Adaptive changes in serotonin neurons of the raphe nuclei in 5-HT4receptor knock-out mouse

The Serotonin 4 Receptor Subtype: A Target of Particular Interest, Especially for Brain Disorders

In recent years, particular attention has been paid to the serotonin 4 receptor, which is well expressed in the brain, but also peripherally in various organs. The cerebral distribution of this receptor is well conserved across species, with high densities in the basal ganglia, where they are expressed by GABAergic neurons. The 5-HT4 receptor is also present in the cerebral cortex, hippocampus, and amygdala, where they are carried by glutamatergic or cholinergic neurons. Outside the central nervous system, the 5-HT4 receptor is notably expressed in the gastrointestinal tract. The wide distribution of the 5-HT4 receptor undoubtedly contributes to its involvement in a plethora of functions. In addition, the modulation of this receptor influences the release of serotonin, but also the release of other neurotransmitters such as acetylcholine and dopamine. This is a considerable asset, as the modulation of the 5-HT4 receptor can therefore play a direct or indirect beneficial role in various disorders. One of the main advantages of this receptor is that it mediates a much faster antidepressant and anxiolytic action than classical selective serotonin reuptake inhibitors. Another major benefit of the 5-HT4 receptor is that its activation enhances cognitive performance, probably via the release of acetylcholine. The expression of the 5-HT4 receptor is also altered in various eating disorders, and its activation by the 5-HT4 agonist negatively regulates food intake. Additionally, although the cerebral expression of this receptor is modified in certain movement-related disorders, it is still yet to be determined whether this receptor plays a key role in their pathophysiology. Finally, there is no longer any need to demonstrate the value of 5-HT4 receptor agonists in the pharmacological management of gastrointestinal disorders.

Functional Dimerization of Serotonin Receptors: Role in Health and Depressive Disorders

Understanding the neurobiological underpinnings of depressive disorder constitutes a pressing challenge in the fields of psychiatry and neurobiology. Depression represents one of the most prevalent forms of mental and behavioral disorders globally. Alterations in dimerization capacity can influence the functional characteristics of serotonin receptors and may constitute a contributing factor to the onset of depressive disorders. The objective of this review is to consolidate the current understanding of interactions within the 5-HT receptor family and between 5-HT receptors and members of other receptor families. Furthermore, it aims to elucidate the role of such complexes in depressive disorders and delineate the mechanisms through which antidepressants exert their effects.

5-HT4 Receptor Agonist Effects on Functional Connectivity in the Human Brain: Implications for Procognitive Action

BACKGROUND

Cognitive deficits are often comorbid with mood disorders and can cause significant functional impairment even after resolution of the primary mood symptoms. We do not currently have pharmacological treatments that adequately address these deficits. 5-HT4 receptor agonists show promise as potential procognitive agents in animal and early human translational studies. Optimal cognitive performance in humans is directly associated with appropriate functional connectivity between specific resting-state neural networks. However, so far the effect of 5-HT4 receptor agonism on resting-state functional connectivity (rsFC) in the brain in humans is unknown.

METHODS

We collected resting-state functional magnetic resonance imaging scans from 50 healthy volunteers, of whom 25 received 6 days × 1 mg prucalopride (a highly selective 5-HT4 receptor agonist) and 25 received placebo in a randomized double-blind design.

RESULTS

Network analyses identified that participants in the prucalopride group had enhanced rsFC between the central executive network and the posterior/anterior cingulate cortex. Seed analyses also showed greater rsFC between the left and right rostral anterior cingulate cortex and the left lateral occipital cortex, and reduced rsFC between the hippocampus and other default mode network regions.

CONCLUSIONS

Similar to other potentially procognitive medications, low-dose prucalopride in healthy volunteers appeared to enhance rsFC between regions involved in cognitive networks and reduce rsFC within the default mode network. This suggests a mechanism for the behavioral cognitive enhancement previously seen with 5-HT4 receptor agonists in humans and supports the potential for 5-HT4 receptor agonists to be used in clinical psychiatric populations.

Hyperthermia and Serotonin: The Quest for a “Better Cyproheptadine”

Fine temperature control is essential in homeothermic animals. Both hyper- and hypothermia can have deleterious effects. Multiple, efficient and partly redundant mechanisms of adjusting the body temperature to the value set by the internal thermostat exist. The neural circuitry of temperature control and the neurotransmitters involved are reviewed. The GABAergic inhibitory output from the brain thermostat in the preoptic area POA to subaltern neural circuitry of temperature control (Nucleus Raphe Dorsalis and Nucleus Raphe Pallidus) is a function of the balance between the (opposite) effects mediated by the transient receptor potential receptor TRPM2 and EP3 prostaglandin receptors. Activation of TRPM2-expressing neurons in POA favors hypothermia, while inhibition has the opposite effect. Conversely, EP3 receptors induce elevation in body temperature. Activation of EP3-expressing neurons in POA results in hyperthermia, while inhibition has the opposite effect. Agonists at TRPM2 and/or antagonists at EP3 could be beneficial in hyperthermia control. Activity of the neural circuitry of temperature control is modulated by a variety of 5-HT receptors. Based on the theoretical model presented the “ideal” antidote against serotonin syndrome hyperthermia appears to be an antagonist at the 5-HT receptor subtypes 2, 4 and 6 and an agonist at the receptor subtypes 1, 3 and 7. Very broadly speaking, such a profile translates in a sympatholytic effect. While a compound with such an ideal profile is presently not available, better matches than the conventional antidote cyproheptadine (used off-label in severe serotonin syndrome cases) appear to be possible and need to be identified.

Memory Disorders Related to Hippocampal Function: The Interest of 5-HT4Rs Targeting

The hippocampus has long been considered as a key structure for memory processes. Multilevel alterations of hippocampal function have been identified as a common denominator of memory impairments in a number of psychiatric and neurodegenerative diseases. For many years, the glutamatergic and cholinergic systems have been the main targets of therapeutic treatments against these symptoms. However, the high rate of drug development failures has left memory impairments on the sideline of current therapeutic strategies. This underscores the urgent need to focus on new therapeutic targets for memory disorders, such as type 4 serotonin receptors (5-HT₄Rs). Ever since the discovery of their expression in the hippocampus, 5-HT₄Rs have gained growing interest for potential use in the treatment of learning and memory impairments. To date, much of the researched information gathered by scientists from both animal models and humans converge on pro-mnesic and anti-amnesic properties of 5-HT₄Rs activation, although the mechanisms at work require more work to be fully understood. This review addresses a fundamental, yet poorly understood set of evidence of the potential of 5-HT₄Rs to re-establish or limit hippocampal alterations related to neurological diseases. Most importantly, the potential of 5-HT₄Rs is translated by refining hypotheses regarding the benefits of their activation in memory disorders at the hippocampal level.

5-HT Receptors and the Development of New Antidepressants

Serotonin modulates several physiological and cognitive pathways throughout the human body that affect emotions, memory, sleep, and thermal regulation. The complex nature of the serotonergic system and interactions with other neurochemical systems indicate that the development of depression may be mediated by various pathomechanisms, the common denominator of which is undoubtedly the disturbed transmission in central 5-HT synapses. Therefore, the deliberate pharmacological modulation of serotonergic transmission in the brain seems to be one of the most appropriate strategies for the search for new antidepressants. As discussed in this review, the serotonergic system offers great potential for the development of new antidepressant therapies based on the combination of SERT inhibition with different pharmacological activity towards the 5-HT system. The aim of this article is to summarize the search for new antidepressants in recent years, focusing primarily on the possibility of benefiting from interactions with various 5-HT receptors in the pharmacotherapy of depression.

