Cultured astrocytes express messenger RNA for multiple serotonin receptor subtypes, without functional coupling of 5-HT1 receptor subtypes to adenylyl cyclase

Serotonergic regulation of cortical neurovascular coupling and hemodynamics upon awakening from sleep in mice

Neurovascular coupling (NVC) is the functional hyperemia of the brain responding to local neuronal activity. It is mediated by astrocytes and affected by subcortical ascending pathways in the cortex that convey information, such as sensory stimuli and the animal condition. Here, we investigate the influence of the raphe serotonergic system, a subcortical ascending arousal system in animals, on the modulation of cortical NVC and cerebral blood flow (CBF). Raphe serotonergic neurons were optogenically activated for 30 s, which immediately awakened the mice from non-rapid eye movement sleep. This caused a biphasic cortical hemodynamic change: a transient increase for a few seconds immediately after photostimulation onset, followed by a large progressive decrease during the stimulation period. Serotonergic neuron activation increased intracellular Ca2+ levels in cortical pyramidal neurons and astrocytes, demonstrating its effect on the NVC components. Pharmacological inhibition of cortical neuronal firing activity and astrocyte metabolic activity had small hypovolemic effects on serotonin-induced biphasic CBF changes, while blocking 5-HT1B receptors expressed primarily in cerebral vasculature attenuated the decreasing CBF phase. This suggests that serotonergic neuron activation leading to animal awakening could allow the NVC to exert a hyperemic function during a biphasic CBF response, with a predominant decrease in the cortex.

The 5-HT1F receptor as the target of ditans in migraine - from bench to bedside

Migraine is a leading cause of disability in more than one billion people worldwide, yet it remains universally underappreciated, even by individuals with the condition. Among other shortcomings, current treatments (often repurposed agents) have limited efficacy and potential adverse effects, leading to low treatment adherence. After the introduction of agents that target the calcitonin gene-related peptide pathway, another new drug class, the ditans - a group of selective serotonin 5-HT_1F receptor agonists - has just reached the international market. Here, we review preclinical studies from the late 1990s and more recent clinical research that contributed to the development of the ditans and led to their approval for acute migraine treatment by the US Food and Drug Administration and the European Medicines Agency.

The Serotonergic System and Amyotrophic Lateral Sclerosis: A Review of Current Evidence

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder characterized by the premature death of motor neurons. Serotonin (5-HT) is a crucial neurotransmitter, and its dysfunction, whether as a contributor or by-product, has been implicated in ALS pathogenesis. Here, we summarize current evidence linking serotonergic alterations to ALS, including results from post-mortem and neuroimaging studies, biofluid testing, and studies of ALS animal models. We also discuss the possible role of 5-HT in modulating some important mechanisms of ALS (i.e. glutamate excitotoxity and neuroinflammation) and in regulating ALS phenotypes (i.e. breathing dysfunction and metabolic defects). Finally, we discuss the promise and limitations of the serotonergic system as a target for the development of ALS biomarkers and therapeutic approaches. However, due to a relative paucity of data and standardized methodologies in previous studies, proper interpretation of existing results remains a challenge. Future research is needed to unravel the mechanisms linking serotonergic pathways and ALS and to provide valid, reproducible, and translatable findings.

Spatiotemporal dynamics of 5-HT6 receptor ciliary localization during mouse brain development

The serotonin 5-HT₆ receptor (5-HT₆R) is a promising target to improve cognitive symptoms of psychiatric diseases of neurodevelopmental origin, such as autism spectrum disorders and schizophrenia. However, its expression and localization at different stages of brain development remain largely unknown, due to the lack of specific antibodies to detect endogenous 5-HT₆R. Here, we used transgenic mice expressing a GFP-tagged 5-HT₆R under the control of its endogenous promoter (Knock-in) as well as embryonic stem cells expressing the GFP-tagged receptor to extensively characterize its expression at cellular and subcellular levels during development. We show that the receptor is already expressed at E13.5 in the cortex, the striatum, the ventricular zone, and to a lesser extent the subventricular zone. In adulthood, it is preferentially found in projection neurons of the hippocampus and cerebral cortex, in striatal medium-sized spiny neurons, as well as in a large proportion of astrocytes, while it is expressed in a minor population of interneurons. Whereas the receptor is almost exclusively detected in the primary cilia of neurons at embryonic and adult stages and in differentiated stem cells, it is located in the somatodendritic compartment of neurons from some brain regions at the neonatal stage and in the soma of undifferentiated stem cells. Finally, knocking-out the receptor induces a shortening of the primary cilium, suggesting that it plays a role in its function. This study provides the first global picture of 5-HT₆R expression pattern in the mouse brain at different developmental stages. It reveals dynamic changes in receptor localization in neurons at the neonatal stage, which might underlie its key role in neuronal differentiation and psychiatric disorders of neurodevelopmental origin.

Astroglial Serotonin Receptors as the Central Target of Classic Antidepressants

Major depressive disorder (MDD) presents multiple clinical phenotypes and has complex underlying pathological mechanisms. Existing theories cannot completely explain the pathophysiological mechanism(s) of MDD, while the pharmacology of current antidepressants is far from being fully understood. Astrocytes, the homeostatic and defensive cells of the central nervous system, contribute to shaping behaviors, and regulating mood and emotions. A detailed introduction on the role of astrocytes in depressive disorders is thus required, to which this chapter is dedicated. We also focus on the interactions between classic antidepressants and serotonin receptors, overview the role of astrocytes in the pharmacological mechanisms of various antidepressants, and present astrocytes as targets for the treatment of bipolar disorder. We provide a foundation of knowledge on the role of astrocytes in depressive disorders and astroglial 5-HT_2B receptors as targets for selective serotonin reuptake inhibitors in vivo and in vitro.

Assessment of astrocytes as a mediator of memory and learning in rodents

The classical view of astrocytes is that they provide supportive functions for neurons, transporting metabolites and maintaining the homeostasis of the extracellular milieu. This view is gradually changing with the advent of molecular genetics and optical methods allowing interrogation of selected cell types in live experimental animals. An emerging view that astrocytes additionally act as a mediator of synaptic plasticity and contribute to learning processes has gained in vitro and in vivo experimental support. Here we focus on the literature published in the past two decades to review the roles of astrocytes in brain plasticity in rodents, whereby the roles of neurotransmitters and neuromodulators are considered to be comparable to those in humans. We outline established inputs and outputs of astrocytes and discuss how manipulations of astrocytes have impacted the behavior in various learning paradigms. Multiple studies suggest that the contribution of astrocytes has a considerably longer time course than neuronal activation, indicating metabolic roles of astrocytes. We advocate that exploring upstream and downstream mechanisms of astrocytic activation will further provide insight into brain plasticity and memory/learning impairment.

Astrocytes promote ethanol-induced enhancement of intracellular Ca2+ signals through intercellular communication with neurons

Ethanol (EtOH) abuse induces significant mortality and morbidity worldwide because of detrimental effects on brain function. Defining the contribution of astrocytes to this malfunction is imperative to understanding the overall EtOH effects due to their role in homeostasis and EtOH-seeking behaviors. Using a highly controllable in vitro system, we identify chemical signaling mechanisms through which acute EtOH exposure induces a modulatory feedback loop between neurons and astrocytes. Neuronally-derived purinergic signaling primed a subpopulation of astrocytes to respond to subsequent acute EtOH exposures (^SEastrocytes: signal enhanced astrocytes) with greater calcium signal strength. Generation of ^SEastrocytes arose from astrocytic hemichannel-derived ATP and accumulation of its metabolite adenosine within the astrocyte microenvironment to modulate adenylyl cyclase and phospholipase C activity. These results highlight an important role of astrocytes in shaping the overall physiological responsiveness to EtOH and emphasize the unique plasticity of astrocytes to adapt to single and multiple exposures of EtOH.

