Nicotine-induced enhancement of responding for conditioned reinforcement in rats: Role of prior nicotine exposure and α4β2 nicotinic receptors

Rationale

Stimuli associated with nicotine can become motivationally significant and may play a role in tobacco dependence. Previous work indicates that nicotine enhances responding for a conditioned reinforcer (CR).

Objectives

These studies examined the effects of prior exposure to nicotine on responding for a CR, persistence of this response, and the role of α4β2 or α7 nicotinic receptor subtypes.

Methods

Water deprived rats were given 13 Pavlovian conditioning sessions where a light/tone conditioned stimulus (CS) was paired with the delivery of water. Then, rats were presented with two levers: one delivered the CS (now a CR), the other was inactive. Experiments examined the effect of nicotine administered prior to Pavlovian conditioning sessions on approach behavior during CS presentations, operant responding for CR in the presence and absence of nicotine, and the persistence of responding for CR. The effects of nicotinic acetylcholine receptor (nAChR) antagonism with mecamylamine and α4β2 or α7 nAChR antagonism with dihydro-beta-erythroidine (DHβE) or methyllycaconitine (MLA) on nicotine-enhanced responding for CR were examined.

Results

Nicotine enhanced approach behavior during CS presentations and potentiated operant responding for CR, an effect sensitized as a result of nicotine exposure during conditioning. Responding for CR and its potentiation by nicotine was stable over multiple tests. Enhanced responding for the CR induced by nicotine was blocked by mecamylamine and DHβE, but not MLA.

Conclusions

Nicotine enhances Pavlovian discriminated approach and shows sensitized nicotine-induced enhancements in responding for CR, an effect depending on α4β2 nAChRs.     

Role of α5* nicotinic acetylcholine receptors in the effects of acute and chronic nicotine treatment on brain reward function in mice

Objective

Allelic variation in the α5 nicotinic acetylcholine receptor (nAChR) subunit gene, CHRNA5, increases vulnerability to tobacco addiction. Here, we investigated the role of α5* nAChRs in the effects of nicotine on brain reward systems.

Materials and methods

Effects of acute (0.03125–0.5 mg/kg SC) or chronic (24 mg/kg per day; osmotic minipump) nicotine and mecamylamine-precipitated withdrawal on intracranial self-stimulation (ICSS) thresholds were assessed in wild-type and α5 nAChR subunit knockout mice. Noxious effects of nicotine were further investigated using a conditioned taste aversion procedure.

Results

Lower nicotine doses (0.03125–0.125 mg/kg) decreased ICSS thresholds in wild-type and α5 knockout mice. At higher doses (0.25–0.5 mg/kg), threshold-lowering effects of nicotine were diminished in wild-type mice, whereas nicotine lowered thresholds across all doses tested in α5 knockout mice. Nicotine (1.5 mg/kg) conditioned a taste aversion to saccharine equally in both genotypes. Mecamylamine (5 mg/kg) elevated ICSS thresholds by a similar magnitude in wild-type and α5 knockout mice prepared with minipumps delivering nicotine. Unexpectedly, mecamylamine also elevated thresholds in saline-treated α5 knockout mice.

Conclusion

α5* nAChRs are not involved in reward-enhancing effects of lower nicotine doses, the reward-inhibiting effects of nicotine withdrawal, or the general noxious effects of higher nicotine doses. Instead, α5* nAChRs regulate the reward-inhibiting effects nicotine doses that oppose the reward-facilitating effects of the drug. These data suggest that disruption of α5* nAChR signaling greatly expands the range of nicotine doses that facilitate brain reward activity, which may help explain the increased tobacco addiction vulnerability associated with CHRNA5 risk alleles.     

Effects of the specific α4β2 nAChR antagonist, 2-fluoro-3-(4-nitrophenyl) deschloroepibatidine, on nicotine reward-related behaviors in rats and mice

Rationale

Alleviating addiction to tobacco products could prevent millions of deaths. Investigating novel compounds selectively targeting α4β2 nAChRs hypothesized to have a key role in the rewarding effects of nicotine may be a useful approach for future treatment.

Objectives

The present study was designed to evaluate 2-fluoro-3-(4-nitrophenyl) deschloroepibatidine (4-nitro-PFEB), a potent competitive antagonist of neuronal α4β2 nAChRs, in several animal models related to nicotine reward: drug discrimination, intracranial self-stimulation (ICSS), conditioned place preference, and limited access to self-administration.

