Determination of behaviorally effective tobacco constituent doses in rats

Acute nicotine vapor normalizes sensorimotor gating and reduces locomotor activity deficits in HIV-1 transgenic rats

Rationale

Despite improved life expectancy of people with HIV (PWH), HIV-associated neurocognitive impairment (NCI) persists, alongside deficits in sensorimotor gating and neuroinflammation. PWH exhibit high smoking rates, possibly due to neuroprotective, anti-inflammatory, and cognitive-enhancing effects of nicotine, suggesting potential self-medication.

Objectives

Here, we tested the effects of acute nicotine vapor exposure on translatable measures of sensorimotor gating and exploratory behavior in the HIV-1 transgenic (HIV-1Tg) rat model of HIV.

Methods

Male and female HIV-1Tg and F344 control rats (n=57) were exposed to acute nicotine or vehicle vapor. Sensorimotor gating was assessed using prepulse inhibition (PPI) of the acoustic startle response, and exploratory behavior was evaluated using the behavioral pattern monitor (BPM).

Results

Vehicle-treated HIV-1Tg rats exhibited PPI deficits at low prepulse intensities compared to F344 controls, as seen previously. No PPI deficits were observed in nicotine-treated HIV1-Tg rats, however. HIV-1Tg rats were hypoactive in the BPM relative to controls, whilst nicotine vapor increased activity and exploratory behavior across genotypes. Cotinine analyses confirmed comparable levels of the primary metabolite of nicotine across genotypes.

Conclusions

Previous findings of PPI deficits in HIV-1Tg rats were replicated and, importantly, attenuated by acute nicotine vapor. Evidence for similar cotinine levels suggest a nicotine-specific effect in HIV-1Tg rats. HIV-1Tg rats had reduced exploratory behavior compared to controls, attenuated by acute nicotine vapor. Therefore, acute nicotine may be beneficial for remediating sensorimotor and locomotor activity deficits in PWH. Future studies should determine the long-term effects of nicotine vapor on similar HIV/NCI-relevant behaviors.

Behavioral and Pharmacokinetic Assessment of Nicotine e-Cigarette Inhalation in Female Rats

INTRODUCTION

Nicotine and tobacco use remain high both globally and in the United States, contributing to large health care expenditures. With a rise in e-cigarette use, it is important to have clinically relevant models of inhaled nicotine exposure.

AIMS AND METHODS

This study aims to extend prior preclinical nicotine inhalation animal data to females and provide both behavior and serum pharmacokinetics. We tested two inhalation doses of nicotine (24 mg/mL and 59 mg/ mL) and compared these to injected doses (0.4 mg/kg and 1 mg/kg). In addition, we assessed locomotor behavior after the same doses. Blood was collected at 10- and 120-minutes post-administration. We assessed nicotine and cotinine serum concentrations by LC-MS/MS.

RESULTS

Showed that while nicotine serum concentrations for the respective high and low-dose administrations were similar between both routes of administration, the route had differential effects on locomotor behavior. Inhaled nicotine showed a dose-dependent decrease in locomotor activity while injected doses showed the opposite trend.

CONCLUSIONS

Our results indicate that the route of administration is an important factor when establishing preclinical models of nicotine exposures. Given that the overall use of e-cigarettes in vulnerable populations is on the rise, our study provides important behavioral and pharmacokinetic information to advance our currently limited understanding of the effects of nicotine vapor exposure.

IMPLICATIONS

This study highlights behavioral differences between different routes of administration of similar doses of nicotine. Using a low and high dose of nicotine, we found that nicotine serum concentrations were similar between the different routes of administration. Our results indicate that different routes of administration have opposing effects on locomotor activity. These findings provide important implications for future behavioral models.

