GBR 12909 and 12935 block dopamine uptake into brain synaptic vesicles as well as nerve endings

Prolonged dopamine D3 receptor stimulation promotes dopamine transporter ubiquitination and degradation through a PKC-dependent mechanism

The dopamine transporter (DAT) is a membrane glycoprotein in dopaminergic neurons, which modulates extracellular and intracellular dopamine levels. DAT is regulated by different presynaptic proteins, including dopamine D₂ (D₂R) and D₃ (D₃R) receptors. While D₂R signalling enhances DAT activity, some data suggest that D₃R has a biphasic effect. However, despite the extensive therapeutic use of D₂R/D₃R agonists in neuropsychiatric disorders, this phenomenon has been little studied. In order to shed light on this issue, DAT activity, expression and posttranslational modifications were studied in mice and DAT-D₃R-transfected HEK cells. Consistent with previous reports, acute treatment with D₂R/D₃R agonists promoted DAT recruitment to the plasma membrane and an increase in DA uptake. However, when the treatment was prolonged, DA uptake and total striatal DAT protein declined below basal levels. These effects were inhibited in mice by genetic and pharmacological inactivation of D₃R, but not D₂R, indicating that they are D₃R-dependent. No changes were detected in mesostriatal tyrosine hydroxylase (TH) protein expression and midbrain TH and DAT mRNAs, suggesting that the dopaminergic system is intact and DAT is posttranslationally regulated. The use of immunoprecipitation and cell surface biotinylation revealed that DAT is phosphorylated at serine residues, ubiquitinated and released into late endosomes through a PKCβ-dependent mechanism. In sum, the results indicate that long-term D₃R activation promotes DAT down-regulation, an effect that may underlie neuroprotective and antidepressant actions described for some D₂R/D₃R agonists.

N-phenylpropyl-N'-substituted piperazines occupy sigma receptors and alter methamphetamine-induced hyperactivity in mice

This study examined the effect of the N-phenylpropyl-N'-substituted piperazine ligands SA4503 (3.4-dimethoxyphenethyl), YZ-067 (4-methoxyphenethyl), YZ-185 (3-methoxyphenethyl) and Nahas-3h (4-methoxybenzyl) on methamphetamine-induced hyperactivity in mice. In a previous study in rats, SA4503 increased methamphetamine-induced hyperactivity at a lower ligand dose and enhanced it at a higher dose. The other ligands have not been investigated in this assay. Presently, mice were administered sigma ligands, and specific [¹²⁵I]E-IA-DM-PE-PIPZE and [¹²⁵I]RTI-121 binding was measured to determine σ1 sigma receptor and dopamine transporter occupancy, respectively. Mice were also administered sigma ligands followed by methamphetamine, and locomotor activity was measured. Each of the ligands occupied σ1 sigma receptors (ED₅₀=0.2-0.6μmol/kg) with similar potency, but none occupied the transporter (ED₅₀>10μmol/kg). At the highest dose tested (31.6μmol/kg) all four sigma ligands significantly attenuated methamphetamine-induced hyperactivity. Interestingly, SA4503, YZ-067 and Nahas-3h, but not YZ-185, enhanced methamphetamine-induced hyperactivity at lower ligand doses (1-3.16μmol/kg). These results suggest that these ligands function as stimulant agonists at lower doses and as antagonists at higher does, with subtle changes in the substitution pattern at the 3- and 4-positions of the phenethyl group contributing to the nature of the interactions. Overall, these data indicate a complex role for σ1 sigma receptor ligands in methamphetamine's behavioral effects.

Olfactory stimulation selectively modulates the OFF pathway in the retina of zebrafish

Cross-modal regulation of visual performance by olfactory stimuli begins in the retina, where dopaminergic interneurons receive projections from the olfactory bulb. However, we do not understand how olfactory stimuli alter the processing of visual signals within the retina. We investigated this question by in vivo imaging activity in transgenic zebrafish expressing SyGCaMP2 in bipolar cell terminals and GCaMP3.5 in ganglion cells. The food-related amino acid methionine reduced the gain and increased sensitivity of responses to luminance and contrast transmitted through OFF bipolar cells but not ON. The effects of olfactory stimulus were blocked by inhibiting dopamine uptake and release. Activation of dopamine receptors increased the gain of synaptic transmission in vivo and potentiated synaptic calcium currents in isolated bipolar cells. These results indicate that olfactory stimuli alter the sensitivity of the retina through the dopaminergic regulation of presynaptic calcium channels that control the gain of synaptic transmission through OFF bipolar cells.