Methods

Long Evans rats were trained in a two-lever discrimination procedure to discriminate 0.4 mg/kg nicotine (s.c.) from saline. Male Sprague–Dawley rats were stereotaxically implanted with electrodes and trained to respond for direct electrical stimulation of the medial forebrain bundle. ICR mice were evaluated using an unbiased place preference paradigm, and finally, male Wistar rats were implanted with intrajugular catheters and tested for nicotine self-administration under limited access (1 h/day).

Results

4-Nitro-PFEB attenuated the discriminative stimulus effects of nicotine, but alone did not produce nicotine-like discriminative stimulus effects. Nicotine-induced facilitation of ICSS reward thresholds was reversed by 4-nitro-PFEB, which alone had no effect on thresholds. 4-Nitro-PFEB also blocked the conditioned place preference produced by nicotine, but alone had no effect on conditioned place preference. Finally, 4-nitro-PFEB dose-dependently decreased nicotine self-administration.

Conclusions

These results support the hypothesis that neuronal α4β2 nAChRs play a key role in mediating the rewarding effects of nicotine and further suggest that targeting α4β2 nAChRs may yield a potential candidate for the treatment of nicotine dependence.     

Substituted heteroaromatic compounds: effect on nicotine self-administration in rats

Rationale

Certain compounds that nonselectively inhibit a prominent human nicotine-metabolizing enzyme (i.e., human cytochrome P-450 2A6, hCYP 2A6) showed inhibition of smoking in humans. However, a comprehensive examination of hCYP 2A6 inhibitors to decrease nicotine self-administration in rats has not been reported.

Objectives

We tested substituted heteroaromatic compounds designed to selectively inhibit hCYP 2A6 in a model system to (a) examine selective hCYP 2A6 inhibitors to decrease cotinine formation in vivo in rats administered with nicotine and (b) examine their efficacy to decrease nicotine self-administration in rats.

Methods

Rats were trained to IV self-administer nicotine in 1-h sessions. Nicotine self-administration was carried out at a unit dose of 0.03?mg/kg/infusion in 0.1?ml/s. Pretreatment with substituted heteroaromatic test compounds (0.5–25?mg/kg, i.p., 30?min prior to nicotine self-administration sessions) resulted in dose-dependent decreases of nicotine self-administration. Using operant conditioning techniques, nicotine- vs. food-reinforced responding was evaluated for compounds 10 and 11.

Results

Compounds 10 and 11 selectively decreased nicotine self-administration with estimated ED₅₀ values 4 and 2.8?mg/kg, respectively. Of the test compounds examined, none showed significant affinity for mammalian α4β2- or α7-neuronal nicotinic acetylcholine (nAChR) receptors and none were inhibitors of the human dopamine transporter (hDAT); thus, neither the endogenous nAChRs nor DAT apparently plays a role in decreasing nicotine self-administration for this series of compounds.

Conclusion

The results indicate that chemical analogs of nicotine can play a role in nicotine self-administration harm reduction but a non-nAChR and a non-hDAT mechanism are likely involved.     

Positive allosteric modulation of α4β2 nicotinic acetylcholine receptors as a new approach to smoking reduction: evidence from a rat model of nicotine self-administration

Rationale

The α4β2 subtype of nicotinic acetylcholine receptors (nAChRs) plays a central role in the mediation of nicotine reinforcement. Positive allosteric modulators (PAMs) at α4β2 nAChRs facilitate the intrinsic efficiency of these receptors, although they do not directly activate the receptors. α4β2 PAMs are hypothesized to reduce nicotine self-administration in subjects engaged in routine nicotine consumption. The present study tested this hypothesis using a rat model of nicotine self-administration.

Methods

Male Sprague–Dawley rats were trained in daily 1-h sessions to intravenously self-administer nicotine (0.03 mg/kg per infusion, free base) on a fixed-ratio 5 schedule. The effects of the α4β2 PAM desformylflustrabromine (dFBr), α4β2 agonist 5-iodo-A-85380, and acetylcholinesterase inhibitor galantamine on nicotine intake were examined. The ability of dFBr and 5-iodo-A-85380 to substitute for nicotine was also assessed.

Results

dFBr and 5-iodo-A-85380 dose-dependently reduced nicotine self-administration without changing lever responses for food. Galantamine decreased the self-administration of nicotine and food at high doses. Unlike 5-iodo-A-85380, dFBr failed to substitute for nicotine in supporting self-administration behavior.