5-HT4 Receptors Are Not Involved in the Effects of Fluoxetine in the Corticosterone Model of Depression

Clinical and preclinical studies report the implication of 5-hydroxytryptamine 4 receptors (5-HT₄Rs) in depression and anxiety. Here, we tested whether the absence of 5-HT4Rs influences the response to the antidepressant fluoxetine in mice subjected to chronic corticosterone administration, an animal model of depression and anxiety. Therefore, the effects of chronic administration of fluoxetine in corticosterone-treated wild-type (WT) and 5-HT₄R knockout (KO) mice were evaluated in the open-field and novelty suppressed feeding tests. As 5-HT_1A receptor (5-HT_1AR) and brain-derived neurotrophic factor (BDNF) are critically involved in depression and anxiety, we further evaluated 5-HT_1A receptor functionality by [³⁵S]GTPγS autoradiography and BDNF mRNA expression by in situ hybridization techniques. We found that 5-HT₄R KO and WT mice displayed anxiety- and depressive-like behavior following chronic administration of corticosterone, as evidenced in the open-field and novelty suppressed feeding tests. In the open-field, a decreased central activity was observed in naïve and corticosterone-treated mice of both genotypes following chronic fluoxetine administration. In the novelty suppressed feeding test, a predictive paradigm of antidepressant activity, chronic treatment with fluoxetine reverted the latency to eat in both genotypes. The antidepressant also potentiated the corticosterone-induced desensitization of the 5-HT_1AR in the dorsal raphe nucleus. Further, chronic fluoxetine increased BDNF mRNA expression in the dentate gyrus of the hippocampus in corticosterone-treated mice of both genotypes. Therefore, our findings indicate that the behavioral effects of fluoxetine in the corticosterone model of depression and anxiety appear not to be dependent on 5-HT₄Rs.

Translating the promise of 5HT4 receptor agonists for the treatment of depression

Animal experimental studies suggest that 5-HT4 receptor activation holds promise as a novel target for the treatment of depression and cognitive impairment. 5-HT4 receptors are post-synaptic receptors that are located in striatal and limbic areas known to be involved in cognition and mood. Consistent with this, 5-HT4 receptor agonists produce rapid antidepressant effects in a number of animal models of depression, and pro-cognitive effects in tasks of learning and memory. These effects are accompanied by molecular changes, such as the increased expression of neuroplasticity-related proteins that are typical of clinically useful antidepressant drugs. Intriguingly, these antidepressant-like effects have a fast onset of their action, raising the possibility that 5-HT4 receptor agonists may be a particularly useful augmentation strategy in the early stages of SSRI treatment. Until recently, the translation of these effects to humans has been challenging. Here, we review the evidence from animal studies that the 5-HT4 receptor is a promising target for the treatment of depression and cognitive disorders, and outline a potential pathway for the efficient and cost-effective translation of these effects into humans and, ultimately, to the clinic.

Multiple facets of serotonergic modulation

The serotonergic system of the central nervous system (CNS) has been implicated in a broad range of physiological functions and behaviors, such as cognition, mood, social interaction, sexual behavior, feeding behavior, sleep-wake cycle and thermoregulation. Serotonin (5-hydroxytryptamine, 5-HT) establishes a plethora of interactions with neurochemical systems in the CNS via its numerous 5-HT receptors and autoreceptors. The facets of this control are multiple if we consider the molecular actors playing a role in the autoregulation of 5-HT neuron activity including the 5-HT_1A, 5-HT_1B, 5-HT_1D, 5-HT_2B, 5-HT₇ receptors as well as the serotonin transporter. Moreover, extrinsic loops involving other neurotransmitters giving the other 5-HT receptors the possibility to impact 5-HT neuron activity. Grasping the complexity of these interactions is essential for the development of a variety of therapeutic strategies for cognitive defects and mood disorders. Presently we can illustrate the plurality of the mechanisms and only conceive that these 5-HT controls are likely not uniform in terms of regional and neuronal distribution. Our understanding of the specific expression patterns of these receptors on specific circuits and neuronal populations are progressing and will expand our comprehension of the function and interaction of these receptors with other chemical systems. Thus, the development of new approaches profiling the expression of 5-HT receptors and autoreceptors should reveal additional facets of the 5-HT controls of neurochemical systems in the CNS.

Serotonin modulation of hippocampal functions: From anatomy to neurotherapeutics

The hippocampal region receives a dense serotoninergic innervation originating from both medial and dorsal raphe nuclei. This innervation regulates hippocampal activity through the activation of distinct receptor families that are expressed in excitatory and inhibitory neurons, terminals of several afferent neurotransmitter systems, and glial cells. Preclinical and clinical studies indicate that hippocampal dysfunctions are involved in learning and memory deficits, dementia, Alzheimer's disease, epilepsy and mood disorders such as anxiety, depression and post-traumatic syndrome disorder, whereas the hippocampus participates also in the therapeutic mechanisms of numerous medicines. Not surprisingly, several drugs acting via 5-HT mechanisms are efficacious to some extent in some diseases and the link between 5-HT and the hippocampus although clear remains difficult to untangle. For this reason, we review reported data concerning the distribution and the functional roles of the 5-HT receptors in the hippocampal region in health and disease. The impact of the 5-HT systems on the hippocampal function is such that the research of new 5-HT mechanisms and drugs is still very active. It concerns notably drugs acting at the 5-HT_1A,_2A,_2C,_4,6 receptor subtypes, in addition to the already existing drugs including the selective serotonin reuptake inhibitors.

Genome sequencing of deep-sea hydrothermal vent snails reveals adaptions to extreme environments

BACKGROUND

The scaly-foot snail (Chrysomallon squamiferum) is highly adapted to deep-sea hydrothermal vents and has drawn much interest since its discovery. However, the limited information on its genome has impeded further related research and understanding of its adaptation to deep-sea hydrothermal vents.

FINDINGS

Here, we report the whole-genome sequencing and assembly of the scaly-foot snail and another snail (Gigantopelta aegis), which inhabits similar environments. Using Oxford Nanopore Technology, 10X Genomics, and Hi-C technologies, we obtained a chromosome-level genome of C. squamiferum with an N50 size of 20.71 Mb. By constructing a phylogenetic tree, we found that these 2 deep-sea snails evolved independently of other snails. Their divergence from each other occurred ∼66.3 million years ago. Comparative genomic analysis showed that different snails have diverse genome sizes and repeat contents. Deep-sea snails have more DNA transposons and long terminal repeats but fewer long interspersed nuclear elements than other snails. Gene family analysis revealed that deep-sea snails experienced stronger selective pressures than freshwater snails, and gene families related to the nervous system, immune system, metabolism, DNA stability, antioxidation, and biomineralization were significantly expanded in scaly-foot snails. We also found 251 H-2 Class II histocompatibility antigen, A-U α chain-like (H2-Aal) genes, which exist uniquely in the Gigantopelta aegis genome. This finding is important for investigating the evolution of major histocompatibility complex (MHC) genes.

CONCLUSION

Our study provides new insights into deep-sea snail genomes and valuable resources for further studies.

