Fig. 2. Opioid self-administration after spinal nerve ligation or sham treatment. The dose effect curves for the self-administration of each opioid were determined across several self-administration sessions as described in the Materials and Methods. The x-axis indicates the dose of opioid per self-administered infusion, and the y-axis represents the number of infusions self-administered per hour. Mechanical allodynia was verified in spinal nerve–ligated rats using von Frey filaments and defined as a paw withdrawal threshold of 4 g or less. n = 6 rats/group with separate groups studied for each drug. * Significantly different from sham , P ≤ 0.05.