Conclusions

These results demonstrated the effectiveness of dFBr in reducing nicotine intake and the inability of dFBr to support self-administration behavior. These findings suggest that positive allosteric modulation of α4β2 nAChRs may be a promising target for the treatment of nicotine addiction. Moreover, α4β2 PAMs, in contrast to agonist medications, may have clinical advantages because they may have little liability for abuse because of their lack of reinforcing actions on their own.     

Mecamylamine elicits withdrawal-like signs in rats following a single dose of nicotine

Rationale

The ability of nicotine to induce dependence (result in a withdrawal syndrome) is typically thought to require long-term, daily smoking. Emerging evidence suggests that symptoms of nicotine withdrawal may occur following only a few cigarettes. Whether acute exposure to nicotine can induce dependence in animals has not been well established.

Objective

The objective of this paper is to examine whether the nicotinic acetylcholine receptor antagonist mecamylamine elicits withdrawal-like signs in rats following acute nicotine exposure.

Methods and Results

Mecamylamine (3.0 mg/kg, s.c.) administered ≈2 h after a single dose of nicotine (0.5 mg/kg, s.c.) elicited increases in intracranial self-stimulation (ICSS) thresholds and somatic signs, two well-established effects of withdrawal from long-term (chronic) nicotine exposure. The magnitude of these effects remained constant across five daily test sessions. A lower dose of mecamylamine (1.5 mg/kg, s.c.) had little or no effect on ICSS thresholds or somatic signs following acute nicotine exposure, but precipitated robust increases in these measures during a chronic nicotine infusion. Finally, rats exhibited a small increase in ICSS thresholds over time following a single nicotine injection (0.5 mg/kg, s.c.), possibly reflecting a modest spontaneous withdrawal-like effect.

Conclusions

Mecamylamine elicited withdrawal-like signs in rats following a single dose of nicotine. The different effects of mecamylamine 1.5 mg/kg following acute versus chronic nicotine exposure supports the notion that these models simulate the early and more advanced stages of nicotine dependence, respectively. While further optimization and validation of these models is necessary, they may provide a novel approach for studying the earliest stages of nicotine dependence.     

Recent advances in understanding nicotinic receptor signaling mechanisms that regulate drug self-administration behavior

Tobacco smoking is one of the leading causes of disease and premature death in the United States. Nicotine is considered the major reinforcing component in tobacco smoke responsible for tobacco addiction. Nicotine acts in the brain through the neuronal nicotinic acetylcholine receptors (nAChRs). The predominant nAChR subtypes in mammalian brain are those containing α4 and β2 subunits. The α4β2 nAChRs, particularly those located in the mesoaccumbens dopamine pathway, play a key role in regulating the reinforcing properties of nicotine. Considering that twelve mammalian nAChR subunits have been cloned, it is likely that nAChRs containing subunits in addition to, or other than, α4 and β2 also play a role in the tobacco smoking habit. Consistent with this possibility, human genome-wide association studies have shown that genetic variation in the CHRNA5–CHRNA3–CHRNB4 gene cluster located in chromosome region 15q25, which encode the α5, α3 and β4 nAChR subunits, respectively, increases vulnerability to tobacco addiction and smoking-related diseases. Most recently, α5-containing nAChRs located in the habenulo-interpeduncular tract were shown to limit intravenous nicotine self-administration behavior in rats and mice, suggesting that deficits in α5-containing nAChR signaling in the habenulo-interpeduncular tract increases vulnerability to the motivational properties of nicotine. Finally, evidence suggests that nAChRs may also play a prominent role in controlling consumption of addictive drugs other than nicotine, including cocaine, alcohol, opiates and cannabinoids. The aim of the present review is to discuss recent preclinical findings concerning the identity of the nAChR subtypes that regulate self-administration of nicotine and other drugs of abuse.     

Effects of smoking on the acoustic startle response and prepulse inhibition in smokers with and without posttraumatic stress disorder

Rationale

Nicotinic acetylcholine receptors (nAChRs) are a critical component of the cholinergic system of neurotransmission in the brain that modulates important physiological processes such as reward, cognition, and mood. Abnormalities in this system are accordingly implicated in multiple psychiatric illnesses, including addiction, schizophrenia, and mood disorders. There is significantly increased tobacco use, and therefore nicotine intake, in patient populations, and pharmacological agents that act on various nicotinic receptor subtypes ameliorate clinical features of these disorders. Better understanding of the molecular mechanisms underlying cholinergic dysfunction in psychiatric disease will permit more targeted design of novel therapeutic agents.

