Serotonin initiates and autoamplifies its own synthesis during mouse central nervous system development

Novel aspects of enteric serotonergic signaling in health and brain-gut disease

Gastrointestinal (GI) comorbidities are common in individuals with mood and behavioral dysfunction. Similarly, patients with GI problems more commonly suffer from co-morbid psychiatric diagnoses. Although the central and enteric nervous systems (CNS and ENS, respectively) have largely been studied separately, there is emerging interest in factors that may contribute to disease states involving both systems. There is strong evidence to suggest that serotonin may be an important contributor to these brain-gut conditions. Serotonin has long been recognized for its critical functions in CNS development and function. The majority of the body's serotonin, however, is produced in the GI tract, where it plays key roles in ENS development and function. Further understanding of the specific impact that enteric serotonin has on brain-gut disease may lay the foundation for the creation of novel therapeutic targets. This review summarizes the current data focusing on the important roles that serotonin plays in ENS development and motility, with a focus on novel aspects of serotonergic signaling in medical conditions in which CNS and ENS co-morbidities are common, including autism spectrum disorders and depression.

Perinatal serotonergic activity: A decisive factor in the control of food intake

ABSTRACT The serotoninergic system controls key events related to proper nervous system development. The neurotransmitter serotonin and the serotonin transporter are critical for this control. Availability of these components is minutely regulated during the development period, and the environment may affect their action on the nervous system. Environmental factors such as undernutrition and selective serotonin reuptake inhibitors may increase the availability of serotonin in the synaptic cleft and change its anorectic action. The physiological responses promoted by serotonin on intake control decrease when requested by acute stimuli or stress, demonstrating that animals or individuals develop adaptations in response to the environmental insults they experience during the development period. Diseases, such as anxiety and obesity, appear to be associated with the body’s response to stress or stimulus, and require greater serotonergic system action. These findings demonstrate the importance of the level of serotonin in the perinatal period to the development of molecular and morphological aspects of food intake control, and its decisive role in understanding the possible environmental factors that cause diseases in adulthood.

Refining the Role of 5-HT in Postnatal Development of Brain Circuits

Changing serotonin (5-hydroxytryptamine, 5-HT) brain levels during critical periods in development has long-lasting effects on brain function, particularly on later anxiety/depression-related behaviors in adulthood. A large part of the known developmental effects of 5-HT occur during critical periods of postnatal life, when activity-dependent mechanisms remodel neural circuits. This was first demonstrated for the maturation of sensory brain maps in the barrel cortex and the visual system. More recently this has been extended to the 5-HT raphe circuits themselves and to limbic circuits. Recent studies overviewed here used new genetic models in mice and rats and combined physiological and structural approaches to provide new insights on the cellular and molecular mechanisms controlled by 5-HT during late stages of neural circuit maturation in the raphe projections, the somatosensory cortex and the visual system. Similar mechanisms appear to be also involved in the maturation of limbic circuits such as prefrontal circuits. The latter are of particular relevance to understand the impact of transient 5-HT dysfunction during postnatal life on psychiatric illnesses and emotional disorders in adult life.

Identification of the role of bone morphogenetic protein (BMP) and transforming growth factor-β (TGF-β) signaling in the trajectory of serotonergic differentiation in a rapid assay in mouse embryonic stem cells in vitro

The mechanism by which extracellular molecules control serotonergic cell fate remains elusive. Recently, we showed that noggin, which inactivates bone morphogenetic proteins (BMPs), induces serotonergic differentiation of mouse embryonic (ES) and induced pluripotent stem cells with coordinated gene expression along the serotonergic lineage. Here, we created a rapid assay for serotonergic induction by generating knock-in ES cells expressing a naturally secreted Gaussia luciferase driven by the enhancer of Pet-1/Fev, a landmark of serotonergic differentiation. Using these cells, we performed candidate-based screening and identified BMP type I receptor kinase inhibitors LDN-193189 and DMH1 as activators of luciferase. LDN-193189 induced ES cells to express the genes encoding Pet-1, tryptophan hydroxylase 2, and the serotonin transporter, and increased serotonin release without altering dopamine release. In contrast, TGF-β receptor inhibitor SB-431542 selectively inhibited serotonergic differentiation, without changing overall neuronal differentiation. LDN-193189 inhibited expression of the BMP signaling target gene Id, and induced the TGF-β target gene Lefty, whereas the opposite effect was observed with SB-431542. This study thus provides a new tool to investigate serotonergic differentiation and suggests that inhibition of BMP type I receptors and concomitant activation of TGF-β receptor signaling are implicated in serotonergic differentiation. Candidate-based screening for serotonergic induction using a rapid assay in mouse embryonic stem cells revealed that the bone morphogenetic protein (BMP) type I receptor kinase inhibitors selectively induce serotonergic differentiation, whereas the TGF-β receptor inhibitor SB-431542 inhibits the differentiation. These results suggest that inhibition of BMP type I receptors and concomitant activation of transforming growth factor-β (TGF-β) receptor signaling are involved in the early trajectory of serotonergic differentiation.

