Mecamylamine blockade of both positive and negative effects of IV nicotine in human volunteers

Comparison of effects produced by nicotine and the α4β2-selective agonist 5-I-A-85380 on intracranial self-stimulation in rats

Intracranial self-stimulation (ICSS) is one type of preclinical procedure for research on pharmacological mechanisms that mediate abuse potential of drugs acting at various targets, including nicotinic acetylcholine receptors (nAChRs). This study compared effects of the nonselective nAChR agonist nicotine (0.032-1.0 mg/kg) and the α4β2-selective nAChR agonist 5-I-A-85380 (0.01-1.0 mg/kg) on ICSS in male Sprague-Dawley rats. Rats were implanted with electrodes targeting the medial forebrain bundle at the level of the lateral hypothalamus and trained to respond under a fixed-ratio 1 schedule for a range of brain stimulation frequencies (158-56 Hz). A broad range of 5-I-A-85380 doses produced an abuse-related increase (or "facilitation") of low ICSS rates maintained by low brain-stimulation frequencies, and this effect was blocked by both the nonselective nAChR antagonist mecamylamine and the selective α4β2 antagonist dihyrdo-β-erythroidine (DHβE). Conversely, nicotine produced weaker ICSS facilitation across a narrower range of doses, and higher nicotine doses decreased high rates of ICSS maintained by high brain-stimulation frequencies. The rate-decreasing effects of a high nicotine dose were blocked by mecamylamine but not DHβE. Chronic nicotine treatment produced selective tolerance to rate-decreasing effects of nicotine but did not alter ICSS rate-increasing effects of nicotine. These results suggest that α4β2 receptors are sufficient to mediate abuse-related rate-increasing effects of nAChR agonists in this ICSS procedure. Conversely, nicotine effects at non-α4β2 nAChRs appear to oppose and limit abuse-related effects mediated by α4β2 receptors, although tolerance can develop to these non-α4β2 effects. Selective α4β2 agonists may have higher abuse potential than nicotine.

Nicotinic receptor modulation to treat alcohol and drug dependence

Alcohol and drug dependence are serious public health problems worldwide. The prevalence of alcohol and drug dependence in the United States and other parts of the world is significant. Given the limitations in the efficacy of current pharmacotherapies to treat these disorders, research in developing alternative pharmacotherapies continues. Preclinical and clinical evidence thus far has indicated that brain nicotinic acetylcholine receptors (nAChRs) are important pharmacological targets for the development of medications to treat alcohol and drug dependence. The nAChRs are a super family of ligand gated ion channels, and are expressed throughout the brain with twelve neuronal nAChR subunits (α2–α10 and β2–β4) identified. Here, we review preclinical and clinical evidence involving a number of nAChR ligands that target different nAChR subtypes in alcohol and nicotine addiction. The important ligands include cytisine, lobeline, mecamylamine, varenicline, sazetidine A and others that target α4β2^* nAChR subtypes as small molecule modulators of the brain nicotinic cholinergic system are also discussed. Taken together, both preclinical and clinical data exist that support nAChR–based ligands as promising therapeutic agents for the treatment of alcohol and drug dependence.

Nicotine aversion: Neurobiological mechanisms and relevance to tobacco dependence vulnerability

Nicotine stimulates brain reward circuitries, most prominently the mesocorticolimbic dopamine system, and this action plays a critical in establishing and maintaining the tobacco smoking habit. Compounds that attenuate nicotine reward are considered promising therapeutic candidates for tobacco dependence, but many of these agents have other actions that limit their potential utility. Nicotine is also highly noxious, particularly at higher doses, and aversive reactions to nicotine after initial exposure can decrease the likelihood of developing a tobacco habit in many first time smokers. Nevertheless, relatively little is known about the mechanisms of nicotine aversion. The purpose of this review is to present recent new insights into the neurobiological mechanisms that regulate avoidance of nicotine. First, the role of the mesocorticolimbic system, so often associated with nicotine reward, in regulating nicotine aversion is highlighted. Second, genetic variation that modifies noxious responses to nicotine and thereby influences vulnerability to tobacco dependence, in particular variation in the CHRNA5-CHRNA3-CHRNB4 nicotinic acetylcholine receptor (nAChR) subunit gene cluster, will be discussed. Third, the role of the habenular complex in nicotine aversion, primarily medial habenular projections to the interpeduncular nucleus (IPN) but also lateral habenular projections to rostromedial tegmental nucleus (RMTg) and ventral tegmental area (VTA) are reviewed. Forth, brain circuits that are enriched in nAChRs, but whose role in nicotine avoidance has not yet been assessed, will be identified. Finally, the feasibility of developing novel therapeutic agents for tobacco dependence that act not by blocking nicotine reward but by enhancing nicotine avoidance will be considered. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Nicotinic receptor antagonists as treatments for nicotine abuse