Results

The objective of this review is to describe the multiple cellular pathways through which chronic nicotine exposure regulates nAChR expression, and to juxtapose these mechanisms with evidence for altered expression of high-affinity nAChRs in human psychiatric illness. Here, we summarize multiple studies from pre-clinical animal models to human in vivo imaging and post-mortem experiments demonstrating changes in nAChR regulation and expression in psychiatric illness.

Conclusions

We conclude that a mechanistic explanation of nAChR abnormalities in psychiatric illness will arise from a fuller understanding of normal nAChR trafficking, along with the detailed study of human tissue, perhaps using novel biotechnological advances, such as induced pluripotent stem cells.     

Modulation of ethanol consumption by genetic and pharmacological manipulation of nicotinic acetylcholine receptors in mice

Rationale

Alcohol and nicotine are commonly co-abused. Genetic correlations between responses to these drugs have been reported, providing evidence that common genes underlie the response to alcohol and nicotine. Nicotinic acetylcholine receptors (nAChRs) in the mesolimbic dopamine system are important in mediating nicotine response, and several studies suggest that alcohol may also interact with these nAChRs.

Objective

The aim of this study was to examine the role of nAChRs containing α7 or β2 subunits in ethanol consumption.

Methods

A two-bottle choice paradigm was used to determine ethanol consumption in wild-type and nAChR subunit knockout mice. Challenge studies were performed using the α4β2 nAChR partial agonist varenicline.

Results

Mice lacking the β2 subunit consumed a similar amount of ethanol compared to their wild-type siblings in an ethanol-drinking paradigm. In contrast, mice lacking the α7 nAChR receptor subunit consumed significantly less ethanol than wild-type mice but consumed comparable amounts of water, saccharin, and quinine. In C57BL/6J mice, varenicline dose-dependently decreased ethanol consumption with a significant effect of 2 mg/kg, without affecting water or saccharin consumption. This effect of varenicline was not reversed in mice lacking either the α7 or β2 subunit, providing evidence that nAChRs containing one of these subunits are not required for this effect of varenicline.

Conclusions

This study provides evidence that α7 nAChRs are involved in ethanol consumption and supports the idea that pharmacological manipulation of nAChRs reduces ethanol intake. Additional nAChRs may also be involved in ethanol intake, and there may be functional redundancy in the nicotinic control of alcohol drinking.     

Ventral tegmental area α6β2 nicotinic acetylcholine receptors modulate phasic dopamine release in the nucleus accumbens core

Rationale

Phasic dopamine (DA) signaling underlies reward learning. Cholinergic and glutamatergic inputs into the ventral tegmental area (VTA) are crucial for modulating burst firing activity and subsequent phasic DA release in the nucleus accumbens (NAc), but the specific VTA nicotinic receptor subtypes that regulate phasic DA release have not been identified.

Objective

The goal was to determine the role of VTA N-methyl-d-aspartate receptors (NMDARs) and specific subtypes of nicotinic acetylcholine receptors (nAChRs) in regulating phasic DA release in the NAc core.

Methods

Fast-scan cyclic voltammetry in anesthetized rats was combined with intra-VTA micro-infusion to evaluate the ability of glutamatergic and cholinergic drugs to modulate stimulated phasic DA release in the NAc core.

Results

VTA NMDAR blockade with AP-5 decreased, while VTA NMDAR activation with NMDA increased NAc peak phasic DA release. Intra-VTA administration of the nonspecific nAChR antagonist mecamylamine produced a persistent decrease in phasic DA release. Infusion of the α6-selective antagonist α-conotoxin MII (α-ctx MII) produced a robust, but transient decrease in phasic DA, whereas infusion of selective doses of either the α4β2-selective antagonist, dihydro-beta-erythroidine, or the α7 antagonist, methyllycaconitine, had no effect. Co-infusion of AP-5 and α-ctx MII produced a similar phasic DA decrease as either drug alone, with no additive effect.

Conclusions

The results suggest that VTA α6β2 nAChRs, but not α4β2 or α7 nAChRs, regulate phasic DA release in the NAc core and that VTA α6β2 nAChRs and NMDA receptors act at a common site or target to regulate NAc phasic DA signaling.     

