Methylphenidate treatment in adolescent rats with an attention deficit/hyperactivity disorder phenotype: cocaine addiction vulnerability and dopamine transporter function

Appropriate animal models of attention deficit/hyperactivity disorder (ADHD) and drug reinforcement allow investigation of possible underlying biological bases of ADHD and its comorbidity with cocaine addiction. Toward this end, spontaneously hypertensive rats (SHRs) exhibiting an ADHD phenotype were compared with Wistar-Kyoto (WKY) and Wistar (WIS) rats. Initially, 1.5 mg/kg oral methylphenidate or vehicle was administered between postnatal days 28 and 55, and acquisition of visual discrimination learning was examined. After discontinuing adolescent treatments, adult rats were evaluated for cocaine self-administration and dopamine transporter (DAT) function in the prefrontal cortex (PFC) and striatum. During adolescence, SHRs showed deficits in visual discrimination relative to WKY and WIS rats when non-medicated. Methylphenidate improved visual discrimination only in SHRs. Compared with WKY and WIS rats, SHRs with previous methylphenidate treatment acquired cocaine self-administration faster, identified cocaine as a highly efficacious reinforcer by displaying an upward shift in the cocaine dose-response function, and showed the greatest motivation to self-administer cocaine by exhibiting the highest progressive ratio breakpoints. In the PFC, the maximal dopamine uptake (V(max)) at DAT was decreased in SHRs and increased in WKY and WIS rats by previous methylphenidate treatment. The affinity (K(m)) for dopamine at DAT in the PFC was not different between strains, nor was V(max) or K(m) altered in the striatum by previous methylphenidate treatment in any strain. Methylphenidate-induced decreases in dopamine clearance by DAT in the PFC may underlie increased cocaine self-administration in SHRs. These preclinical findings suggest that caution should be exercised when methylphenidate is prescribed for first-time treatment of ADHD in adolescent patients, as cocaine addiction vulnerability may be augmented.

Discovery of non-peptide, small molecule antagonists of α9α10 nicotinic acetylcholine receptors as novel analgesics for the treatment of neuropathic and tonic inflammatory pain

A series of azaaromatic quaternary ammonium analogs has been discovered as potent and selective α9α10 nicotinic acetylcholine receptor (nAChR) antagonists. The preliminary structure-activity relationships of these analogs suggest that increased rigidity in the linker units results in higher potency in inhibition of α9α10 nAChRs and greater selectivity over α7 nAChRs. These analogs represent a new class of analgesic for the treatment of neuropathic and tonic inflammatory pain.

Effect of environmental enrichment on methylphenidate-induced locomotion and dopamine transporter dynamics

Rats raised in an enriched condition (EC) are less sensitive to the locomotor effects of stimulant drugs than rats raised in an impoverished condition (IC). Methylphenidate (MPD), a primary pharmacotherapy for attention-deficit/hyperactivity disorder, has abuse potential. This study determined whether environmental enrichment differentially altered the effects of MPD on locomotor activity and dopamine (DA) transporter (DAT) function. Acute and repeated MPD (3 or 10 mg/kg, s.c.) increased locomotion in EC, IC and social condition (SC) rats; however, EC rats showed a blunted response to repeated MPD (3 mg/kg). The maximal velocity (V(max)) of [(3)H]DA uptake in the presence of the combination of phorbol 12-myristate 13-acetate, a protein kinase C (PKC) activator and okadaic acid, a protein phosphatase inhibitor was decreased in EC and IC rats by 68% and 40%, respectively, indicating that DAT in prefrontal cortex (PFC) is more sensitive to PKC-mediated down-regulation in EC rats. Acute MPD (10 mg/kg) administration decreased the V(max) of [(3)H]DA uptake in PFC and striatum in EC rats, but not in IC rats. Furthermore, [(3)H]WIN 35,428 binding density was decreased in PFC of EC and IC rats, and in striatum of EC rats given repeated MPD (10 mg/kg). These results demonstrate that environmental enrichment modulates DAT dynamics in PFC. However, since the change in DAT function was observed only following the high dose of MPH (10 mg/kg), the attenuated locomotor response to repeated MPD (3 mg/kg) in EC rats is not likely due to a specific DAT alteration in the brain regions examined.