•

Olfactory stimuli regulate transmission of signals through retinal bipolar cells

•

Modulation of synaptic gain and sensitivity occur in OFF bipolar cells but not ON

•

An inhibitor of dopamine uptake blocks odor-induced changes in synaptic gain

•

Dopamine potentiates presynaptic calcium channels in isolated bipolar cells

Inhibition of dopamine transporter activity impairs synaptic depression in rat prefrontal cortex through over-stimulation of D1 receptors

In rat prefrontal cortex (PFC), long-term depression induced by low-frequency single stimuli has never been studied. Combined with the well-documented involvement of dopamine transporters (DATs) in the regulation of PFC-dependent cognitive processes, it is important to test whether this form of plasticity can be modulated by DAT activity in the PFC. Here, we show first that prolonged 3-Hz stimuli successfully induced synaptic depression in rat PFC slices whose induction depended on endogenous stimulation of D1-like and D2-like receptors and the activation of extracellular signal-regulated kinase 1/2 (ERK1/2). This depression was found to be significantly impaired by selective inhibition of the DAT by GBR12909 (1-200 nM) or GBR12935 (100 nM). The excess amount of extracellular dopamine caused by DAT inhibition acted critically on D1-like receptors to impair depression. Furthermore, this impairment by GBR12 909 was cancelled by the allosteric-positive mGluR5 modulator CDPPB, the drug known to reverse hyperdopaminergia-induced abnormal PFC activity, and the associated cognitive disturbances. Finally, these induction, impairment, and restoration of synaptic depression were correlated by an inverted-U shape manner with the phosphorylation level of ERK1/2. We suggest that abnormal increases of the extracellular dopamine level by DAT inhibition impair synaptic depression in the PFC through over-stimulation of D1-like receptors.

Novel N-1,2-dihydroxypropyl analogs of lobelane inhibit vesicular monoamine transporter-2 function and methamphetamine-evoked dopamine release

Lobelane, a chemically defunctionalized saturated analog of lobeline, has increased selectivity for the vesicular monoamine transporter 2 (VMAT2) compared with the parent compound. Lobelane inhibits methamphetamine-evoked dopamine (DA) release and decreases methamphetamine self-administration. Unfortunately, tolerance develops to the ability of lobelane to decrease these behavioral effects of methamphetamine. Lobelane has low water solubility, which is problematic for drug development. The aim of the current study was to determine the pharmacological effect of replacement of the N-methyl moiety with a chiral N-1,2-dihydroxypropyl (N-1,2-diol) moiety, which enhances water solubility, altering the configuration of the N-1,2-diol moiety and incorporating phenyl ring substituents into the analogs. To determine VMAT2 selectivity, structure-activity relationships also were generated for inhibition of DA and serotonin transporters. Analogs with the highest potency for inhibiting DA uptake at VMAT2 and at least 10-fold selectivity were evaluated further for ability to inhibit methamphetamine-evoked DA release from superfused striatal slices. (R)-3-[2,6-cis-di(4-methoxyphenethyl)piperidin-1-yl]propane-1,2-diol (GZ-793A), the (R)-4-methoxyphenyl-N-1,2-diol analog, and (R)-3-[2,6-cis-di(1-naphthylethyl)piperidin-1-yl]propane-1,2-diol (GZ-794A), the (R)-1-naphthyl-N-1,2-diol analog, exhibited the highest potency (K(i) ∼30 nM) inhibiting VMAT2, and both analogs inhibited methamphetamine-evoked endogenous DA release (IC(50) = 10.6 and 0.4 μM, respectively). Thus, the pharmacophore for VMAT2 inhibition accommodates the N-1,2-diol moiety, which improves drug-likeness and enhances the potential for the development of these clinical candidates as treatments for methamphetamine abuse.