Relapse to cotinine seeking in rats: differential effects of sex

Relapse is a defining feature of smoking and a significant challenge in cessation management. Elucidation of novel factors underlying relapse may inform future treatments. Cotinine, the major metabolite of nicotine, has been shown to support intravenous self-administration in rats, implicating it as one potential factor contributing to nicotine reinforcement. However, it remains unknown whether cotinine would induce relapse-like behaviors. The current study investigated relapse to cotinine seeking in two relapse models, the reinstatement of drug seeking and incubation of drug craving models. In the reinstatement model, rats were trained to self-administer cotinine, underwent extinction of cotinine-associated responses, and were tested for cue-, drug-, or stress-induced reinstatement. Conditioned cues associated with cotinine self-administration, cotinine (1-2 mg/kg), or the pharmacological stressor yohimbine (1.25-2.5 mg/kg) induced reinstatement of cotinine seeking. Female rats displayed more pronounced cue-induced, but not drug- or stress-induced reinstatement than male rats. In the incubation of the craving model, rats were trained to self-administer cotinine and underwent forced withdrawal in home cages. Rats were tested for cue-induced cotinine-seeking on both withdrawal day 1 and withdrawal day 18. Rats exhibited greater cue-induced cotinine-seeking on withdrawal day 18 compared to withdrawal day 1, with no difference between male and female rats. These findings indicate that cotinine induces sex-specific relapse to drug seeking in rats, suggesting that cotinine may contribute to relapse.

The involvement of mesolimbic dopamine system in cotinine self-administration in rats

Cotinine is the major metabolite of nicotine and has recently been shown to be self-administered intravenously by rats. However, mechanisms underlying cotinine self-administration remained unknown. Mesolimbic dopamine system projecting from the ventral tegmental area (VTA) to nucleus accumbens (NAC) is closely implicated in drug reinforcement, including nicotine. The objective of the current study was to determine potential involvement of mesolimbic dopamine system in cotinine self-administration. An intracranial self-administration experiment demonstrates that cotinine at 0.88 and 1.76 ng/100 nl/infusion was self-infused into the VTA by rats. Rats produced more infusions of cotinine than vehicle and responded more on active than inactive lever during acquisition, reduced responding when cotinine was replaced by vehicle, and resumed responding during re-exposure to cotinine. Microinjection of cotinine at 1.76 ng/100 nl/infusion into the VTA increased extracellular dopamine levels within the NAC. Subcutaneous injection of cotinine at 1 mg/kg also increased extracellular dopamine levels within the NAC. Administration of the D1-like receptor antagonist SCH 23390 attenuated intravenous cotinine self-administration. On the other hand, bupropion, a catecholamine uptake inhibitor, did not significantly alter intravenous cotinine self-administration. These results suggest that activation of mesolimbic dopamine system may represent one cellular mechanism underlying cotinine self-administration. This shared mechanism between cotinine and nicotine suggests that cotinine may play a role in nicotine reinforcement.

Anatabine, Nornicotine, and Anabasine Reduce Weight Gain and Body Fat through Decreases in Food Intake and Increases in Physical Activity

Obesity is a leading cause of preventable death in the United States. Currently approved pharmacotherapies for the treatment of obesity are associated with rebound weight gain, negative side effects, and the potential for abuse. There is a need for new treatments with fewer side effects. Minor tobacco alkaloids (MTAs) are potential candidates for novel obesity pharmacotherapies. These alkaloids are structurally related to nicotine, which can help reduce body weight, but without the same addictive potential. The purpose of the current study was to examine the effects of three MTAs (nornicotine, anatabine, and anabasine) and nicotine on weight gain, body composition, chow intake, and physical activity. We hypothesized that the MTAs and nicotine would reduce weight gain through reductions in chow intake and increases in physical activity. To test this, male Sprague Dawley rats were housed in metabolic phenotyping chambers. Following acclimation to these chambers and to (subcutaneous (sc)) injections of saline, animals received daily injections (sc) of nornicotine, anabasine, anatabine, or nicotine for one week. Compared to saline-injected animals that gained body weight and body fat during the treatment phase, injections of nornicotine and anatabine prevented additional weight gain, alongside reductions in body fat. Rats receiving anabasine and nicotine gained body weight at a slower rate relative to rats receiving saline injections, and body fat remained unchanged. All compounds reduced the intake of chow pellets. Nornicotine and nicotine produced consistent increases in physical activity 6 h post-injection, whereas anabasine’s and anatabine’s effects on physical activity were more transient. These results show that short-term, daily administration of nornicotine, anabasine, and anatabine has positive effects on weight loss, through reductions in body fat and food intake and increases in physical activity. Together, these findings suggest that MTAs are worthy of further investigations as anti-obesity pharmacotherapies.

