The serotonin 5-HT3 receptor: a novel neurodevelopmental target

Serotonergic neuromodulation of synaptic plasticity

Synaptic plasticity constitutes a fundamental process in the reorganization of neural networks that underlie memory, cognition, emotional responses, and behavioral planning. At the core of this phenomenon lie Hebbian mechanisms, wherein frequent synaptic stimulation induces long-term potentiation (LTP), while less activation leads to long-term depression (LTD). The synaptic reorganization of neuronal networks is regulated by serotonin (5-HT), a neuromodulator capable of modify synaptic plasticity to appropriately respond to mental and behavioral states, such as alertness, attention, concentration, motivation, and mood. Lately, understanding the serotonergic Neuromodulation of synaptic plasticity has become imperative for unraveling its impact on cognitive, emotional, and behavioral functions. Through a comparative analysis across three main forebrain structures-the hippocampus, amygdala, and prefrontal cortex, this review discusses the actions of 5-HT on synaptic plasticity, offering insights into its role as a neuromodulator involved in emotional and cognitive functions. By distinguishing between plastic and metaplastic effects, we provide a comprehensive overview about the mechanisms of 5-HT neuromodulation of synaptic plasticity and associated functions across different brain regions.

Structural basis for partial agonism in 5-HT3A receptors

Hyperactivity of serotonin 3 receptors (5-HT3R) underlies pathologies associated with irritable bowel syndrome and chemotherapy-induced nausea and vomiting. Setrons, a class of high-affinity competitive antagonists, are used in the treatment of these conditions. Although generally effective for chemotherapy-induced nausea and vomiting, the use of setrons for treating irritable bowel syndrome has been impaired by adverse side effects. Partial agonists are now being considered as an alternative strategy, with potentially less severe side effects than full antagonists. However, a structural understanding of how these ligands work is lacking. Here, we present high-resolution cryogenic electron microscopy structures of the mouse 5-HT3AR in complex with partial agonists (SMP-100 and ALB-148471) captured in pre-activated and open-like conformational states. Molecular dynamics simulations were used to assess the stability of drug-binding poses and interactions with the receptor over time. Together, these studies reveal mechanisms for the functional differences between orthosteric partial agonists, full agonists and antagonists of the 5-HT3AR.

DNAJC12 in Monoamine Metabolism, Neurodevelopment, and Neurodegeneration

Recent studies show that pathogenic variants in DNAJC12, a co-chaperone for monoamine synthesis, may cause mild hyperphenylalaninemia with infantile dystonia, young-onset parkinsonism, developmental delay and cognitive deficits. DNAJC12 has been included in newborn screening, most revealingly in Spain, and those results highlight the importance of genetic diagnosis and early intervention in combating human disease. However, practitioners may be unaware of these advances and it is probable that many patients, especially adults, have yet to receive molecular testing for DNAJC12. Hence, this review summarizes genotype-phenotype relationships and treatment paradigms for patients with pathogenic variants in DNAJC12. It provides an overview of the structure of DNAJC12 protein, known genetic variants, domains, and binding partners, and elaborates on its role in monoamine synthesis, disease etiology, and pathogenesis. © 2023 International Parkinson and Movement Disorder Society.

5-HT3 Receptors on Mitochondria Influence Mitochondrial Function

The 5-hydroxytryptamine 3 (5-HT₃) receptor belongs to the pentameric ligand-gated cation channel superfamily. Humans have five different 5-HT₃ receptor subunits: A to E. The 5-HT₃ receptors are located on the cell membrane, but a previous study suggested that mitochondria could also contain A subunits. In this article, we explored the distribution of 5-HT₃ receptor subunits in intracellular and cell-free mitochondria. Organelle prediction software supported the localization of the A and E subunits on the inner membrane of the mitochondria. We transiently transfected HEK293T cells that do not natively express the 5-HT₃ receptor with an epitope and fluorescent protein-tagged 5HT3A and 5HT3E subunits. Fluorescence microscopy and cell fractionation indicated that both subunits, A and E, localized to the mitochondria, while transmission electron microscopy revealed the location of the subunits on the mitochondrial inner membrane, where they could form heteromeric complexes. Cell-free mitochondria isolated from cell culture media colocalized with the fluorescent signal for A subunits. The presence of A and E subunits influenced changes in the membrane potential and mitochondrial oxygen consumption rates upon exposure to serotonin; this was inhibited by pre-treatment with ondansetron. Therefore, it is likely that the 5-HT₃ receptors present on mitochondria directly impact mitochondrial function and that this may have therapeutic implications.

Regulation of the E/I-balance by the neural matrisome

In the mammalian cortex a proper excitatory/inhibitory (E/I) balance is fundamental for cognitive functions. Especially γ-aminobutyric acid (GABA)-releasing interneurons regulate the activity of excitatory projection neurons which form the second main class of neurons in the cortex. During development, the maturation of fast-spiking parvalbumin-expressing interneurons goes along with the formation of net-like structures covering their soma and proximal dendrites. These so-called perineuronal nets (PNNs) represent a specialized form of the extracellular matrix (ECM, also designated as matrisome) that stabilize structural synapses but prevent the formation of new connections. Consequently, PNNs are highly involved in the regulation of the synaptic balance. Previous studies revealed that the formation of perineuronal nets is accompanied by an establishment of mature neuronal circuits and by a closure of critical windows of synaptic plasticity. Furthermore, it has been shown that PNNs differentially impinge the integrity of excitatory and inhibitory synapses. In various neurological and neuropsychiatric disorders alterations of PNNs were described and aroused more attention in the last years. The following review gives an update about the role of PNNs for the maturation of parvalbumin-expressing interneurons and summarizes recent findings about the impact of PNNs in different neurological and neuropsychiatric disorders like schizophrenia or epilepsy. A targeted manipulation of PNNs might provide an interesting new possibility to indirectly modulate the synaptic balance and the E/I ratio in pathological conditions.

