Synthesis of symmetrical 1,5-disubstituted granatanines

A general entry into symmetrical 1,5-disubstituted granatanines that involves an alkylative ring-closure on a 2,6-bis enolate piperidine intermediate is described.

Tetrakis-azaaromatic quaternary ammonium salts: novel subtype-selective antagonists at neuronal nicotinic receptors that mediate nicotine-evoked dopamine release

A series of tetrakis-azaaromatic quaternary ammonium salts was synthesized to identify compounds with higher affinity and selectivity as antagonists at neuronal nicotinic receptor subtypes (nAChR) that mediate nicotine-evoked DA release. A high hit rate was achieved in identifying potent analogs that inhibit these nAChRs. Three tetrakis analogs, 11j, 11f, and 11g, were identified as potent (IC(50)=3, 28 and 56nM, respectively) antagonists at these receptors. These compounds represent a novel structural class of nicotinic receptor antagonists.

bis-Pyridinium cyclophanes: novel ligands with high affinity for the blood-brain barrier choline transporter

A series of bis-pyridinium cyclophane analogs designed as conformationally restricted bis-quaternary ammonium compounds were evaluated for their affinity for the blood-brain barrier (BBB) choline transporter. All the cyclophanes investigated exhibited high affinity compared to choline. Of these compounds, N, N'-(1,10-decanediyl)3,3'-(1,9-decadiyn-1,10-diyl)-bis-pyridinium diiodide (5c) and N,N'-(1,9-nonanediyl)3,3'-(1,9-decadiyn-1,10-diyl)-bis-pyridinium dibromide (5b) exhibited highest affinity with K(i) values of 0.8 microM and 1.4 microM, respectively, and constitute some of the most potent BBB choline transporter ligands reported.

Pharmacokinetics of the novel nicotinic receptor antagonist N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide in the rat

Plasma and brain concentrations of the nicotinic acetylcholine receptor antagonist and blood-brain barrier choline transporter substrate, N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide (bPiDDB), were analyzed by liquid beta-scintillation spectrometry after administration of [14CH3]bPiDDB to male Sprague-Dawley rats. Plasma concentrations of [14CH3]bPiDDB were determined at 10 time points over 3 h. Absolute plasma bioavailabilities (1, 3, and 5.6 mg/kg s.c.) were 80.3, 68.2, and 103.7%, respectively. bPiDDB (1, 3, and 5.6 mg/kg) gave Cmax values of 0.13, 0.33, and 0.43 microg/ml, respectively, Tmax values of 5.0, 6.7, and 8.8 min, respectively, and t1/2 values of 76.0, 54.6, and 41.7 min, respectively. Mean area under the plasma concentration versus time curve from time zero to infinity (micrograms per minute per milliliter) and mean Cmax (microg/ml) values were dose-dependent (r2=0.9361 and 0.7968, respectively) over the dose range studied. No metabolism of [14CH3]bPiDDB was detected with any dose of bPiDDB administered. Only moderate protein binding (63-65% in plasma and 59-62% in brain supernatant) was observed, which was reversible. Brain concentrations and brain/plasma ratios of bPiDDB after a single 5.6 mg/kg s.c. dose over 5 to 60 min ranged from 0.09 to 0.33 microg/g brain tissue and were maximal at 10 min after injection, representing approximately 0.6% of the administered dose. Brain/blood ratio (0.18 at 5 min to 0.51 at 60 min after injection) was observed, indicating that clearance from brain is slower than clearance from plasma. The results show that bPiDDB is distributed rapidly from the site of injection into plasma, affords good plasma concentrations, and appears to reach brain tissues via facilitated transport by the blood-brain barrier choline transporter to afford therapeutically relevant concentrations in rat brain.

