Serotonergic Neuronal Function Research Articles

Mouse enteric neurons control intestinal plasmacytoid dendritic cell function via serotonin-HTR7 signaling

Serotonergic neurons in the central nervous system control behavior and mood, but knowledge of the roles of serotonergic circuits in the regulation of immune homeostasis is limited. Here, we employ mouse genetics to investigate the functions of enteric serotonergic neurons in the control of immune responses and find that these circuits regulate IgA induction and boost host defense against oral, but not systemic Salmonella Typhimurium infection. Enteric serotonergic neurons promote gut-homing, retention and activation of intestinal plasmacytoid dendritic cells (pDC). Mechanistically, this neuro-immune crosstalk is achieved through a serotonin-5-HT receptor 7 (HTR7) signaling axis that ultimately facilitates the pDC-mediated differentiation of IgA+ B cells from IgD+ precursors in the gut. Single-cell RNA-seq data further reveal novel patterns of bidirectional communication between specific subsets of enteric neurons and lamina propria DC. Our findings thus reveal a close interplay between enteric serotonergic neurons and gut immune homeostasis that enhances mucosal defense.

Peripheral serotonergic neurons regulate gut motility and anxiety-like behavior

Serotonergic circuits in the central nervous system play important roles in regulating mood and behavior, yet the functions of peripheral serotonergic neurons are less understood. Here, we engineered mice lacking the serotonin-producing enzyme Tph2 in peripheral neurons but with intact Tph2 in central neurons. In contrast to mice lacking Tph2 in all neurons, mice lacking Tph2 in peripheral serotonergic neurons did not exhibit increased territorial aggression. However, similar to the total body Tph2 knockout (KO) mice, the conditional KO animals exhibited reduced gut motility and decreased anxiety-like behavior. These observations reveal that peripheral serotonergic neurons contribute to control of intestinal motility and anxiety-like behavior and suggest that therapeutics targeting this subset of peripheral neurons could be beneficial.

Seizures Cause Prolonged Impairment of Ventilation, CO2 Chemoreception and Thermoregulation.

Sudden unexpected death in epilepsy (SUDEP) has been linked to respiratory dysfunction, but the mechanisms underlying this association remain unclear. Here we found that both focal and generalized convulsive seizures (GCSs) in epilepsy patients caused a prolonged decrease in the hypercapnic ventilatory response (HCVR; a measure of respiratory CO2 chemoreception). We then studied Scn1a R1407X/+ (Dravet syndrome; DS) and Scn8a N1768D/+ (D/+) mice of both sexes, two models of SUDEP, and found that convulsive seizures caused a postictal decrease in ventilation and severely depressed the HCVR in a subset of animals. Those mice with severe postictal depression of the HCVR also exhibited transient postictal hypothermia. A combination of blunted HCVR and abnormal thermoregulation is known to occur with dysfunction of the serotonin (5-hydroxytryptamine; 5-HT) system in mice. Depleting 5-HT with para-chlorophenylalanine (PCPA) mimicked seizure-induced hypoventilation, partially occluded the postictal decrease in the HCVR, exacerbated hypothermia, and increased postictal mortality in DS mice. Conversely, pretreatment with the 5-HT agonist fenfluramine reduced postictal inhibition of the HCVR and hypothermia. These results are consistent with the previous observation that seizures cause transient impairment of serotonergic neuron function, which would be expected to inhibit the many aspects of respiratory control dependent on 5-HT, including baseline ventilation and the HCVR. These results provide a scientific rationale to investigate the interictal and/or postictal HCVR as noninvasive biomarkers for those at high risk of seizure-induced death, and to prevent SUDEP by enhancing postictal 5-HT tone.SIGNIFICANCE STATEMENT There is increasing evidence that seizure-induced respiratory dysfunction contributes to the pathophysiology of sudden unexpected death in epilepsy (SUDEP). However, the cellular basis of this dysfunction has not been defined. Here, we show that seizures impair CO2 chemoreception in some epilepsy patients. In two mouse models of SUDEP we found that generalized convulsive seizures impaired CO2 chemoreception, and induced hypothermia, two effects reported with serotonergic neuron dysfunction. The defects in chemoreception and thermoregulation were exacerbated by chemical depletion of serotonin and reduced with fenfluramine, suggesting that seizure-induced respiratory dysfunction may be due to impairment of serotonin neuron function. These findings suggest that impaired chemoreception because of transient inhibition of serotonergic neurons may contribute to the pathophysiology of SUDEP.

