Reviving through human hippocampal newborn neurons.

Recent contradictory data has renewed discussion regarding the existence of adult hippocampal neurogenesis (AHN) in humans, i.e., the continued production of new neurons in the brain after birth. The present review revisits the debate of AHN in humans from a historical point of view in the face of contradictory evidence, analyzing the methods employed to investigate this phenomenon. Thus, to date, of the 57 studies performed in humans that we reviewed, 84% (48) concluded in favor of the presence of newborn neurons in the human adult hippocampus. Besides quality of the tissue (such as postmortem intervals below 26hours as well as tissue conservation and fixation), considerations for assessing and quantify AHN in the human brain require the use of stereology and toxicological analyses of clinical data of the patient.

[Consequences of the monoaminergic systems cross-talk in the antidepressant activity]. / Conséquences de l'inter-relation des systèmes monoaminergiques dans l'activité antidépressive.

Selective serotonin reuptake inhibitors (SSRIs) are the most prescribed antidepressant treatment for treat major depressive disorders. Despite their effectiveness, only 30% of SSRI-treated patients reach remission of depressive symptoms. SSRIs by inhibiting the serotonin transporter present some limits with residual symptoms. Increasing not only serotonin but also norepinephrine and dopamine levels in limbic areas seems to improve remission. Anatomical relationships across serotoninergic, dopaminergic and noradrenergic systems suggest tight reciprocal regulations among them. This review attempts to present, from acute to chronic administration the consequences of SSRI administration on monoaminergic neurotransmission. The serotonin neurons located in the raphe nucleus (RN) are connected to the locus coeruleus (locus coeruleus), the key structure of norepinephrine synthesis, through GABAergic-inhibiting interneurons. Activation of the 5-HT2A receptors expressed on GABAergic interneurons following SERT-inhibition induces an increase in serotonin leading to inhibitory effect on NE release. Similarly, the serotonin neurons exert negative regulation on dopaminergic neurons from the ventral tegmental area (VTA) through a GABAergic interneuron. These interneurons express the 5-HT2C and 5-HT3 receptors inducing an inhibitory effect of 5-HT on DA release. Positive reciprocal connections are also observed through direct projections from the locus coeruleus to the RN and from the VTA to the RN through α1 and D2 receptors respectively, both stimulating the serotoninergic activity. Acute SSRI treatment induces only a slight increase in 5-HT levels in limbic areas due to the activation of presynaptic 5-HT1A and 5-HT1B autoreceptors counteracting the effects of the transporter blockade. No change in NE levels and a small decrease in the dopaminergic neurotransmission is also observed. These weak changes in monoamine in the limbic areas after acute SSRI treatment seems to be one of key point involved in the onset of action. Following desensitization of the 5-HT1A and 5-HT1B autoreceptors, chronic SSRI treatment induces a large increase in the 5-HT neurotransmission. Changes in 5-HT levels at the limbic areas results in a decrease in NE transmission and an increase in DA transmission through an increase in the post-synaptic D2 receptors sensitivity and not from a change in DA levels, which is mainly due to a desensitization of the 5-HT2A receptor. The observed decrease of NE neurotransmission could explain some limits of the SSRI therapy and the interest to activate NE system for producing more robust effects. On the other hand, the D2 sensitization, especially in the nucleus accumbens, stimulates the motivation behavior as well as remission of anhedonia considering the major role of DA release in this structure. Finally, we need to take into account the key role of each monoaminergic neurotransmission to reach remission. Targeting only one system will limit the therapeutic effectiveness. Clinical evidences, including the STAR*D studies, confirmed this by an increase of the remission rate following the mobilization of several monoaminergic transmissions. However, these combinations cannot constitute first line of treatment considering the observed increase of side effects. Such an approach should be adapted to each patient in regard to its particular symptoms as well as clinical history. The next generation of antidepressant therapy will need to take into consideration the interconnections and the interrelation between the monoaminergic systems.

Glypican-2 levels in cerebrospinal fluid predict the status of adult hippocampal neurogenesis.

Adult hippocampal neurogenesis is a remarkable form of brain plasticity through which new neurons are generated throughout life. Despite its important roles in cognition and emotion and its modulation in various preclinical disease models, the functional importance of adult hippocampal neurogenesis in human health has not been revealed because of a lack of tools for monitoring adult neurogenesis in vivo. Therefore, we performed an unbiased proteomics screen to identify novel proteins expressed during neuronal differentiation using a human neural stem cell model, and we identified the proteoglycan Glypican-2 (Gpc2) as a putative secreted marker of immature neurons. Exogenous Gpc2 binds to FGF2 and inhibits FGF2-induced neural progenitor cell proliferation. Gpc2 is enriched in neurogenic regions of the adult brain. Its expression is increased by physiological stimuli that increase hippocampal neurogenesis and decreased in transgenic models in which neurogenesis is selectively ablated. Changes in neurogenesis also result in changes in Gpc2 protein level in cerebrospinal fluid (CSF). Gpc2 is detectable in adult human CSF, and first pilot experiments with a longitudinal cohort indicate a decrease over time. Thus, Gpc2 may serve as a potential marker to monitor adult neurogenesis in both animal and human physiology and disease, warranting future studies.