Despite the proven efficacy of current pharmacotherapies for tobacco dependence, relapse rates continue to be high, indicating that novel medications are needed. Currently, several smoking cessation agents are available, including varenicline (Chantix®), bupropion (Zyban®), and cytisine (Tabex®). Varenicline and cytisine are partial agonists at the α4β2* nicotinic acetylcholine receptor (nAChR). Bupropion is an antidepressant but is also an antagonist at α3β2* ganglionic nAChRs. The rewarding effects of nicotine are mediated, in part, by nicotine-evoked dopamine (DA) release leading to sensitization, which is associated with repeated nicotine administration and nicotine addiction. Receptor antagonists that selectivity target central nAChR subtypes mediating nicotine-evoked DA release should have efficacy as tobacco use cessation agents with the therapeutic advantage of a limited side-effect profile. While α-conotoxin MII (α-CtxMII)-insensitive nAChRs (e.g., α4β2*) contribute to nicotine-evoked DA release, these nAChRs are widely distributed in the brain, and inhibition of these receptors may lead to nonselective and untoward effects. In contrast, α-CtxMII-sensitive nAChRs mediating nicotine-evoked DA release offer an advantage as targets for smoking cessation, due to their more restricted localization primarily to dopaminergic neurons. Small drug-like molecules that are selective antagonists at α-CtxMII-sensitive nAChR subtypes that contain α6 and β2 subunits have now been identified. Early research identified a variety of quaternary ammonium analogs that were potent and selective antagonists at nAChRs mediating nicotine-evoked DA release. More recent data have shown that novel, nonquaternary bis-1,2,5,6-tetrahydropyridine analogs potently inhibit (IC50<1nM) nicotine-evoked DA release in vitro by acting as antagonists at α-CtxMII-sensitive nAChR subtypes; these compounds also decrease NIC self-administration in rats.

Brain nicotinic receptors as emerging targets for drug addiction: neurobiology to translational research

Drug addiction, a chronic relapsing disorder, is a serious public health problem around the world. A growing body of preclinical and clinical evidence suggests that mammalian brain nicotinic acetylcholine receptors (nAChRs), the heterogeneous family of ion channels, play a pivotal role in drug addiction, including nicotine and alcohol dependence. As a result, there is an increasing interest in developing nAChR-based therapies for the treatment of addictive disorders. The current review summarizes the important preclinical and clinical data, demonstrating the ability of nAChR ligands to modulate nicotine and alcohol-induced biobehavioral and neurochemical changes in laboratory animals and humans. Recent studies suggest that partial agonists and antagonists at nAChRs have therapeutic potential for the management of nicotine and alcohol dependence. The complexity of nAChRs and their regulation for the development of nAChR-based drug candidates as novel pharmacotherapy for other addictive disorders will also be discussed. Taken together, this review will provide new insights into nAChR-based compounds and offer innovative translational strategies for combating drug addictive disorders.

Nicotine's Allure

Both conscious and unconscious memory mechanisms contribute to the rewarding effects of nicotine and other drugs of abuse. In this issue of Neuron, Tang and Dani use in vivo measures of synaptic plasticity in freely moving mice to link nicotine-induced dopamine release in hippocampus to LTP induction and behavioral reinforcement.

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.

