Assessing the effects of chronic sazetidine-A delivery on nicotine self-administration in both male and female rats

Self-administration by female rats of low doses of nicotine alone vs. nicotine in tobacco smoke extract

BACKGROUND

Nicotine has reinforcing effects, but there are thousands of other compounds in tobacco, some of which might interact with nicotine reinforcement.

AIMS

This rat study was conducted to determine if nicotine self-administration is altered by co-administration of the complex mixture of compounds in tobacco smoke extract (TSE).

METHODS

Female Sprague-Dawley rats were tested for self-administration of low doses of nicotine (3 or 10 µg/kg/infusion) at three different rates of reinforcement (FR1, FR3 and FR5) over three weeks either alone or together with the complex mixture of tobacco smoke extract (TSE).

RESULTS

Rats self-administering 3 µg/kg/infusion of nicotine alone showed a rapid initiation on an FR1 schedule, but declined with FR5. Rats self-administering nicotine in TSE acquired self-administration more slowly, but increased responding over the course of the study. With 10 µg/kg/infusion rats self-administered significantly more nicotine alone than rats self-administering the same nicotine dose in TSE. Rats self-administering nicotine alone took significantly more infusions with the 10 than the 3 µg/kg/infusion dose, whereas rats self-administering nicotine in TSE did not. Nicotine in TSE led to a significantly greater locomotor hyperactivity at a dose of 0.1 mg/kg compared to rats that received nicotine alone. Rats self-administering nicotine alone had significantly more responding on the active vs. inactive lever, but rats self-administering the same nicotine doses in TSE did not.

CONCLUSIONS

Self-administration of nicotine in a purer form appears to be more clearly discriminated and dose-related than nicotine self-administered in the complex mixture of TSE.

Nicotine Effects, Body Weight Concerns and Smoking: A Literature Review

How people become addicted to cigarette smoking and remain addicted despite repeated attempts to quit requires piecing together a rather complex puzzle. The present review contextualizes the role of nicotine and smoking sensory stimulation on maintaining smoking, describes nicotine's effects on feeding behavior and body weight, and explores the impact of smoking outcome expectancies, including the belief that nicotine suppresses appetite and body weight, on the decision to smoke or vape (use of e-cigarettes). The analysis concludes with a review of rat models of human nicotine intake that attempt to isolate the effects of nicotine on appetite and weight gain. Animal research replicates with relative closeness phenomena observed in smokers, but the rat model falls short of replicating the long-term weight gain observed post-smoking cessation.

NAChR α4β2 Subtype and their Relation with Nicotine Addiction, Cognition, Depression and Hyperactivity Disorder

BACKGROUND

Neuronal α4β2 nAChRs are receptors involved in the role of neurotransmitters regulation and release, and this ionic channel participates in biological process of memory, learning and attention. This work aims to review the structure and functioning of the α4β2 nAChR emphasizing its role in the treatment of associated diseases like nicotine addiction and underlying pathologies such as cognition, depression and attention-deficit hyperactivity disorder.

METHODS

The authors realized extensive bibliographic research using the descriptors "Nicotine Receptor α4β2" and "cognition", "depression", "attention-deficit hyperactivity disorder", besides cross-references of the selected articles and after analysis of references in the specific literature.

RESULTS

As results, it was that found 179 relevant articles presenting the main molecules with affinity to nAChR α4β2 related to the cited diseases. The α4β2 nAChR subtype is a remarkable therapeutic target since this is the most abundant receptor in the central nervous system.

CONCLUSION

In summary, this review presents perspectives on the pharmacology and therapeutic targeting of α4β2 nAChRs for the treatment of cognition and diseases like nicotine dependence, depression and attention-deficit hyperactivity disorder.

Acute and chronic interactive treatments of serotonin 5HT2C and dopamine D1 receptor systems for decreasing nicotine self-administration in female rats

A variety of neural systems are involved in the brain bases of tobacco addiction. Animal models of nicotine addiction have helped identify a variety of interacting neural systems involved in the pathophysiology of tobacco addiction. We and others have found that drug treatments affecting many of those neurotransmitter systems significantly decrease nicotine self-administration. These treatments include dopamine D₁ receptor antagonist, histamine H1 antagonist, serotonin 5HT_2C agonist, glutamate NMDA antagonist, nicotinic cholinergic α4β2 partial agonist and nicotinic cholinergic α3β4 antagonist acting drugs. It may be the case that combining treatments that affect different neural systems underlying addiction may be more efficacious than single drug treatment. In the current study, we tested the interactions of the D₁ antagonist SCH-23390 and the serotonin 5HT_2c agonist lorcaserin, both of which we have previously shown to significantly reduce nicotine self-administration. In the acute interactions study, both SCH-23390 and lorcaserin significantly reduced nicotine self-administration when given alone and had additive effects when given in combination. In the chronic study, each drug alone caused a significant decrease in nicotine self-administration. No additive effect was seen in combination because SCH-23390 given alone chronically was already highly effective. Chronic administration of the combination was not seen to significantly prolong reduced nicotine self-administration into the post-treatment period. This research shows that unlike lorcaserin and SCH-23390 interactions when given acutely, when given chronically in combination they do not potentiate or prolong each other's effects in reducing nicotine self-administration.

