The contribution of low-affinity transport mechanisms to serotonin clearance in synaptosomes

Although many studies assert that the serotonin (5-HT) transporter (SERT) is the predominant mechanism controlling extracellular 5-HT concentrations, accumulating evidence suggests that low affinity, high capacity transport mechanisms may contribute more to 5-HT clearance than previously thought. The goal of this study was to quantify the contributions of SERT relative to other mechanisms in clearing extracellular 5-HT concentrations ranging from 50 nM to 1 μM in synaptosomes prepared from wild-type and SERT knockout mice using rotating disk electrode voltammetry. SERT inhibitors combined with decynium-22 (D-22), a blocker of several low-affinity transporters, blocked all uptake of 5-HT into synaptosomes. We found that SERT is responsible for the majority of synaptosomal uptake only at relatively low 5-HT concentrations, but comprises a diminishing proportion of 5-HT clearance when extracellular 5-HT increases above 100 nM. The effect of D-22 was similar in wild-type and SERT knockout synaptosomes. Thus, there was no evidence of upregulation of low-affinity mechanisms in knockout mice across the concentrations of 5-HT tested. These are surprising results, in light of the prevailing view that SERT is the primary uptake mechanism for extracellular 5-HT at physiological concentrations. We conclude that non-SERT mediated 5-HT uptake is substantial even at modest 5-HT concentrations. These findings, in conjunction with other studies, have important implications for understanding serotonergic disorders and may explain the variable efficacy and stability of patients' responses to antidepressants, such as the selective serotonin reuptake inhibitors.     

Androgen actions on central serotonin neurotransmission: relevance for mood, mental state and memory.

Sex steroids exert potent effects on mood and mental state in the human. Our previous experimental findings in female rats suggest that these effects may be mediated, in part, by the action of estrogen on the 5-hydroxytryptamine2A receptor (5-HT(2A)R) and serotonin transporter (SERT) in brain. Here we review our recent findings on the effect of acute (approximately 32 h) testosterone manipulation on central 5-HT(2A)R and SERT in male rats. Castration decreased while testosterone or estrogen, but not 5alpha-dihydrotestosterone (5alpha-DHT), increased significantly the content of 5-HT(2A)R mRNA and SERT mRNA in the dorsal raphe nucleus (DR) and the density of 5-HT(2A)R and SERT binding sites in higher centers of the brain. The lack of effect of 5alpha-DHT, a potent androgen which cannot be converted to estrogen, suggests that the action of testosterone depends upon its conversion to estrogen by aromatase. This may also explain why estrogen, but not testosterone or 5alpha-DHT, increased the density of 5-HT(2A)R binding sites in the caudate-putamen, a brain region where aromatase is scarce. The estrogen induction of SERT mRNA is most prominent in the rostral DR and this together with the correlation between sensitivity of DR serotonin neurons to estrogen and neurotoxic amphetamine derivatives provides a potential topochemical handle with which to investigate testosterone/estrogen regulation of SERT gene expression. These findings are discussed in relation to the possible role of interactions between sex steroids and serotonin mechanisms in mood disorders, schizophrenia and Alzheimer's disease.     

Effects of tryptophan depletion on the binding of [11C]-DASB to the serotonin transporter in baboons: response to acute serotonin deficiency.

BACKGROUND: The objective of this study was to evaluate the sensitivity of [(11)C]-N,N-dimethyl-2-(2-amino-4-cyanophenylthio)benzylamine (DASB) binding to the brain serotonin transporter (SERT) to changes in endogenous serotonin (5-hydroxytryptamine [5-HT]) levels. A ligand sensitive to endogenous competition (EC) would enable the measurement of fluctuations of intrasynaptic 5-HT. A ligand insensitive to EC can provide a measure of SERT unaffected by levels of 5-HT. Alternatively, serotonin depletion could accelerate internalization of SERT and reduce binding. METHODS: Eighteen (14 baseline and 9 tryptophan-depleted) positron emission tomography (PET) scans were carried out in two baboons (Papio anubis). A metabolite-corrected arterial input function was used to estimate the binding potential (BP = B(max)/K(D)). RESULTS: Depletion of plasma tryptophan by a mean of 65% from the baseline (p = .03) reduces [(11)C]-DASB BP in the six brain regions of interest (ROI). Lower DASB binding correlated with lower plasma tryptophan levels in the ROIs with higher SERT density. CONCLUSIONS: [(11)C]-DASB binding to SERT in vivo rapidly declines in response to acute reduction in serotonin availability, contrary to what is predicted by a simple competition model. This rapid reduction in SERT availability may be due to accelerated transporter internalization.     