The novel nicotinic receptor antagonist, N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide (bPiDDB), inhibits nicotine-evoked [(3)H]norepinephrine overflow from rat hippocampal slices

Smoking is a significant health concern and strongly correlated with clinical depression. Depression is associated with decreased extracellular NE concentrations in brain. Smokers may be self-medicating and alleviating their depression through nicotine stimulated norepinephrine (NE) release. Several antidepressants inhibit NE transporter (NET) function, thereby augmenting extracellular NE concentrations. Antidepressants, such as bupropion, also inhibit nicotinic receptor (nAChR) function. The current study determined if a recently discovered novel nAChR antagonist, N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide (bPiDDB), inhibits nicotine-evoked NE release from superfused rat hippocampal slices. Previous studies determined that bPiDDB potently (IC(50)=2 nM) inhibits nicotine-evoked striatal [(3)H]dopamine (DA) release in vitro, nicotine-evoked DA release in nucleus accumbens in vivo, and nicotine self-administration in rats. In the current study, nicotine stimulated [(3)H]NE release from rat hippocampal slices (EC(50)=50 microM). bPiDDB inhibited (IC(50)=430 nM; I(max)=90%) [(3)H]NE release evoked by 30 microM nicotine. For comparison, the nonselective nAChR antagonist, mecamylamine, and the alpha7 antagonist, methyllycaconitine, also inhibited nicotine-evoked [(3)H]NE release (IC(50)=31 and 275 nM, respectively; I(max)=91% and 72%, respectively). Inhibition by bPiDDB and mecamylamine was not overcome by increasing nicotine concentrations; Schild regression slope was different from unity, consistent with allosteric inhibition. Thus, bPiDDB was 200-fold more potent inhibiting nAChRs mediating nicotine-evoked [(3)H]DA release from striatum than those mediating nicotine-evoked [(3)H]NE release from hippocampus.

Acute nicotine changes dynorphin and prodynorphin mRNA in the striatum

RATIONALE

Nicotine displays rewarding and aversive effects, and while dopamine has been linked with nicotine's reward, the neurotransmitter(s) involved with aversion remains speculative. The kappa-dynorphinergic system has been associated with negative motivational and affective states, and whether dynorphin (Dyn) contributes to the behavioral pharmacology of nicotine is a pertinent question.

OBJECTIVE

We determined whether administration of a single dose of nicotine alters the biosynthesis of Dyn in the striatum of mice.

RESULTS

Nicotine free base, 1 mg/kg, sc, induced a biphasic, protracted increase of striatal Dyn, an initial rise by 1 h, which declined to control levels by 2 h, and a subsequent increase, between 6 and 12 h, lasting over 24 h. At 1 h, the nicotine effect was dose dependent, with doses>or=0.5 mg/kg inducing a response. Prodynorphin mRNA increased by 30 min for over 24 h, and in situ hybridization demonstrated elevated signal in caudate/putamen and nucleus accumbens. The nicotinic antagonist mecamylamine prevented the Dyn response, and a similar effect was observed with D1- and D2-like dopamine receptor antagonists, SCH 23390, sulpiride, and haloperidol. The glutamate NMDA receptor antagonist MK-801 reversed the nicotine-induced increase of Dyn, while the AMPA antagonist NBQX had a marginal effect.

CONCLUSIONS

We interpret our findings to indicate that acute nicotine enhances the synthesis and release of striatal Dyn. We propose that nicotine influences Dyn primarily through dopamine release and that glutamate plays a modulatory role. A heightened dynorphinergic tone may contribute to the aversive effects of nicotine in naive animals and first-time tobacco smokers.

Differential involvement of glutamatergic mechanisms in the cognitive and subjective effects of smoking

There is growing preclinical evidence for the involvement of glutamate in the behavioral actions of nicotine. The aim of this study, was to investigate the role of N-methyl-D-aspartate (NMDA) receptors in the cognitive and subjective effects of smoking in humans. Sixty regular smokers took part in this double-blind placebo controlled study, that investigated the effect of the NMDA-antagonist memantine (40 mg) and the nicotinic-receptor antagonist mecamylamine (10 mg) on smoking-induced improvement in performance of a task of sustained attention and on smoking-induced changes in subjective effects and craving. Increases in subjective ratings of 'buzzed' following smoking were reversed by memantine, but not by mecamylamine. In contrast, improvement on a Rapid Visual Information Processing task by smoking was opposed by mecamylamine, but not by memantine. Smoking reduced craving for cigarettes, but neither drug altered this effect. Our results suggest that glutamatergic mechanisms may have differential involvement in the subjective and cognitive actions of smoking. Further investigations using different ligands are warranted to fully characterize the role of glutamate underlying the consequences of smoking behavior.