Median raphe serotonergic neurons projecting to the interpeduncular nucleus control preference and aversion

Appropriate processing of reward and aversive information is essential for survival. Although a critical role of serotonergic neurons in the dorsal raphe nucleus (DRN) in reward processing has been shown, the lack of rewarding effects with selective serotonin reuptake inhibitors (SSRIs) implies the presence of a discrete serotonergic system playing an opposite role to the DRN in the processing of reward and aversive stimuli. Here, we demonstrated that serotonergic neurons in the median raphe nucleus (MRN) of mice process reward and aversive information in opposite directions to DRN serotonergic neurons. We further identified MRN serotonergic neurons, including those projecting to the interpeduncular nucleus (5-HTMRN→IPN), as a key mediator of reward and aversive stimuli. Moreover, 5-HT receptors, including 5-HT2A receptors in the interpeduncular nucleus, are involved in the aversive properties of MRN serotonergic neural activity. Our findings revealed an essential function of MRN serotonergic neurons, including 5-HTMRN→IPN, in the processing of reward and aversive stimuli.

Brain-derived neurotrophic factor expression in serotonergic neurons improves stress resilience and promotes adult hippocampal neurogenesis.

The neurotrophin brain-derived neurotrophic factor (BDNF) stimulates adult neurogenesis, but also influences structural plasticity and function of serotonergic neurons. Both, BDNF/TrkB signaling and the serotonergic system modulate behavioral responses to stress and can lead to pathological states when dysregulated. The two systems have been shown to mediate the therapeutic effect of antidepressant drugs and to regulate hippocampal neurogenesis. To elucidate the interplay of both systems at cellular and behavioral levels, we generated a transgenic mouse line that overexpresses BDNF in serotonergic neurons in an inducible manner. Besides displaying enhanced hippocampus-dependent contextual learning, transgenic mice were less affected by chronic social defeat stress (CSDS) compared to wild-type animals. In parallel, we observed enhanced serotonergic axonal sprouting in the dentate gyrus and increased neural stem/progenitor cell proliferation, which was uniformly distributed along the dorsoventral axis of the hippocampus. In the forced swim test, BDNF-overexpressing mice behaved similarly as wild-type mice treated with the antidepressant fluoxetine. Our data suggest that BDNF released from serotonergic projections exerts this effect partly by enhancing adult neurogenesis. Furthermore, independently of the genotype, enhanced neurogenesis positively correlated with the social interaction time after the CSDS, a measure for stress resilience.

PRC2-mediated repression is essential to maintain identity and function of differentiated dopaminergic and serotonergic neurons.

How neurons can maintain cellular identity over an entire life span remains largely unknown. Here, we show that maintenance of identity in differentiated dopaminergic and serotonergic neurons is critically reliant on the Polycomb repressive complex 2 (PRC2). Deletion of the obligate PRC2 component, Eed, in these neurons resulted in global loss of H3K27me3, followed by a gradual activation of genes harboring both H3K27me3 and H3K9me3 modifications. Notably, H3K9me3 was lost at these PRC2 targets before gene activation. Neuronal survival was not compromised; instead, there was a reduction in subtype-specific gene expression and a progressive impairment of dopaminergic and serotonergic neuronal function, leading to behavioral deficits characteristic of Parkinson’s disease and anxiety. Single-cell analysis revealed subtype-specific vulnerability to loss of PRC2 repression in dopamine neurons of the substantia nigra. Our study reveals that a PRC2-dependent nonpermissive chromatin state is essential to maintain the subtype identity and function of dopaminergic and serotonergic neurons.

Dietary yeast influences ethanol sedation in Drosophila via serotonergic neuron function.

