Guidelines on nicotine dose selection for in vivo research

RATIONALE

This review provides insight for the judicious selection of nicotine dose ranges and routes of administration for in vivo studies. The literature is replete with reports in which a dosaging regimen chosen for a specific nicotine-mediated response was suboptimal for the species used. In many cases, such discrepancies could be attributed to the complex variables comprising species-specific in vivo responses to acute or chronic nicotine exposure.

OBJECTIVES

This review capitalizes on the authors' collective decades of in vivo nicotine experimentation to clarify the issues and to identify the variables to be considered in choosing a dosaging regimen. Nicotine dose ranges tolerated by humans and their animal models provide guidelines for experiments intended to extrapolate to human tobacco exposure through cigarette smoking or nicotine replacement therapies. Just as important are the nicotine dosaging regimens used to provide a mechanistic framework for acquisition of drug-taking behavior, dependence, tolerance, or withdrawal in animal models.

RESULTS

Seven species are addressed: humans, nonhuman primates, rats, mice, Drosophila, Caenorhabditis elegans, and zebrafish. After an overview on nicotine metabolism, each section focuses on an individual species, addressing issues related to genetic background, age, acute vs chronic exposure, route of administration, and behavioral responses.

CONCLUSIONS

The selected examples of successful dosaging ranges are provided, while emphasizing the necessity of empirically determined dose-response relationships based on the precise parameters and conditions inherent to a specific hypothesis. This review provides a new, experimentally based compilation of species-specific dose selection for studies on the in vivo effects of nicotine.

Effect of grapefruit juice on cytochrome P450 2A6 and nicotine renal clearance

BACKGROUND AND OBJECTIVE

Grapefruit juice is an inhibitor of the cytochrome P450 (CYP) 3A4 enzyme and transporters such as P-glycoprotein and organic anion transporting polypeptides, leading to clinically important interactions. Our objective was to study the effect of grapefruit juice on the pharmacokinetics of nicotine, which is primarily metabolized by the CYP2A6 enzyme.

METHODS

Ten volunteers were given a 2-mg oral dose of deuterium-labeled nicotine on 3 occasions together with 1 L of water, full-strength grapefruit juice, or half-strength grapefruit juice. Concentrations of nicotine and its metabolites were analyzed in plasma and urine for 8 hours.

RESULTS

Grapefruit juice inhibited the formation of cotinine from nicotine (area under the plasma cotinine concentration-time curve from 0 to 8 hours of 6807 min.ng/mL, 7805 min.ng/mL, and 8007 min.ng/mL for full-strength grapefruit juice, half-strength grapefruit juice, and water, respectively; repeated-measures ANOVA, P=.009). The time to peak plasma concentration of cotinine was delayed (216 minutes, 159 minutes, and 147 minutes, respectively; ANOVA, P=.011), and the peak plasma concentration was lower with grapefruit juice compared with water (18 ng/mL, 21 ng/mL, and 22 ng/mL, respectively; ANOVA, P=.010). Oral clearance, peak plasma concentration, and time to peak plasma concentration of nicotine were not affected. Grapefruit juice increased the renal clearance of nicotine (231 mL/min, 219 mL/min, and 123 mL/min, respectively; ANOVA, P=.045) and cotinine (19 mL/min, 14 mL/min, and 16 mL/min, respectively; ANOVA, P=.002).

CONCLUSIONS

Grapefruit juice inhibits the metabolism of nicotine to cotinine, a pathway mediated by CYP2A6, and increases the renal clearance of nicotine and cotinine. Nicotine oral clearance is not affected by grapefruit juice because the inhibition of hepatic metabolism is offset by the increase in the renal clearance of nicotine. However, other compounds metabolized by CYP2A6, as well as other drugs excreted via renal clearance mechanisms similar to those of nicotine, may be susceptible to significant pharmacokinetic grapefruit juice interactions.

CYP2A6 genotype and the metabolism and disposition kinetics of nicotine

BACKGROUND AND OBJECTIVE

The liver enzyme cytochrome P450 (CYP) 2A6 is primarily responsible for the metabolism of nicotine. Variants in the CYP2A6 gene have been associated with altered nicotine metabolism and with effects on smoking behavior. Our objective was to determine the relationship between variant CYP2A6 genotypes and the disposition and metabolism of nicotine administered intravenously.

METHODS

Intravenous infusions of deuterium-labeled nicotine and cotinine were administered to 278 healthy twin volunteers, most of whom were white. They were genotyped for CYP2A6*1, CYP2A6*2, CYP2A6*4, CYP2A6*7, CYP2A6*8, CYP2A6*9, CYP2A6*10, and CYP2A6*12.

RESULTS

On the basis of the fractional clearance of nicotine to cotinine and on the plasma ratio of 3'-hydroxycotinine to cotinine, both shown to be indicators of CYP2A6 enzymatic activity, subjects were classified into 3 groups. Group 1 included wild-type variant CYP2A6*1/*1 (n=215) and was assumed to have 100% activity. Group 2 included *1/*9 (n=21) and *1/*12 (n=12), which averaged about 80% of normal activity. Group 3 included *1/*2 (n=10), *1/*4 (n=2), *9/*12 (n=3), *9/*4 (n=2), and *9/*9 (n=3), which averaged about 50% of normal activity. The mean total plasma clearance of nicotine (+/-SD) was 18.8+/-6.0, 15.5+/-4.9, and 11.7+/-5.1 mL.min-1.kg-1 in groups 1, 2, and 3, respectively, and group 1 had significantly faster clearance than group 2 (P<.05) and group 3 (P<.01). Overall, groups 2 and 3 also had lower total clearance of cotinine, had longer half-lives for nicotine and cotinine, and excreted in the urine a greater fraction of the nicotine dose as unchanged nicotine and nicotine glucuronide and excreted less as 3'-hydroxycotinine compared with group 1.

CONCLUSIONS

We provide novel pharmacokinetic and metabolic data on nicotine after systemic dosing in relation to common CYP2A6 genotypes. Our data will enhance the interpretation of CYP2A6 genotypic data as used in association studies of smoking behavior and its health consequences.

