Inhibition of monoacylglycerol lipase reduces nicotine reward in the conditioned place preference test in male mice.

Nicotine, the primary psychoactive component in tobacco, plays a major role in the initiation and maintenance of tobacco dependence and addiction, a leading cause of preventable death worldwide. An essential need thus exists for more effective pharmacotherapies for nicotine-use cessation. Previous reports suggest that pharmacological and genetic blockade of CB1 receptors attenuate nicotine reinforcement and reward; while exogenous agonists enhanced these abuse-related behaviors. In this study, we utilized complementary genetic and pharmacologic approaches to test the hypothesis that increasing the levels of the endocannabinoid 2-arachindonoylglycerol (2-AG), will enhance nicotine reward by stimulating neuronal CB1 receptors. Contrary to our hypothesis, we found that inhibition of monoacylglycerol lipase (MAGL), the primary catabolic enzyme of 2-AG, attenuates nicotine conditioned place preference (CPP) in mice, through a non-CB1 receptor-mediated mechanism. MAGL inhibition did not alter palatable food reward or Lithium Chloride (LiCl) aversion. In support of our findings, repeated MAGL inhibition did not induce a reduction in CB1 brain receptor levels or hinder function. To explore the potential mechanism of action, we investigated if MAGL inhibition affected other fatty acid levels in our CPP paradigm. Indeed, MAGL inhibition caused a concomitant decrease in arachidonic acid (AA) levels in various brain regions of interest, suggesting an AA cascade-dependent mechanism. This idea is supported by dose-dependent attenuation of nicotine preference by the selective COX-2 inhibitors valdecoxib and LM-4131. Collectively, these findings, along with our reported studies on nicotine withdrawal, suggest that inhibition of MAGL represents a promising new target for the development of pharmacotherapies to treat nicotine dependence.

Recent findings in the pharmacology of inhaled nicotine: Preclinical and clinical in vivo studies.

INTRODUCTION: The rise of vaping in adolescents, the recent entrance of new inhaled nicotine products such as iQOS on the market and e-cigarette or vaping product use-associated lung injury cases has created concern for the use of inhaled non-combustible nicotine products. This narrative review discusses recent experimental in vivo studies that utilize human, rat and mouse models to understand the pharmacological impact of nicotine from non-combustible products. METHODS: The search engine PubMed was utilized with the following search terms: inhaled nicotine, nicotine e-cigarette, heated tobacco products, iQOS, electronic cigarette, nicotine inhaler, nicotine vaping. This review highlights recent primary in vivo studies of inhaled nicotine administration experimental paradigms that occurred in laboratory settings using human and rodent (rats and mice) models that have been published from January 2017-December 2019. RESULTS: The pharmacokinetics of nicotine via e-cigarettes is influenced by the PG/VG and flavor constituents in e-liquids, the presence of nicotine salts in e-liquids, puff topography of nicotine and tobacco product users and the power of the e-cigarette device. The pharmacodynamic impact of inhaled nicotine has cardiovascular, pulmonary and central nervous system implications. CONCLUSION: The articles reviewed here highlight the importance of both animal and human models to fully understand the impact of inhaled nicotine pharmacology There is a need for more rodent pharmacokinetic inhaled nicotine studies to understand the influences of factors such as flavor and nicotine salts. Additionally, consensus on nicotine measurement in both human and rodent studies is greatly needed.

Preclinical and clinical research on the discriminative stimulus effects of nicotine.

Across species, nicotine can produce robust discriminative stimulus (DS) effects, as with other drugs of abuse, a feature that has been harnessed to advance our understanding on the neuropharmacological mechanisms of nicotine's actions. With the crucial role played by nicotine in supporting tobacco dependence, nicotine DS effects have presented an ideal platform to develop novel generation of smoking cessation compounds. Findings from preclinical strands of research have invigorated the field of human discrimination research to objectively assess nicotine's interoceptive stimulus effects. As such, translation studies provide proof of concept for nicotine DS research as a method to assess the subjective effects of nicotine per se, separate from non-nicotine stimuli involved in smoking. Recent clinical studies with low doses have demonstrated that perceiving nicotine's DS effects is necessary, yet not sufficient, for that dose to be reinforcing. These measures have been instrumental in developing novel strategies with regards to establishing threshold doses of nicotine contained in tobacco products, to then determine subthreshold doses that cannot be discriminated and, therefore, fail to maintain reinforcement. Findings from preclinical and clinical nicotine DS research could substantially inform public health policies aimed at regulating nicotine content of consumer products so that they minimize risks of dependency. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

Reduced Thalamus Volume May Reflect Nicotine Severity in Young Male Smokers.