Gene dose-dependent alterations in extraneuronal serotonin but not dopamine in mice with reduced serotonin transporter expression

Serotonin (5-HT) plays an integral regulatory role in mood, anxiety, cognition, appetite and aggressive behavior. Many therapeutic and illicit drugs that modulate these functions act at the serotonin transporter (SERT), thus a mouse model with reduced transporter expression was created to further investigate the effects of differential serotonin reuptake. In the present study, in vivo microdialysis was used to determine homeostatic alterations in extracellular 5-HT levels in unanesthetized SERT knockout mice. SERT^−/− mice had significantly higher levels of basal dialysate 5-HT than SERT^+/+ mice in striatum and frontal cortex. In addition, although gene-specific increases in 5-HT were evident, neuroadaptive alterations in dialysate dopamine levels were not detected in striatum. Zero net flux microdialysis was utilized to further investigate alterations in extracellular 5-HT. Using this method, a gene dose-dependent increase in extraneuronal 5-HT was observed in striatum (2.8 ± 1, 9.4 ± 1 and 18 ± 3 nM) and frontal cortex (1.4 ± 0.4, 3.5 ± 0.9 and 14 ± 1 nM) in SERT^+/+, SERT^+/− and SERT^−/− mice, respectively. Potassium stimulation revealed greater depolarization-induced increases in striatal 5-HT but not dopamine in SERT^−/− mice. Furthermore, dialysate 5-hydroxyindoleacetic acid (5-HIAA) levels were reduced in striatum in a gene dose-dependent manner, while DOPAC was unchanged in SERT knockout mice. Finally, determination of monoamine oxidase (MAO) activity revealed no significant differences in K_M or V_max of type-A or type-B isozymes indicating that alterations in SERT expression do not cause adaptive changes in the activities of these key catabolic enzymes. Overall, these results demonstrate that constitutive reductions in SERT are associated with increases in 5-HT in the extracellular signaling space in the absence of changes in dopamine neurochemistry. Furthermore, use of zero net flux microdialysis appears warranted in investigations of serotonergic synaptic function where modest changes in extracellular 5-HT are thought to occur in response to altered uptake.     

Differential in vivo clearance of serotonin in rat dorsal raphe nucleus and CA3 region

In vivo chronoamperometric recordings were used to determine if the majority of serotonin transporters (SERTs) in the dorsal raphe nucleus (DRN) are functionally active. This was achieved by comparing the clearance of exogenously applied serotonin (5-HT) from the extracellular fluid (ECF) of the DRN to that in the CA3 region of the hippocampus, an area with lower SERT density. Serotonin was pressure ejected into these regions in anesthetized rats and reproducible electrochemical signals measured by carbon fiber microelectrodes were recorded. Consistent with SERT density as measured by [3H]cyanoimipramine binding in these brain regions (DRN>CA3), clearance of 5-HT was significantly faster in DRN compared to that in the CA3 region. The selective serotonin reuptake inhibitor, fluvoxamine, prolonged 5-HT clearance in both CA3 and DRN. It is known that the norepinephrine transporter (NET) contributes to clearance of 5-HT in the dentate gyrus (DG) but not in CA3. Given that the DRN receives noradrenergic innervation, it was also determined if the NET contributes to 5-HT clearance in the DRN. Destruction of the NET with the neurotoxin 6-hydroxydopamine failed to alter 5-HT clearance parameters in the DRN. These data support the hypothesis that serotonin transporters are functionally active in the DRN, that serotonin clearance is mediated primarily by the SERT in the DRN and that the faster clearance of 5-HT from this region is related to its greater density of functional SERTs.     

Down-regulation of the rat serotonin transporter upon exposure to a selective serotonin reuptake inhibitor.

