Figure 1: Naloxone-induced morphine withdrawal sings in morphine dependent rats. Naloxone administration (2 mg/kg, i.p.) induced morphine withdrawal sings, including chewing, teeth chattering, digging, writhing and sniffing in morphine treated rats (6, 16, 26, 36, 46, 56 and 66 mg/kg, s.c.). Rats receiving morphine (Mor/Nal, n=7) are compared with the ones treated with saline (Sal/Nal, n=6). Data are expressed as mean ± SEM; ** P < 0.01 and **** P < 0.0001.