Serotonin transporter antibodies: production, characterization, and localization in the brain

Wiring and Volume Transmission: An Overview of the Dual Modality for Serotonin Neurotransmission

Serotonin is a neurotransmitter involved in the modulation of a multitude of physiological and behavioral processes. In spite of the relatively reduced number of serotonin-producing neurons present in the mammalian CNS, a complex long-range projection system provides profuse innervation to the whole brain. Heterogeneity of serotonin receptors, grouped in seven families, and their spatiotemporal expression pattern account for its widespread impact. Although neuronal communication occurs primarily at tiny gaps called synapses, wiring transmission, another mechanism based on extrasynaptic diffusion of neuroactive molecules and referred to as volume transmission, has been described. While wiring transmission is a rapid and specific one-to-one modality of communication, volume transmission is a broader and slower mode in which a single element can simultaneously act on several different targets in a one-to-many mode. Some experimental evidence regarding ultrastructural features, extrasynaptic localization of receptors and transporters, and serotonin-glia interactions collected over the past four decades supports the existence of a serotonergic system of a dual modality of neurotransmission, in which wiring and volume transmission coexist. To date, in spite of the radical difference in the two modalities, limited information is available on the way they are coordinated to mediate the specific activities in which serotonin participates. Understanding how wiring and volume transmission modalities contribute to serotonergic neurotransmission is of utmost relevance for the comprehension of serotonin functions in both physiological and pathological conditions.

Exposure to a human relevant mixture of persistent organic pollutants or to perfluorooctane sulfonic acid alone dysregulates the developing cerebellum of chicken embryo

Prenatal exposure to persistent organic pollutants (POPs) is associated with neurodevelopmental disorders. In the present study, we explored whether a human-relevant POP mixture affects the development of chicken embryo cerebellum. We used a defined mixture of 29 POPs, with chemical composition and concentrations based on blood levels in the Scandinavian population. We also evaluated exposure to a prominent compound in the mixture, perfluorooctane sulfonic acid (PFOS), alone. Embryos (n = 7-9 per exposure group) were exposed by injection directly into the allantois at embryonic day 13 (E13). Cerebella were isolated at E17 and subjected to morphological, RNA-seq and shot-gun proteomics analyses. There was a reduction in thickness of the molecular layer of cerebellar cortex in both exposure scenarios. Exposure to the POP mixture significantly affected expression of 65 of 13,800 transcripts, and 43 of 2,568 proteins, when compared to solvent control. PFOS alone affected expression of 80 of 13,859 transcripts, and 69 of 2,555 proteins. Twenty-five genes and 15 proteins were common for both exposure groups. These findings point to alterations in molecular events linked to retinoid X receptor (RXR) signalling, neuronal cell proliferation and migration, cellular stress responses including unfolded protein response, lipid metabolism, and myelination. Exposure to the POP mixture increased methionine oxidation, whereas PFOS decreased oxidation. Several of the altered genes and proteins are involved in a wide variety of neurological disorders. We conclude that POP exposure can interfere with fundamental aspects of neurodevelopment, altering molecular pathways that are associated with adverse neurocognitive and behavioural outcomes.

Localization of the Serotonin Transporter in the Dog Intestine and Comparison to the Rat and Human Intestines

Serotonin is crucial in gastrointestinal functions, including motility, sensitivity, secretion, and the inflammatory response. The serotonin transporter (SERT), responsible for serotonin reuptake and signaling termination, plays a prominent role in gastrointestinal physiology, representing a promising therapeutic target in digestive disorders. Serotonin transporter expression has been poorly investigated in veterinary medicine, under both healthy and pathological conditions, including canine chronic enteropathy, in which the serotonin metabolism seems to be altered. The aim of the present study was to determine the distribution of SERT immunoreactivity (SERT-IR) in the dog intestine and to compare the findings with those obtained in the rat and human intestines. Serotonin transporter-IR was observed in canine enterocytes, enteric neurons, lamina propria cells and the tunica muscularis. Data obtained in dogs were consistent with those obtained in rats and humans. Since the majority of the serotonin produced by the body is synthesized in the gastrointestinal tract, SERT-expressing cells may exert a role in the mechanism of serotonin reuptake.

Immunochemical localization of vesicular monoamine transporter 2 (VMAT2) in mouse brain

Vesicular monoamine transporter 2 (VMAT2, SLC18A2) is a transmembrane transporter protein that packages dopamine, serotonin, norepinephrine, and histamine into vesicles in preparation for neurotransmitter release from the presynaptic neuron. VMAT2 function and related vesicle dynamics have been linked to susceptibility to oxidative stress, exogenous toxicants, and Parkinson's disease. To address a recent depletion of commonly used antibodies to VMAT2, we generated and characterized a novel rabbit polyclonal antibody generated against a 19 amino acid epitope corresponding to an antigenic sequence within the C-terminal tail of mouse VMAT2. We used genetic models of altered VMAT2 expression to demonstrate that the antibody specifically recognizes VMAT2 and localizes to synaptic vesicles. Furthermore, immunohistochemical labeling using this VMAT2 antibody produces immunoreactivity that is consistent with expected VMAT2 regional distribution. We show the distribution of VMAT2 in monoaminergic brain regions of mouse brain, notably the midbrain, striatum, olfactory tubercle, dopaminergic paraventricular nuclei, tuberomammillary nucleus, raphe nucleus, and locus coeruleus. Normal neurotransmitter vesicle dynamics are critical for proper health and functioning of the nervous system, and this well-characterized VMAT2 antibody will be a useful tool in studying neurodegenerative and neuropsychiatric conditions characterized by vesicular dysfunction.

Mapping the connectivity of serotonin transporter immunoreactive axons to excitatory and inhibitory neurochemical synapses in the mouse limbic brain

Serotonin neurons arise from the brainstem raphe nuclei and send their projections throughout the brain to release 5-HT which acts as a modulator of several neuronal populations. Previous electron microscopy studies in rats have morphologically determined the distribution of 5-HT release sites (boutons) in certain brain regions and have shown that 5-HT containing boutons form synaptic contacts that are either symmetric or asymmetric. In addition, 5-HT boutons can form synaptic triads with the pre- and postsynaptic specializations of either symmetrical or asymmetrical synapses. However, due to the labor intensive processing of serial sections required by electron microscopy, little is known about the neurochemical properties or the quantitative distribution of 5-HT triads within whole brain or discrete subregions. Therefore, we used a semi-automated approach that combines immunohistochemistry and high-resolution confocal microscopy to label serotonin transporter (SERT) immunoreactive axons and reconstruct in 3D their distribution within limbic brain regions. We also used antibodies against key pre- (synaptophysin) and postsynaptic components of excitatory (PSD95) or inhibitory (gephyrin) synapses to (1) identify putative 5-HTergic boutons within SERT immunoreactive axons and, (2) quantify their close apposition to neurochemical excitatory or inhibitory synapses. We provide a 5-HTergic axon density map and have determined the ratio of synaptic triads consisting of a 5-HT bouton in close proximity to either neurochemical excitatory or inhibitory synapses within different limbic brain areas. The ability to model and map changes in 5-HTergic axonal density and the formation of triadic connectivity within whole brain regions using this rapid and quantitative approach offers new possibilities for studying neuroplastic changes in the 5-HTergic pathway.

