Central sites and mechanisms of action of nicotine

Low-dose alcohol: Interoceptive and molecular effects and the role of dentate gyrus in rats

Alcohol abuse and dependence are world-wide health problems. Most research on alcohol use focuses on the consequences of moderate to high levels of alcohol. However, even at low concentrations, alcohol is capable of producing effects in the brain that can ultimately affect behavior. The current studies seek to understand the effects of low-dose alcohol (blood alcohol levels of ≤10mM). To do so, these experiments utilize a combination of behavioral and molecular techniques to (1) assess the ability of the interoceptive effects of a low dose of alcohol to gain control over goal-tracking behavior in a Pavlovian discrimination task, (2) determine brain regional differences in cellular activity via expression of immediate early genes (IEGs), and (3) assess the role of the dentate gyrus in modulating sensitivity to the interoceptive effects of a low dose of alcohol. Here, we show that intragastric administration of a dose of 0.8 g/kg alcohol produces blood alcohol levels ≤10mM in both male and female Long-Evans rats and can readily be trained as a Pavlovian interoceptive drug cue. In rats trained on this procedure, this dose of alcohol also modulates expression of the IEGs c-Fos and Arc in brain regions known to modulate expression of alcohol interoceptive effects. Finally, pharmacological inactivation of the dentate gyrus with GABA agonists baclofen and muscimol disrupted the ability of a low dose of alcohol to serve as an interoceptive cue. Together, these findings demonstrate behavioral and molecular consequences of low-dose alcohol.

More than Smoke and Patches: The Quest for Pharmacotherapies to Treat Tobacco Use Disorder

Tobacco use is a persistent public health issue. It kills up to half its users and is the cause of nearly 90% of all lung cancers. The main psychoactive component of tobacco is nicotine, primarily responsible for its abuse-related effects. Accordingly, most pharmacotherapies for smoking cessation target nicotinic acetylcholine receptors (nAChRs), nicotine's major site of action in the brain. The goal of the current review is twofold: first, to provide a brief overview of the most commonly used behavioral procedures for evaluating smoking cessation pharmacotherapies and an introduction to pharmacokinetic and pharmacodynamic properties of nicotine important for consideration in the development of new pharmacotherapies; and second, to discuss current and potential future pharmacological interventions aimed at decreasing tobacco use. Attention will focus on the potential for allosteric modulators of nAChRs to offer an improvement over currently approved pharmacotherapies. Additionally, given increasing public concern for the potential health consequences of using electronic nicotine delivery systems, which allow users to inhale aerosolized solutions as an alternative to smoking tobacco, an effort will be made throughout this review to address the implications of this relatively new form of nicotine delivery, specifically as it relates to smoking cessation. SIGNIFICANCE STATEMENT: Despite decades of research that have vastly improved our understanding of nicotine and its effects on the body, only a handful of pharmacotherapies have been successfully developed for use in smoking cessation. Thus, investigation of alternative pharmacological strategies for treating tobacco use disorder remains active; allosteric modulators of nicotinic acetylcholine receptors represent one class of compounds currently under development for this purpose.

Pharmacological classification of the abuse-related discriminative stimulus effects of trichloroethylene vapor

Inhalants are distinguished as a class primarily based upon a shared route of administration. Grouping inhalants according to their abuse-related in vivo pharmacological effects using the drug discrimination procedure has the potential to provide a more relevant classification scheme to the research and treatment community. Mice were trained to differentiate the introceptive effects of the trichloroethylene vapor from air using an operant procedure. Trichloroethylene is a chlorinated hydrocarbon solvent once used as an anesthetic as well as in glues and other consumer products. It is now primarily employed as a metal degreaser. We found that the stimulus effects of trichloroethylene were similar to those of other chlorinated hydrocarbon vapors, the aromatic hydrocarbon toluene and the vapor anesthetics methoxyflurane and isoflurane. The stimulus effects of trichloroethylene overlapped with those of the barbiturate methohexital, to a lesser extent the benzodiazepine midazolam and to ethanol. NMDA antagonists, the kappa opioid agonist U50,488 and the mixed 5-HT agonist mCPP largely failed to substitute for trichloroethylene. These data suggest that stimulus effects of chlorinated hydrocarbon vapors are mediated at least partially by GABA_A receptor positive modulatory effects.