Disrupting nicotine reinforcement: from cigarette to brain

Cigarette smoking is a tenacious addiction that is maintained to a significant extent by the reinforcing effects of nicotine. An emerging theme in smoking cessation treatment is the development of methods for interfering with these reinforcing effects. By attenuating nicotine reinforcement, treatments may enhance a smoker's chances of successfully remaining abstinent. Several treatment approaches will be described, including the use of denicotinized cigarettes, nicotine vaccines, nicotinic receptor agonists and antagonists, and modulators of brain reinforcement processes. These techniques highlight the numerous sites along the path between the cigarette and the brain that can be targeted for intervention. In addition to unimodal therapies, treatment combinations will be discussed that might more effectively block cigarette reward and thereby further enhance smoking abstinence.

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.

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.

Mecamylamine attenuates cue-induced reinstatement of nicotine-seeking behavior in rats

Mecamylamine, a noncompetitive nicotinic cholinergic antagonist, inhibits nicotine self-administration in animals and may attenuate tobacco smoking in humans trying to quit. Our preliminary data suggested that this agent, at a dose of 2 mg/kg (subcutaneous (s.c.)), also attenuates cue-induced relapse to nicotine-seeking behavior in rats. This study determined whether mecamylamine-induced attenuation can be obtained at doses lower than the high 2 mg/kg dose used in the first study, and whether it is specific to nicotine-associated cues. Male Sprague-Dawley rats were trained to intravenously self-administer nicotine (0.03 mg/kg/infusion) on a fixed-ratio 5 schedule. Each infusion was accompanied by a visual cue (1 s onset of a lever light followed by offset of a house light for 20 s during which time no infusions could be obtained). After the nicotine-maintained responding was extinguished by withholding the delivery of nicotine (saline substitution) and its associated cue, reinstatement tests were conducted. Response-contingent re-presentation of the cue without further availability of nicotine significantly reinstated extinguished responding at the previously nicotine-reinforced lever. Pretreatment with mecamylamine (0.5, 1, and 2 mg/kg, s.c.) dose-dependently attenuated the cue-induced reinstatement of lever responding. Mecamylamine did not change food-taking and -seeking responses, whereas the highest dose (2 mg/kg) decreased nicotine self-administration behavior. The results confirm previous findings that stimuli conditioned to nicotine self-administration effectively elicit reinstatement of nicotine-seeking behavior after extinction and demonstrate that mecamylamine, besides suppressing self-administration of nicotine, effectively attenuates cue-induced nicotine-seeking behavior. These findings suggest that the response-reinstatement procedures used in this study may be useful for studying neurobiological mechanisms of nicotine-seeking behavior and that mecamylamine-like drugs may be potential candidates for pharmacological treatment and prevention of relapse to tobacco smoking in abstinent smokers.

The effects of a novel nicotinic receptor antagonist N,N-dodecane-1,12-diyl-bis-3-picolinium dibromide (bPiDDB) on acute and repeated nicotine-induced increases in extracellular dopamine in rat nucleus accumbens

The present study examined the effects of the novel nicotinic acetylcholine receptor (nAChR) antagonist, N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide (bPiDDB), after acute and repeated nicotine treatment on extracellular dopamine (DA) levels in rat nucleus accumbens (NAcc), using in vivo microdialysis. Acute nicotine (0.4mg/kg, sc) injection produced an increase (232% of basal) in extracellular DA, which was attenuated by pretreatment with the nAChR antagonist mecamylamine (4mg/kg, sc). Pretreatment with bPiDDB (1 or 3mg/kg, sc) dose-dependently reduced the increase in extracellular DA produced by nicotine (0.4mg/kg, sc), but not by amphetamine (0.5mg/kg, sc). Basal levels of NAcc DA increased in animals that had been pretreated with nicotine (0.4mg/kg, sc) for 5 days compared to saline. In addition, nicotine challenge further increased extracellular DA (237% of basal). The increase in DA in NAcc following repeated nicotine was blocked by pretreatment with mecamylamine (4mg/kg, sc) and bPiDDB (1 or 3mg/kg, sc). These results indicate that bPiDDB likely acts as an antagonist at neuronal nAChRs to inhibit DA release in NAcc after acute or repeated nicotine administration. The ability of bPiDDB to inhibit the effect of nicotine in NAcc, combined with previous studies showing decreased nicotine self-administration in rats provides support for bPiDDB as a potential lead compound for the development of a novel pharmacotherapy for nicotine dependence.