A refined high carbohydrate diet is associated with changes in the serotonin pathway and visceral obesity

Consumption of palatable foods high in refined carbohydrate has been implicated as a contributing factor to the epidemic levels of obesity. Such foods may disrupt appetite regulation in the hypothalamus through alterations in hunger and satiety signalling. This investigation examined whether a palatable high refined carbohydrate (HRC) diet with the potential to induce obesity was linked to modulation of serotonin and dopamine signalling within the hypothalamus of rats. Male Wistar rats were allowed ad libitum access to either a palatable refined carbohydrate enriched (HRC) diet or standard chow (SC). Visceral fat percentage was used as a measure of the animals' weight gain during the trial. Real-time PCR was applied to determine any variation in levels of expression of the serotonin (Slc6A4 or Sert) and dopamine transporter (Slc6A3 or Dat) genes. After 29 weeks, the HRC group showed a significant increase in visceral fat percentage accompanied by increased expression of Sert. Higher levels of circulating triglycerides were also seen. This investigation determined that a refined high carbohydrate diet is associated with visceral obesity, increased circulating lipids in the blood and distorted serotonergic signalling, which possibly alters satiety and hunger signals.

Effects of the antidepressant fluoxetine on the subcellular localization of 5-HT1A receptors and SERT

Serotonin (5-HT) 5-HT(1A) autoreceptors (5-HT(1A)autoR) and the plasmalemmal 5-HT transporter (SERT) are key elements in the regulation of central 5-HT function and its responsiveness to antidepressant drugs. Previous immuno-electron microscopic studies in rats have demonstrated an internalization of 5-HT(1A)autoR upon acute administration of the selective agonist 8-OH-DPAT or the selective serotonin reuptake inhibitor antidepressant fluoxetine. Interestingly, it was subsequently shown in cats as well as in humans that this internalization is detectable by positron emission tomography (PET) imaging with the 5-HT(1A) radioligand [(18)F]MPPF. Further immunocytochemical studies also revealed that, after chronic fluoxetine treatment, the 5-HT(1A)autoR, although present in normal density on the plasma membrane of 5-HT cell bodies and dendrites, do not internalize when challenged with 8-OH-DPAT. Resensitization requires several weeks after discontinuation of the chronic fluoxetine treatment. In contrast, the SERT internalizes in both the cell bodies and axon terminals of 5-HT neurons after chronic but not acute fluoxetine treatment. Moreover, the total amount of SERT immunoreactivity is then reduced, suggesting that SERT is not only internalized, but also degraded in the course of the treatment. Ongoing and future investigations prompted by these finding are briefly outlined by way of conclusion.

Differential distribution patterns from medial prefrontal cortex and dorsal raphe to the locus coeruleus in rats

Locus coeruleus (LC) consists of a densely packed nuclear core and a surrounding plexus of dendritic zone, which is further divided into several subregions. Whereas many limbic-related structures topographically target specific subregions of the LC, the precise projections from two limbic areas, that is, medial prefrontal cortex (mPFC) and dorsal raphe (DR), have not been investigated. The goal of the present study is to identify and compare the distribution patterns of mPFC and DR afferent terminals to the LC nuclear core as opposed to specific pericoerulear dendritic regions (Peri-LC). To address these issues, anterograde tracer injections were combined with dopamine-β-hydroxylase (DBH) immunofluorescent staining to reveal the distribution patterns around the LC nuclear complex. Our data suggest that both mPFC-LC and DR-LC projections exhibit selective afferent terminal patterns. More specifically, mPFC-LC projecting fibers mainly target the rostromedial Peri-LC, whereas DR-LC projecting fibers demonstrate a preference to the caudal juxtaependymal Peri-LC. Thus, our present findings provide further evidences that afferents to the LC are topographically organized. Understanding the relationship among different inputs to the LC may help to elucidate the organizing principle which likely governs the interactions between the broad afferent sources of the LC and its global efferent targets.

Shared features of S100B immunohistochemistry and cytochrome oxidase histochemistry in the ventroposterior thalamus and lateral habenula in neonatal rats

The ventroposterior thalamus and the habenular nuclei of the epithalamus are relevant to the monoaminergic system functionally and anatomically. The glia-derived S100B protein plays a critical role in the development of the nervous system including the monoaminergic systems. In this study, we performed an immunohistochemical study of glia-related proteins including S100B, serotonin transporter, and microtubule-associated protein 2, as well as cytochrome oxidase histochemistry in neonatal rats. Results showed the same findings for S100B immunohistochemistry between the ventroposterior thalamus and the lateral habenula at postnatal day 7: intense staining in cell bodies of astrocytes, diffusely spread immunoproduct in the intercellular space, and S100B-free areas as well as a strong reaction to cytochrome oxidase histochemistry. Further common features were the scarcity of glial fibrillary acidic protein-positive astrocytes and the few apoptotic cells observed. The results of the cytochrome oxidase reaction suggested that S100B is released actively into intercellular areas in restricted brain regions showing high neuronal activity at postnatal day 7. Pathology of the ventroposterior thalamus and the habenula is suggested in mental disorders, and S100B might be a key factor for investigations in these areas.

Changes in the serotonergic system and in brain-derived neurotrophic factor distribution in the main olfactory bulb of pcd mice before and after mitral cell loss

The serotonergic centrifugal system innervating the main olfactory bulb (MOB) plays a key role in the modulation of olfactory processing. We have previously demonstrated that this system suffers adaptive changes under conditions of a lack of olfactory input. The present work examines the response of this centrifugal system after mitral cell loss in the Purkinje cell degeneration (pcd) mutant mice. The distribution and density of serotonergic centrifugal axons were studied in the MOB of control and pcd mice, both before and after the loss of mitral cells, using serotonin (5-HT) and 5-HT transporter immunohistochemistry. Studies of the amount of 5-HT and its metabolite, 5-hydroxyindole acetic acid (5-HIAA), were performed by means of high-performance liquid chromatography (HPLC), and the relative amounts of brain-derived neurotrophin factor, BDNF, and its major receptor, tropomyosin-related kinase B (TrkB), were measured by Western blot. Our study revealed that the serotonergic system develops adaptive changes after, but not before, mitral cell loss. The lack of the main bulbar projection cells causes a decrease in the serotonergic input received by the MOB, whereas the number of serotonergic cells in the raphe nuclei remains constant. In addition, one of the molecules directly involved in serotonergic sprouting, the neurotrophin BDNF and its main receptor TrkB, underwent alterations in the MOBs of the pcd animals even before the loss of mitral cells. These data indicate that serotonergic function in the MOB is closely related to olfactory activity and that mitral cell loss induces serotonergic plastic responses.

Organization of somatosensory cortex in the Northern grasshopper mouse (Onychomys leucogaster), a predatory rodent

Northern grasshopper mice (Onychomys leucogaster) are among the most highly carnivorous rodents in North America. Because predatory mammals may have specialization of senses used to detect prey, we investigated the organization of sensory areas within grasshopper mouse neocortex and quantified the number of myelinated axons in grasshopper mouse trigeminal, cochlear, and optic nerves. Multiunit electrophysiological recordings combined with analysis of flattened sections of neocortex processed for cytochrome oxidase were used to determine the topography of primary somatosensory cortex (S1) and the location and size of both the visual and auditory cortex in adult animals. These findings were then related to the distinctive chemoarchitecture of layer IV visible in flattened cortical sections of juvenile grasshopper mice labeled with the serotonin transporter (SERT) antibody, revealing a striking correspondence between electrophysiological maps and cortical anatomy.

