Interaction between a hydroxypiperidine analogue of 4-(2-benzhydryloxy-ethyl)-1-(4-fluorobenzyl)piperidine and Aspartate 68 in the human dopamine transporter

Simultaneous Multiple MS Binding Assays for the Dopamine, Norepinephrine, and Serotonin Transporters

In this work, we present label-free, mass-spectrometry-based binding assays (MS Binding Assays), targeting the human dopamine, norepinephrine, and serotonin transporters (hDAT, hNET, and hSERT) in simultaneous binding experiments. Using a validated LC-ESI-MS/MS method for quantification of the selective dopamine transporter inhibitor (R,R)-4-(2-benzhydryloxyethyl)-1-(4-fluorobenzyl)piperidin-3-ol ((R,R)-D-84), the selective norepinephrine transporter inhibitor (S,S)-reboxetine, and the selective serotonin reuptake inhibitor (S)-citalopram, binding affinities at the three monoamine transporters could be characterized simultaneously in a single binding experiment. The performed simultaneous saturation and competition experiments yielded results that are in good accordance with those determined in MS Binding Assays addressing the monoamine transporters individually. The results obtained from this study underscore the potential of MS Binding Assays for simultaneous affinity determination at different targets, which is difficult to accomplish with conventional radioligand binding assays.

Effects of diet and insulin on dopamine transporter activity and expression in rat caudate-putamen, nucleus accumbens, and midbrain

Food restriction (FR) and obesogenic (OB) diets are known to alter brain dopamine transmission and exert opposite modulatory effects on behavioral responsiveness to psychostimulant drugs of abuse. Mechanisms underlying these diet effects are not fully understood. In this study, we examined diet effects on expression and function of the dopamine transporter (DAT) in caudate-putamen (CPu), nucleus accumbens (NAc), and midbrain regions. Dopamine (DA) uptake by CPu, NAc or midbrain synapto(neuro)somes was measured in vitro with rotating disk electrode voltammetry or with [³ H]DA uptake and was found to correlate with DAT surface expression, assessed by maximal [³ H](-)-2-β-carbomethoxy-3-β-(4-fluorophenyl)tropane binding and surface biotinylation assays. FR and OB diets were both found to decrease DAT activity in CPu with a corresponding decrease in surface expression but had no effects in the NAc and midbrain. Diet treatments also affected sensitivity to insulin-induced enhancement of DA uptake, with FR producing an increase in CPu and NAc, likely mediated by an observed increase in insulin receptor expression, and OB producing a decrease in NAc. The increased expression of insulin receptor in NAc of FR rats was accompanied by increased DA D₂ receptor expression, and the decreased DAT expression and function in CPu of OB rats was accompanied by decreased DA D₂ receptor expression. These results are discussed as partial mechanistic underpinnings of diet-induced adaptations that contribute to altered behavioral sensitivity to psychostimulants that target the DAT.

Insulin enhances striatal dopamine release by activating cholinergic interneurons and thereby signals reward

Insulin activates insulin receptors (InsRs) in the hypothalamus to signal satiety after a meal. However, the rising incidence of obesity, which results in chronically elevated insulin levels, implies that insulin may also act in brain centres that regulate motivation and reward. We report here that insulin can amplify action potential-dependent dopamine (DA) release in the nucleus accumbens (NAc) and caudate-putamen through an indirect mechanism that involves striatal cholinergic interneurons that express InsRs. Furthermore, two different chronic diet manipulations in rats, food restriction (FR) and an obesogenic (OB) diet, oppositely alter the sensitivity of striatal DA release to insulin, with enhanced responsiveness in FR, but loss of responsiveness in OB. Behavioural studies show that intact insulin levels in the NAc shell are necessary for acquisition of preference for the flavour of a paired glucose solution. Together, these data imply that striatal insulin signalling enhances DA release to influence food choices.

The novel trisubstituted pyran derivative D-142 has triple monoamine reuptake inhibitory activity and exerts potent antidepressant-like activity in rodents

