Effects of prenatal 5-methoxytryptamine and parachlorophenylalanine on serotonergic uptake and behavior in the neonatal rat

Effects of Prenatal Exposure to a Mixture of Organophosphate Flame Retardants on Placental Gene Expression and Serotonergic Innervation in the Fetal Rat Brain

There is a growing need to understand the potential neurotoxicity of organophosphate flame retardants (OPFRs) and plasticizers because use and, consequently, human exposure, is rapidly expanding. We have previously shown in rats that developmental exposure to the commercial flame retardant mixture Firemaster 550 (FM 550), which contains OPFRs, results in sex-specific behavioral effects, and identified the placenta as a potential target of toxicity. The placenta is a critical coordinator of fetal growth and neurodevelopment, and a source of neurotransmitters for the developing brain. We have shown in rats and humans that flame retardants accumulate in placental tissue, and induce functional changes, including altered neurotransmitter production. Here, we sought to establish if OPFRs (triphenyl phosphate and a mixture of isopropylated triarylphosphate isomers) alter placental function and fetal forebrain development, with disruption of tryptophan metabolism as a primary pathway of interest. Wistar rat dams were orally exposed to OPFRs (0, 500, 1000, or 2000 μg/day) or a serotonin (5-HT) agonist 5-methoxytryptamine for 14 days during gestation and placenta and fetal forebrain tissues collected for analysis by transcriptomics and metabolomics. Relative abundance of genes responsible for the transport and synthesis of placental 5-HT were disrupted, and multiple neuroactive metabolites in the 5-HT and kynurenine metabolic pathways were upregulated. In addition, 5-HTergic projections were significantly longer in the fetal forebrains of exposed males. These findings suggest that OPFRs have the potential to impact the 5-HTergic system in the fetal forebrain by disrupting placental tryptophan metabolism.

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

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

Assessing behavioural and cognitive domains of autism spectrum disorders in rodents: current status and future perspectives

The establishment of robust and replicable behavioural testing paradigms with translational value for psychiatric diseases is a major step forward in developing and testing etiology-directed treatment for these complex disorders. Based on the existing literature, we have generated an inventory of applied rodent behavioural testing paradigms relevant to autism spectrum disorders (ASD). This inventory focused on previously used paradigms that assess behavioural domains that are affected in ASD, such as social interaction, social communication, repetitive behaviours and behavioural inflexibility, cognition as well as anxiety behaviour. A wide range of behavioural testing paradigms for rodents were identified. However, the level of face and construct validity is highly variable. The predictive validity of these paradigms is unknown, as etiology-directed treatments for ASD are currently not on the market. To optimise these studies, future efforts should address aspects of reproducibility and take into account data about the neurodevelopmental underpinnings and trajectory of ASD. In addition, with the increasing knowledge of processes underlying ASD, such as sensory information processes and synaptic plasticity, phenotyping efforts should include multi-level automated analysis of, for example, representative task-related behavioural and electrophysiological read-outs.

An ecological study on childhood autism

BACKGROUND AND METHODS

Idiopathic autism, suspected to be caused by exposure of genetically susceptible individuals to unknown environmental triggers, has increased dramatically in the past 25 years. The objectives of our study were to determine, using a linear regression model, whether the county prevalence of autism in the Pacific Northwest of the United States was associated with the source of drinking water for that county and whether this relationship was dependent on the level of environmental pollutants and meteorological factors in the county.

RESULTS

We found the previously reported relationship between precipitation and autism in a county was dependent on the amount of drinking water derived from surface sources in the county. We also found a positive association between the EPA's risk of neurological disease and autism, but this relationship was only present in warm areas.

CONCLUSIONS

Our study provides evidence for the hypothesis that environmental factors are associated with autism and that meteorological factors play a role in this relationship.

Developmental exposure to a serotonin agonist produces subsequent behavioral and neurochemical changes in the adult male prairie vole

Autistic spectrum disorders (ASDs) are classified as pervasive developmental disorders characterized by abnormalities in various cognitive and behavioral functions. Although exact underlying causes are still unknown, nearly 30% of autistic patients show elevated blood levels of serotonin (5-HT) and, therefore, various genetic and environmental factors that are known to elevate 5-HT levels may play a role in the development of ASDs. In the present study, we used the socially monogamous male prairie vole (Microtus ochrogaster) as an animal model to examine the effects of perinatal exposure to 5-methoxytryptamine (5-MT), a non-selective serotonin agonist, on subsequent behavioral and neurochemical changes in the brain. 5-MT treated males showed a decrease in affiliation and an increase in anxiety-related behavior, as well as a decrease in the density of 5-HT immunoreactive (ir) fibers in the amygdala and oxytocin-ir and vasopressin-ir cells in the paraventricular nucleus of the hypothalamus, compared to saline treated controls. These data indicate that exposure to 5-HT during early development can induce abnormalities in various neurochemical systems which, in turn, may underlie deficits in social and anxiety-related behaviors. In addition, these data will help to establish the prairie vole model to study the neurobiological underpinnings of complex neuropsychiatric disorders such as ASDs.