Pattern of distribution of serotonergic fibers to the thalamus of the rat

It is well established that serotonergic (5-hydroxytryptamine, 5-HT) fibers, mainly originating from the dorsal and median raphe nuclei of the brainstem, distribute throughout the forebrain, most heavily to 'limbic' forebrain structures. Few reports have examined the distribution of 5-HT fibers to the thalamus and none to our knowledge using immunoprocedures for the detection of the serotonin transporter (SERT)-a very sensitive marker for 5-HT fibers. Using immunohistochemical methods for SERT, we examined the pattern of distribution of 5-HT fibers to the thalamus in the rat. We show that serotonergic fibers are heavily concentrated in midline, intralaminar and association nuclei of the thalamus, and with the exception of the lateral geniculate complex, weakly distributed to principal nuclei of thalamus. Specifically, we demonstrate that 5-HT fibers are densely concentrated in the anteroventral, anteromedial and interanteromedial nuclei of the anterior thalamus, the paraventricular, rhomboid and reuniens nuclei of the midline thalamus, the central medial and central lateral nuclei of the intralaminar thalamus, the intermediodorsal nucleus, the lateral dorsal nucleus, and the dorsal and ventral lateral geniculate nuclei and intergeniculate leaflet of the LGN complex. Less densely innervated sites include the mediodorsal, paracentral, parafascicular, lateral posterior and submedial nuclei of thalamus. Remaining regions of the thalamus, largely consisting of principal nuclei, contained few 5-HT fibers. This pattern of 5-HT innervation indicates that serotonin/ serotonergic fibers mainly affect thalamic nuclei with connections to 'non-principal' or limbic regions of the cortex (or forebrain). This suggests that serotonergic fibers to the thalamus may exert a significant influence on affective and cognitive functions, possibly complementing the actions of 5-HT fibers to other parts of the brain involved in emotional and cognitive behaviors.

Serotoninergic fine-tuning of the excitation-inhibition balance in rat visual cortical networks

Fundamental brain functions depend on a balance between excitation (E) and inhibition (I) that is highly adjusted to a 20-80% set point in layer 5 pyramidal neurons (L5PNs) of rat visual cortex. Dysregulations of both the E-I balance and the serotonergic system in neocortical networks lead to serious neuronal diseases including depression, schizophrenia, and epilepsy. However, no link between the activation of neuronal 5-hydroxytryptamine receptors (5-HTRs) and the cortical E-I balance has yet been reported. Here we used a combination of patch-clamp recordings of composite stimulus-locked responses in L5PN following local electrical stimulations in either layer 2/3 or 6, simultaneous measurement of excitatory and inhibitory conductance dynamics, together with selective pharmacological targeting and single-cell reverse transcriptase-polymerase chain reaction. We show that cortical serotonin shifts the E-I balance in favor of more E and we reveal fine and differential modulations of the E-I balance between 5-HTR subtypes, in relation to whether layer 2/3 or 6 was stimulated and in concordance with the specific expression pattern of these subtypes in pyramidal cells and deep interneurons. This first evidence for the functional segregation of 5-HTR subtypes sheds new light on their coherent functioning in polysynaptic sensory circuits.

Sexual dimorphic stages affect both proliferation and serotonergic innervation in the adult rostral migratory stream

One of the sexual dimorphic differences in adult rodents is neural proliferation. Here we demonstrate that physiological hormone stages can modulate this proliferation in the adult forebrain. Female mice, both pregnant and synchronized in oestrus, exhibited higher proliferating cell percentages than males in both the rostral migratory stream (RMS) and the olfactory bulb (OB). Moreover, although the hormonal component also influenced the subventricular zone (SVZ), no differences in proliferation were observed in this region. In addition, both groups of females had higher numbers of serotonergic fibres in these regions. Serotonin may therefore be related to the mechanism of action by which hormones can affect cell proliferation of this brain region. We also evaluated cell death in the SVZ in males and females, finding that this was higher in the former. Taken together, our results support the idea that in female rodents more neuroblasts are able to reach the RMS and then proliferate, apoptosis being an additional mechanism affecting the low proliferation of cells in the RMS and OB in males. Thus, proliferation in the RMS is influenced by sexual dimorphism.

Organic cation transporter 3: Keeping the brake on extracellular serotonin in serotonin-transporter-deficient mice

Mood disorders cause much suffering and are the single greatest cause of lost productivity worldwide. Although multiple medications, along with behavioral therapies, have proven effective for some individuals, millions of people lack an effective therapeutic option. A common serotonin (5-HT) transporter (5-HTT/SERT, SLC6A4) polymorphism is believed to confer lower 5-HTT expression in vivo and elevates risk for multiple mood disorders including anxiety, alcoholism, and major depression. Importantly, this variant is also associated with reduced responsiveness to selective 5-HT reuptake inhibitor antidepressants. We hypothesized that a reduced antidepressant response in individuals with a constitutive reduction in 5-HTT expression could arise because of the compensatory expression of other genes that inactivate 5-HT in the brain. A functionally upregulated alternate transporter for 5-HT may prevent extracellular 5-HT from rising to levels sufficiently high enough to trigger the adaptive neurochemical events necessary for therapeutic benefit. Here we demonstrate that expression of the organic cation transporter type 3 (OCT3, SLC22A3), which also transports 5-HT, is upregulated in the brains of mice with constitutively reduced 5-HTT expression. Moreover, the OCT blocker decynium-22 diminishes 5-HT clearance and exerts antidepressant-like effects in these mice but not in WT animals. OCT3 may be an important transporter mediating serotonergic signaling when 5-HTT expression or function is compromised.

Dystrophic serotonergic axons in neurodegenerative diseases

Neurodegenerative diseases such as Parkinson's disease (PD), frontal lobe dementia (FLD) and diffuse Lewy-body dementia (DLBD) have diverse neuropathologic features. Here we report that serotonin fibers are dystrophic in the brains of individuals with these three diseases. In neuropathologically normal (control) brains (n=3), serotonin axons immunoreactive (IR) with antibodies against the serotonin transporter (5-HTT) protein were widely distributed in cortex (entorhinal and dorsolateral prefrontal), hippocampus and rostral brainstem. 5-HTT-IR fibers-of-passage appeared thick, smooth, and unbranched in medial forebrain bundle, medial lemniscus and cortex white matter. The terminal branches were fine, highly branched and varicose in substantia nigra, hippocampus and cortical gray matter. In the diseased brains, however, 5-HTT-IR fibers in the forebrain were reduced in number and were frequently bulbous, splayed, tightly clustered and enlarged. Morphometric analysis revealed significant differences in the size distribution of the 5-HTT-IR profiles in dorsolateral prefrontal area between neurodegenerative diseases and controls. Our observations provide direct morphologic evidence for degeneration of human serotonergic axons in the brains of patients with neurodegenerative diseases despite the limited size (n=3 slices for each region (3) from each brain (4), total slices was n=36) and the lack of extensive clinical characterization of the analyzed cohort. This is the first report of dystrophic 5-HTT-IR axons in postmortem human tissue.

Alcohol exposure alters cell cycle and apoptotic events during early neurulation

BACKGROUND

Fetal alcohol exposure causes growth deficits, microencephaly, and neurological abnormalities. Although the effects of alcohol on developmental delay and growth-related deficits have been hypothesized, little is understood about how alcohol alters, in particular, the cyclin pathway within the cell cycle, which is critical to proliferation and apoptotic control. In this study, we examined cell cycle proteins pertinent to the G1-S phase transition and apoptosis, to determine if cell cycle misregulation can be attributed to apoptotic induction and growth defects.

METHODS

We examined cell cycle regulation during G1 and S-phase, and DNA fragmentation damage, using E14 dorsal root ganglia neural stem cells (DRG-NC), and cultured mouse embryos exposed to 200 and 400 mg/dl ethanol.

RESULTS

Alcohol-exposed DRG-NC demonstrated a dose-dependent increase in cells expressing increased cyclin D1 protein, and increased DNA fragmentation. Western blot analysis, using embryos, demonstrated an overexpression of cyclin D1, D2, and E2F1, key G1 to S-phase cell cycle regulatory components, and increases in p53, linking the cell cycle and apoptotic pathways. Bromodeoxyuridine incorporation indicated reduced DNA synthesis and growth in several embryonic regions. Propidium iodide staining demonstrated decreases in DNA content and increases in DNA fragmentation in several embryonic tissues.

CONCLUSIONS

This study indicated that retarded growth of DRG-NC and embryos, induced by alcohol, is associated with altered expression of cell cycle and apoptotic proteins and concurrent inhibition of proliferation and increased DNA fragmentation. We suggest that alcohol induces an increase in cyclin D1 expression, premature S-phase entry, and disjointed DNA synthesis with increased apoptosis.

