Individual self-administration of nicotine by rats

Animal Models to Investigate the Impact of Flavors on Nicotine Addiction and Dependence

BACKGROUND

Tobacco use in humans is a long-standing public health concern. Flavors are common additives in tobacco and alternative tobacco products, added to mask nicotine's harsh orosensory effects and increase the appeal of these products. Animal models are integral for investigating nicotine use and addiction and are helpful for understanding the effects of flavor additives on the use of nicotine delivery products.

OBJECTIVE

This review focuses on preclinical models to evaluate the contribution of flavor additives to nicotine addiction.

MATERIALS AND METHODS

An electronic literature search was conducted by authors up to May 2022. Original articles were selected.

RESULTS

The behavioral models of rodents described here capture multiple dimensions of human flavored nicotine use behaviors, including advantages and disadvantages.

CONCLUSION

The consensus of the literature search was that human research on nicotine use behavior has not caught up with fast-changing product innovations, marketing practices, and federal regulations. Animal models are therefore needed to investigate mechanisms underlying nicotine use and addiction. This review provides a comprehensive overvie.

Impact of menthol on nicotine intake and preference in mice: Concentration, sex, and age differences

Menthol has been shown to contribute to the appeal of tobacco products in humans. However, factors such as sex, age and menthol concentration remain unclear in the interaction between menthol and nicotine. To understand these factors, we utilized a mouse model to determine the impact of menthol on oral nicotine consumption. A range of menthol concentrations (oral and systemic) were tested with or without oral nicotine using the two-bottle choice paradigm in adolescent and adult female and male C57BL/6J mice. Moreover, genetically modified mice were used to investigate the role of α7 nicotinic acetylcholine receptors (nAChRs) on the effects of menthol. Menthol addition to nicotine solution increased oral nicotine consumption in C57BL/6J mice in a sex- and menthol concentration-dependent manner. At lower menthol concentrations, female mice demonstrated an enhancement of nicotine consumption and male mice showed a similar behavior at higher menthol concentrations. Menthol drinking alone was only significantly different by sex at 60 μg/ml menthol concentration where female mice had higher menthol intake than males. Menthol administered both orally and systemically (intraperitoneal) increased oral nicotine consumption. Adolescent female mice had a higher nicotine intake at lower menthol concentrations compared to their adult counterparts. While α7 nAChR wild type mice consumed more mentholated nicotine solution than nicotine only solution, this effect was abolished in KO mice. Effects of menthol are concentration-, sex-, age-, and α7 nAChR-dependent. Oral and intraperitoneal menthol increases nicotine intake, suggesting that sensory, peripheral, and/or central mechanisms are involved in effects of menthol on oral nicotine consumption.

Impact of Menthol on Oral Nicotine Consumption in Female and Male Sprague Dawley Rats

INTRODUCTION

One of the preferable flavors in oral nicotine delivery systems is menthol which masks the harshness of tobacco. However, possible interactions between oral menthol and nicotine on intake and preference remain unclear. Therefore, we aimed to determine the impact of menthol on oral nicotine consumption.

METHODS

Adult Sprague Dawley female and male rats (n = 8 per group) were given a choice of water or drug solution by using two-bottle free choice paradigm for 2 weeks: vehicle (5% ethanol), nicotine (20 mg/L), menthol (1 g/L) and mentholated nicotine groups. At the end of the study, plasma nicotine levels were determined.

RESULTS

When rats were given a choice of nicotine or water, nicotine intake was similar between female and male rats. Menthol addition to nicotine solution significantly increased nicotine intake and preference in male but not female rats without a considerable effect on total fluid intake and body weight change in either sex. The average nicotine intake in male rats was 0.5 ± 0.05 and 1.4 ± 0.12 mg/kg/day for nicotine and menthol-nicotine combination (p < .05), respectively. The average nicotine intake in female rats was 0.6 ± 0.05 and 0.6 ± 0.03 mg/kg/day for nicotine and menthol-nicotine combination (p > .05), respectively. Plasma nicotine levels were not significantly different between the groups in either male (nicotine group: 20.8 ± 4.9, mentholated nicotine group: 31.9 ± 3.2 ng/mL) or female (nicotine group: 24.0 ± 3.3, mentholated nicotine group: 17.8 ± 2.9 ng/mL) rats (p > .05).

