The α4 Nicotinic Acetylcholine Receptor Is Necessary for the Initiation of Organophosphate-Induced Neuronal Hyperexcitability

Acute intoxication with organophosphorus (OP) cholinesterase inhibitors can produce seizures that rapidly progress to life-threatening status epilepticus. Significant research effort has been focused on investigating the involvement of muscarinic acetylcholine receptors (mAChRs) in OP-induced seizure activity. In contrast, there has been far less attention on nicotinic AChRs (nAChRs) in this context. Here, we address this data gap using a combination of in vitro and in vivo models. Pharmacological antagonism and genetic deletion of α4, but not α7, nAChR subunits prevented or significantly attenuated OP-induced electrical spike activity in acute hippocampal slices and seizure activity in mice, indicating that α4 nAChR activation is necessary for neuronal hyperexcitability triggered by acute OP exposures. These findings not only suggest that therapeutic strategies for inhibiting the α4 nAChR subunit warrant further investigation as prophylactic and immediate treatments for acute OP-induced seizures, but also provide mechanistic insight into the role of the nicotinic cholinergic system in seizure generation.

Mifepristone decreases nicotine intake in dependent and non-dependent adult rats

BACKGROUND

Addiction to tobacco and nicotine products has adverse health effects and afflicts more than a billion people worldwide. Therefore, there is an urgent need for new treatments to reduce tobacco and nicotine use. Glucocorticoid receptor blockade shows promise as a novel treatment for drug abuse and stress-related disorders.

AIM

These studies aim to investigate whether glucocorticoid receptor blockade with mifepristone diminishes the reinforcing properties of nicotine in rats with intermittent or daily long access to nicotine.

METHODS

The rats self-administered 0.06 mg/kg/inf of nicotine for 6 h per day, with either intermittent or daily access for 4 weeks before treatment with mifepristone. Daily nicotine self-administration models regular smoking, while intermittent nicotine self-administration models occasional smoking. To determine whether the rats were dependent, they were treated with the nicotinic acetylcholine receptor antagonist mecamylamine, and somatic signs were recorded.

RESULTS

The rats with intermittent access to nicotine had a higher level of nicotine intake per session than those with daily access but only the rats with daily access to nicotine showed signs of physical dependence. Furthermore, mecamylamine increased nicotine intake during the first hour of access in rats with daily access but not in those with intermittent access. Mifepristone decreased total nicotine intake in rats with intermittent and daily access to nicotine. Moreover, mifepristone decreased the distance traveled and rearing in the open field test and operant responding for food pellets.

CONCLUSION

These findings indicate that mifepristone decreases nicotine intake but this effect may be partially attributed to the sedative effects of mifepristone.

Critical roles of nicotinic acetylcholine receptors in olfactory memory formation and retrieval in crickets

Acetylcholine (ACh) is a major excitatory neurotransmitter in the insect central nervous system, and insect neurons express several types of ACh receptors (AChRs). AChRs are classified into two subgroups, muscarinic AChRs and nicotinic AChRs (nAChRs). nAChRs are also divided into two subgroups by sensitivity to α-bungarotoxin (α-BGT). The cricket Gryllus bimaculatus is one of the useful insects for studying the molecular mechanisms in olfactory learning and memory. However, the roles of nAChRs in olfactory learning and memory of the cricket are still unknown. In the present study, to investigate whether nAChRs are involved in cricket olfactory learning and memory, we tested the effects of two different AChR antagonists on long-term memory (LTM) formation and retrieval in a behavioral assay. The two AChR antagonists that we used are mecamylamine (MEC), an α-BGT-insensitive nAChR antagonist, and methyllycaconitine (MLA), an α-BGT-sensitive nAChR antagonist. In crickets, multiple-trial olfactory conditioning induced 1-day memory (LTM), whereas single-trial olfactory conditioning induced 1-h memory (mid-term memory, MTM) but not 1-day memory. Crickets injected with MEC 20 min before the retention test at 1 day after the multiple-trial conditioning exhibited no memory retrieval. This indicates that α-BGT-insensitive nAChRs participate in memory retrieval. In addition, crickets injected with MLA before the multiple-trial conditioning exhibited MTM but not LTM, indicating that α-BGT-sensitive nAChRs participate in the formation of LTM. Moreover, injection of nicotine (an nAChR agonist) before the single-trial conditioning induced LTM. Finally, the nitric oxide (NO)-cGMP signaling pathway is known to participate in the formation of LTM in crickets, and we conducted co-injection experiments with an agonist or inhibitor of the nAChR and an activator or inhibitor of the NO-cGMP signaling pathway. The results suggest that nAChR works upstream of the NO-cGMP signaling system in the LTM formation process.

Pharmacotherapy in Tobacco Cessation: A Narrative Review

A chronic, recurring illness, known as nicotine addiction and dependence, is defined by a person's dependence on the substance up to the extent that their normal day-to-day activities are compromised in the absence of the substance. This paper will highlight first-line smoking cessation treatments, such as nicotine replacement therapy (NRT), bupropion, and varenicline, and second-line medications, such as clonidine, nortriptyline, anxiolytics, mecamylamine, naltrexone, and NicVAX (Nabi Biopharmaceuticals, Rockville, MD, USA). NRT offers many options for nicotine delivery methods, comprising nicotine gum, rapid-release gum, lozenges, transdermal patches, high-dose nicotine patches, oral inhalers, nasal sprays, electronic nicotine delivery systems (ENDS), and sublingual tablets. Pharmacotherapies for quitting tobacco should lessen withdrawal symptoms and stop nicotine's reinforcing effects without having too many side effects.

What We Have Gained from Ibogaine: α3β4 Nicotinic Acetylcholine Receptor Inhibitors as Treatments for Substance Use Disorders

For decades, ibogaine─the main psychoactive alkaloid found in Tabernanthe iboga─has been investigated as a possible treatment for substance use disorders (SUDs) due to its purported ability to interrupt the addictive properties of multiple drugs of abuse. Of the numerous pharmacological actions of ibogaine and its derivatives, the inhibition of α3β4 nicotinic acetylcholine receptors (nAChRs), represents a probable mechanism of action for their apparent anti-addictive activity. In this Perspective, we examine several classes of compounds that have been discovered and developed to target α3β4 nAChRs. Specifically, by focusing on compounds that have proven efficacious in pre-clinical models of drug abuse and have been evaluated clinically, we highlight the promising potential of the α3β4 nAChRs as viable targets to treat a wide array of SUDs. Additionally, we discuss the challenges faced by the existing classes of α3β4 nAChR ligands that must be overcome to develop them into therapeutic treatments.

Outcomes of nicotinic modulation on markers of intestinal IgA antibody response

Acetylcholine (ACh), as a ligand of nicotinic acetylcholine receptors (nAChRs), plays a key role in the cholinergic anti-inflammatory pathway; however, its role in the immunoglobulin A (IgA) response remains unknown. Therefore, the present study aimed to investigate the role of ACh in the intestinal biomarkers involved in IgA synthesis and the polymeric immunoglobulin receptor (pIgR) involved in IgA transcytosis. Groups of mice were administered GTS-21 (an α7nAChR agonist) or mecamylamine (a non-selective nAChR antagonist) intraperitoneally for 7 days. Intestinal fluids were used for antibody concentration assessment by ELISA, cell suspensions from Peyer's patches and the lamina propria were obtained for flow cytometric analysis of plasma cells, and CD4⁺ T-cells expressing intracellular transforming growth factor (TGF)-β and IgA-producing interleukin (IL)-4, -5, -6 and -10, and isolated epithelial cells to determine the levels of pIgR mRNA using reverse transcription-quantitative PCR. Regarding to the untreated control group, the concentration of IgA was reduced in the mecamylamine group and unaltered in the GTS-21 group while IgM levels exhibited no differences; the percentage of IgA⁺ plasma cells from Peyer's patches and the lamina propria, and the percentage of TGF-β⁺/CD4⁺ T-cells from Peyer's patches were greater in the GTS-21-group. In both treatment groups, the percentages of IgM⁺ plasma cells and IL-6⁺/IL-10⁺ CD4⁺ T cells were greater in both compartments; pIgR mRNA expression levels decreased in epithelial cells. The percentage of IL-4 CD4⁺ T-cells were greater in Peyer's patches and lower in the lamina propria in the mecamylamine group, and the percentage of IL-5 CD4⁺ T-cells in the lamina propria were decreased in both treatment groups. These findings require further examination to address the impact of cholinergic modulation on IgA-transcytosis via pIgR. The present study may be an experimental reference for clinical trials that address the role of nicotinic system in intestinal dysfunctions as postoperative ileus.

Estradiol treatment in young postmenopausal women with self-reported cognitive complaints: Effects on cholinergic-mediated cognitive performance

OBJECTIVE

Older women are at increased risk of developing Alzheimer's disease compared to men. One proposed reason is that following menopause there is a decline in estrogens. Estrogens are important for cholinergic functioning and attenuate the impact of cholinergic antagonists on cognitive performance in postmenopausal women. Self-reported or subjective cognitive complaints in middle or older age may represent a harbinger of cognitive decline and those who endorse cognitive complaints appear more likely to develop future cognitive impairment. However, the response of individuals with cognitive complaints after menopause to estrogen and the relationship to cholinergic functioning has not been investigated. This study investigated the effect of estrogen treatment using 17β-estradiol on cognitive performance following anticholinergic blockade in postmenopausal women and the relationship of this interaction with the level of self-reported (subjective) postmenopausal cognitive complaints.

METHODS

Forty postmenopausal women (aged 50-60 years) completed a 3-month treatment regimen of either 1 mg oral estradiol or placebo. Participants then completed four challenge days in which they completed cognitive and behavioral tasks after one of four cholinergic antagonist drug conditions (oral mecamylamine (MECA), intravenous scopolamine, combined MECA and scopolamine, or PLC).