A PET study on regional coexpression of 5-HT1A receptors and 5-HTT in the human brain

RATIONALE

Several lines of evidence suggest inter-dependency between the serotonin transporter (5-HTT) and the 5HT1A receptor, two recognised targets for the treatment of anxiety and depression.

OBJECTIVES

to examine the correlation of regional expression levels for these two serotonergic markers in the human brain in vivo.

METHODS

Twelve male control subjects were examined with PET twice on the same day, using the radioligands [11C]WAY 100635 and [11C]MADAM for quantification of the 5-HT1A receptor and the 5-HTT, respectively. The binding potential (BP) was calculated for raphe nuclei, hippocampus and frontal cortex.

RESULTS

In all regions, the BP for both [11C]WAY 100635 (raphe nuclei 1.85-4.71, hippocampus 2.52-6.17, frontal cortex 2.03-3.79) and [11C]MADAM (2.70-7.65, 0.47-1.76, 0.18-0.51) varied several fold between subjects. In the raphe nuclei, where the two markers are situated on the same neurons, the ratio of [11C]WAY 100635 binding to [11C]MADAM BP binding varied considerably (0.43-1.05). There was a positive correlation between the two markers in the raphe nuclei (rxy=0.68, p<0.05) and in the hippocampus (rxy=0.97, p<0.001) but not in the frontal cortex (rxy=-0.25, p=0.44).

CONCLUSIONS

The results support a correlation between density levels of the 5-HT1A-receptor and the 5-HTT in the raphe nuclei and hippocampus but not in the frontal cortex. A suggested clinical implication is that the inter-individual variability in 5-HT1A-receptor and 5-HTT densities, as well as the ratio of these, is of particular interest in relation to individual responses to selective serotonin reuptake inhibitor treatment.

Changes in the serotonergic system in the main olfactory bulb of rats unilaterally deprived from birth to adulthood

The serotonergic system plays a key role in the modulation of olfactory processing. The present study examined the plastic response of this centrifugal system after unilateral naris occlusion, analysing both serotonergic afferents and receptors in the main olfactory bulb. After 60 days of sensory deprivation, the serotonergic system exhibited adaptive changes. Olfactory deprivation caused a general increase in the number of fibres immunopositive for serotonin but not of those immunopositive for the serotonin transporter. HPLC data revealed an increase in serotonin levels but not in those of its major metabolite, 5-hydroxyindole acetic acid, resulting in a decrease in the 5-hydroxyindole acetic acid/serotonin ratio. These changes were observed not only in the deprived but also in the contralateral olfactory bulb. Double serotonin-tyrosine hydroxylase immunolabelling revealed that the glomerular regions of the deprived olfactory bulb with a high serotonergic fibre density showed a strong reduction in tyrosine hydroxylase. Finally, the serotonin(2A) receptor distribution density and the number of juxtaglomerular cells immunopositive for serotonin(2A) receptor remained unaltered after olfactory deprivation. Environmental stimulation modulated the serotonergic afferents to the olfactory bulb. Our results indicate the presence of a bilateral accumulation of serotonin in the serotonergic axon network, with no changes in serotonin(2A) receptor density after unilateral olfactory deprivation.

Serotonin-, protein kinase C-, and Hic-5-associated redistribution of the platelet serotonin transporter

Emerging data indicate the existence of multiple regulatory processes supporting serotonin (5HT) transporter (SERT) capacity including regulated trafficking and catalytic activation, influenced by post-translational modifications and transporter-associated proteins. In the present study, using differential extraction and sedimentation procedures optimized for the purification of cytoskeletal and membrane-skeletal associated proteins, we analyze SERT localization in platelets. We find that most of the plasma membrane SERT is associated with the membrane skeleton. This association can be enhanced by both transporter activation and 5HT2A receptor activation. Inactivation of transport activity by phorbol ester treatment of intact platelets relocates SERT to the cytoskeleton fraction, consequently leading to transporter internalization. The translocation of SERT between these compartments is correlated with changes in the interaction with the LIM domain adaptor protein Hic-5. Co-immunoprecipitation and uptake activity studies suggest that Hic-5 is a determinant of transporter inactivation and relocation to a compartment subserving endocytic regulation. Associations of SERT with Hic-5 are evident in brain synaptosomes, suggesting the existence of parallel mechanisms operating to regulate SERT at serotonergic synapses.

Immunodetection of the serotonin transporter protein is a more valid marker for serotonergic fibers than serotonin

Tracking serotonergic pathways in the brain through immunodetection of serotonin has widely been used for the anatomical characterization of the serotonergic system. Immunostaining for serotonin is also frequently applied for the visualization of individual serotonin containing fibers and quantification of serotonin positive fibers has been widely used to detect changes in the serotonergic innervation. However, particularly in conditions with enhanced serotonin metabolism the detection level of serotonin may lead to an underestimation of the true number of serotonergic fibers. The serotonin transporter (SERT) protein, on the other hand, is less liable to metabolism and for that reason we hypothetized that SERT immunostaining is a more stable marker of serotonergic fibers. Rats were pretreated with monoamine oxidase (MAO) inhibitor and compared with placebo treated rats. Brains were double immunostained for serotonin and SERT protein and colocalization was quantified in several brain areas by confocal microscopy. In comparison with untreated rats, MAO inhibitor treated rats had a significantly higher number (almost 200% increase) of serotonin immunopositive fibers whereas no difference was observed in the number of the SERT positive fibers. Colocalization between serotonin and SERT positive fibers was close to 100% in MAO inhibitor treated animals but only 30% in untreated rats. We conclude that the rapid metabolism of serotonin leads to an underestimation of immunodetected serotonergic fibers and that in many instances, SERT immunostaining may be a better indicator of serotonergic fibers.

Transitory uptake of serotonin in the developing sensory pathways of the common marmoset

Serotonin (5-HT) affects brain development during sensitive developmental periods. In rodents, transient sites of high affinity capture of 5-HT were demonstrated in the primary sensory neurons and in the sensory thalamocortical afferents. This uptake is required to adjust 5-HT receptor stimulation during the formation of sensory maps. To determine whether similar mechanisms exist in primates, we analyzed staged embryos and postnatal pups in the common marmoset (total gestation time, 142 days). Immunocytochemical analyses were performed using antisera to 5-HT, to the serotonin transporter (SERT), and to the vesicular monoamine transporter (VMAT2). 5-HT, SERT, and VMAT2 labeled the raphe neurons and their terminal network from embryonic day (E)70 to adulthood. In addition, from E70-130 VMAT2 and SERT were observed in all the sensory cranial nerves, the olfactory nerve, the gustatory, the trigeminal, the auditory fibers, in the retinal ganglion cells, and the optic tract up to the lateral geniculate nucleus and the superior colliculus. All the spinal sensory ganglia and their peripheral sensory branches were labeled. Accumulation of 5-HT was observed in all the sensory neurons expressing SERT and the corresponding axon tracts. Since these neurons were missing tryptophan hydroxylase (TPH), the synthesizing enzyme for 5-HT, they most likely accumulated 5-HT through the action of the amine transporters, as has been shown in rodents. No transient expression of 5-HT markers was detectable in the sensory thalamocortical axons at any of the ages examined. Thus, the existence of 5-HT uptake in nonserotoninergic neurons appears to be a conserved feature in primates, although the topographic extent of this transient expression is more restricted than that previously demonstrated in rodents.

