to evaluate responses of enteric neurons to tryptamine-evoked release of endogenous 5-HT. In live mice, we measured total GI transit, gastric emptying, small intestinal transit, and propulsive colorectal motility. To measure colonic migrating motor complexes (CMMCs), we isolated colons and constructed spatiotemporal maps along the proximodistal length to quantify the frequency, velocity, and length of CMMCs. We measured villus height, crypt perimeter, and relative densities of enterochromaffin and enteroendocrine cells in small intestinal tissue.

RESULTS

Levels of 5-HT were significantly lower in enteric neurons from TPH2-R439H mice than from control mice. TPH2-R439H mice had abnormalities in ENS development and ENS-mediated GI functions, including reduced motility and intestinal epithelial growth. Total GI transit and propulsive colorectal motility were slower in TPH2-R439H mice than controls, and CMMCs were slower and less frequent. Villus height and crypt perimeter were significantly decreased in colon tissues from TPH2-R439H mice compared with controls. Administration of 5-HTP SR to adult TPH2-R439H mice restored 5-HT to enteric neurons and reversed these abnormalities. Adult TPH2-R439H mice given oral 5-HTP SR had normalized numbers of enteric neurons, total GI transit, and colonic motility. Intestinal tissue from these mice had normal measures of CMMCs and enteric epithelial growth CONCLUSIONS: In studies of TPH2-R439H mice, we found evidence for reduced release of 5-HT from enteric neurons that results in defects in ENS development and GI motility. Our findings indicate that neuron production of 5-HT links constipation with mood dysfunction. Administration of 5-HTP SR to mice restored 5-HT to the ENS and normalized GI motility and growth of the enteric epithelium. 5-HTP SR might be used to treat patients with intestinal dysfunction associated with low levels of 5-HT.

Tryptophan hydroxylase 2 as a therapeutic target for psychiatric disorders: focus on animal models

Introduction: Tryptophan hydroxylase 2 (TPH2) is the key, rate-limiting enzyme of serotonin (5-HT) synthesis in the brain. Some polymorphic variants of the human Tph2 gene are associated with psychiatric disorders. Area covered: This review focuses on the mechanisms underlying the association between the TPH2 activity and behavioral disturbances in models of psychiatric disorders. Specifically, it discusses: 1) genetic and posttranslational mechanisms defining the TPH2 activity, 2) behavioral effects of knockout and loss-of-function mutations in the mouse Tph2 gene, 3) pharmacological inhibition and the activation of the TPH2 activity and 4) alterations in the brain TPH2 activity in animal models of psychiatric disorders. We show the dual role of the TPH2 activity: both deficit and excess of the TPH2 activity cause significant behavioral disturbances in animal models of depression, anxiety, aggression, obsessive-compulsive disorders, schizophrenia, and catalepsy. Expert opinion: Pharmacological chaperones correcting the structure of the TPH2 molecule are promising tools for treatment of some hereditary psychiatric disorders caused by loss-of-function mutations in the human Tph2 gene; while some stress-induced affective disorders, associated with the elevated TPH2 activity, may be effectively treated by TPH2 inhibitors. This dual role of TPH2 should be taken into consideration during therapy of psychiatric disorders.

Ingestion of Bifidobacterium longum subspecies infantis strain CCFM687 regulated emotional behavior and the central BDNF pathway in chronic stress-induced depressive mice through reshaping the gut microbiota

Probiotics which enhance the biosynthesis of 5-hydroxytryptamine in enterochromaffin cells could alleviate depression symptoms through regulating the CREB-BDNF pathway in the brain.

Pharmacological effects of berberine on mood disorders

Berberine, a natural isoquinoline alkaloid, is used in herbal medicine and has recently been shown to have efficacy in the treatment of mood disorders. Furthermore, berberine modulates neurotransmitters and their receptor systems within the central nervous system. However, the detailed mechanisms of its action remain unclear. This review summarizes the pharmacological effects of berberine on mood disorders. Therefore, it may be helpful for potential application in the treatment of mood disorders.

Omega-3 fatty acids and anxiety: A systematic review of the possible mechanisms at play

Introduction: Anxiety is prevalent, costly, and associated with significant adverse outcomes. The importance of nutrition is underestimated in the management of mental health disorders. In particular, omega-3 fatty acids (ω-3 FAs) are a critical component for healthy development and have been shown to reduce anxiety symptoms. Objective: This paper reviews the current state of the research to identify potential mechanisms underlying the relationship between ω-3 FAs and anxiety reduction. Method: Studies were identified using PubMed, PsycINFO, and CINAHL databases. Results: Of the 197 full-text studies screened, six met criteria for inclusion. Four mechanisms were identified based on primary outcomes reported by each study, Inflammatory Response, Brain-Derived Neurotrophic Factor (BDNF), Cortisol, and Cardiovascular Activity. Conclusion: Five key recommendations are provided to guide future research examining ω-3 FAs and anxiety. They include: (1) standardization of dosage and duration of ω-3 supplementation, (2) more rigorous measurement of variables, (3) effective blinding of participants, (4) designing experiments that test mediation, and (5) increasing sample diversity.

The hypothalamic-pituitary-thyroid axis and personality in a sample of healthy subjects

Thyroid hormones influence various brain pathologies, including psychiatric disorders. However, the relationship between these hormones and the psychological state in the normal, non-clinical population is poorly understood. We aimed to test whether serum levels of thyroid hormones are associated with personality in the healthy population. Thyrotropin (TSH), free T3 (FT4), total and free T4 (TT4 and FT4) concentrations were measured in the blood of 104 healthy participants (44% men) aged 18-59 (M=35±9). Personality traits were assessed using the revised short versions of the Temperament and Character Inventory (TCI-140) and the NEO Five-Factor Inventory (NEO-FFI). The data were analysed by correlational, regression, extreme groups and graphical techniques, which showed significant correlations between inter-individual variations in serum thyroid hormone levels and specific aspects of personality. In particular, high serum TSH was strongly associated with higher Persistence and Self-Directedness, and negatively correlated with Harm Avoidance on the TCI-140 and Neuroticism on the NEO-FFI, thus representing a more adaptive personality profile. Furthermore, increased FT4 was associated with lower Reward-Dependence, and increased TT4 was associated with lower Cooperativeness, representing a deficit in social attachment. Our data indicate that the relationship between thyroid hormones and personality in the healthy population might be rather more complex than the results obtained in clinical samples suggest.

