Mol Neurobiol DOI 10.1007/s12035-015-9152-z

Revisiting the Serotonin Hypothesis: Implications for Major Depressive Disorders Marc Fakhoury 1

Received: 29 January 2015 / Accepted: 19 March 2015 # Springer Science+Business Media New York 2015

Abstract Major depressive disorder (MDD) is a heritable neuropsychiatric disease associated with severe changes at cellular and molecular levels. Its diagnosis mainly relies on the characterization of a wide range of symptoms including changes in mood and behavior. Despite the availability of antidepressant drugs, 10 to 30 % of patients fail to respond after a single or multiple treatments, and the recurrence of depression among responsive patients is very high. Evidence from the past decades suggests that the brain neurotransmitter serotonin (5-HT) is incriminated in MDD, and that a dysfunction of 5-HT receptors may play a role in the genesis of this disease. The 5-HT membrane transporter protein (SERT), which helps regulate the serotonergic transmission, is also implicated in MDD and is one of the main targets of antidepressant therapy. Although a number of behavioral tests and animal models have been developed to study depression, little is known about the neurobiological bases of MDD. Understanding the role of the serotonergic pathway will significantly help improve our knowledge of the pathophysiology of depression and may open up avenues for the development of new antidepressant drugs. The overarching goal of this review is to present recent findings from studies examining the serotonergic pathway in MDD, with a focus on SERT and the serotonin 1A (5-HT1A), serotonin 1B (5-HT1B), and serotonin 2A (5-HT2A) receptors. This paper also describes some of the main molecules involved in the internalization of 5-HT receptors and illustrates the changes in 5-HT neurotransmission in knockout mice and animal model of depression.

* Marc Fakhoury [email protected] 1

Department of Neuroscience, Faculty of Medicine, University of Montreal, Montreal, QC H3C3J7, Canada

Keywords G-protein coupling . Gene knockout . Major depressive disorder . Serotonin . Serotonin receptor . Transporter

Introduction Major depressive disorder (MDD) is one of the leading causes of mental disability worldwide and can affect a person at any point in his life [1]. Also called major depression or clinical depression, this disorder is the most severe form of depression and can lead to a variety of emotional and physical problems [2]. MDD frequently follows a traumatic emotional experience and can be precipitated by the chronic use of pharmacological agents [3]. Individuals affected by this disease exhibit a depressed mood and a generalized loss of interest and display symptoms such as reduced appetite, disturbed sleep, and increased thoughts of suicide [4]. In addition to the devastating effects on the individual, this disease is a heavy burden to the society in terms of health care costs. The ensuing costs of major depression for a society can go up to several billion dollars per year, which are mostly related to the implementation of preventive and rehabilitative programs. Given the complex interaction between genetic and environmental factors, the etiology of MDD is far from being understood [2, 5]. However, accumulating amount of evidence show that mood disorders such as MDD are characterized by a dysfunction of the serotonin (5-HT) neurotransmission [6]. One of the key players in the serotonergic pathway is the neuronal plasma membrane transporter protein (SERT), which plays a major role in the pathophysiology of MDD [7]. Since the isolation of SERT cDNA from rat [8], researchers were able to study its interaction with antidepressant agents at the

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molecular and cellular levels. It was shown that antidepressant drugs, such as the selective 5-HT reuptake inhibitors (SSRIs), directly act on SERT to modulate its activity, resulting in reduced 5-HT uptake by synaptic cells [9]. Despite several decades of research, nothing is known about the consequences of antidepressant drug action on the functional localization of SERT, and the identification of the underlying mechanisms of antidepressant therapy still remains a big challenge. Other players involved in the pathophysiology of MDD include the 5-HT receptors, which are mainly localized on somato-dendritic neurons [9, 10]. There exist two types of 5-HT receptors: the 5-HT autoreceptors, which are largely found on nuclei of origin [11], and the 5-HT heteroreceptors, which are localized on areas of synaptic projections [12]. In animal models of depression, it has been shown that an alteration of5-HT receptors activity produces long-lasting antidepressant effects [9]. Among all 5-HT receptors identified to date, the molecules that are believed to be the most closely associated with mood disorders are the serotonin 1A (5HT1A), serotonin 1B (5-HT1B), and serotonin 2A (5HT2A) receptors [13, 14]. They are the most studied among all other types of receptors, and along with SERT, they constitute the main therapeutic targets of antidepressant therapies. This paper provides an overview of the main players involved in the serotonergic pathway and describes the molecular mechanisms of antidepressant drugs. Behavioral and pharmacological data for SERT and 5-HT1A, 5-HT1B, and 5-HT2A receptors are presented in the following sections and are illustrated in Table 1. Moreover, a closer look is given at the main findings from animal studies and their clinical applications.

Table 1

The 5-HT Hypothesis in the Pathophysiology of MDD From all the hypotheses formulated to date, the 5-HT hypothesis of depression is the most influential and the most studied in attempts to develop new treatment strategies for MDD. This hypothesis states that a reduction in 5-HT level increases the risk of being affected by depression [15]. Since its formulation, many studies have validated the link between the 5-HT system and MDD. It was shown that depressed patients display low level of blood platelet 5-HT [16], as well as low plasma levels of L-tryptophan, a 5-HT precursor [17]. In depressed patients, tryptophan administration seems to provide beneficial effects, whereas depletion of dietary tryptophan induces a recurrence of depressive symptoms [18]. Although deficits in neurotransmitters such as dopamine and norepinephrine may also contribute to the corresponding features of MDD, most studies have focused on the serotonergic system because of the efficacy of SSRIs in treating MDD [19]. SSRIs are a class of compounds typically used as antidepressants and are the most prescribed medications in the treatment of MDD. They increase the extracellular level of 5-HT by inhibiting its reuptake into the presynaptic cell, leading to better neurotransmission and improved mood [20]. Individuals treated with SSRIs such as citalopram and clomipramine can anticipate a high probability of symptom improvement within few weeks of administration [21]. Other SSRIs, including fluoxetine and sertraline, have also shown great promise in the treatment of MDD, because of their ability to effectively block the activity of SERT [22, 23]. Taken together, studies investigating the serotonergic hypothesis in MDD illustrate the importance of maintaining appropriate amount of 5-HT in the brain, and that deviations from normal levels may increase the risk of getting MDD.