Abuse of alcohol is a major clinical problem with far-reaching health consequences. Understanding the environmental and genetic factors that contribute to alcohol-related behaviors is a potential gateway for developing novel therapeutic approaches for patients that abuse the drug. To this end, we have used Drosophila melanogaster as a model to investigate the effect of diet, an environmental factor, on ethanol sedation. Providing flies with diets high in yeast, a routinely used component of fly media, increased their resistance to ethanol sedation. The yeast-induced resistance to ethanol sedation occurred in several different genetic backgrounds, was observed in males and females, was elicited by yeast from different sources, was readily reversible, and was associated with increased nutrient intake as well as decreased internal ethanol levels. Inhibition of serotonergic neuron function using multiple independent genetic manipulations blocked the effect of yeast supplementation on ethanol sedation, nutrient intake, and internal ethanol levels. Our results demonstrate that yeast is a critical dietary component that influences ethanol sedation in flies and that serotonergic signaling is required for the effect of dietary yeast on nutrient intake, ethanol uptake/elimination, and ethanol sedation. Our studies establish the fly as a model for diet-induced changes in ethanol sedation and raise the possibility that serotonin might mediate the effect of diet on alcohol-related behavior in other species.

Selective vulnerability of dorsal raphe-medial prefrontal cortex projection neurons to corticosterone-induced hypofunction.

Glucocorticoid hormones and serotonin (5-HT) are strongly associated with the development and treatment of depression, respectively. Glucocorticoids regulate the function of serotonergic neurons in the dorsal raphe nucleus (DR), which are the major source of 5-HT to the forebrain. DR 5-HT neurons are electrophysiologically heterogeneous, though whether this phenotypic variation aligns with specific brain functions or neuropsychiatric disease states is largely unknown. The goal of this work was to determine if chronic exogenous glucocorticoid administration differentially affects the electrophysiological profile of DR neurons implicated in the regulation of emotion versus visual sensation by comparing properties of cells projecting to medial prefrontal cortex (mPFC) versus lateral geniculate nucleus (LGN). Following retrograde tracer injection into mPFC or LGN, male Sprague-Dawley rats received daily injections of corticosterone (CORT) for 21days, after which whole-cell patch clamp recordings were made from retrogradely labeled DR neurons. CORT-treatment significantly increased the action potential half-width of LGN-projecting DR neurons, but did not significantly affect the firing frequency or excitatory postsynaptic currents of these cells. CORT-treatment significantly reduced the input resistance, evoked firing frequency, and spontaneous excitatory postsynaptic current frequency of mPFC-projecting DR neurons, indicating a concurrent reduction of both intrinsic excitability and excitatory drive. Our results suggest that the serotonergic regulation of cognitive and emotional networks in the mPFC may be more sensitive to the effects of glucocorticoid excess than visual sensory circuits in the LGN and that reduced 5-HT transmission in the mPFC may underlie the association between glucocorticoid excess and depression.

Disconnect Between Brainstem Serotonin Neurons And Prefrontal Cortex Serotonin Receptors In Suicide

Serotonin neurotransmission is widely reported as reduced in suicide attempt and completion. Evidence suggests reduced serotonin innervation of the prefrontal cortex and homeostatic upregulation of postsynaptic 5-HT1A and perhaps 5-HT2A receptors in suicide. However, in the brainstem, we have previously found more tryptophan hydroxylase serotonin biosynthetic enzyme and more serotonin in suicide decedents suggesting serotonergic hyperfunction, but also more autoreceptor binding that could result in reduced neuronal firing. We sought evidence of a disconnect between the brainstem and prefrontal cortex examining the serotonin transporter (SERT) and 5-HT1A receptor in the dorsal and median raphe nucleus and SERT, 5-HT1A and 5-HT2A receptor binding in prefrontal cortex and in anterior cingulate cortex postmortem. Suicide decedents (n=11) and controls (n=18) died suddenly minimizing agonal effects and had a postmortem interval ≤ 24 hour. Autoradiography was performed in right hemisphere coronal sections at a pre-genual level and in transverse sections through the brainstem. In controls, there were correlations between DRN and MRN SERT and 5-HT2A receptor binding throughout prefrontal cortex, and between DRN and MRN 5-HT1A receptor binding and medial and ventral prefrontal cortex. In suicide decedents there were no such relationships. The absence of correlations between brainstem source serotonergic neuron function and receptors on target neurons in prefrontal cortex in suicides suggests a disconnect that may contribute to suicide neuropathology.