Changes in PPARδ expression in a rat model of stress-induced depression

Depression is a common mental disorder that has been linked to a decrease in the expression of serotonin and/or the serotonin transporter in the brain. Antidepressants that target the monoaminergic system are widely used in the clinical setting. Peroxisome proliferator-activated receptor δ (PPAR δ) overexpression or activation is thought to improve depression-like behaviours in rodents. The present study was designed to characterize the changes in PPARδ expression in the hippocampus in rats with stress-induced depression. We used an unpredictable chronic mild stress (CMS) model in rats to study the role of PPARδ in the hippocampus. Behaviour was evaluated via a forced swim test (FST), a tail suspension test (TST), and a sucrose preference test (SPT). Then, the changes in PPARδ expression and other signals were determined using Western blots. We found that PPARδ expression in the hippocampus was markedly reduced in rats with depression. Moreover, the expression of the serotonin transporter was also significantly decreased. Treatment with a PPARδ agonist enhanced the expression of PPARδ and the serotonin transporter in the hippocampus of rats with stress-induced depression. Additionally, treatment with a PPARδ agonist increased the expression of the serotonin transporter in cultured hippocampal (H19-7) cells, and this action was ablated in the absence of PPARδ, which was attenuated with shRNA. Taken together, we found that PPARδ plays an important role in the regulation of serotonin transporter expression and that chronic stress may lower PPARδ expression in the brain via apoptosis and may attenuate serotonin transporter expression, thus inducing depression in rats.

Air pollution, aeroallergens and suicidality: a review of the effects of air pollution and aeroallergens on suicidal behavior and an exploration of possible mechanisms

OBJECTIVE

Risk factors for suicide can be broadly categorized as sociodemographic, clinical and treatment. There is interest in environmental risk and protection factors for suicide. Emerging evidence suggests a link between environmental factors in the form of air pollution and aeroallergens in relation to suicidality.

METHODS

Herein, we conducted a systematic review of 15 articles which have met inclusion criteria on the aforementioned effects.

RESULTS

The majority of the reviewed articles reported an increased suicide risk alongside increased air pollutants or aeroallergens (i.e. pollen) increase; however, not all environmental factors were explored equally. In specific, studies that were delimited to evaluating particulate matter (PM) reported a consistent association with suicidality. We also provide a brief description of putative mechanisms (e.g. inflammation and neurotransmitter dysregulation) that may mediate the association between air pollution, aeroallergens and suicidality.

CONCLUSION

Available evidence suggests that exposure to harmful air quality may be associated with suicidality. There are significant public health implications which are amplified in regions and countries with greater levels of air pollution and aeroallergens. In addition, those with atopic sensitivity may represent a specific subgroup that is at risk.

Decreased brain serotonin turnover rate following administration of Sharbat-e-Ahmed Shah produces antidepressant and anxiolytic effect in rats

Sharbat-e-Ahmed Shah (SAS) has usually been used in Traditional Unani Medicine (TUM) for depression and insomnia but still not evaluated for its anti-depressant and Neuropharmacological activity. In the present study, a Human dose of SAS (0.6 ml/kg/d) was administered orally to the rats for 15 consecutive days. Antidepressant and anxiolytic were screened scientifically in rats by using Forced swim test and light and dark box test. At the end of study high-performance liquid chromatographic (HPLC) method with electrochemical (EC) detector was used for the measurement of blood and brain tryptophan and brain serotonin levels. The present reported results are according to what is known in TUM, where is prescribed as an antidepressant agent. After the administration, SAS (at a human dose for 15 days) reduced the immobility time in rats analogous to Imipramine (positive control) indicating the antidepressant effect of SAS. In the present study, Diazepam or SAS (0.6 ml/kg/day) treated rats stayed in the illuminated side of the light-dark box, as compare to control rats (Veh, 134.62 ± 4.430 s; SAS 0.6 ml/kg, 192.2 ± 8.11 s; DZP 1.0 mg/kg, 205.21.20 ± 10.26 s, p < 0.05). It was also observed that SAS increased the availability of tryptophan in blood and brain and hence increases 5-hydroxytryptamine (Serotonin: 5HT) in the brain. At the end, it was concluded that SAS contains some active principles which increase the availability of neurochemical (tryptophan and 5HT) and decrease the 5HT turnover rate thus causes antidepressant and anxiolytic effects in experimental animals.

Computational and biological evidences on the serotonergic involvement of SeTACN antidepressant-like effect in mice

A series of phenylselanyl-1H-1,2,3-triazole-4-carbonitriles with different substituents were screened for their binding affinity with serotonin transporter (SERT) and dopamine transporter (DAT) by docking molecular. 5-(4methoxyphenyl)-1-(2-(phenylselanyl)phenyl)-1H-1,2,3-triazole-4-carbonitrile (SeTACN) exhibited the best conformation with SERT even higher than fluoxetine and serotonin, suggesting a competitive inhibition. SeTACN demonstrated additional affinity to other serotonergic receptors involved in antidepressant effects: 5HT_1a, 5HT_2a and 5HT₃. In another set of experiments, SeTACN led to significant reductions in the immobility time of mice submitted to forced swimming test (FST) in the dose range of 0.1- 20mg/kg, suggesting an antidepressant-like effect. The possible mechanism of action was investigated using serotonergic and dopaminergic antagonists. The antidepressant-like effect of SeTACN (0.1mg/kg i.g.) was prevented by the pretreatment with WAY100635 (a selective 5HT_1a antagonist), ketanserin (a 5HT2_a/c antagonist) and ondansetron (a selective 5ht₃ antagonist), PCPA (an inhibitor of serotonin synthesis) but not with SCH23390 (dopaminergic D₁ antagonist) and sulpiride (D₂ antagonist). Sub-effective dose of fluoxetine was able to potentiate the effects of a sub-effective dose of SeTACN in FST. None of the treatments affected locomotor activity in open field test (OFT). These results together, suggest that the SeTACN antidepressant-like effect is mediate, at least in parts, by serotonergic system.

