Making and breaking serotonin neurons and autism

Ascorbate and its transporter SVCT2: The dynamic duo's integrated roles in CNS neurobiology and pathophysiology

Ascorbate is a small antioxidant molecule essential for the proper development and function of the brain. Ascorbate is transported into the brain and between brain cells via the Sodium vitamin C co-transporter 2 (SVCT2). This review provides an in-depth analysis of ascorbate's physiology, including how ascorbate is absorbed from food into the CNS, emphasizing cellular mechanisms of ascorbate recycling and release in different CNS compartments. Additionally, the review delves into the various functions of ascorbate in the CNS, including its impact on epigenetic modulation, synaptic plasticity, and neurotransmission. It also emphasizes ascorbate's role on neuromodulation and its involvement in neurodevelopmental processes and disorders. Furthermore, it analyzes the relationship between the duo ascorbate/SVCT2 in neuroinflammation, particularly its effects on microglial activation, cytokine release, and oxidative stress responses, highlighting its association with neurodegenerative diseases, such as Alzheimer's disease (AD). Overall, this review emphasizes the crucial role of the dynamic duo ascorbate/SVCT2 in CNS physiology and pathology and the need for further research to fully comprehend its significance in a neurobiological context and its potential therapeutic applications.

Brain-Enriched Coding and Long Non-coding RNA Genes Are Overrepresented in Recurrent Neurodevelopmental Disorder CNVs

Autism spectrum disorder (ASD) is a neurodevelopmental condition with substantial phenotypic and etiological heterogeneity. Although 10%-20% of ASD cases are attributable to copy number variation (CNV), causative genomic loci and constituent genes remain unclarified. We have developed SNATCNV, a tool that outperforms existing tools, to identify 47 recurrent ASD CNV regions from 19,663 cases and 6,479 controls documented in the AutDB database. Analysis of ASD CNV gene content using FANTOM5 shows that constituent coding genes and long non-coding RNAs have brain-enriched patterns of expression. Notably, such enrichment is not observed for regions identified by using other tools. We also find evidence of sexual dimorphism, one locus uniquely comprising a single lncRNA gene, and correlation of CNVs to distinct clinical and behavioral traits. Finally, we analyze a large dataset for schizophrenia to further demonstrate that SNATCNV is an effective, publicly available tool to define genomic loci and causative genes for multiple CNV-associated conditions.

Median raphe serotonin neurons promote anxiety-like behavior via inputs to the dorsal hippocampus

Anxiety disorders may be mediated in part by disruptions in serotonin (5-hydroxytryptamine, 5-HT) system function. Behavioral measures of approach-avoidance conflict suggest that serotonin neurons within the median raphe nucleus (MRN) promote an anxiogenic state, and some evidence indicates this may be mediated by serotonergic signaling within the dorsal hippocampus. Here, we test this hypothesis using an optogenetic approach to examine the contribution of MRN 5-HT neurons and 5-HT innervation of the dorsal hippocampus (dHC) to anxiety-like behaviours in female mice. Mice expressing the excitatory opsin ChR2 were generated by crossing the ePet-cre serotonergic cre-driver line with the conditional Ai32 ChR2 reporter line, resulting in selective expression of ChR2 in 5-HT neurons. Electrophysiological recordings confirmed that this approach enabled reliable optogenetic stimulation of MRN 5-HT neurons, and this stimulation produced downstream 5-HT release in the dHC as measured by in vivo microdialysis. Optogenetic stimulation of the MRN elicited behavioral responses indicative of an anxiogenic effect in three behavioural tests: novelty-suppressed feeding, marble burying and exploration on the elevated-plus maze. These effects were shown to be behaviourally-specific. Stimulation of 5-HT terminals in the dHC recapitulated the anxiety-like behaviour in the novelty-suppressed feeding and marble burying tests. These results show that activation of 5-HT efferents from the MRN rapidly induces expression of anxiety-like behaviour, in part via projections to the dHC. These findings reveal an important neural circuit implicated in the expression of anxiety in female mice.

GABAergic modulation of serotonergic neurons in the dorsal raphe nucleus

The dorsal raphe nucleus (DRN), located in the brainstem, is involved in several functions such as sleep, temperature regulation, stress responses, and anxiety behaviors. This nucleus contains the largest population of serotonin expressing neurons in the brain. Serotonergic DRN neurons receive tonic γ-aminobutyric acid (GABA)inhibitory inputs from several brain areas, as well as from interneurons within the same nucleus. Serotonergic and GABAergic neurons in the DRN can be distinguished by their size, location, pharmacological responses, and electrophysiological properties. GABAergic neurons regulate the excitability of DRN serotonergic neurons and the serotonin release in different brain areas. Also, it has been shown that GABAergic neurons can synchronize the activity of serotonergic neurons across functions such as sleep or alertness. Moreover, dysregulation of GABA signaling in the DRN has been linked to psychiatric disorders such as anxiety and depression. This review focuses on GABAergic transmission in the DRN. The interaction between GABAergic and serotonergic neurons is discussed considering some physiological implications. Also, the main electrophysiological and morphological characteristics of serotonergic and GABAergic neurons are described.

Actin cytoskeleton dynamics in stem cells from autistic individuals

Several lines of indirect evidence, such as mutations or dysregulated expression of genes related to cytoskeleton, have suggested that cytoskeletal dynamics, a process essential for axons and dendrites development, is compromised in autism spectrum disorders (ASD). However, no study has yet examined whether cytoskeleton dynamics is functionally altered in cells from ASD patients. Here we investigated the regulation of actin cytoskeleton dynamics in stem cells from human exfoliated deciduous teeth (SHEDs) of 13 ASD patients and 8 control individuals by inducing actin filament depolymerization and then measuing their reconstruction upon activation of the RhoGTPases Rac, Cdc42 or RhoA. We observed that stem cells from seven ASD individuals (53%) presented altered dymanics of filament reconstruction, including a patient recently studied by our group whose iPSC-derived neuronal cells show shorten and less arborized neurites. We also report potentially pathogenic genetic variants that might be related to the alterations in actin repolymerization dynamics observed in some patient-derived cells. Our results suggest that, at least for a subgroup of ASD patients, the dynamics of actin polymerization is impaired, which might be ultimately leading to neuronal abnormalities.

Perturbed Developmental Serotonin Signaling Affects Prefrontal Catecholaminergic Innervation and Cortical Integrity

Proper development of the medial prefrontal cortex (mPFC), crucial for correct cognitive functioning, requires projections from, among others, the serotonergic (5-HT) and catecholaminergic systems, but it is unclear how these systems influence each other during development. Here, we describe the parallel development of the 5-HT and catecholaminergic prefrontal projection systems in rat and demonstrate a close engagement of both systems in the proximity of Cajal-Retzius cells. We further show that in the absence of the 5-HT transporter (5-HTT), not only the developing 5-HT but also the catecholaminergic system, including their projections towards the mPFC, are affected. In addition, the layer identity of the mPFC neurons and reelin-positive interneuron number and integration are altered in the absence of the 5-HTT. Together, our data demonstrate a functional interplay between the developing mPFC 5-HT and catecholaminergic systems, and call for a holistic approach in studying neurotransmitter systems-specific developmental consequences for adult behavior, to eventually allow the design of better treatment strategies for neuropsychiatric disorders.