The serotonin transporter (SERT) terminates serotonergic neurotransmission by rapid reuptake of 5-hydroxytryptamine (5-HT) into the nerve terminal or axonal varicosities. SERT represents the target of various antidepressants which inhibit 5-HT transport and are widely used for the pharmacotherapy of depression. Here, we have analyzed the function of SERT stably expressed in HEK 293 cells upon exposure to citalopram, a selective serotonin reuptake inhibitor (SSRI), with respect to 5-HT transport activity and protein expression as estimated by ligand binding experiments. Our results show that long-term exposure to an SSRI causes a down-regulation of transport activity as revealed by a reduction of the maximal transport rate, without affecting substrate affinity, accompanied by a decrease in ligand binding sites.     

High-dose fenfluramine administration decreases serotonin transporter binding, but not serotonin transporter protein levels, in rat forebrain

Administration of D-fenfluramine (D-FEN) or parachloroamphetamine (PCA) can produce long-lasting decreases in serotonin transporter (SERT) binding and tissue levels of serotonin (5-HT) in rat forebrain. These changes have been viewed as evidence for 5-HT neurotoxicity, but no studies have measured SERT protein levels. In the present study, we determined the effect of high-dose D-FEN or PCA, administered according to a "neurotoxic" dosing regimen, on the density of SERT sites using ligand binding methods and on SERT protein levels using Western blots. Rats were sacrificed 2 days and 2 weeks after administration of drug or saline. The density of SERT was determined in homogenates of caudate and whole brain minus caudate. D-FEN and PCA decreased SERT binding by 30-60% in both tissues and at both time points. Similarly, D-FEN and PCA administration profoundly decreased tissue 5-HT and 5-HIAA in frontal cortex. Despite the large decreases in SERT binding and depletion of tissue 5-HT that occurred with D-FEN administration, SERT protein expression, as determined by Western blot analysis, did not change in either tissue or time point. PCA administration decreased SERT protein by about 20% only at the 2-day point in the caudate. Drug treatments did not change expression of glial fibrillary acidic protein (GFAP), a hallmark indicator of neuronal damage, in whole brain minus caudate in the 2-week group. These results support the hypothesis that decreases in tissue 5-HT and SERT binding sites induced by D-FEN and PCA reflect neuroadaptive changes, rather than neurotoxic effects.     

Filtration disrupts synaptosomes during radiochemical analysis of serotonin uptake: comparison with chronoamperometry in SERT knockout mice

Radiochemical methods have failed to reveal decreases in synaptosomal serotonin uptake in mice lacking one functional copy of the serotonin transporter (SERT) gene. By contrast, uptake rates determined by chronoamperometry in synaptosomes from SERT+/- mice show gene-related reductions. We revisited [(3)H]5-HT uptake in SERT knockout mice to determine the effects of inclusion of O(2) in the incubation buffer on the kinetic parameters obtained by this method. In oxygenated synaptosomes prepared from frontal cortex and striatum, modest 25 and 35% reductions in radiolabeled 5-HT uptake were detected in SERT+/- versus SERT+/+ mice. However, even in the presence of O(2), no differences in [(3)H]5-HT uptake were detected between SERT+/- and SERT+/+ mice in brain stem in contrast to 60% reductions determined by chronoamperometry. Moreover, while inclusion of O(2) modestly increased the rates of [(3)H]5-HT uptake, rates determined by chronoamperometry in the presence of O(2) were 40-fold greater than those determined radiochemically. We present evidence that the filtration process used in the radiochemical method leads to substantial loss of transported 5-HT resulting in lower apparent uptake rates. These findings explain the relative insensitivity of radiochemical methods for determining biologically important alterations in uptake such as those occurring between SERT+/- and SERT+/+ mice and in response to O(2).     

Exploring the relationship between serotonin and brain-derived neurotrophic factor: analysis of BDNF protein and extraneuronal 5-HT in mice with reduced serotonin transporter or BDNF expression