Introduction: Nicotine acts as an agonist at presynaptic nicotinic acetylcholine receptors and to facilitate synaptic release of several neurotransmitters including dopamine and glutamate. The thalamus has the highest density of nicotinic acetylcholine receptors in the brain, which may make this area more vulnerable to the addictive effects of nicotine. However, the volume of thalamus abnormalities and the association with smoking behaviors in young smokers remains unknown. Methods: Thirty-six young male smokers and 36 age-, gender- and education-matched nonsmokers participated in the current study. The nicotine dependence severity and cumulative effect were assessed with the Fagerström test for nicotine dependence (FTND) and pack-years. We used subcortical volume analyses method in FreeSurfer to investigate the thalamus volume differences between young smokers and nonsmokers. Correlation analysis was used to investigate the relationship between thalamus volume and smoking behaviors (pack-years and FTND) in young smokers. Results and Conclusions: Relative to nonsmokers, the young smokers showed reduced volume of bilateral thalamus. In addition, the left thalamus volume was correlated with FTND in young smokers. It is hoped that our findings can shed new insights into the neurobiology of young smokers. Implications: In this article, we investigated the changes of thalamus volume in young male smokers compared with nonsmokers. Reduced left thalamus volume was correlated with FTND in young smokers, which may reflect nicotine severity in young male smokers.

Selective CRF2 receptor agonists ameliorate the anxiety- and depression-like state developed during chronic nicotine treatment and consequent acute withdrawal in mice.

The aim of the present study was to investigate the effects of the selective agonists of the corticotropin-releasing factor (CRF) 2 receptor, urocortin 2 (UCN 2) and urocortin 3 (UCN 3), on the anxiety- and depression-like signs induced by acute nicotine withdrawal in mice. In order to do so, male CFLP mice were exposed for 7 days to repeated intraperitoneal (IP) injection with nicotine or saline solution and 1day of acute withdrawal and then a single intracerebroventricular (ICV) injection with UCN 2, UCN 3 or saline solution. After 30min the mice were observed in an elevated plus-maze test or a forced swim test, for anxiety- and depression-like behavior. After 5min of testing, the plasma corticosterone concentration reflecting the activity of the hypothalamic-pituitary-adrenal (HPA) axis was also determined by a chemo-fluorescent method. Half of the animals were treated ICV and evaluated on the 8th day, the other half on the 9th day. On the 8th day, nicotine-treated mice presented signs of anxiolysis and depression, but no significant elevation of the plasma corticosterone concentration. On the 9th day, nicotine-treated mice exhibited signs of anxiety and depression and a significant increase of the plasma corticosterone levels. Central administration of UCN 2 or UCN 3 ameliorated the anxiety- and depression-like state including the hyperactivity of the HPA axis, developed during acute withdrawal following chronic nicotine treatment. The present study suggests that selective CRF2 receptor agonists could be used as a therapy in nicotine addiction.

Decreased Nicotinic Receptor Availability in Smokers with Slow Rates of Nicotine Metabolism.

UNLABELLED: The nicotine metabolite ratio (NMR), a stable measure of hepatic nicotine metabolism via the CYP2A6 pathway and total nicotine clearance, is a predictive biomarker of response to nicotine replacement therapy, with increased quit rates in slower metabolizers. Nicotine binds directly to nicotinic acetylcholine receptors (nAChRs) to exert its psychoactive effects. This study examined the relationship between NMR and nAChR (α4ß2* subtype) availability using PET imaging of the radiotracer 2-(18)F-fluoro-3-(2(S)-azetidinylmethoxy)pyridine (2-(18)F-FA-85380, or 2-(18)F-FA). METHODS: Twenty-four smokers-12 slow metabolizers (NMR < 0.26) and 12 normal metabolizers (NMR ≥ 0.26)-underwent 2-(18)F-FA-PET brain imaging after overnight nicotine abstinence (18 h before scanning), using a validated bolus-plus-infusion protocol. Availability of nAChRs was compared between NMR groups in a priori volumes of interest, with total distribution volume (VT/fP) being the measure of nAChR availability. Cravings to smoke were assessed before and after the scans. RESULTS: Thalamic nAChR α4ß2* availability was significantly reduced in slow nicotine metabolizers (P = 0.04). Slow metabolizers exhibited greater reductions in cravings after scanning than normal metabolizers; however, craving was unrelated to nAChR availability. CONCLUSION: The rate of nicotine metabolism is associated with thalamic nAChR availability. Additional studies could examine whether altered nAChR availability underlies the differences in treatment response between slow and normal metabolizers of nicotine.

Paecilomycies japonica reduces repeated nicotine-induced neuronal and behavioral activation in rats.

