Stimulation of astroglial serotonin receptors produces culture media which regulates growth of serotonergic neurons

Serotonin 1A Receptors on Astrocytes as a Potential Target for the Treatment of Parkinson's Disease

Astrocytes are the most abundant neuron-supporting glial cells in the central nervous system. The neuroprotective role of astrocytes has been demonstrated in various neurological disorders such as amyotrophic lateral sclerosis, spinal cord injury, stroke and Parkinson’s disease (PD). Astrocyte dysfunction or loss-of-astrocytes increases the susceptibility of neurons to cell death, while astrocyte transplantation in animal studies has therapeutic advantage. We reported recently that stimulation of serotonin 1A (5-HT_1A) receptors on astrocytes promoted astrocyte proliferation and upregulated antioxidative molecules to act as a neuroprotectant in parkinsonian mice. PD is a progressive neurodegenerative disease with motor symptoms such as tremor, bradykinesia, rigidity and postural instability, that are based on selective loss of nigrostriatal dopaminergic neurons, and with non-motor symptoms such as orthostatic hypotension and constipation based on peripheral neurodegeneration. Although dopaminergic therapy for managing the motor disability associated with PD is being assessed at present, the main challenge remains the development of neuroprotective or disease-modifying treatments. Therefore, it is desirable to find treatments that can reduce the progression of dopaminergic cell death. In this article, we summarize first the neuroprotective properties of astrocytes targeting certain molecules related to PD. Next, we review neuroprotective effects induced by stimulation of 5-HT_1A receptors on astrocytes. The review discusses new promising therapeutic strategies based on neuroprotection against oxidative stress and prevention of dopaminergic neurodegeneration.

Of rodents and humans: A comparative review of the neurobehavioral effects of early life SSRI exposure in preclinical and clinical research

Selective serotonin reuptake inhibitors (SSRIs) have been a mainstay pharmacological treatment for women experiencing depression during pregnancy and postpartum for the past 25 years. SSRIs act via blockade of the presynaptic serotonin transporter and result in a transient increase in synaptic serotonin. Long-lasting changes in cellular function such as serotonergic transmission, neurogenesis, and epigenetics, are thought to underlie the therapeutic benefits of SSRIs. In recent years, though, growing evidence in clinical and preclinical settings indicate that offspring exposed to SSRIs in utero or as neonates exhibit long-lasting behavioral adaptions. Clinically, children exposed to SSRIs in early life exhibit increased internalizing behavior reduced social behavior, and increased risk for depression in adolescence. Similarly, rodents exposed to SSRIs perinatally exhibit increased traits of anxiety- or depression-like behavior. Furthermore, certain individuals appear to be more susceptible to early life SSRI exposure than others, suggesting that perinatal SSRI exposure may pose greater risks for negative outcome within certain populations. Although SSRIs trigger a number of intracellular processes that likely contribute to their therapeutic effects, early life antidepressant exposure during critical neurodevelopmental periods may elicit lasting negative effects in offspring. In this review, we cover the basic development and structure of the serotonin system, how the system is affected by early life SSRI exposure, and the behavioral outcomes of perinatal SSRI exposure in both clinical and preclinical settings. We review recent evidence indicating that perinatal SSRI exposure perturbs the developing limbic system, including altered serotonergic transmission, neurogenesis, and epigenetic processes in the hippocampus, which may contribute to behavioral domains (e.g., sociability, cognition, anxiety, and behavioral despair) that are affected by perinatal SSRI treatment. Identifying the molecular mechanisms that underlie the deleterious behavioral effects of perinatal SSRI exposure may highlight biological mechanisms in the etiology of mood disorders. Moreover, because recent studies suggest that certain individuals may be more susceptible to the negative consequences of early life SSRI exposure than others, understanding mechanisms that drive such susceptibility could lead to individualized treatment strategies for depressed women who are or plan to become pregnant.

Cellular resilience: 5-HT neurons in Tph2(-/-) mice retain normal firing behavior despite the lack of brain 5-HT

Considerable evidence links dysfunction of serotonin (5-hydroxytryptamine, 5-HT) transmission to neurodevelopmental and psychiatric disorders characterized by compromised "social" cognition and emotion regulation. It is well established that the brain 5-HT system is under autoregulatory control by its principal transmitter 5-HT via its effects on activity and expression of 5-HT system-related proteins. To examine whether 5-HT itself also has a crucial role in the acquisition and maintenance of characteristic rhythmic firing of 5-HT neurons, we compared their intrinsic electrophysiological properties in mice lacking brain 5-HT, i.e. tryptophan hydroxylase-2 null mice (Tph2(-/-)) and their littermates, Tph2(+/-) and Tph2(+/+), by using whole-cell patch-clamp recordings in a brainstem slice preparation and single unit recording in anesthetized animals. We report that the active properties of dorsal raphe nucleus (DRN) 5-HT neurons in vivo (firing rate magnitude and variability; the presence of spike doublets) and in vitro (firing in response to depolarizing current pulses; action potential shape) as well as the resting membrane potential remained essentially unchanged across Tph2 genotypes. However, there were subtle differences in subthreshold properties, most notably, an approximately 25% higher input conductance in Tph2(-/-) mice compared with Tph2(+/-) and Tph2(+/+) littermates (p<0.0001). This difference may at least in part be a consequence of slightly bigger size of the DRN 5-HT neurons in Tph2(-/-) mice (approximately 10%, p<0.0001). Taken together, these findings show that 5-HT neurons acquire and maintain their signature firing properties independently of the presence of their principal neurotransmitter 5-HT, displaying an unexpected functional resilience to complete brain 5-HT deficiency.

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.

Oscillatory serotonin function in depression

Oscillations in brain activities with periods of minutes to hours may be critical for normal mood behaviors. Ultradian (faster than circadian) rhythms of mood behaviors and associated central nervous system activities are altered in depression. Recent data suggest that ultradian rhythms in serotonin (5HT) function also change in depression. In two separate studies, 5HT metabolites in cerebrospinal fluid (CSF) were measured every 10 min for 24 h before and after chronic antidepressant treatment. Antidepressant treatments were associated with enhanced ultradian amplitudes of CSF metabolite levels. Another study used resting-state functional magnetic resonance imaging (fMRI) to measure amplitudes of dorsal raphé activation cycles following sham or active dietary depletions of the 5HT precursor (tryptophan). During depletion, amplitudes of dorsal raphé activation cycles increased with rapid 6 s periods (about 0.18 Hz) while functional connectivity weakened between dorsal raphé and thalamus at slower periods of 20 s (0.05 Hz). A third approach studied MDMA (ecstasy, 3,4-methylenedioxy-N-methylamphetamine) users because of their chronically diminished 5HT function compared with non-MDMA polysubstance users (Karageorgiou et al., 2009). Compared with a non-MDMA using cohort, MDMA users showed diminished fMRI intra-regional coherence in motor regions along with altered functional connectivity, again suggesting effects of altered 5HT oscillatory function. These data support a hypothesis that qualities of ultradian oscillations in 5HT function may critically influence moods and behaviors. Dysfunctional 5HT rhythms in depression may be a common endpoint and biomarker for depression, linking dysfunction of slow brain network oscillators to 5HT mechanisms affected by commonly available treatments. 5HT oscillatory dysfunction may define illness subtypes and predict responses to serotonergic agents. Further studies of 5HT oscillations in depression are indicated.