Role of nicotinic acetylcholine receptors in the effects of cocaine-paired contextual stimuli on impulsive decision making in rats

Rationale

Chronic cocaine exposure produces unconditioned enhancement in impulsive decision making; however, little is known about the effects of cocaine-paired conditioned stimuli on this behavior. Thus, this study explored the effects of cocaine-paired contextual stimuli on impulsive decision making and the contribution of nicotinic acetylcholine receptors (nAChRs) to this phenomenon.

Methods

Rats were trained to achieve stable performance on a delay discounting task, which involved lever press-based choice between a single food pellet (small reward) available immediately and three food pellets (large reward) available after a 10-, 20-, 40-, or 60-s time delay. Rats then received Pavlovian context-cocaine (15?mg/kg, i.p.) and context-saline (1?ml/kg, i.p.) pairings in two other, distinct contexts. Subsequently, delay discounting task performance was assessed in the previously cocaine-paired or saline-paired context following pretreatment with saline or cocaine (15?mg/kg, Experiment 1) or with saline or the nAChR antagonist, mecamylamine (0.2 and 2?mg/kg, Experiment 2), using counterbalanced within-subjects testing designs.

Results

Independent of cocaine pretreatment, rats exhibited greater decrease in preference for the large reward as a function of delay duration in the cocaine-paired context, relative to the saline-paired context. Furthermore, systemic mecamylamine pretreatment dose-dependently attenuated the decrease in preference for the large reward in the cocaine-paired context, but not in the saline-paired context, as compared to saline.

Conclusion

Cocaine-paired contextual stimuli evoke a state of impulsive decision making, which requires nAChR stimulation. Drug context-induced impulsivity likely increases the propensity for drug relapse in cocaine users, making the nAChR an interesting target for drug relapse prevention.     

The effects of chronic versus acute desipramine on nicotine withdrawal and nicotine self-administration in the rat

Rationale

Nicotine withdrawal is characterized by depression-like symptomatology that may be mediated by dysregulations in norepinephrine transmission. These aversive aspects of nicotine withdrawal and the rewarding effects of nicotine play major roles in maintaining nicotine dependence.

Objectives

The aim of this work was to evaluate the effects of desipramine (DMI), a preferential norepinephrine reuptake inhibitor and antidepressant, on preclinical models of nicotine dependence in rats.

Materials and methods

A rate-independent current-intensity discrete-trial threshold intracranial self-stimulation procedure was used to assess brain reward function during nicotine withdrawal induced by cessation of nicotine infusion via subcutaneous osmotic mini pumps (3.16 mg/kg/day, base). Nicotine withdrawal was also measured by somatic signs of withdrawal. DMI was administered acutely (2 or 5 mg/kg, salt) during nicotine/saline withdrawal. In other naïve rats, chronic DMI treatment via mini pump (15 mg/kg/day, salt) began after 7 days of nicotine/saline exposure and continued during administration of nicotine/saline for 14 days and during nicotine/saline withdrawal. Additional rats acquired intravenous nicotine- or food-maintained responding, were prepared with DMI/vehicle-containing mini pumps, and self-administered nicotine or food during 12 days of DMI/vehicle exposure.

Results

Acute DMI administration had no effect on threshold elevations observed in nicotine-withdrawing rats. Chronic DMI administration prevented the reward threshold elevations and the increased somatic signs of nicotine withdrawal. Although chronic DMI significantly decreased nicotine self-administration, it also decreased food-maintained responding.

Conclusions

The results suggest that norepinephrine reuptake inhibitors may be effective anti-smoking treatments that reduce the anhedonic depression-like and somatic components of nicotine withdrawal and may alter the rewarding effects of nicotine and food.

