Pharmacological characterization of dopamine transport in cultured rat astrocytes.

The effects of GBR-12909 (selective DA uptake inhibitor), zimelidine (selective 5-HT uptake inhibitor) and nisoxetine (selective NE uptake inhibitor) on the uptake of 30 nM [3H]DA into cultured rat astrocytes were examined. [3H]DA uptake was inhibited by approximately 50% by GBR-12909 or zimelidine in a concentration-dependent manner (100 nM to approximately 10 microM). Furthermore, the inhibition curves of GBR-12909 were biphasic, and uptake was completely inhibited by a high concentration of GBR-12909 (100 microM). [3H]DA uptake was also inhibited by approximately 50% by nisoxetine in a concentration-dependent manner (0.1 to approximately 100 nM), and nisoxetine was more potent than GBR-12909 or zimelidine. The inhibitory potencies were in the order nisoxetine > GBR-12909 > zimelidine. The uptake of [3H]DA under Na+-free conditions was approximately 50% of that under normal conditions. Thus, DA was taken up by both Na+-dependent and Na+-independent mechanisms. Nisoxetine (100 nM), zimelidine (100 microM) and GBR-12909 (10 microM) inhibited [3H]DA uptake into astrocytes only in the presence of Na+. On the other hand, this uptake was completely inhibited by a high concentration of GBR-12909 (100 microM) in the absence of Na+. The present data suggest that the Na+-dependent uptake of [3H]DA in cultured rat astrocytes may occur in the NE uptake system. Furthermore, astrocytes express the extraneuronal monoamine transporter (uptake2), which is an Na+-independent system, and this transporter is involved in the inactivation of centrally released DA.     

Regional Differences in 5-Hydroxytryptamine and Catecholamine Uptake in Primary Astrocyte Cultures

The uptake of 3H-labelled 5-hydroxytryptamine (5-HT, serotonin) norepinephrine ([3H]NE), and 3,4-dihydroxyphenylethylamine ([ 3H]dopamine, [3H]DA) was studied in primary astrocyte cultures prepared from the cerebral cortex, corpus striatum, and hippocampal regions of neonatal rat brain. Na+-dependent uptake showed marked regional differences. For [3H]5-HT the magnitude of uptake was corpus striatum greater than or equal to cerebral cortex greater than hippocampus, whereas for [3H]NE the order was hippocampus greater than corpus striatum greater than cerebral cortex. For [3H]DA, only the hippocampal cultures showed significant Na+-dependent uptake. [3H]5-HT uptake was specifically inhibited by 10(-7) M fluoxetine whereas [3H]NE uptake was preferentially inhibited by 10(-7) M desipramine. These results may reflect regional brain specialization and/or different developmental patterns of high affinity uptake of serotonin and catecholamines by astrocytes in situ.     

4-Iodotomoxetine: a novel ligand for serotonin uptake sites.

The tomoxetine analog, R-4-iodotomoxetine, binds in vitro to a single site of rat cortical membranes with high affinity (Kd = 0.03 +/- 0.01 nM, n = 4) and can be blocked by a selective serotonin reuptake site inhibitor, paroxetine. The [125I]R-4-iodotomoxetine binding at equilibrium is saturable and is temperature- and Na(+)-dependent. The number of specific [125I]R-4-iodotomoxetine binding sites (Bmax = 356 +/- 20 fmol/mg protein) is similar to that of [3H]citalopram (329 +/- 30 fmol/mg protein), a known serotonin uptake inhibitor. The binding of [125I]R-4-iodotomoxetine is selectively inhibited by several serotonin uptake blockers, and a good correlation is demonstrated between the potency of various drugs to inhibit in vitro binding of [125I]R-4-iodotomoxetine and [3H]citalopram. In addition, lesions performed with the neurotoxin p-chloroamphetamine, which destroys monoamine neurons, including serotonergic neuronal system, result in a 90% reduction of [125I]R-4-iodotomoxetine binding when compared to sham controls. These results indicate that the binding sites labeled by [125I]R-4-iodotomoxetine are associated with the neuronal serotonin uptake sites. However, the in vivo and ex vivo results do not show regional localization corresponding to the distribution of serotonin uptake sites. The nonspecific uptake may be related to this compound's high lipophilicity (octanol-buffer partition coefficient = 1100 - 1400 at pH 7). Although the in vivo properties of [125I]R-4-iodotomoxetine make it an unlikely candidate for mapping serotonin uptake sites with SPECT, the high affinity and selectivity should make it a useful tool for in vitro studies of the serotonin uptake sites.     