Serotonin (5-HT) has been proposed to promote neuronal plasticity during the treatment of mood and anxiety disorders and following neurodegenerative insult by altering the expression of critical genes including brain-derived neurotrophic factor (BDNF). In this study, mice with constitutive reductions in the serotonin transporter (SERT) or BDNF were investigated to further assess the functional relationship between serotonin neurotransmission and BDNF expression. Using a modified extraction procedure and a commercial enzyme-linked immunosorbant assay, 50% decreases in BDNF protein in hippocampus, frontal cortex and brain stem were confirmed in 4-month-old mice lacking one copy of the BDNF gene (BDNF^+/−). By contrast, 4-month-old male and female mice with partial (SERT^+/−) or complete (SERT^−/−) reductions in SERT expression showed no differences in BDNF protein levels compared to SERT^+/+ mice, although male SERT knockout mice of all genotypes had higher BDNF levels in hippocampus, frontal cortex, and brain stem than female animals. Microdialysis also was performed in BDNF^+/− mice. In addition to other phenotypic aspects suggestive of altered serotonin neurotransmission, BDNF^+/− mice show accelerated age-related degeneration of 5-HT forebrain innervation. Nevertheless, extracellular 5-HT levels determined by zero net flux microdialysis were similar between BDNF^+/+ and BDNF^+/− mice in striatum and frontal cortex at 8–12 months of age. These data illustrate that a 50% decrease in BDNF does not appear to be sufficient to cause measurable changes in basal extracellular 5-HT concentrations and, furthermore, that constitutive reductions in SERT expression are not associated with altered BDNF protein levels at the ages and in the brain regions examined in this study.     

Vulnerability to lasting anxiogenic effects of brief exposure to predator stimuli: sex, serotonin and other factors-relevance to PTSD

Lasting anxiogenic effects of predator stress in rodents may model aspects of post-traumatic stress disorder (PTSD). There is a link between genetic variation in the serotonin (5-HT) transporter (SERT) and anxiety in humans, prompting the generation of SERT knockout mice. This review brings together studies of SERT knockout male mice, normal female mice, and different 5-HT receptors in predator stress effects on anxiety. These studies provide for a link between vulnerability to the anxiogenic effects of predator stress and abnormalities of 5-HT transmission induced by a life long reduction in 5-HT reuptake in male mice, which creates a vulnerability like that seen in normal female mice. Data reviewed suggest abnormalities in 5-HT transmission contribute to vulnerability to lasting anxiogenic effects of species relevant stressors. To the extent to which predator stress effects model aspects of PTSD, and in the light of relevant human literature, these considerations implicate abnormalities of 5-HT transmission in vulnerability to PTSD per se, and as a potential contributor to enhanced female vulnerability to PTSD.     

Endogenous 5-HT, released by MDMA through serotonin transporter- and secretory vesicle-dependent mechanisms, reduces hippocampal excitatory synaptic transmission by preferential activation of 5-HT1B receptors located on CA1 pyramidal neurons

A multitude of different serotonin (5-HT) receptor types are expressed in the hippocampus, but the identity of receptors actually mediating the physiological response to endogenous 5-HT has not been determined. We combined pharmacologically induced release of 5-HT with patch-clamp recordings on disinhibited rat CA1 minislices to determine effects of endogenous 5-HT on the excitability of pyramidal neurons and synaptic transmission among them. We found that application of 5-HT releasers, 3,4-methylenedioxy-methamphetamine (MDMA) or p-methylthioamphetamine, at concentrations ranging from 2 to 50 microm, reduced the excitatory synaptic transmission between CA1 pyramidal neurons without altering their basal electrical properties. This effect of MDMA was blocked by the selective 5-HT1B antagonist GR 55562, was dependent on endogenous 5-HT content and was mediated by presynaptically located, pertussis-toxin sensitive mechanisms. We found no other MDMA effects in our preparation, which indicates that the release of endogenous 5-HT preferentially stimulates 5-HT1B receptors on CA1 pyramidal neurons. Therefore, 5-HT1B receptor activation may represent a predominant component of the physiological response to endogenous 5-HT in the CA1. The high sensitivity of the 5-HT1B receptor-mediated reduction of polysynaptic excitatory responses to the extracellular 5-HT level enabled us to study mechanisms of the 5-HT releasing action of MDMA. Block of the serotonin transporter (SERT) with citalopram slowed the time course and reduced overall 5-HT release by MDMA. Depletion of vesicular 5-HT, by inhibition of vesicular monoamine transporter type 2 with tetrabenazine prevented the release. Thus although the SERT reversal contributes, a direct vesicle-depleting action is essential for MDMA release of 5-HT.     