CONCLUSIONS

Menthol increases oral nicotine consumption in male, but not female, rats.

IMPLICATIONS

This study may provide data on the co-use of menthol and nicotine in smokeless tobacco, particularly oral dissolvable tobacco products.

Assessing nicotine dependence using an oral nicotine free-choice paradigm in mice

Models to assess the addictive-like properties of nicotine in mice are limited. Therefore, we aimed to characterize and validate an addiction index by using an oral nicotine free-choice paradigm in mice. Adult C57BL/6J, DBA/2J, or genetically modified mice carrying deletions for nicotinic acetylcholine receptor (nAChR) subunits, (n = 8-10/sex/group) were given a choice of water or nicotine (10-960 μg/ml) solution using a two-bottle free-choice (2BC) paradigm. In general, oral nicotine intake and preference were higher in female mice compared to males. Absence of nicotine led to withdrawal, and intermittent access resulted in an escalation in consumption and greater nicotine withdrawal than continuous exposure. Additionally, oral nicotine consumption increased nucleus accumbens tyrosine hydroxylase levels. While β2 and α6 KO mice showed a significant decrease in nicotine intake, deletion of α5 nAChRs increased nicotine consumption at high concentrations. Deletion of the α7 subunit altered the observed sex difference in nicotine consumption, with females consuming less than males. The α4β2 partial agonist varenicline decreased oral nicotine consumption. Although addition of quinine to the nicotine solution lowered nicotine intake, mice primed with nicotine did not lower their intake after quinine addition. Nicotine deprivation followed by re-exposure showed increased nicotine consumption, and DBA/2J mice consumed less nicotine compared to C57BL/6J. We validated the mouse 2BC paradigm to study nicotine's addictive-like properties including nicotine intake, preference, withdrawal, and escalation of nicotine consumption during binge drinking or after reinstatement of a deprivation period.

Consumption of Substances of Abuse during Pregnancy Increases Consumption in Offspring: Possible Underlying Mechanisms

Correlative human observational studies on substances of abuse have been highly dependent on the use of rodent models to determine the neuronal and molecular mechanisms that control behavioral outcomes. This is particularly true for gestational exposure to non-illicit substances of abuse, such as excessive dietary fat, ethanol, and nicotine, which are commonly consumed in our society. Exposure to these substances during the prenatal period has been shown in offspring to increase their intake of these substances, induce other behavioral changes, and affect neurochemical systems in several brain areas that are known to control behavior. More importantly, emerging studies are linking the function of the immune system to these neurochemicals and ingestion of these abused substances. This review article will summarize the prenatal rodent models used to study developmental changes in offspring caused by prenatal exposure to dietary fat, ethanol, or nicotine. We will discuss the various techniques used for the administration of these substances into rodents and summarize the published outcomes induced by prenatal exposure to these substances. Finally, this review will cover some of the recent evidence for the role of immune factors in causing these behavioral and neuronal changes.

Individual differences in the behavioral effects of nicotine: A review of the preclinical animal literature

Not everyone who tries tobacco or other nicotine-containing products becomes a long-term user. Certain traits or factors that are differentially present in these individuals must be able to help health care providers and researchers determine who is more likely to become chronic users of nicotine-containing products. Some of these factors, particularly sensation-seeking/novelty, impulsivity, and anxiety, lend themselves to the creation of animal models of reactivity to nicotine. These models of reactivity to nicotine can improve the translational aspects of preclinical animal research on nicotine-induced behaviors and treatments in order to help reduce negative outcomes in human populations. The goal of this review is to evaluate the current status of animal models of individual differences that serve to predict the later behavioral effects of nicotine. The limited utility and inconsistency of existing novelty models is considered, as well as the promise of impulsivity and anxiety models in preclinical animal populations. Finally, other models that could be employed to extend the benefit of the current research are examined.

