Effects of varenicline on the reinforcing and discriminative stimulus effects of cocaine in rhesus monkeys.

Varenicline is a low-efficacy, α4β2* subtype-selective nicotinic acetylcholine receptor (nAChR) agonist that has shown success in smoking cessation and promise in preclinical assessments relating to other drugs of abuse. The primary goal of the present study was to examine the effects of varenicline on cocaine self-administration and cocaine discrimination and compare these effects with those of the nAChR agonist nicotine and antagonist mecamylamine. One limitation of agonist treatments is the potential for abuse. Thus, a second goal was to examine the abuse potential of varenicline in rhesus monkeys. In the first experiment, rhesus monkeys (n = 3) were trained to self-administer cocaine (saline, 0.01-0.56 mg/kg) under a progressive-ratio schedule of reinforcement; monkeys also earned all of their food by responding on another lever under a fixed-ratio 50 schedule of reinforcement. Chronic administration of varenicline (0.01-0.56 mg/kg p.o., salt) potentiated the reinforcing effects of cocaine, whereas mecamylamine (0.3-1.7 mg/kg p.o, i.m., i.v., salt) had no significant effects on cocaine self-administration up to doses that disrupted food-maintained responding. Neither varenicline (0.01-0.17 mg/kg, salt) nor nicotine (0.01-0.1 mg/kg, base) functioned as reinforcers when substituted for cocaine. Finally, in monkeys trained to discriminate self-administered 0.3 mg/kg cocaine, varenicline (0.1-0.3 mg/kg i.v.) did not substitute for cocaine but, along with mecamylamine (0.3-1.7 mg/kg i.v.) and nicotine (0.03-0.1 mg/kg i.v.), potentiated the discriminative stimulus effects of cocaine. These results suggest that varenicline has low abuse liability in monkey models of cocaine abuse, but would not be an effective medication for cocaine addiction.