Serotonin neuron development: shaping molecular and structural identities

The continuing fascination with serotonin (5-hydroxytryptamine, 5-HT) as a nervous system chemical messenger began with its discovery in the brains of mammals in 1953. Among the many reasons for this decades-long interest is that the small numbers of neurons that make 5-HT influence the excitability of neural circuits in nearly every region of the brain and spinal cord. A further reason is that 5-HT dysfunction has been linked to a range of psychiatric and neurological disorders many of which have a neurodevelopmental component. This has led to intense interest in understanding 5-HT neuron development with the aim of determining whether early alterations in their generation lead to brain disease susceptibility. Here, we present an overview of the neuroanatomical organization of vertebrate 5-HT neurons, their neurogenesis, and prodigious axonal architectures, which enables the expansive reach of 5-HT neuromodulation in the central nervous system. We review recent findings that have revealed the molecular basis for the tremendous diversity of 5-HT neuron subtypes, the impact of environmental factors on 5-HT neuron development, and how 5-HT axons are topographically organized through disparate signaling pathways. We summarize studies of the gene regulatory networks that control the differentiation, maturation, and maintenance of 5-HT neurons. These studies show that the regulatory factors controlling acquisition of 5-HT-type transmitter identity continue to play critical roles in the functional maturation and the maintenance of 5-HT neurons. New insights are presented into how continuously expressed 5-HT regulatory factors control 5-HT neurons at different stages of life and how the regulatory networks themselves are maintained. WIREs Dev Biol 2018, 7:e301. doi: 10.1002/wdev.301 This article is categorized under: Nervous System Development > Vertebrates: General Principles Gene Expression and Transcriptional Hierarchies > Gene Networks and Genomics Gene Expression and Transcriptional Hierarchies > Cellular Differentiation Nervous System Development > Secondary: Vertebrates: Regional Development.

Sonic hedgehog regulates the pathfinding of descending serotonergic axons in hindbrain in collaboration with Wnt5a and secreted frizzled-related protein 1

Previous studies have demonstrated that both Wnt5a and Sonic hedgehog (Shh) are involved in regulating the pathfinding of descending serotonergic (5-HT, 5-hydroxytryptamine) axons in an opposite manner in the brainstem. Shh and Wnt signaling pathways interact to guide post-crossing commissural axons, where Shh acts as a repellent directly and shaping the Wnt gradient indirectly by regulating the gradient expression of the frizzled-related protein 1 (Sfrp1). Whether such a mechanism functions in descending 5-HT axon guidance remains unknown. Here, we found that the core components of the Shh and Wnt planar cell polarity signaling pathways are expressed in caudal 5-HT neurons, and the expression gradients of Shh, Sfrp1, and Wnt5a exist simultaneously in hindbrain. Dunn chamber assays revealed that Sfrp1 suppressed the attractive Wnt gradient. Moreover, we found that Shh overexpression led to pathfinding defects in 5-HT axon descending, and the axonal pathfinding defects could be partially rescued by administration of an Sfrp1 antagonist in vivo. Biochemical evidence showed Shh overexpression upregulated the expression of the Sfrp1 gene and interrupted Wnt5a binding to Frizzled-3. Taken together, our results indicate that Shh, Sfrp1, and Wnt5a collaborate to direct the pathfinding of descending 5-HT axons in the brainstem.

Regulatory Mechanisms Controlling Maturation of Serotonin Neuron Identity and Function

The brain serotonin (5-hydroxytryptamine; 5-HT) system has been extensively studied for its role in normal physiology and behavior, as well as, neuropsychiatric disorders. The broad influence of 5-HT on brain function, is in part due to the vast connectivity pattern of 5-HT-producing neurons throughout the CNS. 5-HT neurons are born and terminally specified midway through embryogenesis, then enter a protracted period of maturation, where they functionally integrate into CNS circuitry and then are maintained throughout life. The transcriptional regulatory networks controlling progenitor cell generation and terminal specification of 5-HT neurons are relatively well-understood, yet the factors controlling 5-HT neuron maturation are only recently coming to light. In this review, we first provide an update on the regulatory network controlling 5-HT neuron development, then delve deeper into the properties and regulatory strategies governing 5-HT neuron maturation. In particular, we discuss the role of the 5-HT neuron terminal selector transcription factor (TF) Pet-1 as a key regulator of 5-HT neuron maturation. Pet-1 was originally shown to positively regulate genes needed for 5-HT synthesis, reuptake and vesicular transport, hence 5-HT neuron-type transmitter identity. It has now been shown to regulate, both positively and negatively, many other categories of genes in 5-HT neurons including ion channels, GPCRs, transporters, neuropeptides, and other transcription factors. Its function as a terminal selector results in the maturation of 5-HT neuron excitability, firing characteristics, and synaptic modulation by several neurotransmitters. Furthermore, there is a temporal requirement for Pet-1 in the control of postmitotic gene expression trajectories thus indicating a direct role in 5-HT neuron maturation. Proper regulation of the maturation of cellular identity is critical for normal neuronal functioning and perturbations in the gene regulatory networks controlling these processes may result in long-lasting changes in brain function in adulthood. Further study of 5-HT neuron gene regulatory networks is likely to provide additional insight into how neurons acquire their mature identities and how terminal selector-type TFs function in postmitotic vertebrate neurons.

Oxidative stress, serotonergic changes and decreased ultrasonic vocalizations in a mouse model of Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome is an inherited monogenic disorder in which mutations to the 7-dehydrocholesterol (7-DHC) reductase (Dhcr7) gene lead to deficits in cholesterol synthesis. As a result, many patients suffer from gross physiological and neurological deficits. The purpose of this study was to identify a potential abnormal behavioral phenotype in a compound mutant mouse model for Smith-Lemli-Opitz disease (Dhcr7 ^Δ3-5/T93M ) to further validate the model and to provide potential targets for future therapeutic interventions. We also sought to identify some of the underlying changes in brain function that may be responsible for behavioral differences among groups. The Dhcr7 compound mutant mice were smaller than their single mutant littermates. Both single and compound heterozygous mice made fewer ultrasonic vocalizations when separated from the dam, which may suggest a communication deficit in these animals. Striking increases of the highly oxidizable 7-DHC were observed in the compound mutant mice. 7-Dehydrocholesterol is the precursor to cholesterol and builds up because of decreased function of the mutated Dhcr7 enzyme. Additionally, several differences were noted in the serotonergic system including increased expression of the serotonin transporter and increased uptake of serotonin by isolated synaptosomes. We propose that changes to the oxidative environment during development can have a significant impact on the development of serotonergic function and that this contributes to behavioral differences observed in the mutant mice.

Fast Detector/First Responder: Interactions between the Superior Colliculus-Pulvinar Pathway and Stimuli Relevant to Primates

Primates are distinguished from other mammals by their heavy reliance on the visual sense, which occurred as a result of natural selection continually favoring those individuals whose visual systems were more responsive to challenges in the natural world. Here we describe two independent but also interrelated visual systems, one cortical and the other subcortical, both of which have been modified and expanded in primates for different functions. Available evidence suggests that while the cortical visual system mainly functions to give primates the ability to assess and adjust to fluid social and ecological environments, the subcortical visual system appears to function as a rapid detector and first responder when time is of the essence, i.e., when survival requires very quick action. We focus here on the subcortical visual system with a review of behavioral and neurophysiological evidence that demonstrates its sensitivity to particular, often emotionally charged, ecological and social stimuli, i.e., snakes and fearful and aggressive facial expressions in conspecifics. We also review the literature on subcortical involvement during another, less emotional, situation that requires rapid detection and response-visually guided reaching and grasping during locomotion-to further emphasize our argument that the subcortical visual system evolved as a rapid detector/first responder, a function that remains in place today. Finally, we argue that investigating deficits in this subcortical system may provide greater understanding of Parkinson's disease and Autism Spectrum disorders (ASD).

