Reduced Nicotine Self-administration Research Articles

Fluorocitrate inhibition of astrocytes reduces nicotine self-administration and alters extracellular levels of glutamate and dopamine within the nucleus accumbens in male wistar rats

Emerging evidence suggests an important role of astrocytes in mediating behavioral and molecular effects of commonly misused drugs. Passive exposure to nicotine alters molecular, morphological, and functional properties of astrocytes. However, a potential involvement of astrocytes in nicotine reinforcement remains largely unexplored. The overall hypothesis tested in the current study is that astrocytes play a critical role in nicotine reinforcement. Protein levels of the astrocyte marker glial fibrillary acidic protein (GFAP) were examined in key mesocorticolimbic regions following chronic nicotine intravenous self-administration. Fluorocitrate, a metabolic inhibitor of astrocytes, was tested for its effects on behaviors related to nicotine reinforcement and relapse. Effects of fluorocitrate on extracellular neurotransmitter levels, including glutamate, GABA, and dopamine, were determined with microdialysis. Chronic nicotine intravenous self-administration increased GFAP expression in the nucleus accumbens core (NACcr), but not other key mesocorticolimbic regions, compared to saline intravenous self-administration. Both intra-ventricular and intra-NACcr microinjection of fluorocitrate decreased nicotine self-administration. Intra-NACcr fluorocitrate microinjection also inhibited cue-induced reinstatement of nicotine seeking. Local perfusion of fluorocitrate decreased extracellular glutamate levels, elevated extracellular dopamine levels, but did not alter extracellular GABA levels in the NACcr. Fluorocitrate did not alter basal locomotor activity. These results indicate that nicotine reinforcement upregulates the astrocyte marker GFAP expression in the NACcr, metabolic inhibition of astrocytes attenuates nicotine reinforcement and relapse, and metabolic inhibition of astrocytes disrupts extracellular dopamine and glutamate transmission. Overall, these findings suggest that astrocytes play an important role in nicotine reinforcement and relapse, potentially through regulation of extracellular glutamate and dopamine neurotransmission.

A novel, non-opioid, selective orexin-1 receptor antagonist for the treatment of substance use disorders

There is a pressing need for novel treatments for substance use disorders to address increasing rates of addiction and drug overdose. Preclinical studies have shown that the orexin (hypocretin) system in the brain, mediated primarily by the orexin 1 receptor, plays a role in reward-related behaviours and that selective blockade of this receptor is efficacious in rodent models of addiction for a wide range of substances including opioids, cocaine, nicotine, and alcohol. We report here the preclinical profile of C4X3256, a novel, selective oral antagonist of the orexin 1 receptor. In the rat, high levels of receptor occupancy were observed in the tenia tecta region of the brain for up to 8 h following oral dosing. Translation of brain receptor occupancy into pharmacological efficacy was demonstrated in rat models showing significantly reduced nicotine self-administration and diminished cue-induced reinstatement of nicotine seeking following an oral dose of 30 mg/kg C4X3256. In addition, C4X3256 significantly reduced reinstatement of cocaine seeking induced by presentation of tone/light cues in a rat model of relapse. In summary, the nonclinical profile of C4X3256 has supported the progression of this compound into human clinical trials as an investigational medicinal product for the treatment of substance use disorders.

The multitarget FAAH inhibitor/D3 partial agonist ARN15381 decreases nicotine self-administration in male rats