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.

Novel bis-, tris-, and tetrakis-tertiary amino analogs as antagonists at neuronal nicotinic receptors that mediate nicotine-evoked dopamine release

A series of tertiary amine analogs derived from lead azaaromatic quaternary ammonium salts has been designed and synthesized. The preliminary structure-activity relationships of these new analogs suggest that such tertiary amine analogs, which potently inhibit nicotine-evoked dopamine release from rat striatum, represent drug-like inhibitors of α6-containing nicotinic acetylcholine receptors. The bis-tertiary amine analog 7 exhibited an IC(50) of 0.95 nM, while the tris-tertiary amine analog 19 had an IC(50) of 0.35 nM at nAChRs mediating nicotine-evoked dopamine release.

Phenyl ring-substituted lobelane analogs: inhibition of [³H]dopamine uptake at the vesicular monoamine transporter-2

Lobeline attenuates the behavioral effects of methamphetamine via inhibition of the vesicular monoamine transporter (VMAT2). To increase selectivity for VMAT2, chemically defunctionalized lobeline analogs, including lobelane, were designed to eliminate nicotinic acetylcholine receptor affinity. The current study evaluated the ability of lobelane analogs to inhibit [³H]dihydrotetrabenazine (DTBZ) binding to VMAT2 and [³H]dopamine (DA) uptake into isolated synaptic vesicles and determined the mechanism of inhibition. Introduction of aromatic substituents in lobelane maintained analog affinity for the [³H]DTBZ binding site on VMAT2 and inhibitory potency in the [³H]DA uptake assay assessing VMAT2 function. The most potent (K(i) = 13-16 nM) analogs in the series included para-methoxyphenyl nor-lobelane (GZ-252B), para-methoxyphenyl lobelane (GZ-252C), and 2,4-dichlorphenyl lobelane (GZ-260C). Affinity of the analogs for the [³H]DTBZ binding site did not correlate with inhibitory potency in the [³H]DA uptake assay. It is noteworthy that the N-benzylindole-, biphenyl-, and indole-bearing meso-analogs 2,6-bis[2-(1-benzyl-1H-indole-3-yl)ethyl]-1-methylpiperidine hemifumarate (AV-1-292C), 2,6-bis(2-(biphenyl-4-yl)ethyl)piperidine hydrochloride (GZ-272B), and 2,6-bis[2-(1H-indole-3-yl)ethyl]-1-methylpiperidine monofumarate (AV-1-294), respectively] inhibited VMAT2 function (K(i) = 73, 127, and 2130 nM, respectively), yet had little to no affinity for the [³H]DTBZ binding site. These results suggest that the analogs interact at an alternate site to DTBZ on VMAT2. Kinetic analyses of [³H]DA uptake revealed a competitive mechanism for 2,6-bis(2-(4-methoxyphenyl)ethyl)piperidine hydrochloride (GZ-252B), 2,6-bis(2-(4-methoxyphenyl)ethyl)-1-methylpiperidine hydrochloride (GZ-252C), 2,6-bis(2-(2,4-dichlorophenyl)ethyl)piperidine hydrochloride (GZ-260C), and GZ-272B. Similar to methamphetamine, these analogs released [³H]DA from the vesicles, but with higher potency. In contrast to methamphetamine, these analogs had higher potency (>100-fold) at VMAT2 than DAT, predicting low abuse liability. Thus, modification of the lobelane molecule affords potent, selective inhibitors of VMAT2 function and reveals two distinct pharmacological targets on VMAT2.

