21839
Prevention of BTX - A effects on pain and c - Fos expression by opioid antagonists suggest that the central antinociceptive action of BTX - A might be associated with the activity of endogenous opioid system ( involving mu - opioid receptor ) .
 None
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 None