DIFFERENTIAL ROLE OF DELTA-OPIOID RECEPTORS IN THE DEVELOPMENT AND EXPRESSION OF BEHAVIORAL SENSITIZATION TO COCAINE

The present study was designed to determine whether the selective delt a-opioid receptor antagonist naltrindole hydrochloride can prevent the expression and development of sensitization to the locomotor-activati ng effects of cocaine. Rats were sensitized to the motor stimulant eff ects of cocaine (20 mg/kg i.p. x 3 days). 48 h after withdrawal of pre treatment, rats were pretreated with naltrindole (0.1-1.0 mg/kg s.c.) or its vehicle and 15 min later challenged with either saline or the s ensitizing dose of cocaine. In a second set of experiments, naltrindol e (0.1-1.0 mg/kg s.c.) or its vehicle were given in combination with e ither saline or cocaine (20 mg/kg i.p.) for 3 days. Activity in respon se to saline and to cocaine (20 mg/kg i.p.) was assessed on days 4 and 5, respectively. Additional experiments determined whether naltrindol e prevents the development of sensitization to the locomotor-activatin g effects of nicotine: naltrindole (0.3, 1.0 mg/kg s.c.) or its vehicl e were given in combination with nicotine (0.6 mg/kg s.c.) for 3 days. Naltrindole blocked the development but not expression of sensitizati on to the locomotor-activating effects of cocaine. In contrast, naltri ndole failed to modify nicotine-induced sensitization in nicotine-trea ted animals. These data suggest that delta-opioid receptors are involv ed in the development but not expression of behavioral sensitization t o cocaine.