Increased dopamine D3 receptor expression accompanying behavioral sensitization to nicotine in rats

Dopamine D3 receptor antagonists: a patent review (2007 - 2012)

INTRODUCTION

The synthesis and characterization of new highly potent and selective dopamine (DA) D3 receptor antagonists has permitted to characterize the role of the DA D3 receptor in the control of drug-seeking behavior and in the pathophysiology of impulse control disorders and schizophrenia.

AREAS COVERED

In the present review, the authors will first describe most recent classes of DA D3 receptor antagonists by reviewing about 43 patent applications during the 2007 - 2012 period; they will then outline the biological rationale in support of the use of selective DA D3 receptor antagonists in the treatment of drug addiction, impulse control disorders and schizophrenia.

EXPERT OPINION

The strongest clinical application and potential for selective DA D3 receptor antagonists lies in the reduction of drug-induced incentive motivation, the attenuation of drug's rewarding efficacy and the reduction in reinstatement of drug-seeking behavior triggered either by re-exposure to the drug itself, re-exposure to environmental cues that had been previously associated with drug-taking behavior or stress. The selectivity of these antagonists together with reduced lipophilicity (minimizing unspecific binding), increased brain penetration and improved physico-chemical profile are all key factors for clinical efficacy and safety.

Low-dose adolescent nicotine and methylphenidate have additive effects on adult behavior and neurochemistry

Adolescents with Attention Deficit Hyperactivity Disorder (ADHD) have higher rates of smoking than adolescents without ADHD. Since methylphenidate is the primary drug used to treat ADHD, it is likely that many adolescents are exposed to both methylphenidate and nicotine. Recent studies have established that adolescent nicotine induces long-term changes in several neurobehavioral variables. Limited data also suggest that adolescent methylphenidate may affect neural development. Nicotine tolerance is a well-established behavioral phenomenon in rodents, yet the underlying mechanism remains elusive. Recent theories suggest that changes in ventral striatal dopamine indices may relate to nicotine tolerance. As an initial determination of whether nicotine and methylphenidate have additive effects on neurobehavioral development, the present study investigated the combined effects of adolescent nicotine [2mg/kg/d] alone or in conjunction with methylphenidate [1.5mg/kg, 2× daily] following a one-month drug free period on adult behavioral tolerance to nicotine [0.5mg/kg s.c.] and its relation to dopamine receptor mRNA expression in the ventral striatum. Animals with chronic combined (nicotine+methylphenidate) adolescent exposure displayed stronger tolerance as adults to the nicotine-induced locomotor effects in comparison to animals with adolescent exposure to nicotine alone, methylphenidate alone, or controls. Combined chronic adolescent exposure significantly elevated adult D3nf mRNA expression levels in the nucleus accumbens, however a single nicotine injection in adults increased D3nf mRNA levels in naïve animals and decreased D3nf mRNA levels in those that had been previously exposed to combined stimulants during adolescence. Conversely, a single adult nicotine injection increased D1 mRNA levels in the adult nucleus accumbens, particularly in the shell, but only in rats previously exposed to nicotine or methylphenidate as adolescents. To our knowledge this is the first study that has shown long-term behavioral and neurochemical changes stemming from low chronic exposure of these two commonly co-consumed stimulants during adolescence.

Rationale in support of the use of selective dopamine D₃ receptor antagonists for the pharmacotherapeutic management of substance use disorders

Growing evidence indicates that dopamine (DA) D(3) receptors are involved in the control of drug-seeking behavior and may play an important role in the pathophysiology of substance use disorders. First, DA D(3) receptors are distributed in strategic areas belonging to the mesolimbic DA system such as the ventral striatum, midbrain, and pallidum, which have been associated with behaviors controlled by the presentation of drug-associated cues. Second, repeated exposure to drugs of abuse has been shown to produce neuroadaptations in the DA D(3) system. Third, the synthesis and characterization of highly potent and selective DA D(3) receptor antagonists has permitted to further define the role of the DA D(3) receptor in drug addiction. Provided that the available preclinical and preliminary clinical evidence can be translated into clinical proof of concept in human, selective DA D(3) receptor antagonists show promise for the treatment of substance use disorders as reflected by their potential to (1) regulate the motivation to self-administered drugs under schedules of reinforcement that require an increase in work demand and (2) disrupt the responsiveness to drug-associated stimuli that play a key role in the reinstatement of drug-seeking behavior triggered by re-exposure to the drug itself, re-exposure to environmental cues that had been previously associated with drug-taking behavior, or stress.

Chronic Δ⁹-tetrahydrocannabinol exposure induces a sensitization of dopamine D₂/₃ receptors in the mesoaccumbens and nigrostriatal systems

Δ⁹-tetrahydrocannabinol (THC), through its action on cannabinoid type-1 receptor (CB₁R), is known to activate dopamine (DA) neurotransmission. Functional evidence of a direct antagonistic interaction between CB₁R and DA D₂-receptors (D₂R) suggests that D₂R may be an important target for the modulation of DA neurotransmission by THC. The current study evaluated, in rodents, the effects of chronic exposure to THC (1 mg/kg/day; 21 days) on D₂R and D₃R availabilities using the D₂R-prefering antagonist and the D₃R-preferring agonist radiotracers [¹⁸F]fallypride and [³H]-(+)-PHNO, respectively. At 24 h after the last THC dose, D₂R and D₃R densities were significantly increased in midbrain. In caudate/putamen (CPu), THC exposure was associated with increased densities of D₂R with no change in D₂R mRNA expression, whereas in nucleus accumbens (NAcc) both D₃R binding and mRNA levels were upregulated. These receptor changes, which were completely reversed in CPu but only partially reversed in NAcc and midbrain at 1 week after THC cessation, correlated with an increased functionality of D₂/₃R in vivo, based on findings of increased locomotor suppressive effect of a presynaptic dose and enhanced locomotor activation produced by a postsynaptic dose of quinpirole. Concomitantly, the observations of a decreased gene expression of tyrosine hydroxylase in midbrain together with a blunted psychomotor response to amphetamine concurred to indicate a diminished presynaptic DA function following THC. These findings indicate that the early period following THC treatment cessation is associated with altered presynaptic D₂/₃R controlling DA synthesis and release in midbrain, with the concurrent development of postsynaptic D₂/₃R supersensitivity in NAcc and CPu. Such D₂/₃R neuroadaptations may contribute to the reinforcing and habit-forming properties of THC.

Reduced expression of haloperidol conditioned catalepsy in rats by the dopamine D3 receptor antagonists nafadotride and NGB 2904

Haloperidol, a dopamine (DA) D2 receptor-preferring antagonist, produces catalepsy whereby animals maintain awkward posture for a period of time. Sub-threshold doses of haloperidol fail to produce catalepsy initially, however, when the drug is given repeatedly in the same test environment, gradual day-to-day increases in catalepsy are observed. More importantly, if sensitized rats are injected with saline instead of haloperidol they continue to be cataleptic in the test environment suggesting that environment-drug associations may play a role. DA D3 receptors have been implicated in a number of conditioned behaviors. We were interested if DA D3 receptors contribute to catalepsy sensitization and conditioning in rats. We tested this hypothesis using the DA D3 receptor-selective antagonist NGB 2904 (0.5, 1.8 mg/kg) and the DA D3 receptor-preferring antagonist nafadotride (0.1, 0.5 mg/kg). For 10 consecutive conditioning days rats were treated with one of the D3 receptor antagonists alone or in combination with haloperidol (0.25 mg/kg) and tested for catalepsy, quantified by the time a rat remained with its forepaws on a horizontal bar. On test day (day 11), rats were injected with saline or the D3 receptor antagonist and tested for conditioned catalepsy in the previously drug-paired environment. Rats treated with NGB 2904 or nafadotride alone did not develop catalepsy. Rats treated with haloperidol or haloperidol plus NGB 2904 or nafadotride developed catalepsy sensitization with repeated conditioning. When injected with saline they continued to exhibit catalepsy in the test environment--now conditioned. On the other hand, NGB 2904 (1.8 mg/kg) or nafadotride (0.5 mg/kg) given on the test day (after sensitization to haloperidol) significantly attenuated the expression of conditioned catalepsy. Our data suggest that the D3 receptor antagonist NGB 2904 (1.8 mg/kg) and nafadotride (0.5 mg/kg) significantly attenuate conditioned catalepsy in rats when given in test but not when given during sensitization. Results implicate DA D3 receptors in regulating the expression of conditioned catalepsy.