Tobacco use disorder is a worldwide health problem for which available medications show limited efficacy. Nicotine is the psychoactive component of tobacco responsible for its addictive liability. Similar to other addictive drugs, nicotine enhances mesolimbic dopamine transmission. Inhibition of the fatty acid amide hydrolase (FAAH), the enzyme responsible for the degradation of the endocannabinoid anandamide (AEA), palmitoylethanolamide (PEA) and oleoylethanolamide (OEA), reduces nicotine-enhanced dopamine transmission and acquisition of nicotine self-administration in rats. Down-regulation of dopamine transmission by antagonists or partial agonists of the dopamine D3 receptor (DRD3) also reduced nicotine self-administration and conditioned place preference. Based on these premises, we evaluated the effect of ARN15381, a multitarget compound showing FAAH inhibition and DRD3 partial agonist activity in the low nanomolar range, on nicotine self-administration in rats. Pretreatment with ARN15381 dose dependently decreased self-administration of a nicotine dose at the top of the nicotine dose/response (D/R) curve, while it did not affect self-administration of a nicotine dose laying on the descending limb of the D/R curve. Conversely, pretreatment with the selective FAAH inhibitor URB597 and the DRD3 partial agonist CJB090 failed to modify nicotine self-administration independent of the nicotine dose self-administered. Our data indicates that the concomitant FAAH inhibition and DRD3 partial agonism produced by ARN15381 is key to the observed reduction of nicotine self-administration, demonstrating that a multitarget approach may hold clinical importance for the treatment of tobacco use disorder.

Selective Inhibition of Phosphodiesterase 7 Enzymes Reduces Motivation for Nicotine Use through Modulation of Mesolimbic Dopaminergic Transmission.

Approximately 5 million people die from diseases related to nicotine addiction and tobacco use each year. The nicotine-induced increase of corticomesolimbic dopaminergic (DAergic) transmission and hypodopaminergic conditions occurring during abstinence are important for maintaining drug-use habits. We examined the notion of reequilibrating DAergic transmission by inhibiting phosphodiesterase 7 (PDE7), an intracellular enzyme highly expressed in the corticomesolimbic circuitry and responsible for the degradation of cyclic adenosine monophosphate (cAMP), the main second messenger modulated by DA receptor activation. Using selective PDE7 inhibitors, we demonstrated in male rats that systemic PDE7 enzyme inhibition reduced nicotine self-administration and prevented reinstatement to nicotine seeking evoked by cues or by the pharmacological stressor yohimbine. The effect was also observed by direct application of the PDE7 inhibitors into the nucleus accumbens (NAc) shell but not into the core. Inhibition of PDE7 resulted in increased DA- and cAMP-regulated neuronal phosphoprotein and cAMP response element-binding protein and their phosphorylated forms in the NAc. It also enhanced the DA D1 receptor agonism-mediated effects, indicating potentiation of protein kinase A–dependent transmission downstream of D1 receptor activation. In electrophysiological recordings from DA neurons in the lateral posterior ventral tegmental area, the PDE7 inhibitors attenuated the spontaneous activity of DA neurons. This effect was exerted through the potentiation of D1 receptor signaling and the subsequent facilitation of γ-aminobutyric acid transmission. The PDE7 inhibitors did not elicit conditioned place preference and did not induce intravenous self-administration, indicating lack of reinforcing properties. Thus, PDE7 inhibitors have the potential to treat nicotine abuse.SIGNIFICANCE STATEMENT The World Health Organization estimates that there are 1.25 billion smokers worldwide, representing one-third of the global population over the age of 15. Nicotine-induced increase of corticomesolimbic DAergic transmission and hypodopaminergic conditions occurring during abstinence are critical for maintaining drug-use habits. Here, we demonstrate that nicotine consumption and relapse to nicotine seeking are attenuated by reequilibrating DAergic transmission through inhibition of PDE7, an intracellular enzyme responsible for the degradation of cAMP, the main second messenger modulated by DA receptor activation. PDE7 inhibition may represent a novel treatment approach to aid smoking cessation.