Advancing the spontaneous hypertensive rat model of attention deficit/hyperactivity disorder

To advance the spontaneous hypertensive rat (SHR) model of attention deficit/hyperactivity disorder (ADHD), experiments examined the SHR in tasks recognized to assess functioning of the prefrontal cortex or dorsal striatal. Tasks included odor-delayed win-shift (nonspatial working and reference memory), win-stay (habit learning), and attentional set-shifting (attention and behavioral flexibility). In Experiment 1, the SHR strain was compared with Wistar-Kyoto (WKY) and Wistar-Kyoto Hypertensive (WKHT) strains on the first 2 tasks. In Experiment 2, oral methylphenidate (1.5 mg/kg) and vehicle (water) were evaluated on all 3 tasks in SHR and WKY strains. Results demonstrated that the SHR made significantly more errors in the odor-delayed win-shift, win-stay, and attentional set-shifting tasks compared with the WKY. Similar performances in the WKY and WKHT indicated that deficits observed in the SHR were not related solely to hypertension. Treating the SHR with methylphenidate eliminated strain differences in all 3 tasks. These findings provide evidence that the SHR is a valid model for studying ADHD-associated neurocognitive deficits. Moreover, the current behavioral approach is appropriate to assess novel medications developed to target ADHD-associated neurocognitive deficits.

N,N'-Alkane-diyl-bis-3-picoliniums as nicotinic receptor antagonists: inhibition of nicotine-evoked dopamine release and hyperactivity

The current study evaluated a new series of N,N'-alkane-diyl-bis-3-picolinium (bAPi) analogs with C6-C12 methylene linkers as nicotinic acetylcholine receptor (nAChR) antagonists, for nicotine-evoked [3H]dopamine (DA) overflow, for blood-brain barrier choline transporter affinity, and for attenuation of discriminative stimulus and locomotor stimulant effects of nicotine. bAPi analogs exhibited little affinity for alpha4beta2* (* indicates putative nAChR subtype assignment) and alpha7* high-affinity ligand binding sites and exhibited no inhibition of DA transporter function. With the exception of C6, all analogs inhibited nicotine-evoked [3H]DA overflow (IC50 = 2 nM-6 microM; Imax = 54-64%), with N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide (bPiDDB; C12) being most potent. bPiDDB did not inhibit electrically evoked [3H]DA overflow, suggesting specific nAChR inhibitory effects and a lack of toxicity to DA neurons. Schild analysis suggested that bPiDDB interacts in an orthosteric manner at nAChRs mediating nicotine-evoked [3H]DA overflow. To determine whether bPiDDB interacts with alpha-conotoxin MII-sensitive alpha6beta2-containing nAChRs, slices were exposed concomitantly to maximally effective concentrations of bPiDDB (10 nM) and alpha-conotoxin MII (1 nM). Inhibition of nicotine-evoked [3H]DA overflow was not different with the combination compared with either antagonist alone, suggesting that bPiDDB interacts with alpha6beta2-containing nAChRs. C7, C8, C10, and C12 analogs exhibited high affinity for the blood-brain barrier choline transporter in vivo, suggesting brain bioavailability. Although none of the analogs altered the discriminative stimulus effect of nicotine, C8, C9, C10, and C12 analogs decreased nicotine-induced hyperactivity in nicotine-sensitized rats, without reducing spontaneous activity. Further development of nAChR antagonists that inhibit nicotine-evoked DA release and penetrate brain to antagonize DA-mediated locomotor stimulant effects of nicotine as novel treatments for nicotine addiction is warranted.