meso-Transdiene analogs inhibit vesicular monoamine transporter-2 function and methamphetamine-evoked dopamine release

Lobeline, a nicotinic receptor antagonist and neurotransmitter transporter inhibitor, is a candidate pharmacotherapy for methamphetamine abuse. meso-Transdiene (MTD), a lobeline analog, lacks nicotinic receptor affinity, retains affinity for vesicular monoamine transporter 2 (VMAT2), and, surprisingly, has enhanced affinity for dopamine (DA) and serotonin transporters [DA transporter (DAT) and serotonin transporter (SERT), respectively]. In the current study, MTD was evaluated for its ability to decrease methamphetamine self-administration in rats relative to food-maintained responding. MTD specifically decreased methamphetamine self-administration, extending our previous work. Classical structure-activity relationships revealed that more conformationally restricted MTD analogs enhanced VMAT2 selectivity and drug likeness, whereas affinity at the dihydrotetrabenazine binding and DA uptake sites on VMAT2 was not altered. Generally, MTD analogs exhibited 50- to 1000-fold lower affinity for DAT and were equipotent or had 10-fold higher affinity for SERT, compared with MTD. Representative analogs from the series potently and competitively inhibited [(3)H]DA uptake at VMAT2. (3Z,5Z)-3,5-bis(2,4-dichlorobenzylidene)-1-methylpiperidine (UKMH-106), the 3Z,5Z-2,4-dichlorophenyl MTD analog, had improved selectivity for VMAT2 over DAT and importantly inhibited methamphetamine-evoked DA release from striatal slices. In contrast, (3Z,5E)-3,5-bis(2,4-dichlorobenzylidene)-1-methylpiperidine (UKMH-105), the 3Z,5E-geometrical isomer, inhibited DA uptake at VMAT2, but did not inhibit methamphetamine-evoked DA release. Taken together, these results suggest that these geometrical isomers interact at alternate sites on VMAT2, which are associated with distinct pharmacophores. Thus, structural modification of the MTD molecule resulted in analogs exhibiting improved drug likeness and improved selectivity for VMAT2, as well as the ability to decrease methamphetamine-evoked DA release, supporting the further evaluation of these analogs as treatments for methamphetamine abuse.

Effect of dopamine uptake inhibition on brain catecholamine levels and locomotion in catechol-O-methyltransferase-disrupted mice

Two different uptake processes terminate the synaptic action of released catecholamines in brain: the high-affinity uptake to presynaptic nerve terminals (uptake(1), followed by oxidation by monoamine oxidase, MAO) or glial cells uptake (uptake(2), followed by O-methylation by catechol-O-methyltransferase, COMT, and/or oxidation by MAO). For dopaminergic neurons, uptake by the high-affinity dopamine transporter (DAT) is the most effective mechanism, and the contribution of glial COMT remains secondary under normal conditions. In the present study we have characterized the role of COMT using COMT-deficient mice in conditions where DAT is inhibited by 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)-piperazine (GBR 12909) or cocaine. In mice lacking COMT, GBR 12909 results in total brain tissue dopamine levels generally higher than in wild-type mice but no such potentiation was ever seen in striatal extracellular fluid. Dopamine accumulation in nerve endings is more evident in striatum and hypothalamus than in cortex. Both GBR 12909 and cocaine induced hyperlocomotion in mice lacking COMT. Unexpectedly, hyperactivity induced by 20 mg/kg GBR 12909 was attenuated only in male COMT knockout mice, i.e., they had an inability to sustain the hyperactivity induced by DAT inhibition. Furthermore, attenuation of hyperlocomotion was observed also after cocaine treatment in both C57BL/6 (at 5 and 15 mg/kg) and 129/Sv (at 30 mg/kg) genetic background COMT-deficient male mice. Despite the possible interaction between DAT and extraneuronal uptake (and subsequently COMT), the role of COMT in dopamine elimination is still minimal in conditions when DAT is inhibited.

