Effects of opioid receptor antagonists on i.c.v. administered (+)BW373U86 (100 nmol), leu-enkephalin (100 nmol) and met-enkephalin (100 nmol) induced increases in BDNF mRNA expression in rats. Each value represents the mean percent of (veh + veh) ± SEM (n = 4–6). Levels of BDNF mRNA expression were determined 3 h after agonist administration. Subcutaneous administration of sterile water (Veh, 1 mL/kg), naltrindole (NTI, 1 mg/kg) or naltrexone (NTX, 0.1 mg/kg) was conducted 15 min before agonist administration. *P < 0.05 and **P < 0.01 compared with the veh + veh group; #P < 0.05 and ##P < 0.01 compared with the veh + agonist group.