In the previous work

we found that DAD 2 receptors in the spinal cord could me diat e significant anti hyperalgesic effect. But the mechanism is not clear. In the present experiments

the intrathecal injection and fluorescent in situ hybridiza tion (FISH) wi th fluorescentimmunohistochemical (FIH) double staining were used to observe the possible relationship between the anti hyperalgesic effects mediated by D 2 r eceptors and central opioid system. First

the effect of i.t. 5 mM naloxone on the anti hyperalgesia of i.t. D 2 rec eptor agonist LY171555 was examined

the results showed that naloxone produced c omplete antagonism on the anti hyperalgesia mediated by D 2 receptors in the s pinal cord

and suggested that opioid receptors may be activated when the D 2 r eceptors in the spinal cord mediated anti hyperalgesia. FISH with FIH double staining was used to look for the direct relationship betwe en D 2 receptors and the opioid system in the spinal cord. In dorsal horn

we o bserved the colocalization of D 2 mRNA and ENK neurons. The results showed that D 2 receptor mRNA and ENK IR double labeled cells were significantly increas ed at 8 hr after i.t. carrageenan. From these results

we concluded that the anti hyperalgesic effect mediat ed by D 2 receptors in the spinal cord is related to the opioid system

D 2 r eceptors may produce a direct action on ENK interneurons.