Tryptophan-depletion challenge in depressed patients treated with desipramine or fluoxetine: implications for the role of serotonin in the mechanism of antidepressant action

Predictive modeling of antidepressant efficacy based on cognitive neuropsychological theory

BACKGROUND

We aimed to develop a clinical predictive model based on the cognitive neuropsychological (CNP) theory and machine-learning to examine SSRI efficacy in the treatment of MDD.

METHODS

Baseline assessments including clinical symptoms (HAMD, HAMA, BDI, and TEPS scores), negative biases (NEO-PI-R-N and NCPBQ scores), sociodemographic characteristics (social support and SES), and a 5-min eye-opening resting-state EEG were completed by 69 participants with first-episode major depressive disorder (MDD) and 36 healthy controls. The clinical symptoms and negative bias were again assessed after an 8-week treatment of depression with selective serotonin reuptake inhibitors (SSRIs). A multi-modality machine-learning model was developed to predict the effectiveness of SSRI antidepressants.

RESULTS

At baseline, we observed significant differences between MDD patients and healthy controls in terms of social support, clinical symptoms, and negative bias characteristics (p < 0.001). A negative association was found (p < 0.05) between neuroticism and alpha asymmetry in both the central and central-parietal areas, as well as between negative cognitive processing bias and alpha asymmetry in the parietal region. Compared to responders, non-responders exhibited less negative cognitive processing bias and greater alpha asymmetry in both central and central-parietal regions. Importantly, we developed a multi-modality machine-learning model with 83 % specificity using the above salient features.

CONCLUSIONS

Research results support the CNP theory of depression treatment. To some extent, the multimodal clinical model constructed based on the CNP theory effectively predicted the efficacy of this treatment in this population.

LIMITATIONS

Small sample and only focus on the mechanisms of delayed-onset SSRI treatment.

Differences in bacterial taxa between treatment-naive patients with major depressive disorder and non-affected controls may be related to a proinflammatory profile

BACKGROUND

Major depressive disorder (MDD) is characterized by sadness and anhedonia, but also physical symptoms such as changes in appetite and weight. Gut microbiota has been hypothesized to be involved in MDD through gut-brain axis signaling. Moreover, antidepressants display antibacterial properties in the gastrointestinal tract. The aim of this study was to compare the gut microbiota and systemic inflammatory profile of young patients with MDD before and after initiation of antidepressant treatment and/or psychotherapy in comparison with a non-depressed control group (nonMDD).

METHODS

Fecal and blood samples were collected at baseline and at follow-up after four and twelve weeks, respectively. Patients started treatment immediately after collection of the baseline samples. The gut microbiota was characterized by 16 S rRNA gene sequencing targeting the hypervariable V4 region. Plasma levels of 49 unique immune markers were assessed using Mesoscale.

RESULTS

In total, 27 MDD patients and 32 nonMDD controls were included in the study. The gut microbiota in the baseline samples of MDD versus nonMDD participants did not differ regarding α- or β-diversity. However, there was a higher relative abundance of the genera Ruminococcus gnavus group, and a lower relative abundance of the genera Desulfovibrio, Tyzzerella, Megamonas, Olsenella, Gordonibacter, Allisonella and Rothia in the MDD group compared to the nonMDD group. In the MDD group, there was an increase in the genera Rothia, Desulfovibrio, Gordinobacteer and Lactobacillus, while genera belonging to the Firmicutes phylum were found depleted at twelve weeks follow-up compared to baseline. In the MDD group, IL-7, IL-8 and IL-17b levels were elevated compared to the nonMDD group at baseline. Furthermore, MDI score in the MDD group was found to correlate with Bray-Curtis dissimilarity at baseline, and several inflammatory markers at both baseline and after initiation of antidepressant treatment.

CONCLUSION

Several bacterial taxa differed between the MDD group and the nonMDD group at baseline and changed in relative abundance during antidepressant treatment and/or psychotherapy. The MDD group was furthermore found to have a pro-inflammatory profile compared to the nonMDD group at baseline. Further studies are required to investigate the gut microbiota and pro-inflammatory profile of patients with MDD.

Ketamine and rapid antidepressant action: new treatments and novel synaptic signaling mechanisms

Ketamine is an open channel blocker of ionotropic glutamatergic N-Methyl-D-Aspartate (NMDA) receptors. The discovery of its rapid antidepressant effects in patients with depression and treatment-resistant depression fostered novel effective treatments for mood disorders. This discovery not only provided new insight into the neurobiology of mood disorders but also uncovered fundamental synaptic plasticity mechanisms that underlie its treatment. In this review, we discuss key clinical aspects of ketamine's effect as a rapidly acting antidepressant, synaptic and circuit mechanisms underlying its action, as well as how these novel perspectives in clinical practice and synapse biology form a road map for future studies aimed at more effective treatments for neuropsychiatric disorders.

Wistar Kyoto rats exhibit decreased serotonin neuronal firing and increased norepinephrine burst activity but dampened hippocampal α2-adrenoceptor sensitivity

BACKGROUND

Wistar Kyoto (WKY) rats manifest abnormalities in the function of monoamine receptors and transporters, as well as levels of these neurotransmitters in the brain. The present study assessed alterations in the firing activity of serotonin (5-hydroxytryptamine [5-HT]), norepinephrine (NE), and dopamine (DA) neurons, as well as the activity of 5-HT and NE receptors and transporters in the hippocampus.

METHODS

In vivo electrophysiological recordings were conducted in male WKY and Wistar rats. Extracellular single-unit recordings of 5-HT, NE, and DA neurons were performed. Recordings of pyramidal neurons were conducted in the medial prefrontal cortex (mPFC) and the hippocampus, where direct application of 5-HT and NE by iontophoresis was also carried out.

RESULTS

The mean firing rate of 5-HT neurons was significantly decreased in WKY compared to Wistar rats. The burst activity of NE neurons was significantly increased in WKY, while their mean firing activity was not changed. There was no alteration in the firing, burst, and population activity of DA neurons in WKY animals. In the hippocampus, a decrease in sensitivity of α2-adrenoceptors, but not 5-HT receptors, was observed. There was, however, no change in the activity of 5-HT and NE transporters. The firing activity of mPFC pyramidal neurons was similar in WKY versus Wistar rats.

CONCLUSION

In WKY rats, there was a decrease in the firing activity of 5-HT neurons. There was also an enhanced burst activity of NE neurons, accompanied by a reduction in sensitivity of the α2-adrenoceptor in the hippocampus, inferring a decrease in NE transmission.

Essential Amino Acids in the Diet of Children and Adolescents

The increasing requirement for energy and nutrients as a child grows older may result in nutritional deficiencies. The research was to assess the intake level of essential amino acids in the daily diet of children and adolescents living in rural areas. The research was conducted using a questionnaire that analysed food products consumed every day. The questionnaires were completed with the help of the researcher over a period of 7 d. Anthropometric measurements were carried out on every research participant. The participants' financial situation was calculated using a 5-degree scale, where 5 meant very good and 1 very bad. Insufficient body mass was recorded in the study group in 11.1% of boys and 14.7% of girls. Excessive body mass occurred more often among the girls (31%) than the boys (27.9%). Among boys aged 7-15 y old, protein provided 12.8% of the calorie requirement, while for the girls the figure was 13.6%. Among pupils aged 16-18 y old, the figures were 14.06% for boys and 14.33% for girls. Analysis of the results showed that irrespective of age or gender, no insufficient intake of amino acids was recorded among the study participants. In the study group of children and adolescents from rural areas, every third study participant suffered from excess body weight. Due to the fact that the intake of essential amino acids exceeded recommended RDA, it is vital that educational programmes are implemented on how to maintain a correctly balanced diet.