Smoking cessation program and CYP2A6 polymorphism

The relationship between CYP2A6 genotype and smoking status remains unclear although several studies have been reported. In this study, we have investigated the significance of CYP2A6 genotype on smoking habit and treatment of nicotine patch. Sixty-one smokers (1.7%) working in a Japanese company (n = 3585) participated in this smoking cessation program. Forty-four of 61 (72.1%) smokers were treated by nicotine patch. A genotyping analysis was carried out for 41 (40 men and 1 women) of 61 participants (67.2%). The smoking cessation rate at 90 days was 54.1% (33/61). Age and smoking years in re-smoking group are significantly lower than those in smoking cessation group. The smoking cessation rate of participants treated with nicotine patch (63.6%; 28/44) was significantly higher than that of the group non-treated with nicotine patch (29.4%; 5/17) at 90 days (p < 0.05). The incidence of homozygotes of CYP2A6 gene deletion (CYP2A619934/19934) in 41 cases (9.8%; 4/41) could be higher than that in 894 healthy controls (3.7%; 33/894) (p = 0.12), while no other variant alleles (CYP2A619932, CYP2A619933 and CYP2A619936) were found. Age and smoking years of participants with CYP2A619934/19934 are significantly higher than those with CYP2A619931 positive. The scores of Fagerstrom test, an analysis for nicotine-dependence, were slightly different between participants with CYP2A619934/19934 and CYP2A619931 positive. Although treatment of nicotine patch is efficacious to smoking cessation, cases with CYP2A619934/19934 might be more sensitive to nicotine adverse effects and more difficult to quit smoking once they have smoking habit.

Childhood predictors of smoking in adolescence: a follow-up study of Montreal schoolchildren

BACKGROUND

The factors that cause children to become smokers in adolescence remain unclear. Although parental smoking and peer pressure may play a role, physiological factors such as lung volume have also been identified.

METHODS

To investigate these and other possible childhood predictors of teenage smoking, we gathered follow-up data on 191 Montréal schoolchildren, aged 5-12 years (average 9.2 yr) when first examined. At an average age of 13.0 years, they answered further questions on their health and smoking behaviour and provided a second set of spirometric measurements.

RESULTS

At the second survey, 80% of the children had entered high school and 44% had become smokers. Reaching puberty between the surveys was the most significant determinant of becoming a smoker: 56.4% of the 124 children postpubertal at the second survey had taken up smoking, versus 17.9% of the 67 who were still prepubertal (p = 0.001). We found salivary cotinine level, a measure of uptake of environmental tobacco smoke, to be an independent predictor of becoming a teenage smoker; even after adjustment for sex, socioeconomic status of parents, a crowding index, and the numbers at home of siblings, adult smokers and cigarettes smoked, it remained significant for both groups: postpubertal (odds ratio [OR] 1.2, 95% confidence interval [CI] 1.2-3.0) and prepubertal (OR 2.1, 95% CI 1.0- 4.5). The influence of forced vital capacity was marginally significant only in the postpubertal group (OR 5.0, 95% CI 0.88-28.3).

INTERPRETATION

The proportion of nicotine absorbed from that available in environmental tobacco smoke during childhood is associated with subsequent smoking in adolescence. The more efficient absorption of nicotine seen in some children may be related to physiological factors such as lung capacity.

Industry research on the use and effects of levulinic acid: A case study in cigarette additives

Public health officials and tobacco researchers have raised concerns about the possible contributions of additives to the toxicity of cigarettes. However, little attention has been given to the process whereby additives promote initiation and addiction. Levulinic acid is a known cigarette additive. Review of internal tobacco industry documents indicates that levulinic acid was used to increase nicotine yields while enhancing perceptions of smoothness and mildness. Levulinic acid reduces the pH of cigarette smoke and desensitizes the upper respiratory tract, increasing the potential for cigarette smoke to be inhaled deeper into the lungs. Levulinic acid also may enhance the binding of nicotine to neurons that ordinarily would be unresponsive to nicotine. These findings held particular interest in the internal development of ultralight and so-called reduced-exposure cigarette prototypes. Industry studies found significantly increased peak plasma nicotine levels in smokers of ultralight cigarettes following addition of levulinic acid. Further, internal studies observed changes in mainstream and sidestream smoke composition that may present increased health risks. The use of levulinic acid illustrates the need for regulatory authority over tobacco products as well as better understanding of the role of additives in cigarettes and other tobacco products.

Similar uptake of lung carcinogens by smokers of regular, light, and ultralight cigarettes

Cigarette design has changed markedly over the past 60 years and sales-weighed levels of tar and nicotine have decreased. Currently, cigarettes are classified as regular (>14.5 mg tar), light (>6.5-14.5 mg tar), and ultralight (< or =6.5 mg tar), based on a Federal Trade Commission-specified machine-smoking protocol. Epidemiologic studies suggest that there is no difference in lung cancer risk among people who smoke light or ultralight cigarettes compared with regular cigarettes, but the uptake of lung carcinogens in smokers of these types of cigarettes has never been reported. We recruited 175 smokers, who filled out a tobacco use questionnaire in which their current brand was identified as regular, light, or ultralight. Urine samples were collected and analyzed for 1-hydroxypyrene (1-HOP), total 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL plus its glucuronides) and total cotinine (cotinine plus its glucuronides). 1-HOP and total NNAL are biomarkers of uptake of polycyclic aromatic hydrocarbons and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, lung carcinogens in cigarette smoke. Total cotinine is a biomarker of nicotine uptake. There were no statistically significant differences in urinary levels of 1-HOP, total NNAL, and total cotinine in smokers of regular, light, and ultralight cigarettes, whether the results were expressed per mg urinary creatinine, per mL of urine, or per mg creatinine divided by cigarettes per day. Levels of machine measured tar were available for the cigarettes smoked by 149 of the subjects. There was no correlation between levels of tar and any of the biomarkers. These results indicate that lung carcinogen and nicotine uptake, as measured by urinary 1-HOP, total NNAL, and total cotinine is the same in smokers of regular, light, and ultralight cigarettes. The results are consistent with epidemiologic studies that show no difference in lung cancer risk in smokers of these cigarettes.

