A rat model of operant negative reinforcement in opioid-dependent males and females.

Abstract

Avoidance of opioid withdrawal plays a key role in human opioid addiction. Here, we present a procedure for studying operant negative reinforcement in rats that was inspired by primate procedures where opioid-dependent subjects lever-press to prevent naloxone infusions. In Experiment 1, we trained rats (n = 30, 15 females) to lever-press to escape and then avoid mild footshocks (0.13-0.27mA) for 35days (30 trials/d). Next, we catheterized them and implanted minipumps containing methadone (10mg/kg/day) or saline. We then paired (4 times, single session) a light cue (20-s) with a naloxone infusion (20µg/kg, i.v) that precipitated opioid withdrawal. Next, we trained the rats to escape naloxone injections for 10days (30 trials/d). Each trial started with the onset of the opioid-withdrawal cue. After 20-s, the lever extended, and an infusion of naloxone (1 to 2.2µg/kg/infusion) began; a lever-press during an 11-s window terminated the withdrawal-paired cue and the infusion. In Experiment 2, we trained rats (n = 34, 17 females) on the same procedure but decreased the footshock escape/avoidance training to 20days. All rats learned to lever-press to escape or avoid mild footshocks. In both experiments, a subset, 56% (10/18) and 33% (8/24) of methadone-dependent rats learned to lever-press to escape naloxone infusions. We introduce an operant negative reinforcement procedure where a subset of opioid-dependent rats learned to lever-press to escape withdrawal-inducing naloxone infusions. The procedure can be used to study mechanisms of individual differences in opioid negative reinforcement-related behaviors in opioid-dependent rats.