Potentiation of (α4)2(β2)3, but not (α4)3(β2)2, nicotinic acetylcholine receptors reduces nicotine self-administration and withdrawal symptoms

The low sensitivity (α4)3(β2)2 (LS) and high sensitivity (α4)2(β2)3 (HS) nAChR isoforms may contribute to a variety of brain functions, pathophysiological processes, and pharmacological effects associated with nicotine use. In this study, we examined the contributions of the LS and HS α4β2 nAChR isoforms in nicotine self-administration, withdrawal symptoms, antinociceptive and hypothermic effects. We utilized two nAChR positive allosteric modulators (PAMs): desformylflustrabromine (dFBr), a PAM of both the LS and HS α4β2 nAChRs, and CMPI, a PAM selective for the LS nAChR. We found that dFBr, but not CMPI, decreased intravenous nicotine self-administration in male mice in a dose-dependent manner. Unlike dFBr, which fully reverses somatic and affective symptoms of nicotine withdrawal, CMPI at doses up to 15 mg/kg in male mice only partially reduced nicotine withdrawal-induced somatic signs, anxiety-like behavior and sucrose preference, but had no effects on nicotine withdrawal-induced hyperalgesia. These results indicate that potentiation of HS α4β2 nAChRs is necessary to modulate nicotine's reinforcing properties that underlie nicotine intake and to reverse nicotine withdrawal symptoms that influence nicotine abstinence. In contrast, both dFBr and CMPI enhanced nicotine's hypothermic effect and reduced nicotine's antinociceptive effects in male mice. Therefore, these results indicate a more prevalent role of HS α4β2 nAChR isoforms in mediating various behavioral effects associated with nicotine, whereas the LS α4β2 nAChR isoform has a limited role in mediating body temperature and nociceptive responses. These findings will facilitate the development of more selective, efficacious, and safe nAChR-based therapeutics for nicotine addiction treatment.

Amitifadine, a triple reuptake inhibitor, reduces self-administration of the opiate remifentanil in rats.

A variety of neural systems are involved in drug addiction, and some of these systems are shared across different addictive drugs. We have found several different types of drug treatments that successfully reduce nicotine self-administration. The current set of studies is the first in a series to determine if drug treatments that have been found to significantly reduce nicotine self-administration would reduce opiate self-administration. Amitifadine, a triple reuptake inhibitor of dopamine, norepinephrine, and serotonin, was assessed in female Sprague-Dawley rats to determine whether it significantly reduces remifentanil self-administration with either acute or chronic treatment. Acutely, amitifadine doses of 5, 10, and 20mg/kg each significantly reduced remifentanil self-administration. In a chronic study, repeated treatment with 10mg/kg of amitifadine continued to reduce remifentanil self-administration, even after the cessation of treatment. However, amitifadine was not found to attenuate the rise in remifentanil self-administration with continued access. This study and our earlier one showed that the 10mg/kg amitifadine dose did not significantly affect food motivated responding. Amitifadine did not attenuate remifentanil-induced antinociception as measured on the hot plate test but extended and maintained antinociceptive effects. These studies show the promise of amitifadine as a treatment for countering opiate self-administration for adjunctive use with opioids for analgesia. Further studies are needed to determine the possible efficacy of amitifadine for combating opiate addiction or preventing it in humans during adjunctive use with opioids for chronic pain.

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 5HT2C 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 5HT2c 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.

Chronic memantine decreases nicotine self-administration in rats

Neurobehavioral bases of tobacco addiction and nicotine reinforcement are complex, involving more than only nicotinic cholinergic or dopaminergic systems. Memantine is an NMDA glutamate antagonist used to improve cognitive function in people with Alzheimer's disease. Glutamate may be an important component of the reinforcing effects of nicotine, so memantine was evaluated as a potential smoking cessation aid. Two studies were conducted with adult female rats, one testing acute effects of memantine over a range of doses for changing nicotine self-administration and the other testing the chronic effects of memantine to reduce nicotine self-administration. Acute memantine injections slightly, but significantly, increased nicotine self-administration in a dose-related manner. In contrast, chronic memantine treatment significantly reduced nicotine self-administration. During the first day of memantine administration in the chronic study, nicotine self-administration was significantly elevated replicating the acute study. Starting in the second week of treatment there was a significant reduction of nicotine self-administration relative to controls. This was seen because memantine treatment prevented the increase in nicotine self-administration shown by controls. There even continued to be a memantine-induced lowered nicotine self-administration during the week after the cessation of memantine treatment. Memantine or other drugs affecting NMDA glutamate receptors may be useful aids to smoking cessation. Full efficacy for reducing nicotine self-administration was seen as the NMDA drug treatment is given chronically. Importantly, the effect persisted even after treatment is ended, indicating the high potential for NMDA glutamate receptors to impact nicotine addiction.