Constitutive and Acquired Serotonin Deficiency Alters Memory and Hippocampal Synaptic Plasticity

Serotonin (5-HT) deficiency occurs in a number of brain disorders that affect cognitive function. However, a direct causal relationship between 5-HT hypo-transmission and memory and underlying mechanisms has not been established. We used mice with a constitutive depletion of 5-HT brain levels (Pet1KO mice) to analyze the contribution of 5-HT to different forms of learning and memory. Pet1KO mice exhibited a striking deficit in novel object recognition memory, a hippocampal-dependent task. No alterations were found in tasks for social recognition, procedural learning, or fear memory. Viral delivery of designer receptors exclusively activated by designer drugs was used to selectively silence the activity of 5-HT neurons in the raphe. Inhibition of 5-HT neurons in the median raphe, but not the dorsal raphe, was sufficient to impair object recognition in adult mice. In vivo electrophysiology in behaving mice showed that long-term potentiation in the hippocampus of 5-HT-deficient mice was altered, and administration of the 5-HT1A agonist 8-OHDPAT rescued the memory deficits. Our data suggest that hyposerotonergia selectively affects declarative hippocampal-dependent memory. Serotonergic projections from the median raphe are necessary to regulate object memory and hippocampal synaptic plasticity processes, through an inhibitory control mediated by 5-HT1A receptors.

Psychopharmacological interventions in autism spectrum disorder

INTRODUCTION

Individuals with autism spectrum disorder (ASD) commonly present for treatment of emotional and behavioral disturbances associated with ASD's "core" symptoms. Psychotropic medications are widely utilized in alleviating associated emotional and behavioral symptoms.

AREAS COVERED

Emotional and behavioral disturbances associated with ASD include irritability/severely disruptive behavior, which comprises the heaviest symptom burden; hyperactivity and other Attention-Deficit-Hyperactivity-Disorder (ADHD)-type symptoms; repetitive/stereotyped behaviors; and social withdrawal. Existing evidence for medications for each of these symptom clusters will be examined in this review.

EXPERT OPINION

Psychopharmacological treatment of core and associated symptoms in ASD is challenging, in large part because of the heterogeneity in the presentation of ASD. Furthermore, children and adolescents with ASD are more vulnerable to the side effects of psychopharmacological intervention than their age-matched, typically developing counterparts. Currently, risperidone and aripiprazole are the only medications that have been (relatively) reliably shown to help treat certain symptom clusters associated with ASD, namely severely disruptive behavior and hyperactivity. Recent studies have begun to look at medications with mechanisms that are novel in the treatment of ASD and that may address underlying pathophysiology and/or core symptoms such as glutamate-modulating agents. Overall, randomized, placebo-controlled studies of medications for the treatment of ASD are scarce.

Enteric short-chain fatty acids: microbial messengers of metabolism, mitochondria, and mind: implications in autism spectrum disorders

Clinical observations suggest that gut and dietary factors transiently worsen and, in some cases, appear to improve behavioral symptoms in a subset of persons with autism spectrum disorders (ASDs), but the reason for this is unclear. Emerging evidence suggests ASDs are a family of systemic disorders of altered immunity, metabolism, and gene expression. Pre- or perinatal infection, hospitalization, or early antibiotic exposure, which may alter gut microbiota, have been suggested as potential risk factors for ASD. Can a common environmental agent link these disparate findings? This review outlines basic science and clinical evidence that enteric short-chain fatty acids (SCFAs), present in diet and also produced by opportunistic gut bacteria following fermentation of dietary carbohydrates, may be environmental triggers in ASD. Of note, propionic acid, a major SCFA produced by ASD-associated gastrointestinal bacteria (clostridia, bacteroides, desulfovibrio) and also a common food preservative, can produce reversible behavioral, electrographic, neuroinflammatory, metabolic, and epigenetic changes closely resembling those found in ASD when administered to rodents. Major effects of these SCFAs may be through the alteration of mitochondrial function via the citric acid cycle and carnitine metabolism, or the epigenetic modulation of ASD-associated genes, which may be useful clinical biomarkers. It discusses the hypothesis that ASDs are produced by pre- or post-natal alterations in intestinal microbiota in sensitive sub-populations, which may have major implications in ASD cause, diagnosis, prevention, and treatment.

CORTECON: a temporal transcriptome analysis of in vitro human cerebral cortex development from human embryonic stem cells

Many neurological and psychiatric disorders affect the cerebral cortex, and a clearer understanding of the molecular processes underlying human corticogenesis will provide greater insight into such pathologies. To date, knowledge of gene expression changes accompanying corticogenesis is largely based on murine data. Here we present a searchable, comprehensive, temporal gene expression data set encompassing cerebral cortical development from human embryonic stem cells (hESCs). Using a modified differentiation protocol that yields neurons suggestive of prefrontal cortex, we identified sets of genes and long noncoding RNAs that significantly change during corticogenesis and those enriched for disease-associations. Numerous alternatively spliced genes with varying temporal patterns of expression are revealed, including TGIF1, involved in holoprosencephaly, and MARK1, involved in autism. We have created a database (http://cortecon.neuralsci.org/) that provides online, query-based access to changes in RNA expression and alternatively spliced transcripts during human cortical development.

Autism spectrum disorder associated with low serotonin in CSF and mutations in the SLC29A4 plasma membrane monoamine transporter (PMAT) gene

Background

Patients with autism spectrum disorder (ASD) may have low brain serotonin concentrations as reflected by the serotonin end-metabolite 5-hydroxyindolacetic acid (5HIAA) in cerebrospinal fluid (CSF).

Methods

We sequenced the candidate genes SLC6A4 (SERT), SLC29A4 (PMAT), and GCHFR (GFRP), followed by whole exome analysis.

Results

The known heterozygous p.Gly56Ala mutation in the SLC6A4 gene was equally found in the ASD and control populations. Using a genetic candidate gene approach, we identified, in 8 patients of a cohort of 248 with ASD, a high prevalence (3.2%) of three novel heterozygous non-synonymous mutations within the SLC29A4 plasma membrane monoamine transporter (PMAT) gene, c.86A > G (p.Asp29Gly) in two patients, c.412G > A (p.Ala138Thr) in five patients, and c.978 T > G (p.Asp326Glu) in one patient. Genome analysis of unaffected parents confirmed that these PMAT mutations were not de novo but inherited mutations. Upon analyzing over 15,000 normal control chromosomes, only SLC29A4 c.86A > G was found in 23 alleles (0.14%), while neither c.412G > A (<0.007%) nor c.978 T > G (<0.007%) were observed in all chromosomes analyzed, emphasizing the rareness of the three alterations. Expression of mutations PMAT-p.Ala138Thr and p.Asp326Glu in cellulae revealed significant reduced transport uptake activity towards a variety of substrates including serotonin, dopamine, and 1-methyl-4-phenylpyridinium (MPP⁺), while mutation p.Asp29Gly had reduced transport activity only towards MPP⁺. At least two ASD subjects with either the PMAT-Ala138Thr or the PMAT-Asp326Glu mutation with altered serotonin transport activity had, besides low 5HIAA in CSF, elevated serotonin levels in blood and platelets. Moreover, whole exome sequencing revealed additional alterations in these two ASD patients in mainly serotonin-homeostasis genes compared to their non-affected family members.