Sustained recreational use of ecstasy is associated with altered pre and postsynaptic markers of serotonin transmission in neocortical areas: a PET study with [11C]DASB and [11C]MDL 100907

3,4-Methylenedioxymethamphetamine (MDMA), the main psychoactive component of the recreational drug ecstasy, is a potent serotonin (5-HT) releaser. In animals, MDMA induces 5-HT depletion and toxicity in 5-HT neurons. The aim of this study was to investigate both presynaptic (5-HT transporter, SERT) and postsynaptic (5-HT(2A) receptor) markers of 5-HT transmission in recently abstinent chronic MDMA users compared with matched healthy controls. We hypothesized that MDMA use is associated with lower SERT density and concomitant upregulation of 5-HT(2A) receptors. Positron emission tomography studies using the SERT ligand [11C]DASB and the 5-HT(2A) receptor ligand [11C]MDL 100907 were evaluated in 13 current and recently detoxified MDMA users and 13 matched healthy controls. MDMA users reported a mean duration of ecstasy use of 8 years, regular exposure, and at least 2 weeks of abstinence before the scans. SERT and 5-HT(2A) receptor availability (binding potential, BP(ND)) were analyzed with a two-tissue compartment model with arterial input function. Current recreational MDMA use was significantly associated with lower SERT BP(ND) and higher 5-HT(2A) receptor BP(ND) in cortical, but not subcortical regions. Decreased SERT BP(ND) was regionally associated with upregulated 5-HT(2A) receptor BP(ND). In light of the animal literature, the most parsimonious interpretation is that repeated exposure to MDMA in humans, even in moderate amounts, leads to damage in 5-HT neuron terminals innervating the cortex. Alterations in mood, cognition, and impulse control associated with these changes might contribute to sustain MDMA use. The reversibility of these changes upon abstinence remains to be firmly established.     

Deep brain stimulation induces antidepressant-like effects in serotonin transporter knockout mice

Background

Some of the antidepressant-like effects of ventromedial prefrontal cortex (vmPFC) deep brain stimulation (DBS) in rodents have been attributed to the modulation of prefrontal-raphe pathways. This is largely different from selective serotonin reuptake inhibitors (SSRIs), which increase serotonin (5-HT) levels by inhibiting the serotonin transporter (SERT). SSRIs have limited efficacy when given to SERT knockout (KO) mice, or patients with mutations in the serotonin transporter promoter gene (5-HTTLPR).

3,4-methylenedioxymethamphetamine self-administration is abolished in serotonin transporter knockout mice.

BACKGROUND: The neurobiological mechanism underlying the reinforcing effects of 3,4-methylenedioxymethamphetamine (MDMA) remains unclear. The aim of the present study was to determine the contribution of the serotonin transporter (SERT) in MDMA self-administration behavior by using knockout (KO) mice deficient in SERT. METHODS: Knockout mice and wild-type (WT) littermates were trained to acquire intravenous self-administration of MDMA (0, .03, .06, .125, and .25 mg/kg/infusion) on a fixed ratio 1 (FR1) schedule of reinforcement. Additional groups of mice were trained to obtain food and water to rule out operant responding impairments. Microdialysis studies were performed to evaluate dopamine (DA) and serotonin (5-HT) extracellular levels in the nucleus accumbens (NAC) and prefrontal cortex (PFC), respectively, after acute MDMA (10 mg/kg). RESULTS: None of the MDMA doses tested maintained intravenous self-administration in KO animals, whereas WT mice acquired responding for MDMA. Acquisition of operant responding for food and water was delayed in KO mice, but no differences between genotypes were observed on the last day of training. MDMA increased DA extracellular levels to a similar extent in the NAC of WT and KO mice. Conversely, extracellular concentrations of 5-HT in the PFC were increased following MDMA only in WT mice. CONCLUSIONS: These findings provide evidence for the specific involvement of SERT in MDMA reinforcing properties.     

Altered serotonin 2C receptor RNA splicing in suicide: association with editing

In an earlier study, we showed increases in serotonin 2C receptor (5-HT2CR) pre-mRNA editing in prefrontal cortex that were specific to suicide victims irrespective of associated psychiatric diagnoses. Here we demonstrate that the ratio between the two 5-HT2CR splice variants is increased in people who committed suicide, but does not vary among the diagnostic groups. This provides further evidence for suicide-specific neurobiology and suggests that, as it was previously shown in vitro, 5-HT2CR editing modulates its splicing in human brain. The association analysis indicates, however, that the efficiency of 5-HT2CR editing is an imperfect predictor of the splicing outcome, and that splice site selection is only partially controlled by the level of editing in vivo.     