Ketamine treatment involves medial prefrontal cortex serotonin to induce a rapid antidepressant-like activity in BALB/cJ mice.

Unlike classic serotonergic antidepressant drugs, ketamine, an NMDA receptor antagonist, exhibits a rapid and persistent antidepressant (AD) activity, at sub-anaesthetic doses in treatment-resistant depressed patients and in preclinical studies in rodents. The mechanisms mediating this activity are unclear. Here, we assessed the role of the brain serotonergic system in the AD-like activity of an acute sub-anaesthetic ketamine dose. We compared ketamine and fluoxetine responses in several behavioral tests currently used to predict anxiolytic/antidepressant-like potential in rodents. We also measured their effects on extracellular serotonin levels [5-HT]ext in the medial prefrontal cortex (mPFCx) and brainstem dorsal raphe nucleus (DRN), a serotonergic nucleus involved in emotional behavior, and on 5-HT cell firing in the DRN in highly anxious BALB/cJ mice. Ketamine (10 mg/kg i.p.) had no anxiolytic-like effect, but displayed a long lasting AD-like activity, i.e., 24 h post-administration, compared to fluoxetine (18 mg/kg i.p.). Ketamine (144%) and fluoxetine (171%) increased mPFCx [5-HT]ext compared to vehicle. Ketamine-induced AD-like effect was abolished by a tryptophan hydroxylase inhibitor, para-chlorophenylalanine (PCPA) pointing out the role of the 5-HT system in its behavioral activity. Interestingly, increase in cortical [5-HT]ext following intra-mPFCx ketamine bilateral injection (0.25 µg/side) was correlated with its AD-like activity as measured on swimming duration in the FST in the same mice. Furthermore, pre-treatment with a selective AMPA receptor antagonist (intra-DRN NBQX) blunted the effects of intra-mPFCx ketamine on both the swimming duration in the FST and mPFCx [5-HT]ext suggesting that the AD-like activity of ketamine required activation of DRN AMPA receptors and recruited the prefrontal cortex/brainstem DRN neural circuit in BALB/c mice. These results confirm a key role of cortical 5-HT release in ketamine's AD-like activity following the blockade of glutamatergic NMDA receptors. Tight interactions between mPFCx glutamatergic and serotonergic systems may explain the differences in this activity between ketamine and fluoxetine in vivo. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.

[The pharmacological basis of the serotonin system: Application to antidepressant response]. / Les bases de pharmacologie fondamentale du système sérotoninergique : application à la réponse antidépressive.

If serotonin (5-hydroxytryptamin [5-HT]) is well known for its role in mood regulation, it also impacts numerous physiological functions at periphery. Serotonin is synthetized at the periphery into the gut by intestinal enterochromaffin cells and in the central nervous system (CNS) in the raphe nucleus from the essential amino acid tryptophan. Physiological effects of 5-HT are mediated by about 15 serotoninergic receptors grouped into seven broad families (5-HT1, 5-HT2, 5-HT3, 5-HT4, 5-HT5, 5-HT6, 5-HT7 receptor families). Except 5-HT3 receptor, a ligand-gated ion channels, all the others are G protein-coupled receptors. Serotonin's homeostasis involves serotoninergic autoreceptor such as 5-HT1A, 5-HT1B, 5-HT1D, the enzymatic degradation of serotonin by monoamine oxidase A (MAO-A), and a transporter (serotoninergic transporter [SERT]). In the CNS, the SERT is a key target for various antidepressant drugs such as Selective Serotonin Reuptake Inhibitors (SSRI), Serotonin Norepinephrin Reuptake Inhibitors (SNRI) and tricyclics family. However, antidepressant activity of SERT inhibitors is not directly mediated by the SERT inhibition, but a consequence of postsynaptic 5-HT receptor activation following the increase in 5-HT levels in the synaptic cleft. In pharmacology, SSRIs are defined as indirect agonist of postsynaptic receptor. Among all the 5-HT receptors, 5-HT1A, 5-HT1B, 5-HT1D, 5-HT2B and 5-HT4 receptors activation would mediate antidepressant effects. In the meanwhile, 5-HT2A, 5-HT2C, 5-HT3, 5-HT6 and 5-HT7 receptors activation would induce opposite effects. The best serotoninergic antidepressant would directly activate 5-HT1A, 5-HT1B, 5-HT1D, 5-HT2B and 5-HT4 and would block 5-HT2A, 5-HT2C, 5-HT3, 5-HT6 and 5-HT7 receptor. If the chemical synthesis of such a compound may be compromised, SERT inhibition associated with the blockade of some but not all 5-HT receptor could shorten onset of action and/or improve antidepressant efficacy on the overall symptomatology of depression.