Conclusions

Our findings link mutations in SLC29A4 to the ASD population although not invariably to low brain serotonin. PMAT dysfunction is speculated to raise serotonin prenatally, exerting a negative feedback inhibition through serotonin receptors on development of serotonin networks and local serotonin synthesis. Exome sequencing of serotonin homeostasis genes in two families illustrated more insight in aberrant serotonin signaling in ASD.

Postnatal maintenance of the 5-Ht1a-Pet1 autoregulatory loop by serotonin in the raphe nuclei of the brainstem

Background

Despite the importance of 5-HT1A as a major target for the action of several anxiolytics/antidepressant drugs, little is known about its regulation in central serotonin (5-hydroxytryptamine, 5-HT) neurons.

Results

We report that expression of 5-HT1A and the transcription factor Pet1 was impaired in the rostral raphe nuclei of mice lacking tryptophan hydroxylase 2 (Tph2) after birth. The downregulation of Pet1 was recapitulated in 5-Ht1a^-/- mice. Using an explant culture system, we show that reduction of Pet1 and 5-HT1A was rescued in Tph2^-/- brainstem by exogenous 5-HT. In contrast, 5-HT failed to rescue reduced expression of Pet1 in 5-Ht1a^-/- brainstem explant culture.

Conclusions

These results suggest a causal relationship between 5-HT1A and Pet1, and reveal a potential mechanism by which 5-HT1A-Pet1 autoregulatory loop is maintained by 5-HT in a spatiotemporal-specific manner during postnatal development. Our results are relevant to understanding the pathophysiology of certain psychiatric and developmental disorders.

The developmental disruptions of serotonin signaling may involved in autism during early brain development

Autism is a developmental disorder defined by the presence of a triad of communication, social and stereo typical behavioral characteristics with onset before 3years of age. In spite of the fact that there are potential environmental factors for autistic behavior, the dysfunction of serotonin during early development of the brain could be playing a role in this prevalence rise. Serotonin can modulate a number of developmental events, including cell division, neuronal migration, cell differentiation and synaptogenesis. Hyperserotonemia during fetal development results in the loss of serotonin terminals through negative feedback. The increased serotonin causes a decrease of oxytocin in the paraventricular nucleus of the hypothalamus and an increase in calcitonin gene-related peptide (CGRP) in the central nucleus of the amygdale, which are associated with social interactions and vital in autism. However, hyposerotonemia may be also relevant to the development of sensory as well as motor and cognitive faculties. And the paucity of placenta-derived serotonin should have potential importance when the pathogenesis of autism is considered. This review briefly summarized the developmental disruptions of serotonin signaling involved in the pathogenesis of autism during early development of the brain.

Decreased cerebral spinal fluid neurotransmitter levels in Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive, multiple congenital anomaly syndrome with cognitive impairment and a distinct behavioral phenotype that includes autistic features. SLOS is caused by a defect in 3β-hydroxysterol Δ(7)-reductase which leads to decreased cholesterol levels and elevated cholesterol precursors, specifically 7- and 8-dehydrocholesterol. However, the pathological processes contributing to the neurological abnormalities in SLOS have not been defined. In view of prior data suggesting defects in SLOS in vesicular release and given the association of altered serotonin metabolism with autism, we were interested in measuring neurotransmitter metabolite levels in SLOS to assess their potential to be used as biomarkers in therapeutic trials. We measured cerebral spinal fluid levels of serotonin and dopamine metabolites, 5-hydroxyindoleacetic acid (5HIAA) and homovanillic acid (HVA) respectively, in 21 SLOS subjects. Results were correlated with the SLOS anatomical severity score, Aberrant Behavior Checklist scores and concurrent sterol biochemistry. Cerebral spinal fluid (CSF) levels of both 5HIAA and HVA were significantly reduced in SLOS subjects. In individual patients, the levels of both 5HIAA and HVA were reduced to a similar degree. CSF neurotransmitter metabolite levels did not correlate with either CSF sterols or behavioral measures. This is the first study demonstrating decreased levels of CSF neurotransmitter metabolites in SLOS. We propose that decreased levels of neurotransmitters in SLOS are caused by a sterol-related defect in synaptic vesicle formation and that CSF 5HIAA and HVA will be useful biomarkers in development of future therapeutic trials.

Sexual dimorphic effect in the genetic association of monoamine oxidase A (MAOA) markers with autism spectrum disorder

Autism spectrum disorders are heritable and behaviorally-defined neurodevelopmental disorders having skewed sex ratio. Serotonin as modulator of behavior and implication of serotonergic dysfunction in ASD etiology corroborates that serotonergic system genes are potential candidates for autism susceptibility. In the current study X-chromosomal gene, MAOA responsible for degradation of serotonin is investigated for possible association with ASD using population-based approach. Study covers analysis of 8 markers in 421 subjects including cases and ethnically-matched controls from West Bengal. MAOA marker, rs6323 and various haplotypes formed between the markers show significant association with the disorder. Stratification on the basis of sex reveals significant genetic effect of rs6323 with low activity T allele posing higher risk in males, but not in females. Haplotypic association results also show differential effect both in males and females. Contrasting linkage disequilibrium pattern between pair of markers involving rs6323 in male cases and controls further supports the sex-bias in genetic association. Bioinformatic analysis shows presence of Y-encoded SRY transcription factor binding sites in the neighborhood of rs1137070. C allele of rs1137070 causes deletion of GATA-2 binding site and GATA-2 is known to interact with SRY. This is the first study highlighting male-specific effect of rs6323 marker and its haplotypes in ASD etiology and it suggests sexual dimorphic effect of MAOA in this disorder. Overall results of this study identify MAOA as a possible ASD susceptibility locus and the differential genetic effect in males and females might contribute to the sex ratio differences and molecular pathology of the disorder.

Psychopharmacological interventions in autism spectrum disorder

After participating in this educational activity, the physician should be better able to1. Prescribe the appropriate psychotropic medication to treat symptoms of ASD.2. Identify the side effects of the psychotropic medications used to treat ASD.Autism spectrum disorders (ASDs) are characterized by core deficits in social communication and language, and restrictive and repetitive behaviors that cause significant functional impairment and distress for affected individuals and their caregivers. The increasing prevalence of ASD, most recently estimated as 1 in 88 children, presents an ever-increasing burden on families, schools, medical systems, and society at large. Individuals with ASD commonly present for treatment of associated emotional and behavioral disturbances that include anxiety, symptoms of ADHD, compulsions and other repetitive behaviors, mood lability, irritability, aggression, and sleep disturbance. Psychotropic medications are widely utilized in alleviating these symptoms, though rigorous clinical trials in ASD are lacking for most areas of impairment. Strong evidence from randomized, placebo-controlled trials supports the use of atypical antipsychotics, particularly risperidone and aripiprazole, for managing severe irritability and aggression in ASD. Serotonin reuptake inhibitors are commonly used to treat anxiety and compulsions, though reports of efficacy in the literature are mixed, and behavioral side effects in children are common. Minimal evidence supports the utility of anticonvulsants and traditional mood stabilizers in managing mood lability and aggression. Stimulant and nonstimulant ADHD medications can be effective for reducing hyperactivity, inattention, and impulsivity, though to a lesser degree than in ADHD populations without ASD and with greater risk of adverse effects. Psychopharmacological interventions in development for core symptoms of autism include those that target the glutamatergic and GABAergic neurotransmitter systems and the neuropeptide oxytocin. Further research is needed to establish evidence-based interventions in ASD populations.