Metabolism and Disposition Kinetics of Nicotine

Nicotine is of importance as the addictive chemical in tobacco, pharmacotherapy for smoking cessation, a potential medication for several diseases, and a useful probe drug for phenotyping cytochrome P450 2A6 (CYP2A6). We review current knowledge about the metabolism and disposition kinetics of nicotine, some other naturally occurring tobacco alkaloids, and nicotine analogs that are under development as potential therapeutic agents. The focus is on studies in humans, but animal data are mentioned when relevant to the interpretation of human data. The pathways of nicotine metabolism are described in detail. Absorption, distribution, metabolism, and excretion of nicotine and related compounds are reviewed. Enzymes involved in nicotine metabolism including cytochrome P450 enzymes, aldehyde oxidase, flavin-containing monooxygenase 3, amine N-methyltransferase, and UDP-glucuronosyltransferases are represented, as well as factors affecting metabolism, such as genetic variations in metabolic enzymes, effects of diet, age, gender, pregnancy, liver and kidney diseases, and racial and ethnic differences. Also effects of smoking and various inhibitors and inducers, including oral contraceptives, on nicotine metabolism are discussed. Due to the significance of the CYP2A6 enzyme in nicotine clearance, special emphasis is given to the effects and population distributions of CYP2A6 alleles and the regulation of CYP2A6 enzyme.

Rate of nicotine onset from nicotine replacement therapy and acute responses in smokers

The subjective and reinforcing effects of drugs of abuse may depend partly on their rate of onset, with faster acting formulations typically producing stronger effects than slower ones. In this within-subjects study, we examined the acute effects of nicotine replacement therapy via nicotine nasal spray (fast delivery) vs. transdermal nicotine patch (slow delivery) on craving, withdrawal, cardiovascular responses, subjective ratings, and reinforcing effects of smoking. Smokers (N=30) not seeking treatment participated in three sessions, each after overnight smoking abstinence, involving 14-mg nicotine (Nicoderm) or placebo patch, followed 4 hr later by intermittent administration of nicotine (Nicotrol) or placebo nasal spray. Specifically, the three group comparisons were nicotine patch condition (with placebo spray), nicotine spray condition (with placebo patch), and placebo condition (placebo spray and patch). Nicotine patch and nicotine spray were never administered in the same session. Blood nicotine levels were similar between nicotine patch and nicotine spray conditions, by design. Heart rate and systolic blood pressure were higher following nicotine spray vs. the other conditions, as hypothesized. However, other than reductions in craving related to nicotine spray and patch at some points, no differences between conditions were observed in withdrawal, subjective effects of sprays and smoking, or smoking reinforcement assessed by a computer task. Thus, under these acute conditions, the speed of nicotine delivery from nasal spray vs. patch differentially affected cardiovascular responses and perhaps craving but did not influence withdrawal, subjective ratings, and smoking reinforcement.

Surveillance of smokeless tobacco nicotine, pH, moisture, and unprotonated nicotine content

Smokeless tobacco is a complex chemical mixture, including not only the components of the tobacco leaf but also chemicals added during the manufacturing process. Smokeless tobacco contains the addictive chemical nicotine and more than 20 cancer-causing chemicals, including the potent tobacco-specific nitrosamines. The National Toxicology Program of the National Institutes of Health has concluded that oral use of smokeless tobacco is a human carcinogen. Therefore, smokeless tobacco is not a safe alternative to cigarettes. In fact, smokeless tobacco use begins primarily during early adolescence and can lead to nicotine dependence and increased risk of becoming a cigarette smoker. Under the Comprehensive Smokeless Tobacco Health Education Act of 1986 (15 U.S.C. 4401 et seq., Pub. L. 99-252), tobacco manufacturers report annually to the Centers for Disease Control and Prevention (CDC) on the total nicotine, unprotonated nicotine, pH, and moisture content of their smokeless tobacco products. This information is considered "trade secret," or confidential, in accordance with 5 U.S.C. 552(b)(4) and 18 U.S.C. 1905 and cannot be released to the public. In an effort to provide consumers and researchers with information on the nicotine content of smokeless tobacco, CDC arranged for the analysis of popular brands of smokeless tobacco. The results of this CDC study show that pH is a primary factor in the amount of nicotine that is in the most readily absorbable, unprotonated form. Furthermore, this study found that the brands of moist snuff smokeless tobacco with the largest amount of unprotonated nicotine also are the most frequently sold brands.

The effect of methoxsalen on nicotine and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) metabolism in vivo

Nicotine is metabolized to the inactive metabolite cotinine by cytochrome P450 2A6. NNK, or 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, is a potent procarcinogen shown to be activated to a reactive mutagenic metabolite by the enzyme CYP2A6. We studied the effect of inhibiting CYP2A6 on smoking behavior and metabolism of the procarcinogen NNK. In study 1, abstinent smokers (n=7) received methoxsalen (a potent CYP2A6 inhibitor), 30-50 mg orally, one-half hour before three subcutaneous nicotine injections (31 microg/kg) were given at hourly intervals. Methoxsalen increased mean plasma nicotine by 47% (p<.01) and mean nicotine area under the curve (AUC) by 63% (p<.0001); and decreased nicotine clearance by 39% (p<.0001), relative to placebo. In study 2, smokers (n=11) were told to maintain their same number of cigarettes smoked while receiving methoxsalen, 10 mg orally three times daily for 3 days. On day 3 of methoxsalen treatment, a 29% increase in plasma nicotine/expired-air CO (an index of smoke exposure) (p=0.03) was observed. Urinary levels of trans 3'-hydroxycotinine (metabolized by CYP2A6 from cotinine) also were decreased (p<.0001), and significantly more NNK was metabolized to the inactive NNAL-glucuronide (1.04+/-0.54 pmol/mg on day 1 to 1.37+/-0.74 pmol/mg on day 4, p<.01) relative to placebo. Thus, treatment with the CYP2A6 inhibitor methoxsalen in vivo increases the routing of NNK to the inactive NNAL-glucuronide and decreases smoking. CYP2A6 inhibition may have potential as an exposure reduction or cessation strategy in tobacco dependence.

