Relative reinforcing strength of heroin and butorphanol under a concurrent schedule of food and drug availability: effects of varying response requirement

Abstract

The present study determined the effects of changing response cost on the relative reinforcing strength of heroin or butorphanol using a “choice” procedure. Rhesus monkeys (n = 3) were trained to respond on two levers under concurrent fixed-ratio 30 (FR30) schedules of reinforcement during daily 60-min sessions. Responding on one lever resulted in the delivery of IV injections of saline or heroin, 0.3–32.0 ug/kg/inj, and responding on the other lever resulted in the delivery of food pellets. Data were collected on percentage of responses occurring on the injection-associated lever (%ILR), response rate, and numbers of reinforcers earned. After response patterns were stable under training conditions, the self-administration of heroin and the partial μ-opioid receptor agonist butorphanol, 0.1–3.2 ug/kg/inj, was examined when the response requirement for injections was increased to FR100 or decreased to FR10; the response requirement for food was held constant at FR30. %ILR during sessions of saline availability was 11±2% and this increased as a function of unit dose of heroin or butorphanol. Response rates and number of food pellets earned decreased with increasing unit doses of drug, and number of injections earned increased, then decreased, creating an inverted U-shaped function. Increases in response cost produced a trend toward a rightward shift in the dose-response curves for %ILR, rate and food pellets earned, and decreased the number of injections earned. These results indicate that response requirement impacts the relative reinforcing effects of opioids. (Supported by NIH grant DA15723)