BACKGROUND: Many studies have demonstrated that repeated injections of nicotine can produce progressive increases in locomotor activity and enhanced expression of c-fos and tyrosine hydroxylase (TH) in brain dopaminergic areas. Paecilomyces japonica (PJ) is a herbal medicine that is commonly used to treat opiate and other addictions in Eastern Asia. However, its influence on nicotine addiction has not been examined. This study was carried out to investigate the effects of PJ on repeated nicotine-induced behavioral sensitization of locomotor activity and c-Fos and TH expression in the rat brain using immunohistochemistry. METHODS: Rats were pretreated with PJ (10, 25, 50, 100, and 200 mg/kg, intraperitoneally) 30 min before repeated injections of nicotine (0.4 mg/kg, subcutaneously, twice daily for 7 days). Locomotor activity was measured in rats during 7-day nicotine treatments. On the seventh day, c-Fos and TH expression were assessed. RESULTS: Pretreatment with PJ decreased the development of nicotine-induced sensitization, c-Fos expression in the nucleus accumbens and striatum, and TH expression in the ventral tegmental area. PJ decreased nicotine-induced locomotor activity by modulating brain dopaminergic systems. CONCLUSION: The results of the present study suggest that PJ may be a useful agent for preventing and treating nicotine addiction.

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 α4ß2* 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 α4ß2* 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 α4ß2 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.

Serotonin involvement in Rhodiola rosea attenuation of nicotine withdrawal signs in rats.

Rhodiola rosea has been used for centuries in the traditional medicine to stimulate nervous system, to enhance physical and mental performance and to treat fatigue. It is known that administration of Rhodiola rosea extract elicits antidepressant activity, but the mechanism of action still remains unclear. Evidence from animal models and human studies show that nicotine reduces symptoms of depression and that nicotine cessation induces depressive-like symptoms. We investigated the effects of Rhodiola rosea on nicotine withdrawal signs. Nicotine dependence was induced by subcutaneous nicotine injection (2 mg/kg, four times daily) for 14 days. Another group of animals treated with nicotine (for 14 days) and successively with Rhodiola rosea extract was co-administered with selective 5-HT receptorial antagonist WAY 100635 (1 mg/kg). After nicotine withdrawal animals were evaluated for behavioural parameters (locomotor activity, abstinence signs, marble burying test), diencephalic serotonin metabolism and serotonin receptor-1A expression. Results show a significant increase of 5-HT content in N treated with R. rosea, with a significant increase of serotonin receptor 1A, suggesting an involvement of serotonin in beneficial effects of R. rosea on suffering produced by nicotine withdrawal.

Effects of nicotine on the amplitude and gating of the auditory P50 and its influence by dopamine D2 receptor gene polymorphism.

Evidence of normalized auditory P50 suppression with acute nicotine in schizophrenia has supported the contention that elevated smoking rates in this disorder may be an attempt to correct a nicotinic receptor pathophysiology that may underly impaired sensory gating in these patients. There is very little information regarding the neurochemical or genetic pathways through which nicotine regulates P50 amplitude and its suppression in human studies. In a randomized, double-blind, placebo-controlled design with 24 non-smokers, this study examined the influence of TaqIA dopamine D2 receptor gene polymorphisms on P50 and its inhibition during nicotine gum (6 mg) administration. Within a paired click (S(1)-S(2)) paradigm, placebo treated A1(+) and A1(-) allele groups differed with respect to P50 amplitude and gating. While nicotine (relative to placebo) attenuated S(1) P50 amplitude in A1(+) allele carriers, in the A1(-) carriers it increased S(2) P50 amplitude and increased P50 gating as indexed by an augmented gating difference wave (GDW). These findings suggest that nicotine exerts mixed gating properties in healthy nicotine naive volunteers and that dopamine functions to alter both P50 and its gating as well as their response to acute nicotine agonist treatment.

Effects of the selective dopamine D3 receptor antagonist SB-277011A on the reinforcing effects of nicotine as measured by a progressive-ratio schedule in rats.

The dopamine D3 receptor is primarily localized within the mesocorticolimbic system, and may therefore have potential as a pharmacotherapeutic target for the treatment of drug dependence. Studies have shown that the selective dopamine D3 receptor antagonist SB-277011A reduces a variety of dependence-related behavioral effects of cocaine, alcohol and heroin. A previous study examining SB-277011A on nicotine self-administration using relatively low doses of the antagonist and a low response requirement for nicotine found no effect on drug-taking behavior per se, whereas reinstatement of nicotine-seeking was reduced. The purpose of the present study was to further examine the effects of higher doses of SB-277011A on nicotine self-administration in rats under a progressive-ratio (PR) schedule, which imposes relatively high response requirements for nicotine. Rats were trained to respond under a PR schedule of either nicotine or food reinforcement. Once responding was stable, SB-277011A (3-56 mg/kg) or vehicle was administered i.p. 1 h prior to the operant session. The highest dose tested significantly decreased the mean number of reinforcers and mean response rates in the nicotine self-administration group, but had no effect on either the mean number of reinforcers or response rate in the food group. In a separate set of experiments, the effects of SB-277011A on locomotor activity were measured. At the dose that significantly decreased nicotine self-administration, total distance traveled was also significantly decreased, suggesting that the effect on operant responding at the high dose of SB-277011A is at a threshold for motor effects and may not be directly mediated by an action at dopamine D3 receptors.