Prefrontal cortex and drug abuse vulnerability: translation to prevention and treatment interventions

Vulnerability to drug abuse is related to both reward seeking and impulsivity, two constructs thought to have a biological basis in the prefrontal cortex (PFC). This review addresses similarities and differences in neuroanatomy, neurochemistry and behavior associated with PFC function in rodents and humans. Emphasis is placed on monoamine and amino acid neurotransmitter systems located in anatomically distinct subregions: medial prefrontal cortex (mPFC); lateral prefrontal cortex (lPFC); anterior cingulate cortex (ACC); and orbitofrontal cortex (OFC). While there are complex interconnections and overlapping functions among these regions, each is thought to be involved in various functions related to health-related risk behaviors and drug abuse vulnerability. Among the various functions implicated, evidence suggests that mPFC is involved in reward processing, attention and drug reinstatement; lPFC is involved in decision-making, behavioral inhibition and attentional gating; ACC is involved in attention, emotional processing and self-monitoring; and OFC is involved in behavioral inhibition, signaling of expected outcomes and reward/punishment sensitivity. Individual differences (e.g., age and sex) influence functioning of these regions, which, in turn, impacts drug abuse vulnerability. Implications for the development of drug abuse prevention and treatment strategies aimed at engaging PFC inhibitory processes that may reduce risk-related behaviors are discussed, including the design of effective public service announcements, cognitive exercises, physical activity, direct current stimulation, feedback control training and pharmacotherapies. A major challenge in drug abuse prevention and treatment rests with improving intervention strategies aimed at strengthening PFC inhibitory systems among at-risk individuals.

The novel pyrrolidine nor-lobelane analog UKCP-110 [cis-2,5-di-(2-phenethyl)-pyrrolidine hydrochloride] inhibits VMAT2 function, methamphetamine-evoked dopamine release, and methamphetamine self-administration in rats

Both lobeline and lobelane attenuate methamphetamine self-administration in rats by decreasing methamphetamine-induced dopamine release via interaction with vesicular monoamine transporter-2 (VMAT2). A novel derivative of nor-lobelane, cis-2,5-di-(2-phenethyl)-pyrrolidine hydrochloride (UKCP-110), and its trans-isomers, (2R,5R)-trans-di-(2-phenethyl)-pyrrolidine hydrochloride (UKCP-111) and (2S,5S)-trans-di-(2-phenethyl)-pyrrolidine hydrochloride (UKCP-112), were evaluated for inhibition of [(3)H]dihydrotetrabenazine binding and [(3)H]dopamine uptake by using a rat synaptic vesicle preparation to assess VMAT2 interaction. Compounds were evaluated for inhibition of [(3)H]nicotine and [(3)H]methyllycaconitine binding to assess interaction with the major nicotinic receptor subtypes. In addition, compounds were evaluated for inhibition of methamphetamine-evoked endogenous dopamine release by using striatal slices. The most promising compound, UKCP-110, was evaluated for its ability to decrease methamphetamine self-administration and methamphetamine discriminative stimulus cues and for its effect on food-maintained operant responding. UKCP-110, UKCP-111, and UKCP-112 inhibited [(3)H]dihydrotetrabenazine binding (K(i) = 2.66 ± 0.37, 1.05 ± 0.10, and 3.80 ± 0.31 μM, respectively) and had high potency inhibiting [(3)H]dopamine uptake (K(i) = 0.028 ± 0.001, 0.046 ± 0.008, 0.043 ± 0.004 μM, respectively), but lacked affinity at nicotinic receptors. Although the trans-isomers did not alter methamphetamine-evoked dopamine release, UKCP-110 inhibited (IC(50) = 1.8 ± 0.2 μM; I(max) = 67.18 ± 6.11 μM) methamphetamine-evoked dopamine release. At high concentrations, UKCP-110 also increased extracellular dihydroxyphenylacetic acid. It is noteworthy that UKCP-110 decreased the number of methamphetamine self-infusions, while having no effect on food-reinforced behavior or the methamphetamine stimulus cue. Thus, UKCP-110 represents a new lead in the development of novel pharmacotherapies for the treatment of methamphetamine abuse.