The effect of tobacco blend additives on the retention of nicotine and solanesol in the human respiratory tract and on subsequent plasma nicotine concentrations during cigarette smoking

The influence of the tobacco additives diammonium hydrogen phosphate (DAP) and urea on the delivery and respiratory tract retention of nicotine and solanesol and on the uptake of nicotine into venous blood was investigated in 10 smokers under mouth-hold and 75 and 500 mL inhalation conditions. Three cigarettes with identical physical specifications were produced from a common lamina tobacco blend. The control cigarette contained nonammoniated reconstituted tobacco sheet (RTS), whereas DAP and other ammonia compounds were added to the RTS of the second cigarette. Urea was added to the tobacco of the third cigarette. The presence of DAP or urea in the test cigarettes did not significantly influence solanesol retention within the mouth during the mouth-hold condition. Nicotine retention within the mouth during the mouth-hold condition was, however, significantly higher for the DAP cigarette (64.3 +/- 10.5%) than for the urea (53.3 +/- 11.3%) or control cigarette (46.3 +/- 8.6%), but this did not result in an increase in nicotine uptake into venous blood. Solanesol retentions during the 75 and 500 mL inhalation volume conditions and nicotine retentions during the 75 mL inhalation volume condition were not significantly different for the three cigarette types. Although the nicotine retention approached 100% with each cigarette type during the 500 mL inhalation condition, the nicotine retention for the urea-treated cigarette (99.6 +/- 0.2%) was marginally, but statistically, significant, higher than for the control (99.1 +/- 0.5%) and DAP-treated cigarettes (98.8 +/- 0.6%). There were no statistically significant differences between the indices of nicotine uptake into venous blood for the three cigarette types in any of the inhalation conditions.

Role of nicotine pharmacokinetics in nicotine addiction and nicotine replacement therapy: a review

Smoking is a complex behaviour involving both pharmacological and psychological components. Nicotine is the main alkaloid found in tobacco, and is responsible for its addictive potential. Nicotine-positive effects on mood and cognition are strong reinforcements for smokers that contribute to their addiction, and cigarette smoking is particularly addictive because inhaled nicotine is absorbed through the pulmonary venous rather than the systemic venous system, and thus reaches the brain in 10-20 seconds. As the likelihood that a substance will be abused depends on the time between administration and central reinforcement, tobacco smoking can easily become addictive. Nicotine replacement therapy (NRT) is available in different forms (gum, transdermal patch, nasal spray, inhaler, sublingual tablet and lozenge), and has been shown to relieve withdrawal symptoms and to double abstinence rates compared to placebo. Most NRT forms deliver nicotine more slowly than smoking, and the increase in nicotine blood levels is more gradual. Compared to tobacco smoking or even tobacco chewing, few positive (reinforcing) effects are obtained from NRT use. Nasal spray provides faster withdrawal relief than other NRT, but compared to smoking absorption is slower and nicotine blood levels obtained are lower than with smoking. These differences in pharmacokinetic profiles compared with smoking may explain that some smokers still have difficulties quitting smoking even when using NRT (apart from psychological and/or social factors). Combination therapy (e.g., patch+gum, patch+inhaler), higher dosage, temporary abstinence or smoking reduction (using NRT to reduce smoke intake) may be needed to help more smokers to quit.

Assessment of chronic exposure to cigarette smoke and its change during pregnancy by segmental analysis of maternal hair nicotine

This study aimed to investigate the association between biomarkers of fetal exposure to smoking during the whole pregnancy, nicotine in maternal and newborns hair samples, and quantitative measurement of smoking intake and exposure evaluated by maternal self-reported questionnaire. Study subjects were 150 mothers and their newborns from a hospital in Barcelona. A questionnaire including smoking habits was completed in the third trimester of pregnancy and on the day of delivery. Nicotine content was measured in two subsequent segments of maternal hair accounting for the first and last months of pregnancy, and in fetal hair. The geometric mean of nicotine concentration in maternal hair discriminated between nonexposition (3.84 and 2.80 ng/mg in distal and proximal hair segment, respectively) and exposition to cigarette smoke during pregnancy (6.06 and 4.30 ng/mg in distal and proximal hair segment, respectively) (P<0.05), and between these two classes and active smoking (14.40 and 11.08 ng/mg in distal and proximal hair segment, respectively). Maternal hair nicotine was able to differentiate levels of exposure to tobacco smoke and levels of intake. Nicotine concentration in hair from newborns did not differentiate between exposure and nonexposure to environmental tobacco smoke (ETS) in nonsmoking mothers. Finally, chronic exposure to cigarette smoke during pregnancy, assessed by maternal hair nicotine, correlated negatively with anthropometric parameters of newborns.

Diffusion parameters in the striatum of rats with 6-hydroxydopamine-induced lesions and with fetal mesencephalic grafts

Functional recovery after transplantation of dopaminergic cells into the lesioned striatum is dependent on widespread diffusion of the transmitter released by the graft. In the present study, we investigated the diffusion parameters of the extracellular space in the striatum of control, 6-hydroxydopamine-lesioned, intrastriatally grafted, and sham-grafted rats in vivo. We used two types of grafts-single macrografts or multiple micrografts. The real-time iontophoretic tetramethylammonium method enabled us to extract three extracellular space diffusion parameters: volume fraction, alpha, tortuosity, lambda, and nonspecific uptake of tetramethylammonium, k'. Compared with controls (alpha = 0.19, lambda = 1.59), in lesioned animals both alpha and lambda were lower (alpha = 0.14, lambda = 1.50). alpha and lambda were increased inside macro-and micrografts, where alpha = 0.24 and lambda = 1.80, and in sham-grafted areas, where alpha = 0.24 and lambda = 1.72. In regions outside the grafts (alpha = 0.15, lambda = 1.51) or in sham grafts (alpha = 0.14, lambda = 1.49), the values of alpha and lambda were similar to the values observed in lesioned striatum. Nonspecific uptake (k') did not differ among the groups. Our results show that, compared with control, alpha and lambda were decreased in dopamine-depleted areas and increased in areas with grafts. Multiple but smaller graft deposits, in contrast to their enlarged capability for dopaminergic reinnervation, impair the conditions for diffusion and extrasynaptic transmission in a larger area of the striatum than do single macrografts, presumably because of more extensive tissue damage, cell loss, and astrogliosis.

