Uptake of serotonin by adult rat corpus callosum is partially reduced by common antidepressants

Magnetic resonance imaging histogram analysis of corpus callosum in a functional neurological disorder

Background/aim

The aim of the present study was to examine and compare the corpus callosum (CC) via histogram analysis (HA) on T1-weighted MR images of patients diagnosed with Functional Neurological Disorder (FND) and healthy controls.

Materials and methods

The study group included 19 female patients diagnosed with FND, and the control group included 20 healthy subjects. All participants were scanned with a 1.5 T MR scanner. A high-resolution structural image of the entire brain was obtained with sagittal 3D spiral fast spin echo T1-weighted images. Gray level intensity, standard deviation of the histogram, entropy, uniformity, skewness, and kurtosis values were determined with texture analysis. A student’s t-test was used to compare the group data. P < 0.05 was accepted as statistically significant.

Results

It was determined that the mean gray level intensity, standard deviation of the histogram, entropy calculated by the maximum, median and variance and size M percentage values were higher in patients with FND. Kurtosis and size U percentages values were lower in patients with FND.

Conclusion

In the present study, analysis of CC with T1-weighted MR image HA demonstrated significant differences between FND patients and healthy controls. The study findings indicated that HA is a beneficial technique for demonstrating textural variations between the CCs of patients with FND and healthy controls using MR images.

Effect of fluoxetine on proliferation and/or survival of microglia and oligodendrocyte progenitor cells in the fornix and corpus callosum of the mouse brain

BACKGROUND

Fluoxetine is one of the most widely prescribed antidepressants and a selective inhibitor of presynaptic 5-HT transporters. The fornix is the commissural and projection fiber that transmits signals from the hippocampus to other parts of the brain and opposite site of hippocampus. The corpus callosum (CC) is the largest of the commissural fibers that link the cerebral cortex of the left and right cerebral hemispheres. These brain regions play pivotal roles in cognitive functions, and functional abnormalities in these regions have been implicated in the development of various brain diseases. The purpose of the present study was to investigate the effects of fluoxetine on the proliferation and/or survival of microglia and oligodendrocyte progenitor cells (OPCs) in the fornix and CC, the white matter connecting cortical-limbic system, of the adult mouse brain.

METHODS

The effects of fluoxetine on the proliferation and/or survival of microglia and OPCs were examined in lipopolysaccharide (LPS)-treated and normal mice. Proliferating cells were detected in mice that drank water containing the thymidine analog, bromodeoxyuridine (BrdU), using immunohistochemistry.

RESULT

Fluoxetine significantly attenuated LPS-induced increases in the number of BrdU-labeled microglia and morphological activation from the ramified to ameboid shape, and decreased the number of BrdU-labeled OPCs under basal conditions.

CONCLUSIONS

The present results indicate that fluoxetine exerts inhibitory effects on LPS-induced increases in the proliferation and/or survival and morphological activation of microglia and basal proliferation and/or survival of OPCs in the fornix and CC of adult mice.

Glutamic Acid Increased Methotrexate Polyglutamation and Cytotoxicity in a CCRF-SB Acute Lymphoblastic Leukemia Cell Line

Background and Objectives: Acute lymphoblastic leukemia (ALL) is the most common type of cancer in childhood. The majority of patients respond to treatment, but those with resistant phenotypes suffer relapse or death. The antifolate methotrexate (MTX) is the most commonly used drug against ALL due to its efficacy. Once inside leukemic cells, MTX is metabolized into methotrexate polyglutamates (MTX-PG) by action of the enzyme folylpolyglutamate synthetase (FPGS), leading to a longer action compared to that of MTX alone. Materials and Methods: In this work, we demonstrated that the combination treatment of methotrexate and 5 and 10 mM glutamic acid could enhance methotrexate cytotoxicity in CCRF-SB (B-ALL) cells. In addition, MTX plus 20 mM glutamic acid was able to improve the synthesis of MTX-PG₅. Results: All treatments induced an increase in FPGS expression compared to that of the control group. Furthermore, we detected different cellular expression patterns of FPGS in the different treatments. Conclusion: Based on these findings, we demonstrated that levels of methotrexate polyglutamates (MTX-PGs) could be a key determinant of methotrexate-induced cytotoxicity in CCRF-SB acute lymphoblastic leukemia cells.

