The effect of opioid receptor and cannabinoid receptor (CB1) antagonist on fasting and refeeding-induced analgesia. (a) Experimental design and schedule for formalin test. Naloxone (opioid receptor antagonist, 10 mg/kg) and SR 141716 (CB1 receptor antagonist, 10 mg/kg) were intraperitoneally (i.p.) administered 30 min before the formalin injection. (b,c) Time course of spontaneous pain behavior following intraplantar injection of formalin; Free fed-vehicle (n = 9), Free fed-naloxone (n = 7), 24 h fasted-vehicle (n = 13), 24 h fasted-naloxone (n = 11), 2 h refed-vehicle (n = 12), 2 h refed-naloxone (n = 10) / Free fed-vehicle (n = 11), Free fed-SR 141716 (n = 11), 24 h fasted-vehicle (n = 9), 24 h fasted-SR 141716 (n = 9), 2 h refed-vehicle (n = 10), 2 h refed-SR 141716 (n = 9). (d,e) Formalin-induced pain behavior was divided into two phase and the total sum of the licking times for each phase was statistically analyzed. Data are presented as mean ± SEM. **p < 0.01, ***p < 0.001 (d,e one-way ANOVA followed by Bonferroni test).