Biological mechanisms associated with increased perseveration and hyperactivity in a genetic mouse model of neurodevelopmental disorder

Chromosomal deletions at Xp22.3 appear to influence vulnerability to the neurodevelopmental disorders attention deficit hyperactivity disorder (ADHD) and autism. 39,X(Y*)O mice, which lack the murine orthologue of the Xp22.3 ADHD candidate gene STS (encoding steroid sulfatase), exhibit behavioural phenotypes relevant to such disorders (e.g. hyperactivity), elevated hippocampal serotonin (5-HT) levels, and reduced serum levels of dehydroepiandrosterone (DHEA). Here we initially show that 39,X(Y*)O mice are also deficient for the recently-characterised murine orthologue of the Xp22.3 autism candidate gene ASMT (encoding acetylserotonin-O-methyltransferase). Subsequently, to specify potential behavioural correlates of elevated hippocampal 5-HT arising due to the genetic lesion, we compared 39,X(Y*)O MF1 mice to 40,XY MF1 mice on behavioural tasks taxing hippocampal and/or 5-HT function (a 'foraging' task, an object-location task, and the 1-choice serial reaction time task of impulsivity). Although Sts/Asmt deficiency did not influence foraging behaviour, reactivity to familiar objects in novel locations, or 'ability to wait', it did result in markedly increased response rates; these rates correlated with hippocampal 5-HT levels and are likely to index behavioural perseveration, a frequent feature of neurodevelopmental disorders. Additionally, we show that whilst there was no systematic relationship between serum DHEA levels and hippocampal 5-HT levels across 39,X(Y*)O and 40,XY mice, there was a significant inverse linear correlation between serum DHEA levels and activity. Our data suggest that deficiency for genes within Xp22.3 could influence core behavioural features of neurodevelopmental disorders via dissociable effects on hippocampal neurochemistry and steroid hormone levels, and that the mediating neurobiological mechanisms may be investigated in the 39,X(Y*)O model.

Development of hypothalamic serotoninergic neurons requires Fgf signalling via the ETS-domain transcription factor Etv5b

Serotonin is a monoamine neurotransmitter that is involved in numerous physiological functions and its dysregulation is implicated in various psychiatric diseases. In all non-placental vertebrates, serotoninergic (5-HT) neurons are present in several regions of the brain, including the hypothalamus. In placental mammals, however, 5-HT neurons are located in the raphe nuclei only. In all species, though, 5-HT neurons constitute a functionally and molecularly heterogeneous population. How the non-raphe 5-HT populations are developmentally encoded is unknown. Using the zebrafish model we show that, in contrast to the raphe populations, hypothalamic 5-HT neurons are generated independently of the ETS-domain transcription factor Pet1 (Fev). By applying a combination of pharmacological tools and gene knockdown and/or overexpression experiments, we demonstrate that Fgf signalling acts via another ETS-domain transcription factor, Etv5b (Erm), to induce hypothalamic 5-HT neurons. We provide evidence that Etv5b exerts its effects by regulating cell cycle parameters in 5-HT progenitors. Our results highlight a novel role for Etv5b in neuronal development and provide support for the existence of a developmental heterogeneity among 5-HT neurons in their requirement for ETS-domain transcription factors.

Probing the diversity of serotonin neurons

The serotonin (5-HT) system is generally considered as a single modulatory system, with broad and diffuse projections. However, accumulating evidence points to the existence of distinct cell groups in the raphe. Here, we review prior evidence for raphe cell heterogeneity, considering different properties of 5-HT neurons, from metabolism to anatomy, and neurochemistry to physiology. We then summarize more recent data in mice and zebrafish that support a genetic diversity of 5-HT neurons, based on differential transcription factor requirements for the acquisition of the 5-HT identity. In both species, PET1 plays a major role in the acquisition and maintenance of 5-HT identity in the hindbrain, although some 5-HT neurons do not require PET1 for their differentiation, indicating the existence of several transcriptional routes to become serotoninergic. In mice, both PET1-dependent and -independent 5-HT neurons are located in the raphe, but have distinct anatomical features, such as the morphology of axon terminals and projection patterns. In zebrafish, all raphe neurons express pet1, but Pet1-independent 5-HT cell groups are present in the forebrain. Overall, these observations support the view that there are a number of distinct 5-HT subsystems, including within the raphe nuclei, with unique genetic programming and functions.

Emotion, intent and voluntary movement in children with autism. An example: the goal directed locomotion

This article focuses on the impact of intentionality on goal directed locomotion in healthy and autistic children. Closely linked with emotions and motivation, it is directly connected with movement planning. Is planning only preserved when the goal of the action appears motivating for healthy and autistic children? Is movement programming similar for autistic and healthy children, and does it vary according to the emotional valence of the object? Moving in a straight line, twenty autistic and healthy children had to retrieve a positive or aversive emotional valence object. The results suggest planning and programming are preserved in an emotionally positive situation. However, in an aversive situation, autistic children appear to have a deficit in terms of planning and sometimes programming.

The anxiogenic-like effects of dehydration in a semi-desert rodent Meriones shawi indicating the possible involvement of the serotoninergic system

Dehydration is a powerful stimulus causing disequilibrium in homeostasis of water and electrolytes resulting from depletion in total body water. Most studies have focused on domestic and laboratory animals; however, the study of desert animals allows improved understanding about water balance and resistance to dehydration and associated behavioral changes, including those related to mood disorders. Meriones shawi (Shaw's Jird) is a desert rodent characterized by its resistance to long periods of thirst that can extend for several months. In the present study, M. shawi were subjected to water deprivation for 1 and 3 months. We used 5-HT immunohistochemistry to evaluate the effects of prolonged dehydration on the serotoninergic system in both dorsal and median raphe nuclei (DRN, MRN), which are the main sources of 5-HT input to several brain areas. In addition, a dark/light box was used to evaluate the anxiolytic-like or anxiogenic-like effects of dehydration on M. shawi. The results showed a reduction in the 5-HT immunolabelling in both DRN and MRN following 1 and 3 months of dehydration. This diminution of serotonin immunoreactivity was accompanied by noticeable changes in anxiety behavior of Meriones, with animals spending more time in the light box, suggesting anxiogenic-like effects caused by dehydration. Overall, the results indicate that dehydration is able to reduce serotoninergic neurotransmission, which might be involved in generating anxiety behavior in this desert animal.