Enriched environment attenuates nicotine self-administration and induces changes in ΔFosB expression in the rat prefrontal cortex and nucleus accumbens

Environment enrichment conditions have important consequences on subsequent vulnerability to drugs of abuse. The present work examined whether exposure to an enriched environment (EE) decreases oral self-consumption of nicotine. Wistar rats were housed either in a standard environment (SE, four rats per standard cage) or in an EE during 60 days after weaning. EE consisted of eight animals housed in larger cages containing a variety of objects such as boxes, toys, and burrowing material that were changed three times a week. After this period, animals were exposed to nicotine for 3 weeks, where animals chose freely between water and a nicotine solution (0.006% in water). Fluid consumption was evaluated on a daily basis. ΔFosB immunohistochemistry in the prefrontal cortex and nucleus accumbens was also performed. Rats of the EE group consumed less nicotine solution (0.25±0.04 mg/kg/day) than SE rats (0.54±0.05 mg/kg/day). EE increased the number of ΔFos-immunoreactive (ΔFos-ir) cells in the nucleus accumbens core and shell and in the prefrontal cortex, compared with animals in the standard condition. However, rats exposed to nicotine in the SE showed higher ΔFos-ir cells in the nucleus accumbens core and shell than nonexposed rats. Nicotine consumption did not modify ΔFos-ir cells in these brain areas in EE animals. These results support the idea of a possible protective effect of the EE on reward sensitivity and the development of an addictive behavior to nicotine.

Prenatal exposure to nicotine stimulates neurogenesis of orexigenic peptide-expressing neurons in hypothalamus and amygdala

Animal and clinical studies show that gestational exposure to nicotine increases the propensity of offspring to consume nicotine, but the precise mechanism mediating this behavioral phenomenon is unclear. The present study in Sprague Dawley rats examined the possibility that the orexigenic peptide systems, enkephalin (ENK) and orexin (OX), which are stimulated by nicotine in adult animals and promote consummatory behavior, may be similarly responsive to nicotine's stimulatory effect in utero while having long-term behavioral consequences. The results demonstrated that nicotine exposure during gestation at low doses (0.75 or 1.5 mg/kg/d) significantly increased mRNA levels and density of neurons that express ENK in the hypothalamic paraventricular nucleus and central nucleus of the amygdala, OX, and another orexigenic peptide, melanin-concentrating hormone, in the perifornical lateral hypothalamus in preweanling offspring. These effects persisted in the absence of nicotine, at least until puberty. Colabeling of the cell proliferation marker BrdU with the neuronal marker NeuN and peptides revealed a marked stimulatory effect of prenatal nicotine on neurogenesis, but not gliogenesis, and also on the number of newly generated neurons expressing ENK, OX, or melanin-concentrating hormone. During adolescence, offspring also exhibited significant behavioral changes, increased consumption of nicotine and other substances of abuse, ethanol and a fat-rich diet, with no changes in chow and water intake or body weight. These findings reveal a marked sensitivity during gestation of the orexigenic peptide neurons to low nicotine doses that may increase the offspring's propensity to overconsume substances of abuse during adolescence.

The multiple bottle effect is overridden in male and female rats by simultaneous presentation of two oral nicotine solutions

BACKGROUND

Studies on the oral route of nicotine administration in rodents make important contributions to our understanding of human nicotine use, and alternative approaches to smoking cessation. While environmental availability of oral nicotine contributes to voluntary intake and appears to drive consumption initially, solution concentration may exert more control over intake with continued exposure. Further, it is believed that female rodents consume more nicotine and show greater motivation to obtain it than males.