Oral sazetidine-A, a selective α4β2* nicotinic receptor desensitizing agent, reduces nicotine self-administration in rats

Sazetidine-A selectively desensitizes α4β2 nicotinic receptors and also has partial agonist effects. We have shown that subcutaneous acute and repeated injections as well as chronic infusions of sazetidine-A significantly reduce intravenous (IV) nicotine self-administration in rats. To further investigate the promise of sazetidine-A as a smoking cessation aid, it is important to determine sazetidine-A effects with oral administration and the time-effect function for its action on nicotine self-administration. Young adult female Sprague-Dawley rats were trained to self-administer IV nicotine at the benchmark dose of 0.03 mg/kg/infusion dose in an operant FR1 schedule in 45-min sessions. After five sessions of training, they were tested for the effects of acute oral doses of sazetidine-A (0, 0.3, 1 and 3 mg/kg) given 30 min before testing. To determine the time-effect function, these rats were administered 0 or 3 mg/kg of sazetidine-A 1, 2, 4 or 23 h before the onset of testing. Our previous study showed that with subcutaneous injections, only 3 mg/kg of sazetidine-A significantly reduced nicotine self-administration, however, with oral administration of sazetidine-A lower dose of 1 mg/kg was also effective in reducing nicotine intake. A similar effect was seen in the time-effect study with 3 mg/kg of oral sazetidine-A causing a significant reduction in nicotine self-administration across all the time points of 1, 2, 4 or 23 h after oral administration. These results advance the development of sazetidine-A as a possible aid for smoking cessation by showing effectiveness with oral administration and persistence of the effect over the course of a day.

α4β2 Nicotinic receptor desensitizing compounds can decrease self-administration of cocaine and methamphetamine in rats

Sazetidine-A [6-(5(((S)-azetidine-2-yl)methoxy)pyridine-3-yl)hex-5-yn-1-ol] is a selective α4β2 nicotinic receptor desensitizing agent and partial agonist. Sazetidine-A has been shown in our previous studies to significantly reduce nicotine and alcohol self-administration in rats. The question arises whether sazetidine-A would reduce self-administration of other addictive drugs as well. Nicotinic receptors on the dopaminergic neurons in the ventral tegmental area play an important role in controlling the activity of these neurons and release of dopamine in the nucleus accumbens, which is critical mechanism for reinforcing value of drugs of abuse. Previously, we showed that the nonspecific nicotinic antagonist mecamylamine significantly reduces cocaine self-administration in rats. In this study, we acutely administered systemically sazetidine-A and two other selective α4β2 nicotinic receptor-desensitizing agents, VMY-2-95 and YL-2-203, to young adult female Sprague-Dawley rats and determined their effects on IV self-administration of cocaine and methamphetamine. Cocaine self-administration was significantly reduced by 0.3 mg/kg of sazetidine-A. In another set of rats, sazetidine-A (3 mg/kg) significantly reduced methamphetamine self-administration. VMY-2-95 significantly reduced both cocaine and methamphetamine self-administration with threshold effective doses of 3 and 0.3 mg/kg, respectively. In contrast, YL-2-203 did not significantly reduce cocaine self-administration at the same dose range and actually significantly increased cocaine self-administration at the 1 mg/kg dose. YL-2-203 (3 mg/kg) did significantly decrease methamphetamine self-administration. Sazetidine-A and VMY-2-95 are promising candidates to develop as new treatments to help addicts successfully overcome a variety of addictions including tobacco, alcohol as well as the stimulant drugs cocaine and methamphetamine.

Nicotine Self-Administration as Paradigm for Medication Discovery for Smoking Cessation: Recent Findings in Medications Targeting the Cholinergic System

Tobacco kills every year approximately six million people as a direct result of direct use, and it is still considered one of the most excruciating threats for human health worldwide. The low successful rates of the currently available pharmacotherapies to assist in quitting tobacco use suggest there is a need for more effective treatments.The intravenous self-administration (IVSA) paradigm is considered the gold standard to study voluntary drug intake in animal models, including nicotine. The IVSA paradigm has been used to identify key mechanisms involved in the addictive properties of nicotine in both rodents and nonhuman primates. In this chapter we describe how the IVSA paradigm has served to further investigate the role of nicotinic acetylcholine receptors (nAChRs) in the reinforcing properties of nicotine. Notably, this review will cover recent advances (i.e., research carried out during the past decade) using the IVSA paradigm, with a focus on the status of research on current smoking cessation medications (such as varenicline and bupropion) and of other nAChR ligands.The combination of the IVSA paradigm with pharmacological and genetic tools (e.g., knockout animals) has greatly contributed to our understanding of the role of specific subtype nAChRs in nicotine reinforcement processes. We also discuss some of the limitations of the IVSA paradigm so these can be taken into consideration when interpreting and designing new studies.

The nicotinic receptor drug sazetidine-A reduces alcohol consumption in mice without affecting concurrent nicotine consumption

Alcohol and nicotine addiction are frequently co-morbid. The nicotinic acetylcholine receptors (nAChRs) are critical for both alcohol and nicotine addiction mechanisms, since nAChR drugs that reduce nicotine consumption have been shown to also reduce alcohol consumption. Sazetidine-A, a pre-clinical nAChR drug with agonist and desensitizing effects at α4β2 and α7 nAChRs, has been reported to reduce alcohol consumption and nicotine self-administration in rats when administered at high doses. However, this effect has not been replicated in mice. In this study, we examined the effect of sazetidine-A on alcohol and nicotine consumption in male and female mice utilizing voluntary oral consumption procedures previously developed in our lab. We found that sazetidine-A (1 mg/kg, i.p) reduced overnight alcohol consumption, but did not affect nicotine consumption when presented either alone or concurrently with alcohol. Sazetidine-A did not reduce water or saccharin consumption at any dose tested. In a chronic co-consumption experiment in which either alcohol or nicotine was re-introduced after one week of forced abstinence, sazetidine-A attenuated post-abstinence consumption of alcohol but not nicotine. Sazetidine-A also significantly reduced alcohol consumption in an acute, binge drinking-in-the-dark procedure. Finally, we tested the effect of sazetidine-A on alcohol withdrawal, and found that sazetidine-A significantly reduced handling-induced convulsions during alcohol withdrawal. Collectively, these data suggest a novel role for the nAChR targets of sazetidine-A in specifically mediating alcohol consumption, separate from the involvement of nAChRs in mediating nicotine consumption. Delineation of this pathway may provide insight into novel therapies for the treatment of alcohol use disorders.