A randomized, double-blind, placebo-controlled study of the kappa opioid receptor antagonist, CERC-501, in a human laboratory model of smoking behavior.

Preclinical data indicate that selective kappa opioid receptor antagonists reduce nicotine self-administration and withdrawal symptoms. The aim of the current study was to determine whether treatment with CERC-501, an orally available, potent, and selective kappa opioid receptor antagonist, could alleviate nicotine withdrawal and craving and mitigate mood alterations associated with nicotine withdrawal in humans. Healthy, adult cigarette smokers were enrolled into this randomized, multisite, double-blind, placebo-controlled, crossover study. Participants completed two 8-day treatment phases during which they received either CERC-501 (15 mg, p.o., once daily) or placebo. On the seventh day of each dosing phase, participants were admitted as inpatients for an 18-hour cigarette abstinence period followed by experimental testing. The primary outcome measures were (a) performance on the McKee Smoking Lapse test (ie, latency to smoke in exchange for money) and (b) number of cigarettes self-administered during a 60-minute ad lib smoking period. Other outcomes included measures of craving, mood, anxiety, nicotine withdrawal, and subjective effects of cigarette smoking. A total of 71 participants who smoked an average of approximately 23 cigarettes per day were enrolled, and 56 subjects completed the study. CERC-501 was well tolerated, but it did not significantly alter the latency to start smoking (CERC-501: 16.5 min vs placebo: 17.7 min) or the number of cigarettes smoked (CERC-501: 3.3 cigarettes vs placebo: 3.1 cigarettes). Compared with placebo, CERC-501 also did not affect cigarette craving, mood, anxiety, nicotine withdrawal, or subjective effects of smoking. These findings do not support a role for CERC-501 in the treatment of nicotine use disorder.

Prolonging the Reduction of Nicotine Self-Administration in Rats by Coadministering Chronic Nicotine With Amitifadine, a Triple Monoamine Reuptake Inhibitor With CYP2B6 Inhibitory Actions.

Existing treatments can aid tobacco smoking cessation, but they have low efficacy. Because there is a network of neural systems involved in tobacco addiction, combination treatments may provide greater efficacy. Chronic nicotine and amitifadine have each been shown to significantly reduce nicotine self-administration in rats. This study was conducted to determine if the combination of chronic nicotine with amitifadine, a triple monoamine reuptake inhibitor with CYP2B inhibitory effects, would reduce nicotine self-administration to a greater extent than either alone or placebo. This study tested the combination of nicotine plus amitifadine in young adult female Sprague-Dawley rats self-administering nicotine (0.03 mg/kg/infusion). This combination was compared with each treatment alone and the vehicle during continuing nicotine self-administration as well as during resumption of self-administration after a week of enforced abstinence, modeling a quit attempt. Finally, we studied the residual effects of these therapies after discontinuation of treatment. Treatment with either chronic nicotine or amitifadine alone significantly reduced nicotine self-administration relative to controls. The combination of the treatments significantly enhanced this effect. After treatment withdrawal, all of the groups showed increases in nicotine self-administration, but only the combined treatment group remained significantly below control rates of nicotine self-administration. This study showed the promise of amitifadine as a possible new treatment for smoking cessation and suggested that amitifadine is more effective when given with chronic nicotine. The improved efficacy of the amitifadine and nicotine combination may be potentiated by amitifadine's inhibitory effects on CYP2B, which slows nicotine metabolism. This study replicated the effects that chronic nicotine or chronic amitifadine, a triple reuptake inhibitor, significantly reduces nicotine self-administration in rats. It extends those findings by showing that the combination of chronic nicotine plus amitifadine causes significantly greater reduction in nicotine self-administration than either drug treatment alone. The combination of chronic amitifadine and chronic nicotine also causes a persistent significant reduction in nicotine self-administration after the end of treatment. The amitifadine and nicotine treatment should be assessed in humans to determine whether this combination provides greater efficacy in smoking cessation than transdermal nicotine treatment alone.