RESULTS

Compared to PLC, the estradiol treated group performed worse on attention tasks under cholinergic challenge including the choice reaction time task and the critical flicker fusion task. In addition, participants who endorsed greater cognitive complaints showed reduced performance on the N-back working memory task, regardless of whether they received estradiol treatment.

CONCLUSIONS

The findings of this study indicate that estradiol treatment was unable to mitigate anticholinergic blockade in postmenopausal women with subjective cognitive complaints, and worsened performance on attention tasks. Moreover, the present study suggests that greater levels of cognitive complaints following menopause may be associated with an underlying decline in cholinergic function that may manifest as an inability to compensate during working memory tasks.

Co-administration of morphine and levamisole increases death risk, produces neutropenia and modifies antinociception in mice

Levamisole is a veterinary anthelmintic drug and a common adulterant of misused drugs. This study analyses the lethal, antinociceptive and haematological effects produced by acute or repeated levamisole administration by itself or combined with morphine. Independent groups of male Swiss Webster mice were i.p. injected with 100 mg/kg morphine, 31.6 mg/kg levamisole (lethal doses at 10%, LD₁₀ ) or the same doses combined. Naloxone pretreatment (10 mg/kg, i.p.) prevented morphine-induced death, as did 2.5 mg/kg, i.p. mecamylamine with levamisole. Co-administration of levamisole and morphine (Lvm + Mor) increased lethality from 10% to 80%. This augmented effect was prevented by 30 mg/kg, i.p. naloxone and reduced with 10 mg/kg naloxone plus 2.5 mg/kg, i.p. mecamylamine. In independent groups of mice, 17.7 mg/kg, i.p. levamisole antagonized the acute morphine's antinociceptive effect evaluated in the tail-flick test. Repeated 17.7 mg/kg levamisole administration (2×/day/3 weeks) did not affect tolerance development to morphine (10 mg/kg, 3×/day/1 week). Blood samples obtained from mice repeatedly treated with levamisole showed leukopenia and neutropenia. Morphine also produced neutropenia, increased erythrocyte count and other related parameters (e.g. haemoglobin). Lvm + Mor had similar effects on leukocyte and neutrophil counts to those seen with levamisole only, but no erythrocyte-related alterations were evident. Blood chemistry analysis did not indicate liver damage but suggested some degree of electrolyte balance impairment. In conclusion, Lvm + Mor increased death risk, altered morphine-induced antinociceptive effects and produced haematologic abnormalities. The importance of studying combinations of drugs of abuse lies in the fact that drug users frequently combine drugs, which are commonly adulterated.

Melatonin-Activated Receptor Signaling Pathways Mediate Protective Effects on Surfactant-Induced Increase in Jejunal Mucosal Permeability in Rats

A well-functional intestinal mucosal barrier can be compromised as a result of various diseases, chemotherapy, radiation, and chemical exposures including surfactants. Currently, there are no approved drugs targeting a dysfunctional intestinal barrier, which emphasizes a significant medical need. One candidate drug reported to regulate intestinal mucosal permeability is melatonin. However, it is still unclear if its effect is primarily receptor mediated or antioxidative, and if it is associated with enteric neural pathways. The aim of this rat intestinal perfusion study was to investigate the mechanisms of melatonin and nicotinic acetylcholine receptors on the increase in intestinal mucosal clearance of 51Cr-labeled ethylenediaminetetraacetate induced by 15 min luminal exposure to the anionic surfactant, sodium dodecyl sulfate. Our results show that melatonin abolished the surfactant-induced increase in intestinal permeability and that this effect was inhibited by luzindole, a melatonin receptor antagonist. In addition, mecamylamine, an antagonist of nicotinic acetylcholine receptors, reduced the surfactant-induced increase in mucosal permeability, using a signaling pathway not influenced by melatonin receptor activation. In conclusion, our results support melatonin as a potentially potent candidate for the oral treatment of a compromised intestinal mucosal barrier, and that its protective effect is primarily receptor-mediated.

Nicotinic aspects of the discriminative stimulus effects of arecoline

Despite the evidence that the muscarinic agonist arecoline is a drug of abuse throughout Southeast Asia, its stimulus characteristics have not been well studied. The goal of this work was to understand more about the mediation of discriminative stimulus effects of arecoline. Arecoline (1.0 mg/kg s.c.) was trained as a discriminative stimulus in a group of eight rats. The ability of various cholinergic agonists and antagonists to mimic or antagonize the discriminative stimulus effects of arecoline and to modify its rate-suppressing effects was evaluated. A muscarinic antagonist, but neither of two nicotinic antagonists, was able to modify the discriminative stimulus effects of arecoline, suggesting a predominant muscarinic basis of arecoline's discriminative stimulus effects in this assay. However, both nicotine itself and two nicotine agonists with selective affinity for the α4β2* receptor (ispronicline and metanicotine) produced full arecoline-like discriminative stimulus effects in these rats. The discriminative stimulus effects of the selective nicotine agonists were blocked by both the general nicotine antagonist mecamylamine and by the selective α4β2* antagonist, dihydro-beta-erythroidine (DHβE). Surprisingly, only DHβE antagonized the rate-suppressing effects of the selective nicotine agonists. These data indicate a selective α4β2* nicotine receptor component to the behavioral effects of arecoline. Although the nicotinic aspects of arecoline's behavior effects could suggest that abuse of arecoline-containing material (e.g. betel nut chewing) is mediated through nicotinic rather than muscarinic actions, further research, specifically on the reinforcing effects of arecoline, is necessary before this conclusion can be supported.

Rodent models for nicotine withdrawal

BACKGROUND

Animal models are critical to improve our understanding of the neuronal mechanisms underlying nicotine withdrawal. Nicotine dependence in rodents can be established by repeated nicotine injections, chronic nicotine infusion via osmotic minipumps, oral nicotine intake, tobacco smoke exposure, nicotine vapor exposure, and e-cigarette aerosol exposure. The time course of nicotine withdrawal symptoms associated with these methods has not been reviewed in the literature.

AIM

The goal of this review is to discuss nicotine withdrawal symptoms associated with the cessation of nicotine, tobacco smoke, nicotine vapor, and e-cigarette aerosol exposure in rats and mice. Furthermore, age and sex differences in nicotine withdrawal symptoms are reviewed.

RESULTS

Cessation of nicotine, tobacco smoke, nicotine vapor, and e-cigarette aerosol exposure leads to nicotine withdrawal symptoms such as somatic withdrawal signs, changes in locomotor activity, anxiety- and depressive-like behavior, learning and memory deficits, attention deficits, hyperalgesia, and dysphoria. These withdrawal symptoms are most pronounced within the first week after cessation of nicotine exposure. Anxiety- and depressive-like behavior, and deficits in learning and memory may persist for several months. Adolescent (4-6 weeks old) rats and mice display fewer nicotine withdrawal symptoms than adults (>8 weeks old). In adult rats and mice, females show fewer nicotine withdrawal symptoms than males. The smoking cessation drugs bupropion and varenicline reduce nicotine withdrawal symptoms in rodents.

CONCLUSION

The nicotine withdrawal symptoms that are observed in rodents are similar to those observed in humans. Tobacco smoke and e-cigarette aerosol contain chemicals and added flavors that enhance the reinforcing properties of nicotine. Therefore, more valid animal models of tobacco and e-cigarette use need to be developed by using tobacco smoke and e-cigarette aerosol exposure methods to induce dependence.

Mirtazapine decreased induction and expression of cocaine + nicotine-induced locomotor sensitisation in rats

Objectives: Concurrent abuse of cocaine and nicotine is considered a public health problem. To date, no effective therapy has been known to reduce the reinforcing effects of concurrent use of cocaine and nicotine. Mirtazapine, an antagonist of the α₂-adrenoceptor and the 5-HT_2A/C and the 5-HT₃ receptors has proven effective in reducing the cocaine, nicotine and methamphetamine behavioural effects in humans and animals. Our study evaluated the effect of mirtazapine on enhancing locomotor activity during the induction and expression of locomotor sensitisation induced by a cocaine + nicotine mixture.Methods: Wistar rats were dosed with cocaine, nicotine or cocaine + nicotine combination. Mirtazapine (30 mg/kg, i.p.) was administered during the extinction phase.Results: Mirtazapine decreased cocaine + nicotine-induced locomotor activity and induction and expression of locomotor sensitisation. In addition, we found that co-administration of mecamylamine and mirtazapine significantly enhanced the effect of mirtazapine on cocaine + nicotine-induced locomotor activity during induction and expression of behavioural sensitisation.Conclusions: Our results suggest that mirtazapine demonstrated efficacy in decreasing the psycho-stimulant effects of concurrent use of cocaine and nicotine.

Unexpected loss of sensitivity to the nicotinic acetylcholine receptor antagonist activity of mecamylamine and dihydro-β-erythroidine in nicotine-tolerant mice

OBJECTIVES

There is a long-standing interest in developing nicotinic acetylcholine receptor (nAChR) antagonists for concomitant use with nAChR agonists (e.g., nicotine replacement) as complementary smoking cessation aids. Previous studies demonstrate that daily nicotine treatment confers tolerance to some effects of nicotine, as well as cross-tolerance to other nAChR agonists. The current study assessed the extent to which antagonism of nicotine varies as a function of daily nicotine treatment.

METHODS

Schedule-controlled responding and hypothermia were selected for study because they have been previously used to examine the pharmacology of nicotine, and both are sensitive to the development nicotine tolerance. The rate-decreasing and hypothermic effects of nicotine, as well as antagonism of those effects, were examined in C57BL/6J mice before, during treatment with, and after discontinuation of three daily injections of 1.78 mg/kg nicotine. The nonselective nAChR antagonist mecamylamine and the β2 nAChR antagonist dihydro-β-erythroidine (DHβE) were studied in combination with nicotine.