Lipid Raft Facilitated Receptor Organization and Signaling: A Functional Rheostat in Embryonic Development, Stem Cell Biology and Cancer

Molecular views of plasma membrane organization and dynamics are gradually changing over the past fifty years. Dynamics of plasma membrane instigate several signaling nexuses in eukaryotic cells. The striking feature of plasma membrane dynamics is that, it is internally transfigured into various subdomains of clustered macromolecules. Lipid rafts are nanoscale subdomains, enriched with cholesterol and sphingolipids, reside as floating entity mostly on the exoplasmic leaflet of the lipid bilayer. In terms of functionality, lipid rafts are unique among other membrane subdomains. Herein, advances on the roles of lipid rafts in cellular physiology and homeostasis are discussed, precisely, on how rafts dynamically harbor signaling proteins, including GPCRs, catalytic receptors, and ionotropic receptors within it and orchestrate multiple signaling pathways. In the developmental proceedings signaling are designed for patterning of overall organism and they differ from the somatic cell physiology and signaling of fully developed organisms. Some of the developmental signals are characteristic in maintenance of stemness and activated during several types of tumor development and cancer progression. The harmony between extracellular signaling and lineage specific transcriptional programs are extremely important for embryonic development. The roles of plasma membrane lipid rafts mediated signaling in lineage specificity, early embryonic development, stem cell maintenance are emerging. In view of this, we have highlighted and analyzed the roles of lipid rafts in receptor organization, cell signaling, and gene expression during embryonic development; from pre-implantation through the post-implantation phase, in stem cell and cancer biology.

5-HT-dependent synaptic plasticity of the prefrontal cortex in postnatal development

Important functions of the prefrontal cortex (PFC) are established during early life, when neurons exhibit enhanced synaptic plasticity and synaptogenesis. This developmental stage drives the organization of cortical connectivity, responsible for establishing behavioral patterns. Serotonin (5-HT) emerges among the most significant factors that modulate brain activity during postnatal development. In the PFC, activated 5-HT receptors modify neuronal excitability and interact with intracellular signaling involved in synaptic modifications, thus suggesting that 5-HT might participate in early postnatal plasticity. To test this hypothesis, we employed intracellular electrophysiological recordings of PFC layer 5 neurons to study the modulatory effects of 5-HT on plasticity induced by theta-burst stimulation (TBS) in two postnatal periods of rats. Our results indicate that 5-HT is essential for TBS to result in synaptic changes during the third postnatal week, but not later. TBS coupled with 5-HT_2A or 5-HT_1A and 5-HT₇ receptors stimulation leads to long-term depression (LTD). On the other hand, TBS and synergic activation of 5-HT_1A, 5-HT_2A, and 5-HT₇ receptors lead to long-term potentiation (LTP). Finally, we also show that 5-HT dependent synaptic plasticity of the PFC is impaired in animals that are exposed to early-life chronic stress.

Transcriptomes of Clusterin- and S100B-transfected neuronal cells elucidate protective mechanisms against hypoxia and oxidative stress in the hooded seal (Cystophora cristata) brain

BACKGROUND

The hooded seal (Cystophora cristata) exhibits impressive diving skills and can tolerate extended durations of asphyxia, hypoxia and oxidative stress, without suffering from irreversible neuronal damage. Thus, when exposed to hypoxia in vitro, neurons of fresh cortical and hippocampal tissue from hooded seals maintained their membrane potential 4-5 times longer than neurons of mice. We aimed to identify the molecular mechanisms underlying the intrinsic neuronal hypoxia tolerance. Previous comparative transcriptomics of the visual cortex have revealed that S100B and clusterin (apolipoprotein J), two stress proteins that are involved in neurological disorders characterized by hypoxic conditions, have a remarkably high expression in hooded seals compared to ferrets. When overexpressed in murine neuronal cells (HN33), S100B and clusterin had neuroprotective effects when cells were exposed to hypoxia. However, their specific roles in hypoxia have remained largely unknown.

METHODS

In order to shed light on potential molecular pathways or interaction partners, we exposed HN33 cells transfected with either S100B, soluble clusterin (sCLU) or nuclear clusterin (nCLU) to normoxia, hypoxia and oxidative stress for 24 h. We then determined cell viability and compared the transcriptomes of transfected cells to control cells. Potential pathways and upstream regulators were identified via Gene Ontology (GO) and Ingenuity Pathway Analysis (IPA).

RESULTS

HN33 cells transfected with sCLU and S100B demonstrated improved glycolytic capacity and reduced aerobic respiration at normoxic conditions. Additionally, sCLU appeared to enhance pathways for cellular homeostasis to counteract stress-induced aggregation of proteins. S100B-transfected cells sustained lowered energy-intensive synaptic signaling. In response to hypoxia, hypoxia-inducible factor (HIF) pathways were considerably elevated in nCLU- and sCLU-transfected cells. In a previous study, S100B and sCLU decreased the amount of reactive oxygen species and lipid peroxidation in HN33 cells in response to oxidative stress, but in the present study, these functional effects were not mirrored in gene expression changes.

CONCLUSIONS

sCLU and S100B overexpression increased neuronal survival by decreasing aerobic metabolism and synaptic signaling in advance to hypoxia and oxidative stress conditions, possibly to reduce energy expenditure and the build-up of deleterious reactive oxygen species (ROS). Thus, a high expression of CLU isoforms and S100B is likely beneficial during hypoxic conditions.

Selective Targeting of Serotonin 5-HT1a and 5-HT3 Receptors Attenuates Acute and Long-Term Hypersensitivity Associated With Neonatal Procedural Pain

Neonatal painful procedures causes acute pain and trigger long-term changes in nociceptive processing and anxiety behavior, highlighting the need for adequate analgesia during this critical time. Spinal serotonergic receptors 5-HT1a and 5-HT3 play an important role in modulating incoming nociceptive signals in neonates. The current study aims to attenuate acute and long-term hypersensitivity associated with neonatal procedural pain using ondansetron (a 5-HT3 antagonist) and buspirone (a 5-HT1a agonist) in a well-established rat model of repetitive needle pricking. Sprague-Dawley rat pups of both sexes received ondansetron (3 mg/kg), buspirone (3 mg/kg) or saline prior to repetitive needle pricks into the left hind-paw from postnatal day 0-7. Control animals received tactile stimulation or were left undisturbed. Acute, long-term, and post-operative mechanical sensitivity as well as adult anxiety were assessed. Neonatal 5-HT1a receptor agonism completely reverses acute hypersensitivity from P0-7. The increased duration of postoperative hypersensitivity after re-injury in adulthood is abolished by 5-HT3 receptor antagonism during neonatal repetitive needle pricking, without affecting baseline sensitivity. Moreover, 5-HT1a and 5-HT3 receptor modulation decreases adult state anxiety. Altogether, our data suggests that targeted pharmacological treatment based on the modulation of spinal serotonergic network via the 5-HT1a and 5-HT3 receptors in neonates may be of use in treatment of neonatal procedural pain and its long-term consequences. This may result in a new mechanism-based therapeutic venue in treatment of procedural pain in human neonates.

