Figure 2. The CPP coefficient value indicated the relative preference between Rooms 1 and 2 (low-pain-paired [LPP] vs. high-pain-paired [HPP] cues in the Placebo groups, non-paired cues in the Control group).

The CPP coefficient = (Time spent in Room 1–Time spent in Room 2)/(Time spent in Room 1 + Time spent in Room 2) × 100. The preference for cues between the pre- and post-conditioning tests did not significantly differ in the Control group. In the Placebo group, the non-preferred cue (Room 1) from the pre-conditioning test was paired with low level-pain in the conditioning session; the rats’ preference for the LPP cue significantly increased after conditioning (p < 0.01). Increased preference to the LPP cue was also significant in the P + N group (p < 0.001), but not in the P + H group. Values are presented as means ± standard errors (SEs). CPP: conditioned place preference, P + H: Placebo+Haloperidol, P + N: Placebo + Naloxone.