Animal models of nicotine exposure: relevance to second-hand smoking, electronic cigarette use, and compulsive smoking

Much evidence indicates that individuals use tobacco primarily to experience the psychopharmacological properties of nicotine and that a large proportion of smokers eventually become dependent on nicotine. In humans, nicotine acutely produces positive reinforcing effects, including mild euphoria, whereas a nicotine abstinence syndrome with both somatic and affective components is observed after chronic nicotine exposure. Animal models of nicotine self-administration and chronic exposure to nicotine have been critical in unveiling the neurobiological substrates that mediate the acute reinforcing effects of nicotine and emergence of a withdrawal syndrome during abstinence. However, important aspects of the transition from nicotine abuse to nicotine dependence, such as the emergence of increased motivation and compulsive nicotine intake following repeated exposure to the drug, have only recently begun to be modeled in animals. Thus, the neurobiological mechanisms that are involved in these important aspects of nicotine addiction remain largely unknown. In this review, we describe the different animal models available to date and discuss recent advances in animal models of nicotine exposure and nicotine dependence. This review demonstrates that novel animal models of nicotine vapor exposure and escalation of nicotine intake provide a unique opportunity to investigate the neurobiological effects of second-hand nicotine exposure, electronic cigarette use, and the mechanisms that underlie the transition from nicotine use to compulsive nicotine intake.

Effects of nicotine in experimental animals and humans: an update on addictive properties

Tobacco use through cigarette smoking is the leading preventable cause of death in the developed world. Nicotine, a psychoactive component of tobacco, appears to play a major role in tobacco dependence, but the reinforcing effects of nicotine have often been difficult to demonstrate directly in controlled studies with laboratory animals or human subjects. Here we update our earlier review published in Psychopharmacology (Berl) in 2006 on findings obtained with various procedures developed to study dependence-related behavioral effects of nicotine in experimental animals and humans. Results obtained with drug self-administration, conditioned place preference, subjective reports of nicotine effects and nicotine discrimination indicate that nicotine can function as an effective reinforcer of drug-seeking and drug-taking behavior both in experimental animals and humans under appropriate conditions. Interruption of chronic nicotine exposure produces ratings of drug withdrawal and withdrawal symptoms that may contribute to relapse. Difficulties encountered in demonstrating reinforcing effects of nicotine under some conditions, relative to other drugs of abuse, may be due to weaker primary reinforcing effects of nicotine, to aversive effects produced by nicotine, or to a more critical contribution of environmental stimuli to the maintenance of drug-seeking and drug-taking behavior with nicotine than with other drugs of abuse. Several recent reports suggest that other chemical substances inhaled along with nicotine in tobacco smoke may play a role in sustaining smoking behavior. However, conflicting results have been obtained with mice and rats and these findings have not yet been validated in nonhuman primates or human subjects. Taken together, these findings suggest that nicotine acts as a typical drug of abuse in experimental animals and humans in appropriate situations.

Differential involvement of glutamatergic mechanisms in the cognitive and subjective effects of smoking

There is growing preclinical evidence for the involvement of glutamate in the behavioral actions of nicotine. The aim of this study, was to investigate the role of N-methyl-D-aspartate (NMDA) receptors in the cognitive and subjective effects of smoking in humans. Sixty regular smokers took part in this double-blind placebo controlled study, that investigated the effect of the NMDA-antagonist memantine (40 mg) and the nicotinic-receptor antagonist mecamylamine (10 mg) on smoking-induced improvement in performance of a task of sustained attention and on smoking-induced changes in subjective effects and craving. Increases in subjective ratings of 'buzzed' following smoking were reversed by memantine, but not by mecamylamine. In contrast, improvement on a Rapid Visual Information Processing task by smoking was opposed by mecamylamine, but not by memantine. Smoking reduced craving for cigarettes, but neither drug altered this effect. Our results suggest that glutamatergic mechanisms may have differential involvement in the subjective and cognitive actions of smoking. Further investigations using different ligands are warranted to fully characterize the role of glutamate underlying the consequences of smoking behavior.

Acute nicotine reduces and repeated nicotine increases spontaneous activity in male and female Lewis rats

The Lewis (LEW) strain of rat appears more sensitive to nicotine than other strains in self-administration, conditioned place preference, and drug discrimination behavioral studies. The present study sought to further evaluate the behavioral effects of chronic nicotine treatment in the LEW strain by assessing spontaneous activity, which has consistently revealed sensitization to chronic nicotine administration in Sprague Dawley (SD) rats. High active and low active male and female LEW rats (N=8 per group) were treated twice daily with either nicotine (0.4 mg/kg, sc) or vehicle for 14 consecutive days. Regardless of baseline activity level or sex, spontaneous activity was significantly decreased, compared to saline-treated rats, after a single nicotine injection. However, spontaneous activity increased in both low- and high-activity rats (both sexes) over the two weeks of nicotine administration to levels that were significantly higher than saline-treated rats. Based on these findings, acute and chronic nicotine administration had greater suppressive and enhancing effects on spontaneous activity in LEW rats compared to other strains of rats previously studied. These results further clarify the behavioral sensitivity of the LEW strain of rat to nicotine exposure and lend credence to the role of genetics in the individual susceptibility to nicotine dependence.