Genetics of novelty seeking, amphetamine self-administration and reinstatement using inbred rats

Previous research using outbred rats indicates that individual differences in activity in a novel environment predict sensitivity to the reinforcing effect of psychostimulant drugs. The current study examined if the link between responses related to novelty and amphetamine self-administration is heritable. Twelve inbred rat strains were assessed for locomotor activity in a novel environment, preference for a novel environment, and intravenous amphetamine self-administration (acquisition, extinction and amphetamine-induced reinstatement). Strain differences were observed in activity in a novel environment, novelty preference and amphetamine self-administration, indicating a genetic influence for each of these behaviors. While there was no relation between activity in an inescapable novel environment and amphetamine self-administration, strain-dependent differences in novelty preference were positively correlated with the amount of amphetamine self-administered. There was also a positive correlation between the dose-dependent rate of amphetamine self-administration and magnitude of reinstatement. These results show that the activity in an inescapable novel environment and the preference for a novel environment are different genetically, and thus likely to reflect different behavioral constructs. Moreover, these results implicate a genetic influence on the relation between novelty seeking and stimulant self-administration, as well as on the relation between stimulant reward and reinstatement.

Bis-azaaromatic quaternary ammonium salts as ligands for the blood-brain barrier choline transporter

A series of bis-azaaromatic quaternary ammonium compounds containing flexible polymethylenic linkers as well as conformationally restricted linkers were evaluated for their affinity for the blood-brain barrier choline transporter (BBB-ChT). The preliminary structure-activity relationships obtained from this study suggest that incorporating a linear, conformationally restricted linker into the molecule improves affinity for the BBB-ChT.

Repeated nicotine administration robustly increases bPiDDB inhibitory potency at alpha6beta2-containing nicotinic receptors mediating nicotine-evoked dopamine release

The novel nicotinic receptor (nAChR) antagonist, N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide (bPiDDB), and its chemically reduced analog, r-bPiDDB, potently inhibit nicotine-evoked dopamine (DA) release from rat striatal slices. Since tobacco smokers self-administer nicotine repeatedly, animal models incorporating repeated nicotine treatment allow for mechanistic evaluation of therapeutic candidates following neuroadaptive changes. The current study determined the ability of bPiDDB, r-bPiDDB and alpha-conotoxin MII (alpha-CtxMII), a peptide antagonist selective for alpha6beta2-containing nAChRs, to inhibit nicotine-evoked [(3)H]DA release from striatal slices from rats repeatedly administered nicotine (0.4mg/kg for 10 days) or saline (control). Concomitant exposure to maximally effective concentrations of r-bPiDDB (1nM) and alpha-CtxMII (1nM) resulted in inhibition of nicotine-evoked [(3)H]DA release no greater than that produced by either antagonist alone, suggesting that r-bPiDDB inhibits alpha6beta2-containing nAChRs. Repeated nicotine treatment increased locomotor activity, demonstrating behavioral sensitization. Concentration-response curves for nicotine-evoked [(3)H]DA release were not different between nicotine-treated and control groups. Maximal inhibition for alpha-CtxMII was greater following repeated nicotine compared to control (I(max)=90% vs. 62%), with no change in potency. bPiDDB was 3-orders of magnitude more potent in inhibiting nicotine-evoked [(3)H]DA release in nicotine-treated rats compared to control rats (IC(50)=5pM vs. 6nM), with no change in maximal inhibition. Neither a shift to the left in the concentration response nor a change in maximal inhibition was observed for r-bPiDDB following repeated nicotine. Thus, repeated nicotine treatment may differentially regulate the stoichiometry, conformation and/or composition of alpha6beta2-containing nAChRs mediating nicotine-evoked striatal DA release. Therefore, bPiDDB and r-bPiDDB appear to target different alpha6beta2-containing nAChR subtypes.

Predictive screening model for potential vector-mediated transport of cationic substrates at the blood-brain barrier choline transporter

A set of semi-rigid cyclic and acyclic bis-quaternary ammonium analogs, which were part of a drug discovery program aimed at identifying antagonists at neuronal nicotinic acetylcholine receptors, were investigated to determine structural requirements for affinity at the blood-brain barrier choline transporter (BBB CHT). This transporter may have utility as a drug delivery vector for cationic molecules to access the central nervous system. In the current study, a virtual screening model was developed to aid in rational drug design/ADME of cationic nicotinic antagonists as BBB CHT ligands. Four 3D-QSAR comparative molecular field analysis (CoMFA) models were built which could predict the BBB CHT affinity for a test set with an r(2) <0.5 and cross-validated q(2) of 0.60, suggesting good predictive capability for these models. These models will allow the rapid in silico screening of binding affinity at the BBB CHT of both known nicotinic receptor antagonists and virtual compound libraries with the goal of informing the design of brain bioavailable quaternary ammonium analogs that are high affinity selective nicotinic receptor antagonists.