[Pharmacological properties of vortioxetine and its pre-clinical consequences]. / Propriétés pharmacologiques de la vortioxetine et leurs conséquences pré-cliniques.

Selective Serotonin Reuptake Inhibitors (SSRIs) are extensively used for the treatment of major depressive disorder (MDD). SSRIs are defined as indirect receptor agonists since the activation of postsynaptic receptors is a consequence of an increase in extracellular concentrations of serotonin (5-HT) mediated by the blockade of serotonin transporter. The activation of some serotoninergic receptors (5-HT1A, post-synaptic, 5-HT1B post-synaptic, 5-HT2B, and 5-HT4), but not all (5-HT1A, pre-synaptic, 5-HT1B pre-synaptic, 5-HT2A, 5-HT2C, 5-HT3, and probably 5-HT6), induces anxiolytic/antidepressive - like effects. Targetting specifically some of them could potentially improve the onset of action and/or efficacy and/or prevent MD relapse. Vortioxetine (Brintellix, 1- [2-(2,4-dimethylphenyl-sulfanyl)-phenyl]-piperazine) is a novel multi-target antidepressant drug approved by the Food and Drug Administration (FDA) and by European Medicines Agency. Its properties are markedly different from the extensively prescribed SSRIs. Compared to the SSRIs, vortioxetine is defined as a multimodal antidepressant drug since it is not only a serotonin reuptake inhibitor, but also a 5-HT1D, 5-HT3, 5-HT7 receptor antagonist, 5-HT1B receptor partial agonist and 5-HT1A receptor agonist. This specific pharmacological profile enables vortioxetine to affect not only the serotoninergic and noradrenergic systems, but also the histaminergic, cholinergic, gamma-butyric acid (GABA) ergic and glutamatergic ones. Thus, vortioxetine not only induces antidepressant-like or anxiolytic-like activity but also improves cognitive parameters in several animal models. Indeed, vortioxetine was shown to improve working memory, episodic memory, cognitive flexibility and spatial memory in young adult rodents and also in old animal models. These specific effects of the vortioxetine are of interest considering that cognitive dysfunction is a common comorbidity to MDD. Altogether, even though this molecule still needs to be investigated further, especially in the insufficient-response to antidepressant drugs, vortioxetine is already an innovative therapeutic option for the treatment of major depression.

Brain organic cation transporter 2 controls response and vulnerability to stress and GSK3ß signaling.

Interactions between genetic and environmental factors, like exposure to stress, have an important role in the pathogenesis of mood-related psychiatric disorders, such as major depressive disorder. The polyspecific organic cation transporters (OCTs) were shown previously to be sensitive to the stress hormone corticosterone in vitro, suggesting that these transporters might have a physiologic role in the response to stress. Here, we report that OCT2 is expressed in several stress-related circuits in the brain and along the hypothalamic-pituitary-adrenocortical (HPA) axis. Genetic deletion of OCT2 in mice enhanced hormonal response to acute stress and impaired HPA function without altering adrenal sensitivity to adrenocorticotropic hormone (ACTH). As a consequence, OCT2(-/-) mice were potently more sensitive to the action of unpredictable chronic mild stress (UCMS) on depression-related behaviors involving self-care, spatial memory, social interaction and stress-sensitive spontaneous behavior. The functional state of the glycogen synthase kinase-3ß (GSK3ß) signaling pathway, highly responsive to acute stress, was altered in the hippocampus of OCT2(-/-) mice. In vivo pharmacology and western blot experiments argue for increased serotonin tonus as a main mechanism for impaired GSK3ß signaling in OCT2(-/-) mice brain during acute response to stress. Our findings identify OCT2 as an important determinant of the response to stress in the brain, suggesting that in humans OCT2 mutations or blockade by certain therapeutic drugs could interfere with HPA axis function and enhance vulnerability to repeated adverse events leading to stress-related disorders.

Antinociceptive activity of the new triple reuptake inhibitor NS18283 in a mouse model of chemotherapy-induced neuropathic pain.

