Effects of compounding drug-related stimuli: Escalation of heroin self-administration

Previous experiments have demonstrated that presenting independently establ ished discriminative stimuli in compound can substantially increase operant responding maintained by food reinforcement or shock avoidance. Recently, this phenomenon was also shown to occur with cocaine selfadministration. Th e present study further assessed the generality of these stimulus-compoundi ng effects by systematically replicating them with heroin self-administrati on. Rats' nose-poke responses produced intravenous heroin (0.025 mg/kg per infusion) on a variable-ratio schedule when either a tone or a light was pr esent. In the absence of these stimuli, responding was not reinforced. Once discriminative control by the tone and light had been established, the sti muli were presented in compound under extinction (with heroin discontinued) or maintenance conditions (with heroin available during test-stimulus pres entations). In extinction, the tone-light compound increased responding app roximately threefold compared to tone or light alone. Under maintenance con ditions, compounding increased heroin intake approximately twofold. These e ffects closely matched those obtained earlier with cocaine. This consistenc y across pharmacological classes and across drug and nondrug reinforcers fu rther confirms that (a) self-administered drugs support conditioning and le arning in a manner similar to that supported by other reinforcers; and (b) multiple drug-related cues interact in lawful and predictable ways to affec t drug seeking and consumption.