The pharmacological activity of nicotine and nornicotine on nAChRs subtypes: relevance to nicotine dependence and drug discovery

Cigarette smoking and other forms of tobacco use deliver an array of pharmacologically active alkaloids, including nicotine and ultimately various metabolites of these substances. While nornicotine is a significant component in tobacco as well as a minor systemic metabolite of nicotine, nornicotine appears to be N-demethylated locally in the brain where it accumulates at relatively high levels after chronic nicotine administration. We have now examined the effects of nornicotine on specific combinations of neuronal nicotinic acetylcholine receptor (nAChR) subunits expressed in Xenopus oocytes and compared these responses to those evoked by acetylcholine and nicotine. Of the nAChR subtypes studied, we have found that alpha7 receptors are very responsive to nornicotine (EC50 approximately 17 micromol/L I(max) 50%, compared with acetylcholine (ACh)). nAChRs containing the ligand-binding domain of the alpha6 subunits (in the form of an alpha6/alpha3 chimera) are also strongly responsive to nornicotine (EC50 approximately 4 micromol/L I(max) 50%, compared with ACh). Alpha7-type nAChRs have been suggested to be potential therapeutic targets for Alzheimer's disease, schizophrenia and possibly other pathologies. nAChRs containing alpha6 subunits have been suggested to have a role in nicotine-evoked dopamine release. Thus, understanding the actions of nornicotine in the brain may have significance for both emerging therapeutics and the management of nicotine dependence.

Nicotine as a typical drug of abuse in experimental animals and humans

RATIONALE AND BACKGROUND

Tobacco use through cigarette smoking is the leading preventable cause of death in the developed world. Nicotine, a psychoactive component of tobacco, appears to play a major role in tobacco dependence, but reinforcing effects of nicotine often are difficult to demonstrate directly in controlled laboratory studies with animal or human subjects.

OBJECTIVE

To review the major findings obtained with various procedures developed to study dependence-related behavioral effects of nicotine in experimental animals and humans, i.e., drug self-administration, conditioned place preference, subjective reports of nicotine effects and nicotine discrimination, withdrawal signs, and ratings of drug withdrawal.

RESULTS

Nicotine can function as an effective reinforcer of drug-seeking and drug-taking behavior both in experimental animals and humans under appropriate conditions. Interruption of chronic nicotine exposure produces withdrawal symptoms that may contribute to relapse. Difficulties encountered in demonstrating reinforcing effects of nicotine under some conditions, relative to other drugs of abuse, may be due to weaker primary reinforcing effects of nicotine or to a more critical contribution of environmental stimuli to the maintenance of drug-seeking and drug-taking behavior with nicotine than with other drugs of abuse. Further experiments are also needed to delineate the role other chemical substances inhaled along with nicotine in tobacco smoke play in sustaining smoking behavior.

CONCLUSION

Nicotine acts as a typical drug of abuse in experimental animals and humans.

Is dopamine important in nicotine dependence?

Nicotine, like other drugs when abused, can produce a wide array of behaviours, some of which collectively propel 'drug-seeking behaviour'. This review focuses on three stimulus properties of nicotine and examines the role of dopamine in mediating each effect with respect to D1 and D2 receptor subtypes. Dopamine appears to be critical in mediating the reinforcing effects of nicotine, which is in line with other commonly abused psychomotor stimulants. However, evidence derived from studies with local microinjections of nicotine suggests that the origin of nicotine action to produce its other stimulus properties may be via multiple neuroanatomical substrates. The aversive stimulus effects are resistant to dopamine receptor antagonists. The discriminative stimulus effects of nicotine, despite showing some modification with dopaminergic compounds, appear not to be solely mediated via the mesolimbic dopamine system. Taken together, the neurobiology of nicotine dependence remains complex. Nonetheless, such dissociation between stimulus properties may permit the development of more effective therapies in combating tobacco dependence.

Brain sites mediating the discriminative stimulus effects of nicotine in rats

Pharmacological studies suggest that the discriminative stimulus (DS) produced by nicotine is mediated centrally. The aim of the present study was to identify neuroanatomical substrates that mediate the DS properties of nicotine. Specifically, the nucleus accumbens, a brain region known to mediate the DS effects of amphetamine and cocaine, was investigated using a two-lever operant drug discrimination paradigm. Male hooded rats were trained to discriminate nicotine (0.2 mg/kg s.c.) from saline with a tandem schedule of food reinforcement. Once stimulus control was attained, a randomised sequence of nicotine microinjections (2-8 micrograms) was tested for generalisation during brief extinction tests. It was confirmed that the stimulus produced by the systemic administration of nicotine generalized to nicotine administered bilaterally into the dorsal hippocampus, with significant decreases in overall response rates. Microinjections of nicotine (1-8 micrograms) into the nucleus accumbens failed to produce any dose-related increases in responding on the nicotine-appropriate lever although these microinjections also produced significant decreases in response rates. Smaller doses (1-4 micrograms) of nicotine administered into the fourth ventricle produced characteristic prostration responses but these microinjections failed to produce generalization in tests carried out 20 min later, when the disabling effects of prostration had dissipated. These results suggest that the DS effects of nicotine may be mediated, at least in part, through the dorsal hippocampus. Results from intra-accumbens and intraventricular injections suggest that these regions may not be important in mediating the DS effects of nicotine.

