Outline of the experimental paradigm. Following a habituation period, baseline mechanical sensitivity and facial grimace score readings were taken 2 days before injury. Post-CCI, animals were allowed to recover for 5–7 days in their home cage. After the recovery period, scores were recorded again. For conditioning, 10 days post CCI, animals were divided into four groups and treated with fentanyl, saline or no treatment respectively. Grimace scores alone were recorded on each of the 7 conditioning days. On test day (the day after conditioning day 7), scores were recorded from all four groups.