Background sensitivity to chemotherapy-induced nausea and vomiting and response to antiemetics in paediatric patients: a genetic association study

Chemotherapy-induced nausea and vomiting (CINV) remains a common adverse effect for children with cancer. In children, chemotherapy emetogenicity and patient factors such as susceptibility to motion sickness and age group determine a patient's risk of CINV. Besides known risk factors, genetic factors may play a role in interindividual variation in the occurrence of CINV. We investigated the influence of candidate gene polymorphisms on the efficacy of antiemetics and on the background sensitivity to CINV in children. This prospective study included 100 children with cancer (median age 6.4 years, range 0.8-17.9) who received moderately to highly emetogenic chemotherapy. Participants registered nausea and vomiting episodes in a mobile app. Genotypes were determined by whole-genome sequencing (n = 79) or Sanger sequencing (n = 21) for 71 genetic polymorphisms involved in motion sickness and antiemetic pathways. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated to estimate associations between acute CINV and genotypes adjusting for susceptibility to motion sickness and age group. Rs3782025 in the 5-hydroxytryptamine type 3 (5-HT3) receptor gene (HTR3B) [minor allele frequency (MAF): 0.48] affected response to 5-HT3 receptor antagonists; acute CINV occurred in 76% of patients with GA/AA genotypes and in 41% of patients with GG genotype (OR 5.59; 95% CI 1.74-17.9, dominant genetic model). Rs2975226 in the dopamine transporter gene (SLC6A3) (MAF: 0.54) was associated with acute CINV (OR 5.79; 95% CI 1.09-30.67, recessive genetic model). Polymorphisms in HTR3B and SLC6A3 may contribute to the variability in response to antiemetic prophylaxis for CINV in children.

The interplay between 5-HT2C and 5-HT3A receptors in the dorsal periaqueductal gray mediates anxiety-like behavior in mice

The monoamine neurotransmitter serotonin (5-HT) modulates anxiety by its activity on 5-HT_2C receptors (5-HT_2CR) expressed in the dorsal periaqueductal gray (dPAG). Here, we investigated the presence of 5-HT_3A receptors (5-HT_3AR) in the dPAG, and the interplay between 5-HT_2CR and 5-HT_3AR in the dPAG in mediating anxiety-like behavior in mice. We found that 5-HT_3AR is expressed in the dPAG and the blockade of these receptors using intra-dPAG infusion of ondansetron (5-HT_3AR antagonist; 3.0 nmol) induced an anxiogenic-like effect. The activation of 5-HT_3ABR by the infusion of mCPBG [1-(m-Chlorophenyl)-biguanide; 5-HT₃R agonist] did not alter anxiety-like behaviors. In addition, blockade of 5-HT_3AR (1.0 nmol) prevented the anxiolytic-like effect induced by the infusion of the 5-HT_2CR agonist mCPP (1-(3-chlorophenyl) piperazine; 0.03 nmol). None of the treatment effects on anxiety-like behaviors altered the locomotor activity levels. The present results suggest that the anxiolytic-like effect exerted by serotonin activity on 5-HT_2CR in the dPAG is modulated by 5-HT_3AR expressed in same region.

Crosstalk Between Intestinal Serotonergic System and Pattern Recognition Receptors on the Microbiota-Gut-Brain Axis

Disruption of the microbiota-gut-brain axis results in a wide range of pathologies that are affected, from the brain to the intestine. Gut hormones released by enteroendocrine cells to the gastrointestinal (GI) tract are important signaling molecules within this axis. In the search for the language that allows microbiota to communicate with the gut and the brain, serotonin seems to be the most important mediator. In recent years, serotonin has emerged as a key neurotransmitter in the gut-brain axis because it largely contributes to both GI and brain physiology. In addition, intestinal microbiota are crucial in serotonin signaling, which gives more relevance to the role of the serotonin as an important mediator in microbiota-host interactions. Despite the numerous investigations focused on the gut-brain axis and the pathologies associated, little is known regarding how serotonin can mediate in the microbiota-gut-brain axis. In this review, we will mainly discuss serotonergic system modulation by microbiota as a pathway of communication between intestinal microbes and the body on the microbiota-gut-brain axis, and we explore novel therapeutic approaches for GI diseases and mental disorders.

Lateralized Decrease of Parvalbumin+ Cells in the Somatosensory Cortex of ASD Models Is Correlated with Unilateral Tactile Hypersensitivity

Inhibitory control of excitatory networks contributes to cortical functions. Increasing evidence indicates that parvalbumin (PV+)-expressing basket cells (BCs) are a major player in maintaining the balance between excitation (E) and inhibition (I). Disruption of E/I balance in cortical networks is believed to be a hallmark of autism spectrum disorder (ASD). Here, we report a lateralized decrease in the number of PV+ BCs in L2/3 of the somatosensory cortex in the dominant hemisphere of Shank3-/- and Cntnap2-/- mouse models of ASD. The dominant hemisphere was identified during a reaching task to establish each animal's dominant forepaw. Double labeling with anti-PV antibody and a biotinylated lectin (Vicia villosa lectin [VVA]) showed that the number of BCs was not different but rather, some BCs did not express PV (PV-), resulting in an elevated number of PV- VVA+ BCs. Finally, we showed that dominant hindpaws had higher mechanical sensitivity when compared with the other hindpaws. This mechanical hypersensitivity in the dominant paw strongly correlated with the decrease in the number of PV+ interneurons and reduced PV expression in the corresponding cortex. Together, these results suggest that the hypersensitivity in ASD patients could be due to decreased inhibitory inputs to the dominant somatosensory cortex.