Brain Serotonin Release Is Reduced in Patients With Depression: A [11C]Cimbi-36 Positron Emission Tomography Study With a d-Amphetamine Challenge

BACKGROUND

The serotonin hypothesis of depression proposes that diminished serotonergic (5-HT) neurotransmission is causal in the pathophysiology of the disorder. Although the hypothesis is over 50 years old, there is no firm in vivo evidence for diminished 5-HT neurotransmission. We recently demonstrated that the 5-HT_2A receptor agonist positron emission tomography (PET) radioligand [¹¹C]Cimbi-36 is sensitive to increases in extracellular 5-HT induced by an acute d-amphetamine challenge. Here we applied [¹¹C]Cimbi-36 PET to compare brain 5-HT release capacity in patients experiencing a major depressive episode (MDE) to that of healthy control subjects (HCs) without depression.

METHODS

Seventeen antidepressant-free patients with MDE (3 female/14 male, mean age 44 ± 13 years, Hamilton Depression Rating Scale score 21 ± 4 [range 16-30]) and 20 HCs (3 female/17 male, mean age 32 ± 9 years) underwent 90-minute dynamic [¹¹C]Cimbi-36 PET before and 3 hours after a 0.5-mg/kg oral dose of d-amphetamine. Frontal cortex (main region of interest) 5-HT_2A receptor nondisplaceable binding was calculated from kinetic analysis using the multilinear analysis-1 approach with the cerebellum as the reference region.

RESULTS

Following d-amphetamine administration, frontal nondisplaceable binding potential (BP_ND) was significantly reduced in the HC group (1.04 ± 0.31 vs. 0.87 ± 0.24, p < .001) but not in the MDE group (0.97 ± 0.25 vs. 0.92 ± 0.22, not significant). ΔBP_ND of the MDE group was significantly lower than that of the HC group (HC: 15% ± 14% vs. MDE: 6.5% ± 20%, p = .041).

CONCLUSIONS

This first direct assessment of 5-HT release capacity in people with depression provides clear evidence for dysfunctional serotonergic neurotransmission in depression by demonstrating reduced 5-HT release capacity in patients experiencing an MDE.

Involvement of serotonergic neurotransmission in the antidepressant-like effect elicited by cholecalciferol in the chronic unpredictable stress model in mice

Cholecalciferol deficiency has been associated with stress-related psychiatric disorders, particularly depression. Therefore, the present study investigated the antidepressant-like effect of cholecalciferol in female mice and the possible role of the serotonergic system in this response. The ability of cholecalciferol to elicit an antidepressant-like effect and to modulate serotonin levels in the hippocampus and prefrontal cortex of mice subjected to chronic unpredictable stress (CUS) was also investigated. The administration of cholecalciferol (2.5, 7.5, and 25 µg/kg, p.o.) for 7 days, similar to fluoxetine (10 mg/kg, p.o., serotonin reuptake inhibitor), reduced the immobility time in the tail suspension test, without altering the locomotor performance in the open-field test. Moreover, the administration of p-chlorophenylalanine methyl ester (PCPA - 100 mg/kg, i.p., for 4 days, a selective inhibitor of tryptophan hydroxylase, involved in the serotonin synthesis) abolished the antidepressant-like effect of cholecalciferol and fluoxetine in the tail suspension test, demonstrating the involvement of serotonergic system. Additionally, CUS protocol (21 days) induced depressive-like behavior in the tail suspension test and decreased serotonin levels in the prefrontal cortex and hippocampus of mice. Conversely, the administration of cholecalciferol and fluoxetine in the last 7 days of CUS protocol completely abolished the stress-induced depressive-like phenotype. Cholecalciferol was also effective to abrogate CUS-induced reduction on serotonin levels in the prefrontal cortex, but not in the hippocampus. Our results indicate that cholecalciferol has an antidepressant-like effect in mice by modulating the serotonergic system and support the assumption that cholecalciferol may have beneficial effects for the management of depression.

Sex-based changes in rat brain serotonin and behavior in a model of altitude-related vulnerability to treatment-resistant depression

RATIONALE

Rates of depression and suicide increase with altitude. In our animal model, rats housed at moderate altitude vs. at sea level exhibit increased depressive symptoms in the forced swim test (FST) and lack of response to selective serotonin reuptake inhibitors (SSRIs). Depression and SSRI resistance are linked to disrupted serotonergic function, and hypobaric hypoxia may reduce the oxygen-dependent synthesis of serotonin. We therefore tested brain serotonin in rats housed at altitude.

METHODS

Sprague-Dawley rats were housed at altitude (4,500 ft, 10,000 ft) vs. sea level for 7-36 days. Brain serotonin was measured by ELISA, or behavior evaluated in the FST, sucrose preference (SPT), or open-field tests (OFT).

RESULTS

After 2 weeks at 4,500 ft or 10,000ft vs. sea level, serotonin levels decreased significantly at altitude in the female prefrontal cortex, striatum, hippocampus, and brainstem, but increased with altitude in the male hippocampus and brainstem. Female brain serotonin decreased from 7 to 36 days at 4,500 ft, but males did not vary. At 2 weeks and 24 days, females at altitude exhibit lower brain serotonin and increased depressive symptoms in the FST and SPT, with motor behavior unaltered. In males, serotonin, passive coping in the FST and OFT immobility increased with altitude at 2 weeks, but not at 24 days. Male SPT behavior did not change with altitude.

CONCLUSIONS

Females may be more vulnerable to depressive symptoms at altitude, while males may be resilient. Chronic hypoxic stress at altitudes as low as 4,500 ft may cause a brain serotonin imbalance to worsen vulnerability to depression and SSRI resistance, and potentially worsen suicide risk.

Depression and Its Effect on the Menstrual Cycle

A strong association is noted between depression and early perimenopause as well as menopause. The association was found to be the greatest in women with natural menopause at the age less than 40 years. Excessive corticotropin-releasing hormone (CRH) levels in depression lead to inhibition of the hypothalamic-pituitary-gonadal (HPG) axis and increased cortisol levels which further inhibits the action of gonadotropin-releasing hormone (GnRH) neurons, gonadotrophs, and gonads. The resulting changes in luteinizing hormone (LH) amplitude, follicle-stimulating hormone (FSH) levels, and LH pulse frequency were noted in patients with depression.

Besides depression, earlier surgical menopause is associated with cognitive decline. In addition, it is seen that menopausal changes predisposed females to an increased risk of depression. The association between dysmenorrhea and depression was found to be bidirectional and congruent in most studies. Patients with dysmenorrhea and coexisting depression had enhanced pain perception along with a poor response to pain relief measures. Even the treatment of underlying depression has been shown to cause menorrhagia. On the other hand, amenorrhea has also been reported as a side effect of sertraline and electroconvulsive therapy. Menstrual disorders contribute to a significant number of outpatient gynecological visits per year in the United States. Co-existing or history of depression can either be the cause of or interfere in the treatment of these disorders. Furthermore, the treatment of depression can be the etiology of various menstrual abnormalities, while menstrual disorders themselves could be the cause of depression. The increasing prevalence of depression, women’s health, multiple female-specific subtypes, and the preexisting burden of menstrual disorders necessitates more detailed studies on the effects of depression on the menstrual cycle. 

Modulation of depression-related behaviors by adiponectin AdipoR1 receptors in 5-HT neurons

The adipocyte-derived hormone adiponectin has a broad spectrum of functions beyond metabolic control. We previously reported that adiponectin acts in the brain to regulate depression-related behaviors. However, its underlying neural substrates have not been identified. Here we show that adiponectin receptor 1 (AdipoR1) is expressed in the dorsal raphe nucleus (DRN) and colocalized with tryptophan hydroxylase 2 (TPH2), a marker of serotonin (5-HT) neurons. Selective deletion of AdipoR1 in 5-HT neurons induced anhedonia in male mice, as indicated by reduced female urine sniffing time and saccharin preference, and behavioral despair in female mice and enhanced stress-induced decrease in sucrose preference in both sexes. The expression levels of TPH2 were downregulated with a concurrent reduction of 5-HT-immunoreactivity in the DRN and its two major projection regions, the hippocampus and medial prefrontal cortex (mPFC), in male but not female mice lacking AdipoR1 in 5-HT neurons. In addition, serotonin transporter (SERT) expression was upregulated in both DRN projection fields of male mice but only in the mPFC of female mice. These changes presumably lead to decreased 5-HT synthesis and/or increased 5-HT reuptake, thereby reducing 5-HT transmission. The augmented behavioral responses to the selective serotonin reuptake inhibitor fluoxetine but not desipramine, a selective norepinephrine reuptake inhibitor, observed in conditional knockout male mice supports deficient 5-HT transmission underlying depression-related phenotypes. Our results indicate that adiponectin acts on 5-HT neurons through AdipoR1 receptors to regulate depression-related behaviors in a sex-dependent manner.