Extending the analysis of nicotinic receptor antagonists with the study of alpha6 nicotinic receptor subunit chimeras

Heterologous expression systems have increased the feasibility of developing selective ligands to target nicotinic acetylcholine receptor (nAChR) subtypes. However, the alpha6 subunit, a component in nAChRs that mediates some of the reinforcing effects of nicotine, is not easily expressed in systems such as the Xenopus oocyte. Certain aspects of alpha6-containing receptor pharmacology have been studied by using chimeric subunits containing the alpha6 ligand-binding domain. However, these chimeras would not be sensitive to an alpha6-selective channel blocker; therefore we developed an alpha6 chimera (alpha4/6) that has the transmembrane and intracellular domains of alpha6 and the extracellular domain of alpha4. We examined the pharmacological properties of alpha4/6-containing receptors and other important nAChR subtypes, including alpha7, alpha4beta2, alpha4beta4, alpha3beta4, alpha3beta2, and alpha3beta2beta3, as well as receptors containing alpha6/3 and alpha6/4 chimeras. Our data show that the absence or presence of the beta4 subunit is an important factor for sensitivity to the ganglionic blocker mecamylamine, and that dihydro-beta-erythroidine is most effective on subtypes containing the alpha4 subunit extracellular domain. Receptors containing the alpha6/4 subunit are sensitive to alpha-conotoxin PIA, while receptors containing the reciprocal alpha4/6 chimera are insensitive. In experiments with novel antagonists of nicotine-evoked dopamine release, the alpha4/6 chimera indicated that structural rigidity was a key element of compounds that could result in selectivity for noncompetitive inhibition of alpha6-containing receptors. Our data extend the information available on prototypical nAChR antagonists, and establish the alpha4/6 chimera as a useful new tool for screening drugs as selective nAChR antagonists.

Tris-azaaromatic quaternary ammonium salts: Novel templates as antagonists at nicotinic receptors mediating nicotine-evoked dopamine release

A series of tris-azaaromatic quaternary ammonium salts has been synthesized and evaluated for their ability to inhibit neuronal nicotinic acetylcholine receptors (nAChRs) mediating nicotine-evoked [(3)H]dopamine release from superfused rat striatal slices and for inhibition of [(3)H]nicotine and [(3)H]methyllycaconitine binding to whole rat brain membranes. The 3-picolinium compound 1,3,5-tri-{5-[1-(3-picolinium)]-pent-1-ynyl}benzene tribromide (tPy3PiB), 3b, exhibited high potency and selectivity for nAChR subtypes mediating nicotine-evoked [(3)H]dopamine release with an IC(50) of 0.2 nM and I(max) of 67%.

Bis-azaaromatic quaternary ammonium salts as antagonists at nicotinic receptors mediating nicotine-evoked dopamine release: An investigation of binding conformation

A series of conformationally restricted bis-azaaromatic quaternary ammonium salts (3 and 4) have been designed and synthesized in order to investigate the possible binding conformations of N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide (bPiDDB; 2), a compound which potently inhibits neuronal nicotinic acetylcholine receptors (nAChRs) mediating nicotine-evoked dopamine release. The preliminary structure-activity relationships of these new analogues suggest that bPiDDB binds in an extended conformation at the nAChR binding site, and that flexibility of the linker may be important for its high potency in inhibiting nAChRs mediating nicotine-evoked dopamine release.

Region-specific effects of N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide on nicotine-induced increase in extracellular dopamine in vivo

BACKGROUND AND PURPOSE

Systemic administration of N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide (bPiDDB), an antagonist of nicotinic acetylcholine receptors (nAChRs) attenuated the nicotine-induced increase in dopamine levels in nucleus accumbens (NAcc).

EXPERIMENTAL APPROACH

Using in vivo microdialysis, we investigated the effects of local perfusion of the novel nAChR antagonist bPiDDB into the NAcc or ventral tegmental area (VTA) on increased extracellular dopamine in NAcc, induced by systemic nicotine. We also examined the concentration-dependent effects of bPiDDB on the acetylcholine (ACh)-evoked response of specific recombinant neuronal nAChR subtypes expressed in Xenopus oocytes, using electrophysiological methods.