RESULTS

The ED₅₀ values of nicotine to produce rate-decreasing and hypothermic effects were, respectively, 0.44 and 0.82 mg/kg prior, 1.6 and 3.2 mg/kg during, and 0.74 and 1.1 mg/kg after discontinuation of daily nicotine treatment. Prior to daily nicotine treatment, mecamylamine decreased response rate and rectal temperature. However, during daily nicotine, mecamylamine (up to 5.6 mg/kg) only decreased rectal temperature. DHβE (up to 5.6 mg/kg) when studied prior to daily nicotine decreased rectal temperature, but that decrease was abolished during chronic nicotine treatment. Mecamylamine and DHβE antagonized the rate-decreasing and hypothermic effects of nicotine before and after daily nicotine; however, during daily nicotine, mecamylamine and DHβE antagonized only the hypothermic effects of nicotine.

CONCLUSIONS

The differential antagonism of rate-decreasing and hypothermic effects implicates differential involvement of nAChR subtypes. The decreased capacity of mecamylamine and DHβE to antagonize nicotine during chronic nicotine treatment may indicate that their effectiveness as smoking cessations might vary as a function of nicotine tolerance and dependence.

Activation and Inactivation of Nicotinic Receptnors in the Dorsal Hippocampal Region Restored Negative Effects of Total (TSD) and REM Sleep Deprivation (RSD) on Memory Acquisition, Locomotor Activity and Pain Perception

Sleep deprivation (SD) is a common issue in today's society. Sleep is essential for proper cognitive functions, including learning and memory. Furthermore, sleep disorders can alter pain information processing. Meanwhile, hippocampal nicotinic receptors have a role in modulating pain and memory. The goal of this study is to investigate the effect of dorsal hippocampal (CA1) nicotinic receptors on behavioral changes induced by Total (TSD) and REM Sleep Deprivation (RSD). A modified water box and multi-platform apparatus were used to induce TSD and RSD, respectively. To investigate the interaction between nicotinic receptors and hippocampus-dependent memory, nicotinic receptor agonist (nicotine) or antagonist (mecamylamine) was injected into the CA1 region. The results showed, nicotine at the doses of 0.001 and 0.1 µg/rat and mecamylamine at the doses of 0.01 and 0.1 µg/rat decreased memory acquisition, while both at the doses of 0.01 and 0.1 µg/rat enhanced locomotor activity. Additionally, all doses used for both drugs did not alter pain perception. Also, 24 h TSD or RSD attenuated memory acquisition with no effect on locomotor activity and only TSD induced an analgesic effect. Intra-CA1 administration of subthreshold dose of nicotine (0.0001 µg/rat) and mecamylamine (0.001 µg/rat) did not alter memory acquisition, pain perception and locomotor activity in sham of TSD/RSD rats. Both drugs reversed all behavioral changes induced by TSD. Furthermore, both drugs reversed the effect of RSD on memory acquisition, while only mecamylamine reversed the effect of RSD on locomotor activity. In conclusion, CA1 nicotinic receptors play a significant role in TSD/RSD-induced behavioral changes.

Green tobacco sickness: mecamylamine, varenicline, and nicotine vaccine as clinical research tools and potential therapeutics

INTRODUCTION

Green tobacco sickness occurs from transdermal absorption of chemicals from freshly harvested, green tobacco leaves. Signs and symptoms include nausea, vomiting, headache, and abdominal cramps. Prevalence has shifted from the United States and Europe to China, India, and Brazil. Worldwide 8 million individuals are afflicted, including women and children. Areas covered: Mecamylamine (Inversine®, Vecamyl®), a nicotinic acetylcholine receptor (nAChR) antagonist, should be tested as a remedy for green tobacco sickness. Mecamylamine is approved as an oral tablet for the treatment of hypertension, is safe, and is off-patent. Mecamylamine attenuates many of the effects of nicotine and tobacco including seizures, thereby supporting its use as an effective pharmacotherapy for tobacco dependence. Varenicline (Chantix®) and cytisine (Tabex®) are low efficacy (i.e. intrinsic activity) nAChR agonists, are used as smoking cessation aids, and are viable options to test as remedies against green tobacco sickness. Nicotine immunization strategies may provide further options for future testing. Expert commentary: Efforts to demonstrate reversal and/or prevention of green tobacco sickness by mecamylamine will underscore the importance of nicotine in this illness and highlight a new medication for effective treatment of tobacco poisoning.

Amino acid modulation of dopamine in the nucleus accumbens mediates sex differences in nicotine withdrawal

The aversive effect of nicotine withdrawal is greater in female versus male rats, and we postulate that this sex difference is mediated in the nucleus accumbens (NAc). Nicotine withdrawal induces decreases in NAc dopamine and increases in acetylcholine (ACh) levels in male rats. To our knowledge, these neurochemical markers of nicotine withdrawal have not been compared in female versus male rats. Given the role of amino acids in modulating NAc dopaminergic and cholinergic transmission, concomitant measures of gamma-aminobutyric acid (GABA) and glutamate levels were also compared across sex. Rats received continuous nicotine exposure for 14 days, and then NAc dialysate was collected during baseline and following administration of the nicotinic receptor antagonist mecamylamine to precipitate withdrawal. Chronic nicotine exposure was associated with larger increases in baseline dopamine, GABA and glutamate levels in the NAc of female versus male rats, whereas baseline ACh was only increased in male rats. During withdrawal, both sexes displayed equivalent increases in NAc ACh levels. As expected, male rats displayed decreases in dopamine, coupled with increases in GABA and decreases in glutamate levels, suggesting the possibility of increased inhibitory tone in the NAc during withdrawal. Relative to males, female rats displayed larger decreases in NAc dopamine and related increases in GABAergic transmission. As female rats also showed elevated glutamate levels that persist during withdrawal, it is suggested that sex differences may arise from increased glutamatergic drive of inhibitory tone in the NAc. The findings provide a potential mechanism whereby the aversive effects of nicotine withdrawal are enhanced in female rats.

Generation and use of tritiated borane in situ for formamide tritiation to an N-methyl-3 H group

The synthesis of [N-methyl-3 H](+/-)-mecamylamine is described as an example of a convenient and mild method to reduce formamide groups using tritiated borane generated in situ. The product was characterized by thin layer chromatography, high performance liquid chromatography, mass spectrometry, and tritium NMR.

Reversal of mecamylamine-induced effects in healthy subjects by nicotine receptor agonists: Cognitive and (electro) physiological responses

AIMS

Establishing a pharmacological challenge model could yield an important tool to understand the complex role of the nicotinic cholinergic system in cognition and to develop novel compounds acting on the nicotinic acetylcholine receptor.

METHODS

This randomized, double-blind, double-dummy, placebo-controlled, four-way crossover study examined the effects of the nicotinic antagonist mecamylamine on a battery of cognitive and neurophysiological test with coadministration of a placebo, nicotine or galantamine in order to reverse the cognitive impairment caused by mecamylamine.

RESULTS

Thirty-three healthy subjects received a single oral dose of 30 mg of mecamylamine (or placebo) in combination with either 16 mg of oral galantamine or 21 mg of transdermal nicotine (or its double-dummy). Mecamylamine 30 mg induced significant disturbances of cognitive functions. Attention and execution of visual (fine) motor tasks was decreased, short- and long-term memory was impaired and the reaction velocity during the test was slower when compared to placebo. Mecamylamine 30 mg produced a decrease in posterior α and β power in the surface electroencephalogram, effects that were reversed by nicotine coadministration. Memory and motor coordination tests could be partially reversed by the coadministration of nicotine.

CONCLUSIONS

Mecamylamine administration induced slowing of the electroencephalogram and produced decrease in performance of tests evaluating motor coordination, sustained attention and short- and long-term memory. These effects could be partially reversed by the coadministration of nicotine, and to a lesser extent by galantamine.

Mecamylamine treatment for alcohol dependence: a randomized controlled trial

BACKGROUND AND AIMS

The nicotinic acetylcholine receptor antagonist, mecamylamine, is a potential novel pharmacotherapy for alcohol use disorder. The aims were to compare alcohol consumption between mecamylamine and placebo and test if smoking status modified treatment effects.

DESIGN

Out-patient, randomized, double-blind clinical trial for 12 weeks of treatment with mecamylamine (10 mg) (n = 65) versus placebo (n = 63).

SETTING

Connecticut, USA.

PARTICIPANTS

Individuals had current alcohol dependence (n = 128), had an average age of 48.5 [standard deviation (SD) = 9.4], 110 (85.9%) were men, and included 74 smokers (57.8%) and 54 non-smokers (42.2%). Participants were randomized to mecamylamine 10 mg per day or placebo. All subjects also received medical management therapy administered by trained research personnel.

MEASUREMENTS

Primary outcome was percentage of heavy drinking days during the last month of treatment; other outcomes included drinking days, drinks per drinking days, alcohol craving, smoking, symptoms of nicotine withdrawal and side effects.

FINDINGS

There were no significant differences in the percentage of heavy drinking days at 3 months between the mecamylamine (mean = 18.4, SD = 29.0) and placebo treatment groups (mean = 20.4, SD = 29.2) [F1, 100  = 1.3, P = 0.25; effect size d = 0.07; mean difference = 2.06, 95% confidence interval (CI) = -8.96 to 13.08]. There were no significant differences in percentage of drinking days or in drinks per drinking day at month 3 between the mecamylamine and placebo groups; there were no significant interactions.

CONCLUSIONS

Mecamylamine 10 mg per day did not reduce alcohol consumption significantly in treatment-seeking smokers and non-smokers with alcohol use disorder.