Dopamine induces ERK activation in renal epithelial cells through H2O2 produced by monoamine oxidase

BACKGROUND

The rat renal proximal tubule cells contain a large amount of monoamine oxidase, which catalyzes the oxidative deamination of catecholamines such as dopamine (DA). The aim of this study is to investigate the potential role of hydrogen peroxide (H2O2) produced by monoamine oxidase (MAO) isoform on regulation of cell signaling and function.

METHODS

Primary rat proximal tubular cells, which contain almost exclusively MAO-A, and human embryonic kidney 293 (HEK 293) cells stably transfected with human MAO-B cDNA were treated with DA or tyramine in the presence or the absence of some inhibitors. Then, Shc protein tyrosine phosphorylation and extracellular-regulated kinase (ERK) activation were evaluated by immunoprecipitation/immunoblot analysis and cell proliferation by [3H]thymidine incorporation or cell counting.

RESULTS

In rat proximal tubule cells, DA induced tyrosine phosphorylation of Shc, ERK activation, and a significant increase in DNA synthesis. The involvement of MAO-dependent H2O2 generation induced by DA (5 micromol/L) was supported by the demonstration that the DA effects were (1) fully prevented by cell pretreatment with the MAO inhibitor pargyline, the antioxydant N-acetylcysteine (NAC), and the DA uptake inhibitor GBR 12909; (2) not abrogated by the D1 and D2 receptor antagonists; (3) observed in HEK 293 MAO-B cells but not in HEK 293 wild-type cells, which do not express MAO; and (4) similar to those induced by another MAO substrate, tyramine.

CONCLUSIONS

Taken together, these results show that in addition to the effects related to receptor stimulation, DA, and probably the other catecholamines, may induce some of its effects through the MAO-dependent H2O2 production.

Reciprocal autoreceptor and heteroreceptor control of serotonergic, dopaminergic and noradrenergic transmission in the frontal cortex: relevance to the actions of antidepressant agents

The frontal cortex (FCX) plays a key role in processes that control mood, cognition and motor behaviour, functions which are compromised in depression, schizophrenia and other psychiatric disorders. In this regard, there is considerable evidence that a perturbation of monoaminergic input to the FCX is involved in the pathogenesis of these states. Correspondingly, the modulation of monoaminergic transmission in the FCX and other corticolimbic structures plays an important role in the actions of antipsychotic and antidepressant agents. In order to further understand the significance of monoaminergic systems in psychiatric disorders and their treatment, it is essential to characterize mechanisms underlying their modulation. Within this framework, the present commentary focuses on our electrophysiological and dialysis analyses of the complex and reciprocal pattern of auto- and heteroreceptor mediated control of dopaminergic, noradrenergic and serotonergic transmission in the FCX. The delineation of such interactions provides a framework for an interpretation of the influence of diverse classes of antidepressant agent upon extracellular levels of dopamine, noradrenaline and serotonin in FCX. Moreover, it also generates important insights into strategies for the potential improvement in the therapeutic profiles of antidepressant agents.

Protective effects of vanoxeamine (GBR 12909) against ischaemia-induced hyperactivity and neurodegeneration in the gerbil model of cerebral ischaemia

In Mongolian gerbils, bilateral carotid occlusion (BCO) followed by reperfusion causes uniform destruction of the CA1 pyramidal neurons in the hippocampus, and this damage correlates with an increase in locomotor activity. Various drugs, such as NMDA antagonists, calcium channel blockers, and free radical scavengers, have provided neuroprotection against ischaemia-induced damage. More recently, the neuroprotective effects of dopamine have been investigated. A large release of dopamine has been shown to occur at the onset of ischaemia, and dopamine levels return to basal values following reperfusion. In the present study, we investigated the effects of vanoxeamine (GBR 12909) (5 or 10 mg/kg i.p., administered 1 h prior to occlusion) on behavioural and histological changes following global ischaemia in the Mongolian gerbil. Ischaemia was induced by bilateral carotid occlusion for 5 min. Both doses of GBR 12909 significantly potientiated the hyperactivity of the BCO animals measured in the home cage during the first 24 h following surgery and in the locomotor activity arena after 24 h and 48 h. Significant neuroprotection of cells in the CA1 region of the bippocampus was observed in drug-treated animals 96 h postsurgery. The neuroprotective effect of GBR 12909 may be ascribed to sensitisation of the dopamine D, autoreceptor, consequently reducing the release of dopamine that occurs following ischaemia. Alternatively, GBR 12909 may have a direct interaction with the Na+ ion channel-glutamate complex, resulting in reduced release of glutamate and thereby reducing NMDA receptor activation and neuronal damage.