Rapid Anxiolytic Effects of RS67333, a Serotonin Type 4 Receptor Agonist, and Diazepam, a Benzodiazepine, Are Mediated by Projections From the Prefrontal Cortex to the Dorsal Raphe Nucleus

BACKGROUND

Activation of serotonin (5-HT) type 4 receptors (5-HT₄Rs) has been shown to have anxiolytic effects in a variety of animal models. Characterizing the circuits responsible for these effects should offer insights into new approaches to treat anxiety.

METHODS

We evaluated whether acute 5-HT₄R activation in glutamatergic axon terminals arising from the medial prefrontal cortex (mPFC) to the dorsal raphe nucleus (DRN) induced fast anxiolytic effects. Anxiolytic effects of an acute systemic administration (1.5 mg/kg, intraperitoneally) or intra-mPFC infusion with the 5-HT₄R agonist, RS67333 (0.5 μg/side), were examined in mice. To provide evidence that anxiolytic effects of RS67333 recruited an mPFC-DRN neural circuit, in vivo recordings of firing rate of DRN 5-HT neurons, cerebral 5-HT depletion, and optogenetic activation and silencing were performed.

RESULTS

Acute systemic administration and intra-mPFC infusion of RS67333 produced fast anxiolytic effects and increased DRN 5-HT cell firing. Serotonin depletion prevented anxiolytic effects induced by mPFC infusion of RS67333. Surprisingly the anxiolytic effects of mPFC infusion diazepam (1.5 μg/side) were also blocked by 5-HT depletion. Optogenetically activating mPFC terminals targeting the DRN reduced anxiety, whereas silencing this circuit blocked RS67333 and diazepam mPFC infusion-induced anxiolytic effects. Finally, anxiolytic effects induced by an acute systemic RS67333 or diazepam administration were partially blocked after optogenetically inhibiting cortical glutamatergic terminals in the DRN.

CONCLUSIONS

Our findings suggest that activating 5-HT₄R acutely in the mPFC or targeting mPFC pyramidal cell terminals in the DRN might constitute a strategy to produce a fast anxiolytic response.

Therapeutic potential of serotonin 4 receptor for chronic depression and its associated comorbidity in the gut

The latest estimates from world health organization suggest that more than 450 million people are suffering from depression and other psychiatric conditions. Of these, 50-60% have been reported to have progression of gut diseases. In the last two decades, researchers introduced incipient physiological roles for serotonin (5-HT) receptors (5-HTRs), suggesting their importance as a potential pharmacological target in various psychiatric and gut diseases. A growing body of evidence suggests that 5-HT systems affect the brain-gut axis in depressive patients, which leads to gut comorbidity. Recently, preclinical trials of 5-HT4R agonists and antagonists were promising as antipsychotic and prokinetic agents. In the current review, we address the possible pharmacological role and contribution of 5-HT4R in the pathophysiology of chronic depression and associated gut abnormalities. Physiologically, during depression episodes, centers of the sympathetic and parasympathetic nervous system couple together with neuroendocrine systems to alter the function of hypothalamic-pituitary-adrenal (HPA) axis and enteric nervous system (ENS), which in turn leads to onset of gastrointestinal tract (GIT) disorders. Consecutively, the ENS governs a broad spectrum of physiological activities of gut, such as visceral pain and motility. During the stages of emotional stress, hyperactivity of the HPA axis alters the ENS response to physiological and noxious stimuli. Consecutively, stress-induced flare, swelling, hyperalgesia and altered reflexes in gut eventually lead to GIT disorders. In summary, the current review provides prospective information about the role and mechanism of 5-HT4R-based therapeutics for the treatment of depressive disorder and possible consequences for the gut via brain-gut axis interactions. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Role of serotonin 4 receptor in the growth of hippocampal neurons during the embryonic development in mice

Serotonin (5-HT) homeostasis is critical for the brain development which influences neurogenesis, neuronal migration, and circuit formation. Distinctive distribution patterns of serotonin receptors (5-HTRs) in the brain govern various physiological activities. Amongst the 5-HTRs, serotonin 4 receptor (5-HT4R) is widely expressed in embryonic forebrain and affects neuronal development, synaptogenesis, and behavior, but its specific role in brain development is still not completely understood. Therefore, in the present study, we addressed the roles of 5-HT4R in the growth of hippocampal neurons during the development of mice brain. We cultured hippocampal neurons of the mouse at embryonic day 18 and then treatment of 5-HT4R agonist RS67333 was employed. We found RS67333 significantly increased the axonal length, diameter and branching along with total dendritic length, number of primary dendrites and their branching. In addition, these effects were neutralized by the concomitant treatment of 5-HT4R antagonist GR125487, which confirmed the specific role of the 5-HT4R in the growth of axon and dendrites. Further, the treatment of RS67333 upregulated the mRNA expression of collapsin response mediator protein-2 (CRMP2) and non-phosphorylated CRMP2 (npCRMP2) together with neurotrophic factors (BDNF, NT-3, NGF) and TRK-A. Additionally, the current research findings reveal that the knockdown of CRMP2 inhibited RS67333-induced growth of the axons and dendrites, which indicates that CRMP2 is required for the 5-HT4R-mediated growth of the axons and dendrites. Overall, the findings of the present in vitro study enrich the understanding and provide insight roles of 5-HT4R in embryonic brain development by promoting the growth of hippocampal neurons.

Alterations of Expression of the Serotonin 5-HT4 Receptor in Brain Disorders

The serotonin 4 receptor, 5-HT₄R, represents one of seven different serotonin receptor families and is implicated in a variety of physiological functions and their pathophysiological variants, such as mood and depression or anxiety, food intake and obesity or anorexia, or memory and memory loss in Alzheimer's disease. Its central nervous system expression pattern in the forebrain, in particular in caudate putamen, the hippocampus and to lesser extent in the cortex, predispose it for a role in executive function and reward-related actions. In rodents, regional overexpression or knockdown in the prefrontal cortex or the nucleus accumbens of 5-HT₄R was shown to impact mood and depression-like phenotypes, food intake and hypophagia; however, whether expression changes are causally involved in the etiology of such disorders is not clear. In this context, more data are emerging, especially based on PET technology and the use of ligand tracers that demonstrate altered 5-HT₄R expression in brain disorders in humans, confirming data stemming from post-mortem tissue and preclinical animal models. In this review, we would like to present the current knowledge of 5-HT₄R expression in brain regions relevant to mood/depression, reward and executive function with a focus on 5-HT₄R expression changes in brain disorders or caused by drug treatment, at both the transcript and protein levels.

Phytochemistry and pharmacology of anti-depressant medicinal plants: A review

Stress renders an individual to experience mental pressure and exhaustion which brings about feelings of anxiety, depression, anger and/or other negative emotions. Depression affects a person's state of mind, behaviour, health and is often associated with suicide. The use of anti-depressant drugs as therapeutic agents is associated with symptoms such as, delayed onset of action, side-effects, drug-drug and dietary interactions, sexual dysfunction, cardiac toxicity, etc. Thus, there is need to target these issues and improve current treatment options. Medicinal plants have long been used in discovering novel treatment strategies and compounds with promising roles in treating various disease conditions. There has been an increase, worldwide, in the use of medicinal plants and herbs for developing nutraceuticals for treatment of depression and other psychiatric disorders. Medicinal plants in their natural forms are valuable as they are rich in various phytochemical compounds. These phytochemical compounds have pharmacological roles in treating various diseases conditions; apart from being widely available in nature and commercially beneficial. The phytochemical compounds in plants are constantly being explored through various experimental studies to determine the molecular basis of how medicinal plants work in relation to drugs and diseases and to develop neutraceuticals for improving conditions. This review summarizes 110 medicinal plants and their phytochemical constituents that have been shown to possess anti-depressant activity. This review also highlights the various mechanisms of anti-depressant action of some of these plants and their plant parts like roots, stem, leaves, flowers, fruit or whole plant; phytochemical compounds showing anti-depressant activity such flavanoids, steroids, saponins, sugars, lectins, alkaloids, etc.; and various anti-depressant screening models used such as tail suspension test, forced swim test, chronic unpredictable stress test, sucrose preference test, monoamine oxidase inhibition assay, learned helplessness test, open field test, hole board test, etc. However, mechanistic evaluation of many of these plants still needs to be investigated and explored.