Phenylpiperazine derivatives with selectivity for dopamine D3 receptors modulate cocaine self-administration in rats

This study examined cocaine self-administration after pretreatments with three structurally related compounds that bind selectively to dopamine D3 receptors (D3Rs) relative to the D2 receptor subtype (D2Rs) and exhibit varying intrinsic activities in the forskolin-stimulated adenylyl cyclase assay. The compounds are: a) WC10, a D3R weak partial agonist/antagonist with 42-fold D3R:D2R selectivity, b) WC26, a 51-fold selective D3R partial agonist, c) WC44, a 23-fold selective D3R agonist. Rats were stabilized on a multiple variable-interval 60-s (VI60) schedule with alternating components of sucrose (45 mg pellets) or cocaine reinforcement (0.375 mg/kg, IV) and then tested for effects of the WC compounds (0.0, 1.0, 3.0, 5.6, or 10.0 mg/kg, IP). Another cohort was trained to self-administer cocaine (0.75 mg/kg, IV) on a VI60 schedule then tested with various doses of cocaine available (0.0-1.5 mg/kg, IV) following pretreatment with WC10 (5.6 or 10.0 mg/kg) or WC44 (10.0 mg/kg). WC10 and WC26 decreased both cocaine and sucrose reinforcement rates at the 10.0 mg/kg dose, whereas WC44 decreased only cocaine reinforcement rate at this dose. Furthermore, WC26 and WC44 increased response latency for cocaine but not sucrose. In the cocaine dose-response experiment, WC10 and WC44 flattened the dose-effect function of cocaine reinforcement rate. All compounds decreased spontaneous locomotion. WC10 and WC26 also reduced cocaine-induced locomotion. These results support the targeting of D3Rs for treatments for cocaine dependence. WC26 and WC44, in particular, show promise as they increased the latency to respond for cocaine but not sucrose, suggesting selective reduction of the motivation for cocaine.

Higher binding of the dopamine D3 receptor-preferring ligand [11C]-(+)-propyl-hexahydro-naphtho-oxazin in methamphetamine polydrug users: a positron emission tomography study

Positron emission tomography (PET) findings suggesting lower D2-type dopamine receptors and dopamine concentration in brains of stimulant users have prompted speculation that increasing dopamine signaling might help in drug treatment. However, this strategy needs to consider the possibility, based on animal and postmortem human data, that dopaminergic activity at the related D3 receptor might, in contrast, be elevated and thereby contribute to drug-taking behavior. We tested the hypothesis that D3 receptor binding is above normal in methamphetamine (MA) polydrug users, using PET and the D3-preferring ligand [11C]-(+)-propyl-hexahydro-naphtho-oxazin ([11C]-(+)-PHNO). Sixteen control subjects and 16 polydrug users reporting MA as their primary drug of abuse underwent PET scanning after [11C]-(+)-PHNO. Compared with control subjects, drug users had higher [11C]-(+)-PHNO binding in the D3-rich midbrain substantia nigra (SN; +46%; p<0.02) and in the globus pallidus (+9%; p=0.06) and ventral pallidum (+11%; p=0.1), whereas binding was slightly lower in the D2-rich dorsal striatum (approximately -4%, NS; -12% in heavy users, p=0.01) and related to drug-use severity. The [11C]-(+)-PHNO binding ratio in D3-rich SN versus D2-rich dorsal striatum was 55% higher in MA users (p=0.004), with heavy but not moderate users having ratios significantly different from controls. [11C]-(+)-PHNO binding in SN was related to self-reported "drug wanting." We conclude that the dopamine D3 receptor, unlike the D2 receptor, might be upregulated in brains of MA polydrug users, although lower dopamine levels in MA users could have contributed to the finding. Pharmacological studies are needed to establish whether normalization of D3 receptor function could reduce vulnerability to relapse in stimulant abuse.

Exposure to nicotine produces an increase in dopamine D2(High) receptors: a possible mechanism for dopamine hypersensitivity

Dopamine D2 receptors exist in both low- and high-affinity states (D2(High)), the latter being the functionally relevant state. Cocaine self-administration produces an increase in D2(High), a phenomenon that could explain why cocaine administration results in hypersensitivity to dopamine, even though drug addicts were found to have a decreased number of striatal dopamine D2 receptors. As nicotine acts through the same mesocortical dopaminergic signaling pathways as other stimulant drugs, which are known to increase the levels of D2(High), we hypothesized that nicotine exposure could produce an increase in D2(High) levels. We determined D2(High) levels in rats after nicotine administration (1.5 mg/kg/day; 14 days), in rats voluntarily self-administering nicotine using an intravenous self-administration (IVSA) protocol (mean dose 0.5 mg/kg/day; 14 days), as well as after a prolonged withdrawal. An increase in the levels of D2(High) was found in rats who had nicotine administered at a uniform dose, as well as in rats who self-administered nicotine via IVSA, but these changes appear to normalize over time, as indicated by lower D2(High) levels in rats after a prolonged withdrawal period. We suggest that nicotine-induced elevation in D2(High) levels could be participating in hypersensitivity to dopamine following nicotine exposure.

Nicotine: alcohol reward interactions

It is well established that the continued intake of drugs of abuse is reinforcing-that is repeated consumption increases preference. This has been shown in some studies to extend to other drugs of abuse; use of one increases preference for another. In particular, the present review deals with the interaction of nicotine and alcohol as it has been shown that smoking is a risk factor for alcoholism and alcohol use is a risk factor to become a smoker. The review discusses changes in the brain caused by chronic nicotine and chronic alcohol intake to approach the possible mechanisms by which one drug increases the preference for another. Chronic nicotine administration was shown to affect nicotine receptors in the brain, affecting not only receptor levels and distribution, but also receptor subunit composition, thus affecting affinity to nicotine. Other receptor systems are also affected among others catecholamine, glutamate, GABA levels and opiate and cannabinoid receptors. In addition to receptor systems and transmitters, there are endocrine, metabolic and neuropeptide changes as well induced by nicotine. Similarly chronic alcohol intake results in changes in the brain, in multiple receptors, transmitters and peptides as discussed in this overview and also illustrated in the tables. The changes are sex and age-dependent-some changes in males are different from those in females and in general adolescents are more sensitive to drug effects than adults. Although nicotine and alcohol interact-not all the changes induced by the combined intake of both are additive-some are opposing. These opposing effects include those on locomotion, acetylcholine metabolism, nicotine binding, opiate peptides, glutamate transporters and endocannabinoid content among others. The two compounds lower the negative withdrawal symptoms of each other which may contribute to the increase in preference, but the mechanism by which preference increases-most likely consists of multiple components that are not clear at the present time. As the details of induced changes of nicotine and alcohol differ, it is likely that the mechanisms of increasing nicotine preference may not be identical to that of increasing alcohol preference. Stimulation of preference of yet other drugs may again be different -representing one aspect of drug specificity of reward mechanisms.