OBJECTIVES

The purpose of our study was to determine voluntary oral nicotine intake patterns following continuous exposure to relatively high concentrations in male and female rats, employing a multiple bottle approach, and to describe the relationship between oral nicotine consumption and sera cotinine.

METHODS

Using five bottles, adult Sprague-Dawley rats were given continuous access to water and 15 μg/ml nicotine solutions or water and 15 and 30 μg/ml nicotine solutions for 2 weeks; blood serum was analyzed for cotinine.

RESULTS

Rats consistently consumed oral nicotine and female rats ingested more nicotine than males, even at relatively high concentrations. Yet, when both concentrations were presented simultaneously, oral nicotine intake did not exceed that of water, thus overriding an environmental, or multiple-bottle, effect. Cotinine was systemically circulated following first-pass hepatic metabolism of nicotine at early, but not at later stages of nicotine exposure.

CONCLUSIONS

Our findings suggest rats will readily and voluntarily ingest considerably higher doses of nicotine than previously reported resulting in initial systemic cotinine, and trends toward sex differences are mitigated by solution concentration.

Development of an oral operant nicotine/ethanol co-use model in alcohol-preferring (p) rats

BACKGROUND

Alcohol abuse is frequently associated with nicotine (Nic) use. The current experiments were conducted to establish an oral operant ethanol + Nic (EtOH + Nic) co-use model and to characterize some aspects of EtOH + Nic co-use.

METHODS

Rats were allowed to choose between EtOH alone or EtOH + Nic solutions. Additionally, alcohol-preferring (P) rats were allowed to concurrently self-administer 3 distinct EtOH solutions (10, 20, and 30%) with varying amounts of Nic (0.07, 0.14, or 0.21 mg/ml) under operant conditions. P rats were also allowed to concurrently self-administer 2 distinct amounts of Nic (0.07 and 0.14 mg/ml) added to saccharin (Sacc; 0.025%) solutions.

RESULTS

During acquisition, P rats responded for the EtOH + Nic solutions at the same level as for EtOH alone, and responding for EtOH + Nic solutions was present throughout all drinking conditions. P rats also readily maintained stable self-administration behaviors for Nic + Sacc solutions. The results demonstrated that P rats readily acquired and maintained stable self-administration behaviors for EtOH + 0.07 and EtOH + 0.14 mg/ml Nic solutions. Self-administration of EtOH + 0.21 mg/ml Nic was established in only 50% of the subjects. P rats readily expressed seeking behaviors for the EtOH + Nic solutions and reacquired EtOH + Nic self-administration during relapse testing. In addition, tail blood samples indicated that EtOH + Nic co-use resulted in pharmacologically relevant levels of both EtOH and Nic in the blood.

CONCLUSIONS

Overall, the results indicate that P rats readily consume EtOH + Nic solutions concurrently in the presence of EtOH alone, express drug-seeking behaviors, and will concurrently consume physiologically relevant levels of both drugs. These results support the idea that this oral operant EtOH + Nic co-use model would be suitable for studying the development of co-abuse and the consequences of long-term chronic co-abuse.

Oral Nicotine Self-Administration in Rodents

Nicotine addiction is a complex process that begins with self-administration. Consequently, this process has been studied extensively using animal models. A person is usually not called "smoker" if s/he has smoked for a week or a month in a lifetime; in general, a smoker has been smoking for many years. Furthermore, a smoker has free access to cigarettes and can smoke whenever she/he wants, provided there are no social/legal restraints. Subsequently, in an animal model of tobacco addiction, it will be desirable to expose the animal to free access nicotine for 24 hours/day for many weeks, starting at different stages of development.

