Delivery of nicotine in an extract of a smokeless tobacco product reduces its reinforcement-attenuating and discriminative stimulus effects in rats

Abstract

Rationale: Animal models of tobacco addiction rely on administration of nicotine alone or nicotine combined with isolated constituents. Models using tobacco extracts derived from tobacco products and containing a range of tobacco constituents might more accurately simulate tobacco exposure in humans.

Objective: To compare the effects of nicotine alone and an aqueous smokeless tobacco extract in several addiction-related animal behavioral models.

Methods: Nicotine alone and nicotine dose-equivalent concentrations of extract were compared in terms of their acute effects on intracranial self-stimulation (ICSS) thresholds, discriminative stimulus effects, and effects on locomotor activity.

Results: Similar levels of nicotine and minor alkaloids were achieved using either artificial saliva or saline for extraction, supporting the clinical relevance of the saline extracts used in these studies. Extract produced reinforcement-enhancing (ICSS threshold-decreasing) effects similar to those of nicotine alone at low to moderate nicotine doses, but reduced reinforcement-attenuating (ICSS threshold-increasing) effects at a high nicotine dose. In rats trained to discriminate nicotine alone from saline, intermediate extract doses did not substitute for the training dose as well as nicotine alone. Locomotor stimulant effects and nicotine distribution to brain were similar following administration of extract or nicotine alone.

Conclusions: The reinforcement-attenuating and discriminative stimulus effects of nicotine delivered in an extract of a commercial smokeless tobacco product differed from those of nicotine alone. Extracts of tobacco products may be useful for evaluating the abuse liability of those products and understanding the role of non-nicotine constituents in tobacco addiction.

Figure 1

ICSS thresholds (A) and response latencies (B) (expressed as percent of baseline, mean ± SEM) following s.c. injection of nicotine alone or extract (0 – 0.75 mg/kg). Threshold and latency data from the two rats administered s.c. extract 1.25 mg/kg are also shown. * Significantly different from saline (0 mg/kg), p < 0.05 or 0.01. ^# Significantly different from nicotine alone at that dose, p < 0.05 or 0.01.

Figure 2

Percent of total responses on the nicotine-appropriate lever (mean ± SEM) and overall response rate (total responses/sec) following s.c. injection of nicotine alone or extract in Experiment 3a (0.4 mg/kg nicotine training dose, A and B) and Experiment 3b (0.1 mg/kg nicotine training dose, C and D). * Significantly different from saline (0 mg/kg), p < 0.05 or 0.01. ^# Significantly different from nicotine alone at that dose, p < 0.05. For clarity, significant main effects of formulation (extract versus nicotine alone) in Fig 2A and 2C are not shown.

Figure 3

Total distance traveled (Mean ± SEM) during each 30 min post-test during habituation, sensitization, and challenge phases (see text). * Significantly different from the s.c. saline group (negative control) on that test day, p < 0.05 or 0.01. ^# Significantly different from test day 1 for that group, p < 0.05 or 0.01.

Figure 4

Serum (A) and brain (B) nicotine levels and brain:serum nicotine concentration ratios (C) (Mean ± SD) following 10-min pretreatment with s.c. nicotine alone or extract (0.4 mg/kg).