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.

NOP-Related Mechanisms in Substance Use Disorders.

Nociceptin/orphanin FQ (N/OFQ) is a 17 amino acid peptide that was deorphanized in 1995 and has been widely studied since. The role of the N/OFQ system in drug abuse has attracted researchers' attention since its initial discovery. The first two scientific papers describing the effect of intracranial injection of N/OFQ appeared 20years ago and reported efficacy of the peptide in attenuating alcohol intake, whereas heroin self-administration was insensitive. Since then more than 100 scientific articles investigating the role of the N/OFQ and N/OFQ receptor (NOP) system in drug abuse have been published. The present article provides an historical overview of the advances in the field with focus on three major elements. First, the most robust data supportive of the efficacy of NOP agonists in treating drug abuse come from studies in the field of alcohol research, followed by psychostimulant and opioid research. In contrast, activation of NOP appears to facilitate nicotine consumption. Second, emerging data challenge the assumption that activation of NOP is the most appropriate strategy to attenuate consumption of substances of abuse. This assumption is based first on the observation that animals carrying an overexpression of NOP system components are more prone to consume substances of abuse, whereas NOP knockout rats are less motivated to self-administer heroin, alcohol, and cocaine. Third, administration of NOP antagonists also reduces alcohol consumption. In addition, NOP blockade reduces nicotine self-administration. Hypothetical mechanisms explaining this apparent paradox are discussed. Finally, we focus on the possibility that co-activation of NOP and mu opioid (MOP) receptors is an alternative strategy, readily testable in the clinic, to reduce the consumption of psychostimulants, opiates, and, possibly, alcohol.

An improved model of ethanol and nicotine co-use in female P rats: Effects of naltrexone, varenicline, and the selective nicotinic α6β2* antagonist r-bPiDI

Background Although pharmacotherapies are available for alcohol (EtOH) or tobacco use disorders individually, it may be possible to develop a single pharmacotherapy to treat heavy drinking tobacco smokers by capitalizing on the commonalities in their mechanisms of action.Methods Female alcohol-preferring (P) rats were trained for EtOH drinking and nicotine self-administration in two phases: (1) EtOH alone (0 vs. 15% EtOH, 2-bottle choice) and (2) concomitant access, during which EtOH access continued with access to nicotine (0.03 mg/kg/infusion, i.v.) using a 2-lever choice procedure (active vs. inactive lever) in which the fixed ratio (FR) requirement was gradually increased to FR30. When stable co-use was obtained, rats were pretreated with varying doses of naltrexone, varenicline, or r-bPiDI, an α6β2* subtype-selective nicotinic acetylcholine receptor antagonist shown previously to reduce nicotine self-administration.Results While EtOH intake was initially suppressed in phase 2 (co-use), pharmacologically relevant intake for both substances was achieved by raising the “price” of nicotine to FR30. In phase 2, naltrexone decreased EtOH and water consumption but not nicotine intake; in contrast, naltrexone in phase 1 (EtOH only) did not significantly alter EtOH intake. Varenicline and r-bPiDI in phase 2 both decreased nicotine self-administration and inactive lever pressing, but neither altered EtOH or water consumption.Conclusions These results indicate that increasing the “price” of nicotine increases EtOH intake during co-use. Additionally, the efficacy of naltrexone, varenicline, and r-bPiDI was specific to either EtOH or nicotine, with no efficacy for co-use. Nevertheless, future studies on combining these treatments may reveal synergistic efficacy.

Differential regulation of alcohol taking and seeking by antagonism at α4β2 and α3β4 nAChRs.