Recent progress in understanding the pathophysiology of post-traumatic stress disorder: implications for targeted pharmacological treatment

Post-traumatic stress disorder (PTSD) is a common and chronic anxiety disorder that can result after exposure to a traumatic event. Though our understanding of the aetiology of PTSD is incomplete, several neurobiological systems have been implicated in the pathophysiology and vulnerability towards developing PTSD after trauma exposure. We aimed to provide a concise review of benchmark findings in important neurobiological systems related to the aetiology and maintenance of PTSD symptomology. Specifically, we discuss functional aetiologies in the noradrenergic, serotonergic, endogenous cannabinoid and opioid systems as well as the hypothalamic-pituitary adrenal (HPA) axis. This article provides a succinct framework to appreciate the current understanding of neurobiological mechanisms related to the pathophysiology of PTSD and how these findings may impact the development of future, targeted pharmacological treatments for this debilitating disorder.

The effects of ecstasy (MDMA) on brain serotonin transporters are dependent on age-of-first exposure in recreational users and animals

Rationale and Objective

Little is known on the effects of ecstasy (MDMA, a potent 5-HT-releaser and neurotoxin) exposure on brain development in teenagers. The objective of this study was to investigate whether in humans, like previous observations made in animals, the effects of MDMA on the 5-HT system are dependent on age-of-first exposure.

Methods

5-HT transporter (SERT) densities in the frontal cortex and midbrain were assessed with [¹²³I]β-CIT single photon emission computed tomography in 33 users of ecstasy. Subjects were stratified for early-exposed users (age-at-first exposure 14–18 years; developing brain), and late-exposed users (age-at-first exposure 18–36 years; mature brain). In parallel, we investigated the effects of age experimentally with MDMA in early-exposed (adolescent) rats and late-exposed (adult) rats using the same radioligand.

Results

On average, five years after first exposure, we found a strong inverse relationship, wherein age-at-first exposure predicted 79% of the midbrain SERT variability in early (developing brain) exposed ecstasy users, whereas this was only 0.3% in late (mature brain) exposed users (p = 0.007). No such effect was observed in the frontal cortex. In rats, a significant age-BY-treatment effect (p<0.01) was observed as well, however only in the frontal cortex.

Conclusions

These age-related effects most likely reflect differences in the maturational stage of the 5-HT projection fields at age-at-first exposure and enhanced outgrowth of the 5-HT system due to 5-HT’s neurotrophic effects. Ultimately, our findings stress the need for more knowledge on the effects of pharmacotherapies that alter brain 5-HT levels in the pediatric population.

The impact of increased blood lactate on serum S100B and prolactin concentrations in male adult athletes

S100B is an astroglial protein that is increased in the peripheral bloodstream after traumatic brain injury (TBI). Elevated serum levels of S100B have been shown to be predictive of mild TBI. Furthermore, physical activity (PA) can affect S100B levels. Interestingly, increased serum S100B concentrations have been detected in athletes without apparent TBI. Such increases could be attributed to tissue hypoperfusion reflected by blood lactate concentrations [BLa(-)] and/or increased serotonergic activity reflected by prolactin (PRL). The impact of increased blood lactates on peripheral S100B levels per se are yet unknown. The purpose of our study was to investigate if increased blood lactate induced by sodium lactate infusion, without the "side effects" of PA, resulted in changes in serum S100B and PRL. Twelve male adults were given a sodium lactate infusion for a period of 24 min by a perfusor with an infusion rate of 0.01 mL kg(-1) min(-1), increased every 3 min. The main outcome measures showed no increase in serum S100B (p > 0.05). Prolactin increased significantly (p < 0.05) after [BLa(-)] exceeded a concentration of 4 mmol L(-1). Furthermore, the expected values of blood lactate achieved peak values ranging from 11 to 15 mmol L(-1). We conclude that neither increased blood lactate nor serum PRL play an exclusive role in the regulation of S100B. Nevertheless, PA should be surveyed in medical history and critically assessed in determining the severity of TBI, especially in sports. Further studies are needed to clarify the impact of PA on the biomarker S100B.

Age-dependent effects of chronic fluoxetine treatment on the serotonergic system one week following treatment

RATIONALE

Selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine are increasingly used for the treatment of depression in children. Limited data are, however, available on their effects on brain development and their efficacy remains debated. Moreover, previous experimental studies are seriously hampered in their clinical relevance.

OBJECTIVES

The aim of the present study was to investigate putative age-related effects of a chronic treatment with fluoxetine (5 mg/kg, either orally or i.p. for 3 weeks, 1 week washout) using conventional methods (behavioral testing and binding assay using [(123)I]β-CIT) and a novel magnetic resonance imaging (MRI) approach.

METHODS

Behavior was assessed, as well as serotonin transporter (SERT) availability and function through ex vivo binding assays and in vivo pharmacological MRI (phMRI) with an acute fluoxetine challenge (10 mg/kg oral or 5 mg/kg i.v.) in adolescent and adult rats.

RESULTS

Fluoxetine caused an increase in anxiety-like behavior in treated adult, but not adolescent, rats. On the binding assays, we observed increased SERT densities in most cortical brain regions and hypothalamus in adolescent, but not adult, treated rats. Finally, reductions in brain activation were observed with phMRI following treatment, in both adult and adolescent treated animals.

CONCLUSION

Collectively, our data indicate that the short-term effects of fluoxetine on the 5-HT system may be age-dependent. These findings could reflect structural and functional rearrangements in the developing brain that do not occur in the matured rat brain. phMRI possibly will be well suited to study this important issue in the pediatric population.

The influence of serotonin transporter polymorphisms on cortical activity: a resting EEG study

BACKGROUND

The serotonin transporter gene (5-HTT) is a key regulator of serotonergic neurotransmission and has been linked to various psychiatric disorders. Among the genetic variants, polymorphisms in the 5-HTT gene-linked polymorphic region (5-HTTLPR) and variable-number-of-tandem-repeat in the second intron (5-HTTVNTR) have functional consequences. However, their genetic impact on cortical oscillation remains unclear. This study examined the modulatory effects of 5-HTTLPR (L-allele carriers vs. non-carriers) and 5-HTTVNTR (10-repeat allele carriers vs. non-carriers) polymorphism on regional neural activity in a young female population.

METHODS

Blood samples and resting state eyes-closed electroencephalography (EEG) signals were collected from 195 healthy women and stratified into 2 sets of comparisons of 2 groups each: L-allele carriers (N=91) vs. non-carriers for 5-HTTLPR and 10-repeat allele carriers (N=25) vs. non-carriers for 5-HTTVNTR. The mean power of 18 electrodes across theta, alpha, beta, gamma, gamma1, and gamma2 frequencies was analyzed. Between-group statistics were performed by an independent t-test, and global trends of regional power were quantified by non-parametric analyses.

RESULTS

Among 5-HTTVNTR genotypes, 10-repeat allele carriers showed significantly low regional power at gamma frequencies across the brain. We noticed a consistent global trend that carriers with low transcription efficiency of 5-HTT possessed low regional powers, regardless of frequency bands. The non-parametric analyses confirmed this observation, with P values of 3.071×10-8 and 1.459×10-12 for 5-HTTLPR and 5-HTTVNTR, respectively.

CONCLUSIONS AND LIMITATIONS

Our analyses showed that genotypes with low 5-HTT activity are associated with less local neural synchronization during relaxation. The implication with respect to genetic vulnerability of 5-HTT across a broad range of psychiatric disorders is discussed. Given the low frequency of 10-repeat allele of 5-HTTVNTR in our research sample, the possibility of false positive findings should also be considered.

Neonatal S100B protein levels after prenatal exposure to selective serotonin reuptake inhibitors

OBJECTIVE

This study investigated neonatal S100B levels as a biomarker of prenatal selective serotonin reuptake inhibitor (SSRI) exposure.