Early exposure to nicotine during critical periods of brain development: Mechanisms and consequences

Tobacco use during pregnancy continues to be a major problem with more than 16% of pregnant women in the United States continuing to smoke during pregnancy. Tobacco smoke is known to contain more than 4,000 different chemicals, and while many of these compounds have the potential to interfere with proper neurodevelopment, there is direct evidence that nicotine, the major psychoactive substance present in tobacco, acts as a neuroteratogen. Nicotine activates, and subsequently desensitizes, neuronal nicotinic acetylcholine receptor subtypes (AChRs), which are expressed in the developing central nervous system (CNS) prior to the in-growth of cholinergic neurons. Nicotinic AChRs are present by the first trimester of development in both humans and rodents, and activation of these receptors by acetylcholine is thought to play a critical role in CNS development. The purpose of the current review is to provide an overview of the role that nicotinic AChRs play in the developing CNS and to describe the effects of nicotine exposure during early development on neuronal cell biology, nicotinic AChR expression and neurotransmitter system (e.g., dopamine, norepinephrine, serotonin) function. In particular, differences that occur as a result of the timing and duration of nicotine exposure will be discussed. Emphasis will be placed on preclinical studies examining particular periods of time which correspond to periods of prenatal development in humans (i.e., first, second and third trimesters). Finally, the effects of early nicotine exposure on neurobehavioral development as it pertains to specific disorders, i.e., attention deficit hyperactivity disorder (ADHD), depression and addiction, will be discussed.

Pyrrolidine analogues of lobelane: relationship of affinity for the dihydrotetrabenazine binding site with function of the vesicular monoamine transporter 2 (VMAT2)

Ring size reduction of the central piperidine ring of lobelane yielded pyrrolidine analogues that showed marked inconsistencies in their ability to bind to the dihydrotetrabenazine (DTBZ) binding site on the vesicular monoamine transporter-2 (VMAT2) and their ability to inhibit VMAT2 function. The structure-activity relationships indicate that structural modification within the pyrrolidine series resulted in analogues that interact with two different sites, i.e., the DTBZ binding site and an alternative site on VMAT2 to inhibit transporter function.

(2S,6S)-1-Methyl-2,6-trans-distyryl-piperidinium chloride

In the crystal structure of the title compound, C(22)H(26)N(+)·Cl(-), the piperidine ring is in a chair conformation and the two styryl groups are in axial and equatorial positions. The mol-ecule has a hydrogen bond between the NH group and the chloride anion.

1-Methyl-2,6-cis-distyrylpiperidine

The complete molecule of the title compound, C₂₂H₂₅N, is generated by crystallographic mirror symmetry, with two C atoms and the N atom lying on the mirror plane. The central ring adopts a chair conformation and the dihedral angle between the aromatic rings is 56.69 (4)°.

Lobeline esters as novel ligands for neuronal nicotinic acetylcholine receptors and neurotransmitter transporters