Mutually augmenting interactions of dextromethorphan and sazetidine-A for reducing nicotine self-administration in rats

A variety of nicotinic drug treatments have been found to decrease nicotine self-administration. However, interactions of drugs affecting different nicotinic receptor subtypes have not been much investigated. This study investigated the interactions between dextromethorphan, which blocks nicotinic α3β2 receptors as well as a variety of other receptors with sazetidine-A which is a potent and selective α4β2 nicotinic receptor partial agonist with desensitizing properties. This interaction was compared with dextromethorphan combination treatment with mecamylamine, which is a nonspecific nicotinic channel blocker. Co-administration of dextromethorphan (either 0.5 or 5 mg/kg) and lower dose of sazetidine-A (0.3 mg/kg) caused a significant reduction in nicotine SA. With regard to food-motivated responding, 3 mg/kg of sazetidine-A given alone caused a significant decrease in food intake. However, the lower 0.3 mg/kg sazetidine-A dose did not significantly affect food-motivated responding even when given in combination with the higher 5 mg/kg dextromethorphan dose which itself caused a significant decrease in food motivated responding. Interestingly, this higher dextromethorphan dose significantly attenuated the decrease in food motivated responding caused by 3 mg/kg of sazetidine-A. Locomotor activity was increased by the lower 0.3 mg/kg sazetidine-A dose and decreased by the 5 mg/kg dextromethorphan dose. Mecamylamine at the doses (0.1 and 1 mg/kg) did not affect nicotine SA, but at 1 mg/kg significantly decreased food-motivated responding. None of the mecamylamine doses augmented the effect of dextromethorphan in reducing nicotine self-administration. These studies showed that the combination of dextromethorphan and sazetidine-A had mutually potentiating effects, which could provide a better efficacy for promoting smoking cessation, however the strength of the interactions was fairly modest.

Sex differences in nicotine intravenous self-administration: A meta-analytic review

OBJECTIVE

This report reflects a meta-analysis that systematically reviewed the literature on intravenous self-administration (IVSA) of nicotine in female and male rats. The goal was to determine if sex differences in nicotine IVSA exist, estimate the magnitude of the effect, and identify potential moderators of the relationship between sex differences and nicotine consumption.

METHODS

Extensive search procedures identified 20 studies that met the inclusion criteria of employing both female and male rats in nicotine IVSA procedures. The meta-analysis was conducted on effect size values that were calculated from mean total intake or nicotine deliveries using the Hedges' unbiased g^u statistic.

RESULTS

A random effects analysis revealed that overall females self-administered more nicotine than males (weighted g^u=0.18, 95% CI [0.003, 0.34]). Subsequent moderator variable analyses revealed that certain procedural conditions influenced the magnitude of sex differences in nicotine IVSA. Specifically, higher reinforcement requirements (>FR1) and extended-access sessions (23h) were associated with greater nicotine IVSA in females versus males. Females also displayed higher nicotine intake than males when the experiment included a light cue that signaled nicotine delivery. Sex differences were not influenced by the diurnal phase of testing, dose of nicotine, or prior operant training.

CONCLUSION

Overall, the results revealed that female rats display higher levels of nicotine IVSA than males, suggesting that the strong reinforcing effects of nicotine promote tobacco use in women.

Evaluating oral flavorant effects on nicotine self-administration behavior and phasic dopamine signaling

Understanding how tobacco product flavor additives, such as flavorants in electronic cigarettes, influence smoking behavior and addiction is critical for informing public health policy decisions regarding tobacco product regulation. Here, we developed a combined intraoral (i.o.) and intravenous (i.v.) self-administration paradigm in rats to determine how flavorants influence self-administration behavior. By combining i.o. flavorant delivery with fast scan cyclic voltammetry (FSCV) or i.v. nicotine self-administration in adult, male rats, we examined whether flavors alter phasic dopamine (DA) signaling and nicotine self-administration. Oral administration of 10% sucrose or 0.32% saccharin, but not 0.005% menthol, increased phasic DA release in the nucleus accumbens (NAc). Oral sucrose or saccharin, when combined with i.v. nicotine delivery, also led to increased self-administration behavior. Specifically, combined i.o. sucrose and i.v. nicotine decreased responding compared to sucrose alone, and increased responding compared to nicotine alone. In contrast, i.o. flavorants did not alter motivational breakpoint in a progressive ratio task. Oral menthol, which did not alter i.v. nicotine administration, reversed oral nicotine aversion (50 and 100 mg/L) in a two-bottle choice test. Here, we demonstrate that i.o. appetitive flavorants that increase phasic DA signaling also increase self-administration behavior when combined with i.v. nicotine delivery. Additionally, oral menthol effects were specific to oral nicotine, and were not observed with i.v. nicotine-mediated reinforcement. Together, these preclinical findings have important implications regarding menthol and sweet flavorant additive effects on tobacco product use and can be used to inform policy decisions on tobacco product flavorant regulation.

Differential efficacies of the nicotinic α4β2 desensitizing agents in reducing nicotine self-administration in female rats

RATIONALE AND OBJECTIVES

Desensitization of neuronal nicotinic acetylcholine receptors holds promise as an effective treatment of tobacco addiction. Previously, we found that sazetidine-A (Saz-A), which selectively desensitizes α4β2 nicotinic receptors, significantly decreased intravenous (IV) nicotine self-administration (SA) in rats with an effective dose of 3 mg/kg in acute and repeated injection studies. We also found that chronic infusions of Saz-A at doses of 2 and 6 mg/kg/day significantly reduced nicotine SA in rats. In continuing studies, we have characterized other Saz-A analogs, YL-2-203 and VMY-2-95, to determine their efficacies in reducing nicotine SA in rats.