The role of 5-HT receptors in depression

Depression is a polygenic and highly complex psychiatric disorder that remains a major burden on society. Antidepressants, such as selective serotonin reuptake inhibitors (SSRIs), are some of the most commonly prescribed drugs worldwide. In this review, we will discuss the evidence that links serotonin and serotonin receptors to the etiology of depression and the mechanisms underlying response to antidepressant treatment. We will then revisit the role of serotonin in three distinct hypotheses that have been proposed over the last several decades to explain the pathophysiology of depression: the monoamine, neurotrophic, and neurogenic hypotheses. Finally, we will discuss how recent studies into serotonin receptors have implicated specific neural circuitry in mediating the antidepressant response, with a focus being placed on the hippocampus.

Vagal innervation is required for pulmonary function phenotype in Htr4-/- mice

Human genome-wide association studies have identified over 50 loci associated with pulmonary function and related phenotypes, yet follow-up studies to determine causal genes or variants are rare. Single nucleotide polymorphisms in serotonin receptor 4 (HTR4) are associated with human pulmonary function in genome-wide association studies and follow-up animal work has demonstrated that Htr4 is causally associated with pulmonary function in mice, although the precise mechanisms were not identified. We sought to elucidate the role of neural innervation and pulmonary architecture in the lung phenotype of Htr4^-/- animals. We report here that the Htr4^-/- phenotype in mouse is dependent on vagal innervation to the lung. Both ex vivo tracheal ring reactivity and in vivo flexiVent pulmonary functional analyses demonstrate that vagotomy abrogates the Htr4^-/- airway hyperresponsiveness phenotype. Hyperpolarized ³He gas magnetic resonance imaging and stereological assessment of wild-type and Htr4^-/- mice reveal no observable differences in lung volume, inflation characteristics, or pulmonary microarchitecture. Finally, control of breathing experiments reveal substantive differences in baseline breathing characteristics between mice with/without functional HTR4 in breathing frequency, relaxation time, flow rate, minute volume, time of inspiration and expiration and breathing pauses. These results suggest that HTR4's role in pulmonary function likely relates to neural innervation and control of breathing.

Serotonin 5-HT4 receptors modulate the development of glutamatergic input to the dorsal raphe nucleus

The dorsal raphe nucleus (DRN) is a major serotonin (5-hydroxytryptamine, 5-HT)-producing region in the central nervous system. It receives glutamatergic inputs from several brain regions, which are reciprocally modulated by serotonergic signals. We investigated whether serotonin 5-HT₄ receptors (5-HT₄Rs) play a role in the development of glutamatergic control of the DRN, with an emphasis on cortical inputs. Double-label immunohistochemistry and confocal microscopy were used to quantify vesicular glutamate transporter 1 (vGluT1)-immunoreactive terminals in the DRN of mice with a null-mutation in the 5-HT₄R gene. We found no significant change in the overall density of vGluT1-positive terminals in homozygous and heterozygous mice, but heterozygous mice had a significantly higher density of vGluT1-positive terminals contacting serotonergic neurons. These results suggest that altered 5-HT₄R expression may affect the development of cortical glutamatergic control of the DRN.

The absence of 5-HT4 receptors modulates depression- and anxiety-like responses and influences the response of fluoxetine in olfactory bulbectomised mice: Adaptive changes in hippocampal neuroplasticity markers and 5-HT1A autoreceptor

Preclinical studies support a critical role of 5-HT₄ receptors (5-HT₄Rs) in depression and anxiety, but their influence in depression- and anxiety-like behaviours and the effects of antidepressants remain partly unknown. We evaluated 5-HT₄R knockout (KO) mice in different anxiety and depression paradigms and mRNA expression of some neuroplasticity markers (BDNF, trkB and Arc) and the functionality of 5-HT_1AR. Moreover, the implication of 5-HT₄Rs in the behavioural and molecular effects of chronically administered fluoxetine was assessed in naïve and olfactory bulbectomized mice (OBX) of both genotypes. 5-HT₄R KO mice displayed few specific behavioural impairments including reduced central activity in the open-field (anxiety), and decreased sucrose consumption and nesting behaviour (anhedonia). In these mice, we measured increased levels of BDNF and Arc mRNA and reduced levels of trkB mRNA in the hippocampus, and a desensitization of 5-HT_1A autoreceptors. Chronic administration of fluoxetine elicited similar behavioural effects in WT and 5-HT₄R KO mice on anxiety-and depression-related tests. Following OBX, locomotor hyperactivity and anxiety were similar in both genotypes. Interestingly, chronic fluoxetine failed to reverse this OBX-induced syndrome in 5-HT₄R KO mice, a response associated with differential effects in hippocampal neuroplasticity biomarkers. Fluoxetine reduced hippocampal Arc and BDNF mRNA expressions in WT but not 5-HT₄R KO mice subjected to OBX. These results demonstrate that the absence of 5-HT₄Rs triggers adaptive changes that could maintain emotional states, and that the behavioural and molecular effects of fluoxetine under pathological depression appear to be critically dependent on 5-HT₄Rs.

Probable involvement of p11 with interferon alpha induced depression

Depression is one of the major side effects of interferon alpha (IFN-α) treatment, but the molecular mechanism underlying IFN-α-induced depression remains unclear. Several studies have shown that the serotonin receptors 5-HTR1b and 5-HTR4 play key roles in the anti-depression effects associated with p11 (S100A10). We investigated the effects of IFN-α on the regulation of p11, 5-HTR1b and 5-HTR4 in mice and human neuroblastoma cells (SH-sy5y). We found that intraperitoneal injection with IFN-α in Balb/c mice resulted in an increased immobility in FST and TST, and potently lowered the protein levels of p11, 5-HTR1b and 5-HTR4 in the hippocampus or cingulate gyrus. IFN-α significantly down-regulated the protein levels of p11, 5-HTR1b and 5-HTR4 in SH-sy5y cells, in a time- and dose-dependent manner. Our study revealed that over-expression of p11 could prevent the IFN-α-induced down-regulation of 5-HTR1b and 5-HTR4. The results indicated that IFN-α treatment resulted in p11 down-regulation, which subsequently decreased 5-HTR1b and 5-HTR4 in vitro or in vivo. Our findings suggested that p11 might be a potential regulator on 5-HTR1b and 5-HTR4 as well as a predictor of or a therapeutic target for IFN-α-induced depression.

How Does the Brain Implement Adaptive Decision Making to Eat?