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

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

Serotonergic vulnerability and depression: assumptions, experimental evidence and implications

In recent years, the term serotonergic vulnerability (SV) has been used in scientific literature, but so far it has not been explicitly defined. This review article attempts to elucidate the SV concept. SV can be defined as increased sensitivity to natural or experimental alterations of the serotonergic (5-HTergic) system. Several factors that may disrupt the 5-HTergic system and hence contribute to SV are discussed, including genetic factors, female gender, personality characteristics, several types of stress and drug use. It is explained that SV can be demonstrated by means of manipulations of the 5-HTergic system, such as 5-HT challenges or acute tryptophan depletion (ATD). Results of 5-HT challenge studies and ATD studies are discussed in terms of their implications for the concept of SV. A model is proposed in which a combination of various factors that may compromise 5-HT functioning in one person can result in depression or other 5-HT-related pathology. By manipulating 5-HT levels, in particular with ATD, vulnerable subjects may be identified before pathology initiates, providing the opportunity to take preventive action. Although it is not likely that this model applies to all cases of depression, or is able to identify all vulnerable subjects, the strength of the model is that it may enable identification of vulnerable subjects before the 5-HT related pathology occurs.

Plasma serotonin in autism

Serotonin is necessary for normal fetal brain development. Administration of serotonin inhibitors to pregnant rats results in offspring with abnormal behaviors, brain morphology, and serotonin receptor numbers. Low maternal plasma serotonin may contribute to abnormal brain development in autism. In this study, plasma serotonin levels in autism mothers and control mothers of typically developing children were compared, and plasma serotonin levels in children with autism (n = 17) and their family members were measured. Plasma serotonin levels in autism mothers were significantly lower than in mothers of normal children (P = 0.002). Plasma serotonin levels correlated between autism mothers and their children, but differed between autistic children and their fathers (P = 0.028) and siblings (P = 0.063). Low maternal plasma serotonin may be a risk factor for autism through effects on fetal brain development.

Behavioral and cellular consequences of increasing serotonergic activity during brain development: a role in autism?

The hypothesis explored in this review is that the high levels of serotonin in the blood seen in some autistic children (the so-called hyperserotonemia of autism) may lead to some of the behavioral and cellular changes also observed in the disorder. At early stages of development, when the blood-brain Barrier is not yet fully formed, the high levels of serotonin in the blood can enter the brain of a developing fetus and cause loss of serotonin terminals through a known negative feedback function of serotonin during development. The loss of serotonin innervation persists throughout subsequent development and the symptoms of autism appear. A review of the basic scientific literature on prenatal treatments affecting serotonin is given, in support of this hypothesis, with an emphasis on studies using the serotonin agonist, 5-methoxytryptamine (5-MT). In work using 5-MT to mimic hyperserotonemia, Sprague-Dawley rats are treated from gestational day 12 until postnatal 20. In published reports, these animals have been found to have a significant loss of serotonin terminals, decreased metabolic activity in cortex, changes in columnar development in cortex, changes in serotonin receptors, and "autistic-like" behaviors. In preliminary cellular findings given in this review, the animals have also been found to have cellular changes in two relevant brain regions: 1. Central nucleus of the amygdala, a brain region involved in fear-responding, where an increase in calcitonin gene related peptide (CGRP) was found 2. Paraventricular nucleus of the hypothalamus, a brain region involved in social memory and bonding, where a decrease in oxytocin was found. Both of these cellular changes could result from loss of serotonin innervation, possibly due to loss of terminal outgrowth from the same cells of the raphe nuclei. Thus, increased serotonergic activity during development could damage neurocircuitry involved in emotional responding to social stressors and may have relevance to the symptoms of autism.

para-Chlorophenylalanine induces lenticular opacities by prenatal, neonatal, and juvenile treatments, but not by adult treatment, in rats

The lenticular opacities induced by the administration of para-chlorophenylalanine (PCPA) during the different developmental periods were investigated in rats. Rats were given PCPA (100 or 200 mg/kg) during prenatal, neonatal, juvenile, and adult periods, and their lenses were observed ophthalmologically. The prenatal treatment with PCPA on gestation days 14-20 (GD 14-20) produced lenticular opacities that were detected in the area of the lens nucleus (pin-head opacity), and the neonatal treatment on postnatal days 0-40 (PD 0-40) produced mature cataracts. The juvenile treatment on PD 14-40 produced opacities in the posterior area as early as the day following the first treatment (PD 15). When the administration was continued, mature cataract was developed. However, we did not detect any changes in the lens of the adult rat (over 11 weeks of age) treated with the same dose of PCPA. These results suggest that the incidence of a PCPA-induced cataract depends on the age of the animals when PCPA is administered.

