Blockade of 5-HT1A receptors by (+/-)-pindolol potentiates cortical 5-HT outflow, but not antidepressant-like activity of paroxetine: microdialysis and behavioral approaches in 5-HT1A receptor knockout mice

Ventral hippocampus is more sensitive to fluoxetine-induced changes in extracellular 5-HT concentration, membrane 5-HT transporter level and immobility times

Hippocampal responses to selective 5-HT reuptake inhibitor (SSRI) have long been studied. However, its sub-regional involvements in mediating SSRI's pharmacological effects have not been fully addressed. The current study sought to investigate neurochemical, neurobiological and neurobehavioral changes in response to direct fluoxetine perfusion into the ventral and dorsal sub-regions of the hippocampus in C57BL/6 mice. Following fluoxetine perfusion, time courses of dialysate 5-HT, 5-HT transporter (5-HTT) protein (total, membrane and cytoplasmic fractions), locomotion, and immobility times in the forced swim test (FST) and tail suspension test (TST) were determined. At baseline, 5-HT uptake efficiency assessed by the no-net-flux microdialysis, and 5-HTT protein were measured as well. Results show that fluoxetine dose-dependently increased dialysate 5-HT, lowered membrane 5-HTT protein and increased cytoplasmic fraction without changing the total level, decreased immobility times in both the FST and TST, with greater responses all detected in the ventral sub-region compared to the dorsal sub-region. Fluoxetine didn't affect locomotor activity, ruling out the possibility that fluoxetine's effects on immobility maybe due to alteration in locomotion. Besides, lower 5-HT uptake efficiency and lower membrane 5-HTT protein level were found in the ventral sub-region at baseline. Together, the sub-regional differences at baseline and in responses to fluoxetine added powerful evidence to support the existence of two distinct 5-HT sub-systems in the hippocampus, with greater changes to fluoxetine detected in the ventral sub-system.

Role of 5-HT1A-mediated upregulation of brain indoleamine 2,3 dioxygenase 1 in the reduced antidepressant and antihyperalgesic effects of fluoxetine during maintenance treatment

The reduced antidepressant and antihyperalgesic effects of selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine during maintenance treatment has been reported, but little is known about the molecular mechanism of this phenomenon. In three comorbid pain and depression animal models (genetic predisposition, chronic social stress, arthritis), we showed that the fluoxetine's antidepressant and antihyperalgesic effects were reduced during the maintenance treatment. Fluoxetine exposure induced upregulation of the 5-hydroxytryptamine 1A (5-HT1A) auto-receptor and indoleamine 2,3 dioxygenase 1 (IDO1, a rate-limiting enzyme of tryptophan metabolism) in the brainstem dorsal raphe nucleus (DRN), which shifted the tryptophan metabolism away from the 5-HT biosynthesis. Mechanistically, IDO1 upregulation was downstream to fluoxetine-induced 5-HT1A receptor expression because 1) antagonism of the 5-HT1A receptor with WAY100635 or 5-HT1A receptor knockout blocked the IDO1 upregulation, and 2) inhibition of IDO1 activity did not block the 5-HT1A receptor upregulation following fluoxetine exposure. Importantly, inhibition of either the 5-HT1A receptor or IDO1 activity sustained the fluoxetine's antidepressant and antihyperalgesic effects, indicating that 5-HT1A-mediated IDO1 upregulation in the brainstem DRN contributed to the reduced antidepressant and antihyperalgesic effects of fluoxetine. These results suggest a new strategy to improving the therapeutic efficacy of SSRI during maintenance treatment.

[Consequences of the monoaminergic systems cross-talk in the antidepressant activity]

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-HT_2A 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-HT_2C 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-HT_1A and 5-HT_1B 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-HT_1A and 5-HT_1B 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-HT_2A 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.

[Pharmacological properties of vortioxetine and its pre-clinical consequences]

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.

Alterations in alpha5* nicotinic acetylcholine receptors result in midbrain- and hippocampus-dependent behavioural and neural impairments

RATIONALE

Evidence links alterations in α5-containing nicotinic receptors (α5*-nAChRs) to nicotine addiction. Notably, the rs16969968 polymorphism in the α5 gene (α5SNP) increases the risk for heavy smoking and impairs nicotine-rewarding properties in mice. Additional work is needed to understand how native and polymorphic α5*-nAChRs contribute to processes associated with the risk for nicotine addiction.

OBJECTIVES

We aimed at understanding the contribution of α5*-nAChRs to endophenotypes like increased responses to novelty and anxiety, known to promote vulnerability to addiction, and to the response of the dopamine and serotonin systems to nicotine.

METHODS

Behavioural phenotypes were investigated in mice lacking the α5 gene (α5(-/-)). Nicotine injections were performed to test the consequences of nicotine exposure on the phenotypes identified. Dopamine and serotonin signalling were assessed using in vivo microdialysis and electrophysiology. We used lentiviral vectors to compare the consequences of re-expressing either the α5 wild-type allele or the α5SNP in specific brain areas of α5(-/-) mice.

RESULTS

α5(-/-) mice did not exhibit high responses to novelty but showed decreased novelty-induced rearing behaviour together with high anxiety. Exposure to high doses of nicotine rescued these phenotypes. We identified altered spontaneous and nicotine-elicited serotonin and dopamine activity in α5(-/-) mice. Re-expression of α5 in the ventral tegmental area and hippocampus rescued rearing and anxiety levels in α5(-/-) mice, respectively. When expressing the α5SNP instead, this resulted in a knockout-like phenotype for both behaviours.

CONCLUSIONS

We propose that altered α5*-nAChR cholinergic signalling contributes to emotional/behavioural impairments that may be alleviated by nicotine consumption.