The selective dopamine D3 receptor antagonist SB 277011-A, but not the partial agonist BP 897, blocks cue-induced reinstatement of nicotine-seeking

The dopamine D3 receptor (DRD3) has been suggested to be involved in the mechanisms underlying stimulus-controlled drug-seeking behaviour. Ligands acting as DRD3 antagonists (SB 277011-A) or DRD3 partial agonists (BP 897) have shown some promise for reducing the influence of drug-associated cues on motivational behaviour. Here, effects of SB 277011-A and BP 897 were evaluated on cue-induced reinstatement of nicotine-seeking in rats. The effects of BP 897 on nicotine self-administration under a fixed-ratio 5 (FR5) schedule of reinforcement were also evaluated. SB 277011-A (1-10 mg/kg) was able to block cue-induced reinstatement of nicotine-seeking, indicating that DRD3 selective antagonism may be an effective approach to prevent relapse for nicotine. In contrast, BP 897 did not block the cue-induced reinstatement of nicotine-seeking or nicotine-taking under the FR5 schedule. In a control study, rats did not respond to the light stimuli without nicotine delivery, indicating that the responding for the drug-associated cues was induced by the previous pairing of light stimuli with nicotine's effects. These findings validate the role of DRD3 on reactivity to drug-associated stimuli and suggest that the DRD3 antagonist, but perhaps not the DRD3 partial agonist, could be used to prevent relapse in tobacco smokers.

Current perspectives on selective dopamine D(3) receptor antagonists as pharmacotherapeutics for addictions and related disorders

Repeated exposure to drugs of abuse produces long-term molecular and neurochemical changes that may explain the core features of addiction, such as the compulsive seeking and taking of the drug, as well as the risk of relapse. A growing number of new molecular and cellular targets of addictive drugs have been identified, and rapid advances are being made in relating those targets to specific behavioral phenotypes in animal models of addiction. In this context, the pattern of expression of the dopamine (DA) D(3) receptor in the rodent and human brain and changes in this pattern in response to drugs of abuse have contributed primarily to direct research efforts toward the development of selective DA D(3) receptor antagonists. Growing preclinical evidence indicates that these compounds may actually regulate the motivation to self-administer drugs and disrupt drug-associated cue-induced craving. This report will be divided into three parts. First, preclinical evidence in support of the efficacy of selective DA D(3) receptor antagonists in animal models of drug addiction will be reviewed. The effects of mixed DA D(2)/D(3) receptor antagonists will not be discussed here because most of these compounds have low selectivity at the D(3) versus D(2) receptor, and their efficacy profile is related primarily to functional antagonism at D(2) receptors and possibly interactions with other neurotransmitter systems. Second, major advances in medicinal chemistry for the identification and optimization of selective DA D(3) receptor antagonists and partial agonists will be analyzed. Third, translational research from preclinical efficacy studies to so-called proof-of-concept studies for drug addiction indications will be discussed.

A functional role for the dopamine D3 receptor in the induction and expression of behavioural sensitization to ethanol in mice

RATIONALE

We previously reported that mice lacking dopamine D3 receptors (D3R) were resistant to behavioural sensitization to ethanol (EtOH). Since knockout mice have permanent receptor inactivation, we investigated how temporary pharmacological blockade and activation of D3Rs affected the induction or expression of EtOH sensitization.

MATERIALS AND METHOD

In induction studies, DBA/2 mice received 0, 10 or 75 mg/kg, subcutaneous (s.c.) of the D3R antagonist, U99194A ,before each of seven EtOH (2.2 g/kg, intraperitoneal) or saline injections. Locomotor activity (LMA) was assessed in activity chambers. In expression studies, mice received seven injections of EtOH or saline, followed 14 days later with U99194A or vehicle immediately before a test dose of EtOH (1.8 g/kg). In separate experiments, the effects of the D3R agonist PD128907 (0.01 mg/kg, s.c.) were similarly examined during and after EtOH sensitization.

RESULTS

Chronic co-administration of low-dose U99194A blocked the induction of EtOH sensitization, while acute U99194A had no effect in mice that were already sensitized. Chronic co-administration of PD128907 resulted in decreased LMA but this effect was also seen in saline-treated mice, suggesting a simple subtractive effect. In previously sensitized mice, acute PD128907 significantly attenuated the expression of EtOH sensitization, indicating an enhanced response to the drug.

CONCLUSIONS

Results suggest a modulatory role for the D3R in behavioural sensitization to EtOH, where D3R blockade is associated with induction and D3R stimulation is associated with expression of EtOH sensitization. A model is suggested to account for these complementary functions of the D3R at different stages of EtOH sensitization.

Dopamine receptors in human peripheral blood lymphocytes: changes in mRNA expression in opioid addiction

Gradual adaptations of the brain to repeated drug exposure may induce addiction. Brain mesolimbic dopaminergic pathway is the site of the effect of addictive drugs. The dopamine receptors in peripheral blood lymphocytes may reflect the status of homologous brain receptors. In the present study, the effects of opioid addiction on mRNA expression of dopamine D(3), D(4) and D(5) receptors in human peripheral blood lymphocytes were investigated, using a real-time PCR method. Four groups each comprising 30 individuals were enrolled in the study: opioid addicted, methadone maintained, long-term abstinent and normal subjects. The results indicated that dopamine D(3) receptor mRNA expression was increased in addicted and methadone maintained subjects by a factor of 1.74 and 1.98, respectively, but no change was observed in the abstinent group. The dopamine D(4) receptor mRNA expression was reduced in abstinent and addicted subjects (but not in the methadone group) and reached 0.44 and 0.53 the amount of the control group, respectively. Expression of dopamine D(5) receptor mRNA showed a significant reduction in abstinent subjects (0.41 the amount of the control group). However, in the addicted and methadone maintained groups, the change of expression level was not statistically significant. It can be concluded that persisting deficiency of dopamine D(4) and D(5) receptors may be a risk factor urging individuals to addiction, and methadone may exert its therapeutic effects through normalizing mRNA expression of these receptors. The dopamine D(3) receptor may have a negative feedback role in addiction; however, we have no explanation for the persisting up-regulation of this receptor in methadone subjects.

Differential susceptibility to ethanol and amphetamine sensitization in dopamine D3 receptor-deficient mice

RATIONALE

Dopamine D3 receptors (D3Rs) have been implicated in behavioral sensitization to various drugs of abuse, but their role in ethanol (EtOH) sensitization has not been directly examined. We used D3R knockout (D3 KO) mice to examine whether the D3R plays a permissive role in EtOH and amphetamine (AMPH) sensitization. We also investigated whether EtOH sensitization is accompanied by alterations in D3R mRNA expression or binding.

MATERIALS AND METHODS

After comparing EtOH sensitization in C57Bl/6 mice and DBA/2 mice, D3 KO, wild type (WT), and for comparison, D1 and D2 KOs received five biweekly injections of EtOH (2.2 g/kg, i.p.) or saline. Another group of D3 KOs and WT controls received six times AMPH (1.5 mg/kg, i.p.). D3R mRNA and binding were measured in EtOH-sensitized DBA/2 mice with in situ hybridization and [(125)I]-7-OH-PIPAT autoradiography, respectively.

RESULTS

C57Bl/6 mice expressed EtOH sensitization albeit to a lesser extent than DBA/2 mice. Compared to WT mice, D3 KOs were resistant to EtOH sensitization. The behavioral profile of D3 KOs was more similar to D1 KOs than D2 KOs, which also failed to develop EtOH sensitization. However, D3 KOs developed AMPH sensitization normally. EtOH sensitization was not accompanied by changes in either D3R mRNA or D3R binding in the islands of Calleja, nucleus accumbens, dorsal striatum, or cerebellum.

CONCLUSIONS

These results suggest a necessary role for the D3R in EtOH but not AMPH sensitization, possibly through postreceptor intracellular mechanisms. Results also suggest that different neurochemical mechanisms underlie sensitization to different drugs of abuse.

Cellular events in nicotine addiction

In the 25 years since the observation that chronic exposure to nicotine could regulate the number and function of high affinity nicotine binding sites in the brain there has been a major effort to link alterations in nicotinic acetylcholine receptors (nAChRs) to nicotine-induced behaviors that drive the addiction to tobacco products. Here we review the proposed roles of various nAChR subtypes in the addiction process, with emphasis on how they are regulated by nicotine and the implications for understanding the cellular neurobiology of addiction to this drug.