In vivo effects of (-)-nicotine on ethanol-induced increase in glucose utilization in the mouse cerebellum

The purpose of this study was to investigate the possible in vivo effects of (-)-nicotine, ethanol, and an adenosine agonist N6-cyclohexyladenosine (CHA) when injected individually as well as in various combinations on glucose utilization in the fresh cerebellar slices of mice. Mice received ICV (-)-nicotine or CHA followed 5 min later by a test dose of ethanol (2 g/kg; IP). Animals were killed 20 min postethanol treatment and fresh slices (300 microns) of cerebellum were incubated in a glucose medium in Warburg flasks using 14C-glucose as a tracer. Trapped 14CO2 was counted to estimate glucose utilization. Ethanol treatment markedly accentuated glucose utilization, whereas the pretreatment with (-)-nicotine (125 and 250 ng, ICV) resulted in a significant attenuation in the ethanol-induced increase in glucose utilization. However, ICV (-)-nicotine (125 ng) alone did not produce any change in the cerebellar glucose utilization. The attenuation of ethanol-induced increase in glucose utilization by (-)-nicotine was nearly totally blocked by ICV hexamethonium, a purported nicotinic antagonist, suggesting participation of cholinergic-nicotinic receptors. The (-)-nicotine pretreatment also significantly attenuated both the ICV CHA (25 ng)-induced increase in glucose utilization and the accentuation of ethanol-induced increase in glucose utilization by CHA. The antagonistic effect of (-)-nicotine on CHA- and ethanol-induced increase in glucose utilization indicating an interaction between (-)-nicotine and ethanol and between (-)-nicotine and adenosine may suggest involvement of postreceptor (nicotinic and adenosine) mechanisms including ionic channels.

Individual differences in sensitivity to nicotine in mice: response to six generations of selective breeding

Four hundred seventeen heterogeneous stock mice were tested for their relative sensitivity to a low dose of nicotine (0.75 mg/kg) using activity in an automated Y-maze and body temperature as response measures. A wide spectrum of individual responsiveness to nicotine, ranging from complete suppression of activity to stimulation above baseline activity, was found. Replicate measures taken 1 week later on the same animals showed the responses to nicotine to be reliable and reproducible. Activity levels and body temperatures following nicotine administration were highly correlated (r = 0.60, df = 415). From analysis of between-litter proportions of variance, the heritability of nicotine-influenced activity was estimated to be 0.12, indicating that selective breeding for differential responsiveness to nicotine would be possible. The 10 most activated and 10 most depressed male and female mice were chosen as breeders for replicate nicotine activated (NA) and nicotine depressed (ND) lines, respectively. The selection criterion was nicotine-induced activity corrected for baseline activity using regression residuals. After six generations of selective breeding a good response to selection was obtained, although the response was better for the ND than for the NA lines. Realized heritability for responsiveness to nicotine calculated from the six selected generations was found to be 0.20, or slightly greater than that estimated from the foundation population. There were no significant differences in response to selection between the replicate NA or ND lines. Nicotine-induced body temperature was measured as a correlated response to selection, and was found to remain highly correlated with nicotine-induced locomotor activity. The response was more robust for the ND lines than it was for the NA lines. In contrast to the large differences between the ND and NA lines in locomotor activity and body temperatures following nicotine administration, mean baseline activities and body temperatures remained nearly identical throughout. This indicates that selection acted specifically on nicotine-induced responses, and not on baseline measurements, as predicted for response to a selection criterion based on regression residuals.

Intracerebellar nicotinic-cholinergic participation in the cerebellar adenosinergic modulation of ethanol-induced motor incoordination in mice

