Changes in hypothalamic met-enkephalin levels of rats induced by painful stimulation and morphine treatment. 
The possible influence of painful stimulation and morphine analgesia on hypothalamic met-enkephalin levels, and the possible correlation between this biochemical parameter and animal behaviour was studied.  The results indicate that both painful stimulation and morphine treatment induce an increase in hypothalamic metenkephalin.  On the other hand, naloxone did not cause any variation.  In animals submitted to painful stimulus, morphine treatment induced a decrease in hypothalamic met-enkephalin levels.  Finally, when both morphine and naloxone were administrated before nociceptive stimulation, much higher levels were measured.  There was no correlation between the level of hypothalamic met-enkephalin and the pain rating.