BACKGROUND: Chronic neuropathic pain can lead to anxiety and depression. Drugs that block reuptake of serotonin, norepinephrine and/or dopamine are widely used to treat depression, and have emerged as useful drugs in the treatment of neuropathic pain. This study compared the acute antinociceptive effects of NS18283, a novel triple monoamine reuptake inhibitor (MRI) with indatraline, venlafaxine and escitalopram in a mouse model of neuropathic pain. METHOD: Neuropathic pain-like behaviours were induced in mice by repeated injections of oxaliplatin (OXA), and assessed using the von Frey hair test, the cold plate test and the thermal preference plate test. Anxio/depressive phenotype and antidepressant-like properties of compounds were assessed by the novelty suppressed feeding test and the tail suspension test, respectively. RESULTS: In vivo microdialysis experiments showed that each MRI increased extracellular serotonin, norepinephrine and/or dopamine levels in the cingulate cortex, in agreement with their in vitro reuptake inhibitory properties. Indatraline (3 mg/kg) reversed the full repertoire of OXA-induced neuropathic hypersensitivity. NS18283 (10 mg/kg) reversed OXA-induced mechano-hypersensitivity and cold allodynia. Venlafaxine (16 mg/kg) and escitalopram (4 mg/kg) only reversed cold allodynia and mechano-hypersensitivity, respectively. All MRIs produced antidepressant-like activity in anxio/depressive phenotype of OXA mice. CONCLUSIONS: Acute administration of drugs that enhance the activity of serotonin, norepinephrine and dopamine neurotransmission within nociceptive pathways may provide a broader spectrum of antinociception than dual or selective reuptake inhibitors in animal models of neuropathic pain. Whether similar observations would occur after repeated administration of such compounds in an attempt to simulate dosing in humans, or be compromised by dopaminergic-mediated adverse effects warrants further investigation.

BDNF overexpression in mouse hippocampal astrocytes promotes local neurogenesis and elicits anxiolytic-like activities.

The therapeutic activity of selective serotonin (5-HT) reuptake inhibitors (SSRIs) relies on long-term adaptation at pre- and post-synaptic levels. The sustained administration of SSRIs increases the serotonergic neurotransmission in response to a functional desensitization of the inhibitory 5-HT1A autoreceptor in the dorsal raphe. At nerve terminal such as the hippocampus, the enhancement of 5-HT availability increases brain-derived neurotrophic factor (BDNF) synthesis and signaling, a major event in the stimulation of adult neurogenesis. In physiological conditions, BDNF would be expressed at functionally relevant levels in neurons. However, the recent observation that SSRIs upregulate BDNF mRNA in primary cultures of astrocytes strongly suggest that the therapeutic activity of antidepressant drugs might result from an increase in BDNF synthesis in this cell type. In this study, by overexpressing BDNF in astrocytes, we balanced the ratio between astrocytic and neuronal BDNF raising the possibility that such manipulation could positively reverberate on anxiolytic-/antidepressant-like activities in transfected mice. Our results indicate that BDNF overexpression in hippocampal astrocytes produced anxiolytic-/antidepressant-like activity in the novelty suppressed feeding in relation with the stimulation of hippocampal neurogenesis whereas it did not potentiate the effects of the SSRI fluoxetine on these parameters. Moreover, overexpressing BDNF revealed the anxiolytic-like activity of fluoxetine in the elevated plus maze while attenuating 5-HT neurotransmission in response to a blunted downregulation of the 5-HT1A autoreceptor. These results emphasize an original role of hippocampal astrocytes in the synthesis of BDNF, which can act through neurogenesis-dependent and -independent mechanisms to regulate different facets of anxiolytic-like responses.

Adult hippocampal neurogenesis: an actor in the antidepressant-like action.

Depression and anxiety are psychiatric illnesses that are major burdens in society and affect as much as 7% of the world's population. The heterogeneous nature of depression suggests an involvement of multiple distinct brain regions including amygdala, prefrontal cortex and the hippocampus, which may be responsible for the diversity of the symptoms. Besides its critical role in learning and memory, the hippocampus is one of only two areas in mammalian brain where adult neurogenesis occurs. Of the current leading hypotheses of the pathophysiology and treatment of depression, the neurogenesis hypothesis of depression deserves particular attention because changes in neurogenesis are only seen after chronic, but not acute, antidepressant treatment. This review revisits the role of adult hippocampal neurogenesis in the pathophysiology of mood disorders, especially anxiety/depression, and also in the antidepressant-like responses, especially in stressed rodents.

5-HT2A receptor inactivation potentiates the acute antidepressant-like activity of escitalopram: involvement of the noradrenergic system.

