Fig. 1.

EP stress impairs memory retrieval in the MWM task by activating hippocampal µRs. a The effects of systemic injection of naloxone or saline on time spent in the target/opposite quadrants and the target time ratio during probe test of the stressed and unstressed mice. *, target vs. opposite within-group, paired Student’s t-test. The target time ratio F_3,36 = 3.59, p = 0.023, one-way ANOVA; #, vs. S-Sal. b The effects of bilateral intra-hippocampal infusion of CTAP (0.5 µg/µl), naltrindole (2.26 µg/µl), nor-binaltorphimine (3.85 µg/µl), DAMGO (0.5 μg/μl), or saline on memory retrieval. The target time ratio F_8,81 = 7.00, p = 0.000, one-way ANOVA; #, vs. S-Sal. c The effects of EP stress on the levels of hippocampal µR proteins and their phosphorylation. Total µRs F_2,15 = 0.25, p = 0.784; cell surface µRs F_2,12 = 4.09, p = 0.044; phosphorylated µRs F_2,12 = 4.75, p = 0.030; one-way ANOVA; *, vs. unstressed. d Hippocampal injection of β-endorphin antiserum does not affect stress-induced memory impairment. e Hippocampal injection of enkephalin antiserum at a concentration of 1:10 before stress abolishes memory impairment. f The level of Met/Leu-enkephalin in hippocampal tissues. g The effects of systemic application of naloxone or bilateral intra-hippocampal infusion with CTAP on the level of serum corticosterone immediately after 50-min stress. F_3,12 = 4.22, p = 0.030 for naloxone groups, and F_3,12 = 8.74, p = 0.000 for CTAP groups, one-way ANOVA; *, vs. stress. One symbol, p < 0.05; two symbols, p < 0.01; three symbols, p < 0.001. US un-stress, S stress, Sal saline, Nal naloxone, Naltr naltrindole, nor-BNI nor-binaltorphimine, Cort corticosterone, Mem-µR cell surface µRs, p-µR phosphorylated µRs, EKN-Ab enkephalin antiserum, EP-Ab β-endorphin antiserum, Ser control serum