3-hydroxyglutaric acid enhances glutamate uptake into astrocytes from cerebral cortex of young rats

A predominantly neurological presentation is common in patients with glutaric acidemia type I (GA-I). 3-hydroxyglutaric acid (3-OHGA), which accumulates in affected patients, has recently been demonstrated to play a central role in the neuropathogenesis of this disease. In the present study, we investigated the in vitro effects of 3-OHGA at concentrations ranging from 10 to 1000 microM on various parameters of the glutamatergic system, such as the basal and potassium-induced release of [3H]glutamate by synaptosomes, as well as on Na+-dependent [3H]glutamate uptake by synaptosomes and astrocytes and Na+-independent [3H]glutamate uptake by synaptic vesicles from cerebral cortex of 30-day-old Wistar rats. First, we observed that exposure of cultured astrocytes to 3-OHGA for 20 h did not reduce their viability. Furthermore, 3-OHGA significantly increased Na+-dependent [3H]glutamate uptake by astrocytes by up to 80% in a dose-dependent manner at doses as low as 30 microM. This effect was not dependent on the presence of the metabolite during the uptake assay, since it occurred even when 3-OHGA was withdrawn from the medium after cultured cells had been exposed to the acid for approximately 1 h. All other parameters investigated were not influenced by this organic acid, indicating a selective action of 3-OHGA on astrocyte transporters. Although the exact mechanisms involved in 3-OHGA-stimulatory effect on astrocyte glutamate uptake are unknown, the present findings contribute to the understanding of the pathophysiology of GA-I, suggesting that astrocytes may protect neurons against excitotoxic damage caused by 3-OHGA by increasing glutamate uptake and therefore reducing the concentration of this excitatory neurotransmitter in the synaptic cleft.     

KCl stimulation increases norepinephrine transporter function in PC12 cells

The norepinephrine transporter (NET) plays a pivotal role in terminating noradrenergic signaling and conserving norepinephrine (NE) through the process of re-uptake. Recent evidence suggests a close association between NE release and regulation of NET function. The present study evaluated the relationship between release and uptake, and the cellular mechanisms that govern these processes. KCl stimulation of PC12 cells robustly increased [3H]NE uptake via the NET and simultaneously increased [3H]NE release. KCl-stimulated increases in uptake and release were dependent on Ca2+. Treatment of cells with phorbol-12-myristate-13-acetate (PMA) or okadaic acid decreased [3H]NE uptake but did not block KCl-stimulated increases in [3H]NE uptake. In contrast, PMA increased [3H]NE release and augmented KCl-stimulated release, while okadaic acid had no effects on release. Inhibition of Ca2+-activated signaling cascades with KN93 (a Ca2+ calmodulin-dependent kinase inhibitor), or ML7 and ML9 (myosin light chain kinase inhibitors), reduced [3H]NE uptake and blocked KCl-stimulated increases in uptake. In contrast, KN93, ML7 and ML9 had no effect on KCl-stimulated [3H]NE release. KCl-stimulated increases in [3H]NE uptake were independent of transporter trafficking to the plasma membrane. While increases in both NE release and uptake mediated by KCl stimulation require Ca2+, different intracellular mechanisms mediate these two events.     