METHODS

Young adult female Sprague-Dawley rats were fitted with IV catheters and were trained for nicotine SA (0.03 mg/kg/infusion) on a fixed ratio 1 schedule for ten sessions. The same rats were also implanted subcutaneously with osmotic minipumps to continually deliver 2 or 6 mg/kg body weight YL-2-203, VMY-2-95, or saline for four consecutive weeks.

RESULTS

Chronic administration of VMY-2-95 at doses of 2 and 6 mg/kg/day caused significant (p < 0.01) decreases in nicotine SA over the 2 weeks of continued nicotine SA and for the 1-week period of resumed access after a week of enforced abstinence, whereas chronic administration of YL-2-203 at the same doses was not found to be effective.

CONCLUSIONS

These studies, together with our previous studies of Saz-A, revealed a spectrum of efficacies for these α4β2 nicotinic receptor desensitizing agents and provide a path forward for the most effective compounds to be further developed as possible aids to smoking cessation.

The effect of sazetidine-A and other nicotinic ligands on nicotine controlled goal-tracking in female and male rats

Nicotine is the primary addictive component of tobacco products and its complex stimulus effects are readily discriminated by human and non-human animals. Previous research with rodents directly investigating the nature of the nicotine stimulus has been limited to males. The current study began to address this significant gap in the literature by training female and male rats to discriminate 0.4 mg/kg nicotine from saline in the discriminated goal-tracking task. In this task, access to sucrose was intermittently available on nicotine session. On interspersed saline session, sucrose was not available. Both sexes acquired the discrimination as evidenced by increased head entries into sucrose receptacle (goal-tracking) evoked by nicotine; the nicotine generalization curves were also similar between females and males. The pharmacological profile of the nicotine stimulus was assessed using substitution and targeted combination tests with the following ligands: sazetidine-A, PHA-543613, PNU-120596, bupropion, nornicotine, and cytisine. For females and males, nornicotine fully substituted for the nicotine stimulus, whereas sazetidine-A, bupropion, and cytisine all evoked partial substitution. Female and male rats responded in a similar manner to interaction tests where a combination of 1 mg/kg of sazetidine-A plus nicotine or nornicotine shifted the nicotine dose-effect curve to the left. The combination of sazetidine-A plus bupropion or cytisine failed to do so. These findings begin to fill a significant gap the in scientific literature by studying the nature of the nicotine stimulus and response to therapeutically interesting combinations using a model that includes both sexes.

Sex differences in neurotensin and substance P following nicotine self-administration in rats

Investigator-administered nicotine alters neurotensin and substance P levels in Sprague-Dawley rats. This finding suggested a role of the dopamine-related endogenous neuropeptides in nicotine addiction. We sought to extend this observation by determining the responses of neurotensin and substance P systems (assessed using radioimmunoassay) in male and female rats following nicotine self-administration (SA). Male and female Sprague-Dawley were trained to self-administer nicotine, or receive saline infusions yoked to a nicotine-administering rat during daily sessions (1-h; 21 days). Brains were extracted 3 h after the last SA session. Nicotine SA increased tissue levels of neurotensin in the males in the anterior and posterior caudate, globus pallidus, frontal cortex, nucleus accumbens core and shell, and ventral tegmental area. Nicotine SA also increased tissue levels of neurotensin in the females in the anterior caudate, globus pallidus, nucleus accumbens core and shell, but not in the posterior caudate, frontal cortex, or ventral tegmental area. There were fewer sex differences observed in the substance P systems. Nicotine SA increased tissue levels of substance P in both the males and females in the posterior caudate, globus pallidus, frontal cortex, nucleus accumbens shell, and ventral tegmental area. A sex difference was observed in the nucleus accumbens core, where nicotine SA increased tissue levels of substance P in the males, yet decreased levels in the females. The regulation of neuropeptides following nicotine SA may play a role in the susceptibility to nicotine dependence in females and males. Synapse 70:336-346, 2016. © 2016 Wiley Periodicals, Inc.

Estradiol promotes the rewarding effects of nicotine in female rats

It is presently unclear whether ovarian hormones, such as estradiol (E2), promote the rewarding effects of nicotine in females. Thus, we compared extended access to nicotine intravenous self-administration (IVSA) in intact male, intact female, and OVX female rats (Study 1) as well as OVX females that received vehicle or E2 supplementation (Study 2). The E2 supplementation procedure involved a 4-day injection regimen involving 2 days of vehicle and 2 days of E2 administration. Two doses of E2 (25 or 250μg) were assessed in separate groups of OVX females in order to examine the dose-dependent effects of this hormone on the rewarding effects of nicotine. The rats were given 23-hour access to nicotine IVSA using an escalating dose regimen (0.015, 0.03, and 0.06mg/kg/0.1mL). Each dose was self-administered for 4 days with 3 intervening days of nicotine abstinence. The results revealed that intact females displayed higher levels of nicotine intake as compared to males. Also, intact females displayed higher levels of nicotine intake versus OVX females. Lastly, our results revealed that OVX rats that received E2 supplementation displayed a dose-dependent increase in nicotine intake as compared to OVX rats that received vehicle. Together, our results suggest that the rewarding effects of nicotine are enhanced in female rats via the presence of the ovarian hormone, E2.

PKCε phosphorylates α4β2 nicotinic ACh receptors and promotes recovery from desensitization

BACKGROUND AND PURPOSE

Nicotinic (ACh) receptor recovery from desensitization is modulated by PKC, but the PKC isozymes and the phosphorylation sites involved have not been identified. We investigated whether PKCε phosphorylation of α4β2 nAChRs regulates receptor recovery from desensitization.