Heterogeneous targeting of centrifugal inputs to the glomerular layer of the main olfactory bulb

The centrifugal systems innervating the olfactory bulb are important elements in the functional regulation of the olfactory pathway. In this study, the selective innervation of specific glomeruli by serotonergic, noradrenergic and cholinergic centrifugal axons was analyzed. Thus, the morphology, distribution and density of positive axons were studied in the glomerular layer of the main olfactory bulb of the rat, using serotonin-, serotonin transporter- and dopamine-beta-hydroxylase-immunohistochemistry and acetylcholinesterase histochemistry in serial sections. Serotonin-, serotonin transporter-immunostaining and acetylcholinesterase-staining revealed a higher heterogeneity in the glomerular layer of the main olfactory bulb than previously reported. In this sense, four types of glomeruli could be identified according to their serotonergic innervation. The main distinctive feature of these four types of glomeruli was their serotonergic fibre density, although they also differed in their size, morphology and relative position throughout the rostro-caudal main olfactory bulb. In this sense, some specific regions of the glomerular layer were occupied by glomeruli with a particular morphology and a characteristic serotonergic innervation pattern that was consistent from animal to animal. Regarding the cholinergic system, we offer a new subclassification of glomeruli based on the distribution of cholinergic fibres in the glomerular structure. Finally, the serotonergic and cholinergic innervation patterns were compared in the glomerular layer. Sexual differences concerning the density of serotonergic fibres were observed in the atypical glomeruli (characterized by their strong cholinergic innervation). The present report provides new data on the heterogeneity of the centrifugal innervation of the glomerular layer that constitutes the morphological substrate supporting the existence of differential modulatory levels among the entire glomerular population.

Study of tyrosine hydroxylase immunoreactive neurons in neonate rats lactationally exposed to 2,4-dichlorophenoxyacetic Acid

Dopaminergic neurons from the midbrain nuclei substantia nigra (SN; A9) and ventral tegmental area (VTA; A10) were investigated by tyrosine hydroxylase (TH) immunostaining in neonate rat brains exposed to 2,4-dichlorophenoxyacetic acid (2,4-D) through lactation. Dorsal raphe serotonin (5-HT) projections to SN and VTA were also studied by 5-HT transporter (5-HTT) immunostaining and results were quantified by image analysis. Twenty-five-day-old pups exposed to 2,4-D through mothers milk were used. Dams were intraperitoneally administered 70 or 100mg/kg/day of 2,4-D from the 9th to the 25th postpartum day. After 100mg/kg of 2,4-D exposure, a 25% diminution in the SN and a 33% diminution in the VTA neurons' TH immunostaining along with a significantly 5-HT fiber density diminution were observed. The present work supports previous reports which suggest that exposure to 2,4-D during development has multiple effects on CNS.

Selective serotonin reuptake inhibitor disrupts organization of thalamocortical somatosensory barrels during development

To further investigate the role of the transiently expressed serotonin (5-HT) transporter (5-HTT) in the development of thalamic fibers projecting to cortical barrels and the potential developmental changes in neuronal circuitry caused by a selective serotonin reuptake inhibitor (SSRI), paroxetine (5 mg/kg, twice daily, s.c.) or saline was administered to rat pups from postnatal day 0 (P0) to P8. Pups were perfused on P8 for 5-HT immunostaining (-im) to confirm the 5-HT uptake blockade, and 5-HTT-im and phospholipase C-beta1 (PLC-beta1)-im to label the thalamic afferents to barrels and barrel cells respectively. Paroxetine treatment completely blocked 5-HT uptake into the thalamocortical fibers as indicated by the negative 5-HT-im in cortical barrel areas. Organization of thalamic afferents to barrels, indicated by 5-HTT-im or PLC-beta1, was altered in paroxetine-treated pups in the following manners: (1) segregation of thalamocortical fibers was partially disrupted and thalamocortical fibers corresponding to anterior snouts and row A mystacial vibrissae were fused; (2) sizes of the unfused thalamocortical fiber patches related to the long caudal vibrissae in rows B, C, D and E were significantly decreased without changes in the brain weights and cortical areas representing these vibrissae; and (3) thalamocortical fibers corresponding to C4 and D4 vibrissae tended to be closer to each other along the arc while the relative positions of thalamocortical fibers related to the rest of the vibrissae were normal. Our study demonstrated that 5-HTT plays an important role in the refinement, but not the formation, of barrel-like clusters of thalamocortical fibers and that the development of neural circuitry in rodent somatosensory cortex was affected by exposure to a SSRI during thalamocortical synaptic formation.

Serotonin uptake and metabolism by cultured guinea pig airway smooth muscle cells

Serotonin (5-hydroxytryptamine, 5-HT) is synthesized and released in the airways by pulmonary neuroendocrine cells located in the vicinity of airway smooth muscle (ASM). The aim of this study was to determine whether ASM cells contribute to the inactivation of serotonin, and investigate the role of the serotonin transporter (SERT) and monoamine oxidase (MAO) in this process. Cultured guinea pig tracheal smooth muscle cells, maintained in culture medium containing serotonin for 1-4 days, induced a decrease in 5-HT and increase in 5-HIAA in the culture medium. Changes in indole concentrations were prevented by fluvoxamine and iproniazid. Na+-sensitive [3H]-serotonin uptake into cultured ASM cells was time- and concentration-dependent (Km, 561 nM; Vmax, 1.06 pmol/mg protein/min), and inhibited by clomipramine (IC50, 13.7 nM), fluvoxamine (IC50, 0.16 microM) and fluoxetine (IC50, 0.32 microM). Western blot analysis with an anti-SERT antibody revealed a single 115 kDa immunoreactive band in ASM cell lysates. The results of this study suggest that ASM contributes to the uptake and metabolism of serotonin via SERT and MAO, respectively, and may therefore play a role in the inactivation of endogenous serotonin generated within the airway wall.

The human raphe nuclei and the serotonergic system

The raphe nuclei are distributed near the midline of the brainstem along its entire rostro-caudal extension. The serotonergic neurons are their main neuronal components, although a proportion of them lie in subdivisions of the lateral reticular formation. They develop from mesopontine and medullary primordia, and the resulting grouping into rostral and caudal clusters is maintained into adulthood, and is reflected in the connectivity. Thus, the mesencephalon and rostral pons, neurons within the rostral raphe complex (caudal linear, dorsal raphe, and median raphe nuclei) project primarily to the forebrain. By contrast, in the caudal pons and medulla oblongata, neurons within the caudal raphe complex (raphe magnus, raphe obscurus, raphe pallidus nuclei and parts of the adjacent lateral reticular formation) project to the brainstem nuclei and to the spinal cord. The median raphe and dorsal raphe nuclei provide parallel and overlapping projections to many forebrain structures with axon fibers exhibiting distinct structural and functional characteristics. The caudal group of the serotonergic system projects to the brainstem, and, by three parallel projections, to the dorsal, intermediate and ventral columns in the spinal cord. The serotonergic axons arborize over large areas comprising functionally diverse targets. Some projections form classical chemical synapses while many do not, thus contributing to the so-called paracrine or volume transmission. The serotonergic projections participate in the regulation of different functional (motor, somatosensory, limbic) systems; and have been associated with a wide range of neuropsychiatric and neurological disorders. Finally, recent experimental data support the role of serotonin in modulating brain development, such that a dysfunction in serotonergic transmission during early life could lead to long lasting structural and functional alterations.