Distinct developmental origins and regulatory mechanisms for GABAergic neurons associated with dopaminergic nuclei in the ventral mesodiencephalic region

GABAergic neurons in the ventral mesodiencephalic region are highly important for the function of dopaminergic pathways that regulate multiple aspects of behavior. However, development of these neurons is poorly understood. We recently showed that molecular regulation of differentiation of the GABAergic neurons associated with the dopaminergic nuclei in the ventral midbrain (VTA and SNpr) is distinct from the rest of midbrain, but the reason for this difference remained elusive. Here, we have analyzed the developmental origin of the VTA and SNpr GABAergic neurons by genetic fate mapping. We demonstrate that the majority of these GABAergic neurons originate outside the midbrain, from rhombomere 1, and move into the ventral midbrain only as postmitotic neuronal precursors. We further show that Gata2, Gata3 and Tal1 define a subpopulation of GABAergic precursors in ventral rhombomere 1. A failure in GABAergic neuron differentiation in this region correlates with loss of VTA and SNpr GABAergic neurons in Tal1 mutant mice. In contrast to midbrain, GABAergic neurons of the anterior SNpr in the diencephalon are not derived from the rhombomere 1. These results suggest unique migratory pathways for the precursors of important GABAergic neuron subpopulations, and provide the basis for understanding diversity within midbrain GABAergic neurons.

Neurotransmitter phenotype plasticity: an unexpected mechanism in the toolbox of network activity homeostasis

The transmitter phenotype of a neuron has long been thought to be stable for the lifespan. Much as eyes have one color and do not change it over time, neurons have been thought to have one neurotransmitter and retain it for their lifetime. Both principles, exclusivity and stability, are challenged by recent data. More and more neurons in different regions of the brain appear to coexpress two or more neurotransmitters. Moreover, the profile of neurotransmitter expression of a given neuron has been shown to change over time, both during development and in response to changes in activity. The present review summarizes recent studies of this neurotransmitter phenotype plasticity (NPP). Homeostatic mechanisms of plasticity are aimed at maintaining the system within a functional range. They appear to be critical for optimal network operations and have been thought to operate largely by regulating intrinsic excitability, synapse number and synaptic strength. NPP provides a new and unexpected level of regulation of network homeostasis. We propose that it provides the basis for NT coexpression and discuss emerging issues and new questions for further studies in coming years.

Enriched rearing improves behavioral responses of an animal model for CNV-based autistic-like traits

Potocki-Lupski syndrome (PTLS; MIM #610883), characterized by neurobehavioral abnormalities, intellectual disability and congenital anomalies, is caused by a 3.7-Mb duplication in 17p11.2. Neurobehavioral studies determined that ∼70-90% of PTLS subjects tested positive for autism or autism spectrum disorder (ASD). We previously chromosomally engineered a mouse model for PTLS (Dp(11)17/+) with a duplication of a 2-Mb genomic interval syntenic to the PTLS region and identified consistent behavioral abnormalities in this mouse model. We now report extensive phenotyping with behavioral assays established to evaluate core and associated autistic-like traits, including tests for social abnormalities, ultrasonic vocalizations, perseverative and stereotypic behaviors, anxiety, learning and memory deficits and motor defects. Alterations were identified in both core and associated ASD-like traits. Rearing this animal model in an enriched environment mitigated some, and even rescued selected, neurobehavioral abnormalities, suggesting a role for gene-environment interactions in the determination of copy number variation-mediated autism severity.

Serotonergic transcriptional networks and potential importance to mental health

Transcription regulatory networks governing the genesis, maturation and maintenance of vertebrate brain serotonin (5-HT) neurons determine the level of serotonergic gene expression and signaling throughout an animal's lifespan. Recent studies suggest that alterations in these networks can cause behavioral and physiological pathogenesis in mice. Here, we synthesize findings from vertebrate loss-of-function and gain-of-function studies to build a new model of the transcriptional regulatory networks that specify 5-HT neurons during fetal life, integrate them into CNS circuitry in early postnatal life and maintain them in adulthood. We then describe findings from animal and human genetic studies that support possible alterations in the activity of serotonergic regulatory networks in the etiology of mental illness. We conclude with a discussion of the potential utility of our model, as an experimentally well-defined molecular pathway, to predict and interpret the biological effect of genetic variation that may be discovered in the orthologous human network.

Investigating anxiety and depressive-like phenotypes in genetic mouse models of serotonin depletion

Emotional disorders such as depression, panic attacks, generalized anxiety, phobias and post-traumatic stress have been associated to decreased serotonin (5-HT) function, based on the positive effects of treatments that enhance 5-HT neurotransmission. However, it has been difficult to establish a primary role for 5-HT deficiency in these diseases, making preclinical models particularly useful. Over the last ten years a variety of genetic mouse models of 5-HT depletion have been produced, complementing previous pharmacologically-based models. Initial models hindered the differentiation of the raphe 5-HT neurons, while more recently produced models suppressed 5-HT production or incapacitated 5-HT vesicular packaging and release in normally developed raphe neurons. Here, we provide an overview of 11 genetic mouse models with lowered 5-HT transmission and summarize the available behavioural investigations concerning their anxiety and depression phenotypes. Although these studies are still ongoing, some common anxiety-related traits and behavioural phenotypes have emerged. Most studies have reported decreased innate anxiety to novelty but heightened fear responses to conditioned aversive cues. This complex phenotype is in general agreement with the proposed dual function of 5-HT in modulating different defensive behaviours. Surprisingly, the depressive-like behaviours have been less studied and, so far, did not yield a consistent phenotype in standard tests. Future studies should be conducted using more ethological relevant models to conclude on the causal role of 5-HT depletion in depression. This review also describes the differences in level and regional distribution of 5-HT depletion among the available mouse models, which could contribute to the diverse phenotypes observed. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Directed expression of Gata2, Mash1, and Foxa2 synergize to induce the serotonergic neuron phenotype during in vitro differentiation of embryonic stem cells

Investigation of serotonergic neuronal activity and its relationship to disease has been limited by a lack of physiologically relevant in vitro cell models. Serotonergic neurons derived from embryonic stem cells (ESCs) could provide a platform for such studies and provide models for use in drug discovery. Here, we report enhancement of serotonergic differentiation using a genetic approach. Expression of Gata2 increased the yield of serotonergic neurons. Enhancement was only achieved when Gata2 was expressed under the control of the tissue-specific promoter of the transcription factor Nkx6.1. High levels of Gata2 expression in ESCs compromised pluripotency and induced non-neuronal differentiation. Combined directed expression of Gata2, proneural gene Mash1, and forkhead transcription factor Foxa2 further enhanced serotonergic neural differentiation, resulting in a 10-fold increase in serotonin content. These neurons were also capable of depolarization (KCl, 30 mM)-induced elevations of intracellular Ca(2+) . The presence of sonic hedgehog during differentiation produced a further modest increase in numbers (1.5-fold). Transgene expression did not influence the number of tyrosine hydroxylase positive neurons in the cultures after 20 days, implying that Gata2, Mash1, and Foxa2 modulate in vitro differentiation at a time beyond the decision-point for dopaminergic or nondopaminergic commitment. This study demonstrates that the directed expression of specific transcription factors enhances serotonergic neuron differentiation in vitro and highlights the importance of transgene expression at the right stage of ESC differentiation to effect the generation of a desired neural subtype.

