Figure 2.

Timeline for experimental procedures. All animals underwent 20 sessions of SRR conditioning or paired yoked control procedures (n = 11 per group). A subset of rats (n = 8 per group) also underwent 3-5 SRR retention tests with no reinforcement and only background red light (“dark sessions”), intermittent with conditioning sessions. In this same subgroup, open field testing occurred following Session 20 of conditioning. Nociception assays then took place over 4 days, 5–6 days after conditioning (except for one cohort of n = 2 pairs that was delayed approximately a month and underwent several SRR reinstatement sessions beforehand); a subset of rats were injected with formalin on Day 4 to evaluate the development of mechanical allodynia in a von Frey Test conducted 75 min later (n = 4 per group). Acute restraint sessions and blood collection for plasma corticosterone assays were performed during the first 3 days of nociception assays. Not shown here are acclimation and nose poke sessions (described in Materials and methods). A separate subgroup of FRR conditioned rats and their yoked controls (n = 4 per group) underwent identical testing procedures to those shown here but received reinforcement for fast vs. slow RRs.