Nicotine addiction: insights from recent animal studies

RATIONALE

Recent preclinical behavioral and neurobiological research has characterized important behavioral features and has identified neurobiological substrates that may underlie nicotine reinforcement and addiction.

OBJECTIVE

To examine recent advances on nicotine exposure in preclinical models, from three perspectives: (a) the chronopharmacokinetics of nicotine, (b) behavioral studies on nicotine reinforcement, withdrawal, and reinstatement/relapse, and (c) effects of nicotine on neurobiological substrates after repeated exposure.

RESULTS

Preclinical studies can be used to operationally model selected aspects of nicotine reinforcement, withdrawal, and reinstatement or relapse. These may be used to investigate the functional in vivo consequences of acute and long-term changes in neuronal acetylcholine receptor populations that follow nicotine exposure. Behavioral studies focusing on distinct stages of nicotine exposure (e.g., active reinforcement vs. cessation or reinstatement) may also be used in parallel with studies on dopaminergic function, a proposed substrate for the reinforcing effects of nicotine, and of opioid receptor function, a possible site of neuroadaptations secondary to nicotine exposure.

CONCLUSIONS

While no single current animal model may capture the experience of human smoking or nicotine addiction, increasingly, separate animal models are capturing the full spectrum of behavioral and neurobiological dimensions of this complex condition.

A consideration of the role of gas/particle partitioning in the deposition of nicotine and other tobacco smoke compounds in the respiratory tract

Tobacco smoke is an aerosol that contains both gaseous and suspended particulate material (PM). The particles are largely liquid droplets containing a wide variety of condensed organic compounds. Each compound in the smoke will partition between the gas and PM phases and will always seek a state of gas/particle equilibrium. When tobacco smoke is inhaled, a compound such as nicotine can deposit in the respiratory tract (RT) by four different mechanisms: (1) direct gas deposition (DGD) of the portion of the compound that is initially in the gas phase of the inhaled smoke; (2) evaporative gas deposition (EGD) of PM-phase compound by evaporation to the gas phase, then deposition; (3) particle deposition, evaporation from the deposited particle, then deposition from the gas phase (PDE); and (4) particle deposition with diffusion (PDD) into RT tissue. Three of the mechanisms (DGD, EGD, and PDE) involve volatilization from the PM phase. The relative importance of all the mechanisms is therefore greatly affected by the volatility of the compound from the PM phase as it is set by the compound's gas/particle partitioning constant K(p) through the compound's vapor pressure. For a largely nonvolatile compound such as benzo[a]pyrene, only PDD will likely be important. For a semivolatile compound such as nicotine, all four mechanisms can be important. Because tobacco smoke alkaloids such as nicotine can exist in protonated as well as free-base form, the fraction alpha(fb) of the compound that is in the neutral free-base form in the PM phase plays a critical, pH-dependent role in determining the relative importance of the four mechanisms. Equations are developed that can be used to ascertain the importance of the DGD and EGD mechanisms. The value of alpha(fb) for nicotine in a tobacco smoke PM is set by pH(eff), the effective pH of the PM phase. Historically, a primary method for measuring "smoke pH" has involved the direct exposure of a pH electrode to tobacco smoke. This method cannot yield direct insight into pH(eff) or alpha(fb) values because (1) problems exist in using such an electrode to measure smoke PM-phase pH, and (2) by itself, a measurement of the pH of tobacco smoke PM says nothing about the effects of PM-phase activity coefficients of protonated and free-base nicotine on the nicotine species distribution. The "acidic" values that have typically been measured for cigarette "smoke pH" by the direct pH electrode method are therefore neither reliable nor useful in determining the relative distribution of PM-phase nicotine among the protonated and free-base forms. The dependence of the volatility of nicotine from tobacco smoke PM on alpha(fb) means that measuring the gas/particle distribution of nicotine under equilibrium conditions in a tobacco smoke by denuder samplers (or by another method) can yield information about the nicotine K(p) for that smoke. Knowledge of K(p,fb), the partitioning constant for nicotine in the free-base form, then allows calculation of alpha(fb) through the relation K(p) = K(p,fb)/alpha(fb). The available data suggest that the smoke PM from some commercial cigarettes can be characterized by alpha(fb) > or = 0.4, i.e., 40% or more of the nicotine in the free-base form. This conclusion is consistent with (1) the gas-sampling denuder results obtained by Philip Morris in which significant tobacco smoke nicotine was observed to deposit in acid-coated denuder tubes, with more depositing when the cigarette tobacco blend was treated with ammonia; (2) the view that the sensory "impact" exhibited by some tobacco smokes is caused by the deposition of gaseous nicotine in the pharynx; (3) the observed throat irritation caused by nicotine inhalers; and (4) the high overall respiratory tract deposition efficiencies for nicotine of 0.9 and greater that have been reported for some cigarette smokes. The available information combines to create a picture of nicotine in cigarette smoke that contradicts the traditional view that cigarette smoke PM is typically acidic, with little free-base nicotine typically present in the smoke PM phase. Government agencies interested in establishing a framework for the testing and monitoring of nicotine delivery may wish to consider requiring the measurement and publication of the PM-phase alpha(fb) values for the cigars and cigarettes marketed in their jurisdictions.

3H-nicotine, 3H-flunitrazepam, and 3H-cocaine incorporation into melanin: a model for the examination of drug-melanin interactions

To explore drug-melanin interactions, we examined the in vitro tyrosinase-mediated formation of melanin from tyrosine in the presence of the 3H-cocaine (3H-COC), 3H-flunitrazepam (3H-FLU), and 3H-nicotine (3H-NIC) at 10-100,000 ng/mL. Polymerization in the presence of 10 or 100 ng/mL of each drug resulted in almost complete drug incorporation into the melanin pellet. Only 12% (3H-NIC) to 28% (3H-FLU) of the pellet-associated radioactivity could be released upon treatment with 6 M HCl. At 1000-100,000 ng/mL, between 20 and 50% of label became melanin-associated. In each case a significant percentage of melanin-associated radioactivity was resistant to treatment with 6 M HCl. Nicotine-associated radioactivity in the polymer was subject to much greater quenching than was 3H-COC or 3H-FLU, suggesting a much tighter association with the melanin. The subsequent demonstration of a covalent adduct of a melanin intermediate and nicotine has demonstrated the utility of this polymerization system as a model for further chemical characterization of drug-melanin interactions.