METHODS

Maternal (delivery; N = 53) and neonatal (cord; N = 52) serum S100B levels were compared between prenatally SSRI-exposed (maternal, N = 36; neonatal, N = 37; duration: 230 +/- 71 days) and nonexposed (maternal, N = 17; neonatal, N = 15) groups. Measures of maternal depression and anxiety symptoms were assessed during the third trimester (33-36 weeks), and neonatal outcomes, including Apgar scores, birth weight, gestational age at birth, and symptoms of poor neonatal adaptation, were recorded.

RESULTS

S100B levels were significantly lower in prenatally SSRI-exposed neonates than in nonexposed neonates, controlling for gestational age and third-trimester maternal mood (P = .036). In contrast, SSRI-exposed mothers had significantly higher maternal serum S100B levels, compared with nonexposed mothers (P = .014), even controlling for maternal mood in the third trimester. S100B levels were not associated with maternal or neonatal drug levels, duration of prenatal exposure, demographic variables, or risk for poor neonatal adaptation.

CONCLUSIONS

Prenatal SSRI exposure was associated with decreased neonatal serum S100B levels, controlling for prenatal maternal mood. Neonatal S100B levels did not reflect neonatal behavioral outcomes and were not related to pharmacologic indices. These findings are consistent with prenatal alcohol and cocaine exposures, which also alter central serotonin levels.

Noradrenergic and serotonergic mechanisms in the neurobiology of posttraumatic stress disorder and resilience

Posttraumatic stress disorder (PTSD) is characterized mainly by symptoms of re-experiencing, avoidance and hyperarousal as a consequence of catastrophic and traumatic events that are distinguished from ordinary stressful life events. Although extensive research has already been done, the etiology of PTSD remains unclear. Research on the impact of trauma on neurobiological systems can be expected to inform the development of treatments that are directed specifically to symptoms of PTSD. During the past 25 years there has been a dramatic increase in the knowledge about noradrenergic and serotonergic mechanisms in stress response, PTSD and more recently in resilience and this knowledge has justified the use of antidepressants with monoaminergic mechanisms of action for patients with PTSD. Nevertheless, available treatments of PTSD are only to some extent effective and enhanced understanding of the neurobiology of PTSD may lead to the development of improved treatments for these patients. In the present review, we aim to close existing gaps between basic research in psychopathology, neurobiology and treatment development with the ultimate goal to translate basic research into clinically relevant findings which may directly benefit patients with PTSD.

Vitamin E increases S100B-mediated microglial activation in an S100B-overexpressing mouse model of pathological aging

S100B is a calcium-binding protein released by astroglial cells of the brain capable of producing numerous extracellular effects. Although the direct molecular mechanism remains unknown, these effects can be trophic including differentiation, growth, recovery, and survival of neurons when the S100B protein is mainly oxidized and neurotoxic including apoptosis and neuroinflammatory processes marked by microglial activation when in a reduced state. S100B and its receptor RAGE (receptor for advanced glycation end products) have been found to be increased in Alzheimer's disease, Down syndrome, with tissue trauma and ischemia. In the current study, we examined the binding of the S100B receptor (RAGE) on microglial cells and the developmental effects of the antioxidant vitamin E on microglial activation and the upregulation of RAGE in an S100B over-expressing mouse model of pathological aging. We report that RAGE is co-localized on activated microglial cells and vitamin E induced dramatic increases in microglial activation as well as total microglial relative optical density that was accompanied by upregulation of the RAGE receptor, particularly in the CA1 region of the hippocampus. Our findings suggest further investigation into the potential role of vitamin E in reducing the oxidation state of the S100B protein and its influence on neuroinflammatory processes marked by microglial activation in vivo.

Serotonin-1A receptor imaging in recurrent depression: replication and literature review

INTRODUCTION

Serotonin-1A receptor (5-HT1AR) function appears to be decreased in major depressive disorder (MDD) based on physiological responses to 5-HT1AR agonists in vivo and to 5-HT1AR binding in brain tissues postmortem or antemortem. We have previously assessed 5-HT1AR binding potential (BP) in depression using positron emission tomography (PET) and [carbonyl-(11)C]WAY-100635, and we have demonstrated reduced 5-HT1AR BP in the mesiotemporal cortex (MTC) and raphe in depressives with primary recurrent familial mood disorders (n=12) versus controls (n=8) [Drevets WC, Frank E, Price JC, Kupfer DJ, Holt D, Greer PJ, Huang Y, Gautier C, Mathis C. PET imaging of serotonin 1A receptor binding in depression. Biol Psychiatry 1999;46(10):1375-87]. These findings were replicated by some, but not other, studies performed in depressed samples that were more generally selected using criteria for MDD. In the current study, we attempted to replicate our previous findings in an independent sample of subjects selected according to the criteria for primary recurrent depression applied in our prior study.

METHODS

Using PET and [carbonyl-(11)C]WAY-100635, 5-HT1AR BP was assessed in 16 depressed subjects and 8 healthy controls.

RESULTS

Mean 5-HT1AR BP was reduced by 26% in the MTC (P<.005) and by 43% in the raphe (P<.001) in depressives versus controls.

CONCLUSIONS

These data replicate our original findings, which showed that BP was reduced by 27% in the MTC (P<.025) and by 42% in the raphe (P<.02) in depression. The magnitudes of these reductions in 5-HT1AR binding were similar to those found postmortem in 5-HT1AR mRNA concentrations in the hippocampus in MDD [López JF, Chalmers DT, Little KY, Watson SJ. Regulation of serotonin 1A, glucocorticoid, and mineralocorticoid receptor in rat and human hippocampus: implications for neurobiology of depression. Biol Psychiatry 1998;43:547-73] and in 5-HT1AR-binding capacity in the raphe in depressed suicide victims [Arango V, Underwood MD, Boldrini M, Tamir H, Kassir SA, Hsiung S, Chen JJ, Mann JJ. Serotonin 1A receptors, serotonin transporter binding and serotonin transporter mRNA expression in the brainstem of depressed suicide victims. Neuropsychopharmacology 2001;25(6):892-903]. There exists disagreement within the literature, however, regarding the presence and direction of 5-HT1AR-binding abnormalities in depression, which may be explained in some cases by differences in anatomical location (e.g., [Stockmeier CA, Shapiro LA, Dilley GE, Kolli TN, Friedman L, Rajkowska G. Increase in serotonin-1A autoreceptors in the midbrain of suicide victims with major depression--postmortem evidence for decreased serotonin activity. J Neurosci 1998;18(18):7394-401]) and in other cases by pathophysiological heterogeneity within MDD (e.g., some depressives hypersecrete cortisol, which would be expected to down-regulate 5-HT1AR expression [López JF, Chalmers DT, Little KY, Watson SJ. Regulation of serotonin 1A, glucocorticoid, and mineralocorticoid receptor in rat and human hippocampus: implications for neurobiology of depression. Biol Psychiatry 1998;43:547-73]). Antidepressant drug treatment does not alter these abnormalities in 5-HT1AR binding [Sargent PA, Kjaer KH, Bench CJ, Rabiner EA, Messa C, Meyer J, Gunn RN, Grasby PM, Cowen PJ. Brain serotonin1A receptor binding measured by positron emission tomography with [11C]WAY-100635: effects of depression and antidepressant treatment. Arch Gen Psychiatry 2000;57(2):174-80; Moses-Kolko EL, Price JC, Thase ME, Meltzer CC, Kupfer DJ, Mathis CA, Bogers WD, Berman SR, Houck PR, Schneider TN, Drevets WC. Measurement of 5-HT1A receptor binding in depressed adults before and after antidepressant drug treatment using positron emission tomography and [11C]WAY-100635. Synapse 2007;61(7):523-30] but may compensate for blunted 5-HT1AR function by increasing post-synaptic 5-HT1AR transmission [Chaput Y, de Montigny C, Blier P. Presynaptic and postsynaptic modifications of the serotonin system by long-term administration of antidepressant treatments. An in vivo electrophysiologic study in the rat. Neuropsychopharmacology 1991;5(4):219-29].