Lack of brain serotonin affects postnatal development and serotonergic neuronal circuitry formation

Despite increasing evidence suggests that serotonin (5-HT) can influence neurogenesis, neuronal migration and circuitry formation, the precise role of 5-HT on central nervous system (CNS) development is only beginning to be elucidated. Moreover, how changes in serotonin homeostasis during critical developmental periods may have etiological relevance to human mental disorders, remains an unsolved question. In this study we address the consequences of 5-HT synthesis abrogation on CNS development using a knock-in mouse line in which the tryptophan hydroxylase 2 (Tph2) gene is replaced by the eGFP reporter. We report that lack of brain 5-HT results in a dramatic reduction of body growth rate and in 60% lethality within the first 3 weeks after birth, with no gross anatomical changes in the brain. Thanks to the specific expression of the eGFP, we could highlight the serotonergic system independently of 5-HT immunoreactivity. We found that lack of central serotonin produces severe abnormalities in the serotonergic circuitry formation with a brain region- and time- specific effect. Indeed, we observed a striking reduction of serotonergic innervation to the suprachiasmatic and thalamic paraventricular nuclei, while a marked serotonergic hyperinnervation was found in the nucleus accumbens and hippocampus of Tph2∷eGFP mutants. Finally, we demonstrated that BDNF expression is significantly up-regulated in the hippocampus of mice lacking brain 5-HT, mirroring the timing of the appearance of hyperinnervation and thus unmasking a possible regulatory feedback mechanism tuning the serotonergic neuronal circuitry formation. On the whole, these findings reveal that alterations of serotonin levels during CNS development affect the proper wiring of the brain that may produce long-lasting changes leading to neurodevelopmental disorders.

Genetic and pharmacological manipulations of the serotonergic system in early life: neurodevelopmental underpinnings of autism-related behavior

Serotonin, in its function as neurotransmitter, is well-known for its role in depression, autism and other neuropsychiatric disorders, however, less known as a neurodevelopmental factor. The serotonergic system is one of the earliest to develop during embryogenesis and early changes in serotonin levels can have large consequences for the correct development of specific brain areas. The regulation and functioning of serotonin is influenced by genetic risk factors, such as the serotonin transporter polymorphism in humans. This polymorphism is associated with anxiety-related symptoms, changes in social behavior, and cortical gray and white matter changes also seen in patients suffering from autism spectrum disorders (ASD). The human polymorphism can be mimicked by the knockout of the serotonin transporter in rodents, which are as a model system therefore vital to explore the precise neurobiological mechanisms. Moreover, there are pharmacological challenges influencing serotonin in early life, like prenatal/neonatal exposure to selective serotonin reuptake inhibitors (SSRI) in depressed pregnant women. There is accumulating evidence that this dysregulation of serotonin during critical phases of brain development can lead to ASD-related symptoms in children, and reduced social behavior and increased anxiety in rodents. Furthermore, prenatal valproic acid (VPA) exposure, a mood stabilizing drug which is also thought to interfere with serotonin levels, has the potency to induce ASD-like symptoms and to affect the development of the serotonergic system. Here, we review and compare the neurodevelopmental and behavioral consequences of serotonin transporter gene variation, and prenatal SSRI and VPA exposure in the context of ASD.

Monoamine oxidase A regulates neural differentiation of murine embryonic stem cells

Monoamine oxidase (MAO) A is the major metabolizing enzyme of serotonin (5-hydroxytryptamine, 5-HT) which regulates early brain development. In this study, wild-type (WT) and MAO A(neo) embryonic stem (ES) cell lines were established from the inner cell mass of murine blastocysts and their characteristics during ES and differentiating stages were studied. Our results show that the differentiation to neural cells in MAO A(neo) ES cells was reduced compared to WT, suggesting MAO A played a regulatory role in stem cells neural differentiation.

Developmental role of tryptophan hydroxylase in the nervous system

The serotonin 5-hydroxytryptamine (5-HT) neurotransmitter system contributes to various physiological and pathological conditions. 5-HT is the first neurotransmitter for which a developmental role was suspected. Tryptophan hydroxylase (TPH) catalyzes the rate-limiting reaction in the biosynthesis of 5-HT. Both TPH1 and TPH2 have tryptophan hydroxylating activity. TPH2 is abundant in the brain, whereas TPH1 is mainly expressed in the pineal gland and the periphery. However, TPH1 was found to be expressed predominantly during the late developmental stage in the brain. Recent advances have shed light on the kinetic properties of each TPH isoform. TPH1 showed greater affinity for tryptophan and stronger enzymic activity than TPH2 under conditions reflecting those in the developing brain stem. Transient alterations in 5-HT homeostasis during development modify the fine wiring of brain connections and cause permanent changes to adult behavior. An increasing body of evidence suggests the involvement of developmental brain disturbances in psychiatric disorders. These findings have revived a long-standing interest in the developmental role of 5-HT-related molecules. This article summarizes our understanding of the kinetics and possible neuronal functions of each TPH during development and in the adult.