Doses and lung burdens of environmental tobacco smoke constituents in nonsmoking workplaces

This paper models nicotine dose and ultraviolet-absorbing particulate matter (UVPM) alveolar lung burden resulting from exposure to environmental tobacco smoke (ETS) for nonsmokers in workplaces where smoking was reported not to occur. Data were obtained from personal monitoring of ETS in 16 U.S. cities [Jenkins R.A., Guerin M.R., Palausky A., Counts R.W., Bayne C.K., and Dindal A.B. Determination of human exposure to environmental tobacco smoke (ETS): a study conducted in 16 U.S. cities. Draft final report by Oak Ridge National Laboratory for Center for Indoor Air Research, Linthicum, MD, 1996a; Jenkins R.A., Palausky A., Counts R.W., Bayne C.K., Dindal A.B., and Guerin M.R. Exposure to environmental tobacco smoke in sixteen cities in the United States as determined by personal breathing zone air sampling. J. Expos. Anal. Environ. Epidemiol. 1996b: 6(4): 473-502.]. This is a continuation of earlier analyses focusing on nonsmokers in smoking workplaces (SWs) [LaKind J.S., Graves C.G., Ginevan M.E., Jenkins R.A., Naiman D.Q., and Tardiff R.G. Exposure to environmental tobacco smoke in the workplace and the impact of away - from - work exposure. Risk Anal. 1999a: 19(3): 349-358; LaKind J.S., Jenkins R.A., Naiman D.Q., Ginevan M.E., Graves C.G., and Tardiff R.G. Use of environmental tobacco smoke constituents as markers for exposure. Risk Anal. 1999b: 19 (3): 359-373; LaKind J.S., Ginevan M.E., Naiman D.Q., James A.C., Jenkins R.A., Dourson M.L., Felter S.P., Graves C.G., and Tardiff R.G. Distribution of exposure concentrations and doses for constituents of environmental tobacco smoke. RiskAnal. 1999c: 19 (3): 375-390.]. Even though study participants characterized their workplaces as nonsmoking, some individuals reported observing cigarettes in the workplace. Individuals observing six or more cigarettes were excluded from the analysis on the grounds that they were in defacto SWs. Exposure to ETS was lower in nonsmoking than SWs, but even with this exclusion, exposure was not zero. Distributions were selected for each model input, and at least 2000 iterations of the model were made for each dose or lung burden characterization (e.g., for females, for males). In these nonsmoking workplaces (NSWs), neither nicotine nor UVPM concentrations were lognormally distributed. Hence, observed concentrations were used directly via bootstrap sampling (nicotine) or a constant number of times (UVPM) as input to the models. As in SWs, individuals from smoking homes (SHs) experienced greater exposure in NSWs to both nicotine and UVPM than did individuals from nonsmoking homes (NSH; P<0.001 ). The distributions of modeled nicotine dose and UVPM lung burden were highly skewed, with most individuals receiving relatively low exposure to ETS in the workplace. Comparing doses from NSWs modeled here to doses from SWs modeled previously, less difference between smoking and NSWs was apparent in UVPM levels than in nicotine levels. For average exposure, UVPM alveolar lung burdens were approximately 10-fold higher in smoking than NSWs, while average nicotine doses were 20-25 times higher in smoking than NSWs. These findings are in the range observed by other investigators and are partly explained by very low denominators in the ratios (i.e., very low levels experienced in NSWs). For upper bound exposure, the nonsmoking-to-smoking ratios remained about the same for UVPM. For nicotine, the upper bound ratios remained the same for people from NSHs but were halved for people from SHs.

Questionnaire and hair measurement of exposure to tobacco smoke

To assess the relation between nicotine and cotinine levels in hair and reported exposure to environmental tobacco smoke (ETS), hair samples from 112 children (aged 3 months to 10 years) and 76 of their mothers were analyzed and information on the smoking habits of household adults in the preceding 6 months recorded. It was found that the levels of nicotine in children's hair were related to the number of smokers in the house, and increased with the total number of cigarettes smoked by all household adults (P<0.0001). In a multiple regression analysis, mother's smoking was much more a contributor to children's nicotine levels than smoking by the father or other household adults. Cotinine levels were less strongly associated with reported ETS exposure than nicotine. There was a strong correlation between nicotine hair levels in children and mothers (r(s)=0.7, P<0.0001). However, nicotine levels in the hair of active smokers were not correlated with the reported number of cigarettes they smoked per day. In this population, there was a consistent relation between exposure to ETS (assessed by questionnaire) and dose (as measured by nicotine in hair). We conclude that hair nicotine levels rather than hair cotinine levels provide an informative and objective measure of ETS exposure. The number of cigarettes smoked by active smokers may not be an accurate measure of the total nicotine levels in their bodies.

Behavioral sensitization following repeated intravenous nicotine administration: gender differences and gonadal hormones

Repeated intermittent administration of stimulants is well known to produce behavioral sensitization in male animals. The present studies explored whether 1) behavioral sensitization occurred with the i.v. route of administration, 2) sensitization was greater in females than in males, 3) sensitization was modulated by gonadectomy, 4) intact adult female rats maintained normal estrous cytology patterns in response to repeated nicotine administration, and 5) the pharmacokinetics of i.v. nicotine dosing. Adult male, female, castrated, and ovariectomized Sprague-Dawley rats (n = 48) were surgically implanted with an intravenous access port. Animals received 50 microg/kg i.v. nicotine once/day for 14 days. Immediately after the initial nicotine injection and the final day 14 nicotine injection, animals were placed in IR photocell activity chambers for 60 min. Observational time sampling of behavior was also simultaneously performed by an observer blind to treatment condition. An increase in behavioral activity of greater than 120% occurred across the 14-day time course of i.v. nicotine injections. The magnitude of the increase, however, varied as a function of component of activity, gender, and gonadectomy. The behavioral observation data further suggested that the females demonstrated an increased sensitivity to repeated nicotine, as evidenced in a more rapid response, for example, grooming. These behavioral observations were associated with peak arterial levels of nicotine (approximately 25 ng/ml) no greater than the average venous levels of nicotine commonly maintained by cigarette smokers. Repeated i.v. nicotine, at a dose of 50 microg/kg, did not interfere with intact female vaginal cytology or body weight; the failure to detect such alterations were not due to inadequate statistical power. Moreover, no nicotine-treated animals displayed persistent vaginal estrous or were acyclic. Collectively, these data suggest that the i.v. nicotine model may be particularly useful in exploring the gender-dependent effects of nicotine.

