Glucocorticoid receptor antagonism augments fluoxetine-induced downregulation of the 5-HT transporter

Advances in novel molecular targets for antidepressants

Depression is the most common psychiatric illness affecting numerous people world-wide. The currently available antidepressant treatment presents low response and remission rates. Thus, new effective antidepressants need to be developed or discovered. Aiming to give an overview of novel possible antidepressant drug targets, we summarized the molecular targets of antidepressants and the underlying neurobiology of depression. We have also addressed the multidimensional perspectives on the progress in the psychopharmacological treatment of depression and on the new potential approaches with effective drug discovery.

Chronic SSRI stimulation of astrocytic 5-HT2B receptors change multiple gene expressions/editings and metabolism of glutamate, glucose and glycogen: a potential paradigm shift

It is firmly believed that the mechanism of action of SSRIs in major depression is to inhibit the serotonin transporter, SERT, and increase extracellular concentration of serotonin. However, this undisputed observation does not prove that SERT inhibition is the mechanism, let alone the only mechanism, by which SSRI's exert their therapeutic effects. It has recently been demonstrated that 5-HT2B receptor stimulation is needed for the antidepressant effect of fluoxetine in vivo. The ability of all five currently used SSRIs to stimulate the 5-HT2B receptor equipotentially in cultured astrocytes has been known for several years, and increasing evidence has shown the importance of astrocytes and astrocyte-neuronal interactions for neuroplasticity and complex brain activity. This paper reviews acute and chronic effects of 5-HT2B receptor stimulation in cultured astrocytes and in astrocytes freshly isolated from brains of mice treated with fluoxetine for 14 days together with effects of anti-depressant therapy on turnover of glutamate and GABA and metabolism of glucose and glycogen. It is suggested that these events are causally related to the mechanism of action of SSRIs and of interest for development of newer antidepressant drugs.

Meta-analysis of molecular imaging of serotonin transporters in major depression

The success of serotonin-selective reuptake inhibitors has lent support to the monoamine theory of major depressive disorder (MDD). This issue has been addressed in a number of molecular imaging studies by positron emission tomography or single-photon emission computed tomography of serotonin reuptake sites (5-HTT) in the brain of patients with MDD, with strikingly disparate conclusions. Our meta-analysis of the 18 such studies, totaling 364 MDD patients free from significant comorbidities or medication and 372 control subjects, revealed reductions in midbrain 5-HTT (Hedges' g=-0.49; 95% CI: (-0.84, -0.14)) and amygdala (Hedges' g=-0.50; 95% CI: (-0.78, -0.22)), which no individual study possessed sufficient power to detect. Only small effect sizes were found in other regions with high binding (thalamus: g=-0.24, striatum: g=-0.32, and brainstem g=-0.22), and no difference in the frontal or cingulate cortex. Age emerged as an important moderator of 5-HTT availability in MDD, with more severe reductions in striatal 5-HTT evident with greater age of the study populations (P<0.01). There was a strong relationship between severity of depression and 5-HTT reductions in the amygdala (P=0.01). Thus, molecular imaging findings indeed reveal widespread reductions of ∼10% in 5-HTT availability in MDD, which may predict altered spatial-temporal dynamics of serotonergic neurotransmission.

Characterization of electrically evoked field potentials in the medial prefrontal cortex and orbitofrontal cortex of the rat: modulation by monoamines