Omega-3 fatty acid deficiencies in neurodevelopment, aggression and autonomic dysregulation: opportunities for intervention

Mechanisms by which aggressive and depressive disorders may be exacerbated by nutritional deficiencies in omega-3 fatty acids are considered. Early developmental deficiencies in docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) may lower serotonin levels at critical periods of neurodevelopment and may result in a cascade of suboptimal development of neurotransmitter systems limiting regulation of the limbic system by the frontal cortex. Residual developmental deficits may be manifest as dysregulation of sympathetic responses to stress including decreased heart rate variability and hypertension, which in turn have been linked to behavioral dysregulation. Little direct data are available to disentangle residual neurodevelopmental effects from reversible adult pathologies. Ensuring optimal intakes of omega-3 fatty acids during early development and adulthood shows considerable promise in preventing aggression and hostility.

Signaling pathways involved with serotonin1A agonist-mediated neuroprotection against ethanol-induced apoptosis of fetal rhombencephalic neurons

Previously, this laboratory demonstrated that developing serotonin (5-HT) neurons and other fetal rhombencephalic neurons are reduced by in vivo and in vitro exposure to ethanol, effects that are related to ethanol's augmentation of apoptosis. We also found that 5-HT1A agonists diminished the ethanol-associated reduction of 5-HT neurons and other fetal rhombencephalic neurons by attenuating the pro-apoptotic effects of ethanol. Presently, we investigated the hypothesis that the protective/anti-apoptotic effects of a 5-HT1A agonist on fetal rhombencephalic neurons are mediated by activation of the phosphatidylinositol 3' kinase (PI-3K) and/or the mitogen-activated protein kinase kinase (MAPKK) pathway. Apoptotic and non-apoptotic fetal rhombencephalic neurons were quantitated in primary cultures that were treated with 50 mM ethanol and with 100 nM of a 5-HT1A agonist such as 8-OH-DPAT [8-hydroxy 2-(di-n-propylamino)tetralin], ipsapirone, or buspirone. Analysis of neurons stained with Hoechst 33342 demonstrated the anti-apoptotic effects of 5-HT1A agonists and implicated the involvement of the PI-3K pathway and possibly the MAPKK pathway with the protective effects of these drugs. The protective effects were blocked by a 5-HT1A antagonist (WAY 100635), an inhibitor of PI-3K (LY294002), and an inhibitor of MAPKK (PD98059). Western blot analyses showed that ethanol treatment reduces basal pAkt levels. These analyses also provide support for the involvement of the PI-3K pathway; ipsapirone stimulated the phosphorylation of Akt in control and ethanol-treated neurons, and these effects were antagonized by LY294002.

Developmental expression of 5-HT 5A receptor mRNA in the rat brain

In the central nervous system, serotonin (5-HT) may function as a mitogen as well as a neurotransmitter; and its early appearance suggests a potential role in development. The present experiments were done to determine the localization of the mRNA coding for the 5-HT 5A receptor during development of the rat brain. 5-HT 5A gene transcription was assessed by in situ hybridization, from E18 and during postnatal (PN) development. An intense signal of 5-HT 5A mRNA was found in the cerebral cortex and olfactory nucleus at E18, PN0 and PN5. A sharp decrease at PN11 was followed by an increase until reaching the adult level in the cerebral cortex; whereas in the olfactory nucleus, transcription remained weak. In contrast, in the hippocampal formation the signal was weak in the CA1, CA2 and CA3 regions at E18 and PO; increased at P5 and then decreased at P11 before attaining the adult level. We conclude that the gene coding for the 5-HT 5A receptor is already active in the embryonic rat brain and is differentially expressed during development.

The developmental role of serotonin: news from mouse molecular genetics

New genetic models that target the serotonin system show that transient alterations in serotonin homeostasis cause permanent changes to adult behaviour and modify the fine wiring of brain connections. These findings have revived a long-standing interest in the developmental role of serotonin. Molecular genetic approaches are now showing us that different serotonin receptors, acting at different developmental stages, modulate different developmental processes such as neurogenesis, apoptosis, axon branching and dendritogenesis. Our understanding of the specification of the serotonergic phenotype is improving. In addition, studies have revealed that serotonergic traits are dissociable, as there are populations of neurons that contain serotonin but do not synthesize it.

The effects of serotonin on the differentiation of neurons producing vasoactive intestinal polypeptide in the suprachiasmatic nucleus of the rat

The morphogenetic influences of serotonin on the differentiation of neurons synthesizing vasoactive intestinal polypeptide (VIP) in the suprachiasmatic nucleus were studied in rats. This was addressed by comparative morphofunctional analysis of VIP neurons in adult rats whose brains developed prenatally in conditions of normal and deficient serotonin metabolism. Serotonin deficiency was created in fetuses by treatment of their mothers with p-chlorophenylalanine (PCPA). Pregnant females in controls were treated with 0.9% NaCl. VIP neurons in experimental and control animals were found to show no differences in VIP mRNA concentrations and, probably, in the level of VIP synthesis. However, inhibition of serotonin synthesis led to an increase in the number of VIP-immunoreactive neurons and an increase in the VIP concentration within these cells. This was not associated with any change in neuron size, which was an indicator of the absence of functional hypertrophy accompanying activation of specific synthesis. Comparison of the data obtained here showed that during prenatal ontogenesis, serotonin has an imprinting influence on the differentiation of VIP neurons and is probably involved in the formation of the mechanism of VIP secretion.