Blockade of cholinergic transmission elicits somatic signs in nicotine-naïve adolescent rats

High doses of the nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine can elicit somatic signs resembling those associated with nicotine withdrawal in nicotine-naïve adult rats. Understanding this phenomenon, and its possible modulation by acute nicotine and age, could inform the use of mecamylamine as both an experimental tool and potential pharmacotherapy for tobacco dependence and other disorders. This study evaluated the ability of high-dose mecamylamine to elicit somatic signs in adolescent rats, and the potential for acute nicotine pretreatment to potentiate this effect as previously reported in adults. Single or repeated injections of mecamylamine (1.5 or 3.0 mg/kg, s.c.) elicited somatic signs in nicotine-naïve adolescents, but this effect was not influenced by 2 h pretreatment with acute nicotine (0.5 mg/kg, s.c.). In an initial evaluation of the effects of age in this model, mecamylamine (2.25 mg/kg, s.c.) elicited somatic signs in nicotine-naïve adolescents and adults. This effect was modestly enhanced following acute nicotine injections in adults but not in adolescents, even when a higher nicotine dose (1.0 rather than 0.5 mg/kg, s.c.) was used in adolescents to account for age differences in nicotine pharmacokinetics. These studies are the first to show that mecamylamine elicits somatic signs in nicotine-naïve adolescent rats, an effect that should be considered when designing and interpreting studies examining effects of high doses of mecamylamine in adolescents. Our findings also provide preliminary evidence that these signs may be differentially modulated by acute nicotine pretreatment in adolescents versus adults.

Effects of continuous nicotine treatment and subsequent termination on cocaine versus food choice in male rhesus monkeys

One complicating factor in cocaine addiction may be concurrent exposure and potential dependence on nicotine. The aim of the present study was to determine the effects of continuous nicotine treatment and subsequent termination on cocaine versus food choice in rhesus monkeys (Macaca mulatta). For comparison, we also determined effects of the nicotinic receptor antagonist mecamylamine on cocaine versus food choice during continuous saline and nicotine treatment. Rhesus monkeys (N = 3) responded under a concurrent schedule of food pellet (1 g) and intravenous cocaine (0-0.1 mg/kg/injection) availability. Saline and ascending nicotine doses (0.1-1.0 mg/kg/hr, intravenous) were continuously infused for 7-day treatment periods and separated by 24-hr saline treatment periods. Acute effects of mecamylamine (0.32-1.8 mg/kg, intramuscular, 15 min pretreatment) were determined during continuous saline and 0.32-mg/kg/hr nicotine treatments. During saline treatment, cocaine maintained a dose-dependent increase in cocaine choice. Nicotine treatment did not alter cocaine versus food choice. In contrast, preference of 0.032 mg/kg/injection cocaine was attenuated 24 hr following termination of 0.32-mg/kg/hr nicotine treatment, despite no somatic abstinence signs being observed. Acute mecamylamine enhanced cocaine choice during saline treatment and mainly suppressed rates of behavior during nicotine treatment. Overall, continuous nicotine exposure, up to 1 mg/kg/hr, does not enhance cocaine choice and does not produce nicotine dependence, as demonstrated by the lack of abstinence signs.

Cigarette smoke has sensory effects through nicotinic and TRPA1 but not TRPV1 receptors on the isolated mouse trachea and larynx

Cigarette smoke (CS) exposes chemosensory nerves in the airways to a multitude of chemicals, some acting through the irritant receptors TRPV1 and TRPA1 but potentially also through nicotinic acetylcholine receptors (nAChR). Our aim was to characterize the differences in sensory neuronal effects of CS, gas phase, and particulate matter as well as of typical constituents, such as nicotine and reactive carbonyls. Isolated mouse trachea and larynx were employed to measure release of calcitonin gene-related peptide (CGRP) as an index of sensory neuron activation evoked by CS, by filtered CS gas phase essentially free of nicotine, and by dilute total particulate matter (TPM) containing defined nicotine concentrations. With CS stimulation of the superfused trachea, TRPV1 null mutants showed about the same large responses as wild-type mice, whereas both TRPA1(-/-) and double knockouts exhibited 80% reduction; the retained 20% response was abolished by mecamylamine (10 μM), indicating a distinct contribution of nAChRs. These phenotypes were accentuated by using TPM to stimulate the immersed trachea; 50% of response was retained in TRPA1(-/-) and abolished by mecamylamine. In contrast, the gas phase acted like a sheer TRPA1 agonist, consistent with its composition, among other compounds, of volatile reactive carbonyls like formaldehyde and acrolein. In the trachea, the gas phase and CS were equally effective in releasing CGRP, whereas the larynx showed much larger CS than gas phase responses. Thus nicotinic receptors contribute to the sensory effects of cigarette smoke on the trachea, which are dominated by TRPA1. How this translates to human perception affords future research.

Density of α4β2* nAChR on the surface of neurons is modulated by chronic antagonist exposure

The expression of high-affinity α4β2* nicotinic acetylcholine receptors (nAChR) increases following chronic exposure to nicotinic agonists. While, nAChR antagonists can also produce upregulation, these changes are often less pronounced than achieved with agonists. It is unknown if nAChR agonists and antagonists induce receptor upregulation by the same mechanisms. In this study, primary neuronal cultures prepared from cerebral cortex, hippocampus, diencephalon, and midbrain/hindbrain of C57BL/6J mouse embryos were treated chronically with nicotine (agonist), mecamylamine (noncompetitive antagonist) or dihydro-β-erythroidine (competitive antagonist) or the combination of nicotine with each antagonist. The distribution of intracellular and surface [(125)I]epibatidine-binding sites were subsequently measured. Treatment with 1 μmol/L nicotine upregulated intracellular and cell surface [(125)I]epibatidine binding after 96 h. Chronic dihydro-β-erythroidine (10 μmol/L) treatment also increased [(125)I]epibatidine binding on the cell surface; however, mecamylamine was ineffective in upregulating receptors by itself. The combination of 1 μmol/L nicotine plus 10 μmol/L mecamylamine elicited a significantly higher upregulation than that achieved by treatment with nicotine alone due to an increase of [(125)I]epibatidine binding on the cell surface. This synergistic effect of mecamylamine and nicotine was found in neuronal cultures from all four brain regions. Chronic treatment with nicotine concentrations as low as 10 nmol/L produced upregulation of [(125)I]epibatidine binding. However, the effect of mecamylamine was observed only after coincubation with nicotine concentrations equal to or greater than 100 nmol/L. Vesicular trafficking was required for both nicotine and nicotine plus mecamylamine-induced upregulation. Results presented here support the idea of multiple mechanisms for nAChR upregulation.

Central administration of nicotine suppresses tracheobronchial cough in anesthetized cats

We tested the hypothesis that nicotine, which acts peripherally to promote coughing, might inhibit reflex cough at a central site. Nicotine was administered via the vertebral artery [intra-arterial (ia)] to the brain stem circulation and by microinjections into a restricted area of the caudal ventral respiratory column in 33 pentobarbital anesthetized, spontaneously breathing cats. The number of coughs induced by mechanical stimulation of the tracheobronchial airways; amplitudes of the diaphragm, abdominal muscle, and laryngeal muscles EMGs; and several temporal characteristics of cough were analyzed after administration of nicotine and compared with those during control and recovery period. (-)Nicotine (ia) reduced cough number, cough expiratory efforts, blood pressure, and heart rate in a dose-dependent manner. (-)Nicotine did not alter temporal characteristics of the cough motor pattern. Pretreatment with mecamylamine prevented the effect of (-)nicotine on blood pressure and heart rate, but did not block the antitussive action of this drug. (+)Nicotine was less potent than (-)nicotine for inhibition of cough. Microinjections of (-)nicotine into the caudal ventral respiratory column produced similar inhibitory effects on cough as administration of this isomer by the ia route. Mecamylamine microinjected in the region just before nicotine did not significantly reduce the cough suppressant effect of nicotine. Nicotinic acetylcholine receptors significantly modulate functions of brain stem and in particular caudal ventral respiratory column neurons involved in expression of the tracheobronchial cough reflex by a mecamylamine-insensitive mechanism.

Greater ethanol inhibition of presynaptic dopamine release in C57BL/6J than DBA/2J mice: Role of nicotinic acetylcholine receptors

The mesolimbic dopamine system, originating in the ventral tegmental area (VTA) and projecting to the nucleus accumbens (NAc), has been heavily implicated in the reinforcing effects of ethanol. Recent slice voltammetry studies have shown that ethanol inhibits dopamine release selectively during high-frequency activity that elicits phasic dopamine release shown to be important for learning and reinforcement. Presently, we examined ethanol inhibition of electrically evoked NAc dopamine in two mouse strains with divergent dopamine responses to ethanol, C57BL/6 (C57) and DBA/2J (DBA) mice. Previous electrophysiology and microdialysis studies have demonstrated greater ethanol-induced VTA dopaminergic firing and NAc dopamine elevations in DBA compared to C57 mice. Additionally, DBA mice have greater ethanol responses in dopamine-related behaviors, including hyperlocomotion and conditioned place preference. Currently, we demonstrate greater sensitivity of ethanol inhibition of NAc dopamine signaling in C57 compared to DBA mice. The reduced sensitivity to ethanol inhibition in DBA mice may contribute to the overall greater ethanol-induced dopamine signaling and related behaviors observed in this strain. NAc cholinergic activity is known to potently modulate terminal dopamine release. Additionally, ethanol is known to interact with multiple aspects of nicotinic acetylcholine receptor activity. Therefore, we examined ethanol-mediated inhibition of dopamine release at two ethanol concentrations (80 and 160 mM) during bath application of the non-selective nicotinic receptor antagonist mecamylamine, as well as compounds selective for the β2-(dihydro-β-erythroidine hydrobromide; DhβE) and α6-(α-conotoxin MII [H9A; L15A]) subunit-containing receptors. Mecamylamine and DhβE decreased dopamine release and reduced ethanol's inhibitory effects on dopamine in both DBA and C57 mice. Further, α-conotoxin also reduced the dopamine release and the dopamine-inhibiting effects of ethanol at the 80 mM, but not 160 mM, concentration. These data suggest that ethanol is acting in part through nicotinic acetylcholine receptors, or downstream effectors, to reduce dopamine release during high-frequency activity.