The Roles of Serotonin in Neuropsychiatric Disorders

The serotonergic system extends throughout the central nervous system (CNS) and the gastrointestinal (GI) tract. In the CNS, serotonin (5-HT, 5-hydroxytryptamine) modulates a broad spectrum of functions, including mood, cognition, anxiety, learning, memory, reward processing, and sleep. These processes are mediated through 5-HT binding to 5-HT receptors (5-HTRs), are classified into seven distinct groups. Deficits in the serotonergic system can result in various pathological conditions, particularly depression, schizophrenia, mood disorders, and autism. In this review, we outlined the complexity of serotonergic modulation of physiologic and pathologic processes. Moreover, we provided experimental and clinical evidence of 5-HT's involvement in neuropsychiatric disorders and discussed the molecular mechanisms that underlie these illnesses and contribute to the new therapies.

Recombinant expression and purification of pentameric ligand-gated ion channels for Cryo-EM structural studies

Pentameric ligand-gated ion channels (pLGICs) are central players in synaptic neurotransmission and are targets to a range of drugs used to treat neurological disorders and pain. pLGICs are intrinsically dynamic membrane proteins that upon stimulation by neurotransmitters, undergo global conformational changes across multiple domains spanning a distance of over 165Å. The inter-domain flexibility, a feature crucial for their function as signal transducers in chemical synapses, has been problematic in the efforts toward determining high-resolution structures. Earlier structural studies tackled this issue with a variety of strategies that included partial truncation of flexible domains and the use of antibodies and small-molecule inhibitors to restrict domain movement. With the recent advances in cryo-electron microscopy and single-particle analysis, many of these limitations have been overcome. Here, we describe the methods used in the recombinant expression and purification of full-length constructs of two members of the pentameric ligand-gated ion channel family and the approaches used for capturing multiple conformations in cryo-EM imaging.

Rewiring of the Serotonin System in Major Depression

Serotonin is a key neurotransmitter that is implicated in a wide variety of behavioral and cognitive phenotypes. Originating in the raphe nuclei, 5-HT neurons project widely to innervate many brain regions implicated in the functions. During the development of the brain, as serotonin axons project and innervate brain regions, there is evidence that 5-HT plays key roles in wiring the developing brain, both by modulating 5-HT innervation and by influencing synaptic organization within corticolimbic structures. These actions are mediated by 14 different 5-HT receptors, with region- and cell-specific patterns of expression. More recently, the role of the 5-HT system in synaptic re-organization during adulthood has been suggested. The 5-HT neurons have the unusual capacity to regrow and reinnervate brain regions following insults such as brain injury, chronic stress, or altered development that result in disconnection of the 5-HT system and often cause depression, anxiety, and cognitive impairment. Chronic treatment with antidepressants that amplify 5-HT action, such as selective serotonin reuptake inhibitors (SSRIs), appears to accelerate the rewiring of the 5-HT system by mechanisms that may be critical to the behavioral and cognitive improvements induced in these models. In this review, we survey the possible 5-HT receptor mechanisms that could mediate 5-HT rewiring and assess the evidence that 5-HT-mediated brain rewiring is impacting recovery from mental illness. By amplifying 5-HT-induced rewiring processes using SSRIs and selective 5-HT agonists, more rapid and effective treatments for injury-induced mental illness or cognitive impairment may be achieved.

Paradoxical Effects of a Cytokine and an Anticonvulsant Strengthen the Epigenetic/Enzymatic Avenue for Autism Research

Maternal exposure to the valproate short-chain fatty acid (SCFA) during pregnancy is known to possibly induce autism spectrum disorders (ASDs) in the offspring. By contrast, case studies have evidenced positive outcomes of this anticonvulsant drug in children with severe autism. Interestingly, the same paradoxical pattern applies to the IL-17a inflammatory cytokine involved in the immune system regulation. Such joint apparent contradictions can be overcome by pointing out that, among their respective signaling pathways, valproate and IL-17a share an enhancement of the “type A monoamine oxidase” (MAOA) enzyme carried by the X chromosome. In the Guided Propagation (GP) model of autism, such enzymatic rise triggers a prenatal epigenetic downregulation, which, without possible X-inactivation, and when coinciding with genetic expression variants of other brain enzymes, results in the delayed onset of autistic symptoms. The underlying imbalance of synaptic monoamines, serotonin in the first place, would reflect a mismatch between the environment to which the brain metabolism was prepared during gestation and the postnatal actual surroundings. Following a prenatal exposure to molecules that significantly elicit the MAOA gene expression, a daily treatment with the same metabolic impact would tend to recreate the fetal environment and contribute to rebalance monoamines, thus allowing proper neural circuits to gradually develop, provided behavioral re-education. Given the multifaceted other players than MAOA that are involved in the regulation of serotonin levels, potential compensatory effects are surveyed, which may underlie the autism heterogeneity. This explanatory framework opens up prospects regarding autism prevention and treatment, strikingly in line with current advances along the gut microbiome–brain axis.

High-resolution structures of multiple 5-HT3AR-setron complexes reveal a novel mechanism of competitive inhibition

Serotonin receptors (5-HT3AR) play a crucial role in regulating gut movement, and are the principal target of setrons, a class of high-affinity competitive antagonists, used in the management of nausea and vomiting associated with radiation and chemotherapies. Structural insights into setron-binding poses and their inhibitory mechanisms are just beginning to emerge. Here, we present high-resolution cryo-EM structures of full-length 5-HT3AR in complex with palonosetron, ondansetron, and alosetron. Molecular dynamic simulations of these structures embedded in a fully-hydrated lipid environment assessed the stability of ligand-binding poses and drug-target interactions over time. Together with simulation results of apo- and serotonin-bound 5-HT3AR, the study reveals a distinct interaction fingerprint between the various setrons and binding-pocket residues that may underlie their diverse affinities. In addition, varying degrees of conformational change in the setron-5-HT3AR structures, throughout the channel and particularly along the channel activation pathway, suggests a novel mechanism of competitive inhibition.