Genetic variation in the dopamine pathway and smoking cessation

Twin and family studies have established that genetic factors account for much of the variation in tobacco dependence. Therefore, identification of genetic variants predictive of successful smoking cessation has implications for the future prospect of personalized smoking cessation therapies. Converging data implicate the dopamine pathway as an important neural substrate for tobacco dependence. Several candidate genes within the dopamine pathway (e.g., DRD2 and COMT) have been reported to be associated with the efficacy of bupropion and nicotine replacement therapy, and others (e.g., SLC6A3 and DRD4) have been reported to be associated with smoking cessation independent of pharmacotherapy. However, few of these candidate genes are present within regions of suggestive or significant linkage or overlap with genome-wide linkage or association studies of tobacco dependence or smoking cessation. Future studies should seek to replicate genome-wide association analyses with individual-level genotyping, and use better-defined smoking cessation phenotypes. Once robust evidence for association is established, which may take several more years, further research into the likely cost-effectiveness, feasibility and acceptability of personalized medicine for smoking cessation will be necessary before it can be translated into practice.

Effects of nicotine in experimental animals and humans: an update on addictive properties

Tobacco use through cigarette smoking is the leading preventable cause of death in the developed world. Nicotine, a psychoactive component of tobacco, appears to play a major role in tobacco dependence, but the reinforcing effects of nicotine have often been difficult to demonstrate directly in controlled studies with laboratory animals or human subjects. Here we update our earlier review published in Psychopharmacology (Berl) in 2006 on findings obtained with various procedures developed to study dependence-related behavioral effects of nicotine in experimental animals and humans. Results obtained with drug self-administration, conditioned place preference, subjective reports of nicotine effects and nicotine discrimination indicate that nicotine can function as an effective reinforcer of drug-seeking and drug-taking behavior both in experimental animals and humans under appropriate conditions. Interruption of chronic nicotine exposure produces ratings of drug withdrawal and withdrawal symptoms that may contribute to relapse. Difficulties encountered in demonstrating reinforcing effects of nicotine under some conditions, relative to other drugs of abuse, may be due to weaker primary reinforcing effects of nicotine, to aversive effects produced by nicotine, or to a more critical contribution of environmental stimuli to the maintenance of drug-seeking and drug-taking behavior with nicotine than with other drugs of abuse. Several recent reports suggest that other chemical substances inhaled along with nicotine in tobacco smoke may play a role in sustaining smoking behavior. However, conflicting results have been obtained with mice and rats and these findings have not yet been validated in nonhuman primates or human subjects. Taken together, these findings suggest that nicotine acts as a typical drug of abuse in experimental animals and humans in appropriate situations.

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.

A functional polymorphism, rs6280, in DRD3 is significantly associated with nicotine dependence in European-American smokers

Nicotine enhances reward functions in the mesocorticolimbic dopamine system in general and the nucleus accumbens in particular. The genes encoding dopamine receptors are thus plausible candidates for involvement in nicotine dependence (ND). We investigated 13 single nucleotide polymorphisms (SNPs) spanning a region of the dopamine D(3) receptor gene (DRD3) to determine whether DRD3 is associated with ND. We studied a set of 2,037 subjects in 602 nuclear families representing two distinct American populations using three ND measures, namely, smoking quantity (SQ), the Heaviness of Smoking Index (HSI), and the Fagerström Test for ND (FTND). In the family based association study, we found that SNP rs6280 showed a strong association with ND in European-Americans (EAs) and the pooled sample, whereas SNPs rs1486012 and rs963468 had weak associations with ND in African-Americans (AAs) and the pooled sample. Further haplotype analysis with all contiguous 3-SNP groups revealed relatively weak evidence for association of DRD3 with ND in the EA and pooled samples. The results indicate that DRD3 is significantly associated with ND in EAs, and that rs6280, a functional polymorphism causing an amino acid change of serine to glycine (Ser9Gly) in the N-terminal extracellular domain of the D(3) receptor, likely is causative of the association between DRD3 and ND.

Dopamine signaling through D1-like versus D2-like receptors in the nucleus accumbens core versus shell differentially modulates nicotine reward sensitivity

Considerable evidence implicates the mesolimbic dopamine (DA) system in the processing of nicotine's reinforcing properties, specifically the ventral tegmental area (VTA) and the terminal fields of VTA DAergic projections to the "core" (NAcore) and "shell" (NAshell) subdivisions of the nucleus accumbens (NAc). However, the specific roles of DA D(1)-like and D(2)-like receptor subtypes in nicotine reward processing within these NAc subregions have not been elucidated. We report that microinfusions of DA D(1)-like or D(2)-like receptor-specific antagonists into NAcore or NAshell double dissociate the rewarding and aversive properties of systemic or intra-VTA nicotine, and differentially regulate sensitivity to the rewarding properties as well as the motivational valence of either intra-VTA or systemic nicotine administration. Using a place conditioning procedure, NAshell infusions of a D(2)-like receptor antagonist switched the motivational valence of intra-VTA nicotine from aversive to rewarding and potentiated nicotine reward sensitivity to sub-reward threshold intra-VTA nicotine doses. In contrast, NAcore infusions of a D(1)-like receptor antagonist switched intra-VTA nicotine aversion to reward, and potentiated reward sensitivity to sub-reward threshold nicotine doses. Thus, D(1)-like versus D(2)-like receptors in NAcore versus NAshell subdivisions play functionally dissociable roles in modulating systemic or intra-VTA nicotine motivational processing.

The dopamine D3/D2 agonist (+)-PD-128,907 [(R-(+)-trans-3,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano[4,3-b]-1,4-oxazin-9-ol)] protects against acute and cocaine-kindled seizures in mice: further evidence for the involvement of D3 receptors

Previous findings have demonstrated a protective role for dopamine D(3)/D(2) receptor agonists in the convulsant and lethal effects of acutely administered cocaine. Data are provided here to establish that the protection occurs through a D(3)-linked mechanism and that protection is extended to seizure kindling. The D(3) antagonist SB-277011-A [4-quinolinecarboxamide,N-[trans-4-[2-(6-cyano-3,4-dihydro-2(1H)-isoquinolinyl)ethyl]-cyclohexyl]-(9CI)] prevented the anticonvulsant effect of the D(3)/D(2) receptor agonist (+)-PD-128,907 [(R-(+)-trans-3,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano[4,3-b]-1,4-oxazin-9-ol)] on cocaine-induced seizures. The protection afforded by the D(3)/D(2) agonist, (+)-PD-128,907, was eliminated in D(3) receptor-deficient mice. In D(2) receptor knockout mice, the anticonvulsant effects of (+)-PD-128,907 were preserved. (+)-PD-128,907 also prevented the acquisition and expression of cocaine-kindled seizures engendered by repeated daily dosing with 60 mg/kg cocaine. (+)-PD-128,907 also blocked the seizures induced in mice fully seizure kindled to cocaine. Although repeated dosing with cocaine increased the potency of cocaine to produce seizures and lethality (decreased ED(50) values), daily coadministration of (+)-PD-128,907 significantly prevented this potency shift. In mice treated daily with cocaine and (+)-PD-128,907, the density, but not the affinity, of D(3) receptors was increased. The specificity with which (+)-PD-128,907 acts upon this cocaine-driven process was demonstrated by the lack of a significant effect of (+)-PD-128,907 on seizure kindling to a GABA(A) receptor antagonist, pentylenetetrazol. Taken together and with literature findings, the data indicate that dopamine D(3) receptors function in the initiation of a dampening mechanism against the toxic effects of cocaine, a finding that might have relevance to psychiatric disorders of drug dependence, schizophrenia, and bipolar depression.