Brain-derived neurotrophic factor in 5-HT neurons regulates susceptibility to depression-related behaviors induced by subchronic unpredictable stress

Chronic stress is a major risk factor for the development of depression. Brain-derived neurotrophic factor (BDNF) plays an important role in neural functions and exhibits antidepressant effects. However, studies on depression-related behavioral response to BDNF have mainly focused on the limbic system, whereas other regions of the brain still require further exploration. Here, we report that exposure to chronic unpredictable stress (CUS) can induce depression-associated behaviors in mice. CUS could decrease total Bdnf mRNA and protein levels in the dorsal raphe nucleus (DRN), which correlated with depression-related behaviors. A corresponding reduction in exon-specific Bdnf mRNA was observed in the DRN of CUS mice. Bdnf was highly expressed in 5- Hydroxytryptamine (5-HT) neurons from the DRN. Selective deletion of Bdnf in 5-HT neurons alone could not induce anhedonia and behavioral despair in male or female mice, as indicated by the unchanged female urine sniffing time and preference for sucrose/saccharin. However, it could increase the latency to food in female mice, but not in male mice as shown by novelty-suppressed food test. Nevertheless, enhanced stress-induced susceptibility is observed in these male mice as suggested by the decrease in female urine sniffing time, and for female mice by the reduced sucrose preference and increased immobility in forced swim test. Furtherly, total Bdnf mRNA levels in DRN were correlated with depression-related behaviors of female, but not male 5-HT neurons specific Bdnf knockout mice. Our results indicate that BDNF might act on 5-HT neurons to regulate depression-related behaviors and stress vulnerability in a sex-dependent manner.

Potential Role of Neuroactive Tryptophan Metabolites in Central Fatigue: Establishment of the Fatigue Circuit

Central fatigue leads to reduced ability to perform mental tasks, disrupted social life, and impaired brain functions from childhood to old age. Regarding the neurochemical mechanism, neuroactive tryptophan metabolites are thought to play key roles in central fatigue. Previous studies have supported the “tryptophan-serotonin enhancement hypothesis” in which tryptophan uptake into extensive brain regions enhances serotonin production in the rat model of exercise-induced fatigue. However, serotonin was transiently released after 30 minutes of treadmill running to exhaustion, but this did not reflect the duration of fatigue. In addition, as the vast majority of tryptophan is metabolized along the kynurenine pathway, possible involvement of the tryptophan-kynurenine pathway in the mechanism of central fatigue induction has been pointed out. More recently, our study demonstrated that uptake of tryptophan and kynurenine derived from the peripheral circulation into the brain enhances kynurenic acid production in rat brain in sleep deprivation–induced central fatigue, but without change in serotonin activity. In particular, dynamic change in glial-neuronal interactive processes within the hypothalamus-hippocampal circuit causes central fatigue. Furthermore, increased tryptophan-kynurenine pathway activity in this circuit causes reduced memory function. This indicates a major potential role for the endogenous tryptophan-kynurenine pathway in central fatigue, which supports the “tryptophan-kynurenine enhancement hypothesis.” Here, we review research on the basic neuronal mechanism underlying central fatigue induced by neuroactive tryptophan metabolites. Notably, these basic findings could contribute to our understanding of latent mental problems associated with central fatigue.

Thyroid hormone treatment in the management of treatment-resistant unipolar depression: a systematic review and meta-analysis

BACKGROUND

Treatment with thyroid hormones is occasionally used in the management of treatment-resistant depression. However, the evidence supporting this treatment is not fully established. The aim of this study was to systematically review the literature on the effect of adjunctive thyroid hormone in the management of treatment-resistant unipolar depression and to provide a pooled estimate of its efficacy.

METHODS

The study is registered with PROSPERO (reg. no. CRD42018108088) and followed the PRISMA guidelines. Searches were carried out on June 24, 2019, in PubMed, EMBASE, and PsycINFO. Each record was screened independently by at least two reviewers. In instances of discrepancies, consensus was reached upon discussion. Pooled efficacy estimates were calculated based on response rates (decrease of ≥50% on the primary outcome measure) using random effect statistics.

RESULTS

The search yielded 1355 records. Based on the screening of these records, we identified 10 studies that met the inclusion criteria (total number of patients = 663). The comparison of response to thyroid hormones vs. placebo resulted in an odds ratio of 1.56 (95% CI: 0.50-4.84). Similarly, the comparison of response to thyroid hormones vs. lithium resulted in an odds ratio of 1.91 (95% CI: 0.85-4.26). Thus, adjunctive therapy with thyroid hormones was not superior to placebo or lithium in the management of treatment-resistant unipolar depression.

CONCLUSION

According to this review and meta-analysis, there is not sufficient evidence to support the use of adjunctive thyroid hormones for treatment-resistant unipolar depression.

Bipolar oscillations between positive and negative mood states in a computational model of Basal Ganglia

Bipolar disorder is characterized by mood swings-oscillations between manic and depressive states. The swings (oscillations) mark the length of an episode in a patient's mood cycle (period), and can vary from hours to years. The proposed modeling study uses decision making framework to investigate the role of basal ganglia network in generating bipolar oscillations. In this model, the basal ganglia system performs a two-arm bandit task in which one of the arms (action responses) leads to a positive outcome, while the other leads to a negative outcome. We explore the dynamics of key reward and risk related parameters in the system while the model agent receives various outcomes. Particularly, we study the system using a model that represents the fast dynamics of decision making, and a module to capture the slow dynamics that describe the variation of some meta-parameters of fast dynamics over long time scales. The model is cast at three levels of abstraction: (1) a two-dimensional dynamical system model, that is a simple two variable model capable of showing bistability for rewarding and punitive outcomes; (2) a phenomenological basal ganglia model, to extend the implications from the reduced model to a cortico-basal ganglia setup; (3) a detailed network model of basal ganglia, that incorporates detailed cellular level models for a more realistic understanding. In healthy conditions, the model chooses positive action and avoids negative one, whereas under bipolar conditions, the model exhibits slow oscillations in its choice of positive or negative outcomes, reminiscent of bipolar oscillations. Phase-plane analyses on the simple reduced dynamical system with two variables reveal the essential parameters that generate pathological 'bipolar-like' oscillations. Phenomenological and network models of the basal ganglia extend that logic, and interpret bipolar oscillations in terms of the activity of dopaminergic and serotonergic projections on the cortico-basal ganglia network dynamics. The network's dysfunction, specifically in terms of reward and risk sensitivity, is shown to be responsible for the pathological bipolar oscillations. The study proposes a computational model that explores the effects of impaired serotonergic neuromodulation on the dynamics of the cortico basal ganglia network, and relates this impairment to abstract mood states (manic and depressive episodes) and oscillations of bipolar disorder.