A persistent deficit of serotonin neurons in the offspring of ethanol-fed dams: protective effects of maternal ipsapirone treatment

An earlier study from this laboratory found a significant reduction in the density of serotonin (5-HT) neurons in the dorsal and median raphe and in the B9 complex of postnatal day 5 (PN5) offspring of female rats that consumed ethanol on a chronic basis prior to parturition. In addition, we demonstrated that maternal treatment with the 5-HT(1A) agonist ipsapirone (3 mg/kg) prevented the ethanol-associated reduction in 5-HT neurons. The present investigation examined whether there was a persistent deficit of 5-HT-immunopositive neurons in the dorsal and median raphe of the offspring of ethanol-fed dams. We also evaluated whether a lower ipsapirone dose (1 mg/kg) was protective to developing 5-HT neurons in the offspring of ethanol-fed dams. The offspring of ethanol-fed dams exhibited an apparent lasting reduction in the density of 5-HT neurons in the dorsal and median raphe. The density of 5-HT neurons in control offspring was comparable at PN5 and PN19, but at both ages the offspring of ethanol-fed dams had a significant deficit of 5-HT neurons in the dorsal and median raphe. The lack of recovery in the density of 5-HT-immunopositive neurons in the offspring of ethanol-fed dams between PN5 and PN19 suggests and that the reduction was long lasting. The protective effects of ipsapirone appeared to be dose dependent. The density of 5-HT neurons in the dorsal and median raphe of PN5 (prior study) and PN19 offspring of ethanol-fed dams that were treated with 3 mg/kg of ipsapirone between gestational day 13 (G13) and G20 was comparable to that of control offspring. However, the effects of maternal treatment of ethanol-fed dams with the 1 mg/kg dose were variable, and some abnormalities were detected in the offspring of ipsapirone-treated control dams.

Identification of the aromatic L-amino acid decarboxylase (AADC) gene and its expression in the attachment and metamorphosis of the barnacle, Balanus amphitrite

Serotonin and dopamine are involved in the attachment and metamorphosis of cypris larvae of barnacles. Aromatic L-amino acid decarboxylase (AADC) gene, the product of which catalyzes the synthesis of serotonin and dopamine from L-5-hydroxytryptophan and L-3,4-dihydroxyphenylalanine, respectively, was characterized. A DNA clone containing part of an AADC sequence was obtained from the genomic DNA library of the barnacle, Balanus amphitrite. This clone had four putative exons consisting of 226 amino acids with an identity of 63.2% and a similarity of 92.1% with human AADC. Northern blot analysis showed that AADC mRNA was expressed at all stages of barnacles: naupliar larvae, cypris larvae and adult barnacles. Two inducers of larval attachment and metamorphosis; that is, serotonin and extract of adult barnacles, obviously increased the expression of AADC mRNA at an early cypris larval stage. These results suggest that intracellular biosynthesis of serotonin, or dopamine, or both is at least partly involved in the control of the attachment and metamorphosis of cypris larvae.

Alcohol deters the outgrowth of serotonergic neurons at midgestation

We have previously demonstrated that treatment of pregnant C57BL mice from gestation days 8 to 14 with alcohol with 20% ethanol-derived calories (EDC) reduced the number of serotonin (5-HT) neurons and retarded their migration in the fetal brains. In the present study, we obtained similar results with the use of 25% EDC and extended our previous findings by demonstrating that besides the alteration of the number of 5-HT neurons, prenatal alcohol exposure also affects their projecting fibers in their early development. Pregnant C57BL mice were divided into an alcohol-exposed (ALC) group given 25% EDC (4.49%, v/v), a pair-fed group to the ethanol-fed group (PF) and a chow-fed group (Chow). The PF and Chow groups served as controls. Our results showed that in the ALC group, when compared with the control groups, prenatal alcohol exposure with 25% EDC reduced the number of 5-HT-immunoreactive neurons in both the median and dorsal raphe, and the amount of 5-HT-immunoreactive fibers in the medial forebrain bundle (MFB). The diameter of the 5-HT-immunoreactive MFB was also reduced as a result of treatment. No significant differences of the above parameters were found between the PF and Chow groups. The previous and present work confirmed that alcohol reduces the normal formation and growth of 5-HT neurons in the midbrain. Furthermore, the projection of 5-HT fibers, in density as well as in distribution, is reduced in the major trajectory bundle. This may affect the amount of 5-HT fibers available to the forebrain. In light of the importance of the 5-HT system in brain development, alcohol may affect the growth of the forebrain through its effect on 5-HT signaling.