Topiramate does not alter nicotine or cocaine discrimination in rats

The effects of topiramate, a potential treatment for drug dependence, were evaluated in two groups of rats trained to discriminate the administration of either 0.4 mg/kg nicotine or 10 mg/kg cocaine from that of saline, under a fixed-ratio 10 schedule of food delivery. Topiramate (1-60 mg/kg, intraperitoneal) did not produce any nicotine-like or cocaine-like discriminative effects by itself and did not produce any shift in the dose-response curves for nicotine or cocaine discrimination. Thus, the ability to discriminate the effects of nicotine or cocaine does not appear to be altered by topiramate administration. Furthermore, topiramate, given either alone or in combination with nicotine or cocaine, did not depress rates of responding. These experiments indicate that topiramate does not enhance or reduce the ability of rats to discriminate the effects of nicotine or cocaine.

Molecular mechanisms underlying the motivational effects of nicotine

In addition to the primary rewarding properties of nicotine and the alleviation of withdrawal symptoms, cues associated with smoking are critical contributors to maintenance of smoking behavior. Nicotine-paired cues are also critical for precipitating relapse after smoking cessation. An accumulation of evidence suggests that repeated exposure to tobacco, including the primary psychoactive ingredient, nicotine, changes brain neurochemistry in a way that promotes the control that cues associated with smoking or other rewards have over behavior. This chapter will consider the neurochemical mechanisms underlying these neuroadaptations. Targeting these molecular alterations may provide novel treatments for smoking cessation.

Nicotine improves cognitive deficits of dopamine transporter knockout mice without long-term tolerance

Various studies suggest a dysfunction of nicotinic neurotransmission in schizophrenia and establish that patients suffering from schizophrenia and attention deficit hyperactivity disorder (ADHD) have a high tobacco consumption, potentially for the purpose of self-medication. Owing to its neuroprotective and procognitive effects, transdermal nicotine was proposed to be an effective treatment of some neurodegenerative and psychiatric diseases. Mice deficient in the dopamine transporter (DAT KO) exhibit a phenotype reminiscent of schizophrenia and ADHD, including hyperdopaminergia, hyperactivity, paradoxical calming by methylphenidate and cognitive deficits, some of which being improved by antipsychotic agents. We recently demonstrated that nicotinic receptor content and function were profoundly modified in DAT KO mice. In this study, we assessed the effects of a chronic nicotine treatment in the drinking water on the nicotine-induced locomotion, anxiety status and learning performance. Chronically nicotine-treated DAT KO mice were always hypersensitive to the hypolocomotor effect of nicotine without tolerance and did not exhibit the anxiogenic effect of nicotine treatment observed in WT mice. Very interestingly, both acute and chronic nicotine treatments greatly improved their deficits in the cued and spatial learning, without eliciting tolerance. We speculate that the procognitive effects of nicotine in DAT KO mice are related to the upregulation of alpha7 nicotinic receptors in the hippocampus, amygdala, and prelimbic cortex, all areas involved in cognition. Data from our studies on DAT KO mice shed light on the nicotine self-medication in psychiatric patients and suggest that nicotinic agonists could favorably lead to additional therapy of psychiatric diseases.

Exposure to nicotine and sensitization of nicotine-induced behaviors

Evidence for an important link between sensitization of midbrain dopamine (DA) neuron reactivity and enhanced self-administration of amphetamine and cocaine has been reported. To the extent that exposure to nicotine also sensitizes nucleus accumbens DA reactivity, it is likely that it will also impact subsequent drug taking. It is thus necessary to gain an understanding of the long-term effects of exposure to nicotine on nicotinic acetylcholine receptors (nAChRs), neuronal excitability and behavior. A review of the literature is presented in which different regimens of nicotine exposure are assessed for their effects on upregulation of nAChRs, induction of LTP in interconnected midbrain nuclei and development of long-lasting locomotor and DA sensitization. Exposure to nicotine upregulates nAChRs and nAChR currents and produces LTP of excitatory inputs to midbrain DA neurons. These effects appear in the hours to days following exposure. Exposure to nicotine also leads to long-lasting sensitization of nicotine's nucleus accumbens DA and locomotor activating effects. These effects appear days to weeks after drug exposure. A model is proposed in which nicotine exposure regimens that produce transient nAChR upregulation and LTP consequently produce long-lasting sensitization of midbrain DA neuron reactivity and nicotine-induced behaviors. These neuroadaptations are proposed to constitute critical components of the mechanisms underlying the initiation, maintenance and escalation of drug use.

Chronic high dose transdermal nicotine in Parkinson's disease: an open trial

Whether nicotine has therapeutic effects on Parkinson's disease (PD) symptoms is controversial, but high doses and chronic treatment have never been tested. We report the results of a pilot, open-label trial to assess the safety and possible efficacy of chronic high doses of nicotine. Six patients with advanced idiopathic PD received increasing daily doses of transdermal nicotine up to 105 mg/day over 17 weeks. All patients but one accepted the target dose. Nausea and vomiting were frequent but moderate, and occurred in most of the patients (four of six) who received over 90 mg/day and 14 weeks of nicotine treatment. During the plateau phase, patients improved their motor scores and dopaminergic treatment was reduced. These results confirm the feasibility of chronic high dose nicotinic treatment in PD but warrant validation of the beneficial effects by a randomized controlled trial.

Ventral Tegmental Transcriptome Response to Intermittent Nicotine Treatment and Withdrawal in BALB/cJ, C57BL/6ByJ, and Quasi-Congenic RQI Mice

The aim of this study was to identify neurochemical pathways and candidate genes involved in adaptation to nicotine treatment and withdrawal. Locomotor sensitization was assessed in a nicotine challenge test after exposure to intermittent nicotine treatment and withdrawal. About 24 h after the challenge test the ventral tegmentum of the mesencephaion was dissected and processed using oligonucleotide microarrays with 22,690 probe sets (Affymetrix 430A 2.0). Quasi-congenic RQI, and donor BALB/cJ mice developed significant locomotor sensitization, while sensitization was not significant in the background partner, C57BL/6By. Comparing saline treated controls of C57BL/6ByJ and BALB/cJ by a rigorous statistical microarray analysis method we identified 238 differentially expressed transcripts. Quasi-congenic strains B6.Cb4i5-alpha4/Vad and B6.Ib5i7-beta25A/Vad significantly differed from the background strain in 11 and 11 transcripts, respectively. Identification of several cis- and trans-regulated genes indicates that further work with quasi-congenic strains can quickly lead to mapping of Quantitative Trait Loci for nicotine susceptibility because donor chromosome regions have been mapped in quasi-congenic strains. Nicotine treatment significantly altered the abundance of 41, 29, 54, and 14 ventral tegmental transcripts in strains C57BL/6ByJ, BALB/cJ, B6.Cb4i5-alpha4/Vad, and B6.Ib5i7-beta25A/Vad, respectively. Although transcript sets overlapped to some extent, each strain showed a distinct profile of nicotine sensitive genes, indicating genetic effects on nicotine-induced gene expression. Nicotine-responsive genes were related to processes including regulation of signal transduction, intracellular protein transport, proteasomal ubiquitin-dependent protein catabolism, and neuropeptide signaling pathway. Our results suggest that while there are common regulatory mechanisms across inbred strains, even relatively small differences in genetic constitution can significantly affect transcriptome response to nicotine.

Sex differences in nicotine levels following repeated intravenous injection in rats are attenuated by gonadectomy

Previous research demonstrates that repeated intravenous (i.v.) nicotine injection resulted in increased locomotor sensitization in female relative to male rats. In order to determine if increased nicotine levels are detected in females compared to males the present experiment examined the plasma nicotine levels of male, female, castrated (CAST), and ovariectomized (OVX) rats (n=7-11 rats/group) following repeated i.v. nicotine injection (50 microg/kg/injection). All rats received 14 i.v. nicotine injections, one/day. Approximately 1 min after the 14th nicotine injection, rats were rapidly decapitated and trunk blood was collected immediately. Gas chromatography revealed a sex difference in nicotine content: higher plasma nicotine levels were measured from female rats (>10 x increase) relative to males, and the sex difference was attenuated by gonadectomy. These data suggest that the sex difference in plasma nicotine levels is due to alteration in distribution or nicotine metabolism as a function of circulating gonadal hormones. These findings indicate that gonadal hormones may influence nicotine pharmacokinetics and therefore nicotine-induced sex differences in behavior.