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.     

In vitro regulation of serotonin transporter activity by protein kinase A and nicotinic acetylcholine receptors in the prefrontal cortex of rats

We investigated the effect of in vitro exposure to nicotinic acetylcholine receptors (nAChRs), agonists, antagonists, and protein kinase A (PKA) modulators on the activity of the serotonin transporter (SERT) in prefrontocortical (PFC) synaptosomes. The plasma membrane SERT is an active transport mechanism specific for serotonin. Receptors and second messengers capable of altering transporter activity would be expected to have profound effects on serotonergic neurotransmission and on functions involving serotonergic input, such as cognition, anxiety, and mood. Our data suggest that activation of nAChRs, quite likely via PKA, increase the activity of the SERT in the PFC and, thereby, can alter 5-HT levels in a region important in the behavioral effects of nicotine and 5-HT. Nicotine at 4 microM increased [(3)H]5-HT uptake by 75%. Because the nAChR antagonists mecamylamine and dihydro-beta-erythrodine (DHbetaE) both decreased [(3)H]5-HT uptake into synaptosomes, it appeared that the SERT might be tonically activated by acetylcholine present within our synaptosomal preparations. Blocking PKA significantly decreased [(3)H]5-HT, while stimulation of PKA activity significantly increased the uptake. A 66% decrease compared with control was produced by 100 microM Rp-cAMP, and a 41% increase in 5-HT uptake over control was observed with 30 microM Sp-cAMPs. Furthermore, the enhancement in uptake produced by 4 microM nicotine was inhibited in a time-dependent fashion by preincubation with 10 microM Rp-cAMP. A better understanding of the influence of the cholinergic system and the receptors involved in the trafficking of SERT would help clarify the important relationship between the cholinergic and serotonergic systems and the role these systems play in behavior.     

Excess of serotonin affects neocortical pyramidal neuron migration

The serotonin transporter (SERT) is a key molecule involved in the homeostasis of extracellular levels of serotonin and is regulated developmentally. Genetic deletion of SERT in rodents increases extracellular levels of serotonin and affects cellular processes involved in neocortical circuit assembly such as barrel cortex wiring and cortical interneuron migration. Importantly, pharmacological blockade of SERT during brain development leads to phenotypes relevant to psychiatry in rodents and to an increased risk for autism spectrum disorders in humans. Furthermore, developmental adversity interacts with genetically-driven variations of serotonin function in humans and nonhuman primates to increase the risk for a variety of stress-related phenotypes. In this study, we investigate whether an excess of serotonin affects the migration of neocortical pyramidal neurons during development. Using in utero electroporation combined with time-lapse imaging to specifically monitor pyramidal neurons during late mouse embryogenesis, we show that an excess of serotonin reversibly affects the radial migration of pyramidal neurons. We further identify that the serotonin receptor 5-HT₆ is expressed in pyramidal neuron progenitors and that 5-HT₆ receptor activation replicates the effects of serotonin stimulation. Finally, we show that the positioning of superficial layer pyramidal neurons is altered in vivo in SERT knockout mice. Taken together, these results indicate that a developmental excess of serotonin decreases the migration speed of cortical pyramidal neurons, affecting a fundamental step in the assembly of neural circuits. These findings support the hypothesis that developmental dysregulation of serotonin homeostasis has detrimental effects on neocortical circuit formation and contributes to increased vulnerability to psychiatric disorders.     

The interaction of serotonin transporter gene polymorphisms and early adverse life events on vulnerability for major depression

Considerable literature supports the hypothesis of dysfunction in central nervous system serotonergic circuits in the pathophysiology of mood disorders, specifically major depression. Since the development of the selective serotonin (5-HT) reuptake inhibitors, a putative role for the 5-HT transporter (SERT) in the etiology of depression has been explored. The discovery of a functional SERT polymorphism has provided a novel tool to further scrutinize the role of serotonergic neurons in depression. This article reviews the burgeoning evidence of an interaction between early life stress and an SERT polymorphism on vulnerability to depression.