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/dopamine interaction: Electrophysiological and neurochemical evidence

The interaction between serotonin (5-HT) and dopamine (DA) in the central nervous system (CNS) plays an important role in the adaptive properties of living animals to their environment. These are two modulatory, divergent systems shaping and regulating in a widespread manner the activity of neurobiological networks and their interaction. The concept of one interaction linking these two systems is rather elusive when looking at the mechanisms triggered by these two systems across the CNS. The great variety of their interacting mechanisms is in part due to the diversity of their neuronal origin, the density of their fibers in a given CNS region, the distinct expression of their numerous receptors in the CNS, the heterogeneity of their intracellular signaling pathway that depend on the cellular type expressing their receptors, and the state of activity of neurobiological networks, conditioning the outcome of their mutual influences. Thus, originally conceptualized as inhibition of 5-HT on DA neuron activity and DA neurotransmission, this interaction is nowadays considered as a multifaceted, mutual influence of these two systems in the regulation of CNS functions. These new ways of understanding this interaction are of utmost importance to envision the consequences of their dysfunctions underlined in several CNS diseases. It is also essential to conceive the mechanism of action of psychotropic drugs directly acting on their function including antipsychotic, antidepressant, antiparkinsonian, and drug of abuse together with the development of therapeutic strategies of Alzheimer's diseases, epilepsy, obsessional compulsive disorders. The 5-HT/DA interaction has a long history from the serendipitous discovery of antidepressants and antipsychotics to the future, rationalized treatments of CNS disorders.

Serotoninergic system targeting in multiple sclerosis: the prospective for pathogenetic therapy

Serotonin (5-hydroxytryptamine) (5-HT) is a neurotransmitter, which mediates neuropsychological functions of the central nervous system (CNS). Recent studies have shown the modulatory effect of 5-HT on gut microbiota functions, which play an essential role in developing CNS inflammatory diseases. Finally, 5-HT is a direct mediator of neuroimmune interaction. The article reviews the literature data on the role of 5-HT in the regulation of neuroinflammation in multiple sclerosis (MS). The influence of 5-HT and selective serotonin reuptake inhibitors (SSRIs) on experimental autoimmune encephalomyelitis (EAE) and MS pathogenesis, as well as the therapeutic potential of serotoninergic drugs as a pathogenetic therapy of MS, are discussed.

Neuromodulation of Astrocytic K+ Clearance

Potassium homeostasis is fundamental for brain function. Therefore, effective removal of excessive K+ from the synaptic cleft during neuronal activity is paramount. Astrocytes play a key role in K+ clearance from the extracellular milieu using various mechanisms, including uptake via Kir channels and the Na⁺-K⁺ ATPase, and spatial buffering through the astrocytic gap-junction coupled network. Recently we showed that alterations in the concentrations of extracellular potassium ([K⁺]_o) or impairments of the astrocytic clearance mechanism affect the resonance and oscillatory behavior of both the individual and networks of neurons. These results indicate that astrocytes have the potential to modulate neuronal network activity, however, the cellular effectors that may affect the astrocytic K+ clearance process are still unknown. In this study, we have investigated the impact of neuromodulators, which are known to mediate changes in network oscillatory behavior, on the astrocytic clearance process. Our results suggest that while some neuromodulators (5-HT; NA) might affect astrocytic spatial buffering via gap-junctions, others (DA; Histamine) primarily affect the uptake mechanism via Kir channels. These results suggest that neuromodulators can affect network oscillatory activity through parallel activation of both neurons and astrocytes, establishing a synergistic mechanism to maximize the synchronous network activity.

The Association Between Antidepressant Effect of SSRIs and Astrocytes: Conceptual Overview and Meta-analysis of the Literature

Major depressive disorders (MDD) a worldwide psychiatric disease, is yet to be adequately controlled by therapies; while the mechanisms of action of antidepressants are yet to be fully characterised. In the last two decades, an increasing number of studies have demonstrated the role of astrocytes in the pathophysiology and therapy of MDD. Selective serotonin reuptake inhibitors (SSRIs) are the most widely used antidepressants. It is generally acknowledged that SSRIs increase serotonin levels in the central nervous system by inhibiting serotonin transporters, although the SSRIs action is not ideal. The SSRIs antidepressant effect develops with considerable delay; their efficacy is low and frequent relapses are common. Neither cellular nor molecular pharmacological mechanisms of SSRIs are fully characterised; in particular their action on astrocytes remain underappreciated. In this paper we overview potential therapeutic mechanisms of SSRIs associated with astroglia and report the results of meta-analysis of studies dedicated to MDD, SSRIs and astrocytes. In particular, we argue that fluoxetine, the representative SSRI, improves depressive-like behaviours in animals treated with chronic mild stress and reverses depression-associated decrease in astrocytic glial fibrillary acidic protein (GFAP) expression. In addition, fluoxetine upregulates astrocytic mRNA expression of 5-hydroxytriptamin/serotonin_2B receptors (5-HT_2BR). In summary, we infer that SSRIs exert their anti-depressant effect by regulating several molecular and signalling pathways in astrocytes.

Imidazopyridine-Based 5-HT6 Receptor Neutral Antagonists: Impact of N1-Benzyl and N1-Phenylsulfonyl Fragments on Different Receptor Conformational States

G-protein coupled receptors (GPCRs) exist in an equilibrium of multiple conformational states, including different active states, which depend on the nature of the bound ligand. In consequence, different conformational states can initiate specific signal transduction pathways. The study identified compound 7e, which acts as a potent 5-hydroxytryptamine type 6 receptor (5-HT₆R) neutral antagonist at Gs and does not impact neurite growth (process controlled by Cdk5). MD simulations highlighted receptor conformational changes for 7e and inverse agonist PZ-1444. In cell-based assays, neutral antagonists of the 5-HT₆R (7e and CPPQ), but not inverse agonists (SB-258585, intepirdine, PZ-1444), displayed glioprotective properties against 6-hydroxydopamine-induced and doxorubicin-induced cytotoxicity. These suggest that targeting the activated conformational state of the 5-HT₆R with neutral antagonists implicates the protecting properties of astrocytes. Additionally, 7e prevented scopolamine-induced learning deficits in the novel object recognition test in rats. We propose 7e as a probe for further understanding of the functional outcomes of different states of the 5-HT₆R.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Neuromodulation of Glial Function During Neurodegeneration

Glia, a non-excitable cell type once considered merely as the connective tissue between neurons, is nowadays acknowledged for its essential contribution to multiple physiological processes including learning, memory formation, excitability, synaptic plasticity, ion homeostasis, and energy metabolism. Moreover, as glia are key players in the brain immune system and provide structural and nutritional support for neurons, they are intimately involved in multiple neurological disorders. Recent advances have demonstrated that glial cells, specifically microglia and astroglia, are involved in several neurodegenerative diseases including Amyotrophic lateral sclerosis (ALS), Epilepsy, Parkinson's disease (PD), Alzheimer's disease (AD), and frontotemporal dementia (FTD). While there is compelling evidence for glial modulation of synaptic formation and regulation that affect neuronal signal processing and activity, in this manuscript we will review recent findings on neuronal activity that affect glial function, specifically during neurodegenerative disorders. We will discuss the nature of each glial malfunction, its specificity to each disorder, overall contribution to the disease progression and assess its potential as a future therapeutic target.