Evidence suggests that the serotonin 2A receptor (5-HT2AR) modulates the therapeutic activity of selective serotonin reuptake inhibitors (SSRIs). Indeed, among the genetic factors known to influence the individual response to antidepressants, the HTR2A gene has been associated with SSRIs response in depressed patients. However, in these pharmacogenetic studies, the consequences of HTR2A gene polymorphisms on 5-HT2AR expression or function are lacking and the precise role of this receptor is still matter of debate. This study examined the effect of 5-HT2AR agonism or antagonism with DOI and MDL100907, respectively, on the serotonergic system and the antidepressant-like activity of the SSRI escitalopram in mouse. The 5-HT2AR agonist DOI decreased the firing rate of 5-HT neurons in the dorsal raphe (DR) nucleus of 5-HT2AR(+/+) anesthetized mice. This inhibitory response persisted in 5-HT2CR(-/-) but was completely blunted in 5-HT2AR(-/-) mutants. Moreover, the suppressant effect of DOI on DR 5-HT neuronal activity in 5-HT2AR(+/+) mice was attenuated by the loss of noradrenergic neurons induced by the neurotoxin DSP4. Conversely, in 5-HT2AR(+/+) mice, the pharmacological inactivation of the 5-HT2AR by the selective antagonist MDL100907 reversed escitalopram-induced decrease in DR 5-HT neuronal activity. Remarkably, in microdialysis experiments, a single injection of escitalopram increased cortical extracellular 5-HT, but not NE, levels in awake 5-HT2AR(+/+) mice. Although the addition of MDL100907 did not potentiate 5-HT neurotransmission, it allowed escitalopram to increase cortical NE outflow and consequently to elicit an antidepressant-like effect in the forced swimming test. These results suggest that the blockade of the 5-HT2AR may strengthen the antidepressant-like effect of escitalopram by facilitating the enhancement of the brain NE transmission. They provide support for the use of atypical antipsychotics with SSRIs as a relevant antidepressant augmentation strategy.

Co-activation of VTA DA and GABA neurons mediates nicotine reinforcement.

Smoking is the most important preventable cause of mortality and morbidity worldwide. This nicotine addiction is mediated through the nicotinic acetylcholine receptor (nAChR), expressed on most neurons, and also many other organs in the body. Even within the ventral tegmental area (VTA), the key brain area responsible for the reinforcing properties of all drugs of abuse, nicotine acts on several different cell types and afferents. Identifying the precise action of nicotine on this microcircuit, in vivo, is important to understand reinforcement, and finally to develop efficient smoking cessation treatments. We used a novel lentiviral system to re-express exclusively high-affinity nAChRs on either dopaminergic (DAergic) or γ-aminobutyric acid-releasing (GABAergic) neurons, or both, in the VTA. Using in vivo electrophysiology, we show that, contrary to widely accepted models, the activation of GABA neurons in the VTA plays a crucial role in the control of nicotine-elicited DAergic activity. Our results demonstrate that both positive and negative motivational values are transmitted through the dopamine (DA) neuron, but that the concerted activity of DA and GABA systems is necessary for the reinforcing actions of nicotine through burst firing of DA neurons. This work identifies the GABAergic interneuron as a potential target for smoking cessation drug development.

Molecular evidence for BDNF- and GABA-related dysfunctions in the amygdala of female subjects with major depression.

Women are twice as likely as men to develop major depressive disorder (MDD) and are more prone to recurring episodes. Hence, we tested the hypothesis that the illness may associate with robust molecular changes in female subjects, and investigated large-scale gene expression in the post-mortem brain of MDD subjects paired with matched controls (n=21 pairs). We focused on the lateral/basolateral/basomedian complex of the amygdala as a neural hub of mood regulation affected in MDD. Among the most robust findings were downregulated transcripts for genes coding for γ-aminobutyric acid (GABA) interneuron-related peptides, including somatostatin (SST), tachykinin, neuropeptide Y (NPY) and cortistatin, in a pattern reminiscent to that previously reported in mice with low brain-derived neurotrophic factor (BDNF). Changes were confirmed by quantitative PCR and not explained by demographic, technical or known clinical parameters. BDNF itself was significantly downregulated at the RNA and protein levels in MDD subjects. Investigating putative mechanisms, we show that this core MDD-related gene profile (including SST, NPY, TAC1, RGS4 and CORT) is recapitulated by complementary patterns in mice with constitutive (BDNF-heterozygous) or activity-dependent (exon IV knockout) decreases in BDNF function, with a common effect on SST and NPY. Together, these results provide both direct (low RNA/protein) and indirect (low BDNF-dependent gene pattern) evidence for reduced BDNF function in the amygdala of female subjects with MDD. Supporting studies in mutant mice models suggest a complex mechanism of low constitutive and activity-dependent BDNF function in MDD, particularly affecting SST/NPY-related GABA neurons, thus linking the neurotrophic and GABA hypotheses of depression.