SERT and 5-HT1A, 5-HT1B, and 5-HT2A receptors: behavioral and pharmacological data Agonist/activator

Antagonist

Location

Effect of gene knockout

SERT

Bis I

ADAM McN5652

5-HT/thalamo-cortical neurons

5-HT1A

8-OH-DPAT Buspirone Clozapine Flesinoxan Anpirtoline CP 94253

WAY-100635

DOI

M100907 Metergoline Nefazodone

DRN Hippocampus Prefrontal cortex Limbic system Basal ganglia Substantia nigra Nucleus accumbens DRN Frontal cortex Nucleus accumbens

Anxiogenic-like phenotype Increased sensitivity to stress Decreased exploratory activity Anxiety-like behavior Reduced exploratory activity Increased 5-HT firing activity

5-HT1B

5-HT2A

SB 216641 GR127935

Aggressive behavior Increased exploratory activity Improved spatial learning Altered sleep pattern Reduced locomotor activity

This table provides a nonexhaustive list of the agonists/activators and antagonists for SERT and 5-HT receptors in the brain. It also illustrates the main behavioral and neurobiological effects observed in knockout animal models

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SERT, the Main Target of Antidepressant Therapy SERT is a type of monoamine transporter protein that regulates the serotonergic neurotransmission by removing 5-HT from the extracellular fluid [24]. It co-transports one sodium and chloride ion with each molecule of 5-HT, followed by the counter-transport of one potassium ion [25]. This neuronal transporter has been cloned in human and was identified as a protein of 630 amino acids with 12 putative transmembrane domains [26, 27]. SERT is mainly expressed by 5-HT neurons, but also by thalamo-cortical neurons during postnatal development in the rat [28, 29]. In adult rat and human, its messenger RNA (mRNA) is confined to 5-HT neurons [30]. Studies showed that individuals with functional polymorphisms in the promoter region of SERT gene often display more depressive symptoms in relation to stressful life events, illustrating the important role of SERT in the regulation of mood disorders [31, 32]. A growing number of investigators are pushing their way through the characterization of the molecular effects of selective ligands on SERT. Use of selective inhibitory ligands has been extensively studied for the visualization and quantification of SERT in the brain [7]. Examples of such molecules include ADAM and McN5652 [7, 33]. They exhibit stronger affinity for SERT compared to other monoamine transporters and can act as antagonists by blocking the 5-HT reuptake mechanism in the brain [7]. By contrast, bisindolylmaleimide I (Bis I), which inhibits SERT phosphorylation, has been found to upregulate the serotonergic uptake in the brain [34]. Several studies have also investigated the pharmacological effects of antidepressant drugs on the activity of SERT in vivo. Antidepressants such as SSRIs directly bind to SERT and inhibit 5-HTreuptake [35]. By doing so, they increase the synaptic availability of 5-HT, which makes them beneficial for treating MDD (Fig. 1). Strong evidence suggests that upon SSRI treatment, SERT is highly regulated and

Fig. 1 Overview of the molecular mechanisms of 5-HT reuptake in the brain. SERT is mostly found in presynaptic neurons, whereas 5-HT receptors are G-protein coupled receptors mainly located on postsynaptic cells. a In the absence of SSRIs, 5-HT binds to SERT and is recycled to the interior of the presynaptic neuron. b SSRIs downregulate the activity of SERT and increase the extracellular level of 5-HT. 5-HTR 5-HT receptor

undergoes adaptive changes [36]. In vitro studies indicate that following SSRI treatment, SERT undergoes internalization into the cytoplasm in heterologous cell lines and 5-HT neurons, leading to reduced plasmalemmal density [37]. Interestingly, upon drug cessation, internalized SERT molecules reappear on the cell surface of neurons, suggesting that this process is transient and reversible [37]. Clearly, more work is needed to develop better ways to produce longerlasting internalization of SERT and understand the role of this regulatory step in the pathophysiology of MDD.

5-HT Receptors: Functional Localization and Internalization 5-HT1A Receptors There is enough evidence supporting the notion that a dysfunction of 5-HT1A receptors may alter the risk of developing one or more depressive episodes. The 5-HT1A receptor is a 422 amino acid protein that belongs to the family of G proteincoupled receptors (GPCRs) with seven putative transmembrane domains [38]. It has been associated with anxiety, depression, and cognitive functions such as learning and memory [9]. It is a somato-dendritic receptor, particularly abundant in the dorsal nucleus raphe (DRN) and the hippocampus, but also present in the prefrontal cortex and most areas of the limbic system [39]. 5-HT1A receptors exert distinct functions depending on their location. In nuclei of origin, such as the DRN, they function as autoreceptors that regulate the release of 5-HT from neurons [11], whereas in territories of projection such as the hippocampus, they function as heteroreceptors that mediate the inhibition of nonserotonergic neurons [12]. While 5-HT1A autoreceptors are known to undergo a rapid desensitization after activation, 5-HT1A heteroreceptors do not desensitize [40, 41].