     

Preadolescent tobacco smoke exposure leads to acute nicotine dependence but does not affect the rewarding effects of nicotine or nicotine withdrawal in adulthood in rats

Epidemiological studies indicate that parental smoking increases the risk for smoking in children. However, the underlying mechanisms by which parental smoking increases the risk for smoking are not known. The aim of these studies was to investigate if preadolescent tobacco smoke exposure, postnatal days 21-35, affects the rewarding effects of nicotine and nicotine withdrawal in adult rats. The rewarding effects of nicotine were investigated with the conditioned place preference procedure. Nicotine withdrawal was investigated with the conditioned place aversion procedure and intracranial self-stimulation (ICSS). Elevations in brain reward thresholds in the ICSS paradigm reflect a dysphoric state. Plasma nicotine and cotinine levels in the preadolescent rats immediately after smoke exposure were 188 ng/ml and 716 ng/ml, respectively. Preadolescent tobacco smoke exposure led to the development of nicotine dependence as indicated by an increased number of mecamylamine-precipitated somatic withdrawal signs in the preadolescent tobacco smoke exposed rats compared to the control rats. Nicotine induced a similar place preference in adult rats that had been exposed to tobacco smoke or air during preadolescence. Furthermore, mecamylamine induced place aversion in nicotine dependent rats but there was no effect of preadolescent tobacco smoke exposure. Finally, preadolescent tobacco smoke exposure did not affect the elevations in brain reward thresholds associated with precipitated or spontaneous nicotine withdrawal. These studies indicate that passive exposure to tobacco smoke during preadolescence leads to the development of nicotine dependence but preadolescent tobacco smoke exposure does not seem to affect the rewarding effects of nicotine or nicotine withdrawal in adulthood.     

The effects of lobeline on nicotine withdrawal-induced depression-like behavior in mice

Rationale

Evidence suggests that neuronal nicotinic acetylcholine receptor (nAChR) ligand lobeline has antidepressant-like properties.

Objectives

The present study investigated the effects of lobeline on nicotine withdrawal-induced depression-like behavior.

Methods

Adult C57BL/6J mice were exposed to nicotine (200 μg/ml) in drinking solution for 3 weeks. During withdrawal, depression-like behavior was measured by the forced swim test (FST). We also determined norepinephrine (NE) levels in the prefrontal cortex (PFC) and hippocampus during nicotine withdrawal. Furthermore, we determined the effects of repeated treatment with lobeline or a selective α4β2 nAChR ligand 3-(pyridine-3?-yl)-cytisine on brain-derived neurotrophic factor (BDNF) and phosphorylated cAMP-responsive element binding (p-CREB) protein expression in the hippocampus.

Results

Withdrawal from chronic nicotine increased immobility time in the FST, a measure for depression-like behavior. Pretreatment with lobeline significantly decreased immobility time during nicotine withdrawal. In addition, pretreatment with lobeline attenuated nicotine withdrawal-induced increased NE levels in the PFC and hippocampus. Further, repeated treatment with lobeline or 3-(pyridine-3?-yl)-cytisine decreased immobility time in the FST and reduced withdrawal-induced increased BDNF and p-CREB expression in the hippocampus.

Conclusions

Taken together, our results indicate that lobeline attenuated nicotine withdrawal-induced depression-like behavior likely by targeting brain nAChRs, noradrenergic neurotransmission, and/or hippocampal BDNF. Thus, lobeline may have some potential to prevent smoking relapse by counteracting nicotine withdrawal-induced depression in humans.     

Affective and somatic aspects of spontaneous and precipitated nicotine withdrawal in C57BL/6J and BALB/cByJ mice

The aversive aspects of nicotine withdrawal are powerful motivational forces contributing to the tobacco smoking habit. We evaluated measures of affective and somatic aspects of nicotine withdrawal in C57BL/6J and BALB/cByJ mice. Nicotine withdrawal was induced by termination of chronic nicotine delivery through osmotic minipumps or precipitated with the nicotinic acetylcholine receptor (nAChR) antagonists mecamylamine or dihydro-beta-erythroidine (DHbetaE). A rate-independent discrete-trial intracranial self-stimulation threshold procedure was used to assess brain reward function. Anxiety-like behavior and sensorimotor gating were assessed in the light-dark box and prepulse inhibition (PPI) tests, respectively. Acoustic startle response and somatic signs of withdrawal were also evaluated. Spontaneous nicotine withdrawal after 14-day exposure to 10-40 mg/kg/day nicotine induced no alterations in anxiety-like behavior, startle reactivity, PPI, or somatic signs in either strain, and no changes in thresholds in C57BL/6J mice. Extended 28-day exposure to 40 mg/kg/day nicotine induced threshold elevations, increased somatic signs, and anxiety-like behavior 24 h post-nicotine in C57BL/6J mice; thresholds returned to baseline levels by day 4 in nicotine-exposed mice. Mecamylamine or DHbetaE administration induced threshold elevations in nicotine-exposed C57BL/6J mice compared with saline-exposed mice. In conclusion, administration of relatively high nicotine doses over prolonged periods of time induces both the affective and somatic aspects of spontaneous nicotine withdrawal in the mouse, while exposure to nicotine for shorter periods of time is sufficient for nAChR antagonist-precipitated nicotine withdrawal. The current study is one of the first to demonstrate reward deficits associated with both spontaneous and nAChR antagonist-precipitated nicotine withdrawal in C57BL/6J mice.     