Cotinine: Pharmacologically Active Metabolite of Nicotine and Neural Mechanisms for Its Actions

Tobacco use disorder continues to be a leading public health issue and cause of premature death in the United States. Nicotine is considered as the major tobacco alkaloid causing addiction through its actions on nicotinic acetylcholine receptors (nAChRs). Current pharmacotherapies targeting nicotine's effects produce only modest effectiveness in promoting cessation, highlighting the critical need for a better understanding of mechanisms of nicotine addiction to inform future treatments. There is growing interest in identifying potential contributions of non-nicotine components to tobacco reinforcement. Cotinine is a minor alkaloid, but the major metabolite of nicotine that can act as a weak agonist of nAChRs. Accumulating evidence indicates that cotinine produces diverse effects and may contribute to effects of nicotine. In this review, we summarize findings implicating cotinine as a neuroactive metabolite of nicotine and discuss available evidence regarding potential mechanisms underlying its effects. Preclinical findings reveal that cotinine crosses the blood brain barrier and interacts with both nAChRs and non-nAChRs in the nervous system, and produces neuropharmacological and behavioral effects. Clinical studies suggest that cotinine is psychoactive in humans. However, reviewing evidence regarding mechanisms underlying effects of cotinine provides a mixed picture with a lack of consensus. Therefore, more research is warranted in order to provide better insight into the actions of cotinine and its contribution to tobacco addiction.

Rodent models for nicotine withdrawal

BACKGROUND

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

AIM

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

RESULTS

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

CONCLUSION

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

Exposure to Commercial Cigarette Smoke Produces Psychomotor Sensitization Via Hyperstimulation of Glutamate Response in the Dorsal Striatum

Cigarette smoke is a highly complex mixture of nicotine and non-nicotine constituents. Exposure to cigarette smoke enhances tobacco dependence by potentiating glutamatergic neurotransmission via stimulation of nicotinic acetylcholine receptors (nAChRs). We investigated the effects of nicotine and non-nicotine alkaloids in the cigarette smoke condensates extracted from two commercial cigarette brands in South Korea (KCSC A and KCSC B) on psychomotor behaviors and glutamate levels in the dorsal striatum. Repeated and challenge administration of KCSCs (nicotine content: 0.4 mg/kg, subcutaneous) increased psychomotor behaviors (ambulatory, rearing, and rotational activities) and time spent in psychoactive behavioral states compared to exposure to nicotine (0.4 mg/kg) alone. The increase in psychomotor behaviors lasted longer when exposed to repeated and challenge administration of KCSCs compared to nicotine alone. In parallel with sustained increase in psychomotor behaviors, repeated administration of KCSCs also caused long-lasting glutamate release in the dorsal striatum compared to nicotine alone. KCSC-induced changes in psychomotor behaviors and glutamate levels in the dorsal striatum were found to be strongly correlated. These findings suggest that non-nicotine alkaloids in commercial cigarette smoke synergistically act with nicotine on nAChRs, thereby upregulating glutamatergic response in the dorsal striatum, which contributes to the hypersensitization of psychomotor behaviors.

Effects of nicotine, nornicotine and cotinine, alone or in combination, on locomotor activity and ultrasonic vocalization emission in adult rats

RATIONALE

The behavioral effects of the nicotine metabolites nornicotine and cotinine have not been investigated extensively.

OBJECTIVES

To evaluate the effects of nicotine, cotinine, and nornicotine, given alone or in combination, on locomotor activity and emission of ultrasonic vocalizations in male adult rats.

METHODS

Rats were first given home cage nicotine injections to make them tolerant to the drug's locomotor depressant effects. On subsequent days, locomotor activity (LMA) and ultrasonic vocalizations were recorded in an open field, for 60 min after challenge injection, using repeated measures designs. In single-drug experiments, subjects were tested with nicotine 0.05-0.4 mg/kg, cotinine 0.03-3 mg/kg, or nornicotine 0.1-10 mg/kg. In drug-combination experiments, saline or nicotine 0.2 mg/kg challenge was preceded by cotinine (0, 0.3, 3 mg/kg) or nornicotine (0, 0.1, 0.3, 1, 3 mg/kg) injection.

RESULTS

High doses of nornicotine increased LMA and blunted the locomotor stimulant effect of nicotine. Less consistently, nicotine and high doses of nornicotine decreased the 50-kHz call rate, with no clear evidence of a nornicotine × nicotine interaction. Cotinine, given alone or before nicotine injection, altered neither LMA nor the call rate. No drug altered the relative prevalence of flat vs. trill 50-kHz call subtypes, except that the highest dose of nornicotine promoted flat calls over trills. No drug evoked 22-kHz calls.