Dopamine D3receptor ligands for the treatment of tobacco dependence

This review considers the potential use of the dopamine D(3) receptor (DRD3) as a novel therapeutic target for the treatment of tobacco dependence. Among the 5 dopamine receptors identified, the DRD3 is located in the nucleus accumbens, ventral tegmental area and amygdala: 3 brain structures that are implicated in the motivational control of drug-seeking behaviour and drug-conditioning processes. Although it has been proposed that modulating dopamine transmission would be effective in the treatment of drug dependence, no validation has been provided in humans so far. Several highly selective DRD3 ligands have recently been evaluated in preclinical models of drug dependence. These ligands act as DRD3 antagonists in vivo and are able to decrease the motivation to take various drugs of abuse and reduce the influence of associated drug-conditioned behaviour. Of note is that these effects have been found with nicotine-seeking behaviour and nicotine relapse in rodents, suggesting a potential use of these ligands for the treatment of tobacco smokers. In contrast to nicotine replacement therapy, varenicline and bupropion (which are currently used for the treatment of smokers), DRD3 antagonists do not seem to produce nicotine-like effects in experimental animals and, therefore, may not substitute for nicotine or alleviate nicotine withdrawal symptoms in human smokers. This behavioural profile, which was also reported recently with cannabinoid CB(1) receptor antagonists, may result from effects on specific brain pathways that express DRD3 receptors and are involved in relapse and conditioning processes. These preclinical studies suggest that the clinical evaluation of DRD3 ligands should be performed with clinical trials designed specifically to evaluate the relapse phenomena.

The dopamine D3 receptor plays an essential role in alcohol‐seeking and relapse

Our study aimed to identify new candidate genes, which might be involved in alcohol craving and relapse. To find changes in gene expression after long-term alcohol consumption, we studied gene expression profiles in the striatal dopamine system by using DNA microarrays of two different alcohol-preferring rat lines (HAD and P). Our data revealed an up-regulation of the dopamine D3 receptor (D3R) after 1 yr of voluntary alcohol consumption in the striatum of alcohol preferring rats that was confirmed by qRT-polymerase chain reaction. This finding was further supported by the finding of up-regulated striatal D3R mRNA in nonselected Wistar rats after long-term alcohol consumption when compared with age-matched control animals. We further examined the role of the D3R in mediating alcohol relapse behavior using the alcohol deprivation effect (ADE) model in long-term alcohol drinking Wistar rats and the model of cue-induced reinstatement of alcohol-seeking behavior using the selective D3R antagonist SB-277011-A (0, 1, 3, and 10 mg/kg) and the partial agonist BP 897 (0, 0.1, 1, and 3 mg/kg). Both treatments caused a dose-dependent reduction of relapse-like drinking in the ADE model as well as a decrease in cue-induced ethanol-seeking behavior. We conclude that long-term alcohol consumption leads to an up-regulation of the dopamine D3R that may contribute to alcohol-seeking and relapse. We therefore suggest that selective antagonists of this pharmacological target provide a specific treatment approach to reduce alcohol craving and relapse behavior.

The selective dopamine D3 receptor antagonist SB-277011A reduces nicotine-enhanced brain reward and nicotine-paired environmental cue functions

Increasing evidence suggests that enhanced dopamine (DA) neurotransmission in the nucleus accumbens (NAc) may play a role in mediating the reward and reinforcement produced by addictive drugs and in the attentional processing of drug-associated environmental cues. The meso-accumbens DA system is selectively enriched with DA D3 receptors, a DA receptor subtype increasingly implicated in reward-related brain and behavioural processes. From a variety of evidence, it has been suggested that selective DA D3 receptor antagonism may be a useful pharmacotherapeutic approach for treating addiction. The present experiments tested the efficacy of SB-277011A, a selective DA D3 receptor antagonist, in rat models of nicotine-enhanced electrical brain-stimulation reward (BSR), nicotine-induced conditioned locomotor activity (LMA), and nicotine-induced conditioned place preference (CPP). Nicotine was given subcutaneously within the dose range of 0.25-0.6 mg/kg (nicotine-free base). SB-277011A, given intraperitoneally within the dose range of 1-12 mg/kg, dose-dependently reduced nicotine-enhanced BSR, nicotine-induced conditioned LMA, and nicotine-induced CPP. The results suggest that selective D3 receptor antagonism constitutes a new and promising pharmacotherapeutic approach to the treatment of nicotine dependence.

Selective dopamine D3 receptor antagonists enhance cortical acetylcholine levels measured with high-performance liquid chromatography/tandem mass spectrometry without anti-cholinesterases

The present study compared the effects of two selective dopamine (DA) D(3) receptor antagonists, SB-277011A (3, 10 and 30 mg/kg i.p.) and SB-414796A (3, 10 and 30 mg/kg i.p.) on extracellular levels of acetylcholine (ACh) in the rat medial prefrontal cortex (mPFC) by using a LC/MS-MS analytical method that permitted the detection of ACh without the necessity of adding acetylcholinesterase inhibitors to the perfusate. Furthermore, the present LC/MS-MS method permitted the simultaneous measurement of the respective concentrations of SB-277011A and SB-414796A in the same extracellular samples from the mPFC. The systemic administration of both selective DA D(3) receptor antagonists produced a significant increase in extracellular levels of Ach compared to vehicle-treated animals, which was associated with increases in extracellular concentrations of SB-277011A and SB-414796. Overall, the present findings further strengthen the likelihood of a modulation of cortical cholinergic function through a DA D(3)-mediated mechanism and suggest that selective DA D(3) receptor antagonism may be beneficial in the treatment of psychiatric diseases, such as schizophrenia, which are characterized by cognitive dysfunction.

Neuropharmacology and potential efficacy of new treatments for tobacco dependence

This review considers some of the novel therapies that are under development for the treatment of tobacco dependence, outlines their efficacy in clinical studies and explains their mechanisms of action in terms of contemporary theories for the psychobiology of the dependence. It focuses on three treatments with differing mechanisms of action that are at different stages of clinical development. The first is varenicline, a partial agonist at the alpha4beta2 nicotinic receptors, which are thought to play a central role in the addiction to nicotine. Preclinically, this drug mimics the effects of nicotine on dopamine (DA) release in the nucleus accumbens when given alone but attenuates this response to a subsequent nicotine challenge and reduces nicotine self administration. Very encouraging results have been seen in the five clinical studies that have been reported with this drug. The second compound, rimonabant, is a cannabinoid CB1 receptor antagonist. Preclinically, this compound reduces nicotine self administration, DA turnover in nucleus accumbens and attenuates reinstatement of nicotine-seeking behaviour. Clinically, the drug is well tolerated but its effects on smoking cessation are equivocal. However, it has the valuable additional property of inhibiting post-cessation weight gain. Nicotine 'vaccines' are the final group of treatments considered, which involves raising antibodies in the blood that limit the amount of nicotine that penetrates into the brain, thereby reducing the psychopharmacological responses to the drug. The vaccines also reduce DA turnover in nucleus accumbens and reinstatement of nicotine-seeking behaviour after nicotine readministration. The three vaccines discussed are well tolerated and show signs of good efficacy; however, the increase in antibody titre, evoked by the treatment, shows significant inter-individual variation and is generally short lived. Thus, although this approach may provide a valuable aid to smoking cessation, it seems unlikely that it can be used for primary prevention.

