Pharmacological Blockade of α2-Adrenoceptors Induces Reinstatement of Cocaine-Seeking Behavior in Squirrel Monkeys

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Abstract
Converging evidence suggests a role for noradrenergic mechanisms in stress-induced reinstatement of cocaine seeking in animals. Yohimbine, an α2-adrenoceptor antagonist, is known to be anxiogenic and induce stress-related responses in humans and animals. Here, we tested the ability of yohimbine to reinstate cocaine-seeking behavior and induce behavioral and physiological signs characteristic of stress in squirrel monkeys. Monkeys were trained to self-administer cocaine under a second-order schedule of i.v. drug injection. Drug seeking subsequently was extinguished by substituting saline for cocaine injections and omitting the cocaine-paired stimulus. The ability of yohimbine and the structurally distinct α2-adrenoceptor antagonist RS-79948 to reinstate cocaine-seeking behavior was assessed by administering priming injections immediately before test sessions in which the cocaine-paired stimulus was either present or absent. Priming injections of yohimbine (0.1–0.56 mg/kg, i.m.) or RS-79948 (0.01–0.1 mg/kg, i.m.) induced dose-related reinstatement of cocaine-seeking behavior. The magnitude of yohimbine-induced reinstatement was similar regardless of the presence or absence of the cocaine-paired stimulus. Yohimbine also significantly increased salivary cortisol levels, a physiological marker of stress, as well as scratching and self-grooming, behavioral markers of stress in nonhuman primates. In drug interaction experiments, pretreatment with the α2-adrenoceptor agonist clonidine (0.1–0.3 mg/kg, i.m.) dose-dependently inhibited yohimbine-induced reinstatement of cocaine seeking. In contrast, pretreatment with the dopamine receptor antagonist flupenthixol failed to inhibit yohimbine-induced reinstatement of cocaine seeking. The results show that pharmacological blockade of α2-adrenoceptors can induce reinstatement of cocaine-seeking behavior and characteristic stress responses in squirrel monkeys, providing a potentially useful model of stress-induced relapse to drug seeking.