GABAergic basket cells expressing cholecystokinin contain vesicular glutamate transporter type 3 (VGLUT3) in their synaptic terminals in hippocampus and isocortex of the rat

Vesicular glutamate transporter type 3 (VGLUT3) containing neuronal elements were characterized using antibodies to VGLUT3 and molecular cell markers. All VGLUT3-positive somata were immunoreactive for CCK, and very rarely, also for calbindin; none was positive for parvalbumin, calretinin, VIP or somatostatin. In the CA1 area, 26.8 +/- 0.7% of CCK-positive interneuron somata were VGLUT3-positive, a nonoverlapping 22.8 +/- 1.9% were calbindin-positive, 10.7 +/- 2.5% VIP-positive and the rest were only CCK-positive. The patterns of coexpression were similar in the CA3 area, the dentate gyrus and the isocortex. Immunoreactivity for VGLUT3 was undetectable in pyramidal and dentate granule cells. Boutons colabelled for VGLUT3, CCK and GAD were most abundant in the cellular layers of the hippocampus and in layers II-III of the isocortex. Large VGLUT3-labelled boutons at the border of strata radiatum and lacunosum-moleculare in the CA1 area were negative for GAD, but were labelled for vesicular monoamine transporter type 2, plasmalemmal serotonin transporter or serotonin. No colocalization was found in terminals between VGLUT3 and parvalbumin, vesicular acetylcholine transporter and group III (mGluR7a,b; mGluR8a,b) metabotropic glutamate receptors. In stratum radiatum and the isocortex, VGLUT3-positive but GAD-negative boutons heavily innervated the soma and proximal dendrites of some VGLUT3- or calbindin-positive interneurons. The results suggest that boutons coexpressing VGLUT3, CCK and GAD originate from CCK-positive basket cells, which are VIP-immunonegative. Other VGLUT3-positive boutons immunopositive for serotonergic markers but negative for GAD probably originate from the median raphe nucleus and innervate select interneurons. The presumed amino acid substrate of VGLUT3 may act on presynaptic kainate or group II metabotropic glutamate receptors.

Differential expression of nitric oxide in serotonergic projection neurons: neurochemical identification of dorsal raphe inputs to rodent trigeminal somatosensory targets

The dorsal raphe (DR) is invested with nitric oxide synthase (NOS)-expressing profiles. To characterize the connections of NO-containing cells and further assess neurochemical relationships maintained by DR, the transmitter identity of the raphe projection to the trigeminal somatosensory system was examined. Rats were injected with retrograde tracer into vibrissae-related target areas or with anterograde tracer into DR. NADPH-diaphorase (NADPHd) histochemistry or NOS-immunostaining was combined with serotonin (5HT)- or serotonin transporter (SERT)-immunolabeling to examine: 1) the presence of NO in 5HT-containing axons from DR; 2) the distribution of NO-containing fibers with respect to other nitrergic profiles in the somatosensory system; and 3) the propensity for individual projection neurons in specific subdivisions of DR to colocalize 5HT and NO. Results confirm that "barrel-like" patches can be identified in several adult trigeminal relay nuclei by NADPHd histochemistry and demonstrate that fibers from DR contain 5HT and NO. Observations include a high percentage of cortical midline projection neurons which contained NADPHd (70-80%) and coexpressed 5HT. In contrast, approximately 40% of retrogradely labeled DR-thalamus cells in the lateral wing demonstrated NADPHd or 5HT expression, but not both in the same neuron. Colocalization of NADPHd and 5HT within individual DR projection neurons indicates that: i) DR is a source of nitrergic input to trigeminal structures, and ii) NO and 5HT may be simultaneously released to influence information-processing within somatosensory targets. Disparities in NADPHd expression between retrogradely labeled DR neuronal subpopulations further suggest functional differences in the impact of NO on cortical and subcortical targets.

Serotonin and its transporter on proliferation of fetal heart cells

Besides neuronal transmission, serotonin (5-HT) also acts as a trophic signal during the development of the central nervous and neural crest systems. In this study, we report that in addition to trophic effect, 5-HT increases the proliferation of fetal heart cells. We showed for the first time that the cultured heart cells, express serotonin transporter (5-HTT), which confirmed the previously observed accumulation of 5-HT in developing heart. The influence of 5-HT on developing heart cells is studied throughout the dosage. We found that 5-HT concentration at physiological level, 4 microM, permits an optimal proliferation of heart cells as indicated by the number of 5-bromo-deoxyuridine immunoreactive (BrdU-im) cells and myosin heavy chain immunoreactive cells (MF20-im); fluctuation towards either concentrations reduce the proliferation. We hypothesized that 5-HTT plays a role in the heart development. Our study indicated that the blockade of 5-HT uptake by paroxetine decreased the number of BrdU-im cells and MF20-im cells. These data indicate a role of 5-HT and 5-HTT on heart development. Abnormal 5-HT level or misuse of 5-HT uptake blocker may alter the heart development.

Functional interactions between native Gs-coupled 5-HT receptors in HEK-293 cells and the heterologously expressed serotonin transporter

In HEK-293 cells, serotonin (5-hydroxytryptamine, 5-HT) was found to induce cAMP production showing pharmacological characteristics consistent with the 5-HT(7) receptor. The presence of 5-HT(7) (and 5-HT(6)) receptor mRNA was confirmed by RT-PCR. Stable HEK-293 cell lines expressing either wild-type or haemagglutinin (HA)-tagged human 5-HT transporter (SERT) were selected and SERT function was confirmed using [3H]5-HT transport. The presence of SERT caused a 10-fold reduction in the potency of 5-HT-induced cAMP production compared to control cells. Downstream signalling by 5-HT(6/7) receptors could be detected as 5-HT-induced protein kinase A activation and phosphorylation of MAP kinase and CREB using phospho-specific antibodies. SERT inhibitors reversed the reduction in potency of 5-HT-induced cAMP production caused by the presence of SERT, resulting in a concentration-dependent left shift in EC(50) values but also a progressive decrease in the maximal response. Thus, when antidepressants were used to block SERT activity, 5-HT receptor signalling was effectively clamped within a mid-range.

Electroconvulsive shock increases serotonin transporter in the rat frontal cortex

The antidepressive action of electroconvulsive shock (ECS) is thought to involve the alteration in serotonin (5-HT) neurotransmission, including the increase in 5-HT release and uptake. In our previous study, 5-HT transporter (5-HTT) mRNA expression was decreased after single and repetitive ECS in rat raphe nucleus. In the present study, we investigated the effects of single and repetitive ECS on the protein levels of 5-HTT in the frontal cortex, hippocampus and raphe nucleus of rat brain using quantitative Western blot analysis. Single ECS did not alter 5-HTT protein expression in any brain regions examined. Repetitive ECS stably increased 5-HTT protein in the frontal cortex, but not in the hippocampus and raphe nucleus. Because ECS is known to facilitate the release of neurotransmitters, our results suggest that the increased 5-HTT protein expression in the frontal cortex might be a compensatory change against the enhanced 5-HT release by ECS in presynaptic terminals.

Overexpression of an epitope-tagged serotonin transporter in serotonin neurons of the dorsal raphe nucleus using a defective HSV-1 vector

The serotonin transporter (5HTT) plays a central role in serotonin neurotransmission. Abnormalities of 5HTT function have been implicated in depression, anxiety and alcohol intake. To better understand the functional role of this important molecule, we have utilized a viral vector approach to overexpress the 5HTT in regions of the rat brain. We have constructed a bicistronic defective herpes virus (HSV-1) vector that expresses both an epitope-tagged 5HTT as well as beta-galactosidase (beta-GAL) as a marker for infected cells. The vector was capable of conferring serotonin uptake activity to Vero cells in culture, indicating transfer of a functional 5HTT. Injection of the 5HTT virus into the rat brain resulted in a dense focus of specific 125I RTI-55 binding at the injection site, indicating that the virally expressed 5HTT can also bind ligand when expressed in the brain. We were also able to overexpress an epitope tagged 5HTT in serotonergic neurons in the dorsal raphe nucleus (DRN) using this approach. These data demonstrate that the levels of the 5HTT in 5HT neurons can be manipulated in the adult rodent brain using an HSV-1 vector.