Reserpine-induced reduction in norepinephrine transporter function requires catecholamine storage vesicles

Treatment of rats with reserpine, an inhibitor of the vesicular monoamine transporter (VMAT), depletes norepinephrine (NE) and regulates NE transporter (NET) expression. The present study examined the molecular mechanisms involved in regulation of the NET by reserpine using cultured cells. Exposure of rat PC12 cells to reserpine for a period as short as 5 min decreased [³H]NE uptake capacity, an effect characterized by a robust decrease in the V_max of the transport of [³H]NE. As expected, reserpine did not displace the binding of [³H]nisoxetine from the NET in membrane homogenates. The potency of reserpine for reducing [³H]NE uptake was dramatically lower in SK-N-SH cells that have reduced storage capacity for catecholamines. Reserpine had no effect on [³H]NE uptake in HEK-293 cells transfected with the rat NET (293-hNET), cells that lack catecholamine storage vesicles. NET regulation by reserpine was independent of trafficking of the NET from the cell surface. Pre-exposure of cells to inhibitors of several intracellular signaling cascades known to regulate the NET, including Ca²⁺/Ca²⁺–calmodulin dependent kinase and protein kinases A, C and G, did not affect the ability of reserpine to reduce [³H]NE uptake. Treatment of PC12 cells with the catecholamine depleting agent, α-methyl-p-tyrosine, increased [³H]NE uptake and eliminated the inhibitory effects of reserpine on [³H]NE uptake. Reserpine non-competitively inhibits NET activity through a Ca²⁺-independent process that requires catecholamine storage vesicles, revealing a novel pharmacological method to modify NET function. Further characterization of the molecular nature of reserpine's action could lead to the development of alternative therapeutic strategies for treating disorders known to be benefitted by treatment with traditional competitive NET inhibitors.     

Presynaptic Mechanisms Underlying the γ-Aminobutyric Acid-Evoked Receptor-Independent Release of [3H]Norepinephrine in Rat Hippocampus

The effects of gamma-aminobutyric acid (GABA) on the spontaneous efflux of [3H]norepinephrine ([3H]NE) were studied in synaptosomes prepared from rat hippocampus and prelabelled with [3H]NE. It had been observed previously that, when synaptosomes were exposed in superfusion to GABA, the basal release of the tritiated catecholamine was enhanced, apparently with no involvement of the known GABA receptors. The mechanisms underlying this effect have now been investigated. The potency of GABA as a releaser of [3H]NE was decreased by lowering the Na+ content of the superfusion medium, and its effect disappeared at 23 mM Na+. The GABA-induced [3H]NE release was counteracted by the GABA uptake inhibitor N-(4,4-diphenyl-3-butenyl)nipecotic acid (SKF 89976A), but it was unaffected by the NE uptake blockers desmethylimipramine and nisoxetine. The GABA-induced release of [3H]NE was Ca2+-dependent and tetrodotoxin-sensitive. The data support the hypothesis that GABA provoked [3H]NE release by a novel mechanism which involves penetration into the noradrenergic nerve terminals through a GABA carrier located on the NE terminals themselves. This uptake process might be electrogenic and provoke depolarization of the nerve terminals, causing an exocytotic release of [3H]NE.     

Acidic amino acid transport characteristics of a newly developed conditionally immortalized rat type 2 astrocyte cell line (TR-AST).

To characterize acidic amino acid transport in type 2 astrocytes, we established conditionally immortalized rat astrocyte cell lines (TR-AST) from newly developed transgenic rats harboring temperature-sensitive SV40 large T-antigen gene. TR-AST exhibited positive immunostaining for anti-GFAP antibody and A2B5 antibody, characteristics associated with type 2 astrocytes, and expressed glutamine synthetase. Acidic amino acid transporters, GLT-1 and system xc-, which consists of xCT and 4F2hc, were expressed in all TR-ASTs by RT-PCR. On the other hand, GLAST expression was found in TR-AST3 and 5. The characteristics of [3H]L-glutamic acid (L-Glu) uptake by TR-AST5 include an Na+-dependent and Na+-independent manner, concentration-dependence, and inhibition by L-aspartic acid (L-Asp) and D-aspartic acid (D-Asp). The corresponding Michaelis-Menten constants for the Na+-dependent and Na+-independent process were 36.3 microM and 155 microM, respectively. [3H]L-Asp and [3H]D-Asp uptake by TR-AST5 had an Na+-dependent and Na+-independent manner. This study demonstrated that GLT-1, system xc-, and GLAST were expressed in TR-AST, which has the characteristics of type 2 astrocytes and is able to transport acidic amino acids.     