Neurological mechanism of Xiaochaihutang's antidepressant-like effects to socially isolated adult rats

Objectives

Xiaochaihutang (XCHT) has antidepressant effects in multiple animal models of depression in our previous studies. But the antidepressant effects and exact mechanisms of XCHT in a rat model of chronic social isolation stress (CSIS) have never been studied. We therefore aimed to investigate the effects of XCHT on depressive/anxiety-related behaviours of CSIS-exposed rats and understand the neurological mechanism involving neurogenesis.

Methods

We established the CSIS model and then investigated the effects of XCHT on behavioural change. HPLC-MS/MS was adopted to quantify neurotransmitter levels in the cerebrospinal fluid (CSF). Immunofluorescence technology was used to study the effects of XCHT on neurogenesis; while expressions of 5-HT1A receptor signalling pathway in the hippocampus were measured using Western blotting.

Key Findings

Xiaochaihutang significantly alleviated depressive/anxiety-like behaviours of CSIS-exposed rats. XCHT significantly regulated levels of monoamine neurotransmitters in the CSF without affecting Glu, GABA and ACh. XCHT also significantly increased neurogenesis in CSIS-exposed rats. Additionally, XCHT reversed CSIS-induced decrease of 5-HT1A receptor expression and promoted the expression of BDNF in the hippocampus.

Conclusion

Our results suggest that XCHT could significantly regulate the depressive/anxiety-like behaviours induced by CSIS, which are likely attributed to the promotion of hippocampal neurogenesis and neurotrophin expressions through the activation of serotonergic system.

New advances in the treatment of generalized anxiety disorder: the multimodal antidepressant vortioxetine

Generalized Anxiety Disorder (GAD) is a persistent condition characterized by chronic anxiety, exaggerated worry and tension, mainly comorbid with Major Depressive Disorder (MDD). Currently, selective serotonin reuptake inhibitors and serotonin-norepinephrine reuptake inhibitors are recommended as first-line treatment of GAD. However, some patients may not respond to the treatment or discontinue due to adverse effects. Vortioxetine (VRX) is a multimodal antidepressant with a unique mechanism of action, by acting as 5-HT3A, 5-HT1D and 5-HT7 receptor antagonist, partial agonist at the 5-HT1A and 5-HT1B receptors and inhibitor at the 5-HT transporter. Preliminary clinical trials showed contrasting findings in terms of improvement of the anxiety symptomatology and/or cognitive impairment. Here, we aim to systematically review the evidence currently available on the efficacy, safety and tolerability of VRX in the treatment of GAD. The generalizability of results on the efficacy of VRX in patients with anxiety symptomatology and GAD is limited due to few and contrasting RCTs so far available. Only two studies, of which one prevention relapse trial, reported a significant improvement in anxiety symptomatology compared to three with negative findings.

A review of flux considerations for in vivo neurochemical measurements

The mass transport or flux of neurochemicals in the brain and how this flux affects chemical measurements and their interpretation is reviewed. For all endogenous neurochemicals found in the brain, the flux of each of these neurochemicals exists between sources that produce them and the sites that consume them all within μm distances. Principles of convective-diffusion are reviewed with a significant emphasis on the tortuous paths and discrete point sources and sinks. The fundamentals of the primary methods of detection, microelectrodes and microdialysis sampling of brain neurochemicals are included in the review. Special attention is paid to the change in the natural flux of the neurochemicals caused by implantation and consumption at microelectrodes and uptake by microdialysis. The detection of oxygen, nitric oxide, glucose, lactate, and glutamate, and catecholamines by both methods are examined and where possible the two techniques (electrochemical vs. microdialysis) are compared. Non-invasive imaging methods: magnetic resonance, isotopic fluorine MRI, electron paramagnetic resonance, and positron emission tomography are also used for different measurements of the above-mentioned solutes and these are briefly reviewed. Although more sophisticated, the imaging techniques are unable to track neurochemical flux on short time scales, and lack spatial resolution. Where possible, determinations of flux using imaging are compared to the more classical techniques of microdialysis and microelectrodes.

Serotonin 1A and Serotonin 4 Receptors: Essential Mediators of the Neurogenic and Behavioral Actions of Antidepressants

Selective serotonin reuptake inhibitors are the mostly widely used treatment for major depressive disorders and also are prescribed for several anxiety disorders. However, similar to most antidepressants, selective serotonin reuptake inhibitors suffer from two major problems: They only show beneficial effects after 2 to 4 weeks and only about 33% of patients show remission to first-line treatment. Thus, there is a considerable need for development of more effective antidepressants. There is a growing body of evidence supporting critical roles of 5-HT1A and 5-HT4 receptor subtypes in mediating successful depression treatments. In addition, appropriate activation of these receptors may be associated with a faster onset of the therapeutic response. This review will examine the known roles of 5-HT1A and 5-HT4 receptors in mediating both the pathophysiology of depression and anxiety and the treatment of these mood disorders. At the end of the review, the role of these receptors in the regulation of adult hippocampal neurogenesis will also be discussed. Ultimately, we propose that novel antidepressant drugs that selectively target these serotonin receptors could be developed to yield improvements over current treatments for major depressive disorders.

Blockade of the high-affinity noradrenaline transporter (NET) by the selective 5-HT reuptake inhibitor escitalopram: an in vivo microdialysis study in mice

BACKGROUND AND PURPOSE

Escitalopram, the S(+)-enantiomer of citalopram is the most selective 5-HT reuptake inhibitor approved. Although all 5-HT selective reuptake inhibitors (SSRIs) increase extracellular levels of 5-HT ([5-HT](ext)). some also enhance, to a lesser extent, extracellular levels of noradrenaline ([NA](ext)). However, the mechanisms by which SSRIs activate noradrenergic transmission in the brain remain to be determined.

