Acquisition of nicotine discrimination and discriminative stimulus effects of nicotine in rats chronically exposed to caffeine

Caffeine and nicotine are the main psychoactive ingredients of coffee and tobacco, with a high frequency of concurrent use in humans. This study examined the effects of chronic caffeine exposure on 1) rates of acquisition of a nicotine discrimination (0.1 or 0.4 mg/kg, s.c., training doses) and 2) the pharmacological characteristics of the established nicotine discrimination in male Sprague-Dawley rats. Once rats learned to lever-press reliably under a fixed ratio of 10 schedule for food pellets, they were randomly divided into two groups; 12 animals were maintained continuously on caffeine added to the drinking water (3 mg/ml) and another 12 control rats continued to drink tap water. In each group of water- and caffeine-drinking rats, there were six rats trained to discriminate 0.1 mg/kg of nicotine from saline and six rats trained to discriminate 0.4 mg/kg of nicotine from saline. Regardless of the training dose of nicotine, both water- and caffeine-drinking groups required a comparable number of training sessions to attain reliable stimulus control, although there was a trend for a slower acquisition in the caffeine-drinking group trained with 0.1 mg/kg of nicotine. Tests for generalization to different doses of nicotine revealed no significant differences in potency of nicotine between water- and caffeine-drinking groups. The nicotinic-receptor antagonist mecamylamine blocked the discriminative effects of 0.1 and 0.4 mg/kg nicotine with comparable potency and efficacy in water- and caffeine-drinking groups. There was a dose-related generalization to both the 0.1 and 0.4 mg/kg nicotine cue (maximum average of 51-83%) in water-drinking rats after i.p. treatment with d-amphetamine, cocaine, the selective dopamine uptake inhibitor GBR-12909, apomorphine, and the selective dopamine D1 receptor agonist SKF-82958, but not in caffeine-drinking rats (0-22%). There was no generalization to the nicotine cues after i.p. treatment with caffeine or the selective D2 (NPA) and D3 (PD 128,907) dopamine-receptor agonists in water- and caffeine-drinking rats. The dopamine-release inhibitor CGS 10746B reduced the discriminative effects of 0.4 mg/kg nicotine in water-drinking rats, but not in caffeine-drinking rats. There was no evidence of development of tolerance or sensitization to nicotine's effects throughout the study. in conclusion, chronic caffeine exposure (average, 135 mg/kg/day) did not affect the rate of acquisition of the nicotine discrimination, but it did reduce the dopaminergic component of the nicotine-discriminative cue. The reduction of the dopaminergic component of the nicotine cue was permanent, as this effect was still evident after the caffeine solution was replaced with water in caffeine-drinking rats. That nicotine could reliably serve as a discriminative stimulus in the absence of the dopaminergic component of its discriminative cue may differentiate nicotine from "classical dopaminergic" drugs of abuse such as cocaine and amphetamine.