Reinstatement of nicotine-seeking behavior by drug-associated stimuli after extinction in rats

RATIONALE

Smoking-related environmental stimuli have been implicated as an important factor in triggering relapse in abstinent tobacco smokers, and recent evidence indicates that drug-associated stimuli can reinstate nicotine-seeking in rats. However, there is little investigation on the factors that contribute to the latter effect.

OBJECTIVE

This study examined whether a nicotine-associated visual stimulus (VS) can reinstate nicotine-seeking after extinction in a response-reinstatement model of relapse, and whether the behavioral effects of the VS are sensitive to pharmacological blockade of nicotinic neurotransmission. It also determined whether active lever reassignment after food training influences nicotine self-administration and the VS-induced reinstatement.

METHODS

Male Sprague-Dawley rats were trained to self-administer nicotine (0.03 mg/kg/infusion, IV) and associate a VS with each nicotine infusion in 30 daily 1-h sessions. Half of the animals received nicotine infusions for responding at the same lever that previously delivered food; for the other half, infusions resulted from pressing the previously inactive lever during food training. Then, the nicotine-maintained response was extinguished by saline substitution and withholding the VS. One day after rats reached extinction criterion, the reinstatement tests were conducted where the VS was response-contingent represented without further delivery of nicotine. In pharmacological tests, a nicotinic antagonist, mecamylamine, was subcutaneously administered 30 min before reinstatement sessions.

RESULTS

Presentation of the nicotine-associated VS significantly reinstated responding at the previously drug-reinforced lever and pretreatment with mecamylamine effectively attenuated the response-reinstating effect of the VS. Additionally, animals showed similar profiles of nicotine-taking and nicotine-seeking behavior regardless of reassignment of the active lever after food training.

CONCLUSIONS

Nicotine self-administration and the VS-induced reinstatement of nicotine-seeking do not result from a lever bias due to prior experience for food reinforcement. Significantly, these results suggest that environmental stimuli associated with nicotine self-administration can effectively elicit nicotine-seeking behavior in abstinent subjects, that this effect is blocked by nicotine antagonism, and that the present procedures may be useful for studying neurobiological mechanisms of nicotine-seeking behavior and relapse.

Effect of a novel nicotinic receptor antagonist, N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide, on nicotine self-administration and hyperactivity in rats

RATIONALE AND OBJECTIVE

Recent work has shown that the novel compound N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide (bPiDDB) may selectively block nicotinic acetylcholine receptors involved in regulating dopamine release. The current experiments examined the acute effect of bPiDDB on nicotine self-administration, sucrose-maintained responding, and nicotine-induced changes in acute and sensitized locomotor activity.

METHODS

Rats were first trained to respond for either nicotine (i.v.) or sucrose pellets using a standard two-lever operant conditioning procedure using a fixed ratio 5 schedule of reinforcement and were then pretreated with bPiDDB (0, 0.3, 1, or 3 mg kg(-1)) 15 min prior to the session. In separate experiments, rats were assessed for nicotine-induced changes in locomotor activity following pretreatment with bPiDDB (1 or 3 mg kg(-1)) or mecamylamine (1 mg kg(-1)); pretreatments were assessed with both acute and repeated nicotine (0.4 mg kg(-1)) treatment.

RESULTS

Results showed that bPiDDB dose-dependently decreased nicotine self-administration, but not sucrose-maintained responding. In the locomotor experiments, bPiDDB attenuated the hyperactivity produced by acute and repeated nicotine; however, this effect was not robust compared to mecamylamine. In contrast to mecamylamine, bPiDDB did not block the initial hypoactivity produced by acute nicotine.

CONCLUSION

Since bPiDDB decreased nicotine self-administration specifically, this novel nicotinic receptor antagonist may constitute a lead for the development of a clinically useful treatment for tobacco dependence.