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Effects of 5-HT1A receptor ligands on depression have been extensively investigated in vivo. Administration of specific agonists, such as buspirone, induces anxiolytic-like effects in rodents [42]. Long-term treatment with buspirone acts through a combination of their desensitizing effects on 5HT1A autoreceptors and their activation of normally sensitive postsynaptic receptors [10]. Other agonists known to directly act with 5-HT1A receptors and influence the serotonergic neurotransmission include 8-OH-DPAT, clozapine, and flesinoxan [43]. By contrast, 5HT1A antagonists have been extensively used to quantify the amount 5HT1A receptors in vivo using specific neuroimaging techniques. An imaging study using WAY100635, a 5HT1A antagonist, showed that the 5-HT1A receptor binding is significantly different between healthy subjects and individuals with MDD [44]. Evidence also suggests that administration of SSRIs leads to a rapid desensitization of 5-HT1A autoreceptors, followed by their internalization in DRN neurons [45]. As a result, the level of 5-HT neurotransmission increases in forebrain territories of serotonergic innervation, which may explain the therapeutic effect of agonists and antidepressant drugs. Furthermore, findings from genetic studies reported that a functional variant of the 5-HT1A receptor gene could alter the response to antidepressant treatment in melancholic MDD [46]. Taken together, these studies strongly support the notion that 5-HT1A receptors play a key regulatory role in the pathophysiology of MDD and that their activity and location could predict the behavioral effects of antidepressant drugs. 5-HT1B Receptors 5-HT1B receptors contribute to the dynamic regulation of the serotonergic pathway, impacting on several functions including cognition and emotion [13, 47]. They are one of the main targets of antidepressant drugs and one of the most studied among other types of 5-HT receptors [48]. The 5-HT1B receptor is a protein encoded by the HTR1B gene and has been cloned in the rat, mouse, and human [49–51]. It is highly dense in the basal ganglia, nucleus accumbens, and substantial nigra, whereas its mRNA level is significantly high in the DRN, cingulate cortex, hippocampus, striatum, and amygdala [9]. This anatomical distribution is explained by the fact that 5-HT1B receptors get transported to the terminal parts of axons either as heteroreceptors or autoreceptors [52]. This concept is crucial in understanding their role in the pathophysiology of MDD. The activity of 5-HT1B receptors can be altered following administration of antidepressant drugs, but also upon exposure to environmental factors. For instance, exposure to stress causes a significant upregulation of the 5-HT1B receptor in the DRN, hippocampus, and septum [53]. It has been shown that 5-HT1B agonists, such as anpirtoline and CP 94253, produce antidepressant-like effects in an animal model of depression

[9]. On the other hand, studies have shown that 5-HT1B receptors antagonists, such as SB 216641 and GR127935, produce anxiolytic-like effects in animal models of depression [54]. Effect of antidepressant drugs on 5-HT1B have been investigated in vivo and have yielded useful information regarding the pathophysiology of MDD. It was shown that antidepressants produce a notable reduction in 5-HT1B mRNA in the DRN, leading to increased release of 5-HT and reduced level of depressive symptoms [9, 13]. However, it is still not known whether 5-HT1B autoreceptors are subject to the same desensitization as 5-HT1A autoreceptors, and there is not enough evidence demonstrating the ability of 5-HT1B autoreceptors to internalize following treatment with antidepressants. Continued research is needed to better understand the effect of antidepressant drugs on 5-HT1B autoreceptors internalization and localization during MDD. 5-HT2A Receptors 5-HT2A receptor is specifically distributed in brain structures like the frontal cortex and the nucleus accumbens, but also in several brainstem nuclei like the DRN [55, 56]. Studies showed that depressed patients often have increased 5-HT2A receptor densities, suggesting that this receptor might play a role in the pathophysiology of MDD [57, 58]. Moreover, a polymorphism in the promoter region of the 5-HT2A receptor gene confers increased risk of depression [32]. Although 5-HT1A receptors are more strongly associated with the antidepressant effects of SSRIs, the regulation of 5-HT2A receptors is also part of SSRIs’ mode of action [14]. Downregulation of 5-HT2A receptors occurs following treatment with some SSRIs [59], but also after administration of specific antagonists, the most common ones being M100907, nefazodone, and metergoline [14, 60]. 5-HT2A receptor antagonists generate antidepressant-like effects by inhibiting the 5-HT reuptake mechanism and by modulating the release of other neurotransmitters in the prefrontal cortex [61]. On the other hand, 5-HT2A receptor agonists have been reported to result in different molecular effects. Upon administration of selective agonists, such as 2,5-dimethoxy-4iodoamphetamine (DOI), 5-HT2A receptors lose their ability to couple to G proteins and undergo internalization from the plasma membrane to the cytoplasm [62, 63]. This phenomenon is thought to occur as part of the overall process of desensitization and may result in increased 5-HT level. However, this internalization is important for the resensitization of 5HT2A receptors and rescues their ability to couple to G proteins. Therefore, activation of 5-HT2A receptors with selective agonists may not fully cause their downregulation. Overall, there is enough evidence suggesting that a high density of 5-HT2A receptors may be involved in the pathogenesis of MDD and that their downregulation with selective

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antagonists may lead to antidepressant-like effects. However, more work needs to be done to fully characterize the effect of selective antagonists on 5-HT2A receptors. This information will improve our understanding of the role that 5-HT2A receptors play in MDD and will give us more insights into the therapeutic mechanisms of antidepressant drugs.

Signaling Molecules Involved in 5-HT Autoreceptors Internalization Over the past decades, several studies have been conducted in attempts to characterize the molecular machinery underlying the internalization of 5-HT autoreceptors. Massive amount of evidence illustrates the role of G protein-coupled receptor kinases (GRKs) and beta-arrestins (β-arrestins) in the internalization of GPCRs [64, 65]. Upon activation of GPCRs by selective agonists, GRKs rapidly move from the cytosol to the plasma membrane and phosphorylate the GPCRs [66, 67]. This process increases the affinity of the receptors for β-arrestins, which are cytosolic in the basal state but translocate to the plasma membrane to interact with phosphorylated receptors [68]. The interaction of β-arrestins with GPCRs prevents coupling of these receptors to new G proteins and inhibits the transduction of the neuronal signal, which leads to the process of endocytosis [64]. Once entered in the endosomal compartment, internalized receptors are either dephosphorylated and recycled to the plasma membrane or directed to the lysosomal compartment to be degraded [67, 69]. GRK-2/3 and β-arrestin-1/2 are the members of their respective families that are the most likely implicated in the internalization of GPCRs [70, 71]. Postmortem brain analysis of suicide victims showed that membrane associated GRK-2 is upregulated in the prefrontal cortex of drug-free depressed subjects and downregulated in subjects treated with antidepressants [70]. Also noteworthy were reports indicating that β-arrestin-1 levels are significantly elevated in the cerebral cortex and hippocampus of rats chronically treated with different antidepressants and that its expression is reduced in leukocytes of depressed subjects [72, 73]. Moreover, a study showed that mice deficient in βarrestin-2 display a reduced response to antidepressants, supporting the fact that β-arrestins are important for the regulation of drug-based antidepressant action [74]. Overall, there is enough evidence suggesting that responsiveness to antidepressants depends on concomitant changes in the subcellular localization of GRKs and βarrestins in 5-HT neurons. Understanding how these changes are associated with the desensitization and internalization of 5HT receptors in the DRN could help elucidate the neurobiological mechanisms of MDD and promote the development of new therapeutic strategies.