5-HT2A/B receptor expression in the phrenic motor nucleus in a rat model of ALS (SOD1G93A)

Despite respiratory motor neuron death, ventilation is preserved in SOD1G93A rats. Compensatory respiratory plasticity may counterbalance the loss of these neurons. Phrenic long-term facilitation (pLTF; a form of respiratory plasticity) in naïve rats is 5-HT2 and NADPH oxidase-dependent. Furthermore, 5-HT2A, not 5-HT2B, receptor-induced phrenic motor facilitation is NADPH oxidase-independent in naïve rats. pLTF is NADPH oxidase-dependent in pre-symptomatic, but not end-stage, SOD1G93A rats. Here, we hypothesized that in the putative phrenic motor nucleus (PMN) of SOD1G93A rats vs. wild-type littermates: 1) pre-symptomatic rats would have greater 5-HT2B receptor expression that decreases at end-stage; and 2) 5-HT2A receptor expression would increase from pre-symptomatic to end-stage. Putative PMN 5-HT2A receptor expression was reduced when comparing across (but not within) pre-symptomatic vs. end-stage groups (p < 0.05). In contrast, putative PMN 5-HT2B receptor expression was increased when comparing across pre-symptomatic vs. end-stage groups, and within end-stage groups (p < 0.05). These data suggest a potential role for 5-HT2 receptors in pLTF and breathing in SOD1G93A rats.

Central 5-HT receptors and their function; present and future

Since our review of central 5-HT receptors and their function twenty years ago, no new 5-HT receptor has been discovered and there is little evidence that this situation will change in the near future. Nevertheless, over this time significant progress has been made in our understanding of the properties of these receptors and in the clinical translation of this information, and some of these developments are highlighted herein. Such highlights include extensive mapping of 5-HT receptors in both animal and human brain, culminating in readily-accessible brain atlases of 5-HT receptor distribution, as well as emerging data on how 5-HT receptors are distributed within complex neural circuits. Also, a range of important pharmacological and genetic tools have been developed that allow selective 5-HT receptor manipulation, in cells through to whole organism models. Moreover, unexpected complexity in 5-HT receptor function has been identified including agonist-dependent signalling that goes beyond the pharmacology of canonical 5-HT receptor signalling pathways set down in the 1980s and 1990s. This new knowledge of 5-HT signalling has been extended by the discovery of combined signalling of 5-HT and co-released neurotransmitters, especially glutamate. Another important advance has been the progression of a large number of 5-HT ligands through to experimental medicine studies and clinical trials, and some such agents have already become prescribed therapeutic drugs. Much more needs to be discovered and understood by 5-HT neuropharmacologists, not least how the diverse signalling effects of so many 5-HT receptor types interact with complex neural circuits to generate neurophysiological changes which ultimately lead to altered cognitions and behaviour. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Topologically Guided Prioritization of Candidate Gene Transcripts Coexpressed with the 5-HT1A Receptor by Combining In Vivo PET and Allen Human Brain Atlas Data

The serotonin-1A receptor (5-HT_1AR) represents a viable target in the treatment of disorders of the brain. However, development of psychiatric drugs continues to be hindered by the relative inaccessibility of brain tissue. Although the efficacy of drugs selective for the 5-HT_1AR has not been proven, research continues to focus on drugs that influence this receptor subtype. To further knowledge on this topic, we investigated the topological coexpression patterns of the 5-HT_1AR. We calculated Spearman’s rho for the correlation of positron emission tomography-binding potentials (BP_ND) of the 5-HT_1AR assessed in 30 healthy subjects using the tracer [carbonyl-¹¹C]WAY-100635 and predicted whole-brain mRNA expression of 18 686 genes. After applying a threshold of r > 0.3 in a leave-one-out cross-validation of the prediction of mRNA expression, genes with ρ ≥ 0.7 were considered to be relevant. In cortical regions, 199 genes showed high correlation with the BP_ND of the 5-HT_1AR, in subcortical regions 194 genes. Using our approach, we could consolidate the role of BDNF and implicate new genes (AnxA8, NeuroD2) in serotonergic functioning. Despite its explorative nature, the analysis can be seen as a gene prioritization approach to reduce the number of genes potentially connected to 5-HT_1AR functioning and guide future in vitro studies.

Monoamines in glioblastoma: complex biology with therapeutic potential

Glioblastoma (GBM) is characterized by extremely poor prognoses, despite the use of gross surgical resection, alkylating chemotherapeutic agents, and radiotherapy. Evidence increasingly highlights the role of the tumor microenvironment in enabling this aggressive phenotype. Despite this interest, the role of neurotransmitters, brain-specific messengers underlying synaptic transmission, remains murky. These signaling molecules influence a complex network of molecular pathways and cellular behaviors in many CNS-resident cells, including neural stem cells and progenitor cells, neurons, and glia cells. Critically, available data convincingly demonstrate that neurotransmitters can influence proliferation, quiescence, and differentiation status of these cells. This ability to affect progenitors and glia-GBM-initiating cells-and their availability in the CNS strongly support the notion that neurotransmitters participate in the onset and progression of GBM. This review will focus on dopamine and serotonin, as studies indicate they contribute to gliomagenesis. Particular attention will be paid to how these neurotransmitters and their receptors can be utilized as novel therapeutic targets. Overall, this review will analyze the complex biology governing the interaction of GBM with neurotransmitter signaling and highlight how this interplay shapes the aggressive nature of GBM.

Mirtazapine increases glial cell line-derived neurotrophic factor production through lysophosphatidic acid 1 receptor-mediated extracellular signal-regulated kinase signaling in astrocytes

Different classes of antidepressants, such as tricyclic antidepressants, selective serotonin reuptake inhibitor (SSRI), and serotonin and norepinephrine reuptake inhibitor (SNRI), have been shown to increase GDNF production in astrocytes, which could be a key mechanism of the psychotropic effect of antidepressants. The antidepressant mirtazapine is a noradrenaline and specific serotonergic antidepressant (NaSSA) and does not block reuptake of catecholamines and serotonin. The present study examined the effect of mirtazapine on GDNF expression in rat C6 astroglial cells (C6 cells) and rat primary cultured cortical astrocytes (primary astrocytes). Mirtazapine treatment significantly increased GDNF mRNA expression and GDNF release in both C6 cells and primary astrocytes. In primary astrocytes, mirtazapine also increased the expressions of brain-derived neurotrophic factor mRNA. To mimic mirtazapine's putative mechanism of action, cells were treated with either a α₂-adrenoceptor antagonist (yohimbine), 5-HT₂ receptor antagonist (ketanserin), 5-HT₃ receptor antagonist (ondansetron), or a mixture of these--no effect on GDNF mRNA expression was observed. Mirtazapine treatment increased phosphorylation of extracellular signal-regulated kinase (ERK) 1/2, and the mirtazapine-induced GDNF and BDNF expression were blocked by MAPK/ERK kinase (MEK) inhibitor (U0126). Furthermore, the effect of mirtazapine on ERK phosphorylation and expressions of GDNF and BDNF was antagonized by Gi/o inhibitor (pertussis toxin), lysophosphatidic acid-1 (LPA₁) receptor antagonist (AM966), and LPA₁/LPA₃ receptors antagonist (Ki16425). The current findings demonstrate that the NaSSA mirtazapine, similar to other classes of antidepressants, increases GDNF expression through a Gi/o coupled LPA₁ receptor-mediated ERK pathway. The current findings suggest a general mechanism underlying the psychotropic effect antidepressants.