Arterial/venous plasma nicotine concentrations following nicotine nasal spray

BACKGROUND AND OBJECTIVES

Arterial (A) and venous (V) plasma nicotine and cotinine concentrations were measured after nasal nicotine spray in tobacco smokers of both genders. The hypothesis for this research was that a greater A/V difference in plasma nicotine would be present in males than females because males have greater skeletal muscle mass to bind nicotine.

SUBJECTS AND METHODS

Nine male and nine female healthy adult smokers were studied. They all abstained from use of tobacco overnight for 10 h or more prior to the study. Nicotine nasal spray was given in doses of 1-2.5 mg total, with half in each nostril while the subject was supine. Both A and V blood samples were obtained prior to and 3, 6, 10, 15, 20, and 30 min post-nasal nicotine spray.

RESULTS AND CONCLUSIONS

Nasal nicotine administration produced greater A than V plasma levels. There were no gender differences in A/V nicotine concentrations, disproving the above hypothesis, suggesting that other physiochemical factors besides skeletal muscle mass must be involved. Heart rate increases correlated well with arterial plasma nicotine levels (r = 0.77). Males had less variance than females in the expected increase in arterial plasma nicotine concentrations with increased number of nasal sprays. Although there was considerable overlap, mean A cotinine concentrations were consistently slightly larger than V concentrations.

An oral formulation of nicotine for release and absorption in the colon: its development and pharmacokinetics

AIMS

Ulcerative colitis is predominantly a disease of nonsmokers and transdermal nicotine has therapeutic value in active disease; however side-effects are troublesome. The aim of this study was to develop an oral formulation of nicotine which would be slowly released in the colon over 6 h, and to examine its pharmacokinetic profile in 12 healthy volunteers, with measurements of serum nicotine and cotinine, its principal metabolite.

METHODS

Nicotine was combined with a polyacrylic carbomer, Carbopol 974P which was incorporated into 13 different vehicles and their release profiles examined in vitro. The polyglycolized glyceride, Gelucire 50/13, was chosen for subsequent kinetic studies because it consistently produced a suitable release pattern which was linear. Capsules containing 3 mg nicotine, combined with carbomer in Gelucire 50/13, were coated with an acrylic resin Eudragit L; this ensured the capsule would remain intact until the ileum. On 2 separate days, 6 and 15 mg nicotine, contained in 2 and 5 capsules, respectively, were administered to 12 subjects, all nonsmokers, mean age 28 years. Serial blood measurements were taken for 36 h, serum nicotine and cotinine concentrations were measured by gas liquid chromatography.

RESULTS

There was considerable intersubject variability in the nicotine and cotinine values. Plasma nicotine levels began to rise about 4 h after ingestion of the capsules, corresponding with the oro-caecal transit time. Cmax nicotine values were 2.2 and 5 ng ml-1, obtained 7 h after the ingestion of 6 and 15 mg, respectively, of the formulation. The corresponding Cmax values for cotinine were 37 and 94.4 ng ml-1, occurring after 9-10 h. The mean for elimination half-lives in the 24 studies, including the 6 and 15 mg doses, for nicotine were 4.3+/-2.7 h and for cotinine 16.8+/-7.5 h. With 6 mg nicotine-carbomer, only 1 of 12 volunteers had possible side-effects, but with the 15 mg dose 11 out of the 12 reported adverse effects which were systemic or gastrointestinal in nature-their timing corresponded with peak serum concentrations of nicotine.

CONCLUSIONS

An oral formulation of nicotine has been developed; in the ileum and colon, this becomes available for slow linear release over 6 h and delivers high concentrations of nicotine for topical effect on the colon. 6 mg Nicotine was well tolerated, whilst 15 mg gave both systemic and gastrointestinal side-effects. High concentrations of topical nicotine in the colon are achieved with relatively low systemic bioavailablity-reflected by the Cmax and AUC values for nicotine. This, or comparable formulations, may be of therapeutic value in ulcerative colitis.

Altered disposition of repeated nicotine doses in rats immunized against nicotine

Active immunization of rats against nicotine reduces the distribution of a single intravenous dose of nicotine to brain. The current study examined the effects of immunization on the distribution of repeated doses of nicotine, and on estimates of nicotine pharmacokinetic parameters. Groups of rats received five repeated doses of nicotine over 80 min (total dose equivalent to the nicotine intake from one, three or 10 cigarettes in a human). The serum nicotine concentration in immunized rats was 160-430% higher than controls after the fifth dose, demonstrating binding of nicotine to antibody. Brain nicotine concentration in immunized rats was reduced by 30-46%. The reduction in distribution of nicotine to brain correlated with the serum hapten-specific antibody concentration, the percentage of nicotine bound in serum, and with the unbound nicotine concentration in serum. In immunized rats, nicotine had a smaller steady state volume of distribution, lower systemic clearance, and longer terminal half-life than in controls. These data demonstrate that immunization against nicotine reduces nicotine distribution to brain, even after multiple nicotine doses at rates approximating heavy cigarette smoking. Whether this reduction in nicotine distribution is large enough to alter nicotine's physiological or behavioral effects remains to be studied.