KEY RESULTS

Nicotine (0.4 mg kg(-1), s.c.) increased extracellular dopamine in NAcc, which was attenuated by intra-VTA perfusion of mecamylamine (100 microM). Intra-VTA perfusion of bPiDDB (1 and 10 microM) reduced nicotine-induced increases in extracellular dopamine in NAcc. In contrast, intra-NAcc perfusion of bPiDDB (1 or 10 microM) failed to alter the nicotine-induced increase in dopamine in NAcc. Intra-VTA perfusion of bPiDDB alone did not alter basal dopamine levels, compared to control, nor the increased dopamine in NAcc following amphetamine (0.5 mg kg(-1), s.c.). Using Xenopus oocytes, bPiDDB (0.01-100 microM) inhibited the response to ACh on specific combinations of rat neuronal nAChR subunits, with highest potency at alpha3beta4beta3 and lowest potency at alpha6/3beta2beta3.

CONCLUSIONS AND IMPLICATIONS

bPiDDB-Sensitive nAChRs involved in regulating nicotine-induced dopamine release are located in the VTA, rather than in the NAcc. As bPiDDB has properties different from the prototypical nAChR antagonist mecamylamine, further development may lead to novel nAChR antagonists for the treatment of tobacco dependence.

The promiscuity of the dopamine transporter: implications for the kinetic analysis of [3H]serotonin uptake in rat hippocampal and striatal synaptosomes

Evidence indicates that monoaminergic neurotransmitter transporters are promiscuous, transporting substrates other than their cognate neurotransmitters. For example, serotonin is transported by the dopamine transporter (DAT) under conditions in which serotonin transporter (SERT) activity is eliminated (e.g., pharmacological inhibition). We performed a kinetic analysis of [(3)H]serotonin uptake in rat striatal synaptosomes (expressing DAT and SERT) and hippocampal synaptosomes (expressing SERT, but not DAT). Nonspecific [(3)H]serotonin uptake was defined as the amount of uptake remaining in the presence of fluoxetine (10microM) or paroxetine (0.05microM). In hippocampal synaptosomes, K(m) and V(max) values for [(3)H]serotonin uptake did not differ whether fluoxetine or paroxetine was used to define nonspecific uptake. However, in striatal synaptosomes, both K(m) and V(max) values for [(3)H]serotonin uptake were greater when fluoxetine, rather than paroxetine, was used to define nonspecific uptake. These data suggest that, at the concentrations employed, fluoxetine inhibits serotonin uptake at both DAT and SERT, whereas paroxetine only inhibits serotonin uptake at SERT. Thus, when DAT is inhibited by GBR 12909, kinetic parameters for serotonin uptake via SERT in striatum are not different from those obtained in hippocampus. These findings have important implications regarding the analysis of monoaminergic reuptake in brain regions exhibiting heterogeneous transporter expression.

Individual differences in response to novelty predict prefrontal cortex dopamine transporter function and cell surface expression

Previous research has shown that individual differences in response to novelty predict self-administration and the locomotor response to psychostimulant drugs of abuse. The aim of the present study was to determine if individual differences in response to novelty based on inescapable or free-choice novelty tests predict dopamine transporter (DAT) function and trafficking as well as nicotine-induced modulation of DAT. Results show that the maximal velocity (Vmax) of [3H]dopamine uptake into prefrontal cortex (PFC) synaptosomes correlated negatively with the locomotor response to inescapable novelty. In contrast, Vmax correlated positively with novelty place preference in the free-choice novelty test. The divergent correlations between DAT and the two behavioral phenotypes suggest a differential contribution of DAT in these phenotypes, which are known not to be isomorphic. Furthermore, rats categorized as high responders to inescapable novelty had lower Vmax values, which were accompanied by less DAT expression at the cell surface in PFC compared with low responders, suggesting that inherent individual differences in DAT cellular localization may underlie the differential response to novelty. Compared with the saline control, nicotine increased Vmax and cell surface DAT immunoreactivity in PFC from high responders but not from low responders. Similarly, nicotine increased Vmax and cell surface DAT in PFC in rats classified as low in novelty place preference but not in rats classified as high in novelty place preference. Thus, despite the different behavioral phenotypes, the pharmacological effect of nicotine to increase DAT function and cell surface expression was apparent, such that rats with inherently lower DAT function show a greater sensitivity to the neurochemical effect of nicotine.