Serotonin uptake into dopamine neurons via dopamine transporters: a compensatory alternative

Monoamine neurons are believed to use neuronal-specific transporters to remove their own transmitters from the extracellular space and thus terminate transmission to postsynaptic neurons. We report here, for the first time, conclusive evidence that a cross clearance of serotonin into dopamine neurons exists. Such alternative uptake by different neurons is adopted under circumstances when their own transporter function is no longer adequate. When the serotonin transporter (5-HTT) is disrupted in 5-HTT knockout mice, serotonin (5-HT) is found in dopamine (DA) neurons of homozygous (-/-) but not of heterozygous (+/-) mutant mice or their normal littermates. DA neurons containing 5-HT are seen in the substantia nigra and ventral tegmental area (VTA), but not in other brain areas of 5-HTT -/- mice. Normal rats treated with a 5-HT uptake blocker paroxetine also showed similar result. To verify the role of the DA transporter in such ectopic uptake, 5-HTT -/- mice were treated with DA uptake blocker GBR-12935, ectopic 5-HT in DA neurons was disappeared. These data indicate that: (a) 5-HT can be taken into DA neurons in rats and mice when the 5-HTT is not functionally adequate to remove extracellular 5-HT; (b) the 5-HT uptake into DA neurons is not affected by the 5-HT uptake blocker paroxetine; and (c) the DA transporter is responsible for the 5-HT uptake into DA neurons. This study thus demonstrates that cross neuronal type uptake exists and serves as a compensatory backup when a specific transporter is dysfunctional. This study also demonstrates that DA neurons can store 5-HT for possible "false neurotransmitter" or other usage.

Peptide dendrimers: applications and synthesis

Peptide dendrimers are radial or wedge-like branched macromolecules consisting of a peptidyl branching core and/or covalently attached surface functional units. The multimeric nature of these constructs, the unambiguous composition and ease of production make this type of dendrimer well suited to various biotechnological and biochemical applications. Applications include use as biomedical diagnostic reagents, protein mimetics, anticancer and antiviral agents, vaccines and drug and gene delivery vehicles. This review focuses on the different types of peptide dendrimers currently in use and the synthetic methods commonly employed to generate peptide dendrimers ranging from stepwise solid-phase synthesis to chemoselective and orthogonal ligation.

Parabrachio-cortical connections with the lateral hemisphere in the madagascan hedgehog tenrec: prominent projections to layer 1, weak projections from layer 6

The present study was undertaken to further characterize and subdivide the rhinal cortex (insular and perirhinal areas) in the hedgehog tenrec (Echinops telfairi), a placental mammal with a rather low encephalisation index. Injections of wheat germ agglutinin-horseradish peroxidase into the dorsolateral pontine tegmentum revealed a prominent layer 1 projection to several rhinal target areas, while the rhinal cortex only stained weakly for the calcitonin gene-related peptide. Among the regions retrogradely labeled following tracer injections into the rhinal cortex, the parabrachial nucleus was considered the main origin of the tegmento-cortical projection. This conclusion was based on the circumscribed pattern of termination, as well as the differences noted between the pattern of anterograde labeling and the pattern obtained by thyrosine hydroxylase immunohistochemistry. The tracer injections into the dorsolateral tegmentum also revealed numerous retrogradely labeled cells in the layer 5 of the dorsomedial frontal cortex. In contrast, the rhinal cortex only showed few labeled cells and most of these cells were located in the layer 6/7. A comparison with other species indicates that the tenrec's parabrachial nucleus gives rise to the most extensive cortical projections but receives the least prominent input from the lateral cerebral hemisphere. The layer 6/7 projection may be a common mammalian feature but it is overshadowed by the layer 5 projection in higher mammals.

Methamphetamine concentrations in fetal and maternal brain following prenatal exposure

Levels of methamphetamine in maternal striatum and whole fetal mouse brain were assessed at 0.5, 1, 2, and 4 h postinjection on gestational day 14 (GD14) following a single, subcutaneous injection of 40 mg/kg (+)-methamphetamine hydrochloride to pregnant mice. In the dams, striatal concentrations of methamphetamine peaked at 1 h postinjection, reaching levels of approximately 510 ng/mg protein. Amphetamine, the primary metabolite of methamphetamine, increased to 77 ng/mg protein at 2 h and remained elevated by 4 h postinjection. In the fetal brain, peak methamphetamine concentrations of approximately 122 ng/mg protein were attained at 1 h. Amphetamine was only detectable in fetal brain at 2 and 4 h postinjection. Regional analysis of methamphetamine levels in fetal striatum, cortex, and brainstem revealed that the drug was not uniformly distributed. Maternal administration of methamphetamine results in fetal brain drug concentrations, which approximate those reported in human infants whose mother abused methamphetamine. This dosage regimen, therefore, serves as an appropriate animal model for assessing the potential risks to human offspring exposed to methamphetamine in utero.

Pharmacological characterization and visualization of the glial serotonin transporter

Astrocytes contain transport systems that are capable of removing various neurotransmitters from the synaptic cleft by transporters present in the plasma membrane. Glial serotonin transporter (SERT) plays an important role in the re-uptake of 5-hydroxytryptamine (5-HT). We examined the pharmacological characterization of 5-HT uptake into rat cortical synaptosomes and cultured rat astrocytes, and the immunodetection of glial SERT proteins using specific site-directed monoclonal antibodies (MoAb). Furthermore, using a reverse transcriptase-polymerase chain reaction (RT-PCR) method, we addressed the expression of SERT mRNA in cultured rat astrocytes. We investigated the inhibitory effects of various monoamine uptake inhibitors on the uptake of [3H]5-HT into cultured astrocytes and cortical synaptosomes. Tricyclic antidepressants (clomipramine and imipramine) as well as selective serotonin re-uptake inhibitors (fluvoxamine, fluoxetine and zimelidine) were very potent inhibitors of [3H]5-HT uptake in both preparations. In contrast, the inhibitory effects of NE uptake inhibitors (nisoxetine and desipramine) and cocaine were weaker than those of 5-HT uptake inhibitors. In addition, dopamine (DA) uptake inhibitors (nomifensine and GBR-12935) exhibited a Ki value in the low micromolar range. The inhibitory potencies were in the order 5-HT uptake inhibitors (clomipramine, fluvoxamine, fluoxetine, imipramine and zimelidine) > NE uptake inhibitors (nisoxetine and desipramine) = cocaine > DA uptake inhibitors (nomifensine and GBR-12935). There was no difference in the order of the inhibitory effects of various monoamine uptake inhibitors between the two preparations. A correlation analysis of the potencies of various monoamine uptake inhibitors in the inhibition of [3H]5-HT into cultured astrocytes and cortical synaptosomes produced a highly significant correlation coefficient of 0.9893 (P < 0.0001). Immunocytochemical staining using anti-SERT MoAb in cultured astrocytes revealed that the plasma membrane, as well as intracellular, perinuclear compartments, presumably endoplasmic reticulum or golgi membranes, showed a considerable level of immunoreactivity. Extracts of astrocytes and synaptosomes from the cortex were immunoblotted with anti-SERT MoAb. SDS-PAGE/Western blots indicate that anti-SERT MoAb recognized two bands of 120 and 73 kDa in both preparations. RT-PCR demonstrated that astrocytes in cultured expressed mRNA for the cloned SERT protein, which has been characterized as the neuronal SERT. These pharmacological experiments indicate that this uptake process takes place through glial SERT that is very similar to neuronal SERT. Furthermore, the present data also indicate that the presence of the mRNA and protein for the neuronal SERT were established in cultured rat astrocytes, and the polypeptide portion of SERT in astrocytes and frontal cortex could be the same gene product.

