Effects of chronic treatment with 10 mg/kg of morphine (10 MS) or with NMDAR antagonists on antinociceptive effects of MS. Rats received chronic treatments, identified in column titles, b.i.d., for 6 to 7 days and were tested with cumulative doses of MS in a 55 °C water tail-withdrawal assay both before treatment (circles) and 12 h after (squares) the last injection on Day 6 or 7 of treatment; i.e., on Day 7 (D7) or Day 8 (D8). On test days, rats received saline or an appropriate dose of NMDAR antagonist 30 min before cumulative doses of MS. Other details as in Fig. 2. Text in italics identifies the −30-min pretreatment: SAL saline, 0.1 DZ 0.1 mg/kg dizocilpine, 10 MEM 10 mg/kg memantine