Enhanced alcohol-seeking behavior by nicotine in the posterior ventral tegmental area of female alcohol-preferring (P) rats: modulation by serotonin-3 and nicotinic cholinergic receptors

RATIONALE

Alcohol and nicotine co-use can reciprocally promote self-administration and drug-craving/drug-seeking behaviors. To date, the neurocircuitry in which nicotine influences ethanol (EtOH) seeking has not been elucidated. Clinical and preclinical research has suggested that the activation of the mesolimbic dopamine system is involved in the promotion of drug seeking. Alcohol, nicotine, and serotonin-3 (5-HT3) receptors interact within the posterior ventral tegmental area (pVTA) to regulate drug reward. Recently, our laboratory has reported that systemic administration of nicotine can promote context-induced EtOH seeking.

OBJECTIVES

The goals of the current study were to (1) determine if microinjections of pharmacologically relevant levels of nicotine into the pVTA would enhance EtOH seeking, (2) determine if coadministration of nicotinic cholinergic receptor antagonist (nACh) or 5-HT3 receptor antagonists would block the ability of nicotine microinjected into the pVTA to promote EtOH seeking, and (3) determine if 5-HT3 receptors in the pVTA can modulate EtOH seeking.

RESULTS

Nicotine (100 and 200 μM) microinjected into the pVTA enhanced EtOH seeking. Coinfusion with 200 μM mecamylamine (nACh antagonist) or 100 and 200 μM zacopride (5-HT3 receptor antagonist) blocked the observed nicotine enhancement of EtOH seeking. The data also indicated that microinjection of 1 μM CPBG (5-HT3 receptor agonist) promotes context-induced EtOH seeking; conversely, microinjection of 100 and 200 μM zacopride alone reduced context-induced EtOH seeking.

CONCLUSIONS

Overall, the results show that nicotine-enhanced EtOH-seeking behavior is modulated by 5-HT3 and nACh receptors within the pVTA and that the 5-HT3 receptor system within pVTA may be a potential pharmacological target to inhibit EtOH-seeking behaviors.

Nicotine vapor inhalation escalates nicotine self-administration

Humans escalate their cigarette smoking over time, and a major obstacle in the field of pre-clinical nicotine addiction research has been the inability to produce escalated nicotine self-administration in rats. In experiment 1, male Wistar rats were trained to respond for nicotine in 2-hour operant sessions, then exposed to chronic intermittent (12 hours/day) nicotine vapor and repeatedly tested for nicotine self-administration at 8-12 hours of withdrawal. Rats were tested intermittently on days 1, 3 and 5 of the vapor exposure procedure, then tested with nicotine vapor exposure on 6-15 consecutive days. Rats exhibited transient increases in operant nicotine responding during intermittent testing, regardless of vapor condition, and this responding returned to baseline levels upon resumption of consecutive-days testing (i.e. nicotine deprivation effect). Nicotine vapor-exposed rats then escalated nicotine self-administration relative to both their own baseline (∼200% increase) and non-dependent controls (∼3× higher). In experiment 2, rats were exposed or not exposed to chronic intermittent nicotine vapor, then tested for spontaneous and precipitated somatic signs of nicotine withdrawal. Eight hours following removal from nicotine vapor, rats exhibited robust mecamylamine-precipitated somatic signs of withdrawal. There was a strong correlation between nicotine flow rate and air-nicotine concentration, and the air-nicotine concentrations used in experiments 1 and 2 resemble concentrations experienced by human smokers. Collectively, these results suggest that chronic intermittent nicotine vapor inhalation produces somatic and motivational signs of nicotine dependence, the latter of which is evidenced by escalation of nicotine self-administration.

Effects of the nicotinic acetylcholine receptor antagonist mecamylamine on the discriminative stimulus effects of cocaine in male rhesus monkeys

Preclinical drug discrimination procedures have been useful in understanding the pharmacological mechanisms of the subjective-like effects of abused drugs. Converging lines of evidence from neurochemical and behavioral studies implicate a potential role of nicotinic acetylcholine (nACh) receptors in the abuse-related effects of cocaine. The aim of the present study was to determine the effects of the nACh receptor antagonist mecamylamine on the discriminative stimulus effects of cocaine in nonhuman primates. The effects of mecamylamine on the cocaine-like discriminative stimulus effects of nicotine were also examined. Male rhesus monkeys (n = 5) were trained to discriminate 0.32 mg/kg, IM cocaine from saline in a 2-key, food-reinforced discrimination procedure. Initially, potency and time course of cocaine-like discriminative stimulus effects were determined for nicotine and mecamylamine alone. Test sessions were then conducted examining the effects of mecamylamine on cocaine or the cocaine-like discriminative stimulus effects of nicotine. Curiously, mecamylamine produced partial cocaine-like discriminative stimulus effects. Mecamylamine did not significantly alter the discriminative stimulus effects of cocaine up to doses that significantly decreased rates of operant responding. Mecamylamine and nicotine combinations were not different than saline. These results confirm previous nonhuman primate studies of partial substitution with nicotine and extend these findings with mecamylamine. Furthermore, these results extend previous results in rats suggesting cocaine may have nACh receptor antagonist properties.

Emerging drugs for the treatment of tobacco dependence: 2014 update

INTRODUCTION

Tobacco dependence remains a global epidemic and the largest preventable cause of morbidity and mortality around the world. Smoking cessation has benefits at all ages but remains challenging for several reasons, among which are the complexities of nicotine addiction and limitations of available pharmacotherapies.

AREAS COVERED

This review summarizes current and emerging pharmacotherapies for the treatment of tobacco dependence, including first- and second-line recommended agents. Medications with alternative primary indications that have been investigated as potential treatments for tobacco dependence are also discussed. Articles reviewed were obtained through searches of PubMed, Ovid MEDLINE, ClinicalTrials.gov and the Pharmaprojects database.

EXPERT OPINION

Current evidence suggests that the two most effective pharmacotherapies to treat tobacco dependence are varenicline and combination nicotine replacement therapy. Alternative agents investigated demonstrate mixed rates of success in achieving long-term abstinence from smoking. No single pharmacotherapy will serve as a universally successful treatment given the complex underpinnings of tobacco dependence and individuality of smokers. The ultimate goal of tobacco research with respect to pharmacotherapeutic development continues to be providing clinicians with an armamentarium of drugs to choose from allowing for tailoring of treatment for smokers.

Acute effects of mecamylamine and varenicline on cognitive performance in non-smokers with and without schizophrenia

RATIONALE

Nicotinic acetylcholine receptors (nAChRs) have been implicated in the pathophysiology of cognitive deficits in the domains of attention and memory in schizophrenia. While nicotinic agonists and antagonists have been proposed as smoking cessation aids, few comparisons have been made of these agents on cognitive performance in individuals with schizophrenia.

OBJECTIVES

This study investigated the acute effects of a nAChR antagonist, mecamylamine, and partial agonist, varenicline, on cognitive function in non-smokers with and without schizophrenia.

METHODS

Single oral doses of mecamylamine 10 mg, varenicline 1 mg, and placebo were administered 1 week apart in random order to adults with schizophrenia (n = 30) and to healthy volunteers (n = 41) in a double-blind, crossover design. The primary outcome of interest was sustained attention as assessed with hit reaction time variability (HRT-SD) on the identical pairs continuous performance test (CPT-IP).

RESULTS

Mecamylamine worsened performance on CPT-IP HRT-SD, a measure of attention, compared to varenicline in both groups. Performance on mecamylamine was worse than performance on both placebo and varenicline on several additional measures of attention, including CPT-IP hit reaction time (HRT) and random errors at various levels of task difficulty. There was a treatment by diagnosis interaction, such that mecamylamine worsened performance on CPT-IP 2-digit HRT, 3-digit random errors, and 4-digit hit rate compared to placebo and varenicline in participants with schizophrenia; effects not observed in controls.

CONCLUSIONS

These findings support a role for nAChRs in attention and suggest that those with schizophrenia may be particularly sensitive to nAChR blockade.

Tamoxifen improves cholinergically modulated cognitive performance in postmenopausal women

Tamoxifen (TMX) is a selective estrogen receptor modulator that is used as an estrogen receptor antagonist for the treatment and prevention of breast cancer. Whether TMX has antagonist activities in the human brain is less clear and its effects on cognitive function have not been experimentally explored. This study examined how TMX affected cognitive performance in older women using a model of anticholinergic drug-induced cognitive dysfunction. Twenty-one postmenopausal women were administered 20 mg of oral TMX or placebo for 3 months. Participants then took part in five drug challenges using the anticholinergic antinicotinic agent mecamylamine (MECA) and antimuscarinic agent scopolamine (SCOP) and were tested on a comprehensive battery including tasks of attention and psychomotor function, verbal episodic memory, and spatial navigation. After a 3-month placebo washout, participants were then crossed over to the alternate treatment and repeated the drug challenges after 3 months. Compared with placebo treatment, TMX significantly attenuated the impairment from cholinergic blockade on tasks of verbal episodic memory and spatial navigation, but effects on attentional/psychomotor tasks were more variable. Analysis by APOE genotype showed that APO ɛ4+ women showed a greater beneficial effect of TMX on reversing the cholinergic impairment than APO ɛ4- women on most tasks. This study provides evidence that TMX may act as an estrogen-like agonist to enhance cholinergic system activity and hippocampally mediated learning.

The cannabinoid CB2 receptor is necessary for nicotine-conditioned place preference, but not other behavioral effects of nicotine in mice

RATIONALE

Whereas cannabinoid CB1 receptors have long been known to contribute to the rewarding effects and dependence liability of many drugs of abuse, recent studies have implicated the involvement of cannabinoid CB2 receptors.

OBJECTIVE

Here, we evaluated the role of CB2 receptors in the rewarding properties of nicotine, as assessed in the conditioned place preference (CPP) paradigm and mecamylamine-precipitated withdrawal in nicotine dependent mice.