Adaptive decision making to eat is crucial for survival, but in anorexia nervosa, the brain persistently supports reduced food intake despite a growing need for energy. How the brain persists in reducing food intake, sometimes even to the point of death and despite the evolution of multiple mechanisms to ensure survival by governing adaptive eating behaviors, remains mysterious. Neural substrates belong to the reward-habit system, which could differ among the eating disorders. The present review provides an overview of neural circuitry of restrictive food choice, binge eating, and the contribution of specific serotonin receptors. One possibility is that restrictive food intake critically engages goal-directed (decision making) systems and "habit," supporting the view that persistent caloric restriction mimics some aspects of addiction to drugs of abuse.

SIGNIFICANCE STATEMENT

An improved understanding of the neural basis of eating disorders is a timely challenge because these disorders can be deadly. Up to 70 million of people in the world suffer from eating disorders. Anorexia nervosa affects 1-4% of women in United States and is the first cause of death among adolescents in Europe. Studies relying on animal models suggest that decision making to eat (or not) can prevail over actual energy requirements due to emotional disturbances resulting in abnormal habitual behavior, mimicking dependence. These recent studies provide a foundation for developing more specific and effective interventions for these disorders.

Treatment-resistant depression: are animal models of depression fit for purpose?

BACKGROUND

Resistance to antidepressant drug treatment remains a major health problem. Animal models of depression are efficient in detecting effective treatments but have done little to increase the reach of antidepressant drugs. This may be because most animal models of depression target the reversal of stress-induced behavioural change, whereas treatment-resistant depression is typically associated with risk factors that predispose to the precipitation of depressive episodes by relatively low levels of stress. Therefore, the search for treatments for resistant depression may require models that incorporate predisposing factors leading to heightened stress responsiveness.

METHOD

Using a diathesis-stress framework, we review developmental, genetic and genomic models against four criteria: (i) increased sensitivity to stress precipitation of a depressive behavioural phenotype, (ii) resistance to chronic treatment with conventional antidepressants, (iii) a good response to novel modes of antidepressant treatment (e.g. ketamine; deep brain stimulation) that are reported to be effective in treatment-resistant depression and (iv) a parallel to a known clinical risk factor.

RESULTS

We identify 18 models that may have some potential. All require further validation. Currently, the most promising are the Wistar-Kyoto (WKY) and congenital learned helplessness (cLH) rat strains, the high anxiety behaviour (HAB) mouse strain and the CB1 receptor knockout and OCT2 null mutant mouse strains.

CONCLUSION

Further development is needed to validate models of antidepressant resistance that are fit for purpose. The criteria used in this review may provide a helpful framework to guide research in this area.

The role of serotonergic, adrenergic and dopaminergic receptors in antidepressant-like effect

Depression is a serious global illness, becoming more and more common in developed countries. Because of specific symptoms it is considered as a leading cause of disability all over the world with a high death factor due to suicides. There are many antidepressants used in the therapy, but still more than 30% of patients do not respond to the treatment. The heterogeneous nature of the illness and its complex, unclear aetiology may be responsible for these difficulties. Next to the main monoaminergic hypothesis of depression there are also many other approaches connected with the pathophysiology of the disease, including hypothalamic-pituitary-adrenal axis dysregulation, dopaminergic, cholinergic, glutamatergic or GABA-ergic neurotransmission. Nevertheless, it can be unambiguously stated that serotonergic, noradrenergic and dopaminergic systems are precisely connected with pathogenesis of depression, and should be therefore considered as valuable targets in patients' treatment. Bearing that in mind, this review presents the role of serotonergic, adrenergic and dopaminergic receptors in antidepressant-like effect.

Pharmacological Evidence for 5-HT6 Receptor Modulation of 5-HT Neuron Firing in Vivo

5-Hydroxytryptamine (5-HT) neurons in the midbrain dorsal raphe nucleus (DRN) are implicated in the drug treatment and pathophysiology of a wide variety of neuropsychiatric disorders. Accumulating evidence suggests that 5-HT6 receptors may be located and functional in the DRN; therefore, 5-HT6 receptor ligands may have potential as novel modulators of 5-HT neurotransmission. The current study investigated the effect of intravenous (i.v.) administration of the selective 5-HT6 receptor agonist, WAY-181187, and antagonist, SB-399885, on the firing of 5-HT neurons in the DRN in vivo. Extracellular recordings were made in the DRN of anesthetized rats, and single 5-HT neurons were identified on the basis of electrophysiological properties combined with juxtacellular labeling and postmortem immunohistochemical analysis. WAY-181187 (1-4 mg/kg i.v.) caused a dose-dependent increase in 5-HT neuron firing rate. In comparison, SB-399885 (0.125-1 mg/kg i.v.) caused a dose-dependent decrease in 5-HT neuron firing rate, an effect reversed by WAY-181187 (3 mg/kg i.v.). These effects of WAY-181187 and SB-399885 were observed in two separate sets of experiments. In summary, the current data show the modulation of 5-HT neuronal firing by the 5-HT6 ligands WAY-181187 and SB-399885 and are consistent with the presence of 5-HT6 receptor-mediated positive feedback control of 5-HT neurons.

Role of the 5-HT4 receptor in chronic fluoxetine treatment-induced neurogenic activity and granule cell dematuration in the dentate gyrus

Background

Chronic treatment with selective serotonin (5-HT) reuptake inhibitors (SSRIs) facilitates adult neurogenesis and reverses the state of maturation in mature granule cells (GCs) in the dentate gyrus (DG) of the hippocampus. Recent studies have suggested that the 5-HT₄ receptor is involved in both effects. However, it is largely unknown how the 5-HT₄ receptor mediates neurogenic effects in the DG and, how the neurogenic and dematuration effects of SSRIs interact with each other.

Results

We addressed these issues using 5-HT₄ receptor knockout (5-HT4R KO) mice. Expression of the 5-HT₄ receptor was detected in mature GCs but not in neuronal progenitors of the DG. We found that chronic treatment with the SSRI fluoxetine significantly increased cell proliferation and the number of doublecortin-positive cells in the DG of wild-type mice, but not in 5-HT4R KO mice. We then examined the correlation between the increased neurogenesis and the dematuration of GCs. As reported previously, reduced expression of calbindin in the DG, as an index of dematuration, by chronic fluoxetine treatment was observed in wild-type mice but not in 5-HT4R KO mice. The proliferative effect of fluoxetine was inversely correlated with the expression level of calbindin in the DG. The expression of neurogenic factors in the DG, such as brain derived neurotrophic factor (Bdnf), was also associated with the progression of dematuration. These results indicate that the neurogenic effects of fluoxetine in the DG are closely associated with the progression of dematuration of GCs. In contrast, the DG in which neurogenesis was impaired by irradiation still showed significant reduction of calbindin expression by chronic fluoxetine treatment, suggesting that dematuration of GCs by fluoxetine does not require adult neurogenesis in the DG.

Conclusions

We demonstrated that the 5-HT₄ receptor plays an important role in fluoxetine-induced adult neurogenesis in the DG in addition to GC dematuration, and that these phenomena are closely associated. Our results suggest that 5-HT₄ receptor-mediated phenotypic changes, including dematuration in mature GCs, underlie the neurogenic effect of SSRIs in the DG, providing new insight into the cellular mechanisms of the neurogenic actions of SSRIs in the hippocampus.

Developments in the field of antidepressants, where do we go now?