Alcoholism is a serious public health problem throughout the world. Current pharmacotherapies for the treatment of this disorder are poorly effective. Preclinical and clinical findings point to nicotinic acetylcholine receptors (nAChRs) as a promising target for the development of novel and effective medications. Assuage Pharmaceuticals, in collaboration with Torrey Pines Institute for Molecular Studies, has discovered a new class of potent and selective α4β2 nAChR antagonists. Here, it was hypothesized that α4β2 nAChR antagonism is a viable approach for treatment of alcohol use disorders. When tested in rats, one lead compound, AP-202, attenuated both operant alcohol and nicotine self-administration in a paradigm in which the two reinforcers were concurrently available. The conotoxin TP2212-59, a selective α3β4 nAChR antagonist, was only effective in reducing nicotine self-administration. AP-202 also reduced alcohol but not food responding when alcohol was presented as the only reinforcer, whereas the commercially available α4β2 nAChR antagonist dihydro-β-erythroidine failed to alter alcohol self-administration. AP-202 did not block relapse-like behavior induced by previously alcohol-associated stimuli or yohimbine stress. In a reinstatement paradigm, in which alcohol seeking was triggered by a nicotine challenge, a behavior successfully inhibited by the nonselective nAChR antagonist mecamylamine, AP-202 was not effective, while pretreatment with TP2212-59 abolished nicotine-induced reinstatement of alcohol seeking. These findings suggest differential roles for α4β2 and α3β4 nAChR on alcohol taking and seeking with selective blockade of α4β2 nAChR being more implicated in modulating alcohol taking while selective blockade of α3β4 nAChR is involved in nicotine-induced alcohol seeking.

Preclinical evidence for combining the 5-HT2C receptor agonist lorcaserin and varenicline as a treatment for nicotine dependence.

Varenicline, a nicotinic acetylcholine receptor partial agonist, is used to treat nicotine dependence. Lorcaserin, a 5-HT2C receptor agonist has been approved in some countries to treat obesity. Based on preclinical and preliminary clinical evidence, lorcaserin may have potential to treat nicotine dependence. These experiments examined in rats the effects of combining varenicline (0.5 or 1mg/kg) and lorcaserin (0.3, 0.6 and 1mg/kg) on nicotine self-administration, reinstatement of nicotine seeking, responding for food and impulsive action. Both drugs alone reduced nicotine self-administration. Combining varenicline and 0.6 mg/kg lorcaserin reduced responding to a greater extent than either drug alone. In a relapse model, extinguished nicotine seeking was reinstated by a priming injection of nicotine and nicotine-associated cues. Reinstatement was reduced by varenicline (1mg/kg) and by lorcaserin (0.3mg/kg). Combining lorcaserin (0.3mg/kg) with varenicline (0.5 or 1mg/kg) reduced reinstatement to a greater degree than either drug alone. Both drugs had minimal effects on responding for food, alone or in combination. In the five-choice serial reaction time test, varenicline (0.5 or 1mg/kg) increased impulsivity, measured as increased premature responding. This effect was reduced by lorcaserin (0.3mg/kg). Plasma levels of varenicline or lorcaserin were not altered by co-administration of the other drug. Varenicline and lorcaserin have additive effects on nicotine self-administration, and on nicotine seeking. Lorcaserin prevents impulsivity induced by varenicline. This pattern of effects suggests that co-administration of varenicline and lorcaserin has potential as a treatment for nicotine dependence that may exceed the value of either drug alone.

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.

Sub-anesthetic doses of ketamine attenuate nicotine self-administration in rats

Smoking cessation strategies are of prime medical importance. Despite availability of various pharmacological agents in combating addiction to nicotine, more effective medications are needed. Based on recent findings, the glutamatergic system in the brain may provide novel targets. Here, we evaluated the effects of acute administration of sub-anesthetic doses of ketamine, an NMDA receptor antagonist, in both male and female Sprague-Dawley rats trained to self-administer nicotine. Animals were injected subcutaneously with 5, 7.5 and 10 mg/kg ketamine or saline and the effects on the number of intravenous nicotine infusions during a 45 min session was measured. Ketamine treatment significantly reduced nicotine self-administration in a dose-dependent manner. Moreover, a differential sensitivity between the sexes was observed as male rats responded to a lower dose of ketamine and with higher magnitude of effect than female rats. It is concluded that glutamatergic receptor manipulations may offer a novel and potentially sex-dependent intervention in nicotine addiction.