METHODS

Using complementary pharmacological and genetic approaches, we investigated the involvement of CB2 receptors in nicotine- and cocaine-induced CPP in mice and mecamylamine-precipitated withdrawal in nicotine-dependent mice. We also determined whether deletion of CB2 receptors affects nicotine-induced hypothermia and hypoalgesia.

RESULTS

Nicotine-induced (0.5 mg/kg) CPP was completely blocked by selective CB2 antagonist, SR144528 (3 mg/kg) in wild-type mice, and was absent in CB2 (-/-) mice. Conversely, the CB2 receptor agonist, O-1966 (1, 3, 5, 10, 20 mg/kg) given in combination with a subthreshold dose of nicotine (0.1 mg/kg) elicited a place preference. In contrast, O-1966 (20 mg/kg) blocked cocaine (10 mg/kg)-induced CPP in wild type mice, while CB2 (-/-) mice showed unaltered cocaine CPP. CB2 (+/+) and (-/-) nicotine-dependent mice showed almost identical precipitated withdrawal responses and deletion of CB2 receptor did not alter acute somatic effects of nicotine.

CONCLUSIONS

Collectively, these results indicate that CB2 receptors are required for nicotine-induced CPP in the mouse, while it is not involved in nicotine withdrawal or acute effects of nicotine. Moreover, these results suggest that CB2 receptors play opposing roles in nicotine- and cocaine-induced CPP.

Nicotinic acetylcholine receptor antagonists alter the function and expression of serine racemase in PC-12 and 1321N1 cells

Western blot analysis demonstrated that PC-12 cells express monomeric and dimeric forms of serine racemase (m-SR, d-SR) and that 1321N1 cells express m-SR. Quantitative RT-PCR and functional studies demonstrated that PC-12 cells express homomeric and heteromeric forms of nicotinic acetylcholine receptors (nAChR) while 1321N1 cells primarily express the α7-nAChR subtype. The effect of nAChR agonists and antagonists on SR activity and expression was examined by following concentration-dependent changes in intracellular d-Ser levels and SR protein expression. Incubation with (S)-nicotine increased d-Ser levels, which were attenuated by the α7-nAChR antagonist methyllycaconitine (MLA). Treatment of PC-12 cells with mecamylamine (MEC) produced a bimodal reduction of d-Ser reflecting MEC inhibition of homomeric and heteromeric nAChRs, while a unimodal curve was observed with 1321N1 cells, reflecting predominant expression of α7-nAChR. The nAChR subtype selectivity was probed using α7-nAChR selective inhibitors MLA and (R,S)-dehydronorketamine and α3β4-nAChR specific inhibitor AT-1001. The compounds reduced d-Ser in PC-12 cells, but only MLA and (R,S)-dehydronorketamine were effective in 1321N1 cells. Incubation of PC-12 and 1321N1 cells with (S)-nicotine, MEC and AT-1001 did not affect m-SR or d-SR expression, while MLA and (R,S)-dehydronorketamine increased m-SR expression but not SR mRNA levels. Treatment with cycloheximide indicated that increased m-SR was due to de novo protein synthesis associated with phospho-active forms of ERK1/2, MARCKS, Akt and rapamycin-sensitive mTOR. This effect was attenuated by treatment with the pharmacological inhibitors U0126, LY294002 and rapamycin, which selectively block the activation of ERK1/2, Akt and mTOR, respectively, and siRNAs directed against ERK1/2, Akt and mTOR. We propose that nAChR-associated changes in Ca(2+) flux affect SR activity, but not expression, and that MLA and (R,S)-dehydronorketamine bind to allosteric sites on the α7-nAChR and promote multiple signaling cascades that converge at mTOR to increase m-SR levels.

Angiotensin AT1 and AT2 receptor antagonists modulate nicotine-evoked [³H]dopamine and [³H]norepinephrine release

Tobacco smoking is the leading preventable cause of death in the United States. A major negative health consequence of chronic smoking is hypertension. Untoward addictive and cardiovascular sequelae associated with chronic smoking are mediated by nicotine-induced activation of nicotinic receptors (nAChRs) within striatal dopaminergic and hypothalamic noradrenergic systems. Hypertension involves both brain and peripheral angiotensin systems. Activation of angiotensin type-1 receptors (AT1) release dopamine and norepinephrine. The current study determined the role of AT1 and angiotensin type-2 (AT2) receptors in mediating nicotine-evoked dopamine and norepinephrine release from striatal and hypothalamic slices, respectively. The potential involvement of nAChRs in mediating effects of AT1 antagonist losartan and AT2 antagonist, 1-[[4-(dimethylamino)-3-methylphenyl]methyl]-5-(diphenylacetyl)-4,5,6,7-tetrahydro-1H-imidazo[4,5-c]pyridine-6-carboxylic acid (PD123319) was evaluated by determining their affinities for α4β2* and α7* nAChRs using [³H]nicotine and [³H]methyllycaconitine binding assays, respectively. Results show that losartan concentration-dependently inhibited nicotine-evoked [³H]dopamine and [³H]norepinephrine release (IC₅₀: 3.9 ± 1.2 and 2.2 ± 0.7 μM; Imax: 82 ± 3 and 89 ± 6%, respectively). In contrast, PD123319 did not alter nicotine-evoked norepinephrine release, and potentiated nicotine-evoked dopamine release. These results indicate that AT1 receptors modulate nicotine-evoked striatal dopamine and hypothalamic norepinephrine release. Furthermore, AT1 receptor activation appears to be counteracted by AT2 receptor activation in striatum. Losartan and PD123319 did not inhibit [³H]nicotine or [³H]methyllycaconitine binding, indicating that these AT1 and AT2 antagonists do not interact with the agonist recognition sites on α4β2* and α7* nAChRs to mediate these effects of nicotine. Thus, angiotensin receptors contribute to the effects of nicotine on dopamine and norepinephrine release in brain regions involved in nicotine reward and hypertension.

Potential therapeutic uses of mecamylamine and its stereoisomers

Mecamylamine (3-methylaminoisocamphane hydrochloride) is a nicotinic parasympathetic ganglionic blocker, originally utilized as a therapeutic agent to treat hypertension. Mecamylamine administration produces several deleterious side effects at therapeutically relevant doses. As such, mecamylamine's use as an antihypertensive agent was phased out, except in severe hypertension. Mecamylamine easily traverses the blood-brain barrier to reach the central nervous system (CNS), where it acts as a nicotinic acetylcholine receptor (nAChR) antagonist, inhibiting all known nAChR subtypes. Since nAChRs play a major role in numerous physiological and pathological processes, it is not surprising that mecamylamine has been evaluated for its potential therapeutic effects in a wide variety of CNS disorders, including addiction. Importantly, mecamylamine produces its therapeutic effects on the CNS at doses 3-fold lower than those used to treat hypertension, which diminishes the probability of peripheral side effects. This review focuses on the pharmacological properties of mecamylamine, the differential effects of its stereoisomers, S(+)- and R(-)-mecamylamine, and the potential for effectiveness in treating CNS disorders, including nicotine and alcohol addiction, mood disorders, cognitive impairment and attention deficit hyperactivity disorder.

Multiple nicotine training doses in mice as a basis for differentiating the effects of smoking cessation aids

RATIONALE

Receptor mechanisms underlying the behavioral effects of clinically used nicotinic acetylcholine receptor agonists have not been fully established.

OBJECTIVE

Drug discrimination was used to compare receptor mechanisms underlying the effects of smoking cessation aids.

METHODS

Separate groups of male C57BL/6J mice discriminated 0.56, 1, or 1.78 mg/kg of nicotine base. Nicotine, varenicline, and cytisine were administered alone, in combination with each other, and in combination with mecamylamine and dihydro-β-erythroidine (DHβE). Midazolam and morphine were tested to examine sensitivity to non-nicotinics.

RESULTS

The ED50 value of nicotine to produce discriminative stimulus effects systematically increased as training dose increased. Varenicline and cytisine did not fully substitute for nicotine and, as compared with nicotine, their ED50 values varied less systematically as a function of nicotine training dose. Morphine did not substitute for nicotine, whereas midazolam substituted for the low and not the higher training doses of nicotine. As training dose increased, the dose of mecamylamine needed to produce a significant rightward shift in the nicotine dose-effect function also increased. DHβE antagonized nicotine in animals discriminating the smallest dose of nicotine. Varenicline did not antagonize the effects of nicotine, whereas cytisine produced a modest though significant antagonism of nicotine.

CONCLUSIONS

These results suggest that differences in pharmacologic mechanism between nicotine, varenicline, and cytisine include not only differences in efficacy at a common subtype of nicotinic acetylcholine receptor, but also differential affinity and/or efficacy at multiple receptor subtypes.

Simultaneous quantification of nicotine, cotinine, trans-3'-hydroxycotinine, norcotinine and mecamylamine in human urine by liquid chromatography-tandem mass spectrometry

BACKGROUND

Mecamylamine is a nicotine antagonist under investigation in combination with nicotine replacement for smoking treatment.

METHODS

A simple, rapid and reliable liquid chromatography tandem mass spectrometry (LCMSMS) method was developed and validated for quantifying nicotine, cotinine, trans-3'-hydroxycotinine, norcotinine and mecamylamine in human urine. Chromatography was performed on a Synergi PolarRP column with a gradient of 0.1% formic acid and 0.1% formic acid in acetonitrile at 0.25 ml/min with an 8-min total runtime. Analytes were monitored by positive mode electrospray ionization and multiple reaction monitoring mass spectrometry.

RESULTS

Linear dynamic ranges were 1-500 ng/ml for nicotine and norcotinine, 0.5-500 ng/ml for trans-3'-hydroxycotinine, 0.2-500 ng/ml for cotinine, and 0.1-100 ng/ml for mecamylamine; correlation coefficients were consistently greater than 0.99, and all calibrator concentrations were within 20% of target. Extensive endogenous and exogenous interferences were evaluated. At 3 concentrations spanning the linear dynamic range of the assay, mean extraction efficiencies from urine were 55.1-109.1% with analytical recovery (bias) 82.0-118.7% and total imprecision of 0.7-9.1%. Analytes were stable for 24h at room temperature, 72 h at 4 °C, 72 h in autosampler at 15 °C and after three freeze/thaw cycles.