Organic cation transporter 2 controls brain norepinephrine and serotonin clearance and antidepressant response.

High-affinity transporters for norepinephrine (NE) and serotonin (5-HT), which ensure neurotransmitter clearance at the synapse, are the principal targets of widely used antidepressant drugs. Antidepressants targeting these high-affinity transporters, however, do not provide positive treatment outcomes for all patients. Other monoamine transport systems, with lower affinity, have been detected in the brain, but their role is largely unknown. Here we report that OCT2, a member of the polyspecific organic cation transporter (OCT) family, is expressed notably in the limbic system and implicated in anxiety and depression-related behaviors in the mouse. Genetic deletion of OCT2 in mice produced a significant reduction in brain tissue concentrations of NE and 5-HT and in ex vivo uptake of both these neurotransmitters in the presence of the dual 5-HT-NE transport blocker, venlafaxine. In vivo clearance of NE and 5-HT evaluated using microiontophoretic electrophysiology was diminished in the hippocampus of OCT2(-/-) mice in the presence of venlafaxine, thereby affecting postsynaptic neuronal activity. OCT2(-/-) mice displayed an altered sensitivity to acute treatments with NE- and/or 5-HT-selective transport blockers in the forced-swim test. Moreover, the mutant mice were insensitive to long-term venlafaxine treatment in a more realistic, corticosterone-induced, chronic depression model. Our findings identify OCT2 as an important postsynaptic determinant of aminergic tonus and mood-related behaviors and a potential pharmacological target for mood disorders therapy.

Behavioral and serotonergic consequences of decreasing or increasing hippocampus brain-derived neurotrophic factor protein levels in mice.

Antidepressants such as Selective Serotonin Reuptake Inhibitors (SSRI) act as indirect agonists of serotonin (5-HT) receptors. Although these drugs produce a rapid blockade of serotonin transporters (SERTs) in vitro, several weeks of treatment are necessary to observe clinical benefits. This paradox has not been solved yet. Recent studies have identified modifications of intracellular signaling proteins and target genes that could contribute to antidepressant-like activity of SSRI (e.g., increases in neurogenesis and BDNF protein levels), and may explain, at least in part, their long delay of action. Although these data suggest a positive regulation of 5-HT on the expression of the gene coding for BDNF, the reciprocal effects of BDNF on brain 5-HT neurotransmission remains poorly documented. To study the impact of BDNF on serotonergic activity, a dual experimental strategy was used to analyze neurochemical and behavioral consequences of its decrease (strategy 1) or increase (strategy 2) in the brain of adult male mice. (1) In heterozygous BDNF+/- mice in which brain BDNF protein levels were decreased by half, an enhancement of basal extracellular 5-HT levels (5-HText) that induced a down-regulation of SERT, i.e., a decrease in its capacity to reuptake 5-HT, was found in the hippocampus. In addition, the SSRI, paroxetine, failed to increase hippocampal 5-HText in BDNF+/- mice, while it produces robust effects in wild-type littermates. Thus, BDNF+/- mice can be viewed as an animal model of genetic resistance to serotonergic antidepressant drugs. (2) In wild-type BDNF+/+ mice, the effects of intra-hippocampal (vHi) injection of BDNF (100 ng) in combination with a SSRI was examined by using intracerebral microdialysis and behavioral paradigms that predict an antidepressant- and anxiolytic-like activity of a molecule [the forced swim test (FST) and the open field paradigm (OF) respectively]. BDNF induced a rapid and transient increase in paroxetine response on 5-HText in the adult hippocampus, which was correlated with a potentiation of its antidepressant-like activity in the FST. The effects of BDNF were selectively blocked by K252a, an antagonist of its high-affinity TrkB receptor. Such a correlation between neurochemical and behavioral effects of [BDNF+SSRI] co-administration suggests that its antidepressant-like activity is linked to the activation of 5-HT neurotransmission in the adult hippocampus. BDNF also had a facilitatory effect on anxiety-like behavior in the OF test, and paroxetine prevented this anxiogenesis. What was the mechanism by which BDNF exerted these latter effects? Surprisingly, by using zero net flux method of quantitative microdialysis in vivo, we found that an intra-hippocampal BDNF injection in wild-type mice decreased the functional activity of SERT as observed in BDNF+/- mice. However, the decreased capacity of SERT to reuptake 5-HT was not associated to an increase in basal 5-HText in the hippocampus of WT mice. Interestingly, using in situ hybridization experiments indicated that TrkB receptor mRNA was expressed in the hippocampus and dorsal raphe nucleus in adult mice suggesting that the neurochemical and behavioral effects of intra-hippocampal BDNF injection can mobilize both pre- and post-synaptic elements of the brain 5-HT neurotransmission. Taken together, these set of experiments unveiled a relative opposition of neurochemical and behavioral responses following either a decrease (in BDNF+/- mutant mice) or an increase in brain BDNF levels (bilateral intra-hippocampal injection) in adult mice. In view of developing new antidepressant drug strategy, a poly-therapy combining BDNF with a chronic SSRI treatment could thus improve the efficacy of current medications.

