[MK-801 suppresses dynorphin A (1-17)-induced facilitation of nociceptive responses to formalin in rats].

To explore the facilitation of nociceptive response by dynorphin (Dyn ) A in a model of formalin test in rats, the effects of single intrathecal injection (i.t.) of normal saline (NS), MK-801 (antagonist of NMDA receptor), naloxone (antagonist of opioid receptor), or Dyn A (1-17) were observed, and the effects of i.t. MK-801 or naloxone followed by i.t. Dyn A (1-17) were observed as well. The nociceptive licking and biting induced by injection of formalin exhibited two phases. The first phase lasted for a relatively short period of 3-9 min, and the second phase lasted for a relatively longer period after a 3 to 6- min quietness. The results showed that there were no differences in the first phase in all groups; however, there were differences in the second phase as follows: (1) the duration of nociceptive response was significantly increased in Dyn A (1-17) group (489.5+/-22.5 s) as compared to that of NS group (344.7+/-12.9 s), MK-801 group (331.4+/-20.7 s) or naloxone group (352.5+/-18.4 s) (P<0.01 in three cases); (2) the duration of nociceptive response was significantly shortened in MK-801 plus Dyn A (1-17) group (285.7+/-19.4 s) as compared to that of Dyn A (1-17) group (P<0.01), but there were no significant differences as compared to that of MK-801 group; and (3) there was no significant difference in the second phase between naloxone plus Dyn A (1-17) group (473.8+/-17.8 s) and Dyn A (1-17) group, but the duration of nociceptive response was longer than that of NS group or naloxone group (P<0.01 in both). The results obtained suggest: (1) at the spinal cord, Dyn A (1-17) facilitates nociceptive responses; (2) NMDA receptors, but not opioid receptors, are possibly involved in the nociception by Dyn A (1-17).