An Update on the Role of Serotonin and its Interplay with Dopamine for Reward

The specific role of serotonin and its interplay with dopamine (DA) in adaptive, reward guided behavior as well as drug dependance, still remains elusive. Recently, novel methods allowed cell type specific anatomical, functional and interventional analyses of serotonergic and dopaminergic circuits, promising significant advancement in understanding their functional roles. Furthermore, it is increasingly recognized that co-release of neurotransmitters is functionally relevant, understanding of which is required in order to interpret results of pharmacological studies and their relationship to neural recordings. Here, we review recent animal studies employing such techniques with the aim to connect their results to effects observed in human pharmacological studies and subjective effects of drugs. It appears that the additive effect of serotonin and DA conveys significant reward related information and is subjectively highly euphorizing. Neither DA nor serotonin alone have such an effect. This coincides with optogenetically targeted recordings in mice, where the dopaminergic system codes reward prediction errors (PE), and the serotonergic system mainly unsigned PE. Overall, this pattern of results indicates that joint activity between both systems carries essential reward information and invites parallel investigation of both neurotransmitter systems.

TSH cut off point based on depression in hypothyroid patients

BACKGROUND

The prevalence of depressive symptoms in hypothyroidism is high. Considering that hypothyroidism and depression share some clinical features, some researchers use the "brain hypothyroidism" hypothesis to explain the pathogenesis of depression. We aimed to detect a new TSH cut-off value in hypothyroidism based on depression symptoms.

METHODS

A cross-sectional study was conducted on hypothyroid patients referred to endocrine clinics. Individuals who had developed euthyroid state under treatment with levothyroxine with TSH levels of 0.5-5 MIU/L with no need for dosage change were included in the study. After comprehensive history taking, laboratory tests including TSH, T4 and T3 were performed. Beck depression questionnaire was completed for all patients by trained interviewers. TSH cut-off values based on depression was determined by Roc Curve analysis.

RESULTS

The participants were 174 hypothyroid patients (Female; 116: 66.7%, Male; 58: 33.3%) with mean age 45.5 ± 11.7 (19-68) years old. Based on Beck depression test, scores less than 10 was considered healthy and more than 10 were considered depressed. According to Roc curve analysis, the optimal cut- off value of TSH was 2.5 MIU/L with 89.66% sensitivity. The optimal TSH cut- off based on severe depression was 4 MIU/L.

CONCLUSION

The present study suggests that a clinically helpful TSH cut-off value for hypothyroidism should be based on associated symptoms, not just in population studies. Based on the assessment of depression, our study concludes that a TSH cutofff value of 2.5 MIU/L is optimal.

Disconnectivity between Dorsal Raphe Nucleus and Posterior Cingulate Cortex in Later Life Depression

The dorsal raphe nucleus (DRN) has been repeatedly implicated as having a significant relationship with depression, along with its serotoninergic innervation. However, functional connectivity of the DRN in depression is not well understood. The current study aimed to isolate functional connectivity of the DRN distinct in later life depression (LLD) compared to a healthy age-matched population. Resting state functional magnetic resonance imaging (rsfMRI) data from 95 participants (33 LLD and 62 healthy) were collected to examine functional connectivity from the DRN to the whole brain in voxel-wise fashion. The posterior cingulate cortex (PCC) bilaterally showed significantly smaller connectivity in the LLD group than the control group. The DRN to PCC connectivity did not show any association with the depressive status. The findings implicate that the LLD involves disruption of serotoninergic input to the PCC, which has been suggested to be a part of the reduced default mode network in depression.

miR-16 and Fluoxetine Both Reverse Autophagic and Apoptotic Change in Chronic Unpredictable Mild Stress Model Rats

In the clinic selective serotonin reuptake inhibitors (SSRIs), like Fluoxetine, remain the primary treatment for major depression. It has been suggested that miR-16 regulates serotonin transporters (SERT) via raphe nuclei and hippocampal responses to antidepressants. However, the underlying mechanism and regulatory pathways are still obtuse. Here, a chronic unpredicted mild stress (CUMS) depression model in rats was established, and then raphe nuclei miR-16 and intragastric Fluoxetine injections were administered for a duration of 3 weeks. An open field test and sucrose preference quantification displayed a significant decrease in the CUMS groups when compare to the control groups, however these changes were attenuated by both miR-16 and Fluoxetine treatments. A dual-luciferase reporter assay system verified that hsa-miR-16 inhibitory effects involve the targeting of 3′UTR on the 5-HTT gene. Expression levels of miR-16 and BDNF in the hippocampus were examined with RT-PCR, and it was found that increased 5-HT2a receptor expression induced by CUMS can be decreased by miR-16 and Fluoxetine administration. Immunofluorescence showed that expression levels of neuron NeuN and MAP-2 in CUMS rats were lower. Apoptosis and autophagy levels were evaluated separately through relative expression of Bcl-2, Caspase-3, Beclin-1, and LC3II. Furthermore, CUMS was found to decrease levels of hippocampal mTOR, PI3K, and AKT. These findings indicate that apoptosis and autophagy related pathways could be involved in the effectiveness of antidepressants, in which miR-16 participates in the regulation of, and is likely to help integrate rapid therapeutic strategies to alleviate depression clinically. These findings indicate that miR-16 participates in the regulation of apoptosis and autophagy and could account for some part of the therapeutic effect of SSRIs. This discovery has the potential to further the understanding of SSRIs and accelerate the development of new treatments for depression.

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.

Constitutive and Acquired Serotonin Deficiency Alters Memory and Hippocampal Synaptic Plasticity

Serotonin (5-HT) deficiency occurs in a number of brain disorders that affect cognitive function. However, a direct causal relationship between 5-HT hypo-transmission and memory and underlying mechanisms has not been established. We used mice with a constitutive depletion of 5-HT brain levels (Pet1KO mice) to analyze the contribution of 5-HT to different forms of learning and memory. Pet1KO mice exhibited a striking deficit in novel object recognition memory, a hippocampal-dependent task. No alterations were found in tasks for social recognition, procedural learning, or fear memory. Viral delivery of designer receptors exclusively activated by designer drugs was used to selectively silence the activity of 5-HT neurons in the raphe. Inhibition of 5-HT neurons in the median raphe, but not the dorsal raphe, was sufficient to impair object recognition in adult mice. In vivo electrophysiology in behaving mice showed that long-term potentiation in the hippocampus of 5-HT-deficient mice was altered, and administration of the 5-HT1A agonist 8-OHDPAT rescued the memory deficits. Our data suggest that hyposerotonergia selectively affects declarative hippocampal-dependent memory. Serotonergic projections from the median raphe are necessary to regulate object memory and hippocampal synaptic plasticity processes, through an inhibitory control mediated by 5-HT1A receptors.