The 5-HT Pathway: Insights from Animal Studies Animal Models of Depression An ideal animal model of depression should reproduce the behavioral and molecular symptoms of the disease observed in humans and should have a similar response to antidepressants when compared to individuals with MDD [75]. One of the most commonly used animal models of depression is the olfactory bulbectomized (OBX) rat [76]. In OBX rats, the olfactory bulbs are surgically removed, resulting in behavioral and biochemical changes that are similar to those observed in patients with MDD [77]. The advantage of using the OBX model is that it is very well characterized and easy to reproduce [78]. More importantly, the behavioral and neurochemical changes observed in this model are relevant to MDD and include changes in food-motivated behavior, increased responsiveness to stress, disturbance of sleep patterns, and deficits in memory and spatial learning [79]. Using autoradiographic techniques, a study found a significantly higher amount of 5-HT in OBX rats, more particularly in the sensory motor cortex [79]. This is consistent with other reports supporting the notion that the olfactory bulbectomy causes an imbalance in 5-HT synthesis, which may be responsible for the development of depressive symptoms [79, 80]. Other animal models that have shown great promise in the study of MDD include the chronic mild stress (CMS), the learned helplessness (LH), and the chronic social defeat (CSD) model of depression [9]. In the CMS model, depressive symptoms are caused by biochemical alterations following exposure to environmental stressors [81]. CMS has been shown to result in altered aggressive and sexual behavior and in a variety of emotional changes such as grooming deficits [82]. Moreover, this animal model of depression displays abnormal level of SERT and 5-HT1A receptors, which can be reversed upon treatment with SSRIs [81, 83]. In the LH model, depression is induced by exposure to uncontrollable and inescapable stress such as inescapable electric shocks [82, 84]. Studies using the LH model have shown that 5-HT2A receptors are differentially regulated and severely impaired after exposure to single or repeated stress [85, 86]. Defects of 5HT neurotransmission, including upregulation of SERT in the DRN, have also been observed in the CSD model, illustrating the importance of the serotonergic pathway in the pathophysiology of MDD [87, 88]. Clearly, the past few decades have seen a tremendous growth in the number of investigations within the field of depressive disorders. The development of animal models of depression has significantly enhanced our understanding of the neurobiological bases of depression and will continue to have an impact on the development of new therapeutic strategies for MDD.

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5-HT Neurotransmission in Knockout Mice Genetic knockout of SERT and 5-HT receptors have provided interesting insights on the molecular mechanisms of MDD. Mice harboring a genetic deletion of SERT display anxiogenic-like phenotype and increased sensitivity to stress [75]. Behavioral symptoms of SERT knockout mice include decreased exploratory activity in the open-field test and reduced time spent in the open arms during the elevated plus maze test [89]. At the molecular level, a genetic deletion of SERT is associated with increased 5-HT synthesis and turnover, leading to a more pronounced downregulation of 5-HT receptors [90–92]. Similar to SERT knockout mice, 5-HT1A deletion leads to higher levels of anxiety-like behavior, as well as decreased exploratory activity [6, 93]. It also leads to increased 5-HT firing activity [6]. Although 5-HT1A knockout mice have on average a lower amount of brain 5-HT levels [94], the 5-HT tissue concentration in the DRN and several forebrain areas remains unchanged [6, 95]. Evidence from knockout mice also show that conditional suppression of 5-HT1A heteroreceptors in the forebrain leads to increased depressive-like but not anxiety-like behaviors, whereas suppression of 5-HT1A autoreceptors in the DRN leads to increased anxiety-like behaviors, indicating that 5-HT 1A autoreceptors and heteroreceptors play different pathological roles [96]. On the other hand, 5-HT1B knockout mice display a variety of phenotypes and behavioral symptoms that are not observed after deletion of the 5-HT1A receptor gene [95]. They are generally more aggressive and less anxious than the 5-HT1A knockout mice [95]. It was found that 5-HT1B knockout mice exhibit increased exploratory activity and improved spatial learning compared to wild-type mice [96]. Finally, studies investigating the 5-HT2A knockout mice found that deletion of this receptor causes behavioral abnormalities such as altered sleep pattern, which correlate well with depressive symptoms [97, 98]. Inactivation of the 5HT2A receptor gene also leads to reduced locomotor activity and poor control of homeostatic behaviors [98, 99]. Taken all together, studies using genetic knockout mice have provided significant insights into the contribution of the serotonergic pathway to abnormalities found in depressive disorders and enabled us to better understand the neurological bases of MDD.

the serotonergic system, 5-HT receptors and SERT are the most studied because of the ability of SSRIs to regulate their activity. Over the last decades, electrophysiological data from knockout mice and animal models of depression enabled scientist to depict the relationship between genes, behavior, and components of the 5-HT system. The development of such animal models has helped us gain more insight into the etiology of MDD and enabled the investigation of SSRI’s efficacy in vivo. Upon treatment with SSRIs, SERT and 5-HT autoreceptors internalize in nuclei of origin such as the DRN. This step is often correlated with antidepressant-like behavior in rodents and depends on concomitant changes in the subcellular localization of GRKs and β-arrestins. Despite the large amount of research trying to understand the neurobiological bases of depression, there are still serious limitations to current treatments of MDD. More effort needs to be devoted to the development of new therapeutic strategies that could efficiently reduce depressive symptomatology and prevent relapse in patients with MDD. More particularly, a better understanding of the 5-HT neurotransmission and its adaptive changes under antidepressant treatment is needed. So far, not much is known of the functional localization of 5-HT receptors and SERT at cellular and subcellular levels in MDD, notably in an animal model of depression. Future research needs to focus on the characterization of the molecular machinery that regulates the structural and functional changes of SERT and 5-HT receptors, as well as other players involved in the serotonergic pathway. This could be a major determinant of remission versus relapse in depressed individuals. The changes observed in animal models of MDD could be of interest in the clinic, if they are sufficiently important to be measurable by brain imaging techniques in human such as positron emission tomography (PET) or computed tomography (CT). They might then serve to establish the existence of a deficiency in the 5-HT system in depressed individuals and perhaps even serve to detect predispositions to MDD.