Effects of in utero ethanol exposure and maternal treatment with a 5-HT(1A) agonist on S100B-containing glial cells

This laboratory previously showed that in utero ethanol exposure severely impairs the development of the cell bodies and projections of serotonin (5-HT) neurons, and that maternal treatment with a 5-HT(1A) agonist prevents many of these abnormalities. Others demonstrated that stimulation of fetal astroglial 5-HT(1A) receptors increases production and release of S100B, a glial trophic factor that is essential for the development of 5-HT neurons. The present study investigated a potential mechanism by which ethanol hinders development of 5-HT neurons, and by which maternal 5-HT(1A) agonist treatment prevents this damage. This study tested the hypothesis that in utero ethanol exposure reduces the number of S100B immunopositive glia and that maternal 5-HT(1A) agonist treatment prevents ethanol-associated changes in S100B. To test our hypothesis, we determined the effects of in utero ethanol exposure and maternal treatments with the 5-HT(1A) agonists ipsapirone and buspirone on S100B immunopositive glial cells. On gestation day 20 (G20), S100B immunopositive cells were quantified in the midline raphe glial structure (MRGS), a large transient structure that contains substantial numbers of S100B-positive glial cells and that spans the dorsal raphe, median raphe, and B9 complex of 5-HT neurons. S100B immunopositive glial cells were also determined in an area proximal to the dorsal raphe in postnatal day 2 (PN2) rats. In utero ethanol exposure significantly reduced S100B immunopositive glial cells in the MRGS at G20 and in the dorsal raphe at PN2. In addition, treatment of pregnant rats with a 5-HT(1A) agonist between G13 and G20 prevented the ethanol-associated reduction in S100B immunopositive glial cells. These studies demonstrated that part of ethanol's damaging effects on developing 5-HT neurons is mediated by a reduction of S100B and that some of the protective effects of maternal 5-HT(1A) agonist treatment are related to the actions of these drugs on glial cells.

Hippocampal cholinergic neurostimulating peptides (HCNP)

Neuronal development and differentiation require a variety of cell interactions. Diffusible molecules from target neurons play an important part in mediating such interactions. Our early studies used explant culture technique to examine the factors that enhance the differentiation of septo-hippocampal cholinergic neurons, and they revealed that several components resident in the hippocampus are involved in the differentiation of presynaptic cholinergic neurons in the medial septal nucleus. One of these components, originally purified from young rat hippocampus, is a novel undecapeptide (hippocampal cholinergic neurostimulating peptide; HCNP); this enhances the production of ChAT, but not of AchE. Later experiments revealed that: (1) a specific receptor appears to mediate this effect; (2) NGF and HCNP act cooperatively to regulate cholinergic phenotype development in the medial septal nucleus in culture; and (3) these two molecules differ both in their mechanism of release from the hippocampus and their mechanism of action on cholinergic neurons. The amino acid sequence deduced from base sequence analysis of cloned HCNP-precursor protein cDNA shows that HCNP is located at the N-terminal domain of its precursor protein. The 21 kDa HCNP precursor protein shows homology with other proteins, and it functions not only as an HCNP precursor, but also as a binding protein for ATP, opioids and phosphatidylethanolamine. The distribution and localization of HCNP-related components and the expression of their mRNAs support the notion that the precursor protein is multifunctional. In keeping with its multiple functions, the multiple enhancers and promoters found in the genomic DNA for HCNP precursor protein may be involved in the regulation of its gene in a variety of cells and at different stages of development. Furthermore, several lines of evidence obtained from studies of humans and animal models suggest that certain types of memory and learning disorders are associated with abnormal accumulation and expression of HCNP analogue peptide and/or its precursor protein mRNA in the hippocampus.

In utero ethanol exposure decreased the density of serotonin neurons. Maternal ipsapirone treatment exerted a protective effect

Prior studies from this laboratory showed that in utero ethanol exposure severely retards the development of the serotonin (5-HT) system; we demonstrated a reduced concentration of 5-HT and 5-HT reuptake sites and alterations in the concentration of 5-HT(1A) receptors in ethanol-exposed offspring. These investigations also found that maternal treatment with a 5-HT(1A) agonist, buspirone, prevented most of the ethanol-associated damage to the developing 5-HT system. In the present investigation, we investigated whether the ethanol-associated changes in the 5-HT system are due to a reduction of 5-HT neurons and whether any changes in the density of 5-HT neurons can be prevented by maternal treatment with another 5-HT(1A) agonist, ipsapirone. Using immunocytochemistry, we found that in utero ethanol exposure reduced the density of 5-HT immunopositive neurons in the dorsal raphe, median raphe and B9 neurons of postnatal day 5 (PN5) rats. In all three brain areas, the offspring of ethanol-fed, saline-treated dams exhibited a 28%-40% reduction in 5-HT neurons. Ipsapirone prevented the ethanol-induced reduction in 5-HT immunopositive neurons in the dorsal raphe, median raphe and B9 neurons. In the dorsal and median raphe of control offspring, ipsapirone did not alter the concentration of 5-HT neurons. However, this drug did reduce 5-HT neurons in the B9 region of the offspring of control-fed rats.