Abnormal trafficking and subcellular localization of an N-terminally truncated serotonin transporter protein

We report here that a truncated 5-HTT protein is produced in the neurons of the raphe, in serotonin transporter (5-HTT) knockout (KO) mice. The 5-HTT gene has exon 2 deleted and we found that one main transcript, shortened by 450 bp, is produced in these KO mice. The mutated 5-HTT protein is only recognized by antibodies against the C-terminal portion of 5-HTT. This protein is not functional as there is no high-affinity serotonin uptake in 5-HTT KO mice, in adults or during development. Conversely, low-affinity serotonin uptake was detected in vitro, and in dopaminergic neurons of the substantia nigra in vivo. The truncated 5-HTT, recognized by antibodies to the C-terminus, is present exclusively in the somatodendritic compartment of the raphe neurons instead of being exported to axons. As shown with confocal and electron microscopy, the truncated 5-HTT does not reach the plasma membrane and is essentially retained in the endoplasmic reticulum. However, this does not seem to trigger refolding or degradation responses, as no upregulation of the chaperone BiP or of the degradation signal ubiquitin was detected. Last, as observed in heterozygous mice, the presence of the truncated 5-HTT protein, although produced in large quantities, does not disturb the normal trafficking of the wild-type protein. This study therefore validates the 5-HTT KO model despite the occurrence of an incomplete translation, and brings novel information on the in vivo 5-HT uptake and cellular processing of an abnormal 5-HTT protein.

Isolation-rearing of rats produces deficits as adults in the serotonergic innervation of hippocampus

Isolation-rearing of rats causes a variety of behavioral changes, including anxiety, learning deficits and sensory changes related to schizophrenia. Similar changes are seen following loss of serotonin during development. Thus, the effects of isolation-rearing on behavior may be due to changes in serotonin. Sprague-Dawley rats were raised in groups of four (social animals) or in isolation, from postnatal day 22 until postnatal day 64. The hippocampi were examined immunochemically for changes in serotonin. Our findings show that serotonin terminals are lost throughout the CA regions of hippocampus, where there is also an associated loss of dendrites, but not in the molecular layer of the dentate gyrus. Thus, some of the brain and behavioral changes seen in isolation-reared animals could be due to loss of serotonin.

Central serotonin system in Dystonia musculorum mutant mice: biochemical, autoradiographic and immunocytochemical data

The autosomal recessive mutation dystonia musculorum (dt(J)/dt(J)) causes degenerative alterations of peripheral and central sensory pathways that lead to ataxia. To investigate possible changes in the central serotonin system of these mice, HPLC measurements of 5-hydroxytryptophan, 5-hydroxy-tryptamine (serotonin; 5-HT), and 5-HT metabolites were obtained from 22 brain regions and the spinal cord of wild type and dt(J)/dt(J) mutant mice. Also, 5-HT transporters were quantified by [(3)H]citalopram autoradiography in 72 brain regions, subregions, and nuclei, and the 5-HT innervation visualized by immunocytochemistry throughout the brain and spinal cord. In all brain regions measured for indoleamine content, there were no significant differences between the two genotypes. In the spinal cord, an increased tissue concentration of 5-HT (+34%), 5-hydroxyindole-3-acetic acid (+33%), 5-hydroxytryptophol (+21%), and 5-hydroxytryptophan (+45%) in dt(J)/dt(J) actually corresponded to the same total amount of each of these indoleamines in the entire spinal cord, when taking into account its reduced size in the mutants. Quantification of the binding to 5-HT transporters showed increases in the medial geniculate nucleus (+14%), medial (+24%) and lateral (+18%) hypothalamus, interpeduncular (+13%), vestibular (+22%), and deep cerebellar nuclei (+37%) of dt(J)/dt mice, and decreases in the ventral tegmental area (-13%), median and linear raphe nuclei (-20%), as well as in the solitary complex (-35%). There were no apparent differences in the distribution of 5-HT-immunostained fibers in these and other regions of brain and in the spinal cord of dt(J)/dt(J) compared to wild type mice. The bulk of these results indicates a relative sparing of the central 5-HT system in the dt(J)/dt(J) mice, even though alterations in 5-HT transporters could justify attempts at improving the sensorimotor dysfunction by administration of serotoninergic agents in these mice.

Homeostatic regulation of serotonergic function by the serotonin transporter as revealed by nonviral gene transfer

With the aim of exploring the relationship between the serotonin transporter (5-HTT or SERT) and the activity level of serotonin (5-HT) neurotransmission, in vivo expression of this protein was specifically altered using a nonviral DNA transfer method. Plasmids containing the entire coding sequence or a partial antisense sequence of the 5-HTT gene were complexed with the cationic polymer polyethylenimine and injected into the dorsal raphe nucleus of adult male rats. Significant increase or decrease in both [(3)H]citalopram binding and [(3)H]5-HT synaptosomal uptake were observed in various brain areas up to 2 weeks after a single administration of the sense plasmid or 7 d after injection of the short antisense plasmid, respectively. Such changes in 5-HTT expression were associated with functional alterations in 5-HT neurotransmission, as shown by the increased capacity of 5-HT(1A) receptor stimulation to enhance [(35)S]GTP-gamma-S binding onto the dorsal raphe nucleus in sections from rats injected with the sense plasmid. Conversely, both a decrease in 5-HT(1A)-mediated [(35)S]GTP-gamma-S binding and a reduced potency of the 5-HT(1A) receptor agonist ipsapirone to inhibit neuronal firing were observed in the dorsal raphe nucleus of antisense plasmid-injected rats. Furthermore, changes in brain 5-HT and/or 5-HIAA levels, and sleep wakefulness circadian rhythm in the latter animals demonstrated that altered expression of 5-HTT by recombinant plasmids has important functional consequences on central 5-HT neurotransmission in adult rats.

Serotonin, dopamine and norepinephrine transporters in the central nervous system and their inhibitors

An overview is presented on progress made in the research on neuronal transporters of serotonin, dopamine and norepinephrine in the central nervous system. Tools developed by molecular biology, such as expression of cloned transporters, their mutants and chimera in non-neuronal cells offered the opportunity to study the putative domains for binding of substrates and uptake inhibitors and discover factors in the regulation of the transporter function. The study of the distribution of monoamine transporters in human brain became possible by the development of selective radiolabelled transport inhibitors. The relationships between the chemical structure of the uptake inhibitors and the affinity for the monoamine transporters is reported, and the (potential) therapeutic applications of the compounds are discussed.

Dexfenfluramine and norfenfluramine: comparison of mechanism of action in feeding and brain Fos-ir studies

Dexfenfluramine (dF) and dexnorfenfluramine (dNF), its metabolite, are anorectic agents that release serotonin (5-HT) and may have a direct postsynaptic action. The effects on the anorectic effects of dF and dNF of either acute (p-chlorophenylalanine, PCPA) or chronic (5,7-dihydroxytryptamine, 5,7-DHT) brain 5-HT depletions were studied in rats and compared with the actions of a 5-HT uptake inhibitor (fluoxetine) and 5-HT(1B/2C) receptor agonists [1-(3-trifluoromethyl-phenyl)-piperazine and 1-(3-chlorophenyl) piperazine]. The anorexia caused by these agonists was enhanced in rats with 5,7-DHT lesions, possibly a result of receptor supersensitivity. In contrast, fluoxetine anorexia was somewhat reduced in one study and was unchanged in a second. Both dF and dNF anorexias were enhanced in rats with 5,7-DHT lesions. In contrast, the anorectic effects of either dF or dNF were unchanged in PCPA-treated rats relative to controls. Compared with controls, 5, 7-DHT-lesion rats showed greatly increased dF- and dNF-induced Fos-like immunoreactivity (ir) in the paraventricular (PVN) and supraoptic (SON) hypothalamic nuclei, and in the median preoptic area (MnPO), but were similar to controls in most other areas. PCPA pretreatment increased dF- and dNF-induced Fos-ir in the PVN, SON, and MnPO. In controls, equianorectic doses of dF and dNF induced Fos-ir in similar brain regions, but dNF produced relatively larger effects than dF in SON, PVN, and MnPO. The data are discussed in terms of multiple pathways in the anorectic actions of dF and dNF.