Many epidemiological studies have suggested a high correlation between the use of tobacco and ethanol, the two most frequently abused psychoactive drugs. Recently, we reported behavioral interactions between (-)-nicotine, (-)-cotinine and ethanol within the CNS. The present report is a confirmation and an extension of that study. Using a 2 g/kg ethanol-induced motor incoordination (EIMI) as the test response, possible behavioral interactions between (-)-nicotine, (-)-cotinine and ethanol and between (-)-nicotine, (-)-cotinine and adenosine agonist + ethanol in the cerebellum were investigated. (-)-Nicotine, 0.625, 1.25 and 5 ng intracerebellarly (ICB) significantly attenuated EIMI in a dose-related manner. Likewise, ICB injection of 1.25, 2.5, and 5 ng (-)-cotinine, a major metabolite of nicotine, significantly attenuated EIMI after the same i.p. dose of ethanol as in case of (-)-nicotine but less markedly compared to (-)-nicotine. No change in normal motor coordination was observed when the highest dose of (-)-nicotine or (-)-cotinine was injected ICB followed by saline control, suggesting selectivity of their behavioral interactions with ethanol. The attenuation of EIMI by (-)-nicotine and (-)-cotinine was blocked by ICB hexamethonium (1 microgram) and trimethaphan (100 ng), the purported nicotinic-cholinergic antagonists. Finally, the ICB injection of adenosine agonists, N6-cyclohexyladenosine (CHA) or 5'-N-ethylcarboxamidoadenosine (NECA), produced marked accentuation of EIMI which was significantly antagonized by ICB (-)-nicotine and (-)-cotinine. The data obtained in the present study suggested, for the first time, a cerebellar adenosinergic-nicotinic cholinergic interaction and modulation of EIMI. The data also suggested participation of cerebellar nicotinic-cholinergic receptors in EIMI.

Mecamylamine reverses physostigmine-induced attenuation of scopolamine-induced hyperactivity

It has previously been demonstrated that the muscarinic antagonist scopolamine induces hyperactivity in rodents, which is reversed by physostigmine but not by directly acting agonists such as pilocarpine. This may suggest that non-muscarinic actions of physostigmine may be responsible for its reversal of scopolamine-induced hyperactivity. We have found, in male Wistar rats, whose activity was measured on electromagnetic detector plates, that the central nicotinic receptor antagonist mecamylamine (3 mg/kg) reverses the blockade of scopolamine-induced behavioural activation induced by physostigmine. This suggests that activation of nicotinic receptors can counteract the effects of muscarinic blockade. Interestingly, however, treatment with nicotine does not block scopolamine-induced hyperactivity, suggesting that the exogenous and endogenous ligands may have different receptor or neuronal substrates.

Neurobehavioral mechanisms of nicotine action: role in the initiation and maintenance of tobacco dependence

Basic neuroscience research conducted over the last quarter century has provided us with much information concerning potential biobehavioral and neuromolecular mechanisms involved in the initiation and maintenance of tobacco dependence. Nicotinic-acetylcholinergic receptors (AChRs), in addition to having a primary locus on cholinergic neurons, appear to be also located on a variety of noncholinergic neurons (presynaptic and/or postsynaptic sites). Nicotine therefore appears to be able to affect a variety of neuronal pathways involved in behavioral reward and arousal processes, which appear paramount to tobacco dependence. Nicotine appears to have several unique properties at the cellular level that allow it to act both as an agonist and as a potential antagonist at select AChRs. Nicotine's ability to act as an agonist appears to be contingent on an action at nAChRs, which initially open a receptor-linked cation channel, eliciting the entrance of CA++ (or other cations) into the cell. Cation entrance into the cell, therefore, may be the cellular transducer of nicotine's behavioral and dependence-producing effects. Subsequent to this initial agonist effect, the nicotinic receptor is believed to undergo a refractory period, via a desensitization process, during which Ca++ is prevented from further entrance into the cell. It is this ability to induce receptor desensitization which seems central to nicotine's ability to act as an antagonist. The duration of nAChR desensitization may also be useful in explaining individual variability to nicotine's behavioral effects and may be related to the induction of acute and/or chronic tolerance in both animals and man. Nicotine-induced desensitization may also be important to relapse in the smoker if conditioned stimuli are able to provoke such mechanisms, which could lead to the need to smoke. Finally, a model is presented to account for individual smoking patterns and level of tobacco dependence which is partially based on the proposed cellular mechanisms of nicotine action and desensitization at the nAChR.

Chronic effects of nicotine on mesolimbic dopaminergic system in rats

Rats were pretreated with saline or nicotine (1.5 mg/kg/day) by subcutaneously implanting each animal with an Alzet osmotic minipump which continuously released saline or nicotine (1.5 mg/kg/day) for 14 days. The behavioral and biochemical effects of nicotine on the dopaminergic neuronal system in rat nucleus accumbens were examined. It was found that chronic nicotine treatment increased the affinity of L-[3H]nicotine binding site in the nucleus accumbens. This treatment also potentiated the ability of (+)-amphetamine, but not high potassium, to stimulate formation and release of [3H]dopamine in tissue slices from rat nucleus accumbens. Chronic nicotine treatment did not alter the characteristics of [3H]spiperone binding site, the rate of dopamine turnover and the concentrations of gamma-aminobutyric acid in the nucleus accumbens.