Overcoming Resistance to Selective Serotonin Reuptake Inhibitors: Targeting Serotonin, Serotonin-1A Receptors and Adult Neuroplasticity

Major depressive disorder (MDD) is the most prevalent mental illness contributing to global disease burden. Selective serotonin (5-HT) reuptake inhibitors (SSRIs) are the first-line treatment for MDD, but are only fully effective in 30% of patients and require weeks before improvement may be seen. About 30% of SSRI-resistant patients may respond to augmentation or switching to another antidepressant, often selected by trial and error. Hence a better understanding of the causes of SSRI resistance is needed to provide models for optimizing treatment. Since SSRIs enhance 5-HT, in this review we discuss new findings on the circuitry, development and function of the 5-HT system in modulating behavior, and on how 5-HT neuronal activity is regulated. We focus on the 5-HT1A autoreceptor, which controls 5-HT activity, and the 5-HT1A heteroreceptor that mediates 5-HT actions. A series of mice models now implicate increased levels of 5-HT1A autoreceptors in SSRI resistance, and the requirement of hippocampal 5-HT1A heteroreceptor for neurogenic and behavioral response to SSRIs. We also present clinical data that show promise for identifying biomarkers of 5-HT activity, 5-HT1A regulation and regional changes in brain activity in MDD patients that may provide biomarkers for tailored interventions to overcome or bypass resistance to SSRI treatment. We identify a series of potential strategies including inhibiting 5-HT auto-inhibition, stimulating 5-HT1A heteroreceptors, other monoamine systems, or cortical stimulation to overcome SSRI resistance.

5-HTT independent effects of fluoxetine on neuroplasticity

Selective serotonin reuptake inhibitors are among the most prescribed antidepressants. Fluoxetine is the lead molecule which exerts its therapeutic effects, at least in part, by promoting neuroplasticity through increased brain-derived neurotrophic factor (BDNF)/tropomyosin-related receptor kinase B (TrkB) signalling. It is unclear however, to which extent the neuroplastic effects of fluoxetine are solely mediated by the inhibition of the serotonin transporter (5-HTT). To answer this question, the effects of fluoxetine on neuroplasticity were analysed in both wild type (WT) and 5-Htt knock-out (KO) mice. Using Western blotting and RT-qPCR approaches, we showed that fluoxetine 10 µM activated BDNF/TrkB signalling pathways in both CD1 and C57BL/6J mouse primary cortical neurons. Interestingly, effects on BDNF signalling were observed in primary cortical neurons from both 5-Htt WT and KO mice. In addition, a 3-week in vivo fluoxetine treatment (15 mg/kg/d; i.p.) increased the expression of plasticity genes in brains of both 5-Htt WT and KO mice, and tended to equally enhance hippocampal cell proliferation in both genotypes, without reaching significance. Our results further suggest that fluoxetine-induced neuroplasticity does not solely depend on 5-HTT blockade, but might rely, at least in part, on 5-HTT-independent direct activation of TrkB.

Molecular aspects of depression: A review from neurobiology to treatment

Major depressive disorder (MDD), also known as unipolar depression, is one of the leading causes of disability and disease worldwide. The signs and symptoms are low self‑esteem, anhedonia, feeling of worthlessness, sense of rejection and guilt, suicidal thoughts, among others. This review focuses on studies with molecular-based approaches involving MDD to obtain an integrated, more detailed and comprehensive view of the brain changes produced by this disorder and its treatment and how the Central Nervous System (CNS) produces neuroplasticity to orchestrate adaptive defensive behaviors. This article integrates affective neuroscience, psychopharmacology, neuroanatomy and molecular biology data. In addition, there are two problems with current MDD treatments, namely: 1) Low rates of responsiveness to antidepressants and too slow onset of therapeutic effect; 2) Increased stress vulnerability and autonomy, which reduces the responses of currently available treatments. In the present review, we encourage the prospection of new bioactive agents for the development of treatments with post-transduction mechanisms, neurogenesis and pharmacogenetics inducers that bring greater benefits, with reduced risks and maximized access to patients, stimulating the field of research on mood disorders in order to use the potential of preclinical studies. For this purpose, improved animal models that incorporate the molecular and anatomical tools currently available can be applied. Besides, we encourage the study of drugs that do not present "classical application" as antidepressants, (e.g., the dissociative anesthetic ketamine and dextromethorphan) and drugs that have dual action mechanisms since they represent potential targets for novel drug development more useful for the treatment of MDD.

The Neurobiology and Pharmacotherapy of Posttraumatic Stress Disorder

New approaches to the neurobiology of posttraumatic stress disorder (PTSD) are needed to address the reported crisis in PTSD drug development. These new approaches may require the field to move beyond a narrow fear-based perspective, as fear-based medications have not yet demonstrated compelling efficacy. Antidepressants, particularly recent rapid-acting antidepressants, exert complex effects on brain function and structure that build on novel aspects of the biology of PTSD, including a role for stress-related synaptic dysconnectivity in the neurobiology and treatment of PTSD. Here, we integrate this perspective within a broader framework-in other words, a dual pathology model of ( a) stress-related synaptic loss arising from amino acid-based pathology and ( b) stress-related synaptic gain related to monoamine-based pathology. Then, we summarize the standard and experimental (e.g., ketamine) pharmacotherapeutic options for PTSD and discuss their putative mechanism of action and clinical efficacy.

An extract of Synedrella nodiflora (L) Gaertn exhibits antidepressant properties through monoaminergic mechanisms

Synedrella nodiflora (SNE) has been used traditionally for many neurological conditions and some of these neuroactive effects have been scientifically substantiated. The usefulness of SNE in depression has however not been investigated despite the availability of data in other disease models indicating it may be useful. The present study therefore examined the effect of SNE in acute murine models of depression and the possible mechanisms mediating its activities in these models. Preliminary qualitative phytochemical and high performance liquid chromatography (HPLC) screening were conducted on SNE. The behavioural effects of SNE (100, 300 and 1000 mg/kg) pre-treated mice were examined in the forced swimming (FST) and tail suspension (TST) tests. Behavioural events such as mobility (swimming, climbing, curling and climbing), and immobility, were scored. The possible involvement of monoamines in the effects of SNE was assessed in the TST by pre-treating mice with α-methyldopa, reserpine and para-chlorophenylalanine (pCPA) in separate experiments. Flavonoids, tannins, saponins, alkaloids, cardiac glycosides, coumarins, triterpenes, sterols, anthraquinones and phenolic compounds were present in SNE. HPLC analysis revealed the presence of two major constituents observed at retention times 42.56 and 46.51 min, with percentage composition of 45.72% and 36.88% respectively. SNE significantly reduced immobility scores in both FST and TST, suggesting antidepressant effects. The antidepressant properties of SNE were reversed by the pre-treatment of α-methyldopa, reserpine and pCPA, suggesting a possible involvement of monoamines (noradrenaline and serotonin) in its mechanism(s) of actions. SNE exhibits antidepressant effects, possibly mediated through an interplay of enhancement of noradrenergic and serotoninergic mechanisms.

Abnormal Serotonin Levels During Perinatal Development Lead to Behavioral Deficits in Adulthood

Serotonin (5-HT) is one of the best-studied modulatory neurotransmitters with ubiquitous presynaptic release and postsynaptic reception. 5-HT has been implicated in a wide variety of brain functions, ranging from autonomic regulation, sensory perception, feeding and motor function to emotional regulation and cognition. The role of this neuromodulator in neuropsychiatric diseases is unquestionable with important neuropsychiatric medications, e.g., most antidepressants, targeting this system. Importantly, 5-HT modulates neurodevelopment and changes in its levels during development can have life-long consequences. In this mini-review, we highlight that exposure to both low and high serotonin levels during the perinatal period can lead to behavioral deficits in adulthood. We focus on three exogenous factors that can change 5-HT levels during the critical perinatal period: dietary tryptophan depletion, exposure to serotonin-selective-reuptake-inhibitors (SSRIs) and poor early life care. We discuss the effects of each of these on behavioral deficits in adulthood.

