Figure 4: (a) Naltrexone blocks U50,488-induced reinstatement of ethanol seeking. Rats () were trained to self-administer a 10% ethanol solution using standard operant procedures, and then lever pressing for ethanol was subsequently extinguished. Following extinction, rats were pretreated with naltrexone (0 or 3.0 mg/kg, s.c.) 30 min prior to reinstatement testing. Prior to testing, rats received priming injections of U50,488 (0–1.0 mg/kg, i.p.) and were tested for reinstatement of responding on the previously ethanol-associated lever. Tests were conducted every 3-4 days, with rats experiencing extinction sessions daily between test days. Data are expressed as the mean number of lever responses ± SEM during ethanol self-administration sessions. compared to saline-injected rats receiving the same dose of U50,488, Sidak’s test for multiple comparisons. (b) Responding on an inactive lever was measured as an indication of general motor activity. No effects on inactive lever responding were observed.