Discovery of a novel nicotinic receptor antagonist for the treatment of nicotine addiction: 1-(3-Picolinium)-12-triethylammonium-dodecane dibromide (TMPD)

Limitations in efficacy and high relapse rates of currently available smoking cessation agents reveal the need for more efficacious pharmacotherapies. One strategy is to develop subtype-selective nicotinic receptor (nAChR) antagonists that inhibit nicotine-evoked dopamine (DA) release, the primary neurotransmitter involved in nicotine reward. Simple alkylation of the pyridino N-atom converts nicotine from a potent agonist into a potent antagonist. The classical antagonists, hexamethonium and decamethonium, differentiate between peripheral nAChR subtypes. Using a similar approach, we interconnected varying quaternary ammonium moieties with a lipophilic linker to provide N,N'-bis-nicotinium analogs, affording a lead compound, N,N'-dodecyl-1,12-diyl-bis-3-picolinium dibromide (bPiDDB), which inhibited nicotine-evoked DA release and decreased nicotine self-administration. The current work describes a novel compound, 1-(3-picolinium)-12-triethylammonium-dodecane dibromide (TMPD), a hybrid of bPiDDB and decamethonium. TMPD completely inhibited (IC(50)=500 nM) nicotine-evoked DA release from superfused rat striatal slices, suggesting that TMPD acts as a nAChR antagonist at more than one subtype. TMPD (1 microM) inhibited the response to acetylcholine at alpha3beta4, alpha4beta4, alpha4beta2, and alpha1beta1varepsilondelta receptors expressed in Xenopus oocytes. TMPD had a 2-fold higher affinity than choline for the blood-brain barrier choline transporter, suggesting brain bioavailability. TMPD did not inhibit hyperactivity in nicotine sensitized rats, but significantly and specifically decreased nicotine self-administration. Together, the results suggest that TMPD may have the ability to reduce the rewarding effect of nicotine with minimal side effects, a pharmacological profile indicative of potential clinical utility for the treatment of tobacco dependence.

Lobelane decreases methamphetamine self-administration in rats

Lobelane, a minor alkaloid of Lobelia inflata and a synthetic, des-oxy analog of lobeline, has good affinity for the vesicular monoamine transporter and the dopamine transporter. The current study examined the ability of lobelane to specifically decrease methamphetamine self-administration. Rats were trained on a fixed ratio 5 schedule of reinforcement to self-administer methamphetamine (0.05 mg/kg/infusion, i.v.) or to respond for sucrose pellets. Upon reaching stable responding, rats were pretreated with lobelane (0.1, 1, 3, 5.6, or 10 mg/kg, s.c.) or saline, 15 min prior to the operant session. To assess the effect of repeated lobelane on methamphetamine self-administration, rats were pretreated with lobelane (5.6 or 10 mg/kg, s.c.) for 7 sessions. Behavioral specificity was further investigated by assessing the effects of lobelane (0.1, 1, 3, 5, or 10 mg/kg, s.c.) or saline on locomotor activity. Within the dose range tested, lobelane dose-dependently decreased methamphetamine self-administration, while having no effect on sucrose-maintained responding. Locomotor activity was decreased following only the highest dose of lobelane (10 mg/kg). Across repeated pretreatments, tolerance developed to the effect of lobelane on methamphetamine self-administration, demonstrating that the ability of lobelane to specifically decrease methamphetamine self-administration is a transient effect. Thus, taken together, the results show that although lobelane interacts with the pharmacological targets believed to be responsible for its ability to decrease methamphetamine self-administration, removal of the oxygen functionalities from the lobeline molecule may have afforded a compound with an altered pharmacokinetic and/or pharmacodynamic profile.