Medial prefrontal cortex (mPFC) and orbitofrontal cortex (OFC) play critical roles in cognition and behavioural control. Glutamatergic, GABAergic, and monoaminergic dysfunction in the prefrontal cortex has been hypothesised to underlie symptoms in neuropsychiatric disorders. Here we characterised electrically-evoked field potentials in the mPFC and OFC. Electrical stimulation evoked field potentials in layer V/VI of the mPFC and layer V of the OFC. The earliest component (approximately 2 ms latency) was insensitive to glutamate receptor blockade and was presumed to be presynaptic. Later components were blocked by 6,7-dinitroquinoxaline-2,3-dione (DNQX (20 µM)) and were assumed to reflect monosynaptic (latency 4-6 ms) and polysynaptic activity (latency 6-40 ms) mediated by glutamate via AMPA/kainate receptor. In the mPFC, but not the OFC, the monosynaptic component was also partly blocked by 2-amino-5-phosphonopentanoic acid (AP-5 (50-100µM)) indicating the involvement of NMDA receptors. Bicuculline (3-10 µM) enhanced the monosynaptic component suggesting electrically-evoked and/or glutamate induced GABA release inhibits the monosynaptic component via GABAA receptor activation. There were complex effects of bicuculline on polysynaptic components. In the mPFC both the mono- and polysynaptic components were attenuated by 5-HT (10-100 µM) and NA (30 and 60 µM) and the monosynaptic component was attenuated by DA (100 µM). In the OFC the mono- and polysynaptic components were also attenuated by 5-HT (100 µM), NA (10-100 µM) but DA (10-100 µM) had no effect. We propose that these pharmacologically characterised electrically-evoked field potentials in the mPFC and OFC are useful models for the study of prefrontal cortical physiology and pathophysiology.

Effect of selective serotonin reuptake inhibitors and immunomodulator on cytokines levels: an alternative therapy for patients with major depressive disorder

Major depressive disorder (MDD) is a psychiatric illness that presents as a deficit of serotonergic neurotransmission in the central nervous system. MDD patients also experience alterations in cortisol and cytokines levels. Treatment with selective serotonin reuptake inhibitors (SSRIs) is the first-line antidepressant regimen for MDD. The aim of this study was to determine the effect of a combination of SSRIs and an immunomodulator-human dialyzable leukocyte extract (hDLE)-on cortisol and cytokines levels. Patients received SSRIs or SSRIs plus hDLE. The proinflammatory cytokines IL-1 β , IL-2, and IFN- γ ; anti-inflammatory cytokines IL-13 and IL-10; and 24-h urine cortisol were measured at weeks (W) 0, 5, 20, 36, and 52 of treatment. The reduction in cortisol levels in the SSRI-treated group was 30% until W52, in contrast, the combined treatment induced a 54% decrease at W36. The decline in cortisol in patients who were treated with SSRI plus hDLE correlated with reduction of anti-inflammatory cytokines and increases levels of proinflammatory cytokines at the study conclusion. These results suggest that the immune-stimulating activity of hDLE, in combination with SSRIs, restored the pro- and anti-inflammatory cytokine balance and cortisol levels in depressed patients versus those who were given SSRIs alone.

Nongenomic, glucocorticoid receptor-mediated regulation of serotonin transporter cell surface expression in embryonic stem cell derived serotonergic neurons

Depressive disorders have been linked to the combined dysregulation of the hypothalamus-pituitary-adrenal (HPA)-axis and the serotonergic system. The HPA-axis and serotonergic (5-HT) neurons exert reciprocal regulatory actions. It has been reported that glucocorticoid-glucocorticoid receptor (GR) signaling influences serotonin transporter (5-HTT) transcription but data also points to the fact that 5-HTT expression is regulated nongenomically via redistribution of 5-HTT from the cell surface into intracellular compartments. In order to analyze the acute effects of glucocorticoids on 5-HTT cell surface localization we differentiated serotonergic neurons from mouse embryonic stem (ES) cells derived from the C57BL/6N blastocysts. These postmitotic 5-HT neurons express all relevant serotonergic markers following the application of a growth factor-based differentiation protocol. Increasing concentrations of the GR agonist dexamethasone (Dex) resulted in enhanced, dose-dependent 5-HTT cell surface localization in the presence of the protein synthesis inhibitor cycloheximide already 1h after incubation. Inhibition of GR function by the specific GR-antagonist mifepristone abolished the increase in 5-HTT cell surface localization. Hence, our data account for a nongenomic upregulation of 5-HTT cell surface expression by glucocorticoid-GR interaction which likely constitutes a rapid physiological response to increased levels of glucocorticoids as seen during stress. Taken together, we provide a cellular model to analyze and dissect glucocorticoid-5HTT interactions on a molecular level that corresponds to in vivo animal models using C57BL/6N mice.