Early postnatal allopregnanolone levels alteration and adult behavioral disruption in rats: Implication for drug abuse

Several studies have highlighted the role that early postnatal levels of allopregnanolone play in the development of the CNS and adult behavior. Changes in allopregnanolone levels related to stress have been observed during early postnatal periods, and perinatal stress has been linked to neuropsychiatric disorders. The alteration of early postnatal allopregnanolone levels in the first weeks of life has been proven to affect adult behaviors, such as anxiety-related behaviors and the processing of sensory inputs. This review focuses on the first studies about the possible relationship between the early postnatal allopregnanolone levels and the vulnerability to abuse of drugs such as alcohol in adulthood, given that (1) changes in neonatal allopregnanolone levels affect novelty exploration and novelty seeking has been linked to vulnerability to drug abuse; (2) early postnatal administration of progesterone, the main allopregnanolone precursor, affects the maturation of dopaminergic meso-striatal systems, which have been related to novelty seeking and drug abuse; and (3) alcohol consumption increases plasma and brain allopregnanolone levels in animals and humans. Manipulating neonatal allopregnanolone by administering finasteride, an inhibitor of the 5α-reductase enzyme that participates in allopregnanolone synthesis, increases alcohol consumption and decreases the locomotor stimulant effects of low alcohol doses. At a molecular level, finasteride decreases dopamine and serotonin in ventral striatum and dopamine release in nucleus accumbens. Preliminary results suggest that serotonin 5HT3 receptors could also be affected. Although an in-depth study is necessary, evidence suggests that there is a relation between early postnatal allopregnanolone and vulnerability to drug use/abuse.

•

Early postnatal AlloP levels alteration affects brain maturation and adult behavior.

•

Early stress interacts to AlloP influencing neuropsychiatric disorders vulnerability.

•

Fluctuations in neonatal AlloP levels play a role in alcohol abuse vulnerability.

•

Neonatal finasteride induces novelty-seeking profile and increases ethanol intake.

Optogenetic stimulation of serotonin nuclei retrieve the lost memory in Alzheimer's disease

How are memories stored and retrieved? It was one of the most discussed questions in the past century by neuroscientists. Leading studies of the period brought two different explanations to this question: The first statement considers memory as a physiological change in the brain and suggest that the retrieval of memory is only occurred by the same physiologic changes observed during the memory formation, while the second suggests that memory is a psychic mood stored in mind and the retrieval of memory is occurred by mystical energy fluctuations. Although the exact reason and the pathogenesis of Alzheimer's disease have not yet been fully understood, the approaches that centered the retrieval strategy of lost memory constitutes the basis of the treatment strategies in Alzheimer's disease today. The majority of treatment studies has based on the manipulation of the cholinergic system; however, although serotonin has mnemonic effects, its role in the pathogenesis of Alzheimer's disease has not been investigated as much as the cholinergic system. Here we show how serotonin affects the pathogenesis of Alzheimer's disease in a comprehensive perspective and we suggest that the optogenetics manipulation of serotonin nuclei retrieve the lost memory by closing the inward-rectifier potassium channel Kir2 on the memory engram cells. Also, we raise the possible effects of serotonin on the memory engram cells and the interactions between the amyloid-centric hypothesis of Alzheimer's disease and the memory engram hypothesis to explain the pathophysiology of memory loss in Alzheimer's disease.

Molecular mechanism of setron-mediated inhibition of full-length 5-HT3A receptor

Serotonin receptor (5-HT_3AR) is the most common therapeutic target to manage the nausea and vomiting during cancer therapies and in the treatment of irritable bowel syndrome. Setrons, a class of competitive antagonists, cause functional inhibition of 5-HT_3AR in the gastrointestinal tract and brainstem, acting as effective anti-emetic agents. Despite their prevalent use, the molecular mechanisms underlying setron binding and inhibition of 5-HT_3AR are not fully understood. Here, we present the structure of granisetron-bound full-length 5-HT_3AR solved by single-particle cryo-electron microscopy to 2.92 Å resolution. The reconstruction reveals the orientation of granisetron in the orthosteric site with unambiguous density for interacting sidechains. Molecular dynamics simulations and electrophysiology confirm the granisetron binding orientation and the residues central for ligand recognition. Comparison of granisetron-bound 5-HT_3AR with the apo and serotonin-bound structures, reveals key insights into the mechanism underlying 5-HT_3AR inhibition.

Serotonin Regulation of the Prefrontal Cortex: Cognitive Relevance and the Impact of Developmental Perturbation

The prefrontal cortex is essential for both executive function and emotional regulation. The interrelationships among these behavioral domains are increasingly recognized, as well as their sensitivity to serotonin (5-hydroxytryptamine, 5-HT). Prefrontal cortex receives serotonergic inputs from the dorsal and median raphe nuclei and is modulated by multiple subtypes of 5-HT receptor across its layers and cell types. Extremes of serotonergic modulation alter mood regulation in vulnerable individuals, yet the impact of serotonin under more typical physiological parameters remains unclear. In this regard, new tools are permitting a closer examination of the behavioral impact of the serotonin system. Optogenetic and chemogenetic manipulations of dorsal raphe 5-HT neurons reveal that serotonin has a greater impact on executive function than previously appreciated. Domains that appear sensitive to fluctuations in 5-HT neuronal excitability include patience and cognitive flexibility. This work is broadly consistent with ex vivo research investigating how 5-HT regulates prefrontal cortex and its output projections. A growing literature suggests 5-HT modulation of these prefrontal circuits is unexpectedly flexible to alteration during development by genetic, behavioral, environmental or pharmacological manipulations, with lasting repercussions for cognition and emotional regulation. Here, we review the cellular and circuit mechanisms of prefrontal serotonergic modulation, investigate recent research into the cognitive consequences of the serotonergic system, and probe the lasting consequences of developmental perturbations. Understanding both the complexity of the prefrontal serotonin system and its sensitivity during development are essential to learn more about the vulnerabilities of this system in mood and anxiety disorders and the underappreciated cognitive consequences of these disorders and their treatment.