EXPERIMENTAL APPROACH

Receptor recovery from desensitization was investigated by electrophysiological characterization of human α4β2 nAChRs. Phosphorylation of the α4 nAChR subunit was assessed by immunoblotting of mouse synaptosomes. Hypothermia induced by sazetidine-A and nicotine was measured in Prkce(-/-) and wild-type mice.

KEY RESULTS

Inhibiting PKCε impaired the magnitude of α4β2 nAChR recovery from desensitization. We identified five putative PKCε phosphorylation sites in the large intracellular loop of the α4 subunit, and mutating four sites to alanines also impaired recovery from desensitization. α4 nAChR subunit phosphorylation was reduced in synaptosomes from Prkce(-/-) mice. Sazetidine-A-induced hypothermia, which is mediated by α4β2 nAChR desensitization, was more severe and prolonged in Prkce(-/-) than in wild-type mice.

CONCLUSIONS AND IMPLICATIONS

PKCε phosphorylates the α4 nAChR subunit and regulates recovery from receptor desensitization. This study illustrates the importance of phosphorylation in regulating α4β2 receptor function, and suggests that reducing phosphorylation prolongs receptor desensitization and decreases the number of receptors available for activation.

Amitifadine, a triple monoamine re-uptake inhibitor, reduces nicotine self-administration in female rats

A wider diversity of drug treatments to aid smoking cessation is needed to help tailor the most efficacious treatment for different types of smokers. This study was conducted to determine whether amitifadine, which inhibits re-uptake of dopamine, norepinephrine and serotonin, would decrease nicotine self-administration at doses that do not cause adverse side effects. Adult female Sprague-Dawley rats were trained to self-administer nicotine intravenous (IV) and were given acute doses of amitifadine in a repeated measures counterbalanced design. Effects of amitifadine on locomotor activity and food motivated responding were also evaluated. Chronic amitifadine effects were also examined. The 30 mg/kg amitifadine dose significantly reduced nicotine self-administration. The 5 and 10 mg/kg doses reduced nicotine self-administration during the first 15 min of the session when the greatest amount of nicotine was self-administered. The 30 mg/kg amitifadine dose, but not the lower doses caused a significant reduction in locomotor activity averaged over the one-hour session and reduced food motivated responding. The 10 mg/kg dose caused hypoactivity at the beginning of the session, but 5 mg/kg did not cause any hypoactivity. The effects of chronic amitifadine treatment (10 mg/kg) over the course of 15 sessions was also determined. Amitifadine caused a significant reduction in nicotine self-administration, which was not seen to diminish over two consecutive weeks of treatment and a week after enforced abstinence. Amitifadine significantly reduced nicotine self-administration. This prompts further research to determine if amitifadine might be an effective treatment for smoking cessation.

Bupropion-varenicline interactions and nicotine self-administration behavior in rats

Varenicline and bupropion each have been shown to significantly improve cessation of tobacco addiction in humans. They act through different mechanisms and the question about the potential added efficacy with their combined used has arisen. Preclinical animal models of nicotine addiction can help with the evaluation of this combined approach and what dose combinations of varenicline and bupropion may be useful for enhancing tobacco cessation. In this study, we investigated the interacting dose-effect functions of varenicline and bupropion in a rat model of nicotine self-administration. Young adult female Sprague-Dawley rats were allowed to self-administer nicotine in 1-h sessions under an FR1 reinforcement schedule. Varenicline (0.3, 1. 3 mg/kg) and bupropion (8.33, 25, 75 mg/kg) were administered alone or together 15 min before each session. The vehicle saline was the control. Higher doses of each drug alone reduced nicotine self-administration compared to control with reductions of 62% and 75% with 3 mg/kg varenicline and 75 mg/kg bupropion respectively. Lower dose varenicline which does not by itself reduce nicotine self-administration, significantly augmented bupropion effects. The 0.3 mg/kg varenicline dose combined with the 25 and 75 mg/kg bupropion doses caused greater reductions of nicotine self-administration than either dose of bupropion given alone. However, higher dose varenicline did not have this effect. Lower dose bupropion did not augment varenicline effects. Only the high bupropion dose significantly enhanced the varenicline effect. Likewise, combining 1 mg/kg varenicline with 75 mg/kg bupropion reduced self-administration to a greater extent than either dose alone. These results demonstrate that combination therapy with varenicline and bupropion may be more beneficial than monotherapy with either drug alone.

Heterogeneity across brain regions and neurotransmitter interactions with nicotinic effects on memory function

Nicotinic acetylcholine receptors have been shown in many studies to be critically involved in memory function. The precise roles these receptors play depend on the receptor subtype, their anatomic localization, their interactions with other parts of the neural systems underlying cognition and the particular domain of cognitive function. Nicotinic agonists can significantly improve learning, memory, and attention. Nicotinic receptors in the hippocampus are innervated by cholinergic projections from the medial septum and diagonal band. Local infusions of either α7 or α4β2 nicotinic antagonists into either the dorsal or ventral hippocampus produce amnestic effects in rats navigating about a radial arm maze. There is cholinergic innervation of nicotinic receptors in other components of the limbic system as well. In the basolateral amygdala and the anterior thalamus, similar amnestic effects of nicotinic α7 and α4β2 antagonists are seen. Interestingly, there are no additive amnestic effects observed in these limbic areas when α7 and α4β2 receptor antagonists are combined. The particular expression patterns of α7 and α4β2 nicotinic receptors in these limbic and cortical areas may explain this nonadditivity, but further research is needed to determine the specific cause of this phenomenon. Nicotinic receptor mechanisms in the limbic system play an important role in cognitive impairment for a variety of neurological disorders, including Alzheimer's disease and schizophrenia. Alzheimer's disease results in a dramatic decrease in hippocampal nicotinic receptor density, affecting α4β2 receptor expression most prominently. In schizophrenia, there are anomalies in α7 nicotinic receptor expression, which seem to be crucial for the cognitive impairment of the disorder. Chronic nicotine exposure, such as seen with tobacco use, results in an increase in nicotinic receptor density in the limbic system. This effect appears to be related to the desensitization of nicotinic receptors seen after agonist application. Open questions remain concerning the role of desensitization versus activation of nicotinic receptors in cognitive improvement.