Protective effects of maternal buspirone treatment on serotonin reuptake sites in ethanol-exposed offspring

Previous work in this laboratory demonstrated that in utero ethanol exposure is associated with abnormal development of the serotonergic system. Specific abnormalities included deficiencies of serotonin (5-HT) and its metabolites, and cortical 5-HT reuptake sites. The concentration of 5-HT1A receptors was also altered. The serotonin deficit was detected in the fetal ethanol-exposed brain, at an age when 5-HT would normally function as an essential trophic factor. Thus, it was hypothesized that the early 5-HT ethanol-associated deficit of an essential trophic factor (e.g. 5-HT) could contribute to subsequent developmental abnormalities in serotonergic neurons. In the present investigation we used quantitative autoradiography (QAR) to more fully characterize the developmental abnormalities in 5-HT reuptake sites in developing offspring of ethanol-fed rats. In addition, we attempted to overcome the potential negative impact of the ethanol-associated deficit of fetal 5-HT, by administering a 5-HT1A agonist, buspirone, to pregnant rats. These investigations demonstrated that postnatal (PN) 19 and/or 35 day ethanol-exposed offspring had a significant decrease in [3H]citalopram binding to 5-HT reuptake sites in the frontal cortex, parietal cortex, lateral hypothalamus, substantia nigra, medial septum, and striatum. In contrast, [3H]citalopram binding was increased in the dorsal raphe on PN5 and in the median raphe on PN19. No significant ethanol-associated changes were detected in the hippocampus CA3 region or in the amygdala. When [3H]citalopram binding was compared in the offspring of saline- and buspirone-treated dams, it appeared that maternal treatment with buspirone prevented or reversed most of the ethanol-associated developmental abnormalities in 5-HT reuptake sites. Buspirone prevented the decline in binding of [3H]citalopram in the frontal cortex, lateral hypothalamus, substantia nigra and medial septum. Similarly, buspirone treatment prevented the ethanol-associated increase in binding in the dorsal and median raphe. Additional experiments are needed to elucidate the impact of maternal buspirone treatment on the development of other neurotransmitter systems in offspring.

Dose-related effects of prenatal 5-methoxytryptamine (5-MT) on development of serotonin terminal density and behavior

Our previous studies with a tissue culture model of neuronal development have shown that the development of serotonin neurons is dependent, at least in part, on the stimulation of high affinity serotonin receptors. One receptor inhibits the outgrowth of neurons, while the other promotes it. The present study was therefore undertaken to replicate these findings in a whole animal model system. Pregnant Sprague-Dawley rats were treated from gestational day 12 until birth with 0.1, 1.0 or 3.0 mg/kg 5-methoxytryptamine (5-MT). The pups were assessed for serotonin outgrowth by the selective synaptosomal uptake of [3H]serotonin at postnatal days 1, 15 and 30 (D1, D15, D30). In addition, the pups were tested behaviorally for the neonatal serotonin syndrome at D5 (induced by quipazine), spontaneous alternation and open field activity at day 15 and lick suppression at day 30. At 1.0 mg/kg, the terminal outgrowth of serotonin neurons was inhibited, while the highest dose, 3.0 mg/kg, showed stimulation of outgrowth. The highest dose caused behavioral alterations which had abated by 30 days, while the intermediate dose (1.0 mg/kg) showed behavioral changes throughout. Interestingly, the lowest dose, 0.1 mg/kg, showed changes in uptake only at D1 and behavioral changes only at later timepoints, principally at D30. This suggests that serotonin not only plays a role in regulating the development of the neurons which produce it, but that it may also play a role in neurochemical imprinting--that is, changes in behavior in the adult may be due to changes in neurochemistry during development, even though that neurochemistry may have been corrected by the time the animal becomes an adult.