Initial Cross-Over Test of A Positive Allosteric Modulator of Alpha-7 Nicotinic Receptors to Aid Cessation in Smokers With Or Without Schizophrenia.

Preclinical research shows that compounds acting at α7 nicotinic receptors (nAChRs) can reduce nicotine self-administration, suggesting that a positive allosteric modulator (PAM) of α7 receptors, JNJ-39393406, may aid smoking cessation. Moreover, individuals with schizophrenia, who have very high rates of smoking, have reduced expression of α7 nAChRs and may particularly benefit from this compound. In two parallel studies using a within-subject cross-over design, 36 healthy smokers (Study 1) and 62 smokers with schizophrenia (Study 2), both groups high in quit interest, attempted to initiate quitting temporarily during each of two 3-week phases. Treatments were the α7 nicotinic receptor PAM JNJ-39393406 (100 mg b.i.d.) or placebo (double-blind, counter-balanced). In each phase, all smoked ad lib with no drug on week 1 or during dose run-up on week 2, and then tried to quit every day during week 3. Abstinence (confirmed by CO <5 p.p.m.) and smoking reduction (CO <8), as well as cigarettes/day (in Study 1), were assessed daily (Monday-Friday) each quit week and compared between conditions. Secondary outcomes included abstinence symptoms (withdrawal and craving) and cognitive test responding (N-back; continuous performance task). In both studies, compared with placebo, active JNJ-39393406 did not increase the number of abstinent days nor reduce total smoking exposure. We also found no significant improvements in craving, withdrawal, or cognitive function. With this dose and study duration, our findings do not support further testing of this α7 nAChR PAM compound for possible efficacy in smoking cessation, in smokers with or without schizophrenia.

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

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 3mg/kg in acute and repeated injection studies. We also found that chronic infusions of Saz-A at doses of 2 and 6mg/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. Young adult female Sprague-Dawley rats were fitted with IV catheters and were trained for nicotine SA (0.03mg/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 6mg/kg body weight YL-2-203, VMY-2-95, or saline for four consecutive weeks. Chronic administration of VMY-2-95 at doses of 2 and 6mg/kg/day caused significant (p<0.01) decreases in nicotine SA over the 2weeks 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. 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.

Diacylglycerol lipase disinhibits VTA dopamine neurons during chronic nicotine exposure

Chronic nicotine exposure (CNE) alters synaptic transmission in the ventral tegmental area (VTA) in a manner that enhances dopaminergic signaling and promotes nicotine use. The present experiments identify a correlation between enhanced production of the endogenous cannabinoid 2-arachidonoylglycerol (2-AG) and diminished release of the inhibitory neurotransmitter GABA in the VTA following CNE. To study the functional role of on-demand 2-AG signaling in GABAergic synapses, we used 1,2,3-triazole urea compounds to selectively inhibit 2-AG biosynthesis by diacylglycerol lipase (DAGL). The potency and selectivity of these inhibitors were established in rats in vitro (rat brain proteome), ex vivo (brain slices), and in vivo (intracerebroventricular administration) using activity-based protein profiling and targeted metabolomics analyses. Inhibition of DAGL (2-AG biosynthesis) rescues nicotine-induced VTA GABA signaling following CNE. Conversely, enhancement of 2-AG signaling in naive rats by inhibiting 2-AG degradation recapitulates the loss of nicotine-induced GABA signaling evident following CNE. DAGL inhibition reduces nicotine self-administration without disrupting operant responding for a nondrug reinforcer or motor activity. Collectively, these findings provide a detailed characterization of selective inhibitors of rat brain DAGL and demonstrate that excessive 2-AG signaling contributes to a loss of inhibitory GABAergic constraint of VTA excitability following CNE.