Loss of MeCP2 in adult 5-HT neurons induces 5-HT1A autoreceptors, with opposite sex-dependent anxiety and depression phenotypes

The 5-HT1A autoreceptor mediates feedback inhibition of serotonin (5-HT) neurons, and is implicated in major depression. The human 5-HT1A gene (HTR1A) rs6295 risk allele prevents Deaf1 binding to HTR1A, resulting in increased 5-HT1A autoreceptor transcription. Since chronic stress alters HTR1A methylation and expression, we addressed whether recruitment of methyl-binding protein MeCP2 may alter Deaf1 regulation at the HTR1A locus. We show that MeCP2 enhances Deaf1 binding to its HTR1A site and co-immunoprecipitates with Deaf1 in cells and brain tissue. Chromatin immunoprecipitation assays showed Deaf1-dependent recruitment of MeCP2 to the mouse HTR1A promoter, and MeCP2 modulated human and mouse HTR1A gene transcription in a Deaf1-dependent fashion, enhancing Deaf1-induced repression at the Deaf1 site. To address the role of MeCP2 in HTR1A regulation in vivo, mice with conditional knockout of MeCP2 in adult 5-HT neurons (MeCP2 cKO) were generated. These mice exhibited increased 5-HT1A autoreceptor levels and function, consistent with MeCP2 enhancement of Deaf1 repression in 5-HT neurons. Interestingly, female MeCP2-cKO mice displayed reduced anxiety, while males showed increased anxiety and reduced depression-like behaviors. These data uncover a novel role for MeCP2 in 5-HT neurons to repress HTR1A expression and drive adult anxiety- and depression-like behaviors in a sex-specific manner.

Effects of tryptophan depletion on selective serotonin reuptake inhibitor-remitted patients with obsessive compulsive disorder

BACKGROUND

Serotonergic antidepressants are first-line medication therapies for obsessive-compulsive disorder, however it is not known if synaptic serotonin availability is important for selective serotonin reuptake inhibitor efficacy. The present study tested the hypothesis that temporary reduction in central serotonin transmission, through acute tryptophan depletion, would result in an increase in anxiety in selective serotonin reuptake inhibitor-remitted obsessive-compulsive disorder patients.

METHODS

Eight patients (four males) with obsessive-compulsive disorder who showed sustained clinical improvement with selective serotonin reuptake inhibitor treatment underwent acute tryptophan depletion in a randomized, double-blind, placebo-controlled, within-subjects design, over two days one week apart. Five hours after consumption of the depleting/sham drink the participants performed a personalized obsessive-compulsive disorder symptom exposure task. Psychological responses were measured using the Spielberger State Anxiety Inventory, Yale-Brown Obsessive Compulsive Scale and Visual Analogue Scales.

RESULTS

Free plasma tryptophan to large neutral amino acid ratio decreased by 93% on the depletion day and decreased by 1% on the sham day, as anticipated. Psychological rating scores as measured by Visual Analogue Scale showed a significant decrease in perceived control and increase in interfering thoughts at the time of provocation on the depletion day but not on the sham day. A measure of convergent validity, namely Visual Analogue Scale Similar to past, was significantly higher at the time of provocation on both the depletion and sham days. Both the depletion and time of provocation scores for Visual Analogue Scale Anxiety, Spielberger State Anxiety Inventory, Yale-Brown Obsessive Compulsive Scale and blood pressure were not significant.

CONCLUSIONS

Acute tryptophan depletion caused a significant decrease in perceived control and increase in interfering thoughts at the time of provocation. Acute tryptophan depletion had no effect on the Spielberger State Anxiety Inventory or Visual Analogue Scale Anxiety measures, which suggests that the mechanism of action of selective serotonin reuptake inhibitors may be different to that seen in panic, social anxiety and post-traumatic stress disorder. Successful selective serotonin reuptake inhibitor treatment of obsessive-compulsive disorder may involve the ability of serotonin to switch habitual responding to goal-directed behaviour.

Psoriasis, Depression, and Inflammatory Overlap: A Review

Psoriasis has an enormous impact on patients' lives and is frequently associated with depression. Depression in psoriasis may be attributed, at least in part, to elevated proinflammatory cytokines rather than the psychosocial impact of psoriasis itself. Biologics that target inflammatory cytokines treat the clinical manifestations of psoriasis, but may also play a role in reducing associated depression. Multiple biologics have decreased symptoms of depression during clinical trials in psoriasis; however, these studies used a variety of depression screening tools, which limits comparison. Furthermore, it is difficult to distinguish whether improved depression is the result of the direct anti-inflammatory effect of the biologic, or the indirect effect of improved psoriasis leading to better psychological status. Future studies evaluating depression in patients with psoriasis could benefit from a standardized depression screening tool to mitigate discrepancies and facilitate comparison across treatment types. Here, we highlight the inflammatory overlap between psoriasis and depression by examining the pathophysiology of depression, and reviewing psoriasis clinical studies that assessed depression as an outcome measure.

Effect of Physical Exercise and Acute Escitalopram on the Excitability of Brain Monoamine Neurons: In Vivo Electrophysiological Study in Rats

Background

The antidepressant effect of physical exercise has been reported in several clinical and animal studies. Since serotonin, norepinephrine, and dopamine play a central role in depression, it is possible that the beneficial effects of physical exercise are mediated via monoamine pathways. This study investigates the effects of voluntary wheel running on the excitability of monoamine neurons.

Materials and Methods

Male Sprague-Dawley rats were used in the study. Voluntary wheel running (VWR) rats were housed in individual cages with free access to a running wheel, while control animals were housed in standard laboratory cages. After three weeks, the rats were anesthetized, and in vivo electrophysiological recordings were taken from dorsal raphe nucleus serotonin neurons, locus coeruleus norepinephrine neurons, and ventral tegmental dopamine neurons.

Results

VWR stimulated activity in serotonin, but not in norepinephrine or dopamine neurons. Subsequently, acute administration of the selective serotonin reuptake inhibitor escitalopram in control rats led to complete suppression of serotonin neurons; this suppression was reversed by subsequent administration of selective antagonist of serotonin-1A receptors, WAY100135. Escitalopram induced only partial inhibition of serotonin neurons in the VWR rats while WAY100135 increased the firing activity of serotonin neurons above the baseline value.

Conclusions

The beneficial effect of physical exercise on mood is mediated, at least in part, via activation of serotonin neurons. Physical exercise can potentiate the response to selective serotonin reuptake inhibitors by increasing the basal firing activity and diminishing selective serotonin reuptake inhibitor-induced inhibition of serotonin neurons.

The hippocampus and dorsal raphe nucleus are key brain areas associated with the antidepressant effects of lithium augmentation of desipramine

Approximately 50% of depressed individuals fail to achieve remission with first-line antidepressant drugs and a third remain treatment-resistant. When first-line antidepressant treatment is unsuccessful, second-line strategies include dose optimisation, switching to another antidepressant, combination with another antidepressant, or augmentation with a non-antidepressant medication. Much of the evidence for the efficacy of augmentation strategies comes from studies using lithium to augment the effects of tricyclic antidepressants. The neural circuitry underlying the therapeutic effects of lithium augmentation is not yet fully understood. Recently, we reported that chronic treatment with a combination of lithium and the antidepressant desipramine, exerted antidepressant-like behavioural effects in a mouse strain (BALB/cOLaHsd) that did not exhibit an antidepressant-like behavioural response to either drug alone. In the present study, we used this model in combination with ΔFosB/FosB immunohistochemistry to identify brain regions chronically affected by lithium augmentation of desipramine when compared to either treatment alone. The data suggest that the dorsal raphe nucleus and the CA3 regions of the dorsal hippocampus are key nodes in the neural circuitry underlying antidepressant action of lithium augmentation of desipramine. These data give new insight into the neurobiology underlying the mechanism of lithium augmentation in the context of treatment-resistant depression.