Lack of serotonin reuptake during brain development alters rostral raphe-prefrontal network formation

Besides its "classical" neurotransmitter function, serotonin (5-HT) has been found to also act as a neurodevelopmental signal. During development, the 5-HT projection system, besides an external placental source, represents one of the earliest neurotransmitter systems to innervate the brain. One of the targets of the 5-HT projection system, originating in the brainstem raphe nuclei, is the medial prefrontal cortex (mPFC), an area involved in higher cognitive functions and important in the etiology of many neurodevelopmental disorders. Little is known, however, about the exact role of 5-HT and its signaling molecules in the formation of the raphe-prefrontal network. Using explant essays, we here studied the role of the 5-HT transporter (5-HTT), an important modulator of the 5-HT signal, in rostral raphe-prefrontal network formation. We found that the chemotrophic nature of the interaction between the origin (rostral raphe cluster) and a target (mPFC) of the 5-HT projection system was affected in rats lacking the 5-HTT (5-HTT(-/-)). While 5-HTT deficiency did not affect the dorsal raphe 5-HT-positive outgrowing neurites, the median raphe 5-HT neurites switched from a strong repulsive to an attractive interaction when co-cultured with the mPFC. Furthermore, the fasciculation of the mPFC outgrowing neurites was dependent on the amount of 5-HTT. In the mPFC of 5-HTT(-/-) pups, we observed clear differences in 5-HT innervation and the identity of a class of projection neurons of the mPFC. In the absence of the 5-HTT, the 5-HT innervation in all subareas of the early postnatal mPFC increased dramatically and the number of Satb2-positive callosal projection neurons was decreased. Together, these results suggest a 5-HTT dependency during early development of these brain areas and in the formation of the raphe-prefrontal network. The tremendous complexity of the 5-HT projection system and its role in several neurodevelopmental disorders highlights the need for further research in this largely unexplored area.

Genetic and pharmacological manipulations of the serotonergic system in early life: neurodevelopmental underpinnings of autism-related behavior

Serotonin, in its function as neurotransmitter, is well-known for its role in depression, autism and other neuropsychiatric disorders, however, less known as a neurodevelopmental factor. The serotonergic system is one of the earliest to develop during embryogenesis and early changes in serotonin levels can have large consequences for the correct development of specific brain areas. The regulation and functioning of serotonin is influenced by genetic risk factors, such as the serotonin transporter polymorphism in humans. This polymorphism is associated with anxiety-related symptoms, changes in social behavior, and cortical gray and white matter changes also seen in patients suffering from autism spectrum disorders (ASD). The human polymorphism can be mimicked by the knockout of the serotonin transporter in rodents, which are as a model system therefore vital to explore the precise neurobiological mechanisms. Moreover, there are pharmacological challenges influencing serotonin in early life, like prenatal/neonatal exposure to selective serotonin reuptake inhibitors (SSRI) in depressed pregnant women. There is accumulating evidence that this dysregulation of serotonin during critical phases of brain development can lead to ASD-related symptoms in children, and reduced social behavior and increased anxiety in rodents. Furthermore, prenatal valproic acid (VPA) exposure, a mood stabilizing drug which is also thought to interfere with serotonin levels, has the potency to induce ASD-like symptoms and to affect the development of the serotonergic system. Here, we review and compare the neurodevelopmental and behavioral consequences of serotonin transporter gene variation, and prenatal SSRI and VPA exposure in the context of ASD.

Functional and structural neural network characterization of serotonin transporter knockout rats

