Varenicline and Cytisine Diminish the Dysphoric-Like State Associated with Spontaneous Nicotine Withdrawal in Rats

Tobacco addiction is characterized by a negative mood state upon smoking cessation and relapse after periods of abstinence. Clinical studies indicate that negative mood states lead to craving and relapse. The partial alpha 1/alpha 6/beta 2(star) nicotinic acetylcholine receptor (nAChR) agonists varenicline and cytisine are widely used as smoking cessation treatments. Varenicline has been approved in the United States for smoking cessation and cytisine is used in Eastern European countries. Despite the widespread use of these compounds, very little is known about their effects on mood states. These studies investigated the effects of varenicline, cytisine, and the cytisine-derivative 3-(pyridin-3'-yl)-cytisine (3-pyr-Cyt) on brain reward function in nicotine-naive and nicotine-withdrawing rats. The cytisine-derivative 3-pyr-Cyt is a very weak alpha 4 beta 2(star) nAChR partial agonist and like cytisine and varenicline has antidepressant-like effects in animal models. The intracranial self-stimulation (ICSS) procedure was used to investigate the effects of these compounds on brain reward function. Elevations in ICSS thresholds reflect a dysphoric state and a lowering of thresholds is indicative of a potentiation of brain reward function. It was shown that acute administration of nicotine and varenicline lowered ICSS thresholds. Acute administration of cytisine or 3-pyr-Cyt did not affect ICSS thresholds. Discontinuation of chronic, 14 days, nicotine administration led to elevations in ICSS thresholds that lasted for about 2 days. Varenicline and cytisine, but not 3-pyr-Cyt, diminished the nicotine withdrawal-induced elevations in ICSS thresholds. In conclusion, these studies indicate that varenicline and cytisine diminish the dysphoric-like state associated with nicotine withdrawal and may thereby prevent relapse to smoking in humans.