Experiment 2.

(see Figure 1B). In order to test for conditioning using an immune challenge, all rats were given an LPS injection (100 μg/kg i.p.) and then immediately placed either into the lemon-odorized context for 6 hrs (paired and unpaired groups) or back into the home cage (unmanipulated control group). We hypothesized the conditioning of the HPA axis would most likely be revealed during the rise and/or peak of the response to LPS, which is typically 90-180 minutes, depending on the dose administered (25,26). Tail blood samples were collected using the tail nick method as previously described (26,27). Plexiglas restraint tubes were used briefly in order to acquire the blood samples and blood collection took under 2 min for each timepoint. A baseline sample was collected immediately prior to LPS injection (0 min) and then at 45, 90, 180 and 360 min post-LPS injection. The 360 min time point was an exploratory measure to determine if conditioning would impact recovery of the CORT response to LPS. The tail blood samples underwent refrigerated centrifugation (15 min at 10,000 rpm) and isolated serum was stored at −20°C until used for assessment of CORT levels.