Behavioural pharmacology of nicotine: multiple mechanisms

It is widely accepted that nicotine serves as a major reinforcer of tobacco use, but how it does so is obscure. Little is known about the nature of the reinforcing effect of nicotine in psychobiological terms, or about the mechanism of reinforcement at the level of neuropharmacology. Several ideas about the nature of the reinforcement are current, including mood changes such as euphoria, learning, memory and attention improvements, ability to help smokers deal with or adapt to stressors, and capacity to terminate symptoms of a nicotine withdrawal syndrome. These psychobiological effects are not mutually exclusive; different effects may be important for different people, or for the same person under different circumstances. Similarly, the neuropharmacology of nicotine reinforcement remains largely unexplored; nicotinic-cholinergic receptors seem to play a primary role but evidence implicating particular subtypes of nicotinic receptor is not available. Evidence points to the release of dopamine as an important link in the chain of events that transduce effects at nicotinic receptors into behaviour and nicotine-seeking responses, but the dopamine hypothesis addresses only one part of a complex system. The possibility has to be faced that neuropharmacologically, several mechanisms are involved that parallel the several psychobiological effects mentioned above; multiple types of receptor, brain structures and indirectly activated neurotransmitter systems may contribute to the maintenance of nicotine-seeking behaviour.

Nicotine dependence: a preventable risk factor for other diseases

Nicotine meets all critical criteria for an addictive drug. Furthermore, there is no evidence that there would be widespread compulsive use of tobacco without nicotine. These findings have led to consideration of the cigarette as a contaminated vehicle for an addictive drug (nicotine). Nevertheless, nicotine itself may also be used therapeutically to reduce exposure to carcinogens and other tobacco toxins. Nicotine replacement is a useful adjunct in treating tobacco dependence. For example, nicotine replacement in the form of a polacrilex resin (chewing gum) can alleviate physically based signs and symptoms of tobacco abstinence. The fact that this form of nicotine replacement is not attractive to non-users of tobacco has opened the door to the use of nicotine in a therapeutic modality, permitting hope of eliminating tobacco dependence.

An interpretative review of smokeless tobacco research in the United States: Part II

This is the second part of a two-part series reviewing the published literature of smokeless tobacco in the United States. The article explores smokeless tobacco as a pharmacologically addicting substance, educational interventions designed to prevent use or help users quit, and outlines areas of future research.

Neuroregulatory effects of nicotine

The impact of nicotine on the central nervous system is, in an important sense, neuroregulatory, with cascading effects on physiological and biochemical function as well as on behavioral activity. Accordingly, the neurotransmitter and neuroendocrine effects of nicotine constitute a critical part of its biological action, which includes reinforcing as well as pathophysiological consequences. This review focuses on nicotine's effects on cholinergic and non-cholinergic nicotine receptors and on the responses of catecholamines, monoamines, hypophyseal hormones, and cortisol. The contribution of critical variables, such as timing and duration of neuroregulator release and the patterns that make up the total response, is still largely unknown, particularly with regard to the effects of environmental context, history of nicotine use, and mode of administration. The evidence suggests that by altering the bioavailability of the above-listed neuroregulators, nicotine serves as a pharmacological "coping response", providing immediate though temporary improvement in affect or performance in response to environmental demands. Much of what is known to date is based on studies involving the administration of agonists and antagonists under different environmental conditions. Newer technological approaches such as autoradiography and positron emission tomography show potential for determining the neuroregulatory patterns involved and specifying nicotine's locus of action relevant to its behavioral and physiological effects.

Mecamylamine reduces some EEG effects of nicotine chewing gum in humans

Spontaneous EEG was recorded in nine cigarette smokers who had been abstinent from tobacco for 12 hr. Subjects were treated with a capsule containing either centrally acting nicotine blocker, mecamylamine (10 mg), or placebo. At each of three 60-min intervals after the capsule was ingested, the subjects chewed two pieces of gum containing a total of 0, 4 or 8 mg of nicotine. Nicotine and mecamylamine dose combinations were randomized across subjects. Two three-minute periods of spontaneous EEG were recorded before the capsule and before and after gum chewing from bipolar electrode montages at the following positions: Cz-T5, Cz-T6, Cz-F7 and Cz-F8. During one period the subjects relaxed with eyes closed, in the other period they performed a math task with eyes open. When the drugs were given individually, mecamylamine decreased beta power and nicotine gum (4 and 8 mg) increased alpha frequency. Mecamylamine pretreatment prevented the increase in alpha frequency caused by the 4 mg gum dose but not the 8 mg dose. Alpha power was increased by the 8 mg gum dose and that increase was prevented by mecamylamine. Self-reported ratings of the "strength" of the gum were significantly diminished by mecamylamine pretreatment. The data are consistent with the results of earlier studies which indicate that the effects of tobacco administration and withdrawal are mediated by central actions of nicotine.