Antidepressant-like effect of ethanol extract from Zuojin Pill, containing two herbal drugs of Rhizoma Coptidis and Fructus Evodiae, is explained by modulating the monoaminergic neurotransmitter system in mice

ETHNOPHARMACOLOGICAL RELEVANCE

Zuojin Pill (ZJP), a traditional Chinese medicinal decoction, contains two herbal drugs: Coptis chinensis Franch. and Evodia rutaecarpa (Juss.) Benth. in the ratio of 6:1 (w/w). Previous pharmacological studies have shown that two herbs in ZJP have the antagonistic effects on catecholamine secretion in bovine adrenal medullary cells. Furthermore, the alkaloids from the two herbs in ZJP may provide a protective effect for depression in individuals with a low expressing 5-HTT allele by increasing receptor concentration in serotonergic neurons. However, antidepressant effect has not been reported before and has not been fully clarified.

AIM OF THE STUDY

The present study aimed to investigate the antidepressant potential of ethanol extract from ZJP and its monoaminergic mechanism in mice.

MATERIALS AND METHODS

Seven alkaloids were determined from the ethanol extract of ZJP using High Performance Liquid Chromatography (HPLC) with the gradient mobile phase. The ethanol extract from ZJP was used to evaluate the antidepressant potential in mice. Mouse models of depression including the tail suspension test (TST) and the forced swim test (FST) were used to evaluate the effects of the ethanol extract from ZJP. A possible mechanism was explored in the tests of antagonism of reserpine-induced ptosis and hypothermia, and 5-HTP induced head twitch response in mice. The contents of monoamine neurotransmitters including norepinephrine (NE), serotonin (5-hydroxytryptamine or 5-HT) in hippocampus of mice and NE, 5-HT, dopamine (DA) in striatum of mice were determined by HPLC system with Electrochemical Detector (ECD).

RESULTS

The results showed that intragastric administration of the ethanol extract from ZJP (5, 10, 20mg/kg) or fluoxetine (7.5mg/kg) significantly reduced the duration of immobility in TST and FST. However, the effect was not dose-dependent. Ethanol extract from ZJP (5, 10, 20mg/kg) also increased the accumulative number of the 5-HTP-induced head twitch response in mice. The mice were treated with the ethanol extract from ZJP (5, 10, 20mg/kg) or fluoxetine (7.5mg/kg), which could antagonize reserpine-induced ptosis and hypothermia, moreover, both of them could elevate the contents of NE, 5-HT in hippocampus as well as NE, 5-HT, DA in striatum significantly.

CONCLUSION

These results indicate that the ethanol extract from ZJP produced antidepressant-like effect and the possible mechanism, at least in part, is via the central monoaminergic neurotransmitter system and 5-HT plays a major role.

Mineralocorticoid receptor antagonist spironolactone prevents chronic corticosterone induced depression-like behavior

High level of serum corticosteroid is frequently associated with depression, in which a notable HPA (hypothalamus-pituitary-adrenal) axis hyperactivity is often observed. There are two types of corticosteroid receptors expressed in the hippocampus that provide potent negative feedback regulation on the HPA axis but dysfunction during depression, i.e. the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR). The balance between hippocampal MR and GR during chronic stress plays an important role in the occurrence of depression. The aim of this study is to explore if chronic corticosterone administration would induce depression-like behavior and affect the expression and function of hippocampal MR and GR, in addition to assess whether manipulation of corticosteroid receptors would modulate depressive behaviors. Hence, mice were treated with corticosterone (40 mg/kg) for 21 days followed by assessment in a battery of depression-like behaviors. The results show that chronic corticosterone-treated animals displayed an increased immobility time in a forced-swimming test, decreased preference to sucrose solution and novel object recognition performance, and enhanced hippocampal serotonin but decreased MR expression in both hippocampus and hypothalamus. On the other hand, co-administration of MR antagonist, spironolactone (25mg/kg, i.p. × 7 days) in corticosteroid-treated animals reduced immobility time in a forced-swimming test and improved performance in a novel object recognition test. In conclusion, we demonstrate that chronic corticosterone treatment triggers several depression-like behaviors, and in parallel, down-regulates MR expression in the hippocampus and hypothalamus. Administration of an MR antagonist confers an anti-depressant effect in chronic corticosterone-treated animals.