Nicotine effects on learning in zebrafish: the role of dopaminergic systems

Rationale

Nicotine improves cognitive function in a number of animal models including rats, mice, monkeys, and recently, zebrafish. The zebrafish model allows higher throughput and ease in discovering mechanisms of cognitive improvement.

Materials and methods

To further characterize the neural bases of nicotine effects on learning in zebrafish, we determined changes in dopaminergic systems that accompany nicotine-enhanced learning.

Results

Nicotine improved learning and increased brain levels of dihydroxyphenylacetic acid (DOPAC), the primary dopamine metabolite. There was a significant correlation between choice accuracy and DOPAC levels. The nicotinic antagonist mecamylamine blocked the nicotine-induced increase in DOPAC concentrations, in line with our previous finding that mecamylamine reversed nicotine-induced learning improvement.

Conclusions

Dopamine systems are related to learning in zebrafish; nicotine exposure increases both learning rates and DOPAC levels; and nicotinic antagonist administration blocks nicotine-induced rises in DOPAC concentrations. Rapid cognitive assessment of drugs with zebrafish could serve as a useful screening tool for the development of new therapeutics for cognitive dysfunction.     

Varenicline blocks nicotine intake in rats with extended access to nicotine self-administration

Rationale

Much evidence indicates that individuals use tobacco primarily to experience the psychopharmacological properties of nicotine. Varenicline, a partial ??4??2 nicotinic acetylcholine receptor (nAChR) agonist, is effective in reducing nicotine craving and relapse in smokers, suggesting that ??4??2 nAChRs may play a key role in nicotine dependence. In rats, the effect of varenicline on nicotine intake has only been studied with limited access to the drug, a model of the positive reinforcing effect of nicotine. Varenicline has not been tested on the increase in motivation to take nicotine in nicotine-dependent rats.

Objectives

The present study evaluated the effects of varenicline on nicotine intake in rats with extended access to nicotine self-administration (23?h/day), a condition leading to the development of nicotine dependence. We hypothesized that varenicline??s effects on nicotine self-administration would be greater in rats with extended than limited access to the drug and after forced abstinence rather than during baseline self-administration.

Results

Varenicline dose-dependently decreased nicotine self-administration in rats with limited (1?h/day) and extended (23?h/day) access. Despite an increased sensitivity to the motivational effects of abstinence on nicotine intake compared with limited-access rats, varenicline was equally effective in decreasing nicotine intake in dependent rats with extended access to nicotine.

Conclusion

These results suggest that ??4??2 nAChRs are critical in mediating the positive reinforcing effects of nicotine but may not be a key element underlying the negative reinforcement process responsible for the increased nicotine intake after abstinence in dependent subjects.     

Genetic background influences the effects of withdrawal from chronic nicotine on learning and high-affinity nicotinic acetylcholine receptor binding in the dorsal and ventral hippocampus

Rationale

The effects of nicotine on cognitive processes may play an important role in nicotine addiction. Nicotine withdrawal impairs hippocampus-dependent learning and genetic factors influence this effect. However, the neural changes that contribute to these impairments are unknown. Chronic nicotine upregulates hippocampal nicotinic acetycholine receptors (nAChRs), which may contribute to cognitive deficits when nicotine administration ceases. If nAChR upregulation underlies withdrawal deficits in learning, then strains of mice exhibiting withdrawal deficits in hippocampus-dependent learning should also show upregulation of hippocampal nAChRs.

Objectives

Here, we examined the effects of nicotine withdrawal on fear conditioning and [³H]epibatidine binding in the dorsal and ventral hippocampus in two inbred mouse strains and their F1 hybrids.

Methods

Male C57BL/6NTac, 129S6/SvEvTac, and B6129SF1/Tac mice were administered chronic nicotine (18 mg/kg/day) for 12 days through osmotic pumps and then were trained and tested in fear conditioning 24 h after cessation of nicotine treatment.