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.

Prenatal stress reduces S100B in the neonatal rat hippocampus

Prenatal stress has been shown to disturb neonatal rat brain development. The astroglial-specific neurotrophic factor S100B is known to play an important role in normal brain development. In the present study, we investigated the effects of prenatal stress on S100B concentrations in the hippocampus of 1-day-old Fischer 344 rats. Overall, prenatal stress resulted in a 25% reduction in hippocampal S100B content. Further, male hippocampal S100B content was negatively correlated with plasma corticosterone levels. Positive correlations were found between female S100B levels and fetal growth, and hippocampal brain-derived neurotrophic factor content. In conclusion, the observed reduction in neonatal hippocampal S100B levels, as a consequence of prenatal stress, may be involved in affecting postnatal brain development.

Gender-specific 5-HT1A receptor changes in BrdU nuclear labeling patterns in neonatal dentate gyrus

The actions of 5-HT1A receptors on cell proliferation in the rat neonatal dentate gyrus are unknown. We injected a 5-HT1A receptor agonist (ipsapirone) or antagonist (Way 100635) 1 h before injections of BrdU in neonates of both genders between days 2-4, a peak time of dentate gyrus granule cell proliferation. The BrdU immunoreactive (IR) nuclei in the granule cell layer and subgranular zone were examined after 2 weeks. The BrdU-IR nuclear staining patterns were classified as being either diffuse (homogenous dark BrdU-staining throughout the nucleus) or punctate (multiple distinct small stained spots within the nucleus). Most BrdU-labeled nuclei with a diffuse pattern were seen in the subgranular zone while the punctate pattern nuclei were seen within the granular cell layer of the dentate gyrus. 5-HT1A antagonist showed no overall change in absolute number or pattern of labeled nuclei compared to control animals. After a 5-HT1A agonist, there was also no differences in the total number of BrdU-IR nuclei (punctate and diffuse pattern). However, in both genders, the proportion of the BrdU-labeled nuclei showing a punctate compared to diffuse pattern increased: 33% in females and 18% in males. In females, the 5-HT1A receptor agonist increased the number of nuclei showing a punctate pattern by 41%, while in males the 5-HT1A receptor agonist decreased the number of nuclei showing a diffuse pattern by 29%. These results indicate gender-specific 5-HT1A receptor action on the state of nuclear DNA in the cells of the dentate gyrus, without increasing the total number of BrdU-labeled nuclei.

Expression levels of cytoskeletal proteins indicate pathological aging of S100B transgenic mice: an immunohistochemical study of MAP-2, drebrin and GAP-43

S100B is a calcium-binding protein found within astroglial cells. When released, S100B has extracellular neurotrophic effects involving the neuronal cytoskeleton. The gene for S100B is located on chromosome 21 and levels of the protein are elevated in Down Syndrome (DS) and Alzheimer's Disease (AD). Thus, overexpression of S100B may be related to the cytoskeletal abnormalities seen in these disorders. Transgenic mice overexpressing human S100B were examined for cytoskeletal changes as young (70 days) and aged (200 days) adults, using immunochemical staining of the dendritic associated protein, MAP-2, the growth-associated protein-43 (GAP-43) and the dendritic spine marker, drebrin. As young adults, the S100B transgenic mice exhibited significantly greater MAP-2-immunoreactivity in the hippocampus, however as older adults, the animals exhibited less staining. In both the CD1 control animals and the S100B animals, the immunoreactivity of drebrin increased with age, however there were no significant between group differences. Finally, the older S100B animals showed more GAP-43 staining than the control animals, suggesting that synaptic remodeling could take place, possibly in response to the loss of MAP-2-ir dendrites. Overall, the data suggest that S100B overexpression leads to changes in cytoskeletal markers. The longitudinal effects of S100B overexpression are discussed with relevance to aging and pathology.

Juvenile 5HT 1B receptor knockout mice exhibit reduced pharmacological sensitivity to 5HT 1A receptor activation

Serotonin is an important modulator of anxiety and thus drugs that act on this system have frequently been shown to be either anxiogenic or anxiolytic. In addition serotonin has important trophic functions during early development and disruption of serotonin homeostasis is likely to have long-lasting repercussions in the adult. In the present study we examined the contribution of two serotonin receptor subtypes (5HT(1A) and 5HT(1B)) to the pathophysiology of anxiety during development. For this, we have studied homozygous knockout mice lacking the 5HT(1B) receptor and examined the effect of pharmacological manipulations of 5HT(1A) and 5HT(1B) receptors on locomotor activity and emission of ultrasonic vocalization (USV) in 7-8 days old mice. As shown before, drug naïve 5HT(1B) knockout pups showed reduced USV and were hyperactive, in comparison to wild type controls. The administration of RU24969 (a 5HT(1A/1B) agonist) showed a dose-dependent decrease in USV in the wild type and a biphasic effect in the mutants and resulted in dose-dependent increase in activity in the wild type and, to a lesser extent, in the knockouts. The selective 5HT(1A) agonist, 8OH-DPAT, dose-dependently blocked vocalization in both genotypes and also increased locomotion. To differentially activate 5HT(1B) receptors we first blocked 5HT(1A) receptors with WAY100315 and then treated with RU24969. At a high testing temperature, pretreatment with WAY100315 resulted in an anxiogenic effect in wild type pups but not in the knockouts. In agreement with our findings that 5HT(1B) knockout mice were in general less sensitive to 5HT(1A) activation, 5HT(1A) receptor binding was reduced in the knockouts in comparison to controls. Finally, treatment with diazepam dose-dependently decreased USVs in both group with the knockouts showing enhanced sensitivity to this drug. Our results show that important adaptations to a disturbance of serotonin homeostasis occur during the first week of life within the serotonergic system. The observed decreased in sensitivity of 5HT(1B) knockout mice to 5HT(1A) and increased to GABA(A) manipulations are discussed within the context of serotonergic plasticity during development and the implication for clinical treatment of anxiety in genetically predisposed individuals.

Relationship between stress and circulating levels of S100B protein

It has been proposed that S100B can be a marker for several pathological conditions including brain traumas, blood-brain barrier disruption, and ischemia. Because the hypothalamo-pituitary-adrenal axis is activated in these conditions, we investigated the role of glucocorticoids in the effects of stress on serum S100B. Restraint stress increased S100B levels in control and in adrenalectomized but not in corticosterone-injected rats. Adrenalectomy did not alter basal S100B. These results indicate a glucocorticoid-independent relationship between stress and S100B.

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.

Altered dendritic spine density in animal models of depression and in response to antidepressant treatment

Olfactory bulbectomy, neonatal clomipramine administration, and maternal deprivation have been employed as animal models of depression. Each model is unique with respect to the experimental manipulations required to produce "depressive" signs, expression and duration of these signs, and response to antidepressant treatments. Dendritic spines represent a possible anatomical substrate for the enduring changes seen with depression and we have previously shown that chronic antidepressant drug exposure alters the density of hippocampal dendritic spines in an enduring fashion. The purpose of the present study was to determine whether persistent alteration of hippocampal spine density is a common element in each of these different models of depression and whether such alterations could be reversed with chronic antidepressant treatment. The results show that olfactory bulbectomy reduced spine density in CA1, CA3, and dentate gyrus compared to sham-operated controls. Chronic treatment with amitriptyline, a tricyclic antidepressant, reversed the bulbectomy- induced reduction in dendritic spine density in CA1, CA3, and dentate gyrus, whereas treatment with mianserin, an atypical antidepressant, reversed this reduction only in dentate gyrus. On the other hand, neither neonatal clomipramine administration nor maternal deprivation affected hippocampal dendritic spine density. Repeated neonatal handling, however, as a control or as part of the maternal deprivation procedure, elevated spine density in dentate gyrus. These data suggest that long-lasting alterations in hippocampal dendritic spine density contribute to the neural mechanism underlying the olfactory bulbectomy model of depression, but not the neonatal clomipramine or maternal deprivation models.

