The dose-response effect of intrathecal injection of the δ-opioid agonist DPDPE on the nociceptive withdrawal threshold in vehicle- and RTX-treated rats. A: time course of the effect of intrathecal injection of 5 (n = 6), and 10 (n = 7), and 20 µg (n = 7) DPDPE on the nociceptive withdrawal threshold in the vehicle-treated rats. B: time course of the effect of intrathecal injection of 5 (n = 7), and 10 (n = 7), and 20 µg (n = 8) DPDPE on the nociceptive threshold in RTX-treated rats. Note that intrathecal injection of 20 µg DPDPE failed to alter significantly the withdrawal threshold in the presence of 25 ìg naltrindole (n = 7). The nociceptive threshold was assessed by testing the withdrawal response of the hindpaw to a noxious pressure stimulus. Data are presented as means ± SEM. *, P < 0.05 compared with the respective baseline control. NLT, naltrindole.