Neurochemical and Neurotoxic Effects of MDMA (Ecstasy) and Caffeine After Chronic Combined Administration in Mice

MDMA (3,4-methylenedioxymethamphetamine) is a psychostimulant popular as a recreational drug because of its effect on mood and social interactions. MDMA acts at dopamine (DA) transporter (DAT) and serotonin (5-HT) transporter (SERT) and is known to induce damage of dopamine and serotonin neurons. MDMA is often ingested with caffeine. Caffeine as a non-selective adenosine A1/A2A receptor antagonist affects dopaminergic and serotonergic transmissions. The aim of the present study was to determine the changes in DA and 5-HT release in the mouse striatum induced by MDMA and caffeine after their chronic administration. To find out whether caffeine aggravates MDMA neurotoxicity, the content of DA and 5-HT, density of brain DAT and SERT, and oxidative damage of nuclear DNA were determined. Furthermore, the effect of caffeine on MDMA-induced changes in striatal dynorphin and enkephalin and on behavior was assessed. The DA and 5-HT release was determined with in vivo microdialysis, and the monoamine contents were measured by HPLC with electrochemical detection. DNA damage was assayed with the alkaline comet assay. DAT and SERT densities were determined by immunohistochemistry, while prodynorphin (PDYN) and proenkephalin were determined by quantitative PCR reactions. The behavioral changes were measured by the open-field (OF) test and novel object recognition (NOR) test. Caffeine potentiated MDMA-induced DA release while inhibiting 5-HT release in the mouse striatum. Caffeine also exacerbated the oxidative damage of nuclear DNA induced by MDMA but diminished DAT decrease in the striatum and worsened a decrease in SERT density produced by MDMA in the frontal cortex. Neither the striatal PDYN expression, increased by MDMA, nor exploratory and locomotor activities of mice, decreased by MDMA, were affected by caffeine. The exploration of novel object in the NOR test was diminished by MDMA and caffeine. Our data provide evidence that long-term caffeine administration has a powerful influence on functions of dopaminergic and serotonergic neurons in the mouse brain and on neurotoxic effects evoked by MDMA.

Interaction between estradiol and 5-HT1A receptors in the median raphe nucleus on acquisition of aversive information and association to the context in ovariectomized rats

The median raphe nucleus (MRN) is related to stress resistance and defensive responses, a crucial source of serotonergic neurons that project to prosencephalic structures related to stress and anxiety. Estrogen receptors were identified in this mesencephalic structure. It is possible that the estrogen action is related to serotonin effect on somatodendritic 5-HT1A receptors, inhibiting the function of serotonergic neurons and thus preventing of the stress effect and inducing anxiolysis. So, in order to evaluate these aspects, female Wistar rats were ovariectomized and 21 days later were given a direct microinjection of estradiol benzoate (EB) (1200 ng) into the MRN, preceded by microinjections of saline or WAY100.635 (100 ng), a 5-HT1A receptor antagonist. Immediately after the two microinjections, the ovariectomized rats were conditioned with an aversive event (foot shock) session in a Skinner box. Twenty-four hours later, they were exposed to the same context in a test session for 5 min for behavioral assessment: freezing, rearing, locomotion, grooming, and autonomic responses (fecal boluses and micturition). EB microinjection in the MRN prior to the exposure of animals to the foot shocks in the conditioning session did not alter their behavior in this session, but neutralized the association of the aversive experience to the context: there was a decrease in the expression of freezing and an increased rearing activity in the test session. This effect was reversed by prior microinjection of WAY100.635. In conclusion, EB acted on serotonergic neurons in the MRN of the ovariectomized rats, impairing the association of the aversive experience to the context, by co-modulating the functionality of somatodendritic 5-HT1A.