Radioligand binding and immunoautoradiographic evidence for a lack of toxicity to dopaminergic nerve terminals in human cocaine overdose victims

Radioligand binding to and immunolabeling of transport sites associated with monoamine-containing synaptic vesicles affords a novel approach for mapping the integrity of dopaminergic (DAergic) nerve terminals. The present study used [125I]iodovinyltetrabenazine ([125I]TBZ) and a fusion protein antibody directed at the large intraluminal loop of the neuronal vesicular monoamine transporter (hVMAT2-loop) as probes to assess the effects of chronic cocaine use on the integrity of DAergic nerve terminals in the striatum of cocaine fatalities. Visualization of [125I]TBZ binding in human brain revealed a distinct pattern of labeling throughout the rostral-caudal extent of the striatum. Saturation binding of [125I]TBZ in striatal membranes demonstrated a single high affinity site (Kd = 2.3 +/- 0.9 nM and Bmax = 55.5 +/- 8.1 pmol/g tissue) with a pharmacological profile (tetrabenazine > or = iodovinyltetrabenazine > ketanserin > or = reserpine > haloperidol > GBR 12909) consistent with the specific labeling of hVMAT2. Quantitative in vitro autoradiography demonstrated no significant alteration in the density of [125I]TBZ binding sites in the anterior and posterior sectors of the striatum in cocaine fatalities with and without preterminal excited delirium as compared to drug-free and age-matched control subjects. Similarly, the levels of hVMAT2-loop immunoreactivity were not significantly different across control and cocaine fatality groups. The results demonstrate the lack of an alteration in [125I]TBZ binding sites and hVMAT2 protein in the striatum from a young cohort of cocaine fatalities. Since striatal VMAT2 is primarily associated with DAergic nerve terminals, these results suggest that chronic cocaine use failed to affect the integrity of striatal DAergic nerve terminals.

Foetal ventral mesencephalic cell suspension grafts to the 6-hydroxydopamine-lesioned rat reduce the rate of dopamine uptake in the contralateral striatum

The present study employed fast cyclic voltammetry, at carbon-fibre microelectrodes, to monitor and compare the rate of dopamine uptake in the rat striatum contralateral to (a) the 6-hydroxydopamine (6-OHDA)-lesioned/grafted striatum and (b) the 6-OHDA-lesioned/ sham grafted striatum. Cell suspensions of foetal rat ventral mesencephalic tissue were grafted into the dopamine-depleted striatum of unilaterally 6-OHDA-lesioned rats. Six weeks after grafting, animals with functional, mature grafts were monitored for dopamine elimination in the contralateral striatum following electrical stimulation of the median forebrain bundle, before and after treatment with the dopamine uptake inhibitor GBR 12909. Compared to animals with sham grafts, amphetamine-amplified rotational behaviour was significantly reduced in animals with grafts of foetal ventral mesencephalic tissue. Fast cyclic voltammetric measurements followed by evaluation with the aid of a kinetic model revealed that in grafted animals, the rate of dopamine uptake via the high affinity uptake mechanism, following treatment with GBR 12909, was significantly reduced when compared to sham grafted animals.