Prucalopride and donepezil act synergistically to reverse scopolamine-induced memory deficit in C57Bl/6j mice.

It is known that 5-HT(4) receptor agonists increase sAPPalpha levels in the cortex and hippocampus of mice as well as in a model of Alzheimer's disease (AD). As sAPPalpha is thought to have pro-mnesic properties, we assessed whether its increase induces cognitive improvement in a spatial memory task and whether it reverses a scopolamine-induced memory deficit. Mice treated or not treated with scopolamine were trained in the Morris water maze for 3 days. Before the probe test, they received an injection of either a 5-HT(4) receptor agonist (prucalopride or RS 67333), or an acetylcholinesterase inhibitor (donepezil), or both drugs. As expected, scopolamine decreased performance, an effect that was not reversed by the drugs tested when injected alone. However, prucalopride (5 mg kg(-1), s.c.) acted synergistically with donepezil (0.75 mg kg(-1), s.c.) to counteract completely scopolamine-induced amnesia. Western blot analysis of tissue homogenates in the cortex and hippocampus shows that sAPPalpha levels did not differ between saline- and scopolamine-treated mice. Furthermore, a region-dependent drug action was observed since the scopolamine-treated mice display a tendency to increase sAPPalpha levels in the hippocampus after donepezil or in the cortex after prucalopride. Our results suggest that a combined treatment with a 5-HT(4) receptor agonist with an acetylcholinesterase inhibitor has beneficial effects on memory in mice. Moreover, it seems to enhance sAPPalpha levels in two brain regions highly affected in AD. Thus, a drug polytherapy could be interesting not only to enhance cognitive performance and decrease drawbacks but also to get the best action in each brain region.

[Escitalopram and citalopram: the unexpected role of the R-enantiomer]. / Escitalopram et Citalopram: le rôle inattendu de l'énantiomère R.

Citalopram, a selective serotonin reuptake inhibitor, is composed of 2 enantiomers, R-citalopram and S-citalopram, 2 different non-superimposable mirror image forms of the same molecule. Separating these 2 enantiomers has enabled studying their individual properties. Citalopram's pharmacologic activity is centered on the S enantiomer's high affinity for the serotonin transporter which is twice as high as citalopram's and 30 to 40 times higher than R-citalopram. This leads to an inhibition of serotonin reuptake two times higher for escitalopram compared with citalopram and confirms that citalopram's pharmacologic activity is due to the S-enantiomer. Contrary to what might be expected, the effect of escitalopram (DCI of S-citalopram) is not superimposable on an equivalent dose of citalopram but is superior. Several hypotheses could explain this superiority. First, conversions of the S-enantiomer into the R-enantiomer may occur, but there is no reason why this phenomenon would happen more when both enantiomers are present than when escitalopram is alone. Furthermore, pharmacokinetic studies have shown that S or R configurations are stable in vivo. Second, a particular action of R-citalopram may influence the S-enantiomer's kinetic from intestinal absorption to blood-brain barrier. But concentrations of both enantiomers in the frontal cortex are the same. Therefore, R-citalopram does not interfere with escitalopram's kinetic. Finally, interactions may appear at the synaptic level. Results of experimentation, after in situ injection to the cortex level, confirm that an interaction between the 2 enantiomers takes place at that level. A direct negative interaction of R-citalopram on one or several effectors that create the antidepressive effect seems justified. This negative interaction has been studied in depth. Animal models have shown that the R-enantiomer has no antidepressive potential and when associated with escitalopram prohedonic effects disappear. Escitalopram is more powerful than citalopram in reducing anxiety but the presence of R-citalopram reduces the positive effects of escitalopram. We then may conclude that R-citalopram antagonizes the antidepressive effects of escitalopram and that its presence limits the therapeutic effect and reduces the speed of action of citalopram. The antagonism of escitalopram by R-citalopram was not expected and one hypothesis is that a direct interaction between the 2 enantiomers may occur on a particular site of the serotonin transporter. Results have shown that R-citalopram has a significant affinity only for the allosteric site of the transporter, which regulates the affinity of the ligand for the active site at the origin of serotonin reuptake inhibition. Unlike citalopram, escitalopram's pharmacologic action is not blocked by R-citalopram explaining its greater therapeutic efficacy and more rapid mode of action.

5-HT4 receptor agonists increase sAPPalpha levels in the cortex and hippocampus of male C57BL/6j mice.