Serotonin and brain development: role in human developmental diseases

Serotonin is known to play a role in brain development prior to the time it assumes its role as a neurotransmitter in the mature brain. Serotonin regulates both the development of serotonergic neurons (termed autoregulation of development) and the development of target tissues. In both cases, the astroglial-derived protein, S-100beta plays a role. Disruption of serotonergic development can leave permanent alterations in brain function and behavior. This may be the case in such human developmental illnesses as autism and Down Syndrome.

Modern views on an ancient chemical: serotonin effects on cell proliferation, maturation, and apoptosis

Evolutionarily, serotonin existed in plants even before the appearance of animals. Indeed, serotonin may be tied to the evolution of life itself, particularly through the role of tryptophan, its precursor molecule. Tryptophan is an indole-based, essential amino acid which is unique in its light-absorbing properties. In plants, tryptophan-based compounds capture light energy for use in metabolism of glucose and the generation of oxygen and reduced cofactors. Tryptophan, oxygen, and reduced cofactors combine to form serotonin. Serotonin-like molecules direct the growth of light-capturing structures towards the source of light. This morphogenic property also occurs in animal cells, in which serotonin alters the cytoskeleton of cells and thus influences the formation of contacts. In addition, serotonin regulates cell proliferation, migration and maturation in a variety of cell types, including lung, kidney, endothelial cells, mast cells, neurons and astrocytes). In brain, serotonin has interactions with seven families of receptors, numbering at least 14 distinct proteins. Of these, two receptors are important for the purposes of this review. These are the 5-HT1A and 5-HT2A receptors, which in fact have opposing functions in a variety of cellular and behavioral processes. The 5-HT1A receptor develops early in the CNS and is associated with secretion of S-100beta from astrocytes and reduction of c-AMP levels in neurons. These actions provide intracellular stability for the cytoskeleton and result in cell differentiation and cessation of proliferation. Clinically, 5-HT1A receptor drugs decrease brain activity and act as anxiolytics. The 5-HT2A receptor develops more slowly and is associated with glycogenolysis in astrocytes and increased Ca(++) availability in neurons. These actions destabilize the internal cytoskeleton and result in cell proliferation, synaptogenesis, and apoptosis. In humans, 5-HT2A receptor drugs produce hallucinations. The dynamic interactions between the 5-HT1A and 5-HT2A receptors and the cytoskeleton may provide important insights into the etiology of brain disorders and provide novel strategies for their treatment.

Colchicine-induced cytoskeletal collapse and apoptosis in N-18 neuroblastoma cultures is rapidly reversed by applied S-100beta

Brain connections depend on a stable association between dendrites and axons whose cytoskeleton is stabilized by the proteins MAP-2 and tau, respectively. The glial protein S-100beta inhibits the phosphorylation by PKC of these two microtubule-associated proteins. In order to determine if exogenous S-100beta can directly influence the cytoskeleton of living cells, cultures of N-18 cells (neuroblastoma clonal cell line) are treated for 30 min in serum-free medium with 10(-6) M colchicine. In normal media, colchicine induces a rapid retraction of processes, membrane blebbing, nuclear collapse, and cell death. The observed cellular changes, due to cytoskeletal collapse after exposure to colchicine, are similar and consistent with the loss of processes and cytoplasmic blebbing seen in cells undergoing apoptosis. The addition of 20 ng/ml of S-100beta after the initial 30-min exposure to colchicine prevents apoptosis, nuclear collapse and induces the regrowth of retracted processes. Cells were treated with the Hoechst Stain, a fluorescent marker that binds to nuclear material, to determine the occurrence of apoptosis in our cultures. In our control cultures, receiving no drugs, we found that 15.1% of the cells were apoptotic. When colchicine was added to the culture medium we found that 31.6% of the cells became apoptotic. However, when colchicine was followed by exposure to S-100beta we found that only 5.4% of the cells were apoptotic. Our results suggest that extracellular application of the glial protein S-100beta is sufficient to reverse colchicine-induced cytoskeletal collapse and prevent the resultant apoptosis of the cells. The increased levels of S-100beta seen after brain injury and in certain neurological and psychiatric disorders may be considered as beneficial for brain recovery.

Effects of methamphetamine-induced neurotoxicity on the development of neural circuitry: a hypothesis

Exposure of the developing brain to methamphetamine has well-studied biochemical and behavioral consequences. We review: (1) the effects of methamphetamine on mature serotonergic and dopaminergic pathways; (2) the mechanisms of methamphetamine neurotoxicity and (3) the role of serotonergic and dopaminergic signaling in sculpting developing neural circuitry. Consideration of these data suggest the types of neural circuit alterations that may result from exposure of the developing brain to methamphetamine and that may underlie functional defects.

Chronic fluoxetine administration to juvenile rats prevents age-associated dendritic spine proliferation in hippocampus

The density of dendritic spines, the postsynaptic sites of most excitatory synapses, increases during the first 2 postnatal months in rat hippocampus. Significant alterations in hippocampal levels of serotonin and norepinephrine impact synaptic development during this time period. In the present study, dendritic spine density was studied in the hippocampus (CA1) and dentate gyrus of juvenile rats acutely and chronically exposed to antidepressant drugs that act on serotonin and norepinephrine. One group of 21-day-old rats was given a single injection of a serotonin specific re-uptake inhibitor (fluoxetine or fluvoxamine), a norepinephrine-specific re-uptake inhibitor (desipramine), or saline and killed after 24 h. A second group of rats was injected daily, beginning on postnatal day (PN) 21, for 3 weeks. This group was further subdivided into rats that were killed 1 day or 21 days after the last injection. Golgi analysis showed that a single injection of fluvoxamine produced a significant increase in dendritic spine density in stratum radiatum of CA1 and in the dentate gyrus. Further, acute treatment with all three antidepressants increased the total length of secondary dendrites in CA1, with fluoxetine and desipramine increasing the number of secondary dendrites as well. In fluoxetine-treated animals killed on days 42 or 62 (1 or 21 days post-treatment, respectively), dendritic spine density remained at levels present in CA1 at 21 days. These results show that acute antidepressant treatment can impact dendritic length and spine density, and raise the possibility that chronic fluoxetine treatment arrests spine development into young adulthood.

