Animal models to assess the abuse liability of tobacco products: effects of smokeless tobacco extracts on intracranial self-stimulation

Abstract

Background: Preclinical models are needed to inform regulation of tobacco products by the Food and Drug Administration (FDA). Typically, animal models of tobacco addiction involve exposure to nicotine alone or nicotine combined with isolated tobacco constituents (e.g. minor alkaloids). The goal of this study was to develop a model using extracts derived from tobacco products that contain a range of tobacco constituents to more closely model product exposure in humans.

Methods: This study compared the addiction-related effects of nicotine alone and nicotine dose-equivalent concentrations of aqueous smokeless tobacco extracts on intracranial self-stimulation (ICSS) in rats. Extracts were prepared from Kodiak Wintergreen, a conventional product, or Camel Snus, a potential "modified risk tobacco product". Binding affinities of nicotine alone and extracts at various nicotinic acetylcholine receptor (nAChR) subtypes were also compared.

Results: Kodiak and Camel Snus extracts contained levels of minor alkaloids within the range of those shown to enhance nicotine's behavioral effects when studied in isolation. Nonetheless, acute injection of both extracts produced reinforcement-enhancing (ICSS threshold-decreasing) effects similar to those of nicotine alone at low to moderate nicotine doses, as well as similar reinforcement-attenuating/aversive (ICSS threshold-increasing) effects at high nicotine doses. Extracts and nicotine alone also had similar binding affinity at all nAChRs studied.

Conclusions: Relative nicotine content is the primary pharmacological determinant of the abuse liability of Kodiak and Camel Snus as measured using ICSS. These models may be useful to compare the relative abuse liability of other tobacco products and to model FDA-mandated changes in product performance standards.

Keywords: Extract; Intracranial self-stimulation; Nicotine; Non-nicotine tobacco constituents; Policy; Smokeless tobacco.

Figure 1

ICSS thresholds (A) and response latencies (B) (expressed as percent of baseline, mean ± SEM) following injection of nicotine alone or Kodiak extract (0 - 1.25 mg/kg) in Experiment 2a. Fig (C) and (D) show ICSS threshold and latency data from Experiment 2b (second assessment). ^*,** Significantly different from saline (0 mg/kg) for nicotine alone, p < 0.05 or 0.01. ^#,## Significantly different from saline (0 mg/kg) for Kodiak extract, p < 0.05 or 0.01.

Figure 2

ICSS thresholds (A) and response latencies (B) (expressed as percent of baseline, mean ± SEM) following injection of nicotine alone or Camel Snus extract (0 - 1.25 mg/kg) in Experiment 3a. ^*,** Significantly different from saline (0 mg/kg) for nicotine alone, p < 0.05 or 0.01. ^#,## Significantly different from saline (0 mg/kg) for Camel Snus, p ≤ 0.05 or 0.01.

Figure 3

Competition by nicotine alone, Kodiak extract, and Camel Snus extract for α4ß2 (A) or α7 (B) nAChR binding sites expressed in rat brain and labeled by [³H]-epibatidine. Binding data (mean DPM ± SEM) are pooled across 2-3 experiments. K_i values for formulations at these and other nAChRs are shown in Table 3.