BACKGROUND AND PURPOSE: A strategy to treat Alzheimer's disease (AD) is to increase the soluble form of amyloid precursor protein (sAPPalpha), a promnesic protein, in the brain. Because strong evidence supports beneficial effects of 5-hydroxytryptamine 5-HT(4) receptor agonists in memory and learning, we investigated the role of 5-HT(4) receptors on APP processing in 8 weeks-old male C57BL/6j mice. EXPERIMENTAL APPROACH: Mice were given, subcutaneously, prucalopride or ML 10302 (s.c.), two highly selective 5-HT(4) receptor agonists and, up to 240 min later, the hippocampus and cortex were analysed by Western blot for sAPPalpha determination. KEY RESULTS: Prucalopride (5 or 10 mg kg(-1)) significantly increased sAPPalpha levels in the hippocampus and cortex, but did not modify the expression level of APP mRNA as detected by quantitative RT-PCR. A selective 5-HT(4) receptor antagonist, GR125487 (1 mg kg(-1), s.c.) inhibited prucalopride induced- increase in sAPPalpha levels. In addition, levels of sAPPalpha were increased by ML10302 only at 20 mg kg(-1) and was limited to the cortex. Also, prucalopride increased sAPPalpha levels in the cortex of a transgenic mouse model of AD, expressing the London mutation of APP. Furthermore, the combined injection of a selective acetylcholinesterase inhibitor, donepezil and prucalopride induced a synergic increase in sAPPalpha levels in the cortex and hippocampus. CONCLUSIONS AND IMPLICATIONS: Our results demonstrate that the 5-HT(4) receptor plays a key role in the non-amyloidogenic pathway of APP metabolism in vivo and give support to the beneficial use of 5-HT(4) agonists for AD treatment.

Targeted in vivo expression of nicotinic acetylcholine receptors in mouse brain using lentiviral expression vectors.

Nicotinic acetylcholine receptors (nAChRs) in the brain exhibit diverse functional properties and ubiquitous distribution. Yet, except for providing a receptor for the exogenously applied nicotine of tobacco products, their role in the normal functioning of the brain has remained elusive. We have used a lentiviral expression vector to re-express the beta2 subunit specifically in the ventral tegmental area (VTA) of beta2-/- mice. The viral vector efficiently expresses beta2- subunit protein leading to new nAChR-binding sites. VTA neurons transduced by the lentiviral vector are responsive to intravenous nicotine when analyzed using in vivo electrophysiology. Nicotine-induced dopamine release from the nucleus accumbens (NuAcc) was also restored in re-expressing beta2-/- mice. Intra-VTA injection of nicotine was found to be reinforcing in both wild-type and beta2-subunit re-expressing beta2-/- mice, but not in beta2-/- mice. Furthermore, in the absence of applied nicotine, the spontaneous slow exploratory behavior of the mice was restored, whereas fast navigation did not change. This latter behavioral analysis suggests a role for beta2* nAChR, specifically expressed in the VTA, in mammalian cognitive function.

Antidepressant-like activity of selective serotonin reuptake inhibitors combined with a NK1 receptor antagonist in the mouse forced swimming test.

Substance P antagonists of the neurokinin-1 receptor type (NK1) have growing interest as new antidepressant therapies. It has been postulated that these drugs exert this putative therapeutic effect without direct interactions with serotonin (5-HT) neurons. In line with this assumption, previous intracerebral in vivo microdialysis experiments provided evidence that the NK1 receptor antagonists did not change basal cortical 5-HT levels. However, we found that increases in cortical 5-HT overflow caused by systemic injection of the selective serotonin reuptake inhibitor (SSRI), paroxetine was higher in freely moving (C57BL/6x129sv) NK1-/- mutants than in wild-type NK1+/+ mice. More recently, a pharmacological study has led to a similar conclusion since GR205171, a NK1 receptor antagonist, potentiated paroxetine-induced increases in cortical 5-HT dialysate following its acute systemic or intra-raphe administration to wild-type mice . In the present study, we tested whether an acute combination of SSRI and NK1 receptor antagonist could display antidepressant-like activity using the forced swimming test in Swiss mice. We found that a single systemic dose of GR205171 (10 and 30 mg/kg, i.p.) had no effect by itself. However, it selectively potentiated the antidepressant-like activity of subactive doses of two serotonergic antidepressant drugs, citalopram and paroxetine (without psychomotor stimulant activity), but not that of noradrenaline reuptake inhibitor, desipramine. In agreement with neurochemical data, the present study confirms that co-administration of a NK1 receptor antagonist with an antidepressant drug such as a SSRI may have a therapeutic potential to improve the treatment of major depressive episodes in human compared to SSRI alone.