Acetylcholine induces neuritic outgrowth in rat primary olfactory bulb cultures

The rat olfactory bulb is innervated by basal forebrain cholinergic neurons and is endowed with both nicotinic and muscarinic receptors. The development of this centrifugal cholinergic innervation occurs mainly in early postnatal stages. This developmental time-course and the demonstration that acetylcholine can modulate some aspects of neuronal proliferation, differentiation or death, suggests the possible involvement of cholinergic afferents in the morphogenesis and/or plasticity of the olfactory bulb. The purpose of the present work was to assess whether acetylcholine could modulate neuronal morphogenesis in the olfactory bulb. Toward this aim, we developed a primary culture model of rat olfactory bulbs. Three major cell types were identified on the basis of their morphological and immunocytochemical phenotype: neuronal-shaped cells expressing the neuronal markers neuron specific enolase, microtubule associated protein 2, neural cell adhesion molecule and beta-tubulin III; glial-like cells immunoreactive for glial fibrillary acidic protein and flattened cells immunolabelled with antibodies against beta-tubulin III and nestin, most likely neuronal precursors. After three to six days of treatment with 100-microM carbachol, a cholinergic agonist, significant increase in neuritic length was observed in cultured olfactory bulb neurons. The neurite outgrowth effect of carbachol was abolished by co-treatment with 1 microM alpha-bungarotoxin, an alpha 7 subunit nicotinic receptor antagonist, but was not affected by the addition of 10 microM atropine, a general muscarinic antagonist. The effect of carbachol was also mimicked by the nicotinic agonists, nicotine (100 microM) and epibatidine (10 microM). This pharmacological profile suggested the involvement of nicotinic receptors of the alpha 7-like subtype as confirmed using 125I-alpha-bungarotoxin receptor autoradiography.Taken together, these data argue for a role for nicotinic receptors in neuritic outgrowth in the rat olfactory bulb and provide a cellular support to the previously described effects of acetylcholine on olfactory bulb plasticity in vivo.

Neurochemical changes in brain serotonin neurons in immature and adult offspring prenatally exposed to cocaine

The present study investigates the age-dependent effects of prenatal cocaine exposure on changes in the neurochemical and functional status of brain serotonin neurons. Pregnant rats were administered either saline or (-)cocaine HCl (15 mg/kg, subcutaneously), twice daily from gestational days 13 through 20. Neurochemical changes in frontal cortex, hypothalamus, hippocampus, striatum and midbrain of prepubescent and adult offspring were determined by measuring: (1) the content of serotonin (5-HT) and its major metabolite 5-hydroxyindolacetic acid (5-HIAA), and (2) the ability of the serotonin releasing drug p-chloroamphetamine (PCA) to reduce brain serotonin levels. Brain catecholamine content was determined in progeny for comparative purposes. Prior to maturation, prenatal exposure to cocaine did not alter basal levels of brain 5-HT or 5-HIAA in any brain region examined. However, in adult progeny prenatally exposed to cocaine, basal 5-HT content was significantly reduced in the frontal cortex (-32%) and hippocampus (-40%), suggesting maturation-dependent effects of prenatal cocaine exposure on brain 5-HT neurons. Consistent with the maturational onset of changes in 5-HT, striatal dopamine was significantly reduced (-10%) by prenatal exposure to cocaine only in adult offspring. Reductions in 5-HT in most brain regions, produced by pharmacological challenge with p-chloroamphetamine (PCA), were comparable in prenatal saline versus cocaine offspring. One notable exception was the markedly greater reduction (-40%) in 5-HT in the midbrain of immature offspring prenatally exposed to cocaine, suggesting alterations in midbrain 5-HT neurons prior to maturation. Overall, these data demonstrate prenatal cocaine exposure produces region-specific changes in 5-HT neurons in offspring with some deficits occurring only following maturation.

S-100beta protects cultured neurons against glutamate- and staurosporine-induced damage and is involved in the antiapoptotic action of the 5 HT(1A)-receptor agonist, Bay x 3702

The serotonin (5-HT)(1A) receptor agonists have already been shown to protect cultured neurons from excitotoxic as well as from apoptotic damage [B. Ahlemeyer, J. Krieglstein, Stimulation of 5-HT(1A) receptors inhibits apoptosis induced by serum deprivation in cultured neurons from chick embryo, Brain Res. 777 (1997) 179-186. ; B. Ahlemeyer, A. Glaser, C. Schaper, I. Semkova, J. Krieglstein, The 5-HT(1A) receptor agonist, Bay x 3702, inhibited apoptosis induced by serum deprivation in cultured neurons, Eur. J. Pharmacol. 370 (1999) 211-216.; J.H.M. Prehn, M. Welsch, C. Backhauss, J. Nuglisch, F. Ausmeier, C. Karkoutly, J. Krieglstein, Effects of serotonergic drugs in experimental brain ischemia: evidence for a protective role of serotonin in cerebral ischemia, Brain Res. 630 (1993) 110-120.; I. Semkova, P. Wolz, J. Krieglstein, Neuroprotective effect of 5-HT(1A) receptor agonist, Bay x 3702, demonstrated in vitro and in vivo, Eur. J. Pharmacol. 359 (1998) 251-260.; B. Suchanek, H. Struppeck, T. Fahrig, The 5-HT(1A) receptor agonist, Bay x 3702, prevents staurosporine-induced apoptosis, Eur. J. Pharmacol. 355 (1998) 95-101.] and to increase the release of the neurotrophic protein, S-100beta [P.M. Whitaker-Azmitia, R. Murphy, E.C. Azmitia, Stimulation of astroglial 5-HT(1A) receptors releases the serotonergic growth factor, protein S-100, and alters astroglial morphology, Brain Res. 497 (1989) 80-86. ; P.M. Whitaker-Azmitia, R. Murphy, E.C. Azmitia, S-100 protein is released from astroglial cells by stimulation of 5-HT(1A) receptors, Brain Res. 528 (1990) 155-158.]. In this study, we tried to find out whether S-100beta can protect cultured neurons from glutamate- and staurosporine-induced damage and whether the neuroprotective activity of the highly selective 5-HT(1A) receptor agonist, Bay x 3702, is mediated by an induction of S-100beta. Extracellularly added S-100beta (1-10 ng/ml) reduced staurosporine-induced damage in pure neuronal cultures from chick embryo telencephalon as well as in mixed neuronal/glial cultures from neonatal rat hippocampus. In addition, S-100beta (1 ng/ml) reduced neuronal death induced by exposure to glutamate (0.25 mM, 30 min) in mixed neuronal/glial cultures from neonatal rat hippocampus. In cultured rat cortical astrocytes, a 24 h-treatment with Bay x 3702 (1 nM) increased the S-100beta content in the culture medium from 2.2+/-0.3 (controls) to 6.2+/-0.7 ng/ml. In the adult rat, a 4 h-infusion of 4 microg/kg Bay x 3702 (i.v.) was found to increase the S-100beta content in the striatum 6 h after the beginning of the infusion to 153+/-37 microg/g compared with 60+/-20 microg/g in vehicle-treated rats. Bay x 3702 had no effect on the S-100beta content in the rat hippocampus. Finally, we tried to block the protective effect of Bay x 3702 against staurosporine-induced damage in mixed neuronal/glial cultures from rat neonatal hippocampus by anti-S-100beta antibodies. We found only a partial blockade, although the antibodies fully blocked the antiapoptotic effect of S-100beta itself demonstrating that the antibody was effective in blocking neuroprotection by S-100beta. Thus, we conclude that S-100beta was able to protect cultured neurons against glutamate- and staurosporine-induced damage. Furthermore, S-100beta mediated partially the protective effect of the 5-HT(1A) receptor agonist, Bay x 3702, against staurosporine-induced apoptosis in mixed neuronal/glial cultures from neonatal rat hippocampus.

A serotonin transporter gene intron 2 polymorphic region, correlated with affective disorders, has allele-dependent differential enhancer-like properties in the mouse embryo

Polymorphic regions consisting of a variable number of tandem repeats within intron 2 of the gene coding for the serotonin transporter protein 5-HTT have been associated with susceptibility to affective disorders. We have cloned two of these intronic polymorphisms, Stin2.10 and Stin2.12, into an expression vector containing a heterologous minimal promoter and the bacterial LacZ reporter gene. These constructs were then used to produce transgenic mice. In embryonic day 10.5 embryos, both Stin2.10 and Stin2.12 produced consistent beta-galactosidase expression in the embryonic midbrain, hindbrain, and spinal cord floor plate. However, we observed that the levels of beta-galactosidase expression produced by both the Stin2.10 and Stin2.12 within the rostral hindbrain differed significantly at embryonic day 10.5. Our data suggest that these polymorphic variable number of tandem repeats regions act as transcriptional regulators and have allele-dependent differential enhancer-like properties within an area of the hindbrain where the 5-HTT gene is known to be transcribed at this stage of development.