The dopamine D3 receptor is part of a homeostatic pathway regulating ethanol consumption

We recently identified a homeostatic pathway that inhibits ethanol intake. This protective pathway consists of the scaffolding protein RACK1 and brain-derived neurotrophic factor (BDNF). RACK1 translocates to the nucleus after exposure of neurons to ethanol and increases expression of BDNF (McGough et al., 2004). We also found that increasing the levels of BDNF via systemic administration of RACK1 expressed as a Tat-fusion protein (Tat-RACK1) reduces ethanol consumption, whereas reduction of BDNF levels augments this behavior (McGough et al., 2004). Based on these results, we hypothesized that activation of the BDNF receptor TrkB is necessary for the effects of BDNF on ethanol intake and that gene products downstream of BDNF negatively regulate ethanol consumption. Here, we show that inhibition of the BDNF receptor TrkB increases voluntary ethanol consumption in wild-type mice but not in mice lacking one copy of the BDNF gene (BDNF(+/-)). We also find that increases in the levels of BDNF, mediated by ethanol or RACK1, lead to increased dorsal striatal levels of the dopamine D3 receptor (D3R), a gene downstream of BDNF, via activation of the TrkB receptor. Finally, we show that the Tat-RACK1-mediated reduction of ethanol consumption is attenuated by coinjection with either the Trk inhibitor K252a or the dopamine D3R-prefering antagonist U-99194A [5, 6-dimethoxy-2-(di-n-propylamino)indan], suggesting that activation of the BDNF pathway via RACK1 leads to increased expression of the dopamine D3R, which in turn mediates the attenuation of ethanol consumption.

Nicotine as a typical drug of abuse in experimental animals and humans

RATIONALE AND BACKGROUND

Tobacco use through cigarette smoking is the leading preventable cause of death in the developed world. Nicotine, a psychoactive component of tobacco, appears to play a major role in tobacco dependence, but reinforcing effects of nicotine often are difficult to demonstrate directly in controlled laboratory studies with animal or human subjects.

OBJECTIVE

To review the major findings obtained with various procedures developed to study dependence-related behavioral effects of nicotine in experimental animals and humans, i.e., drug self-administration, conditioned place preference, subjective reports of nicotine effects and nicotine discrimination, withdrawal signs, and ratings of drug withdrawal.

RESULTS

Nicotine can function as an effective reinforcer of drug-seeking and drug-taking behavior both in experimental animals and humans under appropriate conditions. Interruption of chronic nicotine exposure produces withdrawal symptoms that may contribute to relapse. Difficulties encountered in demonstrating reinforcing effects of nicotine under some conditions, relative to other drugs of abuse, may be due to weaker primary reinforcing effects of nicotine or to a more critical contribution of environmental stimuli to the maintenance of drug-seeking and drug-taking behavior with nicotine than with other drugs of abuse. Further experiments are also needed to delineate the role other chemical substances inhaled along with nicotine in tobacco smoke play in sustaining smoking behavior.

CONCLUSION

Nicotine acts as a typical drug of abuse in experimental animals and humans.

Effect of quinpirole on striatal dopamine release and locomotor activity in nicotine-treated mice

The effect of chronic oral nicotine treatment which in its intermittent delivery resembles human smoking was studied on the sensitivity of dopamine autoreceptors in mice. On the 50th day of nicotine administration in the drinking water or after 23-25 h withdrawal quinpirole (D2/D3 agonist, 0.01-0.1 mg/kg s.c.) was given, and accumbal and dorsal striatal dopamine outflow, locomotor activity and body temperature were measured. Dorsal striatal extracellular dopamine concentration and locomotor activity were found to be elevated during nicotine administration. Chronic nicotine did not alter the effects of small, autoreceptor preferring doses of quinpirole on accumbal or dorsal striatal dopamine, locomotor activity or body temperature. However, quinpirole's locomotor activity reducing effect was slightly diminished in mice treated repeatedly with nicotine (0.4 mg/kg twice daily for 10 days s.c.). Thus, although repeated nicotine treatment for 5-14 days decreases dopamine autoreceptor sensitivity, after long-term oral nicotine treatment such a decrease is not seen. Thus, the changes occurring in the sensitivity of D2-like dopamine receptors probably play a minor role in regulating the dopaminergic transmission during long-term nicotine administration.

Nicotine-induced sensitization in mice: changes in locomotor activity and mesencephalic gene expression

It is believed that drug-induced behavioral sensitization is an important process in the development of substance dependence. In order to explore mechanisms of sensitization, a mouse model of nicotine-induced locomotor sensitization was established, and effects of the sensitization process on mesencepahlic gene expression were examined. A schedule, which included 3 weeks of intermittent nicotine exposure (0.5 mg/kg, s.c.) and 3 weeks of withdrawal, resulted in locomotor sensitization. Effects of sensitization on mesencephalic expression of approximately 14,000 genes were assessed using oligonucleotide microarrays. Signal intensity differences in samples obtained from repeated nicotine- and saline-exposed animals were analyzed with z-test after False Discovery Rate (FDR) multiple test correction. Genes related to GABA-A receptors and protein phosphatases were among 68 genes showing significantly different expression levels between the saline and the nicotine groups. We hypothesize that some of the gene expression changes in the mesencephalon are involved in pathways leading to nicotine-induced sensitization. Down-regulation of GABA-A receptors induced by repeated nicotine exposure may facilitate dopaminergic neuronal transmission and may contribute to increased locomotor activity.

Ethanol does not affect discriminative-stimulus effects of nicotine in rats

The effects of ethanol were evaluated in rats trained to discriminate 0.4 mg/kg of nicotine from saline under a fixed-ratio 10 schedule of food delivery. Ethanol (0.1-1 g/kg, i.p.) did not produce any nicotine-like discriminative effects and did not produce any shift in the dose-response curve for nicotine discrimination. Thus, the ability to discriminate nicotine's effects does not appear to be altered by ethanol administration. However, the high dose of 1 g/kg ethanol, given either alone or in combination with nicotine, markedly depressed food-maintained responding. This later effect was associated in some rats with an attenuation of the discriminative-stimulus effects of the training dose of nicotine. This suggests that previous reports of increased tobacco smoking following ethanol consumption in humans are connected, in some way, with an increase in motivation to consume nicotine that is produced by ethanol, rather than with a decrease in the subjective response to nicotine.

The role of central dopamine D3 receptors in drug addiction: a review of pharmacological evidence

The cDNA for the dopamine D3 receptor was isolated and characterized in 1990. Subsequent studies have indicated that D3 receptors, as well as D3 receptor mRNA, are primarily localized in limbic regions in mammals. This finding led to the postulate that D3 receptors may be involved in drug dependence and addiction. However, this hypothesis has been difficult to test due to the lack of compounds with high selectivity for central D3 receptors. The interpretation of results from studies using mixed D2/D3 agonists and/or antagonists is problematic because these agents have low selectivity for D3 over D2 receptors and it is likely that their actions are primarily related to D2 receptor antagonism and possibly interaction with other neurotransmitter receptors. Currently, with the synthesis and characterization of new highly selective D3 receptor antagonists such as SB-277011-A this difficulty has been surmounted. The purpose of the present article is to review, for the first time, the effects of various putative D3 receptor selective compounds in animal models of drug dependence and addiction. The results obtained with highly selective D3 receptor antagonists such as SB-277011-A, SB-414796, and NGB-2904 indicate that central D3 receptors may play an important role in drug-induced reward, drug-taking, and cue-, drug-, and stress-induced reinstatement of drug-seeking behavior. Provided these results can be extrapolated to human drug addicts, they suggest that selective DA D3 receptor antagonists may prove effective as potential pharmacotherapeutic agents to manage drug dependence and addiction.