Acknowledgments The author is recipient of an award from the Natural Sciences and Engineering Research Council (NSERC) of Canada. Conflict of Interest The author declares that the present manuscript presents no conflict of interest. Compliance with Ethical Standards For this type of study, formal consent is not required.

Conclusion MDD is a debilitating disease that can lead to a variety of emotional and physical problems. The serotonergic system, which is implicated in mood, emotion, sleep, and the control of behavioral functions, plays an important role in the pathophysiology of this disease. Among all the players involved in

References 1.

Pae CU, Wang SM, Han C, Lee SJ, Patkar AA, Masand PS et al (2014) Vortioxetine: a meta-analysis of 12 short-term, randomized, placebo-controlled clinical trials for the treatment of major depressive disorder. J Psychiatry Neurosci 39:140120

Mol Neurobiol 2.

3.

4.

5. 6.

7.

8. 9.

10. 11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

Fakhoury M (2015) New insights into the neurobiological mechanisms of major depressive disorders. GenHosp Psychiatry 37:172– 177 Kenneson A, Funderburk JS, Maisto SA (2013) Substance use disorders increase the odds of subsequent mood disorders. Drug Alcohol Depend 133:338–343 AmericanPsychiatric Association (2003) Diagnostic and statistical manual of mental disorders, 5th edn. American Psychiatric Press, Washington, DC Saveanu RV, Nemeroff CB (2012) Etiology of depression: genetic and environmental factors. Psychiatr Clin N Am 35:51–71 Domínguez-López S, Howell R, Gobbi G (2012) Characterization of serotonin neurotransmission in knockout mice: implications for major depression. Rev Neurosci 23:429–443 Meyer JH (2007) Imaging the serotonin transporter during major depressive disorder and antidepressant treatment. J Psychiatry Neurosci 32:86–102 Hoffman BJ, Mezey E, Brownstein MJ (1991) Cloning of a serotonin transporter affected by antidepressants. Science 254:579–580 Carr GV, Lucki I (2011) The role of serotonin receptor subtypes in treating depression: a review of animal studies. Psychopharmacology (Berl) 213:265–287 Savitz J, Lucki I, Drevets WC (2009) 5-HT(1A) receptor function in major depressive disorder. Prog Neurobiol 88:17–31 Stamford JA, Davidson C, McLaughlin DP, Hopwood SE (2000) Control of dorsal raphé 5-HT function by multiple 5-HT(1) autoreceptors: parallel purposes or pointless plurality? Trends Neurosci 23:459–465 Kasamo K, Suzuki T, Tada K, Ueda N, Matsuda E, Ishikawa K et al (2001) Endogenous 5-HT tonically inhibits spontaneous firing activity of dorsal hippocampus CA1 pyramidal neurons through stimulation of 5-HT(1A) receptors in quiet awake rats: in vivo electrophysiological evidence. Neuropsychopharmacology 24:141–151 Anthony JP, Sexton TJ, Neumaier JF (2000) Antidepressantinduced regulation of 5-HT(1b) mRNA in rat dorsal raphe nucleus reverses rapidly after drug discontinuation. J Neurosci Res 61:82– 87 Celada P, Puig M, Amargós-Bosch M, Adell A, Artigas F (2004) The therapeutic role of 5-HT1A and 5-HT2A receptors in depression. J Psychiatry Neurosci 29:252–265 Albert PR, Benkelfat C (2013) The neurobiology of depression— revisiting the serotonin hypothesis. II. Genetic, epigenetic and clinical studies. Philos Trans R Soc Lond B Biol Sci 368:20120535 Maurer-Spurej E, Pittendreigh C, Misri S (2007) Platelet serotonin levels support depression scores for women with postpartum depression. J Psychiatry Neurosci 32:23–29 Ogawa S, Fujii T, Koga N, Hori H, Teraishi T, Hattori K et al (2014) Plasma L-tryptophan concentration in major depressive disorder: new data and meta-analysis. J Clin Psychiatry 75:e906–e915 Booij L, Van der Does W, Benkelfat C, Bremner JD, Cowen PJ, Fava M et al (2002) Predictors of mood response to acute tryptophan depletion. A reanalysis. Neuropsychopharmacology 27:852– 861 El Mansari M, Guiard BP, Chernoloz O, Ghanbari R, Katz N et al (2010) Relevance of norepinephrine-dopamine interactions in the treatment of major depressive disorder. CNS Neurosci Ther 16:e1– e17 Vaswani M, Linda FK, Ramesh S (2003) Role of selective serotonin reuptake inhibitors in psychiatric disorders: a comprehensive review. Prog Neuropsychopharmacol Biol Psychiatry 27:85–102 Marazziti D, Golia F, Consoli G, Presta S, Pfanner C, Carlini M et al (2008) Effectiveness of long-term augmentation with citalopram to clomipramine in treatment-resistant OCD patients. CNS Spectr 13: 971–976 Emslie GJ, Heiligenstein JH, Wagner KD, Hoog SL, Ernest DE, Brown E et al (2002) Fluoxetine for acute treatment of depression in