CONCLUSION

Nornicotine can exert an acute anti-nicotine effect in vivo, as previously reported in vitro. The finding that nicotine did not detectably alter the 50-kHz call profile appears consistent with this drug's mild subjective effects in human subjects.

Preliminary evidence from planarians that cotinine establishes a conditioned place preference

While the psychoactive stimulant nicotine has been the subject of extensive research, considerably less attention has focused on other compounds found in either tobacco smoke or that are nicotine metabolites. Recent papers have suggested that some of the compounds in question may either alter nicotine's effects or have reinforcing properties themselves, although they would only be experienced after consumption of tobacco. The potential for these compounds to function as reinforcers or to potentiate the reinforcing properties of nicotine merits investigation. To pursue this line of inquiry, we examined cotinine in a planarian model of environmental place preference. In the present study, planarians demonstrated that the compound cotinine, which is present in tobacco smoke, and is also the principal nicotine metabolite, establishes a conditioned place preference. These data represent the first ever demonstration that cotinine will establish a conditioned place preference in planarians and possibly contribute to the addictive properties of nicotine.

Repeated Administration of Cigarette Smoke Condensate Increases Glutamate Levels and Behavioral Sensitization

Nicotine, a nicotinic acetylcholine receptor agonist, produces the reinforcing effects of tobacco dependence by potentiating dopaminergic and glutamatergic neurotransmission. Non-nicotine alkaloids in tobacco also contribute to dependence by activating the cholinergic system. However, glutamatergic neurotransmission in the dorsal striatum associated with behavioral changes in response to cigarette smoking has not been investigated. In this study, the authors investigated alterations in glutamate levels in the rat dorsal striatum related to behavioral alterations after repeated administration of cigarette smoke condensate (CSC) using the real-time glutamate biosensing and an open-field behavioral assessment. Repeated administration of CSC including 0.4 mg nicotine (1.0 mL/kg/day, subcutaneous) for 14 days significantly increased extracellular glutamate concentrations more than repeated nicotine administration. In parallel with the hyperactivation of glutamate levels, repeated administration of CSC-evoked prolonged hypersensitization of psychomotor activity, including locomotor and rearing activities. These findings suggest that the CSC-induced psychomotor activities are closely associated with the elevation of glutamate concentrations in the rat dorsal striatum.

Behavioral changes after nicotine challenge are associated with α7 nicotinic acetylcholine receptor-stimulated glutamate release in the rat dorsal striatum

Neurochemical alterations associated with behavioral responses induced by re-exposure to nicotine have not been sufficiently characterized in the dorsal striatum. Herein, we report on changes in glutamate concentrations in the rat dorsal striatum associated with behavioral alterations after nicotine challenge. Nicotine challenge (0.4 mg/kg/day, subcutaneous) significantly increased extracellular glutamate concentrations up to the level observed with repeated nicotine administration. This increase occurred in parallel with an increase in behavioral changes in locomotor and rearing activities. In contrast, acute nicotine administration and nicotine withdrawal on days 1 and 6 did not alter glutamate levels or behavioral changes. Blockade of α7 nicotinic acetylcholine receptors (nAChRs) significantly decreased the nicotine challenge-induced increases in extracellular glutamate concentrations and locomotor and rearing activities. These findings suggest that behavioral changes in locomotor and rearing activities after re-exposure to nicotine are closely associated with hyperactivation of the glutamate response by stimulating α7 nAChRs in the rat dorsal striatum.

Tobacco's minor alkaloids: Effects on place conditioning and nucleus accumbens dopamine release in adult and adolescent rats