Brain serotonin homeostasis is crucially maintained by the serotonin transporter (5-HTT), and its down-regulation has been linked to increased vulnerability for anxiety- and depression-related behavior. Studies in 5-HTT knockout (5-HTT^-/-) rodents have associated inherited reduced functional expression of 5-HTT with increased sensitivity to adverse as well as rewarding environmental stimuli, and in particular cocaine hyperresponsivity. 5-HTT down-regulation may affect normal neuronal wiring of implicated corticolimbic cerebral structures. To further our understanding of its contribution to potential alterations in basal functional and structural properties of neural network configurations, we applied resting-state functional MRI (fMRI), pharmacological MRI of cocaine-induced activation, and diffusion tensor imaging (DTI) in 5-HTT^-/- rats and wild-type controls (5-HTT^+/+). We found that baseline functional connectivity values and cocaine-induced neural activity within the corticolimbic network was not significantly altered in 5-HTT^-/- versus 5-HTT^+/+ rats. Similarly, DTI revealed mostly intact white matter structural integrity, except for a reduced fractional anisotropy in the genu of the corpus callosum of 5-HTT^-/- rats. At the macroscopic level, analyses of complex graphs constructed from either functional connectivity values or structural DTI-based tractography results revealed that key properties of brain network organization were essentially similar between 5-HTT^+/+ and 5-HTT^-/- rats. The individual tests for differences between 5-HTT^+/+ and 5-HTT^-/- rats were capable of detecting significant effects ranging from 5.8% (fractional anisotropy) to 26.1% (pharmacological MRI) and 29.3% (functional connectivity). Tentatively, lower fractional anisotropy in the genu of the corpus callosum could indicate a reduced capacity for information integration across hemispheres in 5-HTT^-/- rats. Overall, the comparison of 5-HTT^-/- and wild-type rats suggests mostly limited effects of 5-HTT genotype on MRI-based measures of brain morphology and function.

Rotating disk electrode voltammetric measurements of serotonin transporter kinetics in synaptosomes

Altered serotonin (5-HT) signaling is implicated in several neuropsychiatric disorders, including depression, anxiety, obsessive-compulsive disorder, and autism. The 5-HT transporter (SERT) modulates 5-HT neurotransmission strength and duration. This is the first study using rotating disk electrode voltammetry (RDEV) to measure 5-HT clearance. SERT kinetics were measured in whole brain synaptosomes. Uptake kinetics of exogenous 5-HT were measured using glassy carbon electrodes rotated in 500 μL glass chambers containing synaptosomes from SERT-knockout (-/-), heterozygous (+/-), or wild-type (+/+) mice. RDEV detected 5-HT concentrations of 5nM and higher. Initial velocities were kinetically resolved with K(m) and V(max) values of 99±35 standard error of regression (SER) nM and 181±11 SER fmol/(s×mg protein), respectively in wild-type synaptosomes. The method enables control over drug and chemical concentrations, facilitating interpretation of results. Results are compared in detail to other techniques used to measure SERT kinetics, including tritium labeled assays, chronoamperometry, and fast scan cyclic voltammetry. RDEV exhibits decreased 5-HT detection limits, decreased vulnerability to 5-HT oxidation products that reduce electrode sensitivity, and also overcomes diffusion limitations via forced convection by providing a continuous, kinetically resolved signal. Finally, RDEV distinguishes functional differences between genotypes, notably, between wild-type and heterozygous mice, an experimental problem with other experimental approaches.

Distribution of organic cation transporter 3, a corticosterone-sensitive monoamine transporter, in the rat brain

Organic cation transporter 3 (OCT3) is a high-capacity, low-affinity transporter that mediates bidirectional, sodium-independent transport of dopamine, norepinephrine, epinephrine, serotonin, and histamine. OCT3-mediated transport is directly inhibited by corticosterone, suggesting a potential role for the transporter in mediating some of the effects of stress and glucocorticoids on monoaminergic neurotransmission. To elucidate the importance of OCT3 in clearance of extracellular monoamines in the brain, we used immunohistochemical techniques to describe the distribution of OCT3-like-immunoreactive (OCT3-ir) cells throughout the rostrocaudal extent of adult male rat brains. OCT3-ir cell bodies were widely distributed throughout the brain, with the highest densities observed in the superior and inferior colliculi, islands of Calleja, subiculum, lateral septum, lateral and dorsomedial hypothalamic nuclei, and granule cell layers of the main and accessory olfactory bulbs, the cerebellum, and the retrosplenial granular cortex. OCT3-ir cells and/or fibers were also observed in circumventricular organs, and OCT3-ir ependymal cells were observed in the linings of all cerebral ventricles. The widespread distribution of OCT3-ir cell bodies, including regions receiving dense monoaminergic projections, suggests an important role for this transporter in regulating extracellular concentrations of monoamines in the rat brain and is consistent with the hypothesis that corticosterone-induced inhibition of OCT3-mediated transport may contribute to effects of acute stress or corticosterone on monoaminergic neurotransmission.