CRISPR/Cas9-mediated in vivo gene editing reveals that neuronal 5-HT1A receptors in the dorsal raphe nucleus contribute to body temperature regulation in mice

Serotonin (5-HT) in the central nervous system regulates a variety of biological functions, from the basic homeostatic control to higher brain functions, by acting on fourteen known receptor subtypes. However, it is still usually unclear which receptor subtype is responsible for a specific function due to the lack of highly selective ligands for most of these receptors. Although 5-HT receptor knockout mice are useful, the brain-wide distribution of various receptors makes it difficult to dissect receptor functions in specific and brain regions and cell types. Recent advances in CRISPR/Cas9-mediated in vivo genome editing technology may overcome this problem. In this study, we constructed a viral vector expressing a single guide (sg)RNA targeting Htr1a (sgHtr1a) and Cre recombinase under the control of a neuron-specific promoter. Injection of the viral vector into the dorsal raphe nucleus (DRN) of Cre-dependent Cas9 knock-in mice induced Cre-dependent Cas9 expression mainly in DRN serotonin and GABA neurons. Mismatch cleavage assay and Sanger sequencing showed insertion or deletion formation at the target site. 5-HT_1A receptor agonist-induced hypothermia was attenuated and antidepressant effect of a selective serotonin reuptake inhibitor (SSRI) was enhanced by microinjection of the viral vector expressing sgHtr1a into the DRN of Cre-dependent Cas9 knock-in mice. These results suggest that this in vivo CRISPR/Cas9-mediated 5-HT receptor gene knockout strategy provides a reliable and low-cost method for elucidating 5-HT receptor functions in specific cell types and brain regions. Further, we demonstrate that the neuronal 5-HT_1A receptor in the DRN regulates body temperature and antidepressant effect of SSRI.

Astroglial 5-HT2B receptor in mood disorders

Introduction: Astroglia represent the main cellular homeostatic system of the central nervous system (CNS). Astrocytes are intimately involved in regulation and maintenance of neurotransmission by regulating neurotransmitters removal and turnover and by supplying neurons with neurotransmitters precursors. Astroglial cells are fundamental elements of monoaminergic transmission in the brain and in the spinal cord. Astrocytes receive monoaminergic inputs and control catabolism of monoamines through dedicated transporters and intracellular enzymatic pathways.Areas covered: Astroglial cells express serotonergic receptors; in this review, we provide an in-depth characterization of 5-HT_2B receptors. Activation of these receptors triggers numerous intracellular signaling cascades that regulate expression of multiple genes. Astroglial 5-HT_2B receptors are activated by serotonin-specific reuptake inhibitors, such as major anti-depressant fluoxetine. Expression of astroglial serotonin receptors undergoes remarkable changes in depression disorders, and these changes can be corrected by chronic treatment with anti-depressant drugs.Expert commentary: Depressive behaviors, which occur in rodents following chronic stress or in neurotoxic models of Parkinson disease, are associated with significant changes in the expression of astroglial, but not neuronal 5-HT_2B receptors; while therapy with anti-depressants normalizes both receptors expression and depressive behavioral phenotype. In summary, astroglial serotonin receptors are linked to mood disorders and may represent a novel target for cell- and molecule-specific therapies of depression and mood disorders.

Monoaminergic Mechanisms in Epilepsy May Offer Innovative Therapeutic Opportunity for Monoaminergic Multi-Target Drugs

A large body of experimental and clinical evidence has strongly suggested that monoamines play an important role in regulating epileptogenesis, seizure susceptibility, convulsions, and comorbid psychiatric disorders commonly seen in people with epilepsy (PWE). However, neither the relative significance of individual monoamines nor their interaction has yet been fully clarified due to the complexity of these neurotransmitter systems. In addition, epilepsy is diverse, with many different seizure types and epilepsy syndromes, and the role played by monoamines may vary from one condition to another. In this review, we will focus on the role of serotonin, dopamine, noradrenaline, histamine, and melatonin in epilepsy. Recent experimental, clinical, and genetic evidence will be reviewed in consideration of the mutual relationship of monoamines with the other putative neurotransmitters. The complexity of epileptic pathogenesis may explain why the currently available drugs, developed according to the classic drug discovery paradigm of "one-molecule-one-target," have turned out to be effective only in a percentage of PWE. Although, no antiepileptic drugs currently target specifically monoaminergic systems, multi-target directed ligands acting on different monoaminergic proteins, present on both neurons and glia cells, may represent a new approach in the management of seizures, and their generation as well as comorbid neuropsychiatric disorders.

Agonist and antagonist bind differently to 5-HT1A receptors during Alzheimer's disease: A post-mortem study with PET radiopharmaceuticals

PET imaging studies using 5-HT1A receptor radiotracers show a decreased density of this receptor in hippocampi of patients with Alzheimer's disease (AD) at advanced stages. However, current 5-HT1A receptor radiopharmaceuticals used in neuroimaging are antagonists, thought to bind to 5-HT1A receptors in different functional states (i.e., both the one which displays high affinity for agonists and is thought to mediate receptor activation, as well as the state which has low affinity for agonists). Comparing the PET imaging obtained using an agonist radiotracer, which binds selectively to functional receptors, with the PET imaging obtained using an antagonist radiotracer would therefore provide original information on 5-HT1A receptor impairment during AD. Quantitative autoradiography using [(18)F]F13640 and [(18)F]MPPF, a 5-HT1A agonist and antagonist, respectively, was measured in hippocampi of patients with AD (n = 25, at different Braak stages) and control subjects (n = 9). The neuronal density was measured in the same tissues by NeuN immunohistochemistry. The specific binding of both radiotracers was determined by addition of WAY-100635, a selective 5-HT1A receptor antagonist. The autoradiography distribution of both 5-HT1A PET radiotracers varied across hippocampus regions. The highest binding density was in the pyramidal layer of CA1. Incubation with Gpp(NH)p, a non-hydrolysable analogue of GTP, reduced significantly [(18)F]F13640 binding in hippocampal regions, confirming its preferential interaction with G-coupled receptors, and slightly increased [(18)F]MPPF binding. In the CA1 subfield, [(18)F]F13640 binding was significantly decreased at Braak stages I/II (-19%), Braak stages III/IV (-23%), and Braak stages V/VI (-36%) versus control. In contrast, [(18)F]MPPF binding was statistically reduced only at the most advanced Braak stages V/VI compared to control (-33%). Since [(18)F]F13640 and [(18)F]MPPF can be used in vivo in humans, this neuropharmacological paradigm supports testing the concept of functional imaging using agonist radiopharmaceuticals in future clinical studies.

Serotonergic modulation of the activity of mesencephalic dopaminergic systems: Therapeutic implications

Since their discovery in the mammalian brain, it has been apparent that serotonin (5-HT) and dopamine (DA) interactions play a key role in normal and abnormal behavior. Therefore, disclosure of this interaction could reveal important insights into the pathogenesis of various neuropsychiatric diseases including schizophrenia, depression and drug addiction or neurological conditions such as Parkinson's disease and Tourette's syndrome. Unfortunately, this interaction remains difficult to study for many reasons, including the rich and widespread innervations of 5-HT and DA in the brain, the plethora of 5-HT receptors and the release of co-transmitters by 5-HT and DA neurons. The purpose of this review is to present electrophysiological and biochemical data showing that endogenous 5-HT and pharmacological 5-HT ligands modify the mesencephalic DA systems' activity. 5-HT receptors may control DA neuron activity in a state-dependent and region-dependent manner. 5-HT controls the activity of DA neurons in a phasic and excitatory manner, except for the control exerted by 5-HT_2C receptors which appears to also be tonically and/or constitutively inhibitory. The functional interaction between the two monoamines will also be discussed in view of the mechanism of action of antidepressants, antipsychotics, anti-Parkinsonians and drugs of abuse.