Computational neural network analysis of the affinity of lobeline and tetrabenazine analogs for the vesicular monoamine transporter-2

Back-propagation artificial neural networks (ANNs) were trained on a dataset of 104 VMAT2 ligands with experimentally measured log(1/K(i)) values. A set of related descriptors, including topological, geometrical, GETAWAY, aromaticity, and WHIM descriptors, was selected to build nonlinear quantitative structure-activity relationships. A partial least squares (PLS) regression model was also developed for comparison. The nonlinearity of the relationship between molecular descriptors and VMAT2 ligand activity was demonstrated. The obtained neural network model outperformed the PLS model in both the fitting and predictive ability. ANN analysis indicated that the computed activities were in excellent agreement with the experimentally observed values (r(2)=0.91, rmsd=0.225; predictive q(2)=0.82, loormsd=0.316). The generated models were further tested by use of an external prediction set of 15 molecules. The nonlinear ANN model has r(2)=0.93 and root-mean-square errors of 0.282 compared with the experimentally measured activity of the test set. The stability test of the model with regard to data division was found to be positive, indicating that the generated model is predictive. The modeling study also reflected the important role of atomic distribution in the molecules, size, and steric structure of the molecules when they interact with the target, VMAT2. The developed models are expected to be useful in the rational design of new chemical entities as ligands of VMAT2 and for directing synthesis of new molecules in the future.

Nornicotine inhibition of dopamine transporter function in striatum via nicotinic receptor activation

Nornicotine, a tobacco alkaloid and N-demethylated nicotine metabolite, releases DA from superfused rat striatal slices in a mecamylamine-sensitive manner, indicating nicotinic receptor (nAChR) modulation of this response. The current study determined the effect of nornicotine on rat striatal dopamine transporter (DAT) function using in vivo voltammetry. In a dose-related and mecamylamine-sensitive manner, nornicotine (0.35-12.0 mg/kg, s.c.) decreased DA clearance, suggesting that nornicotine inhibits striatal DAT function via a nAChR-mediated mechanism. Furthermore, the nAChRs mediating the nornicotine-induced inhibition of DAT function appear to be different from those activated by nicotine which increases DA clearance. Understanding the actions of nornicotine in brain may have significance for emerging therapeutics and for the management of nicotine dependence.

Review of the pharmacology and clinical profile of bupropion, an antidepressant and tobacco use cessation agent

Bupropion hydrochloride ((+/-)-2-tert-butylamino)-3'-chloropropiophenone x HCl) is a nonselective inhibitor of the dopamine transporter (DAT) and the norepinephrine transporter (NET) and is also an antagonist at neuronal nicotinic acetylcholine receptors (nAChRs). In animal models used commonly to screen for antidepressant activity, bupropion shows a positive response. Also using animal models, bupropion has been shown to attenuate nicotine-induced unconditioned behaviors, to share or enhance discriminative stimulus properties of nicotine and to have a complex effect on nicotine self-administration, i.e., low doses augmenting nicotine self-administration and high doses attenuating self-administration. Current studies show that bupropion facilitates the acquisition of nicotine conditioned place preference in rats, further suggesting that bupropion enhances the rewarding properties of nicotine. Bupropion has been shown to attenuate the expression of nicotine withdrawal symptoms in both animal models and human subjects. With respect to relapse, current studies show that bupropion attenuates nicotine-induced reinstatement in rats, but large individual differences are apparent. Clinically, bupropion is used as a treatment for two indications, as an antidepressant, the indication for which it was developed, and as a tobacco use cessation agent. In clinical trials, bupropion is being tested as a candidate treatment for psychostimulant drug abuse, attention-deficit hyperactivity disorder (ADHD) and obesity. Bupropion is available in three bioequivalent oral formulations, immediate release (IR), sustained release (SR), and extended release (XL). Extensive hepatic metabolism of bupropion produces three pharmacologically active metabolites, which may contribute to its clinical profile.