Decreasing nicotinic receptor activity and the spatial learning impairment caused by the NMDA glutamate antagonist dizocilpine in rats

Nicotinic systems have been shown by a variety of studies to be involved in cognitive function. Nicotinic receptors have an inherent property to become desensitized after activation. The relative role of nicotinic receptor activation vs. net receptor inactivation by desensitization in the cognitive effects of nicotinic drugs remains to be fully understood. In these studies, we tested the effects of the α7 nicotinic receptor antagonist methyllycaconitine (MLA), the α4β2 nicotinic receptor antagonist dihydro-β-erythroidine (DHβE), the nonspecific nicotinic channel blocker mecamylamine and the α4β2 nicotinic receptor desensitizing agent sazetidine-A on learning in a repeated acquisition test. Adult female Sprague-Dawley rats were trained on a repeated acquisition learning procedure in an 8-arm radial maze. MLA (1-4mg/kg), DHβE (1-4mg/kg), mecamylamine (0.125-0.5mg/kg) or sazetidine-A (1 and 3mg/kg) were administered in four different studies either alone or together with the NMDA glutamate antagonist dizocilpine (0.05 and 0.10mg/kg). MLA significantly counteracted the learning impairment caused by dizocilpine. The overall choice accuracy impairment caused by dizocilpine was significantly attenuated by co-administration of DHβE. Low doses of the non-specific nicotinic antagonist mecamylamine also reduced dizocilpine-induced repeated acquisition impairment. Sazetidine-A reversed the accuracy impairment caused by dizocilpine. These studies provide evidence that a net decrease in nicotinic receptor activity can improve learning by attenuating learning impairment induced by NMDA glutamate blockade. This adds to evidence in cognitive tests that nicotinic antagonists can improve cognitive function. Further research characterizing the efficacy and mechanisms underlying nicotinic antagonist and desensitization induced cognitive improvement is warranted.

Recent developments in novel antidepressants targeting α4β2-nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) have been investigated for developing drugs that can potentially treat various central nervous system disorders. Considerable evidence supports the hypothesis that modulation of the cholinergic system through activation and/or desensitization/inactivation of nAChR holds promise for the development of new antidepressants. The introductory portion of this Miniperspective discusses the basic pharmacology that underpins the involvement of α4β2-nAChRs in depression, along with the structural features that are essential to ligand recognition by the α4β2-nAChRs. The remainder of this Miniperspective analyzes reported nicotinic ligands in terms of drug design considerations and their potency and selectivity, with a particular focus on compounds exhibiting antidepressant-like effects in preclinical or clinical studies. This Miniperspective aims to provide an in-depth analysis of the potential for using nicotinic ligands in the treatment of depression, which may hold some promise in addressing an unmet clinical need by providing relief from depressive symptoms in refractory patients.

Chronic sazetidine-A maintains anxiolytic effects and slower weight gain following chronic nicotine without maintaining increased density of nicotinic receptors in rodent brain

Chronic nicotine administration increases the density of brain α4β2* nicotinic acetylcholine receptors (nAChRs), which may contribute to nicotine addiction by exacerbating withdrawal symptoms associated with smoking cessation. Varenicline, a smoking cessation drug, also increases these receptors in rodent brain. The maintenance of this increase by varenicline as well as nicotine replacement may contribute to the high rate of relapse during the first year after smoking cessation. Recently, we found that sazetidine-A (saz-A), a potent partial agonist that desensitizes α4β2* nAChRs, does not increase the density of these receptors in brain at doses that decrease nicotine self-administration, increase attention in rats, and produce anxiolytic effects in mice. Here, we investigated whether chronic saz-A and varenicline maintain the density of nAChRs after their up-regulation by nicotine. In addition, we examined the effects of these drugs on a measure of anxiety in mice and weight gain in rats. After increasing nAChRs in the rodent brain with chronic nicotine, replacing nicotine with chronic varenicline maintained the increased nAChR binding, as well as the α4β2 subunit proteins measured by western blots. In contrast, replacing nicotine treatments with chronic saz-A resulted in the return of the density of nAChRs to the levels seen in saline controls. Nicotine, saz-A and varenicline each demonstrated anxiolytic effects in mice, but only saz-A and nicotine attenuated the gain of weight over a 6-week period in rats. These findings suggest that apart from its modest anxiolytic and weight control effects, saz-A, or drugs like it, may be useful in achieving long-term abstinence from smoking.

Sazetidine-A Activates and Desensitizes Native α7 Nicotinic Acetylcholine Receptors

The aim of this study was to investigate the ability of sazetidine-A, a novel partial agonist at α4β2 nicotinic acetylcholine receptors (nAChRs), to affect the function of native α7 nAChRs in SH-SY5Y cells and primary cortical cultures. The α7-selective positive allosteric modulator PNU-120596 was used to reveal receptor activation, measured as an increase in intracellular calcium using fluorescent indicators. In the absence of PNU-120596, sazetidine-A elicited mecamylamine-sensitive increases in fluorescence in SH-SY5Y cells (EC₅₀ 4.2 µM) but no responses from primary cortical neurons. In the presence on PNU-120596, an additional response to sazetidine-A was observed in SH-SY5Y cells (EC₅₀ 0.4 µM) and robust responses were recorded in 14 % of cortical neurons. These PNU-120596-dependent responses were blocked by methyllycaconitine, consistent with the activation of α7 nAChRs. Preincubtion with sazetidine-A concentration-dependently attenuated subsequent responses to the α7-selective agonist PNU-282987 in SH-SY5Y cells (IC₅₀ 476 nM) and cortical cultures. These findings support the ability of sazetidine-A to interact with α7 nAChRs, which may contribute to sazetidine-A’s actions in complex physiological systems.

