Effects of contingent and noncontingent nicotine on lever pressing for liquids and consumption in water-deprived rats

Nicotine has been proposed to be a primary reinforcer and a reinforcement enhancer. To date, no studies have examined whether nicotine enhances consummatory behaviors or only operant responding (appetitive behaviors). Experiments were designed to test whether contingent and noncontingent nicotine enhance lever pressing for and consumption of fluids in water-deprived rats. Animals were water-deprived throughout all experiments. They were trained to press two levers under a variable interval (VI-20, 1–35 s). Their lever pressing and water consumption were measured after noncontingent subcutaneous (s.c.) injection of nicotine (1 mg/kg), and in 3 choice conditions (water and quinine solution (18 µg/ml); water and nicotine (32 µg/ml) solution; quinine (18 µg/ml) and nicotine (32 µg/ml) solutions) where nicotine was thus delivered contingently upon lever pressing. The effects of nicotine (1 mg/kg; s.c.) on the consumption of water in a time-limited free access (1 h) paradigm were assessed. Nicotine significantly increased lever pressing and the number of earned reinforcements on both levers in the two choice conditions and when administered s.c. compared to all groups that did not receive nicotine. However, under no condition did animals consume more fluids than baseline. Under the time-limited free access condition nicotine reduced water consumption. Although our findings do not support a reinforcing effect for nicotine, they are consistent with the incentive-amplification hypothesis. Its relevance for human smoking is yet unclear.