Fig. 3. Effect of an acute injection of MK-801 on tail withdrawal latencies in CD-1 mice given morphine-6β-glucuronide (M6G) by acute injection (10 mg/kg) (  A ) or CD-1 mice on the fourth day of a continuous infusion of M6G (infusion rate: 1.6 mg/kg per 24 h) (  B ). All mice were implanted with pellets containing the opioid receptor antagonist naltrexone. (  A ) Acute: Thirty minutes after 0.05 mg/kg subcutaneous MK-801 or saline injection, 10 mg/kg M6G was injected at t = 0. Subsequently, tail-withdrawal latencies were followed for 2 h.  Squares = MK-801 (n = 6);  triangles = saline (n = 6). (  B ) Chronic: During the continuous subcutaneous infusion of M6G, 0.05 mg/kg subcutaneous MK-801 (  up triangles, n = 6) or saline (  down triangles, n = 6) was injected just after the latency measurement on day 4 (pre–MK-801 latencies here shown at  t = 0). Significant treatment, time, and time × treatment effects were observed (all  P < 0.01). Data are mean ± SEM.  Post hoc comparisons: *  P < 0.01  versus pre-M6G baseline (BL). #  P < 0.01  versus t = 0.