Dynamic SERS nanosensor for neurotransmitter sensing near neurons

Current electrophysiology and electrochemistry techniques have provided unprecedented understanding of neuronal activity. However, these techniques are suited to a small, albeit important, panel of neurotransmitters such as glutamate, GABA and dopamine, and these constitute only a subset of the broader range of neurotransmitters involved in brain chemistry. Surface-enhanced Raman scattering (SERS) provides a unique opportunity to detect a broader range of neurotransmitters in close proximity to neurons. Dynamic SERS (D-SERS) nanosensors based on patch-clamp-like nanopipettes decorated with gold nanoraspberries can be located accurately under a microscope using techniques analogous to those used in current electrophysiology or electrochemistry experiments. In this manuscript, we demonstrate that D-SERS can measure in a single experiment ATP, glutamate (glu), acetylcholine (ACh), GABA and dopamine (DA), among other neurotransmitters, with the potential for detecting a greater number of neurotransmitters. The SERS spectra of these neurotransmitters were identified with a barcoding data processing method and time series of the neurotransmitter levels were constructed. The D-SERS nanosensor was then located near cultured mouse dopaminergic neurons. The detection of neurotransmitters was performed in response to a series of K⁺depolarisations, and allowed the detection of elevated levels of both ATP and dopamine. Control experiments were also performed near glial cells, showing only very low basal detection neurotransmitter events. This paper demonstrates the potential of D-SERS to detect neurotransmitter secretion events near living neurons, but also constitutes a strong proof-of-concept for the broad application of SERS to the detection of secretion events by neurons or other cell types in order to study normal or pathological cell functions.

Adjunctive 5-Hydroxytryptophan Slow-Release for Treatment-Resistant Depression: Clinical and Preclinical Rationale

Serotonin transporter (SERT) inhibitors treat depression by elevating brain extracellular 5-hydroxytryptamine (5-HT_Ext). However, only one-third of patients respond adequately. Treatment-resistant depression (TRD) is a major unmet need. Interestingly, elevating 5-HT_Ext beyond what is achieved by a SERT inhibitor appears to treat TRD. Adjunctive administration of 5-hydroxytryptophan (5-HTP) safely elevates 5-HT_Ext beyond the SERT inhibitor effect in humans; however, 5-HTP cannot be a clinically viable drug because of its poor pharmacokinetics. A slow-release (SR) delivery mode would be predicted to overcome the pharmacokinetic limitations of 5-HTP, substantially enhancing the pharmacological action and transforming 5-HTP into a clinically viable drug. Animal studies bear out this prediction. Thus, adjunct 5-HTP SR could be an important new treatment for TRD. Here, we review the clinical and preclinical evidence for this treatment.

Sex-dependent adaptive changes in serotonin-1A autoreceptor function and anxiety in Deaf1-deficient mice

The C (-1019) G rs6295 promoter polymorphism of the serotonin-1A (5-HT1A) receptor gene is associated with major depression in several but not all studies, suggesting that compensatory mechanisms mediate resilience. The rs6295 risk allele prevents binding of the repressor Deaf1 increasing 5-HT1A receptor gene transcription, and the Deaf1-/- mouse model shows an increase in 5-HT1A autoreceptor expression. In this study, Deaf1-/- mice bred on a mixed C57BL6-BALB/c background were compared to wild-type littermates for 5-HT1A autoreceptor function and behavior in males and females. Despite a sustained increase in 5-HT1A autoreceptor binding levels, the amplitude of the 5-HT1A autoreceptor-mediated current in 5-HT neurons was unaltered in Deaf1-/- mice, suggesting compensatory changes in receptor function. Consistent with increased 5-HT1A autoreceptor function in vivo, hypothermia induced by the 5-HT1A agonist DPAT was augmented in early generation male but not female Deaf1-/- mice, but was reduced with succeeding generations. Loss of Deaf1 resulted in a mild anxiety phenotype that was sex-and test-dependent, with no change in depression-like behavior. Male Deaf1 knockout mice displayed anxiety-like behavior in the open field and light-dark tests, while female Deaf1-/- mice showed increased anxiety only in the elevated plus maze. These data show that altered 5-HT1A autoreceptor regulation in male Deaf1-/- mice can be compensated for by generational adaptation of receptor response that may help to normalize behavior. The sex dependence of Deaf1 function in mice is consistent with a greater role for 5-HT1A autoreceptors in sensitivity to depression in men.

SSRI Augmentation by 5-Hydroxytryptophan Slow Release: Mouse Pharmacodynamic Proof of Concept

Drugs, notably SSRIs, that elevate brain extracellular 5-HT (5-HTExt) are antidepressants. Unfortunately, most patients fail to remit. Multipronged clinical evidence suggests that elevating 5-HTExt beyond the SSRI effect enhances antidepressant efficacy, but previous such drug strategies had prohibitive limitations. In humans, adjunct treatment with the 5-HT precursor 5-hydroxytryptophan (5-HTP) elevates 5-HTExt beyond the SSRI effect. Small pilot trials suggest that adjunct 5-HTP can confer antidepressant response in treatment-resistant depression (TRD). However, sustained, stable 5-HTExt elevation is required for antidepressant effect; therefore, the rapid absorption and elimination of standard 5-HTP immediate release (IR) likely curtail 5-HTP IR's antidepressant potential. Slow-release (SR) drug delivery can crucially improve efficacy and safety of rapidly absorbed and eliminated compounds. Here we tested in mice the hypothesis that SR delivery will substantially improve 5-HTP's drug properties, by minimizing adverse effects and securing sustained 5-HTExt elevation beyond the SSRI effect. We modeled 5-HTP SR with minipumps, 5-HTP IR with injections, and chronic SSRI with dietary fluoxetine. We tested adjunct 5-HTP SR in wild-type mice and in mice with low brain 5-HT owing to expression of a mutant form of the brain 5-HT synthesis enzyme, tryptophan hydroxylase 2. In both lines of mice, adjunct 5-HTP SR synergized with SSRI to elevate 5-HTExt beyond the SSRI effect. We observed no adverse effect. Adjunct 5-HTP IR could not produce this therapy-like profile, producing transient 5-HTExt spikes and marked adverse effects. Integrated with a body of clinical data, our mouse data suggest that an adjunct 5-HTP SR drug could safely and effectively elevate 5-HTExt beyond the SSRI effect and represent a novel treatment for TRD.

Correlation between plasma galanin and 5-hydroxytryptamine and psychological status in patients with gastroesophageal reflux disease

AIM: To investigate the difference in plasma galanin (GAL) and 5-hydroxytryptamine (5-HT) in patients with different subtypes of gastroesophageal reflux disease (GERD) and their relationship with patients' anxiety and depression.