Vesicular amine transporter expression and isoform selection in developing brain, peripheral nervous system and gut

The vesicular monoamine transporters VMAT1 and VMAT2 are essential components of monoaminergic neurons and endocrine cells whose expression in development may provide insight into lineage pathways for chemical coding in the diffuse neuroendocrine system. Thus, the brain is a compartment in which only monoaminergic neurons are generated, the gut epithelium generates only endocrine monoamine-containing cells, and the neural crest produces both autonomic monoaminergic neurons and endocrine/paracrine monoaminergic cells. Selection of either the VMAT1 or VMAT2 isoform was examined in these three compartments during development. In the central nervous system VMAT2, but not VMAT1, was expressed in neuroepithelial cells by embryonic day 12 (E12), and all major monoaminergic cell groups by E14. Thalamocortical and hypothalamic neurons that do not express VMAT2 in adulthood were transiently VMAT2-positive from E16 to postnatal day 6 (P6). EC cells of the gut expressed exclusively VMAT1 from E19 on, while histamine-containing enterochromaffin-like (ECL) cells of the stomach expressed only VMAT2 by E19 and throughout postnatal development. VMAT2 and the vesicular acetylcholine transporter VAChT were co-expressed in early development of the primary sympathetic chain as well as in the cranial parasympathetic ganglia. VAChT was progressively restricted to a small population of VMAT2-negative post-ganglionic neurons in the adult sympathetic chain, while VMAT2 expression persisted in sympathetic principal ganglion and SIF cells but was eventually extinguished in cranial parasympathetic ganglia. VMAT1 was co-expressed with VAChT and VMAT2 mRNA in the primary sympathetic chain on E12, but progressively restricted to small intensely fluorescent (SIF) and chromaffin cells thereafter. Thus, expression of the vesicular amine transporters appropriate for chemical coding of brain neurons and gut endocrine cells are pre-determined developmentally. In contrast, the neural crest-derived sympathoadrenal and neural crest-derived parasympathetic cell groups examined here initially co-express two or more vesicular amine transporters, followed by extinction of the inappropriate transporter(s) later in development. Some neural crest-derived neuroendocrine cell populations continue to express both isoforms of VMAT even in adulthood. Lineage distinctions in ontogeny of vesicular amine transporter expression in brain, gut and autonomic nervous system make it likely that the same genes are regulated differently in the autonomic nervous system compared to brain and gut.

Tryptophan hydroxylase mRNAs analysis by RT-PCR: preliminary report on the effect of noradrenaline in the neonatal rat pineal gland

The levels of mRNAs coding for tryptophan hydroxylase (TPOH), the first enzyme in melatonin synthesis, have been investigated by quantitative reverse transcription of RNA, followed by polymerase chain reaction (RT-PCR), after stimulation of neonatal pineal organ cultures with Noradrenaline (NA). TPOH mRNAs were specifically amplified from various adult tissues, namely the pineal gland, raphe, retina, and kidney, but not the lung. PCR signals for TPOH were detected in the neonatal pineal gland in the absence of stimulation. Stimulation of neonatal pineal organ culture with 0.1 microM NA resulted in a significant increase (x2.5) in expression of TPOH mRNAs, whereas higher doses (1 and 10 microM) had no effect. All concentrations of NA enhanced melatonin secretion. Our results suggest that the level of TPOH mRNAs can be controlled by NA and that this effect might be implicated in the gene level regulation of the daily enzyme rhythm in the rat pineal gland.

Cloning and developmental expression of 5-HT1A receptor gene in Xenopus laevis

The aim of our work is to investigate the potential involvement of serotonin and its G-protein-coupled receptors in neural differentiation or other developmental processes in Xenopus laevis. By using a RT-PCR strategy, we isolated a cDNA fragment from X. laevis brain showing high amino-acid similarity with the mammalian 5-HT1A receptor. We used this fragment to isolate a cDNA clone containing a single ORF of 408 amino-acids with an overall amino-acid identity of 73% with the human and rat 5-HT1A receptor. This structural similarity suggests that this clone encodes the Xenopus homolog of the mammalian 5-HT1A receptor (X5-HT1A). In order to establish a possible role for this receptor in development, we analyzed the pattern of its gene expression during embryogenesis, larval stages and in adult brain by in situ hybridization. The first signal of mRNA expression appears in the rostral part of brain stem at stage 22, when the first neurons start differentiation [38,21]. In later stages of development, the cells expressing X5-HT1A transcripts appear to correspond to serotonergic neurons. By stage 41, X5-HT1A mRNA is also detected in the inner nuclear layer (INL) of the developing retina. This pattern of expression is maintained until stage 46, i.e. at the beginning of metamorphosis. In adult, additional brain areas express X5-HT1A mRNA, particularly in telencephalon, diencephalon and mesencephalon. On the whole, our data show that the X5-HT1A receptor mRNA is developmentally regulated, with expression first appearing in differentiating serotonergic neurons, where this receptor may mediate, through an autocrine regulatory pathway, the trophic action of serotonin on developing serotonergic system.

Expression of 5-HT7 receptor mRNA in rat brain during postnatal development

The present study is the first one to demonstrate the expression of 5-HT7 receptor mRNA by in situ hybridization during postnatal development. No quantitative developmental changes in the 5-HT7 gene expression was observed in neocortex, pyramidal layers of CA1 and CA2, dentate gyrus, most of thalamic nuclei, mammillary region, superior colliculus and central gray. However, in retrosplenial cortex, subiculum and medial habenula an increase of labeling is observed between postnatal days (PN) PN15 and PN21. Striatum showed a transient expression during the first stages of development to be undetectable in adults. CA3 pyramidal cell layer, intramediodorsal thalamic nucleus and lateral habenula displayed a high mRNA expression at PN5 and PN8 which decreased throughout development but it was still present in adults. A possible non-neurotransmitter trophic function of 5-HT mediated through 5-HT7 receptors could be suggested.

