22702
Intraperitoneal ( i . p . ) pretreatment with naloxone ( opioid receptor antagonist ) or methysergide ( 5 - HT serotonergic receptor antagonist ) attenuated antinociceptive effect induced by oleanolic acid in the writhing test .
 naloxone, methysergide, 5 - HT, oleanolic acid
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 naloxone, methysergide, 5 - HT, oleanolic acid