Major depression is a severe psychiatric syndrome with very high prevalence and socio-economic impact. Its pathophysiology is poorly known, yet several neurotransmitter systems and brain areas have been implicated. Selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitors (SSRI) and serotonin and norepinephrine reuptake inhibitors (SNRI) are most used antidepressant treatments. However, these drugs show slow onset of action and limited efficacy, making necessary the use of drug augmentation strategies or more aggressive interventions. Two important observations have emerged in recent years indicating that more rapid and effective antidepressant treatments are possible. Hence, the deep brain stimulation (DBS) of ventral anterior (subgenual) cingulate cortex (Cg25) evokes rapid mood improvements in subgroups of treatment-resistant depressive patients, likely mediated by a functional remodelling of cortico-limbic circuits. On the other hand, the non-competitive NDMA receptor antagonist ketamine can also evoke rapid (e.g., 2h) and persistent (up to 1 wk) improvements in some treatment-resistant patients. Moreover, recent preclinical observations indicate the antidepressant capacity of mGluR agents. Overall, this supports the usefulness of glutamatergic transmission as a new area in antidepressant drug development. On the monoamine side, new preclinical and clinical research should clarify the different roles played by 5-HT receptors in depression as well as the brain areas and circuits responsible for therapeutic improvement. This will lead to the synthesis of new agents blocking the serotonin (and possibly norepinephrine) transporter which will also activate or block 5-HT receptors playing respectively positive (e.g., postsynaptic 5-HT1A, 5-HT4) or negative (e.g., presynaptic 5-HT1A,/1B, 5-HT2A, 5-HT2C,5-HT3, etc.) roles in antidepressant effects.

Genetic variation in HTR4 and lung function: GWAS follow-up in mouse

Human genome-wide association studies (GWASs) have identified numerous associations between single nucleotide polymorphisms (SNPs) and pulmonary function. Proving that there is a causal relationship between GWAS SNPs, many of which are noncoding and without known functional impact, and these traits has been elusive. Furthermore, noncoding GWAS-identified SNPs may exert trans-regulatory effects rather than impact the proximal gene. Noncoding variants in 5-hydroxytryptamine (serotonin) receptor 4 (HTR4) are associated with pulmonary function in human GWASs. To gain insight into whether this association is causal, we tested whether Htr4-null mice have altered pulmonary function. We found that HTR4-deficient mice have 12% higher baseline lung resistance and also increased methacholine-induced airway hyperresponsiveness (AHR) as measured by lung resistance (27%), tissue resistance (48%), and tissue elastance (30%). Furthermore, Htr4-null mice were more sensitive to serotonin-induced AHR. In models of exposure to bacterial lipopolysaccharide, bleomycin, and allergic airway inflammation induced by house dust mites, pulmonary function and cytokine profiles in Htr4-null mice differed little from their wild-type controls. The findings of altered baseline lung function and increased AHR in Htr4-null mice support a causal relationship between genetic variation in HTR4 and pulmonary function identified in human GWAS.

Rapid anxiolytic effects of a 5-HT₄ receptor agonist are mediated by a neurogenesis-independent mechanism

Selective serotonin reuptake inhibitors (SSRIs) display a delayed onset of action of several weeks. Past work in naive rats showed that 5-HT₄ receptor agonists had rapid effects on depression-related behaviors and on hippocampal neurogenesis. We decided to investigate whether 5-HT₄ receptor stimulation was necessary for the effects of SSRIs in a mouse model of anxiety/depression, and whether hippocampal neurogenesis contributed to these effects. Using the mouse corticosterone model of anxiety/depression, we assessed whether chronic treatment with a 5-HT₄ receptor agonist (RS67333, 1.5 mg/kg/day) had effects on anxiety- and depression-related behaviors, as well as on hippocampal neurogenesis in comparison with chronic fluoxetine treatment (18 mg/kg/day). Then, using our anxiety/depression model combined with ablation of hippocampal neurogenesis, we investigated whether neurogenesis was necessary for the behavioral effects of subchronic (7 days) or chronic (28 days) RS67333 treatment. We also assessed whether a 5-HT₄ receptor antagonist (GR125487, 1 mg/kg/day) could prevent the behavioral and neurogenic effects of fluoxetine. Chronic treatment with RS67333, similar to fluoxetine, induced anxiolytic/antidepressant-like activity and stimulated adult hippocampal neurogenesis, specifically facilitating maturation of newborn neurons. However, unlike fluoxetine, anxiolytic effects of RS67333 were already present after 7 days and did not require hippocampal neurogenesis. Chronic treatment with GR125487 prevented both anxiolytic/antidepressant-like and neurogenic effects of fluoxetine, indicating that 5-HT₄ receptor activation is necessary for these effects of SSRIs. 5-HT₄ receptor stimulation could represent an innovative and rapid onset therapeutic approach to treat depression with comorbid anxiety.

Serotonergic pharmacology in animal models: from behavioral disorders to dyskinesia

Serotonin (5-HT) dysfunction has been involved in both movement and behavioral disorders. Serotonin pharmacology improves dyskinetic movements as well as depressive, anxious, aggressive and anorexic symptoms. Animal models have been useful to investigate more precisely to what extent 5-HT is involved and whether drugs targeting the 5-HT system can counteract the symptoms exhibited. We review existing rodent and non-human primate (NHP) animal models in which selective 5-HT or dual 5-HT-norepinephrine (NE) transporter inhibitors, as well as specific 5-HT receptors agonists and antagonists, monoamine oxidase A inhibitors (IMAO-A) and MDMA (Ecstasy) have been used. We review overlaps between the various drug classes involved. We confront behavioral paradigms and treatment regimen. Some but not all animal models and associated pharmacological treatments have been extensively studied in the litterature. In particular, the impact of selective serotonin reuptake inhibitors (SSRI) has been extensively investigated using a variety of pharmacological or genetic rodent models of depression, anxiety, aggressiveness. But the validity of these rodent models is questioned. On the contrary, few studies did address the potential impact of targeting the 5-HT system on NHP models of behavioral disorders, despite the fact that those models may match more closely to human pathologies. Further investigations with carefull behavioral analysis will improve our understanding of neural bases underlying the pathophysiology of movement and behavioral disorders.

Under- to over-eating: how do serotonin receptors contribute?

In numerous pathological states, the brain can restrict food intake to a lethal level despite mounting requirements for energy as seen in adolescents with anorexia nervosa. How the brain reduces food intake to the point of death while eating is a cornerstone of survival that remains just as ‘cryptic’ as the association between anorexia and overeating. This review provides a recent snapshot of the neural underpinnings of the rewarding effects of anorexia that may compete with the adaptive decision-making process to eat, and with survival instinct. Among a plethora of factors, impaired activity of the serotonin receptors in the reward system underlies the ability of animals to self-impose food restriction, and the transition from under- to over-eating. However, the triumvirate association between serotonin, overeating and addiction appears unlikely. Considering the implication of the serotonin receptors in the hypothalamus, anorexia and bulimia nervosa could result from an impairment of a ‘synchronic activity’ between the autonomic and voluntary nervous systems.