Molecular genetics of mouse serotonin neurons across the lifespan

New molecular genetics approaches have been developed over the past several years to study brain serotonin (5-HT) neuron development and the roles of 5-HT neurons in behavior and physiology. These approaches were enabled by manipulation of the gene encoding the Pet-1 ETS transcription factor whose expression in the brain is restricted to developing and adult 5-HT neurons. Targeting of the Pet-1 gene led to the development of a mouse line with a severe and stable deficiency of embryonic 5-HT-synthesizing neurons. The Pet-1 transcription regulatory region has been used to create several new 5-HT neuron-type transgenic tools that have greatly increased the experimental accessibility of the small number of brain 5-HT neurons. Permanent and specific marking of 5-HT neurons with Pet-1-based transgenic tools have now been used for flow cytometry, whole cell electrophysiological recordings, progenitor fate mapping, and live time lapse imaging of these neurons. Additional tools provide multiple strategies for conditional temporal targeting of gene expression in 5-HT neurons at different stages of life. Pet-1-based approaches have led to advances in understanding the role of 5-HT neurons in respiration, thermoregulation, emotional behaviors, maternal behavior, and the mechanism of antipsychotic drug actions. In addition, these approaches have begun to reveal the molecular basis of 5-HT neuron heterogeneity and the transcriptional mechanisms that direct 5-HT neuron-type identity, maturation, and maintenance.

Freud-2/CC2D1B mediates dual repression of the serotonin-1A receptor gene

The serotonin-1A (5-HT1A) receptor functions as a pre-synaptic autoreceptor in serotonin neurons that regulates their activity, and is also widely expressed on non-serotonergic neurons as a post-synaptic heteroreceptor to mediate serotonin action. The 5-HT1A receptor gene is strongly repressed by a dual repressor element (DRE), which is recognized by two proteins: Freud-1/CC2D1A and another unknown protein. Here we identify mouse Freud-2/CC2D1B as the second repressor of the 5-HT1A-DRE. Freud-2 shares 50% amino acid identity with Freud-1, and contains conserved structural domains. Mouse Freud-2 bound specifically to the rat 5-HT1A-DRE adjacent to, and partially overlapping, the Freud-1 binding site. By supershift assay using nuclear extracts from L6 myoblasts, Freud-2-DRE complexes were distinguished from Freud-1-DRE complexes. Freud-2 mRNA and protein were detected throughout mouse brain and peripheral tissues. Freud-2 repressed 5-HT1A promoter-reporter constructs in a DRE-dependent manner in non-neuronal (L6) or 5-HT1A-expressing neuronal (NG108-15, RN46A) cell models. In NG108-15 cells, knockdown of Freud-2 using a specific short-interfering RNA reduced endogenous Freud-2 protein levels and decreased Freud-2 bound to the 5-HT1A-DRE as detected by chromatin immunoprecipitation assay, but increased 5-HT1A promoter activity and 5-HT1A protein levels. Taken together, these data show that Freud-2 is the second component that, with Freud-1, mediates dual repression of the 5-HT1A receptor gene at the DRE.

Pet-1 is required across different stages of life to regulate serotonergic function

Transcriptional cascades are required for specification of 5-HT neurons and 5-HT modulated behaviors. Expression of several cascade factors extends across lifespan suggesting their control of behavior may not be temporally restricted to programming normal numbers of 5-HT neurons. We applied new mouse conditional targeting approaches to investigate ongoing requirements for Pet-1, a cascade factor required for the initiation of 5-HT synthesis but whose expression persists into adulthood. We found that Pet-1 was required after 5-HT neuron generation, for multiple steps in 5-HT neuron maturation including axonal innervation to the somatosensory cortex, firing properties, and 5-HT_1A and 5-HT_1B autoreceptor expression. Targeting Pet-1 in adult 5-HT neurons showed that it was still needed to preserve normal anxiety-related behaviors through direct autoregulated control of serotonergic gene expression. These findings show that Pet-1 is required across lifespan and therefore behavioral pathogenesis can result from both developmental and adult-onset alterations in serotonergic transcription.

Activity-dependent expression of Lmx1b regulates specification of serotonergic neurons modulating swimming behavior

Genetic programs, environmental factors, and electrical activity interact to drive the maturation of the brain. Although the cascade of transcription factors that leads to specification of the serotonergic phenotype has been well characterized, its interactions with electrical activity are not known. Here we show that spontaneous calcium spike activity in the hindbrain of developing Xenopus laevis larvae modulates the specification of serotonergic neurons via regulation of expression of the Lmx1b transcription factor. Activity acts downstream of Nkx2.2 but upstream of Lmx1b, leading to regulation of the serotonergic phenotype. Using global manipulation of activity and targeted alteration of Lmx1b expression, we also demonstrate that changes in the number of serotonergic neurons change larval swimming behavior. The results link activity-dependent regulation of a transcription factor to transmitter specification and altered behavior.

Nanostructure-initiator mass spectrometry (NIMS) imaging of brain cholesterol metabolites in Smith-Lemli-Opitz syndrome

Cholesterol is an essential component of cellular membranes that is required for normal lipid organization and cell signaling. While the mechanisms associated with maintaining cholesterol homeostasis in the plasma and peripheral tissues have been well studied, the role and regulation of cholesterol biosynthesis in normal brain function and development have proven much more challenging to investigate. Smith-Lemli-Opitz syndrome (SLOS) is a disorder of cholesterol synthesis characterized by mutations of 7-dehydrocholesterol reductase (DHCR7) that impair the reduction of 7-dehydrocholesterol (7DHC) to cholesterol and lead to neurocognitive deficits, including cerebellar hypoplasia and austism behaviors. Here we have used a novel mass spectrometry-based imaging technique called cation-enhanced nanostructure-initiator mass spectrometry (NIMS) for the in situ detection of intact cholesterol molecules from biological tissues. We provide the first images of brain sterol localization in a mouse model for SLOS (Dhcr7(-/-)). In SLOS mice, there is a striking localization of both 7DHC and residual cholesterol in the abnormally developing cerebellum and brainstem. In contrast, the distribution of cholesterol in 1-day old healthy pups was diffuse throughout the cerebrum and comparable to that of adult mice. This study represents the first application of NIMS to localize perturbations in metabolism within pathological tissues and demonstrates that abnormal cholesterol biosynthesis may be particularly important for the development of these brain regions.

Serotonin: a regulator of neuronal morphology and circuitry

Serotonin is an important neuromodulator associated with a wide range of physiological effects in the central nervous system. The exact mechanisms whereby serotonin influences brain development are not well understood, although studies in invertebrate and vertebrate model organisms are beginning to unravel a regulatory role for serotonin in neuronal morphology and circuit formation. Recent data suggest a developmental window during which altered serotonin levels permanently influence neuronal circuitry, however, the temporal constraints and molecular mechanisms responsible are still under investigation. Growing evidence suggests that alterations in early serotonin signaling contribute to a number of neurodevelopmental and neuropsychiatric disorders. Thus, understanding how altered serotonin signaling affects neuronal morphology and plasticity, and ultimately animal physiology and pathophysiology, will be of great significance.