Role of spinal 5-HT2 receptors subtypes in formalin-induced long-lasting hypersensitivity

BACKGROUND

The purpose of this study was to determine the role of spinal 5-HT2A, 5-HT2B and 5-HT2C receptors in the development and maintenance of formalin-induced long-lasting secondary allodynia and hyperalgesia in rats, as well as their expression in the dorsal root ganglia (DRG) during this process.

METHODS

0.5-1% formalin was used to produce long-lasting secondary allodynia and hyperalgesia in rats. Western blot was used to determine 5-HT2 receptors expression in DRG.

RESULTS

Formalin (0.5-1%) injection produced long-lasting (1-12 days) secondary allodynia and hyperalgesia in both ipsilateral and contralateral hind paws. Intrathecal pre-treatment or post-treatment with the 5-HT2 receptor agonist, DOI (1-10nmol), increased 0.5% formalin-induced secondary allodynia and hyperalgesia in both paws. In contrast, intrathecal pre-treatment with the selective 5-HT2A (ketanserin 1-100nmol), 5-HT2B (RS 127445 1-100nmol) or 5-HT2C (RS 102221 1-100nmol) receptor antagonists prevented and reversed, respectively, 1% formalin-induced secondary allodynia and hyperalgesia in both paws. Likewise, the pronociceptive effect of DOI (10nmol) was blocked by ketanserin, RS 127445 or RS 102221 (0.01nmol). 5-HT2A/2B/2C receptors were expressed in DRG of naïve rats. Formalin injection (1%) increased bilaterally 5-HT2A/2B receptors expression in DRG. In contrast, formalin injection decreased 5-HT2C receptors expression bilaterally in DRG.

CONCLUSION

Data suggest that spinal 5-HT2A/2B/2C receptors have pronociceptive effects and participate in the development and maintenance of formalin-induced long-lasting hypersensitivity. These receptors are expressed in DRG and their expression is modulated by formalin.

Pronociceptive and Antinociceptive Effects of Buprenorphine in the Spinal Cord Dorsal Horn Cover a Dose Range of Four Orders of Magnitude

Due to its distinct pharmacological profile and lower incidence of adverse events compared with other opioids, buprenorphine is considered a safe option for pain and substitution therapy. However, despite its wide clinical use, little is known about the synaptic effects of buprenorphine in nociceptive pathways. Here, we demonstrate dose-dependent, bimodal effects of buprenorphine on transmission at C-fiber synapses in rat spinal cord dorsal horn in vivo. At an analgesically active dose of 1500 μg·kg(-1), buprenorphine reduced the strength of spinal C-fiber synapses. This depression required activation of spinal opioid receptors, putatively μ1-opioid receptors, as indicated by its sensitivity to spinal naloxone and to the selective μ1-opioid receptor antagonist naloxonazine. In contrast, a 15,000-fold lower dose of buprenorphine (0.1 μg·kg(-1)), which caused thermal and mechanical hyperalgesia in behaving animals, induced an enhancement of transmission at spinal C-fiber synapses. The ultra-low-dose buprenorphine-induced synaptic facilitation was mediated by supraspinal naloxonazine-insensitive, but CTOP-sensitive μ-opioid receptors, descending serotonergic pathways, and activation of spinal glial cells. Selective inhibition of spinal 5-hydroxytryptamine-2 receptors (5-HT2Rs), putatively located on spinal astrocytes, abolished both the induction of synaptic facilitation and the hyperalgesia elicited by ultra-low-dose buprenorphine. Our study revealed that buprenorphine mediates its modulatory effects on transmission at spinal C-fiber synapses by dose dependently acting on distinct μ-opioid receptor subtypes located at different levels of the neuraxis.

Lasmiditan for acute migraine treatment

Lasmiditan is a new neurally acting antimigraine drug that acts as a highly selective serotonin1F receptor agonist. Its chemical structure is distinctly different from that of triptans. Its dose-dependent clinical efficacy was proven in two clinical Phase II trials using intravenous and oral formulations. Oral lasmiditan showed significant headache relief after 2 h at doses of 50, 100, 200 and 400 mg. Adverse events of lasmiditan were dose-dependent and mainly associated with the CNS. They predominantly included dizziness, paresthesia and fatigue, and were mostly mild-to-moderate in intensity. Preclinical studies showed that, unlike triptans, lasmiditan lacks any vasoconstrictive activity, which makes it a promising drug for patients with contraindications to triptans owing to cardio- or cerebro-vascular diseases, or an elevated vascular risk profile.

Astrocyte pathology in major depressive disorder: insights from human postmortem brain tissue

The present paper reviews astrocyte pathology in major depressive disorder (MDD) and proposes that reductions in astrocytes and related markers are key features in the pathology of MDD. Astrocytes are the most numerous and versatile of all types of glial cells. They are crucial to the neuronal microenvironment by regulating glucose metabolism, neurotransmitter uptake (particularly for glutamate), synaptic development and maturation and the blood brain barrier. Pathology of astrocytes has been consistently noted in MDD as well as in rodent models of depressive-like behavior. This review summarizes evidence from human postmortem tissue showing alterations in the expression of protein and mRNA for astrocyte markers such as glial fibrillary acidic protein (GFAP), gap junction proteins (connexin 40 and 43), the water channel aquaporin-4 (AQP4), a calcium-binding protein S100B and glutamatergic markers including the excitatory amino acid transporters 1 and 2 (EAAT1, EAAT2) and glutamine synthetase. Moreover, preclinical studies are presented that demonstrate the involvement of GFAP and astrocytes in animal models of stress and depressive-like behavior and the influence of different classes of antidepressant medications on astrocytes. In light of the various astrocyte deficits noted in MDD, astrocytes may be novel targets for the action of antidepressant medications. Possible functional consequences of altered expression of astrocytic markers in MDD are also discussed. Finally, the unique pattern of cell pathology in MDD, characterized by prominent reductions in the density of astrocytes and in the expression of their markers without obvious neuronal loss, is contrasted with that found in other neuropsychiatric and neurodegenerative disorders.

Serotonin 1A receptor agonist increases species- and region-selective adult CNS proliferation, but not through CNTF

Endogenous ciliary neurotrophic factor (CNTF)(1) regulates neurogenesis of the adult brain in the hippocampal subgranular zone (SGZ)(2) and the subventricular zone (SVZ)(3). We have previously shown that the cAMP-inhibiting D2 dopamine receptor increases neurogenesis by inducing astroglial CNTF expression. Here, we investigated the potential role of CNTF in the proliferative response to pharmacological stimulation of the serotonin 1A (5-HT1A)(4) receptor, which also inhibits cAMP, in adult mice and rats. Like others, we show that systemic treatment with the active R-enantiomer of the 5-HT1A agonist 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT)(5) induces proliferation in the SGZ in rats using unbiased stereology of 5-Bromo-2'-deoxyuridine (BrdU)(6) positive nuclei. However, despite the bioactivity of R-8-OH-DPAT, as also shown by a decrease in hippocampal nNOS(7) mRNA levels, it did not increase CNTF mRNA as shown by highly specific quantitative RT-PCR (qPCR)(8). Surprisingly, R-8-OH-DPAT did not cause an increase in SVZ proliferation in rats or in either the SVZ or SGZ of two different strains of mice, C57BL/6J, and 129SvEv, using acute or chronic treatments. There also were no changes in CNTF mRNA, and also not in mice treated with a widely used racemic mixture of 8-OH-DPAT, higher doses or after intracerebral injection, which reduced nNOS. In contrast to the others, we propose that the 5-HT1A receptor might be non-functional in mice with regards to regulating normal neurogenesis and has region-selective activities in rats. These species- and region-specific actions raise important questions about the role of the 5-HT1A receptor in human neurogenesis and its implications for the field of depression.