Serotonin transporter messenger RNA expression in neural crest-derived structures and sensory pathways of the developing rat embryo

A growing body of evidence suggests that serotonin plays an important role in the early development of both neural and non-neural tissues from vertebrate and invertebrate species. Serotonin is removed from the extracellular space by the cocaine- and antidepressant-sensitive serotonin transporter, thereby limiting its action on receptors. In situ hybridization histochemistry was used to delineate serotonin transporter messenger RNA expression during rat embryonic development. Serotonin transporter messenger RNA was widely expressed beginning prior to organogenesis and throughout the second half of gestation. Strikingly, serotonin transporter messenger RNA was detected in neural crest cells, some of which respond to serotonin in vitro, and neural crest-derived tissues, such as autonomic ganglia, tooth primordia, adrenal medulla, chondrocytes and neuroepithelial cells, in the skin, heart, intestine and lung. Within the peripheral sensory pathways, two major cells types were serotonin transporter messenger RNA-positive: (i) sensory ganglionic neurons and (ii) neuroepithelial cells which serve as targets for the outgrowing sensory neurons. Several sensory organs (cochlear and retinal ganglionic cells, taste buds, whisker and hair follicles) contained serotonin transporter messenger RNA by late gestation. The expression of serotonin transporter messenger RNA throughout the sensory pathways from central nervous system relay stations [Hansson S. R. et al. (1997) Neuroscience 83, 1185-1201; Lebrand C. et al. (1996) Neuron 17, 823-835] to sensory nerves and target organs as shown in this study suggests that serotonin may regulate peripheral synaptogenesis, and thereby influence later processing of sensory stimuli. If the early detection of serotonin transporter messenger RNA in skin and gastrointestinal and airway epithelia correlates with protein activity, it may permit establishment of a serotonin concentration gradient across epithelia, either from serotonin in the amniotic fluid or from neuronal enteric serotonin, as a developmental cue. Our results demonstrating serotonin transporter messenger RNA in the craniofacial and cardiac areas identify this gene product as the transporter most likely responsible for the previously identified accumulation of serotonin in skin and tooth germ [Lauder J. M. and Zimmerman E. F. (1988) J. craniofac. Genet. devl Biol. 8, 265-276], and the fluoxetine-sensitive effects on craniofacial [Lauder J. M. et al. (1988) Development 102, 709-720; Shuey D. L. et al. (1992) Teratology 46, 367-378; Shuey D. L. et al. (1993) Anat. Embryol., Berlin 187, 75-85] and cardiac [Kirby M. L. and Waldo K. L. (1995) Circulation Res. 77, 211-215; Yavarone M. S. et al. (1993) Teratology 47, 573-584] malformations. Serotonin transporter messenger RNA was detected in several neural crest cell lineages and may be useful as an early marker for the sensory lineage in particular. The distribution of serotonin transporter messenger RNA in early development supports the hypothesis that serotonin may play a role in neural crest cell migration and differentiation [Lauder J. M. (1993) Trends Neurosci. 16, 233-240], and that the morphogenetic actions of serotonin may be regulated by transport. The striking pattern of serotonin transporter messenger RNA throughout developing sensory pathways suggests that serotonin may play a role in establishing patterns of connectivity critical to processing sensory stimuli. As a target for drugs, such as cocaine, amphetamine derivatives and antidepressants, expression of serotonin transporter during development may reflect critical periods of vulnerability for fetal drug exposure. The widespread distribution of serotonin transporter messenger RNA during ontogeny suggests a previously unappreciated role of serotonin in diverse physiological systems during embryonic development.

Infraorbital nerve transection alters serotonin transporter expression in sensory pathways in early postnatal rat development

The serotonin transporter MRNA has been found throughout the trigeminal sensory system late in gestation and during early postnatal development, a period known to be critical for maturation of the sensory circuitry. The purpose of the present study was to determine whether sensory denervation in newborn rat pups would alter either the density or pattern of expression of the 5-HT transporter (5-HTT) within the trigeminal system. We combined autoradiographic localization of 5-HT transporters and in situ hybridization techniques to visualize both the transporter protein and mRNA in thalamic sensory neurons and in the somatosensory cortex following unilateral infraorbital nerve transection at postnatal day 1. For comparative purposes, similar measurements were conducted in thalamic visual neurons as well as in the visual cortex. Lesion of the infraorbital nerve decreased the [3H]citalopram labelling of 5-HT transporters in the ventral basal and ventral medial areas of the thalamus contralateral to the lesion, while labelling of 5-HT transporters was decreased in both contralateral and ipsilateral sides of the lateral genicuate (visual thalamus). Citalopram labelling of 5-HT transporters was not significantly altered in somatosensory or in cingulate cortex, however a significant decrease was observed in the visual cortex. In contrast, there were no obvious changes in the intensity of the 5-HT mRNA hybridization signal in sensory or visual thalamic areas. Given that the serotonin transporter regulates extracellular concentrations of 5-HT, the present data suggest that altered peripheral innervation and thereby altered sensory inputs to the thalamus during fetal development can potentially influence 5-HT transporter densities and thus, may influence extracellular levels of 5-HT in thalamus and cortex during a critical period of synapse formation. In turn, modulation of 5-HT transporter levels may influence extracellular concentrations of 5-HT in thalamus and cortex during a critical period of synapse formation.

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

The literature describing the expression of 5-HT receptor subtypes by astrocytes is controversial and incomplete. It is clear that primary cultures of astrocytes express receptors of the 5-HT2 family coupled to phospholipase C and of the 5-HT7 receptor family positively coupled to adenylyl cyclase. Cultured astrocytes have also been reported to express receptors of the 5-HT1 family, although the exact subtypes present are unknown. In the present study we have investigated which of the known rat G-protein coupled 5-HT receptor mRNAs are expressed by cultured astrocytes. Reverse transcriptase-polymerase chain reaction (RT-PCR) revealed expression of 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1F, 5-HT2A, 5-HT2B, 5-HT2C, 5-HT5B, 5-HT6 and 5-HT7 receptor mRNAs in astrocytes derived from 2-day old rats and cultured for 10-12 days. Messenger RNAs for 5-HT4 and 5-HT5A receptors were not detected. The functional expression of 5-HT1 receptor subtypes was investigated by measuring the ability of 5-HT1 receptor agonists: 8-OH-DPAT (5-HT1A receptors), RU24969 (5-HT1A, 5-HT1B, 5-HT1D, and 5-HT1F receptors) or sumatriptan (5-HT1B, 5-HT1D, and 5-HT1F receptors) to modulate forskolin or isoproterenol stimulated cAMP production. These compounds, at concentrations up to 10 microM, did not significantly attenuate cAMP production. These results indicate that although astrocytes express mRNA for each of the five 5-HT1 receptor subtypes which have been isolated from the rat, these receptors are not coupled to the inhibition of adenylyl cyclase.