METHODS: The Self-rating Anxiety Scale (SAS) and Self-rating Depression Scale (SDS) were used to assess anxiety and depression in GERD patients and healthy controls. Enzyme linked immunosorbent assay (ELISA) was used to analyze the contents of 5-HT and GAL in plasma of GERD patients and healthy controls.

RESULTS: Compared with the healthy control group, GERD patients without anxiety or depression had increased 5-HT and decreased GAL (t = 1.97, P < 0.05). Compared with reflux esophagitis (RE) patients, non-erosive reflux disease (NERD) patients had more serious anxiety and depression (F = 55.92, 36.51, P < 0.05). Compared with the control group, 5-HT decreased and GAL increased in NERD patients (F = 54.53, 8.00, P < 0.05). 5-HT had a negative correlation with SAS and SDS scores (r = -0.789, -0.787, P < 0.05), but GAL had a positive correlation with SAS and SDS scores (r = 0.688, 0.705, P< 0.05). 5-HT was negatively correlated with GAL (r = -0.744, P < 0.05).

CONCLUSION: In GERD patients without anxiety or depression, 5-HT increases and GAL decreases. NERD patients have more serious anxiety and depression problems than RE patients, and 5-HT decreases and GAL increases. The more serious anxiety and depression problems, the lower 5-HT and the higher GAL.

Changes of Serotonin (5-HT), 5-HT2A Receptor, and 5-HT Transporter in the Sprague-Dawley Rats of Depression, Myocardial Infarction and Myocardial Infarction Co-exist with Depression

BACKGROUND

To evaluate whether serotonin (5-HT), 5-HT2A receptor (5-HT2AR), and 5-HT transporter (serotonin transporter [SERT]) are associated with different disease states of depression, myocardial infarction (MI) and MI co-exist with depression in Sprague-Dawley rats.

METHODS

After established the animal model of four groups include control, depression, MI and MI with depression, we measured 5-HT, 5-HT2AR and SERT from serum and platelet lysate.

RESULTS

The serum concentration of 5-HT in depression rats decreased significantly compared with the control group (303.25 ± 9.99 vs. 352.98 ± 13.73; P = 0.000), while that in MI group increased (381.78 ± 14.17 vs. 352.98 ± 13.73; P = 0.000). However, the depression + MI group had no change compared with control group (360.62 ± 11.40 vs. 352.98 ± 13.73; P = 0.036). The changes of the platelet concentration of 5-HT in the depression, MI, and depression + MI group were different from that of serum. The levels of 5-HT in above three groups were lower than that in the control group (380.40 ± 17.90, 387.75 ± 22.28, 246.40 ± 18.99 vs. 500.29 ± 20.91; P = 0.000). The platelet lysate concentration of 5-HT2AR increased in depression group, MI group, and depression + MI group compared with the control group (370.75 ± 14.75, 393.47 ± 15.73, 446.66 ± 18.86 vs. 273.66 ± 16.90; P = 0.000). The serum and platelet concentration of SERT in the depression group, MI group and depression + MI group were all increased compared with the control group (527.51 ± 28.32, 602.02 ± 23.32, 734.76 ± 29.59 vs. 490.56 ± 16.90; P = 0.047, P = 0.000, P = 0.000 in each and 906.38 ± 51.84, 897.33 ± 60.34, 1030.17 ± 58.73 vs. 708.62 ± 51.15; P = 0.000 in each).

CONCLUSIONS

The concentration of 5-HT2AR in platelet lysate and SERT in serum and platelet may be involved in the pathway of MI with depression. Further studies should examine whether elevated 5-HT2AR and SERT may contribute to the biomarker in MI patients with depression.

Acute selective serotonin reuptake inhibitors regulate the dorsal raphe nucleus causing amplification of terminal serotonin release

Selective serotonin reuptake inhibitors (SSRIs) were designed to treat depression by increasing serotonin levels throughout the brain via inhibition of clearance from the extracellular space. Although increases in serotonin levels are observed after acute SSRI exposure, 3–6 weeks of continuous use is required for relief from the symptoms of depression. Thus, it is now believed that plasticity in multiple brain systems that are downstream of serotonergic inputs contributes to the therapeutic efficacy of SSRIs. The onset of antidepressant effects also coincides with desensitization of somatodendritic serotonin autoreceptors in the dorsal raphe nucleus (DRN), suggesting that disrupting inhibitory feedback within the serotonin system may contribute to the therapeutic effects of SSRIs. Previously, we showed that chronic SSRI treatment caused a frequency‐dependent facilitation of serotonin signaling that persisted in the absence of uptake inhibition. In this work, we use in vivo fast‐scan cyclic voltammetry in mice to investigate a similar facilitation after a single treatment of the SSRI citalopram hydrobromide. Acute citalopram hydrobromide treatment resulted in frequency‐dependent increases of evoked serotonin release in the substantia nigra pars reticulata. These increases were independent of changes in uptake velocity, but required SERT expression. Using microinjections, we show that the frequency‐dependent enhancement in release is because of SERT inhibition in the DRN, demonstrating that SSRIs can enhance serotonin release by inhibiting uptake in a location distal to the terminal release site. The novel finding that SERT inhibition can disrupt modulatory mechanisms at the level of the DRN to facilitate serotonin release will help future studies investigate serotonin's role in depression and motivated behavior.

In this work, stimulations of the dorsal raphe nucleus (DRN) evoke serotonin release that is recorded in the substantia nigra pars reticulata (SNpr) using in vivo fast‐scan cyclic voltammetry. Systemic administration of a selective serotonin reuptake inhibitor (SSRI) causes both an increase in t_1/2 and an increase in [5‐HT]_max in the SNpr. Local application of SSRI to the DRN recapitulates the increase in [5‐HT]_max observed in the SNpr without affecting uptake. Thus, SSRIs increase serotonin signaling via two distinct SERT‐mediated mechanisms.