Major depression disorder is a significant health problem with 10-20% of all adults suffering from this disease. The underlying causes of depression are still unclear and 15% of depressed patients are resistant to all known therapies. Monoamine therapies have so far been the most successful approach for treating depression. Triple monoamine reuptake inhibitors have recently been implicated in generation of potent antidepressant activity while possibly exhibiting a low side-effect profile in addition to treating anhedonia. The additional, previously under-appreciated involvement of dopaminergic systems in depression prompted our efforts to develop novel asymmetric trisubstituted and disubstituted pyran derivatives as triple reuptake inhibitors. One of the lead compounds, D-142, exhibited uptake inhibition (K(i)) values of 29.3 nM, 14.7 nM and 59.3 ± 13.7 nM for norepinephrine, serotonin and dopamine transporters, respectively. Its affinity for serotonin transporter was comparable to fluoxetine, a well known SSRI. In the rat forced swimming test, compound D-142 exhibited potent antidepressant activity in the dose range tested (2.5, 5 and 10mg/kg) and was far more efficacious than the reference compound imipramine. In the mouse tail suspension test, compound D-142 reduced immobility in a dose (2.5, 5 and 10mg/kg) dependent manner, indicating a potent antidepressant effect. In locomotor activity tests, compound D-142 did not exhibit any stimulation in the same dose ranges. In the extended CNS receptors screening assay this molecule exhibited little or no non-specific interaction in the CNS, indicating high specificity for monoamine transporters. These results advance D-142 as a potential potent antidepressant.

Concentration of receptor and ligand revisited in a modified receptor binding protocol for high-affinity radioligands: [3H]Spiperone binding to D2 and D3 dopamine receptors

In receptor binding assays with ultra-high-affinity radioligands, it is difficult, in practice, to adhere the golden rule that the receptor concentration in the assay should be substantially (at least 10-fold) lower than the dissociation constant (K(d)) of the radioligand and inhibition constant (K(i)) of compound. Especially for low specific activity radioligands (usually tritiated ligands of a couple of TBq/mmol), routinely applied in concentrations at around or below the K(d), the use of extremely small amounts of receptor protein per assay will result in low levels of bound radioactivity; the alternative use of larger assay volumes will make it difficult to apply 96-well filtration devices. For assessing the inhibition constant (K(i)) of competitive inhibitors under conditions violating the above golden rule, equations are available incorporating both [receptor] and [ligand] versus K(d); however, their application requires precise knowledge of [receptor] or initial bound/free [radioligand] ratio. In this study, we present the theoretical basis for determining the K(i) for a competitive inhibitor in a new protocol at high [protein] and high [radioligand] with the simple Cheng-Prusoff correction without the need to correct for [receptor] or initial bound/free [radioligand] ratio. In addition, we present results on the binding of the ultra-high-affinity ligand [(3)H]spiperone to dopamine D(2) and D(3) receptors validating the K(i) values calculated with the new protocol for competitive inhibitors as compared with those calculated with the most comprehensive equation available to date, that of Munson and Rodbard (1988). Binding was measured at varying [radioligand] and [receptor], test compounds (including (-)5-OH-DPAT, (+/-)7-OH-DPAT, and ropinirole) were run with varying [receptor], and simulations were done at vastly varying [radioligand] for inhibitors with vastly different K(i)s. The modified high [radioligand] protocol presented here removes a major hindrance in the proper execution of binding assays with ultra-high-affinity tritiated ligands with K(d) values in the sub-nanomolar range, allowing the use of 96-well plates with small volumes of 100-200 microl per binding assay.

Synthesis and biological characterization of (3R,4R)-4-(2-(benzhydryloxy)ethyl)-1-((R)-2-hydroxy-2-phenylethyl)-piperidin-3-ol and its stereoisomers for activity toward monoamine transporters

A novel series of optically active molecules based on a 4-(2-(benzhydryloxy)ethyl)-1-((R)-2-hydroxy-2-phenylethyl)-piperidin-3-ol template were developed. Depending on stereochemistry, the compounds exhibit various degrees of affinity for three dopamine, serotonin, and norepinephrine transporters. These molecules have the potential for treating several neurological disorders such as drug abuse, depression, and attention deficit hyperactivity disorder.Herein we describe the synthesis and biological evaluation of a series of asymmetric 4-(2-(benzhydryloxy)ethyl)-1-((R)-2-hydroxy-2-phenylethyl)-piperidin-3-ol-based dihydroxy compounds in which the hydroxy groups are located on both the piperidine ring and the N-phenylethyl side chain. In vitro uptake inhibition data of these molecules indicate high affinity for the dopamine transporter (DAT) in addition to moderate to high affinity for the norepinephrine transporter (NET). Interestingly, compounds 9 b and 9 d exhibit affinities for all three monoamine transporters, with highest potency at DAT and NET, and moderate potency at the serotonin transporter (SERT) (K(i): 2.29, 78.4, and 155 nM for 9 b and 1.55, 14.1, and 259 nM for 9 d, respectively). Selected compounds 9 a, 9 d, and 9 d' were tested for their locomotor activity effects in mice and for their ability to occasion the cocaine-discriminative stimulus in rats. These test compounds generally exhibit a much longer duration of action than cocaine for elevating locomotor activity, and completely generalize the cocaine-discriminative stimulus in a dose-dependent manner.