children and adolescents: a placebo-controlled, randomized clinical trial. J Am Acad Child Adolesc Psychiatry 41:1205–1215 23. Muijsers RB, Plosker GL, Noble S (2002) Spotlight on sertraline in the management of major depressive disorder in elderly patients. CNS Drugs 16:789–794 24. Montañez S, Daws LC, Gould GG, Frazer A (2003) Serotonin (5HT) transporter (SERT) function after graded destruction of serotonergic neurons. J Neurochem 87:861–867 25. Schloss P, Williams DC (1998) The serotonin transporter: a primary target for antidepressant drugs. J Psychopharmacol 12:115–121 26. Lesch KP, Wolozin BL, Murphy DL, Riederer P (1993) Isolation of a cDNA encoding the human brain serotonin transporter. J Neurochem 60:2319–2322 27. Ramamoorthy S, Bauman AL, Moore KR, Han H, Yang-Feng T, Chang AS et al (1993) Antidepressant- and cocaine-sensitive human serotonin transporter: molecular cloning, expression, and chromosomal localization. Proc Natl Acad Sci USA 90:2542–2546 28. Lebrand C, Cases O, Adelbrecht C, Doye A, Alvarez C, El Mestikawy S et al (1996) Transient uptake ans storage of serotonin in developing thalamic neurons. Neuron 17:823–835 29. Hansson SR, Mezey É, Hoffman BJ (1998) Serotonin transporter messenger RNA in the developing rat brain: early expression in serotonergic neurons and transient expression in non-serotonergic neurons. Neuroscience 83:1185–1201 30. Austin MC, Bradley CC, Mann JJ, Blakely RD (1994) Expression of serotonin transporter messenger mRNA in the human brain. J Neurochem 62:2362–2367 31. Caspi A, Sugden K, Moffitt TE, Taylor A, Craig IW, Harrington H et al (2003) Influence of life stress on depression: moderation by a polymorphism in the 5-HTT gene. Science 301:386–389 32. Eley TC, Sugden K, Corsico A, Gregory AM, Sham P, McGuffin P et al (2004) Gene-environment interaction analysis of serotonin system markers with adolescent depression. Mol Psychiatry 9: 908–915 33. Oya S, Choi SR, Hou C, Mu M, Kung MP, Acton PD et al (2000) 2((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine (ADAM): an improved serotonin transporter ligand. Nucl Med Biol 27:249–254 34. Holley A, Simonson B, Kivell BM (2013) MDMA regulates serotonin transporter function via a Protein kinase C dependent mechanism. J Addict Prev 1:5 35. Zhou Z, Zhen J, Karpowich NK, Law CJ, Reith ME, Wang DN (2009) Antidepressant specificity of serotonin transporter suggested by three LeuT-SSRI structures. Nat Struct Mol Biol 16:652–657 36. Haase J, Killian AM, Magnani F, Williams C (2001) Regulation of the serotonin transporter by interacting proteins. Biochem Soc Trans 29:722–728 37. Lau T, Horschitz S, Berger S, Bartsch D, Schloss P (2008) Antidepressant-induced internalization of the serotonin transporter in serotonergic neurons. FASEB J 22:1702–1714 38. Raymond JR, Mukhin YV, Gettys TW, Garnovskaya MN (1999) The recombinant 5-HT1A receptor: G protein coupling and signalling pathways. Br J Pharmacol 127:1751–1764 39. Riad M, Garcia S, Watkins KC, Jodoin N, Doucet E, Langlois X et al (2000) Somatodendritic localization of 5-HT1A and preterminal axonal localization of 5-HT1B serotonin receptors in adult rat brain. J Comp Neurol 417:181–194 40. Le Poul E, Boni C, Hanoun N, Laporte AM, Laaris N, Chauveau J et al (2000) Differential adaptation of brain 5-HT1A and 5-HT1B receptors and 5-HT transporter in rats treated chronically with fluoxetine. Neuropharmacology 39:110–122 41. Riad M, Watkins KC, Doucet E, Hamon M, Descarries L (2001) Agonist-induced internalization of serotonin-1A receptors in the dorsal raphe nucleus (autoreceptors) but not hippocampus (heteroreceptors). J Neurosci 21:8378–8386

Mol Neurobiol 42.

43. 44.

45.

46.

47.

48.

49.

50.

51.

52.

53. 54.

55. 56.

57.

58.

59.

60.