Metabolomics of Neurotransmitters and Related Metabolites in Post-Mortem Tissue from the Dorsal and Ventral Striatum of Alcoholic Human Brain

We report on changes in neurotransmitter metabolome and protein expression in the striatum of humans exposed to heavy long-term consumption of alcohol. Extracts from post mortem striatal tissue (dorsal striatum; DS comprising caudate nucleus; CN and putamen; P and ventral striatum; VS constituted by nucleus accumbens; NAc) were analysed by high performance liquid chromatography coupled with tandem mass spectrometry. Proteomics was studied in CN by two-dimensional gel electrophoresis followed by mass-spectrometry. Proteomics identified 25 unique molecules expressed differently by the alcohol-affected tissue. Two were dopamine-related proteins and one a GABA-synthesizing enzyme GAD65. Two proteins that are related to apoptosis and/or neuronal loss (BiD and amyloid-β A4 precursor protein-binding family B member 3) were increased. There were no differences in the levels of dopamine (DA), 3,4-dihydrophenylacetic acid (DOPAC), serotonin (5HT), homovanillic acid (HVA), 5-hydroxyindoleacetic acid (HIAA), histamine, L-glutamate (Glu), γ-aminobutyric acid (GABA), tyrosine (Tyr) and tryptophan (Tryp) between the DS (CN and P) and VS (NAc) in control brains. Choline (Ch) and acetylcholine (Ach) were higher and norepinephrine (NE) lower, in the VS. Alcoholic striata had lower levels of neurotransmitters except for Glu (30 % higher in the alcoholic ventral striatum). Ratios of DOPAC/DA and HIAA/5HT were higher in alcoholic striatum indicating an increase in the DA and 5HT turnover. Glutathione was significantly reduced in all three regions of alcohol-affected striatum. We conclude that neurotransmitter systems in both the DS (CN and P) and the VS (NAc) were significantly influenced by long-term heavy alcohol intake associated with alcoholism.

Combination Therapy of Salvianolic Acid and Fluoxetine Improves the Cognitive Function of Rats with Chronic Stress-induced Depression

OBJECTIVE

To establish the beneficial effects of salvianolic acid and fluoxetine on the improvement of cognitive function and amelioration of depression-like symptoms of rats with chronic stress-induced depression.

METHODS

Ninety-nine male Sprague-Dawley rats were randomly divided into 5 groups--a control group with no stress challenge and 4 chronic stress groups. Rats assigned to chronic stress groups were exposed to stress for 3 weeks, and then were given placebo, fluoxetine (20 mg/kg), salvianolic acid (40 mg/kg), or combined fluoxetine and salvianolic acid. Body weight of each rat was recorded throughout the study. Sucrose preference test and water maze experiment were performed after chronic stress challenge and after drug treatment to assess the effect of drug treatments on depressive-like symptoms and cognitive function. The sucrose preference test was also performed before chronic stress exposure for baseline measurement.

RESULTS

Exposure of rats to chronic stress for 3 weeks significantly reduced body weight and sucrose preference values compared with the no stress control. The water maze experiment showed that chronic stress impaired the spatial learning of rats as well. Treatment of stress-challenged rats with fluoxetine and fluoxetine combined with salvianolic acid resulted in shorter training latency and longer time spent in the target quadrant during the exploration stage of the water maze experiment compared with placebo treatment. Effect of the combined regimen was found more obvious.

CONCLUSIONS

Combination therapy of salvianolic acid and fluoxetine could alleviate depression-like symptoms and cognitive deficit induced by chronic stress.

Neuroinflammation and Depression: Microglia Activation, Extracellular Microvesicles and microRNA Dysregulation

Patients with chronic inflammation are often associated with the emergence of depression symptoms, while diagnosed depressed patients show increased levels of circulating cytokines. Further studies revealed the activation of the brain immune cell microglia in depressed patients with a greater magnitude in individuals that committed suicide, indicating a crucial role for neuroinflammation in depression brain pathogenesis. Rapid advances in the understanding of microglial and astrocytic neurobiology were obtained in the past 15–20 years. Indeed, recent data reveal that microglia play an important role in managing neuronal cell death, neurogenesis, and synaptic interactions, besides their involvement in immune-response generating cytokines. The communication between microglia and neurons is essential to synchronize these diverse functions with brain activity. Evidence is accumulating that secreted extracellular vesicles (EVs), comprising ectosomes and exosomes with a size ranging from 0.1–1 μm, are key players in intercellular signaling. These EVs may carry specific proteins, mRNAs and microRNAs (miRNAs). Transfer of exosomes to neurons was shown to be mediated by oligodendrocytes, microglia and astrocytes that may either be supportive to neurons, or instead disseminate the disease. Interestingly, several recent reports have identified changes in miRNAs in depressed patients, which target not only crucial pathways associated with synaptic plasticity, learning and memory but also the production of neurotrophic factors and immune cell modulation. In this article, we discuss the role of neuroinflammation in the emergence of depression, namely dynamic alterations in the status of microglia response to stimulation, and how their activation phenotypes may have an etiological role in neurodegeneneration, in particular in depressive-like behavior. We will overview the involvement of miRNAs, exosomes, ectosomes and microglia in regulating critical pathways associated with depression and how they may contribute to other brain disorders including amyotrophic lateral sclerosis (ALS), Alzheimer’s disease (AD) and Parkinson’s disease (PD), which share several neuroinflammatory-associated processes. Specific reference will be made to EVs as potential biomarkers and disease monitoring approaches, focusing on their potentialities as drug delivery vehicles, and on putative therapeutic strategies using autologous exosome-based delivery systems to treat neurodegenerative and psychiatric disorders.

Activity of Raphé Serotonergic Neurons Controls Emotional Behaviors

Despite the well-established role of serotonin signaling in mood regulation, causal relationships between serotonergic neuronal activity and behavior remain poorly understood. Using a pharmacogenetic approach, we find that selectively increasing serotonergic neuronal activity in wild-type mice is anxiogenic and reduces floating in the forced-swim test, whereas inhibition has no effect on the same measures. In a developmental mouse model of altered emotional behavior, increased anxiety and depression-like behaviors correlate with reduced dorsal raphé and increased median raphé serotonergic activity. These mice display blunted responses to serotonergic stimulation and behavioral rescues through serotonergic inhibition. Furthermore, we identify opposing consequences of dorsal versus median raphé serotonergic neuron inhibition on floating behavior, together suggesting that median raphé hyperactivity increases anxiety, whereas a low dorsal/median raphé serotonergic activity ratio increases depression-like behavior. Thus, we find a critical role of serotonergic neuronal activity in emotional regulation and uncover opposing roles of median and dorsal raphé function.

