Effects of nicotine on responding maintained by environmental stimuli in rats
Past research suggests that nicotine-induced increases in reinforced responding are due to its reinforcer-enhancing effects. It is unclear whether this role is specific to certain kinds of reinforcing consequences (e.g., sensory stimuli, edible reinforcers, drug reinforcers). Furthermore, it is possible that nicotine merely increases behavior that has been trained in the past (i.e., responding reinforced with food consequences). The objectives of Experiments 1 and 2 were to test the generality of the motivating establishing operation (MEO) account of nicotine-induced increases in reinforced responding and to determine whether the history of training (responses that have previously been reinforced with food versus those that have not) would augment the effects.
Experiment 1 used an observing-response procedure to investigate responding maintained by food reinforcers, conditioned reinforcers (i.e., visual stimuli), and responding during extinction. Rats in Experiment 1 received pre-session subcutaneous injections of vehicle (n = 5), 0.3 (n = 6) or 0.56 (n = 6) mg/kg nicotine for 70 sessions. Resistance to extinction was also assessed by removing food for five sessions. Nicotine did not consistently affect food or extinction responding. Both doses of nicotine produced increases in responding maintained by conditioned reinforcers, but did not increase resistance to extinction. Pre-drug response rates accounted for a small but significant percentage of the variance in the drug effect.
In Experiment 1, contingent houselight presentations alone were shown to slightly increase response rates from operant levels. Experiment 2 further evaluated the putative primary reinforcing functions of turning on and turning off a houselight. One group of rats (n=4) was initially trained to press both levers (one was later designated as the active lever and the other the inactive lever), while a different group of rats (n =4) was only trained to press the lever that was later designated as the active lever. Across two phases, five responses on the active lever resulted in the houselight either turning on (Lights On) or turning off (Lights Off). All subjects made more responses on the active lever, regardless of lever training history and the type of stimulus change, suggesting that both stimuli served as primary reinforcers. Nicotine only increased responding on the active lever, again regardless of lever training history, which further supported the MEO role, and refuted the alternative hypothesis that nicotine generally increases behavior that has been trained.
Although there was a tendency for nicotine to increase low pre-drug response rates in both experiments, nicotine systematically increased responding maintained by conditioned reinforcers in Experiment 1, and only increased responding on an active lever in Experiment 2. The results of both experiments are in accord with the MEO account of nicotine – that it increases responding maintained by moderately reinforcing stimuli, such as the conditioned reinforcers and visual stimuli used in the present studies.