Sex differences in nicotine self-administration in rats during progressive unit dose reduction: implications for nicotine regulation policy

Reducing the nicotine content in tobacco products is being considered by the FDA as a policy to reduce the addictiveness of tobacco products. Understanding individual differences in response to nicotine reduction will be critical to developing safe and effective policy. Animal and human research demonstrating sex differences in the reinforcing effects of nicotine suggests that males and females may respond differently to nicotine-reduction policies. However, no studies have directly examined sex differences in the effects of nicotine unit-dose reduction on nicotine self-administration (NSA) in animals. The purpose of the present study was to examine this issue in a rodent self-administration model. Male and female rats were trained to self-administer nicotine (0.06mg/kg) under an FR 3 schedule during daily 23h sessions. Rats were then exposed to saline extinction and reacquisition of NSA, followed by weekly reductions in the unit dose (0.03 to 0.00025mg/kg) until extinction levels of responding were achieved. Males and females were compared with respect to baseline levels of intake, resistance to extinction, degree of compensatory increases in responding during dose reduction, and the threshold reinforcing unit dose of nicotine. Exponential demand-curve analysis was also conducted to compare the sensitivity of males and females to increases in the unit price (FR/unit dose) of nicotine (i.e., elasticity of demand or reinforcing efficacy). Females exhibited significantly higher baseline intake and less compensation than males. However, there were no sex differences in the reinforcement threshold or elasticity of demand. Dose-response relationships were very well described by the exponential demand function (r(2) values>0.96 for individual subjects). These findings suggest that females may exhibit less compensatory smoking in response to nicotine reduction policies, even though their nicotine reinforcement threshold and elasticity of demand may not differ from males.

Design, synthesis and discovery of picomolar selective α4β2 nicotinic acetylcholine receptor ligands

Developing novel and selective compounds that desensitize α4β2 nicotinic acetylcholine receptors (nAChRs) could provide new effective treatments for nicotine addiction, as well as other disorders. Here we report a new class of nAChR ligands that display high selectivity and picomolar binding affinity for α4β2 nicotinic receptors. The novel compounds have Ki values in the range of 0.031-0.26 nM and properties that should make them good candidates as drugs acting in the CNS. The selected lead compound 1 (VMY-2-95) binds with high affinity and potently desensitizes α4β2 nAChRs. At a dose of 3 mg/kg, compound 1 significantly reduced rat nicotine self-administration. The overall results support further characterizations of compound 1 and its analogues in preclinical models of nicotine addiction and perhaps other disorders involving nAChRs.

A mechanistic hypothesis of the factors that enhance vulnerability to nicotine use in females

Women are particularly more vulnerable to tobacco use than men. This review proposes a unifying hypothesis that females experience greater rewarding effects of nicotine and more intense stress produced by withdrawal than males. We also provide a neural framework whereby estrogen promotes greater rewarding effects of nicotine in females via enhanced dopamine release in the nucleus accumbens (NAcc). During withdrawal, we suggest that corticotropin-releasing factor (CRF) stress systems are sensitized and promote a greater suppression of dopamine release in the NAcc of females versus males. Taken together, females display enhanced nicotine reward via estrogen and amplified effects of withdrawal via stress systems. Although this framework focuses on sex differences in adult rats, it is also applied to adolescent females who display enhanced rewarding effects of nicotine, but reduced effects of withdrawal from this drug. Since females experience strong rewarding effects of nicotine, a clinical implication of our hypothesis is that specific strategies to prevent smoking initiation among females are critical. Also, anxiolytic medications may be more effective in females that experience intense stress during withdrawal. Furthermore, medications that target withdrawal should not be applied in a unilateral manner across age and sex, given that nicotine withdrawal is lower during adolescence. This review highlights key factors that promote nicotine use in females, and future studies on sex-dependent interactions of stress and reward systems are needed to test our mechanistic hypotheses. Future studies in this area will have important translational value toward reducing health disparities produced by nicotine use in females. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Varenicline and cytisine: two nicotinic acetylcholine receptor ligands reduce ethanol intake in University of Chile bibulous rats

RATIONALE

Neuronal nicotinic acetylcholine receptors (nAChRs) are pharmacological targets that have recently been implicated in the reinforcing effects of many drugs of abuse, including ethanol. Varenicline and cytisine are nAChR partial agonists in clinical use as smoking cessation aids. However, their efficacies to reduce alcohol consumption have not been fully studied.

OBJECTIVES

This study aims to compare the effects of varenicline and cytisine on ethanol consumption by rats bred for many generations as high ethanol drinkers (UChB).

RESULTS

Repeated dosing (0.5 or 1.0 mg/kg/day i.p.) of varenicline or cytisine, for three consecutive days, to male UChB rats pre-exposed to 10 % (v/v) ethanol and water 24 h/day for 4 weeks, significantly reduced alcohol intake and preference of ethanol over water during 1- and 24-h ethanol access periods. This effect was specific for ethanol intake and was not observed for 0.2 % saccharin or water consumption. Varenicline appears to be more effective than cytisine, probably due to its more favorable pharmacokinetic and pharmacodynamic properties. Long-term use of both nAChRs ligands for more than 8-10 days induced tolerance to their effects on ethanol consumption.