Effects of maternal ethanol consumption and buspirone treatment on dopamine and norepinephrine reuptake sites and D1 receptors in offspring

Previously, it was shown that in utero ethanol exposure results in decreased serotonin (5-HT) and altered concentrations of 5-HT reuptake sites and 5-HT1A receptors in fetal and/or postnatal rats. Because fetal 5-HT is an essential trophic factor, this laboratory previously investigated the hypotheses that the early ethanol-associated 5-HT deficit contributed to subsequent development abnormalities in the serotonergic system and that the effects of the fetal 5-HT deficit could be prevented by maternal treatment with buspirone, a 5-HT1A receptor agonist. The present report determined the effects of maternal treatment with buspirone on two other neurotransmitter systems in the developing offspring of ethanol-fed dams: dopamine (DA) and norepinephrine reuptake sites and D1 receptors in postnatal day 19 offspring of control and ethanol-fed dams, that received daily injections of saline or 4.5 mg/kg buspirone. These investigations found that in utero ethanol exposure significantly decreased norepinephrine reuptake sites in the dorsomedial hypothalamic nucleus and anteroventral thalamic nucleus. There was also an ethanol effect in the dorsal raphe. D1 receptors were moderately increased (5-10% increase) in the striatum, and DA reuptake sites were unchanged in PN19 ethanol-exposed offspring. No other significant ethanol-related effects were noted. Maternal buspirone treatment did not adversely affect the concentration of DA reuptake sites or D1 receptors in control rats. Thus, whereas buspirone exerts protective effects on the developing 5-HT system of ethanol-exposed rats, it does not appear to damage the development of the DA system. Maternal buspirone produced only one significant abnormality in control offspring; it resulted in significant reduction of norepinephrine reuptake sites in the DR.

From oocyte to neuron: do neurotransmitters function in the same way throughout development?

1. Classical neurotransmitters (such as acetylcholine, biogenic amines, and GABA) are functionally active throughout ontogenesis. 2. Based on accumulated evidence, reviewed herein, we present an hypothetical scheme describing developmental changes in this functional activity, from the stage of maturing oocytes through neuronal differentiation. This scheme reflects not only the spatio-temporal sequence of these changes, but also the genesis of neurotransmitter functions, from "protosynapses" in oocytes and cleaving embryos to the development of functional neuronal synapses. 3. Thus, it appears that neurotransmitters participate in various forms of intra- and intercellular signalling throughout all stages of ontogenesis.

The neuroepithelial cells of the fish gill filament: indolamine-immunocytochemistry and innervation

The neuroepithelial cells (NECs) of the fish gill filament share several morphofunctional features with the cells of the neuroepithelial bodies in the lungs of air-breathing vertebrates. In the present study, a detailed indolamine-immunocytochemical analysis of the branchial neuroepithelial cells and nerves was undertaken in non-teleost and teleost species, with particular emphasis on the latter. In the rainbow trout, Oncorhynchus mykiss, the chemical degeneration of either catecholaminergic (by 5- and 6-hydroxydopamines) or indolaminergic (by 5,6-dihydroxy-tryptamine) innervations associated with the NECs was studied using electron microscopy. In teleosts, the NECs are located primarily on the distal half of the filament. In the trout particularly, these cells are innervated mainly by non-indolaminergic nerves taking up sympathetic neurotoxins. The proximal half of the filament contains isolated NECs innervated additionally by intrinsic indolaminergic neurons. Serotonin-like immunoreactivity of the NECs is evident in the granular vesicles packed within the basal soma and processes which surround non-vascular and vascular smooth muscles in the filament. Apical processes from the neuroepithelial cells occasionally contact the water on the surface of the filament epithelium. The secretory function of the NECs is discussed with reference to the probable involvement of serotonin in the modulation of fish gill function. In addition, their connections with both central and branchial nervous systems suggest a possible chemoreceptor role.

Population-specific modulation of 5-HT expression in cultures of embryonic rat rhombencephalon

This study aimed at analyzing the regulation of in vitro serotonin expression by neurons taken from different regions of the embryonic rat rhombencephalon. We studied the influence of co-culture with alarplate tissue using immunocytochemical and biochemical methods. Computer-assisted densitometry was used to estimate the co-culture effects on the serotonin content of the cell bodies. The more dynamic aspects of serotonin expression, such as synthesis and release, were studied by measuring (3H)serotonin newly synthesized from (3H)tryptophan. The density of the immunostaining was significantly decreased in B1,B2 cells by co-culture with both caudal and rostral alar-plate tissue. For B4-B9 cells, only co-culture with rostral alar-plate tissue produced a significant decrease. The de novo synthesis of serotonin was significantly decreased in B1,B2 neurons co-cultured with caudal alar-plate tissue only. Once again, the B4-B9 cells proved to be less influenced by the experimental conditions, as co-culture with both types of alar-plate tissue produced no significant effect. We concluded that the in vitro expression of serotonin can be modulated by environmental factors, but the relative influence of these factors is very different in rostral versus caudal serotonin expressing cell populations.