Mouse plasmacytoma-expressed transcript 1 knock out induced 5-HT disruption results in a lack of cognitive deficits and an anxiety phenotype complicated by hypoactivity and defensiveness

Serotonin (5-HT) is involved in many developmental processes and influences behaviors including anxiety, aggression, and cognition. Disruption of the serotonergic system has been implicated in human disorders including autism, depression, schizophrenia, and ADHD. Although pharmacological, neurotoxin, and dietary manipulation of 5-HT and tryptophan hydroxylase has added to our understanding of the serotonergic system, the results are complicated by multiple factors. A newly identified ETS domain transcription factor, Pet-1, has direct control of major aspects of 5-HT neuronal development. Pet-1 is the only known factor that is restricted in the brain to 5-HT neurons during development and adulthood and exerts dominant control over 5-HT neuronal phenotype. Disruption of Pet-1 produces an approximately 80% loss of 5-HT neurons and content and results in increased aggression in male Pet-1(-/-) mice [Hendricks TJ, Fyodorov DV, Wegman LJ, Lelutiu NB, Pehek EA, Yamamoto B, Silver J, Weeber EJ, Sweatt JD, Deneris ES (2003) Neuron 37:233-247]. We hypothesized that Pet-1(-/-) mice would also exhibit changes in anxiety and cognition. Pet-1(-/-) mice were hypoactive which may have affected the observed lack of anxious behavior in the elevated zero maze and light-dark test. Pet-1(-/-) mice, however, were more defensive during marble burying and showed acoustic startle hyper-reactivity. No deficits in spatial, egocentric, or novel object recognition learning were found in Pet-1(-/-) mice. These findings were unexpected given that 5-HT depleting drugs given to adult or developing animals result in learning deficits [Mazer C, Muneyyirci J, Taheny K, Raio N, Borella A, Whitaker-Azmitia P (1997) Brain Res 760:68-73; Morford LL, Inman-Wood SL, Gudelsky GA, Williams MT, Vorhees CV (2002) Eur J Neurosci 16:491-500; Vorhees CV, Schaefer TL, Williams MT (2007) Synapse 61:488-499]. Lack of differences may be the result of compensatory mechanisms in reaction to a constitutive knock out of Pet-1 or 5-HT may not be as important in learning and memory as previously suspected.

The neurobiology of autism

Improving clinical tests are allowing us to more precisely classify autism spectrum disorders and diagnose them at earlier ages. This raises the possibility of earlier and potentially more effective therapeutic interventions. To fully capitalize on this opportunity, however, will require better understanding of the neurobiological changes underlying this devastating group of developmental disorders. It is becoming clear that the normal trajectory of neurodevelopment is altered in autism, with aberrations in brain growth, neuronal patterning and cortical connectivity. Changes to the structure and function of synapses and dendrites have also been strongly implicated in the pathology of autism by morphological, genetic and animal modeling studies. Finally, environmental factors are likely to interact with the underlying genetic profile, and foster the clinical heterogeneity seen in autism spectrum disorders. In this review we attempt to link the molecular pathways altered in autism to the neurodevelopmental and clinical changes that characterize the disease. We focus on signaling molecules such as neurotrophin, Reelin, PTEN and hepatocyte growth factor, neurotransmitters such as serotonin and glutamate, and synaptic proteins such as neurexin, SHANK and neuroligin. We also discuss evidence implicating oxidative stress, neuroglial activation and neuroimmunity in autism.

Movement disorders and neurochemical changes in zebrafish larvae after bath exposure to fluoxetine (PROZAC)

This study examines the effects of the selective serotonin reuptake inhibitor (SSRI), fluoxetine (PROZAC), on the ontogeny of spontaneous swimming activity (SSA) in developing zebrafish. The development of zebrafish motor behavior consists of four sequential locomotor patterns that develop over 1-5 days post fertilization (dpf), with the final pattern, SSA, established at 4-5 dpf. In stage specific experiments, larvae were exposed to 4.6 microM fluoxetine for 24 h periods beginning at 24 h post fertilization (hpf) and extending through 5 dpf. From 1-3 dpf, there was no effect on SSA or earlier stages of motor development, i.e., spontaneous coiling, evoked coiling and burst swimming. Fluoxetine exposure at 3 dpf for 24 h resulted in a transient decrease in SSA through 7 dpf with a complete recovery by 8 dpf. Larvae exposed to 4.6 microM fluoxetine for 24 h on 4 or 5 dpf showed a significant decrease in SSA by day 6 with no recovery through 14 dpf. Although SSA was significantly affected 24 h after fluoxetine exposure, there was little or no effect on pectoral fin movement. These results demonstrate both a stage specific and a long term effect of 4.6 microM fluoxetine exposure in 4 and 5 dpf larvae. Reverse transcriptase polymerase chain reaction (RT-PCR) was performed to determine the relative levels of a serotonin transporter protein (SERT) transcript and the serotonin 1A (5-HT(1A)) receptor transcript in developing embryos/larvae over 1-6 dpf. Both transcripts were present at 24 hpf with the relative concentration of SERT transcript showing no change over the developmental time range. The relative concentration of the 5-HT(1A) receptor transcript, however, showed a two-tiered pattern of concentration. RT-PCR was also used to detect potential changes in the SERT and 5-HT(1A) receptor transcripts in 6 dpf larvae after a 24 h exposure to 4.6 microM fluoxetine on 5 dpf. Three separate regions of the CNS were individually analyzed, two defined brain regions and spinal cord. The two brain regions showed no effect on transcript levels subsequent to fluoxetine exposure, however, the spinal cord showed a significant decrease in both transcripts. These results suggest a correlation between decreased concentration of SERT and 5-HT(1A) receptor transcripts in spinal cord and decreased SSA subsequent to fluoxetine exposure.

Altered development of neuronal progenitor cells after stimulation with autistic blood sera

Changes of brain structure and functions in people with autism may result from altered neuronal development, however, no adequate cellular or animal models are available to study neurogenesis in autism. Neuronal development can be modeled in culture of neuronal progenitor cells (NPCs) stimulated with serum to differentiate into neurons. Because sera from people with autism and age-matched controls contain different levels of numerous biologically active factors, we hypothesized that development of human NPCs induced to differentiate into neurons with sera from children with autism reflects the altered early neuronal development that leads to autism. The control and autistic sera were collected from siblings aged below 6 years that lived in the same environment. The effect of sera on differentiation of NPC neurospheres into neuronal colonies was tested in 72-h-long cultures by morphometry, immunocytochemistry and immunoblotting. We found that sera from children with autism significantly reduced NPCs' proliferation, but stimulated cell migration, development of small neurons with processes, length of processes and synaptogenesis. These results suggest that development of network of processes and synaptogenesis--the specific events in the brain during postnatal ontogenesis--are altered in autism. Further studies in this cell culture model may explain some of the cellular alterations described in autistic patients.

Neonatal serotonin depletion alters behavioral responses to spatial change and novelty

Multiple brain disorders that show serotonergic imbalances have a developmental onset. Experimental models indicate a role for serotonin as a morphogen in brain development. To selectively study the effects of serotonin depletions on cortical structural development and subsequent behavior, we developed a mouse model in which a serotonin neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), is injected into the medial forebrain bundle (mfb) on the day of birth. Littermates with saline injections into the mfb and age matched mice served as controls. This study characterized the extent and duration of serotonergic denervation after the selective neonatal lesion and investigated effects on exploratory behavior, spatial learning and anxiety in mice of both sexes. We report significant decreases in the serotonergic (5-HT) innervation to cortex and hippocampus, but not to subcortical forebrain structures in 5,7-DHT-lesioned mice. The depletion of 5-HT fibers in cortical areas was long lasting in lesioned mice but autoradiographic binding to high affinity 5-HT transporters was only transiently reduced. Male but not female lesioned mice reduced their exploration significantly in response to spatial rearrangement and object novelty, suggesting increased anxiety in response to change but normal spatial cognition. Our data show that developmental disruptions in the serotonergic innervation of cortex and hippocampus are sufficient to induce permanent, sex specific, behavioral alterations. These results may have significant implications for understanding brain disorders presenting with cortical morphogenetic abnormalities and altered serotonin neurotransmission, such as autism, schizophrenia and affective disorders.