A randomized trial to examine the effect of mifepristone on neuropsychological performance and mood in patients with bipolar depression

BACKGROUND

Deficits in neuropsychological performance are found in patients with bipolar disorder and represent a potential treatment target for novel therapeutic strategies. We have previously demonstrated a beneficial effect on spatial working memory (SWM) of treatment for 1 week with the progesterone and glucocorticoid receptor antagonist mifepristone, evident 2 weeks after the cessation of treatment.

METHODS

We examined the longer-term efficacy of 600 mg/day of mifepristone as an adjunctive treatment, for 1 week, in a placebo-controlled, randomized, double-blind trial in 60 patients with bipolar depression, with SWM as the primary outcome measure. A comparator group of healthy control subjects was also recruited.

RESULTS

At baseline, neuropsychological performance of patients was impaired, but hypothalamic-pituitary-adrenal axis function did not differ from that of control subjects. Mifepristone treatment was associated with a time-limited increase in cortisol awakening response and with a sustained improvement in SWM performance, which was evident 7 weeks after the cessation of treatment. The magnitude of this neuropsychological response was predicted by the magnitude of the cortisol response to mifepristone. The response occurred in the absence of a significant improvement in depressed mood.

CONCLUSIONS

These data accord with the findings of animal studies and demonstrate that brief treatment with mifepristone is associated with a sustained improvement in SWM, an effect that might be mediated by a persistent enhancement in hippocampal mineralocorticoid receptor function.

The genetics of selective serotonin reuptake inhibitors

Selective serotonin reuptake inhibitors (SSRIs) are among the most widely prescribed drugs in psychiatry. Based on the fact that SSRIs increase extracellular monoamine levels in the brain, the monoamine hypothesis of depression was introduced, postulating that depression is associated with too low serotonin, dopamine and noradrenaline levels. However, several lines of evidence indicate that this hypothesis is too simplistic and that depression and the efficacy of SSRIs are dependent on neuroplastic changes mediated by changes in gene expression. Because a coherent view on global gene expression is lacking, we aim to provide an overview of the effects of SSRI treatment on the final targets of 5-HT receptor signal transduction pathways, namely the transcriptional regulation of genes. We address gene polymorphisms in humans that affect SSRI efficacy, as well as in vitro studies employing human-derived cells. We also discuss the molecular targets affected by SSRIs in animal models, both in vivo and in vitro. We conclude that serotonin transporter gene variation in humans affects the efficacy and side-effects of SSRIs, whereas SSRIs generally do not affect serotonin transporter gene expression in animals. Instead, SSRIs alter mRNA levels of genes encoding serotonin receptors, components of non-serotonergic neurotransmitter systems, neurotrophic factors, hypothalamic hormones and inflammatory factors. So far little is known about the epigenetic and age-dependent molecular effects of SSRIs, which might give more insights in the working mechanism(s) of SSRIs.

Hypophagia and induction of serotonin transporter gene expression in raphe nuclei of male and female rats after short-term fluoxetine treatment

Serotonin (5-HT) is one of the regulators of feeding in humans. Drugs acting on the serotoninergic system are used to treat bulimia nervosa and to enhance the effect of hypocaloric diets in overweight subjects. They act rapidly to normalise feeding when used to treat eating-related problems. To explore the role of the 5-HT transporter (serotonin transporter (SERT)) in the short-term action of serotonin selective reuptake inhibitor fluoxetine, rats were i.p. given the drug for five consecutive days. Acute administration of fluoxetine in male and female rats produced a strong reduction in food intake, an effect that held up when daily treatment was maintained for five consecutive days. This reduction translated into a diminution of body weight that was statistically significant in the case of the males. As a reflection of the body weight change in rats killed after the fifth daily drug injection, retroperitoneal fat pad also decreased; a diminution that was statistically significant in the case of male rats. In these conditions, plasma leptin levels of both male and female rats were lower than in untreated animals. While acute fluoxetine administration did not modify SERT gene expression, subchronic drug treatment increased the content of SERT mRNA in the midbrain raphe complex of both rat genders. These findings may contribute to explain the role of SERT in fluoxetine action on binging and as an adjunct to hypocaloric diets.