Azologization of serotonin 5-HT3 receptor antagonists

The serotonin 5-hydroxytryptamine 3 receptor (5-HT₃R) plays a unique role within the seven classes of the serotonin receptor family, as it represents the only ionotropic receptor, while the other six members are G protein-coupled receptors (GPCRs). The 5-HT₃ receptor is related to chemo-/radiotherapy provoked emesis and dysfunction leads to neurodevelopmental disorders and psychopathologies. Since the development of the first serotonin receptor antagonist in the early 1990s, the range of highly selective and potent drugs expanded based on various chemical structures. Nevertheless, on-off-targeting of a pharmacophore’s activity with high spatiotemporal resolution as provided by photopharmacology remains an unsolved challenge bearing additionally the opportunity for detailed receptor examination. In the presented work, we summarize the synthesis, photochromic properties and in vitro characterization of azobenzene-based photochromic derivatives of published 5-HT₃R antagonists. Despite reported proof of principle of direct azologization, only one of the investigated derivatives showed antagonistic activity lacking isomer specificity.

Natural Negative Allosteric Modulators of 5-HT₃ Receptors

Chemotherapy-induced nausea and vomiting (CINV) remain the most common and devastating side-effects associated with cancer chemotherapy. In recent decades, several lines of research emphasize the importance of 5-hydroxytryptamine3 (5-HT3; serotonin) receptors in the pathogenesis and treatment of CINV. 5-HT₃ receptors are members of ligand-gated ion channels that mediate the rapid and transient membrane-depolarizing effect of 5-HT in the central and peripheral nervous system. These receptors play important roles in nausea and vomiting, as well as regulation of peristalsis and pain transmission. The development of antagonists for 5-HT₃ receptor dramatically improved the treatment of CINV in cancer patients. In fact, the most common use of 5-HT₃ receptor antagonists to date is the treatment of nausea and vomiting. In recent years, there has been an increasing tendency to use natural plant products as important therapeutic entities in the treatment of various diseases. In this article, we examined the results of earlier studies on the actions of natural compounds on the functional properties of 5-HT₃ receptors. It is likely that these natural modulators of 5-HT₃ receptors can be employed as lead structures for the synthesis of therapeutic agents for treating CINV in future clinical studies.

5-HT serotonin receptors modulate mitogenic signaling and impact tumor cell viability

Symptoms of depression are present in over half of all cancer patients, and selective serotonin reuptake inhibitor (SSRI) anti-depressant medications are prescribed to nearly a quarter of these individuals in order to cope with their disease. Previous studies have provided evidence that elevated serotonin (5-HT) and serotonin receptor levels may contribute to oncogenic progression, yet little is known regarding the mechanism by which this occurs. The data demonstrated that serotonin receptor mRNAs and proteins are expressed across diverse cancer types, and that serotonin stimulation of tumor cells activates oncogenic signaling mediators including components of the AKT, CREB, GSK3, and MAPK pathways. Selective pharmacological inhibition of the seven known classes of 5-HT receptors in sarcoma and breast cancer cells resulted in dose dependent decreases in tumor cell viability, activation of the p53 DNA damage pathway, suppression of MAPK activity, and significantly reduced tumor volume in an in ovo model. Based on a retrospective clinical analysis of 419 patients diagnosed with breast cancer, we discovered that use of SSRIs was associated with a 2.3-fold increase in tumor proliferation rates for late stage patients based on their Ki-67 index (P=0.03). These data provide evidence that serotonin signaling pathways, which treating oncologists often pharmacologically target to assist cancer patients to psychologically cope with their illness, activate signaling pathways known to promote tumor growth and survival.

Neurobiology and consequences of social isolation stress in animal model-A comprehensive review

The brain is a vital organ, susceptible to alterations under genetic influences and environmental experiences. Social isolation (SI) acts as a stressor which results in alterations in reactivity to stress, social behavior, function of neurochemical and neuroendocrine system, physiological, anatomical and behavioral changes in both animal and humans. During early stages of life, acute or chronic SIS has been proposed to show signs and symptoms of psychiatric and neurological disorders such as anxiety, depression, schizophrenia, epilepsy and memory loss. Exposure to social isolation stress induces a variety of endocrinological changes including the activation of hypothalamic-pituitary-adrenal (HPA) axis, culminating in the release of glucocorticoids (GCs), release of catecholamines, activation of the sympatho-adrenomedullary system, release of Oxytocin and vasopressin. In several regions of the central nervous system (CNS), SIS alters the level of neurotransmitter such as dopamine, serotonin, gamma aminobutyric acid (GABA), glutamate, nitrergic system and adrenaline as well as leads to alteration in receptor sensitivity of N-methyl-D-aspartate (NMDA) and opioid system. A change in the function of oxidative and nitrosative stress (O&NS) mediated mitochondrial dysfunction, inflammatory factors, neurotrophins and neurotrophicfactors (NTFs), early growth response transcription factor genes (Egr) and C-Fos expression are also involved as a pathophysiological consequences of SIS which induce neurological and psychiatric disorders.

Early post-natal neuroactive steroid manipulation modulates ondansetron effects on initial periods of alcohol consumption in rats

Neuroactive steroids (NS) such as allopregnanolone are crucial for brain development and adult behaviour. Early post-natal alterations of NS by administering finasteride induce a decrease in the sensitivity to stimulant effects of low alcohol doses, an increase in alcohol consumption, and a decrease in ventrostriatal dopamine and serotonin levels. The aim of the present study is to observe if the effects of the 5HT3 receptor antagonist ondansetron on initial alcohol consumption are modulated by post-natal NS manipulation. For this purpose, allopregnanolone, finasteride, or vehicle was injected from day 5 to 9. In adulthood, a novel object preference test was carried out in order to detect a possible novelty-seeking pattern in our animals, which has been related to vulnerability to drug abuse. The subjects then had access to two bottles (alcohol or control solutions) one hour daily for two consecutive weeks. Ondansetron (0.01 mg/kg, 0.1 mg/kg or vehicle) was administered before the hour of consumption in the initial phase (days 1, 2, 3) of the procedure, and after prolonged alcohol intake (days 11, 12, 13). Results indicated that finasteride animals showed a higher preference to explore the new object, as well as a higher alcohol consumption than the rest of the groups. Moreover, 0.1 mg/kg of ondansetron decreased alcohol consumption, but only in the post-natal finasteride group, suggesting a possible increase in 5HT3 receptor sensitivity in these animals. In conclusion, NS manipulation in crucial stages of development, such as early post-natal periods, seems to play an important role on the effects of ondansetron on alcohol intake and in the vulnerability to develop drug use or abuse.