Pharmacological treatment of tobacco dependence

Pharmacologically based approaches for the treatment of tobacco dependence are reviewed. The rational basis for pharmacologic treatment approaches is that tobacco dependence is partially, and critically, mediated by the actions of tobacco-delivered nicotine to the central nervous system. These actions include direct reinforcing properties of nicotine itself, tolerance and physiologic dependence, possible beneficial effects of nicotine in the alleviation of anxiety and control of weight, and neurohormonal regulation which can become important to the maintenance of emotional well-being and performance at work. Insofar as tobacco abstinence leads to negative consequences, via these biobehavioral mechanisms, pharmacologic intervention should be able to assist in initial tobacco detoxification and help tobacco abstinent persons to avoid subsequent relapse. The purpose of this review is to survey some of the efforts to develop such interventions, as well as to elucidate some of the issues relevant to such development. Four distinct approaches are discussed: (1) Nicotine replacement, in which physiologic dependence is transferred to a safer and more therapeutically manageable nicotine delivering formulation; this category includes nicotine polacrilex gum; (2) Blockade therapy, in which a drug is taken that blocks the reinforcing properties of nicotine should relapse occur; (3) Nonspecific pharmacotherapy, in which the biobehaviorally mediated correlates of tobacco abstinence are treated on a symptomatic basis; (4) Deterrent therapy, in which a drug is taken prior to smoking such that any tobacco use would produce reliable aversive effects.

Nicotine and arecoline as discriminative stimuli: involvement of a non-cholinergic mechanism for nicotine

The cholinergic innervation of central muscarinic (M-Ch) and nicotinic (N-Ch) receptors was evaluated by studying the interaction of physostigmine with the discriminative stimulus (DS) effects of arecoline and nicotine. Rats were trained to discriminate either arecoline (1.74 mg/kg) or nicotine (1.14 mg/kg) from saline using a two-lever, milk reinforced, operant task. Physostigmine (0.125 mg/kg) pretreatment potentiated, and when administered alone (0.25 mg/kg), generalized with the DS induced by arecoline. In contrast, physostigmine, at the same dose, neither potentiated nor generalized with the DS effects of nicotine. These findings provide evidence that central muscarinic receptors are cholinergically innervated (physiologic) while central nicotinic receptors are not cholinergically innervated but are cholinoceptive (pharmacological).

Stimulation of [3H]dopamine release by nicotine in rat nucleus accumbens

The mesolimbic system of the brain has been shown to be involved in the reward properties of a number of agents. It is possible that release of monoamines by nicotine in this brain area could be related to the pleasurable aspects related to cigarette smoking. In this investigation, the effect of nicotine on the release of [3H]dopamine in the nucleus accumbens of the rat was studied. It was shown that nicotine produced a concentration-dependent increase in [3H]dopamine release at concentrations of 0.1 microM and above. The increase in release was found to be almost completely calcium dependent. The nicotine-induced release was only partially blocked by the nicotinic antagonists hexamethonium and d-tubocurarine. A number of cholinergic agonists, as well as other compounds, were tested for their capacity to mimic the effect of nicotine. At equimolar concentrations there was, at most, only 50% of the activity of nicotine. The results of this study demonstrate that nicotine stimulates the release of dopamine in the nucleus accumbens at concentrations similar to those in the blood of cigarette smokers. This suggests that the release of monoamines in specific nuclei of the mesolimbic system may be an important determinant of the desire to smoke cigarettes.

Supra-additive toxic interaction of nicotine with antihistamines, and enhancement by the proconvulsant pentylenetetrazole

Antihistamines are being increasingly administered in combination with various other agents, with adverse drug reactions the frequent result. The present study consisted of two experiments. Experiment 1 examined the toxicological response of rats to nicotine tartrate (0.0, 2.0, 4.0, and 8.0 mg/kg) in combination with either of two H1-histamine receptor antagonists, the ethylene diamine tripelennamine HCl (0.0, 16.0, 32.0, and 64.0 mg/kg) or the aminoethyl ether diphenhydramine HCl (0.0, 32.0, 64.0, and 96.0 mg/kg). Adult female rats received intraperitoneal injections when housed 12 per cage and toxicological response (number dead per group) was assessed 24 hours post-treatment. The results showed that over the dose ranges employed, and when given alone, nicotine was completely non-lethal, tripelennamine was virtually non-lethal and diphenhydramine was toxic only at the highest dose (5 of 12, at 96.0 mg/kg). However, when nicotine and the antihistamines were delivered in combinations, the toxicological response was markedly altered. Tripelennamine in combination with nicotine yielded supra-additive interaction, with the degree of potentiation being a simple linear function of nicotine within each dose of tripelennamine. The interaction between nicotine and diphenhydramine was more complicated, with certain dose combinations yielding supra-additivity, yet with others yielding antagonism. It was suggested that seizure-precipitated cardiopulmonary collapse was the immediate cause of death, plausibly mediated by central mechanisms. As such, Experiment 2 examined the influence of adding the proconvulsant pentylenetetrazole (PTZ) (0.0, 10.0, and 20.0 mg/kg) to nicotine (0.0, 2.0, 4.0, and 8.0 mg/kg)-tripelennamine (0.0 and 32.0 mg/kg) combination treatments. Effects were assessed both at 1.0 and 24.0 hours post-injection.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of nicotine on beta-endorphin, alpha MSH, and ACTH secretion by isolated perfused mouse brains and pituitary glands, in vitro