Interaction of cocaine-, benztropine-, and GBR12909-like compounds with wild-type and mutant human dopamine transporters: molecular features that differentially determine antagonist-binding properties

The widely abused psychostimulant cocaine is thought to elicit its reinforcing effects primarily via inhibition of the neuronal dopamine transporter (DAT). However, not all DAT inhibitors share cocaine's behavioral profile, despite similar or greater affinity for the DAT. This may be due to differential molecular interactions with the DAT. Our previous work using transporter mutants with altered conformational equilibrium (W84L and D313N) indicated that benztropine and GBR12909 interact with the DAT in a different manner than cocaine. Here, we expand upon these previous findings, studying a number of structurally different DAT inhibitors for their ability to inhibit [(3)H]CFT binding to wild-type, W84L and D313N transporters. We systematically tested structural intermediates between cocaine and benztropine, structural hybrids of benztropine and GBR12909 and a number of other structurally heterologous inhibitors. Derivatives of the stimulant desoxypipradrol (2-benzhydrylpiperidine) exhibited a cocaine-like binding profile with respect to mutation, whereas compounds possessing the diphenylmethoxy moiety of benztropine and GBR12909 were dissimilar to cocaine-like compounds. In tests with specific isomers of cocaine and tropane analogues, compounds with 3alpha stereochemistry tended to exhibit benztropine-like binding, whereas those with 3beta stereochemistry were more cocaine-like. Our results point to the importance of specific molecular features--most notably the presence of a diphenylmethoxy moiety--in determining a compound's binding profile. This study furthers the concept of using DAT mutants to differentiate cocaine-like inhibitors from atypical inhibitors in vitro. Further studies of the molecular features that define inhibitor-transporter interaction could lead to the development of DAT inhibitors with differential clinical utility.

Labeling of dopamine transporter transmembrane domain 1 with the tropane ligand N-[4-(4-azido-3-[125I]iodophenyl)butyl]-2beta-carbomethoxy-3beta-(4-chlorophenyl)tropane implicates proximity of cocaine and substrate active sites

The novel photoaffinity ligand N-[4-(4-azido-3-(125)I-iodophenyl)-butyl]-2-beta-carbomethoxy-3beta-(4-chlorophenyl) tropane ([(125)I]MFZ 2-24) was used to investigate the site for cocaine binding on the dopamine transporter (DAT). [(125)I]MFZ 2-24 irreversibly labeled both rat striatal and expressed human DAT with high affinity and appropriate pharmacological specificity. Tryptic proteolysis of [(125)I]MFZ 2-24 labeled DAT followed by epitope-specific immunoprecipitation demonstrated that the ligand becomes adducted almost exclusively to transmembrane domains (TMs) 1-2. Further localization of [(125)I]MFZ 2-24 incorporation achieved by proteolyzing labeled wild-type and methionine mutant DATs with cyanogen bromide identified the sequence between residues 68 and 80 in TM1 as the ligand adduction site. This is in marked contrast to the previously identified attachment of the photoaffinity label [(125)I]RTI 82 in TM6. Because [(125)I]MFZ 2-24 and [(125)I]RTI 82 possess identical tropane pharmacophores and differ only in the placement of the reactive azido moieties, their distinct incorporation profiles identify the regions of the protein adjacent to different aspects of the cocaine molecule. These findings thus strongly support the direct interaction of cocaine on DAT with TM1 and TM6, both of which have been implicated by mutagenesis and homology to a bacterial leucine transporter as active sites for substrates. These results directly establish the proximity of TMs 1 and 6 in DAT and suggest that the mechanism of transport inhibition by cocaine involves close interactions with multiple regions of the substrate permeation pathway.

Chronic food restriction and dopamine transporter function in rat striatum

The present communication reports on DA uptake in rat striatum in a model of chronic food restriction. The K(m) for DA uptake was unaltered, but the V(max) was reduced by 32%, not supporting the idea that the enhanced behavioral sensitivity to cocaine or d-amphetamine upon chronic food restriction is due to a greater density of DAT at the plasma membrane for drug interaction. Chronic food restriction did not alter the potency of cocaine or D-amphetamine in inhibiting DA uptake in the striatum, suggesting that the enhanced behavioral sensitivity to these drugs upon chronic food restriction is not due to their enhanced affinity for DAT. These results point to factors other than DAT density or affinity underlying the sensitized response to psychostimulants in food restriction.