EXPERIMENTAL APPROACH

This study examined the effects of escitalopram, on both [5-HT](ext) and [NA](ext) in the frontal cortex (FCx) of freely moving wild-type (WT) and mutant mice lacking the 5-HT transporter (SERT(-/-)) by using intracerebral microdialysis. We explored the possibilities that escitalopram enhances [NA](ext), either by a direct mechanism involving the inhibition of the low- or high-affinity noradrenaline transporters, or by an indirect mechanism promoted by [5-HT](ext) elevation. The forced swim test (FST) was used to investigate whether enhancing cortical [5-HT](ext) and/or [NA](ext) affected the antidepressant-like activity of escitalopram.

KEY RESULTS

In WT mice, a single systemic administration of escitalopram produced a significant increase in cortical [5-HT](ext) and [NA](ext). As expected, escitalopram failed to increase cortical [5-HT](ext) in SERT(-/-) mice, whereas its neurochemical effects on [NA](ext) persisted in these mutants. In WT mice subjected to the FST, escitalopram increased swimming parameters without affecting climbing behaviour. Finally, escitalopram, at relevant concentrations, failed to inhibit cortical noradrenaline and 5-HT uptake mediated by low-affinity monoamine transporters.

CONCLUSIONS AND IMPLICATIONS

These experiments suggest that escitalopram enhances, although moderately, cortical [NA](ext) in vivo by a direct mechanism involving the inhibition of the high-affinity noradrenaline transporter (NET).

5-HT(1A) [corrected] receptors in mood and anxiety: recent insights into autoreceptor versus heteroreceptor function

RATIONALE

Serotonin (5-HT) neurotransmission is intimately linked to anxiety and depression and a diverse body of evidence supports the involvement of the main inhibitory serotonergic receptor, the serotonin-1A (5-HT(1A)) subtype, in both disorders.

OBJECTIVES

In this review, we examine the function of 5-HT(1A) receptor subpopulations and re-interpret our understanding of their role in mental illness in light of new data, separating both spatial (autoreceptor versus heteroreceptor) and the temporal (developmental versus adult) roles of the endogenous 5-HT(1A) receptors, emphasizing their distinct actions in mediating anxiety and depression-like behaviors.

RESULTS

It is difficult to unambiguously distinguish the effects of different populations of the 5-HT(1A) receptors with traditional genetic animal models and pharmacological approaches. However, with the advent of novel genetic systems and subpopulation-selective pharmacological agents, direct evidence for the distinct roles of these populations in governing emotion-related behavior is emerging.

CONCLUSIONS

There is strong and growing evidence for a functional dissociation between auto- and heteroreceptor populations in mediating anxiety and depressive-like behaviors, respectively. Furthermore, while it is well established that 5-HT(1A) receptors act developmentally to establish normal anxiety-like behaviors, the developmental role of 5-HT(1A) heteroreceptors is less clear, and the specific mechanisms underlying the developmental role of each subpopulation are likely to be key elements determining mood control in adult subjects.

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.

Overview of experimental models of the blood-brain barrier in CNS drug discovery

The blood-brain barrier (BBB) is a physical and metabolic entity that isolates the brain from the systemic circulation. The barrier consists of tight junctions between endothelial cells that contain egress transporters and catabolic enzymes. To cross the BBB, a drug must possess the appropriate physicochemical properties to achieve a sufficient time-concentration profile in brain interstitial fluid (ISF). In this overview, we review techniques to measure BBB permeation, which is evidenced by the free concentration of compound in brain ISF over time. We consider a number of measurement techniques, including in vivo microdialysis and brain receptor occupancy following perfusion. Consideration is also given to the endothelial and nonendothelial cell systems used to assess both the BBB permeation of a test compound and its interactions with egress transporters, and computer models employed for predicting passive permeation and the probability of interactions with BBB transporters.

Antidepressant activity: contribution of brain microdialysis in knock-out mice to the understanding of BDNF/5-HT transporter/5-HT autoreceptor interactions

Why antidepressants vary in terms of efficacy is currently unclear. Despite the leadership of selective serotonin reuptake inhibitors (SSRIs) in the treatment of depression, the precise neurobiological mechanisms involved in their therapeutic action are poorly understood. A better knowledge of molecular interactions between monoaminergic system, pre- and post-synaptic partners, brain neuronal circuits and regions involved may help to overcome limitations of current treatments and identify new therapeutic targets. Intracerebral in vivo microdialysis (ICM) already provided important information about the brain mechanism of action of antidepressants first in anesthetized rats in the early 1990s, and since then in conscious wild-type or knock-out mice. The principle of ICM is based on the balance between release of neurotransmitters (e.g., monoamines) and reuptake by selective transporters [e.g., serotonin transporter for serotonin 5-hydroxytryptamine (5-HT)]. Complementary to electrophysiology, this technique reflects pre-synaptic monoamines release and intrasynaptic events corresponding to ≈80% of whole brain tissue content. The inhibitory role of serotonergic autoreceptors infers that they limit somatodendritic and nerve terminal 5-HT release. It has been proposed that activation of 5-HT_1A and 5-HT_1B receptor sub-types limits the antidepressant-like activity of SSRIs. This hypothesis is based partially on results obtained in ICM experiments performed in naïve, non-stressed rodents. The present review will first remind the principle and methodology of ICM performed in mice. The crucial need of developing animal models that display anxiety and depression-like behaviors, neurochemical and brain morphological phenotypes reminiscent of these mood disorders in humans, will be underlined. Recently developed genetic mouse models have been generated to independently manipulate 5-HT_1A auto and heteroreceptors and ICM helped to clarify the role of the pre-synaptic component, i.e., by measuring extracellular levels of neurotransmitters in serotonergic nerve terminal regions and raphe nuclei. Finally, we will summarize main advantages of using ICM in mice through recent examples obtained in knock-outs (drug infusion through the ICM probe allows the search of a correlation between changes in extracellular neurotransmitter levels and antidepressant-like activity) or alternatives (infusion of a small-interfering RNA suppressing receptor functions in the mouse brain). We will also focus this review on post-synaptic components such as brain-derived neurotrophic factor in adult hippocampus that plays a crucial role in the neurogenic and anxiolytic/antidepressant-like activity of chronic SSRI treatment. Limitations of ICM will also be considered.