Results

Nicotine withdrawal impaired hippocampus-dependent contextual conditioning in C57BL/6NTac mice but not 129S6/SvEvTac or B6129SF1/Tac mice; no changes were observed in hippocampus-independent cued fear conditioning. Upregulated [³H]epibatidine binding was found in the dorsal, but not ventral, hippocampus of C57BL/6NTac mice and in the ventral hippocampus of B6129SF1/Tac mice after chronic nicotine.

Conclusions

Upregulation of high-affinity binding sites in the dorsal hippocampus of C57BL/6NTac mice, the only strain that exhibited nAChR upregulation in this region and withdrawal deficits in contextual conditioning, suggests that upregulation of high-affinity binding sites in the dorsal hippocampus mediates, in part, nicotine withdrawal deficits in contextual conditioning and genetic background modulates these effects.     

Effects of the beta-lactam antibiotic ceftriaxone on nicotine withdrawal and nicotine-induced reinstatement of preference in mice

Rationale

Several studies suggest that repeated nicotine administration causes alterations in glutaminergic transmission that may play an important role in developing and maintaining nicotine addiction. Chronic nicotine administration in rats decreases the expression of the glutamate transporter-1 (GLT-1) and cysteine–glutamate exchanger (system x_C?) in the nucleus accumbens. We hypothesized that ceftriaxone, a GLT-1 and system x_C? activator, would decrease murine behavioral aspects of nicotine dependence.

Objective

This study aimed to investigate the effect of repeated ceftriaxone administration on the behavioral effects of nicotine using mouse models of conditioned reward and withdrawal.

Method

Using male ICR mice, the ability of repeated ceftriaxone injections to modulate the development and reinstatement of a nicotine-conditioned place preference (CPP) was evaluated. Additionally, nicotine withdrawal-associated signs were assessed. These included both physical (somatic signs and hyperalgesia) and affective (anxiety-related behaviors) withdrawal signs in mice. Finally, the effects of ceftriaxone on nicotine-induced antinociception and hypothermia after acute nicotine injection were measured.

Result

Ceftriaxone had no effect on the development of nicotine preference but significantly attenuated nicotine-induced reinstatement of CPP. Furthermore, ceftriaxone reversed all nicotine withdrawal signs measured in mice.

Conclusion

Altogether, these findings show that a β-lactam antibiotic reduces nicotine withdrawal and nicotine-seeking behavior. Our results suggest that the documented efficacy of ceftriaxone against cocaine and morphine dependence-related behaviors effects extends to nicotine.     

Role of neuronal nicotinic acetylcholine receptors (nAChRs) on learning and memory in zebrafish

Rationale

Neuronal nicotinic acetylcholine receptors (nAChRs) play a modulatory role in cognition, and zebrafish provide a preclinical model to study learning and memory.

Objectives

We investigated the effect of nicotine (NIC) and some new cytisine-derived partial agonists (CC4 and CC26) on spatial memory in zebrafish using a rapid assay on T-maze task. The role of α4/α6β2 and the α7 nAChRs in NIC-induced memory enhancement was evaluated using selective nAChR antagonists.

Results

Low and high doses of NIC, cytisine (CYT), CC4 and CC26 respectively improved and worsened the mean running time, showing an inverted U dose–response function. The effective dose (ED50) (×10^?5 mg/kg) was 0.4 for CC4, 4.5 for CYT, 140 for NIC and 200 for CC26. NIC-induced cognitive enhancement was reduced by the selective nAChR subtype antagonists: methyllycaconitine (MLA) for α7, α-conotoxin (MII) for α6β2, dihydro-β-erythroidine (DhβE) for α4β2, the nonselective antagonist mecamylamine (MEC) and the muscarinic antagonist scopolamine (SCOP), with DhβE being more active than MLA or MII. All the partial agonists blocked the cognitive enhancement. The improvement with the maximal active dose of each partial agonist was blocked by low doses of DhβE (0.001 mg/kg) and MII (0.01 mg/kg). MLA reduced the effects of CC26 and CC4 at doses of 0.01 and 1 mg/kg, respectively, but did not antagonize CYT-induced memory improvement at any of the tested dose. No change in swimming activity was observed.

Conclusions

Our findings demonstrate that zebrafish make a useful model for the rapid screening of the effect of new α4β2 nAChR compounds on spatial memory.