Reduced vesicular monoamine transport disrupts serotonin signaling but does not cause serotonergic degeneration

We previously demonstrated that mice with reduced expression of the vesicular monoamine transporter 2 (VMAT2 LO) undergo age-related degeneration of the catecholamine-producing neurons of the substantia nigra pars compacta and locus ceruleus and exhibit motor disturbances and depressive-like behavior. In this work, we investigated the effects of reduced vesicular transport on the function and viability of serotonin neurons in these mice. Adult (4-6 months of age), VMAT2 LO mice exhibit dramatically reduced (90%) serotonin release capacity, as measured by fast scan cyclic voltammetry. We observed changes in serotonin receptor responsivity in in vivo pharmacological assays. Aged (months) VMAT2 LO mice exhibited abolished 5-HT1A autoreceptor sensitivity, as determined by 8-OH-DPAT (0.1 mg/kg) induction of hypothermia. When challenged with the 5HT2 agonist, 2,5-dimethoxy-4-iodoamphetamine (1 mg/kg), VMAT2 LO mice exhibited a marked increase (50%) in head twitch responses. We observed sparing of serotonergic terminals in aged mice (18-24 months) throughout the forebrain by SERT immunohistochemistry and [(3)H]-paroxetine binding in striatal homogenates of aged VMAT2 LO mice. In contrast to their loss of catecholamine neurons of the substantia nigra and locus ceruleus, aged VMAT2 LO mice do not exhibit a change in the number of serotonergic (TPH2+) neurons within the dorsal raphe, as measured by unbiased stereology at 26-30 months. Collectively, these data indicate that reduced vesicular monoamine transport significantly disrupts serotonergic signaling, but does not drive degeneration of serotonin neurons.

Cocaine self-administration in Wistar-Kyoto rats: a behavioral and biochemical analysis

Depression and cocaine abuse disorders are common concurrent diagnoses. In the present study, we employed Wistar-Kyoto (WKY) rats that showed a depressive-like phenotype to study intravenous cocaine self-administration and extinction/reinstatement procedures. We also investigated the basal tissue level of neurotransmitters, their metabolites and plasma corticosterone (CORT) concentrations in WKY rats, bulbectomized (OBX) rats, and control rats. The WKY rats exhibited an attenuation of the cocaine-associated lever presses and cocaine intake during the acquisition/maintenance of cocaine self-administration only under specific conditions. Active lever presses exhibited by the WKY rats and control animals did not differ during the extinction training and cocaine-seeking behaviors. The WKY rats demonstrated alterations in the basal levels of dopamine, norepinephrine, and serotonin in selected brain structures involved in depression and drug addiction. The changes in the level of neurotransmitters in these animals refer not only to the control (Wistar) rats but also to bulbectomized animals, which represent another depression model. Furthermore, we identified unchanged levels of CORT in the WKY and OBX rats during the light phase and free-stress conditions. This finding suggests that WKY rats should not be used to investigate the co-occurrence of depression and cocaine addiction, as this rat strain does not show an enhanced risk of relapse.

Metabolomics Approach Reveals Integrated Metabolic Network Associated with Serotonin Deficiency

Serotonin is an important neurotransmitter that broadly participates in various biological processes. While serotonin deficiency has been associated with multiple pathological conditions such as depression, schizophrenia, Alzheimer's disease and Parkinson's disease, the serotonin-dependent mechanisms remain poorly understood. This study therefore aimed to identify novel biomarkers and metabolic pathways perturbed by serotonin deficiency using metabolomics approach in order to gain new metabolic insights into the serotonin deficiency-related molecular mechanisms. Serotonin deficiency was achieved through pharmacological inhibition of tryptophan hydroxylase (Tph) using p-chlorophenylalanine (pCPA) or genetic knockout of the neuronal specific Tph2 isoform. This dual approach improved specificity for the serotonin deficiency-associated biomarkers while minimizing nonspecific effects of pCPA treatment or Tph2 knockout (Tph2-/-). Non-targeted metabolic profiling and a targeted pCPA dose-response study identified 21 biomarkers in the pCPA-treated mice while 17 metabolites in the Tph2-/- mice were found to be significantly altered compared with the control mice. These newly identified biomarkers were associated with amino acid, energy, purine, lipid and gut microflora metabolisms. Oxidative stress was also found to be significantly increased in the serotonin deficient mice. These new biomarkers and the overall metabolic pathways may provide new understanding for the serotonin deficiency-associated mechanisms under multiple pathological states.

CSF miR-16 is decreased in major depression patients and its neutralization in rats induces depression-like behaviors via a serotonin transmitter system

BACKGROUND

Animal and cell line studies demonstrated that miR-16 may be associated with major depressive disorder (MDD) via regulation of the expression of serotonin transporter (SERT) gene. However, human studies about miR-16 of patients with MDD are still lacking. The aim of this study was to investigate the possible involvement of miR-16 in the mechanism of MDD in humans.

METHODS

Thirty-six drug-free patients with MDD and 30 healthy controls aged between 18 and 45 years old were recruited. 24-item Hamilton depression scale test was performed for each subject. MiR-16 in cerebrospinal fluid (CSF) and blood, as well as serotonin in CSF were assayed by the qRT-PCR or ELISA method. To confirm the role of CSF miR-16 in MDD, animal study about intracerebroventricular injection of anti-miR-16 was also performed. Depression-like behaviors, CSF miR-16 and serotonin, blood miR-16, and raphe SERT protein of rats were also tested.

RESULTS

CSF miR-16 in MDD patients was significantly lower than that in controls. It was negatively correlated with Hamilton scores and positively associated with CSF serotonin. However, blood miR-16 was not significantly different between two groups and it was not statistically correlated with CSF miR-16. In animal study, anti-miR-16-treated rats were evaluated to exhibit depression-like behaviors, extremely lower CSF miR-16, significantly higher CSF serotonin, and obviously higher raphe SERT protein than control rats.

LIMITATION

We did not detect SERT protein in human brain due to the impossibility of sample collection.

CONCLUSION

Our study suggested that CSF miR-16 participated in the physiopathology of MDD via the modulation of serotonin transmitter system in brain.

Pathogenesis of depression: Insights from human and rodent studies

Major depressive disorder (MDD) will affect one out of every five people in their lifetime and is the leading cause of disability worldwide. Nevertheless, mechanisms associated with the pathogenesis of MDD have yet to be completely understood and current treatments remain ineffective in a large subset of patients. In this review, we summarize the most recent discoveries and insights for which parallel findings have been obtained in human depressed subjects and rodent models of mood disorders in order to examine the potential etiology of depression. These mechanisms range from synaptic plasticity mechanisms to epigenetics and the immune system where there is strong evidence to support a functional role in the development of specific depression symptomology. Ultimately we conclude by discussing how novel therapeutic strategies targeting central and peripheral processes might ultimately aid in the development of effective new treatments for MDD and related stress disorders.