Independent of 5-HT1A receptors, neurons in the paraventricular hypothalamus mediate ACTH responses from MDMA

Acute and chronic complications from the substituted amphetamine 3,4-methylenedioxymethamphetamine (MDMA) are linked to activation of the hypothalamic-pituitary-adrenal (HPA) axis. How MDMA activates the HPA axis is not known. HPA responses to stress are known to be mediated through the paraventricular (PVH) hypothalamus and to involve serotonin-1a (5-HT1A) receptors. We sought to determine if the PVH and 5-HT1A receptors were also involved in mediating HPA responses to MDMA. Rats were pretreated with either saline or a 5-HT1A antagonist, WAY-100635 (WAY), followed by a systemic dose of MDMA (7.5mg/kg i.v.). Animals pretreated with WAY had significantly lower plasma ACTH concentrations after MDMA. To determine if neurons in the PVH were involved, and if their involvement was mediated by 5-HT1A receptors, rats implanted with guide cannulas targeting the PVH were microinjected with the GABAA receptor agonist muscimol, aCSF, or WAY followed by MDMA. Compared to aCSF, microinjections of muscimol significantly attenuated the MDMA-induced rise in plasma ACTH (126 vs. 588pg/ml, P=<0.01). WAY had no effect. Our data demonstrates that neurons in the PVH, independent of 5-HT1A receptors, mediate ACTH responses to MDMA.

The role of the serotonergic and GABA system in translational approaches in drug discovery for anxiety disorders

There is ample evidence that genetic factors play an important role in anxiety disorders. In support, human genome-wide association studies have implicated several novel candidate genes. However, illumination of such genetic factors involved in anxiety disorders has not resulted in novel drugs over the past decades. A complicating factor is the heterogeneous classification of anxiety disorders in the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR) and diverging operationalization of anxiety used in preclinical and clinical studies. Currently, there is an increasing focus on the gene × environment (G × E) interaction in anxiety as genes do not operate in isolation and environmental factors have been found to significantly contribute to the development of anxiety disorders in at-risk individuals. Nevertheless, extensive research on G × E mechanisms in anxiety has not resulted in major breakthroughs in drug discovery. Modification of individual genes in rodent models has enabled the specific study of anxiety in preclinical studies. In this context, two extensively studied neurotransmitters involved in anxiety are the gamma-aminobutyric acid (GABA) and 5-HT (5-hydroxytryptamine) system. In this review, we illustrate the complex interplay between genes and environment in anxiety processes by reviewing preclinical and clinical studies on the serotonin transporter (5-HTT), 5-HT1A receptor, 5-HT2 receptor, and GABAA receptor. Even though targets from the serotonin and GABA system have yielded drugs with known anxiolytic efficacy, the relation between the genetic background of these targets and anxiety symptoms and development of anxiety disorders is largely unknown. The aim of this review is to show the vast complexity of genetic and environmental factors in anxiety disorders. In light of the difficulty with which common genetic variants are identified in anxiety disorders, animal models with translational validity may aid in elucidating the neurobiological background of these genes and their possible role in anxiety. We argue that, in addition to human genetic studies, translational models are essential to map anxiety-related genes and to enhance our understanding of anxiety disorders in order to develop potentially novel treatment strategies.

Antidepressant and anxiolytic potential of the multimodal antidepressant vortioxetine (Lu AA21004) assessed by behavioural and neurogenesis outcomes in mice

Vortioxetine (Lu AA21004) is an investigational novel antidepressant with multimodal activity that functions as a 5-HT3, 5-HT7 and 5-HT(1D) receptor antagonist, 5-HT(1B) receptor partial agonist, 5-HT(1A) receptor agonist and inhibitor of the 5-HT transporter in vitro. Here we explore its anxiolytic and antidepressant potential in adult mice. Vortioxetine was assessed in BalB/cJ@RJ mice using the open-field and forced-swim tests (acute: p.o. 1 h, repeated: daily p.o. 21 days), and in 129S6/SvEvTac mice using the novelty suppressed feeding paradigm (acute: p.o. 1 h, sustained: daily p.o. 14 or 21 days). Fluoxetine and diazepam were controls. Acute and repeated dosing of vortioxetine produced more pronounced anxiolytic- and antidepressant-like activities than fluoxetine. Vortioxetine significantly increased cell proliferation and cell survival and stimulated maturation of immature granule cells in the subgranular zone of the dentate gyrus of the hippocampus after 21 days of treatment. After 14 days, a high dose of vortioxetine increased dendritic length and the number of dendrite intersections, suggesting that vortioxetine accelerates the maturation of immature neurons. Vortioxetine displays an antidepressant and anxiolytic profile following repeated administration associated with increased neurogenesis at several stages. Vortioxetine effects were observed at low levels of 5-HT transporter occupancy, suggesting an alternative mechanism of action to 5-HT reuptake inhibition.

Benzothiazoles as probes for the 5HT1A receptor and the serotonin transporter (SERT): a search for new dual-acting agents as potential antidepressants

The synthesis and evaluation of several benzothiazole-based compounds are described in an attempt to identify novel dual-acting 5HT(1A) receptor and SERT inhibitors as new antidepressants. Binding affinities at the 5HT(1A) receptor and the serotonin transporter do not appear to be congruent and other areas of the binding sites would need to be explored in order to improve binding simultaneously at both sites. Compounds 20 and 23 show moderate binding affinity at the 5HT(1A) receptor and the SERT site and thus, have the potential to be further explored as dual-acting agents. In addition, compound 20 binds with low affinity to the dopamine transporter (DAT), the norepinephrine transporter (NET) and 5HT(2C) receptor, which are desirable properties as selectivity for SERT (and not DAT or NET) is associated with an absence of cardiovascular side effects.

