The effect of apelin-13 on pain modulation at the supraspinal level was investigated in mice using the tail immersion test. Intracerebroventricular (i.c.v.) administration of apelin-13 (0.3, 0.5, 0.8 and 3 microg/mouse) produced a dose- and time-related antinociceptive effect. This effect was significantly antagonized by the APJ receptor antagonist apelin-13(F13A), indicating an APJ receptor-mediated mechanism. Furthermore, naloxone, beta-funaltrexamine and naloxonazine, could reverse the analgesic effect. However, naltrindole or nor-binaltorphimine could not reverse the effect, suggesting that mu opioid receptor (primarily mu(1) opioid receptor subtype) is involved in the analgesic response evoked by apelin-13. Moreover, i.c.v. administration of apelin-13 potentiated the analgesic effect induced by morphine (i.c.v., 5 microg/kg) and this potentiated effect can be also reversed by naloxone.