Tobacco products are some of the most commonly used psychoactive drugs worldwide. Besides nicotine, alkaloids in tobacco include cotinine, myosmine, and anatabine. Scientific investigation of these constituents and their contribution to tobacco dependence is less well developed than for nicotine. The present study evaluated the nucleus accumbens dopamine-releasing properties and rewarding and/or aversive properties of nicotine (0.2-0.8mg/kg), cotinine (0.5-5.0mg/kg), anatabine (0.5-5.0mg/kg), and myosmine (5.0-20.0mg/kg) through in vivo microdialysis and place conditioning, respectively, in adult and adolescent male rats. Nicotine increased dopamine release at both ages, and anatabine and myosmine increased dopamine release in adults, but not adolescents. The dopamine release results were not related to place conditioning, as nicotine and cotinine had no effect on place conditioning, whereas anatabine and myosmine produced aversion in both ages. While the nucleus accumbens shell is hypothesized to play a role in strengthening drug-context associations following initiation of drug use, it may have little involvement in the motivational effects of tobacco constituents once these associations have been acquired. Effects of myosmine and anatabine on dopamine release may require a fully developed dopamine system, since no effects of these tobacco alkaloids were observed during adolescence. In summary, while anatabine and myosmine-induced dopamine release in nucleus accumbens may play a role in tobacco dependence in adults, the nature of that role remains to be elucidated.

Nicotinic effects of tobacco smoke constituents in nonhuman primates

RATIONALE

Recent studies in rodents suggest that non-nicotine constituents of tobacco smoke (e.g., minor tobacco alkaloids) may promote tobacco consumption-either through their own pharmacological effects or by augmenting the effects of nicotine. However, there is scant information on the behavioral pharmacology of minor tobacco alkaloids in primate species.

OBJECTIVE

The present studies were conducted to determine whether the minor tobacco alkaloids nornicotine, anabasine, anatabine, myosmine, and cotinine exhibit nicotine-like behavioral effects in squirrel monkeys.

METHODS

Initial experiments were conducted to determine the effects of nicotine (0.032-1.0 mg/kg) and the minor tobacco alkaloids nornicotine (1-1.8 mg/kg), anabasine (0.1-1.0 mg/kg), anatabine (10-32 mg/kg), myosmine (0.32-1.8 mg/kg), and cotinine (10-180 mg/kg) on food-maintained performance (n = 4). Next, the ability of tobacco alkaloids to substitute for the α4β2-selective nicotinic agonist (+)-epibatidine in drug discrimination experiments was evaluated in a separate group of monkeys (n = 4).

RESULTS

Results show that nicotine and each minor tobacco alkaloid except cotinine (a) produced dose-related decreases in food-maintained responding; (b) substituted for (+)-epibatidine and, in additional experiments, produced additive effects when combined with nicotine; (c) induced emesis or tremor at doses that reduced food-maintained responding and had (+)-epibatidine-like discriminative-stimulus effects; and (d) based on correlation with reported receptor binding affinities, likely produced their behavioral effects through α4β2 receptor mechanisms.

CONCLUSION

Selected minor tobacco alkaloids have nicotinic-like effects that may contribute to tobacco consumption and addiction.

Effects of nicotine and minor tobacco alkaloids on intracranial-self-stimulation in rats

BACKGROUND

While nicotine is the primary addictive compound in tobacco, other tobacco constituents including minor alkaloids (e.g., nornicotine, anabasine) may also contribute to tobacco addiction by mimicking or enhancing the effects of nicotine. Further evaluating the behavioral effects of minor alkaloids is essential for understanding their impact on tobacco addiction and informing development of tobacco product standards by the FDA.

METHODS

This study compared the addiction-related effects of nicotine and the minor alkaloids nornicotine, anabasine, myosmine, anatabine, and cotinine on intracranial self-stimulation (ICSS) thresholds in rats.

RESULTS

Acute injection of nicotine produced reinforcement-enhancing (ICSS threshold-decreasing) effects at low to moderate doses, and reinforcement-attenuating/aversive (ICSS threshold-increasing) effects at high doses. Nornicotine and anabasine produced similar biphasic effects on ICSS thresholds, although with lower potency compared to nicotine. Myosmine only elevated ICSS thresholds at relatively high doses, while anatabine and cotinine did not influence ICSS thresholds at any dose. None of the alkaloids significantly influenced ICSS response latencies, indicating a lack of nonspecific motoric effects.

CONCLUSIONS

These findings indicate that some minor tobacco alkaloids can either fully (nornicotine, anabasine) or partially (myosmine) mimic nicotine's addiction-related effects on ICSS, albeit at reduced potency. These findings emphasize the need for further study of the abuse potential of minor alkaloids, including evaluation of their effects when combined with nicotine and other tobacco constituents to better simulate tobacco exposure in humans. Such work is essential for informing FDA regulation of tobacco products and could also lead to the development of novel pharmacotherapies for tobacco addiction.