Dopamine, serotonin and tachykinin in self-injurious behavior

The neurobiologic basis of self-injurious behavior (SIB) in Lesch-Nyhan syndrome and in other neuropsychiatric conditions remains unclear. The purpose of this review is to summarize recent data concerning SIB induced by the dopamine (DA) uptake inhibitor, GBR-12909 (GBR) and to compare the neurochemical data that have accumulated over the years on SIB in neonatal 6-hydroxydopamine (6OHDA) lesioned rats. The DA uptake inhibitor, GBR, upon repeated administration to adult rats elicits SIB that is temporally associated with a reduction of striatal DA (approximately 30%), increased turnover of serotonin and a robust induction of tachykinin transcription resulting in enhanced biosynthesis and presumably release of tachykinins (substance P and neurokinin A). GBR-induced SIB could be blocked by dopaminergic lesions or by D1 or D2 antagonists. Neonatal dopaminergic lesions result in a high degree of DA loss (> 90%) and elevated levels of serotonin. In this model, SIB is precipitated by DA agonists via activation of D1 DA receptors which are in turn linked to an induction of tachykinin biosynthesis and release. The data taken together suggest that (a) a substantial reduction of DA accompanied by an increase in serotonin turnover may be essential conditions that are conducive to the occurrence of SIB, and (b) this phase is either superimposed with, or followed by a D1 and/or D2 DA receptor-linked activation of striatonigral tachykinin neurons resulting in enhanced tachykinin biosynthesis and release that may sustain the SIB. Thus, a dynamic interplay between DA, serotonin and tachykinin neuronal systems of the basal ganglia appear to influence the genesis and/or expression of SIB.

Fetal mesencephalic grafts decrease the rate of dopamine uptake in the non-lesioned striatum of unilaterally 6-OHDA lesioned rats: an in vivo voltammetric study

In the present study the influence of intraventricular fetal mesencephalic grafts on the elimination rate of extracellular dopamine (DA) in the non-lesioned striatum of previously unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats was investigated. The elimination of DA was measured after electrical stimulation of the medial forebrain bundle in vivo before and after treatment with the high affinity uptake inhibitor GBR 12909 (20 mg/kg i.p.) using fast cyclic voltammetry (FCV). Rotational behavior induced by amphetamine (AMPH, 2 mg/kg i.p.) and tyrosine hydroxylase (TH) immunohistochemistry were used to prove the functional recovery and the ingrowth of the graft, respectively. After grafting, the number of rotations was decreased and TH-positive cells and fibers were found in the grafted striatum. Voltammetric measurements with the aid of a kinetic model revealed a smaller rate constant for the in vivo elimination of extracellular DA in the non-lesioned striatum of grafted rats compared to that of non-grafted controls. This effect was abolished after treatment with GBR 12909. Our results will be discussed in relation to the method used and according to recent investigations of the specific [3H]DA uptake into striatal synaptosomes in vitro. Based on these data we conclude that grafts, placed to the lesioned striatum, reduce the DA uptake rate in the non-lesioned striatum due to the reduction of the number of functional DA transporters.

GBR-12909-induced self-injurious behavior: role of dopamine

A regimen of repeated administration of GBR (10 or 20 mg/kg/day, i.p., for 4 days) to female Sprague-Dawley rats induced a dose-and time-related increase in the incidence of self-injurious behavior (SIB) that consisted of injury to body areas, paws and tail. The treatment regimen decreased striatal DA and DOPAC levels. Dopaminergic denervation with 6-hydroxydopamine (6-OHDA) or D1 DA antagonist, SCH-23390 or D2 DA antagonist, spiperone, blocked the GBR-induced SIB. Male rats were less sensitive than female rats to exhibit a comparable incidence of SIB. Taken together, the study reveals that repeated administration of GBR induces SIB that is dependent on the integrity of nigrostriatal dopaminergic system and the presence of D1 and/or D2 DA receptors.

Dopaminergic modulation of serotonin metabolism in rat striatum: a study with dopamine uptake inhibitor GBR-12909

The dopamine (DA) uptake inhibitor, GBR 12909 (GBR) and a neonatal dopaminergic denervated rat model were used as tools to study the influence of DA on the serotonin (5HT) system in the striatum. The striatal levels of the amines and their acid metabolites (dihydroxyphenylacetic acid, DOPAC; 5-hydroxyindoleacetic acid, 5HIAA) were determined by HPLC. The administration of a single dose (20 mg/kg) of GBR failed to affect the steady-state levels of the amines or metabolites. Repeated administration of GBR (20 mg/kg/day) for 2 or 4 days decreased DA and DOPAC; only the 4-day regimen decreased 5HT and increased 5HIAA levels. The neonatal dopaminergic lesion with 6-hydroxydopamine (6OHDA) depleted (> 95%) DA and DOPAC and increased 5HT and 5HIAA levels in the striatum. Administration of GBR (20 mg/kg/day for 4 days) to lesioned animals failed to influence the lesion-induced increases in 5HT and 5HIAA levels. The results suggest GBR decreases the steady-state levels of DA, resulting in a compensatory increase in the turnover of 5HT that is dependent on the presence of intact dopaminergic terminals. Thus, the effect of GBR on 5HT turnover is indirect. The studies provide further support for a prominent dopaminergic influence on striatal 5HT metabolism.