Sodium Dependence of [3H]]Paroxetine Binding and 5- [3H]]Hydroxytryptamine Uptake in Rat Diencephalon

The sodium dependence of binding of [3H]-paroxetine, a selective serotonin uptake inhibitor, to the serotonin transporter in rat diencephalon was studied in both brain membranes and tissue sections and compared with that of 5-[3H]hydroxytryptamine ([3H]5-HT) uptake by synaptosomes from the same region. Binding of [3H]-paroxetine in both the membranes and sections displayed clear sodium dependence until a plateau occurring at 60 nM NaCl, the EC50 for sodium being 8 and 25 mM, respectively. The affinity (1/KD) of [3H]paroxetine binding was a simple hyperbolic function of sodium concentration. In contrast, the density of [3H]paroxetine sites was not affected by external Na+ concentration. The uptake of [3H]5-HT showed a similar pattern of sodium dependence with an EC50 for Na+ of 25 mM. Both the affinity (1/Km) and the rate (Vmax) of [3H]5-HT uptake were dependent on external [Na+] with sodium-dependence curves fitting a rectangular hyperbola. The kinetic analysis of results indicates that one sodium ion is required for the binding of [3H]paroxetine as well as for the binding and translocation of each [3H]5-HT molecule. The results concur with a single-site model of the sodium-dependent serotonin transporter with common or overlapping domains for 5-HT and 5-HT uptake inhibitors.     

Activation of P2X(7) receptors decreases glutamate uptake and glutamine synthetase activity in RBA-2 astrocytes via distinct mechanisms

Glutamate clearance by astrocytes is critical for controlling excitatory neurotransmission and ATP is an important mediator for neuron-astrocyte interaction. However, the effect of ATP on glutamate clearance has never been examined. Here we report that treatment of RBA-2 cells, a type-2-like astrocyte cell line, with ATP and the P2X(7) receptor selective agonist 3'-O-(4-benzoylbenzoyl) adenosine 5'-triphosphate (BzATP) decreased the Na+-dependent [3H]glutamate uptake within minutes. Mechanistic studies revealed that the decreases were augmented by removal of extracellular Mg2+ or Ca2+, and was restored by P2X7 selective antagonist , periodate-oxidized 2',3'-dialdehyde ATP (oATP), indicating that the decreases were mediated through P2X(7) receptors. Furthermore, stimulation of P2X7 receptors for 2 h inhibited both activity and protein expression of glutamine synthetase (GS), and oATP abolished the inhibition. In addition, removal of extracellular Ca(2+) and inhibition of protein kinase C (PKC) restored the ATP-decreased GS expression but failed to restore the P2X(7)-decreased [3H]glutamate uptake. Therefore, P2X7-mediated intracellular signals play a role in the down-regulation of GS activity/expression. Activation of P2X7 receptors stimulated increases in intracellular Na+ concentration ([Na+](i)) suggesting that the P2X(7)-induced increases in [Na+](i) may affect the local Na+ gradient and decrease the Na+-dependent [3H]glutamate uptake. These findings demonstrate that the P2X7-mediated decreases in glutamate uptake and glutamine synthesis were mediated through distinct mechanisms in these cells.     

Na(+)-dependent high-affinity uptake of L-glutamate in cultured fibroblasts.

Uptake of 1 microM [3H]L-glutamate by cultured 3T3 fibroblasts was strongly dependent on extracellular Na+; it was reduced by elevated concentrations of K+ (60 mM) but it was not influenced by variations in the concentration of Ca2+ (0-9.6 mM). D- and L-Asparate, D- and L-threo-3-hydroxyaspartate DL-threo-3-methylaspartate and a few other glutamate derivatives and analogues inhibited the uptake but several close analogues of L-glutamate (including D-glutamate) had no effect, implying that the uptake system is highly structurally selective. The recently identified inhibitor of glutamate uptake in synaptosomal preparations, L-trans-pyrrolidine-2,4-dicarboxylate, was also among the inhibitors. Apparent Km of the uptake was found to be less than 10 microM. The present observations indicate that Na(+)-dependent 'high-affinity' uptake of L-glutamate may appear in structures which are apparently unrelated to glutamatergic synaptic transmission in the CNS.     

