Evaluation of the inflammation-induced depression model. A, Timelines of the experiments. LPS was added intraperitoneally to induce inflammation. PrRP was injected intracerebroventricularly. Control rats received saline. B, C, Exp. 1. and 2, respectively: Student's t-tests, df = 9, saline: n = 5, treated: n = 6. Percentage of the time spent passively (“floating”) and actively (“struggling”) in the FST 30 min after (B) PrRP (floating: *p = 0.036, struggling: *p = 0.035, t = −2.459 and t = 2.484, respectively) or 24 h after (C) LPS treatments (floating: *p = 0.034, struggling: *p = 0.030, t = −2.499 and t = 2.576, respectively). D, Bodyweight change (t = 5.213, *p = 0.001) and (E) food intake (p = 0.056) during the 24 h after the LPS injections. F, PrRP cell (brown) activation 3 h after LPS treatment. The cFos nuclear signal (black) appears in the activated neurons. The inserts show PrRP neurons at higher magnification. DMX: dorsal motor nucleus of the vagus nerve. G, Exp. 3: Student's t-tests, df = 8, n = 5/group. Relative mRNA levels of TNFα (*p = 0.001, t = 37.153, measured in the A2 cell group), PrRP (DM: *p = 0.001, t = 4.759, A1: *p = 0.008, t = 3.501, NTS: *p = 0.005, t = 3.814), PrRP receptors (PRLHR and NPFF2R: *p = 0.02, t = −2.898 and t = −2.890, respectively) and pro-MCH (*p = 0.001, t = −6.854) 3 h post-LPS treatment. H, Exp. 3: Student's t-tests, df = 4, n = 3/group. PrRP immunopositivity in DLH axons 24 h after treatments. The image illustrates the PFA. The bar graph shows the density of PrRP⁺ fibers in the area (PFA: *p = 0.021, LH: *p = 0.034, t = 3.675 and t = 3.168). I, MCH immunopositive cells in the PFA 24 h after treatments. The arrowheads point to axonal processes. The bar graph shows the total cellular fluorescence measured in the MCH neurons, Student's t-test, t = –3.989 df = 4, *p = 0.016, n = 3/group, error bars indicate mean ± SEM.