The chronic mild stress (CMS) model of depression: History, evaluation and usage

Now 30 years old, the chronic mild stress (CMS) model of depression has been used in >1300 published studies, with a year-on-year increase rising to >200 papers in 2015. Data from a survey of users show that while a variety of names are in use (chronic mild/unpredictable/varied stress), these describe essentially the same procedure. This paper provides an update on the validity and reliability of the CMS model, and reviews recent data on the neurobiological basis of CMS effects and the mechanisms of antidepressant action: the volume of this research may be unique in providing a comprehensive account of antidepressant action within a single model. Also discussed is the use of CMS in drug discovery, with particular reference to hippocampal and extra-hippocampal targets. The high translational potential of the CMS model means that the neurobiological mechanisms described may be of particular relevance to human depression and mechanisms of clinical antidepressant action.

Behavioral Disturbances: An Innovative Approach to Monitor the Modulatory Effects of a Nutraceutical Diet

In dogs, diets are often used to modulate behavioral disturbances related to chronic anxiety and stress caused by intense and restless activity. However, the traditional ways to monitor behavioral changes in dogs are complicated and not efficient. In the current clinical evaluation, a new, simple monitoring system was used to assess the effectiveness of a specific diet in positively modulating the intense and restless activity of 24 dogs of different ages and breeds. This protocol describes how to easily and rapidly evaluate improvement in a set of symptoms related to generalized anxiety by using a specific sensor, a mobile phone app, a wireless router, and a computer. The results showed that dogs treated with specific diets showed significant improvement in the times spent active and at rest after 10 days (p < 0.01 and p < 0.05, respectively). These dogs also showed an overall significant improvement in clinical and behavioral symptoms. A specific sensor, along with its related hardware, was demonstrated to successfully monitor behavioral changes relating to movement in dogs.

Mismatch in cerebral blood flow and glucose metabolism after the forced swim stress in rats

OBJECTIVE

Neuroimaging studies of depression considered as a stress-related disorder have shown uncoupling in regional cerebral blood flow (rCBF) and regional cerebral metabolic rate for glucose (rCMRglc). We hypothesised that the mismatch change of rCBF and rCMRglc could be a stress-related phenomenon.

METHODS

We exposed male rats to 15-min period of forced swim (FS), followed by the measurement of rCBF using N-isopropyl-4-[123I] iodoamphetamine (123I-IMP) and rCMRglc using 2-deoxy-2-[18F] fluoro-D-glucose (18F-FDG).

RESULTS

The uptake rate of 18F-FDG in the FS group showed a significant decrease in the prefrontal cortex (0.86±0.20%ID/g, p<0.01) and thalamus (0.77±0.17%ID/g, p<0.05) and tended to be lower in the hippocampus (0.58±0.13%ID/g) and cerebellum (0.59±0.13%ID/g) without overt alteration in the uptake rate of 123I-IMP.

CONCLUSIONS

The FS stress can cause mismatch change of rCBF and rCMRglc, which reflect a stress-related phenomenon.

Effects in dogs with behavioural disorders of a commercial nutraceutical diet on stress and neuroendocrine parameters

The well-being of dogs can be affected by changes in human lifestyle, eating habits and increased stressors that lead to behavioural disorders including fear, hyperactivity and anxiety, followed by negative affective moods and poor welfare. This randomised, controlled clinical evaluation involved 69 dogs, 38 males and 31 females, of different breeds, with behavioural disorders related to anxiety and chronic stress. They were fed a control diet or a nutraceutical diet (ND group) for 45 days. Neuroendocrine (serotonin, dopamine, β-endorphins, noradrenaline and cortisol) and stress (derivatives of reactive oxygen metabolites (dROMs) and biological antioxidant potential (BAP)) parameters related to behavioural disorders were evaluated at the beginning and end of the study period. Results showed a significant increase in serotonin, dopamine and β-endorphins plasma concentrations (*P<0.05, *P<0.05 and **P<0.01, respectively) and a significant decrease in noradrenaline and cortisol plasma concentrations in the ND group (*P<0.05). dROMs significantly decreased in the ND group (*P<0.05) while BAP was not affected. This study demonstrated for the first time that a specific diet significantly and positively affected neuroendocrine parameters and dROMs. These results open significant perspectives concerning the use of diet and nutraceuticals in the treatment of behavioural disorders.

Blockade of uptake for dopamine, but not norepinephrine or 5-HT, increases selection of high effort instrumental activity: Implications for treatment of effort-related motivational symptoms in psychopathology

Deficits in behavioral activation, exertion of effort, and other psychomotor/motivational symptoms are frequently seen in people with depression and other disorders. Depressed people show a decision bias towards selection of low effort activities, and animal tests of effort-related decision making are being used as models of motivational dysfunctions seen in psychopathology. The present studies investigated the ability of drugs that block dopamine transport (DAT), norepinephrine transport (NET), and serotonin transport (SERT) to modulate work output in rats responding on a test of effort-related decision making (i.e., a progressive ratio (PROG)/chow feeding choice task). With this task, rats choose between working for a preferred food (high carbohydrate pellets) by lever pressing on a PROG schedule vs. obtaining a less preferred lab chow that is freely available in the chamber. The present studies focused on the effects of the selective DAT inhibitor GBR12909, the selective SERT inhibitor fluoxetine, and the selective NET inhibitors desipramine and atomoxetine. Acute and repeated administration of GBR12909 shifted choice behavior, increasing measures of PROG lever pressing but decreasing chow intake. In contrast, fluoxetine, desipramine and atomoxetine failed to increase lever pressing output, and actually decreased it at higher doses. In the behaviorally effective dose range, GBR12909 elevated extracellular dopamine levels in accumbens core as measured by microdialysis, but fluoxetine, desipramine and atomoxetine decreased extracellular dopamine. Thus, blockade of DAT increases selection of the high effort instrumental activity, while inhibition of SERT or NET does not. These results have implications for the use of monoamine uptake inhibitors for the treatment of effort-related psychiatric symptoms in humans.

Effects of acute tryptophan depletion on raphé functional connectivity in depression

Depression remains a great societal burden and a major treatment challenge. Most antidepressant medications target serotonergic raphé nuclei. Acute tryptophan depletion (ATD) modulates serotonin function. To better understand the raphé's role in mood networks, we studied raphé functional connectivity in depression. Fifteen depressed patients were treated with sertraline for 12 weeks and scanned during ATD and sham conditions. Based on our previous findings in a separate cohort, resting state MRI functional connectivity between raphé and other depression-related regions (ROIs) was analyzed in narrow frequency bands. ATD decreased raphé functional connectivity with the bilateral thalamus within 0.025-0.05 Hz, and also decreased raphé functional connectivity with the right pregenual anterior cingulate cortex within 0.05-0.1 Hz. Using the control broadband filter 0.01-0.1 Hz, no significant differences in raphé-ROI functional connectivity were observed. Post-hoc analysis by remission status suggested increased raphé functional connectivity with left pregenual anterior cingulate cortex in remitters (n=10) and decreased raphé functional connectivity with left thalamus in non-remitters (n=5), both within 0.025-0.05 Hz. Reducing serotonin function appears to alter coordination of these mood-related networks in specific, low frequency ranges. For examination of effects of reduced serotonin function on mood-related networks, specific low frequency BOLD fMRI signals can identify regions implicated in neural circuitry and may enable clinically-relevant interpretation of functional connectivity measures. The biological significance of these low frequency signals detected in the raphé merits further study.

In psychiatrically healthy individuals, overweight women but not men have lower tryptophan levels

Gender differences in tryptophan (TRP) breakdown in obese individuals have been previously reported. This could be both contributory to, as well as a consequence of, gender differences in mood changes among obese people. To exclude the potential effect of depression on TRP breakdown and its levels in obesity, we replicated analyses in psychiatrically healthy individuals. In 1000 participants, plasma kynurenine (KYN), TRP, and the KYN/TRP ratio were compared between overweight/obese and normal-weight individuals using analysis of covariance, with adjustment for age and gender. Bivariate post hoc tests were also conducted. There were no significant relationships between KYN, TRP, or the KYN/TRP ratio and overall overweight/obese status. However, a significant gender by weight category interaction was identified for TRP only, with overweight/obese women having lower TRP than overweight/obese men (p = 0.02). No gender differences in TRP were found in non-obese participants. Our study in psychiatrically healthy individuals suggested that lower TRP levels in obese women were not secondary to depression, strengthening the possibility that TRP levels could mediate depression in vulnerable women. Thus experimental manipulations of TRP levels could be used to advance theoretical knowledge, prevention, and clinical control of depression in obese women.