Recognition of psychostimulants, antidepressants, and other inhibitors of synaptic neurotransmitter uptake by the plasma membrane monoamine transporters

The plasma membrane monoamine transporters terminate neurotransmission by removing dopamine, norepinephrine, or serotonin from the synaptic cleft between neurons. Specific inhibitors for these transporters, including the abused psychostimulants cocaine and amphetamine and the tricyclic and SSRI classes of antidepressants, exert their physiological effects by interfering with synaptic uptake and thus prolonging the actions of the monoamine. Pharmacological, biochemical, and immunological characterization of the many site-directed, chimeric, and deletion mutants generated for the plasma membrane monoamine transporters have revealed much about the commonalities and dissimilarities between transporter substrate, ion, and inhibitor binding sites. Mutations that alter the binding affinity or substrate uptake inhibition potency of inhibitors by at least 3-fold are the focus of this review. These findings are clarifying the picture regarding substrate uptake inhibitor/transporter protein interactions at the level of the drug pharmacophore and the amino acid residue, information necessary for rational design of novel medications for substance abuse and a variety of psychiatric disorders.

Discovery of Novel Trisubstituted Asymmetric Derivatives of (2S,4R,5R)-2-benzhydryl-5-benzylaminotetrahydropyran-4-ol, Exhibiting High Affinity for Serotonin and Norepinephrine Transporters in a Stereospecific Manner

In our structure-activity relationship study on 3,6-disubstituted pyran derivatives, we have carried out asymmetric synthesis and biological characterization of trisubstituted (2S,4R,5R)-2-benzhydryl-5-benzylaminotetrahydropyran-4-ol and (3S,4R,6S)-6-benzhydryl-4-benzylaminotetrahydropyran-3-ol derivatives and their enantiomers. All synthesized derivatives were tested for their affinities for the dopamine transporter (DAT), serotonin transporter (SERT), and norepinephrine transporter (NET) in the brain by measuring their potency in inhibiting the uptake of [(3)H]DA, [(3)H]-5-HT, and [(3)H]NE, respectively. Compounds were also tested for their binding affinity at the DAT by their inhibition of [(3)H]WIN 35,428. Biological results indicated that regioselectivity and stereoselectivity played important roles in determining activity for monoamine transporters as only (-)-isomers of 2-benzhydryl-5-benzylaminotetrahydropyran-4-ol derivatives exhibited appreciable potency for the monoamine transporters, in particular for the SERT and NET. Among the active analogues, (-)-9d exhibited potent and selective affinity at the NET (K(i), [(3)H]NE = 4.92 nM; DAT/NET = 91 and SERT/NET = 140). One of the derivatives with p-methoxybenzyl substitution, (-)-9a, was potent at both SERT and NET (K(i), [(3)H]-5-HT = 25.9 and [(3)H]NE = 15.8 nM, respectively). In the active analogue series ((-)-9a-(-)-9e), a cis-relationship between the biphenyl and the amino moiety was maintained for the SERT and NET interactions, as was observed with our earlier 3,6-disubstituted pyran compounds for the DAT interaction. To the best of our knowledge, this current series of compounds represents a novel class of pyran derivatives as blockers for monoamine transporters.

A comprehensive atlas of the topography of functional groups of the dopamine transporter

The neuronal dopamine transporter (DAT) is a transmembrane transporter that clears DA from the synaptic cleft. Knowledge of DAT functional group topography is a prerequisite for understanding the molecular basis of transporter function, the actions of psychostimulant drugs, and mechanisms of dopaminergic neurodegeneration. Information concerning the molecular interactions of drugs of abuse (such as cocaine, amphetamine, and methamphetamine) with the DAT at the functional group level may also aid in the development of compounds useful as therapeutic agents for the treatment of drug abuse. This review will provide a cumulative and comprehensive focus on the amino acid functional group topography of the rat and human DATs, as revealed by protein chemical modification and the techniques of site-directed mutagenesis. The results from these studies, represented mostly by site-directed mutagenesis, can be classified into several main categories: modifications without substantial affects on substrate transport, DAT membrane expression, or cocaine analog binding; those modifications which alter both substrate transport and cocaine analog binding; and those that affect DAT membrane expression. Finally, some modifications can selectively affect either substrate transport or cocaine analog binding. Taken together, these literature results show that domains for substrates and cocaine analogs are formed by interactions with multiple and sometimes distinct DAT functional groups.