Fig. 6.

Mean ± S.E.M. concentration of plasma β-endorphin during forced abstinence. Plasma was obtained 30 minutes directly following last experimental session on day 23. There was a significant difference in β-endorphin concentration between the cocaine plus 3 mg/kg l-THP and 0.1 mg/kg LDN group and the cocaine plus vehicle group (*P < 0.05), as well as between the cocaine plus 3 mg/kg l-THP and 0.1 mg/kg LDN group and the cocaine plus 3 mg/kg l-THP group. *P < 0.05. Samples were analyzed in duplicates; n = 5 for all groups.