Control of the reinforcing effects of nicotine by associated environmental stimuli in animals and humans

Tobacco dependence through cigarette smoking is the leading preventable cause of death in the world and kills nearly 4 million people annually. Nicotine, a psychoactive component of tobacco, is thought to have a major role in tobacco dependence by acting directly as a reinforcer of drug-seeking and drug-taking behavior. However, recent findings obtained with two procedures that are used widely to assess reinforcing effects of drugs in experimental animals, intravenous drug self-administration and conditioned place-preference procedures, demonstrate that environmental factors have a major influence on the reinforcing effects of nicotine. Under some experimental conditions, nicotine is also self-administered reliably by humans. Environmental stimuli that have been associated previously with the self-administration of nicotine can reinstate extinguished drug-seeking behavior in animals and precipitate relapse to smoking behavior in ex-smokers. Innovative medications that target cannabinoid CB(1) and dopamine D(3) receptors and might block specifically the influence of such conditioned environmental stimuli in smokers are in development.

Neurochemical alterations produced by daily nicotine exposure in periadolescent vs. adult male rats

Chronic treatment with nicotine differentially alters behavior in adolescent rats compared to adult rats. It is not known, however, whether the effects of nicotine on the neurochemical pathways with which it interacts differ in adolescents vs. adults. In the current study, the effects of a 7-day treatment with nicotine on nicotinic, dopaminergic, and serotonergic neurochemistry were examined in the caudate putamen and nucleus accumbens in periadolescent vs. adult male rats. Nicotine treatment increased dopamine transporter densities and decreased serotonin transporter densities in periadolescent rats. There was no change in nicotinic acetylcholine receptor densities or dopamine D1 or D2 receptor densities in nicotine-pretreated periadolescent rats. In adult rats pretreated with nicotine, there was an increase in nicotinic acetylcholine densities, but no change in dopamine transporter, dopamine D1 or D2 receptor, or serotonin transporter densities. Overall, these findings show that periadolescent rats have neurochemical adaptations to nicotine different from adult rats. These alterations may explain, at least in part, the differential behavioral effects of chronic nicotine in adult and adolescent male rats.

Dopamine D3 receptor ligands block nicotine-induced conditioned place preferences through a mechanism that does not involve discriminative-stimulus or antidepressant-like effects

Environmental stimuli previously paired with drug taking appear to play a critical role in nicotine dependence. Converging anatomical, pharmacological, and behavioral evidence implicates dopamine D3 receptors (D3Rs) in the mechanisms underlying stimulus-controlled drug-seeking behavior. This study assessed the effects of BP 897, a D3R partial agonist and ST 198, a D3R antagonist, on nicotine-induced conditioned place preferences (CPPs), used as a measure of drug-seeking behavior, on food-maintained responding and on discrimination performance under a two-lever-choice nicotine discrimination procedure. BP 897 and ST 198 both blocked the expression of nicotine-induced CPP at doses selective for D3R. They had no effect on locomotor activity in the CPP apparatus and no significant effect on nicotine discrimination performance or food-maintained responding under the discrimination procedure. Involvement of antidepressant actions in the effects of BP 897 and ST 198 on CPP is unlikely, since we found no effect of D3R blockade with BP 897 or genetic depletion of D3Rs in a forced swimming test, used as a behavioral test for antidepressant activity. This suggests that D3R ligands reduce the motivational effects of nicotine by a mechanism distinct from those of nicotine replacement therapy and bupropion, the two currently used aids for smoking cessation in humans. These findings support the use of D3R ligands as aids for smoking cessation and indicate that their effects would be selective for those rewarding or reinforcing effects of nicotine that contribute to the maintenance of tobacco-smoking behavior, without affecting subjective responses to nicotine or producing any antidepressant-like effects.

Nicotine induces conditioned place preferences over a large range of doses in rats

RATIONALE

Conditioned place preference (CPP) procedures provide one measure of potential rewarding effects of abused drugs. Many attempts to induce CPP with nicotine have been unsuccessful.

OBJECTIVES

To assess the influence of nicotine dose and stimulus assignment procedure on development of nicotine-induced CPP.

METHODS

Initial preferences for one side of a two-compartment apparatus were first determined in Sprague-Dawley rats. In subsequent conditioning trials, the compartment paired with nicotine was the initially preferred side for half of the rats, and the initially non-preferred side for the other half. Rats received either an injection of nicotine (0.01-2 mg/kg SC) before being placed in one compartment (three trials) or saline before being placed in the other compartment (three trials). Control rats had saline injections associated with both compartments. A final test trial with no injection assessed final place preference.

RESULTS

Significant CPP were induced by 0.1-1.4 mg/kg doses of nicotine. Nicotine-induced CPP were only apparent when nicotine was paired with the initially non-preferred side. Moreover, a very high dose of nicotine (2 mg/kg) induced conditioned place aversion when paired with the initially preferred side of the apparatus.

CONCLUSIONS

Nicotine induced significant CPP across a wide range of doses, in accordance with its role as the primary addictive component of tobacco. Small preferences for one side of the apparatus played a major role in the development of nicotine-induced CPP. These findings suggest that biased procedures may be more suitable than unbiased procedures for evaluation of rewarding effects of nicotine using CPP paradigms.

Repeated nicotine treatment in rats with high versus low rearing activity: analyses of behavioural sensitisation and place preference

RATIONALE

Repeated treatment with the cholinergic agonist nicotine can sensitise rats to its psychomotor stimulant effects, which is largely due to changes within the mesolimbic and mesostriatal dopamine system. Since this brain system also plays a critical role in motivational processes, changes of motivational functions may also be expected with repeated nicotine experiences.

OBJECTIVE

Our previous work has shown that normal male Wistar rats can differ systematically with respect to rearing activity in a novel open field: animals with high rearing activity (HRA) differed from those with low rearing activity (LRA) with respect to dopaminergic and cholinergic brain activity. In this study, we asked whether HRA and LRA rats might respond differentially to repeated nicotine treatment, which we tested in terms of behavioural sensitisation and place preference.

METHODS

Nicotine hydrogen tartrate (0.4 mg/kg) or saline was administered on eight alternate days (drug treatment). After each injection, the rats had access to one specific quadrant of a circular unbiased place preference apparatus. Sensitisation to nicotine was assessed by measuring locomotion and rearing during drug treatment. On the days after each drug treatment, rats had free access to the entire apparatus without prior drug treatment. Here, we tested for preference for the previously drug-paired quadrant. One week after this procedure, all animals were tested again for sensitisation and place preference after injection of saline or nicotine.

RESULTS

Overall, sensitisation occurred earlier during locomotor than rearing activity. Both, HRA and LRA rats treated with nicotine showed sensitisation, but with different profiles. Rearing sensitised earlier in HRA than LRA rats, and a sensitised locomotor response to nicotine was observed only in HRA rats when compared with baseline. When re-tested again 1 week later, expression of sensitisation to nicotine was detected in rearing and locomotor activity in both HRA and in LRA rats. In the place preference tests, nicotine-treated and saline-treated rats spent more time in the treatment quadrant, but nicotine did not lead to place preference compared to saline. Furthermore, there was no substantial evidence that nicotine might lead to place preference in only HRA or LRA rats. However, we obtained other evidence that HRA versus LRA rats responded differently to the procedure of place preference testing.

CONCLUSIONS

These data supplement previous findings that different levels of psychomotor activity can affect the reactivity to psychostimulant drugs and add new evidence with respect to nicotine.

A single cocaine exposure increases BDNF and D3 receptor expression: implications for drug-conditioning

Environmental stimuli associated with a single cocaine exposure acquire long-lasting motivational properties that are able to induce relapse. We measured Brain-derived neurotrophic factor (BDNF) and dopamine D3 receptor (Drd3) expressions in rat brain regions that have been involved in drug-conditioning. Acute cocaine produced a transient increase in BDNF mRNA in the prefrontal cortex, associated with a long-lasting increase in drd3 mRNA, and a delayed and long-lasting increase in Drd3 protein in the nucleus accumbens. Methamphetamine and morphine, two drugs known to easily induce drug-conditioning, also markedly elevated BDNF mRNA. Nicotine had more limited effects. Abused drugs increase acutely BDNF expression, which leads to subsequent long-lasting elevation of Drd3 in the nucleus accumbens that may facilitate responding to drug-associated stimuli.