Serotonin-induced brain glycogenolysis in rainbow trout (Oncorhynchus mykiss)

In this study, we evaluated the serotonin-mediated control of cerebral glycogen levels in the rainbow trout, Oncorhynchus mykiss. Intracerebroventricular (i.c.v.) administration of serotonin (5-HT) to normoglycemic trout (time and dose response) decreased glycogen levels in the brain and increased brain glycogen phosphorylase activity (time response). In hypoglycemic fish (that had been fasted for 5 and 10 days), there was a time-dependent decrease in brain glycogen levels; under these conditions, i.c.v. administration of 5-HT also reduced the brain glycogen content in fish that had been fasted for 5 days. In fish with local cerebral hypoglycemia (induced by 2-DG administration), the glycogen levels decreased and, as above, i.c.v. administration of 5-HT also lowered the glycogen content. In hyperglycemic fish, 5-HT did not affect glycogen levels. Administration of receptor agonists 5-HT1A (8-OH-DPAT), 5-HT1B (anpirtoline and CP93129) or 5-HT2 (α-m-5-HT) decreased the brain glycogen levels. This effect was antagonized by the administration of receptor antagonists 5-HT1A (WAY100135 and NAN190), 5-HT1B (NAS181) and 5-HT2B/C (SB206553). Administration of the receptor agonists (±)-DOI (5-HT2A/2C), m-CPP (5-HT2B/2C), BW723C86 (5-HT2B) and WAY 161503 (5-HT2C) led to decreases in the levels of brain glycogen. We found that 5-HT is involved in the modulation of brain glycogen homeostasis in the rainbow trout, causing a glycogenolytic effect when fish are in a normoglycemic or hypoglycemic state, but not when they are in a hyperglycemic state. 5-HT1A, 5-HT1B, 5HT2B and 5-HT2C-like receptors appeared to be involved in the glycogenolytic action of 5-HT, although the effect mediated by 5-HT1A or 5-HT1B was apparently stronger.

Effects of the putative antipsychotic alstonine on glutamate uptake in acute hippocampal slices

A dysfunctional glutamatergic system is thought to be central to the negative symptoms and cognitive deficits recognized as determinant to the poor quality of life of people with schizophrenia. Modulating glutamate uptake has, thus, been suggested as a novel target for antipsychotics. Alstonine is an indole alkaloid sharing with atypical antipsychotics the profile in animal models relevant to schizophrenia, though divergent in its mechanism of action. The aim of this study was to evaluate the effects of alstonine on glutamate uptake. Additionally, the effects on glutathione content and extracellular S100B levels were assessed. Acute hippocampal slices were incubated with haloperidol (10μM), clozapine (10 and 100μM) or alstonine (1-100μM), alone or in combination with apomorphine (100μM), and 5-HT(2) receptor antagonists (0.01μM altanserin and 0.1μM SB 242084). A reduction in glutamate uptake was observed with alstonine and clozapine, but not haloperidol. Apomorphine abolished the effect of clozapine, whereas 5-HT(2A) and 5-HT(2C) antagonists abolished the effects of alstonine. Increased levels of glutathione were observed only with alstonine, also the only compound that failed to decrease the release of S100B. This study shows that alstonine decreases glutamate uptake, which may be beneficial to the glutamatergic deficit observed in schizophrenia. Noteworthily, the decrease in glutamate uptake is compatible with the reversal of MK-801-induced social interaction and working memory deficits. An additional potential benefit of alstonine as an antipsychotic is its ability to increase glutathione, a key cellular antioxidant reported to be decreased in the brain of patients with schizophrenia. Adding to the characterization of the novel mechanism of action of alstonine, the lack of effect of apomorphine in alstonine-induced changes in glutamate uptake reinforces that D(2) receptors are not primarily implicated. Though clearly mediated by 5-HT(2A) and 5-HT(2C) serotonin receptors, the precise mechanisms that result in the effects of alstonine on glutamate uptake warrant elucidation.

Secondary mechanical allodynia and hyperalgesia depend on descending facilitation mediated by spinal 5-HT₄, 5-HT₆ and 5-HT₇ receptors

In the present study we determined the role of spinal 5-hydroxytriptamine (5-HT) and 5-HT(4/6/7) receptors in the long-term secondary mechanical allodynia and hyperalgesia induced by formalin in the rat. Formalin produced acute nociceptive behaviors (flinching and licking/lifting) followed by long-term secondary mechanical allodynia and hyperalgesia in both paws. In addition, formalin increased the tissue content of 5-HT in the ipsilateral, but not contralateral, dorsal part of the spinal cord compared to control animals. Intrathecal (i.t.) administration of 5,7-dihydroxytriptamine (5,7-DHT), a serotonergic neurotoxin, diminished tissue 5-HT content in the ipsilateral and contralateral dorsal parts of the spinal cord. Accordingly, i.t. 5,7-DHT prevented formalin-induced secondary allodynia and hyperalgesia in both paws. I.t. pre-treatment (-10 min) with ML-10302 (5-HT(4) agonist), EMD-386088 (5-HT(6) agonist) and LP-12 (5-HT(7) agonist) significantly increased secondary mechanical allodynia and hyperalgesia in both paws. In contrast, i.t. pre-treatment (-20 min) with GR-125487 (5-HT(4) antagonist), SB-258585 (5-HT(6) antagonist) and SB-269970 (5-HT(7) antagonist) significantly prevented formalin-induced long-term effects in both paws. In addition, these antagonists prevented the pro-nociceptive effect of ML-10302, EMD-386088 and LP-12, respectively. The i.t. post-treatment (6 days after formalin injection) with GR-125487, SB-258585 and SB-269970 reversed formalin-induced secondary allodynia and hyperalgesia in both paws. These results suggest that spinal 5-HT, released from the serotonergic projections in response to formalin injection, activates pre- or post-synaptic 5-HT(4/6/7) receptors at the dorsal root ganglion/spinal cord promoting the development and maintenance of secondary allodynia and hyperalgesia.

A new approach for studying GPCR dimers: drug-induced inactivation and reactivation to reveal GPCR dimer function in vitro, in primary culture, and in vivo

GPCRs are a major family of homologous proteins and are key mediators of the effects of numerous endogenous neurotransmitters, hormones, cytokines, therapeutic drugs, and drugs-of-abuse. Despite the enormous amount of research on the pharmacological and biochemical properties of GPCRs, the question as to whether they exist as monomers, dimers, or higher order structures in the body is unanswered. The GPCR dimer field has been dominated by techniques involving recombinant cell lines expressing mutant receptors, often involving the solubilization of the receptors. These techniques cannot be applied in vivo or even to primary cell cultures. This review will focus on a novel approach to exploring the functional properties of homodimers. Studies of the 5-HT(7) and 5-HT(2A) serotonin receptors have revealed that binding of a pseudo-irreversible antagonist ("inactivator") to one of the orthosteric sites of a homodimer abolishes all receptor activity, and subsequent binding of a competitive antagonist to the orthosteric site of the second protomer releases the inactivator, allowing the receptor to return to an active state. This approach demonstrates allosteric crosstalk between protomers of native GPCR homodimers, indicating that GPCRs do exist and function as homodimers in both recombinant cells and rat primary astrocytes. This technique can be applied universally using intact recombinant or primary cells in culture, membrane homogenate preparations and, potentially, in vivo. The data obtained using the 5-HT(7) and 5-HT(2A) receptors are strongly supportive of a GPCR homodimer structure, with little evidence of monomer involvement in the function of these receptors.