Hesselgrave N, Parsey RV (2013) Imaging the serotonin 1A receptor using [11C]WAY100635 in healthy controls and major depression. Philos Trans R Soc Lond B Biol Sci 368:20120004 Toth M (2003) 5-HT1A receptor knockout mouse as a genetic model of anxiety. Eur J Pharmacol 463:177–184 Dekeyne A, Rivet JM, Gobert A, Millan MJ (2001) Generalization of serotonin (5-HT)1A agonists and the antipsychotics, clozapine, ziprasidone and S16924, but not haloperidol, to the discriminative stimuli elicited by PD128,907 and 7-OH-DPAT. Neuropharmacology 40:899–910 Riad M, Zimmer L, Rbah L, Watkins KC, Hamon M, Descarries L (2004) Acute treatment with the antidepressant fluoxetine internalizes 5-HT1A autoreceptors and reduces the in vivo binding of the PET radioligand [18F]MPPF in the nucleus raphe dorsalis of rat. J Neurosci 24:5420–5426 Baune BT, Hohoff C, Roehrs T, Deckert J, Arolt V, Domschke K (2008) Serotonin receptor 1A–1019C/G variant: impact on antidepressant pharmacoresponse in melancholic depression? Neurosci Lett 436:111–115 Eriksson TM, Alvarsson A, Stan TL, Zhang X, Hascup KN, Hascup ER et al (2013) Bidirectional regulation of emotional memory by 5-HT1B receptors involves hippocampal p11. Mol Psychiatry 18:1096–1105 Ruf BM, Bhagwagar Z (2009) The 5-HT1B receptor: a novel target for the pathophysiology of depression. Curr Drug Targets 10:1118– 1138 Hamblin MW, Metcalf MA, McGuffin RW, Karpells S (1992) Molecular cloning and functional characterization of a human 5HT1B serotonin receptor: a homologue of the rat 5-HT1B receptor with 5-HT1D-like pharmacological specificity. Biochem Biophys Res Commun 184:752–759 Maroteaux L, Saudou F, Amlaiky N, Boschert U, Plassat JL, Hen R (1992) Mouse 5HT1B serotonin receptor: cloning, functional expression, and localization in motor control centers. Proc Natl Acad Sci U S A 89:3020–3304 Voigt MM, Laurie DJ, Seeburg PH, Bach A (1991) Molecular cloning and characterization of a rat brain cDNA encoding a 5hydroxytryptamine1B receptor. EMBO J 10:4017–4023 Morikawa H, Manzoni OJ, Crabbe JC, Williams JT (2000) Regulation of central synaptic transmission by 5-HT(1B) autoand heteroreceptors. Mol Pharmacol 58:1271–1278 Clark MS, Neumaier JF (2001) The 5-HT1B receptor: behavioral implications. Psychopharmacol Bull 35:170–185 Tatarczyńska E, Kłodzińska A, Stachowicz K, Chojnacka-Wójcik E (2004) Effects of a selective 5-HT1B receptor agonist and antagonists in animal models of anxiety and depression. Behav Pharmacol 15:523–534 Hannon J, Hoyer D (2008) Molecular biology of 5-HT receptors. Behav Brain Res 195:198–213 Yeung LY, Kung HF, Yew DT (2010) Localization of 5-HT1A and 5-HT2A positive cells in the brainstems of control age-matched and Alzheimer individuals. Age (Dordr) 32:483–495 Meyer JH, McMain S, Kennedy SH, Korman L, Brown GM, DaSilva JN et al (2003) Dysfunctional attitudes and 5-HT2 receptors during depression and self-harm. Am J Psychiatry 160:90–99 Shelton RC, Sanders-Bush E, Manier DH, Lewis DA (2009) Elevated 5-HT 2A receptors in postmortem prefrontal cortex in major depression is associated with reduced activity of protein kinase A. Neuroscience 158:1406–1415 Pullar IA, Carney SL, Colvin EM, Lucaites VL, Nelson DL, Wed l e y S ( 2 0 0 0 ) LY 3 6 7 2 6 5 , a n i n h i b i t o r o f t h e 5 hydroxytryptamine transporter and 5-hydroxytryptamine(2A) receptor antagonist: a comparison with the antidepressant, nefazodone. Eur J Pharmacol 407:39–46 Kettle CJ, Cheetham SC, Martin KF, Prow MR, Heal DJ (1999) The effects of the peptide-coupling agent, EEDQ, on 5-HT2A

61.

62.

63.

64. 65.

66. 67.

68.

69.

70.

71.

72.

73.

74.

75.

76. 77.

78.

receptor b inding and func tion in rat frontal cortex. Neuropharmacology 38:1421–1430 Pehek EA, Nocjar C, Roth BL, Byrd TA, Mabrouk OS (2006) Evidence for the preferential involvement of 5-HT2A serotonin receptors in stress- and drug-induced dopamine release in the rat medial prefrontal cortex. Neuropsychopharmacology 31:265–277 Ohkura M, Tanaka N, Kobayashi H, Wada A, Nakai J, Yamamoto R (2005) Insulin induces internalization of the 5-HT2A receptor expressed in HEK293 cells. Eur J Pharmacol 518:18–21 Roth BL, Palvimaki EP, Berry S, Khan N, Sachs N, Uluer A et al (1995) 5-Hydroxytryptamine2A (5-HT2A) receptor desensitization can occur without down-regulation. J Pharmacol Exp Ther 275: 1638–1646 Lefkowitz RJ, Shenoy SK (2005) Transduction of receptor signals by beta-arrestins. Science 308:512–517 Zheng H, Loh HH, Law PY (2008) Beta-arrestin-dependent muopioid receptor-activated extracellular signal-regulated kinases (ERKs) translocate to nucleus in contrast to G protein-dependent ERK activation. Mol Pharmacol 73:178–190 Foreman JC, Johansen T, Gibb AJ (2010) Textbook of receptor pharmacology, 3rd edn. CRC, Boca Raton Premont RT, Gainetdinov RR (2007) Physiological roles of G protein-coupled receptor kinases and arrestins. Annu Rev Physiol 69:511–534 Ferguson SSG, Downey WER, Colapietro AM, Barak LS, Menard L, Caron MG (1996) Role of β-arrestins in mediating agonistpromoted G protein-coupled receptor internalization. Science 271: 363–366 Laporte SA, Oakley RH, Zhang J, Holt JA, Ferguson SSG, Caron MG et al (1999) The beta2-adrenergic receptor/betaarrestin complex recruits the clathrin adaptor AP-2 during endocytosis. Proc Natl Acad Sci U S A 96:3712–3717 Grange-Midroit M, García-Sevilla JA, Ferrer-Alcón M, La Harpe R, Walzer C, Guimón J (2002) G protein-coupled receptor kinases, β-arrestin-2 and associated regulatory proteins in the human brain: postmortem changes, effect of age and subcellular distribution. Mol Brain Res 101:39–51 Oakley RH, Laporte SA, Holt JA, Caron MG, Barak LS (2000) Differential affinities of visual arrestin, beta arrestin1, and beta arrestin2 for G protein-coupled receptors delineate two major classes of receptors. J Biol Chem 275:17201–17210 Avissar S, Matuzany-Ruban A, Tzukert K, Schreiber G (2004) Beta-arrestin-1 levels: reduced in leukocytes of patients with depression and elevated by antidepressants in rat brain. Am J Psychiatry 161:2066–2072 Matuzany-Ruban A, Avissar S, Schreiber G (2005) Dynamics of beta-arrestin1 protein and mRNA levels elevation by antidepressants in mononuclear leukocytes of patients with depression. J Affect Disord 88:307–312 David DJ, Samuels BA, Rainer Q, Wang J-W, Marsteller D, Mendez I et al (2009) Neurogenesis-dependent and -independent effects of fluoxetine in an animal model of anxiety/depression. Neuron 62:479–493 Haenisch B, Bonisch H (2011) Depression and antidepressants: insights from knockout of dopamine, serotonin or noradrenaline re-uptake transporters. Pharmacol Ther 129:352–368 Song C, Leonard BE (2005) The olfactory bulbectomised rat as a model of depression. Neurosci Biobehav Rev 29:627–647 Romeas T, Morissette MC, Mnie-Filali O, Piñeyro G, Boye SM (2009) Simultaneous anhedonia and exaggerated locomotor activation in an animal model of depression. Psychopharmacology (Berl) 205:293–303 Harkin A, Kelly JP, Leonard BE (2003) A review of the relevance and validity of olfactory bulbectomy as a model of depression. Clin Neurosci Res 3:253–262

Mol Neurobiol 79.