Angiotensin-(1-7) through Mas receptor up-regulates neuronal norepinephrine transporter via Akt and Erk1/2-dependent pathways

As angiotensin (Ang) (1-7) decreases norepinephrine (NE) content in the synaptic cleft, we investigated the effect of Ang-(1-7) on NE neuronal uptake in spontaneously hypertensive rats. [(3)H]-NE neuronal uptake was measured in isolated hypothalami. NE transporter (NET) expression was evaluated in hypothalamic neuronal cultures by western-blot. Ang-(1-7) lacked an acute effect on neuronal NE uptake. Conversely, Ang-(1-7) caused an increase in NET expression after 3 h incubation (40 ± 7%), which was blocked by the Mas receptor antagonist, a PI3-kinase inhibitor or a MEK1/2 inhibitor suggesting the involvement of Mas receptor and the PI3-kinase/Akt and MEK1/2-ERK1/2 pathways in the Ang-(1-7)-stimulated NET expression. Ang-(1-7) through Mas receptors stimulated Akt and ERK1/2 activities in spontaneously hypertensive rat neurons. Cycloheximide attenuated Ang-(1-7) stimulation of NET expression suggesting that Ang-(1-7) stimulates NET synthesis. In fact, Ang-(1-7) increased NET mRNA levels. Thus, we evaluated the long-term effect of Ang-(1-7) on neuronal NE uptake after 3 h incubation. Under this condition, Ang-(1-7) increased neuronal NE uptake by 60 ± 14% which was blocked by cycloheximide and the Mas receptor antagonist. Neuronal NE uptake and NET expression were decreased after 3 h incubation with an anti-Ang-(1-7) antibody. Ang-(1-7) induces a chronic stimulatory effect on NET expression. In this way, Ang-(1-7) may regulate a pre-synaptic mechanism in maintaining appropriate synaptic NE levels during hypertensive conditions.     

Characterization of carrier-mediated efflux in human embryonic kidney 293 cells stably expressing the rat serotonin transporter: a superfusion study

Human embryonic kidney 293 cells stably transfected with the rat plasmalemmal serotonin transporter (rSERT) were incubated with 5-[3H]hydroxytryptamine ([3H]5-HT) and superfused. Substrates of the rSERT, such as p-chloroamphetamine (PCA) or methylenedioxymethamphetamine, concentration-dependently increased basal efflux of [3H]5-HT. 5-HT reuptake blockers (e.g., imipramine, citalopram) also caused an enhancement of [3H]5-HT efflux, reaching about half the maximal effect of the rSERT substrates. In uptake experiments, both groups of substances concentration-dependently inhibited 5-HT uptake. EC50 values obtained in superfusion experiments significantly correlated with IC50 values from uptake studies (r2 = 0.92). Addition of the Na+,K(+)-ATPase inhibitor ouabain (100 microM) to or the omission of K+ from the superfusion buffer accelerated basal efflux. The effect of PCA (10 microM) was markedly enhanced by both measures, whereas the effect of uptake inhibitors remained unchanged. When [3H]MPP+, a substrate with low affinity for the rSERT, was used instead of [3H]5-HT for labeling the cells, uptake inhibitors failed to augment efflux. By contrast, PCA accelerated [3H]MPP+ efflux, and its effect was strongly enhanced in the presence of ouabain. The results suggest that the [3H]5-HT efflux caused by substrates of rSERT is carrier-mediated, whereas efflux induced by uptake inhibitors is a consequence of interrupted high-affinity reuptake that is ongoing even under superfusion conditions.     