Prenatal and lactational bisphenol A exposure does not alter serotonergic neurons morphologically in the murine dorsal raphe nucleus

ObjectiveThere is concern that bisphenol A (BPA), an endocrine-disrupting chemical, affects brain development when exposed to a fetus and/or infant. We previously reported that increased serotonin (5-HT) and its metabolite (5-HIAA) in the dorsal raphe nucleus (DRN) in murine adult brains when they were prenatally exposed to low doses of BPA. This study investigates the morphological alteration of the dorsal raphe nucleus (DRN) in order to explain the disrupted serotonergic system after prenatal and lactational exposure to bisphenol A (BPA). MethodsThe murine dams were orally administrated with 500μg/kg/day of BPA from embryonic day 0 to postnatal 3weeks. The DRN, the main region of serotonin production, was morphometrically analyzed at 14weeks, using immunohistochemistry and image analysis combined with 3-dimensional reconstruction. ResultsNo significant differences were revealed in the number of tryptophan hydroxylase 2-immunoreactive neurons in any of the DRN sub-regions or the morphometric parameters, including the whole volume, ventrodorsal, longitudinal, and wing lengths of the DRN among the BPA treatment and sex groups. ConclusionsThe murine DRN was not morphologically affected by prenatal and lactational exposure to low doses of BPA. Further studies are necessary regarding the function of serotonergic neurons and the activity of different kinds of related receptors in the brain.

A Tph2GFP Reporter Stem Cell Line To Model in Vitro and in Vivo Serotonergic Neuron Development and Function.

Modeling biological systems in vitro has contributed to clarification of complex mechanisms in simplified and controlled experimental conditions. Mouse embryonic stem (mES) cells can be successfully differentiated toward specific neuronal cell fates, thus representing an attractive tool to dissect, in vitro, mechanisms that underlie complex neuronal features. In this study, we generated and characterized a reporter mES cell line, called Tph2GFP, in which the vital reporter GFP replaces the tryptophan hydroxylase 2 (Tph2) gene. Tph2GFP mES cells selectively express GFP upon in vitro differentiation toward the serotonergic fate, they synthesize serotonin, possess excitable membranes, and show the typical morphological, morphometrical, and molecular features of in vivo serotonergic neurons. Thanks to the vital reporter GFP, we highlighted by time-lapse video microscopy several dynamic processes such as cell migration and axonal outgrowth in living cultures. Finally, we demonstrated that predifferentiated Tph2GFP cells are able to terminally differentiate, integrate, and innervate the host brain when grafted in vivo. On the whole, the present study introduces the Tph2GFP mES cell line as a useful tool allowing accurate developmental and dynamic studies and representing a reliable platform for the study of serotonergic neurons in health and disease.

No association between BDNF Val66Met polymorphism and treatment response in obsessive-compulsive disorder in the Japanese population.

AimBrain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family, and it promotes the development and function of dopaminergic and serotonergic neurons. The Met allele of the BDNF Val66Met polymorphism is associated with a decrease in activity-dependent secretion of BDNF compared with the Val allele, and a number of studies have provided evidence for the association between this polymorphism and obsessive-compulsive disorder (OCD). The purpose of this study was to investigate whether this functional variant of the BDNF gene is associated with OCD and treatment response in patients with OCD in the Japanese population.MethodsWe first performed a case–control association study between the BDNF Val66Met polymorphism and OCD (175 cases and 2,027 controls). Then, we examined an association between this polymorphism and treatment response in 96 patients with OCD.ResultsWe found no significant association between the Met allele and OCD risk or between the Met allele and treatment responses to selective serotonin reuptake inhibitors or serotonin reuptake inhibitor with an atypical antipsychotic (P>0.05).ConclusionOur results suggest that the BDNF Val66Met polymorphism may not be associated as a risk factor for developing OCD or with therapeutic response in patients with OCD in the Japanese population.

The ubiquitination of serotonin transporter in lymphoblasts derived from fluvoxamine-resistant depression patients