Serotonin versus catecholamine deficiency: behavioral and neural effects of experimental depletion in remitted depression

Despite immense efforts into development of new antidepressant drugs, the increases of serotoninergic and catecholaminergic neurotransmission have remained the two major pharmacodynamic principles of current drug treatments for depression. Consequently, psychopathological or biological markers that predict response to drugs that selectively increase serotonin and/or catecholamine neurotransmission hold the potential to optimize the prescriber's selection among currently available treatment options. The aim of this study was to elucidate the differential symptomatology and neurophysiology in response to reductions in serotonergic versus catecholaminergic neurotransmission in subjects at high risk of depression recurrence. Using identical neuroimaging procedures with [(18)F] fluorodeoxyglucose positron emission tomography after tryptophan depletion (TD) and catecholamine depletion (CD), subjects with remitted depression were compared with healthy controls in a double-blind, randomized, crossover design. Although TD induced significantly more depressed mood, sadness and hopelessness than CD, CD induced more inactivity, concentration difficulties, lassitude and somatic anxiety than TD. CD specifically increased glucose metabolism in the bilateral ventral striatum and decreased glucose metabolism in the bilateral orbitofrontal cortex, whereas TD specifically increased metabolism in the right prefrontal cortex and the posterior cingulate cortex. Although we found direct associations between changes in brain metabolism and induced depressive symptoms following CD, the relationship between neural activity and symptoms was less clear after TD. In conclusion, this study showed that serotonin and catecholamines have common and differential roles in the pathophysiology of depression.

Serotonergic and noradrenergic systems are implicated in the antidepressant-like effect of ursolic acid in mice

Ursolic acid (UA) is a natural pentacyclic triterpenoid carboxylic acid that exerts antidepressant-like effects in the tail suspension test (TST) and in the forced swimming test, and this effect was reported to be mediated by the dopaminergic system. Many studies show that currently available antidepressant agents have effects on multiple neurotransmitter systems which account for their efficacy. Therefore, this study was aimed at investigating the possible involvement of the serotonergic, noradrenergic, glutamatergic and opioid systems in the antidepressant-like effect of UA. To this end, several pharmacological agents were administered to verify their ability to influence the antidepressant-like responses elicited by UA in the TST in mice. The open-field test was used to assess the locomotor activity. The results show that the pre-treatment of mice with ρ-chlorophenylalanine (100mg/kg, i.p., 4 days) or α-methyl-ρ-tyrosine (100mg/kg, i.p.) but not with N-methyl-d-aspartate (0.1 pmol/mouse, i.c.v.) or naloxone (1mg/kg, i.p.), was able to prevent the antidepressant-like effect of UA (0.1mg/kg, p.o.). Sub-effective doses of fluoxetine (5mg/kg, p.o.) or reboxetine (2mg/kg, p.o.), but not ketamine (0.1mg/kg, i.p.) or MK-801 (0.001 mg/kg, p.o.), was capable of potentiating the effect of a sub-effective dose of UA (0.001 mg/kg, p.o.) in the TST. None of the treatments affected locomotor activity. Altogether, the results show an involvement of the serotonergic and noradrenergic systems, but not the glutamatergic or opioid systems, in the antidepressant-like effect of UA.

Faster, better, stronger: towards new antidepressant therapeutic strategies

Major depression is a highly prevalent disorder and is predicted to be the second leading cause of disease burden by 2020. Although many antidepressant drugs are currently available, they are far from optimal. Approximately 50% of patients do not respond to initial first line antidepressant treatment, while approximately one third fail to achieve remission following several pharmacological interventions. Furthermore, several weeks or months of treatment are often required before clinical improvement, if any, is reported. Moreover, most of the commonly used antidepressants have been primarily designed to increase synaptic availability of serotonin and/or noradrenaline and although they are of therapeutic benefit to many patients, it is clear that other therapeutic targets are required if we are going to improve the response and remission rates. It is clear that more effective, rapid-acting antidepressants with novel mechanisms of action are required. The purpose of this review is to outline the current strategies that are being taken in both preclinical and clinical settings for identifying superior antidepressant drugs. The realisation that ketamine has rapid antidepressant-like effects in treatment resistant patients has reenergised the field. Further, developing an understanding of the mechanisms underlying the rapid antidepressant effects in treatment-resistant patients by drugs such as ketamine may uncover novel therapeutic targets that can be exploited to meet the Olympian challenge of developing faster, better and stronger antidepressant drugs.

Behavioural and transcriptional changes in the amphipod Echinogammarus marinus exposed to two antidepressants, fluoxetine and sertraline

In the past decade, there have been increasing concerns over the effects of pharmaceutical compounds in the aquatic environment, however very little is known about the effects of antidepressants such as the selective serotonin re-uptake inhibitors (SSRIs). Many biological functions within invertebrates are under the control of serotonin, such as reproduction, metabolism, moulting and behaviour. The effects of serotonin and fluoxetine have recently been shown to alter the behaviour of the marine amphipod, Echinogammarus marinus (Leach, 1815). The purpose of this study was to observe behavioural and transcriptional modifications in this crustacean exposed to the two most prescribed SSRIs (fluoxetine and sertraline) and to develop biomarkers of neurological endocrine disruption. The animals were exposed to both drugs at environmentally relevant concentrations from 0.001 to 1μg/L during short-term (1h and 1day) and medium-term (8 days) experiments. The movement of the amphipods was tracked using the behavioural analysis software during 12min alternating dark/light conditions. The behavioural analysis revealed a significant effect on velocity which was observed after 1h exposure to sertraline at 0.01μg/L and after 1 day exposure to fluoxetine as low as 0.001μg/L. The most predominant effect of drugs on velocity was recorded after 1 day exposure for the 0.1 and 0.01μg/L concentrations of fluoxetine and sertraline, respectively. Subsequently, the expression (in this article gene expression is taken to represent only transcription, although it is acknowledged that gene expression can also be regulated at translation, mRNA and protein stability levels) of several E. marinus neurological genes, potentially involved in the serotonin metabolic pathway or behaviour regulation, were analysed in animals exposed to various SSRIs concentrations using RT-qPCR. The expression of a tryptophan hydroxylase (Ph), a neurocan core protein (Neuc), a Rhodopsin (Rhod1) and an Arrestin (Arr) were measured following exposure to fluoxetine or sertraline for 8 days. The levels of Neuc, Rhod1 and Arr were significantly down-regulated to approximately 0.5-, 0.29- and 0.46-fold, respectively, for the lower concentrations of fluoxetine suggesting potential changes in the phototransduction pathway. The expression of Rhod1 tended to be up-regulated for the lower concentration of sertraline but not significantly. In summary, fluoxetine and sertraline have a significant impact on the behaviour and neurophysiology of this amphipod at environmentally relevant concentrations with effects observed after relatively short periods of time.

Antidepressant Effects of Mallotus oppositifolius in Acute Murine Models

Objective. Hydroalcoholic extract of leaves of Mallotus oppositifolius (MOE), a plant used for CNS conditions in Ghana, was investigated for acute antidepressant effects in the forced swimming (FST) and tail suspension tests (TST). Results. In both FST and TST, MOE (10, 30, and 100 mg kg(-1)) significantly decreased immobility periods and frequencies. A 3-day pretreatment with 200 mg kg(-1), i.p., para-chlorophenylalanine (PCPA), a tryptophan hydroxylase inhibitor, reversed the decline in immobility and the increase of swimming score induced by MOE in the modified FST. Pretreatment with reserpine alone (1 mg kg(-1)), α -methyldopa alone (400 mg kg(-1), i.p.), or a combination of both drugs failed to reverse the decline in immobility or the increase in swimming score caused by the extract in the modified FST. The extract potentiated the frequency of head twitch responses induced by 5-hydroxytryptamine. Pretreatment with d-serine (600 mg kg(-1), i.p.), glycine/NMDA agonist, abolished the behavioural effects of MOE while d-cycloserine (2.5 mg kg(-1), i.p.), a glycine/NMDA partial agonist, potentiated it in both TST and modified FST. Conclusion. The extract exhibited antidepressant effects in mice which is mediated by enhancement of serotoninergic neurotransmission and inhibition of glycine/NMDA receptor activation.