CONCLUSIONS

This preclinical study in UChB rats demonstrated that both varenicline and cytisine reduce alcohol intake, with varenicline producing a greater and longer-lasting reduction than cytisine. However, dose adjustment will have to be considered as a possible way to counter tolerance arising after continued use.

Chemistry and pharmacological studies of 3-alkoxy-2,5-disubstituted-pyridinyl compounds as novel selective α4β2 nicotinic acetylcholine receptor ligands that reduce alcohol intake in rats

Neuronal acetylcholine receptors mediate the addictive effects of nicotine and may also be involved in alcohol addiction. Varenicline, an approved smoking cessation medication, showed clear efficacy in reducing alcohol consumption in heavy-drinking smokers. More recently, sazetidine-A, which selectively desensitizes α4β2 nicotinic receptors, was shown to significantly reduce alcohol intake in a rat model. To develop novel therapeutics for treating alcohol use disorder, we designed and synthesized novel sazetidine-A analogues containing a methyl group at the 2-position of the pyridine ring. In vitro pharmacological studies revealed that some of the novel compounds showed overall pharmacological property profiles similar to that of sazetidine-A but exhibited reduced agonist activity across all nicotinic receptor subtypes tested. In rat studies, compound (S)-9 significantly reduced alcohol uptake. More importantly, preliminary results from studies in a ferret model indicate that these novel nAChR ligands have an improved adverse side-effect profile in comparison with that of varenicline.

Effects of chronic sazetidine-A, a selective α4β2 neuronal nicotinic acetylcholine receptors desensitizing agent on pharmacologically-induced impaired attention in rats

RATIONALE

Nicotine and nicotinic agonists have been shown to improve attentional function. Nicotinic receptors are easily desensitized, and all nicotinic agonists are also desensitizing agents. Although both receptor activation and desensitization are components of the mechanism that mediates the overall effects of nicotinic agonists, it is not clear how each of the two opposed actions contributes to attentional improvements. Sazetidine-A has high binding affinity at α4β2 nicotinic receptors and causes a relatively brief activation followed by a long-lasting desensitization of the receptors. Acute administration of sazetidine-A has been shown to significantly improve attention by reversing impairments caused by the muscarinic cholinergic antagonist scopolamine and the NMDA glutamate antagonist dizocilpine.

METHODS

In the current study, we tested the effects of chronic subcutaneous infusion of sazetidine-A (0, 2, or 6 mg/kg/day) on attention in Sprague-Dawley rats. Furthermore, we investigated the effects of chronic sazetidine-A treatment on attentional impairment induced by an acute administration of 0.02 mg/kg scopolamine.

RESULTS

During the first week period, the 6-mg/kg/day sazetidine-A dose significantly reversed the attentional impairment induced by scopolamine. During weeks 3 and 4, the scopolamine-induced impairment was no longer seen, but sazetidine-A (6 mg/kg/day) significantly improved attentional performance on its own. Chronic sazetidine-A also reduced response latency and response omissions.

CONCLUSIONS

This study demonstrated that similar to its acute effects, chronic infusions of sazetidine-A improve attentional performance. The results indicate that the desensitization of α4β2 nicotinic receptors with some activation of these receptors may play an important role in improving effects of sazetidine-A on attention.

A new radioligand binding assay to measure the concentration of drugs in rodent brain ex vivo

We have developed a new radioligand binding assay method to measure the concentration of nonradiolabeled drugs in the brain ex vivo. This new method fuses the concepts of standard competition and saturation binding assays and uses a transformed version of the Cheng-Prusoff equation (Biochem Pharmacol 22:3099-3108, 1973) to calculate the drug concentration. After testing the validity of this method, we demonstrated its utility by measuring the brain concentration of sazetidine-A, a newly developed nicotinic receptor ligand, and its elimination rate after a single subcutaneous administration. Our results indicate that sazetidine-A reaches brain concentrations that are known to occupy and desensitize the majority of neuronal nicotinic acetylcholine receptor binding sites. Furthermore, using this method, we estimated the half-life of sazetidine-A in the rat brain to be ∼65 min. It is important to note that the method described here to measure sazetidine-A in brain should be generalizable to other drugs acting at any receptor that can be reliably measured with a radiolabeled ligand.

Chronic sazetidine-A at behaviorally active doses does not increase nicotinic cholinergic receptors in rodent brain

Chronic nicotine administration increases α4β2 neuronal nicotinic acetylcholine receptor (nAChR) density in brain. This up-regulation probably contributes to the development and/or maintenance of nicotine dependence. nAChR up-regulation is believed to be triggered at the ligand binding site, so it is not surprising that other nicotinic ligands also up-regulate nAChRs in the brain. These other ligands include varenicline, which is currently used for smoking cessation therapy. Sazetidine-A (saz-A) is a newer nicotinic ligand that binds with high affinity and selectivity at α4β2* nAChRs. In behavioral studies, saz-A decreases nicotine self-administration and increases performance on tasks of attention. We report here that, unlike nicotine and varenicline, chronic administration of saz-A at behaviorally active and even higher doses does not up-regulate nAChRs in rodent brains. We used a newly developed method involving radioligand binding to measure the concentrations and nAChR occupancy of saz-A, nicotine, and varenicline in brains from chronically treated rats. Our results indicate that saz-A reached concentrations in the brain that were ∼150 times its affinity for α4β2* nAChRs and occupied at least 75% of nAChRs. Thus, chronic administration of saz-A did not up-regulate nAChRs despite it reaching brain concentrations that are known to bind and desensitize virtually all α4β2* nAChRs in brain. These findings reinforce a model of nicotine addiction based on desensitization of up-regulated nAChRs and introduce a potential new strategy for smoking cessation therapy in which drugs such as saz-A can promote smoking cessation without maintaining nAChR up-regulation, thereby potentially increasing the rate of long-term abstinence from nicotine.