Individual differences in oral nicotine intake in rats

To study individual differences in nicotine preference and intake, male and female rats were given free access to a choice of oral nicotine (10 or 20 mg/L) or water for 24 h/day for periods of at least six weeks, starting at adolescence or adulthood. A total of 341 rats, were used in four different experiments; weight, nicotine intake and total liquid consumption were recorded weekly. Results show that rats can discriminate nicotine from water, can regulate their intake, and that there are readily detected individual differences in nicotine preference. Ward analyses indicated that the animals could be divided into minimum, median and maximum preferring subgroups in all experiments. The effect of saccharine on nicotine intake was also evaluated; although the addition of saccharine increased total intake, rats drank unsweetened nicotine solutions and those with higher preferences for nicotine, preferred nicotine over water with or without saccharine added. Nicotine reduced weight gain and the effect was more pronounced in females than males. The average nicotine consumption of adolescent rats was higher than adults and nicotine exposure during adolescence reduced nicotine intake in adult rats. About half of the rats which had access to nicotine as adolescents and also as adults had a persistent pattern of consumption; the behavior was very stable in the female minimum preferring groups and a much higher ratio of rats sustained their adolescent behavior as adults. The change in preference was more pronounced when there was an interval between adolescent and adult exposure; female rats showed a more stable behavior than males suggesting a greater role for environmental influences on males. In conclusion, marked individual differences were observed in oral nicotine intake as measured in a continuous access 2-bottle choice test. Age and sex of the subjects and previous exposure to nicotine are significant factors which affect preference in rats.

Solution concentration influences voluntary consumption of nicotine under multiple bottle conditions

Female Sprague Dawley rats were given a choice between two concentrations of nicotine solution (5 microg/ml and 8 microg/ml) and water in a 5-bottle arrangement for 25 days. Rats developed clear bottle discrimination, drinking more of the 5 microg/ml nicotine solution than water or the higher concentration nicotine solution. Further, intake patterns were sensitive to exposure. Differences in consumption of the three solutions (5 microg/ml vs. 8 microg/ml vs. water) were minimal during initial exposure days but became clear and stable with chronic exposure. Control rats given 5 bottles of water drank equally from all bottles and showed no development of preference for bottle position. Results suggest that both environmental availability and post-ingestional effects of nicotine contribute to the voluntarily oral consumption of nicotine solutions by rats. The influence of these two factors, however, is modulated by exposure. Availability appears to drive consumption initially, but the impact of concentration exerts more control over consumption with continued exposure. These data support the utility of oral methods of nicotine self-administration in the laboratory rat and suggest the need for further investigations into the biological impact of nicotine consumed orally.

Anabolic-androgenic steroid dependence? Insights from animals and humans

Anabolic-androgenic steroids (AAS) are drugs of abuse. They are taken in large quantities by athletes and others to increase performance, with negative health consequences. As a result, in 1991 testosterone and related AAS were declared controlled substances. However, the relative abuse and dependence liability of AAS have not been fully characterized. In humans, it is difficult to separate the direct psychoactive effects of AAS from reinforcement due to their systemic anabolic effects. However, using conditioned place preference and self-administration, studies in animals have demonstrated that AAS are reinforcing in a context where athletic performance is irrelevant. Furthermore, AAS share brain sites of action and neurotransmitter systems in common with other drugs of abuse. In particular, recent evidence links AAS with opioids. In humans, AAS abuse is associated with prescription opioid use. In animals, AAS overdose produces symptoms resembling opioid overdose, and AAS modify the activity of the endogenous opioid system.

Availability influences initial and continued ingestion of nicotine by adolescent female rats

BACKGROUND

The availability of tobacco products is associated with the likelihood that adolescents will begin using tobacco. Yet the relationship between nicotine availability and voluntary consumption has not been tested experimentally in developing rats.

METHOD

The impact of environmental availability on adolescent female rats' initial reaction to and continued ingestion of a novel solution (nicotine or control) was compared using a standard 2-bottle free-choice method and a multiple-bottle method.

RESULTS

Manipulating the ratio of bottles containing novel solution to water directly influenced the amount of both nicotine and control solutions consumed. Although subtle differences emerged in intake patterns with prolonged exposure, overall intake patterns were remarkably similar for nicotine and control solutions.