The effects of nicotine on secretion of the pituitary peptides beta-endorphin, alpha MSH, and ACTH were studied using the isolated perfused mouse brain (IPMB) and isolated superfused pituitaries of C3H mice. Nicotine (6.1 microM) stimulated secretion of beta-endorphin immunoreactivity from C3H IPMB approximately twofold. Secretion of alpha MSH immunoreactivity was stimulated approximately two- and sixfold by 6.1 microM and 12.2 microM nicotine, respectively. However, nicotine (6.1 microM) had no direct effect on the secretion of beta-endorphin, alpha MSH, or ACTH immunoreactivities from the isolated superfused pituitaries. The data suggest nicotine acts in the brain to stimulate pituitary secretion of alpha MSH and beta-endorphin. Electrocorticographic (ECoG) activity of the IPMB was monitored. Nicotine induced characteristic ECoG changes including a reduction of input voltage, a biphasic response of rapid desynchronization followed by prolonged synchronization, and seizure at high doses (12.2 microM).

Discriminative stimulus properties of cocaine in pigeons

The present study was designed to assess the discriminative stimulus properties of cocaine in pigeons. Six pigeons were trained to discriminate IM injections of cocaine (2 mg/kg) from saline with responding maintained under a fixed-ratio 30 schedule of food delivery. Cocaine, d-amphetamine, and l-cathinone substituted completely for the training dose of cocaine in all pigeons. When nicotine (0.25-4.0 mg/kg), apomorphine (0.03-1.0 mg/kg), procaine (4-32 mg/kg), and lidocaine (4-16 mg/kg) were substituted, both partial substitutions and individual differences between pigeons were observed. Oxazepam (0.5-4.0 mg/kg) and pentobarbital (2-8 mg/kg) failed to substitute for the training dose of cocaine. Discriminative stimulus control by cocaine was greatest when the drug was administered 10-40 min prior to the session and the effects disappeared after 2 h. The substitution results indicate drug class specificity of the cocaine cue but, in addition, suggest its multidimensional nature.

The effects of smoking on ACTH and cortisol secretion

The relationship among changes in plasma nicotine, ACTH, and cortisol secretion after smoking were investigated. Ten male subjects smoked cigarettes containing 2.87 mg nicotine and 0.48 mg nicotine. No rises in cortisol or ACTH were detected after smoking 0.48 mg nicotine cigarettes. Cortisol rises were significant in 11 of 15 instances after smoking 2.87 mg nicotine cigarettes, but ACTH rose significantly in only 5 of the 11 instances where cortisol increased. Each ACTH rise occurred in a subject who reported nausea and was observed to be pale, sweaty, and tachycardic. Peak plasma nicotine concentrations were not significantly different in sessions when cortisol rose with or without ACTH increases, but cortisol increases were significantly greater in nauseated than in non-nauseated smokers. Our data suggest that smoking-induced nausea stimulates cortisol release by stimulating ACTH secretion and that cortisol secretion in non-nauseated smokers may occur through a non-ACTH mechanism. It is not clear whether nicotine or some other stimulus inherent in smoking is responsible for cortisol secretion without ACTH secretion.

Isolation of a nicotine binding site from rat brain by affinity chromatography

With the use of affinity chromatography, a [3H]-nicotine binding site was purified almost 1,000-fold from a Triton X-100-solubilized extract of rat brain neural membranes. The affinity column was prepared by conjugation of (R,S)-6-(2-hydroxyethyl)nicotine to epoxy-activated Sepharose. Further purification of the material from the affinity column was resolved by using another column of the same affinity gel, resulting in the isolation of a major protein (about 95% purity) that had a Mr of 56,000, as determined by NaDodSO4/polyacrylamide gel electrophoresis, with very minor components ranging in Mr from 47,000 to 83,000. With the use of various nicotine analogues, it was shown that the purified material exhibited nearly identical binding characteristics to rat brain membrane preparations, including stereoselectivity for the nicotine enantiomers. The Kd of the purified site, 3.5 x 10(-9) M, was similar to that observed with membrane and Triton X-100-soluble preparations, whereas the binding capacity was greater than 25 pmol/mg of protein, as compared to 0.07 pmol/mg of protein in the starting material. The results are discussed in relation to the purified nicotinic cholinergic receptor from electroplax. It was concluded that the nicotine site in rat brain was different from the cholinergic receptor of electroplax or calf skeletal muscle.