Functional status of somatodendritic serotonin 1A autoreceptor after long-term treatment with fluoxetine in a mouse model of anxiety/depression based on repeated corticosterone administration

Most preclinical studies investigating the effects and the mechanism of action of antidepressants have been performed in naive rodents. This is inappropriate because antidepressants act on specific symptoms of the pathological condition, such as distress and anxiety. We have developed a mouse model of anxiety/depression based on addition of corticosterone to drinking water. This model is highly reproducible and easy to set up compared with unpredictable chronic mild stress. The serotonin 1A (5-HT(1A)) autoreceptor is known to play a role in mood disorders and their treatments. An increase in somatodendritic 5-HT(1A) autoreceptor density in the dorsal raphe (DR) attenuates the therapeutic activity of selective serotonin-reuptake inhibitors (SSRIs), whereas their functional desensitization promotes activation of brain serotonergic transmission, thereby representing an adaptive change relevant to their therapeutic effect. Here we assessed the effects of sustained administration of the SSRI fluoxetine on 5-HT(1A) autoreceptor sensitivity in mice administered with corticosterone. Fluoxetine attenuated hypothermia induced by the 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin, decreased DR 5-HT neuronal activity, and decreased 5-HT release in both vehicle- and corticosterone-pretreated mice. However, such desensitization was more pronounced in corticosterone-pretreated mice. This change had an overall effect on serotonergic tone because we found a greater firing rate of 5-HT neurons associated with an enhancement of 5-HT outflow in the DR of corticosterone-pretreated mice in response to fluoxetine compared with the corresponding group of vehicle-pretreated mice. These results provide cellular explanations for the antidepressant effects produced by SSRIs in subjects with pathological conditions but not in naive animals or healthy volunteers.

Rodent models in depression research: classical strategies and new directions

Depression, among other mood disorders, represents one of the most common health problems worldwide, with steadily increasing incidence and major socio-economic consequences. However, since the knowledge about the underlying pathophysiological principles is still very scanty, depression and other mood disorders are currently diagnosed solely on clinical grounds. Currently used treatment modalities would therefore benefit enormously from the development of alternative therapeutic interventions. The implementation of proper animal models is a prerequisite for increasing the understanding of the neurobiological basis of mood disorders and is paving the way for the discovery of novel therapeutic targets. In the past thirty years, since the seminal description of the Forced Swim Test as a system to probe antidepressant activity in rodents, the use of animals to model depression and antidepressant activity has come a long way. In this review we describe some of the most commonly used strategies, ranging from screening procedures, such as the Forced Swim Test and the Tail Suspension Test and animal models, such as those based upon chronic stress procedures, to genetic approaches. Finally we also discuss some of the inherent limitations and caveats that need to be considered when using animals as models for mental disorders in basic research.

VGLUT3 (vesicular glutamate transporter type 3) contribution to the regulation of serotonergic transmission and anxiety

Three different subtypes of H(+)-dependent carriers (named VGLUT1-3) concentrate glutamate into synaptic vesicles before its exocytotic release. Neurons using other neurotransmitter than glutamate (such as cholinergic striatal interneurons and 5-HT neurons) express VGLUT3. It was recently reported that VGLUT3 increases acetylcholine vesicular filling, thereby, stimulating cholinergic transmission. This new regulatory mechanism is herein designated as vesicular-filling synergy (or vesicular synergy). In the present report, we found that deletion of VGLUT3 increased several anxiety-related behaviors in adult and in newborn mice as early as 8 d after birth. This precocious involvement of a vesicular glutamate transporter in anxiety led us to examine the underlying functional implications of VGLUT3 in 5-HT neurons. On one hand, VGLUT3 deletion caused a significant decrease of 5-HT(1A)-mediated neurotransmission in raphe nuclei. On the other hand, VGLUT3 positively modulated 5-HT transmission of a specific subset of 5-HT terminals from the hippocampus and the cerebral cortex. VGLUT3- and VMAT2-positive serotonergic fibers show little or no 5-HT reuptake transporter. These results unravel the existence of a novel subset of 5-HT terminals in limbic areas that might play a crucial role in anxiety-like behaviors. In summary, VGLUT3 accelerates 5-HT transmission at the level of specific 5-HT terminals and can exert an inhibitory control at the raphe level. Furthermore, our results suggest that the loss of VGLUT3 expression leads to anxiety-associated behaviors and should be considered as a potential new target for the treatment of this disorder.

A longitudinal study of 5-HT outflow during chronic fluoxetine treatment using a new technique of chronic microdialysis in a highly emotional mouse strain

The onset of a therapeutic response to antidepressant treatment exhibits a delay of several weeks. The present study was designed to know whether extracellular serotonin (5-HT) levels need to be increased in territories of 5-HT innervation in order to obtain beneficial effects from a chronic treatment with a serotonin-selective reuptake inhibitor (SSRI). Thus, we performed a longitudinal study of a chronic fluoxetine treatment in a model of highly emotional mice (BALB/cJ). The function of the 5-HT system in the raphe nuclei and hippocampus, was assessed by using repeated in vivo microdialysis sessions in awake freely moving mice, then studying its relation with behavior, analyzed mainly with open field paradigm. One of the neural mechanisms underlying such delay has been proposed to be the functional status of 5-HT1A autoreceptors in raphe nuclei. Thus, we also assessed the degree of 5-HT1A autoreceptor desensitization by using a local infusion in the raphe of the antagonist, WAY 100635 via reverse microdialysis. We report that the anxiolytic-like effects of fluoxetine correlate in time and amplitude with 5-HT1A autoreceptor desensitization, but neither with the extracellular levels of 5-HT in the raphe nuclei, nor in the hippocampus. Our study suggests that the beneficial anxiolytic/antidepressant-like effects of chronic SSRI treatment indeed depend on 5-HT1A autoreceptor internalization, but do not require a sustained increase in extracellular 5-HT levels in a territory of 5-HT projection such as hippocampus.