Similarities and differences in mechanisms of phrenic and hypoglossal motor facilitation

Intermittent hypoxia-induced long-term facilitation (LTF) is variably expressed in the motor output of several inspiratory nerves, such as the phrenic and hypoglossal. Compared to phrenic LTF (pLTF), less is known about hypoglossal LTF (hLTF), although it is often assumed that cellular mechanisms are the same. While fundamental mechanisms appear to be similar, potentially important differences exist in the modulation of pLTF and hLTF. The primary objectives of this paper are to: (1) review similarities and differences in pLTF and hLTF, pointing out knowledge gaps and (2) present new data suggesting that reduced respiratory neural activity elicits differential plasticity in phrenic and hypoglossal output (inactivity-induced phrenic and hypoglossal motor facilitation, iPMF and iHMF), suggesting that these motor pool-specific differences are not unique to LTF. Differences in fundamental mechanisms or modulation of plasticity among motor pools may confer the capacity to mount a complex ventilatory response to specific challenges, particularly in motor pools with different "jobs" in the control of breathing.

Antidepressants act on glial cells: SSRIs and serotonin elicit astrocyte calcium signaling in the mouse prefrontal cortex

One important target in the treatment of major depressive disorder (MDD) is the serotonin (5-hydroxytryptamine, 5-HT) system. Selective serotonin reuptake inhibitors (SSRI) are used to treat MDD. Yet, the mode of action of these drugs is not completely understood. There is evolving evidence for a role of glutamate in mood disorder and its signaling. Astrocytes are involved in glutamate metabolism and play an active role in memory processing but their role in mood disorders is still largely unknown. A modulation of astrocytic signaling by SSRIs or 5-HT has not been investigated up to now. We investigated astrocytic calcium signaling with the calcium indicator dye Fluo-4. Using a confocal microscope, we imaged astrocytes in the medial prefrontal cortex of acute mouse brain slices after the application of the SSRIs citalopram and fluoxetine. In the same way, we studied the effects of serotonin and the modulation of this signaling by glutamate in astrocytes. We found that astrocyte calcium signaling can be elicited by 5-HT. Also, the SSRIs citalopram and fluoxetine induce calcium signals in about 1/3 of all astrocytes, even when neuronal signal propagation is inhibited. Astrocytic responses to 5-HT have a unique pattern and they could mostly not be evoked twice. We determined that glutamate is a substance that can interfere with 5-HT-induced calcium signals in astrocytes since after stimulation by glutamate, astrocytes did not show a response to 5-HT. Astrocytic calcium signaling is elicited by SSRIs and 5-HT. They may serve as integrators, linking the serotonergic and glutamatergic signaling pathways.

Risperidone-induced inactivation and clozapine-induced reactivation of rat cortical astrocyte 5-hydroxytryptamine₇ receptors: evidence for in situ G protein-coupled receptor homodimer protomer cross-talk

We have reported previously novel drug-induced inactivation and reactivation of human 5-hydroxytryptamine₇ (5-HT₇) receptors in a recombinant cell line. To explain these novel observations, a homodimer structure displaying protomer-protomer cross-talk was proposed. To determine whether these novel observations and interpretations are due to an artifactual G protein-coupled receptor (GPCR) mechanism unique to the recombinant cell line, we explored the properties of r5-HT₇ receptors expressed by cortical astrocytes in primary culture. As in the recombinant cell line, risperidone, 9-OH-risperidone, methiothepin, and bromocriptine were found to potently inactivate r5-HT₇ receptors. As in the recombinant cell line, exposure of risperidone-inactivated astrocyte r5-HT₇ receptors to competitive antagonists resulted in the reactivation of r5-HT₇ receptors. The potencies of the reactivating drugs closely correlated with their affinities for h5-HT₇ receptors. These results indicate the novel inactivating and reactivating property of drugs is not due to an artifact of the recombinant cell line expressing h5-HT₇ receptors but is an intrinsic property of 5-HT₇ receptors in vitro and ex vivo. This evidence suggests that a native (nonmutated) GPCR, in its native membrane environment (cortical astrocyte primary culture), can function as a homodimer with protomer-protomer cross-talk. Homodimers may be a common GPCR structure. The experimental design used in our studies can be used to explore the properties of other GPCRs in their native forms in recombinant cells, primary cultures expressing the endogenous GPCRs, and possibly in vivo. The homodimer structure and protomer-protomer cross-talk offer new avenues of research into receptor dysfunction in disease states and the development of novel drugs.

5-HT(1F) Receptor agonists: a new treatment option for migraine attacks?

Migraine is a debilitating disorder of the CNS. Although therapeutic options for migraine attacks have tremendously advanced with the development of triptans more than a decade ago, several conditions (such as vascular disease) restrict their use. Moreover, some patients do not respond to triptans and other currently available medications. Therefore, treatment alternatives are needed. Study data show that 5-HT(1F) receptor agonists successfully abort migraine attacks. These data also suggest a favorable vascular side-effect profile of these substances, which could be beneficial for migraine treatment in subjects with cardiac or vascular disease. We discuss the current knowledge of 5-HT(1F) receptor-mediated effects, in part by comparing them to triptans, and we also summarize data from basic research and clinical trials.

Metabolic biomarkers related to energy metabolism in Saudi autistic children

OBJECTIVES

Energy metabolism is usually manipulated in many neurodegenerative diseases. Autism is considered a definable systemic disorder resulting in a number of diverse factors that may affect the brain development and functions both pre and post natal. The increased prevalence of autism will have enormous future public implications and has stimulated intense research into potential etiologic factors. This study aims to establish a connection between autism and the deterioration accompanied it, especially in the brain cognitive areas through a postulation of energy manipulation.

MATERIALS AND METHODS

The biochemical changes in activities of enzymes and pathways that participate in the production of ATP as the most important high-energy compound needed by the human brain were measured in Saudi autistic children. Na(+)/K(+)ATPase, ectonucleotidases (NTPDases) (ADPase and ATPase) and creatine kinase (CK), were assessed in plasma of 30 Saudi autistic patients and compared to 30 age-matching control samples. In addition, adenosine mono, di and trinucleotides (ATP, ADP, and AMP) were measured calorimetrically in the red blood cells of both groups and the adenylate energy charge (AEC) was calculated. Moreover, lactate concentration in plasma of both groups was monitored.

RESULTS

The obtained data recorded 148.77% and 72.35% higher activities of Na(+)/K(+)ATPase and CK respectively in autistic patients which prove the impairment of energy metabolism in these children compared to age and sex matching healthy controls. While ADPase was significantly higher in autistic patients, ATPase were non-significantly elevated compared to control. In spite of the significant increase of Na(+)/K(+)ATPase activity in autistic patients, there was no significant difference in the levels of ATP, ADP, and AMP in both groups and the calculated AEC values were 0.814+/-0.094 and 0.806+/-0.081 for autistic and control groups respectively. The unchanged AEC value in autistic patients was easily correlated with the induced activity of CK and ADPase as two enzymes playing a critical role in the stabilization of AEC. Lactate as an important energy metabolite for the brain was significantly higher in autistic patients compared to control showing about 40% increase.

CONCLUSION

The present study confirmed the impairment of energy metabolism in Saudi autistic patients which could be correlated to the oxidative stress previously recorded in the same investigated samples. The identification of biochemical markers related to autism would be advantageous for earlier clinical diagnosis and intervention.