CONCLUSION

This method is useful for monitoring mecamylamine, nicotine and nicotine metabolites in smoking cessation and other clinical nicotine research.

Manipulation of nicotinic acetylcholine receptors differentially affects behavioral inhibition in human subjects with and without disordered baseline impulsivity

RATIONALE

Evidence for a relationship between cigarette smoking and attention-deficit/hyperactivity disorder (ADHD) has prompted investigations into nicotinic treatments for this disorder. Impulsivity is a hallmark of ADHD and is measured in the laboratory as behavioral inhibition (BI) using the stop signal task (SST). Acute nicotine improves SST performance in adolescents and young adults who have both ADHD and impaired baseline SST performance, raising questions about the role of nicotinic acetylcholine receptor function in BI. The specificity of this effect to those with ADHD, the component processes of the SST affected by nicotine, and the effects of nicotinic antagonism are yet unknown.

OBJECTIVES

This study investigated the effects of both a nicotinic receptor agonist and antagonist on the SST and choice reaction time task (CRT) in highly impulsive (HI) and control (CTRL) subjects.

METHODS

This was a within-subjects, double-blind study of: 7 mg transdermal nicotine, 20 mg oral mecamylamine, and placebo. Subjects were recruited into HI (n = 11) and CTRL (n = 14) groups based on both SST and clinical criteria.

RESULTS

BI was significantly improved by nicotine compared with placebo in the HI group and impaired by mecamylamine in the CTRL group. Go signal reaction time on the SST was improved by nicotine compared with placebo in the CTRL group and was unchanged in both groups on the CRT.

CONCLUSIONS

These findings demonstrate nicotinic modulation of BI in subjects with both normal and disordered baseline performance. The effects on BI are consistent with cholinergic enhancement of signal detection processes and/or modulation of noradrenaline by nicotine.

Nicotine-induced impulsive action: sensitization and attenuation by mecamylamine

A conjunctive variable-interval differential-reinforcement-of-low-rate (VI-DRL, n=18) responding schedule and a stop-signal task (n=18) were used to evaluate the disinhibiting effects of nicotine on response withholding in rats. Sucrose solution was used to reinforce responding, and after a stable baseline was achieved under saline-administration conditions, 0.3 mg/kg nicotine was delivered before each session. Experiment 1 showed that repeated, but not the initial, administration of nicotine decreased performance on both tasks, and the effect of sensitization followed a similar timeline; 10 consecutive doses resulted in poorer proportion-correct VI-DRL trials and percent correct stop trials than the initial dose of nicotine. Furthermore, sensitization to 0.3 mg/kg nicotine decreased performance regardless of whether a spaced or consecutive-dosing regimen was followed. Experiment 2 was designed to test whether mecamylamine hydrochloride (0.1-1.0 mg/kg) could attenuate the effects of repeated 0.3 mg/kg nicotine administration, and the degree to which mecamylamine attenuation of the effect of nicotine to produce impulsive action was relative to dose. Results from experiment 2 showed that response disinhibition, as evaluated using the VI-DRL and stop-signal tasks, is related in a systematic manner to nicotinic-acetylcholine receptor activation.

Genome-wide expression analysis reveals diverse effects of acute nicotine exposure on neuronal function-related genes and pathways

Previous human and animal studies demonstrate that acute nicotine exposure has complicated influences on the function of the nervous system, which may lead to long-lasting effects on the behavior and physiology of the subject. To determine the genes and pathways that might account for long-term changes after acute nicotine exposure, a pathway-focused oligoarray specifically designed for drug addiction research was used to assess acute nicotine effect on gene expression in the neuron-like SH-SY5Y cells. Our results showed that 295 genes involved in various biological functions were differentially regulated by 1 h of nicotine treatment. Among these genes, the expression changes of 221 were blocked by mecamylamine, indicating that the majority of nicotine-modulated genes were altered through the nicotinic acetylcholine receptors (nAChRs)-mediated signaling process. We further identified 14 biochemical pathways enriched among the nicotine-modulated genes, among which were those involved in neural development/synaptic plasticity, neuronal survival/death, immune response, or cellular metabolism. In the genes significantly regulated by nicotine but blocked by mecamylamine, 13 enriched pathways were detected. Nine of these pathways were shared with those enriched in the genes regulated by nicotine, including neuronal function-related pathways such as glucocorticoid receptor signaling, p38 MAPK signaling, PI3K/AKT signaling, and PTEN signaling, implying that nAChRs play important roles in the regulation of these biological processes. Together, our results not only provide insights into the mechanism underlying the acute response of neuronal cells to nicotine but also provide clues to how acute nicotine exposure exerts long-term effects on the nervous system.

An examination of NMDA receptor contribution to conditioned responding evoked by the conditional stimulus effects of nicotine

RATIONALE

Research using a drug discriminated goal-tracking (DGT) task showed that the N-methyl-D: -aspartate (NMDA) channel blocker MK-801 (dizocilpine) reduced the nicotine-evoked conditioned response (CR).

OBJECTIVES

Given the unknown mechanism of the effect, Experiment 1 replicated the MK-801 results and included tests with NMDA receptor ligands. Experiments 2a and 2b tested whether MK-801 pretreatment blocked DGT via a state-dependency effect.

METHODS

In Experiment 1, adult male Sprague-Dawley rats received intermittent access to liquid sucrose following nicotine (0.4 mg base/kg); no sucrose was delivered on intermixed saline sessions. Conditioning was indicated by increased anticipatory dipper entries (goal-tracking) on nicotine compared to saline sessions. Antagonism and/or substitution tests were conducted with MK-801, phencyclidine, CGP 39551, d-CPPene (SDZ EAA 494), Ro 25,6981, L-701,324, ACPC, and NMDA. In Experiment 2a, rats received nicotine and sucrose on every session-no intermixed saline sessions without sucrose. Tests combined MK-801 or the non-competitive nicotinic acetylcholine receptor antagonist, mecamylamine with either nicotine or saline. Experiment 2b had sucrose delivered on saline sessions and no sucrose on intermixed nicotine sessions followed by MK-801 antagonism tests of the saline CS.

RESULTS

MK-801 and phencyclidine dose-dependently attenuated the CR in Experiment 1. Ro-25,6981 enhanced the CR, but did not substitute for nicotine. Other ligands showed inconsistent effects. In Experiment 2a, MK-801 pretreatment reduced goal-tracking when given before nicotine and saline test sessions; mecamylamine pretreatment had no effect. In Experiment 2b, MK-801 dose-dependently attenuated the saline-evoked CR.

CONCLUSIONS

Combined, the results suggest that MK-801 blocks discriminated goal-tracking by virtue of state-changing properties.

Nicotine receptors and depression: revisiting and revising the cholinergic hypothesis

There is a well-established connection between smoking and depression. Depressed individuals are over-represented among smokers, and ex-smokers often experience increased depressive symptoms immediately after stopping smoking. Nicotine in tobacco binds, activates and desensitizes nicotinic acetylcholine receptors (nAChRs), but it is not known whether activation or desensitization is more important for the effects of nicotine on depressive symptoms. Here we review, based on clinical and preclinical studies of nicotinic drugs, the hypothesis that blockade (rather than activation) of neuronal nAChRs might be important for the effects of nicotinic agents on depressive symptoms. The endogenous neurotransmitter for nAChRs is acetylcholine, and the effects of nicotine on depression-like behaviors support the idea that dysregulation of the cholinergic system might contribute to the etiology of major depressive disorder. Thus, pharmacological agents that limit acetylcholine signaling through neuronal nAChRs might be promising for the development of novel antidepressant medications.

Nicotinic acetylcholine receptors and depression: a review of the preclinical and clinical literature

Many patients with depression fail to derive sufficient benefit from available treatment options, with up to a third never reaching remission despite multiple trials of appropriate treatment. Novel antidepressant agents are needed, and drugs targeting nicotinic acetylcholine receptors (nAChRs) appear to hold promise in this regard. nAChRs are involved in a variety of neurobiological systems implicated in the pathophysiology of depression. In addition to their role in cholinergic neurotransmission, they modulate dopamine function and influence inflammation and hypothalamic-pituitary-adrenal axis activity. Preclinical studies have suggested antidepressant-like effects of drugs targeting nAChRs, with the most consistent results observed with alpha4beta2 nAChR modulators such as varenicline and nonspecific nAChR antagonists such as mecamylamine. These agents appear to offer the most potential antidepressant-like efficacy when used in conjunction with other established antidepressant treatments. nAChR modulators also influence neural processes that appear to mediate the behavioral effects of antidepressants, such as hippocampal cell proliferation. Clinical evidence, while limited, shows preliminary efficacy for mecamylamine and varenicline. Taken together, the preclinical and clinical evidence suggests that drugs targeting nAChRs may represent an important new approach to the treatment of depression.

Nicotine's enhancing effects on responding maintained by conditioned reinforcers are reduced by pretreatment with mecamylamine, but not hexamethonium, in rats

Several studies have indicated that nicotine increases responding maintained by conditioned reinforcers. We assessed the effects of subcutaneous injections of 0.3 mg/kg nicotine and two nicotinic antagonists on responding maintained by conditioned and primary reinforcers and responding during extinction in 8 Long Evans rats. Mecamylamine, a central and peripheral nicotinic antagonist, and hexamethonium, a peripheral nicotinic antagonist, were administered prior to a subset of the experimental sessions. Nicotine selectively increased responding maintained by conditioned reinforcers and mecamylamine, but not hexamethonium, attenuated this effect. These results suggest that nicotine's enhancing effect on responding maintained by conditioned reinforcers is mediated in the central nervous system.