Vesicular monoamine transporter-2 (VMAT2) is a viable target for development of pharmacotherapies for psychostimulant abuse. Lobeline (1) is a potent antagonist at alpha4beta2 * nicotinic acetylcholine receptors, has moderate affinity (K(i)=5.46microM) for VMAT2, and is being investigated currently as a clinical candidate for treatment of psychostimulant abuse. A series of carboxylic acid and sulfonic acid ester analogs 2-20 of lobeline were synthesized and evaluated for interaction with alpha4beta2 * and alpha7 * neuronal nicotinic acetylcholine receptors (nAChRs), the dopamine transporter (DAT), serotonin transporter (SERT) and VMAT2. Both carboxylic acid and sulfonic acid esters had low affinity at alpha7 * nAChRs. Similar to lobeline (K(i)=4nM), sulfonic acid esters had high affinity at alpha4beta2 * (K(i)=5-17nM). Aromatic carboxylic acid ester analogs of lobeline (2-4) were 100-1000-fold less potent than lobeline at alpha4beta2 * nAChRs, whereas aliphatic carboxylic acid ester analogs were 10-100-fold less potent than lobeline at alpha4beta2 *. Two representative lobeline esters, the 10-O-benzoate (2) and the 10-O-benzenesulfonate (10) were evaluated in the (36)Rb(+) efflux assay using rat thalamic synaptosomes, and were shown to be antagonists with IC(50) values of 0.85microM and 1.60microM, respectively. Both carboxylic and sulfonic acid esters exhibited a range of potencies (equipotent to 13-45-fold greater potency compared to lobeline) for inhibiting DAT and SERT, respectively, and like lobeline, had moderate affinity (K(i)=1.98-10.8microM) for VMAT2. One of the more interesting analogs, p-methoxybenzoic acid ester 4, had low affinity at alpha4beta2 * nAChRs (K(i)=19.3microM) and was equipotent with lobeline, at VMAT2 (K(i)=2.98microM), exhibiting a 6.5-fold selectivity for VMAT2 over alpha4beta2 nAChRs. Thus, esterification of the lobeline molecule may be a useful structural modification for the development of lobeline analogs with improved selectivity at VMAT2.

Lobelane inhibits methamphetamine-evoked dopamine release via inhibition of the vesicular monoamine transporter-2

Lobeline is currently being evaluated in clinical trials as a methamphetamine abuse treatment. Lobeline interacts with nicotinic receptor subtypes, dopamine transporters (DATs), and vesicular monoamine transporters (VMAT2s). Methamphetamine inhibits VMAT2 and promotes dopamine (DA) release from synaptic vesicles, resulting ultimately in increased extracellular DA. The present study generated structure-activity relationships by defunctionalizing the lobeline molecule and determining effects on [(3)H]dihydrotetrabenazine binding, inhibition of [(3)H]DA uptake into striatal synaptic vesicles and synaptosomes, the mechanism of VMAT2 inhibition, and inhibition of methamphetamine-evoked DA release. Compared with lobeline, the analogs exhibited greater potency inhibiting DA transporter (DAT) function. Saturated analogs, lobelane and nor-lobelane, exhibited high potency (K(i) = 45 nM) inhibiting vesicular [(3)H]DA uptake, and lobelane competitively inhibited VMAT2 function. Lobeline and lobelane exhibited 67- and 35-fold greater potency, respectively, in inhibiting VMAT2 function compared to DAT function. Lobelane potently decreased (IC(50) = 0.65 microM; I(max) = 73%) methamphetamine-evoked DA overflow, and with a greater maximal effect compared with lobeline (IC(50) = 0.42 microM, I(max) = 56.1%). These results provide support for VMAT2 as a target for inhibition of methamphetamine effects. Both trans-isomers and demethylated analogs of lobelane had reduced or unaltered potency inhibiting VMAT2 function and lower maximal inhibition of methamphetamine-evoked DA release compared with lobelane. Thus, defunctionalization, cis-stereochemistry of the side chains, and presence of the piperidino N-methyl are structural features that afford greatest inhibition of methamphetamine-evoked DA release and enhancement of selectivity for VMAT2. The current results reveal that lobelane, a selective VMAT2 inhibitor, inhibits methamphetamine-evoked DA release and is a promising lead for the development of a pharmacotherapeutic for methamphetamine abuse.