The nucleus accumbens 5-HTR₄-CART pathway ties anorexia to hyperactivity

In mental diseases, the brain does not systematically adjust motor activity to feeding. Probably, the most outlined example is the association between hyperactivity and anorexia in Anorexia nervosa. The neural underpinnings of this 'paradox', however, are poorly elucidated. Although anorexia and hyperactivity prevail over self-preservation, both symptoms rarely exist independently, suggesting commonalities in neural pathways, most likely in the reward system. We previously discovered an addictive molecular facet of anorexia, involving production, in the nucleus accumbens (NAc), of the same transcripts stimulated in response to cocaine and amphetamine (CART) upon stimulation of the 5-HT(4) receptors (5-HTR(4)) or MDMA (ecstasy). Here, we tested whether this pathway predisposes not only to anorexia but also to hyperactivity. Following food restriction, mice are expected to overeat. However, selecting hyperactive and addiction-related animal models, we observed that mice lacking 5-HTR(1B) self-imposed food restriction after deprivation and still displayed anorexia and hyperactivity after ecstasy. Decryption of the mechanisms showed a gain-of-function of 5-HTR(4) in the absence of 5-HTR(1B), associated with CART surplus in the NAc and not in other brain areas. NAc-5-HTR(4) overexpression upregulated NAc-CART, provoked anorexia and hyperactivity. NAc-5-HTR(4) knockdown or blockade reduced ecstasy-induced hyperactivity. Finally, NAc-CART knockdown suppressed hyperactivity upon stimulation of the NAc-5-HTR(4). Additionally, inactivating NAc-5-HTR(4) suppressed ecstasy's preference, strengthening the rewarding facet of anorexia. In conclusion, the NAc-5-HTR(4)/CART pathway establishes a 'tight-junction' between anorexia and hyperactivity, suggesting the existence of a primary functional unit susceptible to limit overeating associated with resting following homeostasis rules.

A novel regulator of angiogenesis in endothelial cells: 5-hydroxytriptamine 4 receptor

The 5-hydroxytryptamine type 4 receptor (5-HT(4)R) regulates many physiological processes, including learning and memory, cognition, and gastrointestinal motility. Little is known about its role in angiogenesis. Using mouse hindlimb ischemia model of angiogenesis, we observed a significant reduction of limb blood flow recovery 14 days after ischemia and a decrease in density of CD31-positive vessels in adductor muscles in 5-HT(4)R(-/-) mice compared to wild type littermates. Our in vitro data indicated that 5-HT(4)R endogenously expressed in endothelial cells (ECs) may promote angiogenesis. Inhibition of the receptor with 5-HT(4)R antagonist RS 39604 reduced EC capillary tube formation in the reconstituted basement membrane. Using Boyden chamber migration assay and wound healing "scratch" assay, we demonstrated that RS 39604 treatment significantly suppressed EC migration. Transendothelial resistance measurement and immunofluorescence analysis showed that a 5-HT(4)R agonist RS 67333 led to an increase in endothelial permeability, actin stress fiber and interendothelial gap formation. Importantly, we provided the evidence that 5-HT(4)R-regulated EC migration may be mediated by Gα13 and RhoA. Our results suggest a prominent role of 5-HT(4)R in promoting angiogenesis and identify 5-HT(4)R as a potential therapeutic target for modulating angiogenesis under pathological conditions.

5-HT(4) receptors, a place in the sun: act two

5-HT(4) receptors control brain physiological functions such as learning and memory, feeding and mood behaviour as well as gastro-intestinal transit. 5-HT(4) receptors are one of the 5-HT receptors for which the available drugs and signalling knowledge are the most advanced. Several therapeutic 5-HT(4) receptor drugs have been commercialized. Therefore, the hope that 5-HT(4) receptors could also be the target for brain diseases is reasonable. Several major devastating illnesses could benefit from 5-HT(4) receptors-directed therapy such as Alzheimer's disease, feeding-associated diseases such as anorexia and major depressive disorders.

The role of serotonin receptor subtypes in treating depression: a review of animal studies

RATIONALE

Serotonin reuptake inhibitors (SSRIs) are effective in treating depression. Given the existence of different families and subtypes of 5-HT receptors, multiple 5-HT receptors may be involved in the antidepressant-like behavioral effects of SSRIs.

OBJECTIVE

Behavioral pharmacology studies investigating the role of 5-HT receptor subtypes in producing or blocking the effects of SSRIs were reviewed.

RESULTS

Few animal behavior tests were available to support the original development of SSRIs. Since their development, a number of behavioral tests and models of depression have been developed that are sensitive to the effects of SSRIs, as well as to other types of antidepressant treatments. The rationale for the development and use of these tests is reviewed. Behavioral effects similar to those of SSRIs (antidepressant-like) have been produced by agonists at 5-HT(1A), 5-HT(1B), 5-HT(2C), 5-HT(4), and 5-HT(6) receptors. Also, antagonists at 5-HT(2A), 5-HT(2C), 5-HT(3), 5-HT(6), and 5-HT(7) receptors have been reported to produce antidepressant-like responses. Although it seems paradoxical that both agonists and antagonists at particular 5-HT receptors can produce antidepressant-like effects, they probably involve diverse neurochemical mechanisms. The behavioral effects of SSRIs and other antidepressants may also be augmented when 5-HT receptor agonists or antagonists are given in combination.

CONCLUSIONS

The involvement of 5-HT receptors in the antidepressant-like effects of SSRIs is complex and involves the orchestration of stimulation and blockade at different 5-HT receptor subtypes. Individual 5-HT receptors provide opportunities for the development of a newer generation of antidepressants that may be more beneficial and effective than SSRIs.

Reduced signal transduction by 5-HT4 receptors after long-term venlafaxine treatment in rats

BACKGROUND AND PURPOSE

The 5-HT(4) receptor may be a target for antidepressant drugs. Here we have examined the effects of the dual antidepressant, venlafaxine, on 5-HT(4) receptor-mediated signalling events.

EXPERIMENTAL APPROACH

The effects of 21 days treatment (p.o.) with high (40 mg·kg(-1)) and low (10 mg·kg(-1)) doses of venlafaxine, were evaluated at different levels of 5-HT(4) receptor-mediated neurotransmission by using in situ hybridization, receptor autoradiography, adenylate cyclase assays and electrophysiological recordings in rat brain. The selective noradrenaline reuptake inhibitor, reboxetine (10 mg·kg(-1), 21 days) was also evaluated on 5-HT(4) receptor density.

KEY RESULTS

Treatment with a high dose (40 mg·kg(-1)) of venlafaxine did not alter 5-HT(4) mRNA expression, but decreased the density of 5-HT(4) receptors in caudate-putamen (% reduction = 26 ± 6), hippocampus (% reduction = 39 ± 7 and 39 ± 8 for CA1 and CA3 respectively) and substantia nigra (% reduction = 49 ± 5). Zacopride-stimulated adenylate cyclase activation was unaltered following low-dose treatment (10 mg·kg(-1)) while it was attenuated in rats treated with 40 mg·kg(-1) of venlafaxine (% reduction = 51 ± 2). Furthermore, the amplitude of population spike in pyramidal cells of CA1 of hippocampus induced by zacopride was significantly attenuated in rats receiving either dose of venlafaxine. Chronic reboxetine did not modify 5-HT(4) receptor density.

CONCLUSIONS AND IMPLICATIONS

Our data indicate a functional desensitization of 5-HT(4) receptors after chronic venlafaxine, similar to that observed after treatment with the classical selective inhibitors of 5-HT reuptake.