Behavioral and neurochemical effects of prenatal methylenedioxymethamphetamine (MDMA) exposure in rats

MDMA is a hallucinogenic drug that is used by the general public as a recreational drug of abuse. The neurobehavioral consequences of prenatal MDMA exposure are unknown. Groups of pregnant rats were gavaged with 0, 2.5, or 10 mg/kg MDMA during gestation on alternate gestational days 6-18. Gestational duration, litter size, neonatal birth weights and physical appearance at birth were unaffected by MDMA treatments. Pregnancy weight gain was significantly reduced by MDMA treatment. Progeny growth, maturational parameters (eye opening and incisor eruption times), surface righting reflex, swimming performance, forelimb grip strength, milk-induced behaviors, passive avoidance behavior, figure-8 maze activity over 48 hours, the density of brain serotonin (5-HT) uptake sites, and brain 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) levels were unaffected by MDMA treatments. Olfactory discrimination on postnatal days (PND) 9-11 was enhanced in both male and female MDMA-treated progeny, while negative geotaxis (PND 7-10) was delayed in female pups. In contrast to progeny, MDMA caused dose-dependent decreases in 5-HT and 5-HIAA levels in discrete brain areas of the dam. It is concluded that prenatal exposure to MDMA at the levels used here produces only subtle behavioral alterations in developing rats. The dam is more at risk for MDMA-induced 5-HT depletion than is the conceptus.

Prenatal treatment with a selective D1 receptor agonist (SKF 38393) alters adult [3H]paroxetine binding and dopamine and serotonin behavioral sensitivity

We have previously shown that the development of serotonin neurons can be affected by various pharmacological agents acting on the serotonin system. Receptor stimulation by high doses of 5-methoxytryptamine (5-MT) causes increased outgrowth, through release of an astroglial growth factor, while a low concentration of 5-MT has a direct inhibitory effect on neuronal outgrowth. Since 5-MT is known to be a release-regulating autoreceptor agonist, the present study was aimed at testing the hypothesis that inhibition of serotonin release causes the inhibition of outgrowth. We used the D1 receptor agonist SKF 38393 as an inhibitor of serotonin release. Pregnant Sprague-Dawley rats were treated with SKF 38393 (1 mg/kg; subcutaneously) from gestational day 12 until parturition. Development of serotonin terminal outgrowth was evaluated in the offspring using the selective uptake marker [3H]paroxetine in brainstem and frontal cortex. In saline and SKF 38393 treated animals, the developmental pattern in the frontal cortex showed the highest terminal density at day 60 and a return to normal by day 90, with no statistically significant differences between the groups. Conversely, in the brainstem, [3H]paroxetine binding developed normally until postnatal day 90, when the SKF 38393 pretreated animals showed only 58% of the binding observed in saline animals. In a fixed interval responding task, given at day 90, both dopamine and serotonin receptor systems have significantly decreased sensitively after the SKF 38393 pretreatment. In conclusion, our results show that the inhibitory effects of serotonin on the growth of serotonin neurons, may be through inhibition of neurotransmitter release. Moreover, we propose a means by which serotonin and dopamine systems could be interdependent during development.

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

Our work has been concerned with the role of high affinity serotonin receptors in regulating the development of the serotonergic system. In previous studies, we have found evidence that these receptors occur on astroglial cells and that their number is developmentally linked. The current work is aimed at investigating the mechanism by which these receptors may regulate serotonin neuronal growth. Primary cultures of astroglial cells were exposed to serotonin (5-HT) or the selective receptor agonists 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-D-PAT, for 5-HT1a receptors) or trifluoro-methyl-phenyl-piperazine (TFMPP) and m-chlorophenylpiperazine (mCPP) (for 5-HT1b receptors). Media was collected after 4 or 24 h, and added to primary cultures of serotonergic neurons. Growth was determined by specific uptake of radiolabeled serotonin into the cultures. Our results show the presence of a factor(s) in the glial-conditioned media which can be stimulatory or toxic to serotonin neurons, depending on the neuronal plating density. This factor is significantly present after 24 h, is found in both brainstem and cortical astroglial-conditioned media and appears to be linked to the 5-HT1a receptor. Thus, it appears possible that the serotonergic neuronal system can regulate its own development through an action on astroglial cells.

Use of tissue culture models to study neuronal regulatory trophic and toxic factors in the aged brain

Dementia is believed to result from the loss of selective neurons within the brain, but approaches for systematic study of that degenerative process are hampered by the complexity of the neuronal milieu. Tissue culture models provide a means to reduce dramatically the variables inherent in the study of neuronal plasticity. Three levels of complexity can be described: cellular and molecular diversity; primary and secondary interconnections; and finally, the dynamics influenced by age. The following review discusses the advantages and disadvantages of tissue culture models for the detailed study of neuronal trophic and toxic factors. Our selection of factors is broadened to include ions, intermediate metabolites, antioxidants, steroids, neuropeptides, gangliosides, metals, neurotransmitters, brain extracts, and protein molecules. Most of these factors have been shown to be altered in the aged brain, to have a significant effect on cultured neurons, or both. This multilevel analysis provides the reader with an overview of the events regulating neuronal survival, differentiation and death. An understanding of these basic questions is necessary to sequence the molecular events resulting in neuronal death.