Long-term adaptive changes induced by serotonergic antidepressant drugs

The development of conventional antidepressants has been largely based on the hypothesis of monoaminergic dysfunctions and focuses particularly on the serotonin 5-hydroxytryptamine (5-HT) system. Hence, various classes of antidepressant treatments enhance 5-HT neurotransmission with a time course consistent with their delayed therapeutic effect. This delayed onset appears to be associated with the gradual development of specific adaptive changes of functional 5-HT receptors. However, recent theories suggest that major depressive disorders may be associated with impairments of functional plasticity and cellular flexibility. This review discusses several physiological mechanisms by which 5-HT function and hippocampal neuroplasticity are regulated. Knowledge of these long-term adaptations will increase not only our understanding of pathological processes underlying affective disorders, but could also lead to the development of new strategies to treat these devastating illnesses.

Possible role of more positive social behaviour in the clinical effect of antidepressant drugs

Increasing serotonin decreases quarrelsome behaviours and enhances agreeable behaviours in humans. Antidepressants, even those whose primary action is not on serotonin, seem to increase serotonin function. We suggest that antidepressants act in part by effects on social behaviour, which leads to a gradual improvement in mood. We review the evidence supporting the idea that antidepressants may be moving behaviour from quarrelsome to agreeable. The more positive social responses of interaction partners would initiate a cycle of more positive social behaviour, and this iterative process would result in a clinically significant improvement in mood.

Stress, serotonin, and hippocampal neurogenesis in relation to depression and antidepressant effects

Chronic stressful life events are risk factors for developing major depression, the pathophysiology of which is strongly linked to impairments in serotonin (5-HT) neurotransmission. Exposure to chronic unpredictable stress (CUS) has been found to induce depressive-like behaviours, including passive behavioural coping and anhedonia in animal models, along with many other affective, cognitive, and behavioural symptoms. The heterogeneity of these symptoms represents the plurality of corticolimbic structures involved in mood regulation that are adversely affected in the disorder. Chronic stress has also been shown to negatively regulate adult hippocampal neurogenesis, a phenomenon that is involved in antidepressant effects and regulates subsequent stress responses. Although there exists an enormous body of data on stress-induced alterations of 5-HT activity, there has not been extensive exploration of 5-HT adaptations occurring presynaptically or at the level of the raphe nuclei after exposure to CUS. Similarly, although hippocampal neurogenesis is known to be negatively regulated by stress and positively regulated by antidepressant treatment, the role of neurogenesis in mediating affective behaviour in the context of stress remains an active area of investigation. The goal of this review is to link the serotonergic and neurogenic hypotheses of depression and antidepressant effects in the context of stress. Specifically, chronic stress significantly attenuates 5-HT neurotransmission and 5-HT1A autoreceptor sensitivity, and this effect could represent an endophenotypic hallmark for mood disorders. In addition, by decreasing neurogenesis, CUS decreases hippocampal inhibition of the hypothalamic-pituitary-adrenal (HPA) axis, exacerbating stress axis overactivity. Similarly, we discuss the possibility that adult hippocampal neurogenesis mediates antidepressant effects via the ventral (in rodents; anterior in humans) hippocampus' influence on the HPA axis, and mechanisms by which antidepressants may reverse chronic stress-induced 5-HT and neurogenic changes. Although data are as yet equivocal, antidepressant modulation of 5-HT neurotransmission may well serve as one of the factors that could drive neurogenesis-dependent antidepressant effects through these stress regulation-related mechanisms.

Mechanisms of antidepressant resistance

Depression is one of the most frequent and severe mental disorder. Since the discovery of antidepressant (AD) properties of the imipramine and then after of other tricyclic compounds, several classes of psychotropic drugs have shown be effective in treating major depressive disorder (MDD). However, there is a wide range of variability in response to ADs that might lead to non response or partial response or in increased rate of relapse or recurrence. The mechanisms of response to AD therapy are poorly understood, and few biomarkers are available than can predict response to pharmacotherapy. Here, we will first review markers that can be used to predict response to pharmacotherapy, such as markers of drug metabolism or blood-brain barrier (BBB) function, the activity of specific brain areas or neurotransmitter systems, hormonal dysregulations or plasticity, and related molecular targets. We will describe both clinical and preclinical studies and describe factors that might affect the expression of these markers, including environmental or genetic factors and comorbidities. This information will permit us to suggest practical recommendations and innovative treatment strategies to improve therapeutic outcomes.

Monoamine neurocircuitry in depression and strategies for new treatments

Extensive studies showed that monoaminergic neurotransmission that involves serotonin (5-HT), norepinephrine (NE) and dopamine (DA) exerts major influence on brain circuits concerned by the regulation of mood, reactivity to psychological stress, self-control, motivation, drive, and cognitive performance. Antidepressants targeting monoamines directly affect the functional tone of these circuits, notably in limbic and frontocortical areas, and evidence has been provided that this action plays a key role in their therapeutic efficacy. Indeed, at least some of functional changes detected by functional magnetic resonance imaging in emotion- and cognitive-related circuits such as the one involving limbic-cortical-striatal-pallidal-thalamic connections in depressed patients can be reversed by monoamine-targeted antidepressants. However, antidepressants acting selectively on only one monoamine, such as selective inhibitors of 5-HT or NE reuptake, alleviate depression symptoms in a limited percentage of patients, and are poorly effective to prevent recurrence. Thorough investigations for the last 30 years allowed the demonstration of the existence of functional interactions between 5-HT, NE and DA systems, and the identification of the specific receptors involved. In particular, 5-HT systems were shown to exert negative influence on NE and DA systems through 5-HT2A and 5-HT2C receptor- mediated mechanisms, respectively. On the other hand, complex positive and negative influences of NE system on 5-HT neurotransmission are mediated through α1- and α2-adrenergic receptors, respectively. These data provided a rationale for the design of new, multimodal, therapeutic strategies involving drugs acting not only at the "historical" targets such as the 5-HT and/or the NE transporter, but also at other molecular targets to improve their efficacy and their tolerability.

The role of serotonin in cognitive function: evidence from recent studies and implications for understanding depression

BACKGROUND

Symptoms of cognitive impairment such as poor concentration, memory loss and difficulty with decision making are prevalent in patients with depression, but currently are not specific targets for treatment. However, patients can continue to demonstrate cognitive impairments even when apparently clinically recovered. Drugs that potentiate serotonin (5-HT) function, such as selective serotonin reuptake inhibitors (SSRIs), are the mainstay of treatment for depression. Nevertheless, our understanding of the effects of SSRIs and other conventional antidepressant therapy on cognitive function in healthy humans and depressed patients remains limited.

OBJECTIVE

The purpose of this article is to provide a concise overview for clinicians on the impact of pharmacological manipulation of 5-HT on cognitive function in healthy humans with additional reference to animal models where human data are lacking, particularly regarding specific 5-HT receptor subtype modulation.

FINDINGS

The most consistent observation following manipulation of serotonin levels in humans is that low extracellular 5-HT levels are associated with impaired memory consolidation. Preclinical data show that agonism and antagonism at specific 5-HT receptors can exert effects in animal models of cognition.

CONCLUSIONS

Larger, consistently designed studies are needed to understand the roles of 5-HT in cognition in healthy and depressed individuals. Efforts to target specific 5-HT receptors to improve cognitive outcomes are warranted.