Activity of hippocampal extract on development of [3H]5-HT high-affinity uptake in dissociated microcultures

Specific and localized lesions of the 5-HT fibers in the hippocampus induce homotypic collateral sprouting and enhance serotonergic fiber outgrowth from adult neurons and transplanted fetal tissue. In this study, hippocampal extracts were prepared and applied to primary cultures of fetal serotonergic neurons. The effects of plating density and serum additives were examined. The growth of the serotonergic neurons in the rostral brainstem dissociated cultures were estimated by measuring the specific uptake of [3H]5-HT. The results indicate the presence of a trypsin-sensitive factor which is active when prepared fresh at dilutions up to 1/10,000. The factor is higher in hippocampus than cerebellum. Young male tissue contained more activity than either female or aged hippocampus. Although both positive and negative effects are described, higher dilutions of factor (1/1,000) were generally stimulatory in high density cultures while lower dilutions (1/10) were inhibitory in low density cultures. Specific removal of 5-HT hippocampal afferents with fornix-fimbria microinjections of 5,7-dihydroxytryptamine resulted in an initial loss of activity (2 days and 2 weeks) followed by an enhanced activity (2 months) compared to normal hippocampal extract. Several possibilities are discussed as to the identity of the serotonergic growth factor from hippocampal supernatant.

Vasoactive intestinal peptide regulates mitosis, differentiation and survival of cultured sympathetic neuroblasts

Although acute, millisecond-to-millisecond actions of neurotransmitters are well documented, diverse longer-term effects have been discovered only recently. Emerging evidence indicates that these signals regulate a variety of neuronal processes, from phenotypic expression to neurite outgrowth. Here we show that a single putative transmitter, vasoactive intestinal peptide, can exert multiple, long-term effects simultaneously: it stimulates mitosis, promotes neurite outgrowth and enhances survival of sympathetic neuron precursors in culture. As the peptide seems to be a normal presynaptic transmitter in the sympathetic system, synaptic transmission may exert hitherto unexpected effects.

Chromosomal localization of the gene for the human trifunctional enzyme, methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase-formyltetrahydrofolate synthetase

A trifunctional protein in man, 5,10-methylenetetrahydrofolate dehydrogenase-5,10-methenyltetrahydrofolate cyclohydrolase-10-formyltetrahydrofolate synthetase, catalyzes three consecutive steps in the interconversion of tetrahydrofolate derivatives; these derivatives supply one-carbon units for intermediary metabolism. Somatic cell hybridization and in situ hybridization were used to localize the functional gene coding for this protein--to human chromosome 14q24, near the c-fos and TGF-beta 3 loci. A second hybridizing sequence, possibly a pseudogene, was identified near the centromere of the X chromosome, at Xp11.

Early ontogeny of serotonin-immunoreactivity in the sheep brain. An immunohistochemical study

Using immunohistochemistry with specific antiserotonin anti-sera, the ontogeny of serotonergic neurons was studied in the foetal sheep brain. Serotonergic-immunoreactive perikarya first appeared rostrally on day 25 of pregnancy, in the medio-ventral part of the mesencephalic flexure, and caudally, on day 28, in the medio-ventral part of the cervical flexure. The development of this system is very rapid, because on day 40 of gestation, all serotonergic nuclei present in the adult were visible. Compared with other species such as rodents or primates, serotonin appears early in the sheep nervous system, and the development of the serotonergic system is even more rapid. Serotonergic immunoreactivity was seen in some cell bodies in the growing adenohypophysis between days 40 and 50. This phenomenon has not been observed in other species. Because serotonin appears very early and is present in growing areas of the nervous system, it could play a trophic role in the development and maturation of the sheep central nervous system, as has been described previously in other species.

Serotonin-containing neurons in Drosophila melanogaster: development and distribution

Antibodies made against serotonin (5HT) were used to identify the serotonin neuronal system in the developing and adult nervous system of Drosophila melanogaster. The 5HT neuronal pattern is composed of a small number of neurons, 84 in larvae and 106 in adults, distributed in clusters composed of one to five neurons in the CNS; 5HT immunoreactive (5HT-IR) neurons appear to be predominantly intrasegmental interneurons; however, intersegmental 5HT-IR fibers are observed and at least some neurons send peripheral fibers. Acquisition of 5HT immunoreactivity in the CNS occurs late in embryogenesis, by 16-18 hours, and most if not all the 5HT neurons appear to persist into adulthood. During early metamorphosis, the intensity of 5HT-IR neuropil transiently decreases. Other changes in the CNS during this period are reflected in the appearance of two new 5HT clusters and 5HT-IR neuropil in the developing optic lobes. Comparison of the 5HT-IR pattern with other transmitter systems in Drosophila as well as comparison of the 5HT-IR pattern within different insect species is presented.