Nicotinic receptor-based therapeutics and candidates for smoking cessation

Tobacco dependence is the most preventable cause of death and is a chronic, relapsing disorder in which compulsive tobacco use persists despite known negative health consequences. All currently available cessation agents (nicotine, varenicline and bupropion) have limited efficacy and are associated with high relapse rates, revealing a need for more efficacious, alternative pharmacotherapies. The major alkaloid in tobacco, nicotine, activates nicotinic receptors (nAChRs) which increase brain extracellular dopamine producing nicotine reward leading to addiction. nAChRs are located primarily presynaptically and modulate synaptic activity by regulating neurotransmitter release. Subtype-selective nAChR antagonists that block reward-relevant mesocorticolimbic and nigrostriatal dopamine release induced by nicotine may offer advantages over current therapies. An innovative approach is to provide pharmacotherapies which are antagonists at nAChR subtypes mediating nicotine evoked dopamine release. In addition, providing multiple medications with a wider array of targets and mechanisms should provide more treatment options for individuals who are not responsive to the currently available pharmacotherapies. This review summarizes the currently available smoking cessation therapies and discusses emerging potential therapeutic approaches employing pharmacological agents which act as antagonists at nicotinic receptors.

Stereocontrolled synthesis and pharmacological evaluation of cis-2,6-diphenethyl-1-azabicyclo[2.2.2]octanes as lobelane analogues

An efficient and highly stereocontrolled approach for the synthesis of the quinuclidine incorporated lobelane analogues, endo,endo- and exo,exo-2,6-cis-diphenethyl-1-azabicyclo-[2.2.2]octane (2 and 3), has been developed. Analogues 2 and 3 were designed to mimic the axial and equatorial geometry, respectively, of the vesicular monoamine transporter-2 (VMAT2) inhibitor, lobelane. The exo,exo analogue 2 had comparable affinity to lobelane and had greater affinity than the endo,endo analogue 3 at the tetrabenazine binding site on VMAT2, indicating that the preferred binding mode of lobelane is likely the extended conformation.

Selective inhibition of acetylcholine-evoked responses of alpha7 neuronal nicotinic acetylcholine receptors by novel tris- and tetrakis-azaaromatic quaternary ammonium antagonists

A family of 20 tris-azaaromatic quaternary ammonium (AQA) compounds were tested for their inhibition of alpha7 nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus laevis oocytes. The potency of inhibitory activity was related to the hydrophobic character of the tris head groups. Two tris-AQA compounds were studied in detail: the highly effective inhibitor 1,3,5-tri-[5-(1-quinolinum)-pent-1-yn-1-yl]-benzene tribromide (tPyQB) and the less potent antagonist 1,3,5,-tri-{5-[1-(2-picolinium)]-pent-1-yn-1-yl}benzene tribromide (tPy2PiB). In addition, we evaluated 1,2,4,5-tetra-{5-[1-(3-benzyl)pyridinium]pent-1-yl}benzene tetrabromide (tkP3BzPB), a tetrakis-AQA with very hydrophobic headgroups. We compared the activity of the AQA compounds to the frequently used alpha7-antagonist methyllycaconitine (MLA). Both tPyQB and tkP3BzPB were selective antagonists of alpha7. However, although inhibition by tPyQB was reversible within 5 min, the recovery time constant for tkP3BzPB inhibition was 26.6 +/- 0.8 min, so that the equilibrium inhibition in the prolonged presence of nanomolar concentrations of tkP3BzPB was nearly 100%. The potency, selectivity, and slow reversibility of tkP3BzPB were comparable with or greater than that of MLA. The inhibitory actions of tPyQB, tPy2PiB, and tkP3BzPB were evaluated on the acetylcholine (ACh)-evoked responses of native nAChRs in rat brain slices. The alpha7-mediated responses of hippocampal interneurons were effectively reduced by 1 microM tPyQB and tkP3BzPB but not tPy2PiB. In rat medial septum, tkP3BzPB produced a greater inhibition of ACh-evoked responses of cells with fast inward currents (type I) than of cells with predominantly slow kinetics (type II), suggesting that tkP3BzPB can block alpha7 yet preserve the responsiveness of non-alpha7 receptors. These agents might be helpful in elucidating complex receptor responses in brain regions with mixed populations of nAChRs.