Biochemical characterization of [3H]norepinephrine uptake in dissociated brain cell cultures from chick embryos

The uptake of [³H]norepinephrine ([³H]NE) was studied in dissociated brain cell cultures prepared from 8-day-old chick embryos using the whole brain (minus optic lobes). Uptake of [³H]NE, 5×10^–9 M, 10 min incubation, in freshly dissociated noncultured embryonic chick brain cells, was detected in 6-day-old embryos; it was temperature and drug (cocaine, metanephrine) sensitive and increased with brain development. In cultured cells, which were assayed at various days in culture, the increase in [³H]NE accumulation per culture was less than that seen in freshly dissociated noncultured embryonic cells. When [³H]NE uptake was expressed per mg protein, a decrease with days in culture was observed, reflecting perhaps a dilution of growth or proliferation of cells not accumulating NE. Metanephrine, 5×10^–6 M, an inhibitor of extraneuronal uptake, inhibited [³H]NE in 5-day-old cultures whereas desmethylimipramine, an inhibitor of neuronal uptake, inhibited [³H]NE uptake in 15- and 20-day-old cultures. Cocaine, another neuronal inhibitor, inhibited [³H]NE at 10 and 15 days only. We interpret these findings to suggest that during early growth in culture most neuroblasts accumulate NE nonspecifically and, as neuronal maturation proceeds, NE accumulation becomes specific.     

Functional and molecular identification of sodium-coupled dicarboxylate transporters in rat primary cultured cerebrocortical astrocytes and neurons

Na+-coupled carboxylate transporters (NaCs) mediate the uptake of tricarboxylic acid cycle intermediates in mammalian tissues. Of these transporters, NaC3 (formerly known as Na+-coupled dicarboxylate transporter 3, NaDC3/SDCT2) and NaC2 (formerly known as Na+-coupled citrate transporter, NaCT) have been shown to be expressed in brain. There is, however, little information available on the precise distribution and function of both transporters in the CNS. In the present study, we investigated the functional characteristics of Na+-dependent succinate and citrate transport in primary cultures of astrocytes and neurons from rat cerebral cortex. Uptake of succinate was Na+ dependent, Li+ sensitive and saturable with a Michaelis constant (Kt) value of 28.4 microM in rat astrocytes. Na+ activation kinetics revealed that the Na+ to succinate stoichiometry was 3:1 and the concentration of Na+ necessary for half-maximal transport was 53 mM. Although uptake of citrate in astrocytes was also Na+ dependent and saturable, its Kt value was significantly higher (approximately 1.2 mM) than that of succinate. Unlabeled succinate (2 mM) inhibited Na+-dependent [14C]succinate (18 microM) and [14C]citrate (4.5 microM) transport completely, whereas unlabeled citrate inhibited Na+-dependent [14C]succinate uptake more weakly. Interestingly, N-acetyl-L-aspartate, which is the second most abundant amino acid in the nervous system, also completely inhibited Na+-dependent succinate transport in rat astrocytes. The inhibition constant (Ki) for the inhibition of [14C]succinate uptake by unlabeled succinate, N-acetyl-L-aspartate and citrate was 15.9, 155 and 764 microM respectively. In primary cultures of neurons, uptake of citrate was also Na+ dependent and saturable with a Kt value of 16.2 microM, which was different from that observed in astrocytes, suggesting that different Na+-dependent citrate transport systems are expressed in neurons and astrocytes. RT-PCR and immunocytochemistry revealed that NaC3 and NaC2 are expressed in cerebrocortical astrocytes and neurons respectively. These results are in good agreement with our previous reports on the brain distribution pattern of NaC2 and NaC3 mRNA using in situ hybridization. This is the first report of the differential expression of different NaCs in astrocytes and neurons. These transporters might play important roles in the trafficking of tricarboxylic acid cycle intermediates and related metabolites between glia and neurons.     

Uptake of [3H]serotonin into plasma membrane vesicles from mouse cerebral cortex

Preparations of plasma membrane vesicles were used as a tool to study the properties of the serotonin transporter in the central nervous system. The vesicles were obtained after hypotonic shock of synaptosomes purified from mouse cerebral cortex. Uptake of [3H]serotonin had a Na+-dependent and Na+-independent component. The Na+-dependent uptake was inhibited by classical blockers of serotonin uptake and had a Km of 63-180 nM, and a Vmax of 0.1-0.3 pmol mg-1 s-1 at 77 mM Na+. The uptake required the presence of external Na+ and internal K+. It required a Na+ gradient ([Na+]out greater than [Na+]in) and was stimulated by a gradient of K+ ([K+]in greater than [K+]out). Replacement of Cl- by other anions (NO2-, S2O3-(2-)) reduced uptake appreciably. Gramicidin prevented uptake. Although valinomycin increased uptake somewhat, the membrane potential per se could not drive uptake because no uptake was observed when a membrane potential was generated by the SCN- ion in the absence of internal K+ and with equal [Na+] inside and outside. The increase of uptake as a function of [Na+] indicated a Km for Na+ of 118 mM and a Hill number of 2.0, suggesting a requirement of two sodium ions for serotonin transport. The present results are accommodated very well by the model developed for porcine platelet serotonin transport (Nelson, P. J., and Rudnick, G. (1979) J. Biol. Chem. 254, 10084-10089), except for the number of sodium ions that are required for transport.     