CONCLUSION

The intake of oral nicotine was directly influenced by the availability of nicotine solution relative to water. The results indicate that simply providing the right environment is sufficient, and perhaps necessary, to increase rats' voluntary consumption of a nicotine solution without relying on deprivation or sweetening of the solution. That overall intake patterns were comparable for nicotine and control solutions suggests that this may be a general principle of rodent behavior rather than an effect specific to nicotine.

The nicotinic acetylcholine receptor antagonist mecamylamine prevents escalation of cocaine self-administration in rats with extended daily access

RATIONALE

Escalation from moderate to excessive drug intake is a hallmark of human addiction that can be modeled in rats by giving them longer daily access time to self-administer cocaine. Nicotine and cocaine are commonly coabused drugs in humans and recent work in animals suggests that activation of nicotinic acetylcholine receptors (nAChR) can increase cocaine self-administration.

OBJECTIVES

Determine the role of nAChR in the escalation of cocaine self-administration.

METHODS

Control rats self-administered cocaine (0.75 mg/kg/infusion) for either 1 or 6 h per day. Experimental groups had the nAChR antagonist mecamylamine (MEC) added to the cocaine solution for 5 days after the transition from short (1 h per day) to long access (6 h per day) for cocaine self-administration. After 5 days, MEC was removed from the cocaine solution.

RESULTS

Control rats and rats that received a low dose of MEC (7 microg/infusion) with cocaine increased their average hourly intake over 5 days of 6 h per day cocaine access. Rats that received a higher dose of MEC (70 microg/infusion) did not increase their intake of cocaine during 6 h access but continued to self-administer cocaine. When MEC was removed, this group showed an escalation in cocaine self-administration. MEC did not alter cocaine intake in a group that had continuous 1 h access.

CONCLUSIONS

Antagonism of nAChRs during the initial exposure to extended cocaine self-administration access time prevented escalation of, but did not eliminate, drug intake. These findings indicate that MEC-sensitive nAChRs are critical for determining cocaine intake as a function of longer access time.

Region-specific mechanisms for testosterone-induced Fos in hamster brain

Hamsters self-administer androgens. Previously, we determined that testosterone (T) activates select steroid- and opiate-sensitive brain regions. Is T-stimulated neuronal activation androgenic? Thirty-five castrated males with physiologic T replacement (n=7/group) were pre-treated with the androgen antagonist flutamide (15 mg/kg sc) or ethanol (0.25 ml) and infused into the lateral ventricle (ICV) for 4 h with 40 microg T (TF and TE, respectively) or 40 microl vehicle (VF and VE). To determine if androgens and opiates activate overlapping brain areas, 7 additional males received 20 mug morphine sulfate ICV following ethanol injection (ME). Immediately after ICV infusion, animals were perfused. Sixty-micrometer coronal brain slices were stained for Fos. Fos-positive neurons were counted in a 0.3-mm(2) area from 5 regions previously shown to express T-induced Fos: the posteromedial bed nucleus of the stria terminalis (BSTPM), posteromedial amygdala (MeP), lateral habenula (LHb), ventral tegmental area, and lateral pontine nucleus. T induced Fos in all areas reported previously (TE vs. VE, p<0.05), except LHb (p>0.05). Morphine induced Fos in all 5 brain regions (ME vs. VE, p<0.05), indicating that androgens and opiates activate overlapping brain regions. Flutamide alone did not induce Fos (VF vs. VE, p>0.05). Moreover, flutamide treatment blocked T-induced Fos expression only in the steroid-sensitive BSTPM, suggesting that androgens mediate neuronal activation in this area (mean+/-SEM: TF: 68.4+/-13.2 vs. TE: 137.9+/-17.6, p<0.05). The absence of flutamide effects on T-induced Fos in the steroid-sensitive MeP (TE: 210.6+/-50.0 vs. TF: 215.3+/-28.2, p>0.05) suggests that distinct mechanisms activate Fos in individual androgen-responsive nuclei.