QSAR study on maximal inhibition (Imax) of quaternary ammonium antagonists for S-(-)-nicotine-evoked dopamine release from dopaminergic nerve terminals in rat striatum

Maximal inhibition (I(max)) of the agonist effect is an important pharmacological property of inhibitors that interact with multiple receptor subtypes that are activated by the same agonist and which elicit the same functional response. This report represents the first QSAR study on a set of 66 mono- and bis-quaternary ammonium salts that act as antagonists at neuronal nicotinic acetylcholine receptors mediating nicotine-evoked dopamine release, conducted using multi-linear regression (MLR) and neural network (NN) analysis with the maximal inhibition (I(max)) values of the antagonists as target values. The statistical results for the generated MLR model were: r(2)=0.89, rmsd=9.01, q(2)=0.83 and loormsd=11.1; the statistical results for the generated NN model were: r(2)=0.89, rmsd=8.98, q(2)=0.83 and loormsd=11.2. The maximal inhibition values of the compounds exhibited a good correlation with the predictions made by the QSAR models developed, which provide a basis for rationalizing selection of compounds for synthesis in the discovery of effective and selective second generation inhibitors of nAChRs mediating nicotine-evoked dopamine release.

Nicotinic receptor activation increases [3H]dopamine uptake and cell surface expression of dopamine transporters in rat prefrontal cortex

Previous research shows that nicotine increases dopamine (DA) clearance in rat prefrontal cortex (PFC) and striatum via a nicotinic receptor (nAChR)-mediated mechanism. The present study investigated whether activation of nAChRs regulates DA transporter (DAT) function through a trafficking-dependent mechanism. After nicotine administration (0, 0.3, and 0.8 mg/kg s.c., 15-1440 min after injection), DAT function and trafficking in synaptosomes of PFC and striatum were determined. nAChR mediation of the effect of nicotine on DAT function and trafficking in PFC was determined by pretreatment with mecamylamine, dihydro-beta-erythroidine, or methyllycaconitine. Nicotine (0.8 mg/kg, 15 and 30 min after injection) increased the maximal velocity (V(max)) of [3H]DA uptake in PFC with no change in K(m), compared with control. Biotinylation and Western blot assays showed that nicotine (0.8 mg/kg; 30 min) increased DAT cell surface expression in PFC. In contrast, a lower dose of nicotine (0.3 mg/kg; 30 min) did not alter DAT function and trafficking in PFC. Pretreatment with mecamylamine, dihydro-beta-erythroidine, or methyllycaconitine (1.5, 8.0, and 10.0 mg/kg s.c., respectively) completely blocked the nicotine-induced increase in V(max) in PFC. In addition, mecamylamine completely blocked the nicotine-induced increase in DAT cell surface expression in PFC. Nicotine did not increase DAT function and cell surface expression in striatum, indicating that nicotine modulates DAT function in a brain region-specific manner. Thus, results from the present study suggest that the nicotine-induced increases in DAT function and cell surface expression in PFC may mediate some of the behavioral effects of nicotine.

Computational neural network analysis of the affinity of N-n-alkylnicotinium salts for the alpha4beta2* nicotinic acetylcholine receptor

Based on an 85 molecule database, linear regression with different size datasets and an artificial neural network approach have been used to build mathematical relationships to fit experimentally obtained affinity values (K(i)) of a series of mono- and bis-quaternary ammonium salts from [(3)H]nicotine binding assays using rat striatal membrane preparations. The fitted results were then used to analyze the pattern among the experimental K(i) values of a set of N-n-alkylnicotinium analogs with increasing n-alkyl chain length from 1 to 20 carbons. The affinity of these N-n-alkylnicotinium compounds was shown to parabolically vary with increasing numbers of carbon atoms in the n-alkyl chain, with a local minimum for the C(4) (n-butyl) analogue. A decrease in K(i) value between C(12) and C(13) was also observed. The statistical results for the best neural network fit of the 85 experimental K(i) values are r(2) = 0.84, rmsd = 0.39; r(cv)(2) = 0.68, and loormsd = 0.56. The generated neural network model with the 85 molecule training set may also be of value for future predictions of K(i) values for new virtual compounds, which can then be identified, subsequently synthesized, and tested experimentally.