The serotonin-1A receptor in anxiety disorders

The serotonin system plays an important role in the neural processing of anxiety. The involvement of the main inhibitory serotonergic receptor, the serotonin-1A (5-HT1A) subtype, in dysfunctional forms of anxiety has been supported by findings from a wide range of preclinical research and clinical trials, including treatment studies, genetic research, and neuroimaging data. The following article summarizes preclinical results with a focus on 5-HT1A receptor knockout and transgenic mice as genetic models of anxiety. Behavioral, autonomic, and endocrinological changes in these mice are reported. This article also presents genetic polymorphisms in humans associated with increased anxiety scores and pharmacological data focused on 5-HT1A receptor agonists and antagonists. Furthermore, molecular neuroimaging results are presented. Recent positron emission tomography (PET) studies have reported reduced 5-HT1A receptor binding in patients with panic disorder and social anxiety disorder, but not in posttraumatic stress disorder. In healthy subjects, increased anxiety scores might be associated with lower 5-HT1A receptor binding. This overview of preclinical and clinical data provides strong evidence for the key role of the 5-HT1A receptor in the serotonergic dysregulation of anxiety disorders.

Synergistic neurochemical and behavioural effects of acute intrahippocampal injection of brain-derived neurotrophic factor and antidepressants in adult mice

Preclinical data support the view that brain-derived neurotrophic factor (BDNF) and serotonergic systems regulate circuits involved in affective disorders. The present study examined neurochemical and behavioural consequences of an acute intrahippocampal injection of BDNF combined with an antidepressant by using in-vivo intracerebral microdialysis in the ventral hippocampus (vHi) in conscious mice and behavioural paradigms predictive of antidepressant and anxiolytic-like effects [the mouse forced swim test (FST), the open-field (OF) paradigm and the elevated plus maze (EPM)]. Neurochemical data revealed that BDNF (100 ng) potentiated the effects of the systemic administration of a serotonin selective reuptake inhibitor (SSRI; paroxetine 4 mg/kg i.p.) and that of a locally applied citalopram perfusion on dialysate 5-HT levels in the vHi. These neurochemical changes correlated with behavioural data since, in the FST, antidepressant-like activity of paroxetine as measured on swimming behaviour was potentiated by BDNF. These data suggest an interesting synergy between BDNF and SSRI on antidepressant-like activity. Furthermore, in both the OF and EPM paradigms BDNF induced an anxiogenic-like activity, whereas paroxetine prevented this effect. Finally, the neurochemical and behavioural effects of BDNF on the serotonergic system might occur at both pre- and post-synaptic levels since by using in-situ hybridization, we showed that TrkB-R mRNA was expressed in the hippocampus and the dorsal raphe nucleus in adult mice. Taken together the neurochemical and behavioural effects of BDNF suggest that these behavioural changes were mediated by increases in 5-HT neurotransmission in vHi. Thus a BDNF+SSRI combination may offer new alternatives to treat mood disorders.

Enhancement of cortical extracellular 5-HT by 5-HT1A and 5-HT2C receptor blockade restores the antidepressant-like effect of citalopram in non-responder mice

We recently found that the response of DBA/2 mice to SSRIs in the forced swim test (FST) was impaired and they also had a smaller basal and citalopram-stimulated increase in brain extracellular serotonin (5-HT) than 'responder' strains. We employed intracerebral microdialysis, FST and selective antagonists of 5-HT1A and 5-HT2C receptors to investigate whether enhancing the increase in extracellular 5-HT reinstated the anti-immobility effect of citalopram in the FST. WAY 100635 (0.3 mg/kg s.c.) or SB 242084 (1 mg/kg s.c.), respectively a selective 5-HT1A and 5-HT2C receptor antagonist, raised the effect of citalopram (5 mg/kg) on extracellular 5-HT in the medial prefrontal cortex of DBA/2N mice (citalopram alone 5.2+/-0.3 fmol/20 microl, WAY 100635+citalopram 9.9+/-2.1 fmol/20 microl, SB 242084+ citalopram 7.6+/-1.0 fmol/20 microl) to the level reached in 'responder' mice given citalopram alone. The 5-HT receptor antagonists had no effect on the citalopram-induced increase in extracellular 5-HT in the dorsal hippocampus. The combination of citalopram with WAY 100635 or SB 242084 significantly reduced immobility time in DBA/2N mice that otherwise did not respond to either drug singly. Brain levels of citalopram in mice given citalopram alone or with 5-HT antagonists did not significantly differ. The results confirm that impaired 5-HT transmission accounts for the lack of effect of citalopram in the FST and suggest that enhancing the effect of SSRIs on extracellular 5-HT, through selective blockade of 5-HT1A and 5-HT2C receptors, could be a useful strategy to restore the response in treatment-resistant depression.