Disturbed neurotransmitter transporter expression in Alzheimer's disease brain

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by memory loss and behavioral and psychological symptoms of dementia. An imbalance of different neurotransmitters--glutamate, acetylcholine, dopamine, and serotonin--has been proposed as the neurobiological basis of behavioral symptoms in AD. The molecular changes associated with neurotransmission imbalance in AD are not clear. We hypothesized that altered reuptake of neurotransmitters by vesicular glutamate transporters (VGLUTs), excitatory amino acid transporters (EAATs), the vesicular acetylcholine transporter (VAChT), the serotonin reuptake transporter (SERT), or the dopamine reuptake transporter (DAT) are involved in the neurotransmission imbalance in AD. We tested this hypothesis by examining protein and mRNA levels of these transporters in postmortem prefrontal cortex from 10 AD patients and 10 matched non-AD controls. Compared with controls, protein and mRNA levels of VGLUTs, EAAT1-3, VAChT, and SERT were reduced significantly in AD. Expression of DAT and catechol O-methyltransferase was unchanged. Reduced VGLUTs and EAATs may contribute to an alteration in glutamatergic recycling, and reduced SERT could exacerbate depressive symptoms in AD. The reduced VAChT expression could contribute to the recognized cholinergic deficit in AD. Altered neurotransmitter transporters could contribute to the pathophysiology of AD and are potential targets for therapy.

Genetic deletion of fatty acid amide hydrolase alters emotional behavior and serotonergic transmission in the dorsal raphe, prefrontal cortex, and hippocampus

Pharmacological blockade of the anandamide-degrading enzyme, fatty acid amide hydrolase (FAAH), produces CB(1) receptor (CB(1)R)-mediated analgesic, anxiolytic-like and antidepressant-like effects in murids. Using behavioral and electrophysiological approaches, we have characterized the emotional phenotype and serotonergic (5-HT) activity of mice lacking the FAAH gene in comparison to their wild type counterparts, and their response to a challenge of the CB(1)R antagonist, rimonabant. FAAH null-mutant (FAAH(-/-)) mice exhibited reduced immobility in the forced swim and tail suspension tests, predictive of antidepressant activity, which was attenuated by rimonabant. FAAH(-/-) mice showed an increase in the duration of open arm visits in the elevated plus maze, and a decrease in thigmotaxis and an increase in exploratory rearing displayed in the open field, indicating anxiolytic-like effects that were reversed by rimonabant. Rimonabant also prolonged the initiation of feeding in the novelty-suppressed feeding test. Electrophysiological recordings revealed a marked 34.68% increase in dorsal raphe 5-HT neural firing that was reversed by rimonabant in a subset of neurons exhibiting high firing rates (33.15% mean decrease). The response of the prefrontocortical pyramidal cells to the 5-HT(2A/2C) agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane ((+/-)-DOI) revealed desensitized 5-HT(2A/2C) receptors, likely linked to the observed anxiolytic-like behaviors. The hippocampal pyramidal response to the 5-HT(1A) antagonist, WAY-100635, indicates enhanced tonus on the hippocampal 5-HT(1A) heteroreceptors, a hallmark of antidepressant-like action. Together, these results suggest that FAAH genetic deletion enhances anxiolytic-like and antidepressant-like effects, paralleled by altered 5-HT transmission and postsynaptic 5-HT(1A) and 5-HT(2A/2C) receptor function.

Reversal of hippocampal neuronal maturation by serotonergic antidepressants

Serotonergic antidepressant drugs have been commonly used to treat mood and anxiety disorders, and increasing evidence suggests potential use of these drugs beyond current antidepressant therapeutics. Facilitation of adult neurogenesis in the hippocampal dentate gyrus has been suggested to be a candidate mechanism of action of antidepressant drugs, but this mechanism may be only one of the broad effects of antidepressants. Here we show a distinct unique action of the serotonergic antidepressant fluoxetine in transforming the phenotype of mature dentate granule cells. Chronic treatments of adult mice with fluoxetine strongly reduced expression of the mature granule cell marker calbindin. The fluoxetine treatment induced active somatic membrane properties resembling immature granule cells and markedly reduced synaptic facilitation that characterizes the mature dentate-to-CA3 signal transmission. These changes cannot be explained simply by an increase in newly generated immature neurons, but best characterized as "dematuration" of mature granule cells. This granule cell dematuration developed along with increases in the efficacy of serotonin in 5-HT(4) receptor-dependent neuromodulation and was attenuated in mice lacking the 5-HT(4) receptor. Our results suggest that serotonergic antidepressants can reverse the established state of neuronal maturation in the adult hippocampus, and up-regulation of 5-HT(4) receptor-mediated signaling may play a critical role in this distinct action of antidepressants. Such reversal of neuronal maturation could affect proper functioning of the mature hippocampal circuit, but may also cause some beneficial effects by reinstating neuronal functions that are lost during development.

Hyperfunction of muscarinic receptor maintains long-term memory in 5-HT4 receptor knock-out mice

Patients suffering from dementia of Alzheimer's type express less serotonin 4 receptors (5-HTR₄), but whether an absence of these receptors modifies learning and memory is unexplored. In the spatial version of the Morris water maze, we show that 5-HTR₄ knock-out (KO) and wild-type (WT) mice performed similarly for spatial learning, short- and long-term retention. Since 5-HTR₄ control mnesic abilities, we tested whether cholinergic system had circumvented the absence of 5-HTR₄. Inactivating muscarinic receptor with scopolamine, at an ineffective dose (0.8 mg/kg) to alter memory in WT mice, decreased long-term but not short-term memory of 5-HTR₄ KO mice. Other changes included decreases in the activity of choline acetyltransferase (ChAT), the required enzyme for acetylcholine synthesis, in the septum and the dorsal hippocampus in 5-HTR₄ KO under baseline conditions. Training- and scopolamine-induced increase and decrease, respectively in ChAT activity in the septum in WT mice were not detected in the 5-HTR₄ KO animals. Findings suggest that adaptive changes in cholinergic systems may circumvent the absence of 5-HTR₄ to maintain long-term memory under baseline conditions. In contrast, despite adaptive mechanisms, the absence of 5-HTR₄ aggravates scopolamine-induced memory impairments. The mechanisms whereby 5-HTR₄ mediate a tonic influence on ChAT activity and muscarinic receptors remain to be determined.

Specific knock-down of GAD67 in the striatum using naked small interfering RNAs

Small interfering RNAs (siRNA) are double-stranded RNAs of 9-29 nucleotides designed to reduce the expression of homologous genes by a process known as RNA interference (RNAi). Most studies using siRNA in neurons have been performed in mammalian cell cultures. Only few reports have reported the effects of in vivo infusion of siRNA into the brain. In the present study, we performed local intracerebral infusions of naked siRNA against glutamic acid decarboxylase 67 (GAD67) mRNA to engineer specific knock-down of GAD67 protein in the striatum of adult rats. Directly injecting a mix of GAD67 siRNAs into the striatum decreased the levels of corresponding mRNA, evaluated by quantitative real-time PCR. In particular, we show that GAD67 mRNA expression is reduced in the striatum for 3, 6, and 24h following intrastriatal injection of GAD67 siRNA and is restored at 72h. Relative to controls, the levels of GAD67 protein were also lower in the striatum for 6 and 24h after injection. No changes in GAD65 expression, one of the two isoforms of GAD, were detected in the striatum, which further validates the specificity of the siRNA. We demonstrate the efficiency of the RNAi strategy for producing a specific and selective down-regulation of GAD67 in the adult rat brain. This suggests that siRNA-mediated gene knock-down constitute a valid methodological approach for studying the functional consequences of a transient decrease of a gene expression in a brain structure.

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.