Effect of short-term cigarette smoke exposure on body weight, appetite and brain neuropeptide Y in mice.

Although nicotinic receptors have been demonstrated in hypothalamic appetite-regulating areas and nicotine administration alters food intake and body weight in both animals and humans, the mechanisms underlying the effects of smoking on appetite circuits remain unclear. Conflicting effects of nicotine on the major orexigenic peptide, neuropeptide Y (NPY), have been observed in the brain, but the effects of smoking are unknown. Thus, we aimed to investigate how cigarette smoking affects body weight, food intake, plasma leptin concentration, hypothalamic NPY peptide, adipose mass and mRNA expression of uncoupling proteins (UCP), and tumor necrosis factor (TNF) alpha. Balb/C mice (8 weeks) were exposed to cigarette smoke (three cigarettes, three times a day for 4 consecutive days) or sham exposed. Body weight and food intake were recorded. Plasma leptin and brain NPY were measured by radioimmunoassay. UCPs and TNF alpha mRNA were measured by real-time PCR. Food intake dropped significantly from the first day of smoking, and weight loss became evident within 2 days. Brown fat and retroperitoneal white fat masses were significantly reduced, and plasma leptin concentration was decreased by 34%, in line with the decreased fat mass. NPY concentrations in hypothalamic subregions were similar between two groups. UCP1 mRNA was decreased in white fat and UCP3 mRNA increased in brown fat in smoking group. Short-term cigarette smoke exposure led to reduced body weight, food intake, and fat mass. The reduction in plasma leptin concentration may have been too modest to increase NPY production; alternatively, change in NPY or its function might have been offset by nicotine or other elements in cigarette smoke.

Characterization of the effects of bupropion on the reinforcing properties of nicotine and food in rats.

Bupropion is an atypical antidepressant drug that is the only nonnicotine-based prescription medicine approved for smoking cessation by the Food and Drug Administration. The aim of the present experiments was to investigate the effects of bupropion (5-40 mg/kg) on the reinforcing properties of nicotine and food in rats. The effects of bupropion were studied under two schedules of reinforcement: a fixed ratio 5 time-out 20-sec (FR5 TO20 s) and a progressive ratio (PR). Rats were trained to respond for nicotine (0.01 or 0.03 mg/kg/infusion, free base) or food under the FR5 TO20 s schedule. Pretreatment with the highest dose of bupropion (40 mg/kg) resulted in a significant reduction (approximately 50%) of nicotine intake in rats self-administering 0.03 mg/kg/infusion of nicotine. The same dose of bupropion also decreased (approximately 40%) the self-administration of 0.01 mg/kg/infusion of nicotine, but this effect did not reach statistical significance. Pretreatment with bupropion slightly (approximately 15%) reduced responding for food under the FR5 TO20 s schedule. Finally, pretreatment with bupropion did not affect the self-administration of nicotine (0.03 mg/kg/infusion) under a PR schedule, but dose-dependently increased responding for food under the same PR schedule. These findings indicate that a high dose of bupropion decreases the reinforcing properties of nicotine as measured under an FR schedule, while having no apparent effects on breaking points for nicotine under a PR schedule that reflects both the reinforcing properties and the motivation to obtain nicotine.

Nicotine administration decreases nitric oxide synthase expression in the hypothalamus of food-deprived rats.

Effect of nicotine on nitric oxide synthase (NOS) expression in various hypothalamic regions was investigated in rats via nicotineamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry. Sprague-Dawley rats were divided into the fed group, the fed and nicotine-treated group, the food-deprived group, and the food-deprived and nicotine-treated group. The fed groups received abundant food and water, while food was withheld from the food-deprived groups for 48 h. The nicotine-treated groups were injected with nicotine. Following food deprivation, enhanced NAPDH-d expression was detected in the paraventricular nucleus, ventromedial hypothalamic nucleus, and lateral hypothalamic area of the hypothalamus. Nicotine administration to the food-deprived rats resulted in decreased NADPH-d positivity. The present results indicate that nicotine administration is effective in limiting the enhancement in NOS expression following food restriction.