Evidence for significant contribution of a newly identified monoamine transporter (PMAT) to serotonin uptake in the human brain

The high affinity serotonin transporter (SERT) constitutes the principal pathway for removal of serotonin (5-HT) from extracellular fluid of brain, but evidence indicates that other transporters may also be involved in this process. We recently reported the cloning of a novel plasma membrane monoamine transporter (PMAT), which is abundantly expressed in the human brain and avidly transports 5-HT [Engel K, Zhou M, Wang J. Identification and characterization of a novel monoamine transporter in the human brain. J Biol Chem 2004;279:50042-9]. In this study, we evaluated whether PMAT contributes to total human brain uptake of 5-HT using a hybrid depletion approach in Xenopus laevis oocytes. We also examined whether PMAT interacts with selective serotonin reuptake inhibitors (SSRIs) using MDCK cells stably expressing recombinant human PMAT. Microinjection of total human brain poly(A)(+) mRNA into oocytes elicited approximately 2.5-3-fold increase in 5-HT uptake. Pre-hybridization of poly(A)(+) mRNA with PMAT or SERT antisense oligonucleotides significantly reduced mRNA-induced 5-HT uptake. An additive inhibitory effect was observed when poly(A)(+) mRNA was co-hybridized with both PMAT and SERT antisense oligonucleotides. In contrast, mRNA-induced 5-HT uptake was not affected by pre-hybridization with sense oligonucleotides. These data suggest that functional transcripts of PMAT are present in the human brain, and the PMAT transporter may be significantly involved in brain uptake of 5-HT. All five tested SSRIs inhibited PMAT with IC(50) values ranging from 11 to 116 microM, which are much greater than clinically encountered concentrations, suggesting that PMAT activity is minimally affected by SSRI therapies.

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).

Chronoamperometry to determine differential reductions in uptake in brain synaptosomes from serotonin transporter knockout mice

The serotonin transporter (SERT) is a neuronal plasma membrane protein whose primary function is to take up the neurotransmitter serotonin from the extracellular space, thereby controlling the spatial and temporal aspects of serotonergic signaling in the brain. In humans, a commonly expressed genetic variant of the serotonin transporter gene results in 40% reductions in SERT expression that have been linked to increases in anxiety-related personality traits and susceptibility to stress-associated depression. Mice have been engineered to express similar reductions in SERT expression to investigate transporter-mediated control of serotonin neurotransmission and behavior. We employed carbon fiber microelectrode voltammetry (chronoamperometry) to examine serotonin clearance rates in brain liposomes (synaptosomes) prepared from mice with 50% (SERT(+/)(-)) or complete (SERT(-)(/)(-)) loss of SERT expression. Initial characterization of uptake showed that transport of serotonin was enhanced in the presence of oxygen and abolished when synaptosomes were stirred. Additionally, uptake was prevented by inclusion of the serotonin-selective reuptake inhibiting drug paroxetine in the incubation medium. Most notably, unlike prior studies using established radiochemical methods in synaptosomes, we determined 60% reductions in serotonin uptake rates in SERT(+/)(-) mice in two different brain regions-striatum and frontal cortex. Serotonin uptake was not detected in either brain region in SERT(-)(/)(-) mice. Thus, electroanalytical methods offer distinct advantages stemming from excellent temporal resolution for determining transporter kinetics. Moreover, these appear necessary for delineating moderate but biologically important changes in neurotransmitter transporter function.