Translocation of dopamine and binding of 2 beta-carbomethoxy-3 beta-(4-fluorophenyl) tropane (WIN 35,428) measured under identical conditions in rat striatal synaptosomal preparations. Inhibition by various blockers

Translocation of [3H]dopamine and binding of 2 beta-carbomethoxy-3 beta-(4-fluorophenyl)[3H]-tropane ([3H]WIN 35,428) were measured in crude synaptosomal preparations from rat striatum under identical conditions of assay buffer (phosphate-Krebs) and temperature (25 degrees). [3H]Dopamine uptake as a function of time was close to linear for at least 8 min, whereas [3H]WIN 35,428 binding had reached equilibrium within 1 min and remained at its plateau value for at least 20 min. The following inhibitors were tested in uptake and binding assays run in parallel with the same synaptosomal preparation: cocaine, WIN 35,428, benztropine, nomifensine, mazindol, methylphenidate, N-[1-(2-benzo[b]-thiophenyl)cyclohexyl]piperidine (BTCP), Lu 19-005 (Indatraline), 1-(2-(di(4-fluorophenyl)-methoxy)-ethyl)-4-(3-phenyl-2-propyl)piperazine (GBR 12909), 1-(2-(diphenylmethoxy)-ethyl)-4-(3-phenyl-2-propyl)piperazine (GBR 12935) and 7-trifluoromethyl-4-(4-methyl-1-piperazinyl)-pyrrolo [1,2-a]quinoxaline (CGS 12066B). When present together with [3H]dopamine or [3H]WIN 35,428 for 8 min, the observed binding IC50 values were generally higher (average 1.4-fold) than the uptake IC50 values, with a significant y-axis intercept in linear regression analysis of binding on uptake IC50. For slowly equilibrating inhibitors, estimates of uptake IC50 values were overestimates, and relatively lower values were obtained by monitoring [3H]dopamine uptake for 1 min only during the last minute of the 8-min presence of inhibitor; under these conditions, binding over uptake IC50 ratios were on the average 2.3. Kinetic calculations, taking into account both radioligand and inhibitor equilibration kinetics, indicated that the latter comparison between binding and uptake measurements was most relevant, and suggested the involvement of complexities beyond simple competitive inhibition of dopamine transport, such as different binding domains for substrate and blocker recognition, or spare receptors for blockers. The present data indicate that binding over uptake IC50 ratios should be interpreted with caution, depending on the experimental conditions used to measure these ratios.

Evidence that pure uptake inhibitors including cocaine interact slowly with the dopamine neuronal carrier

We have studied the ability of various uptake blockers to protect the dopamine neuronal carrier labeled with [3H]GBR 12783 (1-[2-(diphenylmethoxy)ethyl]-4-(3-phenyl-2-(propenyl)-piperazine) against N-ethylmaleimide-induced alkylation, using membrane preparations obtained from rat striatum. Pure uptake inhibitors such as mazindol, pyrovalerone, nomifensine and methylphenidate, and substrates (dopamine, d-amphetamine, m-tyramine) protected the [3H]GBR 12783 binding site in a concentration-dependent manner. Preincubation of the membranes with these agents prior to N-ethylmaleimide treatment did not modify the protecting ability of substrates, whereas it significantly improved that of pure uptake inhibitors including cocaine. When the preincubation was omitted, the concentration dependence of the protection observed with pure uptake inhibitors decreased and a maximal 40% protection was observed for 10 microM to 1 mM cocaine concentrations. Effective protecting concentrations of blockers are correlated with their Ki determined in standard binding studies. These results reveal that all pure uptake inhibitors bind slowly to the dopamine neuronal carrier whereas substrates interact with it rapidly.