In vitro study of transdermal nicotine delivery: influence of rate-controlling membranes and adhesives

The objective of this study was to evaluate the influence of a rate-controlling membrane and adhesive on the in vitro permeation of nicotine. The physicochemical properties of the adhesive, including adhesion and rheology (viscosity), were also detected. Higher permeability of nicotine was observed through a hydrophilic membrane than through a hydrophobic membrane. Natural rubber and silicone were used as the adhesive bases, respectively. The silicone adhesive showed the highest adhesion among all adhesive formulations. To increase the adhesion of natural rubber, a tackifier (polyisoprene) and a secondary tackifier (terpene polymer; Px 1150) were incorporated into the formulations to achieve acceptable adhesion. The nicotine permeation through silicone adhesive and three natural rubber adhesives with the secondary tackifier (2%, 4%, and 6% Px 1150) was close to that from a commercially available patch (Habitrol), although the loading amount of nicotine was not the same. A longer lag time during the in vitro permeation study of nicotine was required for the adhesives prepared in our laboratory than for the commercially available patch.

Evolution of anisotropic diffusion in the developing rat corpus callosum

Diffusion anisotropy was investigated in the developing rat brain [postnatal day (P)6-29] with the use of ion-selective microelectrodes to measure the three-dimensional distribution of tetramethylammonium (TMA+) iontophoresed into the extracellular space (ECS). The diffusion parameters, ECS volume fraction alpha (alpha = ECS volume/total tissue volume), tortuosity lambda (lambda2 = apparent diffusion coefficient/free diffusion coefficient), and nonspecific TMA+ uptake (k'), were studied in cortical gray matter (layer V) and corpus callosum (CC) of anesthetized rats. ECS volume fraction in cortex and CC was about twice as large in the newborn rat as in adults. In this study, more detailed analysis revealed that alpha in CC gradually decreased from P4, when alpha ranged between 0.42 and 0.45, and reached a final value of 0.26 +/- 0.01 (SE, n = 12 measurements, 6 animals) at about P21. Diffusion in the ECS of CC was isotropic until about P12, i.e., there was no significant difference in the tortuosity factor, lambda, between the three perpendicular axes. From P13 to P17 anisotropy greatly increased as a result of preferential diffusion along the myelinated axons (X-axis). At P21-23 the tortuosity values were lambda(x) = 1.46 +/- 0.03, lambda(y) = 1.70 +/- 0.01, and lambda(z) = 1.72 +/- 0.02 (n = 12), and there were no further changes up to the last postnatal day studied, P29. In contrast to the myelinated CC, cortical gray matter remained isotropic up to P29, with a tortuosity of 1.54 +/- 0.02 (n = 12). The results suggest that diffusion anisotropy in the rat CC is related to myelination; it reaches a maximum at P17, when myelination is well advanced. In myelinated pathways, preferential diffusion of ions and transmitters occurs along the axons. These results are relevant to volume transmission and the interpretation of diffusion-weighted magnetic resonance imaging.

Pharmacology of nicotine: addiction and therapeutics

Nicotine maintains tobacco addiction and has therapeutic utility to aid smoking cessation and possibly to treat other medical diseases. Nicotine acts on nicotinic cholinergic receptors, which demonstrate diversity in subunit structure, function, and distribution within the nervous system, presumably mediating the complex actions of nicotine described in tobacco users. The effects of nicotine in people are influenced by the rate and route of dosing and by the development of tolerance. The metabolism of nicotine is now well characterized in humans. A few individuals with deficient C-oxidation of nicotine, unusually slow metabolism of nicotine, and little generation of cotinine have been described. Nicotine affects most organ systems in the body, although its contribution to smoking-related disease is still unclear. Nicotine as a medication is currently available as a gum, a transdermal delivery device, and a nasal spray, all of which are used for smoking cessation. Nicotine is also being investigated for therapy of ulcerative colitis, Alzheimer's disease, Parkinson's disease, Tourette's syndrome, sleep apnea, and attention deficit disorder.

Anisotropic and heterogeneous diffusion in the turtle cerebellum: implications for volume transmission

1. Measurements of extracellular diffusion properties were made in three orthogonal axes of the molecular and granular layers of the isolated turtle cerebellum with the use of iontophoresis of tetramethylammonium (TMA+) combined with ion-selective microelectrodes. 2. Diffusion in the extracellular space of the molecular layer was anisotropic, that is, there was a different value for the tortuosity factor, lambda i, associated with each axis of that layer. The x- and y-axes lay in the plane parallel to the pial surface of this lissencephalic cerebellum with the x-axis in the direction of the parallel fibers. The z-axis was perpendicular this plane. The tortuosity values were lambda x = 1.44 +/- 0.01, lambda y = 1.95 +/- 0.02, and lambda z = 1.58 +/- 0.01 (mean +/- SE). By contrast, the granular layer was isotropic with a single tortuosity value, lambda Gr = 1.77 +/- 0.01. 3. These data confirm the applicability of appropriately extended Fickian equations to describe diffusion in anisotropic porous media, including brain tissue. 4. Heterogeneity between the molecular and granular layer was revealed by a striking difference in extracellular volume fraction, alpha, for each layer. In the molecular layer alpha = 0.31 +/- 0.01, whereas in the granular layer alpha = 0.22 +/- 0.01. 5. Volume fraction and tortuosity affected the time course and amplitude of extracellular TMA+ concentration after iontophoresis. This was modeled by the use of the average parameters determined experimentally, and the nonspherical pattern of diffusion in the molecular layer was compared with the spherical distribution in the granular layer and agarose gel by computing isoconcentration ellipsoids. 6. One functional consequence of these results was demonstrated by measuring local changes in [K+]o and [Ca2+]o after microiontophoresis of a cerebellar transmitter, glutamate. The ratios of ion shifts in the x- and y-axes in the granular layer were close to unity, with a ratio of 1.04 +/- 0.08 for the rise in [K+]o and 1.03 +/- 0.17 for the decrease in [Ca2+]o. In contrast, ion shifts in the molecular layer had an x:y ratio of 1.44 +/- 0.14 for the rise in [K+]o and 2.10 +/- 0.42 for the decrease in [Ca2+]o. 7. These data demonstrate that the structure of cellular aggregates can channel the migration of substances in the extracellular microenvironment, and this could be a mechanism for volume transmission of chemical signals. For example, the preferred diffusion direction of glutamate along the parallel fibers would help constrain an incoming excitatory stimulus to stay "on-beam."