Transport mechanisms governing serotonin clearance in vivo revealed by high-speed chronoamperometry

High-speed chronoamperometry was used to determine the kinetics of clearance of exogenously applied serotonin (5-HT) in the dorsal raphe nucleus (DRN), dentate gyrus, CA3 region of the hippocampus or corpus callosum of anesthetized rats. Maximal velocity (Vmax) for 5-HT clearance was greatest in the DRN > dentate gyrus > CA3 > corpus callosum. Apparent affinity (K(T)) of the serotonin transporter (5-HTT) was similar in DRN and CA3 but greater in dentate gyrus and corpus callosum. A 90% loss of norepinephrine transporters (NET) produced by 6-hydroxydopamine pretreatment, resulted in a two-fold reduction in Vmax and a 30% decrease in K(T) in the dentate gyrus, but no change in kinetic parameters in the CA3 region. Pretreatment with 5,7-dihydroxytryptamine that resulted in a 90% reduction in 5-HTT density, modestly reduced Vmax in dentate gyrus but not in CA3. The same treatment had no effect on K(T) in the dentate gyrus but increased K(T) two-fold in the CA3. Neurotoxin treatments had no effect on 5-HT clearance in the corpus callosum. In hippocampal regions of intact rats, local application of the selective serotonin reuptake inhibitor, fluvoxamine, inhibited 5-HT clearance most robustly when the extracellular concentration of 5-HT was less than the K(T) value. By contrast, the NET antagonist, desipramine, significantly inhibited 5-HT clearance when extracellular concentrations of 5-HT were greater than the K(T) value. These data indicate that hippocampal uptake of 5-HT may be mediated by two processes, one with high affinity but low capacity (i.e. the 5-HTT) and the other with low affinity but a high capacity (i.e. the NET). These data show for the first time in the whole animal that 5-HT clearance in brain is regionally distinct with regard to rate and affinity.

Zinc modulation of serotonin uptake in the adult rat corpus callosum

Antidepressants partially inhibit the uptake of 5-hydroxytryptamine (5-HT; serotonin) in the rat corpus callosum (CC), a white matter commissure involved in interhemispheric brain communication. It is also known that zinc modulates many proteins, including neurotransmitter transporters. We examined the effects of zinc on the uptake of 5-HT into slices of the adult rat CC, in the absence or presence of some antidepressants. Zinc increased 5-HT uptake in a concentration-dependent manner when the CC slices were incubated in a solution buffered with sodium bicarbonate; however, zinc exerted no effect on 5-HT transport when HEPES was the buffer. Potentiation of 5-HT uptake by zinc was maximal with 1 microM (45% over the control uptake). Moreover, 1 microM zinc potentiated 5-HT uptake in the cingulate cortex by 58% and in the Raphe nucleus by 65%. The antidepressants fluoxetine and imipramine inhibited 5-HT uptake in the CC by approximately 50%, whereas 6-nitroquipazine, a potent 5-HT uptake blocker, inhibited uptake by only 23%. Interestingly, inhibition of 5-HT uptake by all three substances, fluoxetine, imipramine, and 6-nitroquipazine, was counteracted by the presence of 1 microM zinc. Free zinc may thus contribute to modulation of extracellular levels of 5-HT and its removal. These actions should be considered in the treatment of mental depression with antidepressants.

Identification and characterization of a novel monoamine transporter in the human brain

Precise control of monoamine neurotransmitter levels in the extracellular fluids of the brain is critical in maintaining efficient and robust neurotransmission. High affinity transporters in the solute carrier SLC6A family function in removing monoamines from the neurosynaptic cleft. Emerging evidence suggests that these transporters are only one part of a system of transporters that work in concert to maintain brain homeostasis of monoamines. Here we report the cloning and characterization of a new human plasma membrane monoamine transporter, PMAT. The PMAT cDNA encodes a protein of 530 amino acid residues with 10-12 transmembrane segments. PMAT is not homologous to known neurotransmitter transporters but exhibits low homology to members of the equilibrative nucleoside transporter family. When expressed in Madin-Darby canine kidney cells and Xenopus laevis oocytes, PMAT efficiently transports serotonin (K(m) = 114 mum), dopamine (K(m) = 329 mum), and the neurotoxin 1-methyl-4-phenylpyridinium (K(m) = 33 mum). In contrast, there is no significant interaction of PMAT with nucleosides or nucleobases. PMAT-mediated monoamine transport does not require Na(+) or Cl(-) but appears to be sensitive to changes in membrane potential. Northern blot analysis showed that PMAT is predominantly expressed in the human brain and widely distributed in the central nervous system. These studies demonstrate that PMAT may be a novel low affinity transporter for biogenic amines, which, under certain conditions, might supplement the role of the high affinity transporters in the brain.