Cryo-EM structure of 5-HT3A receptor in its resting conformation

Serotonin receptors (5-HT_3AR) directly regulate gut movement, and drugs that inhibit 5-HT_3AR function are used to control emetic reflexes associated with gastrointestinal pathologies and cancer therapies. The 5-HT_3AR function involves a finely tuned orchestration of three domain movements that include the ligand-binding domain, the pore domain, and the intracellular domain. Here, we present the structure from the full-length 5-HT_3AR channel in the apo-state determined by single-particle cryo-electron microscopy at a nominal resolution of 4.3 Å. In this conformation, the ligand-binding domain adopts a conformation reminiscent of the unliganded state with the pore domain captured in a closed conformation. In comparison to the 5-HT_3AR crystal structure, the full-length channel in the apo-conformation adopts a more expanded conformation of all the three domains with a characteristic twist that is implicated in gating.

Serotonin receptor (5-HT_3AR), a pentameric ligand-gated ion channel, regulates numerous gastrointestinal functions. Here the authors provide a cryo-electron microscopic structure from the full-length 5-HT_3AR in the apo-state which corresponds to a resting conformation of the channel.

Serotonin Receptor 5-HT3A Affects Development of Bladder Innervation and Urinary Bladder Function

The autonomic and sensory nervous systems are required for proper function of all visceral organs, including the lower urinary tract (LUT). Despite the wide prevalence of bladder dysfunction, effective treatment options remain limited. Pelvic innervation regenerative strategies are promising, but surprisingly little is known about the molecular factors driving the development of bladder innervation. Given prior evidence that serotonin receptor 5-HT3A is expressed early in LUT development and is an important mediator of adult bladder function, we sought to determine if 5-HT3A is required for the development of autonomic innervation of the bladder. We found that 5-HT3A is expressed early in fetal mouse pelvic ganglia and is maintained through adulthood. Htr3a knockout male mice, but not females, exhibit increased urinary voiding frequency compared to wild type littermates. Analysis of LUT function via anesthetized cystometry revealed decreased voiding efficiency in male Htr3a mutants. Htr3a^−/− mutant animals exhibit a transient disturbance of autonomic neuronal subtype markers (tyrosine hydroxylase and choline acetyl transferase) within the fetal pelvic ganglia, although the imbalance of neuronal subtype markers assayed is no longer apparent in adulthood. Loss of 5-HT3A activity results in a higher density of autonomic and sensory neuronal fibers supplying bladder smooth muscle in both fetal and adult mice. Collectively, our findings highlight 5-HT3A as a critical component in the autonomic control of micturition and identify a novel role for this serotonin receptor in peripheral nervous system development.

Cellular and Molecular Targets of Menthol Actions

Menthol belongs to monoterpene class of a structurally diverse group of phytochemicals found in plant-derived essential oils. Menthol is widely used in pharmaceuticals, confectionary, oral hygiene products, pesticides, cosmetics, and as a flavoring agent. In addition, menthol is known to have antioxidant, anti-inflammatory, and analgesic effects. Recently, there has been renewed awareness in comprehending the biological and pharmacological effects of menthol. TRP channels have been demonstrated to mediate the cooling actions of menthol. There has been new evidence demonstrating that menthol can significantly influence the functional characteristics of a number of different kinds of ligand and voltage-gated ion channels, indicating that at least some of the biological and pharmacological effects of menthol can be mediated by alterations in cellular excitability. In this article, we examine the results of earlier studies on the actions of menthol with voltage and ligand-gated ion channels.

A rapid chemical-genetic screen utilizing impaired movement phenotypes in C. elegans: Input into genetics of neurodevelopmental disorders

Autism spectrum disorder (ASD) is the most common neurodevelopmental disorder with a constantly increasing prevalence. Model organisms may be tools to identify underlying cellular and molecular mechanisms, as well as aid the discovery and development of novel therapeutic approaches. A simple animal such as the nematode Caenorhabditis elegans may provide insights into the extreme complexity of ASD genetics. Despite its potential, using C. elegans in ASD research is a controversial approach and has not yet been used extensively in this context. In this study, we present a screening approach of potential C. elegans mutants as potential ASD models. We screened these mutants for motor-deficiency phenotypes, which can be exploited to study underlying mechanisms of the disorder. Selected motor-deficient mutants were then used in a comprehensive drug screen of over 3900 compounds, including many FDA-approved and natural molecules, that were analyzed for their ability to suppress motility defects caused by ASD-associated gene orthologues. This genetic-chemical approach, i.e. establishing C. elegans models for ASD and screening of a well-characterized compound library, might be a promising first step to understand the mechanisms of how gene variations cause neuronal dysfunction, leading to ASD and other neurological disorders. Positively acting compounds could also be promising candidates for preclinical studies.

Monoamine Transporters as Ionotropic Receptors

It is well established that glutamate and GABA signal through both ionotropic and metabotropic receptors. Conversely, it is thought that, with one exception, monoamines (dopamine, serotonin, and norepinephrine) signal via metabotropic receptors. Given their capacity to generate fast-acting currents, I suggest that the monoamine transporters should be considered as ionotropic receptors.

