MOR1K activation stimulates NO production. Be2C cells were transiently transfected with MOR1K or MOR1 expressing constructs. (A) MOR1K produced robust concentration-dependent increases in the release of NO. (B) The time course associated with morphine-induced (1 μM) NO release was markedly different in response to stimulating MOR1K and MOR1. NO production was blocked by pretreatment with naloxone (0.1 μM) and was significantly different from the respective controls. Data are presented as mean + SEM. *P < 0.05 different from controls.