Watanabe A, Tohyama Y, Nguyen KQ, Hasegawa S, Debonnel G, Diksic M (2003) Regional brain serotonin synthesis is increased in the olfactory bulbectomy rat model of depression: an autoradiographic study. J Neurochem 85:469–475 80. Slotkin TA, Cousins MM, Tate CA, Seidler FJ (2005) Serotonergic cell signaling in an animal model of aging and depression: olfactory bulbectomy elicits different adaptations in brain regions of young adult vs aging rats. Neuropsychopharmacology 30:52–57 81. Yalcin I, Belzung C, Surget A (2008) Mouse strain differences in the unpredictable chronic mild stress: a four-antidepressant survey. Behav Brain Res 193:140–143 82. Krishnan V, Nestler EJ (2011) Animal models of depression: molecular perspectives. Curr Top Behav Neurosci 7:121–147 83. Liang BF, Huang F, Wang HT, Wang GH, Yuan X, Zhang MZ et al (2014) Involvement of norepinephrine and serotonin system in antidepressant-like effects of hederagenin in the rat model of unpredictable chronic mild stress-induced depression. Pharm Biol 4:1–10 84. Chourbaji S, Zacher C, Sanchis-Segura C, Dormann C, Vollmayr B, Gass P (2005) Learned helplessness: validity and reliability of depressive-like states in mice. Brain Res Brain Res Protoc 16:70–78 85. Dwivedi Y, Mondal AC, Payappagoudar GV, Rizavi HS (2005) Differential regulation of serotonin (5HT)2A receptor mRNA and protein levels after single and repeated stress in rat brain: role in learned helplessness behavior. Neuropharmacology 48:204–214 86. Jiang X, Xing G, Yang C, Verma A, Zhang L, Li H (2009) Stress impairs 5-HT2A receptor-mediated serotonergic facilitation of GABA release in juvenile rat basolateral amygdala. Neuropsychopharmacology 34:410–423 87. Venzala E, García-García AL, Elizalde N, Delagrange P, Tordera RM (2012) Chronic social defeat stress model: behavioral features, antidepressant action, and interaction with biological risk factors. Psychopharmacology (Berl) 224:313–325 88. Zhang J, Fan Y, Li Y, Zhu H, Wang L, Zhu MY (2012) Chronic social defeat up-regulates expression of the serotonin transporter in rat dorsal raphe nucleus and projection regions in a glucocorticoiddependent manner. J Neurochem 123:1054–1068 89. Kalueff AV, Jensen CL, Murphy DL (2007) Locomotory patterns, spatiotemporal organization of exploration and spatial memory in serotonin transporter knockout mice. Brain Res 1169:87–97

90.

91.

92.

93.

94.

95.

96.

97.

98.

99.

Fabre V, Beaufour C, Evrard A, Rioux A, Hanoun N, Lesch KP et al (2000) Altered expression and functions of serotonin 5-HT1A and 5-HT1B receptors in knock-out mice lacking the 5-HT transporter. Eur J Neurosci 12:2299–2310 Li Q, Wichems CH, Ma L, Van de Kar LD, Garcia F, Murphy DL (2003) Brain region-specific alterations of 5-HT2A and 5-HT2C receptors in serotonin transporter knockout mice. J Neurochem 84:1256–1265 Gobbi G, Murphy DL, Lesch K, Blier P (2001) Modifications of the serotonergic system in mice lacking serotonin transporters: an in vivo electrophysiological study. J Pharmacol Exp Ther 296: 987–995 Pattij T, Groenink L, Hijzen TH, Oosting RS, Maes RA, van der Gugten J et al (2002) Autonomic changes associated with enhanced anxiety in 5-HT(1A) receptor knockout mice. Neuropsychopharmacology 27:380–390 Janusonis S, Anderson GM, Shifrovich I, Rakic P (2006) Ontogeny of brain and blood serotonin levels in 5-HT receptor knockout mice: potential relevance to the neurobiology of autism. J Neurochem 99: 1019–1031 Ase AR, Reader TA, Hen R, Riad M, Descarries L (2000) Altered serotonin and dopamine metabolism in the CNS of serotonin 5HT(1A) or 5-HT(1B) receptor knockout mice. J Neurochem 75: 2415–2426 Richardson-Jones JW, Craige CP, Nguyen TH, Kung HF, Gardier AM, Dranovsky A et al (2011) Serotonin-1A autoreceptors are necessary and sufficient for the normal formation of circuits underlying innate anxiety. J Neurosci 31:6008–6018 Adrien J (2004) Implication of serotonin in the control of vigilance states as revealed by knockout-mouse studies. J Soc Biol 198:30–36 Popa D, Léna C, Fabre V, Prenat C, Gingrich J, Escourrou P et al (2005) Contribution of 5-HT2 receptor subtypes to sleepwakefulness and respiratory control, and functional adaptations in knock-out mice lacking 5-HT2A receptors. J Neurosci 25:11231– 11238 Halberstadt AL, van der Heijden I, Ruderman MA, Risbrough VB, Gingrich JA, Geyer MA et al (2009) 5-HT(2A) and 5-HT(2C) receptors exert opposing effects on locomotor activity in mice. Neuropsychopharmacology 34:1958–1967