Transgenic mice overexpressing the neurotrophic factor S-100 beta show neuronal cytoskeletal and behavioral signs of altered aging processes: implications for Alzheimer's disease and Down's syndrome

S-100 beta is a neurotrophic factor released by astroglial cells and localized to chromosome 21, within the region which is considered obligate for Down's syndrome (DS). S-100 beta is increased in the postmortem brains of both DS and Alzheimer's disease. Transgenic mice, produced by insertion of the human gene for S-100 beta, were examined for dendritic development at two ages, using an antibody against microtubule associated protein-2 (MAP-2). At the earliest stages, the density of dendrites within the hippocampus of transgenic animals exceeded that of controls. Also, MAP-2 immunostaining was evident in the region of the cell body. By 1 year of age, the transgenic animals had significant loss of dendrites compared to controls and the number of cells showing cell body staining was further increased. These pathological changes could be indicative of the presence of neurofibrillary tangles and cytoskeletal collapse. Behaviorally, younger transgenic animals could not perform in a learning task as well as controls. Together, these findings suggest that increased S-100 beta in brain may lead to accelerated development, followed by increased aging. The pathological changes may prove useful as an animal model of Down's syndrome and Alzheimer's disease.

Serotonin promotes the survival of cortical glutamatergic neurons in vitro

The appearance of 5-hydroxytryptamine (serotonin; 5-HT) in the cerebral cortex coincides with developmental events such as cell proliferation, survival, and differentiation. We tested the hypothesis that 5-HT plays a role in these events by examining rat cortical progenitor cells in vitro. Using bromodeoxyuridine incorporation we found that 5-HT did not affect the proliferation of these cells, but a cell survival assay indicated that it promoted their survival. The observed survival effect was mimicked by the 5-HT2a/2c receptor agonist alpha-methyl-5-HT and blocked by the 5-HT2a receptor antagonist cinanserin. Consistent with increased survival was the finding, using the terminal transferase nick end labeling method, of reduced cell death in cultures exposed to 5-HT. Immunohistochemical analysis with cell-specific markers revealed that the effect of 5-HT was directed specifically to the glutamate-containing neuronal population and not to any other cortical cell types. These results indicate that 5-HT does not exert its effects on dividing neuroepithelial cells in the developing cortex, but rather on postmitotic neurons.

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.

Mobilization of arachidonate and docosahexaenoate by stimulation of the 5-HT2A receptor in rat C6 glioma cells

In this study, we demonstrate that astroglial 5-HT2A receptors are linked to the mobilization of polyunsaturated fatty acids (PUFA). Stimulation of C6 glioma cells, prelabeled with [3H]arachidonate (AA, 20:4n6) and [14C]docosahexaenoate (DHA, 22:6n3), with serotonin and the 5-HT(2A/2C) receptor agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) resulted in the mobilization of both [3H] and [14C] into the supernatant of the cell monolayers. The increased radioactivity in the supernatant was mainly associated with free fatty acids. Experiments using inhibitors of phosphoinositide-specific phospholipase C and PLA2, inhibited the DOI-stimulated mobilization of AA and DHA, suggesting the involvement of both phospholipases. Ketanserin (1 microM), a 5-HT(2A/2C) receptor antagonist, and MDL 100,907 (R(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-pi peridine-methanol) (1 microM), a highly selective antagonist for 5-HT2A receptors, significantly decreased the DOI-stimulated release of AA and DHA. These results indicate that the 5-HT2A receptor is coupled to the mobilization of PUFA. The release of AA and DHA in response to serotonin may represent a mechanism through which astroglia provide these polyunsaturated fatty acids to neurons.

Alcohol, astroglia, and brain development

Glial cells constitute one of the most common cell types in the brain. They play critical roles in central nervous system (CNS) development. Recent evidence demonstrates that glial cells are profoundly affected by prenatal alcohol exposure, suggesting that alterations in these cells may participate in CNS abnormalities associated with ethanol-induced teratogenesis. In vivo studies show that prenatal exposure to alcohol hampers myelinogenesis and is associated with neuroglial heterotopias and abnormal astrogliogenesis. Studies using primary cultures of rat cortical astrocytes show that ethanol affects DNA, RNA, and protein synthesis, decreases the number of mitotic cells, alters the content and distribution of several cytoskeletal proteins including the astroglial marker, glial fibrillary acidic protein (GFAP), and the levels of plasma-membrane glycoproteins, reduces the capacity of astrocytes to secrete growth factors, and induces oxidative stress. Furthermore, ethanol exposure during early embryogenesis alters the normal development of radial glia cells (the main astrocytic precursors), delays the onset of GFAP expression, and decreases mRNA GFAP levels in fetal and postnatal brains and in radial glia and astrocytes in primary culture. Recent evidence suggests that ethanol interferes with the transcription process of GFAP, thus leading to a reduction in GFAP-gene expression during astrogliogenesis. However, brief exposure of rats to high levels of ethanol during the neonatal period (the period of astrocyte differentiation) causes a transient gliosis, with an increase in GFAP and its mRNA levels. These findings indicate that astroglial cells are an important target of ethanol toxicity during central nervous system (CNS) development.

Serotonin depletion during synaptogenesis leads to decreased synaptic density and learning deficits in the adult rat: a possible model of neurodevelopmental disorders with cognitive deficits

Studies in the past have revealed serotonin to play a role in regulating the development and maturation of the mammalian brain, largely through the release of the astroglial protein S-100beta. S-100beta plays a role in neurite extension, microtubule and dendritic stabilization and regulation of the growth associated protein GAP-43, all of which are key elements in the production of synapses. Depletion of serotonin, and thus of S-100beta, during synaptogenesis should lead to a loss of synapses and the behaviors dependent on those synapses. The current study was undertaken to test this hypothesis. In order to assess the influence of serotonin we have looked at the synaptic density in the adult after depletion, by using immunodensitometry of synaptic markers (synaptophysin and MAP-2) and by studying behaviors thought to be highly dependent on synaptic plasticity and density. Male Sprague-Dawley rats were depleted of serotonin on postnatal days (PND) 10-20 by treating with the tryptophan hydroxylase inhibitor parachlorophenylalanine (PCPA; 100 mg/kg, s.c.). On PND's 30 and 62, animals were perfused for immunodensitometry. Littermates were used for behavioral testing. At PND 55-62, the animals were tested in an interchangeable maze with olfactory cues and in an eight-arm radial maze. Our results show a loss of both synaptic markers in the hippocampus on PND 30. At PND 62, the only remaining loss was of the dendritic marker MAP-2. The animals had deficits in both behaviors tested, suggestive of spacial learning deficits and of the failure to extinguish learned behaviors or to re-learn in a new set. Our findings show the long-term consequences of interfering with the role of serotonin in brain development on the morphology and function of the adult brain. These findings may have implications for human diseases, including schizophrenia, thought to be related to neurodevelopmental insults such as malnutrition, hypoxia, viruses or in utero drug exposure. Moreover, they provide further insights into the functioning of serotonin and S-100beta in development and aging.

Role of the 5-HT1A receptor in development of the neonatal rat brain: preliminary behavioral studies

Serotonin exerts an influence on the prenatal development of rat brain. However, later developmental times may be more applicable to the understanding of the role of serotonin in human developmental disorders. Therefore, the current study was undertaken to gain preliminary information on the postnatal effects of serotonin on rat brain development. As the 5-HT1A receptor has been shown to be involved in much of the developmental functions of serotonin, an agonist for this receptor, 8-hydroxy-DPAT (8-OH-DPAT), was used. Neonatal rat pups at three ages (postnatal days, PNDs) 3-10, 10-17 or 17-24) were injected daily with 1 mg/kg 8-OH-DPAT and evaluated for behavioral consequences. The youngest group showed accelerated incisor eruption and eye-opening, a possible consequence of 5-HT1A receptor interactions with epidermal growth factor (EGF). Behaviorally, the animals were more anxious. Animals treated from PND 10-17, showed no change in craniofacial development but showed greater behavioral maturity in measures of spontaneous alternation and activity in the open field. The oldest animals (PND 17-24) showed no behavioral alterations, suggesting that this time length is beyond the critical period for serotonin's influence in brain development.