Expanding treatment of tobacco dependence

Nicotine dependence is the leading preventable cause of adult morbidity and mortality in the world. New research on the treatment of this disorder ranges from studies evaluating access to treatment to studies elucidating the molecular mechanisms of nicotine addiction. As our understanding of the neurobiology of tobacco addiction grows, the number of potential therapeutic targets by which we can intervene in this pernicious disorder also increases. This paper presents an overview of recent research trends in the treatment of tobacco dependence. We review several novel mechanisms of action that may serve as therapeutic targets for the pharmacologic treatment of tobacco dependence, including drugs that affect monamine oxidase, selective nicotinic receptors, glutamate and gamma-aminobutyric acid receptors, and the endocannabinoid system. For each of these therapeutic targets, we discuss medications in development that affect these pathophysiologic mechanisms.

Mecamylamine and Ethanol Preference in Healthy Volunteers

BACKGROUND

Recent evidence suggests that some of the behavioral effects of alcohol may be mediated through actions on nicotinic acetylcholine receptors. Mecamylamine, a nicotinic acetylcholine receptor antagonist, reduces alcohol preference and consumption in alcohol-preferring rats, and in humans, mecamylamine dampens some of the subjective, or mood-altering, effects of alcohol. This experiment was designed to investigate the effects of mecamylamine on consumption of alcohol in healthy social drinkers.

METHODS

Healthy volunteers (12 men, 12 women) participated in a choice procedure in which they chose between an alcoholic beverage and money (low, medium, or high amounts) after pretreatment with mecamylamine (7.5 or 15 mg) or placebo. Outcome measures were the number of alcoholic beverages consumed and the subjective effects of alcohol.

RESULTS

Mecamylamine (15 mg) decreased blood alcohol levels (BALs) after a small fixed dose of alcohol (0.2 g/kg). Even when the lower BALs were taken into account, mecamylamine reduced ratings of stimulation after alcohol (Addiction Research Center Inventory A scale). Mecamylamine did not significantly reduce choice for alcohol versus money. However, there was a tendency for the drug to decrease alcohol choice among participants who reported the greatest stimulant-like effects from alcohol.

CONCLUSION

These results provide only limited support for the idea that nicotinic acetylcholine receptors are involved in the rewarding effects of alcohol.

Local perfusion of nicotine differentially modulates somatodendritic dopamine release in the rat ventral tegmental area after nicotine preexposure

We examined the effects of nicotine perfusion into the ventral tegmental area (VTA) on extracellular dopamine (DA) levels in rats using in vivo microdialysis. Local perfusion with nicotine for 80 min (10-100 microM) modestly increased (approximately 105-131% of basal) the extracellular DA levels in the VTA of rats that had been pretreated with saline for 5 days. In animals that had been pretreated with nicotine for 5 days (0.3 mg/kg, s.c.), perfusion with nicotine for 80 min (10-100 microM) dose-dependently increased the extracellular DA levels in the VTA of rats and did so to a greater extent than in saline-pretreated animals (125-171% of basal). Co-perfusion through the dialysis probe with 100 microM mecamylamine, a nonselective nicotinic acetylcholine receptor (nAChR) antagonist, or 100 microM dihydro-beta-erythroidine, a high affinity and competitive nAChR antagonist, attenuated the enhancement of extracellular DA levels produced by 100 microM nicotine alone. These results suggest that local nicotine challenge potentiated the somatodendritic DA release after nicotine preexposure by stimulation of high-affinity nAChRs in the VTA.

Clinical neuropharmacology of drugs of abuse: a comparison of drug-discrimination and subject-report measures

Advances in molecular pharmacology and behavioral science have helped elucidate the structure and function of the central nervous system and its relationship to behavior and has sparked the development of pharmacological agents that have increasingly selective and potent effects with fewer adverse side effects. The sensitivity and predictive validity of the two most commonly used methodologies for assessing the neuropharmacological effects of centrally active drugs, subject report of drug effects and drug discrimination, were examined. The sensitivity of the measures was comparable across stimulant, sedative, and opioid drugs. Results with drug-discrimination methodologies were generally consistent with hypothesized neuropharmacological mechanisms across all drug classes, whereas subject reports conformed under more limited testing conditions. Firm conclusions regarding the relative utility of drug-discrimination and subject-report measures for clinical studies of neuropharmacological mechanisms are limited by the small number of studies in which the two methodologies have been tested using identical pharmacological pretreatment manipulations.