Interest of using genetically manipulated mice as models of depression to evaluate antidepressant drugs activity: a review

Among the multiple possibilities to study human depressive disorders, animal models remain important preclinical tools. They allow the understanding of the mechanisms of action of antidepressant drugs. Primarily developed in rat, animal models of depression have been adapted to the mouse, an easy-to-use mammal with better genetic possibilities than rats. As an example, genetic manipulation of the serotoninergic 5-hydroxytryptamine-HT; (5-HT) system provided important opportunities to investigate the role of this monoamine in mood disorders. The contribution of either constitutive knockout (KO), tissue specific, or inducible KO mice and animal models in the current knowledge of the pathophysiology and treatment of depression is unanimously recognized. The phenotype of genetically manipulated animals is strongly influenced by both the genetic background of the animal as well as environmental factors. For these reasons, it is necessary to underline that KO mice have been generated on various genetic backgrounds, which strongly influence the behavioral and neurochemical responses to the tests. The present review will thus focus on KO mice lacking G protein-coupled monoaminergic receptors (e.g; 5-HT1B, 5-HT1A, and 5-HT4 receptors) and the 5-HT serotonin transporter, which is the main target of antidepressant drugs (or strategies). The importance of KO mice for neurotrophic factors, particularly for brain-derived neurotrophic factor and its main receptor displaying a tyrosine kinase activity, will also be addressed to illustrate the fact that in preclinical studies, combination of genetic manipulations with pharmacological ones should allow further progress in the field of neuropsychopharmacology.

Rodent models of nicotine withdrawal syndrome

Simple, rapid and inexpensive rodent models of nicotine physical dependence and withdrawal syndrome have proved useful for preliminary screening of smoking cessation treatments. They have led to an exponential increase of knowledge regarding the underlying neurobiological mechanisms of dependence and withdrawal syndrome. The human nicotine withdrawal syndrome in smoking cessation is variable and multidimensional, involving irritability, anxiety, depression, cognitive and attentional impairments, weight gain, sleep disturbances, and craving for nicotine. Aside from sleep disturbances, analogous phenomena have been seen in rodent models using different measures of withdrawal intensity. It appears likely that different withdrawal phenomena may involve some partially divergent mechanisms. For example, depression-like phenomena may involve alterations in mechanisms such as the mesolimbic dopamine pathway from the ventral tegmental area to the nucleus accumbens. Irritability and anxiety may involve alterations in endogenous opioid systems and other regions, such as the amygdala. This chapter reviews many additional anatomical, neurochemical, and developmental elements that impact nicotine physical dependence.

Mutant mouse models and antidepressant drug research: focus on serotonin and brain-derived neurotrophic factor

Several lines of knockout (KO) mice have been evaluated as models of depression-related behavioral and neurobiological changes, and used to investigate molecular and cellular mechanisms underlying the activity of antidepressant drugs. Adult neurogenesis and brain 5-hydroxytryptamine (5-HT)/neurotrophic factor interactions have recently attracted great interest in relation to the mechanism of action of antidepressant drugs. The present review focuses primarily on genetic manipulation of the serotoninergic (5-HT) system. Basal neurochemical and behavioral changes occurring in mice lacking the 5-HT transporter (SERT), which is the main target of antidepressant drugs, as well as in those lacking G protein-coupled serotonin receptors (e.g. 5-HT1B, 5-HT1A, and 5-HT4 receptors) are described and evaluated. The importance of KO mice for neurotrophic factors, particularly for brain-derived neurotrophic factor and its high-affinity receptor (R-TrkB), is also addressed. Constitutive KO, tissue specific, or inducible KO mice targeting both 5-HT and brain-derived neurotrophic factor systems may potentially make an important contribution to knowledge of the pathophysiology and treatment of depression.

Neurokinin1 antagonists potentiate antidepressant properties of serotonin reuptake inhibitors, yet blunt their anxiogenic actions: a neurochemical, electrophysiological, and behavioral characterization

Though neurokinin(1) (NK(1)) receptor antagonists are active in experimental models of depression, clinical efficacy has proven disappointing. This encourages interest in association of NK(1) receptor blockade with inhibition of serotonin (5-HT) reuptake. The selective NK(1) antagonist, GR205171, dose-dependently enhanced citalopram-induced elevations of extracellular levels of 5-HT in frontal cortex, an action expressed stereospecifically vs its less active distomer, GR226206. Further, increases in 5-HT levels in dorsal hippocampus, basolateral amygdala, nucleus accumbens, and striatum were likewise potentiated, and GR205171 similarly facilitated the influence of fluoxetine upon levels of 5-HT, as well as dopamine and noradrenaline. In parallel electrophysiological studies, the inhibitory influence of citalopram and fluoxetine upon raphe-localized serotonergic neurones was stereospecifically blunted by GR205171. Antidepressant actions of citalopram in a forced-swim test in mice were stereospecifically potentiated by GR205171, and it also enhanced attenuation by citalopram of stress-related ultrasonic vocalizations in rats. Further, GR205171 and citalopram additively abrogated the advance in circadian rhythms provoked by exposure to light in hamsters. By contrast, GR205171 stereospecifically blocked anxiogenic actions of citalopram in social interaction procedures in rats and gerbils, and stereospecifically abolished facilitation of fear-induced foot tapping by fluoxetine in gerbils. By analogy to GR205171, a further NK(1) antagonist, RP67580, enhanced the influence of citalopram upon frontocortical levels of 5-HT and potentiated its actions in the forced swim test. In conclusion, NK(1)receptor blockade differentially modulates functional actions of SSRIs: antidepressant properties are reinforced, whereas anxiogenic effects are attenuated. Combined NK(1) receptor antagonism/5-HT reuptake inhibition may offer advantages in the management of depressed and anxious states.

