Exposure to chronic intermittent nicotine vapor induces nicotine dependence

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Abstract

Animal models of drug exposure are important tools for the study of the neurobiological mechanisms of nicotine dependence and as preclinical models for medication development. There are few non-invasive animal models of nicotine exposure and currently there is no known animal model of second-hand exposure to nicotine. We hypothesized that chronic administration of nicotine vapors would produce blood levels of nicotine in rodents that are clinically relevant to those observed in human smoking and that rodents exposed to nicotine vapors would develop dependence to nicotine. We developed a system that vaporizes nicotine in the air in a stable, reliable and consistent manner. Intermittent exposure to nicotine vapor (0.2 mg/m3) for 8 or 14 h per day for 7 days produced a concentration of nicotine in the blood of 22 ng/mL. Sixteen hours after removal from nicotine vapors, rats showed significant somatic withdrawal signs precipitated by mecamylamine (1.5 mg/kg). These results provide a new rodent model of nicotine dependence using vapor administration that produces consistent levels of nicotine in the blood that are relevant for both heavy smoking and second-hand smoking, using a non-invasive technique that mimics the intermittent aspect and route of administration in humans.

Introduction

An increasing amount of nicotine addiction research is undertaken each year, justified by the estimated 3 million yearly deaths from the consequences of nicotine addiction (Maskos et al., 2005). Animal models of drug exposure are important tools for the study of the neurobiological mechanisms of nicotine dependence and as preclinical models for medication development. The most commonly used methods for examining the effects of chronic nicotine administration are osmotic minipumps, repeated subcutaneous injections, intravenous self-administration and cigarette smoke exposure (Koob and Le Moal, 2006). Although these methods are extremely useful and have demonstrated robust predictive validity for tobacco addiction, they also have important limitations. Exposure to nicotine using osmotic minipumps cannot mimic the intermittent aspect of nicotine exposure during cigarette smoking and repeated injections and self-administration cannot mimic the route of human exposure and are labor intensive and relatively invasive techniques. In addition, there is currently no known animal model to study the effect of exposure to very low levels of nicotine in the air, such as those observed after second-hand exposure to cigarette smoke. Although tobacco smoke exposure is a highly relevant and validated technique to model the effect of second hand smoking (Seymour et al., 1997), this technique cannot discriminate the effects of nicotine from the effects of the approximately 4000 additional compounds known to be present in cigarette smoke (Wynder and Hoffmann, 1979).

The purpose of the present study was to validate a rodent model of nicotine administration that produces consistent levels of nicotine in the blood that are relevant for both heavy smoking and second-hand smoking, using a non-invasive and high throughput technique that could mimic both the intermittent aspect and route of administration in humans. To this end, we developed a system based on previous nicotine inhalation research (Waldum et al., 1996) that allowed us to vaporize nicotine in the air in a stable, reliable and consistent manner. We hypothesized that the administration of nicotine vapors would produce blood levels of nicotine that are clinically relevant to those observed in second-hand smoking (< 5 ng/mL; Argacha et al., 2008) and heavy smoking (10–50 ng/mL; Matta et al., 2007). We further hypothesized that rodents exposed to nicotine vapors would develop dependence to nicotine, defined here as the manifestation of a somatic withdrawal syndrome similar to that observed in other studies of rodent nicotine exposure (Epping-Jordan et al., 1998, Grieder et al., 2010, Malin et al., 1992).

Section snippets

Animals

Male Wistar rats (Charles River) weighed 200–250 g at the time of testing. Animals were housed in groups of 3 or 4 in Plexiglas cages in humidity- and temperature-controlled (22 °C) rooms on a 12 h light/dark cycle with lights off from 10:00 AM to 10:00 PM. Each rat was handled for 2–3 days before training by placing groups of animals on a table top and repetitively handling each animal during a 5 min period. Animals had ad libitum access to food and water throughout the course of the studies. All

Results

To evaluate the stability of the concentration of nicotine vapor in air, nicotine was bubbled using a gas-washing bottle at a flow rate of 20 LPM and the levels of nicotine in the air produced by the gas-washing bottle were monitored during 4 subsequent hours using the ethanol-trapping technique and UV spectrophotometry at 260 nm (Waldum et al., 1996). The concentration of nicotine in the vapor chamber air over 4 h was stable and constant, with an average concentration of 1.09 ± 0.02 mg/m3 (1 h = 1.07 

Discussion

Currently there is a gap in animal models of nicotine exposure such that the most commonly used methods for examining the effects of chronic nicotine administration are invasive and cannot mimic intermittent nicotine exposure or exposure to very low levels of nicotine in the air as in second-hand exposure. Here we have provided a new rodent model of nicotine vapor administration that produces consistent levels of nicotine in the blood that are relevant for both heavy smoking and second-hand

Conflict of interest

Maury Cole is an owner of La Jolla Alcohol Research Inc, a company focused on the development of inhalation chambers. No La Jolla Alcohol Research Inc funds were used in this work. Olivier George, Taryn Grieder and George Koob declare that no financial support has been received from any individual or corporate entity for research or professional service, and there are no personal financial holdings that could be perceived as constituting a potential conflict of interest.

Acknowledgements

The authors would like to thank Yanabel Grant and Molly Brennan for their technical support and Michael Arends for editing. This is publication number 20519 — Committee on the Neurobiology of Addictive Disorders from The Scripps Research Institute. This work was supported by the Tobacco Related Disease Research Program (TRDRP) of the State of California (Grant 12RT-0099), the National Institute of Diabetes and Digestive and Kidney Diseases (Grant DK26741), the National Institute on Drug of Abuse

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These authors contributed equally to this work.

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