5HTR3A-driven GFP labels immature olfactory sensory neurons

The ionotropic serotonin receptor, 5-HT₃ , is expressed by many developing neurons within the central nervous system. Since the olfactory epithelium continues to generate new olfactory sensory neurons (OSNs) throughout life, we investigated the possibility that 5-HT₃ is expressed in the adult epithelium. Using a transgenic mouse in which the promoter for the 5-HT_3a subunit drives expression of green fluorescent protein (GFP), we assessed the expression of this marker in the olfactory epithelium of adult mice. Both the native 5-HT_3a mRNA and GFP are expressed within globose basal cells of the olfactory and vomeronasal epithelium in adult mice. Whereas the 5-HT_3a mRNA disappears relatively quickly after final cell division, the GFP label persists for about 5 days, thereby labeling immature OSNs in both the main olfactory system and vomeronasal organ. The GFP-labeled cells include both proliferative globose basal cells as well as immature OSNs exhibiting the hallmarks of ongoing differentiation including GAP43, PGP9.5, but the absence of olfactory marker protein. Some of the GFP-labeled OSNs show characteristics of more mature yet still developing OSNs including the presence of cilia extending from the apical knob and expression of NaV1.5, a component of the transduction cascade. These findings suggest that 5-HT_3a is indicative of a proliferative or developmental state, regardless of age, and that the 5-HT_3A GFP mice may prove useful for future studies of neurogenesis in the olfactory epithelium. J. Comp. Neurol. 525:1743-1755, 2017. © 2016 Wiley Periodicals, Inc.

Neurite outgrowth enhancement by jiadifenolide: possible targets

Covering: 1860-2016A mechanistic link may exist between convulsant plant substances typified by picrotoxinin, and 'neurotrophic' sesquiterpenes like jiadifenolide. Picrotoxinin elicits convulsion by anion blockade of the Cys-loop family of neurotransmitter-gated ion channels. These same receptors mediate neuronal development and neurite outgrowth prior to synapse formation. Due to its structural homology with picrotoxin and anisatin, it is possible that jiadifenolide enhances NGF-stimulated neurite outgrowth by modulation of the Cys-loop family of receptors.

Molecular control of two novel migratory paths for CGE-derived interneurons in the developing mouse brain

GABAergic interneurons are highly heterogenous and originate in the subpallium mainly from the medial (MGE) and caudal (CGE) ganglionic eminences according to a precise temporal sequence. While MGE-derived cells disperse dorsally and migrate towards all regions of the cortex, little is known on how CGE-derived cells reach their targets during development. Here, we unravel the existence of two novel CGE caudo-rostral migratory streams, one located laterally (LMS) and the other one more medially (MMS) that, together with the well-known caudal migratory stream (CMS), contribute to populate the neocortex, hippocampus and amygdala. These paths appear in a precise temporal sequence and express a distinct combination of transcription factors, such as Sp8, Prox1, COUP-TFI and COUP-TFII. By inactivating COUP-TFI in developing interneurons, the lateral and medial streams are perturbed and expression of Sp8 and COUP-TFII affected. As a consequence, adult mutant neocortices have laminar-specific alterations of distinct cortical interneuron subtypes. Overall, we propose that the existence of spatially and temporally regulated migratory paths in the subpallium contributes to the laminar distribution and specification of distinct interneuron subpopulations in the adult brain.

Effects of monoterpenes on ion channels of excitable cells

Monoterpenes are a structurally diverse group of phytochemicals and a major constituent of plant-derived 'essential oils'. Monoterpenes such as menthol, carvacrol, and eugenol have been utilized for therapeutical purposes and food additives for centuries and have been reported to have anti-inflammatory, antioxidant and analgesic actions. In recent years there has been increasing interest in understanding the pharmacological actions of these molecules. There is evidence indicating that monoterpenes can modulate the functional properties of several types of voltage and ligand-gated ion channels, suggesting that some of their pharmacological actions may be mediated by modulations of ion channel function. In this report, we review the literature concerning the interaction of monoterpenes with various ion channels.

Principles of agonist recognition in Cys-loop receptors

Cys-loop receptors are ligand-gated ion channels that are activated by a structurally diverse array of neurotransmitters, including acetylcholine, serotonin, glycine, and GABA. After the term "chemoreceptor" emerged over 100 years ago, there was some wait until affinity labeling, molecular cloning, functional studies, and X-ray crystallography experiments identified the extracellular interface of adjacent subunits as the principal site of agonist binding. The question of how subtle differences at and around agonist-binding sites of different Cys-loop receptors can accommodate transmitters as chemically diverse as glycine and serotonin has been subject to intense research over the last three decades. This review outlines the functional diversity and current structural understanding of agonist-binding sites, including those of invertebrate Cys-loop receptors. Together, this provides a framework to understand the atomic determinants involved in how these valuable therapeutic targets recognize and bind their ligands.

The neuron identity problem: form meets function

A complete understanding of nervous system function cannot be achieved without the identification of its component cell types. In this Perspective, we explore a series of related issues surrounding cell identity and how revolutionary methods for labeling and probing specific neuronal types have clarified this question. Specifically, we ask the following questions: what is the purpose of such diversity, how is it generated, how is it maintained, and, ultimately, how can one unambiguously identity one cell type from another? We suggest that each cell type can be defined by a unique and conserved molecular ground state that determines its capabilities. We believe that gaining an understanding of these molecular barcodes will advance our ability to explore brain function, enhance our understanding of the biochemical basis of CNS disorders, and aid in the development of novel therapeutic strategies.

Serotonin induces the migration of PC12 cells via the serotonin receptor 6/cAMP/ERK pathway

Serotonin (5-HT) functions as a chemoattractant that modulates neural migration during prenatal and early postnatal development. However, its molecular mechanism remains to be elucidated. The effect of 5-HT on neural cell migration was examined using PC12 neuron-like cell line. Transwell migration assay was used to determine the effect of 5-HT on PC12 cell migration. The results demonstrated that 5-HT and nerve growth factor (NGF) induced PC12 cell migration in a dose-dependent manner. Additionally, 5-HT receptor antagonists suggest that 5-HT-induced migration was mediated by serotonin receptor 6 (5-HT₆), a Gs-protein coupled receptor that elevates the intercellular cAMP level. By contrast, antagonists of serotonin receptor 3 (5-HT₃) did not show any effects on PC12 cell migration. Clozapine, an inhibitor of cAMP accumulation mediated by 5-HT₆, significantly reduced the effect of 5-HT on the PC12 cell migration. An inhibitor of extracellular signal-regulated kinase (ERK) also suppressed migration. These results suggest that 5-HT induces PC12 cell migration by activating cAMP/ERK signaling pathways, which is mediated by 5-HT₆ receptor.