Fig. 1. Effects of naltrexone, (±)VK4-116, and (±)VK4-40 on oxycodone self-administration under a fixed-ratio schedule of reinforcement.

This figure shows the effects of (A) naltrexone (NLX, striped bars), (B) ±VK4-116 (gray bars), and (C) (±)VK4-40 (white bars) on oxycodone self-administration (black bars) under an FR schedule of reinforcement. Each bar represents mean (N = 8) ± the standard error of the mean (SEM). *p < 0.05; **p < 0.01.