WY14643 produces anti-depressant-like effects in mice via the BDNF signaling pathway

RATIONALE

Current anti-depressants are clinically effective only after several weeks of administration and always produce side effects.

OBJECTIVES

WY14643 is a selective agonist of peroxisome proliferator-activated receptor-α with neuroprotective and neurotrophic effects. Here, we investigated the anti-depressant effects of WY14643 in mice models of depression.

METHODS

We assessed the anti-depressant effects of WY14643 in the forced swim test (FST), tail suspension test (TST) and chronic social defeat stress (CSDS) model. Western blotting and immunohistochemistry studies were further performed to detect the effects of WY14643 on the brain-derived neurotrophic factor (BDNF) signaling pathway and hippocampal neurogenesis. The anti-BDNF antibody, BDNF signaling inhibitor, and tryptophan hydroxylase inhibitor were also used to explore the anti-depressant mechanisms of WY14643.

RESULTS

WY14643 exhibited robust anti-depressant effects in the FST and TST and also protected against the CSDS stress in mice models. Moreover, WY14643 reversed the stress-induced elevation of corticosterone, deficiency of BDNF signaling pathway, and hippocampal neurogenesis. Blockade of BDNF signaling cascade, not the monoaminergic system, abolished all the anti-depressant effects of WY14643.

CONCLUSIONS

These data provide the first evidence that WY14643 exerts anti-depressant-like activity through promoting the BDNF signaling pathway.

Early life environmental and pharmacological stressors result in persistent dysregulations of the serotonergic system

Dysregulations in the brain serotonergic system and exposure to environmental stressors have been implicated in the development of major depressive disorder. Here, we investigate the interactions between the stress and serotonergic systems by characterizing the behavioral and biochemical effects of chronic stress applied during early-life or adulthood in wild type (WT) mice and mice with deficient tryptophan hydroxylase 2 (TPH2) function. We showed that chronic mild stress applied in adulthood did not affect the behaviors and serotonin levels of WT and TPH2 knock-in (KI) mice. Whereas, maternal separation (MS) stress increased anxiety- and depressive-like behaviors of WT mice, with no detectable behavioral changes in TPH2 KI mice. Biochemically, we found that MS WT mice had reduced brain serotonin levels, which was attributed to increased expression of monoamine oxidase A (MAO A). The increased MAO A expression was detected in MS WT mice at 4 weeks old and adulthood. No change in TPH2 expression was detected. To determine whether a pharmacological stressor, dexamethasone (Dex), will result in similar biochemical results obtained from MS, we used an in vitro system, SH-SY5Y cells, and found that Dex treatment resulted in increased MAO A expression levels. We then treated WT mice with Dex for 5 days, either during postnatal days 7–11 or adulthood. Both groups of Dex treated WT mice had reduced basal corticosterone and glucocorticoid receptors expression levels. However, only Dex treatment during PND7–11 resulted in reduced serotonin levels and increased MAO A expression. Just as with MS WT mice, TPH2 expression in PND7–11 Dex-treated WT mice was unaffected. Taken together, our findings suggest that both environmental and pharmacological stressors affect the expression of MAO A, and not TPH2, when applied during the critical postnatal period. This leads to long-lasting perturbations in the serotonergic system, and results in anxiety- and depressive-like behaviors.

Postnatal loss of hap1 reduces hippocampal neurogenesis and causes adult depressive-like behavior in mice

Depression is a serious mental disorder that affects a person’s mood, thoughts, behavior, physical health, and life in general. Despite our continuous efforts to understand the disease, the etiology of depressive behavior remains perplexing. Recently, aberrant early life or postnatal neurogenesis has been linked to adult depressive behavior; however, genetic evidence for this is still lacking. Here we genetically depleted the expression of huntingtin-associated protein 1 (Hap1) in mice at various ages or in selective brain regions. Depletion of Hap1 in the early postnatal period, but not later life, led to a depressive-like phenotype when the mice reached adulthood. Deletion of Hap1 in adult mice rendered the mice more susceptible to stress-induced depressive-like behavior. Furthermore, early Hap1 depletion impaired postnatal neurogenesis in the dentate gyrus (DG) of the hippocampus and reduced the level of c-kit, a protein expressed in neuroproliferative zones of the rodent brain and that is stabilized by Hap1. Importantly, stereotaxically injected adeno-associated virus (AAV) that directs the expression of c-kit in the hippocampus promoted postnatal hippocampal neurogenesis and ameliorated the depressive-like phenotype in conditional Hap1 KO mice, indicating a link between postnatal-born hippocampal neurons and adult depression. Our results demonstrate critical roles for Hap1 and c-kit in postnatal neurogenesis and adult depressive behavior, and also suggest that genetic variations affecting postnatal neurogenesis may lead to adult depression.

Although the majority of the neurons in the brain are generated during embryonic stage, new neurons are continuously being produced postnatally, and at a much lower rate in adulthood. As postnatal neurogenesis is a key component of the brain maturation process that creates dynamic ‘wirings’ in the brain necessary for an individual to grow, learn, and cope with the external world, attenuated postnatal neurogenesis may affect an individual’s mental stability, rendering a higher susceptibility to depression later in life. In the current study, we genetically ablated the expression of huntingtin-associated protein 1 (Hap1) in mice at various ages or in selective brain regions, and found that early loss of Hap1 significantly reduces postnatal hippocampal neurogenesis, and leads to adult depressive-like behavior. We also found c-kit as an effector to mediate the neurogenesis defect and adult depressive-like phenotype in mice lacking Hap1. The results provide the first genetic evidence to demonstrate the importance of postnatal neurogenesis in adult depression, and may offer new avenues in the prevention and treatment of depression. Our study also has potential implications to other adult-onset mental disorders.

An excitatory synapse hypothesis of depression

Depression is a common cause of mortality and morbidity, but the biological bases of the deficits in emotional and cognitive processing remain incompletely understood. Current antidepressant therapies are effective in only some patients and act slowly. Here, we propose an excitatory synapse hypothesis of depression in which chronic stress and genetic susceptibility cause changes in the strength of subsets of glutamatergic synapses at multiple locations, including the prefrontal cortex (PFC), hippocampus, and nucleus accumbens (NAc), leading to a dysfunction of corticomesolimbic reward circuitry that underlies many of the symptoms of depression. This hypothesis accounts for current depression treatments and suggests an updated framework for the development of better therapeutic compounds.