Does high affinity [3H] imipramine binding label serotonin reuptake sites in brain and platelet?

Recent studies have demonstrated the presence of high affinity binding sites for [³H] imipramine in membrane preparations derived from rat brain, human platelet, and human brain. Although initial reports concluded that there was no relationship between these binding sites and the reuptake sites for biogenic amines, subsequent studies in our laboratory suggested a close relationship between the high affinity imipramine binding site and the serotonin uptake or transport site (cf. ref. 9). To further establish whether these binding sites are associated with either platelet or neuronal uptake of serotonin, the relative potencies of a series of tricyclic antidepressants in inhibiting [³H] imipramine binding and serotonin uptake were determined under identical assay conditions. A close correlation between inhibition of serotonin uptake and [³H] imipramine binding was observed (r = 0.99, p < 0.001). In addition, electrolytic lesions of the midbrain raphe produced a decrease in [³H] imipramine binding in hypothalamic synaptosomes that paralleled the decrease in [³H] serotonin uptake. Finally incubation of synaptosomal membranes with 2,8-dinitroimipramine, an irreversible inhibitor of [³H] imipramine binding, produced a dose-dependent decrease in serotonin uptake, without altering the uptake of nonrepinephrine or dopamine. Taken together our results strongly suggest that high affinity binding of [³]] imipramine selectively labels serotonin uptake sites in brain and platelet.     

The Persistent Membrane Retention of Desipramine Causes Lasting Inhibition of Norepinephrine Transporter Function

The present study examined the potential membrane retention of desipramine (DMI) following exposures of 293-hNET cells to DMI, and its effect on [3H]NE uptake. Incubation of cells with 500 nM DMI for 1 h or 1 day persistently inhibited the uptake of [3H]NE up to 7 days, despite daily repeated washing of cells with drug-free medium. Uptake inhibition was paralleled by persistent retention of DMI associated with cells, as determined by HPLC and by radiotracer experiments using [3H]DMI. [3H]DMI trapped in membranes was displaceable by the structurally unrelated NET inhibitor, nisoxetine, in a concentration-dependent manner, implying interaction of retained [3H]DMI with the NET. A similar cellular retention was observed following incubation of cells with nisoxetine. The results demonstrate that DMI and nisoxetine are persistently retained in cell membranes, at least partly in association with the NET. The retention and slow diffusion of DMI and nisoxetine from membranes may contribute to their pharmacological and modulatory action on NET.     

Transport of dopamine and levodopa and their interaction in COS-7 cells heterologously expressing monoamine neurotransmitter transporters and in monoaminergic cell lines PC12 and SK-N-SH

The present study investigated the effects of levodopa, a precursor of dopamine (DA) therapeutically used for the treatment of Parkinson's disease, on DA transport in the two different systems, COS-7 cells heterologously expressing rat monoamine transporter cDNA and in monoaminergic cell lines PC12 and SK-N-SH. Levodopa enhanced uptake of [3H]DA and [3H]norepinephrine (NE) but not [3H]serotonin in the transfected COS-7 cells in a concentration-dependent manner. On the other hand, in PC12 and SK-N-SH cells where NET is functionally expressed, levodopa enhanced [3H]DA and [3H]NE uptake at low concentrations and inhibited the uptake at higher concentrations. The effects of levodopa on catecholamine transporters in the opposite direction suggest a different mechanism at the intra- and extracellular sites in a levodopa transport-dependent and independent manner.