Mecamylamine attenuates the subjective stimulant-like effects of alcohol in social drinkers

BACKGROUND

Recent studies have implicated central nicotinic cholinergic receptor systems in the reinforcing properties of alcohol. In laboratory animals, mecamylamine, a central nicotinic receptor antagonist, reduces the consumption of and preference for alcohol. This study investigated the effect of mecamylamine on the subjective responses to alcohol in humans. It was hypothesized that mecamylamine (7.5 and 15 mg) would attenuate the stimulant-like subjective effects of alcohol (0.8 g/kg) and decrease the self-reported desire to consume additional alcohol beverages.

METHODS

Fourteen male and 13 female nonsmokers participated in 6 laboratory sessions. During each session, subjects received, in randomized order under double-blinded conditions, a capsule containing mecamylamine (7.5 or 15 mg) or placebo followed by a beverage containing alcohol (0.8 g/kg) or placebo. Physiologic and subjective-effect measures were taken at 30-min intervals for 2 hr after beverage consumption.

RESULTS

Mecamylamine attenuated the stimulant and euphoric effects of alcohol and reduced the self-reported desire to consume additional alcohol beverages. This effect was most pronounced in men, even though women exhibited greater physiologic reactions to mecamylamine.

CONCLUSIONS

These findings suggest that nicotinic cholinergic receptors are involved in mediating some of the stimulant-like effects of alcohol.

The motivational valence of nicotine in the rat ventral tegmental area is switched from rewarding to aversive following blockade of the alpha7-subunit-containing nicotinic acetylcholine receptor

RATIONALE

Within the mammalian ventral tegmental area (VTA), nicotine produces both aversive and rewarding motivational effects. However, the specific neuronal nicotinic acetylcholine receptor (nAChR) subtypes responsible for these effects are not clearly understood.

OBJECTIVES

In the present study, we challenged the motivational effects of nicotine directly in the VTA with nAChR subunit specific antagonists.

METHODS

Using an unbiased place-conditioning procedure as a behavioural assay, we performed bilateral microinfusions of nicotine over a wide range of concentrations (0.008, 8, 24 and 48 nmol/0.5 microl) and challenged the aversive and reinforcing behavioural effects of these nicotine doses with co-administration of di-hydro-beta-erythroidine (DHbetaE) (5 or 50 nmol/0.5 microl), a nAChR antagonist with higher relative affinity for the alpha4beta2 nAChR subunit, methyllycaconitine citrate (MLA) (0.4 or 4 nmol/0.5 microl), a nAChR antagonist that displays greater relative affinity for the alpha7 nAChR, and the NMDA receptor antagonist, d-2-amino-7-phosphoheptanoic acid (AP-7; 18 nmol/0.5 microl).

RESULTS

The alpha4beta2 antagonist DHbetaE blocked both the rewarding and aversive properties of intra-VTA nicotine. However, the alpha7 antagonist MLA blocked nicotine reward and switched the motivational valence of higher doses of nicotine (8-48 nmol/0.5 microl) from rewarding to aversive. The NMDA antagonist AP-7 blocked both the aversive and rewarding effects of intra-VTA nicotine.

CONCLUSIONS

These results suggest a functional dissociation between nAChR neural substrates within the VTA that mediate the bivalent motivational properties of nicotine and further suggest that nicotine may produce its motivational effects through a glutamatergic mechanism.

Neurobiology of the nicotine withdrawal syndrome

The aversive aspects of withdrawal from chronic nicotine exposure are thought to be an important motivational factor contributing to the maintenance of the tobacco habit in human smokers. Much emphasis has been placed on delineating the underlying neurobiological mechanisms mediating different components of the nicotine withdrawal syndrome. Recent studies have shown that both central and peripheral populations of nicotinic acetylcholine receptors (nAChRs) are involved in mediating somatic signs of nicotine withdrawal as measured by the rodent nicotine abstinence scale. However, only central populations of nAChRs are involved in mediating affective aspects of nicotine withdrawal, as measured by elevations in brain-stimulation reward thresholds and conditioned place aversion. Nicotine interacts with several neurotransmitter systems, including acetylcholine, dopamine, opioid peptides, serotonin, and glutamate systems. Evidence so far suggests that these neurotransmitters play a role in nicotine dependence and withdrawal processes. The available evidence also suggests that different underlying neurochemical deficits mediate somatic and affective components of nicotine withdrawal. The aim of the present review is to discuss preclinical findings concerning the neuroanatomical and neurochemical substrates involved in these different aspects of nicotine withdrawal.