Effects of the nicotinic receptor antagonist mecamylamine on ad-lib smoking behavior, topography, and nicotine levels in smokers with and without schizophrenia: a preliminary study

Individuals with schizophrenia have higher plasma nicotine levels in comparison to non-psychiatric smokers, even when differences in smoking are equated. This difference may be related to how intensely cigarettes are smoked but this has not been well studied. Mecamylamine (MEC), a non-competitive nicotinic acetylcholine receptor (nAChR) antagonist, which has been shown to increase ad-lib smoking and to affect smoking topography, was used in the current study as a pharmacological probe to increase our understanding of smoking behavior, smoking topography, and resulting nicotine levels in smokers with schizophrenia. This preliminary study used a within-subject, placebo-controlled design in smokers with schizophrenia (n=6) and healthy control smokers (n=8) to examine the effects of MEC (10mg/day) on ad-lib smoking behavior, topography, nicotine levels, and tobacco craving across two smoking deprivation conditions (no deprivation and 12-h deprivation). MEC, compared to placebo, increased the number of cigarettes smoked and plasma nicotine levels. MEC increased smoking intensity and resulted in greater plasma nicotine levels in smokers with schizophrenia compared to controls, although these results were not consistent across deprivation conditions. MEC also increased tobacco craving in smokers with schizophrenia but not in control smokers. Our results suggest that antagonism of high-affinity nAChRs in smokers with schizophrenia may prompt compensatory smoking, increasing the intensity of smoking and nicotine exposure without alleviating craving. Further work is needed to assess whether nicotine levels are directly mediated by how intensely the cigarettes are smoked, and to confirm whether this effect is more pronounced in smokers with schizophrenia.

Corticotropin-releasing factor-1 receptor activation mediates nicotine withdrawal-induced deficit in brain reward function and stress-induced relapse

BACKGROUND

Tobacco addiction is a chronic brain disorder that is characterized by a negative affective state upon smoking cessation and relapse after periods of abstinence. Previous research has shown that blockade of corticotropin-releasing factor (CRF) receptors with a nonspecific CRF1/CRF2 receptor antagonist prevents the deficit in brain reward function associated with nicotine withdrawal and stress-induced reinstatement of extinguished nicotine-seeking in rats. The aim of these studies was to investigate the role of CRF1 and CRF2 receptors in the deficit in brain reward function associated with precipitated nicotine withdrawal and stress-induced reinstatement of nicotine-seeking.

METHODS

The intracranial self-stimulation (ICSS) procedure was used to assess the negative affective state of nicotine withdrawal. Elevations in brain reward thresholds are indicative of a deficit in brain reward function. Stress-induced reinstatement of nicotine-seeking was investigated in animals in which responding for intravenously infused nicotine was extinguished by substituting saline for nicotine.

RESULTS

In the ICSS experiments, the nicotinic receptor antagonist mecamylamine elevated the brain reward thresholds of the nicotine-dependent rats but not those of the control rats. The CRF1 receptor antagonist R278995/CRA0450 but not the CRF2 receptor antagonist astressin-2B prevented the elevations in brain reward thresholds associated with precipitated nicotine withdrawal. Furthermore, R278995/CRA0450 but not astressin-2B prevented stress-induced reinstatement of extinguished nicotine-seeking. Neither R278995/CRA0450 nor astressin-2B affected operant responding for chocolate-flavored food pellets.

CONCLUSIONS

These studies indicate that CRF(1) receptors but not CRF(2) receptors play an important role in the anhedonic-state associated with acute nicotine withdrawal and stress-induced reinstatement of nicotine-seeking.

Nicotinic receptors differentially modulate the induction and expression of behavioral sensitization to methylphenidate in rats

RATIONALE

Nicotinic acetylcholine receptors (nAChRs) regulate sensitization to stimulant drugs such as d-amphetamine and cocaine.

OBJECTIVES

The current study determined if nAChRs modulate the induction and/or expression of behavioral sensitization to high methylphenidate doses.

METHODS

In experiment 1, rats received saline or mecamylamine (3 mg/kg, sc), followed by saline or methylphenidate (5.6 or 10 mg/kg, sc) during 10 daily sessions; the effect of methylphenidate (1-17 mg/kg, sc) alone was determined 14 days later. In experiment 2, rats received saline or dihydro-beta-erythroidine (DHbetaE; 3 mg/kg, sc), followed by saline or 5.6 mg/kg of methylphenidate. In experiment 3, rats received saline or methylphenidate (5.6 or 10 mg/kg, sc) alone for 10 days; the effect of acute mecamylamine (3 mg/kg, sc) on the response to methylphenidate (1-17 mg/kg, sc) was determined 14 days later. Locomotor activity, sniffing, rearing, grooming, and stereotypy ratings were dependent measures.

RESULTS

Methylphenidate produced dose-dependent increases in locomotor activity, sniffing, and stereotypy on day 1 and these effects were enhanced on day 10, indicative of sensitization. Mecamylamine attenuated methylphenidate-induced stereotypy only on day 1, but reduced locomotor activity, sniffing, rearing, and stereotypy on day 10 and during the methylphenidate challenge phase; similar results were obtained with DHbetaE. However, acute mecamylamine did not alter the effects of the methylphenidate challenge following the induction of sensitization to methylphenidate alone.

CONCLUSIONS

Although nAChRs do not appear to regulate the expression of methylphenidate-induced behavioral sensitization, inhibition of high-affinity beta2 subunit nAChRs attenuates the induction of behavioral sensitization to high doses of methylphenidate.

Effects of acute ultra-low dose mecamylamine on cognition in adult attention-deficit/hyperactivity disorder (ADHD)

OBJECTIVE

Nicotinic cholinergic stimulation has known beneficial effects in attention-deficit/hyperactivity disorder (ADHD). Mecamylamine is a non-competitive nicotinic antagonist which is reported in several animal studies to have paradoxical positive effects on cognition at ultra-low doses. Comparable studies in humans have not been conducted. The aim of this study was to determine the effects of acute ultra-low doses of mecamylamine on cognition in adult ADHD.

METHODS

Fifteen (6 female) non-smokers with ADHD completed this acute, within subjects, double blind study of 0.2, 0.5, 1.0 mg of oral mecamylamine and placebo. Behavioral inhibition, recognition memory, and delay aversion were assessed at each dose.

RESULTS

The 0.5 mg dose of mecamylamine significantly improved recognition memory and reduced tolerance for delay. Mecamylamine increased participant rated irritability and investigator rated restlessness. There were no effects on vital signs or physical side effects.

CONCLUSION

This is the first study to find measurable effects of ultra-low doses of mecamylamine in humans. Mecamylamine did not improve core ADHD cognitive symptoms, but significantly improved recognition memory. These effects may represent mixed receptor activity (activation and blockade) at the doses tested. The finding of beneficial effects on memory processes has important clinical implications and further exploration of this effect is warranted.

Acute nicotine reduces brain arachidonic acid signaling in unanesthetized rats

Nicotine exerts its central effects by activating pre- and postsynaptic nicotinic acetylcholine receptors (nAChRs). Presynaptic nAChRs modulate the release of many neurotransmitters that bind to postsynaptic receptors. These may be coupled to the activation of cytosolic phospholipase A(2) (cPLA(2)), which hydrolyzes arachidonic acid (AA) from membrane phospholipids. We hypothesized that nicotine would modify brain signaling involving AA by binding to nAChRs. Nicotine (0.1 mg/kg, subcutaneously) or saline was injected 2 or 10 mins before infusing [1-(14)C]AA in unanesthetized rats. The AA incorporation coefficient k(*) (a marker of the AA signal) was measured in 80 brain regions by quantitative autoradiography. Nicotine, compared to saline, when administrated 2 mins before [1-(14)C]AA infusion, significantly decreased k(*) for AA in 26 regions, including cerebral cortex, thalamus, and habenula-interpeduncular regions, by 13% to 45%. These decreases could be entirely prevented by pretreatment with mecamylamine (1.0 mg/kg, subcutaneously). When administered 10 mins before [1-(14)C]AA infusion, nicotine did not alter any value of k(*). In summary, nicotine given to unanesthetized rats rapidly reduces signaling involving AA in brain regions containing nAChRs, likely by modulating the presynaptic release of neurotransmitters. The effect shows rapid desensitization and is produced at a nicotine dose equivalent to smoking one cigarette in humans.

Responses of limbic and extrapyramidal substance P systems to nicotine treatment

RATIONALE

Neuropeptides are linked to the psychopathology of stimulants of abuse, principally through dopamine mechanisms. Substance P (SP) is one of these neuropeptides and is associated with both limbic and extrapyramidal dopaminergic pathways and likely contributes to the pharmacology of these stimulants. The effects of nicotine on these dopamine systems have also been extensively studied; however, its effects on the associated SP pathways have received little attention.

OBJECTIVES

In the present study, we elucidated the effects of nicotine treatment on limbic and extrapyramidal SP systems by measuring changes in associated SP tissue concentrations.

MATERIALS AND METHODS

Male Sprague-Dawley rats received (+/-)nicotine 4.0 mg/kg/day (0.8 mg/kg, intraperitoneally; five injections at 2-h intervals) in the presence or absence of selective dopamine D1 and D2 receptor antagonists or a nonselective nicotinic acetylcholine receptor antagonist.

RESULTS

The nicotine treatment significantly but temporarily decreased substance P-like immunoreactivity (SPLI) content in the ventral tegmental area (VTA) and substantia nigra 12-18 h after drug exposure. The nicotine-mediated changes in SPLI were selectively blocked by pretreatment with mecamylamine as well as a dopamine D1, D2, or both receptor antagonists. Other brain areas that also selectively demonstrated nicotine-related declines in SPLI content included prefrontal cortex, the nucleus accumbens shell, and the very posterior caudate.

CONCLUSIONS

These findings indicate that some limbic and basal ganglia SP systems are significantly affected by exposure to nicotine through processes mediated by nicotinic and dopaminergic receptors, suggesting a role for SP pathways in nicotine's limbic and extrapyramidal effects.