There is insufficient serotonergic neuronal function in the pathophysiology of major depressive disorder (MDD). Serotonin transporter (SERT) plays a critical role in terminating the function of serotoninergic neurons. SERT is linked to vulnerability to MDD and is an important target for antidepressants. The expression of SERT in lymphocytes and platelets is associated with their expression in central nervous system. Most of the clinical studies that have analyzed the role of SERT in depression have focused on absolute expression of SERT in the brain or peripheral tissue. Our study has shown that the SERT protein is ubiquitinated, which has been implicated through the SERT stability and depressive behaviors in mice. In our study, we have used lymphoblasts derived from the peripheral blood lymphocytes to quantitatively examine SERT protein expression and ubiquitination in fluvoxamine-responsive and fluvoxamine–resistant MDD patients. We found that the protein levels of SERT were higher in the fluvoxamine-resistant MDD patients. Ubiquitinated protein levels of SERT were lower in the fluvoxamine-resistant MDD patients. The proteasome inhibitor failed to increase the protein levels of SERT in both fluvoxamine-responsive and fluvoxamine-resistant MDD patients. In sum, these findings suggest that the downregulation of the ubiquitination of SERT protein induces insufficient degradation of SERT by proteasome, which resulted in the upregulation of SERT protein in fluvoxamine-resistant MDD patients. Although further studies with various populations will be required to generalize results, SERT protein expression, ubiquitination, and the responsiveness of SERT expression to proteasome inhibitor are potential biomarkers for the diagnosis of MDD and antidepressant efficacy.

Tryptophan hydroxylase 2 and Bcl-xL in the rat raphe nucleus after acute and chronic forced swim stress

Stressful events induce changes in the expression of numerous genes and their protein products in the brain. However, the mechanisms mediating these changes, as well as their significance for the development of stress-induced depression or for coping with stress, remain obscure. Evidence for the sensitivity of tryptophan hydroxylase-2 (TPH2), the rate-limiting enzyme of the serotonin (5-HT) pathway, to stress is concisely reviewed, as well as the neuroprotective function of the antiapoptotic protein Bcl-xL in the brain. The goal of our experiments was to investigate the gene and protein expression of TPH2 and Bcl-xL in the dorsal (DRN) and median (MRN) raphe nuclei during repeated stress events. Gene (RT PCR) and protein (immunohistochemistry) expression was assessed 24 hours after the second and fourteenth forced swim sessions. The increase in TPH2 protein expression observed after the second swim stress exposure might reflect the protective action of Bcl-xL. During the subsequent stressful events, the stress-induced increase in Bcl-xL expression decreased. This effect was associated with the weakening of serotonergic neuron function evidenced by the compensatory activation of the TPH2 gene expression without TPH2 protein increase. Thus, shortand long-term forced swimming resulted in qualitatively different alterations in brain expression of TPH2 and Bcl-xL, suggesting their specific roles during acute and chronic stages in the development of stress-induced psychopathology. These changes may constitute a component of the mechanisms underlying elevated tph2 gene expression in depressed patients.

The effects of methamphetamine self-administration on cortical monoaminergic deficits induced by subsequent high-dose methamphetamine administrations.

Preclinical models suggest that repeated high-dose methamphetamine (METH) exposures, administered in a "binge-like" pattern, acutely decrease norepinephrine (NE), and acutely and persistently decrease serotonin (5-hydroxytryptamine; 5HT) content in the frontal cortex. However, the impact of METH self-administration on this region is unknown. Because of the importance of the monoaminergic neurons in the frontal cortex to a variety of cognitive and addictive processes, effects of METH self-administration on cortical NE and 5HT content were assessed. Results revealed several novel findings. First, METH self-administration decreased cortical NE content as assessed 24 h after last exposure. Consistent with previous preclinical reports after a binge METH regimen, this decrease was reversed 8 days after the final METH exposure. Second, and in contrast to our previous reports involving the hippocampus or striatum, METH self-administration caused persistent decreases in 5HT content as assessed 8 days after the final METH exposure. Of note, the magnitude of this decrease (≈ 20%) was less than that observed typically after a binge METH treatment. Third, prior METH self-administration attenuated METH-induced serotonergic deficits as assessed 7 days, but not 1 h, following a neurotoxic METH regimen. No protection was observed when the binge exposure occurred 15 days after the last self-administration session. Taken together, these data demonstrate important and selective alterations in cortical serotonergic neuronal function subsequent to METH self-administration. These data provide a foundation to investigate complex questions involving "resistance" to the persistent deficits caused by neurotoxic METH exposure and frontal cortical function.