Computer assisted video analysis of swimming performance in a forced swim test: simultaneous assessment of duration of immobility and swimming style in mice selected for high and low swim-stress induced analgesia

In behavioral pharmacology, two problems are encountered when quantifying animal behavior: 1) reproducibility of the results across laboratories, especially in the case of manual scoring of animal behavior; 2) presence of different behavioral idiosyncrasies, common in genetically different animals, that mask or mimic the effects of the experimental treatments. This study aimed to develop an automated method enabling simultaneous assessment of the duration of immobility in mice and the depth of body submersion during swimming by means of computer assisted video analysis system (EthoVision from Noldus). We tested and compared parameters of immobility based either on the speed of an object (animal) movement or based on the percentage change in the object's area between the consecutive video frames. We also examined the effects of an erosion-dilation filtering procedure on the results obtained with both parameters of immobility. Finally, we proposed an automated method enabling assessment of depth of body submersion that reflects swimming performance. It was found that both parameters of immobility were sensitive to the effect of an antidepressant, desipramine, and that they yielded similar results when applied to mice that are good swimmers. The speed parameter was, however, more sensitive and more reliable because it depended less on random noise of the video image. Also, it was established that applying the erosion-dilation filtering procedure increased the reliability of both parameters of immobility. In case of mice that were poor swimmers, the assessed duration of immobility differed depending on a chosen parameter, thus resulting in the presence or lack of differences between two lines of mice that differed in swimming performance. These results substantiate the need for assessing swimming performance when the duration of immobility in the FST is compared in lines that differ in their swimming "styles". Testing swimming performance can also be important in the studies investigating the effects of swim stress on other behavioral or physiological parameters because poor swimming abilities displayed by some lines can increase severity of swim stress, masking the between-line differences or the main treatment effects.

Evidence of an enhanced central 5HT response in irritable bowel syndrome and in the rat maternal separation model

Efforts to define either a biomarker for irritable bowel syndrome (IBS) or a valid animal model have proven disappointing. The aims of this study were to determine if buspirone stimulates prolactin release through the 5-hydroxytryptamine (5HT)1a receptor and whether this response is altered in patients with IBS and in the rat maternal separation model. Buspirone (30 mg) was used to stimulate prolactin release in 40 patients with IBS and in 40 healthy controls. In study 1, 10 IBS patients and 10 controls underwent pretreatment with pindolol (5HT1a antagonist) or placebo followed by buspirone. In study 2, 30 patients with IBS and 30 healthy controls had prolactin release stimulated by buspirone. Maternally separated and nonseparated rats were also treated with buspirone and prolactin monitored. Serotonin metabolites were measured together with the expression of the 5HT1a and serotonin transporter (SERT) gene. Pindolol produced a dose-dependent decrease in the buspirone prolactin response. Patients with IBS and maternally separated rats showed an exaggerated release of prolactin in response to buspirone. In the animal model, an increased turnover of 5HT was found in the brainstem together with a trend toward increased activity of the SERT gene. In conclusion altered central serotonin responses are found in both IBS and in an animal model.

Brain-derived neurotrophic factor-deficient mice exhibit a hippocampal hyperserotonergic phenotype

Growing evidence supports the involvement of brain-derived neurotrophic factor (BDNF) in mood disorders and the mechanism of action of antidepressant drugs. However, the relationship between BDNF and serotonergic signalling is poorly understood. Heterozygous mutants BDNF +/- mice were utilized to investigate the influence of BDNF on the serotonin (5-HT) system and the activity of the serotonin transporter (SERT) in the hippocampus. The zero net flux method of quantitative microdialysis revealed that BDNF +/- heterozygous mice have increased basal extracellular 5-HT levels in the hippocampus and decreased 5-HT reuptake capacity. In keeping with these results, the selective serotonin reuptake inhibitor paroxetine failed to increase hippocampal extracellular 5-HT levels in BDNF +/- mice while it produced robust effects in wild-type littermates. Using in-vitro autoradiography and synaptosome techniques, we investigated the causes of attenuated 5-HT reuptake in BDNF +/- mice. A significant decrease in [3H]citalopram-binding-site density in the CA3 subregion of the ventral hippocampus and a significant reduction in [3H]5-HT uptake in hippocampal synaptosomes, revealed mainly a decrease in SERT function. However, 5-HT1A autoreceptors were not desensitized in BDNF +/- mice. These results provide evidence that constitutive reductions in BDNF modulate SERT function reuptake in the hippocampus.

Substance P Neurokinin 1 Receptor Activation within the Dorsal Raphe Nucleus Controls Serotonin Release in the Mouse Frontal Cortex

Preclinical studies suggest that substance P (SP) neurokinin 1 (NK1) receptor antagonists are efficient in the treatment of anxiety and depression. This therapeutic activity could be mediated via stimulation of serotonin (5-HT) neurons located in the dorsal raphe nucleus (DRN), which receive important SP-NK1 receptor immunoreactive innervations. The present study examined the effects of intraraphe injection of SP on extracellular 5-HT levels in the frontal cortex, ventral hippocampus, and DRN by using intracerebral microdialysis in conscious mice. Intraraphe SP injection dose dependently decreased cortical 5-HT release, whereas no effects were detected in the ventral hippocampus. Cortical effects were blocked by the selective NK1 receptor antagonist N-[[2-methoxy-5-[5-(trifluoromethyl)tetrazol-1-yl]phenyl]methyl]-2-phenylpiperidin-3-amine (GR205171) and completely dampened in mice lacking NK1 receptors. Furthermore, genetic (in knockout 5-HT1A(-/-) mice) or pharmacological inactivation of 5-HT1A autoreceptors blocked cortical responses to SP. Contrasting with its cortical effects, intraraphe SP injection increased 5-HT outflow in the DRN in wild-type mice; this effect was potentiated by a local perfusion of the selective 5-HT1A antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY100635). Finally, SP-induced changes in frontal cortex and DRN dialysate 5-HT levels were blocked by the DRN perfusion of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate ionotropic receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). These data support the hypothesis that SP-induced over-activation of 5-HT1A autoreceptors within the DRN limits cortical 5-HT release. A better knowledge of the complex relationship between tachykininergic, serotonergic, and glutamatergic systems within the DRN might help better understand the pathophysiology and subsequent treatment of depression.