N, N -disubstituted piperazines and homopiperazines: synthesis and affinities at alpha4beta2* and alpha7* neuronal nicotinic acetylcholine receptors

A series of N, N- disubstituted piperazines and homopiperazines were prepared and evaluated for binding to natural alpha4beta2* and alpha7* neuronal nicotinic acetylcholine receptors (nAChRs) using whole brain membrane. Some compounds exhibited good selectivity for alpha4beta2* nAChRs and did not interact with the alpha7* nAChRs subtype. The most potent analogs were compounds 8-19 (K(i) = 10.4 microM), 8-13 (K(i) = 12.0 microM), and 8-24 (K(i) = 12.8 microM). Thus, linking together a pyridine pi-system and a cyclic amine moiety via a homopiperazine ring affords compounds with low affinity but with good selectivity for alpha4beta2* nAChRs.

Nicotine increases dopamine transporter function in rat striatum through a trafficking-independent mechanism

In previous in vivo voltammetry studies, acute nicotine administration increased striatal dopamine clearance. The current study aimed to determine whether nicotine also increases [(3)H]dopamine uptake across the time course of the previous voltammetry studies and whether dopamine transporter trafficking to the cell surface mediates the nicotine-induced augmentation of dopamine clearance in striatum. Rats were administered nicotine (0.32 mg/kg, s.c.); striatal synaptosomes were obtained 5, 10, 40 or 60 min later. Nicotine increased (25%) the V(max) of [(3)H]dopamine uptake at 10 and 40 min. To determine whether the increase in V(max) was due to an increase in dopamine transporter density, [(3)H]GBR 12935 (1-(2-[bis(4-fluorophenyl)methoxy]ethyl)-4-(3-phenylpropyl)piperazine dihydrochloride) binding was performed using rat striatal membranes; no differences were found between nicotine and saline-control groups at 5, 10 or 40 min post-injection, indicating that nicotine did not increase striatal dopamine transporter density; however, [(3)H]GBR 12935 binding assays determine both cell surface and intracellular dopamine transporter. Changes in cellular dopamine transporter localization in striatum were determined using biotinylation and subfractionation approaches; no differences between nicotine and saline-control groups were observed at 10 and 40 min post-injection. These results suggest that the nicotine-induced increase in dopamine uptake and clearance in striatum may occur via a trafficking-independent mechanism.

The pharmacological activity of nicotine and nornicotine on nAChRs subtypes: relevance to nicotine dependence and drug discovery

Cigarette smoking and other forms of tobacco use deliver an array of pharmacologically active alkaloids, including nicotine and ultimately various metabolites of these substances. While nornicotine is a significant component in tobacco as well as a minor systemic metabolite of nicotine, nornicotine appears to be N-demethylated locally in the brain where it accumulates at relatively high levels after chronic nicotine administration. We have now examined the effects of nornicotine on specific combinations of neuronal nicotinic acetylcholine receptor (nAChR) subunits expressed in Xenopus oocytes and compared these responses to those evoked by acetylcholine and nicotine. Of the nAChR subtypes studied, we have found that alpha7 receptors are very responsive to nornicotine (EC50 approximately 17 micromol/L I(max) 50%, compared with acetylcholine (ACh)). nAChRs containing the ligand-binding domain of the alpha6 subunits (in the form of an alpha6/alpha3 chimera) are also strongly responsive to nornicotine (EC50 approximately 4 micromol/L I(max) 50%, compared with ACh). Alpha7-type nAChRs have been suggested to be potential therapeutic targets for Alzheimer's disease, schizophrenia and possibly other pathologies. nAChRs containing alpha6 subunits have been suggested to have a role in nicotine-evoked dopamine release. Thus, understanding the actions of nornicotine in the brain may have significance for both emerging therapeutics and the management of nicotine dependence.