Prior methamphetamine self-administration attenuates serotonergic deficits induced by subsequent high-dose methamphetamine administrations

BackgroundPre-clinical studies indicate that high-dose, non-contingent methamphetamine (METH) administration both rapidly and persistently decreases serotonergic neuronal function. Despite research indicating the hippocampus plays an important role in METH abuse and is affected by METH use, effects of METH self-administration on hippocampal serotonergic neurons are not well understood, and were thus an important focus of the current study. Because humans often administer METH in a binge-like pattern, effects of prior METH self-administration on a subsequent “binge-like” METH treatment were also examined. MethodsRats were treated as described above, and sacrificed 1 or 8d after self-administration or 1h or 7d after the final binge METH or saline exposure. Hippocampal serotonin (5-hydroxytryptamine; 5HT) content and transporter (SERT) function were assessed. ResultsMETH self-administration per se had no persistent effect on hippocampal 5HT content or SERT function. However, this treatment attenuated the persistent, but not acute, hippocampal serotonergic deficits caused by a subsequent repeated, high-dose, non-continent METH treatment administered 1 d the last self-administration session. No attenuation in persistent deficits were seen when the high-dose administration of METH occurred 15d after the last self-administration session. ConclusionsThe present findings demonstrate that METH self-administration alters serotonergic neurons so as to engender “tolerance” to the persistent serotonergic deficits caused by a subsequent METH exposure. However, this “tolerance” does not persist. These data provide a foundation to investigate complex questions including how the response of serotonergic neurons to METH may contribute to contingent-related disorders such as dependence and relapse.

Human bone marrow mesenchymal stem cells protect catecholaminergic and serotonergic neuronal perikarya and transporter function from oxidative stress by the secretion of glial-derived neurotrophic factor

In neurodegenerative disorders, including Parkinson's disease (PD), the potential of mesenchymal stem cells (MSCs) to produce neurorestoration via trans-differentiation has garnered much interest. We believe, however, that the paracrine effects of MSCs may have greater utility. MSCs release neurotrophic factors, including glial derived neurotrophic factor (GDNF). The benefits conferred by MSC GDNF release could potentially apply to all degenerating monoaminergic fibre types, throughout the brains of patients with PD, rather than solely affording protection to the dopaminergic neurones of the nigro-striatal pathway alone. Using an in vitro approach, we have investigated the neuroprotective properties of unmodified human MSCs on rat catecholaminergic and serotonergic cell cultures exposed to the damaging effects of nitric oxide. We have shown that post oxidative and inflammatory stress, soluble factors produced by native human MSCs, requiring no direct cell-cell contact or genetic or other manipulation, confer protection not only of cultured monoaminergic perikarya, but also of monoamine neurotransmitter transporter function. Furthermore, we have confirmed that, in part, this MSC mediated neuroprotective effect is due to MSC GDNF release and that such protection is diminished when the action of GDNF is blocked. Trophic factor release may afford a way by which intravenously infused MSCs can offer protection to all of the dopaminergic, noradrenergic and serotonergic fibre types degenerating widely throughout the brains of patients with PD.

Optimized Derivation and Functional Characterization of 5-HT Neurons from Human Embryonic Stem Cells

The ability to study the characteristics of serotonin release from human serotonergic neurons is valuable both in terms of understanding disease pathology and in trying to understand how drugs that affect the serotonergic system alter neurotransmitter release. There is, however, no good in vitro system to model human serotonergic neurons. Although human embryonic stem (hES) cells offer an attractive model system, the derivation of serotonergic neurons from these cells has remained at a low efficiency. To address this problem, Nestin positive precursors from HUES7 hES cell line were first generated. These Nestin positive cells when terminally differentiated gave rise to 20% MAP-2 positive neurons. A high percentage (>40%) of these neurons could be converted to serotonergic neurons. These serotonergic neurons expressed both serotonin and the neuron-specific tryptophan hydroxylase enzyme. In addition, they expressed several of the transcription factors that have been associated with serotonergic differentiation including Mash1 and Pet1. Finally, during the process of neuronal differentiation, the serotonin content, the localization of serotonin vesicles, and their ability to release serotonin following depolarization was characterized using a live cell serotonin imaging technique based on three-photon microscopy. Thus, for the first time, we demonstrate the feasibility of characterizing the development and function of human serotonergic neurons in vitro.