Effects of amyloid-β peptides on the serotoninergic 5-HT1A receptors in the rat hippocampus

A recent [(18)F]MPPF-positron emission tomography study has highlighted an overexpression of 5-HT(1A) receptors in the hippocampus of patients with mild cognitive impairment compared to a decrease in those with Alzheimer's disease (AD) [Truchot, L., Costes, S.N., Zimmer, L., Laurent, B., Le Bars, D., Thomas-Antérion, C., Croisile, B., Mercier, B., Hermier, M., Vighetto, A., Krolak-Salmon, P., 2007. Up-regulation of hippocampal serotonin metabolism in mild cognitive impairment. Neurology 69 (10), 1012-1017]. We used in vivo and in vitro neuroimaging to evaluate the longitudinal effects of injecting amyloid-β (Aβ) peptides (1-40) into the dorsal hippocampus of rats. In vivo microPET imaging showed no significant change in [(18)F]MPPF binding in the dorsal hippocampus over time, perhaps due to spatial resolution. However, in vitro autoradiography with [(18)F]MPPF (which is antagonist) displayed a transient increase in 5-HT(1A) receptor density 7 days after Aβ injection, whereas [(18)F]F15599 (a radiolabelled 5-HT(1A) agonist) binding was unchanged suggesting that the overexpressed 5-HT(1A) receptors were in a non-functional state. Complementary histology revealed a loss of glutamatergic neurons and an intense astroglial reaction at the injection site. Although a neurogenesis process cannot be excluded, we propose that Aβ injection leads to a transient astroglial overexpression of 5-HT(1A) receptors in compensation for the local neuronal loss. Exploration of the functional consequences of these serotoninergic modifications during the neurodegenerative process may have an impact on therapeutics targeting 5-HT(1A) receptors in AD.

Fluoxetine-mediated 5-HT2B receptor stimulation in astrocytes causes EGF receptor transactivation and ERK phosphorylation

RATIONALE

Fluoxetine has relatively high affinity for Gq/11 protein-coupled 5-HT(2) receptors. Part of these receptors in brain are on astrocytes, where fluoxetine causes an increase in free cytosolic calcium concentration ([Ca(2+)](i)) and phosphorylation of extracellular regulated kinase 1 and 2 (ERK(1/2)).

OBJECTIVE

The objectives of the study are to identify subtype of the 5-HT(2) receptor involved, to establish whether ERK(1/2) phosphorylation is a result of 5-HT(2)-mediated transactivation of epidermal growth factor (EGF) receptors (EGFRs), and to determine signaling pathways up- and downstream of ERK(1/2).

MATERIALS AND METHODS

Primary cultures of mouse astrocytes, which express all three subtypes of the 5-HT(2) receptor but no 5-HT(2) transporter, were used. ERK(1/2) phosphorylation and c-Fos and FosB protein expression were determined with Western blotting, and c-fos and fosB mRNA expression with reverse transcription polymerase chain reaction. Receptor subtype was investigated with subtype-specific 5-HT antagonists and 5-HT(2B) receptor depletion and signaling pathways by EGFR phosphorylation, using immunoprecipitation and Western blotting, inhibition of protein kinase C (PKC), and [Ca(2+)](i) chelation by BAPTA/AM.

RESULTS

ERK(1/2) phosphorylation was abolished by SB204741, a universal 5-HT(2) receptor antagonist, and in 5-HT(2B) receptor-depleted cells, but unaffected by 5-HT(2A) or 5-HT(2C) receptor antagonists (M100907 and SB242084). Phosphorylation of ERK(1/2) and EGFRs was abolished by AG 1478, an inhibitor of EGFR tyrosine kinases, and GM 6001, an inhibitor of Zn-dependent metalloproteinases, suggesting growth factor "shedding" and transactivation of EGFRs. Chelation of [Ca(2+)](i) or PKC inhibition with GF 109203X abrogated ERK(1/2) phosphorylation. Up-regulated mRNA and protein expression of c-fos and fosB was abolished by SB204741, AG1478, and by U0126, an inhibitor of ERK phosphorylation by MAP kinase/ERK kinase.

Role of the serotonin system in ADHD: treatment implications

A limited number of studies have considered whether the activity of serotonin (5-hydroxytryptamine [5-HT]) contributes to the problems experienced by youngsters with attention-deficit/hyperactivity disorder (ADHD). The aim of this article is to review this work and propose interpretations. Peripheral measures of 5-HT and its metabolite do not point to a widespread association with the diagnosis. However, separate consideration of the major domains of dysfunction (motor activity, inattention and impulsivity) support a more differentiated assessment. The marked innervation of motor regions of the brain by 5-HT projections and the clear involvement of 5-HT systems in the control of locomotion in animals suggests a likely node for dysfunction in ADHD. The few relevant studies do not show evidence of this, but more attention should be accorded to the issue. The situation is different for attention-related processes; here, there are deficiencies in perceptual sensitivity and the appropriate designation of saliency to stimulation. These are attributable, in part, to altered 5-HT activity. Marked and opposite changes of 5-HT responsivity are associated with behavioral and cognitive impulsivity. There is also a growing series of studies demonstrating preferential transmission of various genetic markers for 5-HT receptors that are expressed in ADHD. Currently, the heterogeneity of methods in this young discipline restricts the possibilities of definition of these markers and the types of ADHD in which they are expressed.

Cultured rat cortical astrocytes synthesize melatonin: absence of a diurnal rhythm

Melatonin not only plays a major role in the regulation of circadian rhythms, but is also involved in antioxidative defense and immunomodulation. Circulating melatonin levels are derived primarily from the pineal gland while other sources of melatonin have also been reported. Here, we show for the first time that astrocytes from the rat cortex and glioma C6 cell line synthesize melatonin in vitro. In addition, we show the presence of serotonin, the precursor of melatonin and the two key enzymes in the pathway of melatonin synthesis, i.e. N-acetyltransferase and hydroxyndole-O-methyltransferase in the cultured rat cortical astrocytes. Release of melatonin into the culture medium showed no diurnal changes. These point to astrocytes as a local source of melatonin in the rat brain. Its exact physiological function remains a topic for future studies.

Gliogenesis and glial pathology in depression

Recent research has changed the perception of glia from being no more than silent supportive cells of neurons to being dynamic partners participating in brain metabolism and communication between neurons. This discovery of new glial functions coincides with growing evidence of the involvement of glia in the neuropathology of mood disorders. Unanticipated reductions in the density and number of glial cells are reported in fronto-limbic brain regions in major depression and bipolar illness. Moreover, age-dependent decreases in the density of glial fibrillary acidic protein (GFAP) - immunoreactive astrocytes and levels of GFAP protein are observed in the prefrontal cortex of younger depressed subjects. Since astrocytes participate in the uptake, metabolism and recycling of glutamate, we hypothesize that an astrocytic deficit may account for the alterations in glutamate/GABA neurotransmission in depression. Reductions in the density and ultrastructure of oligodendrocytes are also detected in the prefrontal cortex and amygdala in depression. Pathological changes in oligodendrocytes may be relevant to the disruption of white matter tracts in mood disorders reported by diffusion tensor imaging. Factors such as stress, excess of glucocorticoids, altered gene expression of neurotrophic factors and glial transporters, and changes in extracellular levels of neurotransmitters released by neurons may modify glial cell number and affect the neurophysiology of depression. Therefore, we will explore the role of these events in the possible alteration of glial number and activity, and the capacity of glia as a promising new target for therapeutic medications. Finally, we will consider the temporal relationship between glial and neuronal cell pathology in depression.