Berberine and evodiamine influence serotonin transporter (5-HTT) expression via the 5-HTT-linked polymorphic region

The aim of this study was to investigate the effect of berberine and evodiamine on serotonin transporter (5-HTT) expression and then test how allelic variations previously identified in the promoter region could modulate that effect in the serotonergic neuronal cell line RN46A. Both berberine and evodiamine, alone and in combination, increased 5-HTT mRNA and protein expression significantly across the various alleles. When tested against the S, XS(11), L(G), L(A), XL(17), and XL(18) alleles, respectively, 100 μM berberine increased 5-HTT promoter activities by 67%, 128.7%, 106.9%, 100.4%, 26.2% and 82%, 2 μM evodiamine increased 5-HTT promoter activities by 216.7%, 81.6%, 305.6%, 181.5%, 175.3% and 102.2%. Berberine and evodiamine increased 5-HTT promoter activity differently depending on the genetic variation of the 5-HTTLPR polymorphism. This study has provided a convincing example of how herbal compounds influence the expression of one of the most intensively studied psychiatric candidate genes, the serotonin transporter.

Stress hormone regulation: biological role and translation into therapy

Stress is defined as a state of perturbed homeostasis following endangerment that evokes manifold adaptive reactions, which are summarized as the stress response. In the case of mental stress, the adaptive response follows the perception of endangerment. Different peptides, steroids, and biogenic amines operate the stress response within the brain and also after they have been released into circulation. We focus in this review on the biological roles of corticosteroids, corticotrophin-releasing hormone (CRH), and arginine vasopressin (AVP), and we evaluate the effects of treatments directed against the actions of these hormones. CRH and AVP are the central drivers of the stress hormone system, but they also act as neuromodulators in the brain, affecting higher mental functions including emotion, cognition, and behavior. When released toward the pituitary, these central neuropeptides elicit corticotrophin into the periphery, which activates corticosteroid release from the adrenal cortex. These stress hormones are essential for the adequate adaptation to stress, but they can also evoke severe clinical conditions once persistently hypersecreted. Depression and anxiety disorders are prominent examples of stress-related disorders associated with an impaired regulation of stress hormones. We summarize the effects of drugs acting at specific targets of the stress hormone axis, and we discuss their potential use as next-generation antidepressant medications. Such treatments require the identification of patients that will optimally benefit from such specific interventions. These could be a first step into personalized medicine using treatments tailored to the specific pathology of the patients.

The brain 5-HT4 receptor binding is down-regulated in the Flinders Sensitive Line depression model and in response to paroxetine administration

The 5-hydroxytryptamine (5-HT(4)) receptor may be implicated in depression and is a new potential target for antidepressant treatment. We have investigated the brain 5-HT(4) receptor [(3)H]SB207145 binding in the Flinders Sensitive Line rat depression model by quantitative receptor autoradiography, and related this to 5-HT transporter (S)-[N-methyl-(3)H]citalopram binding. We also determined the regulation of 5-HT(4) receptor binding by 1, 14, and 21 days of paroxetine administration and subchronic 5-HT depletion, and compared this with changes in 5-HT(2A) receptor [(3)H]MDL100907 binding. In the Flinders Sensitive Line, the 5-HT(4) receptor and 5-HT transporter binding were decreased in the dorsal and ventral hippocampus, and the changes in binding were directly correlated within the dorsal hippocampus. Chronic but not acute paroxetine administration caused a 16-47% down-regulation of 5-HT(4) receptor binding in all regions evaluated including the basal ganglia and hippocampus, while 5-HT depletion increased the 5-HT(4) receptor binding in the dorsal hippocampus, hypothalamus, and lateral globus pallidus. In comparison, the 5-HT(2A) receptor binding was decreased in the frontal and cingulate cortices after chronic paroxetine administration, and markedly reduced in several regions after 5-HT depletion. Thus, the 5-HT(4) receptor binding was decreased in the Flinders Sensitive Line depression model and in response to chronic paroxetine administration.